Sustained activation of the extracellular signal-regulated kinase pathway is required for extracellular calcium stimulation of human osteoblast proliferation

Bone-Like Tissue Formation by Three-Dimensional Culture of MG63 Osteosarcoma Cells in Gelatin Hydrogels Using Calcium-Enriched Medium

The aim of this study was to investigate the effect of Ca(2+) concentration in culture medium on the promotion of osteogenesis by MG63 osteoblast-like cells and to prepare bone-like tissues by supplying Ca(2+)-enriched medium to MG63 cells immobilized in three-dimensional gelatin hydrogels. Human osteosarcoma MG63 cells were cultured on tissue culture dish under various Ca(2+) concentrations to evaluate the effect of Ca(2+) concentration on calcium deposition. When Ca(2+) concentration was 8 mM, the maximum calcium deposition was obtained at day 28. Then MG63 cells were entrapped in gelatin hydrogels cross-linked by transglutaminase and cultured for 28 days, either in a standard culture medium or in medium containing 8 mM Ca(2+). Effects of Ca(2+)-enriched medium on osteoblastic phenotype of MG63 cells in gelatin hydrogels were analyzed in terms of cell number, calcium deposition content, and alkaline phosphatase (ALP) activity. The characteristics of calcified gelatin hydrogels were evaluated by x-ray diffraction (XRD), histological analysis, and scanning electron microscopy (SEM). After 28 days of culture, no significant difference in cell numbers was found between the different culture conditions. However, calcium content of gelatin hydrogels with cells cultured in Ca(2+)-enriched media was significantly higher than that of hydrogels with cells cultured in standard Ca(2+) concentration medium. After 14 days of culture, ALP activity of cells cultured in Ca(2+)-enriched media was down-regulated compared with that of cells cultured in standard Ca(2+) concentration media. XRD analysis indicated the formation of hydroxyapatite in gelatin hydrogels cultured in the Ca(2+)-enriched media at day 14, and the XRD pattern of the composite at day 21 was almost similar to that of mouse tibia. Moreover, histological analysis and SEM analysis revealed that cross-sections of hydrogels cultured in Ca(2+)-enriched media had an organic/mineral layer structure analogous to that of mouse tibia.

ERK1/2-activated de Novo Mapkapk2 Synthesis Is Essential for Osteogenic Growth Peptide Mitogenic Signaling in Osteoblastic Cells

In osteoblasts, the mitogen-activated protein kinases ERK1/2 and p38 as well as the cAMP-response element-binding protein (CREB) have been implicated in the regulation of proliferation and differentiation. The osteogenic growth peptide (OGP) is a 14-mer bone cell mitogen that increases bone formation and trabecular bone density and stimulates fracture healing. OGP-(10-14) is the physiologically active form of OGP. Using gene array analysis, real-time reverse transcription-PCR, and immunoblot and DNA synthesis assays we show here that in MC3T3 E1 and newborn mouse calvarial osteoblastic cultures the OGP-(10-14) mitogenic signaling is critically dependent on de novo synthesis of mitogen-activated protein kinase-activated protein kinase 2 (Mapkapk2) mRNA and protein. The increase in Mapkapk2 occurs following short term (5-60 min) stimulation of ERK1/2 activity by OGP-(10-14); phosphorylation of p38 remains unaffected. Downstream of Mapkapk2, CREB is phosphorylated on Ser(133) leading to its enhanced transcriptional activity. That these events are critical for the OGP-(10-14) mitogenic signaling is demonstrated by blocking the effects of OGP-(10-14) on the ERK1/2 pathway, Mapkapk2, CREB, and DNA synthesis using the MEK inhibitor PD098059. The OGP-(10-14) stimulation of CREB transcriptional activity and DNA synthesis is also blocked by Mapkapk2 siRNA. These data define a novel mitogenic signaling pathway in osteoblasts whereby ERK1/2 stimulation of CREB phosphorylation and transcriptional activity as well as DNA synthesis are critically dependent on de novo Mapkapk2 synthesis.

