The G972R variant of the Insulin Receptor Substrate-1 (IRS-1) gene, body fat distribution and insulin-resistance

Aims/hypothesis. Insulin resistance is recognised as the core factor in the pathogenesis of Type II (non-insulin-dependent) diabetes mellitus, hypertension and atherosclerosis. Several studies indicate the possible role of mutations of the insulin receptor substrate-1 (IRS-1) gene in the pathogenesis of insulin-resistance and suggest a possible interaction between the IRS-1 gene and obesity, either by an effect on the development of obesity or by causing or aggravating the obesity-associated insulin resistance. Therefore, the prevalence of the G972R mutation of the IRS-1 gene was compared in 157 non-diabetic obese subjects (BMI > 30 m/kg²) and in 157 lean subjects (BMI < 28 m/kg²). By investigating the relation between this IRS-1 mutation, measures of obesity and metabolic parameters, we explored the possible influence of this mutation on body fat distribution and insulin resistance. Methods. The G972R mutation was detected by PCR amplification and BstN-1 restriction enzyme digestion. Data were analysed by univariate and multivariate analysis. Results. The G972R allele was significantly more frequent in obese subjects than in lean subjects (p < 0.002); however, no difference was found between centrally and peripherally obese subjects. Obese G972R carriers had significantly higher BMI (p < 0.001), fasting insulin (p < 0.01), triglycerides (p < 0.03) and HOMA_IR (p < 0.001) than obese non-carriers. No differences were observed between G972R carriers and non-carriers among control subjects. Multivariate analysis confirmed that the IRS-1 G972R mutation was significantly and independently associated with reduced insulin sensitivity (p < 0.009) in the obese group. Conclusion/interpretation. The G972R mutation of the IRS-1 gene associates with obesity, but not with fat distribution, in this Italian cohort, and within the obese subjects this IRS-1 variant strongly associates with metabolic parameters suggesting greater insulin-resistance. These findings indicate a possible interaction between the IRS-1 variant and obesity in worsening insulin sensitivity. [Diabetologia (2001) 44: 367–372] Received: 11 August 2000 and in revised form: 25 October 2000     

The Arg972 variant in insulin receptor substrate-1 is associated with an atherogenic profile in offspring of type 2 diabetic patients

The insulin receptor substrate-1 (IRS-1) gene has been considered a candidate for insulin resistance, type 2 diabetes, and coronary artery disease. To investigate the relationship between the common Gly(972)Arg IRS-1 variant and the presence of cardiovascular risk factors, 153 glucose-tolerant, unrelated offspring of type 2 diabetic patients were studied. There were no differences between Arg(972) IRS-1 carriers and noncarriers in age, gender, body mass index, waist/hip ratio, body composition, fasting glucose and insulin levels, and glucose or insulin levels during the oral glucose tolerance test. Insulin sensitivity, assessed by hyperinsulinemic-euglycemic clamp, was significantly reduced in carriers of Arg(972) IRS-1 (P < 0.03). Carriers of Arg(972) IRS-1 displayed many features of the insulin resistance syndrome, including higher values for serum triglycerides (P < 0.01), total/high density lipoprotein cholesterol ratio (P < 0.01), free fatty acid levels (P < 0.04), systolic blood pressure (P < 0.04), microalbuminuria (P < 0.003), and intima-media thickness (P < 0.02). These results suggest that the Arg(972) IRS-1 variant could contribute to the risk for atherosclerotic cardiovascular diseases associated with type 2 diabetes by producing a cluster of insulin resistance-related metabolic abnormalities.     

The G1422A variant of the cannabinoid receptor gene (CNR1) is associated with abdominal adiposity in obese men

Since recent data suggest that the endocannabinoid system controls food intake through central, and lipogenesis via peripheral CB1 receptors, we hypothesized that genetic variation at the cannabinoid receptor-1 (CNR1) locus could have an effect on adiposity. We investigated, whether a specific CNR1 G1422A genotype is associated with anthropometric markers of obesity and fat distribution in adult obese individuals. A total of 1,064 obese subjects (BMI > or = 30 kg/m2) without diabetes, impaired glucose tolerance or other endocrine diseases and 251 healthy control persons were genotyped for the G1422A variant (rs1049353) with a TaqMan assay. Anthropometric measures as body weight, BMI, waist and waist-to-hip ratio (WHR) were assessed by classical methods. Fat mass (FM) was measured by bio-impedance. The prevalence of the G1422A variant was not significantly different between cases and controls (OR = 1.056; P = 0.626). In obese women, no meaningful associations between CNR1 genotype and anthropometric parameters were found. In obese men, CNR1 1422 A/A genotype was significantly associated with higher WHR (P = 0.009) and waist circumference (P = 0.008) after adjusting for age and BMI. Fat mass percentage showed an association (P = 0.011) which disappeared after adjusting for age and BMI. A trend for an association was seen for fat mass (unadjusted P = 0.099; adjusted P = 0.033). Our data indicate that the G1422A polymorphism in the CNR1 gene is associated with increased abdominal adiposity in obese men.     

