Modulation of TNF-alpha expression in bone marrow macrophages: involvement of vitamin D response element

Acute hyperglycemia induces podocyte apoptosis by monocyte TNF-α release, a process attenuated by vitamin D and GLP-1 receptor agonists

Targeting optimal glycemic control based on hemoglobin A1c (A1c) values reduces but does not abolish the onset of diabetic kidney disease and its progression to chronic kidney disease (CKD). This suggests that factors other than the average glucose contribute to the residual risk. Vitamin D deficiency and frequent episodes of acute hyperglycemia (AH) are associated with the onset of albuminuria and CKD progression in diabetes. This study aimed to determine if moderate levels of AH harm podocytes directly or promote a pro-inflammatory monocyte/macrophage phenotype that leads to podocyte apoptosis, and whether vitamin D deficiency accelerates these processes. We found that AH (16.7 mM D- glucose) didn't induce podocyte apoptosis directly, but it did promote a pro-inflammatory response in human monocytes and macrophages, resulting in an increased TNF-α secretion causing podocyte apoptosis. The AH-induced monocyte TNF-α secretion was inversely correlated with healthy donors' serum 25(OH)D levels. AH induced monocyte TNF-α release by increasing oxidative and ER stress, which in turn increased ADAM17 (A Disintegrin And Metalloprotease 17) and iRhom2 (inactive Rhomboid protein 2) expression, both essential for TNF-α secretion. Additionally, monocyte activation of glucagon-like peptide-1 receptor (GLP-1R), using a GLP-1R agonist, downregulated ADAM17/iRhom2 expression, decreasing TNF-α release and reducing podocyte apoptosis. These results show that a normal vitamin D status may attenuate a mechanism by which AH contributes to podocyte apoptosis and CKD progression and might enhance a novel anti-inflammatory role of GLP-1 to prevent AH-driven CKD progression in diabetes.

In vitro evaluation of the immunomodulatory and antibacterial activities of calcitriol (1,25-dihydroxy-vitamin D3) as a potential application for aerobic vaginitis treatment - preliminary study results

BACKGROUND

Aerobic vaginitis (AV) is a state of abnormal vaginal microbiota, which is associated with increased numbers of aerobic, enteric bacteria and inflammation of the vaginal epithelium. Anti-microbial treatment combined with anti-inflammatory therapy could be useful in the treatment of this condition. It is known that calcitriol, the active form of vitamin D, plays an important role in modulating the immune response in several inflammatory diseases. The aim of this preliminary study was to evaluate in vitro the influence of calcitriol on the immune response of human vaginal epithelial cells to bacterial infection. Moreover, we assessed the anti-bacterial properties of calcitriol, as well as its synergistic activity with antibiotics that are used for the treatment of AV.

RESULTS

Human vaginal A431 epithelial cells were treated with calcitriol (100 nM) and then stimulated with thermally inactivated strains of AV-associated bacteria (Escherichia coli, Enterococcus faecalis, Streptococcus agalactiae), or first stimulated with heat-treated bacteria and then incubated with calcitriol (30 nM). After 24 h, 48 h, and 72 h post-infection, culture supernatants were collected, and levels of pro-inflammatory cytokines IL-6 and IL-1β were analyzed using enzyme-linked immunosorbent assays (ELISA). As it turned out, calcitriol treatment pre- or post-bacterial stimulation of vaginal epithelial cells significantly decreased the levels of IL-6 and IL-1β compared to cells stimulated only with bacteria. Downregulation of pro-inflammatory cytokines was more frequently significant when cells were pre-treated with calcitriol. Additionally, this study evaluated the anti-bacterial properties and synergistic activity of calcitriol with antibiotics by determining the Minimal Inhibitory Concentration (MIC) using colorimetric, Resazurin-based microdilution method detecting the active metabolism of bacteria. As a result, calcitriol by itself did not exhibit clinically relevant anti-bacterial activity; however, when combined with antibiotics, calcitriol significantly reduced the MIC values.

CONCLUSIONS

The results of this preliminary study confirm that calcitriol treatment influences cytokine-mediated immune response during infection and indicate that it can be effective in enhancing antibiotic activity. Such properties could be very useful in the treatment of inflammatory diseases. However, conclusions regarding the potential use of calcitriol in the treatment of AV require further confirmation in in vivo studies as well as in well-designed clinical trials.

Innate Immunity in Calcinosis Cutis

Calcinosis cutis is the deposition of calcium salts in the skin and subcutaneous tissue, manifesting as variably shaped papules, nodules, and plaques that can substantially impair quality of life. The pathophysiology of calcinosis cutis involves dysregulation of proinflammatory cytokines, leukocytes, and other components of the innate immune system. In some conditions associated with calcinosis cutis, elevated serum calcium, phosphate, and vitamin D may also perturb innate immunity. The mechanisms by which these lead to cutaneous and subcutaneous calcification likely parallel those seen in vascular calcification. The role of aberrant innate immunity is further supported by the association between various autoantibodies with calcinosis cutis, such as anti-MDA5, anti-NXP2, anti-centromere, and anti-topoisomerase I. Treatments for calcinosis cutis remain limited and largely experimental, although mechanistically many therapies appear to focus on dampening innate immune responses. Further research is needed to better understand the innate immune pathophysiology and establish treatment options based on randomized-controlled trials.

Vitamin D attenuates COVID-19 complications via modulation of proinflammatory cytokines, antiviral proteins, and autophagy

INTRODUCTION

Global emergence of coronavirus disease-19 (COVID-19) has clearly shown variable severity, mortality, and frequency between and within populations worldwide. These striking differences have made many biological variables attractive for future investigations. One of these variables, vitamin D, has been implicated in COVID-19 with rapidly growing scientific evidence.

AREAS COVERED

The review intended to systematically explore the sources, and immunomodulatory role of vitamin D in COVID-19. Search engines and data sources including Google Scholar, PubMed, NCBI, Scopus, and Web of Science were used for data collection. The search terms used were Vitamin D, COVID-19, immune system, and antiviral mechanism. Overall, 232 sources of information were collected and 188 were included in this review.

