Insulin activates p21Ras and guanine nucleotide releasing factor in cells expressing wild type and mutant insulin receptors.

Insulin stimulates the formation of p21RasGTP in Rat-1 fibroblasts overexpressing wild type (HIRc) or mutant (delta CT and Y/F2) insulin receptors. Maximal insulin effect was observed at 7 min in Y/F2 cells, at 10 min in delta CT cells, and at 15 min in HIRc cells. Mutant insulin receptors which display enhanced mitogenic signaling properties stimulated p21Ras.GTP formation to a greater extent than wild type receptors. The amount of p21Ras was not affected by insulin (Western blotting). Tyrosine kinase inhibitor, Lavendustin A (10 nM), completely prevented insulin-induced activation of p21Ras in all cell lines. Insulin did not lead to GAP phosphorylation, or a change in cellular GAP activity, but did result in marked stimulation of Ras guanine nucleotide releasing factor (GRF) activity (by 48% in HIRc, 71% in delta CT, and 120% in Y/F2 cells). These results indicated that p21Ras.GTP is an important signaling molecule in insulin's mitogenic pathway, but may not participate in metabolic signaling and that insulin's stimulatory effects on p21Ras.GTP formation are mediated through Ras GRF.