Metabolic Actions of Adipocyte Hormones: Focus on Adiponectin
University of Pennsylvania School of Medicine, Division of Endocrinology, Diabetes & Metabolism, 764 Clinical Research Building, 415 Curie Blvd., Philadelphia, PA 19104. E-mail: email@example.com
The obesity epidemic has focused attention on the endocrine function of adipose tissue. Adipose tissue secretes leptin, cytokines, complement factors, and components of the coagulation cascade, most of which are increased in obesity. In contrast, a strong negative correlation exists between adiponectin and adiposity, insulin sensitivity, diabetes, vascular inflammation, and atherosclerosis. Adiponectin treatment in rodents increases insulin sensitivity and reduces lipids and atherogenesis. Chronic and central adiponectin treatment reduces weight, glucose, and lipids. The insulin-sensitizing action of thiazolidinediones is mediated, in part, through adiponectin. A causal role of adiponectin in diabetes, dyslipidemia, and atherosclerosis has been established in knockout mice. Therefore, adiponectin seems to be a marker of obesity-related diseases and a potential therapeutic target.
Adipose tissue functions as an endocrine organ by producing a variety of secreted factors including leptin, adiponectin, cytokines, resistin, and proteins involved in coagulation and vascular control (1) (Table 1). Many of these so-called “adipokines” regulate energy balance, glucose, and lipids and may contribute to diseases associated with obesity (1). Leptin is increased in proportion to adipose tissue mass (1, 2). Although initially thought to act as antiobesity hormone, studies indicate that leptin's primary role is to signal energy deficiency to the brain, leading to adaptive responses including hyperphagia, reduced thermogenesis, and suppression of thyroid, reproductive, and immune function (1, 2). Similar responses to low leptin are observed in humans and rodents with congenital leptin deficiency and lipodystrophy (1, 3). Leptin replacement reverses these metabolic and neuroendocrine defects (1).
Table 1. . Actions of adipocyte hormones
|Leptin (produced by adipocytes; subcutaneous expression is higher than visceral adipose; low levels expressed by muscle, gastric fundus, intestine and placenta)||Inhibits feeding; increases energy expenditure; reduces body fat||Lowers glucose, insulin and lipids; stimulates lipid oxidation||Trophic action on hypothalamic feeding circuits; stimulates reproductive and thyroid hormones; stimulates immunity (inhibits bone formation in rodents)|
|Adiponectin (produced by adipocytes; level in visceral adipose is greater than subcutaneous adipose)||Chronic peripheral or central treatment reduces body weight and fat; stimulates thermogenesis||Increases insulin sensitivity; reduces glucose; stimulates lipid catabolism||Anti-inflammatory; anti-atherogenic|
|Resistin (produced by adipocytes in rodents but mononuclear cells in humans)||Inhibits adipogenesis in rodents||Induces resistance in rodents; metabolic role in humans is uncertain|| |
|TNFα (produced by adipocytes; expression in subcutaneous adipose is greater than visceral adipose)||Inhibits feeding; induces cachexia||Induces insulin resistance, hyperglycemia and dyslipidemia in rodents, but role in glucose and lipid metabolism in humans is controversial|| |
|IL-6 (produced by adipocytes and stromovascular cells; expression in visceral adipose is greater than subcutaneous adipose)||Inhibits feeding; increases energy expenditure; decreases weight||Induces insulin resistance, hyperglycemia, and dyslipidemia|| |
Obesity is typically polygenic, associated with excess food intake and sedentary lifestyle and characterized by elevated leptin level (1). In contrast to leptin-deficient states, obesity results in an increase in leptin; however, affected individuals are unresponsive to the rise in endogenous leptin as well as leptin treatment (1). This leptin resistance is rarely caused by defects of the leptin receptor. Instead, studies suggest that reduced leptin transport across the blood—brain barrier, abnormal leptin signaling, possibly involving Janus kinase/signal transducer and activator of transcription, suppressors of cytokine signaling-3, and protein tyrosine phosphatase-1B, or dysregulation of neuropeptide targets in the hypothalamus and other brain regions may contribute to obesity (1, 2). In addition to its role in energy homeostasis, leptin suppresses hepatic glucose production, stimulates lipid catabolism, and regulates the neuroendocrine axis, pancreatic β cells, immune function, hematopoiesis, angiogenesis, and bone development (1, 2).
