A short pulse of mechanical force induces gene expression and growth in MC3T3-E1 osteoblasts via an ERK 1/2 pathway

Type II cGMP-dependent protein kinase mediates osteoblast mechanotransduction

Continuous bone remodeling in response to mechanical loading is critical for skeletal integrity, and interstitial fluid flow is an important stimulus for osteoblast/osteocyte growth and differentiation. However, the biochemical signals mediating osteoblast anabolic responses to mechanical stimulation are incompletely understood. In primary human osteoblasts and murine MC3T3-E1 cells, we found that fluid shear stress induced rapid expression of c-fos, fra-1, fra-2, and fosB/DeltafosB mRNAs; these genes encode transcriptional regulators that maintain skeletal integrity. Fluid shear stress increased osteoblast nitric oxide (NO) synthesis, leading to activation of cGMP-dependent protein kinase (PKG). Pharmacological inhibition of the NO/cGMP/PKG signaling pathway blocked shear-induced expression of all four fos family genes. Induction of these genes required signaling through MEK/Erk, and Erk activation was NO/cGMP/PKG-dependent. Treating cells with a membrane-permeable cGMP analog partly mimicked the effects of fluid shear stress on Erk activity and fos family gene expression. In cells transfected with small interfering RNAs (siRNA) specific for membrane-bound PKG II, shear- and cGMP-induced Erk activation and fos family gene expression was nearly abolished and could be restored by transducing cells with a virus encoding an siRNA-resistant form of PKG II; in contrast, siRNA-mediated repression of the more abundant cytosolic PKG I isoform was without effect. Thus, we report a novel function for PKG II in osteoblast mechanotransduction, and we propose a model whereby NO/cGMP/PKG II-mediated Erk activation and induction of c-fos, fra-1, fra-2, and fosB/DeltafosB play a key role in the osteoblast anabolic response to mechanical stimulation.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.

Periprosthetic strain magnitude-dependent upregulation of type I collagen synthesis in human osteoblasts through an ERK1/2 pathway

Human osteoblasts sense mechanical stimulation and synthesise type I collagen in periprosthetic osseointegration following total hip arthroplasty. However, the regulation of type I collagen synthesis by periprosthetic strain is unclear because the cellular-level strain magnitude remains unknown to date. Fortunately, the tissue-level strain in implanted femurs is measurable. According to the mechanism of strain amplification, the tissue-level strain was amplified 20 times to stretch human osteoblasts in this study. Elongation of 0.8-3.2% enhanced the mRNA level of type I collagen, whereas the release of procollagen type I C propeptide only increased at 2.4% and 3.2% elongation. Type I collagen expression increased with the activation of ERK1/2 phosphorylation in a strain-magnitude-dependent manner, whereas JNK and P38 were unaffected. The responses were completely inhibited by blocking the ERK1/2 pathway with U0126. The results indicate that type I collagen synthesis in human osteoblasts depends on the level of periprosthetic strain and ERK1/2 activation.

Different magnitudes of tensile strain induce human osteoblasts differentiation associated with the activation of ERK1/2 phosphorylation

Mechanical factors are related to periprosthetic osseointegration following total hip arthroplasty. However, osteoblast response to strain in implanted femurs is unclear because of the absence of accurate stress-measuring methods. In our study, finite element analysis was performed to calculate strain distribution in implanted femurs. 0.8-3.2% tensile strain was then applied to human osteoblasts. Higher magnitudes of strain enhanced the expression of osteocalcin, type I collagen, and Cbfa1/Runx2. Lower magnitudes significantly increased ALP activity. Among these, type I collagen expression increased with the activation of ERK1/2 phosphorylation in a strain-magnitude-dependent manner. Our study marks the first investigation of osteoblast response at different magnitudes of periprosthetic strain. The results indicate that the functional status of human osteoblasts is determined by strain magnitude. The strain distribution in the proximal region of implanted femur should be improved for osseointegration.

Egr-1 mRNA induction by medium flow involves mRNA stabilization and is enhanced by the p38 inhibitor SB203580 in osteoblast-like cells

AIM

Mechanical stimuli are important for maintaining organ structure and tissue function. To elucidate signalling pathways activated by mechanical stimuli, the contribution of mRNA stabilization to induction of egr-1 mRNA by medium flow was examined and the mechanisms responsible for stabilization were analysed. An early-response gene that encodes a transcription factor, egr-1, activates transcription of several genes in response to mechanical stimuli, and was therefore selected to resolve how early-induced signals are integrated and connected to subsequent response.

METHODS

Mouse osteoblast-like MC3T3E1 cells were stably transfected with the chloramphenicol acetyltransferase (CAT) gene linked to the egr-1 promoter, and inductions of endogenous egr-1 and transfected CAT mRNA following medium flow were compared using real-time reverse transcriptase PCR. The mechanism of induction was examined using a transcription inhibitor and mitogen-activated protein (MAP) kinase inhibitors. Activation of MAP kinases by medium flow was investigated using western blotting.

RESULTS

Induction of egr-1 mRNA by medium flow was twofold higher than CAT mRNA induction. Induction of egr-1 mRNA was also observed in cells pre-treated with transcription inhibitor. The p38 inhibitor SB203580 enhanced induction of egr-1 mRNA by medium flow. Extracellular signal regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) were activated by medium flow.

CONCLUSION

A considerable part of egr-1 mRNA induction by medium flow may be due to mRNA stabilization. The p38 inhibitor SB203580 enhances induction.

CYP24 promoter activity is affected by mechanical stress and mitogen-activated protein kinase in MG63 osteoblast-like cells

PURPOSE

For bone homeostasis, vitamin D plays an important role in the regulation of calcium. The enzyme CYP24 inactivates vitamin D and is involved in its regulation. However, the mechanism of expression of CYP24 in osteoblastic cells under mechanical stress is not clear. In this study we investigated CYP24 promoter activity in stretched osteoblastic cells and the participation of mitogen-activated protein kinase (MAPK) in expression of CYP24.

