Phosphorylation of serine 208 in the human vitamin D receptor. The predominant amino acid phosphorylated by casein kinase II, in vitro, and identification as a significant phosphorylation site in intact cells

Vitamin D in Cancer Prevention and Treatment: A Review of Epidemiological, Preclinical, and Cellular Studies

BACKGROUND

Inhibition of human carcinomas has previously been linked to vitamin D due to its effects on cancer cell proliferation, migration, angiogenesis, and apoptosis induction. The anticancer activity of vitamin D has been confirmed by several studies, which have shown that increased cancer incidence is associated with decreased vitamin D and that dietary supplementation of vitamin D slows down the growth of xenografted tumors in mice. Vitamin D inhibits the growth of cancer cells by the induction of apoptosis as well as by arresting the cells at the G0/G1 (or) G2/M phase of the cell cycle. Aim and Key Scientific Concepts of the Review: The purpose of this article is to thoroughly review the existing information and discuss and debate to conclude whether vitamin D could be used as an agent to prevent/treat cancers. The existing empirical data have demonstrated that vitamin D can also work in the absence of vitamin D receptors (VDRs), indicating the presence of multiple mechanisms of action for this sunshine vitamin. Polymorphism in the VDR is known to play a key role in tumor cell metastasis and drug resistance. Although there is evidence that vitamin D has both therapeutic and cancer-preventive properties, numerous uncertainties and concerns regarding its use in cancer treatment still exist. These include (a) increased calcium levels in individuals receiving therapeutic doses of vitamin D to suppress the growth of cancer cells; (b) hyperglycemia induction in certain vitamin D-treated study participants; (c) a dearth of evidence showing preventive or therapeutic benefits of cancer in clinical trials; (d) very weak support from proof-of-principle studies; and (e) the inability of vitamin D alone to treat advanced cancers. Addressing these concerns, more potent and less toxic vitamin D analogs have been created, and these are presently undergoing clinical trial evaluation. To provide key information regarding the functions of vitamin D and VDRs, this review provided details of significant advancements in the functional analysis of vitamin D and its analogs and VDR polymorphisms associated with cancers.

Role of Neural Stem Cells and Vitamin D Receptor (VDR)-Mediated Cellular Signaling in the Mitigation of Neurological Diseases

Specific stem cell-based therapies for treating Alzheimer's disease, Parkinson's disease, and schizophrenia are gaining importance in recent years. Accumulating data is providing further support by demonstrating the efficacy of neural stem cells in enhancing the neurogenesis in the aging brain. In addition to stem cells, recent studies have shown the efficacy of supplementing vitamin D in promoting neurogenesis and neuronal survival. Studies have also demonstrated the presence of mutational variants and single-nucleotide polymorphisms of the vitamin D receptor (VDR) in neurological disorders; however, implications of these mutations in the pathophysiology and response to drug treatment are yet to be explored. Hence, in this article, we have reviewed recent reports pertaining to the role of neural stem cells and VDR-mediated cellular signaling cascades that are involved in enhancing the neurogenesis through Wnt/β-catenin and Sonic Hedgehog pathways. This review benefits neurobiologists and pharmaceutical industry experts to develop stem cell-based and vitamin D-based therapies to better treat the patients suffering from neurological diseases.

Vitamin D Receptor Mediates a Myriad of Biological Actions Dependent on Its 1,25-Dihydroxyvitamin D Ligand: Distinct Regulatory Themes Revealed by Induction of Klotho and Fibroblast Growth Factor-23

The hormonal vitamin D metabolite, 1,25‐dihydroxyvitamin D [1,25(OH)₂D], produced in kidney, acts in numerous end organs via the nuclear vitamin D receptor (VDR) to trigger molecular events that orchestrate bone mineral homeostasis. VDR is a ligand‐controlled transcription factor that obligatorily heterodimerizes with retinoid X receptor (RXR) to target vitamin D responsive elements (VDREs) in the vicinity of vitamin D‐regulated genes. Circulating 1,25(OH)₂D concentrations are governed by PTH, an inducer of renal D‐hormone biosynthesis catalyzed by CYP27B1 that functions as the key player in a calcemic endocrine circuit, and by fibroblast growth factor‐23 (FGF23), a repressor of the CYP27B1 renal enzyme, creating a hypophosphatemic endocrine loop. 1,25(OH)₂D/VDR–RXR acts in kidney to induce Klotho (a phosphaturic coreceptor for FGF23) to correct hyperphosphatemia, NPT2a/c to correct hypophosphatemia, and TRPV5 and CaBP28k to enhance calcium reabsorption. 1,25(OH)₂D‐liganded VDR–RXR functions in osteoblasts/osteocytes by augmenting RANK‐ligand expression to paracrine signal osteoclastic bone resorption, while simultaneously inducing FGF23, SPP1, BGLP, LRP5, ANK1, ENPP1, and TNAP, and conversely repressing RUNX2 and PHEX expression, effecting localized control of mineralization to sculpt the skeleton. Herein, we document the history of 1,25(OH)₂D/VDR and summarize recent advances in characterizing their physiology, biochemistry, and mechanism of action by highlighting two examples of 1,25(OH)₂D/VDR molecular function. The first is VDR‐mediated primary induction of Klotho mRNA by 1,25(OH)₂D in kidney via a mechanism initiated by the docking of liganded VDR–RXR on a VDRE at −35 kb in the mouse Klotho gene. In contrast, the secondary induction of FGF23 by 1,25(OH)₂D in bone is proposed to involve rapid nongenomic action of 1,25(OH)₂D/VDR to acutely activate PI3K, in turn signaling the induction of MZF1, a transcription factor that, in cooperation with c‐ets1‐P, binds to an enhancer element centered at −263 bp in the promoter‐proximal region of the mouse fgf23 gene. Chronically, 1,25(OH)₂D‐induced osteopontin apparently potentiates MZF1. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Pomegranate derivative urolithin A enhances vitamin D receptor signaling to amplify serotonin-related gene induction by 1,25-dihydroxyvitamin D

Mediated by the nuclear vitamin D receptor (VDR), the hormonally active vitamin D metabolite, 1,25-dihydroxyvitamin D₃ (1,25D), is known to regulate expression of genes impacting calcium and phosphorus metabolism, the immune system, and behavior. Urolithin A, a nutrient metabolite derived from pomegranate, possibly acting through AMP kinase (AMPK) signaling, supports respiratory muscle health in rodents and longevity in C. elegans by inducing oxidative damage-reversing genes and mitophagy. We show herein that urolithin A enhances transcriptional actions of 1,25D driven by co-transfected vitamin D responsive elements (VDREs), and dissection of this genomic effect in cell culture reveals: 1) urolithin A concentration-dependency, 2) occurrence with isolated natural VDREs, 3) nuclear receptor selectivity for VDR over ER, LXR and RXR, and 4) significant 3- to 13-fold urolithin A-augmentation of 1,25D-dependent mRNA encoding the widely expressed 1,25D-detoxification enzyme, CYP24A1, a benchmark vitamin D target gene. Relevant to potential behavioral effects of vitamin D, urolithin A elicits enhancement of 1,25D-dependent mRNA encoding tryptophan hydroxylase-2 (TPH2), the serotonergic neuron-expressed initial enzyme in tryptophan metabolism to serotonin. Employing quantitative real time-PCR, we demonstrate that TPH2 mRNA is induced 1.9-fold by 10 nM 1,25D treatment in culture of differentiated rat serotonergic raphe (RN46A-B14) cells, an effect magnified 2.5-fold via supplementation with 10 μM urolithin A. This potentiation of 1,25D-induced TPH2 mRNA by urolithin A is followed by a 3.1- to 3.7-fold increase in serotonin concentration in culture medium from the pertinent neuronal cell line, RN46A-B14. These results are consistent with the concept that two natural nutrient metabolites, urolithin A from pomegranate and 1,25D from sunlight/vitamin D, likely acting via AMPK and VDR, respectively, cooperate mechanistically to effect VDRE-mediated regulation of gene expression in neuroendocrine cells. Finally, gedunin, a neuroprotective natural product from Indian neem tree that impacts the brain derived neurotropic factor pathway, similarly potentiates 1,25D/VDR-action.

