Transactivation activity of thyroid hormone receptors in fish (Conger myriaster) in response to thyroid hormones

We previously isolated cDNAs encoding conger eel (Conger myriaster) thyroid hormone (TH) receptors (TRs). In the present study, we investigated the transactivation activities of conger eel TRs treated with THs (3,3′,5-triiodo-l-thyronine [T3], l-thyroxine [T4], and 3,3′,5′-triiodo-l-thyronine [rT3]), or ligands and activators of other nuclear receptors. Following transient transfection into the Japanese eel (Anguilla japonica) hepatocyte cell line Hepa-E1, the conger eel TRs (cTRs) showed TH-dependent activation of transcription from a TH-responsive promoter. However, no transactivation activity of any of the four cTRs was observed with ligands or activators of other nuclear receptors. Although T3 activation for cTRs is stronger than other THs (T3 > T4 > rT3), the transactivation sensitivity was different from the activity of cTRs with THs, respectively. Therefore, we conclude that cTRs can act in concert with THs in fish metamorphosis.