Author: Malathy Shanmugam

Publisher:

Published: 1997

Total Pages: 284

ISBN-13:

Previous evidence in our laboratory demonstrated the ability of estrogen to regulate specifically the protein kinase C (PKC) $\delta$ isoform in the rat and rabbit corpus luteum. The purpose of this study was to evaluate estrogen's ability to regulate PKC, specifically the PKC $\delta$ isoform, in the estrogen responsive, MCF-7 human breast cancer cell line. With the increasingly characterized role of PKC $\delta$ in growth regulation, we were interested to also determine the ability of PKC $\delta$ to regulate growth in the MCF-7 cell line. Treatment of MCF-7 cells with 10$\sp{-10}$ through 10$\sp{-8}$ M estrogen for 7 days down-regulated specifically PKC $\delta$ mRNA and protein; expression of other PKC isoforms was unchanged. These concentrations of estrogen were maximally proliferative for the MCF-7 cells and an inverse correlation between PKC $\delta$ expression and proliferation in MCF-7 cells was observed. PKC $\delta$ protein levels were found to be 10 fold higher in MCF-7 compared to the estrogen receptor-negative, MDA-MB 231 cells which grow aggressively in the absence of estrogen. In view of the inverse correlation between PKC $\delta$ expression and proliferation in MCF-7 cells we wished to determine whether activation of PKC $\delta$ could signal growth inhibition. Phorbol ester treatments that lead to G1 growth arrest and induction of the cyclin-dependent kinase inhibitor p21$\rm\sp{Waf1/Cip1}$ in MCF-7 cells promoted activation of PKC $\delta$, as evidenced in immunecomplex phosphorylation assays. To show that phorbol ester stimulated G1 growth arrest could be mediated by the induction of p21$\rm\sp{Waf1/Cip1}$ by activated PKC $\delta,$ we demonstrated that constitutively active PKC $\delta$ in the absence but not in the presence of dominant negative PKC $\delta$ activated the WAF1/CIP1 promoter-luciferase construct in transient transfection assays. Activated PKC $\delta$ can therefore signal the transcriptional induction of p21$\rm\sp{WAF1/CIP1}.$ Our results are consistent with the hypothesis that modulation of PKC $\delta$ expression and activation state provides a pathway for growth regulation in breast cancer cells.