Arsenic trioxide decreases AKT protein in a caspase-dependent manner

Arsenic trioxide (AS(2)O(3)) is used clinically to treat acute promyelocytic leukemia but is less successful in other malignancies. To identify targets for potential combination therapies, we have begun to characterize signaling pathways leading to AS(2)O(3)-induced cytotoxicity. Previously, we described the requirement for a reactive oxygen species-mediated, SEK1/c-Jun NH2-terminal kinase (JNK) pathway to induce apoptosis. AKT inhibits several steps in this pathway; therefore, we postulated that AS(2)O(3) might decrease its activity. Indeed, AS(2)O(3) decreases not only AKT activity but also total AKT protein, and sensitivity to AS(2)O(3) correlates with the degree of AKT protein decrease. Decreased AKT expression further correlates with JNK activation and the release of AKT from the JNK-interacting protein 1 scaffold protein known to assemble the mitogen-activated protein kinase cascade. We found that AS(2)O(3) regulates AKT protein stability without significant effects on its transcription or translation. We show that AS(2)O(3) decreases AKT protein via caspase-mediated degradation, abrogated by caspase-6, caspase-8, caspase-9, and caspase-3 inhibitors but not proteosome inhibitors. Furthermore, AS(2)O(3) enhances the ability of a heat shock protein 90 inhibitor to decrease AKT expression and increase growth inhibition. This suggests that AS(2)O(3) may be useful in combination therapies that target AKT pathways or in tumors that have constitutively active AKT expression.