Hairless Modulates Ligand-Dependent Activation of the Vitamin D Receptor-Retinoid X Receptor Heterodimer

Abstract

1) VDR acts as an allosteric transcription factor that undergoes a ligand-dependent conformational change in the cofactor binding site and activation function 2 (AF2) domain, structural rearrangements that result in dynamic interaction with the heterodimer partner retinoid X receptor (RXR; NR2B) and exchange of cofactor complexes.2,3) While corepressors bind to the VDR-RXR heterodimer in the absence of ligand, the ligand binding reduces the affinity of corepressors and increases the affinity for coactivators, a structural transition that induces transcription of specific genes. The VDR-RXR heterodimer binds preferentially to a DNA response element that consists of a two hexanucleotide (AGGTCA or a related sequence) direct repeat motif separated by three nucleotides (DR3).4) The DR3 VDRbinding element has been identified in the regulatory regions of many target genes, including cytochrome P450, family 24, subfamily A, polypeptide 1 (CYP24A1), cathelicidin antimicrobial peptide (CAMP), and osteopontin.2,4) An everted repeat of the hexanucleotide motif separated by six nucleotides (ER6) is another VDR-binding element that regulates expression of the human CYP3A4 gene.2) Dynamic and coordinated interactions of VDR with RXR and cofactor complexes are required for the efficient regulation of transcription.3,5) Lithocholic acid (LCA), a secondary bile acid, acts as an additional physiological VDR ligand and interacts with the VDR ligand-binding pocket in a mode distinct from 1,25(OH) 2 D 3 , and alternate contacts are seen particularly with helices 3 and 4/5, components of the AF2 surface. [6][7][8] A statistical coupling analysis has demonstrated that RXR heterodimer allosteric communication is required for effective activation of VDR by LCA but not by 1,25(OH) 2 D 3 . 9) Indeed, LCA induces the interaction of VDR with RXR, steroid receptor coactivator 1 (SRC-1), and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) in a manner distinct from 1,25(OH) 2 D 3 in cells.3)Although the role of VDR in mediating the biological effects of 1,25(OH) 2 D 3 has been investigated for decades, an understanding of the biology of VDR regulation by LCA is only now emerging.Hairless (HR) is a nuclear protein that functions as a corepressor of nuclear receptors, including thyroid hormone receptor, retinoic acid receptor-related orphan receptors and VDR.10-12) HR interacts with VDR via multiple protein-protein interfaces in the absence of ligand and represses VDR transactivation by 1,25(OH) 2 D 3 and LCA.12,13) Interestingly, LCA but not 1,25(OH) 2 D 3 enhances the interaction, 12) suggesting that LCA induces a conformation of the VDR-RXR heterodimer with enhanced affinity for HR binding. In both mice and humans, mutation of HR results in hair-loss phenotypes similar to VDR mutation, suggesting a functional interaction of HR and VDR in vivo. 14) In contrast to the in vitro interaction, 1,25(OH) 2 D 3 treatment decreases HR binding to VDR in keratinocytes. 15) Although the in vivo role of HR in VDR function remains ...

Vitamin D receptor-mediated control of Soggy, Wise, and Hairless gene expression in keratinocytes

“…Moreover, we have on rare occasions observed in select preparations of calcium-differentiated primary human keratinocytes, but never in KERTr cells, that 1,25D induces rather than represses both DKKL1 and SOSTDC1 expression (data not shown), suggesting that both the DKKL1 and SOSTDC1 VDREs should perhaps be referred to as “conditional”. This observation, probably accounted for by incomplete differentiation of occasional primary human keratinocyte lots, is also consistent with the recent finding that HR elicits coactivation of the cathelicidin gene while corepressing CYP24A1 mRNA in the context of HaCaT keratinocytes (Chuma , et al 2012). Finally, although no dramatic epidermal or hair cycle phenotype exists in either DkkL1 - or SOSTDC1 -knockout mice, the two gene products could redundantly regulate the hair cycle, requiring a double knockout to generate a hair cycle phenotype, or both gene products may possibly coordinate with other genes within the skin compartment to control hair growth.…”

Hairless is a histone H3K9 demethylase

“…HR displays demethylase activity on both H3K9mel and H3K9me2, suggesting that HR can potentially exert opposing effects on its target gene expression. These findings explain the context‐dependent mode of gene regulation by HR, as reported previously (16–18). For example, a recent study shows that HR acts not only as a transcriptional repressor but also as an activator in keratinocyte‐derived HaCaT cells in a gene‐selective manner (18).…”

“…S3). Taken together with prior studies showing that HR can exert opposing effects on target gene activity, where it sometimes activates and sometimes represses (18), these results confirm that HR is a versatile transcription factor functioning through H3K9 demethylation, and its impact on target gene activity may vary in a gene‐specific and/or context‐dependent manner.…”

CYP11A1 in skin: An alternative route to photoprotection by vitamin D compounds

“…It should be noted that most of our studies have been carried out in Skh:hr1 (hairless) mice, as this allows us to avoid artefacts associated with shaving. Although these mice have a mutation in hairless, a protein which binds to the VDR [31,32], we have reported similar effectiveness of 1,25D to reduce UVR-induced DNA damage in human keratinocytes and fibroblasts [33,34] and in people [29] as well as in various strains of mice [35]. We have also noted similar protection against UVR-induced reductions in contact hypersensitivity responses with 1,25D, in several other strains of mice, including female C57/ Bl6, BALB/C (in this test model at least) and C3h/HeN [35].…”