Overview of regulatory cytochrome P450 enzymes of the vitamin D pathway

Vitamin D and Its Analogues: From Differences in Molecular Mechanisms to Potential Benefits of Adapted Use in the Treatment of Alzheimer’s Disease

Lifestyle habits and insufficient sunlight exposure lead to a high prevalence of vitamin D hypovitaminosis, especially in the elderly. Recent studies suggest that in central Europe more than 50% of people over 60 years are not sufficiently supplied with vitamin D. Since vitamin D hypovitaminosis is associated with many diseases, such as Alzheimer’s disease (AD), vitamin D supplementation seems to be particularly useful for this vulnerable age population. Importantly, in addition to vitamin D, several analogues are known and used for different medical purposes. These vitamin D analogues differ not only in their pharmacokinetics and binding affinity to the vitamin D receptor, but also in their potential side effects. Here, we discuss these aspects, especially those of the commonly used vitamin D analogues alfacalcidol, paricalcitol, doxercalciferol, tacalcitol, calcipotriol, and eldecalcitol. In addition to their pleiotropic effects on mechanisms relevant to AD, potential effects of vitamin D analogues on comorbidities common in the context of geriatric diseases are summarized. AD is defined as a complex neurodegenerative disease of the central nervous system and is commonly represented in the elderly population. It is usually caused by extracellular accumulation of amyloidogenic plaques, consisting of amyloid (Aβ) peptides. Furthermore, the formation of intracellular neurofibrillary tangles involving hyperphosphorylated tau proteins contributes to the pathology of AD. In conclusion, this review emphasizes the importance of an adequate vitamin D supply and discusses the specifics of administering various vitamin D analogues compared with vitamin D in geriatric patients, especially those suffering from AD.

Interplay of Vitamin D and CYP3A4 Polymorphisms in Endocrine Disorders and Cancer

Vitamin D has received considerable optimistic attention as a potentially important factor in many pathological states over the past few decades. However, the proportion of the active form of vitamin D metabolites responsible for biological activity is highly questionable in disease states due to flexible alterations in the enzymes responsible for their metabolism. For instance, CYP3A4 plays a crucial role in the biotransformation of vitamin D and other drug substances. Food-drug and/or drug-drug interactions, the disease state, genetic polymorphism, age, sex, diet, and environmental factors all influence CYP3A4 activity. Genetic polymorphisms in CYP450-encoding genes have received considerable attention in the past few decades due to their extensive impact on the pharmacokinetic and dynamic properties of drugs and endogenous substances. In this review, we focused on CYP3A4 polymorphisms and their interplay with vitamin D metabolism and summarized the role of vitamin D in calcium homeostasis, bone diseases, diabetes, cancer, other diseases, and drug substances. We also reviewed clinical observations pertaining to CYP3A4 polymorphisms among the aforementioned disease conditions. In addition, we highlighted the future perspectives of studying the pharmacogenetics of CYP3A4, which may have potential clinical significance for developing novel diagnostic genetic markers that will ascertain disease risk and progression.

A pilot-randomized, double-blind crossover trial to evaluate the pharmacokinetics of orally administered 25-hydroxyvitamin D3 and vitamin D3 in healthy adults with differing BMI and in adults with intestinal malabsorption

BACKGROUND

Obese and malabsorptive patients have difficulty increasing serum 25-hydroxyvitamin D [25(OH)D] after taking vitamin D supplementation. Since 25(OH)D is more hydrophilic than vitamin D, we hypothesized that oral 25(OH)D supplementation is more effective in increasing serum 25(OH)D concentrations in these patients.

OBJECTIVES

We aimed to investigate the pharmacokinetics of oral 25-hydroxyvitamin D3 [25(OH)D3] and oral vitamin D3 in healthy participants with differing BMI and malabsorptive patients.

METHODS

A randomized, double-blind crossover trial was performed in 6 malabsorptive patients and 10 healthy participants who were given 900 µg of either vitamin D3 or 25(OH)D3 orally followed by a pharmacokinetic study (PKS). After ≥28 d from the first dosing, each participant returned to receive the other form of vitamin D and undergo another PKS. For each PKS, serum vitamin D3 and 25(OH)D3 were measured at baseline and at 2, 4, 6, 8, and 12 h and days 1, 2, 3, 7, and 14. Pharmacokinetic parameters were calculated.

RESULTS

Data were expressed as means ± SEMs. The PKS of 900 µg vitamin D3 revealed that malabsorptive patients had 64% lower AUC than healthy participants (1177 ± 425 vs. 3258 ± 496 ng · h/mL; P < 0.05). AUCs of 900 µg 25(OH)D3 were not significantly different between the 2 groups (P = 0.540). The 10 healthy participants were ranked by BMI and categorized into higher/lower BMI groups (5/group). The PKS of 900 µg vitamin D3 showed that the higher BMI group had 53% lower AUC than the lower BMI group (2089 ± 490 vs. 4427 ± 313 ng · h/mL; P < 0.05), whereas AUCs of 900 µg 25(OH)D3 were not significantly different between the 2 groups (P = 0.500).

CONCLUSIONS

Oral 25(OH)D3 may be a good choice for managing vitamin D deficiency in malabsorption and obesity. This trial was registered at clinicaltrials.gov as (NCT03401541.

Vitamin D and its therapeutic relevance in pulmonary diseases

Vitamin D is customarily involved in maintaining bone and calcium homeostasis. However, contemporary studies have identified the implication of vitamin D in several cellular processes including cellular proliferation, differentiation, wound healing, repair and regulatory systems inclusive of host defence, immunity, and inflammation. Multiple studies have indicated corelations between low serum levels of vitamin D, perturbed pulmonary functions and enhanced incidences of inflammatory diseases. Almost all of the pulmonary diseases including acute lung injury, cystic fibrosis, asthma, COPD, Pneumonia and Tuberculosis, all are inflammatory in nature. Studies have displayed strong inter-relations with vitamin D deficiency and progression of lung disorders; however, the underlying mechanism is still unknown. Vitamin D has emerged to possess inhibiting effects on pulmonary inflammation while exaggerating innate immune defenses by strongly influencing functions of inflammatory cells including dendritic cells, monocyte/macrophages, T cells, and B cells along with structural epithelial cells. This review dissects the effects of vitamin D on the inflammatory cells and their therapeutic relevance in pulmonary diseases. Although, the data obtained is very limited and needs further corroboration but presents an exciting area of further research. This is because of its ease of supplementation and development of personalized medicine which could lead us to an effective adjunct and cost-effective method of therapeutic modality for highly fatal pulmonary diseases.

Activated vitamin D3 formulations can be safely used as concomitant medication for prevention of denosumab-induced hypocalcemia in women with postmenopausal osteoporosis

AIM

Denosumab prevents osteoporosis by potently inhibiting bone resorption, but requires oral therapy with calcium and vitamin D preparations to prevent the side effects of hypocalcemia. Generally, a combination drug containing calcium, natural vitamin D, and magnesium is used. However, if activated vitamin D has been used before the initiation of denosumab therapy, continued use of activated vitamin D is not uncommon. This study aimed to evaluate the combination vitamin D preparation, alfacalcidol, and eldecalcitol on the therapeutic effect on denosumab therapy, the preventive effect on hypocalcemia, and the effect on renal function, to determine the optimal choice of concomitant medication.

