Adipocyte

The adipocyte, a cell type located in adipose tissue, stores excess fat and makes it available on demand to the body system for use as energy. Adipocytes also secrete signaling messengers, called adipokines. Adipose tissue—which consists of other cell types besides adipocytes—operates as an endocrine organ, secreting more than 50 known adipokines.

There are two different types of adipose tissue. White adipose tissue (WAT) is the fat storage tissue and brown adipose tissue (BAT) produces heat. BAT is sometimes called ‘baby fat’ as it is present in babies but converts to WAT in adulthood. This conversion is performed by mitochondrial uncoupling protein 1 (UCP1), which is activated by the diet, in the mitochondria. Mice that are genetically made to express UCP1 in their WAT are obesity resistant. This has sparked interest in a possible treatment for obesity: by identifying what causes UCP1 to stop converting BAT to WAT, it may be possible to find a way to prevent that, so that excess energy is released as heat rather than being stored in WAT.

Pioneering studies

Fat cells were first considered to be able to sense energy demands and signal to decrease food intake in the 1950s. However, this was not confirmed until 1973, when Coleman, in animal experiments involving parabiosis, provided evidence that a circulating factor existed that signalled information about the body's energy requirements. This idea gained little attention until Siiteri in 1987 examined the role of obesity in cancers of the reproductive tract and found that adipocytes secreted the hormone estrogen which is implicated in breast and endometrial cancers. Estrogen is produced by the aromatase enzyme which is present in many tissues including adipose tissue. The production of estrogen is accelerated in obese people, as more adipose tissue means more estrogen and this is correlated with a higher incidence of cancers in the reproductive tract in obese individuals. Another prospect which supported the adipocyte as a regulator of energy stores came when adipsin, a serine protease, was also discovered to be secreted by adipocytes, and has since been found to be deficient in some animal models of obesity.

Obesity brings associated complications including cardiovascular problems, some cancers and diabetes mellitus type 2 (T2DM). Cases of obesity are increasing in adults, as are cases of T2DM. The reason for the strong relationship between obesity and T2DM is still unclear, but it has been proposed that the physiology of adipocytes may hold the key.

Resistin

In 2001, an article in the Washington Post read, ‘Hormone may be key to diabetes’. The hormone was resistin, secreted from adipocytes of white adipose tissue in rodents, and it had been found to be involved in the emergence of T2DM. Resistin blood serum levels were significantly increased in cases of obesity and it was found to interfere with the actions of insulin and with glucose tolerance, causing insulin resistance in mice. This was also supported by research showing that deliberately induced diabetes, caused by provision of a high fat diet, caused obesity and elevated resistin levels in mice. These effects could be reversed by administration of a resistin antibody. This discovery sparked excitement, as resistin appeared to be a candidate for the missing link between obesity and T2DM. However, studies in humans showed no significant differences between blood serum resistin levels between lean and obese people, and no differences in serum resistin levels between healthy and diabetic people. These conclusions suggest that resistin does not play a role in obesity-related diabetes and does not seem to be governed by adiposity. The differences between humans and mice may be due to differences in energy metabolism and in genetic differences between the species, as the mouse resistin gene is not identical to the human gene, only showing 59% homology.

More recent studies have found supernormal plasma concentrations of resistin in humans (South Koreans) with T2DM though the studies yielded no insight into the role of the higher resistin levels. They were not related to gender, or to the severity of obesity or insulin resistance. Saudi Arabians with T2DM also have increased plasma resistin levels when assessed for the presence of the metabolic syndrome, an array of risk factors for cardiovascular disease (e.g., obesity in and around the abdomen; increased blood levels of lipids that predispose to atherosclerosis; impaired ability to use insulin or blood sugar; indications of inflammation in the body). That association of high resistin levels and the metabolic syndrome has been confirmed by other investigators. In a Turkish study group, investigators found that one specific form of the resistin gene (polymorphism) associated with insulin resistance and obesity in patients with T2DM

Taken together, those findings link resistin and T2DM, but shed no light on the functional role of resistin in humans. Because cells involved in immunity and inflammation also produce resistin, the link between resistin and T2DM might relate to inflammatory reactions, an area needing further investigation.

