Hormone-dependent phosphorylation of the 1,25-dihydroxyvitamin D3 receptor in mouse fibroblasts

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.

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.

Immune Modulation by Vitamin D and Its Relevance to Food Allergy

Apart from its classical function in bone and calcium metabolism, vitamin D is also involved in immune regulation and has been linked to various cancers, immune disorders and allergic diseases. Within the innate and adaptive immune systems, the vitamin D receptor and enzymes in monocytes, dendritic cells, epithelial cells, T lymphocytes and B lymphocytes mediate the immune modulatory actions of vitamin D. Vitamin D insufficiency/deficiency early in life has been identified as one of the risk factors for food allergy. Several studies have observed an association between increasing latitude and food allergy prevalence, plausibly linked to lower ultraviolet radiation (UVR) exposure and vitamin D synthesis in the skin. Along with mounting epidemiological evidence of a link between vitamin D status and food allergy, mice and human studies have shed light on the modulatory properties of vitamin D on the innate and adaptive immune systems. This review will summarize the literature on the metabolism and immune modulatory properties of vitamin D, with particular reference to food allergy.

Regulation of target gene expression by the vitamin D receptor - an update on mechanisms

Virtually all of the known biological actions of the hormonal ligand 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are mediated by the vitamin D receptor (VDR). Following binding and activation by the ligand, the VDR localizes in the nucleus to the regulatory regions of target genes and recruits chromatin-active coregulatory complexes which, in turn, modulate transcriptional output. The failure of the VDR to function due to crippling mutations results in total hereditary resistance to 1,25(OH)(2)D(3) in both mice and humans. In this review, we summarize the structural and functional properties of the VDR and the role of 1,25(OH)(2)D(3) in receptor activation, and then describe the results of recent studies using genome-wide analyses that define the overarching principles through which the VDR modulates genes expression. We also focus on the recent analysis of a specific 1,25(OH)(2)D(3) regulated gene that provides confirmation of the principles identified through these genome-wide methodologies. Taken together, these studies suggest an unanticipated increase in the complexity of the molecular processes that govern gene regulation by hormones and other factors.

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.

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.

Vitamin D and genomic stability

1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] has been shown to act on novel target tissues not related to calcium homeostasis. There have been reports characterizing 1,25(OH)(2)D(3) receptors and activities in diverse tissues such as brain, pancreas, pituitary, skin, muscle, placenta, immune cells and parathyroid. The receptor hormone complex becomes localized in the nucleus, and undergoes phosphorylation by reacting with a kinase. This form of the receptor then interacts with the Vitamin D responsive element of target gene and modifies the transcription of those genes to develop the action. The modulation of gene transcription results in either the induction or repression of specific messenger RNAs (m-RNAs), ultimately resulting in changes in protein expression needed to produce biological responses. Genes for carbonic anhydrase that are expressed at high levels in osteoclast are known to be involved in bone resorption and Id genes role in osteoblast-osteoclast differentiation reflects the genomic effect of Vitamin D on bones. Genomic action of Vitamin D also explains the biosynthesis of oncogenes, polyamines, lymphokines and calcium binding proteins. However, there is a possibility that some of the actions of 1,25(OH)(2)D(3) may be mediated by non-genomic mechanisms and may not require the binding to Vitamin D receptor (VDR). Vitamin D offers a protection from genotoxic effects of Vitamin D deficiency by increasing the insulin receptor gene expression and BSP (bone sialoprotein), bone-remodeling by decreasing the osteopontin (OPN) m-RNAs, maintaining the normal epidermal structure and enamel matrix. Gonadal insufficiency in Vitamin D deficiency was corrected by vitamin mediated direct regulation of the expression of aramotase gene. The supportive role of Vitamin D in placental function is also evident by its influence on human placental lactogen (hpl) gene transcription accompanied by increase hpl m-RNA levels. Further role of Vitamin D is envisaged in identifying cyclin C as an important target for Vitamin D in cell-cycle regulation. Vitamin D at physiological concentration has been found to protect cell proteins and membranes against oxidative stress by inhibiting the peroxidative attack on membrane lipids. Vitamin D, at a concentration range of 2x10(-8)-5x10(-8)M, induces apoptosis in most cancer cells, stabilizes chromosomal structure and prevents DNA double-strand breaks induced either by endogenous or exogenous factors. Vitamin D is also effective in stimulating DNA synthesis in adult alveolar II cells and provides a novel mechanism of modulation of epithelial cell proliferation in the context of lung development and repair against injury. The regulation of various proto-oncogenes (c-myc, c-fos, c-jun), differentiation inducing properties, antiproliferative effects on keratinocytes and inhibitory effects in several human malignancy ranks Vitamin D as a novel hormone that may have physiological and clinical implication in the carcinogenic process.