EXPERT OPINION

Interaction of vitamin D and vitamin D receptor (VDR) triggers the cellular events to modulate the immune system by regulation of many genes. Vitamin D operates as a double-edged sword against COVID-19. First, in macrophages, it promotes the production of antimicrobial and antiviral proteins like β-defensin 2 and cathelicidin, and these proteins inhibit the replication of viral particles and promote the clearance of virus from the cells by autophagy. Second, it suppresses cytokine storm and inflammatory processes in COVID-19.

Anti-tumor effects of vitamin D in glioblastoma: mechanism and therapeutic implications

Glioma is the most prevalent primary brain tumor in adults among which glioblastoma is the most malignant and lethal subtype. Its common resistance to conventional chemotherapeutics calls for the development of alternative or concomitant treatment. Taking advantage of its endocrine function as a neurosteroid, vitamin D has become a target of interest to be used in conjunction with different chemotherapies. In this article, we review the mechanisms through which vitamin D and its analogs induce anti-tumor activity in glioblastoma, and the practical issues relevant to their potential application based on in vitro and in vivo studies. Vitamin D has largely been reported to promote cell cycle arrest and induce cell death to suppress tumor growth in glioblastoma. Glioblastoma cells treated with vitamin D have also shown reduced migratory and invasive phenotypes, and reduced stemness. It is worth noting that vitamin D analogs are able to produce similar inhibitory actions without causing adverse effects such as hypercalcemia in vivo. Upregulation of vitamin D receptors by vitamin D and its analogs may also play a role in enhancing its anti-tumor activity. Based on current findings and taking into consideration its potential cancer-protective effects, the clinical application of vitamin D in glioblastoma treatment and prevention will be discussed. With some study findings subject to controversy, further investigation is warranted to elucidate the mechanism of action of vitamin D and to evaluate relevant issues regarding its treatment efficacy and potential clinical application.

Steroid Hormone Vitamin D: Implications for Cardiovascular Disease

Understanding of vitamin D physiology is important because about half of the population is being diagnosed with deficiency and treated with supplements. Clinical guidelines were developed based on observational studies showing an association between low serum levels and increased cardiovascular risk. However, new randomized controlled trials have failed to confirm any cardiovascular benefit from supplementation in the general population. A major concern is that excess vitamin D is known to cause calcific vasculopathy and valvulopathy in animal models. For decades, administration of vitamin D has been used in rodents as a reliable experimental model of vascular calcification. Technically, vitamin D is a misnomer. It is not a true vitamin because it can be synthesized endogenously through ultraviolet exposure of the skin. It is a steroid hormone that comes in 3 forms that are sequential metabolites produced by hydroxylases. As a fat-soluble hormone, the vitamin D-hormone metabolites must have special mechanisms for delivery in the aqueous bloodstream. Importantly, endogenously synthesized forms are carried by a binding protein, whereas dietary forms are carried within lipoprotein particles. This may result in distinct biodistributions for sunlight-derived versus supplement-derived vitamin D hormones. Because the cardiovascular effects of vitamin D hormones are not straightforward, both toxic and beneficial effects may result from current recommendations.

Vitamins (A&D) and Isoprenoid (Chenodeoxycholic acid) molecules are accompanied by Th1 immunostimulatory response and therapeutic cure in vivo: possible antileishmanial drugs

Investigation of immune modulatory anti-leishmanial molecules is now being strongly encouraged to overcome the immunosuppression manifested during visceral leishmaniasis (VL), resistance, toxicity and high cost associated with conventional therapeutics. In the present study, we explored the protective efficacy of vitamin D₃, retinoic acid and isoprenoid chenodeoxycholic acid (CDCA) combinations against L. donovani infected BALB/c mice. We also probed the immune modulatory response (Th1 & Th2 cytokines) and infection dynamics following experimental infections with drug treated animals. Our results indicate that Vit.D₃/RA and CDCA/RA combination treatment led to significant inhibition of parasite load on days 21 and 28 post treatment. Furthermore, there was a marked inhibition of Th2 type immune responses in IL-4, IL-5 and polarization of Th1 biased immunity along with upregulation of IL-1, IFN-γ, and TNF-α levels on day 28 post treatment. In addition, mice treated with Vit.D₃/RA and CDCA/RA demonstrates here that splenic histological recovery against the virulent challenge of L. donovani by day 28 was comparable to control group. The conclusions derived from this study suggests that a combination of vitamin A, D₃ and isoprenoids may have a potential immunomodulatory therapeutic role against leishmaniasis.

Vitamin D as a Principal Factor in Mediating Rheumatoid Arthritis-Derived Immune Response

Rheumatoid arthritis (RA) is a systemic multifactorial autoimmune disorder. The interactions between diverse environmental and genetic factors lead to the onset of this complex autoimmune disorder. Serum levels of vitamin D (VD) are involved in the regulation of various immune responses. Vitamin D is a key signaling molecule in the human body that maintains calcium as well as phosphate homeostasis. It also regulates the functions of the immune system and, thus, can play a substantial role in the etiology of various autoimmune disorders, including RA. Low serum VD levels have been found to be associated with a higher risk of RA, although this finding has not been replicated consistently. The molecular mechanisms by which VD influences autoimmunity need to be further explored to understand how variation in plasma VD levels could affect the pathogenesis of RA. This mini-review focuses on the influence of VD and its serum levels on RA susceptibility, RA-associated complexities, treatment, and transcriptome products of key proinflammatory cytokines, along with other cytokines that are key regulators of inflammation in rheumatoid joints.

Association of VDR gene polymorphisms and 22 bp deletions in the promoter region of TLR2Δ22 (-196-174) with increased risk of pulmonary tuberculosis: A case-control study in tea garden communities of Assam

BACKGROUND

A high number of pulmonary tuberculosis (PTB) cases have been reported from tea garden communities of Assam. Till date, no molecular epidemiological study was performed to investigate the association of candidate gene(s) with the risk PTB in this region. The present case-control study was aimed to investigate the association of vitamin D receptor (VDR) gene polymorphisms and 22 bp deletion in the promoter region of toll-like receptor 2 (TLR2) gene with the risk of PTB in tea garden communities of Assam.

METHODS

Genotyping of VDR polymorphisms and TLR2Δ22 (-196-174) gene was carried out for 169 PTB cases and 227 apparently healthy community controls using blood samples by PCR-RFLP followed by DNA sequencing. For association study, both univariate and multivariate logistic regression analyses were performed.

