EXPRESSION OF THE VITAMIN-D AND THE RETINOID-X-RECEPTORS IN SACCHAROMYCES-CEREVISIAE - ALTERNATIVE IN-VIVO MODELS FOR LIGAND-INDUCED TRANSACTIVATION

The transcription factors of the nuclear hormone receptor familiy regu late gene expression via a complex network of macromolecular interacti ons. The ligand dependent activity of the vitamin D receptor is of par ticular interest because it modulates gene expression by the heterodim eric interaction with retinoid X receptors. We report here that indivi dual functions of the vitamin D receptor including DNA-binding, homo- and heterodimerization and transactivation can be reconstituted in the yeast Saccharomyces cerevisiae. Interestingly, the simultaneous expre ssion of the native vitamin D receptor and the retinoid X receptor bet a resulted in a ligand independent transactivation of the lacZ reporte r gene coupled to a mouse osteopontin vitamin D response element. Howe ver, homodimerization of the vitamin D receptor and heterodimerization were strongly enhanced upon ligand binding, when the receptors were e xpressed as fusion proteins with the Gal4 transcription factor in a ye ast two-hybrid system. Furthermore, transactivating activity of a Gal4 -fused vitamin D receptor was induced by vitamin D in a one-hybrid sys tem devoid of retinoid X receptors. In addition, both Gal4-based syste ms behaved similar With regard to their dose-dependent response to vit amin D and related compounds when compared to the transcriptional acti vity of the vitamin D receptor in transiently transfected MCF-7 cells. Our results point out that specific ligands strongly enhanced recepto r dimerization and induced transactivation in yeast and in MCF-7 cells . The constitutive transactivation by vitamin D receptor-retinoid X re ceptor heterodimers in yeast, depending on DNA binding of the receptor s, strongly argues for the existence of cofactors, which are absent in yeast, but play a fundamental role in gene regulation in higher eukar yotic organisms. (C) 1997 Wiley-Liss, Inc.