Cloning of adiponectin receptors that mediate antidiabetic metabolic effects

Adiponectin: action, regulation and association to insulin sensitivity

Adiponectin is a novel adipocyte-specific protein, which, it has been suggested, plays a role in the development of insulin resistance and atherosclerosis. Although it circulates in high concentrations, adiponectin levels are lower in obese subjects than in lean subjects. Apart from negative correlations with measures of adiposity, adiponectin levels are also reduced in association with insulin resistance and type 2 diabetes. Visceral adiposity has been shown to be an independent negative predictor of adiponectin. Thus, most features of the metabolic syndrome's negative associations with adiponectin have been shown. Adiponectin levels seem to be reduced prior to the development of type 2 diabetes, and administration of adiponectin has been accompanied by lower plasma glucose levels as well as increased insulin sensitivity. Furthermore, reduced expression of adiponectin has been associated with some degree of insulin resistance in animal studies indicating a role for hypoadiponectinaemia in relation to insulin resistance. The primary mechanisms by which adiponectin enhance insulin sensitivity appears to be through increased fatty acid oxidation and inhibition of hepatic glucose production. Adiponectin levels are increased by thiazoledinedione treatment, and this effect might be important for the enhanced insulin sensitivity induced by thiazolidinediones. In contrast, adiponectin levels are reduced by pro-inflammatory cytokines especially tumour necrosis factor-alpha. In summary, adiponectin in addition to possible anti-inflammatory and anti-atherogenic effects appears to be an insulin enhancer, with potential as a new pharmacologic treatment modality of the metabolic syndrome and type 2 diabetes.

PPARgamma-mediated insulin sensitization: the importance of fat versus muscle

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear hormone receptor that functions as a transcriptional regulator in a variety of tissues. PPARgamma activation, e.g., through binding of the synthetic glitazones or thiazolidinediones (TZD), results in a marked improvement in type 2 diabetic patients of insulin and glucose parameters resulting from an improvement of whole body insulin sensitivity. The role of different metabolic tissues (fat, skeletal muscle, liver) in mediating PPARgamma function in glucose and insulin homeostasis is still unclear. Recently, the function of PPARgamma in adipose tissue and skeletal muscle has been intensively characterized by using targeted deletion of PPARgamma in those tissues. In those studies, adipose PPARgamma has been identified as an essential mediator for the maintainance of whole body insulin sensitivity. Two major mechanisms have been described. 1) Adipose PPARgamma protects nonadipose tissue against excessive lipid overload and maintains normal organ function (liver, skeletal muscle); and 2) adipose PPARgamma guarantees a balanced and adequate production of secretion from adipose tissue of adipocytokines such as adiponectin and leptin, which are important mediators of insulin action in peripheral tissues. In contrast to studies in adipose-specific PPARgamma-deficient mice, the data in muscle-specific PPARgamma(-/-) mice demonstrate that whole body insulin sensitivity is, at least in part, relying on an intact PPARgamma system in skeletal muscle. Finally, these early and elegant studies using tissue-specific PPARgamma knockout mouse models pinpoint adipose tissue as the major target of TZD-mediated improvement of hyperlipidemia and insulin sensitization.

Adiponectin--journey from an adipocyte secretory protein to biomarker of the metabolic syndrome

Adiponectin is an adipocyte-derived hormone that was discovered in 1995. Unlike leptin, which was identified around the same time, the clinical relevance of adiponectin remained obscure for a number of years. However, starting in 2001, several studies were published from different laboratories that highlighted the potential antidiabetic, antiatherosclerotic and anti-inflammatory properties of this protein complex. Methods to measure the protein with high throughput assays in clinical samples were developed shortly thereafter, and as a result hundreds of clinical studies have been published over the past 3 years describing the role of adiponectin in endocrine and metabolic dysfunction. Furthermore, adiponectin research has expanded to include a role for adiponectin in cancer and other disease areas. Although it is an impossible task to summarize the findings from all these studies in a single review, we aim to demonstrate the utility of circulating adiponectin as a biomarker of the metabolic syndrome. Evidence for this relationship will include how decreased levels of plasma adiponectin ('hypoadiponectinaemia') are associated with increased body mass index (BMI), decreased insulin sensitivity, less favourable plasma lipid profiles, increased levels of inflammatory markers and increased risk for the development of cardiovascular disease. Therefore, adiponectin levels hold great promise for use in clinical application serving as a potent indicator of underlying metabolic complications.

Plasma Adiponectin and Gastric Cancer

Background: Recently, increased body weight has been associated with an increased risk of cancers at multiple specific sites, including gastric cancer. Adiponectin is a peptide hormone secreted by adipose tissue, affecting the proliferation and insulin sensitivity of various types of cells. Moreover, the circulating level of adiponectin has been reported to be inversely related to body mass index.

Methods: Fasting plasma levels of adiponectin were determined in 75 patients with gastric cancer and 52 healthy controls using an ELISA. In these patients, we analyzed the association between plasma adiponectin level and gastric cancer risk as well as various clinicopathologic characteristics.

Results: Plasma adiponectin level was significantly lower in patients with gastric cancer than in healthy controls (9.1 ± 6.2 versus 13.3 ± 9.4 ng/mL, P < 0.01) and showed a significant modest inverse relation with the gastric cancer (odds ratio, 0.92; 95% confidence interval, 0.85-0.97; adjusted odds ratio, 0.89; 95% confidence interval, 0.84-0.95], although body mass index was not different. In addition, adiponectin level was extremely low in patients with upper gastric cancers (upper, 5.5 ± 4.1 ng/mL; middle, 9.7 ± 6.4 ng/mL; lower, 10.7 ± 4.1 ng/mL; P = 0.012). Furthermore, adiponectin level tended to decrease as the tumor stage increased (stage I, 9.9 ± 6.9 ng/mL; stage II, 8.7 ± 5.5 ng/mL; stage III, 8.6 ± 4.1 ng/mL; stage IV, 5.2 ± 6.2 ng/mL; P = 0.34). Interestingly, in 32 patients with undifferentiated cancer, serum adiponectin showed a negative correlation with pathologic findings such as tumor size, depth of invasion, as well as tumor stage (P < 0.05), but no correlation in the remaining 43 patients with differentiated cancer.

Conclusions: Our results suggest that a low plasma adiponectin level is associated with an increased risk for gastric cancer and raise the possibility that adiponectin has a potential role in the progression of gastric cancer, especially in undifferentiated type cancers in the upper stomach.

Adipose Tissue and Atherothrombosis

Obesity is associated with increased cardiovascular mortality and morbidity mainly through insulin resistance. Dysregulation of protein secretion by adipose tissue is involved in obesity-related diseases. Adipose tissue contributes to create a subinflammatory status which could explain the disturbances in the haemostatic and fibrinolytic systems observed in obesity. Elevated plasma levels of PAI-1 demonstrated the strongest association with the degree of insulin resistance and could be an underlying mechanism for the thrombotic tendency and the progression of atherothrombosis during obesity. The effect of PAI-1 was examined on adipose tissue growth in several mouse models as well as on adipocyte differentiation in vitro. Most of the data indicate that PAI-1 can effectively modulate weight gain and may be a potential therapeutic target for controlling cardiovascular morbidity in obese subjects.