Effects of FGF-2/-9 in calvarial bone cell cultures: differentiation stage-dependent mitogenic effect, inverse regulation of BMP-2 and noggin, and enhancement of osteogenic potential

Systemically administered fibroblast growth factors (FGFs) show anabolic effects on bone formation in animals, whereas in vitro cell culture studies have demonstrated that FGFs block mineralized bone nodule formation. These apparently contradictory outcomes indicate that the nature of FGF action is complex and that the biological effect of FGFs may depend on the differentiation stage of osteoblasts, interaction with other cytokines, or the length and mode of exposure to factors. Thus, we have utilized primary calvarial bone cell populations at different maturation phases to determine their responses to 2, FGF-9, and BMP-2, the factors expressed in bone. FGF-2 and FGF-9 stimulated proliferation of the cell populations consisting of more mature osteoblasts, but not those with undifferentiated precursor cells. Continuous treatment with FGF-2/-9 inhibited expression of several osteoblast marker genes and mineralization. However, brief pretreatment with FGF-2/-9 or sequential treatment with FGF-2/-9 followed by BMP-2 led to marked stimulation of mineralization, suggesting that FGFs enhance the intrinsic osteogenic potential. Furthermore, FGF-2 and FGF-9 increased expression of other osteogenic factors BMP-2 and TGFbeta-1. Meanwhile, blocking endogenous FGF signaling, using a virally transduced dominant-negative FGF receptor (FgfR), resulted in drastically reduced expression of the BMP-2 gene, demonstrating for the first time that endogenous FGF/FgfR signaling is a positive upstream regulator of the BMP-2 gene in calvarial osteoblasts. In contrast, expression of a BMP antagonist noggin was inhibited by FGF-2 and FGF-9. Thus, collective data from this study suggest that FGF/FgfR signaling enhances the intrinsic osteogenic potential by selectively expanding committed osteogenic cell populations as well as inversely regulating BMP-2 and noggin gene expression.

Growth hormone attenuates the transcriptional activity of Runx2 by facilitating its physical association with Stat3beta

We document that GH controls osteoblast function by modulating the biological activity of the osteospecific transcription factor Runx2. Evidence is provided for a physical interaction between Runx2 and Stat3beta, which is enhanced by GH and downregulates the transcriptional properties of this key osteogenic regulator.

INTRODUCTION

Growth hormone (GH) signals to bone either through insulin-like growth factor-1 or directly by influencing the function of osteoblasts, the bone-forming cells. This study aimed at exploring the molecular events that underlie the direct biological action of GH on osteoblastic cells, and specifically, the effects that it might exert on the function of the bone-specific transcriptional regulator Runx2.

MATERIALS AND METHODS

The GH-responsive human osteoblastic cell line Saos-2 was used as our experimental system. Western blot analyses were used to monitor the presence of several parameters known to be affected by GH in these cells (i.e., downregulation of GH receptor, induction of STATs, and extracellular signal-regulated kinase [ERK] mitogen-activated protein kinase [MAPK] pathways). Electrophoretic mobility shift assays were used to assess Runx2 and Stat3 binding activity on an osteoblast-specific element (OSE2) after GH treatment. A combination of yeast two-hybrid and co-immunoprecipitation assays were performed to test for the existence of a physical Runx2.Stat3beta association. Finally, co-transfection experiments were used to investigate the interplay of the two transcription factors on the activity of a p6OSE2-Luc promoter after GH stimulation.

RESULTS

We show that GH signaling through Stat3/ERK MAPK potentiates the DNA binding activity of Runx2 but, at the same time, restrains its transcriptional potential. Moreover, a novel physical interaction of Runx2 with transcription factor Stat3beta, which is enhanced by GH stimulation, was documented both in vitro and in vivo. Importantly, this interaction impairs the transcriptional activity of Runx2 without affecting its DNA binding capacity.

CONCLUSION

Our data provide the first evidence that GH modulates the transcriptional function of Runx2 in osteoblastic cells by promoting its inhibitory interaction with Stat3beta. Shedding light on such mechanisms will contribute to a better understanding of GH effects on skeletal homeostasis that may impact on decisions at the clinical level, especially in diseases affecting bone quantity and quality (e.g., osteoporosis).

