Vitamin D resistance

Advances in Vitamin D Receptor Function and Evolution Based on the 3D Structure of the Lamprey Ligand-Binding Domain

1α,25-dihydroxyvitamin D3 (1,25D₃) regulates many physiological processes in vertebrates by binding to the vitamin D receptor (VDR). Phylogenetic analysis indicates that jawless fishes are the most basal vertebrates exhibiting a VDR gene. To elucidate the mechanism driving VDR activation during evolution, we determined the crystal structure of the VDR ligand-binding domain (LBD) complex from the basal vertebratePetromyzon marinus, sea lamprey (lVDR). Comparison of three-dimensional crystal structures of the lVDR-1,25D₃ complex with higher vertebrate VDR-1,25D₃ structures suggests that 1,25D₃ binds to lVDR similarly to human VDR, but with unique features for lVDR around linker regions between H11 and H12 and between H9 and H10. These structural differences may contribute to the marked species differences in transcriptional responses. Furthermore, residue co-evolution analysis of VDR across vertebrates identifies amino acid positions in H9 and the large insertion domain VDR LBD specific as correlated.

Relative Expression of 1,25-Dihydroxyvitamin D3 Receptor, Vitamin D 1α-Hydroxylase, Vitamin D 24-Hydroxylase, and Vitamin D 25-Hydroxylase in Endometriosis and Gynecologic Cancers

The authors demonstrate expression of the vitamin D receptor (VDR) and its hydroxylases in the endometrium and ovaries of women with and without endometriosis and endometrial or ovarian cancer. Immunohistochemistry showed strong staining of the VDR in endometriosis and endometrial cancer, with the most intense staining in epithelial cells. The VDR mRNA was significantly increased in patients with endometrial and ovarian cancer compared to the control group. There was a significantly higher 1 alpha-hydroxylase expression in the endometrium of patients with endometriosis compared to healthy controls. The observed differences in VDR and 1 alpha -hydroxylase mRNA levels were maintained at the protein level. The authors found no differences in 25-OH vitamin D levels between the serum of patients with endometriosis (25.7 +/- 2.1 ng/mL, n = 46) and healthy controls (22.6 +/- 2.0 ng/mL, n = 33, P = .31). They hypothesize that vitamin D might influence the local activity of immune cells and cytokines thought to play important pathogenic roles in the development and maintenance of endometriosis.

Role of renal TRP channels in physiology and pathology

Kidneys critically contribute to the maintenance of whole-body homeostasis by governing water and electrolyte balance, controlling extracellular fluid volume, plasma osmolality, and blood pressure. Renal function is regulated by numerous systemic endocrine and local mechanical stimuli. Kidneys possess a complex network of membrane receptors, transporters, and ion channels which allows responding to this wide array of signaling inputs in an integrative manner. Transient receptor potential (TRP) channel family members with diverse modes of activation, varied permeation properties, and capability to integrate multiple downstream signals are pivotal molecular determinants of renal function all along the nephron. This review summarizes experimental data on the role of TRP channels in a healthy mammalian kidney and discusses their involvement in renal pathologies.

Identification of a novel nonsense mutation in the ligand-binding domain of the vitamin d receptor gene and clinical description of two greek patients with hereditary vitamin d-resistant rickets and alopecia

BACKGROUND/AIMS

We analyzed the vitamin D receptor (VDR) gene in 2 Greek patients who exhibited the classical features of hereditary vitamin D-resistant rickets (HVDRR) type II, including severe bone deformities and alopecia. We also describe the clinical phenotypes and the response to treatment of our patients.

METHODS

Genomic DNA was extracted from peripheral blood samples of both patients. Coding region and flanking introns of VDR gDNA was amplified and direct sequenced.

RESULTS

A unique cytosine to thymine (C>T) transition was identified at nucleotide position 1066 (c.1066C>T) in the ligand-binding domain of the VDR gene of both patients, predicting the substitution of a glutamine to a terminal codon at position 356 (Gln356stop).

