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Tyrosine phosphorylation of sam68 by breast tumor kinase regulates intranuclear localization and cell cycle

Kiven Erique Lukong1, Daniel Larocque, Angela L Tyner

  • 1Terry Fox Molecular Oncology Group and Bloomfield Center for Research on Aging, Lady Davis Institute for Medical Research and Department of Oncology, McGill University, Montreal, Quebec H3T 1E2, Canada.

The Journal of Biological Chemistry
|September 24, 2005
PubMed

Insights

Breast tumor kinase (BRK) phosphorylates the Sam68 protein in breast cancer cells, affecting its location and potentially neutralizing its anti-proliferative effects. This phosphorylation is linked to EGF signaling in tumors.

Area of Science:

  • Oncology
  • Molecular Biology
  • Cell Biology

Background:

  • Breast tumor kinase (BRK) is a tyrosine kinase overexpressed in human breast tumors.
  • BRK promotes tumor growth and potentiates the epidermal growth factor (EGF) response.
  • The interaction between BRK and the RNA-binding protein Sam68, specifically Sam68 phosphorylation, is not fully understood.

Purpose of the Study:

  • To investigate the specific sites of Sam68 phosphorylation by BRK.
  • To elucidate the role of Sam68 phosphorylation in breast cancer cell behavior.
  • To confirm Sam68 as a physiological substrate of BRK.

Main Methods:

  • Generation of phospho-specific antibodies against phosphorylated tyrosine residues in Sam68.
  • Indirect immunofluorescence to observe protein relocalization upon BRK and EGF treatment.
  • Use of small interfering RNA (siRNA) to inhibit BRK expression.
  • Assessment of Sam68's effect on BRK-induced cell proliferation.

Main Results:

  • BRK was shown to phosphorylate all three tyrosine residues in the Sam68 nuclear localization signal.
  • BRK and EGF treatment induced Sam68 phosphorylation and relocalization.
  • Tyrosine 440 was identified as a key site for Sam68 localization modulation, phosphorylated upon EGF treatment.
  • BRK siRNA inhibited Sam68 phosphorylation, confirming Sam68 as a downstream substrate of BRK.
  • Sam68 demonstrated anti-proliferative properties against BRK-induced cell growth.

Conclusions:

  • Sam68 is a physiological substrate of BRK in breast cancer cells, phosphorylated by BRK on its nuclear localization signal tyrosines.
  • Phosphorylation of Sam68, particularly at Tyrosine 440, influences its cellular localization and may be a mechanism by which breast cancer cells overcome Sam68's anti-proliferative effects.
  • Targeting BRK-mediated Sam68 phosphorylation could be a therapeutic strategy for breast cancer.

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