The roles of calcium and 1,25-dihydroxyvitamin D3 in the regulation of vitamin D receptor expression by rat parathyroid glands

Mechanism of calcitriol regulating parathyroid cells in secondary hyperparathyroidism

A common consequence of chronic renal disease is secondary hyperparathyroidism (SHPT) and is closely related to the mortality and morbidity of uremia patients. Secondary hyperparathyroidism (SHPT) is caused by excessive PTH production and release, as well as parathyroid enlargement. At present, the mechanism of cell proliferation in secondary hyperparathyroidism (SHPT) is not completely clear. Decreased expression of the vitamin D receptor (VDR) and calcium-sensing receptor (CaSR), and 1,25(OH)2D3 insufficiency all lead to a decrease in cell proliferation suppression, and activation of multiple pathways is also involved in cell proliferation in renal hyperparathyroidism. The interaction between the parathormone (PTH) and parathyroid hyperplasia and 1,25(OH)2D3 has received considerable attention. 1,25(OH)2D3 is commonly applied in the therapy of renal hyperparathyroidism. It regulates the production of parathormone (PTH) and parathyroid cell proliferation through transcription and post-transcription mechanisms. This article reviews the role of 1,25(OH)2D3 in parathyroid cells in secondary hyperparathyroidism and its current understanding and potential molecular mechanism.

Mechanical stress regulates bone regulatory gene expression independent of estrogen and vitamin D deficiency in rats

Mechanical stress determines bone mass and structure. It is not known whether mechanical loading affects expression of bone regulatory genes in a combined deficiency of estrogen and vitamin D. We studied the effect of mechanical loading on the messenger RNA (mRNA) expression of bone regulatory genes during vitamin D and/or estrogen deficiency. We performed a single bout in vivo axial loading with 14 N peak load, 2 Hz frequency and 360 cycles in right ulnae of nineteen weeks old female control Wistar rats with or without ovariectomy (OVX), vitamin D deficiency and the combination of OVX and vitamin D deficiency (N = 10/group). Total bone RNA was isolated 6 hours after loading, and mRNA expression was detected of Mepe, Fgf23, Dmp1, Phex, Sost, Col1a1, Cyp27b1, Vdr, and Esr1. Serum levels of 25(OH)D, 1,25(OH)2 D and estradiol were also measured at this time point. The effect of loading, vitamin D and estrogen deficiency and their interaction on bone gene expression was tested using a mixed effect model analysis. Mechanical loading significantly increased the mRNA expression of Mepe, and Sost, whereas it decreased the mRNA expression of Fgf23 and Esr1. Mechanical loading showed a significant interaction with vitamin D deficiency with regard to mRNA expression of Vdr and Esr1. Mechanical loading affected gene expression of Mepe, Fgf23, Sost, and Esr1 independently of vitamin D or estrogen, indicating that mechanical loading may affect bone turnover even during vitamin D deficiency and after menopause.

Inhibition of vitamin D analog eldecalcitol on hepatoma in vitro and in vivo

Hepatoma is a serious liver cancer with high morbidity and mortality. Eldecalcitol (ED-71), a vitamin D analog, is extensively used as anti-cancer agent in vitro. Hepatocellular carcinoma cell, SMMC-7721 cell lines were used in this study. Transwell assay, cell apoptosis and cell cycle detection assays were investigated after treatment with ED-71 and phosphate buffered saline (PBS) as control. Sizes of tumors were measured after ED-71 treatment in a mouse model. E-cadherin and Akt gene expressions were detected by real-time PCR (RT-PCR). The results showed that cell invasion and migration were decreased markedly after ED-71 treatment compared to control group. Cell cycle detection showed that the G2 stage was 13.18% and total S-stage was 41.16% in the ED-71 group and G2 stage: 22.88%, total S-stage: 27.34% in the control group. Cell apoptosis rate was promoted in the ED-71 group. Size of the tumors reduced more after the ED-71 treatment than the PBS treatment in mice. ED-71 markedly inhibited the expression of Akt and E-cadherin, either detected by immunohistochemistry or RT-PCR. ED-71 treatment can inhibit the hepatoma agent proliferation by increasing the E-cadherin expression and decreasing Akt expression. Therefore, these findings provide novel evidence that ED-71 can be used as an anti-hepatoma agent.

The Use of Vitamin D Metabolites and Analogues in the Treatment of Chronic Kidney Disease

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are associated with abnormalities in bone and mineral metabolism, known as CKD-bone mineral disorder. CKD and ESRD cause skeletal abnormalities characterized by hyperparathyroidism, mixed uremic osteodystrophy, osteomalacia, adynamic bone disease, and frequently enhanced vascular and ectopic calcification. Hyperparathyroidism and mixed uremic osteodystrophy are the most common manifestations due to phosphate retention, reduced concentrations of 1,25-dihydroxyvitamin D, intestinal calcium absorption, and negative calcium balance. Treatment with 1-hydroxylated vitamin D analogues is useful.

