Maternal vitamin D deficiency impairs Treg and Breg responses in offspring mice and deteriorates allergic airway inflammation

Background

Vitamin D (VitD) can regulate immune responses and maternal VitD-deficiency can affect immune responses in the offspring. This study aimed at investigating the effects of maternal VitD-deficiency during pregnancy on Treg and Breg responses in offspring mice with house dust mite (HDM)-induced allergic airway inflammation.

Methods

Female BALB/c mice were randomized and fed with normal chow or VitD-deficient diet until their offspring weaned. The offspring mice were fed with normal chow and injected with vehicle or HDM to induce allergic airway inflammation. The levels of serum 25(OH)D, cytokines and infiltrate numbers as well as percentages of Tregs and Bregs in the bronchoalveolar lavage fluid (BALF) were analyzed. The relative levels of VitD receptor (VDR), VitD-binding protein (VDBP), Cytochromes P450 (CYP) 27b1, and CYP24A1 mRNA transcripts in the lungs of different groups of mice were measured.

Results

Maternal VitD-deficiency significantly reduced serum 25(OH)D levels in offspring mice. VitD-deficiency significantly increased the relative levels of VDR, VDBP and CYP27B1 mRNA transcripts, but decreased CYP24A1 expression in the lungs of mice. In comparison with the control mice, significantly elevated levels of pro-inflammatory cytokines, increased numbers of lymphocytes and eosinophils, but decreased levels of anti-inflammatory cytokines were detected in the BALF of VitD-deficient mice. VitD-deficiency significantly increased the frequency of Th1, Th2, Th9, Th17 cells, but decreased regulatory T (Tregs) and B cells (Bregs) in the BALF of mice with allergic airway inflammation.

Conclusion

Maternal VitD-deficiency lowed serum 25(OH)D levels and enhanced HDM-induced allergic airway inflammation in the offspring by impairing Breg and Treg responses.

Nuclear Receptors, Ligands and the Mammalian B Cell

Questions concerning the influences of nuclear receptors and their ligands on mammalian B cells are vast in number. Here, we briefly review the effects of nuclear receptor ligands, including estrogen and vitamins, on immunoglobulin production and protection from infectious diseases. We describe nuclear receptor interactions with the B cell genome and the potential mechanisms of gene regulation. Attention to the nuclear receptor/ligand regulation of B cell function may help optimize B cell responses, improve pathogen clearance, and prevent damaging responses toward inert- and self-antigens.

Enhanced CD103 Expression and Reduced Frequencies of Virus-Specific CD8+ T Cells Among Airway Lymphocytes After Influenza Vaccination of Mice Deficient in Vitamins A + D

Previous research has evaluated antibody responses toward an influenza virus vaccine in the context of deficiencies for vitamins A and D (VAD+VDD). Results showed that antibodies and antibody-forming cells in the respiratory tract were reduced in VAD+VDD mice. However, effectors were recovered when oral supplements of vitamins A + D were delivered at the time of vaccination. Here we address the question of how vaccine-induced CD8+ T cell responses are affected by deficiencies for vitamins A + D. VAD+VDD and control mice were vaccinated with an intranasal, cold-adapted influenza virus A/Puerto Rico/8/34 vaccine, with or without oral supplements of vitamins A + D. Results showed that the percentages of vaccine-induced CD8+ T cell and total CD4+ T cell responses were low among lymphocytes in the airways of VAD+VDD animals compared to controls. The CD103 membrane marker, a protein that binds e-cadherin (expressed on respiratory tract epithelial cells), was unusually high on virus-specific T cells in VAD+VDD mice compared to controls. Interestingly, when T cells specific for the PA224-233/Db epitope were compared with T cells specific for the NP366-374/Db epitope, the former population was more strongly positive for CD103. Preliminary experiments revealed normal or above-normal percentages for vaccine-induced T cells in airways when VAD+VDD animals were supplemented with vitamins A + D at the time of vaccination and on days 3 and 7 after vaccination. Our results suggest that close attention should be paid to levels of vitamins A and D among vaccine recipients in the clinical arena, as low vitamin levels may render individuals poorly responsive to vaccines.

Vitamin D receptor 2016: novel ligands and structural insights

INTRODUCTION

Vitamin D₃ activates via its hormonal form 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the transcription factor vitamin D receptor (VDR). VDR is expressed in most human tissues and has more than 1,000 target genes. Thus, 1α,25(OH)₂D₃ and its synthetic analogs have a broad physiological impact. The crystal structures of the VDR ligand-binding domain (LBD), and its various ligands, allows further the understanding of the receptor's molecular actions. Areas covered: We discuss the most important novel VDR ligands and the further insight derived from new structural information on VDR. Expert opinion: There is an increasing appreciation of the impact of vitamin D and its receptor VDR not only in bone biology, but also for metabolic diseases, immunological disorders, and cancer. Detailed structural analysis of the interaction of additional novel ligands with VDR highlight helices 6 and 7 of the LBD as being most critical for stabilizing the receptor for an efficient interaction with co-activator proteins, i.e. for efficient agonistic action. This permits the design of even more effective VDR agonists. In addition, chemists took more liberty in replacing major parts of the 1α,25(OH)₂D₃ molecule, such as the A- and CD-rings or the side chain, with significantly different structures, such as carboranes, and still obtained functional VDR agonists.

Associations of vitamin D deficiency and vitamin D receptor (Cdx-2, Fok I, Bsm I and Taq I) polymorphisms with the risk of primary open-angle glaucoma

BACKGROUND

Vitamin D deficiency and vitamin D receptor gene polymorphisms are known to be significantly associated with high myopia. Whether this genetic variant may impact primary open-angle glaucoma is largely unknown. This study investigated whether vitamin D receptor gene polymorphisms are altered in primary open-angle glaucoma subjects carrying the risk allele, and whether vitamin D deficiency is an important factor in the development of glaucoma.

METHODS

Seventy-three POAG patients and 71 age-matched controls from the Han population were enrolled. Serum levels of 1a, 25-Dihydroxyvitamin D3 were measured by enzyme-linked immunoabsorbent assay. Vitamin D receptor polymorphisms (Cdx-2, Fok I, Bsm I and Taq I) were analyzed using real-time polymerase-chain reaction high resolution melting analysis.

RESULTS

Serum levels of 1a, 25-Dihydroxyvitamin in primary open-angle glaucoma patients were lower than in age-matched controls. Statistical analysis revealed a significant difference in the allelic frequencies of the BsmI and TaqI genotypes between primary open-angle glaucoma patients and age-matched controls, while other polymorphisms did not show any significant differences.

CONCLUSIONS

Vitamin D deficiency and the presence of the BsmI 'B' allele and the TaqI 't' allele are relevant risk factors in the development of glaucoma.

