Adiposity is defined as an accumulation of excess adipose tissue composed of adipocytes, or fat cells. Adiposity develops when energy intake exceeds energy expenditure, a balance affected by both orexigenic (hunger inducing) and anorexigenic (satiety inducing) signals—some of which are hormones that adipose tissue itself secretes. This mini-review summarizes key points about the physiologic roles of three important appetite-regulating hormones: leptin, adiponectin, and ghrelin. It also discusses several clinical scenarios in which knowing the concentrations of these hormones might be useful.
Overweight, Obesity, and Abdominal Obesity
Clinicians routinely measure adiposity using anthropometric measures such as the body mass index (BMI; kg/m2). Individuals with a BMI ≥25 kg/m2 are considered overweight, whereas those with a BMI ≥30 kg/m2 are considered obese. While being overweight or obese generally increases the risk of cardiometabolic disease, cardiovascular disease, and cancer, BMI does not distinguish between lean mass and fat mass. Therefore, BMI sometimes misclassifies as obese individuals with a large lean mass (e.g., body builders), or as normal weight those with an abnormally low lean mass but a large fat mass (e.g., sarcopenic obesity). The relationship between BMI and the risk of cardiovascular disease and cardiometabolic disease also varies significantly by ethnicity, partly because of ethnic differences in body composition (1).
Waist circumference, waist-to-hip ratio, and other measures incorporating girth are generally superior to BMI because they better identify those with an elevated fat mass, which in men commonly presents as abdominal obesity. Abdominal obesity increases risk beyond that predicted by BMI because of visceral fat surrounding internal organs (1). However, the relationship between abdominal obesity and chronic disease risk also varies by ethnicity. Consequently, the International Diabetes Federation has developed ethnic-specific waist circumference cutpoints denoting abdominal obesity, an important component of the metabolic syndrome (2).
Both subcutaneous and visceral fat secrete or are affected by numerous polypeptide hormones. The hormones secreted by fat are called adipokines. While both subcutaneous and visceral fat contribute to adiposity, visceral fat has a much greater impact on cardiometabolic disease because of its metabolic activity. Visceral adipocytes secrete inflammatory cytokines that contribute to systemic inflammation and insulin resistance. They also secrete free fatty acids that lead to clinical hyperlipidemias and ectopic fat deposition, as well as angiotensin-converting enzyme components that increase blood pressure (1).
Ectopic fat is particularly damaging because most cells cannot tolerate excess lipid and are sensitive to its toxic metabolites (3). As a result, adipogenesis and adipocyte metabolism exert an important effect on the development of disease, particularly cardiometabolic disease. This can occur through autocrine, paracrine, or endocrine processes.
Leptin
One of the better known appetite-regulating hormones is leptin, with its name derived from the Greek word “leptos” for thin. Leptin is a 167 amino acid (16 kDa) peptide with a single disulfide bond that binds to leptin receptors (LEPRs) in the hypothalamus (4). Encoded by the ob gene on chromosome 7q32 and expressed in adipose tissue, leptin has several splice variants, with the LEPRB isoform thought to be responsible for the majority of its activity (4). The ob gene was cloned in 1994 and subsequently knocked out in mice. These mice eat voraciously, become massively obese, and develop hyperglycemia (5, 6). Researchers also observed this phenotype in mice with non-functional copies of the db gene, which encodes the leptin receptor (5). These initial experiments demonstrated that leptin is an anorexigenic hormone, and that a deficit of leptin would produce an orexigenic signal. This means that ob/ob or db/db homozygote knockout mice remain in states of perceived starvation.
Interestingly, experiments have shown that leptin is also unexpectedly increased in other obese animal models and humans compared to those of normal weight. The cause of this seemingly paradoxical increase is a vicious cycle of leptin resistance, in which the hypothalamus becomes insensitive to leptin. Although adipose tissue actually produces leptin, leptin resistance prompts adipose tissue to produce more in an effort to bring satiety and reduce food intake (5). Unsurprisingly, treating leptin-deficient ob/ob knockout mice with leptin reduces appetite (5). It also rescues thyroid, endocrine, and immune function from the physiologic effects of perceived starvation and reduces hyperglycemia more significantly than untreated ob/ob knockout mice that receive the same amount of food. These findings suggest that exogenous leptin might be useful as a weight-loss and anti-diabetic therapy.
Research has not fully elucidated the mechanism by which leptin affects appetite and glycemia. In obese mouse models, leptin does not fully cross the blood-brain barrier, leading to low concentrations in cerebral spinal fluid. However, reduced blood-brain barrier permeability may not be the cause of leptin resistance, as it does not precede the development of obesity (4). Rather, leptin receptor-expressing neurons in the arcuate nucleus of the hypothalamus have reduced phosphorylation of signal transducer and activator of transcription 3, a signaling path-way necessary for leptin’s appetite suppression. In particular, pro-opiomelanocortin-expressing neurons within the arcuate nucleus appear to be involved, similar to their role in glycemic responses in animal models.