Nur77 regulates lipolysis in skeletal muscle cells. Evidence for cross-talk between the beta-adrenergic and an orphan nuclear hormone receptor pathway

Skeletal muscle is a major mass peripheral tissue that accounts for approximately 40% of total body weight and 50% of energy expenditure and is a primary site of glucose disposal and fatty acid oxidation. Consequently, muscle has a significant role in insulin sensitivity, obesity, and the blood-lipid profile. Excessive caloric intake is sensed by the brain and induces beta-adrenergic receptor (beta-AR)-mediated adaptive thermogenesis. Beta-AR null mice develop severe obesity on a high fat diet. However, the target gene(s), target tissues(s), and molecular mechanism involved remain obscure. We observed that 30-60 min of beta-AR agonist (isoprenaline) treatment of C2C12 skeletal muscle cells strikingly activated (>100-fold) the expression of the mRNA encoding the nuclear hormone receptor, Nur77. In contrast, the expression of other nuclear receptors that regulate lipid and carbohydrate metabolism was not induced. Stable transfection of Nur77-specific small interfering RNAs (siNur77) into skeletal muscle cells repressed endogenous Nur77 mRNA expression. Moreover, we observed attenuation of gene and protein expression associated with the regulation of energy expenditure and lipid homeostasis, for example AMP-activated protein kinase gamma3, UCP3, CD36, adiponectin receptor 2, GLUT4, and caveolin-3. Attenuation of Nur77 expression resulted in decreased lipolysis. Finally, in concordance with the cell culture model, injection and electrotransfer of siNur77 into mouse tibialis cranialis muscle resulted in the repression of UCP3 mRNA expression. This study demonstrates regulatory cross-talk between the nuclear hormone receptor and beta-AR signaling pathways. Moreover, it suggests Nur77 modulates the expression of genes that are key regulators of skeletal muscle lipid and energy homeostasis. In conclusion, we speculate that Nur77 agonists would stimulate lipolysis and increase energy expenditure in skeletal muscle and suggest selective activators of Nur77 may have therapeutic utility in the treatment of obesity.

Adipose tissue gene expression in obese subjects during low-fat and high-fat hypocaloric diets

AIMS/HYPOTHESIS

Adaptation to energy restriction is associated with changes in gene expression in adipose tissue. However, it is unknown to what extent these changes are dependent on the energy restriction as such or on the macronutrient composition of the diet.

METHODS

We determined the levels of transcripts for 38 genes that are expressed in adipose tissue and encode transcription factors, enzymes, transporters and receptors known to play critical roles in the regulation of adipogenesis, mitochondrial respiration, and lipid and carbohydrate metabolism. Two groups of 25 obese subjects following 10-week hypocaloric diet programmes with either 20-25 or 40-45% of total energy derived from fat were investigated. Levels of mRNA were measured by performing real-time RT-PCR on subcutaneous fat samples obtained from the subjects before and after the diets.

RESULTS

The two groups of subjects lost 7 kg over the duration of the diets. Ten genes were regulated by energy restriction; however, none of the genes showed a significantly different response to the diets. Levels of peroxisome proliferator-activated receptor gamma co-activator 1alpha mRNA were increased, while the expression of the genes encoding leptin, osteonectin, phosphodiesterase 3B, hormone-sensitive lipase, receptor A for natriuretic peptide, fatty acid translocase, lipoprotein lipase, uncoupling protein 2 and peroxisome proliferator-activated receptor gamma was decreased. Clustering analysis revealed new potential coregulation of genes. For example, the expression of the genes encoding the adiponectin receptors may be regulated by liver X receptor alpha.

CONCLUSIONS/INTERPRETATION

In accordance with the comparable loss of fat mass produced by the two diets, this study shows that energy restriction and/or weight loss rather than the ratio of fat: carbohydrate in a low-energy diet is of importance in modifying the expression of genes in the human adipose tissue.

Gene expression of adiponectin and adiponectin receptor 1 in type 2 diabetic rats and the relationship with the parameters of glucose and lipid metabolism

In order to confirm whether the mRNA levels of adiponectin in adipose tissue and mRNA levels of AdipoR1 in the skeletal muscles were correlated with the serum parameters of glucose and lipid metabolism and to clarify the regulation of adiponectin receptor gene expression in diabetic states, serum adiponectin, mRNA levels of adiponectin in adipose tissue and mRNA levels of AdipoR1 in the skeletal muscles were examined in type 2 diabetic rats. The model of type 2 diabetes was prepared by feeding high fat diet and injecting low dosage of streptozotocin (STZ). The diabetic rats were screened out by oral glucose tolerance test. One group of type 2 diabetic rats received rosiglitazone. The serum adiponectin concentration was detected by using ELISA and mRNA levels were examined by RT-PCR. The serum adiponectin levels and mRNA levels of adiponectin in adipose tissue of type 2 diabetic rats were significantly decreased as compared with the normal control rats (P<0.05, P<0.01 respectively). No siglificant changes were observed in the expression of adiponectin receptor 1 in the skeletal muscle of type 2 diabetic rats. The mRNA levels of adiponectin in adipose tissue were reversely correlated with serum insulin (r=-0.66, P<0.05), triglyceride (r= -0.58, P<0.05), cholesterol (r=-0.49, P<0.05), interleukin-6 (r=-0.49, P<0. 05) and tumor necrosis factor (r= -0.43, P<0.05). The expression of adiponectin receptors was not altered in the skeletal muscle of Type 2 diabetic rats. The decreased serum adiponectin was caused by the decreased expression of adiponectin mRNA in adipose tissue rather than the adiponectin receptors in the skeletal muscle, which could be improved by rosiglitazone.

Globular adiponectin increases GLUT4 translocation and glucose uptake but reduces glycogen synthesis in rat skeletal muscle cells

AIMS/HYPOTHESIS

The aim of this study was to determine whether adiponectin elicits glucose uptake via increased GLUT4 translocation and to investigate the metabolic fate of glucose in skeletal muscle cells treated with globular adiponectin.

MATERIALS AND METHODS

Basal and insulin-stimulated 2-deoxy-D: -[(3)H]glucose uptake, cell surface myc-tagged GLUT4 content, production of (14)CO(2) by oxidation of D: -[U-(14)C]glucose and [1-(14)C]oleate, and incorporation of D: -[U-(14)C]glucose into glycogen and lactate were measured in the presence and absence of globular adiponectin.

RESULTS

RT-PCR and Western blot analysis revealed that L6 cells and rat skeletal muscle cells express AdipoR1 mRNA and protein. Globular adiponectin increased both GLUT4 translocation and glucose uptake by increasing the transport V(max) of glucose without altering the K(m). Interestingly, the incorporation of D: -[U-(14)C]glucose into glycogen under basal and insulin-stimulated conditions was significantly decreased by globular adiponectin, whereas lactate production was increased. Furthermore, globular adiponectin did not affect glucose oxidation, but enhanced phosphorylation of AMP kinase and acetyl-CoA carboxylase, and fatty acid oxidation.

CONCLUSIONS/INTERPRETATION

The present study is the first to show that globular adiponectin increases glucose uptake in skeletal muscle cells via GLUT4 translocation and subsequently reduces the rate of glycogen synthesis and shifts glucose metabolism toward lactate production. These effects are consistent with the increased phosphorylation of AMP kinase and acetyl-CoA carboxylase and oxidation of fatty acids induced by globular adiponectin.

Nuclear Receptors as Targets for Drug Development: Molecular Mechanisms for Regulation of Obesity and Insulin Resistance by Peroxisome Proliferator-Activated Receptor γ, CREB-Binding Protein, and Adiponectin

Obesity is defined as increased mass of adipose tissue, conferring a higher risk of cardiovascular and metabolic disorders such as diabetes, hyperlipidemia, and coronary heart disease. To investigate the role of transcriptional factors, which are involved in adipocytes differentiation and adiposity, we have generated peroxisome proliferator-activated receptor (PPAR) gamma or CREB-binding protein (CBP)-deficient mice by gene targeting. Heterozygous PPARgamma-deficient mice were protected from the development of insulin resistance due to adipocyte hypertrophy under a high-fat diet. Heterozygous CBP-deficient mice showed increased insulin sensitivity and were completely protected from body weight gain induced by a high-fat diet. PPARgamma or CBP deficiency results in increased effects of hormones such as adiponectin and leptin. Adiponectin was decreased in obesity and lipoatrophy, and replenishment of adiponectin ameliorated insulin resistance. Moreover, adiponectin-deficient mice showed insulin resistance and atherogenic phenotype. Finally, cDNA encoding adiponectin receptors (AdipoR1/R2) have been identified by expression cloning. The expression of AdipoR1/R2 appears to be inversely regulated by insulin in physiological and pathophysiological states such as fasting/refeeding, insulin deficiency, and hyperinsulinemia models, and it is correlated with adiponectin sensitivity. These results facilitate the understanding of molecular mechanisms of adiponectin actions and obesity-linked diseases such as diabetes and atherosclerosis and propose the molecular targets for anti-diabetic and anti-atherogenic drugs.

