Bone re/modeling is more dynamic in the endothelial nitric oxide synthase(-/-) mouse.

Nitric oxide is a ubiquitous estrogen-regulated signaling molecule that has been implicated in the regulation of bone maturation and remodeling. To better understand the role that bone-cell-secreted nitric oxide plays in ovariectomy-induced modifications of bone turnover, we examined the expression of endothelial NO synthase (eNOS) in bone cells and bone progenitor cells at regular intervals up to 10 wk after acute estrogen deprivation. Ovariectomy led to an anticipated initial decline in bone cell eNOS production, but surprisingly, 17 d after ovariectomy, eNOS expression by bone and marrow stromal cells dramatically rebounded and was maintained at high levels for at least 10 wk after surgery. We examined the long-term consequences of eNOS in the process of ovariectomy-induced bone loss by prospectively analyzing bone mineral density in wild-type and eNOS(-/-) mice for 10 wk after ovariectomy. Ovariectomized eNOS(-/-) mice were observed to undergo an exaggerated state of estrogen-deficiency-induced bone remodeling compared with wild-type controls, suggesting that eNOS may act to mitigate this process. Furthermore, we found that whereas bone formation in estrogen-replete wild-type mice slowed between 14 and 20 wk of age, eNOS knockout mice continued to accrue basal bone mass at a high rate and showed no sign of entering a remodeling stage. Our data suggest that eNOS may play an important role in limiting ovariectomy-induced bone remodeling as well as regulating the transition from basal modeling to remodeling.

Prostaglandin E2 induction of monoblastic differentiation utilizes both cAMP and non-cAMP dependent signalling systems.

U937 cells can be induced to express receptor for complement 5a (C5aR) by sequential 2 day treatments of cells with dihydroxyvitamin D-3 (1,25(OH)2D3) followed by prostaglandin E2. We asked whether the action of prostaglandin E2 to cause maximal C5aR expression required only activation of the cAMP-dependent protein kinase (PKA). Prostaglandin E2 dose dependently activated PKA in control and 1,25(OH)2D3 treated cells; by 4 h the PKA did not respond to further prostaglandin E2 challenge. We hypothesized that prostaglandin E2 actions transduced via PKA should be complete by 4 h; i.e., C5aR induction should be equivalent in cells treated with prostaglandin E2 for 4 h and for 2 days. All cells were treated for the first 2 days with 1,25(OH)2D3 and the second 2 days with prostaglandin E2 or cAMP analogs. C5aR number was measured after 4 days total culture. 4 h pulse treatments with agents were given at the end of the 1,25(OH)2D3 treatment. Cells exposed to a 4 h pulse of prostaglandin E2 had only 68.2 +/- 4.4% the amount of C5aR seen in cells continuously exposed to prostaglandin E2. Continuous culture with a cAMP analog pair (50 microM each of 8-thiomethyl-cAMP + N6-benzoyl-cAMP), which caused a 41.7% +/- 10.8% increase PKA activation above basal, resulted in only 51% +/- 16% of the C5aR numbers seen in cells cultured for 2 days with prostaglandin E2, where PKA remained at basal activity. We therefore concluded that C5aR expression caused by prostaglandin E2 could not be ascribed entirely to duration or degree of activation of cAMP-dependent signalling pathways. We investigated the possibility that the calcium sensitive protein kinase C was involved. Cytoplasmic protein kinase C was increased 154% +/- 14% above control in cells treated with sequential 2 days treatments of 1,25(OH)2D3 and prostaglandin E2. A 147% +/- 2% increase in membrane associated protein kinase C was also seen 10 min after phorbol myristate acetate stimulation in the above treatment group. Finally, phorbol myristate acetate augmented the C5aR induction caused by cAMP analog. We propose that the mechanism of prostaglandin E2 synergism with 1,25(OH)2D3 in causing C5aR induction in U937 cells includes signal transduction not only by the cAMP cascade, but also via protein kinase C modulated pathways.

Regulation of complement 5a receptor expression in U937 cells by phorbol ester.

Receptors for the anaphylactic portion of complement, C5a, are not initially expressed in the monoblastic U937 cell line, but appear as the cell is induced to differentiate by the synergistic actions of 1,25(OH)2D and cyclic adenosine monophosphate (cAMP). Phorbol myristate acetate (PMA), which activates the protein kinase C pathway (PKC), does not cause C5a receptor (C5aR) expression when used as a single agent. The induction of C5aR by the synergistic actions of 1,25(OH)2D and cAMP, however, can be augmented as much as 180% by the addition of PMA. C5aR arising in cells exposed to 1,25(OH)2D and 8,4-chlorophenylthio-cAMP have an affinity constant of about 0.4 nM as assessed by cold competition analysis. We show here that when phorbol augmentation of receptor number occurs, the affinity constant is increased by 3.6-fold. In an effort to ascertain whether the change in C5aR Kd involved a PKC-dependent event we examined whether 5-60 min exposure of C5aR-positive cells to PMA would change C5aR Kd. Acutely, PMA caused a downregulation of receptor binding with decreases in apparent receptor number out of proportion to changes in Kd. One hundred nanomolar PMA, which effects nearly complete translocation of PKC to the membrane, consistently caused a 70-90% decrease in C5a surface binding. This downregulation was proportional to PMA dose and exposure time. Micromolar concentrations of the microtubule depolymerizing agents colchicine and vinblastine caused a less drastic downregulation, about 50% of the maximal phorbol effect. Our data suggest that activation of the PKC system might acutely limit the macrophage's ability to respond to C5a; chronically, phorbols upregulate receptor expression, most likely through positive effects on C5aR gene expression.