RESULTS

This study has shown that BsmI and FokI polymorphisms of VDR gene significantly associated with an increased risk of PTB (AOR = 3.58, 95% CI = 1.64-7.80, P < .01 for B/b genotype of BsmI and AOR = 2.44, 95% CI = 1.40-4.24, P < .01 for F/f genotype of FokI polymorphism). No significant association of TaqI and ApaI polymorphism of VDR gene was found with the risk of PTB. Moreover, this study has revealed that person carrying deletion allele in their TLR2Δ22 (-196-174) gene is significantly associated with an increased risk of PTB having b/b or F/f genotypes in BsmI or FokI polymorphisms of VDR gene.

CONCLUSION

This study has revealed that BsmI and FokI polymorphisms of VDR gene significantly associated with an increased risk of PTB.

Paracrine interactions between mesenchymal stem cells and macrophages are regulated by 1,25-dihydroxyvitamin D3

Mesenchymal stem cells (MSC) modulate the macrophage-mediated inflammatory response through the secretion of soluble factors. In addition to its classical effects on calcium homeostasis, 1,25-dihydroxyvitamin D3 (1,25D3) has emerged as an important regulator of the immune system. The present study investigates whether 1,25D3 modulates the paracrine interactions between MSC and macrophages. 1,25D3 stimulated MSC to produce PGE₂ and VEGF and regulated the interplay between macrophages and MSC toward reduced pro-inflammatory cytokine production. Conditioned media (CM) from co-cultures of macrophages and MSC impaired MSC osteogenesis. However, MSC cultured in CM from 1,25D3-treated co-cultures showed increased matrix maturation and mineralization. Co-culturing MSC with macrophages prevented the 1,25D3-induced increase in RANKL levels, which correlated with up-regulation of OPG secretion. MSC seeding in three-dimensional (3D) substrates potentiated their immunomodulatory effects on macrophages. Exposure of 3D co-cultures to 1,25D3 further reduced the levels of soluble factors related to inflammation and chemotaxis. As a consequence of 1,25D3 treatment, the recruitment of monocytes toward CM of 3D co-cultures decreased, while the osteogenic maturation of MSC increased. These data add new insights into the pleiotropic effects of 1,25D3 on the crosstalk between MSC and macrophages and highlight the role of the hormone in bone regeneration.

Protein pathways working in human follicular fluid: the future for tailored IVF?

The human follicular fluid (HFF) contains molecules and proteins that may affect follicle growth, oocyte maturation and competence acquiring. Despite the numerous studies, an integrated broad overview on biomolecular and patho/physiological processes that are proved or supposed to take place in HFF during folliculogenesis and oocyte development is still missing. In this review we report, for the first time, all the proteins unambiguously detected in HFF and, applying DAVID (Database for Annotation, Visualization and Integrated Discovery) and MetaCore bioinformatic resources, we shed new lights on their functional correlation, delineating protein patterns and pathways with reasonable potentialities for oocyte quality estimation in in vitro fertilisation (IVF) programs. Performing a rigorous PubMed search, we redacted a list of 617 unique proteins unambiguously-annotated as HFF components. Their functional processing suggested the occurrence in HFF of a tight and highly dynamic functional-network, which is balanced by specific effectors, primarily involved in extracellular matrix degradation and remodelling, inflammation and coagulation. Metalloproteinases, thrombin and vitamin-D-receptor/retinoid-X-receptor-alpha resulted as the main key factors in the nets and their differential activity may be indicative of ovarian health and oocyte quality. Despite future accurate clinical investigations are absolutely needed, the present analysis may provide a starting point for more accurate oocyte quality estimation and for defining personalised therapies in reproductive medicine.

The Relevance of Vitamin D and Antinuclear Antibodies in Patients with Inflammatory Bowel Disease Under Anti-TNF Treatment: A Prospective Study

BACKGROUND

The importance of vitamin D in inflammatory bowel disease (IBD) has been analyzed in former studies, namely concerning the severity of the disease and the efficacy of anti-tumor necrosis factor (TNF) medications. In several inflammatory conditions, biologics have been associated with an autoimmune response with formation of antinuclear antibodies (ANA). In addition, an inverse relationship between vitamin D levels and ANA has been documented. We aimed to evaluate the clinical importance of the link between vitamin D, ANA, and anti-TNF in patients with IBD.

METHODS

Prospective study including patients with IBD with indication to start anti-TNF, between 2009 and 2014. Deficiency and extreme deficiency of vitamin D were defined as levels of 25-hydroxyvitamin D below 20 and 4 ng/mL, respectively. ANA titers were considered positive if higher or equal to 1/100.

RESULTS

Among 68 patients (56 with Crohn's Disease, 12 with ulcerative colitis), vitamin D deficiency was detected in 93%. Pretreatment positivity for ANA was related to higher failure rates of anti-TNF treatment (P = 0.008). Pretreatment positivity for ANA and extreme vitamin D deficiency were significant risk factors for adverse events associated with anti-TNF therapy. A significant link was found between extreme deficiency of vitamin D and the presence of ANA.

CONCLUSIONS

Our study highlights the association between vitamin D deficiency and pretreatment positivity for ANA with the risk for anti-TNF failure and adverse events, and the inverse relationship between vitamin D levels and ANA. Due to the high prevalence of vitamin D deficiency in IBD and the immune-mediated nature of the disease, these elements should be evaluated before starting biologics.

Vitamin D and respiratory tract infections in childhood

BACKGROUND

Respiratory tract infections (RTIs) remain among of the most important causes of morbidity and mortality among children. Several studies have associated vitamin D deficiency with an increased risk of RTIs, and vitamin D supplementation has been proposed as a possible preventive measure against RTIs in children. The main aim of this review is to summarize the current evidence from the literature about the link between vitamin D and RTIs in children.

DISCUSSION

Several recent studies have shown that vitamin D has different immunomodulatory properties associated with the risk of RTIs in childhood. In this regard, it is very important to understand the definition of deficiency and insufficiency of vitamin D and when and how to treat this condition. Unfortunately, there is no consensus, although a level of at least 10 ng/mL 25-hydroxycholecalciferol (25[OH]D) is thought to be necessary to promote bone mineralization and calcium homeostasis, and a concentration between 20 ng/mL and 50 ng/mL is considered adequate to provide an immunomodulatory effect. Available data support a role for vitamin D deficiency in the risk of pediatric tuberculosis, recurrent acute otitis media, and severe bronchiolitis, whereas further studies are needed to confirm an association in children with recurrent pharyngotonsillitis, acute rhinosinusitis and community-acquired pneumonia.