METHODS

MG63 osteoblastic cells were cultured on silicon-bottomed plates. Cells were transfected with a reporter gene that contained a CYP24 promoter. After activated vitamin D, 1,25(OH)2D3, was added or not added, cells were stretched. Stretched and non-stretched cells were investigated by luciferase dual assay. Cells were also investigated similarly using medium with an ERK1/2 inhibitor or p38 inhibitor.

RESULTS

The CYP24 promoter was activated by 1,25(OH)2D3 and the promoter activity decreased in stretched cells. Inhibitor of MAPK decreased CYP24 promoter activity. However, CYP24 promoter activity decreased with mechanical stress after addition of p38 inhibitor, while it did not decrease with mechanical stress after addition of ERK1/2 inhibitor. The CYP24 promoter was not activated without 1,25(OH)2D3 in any case.

CONCLUSION

Mechanical stress and MAPK control CYP24 promoter activity in the presence of Vitamin D in MG63 osteoblast-like cells.

NMDA enhances stretching-induced differentiation of osteoblasts through the ERK1/2 signaling pathway

Activation of the excitatory neurotransmitter N-methyl-d-aspartate (NMDA) and stretching both increase Ca(2+) influx in osteoblastic cells. We postulated that NMDA would enhance the osteoblastic cell's response to stretching. The goal of this study was to investigate, in the presence of the neurotransmitter NMDA, the effect of mechanical loading on osteoblast's stage of differentiation and the mitogen-activated protein kinase (MAPK) signaling pathway associated with it. Rat primary osteoblastic cells were subjected to cyclic, equibiaxial stretch for 48 h in the presence or absence of NMDA. Pretreatment with 0.5 mM NMDA significantly enhanced the stretching magnitude-dependent increase in osteogenesis markers. MK801, an antagonist of NMDA receptors, abolished those responses. To further study the mechanism of this response, osteoblastic cells were stretched for 5, 15, or 60 min in the absence of NMDA. Cyclic stretch induced a rapid increase in extracellular signal-regulated kinase ERK1/2 phosphorylation with the peak at 15 min, but no changes were noted in p38 and JNK pathway signaling. NMDA could enhance ERK1/2 phosphorylation stimulated by stretching. U0126, an inhibitor of ERK1/2, blocked the increase in osteogenesis markers. In conclusion, the current study demonstrates that there is a synergistic effect between mechanical stimulation and NMDA in osteoblasts. ERK1/2 signaling may be the common pathway in the increased response to stretching in the presence of NMDA in osteoblastic cells.

The additive effects of minoxidil and retinol on human hair growth in vitro

Minoxidil enhances hair growth by prolonging the anagen phase and induces new hair growth in androgenetic alopecia (AGA), whereas retinol significantly improves scalp skin condition and promotes hair growth. We investigated the combined effects of minoxidil and retinol on human hair growth in vitro and on cultured human dermal papilla cells (DPCs) and epidermal keratinocytes (HaCaT). The combination of minoxidil and retinol additively promoted hair growth in hair follicle organ cultures. In addition, minoxidil plus retinol more effectively elevated phosphorylated Erk, phosphorylated Akt levels, and the Bcl-2/Bax ratio than minoxidil alone in DPCs and HaCaT. We found that the significant hair shaft elongation demonstrated after minoxidil plus retinol treatment would depend on the dual kinetics associated with the activations of Erk- and Akt-dependent pathways and the prevention of apoptosis by increasing the Bcl-2/Bax ratio.

Src and focal adhesion kinase mediate mechanical strain-induced proliferation and ERK1/2 phosphorylation in human H441 pulmonary epithelial cells

Pulmonary epithelial cells are exposed to repetitive deformation during physiological breathing and mechanical ventilation. Such deformation may influence pulmonary growth, development, and barotrauma. Although deformation stimulates proliferation and activates extracellular signal-regulated kinases (ERK1/2) in human pulmonary epithelial H441 cells, the upstream mechanosensors that induce ERK activation are poorly understood. We investigated whether c-Src or focal adhesion kinase (FAK) mediates cyclic mechanical strain-induced ERK1/2 activation and proliferation in human pulmonary epithelial (NCI-H441) cells. The H441 and A549 cells were grown on collagen I-precoated membranes and were subjected to an average 10% cyclic mechanical strain at 20 cycles/min. Cyclic strain activated Src within 2 min by increasing phosphorylation at Tyr(418), followed by rapid phosphorylation of FAK at Tyr(397) and Tyr(576) and ERK1/2 at Thr(202)/Tyr(204) (n = 5, P < 0.05). Twenty-four (A549 cells) and 24-72 h (H441 cells) of cyclic mechanical strain increased cell numbers compared with static culture. Twenty-four hours of cyclic strain also increased H441 FAK, Src, and ERK phosphorylation without affecting total FAK, Src, or ERK protein. The mitogenic effect was blocked by Src (10 micromol/l PP2 or short interfering RNA targeted to Src) or MEK (50 micromol/l PD-98059) inhibition. PP2 also blocked strain-induced phosphorylation of FAK-Tyr(576) and ERK-Thr(202)/Tyr(204) but not FAK-Tyr(397). Reducing FAK by FAK-targeted short interfering RNA blocked mechanical strain-induced mitogenicity and significantly attenuated strain-induced ERK activation but not strain-induced Src phosphorylation. Together, these results suggest that repetitive mechanical deformation induced by ventilation supports pulmonary epithelial proliferation by a pathway involving Src, FAK, and then ERK signaling.

