Insights into the biochemical life cycle of the vitamin D receptor: Protein and DNA interactions that transduce the signal for gene expression

Abstract

The biological actions of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) are mediated by the nuclear vitamin D receptor (VDR), which functions as a ligand-dependent transcriptional regulator. We have developed a six-stage molecular model summarizing the VDR transcriptional activation-life cycle, and tested this model using a variety of experimental approaches, including pull-down assays with GST-fusion proteins, as well as assays of the functional activity of VDR and its putative coactivators in transiently transfected mammalian cells. The six stages of the VDR life cycle are: (1) unoccupied VDR binds to a transcriptional corepressor which serves as a chaperone, maintaining the main protein players in close contact in an inactive complex; (2) VDR becomes occupied by 1,25(OH)₂D₃ ligand, enabling the receptor to heterodimerize strongly with a retinoid X receptor (RXR), leading to high affinity DNA binding and recruitment of coactivators with histone acetyl transferase (HAT) activity; (3) coactivator HAT activity promotes chromatin remodeling, rendering the gene promoter free to interact with the transcription preinitiation complex (PIC); (4) dissociation of VDR from the HAT coactivators, followed by association of a second set of coactivators that promote formation of the preinitiation complex (PIC); (5) transcriptional recycling of the liganded receptor and heteropartner to initiate additional rounds of transcription; and (6) ubiquitination and eventual degradation of VDR. Phosphorylation of VDR may influence all six stages. A testable conclusion from our model is that the role of the 1,25(OH)₂D₃ hormonal ligand would be primarily in the transition from stage 1 to stage 2, but the continued presence of 1,25(OH)₂D₃ appears to be necessary also for the progression from stages 3 through 6 to VDR degradation, or alternatively for recycling via stage 5. This characterization of the macromolecular cofactors that transduce the signal of the 1,25(OH)₂D₃ hormone to promote gene expression in vitamin D target tissues should add to our understanding of endocrine control of bone mineral remodeling and of epithelial cell differentiation. The present work also identifies new protein players that are candidates for mutation or dysregulation in the pathophysiology of vitamin D resistant bone disorders (osteoporosis), and in hyperproliferative diseases of vitamin D regulated epithelial tissues such as skin.