Adiponectin receptor gene expression in human skeletal muscle cells is not regulated by fibrates and thiazolidinediones

Preliminary evidence of genetic determinants of adiponectin response to fenofibrate in the Genetics of Lipid Lowering Drugs and Diet Network

BACKGROUND AND AIMS

Adiponectin is an adipose-secreted protein that has been linked to changes in insulin sensitivity, high-density lipoprotein cholesterol levels, and inflammatory patterns. Although fenofibrate therapy can raise adiponectin levels, treatment response is heterogeneous and heritable, suggesting a role for genetic mediators. This is the first genome-wide association study of fenofibrate effects on circulating adiponectin.

METHODS AND RESULTS

Plasma adiponectin was measured in participants of the Genetics of Lipid Lowering Drugs and Diet Network (n = 793) before and after a 3-week daily treatment with 160 mg of fenofibrate. Associations between variants on the Affymetrix Genome-Wide Human SNP Array 6.0 and adiponectin were assessed using mixed linear models, adjusted for age, sex, site, and family. We observed a statistically significant (P = 5 × 10⁻⁸) association between rs2384207 in 12q24, a region previously linked to several metabolic traits, and the fenofibrate-induced change in circulating adiponectin. Additionally, our genome-wide analysis of baseline adiponectin levels replicated the previously reported association with CDH13 and suggested novel associations with markers near the PCK1, ZBP1, TMEM18, and SCUBE1 genes. The findings from the single marker tests were corroborated in gene-based analyses. Biological pathway analyses suggested a borderline significant association between the EGF receptor signaling pathway and baseline adiponectin levels.

CONCLUSIONS

We present preliminary evidence linking several biologically relevant genetic variants to adiponectin levels at baseline and in response to fenofibrate therapy. Our findings provide support for fine-mapping of the 12q24 region to investigate the shared biological mechanisms underlying levels of circulating adiponectin and susceptibility to metabolic disease.

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

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

Adiponectin receptor 2 is negatively associated with lymph node metastasis of colorectal cancer

Adiponectin is a hormone secreted by adipose tissue and has a variety of functions including the inhibition of tumor growth. The expression and function of the two major adiponectin receptors, AdipoR1 and AdipoR2, in malignant tissue have not been well characterized. In the present study, we evaluated the mRNA levels of AdipoR1 and AdipoR2 expression in 48 surgically resected colorectal cancer specimens, as well as normal colonic mucosa, by quantitative RT-PCR. The values obtained were standardized by β-actin mRNA, and the correlation between their relative expression levels and the clinicopathological characteristics of the patients was examined. The relative expression levels of AdipoR1 and AdipoR2 were significantly reduced in cancer tissue compared with normal tissue (AdipoR1: 0.97±0.39 vs. 1.37±0.41, P<0.0001; AdipoR2: 0.92±0.31 vs. 1.60±0.46, P<0.0001). AdipoR1 and AdipoR2 levels were further reduced in tumors with nodal metastases and the difference was statistically significant in the case of AdipoR2 (0.79±0.27 vs. 1.02±0.30, P=0.012). The results of this study demonstrated that the expression levels of adiponectin receptors are reduced in cancer specimens compared to normal tissue, indicating a downregulation in the course of the development and progression of colorectal cancer. Since adiponectin is abundantly present in the whole body and has inhibitory effects on cancer cells, this downregulation of the receptors may be an escape mechanism of malignant cells from the suppressive effects of adiponectin.

Regulation of beta-cell viability and gene expression by distinct agonist fragments of adiponectin