Impaired endothelial function in never-treated hypertensive subjects carrying the Arg972 polymorphism in the insulin receptor substrate-1 gene

Some cardiovascular risk factors, such as hypertension and insulin resistance, are associated with endothelial dysfunction. Insulin regulates both in vitro and in vivo expression of endothelial nitric oxide synthase (eNOS) via a pathway involving insulin receptor substrate-1 (IRS-1) and phosphatidylinositol-3 kinase. Recently, we found that human endothelial cells obtained from carriers of the Arg(972) IRS-1 polymorphism exhibited reduced eNOS expression in response to chronic exposure to insulin. A reduction in eNOS expression would be expected to be associated with impaired endothelium-dependent vasodilation. To investigate a possible relationship between Arg(972) IRS-1 polymorphism and endothelial dysfunction in vivo, we enrolled a cohort of 100 never-treated hypertensive subjects. Endothelium-dependent and endothelium-independent vasodilation were assessed by increasing doses of acetylcholine and sodium nitroprusside. IRS-1 polymorphism was detected by PCR. The allelic frequency of the Arg(972) IRS-1 variant was 8.0%. Stratifying subjects according to IRS-1 genotype, we observed that acetylcholine-stimulated forearm blood flow was significantly (P < 0.0001) lower in Gly/Arg heterozygous carriers than in Gly/Gly carriers (11.3 +/- 4.4 vs. 14.7 +/- 5.9 ml/100 ml(-1) of tissue per min(-1)). Sodium nitroprusside caused comparable increments in forearm blood flow in both groups (12.9 +/- 2.4 vs. 13.3 +/- 3.5 ml/100 ml(-1) of tissue per min(-1)). Our data strongly suggest that, by inducing endothelial dysfunction, the Arg(972) IRS-1 polymorphism may contribute to the genetic predisposition to develop cardiovascular disease.     

The G972R variant of the insulin receptor substrate-1 gene impairs insulin signaling and cell differentiation in 3T3L1 adipocytes; treatment with a PPARgamma agonist restores normal cell signaling and differentiation

The insulin receptor substrate-1 (IRS-1) plays a central role in insulin sensitivity, and association studies have shown that the IRS-1 G972R variant is a risk factor for insulin resistance. However, how this mutation may lead to impaired insulin sensitivity is still to be determined. Our study aimed to evaluate, after transfection of the IRS-1 G972R variant in 3T3L1 adipocytes, the effect of this mutation on insulin signaling and on cell differentiation. The 3T3L1 cells were transfected with pcDNA3 expression vector containing either the human wild-type IRS-1 or the G972R variant. After induction of differentiation, the 3T3L1 transfected with wild-type IRS-1 differentiated in 6-8 days, while the cells transfected with G972R variant did not differentiate. To determine whether the defect in IRS-1 was responsible for this, we analyzed the expression of several genes involved in the insulin signaling pathway. Results showed that PPARgamma expression was significantly reduced in cells transfected with the mutated IRS-1, together with a significant decrease in binding of phosphatidylinositol-3 kinase (PI 3-kinase) to IRS-1 G972R and in PI 3-kinase activity. In addition, we observed that the interaction between the insulin receptor (IR) and the IRS-1 G972R protein was increased and that the autophosphorylation of the IR was significantly inhibited in 3T3L1-G972R cells compared with 3T3L1-WT. Treatment of the 3T3L1-G972R cells with pioglitazone (PIO), a PPARgamma agonist, restored differentiation with higher level of PPARgamma expression and restoration of PI 3-kinase binding to IRS-1 G972R and PI 3-kinase activity. IR autophosphorylation was also increased. Withdrawal of PIO in fully differentiated 3T3L1-G972R cells determined the reappearance of the insulin signaling defect. Finally, we observed higher levels of IRS-2 expression, suggesting that IRS-2 may play a more important role in adipocyte insulin signaling. In conclusion, IRS-1 G972R variant impairs insulin signaling, and treatment with PPARgamma agonist restores the normal phenotype of 3T3L1 cells.     

