Targeted disruption of AdipoR1 and AdipoR2 causes abrogation of adiponectin binding and metabolic actions

The malonyl CoA axis as a potential target for treating ischaemic heart disease

Cardiovascular disease is the leading cause of death and disability for people living in western societies, with ischaemic heart disease accounting for the majority of this health burden. The primary treatment for ischaemic heart disease consists of either improving blood and oxygen supply to the heart or reducing the heart's oxygen demand. Unfortunately, despite recent advances with these approaches, ischaemic heart disease still remains a major health problem. Therefore, the development of new treatment strategies is still required. One exciting new approach is to optimize cardiac energy metabolism, particularly by decreasing the use of fatty acids as a fuel and by increasing the use of glucose as a fuel. This approach is beneficial in the setting of ischaemic heart disease, as it allows the heart to produce energy more efficiently and it reduces the degree of acidosis associated with ischaemia/reperfusion. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibiting carnitine palmitoyl transferase-I, the rate-limiting enzyme in the mitochondrial uptake of fatty acids. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase, which in turn is phosphorylated and inhibited by 5'AMP-activated protein kinase. The degradation of myocardial malonyl CoA occurs via malonyl CoA decarboxylase (MCD). Previous studies have shown that inhibiting MCD will significantly increase cardiac malonyl CoA levels. This is associated with an increase in glucose oxidation, a decrease in acidosis, and an improvement in cardiac function and efficiency during and following ischaemia. Hence, the malonyl CoA axis represents an exciting new target for the treatment of ischaemic heart disease.

Adipokines: the missing link between insulin resistance and obesity

White adipose tissue was believed to be just an energy-storage organ, but it is now recognized to be an active participant in energy homoeostasis and physiological functions such as immunity and inflammation. Macrophages are components of adipose tissue and actively participate in its activities. Adipose tissue is known to express and secrete a variety of products known as 'adipokines', including leptin, adiponectin, resistin and visfatin, as well as cytokines and chemokines such as tumor necrosis factor-alpha, interleukin-6 and monocyte chemoattractant protein-1. The release of adipokines by either adipocytes or adipose tissue-infiltrated macrophages leads to a chronic subinflammatory state that could play a central role in the development of insulin resistance and type 2 diabetes, and the increased risk of cardiovascular disease associated with obesity.

Adiponectin: an update

The discoveries of leptin and adiponectin were breakthroughs in the field of metabolic diseases. Adipose cells produce both proteins and release them into the circulation. Leptin acts as a fundamental signal for the brain to modulate food intake as a function of energy status. Loss of leptin function results in obesity. Although a biological role for adiponectin has not been firmly established, clinical and experimental observations indicate that low plasma levels contribute to the pathogenesis of insulin resistance, type 2 diabetes and cardiovascular diseases in obese or overweight patients. Adiponectin circulates as several multimeric species, including a high-molecular-weight form thought to be the most clinically relevant. Adiponectin exerts anti-atherogenic effects by targeting vascular endothelial cells and macrophages and insulin-sensitizing effects, mainly predominantly in muscle and liver. The best-characterized molecular mechanism mediating adiponectin's metabolic and vascular activities involved stimulation of AMP kinase activity. Adiponectin signaling pathways comprise at least two putative receptors (AdipoR1 and AdipoR2). Ways to enhance adiponectin bioactivity are actively being sought. In obesity, reducing chronic adipose-tissue inflammation and macrophage infiltration into it could be beneficial to reverse downregulation of adiponectin gene expression by pro-inflammatory cytokines. Pharmacologically, thiazolidinediones and cannabinoid-1 receptor blockers (e.g., rimonabant) increase plasma adiponectin and gene expression in adipocytes. Finally, AdipoR activation to mimic adiponectin actions could prove beneficial to reduce metabolic risk factors in conditions, such as obesity, where low adiponectinemia prevails.

Adiponectin and adiponectin receptor gene variants in relation to type 2 diabetes and insulin resistance-related phenotypes

BACKGROUND

Alterations in adiponectin-mediated pathways are known to be associated with glucose intolerance, insulin resistance (IR), obesity, and type 2 diabetes (T2D) mellitus. Genetic variations in adiponectin (ADIPOQ) and adiponectin 1 and 2 receptor (ADIPOR1 and ADIPOR2) could have effects on IR-related phenotypes and T2D. Here we examine whether the polymorphic markers rs2241766 (ADIPOQ), rs22753738 (ADIPOR1), rs11061971 and rs16928751 (both in ADIPOR2) are implicated in susceptibility to T2D in a Russian population.

METHODS

The polymorphic markers were genotyped in 129 T2D patients, and 117 non-diabetic controls, by polymerase chain reaction (PCR) restriction fragment length polymorphism approach. In the subjects, biochemical characteristics including serum insulin, plasma glucose and serum lipids/lipoproteins were measured and compared for correlation with the genetic variations studied.

RESULTS

Allele T of rs11061971 and allele A of rs16928751 showed association with higher risk of diabetes providing odds ratios (OR) of 2.05 (p = 0.0025) and 1.88 (p = 0.018), respectively. Haplotype A-G consisting of allele A of rs11061971 and allele G of rs16928751 was associated with reduced risk of T2D (OR = 0.59, pc = 0.0224). Compared to other variants, diabetic patients double homozygous for A/A of rs16928751 and G/G of rs16928751 had decreased homeostasis model assessment-insulin resistance (pc = 0.0375) and serum triglycerides (pc = 0.0285).

CONCLUSIONS

The variants of ADIPOR2 confer susceptibility to T2D and are associated with some IR-related phenotypes in the Russian study population.

