Aluminum potentiates the effect of fluoride on tyrosine phosphorylation and osteoblast replication in vitro and bone mass in vivo

The potential of a surface pre-reacted glass root canal dressing for treating apical periodontitis in rats

AIM

To evaluate the efficacy of a prototype root canal dressing containing surface pre-reacted glass-ionomer (S-PRG) fillers on repairing induced periapical lesions in a rat model. Calcium hydroxide [Ca(OH)₂ ] was applied as a comparison in the healing process.

METHODOLOGY

The pulp chambers of the maxillary first molars in 64 male Wistar rats aged 16 weeks were opened to induce periapical lesions. After 28 days, the mesial canal of each tooth was prepared, irrigated with 2.5% sodium hypochlorite only (control group: irrigation) or followed by the respective dressing [Ca(OH)₂ group, irrigation + Ca(OH)₂ ; S-PRG group, irrigation + S-PRG] and restored with composite resin for 3 or 7 days (10/group). Four rats with healthy molars were used as blank controls. Descriptive analysis of the periapical radiographs, haematoxylin and eosin staining and immunohistochemical observation was performed 3 and 7 days after treatment. The periapical grey value, CD68 macrophages and osteoclasts (cathepsin-K) were quantified and statistically analysed with Tukey's honest significant difference test. A significant difference was achieved when P values were <0.05.

RESULTS

S-PRG and Ca(OH)₂ dressings were associated with increased periapical grey values and inhibited osteoclast activity at 3 and 7 days; a significant difference in radiographic results and the number of osteoclasts was obtained at 3 and 7 days compared with the control group (P < 0.05). Reparative tissue was observed histologically in the space of the periapical resorbed necrotic area after S-PRG and Ca(OH)₂ treatment for 3 and 7 days. The number of macrophages was significantly decreased at 3 and 7 days in the S-PRG and Ca(OH)₂ specimens when compared with the controls (P < 0.05).

CONCLUSIONS

In a rat experimental model, the S-PRG root canal dressing was comparable to Ca(OH)₂ in promoting the healing of experimentally induced periapical lesions. S-PRG paste has the potential to be used as an alternative intracanal dressing in teeth with apical periodontitis.

Fluoride-Treated Bio-Resorbable Synthetic Hydroxyapatite Promotes Proliferation and Differentiation of Human Osteoblastic MG-63 Cells

When resorbable hydroxyapatite (HA) granules, which are used as a bone supplement material, were treated in neutral 4% sodium fluoride (NaF) solution, formation of a reactant resembling calcium fluoride was observed on the surface of the granules. Immediate and slow release of fluoride from fluoridated HA (HA+F) granules was observed after immersion in culture fluid, and the concentration increased over time to 1.25 ± 0.05 ppm F at 0.5 hours, 1.57 ± 0.12 ppm F at 24 hours, and 1.73 ± 0.15 ppm F at 48 hours. On invasion assay, migration of human osteoblast-like MG-63 cells exposed to the released fluoride was confirmed in comparison to the cells incubated with a nonfluoridated control sample (P &lt; .01). In addition, fluoride added to the medium increased MG-63 cell proliferation in a manner dependent on fluoride concentrations up to 2.0 ppm (P &lt; .05). At 5.0 ppm, however, fluoride significantly inhibited cell proliferation (P &lt; .005). Activity of the osteogenic differentiation marker, alkaline phosphatase (ALP), also increased with fluoride after exposure for 1 week, increasing significantly at 1.0 ppm (P &lt; .05). The promotion of MG-63 cell migration and proliferation, as well as increased ALP activity, suggested that fluoride released from the surface of resorbable HA granules, which were fluoridated by prior treatment with neutral 4% NaF solution, can provide a superb method to supply fluoride and promote osteogenic cell differentiation.

