Vitamin D-influenced gene expression via a ligand-independent, receptor-DNA complex intermediate

Valproate and Short-Chain Fatty Acids Activate Transcription of the Human Vitamin D Receptor Gene through a Proximal GC-Rich DNA Region Containing Two Putative Sp1 Binding Sites

The vitamin D receptor (VDR) mediates 1,25-dihydroxyvitamin D3 pleiotropic biological actions through transcription regulation of target genes. The expression levels of this ligand-activated nuclear receptor are regulated by multiple mechanisms both at transcriptional and post-transcriptional levels. Vitamin D3 is the natural VDR activator, but other molecules and signaling pathways have also been reported to regulate VDR expression and activity. In this study, we identify valproic acid (VPA) and natural short-chain fatty acids (SCFAs) as novel transcriptional activators of the human VDR (hVDR) gene. We further report a comprehensive characterization of VPA/SCFA-responsive elements in the 5' regulatory region of the hVDR gene. Two alternative promoter DNA regions (of 2.4 and 3.8 kb), as well as subsequent deletion fragments, were cloned in pGL4-LUC reporter vector. Transfection of these constructs in HepG2 and human Upcyte hepatocytes followed by reporter assays demonstrated that a region of 107 bp (from -107 to -1) upstream of the transcription start site in exon 1a is responsible for most of the increase in transcriptional activity in response to VPA/SCFAs. This short DNA region is GC-rich, does not contain an apparent TATA box, and includes two bona fide binding sites for the transcription factor Sp1. Our results substantiate the hypothesis that VPA and SCFAs facilitate the activity of Sp1 on novel Sp1 responsive elements in the hVDR gene, thus promoting VDR upregulation and signaling. Elevated hepatic VDR levels have been associated with liver steatosis and, therefore, our results may have clinical relevance in epileptic pediatric patients on VPA therapy. Our results could also be suggestive of VDR upregulation by SCFAs produced by gut microbiota.

Vitamin D receptor targets hepatocyte nuclear factor 4α and mediates protective effects of vitamin D in nonalcoholic fatty liver disease

Epidemiological studies have suggested a link between vitamin D deficiency and increased risk for nonalcoholic fatty liver disease (NAFLD); however, the underlying mechanisms have remained unclear. Here, using both clinical samples and experimental rodent models along with several biochemical approaches, we explored the specific effects and mechanisms of vitamin D deficiency in NAFLD pathology. Serum vitamin D levels were significantly lower in individuals with NAFLD and in high-fat diet (HFD)-fed mice than in healthy controls and chow-fed mice, respectively. Vitamin D supplementation ameliorated HFD-induced hepatic steatosis and insulin resistance in mice. Hepatic expression of vitamin D receptor (VDR) was up-regulated in three models of NAFLD, including HFD-fed mice, methionine/choline-deficient diet (MCD)-fed mice, and genetically obese (ob/ob) mice. Liver-specific VDR deletion significantly exacerbated HFD- or MCD-induced hepatic steatosis and insulin resistance and also diminished the protective effect of vitamin D supplementation on NAFLD. Mechanistic experiments revealed that VDR interacted with hepatocyte nuclear factor 4 α (HNF4α) and that overexpression of HNF4α improved HFD-induced NAFLD and metabolic abnormalities in liver-specific VDR-knockout mice. These results suggest that vitamin D ameliorates NAFLD and metabolic abnormalities by activating hepatic VDR, leading to its interaction with HNF4α. Our findings highlight a potential value of using vitamin D for preventing and managing NAFLD by targeting VDR.