TRIAL REGISTRATION

Clinical Trials.gov: NCT02539745 . The study was registered retrospectively on August 3rd, 2015. The first participant was enrolled on July 4th, 2013.

Association of T-regulatory cells and CD23/CD21 expression with vitamin D in children with asthma

BACKGROUND

Children with asthma have low vitamin D levels; however, we do not know whether low vitamin D is associated with impaired T-regulatory (Treg) cell population or high IgE receptors (CD23 and CD21) on B cells.

OBJECTIVE

To examine Treg cell function, CD23 and CD21 receptors, vitamin D, and vitamin D-regulating enzymes in children with asthma.

METHODS

Sixty children (2-6 years old) with asthma and 60 age-matched healthy children were selected as study participants. After collecting demographic and clinical data, blood samples were collected. Treg cells and CD23/CD21 expressions were evaluated by flow cytometry, 25-hydroxyvitamin D3 (25[OH]D3) was quantified by high-performance liquid chromatography, and cytokines and total IgE were estimated by enzyme-linked immunosorbent assay. Messenger RNA (mRNA) expressions of FOXP3, CD23, CD21, vitamin D receptors, and vitamin D-regulating enzymes were assessed by reverse transcription-polymerase chain reaction.

RESULTS

The 25(OH)D3 concentrations and proportion of Treg cells were lower (P < .05) among children with asthma. In contrast, the proportions of B cells with CD23 and CD21 expression were higher (P < .05) in patients with asthma compared with controls. Interleukin 10 and transforming growth factor β were also altered in asthma. The mRNA expression of CD23, CD21, and vitamin D receptors was up-regulated, whereas mRNA expression of vitamin D-regulating enzymes, CYP2R1, CYP27B1, and vitamin D binding protein (except CYP24A1) were up-regulated among children with asthma.

CONCLUSION

The current study found impaired Treg cell population and high numbers of B cells with IgE receptors (CD23 and CD21) and altered regulatory cytokines in children with asthma, suggesting impaired immune regulation.

Vitamin D Signaling Modulators in Cancer Therapy

The antiproliferative and pro-apoptotic effects of 1α,25-dihydroxycholecalciferol (1,25(OH)2D3, 1,25D3, calcitriol) have been demonstrated in various tumor model systems in vitro and in vivo. However, limited antitumor effects of 1,25D3 have been observed in clinical trials. This may be attributed to a variety of factors including overexpression of the primary 1,25D3 degrading enzyme, CYP24A1, in tumors, which would lead to rapid local inactivation of 1,25D3. An alternative strategy for improving the antitumor activity of 1,25D3 involves the combination with a selective CYP24A1 inhibitor. The validity of this approach is supported by numerous preclinical investigations, which demonstrate that CYP24A1 inhibitors suppress 1,25D3 catabolism in tumor cells and increase the effects of 1,25D3 on gene expression and cell growth. Studies are now required to determine whether selective CYP24A1 inhibitors+1,25D3 can be used safely and effectively in patients. CYP24A1 inhibitors plus 1,25D3 can cause dose-limiting toxicity of vitamin D (hypercalcemia) in some patients. Dexamethasone significantly reduces 1,25D3-mediated hypercalcemia and enhances the antitumor activity of 1,25D3, increases VDR-ligand binding, and increases VDR protein expression. Efforts to dissect the mechanisms responsible for CYP24A1 overexpression and combinational effect of 1,25D3/dexamethasone in tumors are underway. Understanding the cross talk between vitamin D receptor (VDR) and glucocorticoid receptor (GR) signaling axes is of crucial importance to the design of new therapies that include 1,25D3 and dexamethasone. Insights gained from these studies are expected to yield novel strategies to improve the efficacy of 1,25D3 treatment.

Reprint of "Vitamin D deficiency in pregnant women impairs regulatory T cell function"

Regulatory T cells and IgE receptors (CD23 and CD21) on B cells were assessed in vitamin D deficient pregnant women. For this, 153 pregnant women were recruited from a government hospital and were categorized into three groups based on 25-hydroxyvitamin D3 (25(OH)D3) status. Regulatory T cell population (Treg cells) and CD23/CD21 expression on B cells were quantified by FACS ARIA II in maternal blood at third trimester; and the same parameters were evaluated in cord blood soon after delivery. In addition, TGF β and IL-10 were quantified in maternal and cord blood by using Milliplex kits. In a representative sample of eight women from each group (vitamin D sufficient, insufficient and deficient), placental tissues were processed for mRNA expressions of vitamin D receptor (VDR), retinoic acid receptor (RXR), vitamin D binding protein (VDBP) and vitamin D regulating enzymes. Of the 153 pregnant women, 18 were sufficient (≥30ng/mL), 55 were insufficient (20-29ng/mL) and 80 were deficient (≤19ng/mL) for 25(OH)D3 status. The maternal blood Treg cell population (mean (%)±SE) was lower (p<0.05) in 25(OH)D3 deficient (0.2±0.01) pregnant women compared to insufficient (0.34±0.01) and sufficient (0.45±0.02) pregnant women. Similarly, cord blood Treg cell population (mean (%)±SE) was also lower (p<0.05) in 25(OH)D3 deficient (0.63±0.03) pregnant women when compared to insufficient (1.05±0.04) and sufficient (1.75±0.02) pregnant women. Mean (%)±SE of B cells with CD23 and CD21 in maternal blood was higher (p<0.05) in 25(OH)D3 deficient pregnant women (0.35±0.02; 1.65±0.04) when compared to insufficient (0.22±0.02; 0.55±0.05) and sufficient (0.15±0.02; 0.21±0.01) pregnant women. Similarly, mean (%)±SE of B cell population with CD23 and CD21 in cord blood was also higher (p<0.05) in 25(OH)D3 deficient (0.41±0.02; 1.2±0.03) when compared to insufficient (0.32±0.01; 0.6±0.05) and sufficient (0.2±0.01; 0.4±0.02) pregnant women. Regulatory cytokines, TGF β and IL-10 were lower (p<0.05) in 25(OH)D3 insufficient and deficient subjects. In the placenta tissue of women with 25(OH)D3 deficiency, the regulatory T cell transcription factor FOXP3, vitamin D receptor (VDR) and retinoic acid receptor (RXR) expressions were downregulated. In contrast, CD23, CD21 and VDBP expressions were upregulated in 25(OH)D3 deficient and insufficient women. Vitamin D regulating enzymes (CYP24A1, CYP2R1 and CYP27B1) expression were also altered in women with 25(OH)D3 deficiency. The current study shows that impaired maternal 25(OH)D3 during pregnancy influences the spectrum of immune cells such as regulatory T cells and B cells with IgE receptors and this in turn may be linked to allergy and asthma in neonates.

