A possible role of vitamin D receptors in regulating vitamin D activation in the kidney

A lipidome landscape of aging in mice

Understanding the molecular mechanisms of aging is crucial for enhancing healthy longevity. We conducted untargeted lipidomics across 13 biological samples from mice at various life stages (2, 12, 19 and 24 months) to explore the potential link between aging and lipid metabolism, considering sex (male or female) and microbiome (specific pathogen-free or germ-free) dependencies. By analyzing 2,704 molecules from 109 lipid subclasses, we characterized common and tissue-specific lipidome alterations associated with aging. For example, the levels of bis(monoacylglycero)phosphate containing polyunsaturated fatty acids increased in various organs during aging, whereas the levels of other phospholipids containing saturated and monounsaturated fatty acids decreased. In addition, we discovered age-dependent sulfonolipid accumulation, absent in germ-free mice, correlating with Alistipes abundance determined by 16S ribosomal RNA gene amplicon sequencing. In the male kidney, glycolipids such as galactosylceramides, galabiosylceramides (Gal2Cer), trihexosylceramides (Hex3Cer), and mono- and digalactosyldiacylglycerols were detected, with two lipid classes-Gal2Cer and Hex3Cer-being significantly enriched in aged mice. Integrated analysis of the kidney transcriptome revealed uridine diphosphate galactosyltransferase 8A (UGT8a), alkylglycerone phosphate synthase and fatty acyl-coenzyme A reductase 1 as potential enzymes responsible for the male-specific glycolipid biosynthesis in vivo, which would be relevant to sex dependency in kidney diseases. Inhibiting UGT8 reduced the levels of these glycolipids and the expression of inflammatory cytokines in the kidney. Our study provides a valuable resource for clarifying potential links between lipid metabolism and aging.

Klotho and Fibroblast Growth Factor 23 Are Independent of Vitamin D, and Unlike Vitamin D, Are Not Associated With Graft- and Patient Survival After Kidney Transplantation

Short-term survival after kidney transplantation is excellent but long-term survival remains suboptimal. The aim of the study was to explore the relationship between soluble α-Klotho (sKlotho) and intact fibroblast growth factor 23 (iFGF23) measured 8 wk and 1 y posttransplant with long-term graft- and patient survival in a cohort of kidney transplant recipients with deficient and nondeficient vitamin D (25[OH]D) levels.

Methods

Vitamin D, sKlotho, and iFGF23 were measured 8 wk and 1 y posttransplant in 132 recipients transplanted between November 2012 and October 2013.

Results

Of the 132 kidney transplant recipients, 49 had deficient vitamin D levels (<30 nmol/L) and 83 had nondeficient vitamin D levels (≥30 nmol/L) at 8 wk posttransplant. The mean age was 51 y and the median follow-up was 7.4 y. At 1 y posttransplant, vitamin D increased significantly. There were no significant differences in sKlotho or iFGF23 levels between the 2 vitamin D groups neither at 8 wk nor 1 y. sKlotho increased significantly and iFGF23 decreased significantly in the whole cohort. During the follow-up, there were 36 graft losses (27%) and 27 deaths (20%). Ninety-four percent of the transplant recipients with nondeficient vitamin D levels were alive with a well-functioning graft after 5 y using Kaplan-Meier survival estimates, compared with 84% of the patients with deficient vitamin D levels (P = 0.014). Klotho and FGF23 levels did not influence graft- and patient survival.

Conclusions

In this nationwide cohort of kidney transplant recipients, long-term graft- and patient survival were significantly better in patients with vitamin D ≥30 nmol/L 8 wk posttransplant compared with those with vitamin D <30 nmol/L. sKlotho levels increased and iFGF23 levels decreased from 8 wk to 1 y posttransplant. Klotho and FGF23 levels were not associated with graft- and patient survival.

NADH/NAD+ Redox Imbalance and Diabetic Kidney Disease

Diabetic kidney disease (DKD) is a common and severe complication of diabetes mellitus. If left untreated, DKD can advance to end stage renal disease that requires either dialysis or kidney replacement. While numerous mechanisms underlie the pathogenesis of DKD, oxidative stress driven by NADH/NAD⁺ redox imbalance and mitochondrial dysfunction have been thought to be the major pathophysiological mechanism of DKD. In this review, the pathways that increase NADH generation and those that decrease NAD⁺ levels are overviewed. This is followed by discussion of the consequences of NADH/NAD⁺ redox imbalance including disruption of mitochondrial homeostasis and function. Approaches that can be applied to counteract DKD are then discussed, which include mitochondria-targeted antioxidants and mimetics of superoxide dismutase, caloric restriction, plant/herbal extracts or their isolated compounds. Finally, the review ends by pointing out that future studies are needed to dissect the role of each pathway involved in NADH-NAD⁺ metabolism so that novel strategies to restore NADH/NAD⁺ redox balance in the diabetic kidney could be designed to combat DKD.

Hypoparathyroidism and the Kidney

Hypocalcemia and hyperphosphatemia are the pathognomonic biochemical features of hypoparathyroidism, and result directly from lack of parathyroid hormone (PTH) action on the kidney. In the absence of PTH action, the renal mechanisms transporting calcium and phosphate reabsorption deregulate, resulting in hypocalcemia and hyperphosphatemia. Circulating calcium negatively regulates PTH secretion. Hypocalcemia causes neuromuscular disturbances ranging from epilepsy and tetany to mild paresthesia. Circulating phosphate concentration does not directly regulate PTH secretion. Hyperphosphatemia is subclinical, but chronically promotes ectopic mineralization disease. Vitamin D-thiazide treatment leads to ectopic mineralization and renal damage. PTH treatment has the potential for fewer side effects.