Extracellular signal-regulated kinase-1 and -2 are both essential for the shear stress-induced human osteoblast proliferation

Extracellular signal-regulated kinases (Erk)-1 and -2 are key mediators of various mitogenic signaling pathways, including mechanical stress-induced osteoblast proliferation. Mechanical stimuli, such as flow shear stress, simultaneously activate both Erk-1 and -2 in osteoblasts, resulting in stimulation of osteoblast proliferation. This study sought to test whether Erk-1, -2, or both are essential for the fluid flow shear stress-induced osteoblast proliferation. Moloney leukemia virus (MLV)-based vectors expressing wild-type (wt)- or kinase-deficient (kd) Erk-1 and Erk-2, respectively, were constructed and used to transduce human TE85 osteosarcoma cells with an MOI of 30. An MLV-red fluorescent protein (RFP) vector was included as a control. Effects of Erk-1 and -2 overexpression on cell proliferation in response to a 30-min constant fluid flow shear stress at 20 dynes/cm2 were determined with [3H]thymidine incorporation 24 h after the shear stress. The MLV-Erk vector-transduced TE85 cells showed a >10- and approximately 2-fold overexpression of Erk-1 and -2 protein, respectively. The RFP expressing control cells and the parental TE85 cells each showed an approximately twofold increase (P < 0.01) in [3H]thymidine incorporation in response to the shear stress. Cells overexpressing wt-Erk-1 or -2 showed small enhancing effects on the response to the shear stress in the increases in [3H]thymidine incorporation and cell number. Conversely, overexpression of kd-Erk-1 or -2 each alone completely abolished the shear stress-induced osteoblast proliferation. Overexpression of either kd-Erk-1 or kd-Erk-2 alone did not have a significant effect on basal osteoblast proliferation, suggesting that the Erk signaling pathway may not be essential for basal cell proliferation. In summary, this study demonstrates for the first time that Erk-1 and -2 are both required for the mitogenic response to fluid flow shear stress in human osteoblasts and that blocking Erk-1 or -2 each alone is sufficient to completely block the mitogenic response to shear stress-induced proliferation.

Ca2+ as an extracellular signal in bone

Bone is the major sink and store for calcium and it fulfils essential roles in the maintenance of extracellular free ionised calcium concentration ([Ca2+]e) within its homeostatic range (1.1-1.3 mM). In response to acute hypercalcaemia or hypocalcaemia, Ca2+ is rapidly transported into or out of bone. Bone turnover (and therefore bone Ca2+ turnover) achieves the long-term correction of the [Ca2+]e by the metabolic actions of osteoblasts and osteoclasts, as they respectively incorporate or release Ca2+ from bone. These processes are regulated by the actions of hormones, such as parathyroid hormone (PTH), the release of which is a function of the [Ca2+]e, and is regulated by the action of the Ca2+-sensing receptor (CaR) in the parathyroid gland. Tissue culture studies indicate that bone cells also directly respond to increasing and decreasing [Ca2+]e in their vicinity, independently of the systemic factors. Nevertheless, further studies are necessary to identify how the acute and long-term local changes in [Ca2+]e affect bone cells and the physiological processes they are involved in. Also, the molecular mechanisms which enable the bone cells to sense and respond to [Ca2+]e are not clear. Like the parathyroid cells, bone cells also express the CaR, and accumulating evidence indicates the involvement of this receptor in their responses to the changing extracellular ionic environment.

Calcium receptor-mediated intracellular signalling

As a G protein-coupled receptor (GPCR), the extracellular calcium-sensing receptor (CaR) responds to changes in extracellular free calcium concentration by inducing intracellular signalling. These CaR-induced signals then specifically modulate cellular functions such as parathyroid hormone secretion from the parathyroid glands and calcium reabsorption in the kidney and thus to understand how the CaR functions one must understand how it signals. CaR-induced signalling involves intracellular Ca2+ mobilisation/oscillations as well as the activation of various phospholipases and protein kinases and the suppression of cAMP formation. This review will detail the intracellular pathways by which the CaR is believed to elicit its physiological functions and summarises the evidence for cell- and agonist-specific differential signalling.

