Upregulation of Irisin and Vitamin D-Binding Protein Concentrations by Increasing Maternal 25-Hydrovitamin D Concentrations in Combination with Specific Genotypes of Vitamin D-Binding Protein Polymorphisms.

Dyshomeostasis of vitamin D-binding protein (VDBP) has been implicated in the pathogenesis of various pregnancy complications, including preeclampsia, preterm birth, gestational diabetes, and adverse metabolic profiles in the offspring. VDBP polymorphisms have been consistently reported to contribute to this intriguing interplay. Until recently, the effects of VDBP polymorphism heterogeneity on maternal and neonatal adipomyokine profiles have not been investigated, specifically after incorporating the different maternal and neonatal 25-hydroxyvitamin D concentration cut-offs at birth. We aimed to investigate the potential effects of maternal and neonatal VDBP polymorphisms on adiponectin, irisin, and VDBP concentrations at birth, according to different cut-offs of vitamin D status, in maternal-neonatal dyads recruited from the sunny region of Northern Greece. We obtained blood samples from 66 mother-child pairs at birth. Results indicated that (i) Neonatal serum biomarkers were not affected by any included neonatal VDBP polymorphism according to different cut-offs of neonatal vitamin D status at birth, (ii) neonatal VDBP concentration was elevated in neonates with maternal rs7041 GG genotype, (iii) maternal 25(OH)D at ≤75 nmol/L resulted in increased concentrations of maternal VBDP and irisin concentrations in women with CC genotype for rs2298850 and rs4588,whereas this effect was also evident for this cut-off for neonatal VDBP concentrations at birth for GC genotype for rs 7041, and (iv) no significant effect of neonatal VDBP polymorphisms was observed on neonatal VDBP, adiponectin, or irisin levels when stratified according to maternal 25(OH)D cut-offs. In conclusion, these findings confirm that among women with the combination of CC genotype for rs2298850 and rs4588, a specific high cut-off of maternal 25(OH)D results in increasing maternal VBDP concentrations, hence providing a mechanistic rationale for aiming for specific cut-offs of vitamin D after supplementation during pregnancy, in daily clinical practice.

Skeletal Muscle and the Maintenance of Vitamin D Status.

Vitamin D, unlike the micronutrients, vitamins A, E, and K, is largely obtained not from food, but by the action of solar ultraviolet (UV) light on its precursor, 7-dehydrocholesterol, in skin. With the decline in UV light intensity in winter, most skin production of vitamin D occurs in summer. Since no defined storage organ or tissue has been found for vitamin D, it has been assumed that an adequate vitamin D status in winter can only be maintained by oral supplementation. Skeletal muscle cells have now been shown to incorporate the vitamin D-binding protein (DBP) from blood into the cell cytoplasm where it binds to cytoplasmic actin. This intracellular DBP provides an array of specific binding sites for 25-hydroxyvitamin D (25(OH)D), which diffuses into the cell from the extracellular fluid. When intracellular DBP undergoes proteolytic breakdown, the bound 25(OH)D is then released and diffuses back into the blood. This uptake and release of 25(OH)D by muscle accounts for the very long half-life of this metabolite in the circulation. Since 25(OH)D concentration in the blood declines in winter, its cycling in and out of muscle cells appears to be upregulated. Parathyroid hormone is the most likely factor enhancing the repeated cycling of 25(OH)D between skeletal muscle and blood. This mechanism appears to have evolved to maintain an adequate vitamin D status in winter.

Associations of vitamin D binding protein variants with the vitamin D-induced increase in serum 25-hydroxyvitamin D.

