Molecular cloning of cDNA and genomic DNA for human 25-hydroxyvitamin D3 1 alpha-hydroxylase

Enzymatic activation in vitamin D signaling - Past, present and future

Vitamin D signaling is important in regulating calcium homeostasis essential for bone health but also displays other functions in cells of several tissues. Disturbed vitamin D signaling is linked to a large number of diseases. The multiple cytochrome P450 (CYP) enzymes catalyzing the different hydroxylations in bioactivation of vitamin D₃ are crucial for vitamin D signaling and function. This review is focused on the progress achieved in identification of the bioactivating enzymes and their genes in production of 1α,25-dihydroxyvitamin D₃ and other active metabolites. Results obtained on species- and tissue-specific expression, catalytic reactions, substrate specificity, enzyme kinetics, and consequences of gene mutations are evaluated. Matters of incomplete understanding regarding the physiological roles of some vitamin D hydroxylases are critically discussed and the authors will give their view of the importance of each enzyme for vitamin D signaling. Roles of different vitamin D receptors and an alternative bioactivation pathway, leading to 20-hydroxylated vitamin D₃ metabolites, are also discussed. Considerable progress has been achieved in knowledge of the vitamin D₃ bioactivating enzymes. Nevertheless, several intriguing areas deserve further attention to understand the pleiotropic and diverse activities elicited by vitamin D signaling and the mechanisms of enzymatic activation necessary for vitamin D-induced responses.

Physiological regulation of calcium and phosphorus utilization in laying hens

Commercial laying hens can produce one egg approximately every 24 h. During this process, regulatory systems that control vitamin D3 metabolism, calcium and phosphorus homeostasis, and intestinal uptake of these minerals work in concert to deliver components required for eggshell calcification and bone mineralization. Commercial production cycles have been extended in recent years to last through 100 weeks of age, and older hens often exhibit an increased prevalence of skeletal fractures and poor eggshell quality. Issues such as these arise, in part, through imbalances that occur in calcium and phosphorus utilization as hens age. As a result, an in-depth understanding of the mechanisms that drive calcium and phosphorus uptake and utilization is required to develop solutions to these welfare and economic challenges. This paper reviews factors that influence calcium and phosphorus homeostasis in laying hens, including eggshell formation and development and roles of cortical and medullary bone. Metabolism and actions of vitamin D3 and physiological regulation of calcium and phosphorus homeostasis in key tissues are also discussed. Areas that require further research in avian species, such as the role of fibroblast growth factor 23 in these processes and the metabolism and action of bioactive vitamin D3, are highlighted and the importance of using emerging technologies and establishing in vitro systems to perform functional and mechanistic studies is emphasized.

Effect of Vitamin D on Graft-versus-Host Disease

The different cell subsets of the immune system express the vitamin D receptor (VDR). Through the VDR, vitamin D exerts different functions that influence immune responses, as previously shown in different preclinical models. Based on this background, retrospective studies explored the impacts of vitamin D levels on the outcomes of patients undergoing allogeneic hematopoietic stem-cell transplantation, showing that vitamin D deficiency is related to an increased risk of complications, especially graft-versus-host disease. These results were confirmed in a prospective cohort trial, although further studies are required to confirm this data. In addition, the role of vitamin D on the treatment of hematologic malignancies was also explored. Considering this dual effect on both the immune systems and tumor cells of patients with hematologic malignancies, vitamin D might be useful in this setting to decrease both graft-versus-host disease and relapse rates.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

Vitamin D: Newer Concepts of Its Metabolism and Function at the Basic and Clinical Level

The interest in vitamin D continues unabated with thousands of publications contributing to a vast and growing literature each year. It is widely recognized that the vitamin D receptor (VDR) and the enzymes that metabolize vitamin D are found in many cells, not just those involved with calcium and phosphate homeostasis. In this mini review I have focused primarily on recent studies that provide new insights into vitamin D metabolism, mechanisms of action, and clinical applications. In particular, I examine how mutations in vitamin D metabolizing enzymes-and new information on their regulation-links vitamin D metabolism into areas such as metabolism and diseases outside that of the musculoskeletal system. New information regarding the mechanisms governing the function of the VDR elucidates how this molecule can be so multifunctional in a cell-specific fashion. Clinically, the difficulty in determining vitamin D sufficiency for all groups is addressed, including a discussion of whether the standard measure of vitamin D sufficiency, total 25OHD (25 hydroxyvitamin) levels, may not be the best measure-at least by itself. Finally, several recent large clinical trials exploring the role of vitamin D supplementation in nonskeletal diseases are briefly reviewed, with an eye toward what questions they answered and what new questions they raised.

Genetic and Clinical Characteristics of Patients with Vitamin D Dependent Rickets Type 1A

OBJECTIVE

Vitamin D dependent rickets type 1A (VDDR1A) is an autosomal recessive disorder caused by mutations in the 1α-hydroxylase gene (CYB27B1). As it may be confused with nutritional rickets and hypophosphatemic rickets, genetic analysis is important for making a correct diagnosis.

METHODS

We analysed genomic DNA from 11 patients from eight different Turkish families. The patients were recruited for our studies if they presented with a diagnosis of VDDR.

RESULTS

The mean ± standard deviation age at diagnosis was 13.1±7.4 months. Seven patients had mild hypocalcemia at presentation while four patients had normal calcium concentrations. All patients underwent CYP27B1 gene analysis. The most prevalent mutation was the c.195 + 2T>G splice donor site mutation, affecting five out of 11 patients with VDDR1A. Two patients from the fourth family were compound heterozygous for c.195 + 2T>G and c.195 + 2 T>A in intron-1. Two patients, from different families, were homozygous for a previously reported duplication mutation in exon 8 (1319_1325dupCCCACCC, Phe443Profs*24). One patient had a homozygous splice site mutation in intron 7 (c.1215 + 2 T>A) and one patient had a homozygous mutation in exon 9 (c.1474 C>T).

CONCLUSION

Intron-1 mutation was the most common mutation, as previously reported. All patients carrying that mutation were from same city of origin suggesting a “founder” or a “common ancestor” effect. VDDR1A should definitely be considered when a patient with signs of rickets has a normal 25-OHD level or when there is unresponsiveness to vitamin D treatment.