Obesity is an established risk factor for type 2 diabetes. Activation of the adiponectin receptors has a clear role in improving insulin resistance although conflicting evidence exists for its effects on pancreatic beta-cells. Previous reports have identified both adiponectin receptors (ADR-1 and ADR-2) in the beta-cell. Recent evidence has suggested that two distinct regions of the adiponectin molecule, the globular domain and a small N-terminal region, have agonist properties. This study investigates the effects of two agonist regions of adiponectin on insulin secretion, gene expression, cell viability and cell signalling in the rat beta-cell line BRIN-BD11, as well as investigating the expression levels of adiponectin receptors (ADRs) in these cells. Cells were treated with globular adiponectin and adiponectin (15-36) +/-leptin to investigate cell viability, expression of key beta-cell genes and ERK1/2 activation. Both globular adiponectin and adiponectin (15-36) caused significant ERK1/2 dependent increases in cell viability. Leptin co-incubation attenuated adiponectin (15-36) but not globular adiponectin induced cell viability. Globular adiponectin, but not adiponectin (15-36), caused a significant 450% increase in PDX-1 expression and a 45% decrease in LPL expression. ADR-1 was expressed at a higher level than ADR-2, and ADR mRNA levels were differentially regulated by non-esterified fatty acids and peroxisome-proliferator-activated receptor agonists. These data provide evidence of roles for two distinct adiponectin agonist domains in the beta-cell and confirm the potentially important role of adiponectin receptor agonism in maintaining beta-cell mass.

Adiponectin receptors: expression in Zucker diabetic rats and effects of fenofibrate and metformin

The insulin-sensitizing adipokine, adiponectin, acts through 2 receptors, AdipoR1 and AdipoR2. A decreased expression of these receptors could contribute to insulin resistance and diabetes. We determined if the expression of adiponectin receptors is decreased in an experimental model, the Zucker diabetic rat (ZDF), and if a peroxisome proliferator-activated receptor alpha agonist, fenofibrate, and metformin could increase these expressions. The ZDF and control (L) rats were studied at 7, 14, and 21 weeks. After initial study at 7 weeks, ZDF received no treatment (n = 10), metformin (n = 10), or fenofibrate (n = 10) until final studies at 14 or 21 weeks. The L rats received no treatment. AdipoR1 and R2 expressions were measured in liver, muscle, and white adipose tissue (WAT). As expected, ZDF rats were insulin resistant at 7 weeks, had type 2 diabetes mellitus at 14 weeks, and had diabetes with insulin deficiency at 21 weeks. Compared with L rats, AdipoRs messenger RNA was decreased only in the WAT (P < .05) of 7-week-old ZDF rats, but was unchanged in muscle and increased in liver. Metformin and fenofibrate decreased plasma triacylglycerols (P < .01) as expected. The only effect of fenofibrate on AdipoRs was a moderate increase (P < .01) of both receptors' messenger RNA in liver. Metformin increased AdipoR1 and R2 expression in muscle (P < .01) and AdipoR1 (P < .01) in WAT. These results do not support an important role for decreased AdipoRs expression in the development of insulin resistance and diabetes. Parts of the actions of fenofibrate and of metformin could be mediated by a stimulation of the expression of these receptors in liver and in insulin-sensitive, glucose-utilizing tissues (muscle, WAT), respectively.

Insulin regulates the expression of adiponectin and adiponectin receptors in porcine adipocytes

Adiponectin is an adipocyte-derived hormone that can improve insulin sensitivity. Its functions in regulating glucose utilization and fatty acid metabolism in mammals are mediated by two subtypes of adiponectin receptors (AdipoR1 and AdipoR2). This study was conducted to determine the effect of insulin on the expression of adiponectin and its receptors. We demonstrated that in the presence of 10 nM insulin, addition of 1 microM of insulin or rosiglitazone (a peroxisome proliferator-activated receptor gamma (PPARgamma) agonist) had no effect on the expression of adiponectin and AdipoR genes in differentiated porcine adipocytes. However, the addition of 1 microM insulin plus 1 microM rosiglitazone significantly increased the AdipoR2 mRNA in differentiated porcine adipocytes. Using the phosphatidylinositol 3-kinase inhibitor (PI3K inhibitor, LY 294002), we found that insulin inhibited the expression of AdipoR2 through the PI3K pathway and this inhibition was blocked by addition of rosiglitazone. When porcine adipocytes were cultured without insulin, supplementation with 10 nM insulin inhibited the expression of AdipoR2 and this inhibition effect was also blocked by addition of rosiglitazone. Therefore, these data suggest that a PPARgamma agonist increases expression of AdipoR2 and that insulin inhibits the expression of AdipoR2 through the PI3K pathway.