Mitogenic action of calcium-sensing receptor on rat calvarial osteoblasts

The parathyroid calcium-sensing receptor (CaR) plays a nonredundant role in systemic calcium homeostasis. In bone, Ca(2+)(o), a major extracellular factor in the bone microenvironment during bone remodeling, could potentially serve as an extracellular first messenger, acting via the CaR, that stimulates the proliferation of preosteoblasts and their differentiation to osteoblasts (OBs). Primary digests of rat calvarial OBs express the CaR as assessed by RT-PCR, Northern, and Western blot analysis, and immunocolocalization of the CaR with the OB marker cbfa-1. Real-time PCR revealed a significant increase in CaR mRNA in 5- and 7-d cultures compared with 3-d cultures post harvesting. High Ca(2+)(o) did not affect the expression of CaR mRNA during this time but up-regulated cyclin D (D1, D2, and D3) genes, which are involved in transition from the G1 to the S phase of the cell cycle, as well as the early oncogenes, c-fos and early growth response-1; high Ca(2+)(o) did not, however, alter IGF-I expression, a mitogenic factor for OBs. The high Ca(2+)(o)-dependent increase in the proliferation of OBs was attenuated after transduction with a dominant-negative CaR (R185Q), confirming that the effect of high Ca(2+)(o) is CaR mediated. Stimulation of proliferation by the CaR involves the Jun-terminal kinase (JNK) pathway, as high Ca(2+)(o) stimulated the phosphorylation of JNK in a CaR-mediated manner, and the JNK inhibitor SP600125 abolished CaR-induced proliferation. Our data, therefore, show that the parathyroid/kidney CaR expressed in rat calvarial OBs exerts a mitogenic effect that involves activation of the JNK pathway and up-regulation of several mitogenic genes.

Biologic significance of surface microroughing in bone incorporation of porous bioactive glass implants

A novel chemical etching method was recently developed to create a controlled microrough surface on porous bioactive glass implants. Our earlier in vitro studies showed enhanced attachment of osteoblast-like MG63 cells on a microrough bioactive glass surface. The purpose of our current study was to confirm the in vivo significance of surface microroughening for bone bonding of bioactive glass. Porous bioactive glass cones made of sintered microspheres were surgically implanted in the anterior cortex of rabbit femurs. Peripheral quantitative computed tomography (pQCT), biomechanical push-out testing, histomorphometry, and electron microscopy (BEI-SEM) were used to analyze bone ingrowth and osseointegration at 7, 10, 14, 28, 56, and 84 days after implantation. The results showed that microroughening of the bioactive glass surface significantly enhanced the bone-bonding response of the biomaterial. The positive response was seen in one of the three bioactive glass compositions studied. The affinity index of new bone on the glass surface was significantly (p = 0.02) increased with a trend (p = 0.10) toward improved mechanical incorporation. New bone formation was dependent on the glass composition, and it was found to occur not only through the mechanism of bone ingrowth but also based on in situ osteogenesis within implant interstices. Based on these results, the procedure of microroughening could enhance the osteopromotive properties of certain bioactive glass compositions.

Physiological changes in extracellular calcium concentration directly control osteoblast function in the absence of calciotropic hormones

We investigated the direct effects of changes in free ionized extracellular calcium concentrations ([Ca2+]o) on osteoblast function and the involvement of the calcium-sensing receptor (CaR) in mediating these responses. CaR mRNA and protein were detected in osteoblast models, freshly isolated fetal rat calvarial cells and murine clonal osteoblastic 2T3 cells, and in freshly frozen, undecalcified preparations of human mandible and rat femur. In fetal rat calvarial cells, elevating [Ca2+]o and treatment with gadolinium, a nonpermeant CaR agonist, resulted in phosphorylation of the extracellular signal-regulated kinases 1 and 2, Akt, and glycogensynthase kinase 3beta, consistent with signals of cell survival and proliferation. In agreement, cell number was increased under these conditions. Expression of the osteoblast differentiation markers core binding factor alpha1, osteocalcin, osteopontin, and collagen I mRNAs was increased by high [Ca2+]o, as was mineralized nodule formation. Alkaline phosphatase activity was maximal for [Ca2+]o between 1.2 and 1.8 mM. Inhibition of CaR by NPS 89636 blocked responses to the CaR agonists. In conclusion, we show that small deviations of [Ca2+]o from physiological values have a profound impact on bone cell fate, by means of the CaR and independently of systemic calciotropic peptides.

