Adipose tissue

The obese mouse on the left has large stores of adipose tissue. It is unable to produce the hormone leptin. This causes the mouse to be hungry and eat more, which results in obesity. For comparison, a mouse with a normal amount of adipose tissue is shown on the right.

Mice have eight major adipose depots, four of which are within the abdominal cavity.[1] The paired gonadal depots are attached to the uterus and ovaries in females and the epididymis and testes in males; the paired retroperitoneal depots are found along the dorsal wall of the abdomen, surrounding the kidney, and, when massive, extend into the pelvis. The mesenteric depot forms a glue-like web that supports the intestines and the omental depot (which originates near the stomach and spleen) and - when massive - extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots, respectively.

The two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs (underneath the skin) and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is often covered by a "frosting" of white adipose tissue; sometimes these two types of fat (brown and white) are hard to distinguish. The inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial, which surrounds the heart, and the paired popliteal depots, between the major muscles behind the knees, each containing one large lymph node.[7] Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected,[8] comprising about 30% of dissectible fat.[9]

In an obese person, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus. A panniculus complicates surgery of the morbidly obese individual. It may remain as a literal "apron of skin" if a severely obese person loses large amounts of fat (a common result of gastric bypass surgery). Obesity is treated through exercise, diet, and behavioral therapy. Reconstructive surgery is one aspect of treatment.[10]

Abdominal obesity in a man ("beer belly")

Visceral fat or abdominal fat[11] (also known as organ fat or intra-abdominal fat) is located inside the abdominal cavity, packed between the organs (stomach, liver, intestines, kidneys, etc.). Visceral fat is different from subcutaneous fat underneath the skin, and intramuscular fat interspersed in skeletal muscles. Fat in the lower body, as in thighs and buttocks, is subcutaneous and is not consistently spaced tissue, whereas fat in the abdomen is mostly visceral and semi-fluid.[12] Visceral fat is composed of several adipose depots, including mesenteric, epididymal white adipose tissue (EWAT), and perirenal depots. Visceral fat is often expressed in terms of its area in cm² (VFA, visceral fat area).[13]

An excess of visceral fat is known as abdominal obesity, or "belly fat", in which the abdomen protrudes excessively. New developments such as the Body Volume Index (BVI) are specifically designed to measure abdominal volume and abdominal fat. Excess visceral fat is also linked to type 2 diabetes,[14] insulin resistance,[15] inflammatory diseases,[16] and other obesity-related diseases.[17] Likewise, the accumulation of neck fat (or cervical adipose tissue) has been shown to be associated with mortality.[18] Several studies have suggested that visceral fat can be predicted from simple anthropometric measures,[19] and predicts mortality more accurately than body mass index or waist circumference.[20]

Men are more likely to have fat stored in the abdomen due to sex hormone differences. Estrogen (female sex hormone) causes fat to be stored in the buttocks, thighs, and hips in women.[21][22] When women reach menopause and the estrogen produced by the ovaries declines, fat migrates from the buttocks, hips and thighs to the waist;[23] later fat is stored in the abdomen.[12]

Visceral fat can be caused by excess cortisol levels.[24] At least 10 MET-hours per week of aerobic exercise leads to visceral fat reduction in those without metabolic-related disorders.[25] Resistance training and caloric restriction also reduce visceral fat, although their effect may not be cumulative.[26] Both exercise and hypocaloric diet cause loss of visceral fat, but exercise has a larger effect on visceral fat versus total fat.[27] High-intensity exercise is one way to effectively reduce total abdominal fat.[28][29] An energy restricted diet combined with exercise will reduce total body fat and the ratio of visceral adipose tissue to subcutaneous adipose tissue, suggesting a preferential mobilization for visceral fat over subcutaneous fat.[30]

Epicardial adipose tissue (EAT) is a particular form of visceral fat deposited around the heart and found to be a metabolically active organ that generates various bioactive molecules, which might significantly affect cardiac function.[31] Marked component differences have been observed in comparing EAT with subcutaneous fat, suggesting a location-specific impact of stored fatty acids on adipocyte function and metabolism.[32]

