Mitogen-activated protein kinases mediate interleukin-1?-induced receptor activator of nuclear factor-?B ligand expression in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Interleukin-1beta-stimulated receptor activator of nuclear factor-kappaB ligand (RANKL) expression in human periodontal ligament cells is partially mediated by endogenous prostaglandin E2, whereas mitogen-activated protein kinases (MAPKs) are implicated in regulating various interleukin-1-responsive genes. We investigated herein the involvement of MAPKs in interleukin-1beta-stimulated RANKL expression in human periodontal ligament cells.

MATERIAL AND METHODS

Human periodontal ligament cells were pretreated separately with specific inhibitors of MAPKs, including extracellular signal-regulated kinase, p38 MAPK and c-Jun N-terminal kinase, and subsequently treated with interleukin-1beta. Following each treatment, the phosphorylation of each MAPK, the expression of RANKL, and the production of prostaglandin E2 were determined. RANKL activity was evaluated using an assay to determine the survival of prefusion osteoclasts.

RESULTS

Interleukin-1beta induced RANKL expression at the mRNA and protein levels, as well as RANKL activity in human periodontal ligament cells. Interleukin-1beta also activated extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase. Pretreatment with each MAPK inhibitor partially, but significantly, suppressed interleukin-1beta-induced RANKL expression and its activity, as well as prostaglandin E2 production.

CONCLUSION

In human periodontal ligament cells, three types of MAPK inhibitor may abrogate RANKL expression and activity induced by interleukin-1beta, directly or indirectly through partial suppression of prostaglandin E2 synthesis. In addition, extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase signals may co-operatively mediate interleukin-1beta-stimulated RANKL expression and its activity in those cells.

Involvement of Tiam1 in apoptosis induced by bufalin in HeLa cells

BACKGROUND

It has been previously demonstrated that bufalin, an active agent in the Chinese medicine chan'su, induces apoptosis in human leukemia cells by altering the expression of apoptosis-related genes, such as bcl-2 and c-myc. Tiam1 was also found to play a critical role in bufalin-induced apoptosis through the activation of the Rac1, PAK and JNK pathway in human leukemia cell lines. In the present study, the involvement of the Tiam1 gene products in bufalin-induced apoptosis in human solid tumor HeLa cells was examined.

MATERIALS AND METHODS

HeLa cells were treated with 10(-8) M bufalin and apoptosis was measured by ELISA quantification of nucleosomes. Tiam1 mRNA levels were quantified by real-time PCR analysis and inhibited by transfected siRNA specific for Tiam1.

RESULTS

Apoptosis was induced in HeLa cells by treatment with 10(-8) M bufalin. Expression of both Tiam1 mRNA and its protein was induced 0.5 h after the start of the bufalin treatment. Transfection of Tiam1-specific siRNA into HeLa cells markedly inhibited bufalin-induced apoptosis.

CONCLUSION

Our results suggest that Tiam1 is a downstream mediator of bufalin-induced apoptosis in the human solid tumor HeLa cell line, as well as in leukemia cell lines.

Characterization of rat and human CYP2J enzymes as Vitamin D 25-hydroxylases

vitamin D is 25-hydroxylated in the liver, before being activated by 1alpha-hydroxylation in the kidney. Recently, the rat cytochrome P450 2J3 (CYP2J3) has been identified as a principal vitamin D 25-hydroxylase in the rat [Yamasaki T, Izumi S, Ide H, Ohyama Y. Identification of a novel rat microsomal vitamin D3 25-hydroxylase. J Biol Chem 2004;279(22):22848-56]. In this study, we examine whether human CYP2J2 that exhibits 73% amino acid homology to rat CYP2J3 has similar catalytic properties. Recombinant human CYP2J2 was overexpressed in Escherichia coli, purified, and assayed for vitamin D 25-hydroxylation activity. We found significant 25-hydroxylation activity toward vitamin D3 (turnover number, 0.087 min(-1)), vitamin D2 (0.16 min(-1)), and 1alpha-hydroxyvitamin D3 (2.2 min(-1)). Interestingly, human CYP2J2 hydroxylated vitamin D2, an exogenous vitamin D, at a higher rate than it did vitamin D3, an endogenous vitamin D, whereas, rat CYP2J3 hydroxylated vitamin D3 (1.4 min(-1)) more efficiently than vitamin D2 (0.86 min(-1)). Our study demonstrated that human CYP2J2 exhibits 25-hydroxylation activity as well as rat CYP2J3, although the activity of human CYP2J2 is weaker than rat CYP2J3. CYP2J2 and CYP2J3 exhibit distinct preferences toward vitamin D3 and D2.

Analogs of 1α,25-dihydroxyvitamin D3 with high potency in induction of osteoclastogenesis and prevention of dendritic cell differentiation: Synthesis and biological evaluation of 2-substituted 19-norvitamin D analogs

In our previous papers, we found that introduction of a substituent at C(2) into 1alpha,25-dihydroxy-19-norvitamin D(3) (2a) caused dramatic changes in binding affinity for the vitamin D receptor (VDR) and in transcriptional activity compared with the parent compound. To investigate the broad biological activity of 2-substituted 19-norvitamin D analogs, we synthesized two new (20S)-2-hydroxyethylidene-19-norvitamin D derivatives (3b and 4b) and a total of 16 A-ring-modified analogs including 3b and 4b were tested for the following in vitro and in vivo biological activities: (1) affinity for the VDR, (2) transcriptional activity, (3) osteoclast formation, (4) bone calcium mobilization in rats, and (5) effects on differentiation of dendritic cells (DCs). The biological effects of the analogs were compared with those of 1alpha,25-dihydroxyvitamin D(3) (1a) and 2MD, which is being developed for the treatment of osteoporosis. The efficacy of the (20S)-19-norvitamin D analogs with 2-hydroxyethylidene, 2-hydroxyethoxy, and 2-methyl moieties (3b, 5b, 6b, and 9b) was more than 10-fold stronger than that of 1a with respect to transcriptional activity, ability to induce osteoclast formation, and ability to inhibit CD86 expression, a marker of mature DCs, and was similar to that of 2MD. The (20S)-2beta-hydroxyethoxy derivative 6b was 2 orders of magnitude more active than 1a and approximately twice as potent as 2MD in preventing CD86 production. The 2-epoxy derivatives 7 and 8 were relatively poor ligands for the VDR and exhibited activity lower than that of the natural hormone 1a.

Reveromycin A, an agent for osteoporosis, inhibits bone resorption by inducing apoptosis specifically in osteoclasts

Mature bone-resorbing osteoclasts (OCs) mediate excessive bone loss seen in several bone disorders, including osteoporosis. Here, we showed that reveromycin A (RM-A), a small natural product with three carboxylic groups in its structure, induced apoptosis specifically in OCs, but not in OC progenitors, nonfunctional osteoclasts, or osteoblasts. RM-A inhibited protein synthesis in OCs by selectively blocking enzymatic activity of isoleucyl-tRNA synthetase. The proapoptotic effect of RM-A was inhibited by neutralization or disruption of the acidic microenvironment, a prominent characteristic of OCs. RM-A was incorporated in OCs but not in nonfunctional osteoclasts and OC progenitors in neutral culture medium. Effects of RM-A on OC apoptosis increased under acidic culture conditions. RM-A not only was incorporated, but also induced apoptosis in OC progenitors in acidic culture medium. RM-A inhibited osteoclastic pit formation, decreased prelabeled (45)Ca release in organ cultures, and antagonized increased bone resorption in ovariectomized mice. These results suggested that preventive effects of RM-A on bone resorption in vitro and in vivo were caused by apoptosis through inhibition of isoleucyl-tRNA synthetase in OCs and that specific sensitivity of OCs to RM-A was due to the acidic microenvironment, which increased cell permeability of RM-A by suppressing dissociation of protons from carboxylic acid moieties, making them less polar. This unique mechanism suggested that RM-A might represent a type of therapeutic agent for treating bone disorders associated with increased bone loss.

