Regulation of the growth hormone gene by fish thyroid retinoid receptors

Transcription of growth hormone (GH) genes in vertebrates depends, among ot her factors, on triiodothyronine (T-3) and retinoic acid (RA), mediated by nuclear receptors. In vitro studies of carp (Cyprinus carpio) pituitary cel ls have shown that addition of T-3 and RA increase the steady state levels of the GH messenger RNA. In this study various isomers of fish thyroid hormone and retinoic acid rec eptor genes were tested in carp pituitary primary cell culture for transcri ption stimulation of Atlantic salmon (Salmo salar) GH promoter and of a gen eral thyroid hormone-dependent promoter (4TREpTK). Zebrafish (Danio rerio) thyroid hormone receptor (TR) and retinoic acid receptor (RAR), in the pres ence of their ligands, T-3 and RA respectively, increase synergistically ex pression of the reporter gene regulated by GH promoter, while only TR (and T-3) is required for maximal stimulation from the 4TREpTK promoter. RA is e ssential for RAR mediated transcription from the GH promoter, while transcr iption stimulation by RAR from 4TREpTK promoter is ligand independent. Over expression of the pituitary specific Japanese flounder (fl)TR beta isomer a lone, or with either of the RAR isomers is more stimulatory than the TR alp ha isomer in transcription from the pituitary specific endoTRE promoter ele ment, while there is no preference among the TR or RAR isomers in stimulati ng transcription from the general consensus 4TREp. Furthermore, the recombi nant TR alpha and TR alpha/RAR alpha/gamma heterodimers are shown to bind s pecifically to DNA elements. TR alpha binds specifically to endoTRE from At lantic salmon GH promoter and to the consensus 4TREp. The zebrafish TR alph a and RAR(alpha/gamma) preferentially heterodimerize on the consensus 4TREp , and the endoTRE successfully compete the complex formation. It would seem that the specific sequence of the promoter thyroid response elements contr ols the receptor isomer specificity, and the regulation of transcription by their ligands, thyroid hormone and RA.