Post-translational modifications of the four conserved lysine residues within the collagenous domain of adiponectin are required for the formation of its high molecular weight oligomeric complex

Preatherosclerosis and adiponectin subfractions in obese adolescents

We evaluated total adiponectin, high-molecular weight (HMW), medium-molecular weight (MMW), low-molecular weight (LMW) adiponectin subfractions, clinical parameters, routine lab parameters, lipids, metabolic, inflammatory biomarkers, and intima-media thickness (IMT) of common carotid arteries in 70 obese juveniles and adolescents with preatherosclerosis and 55 normal weight controls of similar age and gender distribution. Compared with the controls, the obese probands had a significantly increased IMT (P < 0.001) and elevated ultra-sensitive C-reactive protein (P < 0.001) indicating early vascular burden. Total and HMW adiponectin were significantly decreased in the obese cohort. The ratio between HMW and total adiponectin was significantly decreased in obese probands whereas the LMW/total adiponectin ratio was increased. Overall, total-, HMW, and MMW adiponectin were significantly negatively correlated with carotid IMT. The HMW/total adiponectin ratio correlated significantly negatively, and the LMW/total adiponectin ratio significantly positively with the IMT. Furthermore, HMW adiponectin was significantly positively correlated with high-density lipoprotein (HDL)-cholesterol and serum apolipoprotein A1, and negatively with BMI, triglycerides, homeostatic model assessment (HOMA)-index, leptin, liver transaminases, and uric acid. This remained stable after controlling for gender. Multiple regression analysis of body measures and all other lab parameters showed the strongest correlation between HMW adiponectin and carotid IMT (beta = -0.35, P < 0.001). Taken together, our study provides the first evidence that preatherosclerosis in obese juveniles and adolescents is associated with altered subfractions of adiponectin, whereas after multiple testing the HMW subfraction showed a better correlation to IMT compared with total adiponectin.

A disulfide-bond A oxidoreductase-like protein (DsbA-L) regulates adiponectin multimerization

Impairments in adiponectin multimerization lead to defects in adiponectin secretion and function and are associated with diabetes, yet the underlying mechanisms remain largely unknown. We have identified an adiponectin-interacting protein, previously named GST-kappa, by yeast 2-hybrid screening. The adiponectin-interacting protein contains 2 thioredoxin domains and has very little sequence similarity to other GST isoforms. However, this protein shares high sequence and secondary structure homology to bacterial disulfide-bond A oxidoreductase (DsbA) and is thus renamed DsbA-like protein (DsbA-L). DsbA-L is highly expressed in adipose tissue, and its expression level is negatively correlated with obesity in mice and humans. DsbA-L expression in 3T3-L1 adipocytes is stimulated by the insulin sensitizer rosiglitazone and inhibited by the inflammatory cytokine TNFalpha. Overexpression of DsbA-L promoted adiponectin multimerization while suppressing DsbA-L expression by RNAi markedly and selectively reduced adiponectin levels and secretion in 3T3-L1 adipocytes. Our results identify DsbA-L as a key regulator for adiponectin biosynthesis and uncover a potential new target for developing therapeutic drugs for the treatment of insulin resistance and its associated metabolic disorders.

Adipocytokines and the metabolic complications of obesity

CONTEXT

Adipose tissue is increasingly recognized as an active endocrine organ with many secretory products and part of the innate immune system. With obesity, macrophages infiltrate adipose tissue, and numerous adipocytokines are released by both macrophages and adipocytes. Adipocytokines play important roles in the pathogenesis of insulin resistance and associated metabolic complications such as dyslipidemia, hypertension, and premature heart disease.

EVIDENCE ACQUISITION

Published literature was analyzed with the intent of addressing the role of the major adipose secretory proteins in human obesity, insulin resistance, and type 2 diabetes.

EVIDENCE SYNTHESIS

This review analyzes the characteristics of different adipocytokines, including leptin, adiponectin, pro-inflammatory cytokines, resistin, retinol binding protein 4, visfatin, and others, and their roles in the pathogenesis of insulin resistance.

CONCLUSIONS

Inflamed fat in obesity secretes an array of proteins implicated in the impairment of insulin signaling. Further studies are needed to understand the triggers that initiate inflammation in adipose tissue and the role of each adipokine in the pathogenesis of insulin resistance.

Adiponectin mitigates the severity of arthritis in mice with collagen-induced arthritis

OBJECTIVE

Adiponectin (AD) is considered an inflammation modulator. In this study, we investigated the effect of AD on rheumatoid arthritis (RA) using a collagen-induced arthritis (CIA) mouse model and RA synovial fibroblasts (RASF).

METHODS

Fifteen DBA/1 mice were divided into three groups. All mice, except the control group, were injected with type II collagen. AD was intra-articularly injected in the left hind legs after arthritis development (the AD-treated group). The severity of the arthritis was measured using an arthritis score and paw thickness. A histopathological assessment of joint sections was performed by haematoxylin/eosin (H&E) staining. Tumour necrosis factor (TNF)-alpha, interleukin (IL)-1beta, IL-6, and matrix metalloproteinase (MMP)-3 expression was evaluated by immunohistochemical staining in the CIA mice. Synovial tissue was obtained from four RA patients during total joint replacement. RASF cultures were established from this tissue. RASF were pretreated with AD and stimulated by TNFalpha or IL-1beta. TNFalpha, IL-1beta, IL-6, and MMP-3 production was measured by enzyme-linked immunosorbent assay (ELISA) and reverse transcription polymerase chain reaction (RT-PCR). RASF proliferation was evaluated using the MTT assay.

RESULTS

AD significantly mitigated the severity of the arthritis and histopathological findings indicative of RA in CIA mice. TNFalpha, IL-1beta, and MMP-3 expression decreased, but IL-6 expression in AD-treated joint tissues increased. Moreover, AD reduced TNFalpha, IL-1beta, and MMP-3 expression in stimulated RASF and increased IL-6 expression in IL-1beta-stimulated RASF. AD significantly inhibited IL-1beta-induced RASF proliferation, despite increased IL-6 expression.

CONCLUSION

These data suggest that AD may play an anti-inflammatory role in the pathophysiology of RA.

Mitochondrial dysfunction contributes to the increased vulnerabilities of adiponectin knockout mice to liver injury

Adiponectin is an adipocyte-derived hormone with a wide range of beneficial effects on obesity-related medical complications. Numerous epidemiological investigations in diverse ethnic groups have identified a lower adiponectin level as an independent risk factor for nonalcoholic fatty liver diseases and liver dysfunctions. Animal studies have demonstrated that replenishment of adiponectin protects against various forms of hepatic injuries, suggesting it to be a potential drug candidate for the treatment of liver diseases. This study was designed to investigate the cellular and molecular mechanisms underlying the hepatoprotective effects of adiponectin. Our results demonstrated that in adiponectin knockout (ADN-KO) mice, there was a preexisting condition of hepatic steatosis and mitochondrial dysfunction that might contribute to the increased vulnerabilities of these mice to secondary liver injuries induced by obesity and other conditions. Adenovirus-mediated replenishment of adiponectin depleted lipid accumulation, restored the oxidative activities of mitochondrial respiratory chain (MRC) complexes, and prevented the accumulation of lipid peroxidation products in ADN-KO mice but had no obvious effects on mitochondrial biogenesis. The gene and protein levels of uncoupling protein 2 (UCP2), a mitochondrial membrane transporter, were decreased in ADN-KO mice and could be significantly up-regulated by adiponectin treatment. Moreover, the effects of adiponectin on mitochondrial activities and on protection against endotoxin-induced liver injuries were significantly attenuated in UCP2 knockout mice.