BACKGROUND: Vitamin D deficiency is a global problem that may be improved by vitamin D supplementation; however, the individual's response to the intervention varies. We aimed to investigate possible genetic factors that may modify the impact of environmental exposure on vitamin D status. The candidate gene variant we investigated was the Gc gene-rs4588 polymorphism at the vitamin D receptor (DBP) locus. METHODS: A total of 619 healthy adolescent Iranian girls received 50000 IU of vitamin D3 weekly for 9 weeks. Serum 25(OH) D concentrations, metabolic profiles and dietary intake were measured at baseline and after 9 weeks of supplementation. The genotypes of the DBP variant (rs4588) were analyzed using the TaqMan genotyping assay. RESULTS: Our results revealed that the rs4588 polymorphism might be associated with serum 25-hydroxy vitamin D both at baseline (p value = 0.03) and after intervention (p value = 0.008). It seemed that the outcome of the intervention was gene-related so that the subjects with common AA genotype were a better responder to vitamin D supplementation (Changes (%) 469.5 (427.1) in AA carriers vs. 335.8 (530) in GG holders), and carriers of the less common GG genotype experienced a rise in fasting blood glucose after 9 weeks (Changes (%) 0 (1.5)). Our findings also showed that the statistical interaction between this variant and supplementation was statistically significant (intervention effect p-value<0.001 and p-value SNP effect = 0.03). The regression model also revealed that after adjusted for potential confounders, likelihood of affecting serum 25(OH)D in individuals who were homozygous for the uncommon allele G was less than those homozygous for the more common AA genotype (OR = 4.407 (1.82-8.89); p = 0.001). CONCLUSION: Serum vitamin 25(OH) D following vitamin 25(OH) D3 supplementation appears to be modified by genetic background. The Gc genetic variant, rs4588 encoding the vitamin D receptor seems to influence the response to vitamin D supplementation.

Vitamin D in burn-injured patients.

Recently, many studies have demonstrated pleotropic effects of vitamin D, including immune modulation and cardiovascular system activity. Sufficient vitamin D concentrations and supplementation of vitamin D may be of benefit in burn-injured patients. Low 25(OH)D has been observed in nearly all pediatric and most adult burn patients. Vitamin D has primarily been studied in pediatric burn patients, focusing on bone marker measurements and the incidence of fractures. The preferred vitamin D dose, formulation, and route of administration remain unknown, and there is limited data on the impact of vitamin D status on clinical outcomes. Further research should focus on determining optimal monitoring strategies, supplementation regimens and clinical outcomes like mortality, length of stay and incidence of sepsis.

Association of free vitamin D3 concentrations and asthma treatment failures in the VIDA Trial.

BACKGROUND: Use of vitamin D3 serum concentrations as a biomarker of vitamin D status is questionable because of variation in vitamin D binding protein. OBJECTIVE: To determine associations between free vitamin D3 concentrations and rates of treatment failure and exacerbations in patients with asthma participating in the Vitamin D Add-on Therapy Enhances Corticosteroid Responsiveness in Asthma (VIDA) trial. METHODS: Free concentrations were directly measured by enzyme-linked immunosorbent assay and stratified into low, medium, and high groups: less than 5pg/mL (n = 65), 5 to 9pg/mL (n = 84), and greater than 9pg/mL (n = 48) after 12 weeks of supplementation with oral vitamin D3 and associated with outcomes. RESULTS: Outcomes did not associate with free concentrations: overall treatment failure rates were 0.60 (95% confidence interval [CI] 0.46-0.78), 0.53 (95%CI 0.40- 0.70), and 0.69 (95%CI 0.54-0.90)/person-year (P = .51), respectively; overall exacerbation rates were 0.28 (95%CI 0.17-0.48), 0.15 (95%CI 0.08-0.30) and 0.42 (95%CI 0.27-0.66)/person-year (P = .22). Mean (standard deviation) baseline free concentrations were lower in non-Hispanic blacks and Hispanics compared with non-Hispanic whites: 4.10 (1.33) and 4.38 (1.11) pg/mL vs 5.16 (1.65) pg/ml, (P < .001 and P = 0.038), respectively. Mean (standard deviation) baseline free concentrations differed between females and males: 4.57 (1.58) and 5.08 (1.41) (P = .026); and between non-overweight (body mass index [BMI] < 25) and overweight (BMI > 25): 5.45 (1.86) vs 4.54 (1.39) (P < .001). The free fraction differed by race and sex but not by BMI. CONCLUSION: The use of free concentrations was inferior to total concentrations as a biomarker of efficacy of vitamin D3 supplementation in VIDA trial participants. Future studies of vitamin D status in patients with asthma should measure both free and total concentrations to better understand which marker of vitamin D function is most informative.

