Perturbation of Autophagy by a Beclin 1-Targeting Stapled Peptide Induces Mitochondria Stress and Inhibits Proliferation of Pancreatic Cancer Cells

SIMPLE SUMMARY: Autophagy has long been regarded as playing both pro- and anti-proliferative roles in many types of cancer, including PDAC. As the autophagy inhibitor CQ failed to show therapeutic efficacy in PDAC clinical trials, it is worth exploring whether the alternative approach, i.e., elevation of the autophagic activity, can exert an anti-tumor effect. The aim of our study was to assess whether a Beclin 1-targeting stapled peptide can exert an anti-proliferative effect in PDAC by perturbing the already elevated autophagy process. Our study reports for the first time that the Beclin 1-targeting stapled peptide Tat-SP4 potently inhibited the proliferation of PDAC cells through the combined effect of excessive autophagy, enhanced endolysosomal degradation of EGFR and significant mitochondria stress. Tat-SP4 induced non-apoptotic cell death in PDAC cells, which is in distinct contrast to apoptosis induced by CQ. In summary, the perturbation of the autophagy process by Tat-SP4 may serve as a novel therapeutic for PDAC. ABSTRACT: Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer, with a dismal five-year survival rate of less than 10%. PDAC possesses prominent genetic alterations in the oncogene KRAS and tumor suppressors p53, SMAD4 and CDKN2A. However, efforts to develop targeted drugs against these molecules have not been successful, and novel therapeutic modalities for PDAC treatment are urgently needed. Autophagy is an evolutionarily conserved self-degradative process that turns over intracellular components in a lysosome-dependent manner. The role of autophagy in PDAC is complicated and context-dependent. Elevated basal autophagy activity has been detected in multiple human PDAC cell lines and primary tumors resected from patients. However, clinical trials using chloroquine (CQ) to inhibit autophagy failed to show therapeutic efficacy. Here we show that a Beclin 1-targeting stapled peptide (Tat-SP4) developed in our lab further enhanced autophagy in multiple PDAC cell lines possessing high basal autophagy activity. Tat-SP4 also triggered faster endolysosomal degradation of EGFR and induced significant mitochondria stress as evidenced by partial loss of Δψ, increased level of ROS and reduced OXPHOS activity. Tat-SP4 exerted a potent anti-proliferative effect in PDAC cell lines in vitro and prohibited xenograft tumor growth in vivo. Intriguingly, excessive autophagy has been reported to trigger a unique form of cell death termed autosis. Tat-SP4 does induce autosis-like features in PDAC cells, including mitochondria stress and non-apoptotic cell death. Overall, our study suggests that autophagy perturbation by a Beclin 1-targeting peptide and the resulting autosis may offer a new strategy for PDAC drug discovery.