Synthesis and Biological Activity of Diastereomeric and Geometric Analogs of Calcipotriol, PRI-2202 and PRI-2205, Against Human HL-60 Leukemia and MCF-7 Breast Cancer Cells

Diastereomeric and geometric analogs of calcipotriol, PRI-2202 and PRI-2205, were synthesized as advanced intermediates from vitamin D C-22 benzothiazoyl sulfones and side-chain aldehydes using our convergent strategy. Calcitriol, calcipotriol (PRI-2201) and tacalcitol (PRI-2191) were used as the reference compounds. Among a series of tested analogs the diastereomeric analog PRI-2202 showed the strongest antiproliferative activity on the human breast cancer cell line MCF-7, whereas the geometric analog PRI-2205 was the weakest. Both analogs were less potent in antiproliferative activity against HL-60 cells compared to the reference compounds. The ability to potentiate antiproliferative effect of cisplatin or doxorubicin against HL-60 cells or that of tamoxifen against the MCF-7 cell line was observed at higher doses of PRI-2202 or PRI-2205 than those of the reference compounds. The proapoptotic activity of tamoxifen, expressed as the diminished mitochondrial membrane potential, as well as the increased phosphatidylserine expression, was partially attenuated by calcitriol, PRI-2191, PRI-2201 and PRI-2205. The treatment of the MCF-7 cells with tamoxifen alone resulted in an increase in VDR expression. Moreover, a further increase in VDR expression was observed when the analogs PRI-2201 or PRI-2205, but not PRI-2191, were used in combination with tamoxifen. This observation could partially explain the potentiation of the antiproliferative effect of tamoxifen by vitamin D analogs.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Vitamin D in cutaneous carcinogenesis: part I

Skin cancer is the most common cancer in the United States. Exposure to ultraviolet radiation is a known risk factor for skin cancer but is also the principal means by which the body obtains vitamin D. Several studies have suggested that vitamin D plays a protective role in a variety of internal malignancies. With regard to skin cancer, epidemiologic and laboratory studies suggest that vitamin D and its metabolites may have a similar protective effect. These noncalcemic actions of vitamin D have called into question whether the current recommended intake of vitamin D is too low for optimal health and cancer prevention. Part I will review the role of vitamin D in the epidermis; part II will review the role of vitamin D in keratinocyte-derived tumors to help frame the discussion on the possible role of vitamin D in the prevention of skin cancer.

Calcitriol stimulates prolactin expression in non-activated human peripheral blood mononuclear cells: breaking paradigms

Calcitriol, the hormonal form of vitamin D(3), exerts immunomodulatory effects through the vitamin D(3) receptor (VDR) and increases prolactin (PRL) expression in the pituitary and decidua. Nevertheless, the effects of calcitriol upon lymphocyte PRL have not been evaluated. Therefore, we investigated calcitriol effects upon PRL in resting and phytohemagglutinin-activated human peripheral blood mononuclear cells (PBMNC) and Jurkat T lymphoma cells. Immunoblots showed constitutive expression of the 50-kDa VDR species in activated PBMNC and Jurkat cells, while a 75-kDa species was recognized in both resting and activated-PBMNC. Only in resting PBMNC calcitriol significantly stimulated PRL expression in a dose-dependent manner. The positive control CYP24A1, a highly VDR-responsive gene, was stimulated by calcitriol, effect that was stronger in resting than in activated-PBMNC (P<0.05), and without effect in Jurkat cells. Calcitriol upregulation of PRL and CYP24A1 was significantly inhibited by the VDR antagonist TEI-9647. EMSA showed that resting PBMNC contain a protein that binds to DR3-type VDRE. Cell activation reduced basal CYP24A1 while induced CYP27B1, VDR and pregnane X receptor (PXR) expression. In summary, calcitriol stimulated PRL and CYP24A1 gene expression in quiescent lymphocytes through a VDR-mediated mechanism. Our results suggest that the 75-kDa VDR species could be participating in calcitriol-mediated effects, and that activation induces factors such as PXR that restrain VDR transcriptional processes. This study supports the presence of a functional VDR in quiescent lymphocytes, providing evidence to reevaluate the VDR paradigm that assumes that lymphocytes respond to calcitriol only after activation. Altogether, our results offer new insights into the mechanisms whereby PRL is regulated in immune cells.