Osmotin Is a Homolog of Mammalian Adiponectin and Controls Apoptosis in Yeast through a Homolog of Mammalian Adiponectin Receptor

The antifungal activity of the PR-5 family of plant defense proteins has been suspected to involve specific plasma membrane component(s) of the fungal target. Osmotin is a tobacco PR-5 family protein that induces apoptosis in the yeast Saccharomyces cerevisiae. We show here that the protein encoded by ORE20/PHO36 (YOL002c), a seven transmembrane domain receptor-like polypeptide that regulates lipid and phosphate metabolism, is an osmotin binding plasma membrane protein that is required for full sensitivity to osmotin. PHO36 functions upstream of RAS2 in the osmotin-induced apoptotic pathway. The mammalian homolog of PHO36 is a receptor for the hormone adiponectin and regulates cellular lipid and sugar metabolism. Osmotin and adiponectin, the corresponding "receptor" binding proteins, do not share sequence similarity. However, the beta barrel domain of both proteins can be overlapped, and osmotin, like adiponectin, activates AMP kinase in C2C12 myocytes via adiponectin receptors.

Plasma adiponectin is decreased in nonalcoholic fatty liver disease

OBJECTIVES

Nonalcoholic fatty liver disease (NAFLD) is a major cause of liver-related morbidity and is frequently associated with obesity and metabolic syndrome. The recently discovered hormone adiponectin is produced by adipose tissue, and low plasma adiponectin is considered a key factor in the development of the insulin resistance underlying metabolic syndrome. Animal studies suggest that adiponectin may protect against non-alcoholic steatohepatitis, but direct evidence in humans is lacking. We therefore conducted this study to assess the relationship between plasma adiponectin and nonalcoholic fatty liver disease to explore its role in the pathogenesis of this disease.

DESIGN AND METHODS

We measured plasma adiponectin and anthropometric, biochemical, hormonal and metabolic correlates in a group of 17 NAFLD patients with diagnosis confirmed by biopsy, and 20 controls with comparable age, body-mass index and sex. Furthermore we compared plasma adiponectin in patients with simple steatosis and steatohepatitis.

RESULTS

Plasma adiponectin was significantly lower in NAFLD patients than controls (5.93+/-0.45 vs 15.67+/-1.60ng/ml). Moreover, NAFLD patients were significantly more insulin resistant while having similar serum leptin. Adiponectin was similar in simple steatosis and in steatohepatitis (6.16+/-0.78 vs 5.69+/-0.49ng/ml). An inverse correlation was observed between adiponectin and homeostatic model assessment (HOMA) of insulin resistance (P = 0.008), while adiponectin did not correlate with serum transaminases and lipid values.

CONCLUSIONS

These data support a role for low circulating adiponectin in the pathogenesis of NAFLD and confirm the strict association between reduced adiponectin production by adipose tissue, NAFLD and insulin resistance.

Dietary conjugated linoleic acid alleviates nonalcoholic fatty liver disease in Zucker (fa/fa) rats

Nonalcoholic fatty liver disease (NAFLD) is the preferred term to describe the spectrum of liver damage ranging from hepatic steatosis to steatohepatitis, liver fibrosis, and cirrhosis, and it is emerging as the most common liver disease in industrialized countries. Thus, the discovery of food components that would ameliorate NAFLD is of interest. Conjugated linoleic acid (CLA), a mixture of positional and geometric isomers of linoleic acid, has attracted considerable attention because of its potentially beneficial biological effects both in vitro and in vivo. We tested whether dietary CLA protects Zucker (fa/fa) rats from hepatic injury. After 8 wk of feeding, hepatomegaly, hepatic triglyceride (TG) accumulation, and elevated hepatic injury markers in plasma were markedly alleviated in CLA-fed Zucker rats compared with linoleic acid-fed (control) rats. These effects were attributed in part to the enhanced hepatic activities of carnitine palmitoyltransferase, a key enzyme of fatty acid beta-oxidation, and microsomal TG transfer protein, an important factor for lipoprotein secretion due to the CLA diet. We previously reported that the severe hyperinsulinemia in control Zucker rats was attenuated in CLA-fed rats due to an enhanced level of plasma adiponectin, which improves insulin sensitivity. In the present study, the adiponectin concentration was increased and the mRNA expression of tumor necrosis factor-alpha, an inflammatory cytokine, was markedly suppressed in the liver of CLA-fed Zucker rats. We speculate that the enhanced level of liver adiponectin may prevent the development and progression of NAFLD in CLA-fed Zucker rats.

Adenovirus-mediated high expression of resistin causes dyslipidemia in mice

The adipocyte-derived hormone resistin has been proposed as a possible link between obesity and insulin resistance in murine models. Many recent studies have reported physiological roles for resistin in glucose homeostasis, one of which is enhancement of glucose production from the liver by up-regulating gluconeogenic enzymes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. However, its in vivo roles in lipid metabolism still remain to be clarified. In this study, we investigated the effects of resistin overexpression on insulin action and lipid metabolism in C57BL/6 mice using an adenoviral gene transfer technique. Elevated plasma resistin levels in mice treated with the resistin adenovirus (AdmRes) were confirmed by Western blotting analysis and RIAs. Fasting plasma glucose levels did not differ between AdmRes-treated mice and controls, but the basal insulin concentration was significantly elevated in AdmRes-treated mice. In AdmRes-treated mice, the glucose-lowering effect of insulin was impaired, as evaluated by insulin tolerance tests. Furthermore, total cholesterol and triglyceride concentrations were significantly higher, whereas the high-density lipoprotein cholesterol level was significantly lower. Lipoprotein analysis revealed that low-density lipoprotein was markedly increased in AdmRes-treated mice, compared with controls. In addition, in vivo Triton WR-1339 studies showed evidence of enhanced very low-density lipoprotein production in AdmRes-treated mice. The expressions of genes involved in lipoprotein metabolism, such as low-density lipoprotein receptor and apolipoprotein AI in the liver, were decreased. These results suggest that resistin overexpression induces dyslipidemia in mice, which is commonly seen in the insulin-resistant state, partially through enhanced secretion of lipoproteins.

Adiponectin inhibits LPS-induced NF-kappaB activation and IL-6 production and increases PPARgamma2 expression in adipocytes

Obesity and insulin resistance are often associated with lower circulating adiponectin concentrations and elevated serum interleukin-6 (IL-6) and/or tumor necrosis factor-alpha (TNF-alpha). Adiponectin suppresses activation of nuclear factor-kappaB (NF-kappaB) in aortic endothelial cells and porcine macrophages. Accordingly, we hypothesized that adiponectin is an anti-inflammatory hormone and suppresses activation of NF-kappaB in adipocytes. Because peroxisome proliferator-activated receptor gamma2 (PPARgamma2) antagonizes the transcriptional activity of NF-kappaB, we determined whether adiponectin alters PPARgamma2 expression in pig adipocytes. In addition, we determined whether interferon-gamma alters the expression of PPARgamma2 in the presence or absence of adiponectin. Primary adipocytes from pig subcutaneous adipose tissue were treated with or without lipopolysaccharide (LPS; 10 microg/ml) and adiponectin (30 microg/ml), and nuclear extracts were obtained for gel shift assays to assess nuclear localization of NF-kappaB. Whereas LPS induced an increase in NF-kappaB activation, adiponectin suppressed both NF-kappaB activation and the induction of IL-6 expression by LPS (P<0.05). Similar results were obtained in 3T3-L1 adipocytes. In addition, adiponectin antagonized LPS-induced increase in TNF-alpha mRNA expression (P<0.05) and tended (P<0.065) to diminish its accumulation in the culture media in 3T3-L1 adipocytes. Adiponectin also induced an upregulation of PPARgamma2 mRNA (P<0.05). Although IFN-gamma did not reduce the basal expression of PPARgamma2, it suppressed PPARgamma2 induction by adiponectin (P<0.05). These findings indicate that adiponectin may be a local regulator of inflammation in the adipocyte and adipose tissue via its regulation of the NF-kappaB and PPARgamma2 transcription factors.