cAMP promotion of osteoclast-like cell development from mouse bone marrow cells requires a permissive action of 1,25-(OH)2D3.

Recruitment of osteoclasts from monocytic precursors is modulated by local signals. We previously showed that monoblastic differentiation in U937 cells is stimulated by 1,25-(OH)2D3 and cAMP in series. We investigate here the combined effects of these agents to stimulate differentiation of osteoclast-like cells from mouse marrow. Cells from mouse marrow were harvested and cultured in alpha-MEM with 10% fetal bovine serum. The appearance of tartrate-resistant acid phosphatase-containing multinuclear cells was measured after 8 days in culture by cytochemical staining. Continuous exposure of cultures to 10 nM 1,25-(OH)2D3 positively stimulated development of these cells after 8 days (101 +/- 3 cells per well, n = 74). No osteoclast-like cells were found when 1,25-(OH)2D3 was added for the first 4 days followed by 4 days more with no treatment. PGE2 (1 microM) as a single agent added during the last 4 days of culture was not able to recruit osteoclast-like cells. However, cultures exposed to 1,25-(OH)2D3 during the first 4 days and 1 microM PGE2 during the second 4 days developed osteoclast-like cells at 8 days [66 +/- 8% of the formation seen with 1,25-(OH)2D3 alone, p less than 0.05]. Dibutyryl cAMP (1 microM to 3 mM) was also not effective used as a single agent, but was able to stimulate formation of TRAP-positive multinuclear cells when 1,25-(OH)2D3 preceded its addition to culture medium. cAMP analogs therefore mimicked the effect of 1 microM PGE2, but these experiments do not allow us to assign the PGE2 action entirely to activation of cAMP second messenger.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor necrosis factor alpha decreases 1,25-dihydroxyvitamin D3 receptors in osteoblastic ROS 17/2.8 cells.

Bone remodeling is a complex process regulated by systemic hormones, local cytokines, and growth factors. One cytokine, tumor necrosis factor alpha (TNF-alpha), is known to have potent inhibitory effects on osteoblast matrix protein production and to stimulate osteoclast recruitment. We have previously shown that TNF-alpha inhibits 1,25-(OH)2D3-stimulated synthesis of bone gla protein (BGP), an abundant and osteoblast-specific matrix constituent. We hypothesized that the mechanism of TNF-alpha action included inhibition of intracellular 1,25-(OH)2D3 receptor (VDR) number or function. To test this, the osteoblastic cell line ROS 17/2.8 was cultured in the presence or absence of TNF-alpha (100 ng/ml), and binding of [3H]1,25-(OH)2D3 to 0.3 M KCl extracts of cytosol was measured by equilibrium assay. Specific [3H]1,25-(OH)2D3 binding decreased 70%, 25 h after addition of TNF-alpha. The decrease in [3H]1,25-(OH)2D3 binding was seen by 18 h, was sustained throughout the 72 h culture period, and was greater in low-density cultures. Scatchard analysis confirmed that TNF-alpha (100 ng/ml for 24 h) caused a decrease in the number of binding sites without change in VDR affinity. Northern analysis with a VDR riboprobe revealed that the decrease in VDR occurred without a change in the 4.4 kb steady-state VDR mRNA [VDR/cyclophilin mRNA signal ratio: control, 2.25; TNF-alpha, 2.24 (24 h), 2.17 (40 h), n = 2 flasks/time point]. These results suggest that TNF-alpha action on osteoblastic cells includes an inhibitory effect on VDR number at a point distal to the synthesis of VDR mRNA.

1,25-dihydroxyvitamin D reduces parathyroid hormone receptor number in ROS 17/2.8 cells and prevents the glucocorticoid-induced increase in these receptors: relationship to adenylate cyclase activation.

