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

11β-HSD1 reduces metabolic efficacy and adiponectin synthesis in hypertrophic adipocytes

Mitochondrial dysfunction in hypertrophic adipocytes can reduce adiponectin synthesis. We investigated whether 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) expression is increased in hypertrophic adipocytes and whether this is responsible for mitochondrial dysfunction and reduced adiponectin synthesis. Differentiated 3T3L1 adipocytes were cultured for up to 21 days. The effect of AZD6925, a selective 11β-HSD1 inhibitor, on metabolism was examined. db/db mice were administered 600 mg/kg AZD6925 daily for 4 weeks via gastric lavage. Mitochondrial DNA (mtDNA) content, mRNA expression levels of 11 β -H sd1 and mitochondrial biogenesis factors, adiponectin synthesis, fatty acid oxidation (FAO), oxygen consumption rate and glycolysis were measured. Adipocyte hypertrophy in 3T3L1 cells exposed to a long duration of culture was associated with increased 11 β -Hsd1 mRNA expression and reduced mtDNA content, mitochondrial biogenesis factor expression and adiponectin synthesis. These cells displayed reduced mitochondrial respiration and increased glycolysis. Treatment of these cells with AZD6925 increased adiponectin synthesis and mitochondrial respiration. Inhibition of FAO by etomoxir blocked the AZD6925-induced increase in adiponectin synthesis, indicating that 11β-HSD1-mediated reductions in FAO are responsible for the reduction in adiponectin synthesis. The expression level of 11 β -Hsd1 was higher in adipose tissues of db/db mice. Administration of AZD6925 to db/db mice increased the plasma adiponectin level and adipose tissue FAO. In conclusion, increased 11β-HSD1 expression contributes to reduced mitochondrial respiration and adiponectin synthesis in hypertrophic adipocytes.

Combined Adiponectin Deficiency and Resistance in Obese Patients: Can It Solve Part of the Puzzle in Nonalcoholic Steatohepatitis

BACKGROUND:

Nonalcoholic fatty liver disease (NAFLD) has become the most prevalent cause of liver disease, nonalcoholic steatohepatitis (NASH) and fibrosis in obese patients identifies the risk group with increased incidence of liver-related deaths.

AIM:

To clarify the role of serum adiponectin and its receptor liver gene expression in the progression of liver damage in NAFLD.

METHODS:

Fifty four (54) obese patients with NAFLD preliminary diagnosed by liver ultra-sound were recruited. Full medical history, anthropometric measurement, biochemical studies, serum adiponectin level, liver biopsy for histological examination and NAS score to identify NASH patients, and assessment of adiponectin receptor gene expression by RT-PCR, were conducted for each patients. Fifteen ages matched average weight healthy adult had been chosen as a control for serum adiponectin level.

RESULTS:

According to NAS score, patients were divided into non- NASH (8 patients), and NASH (46 patients). Serum adiponectin level was significantly lower in NAFLD patients compared to normal participants (p < 0.004). Serum adiponectin level was lower in NASH patients (4.437 ± 2.569 ng/dl in NASH vs. 5.138 ± 2.841 ng/dl in non-NASH). Adiponectin receptor liver gene expression was lower in NASH patients (0.8459 ± 0.4671 vs. 1.0688 ± 0.3965 in non-NASH).

CONCLUSION:

Both adiponectin deficiency and resistance had a role in progression of simple liver steatosis to severe injury in obese patients.

Calorie Restriction Prevents Metabolic Aging Caused by Abnormal SIRT1 Function in Adipose Tissues

Adipose tissue is a pivotal organ determining longevity, due largely to its role in maintaining whole-body energy homeostasis and insulin sensitivity. SIRT1 is a NAD-dependent protein deacetylase possessing antiaging activities in a wide range of organisms. The current study demonstrates that mice with adipose tissue-selective overexpression of hSIRT1(H363Y), a dominant-negative mutant that disrupts endogenous SIRT1 activity, show accelerated development of metabolic aging. These mice, referred to as Adipo-H363Y, exhibit hyperglycemia, dyslipidemia, ectopic lipid deposition, insulin resistance, and glucose intolerance at a much younger age than their wild-type littermates. The metabolic defects of Adipo-H363Y are associated with abnormal epigenetic modifications and chromatin remodeling in their adipose tissues, as a result of excess accumulation of biotin, which inhibits endogenous SIRT1 activity, leading to increased inflammation, cellularity, and collagen deposition. The enzyme acetyl-CoA carboxylase 2 plays an important role in biotin accumulation within adipose tissues of Adipo-H363Y. Calorie restriction prevents biotin accumulation, abolishes abnormal histone biotinylation, and completely restores the metabolic and adipose functions of Adipo-H363Y. The effects are mimicked by short-term restriction of biotin intake, an approach potentially translatable to humans for maintaining the epigenetic and chromatin remodeling capacity of adipose tissues and preventing aging-associated metabolic disorders.

Deficient copper concentrations in dried-defatted hepatic tissue from ob/ob mice: A potential model for study of defective copper regulation in metabolic liver disease

Ob/ob mice provide an animal model for non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD/NASH) in patients with obesity and type-2 diabetes. Low liver copper has been linked to hepatic lipid build-up (steatosis) in animals with systemic copper deficiency caused by low-copper diets. However, hepatic copper status in patients with NAFLD or NASH is uncertain, and a validated animal model useful for the study of hepatic copper regulation in common forms of metabolic liver disease is lacking. Here, we report parallel measurements of essential metal levels in whole-liver tissue and defatted-dried liver tissue from ob/ob and non-obese control mice. Measurements in whole-liver tissue from ob/ob mice at an age when they have developed NAFLD/NASH, provide compelling evidence for factitious lowering of copper and all other essential metals by steatosis, and so cannot be used to study hepatic metal regulation in this model. By marked contrast, metal measurements in defatted-dried liver samples reveal that most essential metals were actually normal and indicate specific lowering of copper in ob/ob mice, consistent with hepatic copper deficiency. Thus ob/ob mice can provide a model useful for the study of copper regulation in NAFLD and NASH, provided levels are measured in defatted-dried liver tissue.

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Ob/ob mice develop hepatic copper deficiency.

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Copper deficiency develops alongside NAFLD/NASH.

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Metal measurements in whole tissue are impacted by infiltrating fat.

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Levels of all essential metals are lower in ob/ob mice when measured in whole liver.

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After fat extraction, metal levels are normal, with the exception of copper.

