Toosendanin, A Novel Late-Stage Autophagy Inhibitor Sensitizes Triple-Negative Breast Cancer to Irinotecan cChemotherapy In Vitro and In Vivo

Abstract Background: Triple-negative breast cancer (TNBC) is a highly aggressive subtype of breast cancer that develops resistance to chemotherapy frequently. Autophagy has been regarded as a pro-survival response to chemotherapeutic drugs in TNBC, and suppression of autophagy can be a therapeutic strategy to overcome drug resistance. Methods: We verified the efficacy of TSN in blocking autophagy flux. The co-localization of autophagosomes and lysosomes was analyzed. Then, lysosome function was determined by pH value and the activity of hydrolytic proteases. For preclinical research, human triple-negative breast cancer MDA-MB-231 and MDA-MB-436 were used for evaluating the anti-proliferative effect in vitro. In vivo, the nude mice were intraperitoneal injection of irinotecan (10 mg/kg), TSN (0.5 mg/kg) or a combination. Autophagy activity and cell apoptosis were determined in tumor tissue. The degree of pathological injury of tissue was evaluated by liver index.Results: Here we reported the natural autophagy inhibitor toosendanin, a triterpenoid extracted from Melia toosenda Sieb. et Zucc, potently inhibited late-stage autophagy in TNBC cells. This effect was achieved via elevating lysosome pH rather than blocking the fusion of autophagosomes and lysosomes. We further investigated the effects of toosendanin on the in vitro and in vivo TNBC models, in combination with chemotherapeutic drug irinotecan (or its active metabolite SN-38), a topoisomerase I (TOP1) inhibitor showing therapeutic potential for TNBC. The data showed that toosendanin blocked SN-38/irinotecan-induced protective autophagy, and significantly induced apoptosis in TNBC cells and tumor xenograft models when compared to SN-38/irinotecan or TSN treatment alone group.