Tumor necrosis factor type alpha (cachectin) stimulates mouse osteoblast-like cells (MC3T3-E1) to produce macrophage-colony stimulating activity and prostaglandin E2

To elucidate the mechanism of tumor necrosis factor alpha (TNF-alpha)-induced bone resorption, the effects of recombinant human TNF-alpha on mouse osteoblast-like cells (MC3T3-E1) were studied. TNF-alpha stimulated MC3T3-E1 cells to produce prostaglandin E2 (PGE2) and macrophage colony stimulating activity (M-CSA) in a dose-dependent manner. TNF decreased alkaline phosphatase (AL-P) activity of MC3T3-E1 cells. These TNF effects were observed at 1 ng/ml (approximately 6 X 10(-11)M). The inhibitory effect on AL-P activity was reversible and the cell growth of MC3T3-E1 cells was only slightly affected by TNF. These findings suggest that both PGE2 and M-CSA stimulated by TNF-alpha are possibly involved in osteoblast-mediated osteoclastic bone resorption, whereas inhibition of AL-P activity may lead to a decrease in bone formation.     

An Epstein-Barr virus-transformed B cell line produces autoregulatory interleukin-1 that regulates bone remodeling

In this study, we demonstrate that an Epstein-Barr virus-transformed B cell line, A-11, produced interleukin-1 (IL-1), a cytokine that regulates bone remodeling. A-11 cells produce IL-1 in a cell dose- and culture time-related manner. The IL-1 activity was neutralized by recombinant human IL-1 (rhIL-1) alpha antiserum, but not by rhIL-1 beta antiserum. The IL-1 was semi-purified by (NH4)2SO4 precipitation, Superose prep 12 gel filtration, and anion-exchange chromatography strongly stimulated in vitro bone resorption. The stimulatory effect of the purified IL-1 on bone resorption was prostaglandin independent. Purified IL-1 inhibited DNA and collagen synthesis in the osteoblastic cell line MC3T3-E1. However, it enhanced significantly the cellular activity of alkaline phosphatase (EC 3.1.3.1), a marker enzyme for differentiation of osteoblasts. On the other hand, A-11 cell proliferation was inhibited by addition of rhIL-1 alpha antiserum, but not by rhIL-1 beta antiserum. And cell proliferation was stimulated by exogenous rhIL-1 alpha and -beta.     

IL-6 is produced by osteoblasts and induces bone resorption

To examine the possible involvement of IL-6 in bone metabolism, a mouse osteoblastic cell line (MC3T3-E1) and primary osteoblast-like cells from fetal mouse calvaria were cultured with several systemic and local bone-resorbing agents and their expression of IL-6 mRNA was determined. Local bone-resorbing agents such as IL-1 alpha, IL-1 beta, TNF-alpha, and LPS greatly induced IL-6 mRNA expression in both MC3T3-E1 cells and primary osteoblast-like cells. Parathyroid hormone slightly increased expression of IL-6 mRNA in primary osteoblast-like cells but not in MC3T3-E1 cells. Neither IL-6 nor 1 alpha,25-dihydroxyvitamin D3 increased expression of IL-6 mRNA in either of the osteoblast-like cells. In agreement with the expression of IL-6 mRNA, biologically active IL-6 was produced in response to the treatment with IL-1 alpha, TNF-alpha, and LPS in MC3T3-E1 cells. Adding IL-6 dose dependently stimulated the release of 45Ca from prelabeled fetal mouse calvaria. Simultaneously adding suboptimal concentrations of IL-6 and IL-1 alpha induced bone resorption cooperatively. In accord with the increase in the release of 45Ca by IL-6, there were three times as many osteoclasts in the bone sections of calvaria cultured with IL-6 for 5 days as in the controls. IL-6 slightly suppressed alkaline phosphatase activity and collagen synthesis in MC3T3-E1 cells. These results indicate that IL-6 is also produced by osteoblasts, preferentially in response to local bone-resorbing agents, and it induces bone resorption both alone and in concert with other bone-resorbing agents.     