Measurement of Vitamin D for Epidemiologic and Clinical Research: Shining Light on a Complex Decision.

Vitamin D is a fat-soluble vitamin that is synthesized in the skin with exposure to sunlight or is ingested from dietary supplements or food. There has been a dramatic increase in research on vitamin D, linking it with health outcomes as varied as reproductive function, infection, cardiovascular disease, and cancer. The study of vitamin D has generated much excitement, partly because there is an ideal intervention: Low levels may be common and can be remedied with widely available supplements. Determination of vitamin D status is complex and has advanced dramatically in the past 5 years. In this paper, we begin by describing important considerations for measurement of total 25-hydroxyvitamin D (25(OH)D), the biomarker traditionally assessed in epidemiologic studies. While 25(OH)D remains the most commonly measured biomarker, emerging evidence suggests that other related analytes may contribute to the characterization of an individual's vitamin D status (e.g., vitamin D-binding protein, bioavailable and free 25(OH)D, the C-3 epimer of 25(OH)D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D). The measurement of these analytes is also complex, and there are important considerations for deciding whether their measurement is warranted in new research studies. Herein we discuss these issues and provide the reader with an up-to-date synthesis of research on vitamin D measurement options and considerations.

Effects of vitamin D binding protein phenotypes and vitamin D supplementation on serum total 25(OH)D and directly measured free 25(OH)D.

OBJECTIVE: To determine the relationship between serum total 25-hydroxyvitamin D (25(OH)D), directly measured free 25(OH)D and calculated free 25(OH)D with regard to vitamin D-binding protein (DBP) phenotypes, sex, BMI, age and season, and their interrelationship to vitamin D supplementation. DESIGN, PATIENTS AND INTERVENTIONS: A randomized controlled trial with 20 000 IU of vitamin D3 per week or placebo for 12 months was designed. A total of 472 subjects, 236 in each of the intervention groups, were included in the analyses. MAIN OUTCOME MEASURES: Baseline serum concentrations and increases in serum total 25(OH)D, directly measured free 25(OH)D, calculated free 25(OH)D and DBP. RESULTS: Serum total 25(OH)D and DBP concentrations were significantly lower in subjects with the phenotype Gc2/Gc2 compared to phenotypes with the Gc1S allele, and lower in males compared to females. When using directly measured free 25(OH)D, the differences related to DBP phenotypes and sexes were clearly diminished. All calculated free 25(OH)D concentrations were overestimated compared to the directly measured free 25(OH)D. Serum parathyroid hormone showed an inverse correlation with all vitamin D parameters analyzed. The increases after 12 months of vitamin D supplementation were not significantly different for any of the vitamin D parameters regardless of DBP phenotype, sex or age. Supplementation with vitamin D did not affect serum DBP. CONCLUSION: Direct measurements of free 25(OH)D reduce the differences seen in total 25(OH)D between DBP phenotype groups and sexes, probably caused by differences in DBP concentrations. With conditions affecting serum DBP concentrations, direct measurements of free 25(OH)D should be considered.

Genetic and environmental factors are associated with serum 25-hydroxyvitamin D concentrations in older African Americans.

