Differential effects of bryostatin 1 and phorbol ester on human breast cancer cell lines

The effects of the protein kinase C (PKC) activators, phorbol ester 12-O-tetradecanoyl-13-phorbol acetate (TPA) and the marine natural product, bryostatin 1, on the growth and morphology of human breast cancer cell lines were examined. TPA (1 to 100 nM) inhibited growth of four of six cell lines by up to 75% in 5-day cultures. Bryostatin 1 inhibited growth of only MCF-7 cells and only at a high dose (100 nM). However, bryostatin 1 completely antagonized the growth inhibition and morphological changes induced by TPA in MCF-7 cells. The divergent effects of these two agents are associated with differing effects on PKC activity and isoform expression in MCF-7 cells. TPA induced rapid translocation of the PKC-alpha isozyme and PKC activity to the membrane fraction of MCF-7 cells. In contrast, bryostatin 1 treatment resulted in the loss of the PKC-alpha isozyme and PKC activity from both cytosolic and membrane compartments within 10 min of treatment. In coincubation assays the bryostatin 1 effect was dominant over that of TPA. Similar effects on PKC-alpha isozyme and PKC activity were seen in a second cell line whose growth was inhibited by TPA but not by bryostatin 1, MDA-MB-468. In contrast, in the T47D cell line, where TPA was not growth inhibitory, TPA failed to induce translocation of PKC-alpha to the cell membrane. Bryostatin, however, still caused loss of PKC-alpha isozyme and PKC activity from cytosolic and membrane fractions. Thus, differential actions of bryostatin 1 and TPA on PKC activity and alpha-isoform level in the membrane-associated fraction of MCF-7 and MDA-MB-468 cells may account for the divergent effects of these two agents on cell growth and morphology. These results suggest that the PKC-alpha isoform may specifically play a role in inhibiting growth of human breast cancer cells.