Vitamin D

Lipoproteins and their receptors in embryonic development: more than cholesterol clearance

Previously, the relevance of lipoproteins and their receptors has mainly been discussed in terms of cholesterol clearance in the adult organism. Now, findings from nematodes to fruit flies to mammals all point towards novel and unexpected roles for lipoprotein metabolism in the control of key regulatory pathways in the developing embryo, including signaling through steroid hormones and throughout the hedgehog and Wnt signaling pathways. Here, we discuss the emerging view of how lipoproteins and their receptors regulate embryogenesis.

Functional evolution of the vitamin D and pregnane X receptors

Background

The vitamin D receptor (VDR) and pregnane X receptor (PXR) are nuclear hormone receptors of the NR1I subfamily that show contrasting patterns of cross-species variation. VDR and PXR are thought to have arisen from duplication of an ancestral gene, evident now as a single gene in the genome of the chordate invertebrate Ciona intestinalis (sea squirt). VDR genes have been detected in a wide range of vertebrates including jawless fish. To date, PXR genes have not been found in cartilaginous fish. In this study, the ligand selectivities of VDRs were compared in detail across a range of vertebrate species and compared with those of the Ciona VDR/PXR. In addition, several assays were used to search for evidence of PXR-mediated hepatic effects in three model non-mammalian species: sea lamprey (Petromyzon marinus), zebrafish (Danio rerio), and African clawed frog (Xenopus laevis).

Results

Human, mouse, frog, zebrafish, and lamprey VDRs were found to have similar ligand selectivities for vitamin D derivatives. In contrast, using cultured primary hepatocytes, only zebrafish showed evidence of PXR-mediated induction of enzyme expression, with increases in testosterone 6β-hydroxylation activity (a measure of cytochrome P450 3A activity in other species) and flurbiprofen 4-hydroxylation activity (measure of cytochrome P450 2C activity) following exposure to known PXR activators. A separate assay in vivo using zebrafish demonstrated increased hepatic transcription of another PXR target, multidrug resistance gene (ABCB5), following injection of the major zebrafish bile salt, 5α-cyprinol 27-sulfate. The PXR target function, testosterone hydroxylation, was detected in frog and sea lamprey primary hepatocytes, but was not inducible in these two species by a wide range of PXR activators in other animals. Analysis of the sea lamprey draft genome also did not show evidence of a PXR gene.

Conclusion

Our results show tight conservation of ligand selectivity of VDRs across vertebrate species from Agnatha to mammals. Using a functional approach, we demonstrate classic PXR-mediated effects in zebrafish, but not in sea lamprey or African clawed frog liver cells. Using a genomic approach, we failed to find evidence of a PXR gene in lamprey, suggesting that VDR may be the original NR1I gene.

Vitamin D status in renal transplant recipients

Vitamin D plays an important role in calcium homeostasis. Renal transplant recipients may be more susceptible to reduced levels because of decreased sun exposure and steroid therapy. This study aimed to determine vitamin D status after renal transplantation and its effect on parathyroid hormone (PTH) and bone mineral density (BMD). We measured serum 25-hydroxyvitamin D levels (25-OHD) in 244 renal transplant recipients, divided into two groups, 104 recently transplanted (less than 1 year) and 140 long-term. Vitamin D status was defined according to NKF/KDOQI guidelines. Mean 25-OHD levels were 33 +/- 19 nmol/L and 42 +/- 20 nmol/L, respectively, for the recent and long-term transplant recipients. Vitamin D insufficiency was present in 29% and 43%, deficiency in 56% and 46% and severe deficiency in 12% and 5%, respectively. An inverse correlation was found between logPTH and 25-OHD (r=-0.2, p= 0.019) in long-term but not in recently transplanted patients. No correlation was found between 25-OHD levels and BMD. Hypercalcaemia was present in 40% of the recently transplanted recipients and 25% of the long-term. In conclusion 25-OHD was low in virtually all of our renal transplant recipients and may aggravate secondary hyperparathyroidism, but its correction may be difficult in patients with hypercalcaemia.

Viability and functionality of fresh and cryopreserved human hyperplastic parathyroid tissue tested in vitro

BACKGROUND/AIMS

This study aimed to test the viability and functionality of fresh and cryopreserved human hyperplastic parathyroid glands cultured in vitro.

METHODS

Small fragments of 18 parathyroid glands from 18 patients with secondary hyperparathyroidism were cultured in vitro, freshly or after cryopreservation, during 60 h. Cell viability and functionality of the parathyroid fragments exposed to calcium and calcitriol were studied.

RESULTS

Human parathyroid glands obtained from renal patients with secondary hyperparathyroidism maintained their viability and functionality for 60 h in culture. Sixty percent of the fresh but only 10% of the cryopreserved parathyroid glands showed the expected response with higher intact parathyroid hormone secretion when cultured with 0.6 mM calcium compared to 1.2 mM calcium. On the contrary, 44 of fresh and 40% of cryopreserved glands behaved in the same manner, showing a similar decrease in intact parathyroid hormone synthesis and secretion when cultured with calcitriol (10(-8)M).

CONCLUSION

These results demonstrate differences in the response to calcium between fresh and cryopreserved glands and no differences in the response to calcitriol. This in vitro culture method may be useful to discriminate between responsive and nonresponsive hyperplastic human parathyroid glands.

Vitamin D as a novel nontraditional risk factor for mortality in hemodialysis patients: The need for randomized trials

Vitamin D has been used in the context of secondary hyperparathyroidism in patients with end-stage renal disease. A wave of recent studies suggests that vitamin D treatment may be associated with decreased mortality risk in these patients. The article by Wolf et al. further supports these studies by identifying vitamin D deficiency as a risk factor for early mortality in incident hemodialysis patients.

Effect of 1,25-(OH)2D3 (a vitamin D analogue) on passively sensitized human airway smooth muscle cells

BACKGROUND AND OBJECTIVES

In asthma, airway smooth muscle cell (ASMC) hyperplasia plays an important role in airway remodelling. Increased expression of matrix metalloproteinases-9 (MMP-9), a disintegrin and metalloprotease 33 (ADAM33) in ASMCs are also relevant to asthmatic airway remodelling. 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) has potent antiproliferative properties in vitro in various cell types; however, its role in ASMCs is not well understood. This study investigated the effect of 1,25-(OH)(2)D(3) on passively sensitized human bronchial (airway) smooth muscle cell (HASMC) proliferation and MMP-9 and ADAM33 expressions.

METHODS

The effect of 1,25-(OH)(2)D(3) on cell proliferation was examined by 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium-bromide colorimetry assay; cell cycle analysis by flow cytometry; and immunocytochemical staining for proliferating cell nuclear antigen (PCNA). The expression of MMP-9 and ADAM33 in HASMCs was investigated by real-time quantitative PCR and Western Blot analysis.

RESULTS

1,25-(OH)(2)D(3) effectively suppressed passively sensitized HASMC proliferation, proliferating cell nuclear antigen expression and G(1)/S transition in HASMCs passively sensitized with asthmatic serum. Further analysis showed that 1,25-(OH)(2)D(3) significantly down-regulated the expressions of protein for MMP-9 and ADAM33, as well as their mRNA levels in passively sensitized HASMCs.

CONCLUSIONS

1,25-(OH)(2)D(3) has direct inhibitory effects on passively sensitized HASMCs in vitro, including inhibition of cell proliferation and expression of MMP-9 and ADAM33, suggesting a possible beneficial role for 1,25-(OH)(2)D(3) in preventing and treating asthmatic airway remodelling.

Modulatory effects of 1,25-dihydroxyvitamin D3 on human B cell differentiation

1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) can modulate immune responses, but whether it directly affects B cell function is unknown. Patients with systemic lupus erythematosus, especially those with antinuclear Abs and increased disease activity, had decreased 1,25(OH)(2)D(3) levels, suggesting that vitamin D might play a role in regulating autoantibody production. To address this, we examined the effects of 1,25(OH)(2)D(3) on B cell responses and found that it inhibited the ongoing proliferation of activated B cells and induced their apoptosis, whereas initial cell division was unimpeded. The generation of plasma cells and postswitch memory B cells was significantly inhibited by 1,25(OH)(2)D(3), although the up-regulation of genetic programs involved in B cell differentiation was only modestly affected. B cells expressed mRNAs for proteins involved in vitamin D activity, including 1 alpha-hydroxylase, 24-hydroxylase, and the vitamin D receptor, each of which was regulated by 1,25(OH)(2)D(3) and/or activation. Importantly, 1,25(OH)(2)D(3) up-regulated the expression of p27, but not of p18 and p21, which may be important in regulating the proliferation of activated B cells and their subsequent differentiation. These results indicate that 1,25(OH)(2)D(3) may play an important role in the maintenance of B cell homeostasis and that the correction of vitamin D deficiency may be useful in the treatment of B cell-mediated autoimmune disorders.

