Periodic mechanical stress stimulates the FAK mitogenic signal in rat chondrocytes through ERK1/2 activity.

The biological effects of periodic mechanical stress on chondrocytes have been studied extensively over the past few years. However, the mechanisms underlying chondrocyte mechanosensing and signaling in response to periodic mechanical stress remain to be determined. In the current study, we examined the effects of focal adhesion kinase (FAK) signaling on periodic mechanical stress-induced chondrocyte proliferation and matrix synthesis. Periodic mechanical stress significantly induced sustained phosphorylation of FAK at Tyr(397) and Tyr(576/577). Reduction of FAK with targeted shRNA via transfection of NH2-terminal tyrosine phosphorylation-deficient FAK mutant Y397F or Y576F-Y577F abolished periodic mechanical stress-induced chondrocyte proliferation and matrix synthesis, accompanied by attenuated ERK1/2 phosphorylation. However, activation of Src, PLCγ1 and Rac1 was not prevented upon FAK suppression. Furthermore, pretreatment with the Src-selective inhibitor, PP2, and shRNA targeted to Src or suppression of Rac1 with its selective inhibitor, NSC23766, blocked FAK phosphorylation at Tyr,(576/577) but not Tyr,(397) under periodic mechanical stress. Interestingly, FAK phosphorylation neither at Tyr(397) nor at Tyr(576/577) was affected by PLCγ1 depletion when periodic mechanical stress was applied. In addition, Tyr(397) and Tyr(576/577) phosphorylation levels were reduced upon pretreatment with a blocking antibody against integrin β1 under conditions of periodic mechanical stress. Our findings collectively suggest that periodic mechanical stress promotes chondrocyte proliferation and matrix synthesis through at least two pathways, integrin β1-Src-Rac1-FAK(Tyr(576/577))-ERK1/2 and integrin β1-FAK (Tyr(397))-ERK1/2.

Liang W ，Ren K ，Liu F ，Cui W ，Wang Q ，Chen Z ，Fan W ... -  《-》

Periodic Mechanical Stress Stimulates GIT1-Dependent Mitogenic Signals in Rat Chondrocytes Through ERK1/2 Activity.

The mitogenic effects of periodic mechanical stress on chondrocytes have been studied extensively, but the mechanisms whereby chondrocytes sense and respond to mechanical stimuli remain to be determined. We explored the question and verified the key role of G protein coupled receptor kinase interacting protein 1 (GIT1) signaling in periodic mechanical stress-induced chondrocyte proliferation. Two steps were undertaken in the experiment. In the first step, the cells were maintained under non-pressure conditions or periodic mechanical stress for 1 h prior to Western blot analysis. In the second step, the cells were pretreated with short hairpin RNA (shRNA) targeted to GIT1 or Src or control scrambled shRNA, or transfected with GIT1 wild-type or GIT1 mutant Y321F, or focal adhesion kinase (FAK) wild-type or FAK mutants Y397F or Y576F/Y577, respectively. Moreover, the cells were pretreated with blocking antibody against integrin β1 or PP2. Then the cells were maintained under non-pressure conditions or periodic mechanical stress for 1 h prior to Western blot analysis, and for 3 days, 8 h per day, prior to direct cell counting and CCK-8 assay, respectively. Periodic mechanical stress significantly induced sustained phosphorylation of GIT1 at Tyr321. Reduction of GIT1 with shRNA targeted to GIT1 and GIT1 mutant Y321F inhibited periodic mechanical stress-promoted chondrocyte proliferation, accompanied by attenuated extracellular signal-regulated kinase (ERK)1/2 and FAK phosphorylation at Tyr576/577. However, activation of Src and FAK-Tyr397 was not prevented upon GIT1 suppression. Furthermore, pretreatment with blocking antibody against integrin β1, Src-selective inhibitor, PP2, and shRNA targeted to Src blocked GIT1 activation under periodic mechanical stress. In addition, GIT1 phosphorylation at Tyr321 was not reduced upon pretreatment with FAK mutants Y397F or Y576F/Y577 under conditions of periodic mechanical stress. These findings collectively suggested that periodic mechanical stress promoted chondrocyte proliferation through at least two separate pathways, integrin β1-Src-GIT1-FAK(Tyr576/577)-ERK1/2, and the other parallel GIT1-independent integrin β1-FAK(Tyr397)-ERK1/2.

Ren K ，Tang J ，Jiang X ，Sun H ，Nong L ，Shen N ，Gu Y ... -  《-》

Periodic mechanical stress activates integrinβ1-dependent Src-dependent PLCγ1-independent Rac1 mitogenic signal in rat chondrocytes through ERK1/2.