Regulatory effects of interleukin-1beta and prostaglandin E2 on expression of receptor activator of nuclear factor-kappaB ligand in human periodontal ligament cells

BACKGROUND

Receptor activator of nuclear factor-kappaB ligand (RANKL), which is expressed on the cell membrane of osteoblasts/stromal cells, stimulates osteoclastogenesis. We investigated the regulatory effects of interleukin-1beta (IL-1beta) and prostaglandin E2 (PGE2) on expression of RANKL in human periodontal ligament (HPDL) cells and the mechanisms involved in the PGE2 effect.

METHODS

The HPDL cells were treated with IL-1beta, alone or in combination with indomethacin (INDO) or NS398, a cyclooxygenase-2 (COX-2) inhibitor. The HPDL cells were also pretreated with H89, a protein kinase A (PKA) inhibitor or GF109203X, a protein kinase C (PKC) inhibitor and subsequently treated with PGE2, PGE receptor (EP)2 agonist, EP4 agonist, forskolin, dibutyryl cAMP (db-cAMP), or 3-(isobutyl)-1-methylxantine (IBMX). After each treatment, expression of EP2, EP4, or RANKL mRNA was analyzed by reverse transcription-polymerase chain reaction and Southern hybridization. Expression of RANKL protein was detected by Western blotting, and cAMP accumulation was determined using a cAMP enzyme immunoassay kit.

RESULTS

IL-1beta stimulated the expression of RANKL at messenger RNA (mRNA) and protein levels in HPDL cells. Endogenous PGE2 partially mediated the IL-1beta-induced RANKL mRNA expression. Exogenously added PGE2 also stimulated RANKL expression at mRNA and protein levels in the cells. The PGE2-stimulated RANKL expression was mediated by EP2/4 and cAMP-dependent PKA, while PKC was possibly involved in the PGE2 action.

CONCLUSION

Human periodontal ligament cells activated with inflammatory factors such as IL-1beta and PGE2 may directly stimulate osteoclastogenesis through RANKL, which is stimulated to express by these factors.

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.

[Intestinal calcium absorption and vitamin D]

Vitamin D is required for efficient absorption of dietary Ca. Accumulated observations indicated that active form of vitamin D is involved in the three steps of intestinal Ca absorption:incorporation of Ca by epithelial cells through brush border membrane, intracellular Ca movement, and excretion of Ca into systemic circulation via basolateral membrane. It is also known that activated vitamin D takes part in growth and differentiation of intestinal epithelial cells.

Mechanisms for the reduction of 24,25-dihydroxyvitamin D3 levels and bone mass in 24-hydroxylase transgenic rats

24-Hydroxylase (CYP24) is an enzyme distributed in the target tissues of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. Two functions for this enzyme have been reported: One is production of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] and the other is inactivation of 1alpha,25(OH)2D3. To elucidate other physiologic roles of CYP24 in vivo, we previously generated rats that constitutively express the CYP24 gene. These transgenic (Tg) rats developed unexpected phenotypes, such as low plasma levels of 24,25(OH)2D3, lipidemia, and albuminuria. In this study, we elucidated the mechanisms for inducing low plasma 24,25(OH)2D3 levels and bone loss. Tg rats excreted massive amounts of vitamin D binding protein (DBP), which coincided with the loss of albumin. In Tg rats, the renal expression pattern of megalin, which serves as an endocytotic receptor responsible for the reuptake of urinary proteins such as DBP and albumin, was identical to that of the wild-type rats. Excreted albumin appeared to compete for the binding and reabsorption of the DBP-25-hydroxyvitamin D3 [25(OH)D3] complex with megalin, resulting in a loss of 25(OH)D3 into the urine and subsequent reduction of plasma 24,25(OH)2D3. In this prominent rat model of nephritis, supplementation of 25(OH)D3 was effective in preventing bone loss in an early stage of renal insufficiency.

Vitamin D and bone

It is now well established that supraphysiological doses of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] stimulate bone resorption. Recent studies have established that osteoblasts/stromal cells express receptor activator of NF-kappaB ligand (RANKL) in response to several bone-resorbing factors including 1alpha,25(OH)(2)D(3) to support osteoclast differentiation from their precursors. Osteoclast precursors which express receptor activator of NF-kappaB (RANK) recognize RANKL through cell-to-cell interaction with osteoblasts/stromal cells, and differentiate into osteoclasts in the presence of macrophage-colony stimulating factor (M-CSF). Osteoprotegerin (OPG) acts as a decoy receptor for RANKL. We also found that daily oral administration of 1alpha,25(OH)(2)D(3) for 14 days to normocalcemic thyroparathyroidectomized (TPTX) rats constantly infused with parathyroid hormone (PTH) inhibited the PTH-induced expression of RANKL and cathepsin K mRNA in bone. The inhibitory effect of 1alpha,25(OH)(2)D(3) on the PTH-induced expression of RANKL mRNA occurred only with physiological doses of the vitamin. Supraphysiological doses of 1alpha,25(OH)(2)D(3) increased serum Ca and expression of RANKL in vivo in the presence of PTH. These results suggest that the bone-resorbing activity of vitamin D does not occur at physiological dose levels in vivo. A certain range of physiological doses of 1alpha,25(OH)(2)D(3) rather suppress the PTH-induced bone resorption in vivo, supporting the concept that 1alpha,25(OH)(2)D(3) or its derivatives are useful for the treatment of various metabolic bone diseases such as osteoporosis and secondary hyperparathyroidism.

Characterization of transgenic rats constitutively expressing vitamin D-24-hydroxylase gene

Vitamin D-24-hydroxylase (CYP24) is one of the enzymes responsible for vitamin D metabolism. CYP24 catalyzes the conversion of 25-hydroxyvitamin D(3) [25(OH)D(3)] to 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)] in the kidney. CYP24 is also involved in the breakdown of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], the active form of vitamin D(3). In this study, we generated transgenic (Tg) rats constitutively expressing CYP24 gene to investigate the biological role of CYP24 in vivo. Surprisingly, the Tg rats showed a significantly low level of plasma 24,25(OH)(2)D(3). Furthermore, the Tg rats developed albuminuria and hyperlipidemia shortly after weaning. The plasma lipid profile revealed that all lipoprotein fractions were elevated in the Tg rats. Also, the Tg rats showed atherosclerotic lesions in the aorta, which greatly progressed with high-fat and high-cholesterol feeding. These unexpected results suggest that CYP24 is involved in functions other than the regulation of vitamin D metabolism.