Identification and characterization of a vitamin D₃ decomposition product bactericidal against Helicobacter pylori

This study demonstrated that the vitamin D₃ decomposition product VDP1 exerts an antibacterial action against Helicobacter pylori but not against other bacteria. Treatment with VDP1 induced a collapse of cell membrane structures of H. pylori and ultimately lysed the bacterial cells. A unique dimyristoyl phosphatidylethanolamine in the membrane lipid compositions contributed to the interaction of VDP1 with H. pylori cells. In separate experiments, VDP1 had no influence on the viability of the human cancer cell lines MKN45 and T47D and lacked any vitamin D₃-like hormonal action against the latter. In both ¹H and ¹³C NMR analyses, the spectra patterns of VDP1 corresponded with those of Grundmann's ketone. These results suggest that VDP1 (or Grundmann's ketone-type indene compound) may become a fundamental structure for the development of new antibacterial substances with selective bactericidal action against H. pylori.

Vitamin D deficiency in pregnant women impairs regulatory T cell function

Regulatory T cells and IgE receptors (CD23 and CD21) on B cells were assessed in vitamin D deficient pregnant women. For this, 153 pregnant women were recruited from a government hospital and were categorized into three groups based on 25-hydroxyvitamin D3 (25(OH)D3) status. Regulatory T cell population (Treg cells) and CD23/CD21 expression on B cells were quantified by FACS ARIA II in maternal blood at third trimester; and the same parameters were evaluated in cord blood soon after delivery. In addition, TGF β and IL-10 were quantified in maternal and cord blood by using Milliplex kits. In a representative sample of eight women from each group (vitamin D sufficient, insufficient and deficient), placental tissues were processed for mRNA expressions of vitamin D receptor (VDR), retinoic acid receptor (RXR), vitamin D binding protein (VDBP) and vitamin D regulating enzymes. Of the 153 pregnant women, 18 were sufficient (≥30 ng/mL), 55 were insufficient (20-29 ng/mL) and 80 were deficient (≤19 ng/mL) for 25(OH)D3 status. The maternal blood Treg cell population (mean (%)± SE) was lower (p<0.05) in 25(OH)D3 deficient (0.2 ± 0.01) pregnant women compared to insufficient (0.34 ± 0.01) and sufficient (0.45 ± 0.02) pregnant women. Similarly, cord blood Treg cell population (mean (%)± SE) was also lower (p<0.05) in 25(OH)D3 deficient (0.63 ± 0.03) pregnant women when compared to insufficient (1.05 ± 0.04) and sufficient (1.75 ± 0.02) pregnant women. Mean (%) ± SE of B cells with CD23 and CD21 in maternal blood was higher (p<0.05) in 25(OH)D3 deficient pregnant women (0.35 ± 0.02; 1.65 ± 0.04) when compared to insufficient (0.22 ± 0.02; 0.55 ± 0.05) and sufficient (0.15 ± 0.02; 0.21 ± 0.01) pregnant women. Similarly, mean (%)± SE of B cell population with CD23 and CD21 in cord blood was also higher (p<0.05) in 25(OH)D3 deficient (0.41 ± 0.02; 1.2 ± 0.03) when compared to insufficient (0.32 ± 0.01; 0.6 ± 0.05) and sufficient (0.2 ± 0.01; 0.4 ± 0.02) pregnant women. Regulatory cytokines, TGF β and IL-10 were lower (p<0.05) in 25(OH)D3 insufficient and deficient subjects. In the placenta tissue of women with 25(OH)D3 deficiency, the regulatory T cell transcription factor FOXP3, vitamin D receptor (VDR) and retinoic acid receptor (RXR) expressions were downregulated. In contrast, CD23, CD21 and VDBP expressions were upregulated in 25(OH)D3 deficient and insufficient women. Vitamin D regulating enzymes (CYP24A1, CYP2R1 and CYP27B1) expression were also altered in women with 25(OH)D3 deficiency. The current study shows that impaired maternal 25(OH)D3 during pregnancy influences the spectrum of immune cells such as regulatory T cells and B cells with IgE receptors and this in turn may be linked to allergy and asthma in neonates.

1,25-dihydroxyvitamin D3 and its nuclear receptor repress human α1 (I) collagen expression and type I collagen formation

BACKGROUND

Vitamin D deficiency is common in chronic liver disease particularly in those with severe liver fibrosis.

AIMS

To determine the effect of 1,25-dihydroxyvitamin D3 (1,25-(OH)2 D3 ) on the human α(1) (I) collagen promoter and collagen formation by human stellate LX-2 cells and the mechanism of the effect of the vitamin D receptor (VDR) on the promoter.

METHODS

Type I collagen was assessed by measurements of collagen mRNA and collagen protein and by transfection experiments. Binding of VDR to the α(1) (I) collagen promoter was determined by EMSA and ChIP assays.

RESULTS

1,25-(OH)2 D3 decreased human α(1) (I) collagen mRNA and protein and the secretion of type I collagen by stellate cells after exposure to TGFβ1. Furthermore, 1,25-(OH)2 D3 inhibited TGFβ1-induced activation of the α(1) (I) collagen promoter in transfected LX-2 cells. The effect of 1,25-(OH)2 D3 is mediated by the VDR, which binds at a proximal Sp1 site and also at a newly identified distal site on the collagen promoter. A VDR expression vector reduced the activities of the collagen promoter in transfected LX-2 cells.

CONCLUSIONS

1,25-(OH)2 D3 inhibits type I collagen formation in human stellate cells. The effect of 1,25-(OH)2 D3 is mediated by its receptor which binds at a proximal Sp1.1 site and at a newly identified distal site on the collagen promoter. Correction of vitamin D deficiency in patients with chronic liver disease is a potential therapy to inhibit progression of fibrosis.

Using components of the vitamin D pathway to prevent and treat colon cancer

The objective of this review was to analyze the components of vitamin D and their potential usefulness in preventing and treating colorectal cancer. The active form of vitamin D, 1α,25(OH(2) )D(3) , targets the wnt/β-catenin pathway by upregulating key tumor suppressor genes such as E-cadherin, which promotes an epithelial phenotype, but this is only possible when the vitamin D receptor (VDR) is present. Colorectal cell lines have shown that VDR expression levels decrease in the later stages of colon cancer. In colorectal cancers with low VDR expression, treatments to increase VDR expression could target alterations at the genomic and epigenomic levels by modulating transcription factors such as SNAIL1 and by utilizing histone deacetyltransferase inhibitors, respectively. Finally, epidemiological studies suggest that the current US Recommended Dietary Allowance should be increased to 2,000 IU in order to raise serum 25(OH)D(3) levels above 30 ng/mL. This increase in vitamin D status can be obtained most efficiently from sun exposure or vitamin D supplementation. In summary, vitamin D and its metabolites could be utilized in strategies to treat and prevent colon cancer.

