Cooperative Signaling between a6b4 Integrin and ErbB-2 Receptor Is Required to Promote Phosphatidylinositol 3-Kinase-dependent Invasion*

We previously demonstrated that b4 integrin subunit overexpression increases in vitro invasiveness of NIH3T3 cells that have been transformed by ErbB-2 oncogene. We used this model to identify domains within the large b4 cytoplasmic domain that are involved in the interaction of a6b4 with ErbB-2, invasion, and phosphatidylinositol 3-kinase (PI3K) activation. For this purpose, we expressed deletion mutants of b4 that lacked either all or portions of the b4 cytoplasmic domain in NIH3T3/ErbB-2 cells. We also used an ecto-domain mutant in which most of the extracellular domain of b4 was replaced with a c-Myc tag. These transfectants were examined for their ability to invade Matrigel and their ability to activate PI3K, as well as for the ability of a6b4 to co-immunoprecipitate with ErbB-2. The results obtained revealed that a region of the b4 cytoplasmic domain between amino acids 854 and 1183 is critical for the ability of a6b4 integrin to increase invasion. Interestingly, the extracellular domain of b4 is not necessary for a6b4 to stimulate invasion. The association of a6b4 with ErbB-2 is dependent upon the b4 cytoplasmic domain and can occur in the absence of a6b4 heterodimerization. Finally, we observed strong activation of PI3K with b4 wild type and with those b4 deletion mutants that were able to stimulate invasion upon the expression in NIH3T3/ErbB-2 cells. In conclusion, our results establish that there is cooperation between a6b4 and ErbB-2 in promoting PI3K-dependent invasion and implicate a specific region of the b4 cytoplasmic domain (amino acids 854–1183) in this event.