Reduction of anabolic signals and alteration of osteoblast nuclear morphology in microgravity

Bone loss has been repeatedly documented in astronauts after flight, yet little is known about the mechanism of bone loss in space flight. Osteoblasts were activated during space flight in microgravity (microg) with and without a 1 gravity (1 g) field and 24 genes were analyzed for early induction. Induction of proliferating cell nuclear antigen (PCNA), transforming growth factor beta (TGFbeta), cyclo-oxygenase-2 (cox-2), cpla2, osteocalcin (OC), c-myc, fibroblast growth factor-2 (fgf-2), bcl2, bax, and fgf-2 message as well as FGF-2 protein were significantly depressed in microg when compared to ground (gr). Artificial onboard gravity normalized the induction of c-myc, cox-2, TGFbeta, bax, bcl2, and fgf-2 message as well as FGF-2 protein synthesis in spaceflight samples. In normal gravity, FGF-2 induces bcl2 expression; we found that bcl2 expression was significantly reduced in microgravity conditions. Since nuclear shape is known to elongate in the absence of mitogens like FGF-2, we used high-resolution image-based morphometry to characterize changes in osteoblast nuclear architecture under microgravity, 1 g flight, and ground conditions. Besides changes in cell shape (roundish/elliptic), other high-resolution analyses show clear influences of gravity on the inner nuclear structure. These changes occur in the texture, arrangement, and contrast of nuclear particles and mathematical modeling defines the single cell classification of the osteoblasts. Changes in nuclear structure were evident as early as 24 h after exposure to microgravity. This documented alteration in nuclear architecture may be a direct result of decreased expression of autocrine and cell cycle genes, suggesting an inhibition of anabolic response in microg. Life on this planet has evolved in a normal gravity field and these data suggest that gravity plays a significant role in regulation of osteoblast transcription.

Fibroblast growth factor-2 is an immediate-early gene induced by mechanical stress in osteogenic cells

Fifteen minutes of physiological MS induces FGF-2 in osteogenic cells. Here, we show that MS induced proliferation in both MC3T3-E1 and BMOp cells isolated from Fgf2(+/+) mice; Fgf2(-/-) BMOp cells required exogenous FGF-2 for a normal proliferation response. The induction of fgf-2 is mediated by PKA and ERK pathways.

INTRODUCTION

Mechanical stress (MS) induces gene expression and proliferation of osteogenic MC3T3-E1 cells. We have previously shown that physiological levels of MS in MC3T3-E1 cells causes extracellular signal-regulated kinase (ERK)1/2 phosphorylation. Here we evaluate the induction and importance of fibroblast growth factor-2 (FGF-2) for MS-induced proliferation.

MATERIALS AND METHODS

We characterized the MS induction of fgf-2 using a 15-minute pulse of 120 mustrain and studied the stability of fgf-2 message using actinomycin D. Bone marrow stromal cells (BMOp) isolated from Fgf2(-/-) and Fgf2(+/+) mice were used to study the importance of FGF-2 in MS-induced proliferation.

RESULTS

We found that the induction of fgf-2 by MS is dependent on both protein kinase A (PKA) and ERK pathways. MS transiently induces fgf-2 within 30 minutes. FGF-2 receptor (FGFR2) was also significantly increased within 1 h. All three isoforms of FGF-2 (24, 22, and 18 kDa) were significantly increased by MS. The MS-mediated increase of fgf-2 mRNA was caused by new synthesis and not stabilization. Pretreatment of MC3T3-E1 cells with cycloheximide showed that the induction of fgf-2 did not require new protein synthesis. Pretreating MC3T3-E1 cells with the mitogen-activated protein kinase (MAPK)/ERK kinase 1/2 (MEK1/2) inhibitor, U0126, or H-89, a PKA inhibitor, significantly inhibited the induction of fgf-2, showing that mechanical induction of fgf-2 is dependent on ERK and PKA signaling pathways. The downstream consequence of a single 15-minute stress pulse was a 3.5-fold increase in cell number in MC3T3-E1 compared with growth in nonstressed control cells. In studies using bone marrow osteoprogenitor cells (BMOp) isolated from Fgf2(+/+)and Fgf2(-/-) mice, we found that FGF-2 was necessary for a full proliferative response to MS.

CONCLUSIONS

These studies show that FGF-2 is an immediate-early gene induced by MS, and its expression is mediated by both the PKA and MAPK signal transduction pathways. FGF-2 was required for a full proliferative response.

Runx2 is a target of mechanical unloading to alter osteoblastic activity and bone formation in vivo

Molecular mechanisms underlying unloading-induced reduction of bone formation have not yet been fully understood. In vitro, Runx2 has been suggested to be involved in mechanical signaling in osteoblasts. However, the roles of Runx2 in vivo during the bone response to mechanical stimuli have not yet been known. The purpose of this paper was to examine the roles of Runx2 in unloading-induced bone loss in vivo. Tail suspension was conducted for 2 wk using 9- to 11-wk-old Runx2 heterozygous knockout mice (Runx2(+/-)) and wild-type (Wt) littermates. Bones were subjected to two-dimensional micro-x-ray computed tomography, bone histomorphometry and RT-PCR analyses. Loss of half Runx2 gene dosage-exacerbated unloading-induced bone loss in trabecular and cortical envelopes. Unloading-induced reduction in mineral apposition rate and bone formation rate in cortical bone as well as trabecular bone was exacerbated in Runx2(+/-) mice, compared with Wt mice. Bone resorption parameters were not significantly affected by unloading or Runx2(+/-) genotype. Basal Runx2 and osterix mRNA levels in bone were reduced by 50% in Wt, whereas unloading in Runx2(+/-) mice did not further alter Runx2 and osterix mRNA levels. In contrast, osteocalcin mRNA levels were reduced by unloading, regardless of Runx2 gene dosage. These data demonstrated that full Runx2 gene dosage is required for maintaining normal function of osteoblasts in mechanical unloading or nonphysiological condition. Finally, we propose Runx2 as a critical target gene in unloading to alter osteoblastic activity and bone formation in vivo.

