Polymorphisms in the vitamin D receptor gene and type 1 diabetes mellitus risk: an update by meta-analysis

Four well known polymorphisms (BsmI, FokI, ApaI, TaqI) in the VDR gene have been implicated in susceptibility to type 1 diabetes mellitus (T1DM), but the results to date have been inconclusive. The aim of this study was to investigate the association between polymorphisms in the VDR gene and T1DM risk by meta-analysis. A total of 57 case-control studies in 26 published studies were included. The results indicated that the BsmI polymorphism is associated with increased risk of T1DM (BB+Bb vs. bb: OR=1.30, 95% CI=1.03-1.63), while the FokI, ApaI and TaqI polymorphisms were not. In the subgroup analysis by ethnicity, the increased risk of T1DM remained in the Asian subgroup for the BsmI polymorphism; whereas no significant association was found in other populations for other polymorphisms. Results from the current study suggest that the BsmI polymorphism is associated with increased risk of T1DM, especially in Asians. Further studies are needed to confirm our results.     

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

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

Regulation of the murine renal vitamin D receptor by 1,25-dihydroxyvitamin D3 and calcium

Renal vitamin D receptor (VDR) is required for 1,25-dihydroxyvitamin D3-[1,25(OH)2D3]-induced renal reabsorption of calcium and for 1,25(OH)2D3-induced 1,25(OH)2D3 24-hydroxylase. The long-term effect of vitamin D and dietary calcium on the expression of renal VDR was examined in the nonobese diabetic mouse. Vitamin D-deficient and vitamin D-replete mice were maintained on diets containing 0.02%, 0.25%, 0.47%, and 1.20% calcium with or without 50 ng of 1,25(OH)2D3 per day. Vitamin D-replete mice on a 1.20% calcium diet had renal VDR levels of 165 fmol/mg protein. Calcium restriction caused renal VDR levels to decrease to <30 fmol/mg protein in vitamin D-deficient mice and to approximately 80 fmol/mg protein in vitamin D-replete mice. When dietary calcium was present, 50 ng of 1,25(OH)2D3 elevated the VDR levels 2- to 10-fold, depending on vitamin D status and the level of calcium. In the absence of either vitamin D or calcium, the VDR mRNA was expressed at a basal level. 1,25(OH)2D3 supplementation caused relative VDR mRNA to increase 8- to 10-fold in the vitamin D-deficient mouse when dietary calcium was available. This increase was completely absent in the calcium-restricted mice. This in vivo study demonstrates that 1,25(OH)2D3 and calcium are both required for renal VDR mRNA expression above a basal level, furthering our understanding of the complex regulation of renal VDR by 1,25(OH)2D3 and calcium.     

Regulation of gonadotropin receptor gene expression

The receptors for the gonadotropins differ from the other G protein-coupled receptors by having a large extracellular hormone-binding domain, encoded by nine or ten exons. Alternative splicing of the large pre-mRNA of approximately 100 kb can result in mRNA species that encode truncated receptor proteins. In this review we discuss the regulation of gonadotropin receptor mRNA expression and the possible roles of alternative splicing in gonadotropin receptor function.     

Targeting expression of the human vitamin D receptor to the keratinocytes of vitamin D receptor null mice prevents alopecia.

Vitamin D receptor (VDR) null mice develop hypocalcemia, hyperparathyroidism, rickets, osteomalacia and alopecia. Normalization of mineral ion homeostasis prevents all of these abnormalities except alopecia. Hair reconstitution assays, performed in athymic nude mice, demonstrate that the lack of VDR in keratinocytes leads to a defect in anagen initiation, similar to that observed in VDR null mice. Although these studies demonstrate that expression of the VDR in keratinocytes is necessary, they do not prove that it is sufficient for maintenance of the normal hair cycle. To address this hypothesis, we generated transgenic mice expressing the human VDR under the control of the keratin 14 (K14) promoter. Two highly expressing transgenic lines were mated with VDR null mice to obtain VDR null mice expressing the human VDR transgene (hVDR+/mVDR-). Expression of the transgene in the VDR null mice prevented alopecia. Furthermore, when subjected to anagen initiation, the hair follicle keratinocytes of the hVDR+/mVDR- mice demonstrated an enhanced proliferative response compared to those of control littermates. Restoration of VDR expression in the keratinocytes of VDR null mice, prevents the hair cycle defect that leads to the development of alopecia.     

