Today, around 2.6 billion people worldwide, or about 40% of the world's population, are affected by being overweight or obese. Unless decisive action is taken to curb this growing epidemic, it is expected that by 2035, more than 4 billion people, or half of the world's population, will be affected by being overweight or obese (according to the World Obesity Federation).
Overweight and obesity are characterized by excessive accumulation of fat mass when energy intake exceeds energy expenditure. One possible way to control energy expenditure is to modulate the thermogenic pathways in white adipose tissue (WAT) and/or brown adipose tissue (BAT).
Adipose tissue is divided into white, brown, and beige colors and has different roles in energy storage, thermogenesis, and metabolism in the body. Environmental factors greatly affect energy metabolism, with diet, exercise, and sleep being the main influencing factors.
Among the different environmental factors that can affect host metabolism and energy balance, the gut microbiota is now considered a key factor. Gut bacteria are involved in two-way communication between the gut and adipose tissue, affecting energy metabolism, nutrient absorption, appetite, and adipose tissue function.
Groundbreaking studies have shown that mice lacking gut microbes (i.e., germ-free mice) or depleting gut microbiota (i.e., on antibiotics) produce less adipose tissue, and many studies have investigated the complex interactions that exist between gut bacteria, some of which are their membrane components (i.e., lipopolysaccharides) and their metabolites (i.e., short-chain fatty acids, endocannabinoids, bile acids, aryl hydrocarbon receptor ligands, and tryptophan derivatives) and their contributions to WAT browning and/or beige fats and changes in BAT activity.
Obesity is associated with a variety of adverse health outcomes, including metabolic disorders such as type 2 diabetes, cardiovascular disease, and certain types of cancer. Therefore, there is an urgent need for new treatment strategies to address the growing prevalence of obesity and its associated health problems.
One promising approach is to modulate the thermogenic pathway in white adipose tissue (WAT) and brown adipose tissue (BAT), which can help control energy expenditure and aid in weight loss (figure below). In addition, the gut microbiota has become a key player in regulating host metabolism and energy balance, and modulating it through a targeted approach may be promising for the treatment of overweight and obesity.
Cani PD, et al., Nat Rev Gastroenterol Hepatol. 2023
In addition to storing lipids (white and beige adipose tissue) and producing heat (beige and brown adipose tissue), human adipose tissue is involved in various metabolic functions through the production of adipokines.
In this article, we will learn about the general physiology of white, brown and beige adipose tissue, the differences between humans and mice, and also understand how the gut microbiota and its different metabolites, their receptors and signaling pathways regulate the development of adipose tissue and its metabolic capacity.
01 Obesity and fat
The number of obese people has doubled
The number of obese people worldwide has almost tripled since 1975, with more than 650 million people now classified as obese with a body mass index (BMI) of ≥30 kg/m2.
Obesity-related comorbidities
Obesity causes more than 4.7 million premature deaths worldwide each year due to the strong association between obesity and its comorbidities, including type 2 diabetes, heart disease, stroke, and an increased risk of several types of cancer. Therefore, it is crucial to develop new therapies that not only reduce excessive weight gain but also limit the risk of disease in people with excess body fat.
How is obesity formed?
Obesity occurs due to a prolonged period of positive caloric intake, where energy intake exceeds energy expenditure, and excess nutrients accumulate within white adipose tissue (WAT) in the form of triglycerides (TG).
What is white adipose tissue and what does it do?
White adipose tissue was initially thought to be a poor connective tissue, but it is becoming increasingly clear that white adipose tissue performs a wide range of important biological functions, and that WAT deficiency is just as harmful to metabolic health as excess fat mass.
As a result, adipose tissue is now considered a central metabolic organ that is closely involved in the systemic regulation of energy metabolism through nutrient exchange and secretion of adipose-derived hormones and cytokines (collectively known as adipokines).
Hagberg CE, et al., Nat Rev Mol Cell Biol. 2023
Human adipose tissue can be broadly divided into:
- Subcutaneous adipose tissue (SAT)
- Visceral adipose tissue (VAT)
- Brown adipose tissue (BAT)
BAT is predominantly found in the neck, supraclavicular area, paravertebral and perirenal regions in thin people. In addition, lean people contain breast (pink) adipose tissue, bone marrow adipose tissue, and dermal white adipose tissue (WAT), the latter of which is anatomically separate from the SAT.
In obese individuals, lipids begin to accumulate ectopically in organs such as the liver and muscles, and to a greater extent in the VAT repertoire, including the omentum, mediastinum, retroperitoneal, omentum, and mesenteric repertoire. Fat also begins to accumulate around the blood vessels as perivascular adipose tissue (PVAT) and pericardial locations around the heart.
Obesity-related pathology can be predicted clinically based on medical history, blood tests, and anthropometric metrics, as well as imaging of ectopic fat accumulation and undesirable adipocyte features.
BMI, Body Mass Index, DXA, Dual-Energy X-ray Absorptiometry, MDCT, Multidetector Computed Tomography, MRI, Magnetic Resonance Imaging, MRS, Magnetic Resonance Spectroscopy, WHR, Waist-to-Hip Ratio.
Complexity and functional diversity of adipose tissue
Adipose tissue is far from an even energy storage chamber. Rather, it embodies a dynamic ecosystem of different cell types whose interactions determine their physiological and pathological roles. The interaction between these cell populations, including adipocytes, macrophages, T cells, fibroblasts, endothelial cells, adipose stem and progenitor cells, neutrophils, and B cells, forms a highly integrated network that coordinates adipose tissue function and homeostasis.
Each fat bank shows its unique functional patterns, cell composition, and disease predisposition, as well as changes in adipocyte types.
Characteristics of obese white adipose tissue
Hagberg CE, et al., Nat Rev Mol Cell Biol. 2023
For example, human subcutaneous adipose tissue (SAT) is primarily composed of white uniloculars (containing a single droplet) adipocytes as well as stromal vascular cells, while perivascular fat pads typically include multiloculars (containing multiple smaller fat droplets) thermogenic beige/brite adipocytes and mammary adipose tissue containing pink adipocytes, which are beneficial for lactation.
Although these specific adipocyte subtypes are important for their respective tissue functions, most human adipocytes in adults reside in WAT and play a role in energy storage.
Adipocyte function and metabolic diseases
Importantly, these are also the fat cells most closely associated with the pathology caused by obesity in humans. Due to the large differences in anatomical, physiological, and pathological features between human and rodent fat banks, the current article will focus primarily on evidence from human studies with the aim of elucidating which adipocyte functions are well established and which areas require more research.
The results of genome-wide association studies and transcriptional analysis combined to confirm the critical role of adipocyte and WAT distribution in maintaining metabolic health during obesity and highlight the development of adipocyte dysfunction as the first step leading to metabolic disease. This includes the established role of adipose tissue in insulin resistance and the development of type 2 diabetes, which is briefly discussed above, and we recommend readers to read some excellent recent reviews for a more in-depth analysis.
Hagberg CE, et al., Nat Rev Mol Cell Biol. 2023
Weight gain and obesity are associated with an increased risk of various chronic, physical, and psychiatric diseases, some of which are described.
Effects of obesity and adipocyte dysfunction on cardiovascular disease, cancer, and reproduction.
Cardiovascular: Perivascular adipocyte dysfunction alters the secretion of adipokines and pro-inflammatory cytokines, thereby promoting vasoconstriction, smooth muscle cell proliferation, and endothelial cell leakage, exacerbating many of the pathological changes found in atherosclerosis, thereby increasing the risk of developing symptomatic cardiovascular disease.
Cancer: Cancer cells work with cancer-associated fat cells to promote the release of free fatty acids (FFAs) from neighboring fat cells, which promote cancer cell proliferation and migration. In addition, fat cells promote cancer growth by releasing cytokines, extracellular matrix (ECM) proteins, and hormones. Cancer-associated adipose tissue has also been linked to treatment resistance. Fat cells, as well as the associated fibrous ECM, can hinder the delivery of cancer drugs. Adipose tissue in obese individuals is also characterized by chronic, low-grade inflammation and increased oxidative stress.
Reproduction: Dysfunctional white adipose tissue altered adipokine secretion can directly impair reproductive organ function and lead to polycystic ovary syndrome (PCOS) and decreased fertility. Obesity also increases the risk of gestational diabetes.
IL-6, interleukin-6, PAI1, plasminogen activator inhibitor 1, TNF, tumor necrosis factor.
02 Types of adipose tissue
For a long time, adipose tissue was just a place where excess energy was passively stored in the form of fat. However, studies have shown that it is an active, dynamic endocrine organ that secretes hormones that play a vital role in regulating metabolism and other physiological processes in the body.
Adipose tissue in the human body can be divided according to its location (subcutaneous and visceral) or according to its morphology (WAT or BAT), and each fat bank has its own unique physiological and metabolic characteristics.
What are fat factors used for?
Adipokines can act locally (autocrine, paracrine) or into the bloodstream to act on distant organs and tissues (endocrine).
Adipokines play a vital role in regulating various physiological processes in the body, including energy metabolism, insulin sensitivity, appetite regulation, lipid metabolism, reproduction, and immune and cardiovascular function.
Adipokine dysregulation, which is often observed in obesity and metabolic disorders, contributes to the development of a variety of chronic diseases, including insulin resistance, inflammation-related diseases, cardiovascular disease, and cancer.
Adipokines are differentially expressed in different fat compartments
For example, visceral adipose tissue (VAT) is higher in vistatin, omentin, chemerin, BMP2, BMP3, and RBP4 compared to subcutaneous adipose tissue (SAT), and adiponectin, leptin, resistin, adipsin, and progranulin are higher in SAT.
A variety of cells secrete 600+ adipokines
Adipokines can be secreted by mature adipocytes as well as stromal vascular cells (including adipocyte precursor cells, endothelial cells, macrophages, foam cells, neutrophils, lymphocytes, fibroblasts, etc.). More than 600 potential secreted proteins have been identified from adipose tissue.
The selection of adipokines expressed by adipocytes, their key biological roles, and the changes in expression levels with obesity are shown in the table. To provide a more in-depth review of adipokines, including those expressed by non-adipocytes, and their role in health and pathology.
Hagberg CE, et al., Nat Rev Mol Cell Biol. 2023
Restriction of subcutaneous fat expansion, which may result in ectopic fat deposits, is associated with obese lesions
It has been proposed that when subcutaneous adipose tissue (SAT) expansion is impaired, especially when hyperplasia is restricted, it leads to ectopic fat deposits in the liver and skeletal muscle, leading to the pathogenesis of obesity-related diseases.
Persistent metabolic alterations can lead to a change in adipose tissue from healthy to dysfunctional, with systemic consequences.
The increase in subcutaneous fat in the human body has a neutral or beneficial effect on metabolism
There is a growing recognition that subcutaneous fat may have a protective effect on metabolism. In line with this, human trials have shown that high-volume liposuction of subcutaneous WAT provides little to no metabolic advantage.
Evidence from mouse models further suggests that transplantation of subcutaneous WAT into the visceral cavity of recipient mice results in a decrease in body weight, total fat mass, glucose and insulin levels, and improves insulin sensitivity, whereas transplanted visceral fat has no effect. These data suggest that subcutaneous fat is essentially different from visceral fat.
Different types of adipose tissue and fat cells
Cani PD, et al., Nat Rev Gastroenterol Hepatol. 2023
White adipose tissue contains unilocular, white adipocytes characterized by a single large lipid droplet and a small number of mitochondria.
