25-hydroxycholecalciferol. A biologically active metabolite of vitamin D3

Nuclear Receptors and the Hidden Language of the Metabolome

Nuclear hormone receptors (NHRs) are a family of ligand-regulated transcription factors that control key aspects of development and physiology. The regulation of NHRs by ligands derived from metabolism or diet makes them excellent pharmacological targets, and the mechanistic understanding of how NHRs interact with their ligands to regulate downstream gene networks, along with the identification of ligands for orphan NHRs, could enable innovative approaches for cellular engineering, disease modeling and regenerative medicine. We review recent discoveries in the identification of physiologic ligands for NHRs. We propose new models of ligand-receptor co-evolution, the emergence of hormonal function and models of regulation of NHR specificity and activity via one-ligand and two-ligand models as well as feedback loops. Lastly, we discuss limitations on the processes for the identification of physiologic NHR ligands and emerging new methodologies that could be used to identify the natural ligands for the remaining 17 orphan NHRs in the human genome.

Design, synthesis, and biological activity of D-bishomo-1α,25-dihydroxyvitamin D3 analogs and their crystal structures with the vitamin D nuclear receptor

The biologically active metabolite of vitamin D3 - calcitriol - is a hormone involved in the regulation of calcium-phosphate homeostasis, immunological processes and cell differentiation, being therefore essential for the proper functioning of the human body. This suggests many applications of this steroid in the treatment of diseases such as rickets, psoriasis and some cancers. Unfortunately, using therapeutic doses of calcitriol is associated with high concentrations of this compound which causes hypercalcemia. For this reason, new calcitriol analogs are constantly sought, devoid of calcemic effects but maintaining its beneficial properties. In this study, we present the synthesis of vitamin D derivatives characterized by an enlarged (seven-membered) ring D. Preparation of the designed vitamin D compounds required separate syntheses of crucial building blocks (C/D-rings fragments with side chain and rings A) which were combined by different methods, including Wittig-Horner reaction and Suzuki coupling. Biological activities of the target vitamin D analogs were assessed both in vitro and in vivo, demonstrating their significant potency compared to the natural hormone. Furthermore, the successful crystallization of these compounds with the vitamin D receptor (VDR) enabled us to investigate additional molecular interactions with this protein.

Advances and challenges in the measurement of 1,25-dihydroxyvitamin D: a comprehensive review

Vitamin D has received significant attention from clinical societies, researchers, and the general population in recent years. While 25-hydroxyvitamin D (25(OH)D) is the most commonly-used biomarker of vitamin D status, 1α,25-dihydroxyvitamin D (1,25(OH)2D), its bioactive form, plays a critical role in regulating calcium and phosphorus homeostasis and is also involved in the immune system and cellular differentiation. Consequently, accurate measurements of 1,25(OH)2D can aid in the differential diagnosis of calcium-related disorders such as hypocalcemia in vitamin D-dependent rickets and hypercalcemia due to inappropriate increase of serum 1,25(OH)2D in granulomatous diseases. However, due to its lipophilicity and very low circulating concentration, the measurement of 1,25(OH)2D is particularly challenging. Over the past several decades, numerous efforts have been made to develop sensitive, specific, and practical laboratory methods for measuring 1,25(OH)2D. Methods using radioreceptor assay, radioimmunoassay, enzyme immunoassay, enzyme-linked immunosorbent assay, automated chemiluminescent immunoassay, and liquid chromatography-tandem mass spectrometry have been described. Each of these methods has unique advantages and limitations, and some are no longer used. Despite the sophisticated methods in use today, substantial variations between methods still exist. A concerted effort toward standardization of 1,25(OH)2D measurement is needed to ensure accurate and reliable results across laboratories and methods.

Vitamin D: 100 years of discoveries, yet controversy continues

Over the past 100 years, many major breakthroughs and discoveries have occurred in relation to vitamin D research. These developments include the cure of rickets in 1919, the discovery of vitamin D compounds, advances in vitamin D molecular biology, and improvements in our understanding of endocrine control of vitamin D metabolism. Furthermore, recommended daily allowances for vitamin D have been established and large clinical trials of vitamin D, aimed at clarifying the effect of Vitamin D in the prevention of multiple diseases, have been completed. However, disappointingly, these clinical trials have not fulfilled the expectations many had 10 years ago. In almost every trial, various doses and routes of administration did not show efficacy of vitamin D in preventing fractures, falls, cancer, cardiovascular diseases, type 2 diabetes, asthma, and respiratory infections. Although concerns about side-effects of long-term high-dose treatments, such as hypercalcaemia and nephrocalcinosis, have been around for four decades, some trials from the past 5 years have had new and unexpected adverse events. These adverse events include increased fractures, falls, and hospitalisations in older people (aged >65 years). Several of these clinical trials were powered appropriately for a primary outcome but did not include dose response studies and were underpowered for secondary analyses. Furthermore, more attention should be paid to the safety of high doses of vitamin D supplementation, particularly in older people. In addition, despite universal recommendations by osteoporosis societies for combining calcium supplements with vitamin D there remains insufficient data about their efficacy and effect on fracture risk in the highest risk groups. More trials are needed for people with severe vitamin D deficiency (ie, serum 25-hydroxyvitamin D <25nmol/L [10ng/mL]). In this Personal View, we summarise and discuss some of the major discoveries and controversies in the field of vitamin D.

