Retinoid X Receptor:Vitamin D₃ Receptor Heterodimers Promote Stable Preinitiation Complex Formation and Direct 1,25-Dihydroxyvitamin D₃-Dependent Cell-Free Transcription

Abstract

The numerous members of the steroid/nuclear hormone receptor superfamily act as direct transducers of circulating signals, such as steroids, thyroid hormone, and vitamin or lipid metabolites, and modulate the transcription of specific target genes, primarily as dimeric complexes. The receptors for 9-cis retinoic acid and 1,25-dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ], RXR and VDR, respectively, as members of this superfamily, form a heterodimeric complex and bind cooperatively to vitamin D responsive elements (VDREs) to activate or repress the transcription of a multitude of genes which regulate a variety of physiological functions. To directly investigate RXR-and VDR-mediated transactivation, we developed a cell-free transcription system for 1,25(OH) 2 D 3 signaling by utilizing crude nuclear extracts and a G-free cassette-based assay. Transcriptional enhancement in vitro was dependent on purified, exogenous RXR and VDR and was responsive to physiological concentrations of 1,25(OH) 2 D 3 . We found that RXR and VDR transactivated selectively from VDRE-linked templates exclusively as a heterodimeric complex, since neither receptor alone enhanced transcription in vitro. By the addition of low concentrations of the anionic detergent Sarkosyl to limit cell-free transcription to a single round and the use of agarose gel mobility shift experiments to assay factor complex assembly, we observed that 1,25(OH) 2 D 3 enhanced RXR:VDR-mediated stabilization or assembly of preinitiation complexes to effect transcriptional enhancement from VDRE-linked promoter-containing DNA.The coordinate expression of particular genes within a cell determines its developmental fate, and specific responses to external stimuli enable it to adapt to rapid changes in its environment. Part of this orchestrated response is the regulated transcription of genes which induce or maintain a differentiated state in a cell-type-or stage-specific manner. Many transactivating and transrepressing factors are employed by a cell to modulate the transcription of these genes by the basal transcriptional machinery. Regulation by such factors occurs through DNA binding, protein-protein interactions, and subsequent stimulation or inhibition of transcriptionally competent preinitiation complexes (PICs). In the traditional view, PIC formation involves an ordered sequential assembly of general transcription factors (GTFs) (TFIID [the TATA-binding protein {TBP} and its associated factors], TFIIA, TFIIB, RNA polymerase II [Pol II], TFIIF, TFIIE, and TFIIH) at core promoter sequences (reviewed in references 19, 34, 84, 110). In regulated transcription, the extent of assembly of PICs into productive complexes able to initiate and elongate mRNA synthesis is stimulated by multiple activators through specific interactions at rate-limiting steps with various components or associated cofactors of this machinery (18,23,31,55,56,64,80,94,104,105). Recently, a simpler view of transactivation has been proposed; it involves activator recruitment or stimulation of TFIID, TFIIE, and ...

The Nuclear Vitamin D Receptor: Biological and Molecular Regulatory Properties Revealed

“…4 for simplicity) likely targets the receptor for ubiquitination and subsequent proteolysis by the 26S proteasome complex. (185)(186)(187) The availability of a cell-free transcription system in which VDR-RXR heterodimers activate transcription in a 1,25(OH) 2 D 3 -dependent fashion (188) should facilitate studies of transcriptional control by VDR, such as demonstrating direct effects by coactivators, promoter-specific factors, or corepressors. Ultimately, with such cell-free transcription systems it may also be feasible to incorporate elements of chromatin structure such as the aforementioned histones, in order to develop a system that truly mimics VDR action in vivo while at the same time permitting manipulations, in vitro.…”

20-Epi Analogues of 1,25-Dihydroxyvitamin D3 Are Highly Potent Inducers of DRIP Coactivator Complex Binding to the Vitamin D3 Receptor

“…The volume of receptor-ligand mix was 3 l instead of the 10 l used in previous assays (23), and the volume of nuclear extract was increased to 15 l from 8 l so that the final reaction volume was 25 l, as described previously.…”

Vitamin D Analogue-Specific Recruitment of Vitamin D Receptor Coactivators

“…It has been proposed that vitamin D analogues exert their differential effects by regulating a number of steps in VDR function, including ligand-VDR binding, interaction with vitamin D responsive elements (VDREs), and VDR homodimerization or heterodimerization with the retinoid X receptor (RXR). (1)(2)(3)(4)(5) The discovery of a family of p160 coactivators that regulate nuclear hormone receptor (NHR) transcriptional function has added another level of complexity to this model. Selective recruitment of transcriptional coactivators to target genes may underlie differential effects of vitamin D analogues.…”