Effects of different magnitudes of mechanical strain on Osteoblasts in vitro

In addition to systemic and local factors, mechanical strain plays a crucial role in bone remodeling during growth, development, and fracture healing, and especially in orthodontic tooth movement. Although many papers have been published on the effects of mechanical stress on osteoblasts or osteoblastic cells, little is known about the effects of different magnitudes of mechanical strain on such cells. In the present study, we investigated how different magnitudes of cyclic tensile strain affected osteoblasts. MC3T3-E1 osteoblastic cells were subjected to 0%, 6%, 12% or 18% elongation for 24h using a Flexercell Strain Unit, and then the mRNA and protein expressions of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL) were examined. The results showed that cyclic tensile strain induced a magnitude-dependent increase (0%, 6%, 12%, and 18%) in OPG synthesis and a concomitant decrease in RANKL mRNA expression and sRANKL release from the osteoblasts. Furthermore, the induction of OPG mRNA expression by stretching was inhibited by indomethacin or genistein, and the stretch-induced reduction of RANKL mRNA was inhibited by PD098059. These results indicate that different magnitudes of cyclic tensile strain influence the biological behavior of osteoblasts, which profoundly affects bone remodeling.

Developmental regulation of skull morphology II: ontogenetic dynamics of covariance

Canalization may play a critical role in molding patterns of integration when variability is regulated by the balance between processes that generate and remove variation. Under these conditions, the interaction among those processes may produce a dynamic structure of integration even when the level of variability is constant. To determine whether the constancy of variance in skull shape throughout most of postnatal growth results from a balance between processes generating and removing variation, we compare covariance structures from age to age in two rodent species, cotton rats (Sigmodon fulviventer) and house mice (Mus musculus domesticus). We assess the overall similarity of covariance matrices by the matrix correlation, and compare the structures of covariance matrices using common subspace analysis, a method related to common principal components (PCs) analysis but suited to cases in which variation is so nearly spherical that PCs are ambiguous. We find significant differences from age to age in covariance structure and the more effectively canalized ones tend to be least stable in covariance structure. We find no evidence that canalization gradually and preferentially removes deviations arising early in development as we might expect if canalization results from compensatory differential growth. Our results suggest that (co)variation patterns are continually restructured by processes that equilibrate variance, and thus that canalization plays a critical role in molding patterns of integration.

Signal transduction pathways involved in mechanical regulation of HB-GAM expression in osteoblastic cells

Protein kinase C (PKC), protein kinase A (PKA), prostaglandin synthesis, and various mitogen-activated protein kinases (MAPKs) have been reported to be activated in bone cells by mechanical loading. We studied the involvement of these signal transduction pathways in the downregulation of HB-GAM expression in osteoblastic cells after cyclic stretching. Specific antagonists and agonists of these signal transduction pathways were added to cells before loading and to non-loaded control cells. Quantitative RT-PCR was used to evaluate gene expression. The data demonstrated that the extracellular signal-regulated kinase (ERK) 1/2 pathway, PKC, PKA, p38, and c-Jun N-terminal kinase MAPK participated in the mechanical downregulation of HB-GAM expression, whereas prostaglandin synthesis did not seem to be involved.

Arachidonic Acid Activates Phosphatidylinositol 3-Kinase Signaling and Induces Gene Expression in Prostate Cancer

Essential fatty acids are not only energy-rich molecules; they are also an important component of the membrane bilayer and recently have been implicated in induction of fatty acid synthase and other genes. Using gene chip analysis, we have found that arachidonic acid, an omega-6 fatty acid, induced 11 genes that are regulated by nuclear factor-kappaB (NF-kappaB). We verified gene induction by omega-6 fatty acid, including COX-2, IkappaBalpha, NF-kappaB, GM-CSF, IL-1beta, CXCL-1, TNF-alpha, IL-6, LTA, IL-8, PPARgamma, and ICAM-1, using quantitative reverse transcription-PCR. Prostaglandin E(2) (PGE(2)) synthesis was increased within 5 minutes of addition of arachidonic acid. Analysis of upstream signal transduction showed that within 5 minutes of fatty acid addition, phosphatidylinositol 3-kinase (PI3K) was significantly activated followed by activation of Akt at 30 minutes. Extracellular signal-regulated kinase 1 and 2, p38 and stress-activated protein kinase/c-Jun-NH(2)-kinase were not phosphorylated after omega-6 fatty acid addition. Thirty minutes after fatty acid addition, we found a significant 3-fold increase in translocation of NF-kappaB transcription factor to the nucleus. Addition of a nonsteroidal anti-inflammatory drug (NSAID) caused a decrease in COX-2 protein synthesis, PGE(2) synthesis, as well as inhibition of PI3K activation. We have previously shown that NSAIDs cause an inhibition of arachidonic acid-induced proliferation; here, we have shown that arachidonic acid-induced proliferation is also blocked (P < 0.001) by PI3K inhibitor LY294002. LY294002 also significantly inhibited the arachidonic acid-induced gene expression of COX-2, IL-1beta, GM-CSF, and ICAM1. Taken together, the data suggest that arachidonic acid via conversion to PGE(2) plays an important role in stimulation of growth-related genes and proliferation via PI3K signaling and NF-kappaB translocation to the nucleus.

