Upregulation and activation of PKCα by ErbB2 through Src promotes breast cancer cell invasion that can be blocked by combined treatment with PKCα and Src inhibitors

Although ErbB2 is known to enhance breast cancer metastasis, the signaling events responsible for this remain elusive. α-Isozyme of protein kinase C (PKCα), which is involved in cancer development and progression, has been suggested to be activated by ErbB2 without direct evidence. In addition, the roles of PKCα in ErbB2-mediated cancer cell malignancy have not been clearly identified. In this study, we investigated whether ErbB2 can activate PKCα and determined what role PKCα plays in ErbB2-mediated breast cancer cell invasion. We expressed wild-type and mutant ErbB2 with altered signaling capacities in MDA-MB-435 breast cancer cells and revealed that overexpression or activation of ErbB2 in MDA-MB-435 cells upregulated and activated PKCα and that downregulation of ErbB2 by small-interfering RNA decreased the expression and activity of PKCα in BT474 breast cancer cells. These in vitro results were supported by data from breast cancer patient samples. In 150 breast cancer tumor samples, ErbB2-overexpressing tumors showed significantly higher positive rates of PKCα membrane immunohistochemistry staining than that of ErbB2-low-expressing tumors. Mechanistically, we found that PKCα is co-immunoprecipitated with Src and PKCα expression and activity can be decreased by Src inhibitor PP2 and by the expression of a dominant-negative mutant of Src. Moreover, ErbB2-mediated upregulation of urokinase-type plasminogen activator receptor (uPAR) is reduced by either the PKCα inhibitor Go6976 or the Src inhibitor PP2, and the combination of Go6976 with PP2 is superior to either agent alone in suppressing uPAR expression and cell invasion. These results demonstrate that PKCα is critical for ErbB2-mediated cancer cell invasion and provide valuable insights for current and future PKCα and Src inhibitor clinical trials.