Regulation of adiponectin receptor expression in human liver and a hepatocyte cell line

Nonalcoholic fatty liver disease (NAFLD) is closely associated with obesity. An adipocyte-derived hormone, adiponectin, may play a role in the pathophysiology of NAFLD through insulin-sensitizing and antifibrotic effects. We found that hepatic expression of adiponectin receptor AdipoR2, but not AdipoR1, was down-regulated in 14 patients with NAFLD compared with 7 patients with a normal liver (P < .05). To investigate the significance of the adiponectin system in obesity and NAFLD, we examined the regulation of AdipoR2 expression in a nonmalignant human hepatocyte cell line, the THLE-5b cells. Insulin down-regulated the levels of AdipoR2 messenger RNA (mRNA) and protein, whereas an adipocytokine, tumor necrosis factor alpha, up-regulated them. A thiazolidinedione, pioglitazone, up-regulated the expression of AdipoR2 mRNA and protein in THLE-5b cells. The AdipoR2 mRNA level was decreased in fatty THLE-5b cells induced by coincubating with fatty acids. These findings suggest that down-regulation of AdipoR2 in the liver caused by hyperinsulinemia and steatosis may play a role in the development of NAFLD.

Adiponectin: a link between obesity and cancer

Adiponectin, an insulin-sensitising hormone produced by adipocytes, has direct antidiabetic, antiatherogenic, anti-inflammatory and antiangiogenic properties. Circulating adiponectin levels are lower in obesity, a disease state that is associated with certain malignancies. Recently, accumulating evidence suggests that adiponectin may have an important protective role in carcinogenesis. There is also evidence that at least some, if not most, cancer cell types express adiponectin receptors; thus adiponectin may act on tumour cells directly by binding and activating adiponectin receptors and downstream signalling pathways. Through its antiangiogenic properties, and also possibly through other mechanisms regulating cell proliferation discussed in this review, adiponectin may prove to be an effective novel anticancer agent. Large association and prospective studies to assess adiponectin levels in relation to risk from cancer, as well as mechanistic studies to prove adiponectin's role in the development of malignancies, and interventional trials to address potential roles of adiponectin in cancer pathogenesis and therapeutics are needed.

Adiponectin in relation to malignancies: a review of existing basic research and clinical evidence

Adiponectin, an adipocyte-secreted hormone that plays an important role in diabetes and cardiovascular disease, may also be of importance in the development and progression of several malignancies. Circulating adiponectin concentrations, which are determined mainly by genetic factors, nutrition, and adiposity, are lower in patients with breast, endometrial, prostate, and colon cancer. It has thus been proposed that adiponectin may be a biological link between obesity (especially central obesity) and increased cancer risk. Adiponectin may influence cancer risk through its well-recognized effects on insulin resistance, but it is also plausible that adiponectin acts on tumor cells directly. Several cancer cell types express adiponectin receptors that may mediate the effects of adiponectin on cellular proliferation. Herein, we review recent evidence supporting a role of serum adiponectin concentrations as a novel risk factor and possible diagnostic marker for obesity-related malignancies, including cancers of the breast, endometrium, colon, and prostate. Further studies are needed to fully elucidate the potential role of adiponectin in cancer diagnostics and therapeutics.

Insulin-sensitizing effects of thiazolidinediones are not linked to adiponectin receptor expression in human fat or muscle

Circulating adiponectin levels are increased by the thiazolidinedione (TZD) class of PPARgamma agonists in concert with their insulin-sensitizing effects. Two receptors for adiponectin (AdipoR1 and AdipoR2) are widely expressed in many tissues, but their physiological significance to human insulin resistance remains to be fully elucidated. We examined the expression patterns of AdipoR1 and AdipoR2 in fat and skeletal muscle of human subjects, their relationship to insulin action, and whether they are regulated by TZDs. Expression patterns of both AdipoRs were similar in subcutaneous and omental fat depots, with higher expression in adipocytes than in stromal cells and macrophages. To determine the effects of TZDs on AdipoR expression, subcutaneous fat and quadriceps muscle were biopsied in 14 insulin-resistant subjects with type 2 diabetes mellitus after 45 mg pioglitazone or placebo for 21 days. This duration of pioglitazone improved insulin's suppression of glucose production by 41% and enhanced stimulation of glucose uptake by 27% in concert with increased gene expression and plasma levels of adiponectin. Pioglitazone did not affect AdipoR expression in muscle, whole fat, or cellular adipose fractions, and receptor expression did not correlate with baseline or TZD-enhanced insulin action. In summary, both adiponectin receptors are expressed in cellular fractions of human fat, particularly adipocytes. TZD administration for sufficient duration to improve insulin action and increase adiponectin levels did not affect expression of AdipoR1 or AdipoR2. Although TZDs probably exert many of their effects via adiponectin, changes in these receptors do not appear to be necessary for their insulin-sensitizing effects.