Brown adipose tissue is made up of brown adipocytes with small multilocular lipid droplets and high mitochondrial density.
White adipocytes can take on brown-like features in response to specific stimuli, such as cold exposure, a process called white adipose tissue beige.
White fat
Subcutaneous adipose tissue is the most abundant type of adipose tissue in the body and is where excess energy is stored in the body in the form of triglycerides.
White adipose tissue WAT is made up of adipocytes, which are specialized cells that store and release lipids based on the body's energy needs. In addition to adipocytes, WAT contains stromal cells, immune cells, and extracellular matrix components.
This white adipose tissue is mainly located in two places
- One is subcutaneous, which is the layer of fat under the skin
- One is visceral fat, which surrounds our internal organs
The adipose tissue under the skin is more active, and the adipose tissue that surrounds the internal organs is more directly related to some metabolic problems in the body, such as insulin resistance.
Different types of adipose tissue in humans and rodents
Cani PD, et al., Nat Rev Gastroenterol Hepatol. 2023
Like humans, adipose tissue in mice is made up of multiple reservoirs. Subcutaneous white adipose tissue (WAT) is found under the skin throughout the body, while visceral WAT wraps around the organs of the abdomen.
However, humans have two main subcutaneous fat reservoirs located in the abdominal and glutemoral regions, while mice have two major subcutaneous fat pads located anterior and posterior. In adults, most thermogenic beige adipose tissue (BAT) repositories are located in the supraclavicular region of the neck. In contrast, the interscapular reservoir is the predominant BAT in mice.
Notably, BAT is more pronounced and visible in adult mice compared to adults. The gonadal WAT repertoire is located near the ovaries and testes and is often used as a proxy for visceral WAT in research.
Within these adipose tissue categories (VAT and subcutaneous WAT), there are multiple storage isotypes. Visceral adipose tissue VAT includes epicardial adipose tissue, perirenal adipose tissue, retroperitoneal adipose tissue, and mesenteric adipose tissue located in the gastrointestinal tract.
These libraries have different anatomical locations, cellular characteristics, metabolic functions, and health implications
For example, the adipose tissue around the kidneys primarily acts as a cushion and heat insulator. It also affects kidney function and blood pressure regulation by secreting adipokines and pro-inflammatory cytokines.
On the other hand, mesenteric adipose tissue plays a role in intestinal immunity, barrier function, and nutrient absorption. It also regulates intestinal motility, secretion, and hormone release by interacting with the enteric nervous system and gut microbiota.
WAT repertoire in the human body and its relationship to metabolic pathology
Subcutaneous adipose tissue (SAT) is the preferred site for storing excess fat, a process that helps protect individuals from metabolic diseases, even in obesity.
SAT expansion protects metabolism, while excess can lead to ectopic lipid deposition and disease
A major determinant of metabolic health is the SAT's ability to safely expand in response to energy needs and store excess fat. A decrease in SAT, as occurs in lipodystrophy syndrome (a systemic or partial deficiency of adipose tissue), leads to an increased cardiometabolic risk due to a reduced ability to buffer excess lipids.
Conversely, when SAT storage capacity is exceeded during obesity, free fatty acids (FFAs) accumulate at VAT and ectopic sites.
Ectopic lipid deposition—an independent risk factor for metabolic disease
Ectopic sites include tissue-resident adipocytes (e.g., epicardial and pancreatic adipose tissue), circulating and nonadipocytes (e.g., lipid accumulation in skeletal muscle and hepatocytes). Lipid accumulation at these sites triggers lipotoxicity, leading to abnormal glucose metabolism, systemic insulin resistance, and inflammation. Thus, ectopic lipid deposition, of which VAT expansion is a hallmark, is an independent risk factor for obesity-related cardiovascular and metabolic disease.
WAT libraries differ from each other in their ability to store and release fatty acids and produce adipokines. In addition, their cellular characteristics also vary by regional distribution, including differences in cell composition, innervation, metabolism, vascularization, extracellular matrix (ECM) composition, and the repertoire of secreted factors.
The location and amount of body fat play an important role in determining the risk of disease
Fat accumulation in the upper body (i.e., VAT, but to some extent, abdominal SAT) increases the risk of diabetes, hypertension, atherosclerosis, dyslipidemia, and cancer, while peripheral or lower body fat (subcutaneous hips and femur) may be metabolically protective.
How did these differences come about?
Differences between reservoirs may be due to the microenvironment, such as unique innervation and vascular aspects (e.g., venous drainage of visceral fat into the portal venous circulation and bathing the liver in by-products of fat metabolism and adipokines), and/or due to intracellular differences.
Storage differences may also be due to differences in the phenotype of preadipocytes, as WAT can be expanded by the differentiation of preadipocytes into new adipocytes (hyperplasia) or by pre-existing adipocyte enlargement (hypertrophy). In line with this, the genetic characteristics and differentiation potential of preadipocytes have been shown to differ between different adipose banks, which remain unchanged even after isolation and long-term culture under the same conditions, suggesting strong epigenetic regulation of adipocyte traits.
The relative distribution of WAT between different warehouses is determined by a variety of factors, including sex, genetics, age, disease state, food intake, and response to medications and hormones.
Gender: Women are more likely to be obese?
Obesity rates are generally higher in women than in men, and fat is stored more efficiently in the periphery (female fat distribution) than in the center (robotic fat distribution). Men tend to have more intrahepatic fat accumulation, which is associated with higher insulin resistance in men than in women.
However, this changes after menopause in women, where fat storage becomes more concentrated and metabolic risks become similar to those in men. Testosterone impairs adipogenesis, estrogen stimulates preadipocytes, and progesterone stimulates differentiation, which may be related to the fact that many women are more likely to be obese after giving birth.
Genetic and environmental factors can also affect fat distribution
There are differences in visceral fat distribution between different ethnic groups, with South Asians having more central obesity than people of European descent, whites having more visceral fat than African-Americans and Hispanics, and continental northern populations having more subcutaneous fat than Southern.
Age is associated with a preferential increase in abdominal fat as well as a decrease in SAT in the lower body
As we age, fat deposits in ectopic sites such as the heart, liver, and skeletal muscle increase, increasing the risk of insulin resistance and cardiovascular disease.
In conclusion, although it is still difficult to control the relative distribution of fat between stores, it is clear that the increase in triglyceride stores in VAT is a surrogate marker of ectopic lipid deposition and is therefore strongly associated with metabolic diseases, while SAT accumulation is neutral or can even have some beneficial effects on general health and metabolism.
Features of dysfunctional WA
WAT is a highly dynamic organization. During obesity, several morphological features of WAT are significantly altered, including adipocyte size, tissue inflammation, vascularization, and extracellular matrix composition. These morphological changes are closely related to pathology and are important tissue-level markers of WAT dysfunction.
★ Fat cell size and number
During weight gain, adipose tissue needs to store more energy, which usually manifests as triglycerides. Adipocytes can adapt to this increased energy storage demand in two ways:
- Hypertrophy: The existing fat cells increase in size because they store more fat (triglycerides)
- Hyperplasia: An increase in the number of new fat cells in adipose tissue.
Both of these mechanisms are the body's way of regulating energy reserves and fat tissue size.
Fat cells – hypertrophy first, hyperplasia later?
In some cases, fat cells may first adapt to increased fat storage needs by hypertrophy, but when they reach a certain volume limit, adipose tissue expands further through hyperplasia. This process is complex and regulated by a variety of factors, including genetics, diet, lifestyle, and hormones, among others.
In early weight gain, fat cell size increases linearly with BMI
Studies have shown that early human weight gain is primarily achieved through an increase in the size of fat cells, while morbid (extreme) obesity is further associated with proliferative tissue expansion.
A meta-analysis of more than 89 studies comparing human adipocyte size showed that adipocyte size increased linearly with BMI.
An increase in the size of fat cells – one of the most consistent markers of metabolic dysfunction
Adipocyte hypertrophy is strongly associated with metabolic and cardiovascular disease, and hypertrophy, rather than obesity itself, is a strong predictor of type 2 diabetes. This contribution to pathology is especially pronounced in visceral fat.
In severely obese women, visceral adipose tissue (VAT) hypertrophy is associated with insulin resistance and hypertension, while subcutaneous adipose tissue (SAT) cell size is associated only with hypertension.
The relationship between fat cell size and metabolic disease
Other studies have confirmed the relationship between visceral adipocyte size and metabolic dysfunction, including insulin resistance, glycosylated hemoglobin, and dyslipidemia.
Treatments that increase the number of small fat cells (by promoting lipogenesis and hyperplasia) can significantly improve metabolic function in patients with type 2 diabetes.
Large fat cells secrete more inflammatory factors, including IL-6, IL-8, MCP1, and leptin, which are associated with the development of a variety of inflammation-related diseases, such as diabetes and cardiovascular disease.
In addition, the ability of large fat cells to take up glucose in response to insulin stimulation is also impaired. This damage includes defects in the transport of GLUT4 to the cell membrane.
Note: GLUT4 is an insulin-dependent glucose transporter responsible for transporting glucose from the blood into cells for use. In large adipocytes, impaired GLUT4 function leads to reduced glucose uptake, which is part of the pathogenesis of diabetes.
★ Chronic low-grade inflammation
Obesity is characterized by the presence of chronic low-grade inflammation in adipose tissue. Adipocytes themselves can secrete pro-inflammatory molecules, which are increased in obese individuals with hypertrophic WAT, leading to the recruitment and activation of immune cells, which further amplifies inflammation. This chronic inflammation can lead to insulin resistance, cardiovascular disease, and other comorbidities.
For example, in rodents, tumor necrosis factor (TNF) secretion can directly reduce insulin sensitivity in adipocytes.
Why is "crown-like structure" more common in obese mice and less common in humans?
"Crown-like structures" are ring-like aggregates of macrophages and other immune cells that surround dying fat cells in an attempt to engulf the lipids of these cells in order to prevent them from leaking into the blood circulation.
- In the obese state, the number of macrophages in the adipose tissue around the gonads of mice can account for more than 50% of all cells.
- In the body's white adipose tissue, the proportion of macrophages is usually low, between 10-20%.
Is obesity-related insulin resistance not related to adipose tissue inflammation?
Emerging data also suggest that insulin resistance in obese patients is not associated with adipose tissue inflammation. This may underscore the failure of clinical strategies to treat metabolic diseases by targeting WAT inflammation, which somewhat weakens the view that adipose tissue inflammation is a causative factor that triggers obesity-induced metabolic diseases in humans.
However, recent studies have shown that signaling pathways targeting certain inflammatory mediators such as MCP1 have shown some positive effects in the treatment of diabetic patients. This suggests that it may be too early to completely rule out inflammation as a therapeutic goal to improve white adipose tissue dysfunction.
At the same time, it should also be recognized that WAT inflammation has a physiological tissue modulation role to a certain extent. While this is more evident in mice, there are also examples in humans, such as the presence of macrophages in healthy omentum and mesenteric adipose tissue, which may well contribute to the immune defenses of the gut.
In summary, although the role of adipose tissue inflammation in obesity and metabolic diseases may not be as immediate or important as previously thought, inflammation and immune response still play a role in this process and may be a potential target for future therapeutic strategies. At the same time, the inflammatory response in adipose tissue also has a physiological positive effect, which needs to be considered in the design of treatment strategies.
★ Angiogenesis and vascularization
In the lean state, human adipose tissue is well vascularized, and each adipocyte is adjacent to at least one capillary microvessel. The expansion of adipose tissue requires the simultaneous formation of new blood vessels from existing blood vessels (angiogenesis) in order to provide nutrients and oxygen to the dilated tissue.