Fluoride's effects on the formation of teeth and bones, and the influence of genetics

Fluorides are present in the environment. Excessive systemic exposure to fluorides can lead to disturbances of bone homeostasis (skeletal fluorosis) and enamel development (dental/enamel fluorosis). The severity of dental fluorosis is also dependent upon fluoride dose and the timing and duration of fluoride exposure. Fluoride's actions on bone cells predominate as anabolic effects both in vitro and in vivo. More recently, fluoride has been shown to induce osteoclastogenesis in mice. Fluorides appear to mediate their actions through the MAPK signaling pathway and can lead to changes in gene expression, cell stress, and cell death. Different strains of inbred mice demonstrate differential physiological responses to ingested fluoride. Genetic studies in mice are capable of identifying and characterizing fluoride-responsive genetic variations. Ultimately, this can lead to the identification of at-risk human populations who are susceptible to the unwanted or potentially adverse effects of fluoride action and to the elucidation of fundamental mechanisms by which fluoride affects biomineralization.

Bone cell mitogenic action of fluoroaluminate and aluminum fluoride but not that of sodium fluoride involves upregulation of the insulin-like growth factor system

The fluoroaluminate (AlF(4)(-)) ion and sodium fluoride (NaF) have previously been shown to be bone cell mitogens. This study sought to determine whether the bone cell mitogenic action of AlF(4)(-) and/or NaF would involve the insulin-like growth factor (IGF) regulatory system. We evaluated the effect of mitogenic doses of AlF(4)(-) and NaF on the mRNA levels and the protein level (in conditioned media [CM]) of several components of the IGF system (i.e., IGF-2, IGF binding protein [IGFBP]-4, and IGFBP-5) in human TE85 osteosarcoma cells. Aluminum fluoride (AlF(3)) was included for comparison. NaF, AlF(3), and AlF(4)(-), each at 50-100 micromol/L, increased [3H]thymidine incorporation in TE85 cells. Mitogenic concentrations of AlF(3) and AlF(4)(-): (1) increased the mRNA (up to twofold after 24 h treatment) and protein (in CM) levels (up to 2.5-fold after 48 h treatment) of IGF-2; (2) increased the mRNA level (twofold) and the protein level in CM (up to threefold) of stimulatory IGFBP-5; and (3) either reduced slightly or had no effect on the mRNA and protein (in CM) levels of the inhibitory IGFBP-4. Conversely, mitogenic concentrations of NaF had no significant effects on the protein (in CM) or mRNA level of IGF-2, IGFBP-4, or IGFBP-5. The addition of an inhibitory concentration of IGFBP-4 completely abolished the bone cell mitogenic activity of AlF(3) and AlF(4)(-) but not that of NaF. The findings of this study provide strong evidence that the bone cell mitogenic activity of AlF(4)(-) and AlF(3), but not that of NaF, is mediated by the upregulation of the IGF regulatory system.

Pharmacotherapy of osteoporosis in postmenopausal women: focus on safety

The therapy of osteoporosis has made enormous strides in the last decade. There is now a range of interventions, each with its pros and cons. Calcium and vitamin D supplementation remain the foundation and have few safety issues. Bisphosphonates are widely used, though gastrointestinal tolerance is a problem with some oral preparations. Intravenous administration may circumvent this, although this introduces the smaller problem of acute phase reactions. The side effect profile of hormone replacement therapy (HRT) is still being delineated after 40 years of use, with substantial new information expected in the next few years. This will clarify its place in the medical management of the menopause. Raloxifene appears to have a superior safety profile to HRT, though its efficacy on bone may be less. While none of these options is suitable for everyone, the range of available therapies does mean that most patients can find an intervention that is effective and acceptable.

Directions of drug discovery in osteoporosis

Osteoporosis is a condition of increasing importance and prevalence in all parts of the world and particularly in Asia. Recent advances have led to the introduction of effective drugs that decrease bone resorption and stabilize bone mass. However, these drugs have been identified by serendipity rather than rational drug design and are not ideal because of limited bioavailability, mode of administration, or other unwanted effects. There is still a place for even more suitable and effective resorption inhibitors than those currently available. The more compelling need in this field is an acceptable drug that is anabolic for bone, that safely and acceptably increases bone mass and improves the disturbances in bone microarchitecture that characterize established and advanced osteoporosis. Possible approaches to identifying more effective resorption inhibitors and new anabolic agents are discussed.