CONCLUSIONS

The novel nonsense mutation c.1066C>T (Gln356stop) is expected to result in a VDR protein 71 amino acids shorter and thus to affect the normal VDR function. In particular, the missing protein part alters the VDR heterodimerization with the retinoid X receptor which has been correlated with the presence of alopecia. Both patients were introduced to treatment with supraphysiological doses of 1α-calcidiol which improved their clinical phenotypes except for alopecia.

Vitamin D Metabolites; Protective versus Toxic Properties: Molecular and Cellular Perspectives

Vitamin D plays an essential role in bone metabolism. The discovery that the vitamin D receptor (VDR), a member of the nuclear receptor superfamily, is expressed in most tissues led researchers to investigate other biological actions of vitamin D. These effects were found to include anti-inflammatory effects and anti-atherogenesis, decreased renin activity and biosynthesis, induction of cell differentiation, inhibition of cell growth, and immunomodulation. In spite of the plethora of evidence on the protective effects of vitamin D, the reports on its intoxication still are considerably few. Therefore, in this review we aim to summarize the molecular and cellular bases of the protect-ive and toxic vitamin D actions that are mediated mostly by VDR. This review will also shed light on vitamin D metabolites other than the active metabolite calcitriol and particularly 25-hydroxy vitamin D (25(OH)D), putting emphasis on its magnifying role in vitamin D intoxication. One of the important themes we discuss is defining serum levels of beneficial or toxic effects of other exogenous vitamin D administration and its impact on 25(OH)D serum levels in animals and human subjects.

Topical vitamin D₃ analogues induce thymic stromal lymphopoietin and cathelicidin in psoriatic skin lesions

BACKGROUND

Psoriasis is a chronic inflammatory skin disease of unknown aetiology, and an active form of vitamin D(3) (1α,25-dihydroxyvitamin D(3)) and its analogues (VD3As) are widely used topical reagents for psoriasis treatment. Besides their well-known calcium homeostasis functions, VD3As have been shown to have various immune-modulating effects including the induction of thymic stromal lymphopoietin (TSLP), a master cytokine for inducing Th2 inflammation, in mouse models, but not yet in human psoriasis. VD3As also have been shown to induce cathelicidin, an antimicrobial peptide and strong inducer of innate immunity. Cathelicidin is overexpressed in psoriatic skin lesions; however, its role in this disease seems as yet inconclusive.

OBJECTIVES

To clarify whether topical VD3As induce TSLP and cathelicidin, and to examine the modulation of expression patterns of related cytokines in human psoriatic lesions.

METHODS

Skin biopsy samples from psoriatic lesions with or without VD3A treatment were subjected to immunohistochemical staining and quantitative reverse transcription-polymerase chain reaction analyses to measure the expression levels of various cytokines.

RESULTS

Significantly higher levels of TSLP, thymus and activation-related chemokine and CCR4 expression were observed in VD3A+ skin samples than in VD3A- samples. In contrast, significantly lower levels of interleukin (IL)-12/23 p40, IL-1α, IL-1β and tumour necrosis factor (TNF)-α expression were observed in the VD3A+ samples than in the VD3A- samples. Expression of cathelicidin was elevated in VD3A+ samples.

CONCLUSIONS

Topical VD3As induce TSLP and cathelicidin in psoriatic lesions, resulting in suppression of IL-12/23 p40, IL-1α, IL-1β and TNF-α, thereby ameliorating psoriatic plaques.

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.

Decreased 1-25 dihydroxyvitamin D3 concentration in peripheral blood serum of patients with thyroid cancer

BACKGROUND AND AIMS

Vitamin D(3), in addition to its role in calcium homeostasis, has been recognized as playing a role in human cancer development. However, little is known about the association between vitamin D status and the development of thyroid cancer. This study aimed to investigate vitamin D metabolism by measuring 25(OH) D(3), 1-25 (OH)(2) D(3), PTH and calcium concentrations in the peripheral blood of patients with different forms of thyroid tumors.

METHODS

The 25-hydroxyvitamin D(3) ,1-25- dihydoxyvitamin D(3), PTH and calcium serum levels of 50 consecutive patients with epithelial thyroid cancer 27 cases of papillary cancers (PTC), 16 follicular cancers (FTC), and seven cases of anaplastic cancers (ATC) and 34 multinodular nontoxic goiter (MNG) were measured by specific immunoassay. The control group consisted of 26 healthy volunteers.