The One-Hundred-Year Anniversary of the Discovery of the Sunshine Vitamin D3: Historical, Personal Experience and Evidence-Based Perspectives

The discovery of a fat-soluble nutrient that had antirachitic activity and no vitamin A activity by McCollum has had far reaching health benefits for children and adults. He named this nutrient vitamin D. The goal of this review and personal experiences is to give the reader a broad perspective almost from the beginning of time for how vitamin D evolved to became intimately involved in the evolution of land vertebrates. It was the deficiency of sunlight causing the devastating skeletal disease known as English disease and rickets that provided the first insight as to the relationship of sunlight and the cutaneous production of vitamin D₃. The initial appreciation that vitamin D could be obtained from ultraviolet exposure of ergosterol in yeast to produce vitamin D₂ resulted in the fortification of foods with vitamin D₂ and the eradication of rickets. Vitamin D₃ and vitamin D₂ (represented as D) are equally effective in humans. They undergo sequential metabolism to produce the active form of vitamin D, 1,25-dihydroxyvitamin D. It is now also recognized that essentially every tissue and cell in the body not only has a vitamin D receptor but can produce 1,25-dihydroxyvitamin D. This could explain why vitamin D deficiency has now been related to many acute and chronic illnesses, including COVID-19.

Assessing vitamin D metabolism - four decades of experience

One hundred years ago, the role of vitamin D for bone mineralization and the prevention of rickets was discovered. Vitamin D comprises a group of over 50 metabolites with multiple functions that go far beyond calcium homeostasis and bone mineralization. Approximately 50 years ago, first methods for the measurement of 25-hydroxyvitamin D (25(OH)D) in human blood were developed. Over the years, different analytical principals were employed including competitive protein binding assays, high-performance liquid chromatography, various immunoassay and mass spectrometric formats. Until the recent standardization of serum 25(OH)D measurement, agreement between methods was unsatisfactory. Since then, comparability has improved, but substantial variability between methods remains. With the advent of liquid chromatography tandem mass spectrometry (LC-MS/MS), the accurate determination of 25(OH)D and other metabolites, such as 24,25(OH)2D, becomes increasingly accessible for clinical laboratories. Easy access to 25(OH)D testing has triggered extensive clinical research showing that large parts of the population are vitamin D deficient. The variable response of vitamin D deficient individuals to supplementation indicates that assessing patients' vitamin D stores by measuring 25(OH)D provides limited insight into the metabolic situation. Meanwhile, first evidence has emerged suggesting that the simultaneous measurement of 25(OH)D, 24,25(OH)2D and other metabolites allows a dynamic evaluation of patients' vitamin D status on metabolic principals. This may help to identify patients with functional vitamin D deficiency from those without. It can be expected that research into the assessment vitamin D status will continue for another 50 years and that this will help rationalizing our approach in clinical practice.

The Centennial Collection of VDR Ligands: Metabolites, Analogs, Hybrids and Non-Secosteroidal Ligands

Since the discovery of vitamin D a century ago, a great number of metabolites, analogs, hybrids and nonsteroidal VDR ligands have been developed. An enormous effort has been made to synthesize compounds which present beneficial properties while attaining lower calcium serum levels than calcitriol. This structural review covers VDR ligands published to date.

Enteral Vitamin D Supplementation in Preterm or Low Birth Weight Infants: A Systematic Review and Meta-analysis

BACKGROUND AND OBJECTIVES

Many preterm and low birth weight (LBW) infants have low vitamin D stores. The objective of this study was to assess effects of enteral vitamin D supplementation compared with no vitamin D supplementation in human milk fed preterm or LBW infants.

METHODS

Data sources include Cochrane Central Register of Controlled Trials, Medline, and Embase from inception to March 16, 2021. The study selection included randomized trials. Data were extracted and pooled with fixed and random-effects models.

RESULTS

We found 3 trials (2479 participants) that compared vitamin D to no vitamin D. At 6 months, there was increase in weight-for-age z-scores (mean difference 0.12, 95% confidence interval [CI] 0.01 to 0.22, 1 trial, 1273 participants), height-for-age z-scores (mean difference 0.12, 95% CI 0.02 to 0.21, 1 trial, 1258 participants); at 3 months there was decrease in vitamin D deficiency (risk ratio 0.58, 95% CI 0.49 to 0.68, I2=58%, 2 trials, 504 participants) in vitamin D supplementation groups. However, there was little or no effect on mortality, any serious morbidity, hospitalization, head circumference, growth to 6 years and neurodevelopment. The certainty of evidence ranged from very low to moderate. Fourteen trials (1969 participants) assessed dose and reported no effect on mortality, morbidity, growth, or neurodevelopment, except on parathyroid hormone and vitamin D status. No studies assessed timing. Limitations include heterogeneity and small sample size in included studies.

CONCLUSIONS

Enteral vitamin D supplementation improves growth and vitamin D status in preterm and LBW infants.

There is more to life than serum vitamin D: a lesson from the past

This commentary revisits a paper from Clinical Science in 1972 entitled “The distribution and storage of vitamin D and its metabolites in human tissues” by Barbara Mawer, Bill Stanbury and colleagues. The paper continues to be well cited 50 years later, in part because the study it describes – which includes the use of human autopsy tissue – would be difficult to replicate today. However, the paper also has resonance today because the focus of the study – what is the fate of vitamin D in the body? – is still not clear. This commentary discusses why the Mawer et al. study was a major advance when published and why there is still much to be learned from this paper half a century later.