Vitamin D receptor agonist VS-105 directly modulates parathyroid hormone expression in human parathyroid cells and in 5/6 nephrectomized rats

Vitamin D receptor (VDR) agonists (VDRAs) are commonly used to treat secondary hyperparathyroidism (SHPT) associated with chronic kidney disease (CKD). Current VDRA therapy often causes hypercalcemia, which is a critical risk for vascular calcification. Previously we have shown that a novel VDRA, VS-105, effectively suppresses serum parathyroid hormone (PTH) without affecting serum calcium levels in 5/6 nephrectomized (NX) uremic rats. However, it is not known whether VS-105 directly regulates PTH gene expression. To study the direct effect of VS-105 on modulating PTH, we tested VS-105 and paricalcitol in the spheroid culture of parathyroid cells from human SHPT patients, and examined the time-dependent effect of the compounds on regulating serum PTH in 5/6 NX uremic rats (i.p. 3x/week for 14days). In human parathyroid cells, VS-105 (100nM) down-regulated PTH mRNA expression (to 3.6% of control) and reduced secreted PTH (to 43.9% of control); paricalcitol was less effective. VS-105 effectively up-regulated the expression of VDR (1.9-fold of control) and CaSR (1.8-fold of control) in spheroids; paricalcitol was also less effective. In 5/6 NX rats, one single dose of 0.05-0.2μg/kg of VS-105 or 0.02-0.04μg/kg of paricalcitol effectively reduced serum PTH by >40% on Day 2. Serum PTH remained suppressed during the dosing period, but tended to rebound in the paricalcitol groups. These data indicate that VS-105 exerts a rapid effect on suppressing serum PTH, directly down-regulates the PTH gene, and modulates PTH, VDR and CaSR gene expression more effectively than paricalcitol.

Fibroblast Growth Factor 23 Regulation by Systemic and Local Osteoblast-Synthesized 1,25-Dihydroxyvitamin D

Circulating levels of fibroblast growth factor 23 (FGF23) increase during the early stages of kidney disease, but the underlying mechanism remains incompletely characterized. We investigated the role of vitamin D metabolites in regulating intact FGF23 production in genetically modified mice without and with adenine-induced uremia. Exogenous calcitriol (1,25-dihydroxyvitamin D) and high circulating levels of calcidiol (25-hydroxyvitamin D) each increased serum FGF23 levels in wild-type mice and in mice with global deficiency of the Cyp27b1 gene encoding 25-hydroxyvitamin D 1-α-hydroxylase, which produces 1,25-hydroxyvitamin D. Compared with wild-type mice, normal, or uremic mice lacking Cyp27b1 had lower levels of serum FGF23, despite having high concentrations of parathyroid hormone, but administration of exogenous 1,25-dihydroxyvitamin D increased FGF23 levels. Furthermore, raising serum calcium levels in Cyp27b1-depleted mice directly increased FGF23 levels and indirectly enhanced the action of ambient vitamin D metabolites via the vitamin D receptor. In chromatin immunoprecipitation assays, 25-hydroxyvitamin D promoted binding of the vitamin D receptor and retinoid X receptor to the promoters of osteoblastic target genes. Conditional osteoblastic deletion of Cyp27b1 caused lower serum FGF23 levels, despite normal circulating levels of vitamin D metabolites. In adenine-induced uremia, only a modest increase in serum FGF23 levels occurred in mice with osteoblastic deletion of Cyp27b1 (12-fold) compared with a large increase (58-fold) in wild-type mice. Therefore, in addition to the direct effect of high circulating concentrations of 25-hydroxyvitamin D, local osteoblastic conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D appears to be an important positive regulator of FGF23 production, particularly in uremia.

Effects of 1α,25-(OH)2D3 on the formation and activity of osteoclasts in RAW264.7 cells

The hormonally active form of vitamin D3, 1α,25-(OH)2D3, has an important role in bone metabolism. This study examined the effects of 1α,25-(OH)2D3 on the ability of two cytokines, receptor activator of nuclear factor-κB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF), to induce RAW 264.7 cells to form osteoclasts. A TRAP histochemical staining assay and bone resorption analysis were used to identify the rate of formation and activity of osteoclasts. The numbers of osteoclasts formed, and their bone resorption activity, was enhanced by the addition of 1α,25-(OH)2D3. The expression levels of osteoclast-specific proteins that are essential for bone resorption, integrin β3, V-ATPase, CAII, CTSK, TRAP and MMP-9, were detected by western blotting. During 48 h, the expression levels of all these proteins significantly increased. Quantitative real-time polymerase chain reaction was used to determine the expression levels of the transcription factors, c-Fos and NFATcl. The expression levels of c-Fos and NFATc1 also increased 24h after treatment with 1α,25-(OH)2D3. These results suggest that 1α,25-(OH)2D3 can regulate bone metabolism by directly enhancing the formation and maturation of osteoclasts.

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

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

No difference between alfacalcidol and paricalcitol in the treatment of secondary hyperparathyroidism in hemodialysis patients: a randomized crossover trial

Alfacalcidol and paricalcitol are vitamin D analogs used for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease, but have known dose-dependent side effects that cause hypercalcemia and hyperphosphatemia. In this investigator-initiated multicenter randomized clinical trial, we originally intended two crossover study periods with a washout interval in 86 chronic hemodialysis patients. These patients received increasing intravenous doses of either alfacalcidol or paricalcitol for 16 weeks, until parathyroid hormone was adequately suppressed or calcium or phosphate levels reached an upper threshold. Unfortunately, due to a period effect, only the initial 16-week intervention period for 80 patients was statistically analyzed. The proportion of patients achieving a 30% decrease in parathyroid hormone levels over the last four weeks of study was statistically indistinguishable between the two groups. Paricalcitol was more efficient at correcting low than high baseline parathyroid hormone levels, whereas alfacalcidol was equally effective at all levels. There were no differences in the incidence of hypercalcemia and hyperphosphatemia. Thus, alfacalcidol and paricalcitol were equally effective in the suppression of secondary hyperparathyroidism in hemodialysis patients while calcium and phosphorus were kept in the desired range.

Ethanol extract of Fructus Ligustri Lucidi increases circulating 1,25-dihydroxyvitamin D3 by inducing renal 25-hydroxyvitamin D-1α hydroxylase activity

OBJECTIVE

The present study was designed to determine whether Fructus Ligustri Lucidi (FLL) ethanol extract can directly regulate vitamin D metabolism both in vivo and in vitro.

