GREB1 amplifies androgen receptor output in prostate cancer and contributes to antiandrogen resistance

Genomic amplification of the androgen receptor (AR) is an established mechanism of antiandrogen resistance in prostate cancer. Here we show that the magnitude of AR signaling output, independent of AR genomic alteration or expression level, also contributes to antiandrogen resistance, through upregulation of the coactivator GREB1. We demonstrate 100-fold heterogeneity in AR output within cell lines and show that cells with high AR output have reduced sensitivity to enzalutamide. Through transcriptomic and shRNA knockdown studies, together with analysis of clinical datasets, we identify GREB1 as a gene responsible for high AR output. We show that GREB1 is an AR target gene that amplifies AR output by enhancing AR DNA binding and promoting p300 recruitment. GREB1 knockdown in high AR output cells restores enzalutamide sensitivity in vivo. Thus, GREB1 is a candidate driver of enzalutamide resistance through a novel feed forward mechanism. RNA-seq was performed on following samples, LNCaP prostate cancer cells with 1) low or high AR activity (AR-low vs AR-high), 2) high AR activity infected with shRNA against Renilla or GREB1, treated with vehicle or DHT (AR-high shRenilla veh or DHT vs. AR-high shGREB1 veh or DHT)