Vitamin D receptor ligands: the impact of crystal structures

INTRODUCTION

In the past years, the biologically active form of vitamin D(3), 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)), has received large appreciation due to the broad physiological impact of the hormone and its nuclear receptor, the transcription factor vitamin D receptor (VDR). Recently, the understanding of VDR actions has progressed greatly, due to VDR crystal structures with various ligands.

AREAS COVERED

This review will present and discuss new synthetic agonistic and antagonistic 1α,25(OH)(2)D(3) analogs in the context of the recent insights provided by VDR crystal structures.

EXPERT OPINION

During the last 5 years, a large number of new 1α,25(OH)(2)D(3) analogs, many of which have an interesting functional profile, have been patented. Moreover, for a surprisingly high number of 1α,25(OH)(2)D(3) analogs, the crystal structure data of their complex with the VDR is available. This structural information provides important insight into the functional potential of the VDR ligands and explains their agonistic and antagonistic action. However, so far, only for a few VDR ligands, a rational design, based on crystal structure information, has been applied. The design of future analogs may also take the specificity of co-factor interaction into account, in order to create selective VDR modulators.

The yin and yang of vitamin D receptor (VDR) signaling in neoplastic progression: operational networks and tissue-specific growth control

Substantive evidence implicates vitamin D receptor (VDR) or its natural ligand 1α,25-(OH)₂ D₃ in modulation of tumor growth. However, both human and animal studies indicate tissue-specificity of effect. Epidemiological studies show both inverse and direct relationships between serum 25(OH)D levels and common solid cancers. VDR ablation affects carcinogen-induced tumorigenesis in a tissue-specific manner in model systems. Better understanding of the tissue-specificity of vitamin D-dependent molecular networks may provide insight into selective growth control by the seco-steroid, 1α,25-(OH)₂ D₃. This commentary considers complex factors that may influence the cell- or tissue-specificity of 1α,25-(OH)₂ D₃/VDR growth effects, including local synthesis, metabolism and transport of vitamin D and its metabolites, vitamin D receptor (VDR) expression and ligand-interactions, 1α,25-(OH)₂ D₃ genomic and non-genomic actions, Ca²⁺ flux, kinase activation, VDR interactions with activating and inhibitory vitamin D responsive elements (VDREs) within target gene promoters, VDR coregulator recruitment and differential effects on key downstream growth regulatory genes. We highlight some differences of VDR growth control relevant to colonic, esophageal, prostate, pancreatic and other cancers and assess the potential for development of selective prevention or treatment strategies.

CDK11p58 represses vitamin D receptor-mediated transcriptional activation through promoting its ubiquitin-proteasome degradation

Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily and regulates transcription of target genes. In this study, we identified CDK11(p58) as a novel protein involved in the regulation of VDR. CDK11(p58), a member of the large family of p34cdc2-related kinases, is associated with cell cycle progression, tumorigenesis, and apoptotic signaling. Our study demonstrated that CDK11(p58) interacted with VDR and repressed VDR-dependent transcriptional activation. Furthermore, overexpression of CDK11(p58) decreased the stability of VDR through promoting its ubiquitin-proteasome-mediated degradation. Taken together, these results suggest that CDK11(p58) is involved in the negative regulation of VDR.

5-Lipoxygenase inhibitors potentiate 1alpha,25-dihydroxyvitamin D3-induced monocytic differentiation by activating p38 MAPK pathway

The treatment of human promyelocytic leukemia cell lines HL-60, and to some extent NB-4, with 1alpha,25-dihydroxyvitamin D(3) (VD3) induces differentiation toward the monocytic/macrophage lineage, demonstrated by the increased expression of CD11b and CD14, and the production of opsonized zymosan particles (OZP)-stimulated reactive oxygen species (ROS). Moreover, in more sensitive HL-60 cells, increased expression of 5-lipoxygenase (5-LPO), Mcl-1, IkappaB, and c-Jun, accompanied by the activation of p38 MAPK, was detected. These VD3 effects on HL-60 cell differentiation were significantly potentiated by 5-LPO inhibitors MK-886 and AA-861 and were inverted by SB202190 (SB), a p38 MAPK inhibitor. The inhibition of differentiation by SB was demonstrated by a reduction of CD14 expression and by a decrease in OZP-activated ROS production. These results indicated that p38 MAPK pathway is involved in 5-LPO inhibitors-dependent potentiation of VD3-induced monocytic differentiation.

1,25-dihydroxyvitamin D3 regulates VEGF production through a vitamin D response element in the VEGF promoter

In previous studies we have demonstrated that the active form of vitamin D (1,25(OH)(2)D(3)) increases vascular endothelial growth factor (VEGF) expression and release in vascular smooth muscle cells (VSMC) in vitro. However, the mechanism by which 1,25(OH)(2)D(3) increases VEGF production is currently unknown. In this work, we demonstrated binding of vitamin D receptor to two response elements in the VEGF promoter. We performed promoter transactivation analysis and we observed that, in 293T cells, VEGF promoter was activated after vitamin D treatment. Using site-directed mutagenesis we have shown that both response elements are important for VEGF promoter activity. Therefore, the increase in VEGF expression and secretion induced by 1,25(OH)(2)D(3) in VSMC in vitro could be explained by direct binding of the vitamin D receptor, as a transcription factor, to VEGF promoter. These results could explain part of the beneficial effects of vitamin D treatment in renal patients by a possible VEGF-mediated improvement of the endothelial dysfunction.

The vitamin D receptor

The vitamin D endocrine system is known for its essential role in calcium homeostasis and bone metabolism, and induces cell differentiation, inhibits cell growth, controls other hormonal systems, and modulates the immune response. Vitamin D(3) is a prohormone that is taken up by diet or synthesized in ultraviolet radiation-exposed skin and metabolically converted to the active metabolite, 1alpha,25-dihydroxyvitamin D(3). This nuclear hormone binds with high affinity the nuclear receptor vitamin D receptor. More than 3000 synthetic analogs of 1alpha,25(OH)(2)D(3) are known. This review aims to provide an overview on vitamin D signaling from the skin perspective.

The coregulator Alien

Alien has characteristics of a corepressor for selected members of the nuclear hormone receptor (NHR) superfamily and also for transcription factors involved in cell cycle regulation and DNA repair. Alien mediates gene silencing and represses the transactivation of specific NHRs and other transcription factors to modulate hormone response and cell proliferation. Alien is a highly conserved protein and is expressed in a wide variety of tissues. Knockout of the gene encoding Alien in mice is embryonic lethal at a very early stage, indicating an important evolutionary role in multicellular organisms. From a mechanistic perspective, the corepressor function of Alien is in part mediated by histone deacetylase (HDAC) activity. In addition, Alien seems to modulate nucleosome assembly activity. This suggests that Alien is acting on chromatin not only through recruitment of histone-modifying activities, but also through enhancing nucleosome assembly.

