Abstract 1937: Quinacrine-induced autophagy in ovarian cancer triggers cathepsin-L mediated lysosomal/mitochondrial membrane permeabilization and cell death

Background: Lysosomes are acidic organelles that contain numerous cathepsins and acid hydrolases. Lysosomal membrane permeabilization (LMP) and the presence of cathepsins in the cytosol triggers intrinsic apoptotic pathway through cathepsin L (CTSL) mediated degradation of critical proteins. This process is mediated by the proteolytic activation of the pro-apoptotic molecule Bid, resulting in mitochondrial outer membrane permeability (MOMP), cyt c release and then, cell death. Based on our previous study that quinacrine (QC) induces autophagy in ovarian cancer, the current study is aimed to investigate the potential role of cathepsins in QC induced LMP and MOMP in ovarian cancer. Methods: MagicRed and Mitotracker Red staining was used to assess the QC induced CTSL activation and LMP. To identify the role of specific cathepsin in QC mediated cytotoxicity, different specific cathepsin inhibitors pepstatin A, CA-074Me, Z-FY(tBU)-DMK were used and cell death was assessed by clonogenic assay, PARP cleavage and annexin v-PI staining. Using ATG5 and/or CTSL knockdown cells and pharmacological inhibition with 3-MA/chloroquine and/or CTSL overexpression clones, QC-induced flux in CTSL activation was determined using several different approaches including evaluating autophagy flux using tandem dual labeled RFP/GFP LC3BII by immunoflourescence analysis. Recombinant CTSL was used to validate identify p62 as novel substrate of CTSL. QC-induced autophagy and CTSL activation were validated by western blot analysis in HeyA8MDR xenograft tumors. Results: QC significantly upregulates cathepsin L (CTSL) but not cathepsin B and D levels implicating the specific role of CTSL in promoting QC-induced autophagic flux and apoptotic cell death in OC cells. We found that QC-induced LMP and CTSL activation promotes Bid cleavage, MOMP, and mitochondrial cytochrome c release. Genetic (shRNA) and pharmacological (Z-FY(tBU)-DMK) inhibition of CTSL markedly reduces QC-induced autophagy, LMP, MOMP, apoptosis, and cell death; whereas ectopic expression of CTSL in ovarian cancer cell lines has an opposite effect. Using recombinant CTSL, we identified p62/SQSTM1 as a novel substrate of CTSL suggesting that CTSL plays a role in QC-induced autophagic flux. CTSL activation is specific to QC-induced autophagy since no CTSL activation is seen in ATG5 knockdown cells or with autophagy inhibitor chloroquine. Importantly we show upregulation of CTSL in QC treated HeyA8MDR xenografts corresponds with attenuation of p62, upregulation of LC3BII, cytochrome c, tBid, cleaved PARP, and caspase3. Conclusion: Taken together, the data suggests that QC-induced autophagy and CTSL upregulation promotes a positive feedback loop leading to excessive autophagic flux, LMP, MOMP to promote QC-induced cell death in ovarian cancer cells. Citation Format: Prabhu Thirusangu, Christopher L. Pathoulas, Upasana Ray, Yinan xiao, Julie Staub, Ashwani Khurana, Vijayalakshmi Shridhar. Quinacrine-induced autophagy in ovarian cancer triggers cathepsin-L mediated lysosomal/mitochondrial membrane permeabilization and cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1937.