BACKGROUND: Low circulating 25-hydroxyvitamin D [25(OH)D] is prevalent in African Americans, but predictors of vitamin D status are understudied compared to Caucasian populations. OBJECTIVE: We investigated whether certain environmental and genetic factors are predictors of circulating 25(OH)D in 989 elderly African Americans participating in the Health, Aging, and Body Composition (Health ABC) Study. METHODS: Regression analysis estimated the cross-sectional association of nongenetic (environmental) factors with 25(OH)D. Single nucleotide polymorphisms (SNPs) associated with 25(OH)D in Caucasian genome-wide association studies (GWASs) were analyzed for association with serum 25(OH)D, including analyses of all imputed SNPs in identified genomic regions. Genome-wide complex trait analysis (GCTA) evaluated the association of all (genome-wide) genotyped SNPs with serum 25(OH)D in the Health ABC Study with replication in the Multi-Ethnic Study of Atherosclerosis (MESA) cohort. RESULTS: Gender, study site, season of blood draw, body mass index, dietary supplement use, dairy and cereal consumption, Healthy Eating Index score, and walking >180 min/wk were associated with 25(OH)D (P < 0.05), jointly explaining 25% of the variation in circulating 25(OH)D. Multivitamin supplement use was the strongest predictor of circulating 25(OH)D, and supplement users had a 6.3-µg/L higher serum 25(OH)D concentration compared with nonusers. Previous GWAS-identified gene regions were not replicated in African Americans, but the nonsynonymous rs7041 SNP in group-specific component (vitamin D binding protein) was close to significance thresholds (P = 0.08), and there was evidence for an interaction between this SNP and use of multivitamin supplements in relation to serum 25(OH)D concentration (P = 0.04). Twenty-three percent (95% CI: 0%, 52%) of the variation in serum 25(OH)D was explained by total genetic variation in a pooled GCTA of 2087 Health ABC Study and MESA African-American participants, but population substructure effects could not be separated from other genetic influences. CONCLUSIONS: Modifiable dietary and lifestyle predictors of serum 25(OH)D were identified in African Americans. GCTA confirms that a proportion of 25(OH)D variability is attributable to genetic variation, but genomic regions associated with the 25(OH)D phenotype identified in prior GWASs of European Americans were not replicated in the Health ABC Study in African Americans.

Eldecalcitol reduces osteoporotic fractures by unique mechanisms.

Eldecalcitol shows higher binding affinity for vitamin D-binding protein (DBP), tighter binding to vitamin D receptor (VDR), and resistance to metabolic degradation via 24-hydroxylation. In silico analysis of the mode of binding demonstrated that the 3-hydroxypropyloxy (3-HP) group of eldecalcitol offers additional hydrogen bond and CH-π interaction for the binding to DBP and VDR. However, the 3-HP group interferes with the binding of eldecalcitol to CYP24A1, causing poor metabolic clearance of eldecalcitol by this enzyme. These characteristics may contribute to the stronger effect of eldecalcitol than calcitriol. The present post-hoc analysis also demonstrate that the incidence of hypercalcemia and hypercalciuria is slightly higher in eldecalcitol than in alfacalcidol group especially in patients with CKD stage 3B, that both serum and urinary calcium return to the baseline levels shortly after cessation of the treatment in both treatment groups, that the incidence of urolithiasis is higher in patients with higher eGFR and is similar between alfacalcidol and eldecalcitol groups, and that eGFR is transiently reduced by both alfacalcidol and eldecalcitol treatment especially among patients with higher eGFR but recovers after the end of both treatment. Eldecalcitol can be used for the treatment of osteoporosis without Ca supplementation to reduce the incidence of hypercalcemia and hypercalciuria, and enough hydration is recommended in order to avoid hypercalcemia, urolithiasis and deterioration of renal function.

Common variants in CYP2R1 and GC genes are both determinants of serum 25-hydroxyvitamin D concentrations after UVB irradiation and after consumption of vitamin D3-fortified bread and milk during winter in Denmark.

