Adiponectin in health and disease: evaluation of adiponectin-targeted drug development strategies

Adiponectin self-regulates its expression and multimerization in adipose tissue: an autocrine/paracrine mechanism?

Adiponectin, a 30-kDa peptide hormone discovered in the mid 1990s, is secreted abundantly and exclusively by adipose tissue. Adiponectin exists in three major forms: a low molecular weight (LMW) trimer, a medium molecular weight (MMW) hexamer, and a high molecular weight (HMW) 18-36 oligomer. The HMW oligomer has the most potent insulin-sensitizing activity therefore impaired adiponectin multimerization may lead to impaired glycemic control. Decreased ratio of HMW/total adiponectin has been observed in patients with obesity, type-2 diabetes mellitus, cardiovascular diseases and insulin resistance-related metabolic syndrome. Previous studies have indicated that berberine or aminoimidazole carboxamide ribonucleotide (AICAR)-induced activation of AMP-activated protein kinase (AMPK) suppresses the expression of adiponectin but promotes adiponectin multimerization in adipocytes. Since adiponectin activates AMPK through adiponectin receptors (AdipoRs) in the membranes of adipocytes, we speculate that adiponectin self-regulates its expression and multimerization in adipose tissue. The hypothesis suggests a potential drug target for treating insulin resistance and provides new interpretation of several clinical observations. In addition, we propose a rapid method for one-step detection of the distribution of adiponectin oligomers in approximately 30 min, based on the open sandwich immunoassay and fluorescence resonance energy transfer technology. With the development of this new method, the ratio of HMW/total adiponectin may be applied in clinical diagnosis as a novel biomarker for insulin resistance and metabolic disorders.

Adiponectin regulation of stellate cell activation via PPARγ-dependent and -independent mechanisms

In this study, we elucidated the mechanism by which adiponectin modulates hepatic stellate cell activation and fibrogenesis. Adiponectin-overexpressing transgenic mice receiving thioacetamide were resistant to fibrosis, compared with controls. In contrast, adiponectin-null animals developed severe fibrosis. Expression of collagen α1(I) and α-smooth muscle actin (α-SMA) mRNAs were significantly lower in adiponectin-overexpressing mice, compared with controls. In wild-type stellate cells exposed to a lentivirus encoding adiponectin, expression of peroxisome proliferator-activated receptor-γ (PPARγ), SREBP1c, and CEBPα mRNAs was significantly increased (3.2-, 4.1-, and 2.2-fold, respectively; n = 3; P < 0.05, adiponectin virus versus control), consistent with possible activation of an adipogenic transcriptional program. Troglitazone, a PPARγ agonist, strongly suppressed up-regulation of collagen α1(I) and α-SMA mRNA in stellate cells isolated from wild-type mice; however, stellate cells from adiponectin-null animals failed to respond to troglitazone. Furthermore, in isolated stellate cells in which PPARγ was depleted using an adenovirus-Cre-recombinase system and in which adiponectin was also overexpressed, collagen α1(I) and α-SMA were significantly inhibited. We conclude that the PPARγ effect on stellate cell activation and the fibrogenic cascade appears to be adiponectin-dependent; however, the inhibitory effect of adiponectin on stellate cell activation was not dependent on PPARγ, suggesting the presence of PPARγ-dependent as well as independent pathways in stellate cells.

The metabolic syndrome in critically ill patients

Metabolic support in intensive care is a rapidly evolving field with new information being gathered almost on a daily basis. In endocrine practice, over the last 20 years, researchers have focussed on a new entity, termed the "metabolic syndrome". This describes the constellation of abnormalities which include central adiposity, insulin resistance and inflammation. All of these predispose the individual to a greater risk of cardiovascular events. Of interest is the observation that some of the metabolic abnormalities in sepsis and multiple organ dysfunction syndrome of critical illness share several common features with that of the metabolic syndrome. In this chapter we describe the features of the metabolic syndrome as is understood in endocrine parlance, the metabolic abnormalities of critical illness and explore the common threads underlying the pathophysiology and the treatment of the two syndromes. The role of adiponectin in the metabolic abnormalities in both the metabolic syndrome and in sepsis are reviewed. The potential role of the pleiotropic effects of statins in the therapy of sepsis is also discussed.

Basic Science for the Clinician 52: adipokines

Adipocytes, the cells that maintain fat stores and influence energy metabolism, produce a variety of messengers, called adipokines (or adipocytokines). These proteins have broad reaching effects on glucose and fat metabolism, but also influence inflammation, by modulating the production of inflammatory cytokines and modifying how established cytokines such as interleukin 6 and tumor necrosis factor α themselves induce or modulate inflammation; some of these proteins are produced by synovial tissue adipocytes, suggesting a very direct effect on the modification of local inflammation. Adipokines provide mechanisms that might explain accelerated atherosclerosis and impaired glucose metabolism in some of our chronic inflammatory diseases and offer potential unique therapeutic approaches to control these and other manifestations of inflammation.

Dopamine receptors in human adipocytes: expression and functions

Introduction

Dopamine (DA) binds to five receptors (DAR), classified by their ability to increase (D1R-like) or decrease (D2R-like) cAMP. In humans, most DA circulates as dopamine sulfate (DA-S), which can be de-conjugated to bioactive DA by arylsulfatase A (ARSA). The objective was to examine expression of DAR and ARSA in human adipose tissue and determine whether DA regulates prolactin (PRL) and adipokine expression and release.

