Metabolism of vitamin D3 in human osteoblasts: evidence for autocrine and paracrine activities of 1 alpha,25-dihydroxyvitamin D3

Association of circulating vitamin D concentrations with intestinal but not systemic inflammation in inflammatory bowel disease

BACKGROUND

Vitamin D may mediate immunomodulatory effects in patients with inflammatory bowel disease (IBD). The relationships between disease activity and circulating levels of total, free, and bioavailable 25(OH) vitamin D (25(OH)D) are poorly defined. The aim of this study was to measure circulating components of the vitamin D axis in patients with IBD and healthy controls and to correlate these with markers of disease activity, adjusting for potential confounders.

METHODS

Clinical data were obtained and serum was analyzed for 25(OH)D and vitamin D-binding protein in patients with IBD and controls. Markers of systemic and intestinal (fecal calprotectin) inflammation were measured.

RESULTS

Serum 25(OH)D concentration was similar across 23 controls, 40 patients with Crohn's disease, and 31 with ulcerative colitis. An inverse correlation between 25(OH)D and calprotectin was noted in Crohn's disease (Pearson's r = -0.35, P = 0.040), ulcerative colitis (r = -0.39, P = 0.039), and all IBD together (r = -0.37, P = 0.003), but not with systemic markers. A similar trend was noted for free and bioavailable 25(OH)D. This inverse correlation remained after partial correlation analysis correcting for sunlight exposure, total oral vitamin D intake, and obesity and was also noted among the subgroup without small intestinal involvement.

CONCLUSIONS

Despite total, free, and bioavailable 25(OH)D concentrations being similar to those in a healthy control population, they inversely correlated strongly with intestinal inflammation. This was independent of potential malabsorption, sunlight exposure, and total vitamin D intake and obesity. Vitamin D may play an immunomodulatory role in IBD.

Vitamin D and Crohn's disease in the adult patient: a review

Crohn's disease (CD) is characterized as a chronic immune-mediated inflammatory disorder of the gastrointestinal tract. Current consensus surrounding the cause of the disease suggests a complex interplay between genetic susceptibility, the intestinal microbiome and environmental factors, leading to the aberrant Th1 and Th17 immune cell mediated response. Vitamin D deficiency is common in CD patients, and long-standing deficiency has been associated with reduced bone mineral density (BMD). Accumulating evidence now suggests that in addition to maintaining skeletal integrity, vitamin D also plays an integral role in regulating the general immune response, a function employed via its genomic actions on the vitamin D receptor (VDR). The VDR is expressed in all immune cells and both directly and indirectly targeted by the bioactive form of vitamin D, 1,25-Dihydroxyvitamin D (1,25[OH]2D). Impaired regulation or deficiency of the vitamin has been linked to the promotion of self-reactive T cell development, loss of immune tolerance to self-structures, and experimental colitis in animal models, whereas the subsequent administration of the vitamin in these models resulted in the improvement of immune-mediated symptoms. In addition, low vitamin D has been associated with disease activity in CD patients, and supplementation appears to be beneficial in improving clinical scores and reducing inflammation. Therefore, the primary aims of this article were to review the molecular evidence supporting the immunoregulatory roles of vitamin D and its supplementation in the CD patient, based on existing literature. The physiological processes, accepted serum concentration values, and its well-recognized role in bone health were also summarized.

Regulation of matrix metalloproteinase-9 protein expression by 1α, 25-(OH)₂D₃ during osteoclast differentiation

To investigate 1α,25-(OH)₂D₃ regulation of matrix metalloproteinase-9 (MMP-9) protein expression during osteoclast formation and differentiation, receptor activator of nuclear factor kB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) were administered to induce the differentiation of RAW264.7 cells into osteoclasts. The cells were incubated with different concentrations of 1α,25-(OH)₂D₃ during culturing, and cell proliferation was measured using the methylthiazol tetrazolium method. Osteoclast formation was confirmed using tartrate-resistant acid phosphatase (TRAP) staining and assessing bone lacunar resorption. MMP-9 protein expression levels were measured with Western blotting. We showed that 1α,25-(OH)₂D₃ inhibited RAW264.7 cell proliferation induced by RANKL and M-CSF, increased the numbers of TRAP-positive osteoclasts and their nuclei, enhanced osteoclast bone resorption, and promoted MMP-9 protein expression in a concentration-dependent manner. These findings indicate that 1α,25-(OH)₂D₃ administered at a physiological relevant concentration promoted osteoclast formation and could regulate osteoclast bone metabolism by increasing MMP-9 protein expression during osteoclast differentiation.

Evidence for a specific uptake and retention mechanism for 25-hydroxyvitamin D (25OHD) in skeletal muscle cells

Little is known about the mechanism for the prolonged residence time of 25-hydroxyvitamin D (25OHD) in blood. Several lines of evidence led us to propose that skeletal muscle could function as the site of an extravascular pool of 25OHD. In vitro studies investigated the capacity of differentiated C2 murine muscle cells to take up and release 25OHD, in comparison with other cell types and the involvement of the membrane protein megalin in these mechanisms. When C2 cells are differentiated into myotubes, the time-dependent uptake of labeled 25OHD is 2-3 times higher than in undifferentiated myoblasts or nonmuscle osteoblastic MG63 cells (P < .001). During in vitro release experiments (after 25OHD uptake), myotubes released only 32% ± 6% stored 25OHD after 4 hours, whereas this figure was 60% ± 2% for osteoblasts (P < .01). Using immunofluorescence, C2 myotubes and primary rat muscle fibers were, for the first time, shown to express megalin and cubilin, endocytotic receptors for the vitamin D binding protein (DBP), which binds nearly all 25OHD in the blood. DBP has a high affinity for actin in skeletal muscle. A time-dependent uptake of Alexafluor-488-labeled DBP into mature muscle cells was observed by confocal microscopy. Incubation of C2 myotubes (for 24 hours) with receptor-associated protein, a megalin inhibitor, led to a 40% decrease in 25OHD uptake (P < .01). These data support the proposal that 25OHD, after uptake into mature muscle cells, is held there by DBP, which has been internalized via membrane megalin and is retained by binding to actin.

Calcium induces pro-anabolic effects on human primary osteoblasts associated with acquisition of mature osteocyte markers

Calcium, in combination with vitamin D, is an effective treatment for osteoporosis. Since bone mineralisation occurs concurrently with osteoblast to osteocyte transition, we hypothesised that calcium would stimulate this process. The effect of calcium (1.8-11.8mM) was tested on human primary osteoblast (NHBC) differentiation in vitro. Cultures were assayed for cell-associated mineral and gene expression associated with osteoblast differentiation and mineralisation. Treatment with calcium resulted in a striking dose- and time-dependent increase in cell-associated mineralisation. Calcium appeared to promote osteoblast to osteocyte differentiation, as indicated by increased expression of osteocalcin (OCN), E11, dentin matrix protein 1 (DMP1) and SOST mRNA. The expression of the osteoclast inhibitor, osteoprotegerin, was dramatically enhanced by calcium. Calcium also increased the ratio of PHEX mRNA expression relative to that of MEPE, suggesting a mechanism for the pro-anabolic effect. Consistent with this, calcium-dependent mineralisation was reversed in the presence of MEPE-ASARM peptides. This study suggests that calcium promotes osteoblast to osteocyte transition and concurrent matrix mineralisation, at least in part through the PHEX-MEPE axis.

