ECM Stiffness Regulates Integrin Activity via FAK-Raf Pathway in a Negative Feedback Manner

ECM stiffness has profound effects on cell migration, cell cycle, proliferation, differentiation, especially stem cell fate dictation. The underlying mechanism has not been elucidated so far. As a cell-adhesive protein, integrin is known as a mechanosensor positioned at the beginning of mechanotransduction. Our previous work demonstrates that ECM stiffness regulates the activity level and subcellular distribution of beta 1 integrin. However, the mechanism of beta 1 integrin activity regulation by ECM stiffness is still unclear. In the present paper, by using a monoclonal antibody specifically recognizing the active conformation of beta 1 integrin, we observed that soft ECM significantly enhanced the level of active beta 1 integrin in comparison of stiff ECM. Meanwhile, the activity of FAK-Raf-MEK-ERK was markedly promoted by stiff ECM. The inhibition of FAK-Raf pathway rescued the low activity level of beta 1 integin on stiff ECM, indicating that stiff ECM may inhibits beta 1 integrin activity by FAK-Raf pathway in a negative feedback manner. In summary, we found a mechanism of the regulation of beta 1 integrin activity and downstream signal by ECM stiffness and provide a cue for understanding the mechanosensing of ECM stiffness by cells.