Generation of 1,25-dihydroxyvitamin D3 in Cyp27b1 knockout mice by treatment with 25-hydroxyvitamin D3 rescued their rachitic phenotypes

We have reported that 25-hydroxyvitamin D₃ [25(OH)D₃] binds to vitamin D receptor and exhibits several biological functions directly in vitro. To evaluate the direct effect of 25(OH)D₃ in vivo, we used Cyp27b1 knockout (KO) mice, which had no detectable plasma 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] when fed a diet containing normal Ca and vitamin D. Daily treatment with 25(OH)D₃ at 250 μg kg^-1 day^-1 rescued rachitic phenotypes in the Cyp27b1 KO mice. Bone mineral density, female sexual cycles, and plasma levels of Ca, P, and PTH were all normalized following 25(OH)D₃ administration. An elevated Cyp24a1 mRNA expression was observed in the kidneys, and plasma concentrations of Cyp24a1-dependent metabolites of 25(OH)D₃ were increased. To our surprise, 1,25(OH)₂D₃ was detected at a normal level in the plasma of Cyp27b1 KO mice. The F1 to F4 generations of Cyp27b1 KO mice fed 25(OH)D₃ showed normal growth, normal plasma levels of Ca, P, and parathyroid hormone, and normal bone mineral density. The curative effect of 25(OH)D₃ was considered to depend on the de novo synthesis of 1,25(OH)₂D₃ in the Cyp27b1 KO mice. This suggests that another enzyme than Cyp27b1 is present for the 1,25(OH)₂D₃ synthesis. Interestingly, the liver mitochondrial fraction prepared from Cyp27b1 KO mice converted 25(OH)D₃ to 1,25(OH)₂D₃. The most probable candidate is Cyp27a1. Our findings suggest that 25(OH)D₃ may be useful for the treatment and prevention of osteoporosis for patients with chronic kidney disease.

Genetic Diseases of Vitamin D Metabolizing Enzymes

Vitamin D metabolism involves 3 highly specific cytochrome P450 (CYP) enzymes (25-hydroxylase, 1α-hydroxylase, and 24-hydroxylase) involved in the activation of vitamin D₃ to the hormonal form, 1,25-(OH)₂D₃, and the inactivation of 1,25-(OH)₂D₃ to biliary excretory products. Mutations of the activating enzymes CYP2R1 and CYP27B1 cause lack of normal 1,25-(OH)₂D₃ synthesis and result in rickets whereas mutations of the inactivating enzyme CYP24A1 cause build-up of excess 1,25-(OH)₂D₃ and result in hypercalcemia, nephrolithiasis, and nephrocalcinosis. This article reviews the literature for 3 clinical conditions. Symptoms, diagnosis, treatment, and management of vitamin D-dependent rickets and idiopathic infantile hypercalcemia are discussed.

Down-regulation of CYP27B1 gene expression in Iranian patients with relapsing-remitting multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) as a complex neurological disease can be due to vitamin D deficiency. CYP27B1 is referred to as a vitamin D metabolizing enzyme.

MATERIALS AND METHODS

This study compared the expression level of CYP27B1 in Relapsing-Remitting MS (RRMS) patients with normal individuals in Iran. The RNA was extracted from 50 RRMS patients and 50 normal controls. Quantitative RT-PCR was adopted to measure the expression level of CYP27B1 gene.

RESULTS

The expression level of CYP27B1gene was significantly lower in the RRMS patients than their normal counterparts (P value = 0.04). Also, the RRMS females participating had a significant reduction in CYP27B1 gene expression compared to normal females (P-Value = 0.01). In addition, the correlation between CYP27B1 expression level, and the risk of Expanded Disability Status Scale of Kurtzke (EDSS) was not linear. Additionally, there was no significant correlation between expression status of CYP27B1gene and duration of the disease.

CONCLUSION

A significant decrease in the expression level of CYP27A1 in female patients could indicate their greater vulnerability to MS than the male patients.

Genetic disorders of Vitamin D biosynthesis and degradation

Vitamin D, an inactive secosteroid pro-hormone, is produced by the action of ultraviolet light on 7-dehydrocholesterol in the skin. The active hormone, 1,25(OH)₂D is produced by sequential 25-hydroxylation in the liver, principally by CYP2R1, and 1α-hydroxylation in the kidney by CYP27B1. Mutations in CYP27B1 cause 1α-hydroxylase deficiency, also known as vitamin D dependent rickets type I or hereditary pseudo-vitamin D deficient rickets; very rare mutations in CYP2R1 can cause 25-hydroxylase deficiency. Both deficiencies cause hypocalcemia, secondary hyperparathyroidism, severe rickets in infancy, and low serum concentrations of 1,25(OH)₂D; both disorders respond to hormonal replacement therapy with calcitriol. The inactivation of vitamin D is principally initiated by its 23- and 24-hydroxylation by CYP24A1. Mutations in CYP24A1 can cause both severe neonatal hypercalcemia and a less severe adult hypercalcemic syndrome. Other pathways of vitamin D metabolism are under investigation, notably its 20-hydroxylation by the cholesterol side-chain cleavage enzyme, CYP11A1.

Vitamin D-Mediated Hypercalcemia: Mechanisms, Diagnosis, and Treatment

Hypercalcemia occurs in up to 4% of the population in association with malignancy, primary hyperparathyroidism, ingestion of excessive calcium and/or vitamin D, ectopic production of 1,25-dihydroxyvitamin D [1,25(OH)₂D], and impaired degradation of 1,25(OH)₂D. The ingestion of excessive amounts of vitamin D₃ (or vitamin D₂) results in hypercalcemia and hypercalciuria due to the formation of supraphysiological amounts of 25-hydroxyvitamin D [25(OH)D] that bind to the vitamin D receptor, albeit with lower affinity than the active form of the vitamin, 1,25(OH)₂D, and the formation of 5,6-trans 25(OH)D, which binds to the vitamin D receptor more tightly than 25(OH)D. In patients with granulomatous disease such as sarcoidosis or tuberculosis and tumors such as lymphomas, hypercalcemia occurs as a result of the activity of ectopic 25(OH)D-1-hydroxylase (CYP27B1) expressed in macrophages or tumor cells and the formation of excessive amounts of 1,25(OH)₂D. Recent work has identified a novel cause of non-PTH-mediated hypercalcemia that occurs when the degradation of 1,25(OH)₂D is impaired as a result of mutations of the 1,25(OH)₂D-24-hydroxylase cytochrome P450 (CYP24A1). Patients with biallelic and, in some instances, monoallelic mutations of the CYP24A1 gene have elevated serum calcium concentrations associated with elevated serum 1,25(OH)₂D, suppressed PTH concentrations, hypercalciuria, nephrocalcinosis, nephrolithiasis, and on occasion, reduced bone density. Of interest, first-time calcium renal stone formers have elevated 1,25(OH)₂D and evidence of impaired 24-hydroxylase-mediated 1,25(OH)₂D degradation. We will describe the biochemical processes associated with the synthesis and degradation of various vitamin D metabolites, the clinical features of the vitamin D-mediated hypercalcemia, their biochemical diagnosis, and treatment.