100 YEARS OF VITAMIN D: Historical aspects of vitamin D

Vitamin D has many physiological functions including upregulation of intestinal calcium and phosphate absorption, mobilization of bone resorption, renal reabsorption of calcium as well as actions on a variety of pleiotropic functions. It is believed that many of the hormonal effects of vitamin D involve a 1,25-dihydroxyvitamin D3-vitamin D receptor-mediated transcriptional mechanism involving binding to the cellular chromatin and regulating hundreds of genes in many tissues. This comprehensive historical review provides a unique perspective of the many steps of the discovery of vitamin D and its deficiency disease, rickets, stretching from 1650 until the present. The overview is divided into four distinct historical phases which cover the major developments in the field and in the process highlighting the: (a) first recognition of rickets or vitamin D deficiency; (b) discovery of the nutritional factor, vitamin D and its chemical structure; (c) elucidation of vitamin D metabolites including the hormonal form, 1,25-dihydroxyvitamin D3; (d) delineation of the vitamin D cellular machinery, functions and vitamin D-related diseases which focused on understanding the mechanism of action of vitamin D in its many target cells.

Natural Vitamin D in Food: To What Degree Does 25-Hydroxyvitamin D Contribute to the Vitamin D Activity in Food?

Vitamin D3, vitamin D2, 25-hydroxyvitamin D3 [25(OH)D₃], and 25-hydroxyvitamin D2 [25(OH)D₂]constitute the vitamin D activity in food. In general, vitamin D activity in food depends on the food's fat content, the feed the animals have been fed, and the animal's exposure to ultraviolet B (UVB) light. There are many gaps in our knowledge of 25-hydroxyvitamin D in food, including the amount present in different types of food, and the amount we process in our daily dietary intake. We aimed to assess the vitamin D vitamers in food (eggs, milk, dairy products, chicken, veal, beef, and pork) on the Danish market using accredited analytical methods. We then combined these data with existing Danish data, as well as with the information from the Danish Dietary Survey to estimate the dietary intake of vitamin D3 and of 25(OH)D₃ by Danes. We report the level of vitamin D in 10% minced pork from free-range pigs slaughtered in summer as 1.39 μg vitamin D3/100 g and 0.40 μg 25(OH)D₃/100 g, which are significantly higher amounts (p < 0.001) than in early spring. The levels of vitamin D2 and 25(OH)D₂ are usually <0.05 μg/100 g, though in beef they are up to 0.14 μg/100 g. 25(OH)D₃ accounts for up to 100% in veal and 8% in fat from free-range pigs. In the Danish diet, the share of 25(OH)D₃ is 24% for children (4-17 years) and 18% for adults (18-75 years). Changes in animal-feeding strategy in the agriculture sector could change the share of 25(OH)D₃ to 11% and 12% if extra vitamin D3 is added to the feed, and the animals are exposed to sunlight or UVB lightlight. Replacing vitamin D3 by 25(OH)D₃ in the feed may result in a share of 25(OH)D₃ of 52% and 40%, respectively, in children and adults. These estimates are based on the assumption that vitamin D3 and 25(OH)D₃ contribute equally to the vitamin D activity. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

The association analysis between CYP24A1 genetic polymorphisms and the risk of ischemic stroke in Chinese Han population

AIMS

Stroke is a complicated neurological disease and the second leading cause of death in the world. We aimed to investigate the association between CYP24A1 genetic polymorphisms and ischemic stroke risk.

METHODS

In this case-control study, four single-nucleotide polymorphisms of CYP24A1 were selected and genotyped by MassARRAY platform in Chinese Han population. Odds ratios and 95% confidence intervals were calculated via logistic regression analysis with adjustment in genetic models.

RESULTS

Our results indicated that CYP24A1 variant (rs1570669) was associated with the decreased risk of ischemic stroke (OR = 0.60, p < .001). Stratification analysis showed that the rs6068816 could enhance the ischemic stroke risk by 1.64 times (OR = 1.64, p = .028), while rs1570669 played protective role (OR = 0.63, p = .044) in age >64 years. The rs2762934 had an increased ischemic stroke susceptibility (OR = 1.62, p = .033); however, rs1570669 might reduce stroke risk (OR = 0.61, p = .015) in age ≤64 years. The rs1570669 depressed ischemic stroke susceptibility both in female and male patients (OR = 0.46, p = .002; OR = 0.69, p = .033, respectively), and rs2296241 would weaken the risk in male (OR = 0.63, p = .012). The rs1570669 was associated with decreased risk of ischemic stroke with hypertension (OR = 0.56, p = .042).

CONCLUSION

Our study gave the evidences that CYP24A1 genetic polymorphisms were significantly associated with ischemic stroke patients, which would provide useful information of assessment or possible diagnostic markers for ischemic stroke.

The vitamin D metabolome: An update on analysis and function

Current understanding of vitamin D tends to be focussed on the measurement of the major circulating form 25-hydroxyvitamin D3 (25OHD3) and its conversion to the active hormonal form, 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂ D3) via the enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1). However, whilst these metabolites form the endocrine backbone of vitamin D physiology, it is important to recognise that there are other metabolic and catabolic pathways that are now recognised as being crucially important to vitamin D function. These pathways include C3-epimerization, CYP24A1 hydroxylase, CYP11A1 alternative metabolism of vitamin D3, and phase II metabolism. Endogenous metabolites beyond 25OHD3 are usually present at low endogenous levels and may only be functional in specific target tissues rather than in the general circulation. However, the technologies available to measure these metabolites have also improved, so that measurement of alternative vitamin D metabolic pathways may become more routine in the near future. The aim of this review is to provide a comprehensive overview of the various pathways of vitamin D metabolism, as well as describe the analytical techniques currently available to measure these vitamin D metabolites.