Tuberous sclerosis gene 2 product modulates transcription mediated by steroid hormone receptor family members

Tuberous sclerosis (TSC) is a genetic disorder that results in the development of hamartomatous lesions in a variety of organ systems. Both the prevalence of the disease and the often devastating consequences of these tumors pose a serious health and medical care problem. The disease has been mapped to two distinct genetic loci in humans, and although the genes (TSC1 and TSC2) for both loci have recently been cloned, their function remains an enigma. Data presented here demonstrates that TSC2 protein can bind and selectively modulate transcription mediated by members of the steroid receptor superfamily of genes. These data place TSC2 into a growing list of nuclear receptor coregulators and strengthen the expanding body of evidence that these coregulators may play critical roles in cellular differentiation.

1,25-Dihydroxyvitamin D3 induces the expression of vascular endothelial growth factor in osteoblastic cells

Angiogenesis is a fundamental process in skeletal development and repair, and previous studies indicate that vascular endothelial growth factor (VEGF), an endothelial cell-specific angiogenic factor, may be involved in bone formation and repair. Therefore, we studied the hormonal regulation of VEGF expression in SaOS-2 osteoblast-like cells, both at the protein level, and at the transcriptional level by transient transfection experiments. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3], increased VEGF expression by approximately 3-fold, and the increase was dose dependent, with maximum stimulation between 1.0 and 10 nM of 1,25-(OH)2D3. Up-regulation of VEGF protein was detected already after 6 h of treatment. VEGF up-regulation was also observed in ROS-17/2.8 and OHS-4 osteoblast-like cells but not in MCF-7 and MDA-MB231 breast carcinoma cells. Dexamethasone (Dex) decreased VEGF expression to 40% of the control, but when added together with 1,25-(OH)2D3, had no effects on the up-regulation of VEGF by 1,25-(OH)2D3. PTH1-34 stimulated weakly VEGF expression, but combined with 1,25-(OH)2D3, resulted in a close to 5-fold stimulation. A 4-day pretreatment of the cells with Dex increased the vitamin D3 receptor expression and resulted in a stronger stimulation of VEGF by 1,25-(OH)2D3, alone or in combination with PTH1-34. The results show that the VEGF promoter is a target of 1,25-(OH)2D3 regulation in osteoblasts, despite the lack of classical vitamin D3 responsive elements. The up-regulation of VEGF in osteoblast-like cells by calciotropic hormones provides additional evidence of the involvement of VEGF in bone metabolism.

Effect of 1,25-dihydroxyvitamin D3 on proliferation in senescent IMR-90 human fibroblasts

The response of IMR-90 human fetal lung fibroblasts at high population doubling level (PDL > 42) to 1,25-dihydroxyvitamin D3[1,25(OH)2D3] was investigated to clarify whether some metabolic and molecular parameters of senescent cells are affected by the hormone treatment. Pyruvate kinase, lactate dehydrogenase and glucose-6-phosphate dehydrogenase activity significantly increased after treatment of confluent-phase cells with 10 nM 1,25(OH)2D3 for 24 h. Steroid specificity was established by the failure of 10 nM levels of 25-hydroxyvitamin D3 to affect the enzyme activities, while estradiol-17 beta and progesterone produced a slight increase in glucose-6-phosphate dehydrogenase and lactate dehydrogenase levels, respectively. 1,25(OH)2D3 also affected fibroblast proliferation, protein content/cell and DNA synthesis. The cell number significantly decreased after a 48 h incubation with 1,25(OH)2D3 at various concentrations (0.01-1 nM) when compared with control fibroblasts, while an increase in the protein content/cell was demonstrated. The same experiment, carried out by protracting the incubation with the hormone for 72 h, showed a similar trend, but 10 nM 1,25(OH)2D3 was also able to inhibit cell proliferation and to stimulate protein synthesis. The incorporation of [3H]thymidine into DNA increased after the treatment of high PDL fibroblasts with 0.01-1 nM of hormone for 48 h in comparison with controls.

Genomic mechanisms involved in the pleiotropic actions of 1,25-dihydroxyvitamin D3