Methods

DAR were analyzed by RT-PCR and Western blotting in explants, primary adipocytes and two human adipocyte cell lines, LS14 and SW872. ARSA expression and activity were determined by qPCR and enzymatic assay. PRL expression and release were determined by luciferase reporter and Nb2 bioassay. Analysis of cAMP, cGMP, leptin, adiponectin and interleukin 6 (IL-6) was done by ELISA. Activation of MAPK and PI3 kinase/Akt was determined by Western blotting.

Results

DAR are variably expressed at the mRNA and protein levels in adipose tissue and adipocytes during adipogenesis. ARSA activity in adipocyte increases after differentiation. DA at nM concentrations suppresses cAMP, stimulates cGMP, and activates MAPK in adipocytes. Acting via D2R-like receptors, DA and DA-S inhibit PRL gene expression and release. Acting via D1R/D5R receptors, DA suppresses leptin and stimulates adiponectin and IL-6 release.

Conclusions

This is the first report that human adipocytes express functional DAR and ARSA, suggesting a regulatory role for peripheral DA in adipose functions. We speculate that the propensity of some DAR-activating antipsychotics to increase weight and alter metabolic homeostasis is due, in part, to their direct action on adipose tissue.

Vascular smooth muscle cell-derived adiponectin: a paracrine regulator of contractile phenotype

Adiponectin is a cardioprotective adipokine derived predominantly from visceral fat. We recently demonstrated that exogenous adiponectin induces vascular smooth muscle cell (VSMC) differentiation via repression of mTORC1 and FoxO4. Here we report for the first time that VSMC express and secrete adiponectin, which acts in an autocrine and paracrine manner to regulate VSMC contractile phenotype. Adiponectin was found to be expressed in human coronary artery and mouse aortic VSMC. Importantly, siRNA knock-down of endogenous adiponectin in VSMC significantly reduced the expression of VSMC contractile proteins. Contractile protein deficiency was also observed in primary VSMC isolated from Adiponectin(-/-) mice. This deficiency could be rescued by culturing Adiponectin(-/-) VSMC in conditioned media from wild type (WT) VSMC. Moreover, the paracrine effect of VSMC-derived adiponectin was confirmed as adiponectin neutralizing antibody blocked the rescue. Overexpressed adiponectin also exerted paracrine effects on neighboring untransfected VSMC, which was also blocked by adiponectin neutralizing antibody. Interestingly, adiponectin expression was inducible by the PPARγ agonist rosiglitazone. Our data support an important role for VSMC-derived adiponectin in maintaining VSMC contractile phenotype, contributing to critical cardioprotective functions in the vascular wall. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Cardiovascular complications of diabetes: recent insights in pathophysiology and therapeutics

Cardiovascular complications represent the principal cause of death in patients with Type 2 diabetes. It is therefore of great importance to dissect the genetic determinants and molecular mechanisms responsible for diabetic cardiovascular complications. New research is of particular importance since, somewhat unexpectedly, large-scale clinical trials have indicated that glycemic control does not appear to have the anticipated major influence as a factor dictating cardiovascular outcome in diabetics. Hence, additional pathophysiological factors such as dyslipidemia, as well as proinflammatory and proatherosclerotic mechanisms, need to be more carefully examined. In this article, we will focus on recent studies in both animal models and humans as well as cellular mechanistic studies that advance our knowledge on the role of dyslipidemia, inflammation and atherosclerotic events in the cardiovascular complications of diabetes. We also translate our focus on research insights to related therapeutic opportunities.

Adiponectin expression in subcutaneous adipose tissue is reduced in women with cellulite

BACKGROUND

Cellulite, which appears as orange peel-type or cottage cheese-like dimpling of the skin on the thighs and buttocks, is a complex, multifactorial, cosmetic disorder of the subcutaneous fat layer and the overlying superficial skin. Adiponectin is an adipocyte-derived hormone mainly produced by subcutaneous fat that shows important protective anti-inflammatory and vasodilatory effects. We hypothesized that adiponectin expressed in the subcutaneous adipose tissue (SAT) might play a role in the pathogenesis of cellulite. We reasoned that a reduction in the expression of adiponectin - a humoral vasodilator - in the SAT of cellulite areas might contribute to the altered microcirculation frequently found in these regions.

METHODS

A total of 15 lean (body mass index [BMI] < 25 kg/m(2) ) women with cellulite and 15 age- and BMI-matched women without cellulite participated in this study. Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to assess adiponectin gene expression. Plasma adiponectin levels were measured using a commercial enzyme immunoassay kit.

RESULTS

Adiponectin mRNA expression in the SAT of the gluteal region was significantly lower in areas with cellulite compared with those without (12.6 ± 3.1 AU versus 16.6 ± 4.1 AU; P=0.006). However, plasma adiponectin levels did not differ between women with (20.3 ± 7.3 μg/ml) and without (19.3 ± 6.1 μg/ml) cellulite (P=0.69).

CONCLUSIONS

Adiponectin expression is significantly reduced in the SAT in areas affected by cellulite. Our findings provide novel insights into the nature of cellulite and may give clues to the treatment of this cosmetic issue.

Activation of AMPK by berberine promotes adiponectin multimerization in 3T3-L1 adipocytes

Adiponectin is assembled into trimer (LMW), hexamer (MMW) and high-molecular-weight (HMW) multimer in adipocytes. The HMW adiponectin is more metabolically active and closely associated with peripheral insulin sensitivity. In this study, we reported that berberine, an isoquinoline alkaloid with insulin-sensitizing effect, inhibits the expression of adiponectin, but promotes the assembly of HMW adiponectin and increases the ratio of HMW to total adiponectin. Berberine activates AMPK. Knockdown of AMPKα1 abolishes the effect of berberine. Activation of AMPK by AICAR also increases the level of HMW adiponectin. Our study suggested that activation of AMPK by berberine promotes adiponectin multimerization.