The osteogenic potential of the marine-derived multi-mineral formula aquamin is enhanced by the presence of vitamin D

Bone degenerative diseases are on the increase globally and are often problematic to treat. This has led to a demand to identify supplements that aid bone growth and formation. Aquamin is a natural multi-mineral food supplement, derived from the red algae Lithothamnion species which contains calcium, magnesium and 72 other trace minerals. It has been previously reported to increase bone formation and mineralisation. This study aimed to investigate the 28 day in vitro osteogenic response of Aquamin supplemented with Vitamin D. The osteogenic potential of MC3T3-E1 osteoblast-like cells was analysed in standard osteogenic medium supplemented with Aquamin +/- Vitamin D3, and the controls consisted of osteogenic medium, +/- Vitamin D3. Proliferation of osteoblasts, metabolic activity and cell viability did not differ between Aquamin and the osteogenic control groups. Alkaline phosphatase (ALP) levels and mineralisation were increased by the supplementation of Aquamin, and the addition of Vitamin D3 increased mineralisation for all groups. The combination of Aquamin and Vitamin D3 yielded a significant increase in ALP and mineralisation over Aquamin alone and the standard osteogenic control +/- Vitamin D3. This study demonstrates that Aquamin aids osteogenesis, and that its osteogenic response can be enhanced by combining Aquamin with Vitamin D3.

Modulation of osteoclastic migration by metabolism of 25OH-vitamin D3

We have reported the metabolism of 25(OH) vitamin D3 (25D) into active 1α,25(OH)2 vitamin D3 (1,25D) by osteoclasts derived from human peripheral blood mononuclear cells (PBMC), RAW 264.7cells or giant cell tumor of bone (GCT), which appears to optimize osteoclast differentiation but inhibit their activity. In this study, to elucidate the mechanism by which 25D reduces osteoclast resorption, we further examined the effect of 25D on osteoclast function by using GCT-derived osteoclasts. 25D treated cells on dentine slices resulted in decreased resorption volume and depth in 3D image analysis. Tartrate-resistant acid phosphatase (TRAP) has been reported to enhance the dephosphorylation of substrate binding proteins, resulting in reduced osteoclast attachment. Therefore, we next investigated the effect of 25D on cell migration. Treatment of GCT cells with 25D augmented cell migration, as determined by live cell imaging. These observations suggest that 25D metabolism by osteoclasts reduces their resorptive capacity, in part by modifying their surface adhesion and migration properties. This article is part of a Special Issue entitled "Vitamin D Workshop".

The pleiotropic effects of vitamin D in bone

A current controversial question related to vitamin D supplementation is what level of serum 25-hydroxyvitamin D3 (25(OH)D3) is required to reduce the incidence of osteoporotic fractures. The reasoning behind vitamin D supplementation has been mostly derived from the role of vitamin D to promote intestinal calcium absorption and reduce bone resorption. While minimum 25(OH)D3 levels of 20nmol/L are required for sufficient intestinal calcium absorption to prevent osteomalacia, the mechanistic details of how higher 25(OH)D3 levels, well beyond that required for optimal calcium absorption, are able to prevent fractures and increase bone mineral density is unclear. Substantial evidence has arisen over the past decade that conversion of 25(OH)D3 to 1,25(OH)2D3via the 1-alpha hydroxylase (CYP27B1) enzyme in osteoblasts, osteocytes, chondrocytes and osteoclasts regulates processes such as cell proliferation, maturation and mineralization as well as bone resorption, which are all dependent on the presence the of the vitamin D receptor (VDR). We and others have also shown that increased vitamin D activity in mature osteoblasts by increasing levels of VDR or CYP27B1 leads to improved bone mineral volume using two separate transgenic mouse models. While questions remain regarding activities of vitamin D in bone to influence the anabolic and catabolic processes, the biological importance of vitamin D activity within the bone is unquestioned. However, a clearer understanding of the varied mechanisms by which vitamin D directly and indirectly influences mineral bone status are required to support evidence-based recommendations for vitamin D supplementation to reduce the risk of fractures. This article is part of a Special Issue entitled 'Vitamin D workshop'.

Extrahepatic 25-Hydroxylation of Vitamin D3 in an Engineered Osteoblast Precursor Cell Line Exploring the Influence on Cellular Proliferation and Matrix Maturation during Bone Development

Osteoblastic precursors experience distinct stages during differentiation and bone development, which include proliferation, extracellular matrix (ECM) maturation, and ECM mineralization. It is well known that vitamin D plays a large role in the regulation of bone mineralization and homeostasis via the endocrine system. The activation of vitamin D requires two sequential hydroxylation steps, first in the kidney and then in the liver, in order to carry out its role in calcium homeostasis. Recent research has demonstrated that human-derived mesenchymal stem cells (MSCs) and osteoblasts can metabolize the immediate vitamin D precursor 25-dihydroxyvitamin D3 (25OHD3) to the active steroid lα,25-dihydroxyvitamin D3 (1,25OH2D3) and elicit an osteogenic response. However, reports of extrahepatic metabolism of vitamin D3, the parental vitamin D precursor, have been limited. In this study, we investigated whether osteoblast precursors have the capacity to convert vitamin D3 to 1,25OH2D3 and examined the potential of vitamin D3 to induce 1,25OH2D3 associated biological activities in osteoblast precursors. It was demonstrated that the engineered osteoblast precursor derived from human marrow (OPC1) is capable of metabolizing vitamin D3 to 1,25OH2D3 in a dose-dependent manner. It was also demonstrated that administration of vitamin D3 leads to the increase in alkaline phosphatase (ALP) activity associated with osteoblast ECM maturation and calcium deposits and a decrease in cellular proliferation in both osteoblast precursor cell lines 0PC1 andOMC3T3-E1. These findings provide a two-dimensional culture foundation for future three-dimensional engineered tissue studies using the OPC1 cell line.

Vitamin D metabolism in human bone marrow stromal (mesenchymal stem) cells

There are many human extra-renal tissues and cells that biosynthesize 1α,25-dihydroxyvitamin D (1α,25(OH)(2)D) by the action of CYP27B1/1α-hydroxylase. Human marrow stromal cells (hMSCs), also known as mesenchymal stem cells, were isolated from marrow discarded from well-characterized, consented subjects during common orthopedic procedures. Human MSCs can give rise to osteoblasts, chondrocytes, adipocytes, and other lineages. Their in vitro differentiation to osteoblasts is stimulated by 1α,25(OH)(2)D, and recent evidence indicates that they have the capacity to metabolize vitamin D in a regulated manner. Human MSCs express the vitamin D receptor, 25-hydroxylases, 1α-hydroxylase, and 24-hydroxylase; stimulation of in vitro osteoblastogenesis by 25(OH)D depends on the activity of CYP27B1/1α-hydroxylase. The finding that hMSCs are a both a producer and target of 1α,25(OH)(2)D suggests a potential autocrine/paracrine role of vitamin D metabolism in osteoblast differentiation. Expression and enzyme activity of CYP27B1/1α-hydroxylase are upregulated by substrate 25(OH)D and Parathyroid Hormone (PTH) and are downregulated by 1α,25(OH)(2)D. With subject age, there are decreases in basal osteoblast potential and in stimulation of osteoblastogenesis by 1α,25(OH)(2)D, 25(OH)D, and PTH. In vitro treatment with a combination of 25(OH)D and PTH rejuvenated osteoblastogenesis with hMSCs from elders; this was attributable to increases in CYP27B1/1α-hydroxylase and in receptor for each hormone by the reciprocal factor. Other clinical variables beside age, i.e. low serum 25(OH)D or low estimated glomerular filtration rate, are correlated with reduced osteoblastogenesis. These studies suggest that osteoblastogenesis may not be optimal unless there is sufficient serum 25(OH)D substrate for hMSCs to synthesize and respond to local 1α,25(OH)(2)D.