The biologically active metabolite of vitamin D (cholecalciferol), i.e. 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a secosteroid hormone whose mode of action involves stereospecific interaction with an intracellular receptor protein (vitamin D receptor; VDR). 1,25(OH)2D3 is known to be a principal regulator of calcium homeostasis, and it has numerous other physiological functions including inhibition of proliferation of cancer cells, effects on hormone secretion and suppression of T-cell proliferation and cytokine production. Although the exact mechanisms involved in mediating many of the different effects of 1,25(OH)2D3 are not completely defined, genomic actions involving the VDR are clearly of major importance. Similar to other steroid receptors, the VDR is phosphorylated; however, the exact functional role of the phosphorylation of the VDR remains to be determined. The VDR has been reported to be regulated by 1,25(OH)2D3 and also by activation of protein kinases A and C, suggesting co-operativity between signal transduction pathways and 1,25(OH)2D3 action. The VDR binds to vitamin D-responsive elements (VDREs) in the 5' flanking region of target genes. It has been suggested that VDR homodimerization can occur upon binding to certain VDREs but that the VDR/retinoid X receptor (RXR) heterodimer is the functional transactivating species. Other factors reported to be involved in VDR-mediated transcription include chicken ovalbumin upstream promoter (COUP) transcription factor, which is involved in active silencing of transcription, and transcription factor IIB, which has been suggested to play a major role following VDR/RXR heterodimerization. Newly identified vitamin D-dependent target genes include those for Ca2+/Mg(2+)-ATPase in the intestine and p21 in the myelomonocytic U937 cell line. Elucidation of the mechanisms involved in the multiple actions of 1,25(OH)2D3 will be an active area of future research.

All-trans retinoic acid inhibits binding of 1,25-dihydroxy-vitamin D3 to the vitamin D receptor in cultured human keratinocytes

Psoriasis is characterized by hyperproliferation and impared differentiation of epidermal keratinocytes (KCs). Psoriasis can be treated with derivatives of retinoic acid (RA) and vitamin D3. Analogues of vitamin D3 are able to inhibit proliferation and stimulate differentiation of KCs. In contrast, RA inhibits terminal differentiation of KCs. Interactions are known to occur between RA and vitamin D3 signalling pathways. The purpose of the present study was to determine the effect of all-trans RA on the binding of 1,25-dihydroxy-vitamin D3 (1,25 (OH)2D3) to the vitamin D3 receptor (VDR) of cultured human KCs. Cultured KCs from normal adults were incubated with or without RA (10-9-10-7M) for 4-24 h. Cells were then harvested, homogenized and ultrasonicated. The extracted protein was incubated with 3H-1,25 (OH)2D3 (0.015-1.0 nM) with or without 250-fold excess nonradioactive 1,25 (OH)2D3 for 24 h and specific binding was determined by use of the dextran coated charcoal binding assay. Western blot analysis utilizing the monoclonal antibody 9A7 gamma to VDR was performed on protein extracted from the KCs. The bands resulting from Western blot analysis were visualized by enhanced chemiluminescence. From Scatchard analysis it was found that KCs bind 1,25 (OH)2D3 with high affinity (Kd = 0.175 nM). This binding was dose and time dependently inhibited by RA (60% inhibition at 10-7 M after 24 h of incubation). By Western blot analysis, RA had no effect on the amount of protein extracted from KCs at any of the RA concentrations tested. In conclusion, these results show that binding of vitamin D3 to its receptor of human KCs can be inhibited markedly by RA without effecting the amount of protein. These results are in contrast to results with other cell types in which RA upregulates binding of 1,25 (OH)2D3 to the VDR. Because interaction between retinoids and vitamin D3 may occur at different levels during signal transduction, it is not possible to predict from our results whether RA will inhibit the effects of vitamin D3 in vivo.

Role of vitamin D3 on the activity patterns of hepatic drug metabolizing enzymes in transplantable murine lymphoma

Vitamin D3 (D3) has been found to exert varied pharmacological actions including restriction of cell growth of a number of malignant cell lines in vitro and inhibition of the promotion of chemical carcinogenesis in mouse skin. In an attempt to confirm the efficacy of D3 as an antineoplastic agent, the present investigation aims at characterizing the importance of D3 in modulating hepatic drug metabolizing enzymes, namely, cytosolic glutathione S-transferase (GSHT), microsomal UDP glucuronyl transferase (UDPGT), and cytochrome P-450, which have been reported by us in recent literature as significant neoplastic markers in mice bearing Dalton's lymphoma (DL). Results show that D3 causes a 150% elevation of GSHT activity and the maintenance of normal, near-control UDPGT activity and cytochrome P-450 content, up to almost 30 days following tumor transplantation, along with bringing about a twofold increase in survival of the host mice. In conclusion, we confirm the definite and significant antitumorigenic role of D3 and its involvement with the discussed hepatic tumor markers in monitoring the processes that lead to cell survival.

