Constitutive expression of thrombospondin 1 in MC3T3-E1 osteoblastic cells inhibits mineralization

Thrombospondin 1 (TSP1) is a multifunctional extracellular glycoprotein present mainly in the fetal and adult skeleton. Although an inhibitory effect of TSP1 against pathological mineralization in cultured vascular pericytes has been shown, its involvement in physiological mineralization by osteoblasts is still unknown. To determine the role of TSP1 in biomineralization, mouse osteoblastic MC3T3-E1 cells were cultured in the presence of antisense phosphorothioate oligodeoxynucleotides complementary to the TSP1 sequence. The 18- and 24-mer antisense oligonucleotides caused concentration-dependent increases in the number of mineralized nodules, acid-soluble calcium deposition in the cell/matrix layer, and alkaline phosphatase activity within 9 days, without affecting cell proliferation. The corresponding sense or scrambled oligonucleotides did not affect these parameters. In the antisense oligonucleotide-treated MC3T3-E1 cells, thickened extracellular matrix, well-developed cell processes, increased intracellular organelles, and collagen fibril bundles were observed. On the other hand, the addition of TSP1 to the culture decreased the production of a mineralized matrix by MC3T3-E1 cells. Furthermore, MC3T3-E1 clones overexpressing mouse TSP1 were established and assayed for TSP1 protein and their capacity to mineralize. TSP1 dose-dependently inhibited mineralization by these cells both in vitro and in vivo. These results indicate that TSP1 functions as an inhibitory regulator of bone mineralization and matrix production by osteoblasts to sustain bone homeostasis.     

Involvement of calcium-sensing receptor in osteoblastic differentiation of mouse MC3T3-E1 cells

We have previously shown that the extracellular calcium-sensing receptor (CaR) is expressed in various bone marrow-derived cell lines and plays an important role in stimulating their proliferation and chemotaxis. It has also been reported that the CaR modulates matrix production and mineralization in chondrogenic cells. However, it remains unclear whether the CaR plays any role in regulating osteoblast differentiation. In this study, we found that mineralization of the mouse osteoblastic MC3T3-E1 cells was increased when the cells were exposed to high calcium (2.8 and 3.8 mM) or a specific CaR activator, NPS-R467 (1 and 3 microM). Next, we stably transfected MC3T3-E1 cells with either a CaR antisense vector (AS clone) or a vector containing the inactivating R185Q variant of the CaR (DN clone) that has previously been shown to exert a dominant negative action. Alkaline phosphatase activities were decreased compared with controls in both the AS and DN clones. However, the levels of type I procollagen and osteopontin mRNA in the AS clone, as detected by Northern blotting, were almost the same as in the controls. On the other hand, the expression of osteocalcin, which is expressed at a later stage of osteoblastic differentiation, was significantly reduced in both the AS and DN clones. Mineralization was also decreased in both clones. In conclusion, this study showed that the abolition of CaR function results in diminishing alkaline phosphatase activity, osteocalcin expression, and mineralization in mouse osteoblastic cells. This suggests that the CaR may be involved in osteoblastic differentiation.     

Androgen-induced mineralization by MC3T3-E1 osteoblastic cells reveals a critical window of hormone responsiveness

Despite their clinical importance for skeletal growth and homeostasis, the actions of androgens on osteoblastic cells are not well understood. MC3T3-E1 cells, a nontransformed murine preosteoblastic cell line, that traverse the stages of osteoblastic differentiation within 30 days in vitro, were exposed to mibolerone (an androgen receptor (AR) agonist) or 5alpha-dihydroxytestosterone (DHT) from days 3 to 30 post-plating. Cells exposed to this hormonal regimen exhibited a significant increase in mineralization (calcium deposition) compared to vehicle-treated cells. Delaying treatment for 4-11 days (treatment still completed on day 30 post-plating) enhanced mineralization further. Within 2 days post-plating, AR protein increased 7.2-fold in androgen-treated cells and 2.5-fold in vehicle-treated cells. MC3T3-E1 cells transfected with an androgen- and glucocorticoid-responsive reporter construct on day 1 post-plating followed by a 2 day exposure to DHT, mibolerone, or dexamethasone (dex; a glucocorticoid receptor agonist) exhibited reporter gene activation only with dex treatment. In contrast, delaying transfection and treatment for at least 1 day resulted in comparable androgen- and dex-mediated reporter gene transactivation. Therefore, the ability of MC3T3-E1 cells to respond to androgens is dependent on the timing of androgen administration.     

