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Dual effects of Ral-activated pathways on p27 localization and TGF-β signaling

Keren Tazat1, Meirav Harsat, Ayelet Goldshmid-Shagal

  • 1Department of Neurobiology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel.

Molecular Biology of the Cell
|April 12, 2013
PubMed

Insights

Ras signaling pathways promote cancer by mislocalizing p27(Kip1) (p27). Activated Ral effectors RalBP1 and phospholipase D1 (PLD1) oppositely regulate p27 localization, impacting epithelial cell growth arrest.

Area of Science:

  • Cell Biology
  • Molecular Oncology
  • Signal Transduction

Background:

  • Constitutive activation of Ras signaling pathways is implicated in epithelial tumorigenesis.
  • Cytoplasmic mislocalization of the cyclin-dependent kinase inhibitor p27(Kip1) (p27) is one mechanism by which Ras signaling contributes to cancer.
  • Previous studies demonstrated that oncogenic N-Ras can activate the Ral-GEF pathway, leading to p27 mislocalization, but the downstream mechanisms remained unclear.

Purpose of the Study:

  • To elucidate the mechanisms by which activated Ral signaling pathways regulate the cellular localization of p27.
  • To investigate the opposing roles of Ral effectors RalBP1 and phospholipase D1 (PLD1) in p27 mislocalization.
  • To determine the physiological relevance of Ral-mediated p27 localization in controlling epithelial cell growth.

Main Methods:

  • Utilized specific RalA and p27 mutants to dissect signaling pathways.
  • Employed short hairpin RNA (shRNA)-mediated knockdown to inhibit Ral downstream signaling components.
  • Investigated the role of p27 Ser-10 phosphorylation by protein kinase B/Akt in p27 mislocalization.
  • Assessed the impact of Ral pathway activation on transforming growth factor-β (TGF-β)-mediated growth arrest in epithelial cells.

Main Results:

  • Activation of RalBP1 by RalA leads to cytoplasmic accumulation of p27, dependent on p27 Ser-10 phosphorylation by Akt.
  • Activation of PLD1 by RalA counteracts p27 cytoplasmic accumulation in a Ser-10-independent manner.
  • Ral-mediated activation of the RalBP1 pathway abrogates TGF-β-induced growth arrest in epithelial cells, highlighting its physiological significance.

Conclusions:

  • Ral signaling pathways exert dual regulation on p27 cellular localization through opposing actions of RalBP1 and PLD1.
  • The RalBP1 pathway, via Akt-mediated phosphorylation of p27, promotes p27 cytoplasmic mislocalization and antagonizes growth arrest signals.
  • These findings reveal a novel mechanism linking Ras/Ral signaling to cell cycle control and tumorigenesis, with potential therapeutic implications.

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