Elevated levels of FOXA1 facilitate androgen receptor chromatin binding resulting in a CRPC-like phenotype

Castration-resistant prostate cancer (CRPC) continues to pose a significant clinical challenge with new generation second line hormonal therapies affording limited improvement in disease outcome. As the androgen receptor (AR) remains a critical driver in CRPC, understanding the determinants of its transcriptional activity is important for developing new AR targeted therapies. FOXA1 is a key component of the AR transcriptional complex yet its role in prostate cancer progression and the relationship between AR and FOXA1 are not completely resolved. It is well established that FOXA1 levels are elevated in advanced prostate cancer and metastases. We mimicked these conditions by over-expressing FOXA1 in the androgen-responsive LNCaP prostate cancer cell line and observed a significant increase in AR genomic binding at novel regions that possess increased chromatin accessibility. High levels of FOXA1 resulted in increased proliferation at both sub-optimal and high 5α-dihydrotestosterone (DHT) concentrations. Immunohistochemical staining for FOXA1 in a clinical prostate cancer cohort revealed that high FOXA1 expression is associated with shorter time to biochemical recurrence after radical prostatectomy (HR 5.0, 95% CI 1.2-21.1, p=0.028), positive surgical margins and higher stage disease at diagnosis. The gene expression program that results from FOXA1 over-expression is enriched for PTEN, Wnt and other pathways typically represented in CRPC gene signatures. Together these results suggest that in an androgen-depleted state, elevated levels of FOXA1 enhance AR binding at genomic regions not normally occupied by AR, which in turn facilitates prostate cancer cell growth.