Independent impairment of osteoblast and osteoclast differentiation in klotho mouse exhibiting low-turnover osteopenia.

We recently identified a new gene, klotho, which is involved in the suppression of multiple aging phenotypes. The mouse homozygous for a disruption of the klotho locus (kl/kl) exhibited multiple pathological conditions resembling human aging. Histomorphometric analysis revealed low-turnover osteopenia in kl/kl mice. The decrease in bone formation exceeded that of bone resorption, resulting in a net bone loss. The number of osteoblast progenitors determined by ex vivo bone marrow cultures was reduced in kl/kl mice. In addition, cultured osteoblastic cells derived from kl/kl mice showed lower alkaline phosphatase activity and matrix nodule formation than those from wild-type mice. Osteoclastogenesis in the coculture of marrow cells and osteoblastic cells was decreased only when marrow cells originated from kl/kl mice independently of the origin of osteoblastic cells. We also found that the expression of osteoprotegerin, an osteoclastogenesis inhibitor, was significantly upregulated in kl/kl mice. We conclude that a defect in the klotho gene expression causes the independent impairment of both osteoblast and osteoclast differentiation, leading to low-turnover osteopenia. Because this state represents a characteristic feature of senile osteoporosis in humans, kl/kl mice can be regarded as a useful model for investigating cellular and molecular mechanisms of age-related bone loss.

Estrogen deficiency stimulates B lymphopoiesis in mouse bone marrow.

We have found that an estrogen deficiency causes a marked increase in bone marrow cells. To examine the effect of estrogen on hemopoiesis, we characterized the increased population of bone marrow cells after ovariectomy (OVX). In OVX mice, the percentage of myeloid cells and granulocytes was decreased, whereas that of B220-positive B lymphocytes was selectively increased 2-4 wk after surgery. The total number of myeloid cells and granulocytes did not change appreciably, but that of B220-positive cells was greatly increased by OVX. When OVX mice were treated with estrogen, the increased B lymphopoiesis returned to normal. B220-positive cells were classified into two subpopulations, B220low and B220high. The majority of the B220low cells were negative for the IgM mu chain, whereas most of the B220high cells were mu-positive. OVX selectively increased the precursors of B lymphocytes identified by B220low. mu-negative phenotype, suggesting that an estrogen deficiency stimulates accumulation of B lymphocyte precursors. When bone marrow-derived stromal cells (ST2) were pretreated with estrogen then co-cultured with bone marrow cells in the presence of estrogen, the stromal cell-dependent B lymphopoiesis was greatly inhibited. The present study suggests that estrogen plays an important role in the regulation of B lymphocyte development in mouse bone marrow.

Alternative differentiation of human promyelocytic leukemia cells (HL-60) induced selectively by retinoic acid and 1 alpha,25-dihydroxyvitamin D3.

Induction of hematopoietic differentiation was investigated in human promyelocytic leukemia cells [HL-60] using two lipophilic vitamins, retinoic acid and 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3]. Both vitamins suppressed proliferation and induced differentiation of HL-60 cells, but 1 alpha,25(OH)2D3 was 70- to 100-fold more potent than was retinoic acid on a molar basis. Simultaneous treatment with suboptimal concentrations of 1 alpha,25(OH)2D3 (0.12 to 1.2 nM) and retinoic acid (10 to 100 nM) showed additive effects in reducing nitroblue tetrazolium, a common marker for monocyte-macrophage and granulocyte differentiation. For the study of alternative differentiation of the cells by the two vitamins, we used monoclonal antibodies specific for either human monocyte-macrophages or granulocytes and other markers specific for macrophage differentiation such as alpha-naphthyl acetate esterase activity and adherence to the dish surface. HL-60 cells were induced to differentiate alternatively into macrophages by 1 alpha,25(OH)2D3 or into granulocytes by retinoic acid. When HL-60 cells were treated with various concentrations of 1 alpha,25(OH)2D3 (1.2 to 120 nM) in the presence of 1000 nM retinoic acid which is a concentration sufficient to induce maximal granulocyte differentiation, the appearance of the markers for monocyte-macrophage differentiation by 1 alpha,25(OH)2D3 was not at all affected by the retinoic acid. These results indicate that 1 alpha,25(OH)2D3 and retinoic acid have additive effects in inducing differentiation of HL-60 cells, but monocyte-macrophage differentiation by 1 alpha,25(OH)2D3 occurs much more readily than does granulocyte differentiation by retinoic acid.