Effect of adiponectin gene polymorphisms on circulating adiponectin and insulin resistance indexes in women with polycystic ovary syndrome

BACKGROUND

We examined the possible association of adiponectin gene polymorphisms with polycystic ovary syndrome (PCOS) and their influence on serum adiponectin and insulin resistance indexes in Greek women with PCOS.

METHODS

We genotyped samples from 100 women with PCOS characterized with respect to body mass index (BMI), glucose and insulin concentrations during an oral glucose tolerance test (OGTT), lipid profile, and serum adiponectin concentrations and from 140 healthy controls for the 45T>G and 276G>T polymorphisms in the adiponectin gene.

RESULTS

The distributions of genotypes and alleles of both polymorphisms were no different in women with PCOS and controls, indicating that the individual polymorphisms are not associated with increased risk for PCOS. However, the two polymorphisms were found to be associated with insulin resistance indexes among women with PCOS and to influence adiponectin production. In particular, carriers of the TG genotype at position +45 had greater hyperinsulinemia, as estimated by the area under the curve for insulin (AUC(insulin)) during the OGTT, than those with the TT genotype (P <0.05), and this was independent of age and BMI. In addition, women with PCOS with the GG or GT genotypes at position +276 had a higher BMI (P = 0.01) and greater AUC(insulin) (P = 0.01) than carriers of the TT genotype. The latter genotype was found less frequently among overweight/obese women with PCOS than in normal-weight individuals (P = 0.002). In addition, the presence of the GG or GT genotype was associated with lower serum adiponectin than the TT genotype, independent of age, BMI, and insulin concentrations (P = 0.03). Serum adiponectin was negatively correlated with serum triglycerides and insulin resistance indexes and positively with HDL-cholesterol.

CONCLUSIONS

Adiponectin gene polymorphisms at positions +45 and +276 are not associated with PCOS. However, these genomic variants may influence production of adiponectin and the metabolic variables related to insulin resistance/metabolic syndrome in patients with PCOS.

Plasma adiponectin decrease in women with nonalcoholic Fatty liver

Adiponectin, secreted specifically from adipocytes, is thought to play a key role in the metabolic syndrome. Plasma adiponectin concentrations were studied in 36 typical nonalcoholic fatty liver (NAFL) women which is commonly associated with the metabolic syndrome. They were diagnosed as NAFL by ultrasound brightness, slightly elevated serum ALT levels and the exclusion of history of alcohol abuse and other known liver diseases. Compared with 64 control women, NAFL had a significant increase in the variables of the metabolic syndrome, other hepatic enzymes and leptin levels, while a reduction in AST/ALT ratio and adiponectin before (mean +/- SE: 7.2 +/- 0.5 vs 9.0 +/- 0.4 microg/ml, p < 0.005) and after adjustment for body fat mass (0.24 +/- 0.02 vs 0.34 +/- 0.02, p < 0.0001), atherogenic Index [(total cholesterol - HDLC)/HDLC: 3.2 +/- 0.3 vs 4.6 +/- 0.3, p < 0.005] or calculated insulin resistance (HOMA-R) (6.6 +/- 1.9 vs 8.6 +/- 0.9, p < 0.005). BMI and amylase were positive, and adiponectin/BMI was negative significant independent determinants of ALT value in multiple regression model. In conclusion, while hypoadiponectinemia was observed in NAFL, hypoadiponectinemia provides the possibility of fat accumulation in the liver.

The role of insulin and the adipocytokines in regulation of vascular endothelial function

Vascular integrity in the healthy endothelium is maintained through the release of a variety of paracrine factors such as NO (nitric oxide). Endothelial dysfunction, characterized by reduced NO bioavailability, is associated with obesity, insulin resistance and Type II diabetes. Insulin has been demonstrated to have direct effects on the endothelium to increase NO bioavailability. Therefore altered insulin signalling in the endothelium represents a candidate mechanism underlying the association between insulin resistance and endothelial dysfunction. In recent years, it has become apparent that insulin sensitivity is regulated by the adipocytokines, a group of bioactive proteins secreted by adipose tissue. Secretion of adipocytokines is altered in obese individuals and there is increasing evidence that the adipocytokines have direct effects on the vascular endothelium. A number of current antidiabetic strategies have been demonstrated to have beneficial effects on endothelial function and to alter adipocytokine concentrations in addition to their effects on glucose homoeostasis. In this review we will explore the notion that the association between insulin resistance and endothelial dysfunction is accounted for by adipocytokine action on the endothelium. In addition, we examine the effects of weight loss, exercise and antidiabetic drugs on adipocytokine availability and endothelial function.

Dysregulation of insulin receptor substrate 2 in beta cells and brain causes obesity and diabetes

The molecular link between obesity and beta cell failure that causes diabetes is difficult to establish. Here we show that a conditional knockout of insulin receptor substrate 2 (Irs2) in mouse pancreas beta cells and parts of the brain--including the hypothalamus--increased appetite, lean and fat body mass, linear growth, and insulin resistance that progressed to diabetes. Diabetes resolved when the mice were between 6 and 10 months of age: functional beta cells expressing Irs2 repopulated the pancreas, restoring sufficient beta cell function to compensate for insulin resistance in the obese mice. Thus, Irs2 signaling promotes regeneration of adult beta cells and central control of nutrient homeostasis, which can prevent obesity and diabetes in mice.

Hormones regulating lipid metabolism and plasma lipids in childhood obesity

OBJECTIVE

To review the mechanisms by which leptin, insulin and adiponectin influence lipid metabolism and plasma lipids in obesity, as well as to describe the associations between these hormones in prepubertal children.

METHOD

Revision of relevant papers published in the last 5 y related to the interactions of leptin, insulin and adiponectin, with special emphasis on those reporting potential mechanisms by which these hormones regulate lipid metabolism and plasma lipids. We also provide original results concerning the relationships found between plasma lipids and leptin, and insulin and adiponectin in prepubertal obese children.

RESULTS

Recent data in the literature shed new light to explain the effects of both leptin and adiponectin in the regulation of lipid metabolism in peripheral tissues. Activation of the AMP-dependent kinase pathway and subsequent increased fatty acid oxidation seems to be the main mechanism of action of these hormones in the regulation of lipid metabolism. In addition, we have found that insulin plasma levels are positively associated to leptin but negatively correlated with adiponectin in obese children. Adiponectin is negatively associated to plasma lipid markers of metabolic syndrome but positively related to HDL-cholesterol, whereas insulin and leptin show opposite patterns. These results support the effect of adiponectin in increasing insulin sensitivity and decreasing plasma triglycerides.

CONCLUSION

Leptin, insulin and adiponectin are associated hormones that regulate lipid metabolism in childhood. Adiponectin appears to be the missing link to explain the alterations in lipid metabolism and plasma lipids seen in obesity.