Cellular Expression Levels of the Vitamin D Receptor Are Critical to Its Transcriptional Regulation by the Pituitary Transcription Factor Pit-1

The biological role of 1,25-dihydroxyvitamin D3 has generally been related to calcium homeostasis, but this hormone also has fundamental effects on processes of cellular proliferation and differentiation. The genomic actions of 1,25-dihydroxyvitamin D3 are mediated by the vitamin D receptor (VDR) present in target cells. However, VDR transcriptional regulation is not well understood, probably attributable to the complexity of the VDR gene and its promoter. In the present study, it is demonstrated that administration of the pituitary transcription factor Pit-1 (originally found in the pituitary gland but also present in other nonpituitary cell types and tissues) to the MCF-7 (human breast adenocarcinoma) cell line induces a significant increase in VDR mRNA and protein levels. Conversely, Pit-1-targeted small interference RNA markedly reduced expression of VDR in MCF-7 cells. Reporter gene assays demonstrated that the effect of Pit-1 is mediated by its binding to a region located between −254 and −246 bp from the VDR transcription start site. Selective mutations of this site completely abolished VDR transcription. Chromatin immunoprecipitation analysis showed that binding of Pit-1 to the VDR promoter leads additionally to recruitment of cAMP response element-binding protein binding protein, acetylated histone H4, and RNA polymerase II. Surprisingly, Pit-1 binding also recruits VDR protein to the VDR promoter. Using several cell lines with different levels of VDR expression, it was demonstrated that up-regulation of VDR transcription by Pit-1 is dependent on the presence of VDR protein, suggesting that transcriptional expression of VDR in a given cell type is dependent on, among other factors, its own expression levels.

Mineral metabolism and aging: the fibroblast growth factor 23 enigma

PURPOSE OF REVIEW

The regulation of phosphate homeostasis was thought to be passively mediated by the calciotrophic hormones parathyroid hormone and 1,25(OH)2D3. This article summarizes the emerging trends that show an active regulation of phosphate homeostasis by fibroblast growth factor 23 (FGF-23) - a process fairly independent of calcium homeostasis - and how altered mineral ion metabolism may affect the aging process.

RECENT FINDINGS

A major breakthrough in FGF-23 biology has been achieved by the demonstration of strikingly similar physical/biochemical phenotypes of Fgf-23(-/-) and klotho hypomorph mice, which eventually led to the identification of klotho as a cofactor in FGF-23 and its receptor interactions. Furthermore, FGF-23 has emerged as a counter regulator of the renal 1alpha(OH)ase and sodium-phosphate cotransporter activities to modulate phosphate homeostasis. Finally, studies point towards a role of dentine matrix protein 1 in affecting phosphate homeostasis, in coordination with FGF-23.

SUMMARY

Recent mouse genetic studies have broadened our understanding of biochemical/molecular pathways involved in phosphate homeostasis, and linked FGF-23 to such regulation. Understanding the molecular interactions of essential calcium and phosphate regulators will enhance our knowledge of the coordinated regulation of mineral ion metabolism, and will help to redefine the molecular pathology of age-associated lesions accompanied by abnormal mineral ion metabolism such as vascular calcifications and osteoporosis.

VDR polymorphisms influence the immune response in type 1 diabetic children from Santiago, Chile

OBJECTIVE

To evaluate the influence of ApaI, BsmI and TaqI polymorphisms of the VDR gene and HLA-DQB1* alleles in type 1 diabetic children and to assess their possible relationship with circulating levels of 25-hydroxyvitamin D(3), auto-antibodies, and INFgamma/TGFbeta1 cytokines levels in Chilean cases and controls.

METHODS

DNA and serum samples from 216 newly diagnosed type 1 diabetic and 203 unrelated control children were evaluated for IA-2 and GAD(65) auto-antibodies, 25-hydroxyvitamin D(3) levels, HLA-DQB1* alleles, and VDR gene polymorphisms.

RESULTS

The frequency of the b allele and the bb genotype in type 1 diabetic patients was significantly lower compared with the control group (0.635 versus 0.749, p<0.01 and 0.370 versus 0.567, p<0.04). 25-Hydroxyvitamin D(3) levels showed no differences between type 1 diabetic and healthy children. In cases, 25-hydroxyvitamin D(3) levels were not associated with a special auto-antibodies profile according to the presence or absence of GAD(65)(+) or IA-2(+). The haplotype combination BAT was higher in cases (0.062 versus 0.019, p<0.0022) and bAT was more frequent in controls (0.266 versus 0.180, p<0.003). In cases, the aaBbTT genotype showed the most significant increase in TGFbeta1 level across the VDR categories. Finally, when considering the HLA class II risk genotype (DQB1*0302) and the VDR genotypes (AabbTT and aabbTT), higher levels of GAD(65), IA-2 and TGFbeta1 were observed among diabetic children.

CONCLUSION

We found an association between a VDR polymorphism (BsmI) and type 1 diabetes. An association was found of AabbTT and aabbTT genotypes and the HLA-DQB1*0302 allele with high levels of GAD(65), IA-2 and TGFbeta1.

Changes in serum 25-hydroxyvitamin D and plasma intact PTH levels following treatment with ergocalciferol in patients with CKD

BACKGROUND

Vitamin D insufficiency and deficiency are very common in patients with chronic kidney disease (CKD). The effect of ergocalciferol administration on serum 25-hydroxyvitamin D and plasma intact parathyroid hormone (PTH) levels in these patients is not known.

STUDY DESIGN

Retrospective study.

SETTING & PARTICIPANTS

Patients with CKD stage 3 or 4 who had a serum 25-hydroxyvitamin D level less than 30 ng/mL (<75 nmol/L) and increased plasma intact PTH level were treated with 50,000 IU of ergocalciferol once weekly for 12 weeks and once monthly thereafter for a total of 6 months. Patients were excluded if they had a history of active vitamin D sterol use.

OUTCOME & MEASUREMENTS

25-Hydroxyvitamin D and intact PTH were measured at baseline and follow-up.

RESULTS

66 patients met inclusion criteria. Average age was 70.4 +/- 1.3 (SE) years (range, 40 to 88 years), and 97% were men. There were 44 patients (66%) with CKD stage 3 and 22 patients (33%) with CKD stage 4. After a median follow-up of 6 months, there was a significant increase in 25-hydroxyvitamin D levels from 16.6 +/- 0.7 to 27.2 +/- 1.8 ng/mL (41 +/- 2 to 68 +/- 4 nmol/L; P < 0.05) and a significant decrease in plasma intact PTH levels from 231 +/- 26 to 192 +/- 25 pg/mL (ng/L; P < 0.05). A multivariate logistic regression model showed that an increase in 25-hydroxyvitamin D level greater than 5 ng/mL (>12 nmol/L) is associated with a significant likelihood of a greater than 30% decrease in plasma intact PTH level (odds ratio, 4.5; 95% confidence interval, 1.5 to 15.1; P < 0.05). Although posttreatment 25-hydroxyvitamin D levels were not different between patients with CKD stages 3 and 4, only patients with CKD stage 3 had a significant decrease in plasma intact PTH levels.

LIMITATIONS

This is a retrospective study with mostly male patients.

CONCLUSIONS

Results show that ergocalciferol administration has a favorable effect on PTH levels if therapy results in an increase in 25-hydroxyvitamin D levels; this effect is more evident in patients with CKD stage 3.

25-Hydroxyvitamin D levels and albuminuria in the Third National Health and Nutrition Examination Survey (NHANES III)

BACKGROUND

Albuminuria is a risk factor for chronic kidney disease progression, end-stage renal disease, cardiovascular events, and mortality. Animal studies suggested that vitamin D insufficiency may contribute to the pathogenesis of albuminuria.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

15,068 adults participating in the Third National Health and Nutrition Examination Survey.

