The mTOR-FAK mechanotransduction signaling axis for focal adhesion maturation and cell proliferation.

Background: Mechanotransduction (MTD) is an important physiopathological signalling pathway associated with cardiovascular disease such as hypertension. Phosphorylation of focal adhesion kinase (FAK) is a MTD-sensing protein. This study tested the hypothesis that mTOR-FAK MTD signaling axis was crucial for focal adhesion (FA) maturation and cell proliferation.

Methods: Shock-wave was adopted as a tool for MTD and mTOR-FAK signaling.

Results: After demonstrating a failure in FAK phosphorylation after microfilament depolymerization, we attempted to identify the upstream regulator out of three kinases known to be activated in pressure-stimulated MTD [i.e., GSK-3β, Akt, and mTORC1 (mammalian target of rapamycin complex 1)]. Of the three specific inhibitors, only rapamycin, an inhibitor of mTORC1, was found to inhibit FAK phosphorylation, suggesting that mTORC1 is the upstream regulator in shock-wave-elicited FAK phosphorylation. Moreover, mTOR and its readout protein S6K were found to be activated by shock-wave stimulation. On the other hand, microscopic examination revealed not only MTD-induced increase in the number of actin stress fibers, but also alternative subcellular localization of mTORC1 as vesicle-like inclusions on microfilaments. Besides, rapamycin was found to destruct the granular pattern of mTORC1, while dissociation between F-actin and mTORC1 was noted after cytochalasin D administration. Since mTORC1 and FAK are essential for cell proliferation, we performed proliferation assay for mesenchymal stem cell (MSC) with and without shock-wave administration/rapamycin treatment/FAK depletion. The results demonstrated significant enhancement of cell proliferation after shock-wave stimulation but remarkable suppression after rapamycin and siFAK treatment.

Conclusion: Our findings suggest not only a co-ordinated regulation of FAK phosphorylation by mTORC1 and microfilaments, but also the participation of mTORC1-FAK signalling in MSC proliferation.