METHODS

This is a retrospective and single-center study. Among 39 patients who had used denosumab (60 mg dose) for at least 12 months between November 2013 and October 2015, those who used the combination medication concomitantly as the standard treatment, those who used alfacalcidol concomitantly, and those who used eldecalcitol concomitantly were compared.

RESULTS

Denosumab therapy markedly increased lumbar spine and femoral neck bone densities at 12 months in the three groups, showing no particular difference in the rate of increase of bone density. The three groups had marked decreases in bone metabolism markers, but had no intergroup differences. No hypocalcemia, hypercalcemia, or obvious renal dysfunction occurred over 12 months.

CONCLUSION

This study indicates that the use of activated vitamin D preparations, as concomitant medications with denosumab therapy, is appropriate considering the therapeutic efficacy of denosumab, prevention of hypocalcemia, and influence on renal function.

Dietary nitrogen and calcium modulate CYP27B1 expression in young goats

In livestock, feeding a reduced nitrogen (N) diet is favored for economic and ecological reasons. Ruminants cope more easily with a reduced N diet than monogastric species. However, changes in mineral homeostasis such as a reduction in 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃) concentrations, calcium (Ca), and IGF1 levels were observed in goats kept on a reduced N diet. The decrease in 1,25-(OH)₂D₃ occurred even during a simultaneous reduction in dietary N and Ca, whereas a solitary Ca reduction stimulated 1,25-(OH)₂D₃ synthesis. The aim of the present study was to examine the effects of N- and/or Ca-reduced diets on the expression of 24-hydroxylase (CYP24A1), 1-alpha-hydroxylase (CYP27B1), vitamin D receptor (VDR), retinoid X receptor alpha (RXRα), IGF1 receptor (IGF1R), Klotho, and fibroblast growth factor receptor 1c (FGFR1c) in kidneys of young goats. Four groups were kept on a control diet, an N-reduced diet, a Ca-reduced diet or an N- and a Ca-reduced diet. Renal expression of CYP24A1 was not affected, whereas CYP27B1 expression was significantly diminished in the N-reduced diet fed goats (P < 0.05) and significantly elevated with the Ca reduction (P < 0.001). The VDR expression was not modified, whereas RXRα (P < 0.05) and Klotho expression (P < 0.001) were stimulated during Ca reduction. The IGF1R (P < 0.05) and FGFR1c (P < 0.05) expression were enhanced with the N reduction. From these data, it can be concluded that the downregulation of renal CYP27B1 expression observed with dietary N reduction is probably mediated by a complex interaction between the somatotropic axis and the Klotho/FGF signaling pathway in young goats.

The role of vitamin D in hepatic metastases from colorectal cancer

Colorectal cancer (CRC) represents a significant health burden worldwide, comprising approximately 10% of annual cancer cases globally. Hepatic metastases are the most common site of CRC metastasis, and are the leading cause of death in CRC patients. There is strong epidemiologic evidence for an inverse association between vitamin D status and risk of CRC; however, the role of vitamin D in the natural history of liver metastases has not yet been investigated. Several researchers have proposed hallmarks of metastases; crucially, metastases can be blocked by interrupting just one rate-limiting step. Vitamin D status has been implicated in each proposed hallmark of metastasis. The aim of this review is to examine the potential role for vitamin D in reducing the development of hepatic metastases from CRC and outline the candidate mechanisms by which vitamin D may mediate these effects. The results of ongoing randomised intervention trials are eagerly awaited to determine whether addressing vitamin D insufficiency in CRC patients could reduce the occurrence of liver metastases, and the consequent morbidity and mortality.

TAZ regulates cell proliferation and sensitivity to vitamin D3 in intrahepatic cholangiocarcinoma

The transcriptional coactivator with PDZ binding motif (TAZ) is reported as one of the nuclear effectors of Hippo-related pathways. TAZ is found overexpressed in many primary tumors and could regulate many biological processes. However, little is known about the role of TAZ in Intrahepatic Cholangiocarcinoma (ICC). In this study, we found that TAZ is expressed more in ICC tissues than in peritumoral tissue, and a robust expression of TAZ is correlated with a lower overall survival rate of ICC patients after hepatectomy. TAZ knockdown results in an increase in cell apoptosis, a promotion of cell-cycle arrest and a decrease in tumor size and weight in vivo through an increased expression of p53. Vitamin D3 can also inhibit cell proliferation by promoting p53 expression in ICC cells. A reduction in TAZ can also enhance the sensitivity of tumor cells to vitamin D by regulating the p53/CYP24A1 pathway. In conclusion, TAZ is associated with the proliferation and drug-resistance of ICC cells, and could be a novel therapeutic target for the treatment of ICC.

Vitamin D Signaling Modulators in Cancer Therapy

The antiproliferative and pro-apoptotic effects of 1α,25-dihydroxycholecalciferol (1,25(OH)2D3, 1,25D3, calcitriol) have been demonstrated in various tumor model systems in vitro and in vivo. However, limited antitumor effects of 1,25D3 have been observed in clinical trials. This may be attributed to a variety of factors including overexpression of the primary 1,25D3 degrading enzyme, CYP24A1, in tumors, which would lead to rapid local inactivation of 1,25D3. An alternative strategy for improving the antitumor activity of 1,25D3 involves the combination with a selective CYP24A1 inhibitor. The validity of this approach is supported by numerous preclinical investigations, which demonstrate that CYP24A1 inhibitors suppress 1,25D3 catabolism in tumor cells and increase the effects of 1,25D3 on gene expression and cell growth. Studies are now required to determine whether selective CYP24A1 inhibitors+1,25D3 can be used safely and effectively in patients. CYP24A1 inhibitors plus 1,25D3 can cause dose-limiting toxicity of vitamin D (hypercalcemia) in some patients. Dexamethasone significantly reduces 1,25D3-mediated hypercalcemia and enhances the antitumor activity of 1,25D3, increases VDR-ligand binding, and increases VDR protein expression. Efforts to dissect the mechanisms responsible for CYP24A1 overexpression and combinational effect of 1,25D3/dexamethasone in tumors are underway. Understanding the cross talk between vitamin D receptor (VDR) and glucocorticoid receptor (GR) signaling axes is of crucial importance to the design of new therapies that include 1,25D3 and dexamethasone. Insights gained from these studies are expected to yield novel strategies to improve the efficacy of 1,25D3 treatment.