Effect of Ganoderma lucidum extract on adipocyte differentiation and adiponectin gene expression in the murine pre-adipocyte cell line, 3T3-L1

Ganoderma lucidum (G. lucidum), a traditional Chinese medicine, has been used for the treatment of various diseases including cancer and atherosclerosis. In this study, the positive effect of G. lucidum on metabolic syndrome was investigated in more detail by the use of 3T3-L1 pre-adipocyte cells. Treatment of 3T3-L1 cells with G. lucidum extract (GE) significantly promoted adipocyte differentiation and adiponectin production in a dose-dependent manner, as assessed by Oil-Red O staining, quantitative RT-PCR and ELISA. Treatment with GW9662, an inhibitor for peroxisome proliferator-activated receptor-gamma (PPARgamma), significantly attenuated GE-dependent adipocyte differentiation and adiponectin gene expression, suggesting the involvement of PPARgamma. Moreover, a reporter gene assay using GAL4-PPAR fusion proteins revealed that GE enhances GAL4-PPARgamma and GAL4-PPARalpha activities. These results indicate the presence of natural compounds possessing PPARgamma and PPARalpha activating properties in G. lucidum.

Clinical review: adiponectin biology and its role in inflammation and critical illness

Adiponectin is an adipokine first described just over a decade ago. Produced almost exclusively by adipocytes, adiponectin circulates in high concentrations in human plasma. Research into this hormone has revealed it to have insulin-sensitizing, anti-inflammatory and cardioprotective roles. This review discusses the history, biology and physiological role of adiponectin and explores its role in disease, with specific focus on adiponectin in inflammation and sepsis. It appears that an inverse relationship exists between adiponectin and inflammatory cytokines. Low levels of adiponectin have been found in critically ill patients, although data are limited in human subjects at this stage. The role of adiponectin in systemic inflammation and critical illness is not well defined. Early data suggest that plasma levels of adiponectin are decreased in critical illness. Whether this is a result of the disease process itself or whether patients with lower levels of this hormone are more susceptible to developing a critical illness is not known. This observation of lower adiponectin levels then raises the possibility of therapeutic options to increase circulating adiponectin levels. The various options for modulation of serum adiponectin (recombinant adiponectin, thiazolidinediones) are discussed.

Adiponectin induces vascular smooth muscle cell differentiation via repression of mammalian target of rapamycin complex 1 and FoxO4

OBJECTIVE

The adipocyte-secreted hormone adiponectin exerts important cardioprotective and antidiabetic effects. Little is known about its effect on vascular smooth muscle cells (VSMC), key cells in restenosis, hypertension, and atherosclerosis.

METHODS AND RESULTS

Using human coronary artery VSMC, we found that recombinant adiponectin in the high-molecular-weight or trimeric forms but not the globular form induced VSMC differentiation through a mechanism similar to the classic feedback signaling used by rapamycin, a drug known to effectively inhibit restenosis on drug-eluting stents. Using a combination of pharmacological agents, small interfering RNA, and overexpression approaches, we demonstrated that adiponectin activated 5'-AMP-activated protein kinase α2 isoform, leading to inhibition of mammalian target of rapamycin complex 1 and S6K1. This in turn stabilized insulin receptor substrate-1, driving Akt2-mediated inhibition of FoxO4 and subsequent contractile protein induction. Although adiponectin and rapamycin have similarly beneficial effects on VSMC phenotype in both cell and organ culture, a direct comparison of the effects of rapamycin versus adiponectin on endothelial cells revealed distinct differences: rapamycin inhibited Akt phosphorylation, whereas adiponectin maintained it. Importantly, Akt activity preserves endothelial function.

CONCLUSION

Adiponectin promotes VSMC differentiation and preserves endothelial cell Akt signaling, suggesting that targeting the adiponectin pathway may have advantages over rapamycin in developing new drug-eluting stent therapeutics.

Metabolic regulation by C1q/TNF-related protein-13 (CTRP13): activation OF AMP-activated protein kinase and suppression of fatty acid-induced JNK signaling

Members of the C1q/TNF family play important and diverse roles in the immune, endocrine, skeletal, vascular, and sensory systems. Here, we identify and characterize CTRP13, a new and extremely conserved member of the C1q/TNF family. CTRP13 is preferentially expressed by adipose tissue and the brain in mice and predominantly by adipose tissue in humans. Within mouse adipose tissue, CTRP13 is largely expressed by cells of the stromal vascular compartment. Due to sexually dimorphic expression patterns, female mice have higher transcript and circulating CTRP13 levels than males. CTRP13 transcript and circulating levels are elevated in obese male mice, suggesting a potential role in energy metabolism. The insulin-sensitizing drug rosiglitazone also increases the expression of CTRP13 in adipocytes, which correlates with the insulin-sensitizing action of CTRP13. In a heterologous expression system, CTRP13 is secreted as a disulfide-linked oligomeric protein. When co-expressed, CTRP13 forms heteromeric complexes with a closely related family member, CTRP10. This heteromeric association does not involve conserved N-terminal Cys residues. Functional studies using purified recombinant protein demonstrated that CTRP13 is an adipokine that promotes glucose uptake in adipocytes, myotubes, and hepatocytes via activation of the AMPK signaling pathway. CTRP13 also ameliorates lipid-induced insulin resistance in hepatocytes through suppression of the SAPK/JNK stress signaling that impairs the insulin signaling pathway. Further, CTRP13 reduces glucose output in hepatocytes by inhibiting the mRNA expression of gluconeogenic enzymes, glucose-6-phosphatase and the cytosolic form of phosphoenolpyruvate carboxykinase. These results provide the first functional characterization of CTRP13 and establish its importance in glucose homeostasis.

