Abstract 4890: Arsenic trioxide induces apoptosis through the activation of ASK1 and is enhanced by a thioredoxin reductase inhibitor

Abstract

Abstract Arsenic trioxide (ATO) is an effective treatment for acute promyelocytic leukemia (APL). While it is undergoing clinical trials for numerous malignancies including multiple myeloma, myelodysplastic syndrome, lymphoma and solid tumors, it has demonstrated only limited efficacy as a single agent. However, it may hold promise as part of a combination therapy. Thus investigation to elucidate the mechanisms of action underlying these clinical responses may lead to generation of rational combination therapies to increase its therapeutic spectrum. Previous work has described a pathway required for ATO-induced apoptosis in APL cells involving the generation of reactive oxygen species (ROS), and the subsequent induction of a specific mitogen-activated protein kinase (MAPK) cascade that includes both stress-activated protein kinase (SAPK)/ERK kinase 1 (SEK1) and c-Jun N-terminal kinases (JNK) activation. However, the link between ROS production and activation of SEK1 remains to be elucidated. Apoptosis signaling kinase 1 (ASK1) is a MAP3K upstream of SEK1 and has been implicated in the induction of stress-induced signaling. Here, we show that ATO activates ASK1 in a dose- and time-dependent manner in an APL cell line. ASK1 activation is preceded by ATO-induced ROS accumulation, which is significantly reduced with N-acetyl-L-cysteine antioxidant co-treatment. Murine embryonic fibroblasts (MEFs) from ASK1 deficient mice are less susceptible to ATO-induced apoptosis and growth inhibition. Moreover, they exhibit markedly lower ATO-induced JNK activity compared to wild type cells. One model of ASK1 regulation suggests that ASK1 is kept in an inactive form by reduced thioredoxin-1 (Trx1). During oxidative stress, Trx1 is oxidized and releases ASK1 for activation. Immunoprecipitation of ASK1 followed by immuoblotting for Trx1 in APL cells shows a strong basal association that is lost with ATO treatment. Furthermore, the activity of thioredoxin reductase 1 (TrxR1), an enzyme that converts oxidized Trx1 into reduced Trx1, is significantly decreased following ATO treatment. This suggests that ATO activates ASK1 signaling by ROS-mediated oxidation of Trx1 and by inhibiting the restoration of reduced Trx1 by decreasing TrxR1 activity. In addition, we show that inhibition of TrxR1 with the TrxR1 inhibitor Au(III)Cl sensitizes APL cells to ATO-induced apoptosis. This suggests that regulation of ASK1 is dependent on Trx1 redox states. Overall, our results suggest that targeting Trx1 may enhance ASK1 signaling and ATO-induced apoptosis in a novel combination therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4890. doi:1538-7445.AM2012-4890