Differential induction of apoptosis in human breast tumor cells by okadaic acid and related inhibitors of protein phosphatases 1 and 2A.
To investigate a possible relationship between apoptosis induction and protein phosphorylation in human breast carcinoma cells, we treated three such cell types, MB-231, MCF-7, and AU-565, with okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A, or phorbol 12 myristate 13-acetate, an activator of protein kinase C. We then examined these cells for the appearance of apoptosis markers. While OA caused multiplication arrest and cytotoxicity in all three cell lines, apoptosis was induced in MB-231 and MCF-7 cells but not in AU-565 cells. A similar cell-specific apoptosis induction was also observed after treatment with dinophysistoxin-1 (an active OA analogue) and with calyculin A (a structurally unrelated protein phosphatase inhibitor) but not with analogues that either are inactive or penetrate epithelial cells poorly. Phorbol 12-myristate 13-acetate also inhibited cell multiplication but was without effect in inducing apoptosis in these cells. Levels of the apoptosis-inhibitory protein BCL2 were examined in these cells, but they did not correlate with this differential susceptibility. We additionally treated the three cell types with 1-beta-D-arabinofuranosylcytosine and genistein to determine whether the AU-565 cell line would also be resistant to apoptosis induction by other chemical stimuli. Both of these agents led to the induction of apoptosis in all three cell lines. These results indicate that the AU-565 cells are specifically resistant to apoptosis induction by inhibitors of protein phosphatases 1 and 2A. This cell-specific resistance may thus allow one to identify cellular mediators of apoptosis by comparing protein phosphorylation patterns in these cells before and after treatment with OA or related inhibitors.