Effects of regulatory domains of specific isoforms of protein kinase C on growth control and apoptosis in MCF-7 breast cancer cells

Protein kinase C (PKC) is a multigene family consisting of at least 11 isoforms that play key roles in growth control and tumorigenesis. To understand the roles of specific isoforms of PKC in breast cancer, we generated derivatives of the human breast cancer cell line MCF-7 that stably overexpress dominant negative mutants (REG) of PKC-α, -ε, or -ζ, which encode only the regulatory domains of the respective isoforms. When stimulated to re-enter the cell cycle after serum starvation, the MCF-7/PKC-α-REG cell line exhibited enhanced cell-cycle progression in comparison to the control cell line. These cells also showed increased sensitivity to growth inhibition and induction of apoptosis in response to various cytotoxic stimuli, including serum starvation, tamoxifen, and γ-radiation. Western blot analysis indicated that the MCF-7/PKC-α-REG cell line displayed marked decreases in the levels of the cyclin-dependent kinase inhibitor p21 ^CIP1 and the anti-apoptotic protein bcl-2. Similar, but less striking, effects were seen in the MCF-7/PKC-ε-REG cell line, and the MCF-7/PKC-ζ-REG cell line showed minimal changes, when compared to the control cells. Taken together, these results suggest that the endogenous PKC-α in MCF-7 cells plays a critical role in regulating cell-cycle control and apoptosis, in part through upregulating the expression of p21 ^CIP1 and bcl-2. Therefore, inhibitors of PKC-α may potentiate the activity of cytotoxic agents in the therapy of breast cancer.