Knockdown of microtubule actin crosslinking factor 1 inhibits cell proliferation in MC3T3-E1 osteoblastic cells

Microtubule actin crosslinking factor 1 (MACF1), a widely expressed cytoskeletal linker, plays important roles in various cells by regulating cytoskeleton dynamics. However, its role in osteoblastic cells is not well understood. Based on our previous findings that the association of MACF1 with F-actin and microtubules in osteoblast-like cells was altered under magnetic force conditions, here, by adopting a stable MACF1-knockdown MC3T3-E1 osteoblastic cell line, we found that MACF1 knockdown induced large cells with a binuclear/multinuclear structure. Further, immunofluorescence staining showed disorganization of F-actin and microtubules in MACF1-knockdown cells. Cell counting revealed significant decrease of cell proliferation and cell cycle analysis showed an S phase cell cycle arrest in MACF1-knockdown cells. Moreover and interestingly, MACF1 knockdown showed a potential effect on cellular MTT reduction activity and mitochondrial content, suggesting an impact on cellular metabolic activity. These results together indicate an important role of MACF1 in regulating osteoblastic cell morphology and function. [BMB Reports 2015; 48(10): 583-588]     

Induction of prostaglandin I(2) receptor by tumor necrosis factor alpha in osteoblastic MC3T3-E1 cells.

Mouse osteoblastic cells MC3T3-E1 produced prostaglandin E(2) via the reaction of cyclooxygenase-2 enzyme induced by tumor necrosis factor alpha (TNFalpha). Originally, the mRNA level for prostaglandin I(2) receptor (IP) was low in the cells. However, the addition of TNFalpha brought about a marked increase in the IP mRNA with a lag of about 3 h up to an about 8-fold higher level for 24 h. In addition, the induction of IP was supported by a binding experiment of [(3)H]iloprost (a stable analogue of prostaglandin I(2)). The amount of iloprost bound to the TNFalpha-stimulated cell membranes increased to a saturation level around 30 nM. Dexamethasone, cycloheximide and cyclooxygenase inhibitor suppressed the IP mRNA induction. The finding with the latter two compounds suggested a TNFalpha-dependent de novo synthesis of a protein, which is involved in the IP mRNA induction and may be attributed partially to the induced cyclooxygenase-2.     

The p75 neurotrophin receptor regulates MC3T3-E1 osteoblastic differentiation

While the role of p75^NTR signaling in the regulation of nerve-related cell growth and survival has been well documented, its actions in osteoblasts are poorly understood. In this study, we examined the effects of p75^NTR on osteoblast proliferation and differentiation using the MC3T3-E1 pre-osteoblast cell line. Proliferation and osteogenic differentiation were significantly enhanced in p75^NTR-overexpressing MC3T3-E1 cells (p75GFP-E1). In addition, expression of osteoblast-specific osteocalcin (OCN), bone sialoprotein (BSP), and osterix mRNA, ALP activity, and mineralization capacity were dramatically enhanced in p75GFP-E1 cells, compared to wild MC3T3-E1 cells (GFP-E1). To determine the binding partner of p75^NTR in p75GFP-E1 cells during osteogenic differentiation, we examined the expression of trkA, trkB, and trkC that are known binding partners of p75^NTR, as well as NgR. Pharmacological inhibition of trk tyrosine kinase with the K252a inhibitor resulted in marked reduction in the level of ALPase under osteogenic conditions. The deletion of the GDI binding domain in the p75^NTR-GFP construct had no effect on mineralization. Taken together, our studies demonstrated that p75^NTR signaling through the trk tyrosine kinase pathway affects osteoblast functions by targeting osteoblast proliferation and differentiation.     