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Hormonal 1,25-dihydroxyvitamin D acts in brain to induce tryptophan hydroxylase-2.

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Urolithin A derived from ellagitannins in pomegranates curbs neuroinflammation.

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Urolithin A enhances the transcriptional actions of 1,25-dihydroxyvitamin D.

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Urolithin A raises 1,25-dihydroxyvitamin D-induced tryptophan hydroxylase-2 mRNA.

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Serotonin rises in raphe cells exposed to urolithin A and 1,25-dihydroxyvitamin D.

Fine tuning of vitamin D receptor (VDR) activity by post-transcriptional and post-translational modifications

Vitamin D receptor (VDR) is a member of the nuclear receptor (NR) superfamily of ligand-activated transcription factors. Activated VDR is responsible for maintaining calcium and phosphate homeostasis, and is required for proper cellular growth, cell differentiation and apoptosis. The expression of both phases I and II drug-metabolizing enzymes is also regulated by VDR, therefore it is clinically important.

Post-translational modifications of NRs have been known as an important mechanism modulating the activity of NRs and their ability to drive the expression of target genes. The aim of this mini review is to summarize the current knowledge about post-transcriptional and post-translational modifications of VDR.

The impact of VDR expression and regulation in vivo

The vitamin D receptor (VDR) mediates the pleiotropic biological actions of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). These actions include orchestration of mineral homeostasis which is coordinated by the kidney, intestine, bone and parathyroid gland wherein the VDR transcriptionally regulates expression of the genes involved in this complex process. Mutations in human VDR (hVDR) cause hereditary vitamin D resistant rickets, a genetic syndrome characterized by hypocalcemia, hyperparathyroidism and rickets resulting from dysregulation of mineral homeostasis. Expression of the VDR is regulated by external stimuli in a tissue-specific manner. However, the mechanisms of this tissue-specificity remain unclear. Studies also suggest that phosphorylation of hVDR at serine 208 impacts the receptor's transcriptional activity. These experiments were conducted in vitro, however, and therefore limited in their conclusions. In this report, we summarize (1) our most recently updated ChIP-seq data from mouse tissues to identify regulatory regions responsible for the tissues-specific regulation of the VDR and (2) our studies to understand the mechanism of hormonal regulation of Vdr expression in bone and kidney in vivo using transgenic mouse strains generated by mouse mini-genes that contain comprehensive genetic information capable of recapitulating endogenous Vdr gene regulation and expression. We also defined the functional human VDR gene locus in vivo by using a human mini-gene comparable to that in the mouse to generate a humanized VDR mouse strain in which the receptor is expressed at normal levels (normal expressor). The present report also shows that a humanized mouse model in which the VDR is expressed at levels about 10-fold lower than the normal expressor mouse rescued the VDR-null phenotype despite its reduced transcriptional activity relative to wildtype expression. We also generated an additional humanized mouse model expressing hVDR bearing a mutation converting serine 208 to alanine (hVDR-S208A). In spite of the mutation, target gene expression induced by the ligand was unchanged relative to a mouse strain expressing comparable levels of wildtype hVDR. Further characterization also showed that serum calcium and parathyroid hormone levels were normal and alopecia was not observed in this hVDR-S208A mouse strain as well. Taken together, our in vivo studies using ChIP-seq analyses and the mini-gene transgenic mice improve our understanding of the tissue-specific regulatory mechanisms of controlling VDR expression and the mechanisms of action of the VDR.

Mouse and human BAC transgenes recapitulate tissue-specific expression of the vitamin D receptor in mice and rescue the VDR-null phenotype

The biological actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are mediated by the vitamin D receptor (VDR), which is expressed in numerous target tissues in a cell type-selective manner. Recent studies using genomic analyses and recombineered bacterial artificial chromosomes (BACs) have defined the specific features of mouse and human VDR gene loci in vitro. In the current study, we introduced recombineered mouse and human VDR BACs as transgenes into mice and explored their expression capabilities in vivo. Individual transgenic mouse strains selectively expressed BAC-derived mouse or human VDR proteins in appropriate vitamin D target tissues, thereby recapitulating the tissue-specific expression of endogenous mouse VDR. The mouse VDR transgene was also regulated by 1,25(OH)2D3 and dibutyryl-cAMP. When crossed into a VDR-null mouse background, both transgenes restored wild-type basal as well as 1,25(OH)2D3-inducible gene expression patterns in the appropriate tissues. This maneuver resulted in the complete rescue of the aberrant phenotype noted in the VDR-null mouse, including systemic features associated with altered calcium and phosphorus homeostasis and disrupted production of parathyroid hormone and fibroblast growth factor 23, and abnormalities associated with the skeleton, kidney, parathyroid gland, and the skin. This study suggests that both mouse and human VDR transgenes are capable of recapitulating basal and regulated expression of the VDR in the appropriate mouse tissues and restore 1,25(OH)2D3 function. These results provide a baseline for further dissection of mechanisms integral to mouse and human VDR gene expression and offer the potential to explore the consequence of selective mutations in VDR proteins in vivo.

Inhibition of protein kinase CK2 reduces Cyp24a1 expression and enhances 1,25-dihydroxyvitamin D(3) antitumor activity in human prostate cancer cells

Vitamin D has broad range of physiological functions and antitumor effects. 24-Hydroxylase, encoded by the CYP24A1 gene, is the key enzyme for degrading many forms of vitamin D including the most active form, 1,25D(3). Inhibition of CYP24A1 enhances 1,25D(3) antitumor activity. To isolate regulators of CYP24A1 expression in prostate cancer cells, we established a stable prostate cancer cell line PC3 with CYP24A1 promoter driving luciferase expression to screen a small molecular library for compounds that inhibit CYP24A1 promoter activity. From this screening, we identified, 4,5,6,7-tetrabromobenzimidazole (TBBz), a protein kinase CK2 selective inhibitor as a disruptor of CYP24A1 promoter activity. We show that TBBz inhibits CYP24A1 promoter activity induced by 1,25D(3) in prostate cancer cells. In addition, TBBz downregulates endogenous CYP24A1 mRNA level in TBBz-treated PC3 cells. Furthermore, siRNA-mediated CK2 knockdown reduces 1,25D(3)-induced CYP24A1 mRNA expression in PC3 cells. These results suggest that CK2 contributes to 1,25D(3)-mediated target gene expression. Finally, inhibition of CK2 by TBBz or CK2 siRNA significantly enhances 1,25D(3)-mediated antiproliferative effect in vitro and in vivo in a xenograft model. In summary, our findings reveal that protein kinase CK2 is involved in the regulation of CYP24A1 expression by 1,25D(3) and CK2 inhibitor enhances 1,25D(3)-mediated antitumor effect.

The vitamin D receptor: new paradigms for the regulation of gene expression by 1,25-dihydroxyvitamin D3

This article represents a summary of what is known of the VDR protein and its molecular mechanism of action at target genes. New methodologies now used, such as ChIP-chip and ChIP-seq, as well as novel reporter studies using large BAC clones stably transfected into culture cells or introduced as transgenes in mice, are providing new insights into how 1,25(OH)2D3-activated VDR modulates the expression of genes at single gene loci and at the level of gene networks. Many of these insights are unexpected and suggest that gene regulation is even more complex than previously appreciated. These studies also highlight new technologies and their central role in establishing fundamental biologic principles.

