Abstract LB-260: Role of the glucocorticoid receptor/beta-catenin interaction in prostate cancer stem cell survival and chemoresistance
Authors: Martinez Shannalee R. , Osterman Carlos J. Diaz , Chen Xin S. , Wang Charles , Zhang Lubo , Casiano Carlos A. 
Published: August 2019

Abstract
Prostate cancer (PCa) is the second leading cause of cancer deaths in U.S. men, disproportionately affecting African American men and certain Hispanic/Latino populations. Patients with advanced PCa are treated with androgen deprivation therapy (ADT) and docetaxel (DTX)-based chemotherapy with effective but transient results due to therapy resistance. While the role of glucocorticoid receptor (GR) in promoting ADT resistance has been well characterized, comparatively little is known about its contribution to PCa stemness and chemoresistance. Our group demonstrated recently that DTX-resistance is associated with upregulation of a stemness transcriptomic program and enhanced cancer stem cell (CSC) properties in PCa cells. We also demonstrated that GR signaling upregulates the DTX-resistance associated proteins Clusterin and LEDGF/p75 in a panel of PCa cells. The proto-oncogene β-catenin promotes CSC survival in many cancers, including PCa, and several studies have demonstrated a crosstalk between AR and β-catenin signaling pathways contributing to ADT resistance. Given the structural similarities between AR and GR and the importance of GR in therapy resistance, this study was designed to investigate the interaction between GR and β-catenin and its contribution to CSC survival and DTX resistance. Using a panel of PCa cell lines including PC3, DU145, 22rv1, and their corresponding DTX-resistant cell lines, we showed by quantitative immunoblotting that GR is upregulated and β-catenin is activated in PC3 and 22rv1 DTX-resistant cells compared to sensitive cells. Co-immunoprecipitation studies revealed interaction between endogenous GR and β-catenin in these cells. We next assessed the effect of the potent GR ligand dexamethasone (Dex) on CSC maintenance, and observed that Dex treatment of PC3 cells increased tumorsphere number and expression of established CSC markers measured by multicolor flow cytometry. To identify potential GR target genes that could mediate the acquisition of CSC properties and DTX resistance, we performed RNA-sequencing analysis in four different Dex-treated and -untreated PCa cell lines. This analysis revealed several candidate GR target genes differentially regulated in a cell-type dependent manner. To determine the effects of the GR/β-catenin interaction on downstream GR target genes and β-catenin target genes, we performed immunoblotting of specific candidate target genes on lysates from cells treated with Dex alone or in combination with COR-10829 (selective GR inhibitor, and MSAB (β-catenin inhibitor). Interestingly, we also observed that the dual blockade of GR and β-catenin markedly reduced tumorsphere formation in DTX-resistant cells, a phenomenon not observed with either GR or β-catenin inhibition alone. In summary, our findings revealed a novel interaction between GR and β-catenin in PCa cells, and suggest that co-targeting the GR/β-catenin signaling axis in advanced PCa patients could be a potential strategy to overcome therapy resistance.
Citation Format: Shannalee R. Martinez, Carlos J. Diaz Osterman, Xin S. Chen, Charles Wang, Lubo Zhang, Carlos A. Casiano. Role of the glucocorticoid receptor/beta-catenin interaction in prostate cancer stem cell survival and chemoresistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-260.