Quantitative kinetic analysis of gene expression during human osteoblastic adhesion on orthopaedic materials

Little information was found in the literature about the expression on hydroxyapatite (HA) materials of genes specific of cellular adhesion molecules although more were found on titanium-based substrates. Hence, the goal of this work was to study by a kinetic approach from 30 min to 4 days the adhesion of Saos-2 cells on microporous (mHA) and non-microporous hydroxyapatite (pHA) in comparison to polished titanium. Our strategy associated the visualization of adhesion proteins inside the cells by immunohistochemistry and the quantitative expression of genes at mRNA level by real-time PCR. The cell morphology was assessed using scanning electron microscopy and the number of cells thanks to biochemical techniques. The cellular attachment was the highest on mHA from 30 min to 24 h although the cell growth on mHA was the lowest after 4 days. Generally, the Saos-2 osteoblastic cells morphology on mHA was radically different than on other surfaces with the particularity of the cytoplasmic edge, which appeared un-distinguishable from the surface. The revelation by specific antibodies of proteins of the cytoskeleton (actin) and the focal adhesions (FAK, phosphotyrosine) confirmed that adhesion and spreading were different on the 3 materials. The actin stress fibres were less numerous and shorter on mHA ceramics. Cells had more focal contacts after 4 h on mHA compared to other substrates but less after 24 h. The highest values of total proteins were extracted from mHA at 0.5 and 24 h and from pHA at 1, 4, and 96 h. The alphav and beta1 integrin, actin, FAK, and ERK gene expression were found to be different with adhesion time and with materials. C-jun expression was comparable on mHA, titanium and plastic but was largely higher than on pHA at 0.5 and 1 h. On the contrary, c-fos expression was the highest on pHA after 0.5 h and the lowest after 1h. This difference between c-fos and c-jun expression on pHA after 0.5 h could be related to the fact that these two genes may differ in their signalling pathways. The expression of the alkaline phosphatase gene after 4 days was lower on mHA compared to other materials demonstrating that the microstructure of the mHA ceramic was not favourable to Saos-2 cells differentiation. Finally, it was demonstrated in this study that HA and titanium surfaces influence as well gene expression at early times of adhesion as the synthesis of adhesion proteins but also proliferation and differentiation phases. Indeed, the signal transduction pathways involved in adhesion of Saos-2 cells on HA and titanium were confirmed by the sequential expression of alphav and beta1 integrins, FAK, and ERK genes followed by the expression of c-jun and c-fos genes for proliferation and alkaline phosphatase gene for differentiation.

Long-term loading inhibits ERK1/2 phosphorylation and increases FGFR3 expression in MC3T3-E1 osteoblast cells

Bone tissue homeostasis relies upon the ability of cells to detect and interpret extracellular signals that direct changes in tissue architecture. This study utilized a four-point bending model to create both fluid shear and strain forces (loading) during the time-dependent progression of MC3T3-E1 preosteoblasts along the osteogenic lineage. Loading was shown to increase cell number, alkaline phosphatase (ALP) activity, collagen synthesis, and the mRNA expression levels of Runx2, osteocalcin (OC), osteopontin, and cyclo-oxygenase-2. However, mineralization in these cultures was inhibited, despite an increase in calcium accumulation, suggesting that loading may inhibit mineralization in order to increase matrix deposition. Loading also increased fibroblast growth factor receptor-3 (FGFR3) expression coincident with an inhibition of FGFR1, FGFR4, FGF1, and extracellular signal-related kinase (ERK)1/2 phosphorylation. To examine whether these loading-induced changes in cell phenotype and FGFR expression could be attributed to the inhibition of ERK1/2 phosphorylation, cells were grown for 25 days in the presence of the MEK1/2 inhibitor, U0126. Significant increases in the expression of FGFR3, ALP, and OC were observed, as well as the inhibition of FGFR1, FGFR4, and FGF1. However, U0126 also increased matrix mineralization, demonstrating that inhibition of ERK1/2 phosphorylation cannot fully account for the changes observed in response to loading. In conclusion, this study demonstrates that preosteoblasts are mechanoresponsive, and that long-term loading, whilst increasing proliferation and differentiation of preosteoblasts, inhibits matrix mineralization. In addition, the increase in FGFR3 expression suggests that it may have a role in osteoblast differentiation.

Mechanical modulation of molecular signals which regulate anabolic and catabolic activity in bone tissue

Identifying the molecular mechanisms that regulate bone's adaptive response to alterations in load bearing may potentiate the discovery of interventions to curb osteoporosis. Adult female mice (BALB/cByJ) were subjected to catabolic (disuse) and anabolic (45 Hz, 0.3g vibration for 10 min/day) signals, and changes in the mRNA levels of thirteen genes were compared to altered indices of bone formation. Age-matched mice served as controls. Following 4 days of disuse, significant (P = 0.05) decreases in mRNA levels were measured for several genes, including collagen type I (-55%), osteonectin (-44%), osterix (-36%), and MMP-2 (-36%) all of which, after 21 days, had normalized to control levels. In contrast, expression of several genes in the vibrated group, which failed to show significant changes at 4 days, demonstrated significant increases after 21 days, including inducible nitric oxide synthase (iNOS) (39%, P = 0.07), MMP-2 (54%), and receptor activator of the nuclear factor kB ligand (RANKL) (32%). Correlations of gene expression patterns across experimental conditions and time points allowed the functional clustering of responsive genes into two distinct groups. Each cluster's specific regulatory role (formation vs. resorption) was reinforced by the 60% suppression of formation rates caused by disuse, and the 55% increase in formation rates stimulated by mechanical signals (P < 0.05). These data confirm the complexity of the bone remodeling process, both in terms of the number of genes involved, their interaction and coordination of resorptive and formative activity, and the temporal sensitivity of the processes. More detailed spatial and temporal correlations between altered mRNA levels and tissue plasticity may further delineate the molecules responsible for the control of bone mass and morphology.

