Abstract 4038: Oncogenic ETS transcription factors in prostate cancer and Ewing sarcoma are interchangeable when avoiding repression by a PRC2/FOXO1 complex

Abstract ETS transcription factors play roles throughout development, aiding in hematopoiesis, blood vessel formation and cell fate. The ETS family is composed of 28 members, all of which share an ETS DNA-binding domain. Chromosomal rearrangements that lead to the overexpression of specific ETS promote oncogenic transformation and tumor development. More than half of prostate tumors are driven by aberrant expression of an ETS protein. Our lab has demonstrated that oncogenic ETS form essential interactions with a ubiquitous RNA-binding protein, EWS, to promote prostate tumorigenesis. A similar mechanism exists in Ewing sarcoma (ES). ES tumors are driven by the expression EWS/ETS fusion proteins. This project focuses on how ETS and EWS/ETS act as oncogenic transcription factors; furthermore, we identify a novel ERG/PRC2/FOXO1 regulatory complex that may be exploited for therapeutic benefit. We compared biological functions of oncogenic ETS found in prostate cancer (ERG, ERG(S96E), ETV1, ETV4, ETV5) and EWS/ETS fusions in ES (EWS/FLI1, EWS/ERG). All oncogenic ETS promoted migration and clonogenic survival in normal prostate epithelial cell lines, RWPE1 and PNT2. Knockdown of endogenous EWS/FLI1 in the ES cell line, A673, reduced anchorage-independent colony formation in soft agar. Exogenous expression of EWS/FLI1, ETV1, ETV4, and ETV5 rescued A673colony formation, however ERG and EWS fused to full-length ERG were unable to rescue this phenotype. Our lab has established that ERG can act as a transcriptional repressor through the formation of an ERG/PRC2 complex. Pulldown assays using purified proteins and A673 lysates revealed that ERG and EWS fused to full-length ERG interact with EZH2, a subunit of the PRC2, and FOXO1, a tumor suppressor that is thought to repress ERG transcriptional activity. Both the N-terminal truncation of ERG and phosphorylation of ERG at S96 disrupted these interactions and rescued colony formation in A673. Further immunoprecipitation assays suggest that a FOXO1/PRC2 complex can exist independent of ETS expression. ERG is thought to cooperate with mutations that promote prostate tumorigenesis through activation of the PI3K/AKT pathway. Our lab has observed increases in clonogenic survival and tumorigenesis upon expression of both ERG and a constitutively- active mAKT in RWPE1. AKT negatively regulates FOXO1 expression. We aim to evaluate the effects of FOXO1 expression in facilitating ERG/PRC2 interactions. Furthermore, we aim to explore the effects of Ipatasertib, an AKT inhibitor in clinical trials, on the ERG/PRC2/FOXO1 complex and ERG-dependent phenotypes in prostate cancer cell models. Citation Format: Nicholas Frederick Downing, Katelyn M. Mills, Peter C. Hollenhorst. Oncogenic ETS transcription factors in prostate cancer and Ewing sarcoma are interchangeable when avoiding repression by a PRC2/FOXO1 complex [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 4038.