Controlling the chromatin organization of vitamin D target genes by multiple vitamin D receptor binding sites

An essential prerequisite for the direct modulation of transcription by 1alpha,25-dihydroxy vitamin D(3) (1alpha,25(OH)(2)D(3)) is the location of at least one activated vitamin D receptor (VDR) protein close to the transcription start site of the respective primary 1alpha,25(OH)(2)D(3) target gene. This is achieved through the specific binding of VDR to a 1alpha,25(OH)(2)D(3) response element (VDRE). Although these elements are well characterized in vitro, the function of VDREs in living cells in the context of chromatin is still largely unknown. To resolve this issue, approximately 8kB of the promoter regions of the primary 1alpha,25(OH)(2)D(3) target genes CYP24, cyclin C and p21((Waf1/Cip1)) were screened by chromatin immunoprecipitation (ChIP) assays for VDR binding sites using antibodies against VDR and its partner proteins. This approach identified three to four functional VDREs per gene promoter. In parallel, in silico screening of the extended gene areas (i.e. 10kB of promoter, introns, exons and 10kB of the downstream region) of all six members of the insulin-like growth factor binding protein (IGFBP) gene family was performed. Gel shift, reporter gene and ChIP assays identified in total 10 functional VDREs in the genes IGFBP1, IGFBP3 and IGFBP5. Taken together, both screening approaches suggest that a reasonable proportion of all VDR target genes, if not all, are under the control of multiple VDREs.

Modulation by the steroid/thyroid hormone superfamily of TGF-beta-stimulated VEGF release from vascular smooth muscle cells

We previously reported that transforming growth factor-beta (TGF-beta) stimulates the release of vascular endothelial growth factor (VEGF) from aortic smooth muscle A10 cells via activation of p38 mitogen-activated protein (MAP) kinase. In the present study, we investigated whether nuclear hormone receptor superfamily members affect TGF-beta-stimulated VEGF release from A10 cells. Retinoic acid or 1,25-dihydroxyvitamin D3 enhanced TGF-beta-induced VEGF release in a concentration-dependent manner, whereas dexamethasone or corticosterone suppressed TGF-beta-induced VEGF release. 1,25-Dihydroxyvitamin D3 and TGF-beta stimulated phosphorylation of p38 MAP kinase in an additive manner. SB203580, an inhibitor of p38 MAP kinase, decreased the VEGF release induced by TGF-beta or 1,25-dihydroxyvitamin D3. However, retinoic acid, dexamethasone, or corticosterone did not affect phosphorylation of p38 MAP kinase. These results indicate that retinoic acid, 1,25-dihydroxyvitamin D3, and glucocorticoids affect TGF-beta-stimulated VEGF release from aortic smooth muscle cells. The stimulatory effect of 1,25-dihydroxyvitamin D3 occurs, in part, via modification of TGF-beta-induced activation of p38 MAP kinase.

Analysis of the 5-lipoxygenase promoter and characterization of a vitamin D receptor binding site

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) and transforming growth factor beta (TGFbeta) potently induce 5-lipoxygenase (5-LO) in myeloid cells. We analyzed vitamin D receptor (VDR) binding to putative vitamin D response elements within the 5-LO promoter and analyzed its function by reporter gene analysis. Binding of VDR and retinoid X receptor to the promoter region was shown in DNase I footprinting, electrophoretic mobility shift and chromatin immunoprecipitation assays. However, the identified VDR binding region did not mediate induction of reporter gene activity by 1,25(OH)(2)D(3)/TGFbeta, neither in the 5-LO promoter context nor with the thymidine kinase (tk) promoter. Insertion of the rat atrial natriuretic factor VDRE in reporter plasmids containing the 5-LO promoter diminished induction by 1,25(OH)(2)D(3)/TGFbeta as compared with the tk promoter. Similarly, low inductions were obtained when cells were transiently or stably transfected with constructs containing various 5-LO promoter regions. Concerning basal promoter activity, we identified a positive regulatory region (-779 to -229), which includes the VDR binding region, in 5-LO-positive MonoMac6 cells. In summary, the VDR/RXR complex binds to putative VDREs in the 5-LO promoter, but other sequences outside the 5-LO promoter seem to be responsible or additionally required for the prominent induction of 5-LO mRNA expression by 1,25(OH)(2)D(3) and TGFbeta.

Cyclin D3 interacts with vitamin D receptor and regulates its transcription activity

D-type cyclins are essential for the progression through the G1 phase of the cell cycle. Besides serving as cell cycle regulators, D-type cyclins were recently reported to have transcription regulation functions. Here, we report that cyclin D3 is a new interacting partner of vitamin D receptor (VDR), a member of the superfamily of nuclear receptors for steroid hormones, thyroid hormone, and the fat-soluble vitamins A and D. The interaction was confirmed with methods of yeast two-hybrid system, in vitro binding analysis and in vivo co-immunoprecipitation. Cyclin D3 interacted with VDR in a ligand-independent manner, but treatment of the ligand, 1,25-dihydroxyvitamin D3, strengthened the interaction. Confocal microscopy analysis showed that ligand-activated VDR led to an accumulation of cyclin D3 in the nuclear region. Cyclin D3 up-regulated transcriptional activity of VDR and this effect was counteracted by overexpression of CDK4 and CDK6. These findings provide us a new clue to understand the transcription regulation functions of D-type cyclins.

The therapeutic effects of vitamin D3 and its analogues in psoriasis

Psoriasis is a common skin disease which is characterised by the proliferation and abnormal differentiation of keratinocytes, coupled with complex immune disturbances. The beneficial effects of vitamin D derivatives in this disease are due to their capacity to inhibit proliferation, their ability to induce normal differentiation and their immunomodulatory properties. Since the systemic administration of these compounds is limited by their effect on calcium metabolism, topical preparations have become available in most countries. Topical calcipotriol and/or tacalcitol are now considered as first-line treatment for mild-to-moderate psoriasis and can be taken in combination with other systemic therapies in more severe cases of the disease. Novel orally active vitamin D analogues, with minimal calcitropic effercts, are, however, required for more effective treatment.