Vitamin D-Dependent Rickets Type 1 Caused by Mutations in CYP27B1 Affecting Protein Interactions With Adrenodoxin

CONTEXT

CYP27B1 converts 25-hydroxyvitamin D3 to active 1,25-dihydroxyvitamin D3, playing a vital role in calcium homeostasis and bone growth. Vitamin D-dependent rickets type 1 (VDDR-1) is a rare autosomal recessive disorder caused by mutations in CYP27B1.

OBJECTIVE

The objective of the study was an enzymatic and structural analysis of mutations in a patient with calcipenic rickets. Design, Setting, Patient, and Intervention: Two siblings presented with calcipenic rickets and normal 1,25-dihydroxyvitamin D3 levels. CYP27B1 gene analysis showed compound heterozygous mutations confirming VDDR-1. We studied wild-type CYP27B1 and mutations H441Y and R459L by computational homology modeling, molecular dynamics simulations, and functional studies using a luciferase assay. The patients were successfully treated with calcitriol.

MAIN OUTCOME

The main outcomes of the study were novel mutations leading to a severe loss of CYP27B1 activities for metabolism of 25-hydroxyvitamin D3.

RESULTS

Mitochondrial cytochrome P450s require adrenodoxin (FDX1) and adrenodoxin reductase. We created models of CYP27B1-FDX1 complex, which revealed negative effects of mutations H441Y and R459L. Upon structural analysis, near-identical folds, protein contact areas, and orientations of heme/iron-sulfur cluster suggested that both mutations may destabilize the CYP27B1-FDX1 complex by negating directional interactions with adrenodoxin. This system is highly sensitive to small local changes modulating the binding/dissociation of adrenodoxin, and electron-transporting efficiency might change with mutations at the surface. Functional assays confirmed this hypothesis and showed severe loss of activity of CYP27B1 by both mutations.

CONCLUSIONS

This is the first report of mutations in CYP27B1 causing VDDR-1 by affecting protein-protein interactions with FDX1 that results in reduced CYP27B1 activities. Detailed characterization of mutations in CYP27B1 is required for understanding the novel molecular mechanisms causing VDDR-1.

Long-term clinical outcome and the identification of homozygous CYP27B1 gene mutations in a patient with vitamin D hydroxylation-deficient rickets type 1A

Vitamin D hydroxylation-deficient rickets type 1A (VDDR1A) is an autosomal recessively-inherited disorder caused by mutations in CYP27B1 encoding the 1α-hydroxylase enzyme. We report on a female patient with VDDR1A who presented with hypocalcemic seizure at the age of 13 months. The typical clinical and biochemical features of VDDR1A were found, such as hypocalcemia, increased alkaline phosphatase, secondary hyperparathyroidism and normal 25-hydroxyvitamin D3 (25(OH)D₃). Radiographic images of the wrist showed metaphyseal widening with cupping and fraying of the ulna and distal radius, suggesting rickets. A mutation analysis of the CYP27B1 gene identified a homozygous mutation of c.589+1G>A in the splice donor site in intron 3, which was known to be pathogenic. Since that time, the patient has been under calcitriol and calcium treatment, with normal growth and development. During the follow-up period, she did not develop genu valgum, scoliosis, or nephrocalcinosis.

Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects

1,25-Dihydroxvitamin D3 [1,25(OH)2D3] is the hormonally active form of vitamin D. The genomic mechanism of 1,25(OH)2D3 action involves the direct binding of the 1,25(OH)2D3 activated vitamin D receptor/retinoic X receptor (VDR/RXR) heterodimeric complex to specific DNA sequences. Numerous VDR co-regulatory proteins have been identified, and genome-wide studies have shown that the actions of 1,25(OH)2D3 involve regulation of gene activity at a range of locations many kilobases from the transcription start site. The structure of the liganded VDR/RXR complex was recently characterized using cryoelectron microscopy, X-ray scattering, and hydrogen deuterium exchange. These recent technological advances will result in a more complete understanding of VDR coactivator interactions, thus facilitating cell and gene specific clinical applications. Although the identification of mechanisms mediating VDR-regulated transcription has been one focus of recent research in the field, other topics of fundamental importance include the identification and functional significance of proteins involved in the metabolism of vitamin D. CYP2R1 has been identified as the most important 25-hydroxylase, and a critical role for CYP24A1 in humans was noted in studies showing that inactivating mutations in CYP24A1 are a probable cause of idiopathic infantile hypercalcemia. In addition, studies using knockout and transgenic mice have provided new insight on the physiological role of vitamin D in classical target tissues as well as evidence of extraskeletal effects of 1,25(OH)2D3 including inhibition of cancer progression, effects on the cardiovascular system, and immunomodulatory effects in certain autoimmune diseases. Some of the mechanistic findings in mouse models have also been observed in humans. The identification of similar pathways in humans could lead to the development of new therapies to prevent and treat disease.

Interactions of vitamin D and the proximal tubule

Severe vitamin D deficiency (reduction in serum 25(OH)D concentration) in infants and children can cause features of the Fanconi syndrome, including phosphaturia, glycosuria, aminoaciduria, and renal tubular acidosis. This indicates that vitamin D and its metabolites influence proximal tubule function. Filtered 25(OH)D bound to vitamin D binding protein (DBP) is endocytosed by megalin-cubilin in the apical membrane. Intracellular 25(OH)D is metabolized to 1,25(OH)2D or calcitroic acid by 1-α-hydroxylase or 24-hydroxylase in tubule cell mitochondria. Bone-produced fibroblast growth factor 23 (FGF23) bound to Klotho in tubule cells and intracellular phosphate concentrations are regulators of 1-α-hydroxylase activity and cause proximal tubule phosphaturia. Aminoaciduria occurs when amino acid transporter synthesis is deficient, and 1,25(OH)2D along with retinoic acid up-regulate transporter synthesis by a vitamin D response element in the promoter region of the transporter gene. This review discusses evidence gained from studies in animals or cell lines, as well as from human disorders, that provide insight into vitamin D-proximal tubule interactions.

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'.

Hypercalcemia associated with mineral oil-induced sclerosing paraffinomas

OBJECTIVE

Granuloma-forming diseases such as sarcoidosis are associated with extrarenal synthesis of 1,25-dihydroxyvitamin D [1,25(OH)₂D]. We describe a case of extensive skin lesions associated with mineral oil injections in which we provide evidence for cutaneous granuloma synthesis of 1,25(OH)₂D in the pathogenesis of the patient's hypercalcemia.

