1 ,25-Dihydroxyvitamin D 3 Receptor as a M ediator of Transrepression of Retinoid Signaling Patsie Polly, 1,2 * Carsten Carlberg, 2 John A. Eisman, 1 and Nigel A. Morrison 1 1 Bone and Mineral Program, Garvan Institute of Medical Research, Sydney, Australia 2 Institut fu ¨r Physiologische Chemie I, Heinrich-Heine-Universita ¨t Du ¨sseldorf, Du ¨sseldorf, Germany Abstract The receptorsfor retinoic acid (RA) and for 1,25-dihydroxyvitamin D 3 (VD), RAR, RXR, and VDR are ligand-inducible members of the nuclear receptor superfamily. These receptors mediate their regulatory effects by binding as dimeric complexes to response elements located in regulatory regions of hormone target genes. Sequence scanning of the tumor necrosisfactor- type I receptor (TNF RI) gene identified a 3' enhancer region composed of two directly repeated hexameric core motifsspaced by 2 nucleotides(DR2). On thisnovel DR2-type sequence, but not on a DR5-type RA response element, VD wasshown to act through itsreceptor, the vitamin D receptor (VDR), asa repressor of retinoid signalling. The repression appearsto be mediated by competitive protein–protein interactionsbetween VDR, RAR, RXR, and possibly their cofactors. This VDR-mediated transrepression of retinoid signaling suggests a novel mechanism for the complex regulatory interaction between retinoidsand VD. J. Cell. Biochem. 67:287–296, 1997.  1997 Wiley-Liss, Inc. Key words: vitamin D 3 receptor; regulation of transcription; retinoid signaling transrepression; tumor necrosis factor- receptor type I The biologically active forms of vitamins A and D, retinoic acid (RA), and 1,25 dihy- droxyvitamin D 3 (VD), are pleiotropic effectors that take an active functional role in many aspects of biology, including cellular develop- ment and differentiation, inhibition of prolifera- tion, tumorigenesis, and embryogenesis [Cham- bon, 1994; Gigue `re, 1994; Walters, 1992]. Both the retinoids and VD act as transcriptional regulators via their specific nuclear receptors. There are two classes of retinoid receptors, RA receptors (RARs) and retinoid X receptors (RXRs), each with three subtypes and various splicing variants [Chambon, 1994; Gigue `re, 1994], whereas only one gene for the VD recep- tor (VDR) has been reported [Carlberg, 1996; Pike, 1991]. All-trans RA is a specific ligand of RARs, whereas 9-cis RA is a pan-agonist of all retinoid receptors. RAR, RXR, and VDR, as well as thyroid hormone (T 3 ) receptor (T 3 R) form a subgroup within the nuclear receptor superfamily [Mangelsdorf et al., 1995]. Their common feature is a highly conserved DNA binding domain of 66–70 amino acids and a moderately homologous C-terminal ligand bind- ing domain (LBD). This conservation is consis- tent with the binding of these nuclear receptors to similar hexameric core binding motifs. Simple hormone response elements are formed by an arrangement of two core binding motifs in a directly repeated, palindromic, or inverted pal- indromic orientation [Carlberg, 1995; Glass, 1994]. Specificity in response element recogni- tion preference of different dimeric complexes is largely dictated by the specific core binding motif sequence, motif spacing, and orientation [Carlberg, 1995; Glass, 1994; Umesono et al., 1991]. Various studies have demonstrated that RAR acts mainly through a heterodimeric com- plex with RXR [Ladias and Karathanasis, 1991], which binds preferentially to directly repeated hexameric motifs (PuKKTCA) spaced by either 2 or 5 nucleotides (DR2 and DR5) or to inverted palindromes spaced by 8 nucleotides [Carlberg, 1995; Glass, 1994]. Moreover, RAR has also been shown to form homodimers [Carlberg et al., 1993; Schra ¨der et al., 1993] and het- erodimers with T 3 R [Glass et al., 1989] and Contract grant sponsors: National Health and Medical Research Council of Australia. *Correspondence to: Dr. Patsie Polly, Institut fu ¨ r Physiolo- gische Chemie I, Heinrich-Heine-Universita ¨t Du ¨ sseldorf, Postfach 10 10 07, D-40001 Du ¨sseldorf, Germany. Received 21 July 1997; Accepted 25 July 1997 Journal of Cellular Biochemistry 67:287–296 (1997) FAST TRACK  1997 Wiley-Liss, Inc.