Dexamethasone enhances 1alpha,25-dihydroxyvitamin D3 effects by increasing vitamin D receptor transcription

Calcitriol, the active form of vitamin D, in combination with the glucocorticoid dexamethasone (Dex) has been shown to increase the antitumor effects of calcitriol in squamous cell carcinoma. In this study we found that pretreatment with Dex potentiates calcitriol effects by inhibiting cell growth and increasing vitamin D receptor (VDR) and VDR-mediated transcription. Treatment with actinomycin D inhibits Vdr mRNA synthesis, indicating that Dex regulates VDR expression at transcriptional level. Real time PCR shows that treatment with Dex increases Vdr transcripts in a time- and a dose-dependent manner, indicating that Dex directly regulates expression of Vdr. RU486, an inhibitor of glucocorticoids, inhibits Dex-induced Vdr expression. In addition, the silencing of glucocorticoid receptor (GR) abolishes the induction of Vdr by Dex, indicating that Dex increases Vdr transcripts in a GR-dependent manner. A fragment located 5.2 kb upstream of Vdr transcription start site containing two putative glucocorticoid response elements (GREs) was evaluated using a luciferase-based reporter assay. Treatment with 100 nm Dex induces transcription of luciferase driven by the fragment. Deletion of the GRE distal to transcription start site was sufficient to abolish Dex induction of luciferase. Also, chromatin immunoprecipitation reveals recruitment of GR to distal GRE with Dex treatment. We conclude that Dex increases VDR and vitamin D effects by increasing Vdr de novo transcription in a GR-dependent manner.

Vitamin D receptor (VDR)-mediated actions of 1α,25(OH)₂vitamin D₃: genomic and non-genomic mechanisms

The conformationally flexible secosteroid, 1α,25(OH)₂vitamin D₃ (1α,25(OH)₂D₃) initiates biological responses via binding to the vitamin D receptor (VDR). The VDR contains two overlapping ligand binding sites, a genomic pocket (VDR-GP) and an alternative pocket (VDR-AP), that respectively bind a bowl-like ligand configuration (gene transcription) or a planar-like ligand shape (rapid responses). When occupied by 1α,25(OH)₂D₃, the VDR-GP interacts with the retinoid X receptor to form a heterodimer that binds to vitamin D responsive elements in the region of genes directly controlled by 1α,25(OH)₂D₃. By recruiting complexes of either coactivators or corepressors, activated VDR modulates the transcription of genes encoding proteins that promulgate the traditional genomic functions of vitamin D, including signaling intestinal calcium and phosphate absorption to effect skeletal and calcium homeostasis. 1α,25(OH)₂D₃/VDR control of gene expression and rapid responses also delays chronic diseases of aging such as osteoporosis, cancer, type-1 and -2 diabetes, arteriosclerosis, vascular disease, and infection.

The vitamin D receptor: new paradigms for the regulation of gene expression by 1,25-dihydroxyvitamin D(3)

The actions of the vitamin D hormone 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are mediated by the vitamin D receptor (VDR), a ligand-activated transcription factor that functions to control gene expression. After ligand activation, the VDR binds directly to specific sequences located near promoters and recruits a variety of coregulatory complexes that perform the additional functions required to modify transcriptional output. Recent advances in transcriptional regulation, which permit the unbiased identification of the regulatory regions of genes, are providing new insight into how genes are regulated. Surprisingly, gene regulation requires the orchestrated efforts of multiple modular enhancers often located many kilobases upstream, downstream, or within the transcription units themselves. These studies are transforming our understanding of how 1,25(OH)(2)D(3) regulates gene transcription.

Vitamin D signalling pathways in cancer: potential for anticancer therapeutics

Epidemiological studies indicate that vitamin D insufficiency could have an aetiological role in various human cancers. Preclinical research indicates that the active metabolite of vitamin D, 1alpha,25(OH)2D3, also known as calcitriol, or vitamin D analogues might have potential as anticancer agents because their administration has antiproliferative effects, can activate apoptotic pathways and inhibit angiogenesis. In addition, 1alpha,25(OH)2D3 potentiates the anticancer effects of many cytotoxic and antiproliferative anticancer agents. Here, we outline the epidemiological, preclinical and clinical studies that support the development of 1alpha,25(OH)2D3 and vitamin D analogues as preventative and therapeutic anticancer agents.

Phosphorylation at serine 208 of the 1alpha,25-dihydroxy Vitamin D3 receptor modulates the interaction with transcriptional coactivators

Upon ligand binding the 1alpha,25-dihydroxy Vitamin D3 receptor (VDR) undergoes a conformational change that allows interaction with coactivator proteins including p160/SRC family members and the multimeric DRIP complex through the DRIP205 subunit. Casein kinase II (CKII) phosphorylates VDR both in vitro and in vivo at serine 208 within the hinge domain. This phosphorylation does not affect the ability of VDR to bind DNA, but increases its ability to transactivate target promoters. Here, we have analyzed whether phosphorylation of VDR by CKII modulates the ability of VDR to interact with coactivators in vitro. We find that both mutation of serine 208 to aspartic acid (VDRS208D) or phosphorylation of VDR by CKII enhance the interaction of VDR with DRIP205 in the presence of 1alpha,25-dihydroxy Vitamin D3. We also find that the mutation VDRS208D neither affects the ability of this protein to bind DNA nor to interact with SRC-1 and RXRalpha. Together, our results indicate that phosphorylation of VDR at serine 208 contributes to modulate the affinity of VDR for the DRIP complex and therefore may have a role in vivo regulating VDR-mediated transcriptional enhancement.

Molecular mechanism of 1,25-dihydroxyvitamin D3 inhibition of adipogenesis in 3T3-L1 cells

We have investigated the molecular mechanism whereby 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] inhibits adipogenesis in vitro. 1,25(OH)2D3 blocks 3T3-L1 cell differentiation into adipocytes in a dose-dependent manner; however, the inhibition is ineffective 24-48 h after the differentiation is initiated, suggesting that 1,25(OH)2D3 inhibits only the early events of the adipogenic program. Treatment of 3T3-L1 cells with 1,25(OH)2D3 does not block the mitotic clonal expansion or C/EBPbeta induction; rather, 1,25(OH)2D3 blocks the expression of C/EBPalpha, peroxisome proliferator-activated receptor-gamma (PPARgamma), sterol regulatory element-binding protein-1, and other downstream adipocyte markers. The inhibition by 1,25(OH)2D3 is reversible, since removal of 1,25(OH)2D3 from the medium restores the adipogenic process with only a temporal delay. Interestingly, although the vitamin D receptor (VDR) protein is barely detectable in 3T3-L1 preadipocytes, its levels are dramatically increased during the early phase of adipogenesis, peaking at 4-8 h and subsiding afterward throughout the rest of the differentiation program; 1,25(OH)2D3 treatment appears to stabilize the VDR protein levels. Consistently, adenovirus-mediated overexpression of human (h) VDR in 3T3-L1 cells completely blocks the adipogenic program, confirming that VDR is inhibitory. Inhibition of adipocyte differentiation by 1,25(OH)2D3 is ameliorated by troglitazone, a specific PPARgamma antagonist; conversely, hVDR partially suppresses the transacting activity of PPARgamma but not of C/EBPbeta or C/EBPalpha. Moreover, 1,25(OH)2D3 markedly suppresses C/EBPalpha and PPARgamma mRNA levels in mouse epididymal fat tissue culture. Taken together, these data indicate that the blockade of 3T3-L1 cell differentiation by 1,25(OH)2D3 occurs at the postclonal expansion stages and involves direct suppression of C/EBPalpha and PPARgamma upregulation, antagonization of PPARgamma activity, and stabilization of the inhibitory VDR protein.