Dual action of adiponectin on insulin secretion in insulin-resistant mice

Adiponectin is secreted by adipocytes and has been implicated as a mediator of insulin sensitivity. In this study, the acute effects of adiponectin on islets isolated from normal or diet-induced insulin resistant mice were examined. In normal islets, adiponectin (5 microg/ml) had no significant effect on insulin secretion. In contrast, in islets from mice rendered insulin resistant by high-fat feeding, adiponectin inhibited insulin secretion at 2.8 mM (P < 0.01) but augmented insulin secretion at 16.7 mM glucose (P < 0.05). The augmentation of glucose-stimulated insulin secretion by adiponectin was accompanied by increased glucose oxidation (P < 0.005), but without any significant effect on palmitate oxidation or the islet ATP/ADP ratio. Furthermore, RT-PCR revealed the expression of the adiponectin receptor AdipoR1 mRNA in mouse islets, however, with no difference in the degree of expression level between the two feeding groups. The results thus uncover a potential dual role for adiponectin to modify insulin secretion in insulin resistance.

Genetic manipulation in nutrition, metabolism, and obesity research

We summarize the current standard methods for overexpressing, inactivating, or manipulating genes, with special focus on nutritional and obesity research. These molecular biology procedures can be carried out with the maintenance of the genetic information to subsequent generations (transgenic technology) or devised to exclusively transfer the genetic material to a given target animal, which cannot be transmitted to the future progeny (gene therapy). On the other hand, the RNA interference (RNAi) approach allows for the creation of new experimental models by transient ablation of gene expression by degrading specific mRNA, which can be applied to assess different biological functions and mechanisms. The combination of these technologies contributes to the study of the function and regulation of different metabolism- and obesity-related genes as well as the identification of new pharmacologic targets for nutritional and therapeutic approaches.

Pleiotropic effects of thiazolidinediones: taking a look beyond antidiabetic activity

Thiazolidinediones (TZD) [Troglitazone (TRO), Pioglitazone (PGZ), Rosiglitazone, (RGZ)] are a novel class of antidiabetic drugs for patients with Type-2 diabetes mellitus (T2DM) able to decrease blood glucose, working through a reduction of insulin resistance. The family of TZD exerts its effect specifically bound to peroxisome proliferator-activated receptor y (PPARy). This is a member of the nuclear hormone receptor superfamily of ligand-dependent transcription factors, together with PPARalpha and deltabeta. Although PPARgamma is essentially expressed in adipose tissue, it has also been found in endothelial cells, macrophages, vascular smooth muscle cells, glomerular mesangial cells, hepatic stellate cells and in several cancer cell lines. In these cells, the PPARgamma activation by TZD determines modulatory effects on growth factor release, production of cytokine, cell proliferation and migration, extracellular matrix remodeling and control on cell cycle progression and differentiation. In addition, TZD have been shown to have a potent antioxidant effect. This review, taking a quick look beyond the antidiabetic activity of PPARgamma, shows the dramatic ranging of medical implications that the use of TZD could have modulating the PPARgamma activity in several diseases with a strong social impact, such as insulin resistance syndrome, chronic inflammation, atherosclerosis and cancer.

Adiponectin, obesity, and cardiovascular disease

Several adipocyte-secreted factors have been demonstrated to potentially link obesity, insulin resistance, and cardiovascular disease. Among those, adiponectin is an insulin-sensitizing and anti-inflammatory adipokine, concentrations of which are decreased in obesity-associated metabolic and vascular disorders. Recently, two adiponectin receptors (AdipoR) have been isolated and adenosine monophosphate kinase (AMPK), as well as acetyl coenzyme A carboxylase (ACC), appear to be critical downstream mediators for various effects of this adipokine. In addition to beneficial metabolic effects, adiponectin seems to be vasoprotective by interfering with various atherogenic processes. Of clinical interest, thiazolidinediones (TZDs) which are used in the treatment of type 2 diabetes stimulate adiponectin expression and secretion whereas several hormones dysregulated in insulin resistance and obesity downregulate this adipokine. The current knowledge on regulation and function of adiponectin in obesity, insulin resistance, and cardiovascular disease is summarized in this review and its clinical implications are discussed.

Non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is an underdiagnosed liver disease characterized by steatosis, necroinflammation and fibrosis. This disease may eventually develop into cirrhosis and hepatocellular carcinoma. NASH is highly prevalent among obese individuals and among patients with diabetes mellitus type 2. Non-alcoholic fatty liver (NAFL), a precursor of NASH, is the main cause of elevated serum liver enzymes among the general population. In NASH the liver is programmed to lipogenesis rather than to glycogenesis and herein insulin-resistance plays a major role. Gradual weight loss, physical exercise and drugs that improve insulin sensitivity are potential therapies.

Adiponectin and its receptors are expressed in bone-forming cells

Adiponectin has until now been considered to be synthesized and secreted exclusively by the adipose tissue, and is reported to influence energy homeostasis and insulin sensitivity. It is also known that body weight is positively correlated with increased bone mineral density and decreased fracture risk. The mechanisms explaining this relation, however, are not completely understood. We report a link between adiponectin and bone homeostasis by demonstrating transcription, translation, and secretion of adiponectin, as well as expression of its receptors, AdipoR1 and AdipoR2, in bone-forming cells. We show that adiponectin and the receptors are expressed in primary human osteoblasts from femur and tibia. The phenotype of bone cells was confirmed by the high expression levels of alkaline phosphatase, collagen type 1, osteocalcin, and CD44, and the formation of mineralization nodules. Immunostaining with monoclonal antibodies also demonstrated the presence of adiponectin in human osteosarcoma cells and normal osteoblasts. Both mRNA expression and secretion of adiponectin to the medium increased during differentiation of human osteoblasts in culture. The adiponectin mRNA level increases in osteoblasts cultured 3 and 7 days in the presence of dietary fatty acids and supplementation of culture medium with recombinant adiponectin enhances the proliferation of murine osteoblasts. The regulation and detailed function of adiponectin in bone still remains obscure, but our findings suggest a functional role in bone homeostasis. If so, adiponectin may provide an important signal linking fat and body weight to bone density.

Adiponectin in renal disease: relationship to phenotype and genetic variation in the gene encoding adiponectin

BACKGROUND

The prevalence of cardiovascular disease (CVD) and inflammation is high in patients with end-stage renal disease (ESRD). Adiponectin is an adipocytokine that may have significant anti-inflammatory and anti-atherosclerotic effects. Low adiponectin levels have previously been found in patients with high risk for CVD.

METHODS

In a cohort of 204 (62% males) ESRD patients aged 52 +/- 1 years the following parameters were studied: presence of CVD, body composition, plasma adiponectin (N= 107), cholesterol, triglycerides, HDL-cholesterol, serum leptin, high-sensitivity C-reactive protein (hs-CRP), urinary albumin excretion (UAE), and single-nucleotide polymorphisms (SNPs) in the apM1 gene at positions -11391, -11377, 45, and 276. Thirty-six age- (52 +/- 2 years) and gender-matched (64% males) healthy subjects served as control subjects.

RESULTS

Markedly (P < 0.0001) elevated median plasma adiponectin levels were observed in ESRD patients (22.2 microg/mL), especially type 1 diabetic patients (36.8 microg/mL), compared to control subjects (12.2 microg/mL). Log plasma adiponectin correlated to visceral fat mass (R=-0.29; P < 0.01) and Log hs-CRP (R=-0.26; P < 0.01). In a stepwise (forward followed by backward) multiple regression model only type-1 diabetes (P < 0.001) and visceral fat mass (P < 0.05) were independently associated with plasma adiponectin levels. The adiponectin gene -11377 C/C genotype was associated with a lower prevalence of CVD (25 vs. 42%) compared to the G/C genotype.

CONCLUSION

The present cross-sectional study demonstrates that, whereas genetic variations seem to have a minor impact on circulating adiponectin levels, lower visceral fat mass and type 1 diabetes mellitus are associated with elevated plasma adiponectin levels in ESRD patients. Furthermore, low levels of adiponectin are associated with inflammation in ESRD.