Calcium-sensing receptor induces proliferation through p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase but not extracellularly regulated kinase in a model of humoral hypercalcemia of malignancy

Using H-500 rat Leydig cancer cells as a model of humoral hypercalcemia of malignancy (HHM), we previously showed that high Ca(2+) induces PTH-related peptide (PTHrP) secretion via the calcium-sensing receptor (CaR) and mitogen- and stress-activated kinases, e.g. MAPK kinase 1 (MEK1), p38 MAPK, and stress-activated protein kinase 1/c-Jun N-terminal kinase. Because cellular proliferation is a hallmark of malignancy, we studied the role of the CaR in regulating the proliferation of H-500 cells. Elevated Ca(2+) has a mitogenic effect on these cells that is mediated by the CaR, because the calcimimetic NPS R-467 also induced proliferation. Inhibition of phosphatidylinositol 3-kinase (PI3K) and p38 MAPK but not MEK1 abolished the mitogenic effect. Activation of PI3K by elevated Ca(2+) was documented by phosphorylation of its downstream kinase, protein kinase B. Because protein kinase B activation promotes cell survival, we speculated that elevated Ca(2+) might protect H-500 cells against apoptosis. Using terminal uridine deoxynucleotidyl nick end labeling staining, we demonstrated that high Ca(2+) (7.5 mM) and NPS R-467 indeed protect cells against apoptosis induced by serum withdrawal compared with low Ca(2+) (0.5 mM). Because the CaR induces PTHrP secretion, it is possible that the mitogenic and antiapoptotic effects of elevated Ca(2+) could be indirect and mediated via PTHrP. However, blocking the type 1 PTH receptor with PTH (7-34) peptide did not alter either high Ca(2+)-induced proliferation or protection against apoptosis. Taken together, our data show that activation of PI3K and p38 MAPK but not of MEK1/ERK by the CaR promotes proliferation of H-500 cells as well as affords protection against apoptosis. These effects are likely direct without the involvement of PTHrP in an autocrine mode.

Calcium-sensing Receptor-mediated ERK1/2 Activation Requires Gαi2 Coupling and Dynamin-independent Receptor Internalization

The calcium-sensing receptor (CaR) recently has been shown to activate MAP kinase (ERK1/2) in various cell types as well as in heterologous expression systems. In this study we show that the CaR agonist NPS R-467 (1 microm), which does not activate the CaR by itself, robustly activates ERK1/2 in the presence of a low concentration of Ca(2+) (0.5 mm CaCl(2)) in human embryonic kidney (HEK) cells permanently expressing the human CaR (HEK-hCaR). Ca(2+) (4 mm) also activates ERK1/2 but with differing kinetics. CaR-dependent ERK1/2 activation begins to desensitize to 4 mm Ca(2+) after 10 min, whereas there is no desensitization to NPS R-467/CaCl(2) as late as 4 h. Moreover, recovery from desensitization occurs as rapidly as 30 min with 4 mm CaCl(2). Pretreatment of HEK-hCaR cells with concanavalin A (250 microg/ml) to block CaR internalization completely eliminated the NPS R-467/CaCl(2)-mediated ERK1/2 activation but did not block the 2-min time point of 4 mm Ca(2+)-mediated ERK1/2 activation. Neither dominant-negative dynamin (K44A) nor dominant-negative beta-arrestin inhibited ERK1/2 activation by either CaR agonist treatment, suggesting that CaR-elicited ERK1/2 signaling occurs via a dynamin-independent pathway. Pertussis toxin pretreatment partially attenuated the 4 mm Ca(2+)-ERK1/2 activation; this attenuated activity was completely restored by co-expression of the Galpha(i2) (C351I) but not Galpha(i1) (C351I) or Galpha(i3) (C351I) G proteins, PTX-insensitive G protein mutants. Taken together, these data suggest that both 4 mm Ca(2+) and NPS R-467/CaCl(2) activate ERK1/2 via distinguishable pathways in HEK-hCaR cells and may represent a nexus to differentially regulate differentiation versus proliferation via CaR activation.

Extracellular calcium is a potent inducer of cyclo-oxygenase-2 in murine osteoblasts through an ERK signaling pathway

[Ca2+]e may be important in bone turnover. We found [Ca2+]e induces COX-2 transcription and PGE2 production in primary calvarial osteoblasts through an ERK signaling pathway. Inhibition of PGE2 production inhibited the [Ca2+]e stimulation of osteoblastic differentiation but not the increase in cell number. Hence, some effects of [Ca2+]e on bone may be mediated by COX-2.

INTRODUCTION

Local changes in extracellular calcium ([Ca2+]e) may play an important role in bone turnover. We examined the possibility that prostaglandins produced by cyclo-oxygenase-2 (COX-2) could mediate some of the effects of [Ca2+]e on osteoblasts.