Caloric restriction: from soup to nuts

Caloric restriction (CR), reduced protein, methionine, or tryptophan diets; and reduced insulin and/or IGFI intracellular signaling can extend mean and/or maximum lifespan and delay deleterious age-related physiological changes in animals. Mice and flies can shift readily between the control and CR physiological states, even at older ages. Many health benefits are induced by even brief periods of CR in flies, rodents, monkeys, and humans. In humans and nonhuman primates, CR produces most of the physiologic, hematologic, hormonal, and biochemical changes it produces in other animals. In primates, CR provides protection from type 2 diabetes, cardiovascular and cerebral vascular diseases, immunological decline, malignancy, hepatotoxicity, liver fibrosis and failure, sarcopenia, inflammation, and DNA damage. It also enhances muscle mitochondrial biogenesis, affords neuroprotection; and extends mean and maximum lifespan. CR rapidly induces antineoplastic effects in mice. Most claims of lifespan extension in rodents by drugs or nutrients are confounded by CR effects. Transcription factors and co-activators involved in the regulation of mitochondrial biogenesis and energy metabolism, including SirT1, PGC-1alpha, AMPK and TOR may be involved in the lifespan effects of CR. Paradoxically, low body weight in middle aged and elderly humans is associated with increased mortality. Thus, enhancement of human longevity may require pharmaceutical interventions.

Expression of the vitamin d receptor is increased in the hypertrophic heart

The liganded vitamin D receptor (VDR) is thought to play an important role in controlling cardiac function. Specifically, this system has been implicated as playing an antihypertrophic role in the heart. Despite this, studies of VDR in the heart have been limited in number and scope. In the present study, we used a combination of real-time polymerase chain reaction, Western blot analysis, immunofluorescence, and transient transfection analysis to document the presence of functional VDR in both the myocytes and fibroblasts of the heart, as well as in the intact ventricular myocardium. We also demonstrated the presence of 1-alpha-hydroxylase and 24-hydroxylase in the heart, 2 enzymes involved in the synthesis and metabolism of 1,25 dihydroxyvitamin D. VDR is shown to interact directly with the human B-type natriuretic peptide gene promoter, a surrogate marker of the transcriptional response to hypertrophy. Of note, induction of myocyte hypertrophy either in vitro or in vivo leads to an increase in VDR mRNA and protein levels. Collectively, these findings suggest that the key components required for a functional 1,25 dihydroxyvitamin D-dependent signaling system are present in the heart and that this putatively antihypertrophic system is amplified in the setting of cardiac hypertrophy.

An enhancer 20 kilobases upstream of the human receptor activator of nuclear factor-kappaB ligand gene mediates dominant activation by 1,25-dihydroxyvitamin D3

Receptor activator of nuclear factor-kappaB ligand (RANKL) is a TNF-like factor that is both produced by osteoblasts, mesenchymal cells, and activated T cells and required for osteoclast maturation and survival. The gene is up-regulated by the two primary calcemic hormones, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and PTH. Previous studies have indicated that five enhancer regions located significantly upstream of the mouse Rankl transcriptional start site mediate up-regulation by 1,25(OH)2D3 and PTH. The most distal of these, termed mRLD5, is highly conserved in the human gene at -96 kb where it was also shown to be functionally active. Four additional mouse Rankl upstream enhancers are also highly conserved in the human gene at -20, -25, -75, and -87 kb. In the present studies, we characterized the activity of these regions, explored their capacity to mediate the actions of 1,25(OH)2D3, and identified the vitamin D response elements contained within the two most proximal segments. Interestingly, whereas the most distal of the five enhancers is the dominant mediator of 1,25(OH)2D3 activity in the mouse Rankl gene, that role in the human gene is manifested by the most proximal element at -20 kb. Importantly, activity at this region in response to 1,25(OH)2D3 was associated with a significant increase in histone acetylation as well as the enhanced recruitment of RNA polymerase II. Both likely reflect the primary role of this enhancer in human RANKL gene expression. Our studies confirm the complex nature of RANKL regulation and indicate that although the five enhancers are evolutionarily conserved across several species, their relative contributions to RANKL expression in response to 1,25(OH)2D3 may be different.

