Interaction of the 1α,25-dihydroxyvitamin D3 receptor at the distal promoter region of the bone-specific osteocalcin gene requires nucleosomal remodelling

Abstract

1alpha,25-Dihydroxyvitamin D3-mediated transcriptional control of the bone-specific osteocalcin (OC) gene requires the integration of regulatory signals at the vitamin D-responsive element (VDRE) and flanking tissue-specific sequences. The 1alpha,25-dihydroxyvitamin D3 receptor (VDR) is a member of the nuclear receptor superfamily and forms a heterodimeric complex with the receptor for 9-cis retinoic acid (RXR) that binds to the VDRE sequence. We have demonstrated previously that changes in chromatin structure at the VDRE region of the rat OC gene promoter accompany transcriptional enhancement in vivo, suggesting a requirement for chromatin remodelling. Here we show that the VDRE in the distal region of the OC gene promoter is refractory to binding of the VDR-RXR complex when organized in a nucleosomal context. Addition of the ligand 1alpha,25-dihydroxyvitamin D3 or the presence of other transcription factors, such as YY1 and Runx/Cbfa (core-binding factor alpha), which also bind to sequences partially overlapping or near the VDRE, is not sufficient to render the VDRE accessible. Thus the VDR-RXR, unlike other steroid receptors, such as glucocorticoid receptor, progesterone receptor and thyroid receptor, is unable to bind its target sequence within a nucleosomal context. Taken together these results demonstrate that nucleosomal remodelling is required for in vivo occupancy of binding sites in the distal region of the OC gene promoter by the regulatory factors responsible for 1alpha,25-dihydroxyvitamin D3-dependent enhancement of transcription.

Chromatin Remodeling and Transcriptional Activity of the Bone-specific Osteocalcin Gene Require CCAAT/Enhancer-binding Protein β-dependent Recruitment of SWI/SNF Activity

“…As VDR/RXR is unable to interact with the VDRE in ROSBRG1TA cells expressing mutant BRG1 (Fig. 8A), these results confirm previous reports indicating that a closed chromatin conformation in the OC promoter prevents interaction of the VDR complex upon vitamin D 3 stimulation (24,27).…”

“…This indicates that interaction of both factors with the OC promoter is independent of SWI/SNF activity and therefore prior to the formation of the proximal DNase I-hypersensitive site. In support of this conclusion, we have previously demonstrated that Runx factors are capable of recognizing their cognate elements within the context of a nucleosomal organization (27,39) and independent of the level of core histone acetylation. 4 Together these results indicate that binding of Runx2 and C/EBP␤ is one of the early events during chromatin remodeling and transcriptional activation of the OC gene.…”

An architectural perspective of vitamin D responsiveness

“…The retention of a nucleosome between the proximal and upstream enhancer domains reduces the distance between the basal regulatory elements and the VDRE and supports a promoter configuration that is conducive to proteinprotein interactions between VDR-associated proteins and components of the RNA polymerase II-bound complex ( Figure 2B). Interaction of the VDR at the distal promoter region of the OC gene requires nucleosomal remodeling [40,41].…”

Bone-Specific Transcription Factor Runx2 Interacts with the 1α,25-Dihydroxyvitamin D3 Receptor To Up-Regulate Rat Osteocalcin Gene Expression in Osteoblastic Cells

“…Thus, it has been reported that mutation of Runx2 sites A and B (which flank the VDRE) abolishes 1␣,25-dihydroxyvitamin D 3 enhancement of OC promoter activity (8). It was also shown, both in vitro and in intact osteoblastic cells, that VDR and Runx2 factors interact simultaneously with their cognate sequences located in close proximity within the distal region of the OC promoter (VDRE and Runx2 site B, respectively) (18,24). In addition, we have reported that in intact osteoblastic cells, the transcriptional coactivator p300 is recruited to the OC promoter by Runx2, where it up-regulates both basal and 1␣,25-dihydroxyvitamin D 3 -enhanced OC promoter activity (24).…”