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 17β-estradiol (E2) or E2-BSA, which cannot enter the cell. E2 and E2-BSA rapidly increased PKC-α in both cells via phosphatidylinositol-dependent phospholipase C and G protein, but not phospholipase A2 or arachidonic acid. In MCF-7 cells, E2 and E2-BSA had comparable effects, maximal at 90 min. In HCC38 cells, PKC was maximal at 9 min, with E2-BSA more than E2. 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 ERα and ERβ had no effect. E2 stimulated [3H]thymidine incorporation in MCF-7 only; E2-BSA had no effect. Tamoxifen did not alter E2-dependent increases in MCF-7 cells, whereas ICI 182780 reduced DNA synthesis in control and E2-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 E2 rapidly increases PKC activity via membrane pathways not involving ERα or ERβ and suggests that tamoxifen works by reducing PKC activity through non-ERα/ERβ-dependent mechanisms.
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