Molecular and functional comparison of 1,25-dihydroxyvitamin D(3) and the novel vitamin D receptor ligand, lithocholic acid, in activating transcription of cytochrome P450 3A4

The vitamin D receptor (VDR) binds to and mediates the effects of the 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) hormone to alter gene transcription. A newly recognized VDR ligand is the carcinogenic bile acid, lithocholic acid (LCA). We demonstrate that, in HT-29 colon cancer cells, both LCA and 1,25(OH)(2)D(3) induce expression of cytochrome P450 3A4 (CYP3A4), an enzyme involved in cellular detoxification. We also show that LCA-VDR stimulates transcription of gene reporter constructs containing DR3 and ER6 vitamin D responsive elements (VDREs) from the human CYP3A4 gene. Utilizing gel mobility shift, pulldown, and mammalian two-hybrid assays, we observe that: (i) 1,25(OH)(2)D(3) enhances retinoid X receptor (RXR) heterodimerization with VDR more effectively than LCA, (ii) the 1,25(OH)(2)D(3)-liganded VDR-RXR heterodimer recruits full-length SRC-1 coactivator, whereas this interaction is minimal with LCA unless LXXLL-containing fragments of SRC-1 are employed, and (iii) both 1,25(OH)(2)D(3) and LCA enhance the binding of VDR to DRIP205/mediator, but unlike 1,25(OH)(2)D(3)-VDR, LCA-VDR does not interact detectably with NCoA-62 or TRIP1/SUG1, suggesting a different pattern of LCA-VDR comodulator association. Finally, residues in the human VDR (hVDR) ligand binding domain (LBD) were altered to create mutants unresponsive to 1,25(OH)(2)D(3)- and/or LCA-stimulated transactivation, identifying S237 and S225/S278 as critical for 1,25(OH)(2)D(3) and LCA action, respectively. Therefore, these two VDR ligands contact distinct residues in the binding pocket, perhaps generating unique receptor conformations that determine the degree of RXR and comodulator binding. We propose that VDR is a bifunctional regulator, with the 1,25(OH)(2)D(3)-liganded conformation facilitating high affinity endocrine actions, and the LCA-liganded configuration mediating local, lower affinity cellular detoxification by upregulation of CYP3A4 in the colon.

Enhanced coactivator binding and transcriptional activation of mutant vitamin D receptors from patients with hereditary 1,25-dihydroxyvitamin D-resistant rickets by phosphorylation and vitamin D analogs

In this study, we report that the function of certain mutant VDRs from patients with hereditary HVDRR can at least be partially restored by phosphorylation and hexafluoro 1,25(OH)2D3 analogs. Our study provides new insights into mechanisms involved in enhancement of mutant VDR function.

INTRODUCTION

1,25-Dihydroxyvitamin D-resistant rickets (HVDRR) is a rare genetic disorder caused by inactivating mutations in the vitamin D receptor (VDR). In this study, we examined VDR from patients with HVDRR having mutations in the ligand-binding domain (F251C, I268T, H305Q, E420K). We examined methods of restoring transcriptional activity of these mutants and the mechanisms involved.

MATERIALS AND METHODS

Reporter gene transcriptional assays were used to examine the activation of mutant VDRs. Western-blot analysis, glutathione S-transferase (GST) pull-down assays, and chromatin immunoprecipitation (ChIP) assays were also used in this study.

RESULTS

Using mutant VDRs, H305Q, F251C, I268T, and 10(-8) M 1,25(OH)2D3, only 10-30% of the activity of wildtype (WT) VDR in activating 24(OH)ase transcription was observed. The transcriptional response of mutant VDR mutants was significantly enhanced 2- to 3-fold by co-treatment of VDR mutant transfected COS-7 cells with 1,25(OH)2D3 and okadaic acid (OA; inhibitor of phosphatase; 50 nM). The H305Q mutant was the most responsive (90% of the response exhibited by WT VDR was restored). The E420K mutant was unresponsive to 1,25(OH)2D3 in the presence or absence of OA. The increased transcriptional response correlated with an increase in the interaction between DRIP205 and the mutant VDR. We further provide evidence that OA induces the phosphorylation of CREB-binding protein (CBP), indicating for the first time a correlation between phosphorylation of CBP and enhanced VDR function. Hexafluoro 1,25(OH)2D3 analogs (RO-26-2198 and RO-4383561) also resulted in at least a partial restoration of the transcriptional responsiveness of mutant VDRs I268T, F251C, and H305Q. Our data indicate that the enhanced potency of the hexafluoro analogs may be caused by increased DRIP205 and glucocorticoid receptor interacting protein 1 (GRIP-1) binding to VDRs and enhanced association of VDRs with DNA, as suggested by results of ChIP assays.

CONCLUSION

Our study provides new insights into the mechanisms involved in the enhancement of VDR function by both phosphorylation and hexafluoro analogs and forms a basis for future study of vitamin D analogs or specifically designed kinase activity mediators as potential therapy for the treatment of selected patients with HVDRR.

Phosphorylation of human vitamin D receptor serine-182 by PKA suppresses 1,25(OH)2D3-dependent transactivation

The human vitamin D receptor (hVDR), which is a substrate for several protein kinases, mediates the actions of its 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) ligand to regulate gene expression. To determine the site, and functional impact, of cAMP-dependent protein kinase (PKA)-catalyzed phosphorylation of hVDR, we generated a series of C-terminally truncated and point mutant receptors. Incubation of mutant hVDRs with PKA and [gamma-32P]ATP, in vitro, or overexpressing them in COS-7 kidney cells labeled with [32P]orthophosphate, revealed that serine-182 is the predominant residue in hVDR phosphorylated by PKA. An aspartate substituted mutant (S182D), incorporating a negative charge to mimic phosphorylation, displayed only 50% of the transactivation capacity in response to 1,25(OH)2D3 of either wild-type or an S182A-altered hVDR. When the catalytic subunit of PKA was overexpressed, a similar reduction in wild-type but not S182D hVDR transactivity was observed. In a mammalian two-hybrid system, S182D bound less avidly than wild-type or S182A hVDR to the retinoid X receptor (RXR) heterodimeric partner that co-mediates vitamin D responsive element recognition and transactivation. These data suggest that hVDR serine-182 is a primary site for PKA phosphorylation, an event that leads to an attenuation of both RXR heterodimerization and resultant transactivation of 1,25(OH)2D3 target genes.

The functional consequences of cross-talk between the vitamin D receptor and ERK signaling pathways are cell-specific

The actions of the active metabolite of 1,25-(OH)2D3 (1,25-D) are mediated primarily by the vitamin D receptor (VDR), a member of the nuclear receptor family of ligand-activated transcription factors. Although their ligands cause transcriptional activation, many of the ligands also rapidly activate cellular signaling pathways through mechanisms that have not been fully elucidated. We find that 1,25-D causes a rapid, but sustained activation of ERK (extracellular signal-regulated kinase) in bone cell lines. However, the effect of ERK activation on VDR transcriptional activity was cell line-specific. Inhibition of ERK activation by the MEK inhibitor, U0126, stimulated VDR activity in MC3T3-E1 cells, but inhibited the activity in MG-63 cells as well as in HeLa cells. VDR is not a known target of ERK. We found that the ERK target responsible for reduced VDR activity in MC3T3-E1 cells is RXRalpha. MC3T3-E1 cells express lower levels of RXRbeta and RXRgamma than either HeLa or MG-63 cells. Although overexpression of RXRalpha in MC3T3-E1 cells increased VDR activity, U0126 further enhanced the activity. In contrast, overexpression of RXRgamma stimulated VDR activity but abrogated the stimulation by U0126. Thus, although 1,25-D treatment activates ERK in many cell types, subsequently inducing changes independent of VDR, the effects of treatment with 1,25-D on the transcriptional activity of VDR are RXR isoform-specific. In cells in which RXRalpha is the VDR partner, the transcriptional activation of VDR by 1,25-D is attenuated by the concomitant activation of ERK. In cells utilizing RXRgamma, ERK activation enhances VDR transcriptional activity.

