Prostate cancer (PC) is generally dependent on the androgen signaling axis for growth. Advanced PC is managed by androgen deprivation therapy (ADT). However, the tumors frequently progress by restoring AR signaling that is androgen-independent or hypersensitive to it, leading to castration resistant prostate cancer (CRPC). CRPC is also supported by hormone-independent actions of AR splice variants which lack the ligand binding domain. Additionally, ADT has many undesirable side effects on a variety of normal tissues that depend on androgen for non-growth related functions, including effects on the cardiovascular system, central nervous system, bone and muscle. Therefore, a more strategic therapy approach would be to disrupt a functional arm of AR-signaling that is critical for PC/CRPC growth but not for the essential physiological roles of AR in normal tissues. Previous studies in this laboratory have identified ELK1 as an AR tethering protein essential for activation of a critical set of androgen/AR growth genes in various PC model cell line models, both in vitro and in vivo, including those resistant to castration and enzalutamide. It was also established that the N-terminal A/B domain of AR binds to ELK1 by co-opting the two ERK docking sites on ELK1 to constitutively activate genes enriched for cell growth functions in PC cells. A lead candidate drug molecule (KCI807) was discovered that bound to AR, blocking its association with ELK1 and inhibiting PC/CRPC tumor growth. KCI807 binds to the A/B domain in AR or its splice variants and has a limited target gene set (a subset of AR target genes) that is primarily and highly enriched for functions in cell cycle progression and mitosis. To further our drug development efforts, we must identify and characterize the ELK1 docking site recognition sites in the AR A/B domain and the binding site of KCI807. We applied a mammalian two-hybrid assay to map the ELK1 interacting motifs in the A/B domain of AR using a series of deletion constructs. Overlapping deletion constructs were made systematically to cover the entire A/B domain. For each of these constructs additional double fusion constructs were used as controls. Expression of fusion proteins was monitored by western blot. Our results excluded a role for the amino-terminal half of the A/B domain in ELK1 interactions but did identify repressor motifs within this region. We identified two docking site recognition sites in the AR A/B domain for the two docking sites in ELK1 in a parallel configuration. The KCI807 binding site was located in the downstream site of AR. Further structural and functional studies of the mapped motifs in AR will reveal more about the biology of the AR-ELK1 interaction and directly aid in the design of next generation antagonists to selectively target ELK1-dependent growth signaling by AR in PC/CRPC tumors.

Citation Format: Claire L. Soave, Rayna Rosati, Yanfang Huang, Manohar Ratnam. Elucidating the structure-function relationships in the interactions of the androgen receptor, ELK1 and the platform antagonist KCI807 in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2469.