Mobilising vitamin D from adipose tissue: The potential impact of exercise

Vitamin D is lipophilic and accumulates substantially in adipose tissue. Even without supplementation, the amount of vitamin D in the adipose of a typical adult is equivalent to several months of the daily reference nutrient intake (RNI). Paradoxically, despite the large amounts of vitamin D located in adipose tissue, individuals with obesity are often vitamin D deficient according to consensus measures of vitamin D status (serum 25-hydroxyvitamin D concentrations). Thus, it appears that vitamin D can become 'trapped' in adipose tissue, potentially due to insufficient lipolytic stimulation and/or due to tissue dysfunction/adaptation resulting from adipose expansion. Emerging evidence suggests that exercise may mobilise vitamin D from adipose (even in the absence of weight loss). If exercise helps to mobilise vitamin D from adipose tissue, then this could have important ramifications for practitioners and policymakers regarding the management of low circulating levels of vitamin D, as well as chronically low levels of physical activity, obesity and associated health conditions. This perspective led us to design a study to examine the impact of exercise on vitamin D status, vitamin D turnover and adipose tissue vitamin D content (the VitaDEx project). The VitaDEx project will determine whether increasing physical activity (via exercise) represents a potentially useful strategy to mobilise vitamin D from adipose tissue.

The discovery and synthesis of the nutritional factor vitamin D

Although vitamin D deficiency was first recognized as rickets/osteomalacia in the early 1600s, it was only a century ago that vitamin D, the nutritional factor responsible, was discovered. This discovery was made difficult by the fact that the substance could be synthesized in human skin by exposure to UV light and could also be present in the diet in animal-derived (D₃) and plant-derived forms (D₂). Prior to 1920, the frequency of vitamin D deficiency in the general population of industrialized cities was high. The discovery of vitamin D led to the widespread fortification of foods e.g. milk and the virtual eradication of rickets in developed nations. Vitamin D₃ was first chemically synthesized in the 1930s and its metabolism to the active form 1,25-dihydroxyvitamin D₃ and its mode of action in calcium and phosphate homeostasis were elucidated in the latter half of the 20th century. Synthetic vitamin D analogs that mimic the physiological effects of vitamin D are now used therapeutically in diseases such as bone disease, chronic kidney disease and psoriasis. Thus, a wide range of disciplines played critical roles in the rich history of vitamin D and these are described in this short historical overview.

Genetic polymorphisms of vitamin D3 metabolizing CYP24A1 and CYP2R1 enzymes in Turkish patients with ischemic stroke

Objective Vitamin D deficiency is known as an important risk factor in pathogenesis of atherosclerosis, which contributes to stroke development. Genetic variations including single nucleotide polymorphisms (SNPs) in enzymes involved in vitamin D metabolism can affect susceptibility to the development of stroke. Therefore, the objective of this study was to investigate the association between polymorphisms of vitamin D metabolizing enzymes (rs927650 SNP in CYP24A1, and rs10741657 SNP in CYP2R1 genes,) and ischemic stroke risk in Turkish population. Materials and methods To test this hypothesis, we designed a case-control study which consisted of 256 ischemic stroke patients and 132 controls. Genotypes were determined by PCR-RFLP technique. Results No significant differences were found between patients and controls in terms of CYP24A1 rs927650 and CYP2R1 rs10741657 genotype frequencies. Polymorphic allele frequencies of CYP24A1 rs927650 and CYP2R1 rs10741657 were 0.414 and 0.660 in stroke patients, respectively. Conclusion This is the first study conducted regarding the association of CYP24A1 rs927650 and CYP2R1 rs10741657 genetic polymorphisms and ischemic stroke risk. The polymorphic genotypes of these polymorphisms, together with hypertension, diabetes, smoking, and obesity, were found as significant risk factors for ischemic stroke.

The Use of Vitamin D Metabolites and Analogues in the Treatment of Chronic Kidney Disease

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are associated with abnormalities in bone and mineral metabolism, known as CKD-bone mineral disorder. CKD and ESRD cause skeletal abnormalities characterized by hyperparathyroidism, mixed uremic osteodystrophy, osteomalacia, adynamic bone disease, and frequently enhanced vascular and ectopic calcification. Hyperparathyroidism and mixed uremic osteodystrophy are the most common manifestations due to phosphate retention, reduced concentrations of 1,25-dihydroxyvitamin D, intestinal calcium absorption, and negative calcium balance. Treatment with 1-hydroxylated vitamin D analogues is useful.

Assessment of Micronutrient Status in Critically Ill Children: Challenges and Opportunities

Micronutrients refer to a group of organic vitamins and inorganic trace elements that serve many functions in metabolism. Assessment of micronutrient status in critically ill children is challenging due to many complicating factors, such as evolving metabolic demands, immature organ function, and varying methods of feeding that affect nutritional dietary intake. Determination of micronutrient status, especially in children, usually relies on a combination of biomarkers, with only a few having been established as a gold standard. Almost all micronutrients display a decrease in their serum levels in critically ill children, resulting in an increased risk of deficiency in this setting. While vitamin D deficiency is a well-known phenomenon in critical illness and can predict a higher need for intensive care, serum concentrations of many trace elements such as iron, zinc, and selenium decrease as a result of tissue redistribution in response to systemic inflammation. Despite a decrease in their levels, supplementation of micronutrients during times of severe illness has not demonstrated clear benefits in either survival advantage or reduction of adverse outcomes. For many micronutrients, the lack of large and randomized studies remains a major hindrance to critically evaluating their status and clinical significance.

