Michael A. Galligan

Functionally relevant polymorphisms in the human nuclear vitamin D receptor gene.

The functional significance of two unlinked human vitamin D receptor (hVDR) gene polymorphisms was evaluated in twenty human fibroblast cell lines. Genotypes at both a Fok I restriction site (F/f) in exon II and a singlet (A) repeat in exon IX (L/S) were determined, and relative transcription activities of endogenous hVDR proteins were measured using a transfected, 1,25-dihydroxyvitamin D(3)-responsive reporter gene. Observed activities ranged from 2--100-fold induction by hormone, with higher activity being displayed by the F and the L biallelic forms. Only when genotypes at both sites were considered simultaneously did statistically significant differences emerge. Moreover, the correlation between hVDR activity and genotype segregated further into clearly defined high and low activity groups with similar genotypic distributions. These results not only demonstrate functional relevance for both the F/f and L/S common polymorphisms in hVDR, but also provide novel evidence for a third genetic variable impacting receptor potency.

Retinoid X receptors stimulate and 9-cis retinoic acid inhibits 1,25-dihydroxyvitamin D3-activated expression of the rat osteocalcin gene.

The vitamin D receptor (VDR) binds the vitamin D-responsive element (VDRE) as a heterodimer with an unidentified receptor auxiliary factor (RAF) present in mammalian cell nuclear extracts. VDR also interacts with the retinoid X receptors (RXRs), implying that RAF may be related to the RXRs. Here we demonstrate that highly purified HeLa cell RAF contained RXR beta immunoreactivity and that both activities copurified and precisely coeluted in high-resolution hydroxylapatite chromatography. Furthermore, an RXR beta-specific antibody disrupted VDR-RAF-VDRE complexes in mobility shift assays. These data strongly indicate that HeLa RAF is highly related to or is identical to RXR beta. Consequently, the effect of the 9-cis retinoic acid ligand for RXRs was examined in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-activated gene expression systems. Increasing concentrations of 9-cis retinoic acid (1 nM to 1 microM) markedly reduced 1,25(OH)2D3-dependent accumulation of osteocalcin mRNA in osteoblast-like ROS 17/2.8 cells. All-trans retinoic acid also interfered with vitamin D responsiveness, but it was consistently less potent than the 9-cis isomer. Transient transfection studies revealed that attenuation by 9-cis retinoic acid was at the transcriptional level and was mediated through interactions at the osteocalcin VDRE. Furthermore, overexpression of both RXR beta and RXR alpha augmented 1,25(OH)2D3 responsiveness in transient expression studies. Direct analysis of VDRE binding in mobility shift assays demonstrated that heteromeric interactions between VDR and RXR were enhanced by 1,25(OH)2D3 and were not affected appreciably by 9-cis retinoic acid, except that inhibition was observed at high retinoid concentrations. These data suggest a regulatory mechanism for osteocalcin gene expression that involves 1,25(OH)2D3-induced heterodimerization of VDR and unliganded RXR. 9-cis retinoic acid may attenuate 1,25(OH)2D3 responsiveness by diverting RXRs away from VDR-mediated transcription and towards other RXR-dependent transcriptional pathways.

Isolation of baculovirus-expressed human vitamin D receptor: DNA responsive element interactions and phosphorylation of the purified receptor

Two controversial aspects in the mechanism of human vitamin D receptor (hVDR) action are the possible significance of VDR homodimers and the functional role of receptor phosphorylation. To address these issues, milligram quantities of baculovirus‐expressed hVDR were purified to 97% homogeneity, and then tested for binding to the rat osteocalcin vitamin D responsive element (VDRE) via electrophoretic mobility shift and half‐site competition assays in the presence or absence of a CV‐1 nuclear extract containing retinoid X receptor (RXR). Methylation interference analysis revealed that both the hVDR homodimer and the VDR‐RXR heterodimer display similar patterns of VDRE G‐base protection. However, in competition studies, the relative dissociation of the homodimeric hVDR complex from the VDRE was extremely rapid (t1/2 < 30 s) compared to the dissociation of the heteromeric complex (t1/2 > 5 min), thus illustrating the relative instability and low affinity of homodimeric VDR binding to DNA. These results indicate that VDR‐RXR heterodimers are the preferred VDRE binding species. Further, two dimensional gel electrophoresis of hVDR demonstrated several isoelectric forms of the receptor, suggesting that it is subject to multiple phosphorylation events. In vitro kinase assays confirmed that purified hVDR is an efficient substrate for protein kinases A and Cβ, as well as casein kinase II. In vivo studies of the expressed receptor in intact cells, namely baculovirus vector infected Sf9 insect cells and transfected mammalian COS‐7 cells, demonstrated that hVDR was phosphorylated in a hormone‐enhanced fashion. Functional consequences of hVDR phosphorylation were suggested by the observations that: (i) potato acid phosphatase (PAP)‐treated hVDR no longer interacted with the VDRE as either a homodimer or a heteromeric complex with RXR, and (ii) treatment of transfected COS‐7 cells with a phosphatase inhibitor (okadaic acid) along with 1,25‐dihydroxyvitamin D3 (1,25(OH)2D3) resulted in a synergistic enhancement of both hVDR phosphorylation and transactivation of a VDRE‐linked reporter gene, compared to the effect of treatment with either agent alone. These studies point to a significant role for phosphorylation of VDR in regulating high‐affinity VDR‐RXR interactions with VDREs, and also in modulating 1,25(OH)2D3‐elicited transcriptional activation in target cells. J. Cell. Biochem. 85: 435–457, 2002.