5α-dihydrotestosterone reduces renal Cyp24a1 expression via suppression of progesterone receptor

Androgens act in concert with vitamin D to influence reabsorption of calcium. However, it is unclear whether androgens directly regulate vitamin D homeostasis or control other cellular events that are related to vitamin D metabolism. To examine whether the expression of vitamin D-related genes in mouse kidney is driven by androgens or androgen-dependent effects, the androgen receptor and other sex steroid receptors were monitored in orchidectomized mice treated with 5α-dihydrotestosterone (DHT). Our results revealed that exposing orchidectomized mice to DHT inhibited the expression of progesterone receptor (Pgr) with or without estrogen receptor α expression, the latter was confirmed by ER-positive (MCF7 and T47D) or -negative (PCT) cells analysis. The loss of Pgr in turn decreased the expression of renal 24-hydroxylase via transcriptional regulation because Cyp24a1 gene has a progesterone receptor-binding site on promoter. When male kidneys preferentially hydroxylate 25-hydroxyvitamin D₃ using 24-hydroxylase rather than 25-hydroxyvitamin D₃-1-alpha hydroxylase, DHT suppressed the Pgr-mediated 24-hydroxylase expression, and it is important to note that DHT increased the blood 25-hydroxyvitamin D₃ levels. These findings uncover an important link between androgens and vitamin D homeostasis and suggest that therapeutic modulation of Pgr may be used to treat vitamin D deficiency and related disorders.

Renal mechanisms of calcium homeostasis in sheep and goats

In small ruminants, the renal excretion of calcium (Ca) and phosphate (Pi) is not modulated in response to dietary Ca restriction. Although this lack of adaptation was observed in both sheep and goats, differences in renal function between these species cannot be excluded. Recent studies demonstrated that compared with sheep, goats have a greater ability to compensate for challenges to Ca homeostasis, probably due to a more pronounced increase in calcitriol production. Therefore, the aim of the present study was to examine the effect of 1) dietary Ca restriction, 2) administration of calcitriol, and 3) lactation on Ca and Pi transport mechanisms and receptors as well as enzymes involved in vitamin D metabolism in renal tissues of sheep and goats. Whereas RNA expression of renal transient receptor potential vanilloid channel type 5 was unaffected by changes in dietary Ca content, a significant stimulation was observed with administration of calcitriol in both sheep (P < 0.001) and goats (P < 0.01). Calbindin-D28K was downregulated during dietary Ca restriction in goats (P < 0.05). Expression of the sodium/Ca exchanger type 1 was decreased by low Ca intake in sheep (P < 0.05) and upregulated by calcitriol treatment in goats (P < 0.05). A significant reduction in RNA expression of the cytosolic and the basolateral Ca transporting proteins was also demonstrated for lactating goats in comparison to dried-off animals. Species differences were found for vitamin D receptor expression, which was stimulated by calcitriol treatment in sheep (P < 0.01) but not in goats. As expected, expression of 1α-hydroxylase was upregulated by dietary Ca restriction (P < 0.001; P < 0.05) and inhibited by exogenous calcitriol (P < 001; P < 0.05) in both sheep and goats. However, whereas 24-hydroxylase expression was stimulated to the same extent by calcitriol treatment in sheep, irrespective of the diet (P < 0.001), a modulatory effect of dietary Ca supply on 24-hydroxylase induction was observed in goats (P < 0.05). Taken together, our results confirm observations that modulation of renal Ca excretion does not contribute to maintenance of Ca homeostasis in these ruminants during restricted dietary supply, unlike responses in monogastric animals. The interesting species differences related to vitamin D metabolism might explain the greater capacity of goats to compensate for challenges of Ca homeostasis and should be further investigated.

Ablation of the Stimulatory G Protein α-Subunit in Renal Proximal Tubules Leads to Parathyroid Hormone-Resistance With Increased Renal Cyp24a1 mRNA Abundance and Reduced Serum 1,25-Dihydroxyvitamin D

PTH regulates serum calcium, phosphate, and 1,25-dihydroxyvitamin D (1,25(OH)2D) levels by acting on bone and kidney. In renal proximal tubules (PTs), PTH inhibits reabsorption of phosphate and stimulates the synthesis of 1,25(OH)2D. The PTH receptor couples to multiple G proteins. We here ablated the α-subunit of the stimulatory G protein (Gsα) in mouse PTs by using Cre recombinase driven by the promoter of type-2 sodium-glucose cotransporter (Gsα(Sglt2KO) mice). Gsα(Sglt2KO) mice were normophosphatemic but displayed, relative to controls, hypocalcemia (1.19 ±0.01 vs 1.23 ±0.01 mmol/L; P < .05), reduced serum 1,25(OH)2D (59.3 ±7.0 vs 102.5 ±12.2 pmol/L; P < .05), and elevated serum PTH (834 ±133 vs 438 ±59 pg/mL; P < .05). PTH-induced elevation in urinary cAMP excretion was blunted in Gsα(Sglt2KO) mice (2- vs 4-fold over baseline in controls; P < .05). Relative to baseline in controls, PTH-induced reduction in serum phosphate tended to be blunted in Gsα(Sglt2KO) mice (-0.39 ±0.33 vs -1.34 ±0.36 mg/dL; P = .07). Gsα(Sglt2KO) mice showed elevated renal vitamin D 24-hydroxylase and bone fibroblast growth factor-23 (FGF23) mRNA abundance (∼3.4- and ∼11-fold over controls, respectively; P < .05) and tended to have elevated serum FGF23 (829 ±76 vs 632 ±60 pg/mL in controls; P = .07). Heterozygous mice having constitutive ablation of the maternal Gsα allele (E1(m-/+)) (model of pseudohypoparathyroidism type-Ia), in which Gsα levels in PT are reduced, also exhibited elevated serum FGF23 (474 ±20 vs 374 ±27 pg/mL in controls; P < .05). Our findings indicate that Gsα is required in PTs for suppressing renal vitamin D 24-hydroxylase mRNA levels and for maintaining normal serum 1,25(OH)2D.

