Abstract 6790: Exploring sensitivity and resistance to proteasome inhibitors in <i>TP53</i> mutated non-small cell lung carcinoma

Abstract Background: Non-small cell lung cancer (NSCLC) is the most common histologic subtype of lung cancer (∼80%), with ∼50% of tumors carrying a missense mutation in the TP53 tumor suppressor gene (Onc-p53). These mutants often lead to the acquisition of oncogenic traits such as tumorigenesis and chemoresistance, as well as vulnerabilities that can be targeted for treatment, one of which being elevated basal levels of proteotoxic stress. We have previously shown that NSCLC cells expressing Onc-p53 exhibit elevated 20S proteasome activity which serves as a vulnerability for proteasome inhibitors (PIs) to further augment the proteotoxic stress levels, leading to the accumulation of reactive oxygen species (ROS). However, despite initial effectiveness, these Onc-p53 NSCLC cells develop resistance to PIs with the underlying mechanisms of this resistance remaining largely unknown. Methods and Results: We have elucidated that bortezomib (BTZ)-induced ROS initiates a cascade including the activation and nuclear translocation of NRF2, followed by the induction of ATF3 and NOXA, and ultimately apoptosis. Furthermore, the combination of BTZ with the dual BCL-XL/BCL-2 targeting BH3-mimetic compound (ABT-263) or a novel BH3-mimetic PROTAC (753B) synergizes with carboplatin, a standard of care chemotherapy, to selectively kill Onc-p53 NSCLC cells in vitro and in vivo. A second-generation PROTAC (WH244) also shows efficacy in vitro. We now show that the inductions of NRF2, ATF3, NOXA, and apoptosis are strongly suppressed in BTZ-resistant Onc-p53 NSCLC cells, generated by continuous culture in suboptimal doses of BTZ up to double its IC50. Interestingly, the combination of BH3-mimetics with BTZ showed cytotoxicity even in BTZ-resistant cells, suggesting that higher doses of BTZ can be used to overcome BTZ-resistance. Bulk-RNA sequencing of BTZ-sensitive vs. BTZ-resistant cells further identified the induction of PD-L1. This induction was abrogated by BTZ-resistant cells, suggesting that immune checkpoint activation may be playing a role in the resistance to PIs observed in previous clinical settings. Conclusion: In summary, our findings show that resistance to PIs such as BTZ in NSCLC tumors expressing Onc-p53 involves adaptations that reduce proteotoxic stress and may leverage immune checkpoint pathways. However, combining PIs with BH3-mimetics effectively restores cytotoxicity, even in BTZ-resistant cells. This suggests that targeting anti-apoptotic pathways alongside traditional therapies (e.g. the combination of BTZ, carboplatin, with either ABT-263 or 753B/WH244) could offer a powerful approach to overcome PI resistance in NSCLC, supporting the potential for PI-based precision combination therapies in NSCLC patients. Citation Format: Victoria Neely, Kranthi Kumar Chougoni, Bin Hu, Jennifer E. Koblinski, Guangrong Zheng, Daohong Zhou, Swati Palit Deb, Sumitra Deb, Jorge J. Nieva, Senthil K. Radhakrishnan, Steven R. Grossman, Hisashi Harada. Exploring sensitivity and resistance to proteasome inhibitors in TP53 mutated non-small cell lung carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6790.