Essential roles of insulin, AMPK signaling and lysyl and prolyl hydroxylases in the biosynthesis and multimerization of adiponectin

Post-translational modifications (PTMs) of the adiponectin molecule are essential for its full bioactivity, and defects in PTMs leading to its defective production and multimerization have been linked to the mechanisms of insulin resistance, obesity, and type-2 diabetes. Here we observed that, in differentiated 3T3-L1 adipocytes, decreased insulin signaling caused by blocking of insulin receptors (InsR) with an anti-InsR blocking antibody, increased rates of adiponectin secretion, whereas concomitant elevations in insulin levels counteracted this effect. Adenosine monophosphate-activated protein kinase (AMPK) signaling regulated adiponectin production by modulating the expression of adiponectin receptors, the secretion of adiponectin, and eventually the expression of adiponectin itself. We found that lysyl hydroxylases (LHs) and prolyl hydroxylases (PHs) were expressed in white-adipose tissue of ob/ob mice, wherein LH3 levels were increased compared with controls. In differentiated 3T3-L1 adipocytes, both non-specific inhibition of LHs and PHs by dipyridyl, and specific inhibition of LHs by minoxidil and of P4H with ethyl-3,4-dihydroxybenzoate, caused significant suppression of adiponectin production, more particularly of the higher-order isoforms. Transient gene knock-down of LH3 (Plod3) caused a suppressive effect, especially on the high molecular-weight (HMW) isoforms. These data indicate that PHs and LHs are both required for physiological adiponectin production and in particular are essential for the formation/secretion of the HMW isoforms.

Anti-atherosclerotic effects of sitagliptin in patients with type 2 diabetes mellitus

BACKGROUND

Advanced glycation end products, selectins, and adiponectin play important roles in the development of atherosclerosis in individuals with diabetes. Sitagliptin has been shown to reduce the concentration of glycated hemoglobin in diabetic patients. However, its effects on soluble receptor for advanced glycation end products (sRAGEs), selectins, and adiponectin in these patients are poorly understood. This study was conducted to assess the effects of sitagliptin on the circulating levels of sRAGEs, monocyte chemoattractant protein-1 (MCP-1), selectins, and adiponectin in patients with type 2 diabetes.

METHODS

Diabetic patients eligible for sitagliptin monotherapy or combination therapy (eg, sitagliptin plus a sulfonylurea) were administered sitagliptin (50 mg/day) for 6 months. Levels of soluble P-selectin (sP-selectin), soluble E-selectin (sE-selectin), soluble vascular cell adhesion molecule-1 (sVCAM-1), MCP-1, sRAGEs, and adiponectin were measured by ELISA at baseline and after 3 and 6 months of treatment.

RESULTS

At baseline, the levels of MCP-1, sP-selectin, sE-selectin, and sVCAM-1 were higher and the level of adiponectin was lower in diabetic patients than in nondiabetic patients. Sitagliptin therapy for 3 and 6 months significantly reduced plasma levels of sP-selectin, sE-selectin, sVCAM-1, and MCP-1 relative to baseline, while significantly increasing adiponectin levels. sRAGEs did not exhibit a statistical significance, although there was an increasing tendency. Furthermore, the reductions in sP-selectin, sE-selectin, sVCAM-1, and MCP-1 during sitagliptin therapy were significantly greater in responders, defined as patients with a significant increase in adiponectin levels, than in nonresponders. In contrast, responders showed a significant increase in the plasma concentration of sRAGEs.

CONCLUSION

Sitagliptin shows an adiponectin-dependent anti-atherothrombotic effect, which may be beneficial for primary prevention of atherothrombosis, in patients with type 2 diabetes.

Adiponectin expression in the porcine pituitary during the estrous cycle and its effect on LH and FSH secretion

Female reproductive success is closely associated with nutritional status and energy balance. In this context, adiponectin appears to be a key hormone connecting reproductive system function and metabolism regulation. It is hypothesized that adiponectin expression in the pituitary depends on the phase of the estrous cycle. The effect of adiponectin on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion is also postulated. Changes in the adiponectin gene and protein expression in the porcine anterior (AP) and posterior (NP) pituitaries as well as the effect of in vitro administration of adiponectin on basal and gonadotropin-releasing hormone (GnRH)- and/or insulin-stimulated LH and FSH secretion were investigated on days 2-3, 10-12, 14-16, and 17-19 of the estrous cycle. Adiponectin gene was more pronounced on days 2-3 in AP but on days 10-12 in NP. Protein concentration in AP was the highest on days 10-12 and in NP on days 10-12 and 17-19 of the cycle. In vitro, adiponectin did not affect basal LH secretion but increased FSH release by AP cells. Adiponectin administration affected GnRH- and/or insulin-induced LH and FSH output in a manner dependent on the phase of the estrous cycle. In this study we indicated for the first time adiponectin expression in the porcine AP and NP that was dependent on the phase of the estrous cycle. In vitro studies indicated that adiponectin may affect gonadotropin secretion. The above suggests that the studied adipokine may influence female reproductive functions via its effect on LH and FSH secretion by gonadotrophs, but the cellular mechanism of its action remains unknown.

Evidence that multiple defects in lipid regulation occur before hyperglycemia during the prodrome of type-2 diabetes

BACKGROUND

Blood-vessel dysfunction arises before overt hyperglycemia in type-2 diabetes (T2DM). We hypothesised that a metabolomic approach might identify metabolites/pathways perturbed in this pre-hyperglycemic phase. To test this hypothesis and for specific metabolite hypothesis generation, serum metabolic profiling was performed in young women at increased, intermediate and low risk of subsequent T2DM.

METHODS

Participants were stratified by glucose tolerance during a previous index pregnancy into three risk-groups: overt gestational diabetes (GDM; n = 18); those with glucose values in the upper quartile but below GDM levels (UQ group; n = 45); and controls (n = 43, below the median glucose values). Follow-up serum samples were collected at a mean 22 months postnatally. Samples were analysed in a random order using Ultra Performance Liquid Chromatography coupled to an electrospray hybrid LTQ-Orbitrap mass spectrometer. Statistical analysis included principal component (PCA) and multivariate methods.

FINDINGS

Significant between-group differences were observed at follow-up in waist circumference (86, 95%CI (79-91) vs 80 (76-84) cm for GDM vs controls, p<0.05), adiponectin (about 33% lower in GDM group, p = 0.004), fasting glucose, post-prandial glucose and HbA1c, but the latter 3 all remained within the 'normal' range. Substantial differences in metabolite profiles were apparent between the 2 'at-risk' groups and controls, particularly in concentrations of phospholipids (4 metabolites with p ≤ 0.01), acylcarnitines (3 with p ≤ 0.02), short- and long-chain fatty acids (3 with p<  = 0.03), and diglycerides (4 with p ≤ 0.05).

INTERPRETATION

Defects in adipocyte function from excess energy storage as relatively hypoxic visceral and hepatic fat, and impaired mitochondrial fatty acid oxidation may initiate the observed perturbations in lipid metabolism. Together with evidence from the failure of glucose-directed treatments to improve cardiovascular outcomes, these data and those of others indicate that a new, quite different definition of type-2 diabetes is required. This definition would incorporate disturbed lipid metabolism prior to hyperglycemia.