Interestingly, leptin lowers glucose in both type 1 and type 2 diabetic mouse models, strongly indicating that the effect is independent of insulin levels. Even though the actual mechanism is not known, one possibility is that leptin increases sympathetic or parasympathetic signaling to target tissues (e.g., skeletal muscle, liver, and adipose tissue), causing them to take up glucose, and to pancreatic alpha cells, inducing them to reduce glucagon secretion (4).
Leptin also decreases lipogenesis and increases beta oxidation of fatty acids via inhibitory phosphorylation of acetyl CoA carboxylase 1 (ACC-1)—the rate-limiting enzyme of de novo lipogenesis—through AMP-protein kinase in both liver and skeletal muscle (7). This may protect the body from the damage caused by ectopic fat deposition (3). Animal experiments suggest that leptin treatment interrupts non-alcoholic fatty liver disease (NAFLD) from progressing to highly inflammatory and fibrotic non-alcoholic steatohepatitis (NASH) (7).
Leptin therapy has not proven successful in treating obesity in humans due to leptin resistance (4). Using leptin with a pancreatic β-cell satiety hormone analogue (amylin), however, has helped to overcome leptin resistance and induce weight loss, though questions remain about its long-term efficacy (4). In patients with early-onset obesity, supplemental leptin is useful in restoring normal energy balance and neuroendocrine function (5).
Leptin therapy also benefits patients with hypothalamic amenorrhea, a condition that affects naturally thin women and those who are extremely active. In these individuals, insufficient adipose tissue and low leptin halt menstrual cycles, causing infertility in adults and delaying puberty in children. Without leptin, the body believes it is starving and cannot devote energy to metabolically expensive endocrine processes such as puberty and menstruation. These women also are at high risk for developing osteoporosis. This is why leptin therapy helps to initiate puberty in hypoleptinemic children at the correct bone age, and rescue hypoleptinemic adult women from infertility while also improving their bone mineral density (5).
Leptin also may be useful in treating lipodystrophy and lipotoxicity. In the latter, lipid accumulation in the liver and skeletal muscle leads to massive insulin resistance and hyperglycemia (4). Despite its potential benefits, leptin therapy for controlling type 1 and type 2 diabetes has yielded mixed results in humans (4).
The bottom line: Leptin testing has limited clinical use other than in patients suspected of having hypoleptinemia. However, new clinical applications may arise from controlled trials of leptin, leptin agonists, and combination therapies in different populations. For now, leptin remains a low-volume send-out test for most laboratories. Those that perform this test mainly utilize manual immunoassays like enzyme-linked immunosorbent assays.
Adiponectin
Adiponectin is a 244 amino acid (30 kDa) oligopeptide encoded by the ADIPOQ gene on chromosome 3q27—a chromosome associated with obesity and type 2 diabetes in genome-wide association studies. Discovered in 1995, adiponectin binds mainly to AdipoR1 and AdipoR2 receptors found in the liver (8, 9). As is the case with leptin, adipose tissue secretes adiponectin. However, adiponectin reaches a much higher concentration than leptin and other plasma proteins (0.01% of total protein) (9). The placenta also secretes adiponectin, and levels of this amino acid are higher in females than males. This may help to explain why women are generally at lower risk for type 2 diabetes than men (9).
Unlike leptin, adiponectin’s concentration is inversely associated with amount of adipose tissue, yet it has a similar anorexigenic effect as leptin (9). While counterintuitive, lower adiponectin may in fact be due to insulin resistance or type 2 diabetes that may interfere with adiponectin secretion (7). Interestingly, adiponectin concentration may not change in obese individuals unless a relatively large (e.g. 10%) weight loss occurs (7).
In ob/ob mice, adiponectin overexpression prevents hepatic lipid accumulation by causing adipocytes to proliferate (7). Similar to the effects of leptin, this improves insulin sensitivity because ectopic lipid interferes with insulin signaling and causes inflammation and apoptosis. These processes obviously have important implications for liver, beta cell, and kidney function.
Another interesting characteristic of adiponectin is that, like leptin, it increases beta-oxidation of fatty acids, decreases lipogenesis in liver and skeletal muscle via phosphorylation of ACC-1, and helps prevent NAFLD from progressing to NASH in animal models (7). Other effects observed in animal models include increased angiogenesis and nitric oxide synthesis (9). Most notably, adiponectin is a potent insulin sensitizer that increases glucose uptake in adipocytes by modifying downstream cell signaling cascades.
Adiponectin also reduces gluconeogenesis and hepatic glucose secretion (9). Administering adiponectin intravenously to ob/ob mice increases thermogenesis and weight loss, an effect enhanced when it is co-administered with leptin. Adiponectin is also decreased in several cancers, possibly because insulin resistance and inflammation promote tumor development and growth (10).