Liganded VDR induces CYP3A4 in small intestinal and colon cancer cells via DR3 and ER6 vitamin D responsive elements

The nuclear vitamin D receptor (VDR) mediates the effects of 1,25-dihydroxyvitamin D(3) (1,25D(3)) to alter intestinal gene transcription and promote calcium absorption. Because 1,25D(3) also exerts anti-cancer effects, we examined the efficacy of 1,25D(3) to induce cytochrome P450 (CYP) enzymes. Exposure of human colorectal adenocarcinoma cells (HT-29) to 10(-8)M 1,25D(3) resulted in >/=3-fold induction of CYP3A4 mRNA and protein as assessed by RT-PCR and Western blotting, respectively. Six vitamin D responsive element (VDRE)-like sequences in the promoter region of the CYP3A4 gene were then individually tested for their ability to enhance transcription. A canonical DR3-type element in the distal region of the promoter (-7719-GGGTCAgcaAGTTCA-7733), and a proximal, non-classical everted repeat with a spacer of 6 bp (ER6; -169-TGAACTcaaaggAGGTCA-152) were identified as functional VDREs in this CYP gene. These data suggest that 1,25D(3)-dependent, VDR-mediated induction of CYP3A4 may constitute a chemoprotective mechanism for detoxification of enteric xenobiotics and carcinogens.

Vector-averaged gravity-induced changes in cell signaling and vitamin D receptor activity in MG-63 cells are reversed by a 1,25-(OH)2D3 analog, EB1089

Skeletal unloading in an animal hindlimb suspension model and microgravity experienced by astronauts or as a result of prolonged bed rest causes site-specific losses in bone mineral density of 1%-2% per month. This is accompanied by reductions in circulating levels of 1,25-(OH)(2)D(3), the active metabolite of vitamin D. 1,25-(OH)(2)D(3), the ligand for the vitamin D receptor (VDR), is important for calcium absorption and plays a role in differentiation of osteoblasts and osteoclasts. To examine the responses of cells to activators of the VDR in a simulated microgravity environment, we used slow-turning lateral vessels (STLVs) in a rotating cell culture system. We found that, similar to cells grown in microgravity, MG-63 cells grown in the STLVs produce less osteocalcin, alkaline phosphatase, and collagen Ialpha1 mRNA and are less responsive to 1,25-(OH)(2)D(3). In addition, expression of VDR was reduced. Moreover, growth in the STLV caused activation of the stress-activated protein kinase pathway (SAPK), a kinase that inhibits VDR activity. In contrast, the 1,25-(OH)(2)D(3) analog, EB1089, was able to compensate for some of the STLV-associated responses by reducing SAPK activity, elevating VDR levels, and increasing expression of osteocalcin and alkaline phosphatase. These studies suggest that, not only does simulated microgravity reduce differentiation of MG-63 cells, but the activity of the VDR, an important regulator of bone metabolism, is reduced. Use of potent, less calcemic analogs of 1,25-(OH)(2)D(3) may aid in overcoming this defect.

Enhancement of VDR-mediated transcription by phosphorylation: correlation with increased interaction between the VDR and DRIP205, a subunit of the VDR-interacting protein coactivator complex

When UMR-106 osteoblastic cells, LLCPK1 kidney cells, and VDR transfected COS-7 cells were transfected with the rat 24-hydroxylase [24(OH)ase] promoter (-1,367/+74) or the mouse osteopontin (OPN) promoter (-777/+79), we found that the response to 1,25dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] could be significantly enhanced 2- to 5-fold by the protein phosphatase inhibitor, okadaic acid (OA). Enhancement of 1,25-(OH)(2)D(3)-induced transcription by OA was also observed using a synthetic reporter gene containing either the proximal 24(OH)ase vitamin D response element (VDRE) or the OPN VDRE, suggesting that the VDRE is sufficient to mediate this effect. OA also enhanced the 1,25-(OH)(2)D(3)-induced levels of 24(OH)ase and OPN mRNA in UMR osteoblastic cells. The effect of OA was not due to an up-regulation of VDR or to an increase in VDR-RXR interaction with the VDRE. To determine whether phosphorylation regulates VDR-mediated transcription by modulating interactions with protein partners, we examined the effect of phosphorylation on the protein-protein interaction between VDR and DRIP205, a subunit of the vitamin D receptor-interacting protein (DRIP) coactivator complex, using glutathione-S-transferase pull-down assays. Similar to the functional studies, OA treatment was consistently found to enhance the interaction of VDR with DRIP205 3- to 4-fold above the interaction observed in the presence of 1,25-(OH)(2)D(3) alone. In addition, studies were done with the activation function-2 defective VDR mutant, L417S, which is unable to stimulate transcription in response to 1,25-(OH)(2)D(3) or to interact with DRIP205. However, in the presence of OA, the mutant VDR was able to activate 24(OH)ase and OPN transcription and to recruit DRIP205, suggesting that OA treatment may result in a conformational change in the activation function-2 defective mutant that creates an active interaction surface with DRIP205. Taken together, these findings suggest that increased interaction between VDR and coactivators such as DRIP205 may be a major mechanism that couples extracellular signals to vitamin D action.

Pharmacological modulation of nerve growth factor synthesis: a mechanistic comparison of vitamin D receptor and beta(2)-adrenoceptor agonists

Increasing nerve growth factor (NGF) in the PNS is a rational strategy for treating certain neurodegenerative disorders. The present studies were undertaken to compare two compounds, a vitamin D(3) analogue (CB1093) with minimal calcaemic effects, and clenbuterol, a long-acting beta(2)-adrenoceptor agonist, both of which induce NGF synthesis in vivo. Clenbuterol caused significant increases in both NGF mRNA and protein in 3T3 cells; with maxima at 10 nM and at 8-12 h exposure. Effects of clenbuterol on NGF mRNA were antagonized by propranolol. Mobility shift assays on whole cell extracts showed that clenbuterol increased AP1 binding in 3T3 cells prior to increasing NGF synthesis. Clenbuterol was without effect on NGF mRNA levels in L929 cells, whereas CB1093 caused significant increases in both NGF mRNA and protein levels in both 3T3 and L929 cells. Stimulation was almost maximal at 24 h exposure and was sustained for at least 72 h. The magnitude of the increase was much greater in L929 (700% increase) than in 3T3 cells (80%). Binding to the vitamin D nuclear receptor (VDR), which acts as a transcription factor itself, was increased as early as 30 min after exposure to of CB1093 and maintained up to 24 h. Increased VDR binding preceded increased NGF mRNA. A 150% increase in AP-1 binding was also evident. This study demonstrates that CB1093 and clenbuterol stimulate NGF levels in vitro and that AP-1 binding could be a commonality between the mechanism of NGF induction of these two compounds.