PREDICTOR

Serum 25-hydroxyvitamin D concentration, examined in quartiles.

OUTCOMES & MEASUREMENTS

Albuminuria, defined using established sex-specific cutoff values for urine albumin-creatinine ratio (25 to 2,999 mg/g for women, 17 to 2,999 mg/g for men).

RESULTS

A stepwise increase in the prevalence of albuminuria was observed with decreasing quartiles of vitamin D concentration: 8.9%, 11.5%, 13.7%, and 15.8% (P < 0.001). Adjusting for age, sex, race/ethnicity, region and season of measurement, smoking status, body mass index, and estimated glomerular filtration rate, relative risks for albuminuria by decreasing quartile of vitamin D concentration were 1.00 (reference group), 1.14 (95% confidence interval, 0.95 to 1.37), 1.22 (95% confidence interval, 1.03 to 1.45), and 1.37 (95% confidence interval, 1.10 to 1.71; P = 0.006). Additionally adjusting for blood pressure and diabetes mellitus, these risks were somewhat attenuated and retained statistical significance.

LIMITATIONS

The cross-sectional design of this study does not allow demonstration of temporal or causal relationships between vitamin D and albuminuria.

CONCLUSIONS

Additional studies are needed to clarify the relationship of vitamin D with albuminuria and determine whether vitamin D therapy prevents or improves markers of kidney and cardiovascular disease.

Constitutive androstane receptor-vitamin D receptor crosstalk: consequence on CYP24 gene expression

We previously reported that the pregnane X receptor (PXR) interferes with vitamin D receptor (VDR) target genes, notably CYP24, by targeting the same responsive elements. Since PXR and constitutive androstane receptor (CAR) share responsive elements in the promoter of their target genes, we wondered whether CAR also interferes with CYP24 expression. The current study shows that: (i) CAR-RXR heterodimer binds to and transactivates the proximal promoter of CYP24 (-1200/+22) and both VDRE-1 and VDRE-2 which control its expression in response to 1,25-dihydroxyvitamin D(3), (ii) androstanol an inverse agonist of hCAR inhibits transactivation of VDREs by hCAR, (iii) mutations of either VDRE-1 or -2 half sites inhibit hCAR-mediated transactivation, and (iv) in primary human hepatocytes (n =11) CITCO, a specific hCAR agonist, is an inducer of CYP24 as well as of CYP2B6 and CYP3A4 mRNAs. In conclusion, CAR/PXR and VDR bind to and transactivate the same response elements in CYP24 promoter.

Conversion from conventional to nocturnal hemodialysis improves vitamin D levels

Patients on conventional hemodialysis have low levels of 25-hydroxy-vitamin D probably due to diet and decreased cutaneous synthesis. As 1,25 dihydroxy-vitamin D synthesis is substrate-dependent in end-stage renal disease, this could be a contributing factor to low 1,25 dihydroxy-vitamin D levels in patients undergoing conventional hemodialysis. We converted 35 patients historically on conventional hemodialysis to nocturnal hemodialysis for a minimum of 6 months thereby significantly increasing sessional equilibrated Kt/V from an average of 1.30 to an average of 2.01. Dietary restrictions were also removed. Serum phosphorus significantly fell, whereas the serum calcium, parathyroid hormone, and the mean dose of calcitriol did not change after the conversion. Significant increases in both 25-hydroxy and 1,25-dihydroxy-vitamin D levels were seen after hemodialysis mode conversion. A significant correlation was found between the dialysis dose and the levels of both hydroxylated forms of vitamin D. We suggest that improving uremia by nocturnal hemodialysis in the absence of exogenous supplementation is associated with increased 25 and 1,25-hydroxy-vitamin D levels. Additionally, normalization of serum phosphorus may improve 1alpha-hydroxylation thereby enhancing substrate-dependent generation of 1,25-dihydroxy-vitamin D in chronic dialysis patients.

Vitamin D and aging: old concepts and new insights

Aging is a complex biological process driven by a selective class of molecules and pathways that affect overall deterioration of physiological functions to increase the risk of age-related diseases. A role of vitamin D in mammalian aging is well documented. Since vitamin D has an essential role in bone formation and mineralization, its deficiency results in impaired bone mineralization, such as rickets in children, osteomalacia in adults and osteoporosis in the aged population. Vitamin D replacement therapy therefore is one of the most commonly prescribed treatments for the elderly. Recent studies using genetically altered mouse models, such as in Fgf-23(-/-) and klotho mutant mice, that exhibit altered mineral ion metabolism due to high vitamin D activities showed features of premature aging that include atherosclerosis, emphysema, osteopenia/osteoporosis, hypogonadism, soft tissue calcifications and generalized atrophy of organs; the pathologic effects of vitamin D in these mouse models are obvious, as diminution or genetic ablation of the vitamin D pathway ameliorated most of the above-mentioned phenotypes, by reversing mineral ion metabolism, and the resultant effect being prolonged survival of the mutant mice. These in vivo mouse studies, although subject to further molecular characterization, add new insights into the role of vitamin D in aging.

Combination therapy with an angiotensin-converting enzyme inhibitor and a vitamin D analog suppresses the progression of renal insufficiency in uremic rats

Monotherapy with angiotensin-converting enzyme inhibitors has been shown to be beneficial in suppressing the progression of experimentally induced kidney diseases. Whether such therapy provides additional benefits when combined with vitamin D or an analog of vitamin D has not been established. Rats were made uremic by 5/6 nephrectomy and treated as follows: Uremic + vehicle (UC), uremic + enalapril (30 mg/L in drinking water; E), uremic + paricalcitol (19-nor; 0.8 microg/kg, three times a week), and uremic + enalapril + paricalcitol (E + 19-nor). A group of normal rats served as control (NC). BP was significantly elevated in the UC and 19-nor groups compared with the NC group but was indistinguishable from normal in the E and E + 19-nor groups. The decrease in creatinine clearance and the increase in the excretion of urinary protein that were observed in the UC group were ameliorated by the use of E alone or by E + 19-nor (P < 0.05 versus UC). The glomerulosclerotic index was significantly decreased in both the 19-nor (P < 0.01) and E + 19-nor groups (P < 0.01) compared with the UC group. Tubulointerstitial volume was significantly decreased in both the E (P < 0.05) and E + 19-nor groups (P < 0.01) compared with the UC group. Both macrophage infiltration (ED-1-positive cells) and production of the chemokine monocyte chemoattractant protein-1 were significantly blunted in E + 19-nor compared with E group. TGF-beta1 mRNA and protein expression were increased in the UC group (mRNA: 23.7-fold; protein: 29.1-fold versus NC). These increases were significantly blunted in the 19-nor group (mRNA: 7.1-fold; protein: 8.0-fold versus NC) and virtually normalized in the E + 19-nor group (protein: 0.8-fold versus NC). Phosphorylation of Smad2 was also elevated in the UC group (7.6-fold versus NC) but less so in the 19-nor-treated rats (5.5-fold versus NC). When rats were treated with E + 19-nor, the phosphorylation of Smad2 was normal (1.1-fold versus NC). Thus, 19-nor can suppress the progression of renal insufficiency via mediation of the TGF-beta signaling pathway, and this effect is amplified when BP is controlled via renin-angiotensin system blockade.

Immunohistochemical Demonstration of Vitamin D Receptor Distribution in Goat Intestines

Vitamin D (VD) plays an important role in calcium homeostasis. 1,25-Dihydroxycholecalciferol or calcitriol modulates gene transcription via nuclear VD receptors (VDR). In the intestines, VD promotes calcium resorption via VDR. VDR has not been systematically assessed within the intestine in any species. We therefore present a semiquantitative immunohistochemical study of the distribution patterns of VDR in goat intestines. Intestinal tissue probes were collected from 5 lambs and 5 non-lactating non-pregnant dams, fixed in formalin, embedded in paraffin and assessed for VDR. Nuclear VDR immunoreaction was scored semiquantitatively. VDR exhibited a segment-specific distribution pattern. Goblet cells were always devoid of VDR. Enterocytes within the surface epithelium and the superficial crypts generally demonstrated only a weak immunoreaction along the length of the intestine, while basally and/or intermediately located crypt epithelial cells exhibited strong VDR immunoreactions in the duodenum, jejunum and colon descendens. The difference in VDR staining between deep and superficial locations was most prominent in the duodenum and less evident in the jejunum, ileum and colon descendens. Results demonstrate that VDR distribution exhibits cell type-, segment- and location-specific patterns in the goat. Data may serve as a basis for future experiments on the role of VDR in Ca metabolism.