In 879 non-diabetic humans, Reilly et al. tested for an association between plasma resistin concentrations and metabolic and inflammatory markers, including coronary artery calcification as an index of atherosclerosis, finding positive associations with inflammatory markers (soluble tumor necrosis factor-receptor-2; interleukin-6; and lipoprotein-associated phospholipase A2. They found no association with insulin resistance, but did with coronary calcification after appropriate justments for confounders. They concluded: "Plasma resistin levels are correlated with markers of inflammation and are predictive of coronary atherosclerosis in humans, independent of CRP [C-reactive protein, a marker of inflammation]. Resistin may represent a novel link between metabolic signals, inflammation, and atherosclerosis."

A recent review of the inflammatory-regulator role of resistin concludes:

Increasing evidence indicates [resistin's] important regulatory roles in various biological processes, including several inflammatory diseases. Further studies have shown that resistin in humans, in contrast to its production by adipocytes in mice, is synthesized predominantly by mononuclear cells both within and outside adipose tissue. Possible roles for resistin in obesity-related subclinical inflammation, atherosclerosis and cardiovascular disease, non-alcoholic fatty liver disease, rheumatic diseases, malignant tumors, asthma, inflammatory bowel disease, and chronic kidney disease have already been demonstrated. In addition, resistin can modulate several molecular pathways involved in metabolic, inflammatory, and autoimmune diseases. In this review, current knowledge about the functions and pathophysiological implications of resistin in different human pathologies is summarized, although there is a significant lack of firm evidence regarding the specific role resistin plays in the "orchestra" of the numerous mediators of inflammation.

Visfatin

Another adipokine implicated in the obesity-diabetes link is visfatin. Visfatin is secreted by visceral fat and has glucose-lowering effects like those of insulin. Visfatin and insulin stimulate muscle and adipose cells to take up glucose and restrain glucose release from hepatocytes. However, visfatin is unlike insulin in that its levels are constant regardless of food intake whereas insulin levels change depending on the feeding state. Visfatin also has significantly lower intracellular levels than insulin but interestingly it acts on the insulin receptor but not competitively. This may mean that visfatin is a mimetic of insulin, but its levels are low enough so that it does not interfere with the actions of insulin.

Leptin

Probably the biggest breakthrough for the study of appetite regulation came in 1994 when the molecular geneticist Jeffrey Friedman discovered the adiposity signal leptin. Using the ob/ob mice which were thought to lack a satiety signal, Friedman and colleagues found that ob codes for a gene which they called leptin, from the Greek ‘leptos’ meaning thin. Mice deficient in this gene are morbidly obese and this obesity can be reversed by giving the mice leptin. The leptin receptor was found in 1995 and is a member of the cytokine receptor family.

Leptin is a signalling molecule released from adipocyte cells to signal regarding adiposity levels. It is secreted into the blood, secretion proportional to body fat, where it travels to the brain causing a decrease in appetite through acting on specific neurones in the brain.

Unsurprisingly, there was huge media interest in leptin as a possible treatment for obesity, and the biotechnology company Amgen paid $20 million to license leptin. However the results of clinical trials on obese people were disappointing, with very few of the participants losing weight. It appeared that most people were not obese because of a deficiency in leptin, but because they were unable to respond to it; they had more fat cells and so more circulating leptin and as a result, reduced sensitivity of the leptin receptor. The failure of the trial, however, overshadowed the fact that about 30% of the subjects lost weight and so may have low circulating levels of leptin. Potentially these people might benefit from leptin therapy. The task now though is to determine those people that have a lower circulating level of leptin.

The use of leptin as a drug has proved valuable in the disease lipodystrophy and in people who have a defective leptin gene and so are constantly hungry.

Leptin inhibits food intake by acting in the appetite control centres of the brain. Leptin receptor mRNA is found primarily in the hypothalamic [arcuate nucleus]], ventromedial nucleus and dorsomedial hypothalamic nucleus, regions that are known to be involved in appetite control. The weight loss caused by leptin seems to arise by two effects;

1. Inhibiting appetite through its actions on the appetite-stimulating neuropeptide Y neurones and the appetite-inhibiting proopiomelanocortin (POMC) neurons, located in the hypothalamic arcuate nucleus. Leptin inhibits NPY neurons,causing a decrease in the release of the inhibitory neurotransmitter GABA (which is synthesised by the NPY neurons). This "disinhibits" the POMC neurons which increase their firing rate leading to the release of alpha MSH – a product of POMC which is a potent inhibitor of appetite. Leptin also acts directly on the POMC neurons.