Fluoroaluminate stimulates phosphorylation of p130 Cas and Fak and increases attachment and spreading of preosteoblastic MC3T3-E1 cells

Fluoroaluminate is a G-protein activator, it stimulates osteoblastic cells in culture, and is a bone-forming agent in vivo. To elucidate the mechanisms of G-protein-mediated action of fluoroaluminate in osteoblasts, we studied protein tyrosine phosphorylation in the preosteoblastic cell line MC3T3-E1. Fluoroaluminate, lysophosphatidic acid (LPA; an agonist for G-protein-coupled receptor), or adhesion to type I collagen all stimulated phosphorylation of a similar set of proteins, including p130, p120, p110 (previously identified as proline-rich tyrosine kinase 2, Pyk2), and p70. The phosphorylation of these proteins was sensitive to an Src inhibitor, but not to a Gi-protein inactivator, pertussis toxin. By purification/mass spectrometry and by immunodepletion, p130 protein was identified as p130 Cas (Crk-associated protein), a Src substrate and a protein involved in signaling by cell-adhesion receptors, integrins. Phosphorylation of immunoprecipitated p130 Cas increased upon stimulation with fluoroaluminate and with agonists of G-protein-coupled receptors, but not with growth factors. By immunodepletion, the p120 protein was identified as focal adhesion kinase, Fak. The addition of fluoroaluminate during cell attachment to type I collagen further stimulated phosphorylation of p130 Cas and of Fak. Simultaneously, fluoroaluminate increased the number of attached MC3T3-E1 cells and their spreading. These novel aspects of fluoroaluminate action in cell culture may be important for the bone-forming action of fluoroaluminate in vivo.

Differential effects of bacterial toxins on mitogenic actions of sodium fluoride and those of aluminum fluoride in human TE85 osteosarcoma cells

This study compared the effects of cholera toxin (CTX) and pertussis toxin (PTX) on the actions of sodium fluoride (NaF) and those of aluminum fluoride (AlF3) on cell proliferation and differentiation, as well as tyrosine phosphorylation level of mitogen activated protein kinase (MAPK) in human bone cells. NaF and AlF3 each significantly stimulated the proliferation of human TE85 osteosarcoma cells, increased cellular alkaline phosphatase (ALP) activity, and increased MAPK tyrosine phosphorylation level. CTX completely blocked the bone cell anabolic activities of both NaF and AlF3. While PTX (2 ng/ml) inhibited the bone cell actions of NaF, it had no significant effect on those of AlF3. Both CTX and PTX completely blocked the stimulatory action of AlF3 on MAPK tyrosine phosphorylation, but neither toxin had an effect on the action of NaF on MAPK tyrosine phosphorylation. In conclusion, PTX and CTX had contrasting effects on the anabolic bone cell actions of NaF and AlF3 actions. These findings argue against the hypothesis that the osteogenic activity of NaF is mediated via the formation of AlF3 in human TE85 osteosarcoma cells.

Evidence for the involvement of two pathways in activation of extracellular signal-regulated kinase (Erk) and cell proliferation by Gi and Gq protein-coupled receptors in osteoblast-like cells

The mechanisms by which Gi and Gq protein- coupled receptors mediate mitogenic signaling in osteoblast-like cells are unknown and were investigated in MC3T3-E1 cells using specific receptor agonists such as lysophosphatidic acid (LPA) and prostaglandin F2alpha (PGF2alpha). In contrast to their implication in epidermal growth factor (EGF) receptor tyrosine kinase signaling, the adaptor protein Shc, the Grb2/Sos complex, and the small G protein Ras were not involved in the activation of Erk induced by either LPA or PGF2alpha in MC3T3-E1 cells, suggesting that activation of Erk by Gi and Gq protein-coupled receptors is Ras independent in these cells. Using specific kinase inhibitors and kinetic analyses, we provide evidence for two distinct components in the activation of Erk by Gi and Gq protein-coupled receptors in MC3T3-E1 cells including an Src-like kinase-dependent pathway and a protein kinase C (PKC)-dependent mechanism. Functional analyses suggested that these two components are required for optimal DNA synthesis in response to LPA and PGF2alpha. These results suggest the implication of two pathways in the stimulation of Erk and cell replication by growth factors acting through Gi and Gq protein-coupled receptors in bone-forming cells.

