Retinoid X receptor isotype identity directs human vitamin D receptor heterodimer transactivation from the 24-hydroxylase vitamin D response elements in yeast

Unlike estrogen and progesterone receptors that operate as homodimers on response elements, retinoid X receptors (RXRs) and vitamin D receptors (VDRs) can function as heterodimers. Studies concerning the significance of heterodimeric partnerships are usually performed utilizing mammalian or insect cells. These cells express endogenous nuclear receptors, making it impossible to assign a role for one receptor subtype over another while studying the function of transfected receptor(s). Yeast lacks endogenous VDRs and RXRs and their ligands and provides a unique cellular context to study nuclear receptor function. We examined the interaction between human VDR and human RXR alpha, mouse RXR beta(2), and mouse RXR gamma to identify physiologically important receptor interactions. DNA binding studies on consensus, osteocalcin, or the rat 24-hydroxylase vitamin D response elements (VDREs) indicated that although RXR complexes can form on the consensus DNA elements, RXR:VDR heterodimers preferentially interact with the natural VDREs. The interaction is RXR isotype-specific and affected by ligands. Transactivation studies using the rat 24-hydroxylase VDREs indicated that VDR preferentially associated with RXR alpha or RXR gamma to stimulate transcription, and the activity was potentiated by ligand. Although RXR beta(2):VDR bound tightly to DNA, the resulting heterodimer transactivated poorly. The regulation of the 24-hydroxylase promoter observed in yeast is similar with respect to transactivation potential of specific VDRE and fold activation observed in osteosarcoma cells. Ligand binding to both receptors in a RXR:VDR complex is required for maximal transcriptional activity, indicating that the isotype-specific RXR partner significantly contributes to the ability of RXR:VDR heterodimers to transactivate from target response elements in yeast.