Developmental vitamin D deficiency alters brain protein expression in the adult rat: implications for neuropsychiatric disorders

An increased risk for multiple sclerosis and schizophrenia is observed at increasing latitude and in patients born in winter or spring. To explore a possible link between maternal vitamin D deficiency and these brain disorders, we examined the impact of prenatal hypovitaminosis D on protein expression in the adult rat brain. Vitamin D-deficient female rats were mated with vitamin D normal males. Pregnant females were kept vitamin D-deficient until birth whereupon they were returned to a control diet. At week 10, protein expression in the progeny's prefrontal cortex and hippocampus was compared with control animals using silver staining 2-D gels associated with MS and newly devised data mining software. Developmental vitamin D (DVD) deficiency caused a dysregulation of 36 brain proteins involved in several biological pathways including oxidative phosphorylation, redox balance, cytoskeleton maintenance, calcium homeostasis, chaperoning, PTMs, synaptic plasticity and neurotransmission. A computational analysis of these data revealed that (i) nearly half of the molecules dysregulated in our animal model have also been shown to be misexpressed in either schizophrenia and/or multiple sclerosis and (ii) an impaired synaptic network may be a consequence of mitochondrial dysfunction.

Association of the VDR translation start site polymorphism and fracture risk in older women

We evaluated the association between the VDR translation start site polymorphism and osteoporotic phenotypes among 6698 older white women. Women with the C/C genotype had lower wrist BMD and an increased risk of wrist and all non-spine/low-trauma fractures. The high frequency of this variant confers a population attributable risk that is similar to several established risk factors for fracture.

INTRODUCTION

The vitamin D receptor (VDR) is a nuclear receptor that regulates bone formation, bone resorption, and calcium homeostasis. A common C to T polymorphism in exon 2 of the VDR gene introduces a new translation start site and a protein that differs in length by three amino acids (T = 427aa, C = 424aa; rs10735810).

MATERIALS AND METHODS

We conducted genetic association analyses of this polymorphism, BMD, and fracture outcomes in a prospective cohort of 6698 white American women >or=65 years of age. Incident fractures were confirmed by physician adjudication of radiology reports. There were 2532 incident nontraumatic/nonvertebral fractures during 13.6 yr of follow-up including 509 wrist and 703 hip fractures.

RESULTS

Women with the C/C genotype had somewhat lower distal radius BMD compared with those with the T/T genotype (CC=0.358 g/cm(2), CT=0.361 g/cm(2), TT=0.369 g/cm(2), p=0.003). The C/C genotype was also associated with increased risk of non-spine, low traumatic fractures (HR: 1.18; 95% CI: 1.04, 1.33) and wrist fractures (HR: 1.33; 95% CI: 1.01, 1.75) compared with the T/T genotype in age-adjusted models. Further adjustments for distal radius BMD only slightly attenuated these associations. The VDR polymorphism was not associated with hip fracture. The population attributable risk (PAR) of the C/C genotype for incident fractures was 6.1%. The PAR for established risk factors for fracture were: low femoral neck BMD (PAR=16.3%), maternal history of fracture (PAR=5.1%), low body weight (PAR=5.3%), corticosteroid use (PAR=1.3%), and smoking (PAR=1.6%). Similar PAR results were observed for wrist fractures.

CONCLUSIONS

The common and potentially functional VDR translation start site polymorphism confers a modestly increased relative risk of fracture among older white women. However, the high frequency of this variant confers a population attributable risk that is similar to or greater than several established risk factors for fracture.

Recovery and maintenance of nephrin expression in cultured podocytes and identification of HGF as a repressor of nephrin

Cultured podocytes easily lose expression of nephrin. In this report, we developed optimum media for recovery and maintenance of nephrin gene expression in murine podocytes. Using reporter podocytes, we found that activity of the nephrin gene promoter was enhanced by DMEM/F12 or α-MEM compared with RPMI-1640. In any of these basal media, addition of 1,25-dihydroxyvitamin D3, all- trans-retinoic acid or dexamethasone significantly increased activity of the nephrin promoter. The effects of the supplemental components were synergistic, and the maximum activation was achieved by DMEM/F12 supplemented with three agents. This culture medium was designated as vitamin D3, retinoic acid and dexamethasone-supplemented DMEM/F12 (VRADD). In reporter podocytes that express nephrin, VRADD induced activation of the nephrin gene promoter up to 60-fold. Even in podocytes that have lost nephrin expression during multiple passages, expression of nephrin mRNA was dramatically recovered by VRADD. However, VRADD caused damage of podocytes in prolonged cultures, which was avoided in the absence of dexamethasone (designated as VRAD). VRAD maintained expression of nephrin for extended periods, which was associated with the differentiated phenotype of podocytes. Using the VRAD-primed podocytes, we revealed that expression of nephrin mRNA as well as nephrin promoter activity was suppressed by a putative dedifferentiation factor of podocytes, hepatocyte growth factor.

Vitamin D, insulin resistance, and renal disease

Chonchol and Scragg report the results of a population study on levels of 25-hydroxyvitamin D in patients with renal dysfunction. They demonstrate that these patients do not show vitamin D deficiency unless renal function is severely affected (GFR<29 mL/min/1.73m2), while vitamin D and renal function loss are independently associated with insulin resistance. These data provide more solid evidence than previous available studies on small patient groups, and pose new questions about the mechanisms responsible for progressive renal disease as well as potential effects of vitamin D supplementation.

Diet and risk of ovarian cancer in the California Teachers Study cohort

Dietary phytochemical compounds, including isoflavones and isothiocyanates, may inhibit cancer development but have not yet been examined in prospective epidemiologic studies of ovarian cancer. The authors have investigated the association between consumption of these and other nutrients and ovarian cancer risk in a prospective cohort study. Among 97,275 eligible women in the California Teachers Study cohort who completed the baseline dietary assessment in 1995-1996, 280 women developed invasive or borderline ovarian cancer by December 31, 2003. Multivariable Cox proportional hazards regression, with age as the timescale, was used to estimate relative risks and 95% confidence intervals; all statistical tests were two sided. Intake of isoflavones was associated with lower risk of ovarian cancer. Compared with the risk for women who consumed less than 1 mg of total isoflavones per day, the relative risk of ovarian cancer associated with consumption of more than 3 mg/day was 0.56 (95% confidence interval: 0.33, 0.96). Intake of isothiocyanates or foods high in isothiocyanates was not associated with ovarian cancer risk, nor was intake of macronutrients, antioxidant vitamins, or other micronutrients. Although dietary consumption of isoflavones may be associated with decreased ovarian cancer risk, most dietary factors are unlikely to play a major role in ovarian cancer development.

Vitamin D and parathyroid hormone in outpatients with noncholestatic chronic liver disease

BACKGROUND & AIMS

The liver plays a central role in vitamin D metabolism. Our aim was to determine the prevalence and type of vitamin D-parathyroid hormone (PTH) disturbance in ambulatory patients with noncholestatic chronic liver disease (CLD) and its relationship with disease severity and liver function.

METHODS

We studied 100 consecutive outpatients (63 men, 37 women; mean age, 49.0 +/- 12.1 [SD] y) with noncholestatic CLD caused by alcohol (n = 40), hepatitis C (n = 38), hepatitis B (n = 12), autoimmune hepatitis (n = 4), hemochromatosis (n = 4), and nonalcoholic steatohepatitis (n = 2); 51 patients had cirrhosis. Serum concentrations of 25-hydroxyvitamin D (25[OH]D), PTH, calcium, phosphate, magnesium, creatinine, and liver function tests were determined.