Peptide hormone isoforms: N-terminally branched PYY3-36 isoforms give improved lipid and fat-cell metabolism in diet-induced obese mice

The prevalence of obesity is increasing with an alarming rate worldwide and there is a need for efficacious satiety drugs. PYY3-36 has been shown to play a role in hypothalamic appetite regulation and novel analogs targeting the Y2 receptor have potential as drugs for the treatment of obesity. We have designed a series of novel PYY3-36 isoforms, by first adding the dipeptide Ile-Lys N-terminal to the N(α) of Ser-13 in PYY13-36 and then anchoring the N-terminal segment, e.g. PYY3-12, to the new Lys N(ε)-amine. We hypothesized that such modifications would alter the folding of PYY, due to changes in the turn motif, which could change the binding mode to the Y receptor sub-types and possibly also alter metabolic stability. In structure-affinity/activity relationship experiments, one series of PYY isoforms displayed equipotency towards the Y receptors. However, an increased Y2 receptor potency for the second series of PYY isoforms resulted in enhanced Y receptor selectivity compared to PYY3-36. Additionally, acute as well as chronic mice studies showed body-weight-lowering effects for one of the PYY isoforms, which was also reflected in a reduction of circulating leptin levels. Interestingly, while the stability and pharmacokinetic profile of PYY3-36 and the N-terminally modified PYY3-36 analogue were identical, only mice treated with the branched analogue showed marked increases in adiponectin levels as well as reductions in non-esterified free fatty acids and triglycerides.

Thermogenesis and related metabolic targets in anti-diabetic therapy

Exercise, together with a low-energy diet, is the first-line treatment for type 2 diabetes type 2 diabetes . Exercise improves insulin sensitivity insulin sensitivity by increasing the number or function of muscle mitochondria mitochondria and the capacity for aerobic metabolism, all of which are low in many insulin-resistant subjects. Cannabinoid 1-receptor antagonists and β-adrenoceptor agonists improve insulin sensitivity in humans and promote fat oxidation in rodents independently of reduced food intake. Current drugs for the treatment of diabetes are not, however, noted for their ability to increase fat oxidation, although the thiazolidinediones increase the capacity for fat oxidation in skeletal muscle, whilst paradoxically increasing weight gain.There are a number of targets for anti-diabetic drugs that may improve insulin sensitivity insulin sensitivity by increasing the capacity for fat oxidation. Their mechanisms of action are linked, notably through AMP-activated protein kinase, adiponectin, and the sympathetic nervous system. If ligands for these targets have obvious acute thermogenic activity, it is often because they increase sympathetic activity. This promotes fuel mobilisation, as well as fuel oxidation. When thermogenesis thermogenesis is not obvious, researchers often argue that it has occurred by using the inappropriate device of treating animals for days or weeks until there is weight (mainly fat) loss and then expressing energy expenditure energy expenditure relative to body weight. In reality, thermogenesis may have occurred, but it is too small to detect, and this device distracts us from really appreciating why insulin sensitivity has improved. This is that by increasing fatty acid oxidation fatty acid oxidation more than fatty acid supply, drugs lower the concentrations of fatty acid metabolites that cause insulin resistance. Insulin sensitivity improves long before any anti-obesity effect can be detected.

Synergistic effects of ascorbic acid and thiazolidinedione on secretion of high molecular weight adiponectin from human adipocytes

AIM

To test the hypothesis that ascorbic acid (AA) and thiazolidinedione (TZD) would have additive effects on HMW adiponectin secretion by virtue of different modes of action.

METHODS

We determined the effects of supplementation of AA and/or TZD on expression and secretion of total and HMW adiponectin from human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes in the absence or presence of the proinflammatory cytokine TNFα.

RESULTS

AA supplementation significantly increased secretion of HMW adiponectin (1.7-fold) without altering adiponectin expression or total adiponectin secretion. TZD significantly increased expression (3-fold) and secretion of total (1.4-fold) but not HMW adiponectin. Combined supplementation resulted in a significant increase in expression (3-fold) and secretion of total (1.8-fold) and HMW (5-fold) adiponectin. Similar results were seen in cells co-treated with TNFα.

CONCLUSIONS

These data show that AA and TZD have synergistic rather than simple additive effects on secretion of HMW adiponectin from human adipocytes and raise the possibility that differences in AA levels may contribute to the variability in adiponectin multimer profiles and efficacy of TZD in humans. Our results also provide a rationale for longitudinal clinical trials investigating the effects of AA supplementation with or without TZD on adiponectin and metabolic profiles.

DsbA-L alleviates endoplasmic reticulum stress-induced adiponectin downregulation

OBJECTIVE

Obesity impairs adiponectin expression, assembly, and secretion, yet the underlying mechanisms remain elusive. The aims of this study were 1) to determine the molecular mechanisms by which obesity impairs adiponectin multimerization and stability, and 2) to determine the potential role of disulfide-bond-A oxidoreductase-like protein (DsbA-L), a recently identified adiponectin interactive protein that promotes adiponectin multimerization and stability in obesity-induced endoplasmic reticulum (ER) stress and adiponectin downregulation.