Adiponectin blocks multiple signaling cascades associated with leptin-induced cell proliferation in Apc Min/+ colon epithelial cells

We previously demonstrated that leptin, an adipose-derived hormone, induces cell proliferation in a model of preneoplastic (IMCE (Apc(Min/+)), but not normal (YAMC (Apc(+/+)), colon epithelial cells by inducing autocrine IL-6 production and trans-IL-6 signaling. Low serum adiponectin is associated with colon, prostate and breast cancer. Adiponectin is secreted by white adipose tissue; the levels of adiponectin in the blood decrease as body mass index (and leptin) increases. In our study, we tested whether murine recombinant globular adiponectin (gArcp30) could modulate leptin-induced cell proliferation, autocrine IL-6 production, trans-IL-6 signaling and other leptin-induced cell signaling events previously observed in IMCE cells but not YAMC cells. Under serum-free conditions, adiponectin (1 mug/ml) inhibited leptin-induced autocrine IL-6 production, soluble IL-6 receptor shedding, trans-IL-6 signaling and subsequent STAT3 phosphorylation in IMCE cells. Adiponectin inhibited leptin-induced cell proliferation in the IMCE cells and this inhibition was associated with I kappa B-alpha phosphorylation, I kappa B-alpha degradation and decreased NF-kappaB p65 DNA activation and binding. These data indicate that adiponectin acts on preneoplastic colon epithelial cells to regulate cell growth via 2 distinct pathways inhibiting leptin-induced NF-kappaB-dependent autocrine IL-6 production and trans-IL-6 signaling. We hypothesize that adiponectin may be an important regulator of colon epithelial cell homeostasis by linking the observed reduced risk for cancer in populations with high serum adiponectin concentrations to specific mechanisms of cell number homeostasis in a model of preneoplastic colon epithelial cells. These data may have broad implications for diet and lifestyle strategies for the prevention and treatment of obesity-associated cancers.

Adiponectin and peroxisome proliferator-activated receptor gamma expression in subcutaneous and omental adipose tissue in children

AIM

To compare the expression levels of the adiponectin and peroxisome proliferator -activated receptor gamma (PPARgamma) genes in subcutaneous adipose tissue (SC) and omental adipose tissue (OM) in children with relation to age and anthropometric variables.

METHODS

Paired biopsies (SC and OM) were obtained from 53 children (age 0.2-14 years, BMI 12.5-25.8 kg/m(2)). Adiponectin and PPARgamma mRNA levels in adipose tissue were measured by real-time PCR.

RESULTS

In overweight, but not in normal weight children, the median adiponectin mRNA level was significantly lower in OM [0.51 (0.1-2.17)] compared to SC [1.29 (0.16-5.08)], (p = 0.03). Adiponectin mRNA levels were strongly associated with PPARgamma mRNA levels in both SC (r = 0.73, p < 0.001) and OM (r = 0.78, p < 0.001).

CONCLUSIONS

The lower adiponectin expression in OM relative to SC in overweight children indicates that metabolic-endocrine alterations begin already in childhood. The close association between adiponectin and PPARgamma expression supports the hypothesis this transcription factor is involved in adiponectin gene regulation.

Adiponectin regulates albuminuria and podocyte function in mice

Increased albuminuria is associated with obesity and diabetes and is a risk factor for cardiovascular and renal disease. However, the link between early albuminuria and adiposity remains unclear. To determine whether adiponectin, an adipocyte-derived hormone, is a communication signal between adipocytes and the kidney, we performed studies in a cohort of patients at high risk for diabetes and kidney disease as well as in adiponectin-knockout (Ad(-/-)) mice. Albuminuria had a negative correlation with plasma adiponectin in obese patients, and Ad(-/-) mice exhibited increased albuminuria and fusion of podocyte foot processes. In cultured podocytes, adiponectin administration was associated with increased activity of AMPK, and both adiponectin and AMPK activation reduced podocyte permeability to albumin and podocyte dysfunction, as evidenced by zona occludens-1 translocation to the membrane. These effects seemed to be caused by reduction of oxidative stress, as adiponectin and AMPK activation both reduced protein levels of the NADPH oxidase Nox4 in podocytes. Ad(-/-) mice treated with adiponectin exhibited normalization of albuminuria, improvement of podocyte foot process effacement, increased glomerular AMPK activation, and reduced urinary and glomerular markers of oxidant stress. These results suggest that adiponectin is a key regulator of albuminuria, likely acting through the AMPK pathway to modulate oxidant stress in podocytes.

Reduced response to adiponectin and lower abundance of adiponectin receptor proteins in type 2 diabetic monocytes

The abundance of the adiponectin receptors, AdipoR1 and AdipoR2, and the effects of the antidiabetic adipokine adiponectin in monocytes of normal-weight and overweight controls and type 2 diabetic patients (T2D) were analyzed. AdipoR1 and AdipoR2 mRNAs were increased in monocytes of obese controls and T2D patients when compared to normal-weight controls, and AdipoR1 mRNA positively correlated to AdipoR2 mRNA, the waist to hip ratio and systemic adiponectin. However, AdipoR1 and AdipoR2 proteins were lower in monocytes of T2D compared to normal-weight donors. Induction of IL-6 and IL-8 by adiponectin, an effect involving p38 MAPK, was also reduced in T2D monocytes.

Adiponectin receptor R1 is upregulated by valproic acid but not by topiramate in human hepatoma cell line, HepG2

Valproic acid (VPA) is an effective and widely used anticonvulsant, associated with metabolic adverse effects such as weight gain, hyperinsulinemia, hyperleptinemia and hypoadiponectinemia. The aim of this study was to evaluate the influence of VPA and topiramate (TPM) on adiponectin binding receptors, adipoR1 and adipoR2, in human liver cancer cells, HepG2. AdipoR1 but not adipoR2 gene expression was upregulated by VPA treatment. TPM did neither affect adipoR1 nor adipoR2 gene expression. Given the tight association between VPA treatment, metabolic side effects and the adipocytokine-axis, upregulation of adipoR1 possibly represents a favoured and insulin-sensitizing mechanism.