The insulin receptor substrate-1 Gly972Arg polymorphism is not associated with Type 2 diabetes mellitus in two population-based studies.

AIMS: The insulin receptor substrate-1 (IRS-1) gene is among the most frequently studied candidate genes for Type 2 diabetes, but findings have been inconsistent. This may have been due to generally small study sizes, or to interaction with body fatness as suggested by studies of insulin sensitivity. The aim of this study was to test the hypothesis that the IRS-1 Gly972Arg variant increases risk of Type 2 diabetes. METHODS: We conducted two large population-based studies including a total of 725 cases and 742 control subjects, who were Caucasian Dutch men and women aged 40-70 years. We calculated odds ratios adjusted for body mass index, study centre, sex and age. RESULTS: Carriers of the Arg allele did not have a higher prevalence of newly detected (OR 0.49, 95% CI 0.24-1.01) or treated (OR 0.71, 0.37-1.35) Type 2 diabetes in the first study, or a higher prevalence of glucose intolerance (OR 1.07, 0.71-1.59) in the second study. The summary odds ratio was 0.86 (0.62-1.17) for carrying the Arg allele as compared with the Gly/Gly genotype. Associations did not differ appreciably by degree of obesity. Also, the Arg variant was not associated with detrimental values for body mass index, waist circumference, plasma HDL-and total cholesterol or hypertension. CONCLUSIONS: Our findings indicate that the IRS-1 Gly972Arg variant does not substantially increase risk of common Type 2 diabetes, or Type 2 diabetes in obese persons.     

Molecular scanning for mutations in the insulin receptor substrate-1 (IRS-1) gene in Turkish with type 2 diabetes mellitus

Insulin receptor substrate-1 (IRS-1) is an endogenous substrate for the insulin receptor tyrosine kinase, which plays a key role in insulin signaling. Recent studies have identified several polymorphisms in the human IRS-1 gene (Irs-1) that are increased in prevalence among type 2 diabetic patients. To determine whether variation in the Irs-1 contributes to genetic susceptibility to type 2 diabetes in Turkish people, PCR-RFLP and DNA sequencing method were utilized to analyze the coding region of Irs-1 in 70 subject and 116 control patients. Three missense mutations were detected (Gly972Arg, Ala512Pro, Ser892Gly). There was no significant association found with any of these variants and diabetes. The Gly972Arg mutation, however, was relatively more common in with 10/70 diabetic patients and 15/116 non-diabetic controls being heterozygous and 1/70 being and 0/116 non-diabetic controls being homozygous for this variant. As a conclusion, Ala512Pro, Ser892Gly mutations were rare and Met613Val, Ser1043Tyr and Cys1095Tyr mutations were not found in the populations studied. Gly972Arg is more common than other known mutations in our population but may not be a major determinant in genetic susceptibility to type 2 diabetes.     

Intrahepatic diacylglycerol content is associated with hepatic insulin resistance in obese subjects

Data from studies in animal models indicate that certain lipid metabolites, particularly diacylglycerol, ceramide, and acylcarnitine, disrupt insulin action. We evaluated the relationship between the presence of these metabolites in the liver (assessed by mass spectrometry) and hepatic insulin sensitivity (assessed using a hyperinsulinemic-euglycemic clamp with stable isotope tracer infusion) in 16 obese adults (body mass index, 48 ± 9 kg/m2). There was a negative correlation between insulin-mediated suppression of hepatic glucose production and intrahepatic diacylglycerol (r = -0.609; P = .012), but not with intrahepatic ceramide or acylcarnitine. These data indicate that intrahepatic diacylglycerol is an important mediator of hepatic insulin resistance in obese people with nonalcoholic fatty liver disease.     

Genetic variants of insulin receptor substrate-1 (IRS-1) in syndromes of severe insulin resistance. Functional analysis of Ala513Pro and Gly1158Glu IRS-1.