Potent estrogen agonists based on carborane as a hydrophobic skeletal structure. A new medicinal application of boron clusters.

BACKGROUND: Carboranes (dicarba-closo-dodecaboranes) are a class of carbon-containing polyhedral boron-cluster compounds having remarkable thermal stability and exceptional hydrophobicity. Applications of the unique structural and chemical properties offered by icosahedral carboranes in boron neutron capture therapy have received increasing attention over the past 30 years. However, these features of carboranes may allow another application as a hydrophobic pharmacophore in biologically active molecules that interact hydrophobically with receptors. RESULTS: We have designed candidate estrogen-receptor-binding compounds having carborane as a hydrophobic skeletal structure and synthesized them. The most potent compound bearing a carborane cage exhibited activity at least 10-fold greater than that of 17beta-estradiol in the luciferase reporter gene assay. Estrogen receptor-alpha-binding data for the compound were consistent with the results of the luciferase reporter gene assay. The compound also showed potent in vivo effects on the recovery of uterine weight and bone loss in ovariectomized mice. CONCLUSION: Further development of the potent carborane-containing estrogenic agonists described here, having a new skeletal structure and unique characteristics, should yield novel therapeutic agents, especially selective estrogen receptor modulators. Furthermore, the suitability of the spherical carborane cage for binding to the cavity of the estrogen receptor-alpha ligand-binding domain should provide a basis for a similar approach to developing novel ligands for other steroid receptors.

Expression of reg/PSP, a pancreatic exocrine gene: relationship to changes in islet beta-cell mass.

A cDNA termed reg was recently isolated by differential screening of a library prepared from regenerating islets isolated from pancreatic remnants of rats subjected to 90% pancreatectomy and nicotinamide treatment. This led to speculation that this gene may be involved in expansion of beta-cell mass. In the current study we have measured reg expression after implantation and resection of a solid insulinoma tumor into rats, maneuvers known, respectively, to reduce and reexpand the volume of beta-cells in the islet. Animals with an implanted insulinoma tumor became profoundly hypoglycemic. Islet beta-cells declined from the normal 75% of total islet volume to less than 30%, in concert with a marked reduction in the reg mRNA level. Removal of the tumor resulted in a sharp increase in beta-cell replication, as measured by [3H]thymidine incorporation and a return to normal beta-cell volume within 4 days of tumor resection. This was associated with a transient induction in reg expression compared to that in tumor-bearing animals, effectively returning the amount of reg mRNA to the levels found in normal animals within 48 h; at later time points after tumor removal (3-7 days) reg expression declined, but then rose toward normal. In situ hybridization analysis localized the initial induction in reg mRNA expression to the exocrine pancreas. Continuous infusion of insulin into normal rats for 4 days, a maneuver that does not significantly reduce beta-cell mass, resulted in dramatically reduced insulin mRNA in islets, but no change in the levels of reg mRNA. We conclude that the diminution in pancreatic beta-cell mass caused by subcutaneous implantation of an insulinoma is associated with reduced reg gene expression and that the increase in beta-cell replication after resection of the tumor is preceded by return of reg gene expression toward normal.

Effects of retinoic acid on the activation and fusion of mouse alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3.