CONCLUSIONS

Maintenance of adequate vitamin D status may be an effective and inexpensive prophylactic method against some RTIs, but the supplementation regimen has not been clearly defined. Further clinical trials are needed to determine the 25(OH)D concentrations associated with an increased risk of RTIs and optimal vitamin D supplementation regimen according to the type of RTI while also taking into consideration vitamin D receptor polymorphisms.

Vitamin D3 modulates the function of chicken macrophages

Vitamin D3 is known to modulate both innate and adaptive immune responses in mammals, but there is little information on its effects on avian immune system cells. Here, we studied the effects of vitamin D3 on chicken macrophages. Chicken macrophages expressed vitamin D receptor (VDR) and lipopolysaccharide (LPS) stimulation increased their VDR expression. Macrophages were treated with 1,25(OH)2D3 in the presence or absence of Toll-like receptor ligands, such as LPS and Pam3CSK4. Subsequently, macrophage activation was assessed by measuring nitric oxide (NO) and expression of CXCL8 and interleukin (IL)-1β. In addition, changes in major histocompatibility complex (MHC)-II and CD86 were examined. Treatment of cells with 1,25(OH)2D3 increased the ability of macrophages to respond to stimuli and produce NO, but vitamin D3 alone did not activate macrophages and resulted in the down-regulation of CD86, MHC-II, CXCL8 and IL-1β. These findings suggest that vitamin D3 has an immunomodulatory role in chicken macrophages.

1,25(OH)2D3 prolongs islet graft survival by inflammatory inhibition

OBJECTIVE

This study aimed to determine the protective effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) against islet graft loss.

METHODS

Proliferation of tumor necrosis factor (TNF)-α-induced macrophages was determined in vitro after treatment with different concentrations of 1,25-(OH)2D3. Intraportal islet transplantation (IPIT) was performed with islets harvested from the Sprague-Dawley rats and transplanted to the diabetic rats. The transplanted rats were assigned to receive 1,25-(OH)2D3 or propylene glycol (control). Islet graft survival; inflammatory cytokine (TNF-α and interleukin [IL]-1); numbers and percentages of macrophages, CD4(+), and CD8(+) T cells in bloods; and expression of nuclear factor (NF)-κB and TNF-α were analyzed. Hematoxylin and eosin staining was performed.

RESULTS

We found 100 mg/mL 1,25-(OH)2D3 per day to have the strongest inhibitory effect on macrophages. Survival time of islet grafts significantly increased in the rats receiving 1,25-(OH)2D3. There were fewer infiltrated inflammatory cells in both islet graft and adjacent tissue in the drug-treated rats with lower serum IL-1 and TNF-α. Furthermore, percentage of macrophages and expression of p-NF-κB p65 and TNF-α in graft sites were significantly lower in the treated rats.

CONCLUSION

Our results demonstrated that 1,25(OH)2D3 prolongs islet graft survival by decreasing nonspecific inflammation in syngeneic IPIT through inhibiting TNF-α/NF-κB pathway and macrophage infiltration.

Vitamin D in inflammatory diseases

Changes in vitamin D serum levels have been associated with inflammatory diseases, such as inflammatory bowel disease (IBD), rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis (MS), atherosclerosis, or asthma. Genome- and transcriptome-wide studies indicate that vitamin D signaling modulates many inflammatory responses on several levels. This includes (i) the regulation of the expression of genes which generate pro-inflammatory mediators, such as cyclooxygenases or 5-lipoxygenase, (ii) the interference with transcription factors, such as NF-κB, which regulate the expression of inflammatory genes and (iii) the activation of signaling cascades, such as MAP kinases which mediate inflammatory responses. Vitamin D targets various tissues and cell types, a number of which belong to the immune system, such as monocytes/macrophages, dendritic cells (DCs) as well as B- and T cells, leading to individual responses of each cell type. One hallmark of these specific vitamin D effects is the cell-type specific regulation of genes involved in the regulation of inflammatory processes and the interplay between vitamin D signaling and other signaling cascades involved in inflammation. An important task in the near future will be the elucidation of the regulatory mechanisms that are involved in the regulation of inflammatory responses by vitamin D on the molecular level by the use of techniques such as chromatin immunoprecipitation (ChIP), ChIP-seq, and FAIRE-seq.

Vitamin D inhibits the occurrence of experimental cerebral malaria in mice by suppressing the host inflammatory response

In animal models of experimental cerebral malaria (ECM), neuropathology is associated with an overwhelming inflammatory response and sequestration of leukocytes and parasite-infected RBCs in the brain. In this study, we explored the effect of vitamin D (VD; cholecalciferol) treatment on host immunity and outcome of ECM in C57BL/6 mice during Plasmodium berghei ANKA (PbA) infection. We observed that oral administration of VD both before and after PbA infection completely protected mice from ECM. VD administration significantly dampened the inducible systemic inflammatory responses with reduced circulating cytokines IFN-γ and TNF and decreased expression of these cytokines by the spleen cells. Meanwhile, VD also resulted in decreased expression of the chemokines CXCL9 and CXCL10 and cytoadhesion molecules (ICAM-1, VCAM-1, and CD36) in the brain, leading to reduced accumulation of pathogenic T cells in the brain and ultimately substantial improvement of the blood-brain barriers of PbA-infected mice. In addition, VD inhibited the differentiation, activation, and maturation of splenic dendritic cells. Meanwhile, regulatory T cells and IL-10 expression levels were upregulated upon VD treatment. These data collectively demonstrated the suppressive function of VD on host inflammatory responses, which provides significant survival benefits in the murine ECM model.