Hzf regulates adipogenesis through translational control of C/EBPalpha

Adipocyte differentiation requires a well-defined programme of gene expression in which the transcription factor C/EBPalpha (CCAAT/enhancer-binding protein) has a central function. Here, we show that Hzf (haematopoietic zinc-finger), a previously identified p53 transcriptional target, regulates C/EBPalpha expression. Hzf is induced during differentiation of preadipocyte cell lines, and its suppression by short hairpin RNA disrupts adipogenesis. In Hzf's absence, expression of C/EBPalpha is severely impaired because of reduced translation of its mRNA. Hzf physically interacts with the 3' untranslated region of C/EBPalpha mRNA to enhance its translation. Taken together, these findings underscore a critical role of Hzf in the adipogenesis regulatory cascade.

Obesity and asthma: possible mechanisms

Epidemiologic data indicate that obesity increases the prevalence and incidence of asthma and reduces asthma control. Obese mice exhibit innate airway hyperresponsiveness and augmented responses to certain asthma triggers, further supporting a relationship between obesity and asthma. Here I discuss several mechanisms that may explain this relationship. In obesity, lung volume and tidal volume are reduced, events that promote airway narrowing. Obesity also leads to a state of low-grade systemic inflammation that may act on the lung to exacerbate asthma. Obesity-related changes in adipose-derived hormones, including leptin and adiponectin, may participate in these events. Comorbidities of obesity, such as dyslipidemia, gastroesophageal reflux, sleep-disordered breathing, type 2 diabetes, or hypertension may provoke or worsen asthma. Finally, obesity and asthma may share a common etiology, such as common genetics, common in utero conditions, or common predisposing dietary factors. Novel therapeutic strategies for treatment of the obese patient with asthma may result from an increased understanding of the mechanisms underlying this relationship.

Circulating adiponectin in preterm infants fed long-chain polyunsaturated fatty acids (LCPUFA)-supplemented formula--a randomized controlled study

Adiponectin has potent insulin-sensitizing effects, improves lipid metabolism, and potentially protects against the development of metabolic syndrome. Thus, increasing adiponectin levels in preterm infants at risk for developing metabolic syndrome may be of special interest. The aim of this study was to examine the effects of dietary long-chain polyunsaturated fatty acids (LCPUFA) on serum adiponectin and lipid concentrations in preterm infants. Adiponectin and lipid levels of 60 healthy preterm infants [gestational age 32.7 (1.9) wk] randomly assigned to be fed either 1) a formula containing LCPUFA [arachidonic and docosahexanoic] (+LCPUFA group) or 2) the same formula without LCPUFA (-LCPUFA/control group), were determined at mean (SD) 33.8 (11.7) d. Adiponectin and HDL-C concentrations were significantly higher in the +LCPUFA group than in controls (p = 0.002 and p = 0.01, respectively); whereas, triglyceride levels were lower (p = 0.06). Adiponectin correlated positively with HDL-C levels and negatively with triglyceride levels in the +LCPUFA group but not in the controls. In conclusion, circulating adiponectin concentrations were higher in preterm infants fed a formula containing LCPUFA than infants fed an LCPUFA-free formula and they correlated with lipidemic profile.

Effect of pronounced weight loss on visceral fat, liver steatosis and adiponectin isoforms

BACKGROUND

Weight loss induced by bariatric surgery is an effective method to reverse obesity and comorbidities. The aim of this prospective weight loss study was to investigate changes of body fat distribution in relation to adiponectin and its isoforms and further to investigate the influence of both body fat distribution and adiponectin on the degree of liver steatosis.

DESIGN

Fifteen severely obese female patients (body mass index 43.1 +/- 4.1, mean age 34.5 +/- 8.6 years) were examined before and after surgical treatment. Grading of fatty liver disease and the subcutaneous and visceral fat diameters were determined by abdominal ultrasonography. Metabolic parameters were determined using standard methods; serum total adiponectin and its isoforms were detected by enzyme immuno assay (EIA).

RESULTS

Mean weight loss was 28.3 kg, which was mostly due to a loss in fat mass, accompanied by an increase in total adiponectin and the high molecular weight (HMW) adiponectin isoform. Visceral adipose tissue (VAT) diameter was highly correlated with liver steatosis, even more strongly than the parameters of liver function. In addition, liver steatosis correlated negatively with HMW adiponectin and binary logistic regression revealed that changes in fat mass, HMW adiponectin and alanine aminotransferase (ALT) were the best predictors for changes in the degree of hepatic steatosis.

CONCLUSIONS

Our results suggest that circulating HMW adiponectin is associated with both VAT and liver steatosis. In summary, the major findings were that the VAT diameter is highly correlated with liver steatosis, even stronger than the parameters of liver function and the association of HMW adiponectin with liver steatosis was better than with total adiponectin.

Fine mapping of mouse QTLs for fatness using SNP data

Quantitative trait loci (QTLs), as determined in crossbred studies, are a valuable resource to identify genes responsible for the corresponding phenotypic variances. Due to their broad chromosomal extension of some dozens of megabases, further steps are necessary to bring the number of candidate genes that underlie the detected effects to a reasonable order of magnitude. We use a set of 13,370 SNPs to identify informative haplotype blocks in 22 mouse QTLs for fatness. About half of the genes in a typical QTL overlap with haplotype blocks, which are different for the two base mouse lines, and which, thus, qualify for further analysis. For these genes we collect four more pieces of evidence for association with fat accumulation, namely (1) homology to genes identified in a Caenorhabditis elegans knock-out experiment as fat decreasing or fat increasing, (2) the overexpression of the genes in mouse fat, liver, muscle, or hypothalamus tissues, (3) the occurrence of a gene in several independently found QTLs, and (4) the information provided by gene ontology, to achieve a ranked list of 131 candidate genes. Ten genes fulfill three or four of the above sketched criteria and are discussed briefly, 121 further genes fulfilling two criteria are provided as on-line material. Viewing the genomic region of fatness-related QTLs under several different aspects is appropriate to assess the many thousands of genes that reside in such QTLs and to produce lists of more robust candidate genes.