Vitamin D3 May Ameliorate the Ketoconazole Induced Adrenal Injury: Histological and Immunohistochemical Studies on Albino Rats

Ketoconazole (KZ) is used widely for treating the superficial, systemic fungal activities and hyperandrogenemic states. Its uses are limited by its deleterious effect on histological structure and function of the adrenal cortex. This study investigates whether vitamin D3 supplement can ameliorate the morphological changes induced by KZ. Thirty four adult male albino rats were randomized into control group (Group I) which was subdivided into: control 1 (n=7) and control 2 (n=7): In control 1, rats were intraperitoneal (I.P) injected once with 1 ml of polyethylene glycol-400 for 15 consecutive days and control 2 rats were injected I.P with (1 μg/kg) of vitamin D3 for the same period. Group II (n=10): rats were I.P injected with KZ (10 mg/100 g of body weight) once daily for 15 days; Group III (n=10): rats were I.P concomitantly injected with KZ and vitamin D3 similar doses to animals in groups II and control 2 respectively. Blood samples were collected to determine plasma ACTH, corticosterone and aldosterone levels. The right adrenal specimens sections were stained with Haematoxylin & Eosin and Masson Trichrome for histological studies and treated with Bax, Ubiquitin and vitamin D receptors for immunohistochemical studies. KZ induced adrenal cortical morphological changes in forms of disturbed adrenocorticocyte cytological architecture, nuclear changes, and intracellular lipid accumulation. KZ also increased adrenal Bax and Ub but decreased the vitamin D receptors immunopositive staining expression, in addition to increased plasma ACTH as well as decreased corticosterone and aldosterone levels. These changes were ameliorated by supplementing with vitamin D3.

Management of osteoporosis with calcitriol in elderly Chinese patients: a systematic review

Osteoporosis, a skeletal disorder characterized by a reduction in bone strength, is becoming a major public health problem in the People’s Republic of China, with a rapid increase observed among the population. Chinese guidelines particularly recommend use of active vitamin D in managing osteoporosis. 1,25-(OH)₂D₃ (calcitriol) is an active vitamin D metabolite. It plays a role in many biological processes, especially in bone metabolism and muscle function, and is mediated by vitamin D receptors. Osteoporosis in elderly men and women is characterized by uncoupled bone remodeling, which is induced by sex hormone deficiencies, somatopause, vitamin D deficiency, reduced synthesis of D hormone, and lack of receptors or receptor affinity for D hormone in target organs. Reviewed here are six randomized controlled trials on calcitriol monotherapy and five on calcitriol therapy combined with other antiosteoporotic agents. Evidence from these trials shows that calcitriol monotherapy can improve bone mineral density in elderly osteoporotic Chinese patients but may be insufficient for long-term treatment. Calcitriol can also decrease bone turnover markers and bring about significant improvements in muscle strength. Further, calcitriol in combination with other therapeutic bone agents was shown to be well tolerated and capable of additional bone-preserving effects compared with use of calcitriol alone in areas including bone mineral density, bone turnover markers, bone pain improvement, and fracture incidence. Hypercalcemia and hypercalciuria, the most common side effects of calcitriol therapy, were not documented in the trials reviewed, and might have been the result of the low dosages used. One study showed that treatment with calcitriol can improve quality of life in patients with osteoporosis, although not to the same extent as bisphosphonates.

Vitamin D hydroxylases CYP2R1, CYP27B1 and CYP24A1 in renal cell carcinoma

BACKGROUND

There is increasing evidence that vitamin D metabolites influence carcinogenesis. Besides its role in mineral homoeostasis, calcitriol, the active metabolite of vitamin D (1,25(OH)2 D3 ), is known to possess antiproliferative, proapoptotic and immunomodulatory effects in cancer. Concerning the synthesis of vitamin D, the hydroxylases CYP2R1, CYP27B1 and CYP24A1 play a critical role, and the latter molecule determines the biological half-life of 1,25(OH)2 D3 , which is synthesized in the proximal renal tubules.

MATERIALS AND METHODS

The adjacency of these two biological processes prompted us to investigate the gene expression of CYP2R1, CYP27B1 and CYP24A1 in patients with ccRCC. Using RT-PCR, we retrospectively compared mRNA expression profiles from human ccRCC tumour samples with those derived from the corresponding adjacent healthy tissues (n = 30).

RESULTS

We observed that all three genes (CYP2R1, CYP27B1 and CYP24A1) were upregulated in tumours compared with normal tissue (P < 0·0001). Moreover, CYP24A1 displayed a significantly higher expression in tumours than CYP27B1 (P < 0·05) and CYP2R1 (P < 0·0001), whereas no differences in the expression of these genes were found in healthy renal tissue. Gene expression of CYP2R1, CYP27B1 and CYP24A did not differ between pathological classifications (TNM, grading, presence of metastasis).

CONCLUSION

We thus conclude that upregulated gene expression of the catabolizing CYP24A1 as well as the synthesizing CYP2R1 and CYP27B1 may lead to a misbalance of vitamin D metabolites in ccRCC and thus contributing to its pathogenesis.

Elevated 1- α hydroxylase activity in monocytes from patients with active tuberculosis

A uremic patient developed hypercalcemia after tuberculosis infection, and his ionized calcium levels correlated with 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) levels. We performed further studies to determine whether monocytes are alternative sites of 1,25(OH)₂D₃ conversion beyond renal tubular cells. Using an ex vivo bioassay, in this study, we found that 1-α hydroxylase (CYP27B1) activity in monocytes is significantly higher in patients with active tuberculosis (TB) than in those with frequent TB contact. However, when monocytes from patients with active TB were restimulated with antigen derived from Mycobacterium tuberculosis, less 1,25(OH)₂D₃ was observed. In contrast, the level of 1,25(OH)₂D₃ was unchanged in those with frequent TB contact. We conclude that monocytes may be an alternative source of 1-α hydroxylase that could convert 25-hydroxyvitamin D₃ to the more active 1,25(OH)₂D₃.

[Mechanism of hepatoprotective action of methionine and composition "Metovitan" against a background of antituberculosis drug administration to rats]

Oral administration of antituberculosis drugs to rats for 60 days in doses that are equivalent to clinical ones, causes changes in mRNA levels expression of liver cytochrome P-450 isoforms CYP3A2, CYP2C23, CYP2E1 and pro- and antioxidant state. Experimental composition "Metovitan" given with anti-TB drugs provided a correction of these abnormalities, that is evidenced by modulation of the level of CYP3A2, CYP2C23, CYP2E1 gene expression and antioxidant activity, inhibition of lipid peroxidation. "Metovitan" normalizes the enzymatic activity and content of total billirubin in the blood serum, shows high hepatoprotective properties, exceeding the efficiency of methionine.

