Vitamin D receptor interactions with the murine osteopontin response element

The nature of the DNA binding interactions of the human vitamin D receptor (hVDR) with the murine osteopontin vitamin D response element (mOP VDRE) was examined. Both recombinant hVDR and human retinoid X receptor beta (hRXRbeta) proteins were obtained from baculovirus-infected Sf9 insect cells. Mixing extracts of the two recombinant proteins resulted in the strong, specific formation of a slower migrating complex in the electrophoretic mobility shift assay. Crude extracts of the expressed hVDR alone were also capable of binding with high affinity to the mOP sequence, and this binding was enhanced in the presence of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). Competition experiments confirmed the specificity of this interaction and revealed that the human osteocalcin VDRE was a poor competitor for this binding. Ethylation interference footprint analyses of hVDR/hRXRbeta and hVDR complexes revealed only subtle differences in how these two different VDR-containing complexes interacted with the mOP VDRE. The footprints displayed contact points in both halves of the direct repeat format, confirming the dimeric and major groove interactions of both types of complexes. DNA affinity chromatography of labelled hVDR extracts revealed a peak eluting at ca. 290 mM KC1 that was capable of rebinding to the mOP sequence in gel shift experiments. Ultraviolet (UV) light-crosslinking experiments of hVDR extracts alone to radiolabelled DNA were consistent with the existence of a homodimeric hVDR interaction. Additionally, these experiments confirmed the direct interaction of a hVDR/hRXRbeta heterodimer when mixed extracts were utilized. From these results we infer that homodimers of the hVDR which respond with enhanced DNA binding to particular vitamin D response elements when exposed to 1,25-(OH)2D3 are possible. This may be of functional significance when RXR proteins are limiting or RXR ligand is present within a cell.