Co-localization of cortactin and phosphotyrosine identifies active invadopodia in human breast cancer cells

Invadopodia are filopodia-like projections possessing protease activity that participate in tumor cell invasion. We demonstrate that co-localization of cortactin and phosphotyrosine identifies a subset of cortactin puncta termed "invadopodial complexes" that we find to be closely associated with the plasma membrane at active sites of focal degradation of the extracellular matrix in MDA-MB-231 breast cancer cells. Manipulation of c-Src activity in cells by transfection with kinase activated c-Src(527) or kinase inactive c-Src(295) results in a dramatic increase or decrease, respectively, in the number of these structures associated with changes in the number of sites of active matrix degradation. Overexpression of kinase-inactive c-Src(295) does not prevent localization of cortactin at the membrane; however, co-localized phosphotyrosine staining is decreased. Thus, elevated phosphotyrosine at invadopodial complexes is specifically associated with the proteolytic activity of invadopodia. Further, invadopodial complexes are spatially, morphologically and compositionally distinct from focal adhesions as determined by localization of focal adhesion kinase (FAK), which is not present in invadopodial complexes. Expression of kinase-inactive c-Src(295) blocks invadopodia activity, but does not block filopodia formation. Thus, invadopodia, but not filopodia, are highly correlated with matrix invasion, and sites of invadopodial activity can be identified by the formation of invadopodial complexes.