Presentation Authors: Jordan Vellky*, William Ricke, Madison, WI
Introduction: 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. AR heterogeneity is a negative prognostic marker, and AR negative cell growth has recently been implicated as a mechanism of therapy resistance. DDX3, an RNA helicase, can both aid and prevent translation of target mRNAs depending on localization. While DDX3 is implicated in cancer, its role as a translational regulator in CaP remains unstudied. Our objective was to investigate DDX3-mediated translational regulation of AR, and its contribution to the development of castration-resistant prostate cancer (CRPC).
Methods: Using human tissue microarrays, we quantified expression of AR and DDX3 in CaP progression. To assess functional implications, we utilized the BCaP and LNCaP-C4 models of CaP progression to CRPC. To determine the RNA binding capacity of DDX3, we utilized 1) RNA immunoprecipitation (RIP) and 2) RNAscope. Effects of loss-of-function (siRNA, small molecule inhibitor) and gain-of-function (overexpression, ADT) experiments were assessed with qPCR, Western blot, and IF.
Results: In patient samples and cell line models, DDX3 protein expression increased through progression, concurrent with localization to cytoplasmic puncta. RIP experiments identified AR as an mRNA target of DDX3 in the metastatic and CRPC cell lines. To determine the implications of DDX3 binding to AR mRNA, AR expression was investigated; while AR mRNA expression increased through progression, AR protein expression decreased. Loss-of-function experiments 1) reduced DDX3 punctate localization, 2) increased AR protein, and 3) increased PSA. Conversely, with DDX3 gain-of-function we saw DDX3 localized to puncta, reduction in AR protein, and reduction in PSA. These data suggest DDX3 acts as a translational repressor of AR in metastasis and CRPC; therefore, co-treatment of DDX3 inhibitors with ADT may prevent AR negative cell growth underlying recurrence. in vitro co-treatment with DDX3 inhibitors and antiandrogens showed decreased proliferation and increased apoptosis compared to either treatment alone, suggesting increased efficacy of antiandrogens 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 translationally, and targeting DDX3 could reduce resistance and disease recurrence.
Source of Funding: T32 CA009135 (JEV)1U54DK104310-01 (WAR)