Coupling of the Insulin-like Growth Factor-I Receptor Tyrosine Kinase to Gi2 in Human Intestinal Smooth Muscle

Abstract

Endogenous insulin-like growth factor-1 (IGF-I) stim- ulates growth of cultured human intestinal smooth mus- cle by activating distinct mitogen-activated protein (MAP) kinase-dependent and phosphatidylinositol 3-ki- nase-dependent signaling pathways. In Rat1 and Balb/ c3T3 fibroblasts and in neurons the IGF-I receptor is coupled to an inhibitory G protein, Gi, which mediates G -dependent MAP kinase activation. The present study determined whether in normal human intestinal smooth muscle cells the IGF-I receptor activates a het- erotrimeric G protein and the role of G protein activa- tion in mediating IGF-I-induced growth. IGF-I elicited IGF-I receptor tyrosine phosphorylation, resulting in the specific activation of Gi2.G subunits selectively mediated IGF-I-dependent MAP kinase activation; G i2 subunits selectively mediated IGF-I-dependent inhibi- tion of adenylyl cyclase actvity. IGF-I-stimulated MAP kinase activation and growth were inhibited by pertus- sis toxin, an inhibitor of Gi/Go activation. Cyclic AMP inhibits growth of human intestinal muscle cells. IGF-I inhibited both basal and forskolin-stimulated cAMP lev- els. This inhibition was attenuated in the presence of pertussis toxin. IGF-I stimulated phosphatidylinositol 3-kinase activation, in contrast to MAP kinase activa- tion, occurred independently of Gi2 activation. These data suggest that IGF-I specifically activates Gi2, result- ing in concurrent G -dependent stimulation of MAP kinase activity and growth, and G i2-dependent inhibi- tion of cAMP levels resulting in disinhibition of cAMP- mediated growth suppression.