Assembly of adiponectin oligomers

Adiponectin is among the most studied adipokines, the collection of molecules secreted from adipose tissue. It is also one of the most architecturally complex adipokines with its various oligomeric states that include trimers, hexamers, nonamers (9mers), dodecamers (12mers), and octadecamers (18mers). The importance of adiponectin in metabolic regulation is underscored by its strong positive associations with improvement in insulin action and also decreased risks for developing type 2 diabetes. Understanding the mechanisms involved in maintaining the steady-state concentrations of adiponectin oligomers in circulation is therefore likely to provide important insight into the development of insulin resistance. This review will discuss the current state of knowledge regarding the biochemical composition of adiponectin oligomers, the commonly used techniques to analyze them, and the known post-translational modifications that affect their assembly. Evidence based on in vitro oligomer assembly reactions in support of a "cystine ratchet" model of adiponectin oligomer formation will be considered along with limitations of the evidence. Secretory pathway proteins that have been shown to affect the distribution of adiponectin oligomers will also be discussed along with hypotheses regarding their potential involvement in the cystine ratchet model of adiponectin oligomerization.

Acute and short-term chronic testosterone fluctuation effects on glucose homeostasis, insulin sensitivity, and adiponectin: a randomized, double-blind, placebo-controlled, crossover study

CONTEXT

Low levels of adiponectin and T in men have been shown to predict development of the metabolic syndrome, but the effects of T on glucose metabolism are incompletely understood and may be influenced either directly or indirectly through changes in body composition or in levels of adiponectin.

OBJECTIVE

The aim of the study was to test whether T exerts its effects on glucose metabolism directly or indirectly.

DESIGN, SETTING, AND PARTICIPANTS

In a randomized, double-blind, placebo-controlled, crossover study, 12 healthy young males were studied on four separate occasions. They received GnRH agonist treatment 1 month before 3 of 4 trial days to induce castrate levels of T. On trial days, T gel containing either high or low physiological T dose or placebo was applied to the body. On a fourth trial day, participants constituted their own eugonadal controls.

INTERVENTION

Each study comprised a 5-hour basal period and a 3-hour hyperinsulinemic euglycemic clamp.

MAIN OUTCOME MEASURES

We measured the effect of acute T on peripheral glucose disposal, total adiponectin and subforms, and other indices of glucose metabolism.

RESULTS

Short-term hypogonadism was associated with increased high molecular weight adiponectin levels (P < .03) and increased oxidative glucose disposal (P = .03) but not total glucose disposal (P = .07). Acute T treatment was an independent suppressor of high molecular weight adiponectin levels (P = .04) but did not affect total glucose disposal (P = .17).

CONCLUSIONS

These data show that T can act through putative fast nongenomic pathways to affect adiponectin levels in humans. The early hypogonadal state is characterized by a marked shift in fuel oxidation from lipids toward glucose, which may rely partly on buffering capabilities of adiponectin.

Metabolic function of the CTRP family of hormones

Maintaining proper energy balance in mammals entails intimate crosstalk between various tissues and organs. These inter-organ communications are mediated, to a great extent, by secreted hormones that circulate in blood. Regulation of the complex metabolic networks by secreted hormones (e.g., insulin, glucagon, leptin, adiponectin, FGF21) constitutes an important mechanism governing the integrated control of whole-body metabolism. Disruption of hormone-mediated metabolic circuits frequently results in dysregulated energy metabolism and pathology. As part of an effort to identify novel metabolic hormones, we recently characterized a highly conserved family of 15 secreted proteins, the C1q/TNF-related proteins (CTRP1-15). While related to adiponectin in sequence and structural organization, each CTRP has its own unique tissue expression profile and non-redundant function in regulating sugar and/or fat metabolism. Here, we summarize the current understanding of the physiological functions of CTRPs, emphasizing their metabolic roles. Future studies using gain-of-function and loss-of-function mouse models will provide greater mechanistic insights into the critical role CTRPs play in regulating systemic energy homeostasis.

Direct effects of adipokines on the heart: focus on adiponectin

Cardiovascular disease, including heart failure, is a principal cause of death in individuals with obesity and diabetes. However, the mechanisms of obesity- and diabetes-induced heart disease are multifaceted and remain to be clearly defined. Of relevance to this review, there is currently great research and clinical interest in the endocrine effects of adipokines on the myocardium and their role in heart failure. We will discuss the potential significance of adipokines in the pathogenesis of heart failure via their ability to regulate remodeling events including metabolism, hypertrophy, fibrosis, and cell death. As an excellent example, we will first focus on adiponectin which is best known to confer numerous cardioprotective effects. However, we comprehensively discuss the existing literature that highlights it would be naive to assume that this was always the case. We also focus on lipocalin-2 which mediates pro-inflammatory and pro-apoptotic effects. It is important when studying actions of adipokines to integrate cellular and mechanistic analyses and translate these to physiologically relevant in vivo models and clinical studies. However, assimilating studies on numerous cardiac remodeling events which ultimately dictate cardiac dysfunction into a unifying conclusion is challenging. Nevertheless, there is undoubted potential for the use of adipokines as robust biomarkers and appropriate therapeutic targets in heart failure.

Implications of adiponectin in linking metabolism to testicular function

Obesity is a major health problem, contributing to the development of various diseases with aging. In humans, obesity has been associated with reduced testosterone production and subfertility. Adipose tissue is an important source of hormones having influences on both metabolism and reproduction. Among them, the production and secretion of adiponectin is inversely correlated to the severity of obesity. The purpose of this review of literature is to present the current state of knowledge on adiponectin research to determine whether this hormone affects reproduction in men. Surprisingly, evidences show negative influences of adiponectin on GnRH secretion from the hypothalamus, LH and FSH secretion from the pituitary and testosterone at the testicular level. Thus far, the involvement of adiponectin in the influence of metabolism on reproduction in men is limited. However, adiponectin and its receptors are expressed by different cell types of the male gonad, including Leydig cells, spermatozoa, and epididymis. In addition, actions of adiponectin at the testicular level have been shown to promote spermatogenesis and sperm maturation. Therefore, autocrine/paracrine actions of adiponectin in the testis may contribute to support male reproductive function.

Deamidated lipocalin-2 induces endothelial dysfunction and hypertension in dietary obese mice

BACKGROUND

Lipocalin-2 is a proinflammatory adipokine upregulated in obese humans and animals. A pathogenic role of lipocalin-2 in hypertension has been suggested. Mice lacking lipocalin-2 are protected from dietary obesity-induced cardiovascular dysfunctions. Administration of lipocalin-2 causes abnormal vasodilator responses in mice on a high-fat diet (HFD).

METHODS AND RESULTS

Wild-type and lipocalin-2 knockout mice were fed with standard chow or HFD. Immunoassays were performed for evaluating the circulating and tissue contents of lipocalin-2. The relaxation and contraction of arteries were studied using a wire myograph. Blood pressure was monitored with implantable radio telemetry. Dietary obesity promoted the accumulation of lipocalin-2 protein in blood and arteries. Deficiency of this adipokine protected mice from dietary obesity-induced elevation of blood pressure. Mass spectrometry analysis revealed that human and murine lipocalin-2 were modified by polyamination. Polyaminated lipocalin-2 was rapidly cleared from the circulation. Adipose tissue was a major site for lipocalin-2 deamidation. The circulating levels and the arterial accumulation of deamidated lipocalin-2 were significantly enhanced by treatment with linoleic acid (18:2n-6), which bound to lipocalin-2 with high affinity and prevented its interactions with matrix metalloproteinase 9 (MMP9). Combined administration of linoleic acid with lipocalin-2 caused vascular inflammation and endothelial dysfunction and raised the blood pressure of mice receiving standard chow. A human lipocalin-2 mutant with cysteine 87 replaced by alanine (C87A) contained less polyamines and exhibited a reduced capacity to form heterodimeric complexes with MMP9. After treatment, C87A remained in the circulation for a prolonged period of time and evoked endothelial dysfunction in the absence of linoleic acid.

