Endogenous IGF-I protects human intestinal smooth muscle cells from apoptosis by regulation of GSK-3β activity

We have previously shown that endogenous IGF-I regulates human intestinal smooth muscle cell proliferation by activation of phosphatidylinositol 3 (PI3)-kinase- and Erk1/2-dependent pathways that jointly regulate cell cycle progression and cell division. Whereas insulin-like growth factor-I (IGF-I) stimulates PI3-kinase-dependent activation of Akt, expression of a kinase-inactive Akt did not alter IGF-I-stimulated proliferation. In other cell types, Akt-dependent phosphorylation of glycogen synthase kinase-3beta (GSK-3beta) inhibits its activity and its ability to stimulate apoptosis. The aim of the present study was to determine whether endogenous IGF-I regulates Akt-dependent GSK-3beta phosphorylation and activity and whether it regulates apoptosis in human intestinal muscle cells. IGF-I elicited time- and concentration-dependent GSK-3beta phosphorylation (inactivation) that was measured by Western blot analysis using a phospho-specific GSK-3beta antibody. Endogenous IGF-I stimulated GSK-3beta phosphorylation and inhibited GSK-3beta activity (measured by in vitro kinase assay) in these cells. IGF-I-dependent GSK-3beta phosphorylation and the resulting GSK-3beta inactivation were mediated by activation of a PI3-kinase-dependent, phosphoinositide-dependent kinase-1 (PDK-1)-dependent, and Akt-dependent mechanism. Deprivation of serum induced beta-catenin phosphorylation, increased in caspase 3 activity, and induced apoptosis of muscle cells, which was inhibited by either IGF-I or a GSK-3beta inhibitor. Endogenous IGF- I inhibited beta-catenin phosphorylation, caspase 3 activation, and apoptosis induced by serum deprivation. IGF-I-dependent inhibition of apoptosis, similar to GSK-3beta activity, was mediated by a PI3-kinase-, PDK-1-, and Akt-dependent mechanism. We conclude that endogenous IGF- I exerts two distinct but complementary effects on intestinal smooth muscle cell growth: it stimulates proliferation and inhibits apoptosis. The growth of intestinal smooth muscle cells is regulated jointly by the net effect of these two processes.