24-Hydroxylase in cancer: impact on vitamin D-based anticancer therapeutics

The active vitamin D hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a major role in regulating calcium homeostasis and bone mineralization. 1,25(OH)2D3 also modulates cellular proliferation and differentiation in a variety of cell types. 24-Hydroxylase, encoded by the CYP24A1 gene, is the key enzyme which converts 1,25(OH)2D3 to less active calcitroic acid. Nearly all cell types express 24-hydroxylase, the highest activity being observed in the kidney. There is increasing evidence linking the incidence and prognosis of certain cancers to low serum 25(OH)D3 levels and high expression of vitamin D 24-hydroxylase, supporting the idea that elevated CYP24A1 expression may stimulate degradation of vitamin D metabolites including 25(OH)D3 and 1,25(OH)2D3. The over expression of CYP24A1 in cancer cells may be a factor affecting 1,25(OH)2D3 bioavailability and anti-proliferative activity pre-clinically and clinically. The combination of 1,25(OH)2D3 with CYP24A1 inhibitors enhances 1,25(OH)2D3 mediated signaling and anti-proliferative effects and may be useful in overcoming effects of aberrant CYP24A1 expression. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

π-π Stacking mediated drug-drug interactions in human CYP2E1

Because of having many low molecular mass substrates, CYP2E1 is of particular interests to the pharmaceutical industry. Many evidences showed that this enzyme can adopt multiple substrates to significantly reduce the oxidation rate of the substrates. The detailed mechanism for this observation is still unclear. In the current study, we employed GPU-accelerated molecular dynamics simulations to study the multiple-binding mode of human CYP2E1, with an aim of offering a mechanistic explanation for the unexplained multiple-substrate binding. Our results showed that Thr303 and Phe478 were key factors for the substrate recognition and multiple-substrate binding. The former can form a significant hydrogen bond to recognize and position the substrate in the productive binding orientation in the active site. The latter acted as a mediator for the substrate communications via π-π stacking interactions. In the multiple-binding mode, the aforementioned π-π stacking interactions formed by the aromatic rings of both substrates and Phe478 drove the first substrate far away from the catalytic center, orienting in an additional binding position and going against the substrate metabolism. All these findings could give atomic insights into the detailed mechanism for the multiple-substrate binding in human CYP2E1, providing useful information for the drug metabolism mechanism and personalized use of clinical drugs.

Novel mutations of CYP27B1 gene lead to reduced activity of 1α-hydroxylase in Chinese patients

Pseudovitamin D-deficiency rickets (PDDR) is an autosomal recessive disorder resulting from a defect in renal 25-hydroxyvitamin D 1α-hydroxylase, the key enzyme in the pathway of vitamin D metabolism. We identified ten different mutations in the 1α-hydroxylase gene (CYP27B1) in eight Chinese families with PDDR by DNA-sequence analysis. Six of them are novel missense mutations: G57V, G73W, L333F, R432C, R459C, and R492W; three are novel deletion mutations: c48-60del, c1310delG, and c1446delA; and an insertion mutation c1325-1332insCCCACCC reported previously. Functional assay revealed that the missense mutants identified in this study retain 5.5-12.1% 1α-hydroxylase activity of the wild type. The study describes nine novel mutations in addition to 37 known mutations of CYP27B1 gene and shows the correlation between these mutations and the clinical findings of 1α-hydroxylase deficiency.

A critical analysis of the (near) legendary status of vitamin D

Labels such as food constituent, nutrient and supplement do not convey a sense of being essential. Yet these rather mundane descriptors, even if correct, belie the true significance of vitamin D. Long believed to be merely a functioning cofactor akin to vitamin C, deficiency of this secosteroid hormone is clearly associated with morbid complications of calcium and bone mineral metabolism, and because the hormonal effects are mediated by nuclear receptors that regulate the expression of many subordinate genes, the vitamin's pleiotropic mode of action can influence numerous metabolic pathways and, possibly, a number of different diseases. Although the vitamin is under intensive investigation, much still remains unknown, even in bone health, as the identity of osteoporosis susceptibility genes remains uncertain. This article focuses on various aspects of the basic science and molecular biology of the vitamin D endocrine system. The primary goal is to critically examine the evidence supporting its role in bone metabolism, diabetes and cancer.

Vitamin D metabolism, cartilage and bone fracture repair

The 1,25-(OH)(2)D metabolite mediates the endocrine actions of vitamin D by regulating in the small intestine the expression of target genes that play a critical role in intestinal calcium absorption. The major role of the vitamin D hormone on bone is indirect and mediated through its endocrine function on mineral homeostasis. However, genetic manipulation of the expression of Cyp27b1 or the VDR in chondrocytes strongly support a direct role for locally synthesized 1,25(OH)(2)D, acting through the VDR, in vascular invasion and osteoclastogenesis during endochondral bone development. Cells from the growth plate respond to the 24,25-(OH)(2)D and 1,25-(OH)(2)D metabolites in a cell maturation-dependent manner and the effects of 1,25-(OH)(2)D are thought to be mediated through binding to the membrane-associated receptor PDIA3 (protein disulfide isomerase associated 3). The physiological relevance of membrane-mediated 1,25-(OH)(2)D signaling is emerging and is discussed. Finally, preliminary results suggest that mice deficient for Cyp24a1 exhibit a delay in bone fracture healing and support a role for 24,25-(OH)(2)D in mammalian fracture repair.

Differential vitamin D 24-hydroxylase/CYP24A1 gene promoter methylation in endothelium from benign and malignant human prostate

Epigenetic alterations occur in tumor-associated vessels in the tumor microenvironment. Methylation of the CYP24A1 gene promoter differs in endothelial cells isolated from tumors and non-tumor microenvironments in mice. The epigenetic makeup of endothelial cells of human tumor-associated vasculature is unknown due to difficulty of isolating endothelial cells populations from a heterogeneous tissue microenvironment. To ascertain CYP24A1 promoter methylation in tumor-associated endothelium, we utilized laser microdissection guided by CD31 immunohistochemistry to procure endothelial cells from human prostate tumor specimens. Prostate tissues were obtained following robotic radical prostatectomy from men with clinically localized prostate cancer. Adjacent histologically benign prostate tissues were used to compare endothelium from benign versus tumor microenvironments. Sodium bisulfite sequencing of CYP24A1 promoter region showed that the average CYP24A1 promoter methylation in the endothelium was 20% from the tumor microenvironment compared with 8.2% in the benign microenvironment (p< 0.05). A 2-fold to 17-fold increase in CYP24A1 promoter methylation was observed in the prostate tumor endothelium compared with the matched benign prostate endothelium in four patient samples, while CYP24A1 remained unchanged in two patient sample. In addition, there is no correlation of the level of CYP24A1 promoter methylation in prostate tumor-associated endothelium with that of epithelium/stroma. This study demonstrates that the CYP24A1 promoter is methylated in tumor-associated endothelium, indicating that epigenetic alterations in CYP24A1 may play a role in determining the phenotype of tumor-associated vasculature in the prostate tumor microenvironment.

VITAMIN D AND HUMAN PREGNANCY

At the end of 2007, Time magazine listed the “benefits of vitamin D” as one of its top 10 medical breakthroughs for that year. Since then there has been a remarkable upsurge of interest in vitamin D, with new research advances seemingly published on a weekly basis. In particular, there has been increasing awareness of the variability of vitamin D status in populations across the globe and, significantly, a growing debate about the need for revised parameters for vitamin D supplementation. Although sub-optimal vitamin D is likely to be a widespread problem for 21stcentury societies, it is also clear that some groups are at much greater risk of low vitamin D status. Prominent amongst these are pregnant women and the aim of the following review article will be to discuss this problem in further detail with specific emphasis on its potential physiological and clinical impact.