RESULTS

Serum 25(OH)D levels were inadequate in 91 patients: vitamin D deficiency (<50 nmol/L) was found in 68 patients and vitamin D insufficiency (50-80 nmol/L) was found in 23 patients. Secondary hyperparathyroidism (serum PTH, >6.8 pmol/L) was present in 16 patients. The prevalence of vitamin D deficiency was significantly higher in cirrhotic vs noncirrhotic patients (86.3% vs 49.0%; P = .0001). In Child-Pugh class C patients, 25(OH)D levels were significantly lower than in class A patients (22.7 +/- 10.0 nmol/L vs 45.8 +/- 16.8 nmol/L; P < .001). Serum 25(OH)D independently correlated with international normalized ratio (negatively; P = .018) and serum albumin (positively; P = .007). Serum 25(OH)D levels of less than 25 nmol/L predicted coagulopathy, hyperbilirubinemia, hypoalbuminemia, increased alkaline phosphatase, and anemia and thrombocytopenia.

CONCLUSIONS

Vitamin D inadequacy is common in noncholestatic CLD and correlates with disease severity, but secondary hyperparathyroidism is relatively infrequent. Management of CLD should include assessment of vitamin D status in all patients and replacement when necessary.

Leptin attenuates gene expression for renal 25-hydroxyvitamin D3-1alpha-hydroxylase in mice via the long form of the leptin receptor

Leptin, the ob gene product secreted by adipocytes, controls overall energy balance. We previously showed that leptin administration to leptin-deficient obese (ob/ob) mice suppressed mRNA expression and activity of renal 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP27B1). In leptin receptor-deficient (db/db) mice, we presently examined whether leptin affects 1alpha-hydroxylase expression in renal tubules through the active form of the leptin receptor (ObRb). Elevated serum concentrations of calcium and 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] in untreated ob/ob mice showed sharp reduction with leptin administration (4 mg/kg, i.p. every 12h for 2 days); no such reduction of elevation occurred in db/db mice. ObRb mRNA was expressed in kidney, brain, fat, lung, and bone in wild-type and ob/ob mice, but not db/db mice. The ob/ob and db/db mice showed large increases in renal 1alpha-hydroxylase mRNA expression and activity. Leptin administration (4 mg/kg) completely abrogated these increases in ob/ob but not db/db mice. Renal 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) mRNA synthesis also was greatly elevated in ob/ob and db/db mice; excesses decreased significantly with leptin administration in ob/ob mice, but increased in db/db mice. Renal tubular cells in primary culture expressed mRNAs including proximal tubules markers (1alpha-hydroxylase and megalin), parathyroid hormone receptor, and vitamin D receptor. Calcitonin receptor mRNA, synthesized mainly in distal tubules, was scant, indicating that most cultured cells were from proximal tubules. Cells did not express ObRb mRNA. Forskolin exposure at 10(-6)M for 3 or 6h significantly increased 1alpha-hydroxylase mRNA. Leptin at 10(-6)M did not change mRNA expression in either presence or absence of forskolin. Accordingly, leptin attenuates renal 1alpha-hydroxylase gene expression through ObRb. Furthermore, leptin appears to act indirectly on renal proximal tubules to regulate 1alpha-hydroxylase gene expression.

Estradiol and progesterone synthesis in human placenta is stimulated by calcitriol

Calcitriol exerts a diverse range of biological actions including the control of growth and cell differentiation, modulation of hormone secretion, and regulation of reproductive function. The placenta synthesizes calcitriol through the expression of CYP27B1, but little is known about local actions of this hormone in the fetoplacental unit. The objective of this study was to investigate the effects of calcitriol upon progesterone (P(4)) and estradiol (E(2)) secretion in trophoblasts cultured from term human placenta. Cells were incubated in the presence of calcitriol for 18 h and pregnenolone or androstenedione were subsequently added as substrates for the 3beta-hydroxysteroid dehydrogenase (3beta-HSD) or P450-aromatase (CYP19), respectively. Calcitriol stimulated in a dose-dependent manner E(2) and P(4) secretion. The use of a selective inhibitor of PKA prevented the effects of calcitriol upon E(2) secretion, but not on P(4). These results show that calcitriol is a physiological regulator of placental E(2) and P(4) production and suggest a novel role for calcitriol upon placental steroidogenesis.

Potentiating effects of nonactive/active vitamin D analogues and ketoconazole in parathyroid cells

BACKGROUND AND OBJECTIVE

1,25-dihydroxyvitamin D(3)[1alpha,25(OH)(2)D(3), calcitriol], and its less calcaemic synthetic analogues have therapeutic potential in several diseases, including hyperparathyroidism (HPT). We have suggested that non-1alpha-hydroxylated (nonactive) vitamin D analogues may present an alternative in tumour cells expressing 25-hydroxyvitamin D(3) 1alpha-hydroxylase (1alpha-hydroxylase). The aim of this study was to investigate biological effects of a non-1alpha-hydroxylated vitamin D analogue in normal and tumour parathyroid cells.

PATIENTS AND METHODS

Effects of vitamin D analogues and ketoconazole on parathyroid hormone (PTH) secretion (radioimmunoassay) and PTH mRNA expression (reverse transcription-polymerase chain reaction) were studied in primary bovine parathyroid cells. Proliferation of tumour cells isolated from HPT patients was determined by thymidine incorporation.

RESULTS

EB1285, non-1alpha-hydroxylated precursor of the vitamin D analogue EB1089, suppressed PTH secretion and PTH mRNA level as well as increased expression of 25-hydroxyvitamin D(3)-24-hydroxylase (24-hydroxylase) in bovine parathyroid cells. EB1285 also inhibited cell proliferation of parathyroid tumour cells from primary (pHPT) and secondary HPT (sHPT) patients. Combined treatment with the cytochrome P450-dependent enzyme inhibitor ketoconazole and EB1285 or with active vitamin D compounds potentiated the suppressive effect on PTH secretion from bovine parathyroid cells. Ketaconazole alone displayed PTH suppression and increased 24-hydroxylase expression.

CONCLUSION

The results support the idea that a non-1alpha-hydroxylated vitamin D analogue may elicit vitamin D receptor (VDR) effects in 1alpha-hydroxylase expressing parathyroid tumour cells. Further studies are warranted to elucidate whether precursor vitamin D analogues as well as inhibitors of 24-hydroxylase present therapeutic alternatives in patients suffering from HPT.

B, J. W, M T. Dietary calcium does not affect prostate tumor progression in LPB-Tag transgenic mice

High dietary calcium has been shown in epidemiological studies to be a risk factor for prostate cancer, and it has been postulated that this effect is secondary to calcium induced modulation of the vitamin D axis. In this study, we used LPB-Tag transgenic mice on the CD1 background to examine the impact of dietary calcium on prostate tumor progression. CD1-LPB-Tag mice predictably develop autochthonous, hormone-responsive prostate tumors by 3 months of age. Age matched transgenic and non-transgenic littermates were weaned onto high (2%) or low (0.2%) calcium diets and mice were sacrificed at 5, 7, and 9 weeks of age. The entire urogenital complex was excised, weighed, and processed for histology. There was no significant effect of dietary calcium on tumor weight or on the time course of tumor progression, as monitored using a modified Gleason grade (MGS). Serum calcium was maintained in the normal range in mice on the low and high calcium diet throughout the study. Circulating 1,25(OH)(2)D(3) was elevated by low dietary calcium in 5-week-old mice, but not in older animals. In summary, neither development nor progression of prostate tumors in LPB-Tag mice was accelerated by high dietary calcium.

Metabolism of vitamin D in the human choriocarcinoma cell line JEG-3

Calcitriol is an antiproliferative prodifferentiating secosteroid that exerts a protective role for some kinds of cancer. Alterations in 25-hydroxyvitamin D-1alpha-hydroxylase (CYP27B1) activity have been found in some tumor cells, but there are no studies performed in human choriocarcinoma. In the present work, calcitriol production and CYP27B1 gene regulation were studied in the human choriocarcinoma cell line JEG-3, and compared with normal human syncytiotrophoblasts (hS) in culture. In JEG-3 cells, secretion of [(3)H]calcitriol was significantly less (P<0.001) than in hS (45+/-17fmol/mg protein versus 174+/-87fmol/mg protein, respectively; n=8). CYP27B1 mRNA was similar in both JEG-3 and hS cells; but the protein was detected only in hS extracts. In contrast to the hS, JEG-3 CYP27B1 gene expression was not regulated by calcitriol or by a cAMP analogue. Our results indicate that in JEG-3 cells calcitriol production is diminished due to CYP27B1 dysregulation and low protein content, and suggest that hyperproliferation could be a consequence of these alterations.

