Abstract 569: Targeting UHRF1’s proteasomal destruction as a novel approach for small cell lung cancer treatment

Abstract Small cell lung cancer (SCLC) is an aggressive lung cancer subtype with a five-year survival rate of less than 8% due to rapid tumor growth, early metastasis, and a “cold” immune environment. Genetic alteration at the RB transcriptional corepressor 1 (RB1) is nearly universally present in SCLC patients. However, the therapeutic development was impeded by a lack of druggable oncogenic mutations. Studies in our laboratory identified ubiquitin-like, containing PHD and RING Finger domains 1 (UHRF1) as a critical protein overexpressed in many cancers with RB-pathway inactivation, including SCLC. Despite UHRF1 overexpression correlates with poor survival in SCLC patients, the roles of UHRF1 in SCLC progression remain to be elucidated. We hypothesized that UHRF1 overexpression is essential for SCLC tumor progression and can be targeted through endogenous proteasomal degradation. To define the role of UHRF1 in SCLC, we generated UHRF1 knockout and knockdown human SCLC cell lines. Using these mutants, we assessed changes in proliferation, clonogenicity, migration, and invasion capabilities. We report that UHRF1 overexpression present in human SCLC cell lines contributes to a more aggressive tumor presentation, affecting both tumor growth and metastasis. Besides human cell line models, we also generated genetically engineered mouse models of SCLC to study the role of Uhrf1 in tumor development, especially in an immune environment. Our results indicate that Uhrf1 overexpression promotes SCLC proliferation, angiogenesis, and pro-tumoral macrophage polarization, overall conditioning a tumor microenvironment that supports a more aggressive phenotype. Loss of Uhrf1 in this system results in a robust increase in overall survival. These results strongly support a critical role of UHRF1 in SCLC tumor progression and provide guidance for future SCLC therapeutics targeting UHRF1 or its key downstream effectors. One approach we apply is to utilize UHRF1’s endogenous degradative machinery by inhibiting its deubiquitinase USP7. In normal cells, UHRF1 abundance oscillates through cell cycle due to the regulation of ubiquitin machinery. USP7 cleaves ubiquitin marks on UHRF1, antagonizing UHRF1 degradation. We report that same as UHRF1, knocking out USP7 leads to decreased clonogenicity, migration, and invasion in human SCLC cell lines. Highly specific USP7 chemical inhibitors, XL-177A and FT671, were also tested and the same effects as USP7 knockout were observed. Collectively, our findings describe the oncogenic role of UHRF1 in SCLC proliferation, metastasis, and immune recognition and provide a therapeutic strategy of inhibiting USP7 for UHRF1 destruction. Citation Format: Yijun Gu, Claudia A. Benavente. Targeting UHRF1’s proteasomal destruction as a novel approach for small cell lung cancer treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 569.