CONCLUSION

These results suggest that the hepatoprotective properties of adiponectin are mediated at least in part by an enhancement of the activities of MRC complexes through a mechanism involving UCP2.

Adipokines and insulin resistance

Obesity is associated with an array of health problems in adult and pediatric populations. Understanding the pathogenesis of obesity and its metabolic sequelae has advanced rapidly over the past decades. Adipose tissue represents an active endocrine organ that, in addition to regulating fat mass and nutrient homeostasis, releases a large number of bioactive mediators (adipokines) that signal to organs of metabolic importance including brain, liver, skeletal muscle, and the immune system--thereby modulating hemostasis, blood pressure, lipid and glucose metabolism, inflammation, and atherosclerosis. In the present review, we summarize current data on the effect of the adipose tissue-derived hormones adiponectin, chemerin, leptin, omentin, resistin, retinol binding protein 4, tumor necrosis factor-alpha and interleukin-6, vaspin, and visfatin on insulin resistance.

Identification and characterization of CTRP9, a novel secreted glycoprotein, from adipose tissue that reduces serum glucose in mice and forms heterotrimers with adiponectin

Adiponectin is a major insulin-sensitizing, multimeric hormone derived from adipose tissue that acts on muscle and liver to regulate whole-body glucose and lipid metabolism. Here, we describe a novel and highly conserved paralog of adiponectin designated as C1q/TNF-related protein (CTRP) 9. Of all the CTRP paralogs, CTRP9 shows the highest degree of amino acid identity to adiponectin in its globular C1q domain. CTRP9 is expressed predominantly in adipose tissue and females expresses higher levels of the transcript than males. Moreover, its expression levels in ob/ob mice changed in an age-dependent manner, with significant up-regulation in younger mice. CTRP9 is a secreted glycoprotein with multiple post-translational modifications in its collagen domain that include hydroxylated prolines and hydroxylated and glycosylated lysines. It is secreted as multimers (predominantly trimers) from transfected cells and circulates in the mouse serum with levels varying according to sex and metabolic state of mice. Furthermore, CTRP9 and adiponectin can be secreted as heterooligomers when cotransfected into mammalian cells, and in vivo, adiponectin/CTRP9 complexes can be reciprocally coimmunoprecipitated from the serum of adiponectin and CTRP9 transgenic mice. Biochemical analysis demonstrates that adiponectin and CTRP9 associate via their globular C1q domain, and this interaction does not require their conserved N-terminal cysteines or their collagen domains. Furthermore, we show that adiponectin and CTRP9 form heterotrimers. In cultured myotubes, CTRP9 specifically activates AMPK, Akt, and p44/42 MAPK signaling pathways. Adenovirus-mediated overexpression of CTRP9 in obese (ob/ob) mice significantly lowered serum glucose levels. Collectively, these results suggest that CTRP9 is a novel adipokine, and further study of CTRP9 will yield novel mechanistic insights into its physiological and metabolic function.

The development of microalbuminuria is associated with raised longitudinal adiponectin levels in female but not male adolescent patients with type 1 diabetes

AIMS/HYPOTHESIS

We determined the longitudinal relationship between adiponectin levels and the development of microalbuminuria in an inception cohort of children with type 1 diabetes.

METHODS

Blood samples collected annually over a median of 9.0 (range 1.3-14.9) years were assayed for adiponectin and HbA(1c) in 55 children (36 girls) with type 1 diabetes and microalbuminuria whose age of onset of diabetes was 9.4 years (range 2.2-15.4). Samples were also assayed from normoalbuminuric children (controls) matched for age, sex and duration of diabetes.

RESULTS

Overall, adiponectin levels were higher in girls than in boys, but only after 11 years of age (median [range]: 15.3 [5.8-124.4] vs 11.6 [4.1-26.5] mg/l, p < 0.001). Furthermore, adiponectin levels were higher in girls with microalbuminuria than in control girls, but this was only apparent after the onset of microalbuminuria (p = 0.001, adjusted for BMI, daily insulin dose, HbA(1c) and age). In boys, adiponectin levels did not differ between those with microalbuminuria and controls. Further sex-related discordant associations with adiponectin levels were observed; in girls, adiponectin levels were positively related to HbA(1c) levels (r = 0.2, p = 0.05) and urine albumin excretion (r = 0.3, p < 0.05) and inversely related to BMI (r = -0.2, p < 0.05). These associations were absent in boys.

CONCLUSIONS/INTERPRETATION

In adolescent girls with type 1 diabetes but not in boys, adiponectin levels increase with increasing urine albumin excretion and onset of microalbuminuria. Although causal links cannot be inferred, this sexual dimorphism may reflect interactive effects of hyperglycaemia and sex steroids on risk of complications and adiponectin production.

Adiponectin deficiency promotes endothelial activation and profoundly exacerbates sepsis-related mortality

Sepsis is a multifactorial, and often fatal, disorder typically characterized by widespread inflammation and immune activation with resultant endothelial activation. In the present study, we postulated that the adipokine adiponectin serves as a critical modulator of survival and endothelial activation in sepsis. To this aim, we evaluated both loss-of-function (adiponectin gene-deficient mice) and subsequent gain-of-function (recombinant adiponectin reconstitution) strategies in two well-established inflammatory models, cecal ligation perforation (CLP) and thioglyocollate-induced peritonitis. Adipoq(-/-) mice, subjected to CLP, exhibited a profound ( approximately 8-fold) reduction in survival compared with their wild-type Adipoq(+/+) littermates after 48 h. Furthermore, compared with wild-type controls, thioglycollate challenge resulted in a markedly greater influx of peritoneal neutrophils in Adipoq(-/-) mice accompanied by an excess production of key chemoattractant cytokines (IL-12p70, TNFalpha, MCP-1, and IL-6) and upregulation of aortic endothelial adhesion molecule VCAM-1 and ICAM-1 expressions. Importantly, all of these effects were blunted by recombinant total adiponectin administration given 3 days prior to thioglycollate challenge. The protective effects of adiponectin were ascribed largely to higher-order adiponectin oligomers, since administration of recombinant C39A trimeric adiponectin did not attenuate endothelial adhesion molecule expression in thioglycollate-challenged Adipoq(-/-) mice. These data suggest a critical role of adiponectin as a modulator of survival and endothelial inflammation in experimental sepsis and a potential mechanistic link between adiposity and increased sepsis.