Arachidonic acid, an omega-6 fatty acid, induces cytoplasmic phospholipase A2 in prostate carcinoma cells

For the past 60 years, dietary intake of essential fatty acids has increased. Moreover, the omega-6 fatty acids have recently been found to play an important role in regulation of gene expression. Proliferation of human prostate cells was significantly increased 48 h after arachidonic acid (AA) addition. We have analyzed initial uptake using nile red fluorescence and we found that the albumin conjugated AA is endocytosed into the cells followed by the induction of RNA within minutes, protein and PGE2 synthesis within hours. Here we describe that AA induces expression of cytosolic phospholipase A2 (cPLA2) in a dose-dependent manner and that this upregulation is dependent upon downstream synthesis of PGE2. The upregulation of cox-2 and cPLA2 was inhibited by flurbiprofen, a cyclooxygenase (COX) inhibitor, making this a second feed-forward enzyme in the eicosanoid pathway. Cox-2 specific inhibitors are known to inhibit colon and prostate cancer growth in humans; however, recent findings show that some of these have cardiovascular complications. Since cPLA2 is upstream in the eicosanoid pathway, it may be a good alternative for a pharmaceutical target for the treatment of cancer.

Stretch-induced phosphorylation of ERK1/2 depends on differentiation stage of osteoblasts

The goal of this study was to investigate the effect of mechanical loading on osteoblasts and extracellular signal-regulated kinase (ERK1/2) signaling in relation to osteoblast differentiation and mineralization. A human osteoblast cell line (SV-HFO) was triggered to differentiate to the advanced state of mineralization by addition of the osteogenic factors dexamethasone and beta-glycerophosphate. Osteoblasts were subjected to cyclic, equibiaxial stretch for 5, 15, or 60 min at different stages of differentiation (days 7, 14, and 21). Baseline (static) phosphorylated ERK1/2 and total ERK1/2 levels gradually increased during osteoblast differentiation. Cyclic stretch induced a rapid increase in ERK1/2 phosphorylation with a maximum between 5 and 15 min. Prolonged stretching for 60 min resulted in a decrease of phosphorylated ERK1/2 towards baseline level, suggesting a desensitization mechanism. The effect of stretch on ERK1/2 phosphorylation was strongest at later stages of differentiation (days 14 and 21). At day 21, the increase of ERK1/2 phosphorylation in response to stretch was significantly lower in non-differentiating than in differentiating osteoblasts. This could not be explained by differences in cell density, but did correlate with the formation of extracellular matrix, collagen fibrils. Mineralization of the extracellular matrix did not lead to a further increase of ERK1/2 phosphorylation. In conclusion, the current study demonstrates that the extent of activation of the ERK1/2 pathway is dependent on the differentiation or functional stage of the osteoblast. The presence of an extracellular matrix, but not per se mineralization, seems to be the predominant determinant of osteoblastic response to strain.

Influence of increased mechanical loading by hypergravity on the microtubule cytoskeleton and prostaglandin E2 release in primary osteoblasts

Cells respond to a wide range of mechanical stimuli such as fluid shear and strain, although the contribution of gravity to cell structure and function is not understood. We hypothesized that bone-forming osteoblasts are sensitive to increased mechanical loading by hypergravity. A centrifuge suitable for cell culture was developed and validated, and then primary cultures of fetal rat calvarial osteoblasts at various stages of differentiation were mechanically loaded using hypergravity. We measured microtubule network morphology as well as release of the paracrine factor prostaglandin E2 (PGE2). In immature osteoblasts, a stimulus of 10x gravity (10 g) for 3 h increased PGE2 2.5-fold and decreased microtubule network height 1.12-fold without affecting cell viability. Hypergravity (3 h) caused dose-dependent (5-50 g) increases in PGE2 (5.3-fold at 50 g) and decreases (1.26-fold at 50 g) in microtubule network height. PGE2 release depended on duration but not orientation of the hypergravity load. As osteoblasts differentiated, sensitivity to hypergravity declined. We conclude that primary osteoblasts demonstrate dose- and duration-dependent sensitivity to gravitational loading, which appears to be blunted in mature osteoblasts.

Mechanisms by which exercise improves bone strength

Certain exercises can induce osteogenesis and improve bone strength, yet the biological processes involved in bone mechanotransduction are only beginning to be understood. Several pathways are emerging from current research, including calcium signaling associated with membrane ion channels, adenosine triphosphate signaling, second messengers such as prostaglandins and nitric oxide, and signaling involving mitogen-activated protein kinase. One characteristic of the mechanosensing apparatus that has only recently been studied is the important role of desensitization. Experimental protocols that insert "rest" periods to reduce the effects of desensitization can double anabolic responses to mechanical loading. Exercises that reduce desensitization may provide an effective means to build bone strength.

Regulation of synthesis of osteoprotegerin and soluble receptor activator of nuclear factor-kappaB ligand in normal human osteoblasts via the p38 mitogen-activated protein kinase pathway by the application of cyclic tensile strain

Mechanical stress is thought to play an important role in bone remodeling. However, the correlation between mechanical stress and bone remodeling is poorly understood. In this context, using a model of cyclic tensile strain (CTS) toward human osteoblasts, synthesis of osteoprotegerin (OPG) and soluble receptor activator of nuclear factor-kappaB ligand (sRANKL), and the activation of mitogen-activated protein kinases (MAPKs) were examined. The application of 7%, 0.25-Hz CTS once a day for 4 h for 3 successive days simultaneously caused an increase of OPG synthesis and a decrease of sRANKL release and RANKL mRNA expression in osteoblasts. As for MAPKs activation in osteoblasts with the application of CTS, p38 MAPK was activated 10-20 min after the application of CTS, but extracellular signal-regulated kinase (ERK1/2) and c-Jun NH2-terminal kinase (JNK) were not activated by such application. Furthermore, when CTS was applied once a day for 4 h for 1, 2, or 3 successive days to osteoblasts, p38 MAPK activation was maintained during the 3-day period but ERK1/2 activation was downregulated from day to day, simultaneously. Then, when CTS was applied once a day for 4 h for 3 successive days to osteoblasts pretreated with the p38 MAPK inhibitor SB203580 for 1 h, OPG synthesis was dose-dependently suppressed and inhibition of sRANKL release and RANKL mRNA expression was abrogated. These results indicate that biological responses of OPG and sRANKL synthesis in osteoblasts to the application of CTS are regulated via the p38 MAPK pathway and suggest that CTS might modulate and regulate bone metabolism.

