| Apoptosis is important for tissue homeostasis and is the mechanism of cell death induced by many anticancer agents. The apoptotic machinery is present in all cells, such that tight regulation must exist to prevent untimely cell and tissue death. To avert apoptotic death, cells utilize survival signaling pathways including the PI3 kinase/Akt and MEK/ERK pathways. Inhibitors of serine/threonine protein phosphatases can inhibit drug-induced apoptosis, implicating these phosphatases in regulation of apoptotic pathways. Here, we determined which protein phosphatases (PP) are critical for this protection from apoptosis, and investigated the relationship between phosphatases and survival pathways.; An apoptotic signal can be delivered through a receptor pathway or via drug-induced stress that triggers death via mitochondrial events. Chemical inhibitors of protein phosphatases, calyculin A, okadaic acid and tautomycin, prevented anisomycin-induced apoptosis, a model of the latter pathway. Concentrations of the inhibitors that prevented apoptosis correlated with inhibition of PP2A. The phosphatase inhibitors prevented Bax translocation from cytosol to mitochondria, indicating that the inhibition occurs further upstream in the mitochondrial pathway of apoptosis. Prevention of apoptosis by okadaic acid is Akt-dependent, but independent of ERK activity and correlated with phosphorylation of Bad at serine 136, which prevents its pro-apoptotic activity.; The protein phosphatase inhibitors also inhibited apoptosis induced through the Fas death receptor. Concentrations that prevented apoptosis correlated with inhibition of PP1. The inhibitors prevented DISC assembly, and association of Fas with cytoskeleton. Characterization of a novel PP1-selective inhibitor, tautomycetin, confirmed our conclusions that PP1 inhibition is not required to inhibit chemical-induced apoptosis, but is involved in prevention of Fas-mediated apoptosis.; These results demonstrate that inhibition of PP1 and PP2A can protect from receptor and mitochondrial-mediated pathways of death, respectively. These differential effects have helped to define critical steps in each pathway, and possible regulatory roles for PP1 and PP2A in apoptotic pathways. The results also highlighted the importance of Akt survival signaling in modulating apoptotic responses. These results may help to identify novel targets for therapeutic drug design through better understanding of how cells balance survival and apoptotic signals. |