Physiological, pharmacological, and nutritional regulation of circulating adiponectin concentrations in humans

Adiponectin is an adipocyte hormone that links visceral adiposity with insulin resistance and atherosclerosis. It is unique among adipocyte-derived hormones in that its circulating concentrations are inversely proportional to adiposity, and low adiponectin concentrations predict the development of type 2 diabetes and cardiovascular disease. Consequently, in the decade since its discovery, adiponectin has generated immense interest as a potential therapeutic target for the metabolic syndrome and diabetes. This review summarizes current research regarding the regulation of circulating adiponectin concentrations by physiological, pharmacological, and nutritional factors, with an emphasis on human studies. In humans, plasma adiponectin concentrations are influenced by age and gender, and are inversely proportional to visceral adiposity. In vitro studies suggest that adiponectin production may be determined primarily by adipocyte size and insulin sensitivity, with larger, insulin-resistant adipocytes producing less adiponectin. While adiponectin concentrations are unchanged after meal ingestion, they are increased by significant weight loss, such as after bariatric surgery. In addition, adiponectin production is inhibited by a number of hormones, including testosterone, prolactin, glucocorticoids and growth hormone, and by inflammation and oxidative stress in adipose tissue. Smoking decreases, while moderate alcohol consumption increases, circulating adiponectin concentrations. Dietary fatty acid composition in rodents influences adiponectin production via ligand-activated nuclear receptors (PPARs); however, current evidence in humans is equivocal. In addition to PPAR agonists (such as thiazolidinediones and fibrates), a number of pharmacological agents (angiotensin receptor type 1 blockers, ACE inhibitors, and cannabinoid receptor antagonists) used in treatment of the metabolic syndrome also increase adiponectin concentrations in humans.

Acarbose treatment increases serum total adiponectin levels in patients with type 2 diabetes

Adiponectin is an anti-diabetic and anti-atherogenic adipokine that serves as a major determinant of insulin sensitivity. Thiazolidine derivatives increase circulating adiponectin, particularly the high molecular weight isoform, which has been shown to well correlate with amelioration of insulin resistance by thiazolidines in diabetic patients. alpha-glucosidase inhibitors are another class of anti-diabetic agents that specifically reduce postprandial blood glucose elevations, but its effect on adiponectin is largely unknown. In the present study we investigated effect of an alpha-glucosidase inhibitor, acarbose, together with pioglitazone, the only thiazolidine derivative available in Japan, on serum concentrations of adiponectin. Seventeen patients with type 2 diabetes were treated with acarbose and sixteen with pioglitazone for three months. Treatment with acarbose and pioglitazone decreased HbA1c values by 0.49% and 0.63%, respectively. Pioglitazone, as expected, increased serum levels of total adiponectin by 2.1 fold and its high molecular weight isoform by 3.6 fold. We found that acarbose also caused a small but significant increase in serum concentrations of total adiponectin. However, in contrast to pioglitazone, no appreciable changes were observed in the levels of high molecular weight adiponectin. In conclusion, acarbose increases serum concentrations of total adiponectin without preference of the high molecular weight isoform in type 2 diabetic patients. Clinical relevance of the increased adiponectin to the acarbose effects remains to be elucidated.

Structural polymorphism of oligomeric adiponectin visualized by electron microscopy

Adiponectin, a macromolecular complex similar to the members of the C1q and other collagenous homologues, elicits diverse biological functions, including anti-diabetes, anti-atherosclerosis, anti-inflammation and anti-tumor activities, which have been directly linked to the high molecular weight (HMW) oligomeric structures formed by multiples of adiponectin trimers. Here, we report the 3-D reconstructions of isolated full-length, recombinant murine C39A adiponectin trimer and hexamer of wild-type trimers (the major HMW form) determined by single-particle analysis of electron micrographs. The pleiomorphic ensemble of collagen-like stretches of the trimers leads to a dynamic structure of HMW that partition into two major classes, the fan-shaped (class I) and bouquet-shaped (class II). In both of these, while the N termini cluster into a compact ellipsoid-shaped (approximately 60 Ax45 Ax45 A) volume, the collagenous domains assume a variety of arrangements. The domains are splayed by up to approximately 90 degrees in class I, can form a close-packed, up to approximately 100x40 A cylindrical assembly in class II, which can house about half of the 66 putative collagen-like sequence and the rest, tethered to the trimeric globular domains at the C terminus, are highly dynamic. As a result, the globular domains elaborate a variety of arrangements, covering an area of up to approximately 4.9x10(5) A(2) and up to approximately 320 A apart, some of which were captured in reconstructions of class II. Our reconstructions suggest that the N-terminal structured domain, agreeing approximately with the expected volume for the octadecameric assembly of the terminal 27 amino acids, is crucial to the formation of the functionally active HMW. On the other hand, conformational flexibility of the trimers at the C terminus can allow the HMW to access and cluster disparate target ligands binding to the globular domains, which may be necessary to activate cellular signaling leading to the remarkable functional diversity of adiponectin.

Serum adiponectin and progression of diabetic nephropathy in patients with type 1 diabetes

OBJECTIVE

The purpose of this study was to elucidate whether serum adiponectin is associated with progression of diabetic nephropathy in type 1 diabetic patients.

RESEARCH DESIGN AND METHODS

This was a prospective follow-up study as a part of the nationwide Finnish Diabetic Nephropathy Study; 1,330 type 1 diabetic patients were followed for 5.0 +/- 2.2 years. Patients were divided at baseline into three groups according to their urinary albumin excretion rate (AER) in three consecutive overnight or 24-h urine collections: 818 patients with normoalbuminuria (AER <20 microg/min), 216 patients with microalbuminuria (20 microg/min <or= AER < 200 microg/min), and 296 patients with macroalbuminuria (AER >or=200 microg/min). Progression of albuminuria was the main outcome. Adiponectin was measured by a time-resolved immunofluorometric assay, and the values were log-transformed and adjusted for age, BMI, and sex before analysis.

RESULTS

Progression either to the next albuminuria level or to end-stage renal disease (ESRD) occurred in 193 patients. No difference in adiponectin concentrations was observed between progressors and nonprogressors in patients with normoalbuminuria or microalbuminuria. In the patients with macroalbuminuria, progression to ESRD was associated with higher adiponectin in the entire group (23.4 +/- 17.1 vs. 16.0 +/- 8.5 mg/l, P < 0.001) and in men (P < 0.001) and women (P < 0.001) separately. Progression to ESRD was also associated with systolic blood pressure, insulin dose, A1C, serum cholesterol, serum triglycerides, AER, and estimated glomerular filtration rate (eGFR). When these covariates were inserted in a Cox regression analysis, A1C, triglycerides, eGFR, and adiponectin were significantly associated with progression from macroalbuminuria.