Rational design, synthesis, and biological activity of novel conformationally restricted vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3)

Two new vitamin D analogues, (22R)- and (22S)-22-ethyl-1,25-dihydroxy-23,24-didehydro-24a,24b-dihomo-20-epivitamin D(3) (3 and 4), were rationally designed on the basis of the active space group concept previously proposed by us. The 22R ethyl group of 3 restricts the mobility of the side chain to active space regions, whereas the 22S ethyl group of 4 confines the side chain to an inactive region. The double bond at C(23) further restricts the side chain flexibility. These compounds (3 and 4) were synthesized using ortho ester Claisen rearrangement as the key step. As expected, the 22R isomer 3 has nearly 100 times higher efficacy than 1,25-dihydroxyvitamin D(3) (1) in cell differentiation, although its affinity for the vitamin D receptor (VDR) was one-seventh of that of 1. The 22S isomer 4 has significantly lower efficacy than 3. A docking study in combination with site-directed mutation analysis revealed that two carbon elongated side chain analogue 3 could be fitted in the ligand binding pocket of the VDR by adopting a stable conformation.

Effects of geranylgeranoic acid in bone: induction of osteoblast differentiation and inhibition of osteoclast formation

Retinoids are known to be of special importance for normal bone growth and development. Recently, we reported that retinoids not only induced osteoblast differentiation, but also inhibited osteoclast formation in vitro. In this study, we examined the osteogenic effects of geranylgeranoic acid (GGA), a chemically synthesized acyclic retinoid, in bone in vitro and in vivo. GGA not only suppressed proliferation of osteoblastic MC3T3-E1 cells, but also up-regulated differentiation markers of osteoblasts such as alkaline phosphatase (ALP) activity and expression of osteopontin (OP) messenger RNA (mRNA). In contrast, GGA inhibited osteoclast formation induced by 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] in cocultures of mouse bone marrow cells and primary osteoblasts. Treatment of stromal ST2 cells with GGA restored the 1alpha,25(OH)2D3- or prostaglandin E2 (PGE2)-induced suppression of osteoprotegerin (OPG) mRNA expression. GGA inhibited osteoclast formation induced by macrophage colony-stimulating factor (M-CSF) and soluble receptor activator of nuclear factor kappaB ligand (sRANKL) in the culture of bone marrow macrophages. Thus, it is likely that GGA inhibits osteoclast formation by affecting both osteoblasts and osteoclast progenitors in the coculture system. Furthermore, in vivo, GGA increased bone mineral density (BMD) of total as well as distal femur in a P6 strain of senescence-accelerated mice (SAMP6). These results indicate that GGA increases bone mass by maintaining a positive balance of bone turnover by inducing osteoblast differentiation and suppressing osteoclast formation.

[The regulation of vitamin D metabolism and PTH secretion by phosphorus]

Vitamin D and PTH are major hormones that regulate calcium metabolism. Also, the serum level of phosphorus is known to change according to the level of calcium and to regulate the vitamin D and PTH. Recently, cloning of 1alpha- hydroxylase cDNA which is the key enzyme in vitamin D metabolism, enabled us to examine the effects of phosphorus in vitamin D metabolism at a molecular level. Furthermore, the direct effect of phosphorus in PTH synthesis is being elucidated in recent reports. In this paper, we summarized the regulation of vitamin D and PTH by phosphorus.

[The mechanism which calcitonin activates vitamin D]

Calcitropic hormones, such as calcitonin and vitamin D, are one of the most common drugs used in the treatment of osteoporosis. Recently, three enzymes involved in vitamin D metabolism (25-hydroxylase, 24-hydroxylase, 1alpha-hydroxylase) were cloned and enabled us to analyze the vitamin D metabolism at a molecular level. We summarize the mechanism which CT activates vitamin D.

Osteoprotegerin produced by osteoblasts is an important regulator in osteoclast development and function

Osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL)/osteoclast differentiation factor, inhibits both differentiation and function of osteoclasts. We previously reported that OPG-deficient mice exhibited severe osteoporosis caused by enhanced osteoclastic bone resorption. In the present study, potential roles of OPG in osteoclast differentiation were examined using a mouse coculture system of calvarial osteoblasts and bone marrow cells prepared from OPG-deficient mice. In the absence of bone-resorbing factors, no osteoclasts were formed in cocultures of wild-type (+/+) or heterozygous (+/-) mouse-derived osteoblasts with bone marrow cells prepared from homozygous (-/-) mice. In contrast, homozygous (-/-) mouse-derived osteoblasts strongly supported osteoclast formation in the cocultures with homozygous (-/-) bone marrow cells, even in the absence of bone-resorbing factors. Addition of OPG to the cocultures with osteoblasts and bone marrow cells derived from homozygous (-/-) mice completely inhibited spontaneously occurring osteoclast formation. Adding 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] to these cocultures significantly enhanced osteoclast differentiation. In addition, bone-resorbing activity in organ cultures of fetal long bones derived from homozygous (-/-) mice was markedly increased, irrespective of the presence and absence of bone-resorbing factors, in comparison with that from wild-type (+/+) mice. Osteoblasts prepared from homozygous (-/-), heterozygous (+/-), and wild-type (+/+) mice constitutively expressed similar levels of RANKL messenger RNA, which were equally increased by the treatment with 1alpha,25(OH)2D3. When homozygous (-/-) mouse-derived osteoblasts and hemopoietic cells were cocultured, but direct contact between them was prevented, no osteoclasts were formed, even in the presence of 1alpha,25(OH)2D3 and macrophage colony-stimulating factor. These findings suggest that OPG produced by osteoblasts/stromal cells is a physiologically important regulator in osteoclast differentiation and function and that RANKL expressed by osteoblasts functions as a membrane-associated form.

Importance of membrane- or matrix-associated forms of M-CSF and RANKL/ODF in osteoclastogenesis supported by SaOS-4/3 cells expressing recombinant PTH/PTHrP receptors

SaOS-4/3, a subclone of the human osteosarcoma cell line SaOS-2, established by transfecting the human parathyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) receptor complementary DNA (cDNA), supported osteoclast formation in response to PTH in coculture with mouse bone marrow cells. Osteoclast formation supported by SaOS-4/3 cells was completely inhibited by adding either osteoprotegerin (OPG) or antibodies against human macrophage colony-stimulating factor (M-CSF). Expression of messenger RNAs (mRNAs) for receptor activator of NF-kappaB ligand/osteoclast differentiation factor (RANKL/ODF) and both membrane-associated and secreted forms of M-CSF by SaOS-4/3 cells was up-regulated in response to PTH. SaOS-4/3 cells constitutively expressed OPG mRNA, expression of which was down-regulated by PTH. To elucidate the mechanism of PTH-induced osteoclastogenesis, SaOS-4/3 cells were spot-cultured for 2 h in the center of a culture well and then mouse bone marrow cells were uniformly plated over the well. When the spot coculture was treated for 6 days with both PTH and M-CSF, osteoclasts were induced exclusively inside the colony of SaOS-4/3 cells. Osteoclasts were formed both inside and outside the colony of SaOS-4/3 cells in coculture treated with a soluble form of RANKL/ODF (sRANKL/sODF) in the presence of M-CSF. When the spot coculture was treated with sRANKL/sODF, osteoclasts were formed only inside the colony of SaOS-4/3 cells. Adding M-CSF alone failed to support osteoclast formation in the spot coculture. PTH-induced osteoclast formation occurring inside the colony of SaOS-4/3 cells was not affected by the concentration of M-CSF in the culture medium. Mouse primary osteoblasts supported osteoclast formation in a similar fashion to SaOS-4/3 cells. These findings suggest that the up-regulation of RANKL/ODF expression is an essential step for PTH-induced osteoclastogenesis, and membrane- or matrix-associated forms of both M-CSF and RANKL/ ODF are essentially involved in osteoclast formation supported by osteoblasts/stromal cells.

