25-Hydroxyvitamin D3 1alpha-hydroxylase and vitamin D synthesis

The Role of 25-Hydroxy Vitamin D Deficiency in Iron Deficient Children of North India

Extensive data from animal and human studies indicate a role of vitamin D in erythropoiesis. Iron and vitamin D deficiencies are implicated with adverse health effects in children even if they are asymptomatic. The potential relationship between the two remains poorly understood. A cross-sectional study was performed in the period from 1st May 2012 through 30th April 2013 and subjects were classified into vitamin D deficiency (VDD), vitamin D insufficiency (VDI) and vitamin D sufficiency (VDS) groups according to their 25(OH) D levels. A total of 263 children were included in the analysis. Anaemia was present in 66 % of 25(OH) D deficient subjects compared with 35 % in vitamin D sufficient individuals (p < 0.0001). The association of breast feeding and development of VDD was also significant (p < 0.05). Serum levels of 25(OH) D were found lower in female sex and if the analysis was performed in the winter/spring season. Physicians should therefore assess vitamin D levels in all anaemic children and ensure adequate supplementation to prevent deficiencies.

Mechanistic Effects of Calcitriol in Cancer Biology

Besides its classical biological effects on calcium and phosphorus homeostasis, calcitriol, the active vitamin D metabolite, has a broad variety of actions including anticancer effects that are mediated either transcriptionally and/or via non-genomic pathways. In the context of cancer, calcitriol regulates the cell cycle, induces apoptosis, promotes cell differentiation and acts as anti-inflammatory factor within the tumor microenvironment. In this review, we address the different mechanisms of action involved in the antineoplastic effects of calcitriol.

Is there more to learn about functional vitamin D metabolism?

The state of information on the enzymes responsible for the conversion of vitamin D3 to 1α,25-dhydroxyvitamin D3 (1,25-(OH)2D3), the metabolic active form responsible for the well-known function of vitamin D on calcium metabolism and bone mineralization has been briefly reviewed. There remains an unidentified enzyme responsible for 25% of the 25-hydroxylation of vitamin D3, while 75% of serum 25-hydroxyvitamin D3 (25-OH-D3) arises from CYP2R1. The well-established suppression of multiple sclerosis (MS) by sunlight has been confirmed using the mouse model, experimental autoimmune encephalomyelitis (EAE). This suppression results from a narrow band of ultraviolet light (300-315nm) that does not increase serum 25-OH-D3. Thus, UV light suppresses EAE by a mechanism not involving vitamin D. Vitamin D deficiency unexpectedly suppresses the development of EAE. Further, vitamin D receptor knockout in susceptible mice also prevents the development of EAE. On the other hand, deletion of CYP2R1 and the 1α-hydroxylase, CYP27B1, does not impair the development of EAE. Thus, either vitamin D itself or a heretofore-unknown metabolite is needed for the development of a component of the immune system necessary for development of EAE. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Plasticity of renal endocrine function

The kidneys are important endocrine organs. They secrete humoral factors, such as calcitriol, erythropoietin, klotho, and renin into the circulation, and therefore, they are essentially involved in the regulation of a variety of processes ranging from bone formation to erythropoiesis. The endocrine functions are established by cells, such as proximal or distal tubular cells, renocortical interstitial cells, or mural cells of afferent arterioles. These endocrine cells are either fixed in number, such as tubular cells, which individually and gradually upregulate or downregulate hormone production, or they belong to a pool of cells, which display a recruitment behavior, such as erythropoietin- and renin-producing cells. In the latter case, regulation of humoral function occurs via (de)recruitment of active endocrine cells. As a consequence renin- and erythropoietin-producing cells in the kidney show a high degree of plasticity by reversibly switching between distinct cell states. In this review, we will focus on the characteristics of renin- and of erythropoietin-producing cells, especially on their origin and localization, their reversible transformations, and the mediators, which are responsible for transformation. Finally, we will discuss a possible interconversion of renin and erythropoietin expression.

CYP105-diverse structures, functions and roles in an intriguing family of enzymes in Streptomyces

The cytochromes P450 (CYP or P450) are a large superfamily of haem-containing enzymes found in all domains of life. They catalyse a variety of complex reactions, predominantly mixed-function oxidations, often displaying highly regio- and/or stereospecific chemistry. In streptomycetes, they are predominantly associated with secondary metabolite biosynthetic pathways or with xenobiotic catabolism. Homologues of one family, CYP105, have been found in all Streptomyces species thus far sequenced. This review looks at the diverse biological functions of CYP105s and the biosynthetic/catabolic pathways they are associated with. Examples are presented showing a range of biotransformative abilities and different contexts. As biocatalysts capable of some remarkable chemistry, CYP105s have great biotechnological potential and merit detailed study. Recent developments in biotechnological applications which utilize CYP105s are described, alongside a brief overview of the benefits and drawbacks of using P450s in commercial applications. The role of CYP105s in vivo is in many cases undefined and provides a rich source for further investigation into the functions these enzymes fulfil and the metabolic pathways they participate in, in the natural environment.

Regulation of the extrarenal CYP27B1-hydroxylase

Provided here is a collective review of research on the extrarenal CYP27B1-hydroxylase that shapes our current and expanding vision of the role this enzyme plays in the intracrinology and paracrinology, as opposed to the traditional endocrinology, of vitamin D to regulate the innate and adaptive immune responses, particularly in human granuloma-forming diseases like tuberculosis. Special emphasis is placed on soluble factors (i.e., cytokines) in the local microenvironment of these human diseases that coordinate amplification and feedback inhibition of the macrophage CYP27B1-hydroxylase. Principal among these factors are Type I and Type II interferons (IFNs); the Type II IFN, IFN-γ, stimulates the production of 1,25-dihydroxyvitamin D (1,25(OH)2D) from 25-hydroxyvitamin D (25OHD) by the granuloma-forming disease-activated macrophage, while the Type I IFNs, IFN-α and IFN-β, block the hydroxylation reaction. The Type I IFN response is associated with more aggressive disease, while the Type II IFN response, the one that promotes 1,25(OH)2D production by the macrophage, is associated with more confined disease. Tilting the balance in the human immune response toward a confined disease phenotype is enabled by the presence of sufficient extracellular 25OHD to modulate IFN-γ-promoted and substrate 25OH-driven intracellular synthesis of 1,25(OH)2D. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Vitamin D status in adult critically ill patients in Eastern India: An observational retrospective study

Background:

The prevalence of vitamin D deficiency in critically ill patients has been reported to be as high as 80%. There is insufficient data regarding the relationship between 25-hydroxyvitamin D [25(OH) D] levels and outcomes in medical intensive care unit (MICU). The goal of this study was to evaluate the prevalence of 25(OH) D deficiency in MICU and its relationship with outcomes.