The p38 and JNK pathways cooperate to trans-activate vitamin D receptor via c-Jun/AP-1 and sensitize human breast cancer cells to vitamin D(3)-induced growth inhibition

The signaling connection between mitogen-activated protein kinases(MAPKs) and nuclear steroid receptors is complex and remains mostly unexplored. Here we report that stress-activated protein kinases p38 and JNK trans-activate nuclear steroid vitamin D receptor (VDR) gene and increase vitamin D(3)-dependent growth inhibition in human breast cancer cells. Activation of p38 and JNK by an active MAPK kinase 6 stimulates VDR promoter activity independently of the ligand vitamin D(3) and estrogen receptor expression. Moreover, stimulation of the endogenous stress pathways by adenovirus-mediated delivery of recombinant MAPK kinase 6 also activates VDR and sensitizes MCF-7 cells to vitamin D(3)-dependent growth inhibition. Both the p38 and JNK MAPK pathways and the downstream transcription factor c-Jun/AP-1 are required for the VDR stimulation, as revealed by application of their dominant negatives, the specific p38 inhibitor SB203580, and site-directed mutagenesis of the AP-1 element in the VDR promoter. The essential role of the p38 and JNK stress pathways in up-regulation of VDR expression is further confirmed by using the chemical stimulator arsenite. These results establish a signaling connection between the stress MAPK pathways and steroid hormone receptor VDR expression and thereby offer new insights into regulation of cell growth by the MAPK pathways through regulation of vitamin D(3)/VDR activity.

Differential regulation of heterodimerization by 1alpha,25-dihydroxyvitamin D(3) and its 20-epi analog

Twenty-epi analogs of 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) are 100-1000 times more potent transcriptionally than the natural hormone. To determine to what extent this enhanced activity is mediated through modulation of the dimerization process, we performed quantitative dimerization assays with in vitro translated vitamin D receptor (ivtVDR) and fusion proteins containing glutathione-S-transferase (GST) and either the ligand-binding domain of VDR (GST-VDR) or retinoid X receptor (RXR)alpha (GST-RXR). We found that VDR did not form homodimers in either the presence or absence of ligand, but heterodimerization of the ligand-binding domains of RXRalpha and VDR was primarily deltanoid-dependent. The ED(50) for induction of heterodimerization was 1-2 x 10(-)(9) M for 1alpha,25(OH)(2)D(3) and 0.5 x 10(-)(11) M for 20-epi 1alpha,25(OH)(2)D(3). Mutations in VDR's activation function 2 domain (AF-2) diminished the abilities of 1alpha,25(OH)(2)D(3) to induce a protease-resistant conformation and heterodimerization. These mutations changed neither the potency of 20-epi-1alpha,25(OH)(2)D(3) to induce protease-resistant conformation nor its potency to induce dimerization. Mutations in heptad 9/helix 10 abolished the ability of both 1alpha,25(OH)(2)D(3) and the 20-epi analog to induce dimerization, but not their potency to fold VDR into a protease-resistant conformation. We hypothesize that both the hormone and the analog stabilize receptor conformations that expose VDR's dimerization interface, and that interfaces exposed by these ligands are probably not significantly different. However, the mechanisms by which the two ligands expose the dimerization interface are different with respect to participation of the AF-2 domain.