Micro-anatomy of subcutaneous fat

Most of the remaining nonvisceral fat is found just below the skin in a region called the hypodermis.[33] This subcutaneous fat is not related to many of the classic obesity-related pathologies, such as heart disease, cancer, and stroke, and some evidence even suggests it might be protective.[34] The typically female (or gynecoid) pattern of body fat distribution around the hips, thighs, and buttocks is subcutaneous fat, and therefore poses less of a health risk compared to visceral fat.[35][36]

Like all other fat organs, subcutaneous fat is an active part of the endocrine system, secreting the hormones leptin and resistin.[33]

The relationship between the subcutaneous adipose layer and total body fat in a person is often modelled by using regression equations. The most popular of these equations was formed by Durnin and Wormersley, who rigorously tested many types of skinfold, and, as a result, created two formulae to calculate the body density of both men and women. These equations present an inverse correlation between skinfolds and body density—as the sum of skinfolds increases, the body density decreases.[37]

Factors such as sex, age, population size or other variables may make the equations invalid and unusable, and, as of 2012, Durnin and Wormersley's equations remain only estimates of a person's true level of fatness. New formulae are still being created.[37]

Marrow fat, also known as marrow adipose tissue (MAT), is a poorly understood adipose depot that resides in the bone and is interspersed with hematopoietic cells as well as bony elements. The adipocytes in this depot are derived from mesenchymal stem cells (MSC) which can give rise to fat cells, bone cells as well as other cell types. The fact that MAT increases in the setting of calorie restriction/ anorexia is a feature that distinguishes this depot from other fat depots.[38][39][40] Exercise regulates MAT, decreasing MAT quantity and diminishing the size of marrow adipocytes.[41][42][43] The exercise regulation of marrow fat suggests that it bears some physiologic similarity to other white adipose depots. Moreover, increased MAT in obesity further suggests a similarity to white fat depots.[41]

Ectopic fat is the storage of triglycerides in tissues other than adipose tissue, that are supposed to contain only small amounts of fat, such as the liver, skeletal muscle, heart, and pancreas.[1] This can interfere with cellular functions and hence organ function and is associated with insulin resistance in type-2 diabetes.[44] It is stored in relatively high amounts around the organs of the abdominal cavity, but is not to be confused with visceral fat.

The specific cause for the accumulation of ectopic fat is unknown. The cause is likely a combination of genetic, environmental, and behavioral factors that are involved in excess energy intake and decreased physical activity. Substantial weight loss can reduce ectopic fat stores in all organs and this is associated with an improvement of the function of those organs.[44]

In the latter case, non-invasive weight loss interventions like diet or exercise can decrease ectopic fat (particularly in heart and liver) in overweight or obese children and adults.[45][46]

Brown fat cell

Brown fat or brown adipose tissue (BAT) is a specialized form of adipose tissue important for adaptive thermogenesis in humans and other mammals. BAT can generate heat by "uncoupling" the respiratory chain of oxidative phosphorylation within mitochondria through tissue-specific expression of uncoupling protein 1 (UCP1).[58] BAT is primarily located around the neck and large blood vessels of the thorax, where it may effectively act in heat exchange. BAT is robustly activated upon cold exposure by the release of catecholamines from sympathetic nerves that results in UCP1 activation. Nearly half of the nerves present in adipose tissue are sensory neurons connected to the dorsal root ganglia.[59]

BAT activation may also occur in response to overfeeding.[60] UCP1 activity is stimulated by long chain fatty acids that are produced subsequent to β-adrenergic receptor activation.[58] UCP1 is proposed to function as a fatty acid proton symporter, although the exact mechanism has yet to be elucidated.[61] In contrast, UCP1 is inhibited by ATP, ADP, and GTP.[62]

Attempts to simulate this process pharmacologically have so far been unsuccessful. Techniques to manipulate the differentiation of "brown fat" could become a mechanism for weight loss therapy in the future, encouraging the growth of tissue with this specialized metabolism without inducing it in other organs. A review on the eventual therapeutic targeting of brown fat to treat human obesity was published by Samuelson and Vidal-Puig in 2020.[63]

