Adiponectin is an adipocyte-specific secretory protein that circulates in serum as a hexamer of relatively low molecular weight (LMW) and a larger multimeric structure of high molecular weight (HMW). Serum levels of the protein correlate with systemic insulin sensitivity. The full-length protein affects hepatic gluconeogenesis through improved insulin sensitivity, and a proteolytic fragment of adiponectin stimulates ␤ oxidation in muscle. Here, we show that the ratio, and not the absolute amounts, between these two oligomeric forms (HMW to LMW) is critical in determining insulin sensitivity. We define a new index, S A , that can be calculated as the ratio of HMW/(HMW ؉ LMW). db/db mice, despite similar total adiponectin levels, display decreased S A values compared with wild type littermates, as do type II diabetic patients compared with insulin-sensitive individuals. Furthermore, S A improves with peroxisome proliferator-activated receptor-␥ agonist treatment (thiazolidinedione; TZD) in mice and humans. We demonstrate that changes in S A in a number of type 2 diabetic cohorts serve as a quantitative indicator of improvements in insulin sensitivity obtained during TZD treatment, whereas changes in total serum adiponectin levels do not correlate well at the individual level. Acute alterations in S A (⌬S A ) are strongly correlated with improvements in hepatic insulin sensitivity and are less relevant as an indicator of improved muscle insulin sensitivity in response to TZD treatment, further underscoring the conclusions from previous clamp studies that suggested that the liver is the primary site of action for the full-length protein. These observations suggest that the HMW adiponectin complex is the active form of this protein, which we directly demonstrate in vivo by its ability to depress serum glucose levels in a dose-dependent manner.

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Acrp30/adiponectin is an adipocyte-specific secretory protein that has recently been implicated as a mediator of systemic insulin sensitivity with liver and muscle as target organs. Acrp30 is found as two forms in serum, as a lower molecular weight trimer-dimer and a high molecular weight complex. Little is know about the regulation and significance of these Acrp30 complexes in serum and about the events that lead to the generation of the bioactive ligand. Here, we show that there is a profound sexual dimorphism of Acrp30 levels and complex distribution in serum. Female mice display significantly higher levels of the high molecular weight complex in serum than males. In both females and males, levels of the high molecular weight complex are significantly reduced in response to a systemic increase of insulin. The ratio of the two complexes is restored upon normalization of glucose levels. Structurally, we show that oligomer formation of Acrp30 critically depends on disulfide bond formation mediated by Cys-39. Mutation of Cys-39 results in trimers that are subject to proteolytic cleavage in the collagenous domain. Surprisingly, Acrp30(C39S) or wild-type Acrp30 treated with dithiothreitol are significantly more bioactive than the higher order oligomeric forms of the protein with respect to reduction of serum glucose levels. Furthermore, treatment of primary hepatocytes with trimeric and higher order forms of Acrp30 confirms that the increased bioactivity seen in vivo is reflected in an augmented potency to reduce glucose output in the presence of gluconeogenic stimuli. Combined, these results shed new light on the regulation of this complex protein and suggest a new model for in vivo activation of the protein, implicating a serum reductase activity.Adipose has been under appreciated as an endocrine tissue for decades because of the prevalent opinion that it served merely as storage for lipids. Recently, however, the importance of adipocytes to whole body energy homeostasis and metabolism has been underscored by several reports focusing on secreted products of adipocytes (1-4). There has been increased interest in adipose tissue as an endocrine organ, and several of these secreted proteins, termed adipokines, are currently undergoing extensive study regarding roles as divergent as feeding behavior to cardiovascular protection. For instance, leptin, the gene disrupted in ob/ob mice, has central roles in the hypothalamus, as well as peripheral effects in liver, muscle, and endothelial cells (5). Other adipose-secreted products, such as tumor necrosis factor ␣ and adipsin (complement factor D), have well established functions in innate immunity (6 -9). The recently identified adipokine resistin has been implicated as a modulator of insulin sensitivity and is also being studied for its effects on metabolism (4, 10).Acrp30 (also known as adiponectin, AdipoQ, and GBP28) is an adipokine exclusively synthesized and secreted by adipocytes (11-14). Acrp30 has recently been shown to influence glucose homeostasis and insulin se...

Adiponectin (ADP) is an adipocyte hormone involved in glucose and lipid metabolism. We detected a rise in ADP in cerebrospinal fluid after intravenous (i.v.) injection, consistent with brain transport. In contrast to leptin, intracerebroventricular (i.c.v.) administration of ADP decreased body weight mainly by stimulating energy expenditure. Full-length ADP, mutant ADP with Cys39 replaced with serine, and globular ADP were effective, whereas the collagenous tail fragment was not. Lep(ob/ob) mice were especially sensitive to i.c.v. and systemic ADP, which resulted in increased thermogenesis, weight loss and reduction in serum glucose and lipid levels. ADP also potentiated the effect of leptin on thermogenesis and lipid levels. While both hormones increased expression of hypothalamic corticotropin-releasing hormone (CRH), ADP had no substantial effect on other neuropeptide targets of leptin. In addition, ADP induced distinct Fos immunoreactivity. Agouti (A(y)/a) mice did not respond to ADP or leptin, indicating the melanocortin pathway may be a common target. These results show that ADP has unique central effects on energy homeostasis.