We have previously shown that 1,25-dihydroxyvitamin D [1,25-(OH)2D3] and glucocorticoid modulate adenylate cyclase activation by PTH in osteoblast-like cells. Here we examine whether steroid effects on PTH receptor density explain the modulation of PTH action. Receptor assays were performed on late logarithmicphase monolayers of ROS 17/2.8 cells using human PTH-like peptide (hPLP) as radioligand. Kd and receptor density were computed from competition of tracer amounts of [125I-Tyr36] hPLP-(1-36) with unlabeled hPLP-(1-36) (0.1-30 nM). Steroid treatment had little or no effect on affinity for ligand. Pretreating cells with 10 nM 1,25-(OH)2D3 for 48 h decreased PTH receptor number to 17% of control values. Treating cells with 10 nM of the glucocorticoid triamcinolone acetonide (TRM) increased receptor number 10-fold, but simultaneous treatment with 1,25-(OH)2D3 (10 nM) completely prevented this receptor increase. Steroid effects required 13-18 h of treatment. Dose-response relationships for steroid modulation, determined from binding at 0.17 nM radioligand, indicated an EC50 of 0.3 nM for glucocorticoid augmentation of PTH receptor number and 0.02 nM for 1,25-(OH)2D3 reduction of receptor number in the presence of absence of the maximum TRM effect. The initial rate of cAMP production by receptor-saturating concentrations of PTH was 11,500 molecules per receptor per minute in untreated cells, comparable to reported turnover numbers for mammalian adenylate cyclase. Control experiments were validated measuring cAMP in intact cells as an indicator of adenylate cyclase activity. Cyclic AMP production was reduced 63% by 1,25-(OH)2D3 (10 nM) treatment. Glucocorticoid (10 nM) enhanced cAMP production twofold but reduced cAMP generation per receptor by 80%.(ABSTRACT TRUNCATED AT 250 WORDS)

1,25-Dihydroxyvitamin D3 and phorbol myristate acetate synergistically increase carbonic anhydrase-II expression in a human myelomonocytic cell line.

Expression of carbonic anhydrase-II (CA-II), an enzyme important to osteoclast function, distinguishes osteoclasts from other cells of monocytic lineage. A cell's selection of terminal osteoclast phenotype is controlled by many different factors, which are not well understood and which may also control the expression of CA-II. We studied the control of CA-II expression in the human HL-60 cell to better understand the signal transduction systems involved in progression to the osteoclast phenotype. Both 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3; 10 nM] and phorbol myristate acetate (10 ng/ml), doses that cause monocytic differentiation of the HL-60 cell, induced small increases in CA-II mRNA and CA-II protein, as measured by Northern analysis and Western immunoblotting, respectively. The maximal response was seen at 3 days. Treatment of HL-60 cells with both agents resulted in synergistic increases in CA-II mRNA (80-fold) and protein. The large increase in CA-II mRNA allowed assessment of the dose dependence of both agents, with ED50 values of 1 nM for 1,25-(OH)2D3 and 1 ng/ml for phorbol myristate acetate. In addition, we have shown that this synergistic response was completely inhibited by a potent inhibitor of protein kinase-C activity, staurosporine (0.1 microM), which has not previously been demonstrated in other cell systems. Staurosporine did not inhibit 1,25-(OH)2D3 induction of nonspecific esterase. Thus, 1,25-(OH)2D3 synergistically interacts with protein kinase-C-activated systems to cause a myelomonocytic precursor to express CA-II a marker of the osteoclast phenotype.

The role of decidual tissue in the development of the dependency on luteinizing hormone in the rat corpus luteum.

In pregnant rats, treatment with an antiserum to LH (LHAS) on days 2-5 inclusive (early LHAS treatment; day 1 = insemination) delayed implantation by about 4 days. The appearance of the dependency of the corpora lutea on LH for the maintenance of progesterone secretion (LH dependency), as determined by the rate of fall in progesterone secretion after a single test injection of LHAS, was also delayed by 4 days. Treatment with a small amount of estradiol on either day 4 or days 4-9 prevented the delay in both implantation and LH dependency. Implantation thus prevented early LHAS treatment from delaying LH dependency, but its effect seems to have been due to decidualization, which accompanies implantation in the rat. In decidual tissue (DT)-bearing pseudopregnant rats, early LHAS treatment delayed LH dependency for only 1 day, while it delayed LH dependency for at least 3 days in ordinary pseudopregnant rats and for at least 4 days in hysterectomized pseudopregnant rats. Estrogen itself seems to have prevented the delay in LH dependency only by inducing implantation, since it had no effect in the DT-bearing pseudopregnant rats. How DT affects the corpus luteum's dependency on LH is unknown, but it may be related to whatever effect DT has on prostaglandin production in the endometrium.

Inhibitory effects of tumor necrosis factor-alpha and interferon-gamma on deoxyribonucleic acid and collagen synthesis by rat osteosarcoma cells (ROS 17/2.8).

