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

Although ErbB2 is known to enhance breast cancer metastasis, the signaling events responsible for this remain elusive. alpha-Isozyme of protein kinase C (PKC alpha), which is involved in cancer development and progression, has been suggested to be activated by ErbB2 without direct evidence. In addition, the roles of PKC alpha in ErbB2-mediated cancer cell malignancy have not been clearly identifed. In this study, we investigated whether ErbB2 can activate PKC alpha and determined what role PKCa 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 PKC alpha and that downregulation of ErbB2by small-interfering RNA decreased the expression and activity of PKC alpha 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 PKCa membrane immunohistochemistry staining than that of ErbB2-low-expressing tumors. Mechanistically, we found that PKC alpha is co-immunoprecipitated with Src and PKC alpha expression and activity can be decreased by Src inhibitor PP2 and by the expression of a dominantnegative mutant of Src. Moreover, ErbB2-mediated upregulation of urokinase-type plasminogen activator receptor (uPAR) is reduced by either the PKC alpha 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 PKC alpha is critical for ErbB2-mediated cancer cell invasion and provide valuable insights for current and future PKC alpha and Src inhibitor clinical trials.