Regulation of gene expression by the CYP27B1 promoter-study of a transgenic mouse model

Vitamin D and the Central Nervous System: Causative and Preventative Mechanisms in Brain Disorders

Twenty of the last one hundred years of vitamin D research have involved investigations of the brain as a target organ for this hormone. Our group was one of the first to investigate brain outcomes resulting from primarily restricting dietary vitamin D during brain development. With the advent of new molecular and neurochemical techniques in neuroscience, there has been increasing interest in the potential neuroprotective actions of vitamin D in response to a variety of adverse exposures and how this hormone could affect brain development and function. Rather than provide an exhaustive summary of this data and a listing of neurological or psychiatric conditions that vitamin D deficiency has been associated with, here, we provide an update on the actions of this vitamin in the brain and cellular processes vitamin D may be targeting in psychiatry and neurology.

Anabolic effects of vitamin D and magnesium in aging bone

Decreased bone mass and an increased risk of bone fractures become more common with age. This condition is often associated with osteoporosis and is caused by an imbalance of bone resorption and new bone formation. Lifestyle factors that affect the risk of osteoporosis include alcohol, diet, hormones, physical activity, and smoking. Calcium and vitamin D are particularly important for the age-related loss of bone density and skeletal muscle mass, but other minerals, such as magnesium, also have an important role. Here, we summarize how optimal magnesium and vitamin D balance improve health outcomes in the elderly, the role of magnesium and vitamin D on bone formation, and the implications of widespread deficiency of these factors in the United States and worldwide, particularly in the elderly population.

Molecular actions of vitamin D in reproductive cell biology

Vitamin D (VitD) is an important secosteroid and has attracted attention in several areas of research due to common VitD deficiency in the population, and its potential to regulate molecular pathways related to chronic and inflammatory diseases. VitD metabolites and the VitD receptor (VDR) influence many tissues including those of the reproductive system. VDR expression has been demonstrated in various cell types of the male reproductive tract, including spermatozoa and germ cells, and in female reproductive tissues including the ovaries, placenta and endometrium. However, the molecular role of VitD signalling and metabolism in reproductive function have not been fully established. Consequently, the aim of this work is to review current metabolic and molecular aspects of the VitD–VDR axis in reproductive medicine and to propose the direction of future research. Specifically, the influence of VitD on sperm motility, calcium handling, capacitation, acrosin reaction and lipid metabolism is examined. In addition, we will also discuss the effect of VitD on sex hormone secretion and receptor expression in primary granulosa cells, along with the impact on cytokine production in trophoblast cells. The review concludes with a discussion of the recent developments in VitD–VDR signalling specifically related to altered cellular bioenergetics, which is an emerging concept in the field of reproductive medicine.

Vitamin D and the epigenome

Epigenetic mechanisms play a crucial role in regulating gene expression. The main mechanisms involve methylation of DNA and covalent modifications of histones by methylation, acetylation, phosphorylation, or ubiquitination. The complex interplay of different epigenetic mechanisms is mediated by enzymes acting in the nucleus. Modifications in DNA methylation are performed mainly by DNA methyltransferases (DNMTs) and ten-eleven translocation (TET) proteins, while a plethora of enzymes, such as histone acetyltransferases (HATs), histone deacetylases (HDACs), histone methyltransferases (HMTs), and histone demethylases (HDMs) regulate covalent histone modifications. In many diseases, such as cancer, the epigenetic regulatory system is often disturbed. Vitamin D interacts with the epigenome on multiple levels. Firstly, critical genes in the vitamin D signaling system, such as those coding for vitamin D receptor (VDR) and the enzymes 25-hydroxylase (CYP2R1), 1α-hydroxylase (CYP27B1), and 24-hydroxylase (CYP24A1) have large CpG islands in their promoter regions and therefore can be silenced by DNA methylation. Secondly, VDR protein physically interacts with coactivator and corepressor proteins, which in turn are in contact with chromatin modifiers, such as HATs, HDACs, HMTs, and with chromatin remodelers. Thirdly, a number of genes encoding for chromatin modifiers and remodelers, such as HDMs of the Jumonji C (JmjC)-domain containing proteins and lysine-specific demethylase (LSD) families are primary targets of VDR and its ligands. Finally, there is evidence that certain VDR ligands have DNA demethylating effects. In this review we will discuss regulation of the vitamin D system by epigenetic modifications and how vitamin D contributes to the maintenance of the epigenome, and evaluate its impact in health and disease.