1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] directly induces both fusion and cytotoxicity in murine alveolar macrophages. Unlike 1 alpha,25(OH)2D3, retinoic acid per se did not induce fusion of alveolar macrophages, but it greatly enhanced the 1 alpha,25(OH)2D3-induced fusion every time the macrophages were treated simultaneously with the two vitamins. The giant cells induced by the two vitamins were much larger than those induced by 1 alpha,25(OH)2D3 alone. The macrophages treated with 1 alpha,25(OH)2D3 started to fuse 36 h after the addition of the vitamin, whereas the macrophages pretreated with retinoic acid for 24 h began to fuse immediately after 1 alpha,25(OH)2D3 was added. 1 alpha,25(OH)2D3 and retinoic acid activated alveolar macrophages similarly, measured by the enhancement of glucose consumption and the induction of cytotoxicity against tumor cells, though 1 alpha,25(OH)2D3 was 100 times more potent than retinoic acid on a molar basis. Simultaneous treatment with physiological concentrations of 1 alpha,25(OH)2D3 (0.12 nM) and retinoic acid (10 nM) induced cytotoxicity additively. Morphological examinations revealed that the treated cells were enlarged and flattened with numerous filopodia. These results clearly indicate that both 1 alpha,25(OH)2D3 and retinoic acid similarly activate alveolar macrophages, and the activated state is prerequisite to the fusion of macrophages induced by 1 alpha,25(OH)2D3.

An ultrastructural study on the multinucleation process of mouse alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3.

The multinucleation process of isolated alveolar macrophages induced by 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] was examined using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). At the beginning of culture, most of the macrophages were spherical in shape. During incubation with 1.2 X 10(-8) M 1 alpha,25(OH)2D3, spreading macrophages appeared among the spherical macrophages, and they increased in number. Spreading macrophages extended many cytoplasmic processes toward adjacent macrophages, and interdigitations of these processes between those of neighboring cells were often seen. Two types of cell contact have been observed in the 1 alpha,25(OH)2D3-treated cells. In some, cytoplasmic processes were put into the cytoplasm of the adjacent cells, where clathrinlike structures were observed at the inner membrane of the concave portion. In others, spreading macrophages occasionally came in contact with adjacent cells by a peripheral rim of their cytoplasm with gap junctions. Cytoplasmic continuity was rarely observed at the boundaries between the closely associated cells. The two types of cell contact were also found, though not frequently, in the untreated cells. These results indicate that 1 alpha,25(OH)2D3 promotes multinucleation of alveolar macrophages through spreading forms with the formation of gap junctions and the coated membrane invagination.

Comparative effects of estrogen and raloxifene on B lymphopoiesis and bone loss induced by sex steroid deficiency in mice.

Estrogen deficiency caused by ovariectomy (OVX) results in a marked bone loss because of stimulated bone resorption. We have reported that OVX selectively stimulates B lymphopoiesis in mouse bone marrow, which is somehow related to bone resorption. Estrogen prevents both the increased B lymphopoiesis and the bone resorption caused by estrogen deficiency. Raloxifene also has a potent estrogenic activity for bone with minimal estrogenic activity for the uterus. To examine the effects of raloxifene on B lymphopoiesis and bone resorption, OVX mice were given either estrogen or raloxifene subcutaneously for 2-4 weeks using a miniosmotic pump. Reduced uterine weight in OVX mice was restored completely by 17beta-estradiol (E2). Some 300-fold higher doses of raloxifene increased uterine weight of OVX mice, but only slightly. The number of B220- positive pre-B cells was increased markedly in bone marrow after OVX. The increased B lymphopoiesis was prevented not only by E2 but by raloxifene. In OVX mice, the trabecular bone volume (BV) of the femoral distal metaphysis was reduced markedly, when measured by microcomputed tomography (microCT) scanning and dual-energy X-ray absorptiometry. Both E2 and raloxifene similarly restored it. Like estrogen deficiency, androgen deficiency induced by orchidectomy (ORX) also resulted in a marked bone loss and increased B lymphopoiesis. Both E2 and raloxifene prevented the changes in ORX mice. These results indicate that both estrogen deficiency and androgen deficiency similarly stimulate B lymphopoiesis in mouse bone marrow, which accompany bone loss. Raloxifene exhibits estrogenic actions in bone and bone marrow to prevent bone loss and regulate B lymphopoiesis without inducing estrogenic action in the uterus.