Expression of adiponectin receptor mRNA in human skeletal muscle cells is related to in vivo parameters of glucose and lipid metabolism

The adiponectin receptors, AdipoR1 and AdipoR2, are thought to transmit the insulin-sensitizing, anti-inflammatory, and atheroprotective effects of adiponectin. In this study, we examined whether AdipoR mRNA expression in human myotubes correlates with in vivo measures of insulin sensitivity. Myotubes from 40 metabolically characterized donors expressed 1.8-fold more AdipoR1 than AdipoR2 mRNA (588 +/- 35 vs. 321 +/- 39 fg/microg total RNA). Moreover, the expression levels of both receptors correlated with each other (r = 0.45, P < 0.01). AdipoR1 mRNA expression was positively correlated with in vivo insulin and C-peptide concentrations, first-phase insulin secretion, and plasma triglyceride and cholesterol concentrations before and after adjustment for sex, age, waist-to-hip ratio, and body fat. Expression of AdipoR2 mRNA clearly associated only with plasma triglyceride concentrations. In multivariate linear regression models, mRNA expression of AdipoR1, but not AdipoR2, was a determinant of first-phase insulin secretion independent of insulin sensitivity and body fat. Finally, insulin did not directly modify myotube AdipoR1 mRNA expression in vitro. In conclusion, we provide evidence that myotube mRNA levels of both receptors are associated with distinct metabolic functions but not with insulin sensitivity. AdipoR1, but not AdipoR2, expression correlated with insulin secretion. The molecular nature of this link between muscle and beta-cells needs to be further clarified.

Possible impairment of transcardiac utilization of adiponectin in patients with type 2 diabetes

OBJECTIVE

Adiponectin, an adipocyte-derived protein, has been suggested to enhance insulin sensitivity and prevent atherosclerosis. Circulating adiponecin levels are reduced in states of insulin resistance such as type 2 diabetes. We examined transcardiac utilization of adiponectin in patients with and without type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 17 male type 2 diabetic patients and 17 male nondiabetic patients were investigated. Venous blood samples were taken to measure glucose and lipid variables. Blood samples for the measurement of adiponectin were collected simultaneously from the aortic root and coronary sinus. Angiographic semiquantitative stenosis score of coronary artery was also evaluated.

RESULTS

The adiponectin levels in both the aortic root and coronary sinus in the diabetic patients were significantly lower than those in the nondiabetic patients. The adiponectin level was significantly lower in the coronary sinus than in the aortic root in the nondiabetic patients, but there was no significant difference between adiponectin levels in the aortic root and coronary sinus in the diabetic patients. The total stenosis score, as an index of severity of coronary artery stenosis, was significantly higher in the diabetic patients than in the nondiabetic patients. The stenosis score was correlated with the degree of transcardiac utilization of adiponectin from the aortic root to coronary sinus in the nondiabetic patients but not in the diabetic patients.

CONCLUSIONS

Diabetic patients not only have a decreased adiponectin level in the basal state compared with nondiabetic patients but also have impaired utilization of adiponectin in the coronary artery and/or the heart, which may promote the development of atherosclerosis.

Adiponectin and the development of type 2 diabetes: the atherosclerosis risk in communities study

Adipocyte-derived secretory proteins have been increasingly linked to diabetes. To investigate whether adiponectin, a major adipocyte secretory protein, predicts diabetes, we conducted a case-cohort study representing the approximately 9-year experience of the 10,275 middle-aged, U.S. African-American and white participants of the Atherosclerosis Risk in Communities (ARIC) study. Adiponectin was measured on stored plasma of 581 incident diabetes case subjects and 572 noncase subjects. Overall hazard ratios (95% CIs) for developing diabetes, for those in the second, third, and fourth (versus the first) quartile of adiponectin were 0.57 (0.41-0.78), 0.39 (0.27-0.56), and 0.18 (0.11-0.27), respectively, after adjustment for age, sex, ethnicity, study center, parental history of diabetes, and hypertension and 0.72 (0.48-1.09), 0.67 (0.43-1.04), and 0.58 (0.34-0.99), respectively, after additional adjustment for BMI, waist-to-hip ratio, fasting glucose, insulin, and a score composed of six inflammation markers. The association was of similar magnitude in men and women and in whites and African Americans, but was absent in smokers and in those with a greater inflammation score (interaction P < 0.01 for each). In conclusion, in this community-based sample of U.S. adults, higher adiponectin levels were associated with a lower incidence of diabetes.

Tumor necrosis factor and its potential role in insulin resistance and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a spectrum of hepatic pathology that resembles alcohol-induced fatty liver disease(AFLD), but which develops in individuals who are not heavy drinkers. In people, NAFLD is associated strongly with obesity,insulin resistance, and dysmetabolic syndrome, but the exact mechanisms that promote liver disease in this clinical context remain poorly understood. The proinflammatory cytokine, funor necrosis factor alpha is known to be a key mediator of AFLD. This article discusses clinical and experimental evidence that tumor necrosis factor plays a role in the pathogenesis of insulin resistance syndromes, including nonalcoholic fatty syndromes, including nonalcoholic fatty liver disease.

Endothelial dysfunction, inflammation, and insulin resistance: a focus on subjects at risk for type 2 diabetes

Subjects with obesity, family history of type 2 diabetes, polycystic ovary syndrome, previous gestational diabetes, dyslipidemia, hypertension, impaired glucose tolerance or impaired fasting glucose, and those with metabolic syndrome are at risk for the development of type 2 diabetes. Some of them are also at risk for cardiovascular disease. Some underlying abnormalities such as insulin resistance, endothelial dysfunction, and low-grade chronic inflammation are frequently present and closely associated in all these groups. The flow of substrates, hormones, and cytokines from visceral fat to skeletal muscle and to the endothelial cells, along with some genetic abnormalities that lead to impaired insulin action in the peripheral tissues and to impaired insulin-stimulated nitric oxide production in endothelial cells, may play a role in establishing these shared metabolic and vascular derangements. Weight loss, thiazolidinediones, and metformin improve vascular function in subjects at risk for type 2 diabetes and may prove to reduce cardiovascular events in these individuals.

Inflammation, insulin resistance, and adiposity: a study of first-degree relatives of type 2 diabetic subjects

OBJECTIVE

Inflammatory markers such as C-reactive protein (CRP) are associated with insulin resistance, adiposity, and type 2 diabetes. Whether inflammation causes insulin resistance or is an epiphenomenon of obesity remains unresolved. We aimed to determine whether first-degree relatives of type 2 diabetic subjects differ in insulin sensitivity from control subjects without a family history of diabetes, whether first-degree relatives of type 2 diabetic subjects and control subjects differ in CRP, adiponectin, and complement levels, and whether CRP is related to insulin sensitivity independently of adiposity.

RESEARCH DESIGN AND METHODS

We studied 19 young normoglycemic nonobese first-degree relatives of type 2 diabetic subjects and 22 control subjects who were similar for age, sex, and BMI. Insulin sensitivity (glucose infusion rate [GIR]) was measured by the euglycemic-hyperinsulinemic clamp. Dual-energy X-ray absorptiometry determined total and abdominal adiposity. Magnetic resonance imaging measured abdominal adipose tissue volumes.

RESULTS

First-degree relatives of type 2 diabetic subjects had a 20% lower GIR than the control group (51.8 +/- 3.9 vs. 64.9 +/- 4.6 micromol x min(-1) x kg fat-free mass(-1), P = 0.04). However, first-degree relatives of subjects with type 2 diabetes and those without a family history of diabetes had normal and comparable levels of CRP, adiponectin, and complement proteins. When the cohort was examined as a whole, CRP was inversely related to GIR (r = -0.33, P = 0.04) and adiponectin (r = -0.34, P = 0.03) and positively related to adiposity (P < 0.04). However, CRP was not related to GIR independently of fat mass. In contrast to C3 (r = 0.41, P = 0.009) and factor B (r = 0.43, P = 0.005), CRP was unrelated to factor D.