1,25-dihydroxyvitamin D3 inhibits proliferation of IMR-90 human fibroblasts and stimulates pyruvate kinase activity in confluent-phase cells

This study tested the hypothesis that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a role in regulating some aspects of metabolism in IMR-90 normal human fetal lung fibroblasts. Among the enzymes studied, only pyruvate kinase showed a significant increase after treatment of confluent-phase cells with 1,25(OH)2D3 at various concentrations (0.1-100 nM range) for 24 h. A parallel increase in lactate output was observed. Steroid specificity was established by the failure of 10 nM levels of 25-hydroxyvitamin D3, estradiol-17 beta and progesterone to affect pyruvate kinase activity. The determination of the time course of [3H]-2-deoxy-D-glucose transport indicated that the hormone did not influence the transmembrane transport system of D-glucose. The addition of the inhibitors cycloheximide and actinomycin D to the culture medium abolished, at least in part, the 1,25(OH)2D3 stimulation of pyruvate kinase activity, suggesting the probable dependence of the hormone effect on cellular RNA and protein synthesis. 1,25(OH)2D3 also affected fibroblast growth and DNA synthesis. Cell number significantly decreased after 2-5 days treatment with 10 nM hormone in comparison with control fibroblasts, and also the incorporation of [3H]thymidine into DNA decreased after treatment of the cells with 1 and 10 nM hormone for 48 h. In conclusion, these data demonstrate that 1,25(OH)2D3 stimulates pyruvate kinase activity in confluent-phase IMR-90 human fibroblasts by a mechanism probably dependent on de novo protein synthesis, and also affects cell growth and DNA synthesis in sub-confluent-phase cells.

Phosphorylation and progesterone receptor function

Four phosphorylation sites have been identified in the chicken progesterone receptor. Two of these sites exhibit basal phosphorylation which is enhanced upon treatment with hormone and two of the sites are phosphorylated in response to hormone. Mutation of one of these hormone dependent sites, Ser530 to Ala530, causes a decrease in transcriptional activation at low concentrations of hormone, but the activity is unaffected at high concentrations. However, the hormone binding of the mutant is unaffected suggesting that phosphorylation of Ser530 plays a role in facilitating the response of the receptor to low concentrations of hormone. The chicken progesterone receptor can be activated by modulators of kinases in the absence of hormone. The finding that signals initiated by tyrosine phosphorylation (through treatment with EGF) or through the dopamine receptor suggests that there are multiple means of activating chicken progesterone receptor. In contrast, the human progesterone receptor does not exhibit ligand independent activation; however, its activity in the presence of the agonist R5020 is enhanced by treatment with 8-Br-cAMP, an activator of protein kinase A, and treatment with 8-Br-cAMP causes the antagonist, RU486, to act as an agonist.

Vitamin D-dependent rickets type II: regulation of human osteocalcin gene expression in cells with defective vitamin D receptors by 1,25-dihydroxyvitamin D-3, retinoic acid, and triiodothyronine

The vitamin D receptor (VDR) is a nuclear transcription factor which binds to the vitamin D response element (VDRE) of the human osteocalcin gene and regulates its expression. Humans with VDR gene mutations, ever among those with the same point mutation in their VDR gene, demonstrate clinical heterogeneity. In addition, in some patients with these mutations, rickets has not recurred following cessation of therapy during follow-up ranging from 6 to 24 years. While important, it is likely that the VDR protein is not the sole factor in the development of rickets. To try to understand these clinical findings, the complex formed between the VDRE and one or more proteins in the nuclear extracts of cultured skin fibroblasts treated with 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3), retinoic acid (RA), and/or triiodothyronine (T3) was investigated since such complexes are likely to precede the transcription of the VDR gene. Complex formation in the control cells with an intact VDR was increased by treatment with either 0.1 nM, 1 nM, 10 nM 1,25(OH)2D3, 100 nM RA, or 100 nM T3; however, combinations of these compounds did not produce an additive effect. In cells of affected patients, 1,25(OH)2D3, RA, or T3 increased complex formation, while no combination had an additive effect. These results indicate that 1,25(OH)2D3, RA, and T3 play a role in the regulation of bone remodeling through modulating the formation of protein complexes on the VDRE. Therefore, the clinical observations in patients with a VDR mutation might be explained at least in part by the overlapping control of osteocalcin expression by 1,25(OH)2D3, RA and T3.

Phosphorylation sites in ligand-induced and ligand-independent activation of the progesterone receptor

Steroid hormone receptors are phosphoproteins that undergo hyperphosphorylation upon binding of hormone. The mechanism and the role of this reaction remain poorly understood. Two-dimensional analysis of ligand-free progesterone receptor (PR) tryptic digests showed the existence of seven main phosphopeptides. Incubation of the cells with the progestin R5020 led to a global increase in the levels of PR phosphorylation. However, the same phosphopeptides were seen, and their levels of labeling relative to each other were unchanged. A similar result was observed after incubation of cells with the antiprogestin RU486. The antiprogestin ZK98299 demonstrated only half of the activity of RU486 in terms of receptor hyperphosphorylation, but the same phosphopeptides, proportionally labeled to the same extent, were observed by chromatography electrophoresis. Ligand-induced DNA binding did not play a role in receptor hyperphosphorylation since the mutant delta 547-592, which is devoid of the first zinc finger region, exhibited the same phosphopeptides, labeled to the same extent, as did wild-type receptor after incubation of cells with hormone. These results suggest that the same kinase(s) act in vivo on ligand-free and on agonist or antagonist-bound progesterone receptor. Binding of different ligands produces different conformational changes in the ligand binding domain of the receptor which enhance, to varying extents, affinity of the receptor for the kinase(s). The DNA binding region also plays a role in the interaction with the kinase(s), although binding to DNA per se is not necessary for the hyperphosphorylation of the receptor to take place.(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.