RESEARCH DESIGN AND METHODS

Tauroursodeoxycholic acid (TUDCA), a chemical chaperone that alleviates ER stress, was used to study the mechanism underlying obesity-induced adiponectin downregulation in db/db mice, high-fat diet-induced obese mice, and in ER-stressed 3T3-L1 adipocytes. The cellular levels of DsbA-L were altered by RNAi-mediated suppression or adenovirus-mediated overexpression. The protective role of DsbA-L in obesity- and ER stress-induced adiponectin downregulation was characterized.

RESULTS

Treating db/db mice and diet-induced obese mice with TUDCA increased the cellular and serum levels of adiponectin. In addition, inducing ER stress is sufficient to downregulate adiponectin levels in 3T3-L1 adipocytes, which could be protected by treating cells with the autophagy inhibitor 3-methyladenine or by overexpression of DsbA-L.

CONCLUSIONS

ER stress plays a key role in obesity-induced adiponectin downregulation. In addition, DsbA-L facilitates adiponectin folding and assembly and provides a protective effect against ER stress-mediated adiponectin downregulation in obesity.

Local anesthetics have a major impact on viability of preadipocytes and their differentiation into adipocytes

BACKGROUND

Autologous fat transplantation is a well-established technique in surgery. Moreover, the use of preadipocytes in soft-tissue engineering is currently being intensely investigated. Current efforts focus on identifying maneuvers that may minimize resorption and provide predictable late results. The aim of this study was to investigate the influence of different local anesthetics frequently used in clinical practice on the viability of preadipocytes and their ability to differentiate into adipocytes.

METHODS

Human preadipocytes were isolated from subcutaneous adipose tissue of 15 patients and treated with bupivacaine, mepivacaine, ropivacaine, articaine/epinephrine, and lidocaine for 30 minutes. Viability was determined directly after treatment and during the ensuing cultivation. Differentiation of preadipocytes was determined by expression of the adipocyte marker adiponectin.

RESULTS

Although the immediate effects of mepivacaine and ropivacaine were only moderate, treatment with articaine/epinephrine and lidocaine strongly impaired preadipocyte viability. Cells normally attached to the culture dishes and proliferated irrespective of the previous treatment. During long-term cultivation, articaine/epinephrine-treated cell viability decreased markedly, whereas other local anesthetics had no impact. Despite normal phenotypic appearance of cells treated with bupivacaine, mepivacaine, ropivacaine, and lidocaine, all local anesthetics markedly impaired adipocyte differentiation as determined by adiponectin expression.

CONCLUSIONS

The authors' results show that there is a marked influence of local anesthetics not only on the quantity but also on the quality of viable preadipocytes as determined by their ability to differentiate into mature adipocytes. Therefore, these results should be considered in the context of autologous fat transfer and soft-tissue engineering.

Evolution of inflammation in nonalcoholic fatty liver disease: the multiple parallel hits hypothesis

Whereas in most cases a fatty liver remains free of inflammation, 10%-20% of patients who have fatty liver develop inflammation and fibrosis (nonalcoholic steatohepatitis [NASH]). Inflammation may precede steatosis in certain instances. Therefore, NASH could reflect a disease where inflammation is followed by steatosis. In contrast, NASH subsequent to simple steatosis may be the consequence of a failure of antilipotoxic protection. In both situations, many parallel hits derived from the gut and/or the adipose tissue may promote liver inflammation. Endoplasmic reticulum stress and related signaling networks, (adipo)cytokines, and innate immunity are emerging as central pathways that regulate key features of NASH.

Endoplasmic reticulum stress and inflammation in obesity and diabetes

Obesity is a major problem worldwide that increases risk for a wide range of diseases, including diabetes and heart disease. As such, it is increasingly important to understand how excess adiposity can perturb normal metabolic functions. It is now clear that this disruption involves not only pathways controlling lipid and glucose homeostasis but also integration of metabolic and immune response pathways. Under conditions of nutritional excess, this integration can result in a metabolically driven, low-grade, chronic inflammatory state, referred to as "metaflammation," that targets metabolically critical organs and tissues to adversely affect systemic homeostasis. Endoplasmic reticulum dysfunction is another important feature of chronic metabolic disease that is also linked to both metabolic and immune regulation. A thorough understanding of how these pathways intersect to maintain metabolic homeostasis, as well as how this integration is altered under conditions of nutrient excess, is important to fully understand, and subsequently treat, chronic metabolic diseases.

Crucial role of the central leptin receptor in murine Trypanosoma cruzi (Brazil strain) infection

Mice carrying a defective leptin receptor gene (db/db mice) are metabolically challenged and upon infection with Trypanosoma cruzi (Brazil strain) suffer high mortality. In genetically modified db/db mice, (NSE-Rb db/db mice), central leptin signaling is reconstituted only in the brain, which is sufficient to correct the metabolic defects. NSE-Rb db/db mice were infected with T. cruzi to determine the impact of the lack of leptin signaling on infection in the absence of metabolic dysregulation. Parasitemia levels, mortality rates, and tissue parasitism were statistically significantly increased in infected db/db mice compared with those in infected NSE-Rb db/db and FVB wild-type mice. There was a reduction in fat mass and blood glucose level in infected db/db mice. Plasma levels of several cytokines and chemokines were statistically significantly increased in infected db/db mice compared with those in infected FVB and NSE-Rb db/db mice. These findings suggest that leptin resistance in individuals with obesity and diabetes mellitus may have adverse consequences in T. cruzi infection.