Early weight gain is carried out by increasing the secretion of angiogenesis, growth factors, and vascular budding. However, as the intrinsic adipocytes enlarge, the distance between the microvessels increases, and the vascular density of the tissue gradually decreases.
In addition, enlarged adipocytes reduce the secretion of pro-angiogenic factors, such as vascular endothelial growth factor A (VEGFA), further reducing oxygen diffusion and leading to a decrease in the relative oxygen pressure of tissues during obesity. This leads to reduced angiogenesis potential of obese WAT (white adipose tissue), decreased capillary density (capillary thinning), and even vascular degeneration.
In addition, reduced WAT oxygenation activates hypoxia-inducible factor 1A (HIF1A) and downstream hypoxic response pathways (e.g., NF-κB pathway) and promotes the secretion of pro-inflammatory factors and fibrotic collagen. Thus, adipose tissue inflammation can be seen as a symptomatic manifestation of a tissue-hypoxic environment, which may explain why targeting pro-inflammatory cytokine signaling alone is not enough to improve overall tissue function and metabolic health. In addition to increasing inflammation, an increase in tissue hypoxia activates pro-fibrotic sclerosis of adipose tissue in obese individuals.
★ Adipocellular extracellular matrix (ECM) changes
All adipocytes are surrounded by a rich three-dimensional extracellular matrix (ECM) network that provides mechanical support to adipocytes, helps maintain the structure of adipose tissue, and plays an important role in cell signaling.
Role and structure of the extracellular matrix
In human adipose tissue, the extracellular matrix network is mainly composed of several types of collagen, especially COLI-VII, in addition to components such as laminin, fibronectin, hyaluronic acid, elastin, and glycosaminoglycans, which are produced by adipocytes and tissue stromal cells.
In healthy adipose tissue, the extracellular matrix undergoes a continuous process of remodeling, in which components of the extracellular matrix are synthesized and degraded in order to accommodate changes in fat mass, cell signaling, and tissue vascularization.
The effects of obesity on adipose tissue remodeling
However, in the obesity state, this ability of the extracellular matrix to remodel is impaired, resulting in an excessive accumulation of collagen in the extracellular matrix surrounding certain cells, which may eventually lead to fibrosis of adipose tissue. These include collagen 6 (COL6), which in mouse studies can limit fat cell expansion, and this limiting effect may directly lead to dysfunction of adipose tissue during weight gain.
Obese subjects with high levels of fatty ECM were likewise shown to have a reduction in weight loss after bariatric surgery. In addition to reduced ECM remodeling, obesity is associated with higher levels of cross-linking between ECM components in WAT, which increases tissue stiffness. Higher WAT hardness is associated with insulin resistance, impaired glucose metabolism, and increased inflammation, possibly because it prevents necessary tissue remodeling.
Obesity-related extracellular matrix abnormalities and their consequences
Of note, the degree of ECM cross-linking and tissue stiffness may be more determinant of WAT dysfunction than the level of ECM accumulation. Several studies have found that metabolically unhealthy obese subjects have reduced overall levels of ECM deposition in adipose tissue compared with lean subjects, and expression of certain ECM components (e.g., COL1 and fibronectin) is reduced, while tissue stiffness and associated transcripts are increased in obese people.
This can be explained in part by immune cells that secrete matrix metallopeptidase and other enzymes that promote ECM degradation in obesity, facilitating immune cell invasion of WAT while promoting adipocyte expansion through degradation of restrictive ECM. This hypothesis remains to be thoroughly tested, but could explain why many pathological changes in WAT morphology (hypertrophy, inflammation, ECM remodeling) often occur at the same time. The main upstream triggering and driving factor for these pathological ECM changes is thought to be a decrease in oxygenation in adipose tissue. Hypoxia also directly exacerbates inflammation, further accelerating ECM changes and the development of WAT fibrosis.
In conclusion, extracellular matrix changes in adipose tissue can lead to dysfunction of adipocytes, impaired insulin sensitivity, chronic inflammation, and other metabolic abnormalities associated with obesity-related comorbidities.
Brown Fat (BAT)
Brown adipose tissue (BAT) contains less than white adipose tissue and is mainly located in the supraclavicular and interscapular regions of the body, and its distribution varies widely between individuals. It is made up of multiloculars of adipocytes that contain a large number of cytoplasmic lipid droplets with a central nucleus and mitochondria, giving them their characteristic brown color.
Note: Multilocular adipocytes are those that contain multiple lipid droplets, usually associated with cells in brown adipose tissue. These cells are different from unilocular adipocytes, which refer to cells in white adipose tissue (WAT) that contain a single large lipid droplet.
This particular type of adipose tissue is responsible for generating heat by burning stored lipids through a process called non-shivering thermogenesis. This process is achieved through high expression of uncoupling protein 1 (UCP1), an inner mitochondrial membrane protein responsible for the uncoupling of respiratory and thermogenic activities. BAT is more metabolically active than WAT and has been shown to have a role in regulating energy homeostasis and glucose metabolism.
In addition, inducible non-MYF5-expressing brown adipocyte progenitors were found between the WAT repertoire and muscle fascicles of mice.
In 2023, two supplementary papers demonstrated that adipose progenitors from different human fat repositories, including BAT and WAT, have similar transcriptomes, suggesting that there is a common progenitor cell. These progenitor cells differentiate into one of two major cell fates: adipocytes or pluripotent cells, known as structural WNT-regulated adipose tissue-resident (SWAT) cells, which provide a pool of progenitor cells that are maintained throughout life. Researchers believe that the delicate balance between these two cell fates (differentiated adipocytes and undifferentiated pluripotent progenitor cells) may be a determinant of adipose tissue composition and function.
Beige fat
Beige adipose tissue (sometimes called light adipose tissue) is a type of adipose tissue that sits somewhere between WAT and BAT. Beige adipose tissue is found in some WAT repertoire and has the morphological and functional characteristics of BAT, such as the ability to burn stored lipids and produce heat.
Thermogenesis can be induced within adipose tissue to regulate energy homeostasis and fight the development of obesity, which has led to a high level of interest in the identification of so-called browning agents, i.e., conditions or reagents that can increase the number or activity of UCP1.
Although animal (mouse and rat) studies have yielded promising results that beige adipocytes can be induced in response to a variety of stimuli, including cold exposure, exercise, and certain pharmaceutical preparations, their pathophysiological relevance remains unclear, as the thermogenic capacity associated with beige browning may be of only minor physiological importance compared to classical BAT.
03 Dysfunction of obese adipocytes
While the hallmarks of WAT dysfunction listed above are well established and readily measurable in human WAT biopsies, the functional changes associated with obesity remain less clear.
Reduced signal sensitivity
Functionally, one of the early changes in mast adipocytes is resistance to physiological insulin signaling along with decreased responsiveness to many other exogenous signals, such as adrenergic stimulation and signaling of the beneficial metabolic regulator fibroblast growth factor 21 (FGF21). This has a wide range of effects on adipocyte function, most notably the dysregulation of fatty acids released by adipocytes through lipolysis.
Obese → insulin does not properly limit lipolysis → lipid leakage
In a healthy postprandial state, insulin effectively limits basal lipolysis and hormone-induced lipolysis. Between meals, catecholamines induce the breakdown of fat in order to provide nutrients to muscles and other organs.
The occurrence of insulin resistance in fat cells during obesity reduces the ability of fat cells to hydrolyze after insulin restricts food intake. This dysregulation of lipolysis leads to a sustained low-grade leakage of fatty acids from the fat adipose tissue into the circulatory system and early accumulation in the liver.
As lipid leakage continues, fatty acids released from adipose tissue also begin to accumulate in other organs, such as skeletal muscle, pancreatic β cells, and renal foot cells, as well as in the walls of larger blood vessels as part of oxidized lipoproteins.
The obesity paradox: decreased fat release and weight gain
Paradoxically, the reduced signal sensitivity of fat cells in obesity also makes them insensitive to the opposite signal, i.e., induction of lipolysis by adrenaline stimulation during fasting. This decrease in signal sensitivity is due, at least in part, to the downregulation of adrenergic receptors on the surface of obesity-associated adipocytes, resulting in a decrease in the level of hormone-induced lipolysis, despite increased basal lipolysis.
Hagberg CE, et al., Nat Rev Mol Cell Biol. 2023
The relative importance of these two opposing changes, one due to reduced insulin signaling and the other due to reduced adrenaline signaling, is difficult to determine in humans, but recent studies have provided some insight: longitudinal studies have shown that reduced levels of adrenaline-stimulated lipolysis are associated with an increased tendency to store lipids for longer periods of time, potentially leading to weight gain and the development of impaired glucose tolerance.
Importantly, in women with weight gain, hormone-driven reductions in lipolysis levels are more closely associated with poor basal metabolic health than increases in basal (insulin-controlled) lipolysis levels. Several studies measuring fatty acid flux have also noted that the rate of fatty acid release per kilogram of fat decreases with body weight, again suggesting that the total lipolytic capacity of fat cells in obesity is impaired.
This may explain why, despite the large increase in fat mass, plasma FFA levels in obese subjects increased only slightly compared to lean individuals. In addition, a meta-analysis found no correlation between fatty acid levels and measures of insulin resistance, which calls into question the dogma that insulin resistance leads to adipose tissue lipolysis in obesity. These studies suggest that adrenaline-stimulated decreased lipolysis is a major phenotypic feature of adipose tissue in obese humans, which may explain the paradoxical paradox of sustained expansion of adipose tissue during obesity (indicating reduced fatty acid release), despite higher levels of basal fatty acid release.
Defective lipid storage
Adipose tissue in obese people has problems storing fat
In addition to changes in fatty acid release in the adipose tissue of obese individuals, the storage capacity of WAT is also negatively affected by the reduced rate of lipid uptake in the circulation. Studies using stable isotopes or measuring visceral (abdominal) adipose tissue clearance have shown that obese people have lower fatty acid intake on the SAT.
Decreased levels of lipoprotein lipase, which can lead to obesity and metabolic syndrome
A more comprehensive understanding of the mechanisms by which obesity mediates lipid storage defects is needed, but an established molecular mechanism involves the lipase lipoprotein lipase (LPL). LPL is the primary lipolytic enzyme involved in the utilization of fatty acids from circulating TG-rich lipoproteins, such as chylomicrons and very low-density lipoprotein VLDL.
Women with obesity and metabolic syndrome often have lower levels of lipoprotein lipase (LPL) in their bodies. LPL is an important enzyme that plays a key role in lipid metabolism and can help break down triglycerides in the blood, so that fatty acids can be absorbed by muscle and adipose tissue and used for energy. Higher levels of LPL are generally considered to have a protective effect because it helps maintain the normal functioning of lipid metabolism and reduces fat accumulation, thereby reducing the risk of developing metabolic diseases.
There are significant differences in the rate of lipid metabolism in adipose tissue between different populations and individuals
Studying lipid turnover in adipose tissue by measuring the incorporation of the stable isotope deuterium, a heavy isotope of hydrogen, found racial differences and storage patterns in lipid metabolism in the human body. Specifically, they found that triglyceride (TG) synthesis was significantly lower in insulin-resistant individuals compared to insulin-sensitive individuals.
The study found that obese people had a reduced rate of TG removal in subcutaneous adipose tissue (SAT) compared to lean people, reflecting a slowdown in lipolysis and subsequent fatty acid oxidation processes, while their lipid storage rate increased, i.e., the fat tissue absorbed more lipid mass per year.