Tumor necrosis factor-alpha (TNF alpha) and interferon-gamma (IFN gamma) have potent effects on bone resorption and collagen synthesis in cultured rat long bones. Since the effects of TNF alpha and IFN gamma may result from interaction with multiple cell types, we studied the effects of these cytokines on the synthesis of DNA and collagen in one cell type with osteoblast phenotype, cloned rat osteosarcoma cells (ROS 17/2.8). Recombinant human TNF alpha did not affect DNA synthesis after 48 h with concentrations of 10(-11)-10(-8) M and inhibited DNA synthesis slightly at 10(-6) M. Recombinant rat IFN gamma (5-500 U/ml) caused a dose-dependent inhibition of DNA synthesis. Coincubation with TNF alpha and IFN gamma inhibited DNA synthesis more than maximal doses of either cytokine alone. This enhanced inhibitory effect was due to the induction of a response to TNF alpha by IFN gamma, since preexposure of cells to IFN gamma for 24 h, followed by incubation with TNF alpha alone for an additional 48 h, also resulted in increased inhibition of DNA synthesis. Preexposure to TNF alpha for 24 h, followed by IFN gamma alone, did not increase the inhibition of DNA synthesis. Incubation with either IFN gamma (5-500 U/ml) or TNF alpha (10(-10)-10(-6) M) inhibited the incorporation of [3H]proline into collagen. Coincubation with intermediate concentrations of both cytokines resulted in an inhibitory effect greater than that produced by maximal concentrations of either alone. The results indicate that 1) IFN gamma and TNF alpha have direct actions on osteoblast-like cells in vitro; 2) IFN gamma modulates the DNA response to TNF alpha; and 3) the greater responses to combined cytokines than to high doses of either alone suggest that these cytokines act, at least in part, through different pathways.

Glucocorticoid and 1,25-dihydroxyvitamin D modulate the degree of adenosine 3',5'-monophosphate-dependent protein kinase isoenzyme I and II activation by parathyroid hormone in rat osteosarcoma cells.

Glucocorticoid increases and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] decreases PTH activation of adenylate cyclase and cAMP-dependent protein kinase in rat osteosarcoma cells (ROS 17/2.8). Since selective cAMP-dependent protein kinase isoenzyme activation may account for specific physiological hormonal responses, we investigated steroid effects on activation of isoenzymes I and II in response to PTH using a new ion exchange separation procedure. Pretreatment of cells for 2 days with the glucocorticoid triamcinolone acetonide (TRM) or 1,25-(OH)2D3 altered the degree of cAMP-dependent protein kinase isoenzyme activation by PTH in accordance with their modulation of intracellular cAMP accumulation, but did not alter the amount of each isoenzyme present or the order in which isoenzymes I and II were activated. In all treatment groups isoenzyme I was preferentially activated by low doses of PTH, while high concentrations activated both isoenzymes, as predicted by the relative affinities of each isoenzyme for cAMP. Glucocorticoid reduced the concentration of bovine PTH-(1-34) required for maximal activation of isoenzyme I from 1 to 0.05 ng/ml and that required for activation of isoenzyme II from 10 to 1 ng/ml. This effect was abolished by simultaneous treatment of cells with 1,25-(OH)2D3. At doses of PTH that caused partial activation (0.05-0.1 ng/ml for isoenzyme I; 1 ng/ml for isoenzyme II), 1,25-(OH)2D3 treatment attenuated this activation. In all groups both isoenzymes were fully activated by 100 ng/ml PTH. Control experiments demonstrated that isoenzyme activation is not a result of cell disruption over the range of PTH doses that regulation by steroid hormone was observed. These results extend our studies on modulation of the cAMP pathway by steroid hormones and make it feasible to correlate selective isoenzyme activation with specific responses to PTH.

Interferon-gamma inhibits 1,25-dihydroxyvitamin D3-stimulated synthesis of bone GLA protein in rat osteosarcoma cells by a pretranslational mechanism.

Interferon-gamma (IFN) is produced by lymphocytes in areas of inflammation and connective tissue destruction. IFN inhibits collagen and DNA synthesis in cultured rat long bones and osteoblastic ROS 17/2.8 cells, suggesting that the periarticular loss of bone that occurs in inflammatory joint diseases may be due to IFN inhibition of bone formation. Since serum levels of bone gla protein (BGP) have been correlated with the bone formation rate, we studied the effect of IFN on production of this osteoblast-specific protein and steady state BGP messenger RNA (mRNA) levels in ROS 17/2.8 cells. RIA of BGP was done using an antibody raised against rat BGP peptide. BGP synthesis was stimulated with 10(-8) M 1,25-dihydroxyvitamin D3 24 h before and continuously after addition of recombinant rat IFN. IFN (100 U/ml) inhibited BGP secretion 52%, 78%, and 70% in the first, second, and third 24 h periods after IFN treatment, compared to control cells cultured with 1,25-dihydroxyvitamin D3 alone. The ED50 for IFN inhibition of BGP production was 3.3 U/ml (0.29 nM). Pulse labeling with [14C]leucine or [3H]proline during the last 4 h of culture revealed that IFN (3-100 U/ml) did not inhibit total protein secretion into the medium. The percent inhibition of BGP production by IFN was independent of media serum concentration or cell density. IFN (100 U/ml) decreased the steady state level of BGP mRNA as measured by Northern analysis using an oligomeric probe for rat BGP. The decrease in hybridization signal for BGP mRNA was detectable by 1 h after IFN exposure and continued to decline at 6 and 24 h. Treatment with cycloheximide (5 micrograms/ml) blocked the inhibitory effect of IFN on steady state levels of BGP mRNA. These results suggest that IFN may inhibit bone formation by selective inhibition of osteoblast matrix protein production. The mechanism of IFN inhibition of BGP production is, at least in part, pretranslational.