Subjects and Methods:

This was a retrospective study in a MICU of a teaching medical college hospital of Eastern India. All patients admitted to MICU, who had levels of 25(OH) D available, were included in the study. The discriminative powers of admission and lowest 25(OH) D values regarding day-30 mortality were evaluated by producing receiver operating curves (ROC). Binary end points were analyzed by means of a Fisher's exact test. Continuous variables were compared by using unpaired t-tests, Welch's tests, or Wilcoxon ranksum tests. All odds ratios and their corresponding 95% confidence intervals were calculated according to the profile-likelihood method. The time from inclusion to death in the two groups was compared with the use of the log-rank test, and the results are presented as Kaplan–Meier curves. Hazard ratios for death from hypo 25(OH) D were calculated by logistic regression model. All P values were 2-tailed and P < 0.05 was considered statistically significant.

Results:

Of the 300 patients admitted during the study period, 25(OH) D levels were available in 152 patients (50.6%). Of these 152 patients, 15 patients (9.8%) had 25(OH) D insufficiency (20-29.9 ng/dL), 79 (51.9%) had 25(OH) D deficiency (0-19.9 ng/dL), and the levels were normal (>30 ng/dl) in 58 (38.2%) patients. Most of the patients with deficient 25(OH) D levels were females (P < 0.05). Higher mortality (P = 0.01), increased length of MICU stay, and prolonged ventilation were observed in patients with 25(OH) D deficiency.

Conclusions:

Patients with 25(OH) D deficiency in MICU have increased hospital mortality, longer mechanical ventilation, and longer MICU stay.

Binding domain-driven intracellular trafficking of sterols for synthesis of steroid hormones, bile acids and oxysterols

Steroid hormones, bioactive oxysterols and bile acids are all derived from the biological metabolism of lipid cholesterol. The enzymatic pathways generating these compounds have been an area of intense research for almost a century, as cholesterol and its metabolites have substantial impacts on human health. Owing to its high degree of hydrophobicity and the chemical properties that it confers to biological membranes, the distribution of cholesterol in cells is tightly controlled, with subcellular organelles exhibiting highly divergent levels of cholesterol. The manners in which cells maintain such sterol distributions are of great interest in the study of steroid and bile acid synthesis, as limiting cholesterol substrate to the enzymatic pathways is the principal mechanism by which production of steroids and bile acids is regulated. The mechanisms by which cholesterol moves within cells, however, remain poorly understood. In this review, we examine the subcellular machinery involved in cholesterol metabolism to steroid hormones and bile acid, relating it to both lipid- and protein-based mechanisms facilitating intracellular and intraorganellar cholesterol movement and delivery to these pathways. In particular, we examine evidence for the involvement of specific protein domains involved in cholesterol binding, which impact cholesterol movement and metabolism in steroidogenesis and bile acid synthesis. A better understanding of the physical mechanisms by which these protein- and lipid-based systems function is of fundamental importance to understanding physiological homeostasis and its perturbation.

A stochastic chemical dynamic approach to correlate autoimmunity and optimal vitamin-D range

Motivated by several recent experimental observations that vitamin-D could interact with antigen presenting cells (APCs) and T-lymphocyte cells (T-cells) to promote and to regulate different stages of immune response, we developed a coarse grained but general kinetic model in an attempt to capture the role of vitamin-D in immunomodulatory responses. Our kinetic model, developed using the ideas of chemical network theory, leads to a system of nine coupled equations that we solve both by direct and by stochastic (Gillespie) methods. Both the analyses consistently provide detail information on the dependence of immune response to the variation of critical rate parameters. We find that although vitamin-D plays a negligible role in the initial immune response, it exerts a profound influence in the long term, especially in helping the system to achieve a new, stable steady state. The study explores the role of vitamin-D in preserving an observed bistability in the phase diagram (spanned by system parameters) of immune regulation, thus allowing the response to tolerate a wide range of pathogenic stimulation which could help in resisting autoimmune diseases. We also study how vitamin-D affects the time dependent population of dendritic cells that connect between innate and adaptive immune responses. Variations in dose dependent response of anti-inflammatory and pro-inflammatory T-cell populations to vitamin-D correlate well with recent experimental results. Our kinetic model allows for an estimation of the range of optimum level of vitamin-D required for smooth functioning of the immune system and for control of both hyper-regulation and inflammation. Most importantly, the present study reveals that an overdose or toxic level of vitamin-D or any steroid analogue could give rise to too large a tolerant response, leading to an inefficacy in adaptive immune function.

Effects of vitamin D receptor knockout on cornea epithelium gap junctions

PURPOSE

Gap junctions are present in all corneal cell types and have been shown to have a critical role in cell phenotype determination. Vitamin D has been shown to influence cell differentiation, and recent work demonstrates the presence of vitamin D in the ocular anterior segment. This study measured and compared gap junction diffusion coefficients among different cornea epithelium phenotypes and in keratocytes using a noninvasive technique, fluorescence recovery after photobleaching (FRAP), and examined the influence of vitamin D receptor (VDR) knockout on epithelial gap junction communication in intact corneas. Previous gap junction studies in cornea epithelium and keratocytes were performed using cultured cells or ex vivo invasive techniques. These invasive techniques were unable to measure diffusion coefficients and likely were disruptive to normal cell physiology.

METHODS

Corneas from VDR knockout and control mice were stained with 5(6)-carboxyfluorescein diacetate (CFDA). Gap junction diffusion coefficients of the corneal epithelium phenotypes and of keratocytes, residing in intact corneas, were detected using FRAP.

RESULTS

Diffusion coefficients equaled 18.7, 9.8, 5.6, and 4.2 μm(2)/s for superficial squamous cells, middle wing cells, basal cells, and keratocytes, respectively. Corneal thickness, superficial cell size, and the superficial squamous cell diffusion coefficient of 10-week-old VDR knockout mice were significantly lower than those of control mice (P < 0.01). The superficial cell diffusion coefficient of heterozygous mice was significantly lower than control mice (P < 0.05).

CONCLUSIONS

Our results demonstrate differences in gap junction dye spread among the epithelial cell phenotypes, mirroring the epithelial developmental axis. The VDR knockout influences previously unreported cell-to-cell communication in superficial epithelium.