Mammalian sirtuin 1 is involved in the protective action of dietary saturated fat against alcoholic fatty liver in mice

This study was undertaken to elucidate the mechanism underlying the protective effect of a high saturated fat (HSF) diet against the development of alcoholic fatty liver in mice. We tested the effects of a HSF diet on the ethanol-mediated increase in hepatic sterol regulatory element binding protein 1 (SREBP-1) activity. Thirty-two male mice were divided into 4 groups and fed liquid diets consisting of either a high polyunsaturated fat (40% of energy from corn oil) or a HSF (40% of energy from cocoa butter) diet with or without ethanol for 4 wk. In the ethanol-containing diets, ethanol was substituted for an equivalent amount of carbohydrate to provide 27.5% of the total energy. Control mice were pair-fed the same volume of liquid diets as the ethanol-fed mice. The HSF diet suppressed the increase in mature SREBP-1 protein and prevented increased mRNA of the SREBP-1-regulated lipogenic enzymes in the ethanol-fed mice (P < 0.05). Sirtuins 1 (SIRT1), a NAD+-dependent class III histone deacetylase, was upregulated by ethanol administration in mice fed the HSF diet (P < 0.05). The HSF diet blocked histone H3 at lysine 9 (lys9) hyperacetylation and attenuated association of acetylated histone H3-Lys9 with the promoters of mitochondrial glycerol-3-phosphate acyltransferase and stearoyl-CoA desaturase 1 in the livers of the ethanol-fed mice. These results suggest that the protective effects of HSF diet against the development of alcoholic liver steatosis may occur via regulation of the hepatic SIRT1-SREBP-1-histone H3 axis, suppressing the expression of genes encoding lipogenic enzymes and slowing the synthesis of hepatic fatty acids.

Enhanced muscle by myostatin propeptide increases adipose tissue adiponectin, PPAR-alpha, and PPAR-gamma expressions

Muscle tissue utilizes a large portion of metabolic energy for its growth and maintenance. Previously, we demonstrated that transgenic over-expression of myostatin propeptide in mice fed a high-fat diet enhanced muscle mass and circulating adiponectin while the wild-type mice developed obesity and insulin resistance. To understand the effects of enhanced muscle growth on adipose tissue metabolism, we analyzed adiponectin, PPAR-alpha, and PPAR-gamma mRNA expressions in several fat tissues. Results indicated muscled transgenic mice fed a high-fat diet displayed increased epididymal adiponectin mRNA expression by 12 times over wild-type littermates. These transgenic mice fed either a high or normal fat diet also displayed significantly high levels of PPAR-alpha and PPAR-gamma expressions above their wild-type littermates in epididymal fat while their expressions in mesenteric fats were not significantly different between transgenic mice and their littermates. This study demonstrates that enhanced muscle growth has positive effects on fat metabolisms through increasing adiponectin expression and its regulations.

Vascular effects of adiponectin: molecular mechanisms and potential therapeutic intervention

Adiponectin is a major adipocyte-secreted adipokine abundantly present in the circulation as three distinct oligomeric complexes. In addition to its role as an insulin sensitizer, mounting evidence suggests that adiponectin is an important player in maintaining vascular homoeostasis. Numerous epidemiological studies based on different ethnic groups have identified adiponectin deficiency (hypoadiponectinaemia) as an independent risk factor for endothelial dysfunction, hypertension, coronary heart disease, myocardial infarction and other cardiovascular complications. Conversely, elevation of circulating adiponectin concentrations by either genetic or pharmacological approaches can alleviate various vascular dysfunctions in animal models. Adiponectin exerts its vasculoprotective effects through its direct actions in the vascular system, such as increasing endothelial NO production, inhibiting endothelial cell activation and endothelium-leucocyte interaction, enhancing phagocytosis, and suppressing macrophage activation, macrophage-to-foam cell transformation and platelet aggregation. In addition, adiponectin reduces neointima formation through an oligomerization-dependent inhibition of smooth muscle proliferation. The present review highlights recent research advances in unveiling the molecular mechanisms that underpin the vascular actions of adiponectin and discusses the potential strategies of using adiponectin or its signalling pathways as therapeutic targets to combat obesity-related metabolic and vascular diseases.

Role of fat mass and adipokines in chronic kidney disease

PURPOSE OF REVIEW

As traditional risk factors cannot alone explain the high prevalence and incidence of cardiovascular disease in chronic kidney disease, the complex of insulin resistance, oxidative stress, and endothelial dysfunction has increasingly been studied as an important non-traditional risk factor. Recent studies show that the adipose tissue is a complex organ with pleiotropic functions far beyond the mere storage of energy. Fat tissue secretes a number of adipokines including leptin and adiponectin, as well as cytokines, such as resistin, visfatin, tumor-necrosis factor-alpha and interleukin-6.

RECENT FINDINGS

Adipokine serum levels are markedly elevated in chronic kidney disease, likely due to a decreased renal excretion. Evidence suggests that these pluripotent signaling molecules may have multiple effects modulating insulin signaling, endothelial health and vascular outcome.

SUMMARY

Fat tissue is a storage depot for energy and a source of circulating signaling molecules. It plays an important role in the catabolic uremic milieu, and has been linked to systemic inflammation and uremic anorexia. Further research is needed to investigate the complex interactions between adipokine signaling networks and its effects on vascular health and outcome in chronic kidney disease.

