Strain-Dependent Control of Transforming Growth Factor-?? Function in Osteoblasts in an In Vitro Model: Biochemical Events Associated with Distraction Osteogenesis

Cyclic tensile force modifies calvarial osteoblast function via the interplay between ERK1/2 and STAT3

BACKGROUND

Mechanical therapies, such as distraction osteogenesis, are widely used in dental clinics. During this process, the mechanisms by which tensile force triggers bone formation remain of interest. Herein, we investigated the influence of cyclic tensile stress on osteoblasts and identified the involvement of ERK1/2 and STAT3.

MATERIALS AND METHODS

Rat clavarial osteoblasts were subjected to tensile loading (10% elongation, 0.5 Hz) for different time periods. RNA and protein levels of osteogenic markers were determined using qPCR and western blot after inhibition of ERK1/2 and STAT3. ALP activity and ARS staining revealed osteoblast mineralization capacity. The interaction between ERK1/2 and STAT3 was investigated by immunofluorescence, western blot, and Co-IP.

RESULTS

The results showed that tensile loading significantly promoted osteogenesis-related genes, proteins and mineralized nodules. In loading-induced osteoblasts, inhibition of ERK1/2 or STAT3 decreased osteogenesis-related biomarkers significantly. Moreover, ERK1/2 inhibition suppressed STAT3 phosphorylation, and STAT3 inhibition disrupted the nuclear translocation of pERK1/2 induced by tensile loading. In the non-loading environment, inhibition of ERK1/2 hindered osteoblast differentiation and mineralization, while STAT3 phosphorylation was elevated after ERK1/2 inhibition. STAT3 inhibition also increased ERK1/2 phosphorylation, but did not significantly affect osteogenesis-related factors.

CONCLUSION

Taken together, these data suggested that ERK1/2 and STAT3 interacted in osteoblasts. ERK1/2-STAT3 were sequentially activated by tensile force loading, and both affected osteogenesis during the process.

Mathematical modeling of bone in-growth into undegradable porous periodic scaffolds under mechanical stimulus

Undegradable scaffolds, as a key element in bone tissue engineering, prevail in the present clinical applications, and the bone in-growth into such scaffolds under mechanical stimulus is an important issue to evaluate the bone-repair effect. This work aims to develop a mathematical framework to investigate the effect of mechanical stimulus on the bone in-growth into undegradable scaffolds. First, the osteoclast and osteoblast activities were coupled by their autocrine and paracrine effects. Second, the mechanical stimulus was empirically incorporated into the coupling cell activities on the basis of experimental observations. Third, the effect of mechanical stimulus including intensity and duration on the bone in-growth process was numerically studied; moreover, the homeostasis of scaffold–bone system under the mechanical stimulus was also treated. The results showed that the numbers of osteoblasts and osteoclasts in the scaffold–bone system tended to constants representing the system homeostasis. Both the mechanical intensity and duration optimized the final bone formation. The numerical results of the bone formation were comparable to the experimental results in rats. The findings from this modeling study could be used to explain many physiological phenomena and clinical observations. The developed model integrates both cell and tissue scales, which can be used as a platform to investigate bone remodeling under mechanical stimulus.

Impact of mechanical stretch on the cell behaviors of bone and surrounding tissues

Mechanical loading is recognized to play an important role in regulating the behaviors of cells in bone and surrounding tissues in vivo. Many in vitro studies have been conducted to determine the effects of mechanical loading on individual cell types of the tissues. In this review, we focus specifically on the use of the Flexercell system as a tool for studying cellular responses to mechanical stretch. We assess the literature describing the impact of mechanical stretch on different cell types from bone, muscle, tendon, ligament, and cartilage, describing individual cell phenotype responses. In addition, we review evidence regarding the mechanotransduction pathways that are activated to potentiate these phenotype responses in different cell populations.

