Differential effects of protein kinase C-eta on apoptosis versus senescence

Protein kinase C-eta (PKCη) is considered an anti-apoptotic kinase, which promotes cell survival and chemoresistance in several cancers, including breast cancer. We have recently shown that PKCη positively regulates the anti-apoptotic protein Mcl-1 in breast cancer cells, and depletion of PKCη induced proteasomal degradation of Mcl-1. We therefore examined if depletion of PKCη would enhance cellular sensitivity to chemotherapeutic agents. Silencing of PKCη by siRNA attenuated apoptosis induced by doxorubicin and paclitaxel in both MCF-7 and T47D breast cancer cells. While silencing of Mcl-1 caused a substantial increase in apoptosis induced by doxorubicin, the combined knockdown of PKCη and Mcl-1 was less effective. Depletion of PKCη also caused an increase in the abundance of the cell cycle inhibitor p27 and a decrease in the clonogenic survival of MCF-7 and T47D cells. PKCη knockdown was associated with an increase in senescence-associated β-galactosidase (SA-β-gal) activity but this increase was attenuated by knockdown of p27. The suppression of doxorubicin-induced apoptosis by PKCη knockdown was partially relieved when p27 was depleted. Since loss of proliferative capacity during senescence could cause resistance to chemotherapeutic drugs, our results suggest that PKCη knockdown inhibits apoptosis by inducing p27-mediated senescence.