AIMS: To define further the role of IRS-1 mutations in human syndromes of severe insulin resistance. METHODS: The IRS-1 gene was scanned for mutations in 83 unrelated affected subjects and 47 unaffected individuals using fluorescent single-strand conformation polymorphism (fSSCP) analysis. A novel heterozygous mutation, Gly1158Glu, was found in one affected subject. Four and two subjects were heterozygous for the previously reported variants Gly972Arg and Ala513Pro, respectively. The previously identified variant Gly819Arg was found in one affected and one unaffected subject. While Gly972Arg has been described to alter the signalling properties of IRS-1, no functional studies of Ala513Pro or Gly1158Glu have been reported. RESULTS: Chinese hamster ovary (CHO) cells stably over-expressing the insulin receptor were transiently transfected with vectors expressing either wild-type, Glu1158 or Pro513 IRS-1. A modest increase in insulin-stimulated tyrosine phosphorylation of Glu1158 IRS-1 was observed. However, this did not result in any significant change in the association of Grb2 or the p85 alpha subunit of PI3-kinase or of PI3-kinase activity. In parallel studies, the Pro513 IRS-1 variant was indistinguishable from wild-type IRS-1. CONCLUSIONS: While subtle effects of these variants cannot be excluded in this system, it is unlikely that these variants are responsible for the extreme insulin resistance seen in the subjects harbouring them. Although IRS proteins play a central role in insulin signalling, functionally significant mutations in the IRS-1 gene are a rare cause of human syndromes of severe insulin resistance.     

胰岛素受体底物1（IRS-1）基因Gly972Arg突变与中国人糖尿病的相关性

目的 研究胰岛素受体物１（ＩＲＳ－１）基因Ｇｌｙ９７２Ａｒｇ突变与中国人糖尿病的相关性。方法 随机选取上海地区中国人３５９例,用ＰＣＲ／ＢｓｔＮ１酶解法检测Ｇｌｙ９７２Ａｒｇ突变。结果 本组中国人中仅发现２例该突变且均为非糖尿病者。结论 该突变可能不是中国人糖尿病的重要遗传因素。     

The prevalent Gly1057Asp polymorphism in the insulin receptor substrate-2 gene is not associated with impaired insulin secretion

Disruption of the insulin receptor substrate-2 was shown to cause type 2 diabetes in mice. This could be largely attributed to abnormal beta-cell development. In humans, a prevalent polymorphism in insulin receptor substrate-2 (Gly1057Asp) was not found be associated with type 2 diabetes in linkage and association studies. We tested the hypothesis that an extreme challenge of the beta cell might reveal subtle abnormalities in carriers of this polymorphism undetected by conventional insulin secretion tests. Therefore, in addition to assessing beta-cell function by oral glucose tolerance testing (n = 318, normal glucose tolerance), we measured the secretory response to maximal stimulation by hyperglycemia (10 mM), glucagon-like peptide-1, and arginine administered in an additive fashion (n = 77, nondiabetic). The allelic frequency of the Asp allele was approximately 37%. Neither the beta-cell function indices from the oral glucose tolerance test nor the secretory response during the hyperglycemic clamp differed measurably between carriers and controls. Moreover, maximal plasma C-peptide concentrations in response to the combined glucose, glucagon-like peptide-1, and arginine stimulus was not different between Gly/Gly (10,745 +/- 1,186 pmol/liter) and X/Asp (10,800 +/- 490 pmol/liter, P = 0.99). In conclusion, our findings strongly suggest that the Gly1057Asp polymorphism in insulin receptor substrate-2 is not associated with beta-cell dysfunction. The normal maximal insulin secretory response makes it unlikely that this common polymorphism results in abnormal beta-cell development.     

Protein kinase C (PKC) beta modulates serine phosphorylation of insulin receptor substrate-1 (IRS-1)--effect of overexpression of PKCbeta on insulin signal transduction

In vitro phosphorylation of 180-kDa protein, obtained by immunoprecipitation of adipocyte homogenate with anti-IRS-1 antibody was increased with the addition of conventional PKC in the presence of Ca2+, phosphatidylserine (PS) and diolein (DL). Human purified IRS-1 was phosphorylated by purified conventional PKC (cPKC) in the presence of Ca2+/PS/DL. These results suggest that PKC may have a role in the serine phosphorylation of IRS-1. In order to clarify the inhibitory effect of cPKC on glucose transport mechanism, we examined the overexpression of PKCbeta in cultured adipocytes. Overexpression of PKCbeta in adipocytes markedly induced mobility shift and serine phosphorylation of IRS-1, whereas overexpression of dominant negative PKCbeta (DNPKCbeta) blocked this mobility shift and serine phosphorylation of IRS-1. Insulin (10 nM) increased [3H]2-deoxyglucose (2-DOG) uptake to 200% from basal level (100%) in cultured adipocytes transfected with a vector alone. Overexpression of PKCbeta in adipocytes decreased insulin-induced 2-DOG uptake to 110%, whereas overexpression of DNPKCbeta increased it to 230%. These results suggest that PKCbeta negatively regulates glucose uptake via serine phosphorylation of IRS-1 in rat adipocytes.