CONCLUSIONS

Polyamination facilitates the clearance of lipocalin-2, whereas the accumulation of deamidated lipocalin-2 in arteries causes vascular inflammation, endothelial dysfunction, and hypertension.

The highly expressed methionine synthase gene of Neurospora crassa is positively regulated by its proximal heterochromatic region

In Neurospora crassa, the methionine synthase gene met-8 plays a key role in methionine synthesis. In this study, we found that MET-8 protein levels were compromised in several mutants defective in proper heterochromatin formation. Bioinformatics analysis revealed a 50-kb AT-rich region adjacent to the met-8 promoter. ChIP assays confirmed that trimethylated H3K9 was enriched in this region, indicating that heterochromatin may form upstream of met-8. In an H3K9R mutant strain, the output of met-8 was dramatically reduced, similar to what we observed in mutant strains that had defective heterochromatin formation. Furthermore, the production of ectopically expressed met-8 at the his-3 locus in the absence of a normal heterochromatin environment was inefficient, whereas ectopic expression of met-8 downstream of two other heterochromatin domains was efficient. In addition, our data show that the expression of mig-6 was also controlled by an upstream 4.2-kb AT-rich region similar to that of the met-8 gene, and we demonstrate that the AT-rich regions adjacent to met-8 or mig-6 are required for their peak expression. Our study indicates that met-8 and mig-6 may represent a novel type of gene, whose expression relies on the proper formation of a nearby heterochromatin region.

Porcine adiponectin receptor 1 transgene resists high-fat/sucrose diet-induced weight gain, hepatosteatosis and insulin resistance in mice

Adiponectin and its receptors have been demonstrated to play important roles in regulating glucose and lipid metabolism in mice. Obesity, type II diabetes and cardiovascular disease are highly correlated with down-regulated adiponectin signaling. In this study, we generated mice overexpressing the porcine Adipor1 transgene (pAdipor1) to study its beneficial effects in metabolic syndromes as expressed in diet-induced obesity, hepatosteatosis and insulin resistance. Wild-type (WT) and pAdipor1 transgenic mice were fed ad libitum with a standard chow diet (Chow) or a high-fat/sucrose diet (HFSD) for 24 weeks, beginning at 6 to 7 weeks of age. There were 12 mice per genetic/diet/sex group. When challenged with HFSD to induce obesity, the pAdipor1 transgenic mice resisted development of weight gain, hepatosteatosis and insulin resistance. These mice had lowered plasma adiponectin, triglyceride and glycerol concentrations compared to WT mice. Moreover, we found that (indicated by mRNA levels) fatty acid oxidation was enhanced in skeletal muscle and adipose tissue, and liver lipogenesis was inhibited. The pAdipor1 transgene also restored HFSD-reduced phosphoenolpyruvate carboxykinase 1 (Pck1) and glucose transporter 4 mRNA in the adipose tissues, implying that the increased Pck1 may promote glyceroneogenesis to reduce glucose intolerance and thus activate the flux of glyceride-glycerol to resist diet-induced weight gain in the adipose tissues. Taken together, we demonstrated that pAdipor1 can prevent diet-induced weight gain and insulin resistance. Our findings may provide potential therapeutic strategies for treating metabolic syndromes and obesity, such as treatment with an ADIPOR1 agonist or activation of Adipor1 downstream targets.

A novel ADIPOQ mutation (p.M40K) impairs assembly of high-molecular-weight adiponectin and is associated with early-onset obesity and metabolic syndrome

CONTEXT

The phenotypic effects of ADIPOQ mutations early in life, prior to type 2 diabetes onset, have not been studied.

AIM

The objective of the study was to characterize the impact of a novel ADIPOQ mutation in vitro and in vivo.

DESIGN

The design of the study was ADIPOQ screening, adiponectin oligomerization, and cardiometabolic phenotype assessment.

SUBJECTS

Fourteen hypoadiponectinemic (<3 μg/mL) and 686 normoadiponectinemic young adults (23-25 y) were prospectively followed up since birth.

MAIN OUTCOME MEASURES

Human and recombinant murine mutant adiponectin oligomerization, the proband's ADIPOQ and ADIPOR1/R2 adipose tissue (AT) expression, and cardiometabolic profile were measured.

RESULTS

The heterozygous ADIPOQ p.M40K mutation was identified in one hypoadiponectinemic male (2.4 μg/mL) and three other family members. Carriers presented a marked reduction of serum high-molecular weight to total adiponectin ratio when compared with controls (9.4% ± 1% vs 56.6% ± 13%; P < .05) and family noncarriers (9.4% ± 1% vs 42% ± 0.5%; P = .05). Both mRNA and protein levels of adiponectin were increased in the AT of the proband (2.3- and 1.6-fold, respectively). However, the high-molecular weight to total adiponectin ratio of adiponectin was decreased (3.3-fold). Moreover, the expressions of ADIPOR1 and ADIPOR2 were significantly down-regulated in the AT of the proband (6- and 1.2-fold, respectively). The results were confirmed by in vitro studies on the recombinant murine homologous mutation (p.M43K). The proband's cardiometabolic phenotype progression was further characterized: born small for gestational age and adolescence-onset obesity; insulin resistance (homeostasis assessment model of insulin resistance of 4.7), and dyslipidemia at 25 years; decreased high-molecular weight adiponectin (0.24 μg/mL = 10%), hypertension (180/120 mm Hg), steatosis (fat liver = 40% ± 6%), increased carotid intima-media thickness at 31 years, and type 2 diabetes (glycosylated hemoglobin = 6.6%) at 34 years of age. Of note, all of the affected family members presented features of metabolic syndrome.

CONCLUSION

The newly identified ADIPOQ p.M40K mutation associates with severe cardiometabolic dysfunction due to the impairment of high-molecular weight adiponectin complex formation.

Adipokines and melanocortins in the hepatic manifestation of metabolic syndrome: nonalcoholic fatty liver disease

Metabolic syndrome is associated with nonalcoholic fatty liver disease and its more aggressive form, nonalcoholic steatohepatitis. Adipokines produced by white adipose tissue possess broad physiological activity and play an important autocrine role in obesity-associated complications, including metabolic syndrome, nonalcoholic fatty liver disease and cardiovascular disease. Various adipokines may have beneficial or harmful effects. Other tissues, particularly stomach and intestine, produce active molecules that can influence the function of adipocytes and, possibly, the levels of adipokine secretion. In some cases, the production sites of these molecules remain unknown. The review focuses on our current understanding of the disease-related effects of the adipokines and the melanocortins on various peripheral tissues, and discusses some of their potential interactions with each other. Potential therapeutic applications are also considered.