The rate of lipid turnover in adipocytes is strongly correlated with the risk of metabolic disease
In vitro experiments, the lipolysis of isolated adipocytes when stimulated was positively correlated with the removal of TG measured by radiocarbon dating in vivo. This further confirms that lipolysis determines the rate of lipid turnover in adipocytes by regulating the rate of TG removal. Therefore, high lipid storage and low TG removal promote the accumulation of adipose tissue, which in turn leads to obesity. Conversely, if a person has a low rate of TG removal and storage (as observed in patients with familial mixed hyperlipidemia), this results in a reduced ability of fat cells to store and release fatty acids.
Decreased TG turnover may lead to an abnormal accumulation of fatty acids in the liver, causing lipidemia
Decreased TG turnover in adipose tissue may promote ectopic deposition of fatty acids in the liver, resulting in dyslipidemia. Therefore, lipid turnover in adipocytes has become a new target for the prevention and treatment of metabolic diseases.
Together, these results depict the loss of the ability of obese adipocytes to efficiently take up dietary sources of fatty acids, as well as the ability to retain and release fatty acids from adipose tissue between meals, resulting in an overall severe deficit in the ability to store fatty acids as triglycerides during eating.
It should be noted that, in addition to lipid storage defects, obese adipocytes exhibit pervasive dysfunctions of intracellular energy metabolism and substrate utilization. For example, studies have shown that fat cells in metabolically unhealthy individuals exhibit depletion of metabolites in the citric acid cycle, as well as significant alterations in tissue amino acid levels and expression of amino acid catabolic enzymes.
Defects in lipid signaling
Lipid signaling refers to a complex biochemical process in which lipids, such as fatty acids, play a key role in cellular communication and the regulation of various physiological and pathological processes. Of particular importance is the effect of obesity on lipid signaling pathways, which may pave the way for the emergence of harmful diseases such as fatty liver and hyperlipidemia.
Insulin resistance-induced lipolysis disorders and health effects
As mentioned above, one of the key players in lipid signaling is insulin. Insulin resistance impairs the ability of fat cells to effectively regulate lipolysis in response to insulin. As a result, the level of FFA released by fat cells increases, flooding into the bloodstream, leading to lipotoxic phenomena.
How excess fat can lead to liver disease and dyslipidemia
The excess fatty acids enter the liver, where they are reassembled into triglycerides. This influx overwhelms the liver's ability to process fat, resulting in a buildup of lipids within liver cells, known as fatty liver or hepatic steatosis. In addition, obesity disturbs the balance between different adipogenic factors, such as adiponectin and leptin.
Changes in adiponectin and leptin levels due to obesity
Adiponectin usually has anti-inflammatory and insulin-sensitizing effects, but decreases in obesity. Conversely, leptin, which regulates appetite and energy expenditure, may also be dysregulated, leading to disruption of lipid homeostasis. An intricate network of lipid signaling pathways is also involved in transcription factors such as peroxisome proliferator-activated receptors (PPARs) and sterol regulatory element-binding proteins (SREBPs).
Metabolic Commander Error: Obesity Affects Key Transcription Factors
In obesity, these transcription factors are misexpressed, leading to upregulation of genes involved in fatty acid synthesis and storage, while inhibiting genes responsible for fatty acid oxidation. The end result of these disruptions leads to hyperlipidemia – a condition characterized by elevated levels of lipids in the blood.
Excess Fat, Impaired Health: Hyperlipidemia and Obesity Metabolic Problems
Triglycerides and cholesterol, which are transported by lipoproteins, become abundant, increasing the risk of atherosclerosis and cardiovascular disease. In addition, obesity and associated adipocyte hypertrophy can also trigger the release of pro-inflammatory cytokines and chemokines, laying the foundation for a chronic low-grade inflammatory state and further exacerbating lipid dysregulation.
Therefore, the multifactorial influence of obesity on lipid signaling pathways has a profound impact on metabolic homeostasis, and the resulting lipid dysregulation lays the foundation for the development of fatty liver and hyperlipidemia.
Altered adipokine secretion and aging
Weight gain is known to significantly reduce the secretion of the beneficial adipogenic factor adiponectin while increasing the secretion of leptin. In addition, the size of SAT adipocytes is linearly correlated with increased secretion of a large number of other pro-inflammatory cytokines, including IL-6, IL-8, MCP1, and TNF5. These cytokines can not only promote the infiltration and activation of immune cells (including macrophages, T cells, and neutrophils) in adipose tissue, but also impair adipogenic differentiation, induce insulin resistance in adipocytes and cytokine leakage into the circulation, and promote systemic metabolism.
Risk of induced inflammation and comorbidities
Different adipose tissue banks carry a greater or smaller risk of disease. For example, intramuscular fat carries a greater risk of cardiometabolic disease than SAT (and VAT), which is partly attributed to differences in the metabolic and secretory properties of adipocytes.
One of the emerging mechanisms behind altered adipokine secretion in obesity is premature cellular senescence or senescence, which is also known to affect cellular metabolism. In addition to preadipocyte and endothelial cell senescence that has been well established in human adipose tissue. In addition, immune cell senescence (T cells, macrophages) has also been reported to accumulate VAT in mice and human obesity.
To gain a deeper understanding of these cell types, readers can read recent reports that hyperinsulinemia promotes aging and increases cytokine release from mature human adipocytes as part of the senescence-associated secretory phenotype (SASP).
Reduces fat cell aging – reduces WAT inflammation
Senescent cells avoid apoptosis by enabling the senescence anti-apoptotic pathway. A class of compounds known as senolytics has been found to induce apoptosis in senescent cells and have beneficial effects on the health of mice and humans.
Brief systemic administration of two anti-aging drugs, dasatinib and quercetin, can reduce the burden of senescent cells in the adipose tissue of patients with diabetic nephropathy, which is known to increase the burden of senescent cells in adipose tissue. Others have reported similar results in mouse and human studies. Therefore, reducing adipocyte aging may be an effective way to reduce WAT inflammation during hypertrophic obesity.
04 Environmental factors affecting energy metabolism
Energy metabolism is a complex process that involves converting food into a usable form of energy for the body. Accurate regulation of energy metabolism is essential for maintaining energy balance and preventing the development of obesity and related metabolic disorders. While intrinsic biological factors such as age, gender, and genetics certainly play a role in energy metabolism, environmental factors such as diet, exercise, and sleep also play a large role (figure below).
Notably, the gut microbiota can also play a role in regulating adipose tissue metabolism and thermogenesis, and the composition and function of the gut microbiota may differ between humans and mice. These differences highlight the importance of studying human and mouse models to fully understand the role of adipose tissue in metabolic health and disease.
Cani PD, et al., Nat Rev Gastroenterol Hepatol. 2023
The gut microbiota is a dynamic ecosystem whose composition and function are influenced by a variety of environmental and external factors such as diet, smoking, drug use, sleep, exercise, and emotional stress. However, the degree of regulation caused by changes in these factors can vary greatly between individuals.
This variability is influenced by the initial composition of the gut microbiota as well as intrinsic factors such as age, biological sex, and genetic predisposition. This delicate balance is the result of a complex interplay between our lifestyle choices and inherent traits, and any disturbance can profoundly affect our overall health.
Drink
Diet is one of the most important environmental factors affecting energy metabolism. In addition to the number of calories consumed, studies have shown that diet quality can significantly affect energy intake, energy expenditure, and energy metabolism.
Foods rich in fiber and protein tend to be more satiety and can reduce overall calorie intake. In contrast, highly processed and energy-dense foods tend to lead to excess. Additionally, calorie sources can affect appetite and food choices, and high-fat diets may increase hunger pangs and promote the consumption of high-calorie foods.
The quality of the diet also affects energy expenditure
Physical activity and exercise are important factors in energy expenditure, but the thermal effect (TEF) of food also accounts for about 10% of total energy expenditure.
TEF is the energy needed to digest, absorb, and metabolize food; it varies from person to person and depends on physical activity level and macronutrient composition of the diet. Protein has a higher TEF than carbohydrates or fats, which means that a high-protein diet may increase energy expenditure compared to a low-protein diet.
Diet can also affect energy metabolism
Diets high in sugar and refined carbohydrates are associated with insulin resistance and impaired glucose metabolism, affecting the body's ability to use energy efficiently. Conversely, a diet rich in fiber, whole grains, fruits, and vegetables can improve insulin sensitivity and promote more efficient energy utilization.
Tempering
Exercise can increase energy expenditure and improve metabolic health by promoting the development of lean muscle mass, improving insulin sensitivity, and reducing inflammation. In addition, exercise can increase the expression of genes involved in energy metabolism.
AMPK pathway
One important pathway is the AMP-activated protein kinase (AMPK) pathway, which is activated during exercise and increases glucose uptake and fatty acid oxidation in muscle cells. AMPK also regulates mitochondrial biogenesis and oxidative metabolism, improving energy metabolism and metabolic health.
PGC1α pathway
Another important pathway is peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α) pathway, which is involved in mitochondrial biogenesis and oxidative metabolism. Exercise can increase PGC1α expression, which increases mitochondrial biosynthesis and improves energy metabolism.
Exercise can directly affect the production of specific bioactive lipids in brown adipose tissue
It has been suggested that the substances released by this tissue during exercise are a possible mechanism for some of the health benefits associated with regular physical activity. Through lipidomic analysis, the researchers found that a period of moderate-intensity exercise significantly increased the levels of the circulating linoleic acid metabolite 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME) (P < 0.05) in men, women, young adults (24-42 years), and older adults (65-90 years), as well as those who exercise regularly or lead a sedentary lifestyle.
12,13-DiHOME is a brown adipose tissue-derived metabolite that is also released in response to cold. However, in the context of exercise, studies in mice have shown that both single exercise and regular exercise training directly increase circulating 12,13-diHOME levels from brown adipose tissue. If brown adipose tissue is surgically removed, the increase in 12,13-diHOME disappears. In addition, administration of 12,13-diHOME to mice results in enhanced uptake and oxidation of fatty acids in skeletal muscle, but does not affect glucose uptake.
These findings suggest that this 12,13-diHOME represents a novel class of exercise-induced circulatory factors that may contribute to the metabolic changes that occur during physical activity.
Sleep
Sleep is an environmental factor that is often overlooked, and it affects energy metabolism. For humans, sleep deprivation or poor sleep quality is associated with an increased risk of obesity and metabolic disorders. Lack of sleep can disrupt the regulation of appetite hormones, leading to hunger pangs and increased food intake.
Blood samples from sleep-deprived people show similar metabolic profiles to obese people
In addition, sleep deprivation can impair glucose metabolism and insulin sensitivity, which can lead to the development of type 2 diabetes. Interestingly, long sleep duration has also been linked to an increased risk of type 2 diabetes in humans.
Chronic sleep deprivation – hypothalamic-pituitary-adrenal axis disorder
An important link affected by sleep deprivation is the hypothalamic-pituitary-adrenal axis, which is responsible for the release of the stress hormone cortisol, which regulates glucose metabolism and appetite. Long-term sleep deprivation can lead to dysregulation of the hypothalamic-pituitary-adrenal axis, resulting in increased cortisol release and impaired glucose metabolism in the body.
