Dystrobrevin alpha gene is a direct target of the vitamin D receptor in muscle

The biologically active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (VD3), exerts its tissue-specific actions through binding to its intracellular vitamin D receptor (VDR) which functions as a heterodimer with retinoid X receptor (RXR) to recognize vitamin D response elements (VDRE) and activate target genes. Upregulation of VDR in murine skeletal muscle cells occurs concomitantly with transcriptional regulation of key myogenic factors upon VD3 administration, reinforcing the notion that VD3 exerts beneficial effects on muscle. Herein we elucidated the regulatory role of VD3/VDR axis on the expression of dystrobrevin alpha (DTNA), a member of dystrophin-associated protein complex (DAPC). In C2C12 cells, Dtna and VDR gene and protein expression were upregulated by 1-50 nM of VD3 during all stages of myogenic differentiation. In the dystrophic-derived H2K-mdx52 cells, upregulation of DTNA by VD3 occurred upon co-transfection of VDR and RXR expression vectors. Silencing of MyoD1, an E-box binding myogenic transcription factor, did not alter the VD3-mediated Dtna induction, but Vdr silencing abolished this effect. We also demonstrated that VD3 administration enhanced the muscle-specific Dtna promoter activity in presence of VDR/RXR only. Through site-directed mutagenesis and chromatin immunoprecipitation assays, we have validated a VDRE site in Dtna promoter in myogenic cells. We have thus proved that the positive regulation of Dtna by VD3 observed during in vitro murine myogenic differentiation is VDR mediated and specific. The current study reveals a novel mechanism of VDR-mediated regulation for Dtna, which may be positively explored in treatments aiming to stabilize the DAPC in musculoskeletal diseases.

The transcriptomic response of mixed neuron-glial cell cultures to 1,25-dihydroxyvitamin d3 includes genes limiting the progression of neurodegenerative diseases

Seasonal or chronic vitamin D deficiency and/or insufficiency is highly prevalent in the human population. Receptors for 1,25-dihydroxyvitamin D3, the hormonal metabolite of vitamin D, are found throughout the brain. To provide further information on the role of this hormone on brain function, we analyzed the transcriptomic profiles of mixed neuron-glial cell cultures in response to 1,25-dihydroxyvitamin D3. 1,25-dihydroxyvitamin D3 treatment increases the mRNA levels of 27 genes by at least 1.9 fold. Among them, 17 genes were related to neurodegenerative and psychiatric diseases, or brain morphogenesis. Notably, 10 of these genes encode proteins potentially limiting the progression of Alzheimer's disease. These data provide support for a role of 1,25-dihydroxyvitamin D3 in brain disease prevention. The possible consequences of circannual or chronic vitamin D insufficiencies on a tissue with a low regenerative potential such as the brain should be considered.

Calcitriol upregulates open chromatin and elongation markers at functional vitamin D response elements in the distal part of the 5-lipoxygenase gene

5-Lipoxygenase (5-LO) gene expression is strongly upregulated during induction of myeloid cell differentiation by 1alpha,25-dihydroxyvitamin D(3) (calcitriol) and transforming growth factor-beta (TGFbeta) in a promoter-independent manner. In an activity-guided approach using reporter gene assays where the distal part of the 5-LO gene was included in the reporter gene plasmid, we localized vitamin D response elements (VDREs) within exon 10, exon 12, and intron M. We found that these newly identified VDRE sites are bound by vitamin D receptor both in vitro by gel-shift analysis and in vivo by chromatin immunoprecipitation assays. In reporter gene assays, the distal part of the 5-LO gene has promoter-like activity that is inducible by calcitriol in a vitamin D receptor-dependent manner. The vitamin D effects were attenuated when the VDREs in exon 10, exon 12, and intron M were deleted or mutated. When we analyzed the effects of calcitriol plus TGFbeta on chromatin modifications at exon 10, exon 12, and intron M of the 5-LO gene in Mono Mac 6 cells by chromatin immunoprecipitation analysis, we found an increase in histone H4 K20 monomethylation and a prominent presence of histone H3 K36 trimethylation. Combined treatment with calcitriol and TGFbeta also increased histone H4 acetylation, a marker for open chromatin, and the elongation form of RNA polymerase II at these sites, whereas the transcription initiation marker histone H3 K4 trimethylation was almost undetectable. The data suggest that calcitriol induces chromatin opening and transcript elongation via VDREs located at the 3'-end of the 5-LO gene.