Chemotactic responses of osteoblastic MC3T3-E1 cells toward zinc chloride

Although zinc (Zn) is known to participate in bone formation, its exact role in the remodeling of this tissue has not been fully clarified. The present study was designed to investigate whether Zn has a role at the resorptive sites in vitro. We investigated the migration of osteoblastic MC3T3-E1 cells in response to Zn using a Boyden chamber assay. Exposure of MC3T3-E1 cells to Zn stimulated the migration of MC3T3-E1 cells. Checkerboard analysis revealed that the migration of MC3T3-E1 cells toward Zn was a directional (chemotaxis) rather than a random (chemokinesis) motion. Pretreatment of MC3T3-E1 cells with pertussis toxin completely blocked the chemotactic response of cells to Zn, indicating that it is mediated by G-protein-coupled receptors. Because the bone is one of the major Zn storage sites, we suggest that Zn released from bone-resorptive sites plays an important role in the recruitment of osteoblasts and bone renewal.     

Expression of lysyl oxidase isoforms in MC3T3-E1 osteoblastic cells

Covalent intermolecular cross-linking of collagen is initiated by the action of lysyl oxidase (LOX) on the telopeptidyl lysine and hydroxylysine residues. Recently, several LOX isoforms, i.e., LOX-like proteins 1-4 (LOXL1-4), have been identified but their specific tissue distribution and functions are still largely unknown. In this study, mRNA expression of LOX and LOXL1-4 in MC3T3-E1 osteoblastic cells was screened by RT-PCR and quantitatively analyzed by real-time PCR during cell differentiation and matrix mineralization. The results demonstrated that LOX and all LOXLs, except LOXL2, were expressed in this cell line and that the expression pattern during cell differentiation and matrix mineralization was distinct from one another. This indicates that the expression of LOX and its isoforms is highly regulated during osteoblast differentiation, suggesting their distinct roles in collagen matrix stabilization and subsequent mineralization.     

Soybean ethanol extract increases the function of osteoblastic MC3T3-E1 cells

To investigate the bioactivities of soybean, which act on bone metabolism, we studied the effect of a soybean ethanol extract on the activity of osteoblast MC3T3-E1 cells. Soy extract (0.01-0.1 g/l) dose-dependently increased survival (P<0.05) and DNA synthesis (P<0.05) of MC3T3-E1 cells. In addition, soy extract (0.05 g/l) increased alkaline phosphatase activity (P<0.05) and collagen synthesis (P<0.05) of MC3T3-E1 cells. Moreover, the anti-estrogen tamoxifen eliminated the stimulation of MC3T3-E1 cells on the proliferation, ALP activity and collagen synthesis by soy extract, indicating that the main action of the soy extract on osteoblastic MC3T3-E1 cells is similar to that of estrogen effects. Treatment with soy extract prevented apoptosis, as assessed by a one-step sandwich immunoassay and DNA gel electrophoresis studies. This effect may be associated with the activation of the estrogen receptor, since we observed soy extract-mediated survival against apoptosis was blocked by the estrogen receptor antagonist tamoxifen in cells, further supporting a receptor-mediated mechanism of cell survival. These results suggest that osteoblast function is promoted by soy extract and that the estrogen receptor is involved in the response, thereby playing an important role in bone remodeling. In conclusion, soy extract has a direct stimulatory effect on bone formation in cultured osteoblastic cell in vitro. Presumably, dietary soy products are useful in the prevention of osteoporosis.     

