REGULATION OF PROTEIN KINASE C-ETA IN BREAST CANCER

Purpose: The protein kinase C (PKC) and phosphotidylinositide-3 kinase (PI3K) signaling pathways play critical roles in the development of breast cancer and regulate cell proliferation, differentiation, cell death and tumor promotion. PKCs serve as receptors for tumor-promoting phorbol esters, which are potent activators of conventional and novel members of the PKC family, and can substitute for the physiologic activator diacylglycerol. Prolonged treatment with phorbol esters, however, induces downregulation of these PKCs. Protein kinase C-eta is a novel member of the PKC family but resists downregulation by phorbol esters. This unique regulation of PKC-eta may have implications in tumor promotion. PKCs are regulated not only by cofactors but also by phosphorylation. Phosphorylation and dephosphorylation of PKCs can regulate their activity, stability and function. The objective of this study is to understand the regulation and contribution of PKC-eta in breast cancer. Methods: Established breast cancer cell lines were used in our study. The effect of distinct kinase inhibitors on PKCη protein levels was determined by Western blot analysis. The effect of proteasome and protease inhibitors on PKC-eta levels was also assessed by Western blotting. RNAi technique was utilized to knockdown members of the PKC and PI3K pathways. MTT assay was performed to determine the effect of PKC-eta on cell growth while cell proliferation upon PKC-eta knockdown was monitored by clonogenic assay. Results: Inhibition of PKC and PI3K pathways induced downregulation of PKC-eta via two distinct mechanisms. PKC-eta depletion inhibited the growth and proliferation of breast cancer cells. Conclusions: Our results demonstrate that the distinct regulation of PKC-eta by members of the PKC and PI3K family contributes to breast cancer cell growth.