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.

Three-dimensional structure-function relationship of vitamin D: side chain location and various activities

The various biological activities of side-chain mobility restricted analogs, four diastereomers at C(20) and C(22) of 22-methyl-1alpha,25-dihydroxyvitamin D3, were evaluated. The relationship between structure and the various activities of the analogs was discussed in terms of the active space region concept that we previously suggested.

Mg2+ binding and catalytic function of sphingomyelinase from Bacillus cereus

The modes of Mg2+ binding to SMase from Bacillus cereus were studied on the basis of the changes in the tryptophyl fluorescence intensity. This enzyme was shown to possess at least two binding sites for Mg2+ with low and high affinities. The effects of Mg2+ binding on the enzymatic activity and structural stability of the enzyme molecule were also studied. The results indicated that the binding of Mg2+ to the low-affinity site was essential for the catalysis, but was independent of the substrate binding to the enzyme. It was also indicated that the alkaline denaturation of the enzyme was partly prevented by the Mg2+ binding, whereas no significant protective effect was observed against the denaturation by urea. The pH dependence of the kinetic parameters for the hydrolysis of micellar HNP and mixed micellar SM with Triton X-100 (1:10), catalyzed by SMase from B. cereus, was studied in the presence of a large amount of Mg2+ to saturate both the low- and high-affinity sites. The pH dependence curves of the logarithm of 1/Km for these two kinds of substrates were similar in shape to each other, and showed a single transition. On the other hand, the shapes of the pH dependence curves of the logarithm of kcat for these two kinds of substrates were different from each other. The pH dependence curve for micellar HNP showed three transitions and, counting from the acidic end of the pH region, the first and third transitions having tangent lines with slopes of +1 and -1, respectively. On the other hand, the curve for mixed micellar SM with Triton X-100 showed one large transition with a slope of +1 (the first transition) and a very small transition (the third transition). On the basis of the present results and the three-dimensional structure of bovine pancreatic DNase I, which has a primary structure similar to that of B. cereus SMase, we proposed a catalytic mechanism for B. cereus SMase based on general-base catalysis.

Potential role of cbfa1, an essential transcriptional factor for osteoblast differentiation, in osteoclastogenesis: regulation of mRNA expression of osteoclast differentiation factor (ODF)

The role of Cbfa1 (core binding factor alpha1), an essential transcriptional factor for osteoblast differentiation, in osteoclastogenesis was investigated in vitro and in vivo using Cbfa1-deficient calvarial cells and mice. Co-cultures of calvarial cells isolated from embryos with three different Cbfa1 genotypes (Cbfa1+/+, Cbfa1+/- and Cbfa1-/-) and normal spleen cells generated TRAP-positive multinucleated osteoclast-like cells (OCLs) in response to 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] and dexamethasone, but the number and bone-resorbing activity of OCLs formed in co-culture with Cbfa1-/- calvarial cells were significantly decreased in comparison with those formed in co-cultures with Cbfa1+/+ or Cbfa1+/- calvarial cells. The expression of osteoclast differentiation factor/osteoprotegerin ligand (ODF/OPGL) mRNA was increased by the treatment with 1alpha, 25(OH)2D3 and dexamethasone in calvarial cells from Cbfa1+/+ and Cbfa1+/- mouse embryos, but not from Cbfa1-/- embryos. In contrast, the expression of osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF) mRNA was inhibited by 1alpha,25(OH)2D3 and dexamethasone similarly in all three types of calvarial cells. ODF/OPGL and OPG/OCIF mRNAs were highly expressed in the tibia and femur of Cbfa1+/+ and Cbfa1+/- embryos. In the tibia and femur of Cbfa1-/- embryos, however, ODF/OPGL mRNA was undetectable and the expression of OPG/OCIF mRNA was also decreased compared with those in Cbfa1+/+ and Cbfa1+/- embryos. These results suggested that Cbfa1 is somehow involved in osteoclastogenesis through regulation of ODF/OPGL.

Two novel 1alpha-hydroxylase mutations in French-Canadians with vitamin D dependency rickets type I1

BACKGROUND

Vitamin D dependency rickets type I (VDDR-I) is an autosomal recessive disorder in which 25-hydroxyvitamin D 1alpha-hydroxylase (1alpha-hydroxylase) activity in renal proximal tubules is deficient. VDDR-I is recognized throughout the world, but occurs more frequently in a subset of the French-Canadian population. We and others have recently cloned the human 1alpha-hydroxylase cDNA and gene, making it possible to screen for mutations. The first VDDR-I mutations were reported in one American and four Japanese patients. In this study, we screened for 1alpha-hydroxylase mutations in French-Canadian patients with VDDR-I.

METHODS

The nine exons of the 1alpha-hydroxylase gene were amplified by polymerase chain reaction (PCR) from genomic DNA of four unrelated French-Canadian patients with VDDR-I and their parents, and sequenced.

RESULTS

Three of the patients were homozygous for a single base-pair deletion (G) at position 262 in the cDNA that lies in exon 2, and causes a premature termination codon upstream from the putative ferredoxin- and heme-binding domains. The fourth patient was homozygous for a 7-bp insertion (CCCCCCA) at position 1323 of the cDNA that lies in exon 8, and causes a premature termination upstream from the putative heme-binding domain. In each family, obligate carriers have one copy of the mutant allele. These mutations, which could be detected by PCR-restriction fragment length polymorphism and polyacrylamide gel electrophoresis of the PCR products, were not found in 25 normal French-Canadians.

CONCLUSION

We describe two novel 1alpha-hydroxylase mutations that are consistent with loss of function in four French-Canadian patients with VDDR-I and suggest that the 1alpha-hydroxylase mutations arise from more than one founder in this population.

[Cloning and expression of 25-hydroxyvitamin D3-1 alpha-hydroxylase gene]

A full-length cDNA for the rat kidney mitochondrial cytochrome P450 mixed function oxidase, 25-hydroxyvitamin D3-1 alpha-hyroxylase, was cloned from a vitamin D-deficient rat kidney cDNA library and subcloned into the mammalian expression vector pcDNA 3.1(+). When 1 alpha-hydroxylase cDNA was transfected into COS-7 cells, they expressed 25-hydroxyvitamin D3-1 alpha-hydroxylase activity. The sequence analysis showed that 1 alpha-hydroxylase cDNA consisted of 2469 bp in length and contained an open reading frame encoding 501 amino acids. The expression of 1 alpha-hydroxylase mRNA was greatly increased in the kidney of vitamin D-deficient rats. In rats with the enhanced renal production of 1 alpha, 25-dihydroxyvitamin D3 (rats fed a low Ca diet), expression of 1 alpha-hydroxylase mRNA was greatly enhanced in the renal proximal convoluted tubules. These results clearly demonstrate that the expression of 1 alpha-hydroxylase is regulated at a transcriptional level. The DNA flanking the 5'-sequence of the mouse 1 alpha-hydroxylase gene has been cloned and sequenced. The promoter has 3 potential CRE sites, 2 perfect and 1 imperfect AP-1 sites, while no DR-3 was detected. Parathyroid hormone stimulates this promoter-directed synthesis of luciferase by 17-fold, while forskolin stimulates it by 3-fold. These results indicate that parathyroid hormone stimulates 25-hydroxyvitamin D3-1 alpha-hydroxylase by acting on the promoter of the 1 alpha-hydroxylase gene.