Role of signal transduction pathways in lung inflammatory responses

A variety of cell types participate in lung inflammation. Macrophages and epithelial cells play an important role in the inflammatory process by releasing cytokines in a complex cell to cell network. Interleukins are important mediators of this cell signalling. The interleukins IL-6 and IL-8 are released from epithelial cells in response to noxious agents such as particles, bacterial and fungal toxins and various chemicals. Though the involvement of, e.g. NF-IL-6 (C/EBP-beta) in the regulation of interleukins has been reported, the role of different signal transduction pathways in the regulation of these mediators has not been thoroughly investigated in lung epithelial cells. The involvement of different signal transduction pathways in the release of inflammatory markers is discussed with special emphasis on the effect of lung toxic compounds in human and rat lung epithelial cells.

Molecular mechanism of action of fluoride on bone cells

Fluoride is an effective anabolic agent to increase spinal bone density by increasing bone formation, and at therapeutically relevant (i.e., micromolar) concentrations, it stimulates bone cell proliferation and activities in vitro and in vivo. However, the fluoride therapy of osteoporosis has been controversial, in large part because of a lack of consistent antifracture efficacy. However, information regarding the molecular mechanism of action of fluoride may improve its optimum and correct usage and may disclose potential targets for the development of new second generation drugs that might have a better efficacy and safety profile. Accordingly, this review will address the molecular mechanisms of the osteogenic action of fluoride. In this regard, we and other workers have proposed two competing models, both of which involve the mitogen activated protein kinase (MAPK) mitogenic signal transduction pathway. Our model involves a fluoride inhibition of a unique fluoride-sensitive phosphotyrosine phosphatase (PTP) in osteoblasts, which results in a sustained increase in the tyrosine phosphorylation level of the key signaling proteins of the MAPK mitogenic transduction pathway, leading to the potentiation of the bone cell proliferation initiated by growth factors. The competing model proposes that fluoride acts in coordination with aluminum to form fluoroaluminate, which activates a pertussis toxin-sensitive Gi/o protein on bone cell membrane, leading to an activation of cellular protein tyrosine kinases (PTKs), which in turn leads to increases in the tyrosine phosphorylation of signaling proteins of the MAPK mitogenic signal transduction pathway, ultimately leading to a stimulation of cell proliferation. A benefit of our model, but not the other model, is that it accounts for all the unique properties of the osteogenic action of fluoride. These include the low effective fluoride dose, the skeletal tissue specificity, the requirement of PTK-activating growth factors, the sensitivity to changes in medium phosphate concentration, the preference for undifferentiated osteoblasts, and the involvement of the MAPK. Unlike fluoride, the mitogenic action of fluoroaluminate is not specific for skeletal cells. Moreover, the mitogenic action of fluoroaluminate shows several important, different characteristics than that of fluoride. Thus, it is likely that our model of a fluoride-sensitive PTP represents the actual molecular mechanism of the osteogenic action of fluoride.

Catecholamines stimulate the proliferation and alkaline phosphatase activity of MC3T3-E1 osteoblast-like cells

A number of factors have been shown to influence osteoblastic proliferation, including fluoride. Recent observations suggest that heterotrimeric G proteins are probably involved in the mitogenic response induced by this agent, further suggesting a role of guanosine 5'-triphosphate (GTP)-binding protein-coupled receptors (GPCR) in the regulation of osteoblastic cell growth. We therefore explored what mitogenic factors known to activate GPCR can influence the replication of mouse osteoblast-like MC3T3-E1 cells. Among several candidates, epinephrine was found to be a potent mitogen for these cells, and its effect on the growth and differentiation of these cells was further investigated. Deoxyribonucleic acid (DNA) synthesis was dose dependently enhanced by this catecholamine in the concentration range of 1 nmol/L-10 micromol/L. Stimulation of DNA synthesis by catecholamines was in the order of epinephrine > norepinephrine >> isoproterenol, indicating that alpha adrenergic receptors mediated this cellular response. Further analysis with specific adrenergic receptor agonists and antagonists suggested that the mitogenic response induced by epinephrine in MC3T3-E1 cells is mediated by alpha1 adrenergic receptors. In addition to its effect on cell replication, epinephrine also enhanced alkaline phosphatase (ALP) activity in these cells but had little effect on collagen synthesis and osteocalcin production. As for the mitogenic response, the change in ALP activity was found to be mediated by alpha1 adrenergic receptors. Both effects of epinephrine on cell replication and ALP activity were markedly reduced by pretreatment of the cells with pertussis toxin (PTX), suggesting a role of Gi proteins. These effects were also completely blocked by pretreatment of the cells with 50 micromol/L genistein, a nonselective inhibitor of tyrosine kinase. In conclusion, the results indicate that epinephrine enhances replication and ALP activity of MC3T3-E1 osteoblast-like cells via alpha1 adrenergic receptors coupled to Gi proteins. The signaling mechanism probably involves a tyrosine phosphorylation mechanism. These observations suggest that PTX-sensitive G proteins are potent mediators of cell proliferation and ALP activity of osteoblast-like cells in response to factors acting through G protein-coupled receptors.

