Integrin α ₆β ₄ cooperates with LPA signaling to stimulate Rac through AKAP-LBc-mediated RhoA activation

The α6β4 integrin promotes carcinoma invasion through its ability to promote directed migration and polarization of carcinoma cells. In this study, we explore how the α ₆β ₄ integrin cooperates with lysophosphatidic acid (LPA) to activate Rho and Rac small GTPases. Through the use of dominant negative Rho constructs, C3 exotransferase, and Rho kinase inhibitor, we find that Rho is critical for LPA-dependent chemotaxis and lamellae formation. However, utilization of specific Rho isoforms depends on integrin α ₆β ₄ expression status. Integrin α ₆β ₄-negative MDA-MB-435 cells utilize only RhoC for motility, whereas integrin α ₆β ₄-expressing cells utilize RhoC but additionally activate and utilize RhoA for LPA-dependent cell motility and lamellae formation. Notably, the activation of RhoA by cooperative LPA and integrin α ₆β ₄ signaling requires the Rho guanine nucleotide exchange factor AKAP-Lbc. We also determine that integrin α ₆β ₄ cannot activate Rac1 directly but promotes LPA-mediated Rac1 activation that is dependent on RhoA activity and de novo β1 integrin ligation. Finally, we find that the regulation of Rac1 and RhoA in response to LPA is differentially regulated by phosphodiesterases, PKA, and phosphatidylinositol 3-kinase, thus supporting their spatially distinct compartmentalization. In summary, signaling from integrin α ₆β ₄ facilitates LPA-stimulated chemotaxis through preferential activation of RhoA, which, in turn, facilitates activation of Rac1.