Grb-2-associated binder-1 is involved in insulin-induced egr-1 gene expression through its phosphatidylinositol 3 '-kinase binding site

The Grb2-associated binder-1 (Gab1) is one of the major adapter molecules d ownstream of growth factor receptor signaling. Even though insulin causes t yrosine phosphorylation of Gab1, its role in insulin signaling has not been identified yet. We have demonstrated that insulin increased expression of early growth response gene-1 (egr-1), which is one of the most important tr anscription factors involved in cell proliferation and differentiation. In the present study, the possible role of Gab1 in insulin-induced egr-1 expre ssion was studied using Rat1 fibroblasts expressing human insulin receptors and wildtype Gab1 (HIRc/Gab1(WT)), Gab1 with three tyrosines in the phosph atidylinositol (PI) 3'-kinase binding domain mutated to phenylalanine (HIRc / Gab1(Delta PI3K)), or histidinol resistance only (HIRc/HIS). Insulin-indu ced egr-1 expression in HIRc/Gab1 (Delta PI3K) cells was much lower than in the other cells, as determined by Northern blot analysis, These results su ggest that Gab1 is involved in the signaling pathway for insulin-induced eg r-1 expression through increasing PI3'-kinase activity. The MAP kinase acti vity increased less with insulin treatment in HIRc/Gab1(Delta PI3K) cells t han in other cells. Inhibition of MAP kinase by the MEK inhibitor completel y abolished insulin-induced egr-1 expression. These results suggest that Ga b1 increases MAP kinase activity through its PI3'-kinase binding site, whic h then leads to egr-1 expression. Our results indicate that Gab1 is involve d in the control of egr-1 expression regulated by insulin.