BACKGROUND: Little is known about how the genetic variation in vitamin D modulating genes influences ultraviolet (UV)B-induced 25-hydroxyvitamin D [25(OH)D] concentrations. In the Food with vitamin D (VitmaD) study, we showed that common genetic variants rs10741657 and rs10766197 in 25-hydroxylase (CYP2R1) and rs842999 and rs4588 in vitamin D binding protein (GC) predict 25(OH)D concentrations at late summer and after 6-mo consumption of cholecalciferol (vitamin D3)-fortified bread and milk. OBJECTIVES: In the current study, called the Vitamin D in genes (VitDgen) study, we analyzed associations between the increase in 25(OH)D concentrations after a given dose of artificial UVB irradiation and 25 single nucleotide polymorphisms located in or near genes involved in vitamin D synthesis, transport, activation, or degradation as previously described for the VitmaD study. Second, we aimed to determine whether the genetic variations in CYP2R1 and GC have similar effects on 25(OH)D concentrations after artificial UVB irradiation and supplementation by vitamin D3-fortified bread and milk. DESIGN: The VitDgen study includes 92 healthy Danes who received 4 whole-body UVB treatments with a total dose of 6 or 7.5 standard erythema doses during a 10-d period in winter. The VitmaD study included 201 healthy Danish families who were given vitamin D3-fortified bread and milk or placebo for 6 mo during the winter. RESULTS: After UVB treatments, rs10741657 in CYP2R1 and rs4588 in GC predicted UVB-induced 25(OH)D concentrations as previously shown in the VitmaD study. Compared with noncarriers, carriers of 4 risk alleles of rs10741657 and rs4588 had lowest concentrations and smallest increases in 25(OH)D concentrations after 4 UVB treatments and largest decreases in 25(OH)D concentrations after 6-mo consumption of vitamin D3-fortified bread and milk. CONCLUSION: Common genetic variants in the CYP2R1 and GC genes modify 25(OH)D concentrations in the same manner after artificial UVB-induced vitamin D and consumption of vitamin D3-fortified bread and milk.

Vitamin D-binding protein controls T cell responses to vitamin D.

BACKGROUND: In vitro studies have shown that the active form of vitamin D3, 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), can regulate differentiation of CD4+ T cells by inhibiting Th1 and Th17 cell differentiation and promoting Th2 and Treg cell differentiation. However, the serum concentration of 1,25(OH)2D3 is far below the effective concentration of 1,25(OH)2D3 found in in vitro studies, and it has been suggested that 1,25(OH)2D3 must be produced locally from the inactive precursor 25-hydroxyvitamin D3 (25(OH)D3) to affect ongoing immune responses in vivo. Although it has been reported that activated T cells express the 25(OH)D-1α-hydroxylase CYP27B1 that converts 25(OH)D3 to 1,25(OH)2D3, it is still controversial whether activated T cells have the capacity to produce sufficient amounts of 1,25(OH)2D3 to affect vitamin D-responsive genes. Furthermore, it is not known how the vitamin D-binding protein (DBP) found in high concentrations in serum affects T cell responses to 25(OH)D3. RESULTS: We found that activated T cells express CYP27B1 and have the capacity to produce sufficient 1,25(OH)2D3 to affect vitamin D-responsive genes when cultured with physiological concentrations of 25(OH)D3 in serum-free medium. However, if the medium was supplemented with serum or purified DBP, DBP strictly inhibited the production of 1,25(OH)2D3 and 25(OH)D3-induced T cell responses. In contrast, DBP did not inhibit the effect of exogenous 1,25(OH)2D3. Actin, arachidonic acid and albumin did not affect the sequestration of 25(OH)D3 by DBP, whereas carbonylation of DBP did. CONCLUSIONS: Activated T cells express CYP27B1 and can convert 25(OH)D3 to 1,25(OH)2D3 in sufficiently high concentrations to affect vitamin D-responsive genes when cultured in serum-free medium. However, DBP sequesters 25(OH)D3 and inhibits the production of 1,25(OH)2D3 in T cells. To fully exploit the immune-regulatory potential of vitamin D, future studies of the mechanisms that enable the immune system to exploit 25(OH)D3 and convert it to 1,25(OH)2D3 in vivo are required.

An integrated approach to defining genetic and environmental determinants for major clinical outcomes involving vitamin D.