Role of adiponectin and PBEF/visfatin as regulators of inflammation: involvement in obesity-associated diseases

Obesity and obesity-related disorders play an important role in clinical medicine. Adipose tissue, with its soluble mediators called adipocytokines, has emerged as a major endocrine organ. These adipocytokines comprise many mediators such as adiponectin, PBEF (pre-B-cell-enhancing factor)/visfatin, leptin, resistin, retinol-binding protein-4 and others. They play major roles in key aspects of metabolism, such as insulin resistance, fatty acid oxidation, inflammation and immunity. Adiponectin, a prototypic adipocytokine, is of importance in the regulation of insulin resistance, as circulating levels are decreased in obesity and diseases associated with insulin resistance. Besides its major role in regulation of insulin sensitivity, recent evidence suggests potent anti-inflammatory functions for adiponectin. These effects are paralleled by other immune-regulatory properties, such as regulation of endothelial cell function. The in vitro effects of adiponectin have been corroborated by several studies demonstrating potent in vivo anti-inflammatory effects. Many other adipocytokines, such as PBEF/visfatin, leptin, resistin or retinol binding protein-4, are involved in the physiology and pathophysiology of adipocytes, adipose tissue and related diseases. PBEF/visfatin, another recently characterized adipocytokine, has been linked to several inflammatory disease states beyond insulin resistance, such as acute lung injury or inflammatory bowel diseases. It has been recognized for many decades that obesity is accompanied by an increase in cancer and potentially some immune-mediated diseases. Understanding this new exciting world of adipocytokines will be of importance in the development of novel therapies for obesity-associated diseases.

Control of glycogen synthase through ADIPOR1-AMPK pathway in renal distal tubules of normal and diabetic rats

Diabetic nephropathies are characterized by glycogen accumulation in distal tubular cells, which eventually leads to their apoptosis. The present study aims to determine whether adiponectin and AMPK are involved in the regulation of glycogen synthase (GS) in these structures. Western blots of isolated distal tubules revealed the presence of adiponectin receptor ADIPOR1, catalytic AMPK subunits alpha(1) and alpha(2), their phosphorylated active forms, and the glycogen-binding AMPK subunit beta(2). ADIPOR2 was not detected. Expression levels of ADIPOR1, AMPKalpha(1), AMPKalpha(2), and AMPKbeta(2) were increased in streptozotocin-treated diabetic rats, whereas phosphorylated active AMPK levels were strongly decreased. Immunohistochemistry revealed the presence of ADIPOR1 on the luminal portion of distal tubules and thick ascending limb cells. Catalytic subunits alpha(1) and alpha(2), their phosphorylated active forms, and the glycogen-binding subunit beta(2) were also found in the same cells, confirming immunoblot results. In vitro, 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR; 2 mM) and globular adiponectin (10 mug/ml) activated catalytic AMPK in distal tubules isolated from kidneys of normal rats but much more weakly in those from diabetic rats. GS inhibition paralleled AMPK activation in both groups of animals: active GS levels were low in control animals and elevated in diabetic ones. Finally, glucose-6-phosphate, an allosteric activator of GS, was also increased in diabetic rats. These results demonstrate that in distal tubular cells, adiponectin through luminal ADIPOR1 activates AMPK, leading to the inhibition of GS. During hyperglycemia, this regulation is altered, which may explain, at least in part, the accumulation of large glycogen deposits.

Adenosine monophosphate-activated protein kinase modulates the activated phenotype of hepatic stellate cells

Adiponectin limits the development of liver fibrosis and activates adenosine monophosphate-activated protein kinase (AMPK). AMPK is a sensor of the cellular energy status, but its possible modulation of the fibrogenic properties of hepatic stellate cells (HSCs) has not been established. In this study, we investigated the role of AMPK activation in the biology of activated human HSCs. A time-dependent activation of AMPK was observed in response to a number of stimuli, including globular adiponectin, 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), or metformin. All these compounds significantly inhibited platelet-derived growth factor (PDGF)-stimulated proliferation and migration of human HSCs and reduced the secretion of monocyte chemoattractant protein-1. In addition, AICAR limited the secretion of type I procollagen. Knockdown of AMPK by gene silencing increased the mitogenic effects of PDGF, confirming the negative modulation exerted by this pathway on HSCs. AMPK activation did not reduce PDGF-dependent activation of extracellular signal-regulated kinase (ERK) or Akt at early time points, whereas a marked inhibition was observed 24 hours after addition of PDGF, reflecting a block in cell cycle progression. In contrast, AICAR blocked short-term phosphorylation of ribosomal S6 kinase (p70(S6K)) and 4E binding protein-1 (4EBP1), 2 downstream effectors of the mammalian target of rapamycin (mTOR) pathway, by PDGF. The ability of interleukin-a (IL-1) to activate nuclear factor kappa B (NF-kappaB) was also reduced by AICAR.

CONCLUSION

Activation of AMPK negatively modulates the activated phenotype of HSCs.

Adipokines and the peripheral and neural control of energy balance

Adipokines are secreted by adipose tissue and control various physiological systems. Low leptin levels during fasting stimulate feeding, reduce energy expenditure, and modulate neuroendocrine and immune function to conserve energy stores. On the other hand, rising leptin levels in the overfed state prevent weight gain by inhibiting food intake and increasing energy expenditure. These actions are mediated by neuronal circuits in the hypothalamus and brainstem. Leptin also controls glucose and lipid metabolism by targeting enzymes such as AMP-activated protein kinase and stearoyl-coenzyme A desaturase-1 in liver and muscle. Likewise, adiponectin and resistin control energy balance and insulin sensitivity via central and peripheral targets. As highlighted in this review, there are distinct as well as common signaling pathways for adipokines. Understanding adipokine signaling in the brain and other organs will provide insights into the pathogenesis and treatment of obesity, diabetes and various metabolic disorders.

Obesity in chronic kidney disease: good or bad?