G0S2 is an all-trans-retinoic acid target gene

All-trans-retinoic acid (RA) treatment of acute promyelocytic leukemia (APL) cases expressing the t(15;17) product, PML/RARalpha, is a successful example of differentiation therapy. Uncovering RA target genes is of considerable interest in APL. This study comprehensively examines in APL cells transcriptional and post-transcriptional regulation of the novel candidate RA target gene, G0S2, the G0/G1 switch gene. Reverse transcription (RT)-polymerase chain reaction (PCR) and heteronuclear PCR assays performed +/- treatment with the protein synthesis inhibitor cycloheximide (CHX) revealed G0S2 induction within 3 h of RA-treatment. Treatment with the RNA synthesis inhibitor actinomycin D did not implicate G0S2 transcript stabilization in the RA-mediated increase of G0S2 mRNA expression. Promoter elements of G0S2 were cloned into a reporter plasmid and retinoic acid receptor (RAR) co-transfection assays confirmed transcriptional activation after RA-treatment. Consistent with G0S2 being a direct RA target gene, retinoic acid response element (RARE) half-sites were found in this promoter. Mutation of these sites blocked RA-transcriptional activation of G0S2. To extend analyses to the protein expression level, a polyclonal anti-G0S2 antibody was derived and detected murine and human G0S2 species. G0S2 protein was rapidly induced in cultured NB4-S1 human APL cells and in APL transgenic mice treated with RA. An RAR pan-antagonist confirmed dependence on RARs for this induction. That these findings are clinically relevant was shown by analyses of APL cells derived directly from patients. These leukemic cells induced both a prominent increase in the cellular differentiation marker nitrotetrazolium blue (NBT) staining and marked increase in G0S2 expression. Taken together, these findings indicate G0S2 is an RA target gene. The functional role of G0S2 in retinoid response of APL warrants further study.

Insulin-like growth factor binding protein-5 interacts with the vitamin D receptor and modulates the vitamin D response in osteoblasts

The 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]-induced differentiation of osteoblasts comprises the sequential induction of cell cycle arrest at G0/G1 and the expression of bone matrix proteins. Reports differ on the effects of IGF binding protein (IGFBP)-5 on bone cell growth and osteoblastic function. IGFBP-5 can be growth stimulatory or inhibitory and can enhance or impair osteoblast function. In previous studies, we have shown that IGFBP-5 localizes to the nucleus and interacts with the retinoid receptors. We now show that IGFBP-5 interacts with nuclear vitamin D receptor (VDR) and blocks retinoid X receptor (RXR):VDR heterodimerization. VDR and IGFBP-5 were shown to colocalize to the nuclei of MG-63 and U2-OS cells and coimmunoprecipitate in nuclear extracts from these cells. Induction of osteocalcin promoter activity and alkaline phosphatase activity by 1,25(OH)2D3 were significantly enhanced when IGFBP-5 was down-regulated in U2-OS cells. Moreover, we found IGFBP-5 increased basal alkaline phosphatase activity and collagen alpha1 type 1 expression, and that 1,25(OH)2D3 was unable to further induce the expression of these bone differentiation markers in MG-63 cells. Expression of IGFBP-5 inhibited MG-63 cell growth and caused cell cycle arrest at G0/G1 and G2/M. Furthermore, IGFBP-5 reduced the effects of 1,25(OH)2D3 in blocking cell cycle progression at G0/G1 and decreased the expression of cyclin D1. These results demonstrate that IGFBP-5 can interact with VDR to prevent RXR:VDR heterodimerization and suggest that IGFBP-5 may attenuate the 1,25(OH)2D3-induced expression of bone differentiation markers while having a modest effect on the 1,25(OH)2D3-mediated inhibition of cell cycle progression in bone cells.

Mechanism of vitamin D3-induced transcription of phospholipase D1 in HaCat human keratinocytes

1alpha,25-Dihydroxyvitamin D(3) (VitD(3)) increases protein and gene expression of phospholipase D1 (PLD1), but not PLD2, in HaCaT human keratinocytes. We show that VitD(3) increases PLD1 gene expression through a vitamin D responsive element (VDRE) on the 5' PLD1 promoter (-260 bp to -246 bp from exon 1). Similar results were obtained by transfecting VitD(3) receptor (VDR) into HEK293 cells, which are originally VitD(3)-unresponsive. Electrophoresis mobility shift assays (EMSA) and chromatin immunoprecipitation (CHIP) assays showed that the complex of VitD(3), VDR and retinoid X receptor alpha (RXRalpha) binds to the VDRE and increases PLD1 gene expression in HaCaT cells.