Influence of heating and cyclic tension on the induction of heat shock proteins and bone-related proteins by MC3T3-E1 cells

Stress conditioning (e.g., thermal, shear, and tensile stress) of bone cells has been shown to enhance healing. However, prior studies have not investigated whether combined stress could synergistically promote bone regeneration. This study explored the impact of combined thermal and tensile stress on the induction of heat shock proteins (HSPs) and bone-related proteins by a murine preosteoblast cell line (MC3T3-E1). Cells were exposed to thermal stress using a water bath (44°C for 4 or 8 minutes) with postheating incubation (37°C for 4 hours) followed by exposure to cyclic strain (equibiaxial 3%, 0.2 Hz, cycle of 10-second tensile stress followed by 10-second rest). Combined thermal stress and tensile stress induced mRNA expression of HSP27 (1.41 relative fold induction (RFI) compared to sham-treated control), HSP70 (5.55 RFI), and osteopontin (1.44 RFI) but suppressed matrix metalloproteinase-9 (0.6 RFI) compared to the control. Combined thermal and tensile stress increased vascular endothelial growth factor (VEGF) secretion into the culture supernatant (1.54-fold increase compared to the control). Therefore, combined thermal and mechanical stress preconditioning can enhance HSP induction and influence protein expression important for bone tissue healing.

Regulation of the levels of Smad1 and Smad5 in MC3T3-E1 cells by Icariine in vitro

The purpose of this study was to investigate the role of Icariine on the expression of Smadl and Smad5 mRNA and protein levels in MC3T3-E1 cells in vitro. MC3T3-E1 cells were cultured in the presence of different concentrations of Icariine (0, 10, 40 and 80 ng/ml). Smad1 and Smad5 mRNA levels were detected by reverse transcription-polymerase chain reaction (RT-PCR) and the expression of proteins was determined by western blotting, immunohistochemistry staining and immunofluorescence. Smad1 and Smad5 mRNA levels continuously increased in 10, 40 and 80 ng/ml of Icariine with time and the differences indicated statistical significance. Western blot analysis demonstrated that the Smad1 and Smad5 protein levels in the 10, 40 and 80 ng/ml groups were higher compared with the 0 ng/ml group at 24, 48 and 72 h, and the difference was statistically significant. Immunohistochemistry staining and immunofluorescence showed that the expression of the Smad1 and Smad5 proteins was higher in the cytoplasm and nuclei in the 10, 40 and 80 ng/ml groups compared with the 0 ng/ml group. Icariine has a direct stimulatory function on the differentiation of MC3T3-E1 osteoblastic cells in vitro, which may be mediated by increasing the production of Smad1 and Smad5 in osteoblasts.

Uniaxial mechanical strain: an in vitro correlate to distraction osteogenesis

BACKGROUND

Distraction osteogenesis is a valuable clinical tool; however the molecular mechanisms governing successful distraction remain unknown. We have used a uniaxial in vitro strain device to simulate the uniaxial mechanical environment of the interfragmentary distraction gap.

MATERIALS AND METHODS

Using the Flexcell system, normal human osteoblasts were subjected to different levels of cyclical uniaxial mechanical strain. Cellular morphology, proliferation, migration, and the expression of angiogenic (vascular endothelial growth factor [VEGF] and fibroblast growth factor-2 [FGF-2]) and osteogenic (osteonectin, osteopontin, and osteocalcin) proteins and extracellular matrix molecules (collagen IalphaII) were analyzed in response to uniaxial cyclic strain.

RESULTS

Osteoblasts exposed to strain assumed a fusiform spindle-shaped morphology aligning parallel to the axis of uniaxial strain and osteoblasts exposed to strain or conditioned media had a 3-fold increase in proliferation. Osteoblast migration was maximal (5-fold) in response to 9% strain. Angiogenic cytokine, VEGF, and FGF-2, increased 32-fold and 2.6-fold (P < 0.05), respectively. Osteoblasts expressed greater amounts of osteonectin, osteopontin, and osteocalcin (2.1-fold, 1.8-fold, 1.5-fold respectively, P < 0.01) at lower levels of strain (3%). Bone morphogenic protein-2 production increased maximally at 9% strain (1.6-fold, P < 0.01). Collagen I expression increased 13-, 66-, and 153-fold in response to 3, 6, and 9% strain, respectively.