Vitamin D measurement standardization: The way out of the chaos

Substantial variability is associated with laboratory measurement of serum total 25-hydroxyvitamin D [25(OH)D]. The resulting chaos impedes development of consensus 25(OH)D values to define stages of vitamin D status. As resolving this situation requires standardized measurement of 25(OH)D, the Vitamin D Standardization Program (VDSP) developed methodology to standardize 25(OH)D measurement to the gold standard reference measurement procedures of NIST, Ghent University and CDC. Importantly, VDSP developed protocols for standardizing 25(OH)D values from prior research based on availability of stored serum samples. The effect of such retrospective standardization on prevalence of "low" vitamin D status in national studies reported here for The Third National Health and Nutrition Examination Survey (NHANES III, 1988-1994) and the German Health Interview and Examination Survey for Children and Adolescents (KIGGS, 2003-2006) was such that in NHANES III 25(OH)D values were lower than original values while higher in KIGGS. In NHANES III the percentage with values below 30, 50 and 75 nmol/L increased from 4% to 6%, 22% to 31% and 55% to 71%, respectively. Whereas in KIGGS after standardization the percentage below 30, 50, and 70 nmol/L decreased from 28% to 13%, 64% to 47% and 87% to 85% respectively. Moreover, in a hypothetical example, depending on whether the 25(OH)D assay was positively or negatively biased by 12%, the 25(OH)D concentration which maximally suppressed PTH could vary from 20 to 35ng/mL. These examples underscore the challenges (perhaps impossibility) of developing vitamin D guidelines using unstandardized 25(OH)D data. Retrospective 25(OH)D standardization can be applied to old studies where stored serum samples exist. As a way forward, we suggest an international effort to identify key prior studies with stored samples for re-analysis and standardization initially to define the 25(OH)D level associated with vitamin D deficiency (rickets/osteomalacia). Subsequent work could focus on defining inadequacy. Finally, examples reported here highlight the importance of suspending publication of meta-analyses based on unstandardized 25(OH)D results.

Specific interactions between vitamin-D receptor and its ligands: Ab initio molecular orbital calculations in water

Vitamin D is recognized to play important roles not only in the bone metabolism and the regulation of Ca amount in the blood but also in the onset of immunological diseases. These physiological actions caused by vitamin D are triggered by the specific interaction between vitamin D receptor (VDR) and vitamin D. In the present study, we investigated the interactions between VDR and vitamin D derivatives using ab initio molecular simulation, in order to elucidate the reason for the significant difference in their effects on VDR activity. Based on the results simulated, we elucidated which parts of the derivatives and which residues of VDR mainly contribute to the specific binding between VDR and the derivatives at an electronic level. This finding will be helpful for proposing new vitamin D derivatives as a potent modulator or inhibitor against VDR.

Calcitroic Acid-A Review

Calcitroic acid was isolated and characterized almost four decades ago, but little is known about this important vitamin D metabolite. Four reported synthetic strategies to generate calcitroic acid are presented that highlight the scientific progress in the field of chemistry directed to vitamin D analog synthesis. The most recent synthesis described the generation of calcitroic acid with an overall yield of 12.8% in 13 steps. The endogenous formation of calcitroic acid has been demonstrated in perfused rat kidney using 24,25,26,27-tetranor-1,23(OH)₂D₃. Although, the majority of vitamin D metabolism is mediated by 24-hydoxylase (CYP24A1), it is not clear why the formation of calcitroic acid was not observed in the presence of recombinant CYP24A1 enzyme. Furthermore, it is not known if enzyme 1α-hydroxylase (CYP27B1) can convert calcioic acid into calcitroic acid. In addition to the lack of research investigating the endogenous formation of calcitroic acid, the physiological role of calcitroic acid remains unknown. Only a few reports mentioned the biological activity of calcitroic acid in connection with the vitamin D receptor (VDR). When administered subcutaneously, calcitroic acid has anthracitic properties and elevates calcium blood levels when administered intravenously. In vitro, calcitroic acid at higher concentrations has been shown to bind VDR and induce gene transcription. However, these studies were not carried out in cells derived from target organs of calcitroic acid such as kidney, liver, and intestine. We can conclude that our current knowledge of calcitroic acid is limited, and more studies are needed to identify its physiological role.

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.

Effects of Osteoporosis-Inducing Drugs on Vitamin D-Related Gene Transcription and Mineralization in MG-63 and Saos-2 Cells

Vitamin D₃ is important for calcium and phosphate homeostasis. To exert its effects, vitamin D₃ has to be enzymatically activated into 1,25D₃ (1,25-dihydroxyvitamin D₃ ). Regulation by endogenous vitamin D metabolites of the activation and inactivation of 1,25D₃ is important to maintain adequate amounts of active vitamin D₃ . Vitamin D deficiency and low bone mineral density have been linked to treatments with antiretroviral drugs and glucocorticoids. However, the causes of drug-induced osteoporosis remain unclear. The antiretroviral drugs efavirenz and ritonavir as well as the glucocorticoid dexamethasone were included in this study. Their effects on transcription of vitamin D-regulating enzymes in MG-63 cells were investigated. Ritonavir and dexamethasone both induced transcription of CYP27B1, the enzyme responsible for the formation of 1,25D₃ . Efavirenz, however, suppressed CYP27B1 expression. When administered together with endogenous vitamin D metabolites, dexamethasone and efavirenz counteracted the 1,25D₃ -mediated up-regulation of CYP24A1, which inactivates 1,25D₃ . This suggests that the drugs may interfere with local regulation of the vitamin D metabolizing system in osteoblasts. Studies on mineralization were performed in MG-63 cells and Saos-2 cells by measuring calcium concentrations accumulated over time. The effects of efavirenz, ritonavir and dexamethasone and/or vitamin D metabolites were examined. 1,25D₃ induced mineralization in both cell lines. Efavirenz administered alone did not affect mineralization but suppressed the inducing effects of 1,25D₃ on mineralization in both MG-63 cells and Saos-2 cells. In summary, the results suggest that antiretroviral drugs and glucocorticoids may adversely affect bone by interference with the vitamin D system in osteoblasts.