UV-irradiated 7-dehydrocholesterol coating on polystyrene surfaces is converted to active vitamin D by osteoblastic MC3T3-E1 cells

The aim of the present study was to determine the effects of UV irradiation on the conversion of 7-dehydrocholesterol (7-DHC), which has been coated onto a polystyrene surface, to cholecalciferol (D3), and the resulting effect on the formation of vitamin D (1,25-D3) by MC3T3-E1 cells. The changes in gene expression of the enzymes regulating its hydroxylation, Cyp27b1 and Cyp27a1, were monitored as well as the net effect of the UV-treated 7-DHC coating on cell viability and osteoblast differentiation. MC3T3-E1 cells were found to express the enzymes required for synthesizing active 1,25-D3, and we found a dose-dependent increase in the production of both 25-D3 and 1,25-D3 levels for UV-activated 7-DHC samples unlike UV-untreated ones. Cell viability revealed no cytotoxic effect for any of the treatments, but only for the highest dose of 7-DHC (20 nmol per well) that was UV-irradiated. Furthermore, osteoblast differentiation was increased in cells treated with some of the higher doses of 7-DHC when UV-irradiated, as shown by collagen-I, osterix and osteocalcin relative mRNA levels. The conversion of 7-DHC to preD3 exogenously by UV irradiation and later to 25-D3 by MC3T3-E1 cells was determined for the optimum 7-DHC dose (0.2 nmol per well), i.e. 8.6 ± 0.7% of UV-activated 7-DHC was converted to preD3 and 6.7 ± 2.8% of preD3 was finally converted to 25-D3 under the conditions studied. In conclusion, we demonstrate that an exogenous coating of 7-DHC, when UV-irradiated, can be used to endogenously produce active vitamin D. We hereby provide the scientific basis for UV-activated 7-DHC coating as a feasible approach for implant therapeutics focused on bone regeneration.

UV photoactivation of 7-dehydrocholesterol on titanium implants enhances osteoblast differentiation and decreases Rankl gene expression

Vitamin D plays a central role in bone regeneration, and its insufficiency has been reported to have profound negative effects on implant osseointegration. The present study aimed to test the in vitro biological effect of titanium (Ti) implants coated with UV-activated 7-dehydrocholesterol (7-DHC), the precursor of vitamin D, on cytotoxicity and osteoblast differentiation. Fourier transform infrared spectroscopy confirmed the changes in chemical structure of 7-DHC after UV exposure. High-pressure liquid chromatography analysis determined a 16.5±0.9% conversion of 7-DHC to previtamin D(3) after 15min of UV exposure, and a 34.2±4.8% of the preD(3) produced was finally converted to 25-hydroxyvitamin D(3) (25-D(3)) by the osteoblastic cells. No cytotoxic effect was found for Ti implants treated with 7-DHC and UV-irradiated. Moreover, Ti implants treated with 7-DHC and UV-irradiated for 15min showed increased 25-D(3) production, together with increased ALP activity and calcium content. Interestingly, Rankl gene expression was significantly reduced in osteoblasts cultured on 7-DHC-coated Ti surfaces when UV-irradiated for 15 and 30min to 33.56±15.28% and 28.21±4.40%, respectively, compared with the control. In conclusion, these findings demonstrate that UV-activated 7-DHC is a biocompatible coating of Ti implants, which allows the osteoblastic cells to produce themselves active vitamin D, with demonstrated positive effects on osteoblast differentiation in vitro.

RANKL/OPG ratio and DKK-1 expression in primary osteoblastic cultures from osteoarthritic and osteoporotic subjects

OBJECTIVE

To evaluate the expression of Dickkopf-1 protein factor (DKK-1), DKK-2, and β-catenin, components of the Wnt pathway, in human osteoarthritic (OA) and osteoporotic (OP) osteoblasts and to correlate it to cell metabolic activity, proliferation, and receptor activator of nuclear factor-κB ligand/osteoprotegerin (RANKL/OPG) expression.

METHODS

Primary human osteoblast cultures were obtained from healthy, OA, and OP donors. In each cell population we evaluated DKK-1, DKK-2, nonphosphorylated β-catenin and RANKL/OPG expression, osteocalcin and alkaline phosphatase (ALP) synthesis, and cell proliferation, both in basal condition and after vitamin D3 stimulation.

RESULTS

DKK-1 and DKK-2 showed opposite patterns of expression in OA and OP osteoblasts. The RANKL/OPG ratio was significantly higher in the OP group because of a greater expression of RANKL, whereas it was significantly lower in the OA group because of a higher expression of OPG. Treatment with vitamin D3 increased the RANKL/OPG ratio and DKK-2 expression and reduced DKK-1 expression in each cell population, but did not affect β-catenin levels. Both osteocalcin and ALP production and cell proliferation were enhanced in OA cells and reduced in the OP ones.

CONCLUSION

These data confirm that OA and OP are characterized by opposite bone changes, consisting of reduced bone remodeling processes with increased osteoblast activity in OA, and enhanced bone resorptive activity with reduction of osteoblast metabolism in OP, and suggest that the Wnt pathway is involved in the pathogenesis of both diseases.

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.

Availability of 25-hydroxyvitamin D(3) to APCs controls the balance between regulatory and inflammatory T cell responses

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], the active form of vitamin D, exerts potent effects on several tissues including cells of the immune system, where it affects T cell activation, differentiation and migration. The circulating, inactive form of vitamin D, 25(OH)D(3), is generally used as an indication of vitamin D status. However, use of this precursor depends on its uptake by cells and subsequent conversion by the enzyme 25(OH)D(3)-1α-hydroxylase (CYP27B1) into active 1,25(OH)(2)D(3). Using human T cells, we show in this study that addition of inactive 25(OH)D(3) is sufficient to alter T cell responses only when dendritic cells (DCs) are present. Mechanistically, CYP27B1 is induced in DCs upon maturation with LPS or upon T cell contact, resulting in the generation and release of 1,25(OH)(2)D(3), which subsequently affects T cell responses. In most tissues, vitamin D binding protein acts as a carrier to enhance the use of vitamin D. However, we show that vitamin D binding protein modulates T cell responses by restricting the availability of inactive 25(OH)D(3) to DC. These data indicate that the level of free 25(OH)D(3) available to DCs determines the inflammatory/regulatory balance of ensuing T cell responses.

Vitamin D supplementation protects against bone loss associated with chronic alcohol administration in female mice

Chronic alcohol abuse results in decreased bone mineral density (BMD), which can lead to increased fracture risk. In contrast, low levels of alcohol have been associated with increased BMD in epidemiological studies. Alcohol's toxic skeletal effects have been suggested to involve impaired vitamin D/calcium homeostasis. Therefore, dietary vitamin D supplementation may be beneficial in reducing bone loss associated with chronic alcohol consumption. Six-week-old female C57BL/6J mice were pair-fed ethanol (EtOH)-containing liquid diets (10 or 36% total calories) for 78 days. EtOH exposure at 10% calories had no effects on any measured bone or serum parameter. EtOH consumption at 36% of calories reduced BMD and bone strength (P<0.05), decreased osteoblastogenesis, increased osteoclastogenesis, suppressed 1,25-hydroxyvitamin D3 [1,25(OH)2D3] serum concentrations (P<0.05), and increased apoptosis in bone cells compared with pair-fed controls. In a second study, female mice were pair-fed 30% EtOH diets with or without dietary supplementation with vitamin D3 (cholecalciferol; VitD) for 40 days. VitD supplementation in the EtOH diet protected against cortical bone loss, normalized alcohol-induced hypocalcaemia, and suppressed EtOH-induced expression of receptor of nuclear factor-κB ligand mRNA in bone. In vitro, pretreatment of 1,25(OH)2D3 in osteoblastic cells inhibited EtOH-induced apoptosis. In EtOH/VitD mice circulating 1,25(OH)2D3 was lower compared with mice receiving EtOH alone (P<0.05), suggesting increased sensitivity to feedback control of VitD metabolism in the kidney. These findings suggest dietary VitD supplementation may prevent skeletal toxicity in chronic drinkers by normalizing calcium homeostasis, preventing apoptosis, and suppressing EtOH-induced increases in bone resorption.