CYP24A1 and CYP27B1 Polymorphisms Modulate Vitamin D Metabolism in Colon Cancer Cells

Vitamin D is a well-studied agent for cancer chemoprevention and treatment. Its chief circulating metabolite, 25-hydroxyvitamin D, is converted into the active hormone 1,25-dihydroxyvitamin D (1,25D) by the cytochrome P450 enzyme CYP27B1 in kidney and other tissues. 1,25D is then deactivated by CYP24A1 and ultimately catabolized. Colorectal carcinoma cells express CYP27B1 and CYP24A1 that locally regulate 1,25D with potential implications for its impact on carcinogenesis. While 1,25D inhibits cancer growth, the effects of polymorphic variations in genes encoding proteins involved in 1,25D homeostasis are poorly understood. Using an RXR-VDR mammalian two-hybrid (M2H) biologic assay system, we measured vitamin D metabolite uptake and activation of the vitamin D receptor (VDR) pathway in colon cancer cells that expressed one of five CYP27B1 single-nucleotide polymorphisms (SNP) or four CYP24A1 SNPs. Compared with the wild-type control, four of five CYP27B1 SNPs reduced enzymatic activity, whereas one (V166L) increased activity. For CYP24A1, all tested SNPs reduced enzyme activity. Quantitative real-time PCR analyses supported the results of M2H experiments. The observed SNP-directed variation in CYP functionality indicated that vitamin D homeostasis is complex and may be influenced by genetic factors. A comprehensive understanding of 1,25D metabolism may allow for a more personalized approach toward treating vitamin D-related disorders and evaluating risk for carcinogenesis.

Vitamin D receptor displays DNA binding and transactivation as a heterodimer with the retinoid X receptor, but not with the thyroid hormone receptor†

The vitamin D receptor (VDR) is a transcription factor believed to function as a heterodimer with the retinoid X receptor (RXR). However, it was reported [Schräder et al., 1994] that, on putative vitamin D response elements (VDREs) within the rat 9k and mouse 28k calcium binding protein genes (rCaBP 9k and mCaBP 28k), VDR and thyroid hormone receptor (TR) form heterodimers that transactivate in response to both 1,25‐dihydroxyvitamin D3 (1,25(OH)2D3) and triiodothyronine (T3). We, therefore, examined associations of these receptors on the putative rCaBP 9k and mCaBP 28k VDREs, as well as on established VDREs from the rat osteocalcin (rOC) and mouse osteopontin (mOP) genes, plus the thyroid hormone response element (TRE) from the rat myosin heavy chain (rMHC) gene. In gel mobility shift assays, we found no evidence for VDR‐TR heterodimer interaction with any tested element. Further, employing these hormone response elements linked to reporter genes in transfected cells, VDR and TR mediated responses to their cognate ligands only from the rOC/mOP and rMHC elements, respectively, while the CaBP elements were unresponsive to any combination of ligand(s). Utilizing the rOC and mOP VDREs, two distinct repressive actions of TR on VDR‐mediated signaling were demonstrated: a T3‐independent action, presumably via direct TR‐RXR competition for DNA binding, and a T3‐dependent repression, likely by diversion of limiting RXR from VDR‐RXR toward the formation of TR‐RXR heterodimers. The relative importance of these two mechanisms differed in a response element‐specific manner. These results may provide a partial explanation for the observed association between hyperthyroidism and bone demineralization/osteoporosis. J. Cell. Biochem. 75:462–480, 1999.

Receptor mediated genomic action of the 1,25(OH)2D3 hormone: Expression of the human vitamin D receptor in E. coli