Adipose tissue expresses cytokines, e.g., tumor necrosis factor α (TNFα)1 and interleukin-6, which induce cachexia, insulin resistance, diabetes, and lipid abnormalities (4, 5, 6, 7). Adipose tissue in obese humans and rodents is infiltrated by macrophages that secrete TNFα and interleukin-6, as well as monocyte chemoattractant protein-1, a chemokine that stimulates the recruitment of monocytes (8, 9). Monocyte chemoattractant protein-1 is increased in obesity and thought to increase insulin resistance, vascular inflammation, and atherosclerosis (8, 9). Adipsin (complement factor D) is produced by adipose tissue and regulates the synthesis of acylation stimulating protein through complement factor 3 (10). Acylation stimulating protein has been associated with impairment of insulin sensitivity and dyslipidemia (10). Plasminogen activator inhibitor-1 is increased with visceral adiposity and associated with greater risk for thrombosis and cardiovascular complications (11). Various proteins of the renin-angiotensin system are produced by adipose tissue and may mediate cardiovascular morbidity in obesity (12).
This review will focus on adiponectin, a protein synthesized and secreted exclusively by mature adipocytes (13). Although adiponectin was identified about the same time as leptin, its biology was unclear until recently. Adiponectin is the most abundant adipose-secreted protein, highly conserved across species, and circulates as various complexes. In contrast to most adipocyte hormones, adiponectin is decreased in obesity and inversely correlated with insulin resistance, glucose intolerance, dyslipidemia, and atherosclerosis. These abnormalities are reversible by adiponectin treatment, implying that adiponectin is causally linked to complications of obesity and could be targeted for treatment.
Chemistry of Adiponectin
Adiponectin was independently identified by four laboratories, hence the multiple names (14, 15, 16, 17). The Lodish laboratory first discovered adiponectin in 1995 as a protein synthesized and secreted by differentiated murine 3T3-L1 adipocytes (14). It was named “adipocyte complement-related protein of 30 kDa” (ACRP30) because of homology to complement C1q (14). The same protein was identified independently by the Spiegelman laboratory and named adipoQ (15). Both studies found high levels of murine leptin in plasma, suggesting an endocrine role (14, 15). Adiponectin was found to be highly expressed in human adipose tissue and, thus, given the name “adipose most abundant gene transcript 1” (APM1) (16). Independently, the protein was purified and shown to have a high affinity for gelatin—cellulose resins and was named “gelatin-binding protein of 28 kDa” (GBP28) (17).
The primary sequence of adiponectin contains a signal peptide at the N terminus, a variable region with no homology among different species, a collagenous region containing 22 Gly-X-Pro or Gly-X-Y repeats, and a globular domain with sequence homology to C1q in the C terminus. This sequence of adiponectin is highly conserved among mammals (13). Adiponectin shares a similar modular structure with C1q and other proteins, e.g., saccular collagen of the inner ear, hibernation-related proteins HP-20, 25, and 27 and type VIII and type X collagens (13). Moreover, the crystal structure of the C-terminal globular domain forms a homotrimeric structure that is structurally similar to the TNF superfamily, despite a lack of sequence homology. Native adiponectin exists as a homotrimer that forms a hexamer consisting of two adjacent trimeric globular domains and a single stalk composed of collagen domains from two trimers. The hexamers form high molecular weight multimeric complexes (13, 18, 19). The globular domain is still able to form trimers but does not associate into higher-order forms. All of these forms are present in plasma, although the biological activity is thought to be mediated by higher-order complexes (18, 19). Full-length adiponectin produced by mammalian cells is more potent than bacterially expressed adiponectin, because the latter lacks critical post-translational modifications, including hydroxylation and glycosylation, and is unable to form higher-order complexes (18, 19, 20).
Adiponectin is the most abundant protein secreted by adipose tissue. Its concentration in plasma is two to three orders of magnitude higher than other polypeptide hormones (13). In humans, plasma concentrations range from 5 to 30 μg/mL. The synthesis and secretion of adiponectin is dependent on various factors. In contrast to most adipocyte hormones, adiponectin is decreased in obesity and increased in response to weight reduction (21, 22). Adiponectin is lower in men than women, possibly as a result of suppression by androgens (23, 24). Moreover, women have higher proportions of high molecular weight adiponectin than men (24). Diurnal and pulsatile adiponectin secretion has been shown in humans, although in contrast to leptin, adiponectin peaks in the morning and decreases at night (25). Adiponectin is inhibited by adrenergic activity, glucocorticoids, TNF, and dibutyryl-cAMP (26, 27, 28, 29, 30). Thiazolidinediones (TZDs) increase adiponectin in parallel with their insulin-sensitizing action (31, 32). However, insulin treatment has variable effects on adiponectin mRNA and protein levels (13, 23, 24). Leptin, angiotensin II, growth hormone, and triiodothyronine do not seem to affect adiponectin expression (13).
