IGF-I elicits growth of human intestinal smooth muscle cells by activation of PI3K, PDK-1, and p70S6 kinase.
Endogenous IGF-I regulates growth of human intestinal smooth muscle cells by jointly activating phosphatidylinositol 3-kinase (PI3K) and ERK1/2. The 70-kDa ribosomal S6 kinase (p70S6 kinase) is a key regulator of cell growth activated by several independently regulated kinases. The present study characterized the role of p70S6 kinase in IGF-I-induced growth of human intestinal smooth muscle cells and identified the mechanisms of p70S6 kinase activation. IGF-I-induced growth elicited via either the PI3K or ERK1/2 pathway required activation of p70S6 kinase. IGF-I elicited concentration-dependent activation of PI3K, 3-phosphoinositide-dependent kinase-1 (PDK-1), and p70S6 kinase that was sequential and followed similar time courses. IGF-I caused time-dependent and concentration-dependent phosphorylation of p70S6 kinase on Thr(421)/Ser(424), Thr(389), and Thr(229) that paralleled p70S6 kinase activation. p70S6 kinase(Thr(421)/Ser(424)) phosphorylation was PI3K dependent and PDK-1 independent, whereas p70S6 kinase(Thr(389)) and p70S6 kinase(Thr(229)) phosphorylation and p70S6 kinase activation were PI3K dependent and PDK-1 dependent. IGF-I elicited sequential Akt(Ser(308)), Akt(Ser(473)), and mammalian target of rapamycin(Ser(2448)) phosphorylation; however, transfection of muscle cells with kinase-inactive Akt1(K179M) showed that these events were not required for IGF-I to activate p70S6 kinase and stimulate proliferation of human intestinal muscle cells.
Kuemmerle
John F
JF
Departments of Medicine and Physiology, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, VA 23298-0711, USA. jkuemmerle@hsc.vcu.edu
D064413:3-Phosphoinositide-Dependent Protein Kinases; D015153:Blotting, Western; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects*; D020536:Enzyme Activators / Q000494:pharmacology*; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D007583:Jejunum / Q000166:cytology / Q000187:drug effects; D007700:Kinetics; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects / Q000254:growth & development*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors*; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis / Q000235:genetics; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D013936:Thymidine / Q000378:metabolism
D064413:3-Phosphoinositide-Dependent Protein Kinases; D020536:Enzyme Activators; 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; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D013936:Thymidine; C494918:AKT1 protein, human