New 19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 analogs strongly stimulate osteoclast formation both in vivo and in vitro

2-Methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 (2MD), an analog of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], has been shown to strongly induce bone formation both in vitro and in vivo. We have synthesized four substituents at carbon 2 of 2MD (2MD analogs), four stereoisomers at carbon 20 of the respective 2MD analogs (2MD analog-C20 isomers) and four 2MD analogs with an oxygen atom at carbon 22 (2MD-22-oxa analogs) and examined their ability to stimulate osteoclastogenesis and induce hypercalcemia. 2MD analogs were 100 times as potent as 1alpha,25(OH)2D3 in stimulating the formation of osteoclasts in vitro and in inducing the expression of receptor activator of NF-kappaB ligand (RANKL) and 25-hydroxyvitamin D3-24 hydroxylase mRNAs in osteoblasts. The osteoclast-inducing activities of 2MD analog-C20 isomers and 2MD 22-oxa analogs were much weaker than those of 2MD analogs. In addition, the activity of a 2MD analog in inducing dentine resorption was much stronger than that of 1alpha,25(OH)2D3 in the pit formation assay. Affinities to the vitamin D receptor and transcriptional activities of these compounds did not always correlate with their osteoclastogenic activities. Osteoprotegerin-deficient (OPG-/-) mice provide a suitable model for investigating in vivo effects of 2MD analogs because they exhibit extremely high concentrations of serum RANKL. The same amounts of 2MD analogs and 1alpha,25(OH)2D3 were administered daily to OPG-/- mice for 2 days. The elevation in serum concentrations of RANKL and calcium was much greater in 2MD analog-treated OPG-/- mice than in 1alpha,25(OH)2D3-treated ones. A 2MD analog was much more potent than 1alpha,25(OH)2D3 in causing hypercalcemia and in increasing soluble RANKL with enhanced osteoclastogenesis even in wild-type mice. In contrast, the administration of the 2MD analog to c-fos-deficient mice failed to induce osteoclastogenesis and hypercalcemia. These results suggest that new substituents at carbon 2 of 2MD strongly stimulate osteoclast formation in vitro and in vivo, and that osteoclastic bone resorption is indispensable for their hypercalcemic action of 2MD analogs in vivo.

1alpha,25-dihydroxy-vitamin D3 stimulation of bronchial smooth muscle cells induces autocrine, contractility, and remodeling processes

Genetic variants in the vitamin D receptor (VDR) gene were recently associated with asthma. The biological mechanisms explaining this association are unknown but are likely to involve many cell types given the pleiotropic effect of its ligand, 1alpha,25-dihydroxy-vitamin D3 [1alpha,25(OH)2D3]. Considering the prominent role of bronchial smooth muscle cells (BSMCs) in the pathogenesis of asthma, experiments were conducted to explore the gene regulatory effects of 1alpha,25(OH)2D3 in these cells. Using RT-PCR and Western blot, we showed that VDR is present both at the mRNA transcript and protein levels in human BSMCs. The functionality of the receptor was then demonstrated by showing a >200-fold change in the expression of the 24-hydroxylase (CYP24A1) gene following 1alpha,25(OH)2D3 stimulation. Microarray experiments were then performed to identify differentially regulated genes and pathways in BMSCs treated or not with 1alpha,25(OH)2D3. A total of 729 probe sets on the U133 plus 2.0 Affymetrix GeneChip showed fold-change differences above the 1.5 threshold using the Robust Multichip Average intensities. This corresponds to 231 unique genes that were upregulated and 215 unique genes that were down-regulated following 1alpha,25(OH)2D3 stimulation. A high similarity between microarray and real-time PCR results was observed for 13 random genes, with a concordance correlation coefficient of 0.91. Real-time PCR was also performed to confirm the regulation of asthma candidate genes. To identify the biological relevance of this regulation, biological pathways analyses were performed. The most significant network of upregulated genes included genes involved in morphogenesis, cell growth, and survival as well as genes encoding structural proteins, which are potentially involved in airway remodeling.

Regulation of Vitamin D hydroxylases gene expression by 1,25-dihydroxyvitamin D3 and cyclic AMP in cultured human syncytiotrophoblasts

Human placenta synthesizes and metabolizes 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)/calcitriol] through the activity of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP27B1) and 1,25(OH)(2)D(3)-24-hydroxylase (CYP24A1), the two key enzymes for Vitamin D metabolism. In this study, calcitriol rapidly generated intracellular cAMP accumulation in cultured human syncytiotrophoblast cells, which in turn enhanced hCG secretion, a marker of trophoblast endocrine activity. The effects of 1,25(OH)(2)D(3) upon the expression of CYP27B1 and CYP24A1 were also investigated. 1,25(OH)(2)D(3) and activators of the PKA signaling system decreased the expression of CYP27B1, whereas increased CYP24A1 gene transcription. The use of a selective inhibitor of PKA (H-89) prevented the effects of calcitriol on CYP27B1 gene and hCG secretion, but not on CYP24A1 transcription. Addition of ZK 159222, a Vitamin D receptor (VDR) antagonist, blocked the calcitriol-mediated upregulation of 24-hydroxylase gene expression but did not affect calcitriol-induced downregulation of CYP27B1 gene or hCG stimulation. In addition, our study also demonstrated a role of calcitonin on Vitamin D hydroxylases gene regulation in placenta. The overall data suggest that calcitriol downregulates CYP27B1 expression via a cAMP-dependent signaling pathway, whereas upregulates 24-hydroxylase gene expression through a VDR-dependent mechanism.

Lead and delta-aminolevulinic acid dehydratase polymorphism: where does it lead? A meta-analysis

BACKGROUND

Lead poisoning affects many organs in the body. Lead inhibits delta-aminolevulinic acid dehydratase (ALAD), an enzyme with two co-dominantly expressed alleles, ALAD1 and ALAD2.

OBJECTIVE

Our meta-analysis studied the effects of the ALAD polymorphism on a) blood and bone lead levels and b) indicators of target organ toxicity.

DATA SOURCE

We included studies reporting one or more of the following by individuals with genotypes ALAD1-1 and ALAD1-2/2-2: blood lead level (BLL), tibia or trabecular lead level, zinc protoporphyrin (ZPP), hemoglobin, serum creatinine, blood urea nitrogen (BUN), dimercaptosuccinic acid-chelatable lead, or blood pressure.

DATA EXTRACTION

Sample sizes, means, and standard deviations were extracted for the genotype groups.

DATA SYNTHESIS

There was a statistically significant association between ALAD2 carriers and higher BLL in lead-exposed workers (weighted mean differences of 1.93 microg/dL). There was no association with ALAD carrier status among environmentally exposed adults with BLLs < 10 microg/dL. ALAD2 carriers were potentially protected against adverse hemapoietic effects (ZPP and hemoglobin levels), perhaps because of decreased lead bioavailability to heme pathway enzymes.

CONCLUSION

Carriers of the ALAD2 allele had higher BLLs than those who were ALAD1 homozygous and higher hemoglobin and lower ZPP, and the latter seems to be inversely related to BLL. Effects on other organs were not well delineated, partly because of the small number of subjects studied and potential modifications caused by other proteins in target tissues or by other polymorphic genes.

Nonclassical Aspects of Differential Vitamin D Receptor Activation

The 'classical' effects of vitamin D receptor activator or agonist (VDRA) therapy for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease primarily involves suppressive effects on the parathyroid gland, and regulation of calcium and phosphorus absorption in the intestine and mobilisation in bone. Observational studies in haemodialysis patients report improved cardiovascular and all-cause survival among those receiving VDRA therapy compared with those not on VDRA therapy. Among VDRAs, the selective VDRA paricalcitol has been associated with greater survival than nonselective VDRAs, such as calcitriol (1,25-dihydroxyvitamin D(3)). The survival benefits of paricalcitol appear to be linked, at least in part, to 'nonclassical' actions of VDRAs, possibly through VDRA-mediated modulation of gene expression. In cardiovascular tissues, VDRAs are reported to have beneficial effects such as anti-inflammatory and antithrombotic effects, inhibition of vascular smooth muscle cell proliferation, inhibition of vascular calcification and stiffening, and regression of left ventricular hypertrophy. VDRAs are also reported to negatively regulate the renin-angiotensin system, which plays a key role in hypertension, myocardial infarction and stroke. The selective VDRAs, paricalcitol and maxacalcitol, are associated with direct protective effects on glomerular architecture and antiproteinuric effects in response to renal damage. Paricalcitol regulates several cardiovascular and renal parameters more favourably than nonselective VDRAs. Complex nonclassical effects, which are not clearly understood, possibly contribute to the improved survival seen with VDRAs, especially paricalcitol.