Seasonal and age-related differences in serum 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D and parathyroid hormone in patients from Western Norway

OBJECTIVE

The aim of this study was to investigate the seasonal and age-related variation of vitamin D and PTH serum concentrations in a large general patient population in Western Norway.

DESIGN

A retrospective study was conducted at the Hormone laboratory, Haukeland University Hospital, Bergen, Norway. All analyses of 25-hydroxyvitamin D (25(OH)D) (n = 8325), 1,25-dihydroxyvitamin D (1,25(OH)2D) (n = 4509) and PTH (n = 4203) requested from private practitioners from 2005 to 2008 were included. All three analytes were available in 1551 subjects. Subjects. Mean age of the study population was 49.8 years and 70.9% of the samples were from women.

RESULTS

The highest concentrations of 25(OH)D and 1,25(OH)2D were observed in July-September. In April 43% of the studied population had 25(OH)D concentrations below 50 nmol/L. There was a positive correlation between 25(OH)D and 1,25(OH)2D (p < 0.001). The levels of 25(OH)D and PTH were negatively correlated (p < 0.001) while 1,25(OH)2D and PTH showed a weak positive correlation (p = 0.015). We observed higher concentrations of 25(OH)D (p = 0.003) and lower 1,25(OH)2D levels (p < 0.001) in the older age groups. PTH increased throughout the whole age span (p < 0.001).

CONCLUSION

We observed a seasonal variation in 25(OH)D and 1,25(OH)2D with low serum concentrations during winter/early spring while PTH showed an inverse pattern. Higher levels of PTH in winter and the elderly may reflect an impaired vitamin D status that may affect calcium homeostasis and bone health.

Epigenetic silencing of CYP24 in the tumor microenvironment

Calcitriol (1,25 dihydroxycholecalciferol) has significant anti-tumor activity in vitro and in vivo in a number of tumor model systems. We developed a system for isolation of fresh endothelial cells from tumors and Matrigel environments which demonstrate that CYP24, the catabolic enzyme involved in vitamin D signaling, is epigenetically silenced selectively in tumor-derived endothelial cells (TDEC). TDEC maintain phenotypic characteristics which are distinct from endothelial cells isolated from normal tissues and from Matrigel plugs (MDEC). In TDEC, calcitriol induces G(0)/G(1) arrest, modulates p27 and p21, and induces apoptotic cell death and decreases P-Erk and P-Akt. In contrast, endothelial cells isolated from normal tissues and MDEC are unresponsive to calcitriol-mediated anti-proliferative effects despite intact signaling through the vitamin D receptor (VDR). In TDEC, which are sensitive to calcitriol, the CYP24 promoter is hypermethylated in two CpG island regions located at the 5'end; this hypermethylation may contribute to gene silencing of CYP24. The extent of methylation in these two regions is significantly less in MDEC. Lastly, treatment of TDEC with a DNA methyltransferase inhibitor restores calcitriol-mediated induction of CYP24 and resistance to calcitriol. These data suggest that epigenetic silencing of CYP24 modulates cellular responses to calcitriol.

CYP24A1-deficient mice as a tool to uncover a biological activity for vitamin D metabolites hydroxylated at position 24

The CYP24A1 enzyme (25-hydroxyvitamin D-24-hydroxylase) not only is involved in the catabolic breakdown of 1,25-dihydroxyvitamin D [1,25(OH)2D] but also generates the 24,25-dihydroxyvitamin D [24,25(OH)2D] metabolite. The biological activity of 24,25(OH)2D remains controversial. While in vitro studies suggest that primary cultures of rat rib chondrocytes respond to 24,25(OH)2D in a maturation-specific manner and that the metabolite is necessary for the cells to progress from a proliferating, immature status to a differentiated, 1,25(OH)2D-responsive stage, in vivo evidence to support this putative role remains lacking. Studies in chicken showed increases in serum levels of 24,25(OH)2D and of the renal mRNA levels of Cyp24a1 following fracture, suggesting a role for 24,25(OH)2D in fracture repair. The Cyp24a1-deficient mouse strain represents an invaluable tool to examine the putative role of 24,25(OH)2D in mammalian fracture repair. We have compared fracture repair between Cyp24a1-/- mice and wild-type controls. We have observed a delay in the mineralization of the cartilaginous matrix of the soft callus in Cyp24a1-/- mutant animals, accompanied by reduced expression of chondrocyte marker genes. These results show that Cyp24a1 deficiency delays fracture repair and strongly suggest that vitamin D metabolites hydroxylated at position 24, such as 24,25(OH)2D3, play an important role in the mechanisms leading to normal fracture healing.

Vitamin D deficiency among HIV type 1-infected individuals in the Netherlands: effects of antiretroviral therapy

Vitamin D regulates bone metabolism but has also immunoregulatory properties. In HIV-infected patients bone disorders are increasingly observed. Furthermore, low 1,25(OH)(2)D(3) levels have been associated with low CD4(+) counts, immunological hyperactivity, and AIDS progression rates. Few studies have examined the vitamin D status in HIV-infected patients. This study will specifically focus on the effects of antiretroviral agents on vitamin D status. Furthermore, the effect of vitamin D status on CD4 cell recovery after initiation of HAART will be evaluated. Among 252 included patients the prevalence of vitamin D deficiency (<35 nmol/liter from April to September and <25 nmol/liter from October to March) was 29%. Female sex, younger age, dark skin, and NNRTI treatment were significant risk factors in univariate analysis, although in multivariate analyses skin pigmentation remained the only independent risk factor. Median 25(OH)D(3) levels were significantly lower in white NNRTI-treated patients [54.5(27.9-73.8) nmol/liter] compared to white PI-treated patients [77.3 (46.6-100.0) nmol/liter, p = 0.007], while among nonwhites no difference was observed. Both PI- and NNRTI-treated patients had significantly higher blood PTH levels than patients without treatment. Moreover, NNRTI treatment puts patients at risk of elevated PTH levels (>6.5 pmol/liter). Linear regression analysis showed that vitamin D status did not affect CD4 cell recovery after initiation of HAART. In conclusion, 29% of the HIV-1-infected patients had vitamin D deficiency, with skin color as an independent risk factor. NNRTI treatment may add more risk for vitamin D deficiency. Both PI- and NNRTI-treated patients showed higher PTH levels and might therefore be at risk of bone problems. Evaluation of 25(OH)D(3) and PTH levels, especially in NNRTI-treated and dark skinned HIV-1-infected patients, is necessary to detect and treat vitamin D deficiency early.

