Abstract A072: Dissecting the mechanisms of dependency of stem-cell-like castration resistant prostate cancer on FOS proto-oncogenes and MAPK signaling

Abstract Stem cell-like castration resistant prostate cancer (SCL-CRPC) is a major subtype of double-negative prostate cancer (DNPC) that is resistant to conventional and next-generation of androgen signaling inhibition. SCL-CRPC is characterized by a distinct chromatin accessibility signature enriched with motifs for the AP-1 family of transcription factors. Here, we seek to identify the members of the AP-1 family that are critical to the growth and survival of SCL-CRPC. To inhibit AP-1 activity, we induced the expression of the dominant negative AFOS (constructs kindly provided by Jude Owiredu in Chris Barbieri lab at Weill Cornell) in SCL models and observed growth suppression, suggesting a dependency of this disease subtype on AP-1. Using in-house developed SCL prostate organoids (MSKPCa series) and conventional cell lines, we knocked down AP-1 factors individually or in combination via siRNAs and found that FOS proto-oncogenes FOSL1 and cFOS are required for the rapid growth of these models. Given that there is known crosstalk between MAPK signaling and these FOS proto-oncogenes, we hypothesize that SCL-CPRC also depends on MAPK signaling for growth. Pharmacological inhibition of MEK via trametinib in SCL models resulted in rapid downregulation of cFOS, FOSL1 and TAZ transcripts in as early as 1hrs in our RNA-Seq, suggesting that MAPK signaling positively relates to the core SCL-CRPC transcriptional program. MEK inhibition also reduces proteins levels in 4hrs and 24hrs. Surprisingly, MEK inhibition also led to G1 arrest in DU145 which has RB loss, leading to new investigations in the cell cycle status of our SCL model in response to MAPK inhibition as well as FOS proto-oncogene knock-down. To identify upstream regulators of FOSL1 and cFOS, we will perform a CRISPR sgRNA screen for E3 ligases targeting these FOS proto-oncogenes to the proteosome. In summary, we have determined the role of the FOS proto-oncogenes FOSL1 and cFOS as critical drivers of SCL-CRPC. We will investigate its interaction with MAPK signaling as well as regulators of its protein stability and assess the impact of its inhibition via AFOS on cell cycle in vitro and in vivo. With these mechanisms clearly studied, we would be able to give strategic insights for drug targeting. Citation Format: Hongsu Wang, Chen Khuan Wong, Dana Schoeps, Yu Chen. Dissecting the mechanisms of dependency of stem-cell-like castration resistant prostate cancer on FOS proto-oncogenes and MAPK signaling [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Innovations in Prostate Cancer Research and Treatment; 2026 Jan 20-22; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(2_Suppl):Abstract nr A072.