METHODS

Eleven-month-old, aged Sprague-Dawley sham-operated and ovariectomized (OVX) female rats were fed a normal-calcium (Ca) diet (0.6% Ca, 0.65% phosphorus) and received either FLL (700 mg/kg) or vehicle daily for 12 weeks. The in vitro effects of FLL on vitamin D metabolism were studied using primary cultures of the rat renal proximal tubules. mRNA and protein expressions of 25-hydroxyvitamin D-1α hydroxylase (1-OHase) and vitamin D receptor (VDR) in the kidney and proximal tubule were measured using real-time polymerase chain reaction and Western blotting, respectively. The concentrations of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) synthesized by renal 1-OHase were measured by a competitive enzyme immunoassay.

RESULTS

FLL treatment significantly increased serum 1,25(OH)2D3 levels in both sham (P < 0.01) and OVX (P < 0.05) rats. FLL increased renal 1-OHase and VDR protein and mRNA expressions in sham rats. Protein expression of renal 1-OHase, but not VDR, was also up-regulated in OVX rats during FLL treatment. 1-OHase mRNA and 1-OHase activity were increased by FLL treatment in primary cultures of renal proximal tubule cells.

CONCLUSIONS

FLL could increase the circulating levels of 1,25(OH)2D3 in vivo in aged female rats by directly stimulating 1-OHase activity. Thus, it might be an ideal oral agent that can help to improve the ability to induce 1,25(OH)2D3 synthesis and Ca balance in postmenopausal women who are of high risk of developing osteoporosis.

Calcium-sensing receptor expression is regulated by glial cells missing-2 in human parathyroid cells

Glial cells missing-2 (Gcm2) is the key regulating transcription factor for parathyroid gland development. The continued expression of high levels of Gcm2 in mature parathyroid glands suggests that it is required for maintenance of parathyroid cell differentiation. The role of Gcm2 in parathyroid cell physiology, however, has not been fully studied. In this study, we examined the effects of Gcm2 silencing on cultured human parathyroid cells. Collagenase-dispersed human parathyroid cells from patients with chronic kidney disease were placed in monolayer cultures and infected with lentivirus expressing shRNA for human Gcm2. Seventy-two hours after infection, mRNA was processed and analyzed for Gcm2, PTH, vitamin D receptor (VDR), calcium-sensing receptor (CaR), 25-hydroxyvitamin D(3) 1-alpha-hydroxylase (1-OHase), and proliferating cell nuclear antigen (PCNA) by real-time PCR (qPCR). Protein expression of affected genes was analyzed by immunoblot 72 h after infection. Gcm2 mRNA and protein were decreased by 74.2 +/- 12.2% (SD; n = 3 experiments; p < 0.01) and 67.5 +/- 15.7% (n = 2; p < 0.01), respectively. CaR mRNA and protein were reduced by 47.8 +/- 21.1% (n = 3; p < 0.01) and 48.1 +/- 4.3% (n = 3; p < 0.01), respectively. However, VDR, PTH, 1-OHase, and PCNA were not significantly affected by Gcm2 silencing. Further analysis of CaR mRNA indicated that transcripts containing exon 1B, derived by transcription from CaR promoter 2, were downregulated (58.8 +/- 19.27%; n = 3; p < 0.05) by Gcm2 silencing. Exon 1A-containing transcripts from promoter 1 were expressed at very low levels in the cultures. These results indicate that one function of Gcm2 is to maintain high levels of CaR expression in parathyroid cells.

K+-dependent Na+/Ca2+ exchanger 3 is involved in renal active calcium transport and is differentially expressed in the mouse kidney

Previously, we reported that renal active calcium-transporting genes are highly expressed in female mice and suggested that renal calcium-processing genes play a critical role in normal calcium reabsorption in females (Lee GS, Lee KY, Choi KC, Ryu YH, Paik SG, Oh GT, Jeung EB. J Bone Miner Res 22: 1968-1978, 2007). In the current study, we evaluated the differential expression of renal calcium-processing genes in male and female mice. Using microarray analysis, we identified K(+)-dependent Na(+)/Ca(2+) exchanger 3 (NCKX3) as a gene that was differentially expressed in the kidneys of female and male mice. The expression levels of renal NCKX3 mRNA and protein were higher in female than in male mice, whereas there was no difference between the genders in the levels of NCKX3 expression in the brain. Renal NCKX3 localized to the basolateral layer of distal convoluted tubules, indicating that this protein participates in renal calcium reabsorption. To identify putative regulators in the gender-specific expression of NCKX3, several hormones were injected into mature female and male mice. Although any hormones did not alter NCKX3 expression, adrenal gland-secreted hormones aldosterone and hydrocortisone did downregulate renal NCKX3 mRNA expression in female mice, but they did not change its protein levels. Taken together, the results in this study suggest that a high level of renal NCKX3 expression maintain in distal convoluted tubules may play a role in active calcium transport in the kidneys of female mice.