Nuclear hormone receptors as targets for new drug discovery

There are two basic types of receptor transducing systems: those which utilize membrane bound receptors and are activated at the cell surface by the appropriate hormone and transmit their signal to the internae of the cell via a second messenger (i.e. cAMP), and those that utilize internal, cytoplasmic or nuclear receptors (intracellular receptors) which upon activation by hormones interact directly with DNA and alter the genetic program of a cell. This review focuses on the mechanism of action of these intracellular receptors and discusses how such an understanding is expected to facilitate the discovery of new therapeutic agents.

Chicken progesterone receptor expressed in Saccharomyces cerevisiae is correctly phosphorylated at all four Ser-Pro phosphorylation sites

This study describes the phosphorylation of chicken progesterone receptor (cPR) produced in yeast, Saccharomyces cerevisiae, and examines the dependence of specific phosphorylations on hormone and DNA binding. The chicken progesterone receptor is expressed in vivo as two forms, cPRB and a smaller form, cPRA. Characterization of the phosphorylation sites in the cPRB form expressed in yeast shows that progesterone receptor is phosphorylated on the three serines (Ser211, Ser260, and Ser530) reported previously in chicken oviduct. An additional site which was phosphorylated in response to hormone was also detected and was subsequently identified as Ser367. Although cPRB and cPRA are phosphorylated identically in chicken oviduct, cPRA expressed in yeast is phosphorylated on Ser211, Ser260, and Ser367, but phosphorylation of Ser530 is almost undetectable. In contrast, cPRB expressed in yeast is phosphorylated on all four sites. No phosphorylations were found in or near the region required for hormone binding, indicating that phosphorylation is not required for hormone binding. In order to determine whether any of the phosphorylations were DNA-dependent, phosphorylation was also studied using cPRA containing a partial deletion of the DNA binding domain. Two of the sites, Ser211 and Ser367, showed reduced phosphorylation in this mutant, suggesting a possible requirement for DNA binding activity for the phosphorylation of these sites. To our knowledge, this is one of the first demonstrations that a eucaryotic protein expressed in yeast is correctly phosphorylated.

The 1,25-dihydroxy-vitamin D3 receptor is phosphorylated in response to 1,25-dihydroxy-vitamin D3 and 22-oxacalcitriol in rat osteoblasts, and by casein kinase II, in vitro

We analyzed the endogenous nuclear 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3) receptor (VDR) in rat osteosarcoma (ROS 17/2.8) cells and present biochemical evidence that it is a phosphoprotein. When ROS 17/2.8 cells are labeled metabolically with [35S]methionine, treatment with 10(-8) M 1,25(OH)2D3 elicits a decrease in the electrophoretic mobility of immunoprecipitated VDR in denaturing polyacrylamide gels, a property characteristic of phosphorylated proteins. Similar labeling of cells with [32P]orthophosphate results in a rapid (< or = 30 min), 1,25(OH)2D3-dependent incorporation of 32P into a 54-kDa VDR species that comigrates with the slower migrating receptor species extracted from [35S]methionine-labeled ROS 17/2.8 cells that have been exposed to 1,25(OH)2D3. Alkaline phosphatase treatment of immunoprecipitated VDR from 1,25(OH)2D3-treated cells converts the form of the VDR with reduced mobility to the faster migrating species present in 1,25(OH)2D3-deficient cells. Incubation of ROS 17/2.8 cells with the non-hypercalcemic 1,25(OH)2D3 analog, 22-oxacalcitriol (OCT), produces a level of VDR phosphorylation similar to that elicited by 1,25(OH)2D3 treatment. Transient transfection of osteosarcoma cells with a reporter vector containing a vitamin D responsive element derived from the rat osteocalcin gene yields equivalent transcriptional activation in the presence of either 1,25(OH)2D3 or OCT. Further experiments performed at various 1,25(OH)2D3 concentrations to assess the relationship between receptor phosphorylation and transcriptional activity in intact cells showed a positive correlation between these two parameters, indicating that the 1,25(OH)2D3 hormone stimulates VDR phosphorylation and transcriptional activation in parallel. Finally, highly purified casein kinase II (CK-II) phosphorylates the VDR in a 1,25(OH)2D3-independent, in vitro reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization and hormonal stimulation of phosphorylation sites in the LNCaP-cell androgen receptor

