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 p21WAF1/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 at-rested 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 p21(WAFI/CIPI) 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 delta and eta isoforms were expressed at markedly (greater than or equal to 10-fold) elevated levels in MCF7 versus MDA-MB 231 cells. These results suggested that the differential sensitivity to growth arrest and induction of p21(WAFI/CIPI) could reflect, at least in part, increased expression of PMA-dependent PKC isoforms delta and/or eta. Direct evidence to Support this hypothesis was provided by the ability of transient transfections into MCF-7 cells of constitutively active PKC delta but not of PKC's eta or alpha or epsilon to enhance p21(WAFI/CIPI) promoter activity. These results suggest that PKC delta plays a fundamental role in the regulation of growth in estrogen receptor positive breast cancer cells. (C) 2001 Published by Elsevier Science Ireland Ltd.