RESULTS

Our results revealed significantly lower 1-25 (OH)(2) D(3) concentration in the PTC group (22.67 pg/mL +/- 8.12; p <0.05), FTC group (16.09 pg/mL +/- 6.15; p <0.02) and ATC group (9.48 pg/mL +/- 5.18; p <0.02). Levels of 1-25 (OH)(2) D(3) varied by cancer stage and were also significantly different. A significant decrease in circulating 1-25 (OH)(2) D(3) concentration was found in patients with stage I (24.12 pg/mL +/- 6.77; p <0.05), stage II (16.93 pg/mL +/- 4.55; p <0.05), stage III (12.44 +/- 8.98; p <0.02) and in stage IVa (6.18 +/- 2.22; p <0.01). There were no significant differences when comparing serum levels of 25(OH) D(3), PTH or calcium concentrations among individuals with multinodular goiter, thyroid cancer and age- and sex-matched control volunteers.

CONCLUSIONS

Our study revealed that impaired vitamin D(3) metabolism may play an important role in thyroid follicular cell oncogenesis.

Innate immunity to mycobacteria: vitamin D and autophagy

Autophagy is an ancient mechanism of protein degradation and a novel antimicrobial strategy. With respect to host defences against mycobacteria, autophagy plays a crucial role in antimycobacterial resistance, and contributes to immune surveillance of intracellular pathogens and vaccine efficacy. Vitamin D3 contributes to host immune responses against Mycobacterium tuberculosis through LL-37/hCAP-18, which is the only cathelicidin identified to date in humans. In this review, we discuss recent advances in our understanding of host immune strategies against mycobacteria, including vitamin D-mediated innate immunity and autophagy activation. This review also addresses our current understanding regarding the autophagy connection to principal innate machinery, such as ubiquitin- or inflammasome-involved pathways. Integrated dialog between autophagy and innate immunity may contribute to adequate host immune defences against mycobacterial infection.

The role of transient receptor potential channels in kidney disease

The transient receptor potential (TRP) superfamily consists, in mammals, of six protein subfamilies, TRPC, TRPM, TRPV, TRPA, TRPML and TRPP. TRPs are cation channels involved in many physiological processes and in the pathogenesis of various disorders. In the kidney, TRP channels are expressed along the nephron, and a role for some of these channels in renal function has been proposed. TRPC3 is thought to facilitate calcium ion influx into the principal cells of the collecting duct in response to vasopressin. TRPM3 and TRPV4 might be osmosensors, whereas the TRPP1/TRPP2 complex could function as a mechanosensor in the cilia of renal epithelial cells. A number of kidney diseases have also been linked to dysfunctional activity of TRPs. TRPC6 dysfunction has been associated with the onset of focal segmental glomerosclerosis; TRPP2 dysfunction is linked to autosomal-dominant polycystic kidney disease, TRPM6 mutations underlie hypomagnesemia with secondary hypocalcemia, and TRPV1 dysfunction is implicated in renal hypertension. A link between TRPC1 dysfunction and diabetic nephropathy has also been suggested in an animal model. Animal studies have implicated a role for TRPV5 in idiopathic hypercalciuria and vitamin D-dependent rickets, although these observations have not been confirmed in patients. This Review focuses on the role of renal TRP channels in health and disease.

A girl with a novel splice site mutation in VDR supports the role of a ligand-independent VDR function on hair cycling

Mutations in vitamin D receptor (VDR) cause hereditary vitamin D resistant rickets (HVDRR). We reported a Thai girl with HVDRR, presenting with an early onset of rickets and partial alopecia. She was a product of a consanguineous couple. Mutation analysis showed that she was homozygous for a novel splice site mutation of the VDR gene, 462 + 1 G --> C, resulting in incorporation of the whole 254 bp of the intron 4 into its mRNA. The mutated protein is expected to contain no ligand-binding domain. The fact that she did not develop total alopecia despite of no VDR ligand-binding domain supports that VDR function on hair cycling is ligand independent.