Cytokines and Hormones That Contribute to the Positive Association between Fat and Bone

The positive association between body weight and bone density has been established in numerous laboratory and clinical studies. Apart from the direct effect of soft tissue mass on bone through skeletal loading, a number of cytokines and hormones contribute to the positive association between adipose and bone tissue, acting either locally in sites where cells of the two tissues are adjacent to each other or systemically through the circulation. The current review describes the effects of such local and systemic factors on bone physiology. One class of factors are the adipocyte-secreted peptides (adipokines), which affect bone turnover through a combination of direct effects in bone cells and indirect mechanisms mediated by the central nervous system. Another source of hormones that contribute to the coupling between fat and bone tissue are beta cells of the pancreas. Insulin, amylin, and preptin are co-secreted from pancreatic beta cells in response to increased glucose levels after feeding, and are also found in high circulating levels in obesity. A number of peptide hormones secreted from the gastrointestinal tract in response to feeding affect both fat and bone cells and thus can also act as mediators of the association between the two tissues. The current review focuses on results of laboratory studies investigating possible mechanism involved in the positive association between fat mass and bone mass.

Genetic analysis of adiponectin variation and its association with type 2 diabetes in African Americans

OBJECTIVE

Adiponectin is an adipocytokine that has been implicated in a variety of metabolic disorders, including T2D and cardiovascular disease. Studies evaluating genetic variants in ADIPOQ have been contradictory when testing association with T2D in different ethnic groups.

DESIGN AND METHODS

In this study, 18 SNPs in ADIPOQ were tested for association with plasma adiponectin levels and diabetes status. SNPs were examined in two independent African-American cohorts (nmax = 1,116) from the Insulin Resistance Atherosclerosis Family Study (IRASFS) and the African American-Diabetes Heart Study (AA-DHS).

RESULTS

Five polymorphisms were nominally associated with plasma adiponectin levels in the meta-analysis (P = 0.035-1.02 × 10(-6) ) including a low frequency arginine to cysteine mutation (R55C) which reduced plasma adiponectin levels to <15% of the mean. Variants were then tested for association with T2D in a meta-analysis of these and the Wake Forest T2D case-control study (n = 3,233 T2D, 2645 non-T2D). Association with T2D was not observed (P ≥ 0.08), suggesting limited influence of ADIPOQ variants on T2D risk.

CONCLUSIONS

Despite identification of variants associated with adiponectin levels, a detailed genetic analysis of ADIPOQ revealed no association with T2D risk. This puts into question the role of adiponectin in T2D pathogenesis: whether low adiponectin levels are truly causal for or rather a consequence.

Oligomeric adiponectin forms and their complexes in the blood of healthy donors and patients with type 2 diabetes mellitus

Adiponectin (Adn) is a protein that circulates in the blood in several oligomeric forms, namely low-, medium-, and high-molecular-weight forms. Adn may serve as a risk factor for type 2 diabetes mellitus (T2DM). The aims of this work were (1) to produce monoclonal antibodies (MAbs) specific to different Adn oligomeric forms, (2) to design immunoassays suitable for measuring the Adn forms present in human blood, and (3) to investigate the changes in Adn forms that occur in patients with T2DM. Gel filtration, fluoroimmunoassays, and Western blotting were utilized as major techniques in this study. MAbs recognizing various oligomeric forms of Adn were obtained. Complexes between Adn and complement component C1q and between the low molecular weight form of Adn and albumin were described in human blood. A decrease in the total Adn and Adn-albumin complex levels in the blood of patients with T2DM and no difference in the levels of the Adn-C1q complex in comparison with healthy volunteers were demonstrated. An Adn94-Adn63 fluoroimmunoassay was selected as the technique that most accurately measured the mass ratio of Adn oligomers in blood samples, and an Adn214-Adn27 assay that measured the low-molecular-weight form of Adn only.

Expression of adiponectin and its receptors in avian species

Adipose tissue is a dynamic endocrine organ secreting a variety of hormones that affect physiological functions within the central nervous system, cardiovascular system, reproductive, and immune systems. The endocrine role of avian adipose tissue remains enigmatic as many of the classical hormones found in mammalian adipose tissue have not been found in avians. This mini-review summarizes our current knowledge on avian adiponectin, one of the most abundant adipose tissue hormones, and its receptors. We cloned the genes encoding chicken adiponectin and its receptors, AdipoR1 and AdipoR2. Using anti-chicken adiponectin antibody, we found that chicken adipose tissue and plasma predominantly contain a unique polymer of adiponectin with a mass greater than 669kDa, unlike mammalian adiponectin which is found as three distinct oligomers. Mass spectrometric analyses of chicken adiponectin revealed certain post-translational modifications that are likely to favor the unique multimerization of adiponectin in chickens. Unlike adiponectin, the nucleotide sequences of chicken AdipoR1- and AdipoR2 cDNA are highly similar to that of mammalian adiponectin receptors. Both adiponectin and adiponectin receptors are widely expressed in several tissues in the chicken. Herein, we review the unique biochemistry of adiponectin as well as expression of adiponectin and its receptors in the chicken. Future studies should focus on elucidating the role of adiponectin, AdipoR1, and AdipoR2 on metabolism, steroidogenesis, and adipose tissue remodeling during growth and reproduction in birds.

Role of adiponectin and its receptors in cancer

Adiponectin (APN), a novel hormone/cytokine derived from adipocyte tissue, is involved in various physiological functions. Genetics, nutrition, and adiposity are factors contributing to circulating plasma concentrations of APN. Clinical correlation studies have shown that lower levels of serum APN are associated with increased malignancy of various cancers, such as breast and colon cancers, suggesting that APN has a role in tumorigenesis. APN affects insulin resistance, thus further influencing cancer development. Tumor cells may express receptors for APN. Cellular signaling is the mechanism by which APN exerts its host-protective responses. These factors suggest that serum APN levels and downstream signaling targets of APN may serve as potential diagnostic markers for malignancies. Further research is necessary to clarify the exact role of APN in cancer diagnosis and therapy.

Adiponectin mediates the metabolic effects of FGF21 on glucose homeostasis and insulin sensitivity in mice

Fibroblast growth factor 21 (FGF21) is a metabolic hormone with pleiotropic effects on regulating glucose and lipid homeostasis and insulin sensitivity. However, the mechanisms underlying the metabolic actions of FGF21 remain unknown. Here we show that the insulin-sensitizing adipokine adiponectin is a downstream effector of FGF21. Treatments with FGF21 enhanced both expression and secretion of adiponectin in adipocytes, thereby increasing serum levels of adiponectin in mice. Adiponectin knockout mice were refractory to several therapeutic benefits of FGF21, including alleviation of obesity-associated hyperglycemia, hypertriglyceridemia, insulin resistance, and hepatic steatosis. Furthermore, the effects of FGF21 on attenuation of obesity-induced impairment in insulin signaling in liver and skeletal muscle were abrogated in adiponectin knockout mice, whereas FGF21-mediated activation of ERK1/ERK2 in adipose tissues remained unaffected. Therefore, adiponectin couples FGF21 actions in local adipocytes to liver and skeletal muscle, thereby mediating the systemic effects of FGF21 on energy metabolism and insulin sensitivity.