Zinc finger transcription factors in skeletal development

Cellular and molecular processes that regulate the development of skeletal tissues resemble those required for regeneration. Given the prevalence of degenerative skeletal disorders in an increasingly aging population, the molecular mechanisms of skeletal development must be understood in detail if novel strategies are to be developed in regenerative medicine. Research in this area over the past decade has revealed that cell differentiation is largely controlled at the level of gene transcription, which in turn is regulated by transcription factors. Transcription factors usually recognize and bind to specific DNA sequences in the promoter of target genes via characteristic DNA-binding domains. Although the gene family containing C2H2 zinc fingers as DNA-binding motifs is the largest family of transciptional regulators, with several hundred individual members in mammals, only a small but increasing number of zinc finger genes have been implicated in bone, cartilage, or tooth development. These zinc finger proteins (ZFPs) contain multiple structural motifs that require zinc to maintain their structural integrity and function. Interestingly, zinc deficiency is known to result in skeletal growth retardation and has been identified as a risk factor in the pathogenesis of osteoporosis. This review attempts to summarize our current state of knowledge regarding the role of ZFPs in the molecular regulation of skeletogenesis.

Signal transduction and mechanical stress

Bone undergoes a constant process of remodeling in which mass is retained or lost in response to the relative activity of osteoblasts and osteoclasts. Weight-bearing exercise-which is critical for retaining skeletal integrity-promotes osteoblast function, whereas a lack of mechanical stimulation, as seen during spaceflight or prolonged bed rest, can lead to osteoporosis. Thus, understanding mechanotransduction at the cellular level is key to understanding basic bone biology and devising new treatments for osteoporosis. Various mechanical stimuli have been studied as in vitro model systems and have been shown to act through numerous signaling pathways to promote osteoblast activity. Here, we examine the various types of stress and the sequential response of transduction pathways that result in changes in gene expression and the ensuing proliferation of osteoblasts.

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.

Mechanical strain on osteoblasts activates autophosphorylation of focal adhesion kinase and proline-rich tyrosine kinase 2 tyrosine sites involved in ERK activation

The mechanisms involved in the mechanical loading-induced increase in bone formation remain unclear. In this study, we showed that cyclic strain (CS) (10 min, 1% stretch at 0.25 Hz) stimulated the proliferation of overnight serum-starved ROS 17/2.8 osteoblast-like cells plated on type I collagen-coated silicone membranes. This increase was blocked by MEK inhibitor PD-98059. Signaling events were then assessed 0 min, 30 min, and 4 h after one CS period with Western blotting and coimmunoprecipitation. CS rapidly and time-dependently promoted phosphorylation of both ERK2 at Tyr-187 and focal adhesion kinase (FAK) at Tyr-397 and Tyr-925, leading to the activation of the Ras/Raf/MEK pathway. Cell transfection with FAK mutated at Tyr-397 completely blocked ERK2 Tyr-187 phosphorylation. Quantitative immunofluorescence analysis of phosphotyrosine residues showed an increase in focal adhesion plaque number and size in strained cells. CS also induced both Src-Tyr-418 phosphorylation and Src to FAK association. Treatment with the selective Src family kinase inhibitor pyrazolopyrimidine 2 did not prevent CS-induced FAK-Tyr-397 phosphorylation suggesting a Src-independent activation of FAK. CS also activated proline-rich tyrosine kinase 2 (PYK2), a tyrosine kinase highly homologous to FAK, at the 402 phosphorylation site and promoted its association to FAK in a time-dependent manner. Mutation of PYK2 at the Tyr-402 site prevented the ERK2 phosphorylation only at 4 h. Intra and extracellular calcium chelators prevented PYK2 activation only at 4 h. In summary, our data showed that osteoblast response to mitogenic CS was mediated by MEK pathway activation. The latter was induced by ERK2 phosphorylation under the control of FAK and PYK2 phosphorylation orchestrated in a time-dependent manner.

Mechanical strain induces collagenase-3 (MMP-13) expression in MC3T3-E1 osteoblastic cells

Mechanical strain plays a crucial role in bone remodeling during growth and development and healing of bone besides systemic and local factors. One of the major factors involves in remodeling process is matrix metalloproteinases (MMPs) such as MMP-13 that has been shown to degrade the native interstitial collagens in several tissues. To study how mechanical strain affects extracellular matrix degradation by MMP-13, a biaxial strain was applied to MC3T3-E1 osteoblastic cells plated onto a collagen-coated flexible elastic membrane. The MMP-13 protein and mRNA expression were determined by Western blotting and reverse transcriptase-PCR, respectively. The zymographic activities of MMP-13 increased dramatically at 30 min, reached a peak by 2-fold at 1 h, and maintained up to 4 h. Moreover, the MMP-13 and c-fos mRNA expressed at 5 min, increased to 2.8- and 3-fold at 1 h, respectively, and gradually declined thereafter. Cycloheximide and actinomycin D did not inhibit the MMP-13 and c-fos mRNA expression, suggesting that such expression does not require de novo protein synthesis and not change their stabilities. To investigate which of the mitogen-activated protein kinase (MAPK) pathways involves in the expression of MMP-13, inhibitors such as PD98059, SB203580, and SP600125 were used. However, only PD98059 (an inhibitor of MEK1/2 activation) inhibited MMP-13 and c-fos gene expression; the result was further substantiated by transfecting with the dominant negative mutants of MEK1/2 (MEK K97R) and ERK2. Taken together, our results showed that mechanical strain induces the MMP-13 expression through MEK-ERK signaling pathway to regulate mechanical adaptation.