Parathyroid hormone activation of the 25-hydroxyvitamin D3-1alpha-hydroxylase gene promoter

The DNA flanking the 5' sequence of the mouse 1alpha-hydroxylase gene has been cloned and sequenced. A TATA box has been located at -30 bp and aCCAAT box has been located at -79 bp. The gene's promoter activity has been demonstrated by using a luciferase reporter gene construct transfected into a modified pig kidney cell line, AOK-B50. Parathyroid hormone stimulates this promoter-directed synthesis of luciferase by 17-fold, whereas forskolin stimulates it by 3-fold. The action of parathyroid hormone is concentration-dependent. 1,25-Dihydroxyvitamin D3 does not suppress basal promoter activity and marginally suppresses parathyroid hormone-driven luciferase reporter activity. The promoter has three potential cAMP-responsive element sites, and two perfect and one imperfect AP-1 sites, while no DR-3 was detected. These results indicate that parathyroid hormone stimulates 25-hydroxyvitamin D3-1alpha-hydroxylase by acting on the promoter of the 1alpha-hydroxylase gene.

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.

Molecular cloning of cDNA and genomic DNA for human 25-hydroxyvitamin D3 1 alpha-hydroxylase

The 25-hydroxyvitamin D3 1 alpha-hydroxylase (1 alpha-hydroxylase) is a cytochrome P450 enzyme that catalyzes the conversion of 25-hydroxyvitamin D3 to 1 alpha,25-dihydroxyvitamin D3. This enzyme plays an important role in calcium homeostasis. Here we report the molecular cloning of cDNA and gene for human 1 alpha-hydroxylase. The cDNA clone was obtained from a human kidney cDNA library by cross-hybridization with a previously cloned rat cDNA probe. The cDNA consists of 2469 bp and encodes a protein of 508 amino acids that shows 82.5% sequence identity with the rat enzyme. A computer-aided homology search revealed that 1 alpha-hydroxylase shares a relatively high homology with vitamin D3 25-hydroxylase (about 40% amino acid identity). Northern blot analysis showed that the 2.5-kb mRNA is most abundant in kidney. The gene for human 1 alpha-hydroxylase spans approximately 6 kb, is composed of nine exons, and is present as a single copy. This molecular cloning makes it possible to investigate the genetic mechanism of diseases related to calcium metabolism, including vitamin D-dependency rickets type I.

1alpha,25-dihydroxyvitamin D3-24-hydroxylase (CYP24) hydroxylates the carbon at the end of the side chain (C-26) of the C-24-fluorinated analog of 1alpha,25-dihydroxyvitamin D3

The sequential oxidation and cleavage of the side chain of 1alpha, 25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) initiated by the hydroxylation at C-24 is considered to be the major pathway of this hormone in the target cell metabolism. In this study, we examined renal metabolism of a synthetic analog of 1alpha,25(OH)2D3, 24, 24-difluoro-1alpha,25-dihydroxyvitamin D3 (F2-1alpha,25(OH)2D3), C-24 of which was designed to resist metabolic hydroxylation. When kidney homogenates prepared from 1alpha,25(OH)2D3-supplemented rats were incubated with F2-1alpha,25(OH)2D3, it was mainly converted to a more polar metabolite. We isolated and unequivocally identified the metabolite as 24,24-difluoro-1alpha,25,26-trihydroxyvitamin D3 (F2-1alpha,25,26(OH)3D3) by ultraviolet absorption spectrometry, frit-fast atom bombardment liquid chromatography/mass spectroscopy analysis, and direct comparison with chemically synthesized F2-1alpha,25,26(OH)3D3. Metabolism of F2-1alpha,25(OH)2D3 into F2-1alpha,25,26(OH)3D3 by kidney homogenates was induced by the prior administration of 1alpha,25(OH)2D3 into rats. The C-24 oxidation of 1alpha,25(OH)2D3 in renal homogenates was inhibited by F2-1alpha,25(OH)2D3 in a concentration-dependent manner. Moreover, F2-1alpha,25,26(OH)3D3 was formed in ROS17/2.8 cells transfected with a plasmid expressing 1alpha,25(OH)2D3-24-hydroxylase (CYP24) but not in the cells transfected with that expressing vitamin D3-25-hydroxylase (CYP27) or containing inverted CYP27 cDNA. These results show that CYP24 catalyzes not only hydroxylation at C-24 and C-23 of 1alpha,25(OH)2D3 but also at C-26 of F2-1alpha,25(OH)2D3, indicating that this enzyme has a broader substrate specificity of the hydroxylation sites than previously considered.

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.

Evidence for 54-kD protein in chicken kidney as a cytochrome P450 with a high molecular activity of 25-hydroxyvitamin D3 1 alpha-hydroxylase

Conversion of 25-hydroxyvitamin D3 (25(OH)D3) to the active vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) is catalyzed by 25(OH)D3, 1 alpha-hydroxylase(1 alpha-hydroxylase). It has been suggested that this enzyme is cytochrome P450 (P450). We purified 1 alpha-hydroxylase 430-fold from cholate-solubilized kidney mitochondria of vitamin D-deficient chickens by utilizing hydrophobic and ion-exchange column chromatographies. Enzymatic activity was assessed by measuring on HPLC the formation of 1 alpha,25(OH)2D3 from 25(OH)D3 in the assay mixture containing NADPH, adrenodoxin reductase, adrenodoxin as a reducing system. The purified enzyme showed a CO-difference spectrum characteristic of P450. The molecular activity of this preparation was calculated to be 8.7 pmol/min/pmol P450. This value was higher by more than 87-fold than those reported so far. The present preparation was found to contain several proteins on SDS-PAGE. Among them, only the 54-kD protein became undetectable when kidney mitochondria from normal and vitamin D-replete chickens, where 1 alpha-hydroxylase activities were 15 and 0% of that found in vitamin D-deficient chicken, respectively, were used as the starting enzyme sources. Furthermore, the band intensity of the 54-kD protein accounted for the spectrophotometrically determined amount of P450 in the preparation. These results suggest that the 54-kD protein is 1 alpha-hydroxylase.

Regulation of vitamin D-responsive gene expression by fluorinated analogs of calcitriol in rat osteoblastic ROB-C26 cells

We compared the activation of vitamin D-responsive genes by 24,24-difluorocalcitriol [F2-1 alpha,25(OH)2D3] and 26,26,26,27,27,27-hexafluorocalcitriol [F6-1 alpha,25(OH)2D3] with that by calcitriol [1 alpha,25(OH)2D3] in rat osteoblastic ROB-C26 cells. F2-1 alpha,25(OH)2D3 and F6-1 alpha, 25(OH)2D3 were ten times more potent than 1 alpha,25(OH)2D3 in inducing the expression of 1 alpha, 25(OH)2D3-24-hydroxylase (24-OHase) mRNA 6 h after adding vitamin D compounds. The lower affinity of these two fluorinated analogs compared with that of 1 alpha,25(OH)2D3 for vitamin D binding protein in serum (serum DBP) seemed to be partly involved in their increased ability to activate the 24-OHase gene. A time course study revealed that the expression of the 24-OHase and osteopontin mRNAs in the cells incubated with 1 alpha, 25(OH)2D3 and F2-1 alpha,25(OH)2D3 attained maximal levels at 6 h for 24-OHase mRNA and 18 h for osteopontin mRNA, the both decreased thereafter. On the contrary, F6-1 alpha,25(OH)2D3 increased the expression of 24-OHase and osteopontin exponentially until 72 h. While F2-1 alpha,25(OH)2[1 beta-3H]D3 was catabolized quickly by ROB-C26 cells, F6-1 alpha,25(OH)2[1 beta-3H]D3 was slowly and quantitatively converted into putative 26,26,26,27,27,27-hexafluoro-23S-hydroxy[1 beta-3H]calcitriol (F6-1 alpha,23S,25(OH)3[1 beta-3H]D3). This may explain why the time-course profiles of the accumulation of mRNAs for 24-OHase and osteopontin differed in the cells exposed to the fluorinated analogs. In addition to the longer retention, unknown up-regulating mechanisms appeared to be involved in the exponential activation of the 24-OHase and osteopontin genes induced by F6-1 alpha,25(OH)2D3.