Evidence for the dysregulated expression of TWIST1, TGFβ1 and SMAD3 in differentiating osteoblasts from primary hip osteoarthritis patients

OBJECTIVE

This study compared human primary osteoblasts derived from hip osteoarthritis (OA) cases against controls (CTLs) to investigate candidate OA disease genes, twist homologue 1 (TWIST1), wingless MMTV integration site family member 5B (WNT5B), transforming growth factor-β (TGFβ1) and SMAD family member 3 (SMAD3), during osteoblast differentiation, relative to calcium apposition and elemental mineral composition.

MATERIALS & METHODS

Primary osteoblast cultures were generated from intertrochanteric trabecular bone samples from five female primary hip OA cases and five age-matched female CTLs. During a 42-day differentiation time-course, alizarin red stains, energy-dispersive X-ray spectroscopy and real-time RT-polymerase chain reaction (PCR) were used to quantify calcium, elemental composition and gene expression, respectively. Data were analysed using linear mixed effects models and Pearson correlation matrices.

RESULTS

Significant differences, correlations and associations were found in OA and CTL osteoblasts between gene and mineral measures. The calcium: phosphorous (Ca:P) ratio was significantly more varied in OA compared to CTL. Calcium apposition, mineral composition as well as TWIST1 and TGFβ1 mRNA expression changed significantly over time. TWIST1 mRNA expression was elevated and correlated with SMAD3 mRNA levels in the OA cohort during the time-course. Associations were observed between tissue non-specific alkaline phosphatase (TNAP), osteocalcin (OCN), TWIST1, TGFβ1, SMAD3 mRNA levels and mineral measures in OA against CTL. Temporal differences between SMAD3 mRNA expression and mineral composition were also found in OA.

CONCLUSIONS

Dysregulated expression of TWIST1, TGFβ1 and SMAD3 mRNA observed in OA bone is reflected in the functionality of the osteoblast when these cells are cultured ex vivo. The results presented here are consistent with at least part of the aetiology of primary hip OA deriving from altered intrinsic properties of the osteoblast.

MKP-1 is essential for canonical vitamin D-induced signaling through nuclear import and regulates RANKL expression and function

Vitamin D(3,) and its most active form, 1,25(OH)(2)D(3), are well known to stimulate osteoclastogenesis through stromal cell induction of the receptor activator of nuclear factor-κB ligand (RANKL). MAPK phosphatase-1 (MKP-1) is a phosphatase classically known to negatively regulate the innate immune response through dephosphorylation of p38, ERK, and c-Jun N-terminal kinase activity. This paper describes a new function of MKP-1 in permitting genomic 1,25(OH)(2)D(3) signaling and downstream osteoclastogenesis through RANKL. Initially, quantitative RT-PCR (qRT-PCR) and immunoblot analysis comparing bone marrow stromal cells (BMSC) revealed that 1,25(OH)(2)D(3)-induced vitamin D receptor (VDR), cytochrome P 45024a1, and RANKL mRNA expression and protein were significantly attenuated or absent in MKP-1(-/-) BMSC. Immunoblot analysis from cellular fractions of wild type and MKP-1(-/-) BMSC stimulated with 10(-7) m 1,25(OH)(2)D(3) revealed retinoid X receptor (RXR)α nuclear import was impaired in MKP-1(-/-) BMSC, whereas VDR import was not. Proximity ligation assays revealed that baseline VDR-RXRα heterodimer translocation was unchanged, yet 1,25(OH)(2)D(3)-induced nuclear translocation of VDR-RXRα heterodimers was reduced in MKP-1(-/-) BMSC. A functional consequence was observed as BMSC from MKP-1(-/-) mice treated with 1,25(OH)(2)D(3) and cocultured with RAW 264.7 cells had a 91% decrease in osteoclastogenesis and a 94.5% decrease in mineralized matrix resorption compared with wild-type cocultures (P < 0.01). These results reveal an unexpected, permissive role for MKP-1 in canonical 1,25(OH)(2)D(3) signaling via VDR-RXRα heterodimer nuclear import and downstream osteoclastogenesis through stromal cell RANKL expression.

Review article: vitamin D and inflammatory bowel disease--established concepts and future directions

BACKGROUND

Understanding of the role of vitamin D in health and disease has increased markedly in the past decade, with its involvement extending well beyond traditional roles in calcium and phosphate homeostasis and musculoskeletal health. This conceptual expansion has been underpinned by identification and exploration of components of this axis including vitamin D-binding protein, key enzymes and receptors in multiple cell types, and a greater recognition of nonclassical autocrine and paracrine effects. Its influence in IBD remains uncertain.

AIM

To review the role of vitamin D in bone health, immune regulation and cancer prevention in IBD, and to outline practical issues and limitations of its use.

METHODS

An extensive online literature review including PubMed and Medline.

RESULTS

In patients with IBD, the vitamin D axis provides an important and often underutilised pathway to preserving bone health. Furthermore, an exciting body of clinical and basic science research demonstrates that these pathways may have an integral part to play in regulation of the immune response in IBD, through effects on the intestinal barrier, antigen presenting cells and adaptive T cells. The possibility of chemoprevention requires further study. The optimal target level of 25-hydroxy vitamin D in patients with IBD is currently uncertain, as is the best therapeutic modality.

CONCLUSIONS

Study of vitamin D pathways may result in the development of relatively inexpensive therapeutic options to optimise patient outcomes. Further prospective clinical research is required to address efficacy and long-term safety.

Osteocyte regulation of bone mineral: a little give and take

Osteocytes actively participate in almost every phase of mineral handling by bone. They regulate the mineralisation of osteoid during bone formation, and they are also a major RANKL-producing cell. Osteocytes are thus able to liberate bone mineral by regulating osteoclast differentiation and activity in response to a range of stimuli, including bone matrix damage, bone disuse and mechanical unloading, oestrogen deficiency, high-dose glucocorticoid and chemotherapeutic agents. At least some of these activities may be regulated by the osteocyte-secreted product, sclerostin. There is also mounting evidence that in addition to regulating phosphate homeostasis systemically, osteocytes contribute directly to calcium homeostasis in the mature skeleton. Osteocyte cell death and the local loss of control of bone mineralisation may be the cause of focal hypermineralisation of bone and osteopetrosis, as seen in aging and pathology. The sheer number of osteocytes in bone means that "a little give and take" in terms of regulation of bone mineral content translates into a powerful whole organism effect.

Characterization of the autocrine/paracrine function of vitamin D in human gingival fibroblasts and periodontal ligament cells

BACKGROUND

We previously demonstrated that 25-hydroxyvitamin D(3), the precursor of 1α,25-dihydroxyvitamin D(3), is abundant around periodontal soft tissues. Here we investigate whether 25-hydroxyvitamin D(3) is converted to 1α,25-dihydroxyvitamin D(3) in periodontal soft tissue cells and explore the possibility of an autocrine/paracrine function of 1α,25-dihydroxyvitamin D(3) in periodontal soft tissue cells.