Dietary calcium and 1,25-dihydroxyvitamin D3 regulate transcription of calcium transporter genes in calbindin-D9k knockout mice

The effect(s) of oral calcium and vitamin D(3) were examined on the expression of duodenal and renal active calcium transport genes, i.e., calbindin-D9k (CaBP-9k) and calbindin-D28k (CaBP-28k), transient receptor potential cation channels (TRPV5 and TRPV6), Na(+)/Ca(2+) exchanger 1 (NCX1) and plasma membrane calcium ATPase 1b (PMCA1b), in CaBP-9k KO mice. Wild-type (WT) and KO mice were provided with calcium and vitamin D(3)-deficient diets for 10 weeks. The deficient diet significantly decreased body weights compared with the normal diet groups. The serum calcium concentration of the WT mice was decreased by the deficient diet but was unchanged in the KO mice. The deficient diet significantly increased duodenal transcription of CaBP-9k and TRPV6 in the WT mice, but no alteration was observed in the KO mice. In the kidney, the deficient diet significantly increased renal transcripts of CaBP-9k, TRPV6, PMCA1b, CaBP-28k and TRPV5 in the WT mice but did not alter calcium-relating genes in the KO mice. Two potential mediators of calcium-processing genes, vitamin D receptor (VDR) and parathyroid hormone receptor (PTHR), have been suggested to be useful for elucidating these differential regulations in the calcium-related genes of the KO mice. Expression of VDR was not significantly affected by diet or the KO mutation. Renal PTHR mRNA levels were reduced by the diet, and reduced expression was also seen in the KO mice given the normal diet. Taken together, these results suggest that the active calcium transporting genes in KO mice may have resistance to the deficiency diet of calcium and vitamin D(3).

Direct upregulation of parathyroid calcium-sensing receptor and vitamin D receptor by calcimimetics in uremic rats

To investigate whether the effect of the calcimimetic AMG 641 and calcitriol on CaSR and VDR expression could be separated from their ability to reduce parathyroid cell proliferation, five-sixth nephrectomized (5/6 Nx) rats received vehicle, AMG 641, calcitriol, or AMG 641+calcitriol either daily for 13 days (long-term protocol) or in a single dose (short-term protocol). In the long-term protocol, AMG 641, calcitriol, and their combination significantly reduced the percentage of proliferating parathyroid cells. Proliferation was uncontrolled in the short-term protocol. A significant increase in CaSR mRNA (% vs. beta-actin) was detected in rats treated with both calcitriol (1.60 +/- 0.30) and AMG 641 (1.66 +/- 0.25) for 13 days (P = 0.01 vs. 5/6 Nx+vehicle, 0.89 +/- 0.09); and there was a further increase when both drugs were administered simultaneously (2.46 +/- 0.33). In the short-term protocol, only rats receiving AMG 641 alone (2.01 +/- 0.33, P < 0.001) showed increased expression of CaSR mRNA, whereas the combination (1.81 +/- 0.20) produced no additional benefit. AMG 641 also increased CaSR mRNA expression in vitro. Changes in VDR mRNA paralleled those of CaSR mRNA. In the long-term treatment, both AMG 641 (0.87 +/- 0.14) and calcitriol (0.99 +/- 0.12) increased VDR mRNA (P < 0.05 vs. 5/6 Nx+vehicle, 0.49 +/- 0.10), and the increase was more accentuated when the drugs were combined (1.49 +/- 0.45). In the short-term protocol, only treatment with AMG 641, alone (1.52 +/- 0.41) or combined with calcitriol (1.86 +/- 0.24), increased VDR mRNA. In conclusion, our results demonstrate an acute increase in CaSR mRNA and VDR mRNA in the parathyroid glands of uremic rats treated with AMG 641, in which cell proliferation was uncontrolled, thus supporting a direct effect of calcimimetics on CaSR and VDR expression by hyperplastic parathyroid cells.

Phosphate: known and potential roles during development and regeneration of teeth and supporting structures

Inorganic phosphate (P(i)) is abundant in cells and tissues as an important component of nucleic acids and phospholipids, a source of high-energy bonds in nucleoside triphosphates, a substrate for kinases and phosphatases, and a regulator of intracellular signaling. The majority of the body's P(i) exists in the mineralized matrix of bones and teeth. Systemic P(i) metabolism is regulated by a cast of hormones, phosphatonins, and other factors via the bone-kidney-intestine axis. Mineralization in bones and teeth is in turn affected by homeostasis of P(i) and inorganic pyrophosphate (PPi), with further regulation of the P(i)/PP(i) ratio by cellular enzymes and transporters. Much has been learned by analyzing the molecular basis for changes in mineralized tissue development in mutant and knock-out mice with altered P(i) metabolism. This review focuses on factors regulating systemic and local P(i) homeostasis and their known and putative effects on the hard tissues of the oral cavity. By understanding the role of P(i) metabolism in the development and maintenance of the oral mineralized tissues, it will be possible to develop improved regenerative approaches.

Vitamin D in chronic kidney disease: A systemic role for selective vitamin D receptor activation

Hyperparathyroidism occurs in most patients during the progression of chronic kidney disease (CKD) and one of its initiating events, reduced serum levels of 1,25-dihydroxyvitamin D, results from a decrease in renal 1alpha hydroxylase activity, which converts 25-hydroxyvitamin D to its activated form. The combination of persistently high parathyroid hormone (PTH) and low 1,25-dihydroxyvitamin D is associated with bone loss, cardiovascular disease, immune suppression and increased mortality in patients with end-stage kidney failure. Recent studies in dialysis patients suggest that paricalcitol, a selective activator of the vitamin D receptor (VDR), is associated with a more favorable efficacy to side effect profile than calcitriol, with less morbidity and better survival. One hypothesis derived from such studies suggests that systemic activation of VDRs may have direct effects on the cardiovascular system to decrease mortality in CKD. Although current guidelines for regulating serum calcium, phosphate and PTH recommend specific interventions at the various stages of CKD to prevent or postpone irreversible parathyroid disease and decrease cardiovascular morbidity and mortality, emerging data suggest that vitamin D therapy may prolong survival in this patient population by mechanisms that are independent of calcium, phosphate and PTH. It is suggested that a re-evaluation of current treatment recommendations is needed and that future research should focus on mechanisms that distinguish potential tissue specific benefits of selective VDR activators in patients with CKD.