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

The vitamin D endocrine system is regulated reciprocally by renal 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylases. Previously, we reported that renal proximal convoluted tubules, the major site of 1 alpha, 25-dihydroxyvitamin D3 production, have vitamin D receptors. In the presence of vitamin D receptors, renal proximal convoluted tubules cannot maintain the state of enhanced production of 1 alpha, 25-dihydroxyvitamin D3. To clarify this discrepancy, we proposed a working hypothesis for the reciprocal control of renal 25-hydroxyvitamin D3 1 alpha- and 24-hydroxylase activities. In rat models of enhanced renal production of 1 alpha, 25-dihydroxyvitamin D3, expression of vitamin D receptors and 25-hydroxyvitamin D3 24-hydroxylase mRNAs was strikingly suppressed in renal proximal convoluted tubules but not in the cortical collecting ducts. In vitamin D-deficient rats with up-regulated renal 25-hydroxyvitamin D3 1 alpha-hydroxylase activity, expression of vitamin D receptor mRNA in renal proximal convoluted tubules was also down-regulated, indicating that the down-regulation of vitamin D receptor mRNA is not the result of the enhanced production of 1 alpha, 25-dihydroxyvitamin D3. In Japanese quail models with up-regulated renal 25-hydroxyvitamin D3 1 alpha-hydroxylase activity by sex steroids, expression of vitamin D receptor mRNA was also down-regulated in the kidney but not in the duodenum. These results suggest that the down-regulation of vitamin D receptors plays a critical role in production of 1 alpha, 25-dihydroxyvitamin D3 in renal proximal convoluted tubules.

Microgravity generated by space flight has little effect on the growth and development of chick embryonic bone

Seven days' space flight of fertilized chicken eggs pre- incubated for 7 and 10 days on earth caused no differences in the morphology of osteoblasts, osteoclasts, and osteocytes of humerus and tibia from those of control embryos. Bone-resorbing and -forming activities of the femur were not different between control and flight groups. As a consequence, calcium and phosphorus contents of the femora between control and flight groups were not changed. Alkaline phosphatase activity of 3 different regions (resting cartilage, growth cartilage, and cortical bone) of tibia showed no significant difference between control and flight groups. No significant difference of gene expressions of hepatocyte growth factor and receptors of fibroblast growth factor was observed in perichondrium, trabecula, and skeletal muscles and tendons of hind limbs between control and flight groups. Unlike the results of previous space flight experiments in which young growing mammals were used, these morphological and biochemical results indicate that microgravity has little effect on bone metabolism of the chick embryo.

Mouse primary osteoblasts express vitamin D3 25-hydroxylase mRNA and convert 1 alpha-hydroxyvitamin D3 into 1 alpha,25-dihydroxyvitamin D3

We examined whether 1 alpha-hydroxyvitamin D3 (1 alpha(OH)D3) is metabolized into 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) in bone. Northern blot analysis indicated that the expression of vitamin D3 25-hydroxylase mRNA was highest in the liver, followed by the duodenum, calvaria, lung, kidney, skin and long bone, and lowest in the spleen. Of the bone cell fractions isolated from fetal mouse calvaria by a sequential enzymatic digestion, fraction 3, which consisted of mostly osteoblastic cells, showed the highest expression of vitamin D3 25-hydroxylase mRNA. When either cultured bone cells of fraction 3 or mouse calvaria were incubated with [3H]-1 alpha (OH)D3, a radioactive peak which comigrated at the same position as authentic 1 alpha,25(OH)2D3 was found on an HPLC chromatogram. The radioactive fraction obtained from the conditioned media of fetal mouse calvaria was tentatively identified as 1 alpha,25(OH)2D3 by cochromatography with authentic 1 alpha,25(OH)2D3 on three different HPLC systems and a thermal isomerization analysis. These results indicate that 1 alpha(OH)D3 is hydroxylated at the 25-position in bones, resulting in the local synthesis of 1 alpha,25(OH)2D3 from 1 alpha(OH)D3 in the skeletal tissues.

Biological activities of 24-fluoro-1 alpha,25-dihydroxyvitamin D-2 and its 24-epimer

Biological activities of two epimeric 24-fluorinated vitamin D-2 analogs, 24-fluoro-1 alpha,25-dihydroxyvitamin D-2 [24-F-1,25-(OH)2D2] and its 24-epimer [24-epi-24-F-1,25-(OH)2D2], were studied and compared with 1 alpha,25-dihydroxyvitamin D-3 [1,25-(OH)2D3] and 1 alpha,25-dihydroxyvitamin D-2 [1,25-(OH)2D2]. 24-F-1,25-(OH)2D2 was nearly as active as 1,25-(OH)2D3 and 1,25-(OH)2D2 both in regulating calcium metabolism in vivo including bone mineral mobilization and intestinal calcium transport and in inducing differentiation of HL-60 cells. While 24-epi-24-F-1,25-(OH)2D2 showed distinct properties in these two types of the actions. Though the 24-epimer was nearly as potent as 1,25-(OH)2D3 in inducing differentiation of HL-60 cells, it showed little activity in regulating calcium metabolism in vivo. The fluorine atom introduced at the 24-position of either 1,25-(OH)2D2 or its 24-epimer had no potentiating effect. This is in sharp contrast with the cases of 24- and 26,27-multifluorinated analogs of active vitamin D-3.

Identification of a vitamin D-responsive element in the 5'-flanking region of the rat 25-hydroxyvitamin D3 24-hydroxylase gene

The 5'-flanking region of the rat vitamin D3 24-hydroxylase (P450cc24) gene was examined and a vitamin D-responsive element (VDRE) responsible for the 1 alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) enhancement was identified. Unidirectional deletion analyses of the 5'-flanking region indicated that the region [-167/-102] is involved in vitamin D responsiveness. Further functional analyses showed that the segment [-204/-129] conferred the hormone responsiveness in an orientation-independent manner when it was placed upstream to the heterologous thymidine kinase promoter or the rabbit beta-globin promoter. The segment [-204/-129] contained two direct repeat motifs homologous to other VDREs found in the osteocalcin and osteopontin genes. Synthetic oligonucleotides containing the putative VDRE were used for functional analyses and gel mobility shift assays. The proximal [-151/-137], but not the distal [-169/-155] direct repeat activated the transcription in response to 1,25-(OH)2D3 through the beta-globin promoter. Furthermore, the proximal direct repeat formed a complex with the vitamin D receptor and a nuclear accessory factor(s) from COS cells (or retinoid X receptor) in the presence of 1,25-(OH)2D3. These results indicate that a direct repeat motif, AGGTGAgt-gAGGGCG, located at -151 base pairs upstream in the antisense strand binds to a heterologous dimer consisting of the VDR occupied with 1,25-(OH)2D3 and the nuclear accessory factor and that it plays a critical role in mediating the vitamin D enhancement of the rat P450cc24 gene expression.