Until recently, brown adipose tissue in humans was thought to be primarily limited to infants, but new evidence has overturned that belief. Metabolically active tissue with temperature responses similar to brown adipose was first reported in the neck and trunk of some human adults in 2007,[64] and the presence of brown adipose in human adults was later verified histologically in the same anatomical regions.[65][66][67]

Browning of WAT, also referred to as "beiging", occurs when adipocytes within WAT depots develop features of BAT. Beige adipocytes take on a multilocular appearance (containing several lipid droplets) and increase expression of uncoupling protein 1 (UCP1).[68] In doing so, these normally energy-storing adipocytes become energy-releasing adipocytes.

The calorie-burning capacity of brown and beige fat has been extensively studied as research efforts focus on therapies targeted to treat obesity and diabetes. The drug 2,4-dinitrophenol, which also acts as a chemical uncoupler similarly to UCP1, was used for weight loss in the 1930s. However, it was quickly discontinued when excessive dosing led to adverse side effects including hyperthermia and death.[68] β3 agonists, like CL316,243, have also been developed and tested in humans. However, the use of such drugs has proven largely unsuccessful due to several challenges, including varying species receptor specificity and poor oral bioavailability.[69]

Cold is a primary regulator of BAT processes and induces WAT browning. Browning in response to chronic cold exposure has been well documented and is a reversible process. A study in mice demonstrated that cold-induced browning can be completely reversed in 21 days, with measurable decreases in UCP1 seen within a 24-hour period.[70] A study by Rosenwald et al. revealed that when the animals are re-exposed to a cold environment, the same adipocytes will adopt a beige phenotype, suggesting that beige adipocytes are retained.[71]

Transcriptional regulators, as well as a growing number of other factors, regulate the induction of beige fat. Four regulators of transcription are central to WAT browning and serve as targets for many of the molecules known to influence this process.[72] These include peroxisome proliferator-activated receptor gamma (PPARγ), PRDM16,[73] peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), and Early B-Cell Factor-2 (EBF2).[74][75][76]

The list of molecules that influence browning has grown in direct proportion to the popularity of this topic and is constantly evolving as more knowledge is acquired. Among these molecules are irisin and fibroblast growth factor 21 (FGF21), which have been well-studied and are believed to be important regulators of browning. Irisin is secreted from muscle in response to exercise and has been shown to increase browning by acting on beige preadipocytes.[77] FGF21, a hormone secreted mainly by the liver, has garnered a great deal of interest after being identified as a potent stimulator of glucose uptake and a browning regulator through its effects on PGC-1α.[68] It is increased in BAT during cold exposure and is thought to aid in resistance to diet-induced obesity[78] FGF21 may also be secreted in response to exercise and a low protein diet, although the latter has not been thoroughly investigated.[79][80] Data from these studies suggest that environmental factors like diet and exercise may be important mediators of browning. In mice, it was found that beiging can occur through the production of methionine-enkephalin peptides by type 2 innate lymphoid cells in response to interleukin 33.[81]

Due to the complex nature of adipose tissue and a growing list of browning regulatory molecules, great potential exists for the use of bioinformatics tools to improve study within this field. Studies of WAT browning have greatly benefited from advances in these techniques, as beige fat is rapidly gaining popularity as a therapeutic target for the treatment of obesity and diabetes.

DNA microarray is a bioinformatics tool used to quantify expression levels of various genes simultaneously, and has been used extensively in the study of adipose tissue. One such study used microarray analysis in conjunction with Ingenuity IPA software to look at changes in WAT and BAT gene expression when mice were exposed to temperatures of 28 and 6 °C.[82] The most significantly up- and downregulated genes were then identified and used for analysis of differentially expressed pathways. It was discovered that many of the pathways upregulated in WAT after cold exposure are also highly expressed in BAT, such as oxidative phosphorylation, fatty acid metabolism, and pyruvate metabolism.[82] This suggests that some of the adipocytes switched to a beige phenotype at 6 °C. Mössenböck et al. also used microarray analysis to demonstrate that insulin deficiency inhibits the differentiation of beige adipocytes but does not disturb their capacity for browning.[83] These two studies demonstrate the potential for the use of microarray in the study of WAT browning.