Cardiovascular disease (CVD) remains the major cause of morbidity and mortality in chronic kidney disease (CKD) patients. As traditional risk factors cannot alone explain the high prevalence and incidence of CVD in this high-risk population, the complex of insulin resistance, oxidative stress, and endothelial dysfunction has increasingly been studied as important non-traditional risk factors. Recent studies show that the adipose tissue is a complex organ with functions far beyond the mere storage of energy. Indeed, it has recently been shown that fat tissue secretes a number of adipokines - including leptin, adiponectin and retinol-binding protein, as well as cytokines such as resistin, visfatin, tumor necrosis factor and interleukin-6. Adipokine serum levels are furthermore markedly elevated in CKD, likely due to a decreased renal excretion. Evidence suggests that these pluripotent signaling molecules may have multiple effects modulating insulin signaling, endothelial health and putatively CVD. As fat tissue is also a storage depot for energy, much needed in the catabolic milieu of uremia, further research is still needed to elucidate the likely complex interactions between these signaling networks, vascular health and outcome in this high-risk population.

The physiological and pathophysiological role of adiponectin and adiponectin receptors in the peripheral tissues and CNS

Adiponectin is an abundantly expressed adipokine in adipose tissue and has direct insulin sensitizing activity. A decrease in the circulating levels of adiponectin by interactions between genetic factors and environmental factors causing obesity has been shown to contribute to the development of insulin resistance, type 2 diabetes, metabolic syndrome and atherosclerosis. In addition to its insulin sensitizing actions, adiponectin has central actions in the regulation of energy homeostasis. Adiponectin enhances AMP-activated protein kinase activity in the arcuate hypothalamus via its receptor AdipoR1 to stimulate food intake and decreases energy expenditure. We propose a hypothesis on the physiological role of adiponectin: a starvation gene in the course of evolution by promoting fat storage on facing the loss of adiposity.

Adiponectin and inflammation: consensus and controversy

Circulating levels of adiponectin decrease with increasing visceral obesity and are lower in patients with type 2 diabetes, the metabolic syndrome, and cardiovascular disease compared with controls matched by body mass index. Several reports demonstrated anti-inflammatory effects of adiponectin. Because increased adipose tissue is associated with low-grade chronic inflammation and proinflammatory factors inhibit adiponectin production, the current hypothesis states that chronic inflammation associated with visceral obesity inhibits production of adiponectin, perpetuating inflammation. The negative correlation between adiponectin and markers of inflammation in the aforementioned conditions supports this hypothesis. In contrast with disorders typically associated with excess adiposity and positive energy balance, adiponectin levels are elevated--rather than decreased--in classic chronic inflammatory/autoimmune diseases that are unrelated to increased adipose tissue, such as rheumatoid arthritis, SLE, inflammatory bowel disease, type 1 diabetes, and cystic fibrosis. In these patients, adiponectin levels positively--rather than negatively--correlate with inflammatory markers. Furthermore, proinflammatory effects of adiponectin have been reported in tissues such as joint synovium and colonic epithelium. Thus, adiponectin is regulated in the opposite direction and may exert differential functions in classic versus obesity-associated inflammatory conditions. This article discusses this apparent paradox and presents possible alternative and/or complementary explanations.

Potential therapies based on antidiabetic peptides

Since adipose tissue was shown to be more than a storage organ, the many cytokines it produces have been identified, along with their roles in energy homeostasis, appetite, and insulin resistance. Concurrently, numerous gut hormones with a diversity of effects have been discovered. They include, amongst many others, peptide YY, ghrelin and oxyntomodulin. As these peptides have been investigated, the potential for their use as novel anti-obesity and antidiabetic therapies has been realized. In this chapter we describe the actions of four of the peptides that have been proposed as the basis for promising new therapies for diabetes: leptin, adiponectin, obestatin and peptide YY. They each have an effect on appetite and, directly or indirectly, on glucose metabolism. We synthesize available data for these peptides and consider the therapeutic potential of each.

Gene expression of adiponectin receptors in human visceral and subcutaneous adipose tissue is related to insulin resistance and metabolic parameters and is altered in response to physical training

OBJECTIVE

Adiponectin receptors 1 and 2 (AdipoR1 and AdipoR2, respectively) mediate the effects of adiponectin on glucose and lipid metabolism in vivo. We examined whether AdipoR1 and/or AdipoR2 mRNA expression in human adipose tissue is fat-depot specific. We also studied whether their expression in visceral and subcutaneous fat depots is associated with metabolic parameters and whether their expression is regulated by intensive physical exercise.

RESEARCH DESIGN AND METHODS

We determined metabolic parameters and assessed AdipoR1 and AdipoR2 mRNA expression using quantitative real-time PCR in adipose tissue in an observational study of 153 subjects and an interventional study of 60 subjects (20 each with normal glucose tolerance, impaired glucose tolerance, and type 2 diabetes) before and after intensive physical training for 4 weeks.

RESULTS

AdipoR1 and AdipoR2 mRNA expression is not significantly different between omental and subcutaneous fat, but their expression is several-fold lower in adipose tissue than in muscle. AdipoR2 mRNA expression in visceral fat is highly correlated with its expression in subcutaneous fat. AdipoR2 mRNA expression in both visceral and subcutaneous fat is positively associated with circulating adiponectin and HDL levels but negatively associated with obesity as well as parameters of insulin resistance, glycemia, and other lipid levels before and after adjustment for fat mass. Physical training for 4 weeks resulted in increased AdipoR1 and AdipoR2 mRNA expression in subcutaneous fat.

CONCLUSIONS

AdipoR2 mRNA expression in fat is negatively associated with insulin resistance and metabolic parameters independently of obesity and may mediate the improvement of insulin resistance in response to exercise.

