Thyroid hormone receptors bind to an element in the connexin43 promoter

Thyroid hormones stimulate gap junctional communication in rat liver WB-F34 4 epithelial cells, elevating connexin43 mRNA and protein levels. In the pr esent work we analysed connexin43 expression in liver and heart samples fro m thyroid hormone-treated Wistar rats, Connexin43 mRNA was elevated 2.1-fol d in rat liver samples as compared to controls, while there was no change i n heart. Thyroid hormone response elements in the rat connexin43 promoter r egion were examined; a candidate sequence, including a binding site for lig and-dependent transcription factors, was identified at position - 480 to - 464. This putative regulatory element, rCx(-480), contains a direct repeat structure separated by three base pairs (DR3-type element). In electrophore tic mobility shift assays using in vitro translated proteins, the rCX(-480) element formed stronger complexes with thyroid hormone receptor alpha /ret inoid X receptor alpha heterodimers than with vitamin D receptor/retinoid X receptor or heterodimers. In transfected Cos-7 cells, promoter activation was observed via this element after treatment with 3,3',5-tri-iodo-L-thyron ine. Loss of binding was seen when the 3' half-site or the spacer region of the rCx(-480) element were experimentally mutated, while a stronger bindin g was observed with mutations introduced in the 5' half-site.