Oncogenic Mutant p53 Sensitizes Non-Small Cell Lung Cancer Cells to Proteasome Inhibition via Oxidative Stress-Dependent Induction of Mitochondrial Apoptosis.

Non-small cell lung cancer (NSCLC) cells with oncogenic mutant p53 alleles (Onc-p53) exhibit significantly higher levels of proteasome activity, indicating that Onc-p53 induces proteotoxic stress which may be leveraged as a therapeutic vulnerability. Proteasome inhibitors (PIs) are most active in cells under proteotoxic stress, so we investigated whether PIs exhibit preferential cytotoxicity in Onc-p53 NSCLC cells. Indeed, BTZ and other PIs exhibited IC50 values 6-15-fold lower in Onc-p53 cells vs. wild-type (WT) p53 cells. BTZ cytotoxic effects in Onc-p53 cells were abrogated by antioxidants such as N-acetyl L-cysteine, indicating that oxidative stress is the critical driver of BTZ-dependent cytotoxic effects in Onc-p53 cells. Importantly, we observed oxidative stress-dependent transcriptional induction of the pro-apoptotic BH3-only protein NOXA leading to cleavage of caspase-3, consistent with an apoptotic mechanism of cell death in Onc-p53 but not in WT p53 cells treated with BTZ. BTZ-generated oxidative stress was linked to nuclear translocation of NRF2 and transcriptional activation of ATF3, which in turn was required for NOXA induction. Validating BTZ's translational potential in Onc-p53 NSCLC cells, BTZ and the BH3-mimetic navitoclax were synergistically cytotoxic in Onc-p53 but not WT p53 cells in vitro, and BTZ effectively limited growth of Onc-p53 NSCLC xenografts when combined with navitoclax and carboplatin (a standard of care chemotherapeutic in NSCLC) in vivo. Our data therefore support further investigation of the therapeutic utility of PIs combined with BH3-mimetics and chemotherapy in Onc-p53 human NSCLC as a novel therapeutic strategy.