Endogenous bone-resorbing factors in estrogen deficiency: cooperative effects of IL-1 and IL-6.

Estrogen deficiency causes a marked bone loss by stimulating osteoclastic bone resorption. To explore the endogenous bone-resorbing factors involved in estrogen deficiency, we examined the bone-resorbing activity present in the supernatant fraction of mouse bone marrow collected from ovariectomized (OVX) mice. Adding bone marrow supernatants at 20-80% to organ cultures of mouse long bones dose-dependently stimulated bone resorption. The endogenous bone-resorbing activity present in bone marrow supernatants from OVX mice was much higher than that from sham-operated mice 2-4 weeks after surgery, and it was significantly diminished by indomethacin in vitro. Anti-IL-1 alpha antibody completely neutralized the bone-resorbing activity present in bone marrow supernatants from OVX mice. Antibodies against IL-1 beta, IL-6, and IL-6 receptors also neutralized it, but partially. The concentration of IL-1 alpha measured by ELISA was much higher in bone marrow supernatants than in sera, but it was not appreciably changed before or after OVX. The concentration of IL-1 beta in bone marrow supernatants from OVX mice was less than the detection limit. OVX stimulated IL-1 activity in bone marrow supernatants measured by means of the proliferation of thymocytes. However, the level of IL-1 alpha present in bone marrow supernatants from OVX mice was insufficient to stimulate bone resorption. Compared with the serum concentration, bone marrow supernatants contained a much higher level of IL-6 as well, and it was further increased by OVX. However, IL-6 alone present in bone marrow supernatants from OVX mice again did not stimulate bone resorption.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular calcium is involved in the mechanism of differentiation of mouse myeloid leukemia cells (M1) induced by 1 alpha, 25-dihydroxyvitamin D3.

We have reported that 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3] suppresses proliferation and induces differentiation of murine myeloid leukemia cells (M1) into macrophages. In the current study, M1 cells were cultured either with 2.0 or 0.15 mM total calcium to examine the effect of calcium on the process of differentiation induced by the vitamin. The 0.15 mM calcium medium greatly enhanced 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3]-induced inhibition of cell growth and suppression of [3H]thymidine incorporation. Addition of Verapamil, a calcium antagonist, to the 2.0 mM calcium medium also elicited similar responses. The absolute number of cells with phagocytic activity induced by 1 alpha, 25(OH)2D3 was almost identical in media containing either concentration of calcium, and in cultures with or without Verapamil. Culture in the 0.15 mM calcium medium or addition of Verapamil to the 2.0 mM calcium medium did not suppress cell growth nor induce phagocytic activity in the absence of the vitamin. To confirm the preferential effect of calcium on cell growth, M1 cells were pretreated for 3 days with 1 alpha, 25(OH)2D3 in either the 2.0 or 0.15 mM calcium medium. Then the pretreated cells were washed and subcultured in the absence of 1 alpha, 25(OH)2D3 in either medium. The growth rate was inhibited much more effectively in the subculture with 0.15 mM calcium than with 2.0 mM calcium. These results suggest that the M1 cells' increased requirement of extracellular calcium, caused by the treatment with 1 alpha, 25(OH)2D3, is closely related to cell growth rather than differentiation.