Induction of differentiation by 1alpha-hydroxyvitamin D(5) in T47D human breast cancer cells and its interaction with vitamin D receptors

The role of the active metabolite of vitamin D, 1,25 dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), in cell differentiation is well established. However, its use as a differentiating agent in a clinical setting is precluded due to its hypercalcaemic activity. Recently, we synthesised a relatively non-calcaemic analogue of vitamin D(5), 1alpha-hydroxyvitamin D(5) (1alpha(OH)D(5)), which inhibited the development of carcinogen-induced mammary lesions in culture and suppressed the incidence of chemically induced mammary carcinogmas in rats. In the present study, we determined the differentiating effects of 1alpha-(OH)D(5) in T47D human breast cancer cells and compared its effects with 1,25(OH)(2)D(3). Cells incubated with either 10 or 100 nM of the analogues inhibited cell proliferation in a dose-dependent manner, as measured by the dimethylthiazolyl-2,5-diphenyltetrazolium bromide (MTT) assay. Similar growth-inhibitory effects were also observed for MCF10(neo) cells. Both vitamin D analogues induced cell differentiation, as determined by induction of casein expression and lipid production. However, MCF10(neo) cells failed to respond to either vitamin D analogue and did not undergo cell differentiation. Since the cell differentiating effect of vitamin D is considered to be mediated via the vitamin D receptor (VDR), we examined the induction of VDR using reverse transcriptase-polymerase chain reaction (RT-PCR) in both cells. The results showed that, in T47D cells, both 1,25(OH)(2)D(3) and 1alpha(OH)D(5) induced VDR in a dose-dependent manner. Moreover, both analogues of vitamin D upregulated the expression of vitamin D response element-chloramphenicol acetyl transferase (VDRE-CAT). These results collectively indicate that 1alpha-(OH)D(5) may mediate its cell-differentiating action via VDR in a manner similar to that of 1,25(OH)(2)D(3).

Heterodimeric DNA binding by the vitamin D receptor and retinoid X receptors is enhanced by 1,25-dihydroxyvitamin D3 and inhibited by 9-cis-retinoic acid. Evidence for allosteric receptor interactions

Gel mobility shift analysis was utilized to investigate the molecular function of 1alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and 9-cis-retinoic acid (9-cis-RA) ligands in the binding of the vitamin D receptor (VDR) and retinoid X receptor (RXR) to mouse osteopontin and rat osteocalcin vitamin D-response elements (VDREs). At physiological ionic strength and reduced concentrations of expressed proteins, efficient binding to either VDRE occurs as a VDR. RXR heterodimer, not as a VDR homodimer. 1,25-(OH)2D3 dramatically enhances heterodimer-VDRE interaction, whereas somewhat higher concentrations of 9-cis-RA inhibit this association, perhaps related to the role of this retinoid in facilitating RXR homodimer formation. Interestingly, if VDR is occupied by 1,25-(OH)2D3 prior to complexing with RXR, the resulting heterodimer is relatively resistant to dissociation and diversion to other pathways by 9-cis-RA. Therefore, a proposed molecular action of 1,25-(OH)2D3 is to generate an allosteric switch in VDR to a form that not only binds to the VDRE with high affinity and specificity as a heterodimer with RXR, but also interacts with the RXR partner to conformationally restrict the action of its cognate ligand.

Regulation of transforming growth factor-beta type II receptor expression in human breast cancer MCF-7 cells by vitamin D3 and its analogues

In view of the tumor suppressor role of the transforming growth factor-beta (TGFbeta) type II receptor (RII), the identification and characterization of agents that can induce the expression of this receptor are of potential importance to the development of chemoprevention approaches as well as treatment of cancer. To date, the identification of exogenous agents that control RII expression has been rare. We demonstrated that proliferation of MCF-7 early passage cells (MCF-7 E), which express RII and are sensitive to TGFbeta growth inhibition activity, was significantly inhibited by vitamin D3 and its analogue EB1089. In contrast, proliferation of MCF-7 late passage cells (MCF-7 L), which have lost cell surface RII and are resistant to TGFbeta, was not affected by these two compounds. TGFbeta-neutralizing antibody was able to block the inhibitory effect on MCF-7 E cells by these compounds, indicating that treatment induced autocrine-negative TGFbeta activity. An RNase protection assay showed approximately a 3-fold induction of the RII mRNA, while a receptor cross-linking assay revealed a 3-4-fold induction of the RII protein. In contrast, there was no change in either RII mRNA or protein in the MCF-7 L cells.

