NAC1 is a nuclear factor belonging to the BTB gene family. The BTB domain of NAC1 is responsible for its homodimerization and interaction with other proteins to form a complex that plays an important role in maintaining the pluripotency of ES cells and in the development of chemo-resistance in ovarian cancer cells. Elucidating the molecular mechanisms underlying how NAC1 contributes to drug resistance is fundamental to improve clinical outcome in recurrent post-chemotherapy ovarian cancer patients. The purpose of this study was to determine whether autophagy, a conserved response to several types of stresses including therapeutic stress, was involved in NAC1- mediated resistance to cisplatin, a commonly used chemotherapeutic drug in the treatment of ovarian cancer. We found that treatment with cisplatin caused a robust activation of autophagy in NAC1-expressing ovarian cancer cell lines including A2780, OVCAR3, and SKOV3, based on the alteration of LC3-II and p62 by Western blot analysis, and the presence of GFP-LC3 puncta by microscopic observation. We further demonstrated that knockdown of NAC1 expression by RNA interference or inactivation of NAC1 by inducing the expression of NAC1 deletion mutant that contains only the BTB domain significantly inhibited the cisplatin-induced autophagy, resulting in increased cisplatin cytotoxicity as measured by MTT viability assay. Moreover, we observed that inhibition of autophagy and sensitization to cisplatin by NAC1 knockdown or inactivation were accompanied by induction of apoptosis, as evidenced by increases in Annexing V staining and in the levels of pro-apoptotic proteins, caspase-3 and PARP. To confirm that the sensitizing effect of NAC1 inhibition on the cytotoxicity of cisplatin was mediated through suppression of autophagy, we assessed the effects of autophagy inhibitors, 3-MA and chloroquine, on the cytotoxicity of cisplatin. Treatment of cancer cells with 3-MA and chloroquine also enhanced the sensitivity of SKOV3, A2780 and OVCAR3 cells to cisplatin, indicating that suppression of autophagic response indeed renders the NAC1-expressing ovarian cancer cells more sensitive to cisplatin. The regulation of autophagy by NAC1 appeared to be mediated via HMGB1 (high mobility group box1), as NAC1 could regulate the expression, translocation and release of HMGB1, which is known to activate autophagy by disrupting the interaction between beclin 1 and bcl-2. The results of our study not only revealed a new mechanism governing cisplatin sensitivity but also identified NAC1 as a novel regulator of autophagy. Thus, the NAC1-mediated autophagy may be exploited as a new target for enhancing the efficacy of cisplatin against ovarian cancer or other types of malignancies. Animal studies are in progress to evaluate the impact of NAC1 on antitumor efficacy of cisplatin in tumor xenograft models.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2877. doi:10.1158/1538-7445.AM2011-2877