Fluoroaluminate induces activation and association of Src and Pyk2 tyrosine kinases in osteoblastic MC3T3-E1 cells

Fluoride is known to increase bone mass in vivo, probably through stimulation of osteoblast proliferation; however, the mechanisms of fluoroaluminate action in osteoblasts have not yet been elucidated. We have previously shown that in osteoblastic MC3T3-E1 cells, fluoroaluminate stimulates G protein-mediated protein tyrosine phosphorylation (Scaronuscarona, M., Standke, G. J. R., Jeschke, M., and Rohner, D. (1997) Biochem. Biophys. Res. Commun. 235, 680-684). Although the Ser/Thr kinases Erk1, Erk2, and p70(S6K) were activated in response to fluoroaluminate, the identity of fluoroaluminate-activated tyrosine kinase(s) remained elusive. In this study, we show that in MC3T3-E1 cells, fluoroaluminate induces a 110-kDa tyrosine-phosphorylated protein that we identify as Pyk2, a cytoplasmic tyrosine kinase related to Fak (focal adhesion kinase). The tyrosine phosphorylation of Pyk2 increased in a dose- and time-dependent manner. The autophosphorylation activity of Pyk2 increased 3-fold and reached its maximum within 10 min of fluoroaluminate treatment. Fluoroaluminate also induced activation of Src and the association of Pyk2 with Src. The phosphorylation of Src-associated Pyk2 increased >20-fold in in vitro kinase assays, suggesting that Pyk2 is phosphorylated by Src. Although MC3T3-E1 cells express much more Fak than Pyk2, Src preferentially associated with Pyk2. In vitro, Pyk2 bound to the Src SH2 domain, suggesting that this interaction mediates the Src-Pyk2 association in cells. These data indicate that osteoblastic cells express Pyk2, which is tyrosine-phosphorylated and activated in response to G protein activation by fluoroaluminate, and that the mechanism of Pyk2 activation most likely involves Src. Thus, Src and Pyk2 are tyrosine kinases involved in G protein-mediated tyrosine phosphorylation in osteoblastic cells and may be important for the osteogenic action of fluoroaluminate.

Fluoride potentiates the osteogenic effects of IGF-I in aged ovariectomized rats

The molecular mechanisms whereby fluoride stimulates osteogenic cell proliferation are not clearly established. However, fluoride has been shown to enhance the protein tyrosine phosphorylation of various constituents of intracellular signaling cascades in osteoblastic cells following stimulation of growth factor receptors such as the insulin-like growth factor-I (IGF-I) receptor. Such in vitro findings provided the rationale for testing whether the administration of fluoride could enhance IGF-I effects on bone mass in vivo. Adult ovariectomized osteopenic rats were treated with sodium fluoride at a dose of 6 mg/kg per day in drinking water for 8 weeks in association with IGF-I either at a dose of 2 mg/kg per day, which is capable of increasing bone mass, or at a lower dose without detectable skeletal effects. Bone mineral density (BMD) and content (BMC) were evaluated by dual-energy X-ray absorptiometry at the levels of the lumbar spine and proximal, midshaft, and total tibia before and after 8 weeks of treatment. During this period, fluoride alone did not significantly influence BMD/BMC at any skeletal site. However, it potentiated the effect of the higher dose of IGF-I on bone mass at the level of the proximal tibia. When administered in combination with the lower dose of IGF-I, which per se did not modify bone mass, it appeared to sensitize tibial bone to the effects of IGF-I. These changes were associated with a concomitant increase in osteocalcin, taken as a reflection of bone formation. These results indicate that fluoride could potentiate the osteogenic effects of IGF-I on bone in adult ovariectomized rats.