Vitamin D and depression: a systematic review and meta-analysis comparing studies with and without biological flaws

Efficacy of Vitamin D supplements in depression is controversial, awaiting further literature analysis. Biological flaws in primary studies is a possible reason meta-analyses of Vitamin D have failed to demonstrate efficacy. This systematic review and meta-analysis of Vitamin D and depression compared studies with and without biological flaws. The systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The literature search was undertaken through four databases for randomized controlled trials (RCTs). Studies were critically appraised for methodological quality and biological flaws, in relation to the hypothesis and study design. Meta-analyses were performed for studies according to the presence of biological flaws. The 15 RCTs identified provide a more comprehensive evidence-base than previous systematic reviews; methodological quality of studies was generally good and methodology was diverse. A meta-analysis of all studies without flaws demonstrated a statistically significant improvement in depression with Vitamin D supplements (+0.78 CI +0.24, +1.27). Studies with biological flaws were mainly inconclusive, with the meta-analysis demonstrating a statistically significant worsening in depression by taking Vitamin D supplements (-1.1 CI -0.7, -1.5). Vitamin D supplementation (≥800 I.U. daily) was somewhat favorable in the management of depression in studies that demonstrate a change in vitamin levels, and the effect size was comparable to that of anti-depressant medication.

Vitamin D activities for health outcomes

Reports describing significant health risks due to inadequate vitamin D status continue to generate considerable interest amongst the medical and lay communities alike. Recent research on the various molecular activities of the vitamin D system, including the nuclear vitamin D receptor and other receptors for 1,25-dihydroxyvitamin D and vitamin D metabolism, provides evidence that the vitamin D system carries out biological activities across a wide range of tissues similar to other nuclear receptor hormones. This knowledge provides physiological plausibility of the various health benefits claimed to be provided by vitamin D and supports the proposals for conducting clinical trials. The vitamin D system plays critical roles in the maintenance of plasma calcium and phosphate and bone mineral homeostasis. Recent evidence confirms that plasma calcium homeostasis is the critical factor modulating vitamin D activity. Vitamin D activities in the skeleton include stimulation or inhibition of bone resorption and inhibition or stimulation of bone formation. The three major bone cell types, which are osteoblasts, osteocytes and osteoclasts, can all respond to vitamin D via the classical nuclear vitamin D receptor and metabolize 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D to activate the vitamin D receptor and modulate gene expression. Dietary calcium intake interacts with vitamin D metabolism at both the renal and bone tissue levels to direct either a catabolic action on the bone through the endocrine system when calcium intake is inadequate or an anabolic action through a bone autocrine or paracrine system when calcium intake is sufficient.

Does vitamin D sufficiency equate to a single serum 25-hydroxyvitamin D level or are different levels required for non-skeletal diseases?

Objective: Clarify the concept of vitamin D sufficiency, the relationship between efficacy and vitamin D status and the role of Vitamin D supplementation in the management of non-skeletal diseases. We outline reasons for anticipating different serum vitamin D levels are required for different diseases. Method: Review the literature for evidence of efficacy of supplementation and minimum effective 25-hydroxyvitamin D (25-OHD) levels in non-skeletal disease. Results: Evidence of efficacy of vitamin supplementation is graded according to levels of evidence. Minimum effective serum 25-OHD levels are lower for skeletal disease, e.g., rickets (25 nmol/L), osteoporosis and fractures (50 nmol/L), than for premature mortality (75 nmol/L) or non-skeletal diseases, e.g., depression (75 nmol/L), diabetes and cardiovascular disease (80 nmol/L), falls and respiratory infections (95 nmol/L) and cancer (100 nmol/L). Conclusions: Evidence for the efficacy of vitamin D supplementation at serum 25-OHD levels ranging from 25 to 100 nmol/L has been obtained from trials with vitamin D interventions that change vitamin D status by increasing serum 25-OHD to a level consistent with sufficiency for that disease. This evidence supports the hypothesis that just as vitamin D metabolism is tissue dependent, so the serum levels of 25-OHD signifying deficiency or sufficiency are disease dependent.

