Interstitial fluid flow, generated upon induced movement of extracellular fluid after mechanical loading, activates many signal transduction pathways in bone cells. The mechanisms of mechanobiology in bone tissue are still not clearly understood. Recently focal adhesion kinase (FAK) was shown to be involved in mechanotransduction in a number of cells. This study was designed to characterize the functional roles of FAK in mediating osteoblast response to mechanical steady-state fluid shear stress (FSS). We reported here that FSS (15 dynes/cm(2)) induced activation of FAK and formation of FAK·Grb2·Sos ternary complex in MG-63 cells, which was necessary for activation of the downstream mitogen-activated protein kinase pathway signaling molecules extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). Our results also showed that transfection of FAK (F397Y) plasmid, a negative mutant of FAK, blocked the increased expression of binding factor alpha 1, osterix, osteocalcin and alkaline phosphatase induced by FSS in MG-63 cells. These results demonstrate that FAK signaling is critical for FSS-induced activation of ERK and JNK, and for promotion of osteoblast differentiation and osteogenesis via its association with Grb2/Sos complex.

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