Adiponectin lowers glucose production by increasing SOGA

Adiponectin is a hormone that lowers glucose production by increasing liver insulin sensitivity. Insulin blocks the generation of biochemical intermediates for glucose production by inhibiting autophagy. However, autophagy is stimulated by an essential mediator of adiponectin action, AMPK. This deadlock led to our hypothesis that adiponectin inhibits autophagy through a novel mediator. Mass spectrometry revealed a novel protein that we call suppressor of glucose by autophagy (SOGA) in adiponectin-treated hepatoma cells. Adiponectin increased SOGA in hepatocytes, and siRNA knockdown of SOGA blocked adiponectin inhibition of glucose production. Furthermore, knockdown of SOGA increased late autophagosome and lysosome staining and the secretion of valine, an amino acid that cannot be synthesized or metabolized by liver cells, suggesting that SOGA inhibits autophagy. SOGA decreased in response to AICAR, an activator of AMPK, and LY294002, an inhibitor of the insulin signaling intermediate, PI3K. AICAR reduction of SOGA was blocked by adiponectin; however, adiponectin did not increase SOGA during PI3K inhibition, suggesting that adiponectin increases SOGA through the insulin signaling pathway. SOGA contains an internal signal peptide that enables the secretion of a circulating fragment of SOGA, providing a surrogate marker for intracellular SOGA levels. Circulating SOGA increased in parallel with adiponectin and insulin activity in both humans and mice. These results suggest that adiponectin-mediated increases in SOGA contribute to the inhibition of glucose production.

Access to gram scale amounts of functional globular adiponectin from E. coli inclusion bodies by alkaline-shock solubilization

The adipose tissue derived protein adiponectin exerts anti-diabetic, anti-inflammatory and anti-atherosclerotic effects. Adiponectin serum concentrations are in the microgram per milliliter range in healthy humans and inversely correlate with obesity and metabolic disorders. Accordingly, raising circulating adiponectin levels by direct administration may be an intriguing strategy in the treatment of obesity-related metabolic disorders. However production of large amounts of recombinant adiponectin protein is a primary obstacle so far. Here, we report a novel method for large amount production of globular adiponectin from E. coli inclusion bodies utilizing an alkaline-shock solubilization method without chaotropic agents followed by precipitation of the readily renaturing protein. Precipitation of the mildly solubilized protein capitalizes on advantages of inclusion body formation. This approach of inclusion body protein recovery provides access to gram scale amounts of globular adiponectin with standard laboratory equipment avoiding vast dilution or dialysis steps to neutralize the pH and renature the protein, thus saving chemicals and time. The precipitated protein is readily renaturing in buffer, is of adequate purity without a chromatography step and shows biological activity in cultured MCF7 cells and significantly lowered blood glucose levels in mice with streptozotocin induced type 1 diabetes.

Gluttony, sloth and the metabolic syndrome: a roadmap to lipotoxicity

Once considered divine retribution for sins, comorbidities of obesity (metabolic syndrome) are today attributed to obesity-induced metabolic defects. Here, we propose that obesity and hyperleptinemia protect lipid-intolerant nonadipose organs against lipotoxic lipid spillover during sustained caloric surplus. Metabolic syndrome is ascribed to lipotoxicity caused by age-related resistance to antilipotoxic protection by leptin.

Maternal overweight programs insulin and adiponectin signaling in the offspring

Gestational exposure to maternal overweight (OW) influences the risk of obesity in adult life. Male offspring from OW dams gain greater body weight and fat mass and develop insulin resistance when fed high-fat diets (45% fat). In this report, we identify molecular targets of maternal OW-induced programming at postnatal d 21 before challenge with the high-fat diet. We conducted global transcriptome profiling, gene/protein expression analyses, and characterization of downstream signaling of insulin and adiponectin pathways in conjunction with endocrine and biochemical characterization. Offspring born to OW dams displayed increased serum insulin, leptin, and resistin levels (P < 0.05) at postnatal d 21 preceding changes in body composition. A lipogenic transcriptome signature in the liver, before development of obesity, was evident in OW-dam offspring. A coordinated locus of 20 sterol regulatory element-binding protein-1-regulated target genes was induced by maternal OW. Increased nuclear levels of sterol regulatory element-binding protein-1 and recruitment to the fatty acid synthase promoter were confirmed via ELISA and chromatin immunoprecipitation analyses, respectively. Higher fatty acid synthase and acetyl coenzyme A carboxylase protein and pAKT (Thr(308)) and phospho-insulin receptor-beta were confirmed via immunoblotting. Maternal OW also attenuated AMP kinase/peroxisome proliferator-activated receptor-alpha signaling in the offspring liver, including transcriptional down-regulation of several peroxisome proliferator-activated receptor-alpha-regulated genes. Hepatic mRNA and circulating fibroblast growth factor-21 levels were significantly lower in OW-dam offspring. Furthermore, serum levels of high-molecular-weight adiponectin (P < 0.05) were decreased in OW-dam offspring. Phosphorylation of hepatic AMP-kinase (Thr(172)) was significantly decreased in OW-dam offspring, along with lower AdipoR1 mRNA. Our results strongly suggest that gestational exposure to maternal obesity programs multiple aspects of energy-balance regulation in the offspring.

Decreased serum levels of adiponectin in subjects with autism

The neurobiological basis for autism remains poorly understood. We hypothesized that adipokines, such as adiponectin, may play a role in the pathophysiology of autism. In this study, we examined whether serum levels of adiponectin are altered in subjects with autism. We measured serum levels of adiponectin in male subjects with autism (n=31) and age-matched healthy male subjects (n=31). The serum levels of adiponectin in the subjects with autism were significantly lower than that of normal control subjects. The serum adiponectin levels in the subjects with autism were negatively correlated with their domain A scores in the Autism Diagnostic Interview-Revised, which reflects their impairments in social interaction. This study suggests that decreased levels of serum adiponectin might be implicated in the pathophysiology of autism.