Structural Determinants of the Agonist-independent Association of Human Peroxisome Proliferator-activated Receptors with Coactivators

Lipid homeostasis is controlled by various nuclear receptors (NRs), including the peroxisome proliferator-activated receptors (PPARalpha, delta, and gamma), which sense lipid levels and regulate their metabolism. Here we demonstrate that human PPARs have a high basal activity and show ligand-independent coactivator (CoA) association comparable with the NR constitutive androstane receptor. Using PPARgamma as an example, we found that four different amino acid groups contribute to the ligand-independent stabilization of helix 12 of the PPAR ligand-binding domain. These are: (i) Lys329 and Glu499, mediating a charge clamp-type stabilization of helix 12 via a CoA bridge; (ii) Glu352, Arg425, and Tyr505, directly stabilizing the helix via salt bridges and hydrogen bonds; (iii) Lys347 and Asp503, interacting with each other as well as contacting the CoA; and (iv) His351, Tyr(355), His477, and Tyr501, forming a hydrogen bond network. These amino acids are highly conserved within the PPAR subfamily, suggesting that the same mechanism may apply for all three PPARs. Phylogenetic trees of helix 12 amino acid and nucleotide sequences of all crystallized NRs and all human NRs, respectively, indicated a close relationship of PPARs with constitutive androstane receptor and other constitutive active members of the NR superfamily. Taking together, the ligand-independent tight control of the position of the PPAR helix 12 provides an effective alternative for establishing an interaction with CoA proteins. This leads to high basal activity of PPARs and provides an additional view on PPAR signaling.

Antagonist- and inverse agonist-driven interactions of the vitamin D receptor and the constitutive androstane receptor with corepressor protein

Ligand-dependent signal transduction by nuclear receptors (NRs) includes dynamic exchanges of coactivator (CoA) and corepressor (CoR) proteins. Here we focused on the structural determinants of the antagonist- and inverse agonist-enhanced interaction of the endocrine NR vitamin D receptor (VDR) and the adopted orphan NR constitutive androstane receptor (CAR) from two species with the CoR NR corepressor. We found that the pure VDR antagonist ZK168281 and the human CAR inverse agonist clotrimazole are both effective inhibitors of the CoA interaction of their respective receptors, whereas ZK168281 resembled more the mouse CAR inverse agonist androstanol in its ability to recruit CoR proteins. Molecular dynamics simulations resulted in comparable models for the CoR receptor interaction domain peptide bound to VDR/antagonist or CAR/inverse agonist complexes. A salt bridge between the CoR and a conserved lysine in helix 4 of the NR is central to this interaction, but also helix 12 was stabilized by direct contacts with residues of the CoR. Fixation of helix 12 in the antagonistic/inverse agonistic conformation prevents an energetically unfavorable free floatation of the C terminus. The comparable molecular mechanisms that explain the similar functional profile of antagonist and inverse agonists are likely to be extended from VDR and CAR to other members of the NR superfamily and may lead to the design of even more effective ligands.

Agonist-dependent and agonist-independent transactivations of the human constitutive androstane receptor are modulated by specific amino acid pairs

The constitutive androstane receptor (CAR) is an interesting member of the nuclear receptor superfamily because of its exceptionally high constitutive activity due to ligand-independent interaction of the ligand-binding domain with co-activator proteins. This study compares the agonist-dependent and agonist-independent activities of human CAR with those of mouse CAR and the vitamin D receptor and demonstrates that the constitutive activity of CAR is mediated by at least three contacts between the amino acids of helix 12, partner amino acids in helices 4 and 11, and a charge clamp between helices 12 and 3. The stabilization of helix 12 by a contact between its C terminus and the lysine of helix 4 has the same impact in human and mouse CARs. In addition, the charge clamp between the glutamate in helix 12 and the lysine in helix 3 is also important for the constitutive activity of both receptor orthologs but less critical for the agonist-dependent stabilization of their respective helices 12. Interestingly, Cys-357 in mouse CAR has significantly more impact on the stabilization of helix 12 than does the orthologous position Cys-347 in human CAR. This deficit appears to be compensated by a more dominant role of Ile-330 in human CAR over Leu-340 in mouse CAR because it is more efficient than Cys-347 in controlling the flexibility of helix 12 in the presence of an agonist. The constitutive activity of other members of the nuclear receptor superfamily could be explained by a homologous hydrophobic interaction between large, non-polar amino acids of helices 11 and 12.

Localization and regulation of the epithelial Ca2+ channel TRPV6 in the kidney

The family of epithelial Ca(2+) channels consists of two highly homologues members, TRPV5 and TRPV6, which constitute the apical Ca(2+) entry mechanism in active Ca(2+) (re)absorption in kidney and small intestine. In kidney, TRPV5 expression has been extensively studied, whereas TRPV6 localization and regulation has been largely confined to the small intestine. The present study investigated the renal distribution of TRPV6 and regulation by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). In mouse kidney, TRPV6 was detected by immunohistochemistry at the apical domain of the distal convoluted tubules (DCT2), connecting tubules (CNT), and cortical and medullary collecting ducts (CD). Furthermore, several putative vitamin D-responsive elements were detected upstream of the mouse TRPV6 start codon, and 1,25(OH)(2)D(3) treatment significantly increased renal TRPV6 mRNA and protein expression. In DCT2 and CNT, TRPV6 co-localizes with the other known Ca(2+) transport proteins, including TRPV5 and calbindin-D(28K). Together, these data suggest a role for TRPV6 in 1,25(OH)(2)D(3)-stimulated Ca(2+) reabsorption in these segments. Interestingly, distribution of TRPV6 extended to the CD, where it localized to the apical domain of principal and intercalated cells, which are not generally implicated in active Ca(2+) reabsorption. In addition, TRPV6 mRNA levels were quantified in a large set of tissues, and in the order of decreasing expression level were detected: prostate > stomach, brain > lung > duodenum, kidney, bone, cecum, heart > colon > skeletal muscle > pancreas. Therefore, additional physiologic functions for TRPV6 are feasible. In conclusion, TRPV6 is expressed along the apical domain of DCT2, CNT, and CD, where TRPV6 expression is positively regulated by 1,25(OH)(2)D(3).

Importance of cytochrome P450-mediated metabolism in the mechanism of action of vitamin D analogs

The elucidation of the metabolic pathway for vitamin D, including the delineation of the specific cytochrome P450s (CYPs) involved in activation and catabolism, has emphasized the overall importance of metabolic considerations in vitamin D analog design. This short review attempts to summarize recent findings with isolated CYPs and animal models in which CYPs are genetically manipulated to draw attention to structural features of vitamin D analogs that make them more or less resistant to metabolic enzymes. We conclude by placing metabolic considerations in the context of the other important aspects of vitamin D analogs.