CONCLUSIONS

The insulin-resistant state is not associated with changes in inflammatory markers or complement proteins in subjects at high risk of type 2 diabetes. Our study confirms a strong relationship between CRP and fat mass. Increasing adiposity and insulin resistance may interact to raise CRP levels.

Adiponectin receptor 1 gene (ADIPOR1) as a candidate for type 2 diabetes and insulin resistance

Considerable data support adiponectin as an important adipose-derived insulin sensitizer that enhances fatty acid oxidation and alters hepatic gluconeogenesis. Adiponectin acts by way of two receptors, ADIPOR1 and ADIPOR2. ADIPOR1 is widely expressed in tissues, including muscle, liver, and pancreas, and binds the globular form of adiponectin with high affinity. To test the hypothesis that sequence variations in or near the ADIPOR1 gene contribute to the risk of developing type 2 diabetes and the metabolic syndrome, we screened the eight exons (including the untranslated exon 1) of the ADIPOR1 gene with flanking intronic sequences and the 5' and 3' flanking sequences. We identified 22 single nucleotide polymorphisms (SNPs) in Caucasian and African-American subjects, of which a single nonsynonymous SNP (N44K) in exon 2 was present only in African-American subjects. We typed 14 sequence variants that had minor allele frequencies >5%. No SNP was associated with type 2 diabetes in Caucasians or African Americans, and no SNP was a determinant of insulin sensitivity or insulin secretion among nondiabetic members of high-risk Caucasian families. However, the two alleles of a SNP in the 3' untranslated region were expressed unequally, and ADIPOR1 mRNA levels were significantly lower among transformed lymphocytes from diabetic African-American individuals than among control cell lines. This altered gene expression might suggest a role for ADIPOR1 in the metabolic syndrome.

The potential role of genes in nonalcoholic fatty liver disease

Although most people with obesity and type 2 diabetes will have steatosis, only a minority will ever develop nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Family studies suggest that genetic factors are important in disease progression, although dissecting genetic factors playing a role in NASH and fibrosis from those influencing the development established risk factors is difficult. Several approaches can be used to look for genetic factors playing a role in nonalcoholic fatty liver disease (NAFLD). In the future, genome-wide single nucleotide polymorphism (SNP) scanning of cases and controls may become feasible. To date, however,studies have relied on candidate gene, case control, allele association methodology. Recent, and as yet preliminary, studies have reported associations between steatosis severity, NASH, and fibrosis with genes whose products are involved in lipid metabolism,oxidative stress, and endotoxin-cytokine interactions. If confirmed,these associations will enhance understanding of disease pathogenesis,and accordingly, the ability to design effective therapies.

Obesity and alcoholic liver disease

Obesity potentiates the severity of alcohol-induced liver damage. Ethanol influences adipose tissue production of hormones and cytokines. The mechanisms by which adiposity and ethanol interact to produce hepatic steatosis and steatohepatitis are beginning to be studied. Exacerbation of the proinflammatory state that induces tumor necrosis factor activity and hepatic insulin resistance seems to be involved. However, the precise cellular signals that culminate in hepatocyte dysfunction and death remain controversial. Both hepatocyte apoptosis and necrosis are likely, but further study is needed to develop optimal hepatoprotective strategies. It is currently unclear whether the hepatotoxic consequences of obesity and ethanol ingestion are additive or synergistic. This information has important prognostic implications and might be useful to formulate body mass index-based guidelines for "safe" alcohol consumption. Findings of studies in experimental animals also raise questions about the relation between steatohepatitis and cirrhosis. Despite overwhelming evidence that obesity promotes alcohol-induced steatosis and steatohepatitis, most obese human beings (and mice) who drink alcohol do not become cirrhotic. Moreover, at least in mice, even severe steatohepatitis leads to cirrhosis relatively infrequently. Thus, it is conceivable that, although steatohepatitis is a permissive factor for cirrhosis, it is neither necessary nor sufficient for cirrhosis to occur. The quest to identify the proximal mediators of hepatic fibrosis should probably include an investigation of how various adipokines, neurotransmitters, and cytokines interact to regulate hepatic stellate cells. Armed with such knowledge, further modifying actions of ethanol on these mechanisms can be explored by investigators.

Endocrine regulation of energy metabolism: review of pathobiochemical and clinical chemical aspects of leptin, ghrelin, adiponectin, and resistin

BACKGROUND

Recent studies point to the adipose tissue as a highly active endocrine organ secreting a range of hormones. Leptin, ghrelin, adiponectin, and resistin are considered to take part in the regulation of energy metabolism.

APPROACH

This review summarizes recent knowledge on leptin and its receptor and on ghrelin, adiponectin, and resistin, and emphasizes their roles in pathobiochemistry and clinical chemistry.

CONTENT

Leptin, adiponectin, and resistin are produced by the adipose tissue. The protein leptin, a satiety hormone, regulates appetite and energy balance of the body. Adiponectin could suppress the development of atherosclerosis and liver fibrosis and might play a role as an antiinflammatory hormone. Increased resistin concentrations might cause insulin resistance and thus could link obesity with type II diabetes. Ghrelin is produced in the stomach. In addition to its role in long-term regulation of energy metabolism, it is involved in the short-term regulation of feeding. These hormones have important roles in energy homeostasis, glucose and lipid metabolism, reproduction, cardiovascular function, and immunity. They directly influence other organ systems, including the brain, liver, and skeletal muscle, and are significantly regulated by nutritional status. This newly discovered secretory function has extended the biological relevance of adipose tissue, which is no longer considered as only an energy storage site.

SUMMARY

The functional roles, structures, synthesis, analytical aspects, and clinical significance of leptin, ghrelin, adiponectin, and resistin are summarized.

Adipose tissue and adipokines: for better or worse

It is now recognized that the white adipose tIssue (WAT) produces a variety of bioactive peptIdes, collectively termed "adipokines". Alteration of WAT mass in obesity or lipoatrophy, affects the production of most adipose secreted factors. Since both conditions are associated with multiple metabolic disorders and increased risk of cardiovascular diseases, the Idea has emerged that WAT could be instrumental in these complications, by virtue of its secreted factors. Several adipokines are increased in the obese state and have been implicated in hypertension (angiotensinogen), impaired fibrinolysis (PAI-1) and insulin resistance (ASP, TNFalpha, IL-6, resistin). Conversely, leptin and adiponectin both exert an insulin-sensitizing effect, at least in part, by favoring tIssue fatty-acId oxIdation through activation of AMP-activated kinase. In obesity, insulin resistance has been linked to leptin resistance and decreased plasma adiponectin. In lipoatrophic mice, where leptin and adiponectin circulating levels are low, administration of the two adipokines synergistically reverses insulin resistance. Leptin and adiponectin also have distinct properties: leptin, as a long-term integrative signal of energy store and adiponectin, as a potent anti-atherogenic agent. The thiazolIdinedione anti-diabetic drugs increase endogenous adiponectin production in rodents and humans, supporting the Idea that the development of new drugs targeting adipokines might represent a promising therapeutic approach to protect obese patients from insulin resistance and atherosclerosis.

Recent advances in alcoholic liver disease II. Minireview: molecular mechanisms of alcoholic fatty liver

Alcohol has long been thought to cause fatty liver by way of altered NADH/NAD(+) redox potential in the liver, which, in turn, inhibits fatty acid oxidation and the activity of tricarboxylic acid cycle reactions. More recent studies indicate that additional effects of ethanol both impair fat oxidation and stimulate lipogenesis. Ethanol interferes with DNA binding and transcription-activating properties of peroxisome proliferator-activated receptor-alpha (PPARalpha), as demonstrated with cultured cells and in ethanol-fed mice. Treatment of ethanol-fed mice with a PPARalpha agonist can reverse fatty liver even in the face of continued ethanol consumption. Ethanol also activated sterol regulatory element binding protein 1, inducing a battery of lipogenic enzymes. These effects may be due in part to inhibition of AMP-dependent protein kinase, reduction in plasma adiponectin, or increased levels of TNF-alpha in the liver. The understanding of these ethanol effects provides new therapeutic targets to reverse alcoholic fatty liver.

