Using the organoid culture system and aggressive Pten/Tp53-null mouse model (GEMM), we have previously shown that the prostate cancer cell population harbors two classes of self-renewing luminal progenitors which are resistant to in vivo castration and to androgen receptor (AR) inhibitors (enzalutamide) ex vivo. Understanding signaling pathways governing intrinsic survival/self-renewal ability of luminal progenitors in castrate conditions can highlight pathways that play a role in acquired drug resistance. To identify mechanisms of castration resistance in luminal progenitors, we performed RNAseq analysis of luminal progenitor organoids derived from wild-type(WT) and Pten/Tp53-null mice (intact and castrated (two weeks), n=5; each). Gene enrichment analysis identified key signaling pathways altered in luminal tumor organoids (AR signaling, lipid metabolism, protein secretion, inflammation etc.) that have also been described for FACS-purified human prostate luminal (CD49flo) fraction. Interestingly, we found no difference in transcriptional profiles of intact and castrated tumor organoids, suggesting intrinsic survival ability of luminal progenitors upon castration. Of note, we observed most significant enrichment of STAT1-dependent IRDS in luminal progenitor tumor organoids relative to wild type luminal organoids. IRDS comprises of a subset of STAT1-driven genes that have been previously associated with survival of cancer cells and with breast cancer therapy resistance. Our analysis of human prostate cancer datasets revealed IRDS as a prognostic marker for progression in the TCGA primary prostate cancer cohort (p<0.05). Further, high IRDS-expressing CRPC samples (SU2C dataset) were enriched for low AR signaling (r= -0.33, p< 0.05). CRPC patients in IRDS-hi cohort showed enrichment for cancer stem cell phenotype and for genes associated with drug resistance, Consistent with the bioinformatics analysis real time PCR, immunofluorescence and western blot analysis of ex-vivo organoid cultures of castration-resistant Pten/Tp53-null tumor organoids showed higher protein expression of STAT1 and IRDS genes in luminal tumor organoids relative to luminal WT organoids. In vivo, castrated prostate tumors showed higher STAT1 levels than the intact tumors. Treatment with enzalutamide of luminal tumor organoids resulted in time dependent increase in STAT1 expression. STAT1 KD in tumor organoids decreased number of progeny organoids in subsequent generations suggesting either a direct or indirect effect upon self-renewal. Overall, our initial findings suggest STAT1 dependent signaling as a potential mechanism of androgen-independent survival in prostate cancer.

Citation Format: Supreet Agarwal, Kerry McGowen, Keith Jansson, Mike Beshiri, Fathi Elloumi, Maggie Cam, Kathy Kelly. STAT1 dependent interferon-related DNA damage resistance signature (IRDS) as a survival mechanism in castrate resistant prostate cancer (CRPC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2896. doi:10.1158/1538-7445.AM2017-2896