Obesity as the common soil of non-alcoholic fatty liver disease and diabetes: Role of adipokines

Non‐alcoholic fatty liver disease (NAFLD) describes a spectrum of liver conditions from simple steatosis, steatohepatitis to end‐stage liver disease. The prevalence of NAFLD has been on the rise in many parts of the world, including Asia, and NAFLD is now the liver disease associated with the highest mortality, consequent to the increased risk of cardiovascular diseases and hepatocellular carcinoma. Whereas NAFLD is an independent risk factor for type 2 diabetes, increased hepatic and peripheral insulin resistance contribute to the pathogenesis of both NAFLD and diabetes, which are associated with enhanced cardiovascular risk. Studies in humans and animal models have suggested obesity as the common link of these two diseases, likely mediated by adipose tissue inflammation and dysregulated adipokine production in obesity. In the present review, we discuss recent advances in our understanding of the role of several novel adipokines (adiponectin, adipocyte fatty acid binding protein and fibroblast growth factor‐21) in the pathophysiology of NAFLD and diabetes, as well as their use as potential biomarkers and therapeutic targets for dysglycemia in NAFLD patients.

Extracellular conversion of adiponectin hexamers into trimers

Adiponectin is an adipocyte-secreted hormone that exists as trimers, hexamers and larger species collectively referred to as HMW (high-molecular-weight) adiponectin. Whether hexamers or HMW adiponectin serve as precursors for trimers outside the circulation is currently unknown. Here, we demonstrate that adiponectin trimers can be generated from larger oligomers secreted from primary rat adipose cells or differentiated 3T3-L1 adipocytes. Purified hexameric, but not HMW, adiponectin converted into trimers in conditioned media separated from 3T3-L1 adipocytes or, more efficiently, when enclosed in the dialysis membrane in the presence of adipocytes. Several lines of evidence indicate that the conversion is mediated by an extracellular redox system. First, N-terminal epitope-tagged hexamers converted into trimers without proteolytic removal of the tag. Secondly, appearance of trimers was associated with conversion of disulfide-bonded dimers into monomers. Thirdly, thiol-reactive agents inhibited conversion into trimers. Consistent with a redox-based mechanism, purified hexamers reductively converted into trimers in defined glutathione redox buffer with reduction potential typically found in the extracellular environment while the HMW adiponectin remained stable. In addition, conversion of hexamers into trimers was enhanced by NADPH, but not by NADP+. Collectively, these data strongly suggest the presence of an extracellular redox system capable of converting adiponectin oligomers.

Blood-Based Indicators of Insulin Resistance and Metabolic Syndrome in Bottlenose Dolphins (Tursiops truncatus)

Similar to people with metabolic syndrome, bottlenose dolphins (Tursiops truncatus) can have a sustained postprandial hyperglycemia and hyperinsulinemia, dyslipidemia, and fatty liver disease. A panel of potential postprandial blood-based indicators of insulin resistance and metabolic syndrome were compared among 34 managed collection dolphins in San Diego Bay, CA, USA (Group A) and 16 wild, free-ranging dolphins in Sarasota Bay, FL, USA (Group B). Compared to Group B, Group A had higher insulin (2.1 ± 2.5 and 13 ± 13 μIU/ml), glucose (87 ± 19 and 108 ± 12 mg/dl), and triglycerides (75 ± 28 and 128 ± 45 mg/dl) as well as higher cholesterol (total, high-density lipoprotein cholesterol, and very low density lipoprotein cholesterol), iron, transferrin saturation, gamma-glutamyl transpeptidase (GGT), alanine transaminase, and uric acid. Group A had higher percent unmodified adiponectin. While Group A dolphins were older, the same blood-based differences remained when controlling for age. There were no differences in body mass index (BMI) between the groups, and comparisons between Group B and Group A dolphins have consistently demonstrated lower stress hormones levels in Group A. Group A dolphins with high insulin (greater than 14 μIU/ml) had higher glucose, iron, GGT, and BMI compared to Group A dolphins with lower insulin. These findings support that some dolphin groups may be more susceptible to insulin resistance compared to others, and primary risk factors are not likely age, BMI, or stress. Lower high-molecular weight adiponectin has been identified as an independent risk factor for type 2 diabetes in humans and may be a target for preventing insulin resistance in dolphins. Future investigations with these two dolphin populations, including dietary and feeding differences, may provide valuable insight for preventing and treating insulin resistance in humans.

Sphingolipid metabolism and obesity-induced inflammation

Obesity is a metabolic disorder developed by overnutrition and a major cause for insulin resistance and cardiovascular events. Since adipose tissue is one of the major sites for the synthesis and secretion of cytokines, enlarged adipose tissue in obese condition alters inflammatory state leading to pathophysiological conditions such as type 2 diabetes and increased cardiovascular risk. A plausible theory for development of metabolic dysregulation is that obesity increases secretion of inflammatory cytokines from adipose tissue and causes a chronic inflammation in the whole body. Additionally accumulation of lipids in non-adipose tissues elevates the cellular levels of bioactive lipids that inhibit the signaling pathways implicated in metabolic regulation together with activated inflammatory response. Recent findings suggest that obesity-induced inflammatory response leads to modulation of sphingolipid metabolism and these bioactive lipids may function as mediators for increased risk of metabolic dysfunction. Importantly, elucidation of mechanism regarding sphingolipid metabolism and inflammatory disease will provide crucial information to development of new therapeutic strategies for the treatment of obesity-induced pathological inflammation.

Ratiometric Measurements of Adiponectin by Mass Spectrometry in Bottlenose Dolphins (Tursiops truncatus) with Iron Overload Reveal an Association with Insulin Resistance and Glucagon

High molecular weight (HMW) adiponectin levels are reduced in humans with type 2 diabetes and insulin resistance. Similar to humans with insulin resistance, managed bottlenose dolphins (Tursiops truncatus) diagnosed with hemochromatosis (iron overload) have higher levels of 2 h post-prandial plasma insulin than healthy controls. A parallel reaction monitoring assay for dolphin serum adiponectin was developed based on tryptic peptides identified by mass spectrometry. Using identified post-translational modifications, a differential measurement was constructed. Total and unmodified adiponectin levels were measured in sera from dolphins with (n = 4) and without (n = 5) iron overload. This measurement yielded total adiponectin levels as well as site specific percent unmodified adiponectin that may inversely correlate with HMW adiponectin. Differences in insulin levels between iron overload cases and controls were observed 2 h post-prandial, but not during the fasting state. Thus, post-prandial as well as fasting serum adiponectin levels were measured to determine whether adiponectin and insulin would follow similar patterns. There was no difference in total adiponectin or percent unmodified adiponectin from case or control fasting animals. There was no difference in post-prandial total adiponectin levels between case and control dolphins (mean ± SD) at 763 ± 298 and 727 ± 291 pmol/ml, respectively (p = 0.91); however, percent unmodified adiponectin was significantly higher in post-prandial cases compared to controls (30.0 ± 6.3 versus 17.0 ± 6.6%, respectively; p = 0.016). Interestingly, both total and percent unmodified adiponectin were correlated with glucagon levels in controls (r = 0.999, p  < 0.001), but not in cases, which is possibly a reflection of insulin resistance. Although total adiponectin levels were not significantly different, the elevated percent unmodified adiponectin follows a trend similar to HMW adiponectin reported for humans with metabolic disorders.