CONCLUSIONS

Increased serum adiponectin levels predict the progression from macroalbuminuria to ESRD in type 1 diabetic patients.

Adiponectin is expressed by skeletal muscle fibers and influences muscle phenotype and function

Adiponectin (Ad) is linked to various disease states and mediates antidiabetic and anti-inflammatory effects. While it was originally thought that Ad expression was limited to adipocytes, we demonstrate here that Ad is expressed in mouse skeletal muscles and within differentiated L6 myotubes, as assessed by RT-PCR, Western blot, and immunohistochemical analyses. Serial muscle sections stained for fiber type, lipid content, and Ad revealed that muscle fibers with elevated intramyocellular Ad expression were consistently type IIA and IID fibers with detectably higher intramyocellular lipid (IMCL) content. To determine the effect of Ad on muscle phenotype and function, we used an Ad-null [knockout (KO)] mouse model. Body mass increased significantly in 24-wk-old KO mice [+5.5 +/- 3% relative to wild-type mice (WT)], with no change in muscle mass observed. IMCL content was significantly increased (+75.1 +/- 25%), whereas epididymal fat mass, although elevated, was not different in the KO mice compared with WT (+35.1 +/- 23%; P = 0.16). Fiber-type composition was unaltered, although type IIB fiber area was increased in KO mice (+25.5 +/- 6%). In situ muscle stimulation revealed lower peak tetanic forces in KO mice relative to WT (-47.5 +/- 6%), with no change in low-frequency fatigue rates. These data demonstrate that the absence of Ad expression causes contractile dysfunction and phenotypical changes in skeletal muscle. Furthermore, we demonstrate that Ad is expressed in skeletal muscle and that its intramyocellular localization is associated with elevated IMCL, particularly in type IIA/D fibers.

Post-translational modifications of adiponectin: mechanisms and functional implications

Adiponectin is an insulin-sensitizing adipokine with anti-diabetic, anti-atherogenic, anti-inflammatory and cardioprotective properties. This adipokine is secreted from adipocytes into the circulation as three oligomeric isoforms, including trimeric, hexameric and the HMW (high-molecular-mass) oligomeric complex consisting of at least 18 protomers. Each oligomeric isoform of adiponectin exerts distinct biological properties in its various target tissues. The HMW oligomer is the major active form mediating the insulin-sensitizing effects of adiponectin, whereas the central actions of this adipokine are attributed primarily to the hexameric and trimeric oligomers. In patients with Type 2 diabetes and coronary heart disease, circulating levels of HMW adiponectin are selectively decreased due to an impaired secretion of this oligomer from adipocytes. The biosynthesis of the adiponectin oligomers is a complex process involving extensive post-translational modifications. Hydroxylation and glycosylation of several conserved lysine residues in the collagenous domain of adiponectin are necessary for the intracellular assembly and stabilization of its high-order oligomeric structures. Secretion of the adiponectin oligomers is tightly controlled by a pair of molecular chaperones in the ER (endoplasmic reticulum), including ERp44 (ER protein of 44 kDa) and Ero1-Lalpha (ER oxidoreductase 1-Lalpha). ERp44 inhibits the secretion of adiponectin oligomers through a thiol-mediated retention. In contrast, Ero1-Lalpha releases HMW adiponectin trapped by ERp44. The PPARgamma (peroxisome-proliferator-activated receptor gamma) agonists thiazolidinediones selectively enhance the secretion of HMW adiponectin through up-regulation of Ero1-Lalpha. In the present review, we discuss the recent advances in our understanding of the structural and biological properties of the adiponectin oligomeric isoforms and highlight the role of post-translational modifications in regulating the biosynthesis of HMW adiponectin.

Activation of peroxisome proliferator-activated receptor-alpha inhibits the injurious effects of adiponectin in rat steatotic liver undergoing ischemia-reperfusion

Hepatic steatosis is a major risk factor in ischemia-reperfusion (I/R). Adiponectin acts as an antiobesity and anti-inflammatory hormone. Adiponectin activates peroxisome proliferator-activated receptor-alpha (PPAR-alpha), a transcription factor that regulates inflammation in liver disease. Ischemic preconditioning (PC) based on brief periods of I/R protects steatotic livers against subsequent sustained I/R injury, but just how this is achieved is poorly understood. This study explains the role of PPAR-alpha and adiponectin in the vulnerability shown by steatotic livers to I/R and the benefits of PC in this situation. PPAR-alpha and adiponectin levels in nonsteatotic livers undergoing I/R were similar to those found in the sham group. However, reduced PPAR-alpha and increased adiponectin levels, particularly the high molecular weight isoform, were observed in steatotic livers as a consequence of I/R. Our results suggest that mitogen-activated protein kinases (MAPKs) may be positive regulators of adiponectin accumulation in steatotic livers. The addition of adiponectin small interfering RNA (siRNA) before I/R protected steatotic livers against oxidative stress and hepatic injury. The induction of PC before I/R increased PPAR-alpha and reduced adiponectin levels in steatotic livers. PC, which increased PPAR-alpha, as well as PPAR-alpha agonist pretreatment reduced MAPK expression, adiponectin, oxidative stress, and hepatic injury that follows I/R. In addition, the administration of a PPAR-alpha antagonist in preconditioned steatotic livers eliminated the beneficial effects of PC on MAPKs, adiponectin, oxidative stress, and hepatic injury.

CONCLUSION

Steatotic livers are more predisposed to down-regulate PPAR-alpha and overexpress adiponectin when subjected to I/R. PPAR-alpha agonists and adiponectin siRNA are promising candidates to protect steatotic livers. PPAR-alpha agonists as well as PC, through PPAR-alpha, inhibited MAPK expression following I/R. This in turn inhibited adiponectin accumulation in steatotic livers and adiponectin-worsening effects on oxidative stress and hepatic injury.

Adiponectin binds C1q and activates the classical pathway of complement

The adipose-specific protein adiponectin binds to a number of target molecules, including damaged endothelium and the surface of apoptotic cells. However, the significance of this binding remains unclear. This study demonstrates the binding of purified C1q to recombinant adiponectin under physiological conditions, and the dependence of this upon Ca(++) and Mg(++). Binding was enhanced by metaperiodate-mediated destruction of glucosylgalactosyl sugars on adiponectin. Adiponectin was bound by the globular domain of the A chain of collagenase-digested C1q, and C1q binding induced deposition of C4 and C3 through activation of the classical complement pathway. After Western blotting, affinity-purified adiponectin from human serum bound C1q, whereas adiponectin in whole serum did not, unless pre-treated with metaperiodate. These results suggest adiponectin is member of the pattern-recognition family of defence collagens, able to bind target molecules and activate complement. It may therefore play an important role in innate immunity and autoimmune phenomena.