The renin angiotensin system and the metabolic syndrome

The renin angiotensin system (RAS; most well-known for its critical roles in the regulation of cardiovascular function and hydromineral balance) has regained the spotlight for its potential roles in various aspects of the metabolic syndrome. It may serve as a causal link among obesity and several co-morbidities. Drugs that reduce the synthesis or action of angiotensin-II (A-II; the primary effector peptide of the RAS) have been used to treat hypertension for decades and, more recently, clinical trials have determined the utility of these pharmacological agents to prevent insulin resistance. Moreover, there is evidence that the RAS contributes to body weight regulation by acting in various tissues. This review summarizes what is known of the actions of the RAS in the brain and throughout the body to influence various metabolic disorders. Special emphasis is given to the role of the RAS in body weight regulation. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

The preclinical pharmacology of roflumilast--a selective, oral phosphodiesterase 4 inhibitor in development for chronic obstructive pulmonary disease

After more than two decades of research into phosphodiesterase 4 (PDE4) inhibitors, roflumilast (3-cyclopropylmethoxy-4-difluoromethoxy-N-[3,5-di-chloropyrid-4-yl]-benzamide) may become the first agent in this class to be approved for patient treatment worldwide. Within the PDE family of 11 known isoenzymes, roflumilast is selective for PDE4, showing balanced selectivity for subtypes A-D, and is of high subnanomolar potency. The active principle of roflumilast in man is its dichloropyridyl N-oxide metabolite, which has similar potency as a PDE4 inhibitor as the parent compound. The long half-life and high potency of this metabolite allows for once-daily, oral administration of a single, 500-microg tablet of roflumilast. The molecular mode of action of roflumilast--PDE4 inhibition and subsequent enhancement of cAMP levels--is well established. To further understand its functional mode of action in chronic obstructive pulmonary disease (COPD), for which roflumilast is being developed, a series of in vitro and in vivo preclinical studies has been performed. COPD is a progressive, devastating condition of the lung associated with an abnormal inflammatory response to noxious particles and gases, particularly tobacco smoke. In addition, according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD), significant extrapulmonary effects, including comorbidities, may add to the severity of the disease in individual patients, and which may be addressed preferentially by orally administered remedies. COPD shows an increasing prevalence and mortality, and its treatment remains a high, unmet medical need. In vivo, roflumilast mitigates key COPD-related disease mechanisms such as tobacco smoke-induced lung inflammation, mucociliary malfunction, lung fibrotic and emphysematous remodelling, oxidative stress, pulmonary vascular remodelling and pulmonary hypertension. In vitro, roflumilast N-oxide has been demonstrated to affect the functions of many cell types, including neutrophils, monocytes/macrophages, CD4+ and CD8+ T-cells, endothelial cells, epithelial cells, smooth muscle cells and fibroblasts. These cellular effects are thought to be responsible for the beneficial effects of roflumilast on the disease mechanisms of COPD, which translate into reduced exacerbations and improved lung function. As a multicomponent disease, COPD requires a broad therapeutic approach that might be achieved by PDE4 inhibition. However, as a PDE4 inhibitor, roflumilast is not a direct bronchodilator. In summary, roflumilast may be the first-in-class PDE4 inhibitor for COPD therapy. In addition to being a non-steroid, anti-inflammatory drug designed to target pulmonary inflammation, the preclinical pharmacology described in this review points to a broad functional mode of action of roflumilast that putatively addresses additional COPD mechanisms. This enables roflumilast to offer effective, oral maintenance treatment for COPD, with an acceptable tolerability profile and the potential to favourably affect the extrapulmonary effects of the disease.

Lack of association between adiponectin levels and atherosclerosis in mice

OBJECTIVE

Adiponectin is an adipocyte-derived, secreted protein that is implicated in protection against a cluster of related metabolic disorders. Mice lacking adiponectin display impaired hepatic insulin sensitivity and respond only partially to peroxisome proliferator-activated receptor gamma agonists. Adiponectin has been associated with antiinflammatory and antiatherogenic properties; however, the direct involvement of adiponectin on the atherogenic process has not been studied.

METHODS AND RESULTS

We crossed adiponectin knockout mice (Adn(-/-)) or mice with chronically elevated adiponectin levels (Adn(Tg)) into the low-density lipoprotein receptor-null (Ldlr(-/-)) and the apoliprotein E-null (Apoe(-/-)) mouse models. Adiponectin levels did not correlate with a suppression of the atherogenic process. Plaque volume in the aortic root, cholesterol accumulation in the aorta, and plaque morphology under various dietary conditions were not affected by circulating adiponectin levels. In light of the strong associations reported for adiponectin with cardiovascular disease in humans, the lack of a phenotype in gain- and loss-of-function studies in mice suggests a lack of causation for adiponectin in inhibiting the buildup of atherosclerotic lesions.

CONCLUSIONS

These data indicate that the actions of adiponectin on the cardiovascular system are complex and multifaceted, with a minimal direct impact on atherosclerotic plaque formation in preclinical rodent models.

Gluttony, sloth and the metabolic syndrome: a roadmap to lipotoxicity

Once considered divine retribution for sins, comorbidities of obesity (metabolic syndrome) are today attributed to obesity-induced metabolic defects. Here, we propose that obesity and hyperleptinemia protect lipid-intolerant nonadipose organs against lipotoxic lipid spillover during sustained caloric surplus. Metabolic syndrome is ascribed to lipotoxicity caused by age-related resistance to antilipotoxic protection by leptin.