Inverse association between serum adiponectin level and transaminase activities in Japanese male workers

BACKGROUND/AIMS

Since a novel polypeptide named adiponectin was shown to prevent the development of steatosis and steatohepatitis in animal models, we studied whether it was also possible in a clinical situation.

METHODS

Associations between serum adiponectin levels and serum transaminase activities were studied in 791 Japanese males who were not heavy drinkers, and had no autoimmune or HBV- or HCV-induced liver diseases.

RESULTS

Various markers of metabolic diseases including levels of body mass index (BMI), serum triglyceride, total cholesterol, and insulin resistance assessed by the homeostasis model were significantly higher in subjects with increased transaminase activities when compared to those with normal activities. Single regression analyses demonstrated that the logarithmic serum adiponectin level was inversely correlated with the levels of logarithmic serum AST (r=-0.229, P<0.0001), ALT (r=-0.305, P<0.0001), and gammaGTP (r=-0.278, P<0.0001). Even in multiple regression analyses in which subjects' age and levels of BMI, serum triglyceride, total cholesterol, and insulin resistance were adjusted, the inverse correlations were significant (P=0.0426, 0.0332, and 0.0011, respectively).

CONCLUSIONS

Hypoadiponectinemia may worsen liver diseases associated with metabolic diseases in clinical cases. In addition to aggravation of insulin resistance and hyperlipidemia, some hypoadiponectinemia specific mechanisms may stand behind the association.

Elevated circulating adiponectin levels in liver cirrhosis are associated with reduced liver function and altered hepatic hemodynamics

Adiponectin is a novel adipocytokine negatively correlated with parameters of the metabolic syndrome, such as body mass index (BMI), body fat mass (BFM), and circulating insulin levels. Furthermore, metabolic actions directly on the liver have been described. The aim of the present study was to characterize circulating adiponectin levels, hepatic turnover, and the association of adiponectin with key parameters of hepatic as well as systemic metabolism in cirrhosis, a catabolic disease. Circulating adiponectin levels and hepatic turnover were investigated in 20 patients with advanced cirrhosis. Hepatic hemodynamics [portal pressure, liver blood flow, hepatic vascular resistance, indocyanine green (ICG) half-life], body composition, resting energy expenditure, hepatic free fatty acids (FFA) and glucose turnover, and circulating levels of hormones (catecholamines, insulin, glucagon) and proinflammatory cytokines (IL-1beta, TNF-alpha, IL-6) were also assessed. Circulating adiponectin increased dependently on the clinical stage in cirrhosis compared with controls (15.2 +/- 1.7 vs. 8.2 +/- 1.1 microg/ml, respectively, P < 0.01), whereas hepatic extraction decreased. Adiponectin was negatively correlated with parameters of hepatic protein synthesis (prothrombin time: r = -0.62, P = 0.003; albumin: r = -0.72, P < 0.001) but not with transaminases or parameters of lipid metabolism. In addition, circulating adiponectin increased with portal pressure (r = 0.67, P = 0.003), hepatic vascular resistance (r = 0.60, P = 0.008), and effective hepatic blood flow (ICG half-life: r = 0.69, P = 0.001). Adiponectin in cirrhosis was not correlated with BMI, BFM, parameters of energy metabolism, insulin levels, hepatic FFA and glucose turnover, and circulating proinflammatory cytokines. These results demonstrate that 1) adiponectin plasma levels in cirrhosis are significantly elevated, 2) the liver is a major source of adiponectin extraction, and 3) adiponectin levels in cirrhosis do not correlate with parameters of body composition or metabolism but exclusively with reduced liver function and altered hepatic hemodynamics.

Reduced gene expression of adiponectin in fat tissue from patients with end-stage renal disease

BACKGROUND

Cardiovascular disease (CVD) is the main cause of death in end-stage renal disease (ESRD) patients. It has been suggested that inflammation plays a key role in the development of both atherosclerosis and malnutrition (MIA), a combination of complications associated with poor outcome. Although plasma levels of adiponectin, a recently discovered anti-inflammatory and antiatherogenic adipocytokine, are markedly elevated in ESRD, gene expression of adiponectin (ApM1) has not been analyzed in ESRD patients.

METHODS

We analyzed the ApM1 gene expression in adipose tissue from 18 ESRD patients of whom 9 (7 males, 60 +/- 8 years, BMI 24 +/- 6 kg/m(2)) had a high prevalence of MIA complications, and 9 age- (55 +/- 9 years), gender- (7 males) and BMI- (24 +/- 2 kg/m(2)) matched ESRD patients had few MIA complications. The results were compared with age- (59 +/- 11 years), gender- (7 males), and BMI- (24 +/- 6 kg/m(2)) matched healthy control patients. Information on CVD was obtained at the recruitment based on a detailed medical history. Malnutrition was defined as a subjective global assessment (SGA) score >1. Inflammation was defined as CRP >/=10 mg/L. Gene expression analysis was performed using the in situ hybridization technique.

RESULTS

Gene expression of ApM1 was lower in ESRD patients compared with healthy control patients (P= 0.001). On the other hand, when comparing the gene expression between ESRD patients with and without MIA complications, respectively, no difference in the ApM1 gene expression was detected.

CONCLUSION

Adiponectin gene expression is significantly down-regulated in ESRD patients compared with healthy control patients. We propose that the decrease in expression may be the result of a negative feedback regulation, as a result of elevated levels of circulating adiponectin caused by renal failure.

Direct demonstration of lipid sequestration as a mechanism by which rosiglitazone prevents fatty-acid-induced insulin resistance in the rat: comparison with metformin

AIMS/HYPOTHESIS

Thiazolidinediones can enhance clearance of whole-body non-esterified fatty acids and protect against the insulin resistance that develops during an acute lipid load. The present study used [(3)H]-R-bromopalmitate to compare the effects of the thiazolidinedione, rosiglitazone, and the biguanide, metformin, on insulin action and the tissue-specific fate of non-esterified fatty acids in rats during lipid infusion.

METHODS

Normal rats were treated with rosiglitazone or metformin for 7 days. Triglyceride/heparin (to elevate non-esterified fatty acids) or glycerol (control) were then infused for 5 h, with a hyperinsulinaemic clamp being performed between the 3rd and 5th hours.

RESULTS

Rosiglitazone and metformin prevented fatty-acid-induced insulin resistance (reduced clamp glucose infusion rate). Both drugs improved insulin-mediated suppression of hepatic glucose output but only rosiglitazone enhanced systemic non-esterified fatty acid clearance (plateau plasma non-esterified fatty acids reduced by 40%). Despite this decrease in plateau plasma non-esterified fatty acids, rosiglitazone increased fatty acid uptake (two-fold) into adipose tissue and reduced fatty acid uptake into liver (by 40%) and muscle (by 30%), as well as reducing liver long-chain fatty acyl CoA accumulation (by 30%). Both rosiglitazone and metformin increased liver AMP-activated protein kinase activity, a possible mediator of the protective effects on insulin action, but in contrast to rosiglitazone, metformin had no significant effect on non-esterified fatty acid kinetics or relative tissue fatty acid uptake.

CONCLUSIONS/INTERPRETATION

These results directly demonstrate the "lipid steal" mechanism, by which thiazolidinediones help prevent fatty-acid-induced insulin resistance. The contrasting mechanisms of action of rosiglitazone and metformin could be beneficial when both drugs are used in combination to treat insulin resistance.