Estrogen-dependent rapid activation of protein kinase C in estrogen receptor-positive MCF-7 breast cancer cells and estrogen receptor-negative HCC38 cells is membrane-mediated and inhibited by tamoxifen

We examined protein kinase C ( PKC) in the regulation of breast cancer cells by estrogen. Estrogen receptor ( ER)positive (+) MCF-7 and ER-negative (-) HCC38 cells were treated with 17beta-estradiol (E-2) or E-2-BSA, which cannot enter the cell. E-2 and E-2-BSA rapidly increased PKC-alpha in both cells via phosphatidylinositol-dependent phospholipase C and G protein, but not phospholipase A(2) or arachidonic acid. In MCF-7 cells, E-2 and E-2-BSA had comparable effects, maximal at 90 min. In HCC38 cells, PKC was maximal at 9 min, with E-2-BSA more than E-2. Tamoxifen blocked estrogen-dependent PKC in MCF-7 cells and reduced it in HCC38 cells. ER-antagonist ICI 182780, ER-agonist diethylstilbestrol, and antibodies to ERalpha and ERbeta had no effect. E-2 stimulated [H-3] thymidine incorporation in MCF-7 only; E-2-BSA had no effect. Tamoxifen did not alter E-2-dependent increases in MCF-7 cells, whereas ICI 182780 reduced DNA synthesis in control and E-2-treated cultures. PKC activity was positively correlated with tumor severity in 133 breast cancer specimens and was greater in ER(-) tumors. Tamoxifen treatment reduced recurrence, and recurrent tumors had higher PKC activity. This indicates that E-2 rapidly increases PKC activity via membrane pathways not involving ERalpha or ERbeta and suggests that tamoxifen works by reducing PKC activity through non-ERalpha/ERbeta-dependent mechanisms.