Identification of abnormally expressed genes in skeletal muscle contributing to insulin resistance and type 2 diabetes
Abstract: The metabolic defects of insulin resistance and type 2 diabetes can result from changes in gene expression and protein functions due to genetic and environmental influences. The aim of this study was to identify abnormally expressed genes associated with insulin resistance or type 2 diabetes, and further to test whether possible defects are inherited or acquired. To achieve this, we used cDNA differential display combined with relative quantatitive RT-PCR to examine gene expression in skeletal muscle from patients with IGT and type 2 diabetes, monzygotic twin pairs discordant for type 2 diabetes, and mice of C57BL/6J and NMRI strains fed a high-fat or normal chow diet. We also used the candidate gene approach to examine muscle gene expression of GS, IRS-1, IRS-2 and Shc in relation with insulin resistance in type 2 diabetes. Using cDNA differential display we identified the abnormally expressed mitochondrial-encoded ND1 gene in human diabetic patients and the nuclear-encoded cathepsin L gene in C57BL/6J mice. Subsequent studies showed that insulin infusion increased gene expression of both ND1 and cathepsin L, and the effect was correlated with insulin-stimulated glucose uptake. Further, the effect of insulin on cathepsin L gene expression was impaired in diabetic twins but not in nondiabetic twins. There was no concordance for gene expression of ND1 and cathepsin L between monozygotic twin pairs. We also observed that insulin had a stimulatory effect on GS gene expression. Whereas this effect was normal in nondiabetic twins, it was impaired in diabetic twins, most likely due to hyperglycemia. The GS protein levels, however, were unchanged. Fat feeding resulted in impaired GS activity and insulin resistance in C57BL/6J and NMRI mice but the changes in GS activity did not involve its gene expression. In contrast to the stimulatory effect on gene expression of ND1, cathepsin L and GS, insulin downregulates IRS-1, IRS-2 and Shc gene expression in muscle. As this effect was similar in the diabetic and nondiabetic monozygotic twins as well as the control subjects, it is unlikely that it is involved in the pathogenesis of skeletal muscle insulin resistance. Conclusions: 1) Changes in mitochondrial gene expression are common in skeletal muscle from patients with type 2 diabetes; these changes most likely represent an adaptation to altered intracellular glucose flux. 2) Cathepsin L represents a novel candidate gene for type 2 diabetes, the expression of which is altered in type 2 diabetes. 3) Impaired glycogen synthesis and insulin resistance in human type 2 diabetes does not result from inherited defects in glycogen synthase gene expression in skeletal muscle. Fat feeding induces impaired glycogen synthase activity and insulin resistance in mice but this does not involve changes in glycogen synthase gene expression. 4) Insulin downregulates the expression of IRS-1, IRS-2 and Shc genes in skeletal muscle. As this effect remains intact in insulin resistant type 2 diabetes patients, it is unlikely that it can explain desensitization to insulin seen during prolonged exposure to insulin.
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