Tumor necrosis factor-alpha inhibits 1,25-dihydroxyvitamin D3-stimulated bone Gla protein synthesis in rat osteosarcoma cells (ROS 17/2.8) by a pretranslational mechanism.

Tumor necrosis factor-alpha (TNF alpha), a 17,000 mol wt protein, mediates a variety of immunological and inflammatory events. TNF alpha is a potent inhibitor of bone collagen synthesis and stimulator of osteoclastic bone resorption, the net effect of which is to cause bone loss. We have previously reported that TNF alpha inhibits the synthesis of collagen by osteoblastic cells in culture out of proportion to effects on total protein synthesis, suggesting that inhibition of bone formation by TNF alpha may be due to selective inhibition of matrix protein synthesis. To further test this hypothesis and to evaluate the mechanism of TNF alpha action, we studied the effect of TNF alpha on synthesis of the osteoblast-specific bone Gla protein (BGP) by ROS 17/2.8 cells, which have the osteoblast phenotype. Cells were cultured with 10 nM 1,25-dihydroxyvitamin D3 to stimulate BGP secretion, followed by the addition of TNF alpha (1-100 ng/ml) in 1,25-dihydroxyvitamin D3-containing medium. TNF alpha (10 ng/ml) inhibited BGP secretion to 42 +/- 5%, 19 +/- 10%, and 15 +/- 3% of control values after 24, 48, and 72 h of treatment. After 48 h, inhibition of BGP secretion was observed with 2 ng/ml TNF alpha and was maximum with 100 ng/ml. To determine the effect of TNF alpha on total protein synthesis, cells were pulse labeled with [14C]leucine during the last 4 h of TNF alpha treatment, and incorporation of radioactivity into trichloroacetic acid-precipitable protein in cell layer and medium was determined. The TNF alpha inhibition of BGP secretion was independent of changes in [14C]leucine incorporation, suggesting that TNF alpha did not have a general inhibitory effect on total protein synthesis. Cell number was not affected by TNF alpha. Northern analysis of steady state BGP mRNA revealed a dose-dependent decrease in the BGP/cyclophilin mRNA hybridization signal intensity after 24 h of treatment. The maximum inhibitory effect was 41 +/- 5% of the control value with 100 ng/ml TNF alpha. The effect of TNF alpha on steady state BGP mRNA levels was not prevented by treatment of cells with cycloheximide, suggesting that TNF-induced new protein synthesis was not required for TNF alpha action. These results suggest that the mechanism of TNF alpha inhibition of BGP synthesis includes a pretranslational site and support the hypothesis that TNF alpha inhibits bone formation by a selective inhibition of matrix protein production.

Inhibition of osteoblast differentiation by tumor necrosis factor-alpha.

Tumor necrosis factor-alpha (TNF-alpha) has a key role in skeletal disease in which it promotes reduced bone formation by mature osteoblasts and increased osteoclastic resorption. Here we show that TNF inhibits differentiation of osteoblasts from precursor cells. TNF-alpha treatment of fetal calvaria precursor cells, which spontaneously differentiate to the osteoblast phenotype over 21 days, inhibited differentiation as shown by reduced formation of multilayered, mineralizing nodules and decreased secretion of the skeletal-specific matrix protein osteocalcin. The effect of TNF was dose dependent with an IC50 of 0.6 ng/ml, indicating a high sensitivity of these precursor cells. Addition of TNF-alpha from days 2-21, 2-14, 7-14, and 7-10 inhibited nodule formation but addition of TNF after day 14 had no effect. Partial inhibition of differentiation was observed with addition of TNF on only days 7-8, suggesting that TNF could act during a critical period of phenotype selection. Growth of cells on collagen-coated plates did not prevent TNF inhibition of differentiation, suggesting that inhibition of collagen deposition into matrix by proliferating cells could not, alone, explain the effect of TNF. Northern analysis revealed that TNF inhibited the expression of insulin-like growth factor I (IGF-I). TNF had no effect on expression of the osteogenic bone morphogenic proteins (BMPs-2, -4, and -6), or skeletal LIM protein (LMP-1), as determined by semiquantitative RT-PCR. Addition of IGF-I or BMP-6 to fetal calvaria precursor cell cultures enhanced differentiation but could not overcome TNF inhibition, suggesting that TNF acted downstream of these proteins in the differentiation pathway. The clonal osteoblastic cell line, MC3T3-E1-14, which acquires the osteoblast phenotype spontaneously in postconfluent culture, was also studied. TNF inhibited differentiation of MC3T3-E1-14 cells as shown by failure of mineralized matrix formation in the presence of calcium and phosphate. TNF was not cytotoxic to either cell type as shown by continued attachment and metabolism in culture, trypan blue exclusion, and Alamar Blue cytotoxicity assay. These results demonstrate that TNF-alpha is a potent inhibitor of osteoblast differentiation and suggest that TNF acts distal to IGF-I, BMPs, and LMP-1 in the progression toward the osteoblast phenotype.