1α,24(S)(OH)2D2 normalizes bone morphology and serum parathyroid hormone without hypercalcemia in 25-hydroxyvitamin D-1-hydroxylase (CYP27B1)-deficient mice, an animal model of vitamin D deficiency with secondary hyperparathyroidism

BACKGROUND

Vitamin D compounds are effective in managing elevated PTH levels in secondary hyperparathyroidism (SHPT) of renal failure. However, undesired increases in serum calcium and phosphorus associated with compounds such as calcitriol [1,25(OH)2D3] has prompted a search for compounds with improved safety profiles. 1alpha,24(S)(OH)2D2 (1,24(OH)2D2) is a vitamin D2 metabolite with low calcium-mo bilizing activity in vivo. We studied the efficacy of 1,24(OH)2D2 in mice lacking the CYP27B1 enzyme [25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-OHase)], a novel vitamin D deficiency model with SHPT.

MATERIALS AND METHODS

1alpha-OHase-deficient (-/-) mice and normal (+/-) heterozygous littermates re ceived 1,24(OH)2D2 (100, 300, 1000, and 3000 pg/g/day) or 1,25(OH)2D3 (30, 300, and 500 pg/g/day) for 5 weeks via daily sc injection. Control groups received vehicle.

RESULTS

Vehicle-treated 1alpha-OHase-deficient mice were hypocalcemic and had greatly elevated serum PTH. 1,24(OH)2D2 at doses above 300 pg/g/day normalized serum calcium, serum PTH, bone growth plate morphology, and other bone parameters. No hy percalcemia was observed at any dose of 1,24(OH)2D2 in normal or 1alpha-OHase-deficient animals. In contrast, 1,25(OH)2D3 at only 30 pg/g/day normalized calcemia, serum PTH, and bone parameters, but at higher doses completely suppressed PTH and caused hypercalcemia in both 1alpha-OHase-deficient and normal mice. Treatment with 500 pg/g/day of 1,25(OH)2D3 also induced osteomalacia in normal animals.

CONCLUSION

1,25(OH)2D3 was maximally active at 10-fold lower doses than 1,24(OH)2D2, but induced hypercalcemia and osteomalacia at high doses. 1,24(OH)2D2 normalized serum calcium, serum PTH, and bone histomorphometry without hypercalcemia in 1alpha-OHase-deficient mice with SHPT.

Splice variants of the CYP27b1 gene and the regulation of 1,25-dihydroxyvitamin D3 production

The cytochrome P450 25-hydroxyvitamin D3-1alpha-hydroxylase (CYP27b1) plays a pivotal role in vitamin D physiology by catalyzing synthesis of active 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In common with other P450s, CYP27b1 is known to exhibit alternative splicing. Here we have cloned and sequenced several novel intron 2-containing, noncoding splice variant mRNAs for CYP27b1 in 1,25(OH)2D3-producing HKC-8 human proximal tubule and THP-1 monocytic cells. Regulation of 1,25(OH)2D3 synthesis in these cell lines by calciotropic and noncalciotropic factors was associated with altered expression of the CYP27b1 splice variants. To assess the functional significance of this, HKC-8 cells were transfected with short hairpin RNA (shRNA) to inhibit mRNAs containing sequences from intron 2. This resulted in a significant increase in the expression of CYP27b1 protein and synthesis of 1,25(OH)2D3 by HKC-8 cells compared with control cells for two different intron 2-containing shRNAs (both P<0.001). shRNA to intron 2 had no significant effect on the levels of wild-type CYP27b1 mRNA, suggesting a posttranscriptional mechanism of action. By contrast, shRNA to wild-type CYP27b1 suppressed transcription and activity of the enzyme by 70 and 31%, respectively (both P<0.01). These data indicate that noncoding splice variants of CYP27b1 are functionally active and may play a significant role in the regulation of 1,25(OH)2D3 synthesis during normal physiology.

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.

Role of oncoprotein growth factor independent-1 (GFI1) in repression of 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1): a comparative analysis in human prostate cancer and kidney cells

1,25-Dihydroxyvitamin D (1,25D) inhibits growth of prostate cancer cells and has been proposed to play a protective role in prostate cancer. However, 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1), the enzyme responsible for the cellular synthesis of 1,25D, is repressed in prostate cancer cells. Recently, we have identified a role for the transcription factor, Growth Factor Independent-1 (GFI1) in the repression of the CYP27B1 gene in human prostate cancer cell lines. GFI1 is known to form a large protein complex with co-repressors that recruit histone deacetylases. We have proposed a model for the molecular repression of CYP27B1 gene expression. The formation of such a repressive complex on the inhibitory domain of the CYP27B1 gene in prostate cancer cells could lead to the silencing of gene expression either by inactivating nearby enhancer or proximal promoter domains and lead to cancer progression by reducing local production of 1,25D. These studies demonstrate that GFI1 may play a significant role in the down regulation of endogenous production of 1,25D in prostate cancer cells and could provide a novel insight to future diagnosis and treatment.

[Vitamin D deficiency in adults: to better understand a new presentation of an old disease]

Vitamin D is synthesized in skin through a reaction mediated by sunlight, and it is metabolized to 25-hydroxyvitamin D, in liver, and in 1,25-dihydroxyvitamin D, in kidney. This last reaction has a tight feedback mechanism. 1,25-dihydroxyvitamin D is the active hormone, and its actions are mediated mainly by nuclear receptors. Its major functions are in calcium metabolism and bone mass maintenance. Hypovitaminosis D, as a disease in adult people, manifests itself with hypocalcemia and secondary hyperparathyroidism with subsequent loss of trabecular bone, thinning of cortical bone, and, eventually, a higher risk of fractures. Hypovitaminosis D is a very common condition in Europe, Africa, North America and some South American countries, such as Chile and Argentina. Measurement of serum total 25-hydroxyvitamin D concentration is the gold standard to diagnose vitamin D deficiency. Serum concentrations below 50 nmol/L are associated with an increase in parathyroid hormone concentration, and bone loss. Risk factors for vitamin D deficiency, like poor sunlight exposition, aging skin and factors that interfere with normal vitamin D metabolism, are well established. Oral vitamin D supplementation, an easy and inexpensive treatment, is needed to treat this illness.

Molecular action of 1,25-dihydroxyvitamin D3 and phorbol ester on the activation of the rat cytochrome P450C24 (CYP24) promoter: role of MAP kinase activities and identification of an important transcription factor binding site

Although investigations of the transcriptional regulation of the rat cytochrome P450C24 [CYP24 (25-hydroxyvitamin D3 24-hydroxylase)] gene by 1,25D (1,25-dihydroxyvitamin D3) at either the genomic, or more recently at the non-genomic, level have provided insight into the mechanism of control of 1,25D levels, this regulation is still poorly characterized. Using HEK-293T cells (human embryonic kidney 293T cells), we reported that 1,25D induction of CYP24 requires JNK (c-Jun N-terminal kinase) but not the ERK1/2 (extracellular-signal-regulated kinase 1/2). The phenomenon of synergistic up-regulation of CYP24 expression by PMA and 1,25D is well known and was found to be protein kinase C-dependent. Whereas ERK1/2 was not activated by 1,25D alone, its activation by PMA was potentiated by 1,25D also. The importance of ERK1/2 for transcriptional synergy was demonstrated by transfection of a dominant-negative ERK1(K71R) mutant (where K71R stands for Lys71-->Arg), which resulted in a reduced level of synergy on a CYP24 promoter-luciferase construct. JNK was also shown to be required for synergy. We report, in the present study, the identification of a site located at -171/-163, about 30 bp upstream of the vitamin D response element-1 in the CYP24 proximal promoter. This sequence, 5'-TGTCGGTCA-3', is critical for 1,25D induction of CYP24 and is therefore termed the vitamin D stimulatory element. The vitamin D stimulatory element, a target for the JNK module, and an Ets-1 binding site were shown to be vital for synergy between PMA and 1,25D. This is the first report to identify the DNA binding sequences required for the synergy between PMA and 1,25D and a role for JNK on the CYP24 gene promoter.