The vitamin D receptor in the proximal renal tubule is a key regulator of serum 1α,25-dihydroxyvitamin D₃

It is well established that the mitochondria of proximal convoluted tubule cells of the kidney are the site of production of circulating 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The production of 1,25(OH)2D3 at this site is tightly regulated. Parathyroid hormone markedly stimulates 1,25(OH)2D3 production, whereas 1,25(OH)2D3 itself suppresses production. The mechanism of suppression by 1,25(OH)2D3 has not yet been elucidated. We have now found that in the absence of vitamin D (vitamin D deficiency), the vitamin D receptor (VDR) is found in the interior of the apical brush border of the proximal tubule cells. This is unique for the proximal tubule cells, since this has not been observed in the distal tubule cells or in other epithelial cells, such as intestinal mucosa. Administration of 1,25(OH)2D3 to vitamin D-deficient rats results in the movement of VDR from the brush border to the cytoplasm and nucleus presumably bound to reabsorbed 1,25(OH)2D3. The VDR bound to 1,25(OH)2D3 suppresses expression of 25-hydroxyvitamin D3 1α-hydroxylase and stimulates the 25-hydroxyvitamin D3 24-hydroxylase. Thus, VDR in the apical brush border of the proximal convoluted tubule cells serves to "sense" the level of circulating 1,25(OH)2D3 and modulates the activity of the 1α-hydroxylase and the 24-hydroxylase accordingly.

Rickets-vitamin D deficiency and dependency

Rickets is an important problem even in countries with adequate sun exposure. The causes of rickets/osteomalacia are varied and include nutritional deficiency, especially poor dietary intake of vitamin D and calcium. Non-nutritional causes include hypophosphatemic rickets primarily due to renal phosphate losses and rickets due to renal tubular acidosis. In addition, some varieties are due to inherited defects in vitamin D metabolism and are called vitamin D dependent rickets. This chapter highlights rickets/osteomalacia related to vitamin D deficiency or to inherited defects in vitamin D metabolism. Hypophosphatemic rickets and rickets due to renal tubular acidosis are discussed in other sections of the journal.

Specific regulation of CYP27B1 and VDR in proximal versus distal renal cells

In this study, we utilized murine renal proximal (MPCT-G) and distal (DKC-8) tubular epithelial cell lines to compare the gene expressions and promoter activities of 1,25(OH)(2)D(3) receptor (VDR) and 25-hydroxyvitamin D-1alpha-hydroxylase (CYP27B1) in response to 50 nM of parathyroid hormone (PTH) and changes in extracellular calcium (Ca(2+)) concentration. In MPCT-G cells, VDR gene expression was suppressed by PTH, whereas CYP27B1 gene expression was elevated in response to PTH. In DKC-8 cells, treatment of PTH significantly increased the relative gene expression of VDR by 6.5-fold while CYP27B1 gene expression was unchanged. High Ca(2+) exposure stimulated VDR gene expression and repressed CYP27B1 gene expression in both dose and time-dependent fashion in MPCT-G but not DKC-8 cells. The analysis of promoter activities and VDR protein levels corresponded with the gene expression data. We conclude that PTH-mediated decrease in VDR and increase in renal CYP27B1 is proximal cell-specific.

Extracellular calcium is a direct effecter of VDR levels in proximal tubule epithelial cells that counter-balances effects of PTH on renal Vitamin D metabolism

In renal proximal tubules, VDR is transiently decreased by parathyroid hormone (PTH) during times of hypocalcemia and returns to normal levels with the rise in serum calcium (Ca). In this study we tested the hypothesis that elevated extracellular Ca induces VDR in a human renal proximal cell line (HK-2G) stably expressing PTH receptor type I. Exposure of HK-2G cells to increasing Ca concentration, up to 3mM, induced the expression of VDR. The increase in VDR occurred within 1h and was sustained over 24h. The increase in VDR was also dose-dependently increased using 20-100 nM gadolinium, suggesting the induction of VDR is regulated via the extracellular Ca sensing receptor (CaSR) with is naturally expressed in HK-2G cells. In conclusion, an extracellular Ca concentration in the physiological range is capable of direct increase of renal proximal VDR expression, and the induction mechanism represents a strategy the body may use to counterbalance effects of PTH on renal Vitamin D metabolism.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Gene profiling the effects of calcium deficiency versus 1,25-dihydroxyvitamin D induced hypercalcemia in rat kidney cortex

Determinants involved in the activation and repression of 1,25-dihydroxyvitamin D (1,25(OH)(2)D(3)) synthesis in renal cortex by changes in extracellular Ca were studied. Cortical kidney RNA isolated from hypocalcemic (LC) rats generated by a low Ca diet, and hypercalcemic (HC) rats generated by a normal Ca diet and two sequential 1 microg doses of 1,25(OH)(2)D(3). Among the genes up-regulated were 1alpha-OHase (4.6-fold) in the LC group and high differential gene expression of VDR (4.0-fold) and 24-OHase (10.4-fold) in the HC group. Moreover, the exposure of renal cortex to LC versus HC conditions revealed a high differential expression of a PKA-dominated pathway involving CBP interacting protein, GATA-1 and CREB transcription factors in the LC model. In the HC model, elevated renal cortex gene expression of several growth factors, peptide receptors, and intracellular signaling molecules depicts a role for CaSR activation and receptor tyrosine kinase signaling in 1,25(OH)(2)D(3)-mediated gene activation and repression of 1alpha-OHase.

Vitamin D receptor is required for dietary calcium-induced repression of calbindin-D9k expression in mice

Calbindin (CaBP), the vitamin D-dependent calcium-binding protein, is believed to play an important role in intracellular calcium transport. The aim of this study was to investigate the effect of high dietary calcium on the expression of CaBP-D9k and CaBP-D28k in the presence and absence of a functional vitamin D receptor (VDR). Treatment with the HCa-Lac diet containing 2% calcium, 1.5% phosphorus and 20% lactose reversed the hypocalcemia seen in adult VDR-null mice in 3 weeks but did not significantly change the blood ionized calcium in wild-type mice. This dietary treatment dramatically suppressed both the duodenal and the renal CaBP-D9k expression in wild-type mice at both mRNA and protein levels but had little effect on the expression of the same gene in VDR-null mice. Removal of this diet gradually restored the expression of CaBP-D9k to the untreated level in wild-type mice. Only moderate or little change in CaBP-D28k expression was seen in wild-type and VDR-null mice fed with the HCa-Lac diet. The VDR content in the duodenum or kidney of wild-type mice was not altered by the dietary treatment. These results suggest that calcium regulates CaBP-D9k expression by modulating the circulating 1,25-dihydrxyvitamin D(3) level and that VDR is thus required for the dietary calcium-induced suppression of CaBP-D9k expression. Calcium regulation of the CaBP-D9k level may represent an important mechanism by which animals maintain their calcium balance.

