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Overexpression of the mTOR alpha4 phosphoprotein activates protein phosphatase 2A and increases Stat1alpha binding to

Wei Lun Nien1, Shauna M Dauphinee, Lori D Moffat

  • 1Department of Biochemistry & Molecular Biology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia B3H 1X5, Canada.

Molecular and Cellular Endocrinology
|November 7, 2006
PubMed

Insights

Alpha4 phosphoprotein enhances protein phosphatase 2A (PP2A) activity by increasing its carboxymethylation. This leads to decreased Stat1alpha methylation and increased Stat1alpha-PIAS1 association, ultimately inhibiting prolactin-induced IRF-1 promoter activity.

Area of Science:

  • Cellular signaling pathways
  • Protein-protein interactions
  • Gene regulation

Background:

  • Alpha4 phosphoprotein is a prolactin (PRL)-downregulated gene product.
  • Alpha4 interacts with protein phosphatase 2A catalytic subunit (PP2Ac).
  • Alpha4 overexpression inhibits PRL-inducible IRF-1 promoter activity via an unknown mechanism.

Purpose of the Study:

  • To elucidate the mechanism by which alpha4 inhibits PRL-inducible IRF-1 promoter activity.
  • To investigate the interaction between alpha4 and PP2Ac.
  • To determine the effect of alpha4 on PP2A activity and downstream signaling.

Main Methods:

  • Stable alpha4-PP2Ac complex formation was assessed in COS-1 cells.
  • PP2Ac carboxymethylation and PP2A activity were measured following alpha4 overexpression.
  • Phosphorylation of eukaryotic initiation factor 4E-binding protein (4E-BP1) and Stat was analyzed.
  • Arginine methylation of Stat1alpha and its association with PIAS1 were evaluated.

Main Results:

  • A stable alpha4-PP2Ac complex was observed, unaffected by rapamycin.
  • Alpha4 overexpression increased PP2Ac carboxymethylation and PP2A activity.
  • Increased PP2A activity correlated with decreased 4E-BP1 phosphorylation.
  • Alpha4 overexpression led to Stat1alpha hypomethylation and increased Stat1alpha-PIAS1 binding.

Conclusions:

  • Ectopic alpha4 enhances PP2A activity in COS-1 cells.
  • Alpha4-mediated PP2A activation results in Stat1alpha hypomethylation and increased association with PIAS1.
  • These molecular events contribute to the inhibition of Stat-mediated transcription and PRL-inducible IRF-1 promoter activity.

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