Mechanism of the mitogenic effect of fluoride on osteoblast-like cells: evidences for a G protein-dependent tyrosine phosphorylation process

Recent results indicate that a fluoroalumino complex (AlFx) is probably the molecule responsible for the mitogenic effect of fluoride in MC3T3-E1 osteoblast-like cells. Initial analysis suggested that a tyrosine phosphorylation (tyr phos) process similar to that induced by thrombin and activation of the p42 MAP kinase (ERK 2) mediate this cellular response. In the present study, the signaling mechanism activated by AlFx was further investigated. The results indicated that AlFx dose-dependently enhanced the tyr phos of the cell adhesion proteins FAK and paxillin, as well as of the adaptor molecules p46shc, p52shc, and p66shc and their association with GRB2. Pretreatment of MC3T3-E1 cells with cytochalasin D completely prevented FAK and paxillin tyr phos without any alteration in the tyr phos of Shc proteins and activation of ERK2 induced by AlFx. This observation suggests that in confluent MC3T3-E1 cells, there is no link between the activation of FAK induced by AlFx and the stimulation of ERK2. Pretreatment of the cells with pertussis toxin inhibited Shc phosphorylation, activation of ERK2, and markedly reduced cell replication induced by AlFx. This toxin also significantly reduced the stimulation of Pi transport activity induced by AlFx in these cells. Alteration in tyr phos induced by AlFx was not associated with any detectable inhibition of tyrosine phosphatase activity in MC3T3-E1 cell homogenates, suggesting that enhanced tyr phos induced by AlFx probably resulted from activation of a tyrosine kinase. In conclusion, the results of this study suggest that the mitogenic effect of fluoride in MC3T3-E1 osteoblast-like cells is mediated by the activation of a pertussis toxin-sensitive Gi/o protein and suggest an important role for these heterotrimeric G proteins in controlling the growth and differentiation of bone-forming cells.

Fluoroaluminate induces pertussis toxin-sensitive protein phosphorylation: differences in MC3T3-E1 osteoblastic and NIH3T3 fibroblastic cells

Fluoride is an acknowledged bone-forming agent that may act through stimulation of osteoblast proliferation. Fluoride's action on osteoblasts and bone is potentiated by aluminum, which can form a complex with fluoride (fluoroaluminate) and activate heterotrimeric G proteins. Here we examined signaling pathways activated by fluoroaluminate in MC3T3-E1 osteoblastic and in NIH3T3 fibroblastic cells. In MC3T3-E1 cells, fluoroaluminate induced a decrease in cAMP levels and an increase in MAP and p70 S6 kinase phosphorylations. These responses were partially or completely prevented by pertussis toxin, an inhibitor of G alpha i proteins. In NIH3T3 cells, fluoroaluminate induced weaker tyrosine and MAP kinase phosphorylations. Fluoroaluminate, but not PDGF, induced a long-lasting tyrosine phosphorylation of a 130 kDa protein only in MC3T3-E1 cells. The expression of G alpha i2, but not of G alpha s and G alpha q/11 proteins was about 10-fold higher in MC3T3-E1 cells. Thus, different signaling in osteoblastic and fibroblastic cells may be due to differential expression of G alpha i proteins and tyrosine kinase substrates and could underlie fluoride's pharmacological action in bone.

Fluoride at mitogenic doses induces a sustained activation of p44mapk, but not p42mapk, in human TE85 osteosarcoma cells