Epigenome-wide search for association of serum 25-hydroxyvitamin D concentration with leukocyte DNA methylation in a large cohort of older men

Aim: We aimed for an epigenome-wide identification of vitamin D-associated CpG sites in leukocyte DNA. Materials & methods: Infinium HumanMethylation450BeadChip measurements in 402 Caucasian older men were evaluated for significant association with 25-hydroxy-vitamin (25(OH)D) using Spearman's correlation and median regression to adjust for confounding variables. A cross-validation approach as well as a bootstrapping procedure were implemented to determine the replicability of significant associations. Multiple testing was corrected for by Benjamini–Hochberg or Bonferroni. Results: Although in the screening subcohorts significant associations of DNAm with 25(OH)D were observed in the validation cohorts these associations were not replicated after adjustment for potential confounders. At none of the 361,945 CpGs a significant association of DNAm with 25(OH)D was found in all 100 random bootstrap samples, but in comparison at 462 CpGs for the well-established association with age. Conclusion: Leukocyte DNAm was not associated with 25(OH)D levels after validation and consideration of confounders.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

LC-MS/MS for Identifying Patients with CYP24A1 Mutations

BACKGROUND

Patients have been described with loss-of-function CYP24A1 (cytochrome P450, family 24, subfamily A, polypeptide 1) mutations that cause a high ratio of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D [25(OH)D/24,25(OH)2D], increased serum 1,25-dihydroxyvitamin D, and resulting hypercalcemia, hypercalciuria and nephrolithiasis. A 25(OH)D/24,25(OH)2D ratio that can identify patients who are candidates for confirmatory CYP24A1 genetic testing would be valuable. We validated an LC-MS/MS assay for 24,25(OH)2D (D3 and D2) and determined a 25(OH)D/24,25(OH)2D cutoff to identify candidates for confirmatory genetic testing.

METHODS

After addition of isotope-labeled internal standard, serum samples were extracted by solid-phase extraction, derivatized with 4-phenyl-1,2,4,-triazoline-3,5-dione, and quantified by LC-MS/MS. We measured 25(OH)D/24,25(OH)2D in 91 healthy patients and 34 patients with clinically suspected CYP24A1-mediated hypercalcemia.

RESULTS

The limits of detection and quantification were 0.03 (0.2) and 0.1 (0.24) nmol/L, respectively, for 24,25(OH)2D3, and 0.1 (0.23) and 0.5 (1.16) nmol/L for 24,25(OH)2D2. Intra- and interassay imprecision was 4%–15% across the analytical measurement range of 0.1–25 ng/mL (0.2–60 nmol/L). No interference was observed with 25(OH)D and 1,25(OH)2D. 25(OH)D/24,25(OH)2D of 7–35 was observed in healthy patients, whereas in 2 patients with CYP24A1 mutations, 25(OH)D/24,25(OH)2D was significantly increased (99–467; P &lt; 0.001). A 25(OH)D/24,25(OH)2D ratio ≥99 identified patients who were candidates for CYP24A1 genetic testing.

CONCLUSIONS

Increased 25(OH)D/24,25(OH)2D supports the diagnosis of reduced CYP24A1 activity due to mutations in CYP24A1. Measurement of 25(OH)D/24,25(OH)2D should be considered a part of the clinical workup in patients with hypercalcemia of otherwise unknown etiology.

Vitamin D: Historical Overview

A history of vitamin D has been provided, dating from the earliest description of rickets, the disease resulting from vitamin D deficiency, to a current understanding of vitamin D metabolism and the mechanism of action of its hormonal form in regulating gene expression in target organs. Vitamin D is produced in skin by impact of 280-310 nm light on 7-dehydrocholesterol. The vitamin D is then converted in the liver to a circulating form, 25-hydroxyvitamin D that is converted largely, if not exclusively, in the kidney to the final hormone, 1α,25-dihydroxyvitamin D. This hormone functions through a nuclear receptor that regulates expression of key genes in target organs. Among its many resulting functions are increased intestinal calcium and phosphate absorption, bone calcium mobilization, and renal reabsorption of calcium. The resultant increase in serum calcium and phosphate supports bone mineralization, curing rickets, and osteomalacia. There are many other functions of vitamin D that remain to be described that contribute to its health supporting role.

