INTEGRIN-MEDIATED ACTIVATION OF MAP KINASE IS INDEPENDENT OF FAK - EVIDENCE FOR DUAL INTEGRIN SIGNALING PATHWAYS IN FIBROBLASTS

Integrin-mediated cell adhesion causes activation of MAP kinases and i ncreased tyrosine phosphorylation of focal adhesion kinase (FAK). Auto phosphorylation of FAK leads to the binding of SH2-domain proteins inc luding Src-family kinases and the Grb2-Sos complex, Since Grb2-Sos is a key regulator of the Ras signal transduction pathway, one plausible hypothesis has been that integrin-mediated tyrosine phosphorylation of FAK leads to activation of the Ras cascade and ultimately to mitogen activated protein (MAP) kinase activation. Thus, in this scenario FAK would serve as an upstream regulator of MAP kinase activity. However, in this report we present several lines of evidence showing that integ rin-mediated MAP kinase activity in fibroblasts is independent of FAK, First, a beta 1 integrin subunit deletion mutant affecting the putati ve FAK binding site supports activation of MAP kinase in adhering fibr oblasts but not tyrosine phosphorylation of FAK. Second, fibroblast ad hesion to bacterially expressed fragments of fibronectin demonstrates that robust activation of MAP kinase can precede tyrosine phosphorylat ion of FAK. Finally, we have used FRNK, the noncatalytic COOH-terminal domain of FAK, as a dominant negative inhibitor of FAK autophosphoryl ation and of tyrosine phosphorylation of focal contacts. Using retrovi ral infection, we demonstrate that levels of FRNK expression sufficien t to completely block FAK tyrosine phosphorylation were without effect on integrin-mediated activation of MAP kinase. These re suits strongl y suggest that integrin-mediated activation of MAP kinase is independe nt of FAK and indicate the probable existence of at least two distinct integrin signaling pathways in fibroblasts.