Increased expression of tissue plasminogen activator messenger ribonucleic acid is an immediate response to parathyroid hormone in neonatal rat osteoblasts.

Osteoblasts have been reported to produce tissue-type (t) plasminogen activator (PA), which may be involved in the initiation of bone resorption via plasmin-metalloproteinase degradation of adjacent extracellular matrix. To investigate this cAMP-activated gene, we characterized the PTH regulation of tPA messenger RNA (mRNA) in neonatal rat osteoblast cultures before and after differentiation in vitro. RNA was purified from cultures at confluence or after treatment with glucocorticoid for 1 week and BGJ/ascorbic acid/beta-glycerophosphate for a second week. Northern blots of total or poly(A)+ RNA were hybridized simultaneously with an oligonucleotide or complementary RNA probe for rat tPA and an oligomeric DNA probe for cyclophilin (CYP), an abundant control gene. Differentiation was monitored by expression of rat osteocalcin mRNA and protein. Both bovine PTH1-34 and forskolin caused an increase in tPA/CYP ratio in the presence of phosphodiesterase inhibitor (IBMX) and cycloheximide (CHX). The effect was maximal (16- to 21-fold increase in tPA mRNA and 6- to 8-fold increase in tPA/CYP ratio) with 25 nM hormone for 6 h and was half-maximally stimulated by 0.75-2.5 nM PTH. The tPA response to PTH was present in first passage osteoblast cultures at confluence and after 1 to 2 weeks of glucocorticoid treatment. Exposure of the differentiated cultures of 1,25-dihydroxyvitamin D (10 nM) for 2 days markedly stimulated osteocalcin mRNA while having no effect on tPA. In Northern blots of poly(A)+ RNA from cultures not treated with CHX, IBMX and PTH (2.5 h) independently stimulated tPA mRNA with no significant effect on CYP mRNA levels. The tPA/CYP ratio increased in five consecutive experiments and the effect of IBMX and PTH were additive. These data indicate that PTH acts via cAMP to stimulate tPA expression by a mechanism that is independent of protein synthesis. The enhancement of PTH action by CHX is compatible with feedback inhibition of tPA transcription by a hormone-activated repressor (which has been proposed to occur in granulosa cells) but effects of CHX on tPA mRNA stability may also occur. Expression of tPA mRNA before and after differentiation may indicate that the enzyme has more than one function.

Dexamethasone promotes expression of membrane-bound macrophage colony-stimulating factor in murine osteoblast-like cells.

The mechanisms by which glucocorticosteroids promote osteoclastogenesis in vitro are uncertain. As macrophage colony-stimulating factor (MCSF) is critical for osteoclastogenesis, we hypothesized that glucocorticosteroids might regulate membrane-bound MCSF (mMCSF) and soluble MCSF (sMCSF) production by stromal cells or osteoblasts. ST2 cells or murine calvarial osteoblasts (MOBs) were treated with dexamethasone (Dex; 100 nM) and/or 1,25-dihydroxyvitamin D [1,25(OH)2D; 10 nM] for 3 days. Control values for mMCSF and sMCSF as units per 100,000 cells were 9 +/- 1.4 and 511 +/- 56 in ST2 cells and 5.9 +/- 0.8 and 379 +/- 47 in MOB cells, respectively. Dex increased mMCSF to 156 +/- 16% and 143 +/- 26% compared with the control value in ST2 and MOB cells, respectively, whereas 1,25-(OH)2D caused increases of 195 +/- 16% and 164 +/- 21%. In the presence of both Dex and 1,25-(OH)2D, mMCSF increased to 209 +/- 24% and 216 +/- 26% in the two cell types, respectively. 1,25-(OH)2D caused modest increases in sMCSF, as expected, in both cell types (153 +/- 6% and 122 +/- 4%). Dex inhibited 1,25-(OH)2D-stimulated sMCSF (115 +/- 7% of control) in ST2 cells. Analysis of mMCSF transcript levels by semiquantitative RT-PCR revealed Dex-stimulated increases of 170 +/- 11% in ST2 cells and 126 +/- 16% in MOB cells compared with the control level. The increased expression of the transcript for sMCSF in the presence of Dex and 1,25-(OH)2D, measured by both RT-PCR and Northern analysis (219 +/- 53% and 242%, respectively), despite inhibition of sMCSF protein, indicated that the inhibitory effect of Dex in ST2 cells was posttranscriptional. Half-life studies showed that Dex prolonged MCSF messenger RNA from 2.8 to 7.5 h. These results suggest that Dex influences osteoclastogenesis by increasing the expression of mMCSF by accessory cells in culture.

Glucocorticoid-induced differentiation of fetal rat calvarial osteoblasts is mediated by bone morphogenetic protein-6.

