The ligation of available alpha6beta1 integrin in adherent LOX melanoma cells by laminin G peptides and integrin stimulatory antibodies induced cell invasiveness, independent of adhesion activity of integrins that were pre-bound to extracellular matrix (Nakahara, H., Nomizu, M., Akiyama, S. K., Yamada, Y., Yeh, Y., and Chen, W.-T. (1996) J. Biol. Chem. 271, 27221-27224). Here, we show that this induced invasion involves an increase in tyrosine phosphorylation of a 190-kDa GTPase-activating protein for Rho family members (p190(RhoGAP); p190) and membrane-protrusive activities at invadopodia. This tyrosine phosphorylation does not occur when the adherent cells are treated with non-activating antibody against beta1 integrin, control laminin peptides, or tyrosine kinase inhibitors genistein and herbimycin A. Although p190 and F-actin co-distribute in all cell cortex extensions, tyrosine-phosphorylated proteins including p190 appear to associate with F-actin specifically in invadopodia. In addition, the localized matrix degradation and membrane-protrusive activities were blocked by treatment of LOX cells with tyrosine kinase inhibitors as well as microinjection of antibodies directed against p190 but not by non-perturbing antibodies or control buffers. We suggest that activation of the alpha6beta1 integrin signaling regulates the tyrosine phosphorylation state of p190 which in turn connects downstream signaling pathways through Rho family GTPases to actin cytoskeleton in invadopodia, thus promoting membrane-protrusive and degradative activities necessary for cell invasion.