[Repairing the spinal cord with vitamin D: a promising strategy]

In 2014, a phase II randomised, double blind clinical trial assessing the efficacy of cholecalciferol (vitamin D3) in patients with a cervical trauma will be set up. This trial stems from previous studies showing that vitamin D supplementation improves functional recovery in rat models of peripheral or central nerve injury. In a first series of experiments, we used a rat model of peripheral nerve trauma to demonstrate the therapeutic efficiency of vitamin D. We first demonstrated that ergocalciferol (vitamin D2) increases the number and the diameter of newly formed axons and improves the response of metabosensitive fibers from tibialis muscle, in a model of transected peroneal nerve. Then, we compared vitamin D2 and vitamin D3 and observed that the latter is more efficient. At the dose of 500 IU/kg/day, vitamin D3 induces a dramatic functional recovery. We also demonstrated that vitamin D3 increases the number of preserved or newly formed axons in the proximal end, the mean axon diameter in the distal end, neurite myelination in both the distal and proximal ends as well as the expression of genes involved in axogenesis and myelination. In parallel, we assessed the therapeutic role of vitamin D on the central nervous system. In a first study, using a rat model of spinal cord compression at the T10 thoracic level, we delivered vitamin D3 (cholecalciferol) orally at the dose of 50 IU/kg/day or 200 IU/kg/day. When compared to control animals, vitamin D-treated rats displayed, three months after injury, a significant improvement of ventilatory frequency and a reduction of H reflex indicating functional improvements at three months post-injury. In a second study, we used a rat model of cervical hemisection (C2) with a higher dose of oral vitamin D3 (500 IU/kg/day) delivered weekly, during 12 weeks. We observed an improved locomotor recovery, a reduced spasticity and a significantly higher rate of axons crossing the lesion site in treated animals. However, it must be pointed out that the functional improvement is reduced when vitamin D is provided one week after the trauma.

Vitamin D in the maternal-fetal-neonatal interface: clinical implications and requirements for supplementation

Identification of the current evidence regarding the pathophysiological and clinical facets of vitamin D in the maternal-fetal-neonatal interface is of value because of the significance of the vitamin D endocrine system in human health and high prevalence of vitamin D deficiency in mothers and their infants. Although many questions have still not been answered by the existing literature, we found evidence that: (i) during pregnancy vitamin D participates in fetal skeletal mineralization and growth, (ii) neonatal vitamin D levels are dependent on the maternal vitamin D status at delivery, (iii) a vitamin D sufficient status at birth may decrease the risk for the development of asthma and type 1 diabetes mellitus in later life, (iv) recommendations for maintaining serum 25-hydroxyvitamin D [25(OH)D] levels ≥32 ng/mL to avoid secondary hyperparathyroidism in adults have not been applied to mothers and their infants, (v) American Academy of Pediatrics recommended supplementation of 400 IU of vitamin D per day is sufficient only for infants who are born with normal vitamin D levels and (vii) supplementation of lactating mothers with high doses of vitamin D (4000 IU/d) allows the achievement of optimal 25(OH)D concentrations (>32 ng/mL) in the maternal and infant serum without any risk of hypervitaminosis D in the mother. We believe that inconsistency in the recognition of sufficient levels of vitamin D in mothers and their infants affects the identification of adequate doses for vitamin D supplementation during pregnancy, lactation and infancy.

Peripubertal vitamin D(3) deficiency delays puberty and disrupts the estrous cycle in adult female mice

The mechanism(s) by which vitamin D(3) regulates female reproduction is minimally understood. We tested the hypothesis that peripubertal vitamin D(3) deficiency disrupts hypothalamic-pituitary-ovarian physiology. To test this hypothesis, we used wild-type mice and Cyp27b1 (the rate-limiting enzyme in the synthesis of 1,25-dihydroxyvitamin D(3)) null mice to study the effect of vitamin D(3) deficiency on puberty and reproductive physiology. At the time of weaning, mice were randomized to a vitamin D(3)-replete or -deficient diet supplemented with calcium. We assessed the age of vaginal opening and first estrus (puberty markers), gonadotropin levels, ovarian histology, ovarian responsiveness to exogenous gonadotropins, and estrous cyclicity. Peripubertal vitamin D(3) deficiency significantly delayed vaginal opening without affecting the number of GnRH-immunopositive neurons or estradiol-negative feedback on gonadotropin levels during diestrus. Young adult females maintained on a vitamin D(3)-deficient diet after puberty had arrested follicular development and prolonged estrous cycles characterized by extended periods of diestrus. Ovaries of vitamin D(3)-deficient Cyp27b1 null mice responded to exogenous gonadotropins and deposited significantly more oocytes into the oviducts than mice maintained on a vitamin D(3)-replete diet. Estrous cycles were restored when vitamin D(3)-deficient Cyp27b1 null young adult females were transferred to a vitamin D(3)-replete diet. This study is the first to demonstrate that peripubertal vitamin D(3) sufficiency is important for an appropriately timed pubertal transition and maintenance of normal female reproductive physiology. These data suggest vitamin D(3) is a key regulator of neuroendocrine and ovarian physiology.

Decreased anti-inflammatory responses to vitamin D in neonatal neutrophils

Neutrophil activity is prolonged in newborns, suggesting decreased exposure and/or responses to immunosuppressive modulators, such as 1,25-hydroxyvitamin D(3) (1,25-vit D(3)). We hypothesized that 1,25-vit D(3) suppresses neutrophil activation and that this response is impaired in newborns. Consistent with this, 1,25-vit D(3) decreased LPS-induced expression of macrophage inflammatory protein-1β and VEGF in adult, but not neonatal, neutrophils. Expression of vitamin D receptor (VDR) and 25-hydroxyvitamin D(3)-1α-hydroxylase was reduced in neonatal, relative to adult neutrophils. Moreover, 1,25-vit D(3) induced VDR gene expression in activated adult, but not neonatal, neutrophils. 1,25-vit D(3) also suppressed expression of cyclooxygenase-2 and induced expression of 5-lipoxygenase in LPS-exposed adult neutrophils, while neonatal cells were not affected. 1,25-vit D(3) had no effect on respiratory burst in either adult or neonatal cells. Anti-inflammatory activity of vitamin D is impaired in neonatal neutrophils, and this may be due to decreased expression of VDR and 1α-hydroxylase. Insensitivity to 1,25-vit D(3) may contribute to chronic inflammation in neonates.