METHODS

Analysis of expression of the 25(OH)D-1a-hydroxylase (1-a OHase [CYP27b1]) was carried out by immunohistochemical analysis of involved skin.

RESULTS

In involved skin, expression of CYP27b1 was found in the dermis, where it is not normally expressed. Successful management of hypercalcemia was achieved with glucocorticoids.

CONCLUSIONS

Hypercalcemia associated with mineral oil induced skin lesions is likely driven by unregulated expression of CYP27b1 by inflammatory monocytes and macrophages infiltrating the dermis.

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.

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.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

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.

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.

Vitamin D metabolism in the kidney: regulation by phosphorus and fibroblast growth factor 23

1,25-Dihydroxyvitamin D (1,25(OH)(2)D) plays a critical role in calcium and phosphorus (Pi) metabolism, bone growth, and tissue differentiation. The synthesis of 1,25(OH)(2)D in the proximal renal tubule is the primary determinant of its circulating concentration and is mediated by the mitochondrial enzyme, 25-hydroxyvitamin D-1α-hydroxylase, CYP27B1). Enzyme activity in the kidney is tightly regulated by several factors, of which Pi and fibroblast growth factor 23 (FGF-23) are important determinants. In healthy human subjects and experimental animals, dietary Pi restriction and resultant hypophosphatemia stimulate renal 1,25(OH)(2)D production by transcriptional up regulation of the 1α-hydroxylase gene, and this effect is independent of serum concentrations of PTH. Dietary Pi intake and serum Pi concentration also are important determinants of the circulating concentration of FGF-23, itself a potent regulator of Pi and vitamin D metabolism. In several inherited human hypophosphatemic diseases, including X-linked hypophosphatemia, serum FGF-23 concentrations are increased, resulting in renal Pi wasting, hypophosphatemia, inappropriately low serum concentrations of 1,25(OH)(2)D, and growth retardation and rickets in children. Experimental studies demonstrate that direct administration of recombinant FGF-23 or its over-expression in mice induces a dose-dependent decrease in renal CYP27B1 mRNA expression, an increase in renal 24-hydroxylase mRNA expression, and a consequent decrease in serum 1,25(OH)(2)D concentrations. Studies in vitro and in vivo demonstrate that activation of MEK/ERK1/2 signaling in the kidney is necessary for the suppression of CYP27B1 gene expression by FGF-23. Thus, phosphorus and FGF-23 are important physiologic determinants of the renal metabolism of 1,25(OH)(2)D.

Mutation prediction by PolyPhen or functional assay, a detailed comparison of CYP27B1 missense mutations

Vitamin D-dependent rickets type 1 (VDDR-I) is caused by mutation in CYP27B1. The glycine residue at codon 102 is not conserved between human (G(102)) and rodent (S(102)). G102E mutation results in 80% reduction in its enzymatic activity but PolyPhen predicts benign change. It is not known whether G102S has any damaging effect on 1α-hydroxylase activity. We investigated the effect of CYP27B1 (G102S) on its enzymatic activity and compared mutation prediction accuracy for all known CYP27B1 mutations among three free online protein prediction programs: PolyPhen, PolyPhen-2, and PSIPRED. G102S has no damaging effect on 1α-hydroxylase activity. G102D retained 30% enzymatic activity. All three programs correctly predicted damaging change for G102D. PolyPhen predicted benign change for G102S, whereas PolyPhen-2 and PSIPRED indicated possible damaging effect. Among 24 reported damaging mutations, PSIPRED, PolyPhen-2, and PolyPhen achieved 100%, 91.7% (22/24), and 75% (18/24) accuracy rate, respectively. The residues of incorrectly predicted mutations were not conserved. We conclude that G102D resulted in a significant reduction in 1α-hydroxylase activity, whereas G102S did not. PSIPRED and PolyPhen-2 are superior to PolyPhen in predicting damaging mutations.

Vitamin D dependent rickets type I

Vitamin D is present in two forms, ergocalciferol (vitamin D₂) produced by plants and cholecalciferol (vitamin D₃) produced by animal tissues or by the action of ultraviolet light on 7-dehydrocholesterol in human skin. Both forms of vitamin D are biologically inactive pro-hormones that must undergo sequential hydroxylations in the liver and the kidney before they can bind to and activate the vitamin D receptor. The hormonally active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)₂D], plays an essential role in calcium and phosphate metabolism, bone growth, and cellular differentiation. Renal synthesis of 1,25(OH)₂D from its endogenous precursor, 25-hydroxyvitamin D (25OHD), is the rate-limiting and is catalyzed by the 1α-hydroxylase. Vitamin D dependent rickets type I (VDDR-I), also referred to as vitamin D 1α-hydroxylase deficiency or pseudovitamin D deficiency rickets, is an autosomal recessive disorder characterized clinically by hypotonia, muscle weakness, growth failure, hypocalcemic seizures in early infancy, and radiographic findings of rickets. Characteristic laboratory features are hypocalcemia, increased serum concentrations of parathyroid hormone (PTH), and low or undetectable serum concentrations of 1,25(OH)₂D despite normal or increased concentrations of 25OHD. Recent advances have showed in the cloning of the human 1α-hydroxylase and revealed mutations in its gene that cause VDDR-I. This review presents the biology of vitamin D, and 1α-hydroxylase mutations with clinical findings.

Epigenetic reduction in invariant NKT cells following in utero vitamin D deficiency in mice

Vitamin D status changes with season, but the effect of these changes on immune function is not clear. In this study, we show that in utero vitamin D deficiency in mice results in a significant reduction in invariant NKT (iNKT) cell numbers that could not be corrected by later intervention with vitamin D or 1,25-dihydroxy vitamin D(3) (active form of the vitamin). Furthermore, this was intrinsic to hematopoietic cells, as vitamin D-deficient bone marrow is specifically defective in generating iNKT cells in wild-type recipients. This vitamin D deficiency-induced reduction in iNKT cells is due to increased apoptosis of early iNKT cell precursors in the thymus. Whereas both the vitamin D receptor and vitamin D regulate iNKT cells, the vitamin D receptor is required for both iNKT cell function and number, and vitamin D (the ligand) only controls the number of iNKT cells. Given the importance of proper iNKT cell function in health and disease, this prenatal requirement for vitamin D suggests that in humans, the amount of vitamin D available in the environment during prenatal development may dictate the number of iNKT cells and potential risk of autoimmunity.

