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Modulation of testicular receptor 4 activity by mitogen-activated protein kinase-mediated phosphorylation

M D Mostaqul Huq1, Pawan Gupta, Nien-Pei Tsai

  • 1Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota 55455, USA.

Molecular & Cellular Proteomics : MCP
|August 5, 2006
PubMed

Insights

Testicular receptor 4 (TR4) activity is regulated by phosphorylation, not ligands. Mitogen-activated protein kinase (MAPK) phosphorylation of TR4

Area of Science:

  • Molecular Endocrinology
  • Nuclear Receptor Signaling
  • Signal Transduction

Background:

  • Testicular receptor 4 (TR4) is an orphan nuclear receptor with demonstrated functions.
  • The mechanism regulating TR4 activity in the absence of specific ligands remains unknown.

Purpose of the Study:

  • To investigate the ligand-independent regulation of TR4 biological activity.
  • To identify the role of post-translational modifications, specifically phosphorylation, in TR4 function.

Main Methods:

  • Mass spectrometry-based proteome analysis to identify TR4 phosphorylation sites.
  • Site-directed mutagenesis to assess the functional significance of phosphorylation.
  • Recruitment assays for corepressors and coactivators.
  • Validation using wild-type and mutant TR4 in target gene regulation studies.

Main Results:

  • Identified three MAPK-mediated phosphorylation sites on TR4's AF-1 domain: Ser(19), Ser(55), and Ser(68).
  • Phosphorylation at Ser(19) and Ser(68) is functionally significant.
  • MAPK-mediated phosphorylation converts TR4 into a repressor by recruiting RIP140.
  • Dephosphorylation converts TR4 into an activator by recruiting PCAF.
  • TR4 mutants mimicking constitutive phosphorylation or dephosphorylation demonstrated altered apoE gene regulation.

Conclusions:

  • TR4 activity is regulated by a ligand-independent mechanism involving MAPK-mediated phosphorylation of its AF-1 domain.
  • Phosphorylation status dictates TR4's interaction with corepressors and coactivators, thereby controlling its transcriptional activity.
  • This provides novel insights into the regulation of orphan nuclear receptors.

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