Current understanding of the function of the nuclear vitamin D receptor in response to its natural and synthetic ligands

The vitamin D receptor (VDR), the high affinity receptor for 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3), is a member of the nuclear receptor superfamily. VDR preferentially forms a heterodimeric complex with the retinoid X receptor (RXR) and binds to 1alpha,25(OH)2D3 response elements (VDREs) that consist of two hexameric motifs in a directly repeated (DR) or inverted palindromic (IP) arrangement. DNA-complexed VDR acts as a molecular switch of nuclear 1alpha,25(OH)2D3 signaling by transmitting its activation status to different chromatin loci containing the 1alpha,25(OH)2D3 target genes. Approximately 0.5% of the human genome (about 200 genes) are estimated to be primary targets of 1alpha,25(OH)2D3, but via various mechanisms the VDR appears to interfere in the regulation of even more genes. The molecular basis of the regulatory actions of 1alpha,25(OH)2D3 and its synthetic analogs are ligand-triggered protein-protein interactions of the ligand-binding domain (LBD) of the VDR with coactivator (CoA), corepressor (CoR) and other nuclear proteins. Most analogs have been identified as agonists, a few as antagonists (ZK159222 and TEI-9647) and only Gemini and some of its variations as nonagonists. The positioning of helix 12 of the LBD is of critical importance for the agonistic, antagonistic and nonagonistic conformation of the VDR. In each of the three conformations, the VDR performs different protein-protein interactions, which then result in a characteristic functional profile. The functional profile of some 1alpha,25(OH)2D3 analogs, such as EB1089 and Gemini, can be modulated by protein and DNA interaction partners of the VDR. This provides them with some selectivity for DNA-dependent and -independent signaling pathways and VDRE structures.

Characterization of DNA complexes formed by the nuclear receptor constitutive androstane receptor

The nuclear receptor constitutive androstane receptor (CAR) acts as a xenobiotic sensor and regulates the expression of enzymes, such as several cytochromes P450s and the UDP-glucuronosyltransferase (UGT) type 1A1. CAR binds as a heterodimer with the retinoid X receptor (RXR) to specific DNA sites, called response elements (REs). Clusters of CAR REs, referred to as phenobarbital response enhancer modules (PBREMs), have been identified in several CAR target genes. In this study we confirm that REs formed by direct repeats of two AGTTCA hexamers with 4 spacing nucleotides are optimal for the binding of CAR-RXR heterodimers. In addition, we found that the heterodimers also form complexes on everted repeat-type arrangements with 8 spacing nucleotides. We also observed that CAR is able to bind DNA as a monomer and to interact in this form with different coregulators even in the presence of RXR. Systematic variation of the nucleotides 5'-flanking to both AGTTCA hexamers showed that the dinucleotide sequence modulates the DNA complex formation of CAR monomers and CAR-RXR heterodimer by a factor of up to 20. The highest preference was found for the sequence AG and lowest for CC. The increased DNA affinity of CAR is mediated by the positively charged arginines 90 and 91 located in the carboxyl-terminal extension of the DNA-binding domain of the receptor. Furthermore, we show that one of the three CAR REs of the human UGT1A1 PBREM is exclusively bound by CAR monomers and this is regulated by ligands that bind to this nuclear receptor. This points to a physiological role for CAR monomers. Therefore, both CAR-RXR heterodimers and CAR monomers can contribute to the gene activating function of PBREMs in CAR target genes.

Structural evaluation of the agonistic action of a vitamin D analog with two side chains binding to the nuclear vitamin D receptor

The vitamin D receptor (VDR) is one of the endocrine members of the nuclear receptor superfamily and has a characteristic high affinity for its natural ligand 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. From a mechanistic point of view, the most interesting analog of 1alpha,25(OH)2D3 is the one that carries two side chains, referred to as Gemini. In this study, molecular dynamics (MD) simulations of the Gemini-VDR complex were performed that demonstrated that the binding of a ligand with a 25% increased volume does not disturb the overall structure of the ligand-binding domain (LBD). It was found that one of the two side chains takes exactly the same position as the single side chain of the natural ligand, which suggests that the molecular mechanism of the agonism of Gemini is identical to that of 1alpha,25(OH)2D3. VDR single and double point mutants at L227, A303, I313, and L397 and in vitro and ex vivo assessment of their agonistic action confirmed the predictions of the MD simulations. Moreover, it was found that the second side chain of Gemini can choose between two binding positions within the ligand-binding pocket of the VDR. These two newly identified "corners" were characterized most specifically by the amino acids pairs L227/A303 and I313/L397. Therefore, Gemini is an important model compound that allows further insight into the molecular actions of the VDR but is, in parallel, also a promising precursor for the design of even more potent 1alpha,25(OH)2D3 analogs.

Differential nuclear receptor signalling from DR4-type response elements

Nuclear receptors form a large family of highly related transcription factors that transform an incoming signal in the form of a lipophilic hormone into an activation of the basal transcriptional machinery. The specific recognition of nuclear receptor DNA binding sites, referred to as response elements (REs), determines the genes that can be regulated by nuclear hormones. In this study, it was shown that the complexes of the retinoid X receptor (RXR) with either the vitamin D3 receptor (VDR), the thyroid hormone receptor (T3R) or the liver X receptor (LXR) have comparable functionality on a RE of the rat pit-1 gene that is formed by a direct repeat of two hexameric binding motifs spaced by 4 nucleotides (DR4). The sequence of two nucleotides 5'-flanking the downstream binding motif of this DR4-type RE and, interestingly, also those flanking the upstream motif were shown to have in part rather drastic and receptor-specific effects on heterodimer complex formation on DNA. In particular, a downstream substitution into GA reduced the complex formation for LXR specifically, while upstream substitutions into AA or TA increase complex formation for LXR and, to a lesser extent, T3R. The preference of this in vitro complex formation was shown to correlate well with the functional activity of the nuclear receptors in living cells. The results of this study allow (i) a more detailed understanding of known REs, (ii) a more straightforward search for putative REs in newly identified promoter sequences, for example, of the whole human genome, and (iii) a more precise prediction of the hormone responsiveness of the respective genes.

The critical role of carboxy-terminal amino acids in ligand-dependent and -independent transactivation of the constitutive androstane receptor

The mouse constitutive androstane receptor (CAR) is a unique member of the nuclear receptor superfamily, for which an inverse agonist, the testosterone metabolite 5alpha-androstan-3alpha-ol (androstanol), and an agonist, the xenobiotic 1,4-bis[2-(3, 5-dichloropyridyloxy)] benzene, are known. In this study the role of the transactivation domain 2 (AF-2) of CAR was investigated, which is formed by the seven most carboxy-terminal amino acids of the receptor. The AF-2 domain was shown to be critical for the constitutive activity by mediating a ligand-independent interaction of CAR with coactivator (CoA) proteins. In addition this domain increased and decreased contact with CoAs in the presence of agonist and inverse agonist, respectively. In analogy to classical endocrine nuclear receptors, in CAR the charge clamp between K187 (in helix 3) and E355 (within the AF-2 domain) was expected to be critical for its interaction with CoAs. However, the hydrophobic amino acids L352, L353, and I356 on the surface of the AF-2 domain were found to be more important for this protein-protein interaction. Moreover, these amino acids and C357 were shown to be involved in the response of CAR to androstanol. Interestingly, the cysteine at position 357 appears to block classical endocrine responsiveness of CAR to agonists, since mutagenesis of this amino acid both reduced CoA interaction in the absence of ligand and drastically increased inducibility by 1,4-bis[2-(3, 5-dichloropyridyloxy)] benzene. We showed that this blockade is not due to an intramolecular disulfide bridge, but is probably caused by an interaction between C357 and Y336.