The trans-10, cis-12 isomer of conjugated linoleic acid decreases adiponectin assembly by PPARgamma-dependent and PPARgamma-independent mechanisms

The adipocyte-derived secretory protein adiponectin functions as an insulin-sensitizing agent. In plasma, adiponectin exists as low, medium, and high molecular weight oligomers. Treatment with trans-10, cis-12 conjugated linoleic acid (t-10, c-12 CLA) reduces levels of adiponectin as well as triglyceride (TG) in mice and adipocyte cell culture models. The aim of this study was to determine whether the effects of t-10, c-12 CLA on adiponectin and TG are mediated through modulation of the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma). 3T3-L1 cells were treated either during or after differentiation into adipocytes with 100 microM t-10, c-12 CLA with or without 10 microM troglitazone, a PPARgamma agonist, or 1 microM GW9662, a PPARgamma antagonist, and adiponectin and TG levels were analyzed. Treatment with t-10, c-12 CLA reduced TG as well as cellular and secreted adiponectin levels and impaired the assembly of adiponectin oligomers. These changes were accompanied by decreases in PPARgamma mass. Troglitazone was able to reverse the t-10, c-12 CLA-mediated decrease in TG levels and restore the assembly of adiponectin oligomers but was unable to restore adiponectin synthesis. Conversely, treatment with GW9662 decreased TG mass and impaired adiponectin oligomer assembly but did not decrease total adiponectin mass. In a reporter assay, t-10, c-12 CLA appeared to be a partial PPARgamma agonist and prevented the stimulation of reporter activity by troglitazone. Therefore, the t-10, c-12 CLA isomer appears to alter adipocyte adiponectin metabolism through PPARgamma-dependent and PPARgamma-independent mechanisms.

Adiponectin Mediates the Suppressive Effect of Rosiglitazone on Plasminogen Activator Inhibitor-1 Production

Objective— The purpose of this study was to examine the effects of PPAR-γ agonist rosiglitazone, relative to sulfonylureas, on circulating levels of adiponectin and the prothrombotic factor, plasminogen activator inhibitor (PAI)-1, in type 2 diabetic patients, and to investigate, in animal models, whether the antithrombotic action of rosiglitazone was mediated through adiponectin.

Methods and Results— Our clinical study (n=64) showed that after 24-week add-on therapy, the rosiglitazone group had a greater mean reduction in plasma PAI-1 levels (25%, versus 12% in sulfonylurea group, P =0.002). Stepwise multiple linear regression analysis identified the reduction in plasma fasting glucose and the rise in adiponectin levels to be independently associated with the reduction in PAI-I concentration in the rosiglitazone-treated patients. Rosiglitazone (20 mg/kg/d) reduced adipose tissue PAI-1 mRNA expression and its plasma levels in wild-type C57 mice with diet-induced obesity ( P <0.001), but this suppressive effect was attenuated in adiponectin knockout mice. Adenovirus-mediated overexpression of adiponectin led to a significant suppression of adipose tissue PAI-1 expression and its circulating concentrations in db/db diabetic mice. Our in vitro study demonstrated that recombinant adiponectin directly inhibited PAI-1 production in 3T3-L1 adipocytes.

Conclusions— The antithrombotic effect of rosiglitazone is mediated, at least in part, through the suppressive effect of adiponectin on PAI-1 production.

Adiponectin activates adenosine monophosphate-activated protein kinase and decreases luteinizing hormone secretion in LbetaT2 gonadotropes

Metabolic dysregulation is associated with reproductive disorders, but the underlying mechanisms are not clearly understood. Adiponectin is an adipocyte-derived secretory factor that improves insulin sensitivity. Results from animal models indicate that overexpression of adiponectin impairs female fertility. We hypothesized that adiponectin regulates reproduction by altering the hypothalamic-pituitary axis. Mouse LbetaT2 immortalized gonadotrope cells express both adiponectin receptors 1 and 2. Adiponectin increases phosphorylation of AMP-activated protein kinase (AMPK), a downstream target of adiponectin receptors, and reduces basal and GnRH-stimulated LH secretion, acutely. The repression of LH secretion can be mimicked by 5-aminoimidazole-4-carboxamide-1-beta-riboside, an AMP analog, suggesting the involvement of AMPK. A dominant-negative AMPK mutant or compound C, a selective AMPK inhibitor, potentiates basal LH secretion and abolishes the inhibitory effect of adiponectin. Chronic activation of AMPK by 5-aminoimidazole-4-carboxamide-1-beta-riboside decreases cellular LH levels, and expression of dominant-negative AMPK increases cellular LH levels, suggesting a second effect of AMPK to regulate LH synthesis. Lastly, intravenous injection of an adenovirus expressing adiponectin into male mice reduces serum LH levels without changing FSH levels. In conclusion, our results suggest that adiponectin decreases LH secretion in pituitary gonadotropes in an AMPK-dependent manner.

Preferential increase in high-molecular weight adiponectin after niacin

OBJECTIVE

Recently we have demonstrated that treatment with niacin raises adiponectin between 52% and 95% in patients with the metabolic syndrome. In this study we investigated whether all three adiponectin fractions are increased equally, and, secondly, whether the increase in the biologically most active high-molecular weight (HMW) fraction can prevent the deterioration of insulin sensitivity that was also observed after niacin.

METHODS AND RESULTS

We used sera frozen at -80 degrees C from a randomized double-blind placebo-controlled treatment study in which 20 men with the metabolic syndrome received 1500 mg niacin for 6 weeks. Low- and medium-molecular weight adiponectin increased by 35% and 33%, respectively, but HMW adiponectin by 88% (all p<0.05). The increase in HMW adiponectin was almost two times as large in patients with lower BMI and better insulin sensitivity. However, treatment with niacin induced a deterioration of insulin sensitivity, as assessed by the HOMA-IR, independently of the increase in HMW adiponectin.

CONCLUSION

HMW adiponectin is the fraction most affected by treatment with niacin. The niacin-associated deterioration of insulin sensitivity, however, occurs even in subgroups with the greatest increase of HMW adiponectin.

Activation of nuclear factor-kappaB by high molecular weight and globular adiponectin

Adipose tissue secretes a wide range of hormones named adipokines, and these may play a role in obesity-related inflammation. Adiponectin is an exceptional adipokine because low plasma concentrations are associated with obesity, type 2 diabetes, and cardiovascular diseases. It has been observed that plasma adiponectin concentrations are elevated during inflammatory conditions like preeclampsia and arthritis. Nuclear factor-kappaB (NF-kappaB) is an essential transcription factor for expression of inflammation-related proteins. We have used U937 cells stably transfected to express luciferase under the control of NF-kappaB to examine if adiponectin may modulate NF-kappaB activity. Physiological concentrations of native adiponectin induced NF-kappaB activity. This effect was relatively strong compared with proinflammatory adipokines like leptin, resistin, and IL-6. The enhanced NF-kappaB activity was attributed to the high molecular weight adiponectin isoforms. NF-kappaB was not activated by mutated adiponectin that is unable to form high molecular weight complexes. Furthermore, the C-terminal fragment, globular adiponectin, markedly increased NF-kappaB reporter activity, cytokine release, and mRNA expression of inflammation marker genes, at higher levels than stimulation with TNF-alpha and lipopolysaccharide. NF-kappaB activation by globular adiponectin was not affected by antibody inhibition of toll-like receptor 4 or TNF receptors 1 and 2 but was attenuated by inhibitors of p38 MAPK, phosphatidylinositol 3-kinase, and protein kinase C. Analyses of the p65 subunit of NF-kappaB in different leukocyte cell lines showed activation of two monocytic cell lines (U937 and THP-1) by native and globular adiponectin. Our results indicate that adiponectin has proinflammatory properties in monocytic cells.