Effect of glucocorticoids and 1,25-dihydroxyvitamin D3 on the developmental expression of the rat intestinal vitamin D receptor gene.

In this study the ontogenesis of rat intestinal vitamin D receptor (VDR) gene expression was examined. When Northern and slot blot analyses were used to examine the expression of intestinal VDR mRNA in 15-, 18-, 22-, and 28-day-old rats, induction of VDR mRNA was not observed until 22 days postpartum. Since little is known, particularly in the neonate, concerning the in vivo regulation of VDR gene expression, we examined the possibility that glucocorticoids and/or 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] could affect the developmental expression of the intestinal VDR gene. To examine the effect of glucocorticoids, rat pups received three sequential injections (one per day) of hydrocortisone (5, 2.5, and 2.5 mg/100 g BW). Hydrocortisone administration before day 14 or on days 19-21 was not effective in inducing VDR mRNA. However, a significant 3.8-fold increase in intestinal VDR mRNA was observed in rats injected with hydrocortisone from days 15-17. The hydrocortisone effective period coincides with the glucocorticoid-sensitive period of rat intestinal development. It should be noted, however, that the up-regulation of VDR was accompanied by an increase in actin mRNA, suggesting that the effect is not specific for VDR. Similarly, when rats were bilaterally adrenalectomized on day 17 (killed on day 22), a 4-fold decrease in VDR mRNA was observed, accompanied by a decrease in actin mRNA. However, when rats were injected with 1,25-(OH)2D3 (25 ng/day.100 g BW) from days 15-17, levels of intestinal VDR mRNA were significantly increased by 1.5-fold, and this change was specific for VDR mRNA. In summary, our results indicate that hydrocortisone and 1,25-(OH)2D3 can precociously induce intestinal VDR mRNA, suggesting the involvement of glucocorticoids and 1,25-(OH)2D3 in the regulation of VDR gene expression in the developing rat intestine. However, our results also indicate that the effect of glucocorticoids (unlike the effect of 1,25-(OH)2D3) is not specific for VDR mRNA, but may reflect general effects of glucocorticoids on intestinal maturation.     

Regulation of vitamin D biosynthesis by calcium

Vitamin D is a secosteroid that is made in the skin by the action of sunlight. Vitamin D is biologically insert and must undergo two successive hydroxylations in the liver and kidney to become the biologically active vitamin D [1alpha,25 (OH)(2)D]. Recently it was appreciated that 1alpha,25 (OH)(2)D not only regulates calcium but also is capable of inhibiting the proliferation and inducing terminal differentiation of variety of cells. The renal production of 1alpha,25 (OH)(2)D is tightly regulated by serum calcium levels through the action of parathyroid hormone.     

DNA target selectivity by the vitamin D3 receptor: mechanism of dimer binding to an asymmetric repeat element.

The 1,25-dihydroxyvitamin D3 receptor, like other members of the nuclear receptor superfamily, forms dimers in solution that are probably stabilized by a dyad symmetrical interface formed by the ligand-binding domain. This receptor, however, recognizes DNA targets that are not dyad symmetric but rather are organized as direct repeats of a hexameric sequence with a characteristic 3-bp spacing. Using molecular modeling and site-directed mutagenesis, we have identified regions within the vitamin D3 receptor zinc finger region that confer selectivity for direct repeats with appropriate spacing. Reflecting the organization of the DNA target, these regions, mapping to the tip of the first zinc finger module and the N and C termini of the second finger module, direct asymmetrical protein-protein contacts. A stereochemical model is proposed for these interactions.     