There is substantial genetic and epidemiological evidence implicating vitamin D in the pathogenesis of many common diseases. A number of studies have sought to define an association for disease with sequence variation in the VDR gene, encoding the ligand-activated nuclear hormone receptor for vitamin D. The results of such studies have been difficult to replicate and are likely to need to account for specific environmental exposures. Here, we review recent work that has begun to study the interactions between VDR gene polymorphisms, vitamin D blood levels, and complex disease susceptibility, notably in the context of major clinical outcomes. We highlight the challenges moving forward in this area and its importance for effective clinical translation of current research.

Vitamin D transport proteins megalin and disabled-2 are expressed in prostate and colon epithelial cells and are induced and activated by all-trans-retinoic acid.

Megalin and disabled-2 (Dab2) are essential for uptake of the 25-hydroxycholecalciferol (25D3)-vitamin D binding protein (DBP) complex in tissues. In the kidney, this mechanism regulates serum 25D3 levels and production of 1,25-dihydroxycholecalciferol (1,25D3) by CYP27B1 for systemic use. Previously, we showed that mammary epithelial cells expressing CYP27B1 express megalin and Dab2 and internalize DBP by endocytosis, indicating 25D3 was accessible for conversion to 1,25D3 in extra-renal tissues. Moreover, induction of megalin and Dab2 (protein and mRNA abundance) by all-trans-retinoic acid (RA) enhanced DBP uptake. This suggests megalin and Dab2 play a central role in uptake of vitamin D and may predict actions of vitamin D in extra-renal tissues. Here, we characterized megalin and Dab2 expression and uptake of DBP in transformed human prostate and colon epithelial cells. Megalin and Dab2 were expressed in prostate and colon epithelial cells, which was markedly enhanced following treatment with RA. Furthermore, DBP uptake was stimulated by low-dose RA supplementation in LNCaP, PC-3, and Caco-2 cells. Taken together, these are the first studies to our knowledge that have demonstrated modulated expression of megalin and Dab2, as well as an association between increased expression of endocytic proteins with DBP uptake in prostate and colon cells.

Upregulation of calcitriol during pregnancy and skeletal recovery after lactation do not require parathyroid hormone.

Pregnancy invokes a doubling of intestinal calcium absorption whereas lactation programs skeletal resorption to provide calcium to milk. Postweaning bone formation restores the skeleton's bone mineral content (BMC), but the factors that regulate this are not established. We used Pth-null mice to test whether parathyroid hormone (PTH) is required for postweaning skeletal recovery. On a normal 1% calcium diet, wild-type (WT) and Pth-null mice each gained BMC during pregnancy, declined 15% to 18% below baseline during lactation, and restored the skeleton above baseline BMC within 14 days postweaning. A 2% calcium diet reduced the lactational decline in BMC without altering the gains achieved during pregnancy and postweaning. The hypocalcemia and hyperphosphatemia of Pth-null mice normalized during lactation and serum calcium remained normal during postweaning. Osteocalcin and propeptide of type 1 collagen (P1NP) each rose significantly after lactation to similar values in WT and Pth-null. Serum calcitriol increased fivefold during pregnancy in both genotypes whereas vitamin D binding protein levels were unchanged. Absence of PTH blocked a normal rise in fibroblast growth factor-23 (FGF23) during pregnancy despite high calcitriol. A 30-fold higher expression of Cyp27b1 in maternal kidneys versus placenta suggests that the pregnancy-related increase in calcitriol comes from the kidneys. Conversely, substantial placental expression of Cyp24a1 may contribute significantly to the metabolism of calcitriol. In conclusion, PTH is not required to upregulate renal expression of Cyp27b1 during pregnancy or to stimulate recovery from loss of BMC caused by lactation. A calcium-rich diet in rodents suppresses skeletal losses during lactation, unlike clinical trials that showed no effect of supplemental calcium on lactational decline in BMC.

Vitamin D binding protein and monocyte response to 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D: analysis by mathematical modeling.