Phosphorylation of the androgen receptor in human prostate tumour cells (LNCaP) is increased by addition of androgens to intact cells. Double-label studies, using [35S]methionine incorporation into receptor protein, and [32P]P(i) to label metabolically receptor phosphorylation sites, have enabled us to determine the phosphate content, relative to receptor protein, of both nontransformed and transformed and androgen receptors generated in intact LNCaP cells. No net change in the phosphorylation of the intact 110 kDa steroid-binding component of the androgen-receptor complex was found upon transformation to the tight nuclear binding form in the intact cell. Partial proteolysis of androgen receptor protein metabolically labelled with [32P]P(i) and photolabelled with [3H]R1881 (methyltrienolone) revealed that phosphorylation occurs mainly in the N-terminal trans-activation domain, whereas no phosphorylation was detected in the steroid- and DNA-binding domains. The location of most (> 90%) of the hormonally regulated phosphorylation sites in the N-terminal trans-activation domain suggests a role of phosphorylation of the androgen receptor in transcription regulation.

Phosphorylation is involved in transcriptional activation by the 1,25-dihydroxyvitamin D3 receptor

The 1,25-dihydroxyvitamin D3 receptor becomes phosphorylated upon treatment with 1,25-dihydroxyvitamin D3. We have investigated the role of phosphorylation in the transcriptional activity induced by 1,25-dihydroxyvitamin D3 through its receptor. An active 1,25-dihydroxyvitamin D3-dependent transcription system was reconstituted in CV-1 cells by co-transfection of plasmids containing the rat 1,25-(OH)2D3 receptor DNA and a functional vitamin D response element (DRE) in a reporter gene construct. Treatment of these transiently transfected CV-1 cells with modulators of protein kinase A (8-Br-cAMP, PKIA and H-9) and phosphatases (Okadaic acid) resulted in mimicking or abolishing the transcriptional activity of 1,25-dihydroxyvitamin D3 in a receptor-dependent fashion. These modulators directly altered 1,25-dihydroxyvitamin D3 receptor phosphorylation. Therefore, the present results strongly suggest that phosphorylation plays a central role in the transcriptional activity of the 1,25-dihydroxyvitamin D3 receptor.

Identification of a competitive binding component in vitamin D-resistant New World primate cells with a low affinity but high capacity for 1,25-dihydroxyvitamin D3

Monkeys in a number of different New World primate genera express a form of compensated target organ resistance to steroid hormones, including 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Characterization of these phenotypes has previously relied upon the study of the 1,25-(OH)2D3-receptor (VDR) interaction in cultured dermal fibroblasts from affected primates. In this report, we show that three of these prototypic phenotypes can be faithfully reproduced in previously established cultured cell lines: B95-8, EBV-transformed B lymphoblasts from the marmoset (Callithrix jacchus), a New World primate with recognized vitamin D resistance; OMK, renal tubular epithelial cells from the owl monkey (Aotus trivergatus), a New World primate with an Old World primate-like VDR phenotype; and MLA144, transformed B lymphoblasts from a gibbon (Hylobates), an Old World primate that expresses the wild-type VDR phenotype. The rank order of specific nuclear uptake and binding of [3H]1,25-(OH)2D3 to the VDR was OMK > or = MLA144 >> B95-8. Despite a 7- to 9-fold difference in cellular VDR content according to ligand binding analyses, there was no discernible difference in the internalization constant Kin for specific cellular uptake of [3H]1,25-(OH)2D3 (0.12-0.26 nM) or in the quantity of VDR detected by immunoblot analysis. We now speculate that the discrepancy in VDR quantitation by binding and immunoblot analysis in the B95-8 New World primate cell line results from the presence of an intracellular, vitamin D metabolite binding moiety in this cell line that competes with the VDR for metabolite binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of 1,25-dihydroxyvitamin D3 receptor gene expression by parathyroid hormone and cAMP-agonists

We studied the effect of parathyroid hormone (PTH) and activation of the cAMP signal pathway on vitamin D receptor (VDR) mRNA levels in the phenotypically osteoblast cell line UMR 106. PTH caused a time- and dose-dependent increase of the VDR mRNA content with a maximum after 2 h. After 24 h the VDR mRNA level in PTH-treated cells returned to control level. In contrast, the 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-induced increase in VDR mRNA did not decline after 24 h. Inhibition of transcription with actinomycin D (10 micrograms/ml) completely abolished the PTH-induced increase of VDR mRNA and inhibition of translation with cycloheximide (1 microgram/ml) resulted in superinduction of VDR mRNA. The role of cAMP in the induction of VDR mRNA was studied with several agents acting via the cAMP pathway. Incubation for 2 and 4 h with forskolin, Bt2cAMP, PTHrP or prostaglandin E2 caused an increase in the level of VDR mRNA comparable to that caused by PTH. The calcium ionophore A23187 did not affect VDR mRNA level. The present study demonstrates that PTH and activation of the cAMP signal pathway cause up-regulation of VDR via induction of VDR gene expression. The effect of cAMP on the VDR gene is suggestive for a cAMP responsive element in the VDR gene.