Stimulation of prostaglandin E2 and bone resorption by recombinant human interleukin 1 alpha in fetal mouse bones

Recombinant human interleukin 1 alpha (rhIL-1 alpha) stimulates prostaglandin E2 and bone resorption in cultured forearm bones of fetal mouse in a dose-dependent manner: the minimal rhIL-1 alpha to elicit a significant bone resorption was 1.6 ng/ml (89 pM). The half maximal concentrations to elicit bone resorption and thymocyte proliferation were 3.3 ng/ml (183 pM) and 0.31 ng/ml (17 pM), respectively. The bone resorbing activity induced by IL-1 was partially inhibited by indomethacin and hydrocortisone, and completely inhibited by anti-IL 1-antibody. There was a good correlation between PGE2 production and bone resorption induced by IL-1 alpha. These results suggest that rhIL-1 alpha stimulates bone resorption at approximately 10 times the concentrations necessary for thymocyte proliferation and that PGE2 produced in the bone is at least in part involved in osteoclastic bone resorption.     

Effects of recombinant human interleukin 1 alpha and interleukin 1 beta on cell growth and alkaline phosphatase of the mouse osteoblastic cell line MC3T3-E1

Recombinant human interleukin 1 (rhIL-1)alpha and rhIL-1 beta were examined for their effects on DNA synthesis, cell growth and alkaline phosphatase activity of the mouse osteoblastic cell line MC3T3-E1. The relative activity of rhIL-1 alpha and rhIL-1 beta was compared in terms of the units which induced half-maximal [3H]thymidine uptake into mouse thymocyte cultures exposed to IL-1. Both rhIL-1 alpha and rhIL-1 beta significantly inhibited DNA synthesis and division of the cells in a concentration- and cultivation time-dependent fashion. In contrast, rhIL-1 alpha and rhIL-1 beta markedly increased alkaline phosphatase activity, which is a marker of osteoblastic differentiation. This activity in cells treated with rhIL-1 alpha and rhIL-1 beta increased about 2.0- and 1.7-fold, respectively, compared with that of control cultures. Inhibition of the DNA synthesis and stimulation of alkaline phosphatase activity by both types of rhIL-1 were completely neutralized by treatment with their respective polyclonal antisera. Also, inhibition of DNA synthesis was unaffected by the addition of cyclooxygenase and lipoxygenase inhibitors, and stimulation of alkaline phosphatase activity was unaffected by the addition of indomethacin. These results indicate that both rhIL-1 alpha and rhIL-1 beta have qualitatively similar biological effects on osteoblastic cells. They also suggest that IL-1 is an important modulator of the growth and differentiation of osteoblasts.     

Effects of transforming growth factor-beta and IL-1 alpha on prostaglandin synthesis in serum-deprived osteoblastic cells.

We investigated the effects that the combination of IL-1 alpha and transforming growth factor-beta (TGF-beta) had on PGE2 production in a murine clonal osteoblastic cell line MC3T3-E1 and primary rat calvarial osteoblast-like cells. In serum-supplemented medium, IL-1 alpha was a potent stimulator of PGE2 production in MC3T3-E1 cells (50-fold increase with 0.1 ng/ml). TGF-beta (10 ng/ml) had only a small effect alone and no additional effect on IL-1 alpha-induced responses. In serum-deprived MC3T3-E1 cells, PGE2 responses to IL-1 alpha were either absent or markedly reduced. TGF-beta alone had small effects. However, simultaneous addition of TGF-beta with IL-1 alpha to MC3T3-E1 cells partially restored the ability of IL-1 alpha to generate a PGE2 response (10-fold increase in PGE2 with 0.1 ng/ml of both IL-1 alpha and TGF-beta). As with MC3T3-E1 cells, serum-deprived primary fetal rat calvarial osteoblastic cells also did not respond to IL-1 alpha, unless TGF-beta was present in the medium (sixfold increase in PGE2 with 0.1 ng/ml IL-1 alpha and 10 ng/ml TGF-beta). The synergistic effect of TGF-beta and IL-1 alpha was specific for PGE2 responses, because these factors did not synergistically affect cell proliferation, collagen and noncollagen protein synthesis, or alkaline phosphatase activity. The observed synergy was not associated with changes in the steady state cyclooxygenase (PGH synthase) mRNA levels. However, it did correlate with increased release of [3H]arachidonic acid from prelabeled serum-depleted MC3T3-E1 cells. Hence, the synergistic interactions of IL-1 alpha and TGF-beta on PGE2 appear to occur through an increase in the release of arachidonic acid substrate from phospholipid pools. These effects may be important for both normal bone turnover and the responses of bone to inflammatory and immune stimuli.     