Lack of sleep – disrupting the biological clock
Another important pathway is the circadian clock system, which regulates the timing of physiological processes, including metabolism. Lack of sleep can disrupt the circadian clock system, leading to dysregulation of energy metabolism. In mice, this dysregulation is mediated by several genes, including Clock, Bmal1, Dec, Per1, and Cry1, which are involved in the regulation of circadian rhythms.
Lack of sleep – impairs insulin signaling and metabolism
Sleep deprivation also impairs insulin signaling and glucose metabolism through the AKT pathway. AKT is a key regulator of glucose metabolism, and sleep deprivation has been shown to reduce AKT phosphorylation and impair glucose uptake in adipocytes and muscle cells in humans and mice.
Lack of sleep – affects the regulation of appetite hormones in humans
These include ghrelin and leptin. Ghrelin is an appetite-stimulating hormone, and sleep deprivation has been shown to increase ghrelin levels, leading to hunger pangs and increased food intake. Leptin is a hormone that emits a feeling of fullness, and sleep deprivation has been shown to reduce leptin levels, further promoting increased appetite in humans.
Gut microbiota
The gut microbiota is a complex microbial ecosystem that includes bacteria, viruses, fungi, protozoa, and archaea that reside in the gastrointestinal tract. Gut bacteria are by far the most widely studied and understood members of this microbiota due to their cultureability, relatively large genome size, complex functional diversity, and promising therapeutic potential.
The gut microbiota can have a significant impact on energy metabolism by regulating nutrient absorption and utilization, regulating appetite, and influencing the development and function of adipose tissue.
The gut microbiota can produce a variety of metabolites that affect energy metabolism, including short-chain fatty acids, bile acids, and different bioactive lipids including endocannabinoids (eCBs), oxylipids, and amino acid derivatives.
The gut microbiota has been implicated in the development of obesity and metabolic diseases, and the gut microbiota can regulate the development and function of adipose tissue in mice. For example, specific gut microbiota can promote or eliminate browning of WATs, thereby increasing energy expenditure and improving metabolic health in mice.
The previously mentioned environmental factors that affect energy, such as diet, sleep, and exercise, are all associated with changes in the composition of the gut microbiota. Some preclinical studies have shown that the gut microbiota may be one of the key factors influencing energy metabolism, acting through changes in multiple metabolites such as bile acids, SCFAs, bioactive lipids, etc.
Methods for targeting the microbiota to alter the metabolism of adipose tissue
All of the dietary ingredients listed here have been described as adding beige or brown to the fatty tissue and affecting the microbiota. Both of them prevent diet-induced obesity in mice. Most of these compounds play a role in brown adipose tissue (BAT) and white adipose tissue by altering the same markers of fat browning and fat oxidation, such as increasing levels of uncoupling protein 1 (UCP1), DIO2, CPT1α, Cidea, peroxisome proliferators. Activating receptors-γ coactivator 1α (PGC1α), SIRT1, and BMP7. Some of them increase cold-induced thermogenesis and the number and/or activity of mitochondria.
The most researched dietary ingredients
Resveratrol: also known as trans-3,5,4'-trihydroxystilbelle, is an organic compound that belongs to natural polyphenols. It is mainly found in plants and plant-derived products, such as knotweed, various fruits, including grapes and berries, peanuts, and red wine.
Capsaicin: An alkaloid compound found in chili peppers.
Quercetin: An important flavonoid commonly found in the human diet and found in apples, berries, and onions.
Epigallocatechin-3-gallate: A polyphenolic compound found in the unfermented dry leaves of the Camellia sinensis 264 plant.
Berberine: A naturally derived alkaloid found in specific flowering plants such as Berberaceae, Coptis chinensis, and Canadian hydra, used in traditional Chinese medicine.
Rhubarb Extract: An anthraquinone-rich crude extract derived from rhubarb root.
Camu Camu (Myrciaria dubia): An Amazonian fruit with unique phytochemical characteristics.
Specific bacteria
Akkermansia muciniphila: increases browning, fatty acid oxidation, and BAT activity, and is associated with enhanced intestinal barrier function.
Dysosmobacter welbionis J115 T: is a butyrate producer that has recently been identified and described as reducing BAT albionism and increasing mitochondrial activity by producing a variety of bioactive lipids, including 12,13-diHOME.
For other details, please see our previous article on obesity and gut microbiota.
Extended reading:
Analysis of the latest research on the effects of the microbiome on obesity
05 Microbiota-related compounds that affect the metabolism of adipose tissue
Fasting induces adipokines
Fasting-induced adipokine (FIAF), also known as angiopoietin-like protein 4 (ANGPTL4), is a circulating protein produced in response to fasting in a variety of tissues, including gut, liver, and adipose tissue, and is the site of action of the major peroxisome proliferator-activated receptor (PPAR) protein.
FIAF regulates lipid metabolism in mice by inhibiting lipoprotein lipase (LPL), a rate-limiting enzyme that hydrolyzes triglyceride cores in circulating lipoproteins, thereby reducing the uptake of fatty acids into adipose tissue and muscle.
Interaction of gut microbiota with FIAF expression
Mouse studies have shown that the gut microbiota regulates FIAF production. FIAF is constitutively expressed in germ-free mice, whereas regularization (colonization of the gut microbiota of non-germ-free mice) decreases FIAF expression and increases LPL activity, resulting in an increase in body fat mass.
In addition, germ-free mice with the FIAF gene knocked out lost their resistance to obesity caused by a high-fat diet. However, it is crucial to approach these findings with caution.
The complex relationship between FIAF and obesity resistance
The current findings challenge the widely accepted belief that a lack of gut microbiota inherently creates resistance to obesity, and that different outcomes may be related to the source of dietary fat used. An attempt to replicate a groundbreaking study failed to reflect the original findings, so the impact of a loss of gut microbiota on obesity remains inconclusive.
This evidence highlights the complexity of the relationship between gut bacteria and metabolic diseases and suggests that further exploration is needed. Whether there is a causal relationship between the production of FIAF and gut microbiota-mediated fat storage effects remains controversial, especially in germ-free mice, where high-fat diet-induced obesity only increased protein expression of FIAF in the gut and not in circulation.
Several studies have shown that administration of certain bacteria can increase circulating FIAF levels in mice and increase their expression in human intestinal epithelial cells, suggesting that regulation of the gut microbiota can influence FIAF production.
Although FIAF also appears to play a crucial role in the central regulation of energy metabolism by inhibiting mouse hypothalamic AMPK activity, the exact mechanism by which the gut microbiota regulates FIAF protein expression remains inadequately understood. Whether the gut microbiota regulates hypothalamic FIAF is unclear.
Short-chain fatty acids and key receptors
Humans don't have the digestive enzymes needed to break down dietary fiber. As a result, indigestible carbohydrates remain unaffected as they pass through the upper gastrointestinal tract and reach the large intestine, where they can be fermented by anaerobic bacteria. This fermentation process results in the production of various metabolites, of which short-chain fatty acids are the main ones. The amount and type of fiber consumed has a significant impact on the diversity and composition of the gut microbiota, which in turn affects the production of short-chain fatty acids.
Short-chain fatty acids, of which acetate, butyrate, and propionate are the main forms in the gut, are a great source of extra energy from undigested food. It is estimated that short-chain fatty acids can provide 10% of a person's daily calories, and colon cells use short-chain fatty acids, especially butyric acid, as their preferred source of energy.
In addition, gut-derived short-chain fatty acids can be transported into the bloodstream through colon cells, where they mix with endogenous short-chain fatty acids (produced and released by tissues and organs) and exert various effects on lipid, glucose, and cholesterol metabolism in a variety of tissues by acting as substrates or signaling molecules3 (figure below).
Molecular mechanisms and metabolites produced by the gut microbiota
Acts on specific receptors in the gut or in white and brown adipose tissue
Cani PD, et al., Nat Rev Gastroenterol Hepatol. 2023
Metabolites secreted by certain microorganisms (e.g., lipopolysaccharide LPS, pathogen-associated molecular pattern PAMP, endocannabinoids), which are produced by microbial digestion of dietary components (e.g., short-chain fatty acids) or by transformation to produce host-derived factors (e.g., endocannabinoids and bile acids) can be sensed by various receptors and pathways, altering gut integrity and host health. The upper right figure refers to the specific receptors expressed in colon cells or enteroendocrine cells, and different specific receptors and their ligands are derived from microbial metabolites or components. The lower right panel depicts the receptors expressed in white and brown adipocytes, specific ligands from microbial metabolites or components, and specific metabolic effects resulting from the activation of these receptors.
AHR, Aryl Hydrocarbon Receptor, CB, Cannabinoid Receptor, CD14, Cluster of Differentiation 14, GLP1, Glucagon-like Peptide 1, GPR, G Protein-Coupled Receptor, MYD88, Bone Marrow Differentiation Primary Response 88, PPAR, Peroxisome Proliferator-Activated Receptor, PYY, Peptide YY, TGR5, Takeda G Protein-Coupled Receptor 5, TLR, Toll-like Receptor.
Concentration balance of short-chain fatty acids and health
Both low and excessively high concentrations of short-chain fatty acids can adversely affect the health of both humans and mice. To prevent excessive levels of short-chain fatty acids in the blood, the liver efficiently absorbs most of the circulating short-chain fatty acids. In the liver, acetate is used as an energy source and as a substrate for the synthesis of long-chain fatty acids and cholesterol, and propionate is used as a precursor to gluconeogenesis.
In humans, mice, and rats, low SCFA concentrations have been linked to the development of chronic metabolic disorders such as obesity, insulin resistance, and diabetes, and studies in mice and rats have confirmed that dietary fiber or SCFA supplementation can alleviate the development of obesity induced by a high-fat diet.
Short-chain fatty acids: from appetite control to energy balance
One of these mechanisms is the role of SCFAs as signaling molecules. SCFAs, specifically butyrate and propionate, act as signaling molecules that regulate the secretion of various hormones involved in appetite regulation, satiety, and energy expenditure. For example, SCFAs can stimulate the release of GLP1, PYY, and leptin. GLP1 and PYY are hormones that promote satiety and reduce food intake, while leptin helps regulate energy balance by signaling the brain about energy storage.
In addition, SCFAs can interact with G protein-coupled receptors (GPRs) on the surface of enteroendocrine L cells, specifically GPR41 and GPR43, to stimulate the secretion of intestinal peptides. In addition to directly stimulating intestinal peptide secretion (involved in appetite regulation), SCFAs have also proposed that SCFAs trigger intracellular signaling pathways upon activation of these receptors, ultimately influencing energy metabolism, inflammation, and insulin sensitivity in different cell types (i.e., white and brown adipocytes, hepatocytes, neurons, and immune cells).
The relationship between SCFA and adipose tissue is complex and not fully understood. For example, some studies have shown that elevated SCFA concentrations may contribute to obesity and insulin resistance, while others have found that SCFAs can improve insulin sensitivity and help with weight loss in mice, rats, and humans.
Different SCFAs have different effects on adipose tissue metabolism
For example, butyrate induces adipogenesis through GPR43 activation, while propionate stimulates adipogenesis in mature adipocytes through GPR41 activation.
In fact, in adipose tissue, activation of GPR41 and GPR43 can promote adipocyte differentiation and adipogenesis, leading to the formation (hyperplasia) of new adipocytes and an increase in adipose tissue mass.
Impact of SCFA on BAT
An in vitro study showed that acetate promoted upregulation of gene and protein expression of adipocyte protein 2 (AP2; a marker of adipocyte differentiation), PGC1α, and UCP1 in mouse brown adipocytes, thereby increasing mitochondrial biogenesis, but these effects were compromised in cells with reduced GPR43 expression.
