The human vitamin D receptor (VDR) is a key nuclear receptor that binds nutritionally derived ligands and exerts bioeffects that contribute to bone mineral homeostasis, detoxification of exogenous and endogenous compounds, cancer prevention, and mammalian hair cycling. Liganded VDR modulates gene expression via heterodimerization with the retinoid X receptor and recruitment of coactivators or corepressors. VDR interacts with the corepressor hairless (Hr) to control hair cycling, an action independent of the endocrine VDR ligand, 1,25-dihydroxyvitamin D(3). We report novel dietary ligands for VDR including curcumin, gamma-tocotrienol, and essential fatty acid derivatives that likely play a role in the bioactions of VDR.

Both the vitamin D receptor (VDR) and hairless (hr) genes play a role in the mammalian hair cycle, as inactivating mutations in either result in total alopecia. VDR is a nuclear receptor that functions as a ligand-activated transcription factor, whereas the hairless gene product (Hr) acts as a corepressor of both the thyroid hormone receptor (TR) and the orphan nuclear receptor, ROR␣. In the present study, we show that VDR-mediated transactivation is strikingly inhibited by coexpression of rat Hr. The repressive effect of Hr is observed on both synthetic and naturally occurring VDR-responsive promoters and also when VDR-mediated transactivation is augmented by overexpression of its heterodimeric partner, retinoid X receptor. Utilizing in vitro pull down methods, we find that Hr binds directly to VDR but insignificantly to nuclear receptors that are not functionally repressed by Hr. Coimmunoprecipitation data demonstrate that Hr and VDR associate in a cellular milieu, suggesting in vivo interaction. The Hr contact site in human VDR is localized to the central portion of the ligand binding domain, a known corepressor docking region in other nuclear receptors separate from the activation function-2 domain. Coimmunoprecipitation and functional studies of Hr deletants reveal that VDR contacts a C-terminal region of Hr that includes motifs required for TR and ROR␣ binding. Finally, in situ hybridization analysis of hr and VDR mRNAs in mouse skin demonstrates colocalization in cells of the hair follicle, consistent with a hypothesized intracellular interaction between these proteins to repress VDR target gene expression, in vivo.Nuclear receptors comprise a family of ligand-activated transcription factors that coordinate physiological and developmental processes by regulating specific changes in gene expression (1, 2). The vitamin D receptor (VDR) 1 mediates signaling by 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) and is a member of the thyroid hormone/retinoic acid receptor subfamily of nuclear receptors that heterodimerize with the retinoid X receptor (RXR) on direct repeat hormone-responsive elements in the promoters of regulated genes (3, 4). Binding of liganded VDR⅐RXR to a vitamin D-responsive element (VDRE) in target genes such as osteocalcin, osteopontin, and vitamin D 24-hydroxylase (CYP24) is accompanied by the recruitment of coactivator proteins (5). VDR coactivators such as steroid receptor coactivator-1 (6), NCoA-62 (7), and vitamin D receptor interacting protein 205 (8) stimulate transcriptional activation by facilitating chromatin remodeling and/or attraction of RNA polymerase II. Although some unliganded nuclear receptors (thyroid hormone and retinoic acid receptors) can mediate repression through association with corepressors such as nuclear receptor corepressor (N-CoR) (9) and silencing mediator for retinoic acid and thyroid hormone receptors (SMRT) (10), evidence suggests that VDR does not associate strongly with these corepressors (9 -12).Targeted gene deletion studies in mice (13, 14) and inactivat...

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The mammalian hair cycle requires both the vitamin D receptor (VDR) and the hairless (Hr) corepressor, each of which is expressed in the hair follicle. Hr interacts directly with VDR to repress VDR-targeted transcription. Herein, we further map the VDR-interaction domain to regions in the C-terminal half of Hr that contain two LXXLL-like pairs of motifs known to mediate contact of Hr with the RAR-related orphan receptor alpha and with the thyroid hormone receptor, respectively. Site-directed mutagenesis indicates that all four hydrophobic motifs are required for VDR transrepression by Hr. Point mutation of rat Hr at conserved residues corresponding to natural mutants causing alopecia in mice (G985W and a C-terminal deletion ΔAK) and in humans (P95S, C422Y, E611G, R640Q, C642G, N988S, D1030N, A1040T, V1074M and V1154D), as well as alteration of residues in the C-terminal Jumonji C domain implicated in histone demethylation activity (C1025G/E1027G and H1143G) revealed that all Hr mutants retained VDR association, and that transrepressor activity was selectively abrogated in C642G, G985W, N988S, D1030N, V1074M, H1143G and V1154D. Four of these latter Hr mutants (C642G, N988S, D1030N and V1154D) were found to associate normally with histone deacetylase-3. Finally, we identified three regions of human VDR necessary for association with Hr, namely residues 109-111, 134-201, and 202-303. It is concluded that Hr and VDR interact via multiple protein-protein interfaces, with Hr recruiting histone deacetylases and possibly itself catalyzing histone demethylation to effect chromatin remodeling and repress the transcription of VDR target genes that control the hair cycle.Keywords calcitriol receptors; histone deacetylase; histone demethylase; human HR protein; rat hr protein; Jumonji domainThe hairless protein (Hr), a 130 kDa nuclear transcription factor, is mutated in at least two forms of human alopecia: alopecia universalis congenita and atrichia with papular lesions Ahmad et al., 1999]. Also, mouse models in which hr gene expression has been reduced or eliminated [Zarach et al., 2004] display a phenotype of hair loss, hyperproliferation of skin and dermal cysts. However, the molecular mechanisms by which Hr exerts its effects on the skin and on hair growth/maintenance are still being elucidated. Hr, which is also highly expressed in brain, has been shown to interact with the thyroid hormone receptor (TR) and with the RAR-related orphan receptors (RORs), especially RORα [Moraitis et al., 2002]. In addition, some of the functional domains in Hr mediating these interactions have been mapped (Fig. 1D and Fig. 2A). Hr possesses a nuclear localization signal (NLS) from residues 437-454 [Djabali et al., 2001] and a single zinc-finger motif (amino acids 620-645) [Cachon-Gonzalez et al., 1994] (see also Fig. 2A), as well as four motifs of hydrophobic amino acids, two of the form LXXLL (where L = leucine and X = any amino acid residue) and two of the form ΦXXΦΦ (where Φ = any one of the hydrophobic amino acids leucine, i...