IRS-1 variant: A new risk factor for coronary artery disease?

Imatinib improves insulin resistance and inhibits injury-induced neointimal hyperplasia in high fat diet-fed mice

Imatinib, a PDGF receptor tyrosine kinase inhibitor, has been shown to suppress intimal hyperplasia in different animal models under normal metabolic milieu, diabetic, and/or hypercholesterolemic conditions. However, the impact of imatinib treatment on injury-induced neointimal hyperplasia has not yet been investigated in the setting of insulin resistance without frank diabetes. Using a mouse model of high fat diet (HFD)-induced insulin resistance and guidewire-induced arterial injury, the present study demonstrates that intraperitoneal administration of imatinib (25 mg/kg/day) for ~3 weeks resulted in a marked attenuation of neointimal hyperplasia (intima/media ratio) by ~78% (n = 6-9 per group; P < 0.05). Imatinib treatment also led to significant improvements in key metabolic parameters. In particular, imatinib improved insulin resistance and glucose tolerance, as revealed by complete inhibition of HFD-induced increase in HOMA-IR index and AUC_IPGTT, respectively. In addition, imatinib treatment led to diminutions in HFD-induced increases in plasma total cholesterol and triglycerides by ~73% and ~59%, respectively. Furthermore, imatinib decreased HFD-induced increase in visceral fat accumulation by ~51% (as determined by epididymal white adipose tissue weight). Importantly, imatinib treatment in HFD-fed mice enhanced plasma levels of high-molecular-weight adiponectin by ~2-fold without affecting total adiponectin. However, there were no significant changes in mean arterial pressure in insulin-resistant state or after imatinib exposure, as measured by tail-cuff method. Together, the present findings suggest that targeting PDGF receptor tyrosine kinase using imatinib may provide a realistic treatment option to prevent injury-induced neointimal hyperplasia and diet-induced insulin resistance in obesity.

PDGF-induced vascular smooth muscle cell proliferation is associated with dysregulation of insulin receptor substrates

In vascular smooth muscle cells (VSMCs), platelet-derived growth factor (PDGF) plays a major role in inducing phenotypic switching from contractile to proliferative state. Importantly, VSMC phenotypic switching is also determined by the phosphorylation state/expression levels of insulin receptor substrate (IRS), an intermediary signaling component that is shared by insulin and IGF-I. To date, the roles of PDGF-induced key proliferative signaling components including Akt, p70S6kinase, and ERK1/2 on the serine phosphorylation/expression of IRS-1 and IRS-2 isoforms remain unclear in VSMCs. We hypothesize that PDGF-induced VSMC proliferation is associated with dysregulation of insulin receptor substrates. Using human aortic VSMCs, we demonstrate that prolonged PDGF treatment led to sustained increases in the phosphorylation of protein kinases such as Akt, p70S6kinase, and ERK1/2, which mediate VSMC proliferation. In addition, PDGF enhanced IRS-1/IRS-2 serine phosphorylation and downregulated IRS-2 expression in a time- and concentration-dependent manner. Notably, phosphoinositide 3-kinase (PI 3-kinase) inhibitor (PI-103) and mammalian target of rapamycin inhibitor (rapamycin), which abolished PDGF-induced Akt and p70S6kinase phosphorylation, respectively, blocked PDGF-induced IRS-1 serine phosphorylation and IRS-2 downregulation. In contrast, MEK1/ERK inhibitor (U0126) failed to block PDGF-induced IRS-1 serine phosphorylation and IRS-2 downregulation. PDGF-induced IRS-2 downregulation was prevented by lactacystin, an inhibitor of proteasomal degradation. Functionally, PDGF-mediated IRS-1/IRS-2 dysregulation resulted in the attenuation of insulin-induced IRS-1/IRS-2-associated PI 3-kinase activity. Pharmacological inhibition of PDGF receptor tyrosine kinase with imatinib prevented IRS-1/IRS-2 dysregulation and restored insulin receptor signaling. In conclusion, strategies to inhibit PDGF receptors would not only inhibit neointimal growth but may provide new therapeutic options to prevent dysregulated insulin receptor signaling in VSMCs in nondiabetic and diabetic states.

Gly972Arg variant in the insulin receptor substrate-1 gene and association with Type 2 diabetes: a meta-analysis of 27 studies

AIMS/HYPOTHESIS

Several case-control studies have examined the association between the Gly972Arg variant in the IRS-1 gene and Type 2 diabetes, but most had limited power and results could therefore be conflicting.

METHODS

We systematically reviewed the literature by means of a meta-analysis and investigated sources of heterogeneity in results of different studies.

RESULTS

The summary risk ratio, based on 3408 cases and 5419 control cases from 27 studies, was 1.25 (95% CI 1.05-1.48). The results, however, differed according to the type of study, method of verifying non-diabetic status of the control subjects, and age of the case subjects. Population-based studies reported lower odds ratios than hospital-based studies (OR 0.98, 95% CI 0.74-1.30 vs OR 1.43, 95% CI 1.17-1.74). Also, the diagnostic test to exclude diabetes amongst control subjects interacted with the association between the IRS-1 Gly972Arg variant and Type 2 diabetes (p=0.03). Finally, the odds ratio reduced with increasing age ( p=0.03).

CONCLUSION/INTERPRETATION

Overall, carriers of the 972Arg variant of the IRS-1 gene are at a 25% increased risk of having Type 2 diabetes compared with non-carriers. The odds ratios are generally higher in hospital-based studies, including relatively young, symptomatic, cases.

Intima media thickness of carotid arteries is reduced in heterozygous carriers of the Gly972Arg variant in the insulin receptor substrate-1 gene

BACKGROUND

The Gly972Arg mutation in the IRS-1 gene has been found to be associated with insulin resistance and type II diabetes. A recently published study described an association between the Arg allele and an increased risk for coronary artery disease. In the present study we asked whether the presence of the codon 972 mutation in the IRS-1 gene is associated with higher IMT values of the carotid arteries.

MATERIALS AND METHODS

To address this question, genotypes of the codon 972 polymorphism were determined in 1018 healthy unrelated individuals aged 40-65 years. Three homozygous carriers of the mutation were excluded for statistical analysis. In all subjects, intima media thickness (IMT) and B-scores of carotid arteries as well as a large number of metabolic parameters were determined.

RESULTS

Heterozygous carriers of the Arg972 allele exhibited significantly lower IMT and B-score values than noncarriers. Total cholesterol, LDL-cholesterol and serum levels of apolipoprotein B were significantly lower in the carriers. Furthermore, a significant interaction between Gly972Arg-carrier status and mean daytime 24-h systolic blood pressure with regard to IMT could be observed; carriers with a systolic blood pressure above the median had lower IMT values than carriers with a systolic blood pressure equal or below the median. All these effects were more pronounced in females and remained significant after adjustment for sex, age, BMI, systolic blood pressure and serum apolipoprotein B levels. No significant differences between the carriers and the noncarriers could be found for BMI, insulin sensitivity or frequency of type II diabetes.

CONCLUSIONS

The results of our study demonstrate that the presence of the Arg972 allele is associated with lower IMT values of the carotid arteries. This finding is partly explained by lower serum levels of apolipoprotein B in carriers. The protective effect of the Gly972 Arg mutation seems to be stronger in the presence of a higher systolic blood pressure. Our data contradict previous findings suggesting an increased risk for insulin resistance, type II diabetes and atherosclerotic vascular disease in carriers of the mutation.