Vitamine D : Métabolisme, régulation et maladies associées

Vitamin D is well known as a hormone involved in mineral metabolism and bone growth. Conversion into the active metabolite 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) from the precursor is effected by cytochrome P450 enzymes in the liver (CYP27A1 and CYP2R1) and the kidney (CYP27B1). CYP27A1 has been shown to be transcriptionally regulated by nuclear receptors (PPARalpha, gamma, HNF-4alpha and SHP) which are ligand-dependent transcription factors. CYP27B1 is tightly regulated by the plasma levels of calcium, phosphate, parathyroid hormone (PTH) and 1,25(OH)2D3 itself. In vitamin D target organs, inactivation of vitamin D is attributed to CYP24A1 which is transcriptionally induced by 1,25(OH)2D3 whose action is mediated by binding to its cognate nuclear receptor, the vitamin D receptor (VDR). Diseases associated to Vitamin D deficiency (rickets in children, and osteomalacia or osteoporosis in adults) and autosomal recessive forms of inherited rickets illustrate the key role of vitamin D in calcium homeostasis and bone metabolism. Recently, discovery of 1,25(OH)2D3 new biological actions that include antiproliferative, prodifferentiating effect on many cell types and immunoregulatory properties creates a growing interest for this vitamin. In this way, a best understanding of various actors implicated in vitamin D metabolism and its regulation is of a major importance to optimise the use of vitamin D in disease prevention.

IL-10 signaling is essential for 1,25-dihydroxyvitamin D3-mediated inhibition of experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) results from an aberrant, neuroantigen-specific, T cell-mediated autoimmune response. Because MS prevalence and severity decrease sharply with increasing sunlight exposure, and sunlight supports vitamin D(3) synthesis, we proposed that vitamin D(3) and 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) may protect against MS. In support of this hypothesis, 1,25-(OH)(2)D(3) strongly inhibited experimental autoimmune encephalomyelitis (EAE). This inhibition required lymphocytes other than the encephalitogenic T cells. In this study, we tested the hypothesis that 1,25-(OH)(2)D(3) might inhibit EAE through the action of IL-10-producing regulatory lymphocytes. We report that vitamin D(3) and 1,25-(OH)(2)D(3) strongly inhibited myelin oligodendrocyte peptide (MOG(35-55))-induced EAE in C57BL/6 mice, but completely failed to inhibit EAE in mice with a disrupted IL-10 or IL-10R gene. Thus, a functional IL-10-IL-10R pathway was essential for 1,25-(OH)(2)D(3) to inhibit EAE. The 1,25-(OH)(2)D(3) also failed to inhibit EAE in reciprocal, mixed bone marrow chimeras constructed by transferring IL-10-deficient bone marrow into irradiated wild-type mice and vice versa. Thus, 1,25-(OH)(2)D(3) may be enhancing an anti-inflammatory loop involving hemopoietic cell-produced IL-10 acting on brain parenchymal cells and vice versa. If this interpretation is correct, and humans have a similar bidirectional IL-10-dependent loop, then an IL-10-IL-10R pathway defect could abrogate the anti-inflammatory and neuro-protective functions of sunlight and vitamin D(3). In this way, a genetic IL-10-IL-10R pathway defect could interact with an environmental risk factor, vitamin D(3) insufficiency, to increase MS risk and severity.

Hybrid homology modeling and mutational analysis of cytochrome P450C24A1 (CYP24A1) of the Vitamin D pathway: insights into substrate specificity and membrane bound structure-function

Cytochrome P450C24A1 (CYP24A1), a peripheral inner mitochondrial membrane hemoprotein and candidate oncogene, regulates the side-chain metabolism and biological function of vitamin D and many of its related analog drugs. Rational mutational analysis of rat CYP24A1 based on hybrid (2C5/BM-3) homology modeling and affinity labeling studies clarified the role of key domains (N-terminus, A', A, and F-helices, beta3a strand, and beta5 hairpin) in substrate binding and catalysis. The scope of our study was limited by an inability to purify stable mutant enzyme targeting soluble domains (B', G, and I-helices) and suggested greater conformational flexibility among CYP24A1's membrane-associated domains. The most notable mutants developed by modeling were V391T and I500A, which displayed defective-binding function and profound metabolic defects for 25-hydroxylated vitamin D3 substrates similar to a non-functional F-helix mutant (F249T) that we previously reported. Val-391 (beta3a strand) and Ile-500 (beta5 hairpin) are modeled to interact with Phe-249 (F-helix) in a hydrophobic cluster that directs substrate-binding events through interactions with the vitamin D cis-triene moiety. Prior affinity labeling studies identified an amino-terminal residue (Ser-57) as a putative active-site residue that interacts with the 3beta-OH group of the vitamin D A-ring. Studies with 3-epi and 3-deoxy-1,25(OH)2D3 analogs confirmed interactions between the 3beta-OH group and Ser-57 effect substrate recognition and trafficking while establishing that the trans conformation of A-ring hydroxyl groups (1alpha and 3beta) is obligate for high-affinity binding to rat CYP24A1. Our work suggests that CYP24A1's amphipathic nature allows for monotopic membrane insertion, whereby a pw2d-like substrate access channel is formed to shuttle secosteroid substrate from the membrane to the active-site. We hypothesize that CYP24A1 has evolved a unique amino-terminal membrane-binding motif that contributes to substrate specificity and docking through coordinated interactions with the vitamin D A-ring.

Klotho as a regulator of fibroblast growth factor signaling and phosphate/calcium metabolism

PURPOSE OF REVIEW

This review summarizes the most recent findings on Klotho in the regulation of fibroblast growth factor-23 (FGF23) signaling and phosphate/calcium homeostasis.

RECENT FINDINGS

The klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends life span when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. FGF23 is a hormone that suppresses phosphate reabsorption in renal proximal tubules. Recent studies have shown that Klotho mice and Fgf23 mice exhibit identical phenotypes including hyperphosphatemia and hypercalcemia in addition to the aging-like syndrome. This may be explained by the fact that Klotho binds to multiple FGF receptors and increases their affinity to FGF23. Another Klotho protein function is to activate transient receptor potential vanilloid-5 - a calcium channel involved in calcium reabsorption in the kidney. Klotho protein can modify sugar chains on transient receptor potential vanilloid-5 through its activity as a beta-glucuronidase, preventing the calcium channel from internalization and inactivation.

SUMMARY

Klotho protein binds to fibroblast growth factor receptors and functions as a regulator of FGF23 signaling. It also functions as an enzyme that modifies sugar chains of transient receptor potential vanilloid-5 and regulates its activity. Klotho is a multi-functional protein that regulates phosphate/calcium metabolism as well as aging.

Expression analysis of LacZ gene placed in the locus of Cnot7 exhibits its activity in osteoblasts in vivo and in mineralized nodules in vitro

CCR4-NOT complex 7 (Cnot7) was identified as a regulator of gene expression in yeast and evolutionally conserved in mammals. Cnot7 deficient male mice exhibit abnormality in spermatogenesis. As these mice contained construct to express LacZ, we followed the expression patterning in these animals. LacZ was expressed in osteoblasts located in the primary spongiosa in adult mice. Cellular analysis indicated that LacZ is expressed in osteoblasts but not in osteoclasts. In the mineralized nodules formed in the culture of bone marrow cells obtained from Cnot7 +/- mice, LacZ expression was mainly observed in the cells forming mineralized nodules but not in un-mineralized area scattered around the periphery of the nodules. LacZ blue positive cells were gradually depositing minerals along its time course of the in vitro mineralization assay. Cnot7 expression was enhanced by the treatment with BMP. These data suggest that Cnot7 is expressed in osteoblasts and is associated with mineralization.

