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

Abstract:
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.