Effects of arachidonic acid, docosahexaenoic acid, prostaglandin E(2) and parathyroid hormone on osteoprotegerin and RANKL secretion by MC3T3-E1 osteoblast-like cells

Bone is continuously remodeled through resorption by osteoclasts and the subsequent synthesis of the bone matrix by osteoblasts. Cell-to-cell contact between osteoblasts and osteoclast precursors is required for osteoclast formation. RANKL (receptor activator of nuclear factor-kappaB ligand) expressed on osteoblastic cell membranes stimulates osteoclastogenesis, while osteoprotegerin (OPG) secreted by osteoblasts inhibits osteoclastogenesis. Although polyunsaturated fatty acids (PUFAs) have been implicated in bone homeostasis, the effects thereof on OPG and RANKL secretion have not been investigated. MC3T3-E1 osteoblasts were exposed to the n-6 PUFA arachidonic acid (AA) and the n-3 PUFA docosahexaenoic acid (DHA); furthermore, the bone-active hormone parathyroid hormone (PTH) and the effects thereof were tested on OPG and RANKL secretion. Prostaglandin E(2) (PGE(2)), a product of AA metabolism that was previously implicated in bone homeostasis, was included in the study. AA (5.0-20 microg/ml) inhibited OPG secretion by 25-30%, which was attenuated by pretreatment with the cyclooxygenase blocker indomethacin, suggesting that the inhibitory effect of AA on OPG could possibly be PGE(2)-mediated. MC3T3-E1 cells secreted very low basal levels of RANKL, but AA stimulated RANKL secretion, thereby decreasing the OPG/RANKL ratio. DHA suppressed OPG secretion to a smaller extent than AA. This could, however, be due to endogenous PGE(2) production. No RANKL could be detected after exposing the MC3T3-E1 cells to DHA. PTH did not affect OPG secretion, but stimulated RANKL secretion. This study demonstrates that AA and PTH reduce the OPG/RANKL ratio and may increase osteoclastogenesis. DHA, however, had no significant effect on OPG or RANKL in this model.     

Intercellular signaling between ameloblastoma and osteoblasts

Ameloblastoma is an odontogenic tumor located in the bone jaw with clinical characteristics of extensive bone resorption. It is a locally invasive tumor with a high recurrence rate despite adequate surgical removal. In bone disease, tumors and other cells including osteoblasts, osteoclasts, and osteocytes in the bone microenvironment contribute to the pathogenesis of tumor growth. However, the effect of osteoblasts on ameloblastoma cells is not well-understood, and there has been limited research on interactions between them.This study investigated interactions between ameloblastoma cells and osteoblasts using a human ameloblastoma cell line (AM-3 ameloblastoma cells) and a murine pre-osteoblast cell line (MC3T3-E1 cells). We treated each cell type with the conditioned medium by the other cell type. We analyzed the effect on cytokine production by MC3T3-E1 cells and the production of MMPs by AM-3 cells. Treatment with AM-3-conditioned medium induced inflammatory cytokine production of IL-6, MCP-1, and RANTES from MC3T3-E1 cells. The use of an IL-1 receptor antagonist suppressed the production of these inflammatory cytokines by MC3T3-E1 cells stimulated with AM-3-conditioned medium. The MC3T3-E1-conditioned medium triggered the expression of MMP-2 from AM-3 cells. Furthermore, we have shown that the proliferation and migration activity of AM-3 cells were accelerated by MC3T3-E1 conditioned media.In conclusion, these intercellular signalings between ameloblastoma cells and osteoblasts may play multiple roles in the pathogenesis of ameloblastoma.     

