Transforming growth factor-β (TGF-β) plays an important role in several diseases that characteristically involve changes in tissue rigidity, such as cancer and tissue fibrosis. To determine whether matrix rigidity regulates the effects of TGF-β, we examined NMuMG and MDCK epithelial cells cultured on polyacrylamide gels with varying rigidity and treated with TGF-β1. Decreasing matrix rigidity reduced cell spreading and increased TGF-β1-induced apoptosis, while increasing matrix rigidity resulted in epithelial-mesenchymal transition (EMT). To more carefully control cell spreading, microcontact printing was used to restrict ECM area and revealed that reducing cell spreading also increased apoptosis. Apoptosis on compliant substrates was associated with decreased FAK expression, and FAK overexpression rescued cell survival but not EMT. Further investigation revealed manipulations of FAK activity, using pharmacological inhibitors or expression of FAK mutants, did not affect apoptosis or EMT, suggesting that FAK regulates apoptosis through expression but not activity. Additional investigation into the signaling pathways regulated by rigidity revealed a role for PI3K/Akt. We observed increased Akt activity with increasing rigidity, and that PI3K/Akt activity was necessary for cell survival and EMT on rigid substrates. These findings demonstrate that matrix rigidity regulates a switch in TGF-β-induced cell functions through rigidity-dependent regulation of FAK and PI3K, and suggest that changes in tissue mechanics during disease contribute to the cellular response to TGF-β