Hypoxia suppresses runx2 independent of modeled microgravity

Bone loss is a consequence of skeletal unloading as seen in bed rest and space flight. Unloading decreases oxygenation and osteoblast differentiation/function in bone. Previously we demonstrated that simulation of unloading in vitro, by culturing differentiating mouse osteoblasts in a horizontal rotating wall vessel (RWV), results in suppressed expression of runx2, a master transcriptional regulator of osteoblast differentiation. However, the RWV is able to reproduce in a controlled fashion at least two aspects of disuse that are directly linked, model microgravity and hypoxia. Hypoxia in the RWV is indicated by reduced medium oxygen tension and increased expression of GAPDH and VEGF. To uncouple the role of model microgravity from hypoxia in suppressed runx2 expression, we cultured osteoblasts under modeled microgravity (oxygenated, horizontal RWV rotation), hypoxia (vertical RWV rotation), or both conditions (horizontal RWV rotation). The expression, DNA binding activity and promoter activity of runx2, was suppressed under hypoxic but not normoxic modeled microgravity RWV conditions. Consistent with a role for hypoxia in suppression of runx2, direct exposure to hypoxia alone is sufficient to suppress runx2 expression in osteoblasts grown in standard tissue culture plates. Taken together, our findings indicate that hypoxia associated with skeletal unloading could be major suppressor of runx2 expression leading to suppressed osteoblast differentiation and bone formation.

Ras induction of superoxide activates ERK-dependent angiogenic transcription factor HIF-1alpha and VEGF-A expression in shock wave-stimulated osteoblasts

Vascular endothelial growth factor (VEGF) released by osteoblasts plays an important role in angiogenesis and endochondral ossification during bone formation. In animal studies, we have reported that shock waves (SW) can promote osteogenic differentiation of mesenchymal stem cells through superoxide-mediated signal transduction (Wang, F. S., Wang, C. J., Sheen-Chen, S. M., Kuo, Y. R., Chen, R. F., and Yang, K. D. (2002) J. Biol. Chem. 277, 10931-10937) and vascularization of the bone-tendon junction. Here, we found that SW elevation of VEGF-A expression in human osteoblasts to be mediated by Ras-induced superoxide and ERK-dependent HIF-1alpha activation. SW treatment (0.16 mJ/mm(2), 1 Hz, 500 impulses) rapidly activated Ras protein (15 min) and Rac1 protein (30 min) and increased superoxide production in 30 min and VEGF mRNA expression in 6 h. Early scavenging of superoxide, but not nitric oxide, peroxide hydrogen, or prostaglandin E(2), reduced SW-augmented VEGF-A levels. Inhibition of superoxide production by diphenyliodonium, an NADPH oxidase inhibitor, was found to suppress VEGF-A expression. Transfection of osteoblasts with a dominant negative (S17N) Ras mutant abrogated the SW enhancement of Rac1 activation, superoxide synthesis, and VEGF expression. Further studies demonstrated that SW significantly promoted ERK activation in 1 h and HIF-1alpha phosphorylation and HIF-1alpha binding to VEGF promoter in 3 h. In support of the observation that superoxide mediated the SW-induced ERK activation and HIF-1alpha transactivation, we further demonstrated that scavenging of superoxide by superoxide dismutase and inhibition of ERK activity by PD98059 decreased HIF-1alpha activation and VEGF-A levels. Moreover, culture medium harvested from SW-treated osteoblasts increased vessel number of chick chorioallantoic membrane. Superoxide dismutase pretreatment and anti-VEGF-A antibody neutralization reduced the promoting effect of conditioned medium on angiogenesis. Thus, modulation of redox reaction by SW may have some positive effect on angiogenesis during bone regeneration.

Function of the cytoskeleton in gravisensing during spaceflight

Since astronauts and cosmonauts have significant bone loss in microgravity we hypothesized that there would be physiological changes in cellular bone growth and cytoskeleton in the absence of gravity. Investigators from around the world have studied a multitude of bone cells in microgravity including Ros 17/2.8, Mc3T3-E1, MG-63, hFOB and primary chicken calvaria. Changes in cytoskeleton and extracellular matrix (ECM) have been noted in many of these studies. Investigators have noted changes in shape of cells exposed to as little as 20 seconds of microgravity in parabolic flight. Our laboratory reported that quiescent osteoblasts activated by sera under microgravity conditions had a significant 60% reduction in growth (p<0.001) but a paradoxical 2-fold increase in release of the osteoblast autocrine factor PGE2 when compared to ground controls. In addition, a collapse of the osteoblast actin cytoskeleton and loss of focal adhesions has been noted after 4 days in microgravity. Later studies in Biorack on STS-76, 81 and 84 confirmed the increased release of PGE2 and collapse of the actin cytoskeleton in cells grown in microgravity conditions, however flown cells under 1 g conditions maintained normal actin cytoskeleton and fibronectin matrix. The changes seen in the cytoskeleton are probably not due to alterations in fibronectin message or protein synthesis since no differences have been noted in microgravity. Multiple investigators have observed actin and microtubule cytoskeletal modifications in microgravity, suggesting a common root cause for the change in cell architecture. The inability of the 0 g grown osteoblast to respond to sera activation suggests that there is a major alteration in anabolic signal transduction under microgravity conditions, most probably through the growth factor receptors and/or the associated kinase pathways that are connected to the cytoskeleton. Cell cycle is dependent on the cytoskeleton. Alterations in cytoskeletal structure can block cell growth either in G1 (F-actin microfilament collapse), or in G2/M (inhibition of microtubule polymerization during G2/M-phase). We therefore hypothesize that microgravity would inhibit growth in either G1, or G2/M.