Modulation of CYP27B1 and CYP24 mRNA expression in bone is independent of circulating 1,25(OH)2D3 levels

Circulating levels of 1,25-dihydroxyvitamin D (1,25D) are determined by bioactivation catalyzed by the renal 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1) and degradation through the action of the renal 25-hydroxyvitamin D 24-hydroxylase (CYP24). CYP27B1 and CYP24 are also present in bone cells, but little is known of their physiological role. The purpose of this study was to determine the changes that occur with aging on the expression of CYP27B1 and CYP24 mRNA in whole kidney and femora of female Sprague-Dawley rats. Real-time RT-PCR was used to measure CYP27B1, CYP24 and vitamin D receptor (VDR) mRNA levels in the kidneys and bones of animals aged between 3 weeks and 2 years. Circulating 1,25D levels decreased exponentially with age which was correlated with both reduced kidney CYP27B1 mRNA (R(2) = 0.72) and increased CYP24 mRNA levels (R(2) = 0.71). In the bone, CYP27B1 mRNA levels were maintained at their highest level throughout the ages of 3 to 15 weeks before decreasing in adult animals (P < 0.05). Bone CYP24 mRNA levels were positively correlated with bone CYP27B1 mRNA and not circulating 1,25D levels (R(2) = 0.74). Levels of bone CYP27B1 mRNA were positively correlated with distal femoral epiphyseal trabecular number (Tb.N) (R(2) = 0.74) and negatively with the trabecular thickness (Tb.Th) (R(2) = 0.56) in animals aged between 12 weeks and 2 years. These findings indicate that the regulation of CYP27B1 and CYP24 mRNA expression in the bone is unique from that in the kidney. The synthesis of 1,25D in bone tissue regulates bone CYP24 expression and is associated with bone mineralization suggesting that vitamin D metabolism has an autocrine or paracrine function.

Integration of hormone signaling in the regulation of human 25(OH)D3 24-hydroxylase transcription

The current study sought to define the molecular mechanisms involved in the cross talk between 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and activators of PKC in the regulation of 25(OH)D(3) 24-hydroxlyase [24(OH)ase]. Transfection of the h24(OH)ase promoter construct [-5,500/-22 luciferase; vitamin D response elements at -294/-274 and -174/-151; AP-1 site at -1,167/-1,160] in vitamin D receptor (VDR)-transfected COS-7 cells resulted in strong activation by 1,25(OH)(2)D(3). In these cells, cotreatment with the PKC activator TPA and 1,25(OH)(2)D(3) yielded a 27-fold increase in luciferase activity, which was 2- to 3-fold greater than activation obtained with 1,25(OH)(2)D(3) alone (P < 0.05). Similar results were observed using LLCPK-1 kidney cells, suggesting that the previously observed enhancement of 1,25(OH)(2)D(3)-induced renal 24(OH)ase mRNA and activity by PKC activation occurs at the level of transcription. The functional cooperation between PKC activation and VDR was not found to be mediated by the AP-1 site in the h24(OH)ase promoter or by enhanced binding of GRIP or DRIP205 to VDR and was also not due to PKC-mediated phosphorylation of VDR on Ser(51). Our study demonstrates that, in LLCPK-1 kidney cells, the PKC enhancement of 1,25(OH)(2)D(3)-stimulated transcription may be due, in part, to an increase in VDR concentration. In addition, inhibitors of the MAPK pathway were found to decrease the TPA enhancement (P < 0.05). Because activation of MAPK has been reported to result in the phosphorylation of SRC-1 and in functional cooperation between SRC-1 and CREB binding protein, we propose that the potentiation of VDR-mediated transcription may also be mediated through changes in the phosphorylation of specific VDR coregulators.

In vivo administration of 1,25-dihydroxyvitamin D3 suppresses the expression of RANKL mRNA in bone of thyroparathyroidectomized rats constantly infused with PTH

It is known that pharmacological or toxic doses of vitamin D induce bone resorption both in vivo and in vitro, whereas physiological doses of the vitamin have a protective effect on bone in vivo. To investigate the discrepancies of the dose-dependent effect of vitamin D on bone resorption, we examined the in vivo effect of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] on the expression of the receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG) mRNAs in bone of thyroparathyroidectomized (TPTX) rats infused with or without parathyroid hormone (PTH). Continuous infusion of 50 ng/h of PTH greatly increased the expression of RANKL mRNA in bone of TPTX rats. Expression of OPG mRNA was not altered by PTH infusion. When graded doses of 1,25(OH)(2)D(3) was daily administered orally for 14 days to normocalcemic TPTX rats constantly infused with PTH, 0.01 and 0.1 microg/kg of 1,25(OH)(2)D(3) inhibited the PTH-induced RANKL mRNA expression, but 0.5 microg/kg of the vitamin did not inhibit it. Regulator of G protein signaling-2 (RGS-2) gene expression was suppressed by 1,25(OH)(2)D(3) dose-dependently, but PTH/PTHrP receptor mRNA expression was not altered. Bone morphometric analyses revealed that 1,25(OH)(2)D(3) suppressed PTH-induced osteoclast number in vivo. These results suggest that pharmacological or toxic doses of 1,25(OH)(2)D(3) stimulate bone resorption by inducing RANKL, but a certain range of physiological doses of the vitamin inhibit PTH-induced bone resorption, the latter mechanism appeared to be mediated, at least in part, by the suppression of the PTH/PTHrP receptor-mediated signaling.