Vitamin D modulates prostaglandin E2 synthesis and degradation in human lung fibroblasts

Vitamin D insufficiency has been increasingly recognized in the general population worldwide and has been associated with several lung diseases, including asthma, chronic obstructive pulmonary disease (COPD), and respiratory tract infections. Fibroblasts play a critical role in tissue repair and remodeling, which is a key feature of COPD and asthma. Fibroblasts modulate tissue repair by producing and modifying extracellular matrix components and by releasing mediators that act as autocrine or paracrine modulators of tissue remodeling. The current study was designed to investigate if vitamin D alters fibroblast release of key autocrine/paracrine repair factors. First, we demonstrated that human fetal lung (HFL)-1 cells express the vitamin D receptor (VDR) and that vitamin D, 25-hydroxyvitamin D [25(OH)D], or 1,25-dihydroxyvitamin D [1,25(OH)2D] induce VDR nuclear translocation and increase VDR-DNA binding activity. We next demonstrated that vitamin D, 25(OH)D, and 1,25(OH)2D significantly reduced prostaglandin (PG)E2 production by human lung fibroblasts (HFL-1) but had no effect on transforming growth factor β1, vascular endothelial growth factor, or fibronectin production. Vitamin D, 25(OH)D, and 1,25(OH)2D significantly inhibited IL-1β-induced microsomal PGE synthase (mPGES)-1 expression; in contrast, all three forms of vitamin D stimulated 15-hydroxy PG dehydrogenase, an enzyme that degrades PGE2. Cyclooxygenase-1 and -2 and the other two PGE2 synthases (mPGES-2 and cytosolic PGE synthase) were not altered by vitamin D, 25(OH)D, or 1,25(OH)2D. Finally, the effect of PGE2 inhibition by 25(OH)D was observed in adult lung fibroblasts. These findings suggest that vitamin D can regulate PGE2 synthesis and degradation and by this mechanism can modulate fibroblast-mediated tissue repair function.

Synthesis of 24(28)-Methylene-1α-Hydroxyvitamin D3, a Novel Vitamin D3 Analogue

24(28)-Methylene-1α-hydroxyvitamin D3 was synthesised in 13 steps from vitamin D2. The key step of the synthesis involved the Wittig–Horner olefination of a nor-vitamin D2 aldehyde with diethylphosphono-3-methyl-2-butanone. The resulting enone was followed by reduced methylenation, photoisomerisation and deprotection then gave the target compound.

Placental CYP27B1 and CYP24A1 expression in human placental tissue and their association with maternal and neonatal calcitropic hormones

CONTEXT

Placental CYP27B1 may contribute to circulating maternal calcitriol concentrations across gestation, but determinants of CYP27B1 and CYP24A1 expression in term human placental tissue are not well established.

OBJECTIVE

We hypothesized that higher CYP27B1 protein expression would be associated with increased maternal calcitriol during gestation and that CYP27B1 expression would be impacted by substrate availability.

DESIGN

This was a prospective, longitudinal study.

SETTING

The study was completed in an urban, prenatal clinic located in Rochester, New York.

PATIENTS

The study was undertaken in a cohort of 70 pregnant adolescents (≤18 y of age) and their term neonates.

INTERVENTION

There was no intervention.

MAIN OUTCOMES

Protein and mRNA expressions of CYP27B1, CYP24A1, and vitamin D receptor were measured in term placental tissue and related to 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D, PTH, serum total calcium, IL-6, leptin, and osteoprotegerin measured in maternal serum at midgestation and delivery and in umbilical cord serum at birth.

RESULTS

Placental CYP27B1 protein expression was significantly positively associated with maternal 25(OH)D at both midgestation (n = 68, P = .009) and delivery (n=67, P = .006). Maternal serum 1,25-dihydroxyvitamin D concentrations at midgestation were positively correlated with term placental CYP27B1 mRNA expression (n = 49, P = .002). Significant positive associations were evident between placental CYP27B1 and CYP24A1 protein expression (P = .001, n = 70). Maternal PTH concentrations at midgestation or delivery did not significantly impact placental protein or transcript level of either enzyme. Variability in placental CYP27B1 protein expression was best captured by a model that included maternal midgestation 25(OH)D concentration, placental vitamin D receptor protein expression, and maternal midgestation IL-6 concentrations (P = .002, n = 60, R(2) = 0.22).

CONCLUSIONS

Higher maternal 25(OH)D during pregnancy was associated with significantly higher placental protein expression of CYP27B1 at term supportive of a link between substrate availability and placental production of calcitriol.

Vitamin D metabolism, mechanism of action, and clinical applications

Vitamin D3 is made in the skin from 7-dehydrocholesterol under the influence of UV light. Vitamin D2 (ergocalciferol) is derived from the plant sterol ergosterol. Vitamin D is metabolized first to 25 hydroxyvitamin D (25OHD), then to the hormonal form 1,25-dihydroxyvitamin D (1,25(OH)2D). CYP2R1 is the most important 25-hydroxylase; CYP27B1 is the key 1-hydroxylase. Both 25OHD and 1,25(OH)2D are catabolized by CYP24A1. 1,25(OH)2D is the ligand for the vitamin D receptor (VDR), a transcription factor, binding to sites in the DNA called vitamin D response elements (VDREs). There are thousands of these binding sites regulating hundreds of genes in a cell-specific fashion. VDR-regulated transcription is dependent on comodulators, the profile of which is also cell specific. Analogs of 1,25(OH)2D are being developed to target specific diseases with minimal side effects. This review will examine these different aspects of vitamin D metabolism, mechanism of action, and clinical application.

Vitamin D and systemic lupus erythematosus: state of the art

Systemic lupus erythematosus (SLE) is a systemic inflammatory disease associated with genetic, environmental, hormonal, and immunological factors. One of these factors is vitamin D deficiency. Vitamin D plays many roles in the immune system. Several studies have suggested a potential role in the development of autoimmune diseases. SLE patients have low serum levels of vitamin D, which increase the possibility of an association between vitamin deficiency and disease onset and evolution. This review of the literature presents an analysis of the aspects related to the immunoregulatory effects of vitamin D and its importance for SLE, as well as the recommendations for vitamin D supplementation in these patients.