A novel G102E mutation of CYP27B1 in a large family with vitamin D-dependent rickets type 1

CONTEXT

Mutations in the CYP27B1 gene, which encodes vitamin D 1alpha-hydroxylase, are the genetic basis for vitamin D-dependent rickets type 1 (VDDR-I).

OBJECTIVE

The aim of this study was to investigate the CYP27B1 mutation in a large family with VDDR-I and characterize the genotype-phenotype correlation.

PATIENTS AND METHODS

The index patient was a 23-yr-old female who had a progressive form of rickets and growth retardation since the age of 9 months. Laboratory data showed hypocalcemia, low urine calcium, hypophosphatemia, high serum alkaline phosphatase, elevated PTH, and low serum 1,25-dihydroxyvitamin D(3). Her parents were healthy first-degree cousins, and two of her 12 siblings were affected with similar but milder rickets. Three other siblings were asymptomatic but had biochemical evidence of the disease. The entire coding region of the CYP27B1 gene was sequenced, and the mutation was characterized by functional studies.

RESULTS

We found a novel biallelic c.305G>A sequence variation at codon 102, changing amino acid from glycine to glutamic acid (G102E) in the patient and five affected siblings, whereas a monoallelic c.305G>A variation was present in the mother and five nonaffected siblings. This variation was not present in 100 population controls. Expression of this mutant in CHO cells revealed an 80% reduction in the 1alpha-hydroxylase activity as compared to wild-type activity.

CONCLUSIONS

A novel mutation in the CYP27B1 gene was found in patients with VDDR-I. This mutation resulted in a significant reduction in 1alpha-hydroxylase activity. The residual enzymatic activity may account for the mild phenotype presentation in some affected members.

Bioconversion of vitamin D to its active form by bacterial or mammalian cytochrome P450

Bioconversion processes, including specific hydroxylations, promise to be useful for practical applications because chemical syntheses often involve complex procedures. One of the successful applications of P450 reactions is the bioconversion of vitamin D₃ to 1α,25-dihydroxyvitamin D₃. Recently, a cytochrome P450 gene encoding a vitamin D hydroxylase from the CYP107 family was cloned from Pseudonocardia autotrophica and is now applied in the bioconversion process that produces 1α,25-dihydroxyvitamin D₃. In addition, the directed evolution study of CYP107 has significantly enhanced its activity. On the other hand, we found that Streptomyces griseolus CYP105A1 can convert vitamin D₃ to 1α,25-dihydroxyvitamin D₃. Site-directed mutagenesis of CYP105A1 based on its crystal structure dramatically enhanced its activity. To date, multiple vitamin D hydroxylases have been found in bacteria, fungi, and mammals, suggesting that vitamin D is a popular substrate of the enzymes belonging to the P450 superfamily. A combination of these cytochrome P450s would produce a large number of compounds from vitamin D and its analogs. Therefore, we believe that the bioconversion of vitamin D and its analogs is one of the most promising P450 reactions in terms of practical application.

Genetic disorders and defects in vitamin d action

Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D, the circulating hormone precursor, is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase). When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets. When the VDR is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops. Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. In this article, these 2 genetic childhood diseases, which present similarly with hypocalcemia and rickets in infancy, are discussed and compared.

Immunomodulatory effects of Vitamin D in multiple sclerosis

Although Vitamin D is best known as a modulator of calcium homeostasis, it also has immune modulating potential. A protective effect of Vitamin D on multiple sclerosis is supported by the reduced risk associated with sun exposure and use of Vitamin D supplements. Moreover, high circulating levels of Vitamin D have been associated with lower risk of multiple sclerosis. In this study, we measured 1,25 (OH)(2) Vitamin D and 25 (OH) Vitamin D levels in multiple sclerosis patients separated into different clinical subgroups according to disease status. In addition, direct effects of 1,25 (OH)(2) Vitamin D on ex vivo CD4+ T cells and myelin-peptide specific T cell lines were investigated to gain more insight into putative regulatory mechanisms in the disease pathogenesis. One hundred and thirty-two Hispanic patients with clinically definite multiple sclerosis were studied, 58 with relapsing remitting multiple sclerosis during remission, 34 during relapse and 40 primary progressive multiple sclerosis cases. Sixty healthy individuals matched with respect to place of residence, race/ethnicity, age and gender served as controls. Levels of 25(OH)D(3) and 1,25(OH)(2)D(3), measured by ELISA were significantly lower in relapsing-remitting patients than in controls. In addition, levels in patients suffering relapse were lower than during remissions. In contrast, primary progressive patients showed similar values to controls. Proliferation of both freshly isolated CD4+ T cells and MBP-specific T cells was significantly inhibited by 1,25(OH)(2)D(3). Moreover, activated Vitamin D enhanced the development of IL-10 producing cells, and reduced the number of IL-6 and IL-17 secreting cells. Notably, Vitamin D receptor expression was induced by 1,25(OH)(2)D(3) in both activated and resting cells. Interestingly, T cells were able to metabolize 25(OH)D(3) into biologically active 1,25(OH)(2)D(3), since T cells express alpha1-hydroxylase constitutively. Finally, 1,25(OH)(2)D(3) also increased the expression and biological activity of indoleamine 2,3-dioxygenase, mediating significant increase in the number of CD4+CD25+ T regulatory cells. Collectively, these data suggest that 1,25(OH)(2)D(3) plays an important role in T cell homeostasis during the course of multiple sclerosis, thus making correction of its deficiency may be useful during treatment of the disease.

Inherited disorders of calcium homeostasis

In mammals a complicated homeostatic mechanism has evolved to maintain near consistency of extracellular calcium ion levels. The homeostatic mechanism involves several hormones, which comprise among others, parathyroid hormone and vitamin D. The recent resurge in vitamin D deficiency, as a global health issue, has increased interest in the hormone. In addition to vitamin D deficiency, other causes of rickets are calcium deficiency and inherited disorders of vitamin D and phosphorus metabolism. Vitamin D-resistant syndromes are caused by hereditary defects in metabolic activation of the hormone or by mutations in the vitamin D receptor, which binds the hormone with high affinity and regulates the expression of genes through zinc finger mediated DNA binding and protein-protein interaction. Current interest is to correlate the type/position of mutations that result in disorders of vitamin D metabolism or in vitamin D receptor function with the variable phenotypic features and clinical presentation. The calcium sensing receptor plays a key role in calcium homeostasis. Loss of function mutations in the calcium sensing receptor can cause familial benign hypocalciuric hypercalcemia in heterozygotes and neonatal severe hyperparathyroidism when homozygous mutations occur in the calcium sensing receptor. Gain of function mutation can cause the opposite effect causing autosomal dominant hypocalcemia. Mouse models using targeted gene disruption strategies have been valuable tools to study the effect of mutations on the calcium sensing receptor or in the vitamin D activation pathway. Dysfunctional calcium sensing receptors with function altering mutations may be responsive to treatment with allosteric modulators of the calcium sensing receptor. Vitamin D analogs which induce unusual structural conformations on the vitamin D receptor may have a variety of therapeutic indications. This review summarises recent advances in knowledge of the molecular pathology of inherited disorders of calcium homeostasis.