Gene expression of vitamin D hydroxylase and megalin in the remnant kidney of nephrectomized rats

BACKGROUND

Regulation of vitamin D hydroxylase genes in the early stage of chronic renal failure is not fully understood. Using nephrectomized rats, we examined changes in mRNA levels of CYP27B1 (25-hydroxyvitamin D3-1 alpha-hydroxylase), CYP24 (25-hydroxyvitamin D3-24-hydroxylase), and vitamin D receptor in relation to megalin, recently found to participate in renal vitamin D metabolism.

METHODS

A rat model of moderate renal failure was induced by 3/4 nephrectomy. Plasma parameters, including vitamin D metabolite concentrations, were measured at weeks 2, 4 and 8, and poly(A)+ RNA extracted from the remnant kidneys was subjected to Northern blot hybridization.

RESULTS

Plasma creatinine concentration at week 2 was 0.40 +/- 0.02 mg/dL in the sham-operated and 0.93 +/- 0.15 mg/dL in the nephrectomized rats, and both values remained constant up to week 8. Plasma concentrations of 25(OH)D3, 1 alpha,25(OH)2D3, and 24,25(OH)2D3 were unchanged between nephrectomized and sham-operated rats at week 8. Intact parathyroid hormone (PTH) increased at week 8 in nephrectomized rats. CYP27B1 mRNA in nephrectomized rats did not vary at week 2, but increased approximately two- and four-fold at weeks 4 and 8, respectively, compared to the sham-operated rats. CYP24 and megalin mRNAs, on the other hand, began to decline as early as at week 2 in nephrectomized rats and kept decreasing throughout the experiment. The expression of vitamin D receptor was modestly but significantly decreased only at week 8.

CONCLUSION

Coordinated and reciprocal alterations of the increase in CYP27B1 mRNA and the decrease in CYP24 mRNA may play a pivotal role in maintaining the plasma level of 1 alpha,25(OH)2D3 in the face of reduced nephron mass and/or megalin expression.

Regulation of renal vitamin D receptor is an important determinant of 1alpha,25-dihydroxyvitamin D(3) levels in vivo

The synthesis of 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) is most strongly regulated by dietary calcium and the action of parathyroid hormone to increase 1alpha-hydroxylase (1alpha-OHase) and decrease 24-hydroxylase (24-OHase) in kidney proximal tubules. This study examines the hypothesis that 1,25-(OH)(2)D(3) synthesis, induced by dietary calcium restriction, is also the result of negative feedback regulation blockade. Rats fed a low calcium (0.02%, -Ca) diet and given daily oral doses of vitamin D (0, 0.5, 1.0, 2.0, 4.0, 8.0, and 16.0 microg) remained hypocalcemic despite increasing levels of serum calcium in relation to the vitamin D dose. Plasma levels of 1,25-(OH)(2)D(3) rose to high levels (1200 pg/ml) at the high vitamin D dose levels. As expected, thyroparathyroidectomy caused a rapid fall in serum 1,25-(OH)(2)D(3). In rats fed a 0.47% calcium diet (+Ca) supplemented with vitamin D (4 microg/day), exogenous 1,25-(OH)(2)D(3) suppressed renal 1alpha-OHase and stimulated the 24-OHase. In rats fed the -Ca diet, vitamin D was unable to suppress the renal 1alpha-OHase or stimulate the renal 24-OHase. In contrast, vitamin D was fully able to stimulate intestinal 24-OHase. Intestinal vitamin D receptor (VDR) was present under all circumstances, while kidney VDR was absent under hypocalcemic conditions and present under normocalcemic conditions. It appears that tissue-specific down-regulation of VDR by hypocalcemia blocks the 1,25-(OH)(2)D(3) suppression of the 1alpha-OHase and upregulation of the 24-OHase in the kidney, causing a marked accumulation of 1,25-(OH)(2)D(3) in the plasma.

Hydroxylases involved in vitamin D metabolism are differentially expressed in murine embryonic kidney: application of whole mount in situ hybridization

In this study we examined the expression of 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase) and 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase) by RT-PCR and whole mount in situ hybridization using organ culture of kidney taken from mouse embryo. First, the kidneys of mouse embryo at 11.5-17.5 days gestation were cultured in the presence or absence of forskolin and 1,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)]. Forskolin and 1alpha,25-(OH)(2)D(3) induced the expression of 1alpha-hydroxylase and 24-hydroxylase, respectively, in a dose- and time-dependent manner. In the absence of stimulants, the expression of 1alpha-hydroxylase and 24-hydroxylase was detected from days 13.5-17.5 gestation. The expression of vitamin D receptor and megalin was detected from days 13.5 and 11.5, respectively. Next, signals for the expression of either 1alpha-hydroxylase or 24-hydroxylase were detected by whole mount in situ hybridization in kidney explants taken from embryo at 15.5 days gestation after the appropriate stimulation. However, the localization of signals differed between the two enzymes; 1alpha-hydroxylase messenger RNA was expressed in the inner area of the kidney explants, whereas 24-hydroxylase messenger RNA was expressed in the surface area. The expression of both hydroxylases was restricted to the epithelium of developing renal tubules. The pattern of megalin expression was similar to that of 1alpha-hydroxylase expression. To confirm the difference in distribution of 1alpha-hydroxylase and 24-hydroxylase transcripts, the explants were hybridized with probes for both 1alpha-hydroxylase and 24-hydroxylase using double labeling techniques after simultaneous stimulation with forskolin and 1alpha,25-(OH)(2)D(3), resulting in the detection at different locations of positive signals for the two enzymes. These results suggest that the expression of 1alpha-hydroxylase is induced in a distinct epithelium of renal tubules from that of 24-hydroxylase even at the early stage of kidney development before glomerulogenesis.

