ADDITIONAL PROTEIN FACTORS PLAY A ROLE IN THE FORMATION OF VDR RXR COMPLEXES ON VITAMIN-D RESPONSE ELEMENTS/

The vitamin D receptor (VDR) elicits a transcriptional response to 1,2 5-dihydroxyvitamin D-3 by binding to specific response elements (VDRE) in the promoter of target genes. Retinoic X receptor (RXR) is require d for formation of the VDR-VDRE complex when VDR is supplied at physio logic concentrations. When porcine intestinal nuclear extract is used as a source of VDR, two distinct complexes are always observed with na tive gel electrophoresis. Both complexes contain VDR and RXR. We now s how that the faster-migrating complex requires another heretofore unkn own nuclear factor for its formation. In addition, we provide evidence that the formation of the slower-migrating complex is enhanced by tra nscription factor IIB (TFIIB). Using ligand binding assays, we determi ned that both complexes contain the same ratio of VDR to VDRE. Using R XR subtype-specific antibodies in gel shift assays, we show that the c omplexes contain more than one RXR subtype. Therefore, the present res ults demonstrate VDR-RXR-VDRE complexes formed with pig intestinal nuc lear extracts contain other proteins and that the complexes formed bet ween VDR and VDRE are not simply heterodimers of VDR and RXR. (C) 1998 Wiley-Liss, Inc.