Vitamin D binding protein (DBP) plays a key role in the bioavailability of active 1,25-dihydroxyvitamin D (1,25(OH)(2)D) and its precursor 25-hydroxyvitamin D (25OHD), but accurate analysis of DBP-bound and free 25OHD and 1,25(OH)(2)D is difficult. To address this, two new mathematical models were developed to estimate: 1) serum levels of free 25OHD/1,25(OH)(2)D based on DBP concentration and genotype; 2) the impact of DBP on the biological activity of 25OHD/1,25(OH)(2)D in vivo. The initial extracellular steady state (eSS) model predicted that 50 nM 25OHD and 100 pM 1,25(OH)(2)D), <0.1% 25OHD and <1.5% 1,25(OH)(2)D are 'free' in vivo. However, for any given concentration of total 25OHD, levels of free 25OHD are higher for low affinity versus high affinity forms of DBP. The eSS model was then combined with an intracellular (iSS) model that incorporated conversion of 25OHD to 1,25(OH)(2)D via the enzyme CYP27B1, as well as binding of 1,25(OH)(2)D to the vitamin D receptor (VDR). The iSS model was optimized to 25OHD/1,25(OH)(2)D-mediated in vitro dose-responsive induction of the vitamin D target gene cathelicidin (CAMP) in human monocytes. The iSS model was then used to predict vitamin D activity in vivo (100% serum). The predicted induction of CAMP in vivo was minimal at basal settings but increased with enhanced expression of VDR (5-fold) and CYP27B1 (10-fold). Consistent with the eSS model, the iSS model predicted stronger responses to 25OHD for low affinity forms of DBP. Finally, the iSS model was used to compare the efficiency of endogenously synthesized versus exogenously added 1,25(OH)(2)D. Data strongly support the endogenous model as the most viable mode for CAMP induction by vitamin D in vivo. These novel mathematical models underline the importance of DBP as a determinant of vitamin D 'status' in vivo, with future implications for clinical studies of vitamin D status and supplementation.

Effect of the Gc-derived macrophage-activating factor precursor (preGcMAF) on phagocytic activation of mouse peritoneal macrophages.

BACKGROUND: The 1f1f subtype of the Gc protein (Gc(1f1f) protein) was converted into Gc-derived macrophage-activating factor (GcMAF) by enzymatic processing in the presence of ß-galactosidase of an activated B-cell and sialidase of a T-cell. We hypothesized that preGc(1f1f)MAF, the only Gc(1f1f) protein lacking galactose, can be converted to GcMAF in vivo because sialic acid is cleaved by residual sialidase. Hence, we investigated the effect of preGc(1f1f)MAF on the phagocytic activation of mouse peritoneal macrophages. RESULTS: We examined the sugar moiety of preGc(1f1f)MAF with a Western blot using peanut agglutinin (PNA) and Helix pomatia agglutinin (HPA) lectin. We also found that preGc(1f1f)MAF significantly enhanced phagocytic activity in mouse peritoneal macrophages but only in the presence of the mouse peritoneal fluid; the level of phagocytic activity was the same as that observed for GcMAF. CONCLUSION: PreGc(1f1f)MAF can be used as an effective macrophage activator in vivo.

Vitamin D and diabetes.

Type 1 (T1D) and type 2 (T2D) diabetes are considered multifactorial diseases in which both genetic predisposition and environmental factors participate in their development. Many cellular, preclinical, and observational studies support a role for vitamin D in the pathogenesis of both types of diabetes including: (1) T1D and T2D patients have a higher incidence of hypovitaminosis D; (2) pancreatic tissue (more specifically the insulin-producing beta-cells) as well as numerous cell types of the immune system express the vitamin D receptor (VDR) and vitamin D-binding protein (DBP); and (3) some allelic variations in genes involved in vitamin D metabolism and VDR are associated with glucose (in)tolerance, insulin secretion, and sensitivity, as well as inflammation. Moreover, pharmacologic doses of 1,25-dihydroxyvitamin D (1,25(OH)(2)D), the active form of vitamin D, prevent insulitis and T1D in nonobese diabetic (NOD) mice and other models of T1D, possibly by immune modulation as well as by direct effects on beta-cell function. In T2D, vitamin D supplementation can increase insulin sensitivity and decrease inflammation. This article reviews the role of vitamin D in the pathogenesis of T1D and T2D, focusing on the therapeutic potential for vitamin D in the prevention/intervention of T1D and T2D as well as its complications.