METHODOLOGY/PRINCIPAL FINDINGS

We established primary cultures of human gingival fibroblasts and human periodontal ligament cells from 5 individual donors. We demonstrated that 1α-hydroxylase was expressed in human gingival fibroblasts and periodontal ligament cells, as was cubilin. After incubation with the 1α-hydroxylase substrate 25-hydroxyvitamin D(3), human gingival fibroblasts and periodontal ligament cells generated detectable 1α,25-dihydroxyvitamin D(3) that resulted in an up-regulation of CYP24A1 and RANKL mRNA. A specific knockdown of 1α-hydroxylase in human gingival fibroblasts and periodontal ligament cells using siRNA resulted in a significant reduction in both 1α,25-dihydroxyvitamin D(3) production and mRNA expression of CYP24A1 and RANKL. The classical renal regulators of 1α-hydroxylase (parathyroid hormone, calcium and 1α,25-dihydroxyvitamin D(3)) and Porphyromonas gingivalis lipopolysaccharide did not influence 1α-hydroxylase expression significantly, however, interleukin-1β and sodium butyrate strongly induced 1α-hydroxylase expression in human gingival fibroblasts and periodontal ligament cells.

CONCLUSIONS/SIGNIFICANCE

In this study, the expression, activity and functionality of 1α-hydroxylase were detected in human gingival fibroblasts and periodontal ligament cells, raising the possibility that vitamin D acts in an autocrine/paracrine manner in these cells.

Vitamin D and bone

All cells comprising the skeleton-chondrocytes, osteoblasts, and osteoclasts-contain both the vitamin D receptor and the enzyme CYP27B1 required for producing the active metabolite of vitamin D, 1,25 dihydroxyvitamin D. Direct effects of 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D on these bone cells have been demonstrated. However, the major skeletal manifestations of vitamin D deficiency or mutations in the vitamin D receptor and CYP27B1, namely rickets and osteomalacia, can be corrected by increasing the intestinal absorption of calcium and phosphate, indicating the importance of indirect effects. On the other hand, these dietary manipulations do not reverse defects in osteoblast or osteoclast function that lead to osteopenic bone. This review discusses the relative importance of the direct versus indirect actions of vitamin D on bone, and provides guidelines for the clinical use of vitamin D to prevent/treat bone loss and fractures.

VDR and CYP27B1 are expressed in C2C12 cells and regenerating skeletal muscle: potential role in suppression of myoblast proliferation

1α,25(OH)(2)D(3), the active form of vitamin D(3), has been reported to regulate the cell biology of skeletal muscle. However, there has been some controversy about the expression of the vitamin D receptor (VDR) and thus the potential role of vitamin D(3) in skeletal muscle. In this study, we isolated and sequenced the full-length Vdr and Cyp27b1 transcripts in C2C12 myoblasts and myotubes. Western blots and immunocytochemistry confirmed protein expression in both myoblasts and myotubes clearly demonstrating that C2C12 cells express VDR and CYP27B1. To determine the vitamin D(3) action, we found that C2C12 myoblasts treated with either 1α,25(OH)(2)D(3) or 25(OH)D(3) inhibited cell proliferation and this was associated with increased Vdr expression. The observation that treatment of C2C12 myoblasts with the inactive form of vitamin D(3), [25(OH)D(3)], inhibited proliferation suggested that CYP27B1 was functionally active. We used small interfering RNA to knock down Cyp27b1 in myoblasts, and cells were treated with 25(OH)D(3). The growth-suppressive effects of 25(OH)D(3) were abolished, suggesting that CYP27B1 in myoblasts is necessary for the ability of 25(OH)D(3) to affect cell proliferation. Finally, we analyzed expression of VDR and CYP27B1 in regenerating skeletal muscle in vivo. We found that expression of VDR and CYP27B1 increased significantly at day 7 of regeneration, and these results confirm the expression of Vdr and Cyp27b1 in vivo and suggest a potential role for vitamin D(3) in skeletal muscle regeneration following injury.

Microarray analysis of 1,25(OH)₂D₃ regulated gene expression in human primary osteoblasts

Though extensive studies have been conducted, questions regarding the molecular effectors and pathways underlying the regulatory role of 1,25(OH)(2)D(3) in human osteoblasts other than cell differentiation and matrix protein production remain unanswered. This study aims to identify genes and pathways that are modulated by 1,25(OH)(2)D(3) treatment in human osteoblasts. Primary osteoblast cultures obtained from human bone tissue samples were treated with 1,25(OH)(2)D(3) (10(-7)  M) for 24 h and their transcritptomes were profiled by microarray analysis using the Affymetrix GeneChip. Statistical analysis was conducted to identify genes whose expression is significantly modulated following 1,25(OH)(2)D(3) treatment. One hundred and fifty-eight genes were found to be differentially expressed. Of these, 136 were upregulated, indicating clear transcriptional activation by 1,25(OH)(2)D(3). Biostatistical evaluation of microarray data by Ingenuity Pathways Analysis (IPA) revealed a relevant modulation of genes involved in vitamin D metabolism (CYP24), immune functions (CD14), neurotransmitter transporters (SLC1A1, SLC22A3), and coagulation [thrombomodulin (THBD), tissue plasminogen activator (PLAT), endothelial protein C receptor (PROCR), thrombin receptor (F2R)]. We identified a restricted number of highly regulated genes and confirmed their differential expression by real-time quantitative PCR (RT qPCR). The present genome-wide microarray analysis on 1,25(OH)(2)D(3) -treated human osteoblasts reveals an interplay of critical regulatory and metabolic pathways and supports the hypothesis that 1,25(OH)(2)D(3) can modulate the coagulation process through osteoblasts, activates osteoclastogenesis through inflammation signaling, modulates the effects of monoamines by affecting their reuptake.

Effect of 25-hydroxyvitamin D3 and 1 α,25 dihydroxyvitamin D3 on differentiation and apoptosis of human osteosarcoma cell lines

Osteosarcoma (OS) is a malignant bone tumor predominantly affecting children and adolescents. OS has a 60% survival rate with current treatments; hence, there is a need to identify novel adjuncts to chemotherapeutic regimens. In this pilot study, we investigated the dose-response to 1α,25-dihdroxyvitamin D(3) (1,α 25(OH)(2) D(3)) and 25-hydroxyvitamin D(3) (25(OH)D(3)) by human OS cell lines, SaOS-2, and 143B. We hypothesized that 1,α 25(OH)(2) D(3) and 25(OH)D(3) would stimulate differentiation and induce apoptosis in OS cells in a dose-dependent manner. Human OS cell lines, SaOS-2, and 143B, were treated with 1,α 25(OH)(2)D(3) or 25(OH)D(3) or an ethanol control, respectively, at concentrations ranging from 1 to 1,000 nM. Ki67 (a marker of cellular proliferation) immunocytochemistry revealed no significant changes in the expression of Ki-67 or MIB-1 in 1α,25(OH)(2)D(3) or 25(OH)D(3) treated SaOS-2 or 143B cells. Both control and 1α,25(OH)(2) D(3) treated SaOS-2 and 143B cells expressed vitamin D receptor (VDR). Markers of osteoblastic differentiation in 143B cells and SaOS-2 cells were induced by both 25(OH)D(3) and 1α,25(OH)(2) D, and evident by increases in alkaline phosphatase (ALP) activity, osteocalcin (OCN) mRNA expression, and mineralization of extra-cellular matrix (ECM) by alizarin red staining. An increasing trend in apoptosis in response to 25(OH)D(3), in both SaOS-2 and 143B cells was detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) staining. With 1α,25(OH)(2)D(3) treatment, apoptosis was evident at higher concentrations only. These preliminary findings suggest that OS cells express VDR and respond to 25(OH)D(3) and 1α,25(OH)(2)D(3) by undergoing differentiation and apoptosis.