Effect of vitamin D3 on phagocytic potential of macrophages with live Mycobacterium tuberculosis and lymphoproliferative response in pulmonary tuberculosis

Immune responses are elicited through antigen presentation and recognition by macrophages and T-lymphocytes, respectively. The immunomodulatory effect of vitamin D(3) on macrophage phagocytic potential with live Mycobacterium tuberculosis, spontaneous and M. tuberculosis culture filtrate antigen induced lymphocyte responses were studied in pulmonary tuberculosis patients (PTBPs) ( n = 31) and normal healthy subjects (NHSs) ( n = 43). Vitamin D(3) at a concentration of 10(-7) M significantly enhanced the macrophage phagocytosis of live M. tuberculosis in normal subjects with low phagocytic potential (less than 10%) ( p = 0.015). No such increase was observed in PTBPs. Vitamin D(3) significantly decreased the spontaneous lymphoproliferative response ( p = 0.022) and increased the apoptosis of peripheral blood mononuclear cells in PTBPs ( p = 0.024). In normals, vitamin D(3) increased the spontaneous lymphoproliferative response. An inverse correlation between macrophage phagocytosis and spontaneous response was observed in NHSs, whereas a direct correlation was seen between vitamin D(3)-treated cells in normal subjects under in vitro condition. Vitamin D(3) decreased the M. tuberculosis culture filtrate antigen induced lymphocyte response significantly in normal subjects ( p = 0.0003), while it had no influence on the lymphocyte response in PTBPs. The present study suggests that exposure to vitamin D(3) increases the phagocytic potential and spontaneous lymphoproliferative response but brings down the antigen-induced response in normals. In tuberculosis, addition of vitamin D(3) has no significant effect on antigen-induced lymphoproliferative response. This may be due to the unresponsive nature of the cells to the action of vitamin D(3) by virtue of the disease, which renders them inactive.

Identification of a Vitamin D response element in the human insulin receptor gene promoter

The present study was designed to explore the possible presence and location of Vitamin D response elements (VDREs) in the human insulin receptor (hIR) gene promoter. To this end, the -1819 to -271 bp fragment of the hIR promoter (wild type promoter) and progressive 5' deletions of this promoter (up to -1473 and -876 bp) were linked to the luciferase pGL2-basic vector to construct the reported plasmids: phIR (-1819)-GL2, phIR(-1473)-GL2 and phIR(-876)-GL2, respectively. U-937 cells were transiently transfected with these plasmids, and then the cells were either untreated or treated for 24h with 10(-8) M 1,25-dihydroxyvitamin D(3) (1,25D(3)). Luciferase determinations revealed that, while the activity of the wild promoter was increased 1.6-fold by the hormone, the activities of progressive 5' deletions of this promoter were enhanced 1.7-, and 1.6-fold, respectively. Thus, the region extending from -876 to -271bp of the hIR promoter, appears to contain VDREs, and to be sufficient for induction by 1,25D(3). In order to identify these potential VDREs, we performed a computer search of candidate sequences by homology with a consensus VDRE sequence. This search yielded a sequence located between -761 and -732 bp (5'CGTCGGGCCTGTGGGGCGCCTCCGGGGGTC3'), which includes an overlapping AP-2 like sequence, as a good candidate. Electrophoretic mobility shift assays revealed that the Vitamin D receptor (VDR) specifically recognized this sequence, since a VDR-DNA complex was able to compete with the unlabeled probe and was cleared by the specific anti-VDR antibody 9A7. These data represent the first identification of a VDRE in the hIR gene promoter.

Transcriptional activation of the human insulin receptor gene by 1,25-dihydroxyvitamin D(3)

Treatment with 10(-8) M 1,25-dihydroxyvitamin D(3) for 24 h causes transcriptional activation of the human insulin receptor gene in U-937 human promonocytic cells. The activation seems to potentiate the response to insulin in terms of glucose oxidation. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, causes a greater inhibition of insulin-stimulated glucose oxidation in 1,25-dihydroxyvitamin D(3)-treated cells than in untreated cells. This suggests a stimulation of phosphatidylinositol 3-kinase activity by 1,25-dihydroxyvitamin D(3), which could mediate, at least in part, the potentiation of the insulin response.