The clinical use of vitamin D metabolites and their potential developments: a position statement from the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) and the International Osteoporosis Foundation (IOF)

Several compounds are produced along the complex pathways of vitamin D3 metabolism, and synthetic analogs have been generated to improve kinetics and/or vitamin D receptor activation. These metabolites display different chemical properties with respect to the parental or native vitamin D3, i.e., cholecalciferol, which has been, so far, the supplement most employed in the treatment of vitamin D inadequacy. Hydrophilic properties of vitamin D3 derivatives facilitate their intestinal absorption and their manageability in the case of intoxication because of the shorter half-life. Calcidiol is a more hydrophilic compound than parental vitamin D3. Active vitamin D analogs, capable of binding the vitamin D receptor evoking vitamin D-related biological effects, are mandatorily employed in hypoparathyroidism and kidney failure with impaired 1α-hydroxylation. They have been shown to increase BMD, supposedly ameliorating calcium absorption and/or directly affecting bone cells, although their use in these conditions is jeopardized by the development of hypercalciuria and mild hypercalcemia. Further studies are needed to assess their overall safety and effectiveness in the long-term and new intermittent regimens, especially when combined with the most effective antifracture agents.

A therapeutic role for vitamin D on obesity-associated inflammation and weight-loss intervention

Vitamin D plays an essential role in the regulation of skeletal metabolism as well as calcium and phosphate homeostasis, while vitamin D receptor (VDR) regulates de novo lipid synthesis, thereby contributing to the development of obesity. Furthermore, obese individuals are at a greater risk for vitamin D deficiency which may increase the potential risk for chronic inflammation, insulin resistance, and metabolic syndrome. While acute exercise enhances the activation of inflammatory signaling pathways, chronic exercise training may attenuate elevated pro-inflammatory cytokine production, resulting in the improvement of cardiovascular and metabolic health in obese individuals. Supplementation with vitamin D coupled with exercise or mild caloric restriction has been shown to improve markers of fitness and inflammation as well as cholesterol. Therefore, this review primarily addresses the impact of vitamin D deficiency in obesity-related inflammatory imbalances and how exercise and weight-loss interventions may enhance the beneficial effects on vitamin D-mediated inflammation in obesity.

Cytochrome P450 Vitamin D Hydroxylases in Inflammation and Cancer

Vitamin D insufficiency correlates with increased incidence of inflammatory disorders and cancer of the colon, breast, liver, and prostate. Preclinical studies demonstrated that the hormonally active form of vitamin D, 1,25(OH)2D3, has antiproliferative, proapoptotic, anti-inflammatory, and immunomodulatory effects. Tissue levels of 1,25(OH)2D3 are determined by expression and activity of specific vitamin D hydroxylases expressed at renal and extrarenal sites. In order to understand how perturbations in the vitamin D system affect human health, we need to understand the steps involved in the synthesis and catabolism of the active metabolite. This review provides an overview about recent findings on the altered vitamin D metabolism in inflammatory conditions and carcinogenesis. We will summarize existing data on the pathophysiological regulation of vitamin D hydroxylases and outline the role of adequate levels of 1,25(OH)2D3 on tissue homeostasis.

Vitamin D as a Resilience Factor, Helpful for Survival of Potentially Fatal Conditions: A Hypothesis Emerging from Recent Findings of the ESTHER Cohort Study and the CHANCES Consortium

There is debate on whether vitamin D deficiency is a risk factor for major chronic diseases and premature death or whether observed associations were just confounded by general health status. Here, we review recent results from the Epidemiologische Studie zu Chancen der Verhütung, Früherkennung und optimierten Therapie chronischer Erkrankungen in der älteren Bevölkerung (ESTHER) cohort study and the Consortium on Heatlh and Ageing: Network of Cohorts from Europe and the United States (CHANCES) that suggest that vitamin D deficiency may not be a risk factor for the development of cardiovascular diseases and cancer but may be a risk factor for fatal instances of these diseases. Furthermore, analyses comprehensively adjusted for the health status showed that the association of vitamin D and mortality was very likely not confounded by general health status. These results suggest that vitamin D could be a marker of resilience to fatality of potentially fatal diseases. Sufficient vitamin D serum concentrations may be needed to regulate the response of the immune system when it is challenged by severe diseases to prevent a fatal course of the disease. If this hypothesis can be verified through basic research studies and adequately designed randomized controlled trials, it could have important public health implications because vitamin D deficiency is very common worldwide, and interventions could be implemented easily.

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'.

Synthesis of novel vitamin D3 analog with an additional ring annulated to A and seco-B rings

A simple method for the synthesis of yet unknown 5E-vitamin D3 analogs with an additional six-membered ring connecting C-6 and C-19 was developed. Ring-closing metathesis (RCM) was used for efficient formation thereof from the corresponding 5E-isomers of 6-alkenyl vitamin D3 compounds which in turn were prepared from the 6-oxo-3,5-cyclovitamin D3. Reinvestigation of the Grignard reactions of this latter compound as well as the following acid-catalyzed cycloreversions showed discrepancies with the literature data describing the course of such processes.

History of the discovery of vitamin D and its active metabolites

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

Triennial Growth Symposium--Vitamin D: bones and beyond

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

Calcidiol [25(OH)D3]: from diagnostic marker to therapeutical agent

OBJECTIVE

Osteoporosis is a skeletal disorder characterized by diminished bone strength, which results in an increased risk of fracture. Currently, osteoporosis is a public health priority due to the large number of individuals affected and the detrimental effect on quality of life. Primary osteoporosis, the most common form, usually results from age-related reduction in bone mineral strength. Over time, the individual's capacity to build bone is impaired, as the synthesis of vitamin D, the hormone responsible for calcium absorption, tends to decline. As serum calcium levels decrease, metabolic control serves to increase the removal of calcium from the skeleton to make up for the deficit. The synthesis of the 'hormone' vitamin D and its control therefore become central to intervention in involutional osteoporosis syndromes. In humans, plain vitamin D (cholecalciferol), also called parental or native vitamin D, is photosynthesized in the skin and then hydroxylated in the liver into the vitamin D analog calcidiol [25(OH)D3], which is hydroxylated again in the kidney into the vitamin D analog calcitriol [1,25(OH)2D3]. The advantage of administering vitamin D analogs is that the pro-drug calcidiol avoids the effect of declines in hepatic function, while calcitriol avoids the effect of declines in hepatic and kidney function. A strategy to enhance [25(OH)D3] levels to the optimal threshold of vitamin D is supplementation with the calcidiol metabolite itself. The goal of this paper is to review published studies on the efficacy of the calcidiol metabolite in increasing 25(OH)D3 serum levels and improving skeletal health parameters in humans.