Putative adiponectin receptors (AdipoRs) have been identified (33). AdipoR1 is abundantly expressed in skeletal muscle, whereas AdipoR2 is mainly present in the liver. AdipoRs contain seven transmembrane domains, but are structurally and functionally distinct from G protein—coupled receptors. Activation of AdipoRs does not affect cAMP level; rather, these receptors phosphorylate and activate AMP kinase, a cellular fuel sensor which mediates fatty acid metabolism (33). AdipoR1 is a high-affinity receptor for globular adiponectin and has low affinity for the full-length adiponectin. In contrast, AdipoR2 has intermediate affinity for globular and full-length adiponectin (33). The distribution of AdipoR1 and R2 parallels the differential effects of full-length and globular adiponectin in liver and muscle (33). T-cadherin binds specifically to hexameric and high molecular weight adiponectin (34). Unlike AdipoRs, T-cadherin is expressed widely in endothelial and smooth muscle cells. The precise role in T-cadherin is unknown, although it is has been suggested that this protein could act as a coreceptor in modulating the response to adiponectin (34).
Effects of Adiponectin on Energy Homeostasis, Glucose, and Lipids
An involvement of adiponectin in energy homeostasis was first suggested by the observation that adiponectin levels in adipose tissue and plasma were reduced in Lepob/ob mice (14, 15). Genome-wide scans have mapped a susceptibility locus for the metabolic syndrome X and diabetes to chromosome 3q27, where the adiponectin gene is located (35, 36). Adiponectin is reduced in patients with dominant-negative mutations in the peroxisome proliferator-activated receptor γ gene, who develop severe insulin resistance and type 2 diabetes (37). Plasma adiponectin is inversely related to adiposity, insulin sensitivity, and type 2 diabetes, whereas elevated adiponectin concentration is associated with reduced risk of diabetes (38, 39). Longitudinal studies in monkeys have revealed a decline in adiponectin before the onset of obesity, insulin resistance, and diabetes (40). Importantly, studies in rodents and humans indicate that the insulin-sensitizing action of TZD is mediated partly through adiponectin (31, 32).
Fruebis et al. (41) injected a globular trimeric form of mammalian adiponectin in mice fed a diet rich in fat and sucrose and observed a blunted increase in glucose, fatty acids, and triglycerides. Bacterially produced full-length adiponectin was not effective in this model, confirming the importance of posttranslational modification of adiponectin (41). Chronic infusion of full-length mammalian adiponectin decreased body weight through enhancement of fatty acid oxidation but did not affect food intake (41). Yamauchi et al. (42) administered bacterially expressed adiponectin in wild-type and obese mice and observed potent effects on glucose and lipids. Adiponectin decreased insulin resistance by stimulating fatty acid oxidation and reducing triglyceride content in muscle and liver (42). Moreover, adiponectin improved glucose and lipids in lipoatrophic mice and completely reversed these abnormalities when given in combination with leptin (42). The Scherer laboratory studied the biology of mammalian adiponectin in several models (18, 43, 44). Peripheral injection of full-length adiponectin rapidly decreased glucose in wild-type, Lepob/ob, and insulin-deficient mice by suppressing hepatic gluconeogenesis (43). The biological activity of mammalian adiponectin resides mainly in the hexameric and higher-order forms of adiponectin (43). However, other studies have suggested that a cys39ser mutant adiponectin that is unable to form higher-order complexes is able to reduce glucose in obese and diabetic mice (18, 44). Whether this mutant protein is involved in the normal function of adiponectin is as yet unknown.
The long-term effect of adiponectin on energy balance has been tested in obese rats (45). Chronic exposure to adiponectin expressed by adeno-associated virus decreased food intake and body weight. Adiponectin reduced glucose and lipids by inhibiting hepatic gluconeogenesis and lipogenesis (45). Masaki et al. (46) have compared the effects of peripheral and central adiponectin treatment. Intraperitoneal, but not intracerebroventricular, injection of bacterially produced adiponectin attenuated weight gain and decreased visceral adiposity. Adiponectin increased uncoupling protein expression in brown adipose tissue and muscle, stimulated sympathetic nerve activity in brown adipose tissue, and increased temperature. These thermogenic responses were associated with induction of Fos immunoreactivity in the paraventricular hypothalamic nucleus. We studied the central effects of mammalian adiponectin and observed potent antiobesity and reduction of glucose and lipids in Lepob/ob mice (47). Adiponectin increased energy expenditure but had no effect on food intake (47). In addition, adiponectin potentiated the effects of leptin on glucose and lipid oxidation (47). Moreover, adiponectin and leptin were both ineffective in agouti (Ay/a) mice, implying that the melanocortin 4 receptor could be a common target (47). As reported earlier, adiponectin activated neurons in the paraventricular hypothalamus, a region that integrates peripheral and central signals that control feeding, autonomic, and neuroendocrine function (47). The forms of adiponectin, the nature of the transport mechanisms, and the precise neuronal targets remain to be determined. Nonetheless, these findings raise the possibility that adiponectin and leptin act coordinately in the central nervous system to influence metabolism.