Association of vitamin D receptor gene polymorphisms with insulin resistance and response to vitamin D

The objectives of the study were to determine associations between single nucleotide polymorphisms (SNPs) of the vitamin D receptor (VDR) gene and insulin resistance and the effects of these SNPs on changes in insulin sensitivity in response to vitamin D supplementation. The research described here was an extension of the Surya study. Genotyping of the Cdx-2, FokI, BsmI, ApaI, and TaqI SNPs was carried out on 239 South Asian women in New Zealand using polymerase chain reaction-based techniques. Associations of these genotypes and 3' end haplotypes with insulin resistance were determined using multiple regression analysis. Associations between SNP genotypes and responses in insulin sensitivity to vitamin D supplementation (4000 IU vitamin D(3) per day) were also determined for a subset (81) of these women. BsmI BB, ApaI AA, and TaqI tt genotypes were significantly associated with lower insulin resistance compared with BsmI bb, ApaI aa, and TaqI TT, respectively, in the cohort of 239 women. Furthermore, homozygosity of the haplotypes baT and BAt was associated with higher and lower insulin resistance, respectively, compared with no copies of their respective alleles. Of the 81 subjects who were supplemented with vitamin D, women with the FokI Ff genotype showed a significantly greater improvement in insulin sensitivity (increase of 29.4 [2.9, 38.1]) compared with women with the FokI FF genotype (increase of 2.3 [-11.5, 10.1]). This study has highlighted the association of vitamin D responsiveness and insulin resistance with VDR gene polymorphisms. This is the first study to determine associations between all three. Genotyping of the VDR gene may provide a predictive measure for insulin resistance in response to vitamin D intervention.     

Bufalin alleviates adriamycin- induced podocyte injury by up-regulating the expression of vitamin D receptor

<正>Objective To investigate the role of vitamin D receptor(VDB)in the protection of bufalin on podocyte injury induced by adriamycin(ADR).Methods(1)In vitro:the toxic effect of different concentrations of bufalin(10~(-9),10~(-8),10~(-7),10~(-6)mol/L)on podocyte was evaluated by lactate dehydrogenase(LDH)test;Annexin V-FITC and RT-PCR were utilized for podocyte apoptosis     

Human bladder as a novel target for vitamin D receptor ligands

Human prostate is now considered a target for vitamin D receptor (VDR) ligands, such as BXL-628. Because BXL-628 inhibited prostate growth without interfering with androgen signaling, it represents a new option for benign prostate hyperplasia (BPH) therapy. However, BPH symptoms are related not only to prostate size, but also to compensatory bladder hypertrophy and eventual overactivity. We now report that human bladder expresses VDR (determined by real-time PCR immunohistochemistry and Western blot) and responds to VDR agonists, such as the natural ligand, calcitriol, and its synthetic and less hypercalcemic derivative, BXL-628. Experiments were conducted with stromal cells derived from human bladder neck obtained at surgery from BPH patients. BXL-628 counteracted keratinocyte growth factor (KGF) and androgen-induced cell proliferation and stimulated apoptosis with a parallel reduced expression of the survival oncoprotein Bcl-2. Prolonged serum starvation time-dependently pushed bladder stromal cells to express activated myofibroblast markers, such as desmin and smoothelin, without changing other contractile-related proteins and intermediate filaments, such as vimentin. Chronic exposure to BXL-628 prevented starvation-induced cell phenotype modification. Because hypertrophy and starvation-induced bladder remodeling are supposed to underlie bladder overactivity, it is possible that BXL-628 might be helpful in reducing not only cumbersome symptoms related to prostate overgrowth, but also those related to bladder irritation.     

An update on vitamin D and human immunity

In the last 5 years, there has been a remarkable change in our understanding of the health benefits of vitamin D. The classical actions of vitamin D as a determinant of mineral metabolism and rachitic bone disease have been expanded to include a broader role in skeletal homoeostasis and prevalent bone disorders such as osteoporosis. However, it is the nonskeletal function of vitamin D that has attracted most attention. Although pluripotent responses to vitamin D have been recognized for many years, our new perspective on nonclassical vitamin D function stems from two more recent concepts. The first is that impaired, vitamin D status is common to many populations across the globe. This has prompted studies to explore the health impact of suboptimal circulating levels of vitamin D, with association studies linking vitamin D 'insufficiency' to several chronic health problems including autoimmune and cardiovascular disease, hypertension and common cancers. In support of a broader role for vitamin D in human health, studies in vitro and using animal models have highlighted immunomodulatory and anticancer effects of vitamin D that appear to depend on localized activation of vitamin D. The conclusion from these reports is that many nonclassical actions of vitamin D are independent of conventional vitamin D endocrinology and are therefore more sensitive to variations in vitamin D status. The current review summarizes these developments, with specific reference to the newly identified effects of vitamin D on the immune system, but also highlights the challenges in translating these observations to clinical practice.     