The nuclear vitamin D receptor (VDR) binds the 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) hormone with high affinity and elicits its actions to stimulate gene expression in target cells by binding to the vitamin D-responsive element (VDRE). VDREs in such positively controlled genes as osteocalcin, osteopontin, beta 3 integrin and vitamin D-24-OHase are direct hexanucleotide repeats with a spacer of three nucleotides. The present studies of VDR/VDRE interaction utilized full-length human vitamin D receptor (hVDR) that was overexpressed in E. coli, purified to near homogeneity (> 95%), and its authenticity confirmed by demonstrating high affinity hormone binding and reactivity to monoclonal antibody 9A7 gamma. The expressed hVDR displays strict dependence on the family of retinoid X receptors (RXRs) for binding to the vitamin D-responsive element (VDRE) in the rat osteocalcin gene. Similar overexpression in E. coli of the DNA binding domain (delta 134), containing only residues 4-133 of hVDR, generated a receptor species that possesses intrinsic DNA binding activity. Both full-length and delta 134 hVDRs retain similar DNA binding specificities when tested with several natural hormone responsive elements, indicating that the N-terminal zinc finger region determines hVDR-DNA sequence selectivity. The C-terminal region of the molecule is required for hormone binding and confers the receptor with the property of very high affinity DNA binding, via heterodimerization between hVDR and RXR. A natural ligand for the RXR co-receptor, 9-cis retinoic acid, suppresses both VDR-RXR binding to the VDRE and 1,25(OH)2D3 stimulated transcription, indicating that 9-cis retinoic acid recruits RXR away from VDR to instead form RXR homodimers.

Associations between vitamin D-binding protein isotypes, circulating 25(OH)D levels, and vitamin D metabolite uptake in colon cancer cells.

Vitamin D metabolites have been extensively studied as cancer chemopreventive agents. Gc-globulin (GC) isotypes, based on rs7041 and rs4588 diplotypes, have varying affinities for 1α,25-dihydroxyvitamin D (1,25(OH)2D) and 25-hydroxyvitamin D (25(OH)D), which may affect circulating metabolite concentration as well as delivery at the cellular level. We evaluated associations between GC isotype and circulating vitamin D metabolite concentrations in 403 ursodeoxycholic acid (UDCA) clinical trial participants. Metabolite uptake was evaluated in human colon cancer (HCT-116) cells treated with ethanol vehicle, 1,25(OH)2D, or 25(OH)D, and with plasma from individuals with known GC isotype. Mammalian-2-hybrid and vitamin D–responsive element-based luciferase assays were used to measure the vitamin D receptor pathway activation as a marker for metabolite uptake. Regression analysis demonstrated significantly lower serum 25(OH)D concentration for clinical trial participants with 1F_2, 1S_2, or 2_2 isotypes (P < 0.01) compared with 1S_1S. Consistent with these in vivo observations, cellular data revealed that 25(OH)D uptake varied less by GC isotype only at the higher concentration tested (P = 0.05), while 1,25(OH)2D uptake differed markedly by GC isotype across concentration and assay (P < 0.01). The 1F_1S and 1F_2 isotypes produced the greatest reporter gene induction with 1,25(OH)2D treatment and, while activation varied less with 25(OH)D, the 2_2 isotype demonstrated increased induction at the lower concentration. These results suggest that vitamin D metabolite concentration and delivery to colon cells may vary not only by GC isotype, but also that certain isotypes may more effectively deliver 1,25(OH)2D versus 25(OH)D. Overall, these results may help identify populations at risk for cancer and potential recipients of targeted chemoprevention. Cancer Prev Res; 7(4); 426–34. ©2014 AACR.

SIRT1 enzymatically potentiates 1,25-dihydroxyvitamin D3 signaling via vitamin D receptor deacetylation

The hormonal metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25D), binds to the vitamin D receptor (VDR) and promotes heterodimerization of VDR with a retinoid-X-receptor (RXR) to genomically regulate diverse cellular processes. Herein, it is revealed for the first time that VDR is post-translationally acetylated, and that VDR immunoprecipitated from human embryonic kidney (HEK293) cells displays a dramatic decrease in acetylated receptor in the presence of 1,25D-ligand, sirtuin-1 (SIRT1) deacetylase, or the resveratrol activator of SIRT1. To elucidate the functional significance of VDR deacetylation, vitamin-d-responsive-element (VDRE)-based transcriptional assays were performed to determine if deacetylase overexpression affects VDR/VDRE-driven transcription. In HEK293 kidney and TE85 bone cells, co-transfection of low amounts (1-5ng) of a SIRT1-expression vector elicits a reproducible and statistically significant enhancement (1.3- to 2.6-fold) in transcription mediated by VDREs from the CYP3A4 and cyp24a1 genes, where the magnitude of response to 1,25D-ligand is 6- to 30-fold. Inhibition of SIRT1 via EX-527, or utilization of a SIRT1 loss-of-function mutant (H363Y), resulted in abrogation of SIRT1-mediated VDR potentiation. Studies with a novel, non-acetylatable VDR mutant (K413R) showed that the mutant VDR possesses enhanced responsiveness to 1,25D, in conjunction with reduced, but still significant, sensitivity to exogenous SIRT1, indicating that acetylation of lysine 413 is relevant, but that other acetylated residues in VDR contribute to modulation of its activity. We conclude that the acetylation of VDR comprises a negative feedback loop that attenuates 1,25D-VDR signaling. This regulatory loop is reversed by SIRT1-catalyzed deacetylation of VDR to amplify VDR signaling and 1,25D actions.

Exploration of early-life candidate biomarkers for childhood asthma using antibody arrays

Proteomic approaches identifying biomarkers have been applied to asthma to only a very limited extent.