Vitamin D 1alpha-hydroxylase gene mutations in patients with 1alpha-hydroxylase deficiency

CONTEXT

Vitamin D 1alpha-hydroxylase deficiency, also known as vitamin D-dependent rickets type 1, is an autosomal recessive disorder characterized by the early onset of rickets with hypocalcemia and is caused by mutations of the 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha-hydroxylase, CYP27B1) gene. The human gene encoding the 1alpha-hydroxylase is 5 kb in length, located on chromosome 12, and comprises nine exons and eight introns. We previously isolated the human 1alpha-hydroxylase cDNA and gene and identified 19 different mutations in 25 patients with 1alpha-hydroxylase deficiency. OBJECTIVES, PATIENTS, AND METHODS: We analyzed the 1alpha-hydroxylase gene of 10 patients, five from Korea, two from the United States, and one each from Argentina, Denmark, and Morocco, all from nonconsanguineous families. Each had clinical and radiographic features of rickets, hypocalcemia, and low serum concentrations of 1,25-dihydroxyvitamin D(3).

RESULTS

Direct sequencing identified the responsible 1alpha-hydroxylase gene mutations in 19 of 20 alleles. Four novel and four known mutations were identified. The new mutations included a nonsense mutation in exon 6, substitution of adenine for guanine (2561G-->A) creating a stop signal at codon 328; deletion of adenine in exon 9 (3922delA) causing a frameshift; substitution of thymine for cytosine in exon 2 (1031C-->T) causing the amino acid change P112L; and a splice site mutation, substitution of adenine for guanine in the first nucleotide of intron 7 (IVS7+1 G-->A) causing a frameshift.

CONCLUSIONS

Mutations in the 1alpha-hydroxylase gene previously were identified in 44 patients, to which we add 10 more. The studies show a strong correlation between 1alpha-hydroxylase mutations and the clinical findings of 1alpha-hydroxylase deficiency.

Extra-renal 25-hydroxyvitamin D3-1alpha-hydroxylase in human health and disease

Although ectopic expression of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-OHase) has been recognized for many years, the precise function of this enzyme outside the kidney remains open to debate. Three specific aspects of extra-renal 1alpha-OHase have attracted most attention: (i) expression and regulation in non-classical tissues during normal physiology; (ii) effects on the immune system and inflammatory disease; (iii) expression and function in tumors. The most well-recognized manifestation of extra-renal 1alpha-OHase activity remains that found in some patients with granulomatous diseases where locally synthesized 1alpha,25(OH)(2)D(3) has the potential to spill-over into the general circulation. However, immunohistochemistry and mRNA analyses suggest that 1alpha-OHase is also expressed by a variety of normal human tissues including the gastrointestinal tract, skin, vasculature and placenta. This has promoted the idea that autocrine/paracrine synthesis of 1,25(OH)(2)D(3) contributes to normal physiology, particularly in mediating the potent effects of vitamin D on innate (macrophage) and acquired (dendritic cell) immunity. We have assessed the capacity for synthesis of 1,25(OH)(2)D(3) in these cells and the functional significance of autocrine responses to 1alpha-hydroxylase. Data suggest that local synthesis of 1,25(OH)(2)D(3) may be a preferred mode of response to antigenic challenge in many tissues.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Heterologous expression, purification, and properties of human cytochrome P450 27C1

Cytochrome P450 (P450) 27C1 is one of the "orphan" P450 enzymes without a known biological function. A human P450 27C1 cDNA with a nucleotide sequence modified for Escherichia coli usage was prepared and modified at the N-terminus, based on the expected mitochondrial localization. A derivative with residues 3-60 deleted was expressed at a level of 1350nmol/L E. coli culture and had the characteristic P450 spectra. The identity of the expressed protein was confirmed by mass spectrometry of proteolytic fragments. The purified P450 was in the low-spin iron state, and the spin equilibrium was not perturbed by any of the potential substrates vitamin D(3), 1alpha- or 25-hydroxy vitamin D(3), or cholesterol. P450s 27A1 and 27B1 are known to catalyze the 25-hydroxylation of vitamin D(3) and the 1alpha-hydroxylation of 25-hydroxy vitamin D(3), respectively. In the presence of recombinant human adrenodoxin and adrenodoxin reductase, recombinant P450 27C1 did not catalyze the oxidation of vitamin D(3), 1alpha- or 25-hydroxy vitamin D(3), or cholesterol at detectable rates. P450 27C1 mRNA was determined to be expressed in liver, kidney, pancreas, and several other human tissues.

CYP27B1 null mice with LacZreporter gene display no 25-hydroxyvitamin D3-1alpha-hydroxylase promoter activity in the skin

The hormonally active form of vitamin D(3),1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], is synthesized in the kidney through a tightly regulated reaction catalyzed by 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase), the product of the CYP27B1 gene. Through gene targeting in embryonic stem cells, we engineered a mouse strain in which the coding region of the 1alpha-hydroxylase gene is replaced by the genes for beta-galactosidase (lacZ) and neomycin resistance. Null mice produced no detectable 1alpha-hydroxylase transcript. The mice grew normally when maintained on a balanced diet containing 1,25(OH)(2)D(3) but rapidly developed rickets when phosphorus and 1,25(OH)(2)D(3) were restricted. Rickets was curable through administration of 1,25(OH)(2)D(3) but not its biological precursor, 25-hydroxyvitamin D(3). Upon administration of a diet low in calcium and devoid of any form of vitamin D(3), beta-galactosidase activity was detected in the kidneys of the -/- and +/- mice and in placentas harvested from -/- females bred with -/- males. No beta-galactosidase activity was detected in skin sections or in primary keratinocyte cultures from -/- animals. Our results demonstrate we have generated 1alpha-hydroxylase null mice that display phenotypes characteristic of vitamin D-dependency rickets type I. From the histochemical analysis of reporter gene expression in these mice, we conclude that acute 1,25(OH)(2)D(3) deficiency in otherwise healthy animals does not stimulate local production of 1,25(OH)(2)D(3) in the skin. These findings stand in contrast to previously published reports of 1,25(OH)(2)D(3) production in keratinocytes.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

