ALTERATIONS IN INSULIN SIGNALING PATHWAY INDUCED BY PROLONGED INSULIN-TREATMENT OF 3T3-L1 ADIPOCYTES

Insulin-induced glucose transport stimulation, which results from the translocation of glucose transporter 4 (GLUT 4)-containing vesicles, i s completely blocked after prolonged insulin treatment of 3T3-L1 adipo cytes. Since GLUT 4 expression was reduced by only 30%, we looked at t he insulin signalling pathway in this insulin-resistant model. Insulin -induced tyrosine phosphorylation of the major insulin receptor substr ate IRS1 was reduced by 50+/-7%, while its expression was decreased by 70+/-4%. When cells were treated with wortmannin (a PI3-kinase inhibi tor) together with insulin, the expression of IRS1 diminished to a muc h lower extent. Associated with the decrease in IRS1 expression and ph osphorylation, the activation by insulin of antiphosphotyrosine immuno precipitable PI3-kinase activity and of p44(mapk) and p42(mapk) activi ties was altered. However, the expression of these proteins was normal and p44(mapk) activity remained responsive to the tumour promoter TPA . Those results indicate that prolonged insulin treatment of 3T3-L1 ad ipocytes induces an insulin-resistant state with a reduced ability of insulin to stimulate the PI3-kinase and the MAP-kinases and a blockade of glucose transporter translocation.