The activities of lysyl hydroxylase 3 (LH3) regulate the amount and oligomerization status of adiponectin

Lysyl hydroxylase 3 (LH3) has lysyl hydroxylase, galactosyltransferase, and glucosyltransferase activities, which are sequentially required for the formation of glucosylgalactosyl hydroxylysines in collagens. Here we demonstrate for the first time that LH3 also modifies the lysine residues in the collagenous domain of adiponectin, which has important roles in glucose and lipid metabolism and inflammation. Hydroxylation and, especially, glycosylation of the lysine residues of adiponectin have been shown to be essential for the formation of the more active high molecular weight adiponectin oligomers and thus for its function. In cells that totally lack LH3 enzyme, the galactosylhydroxylysine residues of adiponectin were not glucosylated to glucosylgalactosylhydroxylysine residues and the formation of high and middle molecular weight adiponectin oligomers was impaired. Circulating adiponectin levels in mutant mice lacking the lysyl hydroxylase activity of LH3 were significantly reduced, which indicates that LH3 is required for complete modification of lysine residues in adiponectin and the loss of some of the glycosylated hydroxylysine residues severely affects the secretion of adiponectin. LH mutant mice with reduced adiponectin level showed a high fat diet-induced increase in glucose, triglyceride, and LDL-cholesterol levels, hallmarks of the metabolic syndrome in humans. Our results reveal the first indication that LH3 is an important regulator of adiponectin biosynthesis, secretion and activity and thus might be a potential candidate for therapeutic applications in diseases associated with obesity and insulin resistance.

Hypoadiponectinaemia in diabetes mellitus type 2: molecular mechanisms and clinical significance

1. This review focuses on the regulatory mechanisms of adiponectin (APN) gene expression during physiologic conditions and both the clinical significance and underlying molecular mechanisms of hypoadiponectinaemia during pathologic conditions. 2. Adiponectin is a versatile cardiovascular protective factor. It plays an important role in regulating insulin sensitivity and energy homeostasis, with anti-inflammatory and anti-atherosclerotic properties. 3. Adiponectin gene expression is downregulated in both obesity and diabetes mellitus type 2. Hypoadiponectinaemia is an independent risk factor for coronary artery disease in type 2 diabetic patients. 4. Exogenous supplementation of recombinant APN attenuates insulin resistance, improving metabolic disorders. Therefore, APN-targeted pharmaceutical strategies increasing circulating APN levels may be therapeutic against type 2 diabetes. 5. There is great value in elucidating the regulatory mechanisms of APN gene expression during physiologic and pathologic conditions. APN biosynthesis regulation includes transcriptional expression and post-translational modification, oligomerization, and secretion. Under pathological conditions, including obesity and diabetes mellitus type 2, hypoxia, oxidative stress, and inflammation suppress APN mRNA levels and its secretion.

The role of adiponectin in cancer: a review of current evidence

Excess body weight is associated not only with an increased risk of type 2 diabetes and cardiovascular disease (CVD) but also with various types of malignancies. Adiponectin, the most abundant protein secreted by adipose tissue, exhibits insulin-sensitizing, antiinflammatory, antiatherogenic, proapoptotic, and antiproliferative properties. Circulating adiponectin levels, which are determined predominantly by genetic factors, diet, physical activity, and abdominal adiposity, are decreased in patients with diabetes, CVD, and several obesity-associated cancers. Also, adiponectin levels are inversely associated with the risk of developing diabetes, CVD, and several malignancies later in life. Many cancer cell lines express adiponectin receptors, and adiponectin in vitro limits cell proliferation and induces apoptosis. Recent in vitro studies demonstrate the antiangiogenic and tumor growth-limiting properties of adiponectin. Studies in both animals and humans have investigated adiponectin and adiponectin receptor regulation and expression in several cancers. Current evidence supports a role of adiponectin as a novel risk factor and potential diagnostic and prognostic biomarker in cancer. In addition, either adiponectin per se or medications that increase adiponectin levels or up-regulate signaling pathways downstream of adiponectin may prove to be useful anticancer agents. This review presents the role of adiponectin in carcinogenesis and cancer progression and examines the pathophysiological mechanisms that underlie the association between adiponectin and malignancy in the context of a dysfunctional adipose tissue in obesity. Understanding of these mechanisms may be important for the development of preventive and therapeutic strategies against obesity-associated malignancies.

Suppression of PDGF-induced PI3 kinase activity by imatinib promotes adipogenesis and adiponectin secretion

Improved glucose and lipid metabolism is a unique side effect of imatinib therapy in some chronic myeloid leukaemia (CML) patients. We recently reported that plasma levels of adiponectin, an important regulator of insulin sensitivity, are elevated following imatinib therapy in CML patients, which could account for these improved metabolic outcomes. Adiponectin is secreted exclusively from adipocytes, suggesting that imatinib modulates adiponectin levels directly, by transcriptional upregulation of adiponectin in pre-existing adipocytes, and/or indirectly, by stimulating adipogenesis. In this report, we have demonstrated that imatinib promotes adipogenic differentiation of human mesenchymal stromal cells (MSCs), which in turn secrete high-molecular-weight adiponectin. Conversely, imatinib does not stimulate adiponectin secretion from mature adipocytes. We hypothesise that inhibition of PDGFRα (PDGFRA) and PDGFRβ (PDGFRB) is the mechanism by which imatinib promotes adipogenesis. Supporting this, functional blocking antibodies to PDGFR promote adipogenesis and adiponectin secretion in MSC cultures. We have shown that imatinib is a potent inhibitor of PDGF-induced PI3 kinase activation and, using a PI3 kinase p110α-specific inhibitor (PIK-75), we have demonstrated that suppression of this pathway recapitulates the effects of imatinib on MSC differentiation. Furthermore, using mitogens that activate the PI3 kinase pathway, or MSCs expressing constitutively activated Akt, we have shown that activation of the PI3 kinase pathway negates the pro-adipogenic effects of imatinib. Taken together, our results suggest that imatinib increases plasma adiponectin levels by promoting adipogenesis through the suppression of PI3 kinase signalling downstream of PDGFR.

Synthesis and assembly of functional high molecular weight adiponectin multimers in an engineered strain of Escherichia coli

Adiponectin has many beneficial effects on cardiovascular and obesity-related disorders. It is part of a class of proteins that contains short collagenous domains, along with surfactant proteins A and D, and complement protein C1q. This class of biomacromolecules requires post-translational modifications to form biologically active assemblies. By introducing a set of post-translational modifying enzymes into Escherichia coli , we have created a prokaryotic expression system that functionally assembles adiponectin, as assessed by the ability of produced adiponectin multimers to suppress human endothelial cell apoptosis. This study represents the first example of the assembly of functional high order multimers of any member of this class of proteins outside of eukaryotic cells. Furthermore, the results give fundamental insight into the process of assembly such as the necessity and sufficiency of various post-translational steps for functional assembly. We expect that fine-tuning of the expression system will allow for efficient production and functional assembly of biomolecules that assemble via short collagenous domains.

