Chronic insulin effects on insulin signalling and GLUT4 endocytosis are reversed by metformin

Decreases in insulin-responsive glucose transport and associated levels of cell surface GLUT4 occur in rat adipocytes maintained in culture for 20 h u nder hyperinsulinaemic and hyperglycaemic conditions. We have investigated whether this defect is due to reduced signalling from the insulin receptor, GLUT4 expression or impaired GLUT4 trafficking. The effects of chronic ins ulin treatment on glucose transport and GLUT4 trafficking were ameliorated by inclusion of metformin in the culture medium. In comparison with the acu te effects of insulin, chronic insulin treatment attenuated changes in sign alling processes leading to glucose transport. These included insulin recep tor tyrosine phosphorylation, phosphoinositide 3-kinase activity and Akt ac tivity, which were all reduced by 60-70%. Inclusion of metformin in the cul ture medium prevented the effects of the chronic insulin treatment on these signalling processes. In comparison with cells maintained in culture witho ut insulin, the total expression of GLUT4 protein was not significantly alt ered by chronic insulin treatment, although the level of GLUT1 expression w as increased Trafficking rate constants for wortmannin-induced cell-surface loss of GLUT4 and GLUT1 were assessed by 2-N-4-(1-azi-2,2,2-trifluoroethyl )benzoyl-1,3-bis(D-mannose-4-yloxy)-2-propylamine (ATB-BMPA) photolabelling . In comparison with cells acutely treated with insulin, chronic insulin tr eatment resulted in a doubling of the rate constants for GLUT4 endocytosis. These results suggest that the GLUT4 endocytosis process is very sensitive to the perturbations in signalling that occur under hyperinsulinaemic and hyperglycaemic conditions, and that the resulting elevation of endocytosis accounts for the reduced levels of net GLUT4 translocation observed.