Alteration of lipid metabolism associated with the activation 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] directly induces fusion and tumoricidal activity (activation) in murine alveolar macrophages. In this study we examined lipid metabolism associated with the fusion and activation of alveolar macrophages induced by 1 alpha, 25-(OH)2D3. Treatment of alveolar macrophages with 12 nM 1 alpha, 25-(OH)2D3 for 48 h caused a marked increase in incorporation of [14C]acetic acid and [14C]oleic acid into triacylglycerol. The macrophages treated with the vitamin began to fuse and show cytotoxicity at 48 h, whereas incorporation of the radioactive compounds into triacylglycerol started as early as 12 h after 1 alpha, 25-(OH)2D3 was added. The triacylglycerol synthesis induced by 1 alpha, 25-(OH)2D3 was greatly increased when 14C-labeled unsaturated fatty acids were used as tracers compared with 14C-labeled saturated fatty acids. The activity of diacylglycerol acyltransferase, which catalyzes the last step of the three acylations in triacylglycerol synthesis, was significantly higher in the macrophages treated with 1 alpha, 25-(OH)2D3 than in the control macrophages. Like 1 alpha, 25-(OH)2D3, retinoic acid and lypopolysaccharides also activated alveolar macrophages, but not induce any fusion. The activated macrophages cultured with retinoic acid or lypopolysaccharides also induced synthesis of triacylglycerol. These results indicate that 1 alpha, 25-(OH)2D3 induces the synthesis of triacylglycerol by preferentially incorporating unsaturated fatty acids into diacylglycerol, and that the alteration of lipid metabolism is related to the activation, rather than the fusion, of alveolar macrophages.

Expression of estrogen receptor beta in rat bone.

A novel estrogen receptor, estrogen receptor beta (ERbeta), has recently been cloned from a rat prostate cDNA library. In bone, which is an important target tissue of estrogen, ER alpha has been reported to be present preferentially in osteoblasts, but the mechanism of action of estrogen in bone is still not known. In the present study, we examined expression of ERbeta mRNA in bone. Expression of ERbeta mRNA was evident in primary osteoblastic cells isolated from 1-day-old rat calvaria and rat osteosarcoma cells (ROS 17/2.8), and its level was higher than that of ER alpha mRNA. When osteoblastic cells were cultured for 28 days to induce differentiation into mature osteoblasts capable of forming bone nodules, ERbeta mRNA was constantly and highly expressed during the entire culture period. In contrast, the level of ER alpha mRNA was very low at the beginning of culture and it gradually increased during the differentiation of osteoblastic cells. Various tissues including bone were isolated from 8-week-old rats of both sexes, and total RNA was extracted to compare the tissue distribution of expression levels of ERbeta mRNA. In cancellous bone of the distal femoral metaphysis and lumbar vertebra, expression of ERbeta mRNA was obvious, and its level was equivalent to those in the uterus and testis, but lower than those in the ovary and prostate. The level of ERbeta mRNA in femoral cortical bone was lower than that in cancellous bone. There was no appreciable differences between female and male rats in the distribution and expression levels of ERbeta mRNA in bone. These results indicate that ERbeta mRNA is highly expressed in osteoblasts in rat bone, suggesting that there is a distinct mechanism of estrogen action mediated by ERbeta in bone.

A synthetic analogue of vitamin D3, 22-oxa-1 alpha,25-dihydroxyvitamin D3, is a potent modulator of in vivo immunoregulating activity without inducing hypercalcemia in mice.

The in vivo immunoregulating activity and the hypercalcemic action of 4 synthetic analogues of vitamin D3 with an oxygen atom in the side chain were compared with those of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] in mice. Oral administration of these vitamin D3 compounds augmented the primary immune response, induced by immunization with a suboptimal number of sheep erythrocytes, without inducing hypercalcemia. The order of the in vivo potency to induce the immune response was 22-oxa-1 alpha,25(OH)2D3 greater than 1 alpha,25(OH)2D3 not equal to 20-oxa-1 alpha,25(OH)2D3 not equal to 22-oxa-1 alpha(OH)D3 greater than 1 alpha(OH)D3 not equal to 20-oxa-1 alpha(OH)D3. 22-Oxa-1 alpha,25(OH)2D3 was about 50 times more potent than 1 alpha,25(OH)2D3 in inducing the in vivo primary immune response, but the former was only 1/100 as active as the latter in inducing hypercalcemia. These results suggest that the immunoregulating activity of vitamin D compounds can be separated structurally from their hypercalcemic action in vivo.