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in the human kidney

The secosteroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a vital role in calcium metabolism, tissue differentiation, and normal bone growth. Biosynthesis of 1,25(OH)2D3 is catalyzed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-hydroxylase). Although activity of this enzyme has been described in several tissues, the kidneys are recognized to be the principal site of 1,25(OH)2D3 production. To date, enzyme activity studies using vitamin D-deficient animals have suggested that 1alpha-hydroxylase is expressed exclusively in proximal convoluted tubules. With the recent cloning of 1alpha-hydroxylase, specific cRNA probes and in-house polyclonal antiserum have been used to determine the distribution of 1alpha-hydroxylase along the human nephron. Immunohistochemistry and in situ hybridization studies indicated strong expression of 1alpha-hydroxylase protein and mRNA in the distal convoluted tubule, the cortical and medullary part of the collecting ducts, and the papillary epithelia. Lower expression was observed along the thick ascending limb of the loop of Henle and Bowman's capsule. Weaker and more variable expression of 1alpha-hydroxylase protein and mRNA was seen in proximal convoluted tubules, and no expression was observed in glomeruli or vascular structures. These data show for the first time the distribution of alpha1-hydroxylase expression in normal human kidney. In contrast to earlier enzyme activity studies conducted in vitamin D-deficient animals, our data indicate that the distal nephron is the predominant site of 1alpha-hydroxylase expression under conditions of vitamin D sufficiency.

Calcitonin is a major regulator for the expression of renal 25-hydroxyvitamin D3-1alpha-hydroxylase gene in normocalcemic rats

Regulation of vitamin D metabolism has long been examined by using vitamin D-deficient hypocalcemic animals. We previously reported that, in a rat model of chronic hyperparathyroidism, expression of 25-hydroxyvitamin D3-1alpha-hydroxylase (CYP27B1) mRNA was markedly increased in renal proximal convoluted tubules. It is believed that the major regulator for the expression of renal CYP27B1 is parathyroid hormone (PTH). However, in the normocalcemic state, the mechanism to regulate the renal CYP27B1 gene could be different, since plasma levels of PTH are very low. In the present study, the effect of PTH and calcitonin (CT) on the expression of renal CYP27B1 mRNA was investigated in normocalcemic sham-operated rats and normocalcemic thyroparathyroidectomized (TPTX) rats generated by either PTH or CaCl2 infusion. A single injection of CT dose-dependently decreased the expression of vitamin D receptor mRNA in the kidney of normocalcemic sham-TPTX rats. Concomitantly, CT greatly increased the expression of CYP27B1 mRNA in the kidney of normocalcemic sham-TPTX rats. CT also increased the expression of CYP27B1 mRNA in the kidney of normocalcemic TPTX rats. Conversion of serum [3H]1alpha,25(OH)2D3 from 25-hydroxy[3H]vitamin D3 in vivo was also greatly increased by the injection of CT into sham-TPTX rats and normocalcemic TPTX rats, but not into hypocalcemic TPTX rats. In contrast, administration of PTH did not induce the expression of CYP27B1 mRNA in the kidney of vitamin D-replete sham-TPTX rats and hypocalcemic TPTX rats. PTH increased the expression of renal CYP27B1 mRNA only in vitamin D-deficient hypocalcemic TPTX rats. These results suggest that CT plays an important role in the maintenance of serum 1alpha,25(OH)2D3 under normocalcemic physiological conditions, at least in rats.

Novel findings about 24,25-dihydroxyvitamin D: an active metabolite?

The physiological role of 24,25-dihydroxyvitamin D remains controversial. Recent results suggest that 24,25-dihydroxyvitamin D is essential for fracture healing, and binding sites for 24,25-dihydroxyvitamin D have been identified in fracture callus tissue. Mice deficient in the 25-hydroxyvitamin D-24-hydroxylase enzyme provide novel genetic tools in which to study the role of 24,25-dihydroxyvitamin D in bone development and fracture repair.

The renal function of 25-hydroxyvitamin D3-1alpha-hydroxylase

The active, hormonal form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has numerous pleiotropic actions including the regulation of calcium homeostasis, control of bone cell differentiation and modification of immune responses. Synthesis of 1,25(OH)2D3 from the major circulating metabolite, 25-hydroxyvitamin D3 (25(OH)D3), is catalysed by the mitochondrial cytochrome P450 enzyme 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-HYD). Although 1alpha-HYD activity has been demonstrated at several ectopic sites, circulating levels of 1,25(OH)2D3 appear to reflect the expression of this enzyme in the kidney. The tight regulation of 1alpha-HYD in both renal and ectopic tissues has made studies of the expression and regulation of this enzyme remarkably difficult. However, the recent cloning of mouse, rat and human cDNAs for 1alpha-HYD has stimulated renewed interest in the molecular endocrinology of 1,25(OH)2D3 production. Analysis of the 1alpha-HYD sequence has revealed homology with the liver enzyme vitamin D-25-hydroxylase, and the ubiquitously expressed vitamin D-24-hydroxylase. Furthermore, mutations causing the inherited disorder vitamin D-dependent rickets type 1, also known as pseudo-vitamin D deficiency rickets have been described for the 1alpha-HYD gene and these have been mapped to chromosome 12q14 by linkage analysis. The availability of sequence information for the 1alpha-HYD gene has also facilitated the development of new molecular tools which will help to clarify key functions of the enzyme. Specific issues such as tissue distribution and regulatory pathways are discussed in this review, with particular emphasis on the role of 1alpha-HYD in renal calcium/phosphate homeostasis.

Expression of 25(OH)D3 24-hydroxylase in distal nephron: coordinate regulation by 1,25(OH)2D3 and cAMP or PTH

Previous studies using microdissected nephron segments reported that the exclusive site of renal 25-hydroxyvitamin D3-24-hydroxylase (24OHase) activity is the renal proximal convoluted tubule (PCT). We now report the presence of 24OHase mRNA, protein, and activity in cells that are devoid of markers of proximal tubules but express characteristics highly specific for the distal tubule. 24OHase mRNA was undetectable in vehicle-treated mouse distal convoluted tubule (DCT) cells but was markedly induced when DCT cells were treated with 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]. 24OHase protein and activity were also identified in DCT cells by Western blot analysis and HPLC, respectively. 8-Bromo-cAMP (1 mM) or parathyroid hormone [PTH-(1-34); 10 nM] was found to potentiate the effect of 1, 25(OH)2D3 on 24OHase mRNA. The stimulatory effect of cAMP or PTH on 24OHase expression in DCT cells suggests differential regulation of 24OHase expression in the PCT and DCT. In the presence of cAMP and 1, 25(OH)2D3, a four- to sixfold induction in vitamin D receptor (VDR) mRNA was observed. VDR protein, as determined by Western blot analysis, was also enhanced in the presence of cAMP. Transient transfection analysis in DCT cells with rat 24OHase promoter deletion constructs demonstrated that cAMP enhanced 1, 25(OH)2D3-induced 24OHase transcription but this enhancement was not mediated by cAMP response elements (CREs) in the 24OHase promoter. We conclude that 1) although the PCT is the major site of localization of 24OHase, 24OHase mRNA and activity can also be localized in the distal nephron; 2) both PTH and cAMP modulate the induction of 24OHase expression by 1,25(OH)2D3 in DCT cells in a manner different from that reported in the PCT; and 3) in DCT cells, upregulation of VDR levels by cAMP, and not an effect on CREs in the 24OHase promoter, is one mechanism involved in the cAMP-mediated modulation of 24OHase transcription.