Extrarenal expression of the 25-hydroxyvitamin D-1-hydroxylase

Like the vitamin D receptor (VDR), the CYP27B1-hydroxylase is expressed widely in human tissues. This expression profile establishes the potential for interaction of the VDR with the product of the CYP27B1, 1,25-dihydroxyvitamin D (1,25-(OH)(2)D), in either an intracrine or paracrine mode. This expansive expression profile also suggests that the local production and action of 1,25-(OH)(2)D to regulate VDR-directed gene expression may be similarly wide-ranging and distinct from what occurs in the kidney; the proximal renal tubular epithelial cell is the richest source of the CYP27B1 and the site for production of 1,25-(OH)(2)D destined to function as a hormone. Existence of the CYP27B1 at extrarenal sites has been widely documented, although the functional impact of the enzyme in these tissues has yet to be fully demonstrated. Two notable exceptions are the disease-activated macrophage (e.g., in sarcoidosis or tuberculosis) and the placenta. These two tissues are capable of generating enough 1,25-(OH)(2)D so as to be detectable in the general circulation. As such, this review will focus on CYP27B1 expression only at these two sites, theorizing that 1,25-(OH)(2)D production at these sites is for the purpose of local immunoregulatory function, not for controlling calcium balance in the host or the fetus.

Vitamin D and cancer: a review of molecular mechanisms

The population-based association between low vitamin D status and increased cancer risk can be inconsistent, but it is now generally accepted. These relationships link low serum 25OHD (25-hydroxyvitamin D) levels to cancer, whereas cell-based studies show that the metabolite 1,25(OH)2D (1,25-dihydroxyvitamin D) is a biologically active metabolite that works through vitamin D receptor to regulate gene transcription. In the present review we discuss the literature relevant to the molecular events that may account for the beneficial impact of vitamin D on cancer prevention or treatment. These data show that although vitamin D-induced growth arrest and apoptosis of tumour cells or their non-neoplastic progenitors are plausible mechanisms, other chemoprotective mechanisms are also worthy of consideration. These alternative mechanisms include enhancing DNA repair, antioxidant protection and immunomodulation. In addition, other cell targets, such as the stromal cells, endothelial cells and cells of the immune system, may be regulated by 1,25(OH)2D and contribute to vitamin D-mediated cancer prevention.

RANKL/RANK/OPG system and bone status in females with anorexia nervosa

Minimal data exist concerning the relationship between osteokines of the RANKL/RANK/OPG system, especially RANKL, and bone status in females with anorexia nervosa (AN). For this reason we investigated the relationship between bone metabolism (as assessed based on serum levels of OC and CTx), and OPG and sRANKL concentrations in females with AN. Ninety-one female patients with AN and 29 healthy female subjects aged 13 to 18 years of age participated in the study. Serum OC, CTx, OPG and sRANKL were measured by ELISA. The female patients with AN demonstrated an essential suppression of OC and CTx, increased OPG and sRANKL levels, and a reduced OPG/sRANKL ratio. OC, CTx and the OPG/sRANKL ratio correlated positively with body mass and BMI in these patients, whereas in the case of OPG and sRANKL the relationship was negative. A significant positive correlation was observed between OPG and sRANKL and also between bone markers and the OPG/sRANKL ratio, and negative between CTx and sRANKL. In female patients with AN, the OPG/RANKL ratio was a significant and independent predictor of osteocalcin, a bone formation marker - OC (R(2)=0.065, p=0.012) whereas the OPG/sRANKL ratio and BMI were significant and independent predictors of a bone resorption marker - CTx (R(2)=0.095; p=0.012). In conclusion, the body mass, BMI values, and bone markers suppression observed in female patients with AN might be associated with an increase in OPG and sRANKL levels and a significant decrease of the OPG/sRANKL ratio. Although higher OPG levels may compensate for excessive bone resorption in female patients with AN, the lower OPG/sRANKL ratio seems to indicate that some inadequacies exist regarding this compensation effect, which might contribute to low bone density in these patients. The OPG/sRANKL ratio might prove a more relevant marker to predict bone metabolism in female patients with AN than sRANKL and/or OPG alone.

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

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

Alcohol and bone: review of dose effects and mechanisms

Alcohol is widely consumed across the world. It is consumed in both social and cultural settings. Until recently, two types of alcohol consumption were recognized: heavy chronic alcohol consumption or light consumption. Today, there is a new pattern of consumption among teenagers and young adults namely: binge drinking. Heavy alcohol consumption is detrimental to many organs and tissues, including bones, and is known to induce secondary osteoporosis. Some studies, however, have reported benefits from light alcohol consumption on bone parameters. To date, little is known regarding the effects of binge drinking on bone health. Here, we review the effects of three different means of alcohol consumption: light, heavy, and binge drinking. We also review the detailed literature on the different mechanisms by which alcohol intake may decrease bone mass and strength. The effects of alcohol on bone are thought to be both direct and indirect. The decrease in bone mass and strength following alcohol consumption is mainly due to a bone remodeling imbalance, with a predominant decrease in bone formation. Recent studies, however, have reported new mechanisms by which alcohol may act on bone remodeling, including osteocyte apoptosis, oxidative stress, and Wnt signalling pathway modulation. The roles of reduced total fat mass, increased lipid content in bone marrow, and a hypoleptinemia are also discussed.

Lack of efficacy of ergocalciferol repletion

INTRODUCTION

Vitamin D has become an area of intensive scrutiny, both in medical and lay literature. However, there are limited data to suggest proper repletion regimens for those patients who have hypovitaminosis D. Consequently, various methods are used in clinical practice. The aim of this study was to assess the efficacy of various treatment strategies for hypovitaminosis D in an ambulatory internal medicine practice.

METHODS

A retrospective chart review between October 2005 and June 2010 of a suburban internal medicine practice was performed via query of the electronic medical record (Centricity, General Electric Healthcare, UK). Patients with a 25-hydroxyvitamin D concentration less than 32 mg/dl were identified and treated. Treatment success was defined as 25-hydroxyvitamin D concentrations greater than 32 mg/dl. Statistical analysis to assess changes in vitamin D level controlling for season, comorbidities, and demographics were used.

RESULTS

A total of 607 treatment episodes were identified, with 395 excluded due to lack of follow-up vitamin D level within 16 weeks, no treatment documented, topical treatment, doxercalciferol treatment, or non-compliance. Of the remaining patients, there were 212 treatment instances on 178 patients. Ergocalciferol 50,000 international units (IU) was used most frequently (71.4% of the time.). A higher initial vitamin D level was positively associated with treatment success (adjusted odds ratio = 1.11, p=0.002). Increased doses of ergocalciferol increased the likelihood of treatment success (p=0.0011). Seasonal variation was related to posttreatment 25-hydroxyvitamin D concentration as was body mass index (BMI) (p=0.003 and p=0.044).

CONCLUSION

Pretreatment levels of 25-hydroxyvitamin D, BMI, season, and vitamin D dose are predictors of successful hypovitaminosis D treatment. Our data suggest that patients with initial 25-hydroxyvitamin D concentrations of <20 should be treated with a higher total dose of ergocalciferol than 50,000 IU for 8 weeks. Further studies, including prospective, randomized trials, are needed to determine an optimal treatment protocol to account for the numerous variables.

Vitamin D metabolism within bone cells: effects on bone structure and strength

The endocrine activity of 1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)) contributes to maintaining plasma calcium and phosphate homeostasis through actions on the intestine, kidney and bone. A significant body of evidence has been published over the last 10 years indicating that all major bone cells have the capacity to metabolise 25-hydroxyvitamin D (25(OH)D(3)) to 1,25(OH)(2)D(3), which in turn exerts autocrine/paracrine actions to regulate bone cell proliferation and maturation as well as bone mineralisation and resorption. In vivo and in vitro studies indicate that these autocrine/paracrine activities of 1,25(OH)(2)D(3) in bone tissue contribute to maintaining bone mineral homeostasis and enhancing skeletal health.