In human white adipocytes, the results were different
Preadipocytes isolated from human omental adipose tissue cultured for 13 days and exposed to different GPR43 agonists (i.e., physiological or synthetic) to study the effect on adipocyte differentiation did not show any effect on AP2 gene expression and eventual differentiation.
Conversely, troglitazone, a PPARγ agonist, increased AP2 gene expression in these cells and decreased the tendency of GPR43 gene expression (P = 0.06). This observation suggests that, unlike mice, there is no relationship between GPR43 and human adipocyte differentiation.
In addition, the same researchers also found that GPR43 gene expression in adipose tissue of obese individuals was not increased, but was mainly associated with tumor necrosis factor (TNF)-related inflammatory processes.
Butyrate regulates appetite
If we focus on butyrate, the mechanism by which butyrate confers metabolic benefits in mice and humans is still not fully understood. In 2018, Li et al. investigated the effects of butyrate on appetite and energy expenditure to determine the extent to which these two factors contribute to the beneficial metabolic effects of butyrate, and found that one acute oral infusion of butyrate through gastric tube (rather than intravenous injection) reduced food intake within 1 hour after refeeding in mice starved overnight.
Butyrate also inhibits the activity of appetite neurons in different areas of the brain. The researchers confirmed that long-term butyrate supplementation in drinking water could prevent diet-induced obesity, hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis, but they largely attributed this effect to a reduction in food intake.
Butyrate also modestly enhances fatty acid oxidation and activates BAT
Increased utilization of fatty acids, which is not only due to reduced food intake, but also mainly due to increased sympathetic outflow of BAT. The investigators eventually found that subdiaphragmatic vagotomy eliminated the effects of butyrate on food intake and stimulation of BAT metabolic activity.
Taken together, these findings suggest that butyrate acts on the gut-cranial neural circuit, improving energy metabolism by reducing energy intake and enhancing fatty acid oxidation by activating BAT.
Molecular patterns associated with LPS and other pathogens
Low-grade inflammation is one of the hallmarks of obesity and related metabolic disorders. Due to the occurrence of metabolic endotoxemia, the origin of this inflammation is initially related to the intestinal microbiota. Metabolic endotoxemia, also known as endotoxin-induced metabolic inflammation, is a condition characterized by elevated levels of circulating lipopolysaccharide (LPS; commonly referred to as endotoxin) in the blood, resulting in low-grade chronic inflammation and metabolic dysfunction. LPS is a molecule found on the outer membrane of certain types of bacteria, such as gram-negative bacteria. Under normal conditions, the intestinal barrier prevents endotoxins from translocating from the intestinal lumen into the bloodstream.
However, in addition to typical infections or inflammatory bowel disease, certain factors can compromise the integrity of the intestinal barrier, allowing endotoxins to penetrate the circulatory system. These factors include a high-fat diet, excessive alcohol consumption, obesity, high blood sugar, and a lack of dietary fiber, all of which can lead to significant alterations in the integrity of the intestinal barrier. These alterations involve changes in the arrangement and localization of tight junction proteins, changes in the production of antimicrobial peptides, and modifications in the composition of the mucus layer.
Multiple mechanisms have been proposed through which gut-derived compounds, such as lipopolysaccharides, can influence adipose tissue metabolism. One of them is the stimulation of inflammatory pathways through TLR4 and its coreceptor CD14, which triggers an immune response in adipose tissue.
LPS exposure, inhibition of adipocyte differentiation
When exposed to LPS, adipocytes and preadipocytes undergo changes that interfere with normal adipogenesis. For example, LPS can inhibit the differentiation of mouse preadipocytes into mature adipocytes by disrupting the expression of key transcription factors involved in adipogenesis, such as PPARγ and CEBPA. LPS triggers the release of pro-inflammatory cytokines, such as TNF, which interferes with the differentiation process via the WNT-β-catenin-T cytokine 4 (TCF4) pathway.
Specifically, in vitro, TNF enhances TCF4-dependent transcriptional activity and promotes the stabilization of β-catenin and a pro-inflammatory environment that hinders adipogenesis.
LPS can alter the secretion of different adipokines
In addition to the direct effects of LPS and inflammation on the adipogenesis process, LPS has also been found in mice to alter the secretion of different adipokines, including increased secretion of apelin, adiponectin, and leptin, which play an important role in regulating energy metabolism and inflammation as well as lipogenesis. In vitro, LPS may also play a role in impaired adipogenesis and the occurrence of adipose tissue cell senescence, particularly in the context of obesity and senescence.
However, it is important to note that the effects of LPS on adipogenesis may vary depending on the concentration and duration of exposure, as well as the specific cellular environment. In fact, several in vivo and in vitro studies have shown that LPS can increase preadipocyte proliferation and adipogenesis through JAK-STAT and AMPK-dependent cPLA2 protein expression and CD14-dependent mechanisms.
LPS produced by E. coli affects gut health, glucose metabolism problems
To investigate whether LPS in the gut is sufficient to promote glucose and insulin tolerance and the accumulation of macrophages in WAT, monocolonizing germ-free mice with E. coli found that colonization of the gut by this LPS-producing bacterium resulted in impaired glucose metabolism, increased macrophage accumulation, and polarization of the pro-inflammatory M1 phenotype in WAT.
In contrast, single colonization of germ-free mice with Escherichia coli (i.e., E. coli MLK1067) expressing LPS but with reduced immunogenicity does not induce macrophage accumulation or inflammation in WAT.
Different sources of LPS have different effects on metabolic and immune responses
Similarly, data suggest that LPS from specific bacteria can have an antagonistic effect on TLR4 but still cause endotoxemia as measured in endotoxin units. LPS from E. coli impaired the integrity of the intestinal barrier and exacerbated glycemic control in mice.
However, when comparing the same endotoxin unit dose of LPS from other bacteria (e.g., Rhodobacterium spherocosa), the researchers found that the mice did not have the same negative effects and even offset the blood glucose abnormalities caused by the same amount of E. coli LPS. Lipopolysaccharides in obese mice.
These findings suggest that metabolic endotoxemia should not be limited to LPS burden alone, but should also consider specific characteristics of the LPS molecule, such as lipid A acylation.
Peptidoglycan and lipopeptides have also been associated with intestinal barrier damage and obesity
In addition to lipopolysaccharides, disruption of the intestinal barrier associated with overweight and obesity has been linked to translocation of molecular patterns associated with other pathogens and the development of fat mass. For example, studies have shown that peptidoglycan and lipopeptides may also contribute to the development of metabolic disorders, and individuals affected by obesity have been shown to have increased concentrations of peptidoglycan and lipopeptides in their blood. Peptidoglycan is a component of the bacterial cell wall in gram-positive and gram-negative bacteria.
Receptors such as NOD1 promote lipolysis in obese individuals by activating multiple signaling pathways
Bacterial peptidoglycan can induce lipolysis in adipocytes by activating protein 1 (NOD1) containing a nucleotide-binding oligomeric domain. This NOD1-mediated lipolysis involves stress kinases (ERK1 and ERK2), PKA, and NF-κB pathways, converging on hormone-sensitive lipases. Endoplasmic reticulum stress inositol requirement protein 1 acts as a key regulator of lipolysis and blood triglycerides during inflammation.
These data suggest that receptors with pathogen-associated molecular patterns, such as TLR and NOD-like receptors, are a converging point that can link obesity-related immune responses to hyperlipidemia and insulin resistance, at least in mice.
Defects or alterations in specific receptors such as Tlr5 and Tlr2 are associated with features of metabolic syndrome
Flagellin (the protein component of bacterial flagella), bacterial DNA, and bacterial lipoproteins are also molecules that act on specific TLRs and are released into the bloodstream due to increased intestinal permeability or translocation in obese and diabetic patients. However, the role of these compounds in the development of metabolic disorders remains controversial.
For example, mice with genetic defects in Tlr5 (bacterial flagellin receptor) had altered microbiota composition and exhibited features associated with metabolic syndrome.
Also related to specific alterations in the composition of the gut microbiota, mice lacking Tlr2, a pattern recognition receptor that detects many ligands in bacteria, exhibited a metabolic syndrome phenotype characterized by insulin resistance, glucose intolerance, fat mass and weight gain, and elevated circulating LPS levels and subclinical inflammation.
Finally, mice deficient in Nod2 (which detects peptidoglycan) exhibited higher inflammation in adipose tissue and liver, exacerbated insulin resistance during high-fat diet feeding, and increased translocation of commensal bacteria from gut to adipose tissue and liver.
Taken together, these findings underscore the importance of studying bacterial component detection and better understanding the links between gut microbes, inflammation, and adipose tissue in the context of obesity and type 2 diabetes.
Tryptophan derivatives
Tryptophan can be metabolized into different metabolites in the gut microbiota and tissue cells. Indole, a metabolite of tryptophan of bacterial origin, such as indole 3-propionate (IPA), was present at lower levels in blood samples of obese individuals than in normal weight control samples.
The kynurenine pathway is responsible for the degradation of tryptophan to kynurenine (Kyn), kynureic acid (Kyna), and quinolineic acid. Conversely, plasma levels of Kyn are elevated in obese individuals, which may be attributed to enhanced enzymatic activity of indoleamine 2,3-dioxygenase 1 (IDO1). However, some gut bacteria encode enzymes that are homologous to the eukaryotic Kyn pathway.
AHR signaling pathway
Tryptophan derivatives and indoles from the gut microbiota can regulate adipose tissue development by activating the aryl hydrocarbon receptor (AHR) signaling pathway to promote the differentiation of preadipocytes into mature adipocytes. The AHR signaling pathway is involved in the regulation of adipogenesis and adipocyte metabolism.
Kyna and GPR35
Kyna inhibits weight gain and improves glucose tolerance in high-fat diet-fed mice by activating GPR35 and promoting fatty acid oxidation, thermogenesis, and anti-inflammatory gene expression in adipose tissue.
Kyna and GPR35 enhanced PGC1α expression and cellular respiration in adipocytes and increased gene expression levels of Rgs14, thereby enhancing β-adrenergic receptor signaling. Conversely, deletion of the gene for GPR35 leads to gradual weight gain, glucose intolerance, and increased sensitivity to high-fat diets.
In addition, Gpr35 knockout mice exhibited exercise-induced impairment of adipose tissue browning. These findings reveal a novel pathway through which gut microbiota-derived metabolites communicate to regulate energy homeostasis.
IDO1 enzyme activity
In obesity, increased IDO1 enzyme activity, associated with enhanced activity in the gut, leads to a shift in tryptophan metabolism from the production of indole derivatives and IL-22 to kynurenine. Studies have shown that inhibition or deletion of IDO1 can improve insulin sensitivity, protect the intestinal barrier, reduce metabolic endotoxemia and inflammation, and alter lipid metabolism in the liver and adipose tissue.
Adipose tissue may be the main direct source of KYN
In vivo studies have shown that the IDO1 gene and protein are expressed in adipocytes. Consume Ido1 in adipocytes to prevent the accumulation of Khyn and protect mice from obesity. Interestingly, the mechanism behind this effect still involves the activation of AHR, as the removal of Ahr genes from adipocytes counteracts the effects of Kyn 171.