Identification of the amino acid residue of CYP27B1 responsible for binding of 25-hydroxyvitamin D3 whose mutation causes vitamin D-dependent rickets type 1

We previously reported the three-dimensional structure of human CYP27B1 (25-hydroxyvitamin D3 1alpha-hydroxylase) constructed by homology modeling. Using the three-dimensional model we studied the docking of the substrate, 25-hydroxyvitamin D3, into the substrate binding pocket of CYP27B1. In this study, we focused on the amino acid residues whose point mutations cause vitamin D-dependent rickets type 1, especially unconserved residues among mitochondrial CYPs such as Gln65 and Thr409. Recently, we successfully overexpressed mouse CYP27B1 by using a GroEL/ES co-expression system. In a mutation study of mouse CYP27B1 that included spectroscopic analysis, we concluded that in a 1alpha-hydroxylation process, Ser408 of mouse CYP27B1 corresponding to Thr409 of human CYP27B1 forms a hydrogen bond with the 25-hydroxyl group of 25-hydroxyvitamin D3. This is the first report that shows a critical amino acid residue recognizing the 25-hydroxyl group of the vitamin D3.

Modulation of CYP27B1 and CYP24 mRNA expression in bone is independent of circulating 1,25(OH)2D3 levels

Circulating levels of 1,25-dihydroxyvitamin D (1,25D) are determined by bioactivation catalyzed by the renal 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1) and degradation through the action of the renal 25-hydroxyvitamin D 24-hydroxylase (CYP24). CYP27B1 and CYP24 are also present in bone cells, but little is known of their physiological role. The purpose of this study was to determine the changes that occur with aging on the expression of CYP27B1 and CYP24 mRNA in whole kidney and femora of female Sprague-Dawley rats. Real-time RT-PCR was used to measure CYP27B1, CYP24 and vitamin D receptor (VDR) mRNA levels in the kidneys and bones of animals aged between 3 weeks and 2 years. Circulating 1,25D levels decreased exponentially with age which was correlated with both reduced kidney CYP27B1 mRNA (R(2) = 0.72) and increased CYP24 mRNA levels (R(2) = 0.71). In the bone, CYP27B1 mRNA levels were maintained at their highest level throughout the ages of 3 to 15 weeks before decreasing in adult animals (P < 0.05). Bone CYP24 mRNA levels were positively correlated with bone CYP27B1 mRNA and not circulating 1,25D levels (R(2) = 0.74). Levels of bone CYP27B1 mRNA were positively correlated with distal femoral epiphyseal trabecular number (Tb.N) (R(2) = 0.74) and negatively with the trabecular thickness (Tb.Th) (R(2) = 0.56) in animals aged between 12 weeks and 2 years. These findings indicate that the regulation of CYP27B1 and CYP24 mRNA expression in the bone is unique from that in the kidney. The synthesis of 1,25D in bone tissue regulates bone CYP24 expression and is associated with bone mineralization suggesting that vitamin D metabolism has an autocrine or paracrine function.

Stay tuned to PXR: an orphan actor that may not be D-structive only to bone

Pregnane X receptor (PXR) plays an important role in detoxifying xenobiotics and drugs. In this issue of the JCI, Pascussi et al. provide convincing evidence that PXR can also induce vitamin D deficiency and bone disease because of its ability to cross-talk with the vitamin D-responsive gene that catabolizes 25-hydroxy-vitamin D and 1,25-dihydroxyvitamin D. This cross-talk behavior has important health ramifications and can be mitigated through the identification and treatment of PXR-induced vitamin D deficiency.

Possible involvement of pregnane X receptor-enhanced CYP24 expression in drug-induced osteomalacia

Vitamin D controls calcium homeostasis and the development and maintenance of bones through vitamin D receptor activation. Prolonged therapy with rifampicin or phenobarbital has been shown to cause vitamin D deficiency or osteomalacia, particularly in patients with marginal vitamin D stores. However, the molecular mechanism of this process is unknown. Here we show that these drugs lead to the upregulation of 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) gene expression through the activation of the nuclear receptor pregnane X receptor (PXR; NR1I2). CYP24 is a mitochondrial enzyme responsible for inactivating vitamin D metabolites. CYP24 mRNA is upregulated in vivo in mice by pregnenolone 16alpha-carbonitrile and dexamethasone, 2 murine PXR agonists, and in vitro in human hepatocytes by rifampicin and hyperforin, 2 human PXR agonists. Moreover, rifampicin increased 24-hydroxylase activity in these cells, while, in vivo in mice, pregnenolone 16alpha-carbonitrile increased the plasma concentration of 24,25-dihydroxyvitamin D(3). Transfection of PXR in human embryonic kidney cells resulted in rifampicin-mediated induction of CYP24 mRNA. Analysis of the human CYP24 promoter showed that PXR transactivates the sequence between -326 and -142. We demonstrated that PXR binds to and transactivates the 2 proximal vitamin D-responsive elements of the human CYP24 promoter. These data suggest that xenobiotics and drugs can modulate CYP24 gene expression and alter vitamin D(3) hormonal activity and calcium homeostasis through the activation of PXR.

Homology modeling of human 25-hydroxyvitamin D3 1alpha-hydroxylase (CYP27B1) based on the crystal structure of rabbit CYP2C5

Seventeen missense mutations of 25-hydroxyvitamin D(3) 1alpha-hydroxylase (CYP27B1) that cause Vitamin D-dependent rickets type I (VDDR-I) have been identified. To understand the mechanism by which each mutation disrupts 1alpha-hydroxylase activity and to visualize the substrate-binding site, we performed the homology modeling of CYP27B1. The three-dimensional (3D) structure of CYP27B1 was modeled on the basis of the crystal structure of rabbit CYP2C5, the first solved X-ray structure of a eukaryotic CYP. The 3D structure of CYP27B1 contains 17 helices and 6 beta-strands, and the overall structural folding is similar to the available structures of soluble CYPs as well as to the template CYP2C5. Mapping of the residues responsible for VDDR-I has provided much information concerning the function of each mutant. We have previously reported site-directed mutagenesis studies on several mutants of CYP27B1 causing VDDR-1, and suggested the role of each residue. All these suggestions are in good agreement with our 3D-model of CYP27B1. Furthermore, this model enabled us to predict the function of the other mutation residues responsible for VDDR-I.