METHODS

We examined the [Ca2+]e induction of COX-2 expression and prostaglandin E2 (PGE2) production in primary osteoblasts (POBs) obtained by sequential enzymatic digestion of mouse calvariae. We measured mRNA and protein levels by Northern and Western analyses and PGE2 production in culture medium by radioimmunoassay (RIA). COX-2 promoter activity was measured as luciferase activity in calvarial osteoblasts derived from mice transgenic for 371 bp of the COX-2 promoter fused to a luciferase reporter gene.

RESULTS AND CONCLUSIONS

COX-2 mRNA and protein expression were induced by 3-40 mM of [Ca2+]e. [Ca2+]e (5 mM) induced COX-2 mRNA within 30 minutes; levels peaked at 6-9 h and remained elevated at 24 h. Cumulative medium PGE2 was increased at 3 h, with levels rising to 30 nM at 24 h. PGE2 production in POBs from mice with only COX-1 gene expression was 1/40th of that in POBs from mice with both COX-1 and COX-2 gene expression. [Ca2+]e increased alkaline phosphatase activity and osteocalcin mRNA, and this increase was blocked by inhibiting PGE2 production. [Ca2+]e stimulation of COX-2 promoter activity correlated with the induction of COX-2 mRNA expression. [Ca2+]e induced rapid and transient phosphorylation of extracellular signal-regulated kinase (ERK) in POBs, which peaked at 5-10 minutes. Inhibition of ERK phosphorylation with the specific inhibitors, PD-98059 and U-0126, decreased the [Ca2+]e induction of both COX-2 mRNA and luciferase activity by 70-80%. Although less effective than [Ca2+]e, strontium [Sr2+]e also induced COX-2 mRNA and promoter activity in POBs through an ERK signaling pathway. We conclude that [Ca2+]e is a potent transcriptional inducer of COX-2 expression and PGE2 production in osteoblasts through an ERK signaling pathway.

Requirement of calcium and phosphate ions in expression of sodium-dependent vitamin C transporter 2 and osteopontin in MC3T3-E1 osteoblastic cells

Osteoclasts dissolve mineralized bone matrix at bone resorption sites and release large amounts of calcium (Ca(2+)) and phosphate (PO(4)(3-)) ions into the extracellular fluid. However, the exact nature of Ca(2+) and PO(4)(3-) on osteoblasts remains unclear. We proposed that Ca(2+) and PO(4)(3-) ions are required for the expression of sodium-dependent vitamin C transporter (SVCT) 2 and a differentiation marker, osteopontin (OPN), in osteoblasts as a response to the osteoclastic degradation. Results from Northern blotting indicated that a deficiency of Ca(2+) or PO(4)(3-) inhibited both SVCT2 and OPN expression in a time-dependent manner, whereas elevated Ca(2+) (1 to 4 mM) or PO(4)(3-) (1 to 4 mM) dose-dependently induced SVCT2, OPN expression and OPN promoter activity. In addition, the L-type calcium channel blocker, nifedipine (5 to 20 micro M) and the phosphate transporter inhibitor, foscarnet (0.15 to 0.6 mM), dose-dependently abolished Ca(2+)- and PO(4)(3-)-induced SVCT2, OPN expression and OPN promoter activity. Furthermore, the results from L-ascorbic acid uptake assay and Western blotting indicated that the stimulatory effect of Ca(2+) and PO(4)(3-) on functional SVCT2 protein expression. These findings suggested that Ca(2+) and PO(4)(3-) regulate osteoblastic phenotype by entering into cells to stimulate SVCT2 and OPN expression.

p44/42 MAPK activation is necessary for receptor activator of nuclear factor-kappaB ligand induction by high extracellular calcium

Although extracellular calcium (Ca(2+)(o)) has been suggested to modulate bone remodeling, the exact mechanism is unclear. This study was performed to explore the signaling pathways of high Ca(2+)(o) that are responsible for controlling the expression of receptor activator of NF-kappaB ligand (RANKL) in mouse osteoblastic cells. As previously reported, high Ca(2+)(o) increased RANKL expression. However, the G protein-coupled Ca(2+)(o)-sensing receptor (CaSR) was not detected in the primary cultured mouse osteoblastic cell. The inhibition of the pertussis-sensitive G protein, phospholipase C, protein kinase C, intracellular calcium mobilization, p38 MAPK, or phosphoinositide 3-kinase did not block RANKL induction caused by high Ca(2+)(o). In contrast, the inhibition of p44/42 MAPK pathway reduced the RANKL expression induced by high Ca(2+)(o). Moreover, high Ca(2+)(o) activated p44/42 MAPK and MEK1/2. These results suggest that RANKL induction by high Ca(2+)(o) might not be mediated by CaSR and its putative downstream signaling pathways, but the pathway employing p44/42 MAPK is involved in the high Ca(2+)(o)-induced RANKL expression in mouse osteoblastic cells.

