Differential regulation of the phosphatidylinositol 3-kinase/Akt and p70 S6 kinase pathways by the alpha(1A)-adrenergic receptor in rat-1 fibroblasts

J Biol Chem. 2000 Feb 18;275(7):4803-9. doi: 10.1074/jbc.275.7.4803.


Phosphatidylinositol (PI) 3-kinase and its downstream effector Akt are thought to be signaling intermediates that link cell surface receptors to p70 S6 kinase. We examined the effect of a G(q)-coupled receptor on PI 3-kinase/Akt signaling and p70 S6 kinase activation using Rat-1 fibroblasts stably expressing the human alpha(1A)-adrenergic receptor. Treatment of the cells with phenylephrine, a specific alpha(1)-adrenergic receptor agonist, activated p70 S6 kinase but did not activate PI 3-kinase or any of the three known isoforms of Akt. Furthermore, phenylephrine blocked the insulin-like growth factor-I (IGF-I)-induced activation of PI 3-kinase and the phosphorylation and activation of Akt-1. The effect of phenylephrine was not confined to signaling pathways that include insulin receptor substrate-1, as the alpha(1)-adrenergic receptor agonist also inhibited the platelet-derived growth factor-induced activation of PI 3-kinase and Akt-1. Although increasing the intracellular Ca(2+) concentration with the ionophore A23187 inhibited the activation of Akt-1 by IGF-I, Ca(2+) does not appear to play a role in the phenylephrine-mediated inhibition of the PI 3-kinase/Akt pathway. The differential ability of phenylephrine and IGF-I to activate Akt-1 resulted in a differential ability to protect cells from UV-induced apoptosis. These results demonstrate that activation of p70 S6 kinase by the alpha(1A)-adrenergic receptor in Rat-1 fibroblasts occurs in the absence of PI 3-kinase/Akt signaling. Furthermore, this receptor negatively regulates the PI 3-kinase/Akt pathway, resulting in enhanced cell death following apoptotic insult.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis / radiation effects
  • Calcium / metabolism
  • Cell Line
  • Enzyme Activation
  • Fibroblasts / drug effects
  • Fibroblasts / enzymology
  • Fibroblasts / metabolism
  • Humans
  • Insulin-Like Growth Factor I / metabolism
  • Phenylephrine / pharmacology
  • Phosphatidylinositol 3-Kinases / metabolism*
  • Protein-Serine-Threonine Kinases*
  • Proto-Oncogene Proteins / metabolism*
  • Proto-Oncogene Proteins c-akt
  • Rats
  • Receptors, Adrenergic, alpha-1 / metabolism*
  • Ribosomal Protein S6 Kinases / metabolism*
  • Signal Transduction


  • ADRA1A protein, human
  • Adra1a protein, rat
  • Proto-Oncogene Proteins
  • Receptors, Adrenergic, alpha-1
  • Phenylephrine
  • Insulin-Like Growth Factor I
  • Phosphatidylinositol 3-Kinases
  • AKT1 protein, human
  • Akt1 protein, rat
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases
  • Calcium