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Articles in PresS, published online ahead of print November 20, 2002
Am J Physiol Gastrointest Liver Physiol, 10.1152/ajpgi.00310.2002
Submitted on July 29, 2002
Accepted on November 19, 2002
1 Departments of Medicine and Physiology, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, VA, USA
* To whom correspondence should be addressed. E-mail: jkuemmerle{at}hsc.vcu.edu.
Endogenous Insulin-like growth factor-I (IGF-I) regulates growth of human intestinal smooth muscle cells by jointly activating phosphotidylinositol 3-kinase (PI 3-kinase) 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 PI 3-kinase or Erk1/2 pathway required activation of p70S6 kinase. IGF-I elicited concentration-dependent activation of PI 3-kinase, 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 Thr421/Ser424, Thr389 and Thr229 that paralleled p70S6 kinase activation. p70S6 kinase(Thr421/Ser424) phosphorylation was PI 3-kinase-dependent and PDK-1-independent whereas p70S6 kinase(Thr389) and p70S6 kinase(Thr229) phosphorylation and p70S6 kinase activation were PI 3-kinase-dependent and PDK-1-dependent. IGF-I elicited sequential Akt(Ser308), Akt(Ser473), and mTOR(Ser2448) phosphorylation, however, transfection of muscle cells with kinase-inactive Akt(K179M) showed that these events were not required for IGF-I to activate p70S6 kinase and stimulate proliferation of human intestinal muscle cells.
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