Activation of Src kinase in skeletal muscle cells by 1, 1,25-(OH(2))-vitamin D(3) correlates with tyrosine phosphorylation of the vitamin D receptor (VDR) and VDR-Src interaction

The rapid effect of 1 alpha,25(OH(2))-vitamin D(3) [1 alpha, 25(OH(2))D(3)] on tyrosine kinase Src and its relationship to the vitamin D receptor (VDR) was investigated to further characterize the hormone signaling mechanism in chick muscle cells. Exposure of cultured myotubes to 1 alpha,25(OH(2))D(3) caused a time-dependent increase in Src activity, which was evident at 1 min (one-fold) and reached a maximum at 5 min (15-fold). Immunoblotting with anti-phosphotyrosine antibody of immunoprecipitated Src showed that the hormone decreased Src tyrosine phosphorylation state with maximal effects at 5 min. Using a database for protein consensus motifs we found a putative tyrosine phosphorylation site (amino acids 164-170: KTFDTTY) within the primary sequence of the chick VDR. When the myotube VDR was immunoprecipitated it appeared onto SDS-PAGE gels as a single band of 58 kDa recognized by an anti-phosphotyrosine antibody. Prior treatment of cells with (1)alpha,25(OH(2))D(3) significantly increased tyrosine phosphorylation of the VDR (two- to three-fold above basal levels). In agreement with Src being a SH2-domain containing protein involved in recognition of tyrosine-phosphorylated targets, immunoprecipitation with anti-Src antibody under native conditions followed by blotting with anti-VDR antibody, or using the antibodies in inverse order, showed that the VDR co-precipitates with Src, thus indicating the existence of a VDR/Src complex. Stimulation with the cognate VDR ligand significantly increased formation of the complex with respect to basal conditions. These results altogether provide the first evidence to date for 1 alpha,25(OH(2))D(3) activation involving Src association to tyrosine phosphorylated VDR.

Inhibitory effect of NF-kappaB on 1,25-dihydroxyvitamin D(3) and retinoid X receptor function

Responsiveness to 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] may be diminished in osteoporosis and inflammatory arthritis. The inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) is produced in excess in these disorders and has been shown to decrease osteoblast transcriptional responsiveness to vitamin D and to inhibit the binding of the vitamin D receptor (VDR) and its nuclear partner the retinoid X receptor (RXR) to DNA. Previous studies have shown that a vitamin D (VDRE) or retinoid X DNA response element (RXRE) is sufficient to confer TNF-alpha inhibition of vitamin D or retinoid-stimulated transcription in the absence of known TNF-alpha-responsive DNA sequences. We tested the hypothesis that the TNF-alpha-stimulated transcription factor nuclear factor (NF)-kappaB could, in part, mediate TNF-alpha action by inhibiting the transcriptional potency of the VDR and RXR at their cognate cis regulatory sites. Osteoblastic ROS 17/2.8 cells transfected with a dose of NF-kappaB comparable to that stimulated by TNF-alpha decreased 1,25(OH)(2)D(3)-stimulated transcription. This inhibitory effect of NF-kappaB was not observed on basal transcription of a heterologous reporter in the absence of the VDRE. The effects of NF-kappaB and TNF-alpha were comparable but not additive. COS-7 cells were cotransfected with reporters under the regulation of VDRE or RXRE along with vectors expressing VDR, RXR, and NF-kappaB nuclear proteins. Reconstituted NF-kappaB and the NF-kappaB subunit p65 alone, but not p50, dose dependently suppressed basal and ligand-stimulated transcription. p65 overexpression completely abrogated enhanced VDRE-mediated transcriptional activity in response to 1,25(OH)(2)D(3). Electrophoretic mobility shift experiments did not reveal a direct effect of recombinant NF-kappaB or its individual subunits on the binding of heterodimeric VDR-RXR to DNA. These results suggest that TNF-alpha inhibition of hormone-stimulated transcriptional activation may be mediated by activation of NF-kappaB. In contrast, the inhibitory effect of TNF-alpha on binding of receptors to DNA is unlikely to be mediated by NF-kappaB and is not necessary for inhibition of transcription.

The polymorphic N terminus in human vitamin D receptor isoforms influences transcriptional activity by modulating interaction with transcription factor IIB

The human vitamin D receptor (hVDR) is a ligand-regulated transcription factor that mediates the actions of the 1,25-dihydroxyvitamin D3 hormone to effect bone mineral homeostasis. Employing mutational analysis, we characterized Arg-18/Arg-22, hVDR residues immediately N-terminal of the first DNA binding zinc finger, as vital for contact with human basal transcription factor IIB (TFIIB). Alteration of either of these basic amino acids to alanine also compromised hVDR transcriptional activity. In contrast, an artificial hVDR truncation devoid of the first 12 residues displayed both enhanced interaction with TFIIB and transactivation. Similarly, a natural polymorphic variant of hVDR, termed F/M4 (missing a FokI restriction site), which lacks only the first three amino acids (including Glu-2), interacted more efficiently with TFIIB and also possessed elevated transcriptional activity compared with the full-length (f/M1) receptor. It is concluded that the functioning of positively charged Arg-18/Arg-22 as part of an hVDR docking site for TFIIB is influenced by the composition of the adjacent polymorphic N terminus. Increased transactivation by the F/M4 neomorphic hVDR is hypothesized to result from its demonstrated enhanced association with TFIIB. This proposal is supported by the observed conversion of f/M1 hVDR activity to that of F/M4 hVDR, either by overexpression of TFIIB or neutralization of the acidic Glu-2 by replacement with alanine in f/M1 hVDR. Because the f VDR genotype has been associated with lower bone mineral density in diverse populations, one factor contributing to a genetic predisposition to osteoporosis may be the F/f polymorphism that dictates VDR isoforms with differential TFIIB interaction.

Biochemical evidence for a 170-kilodalton, AF-2-dependent vitamin D receptor/retinoid X receptor coactivator that is highly expressed in osteoblasts

Human vitamin D receptor (hVDR) fused to glutathione S-transferase was utilized to detect a VDR-interacting protein (VIP) of approximately 170 kDa. VIP(170) is expressed in osteoblast-like ROS 17/2.8 cells and, to a lesser extent, in COS-7 and HeLa cells. VIP(170) may be a coactivator because it interacts only with 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) ligand-bound hVDR and because a mutation (E420A) in the activation function-2 (AF-2) of hVDR abolishes both receptor-mediated transactivation and VIP(170) binding. Unlike L254G hVDR, a heterodimerization mutant with an intact AF-2, the E420A mutant is only partially attenuated in its association with the retinoid X receptor (RXR) DNA-binding partner. Finally, the ability of overexpressed hVDR to squelch glucocorticoid receptor-mediated transactivation is lost in both the L254G and E420A mutants. These results suggest that several protein-protein interactions, including VDR association with RXR and VIP(170), are required for stabilization of a multimeric complex that transduces the signal for 1,25(OH)(2)D(3)-elicited transactivation.

Effect of cyclic adenosine 3',5'-monophosphate and protein kinase A on ligand-dependent transactivation via the vitamin D receptor

We examined the effects of cyclic adenosine 3',5'-monophosphate (cAMP) and protein kinase A (PKA) on the ligand-dependent transactivation mediated via the 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptor (VDR). A human VDR expression plasmid was transfected into HeLa, Saos-2 and MG63 cells with a luciferase reporter gene construct containing the vitamin D responsive element. With the addition of 0.5 mM 8 bromo-cAMP, the response to 1,25(OH)2D3 was suppressed to 61 and 78% in the HeLa and Saos-2 cells, respectively. The suppressive effect of 8 bromo-cAMP was observed without the introduction of the VDR expression plasmid in the MG63 cells. In the HeLa cells the co-expression of PKA reduced the ligand-inducible transactivation to 61% and the fold induction by 1,25(OH)2D3 to 89% of that without PKA. The CREB binding protein (CBP) was recently reported to integrate the intracellular signals via the cAMP/PKA cascade and nuclear hormone receptors. However, the suppressive effect of cAMP was not influenced by the co-expression of CBP. Lastly, we introduced point mutations at possible PKA phosphorylation sites into the VDR expression vector at serine-172 and threonine-175, but both mutant receptors still exhibited reduced transactivation with the co-expression of PKA. These results indicate that the phosphorylation of proteins other than the VDR may also be involved in the inhibitory effect mediated by the cAMP/PKA cascade.