Statin reverses reduction of adiponectin receptor expression in infarcted heart and in TNF-α-treated cardiomyocytes in association with improved glucose uptake

Statin treatment improves insulin resistance in skeletal muscle. Thus this study assessed whether statin may affect the myocardial expression levels of AdipoR1 and AdipoR2, receptors of adiponectin that enhance insulin sensitivity, and whether statin may improve insulin resistance in cardiomyocytes. Myocardial infarction (MI) was created by the ligation of the left coronary artery in male mice. Expression levels of mRNA and protein levels of AdipoR1 but not of AdipoR2 were significantly decreased in the remote area as well as in the healed infarcted area in the left ventricles 4 wk after MI. Oral administration of pravastatin (50 mg·kg−1·day−1 for 4 wk after MI) reversed the decrease in myocardial expression levels of AdipoR1 independently of changes in serum lipid profiles and insulin levels. With the use of cultured cardiomyocytes, incubation with tumor necrosis factor (TNF)-α, a mediator of postinfarction myocardial dysfunction, inhibited AdipoR1 mRNA and protein expression levels. Coincubation of the cells with pravastatin reversed the inhibitory effects of TNF-α on AdipoR1 expression. In parallel, pravastatin reversed the TNF-α-induced decrease in globular adiponectin-induced 2-deoxy-d-[3H]glucose uptake in insulin-treated cultured cells. Moreover, this effect of pravastatin was inhibited by the suppression of AdipoR1 expression by small-interfering RNA specific for AdipoR1. Incubation with H2O2 reduced AdipoR1 expression in cultured cardiomyocytes that were attenuated by N-acetyl-l-cysteine or pravastatin. Pravastatin suppressed TNF-α-induced intracellular oxidants in cultured cardiomyocytes. In conclusion, pravastatin reversed the reduction of AdipoR1 expression in postinfarction mouse myocardium and in TNF-α-treated cardiomyocytes partly through an antioxidative mechanism in association with improved glucose uptake.

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.

Diet, energy metabolism and mitochondrial biogenesis

PURPOSE OF REVIEW

This review highlights some recent findings regarding nutritional and endocrine regulators of mitochondrial mass and function and their association with insulin resistance.

RECENT FINDINGS

Insulin resistance is central to many chronic metabolic diseases, including obesity, type 2 diabetes, dyslipidemia, and hypertension. Insulin resistance in skeletal muscle is associated with lower mitochondrial mass and reduced oxidative phosphorylation. Part of the mitochondrial dysfunction can be triggered by adverse nutrition. Increased fatty acid exposure, resulting from high fats diets or overfeeding, is linked to both decreased mitochondrial number and markers of oxidative phosphorylation. Caloric restriction and the adiponectin signaling pathway, however, can stimulate mitochondrial biogenesis by elevating the transcriptional machinery that regulates mitochondrial mass, improving mitochondrial efficiency, activating the peroxisome proliferator-activated receptor coactivator 1alpha mediated reactive oxygen species scavenging mechanism, and lowering reactive oxygen species production.

SUMMARY

States of insulin resistance are characterized by defects in lipid and carbohydrate metabolism. Abnormalities in oxidative capacity, however, can be partially normalized by caloric restriction by modulating mitochondrial mass in an insulin sensitizing manner.

Lipid-induced oxidative stress causes steatohepatitis in mice fed an atherogenic diet

Recently, nonalcoholic steatohepatitis (NASH) was found to be correlated with cardiovascular disease events independently of the metabolic syndrome. The aim of this study was to investigate whether an atherogenic (Ath) diet induces the pathology of steatohepatitis necessary for the diagnosis of human NASH and how cholesterol and triglyceride alter the hepatic gene expression profiles responsible for oxidative stress. We investigated the liver pathology and plasma and hepatic lipids of mice fed the Ath diet. The hepatic gene expression profile was examined with microarrays and real-time polymerase chain reactions. The Ath diet induced dyslipidemia, lipid peroxidation, and stellate cell activation in the liver and finally caused precirrhotic steatohepatitis after 24 weeks. Cellular ballooning, a necessary histological feature defining human NASH, was observed in contrast to existing animal models. The addition of a high-fat component to the Ath diet caused hepatic insulin resistance and further accelerated the pathology of steatohepatitis. A global gene expression analysis revealed that the Ath diet up-regulated the hepatic expression levels of genes for fatty acid synthesis, oxidative stress, inflammation, and fibrogenesis, which were further accelerated by the addition of a high-fat component. Conversely, the high-fat component down-regulated the hepatic gene expression of antioxidant enzymes and might have increased oxidative stress.

CONCLUSION

The Ath diet induces oxidative stress and steatohepatitis with cellular ballooning. The high-fat component induces insulin resistance, down-regulates genes for antioxidant enzymes, and further aggravates the steatohepatitis. This model suggests the critical role of lipids in causing oxidative stress and insulin resistance leading to steatohepatitis.

Circulating adiponectin and adiponectin receptor expression in skeletal muscle: effects of exercise

Excess visceral fat can regulate insulin sensitivity and energy metabolism by releasing adipokines into the circulation which then bind with their cognate receptors in various tissues and alter glucose and lipid metabolism. Circulating levels of adiponectin, which promotes glucose uptake into skeletal muscle and increases fat oxidation rates, are decreased in obesity. Strategies to enhance the insulin-like and insulin-sensitizing actions of adiponectin have been shown to be effective in improving metabolic abnormalities associated with obesity and diabetes. Interestingly, the insulin-sensitizing effects of exercise have similar metabolic effects as adiponectin in that exercise also promotes glucose uptake into muscle and increases rates of fatty acid oxidation. Recent studies have begun to examine the potential role of adiponectin in mediating the insulin-sensitizing action of exercise by investigating changes in plasma adiponectin levels and tissue-specific adiponectin receptor (AdipoR) expression. In this review, we have summarized the key findings to date which suggest that changes in expression of AdipoR isoforms in skeletal muscle, rather than circulating total adiponectin levels, may be of physiological importance.