Platelet-activating factor stimulates production of prostaglandin E2 in murine osteoblast-like cell line MC3T3-E1.

We found that platelet-activating factor (PAF) stimulated the production of prostaglandin (PG) E2 in MC3T3-E1 cells in a time- and dose-dependent manner. 1.0 microM PAF gave a maximal stimulation of PGE2 production by MC3T3-E1 cells after a 4 hr PAF-treatment. Furthermore, the PAF-induced PGE2 production was abolished by the pre-treatment of the cells with a PAF receptor antagonist, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho(N,N,N-trimethyl)hexanolamine, which occupied the same receptor site as PAF. These results suggest that PAF stimulates the PGE2 synthesis through a PAF receptor mediated pathway. Possibly PAF modulates bone metabolism by stimulating PGE2 synthesis.     

Inductive effects of prostaglandins on alkaline phosphatase in osteoblastic cells, clone MC3T3-E1

The effects of prostaglandins (PGs) on the induction of alkaline phosphatase (ALP) were investigated in osteoblastic clone MC3T3-E1 cells cultured in serum-free medium. Prostaglandin E2 (PGE2) stimulated ALP activity in the cells in a dose-dependent fashion with a maximal effect which was about twice that in the control cells at concentrations of 100-500 ng/ml. Actinomycin D and cycloheximide inhibited the stimulative effect of PGE2 on ALP activity in the cells. PGE2-induced and native ALPs in the cells were of the same type as that in adult mouse calvaria, being heat-labile, L-homoarginine- and levamisole-sensitive, and L-phenylalanine-insensitive. Isobutyl methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, stimulated the inductive effect of PGE2 on ALP activity at 0.1 mM, at which concentration IBMX alone had little effect on the activity. PGE2 also increased the intracellular cAMP content in a dose-dependent fashion with a maximal effect at 100 ng/ml. PGE1, PGF1 alpha, and PGF2 alpha (primary PGs like PGE2) increased the activity. Our present results suggest that PGs stimulate the differentiation of osteoblasts and are involved in bone formation in vivo, as well as in bone resorption.     

Leukemia inhibitory factor/differentiation-stimulating factor (LIF/D-factor): regulation of its production and possible roles in bone metabolism.

Leukemia inhibitory factor/differentiation-stimulating factor (LIF/D-factor), expression of its mRNA, and possible roles in bone metabolism were studied in murine primary and clonal osteoblast-like cells. Local bone-resorbing factors such as IL-1, TNF alpha, and LPS strongly induced expression of LIF/D-factor mRNA in both clonal MC3T3-E1 cells and primary osteoblast-like cells. Neither parathyroid hormone nor 1 alpha,25-dihydroxyvitamin D3 stimulated expression of LIF/D-factor mRNA. LIF/D-factor per se did not stimulate expression of its own mRNA. Appreciable amounts of LIF/D-factor were detected in synovial fluids from rheumatoid arthritis (RA) patients but not in those with osteoarthritis (OA). Simultaneous treatment with LIF/D-factor, IL-1, and IL-6 at the concentrations found in synovial fluids from RA patients greatly enhanced bone resorption, though these cytokines did not stimulate bone resorption when separately applied. This suggests that LIF/D-factor produced by osteoblasts is in concert with other bone-resorbing cytokines such as IL-1 and IL-6 involved in the bone resorption seen in the joints of RA patients. LIF/D-factor specifically bound to MC3T3-E1 cells with an apparent dissociation constant of 161 pM and 1,100 binding sites/cell. LIF/D-factor dose-dependently suppressed incorporation of [3H]thymidine into MC3T3-E1 cells. In addition, it potentiated the alkaline phosphatase activity induced by retinoic acid, though LIF/D-factor alone had no effect on enzyme activity. These results suggest that LIF/D-factor is involved in not only osteoclastic bone resorption but also osteoblast differentiation in conjugation with other osteotropic factors.     