VITAMIN D IN HEALTH AND DISEASE: Beyond Minerals and Parathyroid Hormone: Role of Active Vitamin D in End-Stage Renal Disease

Secondary hyperparathyroidism is a common complication of end-stage renal disease (ESRD) that is often treated with activated forms of intravenous vitamin D. The natural course and treatment of secondary hyperparathyroidism in hemodialysis patients is punctuated by episodes of hypercalcemia, hyperphosphatemia, and increased calcium-phosphate product, which in previous studies were linked to increased mortality. Historically these episodes have been attributed to vitamin D, leading some authorities to favor decreased vitamin D use. However, the studies that examined the impact of mineral levels and parathyroid hormone (PTH) on survival did not consistently account for vitamin D therapy itself on hemodialysis patient survival. The current review examines in detail two recent large-scale studies of hemodialysis patients: one that demonstrated a survival advantage of paricalcitol over calcitriol and a second that demonstrated a significant survival advantage of any intravenous vitamin D formulation versus none. In both studies, the effects were independent of mineral and PTH levels, suggesting "nontraditional" actions of vitamin D contributed to the observed survival advantage. Several of these nontraditional actions are reviewed with an emphasis on those that might impact hemodialysis outcomes.

Vitamin D

The vitamin D endocrine system plays an essential role in calcium homeostasis and bone metabolism, but research during the past two decades has revealed a diverse range of biological actions that include induction of cell differentiation, inhibition of cell growth, immunomodulation, and control of other hormonal systems. Vitamin D itself is a prohormone that is metabolically converted to the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. This vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses. This review focuses on several recent developments that extend our understanding of the complexities of vitamin D metabolism and actions: the final step in the activation of vitamin D, conversion of 25-hydroxyvitamin D to 1,25(OH)(2)D in renal proximal tubules, is now known to involve facilitated uptake and intracellular delivery of the precursor to 1alpha-hydroxylase. Emerging evidence using mice lacking the VDR and/or 1alpha-hydroxylase indicates both 1,25(OH)(2)D(3)-dependent and -independent actions of the VDR as well as VDR-dependent and -independent actions of 1,25(OH)(2)D(3). Thus the vitamin D system may involve more than a single receptor and ligand. The presence of 1alpha-hydroxylase in many target cells indicates autocrine/paracrine functions for 1,25(OH)(2)D(3) in the control of cell proliferation and differentiation. This local production of 1,25(OH)(2)D(3) is dependent on circulating precursor levels, providing a potential explanation for the association of vitamin D deficiency with various cancers and autoimmune diseases.

Parathyroid hormone decreases renal vitamin D receptor expression in vivo

The vitamin D receptor (VDR) is a nuclear transcription factor responsible for mediating the biological activities of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. Renal and parathyroid gland VDR content is an important factor in calcium homeostasis, vitamin D metabolism, and the treatment of secondary hyperparathyroidism and renal osteodystrophy. In these tissues, VDR expression is highly regulated by the calcium and vitamin D status. Although 1,25(OH)(2)D(3) up-regulates VDR expression, hypocalcemia and vitamin D deficiency result in drastically reduced expression of the receptor. The generation of 25-hydroxyvitamin D(3)-1alpha-hydroxylase-null mice, which are incapable of endogenously producing 1,25(OH)(2)D(3), has allowed us to investigate the influence of parathyroid hormone (PTH) on VDR expression independent of PTH-mediated increases in 1,25(OH)(2)D(3). Administration of human PTH (1-34) (110 microg/kg per day) for 48 h reduced renal VDR levels from 515 to 435 fmol/mg protein (15%, P < 0.03) in wild-type mice. In the 25-hydroxyvitamin D(3)-1alpha-hydroxylase-null mice, PTH administration strongly reduced renal VDR levels, from 555 to 394 fmol/mg protein (29%, P < 0.001). These results demonstrate that PTH is a potent down-regulator of VDR expression in vivo.

1,25-Dihydroxyvitamin D3 up-regulates the renal vitamin D receptor through indirect gene activation and receptor stabilization

Expression of the vitamin D receptor (VDR) in the kidney and intestine plays a major role in calcium homeostasis and the metabolism of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). Calcium and 1,25(OH)(2)D(3)-mediated regulation of renal and duodenal VDR expression has been analyzed in vivo and the mechanisms responsible for the renal regulation have been studied in mouse kidney TCMK-1 cells. Vitamin D-deficient mice were maintained on diets containing either 0.02 or 0.47% calcium, with or without 50ng of 1,25(OH)(2)D(3) per day. Renal VDR levels were significantly higher in the vitamin D-deficient mice fed the 0.47% calcium diet vs. the calcium-restricted diet, and were increased 5-fold by 1,25(OH)(2)D(3) when dietary calcium was present. The renal VDR transcript was expressed at a basal level in the absence of calcium or 1,25(OH)(2)D(3); 50ng of 1,25(OH)(2)D(3) elevated renal VDR mRNA levels approximately 10-fold in the presence of calcium. Neither calcium nor 1,25(OH)(2)D(3) had any significant effect on duodenal VDR or VDR mRNA expression. In TCMK-1 cells, 1,25(OH)(2)D(3) increased receptor and VDR mRNA content in both low and adequate calcium medium. The 1,25(OH)(2)D(3)-mediated increase in VDR mRNA did not result from increased stability of the transcript. Further, the increase in mRNA was blocked by cycloheximide, indicating a requirement for protein synthesis and an indirect regulation of VDR transcription. Thus, both dietary serum calcium and 1,25-(OH)(2)D(3) are required for VDR expression in kidney but not in intestine where neither is required. The 1,25-(OH)(2)D(3) requirement can also be shown in TCMK-1 cells in vitro, while the calcium requirement was not found.

