The orphan nuclear receptor Ear-2 is a negative coregulator for thyroid hormone nuclear receptor function

Area of Science:

• Molecular Biology
• Endocrinology
• Genetics

Background:

• Thyroid hormone receptors (TR) are crucial ligand-dependent transcription factors regulating vital cellular processes.
• TR's diverse effects are hypothesized to be mediated by a complex network of coregulators.
• TR-mediated transcriptional responses exhibit cell-type-specific variations.

Purpose of the Study:

• To identify novel coregulators of human TR subtype beta1 (TRbeta1) using a yeast two-hybrid system.
• To investigate the functional interaction between TRbeta1 and identified coregulators.
• To elucidate the role of coregulators in TR-mediated transcriptional regulation.

Main Methods:

• Yeast two-hybrid screening using TRbeta1 as bait in RKO colon carcinoma cells.
• Co-immunoprecipitation assays to confirm physical interactions between TRbeta1 and Ear-2.
• Reporter gene assays to assess the impact of Ear-2 on TRbeta1-mediated transcription.

Main Results:

• Ear-2, a member of the orphan nuclear receptor family, was identified as a TRbeta1-interacting protein.
• Physical interaction between Ear-2 and TRbeta1 was confirmed in vitro and in cells.
• Ear-2 inhibited TRbeta1 binding to T3 response elements and repressed both basal and T3-dependent TRbeta1 transcriptional activity.
• Ear-2's repressive effect was cell-type-dependent and reversible by coactivators.

Conclusions:

• Ear-2 acts as a TRbeta1 corepressor, modulating TR-mediated gene expression.
• TR-mediated responses are regulated by a balance between corepressors and coactivators.
• These findings support the hypothesis that TR's diverse functions are orchestrated by an extensive coregulator network, including Ear-2.