The relationship between fusion and proliferation in mouse alveolar macrophages.

We have reported that protein factors separated from conditioned media of Concanavalin A-stimulated spleen cell cultures induce growth and fusion of mouse alveolar macrophages. A macrophages growth factor (MGF) was purified and, at the final step of purification on HPLC, eluted at the same position as a colony-stimulating factor (CSF), suggesting that MGF is identical with CSF. In the present study, we examined the relationship between proliferation and fusion of macrophages using purified CSF (MGF) and 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3]. The latter was used in place of a macrophage fusion factor which is supposed to be contained in the same conditioned medium, since a macrophage fusion factor has not yet been isolated. Adding less than 5% unfractionated conditioned medium from Concanavalin A-stimulated spleen cells markedly induced proliferation of alveolar macrophages without inducing fusion. In contrast, adding the same unfractionated conditioned medium at concentrations of 10% or more suppressed proliferation dose dependently, whereas it induced fusion reciprocally. Proliferation of macrophages was similarly enhanced by adding purified CSF or retinoic acid. Fusion of macrophages was induced by 1 alpha,25-(OH)2D3, but not by purified CSF or retinoic acid. Adding 1 alpha,25-(OH)2D3 together with purified CSF or retinoic acid completely suppressed the increase of proliferation induced by either growth factor, whereas that treatment rather potentiated the fusion induced by 1 alpha,25-(OH)2D3 alone. These results indicate that the fusion and proliferation of macrophages occur in a reciprocal fashion.

Regulation of matrix metalloproteinases (MMP-2, -3, -9, and -13) by interleukin-1 and interleukin-6 in mouse calvaria: association of MMP induction with bone resorption.

Interleukin-1 (IL-1) greatly induces osteoclast formation and stimulates bone resorption of mouse calvaria in culture. In the presence of soluble IL-6 receptor (sIL-6R), IL-6 similarly induces osteoclast formation, but the potency of IL-6 in inducing bone resorption in organ culture is weaker than that of IL-1. To study the differences in bone-resorbing activity between IL-1 and IL-6, we examined the effects of the two cytokines on the induction of matrix metalloproteinases (MMPs). In mouse calvarial cultures, IL-1 markedly enhanced the messenger RNA (mRNA) expression of MMP-13 (collagenase 3), MMP-2 (gelatinase A), MMP-9 (gelatinase B), and MMP-3 (stromelysin 1), which associated with increases in bone matrix degradation. A hydroxamate inhibitor of MMPs significantly suppressed bone-resorbing activity induced by IL-1. Gelatin zymography showed that both pro- and active-forms of MMP-2 and MMP-9 were detected in the conditioned medium collected from calvarial cultures, and IL-1 markedly stimulated both pro- and active-forms of the two gelatinases. IL-6 with sIL-6R also stimulated mRNA expression and biological activities of these MMPs, but the potency was much weaker than that of IL-1. Conditioned medium collected from IL-1-treated calvariae degraded native type I collagen, but 3/4- and 1/4-length collagen fragments were not detected, suggesting that both collagenases and gelatinases synergistically degraded type I collagen into smaller fragments. In mouse osteoblastic cells, the expression ofMMP-2, MMP-3, and MMP-13 mRNAs could be detected, and they were markedly enhanced by IL-1alpha on days 2 and 5. IL-6 with sIL-6R also induced expression of MMP-13 and MMP-2 mRNAs on day 2, but the expression was rather transient. These results demonstrate that the potency of induction of MMPs by IL-1 and IL-6 is closely linked to the respective bone-resorbing activity, suggesting that MMP-dependent degradation of bone matrix plays a key role in bone resorption induced by these cytokines.

Intracellular calcium and protein kinase C mediate expression of receptor activator of nuclear factor-kappaB ligand and osteoprotegerin in osteoblasts.