Vitamin D-binding protein directs monocyte responses to 25-hydroxy- and 1,25-dihydroxyvitamin D.

BACKGROUND: Serum 25-hydroxyvitamin D (25OHD) is a key factor in determining monocyte induction of the antimicrobial protein cathelicidin, which requires intracrine conversion of 25OHD to 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. Both vitamin D metabolites circulate bound to vitamin D-binding protein (DBP), but the effect of this on induction of monocyte cathelicidin remains unclear. METHODS: Human monocytes were cultured in medium containing 1) serum from DBP knockout (DBP(-/-)) or DBP(+/-) mice, 2) serum-free defined supplement reconstituted with DBP or albumin (control), and 3) human serum with different DBP [group-specific component [Gc]] genotypes with varying affinities for vitamin D metabolites. In each case, response to added 1,25(OH)(2)D(3) or 25OHD(3) was determined by measuring expression of mRNA for cathelicidin and 24-hydroxylase. Monocyte internalization of DBP was assessed by fluorescent tagging followed by microscopic and flow cytometric analysis of tagged DBP. RESULTS: Monocytes cultured in DBP(-/-) serum showed more potent induction of cathelicidin by 25OHD(3) or 1,25(OH)(2)D(3) when compared with DBP(+/-) serum. Likewise, DBP added to serum-free medium attenuated 25OHD(3)/1,25(OH)(2)D(3) responses. Fluorescently tagged DBP showed low-level uptake by monocytes, but this did not appear to involve a megalin-mediated mechanism. Human serum containing low-affinity Gc2-1S or Gc2-2, respectively, supported 2.75-fold (P = 0.003) and 2.43-fold (P = 0.016) higher induction of cathelicidin by 25OHD relative to cells cultured with high affinity Gc1F-1F. CONCLUSION: These data indicate that DBP plays a pivotal role in regulating the bioavailablity of 25OHD to monocytes. Vitamin D-dependent antimicrobial responses are therefore likely to be strongly influenced by DBP polymorphisms.

Genetic polymorphisms of the vitamin D binding protein and plasma concentrations of 25-hydroxyvitamin D in premenopausal women.

BACKGROUND: Vitamin D status, determined on the basis of 25-hydroxyvitamin D [25(OH)D] concentrations, is associated with the risk of several diseases. Vitamin D binding protein (DBP) is the major carrier of vitamin D and its metabolites, but the role of DBP single nucleotide polymorphisms (SNPs) on 25(OH)D concentrations is unclear. OBJECTIVE: The objective was to evaluate the association of 2 DBP gene SNPs with 25(OH)D concentrations and explore whether such association varies according to the amount of vitamin D that needs to be transported. DESIGN: This cross-sectional study included 741 premenopausal white women, mostly of French descent. Plasma 25(OH)D concentrations were measured by radioimmunoassay. DBP-1 (rs7041) and DBP-2 (rs4588) were genotyped with a Sequenom MassArray platform. Associations and interactions were modeled by using multivariate linear regression. RESULTS: DBP-1 and DBP-2 SNPs were in strong linkage disequilibrium and were both associated with 25(OH)D concentrations. An additional copy of the rare allele of DBP-1 or DBP-2 was associated with lower 25(OH)D concentrations (beta = -3.29, P for trend = 0.0003; beta = -4.22, P for trend < 0.0001, respectively). These DBP polymorphisms explained as much of the variation in circulating 25(OH)D as did total vitamin D intake (r2 = 1.3% for DBP-1, r2 = 2.0% for DBP-2, and r2 < or = 1.2% for vitamin D intake). CONCLUSION: Circulating 25(OH)D concentrations in premenopausal women are strongly related to DBP polymorphisms. Whether DBP rare allele carriers have a different risk of vitamin D-related diseases and whether such carriers can benefit more or less from dietary interventions, vitamin D supplementation, or sun exposure need to be clarified.