Abstract LB-050: Interferon signaling confers resistance to androgen deprivation therapy in advanced prostate cancer
Authors: Agarwal Supreet , McGowen Kerry , Elloumi Fathi , Cam Maggie , Beshiri Mike , Jansson Keith , Corey Eva , Kelly Kathleen 
Published: August 2018

Abstract
Loss of TP53 and PTEN activity are frequent genetic events in CRPC that are often associated with poorly differentiated, treatment resistant tumors. Using the Pten/Tp53-null mouse prostate cancer (PCa) model (Pb-Cre; Ptenfl/fl Tp53 fl/fl) and organoid cultures we previously have shown that a high proportion of luminal progenitors are intrinsically resistant to androgen deprivation therapy (ADT). To identify mechanisms of ADT resistance in luminal progenitors, we performed RNAseq analysis of luminal progenitor organoids derived from wild-type(WT) and Pten/Tp53-null mice. Pathway analysis identified key signaling alterations in luminal tumor organoids (AR signaling, lipid metabolism, protein secretion, inflammation etc.) that also have been described in FACS-purified human prostate luminal (CD49flo) fractions. Interestingly, we found no difference in transcriptional profiles from intact and previously castrated tumor organoids, suggesting that Pten/Tp53 null luminal progenitors are intrinsically resistance to ADT. Of note, we observed most significant enrichment of interferon(IFN) signaling in luminal progenitor tumor organoids relative to wild type luminal organoids. In other cancers and contrary to the anti-proliferative IFN signaling, expression of a subset of IFN genes contributes to genotoxic therapy resistance. This subset, referred to as IRDS (IFN-related DNA damage signature), includes a group of unphosphorylated STAT1 (U-STAT1)-driven genes that has a pro-survival function and promotes cancer cell intrinsic drug resistance. We hypothesize that the IRDS contributes to survival of PCa cells following ADT or genotoxic therapies. RT-PCR, immunofluorescence, and western blot analysis of luminal progenitor organoids showed higher U-STAT1 levels in tumor- than WT organoids, whereas pSTAT1 (Y701) was undetectable. Similarly, U-STAT1 was upregulated in Pten/Tp53-null tumor tissue relative to normal prostate. Further, for tumor organoids, ADT resulted in higher U-STAT1 levels. STAT1 depletion in tumor organoids decreased the number of progeny organoids in subsequent passages, suggesting a role in self-renewal for luminal tumor stem cells. Altogether, U-STAT1 regulated signaling has pro-survival function in Pten/Tp53-null advanced PCa. Several PDXs originating from metastatic PCa (LuCaPs 23.1, 96 and 141) express high levels of U-STAT1 dependent IFN signaling. STAT1 depletion significantly reduced self-renewing ability of these PDX derived organoids, consistent with mouse luminal tumor organoids. Interestingly, in LuCaP 141, STAT1 depletion synergized with ADT to inhibit growth ex vivo. These findings suggest that U-STAT1 dependent IFN signaling may contribute to castration resistance. We subsequently analyzed human PCa datasets to determine clinical correlates of IRDS. Analysis of the TCGA primary PCa cohort revealed IRDS as a prognostic marker for progression (n= 500, p&lt;0.05). Further, high IRDS-expressing samples in the CRPC dataset were enriched for low AR signaling (n = 150, r= -0.33, p&lt; 0.05). CRPC patients expressing the highest IRDS levels (n=30 of 150) showed enrichment for genes associated with stem cell features and therapy resistance. Overall, our findings suggest U-STAT1 dependent IRDS expression is correlated with poorly differentiated PCa and may contribute to intrinsic therapy resistance.
Citation Format: Supreet Agarwal, Kerry McGowen, Fathi Elloumi, Maggie Cam, Mike Beshiri, Keith Jansson, Eva Corey, Kathleen Kelly. Interferon signaling confers resistance to androgen deprivation therapy in advanced prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-050.