1alpha,25-Dihydroxyvitamin D3-induced monocyte antimycobacterial activity is regulated by phosphatidylinositol 3-kinase and mediated by the NADPH-dependent phagocyte oxidase

We investigated the basis for the induction of monocyte antimycobacterial activity by 1alpha,25-dihydroxyvitamin D(3) (D(3)). As expected, incubation of Mycobacterium tuberculosis-infected THP-1 cells or human peripheral blood, monocyte-derived macrophages with hormone resulted in the induction of antimycobacterial activity. This effect was significantly abrogated by pretreatment of cells with either of the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, wortmannin or LY294002, or with antisense oligonucleotides to the p110 subunit of PI 3-Kalpha. Cells infected with M. tuberculosis alone or incubated with D(3) alone produced little or undetectable amounts of superoxide anion (O(2)). In contrast, exposure of M. tuberculosis-infected cells to D(3) led to significant production of O(2), and this response was eliminated by either wortmannin, LY294002, or p110 antisense oligonucleotides. As was observed for PI 3-K inactivation, the reactive oxygen intermediate scavenger, 4-hydroxy-TEMPO, and degradative enzymes, polyethylene glycol coupled to either superoxide dismutase or catalase, also abrogated D(3)-induced antimycobacterial activity. Superoxide production by THP-1 cells in response to D(3) required prior infection with live M. tuberculosis, since exposure of cells to either killed M. tuberculosis or latex beads did not prime for an oxidative burst in response to subsequent hormone treatment. Consistent with these findings, redistribution of the cytosolic oxidase components p47(phox) and p67(phox) to the membrane fraction was observed in cells incubated with live M. tuberculosis and D(3) but not in response to combined treatment with heat-killed M. tuberculosis followed by D(3). Redistribution of p47(phox) and p67(phox) to the membrane fraction in response to live M. tuberculosis and D(3) was also abrogated under conditions where PI 3-K was inactivated. Taken together, these results indicate that D(3)-induced, human monocyte antimycobacterial activity is regulated by PI 3-K and mediated by the NADPH-dependent phagocyte oxidase.

Ligand affinity, homodimerization, and ligand-induced secondary structural change of the human vitamin d receptor

The intrinsic tryptophan fluorescence signal of the full-length nuclear receptor hVDR was used to directly determine the dissociation constants, K(d), of two ligands yielding K(d) = 32 nM for 1alpha,25(OH)(2)D(3) and K(d) = 322 nM for 25(OH)D(3). Ligand binding was accompanied by a conformational change in the alpha-helical part of hVDR as revealed by CD spectroscopy. In addition, the presence of calcitriol was found to be a necessary prerequisite for the homodimerisation of hVDR which was monitored using fluorescence anisotropy. We conclude that the observed ligand-induced structural change of hVDR is conditional for dimerisation of the protein.

The vitamin D response element-binding protein. A novel dominant-negative regulator of vitamin D-directed transactivation

Vitamin D resistance in certain primate genera is associated with the constitutive overexpression of a non-vitamin D receptor (VDR)-related, vitamin D response element-binding protein (VDRE-BP) and squelching of vitamin d-directed transactivation. We used DNA affinity chromatography to purify proteins associated with non-VDR-VDRE binding activity from vitamin d-resistant New World primate cells. In electrophoretic mobility shift assays, these proteins bound specifically to either single-strand or double-strand oligonucleotides harboring the VDRE. Amino acid sequencing of tryptic peptides from a 34-kDa (VDRE-BP1) and 38-kDa species (VDRE-BP-2) possessed sequence homology with human heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and hnRNPA2, respectively. cDNAs bearing the open reading frame for both VDRE-BPs were cloned and used to transfect wild-type, hormone-responsive primate cells. Transient and stable overexpression of the VDRE-BP2 cDNA, but not the VDRE-BP1 cDNA, in wild-type cells with a VDRE-luciferase reporter resulted in significant reduction in reporter activity. These data suggest that the hnRNPA2-related VDRE-BP2 is a dominant-negative regulator of vitamin D action.