Gut microbes affect miR-181 expression and regulate fat metabolism
The study also showed that tryptophan-derived metabolites produced by the gut microbiota control the expression of the miR-181 family in mouse white adipocytes, thereby regulating energy expenditure and insulin sensitivity. In addition, dysregulation of the gut microbiota-miR-181 axis leads to obesity, insulin resistance, and WAT inflammation in mice. In a group of children classified by body weight percentile, miR-181 expression and plasma abundance of tryptophan-derived metabolites in obese patients were found to be dysregulated.
Bioactive lipids
Bioactive lipids are a class of signaling molecules derived from lipids (fatty acids, phospholipids, and sphingolipids) that are involved in a wide range of biological activities, including inflammation, pain regulation, blood pressure regulation, cell growth and differentiation, apoptosis (programmed cell death), and immune responses.
Bioactive lipids produced by the host and gut microbiota can influence the composition and activity of the microbiota and various host metabolic processes.
★ Bile acids
Bile acid production and regulation
Bile acids are produced by the liver, but are highly regulated by the activity and composition of the microbiota. Bile acids, after binding to glycine or taurine, are stored in the gallbladder and subsequently released into the small intestine when eating.
Lipid digestion and absorption
The release of bile acids aids in the digestion and absorption of dietary fats. They emulsify fats, increase the efficiency of action of lipases, and thus promote the breakdown of lipids and the absorption of fat-soluble vitamins.
Bile acid circulation
Approximately 95% of bile acids are reabsorbed in the ileal portion of the small intestine and transported back to the liver for resecretion, creating a highly efficient circulation. This process affects the metabolism of cholesterol and the total amount of bile acids in the body.
Bile acids act as signaling molecules
Bile acids are more than just digestive aids, they act as signaling molecules, acting as hormones and influencing glucose, lipid and energy metabolism. Bile acids regulate metabolic processes by activating specific receptors, such as G protein-coupled bile acid receptor 1 (TGR5).
Role of TGR5 receptors
The TGR5 receptor is widely distributed in a variety of tissues, especially in brown adipose tissue (BAT). Through the TGR5 receptor, bile acids can activate signaling and gene expression associated with lipid metabolism, energy expenditure, and inflammation.
Effect of bile acids on energy metabolism
Bile acids can affect energy metabolism by increasing lipolysis and substrate availability, improving mitochondrial function and mitochondrial β-oxidation. For example, oral supplementation with CDCA can increase the activity of brown adipose tissue and whole-body energy expenditure.
Interaction of bile acids with enteroendocrine hormones
The TGR5 receptor, expressed on enteroendocrine L cells, is associated with the release of gastrointestinal hormones such as PYY and GLP1, which are essential for maintaining energy balance and metabolic regulation.
★ Endocannabinoids
The eCB system is known for its wide range of physiological roles, including the regulation of appetite (i.e., energy metabolism), glucose, and lipid metabolism, as well as its role in immunity, inflammation, and interactions between the microbiota and the host.
- The first endocannabinoid to be discovered was anandamide (N-arachidonoylethanolamide), which is both a CB1 and CB2 ligand;
- The second endocannabinoid receptor ligand to be identified is 2-arachidonoylglycerol.
Several groundbreaking studies in mice, rats, and humans have shown that eCB is involved in the metabolism of adipose tissue and that activation of the eCB system promotes adipogenesis.
The eCB system plays an important role in gut barrier function, gut microbiota, and adipose tissue metabolism
Specifically, in mice, the presence of anandamide was increased during obesity and diabetes, which triggered intestinal permeability through a CB1-dependent mechanism. In addition, when the eCB system is activated pharmacologically with potent eCB agonists, it increases lipogenesis and disrupts the intestinal barrier. The increased permeability further amplifies the level of LPS (i.e., metabolic endotoxemia) in the bloodstream, disrupting the intestinal barrier and affecting the eCB system throughout the gut and adipose tissue.
In the pathological state of obesity, alterations in eCB tone and increased levels of LPS lead to dysregulation of adipogenesis, perpetuating the initial imbalance and establishing a harmful cycle that leads to alterations in adipose tissue metabolism. This is a novel pathophysiological mechanism that connects the gut microbiota to the gut eCB system and plays an important role in regulating lipogenesis.
Lipogenesis with the eCB system
Adipogenesis is influenced by the feedback loop between the endocannabinoid system and lipopolysaccharide (LPS). Obesity is associated with changes in the eCB system, elevated plasma LPS levels, and disruption of gut microbiota composition.
Gut microbiota and metabolism
Changes in gut microbiota composition in obese and diabetic mice are associated with changes in metabolic function and changes in the function of the eCB system. These findings were also confirmed in a mouse model of diet-induced obesity and in germ-free mice.
THE IMPORTANCE OF THE NAPEPLD ENZYME
NAPEPLD enzyme is involved in the synthesis of bioactive lipids in adipocytes and is essential for maintaining normal metabolic function. Mouse models have shown that lack of NAPEPLD enzyme leads to spontaneous obesity, insulin resistance, and inflammation, even on a normal caloric diet.
The intestinal microbiota of NAPEPLD enzyme-deficient mice can replicate a similar metabolic phenotype, including reduced thermogenic programs and alterations in the gut microbiota after transfer to germ-free mice.
DYSREGULATION OF THE NAPEPLD ENZYME MAY LEAD TO METABOLIC COMPLICATIONS.
In conclusion, all evidence points to bidirectional communication between the host eCB system and the gut microbiota. However, further research is needed to identify several potential new therapeutic targets.
★ Oxylipids
Oxylipids are a diverse class of bioactive lipid molecules derived from the oxidation of polyunsaturated fatty acids. The gut microbiota has an impact on oxylipid-mediated inflammatory processes.
12,13-DiHOME is an oxylipid formed from linoleic acid through the action of cytochrome P450 and soluble epoxide hydrolases. 12,13-DiHOME is mainly produced by BAT or beige adipose tissue, and factors such as exercise, diet, and temperature can affect its concentration in the body. It has the role of regulating the uptake of fatty acids in adipose tissue and thermoregulation during cold exposure.
The study found that 28 obese adolescent men had lower concentrations of 12,13-DiHOME than 28 men of normal weight and increased with strenuous exercise. In high-fat diet-induced obese mice, administration of 12,13-diHOME for two weeks promoted fatty acid transport to BAT, decreased circulating triglyceride concentrations, and increased gene expression of LPL, an enzyme that hydrolyzes triglycerides in lipoproteins, in BAT.
Some intestinal bacteria can produce and secrete 12,13-diHOME. For example, 12,13-diHOME was found in several bioactive lipids produced by Welbionis Dysosmobacter, and administration of this bacterium to mice significantly reduced (P < 0.001) BAT whitening and increased mitochondrial activity induced by a high-fat diet.
★ The role of succinic acid and GPR91
Succinic acid is an intermediate in the tricarboxylic acid cycle (also known as the citric acid cycle or Krebs cycle) and is central to cellular metabolism and energy homeostasis.
Metabolic regulation
Succinic acid participates in metabolic regulation through GPR91 on fat cells, and it can be produced by microorganisms through carbohydrate fermentation and appears as a catabolite.
The importance of microbial products
Succinic acid, as a microbial product, has metabolic health benefits when dietary fiber is consumed, such as increasing the production of succinic acid through the action of Prevonella. Succinic acid producers such as Akkermansia muciniphila are negatively associated with obesity, diabetes, and metabolic disorders.
Succinic acid levels in Crohn's disease
Plasma succinic acid levels were significantly higher in patients with Crohn's disease than in healthy controls, and SUCNR1 expression was higher in adipose tissue of patients with active Crohn's disease.
Succinate may promote the transition of white adipocytes to beige adipocytes in Crohn's disease.
The role of GPR91
GPR91 is highly expressed in mouse white adipose tissue (WAT) and regulates fat mass and glucose homeostasis. In a mouse model of Gpr91 knockout, the absence of GPR91 affects metabolism and body weight, but the specific effect (weight gain/weight loss) depends on the experimental conditions.
Gpr91 knockout mice exhibit smaller WAT compartments, smaller adipocytes, increased energy expenditure, and improved glucose regulation under a regular diet.
GPR91 may be a potential target for the treatment of obesity, hypertension, and diabetes.
These findings reveal the important role of succinic acid and GPR91 in energy metabolism and adipose tissue function, as well as possible pathological roles in disease states. This provides a new direction for future treatment strategies.
06 Adipose tissue microbiota
Current human studies have shown that a microbiota feature is present in an individual's adipose tissue and that this feature may vary depending on the host's metabolic burden. In this section, we discuss this new topic, focusing on the following areas:
1) methods and challenges for detecting and characterizing the adipose tissue microbiota;
2) potential sources and mechanisms of microbial transfer from the gut to adipose tissue;
3) the diversity and functional role of adipose tissue microbiota under different adipose reservoir and metabolic conditions;
and 4) implications and prospects for future research and therapeutic interventions.
Methods and challenges for detecting and characterizing the adipose tissue microbiota
One of the main challenges in studying the adipose tissue microbiota is ensuring the reliability and validity of microbial detection methods. Several studies have used bacterial quantification based on the 16S rRNA gene to identify and compare microbial profiles across different adipose tissue reservoirs and metabolic conditions. However, this method has some limitations, such as the risk of contamination from the environment or reagent source, the low sensitivity and specificity of some primers and probes, and the difficulty of distinguishing between live and dead bacteria.
Possible pathways for the transfer of microorganisms from the gut to adipose tissue
The origin and pathways of microbial transfer from the gut to adipose tissue are not fully understood, but several mechanisms have been proposed.
1.
One possibility is that bacteria or their components cross the intestinal barrier by increasing intestinal permeability, which is often seen in obesity and type 2 diabetes.
2.
Another possibility is that bacteria or their genetic material are actively transported by immune cells, such as macrophages or dendritic cells, from gut-associated lymphoid tissue to adipose tissue.
3.
A third possibility is that bacteria or their components are carried by the portal vein or lymphatic system to the liver or other organs, where they can affect local or systemic inflammation and metabolism.
Adipose tissue microbiota, diversity and functional roles under different adipose reservoir and metabolic conditions
The diversity and functional role of the adipose tissue microbiota may vary depending on a variety of factors, such as the anatomical location of the fat pool, the metabolic status of the host, and interactions with other host factors.
For example, the different adipose tissue pools (subcutaneous, mesenteric, omentum, and liver) of obese individuals with or without type 2 diabetes have different microbial profiles, and these characteristics are not related to BMI.
Tissue-specific quantification, classification, and composition of bacterial signatures correlate with tissue-dependent inflammatory markers and metabolic signatures.
Obese individuals have higher SAT bacterial load and lower bacterial diversity compared to normal-weight individuals, and these differences are associated with alterations in lipid metabolism and inflammation-related gene expression.
The link between the presence of specific bacteria in breast milk and their origin
Another important challenge in the context of the adipose tissue microbiota involves the link between the presence of specific bacteria in breast milk and their origin, and ultimately the possible link to the development of "pink" adipocytes.
"Pink" adipocytes are a unique type of adipocyte that can be found in the subcutaneous fat repertoire of pregnant and lactating mice. These pink fat cells are specialized cells derived from white fat cells under the skin that produce and release milk.
Accumulating evidence suggests that they undergo a process of transdifferentiation and become mammary alveolar epithelial cells. The evidence also supports the hypothesis that transdifferentiation can occur in a reversible manner from white to pink, pink to brown, and brown to myoepithelial cells.
A microbiota with a unique composition is found in breast milk. Healthy women's milk is usually low in bacteria, mainly staphylococcus, streptococcus, lactic acid bacteria, and other gram-positive bacteria such as Corynebacterium, Propionibacterium, and Bifidobacteria, but DNA from strictly anaerobic bacteria can also be found. It consists of a coordinated microbiota and interconnected network.
