GASTRIC-INHIBITORY POLYPEPTIDE ACTIVATES MAP KINASE THROUGH THE WORTMANNIN-SENSITIVE AND WORTMANNIN-INSENSITIVE PATHWAYS

The signal transduction pathways of a cloned human gastric inhibitory polypeptide (GIP) receptor have been investigated in CHO cells stably expressing this receptor. Exposure of GIP receptor expressing cells to GIP significantly increased MAP kinase activity, Time course analysis showed that a rapid and marked increase in MAP kinase activation was detected and that this activation reached maximal levels 10 min after the addition of GIP. Dose-response analysis showed that GIP activated MAP kinase activity in a dose-dependent manner with an ED50 value of 5 .9 x 10(-10) M of GIP. Wortmannin, a potent inhibitor of phosphatidyli nositol 3-kinase (PI3-kinase), partially inhibited GIP-induced MAP kin ase activation, suggesting that GIP activates MAP kinase through two d ifferent, wortmannin-sensitive and -insensitive pathways. It has been demonstrated that in CHO cells cAMP attenuates MAP kinase activity by inhibiting Raf-1. Since GIP elevates intracellular cAMP, we examined t he effects of cAMP on MAP kinase activation. Interestingly, forskolin, which increased intracellular cAMP levels, significantly inhibited MA P kinase activation by GIP, but did not affect MAP kinase activation b y GIP in the presence of wortmannin, suggesting that the wortmannin-se nsitive pathway activates an MAP kinase cascade at or above the level of Raf-1 and that the wortmannin-insensitive pathway activates an MAP kinase cascade below the level of Raf-1. These findings demonstrate th at the GIP receptor is linked to the MAP kinase cascade via at least t wo different pathways. (C) 1997 Academic Press.