Suppression of the human parathyroid hormone promoter by vitamin D involves displacement of NF-Y binding to the vitamin D response element

An earlier report in the literature indicated the vitamin D response element (VDRE) in the human parathyroid hormone (hPTH) promoter could be specifically bound by an unidentified transcription factor in addition to the vitamin D receptor (VDR) complex. We confirmed that OK and HeLa cell nuclear extracts formed a specific complex with the hPTH VDRE that was insensitive to competition with other VDRE sequences. However, this factor could be competed for by a consensus NF-Y DNA-binding site, and an anti-NF-Y antibody was able to supershift the bound band. Mutational analysis indicated that the NF-Y-binding site partially overlapped the 3' portion of the VDRE. Transfection studies using an hPTH promoter construct in Drosophila SL2 cells demonstrated strong synergistic transactivation by NF-Y interactions with both the VDRE site and a previously described distal NF-Y-binding site. Finally, mobility shift studies indicated that the VDR heterodimer competed with NF-Y for binding to the VDRE sequence, and NF-Y-stimulated activity of the hPTH promoter could be suppressed in a hormone-dependent manner when the VDR heterodimer complex was coexpressed in SL2 cells. In summary, these findings establish the presence of a proximal NF-Y-binding site in the hPTH promoter and highlight the potential for synergism between distal and proximal NF-Y DNA elements to strongly enhance transcription. Furthermore, findings suggest that the repressive effects of vitamin D on hPTH gene transcription may involve displacement of NF-Y binding to the proximal site by the VDR heterodimer, which subsequently attenuates synergistic transactivation.

Regulation of the murine renal vitamin D receptor by 1,25-dihydroxyvitamin D3 and calcium

Renal vitamin D receptor (VDR) is required for 1,25-dihydroxyvitamin D3-[1,25(OH)2D3]-induced renal reabsorption of calcium and for 1,25(OH)2D3-induced 1,25(OH)2D3 24-hydroxylase. The long-term effect of vitamin D and dietary calcium on the expression of renal VDR was examined in the nonobese diabetic mouse. Vitamin D-deficient and vitamin D-replete mice were maintained on diets containing 0.02%, 0.25%, 0.47%, and 1.20% calcium with or without 50 ng of 1,25(OH)2D3 per day. Vitamin D-replete mice on a 1.20% calcium diet had renal VDR levels of 165 fmol/mg protein. Calcium restriction caused renal VDR levels to decrease to <30 fmol/mg protein in vitamin D-deficient mice and to approximately 80 fmol/mg protein in vitamin D-replete mice. When dietary calcium was present, 50 ng of 1,25(OH)2D3 elevated the VDR levels 2- to 10-fold, depending on vitamin D status and the level of calcium. In the absence of either vitamin D or calcium, the VDR mRNA was expressed at a basal level. 1,25(OH)2D3 supplementation caused relative VDR mRNA to increase 8- to 10-fold in the vitamin D-deficient mouse when dietary calcium was available. This increase was completely absent in the calcium-restricted mice. This in vivo study demonstrates that 1,25(OH)2D3 and calcium are both required for renal VDR mRNA expression above a basal level, furthering our understanding of the complex regulation of renal VDR by 1,25(OH)2D3 and calcium.

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.

Relationships with serum parathyroid hormone in old institutionalized subjects

OBJECTIVE AND BACKGROUND

Old people in residential care are at the highest risk of any group for hip fracture. This may relate to their high prevalence of hyperparathyroidism. There are few data, however, on relationships with serum parathyroid hormone (PTH) in these individuals. This study therefore examined complex associations with serum PTH in nursing home and hostel residents.

DESIGN

Cross-sectional analysis.

PATIENTS

One hundred and forty-three nursing home and hostel residents of median age 84 years.

MEASUREMENTS

Serum PTH, 25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D (1,25-(OH)2D), plasma creatinine, phosphate, calcium, albumin, Bsm-1 vitamin D receptor genotype, age, weight and use of frusemide or thiazide.

RESULTS

The statistical models determined accounted for half the interindividual variation in serum PTH. Heavier weight was associated with both the prevalence of secondary hyperparathyroidism and the serum concentration of PTH. Novel interactions with serum PTH were identified between: weight and 25OHD; 25OHD and phosphate; and phosphate and thiazide diuretic use. Plasma phosphate was associated with PTH independently of calcium and 1,25-(OH)2D. There was no independent association between PTH and nuclear vitamin D receptor genotype.

CONCLUSIONS

Heavier weight is associated with both the prevalence and severity of secondary hyperparathyroidism and consistent with animal models of secondary hyperparathyroidism, phosphate may relate to serum PTH independently of 1,25-(OH)2D or calcium.

Reduced parathyroid vitamin D receptor messenger ribonucleic acid levels in primary and secondary hyperparathyroidism

Vitamin D, via its receptor (VDR), inhibits the hormone secretion and proliferation of parathyroid cells. Vitamin D deficiency and reduced parathyroid VDR expression has been associated with development of hyperparathyroidism (HPT) secondary to uremia. VDR polymorphisms may influence VDR messenger RNA (mRNA) levels and have been coupled to an increased risk of parathyroid adenoma of primary HPT. VDR mRNA relative to glyceraldehyde-3-phosphate dehydrogenase mRNA levels were determined by RNase protection assay in 42 single parathyroid adenomas of patients with primary HPT, 23 hyperplastic glands of eight patients with uremic HPT, and 15 normal human parathyroid glands. The adenomas and hyperplasias demonstrated similar VDR mRNA levels, which were reduced (42 +/- 2.8% and 44 +/- 4.0%) compared with the normal glands (P < 0.0001). Comparison of parathyroid adenoma with a normal-sized parathyroid gland of the same individual (n = 3 pairs) showed a 20-58% reduction in the tumor. Nodularly enlarged glands represent a more advanced form of secondary HPT and showed greater reduction in the VDR mRNA levels than the diffusely enlarged glands (P < 0.005). The reduced VDR expression is likely to impair the 1,25(OH)2D3-mediated control of parathyroid functions, and to be of importance for the pathogenesis of not only uremic but also primary HPT. Circulating factors like calcium, PTH, and 1,25(OH)2D3 seem to be less likely candidates mediating the decreased VDR gene expression in HPT.