Glucocorticoids (GCs) at physiological concentrations promote osteoblast differentiation from fetal calvarial cells, calvarial organ cultures, and bone marrow stromal cells; however, the cellular pathways involved are not known. Bone morphogenetic proteins (BMPs) are recognized as important mediators of osteoblast differentiation. Specific roles for individual BMPs during postembryonic membranous bone formation have yet to be determined. We recently reported that GC potentiated the osteoblast differentiation effects of BMP-2 and BMP-4, but not of BMP-6, which, by itself, was the most potent of the three. In the present study, we used fetal rat secondary calvarial cultures to study the role of BMP-6 during early osteoblast differentiation. Treatment with the GC triamcinolone (10(-9) M) resulted in a 5- to 8-fold increase in BMP-6 steady-state messenger RNA levels, peaking at 12 h. In contrast, BMPs -2, -4, -5, -7, and transforming growth factor (TGF)-beta1 messenger RNA levels increased by less than 2-fold, after GC treatment, compared with untreated control cultures at 24 h. BMP-6 protein secretion increased 6- to 7-fold by 12 h and 12-fold (from 7.5 to 90 ng/ml) by 24 h, as measured by quantitative Western analysis. Treatment of cells with oligodeoxynucleotides antisense to BMP-6 diminished secretion of BMP-6 protein and significantly inhibited the GC-induced differentiation, as determined by a 10-fold decrease in the number of mineralized bone nodules, compared with controls that were treated with sense oligonucleotides or no oligonucleotides (ANOVA, P < 0.05). The antisense oligonucleotide inhibition of differentiation was rescued by treatment with exogenous recombinant human BMP-6. We conclude that GC-induced differentiation of osteoblasts from the pluripotent precursors is mediated, in part, by BMP-6. These results suggest that BMP-6 has an important and unique role during early osteoblast differentiation.

A single up-stream element confers responsiveness to 1,25-dihydroxyvitamin D3 and tumor necrosis factor-alpha in the rat osteocalcin gene.

Tumor necrosis factor-alpha (TNF alpha) is one of several autocrine/paracrine factors known to exert potent inhibitory effects on bone. We have shown that TNF alpha inhibition of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]-stimulated synthesis of the bone-specific protein osteocalcin (OC) occurs by decreasing steady state levels of OC mRNA, suggesting a pretranslational mechanism. In many genes, TNF alpha action is mediated by the transcription factor NF kappa B. Analysis of OC 5'-flanking DNA revealed a sequence structurally homologous to the previously described NF kappa B-binding site and, thus, a potential TNF alpha response element. Deletion analysis was performed to identify the sequences mediating the response to TNF alpha in osteoblastic ROS 17/2.8 cells by transient transfection with reporter constructs containing rat OC 5'-flanking DNA [chloramphenicol acetyltransferase (CAT)] that retained or deleted homologous NF kappa B sites or a previously defined 1,25-(OH)2D3 response element (VDRE). Transfection with all reporter constructs resulted in low basal CAT activity, measured 72 h after transfection. 1,25-(OH)2D3 stimulated CAT activity 2.8- to 4.5-fold in cells transfected with constructs that included the VDRE. TNF alpha inhibited 1,25-(OH)2D3-stimulated, but not basal, CAT activity. Deletion analysis localized the effect of TNF alpha to a sequence between -522 and -306 relative to the OC transcription start site, an area that included the VDRE but deleted a homologous NF kappa B element. Transfection of cells with a heterologous reporter containing one copy of the OC VDRE inserted in correct orientation or two copies in inverse orientation was sufficient to confer a response to TNF alpha. Gel mobility shift analysis of DNA-nuclear protein interaction revealed that 1,25-(OH)2D3 stimulated an increase in binding of nuclear proteins to an OC 32P-VDRE probe. Preincubation of nuclear extract with specific monoclonal antibodies confirmed that the proteins binding the VDRE included the vitamin D receptor and retinoid-X receptor. TNF alpha treatment of cells inhibited the 1,25-(OH)2D3-stimulated increase in nuclear protein binding to the VDRE. These results suggest 1) the VDRE is sufficient to confer a response to the inhibitory effect of TNF alpha on 1,25-(OH)2D3-stimulated rat OC gene transcription; 2) the action of TNF alpha does not require homologous NF kappa B response elements; and 3) the mechanism of TNF alpha inhibition of 1,25-(OH)2D3-stimulated OC gene expression includes modulation of binding of the vitamin D receptor/retinoid-X receptor heterodimer to the VDRE.

Inhibition of 1,25-dihydroxyvitamin D3 stimulated osteocalcin gene transcription by tumor necrosis factor-alpha: structural determinants within the vitamin D response element.