The bottom line: While adiponectin is associated with several important diseases, it is unclear how clinicians should act in the face of an abnormal result—especially a low result. At present, it is unknown whether treating patients with adiponectin or adiponectin agonists will yield benefits above and beyond conventional therapies such as insulin sensitizers, statins, anti-inflammatory agents, or lifestyle modification. Therefore, conventional lab tests are likely to remain the standard of care until new research and guidelines support routine testing. Several large referral laboratories offer adiponectin testing and, like leptin, perform this testing via manual immunoassay.
Ghrelin
Ghrelin is a small 28 amino acid (3.3 kDa) protein produced by the hypothalamus as well as the stomach, gastrointestinal (GI) system, heart, lung, and adipose tissue (11). Discovered in 1999, ghrelin was named after the Latin word “ghre,” meaning to grow. Ghrelin is encoded by the preproghrelin gene on chromosome 3p25-26. This produces a 117 amino acid product that is cleaved and modified to form ghrelin and a 23 amino acid inhibitory peptide called obestatin (11).
Mainly secreted by X/A cells of the oxyntic gland in the gastric fundus, ghrelin is present as both acylated (5%) and non-acylated / desacylated (95%) forms that may have different physiologic effects. Both cross the blood-brain barrier, but desacylated ghrelin has difficulty traveling back into the circulation. Ghrelin causes growth hormone secretion by binding to the growth hormone secretagogue receptor in the anterior pituitary gland, and is negatively regulated by growth hormone. However, many of ghrelin’s physiologic effects are specific to either acylated or desacylated ghrelin, often represented by a ratio. This difference complicates research in the area as ghrelin assays often measure total ghrelin, which is the sum of both forms (11).
Unlike leptin and adiponectin, ghrelin is a well-known orexigenic hormone. Unsurprisingly, ghrelin causes gastric acid secretion and increases gut motility (11). The desacylated form is secreted during prolonged calorie restriction that stimulates neurons in the arctuate nucleus of the hypothalamus, causing secretion of hunger-inducing proteins neuropeptide Y and agouti-related peptides (11). This stimulation also likely involves vagal nerve activation. After feeding, ghrelin concentration drops back to baseline levels.
Ghrelin is negatively associated with BMI in humans, and several studies indicate that it does not drop back to baseline in obese individuals following food consumption. The lower baseline level of ghrelin is associated with decreased expression and acylation of ghrelin, and a reduced number of ghrelin receptors.
Ghrelin is responsible for severe obesity observed in Prader-Willi syndrome. Patients with this condition have non-functional genes on chromosome 15 inherited from partially deleted paternal forms (11). These patients have hyperghrelinemia, voracious appetites, and are severely obese.
Ghrelin has other effects on adipocytes and lipids. Ghrelin increases the proliferation and differentiation of pre-adipocytes into adipocytes, partially through activating the peroxisome proliferator-activated receptor gamma. Ghrelin also stimulates lipid accumulation and decreases insulin sensitivity and insulin secretion, causing glucose to rise. This likely occurs through increased free fatty acid release from adipocytes, causing ectopic fat deposition. Interestingly, this effect seems more related to acylghrelin than desacylghrelin.
In patients with type 2 diabetes, lower ghrelin levels are associated with abdominal obesity and worsening insulin resistance (11). As ghrelin is produced by the stomach and parts of the GI system, bariatric surgery reduces plasma levels of ghrelin, helping to quell appetite and induce weight loss. Other studies have found ghrelin treatment to be associated with several other unexpected effects, such as reducing adiponectin secretion, lowering blood pressure, and causing tissue-specific pro- or anti-inflammatory effects via expression or suppression of NF-kB (nuclear factor kappa-light-chain-enhancer of activated B cells). Further research is needed to clarify the many possible physiologic effects and clinical applications of ghrelin and ghrelin agonists.
The bottom line: Ghrelin testing is not generally indicated. However, it may be useful to help characterize Prader-Willi syndrome and the impact of bariatric surgery. Labs that perform ghrelin usually do so by manual immunoassay.
Conclusion
Leptin, adiponectin, and ghrelin are highly specialized tests with limited clinical utility. Knowledge of their clinical applications, however, will help clinical laboratory professionals consult effectively with clinical teams in selecting these rare send-out tests. However, an important question to ask is, “Will having this test result change clinical management?” If there is no accepted clinical response to a high or low result, the test likely will not be useful.
References
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Lawrence de Koning, PhD, DABCC, FACB, FCACB, is a clinical biochemist at Calgary Laboratory Services and a clinical associate professor in the departments of pathology and laboratory medicine, pediatrics, and community health sciences at the University of Calgary Cumming School of Medicine in Calgary, Alberta, Canada.+Email: [email protected]