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) produced by osteoblasts/stromal cells are involved as positive and negative regulators in osteoclast formation. Three independent signals have been proposed to induce RANKL expression in osteoblasts/stromal cells: vitamin D receptor-, cAMP-, and gp130-mediated signals. We previously reported that intracellular calcium-elevating compounds such as ionomycin, cyclopiazonic acid, and thapsigargin induced osteoclast formation in cocultures of mouse bone marrow cells and primary osteoblasts. Increases in calcium concentration in culture medium also induced osteoclast formation in cocultures. Treatment of primary osteoblasts with these compounds or with high calcium medium stimulated the expression of both RANKL and OPG messenger RNAs (mRNAs). 1,2-Bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)-tetra(acetoxymethyl)ester, an intracellular calcium chelator, suppressed both ionomycin-induced osteoclast formation in cocultures and expression of RANKL and OPG mRNAs in primary osteoblasts. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, also stimulated osteoclast formation in these cocultures and the expression of RANKL and OPG mRNAs in primary osteoblasts. Protein kinase C inhibitors such as calphostin and staurosporin suppressed ionomycin- and PMA-induced osteoclast formation in cocultures and expression of RANKL and OPG mRNAs in primary osteoblasts. Ionomycin stimulated RANKL mRNA expression in ST2 and MC3T3-G2/PA6 cells, but not in MC3T3-E1 or NIH-3T3 cells. These effects were closely correlated with osteoclast formation in response to ionomycin in cocultures with these stromal cell lines. OPG strongly inhibited osteoclast formation induced by calcium-elevating compounds and PMA in cocultures, suggesting that RANKL expression in osteoblasts is a rate-limiting step for osteoclast induction. Forskolin, an activator of cAMP signals, also stimulated osteoclast formation in cocultures. Forskolin enhanced RANKL mRNA expression but suppressed OPG mRNA expression in primary osteoblasts. These results suggest that the calcium/protein kinase C signal in osteoblasts/stromal cells is the fourth signal for inducing RANKL mRNA expression, which, in turn, stimulates osteoclast formation.

Transcriptional induction of cyclooxygenase-2 in osteoblasts is involved in interleukin-6-induced osteoclast formation.

Interleukin-6 (IL-6) induces osteoclast-like cell (osteoclast) formation in a dose-dependent fashion in cocultures of mouse bone marrow cells and osteoblastic cells when soluble IL-6 receptor (sIL-6R) is present. Simultaneous treatment with submaximal doses of IL-1alpha and IL-6 with sIL-6R caused marked induction of osteoclast formation and PGE2 synthesis. These effects were suppressed by adding neutralizing antibodies against IL-1alpha or IL-6R and were totally abolished by adding nonsteroidal antiinflammatory drugs, such as indomethacin and a selective cyclooxygenase-2 (COX-2) inhibitor (NS398). In mouse osteoblastic cells, both IL-1alpha and IL-6 with sIL-6R markedly induced messenger RNA expression of COX-2, but not COX-1, as determined by Northern blot analysis, and luciferase activity in cells stably transfected with a COX-2 promoter-luciferase fusion construct. IL-6 and sIL-6R, when added separately, did not stimulate COX-2 messenger RNA expression. Simultaneous addition of IL-1alpha and IL-6 with sIL-6R to osteoblast cultures cooperatively induced transcription of COX-2, which was associated with a marked increase in COX activity measured by the conversion of arachidonic acid into PGE2. The increased PGE2 synthesis by osteoblasts may play an important role in osteoclastogenesis induced by submaximal doses of IL-1 and IL-6.

The role of prostaglandin E receptor subtypes (EP1, EP2, EP3, and EP4) in bone resorption: an analysis using specific agonists for the respective EPs.

