Ligand-triggered stabilization of vitamin D receptor/retinoid X receptor heterodimer conformations on DR4-type response elements

Nuclear receptors integrate an incoming signal in the form of a nuclear hormone by undergoing a conformational change that results via co-activator proteins in an activation of the basal transcriptional machinery. The vitamin D-3 receptor is the nuclear receptor for 1 alpha,25-dihydroxyvitamin D-3 (1 alpha,25(OH)(2)D-3) and is known to function as a heterodimer with the retinoid X receptor on DR3-type 1 alpha,25(OH)(2)D-3 response elements. Here, it could be demonstrated that DR4-type response elements are at least as effective as DR3-type 1 alpha,25(OH)(2)D-3 response elements. Gel shift dipping analysis showed that vitamin D-3 receptor-retinoid X receptor heterodimers form in response to 1 alpha,25(OH)(2)D-3 and retinoid X receptor ligands, the pan-agonist 9-cis retinoic acid (9cRA) and the retinoid X receptor-selective retinoid CD2425, different conformations on the DR4-type element of the rat Pit-1 gene. Interestingly, on this response element the heterodimeric complexes of retinoid X receptor with the thyroid hormone receptor, the retinoic acid receptor and the benzoate ester receptor also displayed characteristic individual ligand-dependent complex formation. On the level of complex formation, utilizing DNA affinity and functional assays, only vitamin D-3 receptor-retinoid X receptor heterodimers showed a synergistic interaction of both ligands. However, the sensitivity of vitamin D-3 receptor-retinoid X receptor heterodimers to 1 alpha,25(OH)(2)D-3 was found to be much higher than to retinoid X receptor Ligands. Taken together, this study demonstrates a unique interaction potential of vitamin D-3 receptor and retinoid X receptor but also establishes DR4-type response elements as multi-functional DNA binding sites with a potential to integrate various hormone signalling pathways. (C) 2000 Academic Press.