Changes in cAMP-dependent phosphorylation of specific cytosol protein in the peri- and postmenopausal ovary

Changes in cAMP-dependent phosphorylation of 53 kDa protein in a human ovary with respect to the aging process were studied. The phosphorylation of endogenous proteins in ovarian cytosol decreased with aging. However, ovarian aging was accompanied by an increase in the cAMP-dependent phosphorylation of 53 kDa specific protein while the amount of this protein decreased with aging. The plasma gonadotropin levels of women with regular menstrual periods were significantly different from those of women whose menstrual periods had in the recent past become irregular (FSH: p less than 0.01; LH: p less than 0.05); however, there was no significant difference in the plasma estradiol levels between these two groups. Our data show the possible correlations between the non-steroidal ovarian factor (53 kDa protein) and/or its phosphorylation by cAMP-dependent protein kinase and the ovarian aging process.

TPA decreases 1,25(OH)2D3 binding and calbindin D-28K in renal (MDBK) cells

The effect of the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) on vitamin D receptors (VDRs) was studied in MDBK cells, a normal bovine renal epithelial cell line. 24 h treatment of MDBK cells with TPA resulted in down-regulation of VDR number, with no change in the binding affinity for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or approximate molecular weight determined by fast protein liquid chromatography (FPLC). TPA treatment also reduced the level of calbindin D-28K, a vitamin D-dependent renal protein. 4 alpha-Phorbol 12,13-didecanoate (4 alpha-PDD), an inactive phorbol ester, did not affect either 1,25(OH)2D3 binding or calbindin D-28K levels. TPA elicited a significant decrease in membrane-associated protein kinase C (PKC) activity which coincided with the reduction in VDR number and calbindin D-28K. These data support a link between TPA, PKC activity and vitamin D actions in kidney.

A nuclear protein essential for binding of rat 1,25-dihydroxyvitamin D3 receptor to its response elements

Recombinant 1,25-dihydroxyvitamin D3 receptor from a baculovirus expression system requires a mammalian-derived nuclear accessory protein for binding to a vitamin D response element (DRE). This was established by electrophoretic mobility shift analyses using radiolabeled DNA probes consisting of DREs from two vitamin D-responsive genes. Mammalian nuclear extract was also required for the binding of wild-type porcine vitamin D receptor to a DRE. Surprisingly, the accessory factor-dependent formation of receptor-DRE complex was independent of exogenous 1,25-dihydroxyvitamin D3. A 59- to 64-kDa accessory protein from porcine intestinal nuclear extract was identified by size-exclusion chromatography. Nuclear extracts from rat liver and kidney contained accessory factor, whereas smaller amounts were detected in heart muscle. Spleen and skeletal muscle contained no detectable accessory factor.

Rapid homologous up-regulation of binding capacity of androgen receptors in intact cells

Incubation of MCF 7 cells with 5 alpha-dihydrotestosterone (DHT) at 37 degrees C led to a 70% increase in the Bmax of androgen receptor, as compared to the values measured at 2 degrees C, without detectable changes in equilibrium dissociation constants. When MCF 7 cells were incubated with hormone at 2 degrees C, to reach steady-state levels of androgen-receptor complex, a subsequent temperature shift to 37 degrees C induced a rapid (t 1/2 = 3 min) cycloheximide-insensitive increase in DHT binding to androgen receptor. MCF 7 cell treatments at 37 degrees C either before or after incubation with DHT at 2 degrees C showed that up-regulation of binding capacity of androgen receptor could be observed only if hormone is present during incubation at physiological temperature.

Evidence for two classes of 1,25-dihydroxyvitamin D3 binding sites in classical vs. nonclassical target tissues

Possible differences in 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] binding sites in classical and nonclassical target tissues were tested by Scatchard analysis of [3H]1,25(OH)2D3 binding in parallel chromatin preparations of rat kidney vs. testis. Two distinct binding components were resolved in kidney (p less than 0.005). Moreover, the single binding site in testis exhibited a 10-fold lower Kd (p less than 0.05) than did the principal binding site in kidney (50 +/- 4 vs. 405 +/- 142 pM). Secondly, regulation of [3H]1,25(OH)2D3 binding sites also differed. 1,25(OH)2D3 injection resulted in increased 1,25(OH)2D3 binding (p less than 0.05) in kidney (92%) and intestine (415%), but not in testis, lung or heart. These results suggest that the principal 1,25(OH)2D3 binding sites in classical targets kidney and intestine may be intrinsically different from those in at least some nonclassical targets.

