Y. Berghoferhochheimer et al., EXPRESSION OF THE VITAMIN-D AND THE RETINOID-X-RECEPTORS IN SACCHAROMYCES-CEREVISIAE - ALTERNATIVE IN-VIVO MODELS FOR LIGAND-INDUCED TRANSACTIVATION, Journal of cellular biochemistry, 66(2), 1997, pp. 184-196
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.