Differential effects of 20-epi vitamin D analogs on the vitamin D receptor homodimer

Vitamin D analogs have received increased attention because of their possible therapeutic benefits in treating osteoporosis and various proliferative disorders. Several analogs were examined for their effects on DNA binding of the vitamin D receptor (VDR) homodimer complex with the murine osteopontin vitamin D response element. All of the tested analogs increased complex binding by recombinant human VDR in the electrophoretic mobility shift assay and notable differences in mobility of these complexes were observed. A panel of C-terminal anti-VDR antisera were screened for their ability to interact with analog-bound VDR homodimer complexes or as a heterodimer complex with recombinant human retinoid X receptor alpha (rhRXR alpha). Like calcitriol, analog-bound heterodimer complexes were largely resistant to interaction with these antisera; however, striking differences were observed with the various antisera in an analogous homodimer binding experiment. KH1060 and CB1093, analogs with 20-epi conformations, produced homodimer complexes that were 3- to 6-fold more resistant to supershifting with Ab180 compared with the hormone or EB1089. Chymotrypsin digestion in combination with Western blotting using a C-terminal anti-VDR antiserum revealed similar digestion patterns for all ligands. However, KH1060- and CB1093-bound VDR complexes were more resistant to digestion than either calcitriol or EB1089. Finally, the ability of these compounds to yield stable homodimer complexes was assessed by challenging preformed homodimer with the exogenous addition of rhRXR alpha extracts. Although new heterodimer complexes appeared in a time-dependent fashion, the preformed homodimer complexes exhibited stable binding throughout the time course of the experiment. The results indicate that VDR homodimers are targets of vitamin D analogs with differential effects on C-terminal protein conformation that may partially explain the varied biological responses of these compounds.

Polyenylphosphatidylcholine inhibits PDGF-induced proliferation in rat hepatic stellate cells

Polyenylphosphatidylcholine (PPC), a polyunsaturated phospholipid extract from soy beans, prevents the development of liver cirrhosis in animal models. Its mechanism of action is unknown. Based on the hypothesis that PPC might act by decreasing hepatic stellate cell proliferation, we studied the effect of PPC and its main components, dilinoleoylphosphatidylcholine (DLPC) and palmitoyl-linoleoylphosphatidylcholine (PLPC), on PDGF-induced stellate cell proliferation and intracellular signal transduction. Normal rat hepatic stellate cells in tissue culture were serumstarved, and incubated with 10ng/ml PDGF in the absence or presence of phospholipids. Cell proliferation was measured by 3H-thymidine incorporation. P44MAPK activation was determined by kinase assay, and AP-1 binding by electrophoretic mobility shift assay. PPC (200 ng/ml) significantly inhibited PDGF-induced proliferation (p < 0.05; ANOVA, n = 3) and antagonized PDGF-induced P44MAPK activation and AP-1 binding. This effect was mimicked by DLPC but not by PLPC. Neither DLPC nor PLPC prevented PDGF receptor activation. We conclude that PPC exerts a previously unrecognized effect on mitogen-induced stellate cell proliferation which may be mediated by DLPC. Inhibition of this cascade represents a potential mechanism for the inhibitory effect of PPC on hepatic fibrogenesis.

Identification of the subdomain in the nuclear receptor for the hormonal form of vitamin D3, 1 alpha,25-dihydroxyvitamin D3, vitamin D receptor, that is covalently modified by an affinity labeling reagent

Multiple physiological actions of the hormonal form of vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), are mediated by a genomic pathway which is initiated by the highly specific recognition and binding by its cognate receptor (vitamin D receptor, VDR) in the target cells. Thus, knowledge of the three-dimensional geometries of the ligand, i.e., 1,25(OH)2D3, and the 1,25(OH)2D3-binding domain of VDR is crucial for a better understanding of diverse physiological roles of this hormone. Recently our laboratory has developed 1 alpha,25-dihydroxyvitamin D3-3 beta-bromoacetate (1,25(OH)2 D3-3-BE) as an affinity labeling reagent for covalently modifying the hormone binding domain of native VDRs from calf thymus and rat osteosarcoma cells and baculovirus-expressed recombinant human VDR (hVDR). In the present report, we report affinity labeling of the hormone binding domain of hVDR, expressed in Escherichia coli as a glutathione S-transferase fusion partner, site-specific cleavage of the affinity-labeled VDR with 3-bromo-3-methyl-2-(2-nitrophenylmercapto)- 3H-indole, and identification of the C-terminal subdomain of human VDR containing the putative hormone binding site.

Mutations in the 1,25-dihydroxyvitamin D3 receptor identifying C-terminal amino acids required for transcriptional activation that are functionally dissociated from hormone binding, heterodimeric DNA binding, and interaction with basal transcription factor IIB, in vitro

To investigate a potential ligand-dependent transcriptional activation domain (AF-2) in the C-terminal region of the human vitamin D receptor (hVDR), two conserved residues, Leu-417 and Glu-420, were replaced with alanines by site-directed mutagenesis (L417A and E420A). Transcriptional activation in response to 1, 25-dihydroxyvitamin D3 (1,25-(OH)2D3) was virtually eliminated when either point mutant was transfected into several mammalian cell lines. Furthermore, both mutants exhibited a dominant negative phenotype when expressed in COS-7 cells. Scatchard analysis at 4 degrees C and a ligand-dependent DNA binding assay at 25 degrees C revealed essentially normal 1,25-(OH)2D3 binding for the mutant hVDRs, which were also equivalent to native receptor in associating with the rat osteocalcin vitamin D responsive element as a presumed heterodimer with retinoid X receptor. Glutathione S-transferase-human transcription factor IIB (TFIIB) fusion protein linked to Sepharose equally coprecipitated the wild-type hVDR and the AF-2 mutants. These data implicate amino acids Leu-417 and Glu-420, residing in a putative alpha-helical region at the extreme C terminus of hVDR, as critical in the mechanism of 1, 25-(OH)2D3-stimulated transcription, likely mediating an interaction with a coactivator(s) or a component of the basal transcriptional machinery distinct from TFIIB.

Activation of Raf-mitogen-activated protein kinase signaling pathway by 1,25-dihydroxyvitamin D3 in normal human keratinocytes

The biologic effects of the vitamin D hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are believed to be mediated by an intracellular vitamin D receptor, which after ligand binding acts as a transcription factor modulating expression of a variety of genes. Besides having a well-known role in calcium metabolism, this hormone is an important regulator of proliferation in a majority of normal and neoplastic cells. Keratinocytes provide a convenient model for investigating the growth-related effects of vitamin D in normal cells. Growth of keratinocytes may be either stimulated or inhibited by 1,25(OH)2D3, depending on the degree of cell differentiation. We show here that 1,25(OH)2D3 stimulates DNA synthesis via sequential activation of Raf and the mitogen-activated protein kinase. Activation of these kinases is independent on protein and mRNA synthesis and is preceded by rapid tyrosine phosphorylation of an adaptor protein p66 (Shc) and formation of a complex between p66 Shc, a bridging molecule Grb2, and a Ras activator, mSos. Vitamin D receptor protein associates with Shc, indicating that this steroid hormone is able to signal through the transcription-independent pathways similar to those used by peptide hormones and cytokines.

Phosphorylation of Ser211 in the chicken progesterone receptor modulates its transcriptional activity

The chicken progesterone receptor has been shown to be phosphorylated in vivo at four major sites. Previous studies have shown that mutation of one of the hormone-dependent phosphorylation sites, Ser530, to alanine decreases the transcriptional activity of the receptor under conditions where ligand is limited. Here, we present evidence for the functional significance of another phosphorylation site, Ser211. Mutation of Ser211 to alanine results in a decrease in the transcriptional activity of the receptor and affects the phosphorylation-dependent decrease in mobility of the receptor in SDS-polyacrylamide gel electrophoresis. The degree of reduction in transcriptional activity is dependent on both the cell type and the reporters used in the studies but is independent of hormone concentration, suggesting that phosphorylation at Ser211 regulates the activity of the receptor through a mechanism distinct from Ser530 phosphorylation.