Age-related decline in osteoblastogenesis and 1α-hydroxylase/CYP27B1 in human mesenchymal stem cells: stimulation by parathyroid hormone

With aging, there is a decline in bone mass and in osteoblast differentiation of human mesenchymal stem cells (hMSCs) in vitro. Osteoblastogenesis can be stimulated with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ] and, in some hMSCs, by the precursor 25-hydroxyvitamin D(3) (25OHD(3) ). CYP27B1/1α-hydroxylase activates 25OHD(3) and, to a variable degree, hMSCs express CYP27B1. In this study, we tested the hypotheses (i) that age affects responsiveness to 25OHD(3) and expression/activity of CYP27B1 in hMSCs and (ii) that parathyroid hormone (PTH) upregulates CYP27B1 in hMSCs, as it does in renal cells. There were age-related declines in osteoblastogenesis (n=8, P=0.0286) and in CYP27B1 gene expression (n=27, r= -0.498; P=0.008) in hMSCs. Unlike hMSCs from young subjects (≤50 years), hMSCs from older subjects (≥55 years) were resistant to 25OHD(3) stimulation of osteoblastogenesis. PTH1-34 (100 nm) provided hMSCs with responsiveness to 25OHD(3) (P=0.0313, Wilcoxon matched pairs test) and with two episodes of increased 1,25(OH)(2) D(3) synthesis, of cAMP response element binding protein (CREB) activation, and of CYP27B1 upregulation. Both increases in CYP27B1 expression by PTH were obliterated by CREB-siRNA or KG-501 (which specifically inhibits the downstream binding of activated CREB). Only the second period of CREB signaling was diminished by AG1024, an inhibitor of insulin-like growth factor-I receptor kinase. Thus, PTH stimulated hMSCs from elders with responsiveness to 25OHD(3) by upregulating expression/activity of CYP27B1 and did so through CREB and IGF-I pathways.

Sclerostin stimulates osteocyte support of osteoclast activity by a RANKL-dependent pathway

Sclerostin is a product of mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation. While evidence suggests that sclerostin has an anti-anabolic role, the possibility also exists that sclerostin has catabolic activity. To test this we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin, or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated the expression of receptor activator of nuclear factor kappa B (RANKL) mRNA and down-regulated that of osteoprotegerin (OPG) mRNA, causing an increase in the RANKL∶OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold) compared to untreated co-cultures. The increased resorption was abolished by co-addition of recombinant OPG. In co-cultures of MLO-Y4 cells with PBMC, SCL also increased the number and size of the TRAP-positive multinucleated cells formed. Importantly, rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Further, rhSCL did not induce apoptosis of MLO-Y4 cells, as determined by caspase activity assays, demonstrating that the osteoclastic response was not driven by dying osteocytes. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes, in a RANKL-dependent manner.

Shining D' light on chronic kidney disease: mechanisms that may underpin the cardiovascular benefit of vitamin D

Hypovitaminosis D is a significant health-care burden worldwide, particularly in susceptible populations such as those with chronic kidney disease (CKD). Recent epidemiological studies have identified that both higher serum vitamin D concentrations and use of vitamin D supplements may confer a survival benefit both in terms of all-cause and cardiovascular mortality. There is potential to investigate this inexpensive therapy for the CKD population, which suffers excessive cardiovascular events, although the mechanisms explaining this link have yet to be fully elucidated. This review discusses potential mechanisms identified in the basic science literature that may provide important insights into how vitamin D may orchestrate a change in cardiovascular risk profile through such diverse mechanisms as inflammation, atherogenesis, glucose homeostasis, vascular calcification, renin-angiotensin regulation and alterations in cardiac physiology. Where available, the clinical translation of these concepts to intervention trials in the CKD population will be reviewed.

Effects of 25-hydroxyvitamin D(3) on proliferation and osteoblast differentiation of human marrow stromal cells require CYP27B1/1α-hydroxylase

1,25-Dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] has many noncalcemic actions that rest on inhibition of proliferation and promotion of differentiation in malignant and normal cell types. 1,25(OH)(2)D(3) stimulates osteoblast differentiation of human marrow stromal cells (hMSCs), but little is known about the effects of 25-hydroxyvitamin D(3)[25(OH)D(3)] on these cells. Recent evidence shows that hMSCs participate in vitamin D metabolism and can activate 25(OH)D(3) by CYP27B1/1α-hydroxylase. These studies test the hypothesis that antiproliferative and prodifferentiation effects of 25(OH)D(3) in hMSCs depend on CYP27B1. We studied hMSCs that constitutively express high (hMSCs(hi-1α) ) or low (hMSCs(lo-1α)) levels of CYP27B1 with equivalent expression of CYP24A1 and vitamin D receptor. In hMSCs(hi-1α), 25(OH)D(3) reduced proliferation, downregulated proliferating cell nuclear antigen (PCNA), upregulated p21(Waf1/Cip1), and decreased cyclin D1. Unlike 1,25(OH)(2)D(3), the antiapoptotic effects of 25(OH)D(3) on Bax and Bcl-2 were blocked by the P450 inhibitor ketoconazole. The antiproliferative effects of 25(OH)D(3) in hMSCs(hi-1α) and of 1,25(OH)(2)D(3) in both samples of hMSCs were explained by cell cycle arrest, not by increased apoptosis. Stimulation of osteoblast differentiation in hMSCs(hi-1α) by 25(OH)D(3) was prevented by ketoconazole and upon transfection with CYP27B1 siRNA. These data indicate that CYP27B1 is required for 25(OH)D(3)'s action in hMSCs. Three lines of evidence indicate that CYP27B1 is required for the antiproliferative and prodifferentiation effects of 25(OH)D(3) on hMSCs: Those effects were not seen (1) in hMSCs with low constitutive expression of CYP27B1, (2) in hMSCs treated with ketoconazole, and (3) in hMSCs in which CYP27B1 expression was silenced. Osteoblast differentiation and skeletal homeostasis may be regulated by autocrine/paracrine actions of 25(OH)D(3) in hMSCs.

Bioavailability and efficacy of vitamin D2 from UV-irradiated yeast in growing, vitamin D-deficient rats

New food sources are needed to bridge the gap between vitamin D intake and recommended intake. We assessed the bioavailability and efficacy of vitamin D in an 8 week dose-response study of bread made with vitamin D2-rich yeast compared to vitamin D3 in growing, vitamin D-deficient rats. Plasma 25-hydroxyvitamin D (25OHD) levels increased in a curvilinear, dose-dependent manner with both forms of vitamin D, but rats fed vitamin D2-rich yeast achieved lower levels than rats fed vitamin D3. Rats fed the highest doses of vitamin D had significantly greater (p<0.05) trabecular BMC, BMD, bone volume, and connectivity density, and greater midshaft total cross-sectional area, compared to rats on the vitamin D-deficient diets, with no significant difference due to vitamin D source. Vitamin D2-rich yeast baked into bread is bioavailable and improves bone quality in vitamin D-deficient animals.

Contribution of a common variant in the promoter of the 1-α-hydroxylase gene (CYP27B1) to fracture risk in the elderly

CYP27B1 encodes mitochondrial 1α-hydroxylase, which converts 25-hydroxyvitamin D to its active 1,25-dihydroxylated metabolite. We tested the hypothesis that common variants in the CYP27B1 promoter are associated with fracture risk. The study was designed as a population-based genetic association study, which involved 153 men and 596 women aged 65-101 years, who had been followed for 2.2 years (range 0.1-5.5) between 1999 and 2006. During the follow-up period, the incidence of fragility fractures was ascertained. Bone ultrasound attenuation (BUA) was measured in all individuals, as were serum 25-hydroxyvitamin D and PTH concentrations; 86% subjects had vitamin D insufficiency. Genotypes were determined for the -1260C>A (rs10877012) and +2838T>C (rs4646536) CYP27B1 polymorphisms. A reporter gene assay was used to assess functional expression of the -1260C>A CYP27B1 variants. The association between genotypes and fracture risk was analyzed by Cox's proportional hazards model. We found that genotypic distribution of CYP27B1 -1260 and CYP27B1 +2838 polymorphisms was consistent with the Hardy-Weinberg equilibrium law. The two polymorphisms were in high linkage disequilibrium, with D' = 0.96 and r² = 0.94. Each C allele of the CYP27B1 -1260 polymorphism was associated with increased risk of fracture (hazard ratio = 1.34, 95% CI 1.03-1.73), after adjustment for age, sex, number of falls, and BUA. In transient transfection studies, a reporter gene downstream of the -1260(A)-containing promoter was more highly expressed than that containing the C allele. These data suggest that a common but functional variation within the CYP27B1 promoter gene is associated with fracture risk in the elderly.