Regulation of vitamin D metabolism

Fundamental to understanding the way in which perturbations in the vitamin D endocrine system can affect human health is an appreciation of the steps involved in the production of the well-recognized active hormonal form, 1,25-dihydroxyvitamin D(3). Thus this paper focuses first on the nature and regulation of the two enzymes responsible for the production of 1,25-dihydroxyvitamin D(3), the 25-hydroxylase in the liver and the 1α-hydroxylase in the kidney. The most important regulators of the 1α-hydroxylase in the kidney are 1,25-dihydroxyvitamin D(3) itself, parathyroid hormone and FGF23. The extent and importance of extra-renal, 1,25-dihydroxyvitamin D(3) synthesis is then considered. Finally the features of the 24R-hydroxylase, which produces 24R,25-dihydroxyvitamin D(3) in the kidney and is induced by and inactivated, 1,25-dihydroxyvitamin D(3)in target cells are described.     

维生素D与糖尿病关系的研究进展

维生素D（VitD）与糖尿病的发生发展密切相关,VitD缺乏或不足人群的糖尿病患病率明显高于普通人群。低VitD水平通过损害自身免疫,抑制胰岛素的合成及分泌,促进慢性炎症反应,加重胰岛素抵抗等机制而影响糖尿病的发生发展过程。适当补充VitD有助于更好地控制血糖。     

Fok-I gene polymorphism of vitamin D receptor in patients with beta-thalassemia major and its effect on vitamin D status

Most of the biological actions of vitamin D are mediated by an intracellular receptor (VDR) in which several single nucleotide gene polymorphisms have been identified. Vitamin D deficiency is increasingly identified among thalassemic patients and recent evidence links it with myocardial iron accumulation. The aim of this work was to assess the distribution of the Fok-I polymorphism of the VDR gene among Greek children and young adults with beta-thalassemia major and to investigate its association with 25(OH)D(3) and 1,25(OH)(2)D(3) serum levels. Sixty-nine thalassemic patients (35 females and 34 males), with a mean age of 23·05±6·07?years, participated in the study. Genotype frequencies of Fok-I were similar to those previously reported for other populations; 44·9% of the patients were homozygotes for F allele, 43·5% were heterozygotes and 11·6% were homozygotes for the f allele. Low levels of serum 25(OH)D(3) were recorded, as 41 patients (59·4%) were below the cut-off limit of 50?nmol/l that determines deficiency, whereas, levels of 1,25(OH)(2)D(3) showed wide variability ranging from deficiency (≤50 pmol/l) in 34 patients (49·3%) to excess (≥125 pmol/l) in 13 patients (18·8%). When stratifying patients according to serum 1,25(OH)(2) D(3) concentrations, a higher prevalence of the f allele was observed in the deficiency group (P = 0·03). A comparison of the serum concentrations of the two vitamin D metabolites produced a trend towards a negative correlation (r = -0·204, P = 0·09). Further studies are required to assess the genetic contribution to the regulation of vitamin D metabolites in the serum of patients with beta-thalassemia major.     

Podocytes as target of vitamin D

Vitamin D deficiency is a prominent feature of chronic kidney disease (CKD) even in its early stages. While vitamin D deficiency leads to mineral imbalance and bone problems in CDK patients, it also accelerates the progression of kidney disease. Ever since the observation that vitamin D analogs reduce proteinuria in CKD patients, it has been postulated that podocytes are major target of the reno-protective action of vitamin D. Recent large randomized clinical trials have confirmed the potent anti-proteinuric activity of vitamin D therapy. Studies from various animal models of kidney disease have demonstrated that vitamin D prevents podocyte injury and cell loss, promotes the expression of slit diaphragm proteins and maintains the integrity of the glomerular filtration barrier. Emerging experimental data suggest that vitamin D may protect podocytes by targeting multiple pathways, including the renin-angiotensin system, Wnt/β-catenin pathway and pro-apoptotic pathway.