Regulation of type II renal Na+-dependent inorganic phosphate transporters by 1,25-dihydroxyvitamin D3. Identification of a vitamin D-responsive element in the human NAPi-3 gene

Vitamin D is an important regulator of phosphate homeostasis. The effects of vitamin D on the expression of renal Na+-dependent inorganic phosphate (Pi) transporters (types I and II) were investigated. In vitamin D-deficient rats, the amounts of type II Na+-dependent Pi transporter (NaPi-2) protein and mRNA were decreased in the juxtamedullary kidney cortex, but not in the superficial cortex, compared with control rats. The administration of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) to vitamin D-deficient rats increased the initial rate of Pi uptake as well as the amounts of NaPi-2 mRNA and protein in the juxtamedullary cortex. The transcriptional activity of a luciferase reporter plasmid containing the promoter region of the human type II Na+-dependent Pi transporter NaPi-3 gene was increased markedly by 1,25-(OH)2D3 in COS-7 cells expressing the human vitamin D receptor. A deletion and mutation analysis of the NaPi-3 gene promoter identified the vitamin D-responsive element as the sequence 5'-GGGGCAGCAAGGGCA-3' nucleotides -1977 to -1963 relative to the transcription start site. This element bound a heterodimer of the vitamin D receptor and retinoid X receptor, and it enhanced the basal transcriptional activity of the promoter of the herpes simplex virus thymidine kinase gene in an orientation-independent manner. Thus, one mechanism by which vitamin D regulates Pi homeostasis is through the modulation of the expression of type II Na+-dependent Pi transporter genes in the juxtamedullary kidney cortex.

Cloning and expression of rat 25-hydroxyvitamin D3-1alpha-hydroxylase cDNA

A full-length cDNA for the rat kidney mitochondrial cytochrome P450 mixed function oxidase, 25-hydroxyvitamin D3-1alpha-hydroxylase (P4501alpha), was cloned from a vitamin D-deficient rat kidney cDNA library and subcloned into the mammalian expression vector pcDNA 3.1(+). When P4501alpha cDNA was transfected into COS-7 transformed monkey kidney cells, they expressed 25-hydroxyvitamin D3-1alpha-hydroxylase activity. The sequence analysis showed that P4501alpha was of 2,469 bp long and contained an ORF encoding 501 amino acids. The deduced amino acid sequence showed a 53% similarity and 44% identity to the vitamin D3-25-hydroxylase (CYP27), whereas it has 42.6% similarity and 34% identity with the 25-hydroxyvitamin D3-24-hydroxylase (CYP24). Thus, it composes a new subfamily of the CYP27 family. Further, it is more closely related to the CYP27 than to the CYP24. The expression of P4501alpha mRNA was greatly increased in the kidney of vitamin D-deficient rats. In rats with the enhanced renal production of 1alpha,25-dihydroxyvitamin D3 (rats fed a low Ca diet), P4501alpha mRNA was greatly increased in the renal proximal convoluted tubules.

24R,25-dihydroxyvitamin D3: an essential vitamin D3 metabolite for both normal bone integrity and healing of tibial fracture in chicks

We tested the hypothesis that 24R,25-dihydroxyvitamin D3 [24R,25-(OH)2D3] is an essential vitamin D metabolite for the development of normal bone integrity and the healing of fractures. The natural 24R,25-(OH)2D3 and its synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25-(OH)2D3] were tested alone or in combination with 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], on normal bone development and other related variables of the Ca2+ homeostasis system [serum Ca2+, 25-hydroxyvitamin D3 (25OHD3), 24,25-(OH)2D3, and 1alpha,25-(OH)2D3 levels] in chicks. Mechanical testing of torsional strength was carried out on the femur. 24R,25-(OH)2D3 (80 nmol/kg diet) alone was sufficient for normal bone growth and integrity similar to that achieved by the vitamin D3-replete controls. Next, chicks were fed a 25OHD3-replete diet (75 nmol/kg diet) for 8 days after hatching, and then 25OHD3 was withdrawn to minimize any residual circulating metabolites before the imposition of standardized tibial fractures 14 days later. Vitamin D metabolites were administered for 2 weeks to determine their effects on the mechanical properties of healed tibia. 24S,25-(OH)2D3 combined with 1alpha,25-(OH)2D3 or 1alpha,25-(OH)2D3 alone resulted in poor healing [strength values of 0.158 +/- 0.011 and 0.123 +/- 0.009 Nm (Newton x meter), respectively] compared with that in the 25OHD3-treated control group (0.374 +/- 0.029 Nm). In contrast, the fractured tibia of the birds fed 24R,25-(OH)2D3 in combination with 1alpha,25-(OH)2D3 showed healing equivalent to that in the control group, with strength values of 0.296 +/- 0.043 Nm. These results suggest that when 24R,25-(OH)2D3 is present at normal physiological concentrations, it is an essential vitamin D3 metabolite for both normal bone integrity and healing of fracture in chicks.