Inhibition of the 1,25-dihydroxyvitamin D3-induced increase in vitamin D receptor (VDR) levels and binding of VDR-retinoid X receptor (RXR) to a direct repeat (DR)-3 type response element by an RXR-specific ligand in human keratinocyte cultures

The biological active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), mediates most of its actions through the intracellular vitamin D receptor (VDR). VDR binds to vitamin D responsive elements (VDREs) in the promoter region of responsive genes and regulates transcription. Usually the VDREs consist of a direct repeat of two hexanucleotides spaced by three nucleotides (DR-3), to which VDR preferentially binds as a heterodimer with the retinoid X receptor (RXR). In the present study, we examined the effect of 1,25(OH)2D3 and a specific ligand for RXR, CD2809, on VDR and RXR levels in cultured human keratinocytes and on the binding of RXR-VDR to a DR-3 type response element. Incubation with 1,25(OH)2D3 increased VDR levels as determined by Western blotting, increased VDR-RXR binding to a DR-3 type response element as determined by the electromobility shift assay (EMSA), and induced the 25-OH-D3 24-hydroxylase (24-hydroxylase) gene, containing a DR-3 type response element. CD2809 caused a slight decrease in RXRalpha levels, but had no effect on VDR levels. Addition of both CD2809 and 1,25(OH)2D3 decreased VDR levels as well as the VDR-RXR binding levels to the DR-3 type response element, compared to 1,25(OH)2D3 alone. In conclusion, an RXR-specific ligand interferes with the 1,25(OH)2D3-induced stimulation of VDR levels and VDR-RXR binding to DNA in keratinocyte cultures. It is therefore possible that RXR-specific ligands may counteract certain biological actions of vitamin D3.

Salt concentration determines 1,25-dihydroxyvitamin D3 dependency of vitamin D receptor-retinoid X receptor--vitamin D-responsive element complex formation

The electrophoretic mobility shift assay was used to determine in vitro formation of the vitamin D receptor-retinoid X receptor beta (VDR-RXR beta) heterodimer complex on vitamin D-response elements (VDREs) from rat osteocalcin, mouse osteopontin, rat 25-hydroxyvitamin D3 24-hydroxylase, and human parathyroid hormone (PTH) genes. Baculovirus-expressed rat VDR was used as VDR and the binding reactions were performed at salt concentrations ranging from 50 to 170 mM KCI. Without ligand, optimum complex formation was observed at 50 mM KCI and markedly decreased with increasing KCI for all VDREs. In the presence of 1,25-dihydroxyvitamin D3, optimum complex formation occurred between 110 and 130 mM KCI for positive (enhancer) VDREs. At low salt concentrations (50-70 mM KCI), 1,25-dihydroxyvitamin D3 did not increase complex formation and actually caused a slight decrease. However, above 90 mM KCI, 1,25-dihydroxy-vitamin D3 markedly increased complex formation and at 150-170 mM KCI, a concentration that presumably mimics physiologic nuclear levels, 1,25-dihydroxy-vitamin D3 appeared to be required for complex formation. With the suppressive cis-acting sequence, i.e., PTH-VDRE, optimum detection of VDR complexes in the presence of 1,25-dihydroxyvitamin D3 occurred at a lower salt concentration (90-110 mM KCI). Moreover, no specific complexes were formed at high salt concentrations, even when 1,25-dihydroxyvitamin D3 was added. Thus, when analyzing an effect of ligand on VDR-RXR-VDRE complex formation, it is essential that the reaction be carried out with a range of salt concentrations. Further, 1,25-dihydroxyvitamin D3 appears to be required for formation of the VDR-RXR beta-VDRE complex at salt concentrations approaching physiological.