One of the key unknowns is whether alterations in breast tissue and, eventually, the microbiota in breast milk may affect breast health, mammary adipose tissue, and transdifferentiation from white to pink adipocytes. It is important to note that in addition to colostrum and cow's milk, breast tissue of women who are breastfeeding or not may contain microbiota, which may have an impact on the occurrence, progression, and treatment of breast cancer.
Impact and prospects for future research and therapeutic interventions
The study of adipose tissue microbiota is a novel and promising area of research that may provide new insights into the pathophysiology and treatment of metabolic diseases. However, many outstanding issues and challenges still need to be resolved. For example:
- What is the causal relationship between adipose tissue microbiota and metabolic outcomes?
- How do diet, lifestyle, genetics, medications, or other environmental factors affect the adipose tissue microbiota?
- How can we manipulate or modulate the adipose tissue microbiota to improve metabolic health?
More longitudinal, interventional, and mechanistic studies, as well as standardized protocols for sampling, processing, analyzing, and reporting adipose tissue microbiota data, are needed to answer these questions.
07 The gut-fat axis and the search for biomarkers of obesity and insulin resistance
The study of the complex interactions between the gut microbiota and adipose tissue has revealed an interesting interaction that extends far beyond digestion and metabolism.
The gut microbiota influences a variety of physiological processes, including energy homeostasis, inflammation, and insulin sensitivity. The gut-fat axis represents a two-way communication system involving signaling molecules, metabolites, and immune mediators exchanged between the gut microbiota and adipose tissue.
Once thought to be an inert energy reservoir, adipose tissue is now thought to be an active endocrine organ that releases adipokines, cytokines, and other factors with systemic effects. On the other hand, the gut microbiota produces a range of metabolites that affect host metabolism and immune responses. This dynamic interaction between gut microbiota and adipose tissue opens up new avenues for identifying biomarkers associated with obesity and insulin resistance.
The potential biomarkers generated by this interaction are expected to identify individuals at risk of metabolic disorders, enabling early intervention and personalized strategies to mitigate the effects of obesity and improve insulin sensitivity.
Biomarkers associated with obesity and insulin resistance
Microbial diversity and composition
Alterations in gut microbiota diversity and abundance of specific microbial groups are associated with obesity and insulin resistance. For example, surrogate indicators of adipocyte diameter, glucose, and insulin sensitivity appear to be closely related to the abundance of Ackermansia in humans. The diameter of subcutaneous white adipocytes is similar to that of A. There was a negative correlation between muciniphila abundance, and A. muciniphila abundance was negatively correlated. Individuals with high abundance of muciniphila have lower average adipocyte size. Although there is still a heated debate due to many confounding factors and large individual differences, the identification of certain microbial (core) characteristics can serve as early indicators of metabolic dysfunction.
Metabolites
Microbial metabolites, such as short-chain fatty acids, secondary bile acids, and trimethylamine-N-oxides, can reflect gut microbiota activity and may predict the risk of obesity and insulin resistance.
Increased levels of short-chain fatty acids are also associated with reductions in body weight, fat mass, waist circumference, fasting blood glucose, insulin resistance, and inflammation.
Elevated secondary bile acid levels are associated with reduced body mass index, waist-to-hip ratio, fasting blood glucose, insulin resistance, and inflammation.
Increased levels of trimethylamine-N-oxide are associated with increased body mass index, waist circumference, body fat percentage, fasting blood glucose, insulin resistance, blood pressure, inflammation, and oxidative stress.
Adipokines and inflammatory markers
Circulating levels of a large number of adipogenic factors and inflammatory markers affected by adipose tissue health can serve as indicators of obesity-related insulin resistance.
Metabolic response to diet
Individual differences in gut microbiota response to dietary interventions may be associated with obesity risk and insulin sensitivity, providing the basis for personalized dietary recommendations.
Microbe-host interaction genes
Genetic variants that affect the interaction between gut microbes and hosts may lead to obesity and susceptibility to insulin resistance, providing genetic markers for risk assessment.
08 Moving from the laboratory to the clinic: the main challenges
Although valuable insights have been gained over the past few years into the interactions between the gut microbiota and adipose tissue, translating findings from in vitro and animal studies into humans remains particularly challenging.
Limitations inherent in animal models
Germ-free mice are housed without an intestinal microbiota and can provide insight into the role of certain gut bacteria or combinations of bacteria. However, these mice lack microbial interactions during development, resulting in altered metabolism and impaired immune system function, which may not accurately reflect human physiology.
Hereditary obese mice, such as ob/ob and db/db mice, help us understand the pathophysiology of obesity, but their genetic basis limits their transformation to human obesity because leptin and leptin receptor deficiencies are rare in humans, and mutations lead to disruption of major obesity metabolic regulatory pathways.
Species differences and the complexity of the transformation of research results
On the other hand, obese mice on a high-fat diet mimicked some aspects of human obesity but failed to replicate the multifactorial nature of the disease. Genetic and lifestyle factors play an important role in human obesity and are difficult to replicate in the laboratory.
Translating findings from animal models to humans is also challenging due to the biological differences between species. Genetic variation, diet, gut microbiota composition, and environmental influences vary, making direct translation difficult. Animal models often oversimplify complex human metabolic pathways and fail to account for the heterogeneity observed in populations. This is one of the main reasons why the results of animal studies are often not confirmed in human studies, and one of the reasons why data obtained only from animal experiments must be interpreted with caution.
Individual differences and dynamics of the gut microbiota
Although many animal studies have shown that interventions targeting the gut microbiota and its metabolites promise to combat obesity and metabolic disorders, designing clinical trials to substantiate these findings presents unique challenges. The gut microbiota exhibits significant inter-individual differences, making it difficult to establish standardized interventions that produce consistent effects across different populations. To complicate matters further, the gut microbiota is a highly dynamic and complex ecosystem that can be influenced by a variety of factors such as diet, medications, stress, and other environmental factors, and changes in the gut microbiota can take some time to become apparent.
The lack of standardization in the field of gut microbiome interventions, which extends to study design, sample collection, and data analysis, makes it extremely difficult to compare and evaluate the effectiveness of these interventions.
Variability of intervention parameters
There is considerable variability in the parameters of gut microbiota interventions. This difference includes the type of probiotics and prebiotics used, the dose administered, and the duration and timing of the intervention. Different clinical and preclinical studies use different strains or combinations of strains, so comparing their efficacy can be challenging. In addition, the optimal dose and duration of the intervention have not been determined, leading to inconsistent treatment regimens. The timing of the start of the intervention and the route of administration were also different, bringing additional variability to the studies.
There is a lack of standardization in data collection
Changes in sample collection methods, such as stool collection techniques, storage conditions, and transport protocols, may affect the quality and consistency of gut microbiota data. In addition, the collection and reporting of metadata, including dietary information, lifestyle factors, medication use, and clinical characteristics, is often inconsistent across studies. The lack of standardized data collection procedures hinders the ability to accurately interpret and compare results.
Limitations of stool samples
Due to their non-invasive collection methods and sufficient biomass for analysis, fecal samples remain the primary source of material for most gut microbiota studies. However, it is important to recognize the limitations when relying solely on stool samples, as the microbiota in stool may not accurately represent microbial communities at different locations within the gut, resulting in an incomplete understanding of the effects on the gut microbiota and the impact on health.
Spatial heterogeneity of the gut microbiota
The gut microbiota varies along the length of the gastrointestinal tract due to factors such as environmental changes, nutrient availability, and different oxygen levels from the stomach to the large intestine. Different microbial communities thrive in these different conditions.
Differences in mucosal and intestinal lumen microbiota
In addition, the intestinal microbiota in the intestinal lumen (feces) can be very different from the intestinal microbiota of the mucous membrane close to the intestinal wall. The mucosal layer is the dynamic interface where the host-microbial interaction occurs. Microorganisms attached to the mucous membrane can have different effects and effects than microorganisms that float freely in the lumen. In addition, microbial communities in different parts of the gut may have different metabolic activities. For example, bacteria in the colon produce various metabolites through fermentation, which have systemic effects on host health. Studying fecal metabolites alone may not provide complete information, as they may be affected by interactions between bacteria in different parts of the gut.
Microbial translocation and systemic effects
Finally, certain bacteria or metabolites can travel from the intestinal lumen to other parts of the body, potentially affecting distant organs and systems. Understanding translocation dynamics and the specific microbial populations involved requires a more comprehensive sampling strategy, not just fecal samples. However, to date, there are no clear biomarkers that are easy to use clinically to fully reflect intestinal permeability and its dynamics. Therefore, while feces provide valuable insights, recognizing their limitations and addressing the challenge of accurately describing the gut microbiota throughout the gastrointestinal tract is critical to gaining a more complete understanding of their role in health and disease.
Technical challenges of gut microbiota analysis
Analyzing the gut microbiota presents its own challenges due to the lack of standardized techniques and workflows. Different studies employ different methods to analyze the gut microbiota, such as 16S rRNA gene sequencing, shotgun metagenomics, or metatranscriptomics. Each method has its own advantages and limitations, and the choice of technique can affect the accuracy and comprehensiveness of the results.
Functional metagenomic approaches using shotgun sequencing and biochemical interpretation have become powerful tools for microbiome research by identifying functional bacterial genes and pathways that contribute to various physiological processes, but even this technique has its limitations. In addition to the high cost, complexity of data interpretation, and challenges of functional annotation, shotgun metagenomic sequencing only provides information about the presence of functional genes, but may not fully capture information about gene expression and regulation. In addition, there is no standardized bioinformatics process for processing and analyzing gut microbiota data. Different methods of quality control, categorical allocation, and statistical analysis may lead to differences and difficulties in comparing the results of studies.
Despite the above-mentioned difficulties and challenges, the pace of scientific research will not stop there. With the continuous development and improvement of technology, the methods of gut microbiota analysis will also continue to improve. Standardized techniques and workflows will help improve the reproducibility and accuracy of data, thereby driving breakthroughs in gut microbiota research.
09 Conclusion
Unlike earlier descriptions of fat cells as simple blood vessels that store and release lipids, we are increasingly recognizing the complexity of fat cells.
When we overfeed fat cells, we begin to appreciate the myriad contributions of WAT to whole-body health. Understanding how obesity-related adipocyte dysfunction contributes to disease states may help develop new cell-targeting strategies to improve or restore adipocyte function.
While obesity rates continue to rise, including in children, there are more emerging treatments to address obesity and related comorbidities. Given the strong link between obesity, fat cell size, and fat cell dysfunction, strategies to reduce fat mass (and fat cell size) are good therapeutic targets.
Advances in technology and data integration will continue to provide new insights into how fat cells are affected by weight gain and give us a clearer picture of fat cell dysfunction in obesity and related diseases.
Therefore, despite the challenges, there is still unlimited potential and opportunities for research in this area. The focus of the microbiome era is on understanding and harnessing the potential of the gut microbiota, including its role in different adipose tissues, which is undoubtedly an important part of the paradigm shift in medicine and healthcare in the future.
Key References:
Cani PD, Van Hul M. Gut microbiota in overweight and obesity: crosstalk with adipose tissue. Nat Rev Gastroenterol Hepatol. 2023 Dec 8.
Hagberg CE, Spalding KL. White adipocyte dysfunction and obesity-associated pathologies in humans. Nat Rev Mol Cell Biol. 2023 Dec 12.