Abstract
Adhesion of epithelial cells to basement membranes (BM) occurs through two major structures: actin-associated focal contacts and keratin-associated hemidesmosomes, both of which form on laminin-332 (Ln-332). In epithelial-derived cancer cells, additional actin-linked structures with putative adhesive properties, invadopodia, are frequently present and mediate BM degradation. A recent study proposed that BM invasion requires a proper combination of focal contacts and invadopodia for invading cells to gain traction through degraded BM, and suggested that these structures may compete for common molecular components such as Src kinase. In this study, we tested the role of the Ln-332 in regulating invadopodia in 804G rat bladder carcinoma cells, a cell line that secretes Ln-332 and forms all three types of adhesions. Expression of shRNA to Ln-332 gamma2 chain (gamma2-kd) led to increased numbers of invadopodia and enhanced extracellular matrix degradation. Replating gamma2-kd cells on Ln-332 or collagen-I fully recovered cell spreading and inhibition of invadopodia. Inhibition of alpha3 or beta1, but not alpha6 or beta4, phenocopied the effect of gamma2-kd, suggesting that alpha3beta1-mediated focal contacts, rather than alpha6beta4-mediated hemidesmosome pathways, intersect with invadopodia regulation. gamma2-kd cells exhibited alterations in focal contact-type structures and in activation of focal adhesion kinase (FAK) and Src kinase. Inhibition of FAK also increased invadopodia number, which was reversible with Src inhibition. These data are consistent with a model whereby actin-based adhesions can limit the availability of active Src that is capable of invadopodia initiation and identifies Ln-332-beta1 interactions as a potent upstream regulator that limits cell invasion.