Vitamin D metabolism, mechanism of action, and clinical applications

Vitamin D3 is made in the skin from 7-dehydrocholesterol under the influence of UV light. Vitamin D2 (ergocalciferol) is derived from the plant sterol ergosterol. Vitamin D is metabolized first to 25 hydroxyvitamin D (25OHD), then to the hormonal form 1,25-dihydroxyvitamin D (1,25(OH)2D). CYP2R1 is the most important 25-hydroxylase; CYP27B1 is the key 1-hydroxylase. Both 25OHD and 1,25(OH)2D are catabolized by CYP24A1. 1,25(OH)2D is the ligand for the vitamin D receptor (VDR), a transcription factor, binding to sites in the DNA called vitamin D response elements (VDREs). There are thousands of these binding sites regulating hundreds of genes in a cell-specific fashion. VDR-regulated transcription is dependent on comodulators, the profile of which is also cell specific. Analogs of 1,25(OH)2D are being developed to target specific diseases with minimal side effects. This review will examine these different aspects of vitamin D metabolism, mechanism of action, and clinical application.

History of the discovery of vitamin D and its active metabolites

Before the twentieth century, it was not possible to describe the essentials of a diet that could support life, growth and reproduction of higher animals. The discovery of vitamin A by McCollum and Davis in 1913 ushered in the era of accessory food substances culminating in the achievement of that goal. It included the discovery of vitamin D and its production in skin caused by ultraviolet light. This was followed by a description of its actions at the physiological level that resulted in a healthy skeleton and beyond. To carry out these functions, vitamin D is converted to a hormone that acts through a nuclear receptor. The findings leading to this concept and their importance to biology and medicine are presented.

Vitamin d and its relationship with obesity and muscle

The skin synthesis of vitamin D represents the first step of a metabolic pathway whose features have been extensively studied and clarified in the last decades. In particular, the production of active and inactive forms of the hormone and the actions of the corresponding enzymes have offered new insights into the knowledge of vitamin D metabolism. Additionally, the description of the different organs and tissues expressing the vitamin D receptor and its possible functions, as well as its genetic determinants, have allowed focusing on the interrelationship between vitamin D and many physiological and pathological functions. In this context, many studies reported the association between vitamin D and adipose tissue metabolism, as well as the possible role of the hormone in obesity, weight, and fat mass distribution. Finally, many reports focused on the vitamin D-related effects on skeletal muscle, particularly on the mechanisms by which vitamin D could directly affect muscle mass and strength. This paper is mainly aimed to review vitamin D metabolism and its relationship with obesity and skeletal muscle function.

Triennial Growth Symposium--Vitamin D: bones and beyond

The exploration of vitamin D metabolism and function has led to the discovery of active forms of vitamin D that find great usefulness in treating patients with bone disease or renal failure and also perhaps in topical application for the treatment of skin disorders, such as psoriasis. It may also be effective in some types of autoimmune disease. This warrants our attention to maintaining an adequate vitamin D level in our blood to assure that the expected functions of vitamin D take place. However, we must not get so overenthusiastic as to expect vitamin D to be effective in treating or preventing many diverse diseases and especially caution is urged in considering that vitamin D compounds might be used to suppress cancerous growth.

Murine CD8+ T cells but not macrophages express the vitamin D 1α-hydroxylase

The active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is synthesized by the 1α-hydroxylase, which is encoded by the Cyp27B1 gene. Using transgenic mice that have replaced the Cyp27B1 gene with the bacterial lacZ reporter gene (β-galactosidase), the inflammatory conditions that induce Cyp27B1 in the immune system were probed. A variety of stimuli including lipopolysaccharide, anti-CD3 or PMA/ionomycin were used to stimulate splenocytes and bone marrow derived macrophage in vitro. Only anti-CD3 stimulation resulted in a low induction of β-galactosidase activity in the spleen, indicating that T cells might be a source of Cyp27B1. In vivo, challenge with lipopolysaccharide, α-galactosylceramide, and Listeria monocytogenes failed to induce β-galactosidase activity outside of the kidneys. During more prolonged and severe inflammation there was staining in both the lungs and the gastrointestinal tract for β-galactosidase. Furthermore, wild-type reconstitution of the hematopoietic cell population in Cyp27B1 KO mice protected the mice from experimental colitis. T cell production of Cyp27B1 activity was shown to be from the CD8+ but not the CD4+ T cell population. CD8+ T cells expressed the reporter gene only after 48 h of stimulation. The data is consistent with a model where CD8+ T cells are activated to produce Cyp27B1 and 1,25(OH)2D3 that serves to turn off the local immune response.

Molecular link between vitamin D and cancer prevention

The metabolite of vitamin D, 1α,25-dihydroxyvitamin D₃ (also known as calcitriol), is a biologically active molecule required to maintain the physiological functions of several target tissues in the human body from conception to adulthood. Its molecular mode of action ranges from immediate nongenomic responses to longer term mechanisms that exert persistent genomic effects. The genomic mechanisms of vitamin D action rely on cross talk between 1α,25-dihydroxyvitamin D₃ signaling pathways and that of other growth factors or hormones that collectively regulate cell proliferation, differentiation and cell survival. In vitro and in vivo studies demonstrate a role for vitamin D (calcitriol) in modulating cellular growth and development. Vitamin D (calcitriol) acts as an antiproliferative agent in many tissues and significantly slows malignant cellular growth. Moreover, epidemiological studies have suggested that ultraviolet-B exposure can help reduce cancer risk and prevalence, indicating a potential role for vitamin D as a feasible agent to prevent cancer incidence and recurrence. With the preventive potential of this biologically active agent, we suggest that countries where cancer is on the rise--yet where sunlight and, hence, vitamin D may be easily acquired--adopt awareness, education and implementation strategies to increase supplementation with vitamin D in all age groups as a preventive measure to reduce cancer risk and prevalence.