Vitamin D and barrier function: a novel role for extra-renal 1 alpha-hydroxylase

Much recent attention has focused on the positive health benefits of vitamin D beyond its established role in calcium homeostasis. Epidemiology has highlighted the link between vitamin D deficiency and prevalent diseases such as common cancers and autoimmune disease. Furthermore, studies in vitro have shown that the active form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is a potent antiproliferative and immunosuppressive agent. The net effect of this has been the generation and analysis of synthetic analogues of vitamin D for potential use in the treatment of cancers and other disorders including psoriasis. However, there is increasing interest in the impact that vitamin D may have on normal physiology above and beyond its classical effects on calcium homeostasis and bone metabolism. We have postulated that these 'non-calcemic' effects of vitamin D are dependent on extra-renal synthesis of 1,25(OH)(2)D(3) via the enzyme 1 alpha-hydroxylase at barrier sites throughout the body. Here we present a review of the mechanisms associated with extra-renal 1 alpha-hydroxylase, and we also speculate on how this 'new' physiological role for vitamin D may actually reflect an ancient function for this pluripotent secosteroid.

Combination of vitamin D metabolites with selective inhibitors of vitamin D metabolism

1alpha,25(OH)2D3 exerts antiproliferative, differentiating effects on many cell types, including cancer tissues. In most of its target cells, levels of 1alpha,25(OH)2D3 are regulated by local synthesis via CYP27B and metabolism via CYP24. Rapidly induced by vitamin D, CYP24 repeatedly hydroxylates the vitamin D side chain and ultimately terminates hormonal activity. Aiming at increased hormone levels, lifetime and function, numerous vitamin D analogs have been synthesized with structural modifications, which impede oxidation of the vitamin D side chain. Our group followed a different strategy, namely, blocking 1,25(OH)2D3 metabolism with inhibitors of CYP24. As appropriate inhibitors, we exploited compounds termed azoles, which directly bind to the heme iron of the CYPs via an azole nitrogen and to other parts of the substrate site. We synthesized some 400 azoles and tested their potential to selectively inhibit CYP24, but not hormone synthesis by the related CYP27B. Using primary human keratinocyte cultures as the source of CYP24 and CYP27, we discovered some 50 inhibitors of CYP24 with IC50 values in the nanomole range and selectivities up to 60-fold. As the first representative of selective CYP24 inhibitors, VID400 underwent preclinical development. In human keratinocytes, VID400 stabilized levels of endogenously produced 1alpha,25(OH)2D3, and thereby strongly amplified and prolonged expression of CYP24, a surrogate marker of hormonal function. In parallel, antiproliferative activity showed up at 100-fold or more lower concentrations of 1alpha,25(OH)2D3. This data suggests that CYP24 inhibitors could become attractive drugs in antiproliferative therapy, used as single entities to increase or extend endogenous hormone function or in combination with low doses of potent analogs. Moreover, we used selective inhibitors as valuable tools to (a) elucidate regulatory mechanisms of vitamin D synthesis and metabolism, (b) determine intrinsic activities of the otherwise highly transient vitamin D metabolites and (c) model the active sites of CYP24 and CYP27B.

Molecular basis for pseudo vitamin D-deficiency rickets in the Hannover pig

The molecular basis for pseudo vitamin D deficiency rickets (PDDR) in the Hannover pig model was determined in the current study. Consistent with the inability of Hannover PDDR pigs to maintain ambient levels of 1,25-dihydroxyvitamin D (i.e., 1,25D), the bioactivation enzyme cytochrome P450C1 (or CYP27B1) was determined to contain coding-region deletions that rendered the enzyme ineffective due to frame-shift mutations and expression of a premature termination codon. Expression levels of P450C1mRNA were up-regulated in response to the low-1,25D high-parathyroid hormone state of the PDDR animals. In a complementary manner, cytochrome P450C24 mRNA was not detectable in PDDR pigs. Two different deletions were detected within the Hannover pig strain in which the P450C1 coding region contained either 173 bp or 329 bp deletions that resulted in the expression of non-sense products beginning within the I-helix region and extending through the truncated C-terminal domains. The boundaries for the deletion segments aligned with derived mRNA processing sites. This observation was consistent with an mRNA processing error as the causative factor for the coding-region deletions. Based upon the expression of a non-functional P450C1 enzyme, the Hannover pig model for PDDR was determined to be identical to the human disease in which enzyme-inhibitory mutations are the molecular basis for the calcium disorder.

Inhibitors of vitamin D hydroxylases: structure-activity relationships

Aiming at new drugs to efficiently treat diseases, in which either increased or decreased levels of active vitamin D are desirable, we have designed some 400 structurally different azole-type inhibitors and examined their capacity to selectively block vitamin D metabolism by CYP24 or synthesis by CYP27B, in human keratinocytes. Based on resulting data, we built pharmacophore models of the active sites using commercial software. The overlay of potent selective compounds indicated similar docking modes in the two-substrate pockets and allowed for identification of bioactive conformations. Superimposing these bioactive conformations with low energy conformers of 25(OH)D(3) suggested that the substrate-mimicked by strong inhibitors in size, shape and lipophilic character-binds to both enzymes in 6s-trans configuration. Pharmacophoric models implied a similar geometry of the substrate sites, nevertheless specific features of CYP24 and CYP27B could be defined. Bulky substituents in alpha-position to the azole caused selectivity for CYP24, whereas bulky substituents in beta-position could result in selectivity for CYP27B. Moreover, studies with small sterically restricted inhibitors revealed a probable location of the 3-OH-group of 25(OH)D(3) in CYP27B. In the absence of crystal structures, our inhibitors are valuable tools to model and understand the active sites of vitamin D hydroxylases, resulting in the design of powerful, selective therapeutics.

Expression, structure-function, and molecular modeling of vitamin D P450s

Although vitamin D(3) is a natural product of a sunlight-mediated process in the skin, the secosteroid's biological function is dependent upon specific cytochrome P450 enzymes that mediate the parent vitamin's bioactivation and inactivation. Cytochrome P450C1 (CYP27B1) is the regulatory rate-limiting enzyme that directs the bioactivation process through introduction of a C-1alpha hydroxyl group. The resultant 1,25-dihydroxyvitamin D(3) (1,25D) is the biologically active secosteroid hormone that directs the multitude of vitamin D-dependent actions involved with calcium homeostasis, cellular differentiation and growth, and the immune response. The circulating and cellular level of 1,25D is regulated through a coordinated process involving the hormone's synthesis and degradation. Central to the degradation and turnover of 1,25D is the regulatory multi-catalytic cytochrome P450C24 (CYP24) enzyme that directs the introduction of C-24R groups onto targeted 25-hydroxy substrates. Discussed in this article is the action of the rat CYP24 to catalyze the side-chain oxidation and cleavage of 25-hydroxylated vitamin D metabolites. Expression and characterization of purified recombinant rat CYP24 is discussed in light of mutations directed at the enzyme's active site.