The adipose tissue-derived hormone adiponectin improves insulin sensitivity and its circulating levels are decreased in obesityinduced insulin resistance. Here, we report the generation of a mouse line with a genomic disruption of the adiponectin locus. We aimed to identify whether these mice develop insulin resistance and which are the primary target tissues affected in this model. Using euglycemic/insulin clamp studies, we demonstrate that these mice display severe hepatic but not peripheral insulin resistance. Furthermore, we wanted to test whether the lack of adiponectin magnifies the impairments of glucose homeostasis in the context of a dietary challenge. When exposed to high fat diet, adiponectin null mice rapidly develop glucose intolerance. Specific PPAR␥ agonists such as thiazolidinediones (TZDs) improve insulin sensitivity by mechanisms largely unknown. Circulating adiponectin levels are significantly up-regulated in vivo upon activation of PPAR␥. Both TZDs and adiponectin have been shown to activate AMP-activated protein kinase (AMPK) in the same target tissues. We wanted to address whether the ability of TZDs to improve glucose tolerance is dependent on adiponectin and whether this improvement involved AMPK activation. We demonstrate that the ability of PPAR␥ agonists to improve glucose tolerance in ob/ob mice lacking adiponectin is diminished. Adiponectin is required for the activation of AMPK upon TZD administration in both liver and muscle. In summary, adiponectin is an important contributor to PPAR␥-mediated improvements in glucose tolerance through mechanisms that involve the activation of the AMPK pathway.Adiponectin/ACRP30 (adipocyte complement-related protein of 30 kDa), an adipocyte-specific secretory protein, has been shown to modulate insulin sensitivity; however, the mechanism(s) by which it acts are not fully understood (1). A number of clinical studies revealed a strong link between whole body insulin sensitivity and circulating adiponectin levels (2). Furthermore, circulating adiponectin is negatively correlated with the body mass index (3). Weight reduction leads to a significant increase in adiponectin plasma levels slightly preceding improvements in insulin sensitivity, thus suggesting a causative role of adiponectin in enhancing insulin sensitivity (4). Adiponectin null mouse models were described previously, however, with somewhat varying outcomes regarding their metabolic phenotype. Kubota et al. (5) noted mild insulin resistance under basal conditions in heterozygotes (60% reduction in adiponectin serum levels) and more severe insulin resistance in adiponectin null animals. This report differed from adiponectin null mice described by Maeda et al. (6) that showed nearly normal insulin sensitivity when fed on a standard laboratory diet but developed severe insulin resistance in as few as 2 weeks on a high fat/high sucrose diet. However, a third independent report of adiponectin null mice by Ma et al. (7) described an unexpected increase in fatty acid oxidation in skeletal muscle...

Adiponectin is a plasma protein expressed exclusively in adipose tissue. Adiponectin levels are linked to insulin sensitivity, but a direct effect of chronically elevated adiponectin on improved insulin sensitivity has not yet been demonstrated. We identified a dominant mutation in the collagenous domain of adiponectin that elevated circulating adiponectin values in mice by 3-fold. Adiponectinemia raised lipid clearance and lipoprotein lipase activity, and suppressed insulin-mediated endogenous glucose production. The induction of adiponectin during puberty and the sexual dimorphism in adult adiponectin values were preserved in these transgenic animals. As a result of elevated adiponectin, serum PRL values and brown adipose mass both increased. The effects on carbohydrate and lipid metabolism were associated with elevated phosphorylation of 5'-AMP-activated protein kinase in liver and elevated expression of peroxisomal proliferator-activated receptor gamma2, caveolin-1, and mitochondrial markers in white adipose tissue. These studies strongly suggest that increasing endogenous adiponectin levels has direct effects on insulin sensitivity and may induce similar physiological responses as prolonged treatment with peroxisomal proliferator-activated receptor gamma agonists.

We describe the generation and characterization of the first inducible 'fatless' model system, the FAT-ATTAC mouse (fat apoptosis through targeted activation of caspase 8). This transgenic mouse develops identically to wild-type littermates. Apoptosis of adipocytes can be induced at any developmental stage by administration of a FK1012 analog leading to the dimerization of a membrane-bound, adipocyte-specific caspase 8-FKBP fusion protein. Within 2 weeks of dimerizer administration, FAT-ATTAC mice show near-knockout levels of circulating adipokines and markedly reduced levels of adipose tissue. FAT-ATTAC mice are glucose intolerant, have diminished basal and endotoxin-stimulated systemic inflammation, are less responsive to glucose-stimulated insulin secretion and show increased food intake independent of the effects of leptin. Most importantly, we show that functional adipocytes can be recovered upon cessation of treatment, allowing the study of adipogenesis in vivo, as well as a detailed examination of the importance of the adipocyte in the regulation of multiple physiological functions and pathological states.

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