Obesity, inflammation, and liver cancer

Obesity has become a universal and major public health problem with increasing prevalence in both adults and children in the 21st century, even in developing countries. Extensive epidemiological studies reveal a strong link between obesity and development and progression of various types of cancers. The connection between obesity and liver cancer is particularly strong and obesity often results in liver diseases such as non-alcoholic fatty liver disease (NAFLD) and the more severe non-alcoholic steatohepatitis (NASH). NASH is characterized by fatty liver inflammation and is believed to cause fibrosis and cirrhosis. The latter is a known liver cancer risk factor. In fact due to its much higher prevalence obesity may be a more substantial contributor to overall hepatocellular carcinoma burden than infection with hepatitis viruses. Here we review and discuss recent advances in elucidation of cellular and molecular alterations and signaling pathways associated with obesity and liver inflammation and their contribution to hepatocarcinogenesis.

Molecular tools to characterize adiponectin activity

Within the past years, numerous hormones were found to be secreted by adipose tissue. As these adipokines exert different physiological effects with great importance in obesity, they provide new strategies for the treatment of obesity associated disorders. Adiponectin is one of the most promising targets due to its protective properties in glucose and lipid metabolism, which are mediated by the adiponectin receptor 1 and 2. Within the past decades, substantial progress in understanding the molecular function of this unique ligand-receptor system could be achieved. This review summarizes the most important approaches for the investigation of adiponectin activity. Even though many insights into adipokine function could be achieved, clarification of the detailed mode of action is still challenging. For this reason, this review gives an overview of frequently used methods, which led to the molecular characterization of adiponectin and might help to get more detailed insights into the broad aspects of obesity.

Adiponectin receptor signalling in the brain

Adiponectin is an important adipocyte-derived hormone that regulates metabolism of lipids and glucose, and its receptors (AdipoR1, AdipoR2, T-cadherin) appear to exert actions in peripheral tissues by activating the AMP-activated protein kinase, p38-MAPK, PPARα and NF-kappa B. Adiponectin has been shown to exert a wide range of biological functions that could elicit different effects, depending on the target organ and the biological milieu. There is substantial evidence to suggest that adiponectin receptors are expressed widely in the brain. Their expression has been detected in regions of the mouse hypothalamus, brainstem, cortical neurons and endothelial cells, as well as in whole brain and pituitary extracts. While there is now considerable evidence for the presence of adiponectin and its receptors in the brain, their precise roles in brain diseases still remain unclear. Only a few research studies have looked at this facet of adiponectins in brain disorders. This brief review will describe the evidence for important functions by adiponectin, its structure and known actions, evidence for expression of AdipoRs in the brain, their involvement in brain disorders and the therapeutic potential of agents that could modify AdipoR signalling.

CTRP1 protein enhances fatty acid oxidation via AMP-activated protein kinase (AMPK) activation and acetyl-CoA carboxylase (ACC) inhibition

We previously described the adipokine CTRP1, which has up-regulated expression following exposure to the anti-diabetic drug rosiglitazone and increased circulating levels in adiponectin-null mice (Wong, G. W., Krawczyk, S. A., Kitidis-Mitrokostas, C., Revett, T., Gimeno, R., and Lodish, H. F. (2008) Biochem. J. 416, 161-177). Although recombinant CTRP1 lowers blood glucose in mice, its physiological function, mechanisms of action, and roles in metabolic stress remain unknown. Here, we show that circulating levels of CTRP1 are strikingly reduced in diet-induced obese mice. Overexpressing CTRP1 in transgenic mice improved insulin sensitivity and decreased high-fat diet-induced weight gain. Reduced adiposity resulted from enhanced fatty acid oxidation and energy expenditure, effects mediated by AMP-activated protein kinase (AMPK). In skeletal muscle of transgenic mice, AMPKα and its downstream target, acetyl-CoA carboxylase (ACC), were hyperphosphorylated, indicative of AMPK activation and ACC inhibition. Inactivation of ACC promotes mitochondrial fat oxidation. Consistent with the direct effect of CTRP1 on AMPK signaling, recombinant CTRP1 administration acutely stimulated muscle AMPKα and ACC phosphorylation in vivo. In isolated soleus muscle, recombinant CTRP1 activated AMPK signaling to increase fatty acid oxidation ex vivo, an effect abrogated by an AMPK inhibitor. These results provide the first in vivo evidence that CTRP1 is a novel regulator of fatty acid metabolism.

Adiponectin action: a combination of endocrine and autocrine/paracrine effects

The widespread physiological actions of adiponectin have now been well characterized as clinical studies and works in animal models have established strong correlations between circulating adiponectin level and various disease-related outcomes. Thus, conventional thinking attributes many of adiponectin's beneficial effects to endocrine actions of adipose-derived adiponectin. However, it is now clear that several tissues can themselves produce adiponectin and there is growing evidence that locally produced adiponectin can mediate functionally important autocrine or paracrine effects. In this review article we discuss regulation of adiponectin production, its mechanism of action via receptor isoforms and signaling pathways, and its principal physiological effects (i.e., metabolic and cardiovascular). The role of endocrine actions of adiponectin and changes in local production of adiponectin or its receptors in whole body physiology is discussed.

Novel adiponectin variants identified in type 2 diabetic patients reveal multimerization and secretion defects

ADIPOQ, encoding adiponectin, is a candidate gene for type 2 diabetes (T2D) identified by genome-wide linkage analyses with supporting evidence showing the protein function in sensitizing insulin actions. In an endeavor to characterize candidate genes causing T2D in Thai patients, we identified 10 novel ADIPOQ variations, several of which were non-synonymous variations observed only in the patients. To examine the impact of these non-synonymous variations on adiponectin structure and biochemical characteristics, we conducted a structural analysis of the wild-type and variant proteins by in silico modeling and further characterized biochemical properties of the variants with predicted structural abnormalities from the modeling by molecular and biochemical studies. The recombinant plasmids containing wild-type and variant ADIPOQ cDNAs derived from the variations identified by our study (R55H, R112H, and R131H) and previous work (G90S and R112C) were constructed and transiently expressed and co-expressed in cultured HEK293T cells to investigate their oligomerization, interaction, and secretion. We found that the novel R55H variant impaired protein multimerization but it did not exert the effect over the co-expressed wild-type protein while novel R131H variant impaired protein secretion and also affected the co-expressed wild-type protein in a dominant negative fashion. The R131H variant could traffic from the endoplasmic reticulum to the Golgi, trans-Golgi network, and early endosome but could not be secreted. The R131H variant was likely to be degraded through the lysosomal system and inhibition of its degradation rescued the variant protein from secretion defect. We have shown the possibility of using in silico modeling for predicting the effect of amino acid substitution on adiponectin oligomerization. This is also the first report that demonstrates a dominant negative effect of the R131H variant on protein secretion and the possibility of using protein degradation inhibitors as therapeutic agents in the patients carrying adiponectin variants with secretion defect.