2. Leptin also increases energy expenditure by increasing body temperature and oxygen consumption

Leptin is synthesised in many other tissues including the stomach, ovary, placenta and liver and its receptors are also located in a diverse range of tissues. It has a role in many other physiological functions such as reproduction, where levels of leptin appear to dictate the commencement of puberty and in foetal development where there is a surge in leptin levels in the first week of life, which does not correspond to a decrease in food intake but is thought to be a developmental signal. During development, mice deficient in leptin have disruptions in the arcuate nucleus neural projections, but not other hypothalamic projections, and these effects can be reversed upon leptin treatment.

Leptin’s also has a role in other physiological processes. For example it can be thought of as a signal informing the body when it has sufficient fat to accommodate an ‘expensive’ physiological function like reproduction. Leptin therefore appears to protect the body against starvation by only allowing energy dense processes to occur when the body is ready. Its primary role is unclear, is it a satiety signal to prevent overeating or an evolutionarily efficient signal protecting against starvation?

Adiponectin

Adiponectin alters insulin receptor function, diminishes the action of insulin in the liver, alters the metabolism of free fatty acids by liver cells, and protects against inflammation. Adults and children (humans) who have non-alcoholic fatty liver disease (NAFLD) show decreased plasma concentrations of adiponectin. Inasmuch as insulin resistance in the liver, and hyperinsulinemia, feature in NAFLD, those finding suggest a link between fatty liver and insulin resistance perhaps in part to reduced adiponectin. NAFLD also features elevated levels of leptin, an adipokine that reduces appetite but also interferes with insulin action in the brain.

Adiponectin exhibits the following actions:

• Perhaps by increasing fatty acid oxidation, a smaller increase in plasma FFA occurs after a high fat meal;
• By enhancing lipid metabolism, insulin sensitivity improves;
• Reduced glucose production by the liver;
• Plasma concentrations associate inversely with endogenous glucose production.

In patients who have had an acute myocardial infarction, the risk of subsequent major adverse cardiovascular events is lowest in patients with the highest plasma concentrations of adiponectin.

A complex incompletly elucidated relationship exists between adipose tissue cytokines and bone metabolism. Some workers find that in postmenopausal women plasma adiponectin concentrations associate inversely with bone mineral density.

Table of molecules released by adipose tissue

MOLECULES RELEASED BY ADIPOSE TISSUE
Molecule	Visceral Adipose Tissue	Subcutaneous Adipose Tissue
Acylation stimulating protein	+	++
Adiponectin	++	+
Angiotensinogen	++	+
Atrial Natriuretic Peptide	+	+
Cholesteryl-ester transferase	++	+
Estrogens	+	+
Free Fatty Acids/Glycerol	++	+
IGF-binding protein 3 (IGFBP3)	+	+
Insulin-like growth factor-I	+	+
Interleukin-6	++	+
Leptin	+	++
Lipoprotein lipase	+	++
Monobutyrin	+	+
Plasminogen Activator Inhibitor - factor 1	++	+
Resistin	++	+
Retinol binding protein-4	+	+
Tumor necrosis factor-a	++	+
Visfatin	+	+
Adapted from Garrutti et al. and modified by Citizendium editors.

Other genes of interest

Not all of the genes of interest in adipocytes are secreted proteins. For example, adiponutrin was first identified as a gene highly expressed in adipocytes. Obesity is associated with elevated levels of adiponutrin mRNA in adipose tissue (and in liver) in humans, and so is a potential target for therapies to manage obesity. Adiponutrin/PNPLA3 is a member of the patatin-like phospholipase family, and is strongly associated with membranes and with lipid droplets. but its exact biological function is unclear. A key feature of adiponutrin is that its expression in adipose tissue is nutritionally regulated, decreasing after fasting and strongly up-regulated by feeding. The induction of expression by a meal is rapid, and depends on the macronutrient composition of the meal: induction is most marked after meals that contain high sucrose and high protein, less so on a high lipid diet. This pattern of regulation mirrors that of lipogenic genes, suggesting that adiponutrin might be involved in regulating lipogenesis or lipolysis.