Paricalcitol attenuates renal interstitial fibrosis in obstructive nephropathy

Deficiency in vitamin D and its active metabolites is a pathologic feature of chronic kidney diseases. Despite that tubular epithelial cells are the major sites of active vitamin D synthesis, little is known about the role of vitamin D in maintaining the structural and functional integrity of tubular epithelium. This study investigated the effects of paricalcitol (19-nor-1,25-hydroxy-vitamin D(2)), a synthetic vitamin D analogue, on obstructive nephropathy, a model that is characterized by predominant tubulointerstitial lesions. Compared with vehicle controls, paricalcitol significantly attenuated renal interstitial fibrosis in mouse kidney after ureteral obstruction, as demonstrated by a reduced interstitial volume, decreased collagen deposition, and repressed mRNA expression of fibronectin and type I and type III collagens. Paricalcitol largely preserved E-cadherin and reduced alpha-smooth muscle actin expression in vivo. In addition, paricalcitol suppressed renal TGF-beta1 and its type I receptor expression, restored vitamin D receptor abundance, and inhibited cell proliferation and apoptosis after obstructive injury. In vitro, paricalcitol abolished TGF-beta1-mediated E-cadherin suppression and alpha-smooth muscle actin and fibronectin induction in tubular epithelial cells, underscoring its ability to block directly the epithelial to mesenchymal transition (EMT). It is interesting that paricalcitol almost completely suppressed renal induction of Snail, a critical transcription factor that is implicated in EMT programming. Furthermore, paricalcitol inhibited the TGF-beta1-mediated Snail induction in vitro, and ectopic expression of Snail repressed E-cadherin promoter activity and downregulated E-cadherin expression in tubular epithelial cells. These studies suggest that paricalcitol is able to ameliorate renal interstitial fibrosis in obstructive nephropathy, possibly by preserving tubular epithelial integrity through suppression of EMT.

Regulation of human epidermal keratinocyte differentiation by the vitamin D receptor and its coactivators DRIP205, SRC2, and SRC3

It has long been known that the active metabolite of vitamin D, 1,25 dihydroxyvitamin D(3), stimulates differentiation and inhibits proliferation in epidermal keratinocytes through interaction with the vitamin D receptor (VDR). VDR functions through the coordinate binding of vitamin D response elements in the DNA and specific coactivator proteins which help to initiate transcription. It was recently observed that VDR binds to two major coactivator complexes, DRIP (VDR-interacting protein) and SRC (steroid receptor coactivator), during keratinocyte differentiation. To determine the role of VDR and its coactivators in mediating keratinocyte differentiation, we developed an adenoviral system to knock down, or in the case of VDR, overexpress these genes. In order to study all stages of keratinocyte development, we employed an advanced differentiated normal human keratinocyte culture system that produces a multilayer phenotype similar to that of normal skin. These studies have shown that VDR, DRIP, and SRC are all required for promotion of both early and late keratinocyte differentiation. Additionally, each individual differentiation marker that was assayed has a different specificity for the coactivators that regulate its expression.

Medical therapy of secondary hyperparathyroidism in chronic kidney disease: old and new drugs

Secondary hyperparathyroidism (SHPT), a common complication of chronic kidney disease, is characterised by elevated serum levels of parathyroid hormone, parathyroid hyperplasia, excessive bone resorption and increased risk for cardiovascular morbidity. The stringent metabolic targets proposed by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI) for patients with SHPT are difficult to achieve using conventional treatment regimens. Several new agents, including new vitamin D sterols and phosphate binders, as well as a novel class of compounds--the calcimimetics--have been developed in recent years. This review examines new and traditional therapies for SHPT and how these can best be utilised in order to achieve the new K/DOQI targets.

In vivo effects of chronic contamination with depleted uranium on vitamin D3 metabolism in rat

The extensive use of depleted uranium (DU) in today's society results in the increase of the number of human population exposed to this radionuclide. The aim of this work was to investigate in vivo the effects of a chronic exposure to DU on vitamin D(3) metabolism, a hormone essential in mineral and bone homeostasis. The experiments were carried out in rats after a chronic contamination for 9 months by DU through drinking water at 40 mg/L (1 mg/rat/day). This dose corresponds to the double of highest concentration found naturally in Finland. In DU-exposed rats, the active vitamin D (1,25(OH)(2)D(3)) plasma level was significantly decreased. In kidney, a decreased gene expression was observed for cyp24a1, as well as for vdr and rxralpha, the principal regulators of CYP24A1. Similarly, mRNA levels of vitamin D target genes ecac1, cabp-d28k and ncx-1, involved in renal calcium transport were decreased in kidney. In the brain lower levels of messengers were observed for cyp27a1 as well as for lxrbeta, involved in its regulation. In conclusion, this study showed for the first time that DU affects both the vitamin D active form (1,25(OH)(2)D(3)) level and the vitamin D receptor expression, and consequently could modulate the expression of cyp24a1 and vitamin D target genes involved in calcium homeostasis.

Expression of vitamin D3 receptor in kidney tumors

The kidney is not only a primary vitamin D target organ but also is a key site of vitamin D metabolism. Recent studies have shown that vitamin D has important physiologic effects on proliferation and differentiation in a variety of benign and malignant cells. Our preliminary immunohistochemical study showed that vitamin D receptor (VDR) was highly expressed in renal distal tubules and collecting ducts, whereas the renal proximal tubules and glomeruli did not express VDR. These observations led us to study the expression of VDR in various kidney tumors to determine the possible diagnostic utility of VDR. Paraffin tissue microarray (TMA) blocks were constructed containing core cylinders from clear cell (52), papillary (35), chromophobe (20), sarcomatoid (20), and metastatic (59) renal cell carcinomas (RCCs). Oncocytomas (20), normal adult kidneys (12), and normal adult adrenals (6) were also included. In addition, 30 clear cell RCCs and 3 collecting duct carcinomas were also studied using conventional sections. Furthermore, VDR messenger RNA and protein expression was also quantified using real-time reverse transcriptase-polymerase chain reaction and Western blot analysis. Vitamin D receptor was strongly positive in collecting duct carcinomas (100% [3/3], cytoplasmic), papillary RCCs (94% [33/35], cytoplasmic), chromophobe RCCs (85% [17/20], membranous), and oncocytomas (90% [18/20], cytoplasmic with perinuclear accentuation). In contrast, VDR expression was focal/weak and present only in the peripheral regions of clear cell RCCs. Vitamin D receptor was weakly positive in sarcomatoid variant RCCs (88% [14/16]) regardless of the type of associated original RCC. Overall, VDR is a discriminative marker for renal cell tumors. The preferential expression of VDR in chromophobe RCCs, oncocytomas, and collecting duct carcinomas is in agreement with the concept that these tumors differentiate toward epithelium lining the distal convoluted tubules and collecting ducts. Considering the different VDR expression patterns, VDR is a useful ancillary tool in distinguishing chromophobe RCCs from oncocytomas. In addition, the focal and much weaker VDR expression in clear cell RCCs makes VDR valuable in distinguishing clear cell RCC from other types of RCCs.

Chronic contamination with 137Cesium affects Vitamin D3 metabolism in rats

Twenty years after Chernobyl disaster, many people are still chronically exposed to low dose of (137)Cs, mainly through the food consumption. A large variety of diseases have been described in highly exposed people with (137)Cs, which include bone disorders. The aim of this work was to investigate the biological effects of a chronic exposure to (137)Cs on Vitamin D(3) metabolism, a hormone essential in bone homeostasis. Rats were exposed to (137)Cs in their drinking water for 3 months at a dose of 6500 Bq/l (approximately 150 Bq/rat/day), a similar concentration ingested by the population living in contaminated territories in the former USSR countries. Cytochromes P450 enzymes involved in Vitamin D(3) metabolism, related nuclear receptors and Vitamin D(3) target genes were assessed by real time PCR in liver, kidney and brain. Vitamin D, PTH, calcium and phosphate levels were measured in plasma. An increase in the expression level of cyp2r1 (40%, p<0.05) was observed in the liver of (137)Cs-exposed rats. However a significant decrease of Vitamin D (1,25(OH)D(3)) plasma level (53%, p=0.02) was observed. In brain, cyp2r1 mRNA level was decreased by 20% (p<0.05), while the expression level of cyp27b1 is increased (35%, p<0.05) after (137)Cs contamination. In conclusion, this study showed for the first time that chronic exposure with post-accidental doses of (137)Cs affects Vitamin D(3) active form level and induces molecular modifications of CYPs enzymes involved its metabolism in liver and brain, without leading to mineral homeostasis disorders.