PGE2 functions as a potent stimulator of bone resorption. The action of PGE2 is thought to be mediated by some PGE receptor subtypes present in osteoblastic cells. In this study, we examined the involvement of PGE receptor subtypes, EP1, EP2, EP3, and EP4, in PGE2-induced bone resorption using specific agonists for the respective EPs. In mouse calvaria cultures, EP4 agonist markedly stimulated bone resorption, but its maximal stimulation was less than that induced by PGE2. EP2 agonist also stimulated bone resorption, but only slightly. EP1 and EP3 agonists did not stimulate it at all. RT-PCR showed that osteoblastic cells isolated from newborn mouse calvaria expressed all of the EPs messenger RNA (mRNA). Both EP2 agonist and EP4 agonist induced cAMP production and the expression of osteoclast differentiation factor (ODF) mRNA in osteoblastic cells. Simultaneous addition of EP2 and EP4 agonists cooperatively induced cAMP production and ODF mRNA expression. In mouse bone marrow cultures, EP2 and EP4 agonists moderately induced osteoclast formation, but the simultaneous addition of the two agonists cooperatively induced it, similar to that by PGE2. In calvaria culture from EP4 knockout mice, a marked reduction in bone resorption to PGE2 was found. In EP4 knockout mice, EP4 agonist failed to induce bone resorption, but EP2 agonist slightly, but significantly, induced bone resorption. These findings suggest that PGE2 stimulates bone resorption by a mechanism involving cAMP and ODF, which is mediated mainly by EP4 and partially by EP2.

Selective effects of genistein, a soybean isoflavone, on B-lymphopoiesis and bone loss caused by estrogen deficiency.

Genistein, an isoflavone abundantly present in soybeans, has structural similarity to estrogen, suggesting that genistein may act as a phytoestrogen. To examine the possible role of genistein in hemopoiesis and bone metabolism, female mice were either sham-operated or ovariectomized (OVX), and selected OVX mice were administered genistein for 2-4 weeks (0.1-0.7 mg/day) or 17beta-estradiol (E2; 0.01-0.1 microg/day) s.c., using a miniosmotic pump (Alza Corp., Palo Alto, CA). In OVX mice, uterine weight declined but was completely restored by E2 administration. In contrast, genistein did not demonstrate a reversal of the OVX-induced uterine atrophy. The number of bone marrow cells markedly increased, 2-4 weeks after OVX, and most of these were B220-weakly positive pre-B cells. The increased B-lymphopoiesis was completely restored, not only by E2 but also by genistein administration. In OVX mice, the trabecular bone volume of the femoral distal metaphysis, measured by microcomputed tomography scanning and dual-energy x-ray absorptiometry, was markedly reduced; and genistein restored this, as did E2. These results indicate that genistein exhibits estrogenic action in bone and bone marrow, to regulate B-lymphopoiesis and prevent bone loss, without exhibiting estrogenic action in the uterus. Phytoestrogens may be useful for preventing bone loss caused by estrogen deficiency in females.

Phagocytic cells metabolize 25-hydroxyvitamin D3 to 10-oxo-19-nor-25-hydroxyvitamin D3 and a new metabolite, 8 alpha,25-dihydroxy-9,10-seco-4,6,10(19)-cholestatrien-3-one.

The metabolism of 25-hydroxyvitamin D3 [25(OH)D3] was examined in several phagocytic cells including alveolar macrophages and myeloid leukemia cells (M1, HL-60 and U937). Phagocytic cells converted 25(OH)D3 to 10-oxo-19-nor-25-hydroxyvitamin D3 and a new metabolite. The former metabolite was dominant in shorter incubation periods (1 h), whereas the latter dominated over longer incubation periods (24 h). The new metabolite was produced from 25(OH)D3 directly but not through 10-oxo-19-nor-25-hydroxyvitamin D3. The new metabolite was unequivocally identified as 8 alpha,25-dihydroxy-9-10-seco-4,6,10(19)-cholestatrien-3-one. These results suggest that phagocytic cells somehow promote oxidation of the triene part of vitamin D compounds.