Integrating the immune system with the regulation of growth and efficiency

Muscle growth in meat animals is a complex process governed by integrated signals emanating from multiple endocrine and immune cells. A generalized phenomenon among meat animal industries is that animals commonly fail to meet their genetic potential for growth in commercial production settings. Recent evidence indicates that adipocytes and myofibers are equipped with functional pattern recognition receptors and are capable of responding directly to the corresponding pathogens and other receptor ligands. Thus, these cells are active participants in the innate immune response and, as such, produce a number of immune and metabolic regulators, including proinflammatory cytokines and adiponectin. Specifically, the transcription factor, nuclear factor kappa B, is activated in adipocytes and muscle cells by bacterial lipopolysaccharide and certain saturated fatty acids, which are potent agonists for the Toll-like receptor-4 pattern recognition receptor. Receptor activation results in the local production of interleukin-6 and tumor necrosis factor-alpha, and creates a local environment by which these cytokines regulate both metabolic and immunological pathways. However, adipocytes are also the predominant source of the antiinflammatory hormone, adiponectin, which suppresses the activation of nuclear factor kappa B and the production of proinflammatory cytokines. The molecular ability to recognize antigens and produce regulatory molecules strategically positions adipocytes and myofibers to regulate growth locally and to reciprocally regulate metabolism in peripheral tissues.

Recent advances in the measurement of adiponectin isoform distribution

PURPOSE OF REVIEW

Adiponectin has potent antidiabetic and antiatherosclerotic actions. Recent studies in animals and humans suggest that the high-molecular weight adiponectin complex, consisting of many adiponectin monomers, is the biologically active form of the peptide. This article will present recent methodological approaches for analyzing adiponectin isoform distribution.

RECENT FINDINGS

A handful of methods have been used for the isolation and measurement of high-molecular weight adiponectin, based on velocity gradient centrifugation, gel filtration chromatography and polyacrylamide gel electrophoresis. Recently, two novel sandwich enzyme-linked immuno-sorbent assays have been developed. The first makes use of antibodies raised against human high-molecular weight adiponectin and thus allows for the specific determination of high-molecular weight adiponectin in plasma. The second and more versatile enzyme-linked immuno-sorbent assay system enables the measurement of all adiponectin isoforms (i.e. low, middle and high-molecular weight) by means of selective digestion by proteases.

SUMMARY

The development and commercial availability of novel enzyme-linked immuno-sorbent assay kits enables the easy and rapid measurement of high-molecular weight adiponectin in both research and clinical practice and will undoubtedly advance further our understanding of the role of adiponectin in health and disease.

Adiponectin inversely correlates with high sensitive C-reactive protein and triglycerides, but not with insulin sensitivity, in apparently healthy Japanese men

Adiponectin, an antiatherogenic peptide, has diverse biological actions on insulin sensitivity, inflammation and lipid metabolism. To explore physiological and pathophysiological significance of adiponectin in the Japanese general population, we systematically analyzed the relationship between adiponectin and high sensitive CRP (hsCRP), lipids, insulin sensitivity, and anthropometric parameters in 166 consecutive adult male health examinees. By univariate analysis, serum adiponectin was positively correlated with age and HDL-cholesterol, and inversely correlated with fasting plasma glucose, fasting insulin, homeostasis model assessment insulin-resistance, waist, body mass index, triglycerides and hsCRP. However, multivariate analysis revealed that adiponectin independently correlated with triglycerides (r = -0.243, P = 0.0033) and hsCRP (r = -0.262, P = 0.0015) but not with all other variables. Adiponectin was lower and hsCRP higher in the subjects with metabolic syndrome (n = 22) than in those without it (n = 144) (adiponectin, 5.4 +/- 2.8 vs 7.5 +/- 4.2 microg/ml, p = 0.002; hsCRP, 832 +/- 605 vs 470 +/- 524 ng/ml, p = 0.0004). Current findings suggest that relative importance of hypertriglyceridemia and enhanced inflammation, rather than insulin resistance, as the downstream events of hypoadiponectinemia leading to atherosclerosis in the Japanese general population.

Adiponectin stimulates AMP-activated protein kinase in the hypothalamus and increases food intake

Adiponectin has been shown to stimulate fatty acid oxidation and enhance insulin sensitivity through the activation of AMP-activated protein kinase (AMPK) in the peripheral tissues. The effects of adiponectin in the central nervous system, however, are still poorly understood. Here, we show that adiponectin enhances AMPK activity in the arcuate hypothalamus (ARH) via its receptor AdipoR1 to stimulate food intake; this stimulation of food intake by adiponectin was attenuated by dominant-negative AMPK expression in the ARH. Moreover, adiponectin also decreased energy expenditure. Adiponectin-deficient mice showed decreased AMPK phosphorylation in the ARH, decreased food intake, and increased energy expenditure, exhibiting resistance to high-fat-diet-induced obesity. Serum and cerebrospinal fluid levels of adiponectin and expression of AdipoR1 in the ARH were increased during fasting and decreased after refeeding. We conclude that adiponectin stimulates food intake and decreases energy expenditure during fasting through its effects in the central nervous system.

Metabolic stress in insulin's target cells leads to ROS accumulation - A hypothetical common pathway causing insulin resistance

The metabolic syndrome is a cluster of cardiovascular risk factors, and visceral adiposity is a central component that is also strongly associated with insulin resistance. Both visceral obesity and insulin resistance are important risk factors for the development of type 2 diabetes. It is likely that adipose tissue, particularly in the intra-abdominal depot, is part of a complex interplay involving several tissues and that dysregulated hormonal, metabolic and neural signalling within and between organs can trigger development of metabolic disease. One attractive hypothesis is that many factors leading to insulin resistance are mediated via the generation of abnormal amounts of reactive oxygen species (ROS). There is much evidence supporting that detrimental effects of glucose, fatty acids, hormones and cytokines leading to insulin resistance can be exerted via such a common pathway. This review paper mainly focuses on metabolic and other 'stress' factors that affect insulin's target cells, in particular adipocytes, and it will highlight oxidative stress as a potential unifying mechanism by which these stress factors promote insulin resistance and the development and progression of type 2 diabetes.

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.