Ginsenoside Rh2(S) induces the differentiation and mineralization of osteoblastic MC3T3-E1 cells through activation of PKD and p38 MAPK pathways

As part of the search for biologically active anti-osteoporotic agents that enhance differentiation and mineralization of osteoblastic MC3T3-E1 cells, we identified the ginsenoside Rh2(S), which is an active component in ginseng. Rh2(S) stimulates osteoblastic differentiation and mineralization, as manifested by the up-regulation of differentiation markers (alkaline phosphatase and osteogenic genes) and Alizarin Red staining, respectively. Rh2(S) activates p38 mitogen-activated protein kinase (MAPK) in time- and concentration-dependent manners, and Rh2(S)-induced differentiation and mineralization of osteoblastic cells were totally inhibited in the presence of the p38 MAPK inhibitor, SB203580. In addition, pretreatment with Go6976, a protein kinase D (PKD) inhibitor, significantly reversed the Rh2(S)-induced p38 MAPK activation, indicating that PKD might be an upstream kinase for p38 MAPK in MC3T3-E1 cells. Taken together, these results suggest that Rh2(S) induces the differentiation and mineralization of MC3T3-E1 cells through activation of PKD/p38 MAPK signaling pathways, and these findings provide a molecular basis for the osteogenic effect of Rh2(S).     

Dexamethasone induces caspase activation in murine osteoblastic MC3T3-E1 cells

Glucocorticoids are widely used as anti-inflammatory and chemotherapeutic agents. However, prolonged use of glucocorticoids leads to osteoporosis. This study was designed to examine the mechanism of dexamethasone (DEX)-induced apoptosis in murine osteoblastic MC3T3-E1 cells. Total RNA was extracted from MC3T3-E1 cells treated with 10(-7) M DEX for 6 h. DEX exerted a variety of effects on apoptotic gene expression in osteoblasts. Ribonuclease protection assays (RPA) revealed that DEX upregulated mRNA levels of caspases-1, -3, -6, -8, -11, -12, and bcl-XL. Western blot analysis showed enhanced processing of these caspases, with the appearance of their activated enzymes 8 h after DEX treatment. In addition, DEX also induced the activation of caspase-9. DEX elevated the levels of cleaved poly(ADP-ribose) polymerase and lamin A, a caspase-3 and a caspase-6 substrate, respectively. Expression of bcl-XL protein level was upregulated by DEX. Cytochrome c release was detected in the cytosol of DEX-treated cells. Furthermore, caspase-3 enzyme activity was elevated by 2-fold after DEX treatment for 7 h. Finally, early apoptotic cells were detected in cells treated with DEX for 3 h. Our results demonstrate that DEX-induced apoptosis involves gene activation of a number of caspases.     

Effect of progesterone on apoptosis of murine MC3T3-E1 osteoblastic cells

Progesterone (P) has been suggested as a bone-trophic hormone. Previous studies have shown that P promoted bone formation by stimulating the proliferation and differentiation of osteoblasts. But, the effect of P on apoptosis of osteoblast in vitro has not been reported. We propose that P may promote bone formation by suppressing the apoptosis of osteoblast. The present study was performed to investigate the effect of P on apoptosis of murine MC3T3-E1 osteoblastic cells. Cell apoptosis was measured by acidine orange/ethidium bromide (AO/EB) staining and sandwich-enzyme-immunoassay. Progesterone receptor (PR), cytochrome c, caspase-9 and caspase-3 protein levels were determined by Western blot analysis. The enzyme substrate was also used to assess the activation of caspase-3 and caspase-9. Progesterone suppressed MC3T3-E1 cells apoptosis induced by serum deprivation, and this effect was blocked by a PR antagonist RU486. Furthermore, the suppressive effects of P on cytochrome c release and caspase-9 and caspase-3 activation in serum-deprived MC3T3-E1 cells were also reversed by RU486. Our study demonstrated that P protects osteoblast against apoptosis through PR and the downstream mitochondrial pathway. Thus, the data suggest that the effects of P on osteoblast apoptosis may contribute to the mechanisms by which P exerts its action on bone formation.     

Adiponectin and AMP kinase activator stimulate proliferation,differentiation, and mineralization of osteoblastic MC3T3-E1 cells

Background  

Adiponectin is a key mediator of the metabolic syndrome that is caused by visceral fat accumulation. Adiponectin and its receptors are known to be expressed in osteoblasts, but their actions with regard to bone metabolism are still unclear. In this study, we investigated the effects of adiponectin on the proliferation, differentiation, and mineralization of osteoblastic MC3T3-E1 cells.