Vitamin D deficiency and toxicity in chronic kidney disease: in search of the therapeutic window

Both vitamin D deficiency and vitamin D toxicity are associated with cardiovascular complications in chronic kidney disease (CKD). Clinical and experiment data indicate that the association of vitamin D levels with cardiovascular disease is best illustrated as a biphasic, or U-shaped, curve. Children and adolescents with CKD need vitamin D due to the demands of a growing skeleton, to prevent renal rickets. However, this therapy carries the risk of severe side effects and chronic toxicity. Observational studies show that vitamin D deficiency and toxicity are frequently present in patients with CKD. In view of the importance of cardiovascular complications for the long-term survival of young patients, these findings demand a judicious use of vitamin D preparations. In clinical practice, the therapeutic window is rather small, presenting a therapeutic challenge to avoid both vitamin D deficiency and toxicity.

Vitamin D(3) metabolites induce osteogenic differentiation in human dental pulp and human dental follicle cells

Vitamin D(3) metabolites regulate the bone metabolism and 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)) is known to play an important role in teeth mineralization. However, little is known about the potential of vitamin D as an osteogenic inducer in human dental pulp (hDPCs) and dental follicle cells (hDFCs) in vitro. Therefore, we investigated the effects of vitamin D(3) metabolites 1α,25(OH)(2)D(3) and 25-hydroxyvitamin D(3) (25OHD(3)) on proliferation and osteogenic differentiation of hDPCs and hDFCs in vitro. We also examined whether vitamin D(3) metabolic enzymes were regulated in hDFCs and hDPCs. Cell proliferation was decreased by both metabolites in hDPCs and hDFCs. Vitamin D(3) metabolites increased ALP activity and induced mineralization when osteogenic supplements (OS; l-ascorbic acid-2-phosphate+β-glycerophosphate) were added, though the expression of osteocalcin (OC) and osteopontin (OPN) were regulated without the addition of OS. CYP24 and CYP27B1 expressions were upregulated by vitamin D(3) metabolites and 25OHD(3) was converted into 1α,25(OH)(2)D(3) in the culture medium. These results confirm that 1α,25(OH)(2)D(3) (10 and 100 nM) and 25OHD(3) (500 nM) can be used as osteogenic inducers synergistically with osteogenic supplements for differentiation of hDPCs and hDFCs. Furthermore, our findings strengthen our knowledge about the role of hDPCs and hDFCs as vitamin D(3) target cells.

Osteoclastic metabolism of 25(OH)-vitamin D3: a potential mechanism for optimization of bone resorption

The extrarenal synthesis of 1α,25 dihydroxyvitamin D3 (1,25D) has been demonstrated in a number of cell types including osteoblasts and cells of the monocyte/macrophage lineage. The skeleton appears responsive to serum levels of the 1,25D precursor, 25 hydroxyvitamin D3 (25D), in terms of bone mineralization parameters. The effect of metabolism of 25D into active 1,25D by osteoclast lineage cells is unknown. We found that CYP27B1 mRNA expression increased with exposure of human peripheral blood mononuclear cells (PBMCs) to macrophage colony-stimulating factor in the presence or absence of receptor activator of nuclear factor-κB ligand. Consistent with this, human osteoclast cultures incubated with 25D produced measurable quantities of 1,25D. Osteoclast formation from either mouse RAW264.7 cells or human PBMCs in the presence of physiological concentrations of 25D resulted in significant up-regulation of the key osteoclast transcription factor, nuclear factor of activated T cells-c1 in PBMCs and a number of key osteoclast marker genes in both models. The expression of the osteoblast coupling factor, ephrin-b2, was also increased in the presence of 25D. Levels of CYP27B1 and nuclear factor of activated T cells-1 mRNA correlated during osteoclastogenesis and also in a cohort of human bone samples. CYP27B1 short-hairpin RNA knockdown in RAW264.7 cells decreased their osteoclastogenic potential. 25D dose dependently reduced the resorptive capacity of PBMC-derived osteoclasts without compromising cell viability. 25D also reduced resorption by RAW264.7- and giant cell tumor-derived osteoclasts. Conversely, osteoclasts formed from vitamin D receptor-null mouse splenocytes had increased resorptive activity compared with wild-type cells. We conclude that 25D metabolism is an important intrinsic mechanism for optimizing osteoclast differentiation, ameliorating osteoclast activity, and potentially promoting the coupling of bone resorption to formation.

Experimental evidence for the effects of calcium and vitamin D on bone: a review

Animal models fed low calcium diets demonstrate a negative calcium balance and gross bone loss while the combination of calcium deficiency and oophorectomy enhances overall bone loss. Following oophorectomy the dietary calcium intake required to remain in balance increases some 5 fold, estimated to be approximately 1.3% dietary calcium. In the context of vitamin D and dietary calcium depletion, osteomalacia occurs only when low dietary calcium levels are combined with low vitamin D levels and osteoporosis occurs with either a low level of dietary calcium with adequate vitamin D status or when vitamin D status is low in the presence of adequate dietary calcium intake. Maximum bone architecture and strength is only achieved when an adequate vitamin D status is combined with sufficient dietary calcium to achieve a positive calcium balance. This anabolic effect occurs without a change to intestinal calcium absorption, suggesting dietary calcium and vitamin D have activities in addition to promoting a positive calcium balance. Each of the major bone cell types, osteoblasts, osteoclasts and osteocytes are capable of metabolizing 25 hydroxyvitamin D (25D) to 1,25 dihydroxyvitamin D (1,25D) to elicit biological activities including reduction of bone resorption by osteoclasts and to enhance maturation and mineralization by osteoblasts and osteocytes. Each of these activities is consistent with the actions of adequate circulating levels of 25D observed in vivo.

The effect of dietary calcium on 1,25(OH)2D3 synthesis and sparing of serum 25(OH)D3 levels

Vitamin D depletion in rats causes osteopenia in at least three skeletal sites. However it is unclear whether modulation of dietary calcium intake impacts on the relationship between the level of serum 25-hydroxyvitamin D (25D) and bone loss. Nine-month-old female Sprague-Dawley rats (n=5-6/group) were pair-fed a semi-synthetic diet containing either 0 or 20 IU vitamin D3/day with either low (0.1%) or high (1%) dietary Ca for 6 months. At 15 months of age, fasting bloods were collected for biochemical analyses. Serum 25D levels were lowest in the animals fed 0 IU vitamin D and 0.1% Ca. The animals fed 1% Ca had significantly higher serum 25D levels when compared to animals fed 0.1% Ca (P<0.05). The major determinants of serum 25D were dietary vitamin D and dietary calcium (Multiple R=0.75, P<0.05). Animals fed 0.1% Ca had higher renal CYP27B1 mRNA expression and 12-18-fold increased levels of serum 1,25D. Hence, the reported effects of low calcium diets on bone loss may be, in part, due to the subsequent effects of 25D metabolism leading to reduction in vitamin D status. Such an interaction has significant implications, given the recent evidence for local synthesis of active vitamin D in bone tissue.