Fluoride, at micromolar concentrations, stimulates bone cell proliferation in vitro. In this study, we sought to test whether fluoride at mitogenic doses increases the tyrosyl phosphorylation level and specific activity of a mitogen-activated protein kinase (MAPK) in human TE85 osteosarcoma cells. Analysis by immunoprecipitation with antiphosphotyrosine antibody followed by Western analysis using an anti-pan extracellular signal-regulated kinase antibody revealed that fluoride at the optimal mitogenic dose (i.e. 100 mumol/L) induced a time-dependent increase in the steady state tyrosyl phosphorylation level of p44mapk, but not p42mapk, with the maximal increase (4- to 13-fold) after 1-3 h fluoride treatment. The effect was sustained in that a 9-fold increase was seen after 12 h of the fluoride treatment. The sustained nature of the effect is consistent with an inhibition of dephosphorylation rather than a direct stimulation of phosphorylation. The fluoride effect on the tyrosyl phosphorylation level of p44mapk was dose dependent, with the optimal dose being 100 mumol/L fluoride. The mitogenic dose of fluoride also increased the specific activity and the in-gel kinase activity of p44mapk, but not that of p42mapk, in a time-dependent manner similar to the effect on the p44mapk tyrosyl phosphorylation level. Fluoride at the same micromolar doses did not increase cell proliferation, tyrosyl phosphorylation, or specific activity of any MAPK in human skin foreskin fibroblasts, which are fluoride-nonresponsive cells. Consistent with the interpretation that the effect of fluoride on the steady state tyrosyl phosphorylation level of p44mapk is a consequence of an inhibition of a phosphotyrosyl phosphatase (PTP), mitogenic doses of orthovanadate, a bone cell mitogen and a PTP inhibitor, also increased the steady state tyrosyl phosphorylation level of p44mapk, but not p42mapk, in a time-dependent sustained manner similar to that observed with fluoride. Together, these findings support the concept that inhibition of a PTP activity in bone cells could lead to an activation of MAPK activity.

Platelet-derived growth factor stimulates sodium-dependent Pi transport in osteoblastic cells via phospholipase Cgamma and phosphatidylinositol 3' -kinase

Inorganic phosphate (Pi) is a major regulator of cell metabolism. The Pi transport activity in the plasma membrane is a main determinant of the intracellular level of this ion. In bone-forming cells, Pi transport is important for the calcification of the bone matrix. In this study, the effect of platelet-derived growth factor (PDGF) on Pi transport activity and the signaling mechanism involved in this cellular response were analyzed. The results indicate that PDGF is a potent and selective stimulator of sodium-dependent Pi transport in the mouse calvaria-derived MC3T3-E1 osteoblast-like cells. The change in Pi transport induced by PDGF-BB was dependent on translational processes and affected the Vmax of the Pi transport system. These observations suggested that enhanced Pi transport activity in response to PDGF resulted from insertion of newly synthesized Pi transporters in the plasma membrane. The role of activation of mitogen activated protein (MAP) kinase, phospholipase C (PLC)gamma or phosphatidylinositol 3-kinase (PI-3-kinase), in mediating this effect of PDGF, was investigated. A selective inhibitor of the PDGF receptor tyrosine kinase activity (CGP 53716) completely blocked PDGF-induced protein tyrosine phosphorylation of several proteins including the PDGF receptor, PLCgamma, MAP kinase, and association of the p85 subunit of PI-3'-kinase. Associated with this effect, the increase in Pi transport induced by PDGF was completely blunted by 5 microM CGP 53716. Inhibition of MAP kinase activity by cAMP agonists did not influence Pi transport stimulation induced by PDGF. However, inhibitors of protein kinase C completely blocked this response. A selective inhibitor of PI-3-kinase, LY294002, also significantly reduced this effect of PDGF. In summary, these results indicate that PDGF is a potent and selective stimulator of Pi transport in osteoblastic cells. The mechanism responsible for this effect is not mediated by MAP kinase but involves tyrosine phosphorylation-dependent activation of PLCgamma and PI-3-kinase.

Aluminum potentiates P(i) transport stimulation induced by fluoride in osteoblast-like cells

The effect of aluminum (AI) on inorganic phosphate (P(i)) transport stimulation induced by fluoride (F) was investigated in MC3T3-E1 osteoblast-like cells. Al potentiated the increase in P(i) transport activity induced by F in a dose- and time-dependent manner. Results obtained with deferoxamine mesylate, an Al chelator, suggest that a fluoroalumino complex is probably the active F molecule responsible for the change in P(i) transport observed in this study. The signaling pathway responsible for the stimulation of P(i) transport by F+Al likely involves a tyrosine phosphorylation process but neither a protein kinase C nor a mitogen-activated protein kinase pathway. As previously found in UMR-106 cells for F alone, F+Al potentiated the change in P(i) transport induced by fetal calf serum. A similar interaction was found between F+Al and thrombin acting through a G protein-coupled receptor. These observations are compatible with the hypothesis that F+Al could interact with G protein-coupled receptors associated with a signaling tyrosine phosphorylation process involved in the regulation of P(i), transport in osteoblast-like cells.