Three-fold induction of renal 25-hydroxyvitamin D3-24-hydroxylase activity and increased serum 24,25-dihydroxyvitamin D3 levels are correlated with the healing process after chick tibial fracture

To investigate the possible biological actions of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3), a tibial fracture-healing model was established in White Leghorn chicks. Three-week-old White Leghorn chicks fed a vitamin D3-replete diet were divided into four groups (control, anesthetized, sham, and fractured). On varying days after tibial fracture (F) or sham manipulation (S), renal 25(OH)D3-1 alpha-hydroxylase and 25(OH)D3-24-hydroxylase (24-hydroxylase) activities and serum Ca2+ concentrations were measured. Metofane anesthesia was found to have no effect on the activity of either of the hydroxylases; the activities of the hydroxylases in the control, anesthetized, and sham-operated birds were similar. By 10 days after tibial fracture, the renal 24-hydroxylase activity increased more than 3-fold in F (1.33 +/- 0.07 pmol/mg of protein) as compared with S (0.42 +/- 0.03 pmol/mg of protein) (p < 0.0001). A time-dependent study of the renal 24-hydroxylase activity during the fracture repair process revealed a slow increase from the first day after fracture, a higher activity at 8 days, which peaked at 10-11 days, which is consistent with the formation of the callus. The 24-hydroxylase activity then returned to the same level as the sham group 14 days after fracture. There was no significant difference in serum Ca2+ levels between the F and S groups over the 3-week postfracture period. Serum levels of vitamin D3 metabolites were also measured during the fracture healing process: a 3.4x increase of the 24,25(OH)2D3 level in the fractured group (3.64 +/- 1.16 nM) was observed as compared with the control groups (1.08 +/- 0.49 nM) at 10 days after fracture (p = 0.068). No significant differences were observed in the plasma levels of 25(OH)D3 or 1 alpha, 25(OH)2D3 between the group with a fracture and the controls. Exposure of primary chick kidney cells in culture to serum obtained from chicks with a tibial fracture for 20 h resulted in an approximately 40% increase in the activity of the 24-hydroxylase as compared with cells exposed to serum from control birds. These results suggest that 24,25(OH)2D3 is involved in the early process of fracture repair and that there is some form of physiological communication between the fractured bone and the kidney so as to increase the renal 24-hydroxylase and the circulating concentration of this metabolite.

Functional assessment of two vitamin D-responsive elements in the rat 25-hydroxyvitamin D3 24-hydroxylase gene

Two vitamin D-responsive elements (VDRE-1 and VDRE-2) were recently identified in the 5'-upstream region of the rat 25-hydroxyvitamin D3 24-hydroxylase gene at -151/-137 and -259/-245, respectively. We studied the transcriptional regulation of this gene by vitamin D by means of mutational analysis. Introducing mutations into VDRE-1 and VDRE-2 in the native promoter -291/+9 reduced vitamin D-dependent chloramphenicol acetyltransferase activity by 86 and 41%, respectively. Mutation of the direct repeat -169/-155 located at 3 base pairs upstream of VDRE-1 also caused 50% decrease of chloramphenicol acetyltransferase activity. Connection of the element -169/-155 to VDRE-1 enhanced the vitamin D responsiveness of VDRE-1 5-fold through the heterologous beta-globin promoter. The fragment -291/-102 containing the two VDREs showed two shifted bands in the presence of the vitamin D receptor and retinoid X receptor in gel retardation analysis, and the appearance of the slower migrating band indicates that two sets of receptor complexes bind to this fragment simultaneously. These results demonstrate that VDRE-1 is a stronger mediator of vitamin D function than VDRE-2 due to the presence of the accessory element -169/-155 located adjacent to VDRE-1, although VDRE-2 exhibits a smaller dissociation constant for the vitamin D receptor-retinoid X receptor complex than VDRE-1.

Preferential accumulation in vivo of 24R,25-dihydroxyvitamin D(3) in growth plate cartilage of rats

Vitamin D(3) is metabolized in vivo through 25-(OH)D(3) (25D) to both 1α,25-(OH)(2)D(3) (1,25D) and 24R,25-(OH)(2)D(3) (24,25D). Whereas it is assumed that this metabolism occurs primarily in the kidney, recent studies show that there are extrarenal 1α-and 24R-hydroxylase activities as well, and in chondrocytes, these enzymes are regulated by hormones and growth factors. Furthermore, chondrocytes from the resting zone of growth plate cartilage are a target cell population for 24,25D action, suggesting that this vitamin D metabolite may be targeted to this tissue in vivo. To test this hypothesis, 30 normal male Sprague Dawley rats (120 ±20 g) were divided into three groups of eight animals each, and a control group of six animals, and fed ad libitum for 2 wk, a standard rat chow (Teklad LM-485), which contained 3 IU vitamin D(3)/g. The rats were then injected im daily at 9:00AM: , for 4 consecutive d, with 0.1 mL of either [(3)H]-25D, [(3)H]-1,25D or [(3)H]-24,25D. Each dose contained 13 pmol of hormone (0.36 μCi/dose). The distribution of these metabolites was assessed in tibial bone (B) following ablation of the bone marrow, articular cartilage from the tibia (AC), costochondral growth plate cartilage (GC), serum (S), small intestine (I), and kidney (K). The use of high specific activity tritiated vitamin D metabolites facilitated determining tissue localization and further metabolism without perturbation of the body pools of each major metabolite. Accumulation of [(3)H]-1,25D or [(3)H]-24,25D in each tissue was compared to circulating serum levels. In rats dosed with [(3)H]-25D, the tissue:serum ratios for 1,25D were 4.1 (AC), 35.4 (GC), 1.3 (B), 0.7 (K), and 3.0 (I); and tissue:serum ratios for 24,25D were 1.6 (AC), 9.9 (GC), 0.04 (B), 0.2 (K), and 0.4 (I). In rats dosed with [(3)H]-24,25D alone, GC was the only tissue to accumulate the administered metabolite at a concentration significantly higher than that of serum. Similarly, in rats dosed with [(3)H]-1,25D alone, GC was the only tissue to accumulate 1,25D at a concentration higher than that of serum. These results demonstrate, for the first time, that under in vivo conditions, GC specifically accumulates 24,25D and 1,25D. This suggests that growth plate may be a target organ for these two hormones.