METHODS

A library search of published papers in the area of use of calcidiol in humans from 1967 to 2013 was performed (key words: calcidiol, 25-hydroxy-vitamin D3, vitamin D supplementation, vitamin D metabolism, osteomalacia).

RESULTS AND CONCLUSION

The results of the survey made it possible to conclude that calcidiol is characterized by a number of features that make the compound ideal in conditions that require supplementation with a 25-hydroxylated metabolite.

Synthesis of novel 19-norvitamin D3 analogs with unnatural triene system

9-Alkylidene analogs of 19-nor-1α,25-(OH)2D3 were synthesized, possessing a 'reversed' triene system compared to the natural hormone. The conjugated triene moiety of the novel analogs was constructed by coupling an enyne anion, representing an A-ring synthon, with a 9α-substituted Grundmann ketone derivative. Regioselective dehydration followed by semihydrogenation under Lindlar conditions, provided the desired 9-alkylated 19-norprevitamins which were thermally isomerized to the corresponding 9-methylene and 9-ethylidene analogs of 19-norcalcitriol. It was established that only the former compound had significant binding affinity to the full-length recombinant rat vitamin D receptor. The remaining in vitro studies show very low activity of both analogs. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Serum 25-hydroxyvitamin D levels and overall mortality. A systematic review and meta-analysis of prospective cohort studies

OBJECTIVE

To provide a systematic review and meta-analysis of prospective, population-based cohort studies on the association of serum 25-hydroxyvitamin D (25(OH)D) and all-cause mortality.

METHODS

Relevant studies were identified by systematically searching Medline, EMBASE and ISI Web of Knowledge. Reported hazard ratios (HRs) for 25(OH)D categories were recalculated employing comprehensive trend estimation from summarized dose-response data and pooled in a random effects model meta-analysis.

RESULTS

Overall, 12 original studies were included in the review and meta-analysis comprising 32,142 mainly elderly study participants with measured 25(OH)D of whom 6921 died during follow-up. An inverse association between 25(OH)D levels and all-cause mortality was found in all but two studies that was statistically significant in several of the individual studies. In meta-analysis, 25(OH)D levels were significantly inversely associated with all-cause mortality with a pooled HR of 0.92 (95% confidence interval: 0.89-0.95) for a 20 nmol/l increase in 25(OH)D levels.

CONCLUSION

In this meta-analysis of prospective, population-based cohort studies, a 20 nmol/l increase in 25(OH)D levels was associated with an 8% lower mortality in the general elderly population. This agrees with results from meta-analyses on randomized controlled trials that found a decrease in mortality with vitamin D3 supplementation of a comparable magnitude.

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 pregnancy: skeletal effects, nonskeletal effects, and birth outcomes

The function and requirement of vitamin D during pregnancy for both mother and fetus have remained a mystery. This fact was highlighted by The Cochrane Review in 2000, which reported a lack of randomized controlled trials (RCTs) with respect to vitamin D requirements during pregnancy. Unfortunately, during the past decade only a single RCT has been performed with respect to vitamin D requirements during pregnancy. In this review we will discuss vitamin D metabolism during pregnancy as well as the consequences of vitamin D deficiency on skeletal, nonskeletal, and birth outcomes using birth observational data and data from our recent RCT. New RCT data strongly support previous observational studies in that improving nutritional vitamin D status will improve birth outcomes. The new RCT data indicate that 4,000 IU/day vitamin D(3) during pregnancy will "normalize" vitamin D metabolism and improve birth outcomes including primary cesarean section and comorbidities of pregnancy with no risk of side effects.

The role of mass spectrometry in the analysis of vitamin D compounds

This review highlights the superseding role of mass spectrometry in the structural characterization and quantitation of vitamin D compounds in comparison to other analytical methods (e.g., UV, bioassays) that lack the sensitivity and specificity of mass spectrometry. After a short introduction to the biochemistry of vitamin D compounds, an overview of the current techniques to characterize and quantitate vitamin D compounds is given with emphasis on the contribution of mass spectrometry.

Dietary modification of metabolic pathways via nuclear hormone receptors

Nuclear hormone receptors (NHRs), as ligand-dependent transcription factors, have emerged as important mediators in the control of whole body metabolism. Because of the promiscuous nature of several members of this superfamily that have been found to bind ligand with lower affinity than the classical steroid NHRs, they consequently display a broader ligand selectivity. This promiscuous nature has facilitated various bioactive dietary components being able to act as agonist ligands for certain members of the NHR superfamily. By binding to these NHRs, bioactive dietary components are able to mediate changes in various metabolic pathways, including, glucose, cholesterol and triglyceride homeostasis among others. This review will provide a general overview of the nuclear hormone receptors that have been shown to be activated by dietary components. The physiological consequences of such receptor activation by these dietary components will then be discussed in more detail.