THE TRANSCRIPTION FACTOR AP-1 IS REQUIRED FOR EGF-INDUCED ACTIVATION OF RHO-LIKE GTPASES, CYTOSKELETAL REARRANGEMENTS, MOTILITY, AND IN-VITRO INVASION OF A431 CELLS

Human squamous cell carcinomas (SCC) frequently express elevated level s of epidermal growth factor receptor (EGFR). EGFR overexpression in S CC-derived cell lines correlates with their ability to invade in an in vitro invasion assay in response to EGF, whereas benign epidermal cel ls, which express low levels of EGFR, do not invade. EGF-induced invas ion of SCC-derived A431 cells is inhibited by sustained expression of the dominant negative mutant of c-Jun, TAM67, suggesting a role for th e transcription factor AP-1 (activator protein-1) in regulating invasi on. Significantly, we establish that sustained TAM67 expression inhibi ts growth factor-induced cell motility and the reorganization of the c ytoskeleton and cell-shape changes essential for this process: TAM67 e xpression inhibits EGF-induced membrane ruffling, lamellipodia formati on, cortical actin polymerization and cell rounding. Introduction of a dominant negative mutant of Rac and of the Rho inhibitor C3 transfera se into A431 cells indicates that EGF-induced membrane ruffling and la mellipodia formation are regulated by Rac, whereas EGF-induced cortica l actin polymerization and cell. rounding are controlled by Rho. Const itutively activated mutants of Rac or Rho introduced into A431 or A431 cells expressing TAM67 (TA cells) induce equivalent actin cytoskeleta l rearrangements, suggesting that the effector pathways downstream of Rac and Rho required for these responses are unimpaired by sustained T AM67 expression. However, EGF-induced translocation of Rac to the cell membrane, which is associated with its activation, is defective in TA cells. Our data establish a novel link between AP-1 activity and EGFR activation of Rac and Rho, which in turn mediate the actin cytoskelet al rearrangements required for cell motility and invasion.