Vitamin D receptor interactions with the rat parathyroid hormone gene: synergistic effects between two negative vitamin D response elements

Vitamin D response elements (VDREs) that are required for negative regulation of rat parathyroid hormone (rPTH) gene expression have been characterized. Gel mobility shift assays using DNA restriction enzyme fragments and recombinant proteins for vitamin D and retinoic acid X receptors (VDR/RXR) revealed a sequence between -793 and -779 that bound a VDR/RXR heterodimer with high affinity (VDRE1). Furthermore, a lower affinity site (VDRE2) was detected that acted in combination with VDRE1 to bind a second VDR/RXR complex. As determined by ethylation interference analysis, the nucleotide sequence of VDRE1 consisted of GGTTCA GTG AGGTAC, which is remarkably similar to the sequence of the negative VDRE found in the chicken PTH (cPTH) gene. Using the same technique, VDRE2 was identified between positions -760 and -746 and contained the sequence AGGCTA GCC AGTTCA. Functional analysis was determined by transfection studies with plasmid constructs that expressed the gene for chloramphenicol acetyl transferase (CAT). The ability of the VDREs to regulate gene expression was tested in their native context with the rPTH promoter as well as when positioned immediately upstream from the cPTH promoter. With either plasmid construct, exposure to 10(-8)M 1,25(OH)2D3 resulted in a 60-70% decrease in CAT gene expression when both VDRE1 and VDRE2 were present. Examination of the individual VDREs showed that inhibition by 10(-8) M 1,25(OH)2D3 was only 35-40% when just VDRE1 was present. By itself, VDRE2 was even less effective, as significant inhibition of CAT activity (20%) was observed only in the presence of higher concentrations of 1, 25(OH)2D3 (10(-7)M) or when a plasmid vector that overexpressed the VDR protein was cotransfected. In conclusion, the rPTH gene contains two negative VDREs that act in concert to bind two RXR/VDR heterodimer complexes and that both VDREs are required for maximal inhibition by 1,25(OH)2D3.

Vitamin D

The vitamin D endocrine systems plays a critical role in calcium and phosphate homeostasis. The active form of vitamin D, 1, 25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], binds with high affinity to a specific cellular receptor that acts as a ligand-activated transcription factor. The activated vitamin D receptor (VDR) dimerizes with another nuclear receptor, the retinoid X receptor (RXR), and the heterodimer binds to specific DNA motifs (vitamin D response elements, VDREs) in the promoter region of target genes. This heterodimer recruits nuclear coactivators and components of the transcriptional preinitiation complex to alter the rate of gene transcription. 1,25(OH)(2)D(3) also binds to a cell-surface receptor that mediates the activation of second messenger pathways, some of which may modulate the activity of the VDR. Recent studies with VDR-ablated mice confirm that the most critical role of 1, 25(OH)(2)D(3) is the activation of genes that control intestinal calcium transport. However, 1,25(OH)(2)D(3) can control the expression of many genes involved in a plethora of biological actions. Many of these nonclassic responses have suggested a number of therapeutic applications for 1,25(OH)(2)D(3) and its analogs.

Vitamin D and the parathyroid

Vitamin D's biologically active metabolite, 1,25(OH)2D3, has important effects upon the parathyroid cell that are relevant to both the physiology of mineral metabolism and the regulation of the secondary hyperparathyroidism of chronic renal failure. 1,25(OH)2D3 markedly decreases parathyroid hormone (PTH) gene transcription and thus PTH synthesis and secretion. It also acts to decrease parathyroid cell proliferation. Nonhypercalemic analogs of 1,25(OH)2D3 are being developed that may have a wider therapeutic window than 1,25(OH)2D3 itself. In the situations of chronic hypocalcemia and hypophosphatemia, there are interesting interrelationships between 1,25(OH)2D3 and the post-transcriptional regulation of the PTH gene. In nodular secondary hyperparathyroidism, there is down-regulation of the vitamin D receptor in the parathyroid. Different vitamin D receptor genotypes may be associated with higher levels of serum PTH and a predisposition to autonomous hyperplasia.

The vitamin D receptor (VDR) start codon polymorphism in primary hyperparathyroidism and parathyroid VDR messenger ribonucleic acid levels

Vitamin D regulates parathyroid cell proliferation and secretion of PTH. Increased prevalence of the polymorphic vitamin D receptor (VDR) alleles b, a, and T has been reported in sporadic primary hyperparathyroidism (PHPT), suggesting that these genetic variants may predispose to the disease. Recently, another polymorphism in the VDR gene was related to bone mineral density, and this VDR-FokI polymorphism causes different lengths of the VDR, implying possible functional consequences. The VDR-FokI polymorphism was studied in 182 postmenopausal women with sporadic PHPT and in matched controls. No significant differences in distribution of the VDR-FokI genotypes could be detected between the groups, although there was a tendency toward overrepresentation of the F allele in the PHPT patients (P = 0.05). There were no significant associations with age, serum calcium, serum PTH, bone mineral density, or parathyroid tumor weight. The VDR genotypes were unrelated to VDR and PTH messenger ribonucleic acid levels in the parathyroid adenomas of 42 PHPT patients. In 23 PHPT patients, the Ca2+-PTH set-points were determined in vivo and were unrelated to the VDR alleles. We suggest that the VDR-FokI polymorphism has at most a minor pathogenic importance in the development of PHPT.

