Arsenic trioxide induces a beclin-1-independent autophagic pathway via modulation of SnoN/SkiL expression in ovarian carcinoma cells

Arsenic trioxide (As₂O₃), used to treat promyelocytic leukemia, triggers cell death through unknown mechanisms. To further our understanding of As₂O₃-induced death, we analyzed its effects on transforming growth factor-Β (TGFΒ) signaling mediators in ovarian cells. Dysregulated TGFΒ signaling is a characteristic of ovarian cancers. As₂O₃ reduced the protein expression of EVI1, TAK1, SMAD2/3, and TGFΒRII while increasing SnoN/SkiL. EVI1 protein was modulated by treatment with the proteasome inhibitors, MG132 and PS-341/Velcade, suggesting that degradation occurs through the ubiquitin-proteasome pathway. The sensitivity of ovarian cells to As₂O₃-induced apoptosis correlated with expression of multidrug resistance protein 1. Interestingly, expression of SnoN was similar to LC3-II (autophagy marker), which increased with induction of cytoplasmic vacuolation preceding apoptosis. These vesicles were identified as autophagosomes based on transmission electron microscopy and immunofluorescence staining with EGFP-LC3. The addition of N-acetyl-L-cysteine (ROS scavenger) to As₂O₃-treated cells reversed changes in SnoN protein and the autophagic/apoptotic response. In contrast to beclin-1 knockdown, siRNA targeting ATG5, ATG7, and hVps34 markedly reduced autophagy in As₂O₃-treated ovarian carcinoma cells. Further, treatment with SnoN siRNA markedly decreased LC3-II levels and increased PARP degradation (an apoptosis marker). Collectively, these findings suggest that As₂O₃ induces a beclin-1-independent autophagic pathway in ovarian carcinoma cells and implicates SnoN in promoting As₂O₃-mediated autophagic cell survival.