25‐Hydroxyvitamin D3is an active hormone in human primary prostatic stromal cells

According to the present paradigm, 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] is a biologically active hormone; whereas 25-hydroxyvitamin D3 (25OHD3) is regarded as a prohormone activated through the action of 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-hydroxylase). Although the role of vitamin D3 in the regulation of growth and differentiation of prostatic epithelial cells has been well studied, its action and metabolism in prostatic stroma are still largely unknown. We investigated the effects of 25OHD3 and 1alpha,25-(OH)2D3 on two human stromal primary cultures termed P29SN and P32S. In a cell proliferation assay, 25OHD3 was found at physiological concentrations of 100-250 nM to inhibit the growth of both primary cultures, whereas 1alpha,25-(OH)2D3 at a pharmacological concentration of 10 nM exhibited the growth-inhibitory effects on P29SN cells but not on P32S cells. Quantitative real-time RT-PCR analysis revealed that both 25OHD3 and 1alpha,25-(OH)2D3 induced 25-hydroxyvitamin D3 24-hydroxylase (24-hydroxylase) mRNA in a dose- and time-dependent manner. By inhibiting 1alpha-hydroxylase and/or 24-hydroxylase enzyme activities, the induction of 24-hydroxylase mRNA by 250 nM 25OHD3 was clearly enhanced, suggesting that 1alpha-hydroxylation is not a prerequisite for the hormonal activity of 25OHD3. Altogether our results suggest that 25OHD3 at a high but physiological concentration acts as an active hormone with respect to vitamin D3 responsive gene regulation and suppression of cell proliferation.

Lack of mutations in CYP2D6 and CYP27 in patients with apparent deficiency of vitamin D 25-hydroxylase

Activation of vitamin D requires hepatic 25-hydroxylation and renal 1alpha-hydroxylation. Defects in renal P450c1alpha are well-described, but few patients with defective vitamin D 25-hydroxylation are reported. The cytochrome P450 enzymes CYP2D6 and CYP27 are potential 25-hydroxylases. We sequenced both genes in two reported families with hepatic 25-hydroxylase deficiency and found no mutations. 25-Hydroxylation occurs in both mitochondria and microsomes. The existence genes encoding distinct enzymes would provide genetic redundancy, explaining the rarity of apparent vitamin D 25-hydroxylase deficiency.

Adaptive responses of 25-hydroxyvitamin D3 1-alpha hydroxylase expression to dietary phosphate restriction in young and adult rats

Regulation of vitamin D metabolism alters with age. The present study is undertaken to investigate if the loss of renal 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) production in response to dietary phosphate (P) restriction in adult rats is due to an alteration in the renal expression of 25-hydroxyvitamin D(3) 1-alpha hydroxylase (1-OHase). Young (4-6 weeks old) and adult (12-14 weeks old) male Sprague Dawley rats were fed either normal P (NPD) or low P diet (LPD) for 0-5 days. Basal expression of 1-OHase protein was higher in adult rats. Young rats, but not adult rats, significantly increased 1-OHase protein and mRNA expressions in response to LPD in a time-dependent manner. To determine if the stability of renal 1-OHase protein changes with LPD feeding, young and adult rats fed either NPD or LPD for 5 days were injected intravenously with cycloheximide (CHX), a protein synthesis inhibitor. CHX decreased 1-OHase protein expression in young rats fed NPD. However, CHX did not alter 1-OHase protein expression in young rats fed LPD nor in adult rats fed either diet. The results indicate that the stability of renal 1-OHase protein increased with age and that LPD increased its stability only in young rats.

Metabolism of 25-hydroxyvitamin D3 by microsomal and mitochondrial vitamin D3 25-hydroxylases (CYP2D25 and CYP27A1): a novel reaction by CYP27A1

The metabolism of 25-hydroxyvitamin D(3) was studied with a crude mitochondrial cytochrome P450 extract from pig kidney and with recombinant human CYP27A1 (mitochondrial vitamin D(3) 25-hydroxylase) and porcine CYP2D25 (microsomal vitamin D(3) 25-hydroxylase). The kidney mitochondrial cytochrome P450 catalyzed the formation of 1alpha,25-dihydroxyvitamin D(3), 24,25-dihydroxyvitamin D(3) and 25,27-dihydroxyvitamin D(3). An additional metabolite that was separated from the other hydroxylated products on HPLC was also formed. The formation of this 25-hydroxyvitamin D(3) metabolite was dependent on NADPH and the mitochondrial electron transferring protein components. A monoclonal antibody directed against purified pig liver CYP27A1 immunoprecipitated the 1alpha- and 27-hydroxylase activities towards 25-hydroxyvitamin D(3) as well as the formation of the unknown metabolite. These results together with substrate inhibition experiments indicate that CYP27A1 is responsible for the formation of the unknown 25-hydroxyvitamin D(3) metabolite in kidney. Recombinant human CYP27A1 was found to convert 25-hydroxyvitamin D(3) into 1alpha,25-dihydroxyvitamin D(3), 25,27-dihydroxyvitamin D(3) and a major metabolite with the same retention time on HPLC as that formed by kidney mitochondrial cytochrome P450. Gas chromatography-mass spectrometry (GC-MS) analysis of the unknown enzymatic product revealed it to be a triol different from other known hydroxylated 25-hydroxyvitamin D(3) metabolites such as 1alpha,25-, 23,25-, 24,25-, 25,26- or 25,27-dihydroxyvitamin D(3). The product had the mass spectrometic properties expected for 4beta,25-dihydroxyvitamin D(3). Recombinant porcine CYP2D25 converted 25-hydroxyvitamin D(3) into 1alpha,25-dihydroxyvitamin D(3) and 25,26-dihydroxyvitamin D(3). It can be concluded that both CYP27A1 and CYP2D25 are able to carry out multiple hydroxylations of 25-hydroxyvitamin D(3).