FGF-23 regulates CYP27B1 transcription in the kidney and in extra-renal tissues

The mitochondrial enzyme 25-hydroxyvitamin D 1α-hydroxylase, which is encoded by the CYP27B1 gene, converts 25OHD to the biological active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)₂D). Renal 1α-hydroxylase activity is the principal determinant of the circulating 1,25(OH)₂D concentration and enzyme activity is tightly regulated by several factors. Fibroblast growth factor-23 (FGF-23) decreases serum 1,25(OH)₂D concentrations by suppressing CYP27B1 mRNA abundance in mice. In extra-renal tissues, 1α-hydroxylase is responsible for local 1,25(OH)₂D synthesis, which has important paracrine actions, but whether FGF-23 regulates CYP27B1 gene expression in extra-renal tissues is unknown. We sought to determine whether FGF-23 regulates CYP27B1 transcription in the kidney and whether extra-renal tissues are target sites for FGF-23-induced suppression of CYP27B1. In HEK293 cells transfected with the human CYP27B1 promoter, FGF-23 suppressed promoter activity by 70%, and the suppressive effect was blocked by CI-1040, a specific inhibitor of extracellular signal regulated kinase 1/2. To examine CYP27B1 transcriptional activity in vivo, we crossed fgf-23 null mice with mice bearing the CYP27B1 promoter-driven luciferase transgene (1α-Luc). In the kidney of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity was increased by 3-fold compared to that in wild-type/1α-Luc mice. Intraperitoneal injection of FGF-23 suppressed renal CYP27B1 promoter activity and protein expression by 26% and 60% respectively, and the suppressive effect was blocked by PD0325901, an ERK1/2 inhibitor. These findings provide evidence that FGF-23 suppresses CYP27B1 transcription in the kidney. Furthermore, we demonstrate that in FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA abundance are increased in several extra-renal sites. In the heart of FGF-23 null/1α-Luc mice, CYP27B1 promoter activity and mRNA were 2- and 5-fold higher, respectively, than in control mice. We also observed a 3- to 10-fold increase in CYP27B1 mRNA abundance in the lung, spleen, aorta and testis of FGF-23 null/1α-Luc mice. Thus, we have identified novel extra-renal target sites for FGF-23-mediated regulation of CYP27B1.

An analysis of the association between the vitamin D pathway and serum 25-hydroxyvitamin D levels in a healthy Chinese population

Vitamin D deficiency has been recognized as a major public health issue worldwide. Recent studies have indicated that genetic factors might play an important role in determining serum 25-hydroxyvitamin D [25(OH)D] levels in Caucasians and African Americans. However, the genes that contribute to the variation in serum 25(OH)D levels in Chinese are unknown. In this study, we screened 15 key genes within the vitamin D metabolic pathway using 96 single-nucleotide polymorphism (SNP) markers in a group of 2897 unrelated healthy Chinese subjects. Significant confounding factors that may influence the variability in serum 25(OH)D levels were used as covariates for association analyses. An association test for quantitative traits was performed to evaluate the association between candidate genes and serum 25(OH)D levels. In the present study, variants and/or haplotypes in GC, CYP2R1, and DHCR7/NADSYN1 were identified as being associated with 25(OH)D levels. Participants with three or four risk alleles of the two variants (GC-rs4588 and CYP2R1-rs10766197) had an increased chance of presenting with a 25(OH)D concentration lower than 20 ng/mL (odds ratio 2.121, 95% confidence interval 1.586-2.836, p = 6.1 × 10(-8) ) compared with those lacking the risk alleles. Each additional copy of a risk allele was significantly associated with a 0.12-fold decrease in the log-25(OH)D concentration (p = 3.7 × 10(-12) ). Haplotype TGA of GC rs705117-rs2282679-rs1491710, haplotype GAGTAC of GC rs842999-rs705120-rs222040-rs4588-rs7041-rs10488854, haplotype CA of GC rs1155563-rs222029, and haplotype AAGA of CYP2R1 rs7936142-rs12794714-rs2060793-rs16930609 were genetic risk factors toward a lower 25(OH)D concentration. In contrast, haplotype TGGGCCC of DHCR7/NADSYN1 rs1790349-rs7122671-rs1790329-rs11606033-rs2276360-rs1629220-rs2282618 were genetic protective factors. The results suggest that the GC, CYP2R1, and DHCR7/NADSYN1 genes might contribute to variability in the serum 25(OH)D levels in a healthy Chinese population in Shanghai. These markers could be used as tools in Mendelian randomization analyses of vitamin D, and they could potentially be drug targets in the Chinese population in Shanghai.

Effects of vitamin D on immune disorders with special regard to asthma, COPD and autoimmune diseases: a short review

This paper reviews the recent data on the role of vitamin D (VD) in the genesis of various immunological disorders. It inhibits immune reactions in general, but it enhances the transcription of 'endogenous antibiotics' such as cathelicidin and defensins. VD inhibits the genesis of both Th1- and Th2-cell mediated diseases. The pleiotropic character VD-induced effects are due to the altered transcription of hundreds of genes. VD supplementation in most related studies reduced the prevalence of asthma. Th1-dependent autoimmune diseases (e.g., multiple sclerosis, Type 1 diabetes, Crohn's disease, rheumatoid arthritis and so on) are also inhibited by VD due to inhibition of antigen presentation, reduced polarization of Th0 cells to Th1 cells and reduced production of cytokines from the latter cells. VD seems to also be a useful adjunct in the prevention of allograft rejection. Last but not least, VD supplementation may be useful in the prevention or adjunct treatment of chronic obstructive pulmonary disease.

Adaptive evolution of the myo6 gene in old world fruit bats (family: pteropodidae)

Myosin VI (encoded by the Myo6 gene) is highly expressed in the inner and outer hair cells of the ear, retina, and polarized epithelial cells such as kidney proximal tubule cells and intestinal enterocytes. The Myo6 gene is thought to be involved in a wide range of physiological functions such as hearing, vision, and clathrin-mediated endocytosis. Bats (Chiroptera) represent one of the most fascinating mammal groups for molecular evolutionary studies of the Myo6 gene. A diversity of specialized adaptations occur among different bat lineages, such as echolocation and associated high-frequency hearing in laryngeal echolocating bats, large eyes and a strong dependence on vision in Old World fruit bats (Pteropodidae), and specialized high-carbohydrate but low-nitrogen diets in both Old World and New World fruit bats (Phyllostomidae). To investigate what role(s) the Myo6 gene might fulfill in bats, we sequenced the coding region of the Myo6 gene in 15 bat species and used molecular evolutionary analyses to detect evidence of positive selection in different bat lineages. We also conducted real-time PCR assays to explore the expression levels of Myo6 in a range of tissues from three representative bat species. Molecular evolutionary analyses revealed that the Myo6 gene, which was widely considered as a hearing gene, has undergone adaptive evolution in the Old World fruit bats which lack laryngeal echolocation and associated high-frequency hearing. Real-time PCR showed the highest expression level of the Myo6 gene in the kidney among ten tissues examined in three bat species, indicating an important role for this gene in kidney function. We suggest that Myo6 has undergone adaptive evolution in Old World fruit bats in relation to receptor-mediated endocytosis for the preservation of protein and essential nutrients.