Regulation of messenger ribonucleic acid expression of 1 alpha,25-dihydroxyvitamin D3-24-hydroxylase in rat osteoblasts

We have reported that PTH inhibits 25-hydroxyvitamin D3-24-hydroxylase messenger RNA (mRNA) expression induced by 1 alpha,25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2D3] in rat kidney but not intestine. In the present study, we examined whether the suppression of 24-hydroxylase mRNA expression by PTH occurs commonly in tissues and cells which have PTH receptors. Administration of 1 alpha, 25-(OH)2D3 into rats fed a synthetic vitamin D-repleted diet containing adequate calcium greatly increased serum levels of calcium and 1 alpha, 25-(OH)2D3. Also, there was a 4-fold increase in bone 24-hydroxylase activity in response to 1 alpha, 25-(OH)2D3 administration. In rats fed a low calcium diet, renal 24-hydroxylase activity was suppressed probably due to secondary hyperparathyroidism. In contrast, the low calcium feeding did not suppress bone 24-hydroxylase activity. The expression of 24-hydroxylase mRNA in rat osteoblastic C-26 and C-11 cells was similar and attained maximal levels 24 h after cells were incubated with 10(-8) M 1 alpha, 25-(OH)2D3. Induction of 24-hydroxylase mRNA expression by 1 alpha, 25-(OH)2D3 was much greater and earlier in immature C-26 cells than mature C-11 cells. Simultaneous addition of PTH, prostaglandin E2, or cAMP together with 1 alpha, 25-(OH)2D3 did not down-regulate mRNA expression of 24-hydroxylase induced by the vitamin in both C-26 and C-11 cells. Of the three osteoblastic cells (C-26, C-20, and C-11) examined, C-26 cells showed the least mRNA expression of vitamin D receptors, in spite of the highest expression of 24-hydroxylase mRNA. These results suggest that unlike in the kidney, bone 24-hydroxylase mRNA expression is not down-regulated by PTH despite of the presence of PTH receptors. They also suggest that the degree of the induction of 24-hydroxylase mRNA by 1 alpha, 25-(OH)2D3 is not explained simply by the vitamin D receptors content.

The role of gravity in chick embryogenesis

Thirty fertilized chick eggs preincubated for 0, 7 and 10 days on earth (10 eggs each) were flown in the space shuttle 'Endeavour' and further incubated for 7 days under microgravity. Twenty out of thirty eggs (9/10 ten-day-old; 10/10 seven-day-old; 1/10 zero-day-old) were recovered alive after landing. The only living embryo of the zero-day-old group died 24 days after launch, and was comparable to a 16-day-old embryo. The high mortality of the 0-day-old eggs appeared to be related to the specific inner structure of the egg. Simulation experiments performed on earth indicated that when yolk stayed in the albumen for more than 2 days, most of the embryos died. The subtle difference in specific gravity between the yolk (1.029) and albumen (1.040) plays a critical role in early chick embryogenesis.

The mechanisms of regulation of vitamin D metabolism in the kidney

Recently, the 24-hydroxylase enzyme of 25-hydroxyvitamin D3 was purified from rat and chick kidney, and its complementary DNA was cloned. The length of the rat 24-hydroxylase gene is about 15 kbp with 12 exons. Cloning of the 24-hydroxylase gene made it possible to examine the mechanism of regulation of vitamin D metabolism at the gene level. Expression of renal 24-hydroxylase mRNA is regulated by a balance of plasma parathyroid hormone and 1 alpha,25-dihydroxyvitamin D3 levels, whereas intestinal 24-hydroxylase mRNA is regulated mainly by plasma 1 alpha,25-dihydroxyvitamin D3. This article reviews recent progress in understanding the molecular aspects of vitamin D metabolism.

1 alpha,25-dihydroxyvitamin D3 regulates in vivo production of the third component of complement (C3) in bone

We previously reported that 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3] specifically stimulates production of the third component of complement (C3) by murine osteoblastic cells and marrow-derived stromal cells (ST2) in vitro. In the present study we examined tissue-specific production of C3 in vivo in vitamin D-deficient mice, some of which received supplemental 1 alpha,25-(OH)2D3. Western blot analysis indicated that the C3 protein band in bone was undetectable in vitamin D-deficient mice, but became distinct 48 h after 1 alpha,25-(OH)2D3 administration. The mRNA expression of C3 in bone was also undetectable in vitamin D-deficient mice and appeared as early as 24 h after 1 alpha,25-(OH)2D3 administration. mRNA expression apparently preceded the appearance of C3 protein. In contrast, there was no significant difference in the expression of hepatic C3 mRNA among normal mice fed laboratory chow and vitamin D-deficient mice with and without 1 alpha,25-(OH)2D3 administration. The serum concentration of C3 in vitamin D-deficient mice was almost identical to that in normal mice and was unchanged after 1 alpha,25-(OH)2D3 administration. 1 alpha,25-(OH)2D3 receptor (VDR) mRNAs were detected in the kidney and intestine, whereas no appreciable mRNA expression of VDR occurred in the liver. Osteopontin mRNA was expressed in response to 1 alpha,25-(OH)2D3 in the kidney, but not in the intestine. Immunohistochemical studies showed that in normal mice, the C3 protein was located mainly in the periosteal regions of calvaria and on the surfaces of bone trabeculae in the tibial metaphyses. These results demonstrate that 1 alpha,25-(OH)2D3 tissue-specifically regulates in vivo production of C3 in bone. The production of bone C3 cannot be attributed to the presence of VDR alone, and we speculate that other tissue-specific factors are required.

Parathyroid hormone inhibits 25-hydroxyvitamin D3-24-hydroxylase mRNA expression stimulated by 1 alpha,25-dihydroxyvitamin D3 in rat kidney but not in intestine

Using a cDNA probe for rat renal 24-hydroxylase, expression of its mRNA was compared in the rat kidney and intestine. Vitamin D-deficient rats received a single injection of 1 alpha,25-dihydroxyvitamin D3. Expression of 24-hydroxylase mRNA was first detected in the kidney at 3-h post-injection and increased thereafter. Similarly, 24-hydroxylase mRNA was expressed in the intestine after 1 alpha,25-dihydroxyvitamin D3 injection. However, the dose level of 1 alpha,25-dihydroxyvitamin D3 required to induce the intestinal 24-hydroxylase mRNA expression was only 1/100 the amount required to induce renal 24-hydroxylase mRNA. Induction of intestinal 24-hydroxylase mRNA expression by 1 alpha,25-dihydroxyvitamin D3 was far more rapid than that of renal 24-hydroxylase mRNA. Thyroparathyroidectomy shortened the time required to induce expression of renal, but not intestinal, 24-hydroxylase mRNA. Administration of either parathyroid hormone or cAMP to vitamin D-deficient rats greatly reduced the expression of 24-hydroxylase mRNA in the kidney but not in the intestine. When rats were fed a vitamin D-repleted diet containing 0.7% (adequate) or 0.03% (low) calcium for 2 weeks, intestinal expression of 24-hydroxylase mRNA could be induced only in the low calcium group. In contrast, renal mRNA expression was preferentially stimulated in the adequate calcium group. These results clearly demonstrate that the expression of 24-hydroxylase mRNA is down-regulated by parathyroid hormone in the kidney but not in the intestine.