RNA sequencing (RNA-Seq) is a powerful computational tool that allows for the quantification of RNA expression for all genes within a sample. Incorporating RNA-Seq into browning studies is of great value, as it offers better specificity, sensitivity, and a more comprehensive overview of gene expression than other methods. RNA-Seq has been used in both human and mouse studies in an attempt characterize beige adipocytes according to their gene expression profiles and to identify potential therapeutic molecules that may induce the beige phenotype. One such study used RNA-Seq to compare gene expression profiles of WAT from wild-type (WT) mice and those overexpressing Early B-Cell Factor-2 (EBF2). WAT from the transgenic animals exhibited a brown fat gene program and had decreased WAT specific gene expression compared to the WT mice.[84] Thus, EBF2 has been identified as a potential therapeutic molecule to induce beiging.

Chromatin immunoprecipitation with sequencing (ChIP-seq) is a method used to identify protein binding sites on DNA and assess histone modifications. This tool has enabled examination of epigenetic regulation of browning and helps elucidate the mechanisms by which protein-DNA interactions stimulate the differentiation of beige adipocytes. Studies observing the chromatin landscapes of beige adipocytes have found that adipogenesis of these cells results from the formation of cell specific chromatin landscapes, which regulate the transcriptional program and, ultimately, control differentiation. Using ChIP-seq in conjunction with other tools, recent studies have identified over 30 transcriptional and epigenetic factors that influence beige adipocyte development.[84]

The thrifty gene hypothesis (also called the famine hypothesis) states that in some populations the body would be more efficient at retaining fat in times of plenty, thereby endowing greater resistance to starvation in times of food scarcity. This hypothesis, originally advanced in the context of glucose metabolism and insulin resistance, has been discredited by physical anthropologists, physiologists, and the original proponent of the idea himself with respect to that context, although according to its developer it remains "as viable as when [it was] first advanced" in other contexts.[85][86]

In 1995, Jeffrey Friedman, in his residency at the Rockefeller University, together with Rudolph Leibel, Douglas Coleman et al. discovered the protein leptin that the genetically obese mouse lacked.[87][88][89] Leptin is produced in the white adipose tissue and signals to the hypothalamus. When leptin levels drop, the body interprets this as a loss of energy, and hunger increases. Mice lacking this protein eat until they are four times their normal size.

Leptin, however, plays a different role in diet-induced obesity in rodents and humans. Because adipocytes produce leptin, leptin levels are elevated in the obese. However, hunger remains, and—when leptin levels drop due to weight loss—hunger increases. The drop of leptin is better viewed as a starvation signal than the rise of leptin as a satiety signal.[90] However, elevated leptin in obesity is known as leptin resistance. The changes that occur in the hypothalamus to result in leptin resistance in obesity are currently the focus of obesity research.[91]

Gene defects in the leptin gene (ob) are rare in human obesity.[92] As of July 2010, only 14 individuals from five families have been identified worldwide who carry a mutated ob gene (one of which was the first ever identified cause of genetic obesity in humans)—two families of Pakistani origin living in the UK, one family living in Turkey, one in Egypt, and one in Austria[93][94][95][96][97]—and two other families have been found that carry a mutated ob receptor.[98][99] Others have been identified as genetically partially deficient in leptin, and, in these individuals, leptin levels on the low end of the normal range can predict obesity.[100]

Several mutations of genes involving the melanocortins (used in brain signaling associated with appetite) and their receptors have also been identified as causing obesity in a larger portion of the population than leptin mutations.[101]

Adipose tissue has a density of ~0.9 g/ml.[102] Thus, a person with more adipose tissue will float more easily than a person of the same weight with more muscular tissue, since muscular tissue has a density of 1.06 g/ml.[103]