Studies indicate that adiponectin exerts its effects on energy homeostasis and glucose and lipid metabolism through phosphorylation and activation of adenosine monophosphate-activated protein kinase (AMPK), an enzyme typically activated by cellular stress (48). Both globular and full-length adiponectin stimulate AMPK in skeletal muscle, but only full-length adiponectin activates AMPK in liver (49). AMPK activation stimulates phosphorylation of acetyl coenzyme A carboxylase, fatty-acid oxidation, glucose uptake, and lactate production in myocytes (48). Moreover, activation of AMPK in liver reduces enzymes involved in gluconeogenesis leading to reduction of glucose levels. Blocking AMPK activation by dominant-negative mutant prevents these metabolic changes, confirming the importance of AMPK in the signaling of adiponectin (49). Interestingly, AMP kinase has also been implicated in the actions of leptin, metformin, and TZDs, suggesting that it may be a common target for antidiabetic and insulin-sensitizing effects of various compounds (48).
Maeda et al. (50) examined the role of adiponectin using a gene knock-out approach in mice. Ablation of the adiponectin gene had no obvious metabolic effects in mice fed a chow diet; however, when they were fed a high fat/sucrose diet, the knock-out mice developed severe insulin resistance accompanied by increased lipid deposition in muscle. Adipose and plasma TNFα levels were increased as a result of adiponectin deficiency, and these changes were reversed by adiponectin treatment (50).
Role of Adiponectin in Vascular Biology
Studies in animal models and humans have shown associations among adiponectin, insulin sensitivity, endothelial function, and cardiovascular morbidity (51). A causal role of adiponectin has been established in knock-out mice that develop severe vascular alterations, including reduced vasodilation in response to acetylcholine compared with wild-type mice, neointimal thickening, and increased proliferation of smooth muscle cells in arteries after mechanical injury (52, 53, 54). Importantly, adiponectin treatment attenuates these changes (52, 53, 54). Adiponectin prevents atherosclerotic lesions in apolipoprotein E—deficient mice, in part by suppressing the expression of vascular cell adhesion molecule-1 and class A scavenger receptors, as well as reducing the deleterious effects of TNF-induced adhesion molecules on the surface of endothelial cells. Adiponectin also inhibits TNF-mediated inflammatory changes in endothelial cells including leukocytic colony formation and phagocytic activity. Globular adiponectin inhibits oxidized low-density lipoprotein—mediated endothelial proliferation and formation of reactive oxygen species integral to vascular damage (51). Finally, adiponectin has profound effects on vascular remodeling that limit the development of atherosclerosis and other vascular diseases (51). AdipoR1 and AdipoR2 are present in endothelial cells, although their specific roles in adiponectin signaling and vascular function is unknown. Activation of AMP kinase activation in endothelium increases fatty acid oxidation and is coupled to activation of endothelial NO synthase, thus providing a possible link between adiponectin and NO production and vascular reactivity. Whether AMPK is involved in vascular inflammation as well as adiponectin's effects on angiogenesis, growth, and remodeling remains to be determined.
The discovery of leptin in 1994 was a major milestone toward understanding the metabolic role of adipose tissue (55). Although several adipocyte hormones have since been identified, the literature in filled with seemingly conflicting results, often based on studies in rodents and associations in humans (1). For example, the importance of TNF and resistin in glucose homeostasis in rodents vs. humans is still debated, in part because there are few studies that have examined the primary physiological roles of these hormones or pharmacological treatment in humans and primates (56, 57, 58, 59). Until recently, knowledge of the biology of leptin was largely based on rodents, rare cases of congenital leptin deficiency, and associations in humans (1, 2). It is now clear that leptin has profound effects on metabolism and neuroendocrine function in lipodystrophic and healthy patients (60, 61, 62, 63, 64). Likewise, our understanding of adiponectin will be best served by studies into its kinetics and direct testing of various forms of this adipocyte hormone in humans and appropriate animal models. It will also be important to identify the underlying causes responsible for the decline in adiponectin in obese and diabetic states. Because most ligand—receptor complexes have affinity constants in the nanomolar range, a crucial question is whether the high plasma adiponectin concentration is indeed indicative of its biological activity. The bioactive forms of adiponectin and specific mechanisms that mediate energy homeostasis, lipids, glucose, and vascular biology are poorly understood. Despite these shortcomings, adiponectin and various adipocyte hormones hold great promise in the diagnosis, risk assessment, and pharmacological treatment of obesity and other metabolic diseases.
This study was supported by NIH Grant RO1 DK062348.
Nonstandard abbreviations: TNF, tumor necrosis factor; TZD, thiazolidinedione; AdipoRs, adiponectin receptors; AMPK, adenosine monophosphate-activated protein kinase.