Mechanism of regulation of calcium-pumping activity in chick intestine

The role of the calcium pump in the stimulation of intestinal calcium transport activity by 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] was examined in chicks. The in situ intestinal absorption of calcium increased approximately threefold in the duodenum, jejunum, and ileum 6 h after a single injection of 625 ng of 1 alpha,25(OH)2D3 into vitamin D-deficient chicks. The same treatment also increased approximately twofold the rate of ATP-dependent calcium uptake by the basolateral membrane vesicles (BL) isolated from those three sites. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that a Mg(2+)-dependent calcium-stimulated phosphorylated intermediate with an apparent molecular mass of 105 kDa appeared in the BL. The 1 alpha,25(OH)2D3 treatment gave no change in the levels of the intermediate. Pretreatment of the BL with alkaline phosphatase decreased the calcium uptake by the BL isolated from 1 alpha,25(OH)2D3-treated chicks, but it had little effect on the uptake by the BL from vitamin D-deficient chicks. These results suggest that at an early stage of the 1 alpha,25(OH)2D3-induced intestinal calcium transport process, the vitamin regulates the calcium-pumping activity of chick intestinal BL by phosphorylation and dephosphorylation but not by a stoichiometric change in the pump.

1 alpha,25-dihydroxyvitamin D3 modulation in lipid metabolism in established bone marrow-derived stromal cells, MC3T3-G2/PA6

MC3T3-G2/PA6 (PA6) cells established from newborn mouse calvaria are preadipocytic stromal cells, which differentiate into adipocytes in response to glucocorticoids. We examined the effects of 1 alpha,25-dihydroxyvitamin D3[1 alpha,25(OH)2D3] on adipogenesis in PA6 cells. When PA6 cells were cultured with 10(-8) M dexamethasone, adipocytes containing oil red O-positive droplets first appeared on day 7 (3 days after confluence was attained) and the maximal synthesis of neutral lipids occurred on day 12. Simultaneous addition of 1 alpha,25(OH)2D3 at 10(-9)M completely blocked this dexamethasone-induced neutral lipid synthesis throughout the 14-day culture period. Dose-response studies of vitamin D3 derivatives showed that 1 alpha,25(OH)2D3 was the most potent in inhibiting neutral lipid synthesis in PA6 cells, followed by 1 alpha-hydroxyvitamin D3, 25-hydroxyvitamin D3, and 24R,25-dihydroxyvitamin D3, in that order. Dexamethasone greatly enhanced incorporation of [14C]-acetic acid into triacylglycerol in PA6 cells. The incorporation was markedly inhibited by the addition of 10(-9) M 1 alpha,25(OH)2D3. Instead, 1 alpha,25(OH)2D3 greatly increased incorporation of [14C]-acetic acid into phospholipids, such as phosphatidylcholine and phosphatidylethanolamine, irrespective of the presence or absence of dexamethasone. These results suggest that 1 alpha,25(OH)2D3 modulation of lipid metabolism in bone marrow stromal cells is receptor mediated.

Tissue-specific production of the third component of complement(C3) by vitamin D in bone

The third component of complement (C3) is a protein produced by osteoblastic cells in response to 1 alpha,25(OH)2D3. The bone C3 appears to be involved in differentiation of bone marrow cells into osteoclasts in concert with other vitamin D-dependent factors. Further studies are needed to understand the precise role of C3 in bone.

The specific production of the third component of complement by osteoblastic cells treated with 1 alpha,25-dihydroxyvitamin D3

A 190 kDa protein was purified from conditioned media of mouse marrow-derived stromal cell (ST2) cultures treated with 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) and identified as the third component of mouse complement (C3). Northern and Western blot analysis revealed that the production of C3 by ST2 and primary osteoblastic cells was strictly dependent on 1 alpha,25(OH)2D3, but the production by hepatocytes was not. Adding 1 alpha,25(OH)2D3 together with mouse C3 antibody to bone marrow cultures greatly inhibited the formation of tartrate-resistant acid phosphatase (TRAP)-positive osteoclast-like multinucleated cells. Adding C3 alone induced no TRAP-positive cell formation. These results suggest that, in bone tissues, C3 is specifically produced by osteoblasts in response to 1 alpha,25(OH)2D3 and somehow involved in inducing differentiation of bone marrow cells into osteoclasts in concert with other factors produced by osteoblasts in response to 1 alpha,25(OH)2D3.

Transglutaminase is involved in the fusion of mouse alveolar macrophages induced by 1 alpha, 25-dihydroxyvitamin D3

We have reported that 1 alpha,25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3] induces fusion of mouse alveolar macrophages directly by a mechanism involving spermidine-dependent protein synthesis (Tanaka, H. et. al., 1989, Exp. Cell Res. 180, 72-83). The macrophage fusion induced by 1 alpha,25(OH)2D3 occurred in a calcium-dependent manner (Jin, C.H. et al., 1988, J. Cell. Physiol. 137, 110-116). In the present study, we examined the possibility that transglutaminase, a calcium-dependent enzyme, is involved in the fusion of macrophages induced by 1 alpha,25(OH)2D3. The activity of transglutaminase increased greatly 12 h after 1 alpha,25(OH)2D3 was ended and reached a maximum at 48 h. Western blot analysis of the cell lysate using an anti-transglutaminase antibody showed that 1 alpha,25(OH)2D3 induced a 77-kDa protein corresponding to transglutaminase. When spermidine synthesis was inhibited by adding methylglyoxal bis(guanylhydrazone) (MGBG), an inhibitor of S-adenosylmethionine decarboxylase, the increase in the transglutaminase synthesis by 1 alpha,25(OH)2D3 was markedly inhibited with concomitant inhibition of fusion. Adding more spermidine restored both the synthesis of transglutaminase and the fusion. The treatment of macrophages with cystamine, an inhibitor of transglutaminase, inhibited the fusion in parallel with the suppression of transglutaminase activity, both induced by 1 alpha,25(OH)2D3. These results clearly indicate that 1 alpha,25(OH)2D3 induces transglutaminase by a spermidine-dependent mechanism and that this enzyme is involved in a biological reaction(s) essential for inducing macrophage fusion.

Putrescine is involved in the vitamin D action in chick intestine

We have reported that a single injection of 1 alpha,25-dihydroxyvitamin D3 into vitamin D-deficient chicks produces a marked increase of putrescine accumulation in the duodenum from two different sources, ornithine and spermidine. In the present study, the effects of putrescine depletion and its supplementation on duodenal villus length and calcium absorption were examined in newborn and 5-week-old chicks. Administering either alpha-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, or N1,N4-bis(2,3-butadienyl)-1,4-butanediamine, a specific inhibitor of polyamine oxidase, to newborn chicks significantly decreased the duodenal content of putrescine and calcium transport activity. The putrescine depletion also induced shortening of the duodenal villus length. The inhibition of calcium absorption and villus length in the putrescine-depleted chicks was almost completely restored by administering putrescine to the birds. The effect of the putrescine depletion and its supplementation on the duodenal villus length and the calcium absorption was reproduced in 5-week-old vitamin D-deficient chicks given vitamin D3 or 1 alpha,25-dihydroxyvitamin D3. These results clearly indicate that putrescine is somehow involved in the vitamin D action in maintaining the morphological and functional development of the intestinal villus mucosa.