Abstract 4409: Exploring PBX1's role in enzalutamide resistance in advanced prostate cancer

Aggressive prostate cancer (PCa) is often driven by upscaled activity of the androgen receptor (AR), and second-generation AR antagonists such as Enzalutamide have become standard therapeutics for men with advanced PCa. While patients initially respond to treatment, many develop resistance, with disease progression and poor prognosis. We have focused on a mechanism involving an AR cistrome switch from a canonical to a non-canonical cistrome as PCa becomes Enzalutamide-resistant. As collaborating protein partners drive transcriptional activity, we employed core regulatory circuitry analysis in Enzalutamide-resistant PCa cell line models to identify novel Transcription Factor (TF) candidates that may be involved in non-canonical AR cistrome reprogramming. One identified candidate is Pre B-Cell Leukemia Homeobox 1 (PBX1), a TALE homeodomain TF involved in embryonic development and cellular differentiation. PBX1 is known to work with other TFs to modulate gene expression and has been implicated as an oncogenic driver in hematologic and solid cancers, including PCa. Our group has shown that PBX1 is highly expressed in prostate cancer cell line models of castrate- and Enzalutamide-resistance compared to Enzalutamide-sensitive models. Through multi-ome single-cell ATAC + gene expression analysis of a heterogenous patient-derived xenograft (PDX) model, we identified clusters of cells that are enriched for PBX1, AR, and non-canonical AR target CREB5, while simultaneously expressing low levels of KLK3, a prominent canonical AR target. Based on these findings, we hypothesize that PBX1 works in tandem with AR to drive non-canonical AR cistrome and mediate Enzalutamide resistance in advanced PCa. To test this hypothesis, we first determined the impact of silencing PBX1 activity in Enzalutamide-resistant PCa models. By targeting PBX1 pharmacologically in our cell lines and PDX models, we have observed an anti-proliferative effect specifically in a castration-resistant, Enzalutamide-resistant cell line model. This effect was not observed when treating an Enzalutamide-sensitive cell line model, which has low PBX1 expression. Our observations were consistent across several models and our ongoing mechanistic studies will provide further insight into PBX1’s exclusive role in Enzalutamide-resistant CRPC. In addition, we are currently assessing the impact of inhibition or perturbation of PBX1 in Enzalutamide-resistant PCa cell lines and PDX models on canonical and non-canonical AR cistrome activity using multi-omic approaches. Citation Format: Ephraim J. Gardner, Surendra Gulla, Leslie McClinton, Jonathan E. Bard, Remi Adelaiye-Ogala. Exploring PBX1's role in enzalutamide resistance in advanced prostate cancer [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 4409.