Q. Adiponectin and its receptors are expressed in adult ventricular cardiomyocytes and upregulated by activation of peroxisome proliferator-activated receptor gamma

Adiponectin is a protein hormone involved in maintaining energy homeostasis in metabolically active tissues. It enhances glucose and lipid metabolism via activation of AMP-dependent kinase (AMPK) in skeletal muscle and liver. Energy homeostasis is vital for the heart to work as a pump. In this study, we investigated whether adiponectin and its receptors are expressed in adult ventricular cardiomyocytes. We observed adiponectin transcript and protein in cultured ventricular cardiomyocytes isolated from adult rat, by quantitative real-time PCR, ELISA assays, Western blots, and immunofluorescent staining. In addition, we detected adiponectin receptor (AdipoR1 and AdipoR2) expression in the heart. AdipoR1 was expressed in rat myocardium at a level of approximately 50% of that in skeletal muscle; whereas adipoR2 was expressed at a similar level to that in liver. Rosiglitazone, a Peroxisome proliferator activated receptor gamma (PPARgamma) activator, substantially elevated expression of adiponectin in cultured cardiomyocytes and its secretion into cultured media. Rosiglitazone also increased adipoR1 and adipoR2 expression in cardiomyocytes. Treatment of recombinant globular adiponectin in cultured cardiomyocytes increased fatty acid oxidation and glucose uptake via activation of AMPK, suggesting a role for adiponectin in cardiac energy metabolism. Together, these data establish the existence of a local cardiac-specific adiponectin system that is regulated by PPARgamma. Moreover, these findings indicate a role for adiponectin on normal myocardial energy homeostasis, in part, through the activation of AMPK.

Regulation of adiponectin and its receptors in response to development of diet-induced obesity in mice

Adiponectin and its receptors play an important role in energy homeostasis and insulin resistance, but their regulation remains to be fully elucidated. We hypothesized that high-fat diet would decrease adiponectin but increase adiponectin receptor (AdipoR1 and AdipoR2) expression in diet-induced obesity (DIO)-prone C57BL/6J and DIO-resistant A/J mice. We found that circulating adiponectin and adiponectin expression in white adipose tissue are higher at baseline in C57BL/6J mice compared with A/J mice. Circulating adiponectin increases at 10 wk but decreases at 18 wk in response to advancing age and high-fat feeding. However, adiponectin levels corrected for visceral fat mass and adiponectin mRNA expression in WAT are affected by high-fat feeding only, with both being decreased after 10 wk in C57BL/6J mice. Muscle AdipoR1 expression in both C57BL/6J and A/J mice and liver adipoR1 expression in C57BL/6J mice increase at 18 wk of age. High-fat feeding increases both AdipoR1 and AdipoR2 expression in liver in both strains of mice and increases muscle AdipoR1 expression in C57BL/6J mice after 18 wk. Thus advanced age and high-fat feeding, both of which are factors that predispose humans to obesity and insulin resistance, are associated with decreasing adiponectin and increasing AdipoR1 and/or AdipoR2 levels.

Differential regulation of adiponectin receptor gene expression by adiponectin and leptin in myotubes derived from obese and diabetic individuals

OBJECTIVE

This study aimed to investigate the regulation of adiponectin receptors 1 (AdipoR1) and 2 (AdipoR2) gene expression in primary skeletal muscle myotubes, derived from human donors, after exposure to globular adiponectin (gAd) and leptin.