Cytochrome P450-mediated metabolism of vitamin D

The vitamin D signal transduction system involves a series of cytochrome P450-containing sterol hydroxylases to generate and degrade the active hormone, 1α,25-dihydroxyvitamin D3, which serves as a ligand for the vitamin D receptor-mediated transcriptional gene expression described in companion articles in this review series. This review updates our current knowledge of the specific anabolic cytochrome P450s involved in 25- and 1α-hydroxylation, as well as the catabolic cytochrome P450 involved in 24- and 23-hydroxylation steps, which are believed to initiate inactivation of the vitamin D molecule. We focus on the biochemical properties of these enzymes; key residues in their active sites derived from crystal structures and mutagenesis studies; the physiological roles of these enzymes as determined by animal knockout studies and human genetic diseases; and the regulation of these different cytochrome P450s by extracellular ions and peptide modulators. We highlight the importance of these cytochrome P450s in the pathogenesis of kidney disease, metabolic bone disease, and hyperproliferative diseases, such as psoriasis and cancer; as well as explore potential future developments in the field.

Higher blood 25(OH)D level may reduce the breast cancer risk: evidence from a Chinese population based case-control study and meta-analysis of the observational studies

Experimental data suggest a protective effect of vitamin D on breast cancer; however, epidemiologic results remain inclusive. With a Chinese population-based case-control study and meta-analysis of the observational studies, we here systematically evaluated the association of blood 25(OH)D level and breast cancer risk. With 593 breast cancer cases and 580 cancer-free controls from Shanghai, China, we found that 80% of the normal women had severe vitamin D deficiency (less than 20 ng/mL) and 15.2% had mild deficiency (20 to 30 ng/mL) and only 4.8% of women had sufficient vitamin D level (>30 ng/mL) while the proportion was 96.1%, 3.2% and 0.7% respectively for the breast cancer patients. Compared to those with the lowest quartile of plasma 25(OH)D level, women with highest quartile 25(OH)D level showed a significant decreased breast cancer risk (Q4 vs.Q1: OR = 0.10, 95% CI = 0.06–0.15) and every 1 ng/ml increment of plasma 25(OH)D level led to a 16% lower odds of breast cancer (OR = 0.84, 95% CI = 0.81–0.87; P<0.001). From the meta-analysis of the observational studies, we found that women with highest quantile of blood 25(OH)D level was associated with a significantly reduced breast cancer risk compared to those with lowest quantile of blood 25(OH)D level for the 11 nested case-control and retrospective studies (pooled OR = 0.86, 95% CI = 0.75–1.00) and 10 case-control studies (7 population based, OR = 0.35, 95% CI = 0.24–0.52; 3 hospital based, OR = 0.08, 95% CI = 0.02–0.33). These results suggest that vitamin D may have a chemo-preventive effect against breast cancer.

Human cytochromes P450 in health and disease

There are 18 mammalian cytochrome P450 (CYP) families, which encode 57 genes in the human genome. CYP2, CYP3 and CYP4 families contain far more genes than the other 15 families; these three families are also the ones that are dramatically larger in rodent genomes. Most (if not all) genes in the CYP1, CYP2, CYP3 and CYP4 families encode enzymes involved in eicosanoid metabolism and are inducible by various environmental stimuli (i.e. diet, chemical inducers, drugs, pheromones, etc.), whereas the other 14 gene families often have only a single member, and are rarely if ever inducible or redundant. Although the CYP2 and CYP3 families can be regarded as largely redundant and promiscuous, mutations or other defects in one or more genes of the remaining 16 gene families are primarily the ones responsible for P450-specific diseases-confirming these genes are not superfluous or promiscuous but rather are more directly involved in critical life functions. P450-mediated diseases comprise those caused by: aberrant steroidogenesis; defects in fatty acid, cholesterol and bile acid pathways; vitamin D dysregulation and retinoid (as well as putative eicosanoid) dysregulation during fertilization, implantation, embryogenesis, foetogenesis and neonatal development.

Vitamin D is a regulator of endothelial nitric oxide synthase and arterial stiffness in mice

The vitamin D hormone 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] is essential for the preservation of serum calcium and phosphate levels but may also be important for the regulation of cardiovascular function. Epidemiological data in humans have shown that vitamin D insufficiency is associated with hypertension, left ventricular hypertrophy, increased arterial stiffness, and endothelial dysfunction in normal subjects and in patients with chronic kidney disease and type 2 diabetes. However, the pathophysiological mechanisms underlying these associations remain largely unexplained. In this study, we aimed to decipher the mechanisms by which 1,25(OH)2D3 may regulate systemic vascular tone and cardiac function, using mice carrying a mutant, functionally inactive vitamin D receptor (VDR). To normalize calcium homeostasis in VDR mutant mice, we fed the mice lifelong with the so-called rescue diet enriched with calcium, phosphate, and lactose. Here, we report that VDR mutant mice are characterized by lower bioavailability of the vasodilator nitric oxide (NO) due to reduced expression of the key NO synthesizing enzyme, endothelial NO synthase, leading to endothelial dysfunction, increased arterial stiffness, increased aortic impedance, structural remodeling of the aorta, and impaired systolic and diastolic heart function at later ages, independent of changes in the renin-angiotensin system. We further demonstrate that 1,25(OH)2D3 is a direct transcriptional regulator of endothelial NO synthase. Our data demonstrate the importance of intact VDR signaling in the preservation of vascular function and may provide a mechanistic explanation for epidemiological data in humans showing that vitamin D insufficiency is associated with hypertension and endothelial dysfunction.

Systemic steroid exposure is associated with differential methylation in chronic obstructive pulmonary disease

RATIONALE

Systemic glucocorticoids are used therapeutically to treat a variety of medical conditions. Epigenetic processes such as DNA methylation may reflect exposure to glucocorticoids and may be involved in mediating the responses and side effects associated with these medications.

OBJECTIVES

To test the hypothesis that differences in DNA methylation are associated with current systemic steroid use.

METHODS

We obtained DNA methylation data at 27,578 CpG sites in 14,475 genes throughout the genome in two large, independent cohorts: the International COPD Genetics Network (n(discovery) = 1,085) and the Boston Early Onset COPD study (n(replication) = 369). Sites were tested for association with current systemic steroid use using generalized linear mixed models.

MEASUREMENTS AND MAIN RESULTS

A total of 511 sites demonstrated significant differential methylation by systemic corticosteroid use in all three of our primary models. Pyrosequencing validation confirmed robust differential methylation at CpG sites annotated to genes such as SLC22A18, LRP3, HIPK3, SCNN1A, FXYD1, IRF7, AZU1, SIT1, GPR97, ABHD16B, and RABGEF1. Functional annotation clustering demonstrated significant enrichment in intrinsic membrane components, hemostasis and coagulation, cellular ion homeostasis, leukocyte and lymphocyte activation and chemotaxis, protein transport, and responses to nutrients.