Interleukin (IL)-17 enhances prostaglandin F(2 alpha)-stimulated IL-6 synthesis in osteoblasts

We previously showed that prostaglandin F(2alpha) (PGF(2alpha)) and endothelin-1 (ET-1) induce interleukin (IL)-6 through the activation of protein kinase C-dependent p44/p42 mitogen-activated protein (MAP) kinase in osteoblast-like MC3T3-E1 cells. It has recently been reported that tumor necrosis factor-alpha-induced IL-6 synthesis is amplified by IL-17 in these cells. In the present study, we investigated the effect of IL-17 on the IL-6 synthesis stimulated by PGF(2alpha) in MC3T3-E1 cells. IL-17 significantly enhanced the PGF(2alpha)-induced IL-6 synthesis in a dose-dependent manner in the range between 0.1 and 10 ng/ml. IL-17 also enhanced the IL-6 synthesis stimulated by 12- O -tetradecanoylphorbol-13-acetate, a direct activator of protein kinase C. In addition, IL-17 amplified the IL-6 synthesis induced by ET-1. However, IL-17 hardly affected the phosphorylation of p44/p42 MAP kinase induced by PGF(2alpha) or ET-1. These results strongly suggest that IL-17 enhances the IL-6 synthesis stimulated by PGF(2alpha) as well as ET-1 in osteoblasts, and that the effect is exerted at a point downstream from p44/p42 MAP kinase.     

The neuropeptide VIP potentiates IL-6 production induced by proinflammatory osteotropic cytokines in calvarial osteoblasts and the osteoblastic cell line MC3T3-E1

Skeletal turnover is orchestrated by a complex network of regulatory factors. Lately, regulation of bone metabolism through neuro-osteological interactions has been proposed. Here, we address the question whether IL-6 production can be affected by interactions between the neuropeptide VIP and proinflammatory, bone-resorbing cytokines. By using calvarial osteoblasts, we showed that IL-1beta increased IL-6 production time- and concentration-dependently, and that these effects were potentiated by VIP. Furthermore, IL-1beta stimulated IL-6 promoter activity in the osteoblastic cell line MC3T3-E1 stably transfected with a human IL-6 promoter/luciferase construct, and both VIP, and the related neuropeptide PACAP-38, increased the effect of IL-1beta in a synergistic manner. The IL-6 protein release from calvarial osteoblasts was also stimulated by the osteoclastogenic, proinflammatory cytokines IL-11, LIF, OSM, IL-17, TGF-beta, and TNF-alpha. All effects, except for that of TNF-alpha, were synergistically potentiated by VIP. These findings further support the role of neuropeptides, and the presence of neuro-immunological interactions, in bone metabolism.     

Synthesis of colony-stimulating factor (CSF) and differentiation-inducing factor (D-factor) by osteoblastic cells, clone MC3T3-E1

The role of osteoblasts in inducing the proliferation and differentiation of bone marrow cells was examined. Conditioned medium obtained from mouse osteoblastic cell (MC3T3-E1) cultures stimulated colony formation of mouse bone marrow cells (CSF) and differentiation of mouse myeloid leukemia cells (M1) into macrophage-like cells (D-factor). The CSF activity increased time dependently in parallel with the increase of alkaline phosphatase activity during the culturing of the MC3T3-E1 cells. The activity of the D-factor attained a maximum on days 12 - 15 and decreased thereafter. Both the CSF and the D-factor were eluted in a range of 25,000 to 67,000 daltons on gel filtration. The fraction containing both factors exhibited bone-resorbing activity. These results suggest that osteoblasts are involved in bone resorption at least in part by enhancing the proliferation and differentiation of osteoclast progenitors.     

