IGF-I elicits growth of human intestinal smooth muscle cells by activation of PI3K, PDK-1, and p70S6 kinase

Am J Physiol Gastrointest Liver Physiol. 2003 Mar;284(3):G411-22. doi: 10.1152/ajpgi.00310.2002. Epub 2002 Nov 20.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 3-Phosphoinositide-Dependent Protein Kinases
  • Blotting, Western
  • Cell Division / drug effects
  • Cells, Cultured
  • Enzyme Activation / drug effects*
  • Enzyme Activators / pharmacology*
  • Humans
  • Insulin-Like Growth Factor I / pharmacology*
  • Jejunum / cytology
  • Jejunum / drug effects
  • Kinetics
  • Muscle, Smooth / cytology
  • Muscle, Smooth / drug effects
  • Muscle, Smooth / growth & development*
  • Phosphatidylinositol 3-Kinases / metabolism*
  • Phosphorylation
  • Protein-Serine-Threonine Kinases / antagonists & inhibitors*
  • Proto-Oncogene Proteins / biosynthesis
  • Proto-Oncogene Proteins / genetics
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism*
  • Signal Transduction / physiology
  • Thymidine / metabolism


  • Enzyme Activators
  • Proto-Oncogene Proteins
  • Insulin-Like Growth Factor I
  • Phosphatidylinositol 3-Kinases
  • 3-Phosphoinositide-Dependent Protein Kinases
  • AKT1 protein, human
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases, 70-kDa
  • Thymidine