The effects of periodic mechanical stress on the mitogenesis of chondrocytes have been studied extensively in recent years. However, the mechanisms underlying the ability of chondrocytes to sense and respond to periodic mechanical stress remain a matter of debate. We explored the signal transduction pathways of proliferation and matrix synthesis when chondrocytes were exposed to periodic mechanical stress. We observed that periodic mechanical stress statistically and significantly enhanced the phosphorylation and activation of Rac1 (p<0.05 for each). Pre-treatment with the Rac1 selective inhibitor NSC23766 attenuated periodic mechanical stress-induced chondrocyte proliferation and matrix synthesis (p<0.05 for each) and abrogated ERK1/2 signal activation (p<0.05), but did not block periodic mechanical stressinduced Src and PLCγ1 phosphorylation in this context. In addition, inhibition of Src with its selective inhibitor PP2 and shRNA targeted to Src blocked Rac1 signal activation (p<0.05 for each), but inhibition of the activity of PLCγ1 did not affect the phosphorylation and activation levels of Rac1 under conditions of periodic mechanical stress. The up-regulation of proliferation and matrix synthesis was inhibited in chondrocytes in response to periodic mechanical stress after pretreatment with blocking antibody against integrinβ1 (p<0.05 for each) but not after pretreatment with blocking antibody against integrinβ3. The phosphorylation levels of ERK1/2, Rac1, PLCγ1 and Src, and Rac1 activation level were also reduced when integrinβ1 was blocked in this context (p<0.05 for each). These findings suggest that periodic mechanical stress promotes chondrocyte proliferation and matrix synthesis in part by activating the ERK1/2 mitogenic signal through the integrinβ1-Src-PLCγ1/Rac1-ERK1/2 pathway, which links these important signaling molecules into mitogenic cascades.

Ren K ，Liu F ，Huang Y ，Liang W ，Cui W ，Wang Q ，Fan W ... -  《-》

Periodic Mechanical Stress INDUCES Chondrocyte Proliferation and Matrix Synthesis via CaMKII-Mediated Pyk2 Signaling.

Periodic mechanical stress can promote chondrocyte proliferation and matrix synthesis to improve the quality of tissue-engineered cartilage. Although the integrin β1-ERK1/2 signal cascade has been implicated in periodic mechanical stress-induced mitogenic effects in chondrocytes, the precise mechanisms have not been fully established. The current study was designed to probe the roles of CaMKII and Pyk2 signaling in periodic mechanical stress-mediated chondrocyte proliferation and matrix synthesis. Chondrocytes were subjected to periodic mechanical stress, proliferation was assessed by direct cell counting and CCK-8 assay; gene expressions were analyzed using quantitative real-time PCR, protein abundance by Western blotting. Mechanical stress, markedly enhanced the phosphorylation levels of Pyk2 at Tyr402 and CaMKII at Thr286. Both suppression of Pyk2 with Pyk2 inhibitor PF431396 or Pyk2 shRNA and suppression of CaMKII with CaMKII inhibitor KN-93 or CaMKII shRNA blocked periodic mechanical stress-induced chondrocyte proliferation and matrix synthesis. Additionally, either pretreatment with KN-93 or shRNA targeted to CaMKII prevented the activation of ERK1/2 and Pyk2 under conditions of periodic mechanical stress. Interestingly, in relation to periodic mechanical stress, in the context of Pyk2 inhibition with PF431396 or its targeted shRNA, only the phosphorylation levels of ERK1/2 were abrogated, while CaMKII signal activation was not affected. Moreover, the phosphorylation levels of CaMKII- Thr286 and Pyk2- Tyr402 were abolished after pretreatment with blocking antibody against integrinβ1 exposed to periodic mechanical stress. Our results collectively indicate that periodic mechanical stress promotes chondrocyte proliferation and matrix synthesis through the integrinβ1-CaMKII-Pyk2-ERK1/2 signaling cascade.

Liang W ，Li Z ，Wang Z ，Zhou J ，Song H ，Xu S ，Cui W ，Wang Q ，Chen Z ，Liu F ，Fan W ... -  《-》

Periodic mechanical stress activates MEK1/2-ERK1/2 mitogenic signals in rat chondrocytes through Src and PLCγ1.

Ren K ，Ma Y ，Huang Y ，Liang W ，Liu F ，Wang Q ，Cui W ，Liu Z ，Yin G ，Fan W ... -  《-》