Interleukin 1 and tumor necrosis factor potentiate angiotensin II- and calcium ionophore-stimulated prostaglandin E2 synthesis in rat renal mesangial cells

In resting mesangial cells, angiotensin II and the calcium ionophore A23187 stimulated prostaglandin E2 (PGE2) formation. After pretreatment with interleukin 1 beta (IL-1 beta) or tumor necrosis factor alpha (TNF alpha), which are themselves potent stimuli for PGE2 synthesis, mesangial cells displayed an amplified response to angiotensin II and A23187. The cytokine-induced effects occurred in a time- and dose-dependent manner and were attenuated by actinomycin D, cycloheximide and dexamethasone. IL-1 beta and TNF alpha treatment also increased the amount of arachidonic acid released after stimulation of cells with angiotensin II and A23187. In addition, IL-1 beta but not TNF alpha treatment augmented the formation of PGE2 from exogenous arachidonic acid by mesangial cells. Furthermore, the conversion of prostaglandin H2 to PGE2 was not changed by IL-1 beta and TNF alpha. These results suggest that IL-1 beta and TNF alpha exert a priming effect on PGE2 production in mesangial cells.     

Interleukin (IL)-17 enhances tumor necrosis factor-alpha-stimulated IL-6 synthesis via p38 mitogen-activated protein kinase in osteoblasts

Inflammatory cytokines are well known to play crucial roles in the pathogenesis of rheumatoid arthritis. Among them, interleukin (IL)-17 is a cytokine that is mainly synthesized by activated T cells and its receptors are present in osteoblasts. The synthesis of IL-6, known to stimulate osteoclastic bone resorption, is reportedly responded to bone resorptive agents such as tumor necrosis factor-alpha (TNF-alpha) in osteoblasts. It has been reported that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis in osteoblast-like MC3T3-E1 cells. We previously showed that sphingosine 1-phosphate (S1-P) mediates TNF-alpha-stimulated IL-6 synthesis in these cells. In the present study, we investigated the mechanism of IL-17 underlying enhancement of IL-6 synthesis in MC3T3-E1 cells. IL-17 induced phosphorylation of p38 mitogen-activated protein (MAP) kinase. SB203580 and PD169316, specific inhibitors of p38 MAP kinase, significantly reduced the enhancement by IL-17 of TNF-alpha-stimulated IL-6 synthesis. IL-17 also amplified S1-P-stimulated IL-6 synthesis, and the amplification by IL-17 was suppressed by SB203580. Anisomycin, an activator of p38 MAP kinase, which alone had no effect on IL-6 level, enhanced the IL-6 synthesis stimulated by TNF-alpha. SB203580 and PD169316 inhibited the amplification by anisomycin of the TNF-alpha-induced IL-6 synthesis. Taken together, our results strongly suggest that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis via p38 MAP kinase activation in osteoblasts.     

Carboxyl-terminal propeptide of type I collagen (c-propeptide) modulates the action of TGF-beta on MC3T3-E1 osteoblastic cells

Previously we found that the carboxyl-terminal propeptide of type I collagen (c-propeptide) is a major secretory protein of MC3T3-E1 osteoblastic cells. In this study, we found that c-propeptide suppresses collagen synthesis and alkaline phosphatase activity of MC3T3-E1 osteoblastic cells at the early-differentiated stage in a dose dependent manner. Mature osteoblasts did not respond to c-propeptide. These findings imply that c-propeptide modulates the function of osteoblasts at an early differentiation stage. Transforming growth factor-beta (TGF-beta) is stored in bone and released from bone matrix after the resorption by osteoclasts. We investigated the effect of c-propeptide on the action of TGF-beta, and found that it enhanced the effect of TGF-beta. We conclude that c-propeptide is a physiological modulator of TGF-beta in bone metabolism.     

