Thyroid hormone receptors bind to an element in the connexin43 promoter

Thyroid hormones stimulate gap junctional communication in rat liver WB-F344 epithelial cells, elevating connexin43 mRNA and protein levels. In the present work we analysed connexin43 expression in liver and heart samples from thyroid hormone-treated Wistar rats. Connexin43 mRNA was elevated 2.1-fold in rat liver samples as compared to controls, while there was no change in heart. Thyroid hormone response elements in the rat connexin43 promoter region were examined; a candidate sequence, including a binding site for ligand-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 electrophoretic mobility shift assays using in vitro translated proteins, the rCx-480 element formed stronger complexes with thyroid hormone receptor alpha/retinoid X receptor alpha heterodimers than with vitamin D receptor/retinoid X receptor alpha heterodimers. In transfected Cos-7 cells, promoter activation was observed via this element after treatment with 3,3',5-triiodo-L-thyronine. 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 binding was observed with mutations introduced in the 5' half-site.