Homocysteine thiolactone inhibits insulin signaling, and glutathione has aprotective effect

Hyperhomocysteinemia and insulin resistance are independent factors for car diovascular disease. Most of the angiotoxic effects of homocysteine are rel ated to the formation of homocysteine thiolactone and the consequent increa se in oxidative stress. The oxidative stress has also been shown to impair insulin action, therefore leading to insulin resistance. In order to study a putative direct effect of homocysteine on insulin signaling, we have char acterized the molecular counter-regulation of the early events in the signa l transduction of the insulin receptor, and the metabolic end-point of glyc ogen synthesis. We employed HTC rat hepatoma cells transfected with the hum an insulin receptor. A 10 min exposure to homocysteine thiolactone (50 muM) resulted in a significant inhibition of insulin-stimulated tyrosine phosph orylation of the insulin receptor beta -subunit and its substrates IRS-1 an d p60-70, as well as their association with the p85 regulatory subunit of p hosphatidylinositol 3-kinase. These effects led to impairment of the insuli n-stimulated phosphatidylinositol 3-kinase activity, which plays a central role in regulating insulin action. Thus, insulin-stimulated glycogen synthe sis was also inhibited by homocysteine thiolactone. To investigate whether oxidative stress was mediating the counter-regulatory effect of homocystein e thiolactone on insulin signaling, we preincubated the cells (5 min) with 250 muM glutathione prior to the incubation with homocysteine (10 min) and subsequent insulin challenge. Glutathione completely abolished the effects of homocysteine thiolactone on insulin-receptor signaling and restored the insulin-stimulated glycogen synthesis. In conclusion, these data suggest th at homocysteine thiolactone impairs insulin signaling by a mechanism involv ing oxidative stress, leading to a defect in insulin action.