Molecular basis of the selective activity of vitamin D analogues

More than 2,000 synthetic analogues of the biological active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)), are presently known. Basically, all of them interfere with the molecular switch of nuclear 1alpha,25(OH)(2)D(3) signaling, which is the complex of the vitamin D receptor (VDR), the retinoid X receptor (RXR), and a 1alpha,25(OH)(2)D(3) response element (VDRE). Central element of this molecular switch is the ligand-binding domain (LBD) of the VDR, which can be stabilized by a 1alpha,25(OH)(2)D(3) analogue either in its agonistic, antagonistic, or non-agonistic conformation. The positioning of helix 12 of the LBD is of most critical importance for these conformations. In each of the three conformations, the VDR performs different protein-protein interactions, which then result in a characteristic functional profile. Most 1alpha,25(OH)(2)D(3) analogues have been identified as agonists, a few are antagonists (e.g., ZK159222 and TEI-9647), and only Gemini and some of its derivatives act under restricted conditions as non-agonists. The functional profile of some 1alpha,25(OH)(2)D(3) analogues, such as EB1089 and Gemini, can be modulated by protein and DNA interaction partners of the VDR. This provides them with some selectivity for DNA-dependent and -independent signaling pathways and VDRE structures.

Regulation of the pdx1 gene promoter in pancreatic beta-cells

In the adult pancreas the expression of the transcription factor PDX1 is mainly restricted to the beta-cells of the islets of Langerhans. In this study we have identified a region of the pdx1 promoter between -2715 and -1960 which was essential to direct pancreatic islet-cell-specific expression of PDX1. We have also begun for the first time to understand the complex nutritional and hormonal regulation controlling PDX1 expression. The current study has established the fact that glucose, GLP-1, insulin, T(3), HB-EGF, and TNF-alpha all positively regulate the PDX1 gene promoter in pancreatic beta-cells. This study represents the first detailed exploration of the nutritional and hormonal regulation of this vital beta-cell gene.

Structurally and functionally important amino acids of the agonistic conformation of the human vitamin D receptor

The crystal structures of the ligand binding domain of human vitamin D receptor (VDR) complexed with its natural ligand or the superagonists MC1288 or KH1060 have recently been reported. The crystallized ligand binding domain (LBD) of VDR, however, differs from the full-length VDR with respect to deletion of 50 amino acids between its helices 2 and 3. In this study, we investigated structurally and functionally important amino acid interactions within the ligand binding pocket of the full-length VDR in the presence of several synthetic vitamin D(3) analogs. We used site-directed mutagenesis scanning combined with limited proteolytic digestion, electrophoretic mobility shift assay, and reporter gene assay and correlated the findings with the crystal structures of truncated VDR LBD. Our results suggest that structurally different agonists have distinct ligand-receptor interactions and that the amino acid residues H229, D232, E269, F279, and Y295 are critical for the agonistic conformation of the VDR. Our biological data, which were obtained with the full-length VDR, fit well with the crystal structure of the truncated VDR LBD and suggest that removal of the insertion domain between helices 2 and 3 of the receptor does not markedly influence the functionality of the VDR.

Expression of CYP3A4, CYP2B6, and CYP2C9 is regulated by the vitamin D receptor pathway in primary human hepatocytes

The fully active dihydroxylated metabolite of vitamin D(3) induces the expression of CYP3A4 and, to a lesser extent, CYP2B6 and CYP2C9 genes in normal differentiated primary human hepatocytes. Electrophoretic mobility shift assays and cotransfection in HepG2 cells using wild-type and mutated oligonucleotides revealed that the vitamin D receptor (VDR) binds and transactivates those xenobiotic-responsive elements (ER6, DR3, and DR4) previously identified in CYP3A4, CYP2B6, and CYP2C9 promoters and shown to be targeted by the pregnane X receptor (PXR) and/or the constitutive androstane receptor (CAR). Full VDR response of various CYP3A4 heterologous/homologous promoter-reporter constructs requires both the proximal ER6 and the distal DR3 motifs, as observed previously with rifampicin-activated PXR. Cotransfection of a CYP3A4 homologous promoter-reporter construct (including distal and proximal PXR-binding motifs) and of PXR or CAR expression vectors in HepG2 cells revealed the ability of these receptors to compete with VDR for transcriptional regulation of CYP3A4. In conclusion, this work suggests that VDR, PXR, and CAR control the basal and inducible expression of several CYP genes through competitive interaction with the same battery of responsive elements.

The 5-lipoxygenase promoter is regulated by DNA methylation

5-lipoxygenase (5-LO), the key enzyme in leukotriene biosynthesis, is expressed in a tissue- and cell differentiation-specific manner. The 5-LO core promoter required for basal promoter activity has a unique (G+C)-rich sequence that contains five tandem Sp1 consensus sequences. The mechanisms involved in the regulation of cell type-specific 5-LO expression are unknown. Here we show that 5-LO expression is regulated by DNA methylation. Treatment of the 5-LO-negative cell lines U937 and HL-60TB with the demethylating agent 5-aza-2'-deoxycytidine (AdC) up-regulated expression of 5-LO primary transcripts and mature mRNA in a similar fashion, indicating that AdC stimulates 5-LO gene transcription. Analysis of the methylation status of the 5-LO promoter revealed that the core promoter region was methylated in U937 and HL-60TB cells, whereas it was unmethylated in the 5-LO-positive parent HL-60 cell line. Reporter gene assays with 5-LO promoter constructs gave up to 68- and 655-fold repression of 5-LO promoter activity in HeLa and Mono Mac 6 cells by methylation. 1,25-dihydroxyvitamin D(3) and transforming growth factor-beta (TGFbeta), potent inducers of the 5-LO pathway in myeloid cell lines, increased 5-LO RNA expression in HL-60TB and U937 cells, but co-treatment with AdC was required to achieve 5-LO expression levels in HL-60TB cells that were comparable with wild-type HL-60 cells. In reporter gene assays, 1,25-dihydroxyvitamin D(3) and TGFbeta were unable to induce promoter activity when the 5-LO promoter constructs were methylated, which suggests that 5-LO promoter demethylation is a prerequisite for the high level induction of 5-LO gene expression by 1,25-dihydroxyvitamin D(3) and TGFbeta and that the effects of both agents on 5-LO mRNA expression are not related to DNA methylation.