Protein kinase C-dependent mobilization of the alpha 6 beta 4 integrin from hemidesmosomes and its association with actin-rich cell protrusions drivethe chemotactic migration of carcinoma cells

We explored the hypothesis that the chemo tactic migration of carcinoma cel ls that assemble hemidesmosomes involves the activation of a signaling path way that releases the alpha 6 beta 4 integrin from these stable adhesion co mplexes and promotes its association with F-actin in cell protrusions enabl ing it to function in migration. Squamous carcinoma-derived A431 cells were used because they express alpha 6 beta 4 and migrate in response to EGF st imulation. Using function-blocking antibodies we show that the alpha 6 beta 4 integrin participates in EGF-stimulated chemotaxis and is required for l amellae formation on laminin-1, At concentrations of EGF that stimulate A43 1 chemotaxis (similar to 1 ng/ml), the alpha 6 beta 4 integrin is mobilized from hemidesmosomes as evidenced by indirect immunofluorescence microscopy using mAbs specific for this integrin and hemidesmosomal components and it s loss from a cytokeratin fraction obtained by detergent extraction. EGF st imulation also increased the formation of lamellipodia and membrane ruffles that contained alpha 6 beta 4 in association with F-actin. Importantly, we demonstrate that this mobilization of alpha 6 beta 4 from hemidesmosomes a nd its redistribution to cell protrusions occurs by a mechanism that involv es activation of protein kinase C-alpha and that it is associated with the phosphorylation of the beta 4 integrin subunit on serine residues. Thus, th e chemotactic migration of A431 cells on laminin-1 requires not only the fo rmation of F-actin-rich cell protrusions that mediate alpha 6 beta 4-depend ent cell movement but also the disruption of alpha 6 beta 4-containing hemi desmosomes by protein kinase C.