Total and high molecular weight but not trimeric or hexameric forms of adiponectin correlate with markers of the metabolic syndrome and liver injury in Thai subjects

CONTEXT/OBJECTIVE

Decreased total adiponectin has been associated with metabolic disorders, including obesity, diabetes, fatty liver, and the metabolic syndrome. Although circulating adiponectin is composed of trimers, hexamers, and high molecular weight (HMW) multimers, there has been limited study of the specific metabolic correlates of these isoforms in humans. Thus, our objective was to evaluate the associations of these adiponectin isoforms with metabolic and anthropometric parameters.

DESIGN/PARTICIPANTS/SETTING

A total of 53 diabetic and 68 nondiabetic subjects attending outpatient clinics underwent cross-sectional metabolic characterization. Circulating levels of HMW, hexameric, and trimeric adiponectin were measured using a multimeric adiponectin ELISA based upon selective protease-mediated digestion.

RESULTS

On Spearman univariate analysis, both total and HMW adiponectin levels were inversely associated with body mass index, fasting glucose, homeostasis model of assessment of insulin resistance, triglycerides, and alanine aminotransferase (ALT) (all |r| >or= 0.22; P < 0.05), with the HMW isoform also positively correlated with high-density lipoprotein cholesterol (r = 0.19; P = 0.036). In contrast, hexameric and trimeric adiponectin were significantly associated with only body mass index (r = -0.23; P = 0.0102) and mid-upper arm circumference (r = 0.21; P = 0.039), respectively. On separate forward stepwise multiple linear regression analyses, fasting glucose and ALT emerged as independent, negative covariates of both total and HMW adiponectin, whereas no independent covariates of hexameric and trimeric adiponectin were identified. Furthermore, after adjustment for age, gender, and diabetes, mean ALT was highest in subjects in the lowest tertile of HMW adiponectin, followed in turn by the middle and highest tertiles, respectively (trend P = 0.028).

CONCLUSIONS

HMW adiponectin, but not hexameric or trimeric, tracks with the metabolic correlates of total adiponectin. Furthermore, an independent inverse association exists between ALT and HMW adiponectin.

Determinants of serum adiponectin in persons with and without type 1 diabetes

Low levels of adiponectin have been related to coronary heart disease, but adiponectin is higher in persons with type 1 diabetes who have an increased rate of coronary disease. In the Coronary Artery Calcification in Type 1 Diabetes Study (2000-2002), the authors investigated potential determinants of elevated adiponectin levels in persons with type 1 diabetes and whether a difference exists compared with nondiabetic persons. Serum adiponectin was measured in 1,393 persons (sex: 48% male; age: 38 (standard deviation: 9) years; diabetes duration: 23 (standard deviation: 9) years; 54% nondiabetic and 46% with type 1 diabetes). Determinants of log-transformed adiponectin levels were evaluated by multiple linear regression analysis with interaction terms to determine whether predictors of adiponectin levels differed by diabetes status. Adiponectin levels were higher in type 1 diabetic than nondiabetic persons (13.5 (standard deviation: 1.0) vs. 8.8 (standard deviation: 1.0) microg/ml; p < 0.0001), adjusting for age, gender, body mass index, and glomerular filtration rate. The final regression model explained 67% of the difference in adiponectin levels between type 1 diabetic and nondiabetic persons. The variables explaining this difference included high density lipoprotein cholesterol, albumin excretion rate, plasminogen activator inhibitor-1, and hemoglobin A1c level. Adiponectin is higher in type 1 diabetic than nondiabetic persons. Although some of the difference can be explained, further study is needed to better understand the relation between elevated adiponectin levels and patient outcomes, including coronary heart disease.

Secretion and assembly of type IV and VI collagens depend on glycosylation of hydroxylysines

Most lysines in type IV and VI collagens are hydroxylated and glycosylated, but the functions of these unique galactosylhydroxylysyl and glucosylgalactosylhydroxylysyl residues are poorly understood. The formation of glycosylated hydroxylysines is catalyzed by multifunctional lysyl hydroxylase 3 (LH3) in vivo, and we have used LH3-manipulated mice and cells as models to study the function of these carbohydrates. These hydroxylysine-linked carbohydrates were shown recently to be indispensable for the formation of basement membranes (Ruotsalainen, H., Sipilä, L., Vapola, M., Sormunen, R., Salo, A. M., Uitto, L., Mercer, D. K., Robins, S. P., Risteli, M., Aszodi, A., Fässler, R., and Myllylä, R. (2006) J. Cell Sci. 119, 625-635). Analysis of LH3 knock-out embryos and cells in this work indicated that loss of glycosylated hydroxylysines prevents the intracellular tetramerization of type VI collagen and leads to impaired secretion of type IV and VI collagens. Mice lacking the LH activity of LH3 produced slightly underglycosylated type IV and VI collagens with abnormal distribution. The altered distribution and aggregation of type VI collagen led to similar ultrastructural alterations in muscle to those detected in collagen VI knockout and some Ullrich congenital muscular dystrophy patients. Our results provide new information about the function of hydroxylysine-linked carbohydrates of collagens, indicating that they play an important role in the secretion, assembly, and distribution of highly glycosylated collagen types.

S-adenosylmethionine attenuates hepatic lipid synthesis in micropigs fed ethanol with a folate-deficient diet

BACKGROUND

To demonstrate a causative role of abnormal methionine metabolism in the pathogenesis of alcoholic steatosis, we measured the effects on hepatic lipid synthesis of supplementing ethanol and folate-deficient diets with S-adenosylmethionine (SAM), a metabolite that regulates methionine metabolism.

METHODS

Yucatan micropigs were fed folate-deficient diets as control, with ethanol at 40% of kcal, and with ethanol supplemented with SAM at 0.4 g/1,000 kcal for 14 weeks. Histopathology, triglyceride levels and transcripts, and protein levels of the regulatory signals of hepatic lipid synthesis were measured in terminal omental adipose and liver samples.

RESULTS

Feeding ethanol at 40% of kcal with folate-deficient diets for 14 weeks increased and supplemental SAM maintained control levels of liver and plasma triglyceride. Serum adiponectin, liver transcripts of adiponectin receptor-1 (AdipoR1), and phosphorylated adenosine monophosphate kinase-beta (p-AMPKbeta) were each reduced by ethanol feeding and were sustained at normal levels by SAM supplementation of the ethanol diets. Ethanol feeding activated and SAM supplementation maintained control levels of ER stress-induced transcription factor sterol regulatory element-binding protein-1c (SREBP-1c) and its targeted transcripts of lipid synthesizing enzymes acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and glycerol-3-phosphate acyltransferase (GPAT).