Characterization of Vitamin D Receptor Ligands with Cell-Specific and Dissociated Activity

Although the main role of 1alpha,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] is to regulate calcium homeostasis, the valuable therapeutic applications of this compound have led to the search of new 1,25-(OH)(2)D(3)-vitamin D receptor (VDR) ligands with less side effects. In this work we have characterized seven 1,25-(OH)(2)D(3) derivatives (ZK136607, ZK161422, ZK157202, ZK159222, ZK168492, ZK191732, and ZK168289). ZK157202 is an agonist that gives a pattern similar to that of 1,25-(OH)(2)D(3) or ZK161422 in limited trypsin digestion assays, is able to recruit p160 and VDR-interacting protein 205 coactivators, is as potent as 1,25-(OH)(2)D(3) to stimulate vitamin D response element-dependent transcription in HeLa cells, and acts as a superagonist in human embryonic kidney 293T cells. This compound is also more potent than the natural ligand to transrepress the activation of the retinoic acid receptor beta2 promoter by retinoic acid and the response of the collagenase promoter to 4alpha-12-O-tetradecanoylphorbol 13-acetate. ZK136607, ZK168492, ZK191732, and ZK168289 have a profile similar to that of the partial antagonist ZK159222. They induce an antagonistic-type proteolytic pattern, do not recruit classical coactivators, and have little transactivation potency. However, they act in a cell context-dependent manner because they lack activity in HeLa cells while presenting some agonistic activity in human embryonic kidney 293T cells, or vice versa. Furthermore, some of these compounds have a dissociated activity: they cannot transactivate but they are as potent as 1,25-(OH)(2)D(3) in transrepression assays. Together our results demonstrate the existence of novel VDR ligands with variable biological functions and dissociated activity. They should represent useful tools for studying VDR function and could have therapeutic utility.

Activation of receptor activator of NF-kappaB ligand gene expression by 1,25-dihydroxyvitamin D3 is mediated through multiple long-range enhancers

RANKL is a tumor necrosis factor (TNF)-like factor secreted by mesenchymal cells, osteoblast derivatives, and T cells that is essential for osteoclastogenesis. In osteoblasts, RANKL expression is regulated by two major calcemic hormones, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and parathyroid hormone (PTH), as well as by several inflammatory/osteoclastogenic cytokines; the molecular mechanisms for this regulation are unclear. To identify such mechanisms, we screened a DNA microarray which tiled across the entire mouse RankL gene locus at a 50-bp resolution using chromatin immunoprecipitation (ChIP)-derived DNA precipitated with antibodies to the vitamin D receptor (VDR) and the retinoid X receptor (RXR). Five sites of dimer interaction were observed on the RankL gene centered at 16, 22, 60, 69, and 76 kb upstream of the TSS. These regions contained binding sites for not only VDR and RXR, but also the glucocorticoid receptor (GR). The most distant of these regions, termed the distal control region (RL-DCR), conferred both VDR-dependent 1,25(OH)(2)D(3) and GR-dependent glucocorticoid (GC) responses. We mapped these activities to an unusual but functionally active vitamin D response element and to several potential GC response elements located over a more extensive region within the RL-DCR. An evolutionarily conserved region within the human RANKL gene contained a similar vitamin D response element and exhibited an equivalent behavior. Importantly, hormonal activation of the RankL gene was also associated with chromatin modification and RNA polymerase II recruitment. Our studies demonstrate that regulation of RankL gene expression by 1,25(OH)(2)D(3) is complex and mediated by at least five distal regions, one of which contains a specific element capable of mediating direct transcriptional activation.

Transactivation of rat apical sodium-dependent bile acid transporter and increased bile acid transport by 1alpha,25-dihydroxyvitamin D3 via the vitamin D receptor