CONCLUSIONS

Uniaxial cyclic strain using the Flexcell device under appropriate strain parameters provides a novel in vitro model that induces osteoblast cellular and molecular expression patterns that simulate patterns observed in the in vivo distraction gap.

Prostaglandin E2 increases transforming growth factor-beta type III receptor expression through CCAAT enhancer-binding protein delta in osteoblasts

Variations in individual TGF-beta receptors (TbetaRs) may modify TGF-beta activity and significantly alter its effects on connective tissue growth or repair. Differences in the amount of TbetaR type III (TbetaRIII) relative to signal transducing TbetaRI occur on bone cells during differentiation or in response to other growth regulators. Here we investigated prostaglandin (PG) E2, a potent effector during trauma, inflammation, or mechanical load, on TbetaR expression in primary osteoblast-enriched cultures. PGE2 rapidly increased TbetaRIII mRNA and protein expression and enhanced TbetaRIII gene promoter activity through a discrete region within 0.4 kb of the transcription start site. PGE2 alters osteoblast function through multiple signal-inducing pathways. In this regard, protein kinase A (PKA) activators, PGE1 and forskolin, also enhanced gene expression through the TbetaRIII gene promoter, whereas protein kinase C activators, PGF2alpha and phorbol myristate acetate, did not. The stimulatory effect of PGE2 on TbetaRIII promoter activity was suppressed by a dominant negative PKA-regulatory subunit, but not by dominant negative protein kinase C. PGE2 specifically increased nuclear factor CCAAT enhancer-binding protein delta (C/EBPdelta) binding to a half-binding site upstream of the basal TbetaRIII promoter region, and promoter activity was sensitive to C/EBPdelta overexpression and to dominant-negative C/EBPdelta competition. In parallel with their effect on TbetaRIII expression, activators of PKA decreased TGF-beta-induced activity. In summary, high levels of PGE2 that occur with inflammation or trauma may, through PKA-activated C/EBPdelta, preferentially increase TbetaRIII expression and in this way delay TGF-beta-dependent activation of osteoblasts during the early stabilization phase of bone repair.

Cyclic mechanical strain increases production of regulators of bone healing in cultured murine osteoblasts

BACKGROUND

The adaptive response of bone to mechanical strain, for which angiogenesis is required, is underscored during fracture healing. Vascular endothelial growth factor (VEGF) and transforming growth factor beta-1 (TGF-beta1) are critical regulators of angiogenesis. The purpose of this study was to examine the effect of strain on the production of VEGF and TGF-beta1.

STUDY DESIGN

MC3T3-E1 mouse osteoblasts underwent cyclic strain (low, 0.1 Hz, or high, 0.2 Hz) for 24 or 48 hours. VEGF and TGF-beta1 protein levels were determined by ELISA, and Northern blot analysis was performed for VEGF mRNA. Alkaline phosphatase (an osteoblast differentiation marker) activity was determined by functional enzymatic assay. All measurements were standardized for cell number by crystal violet colorimetric assay. Statistical significance was determined by t-test, ANOVA, and the Tukey-Kramer test.

RESULTS

Protein production of VEGF and TGF-beta1 was dose-dependently elevated by strain (p < 0.05); alkaline phosphatase did not rise significantly. Northern blot analysis of strained osteoblast cells demonstrated increased VEGF mRNA. Cyclic strain was found to be progressively destructive in a dose-dependent manner, causing 51% and 70% decreases in cell number under low and high strain, respectively (p < 0.01).

CONCLUSIONS

We demonstrated simultaneous, dose-dependent increases in VEGF and TGF-beta1 protein production by osteoblastic cells in response to increasing strain. VEGF mRNA also increased in response to strain. This strain-induced increase in angiogenic cytokines suggests a potential mechanism by which injured bone may recruit a new blood supply. But we also found increasing strain to increase cellular toxicity, suggesting that cyclic mechanical strain may select for a subpopulation of osteoblasts.