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.

The pleiotropic effects of vitamin D in bone

A current controversial question related to vitamin D supplementation is what level of serum 25-hydroxyvitamin D3 (25(OH)D3) is required to reduce the incidence of osteoporotic fractures. The reasoning behind vitamin D supplementation has been mostly derived from the role of vitamin D to promote intestinal calcium absorption and reduce bone resorption. While minimum 25(OH)D3 levels of 20nmol/L are required for sufficient intestinal calcium absorption to prevent osteomalacia, the mechanistic details of how higher 25(OH)D3 levels, well beyond that required for optimal calcium absorption, are able to prevent fractures and increase bone mineral density is unclear. Substantial evidence has arisen over the past decade that conversion of 25(OH)D3 to 1,25(OH)2D3via the 1-alpha hydroxylase (CYP27B1) enzyme in osteoblasts, osteocytes, chondrocytes and osteoclasts regulates processes such as cell proliferation, maturation and mineralization as well as bone resorption, which are all dependent on the presence the of the vitamin D receptor (VDR). We and others have also shown that increased vitamin D activity in mature osteoblasts by increasing levels of VDR or CYP27B1 leads to improved bone mineral volume using two separate transgenic mouse models. While questions remain regarding activities of vitamin D in bone to influence the anabolic and catabolic processes, the biological importance of vitamin D activity within the bone is unquestioned. However, a clearer understanding of the varied mechanisms by which vitamin D directly and indirectly influences mineral bone status are required to support evidence-based recommendations for vitamin D supplementation to reduce the risk of fractures. This article is part of a Special Issue entitled 'Vitamin D workshop'.

Alteration of vitamin D metabolic enzyme expression and calcium transporter abundance in kidney involved in type 1 diabetes-induced bone loss

This study aimed to delineate the mechanism involved in type 1 diabetes-induced bone loss. The results revealed the alteration of vitamin D metabolic enzyme expression and the downregulation of renal calcium transporter abundance in type 1 diabetic mice.

INTRODUCTION

The purpose of this study was to investigate the changes of the expression of vitamin D metabolic enzymes and transcellular calcium-transporting proteins in kidneys from mice with experimentally induced diabetes.

METHODS

Male DBA/2J mice were injected with either vehicle (control) or streptozotocin (STZ) daily for five consecutive days. Bone mineral density was measured by peripheral quantitative computerized tomography, and bone histomorphology was analyzed by Safranin O staining. Real-time PCR and Western blotting were applied to determine the expression of target genes and proteins.

RESULTS

Type 1 diabetes produced high urinary calcium excretion and loss of trabecular bone measured at the proximal metaphysis of the tibia and the distal femur. Bone loss was associated with deterioration of trabecular bone microstructure. Quantified PCR results showed that mRNA expression level in the kidney of diabetic mice for 25-hydroxyvitamin D-24-hydroxylase was downregulated at week 10, while those for 25-hydroxyvitamin D-1α-hydroxylase were upregulated at week 20. In addition, mRNA expression levels for renal transient receptor potential V6, plasma membrane Ca-ATPase (PMCA)1b, and vitamin D receptor (VDR) genes were decreased in STZ-treated mice. Western blot analysis showed that protein expression of PMCA1b and VDR was significantly decreased in kidneys from STZ-treated mice compared to that of controls.

CONCLUSIONS

The limitation in this study is the lack of vitamin D, parathyroid hormone, and phosphorus levels in serum. However, the present study supports the conclusion that the underlying mechanism contributing to type 1 diabetes-associated bone loss may be alterations of vitamin D metabolic enzyme expression and associated decreases in expression of renal calcium transporters.