Role of the different mitogen-activated protein kinase subfamilies in the stimulation of dog and human thyroid epithelial cell proliferation by cyclic adenosine 5'-monophosphate and growth factors

We have investigated the role of the different classes of MAPKs, i.e. ERKs, c-Jun N-terminal kinases (JNKs), and p38 MAPK in the proliferation of dog and human thyroid epithelial cells (thyrocytes) in primary cultures. In these cells, TSH, acting through cAMP, epidermal growth factor, hepatocyte growth factor (HGF), and phorbol 12-myristate 13-acetate induce DNA synthesis. With the exception of HGF, all of these factors require the presence of insulin for mitogenic effects to be expressed. We found that TSH and forskolin are without effect on the phosphorylation and activity of the different classes of MAPKs. In contrast, all the cAMP-independent growth factors, whereas without effect on the phosphorylation and activity of JNKs and p38 MAPK, stimulated the ERKs. This effect was strong and sustained in response to HGF, epidermal growth factor and 12-myristate 13-acetate but weak and transient in response to insulin. Moreover, whereas in stimulated cells DNA synthesis was inhibited by PD 098059, an inhibitor of MAPK kinase 1 and consequently of ERKs, it was not modified by SB 203580, an inhibitor of p38 MAPK. Taken together, these data 1) exclude a role of JNKs and p38 MAPK in the proliferation of dog and human thyrocytes; 2) suggest that the mitogenic action of the cAMP-independent agents requires a strong and sustained activation of both ERKs and phosphatidylinositol 3-kinase/protein kinase B as realized by HGF alone or by the other agents together with insulin; and 3) show that TSH and cAMP do not activate ERKs but that the weak activation of ERKs by insulin is nevertheless necessary for DNA synthesis to occur.

Prolonged nuclear retention of activated extracellular signal-regulated kinase 1/2 is required for hepatocyte growth factor-induced cell motility

We examined the signaling pathway by which hepatocyte growth factor (HGF) induces cell motility, with special focus on the role of extracellular signal-regulated kinase (ERK) in the nucleus. We used Madin-Darby canine kidney cells overexpressing ERK2 because of their prominent motility response to HGF. HGF stimulation of the cells induces not only a rapid, marked, and sustained activation and rapid nuclear accumulation of ERK1/2, but also a prolonged nuclear retention of the activated ERK1/2. Interruption of the ERK1/2 activation by PD98059 treatment of the cells 30 min after HGF stimulation abolishes the HGF-induced cell motility. Enforced cytoplasmic retention of the activated ERK1/2 by the expression of an inactive form of MKP-3 cytoplasmic phosphatase inhibits the cell motility response. Although epidermal growth factor stimulation of the cells induces the activation and nuclear accumulation of ERK1/2, it does not induce the prolonged nuclear retention of the activated ERK1/2, and fails to induce cell motility. In the nucleus, activated ERK1/2 continuously phosphorylate Elk-1, leading to the prolonged expression of c-fos, which results in the expression of several genes such as matrix metalloproteinase (mmp)-9; MMP-9 activity is required for the induction of the cell motility response. Our results indicate that the sustained activity of ERK1/2 in the nucleus is required for the induction of HGF-induced cell motility.

