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The architectural organization of the genome and regulatory proteins within the nucleus supports gene expression in a physiologically regulated manner. In osteoblastic cells ligand activation induces a nuclear punctate distribution of the 1alpha,25-dihydroxy vitamin D3 (1alpha,25(OH)2D3) receptor (VDR) and promotes its interaction with transcriptional coactivators such as SRC-1, NCoA-62/Skip, and DRIP205. Here, we discuss evidence demonstrating that in osteoblastic cells VDR binds to the nuclear matrix fraction in a 1alpha,25(OH)2D3-dependent manner. This interaction occurs rapidly after exposure to 1alpha,25(OH)2D3 and does not require a functional VDR DNA binding domain. The nuclear matrix-bound VDR molecules colocalize with the also nuclear matrix-associated coactivator DRIP205. We propose a model where the rapid association of VDR with the nuclear matrix fraction represents an event that follows 1alpha,25(OH)2D3-dependent nuclear localization of VDR, but that precedes 1alpha,25(OH)2D3-dependent transcriptional upregulation at target genes., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

Vitamin D is a principal modulator of skeletal gene expression, thus necessitating an understanding of interfaces between the activity of this steroid hormone and regulatory cascades that are functionally linked to the regulation of skeletal genes. Physiologic responsiveness requires combinatorial control, whereas co-regulatory proteins determine the specificity of biologic responsiveness to physiologic cues. It is becoming increasingly evident that regulatory complexes containing the vitamin D receptor are dynamic rather than static. Temporal and spatial modifications in the composition of these complexes provide a mechanism for integrating regulatory signals to support positive or negative control through synergism and antagonism. Compartmentalization of components of vitamin D control in nuclear microenvironments supports the integration of regulatory activities, perhaps by establishing thresholds for protein activity in time frames that are consistent with the execution of regulatory signaling.

Vitamin D serves as a principal modulator of skeletal gene transcription, thus necessitating an understanding of interfaces between the activity of this steroid hormone and regulatory cascades that are functionally linked to the regulation of skeletal genes. Physiological responsiveness requires combinatorial control where coregulatory proteins determine the specificity of biological responsiveness to physiological cues. It is becoming increasingly evident that the regulatory complexes containing the vitamin D receptor are dynamic rather than static. Temporal and spatial modifications in the composition of these complexes provide a mechanism for integrating regulatory signals to support positive or negative control through synergism and antagonism. Compartmentalization of components of vitamin D control in nuclear microenvironments supports the integration of regulatory activities, perhaps by establishing thresholds for protein activity in time frames that are consistent with the execution of regulatory signaling.

1α,25-dihydroxy vitamin D3 (vitamin D3) has an important role during osteoblast differentiation as it directly modulates the expression of key bone-related genes. Vitamin D3 binds to the vitamin D3 receptor (VDR), a member of the superfamily of nuclear receptors, which in turn interacts with transcriptional activators to target this regulatory complex to specific sequence elements within gene promoters. Increasing evidence demonstrates that the architectural organization of the genome and regulatory proteins within the eukaryotic nucleus support gene expression in a physiological manner. Previous reports indicated that the VDR exhibits a punctate nuclear distribution that is significantly enhanced in cells grown in the presence of vitamin D3. Here, we demonstrate that in osteoblastic cells, the VDR binds to the nuclear matrix in a vitamin D3-dependent manner. This interaction of VDR with the nuclear matrix occurs rapidly after vitamin D3 addition and does not require a functional VDR DNA-binding domain. Importantly, nuclear matrix-bound VDR colocalizes with its transcriptional coactivator DRIP205/TRAP220/MED1 which is also matrix bound. Together these results indicate that after ligand stimulation the VDR rapidly enters the nucleus and associates with the nuclear matrix preceding vitamin D3-transcriptional upregulation. J. Cell. Physiol. 222: 336–346, 2010. © 2009 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

Bone-specific transcription of the osteocalcin (OC) gene is regulated principally by the Runx2 transcription factor and is further stimulated in response to 1α,25-dihydroxyvitamin D3 via its specific receptor (VDR). The rat OC gene promoter contains three recognition sites for Runx2 (sites A, B, and C). Mutation of sites A and B, which flank the 1α,25-dihydroxyvitamin D3-responsive element (VDRE), abolishes 1α,25-dihydroxyvitamin D3-dependent enhancement of OC transcription, indicating a tight functional relationship between the VDR and Runx2 factors. In contrast to most of the members of the nuclear receptor family, VDR possesses a very short N-terminal A/B domain, which has led to the suggestion that its N-terminal region does not contribute to transcriptional enhancement. Here, we have combined transient-overexpression, coimmunoprecipitation, in situ colocalization, chromatin immunoprecipitation, and glutathione S-transferase pull-down analyses to demonstrate that in osteoblastic cells expressing OC, VDR interacts directly with Runx2 bound to site B, which is located immediately adjacent to the VDRE. This interaction contributes significantly to 1α,25-dihydroxyvitamin D3-dependent enhancement of the OC promoter and requires a region located C terminal to the runt homology DNA binding domain of Runx2 and the N-terminal region of VDR. Together, our results indicate that Runx2 plays a key role in the 1α,25-dihydroxyvitamin D3-dependent stimulation of the OC promoter in osteoblastic cells by further stabilizing the interaction of the VDR with the VDRE. These studies demonstrate a novel mechanism for combinatorial control of bone tissue-specific gene expression. This mechanism involves the intersection of two major pathways: Runx2, a "master" transcriptional regulator of osteoblast differentiation, and 1α,25-dihydroxyvitamin D3, a hormone that promotes expression of genes associated with these terminally differentiated bone cells. [ABSTRACT FROM AUTHOR]

Bone-specific transcription of the osteocalcin (OC) gene is principally regulated by the Runx2 transcription factor and further stimulated in response to 1α,25-dihydroxy Vitamin D3 via its specific receptor (VDR). The rat OC gene promoter contains three recognition sites for Runx2 (sites A–C). Mutation of sites A and B, which flank the 1α,25-dihydroxy Vitamin D3-responsive element (VDRE), abolishes 1α,25-dihydroxy Vitamin D3-dependent enhancement of OC transcription, indicating a tight functional relationship between VDR and Runx2 factors. Additionally, the transcriptional co-activator p300 is recruited to the OC promoter by Runx2 where it up-regulates both basal and 1α,25-dihydroxy Vitamin D3-enhanced OC expression. Here, we present an overview of how in osteoblastic cells expressing OC, Runx2 modulates the 1α,25-dihydroxy Vitamin D3-dependent stimulation of the OC promoter by first recruiting transcriptional co-activators and then by further stabilizing the interaction of the VDR with the VDRE. [Copyright &y& Elsevier]