Vitamin D-mediated hypercalcemia in lymphoma: evidence for hormone production by tumor-adjacent macrophages

Nearly one-half of all hypercalcemic patients with lymphoma present with inappropriately elevated circulating concentrations of the active vitamin D metabolite 1,25-dihydroxyvitamin D (1,25(OH)2D3). However, the cellular source of the vitamin D hormone in lymphomas remains unclear. To address this, we report the case of a 75-year-old man with hypercalcemia associated with raised circulating concentrations of 1,25(OH)2D3 and suppressed parathyroid hormone (PTH) levels. Positron emission tomographic (PET) and computed tomographic (CT) imaging revealed the presence of a large lymphoma that was confined to the spleen; subsequent pathological analysis showed that this was an intermediate grade B-cell lymphoma. After surgical removal of the spleen, serum calcium and 1,25(OH)2D3 levels became normalized within 24 h. Immunolocalization of the vitamin D-activating enzyme 25-hydroxyvitamin D3-1alpha-hydroxylase (la-hydroxylase) in sections of resected spleen showed that staining was negative in the lymphoma cells but positive in neighboring macrophages. This case study indicates that the hypercalcemia associated with lymphomas may be due, in some instances, to excessive extrarenal production of 1,25(OH)2D3. Furthermore, by using immunohistochemistry to assess the distribution of la-hydroxylase, we have been able to show for the first time that tissue macrophages, rather than actual tumor cells, are the most likely ectopic source of this enzyme. Based on this case study, we propose that the abnormal synthesis of 1,25(OH)2D3 associated with some lymphomas is because of paracrine regulation of tumor-associated macrophages.

Genistein and vitamin D synergistically inhibit human prostatic epithelial cell growth

We performed studies to test synergism between the growth inhibitory effects of genistein and vitamin D compounds on prostatic epithelial cells. Isobolographic analysis demonstrated that genistein, in combination with the hormonally active form of cholecalciferol, 1alpha,25-dihydroxycholecalciferol, synergistically inhibited the growth of primary human prostatic epithelial cells (HPEC) and prostate cancer cells. Synergistic growth inhibition of HPEC was also observed between genistein and the low-calcemic vitamin D compound 25-hydroxycholecalciferol. Flow cytometry with HPEC indicated that genistein induced arrest in the G(2)M phase, whereas 1alpha,25-dihydroxycholecalciferol or 25-hydroxycholecalciferol induced arrest in the G(1/0) phase of the cell cycle. Combining genistein with either vitamin D compound resulted in both G(2)M and G(1/0) arrest in HPEC. In contrast, flow cytometry of prostate cancer cells indicated that both genistein and 1alpha,25-dihydroxycholecalciferol induced a G(1/0) arrest either alone or in combination. These are the first studies that demonstrate synergism between the prostatic cell growth inhibition elicited by genistein and that elicited by vitamin D compounds.

A novel splicing defect (IVS6+1G>T) in a patient with pseudovitamin D deficiency rickets

A 15-month-old boy with severe rickets, that by clinical analysis was diagnosed as affected by hereditary pseudovitamin D deficiency rickets (PDDR), was evaluated for mutations in the 25OHD3 1alpha-hydroxylase gene. Molecular analysis showed a double heterozygous state for a novel splicing mutation in the invariant dinucleotide of the donor site of IVS6 and a 7 nucleotide insertion in the exon 8, which is common in different ethnical backgrounds.

Identification of a renal proximal tubular cell-specific enhancer in the mouse 25-hydroxyvitamin d 1alpha-hydroxylase gene

The active form of vitamin D is synthesized by 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha-hydroxylase), which is expressed predominantly in renal proximal tubular cells. To clarify the mechanism of cell-specific gene expression of this enzyme, the 5'-flanking region of the mouse 1alpha-hydroxylase gene was investigated. Investigation began with mRNA expression of 1alpha-hydroxylase in cultured cells, including LLC-PK1, NIH/3T3, HepG2, MDCK, and OK cells. Expression of 1alpha-hydroxylase mRNA was restricted in LLC-PK1 cells. Several lengths of the 5'-flanking region of 1alpha-hydroxylase gene were linked to a pGL3-basic luciferase vector and introduced into these cells. Only LLC-PK1 cells had a substantial luciferase activity. Deletion analyses revealed that luciferase activity was detected in constructs extending from the transcription initiation site to -1652 to -105 bp, whereas further deletion to -80 bp resulted in a marked decrease in activity. The region from -105 to -80 bp contained two ternary complex factor-1 (TCF-1) sites, and mutations in the proximal TCF-1 site decreased the activity. Electrophoretic mobility shift assay demonstrated binding of LLC-PK1 nuclear proteins to this region. Tests of enhancer function in LLC-PK1 cells indicated that the 26-bp fragment behaved as a classical enhancer, i.e., independently of position and orientation. Moreover, a decoy oligonucleotide corresponding to this region substantially inhibited the promoter activity of 1alpha-hydroxylase gene. This study suggests that the -105 to -80 bp element of mouse 1alpha-hydroxylase gene contains an enhancer to be necessary for renal proximal tubular cell-specific expression.

Enzymatic studies on the key enzymes of vitamin D metabolism; 1 alpha-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24)

The key enzymes of vitamin D3 metabolism, renal 25-hydroxyvitamin D3 1 alpha-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24) were expressed in Escherichia coli, and their enzymatic properties were revealed. As expected, mouse CYP27B1 and human CYP27B1 showed the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 with the Michaelis constant, Km, value of 2.7 microM. Unexpectedly, both mouse CYP27B1 and human CYP27B1 showed greater Vmax/Km values toward 24,25-dihydroxyvitamin D3 than 25-hydroxyvitamin D3, suggesting that 24, 25-dihydroxyvitamin D3 is a better substrate than 25-hydroxyvitamin D3 for both CYP27B1. Enzymatic studies on substrate specificity of CYP27B1 revealed that 25-hydroxyl group of vitamin D3 was essential for the 1 alpha-hydroxylase activity, and 24-hydroxyl group enhanced the activity, but, 23-hydroxyl group greatly reduced the activity. On rat CYP24, it was demonstrated that CYP24 catalyzed four-step monooxygenation towards 25-hydroxyvitamin D3. Furthermore, in vivo and in vitro metabolic studies on 1 alpha,25-dihydroxyvitamin D3 clearly indicated that CYP24 catalyzed six-step monooxygenation to convert 1 alpha,25-dihydroxyvitamin D3 into calcitroic acid which is known as a final metabolite of 1 alpha,25-dihydroxyvitamin D3 for excretion in bile. These results strongly suggest that CYP24 is highly responsible for the metabolism of both 25-hydroxyvitamin D3 and 1 alpha,25-dihydroxyvitamin D3. In addition, we have succeeded in the construction of mitochondrial P450 electron transport chain consisting of ADR, ADX and each of CYP27B1 and CYP24 in E. coli cells. The coexpression system with CYP27B1 might be useful as a bioreactor to produce 1 alpha,25-dihydroxyvitamin D3. In contrast, the coexpression system with CYP24 would be applied to metabolic studies of vitamin D analogs used as drugs.