Endogenous IGF-I protects human intestinal smooth muscle cells from apoptosis by regulation of GSK-3 beta 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-3 beta (GSK-3 beta) 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-3 beta phosphorylation and activity and whether it regulates apoptosis in human intestinal muscle cells. IGF-I elicited time- and concentration-dependent GSK-3 beta phosphorylation (inactivation) that was measured by Western blot analysis using a phospho-specific GSK-3beta antibody. Endogenous IGF-I stimulated GSK-3 beta phosphorylation and inhibited GSK-3 beta activity (measured by in vitro kinase assay) in these cells. IGF-I-dependent GSK-3 beta phosphorylation and the resulting GSK-3 beta 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-3 beta 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-3 beta 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.
Division of Gastroenterology, Medical College of Virginia Campus, Virginia Commonwealth University, PO Box 980711, Richmond, VA 23298-0711, USA. jkuemmerle@hsc.vcu.edu
John F
JF
Kuemmerle
D064413:3-Phosphoinositide-Dependent Protein Kinases; D017209:Apoptosis / Q000187:drug effects* / Q000235:genetics*; D015153:Blotting, Western; D018929:Cell Culture Techniques; D049109:Cell Proliferation; D003598:Cytoskeletal Proteins / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007583:Jejunum / Q000166:cytology / Q000502:physiology*; D009130:Muscle, Smooth / Q000166:cytology / Q000502:physiology*; D019869:Phosphatidylinositol 3-Kinases / Q000494:pharmacology; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000494:pharmacology; D011518:Proto-Oncogene Proteins / Q000494:pharmacology; D051057:Proto-Oncogene Proteins c-akt; D015534:Trans-Activators / Q000378:metabolism; D051176:beta Catenin
D064413:3-Phosphoinositide-Dependent Protein Kinases; D003598:Cytoskeletal Proteins; D038362:Glycogen Synthase Kinase 3; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D015534:Trans-Activators; D051176:beta Catenin; C495270:CTNNB1 protein, human; C494918:AKT1 protein, human; C000605506:GSK3B protein, human