Recent advances in the molecular biology of vitamin D action

Following the cloning and deletion analysis of the vitamin D receptor, most recent advances have been in the isolation and characterization of the DNA response elements found in the promoter region of target genes of vitamin D. Vitamin D, like the thyroid and retinoid hormones, binds to repeat sequences, but the repeats are separated by three nonspecified bases. The action of the VDR requires the presence of the RXR proteins and evidently other proteins that are involved in regulating transcriptions. A possible role of phosphorylation of the ligand binding domain of the VDR in transcription has also appeared. Very likely, the molecular events involved in vitamin D stimulation or suppression of a target gene will include its interaction with a number of transcription factors, both in the regulation of transcription and in the actual machinery involved in the transcription process through polymerase II. Although likely, it is not entirely clear whether the genomic action of vitamin D can account for all of its biological activities. Nongenomic actions of the vitamin D hormone have been reported, but convincing evidence that this is of biological importance in vivo is lacking. Advances in our understanding of the vitamin D mechanism of action can clearly be expected from physical studies of cloned and expressed vitamin D receptor and its subdomains, elucidation of the transcription factors in vitamin D-modulated transcription of target genes, elucidation of the role of phosphorylation in the transcription process, and the identification of important genes that are regulated in the specific target tissues responsive to vitamin D. This will definitely remain as a very active field of investigation well into the future.

Vitamin D receptor alleles and bone physiology

The vitamin D endocrine system is central to the control of bone and calcium homeostasis. The active hormonal form of vitamin D, 1,25 dihydroxyvitamin D (calcitriol), the circulating level of which is tightly regulated, acts through a specific receptor to mediate its genomic actions on almost every aspect of calcium homeostasis. Because of its transactivation function, it is possible that a small difference in vitamin D receptor level could be amplified into a biologically significant alteration in physiological setpoint. The recent finding that polymorphisms in the vitamin D receptor gene are predictive of bone density (Morrison et al., Nature 367:284-287, 1994) is the first example of an allelic effect in such a homeostatically controlled system. This raises the possibility that such central operators may exist in other regulatory pathways, and could explain a large part of the observed "normal" population distribution that exists for all physiological parameters.

Variations in vitamin D receptor transcription factor complexes associated with the osteocalcin gene vitamin D responsive element in osteoblasts and osteosarcoma cells

Vitamin D responsive transcription of the bone-specific osteocalcin gene differs markedly in osteosarcoma cells and normal diploid osteoblasts. In osteoblasts the osteocalcin gene is transcribed, and upregulated by Vitamin D, only in post-proliferative cells, but in osteosarcoma cells expression is constitutive. This distinction in transcriptional regulation of the osteocalcin gene correlates with striking differences in the relative representation of two principal Vitamin D-dependent protein/DNA complexes designated V1 and V2 at the Vitamin D responsive element in the osteocalcin promoter. Formation of both complexes is Vitamin D dependent and they contain the Vitamin D receptor as well as an RXR related protein. Pore size exclusion and sedimentation velocity analyses suggest that the V1 and V2 complexes represent oligomeric protein assemblies (respectively, tetramers and trimers), and reflect primarily DNA-directed association of the monomeric protein components at the osteocalcin Vitamin D responsive element. UV crosslinking and methylation interference analyses of the V1 and V2 complexes at the osteocalcin Vitamin D responsive element indicate differences in protein/DNA recognition. For example, the V1 complex interacts with both steroid half-elements, whereas the V2 complex appears to recognize the proximal half-element. Our findings suggest variations in protein/protein and protein/DNA interactions of the VDR and RXR related complexes V1 and V2 at the osteocalcin Vitamin D responsive element that reflect unique properties of the osteosarcoma and normal diploid osteoblast phenotype.