Opposite effects of tamoxifen on in vitro protein kinase C activity and endogenous protein phosphorylation in intact MCF-7 cells

We investigated the effects of the antiestrogen tamoxifen on MCF-7 cell protein kinase C either by using the in vitro histone kinase assay or by studying the phosphorylation of its endogenous Mr 28,000 protein substrate in intact cells. In the in vitro assay, tamoxifen inhibited the enzyme competitively with respect to phospholipid, whereas estradiol and morpholinobenzyl phenoxy ethanamine, a specific ligand for antiestrogen binding sites, were considerably less efficient. In contrast, tamoxifen did not affect phosphorylation of the Mr 28,000 protein induced by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate in intact MCF-7 cells. Estradiol and morpholinobenzyl phenoxy ethanamine also had no effect. At high concentration (100 microM), tamoxifen itself stimulated specific phosphorylation of this Mr 28,000 protein. Estradiol and morpholinobenzyl phenoxy ethanamine neither mimicked nor interfered with this effect. Our data suggest that the effect of tamoxifen on protein kinase C activity depends on the phospholipid environment of the enzyme, and opposite effects may be observed in intact cells to those seen in disrupted cells. The action of tamoxifen on endogenous protein phosphorylation was thought to be due to direct interaction with the phospholipid binding domain of the enzyme rather than by interaction with the estrogen receptor or the antiestrogen binding site. Nevertheless, our results do not rule out a possible activation by tamoxifen of specific protein kinase(s) and phosphatase(s). In any case, the antiproliferative activity of tamoxifen on MCF-7 cells cannot be attributed to its effects on protein kinase C.