Control of osteoblast function requires the coordinate activity of systemic and local regulatory factors. We have investigated the mechanism of interaction between the secosteroid 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and the cytokine tumor necrosis factor-alpha (TNF-alpha) by measuring their effects on two 1,25-(OH)2D3 responsive matrix protein genes, osteocalcin (OC) and osteopontin (OP). Our previous studies revealed that an inhibitory effect of TNF-alpha on 1,25-(OH)2D3-stimulated OC gene transcription is conferred by the same 25 base pair region of 5'-flanking DNA that confers a response to vitamin D (VDRE). Gel mobility shift studies of [32P]VDRE binding to ROS 17/2.8 cell nuclear extract revealed that TNF-alpha inhibits 1,25-(OH)2D3 stimulated formation of specific retinoid X receptor/vitamin D receptor (RXR/VDR)-DNA complexes in vitro. To determine if TNF-alpha was inhibiting nuclear protein-VDRE binding by modulation of VDR availability, we measured intranuclear VDR in cells treated with 1,25-(OH)2D3 (10(-8) M), TNF-alpha (100 ng/ml), or both, by western blot. 1,25-(OH)2D3 caused upregulation of the nuclear VDR. Treatment with TNF-alpha inhibited the 1,25-(OH)2D3-stimulated up-regulation of VDR nuclear protein content. However, down-regulation of VDR was unlikely to be the mechanism of TNF-alpha action because TNF-alpha had no effect on 1,25-(OH)2D3 stimulation of steady state OP messenger RNA or transcription of an OP-VDRE-chloramphenicol acetyl transferase reporter construct. These results suggest that decreased VDR alone does not explain the mechanism of TNF-alpha action. VDRE structural requirements for TNF-alpha action were characterized by comparing binding of mutant and hybrid forms of mouse (m)OP-, rat (r)OC-, and human (h)OC-VDRE probes to nuclear protein from cells treated with 1,25-(OH)2D3 and/or TNF-alpha. These homologous vitamin D response elements differ in that an AP-1 sequence is included in the rOC-VDRE and hOC-VDRE but not in the OP-VDRE. Gel mobility shift analysis revealed that TNF-alpha inhibited 1,25-(OH)2D3 stimulation of nuclear protein binding to rOC-VDRE and hOC-VDRE to 59% and 69% of control, respectively, but had no effect on 1,25-(OH)2D3 stimulation of nuclear protein binding to OP-VDRE. The effect of TNF-alpha could not be conferred in a mutant OP-VDRE in which the rOC-VDRE AP-1 sequence was inserted.(ABSTRACT TRUNCATED AT 400 WORDS)

LMP-1, a LIM-domain protein, mediates BMP-6 effects on bone formation.

Glucocorticoids can promote osteoblast differentiation from fetal calvarial cells and bone marrow stromal cells. We recently reported that glucocorticoid specifically induced bone morphogenetic protein-6 (BMP-6), a glycoprotein signaling molecule that is a multifunctional regulator of vertebrate development. In the present study, we used fetal rat secondary calvarial cultures to determine genes induced during early osteoblast differentiation as initiated by glucocorticoid treatment. Glucocorticoid, and subsequently BMP-6, was found to induce a novel rat intracellular protein, LIM mineralization protein-1 (LMP-1), that in turn resulted in synthesis of one or more soluble factors that could induce de novo bone formation. Blocking expression of LMP-1 using antisense oligonucleotide prevented osteoblast differentiation in vitro. Overexpression of LMP-1 using a mammalian expression vector was sufficient to initiate de novo bone nodule formation in vitro and in sc implants in vivo. These data demonstrate that LMP-1 is an essential positive regulator of the osteoblast differentiation program as well as an important intermediate step in the BMP-6 signaling pathway.

Differential effects and glucocorticoid potentiation of bone morphogenetic protein action during rat osteoblast differentiation in vitro.

Bone morphogenetic proteins (BMPs) induce cartilage and bone differentiation in vivo and promote osteoblast differentiation from calvarial and marrow stromal cell preparations. Functional differences between BMP-2, -4, and -6 are not well understood. Recent investigations find that these three closely related osteoinductive proteins may exert different effects in primary rat calvarial cell cultures, suggesting the possibility of unique functions in vivo. In this study, we use a fetal rat secondary calvarial cell culture system to examine the differential effects of BMP-2, -4, and -6 on early osteoblast differentiation. These cells do not spontaneously differentiate into osteoblasts, as do cells in primary calvarial cultures, but rather require exposure to a differentiation initiator such as glucocorticoid or BMP. We determined that BMP-6 is a 2- to 2.5-fold more potent inducer of osteoblast differentiation than BMP-2 or -4. BMP-6 induced the formation of more and larger bone nodules as well as increased osteocalcin secretion. The effects of all three of these BMPs were potentiated up to 10-fold by cotreatment or pretreatment with the glucocorticoid triamcinolone (Trm). The Trm effects were synergistic with those of BMP-2 or -4, suggesting that this glucocorticoid may increase the cell responsiveness to these BMPs. Finally, BMP-6 did not require either cotreatment or pretreatment with Trm to achieve greater amounts of osteoblast differentiation than seen with BMP-2 or BMP-4 treatment, suggesting that BMP-6 may act at an earlier stage of cell differentiation.