A role for protein kinase C δ in the differential sensitivity of MCF-7 and MDA-MB 231 human breast cancer cells to phorbol ester-induced growth arrest and p21WAFI/CIP1 induction

The goal of this study was to investigate the differential sensitivity of estrogen receptor (ER) positive MCF-7 and ER negative MDA-MB 231 breast cancer cells to phorbol myristate acetate (PMA)-dependent growth arrest. MCF-7 cells were growth arrested by 80% while MDA-MB 231 cells were arrested by 20% in response to seven days of treatment with 10 nM PMA. Coincident with the increased sensitivity of MCF-7 cells to be growth arrested by the protein kinase C (PKC) activator PMA, PMA induced 9-fold higher levels of the cyclin dependent kinase (Cdk) inhibitor p21WAF1/GIP1 in MCF-7 compared to MDA-MB 231 cells. A comparison of the PKC isoforms expressed in MCF-7 versus MDA-MB 231 cells showed that only the PMA-sensitive PKC δ and η isoforms were expressed at markedly (≥10-fold) elevated levels in MCF7 versus MDA-MB 231 cells. These results suggested that the differential sensitivity to growth arrest and induction of p2lWAFl/CIPl could reflect, at least in part, increased expression of PMA-dependent PKC isoforms δ and/or η. Direct evidence to support this hypothesis was provided by the ability of transient transfections into MCF-7 cells of constitutively active PKC δ but not of PKCʹs η or α or ϵ to enhance p21WAFl/CIP1 promoter activity. These results suggest that PKC δ plays a fundamental role in the regulation of growth in estrogen receptor positive breast cancer cells.