CONCLUSIONS

Ethanol feeding with a folate-deficient diet stimulates hepatic lipid synthesis by down-regulating adiponectin-mediated pathways of p-AMPK to increase the expression of nSREBP-1c and its targeted lipogenic enzymes. Preventing abnormal hepatic methionine metabolism by supplementing ethanol diets with SAM reduces liver triglyceride levels by up-regulation of adiponectin-mediated pathways to decrease fatty acid and triglyceride synthesis. This study demonstrates that ethanol-induced hepatic lipid synthesis is mediated in part by abnormal methionine metabolism, and strengthens the concept that altered methionine metabolism plays an integral role in the pathogenesis of steatosis.

Adiponectin deficiency increases leukocyte-endothelium interactions via upregulation of endothelial cell adhesion molecules in vivo

This study reports on what we believe are novel mechanism(s) of the vascular protective action of adiponectin. We used intravital microscopy to measure leukocyte-endothelium interactions in adiponectin-deficient (Ad(-/-)) mice and found that adiponectin deficiency was associated with a 2-fold increase in leukocyte rolling and a 5-fold increase in leukocyte adhesion in the microcirculation. Measurement of endothelial NO (eNO) revealed that adiponectin deficiency drastically reduced levels of eNO in the vascular wall. Immunohistochemistry demonstrated increased expression of E-selectin and VCAM-1 in the vascular endothelium of Ad(-/-) mice. Systemic administration of the recombinant globular adiponectin domain (gAd) to Ad(-/-) mice significantly attenuated leukocyte-endothelium interactions and adhesion molecule expression in addition to restoring physiologic levels of eNO. Importantly, prior administration of gAd also protected WT mice against TNF-alpha-induced leukocyte-endothelium interactions, indicating a pharmacologic action of gAd. Mechanistically, blockade of eNOS with N(omega)-nitro-L-arginine methyl ester ( L-NAME) abolished the inhibitory effect of gAd on leukocyte adhesion, demonstrating the obligatory role of eNOS signaling in the antiinflammatory action of gAd. We believe this is the first demonstration that gAd protects the vasculature in vivo via increased NO bioavailability with suppression of leukocyte-endothelium interactions. Overall, we provide evidence that loss of adiponectin induces a primary state of endothelial dysfunction with increased leukocyte-endothelium adhesiveness.

Pathophysiological significance of adiponectin

Adipose tissue, which classically has been considered as an energy-storing organ, is now viewed as a massive source of bioactive substances such as leptin, tumor necrosis factor (TNF)-alpha, and adiponectin. Adiponectin was discovered to be the most abundant adipose-specific transcript. Its function had been unclear, but epidemiological and clinical studies have demonstrated that serum levels of adiponectin are inversely associated with body weight, especially abdominal visceral fat accumulation. In addition, adiponectin was inversely related to cardiovascular risk factors, such as insulin resistance, blood pressure, and low-density lipoprotein (LDL) cholesterol and triglyceride levels, and was positively related to high-density lipoprotein (HDL) cholesterol levels. Moreover, low adiponectin concentration is associated with a high incidence of cardiovascular disease (CVD), diabetes, some kinds of cancer, and other various diseases. These associations suggest the clinical significance of adiponectin, and a number of investigations are now being conducted to clarify the biological functions of adiponectin. Recent studies have revealed that adiponectin exhibits antiinflammatory, antiatherogenic, and antidiabetic properties. In addition, adiponectin has been thought to be a key molecule in "metabolic syndrome," which is an epidemiological target for preventing cardiovascular disease. Various functions of adiponectin may possibly serve to prevent and treat obesity-related diseases and CVD. Furthermore, enhancement of adiponectin secretion or action may become a promising therapeutic target.

Expanding the lysyl hydroxylase toolbox: New insights into the localization and activities of lysyl hydroxylase 3 (LH3)

Hydroxylysine and its glycosylated forms, galactosylhydroxylysine and glucosylgalactosylhydroxylysine, are post-translational modifications unique to collagenous sequences. They are found in collagens and in many proteins having a collagenous domain in their structure. Since the last published reviews, significant new data have accumulated regarding these modifications. One of the lysyl hydroxylase isoforms, lysyl hydroxylase 3 (LH3), has been shown to possess three catalytic activities required sequentially to produce hydroxylysine and its glycosylated forms, that is, the lysyl hydroxylase (LH), galactosyltransferase (GT), and glucosyltransferase (GGT) activities. Studies on mouse models have revealed the importance of these different activities of LH3 in vivo. LH3 is the main molecule responsible for GGT activity in mouse embryos. A lack of this activity causes intracellular accumulation of type IV collagen, which disrupts the formation of basement membranes (BMs) during mouse embryogenesis and leads to embryonic lethality. The specific inactivation of the LH activity of LH3 causes minor alterations in the structure of the BM and collagen fibril organization, but does not affect the lifespan of mutated mice. Recent data from zebrafish demonstrate that growth cone migration depends critically on the LH3 glycosyltransferase domain. LH3 is located in the ER loosely associated with the membranes, but, unlike the other isoforms, LH3 is also found in the extracellular space in some tissues. LH3 is able to adjust the amount of hydroxylysine and hydroxylysine-linked carbohydrates of extracellular proteins in their native conformation, suggesting that it may have a role in matrix remodeling.

Adiponectin-induced endothelial nitric oxide synthase activation and nitric oxide production are mediated by APPL1 in endothelial cells

Adiponectin protects the vascular system partly through stimulation of endothelial nitric oxide (NO) production and endothelium-dependent vasodilation. The current study investigated the role of two recently identified adiponectin receptors, AdipoR1 and -R2, and their downstream effectors in mediating the endothelium actions of adiponectin. In human umbilical vein endothelial cells, adiponectin-induced phosphorylation of endothelial NO synthase (eNOS) at Ser(1177) and NO production were abrogated when expression of AdipoR1 and -R2 were simultaneously suppressed. Proteomic analysis demonstrated that the cytoplasmic tails of both AdipoR1 and -R2 interacted with APPL1, an adaptor protein that contains a PH (pleckstrin homology) domain, a PTB (phosphotyrosine-binding) domain, and a Leucine zipper motif. Suppression of APPL1 expression by RNA interference significantly attenuated adiponectin-induced phosphorylation of AMP-activated protein kinase (AMPK) at Thr(172) and eNOS at Ser(1177), and the complex formation between eNOS and heat shock protein 90, resulting in a marked reduction of NO production. Adenovirus-mediated overexpression of a constitutively active version of AMPK reversed these changes. In db/db diabetic mice, both APPL1 expression and adiponectin-induced vasodilation were significantly decreased compared with their lean littermates. Taken together, these results suggest that APPL1 acts as a common downstream effector of AdipoR1 and -R2, mediating adiponectin-evoked endothelial NO production and endothelium-dependent vasodilation.

