Transcriptional regulation of fish growth hormone gene

Transcription of growth hormone (GH) genes in vertebrates depends, among other factors, on pituitary specific transcription factors (such as Pit-1) and on thyroid hormone (TH) and TH receptors. Gel retardation experiments indicated the presence of proteins in the carp pituitary nuclear extract that specifically bind the Pit-1 DNA response element-and the adjacent putative TH DNA response element (TRE), on the Atlantic salmon (Salmo salar) GH proximal promoter. The putative TRE element competes effectively in vitro in gel retardation assays with 4xTREpal, a consensus TRE element, on zebrafish (Danio rerio) purified recombinant TH receptor (zfTHR). The zfTHR/TRE complex can also be further identified with antibodies to rat THR that causes the complex a supershift in gel retardation assay. Cross competition was observed in nuclear extracts between the TRE and Pit-1 sites. It was found that 100 molar fold excess of Pit-1 binding element can compete effectively with the TH binding element and reduce its binding activity. The endogenous TRE binding element, on the other hand, cannot effectively reduce the binding activity of the Pit-1 DNA binding element. This unexpected competition between the two sites was examined in the presence and absence of the hormones: TH, 9-cis retinal, and TH + 9-cis retinal. The addition of the two hormones together significantly increased the binding properties of the TH response element as compared to its binding in the absence of the hormones, and also improved its binding activity in the presence of the Pit-1 DNA competitor. In summary, the two adjacent sites that control GH gene transcription, Pit-1 and TRE, on the fish GH promoter exhibit some degree of interaction between their respective transcription factors, the Pit-1 protein and the thyroid/retinoic acid receptors.