CONCLUSIONS

Our analyses suggest that systemic steroid use is associated with site-specific differential methylation throughout the genome. Differentially methylated CpG sites were found in biologically plausible and previously unsuspected pathways; these genes and pathways may be relevant in the development of novel targeted therapies.

Sinonasal epithelial cells synthesize active vitamin D, augmenting host innate immune function

BACKGROUND

Vitamin D, long recognized for its role in bone metabolism and calcium homeostasis, has been increasingly shown to augment innate immunity. 1-α-Hydroxylase, the enzyme responsible for the synthesis of active vitamin D, has been shown to have extrarenal expression in multiple cell types, including airway epithelial cells. The purpose of this study is to explore whether sinonasal epithelial cells (SNECs) express 1-α-hydroxylase, allowing for the local production of active vitamin D, thereby augmenting innate immune function.

METHODS

Human SNECs were grown in culture and stimulated by inactive vitamin D. Expression of 1-α-hydroxylase was measured by real-time polymerase chain reaction and immunocytochemistry. Active vitamin D production was measured by enzyme-linked immunosorbent assay (ELISA). The expression of cathelicidin, an antimicrobial peptide, was measured by real-time polymerase chain reaction and immunocytochemistry.

RESULTS

SNECs constitutively express the enzyme 1-α-hydroxylase resulting in active vitamin D production. SNECs exposed to inactive vitamin D had a significant 8-fold increase in cathelicidin expression when compared to controls.

CONCLUSION

SNECs can generate active vitamin D, which significantly increases expression of the antimicrobial peptide cathelicidin. © 2013 ARS-AAOA, LLC.

Enhancing the prioritization of disease-causing genes through tissue specific protein interaction networks

The prioritization of candidate disease-causing genes is a fundamental challenge in the post-genomic era. Current state of the art methods exploit a protein-protein interaction (PPI) network for this task. They are based on the observation that genes causing phenotypically-similar diseases tend to lie close to one another in a PPI network. However, to date, these methods have used a static picture of human PPIs, while diseases impact specific tissues in which the PPI networks may be dramatically different. Here, for the first time, we perform a large-scale assessment of the contribution of tissue-specific information to gene prioritization. By integrating tissue-specific gene expression data with PPI information, we construct tissue-specific PPI networks for 60 tissues and investigate their prioritization power. We find that tissue-specific PPI networks considerably improve the prioritization results compared to those obtained using a generic PPI network. Furthermore, they allow predicting novel disease-tissue associations, pointing to sub-clinical tissue effects that may escape early detection.

Expression of vitamin D-metabolizing enzymes in human adipose tissue -- the effect of obesity and diet-induced weight loss

OBJECTIVE

Low vitamin D (VD) levels are common in obesity. We hypothesized that this may be due to metabolism of VD in adipose tissue (AT). Thus, we studied (1) whether the VD-metabolizing enzymes were expressed differently in AT of lean and obese individuals and in visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT), and (2) whether their expression was influenced by weight loss.

METHODS

Samples of SAT and VAT were analyzed for expression of the vitamin-D-25-hydroxylases CYP2R1, CYP2J2, CYP27A1 and CYP3A4, the 25-vitamin-D-1α-hydroxylase CYP27B1, the catabolic vitamin-D-24-hydroxylase CYP24A1, and the vitamin D receptor, using reverse transcriptase-PCR. Moreover, plasma 25-hydroxy-vitamin D (25OHD) level was measured and related to the expression of these enzymes. Samples of SAT and VAT from 20 lean women and 20 obese women, and samples of SAT from 17 obese subjects before and after a 10% weight loss were analyzed.

RESULTS

A plasma 25OHD level <50 nmol l(-1) was highly prevalent in both lean (45%) and obese (90%) women (P<0.01). Plasma 25OHD increased by 27% after weight loss in the obese individuals (P<0.05). Expression levels of the 25-hydroxylase CYP2J2 and the 1α-hydroxylase CYP27B1 were decreased by 71% (P<0.0001) and 49% (P<0.05), respectively, in SAT of the obese. CYP24A1 did not differ between lean and obese women, but the expression was increased by 79% (P<0.05) after weight loss.

CONCLUSION

Obesity is characterized by a decreased expression of the 25-hydroxylase CYP2J2 and the 1α-hydroxylase CYP27B1 in SAT, whereas the catabolic CYP24A1 does not differ between lean and obese women. However, the expression of CYP24A1 is increased after weight loss. Accordingly, AT has the capacity to metabolize VD locally, and this can be dynamically altered during obesity and weight loss.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Characterization of the autocrine/paracrine function of vitamin D in human gingival fibroblasts and periodontal ligament cells

BACKGROUND

We previously demonstrated that 25-hydroxyvitamin D(3), the precursor of 1α,25-dihydroxyvitamin D(3), is abundant around periodontal soft tissues. Here we investigate whether 25-hydroxyvitamin D(3) is converted to 1α,25-dihydroxyvitamin D(3) in periodontal soft tissue cells and explore the possibility of an autocrine/paracrine function of 1α,25-dihydroxyvitamin D(3) in periodontal soft tissue cells.

METHODOLOGY/PRINCIPAL FINDINGS

We established primary cultures of human gingival fibroblasts and human periodontal ligament cells from 5 individual donors. We demonstrated that 1α-hydroxylase was expressed in human gingival fibroblasts and periodontal ligament cells, as was cubilin. After incubation with the 1α-hydroxylase substrate 25-hydroxyvitamin D(3), human gingival fibroblasts and periodontal ligament cells generated detectable 1α,25-dihydroxyvitamin D(3) that resulted in an up-regulation of CYP24A1 and RANKL mRNA. A specific knockdown of 1α-hydroxylase in human gingival fibroblasts and periodontal ligament cells using siRNA resulted in a significant reduction in both 1α,25-dihydroxyvitamin D(3) production and mRNA expression of CYP24A1 and RANKL. The classical renal regulators of 1α-hydroxylase (parathyroid hormone, calcium and 1α,25-dihydroxyvitamin D(3)) and Porphyromonas gingivalis lipopolysaccharide did not influence 1α-hydroxylase expression significantly, however, interleukin-1β and sodium butyrate strongly induced 1α-hydroxylase expression in human gingival fibroblasts and periodontal ligament cells.

CONCLUSIONS/SIGNIFICANCE

In this study, the expression, activity and functionality of 1α-hydroxylase were detected in human gingival fibroblasts and periodontal ligament cells, raising the possibility that vitamin D acts in an autocrine/paracrine manner in these cells.