Model-based clustering reveals vitamin D dependent multi-centrality hubs in a network of vitamin-related proteins

Background

Nutritional systems biology offers the potential for comprehensive predictions that account for all metabolic changes with the intricate biological organization and the multitudinous interactions between the cellular proteins. Protein-protein interaction (PPI) networks can be used for an integrative description of molecular processes. Although widely adopted in nutritional systems biology, these networks typically encompass a single category of functional interaction (i.e., metabolic, regulatory or signaling) or nutrient. Incorporating multiple nutrients and functional interaction categories under an integrated framework represents an informative approach for gaining system level insight on nutrient metabolism.

Results

We constructed a multi-level PPI network starting from the interactions of 200 vitamin-related proteins. Its final size was 1,657 proteins, with 2,700 interactions. To characterize the role of the proteins we computed 6 centrality indices and applied model-based clustering. We detected a subgroup of 22 proteins that were highly central and significantly related to vitamin D. Immune system and cancer-related processes were strongly represented among these proteins. Clustering of the centralities revealed a degree of redundancy among the indices; a repeated analysis using subsets of the centralities performed well in identifying the original set of 22 most central proteins.

Conclusions

Hierarchical and model-based clustering revealed multi-centrality hubs in a vitamin PPI network and redundancies among the centrality indices. Vitamin D-related proteins were strongly represented among network hubs, highlighting the pervasive effects of this nutrient. Our integrated approach to network construction identified promiscuous transcription factors, cytokines and enzymes - primarily related to immune system and cancer processes - representing potential gatekeepers linking vitamin intake to disease.

Prostate cancer and vitamin D: what does the evidence really suggest?

The optimal approach to vitamin D supplementation for the average healthy person is debatable. In patients with cancer, the role of vitamin D supplementation, possibly in treatment, is even less clear. Vitamin D is shown to play a role in prostate cancer biology; however, the clinical data have not consistently demonstrated a link. Additional studies are needed to determine if higher doses of vitamin D supplements could benefit selected populations (ie, the elderly or patients with cancer) even if they may not be beneficial for the general population.

Calcitriol inhibits TNF-alpha-induced inflammatory cytokines in human trophoblasts

Elevated placental proinflammatory cytokine release is associated with miscarriage, preterm labor and preeclampsia. Specifically, tumor necrosis factor-alpha (TNF-alpha)-induced cytokines may threaten pregnancy outcome. Since trophoblasts produce calcitriol, a hormone with strong immunosuppressive properties, we assessed the effects of this secosteroid on inflammatory cytokines induced in trophoblasts by challenge with TNF-alpha. The effects of calcitriol on synthesis of mRNAs encoding interleukin-6 (IL-6), interferon-gamma (IFN-gamma), and TNF-alpha were measured by real time RT-PCR. Secreted cytokines were quantified by ELISA. The effects of TNF-alpha on CYP24A1, chorionic gonadotropin (hCG), 3beta-hydroxysteroid dehydrogenase (HSD3B1) and P(450)-aromatase (CYP19) mRNA expression were also studied. TNF-alpha stimulated IL-6, IFN-gamma and its own expression more than 3-fold over controls (P<0.05). Calcitriol inhibited the expression profile of inflammatory cytokine genes in a dose-response manner (P<0.05). This effect was prevented by addition of the vitamin D receptor antagonist TEI-9647. TNF-alpha also significantly inhibited expression of hCG, HSD3B1 and CYP19 genes, and stimulated CYP24A1 gene expression. These data show that calcitriol prevents TNF-alpha induction of inflammatory cytokines through a process likely to be mediated by the vitamin D receptor. We conclude that TNF-alpha inhibits placental hormone synthesis and stimulates calcitriol catabolism by regulating enzymes involved in these processes.

Vitamin D and human health: lessons from vitamin D receptor null mice

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)(2)D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1alpha-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype. The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)(2)D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1alpha-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

Effect of 1,25-dihydroxy-vitamin D3 in experimental sepsis

BACKGROUND

In addition to the regulation of calcium homeostasis, vitamin D affects the cellular immune system, targets the TNF-alpha pathway and increases vasoconstrictor response to angiotensin II. We therefore examined the effect of 1,25-dihydroxy-vitamin D(3) on coagulation and organ failure in experimental sepsis in the rat.

METHODS

Three series of placebo-controlled studies were conducted. All rats were pre-treated with daily SC injections of 1,25-dihydroxy-vitamin D(3) 100 ng/kg or placebo vehicle for 3 days. In study 1, sepsis was accomplished by abdominal surgery comprising a coecal ligation and puncture with a 1,2 mm needle, or sham surgery. In study 2, the rats had a single IP injection of lipopolysaccharide from E. Coli 0111:B4 (LPS) 8 mg/kg, or placebo. In study 3, an hour-long IV infusion of LPS 7 mg/kg, or placebo was given.

RESULTS

All three models of sepsis showed significant effects on coagulation and liver function with reduced thrombocyte count and prothrombin time together with elevated ALT and bilirubin (p<0.05) as compared to controls. In study 1, the vitamin D treated rats maintained normal platelet count, whereas the vehicle treated rats showed a significant reduction (p<0.05). This effect of vitamin D on platelets was not found in the LPS-treated groups. We found no significant differences between vitamin D and placebo-treated rats with regards to liver function.

CONCLUSION

The present data suggest a positive modulating effect of 1,25-dihydroxy-vitamin D(3) supplementation on sepsis-induced coagulation disturbances in the coecal ligation and puncture model. No such effect was found in LPS-induced sepsis.

Vitamin D3and the immune system: maintaining the balance in health and disease

1,25-Dihydroxyvitamin D3(1,25(OH)2D3), the active form of vitamin D3, is a central player in Ca and bone metabolism. More recently, important immunomodulatory effects have been attributed to this hormone. By binding to its receptor, the vitamin D receptor, 1,25(OH)2D3regulates the expression of various genes and consequently affects the behaviour of different cell types within the immune system. 1,25(OH)2D3can potently inhibit pathogenic T cells and gives rise to elevated numbers of regulatory T cells via the induction of tolerogenic dendritic cells. These immunomodulatory activities of 1,25(OH)2D3have also been proven usefulin vivo: administration of 1,25(OH)2D3in several animal models can prevent or cure different autoimmune diseases and graft rejection. To overcome the dose-limiting side effects of 1,25(OH)2D3on Ca and bone, less calcaemic structural analogues (alone or in combination with synergistically acting drugs or bone-resorption inhibitors) have been successfully used in animal models. Furthermore, as 1,25(OH)2D3also contributes to host defence against infectious agents by the induction of antimicrobial responses, this molecule might provide a new strategy to deal with drug-resistant infections. According to the pleiotropic effects of 1,25(OH)2D3in the immune system, increasing epidemiological data underline the importance of adequate vitamin D intakes in reducing the risk of several autoimmune diseases and infections such as tuberculosis.