Phosphorylation of the human estrogen receptor by mitogen-activated protein kinase and casein kinase II: consequence on DNA binding

We determined the amino acid and radiolabel sequences of tryptic [32P]phosphopeptides of the purified human estrogen receptor (hER) from MCF-7 cells and Sf9 cells. Serine 118 was identified as a site that was phosphorylated independently of estradiol-binding in MCF-7 cells. Proline is on the carboxy terminus of serine 118, which suggests that the serine-proline may be a consensus phosphorylation site motif for either the mitogen-activated protein (MAP) kinase or p34cdc2 kinase. MAP kinase selectively phosphorylated the recombinant hER in vitro on serine 118 independent of estradiol-binding, whereas p34cdc2 did not phosphorylate the hER. We demonstrated previously that serine 167 of the hER was phosphorylated in an estradiol-dependent manner. We therefore compared the consequence of hER phosphorylation at serine 118 by MAP kinase and phosphorylation at serine 167 by casein kinase II on the receptor's affinity for specific DNA binding. The binding of the hER to an estrogen response element was not altered by phosphorylation with MAP kinase at serine 118 but was significantly increased when phosphorylated at serine 167 by casein kinase II. These data suggest that phosphorylation of the hER by MAP kinase(s) pathways may influence receptor action by a mechanism other than the estradiol-dependent phosphorylation of hER by casein kinase II.

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.

Protein kinase C and mitogen-activated protein kinase are required for 1,25-dihydroxyvitamin D3-stimulated Egr induction

Recent studies have demonstrated that 1,25-dihydroxyvitamin D3 (D3) can activate Raf kinase and induce Egr expression in cultured rat hepatic Ito cells (Lissoos, T. W., Beno, D. W. A., and Davis, B. H. (1993) J. Biol. Chem. 268, 25132-25138). Since Raf is an upstream activator of mitogen-activated protein kinase (MAPK), the current study evaluated the ability of D3 to activate MAPK. D3-activated MAPK and induced its cytoplasmic to perinuclear translocation in Ito cells. MAPK activation was found to be protein kinase C-dependent, which was analogous to previous studies of D3 and Raf activation. To further explore the D3 cascade, a series of transient transfections were performed using dominant negative raf and MAPK mutant plasmids which effectively block Ras-induced Raf and MAPK activity, respectively. D3 induced a marked increase in the expression of a chloramphenicol acetyltransferase reporter gene linked to the Egr promoter (egr-CAT). When the dominant negative Raf plasmid was co-transfected, there was no significant reduction in egr-CAT. In contrast, when the dominant negative MAPK plasmid was co-transfected, egr-CAT induction was completely abolished. These results suggest that 1) D3 stimulates MAPK via a protein kinase C-dependent pathway, 2) D3-induced Egr expression can occur via a pathway independent of Ras-induced Raf, and 3) D3 absolutely requires MAPK activity for Egr expression.

1,25-dihydroxyvitamin D3 regulates the synthesis of nerve growth factor in primary cultures of glial cells

The effect of 1,25-dihydroxyvitamin D3 (1,25-(OH)2 D3) on nerve growth factor (NGF) synthesis was investigated in primary cultures of astrocytes prepared from brain of neonatal rats. 1,25-(OH)2 D3 elicited a dose-dependent increase of NGF mRNA with a maximal effect at 10(-7) M, which persisted for at least 48 h. Northern blot analysis revealed an expression of the vitamin D3 receptor (VDR) gene in primary glial cells. Treatment of cells with 1,25-(OH)2 D3 led to an increase in the VDR mRNA levels. Similar results were obtained in C6 glioma cells. Exposure of primary glial cells to 10(-8) M 1,25-(OH)2 D3 caused only a 2-fold increase of the levels of cell-secreted NGF after 3 days of treatment. However, a 5-fold increase was observed three days after a second addition of vitamin D3. Likewise, a pretreatment with lower doses of hormone such as 10(-10) M or 10(-9) M enhanced the responsiveness of the cells to a 24 h treatment with 10(-8) M hormone. It appears, therefore, that the duration of the treatment influences the level of synthesis of NGF, possibly as a consequence of the increase of the VDR gene expression. The specificity of 1,25-(OH)2 D3 is supported by the fact that a concentration of 10(-7) M of an another vitamin D3 metabolite, 24,25-(OH)2 D3, had no effect on NGF synthesis. Several lines of evidence indicate that astrocytes constitute the major cell type responsive to 1,25-(OH)2 D3 in primary cultures of glial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25-dihydroxyvitamin D3 modulates phosphorylation of serine 205 in the human vitamin D receptor: site-directed mutagenesis of this residue promotes alternative phosphorylation

The vitamin D receptor (VDR) from a variety of animal species is a hormone-modulated substrate for phosphorylation in vivo. In this report, we utilize an expression vector to produce recombinant human VDR (hVDR) in 1,25-dihydroxyvitamin D3-treated COS-1 cells. Immunoprecipitation of the phosphorylated hVDR followed by gel purification and phosphoamino acid analysis revealed modification exclusively on one or more serine residues, consistent with previous studies of the VDR in other species. To identify the region of phosphorylation, immunoprecipitated and gel-purified hVDR from COS-1 cells was first mixed with purified hVDR isolated to homogeneity from Saccharomyces cerevisiae and then digested with trypsin or V8 protease, and the peptides were resolved on HPLC. The single phosphate-containing peptides were recovered and subjected to amino acid sequence analysis, revealing the modification to reside in a region extending from residue 171 to residue 206 common to both the tryptic- and the V8 protease-derived peptides. Sequential cleavage of similar VDR mixtures using trypsin and then CNBr, alpha-chymotrypsin, or thermolysin demonstrated an amino-terminal boundary of the phosphorylated peptide at 202. Selective manual Edman degradation of phosphorylated peptides beginning at 171, 195, and 200 revealed phosphate release only at serine 205. This peptide contained an average of 8-fold less radioactive phosphate in the absence of prior treatment of the culture cells with 1,25(OH)2D3. Site-directed modification of VDR serine 205 to alanine, aspartate, or glutamate each led to fully functional proteins when assessed in a transactivation assay using several VDRE-linked natural promoters. Unexpectedly, evaluation of the serine 205 to alanine hVDR mutant revealed that this protein continued to be phosphorylated in a hormone-dependent manner on an alternative site. These studies show directly that hVDR serine residue 205, a consensus site for casein kinase II, is modified in vivo in response to hormone.

Steroid hormone receptor phosphorylation: is there a physiological role?

All members of the steroid hormone receptor family are phosphoproteins. Additional phosphorylation occurs in the presence of hormone. This hormone-induced phosphorylation, which is 2- to 7-fold more than the basal phosphorylation, is a rapid process. All steroid receptors are phosphorylated at more than one single site. Most phosphorylation sites are located in the N-terminal domain, and phosphorylation occurs mainly on serine residues. Phosphorylation on threonine residues occurs in only a few cases. Phosphorylation on tyrosine residues has been found only for the estrogen receptor. Six different protein kinases are possibly involved in steroid receptor phosphorylation (estrogen receptor kinase; protein kinase A; protein kinase C; casein kinase II; DNA-dependent kinase; Ser-Pro kinases). Steroid receptor phosphorylation has been directly implicated in: activation of hormone binding, nuclear import of steroid receptors, modulation of binding to hormone response elements, and consequently in transcription activation.