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.
Kuemmerle
John F
JF
Division of Gastroenterology, Medical College of Virginia Campus, Virginia Commonwealth University, PO Box 980711, Richmond, VA 23298-0711, USA. jkuemmerle@hsc.vcu.edu
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