Abstract 5904: Post-transcriptional regulation of androgen receptor by DDX3 in prostate cancer progression
Authors: Vellky Jordan E. , Ricke William 
Published: August 2018

Purpose

Prostate cancer (CaP) driven by androgen receptor (AR) can be targeted therapeutically by androgen deprivation therapy (ADT); however, in 10-20% of cases, ADT fails, allowing disease recurrence. Mechanisms for recurrence include dysregulation of AR at the gene, RNA, and protein level. Recently, AR negative cell growth has been implicated as a mechanism of resistance. This study aims to identify mechanisms of drug resistance in castration-resistant prostate cancer (CRPC) that can be exploited therapeutically to reduce disease recurrence.

Experimental Procedures

Using the BCaP model, a novel series of cell lines derived from a single human prostate epithelial cell, we can study the progression of CaP from benign to metastasis. Additionally, using the LNCaP-C4 series we can study progression to CRPC. To determine the RNA binding capacity of DDX3, we utilized 1) RIP-qPRC for identification of mRNA targets and 2) RNAscope for visual colocalization. Changes in RNA and protein expression were determined using qPCR, Western blot, and IF.

Results

DDX3 is an ATP-dependent RNA helicase that can aid translation of target mRNAs, or in stress conditions, form ribonucleoprotein granules that prevent translation. While DDX3 is implicated in several cancers, its role as a translational regulator in CaP remains unstudied. In BCaP and LNCaP-C4, DDX3 protein expression increases through progression, concurrent with localization to cytoplasmic puncta. RIP-qPCR identified AR as an mRNA target of DDX3 in the metastatic and CRPC cell lines. Because of the dual role of DDX3 in translational control, AR expression in these models was investigated; while AR mRNA expression increased through progression, AR protein expression decreased. Treatments in vitro with RK33, a commercially available inhibitor of DDX3, restored the protein expression of AR and downstream AR signaling (PSA, Nkx3.1). These data suggest DDX3 acts as a translational repressor of AR in metastasis and CRPC. To confirm that increased DDX3 expression is sufficient to reduce AR protein expression, we overexpressed DDX3 using two models: induced cellular stress and a DDX3 expression vector. Here, we saw DDX3 localized to puncta, and subsequent reduction in AR and PSA protein. Because DDX3 is sufficient to reduce AR protein expression in metastases and CRPC, co-treatment of DDX3 inhibitors with anti-androgen therapy may prevent AR negative cell growth underlying recurrence. in vitro co-treatments with RK33 and bicalutamide, an AR antagonist, show decreased proliferation compared to either treatment alone, suggesting increased efficacy of bicalutamide with DDX3 inhibition.

Conclusions

DDX3 as a repressor of AR translation could have clinical implications as a mechanism of resistance to ADT. Based on preliminary data, DDX3 could be contributing to the regulation of AR post-transcriptionally, and targeting DDX3 could reduce resistance and disease recurrence.

Citation Format: Jordan E. Vellky, William Ricke. Post-transcriptional regulation of androgen receptor by DDX3 in prostate cancer progression [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 5904.