[Role of beta thyroid hormone receptor in the modulation of the TRH transcription]

Two genes coding for thyroid hormone (T3) receptors (THR) have been identified: THR alpha and THR beta. To determine their transcriptional effects we transfected reporter genes expressing chloramphenicol acetyl transferase (CAT) alone or with vectors coding for THR alpha or THR beta in primary cultures of embryonic chick hypothalami. Transcription from a promotor containing a T3 Response Element (T3RE-CAT), was activated in the presence of tri-iodo thyronine (T3) and much reduced in the absence of the hormone. In contrast, in the absence of T3, transcription from the Thyrotropin releasing hormone (TRH) promoter (TRH-CAT) was much greater than in cultures grown with T3. In cotransfection experiments where THR alpha or THR beta was expressed with the TRH promoter construct, THR beta-dependent transcription, but not THR alpha-dependent transcription, was significantly reduced in the presence of T3.