Adjudication of the alleged role of vitamin d in the antimicrobial pathway

Dynamic interactions between microorganism and host have evolved in such a way that while microbial pathogens are the cause of many human infections, a symbiotic relationship is also known to exist. Another important anomaly is that exposure to pathogenic organisms does not necessarily result in development of clinical disease. The latter conclusion infers that susceptibility to infectious disease can be modified by host-related factors. Arguably the two most prominent factors are genetic variability and immunologic status of the exposed individual. Because of the Human Genome and the HapMap projects, developments in genotyping technology have brought the possibility of identifying associations between specific genetic alterations and common diseases closer to reality. In addition, a growing body of evidence suggests vitamin D has an important contributory role in the antimicrobial pathway.

Vitamin D and polymorphisms of VDR gene in patients with systemic lupus erythematosus

The susceptibility for the development of systemic lupus erythematosus (SLE) is related to environmental, hormonal, genetic, and immunological factors. Numerous genes have been linked to the emergence of SLE, including vitamin D receptor (VDR) gene that synthesizes the receptor of vitamin D. Several polymorphisms have been described since the discovery of this gene, and their effects on VDR activity are still poorly understood. Vitamin D's biological functions are mediated by VDR. Vitamin D exerts many actions on the immune system, and several studies have suggested its role in the pathogenesis of autoimmune diseases. SLE patients have low blood levels of vitamin D, which raises the possibility of association between the deficiency of this vitamin and the onset of the disease. BsmI and FokI polymorphic variants seem to be related to the onset of the disease in Asian patients. In this article, we review the aspects related to the metabolism and immunoregulatory effects of vitamin D, VDR, and main polymorphisms involving the VDR gene and the relationship between vitamin D levels and its receptor with SLE.

A novel CYP27B1 mutation causes a feline vitamin D-dependent rickets type IA

A 12-week-old domestic cat presented at a local veterinary clinic with hypocalcemia and skeletal abnormalities suggestive of rickets. Osteomalacia (rickets) is a disease caused by impaired bone mineralization leading to an increased prevalence of fractures and deformity. Described in a variety of species, rickets is most commonly caused by vitamin D or calcium deficiencies owing to both environmental and or genetic abnormalities. Vitamin D-dependent rickets type 1A (VDDR-1A) is a result of the enzymatic pathway defect caused by mutations in the 25-hydroxyvitamin D(3)-1-alpha-hydroxylase gene [cytochrome P27 B1 (CYP27B1)]. Calcitriol, the active form of vitamin D(3), regulates calcium homeostasis, which requires sufficient dietary calcium availability and correct hormonal function for proper bone growth and maintenance. Patient calcitriol concentrations were low while calcidiol levels were normal suggestive of VDDR-1A. The entire DNA coding sequencing of CYP27B1 was evaluated. The affected cat was wild type for previously identified VDDR-1A causative mutations. However, six novel mutations were identified, one of which was a nonsense mutation at G637T in exon 4. The exon 4 G637T nonsense mutation results in a premature protein truncation, changing a glutamic acid to a stop codon, E213X, likely causing the clinical presentation of rickets. The previously documented genetic mutation resulting in feline VDDR-1A rickets, as well as the case presented in this research, result from novel exon 4 CYP27B1 mutations, thus exon 4 should be the initial focus of future sequencing efforts.

Hormonal gene regulation through DNA methylation and demethylation

Methylation and demethylation of cytosine residues in the genomic DNA play key roles in a wide range of fundamental biological processes such as differentiation and development, genome stability, imprinting, X chromosome inactivation, carcinogenesis and aging. DNA methylation is considered to be a stable modification associated with the epigenetic silencing of genomic loci and maintained through cellular division. Recent studies however, suggest that DNA methylation and demethylation are considerably more dynamic than previously thought and may be involved in repression and derepression of gene activity during the lifespan of a cell. This article is focused on epigenetic mechanisms in the hormonal regulation of the cytochrome p450 27B1 or CYP27B1 gene activity that involve reversible epigenetic modifications to chromatin and DNA methylation profiles.

Genetic disorders and defects in vitamin D action

The biochemical and genetic analysis of the VDR in patients with HVDRR has yielded important insights into the structure and function of the receptor in mediating 1,25(OH)2D3 action. Similarly, study of children affected by HVDRR continues to provide a more complete understanding of the biologic role of 1,25(OH)2D3 in vivo. A concerted investigative approach to HVDRR at the clinical, cellular, and molecular levels has proved valuable in gaining knowledge of the functions of the domains of the VDR and elucidating the detailed mechanism of action of 1,25(OH)2D3. These studies have been essential to promote the well-being of the families with HVDRR and in improving the diagnostic and clinical management of this rare genetic disease.

Extrarenal expression of the 25-hydroxyvitamin D-1-hydroxylase

Like the vitamin D receptor (VDR), the CYP27B1-hydroxylase is expressed widely in human tissues. This expression profile establishes the potential for interaction of the VDR with the product of the CYP27B1, 1,25-dihydroxyvitamin D (1,25-(OH)(2)D), in either an intracrine or paracrine mode. This expansive expression profile also suggests that the local production and action of 1,25-(OH)(2)D to regulate VDR-directed gene expression may be similarly wide-ranging and distinct from what occurs in the kidney; the proximal renal tubular epithelial cell is the richest source of the CYP27B1 and the site for production of 1,25-(OH)(2)D destined to function as a hormone. Existence of the CYP27B1 at extrarenal sites has been widely documented, although the functional impact of the enzyme in these tissues has yet to be fully demonstrated. Two notable exceptions are the disease-activated macrophage (e.g., in sarcoidosis or tuberculosis) and the placenta. These two tissues are capable of generating enough 1,25-(OH)(2)D so as to be detectable in the general circulation. As such, this review will focus on CYP27B1 expression only at these two sites, theorizing that 1,25-(OH)(2)D production at these sites is for the purpose of local immunoregulatory function, not for controlling calcium balance in the host or the fetus.

Rickets-vitamin D deficiency and dependency

Rickets is an important problem even in countries with adequate sun exposure. The causes of rickets/osteomalacia are varied and include nutritional deficiency, especially poor dietary intake of vitamin D and calcium. Non-nutritional causes include hypophosphatemic rickets primarily due to renal phosphate losses and rickets due to renal tubular acidosis. In addition, some varieties are due to inherited defects in vitamin D metabolism and are called vitamin D dependent rickets. This chapter highlights rickets/osteomalacia related to vitamin D deficiency or to inherited defects in vitamin D metabolism. Hypophosphatemic rickets and rickets due to renal tubular acidosis are discussed in other sections of the journal.