RESEARCH METHODS AND PROCEDURES

Four distinct primary cell culture groups were established [Lean, Obese, Diabetic, Weight Loss (Wt Loss); n = 7 in each] from rectus abdominus muscle biopsies obtained from surgical patients. Differentiated myotube cultures were exposed to gAd (0.1 microg/mL) or leptin (2.5 microg/mL) for 6 hours. AdipoR1 and AdipoR2 gene expression was measured by real-time polymerase chain reaction analysis.

RESULTS

AdipoR1 mRNA expression in skeletal muscle myotubes derived from Lean subjects (p < 0.05) was stimulated 1.8-fold and 2.5-fold with gAd and leptin, respectively. No increase in AdipoR1 gene expression was measured in myotubes derived from Obese, Diabetic, or Wt Loss subjects. AdipoR2 mRNA expression was unaltered after gAd and leptin exposure in all myotube groups.

DISCUSSION

Adiponectin and leptin are rapid and potent stimulators of AdipoR1 in myotubes derived from lean healthy individuals. This effect was abolished in myotubes derived from obese, obese diabetic subjects, and obese-prone individuals who had lost significant weight after bariatric surgery. The incapacity of skeletal muscle of obese and diabetic individuals to respond to exogenous adiponectin and leptin may be further suppressed as a result of impaired regulation of the AdipoR1 gene.

Expression of the adiponectin receptors AdipoR1 and AdipoR2 in lean rats and in obese Zucker rats

The adiponectin receptors, AdipoR1 and AdipoR2, are thought to transmit the insulin-sensitizing effects of adiponectin, an adipokine secreted by adipocytes. Modifications of their expression in insulin-sensitive tissues (skeletal muscle, liver, and adipose tissue) could therefore play a role in the control of insulin sensitivity and the development of insulin resistance. Recent data in mice supported this possibility. We examined whether the expression of adiponectin receptors (messenger RNA [mRNA] concentrations) is controlled in vivo in rats (Wistar) by nutritional factors (high-fat [HF] vs high-carbohydrate diet, fasting vs fed state) and whether this expression is decreased in an experimental model of insulin resistance, the obese Zucker rat. In Wistar rats, neither an HF diet nor fasting modified the mRNA concentrations of AdipoR1 in muscle, liver, or adipose tissue; the only modification observed was a decrease (P < .05) in AdipoR2 mRNA level in the liver of rats fed with an HF diet. In obese Zucker rats compared with their lean controls, neither AdipoR1 nor AdipoR2 expression was modified in muscle. AdipoR2 expression was slightly decreased in adipose tissue, whereas the expression of both AdipoR1 and AdipoR2 was increased (P < .05) in the liver of obese Zucker rats. In conclusion, contrary to what was reported in mice, the expression of adiponectin receptors in rats is poorly responsive to changes in nutritional conditions and is not decreased in a model of insulin resistance. These results do not support an important role for the expression of AdipoR1 and AdipoR2 in the modulation of sensitivity to insulin.

Physiological and pathophysiological regulation of regional adipose tissue in the development of insulin resistance and type 2 diabetes

AIM

To survey the latest state of knowledge concerning the regulation of regional adipocytes and their role in the development of insulin resistance and type 2 diabetes.

METHODS

Data from the English-language literature on regional adipocytes, including abdominal, intramyocellular, intrahepatic and intra-islet fat as well as the adipokines and their relations to insulin resistance and type 2 diabetes, were reviewed.

RESULTS

It is not the total amount of fat but the fat that resides within skeletal muscle cell (intramyocellular fat), hepatocytes and intra-abdominally (visceral fat), via systemic and local secretion of several adipokines, that influences insulin resistance. Among the adipokines that relate to insulin resistance, adiponectin and leptin appear to have clinical relevance to human insulin resistance and others may also contribute, but their role is still inconclusive. The intra-islet fat also adversely affects beta-cell function and number (beta-cell apoptosis), eventually leading to deterioration of glucose tolerance. The abnormal location of fat observed in patients with type 2 diabetes and their relatives is conceivably partly the results of the genetically determined, impaired mitochondrial fatty acid oxidative capacity. Restriction or elimination of the fat load by weight control, regular exercise and thiazolidinediones has been shown to improve insulin resistance and beta-cell function and to delay the development of type 2 diabetes.

CONCLUSION

These data support the plausibility of an essential role of regional adipose tissue in the development of insulin resistance and type 2 diabetes.