Adhesion-independent α6β4 integrin clustering is mediated by phosphatidylinositol 3-kinase

Clustering of cell-surface integrins is known to augment integrin-mediated signal transduction, but mechanisms of integrin clustering are poorly understood. Here we report that adhesion-independent clustering of α6β4 integrin, known to be important in mediating tumor cell motility, is driven by phosphatidylinositol 3-kinase (PDK) but does not require activation of the PI3K-Akt pathway. We observed clustering of α6β4 in breast carcinoma cells after adhesion-independent cross-linking of the β4 integrin subunit. Clustering was significantly blocked when cross-linking was performed in the presence of PI3K inhibitors LY294002 and wortmannin. In contrast, no significant inhibition of clustering was observed with protein kinase C inhibitor GF109203X, rapamycin, or heparin. Although α6β4 clustering was blocked by PI3K inhibitors, clustering was not associated with increased PI3K lipid kinase activity or increased phosphorylation of Akt. A novel role for PI3K in α6β4 integrin clustering is proposed.