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 β1 integrin. However, the mechanism of β1 integrin activity regulation by ECMstiffness is still unclear. In the present paper, by using a monoclonal antibody specifically recognizing the active conformation of β1 integrin, we observed that soft ECM significantly enhanced the level of active β1 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 β1 integrin on stiff ECM, indicating that stiff ECM may inhibits β1 integrin activity by FAK-Raf pathway in a negative feedback manner. In summary, we found a mechanismof the regulation of β1 integrin activity and downstream signal by ECM stiffness and provide a cue for understanding the mechanosensing of ECMstiffness by cells.