Adipokines and vascular disease in diabetes

Adipokines, in particular adiponectin, have been highlighted in the pathogenesis of obesity-related illnesses, including type 2 diabetes, because of their role in the regulation of insulin sensitivity as well as vascular endothelial function. Since cardiovascular disease accounts for an overwhelming proportion of the morbidity and mortality suffered by patients with diabetes, researchers are actively seeking a better understanding of the role that adipokines play in the vasculature with the hope that the use of these agents, or activation of their signaling pathways, might help prevent micro- and macrovascular complications. This brief review highlights recent work on the vascular effects of circulating adipokines, focusing on adiponectin, and includes some recent findings with leptin and resistin. This highly active area of investigation has identified novel hormonal mechanisms by which the adipose tissue mass can influence vascular function with important consequences for cardiovascular risk.

Total and high-molecular weight adiponectin in relation to metabolic variables at baseline and in response to an exercise treatment program: comparative evaluation of three assays

OBJECTIVE

Adiponectin, an adipocyte-secreted hormone, circulates in the serum in several multimeric forms. Compared with total adiponectin, high-molecular weight (HMW) adiponectin has been suggested to be a better predictor of metabolic parameters and insulin sensitivity in humans. Our objective was to compare total adiponectin with HMW adiponectin as predictors of metabolic variables and insulin sensitivity at both baseline and after an exercise intervention.

RESEARCH DESIGN AND METHODS

We obtained blood samples from 60 men and women with normal glucose tolerance (n = 20), impaired glucose tolerance (IGT) (n = 20), or type 2 diabetes (n = 20) at baseline and after 4 weeks of training to measure metabolic variables. Using commercially available assays, we measured plasma total adiponectin using LINCO, Mediagnost, and ALPCO assays and HMW adiponectin using an ALPCO assay.

RESULTS

HMW adiponectin and total adiponectin (ALPCO) had similar ability to predict the presence of insulin resistance. Total adiponectin, as measured by radioimmunoassay (LINCO) and enzyme-linked immunosorbent assay (ELISA) (Mediagnost), correlated most strongly with measures of insulin sensitivity (P < 0.01) and lipid profile (P < 0.01) at baseline, showed greater improvements of adiponectin levels (P < 0.001), was more closely associated with improvements of lipid measures with exercise training (P < 0.01), and more accurately predicted insulin resistance and IGT in comparison with total adiponectin or HMW measured with the ALPCO ELISA.

CONCLUSIONS

These results do not support the superiority of HMW over total adiponectin (measured using currently available assays) in assessing metabolic variables at baseline or in response to physical training. Moreover, there are significant differences in the ability of commercially available assays for total adiponectin to predict metabolic variables.

Mechanisms regulating energy metabolism by adiponectin in obesity and diabetes

Nutritional control of molecular events has become of great interest given the increased incidence of diet-induced obesity, and consequently Type 2 (non-insulin-dependent) diabetes, in recent years. The altered adipose tissue content in obese individuals results in an altered profile of circulating adipokines, and here we focus on adiponectin, whose circulating levels decrease in obese individuals. Adiponectin is a 30 kDa protein but circulates primarily as hexameric, oligomeric and, to a lesser extent, trimeric forms. Full-length adiponectin can also be cleaved to produce a fragment containing the globular domain that exerts potent metabolic effects. Adiponectin has insulin-mimetic and -sensitizing actions including stimulation of glucose uptake in skeletal muscle and suppression of glucose production in liver. Hence, adiponectin has attracted great interest as an antidiabetic agent. Adiponectin acts via two receptor isoforms, AdipoR1 (adiponectin receptor 1) and AdipoR2, which have distinct tissue distributions and affinities for recognition of the various adiponectin forms. Expression of AdipoR isoforms can be regulated by hyperinsulinaemia and hyperglycaemia with the consequence of increased sensitivity or resistance to specific forms of adiponectin. In summary, regulation of adiponectin or AdipoR expression may be of great importance in the development of metabolic perturbations characteristic of Type 2 diabetes in obese individuals.

Adiponectin modulates the glycogen synthase kinase-3beta/beta-catenin signaling pathway and attenuates mammary tumorigenesis of MDA-MB-231 cells in nude mice

Adiponectin is an adipokine that has pleiotropic beneficial roles in systemic insulin resistance and inflammation. Several recent clinical studies suggest that low serum levels of adiponectin are associated with increased risks of breast cancer. Here, we investigated the direct effects of adiponectin on breast cancer development in vitro and in vivo. Our results showed that adiponectin significantly attenuated the proliferations of two typical human breast cancer cells, MDA-MB-231 and T47D, in a cell type-specific manner. Further analysis revealed that adiponectin could induce apoptosis and arrest the cell cycle progression at G(0)-G(1) phase in MDA-MB-231 cells. Prolonged treatment with adiponectin in this cell line blocked serum-induced phosphorylation of Akt and glycogen synthase kinase-3beta (GSK-3beta), suppressed intracellular accumulation of beta-catenin and its nuclear activities, and consequently reduced expression of cyclin D1. Adiponectin-mediated suppression of cyclin D1 expression and attenuation of cell proliferation was abrogated by the GSK-3beta inhibitor lithium chloride. These results suggest that the inhibitory role of adiponectin on MDA-MB-231 cell growth might be attributed to its suppressive effects on the GSK-3beta/beta-catenin signaling pathway. Furthermore, our in vivo study showed that both supplementation of recombinant adiponectin and adenovirus-mediated overexpression of this adipokine substantially reduced the mammary tumorigenesis of MDA-MB-231 cells in female nude mice. Taken together, these data support the role of adiponectin as a negative regulator of breast cancer development and also suggest that adiponectin might represent a novel therapeutic target for this disease.

Adipose tissue-derived factors: impact on health and disease

The endocrine functions of the adipose organ are widely studied at this stage. The adipose organ, and in particular adipocytes, communicate with almost all other organs. Although some adipose tissue pads assume the functions as distinct "miniorgans," adipocytes can also be present in smaller numbers interspersed with other cell types. Although fat pads have the potential to have a significant systemic impact, adipocytes may also affect neighboring tissues through paracrine interactions. These local or systemic effects are mediated through lipid and protein factors. The protein factors are commonly referred to as adipokines. Their expression and posttranslational modifications can undergo dramatic changes under different metabolic conditions. Due to the fact that none of the mutations that affect adipose tissue trigger embryonic lethality, the study of adipose tissue physiology lends itself to genetic analysis in mice. In fact, life in the complete absence of adipose tissue is possible in a laboratory setting, making even the most extreme adipose tissue phenotypes genetically amenable to be analyzed by disruption of specific genes or overexpression of others. Here, we briefly discuss some basic aspects of adipocyte physiology and the systemic impact of adipocyte-derived factors on energy homeostasis.