Regulation of renal vitamin D-24-hydroxylase by phosphate: effects of hypophysectomy, growth hormone and insulin-like growth factor I

Dietary phosphate (Pi) regulates the production of 1,25-dihydroxyvitamin D3. This control is blunted in hypophysectomized (HPX) rats but can be restored by growth hormone (GH) or IGF-I. The regulation of the vitamin D catabolism by Pi, GH and IGF-I is less clear. In the present study, we found that the activity and transcript levels of the catabolic enzyme, vitamin D-24-hydroxylase (24-OHase) were decreased 3- and 5-fold, respectively, during Pi restriction in normal rats, but this effect is greatly reduced in HPX rats. Examination of the serum chemistries revealed that HPX rats on the high Pi diet had lower serum Pi levels than normal rats on this diet, presumably due to the known defective reabsorption of Pi by HPX rats. Treatment of HPX rats, adapted to a 0.6% P diet, with GH (150 micrograms) or IGF-I (80 micrograms) suppressed 24-OHase mRNA levels by 88% and 64%, respectively, by 20 hours and these effects were preceded by decreases in serum Pi. Our findings show that the 24-OHase is regulated by dietary Pi and this control is modulated by hypophysectomy, GH and IGF-I.

Vitamin D receptor alleles b, a, and T: risk factors for sporadic primary hyperparathyroidism (HPT) but not HPT of uremia or MEN 1

Polymorphisms in the vitamin D receptor (VDR) gene have been hypothezised to interfere with VDR expression. VDR alleles (Bb, Aa and Tt) were examined in 254 Caucasian patients with sporadic primary hyperparathyroidism (spHPT, n = 206), HPT of multiple endocrine neoplasia type 1 (MEN-1; n = 17), and HPT of uremia (n = 31). In comparison to age- and sex-matched controls, the b, a and T alleles were overrepresented in 100 menopausal females with spHPT (p = 0.006-0.0004), equivalent to an odds ratio of 2.6-3.4 for spHPT in homozygotes for the b, a and, T alleles. The association between VDR genotypes and spHPT was restricted to female patients and those with parathyroid adenoma (p = 0.0006-0.0001), whereas HPT of MEN 1 and uremia seemed unrelated to the VDR polymorphisms (p = 0.26-0.96). The results suggest that the VDR alleles b, a, and T are novel risk factors in the essentially uncharacterized pathogenesis of spHPT.

Depressed expression of calcium receptor in parathyroid gland tissue of patients with hyperparathyroidism

The factors involved in abnormal parathyroid cell secretory function and growth in patients with primary (I degree) and secondary (II degree) hyperparathyroidism are still incompletely understood. We compared the expression of the calcium-sensing receptor (CaR) at the gene message and the protein level in parathyroid tissue obtained from patients with I degree non-uremic or II degree uremic hyperparathyroidism with that in normal parathyroid tissue, using in situ hybridization and immunohistochemistry techniques. The expression of the CaR mRNA and protein was reduced in most cases of I degree adenoma and II degree hyperplasia, compared with strong expression normal parathyroid tissue. In II degree hyperparathyroidism, expression of both receptor mRNA message and protein was often particularly depressed in nodular areas, compared with adjacent non-nodular hyperplasia. Decreased Ca-R expression in adenomatous and hyperplastic parathyroid glands would be compatible with a less efficient control of PTH synthesis and secretion by plasma calcium than in normal parathyroid tissue.

1,25-dihydroxyvitamin D3 and 22-oxacalcitriol prevent the decrease in vitamin D receptor content in the parathyroid glands of uremic rats

Decreased content of the 1,25-dihydroxyvitamin D3 receptor (VDR) in parathyroid glands from patients and animals with chronic renal failure has been implicated in the pathogenesis of secondary hyperparathyroidism. In these studies, we examined the regulation of VDR by 1,25-dihydroxyvitamin D3 (1,25-D3) and 22-oxacalcitriol (OCT) in parathyroid glands of uremic rats. After eight weeks of renal failure, VDR content in parathyroid glands of uremic rats was decreased (400 +/- 42 vs. 729 +/- 47 fmol/mg protein in normal control rats, P < 0.05) and strongly correlated with serum 1,25-D3 levels (r = 0.829, P = 0.0001). Treatment with either 1,25-D3 or OCT prevented the decrease in VDR. We conclude that low serum 1,25-D3 levels, at least in part, account for the decrease in VDR content in parathyroid glands of uremic rats and that treatment with 1,25-D3 or OCT prevents this decrease ameliorating the development of secondary hyperparathyroidism.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

Osteoclasts express the B2 isoform of vacuolar H(+)-ATPase intracellularly and on their plasma membranes

Osteoclasts express high levels of vacuolar H(+)-ATPase (V-ATPase) in their ruffled membranes, driving the secretion of H+ required for normal bone resorption. Previous reports have suggested that the B subunit of the osteoclast V-ATPase differs from those expressed in kidney and other tissues. In this study, B subunit isoform-specific antibodies and cDNA probes were used to examine which B subunit isoform is expressed in osteoclasts and osteoclast-like cells. Immunoblotting and RNA hybridization analysis were used to demonstrate that cells from an osteoclast-rich mouse bone marrow culture model express the B2 but not the B1 subunit isoform. Immunocytochemical staining of murine osteoclasts generated in vitro and of native rat osteoclasts in bone sections showed that the B2 but not the B1 isoform was expressed at high levels and was polarized to the ruffled membrane. Human marrow cultures and monocyte-derived macrophages, used as models for osteoclasts, also expressed the B2 but not the B1 subunit isoform. These results indicate that V-ATPases containing the B2 subunit isoform mediate osteoclast bone resorption.