Bone CYP27B1 gene expression is increased with high dietary calcium and in mineralising osteoblasts

Although the regulation of renal 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1) is reasonably well understood, the same cannot be said about the regulation of bone CYP27B1 expression. We have compared the regulation of kidney and bone CYP27B1 expression with modulation of dietary vitamin D and calcium levels. Vitamin D-deplete and vitamin D-replete female Sprague-Dawley rats were fed either 1% Ca (HC) or 0.1% Ca (LC) diets from 6 months of age. At 9 months of age, animals were killed for mRNA analyses from kidney and bone by real-time RT-PCR. Additionally, primary bone cells were cultured from pCYP27B1-Luc reporter mice in pro-osteogenic media over 15 days and analysed for mRNA for CYP27B1 and other osteogenic markers. In vivo expression of bone CYP27B1 mRNA was independent of changes to kidney CYP27B1 levels with both serum 1,25D and PTH as negative determinants of bone CYP27B1 mRNA levels. Bone cells in pro-mineralising conditions significantly increased CYP27B1 promoter activity over 15 days (P<0.001) which preceded marked increases in alkaline phosphatase, osteocalcin and vitamin D receptor mRNA expression and mineral deposition. These findings confirm that the regulation of bone CYP27B1 is unique from that in the kidney, and may play an important role in bone formation.

Regulation of the 5'-flanking region of the human CYP27B1 gene in osteoblast cells

Synthesis of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is catalysed by the enzyme 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP27B1). Regulation of CYP27B1 gene expression is poorly understood, particularly in non-renal tissues including bone where 1,25(OH)(2)D(3) is hypothesised to serve autocrine/paracrine roles. Transient transfection of ROS 17/2.8 osteoblast-like cells with reporter gene constructs containing deletions of the 5'-flanking region of the human CYP27B1 gene revealed a proximal promoter, enhancer region and strong upstream repressive region. Putative CCAAT and GC boxes, as well as Ets protein binding sites were shown to contribute to promoter and enhancer activities respectively in common with kidney and prostate cells. Inhibition of basal expression was largely attributed to a palindrome 5'-GTCTCAGAC-3' (-1015/-1007bp) that contains two putative canonical Smad binding elements. We conclude that repression of CYP27B1 gene expression may be a common event but the novel inhibitory elements we have identified may be unique to osteoblasts.

Humanized mouse lines and their application for prediction of human drug metabolism and toxicological risk assessment

Cytochrome P450s (P450s) are important enzymes involved in the metabolism of xenobiotics, particularly clinically used drugs, and are also responsible for metabolic activation of chemical carcinogens and toxins. Many xenobiotics can activate nuclear receptors that in turn induce the expression of genes encoding xenobiotic metabolizing enzymes and drug transporters. Marked species differences in the expression and regulation of cytochromes P450 and xenobiotic nuclear receptors exist. Thus, obtaining reliable rodent models to accurately reflect human drug and carcinogen metabolism is severely limited. Humanized transgenic mice were developed in an effort to create more reliable in vivo systems to study and predict human responses to xenobiotics. Human P450s or human xenobiotic-activated nuclear receptors were introduced directly or replaced the corresponding mouse gene, thus creating "humanized" transgenic mice. Mice expressing human CYP1A1/CYP1A2, CYP2E1, CYP2D6, CYP3A4, CY3A7, pregnane X receptor, and peroxisome proliferator-activated receptor alpha were generated and characterized. These humanized mouse models offer a broad utility in the evaluation and prediction of toxicological risk that may aid in the development of safer drugs.

Co-expression of CYP27B1 enzyme with the 1.5kb CYP27B1 promoter-luciferase transgene in the mouse

The renal enzyme 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1), responsible for the synthesis of circulating. 1,25-dihydroxyvitamin D (1,25D), is also expressed in a number of non-renal tissues. The regulation of CYP27B1 expression by the short flanking promoter outside the kidney is, however, largely unknown. We have used a transgenic mice expressing the 1.5kb promoter of the human CYP27B1 gene fused to the firefly luciferase gene in order to investigate tissue-specific CYP27B1 expression. These transgenic animals demonstrated co-localised luciferase and endogenous CYP27B1 expression in kidney proximal convoluted tubular cells. Strong co-expression of luciferase and CYP27B1 also occurred in neurons and Purkinje cells of the cerebellum and in Leydig and Sertoli cells of the testes. Other tissues to exhibit CYP27B1-promoter directed luciferase activity included lung, prostate, trabecular bone and jejunum as well as the choroid epithelium. The tissue specific changes in luciferase activity were age-related. These findings demonstrate that the proximal 1.5kb 5' flanking region of the CYP27B1 gene directs the expression of CYP27B1 in a number of known and novel tissues in a specific manner.