Novel A-ring homodimeric C-3-carbamate analogues of 1α,25-dihydroxyvitamin D3: Synthesis and preliminary biological evaluation

The synthesis of a new class of vitamin D3 analogues in which two units of 1alpha,25-dihydroxyvitamin D3 are linked at the C-3 position by a dicarbamate functionality of variable length is described. The analogues demonstrated no affinity for the vitamin D receptor and possessed no antiproliferative or transactivating properties.

The vitamin D slant on allergy

Oral vitamin D supplementation has been introduced into modern medicine to prevent rickets without the knowledge that this may have profound immunological consequences. The main vitamin D metabolite calcitriol suppresses dendritic cell maturation and consecutive Th(1) cell development, which has independently described as a key mechanism of allergy development. Animal studies and epidemiological surveys now provide a first link of early vitamin D supplementation and later allergy where several vitamin D regulated genes seem to be involved. A randomized clinical trial of vitamin D supplementation could be a further step to follow up the vitamin hypothesis.

1,25-Dihydroxyvitamin D3 regulates the expression of low-density lipoprotein receptor-related protein 5 via deoxyribonucleic acid sequence elements located downstream of the start site of transcription

The skeleton is a direct target of vitamin D action, where the hormone modulates the proliferation of osteoblast precursors, their differentiation into mature osteoblasts, and their functional activity. Some of these effects of vitamin D are reminiscent of those orchestrated by the Wnt signaling pathway wherein stimulation of the membrane receptor Frizzled and its coreceptor LRP5 leads to activation of beta-catenin and subsequent transcription-mediated changes in osteoblast biology. Indeed, LRP5 is now known to play a particularly important role in bone formation such that the loss of this component results in a reduction in osteoblast number, a delay in mineralization, and a reduction in peak bone mineral density. Interestingly, we discovered during the course of a vitamin D receptor (VDR) chromatin immunoprecipitation/DNA microarray analysis that 1,25-(OH)2D3 could induce binding of the VDR to sites within the Lrp5 gene locus. VDR and retinoid X receptor binding was evident both in primary osteoblasts as well as in osteoblasts of cell line origin. Importantly, this interaction between 1,25-(OH)2D3-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of Lrp5 mRNA transcripts in vivo as well as in vitro. One of these sites within the Lrp5 locus was discovered to confer vitamin D response to a heterologous promoter when introduced into osteoblastic cells, permitting both the identification and characterization of the vitamin D response element located within. Interestingly, additional studies revealed that whereas the regulatory region in the mouse Lrp5 gene was highly conserved in the human genome, the vitamin D response element was not. Our studies show that 1,25-(OH)2D3 can enhance the expression of a critical component of the Wnt signaling pathway that is known to impact osteogenesis.

1,25-Dihydroxyvitamin D inhibits lipopolysaccharide-induced immune activation in human endothelial cells

In addition to its well-known role in mineral and skeletal homeostasis, 1,25-dihydroxyvitamin D3 [1,25-(OH)2, D3] regulates the differentiation, growth and function of a broad range of immune system cells, including monocytes, dendritic cells, T and B lymphocytes. Vascular endothelial cells play a major role in the innate immune activation during infections, sepsis and transplant rejection; however, currently there are no data on the effect of 1,25-(OH)2 D3 on microbial antigen-induced endothelial cell activation. Here we show that 1,25-(OH)2 D3 pretreatment of human microvessel endothelial cells (HMEC) inhibited the enteric gram-negative bacterial lipopolysaccharide (LPS) activation of transcription factor NF-kappaB and interleukin (IL)-6, IL-8 and regulated upon activation normal T cell exposed and secreted (RANTES) release. The effect of 1,25-(OH)2 D3 was not due to increased cell death or inhibition of endothelial cell proliferation. 1,25-(OH)2 D3 pretreatment of HMEC did not block MyD88-independent LPS-induced interferon (IFN)-beta promoter activation. 1,25-(OH)2 D3 pretreatment of HMEC did not modulate Toll-like receptor 4 (TLR4) or MD-2 expression. These data suggest that 1,25-(OH)2 D3 may play a role in LPS-induced immune activation of endothelial cells during gram-negative bacterial infections, and a suggest a potential role for 1,25-(OH)2 D3 and its analogues as an adjuvant in the treatment of gram-negative sepsis.

Immunoregulation by 1,25-dihydroxyvitamin D3: basic concepts

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the biologically active metabolite of Vitamin D(3), not only regulates bone and calcium metabolism but also exerts other biological activities, including immunomodulation via the nuclear Vitamin D receptor expressed in antigen-presenting cells and activated T cells. This regulation is mediated through interference with nuclear transcription factors such as NF-AT and NF-kappaB or by direct interaction with Vitamin D responsive elements in the promoter regions of cytokine genes. Dendritic cells (DCs) are primary targets for the immunomodulatory activity of 1,25(OH)(2)D(3), as indicated by inhibited DC differentiation and maturation, leading to down-regulated expression of MHC-II, costimulatory molecules and IL-12. Moreover, 1,25(OH)(2)D(3) enhances IL-10 production and promotes DC apoptosis. Together, these effects of 1,25(OH)(2)D(3) inhibit DC-dependent T cell activation. Immunomodulation by 1,25(OH)(2)D(3) and its analogs in vivo has been demonstrated in different models of autoimmune diseases and transplantation. Moreover, combining analogs with other immunosuppressants leads to synergism in models of autoimmunity and transplantation. The availability of 1,25(OH)(2)D(3) analogs with immunomodulatory activity at non-hypercalcemic doses may allow exploitation of their immunomodulatory effects in a clinical setting of treatment of autoimmune diseases and prevention of allograft rejection.