Transactivation of the rat apical sodium-dependent bile acid transporter (ASBT; Slc10a2) by 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] via the vitamin D receptor (VDR), was studied. Levels of ASBT protein and mRNA were low in the duodenum and high in the ileum, and both were induced by 1,25(OH)(2)D(3). The nuclear receptor protein, VDR, was present uniformly in the duodenum, jejunum, and ileum of the rat small intestine. The physiological relevance of ASBT induction by 1,25(OH)(2)D(3) was assessed by measuring absorption of cholylsarcosine, a non-metabolized synthetic bile acid analog, from duodenal or ileal closed loops of the perfused rat small intestine preparation. Absorption of cholylsarcosine was much greater from the ileal segment (28-fold that of the duodenum under control conditions) and was enhanced with 1,25(OH)(2)D(3) treatment. Transient transfection analysis of the rat ASBT promoter in Caco-2 cells revealed concentration-dependent enhancement of luciferase reporter activity after treatment with 1,25(OH)(2)D(3). The activation by 1,25(OH)(2)D(3) was abrogated after site-directed mutagenesis or deletion of the vitamin D response element (VDRE) in the ASBT promoter. Gel-shift mobility assays of nuclear extracts from rat ileum showed that both rat retinoid X receptor and VDR were bound to the VDRE. The results indicate that rat ASBT gene expression is activated by 1,25(OH)(2)D(3) by specific binding to the VDRE and that such activation enhances ileal bile acid transport. Human ABST mRNA and promoter activity were also increased in Caco-2 cells treated with 1,25(OH)(2)D(3), suggesting a physiological role of VDR in human ileal bile acid homeostasis.

Direct Regulation of HOXA10 by 1,25-(OH)2D3in Human Myelomonocytic Cells and Human Endometrial Stromal Cells

Vitamin D receptor (VDR) and the functionally active form of its ligand, 1,25-(OH)2D3, have been implicated in female reproduction function and myeloid leukemic cell differentiation. HOXA10 is necessary for embryo implantation and fertility, as well as hematopoeitic development. In this study, we identified a direct role of vitamin D in the regulation of HOXA10 in primary human endometrial stromal cells, the human endometrial stromal cell line (HESC), and in the human myelomonocytic cell line, U937. Treatment of primary endometrial stromal cells, or the cell lines HESC and U937 with 1,25-(OH)2D3 increased HOXA10 mRNA and protein expression. VDR mRNA and protein were detected in primary uterine stromal cells as well as HESC and U937 cells. We cloned the HOXA10 upstream regulatory sequence and two putative vitamin D response elements (VDRE) into luciferase reporter constructs and transfected primary stromal cells and HESC. One putative VDRE (P1: -385 to -434 bp upstream of HOXA10) drove reporter gene expression in response to treatment with 1,25-(OH)2D3. In EMSA, VDR demonstrated binding to the HOXA10 VDRE in the presence of 1,25-(OH)2D3. 1,25-(OH)2D3 up-regulates HOXA10 expression by binding VDR and interacting with a VDRE in the HOXA10 regulatory region. Direct regulation of HOXA10 by vitamin D has implications for fertility and myeloid differentiation.

9-cis retinoic acid accelerates calcitriol-induced osteocalcin production and promotes degradation of both vitamin D receptor and retinoid X receptor in human osteoblastic cells

Abstract vitamin D receptor (VDR) and retinoid X receptor (RXR) heterodimerize to mediate the genomic actions of 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3), calcitriol), the biologically active form of vitamin D(3). In this study, we show that 9-cis retinoic acid (9-cisRA), the ligand for RXR, accelerates calcitriol-induced expression of osteocalcin gene, the marker for mature osteoblasts. Calcitriol and its synthetic analog KH1060 (1 nM) induced osteocalcin secretion after a 96-h incubation period as detected by radioimmunoassay. When these compounds were used together with 9-cisRA, osteocalcin protein secretion was, however, detected already after 72 and 48 h, respectively. Detection of osteocalcin mRNA with quantitative PCR revealed elevated mRNA levels already after a 4-h treatment of the cells with calcitriol, KH1060, or 9-cisRA compared with untreated cells. In combination treatments, 9-cisRA rapidly stimulated osteocalcin mRNA synthesis induced by the different vitamin D(3) compounds. In MG-63 cells treated with calcitriol or KH1060, the stimulation was maximal after the first 4 h and diminished thereafter. In fact, after the 48-h incubation 9-cisRA reduced osteocalcin mRNA levels in KH1060-treated cells, the amount of mRNA being only 44% of the levels obtained with KH1060 alone. The reduction was accompanied by an increased degradation rate of both VDR and RXRbeta in the presence of 9-cisRA. Furthermore, 9-cisRA increased the formation of RXRbeta-VDR-VDRE complex on the osteocalcin gene VDRE. These results suggest that 9-cisRA accelerates calcitriol-induced osteocalcin production in human osteoblastic cells through increased formation of transcriptionally active chromatin complexes and, subsequently, promotes degradation of the heterodimeric complex of VDR and RXR.

