Abstract 3224: Identifying the molecular mechanisms underlying progesterone receptor downregulation in endometrial cancer
Authors: Meng Xiangbing , Lamont Lillie , Yang Shujie 
Published: March 2024

Abstract
Background: Endometrial cancer (EC) is the most common gynecologic cancer. Cancer survival has improved over the past four decades for all the most common cancers except cervical and EC, calling for significant treatment advancements. Progesterone receptor (PR) is a favorable prognostic marker for EC and other solid tumors. However, PR expression has been shown to be reduced or lost in these malignant tumors, which correlates with worse patient survival and limits the effectiveness of progestin therapy. Thus, restoring functional PR expression is vital to sensitizing tumor cells to progestin therapy, enhancing EC clinical interventions, and improving patient outcomes. In this project, we aim to restore functional PR expression.
Methods: To visualize PR expression in real-time, we constructed an innovative endogenous PR reporter gene transfected EC cells. This system facilitated screening potential small molecular PR inducers from the FDA-approved oncology drug library. Sixteen drugs were identified to increase PR expression; topoisomerase II inhibitors (Top IIi), mitoxantrone and idarubicin were the top hits. Drug effect of Top IIi, as well as HDAC inhibitor romidepsin, dual PI3k/HDAC inhibitor CUDC-907, and PHB2 inhibitors, fluorizoline (FLZ) and rocaglamide (RocA), were evaluated as single drug or in combination. Potential PR repressors, including G9a, HDAC2, PHB2, and Top2A, were knocked down using siRNA, shRNA, or CRISPR-Cas9 in EC cells and PR expression was analyzed. A chromatin immunoprecipitation (ChIP) assay was also conducted to validate bona fide PR repressors.
Results: Mitoxantrone and idarubicin were validated to be effective PR inducers. G9a, HDAC2, PHB2, and Top2A were confirmed PR repressors. Single treatment with mitoxantrone, idarubicin, romidepsin, CUDC-907, FLZ, and RocA achieved effective inhibition of EC cell proliferation and enhanced PR expression except FLZ and RocA. Combined romidepsin with Topi or PHB2i demonstrated additive or synergistic effects. More importantly, dual treatment of CUDC-907 with mitoxantrone enhanced PR expression compared to a single treatment or no treatment control. Our mass spectrometry and ChIP assay by specific antibodies against HDAC2 and PHB2 revealed that potential mechanisms include PHB2 forming a complex with HDAC2 that binds on the PR promoter, thus inhibiting PR transcription.
Conclusion: Co-treatment of CUDC-907 with Top2Ai increased PR expression and enhanced cell death in EC cell lines, indicating a potential treatment strategy. In vivo studies utilizing our EC-PDX models will further validate the efficacy of restoring PR expression and suppressing EC tumor growth. Further studies of the molecular mechanisms behind this strategy will be conducted.
Citation Format: Xiangbing Meng, Lillie Lamont, Shujie Yang. Identifying the molecular mechanisms underlying progesterone receptor downregulation in endometrial cancer [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 3224.