Mechanical strain and fluid movement both activate extracellular regulated kinase (ERK) in osteoblast-like cells but via different signaling pathways

Extracellular regulated kinases (ERKs)-1 and -2 are members of the MAPK family of protein kinases involved in the proliferation, differentiation, and apoptosis of bone cells. We have shown previously that ROS 17/2.8 cells show increased activation of ERK-1 or -2, which is sustained for 24 h, when the strips onto which they are seeded are subjected to a 10 min period of cyclic four point bending that produces physiological levels of mechanical strain along with associated fluid movement of the medium. Movement of the strips through the medium without bending causes fluid movement without strain. This also increases ERK-1/2 activation, but in a biphasic manner over the same time period. Our present study investigates the role of components of signaling pathways in the activation of ERK-1/2 in ROS 17/2.8 cells in response to these stimuli. Using a range of inhibitors we show specific differences by which ERK-1 and ERK-2 are activated in response to fluid movement alone, compared with those induced in response to strain plus its associated fluid movement. ERK-1 activation induced by fluid movement was markedly reduced by nifedipine, and therefore appears to involve L-type calcium channels, but was unaffected by either L-NAME or indomethacin. This suggests independence from prostacyclin (PGI(2)) and nitric oxide (NO) production. In contrast, ERK-1 activation induced by application of strain (and its associated fluid disturbance) was abrogated by TMB-8 hydrochloride, L-NAME, and indomethacin. This suggests that strain-induced ERK-1 activation is dependent upon calcium mobilization from intracellular stores and production of NO and PGI(2). ERK-2 activation appears to be mediated by a separate mechanism in these cells. Its activation by fluid movement alone involved both PGI(2) and NO production, but its activation by strain was not affected by any of the inhibitors used. The G protein inhibitor, pertussis toxin, did not cause a reduction in the activation of ERK-1 or -2 in response to either stimulus. These results are consistent with earlier observations of ERK activation in bone cells in response to both strain (with fluid movement) and fluid movement alone, and further demonstrate that these phenomena stimulate distinct signaling pathways.

The bone-specific transcriptional regulator Cbfa1 is a target of mechanical signals in osteoblastic cells

A primary goal of bone research is to understand the mechanism(s) by which mechanical forces dictate the cellular and metabolic activities of osteoblasts, the bone-forming cells. Several studies indicate that osteblastic cells respond to physical loading by transducing signals that alter gene expression patterns. Accumulated data have documented the fundamental role of the osteoblast-specific transcription factor Cbfa1 (core-binding factor) in osteoblast differentiation and function. Here, we demonstrate that low level mechanical deformation (stretching) of human osteoblastic cells directly up-regulates the expression and DNA binding activity of Cbfa1. This effect seems to be fine tuned by stretch-triggered induction of distinct mitogen-activated protein kinase cascades. Our novel finding that activated extracellular signal-regulated kinase mitogen-activated protein kinase physically interacts and phosphorylates endogenous Cbfa1 in vivo (ultimately potentiating this transcription factor) provides a molecular link between mechanostressing and stimulation of osteoblast differentiation. Elucidation of the specific modifiers and cofactors that operate in this mechanotranscription circuitry will contribute to a better understanding of mechanical load-induced bone formation which may set the basis for nonpharmacological intervention in bone loss pathologies.

Aminoglycosides increase intracellular calcium levels and ERK activity in proximal tubular OK cells expressing the extracellular calcium-sensing receptor

Aminoglycoside antibiotics (AGAs) are nephrotoxic, with most of the damage confined to the proximal tubule, but the mechanism for cellular toxicity is not clear. It has been previously shown that the extracellular-calcium sensing receptor (CaR) is expressed in intact rat proximal tubule and can be stimulated by the AGA neomycin. To investigate whether CaR could contribute to AGA-induced nephrotoxicity, the acute responses to various AGAs in the proximal tubule-derived opossum kidney (OK) cell line were examined. The presence in OK cells of CaR-related transcripts and protein was demonstrated by northern analyses, reverse transcriptase-PCR, immunocytochemistry, and immunoblotting. OK cells responded to elevated extracellular calcium (Ca(2+)(o)) and neomycin but also to gentamicin and tobramycin with an increase in cytosolic [Ca(2+)]. Ca(2+)(o), neomycin, and gentamicin also activated the extracellular signal-regulated kinases, ERK1 and ERK2. Neomycin-induced ERK activation was both dose- and time-dependent and was attenuated by inhibitors of phosphatidylinositol 3-kinase, phosphatidylinositol bisphosphate (PIP(2))-specific phospholipase C, and MEK1, but not of protein kinase C. Thus, proximal tubular OK cells express a CaR that mediates Ca(2+)(i) mobilization and PIP(2)-PLC-dependent ERK activation in response to AGAs and thus could play a role in AGA-induced nephrotoxicity.