Phosphatidylinositol 3-kinase/Akt auto-regulates PDGF-BB-stimulated interleukin-6 synthesis in osteoblasts

It has been reported that platelet-derived growth factor (PDGF)-BB stimulates the synthesis of interleukin (IL)-6 in osteoblasts. In the present study, we investigated whether the phosphatidylinositol 3-kinase (PI3K)/Akt is involved in the PDGF-BB-induced IL-6 synthesis in osteoblast-like MC3T3-E1 cells. PDGF-BB markedly induced the phosphorylation of Akt and GSK-3beta. Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2-(R)-2-O-methyl-3-O-octadecylcarbonate, significantly amplified the synthesis of IL-6 by PDGF-BB. The PDGF-BB-induced GSK-3beta phosphorylation was suppressed by the Akt inhibitor. The IL-6 synthesis stimulated by PDGF-BB was markedly enhanced by LY294002 and wortmannin, inhibitors of PI3K. Wortmannin and LY294002 suppressed the PDGF-BB-induced phosphorylation of Akt and GSK-3beta. Taken together, these results strongly suggest that PI3K/Akt negatively regulates the PDGF-BB-stimulated IL-6 synthesis in osteoblasts.     

Effects of TGF-beta, TNF-alpha, IL-beta and IL-6 alone or in combination, and tyrosine kinase inhibitor on cyclooxygenase expression, prostaglandin E2 production and bone resorption in mouse calvarial bone cells

Cyclooxygenase-2 (COX-2) and tyrosine kinase, which are involved in the biosynthesis of prostaglandin E(2) (PGE(2)) in mouse calvarial osteoblasts, are stimulated by cytokine interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha) and/or interleukin-6 (IL-6). IL-1beta and IL-6 and, to a lesser extent, TNF-alpha, enhances COX-2 mRNA levels in calvarial osteoblasts. Simultaneous treatment with IL-6 and IL-1beta and TNF-alpha resulted in enhanced COX-2 mRNA levels accompanied by the cooperative stimulation of PGE(2) biosynthesis compared to cells treated with IL-1beta or TNF-alpha or IL-6 alone. In contrast, the presence of TGF-beta reduced COX-2 mRNA level, PGE(2) biosynthesis and bone resorption induced by IL-1beta, TNF-alpha, IL-6 or a combination thereof. However, neither IL-1beta, TNF-alpha, IL-6 nor a combination of IL-1beta, TNF-alpha, IL-6 enhanced COX-1 mRNA levels in calvarial osteoblasts. A novel Src tyrosine kinase inhibitor, Herbimycin A (HERB), reduced COX-2 mRNA levels as well as PGE(2) production induced by IL-1beta, TNF-alpha and IL-6 or a combination of IL-1beta, TNF-alpha, IL-6, whereas COX-1 mRNA levels remained unaffected. Finally, HERB was found to inhibit in vitro bone resorption. These results indicate that the cooperative effects of IL-beta, TNF-alpha, IL-6 on PGE(2) production are due to the enhanced expression of the COX-2 gene and that tyrosine kinase(s) are involved in COX-2 signal transduction in mouse calvarial osteoblasts. Thus, the Src family of kinase inhibitors may be useful in treating diseases associated with elevated bone loss.     

Impact of the microgravity environment in a 3-dimensional clinostat on osteoblast- and osteoclast-like cells

Mechanical unloading conditions result in decreases in bone mineral density and quantity, which may be partly attributed to an imbalance in bone formation and resorption. To investigate the effect of mechanical unloading on osteoblast and osteoclast differentiation, and the expression of RANKL and OPG genes in osteoblasts, we used a three-dimensional (3D) clinostat system simulating microgravity to culture MC3T3-E1 and RAW264.7 cells. Long-term exposure (7 days) of MC3T3-E1 cells to microgravity in the 3D clinostat inhibited the expression of Runx2, Osterix, type I collagen alphaI chain, RANKL and OPG genes. Similarly, 3D clinostat exposure inhibited the enhancement of beta3-integrin gene expression, which normally induced by sRANKL stimulation in RAW264.7 cells. These results, taken together, demonstrate that long-term 3D clinostat exposure inhibits the differentiation of MC3T3-E1 cells together with suppression of RANKL and OPG gene expression, as well as the RANKL-dependent cellular fusion of RAW264.7 cells, suggesting that long-term mechanical unloading suppresses bone formation and resorption.