Adiponectin action from head to toe

Adiponectin, the most abundant protein secreted by white adipose tissue, is known for its involvement in obesity-related disorders such as insulin resistance, type 2 diabetes mellitus and atherosclerosis. Moreover, modulation of the circulating adiponectin concentration is observed in pathologies that are more or less obesity-related, such as cancer and rheumatoid arthritis. The wide distribution of adiponectin receptors in various organs and tissues suggests that adiponectin has pleiotropic effects on numerous physiological processes. Besides its well-known insulin-sensitizing, anti-inflammatory and antiatherosclerotic properties, accumulating evidence suggests that adiponectin may also have anticancer properties and be cardioprotective. A beneficial effect of adiponectin on female reproductive function was also suggested. Since adiponectin has numerous beneficial biological functions, its use as a therapeutic agent has been suggested. However, the use of adiponectin or its receptors as therapeutic targets is complicated by the presence of different adiponectin oligomeric isoforms and production sites, by multiple receptors with differing affinities for adiponectin isoforms, and by cell-type-specific effects in different tissues. In this review, we discuss the known and potential roles of adiponectin in various tissues and pathologies. The therapeutic promise of administration of adiponectin and the use of its circulating levels as a diagnostic biomarker are further discussed based on the latest experimental studies.

Adipose tissue expandability, lipotoxicity and the Metabolic Syndrome--an allostatic perspective

While the link between obesity and type 2 diabetes is clear on an epidemiological level, the underlying mechanism linking these two common disorders is not as clearly understood. One hypothesis linking obesity to type 2 diabetes is the adipose tissue expandability hypothesis. The adipose tissue expandability hypothesis states that a failure in the capacity for adipose tissue expansion, rather than obesity per se is the key factor linking positive energy balance and type 2 diabetes. All individuals possess a maximum capacity for adipose expansion which is determined by both genetic and environmental factors. Once the adipose tissue expansion limit is reached, adipose tissue ceases to store energy efficiently and lipids begin to accumulate in other tissues. Ectopic lipid accumulation in non-adipocyte cells causes lipotoxic insults including insulin resistance, apoptosis and inflammation. This article discusses the links between adipokines, inflammation, adipose tissue expandability and lipotoxicity. Finally, we will discuss how considering the concept of allostasis may enable a better understanding of how diabetes develops and allow the rational design of new anti diabetic treatments.

Enhanced insulin sensitivity after acute exercise is not associated with changes in high-molecular weight adiponectin concentration in plasma

BACKGROUND AND OBJECTIVE

The effect of exercise on the plasma concentration of high-molecular weight (HMW) adiponectin (i.e. the biologically active form of circulating adiponectin) and the possible role of HMW adiponectin in mediating the exercise-induced enhancement of insulin action are not known. The aim of this study was to evaluate the relationship between the post-exercise increase in insulin sensitivity and plasma HMW adiponectin concentration.

DESIGN AND METHODS

We measured total and HMW adiponectin concentrations in plasma using an ELISA kit, and insulin sensitivity using the updated homeostasis model assessment of insulin sensitivity (HOMA2-IS) score in the basal, overnight fasted state, once approximately 12 h after a single bout of moderate-intensity endurance exercise and once after an equivalent period of rest, in 27 healthy men and women (age: 29+/-1 years and body mass index: 24.7+/-0.8 kg/m(2)).

RESULTS

The HOMA2-IS score was 18+/-7% greater after exercise than after rest (229+/-20 and 196+/-17 respectively; P=0.006), whereas the concentrations of total adiponectin (7.8+/-0.5 and 7.7+/-0.5 mg/l respectively; P=0.597) and HMW adiponectin (3.0+/-0.3 and 3.0+/-0.3 mg/l respectively; P=0.625) were not different. The exercise-induced change in HOMA2-IS score was not related to changes in total and HMW adiponectin concentrations (P>0.3).

CONCLUSIONS

Changes in HMW adiponectin concentration are not involved in the acute exercise-induced enhancement of insulin action.

Reversible physiological transdifferentiation in the adipose organ

All mammals are provided with two distinct adipose cells, white and brown adipocytes. White adipocytes store lipids to provide fuel to the organism, allowing intervals between meals. Brown adipocytes use lipids to produce heat. Previous descriptions have implied their localization in distinct sites of the body; however, it has been demonstrated that they are present together in many depots, which has led to the new concept of the adipose organ. In order to explain their coexistence the hypothesis of reversible physiological transdifferentiation has been developed, i.e. they are contained together because they are able to convert, one into the other. In effect, if needed the brown component of the organ could increase at the expense of the white component and vice versa. This plasticity is important because the brown phenotype of the organ is associated with resistance to obesity and its related disorders. A new example of reversible physiological transdifferentiation of adipocytes is offered by the mammary gland during pregnancy, lactation and post-lactation stages. The gravidic hormonal stimulus seems to trigger a transdifferentiation of adipocytes into milk-producing and secreting epithelial glands. In the post-lactation period some of the epithelial cells of the mammary gland seem to transdifferentiate into adipocytes. Recent unpublished results suggest that explanted adipose tissue, as well as explanted isolated mature adipocytes, is able to transdifferentiate into glands with epithelial markers of milk-secreting mammary glands. These findings, if confirmed, seem to suggest new windows into the cell biology frontiers of adipocytes.