A 5'-distal enhanceosome in the PDGF-A gene is activated in choriocarcinoma cells via ligand-independent binding of vitamin D receptor and constitutive jun kinase signaling

Overexpression of platelet-derived growth factor A-chain (PDGF-A) is clearly linked to autocrine and paracrine stimulation of malignant growth in many human cancers. We have shown previously that PDGF-A overexpression in choriocarcinoma, hepatoma and lung carcinoma cell lines is driven by the activity of a 66 bp enhancer element (ACE66) located approximately 7 kb upstream of the PDGF-A transcription start site. In this study, the ACE66 element is shown to be activated in JEG-3 choriocarcinoma cells through synergistic interactions between consensus DNA motifs for binding of vitamin D receptor, AP1 and ELK1. Binding of the vitamin D/retinoid-X receptor (VDR/RXRalpha) heterodimer to the ACE66 element was reconstituted in vitro with recombinant VDR/RXRalpha and with JEG-3 nuclear extract, and was verified in living JEG-3 cells by chromatin immunoprecipitation analysis. Transcriptional activity of the ACE66 element, as well as occupancy of the element by VDR/RXRalpha, was shown to be independent of stimulation with the hormonal VDR ligand, 1,25-dihydroxyvitamin D3. The jun kinase pathway of mitogen-activated protein kinase (MAPK) signaling was shown to activate the ACE66 enhancer, most likely through activation of factors binding to the AP1 element. These results identify a novel mechanism of transcriptional enhancement involving ligand-independent activity of the VDR/RXR heterodimer and MAPK signaling pathways that appears to play an important role in the overexpression of PDGF in many different settings of human malignancy.

Vitamin D-induced up-regulation of tumour necrosis factor alpha (TNF-alpha) in prostate cancer cells

1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3 or calcitriol) is an active hormone that regulates cellular proliferation and induces apoptosis in cancer cells. Here we report on a new calcitriol target gene in prostate cancer cells, tumour necrosis factor alpha (TNF-alpha). Calcitriol and its analogue CB1093 up-regulate TNF-alpha mRNA expression in LNCaP and PC-3 cells. The stimulation is dose-dependent in both of these cell lines, demonstrated by the quantitative real-time polymerase chain reaction. Calcitriol and CB1093 act synergistically with human recombinant TNF-alpha in activation of TNF-alpha mRNA expression in LNCaP but not in PC-3 cells. Transcriptional activation of TNF-alpha gene by calcitriol or CB1093 does not lead to TNF-alpha protein secretion, however calcitriol and CB1093 enhance TPA-stimulated TNF-alpha production in LNCaP cells. We did not observe any significant effect of calcitriol on regulation of TNFR1 at the level of gene expression. Nor does calcitriol affect transcriptional regulation of cytokine (IL-1, IL-6) and cytokine receptor genes in LNCaP and PC-3 prostate cancer cell lines. Calcitriol and its analogue CB1093 at 10 nM concentration induce programmed cell death in LNCaP cells. Combined addition of human recombinant TNF-alpha with calcitriol or CB1093 cause enhanced effect in induction of apoptosis. We conclude that under physiological conditions vitamin D activates only the transcription of TNF-alpha gene, for TNF-alpha protein synthesis additional cofactors are required. Therefore a cooperation of vitamin D and TNF-alpha may play an important role in the control of cell growth in prostate cancer.

TNF-alpha expression is transcriptionally regulated by RANK ligand

Tumor necrosis factor (TNF)-alpha is known for its osteoclastogenic and resorptive activities. Induction of osteoclastogenesis by receptor activator of NF-kappaB ligand (RANKL) is accompanied by increased TNF-alpha expression. In the present study we investigated the mechanism by which RANKL induces expression of TNF-alpha in osteoclast precursors. The macrophage-like cell-line, RAW 264.7 was used as a model for osteoclast precursors. To examine if RANKL-mediated increase in TNF-alpha expression involves increased stability of its transcript, RAW264.7 cells were treated with or without RANKL, and then a transcription inhibitor was added. At different time points, TNF-alpha and L32 mRNA levels were examined. TNF-alpha mRNA stability was not altered by RANKL. We next measured directly the transcription rate of TNF-alpha by a run-on assay and found that RANKL increases TNF-alpha transcription rate by 2.9-fold in RAW264.7 cells. We further characterized this transcriptional induction of TNF-alpha by RANKL. Gel shift assays using nuclear extracts derived from RANKL-treated RAW264.7 cells show increased specific NF-kappaB binding activity on the murine TNF-alpha promoter. Gliotoxin, known for its ability to inhibit NF-kappaB activation blocked RANKL-induced TNF-alpha expression. We finally used 1,260 bp of the murine TNF-alpha promoter fused to luciferase, as well as four mutants of this promoter carrying mutations in each of the four NF-kappaB sites to stably transfect RAW 264.7 cells. Reporter activity was increased in response to RANKL in wild type promoter transfected cells, whereas treatment of the mutants' transfected cells did not elicit reporter activity. In conclusion, RANKL induces TNF-alpha expression via a transcriptional mechanism, depending on the NF-kappaB sites in the TNF promoter.

Calcitriol in cancer treatment: From the lab to the clinic

1,25-Dihydroxyvitamin D (calcitriol), the most active metabolite of vitamin D, has significant antineoplastic activity in preclinical models. Several mechanisms of activity have been proposed. These include inhibition of proliferation associated with cell cycle arrest and, in some models, differentiation, reduction in invasiveness and angiogenesis, and induction of apoptosis. Proposed mechanisms differ between tumor models and experimental conditions, and no unifying hypothesis about the mechanism of antineoplastic activity has emerged. Synergistic and/or additive effects with cytotoxic chemotherapy, radiation, and other cancer drugs have been reported. Significantly supraphysiological concentrations of calcitriol are required for antineoplastic effects. Such concentrations are not achievable in patients when calcitriol is dosed daily due to predictable hypercalcemia and hypercalcuria; however, phase I trials have demonstrated that intermittent dosing allows substantial dose escalation and has produced potentially therapeutic peak calcitriol concentrations. Recently, a phase II study reported encouraging levels of activity for the combination of high-dose calcitriol and docetaxel administered on a weekly schedule in patients with androgen-independent prostate cancer. This regimen is now under study in a placebo-controlled randomized trial in androgen-independent prostate cancer and in phase II studies in several other tumor types. Further work is needed to elucidate the molecular mechanisms of antineoplastic activity and optimal clinical applications of calcitriol in cancer.