1,25-(OH)2-vitamin D3 suppresses the bone-related Runx2/Cbfa1 gene promoter

The steroid hormone 1,25-(OH)2-vitamin D3 (VD3) regulates osteoblast differentiation by either activating or repressing transcription of numerous bone phenotypic genes. We addressed whether VD3 also influences osteogenesis by controlling activity of the runt-related transcription factor Runx2/Cbfa1, a key regulator of bone formation in vivo. Our data showed that expression of Runx2 was downregulated by VD3 within 24 h in MC3T3 and ROS 17/2.8, but not in ROS 24.1 cells, which lack a functional vitamin D receptor (VDR). Transient transfection assays showed that the initial 0.6 kb of the bone-related rat and mouse Runx2 promoters both exhibited a 50% reduction of promoter activity in response to VD3 in osteoblastic cells. Furthermore, VD3 inhibited Runx2 transcription in ROS 24.1 cells only upon forced expression of the VDR. Gel mobility shift assays with antibodies and oligonucleotide competition experiments demonstrated that proximal promoter sequences (-92 to -16) contain a functional VD3-responsive element (VDRE) that binds a VDR/retinoid X receptor heterodimer. Mutation of this VDRE completely abolished responsiveness of the Runx2 promoter to VD3 treatment. Together these studies establish that Runx2 expression is regulated by VD3. This VD3-mediated suppression of Runx2 activity provides regulatory coupling between tissue-specific and steroid hormone-dependent control of genes during bone formation.

1Alpha25(OH)2D3 interferes with retinoic acid-induced inhibition of c-fos gene expression for AP-1 formation in osteoblastic cells

Our previous studies demonstrated retinoic acid (RA) inhibition of activation protein-1 (AP-1) formation in TNF-alpha-treated osteoblastic MC3T3-E1 cells via c-fos suppression. In the present study, we observed that 1alpha25(OH)2D3 was able to interfere at the transcriptional level with RA inhibition of TNF-alpha-induced c-fos gene expression in cells when the cells were incubated with the vitamin for 24 hr before the RA treatment. 22-Oxa-1,25(OH)2D3 (OCT), an analog derivative of 1alpha25(OH)2D3, having high affinity for the vitamin D3 receptor (VDR), also interfered with the RA-induced inhibition of c-fos gene expression in the TNF-alpha-treated cells. In contrast, this was not the case for 24,25(OH)2D3. Moreover, we observed that the interfering effect was clearly blocked by pretreatment with VDR antisense oligonucleotide. 1alpha25(OH)2D3 interfered with RA inhibition of the TPA-response element binding activity of AP-1 in the cytokine-treated cells. Furthermore, 1alpha25(OH)2D3 actually blocked the AP-1-mediated gene expression of monocyte chemoattractant JE/MCP-1 induced in the cytokine-treated cells. The present study suggests a regulatory interference by 1alpha25(OH)2D3 for RA inhibition of TNF-alpha-induced AP-1 activity in osteoblasts.

Vitamin D(3) augments osteoclastogenesis via vitamin D-responsive element of mouse RANKL gene promoter

Receptor activator of NF-kappaB ligand (RANKL) is a membrane-bound signal transducer necessary for the induction and maintenance of osteoclasts. To clarify the molecular mechanism by which 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) augments osteoclasts, we characterized the promoter region of the mouse RANKL gene. Mirroring in vitro osteoclastogenesis demonstrated by a coculture of bone marrow macrophages with ST2 stromal cells, Northern blot, and nuclear run-on analyses showed that 1,25-(OH)(2)D(3) upregulate RANKL gene expression at the transcriptional level. Using a series of deletion mutants of mouse RANKL promoter-luciferase reporter gene constructs, transient transfection studies revealed that the inductive effect of 1,25-(OH)(2)D(3) was abolished when the region up to -723 was deleted. An electrophoretic motility shift assay demonstrated that the VDR-RXRbeta heterodimer bound to AGGTCAGCCTGGTTCA (-937/-922), and VDRE/nuclear protein supershift complexes that bound to anti-VDR and -RXRbeta antibodies were detected in the nuclear extract of 1,25-(OH)(2)D(3)-treated ST2 cells. Furthermore, induction of mutation to the putative VDRE also diminished the inductive effect of 1,25-(OH)(2)D(3). We therefore concluded that mouse RANKL gene is one of the target genes of 1,25-(OH)(2)D(3) containing a functional VDRE in the promoter region.