Human vitamin D receptor is selectively phosphorylated by protein kinase C on serine 51, a residue crucial to its trans-activation function

The vitamin D receptor (VDR) is known to be a phosphoprotein and inspection of the deduced amino acid sequence of human VDR (hVDR) reveals the conservation of three potential sites of phosphorylation by protein kinase C (PKC)--namely, Ser-51, Ser-119, and Ser-125. Immunoprecipitated extracts derived from a rat osteoblast-like osteosarcoma cell line that contains the VDR in high copy number were incubated with the alpha, beta, and gamma isozymes of PKC, and VDR proved to be an effective substrate for PKC-beta, in vitro. When hVDR cDNAs containing single, double, and triple mutations of Ser-51, Ser-119, and Ser-125 were expressed in CV-1 monkey kidney cells, immunoprecipitated and phosphorylated by PKC-beta, in vitro, the mutation of Ser-51 selectively abolished phosphorylation. Furthermore, when transfected CV-1 cells were treated with phorbol 12-myristate 13-acetate, a PKC activator, phosphorylation of wild-type hVDR was enhanced, whereas that of the Ser-51 mutant hVDR was unaffected. Therefore, Ser-51 is the site of hVDR phosphorylation by PKC, both in vitro and in vivo. To evaluate the functional role of Ser-51 and its potential phosphorylation, hVDR-mediated transcription was tested using cotransfection with expression plasmids and a reporter gene that contained a vitamin D response element. Mutation of Ser-51 markedly inhibited transcriptional activation by the vitamin D hormone, suggesting that phosphorylation of Ser-51 by PKC could play a significant role in vitamin D-dependent transcriptional activation. Therefore, the present results link the PKC signal transduction pathway of growth regulation and tumor promotion to the phosphorylation and function of VDR.

Overproduction of rat 1,25-dihydroxyvitamin D3 receptor in insect cells using the baculovirus expression system

The rat 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor has been expressed at elevated levels in Spodoptera frugiperda cells using the baculovirus expression vector system. The recombinant 1,25-(OH)2D3 receptor is full-length, binds 1,25-(OH)2D3, and is recognized by a monoclonal antibody specific for 1,25-(OH)2D3 receptor. Densitometric scanning of Coomassie brilliant blue-stained SDS/polyacrylamide gels indicated a recombinant receptor protein level comprising 5% of the total soluble protein from the insect cells. The hydroxylapatite binding assay revealed average levels of 2 nmol of unoccupied 1,25-(OH)2D3 receptor per mg of protein in insect cells at 72 hr after infection with recombinant baculovirus. A measure of total 1,25-(OH)2D3 receptor using a ligand-independent, immunoradiometric assay disclosed average levels of 2.3 nmol of receptor per mg of protein produced by these same cells. A monoclonal antibody directed against the 1,25-(OH)2D3 receptor, and reported to cross-react with this receptor derived from several species, recognized the recombinant rat 1,25-(OH)2D3 receptor upon Western analysis. A monoclonal antibody directed specifically against the porcine receptor failed to recognize the recombinant rat 1,25-(OH)2D3 receptor protein. The cytosolic preparation of insect cells infected with recombinant baculovirus exhibited an equilibrium dissociation constant of 1 x 10(-11) M as determined by a 1,25-(OH)2D3 saturation analysis plotted by the method of Scatchard. This expression system provides an adequate source from which abundant quantities of 1,25-(OH)2D3 receptor can be purified for subsequent x-ray crystallographic analyses.

Sites of phosphorylation and photoaffinity labeling of the 1,25-dihydroxyvitamin D3 receptor

The 1,25-dihydroxyvitamin D3 receptor is a member of the steroid/thyroid hormone receptor gene family and is thought to act by regulating transcription of specific genes. In this report, we have used peptide mapping of porcine 1,25-dihydroxyvitamin D3 receptor to localize the sites of phosphorylation, photoaffinity labeling, and monoclonal antibody binding. Receptor was immunoprecipitated from [32P]orthophosphate-labeled pig kidney LLC-PK1 cells grown in the absence and presence of 1,25-dihydroxyvitamin D3. Phosphorylation of receptor was induced by 1,25-dihydroxyvitamin D3. The phosphorylated receptor was digested with Staph A V8 protease within Cleveland gels and the 32P label was found entirely in a 23-kDa fragment. Similarly, receptor that was photoaffinity labeled with 1,25-dihydroxy-[26,27-3H]vitamin D3 was subjected to peptide mapping by Cleveland gels. The primary site of photoaffinity label incorporation was in the same 23-kDa peptide. This peptide was localized to a region in the center of the receptor protein, spanning part of the previously designated hinge region and roughly one-half the proposed steroid binding domain. Because phosphorylation did not occur in the DNA binding domain, it may not be involved in the binding of receptor to DNA. The localization of phosphorylation sites to this 23-kDa peptide may suggest that phosphorylation is involved in steroid binding and/or activation of the receptor.