The osmotic shock-induced glucose transport pathway in 3T3-L1 adipocytes is mediated by Gab-1 and requires Gab-1 associated phosphatidylinositol 3-kinase activity for full activation

Osmotic shock treatment of 3T3-L1 adipocytes causes an increase in glucose transport activity and translocation of GLUT4 protein similar to that elici ted by insulin treatment. Insulin stimulation of GLUT4 translocation and gl ucose transport activity was completely inhibited by wortmannin, however, a ctivation by osmotic shock was only partially blocked. Additionally, we hav e found that the newly identified insulin receptor substrate Gab-l (Grb2-as sociated binder-1) is tyrosine-phosphorylated following sorbitol stimulatio n. Treatment of cells with the tyrosine kinase inhibitor genistein inhibite d osmotic shock-stimulated Gab-l phosphorylation as well as shock-induced g lucose transport. Furthermore, pretreatment with the selective Src family k inase inhibitor PP2 completely inhibited the ability of sorbitol treatment to cause tyrosine phosphorylation of Gab-1. We have also shown that microin jection of anti-Gab-1 antibody inhibits osmotic shock-induced GLUT4 translo cation. Furthermore, phosphorylated Gab-l binds and activates phosphatidyli nositol 3-kinase (PI3K) in response to osmotic shock. The PI3K activity ass ociated with Gab-1 was 82% of that associated with anti-phosphotyrosine ant ibodies, indicating that Gab-l is the major site for PI3K recruitment follo wing osmotic shock stimulation, Although wortmannin only causes a partial b lock of osmotic shock-stimulated glucose uptake, wortmannin completely abol ishes Gab-l associated PI3K activity. This suggests that other tyrosine kin ase-dependent pathways, in addition to the Gab-1-PI3K pathway, contribute t o osmotic shock-mediated glucose transport. To date, Gab-l is the first pro tein identified as a member of the osmotic shock signal transduction pathwa y.