Inhibition of androgen receptor transactivation function by adenovirus type 12 E1A undermines prostate cancer cell survival.

BACKGROUND: Mutations or truncation of the ligand-binding domain (LBD) of androgen receptor (AR) underlie treatment resistance for prostate cancer (PCa). Thus, targeting the AR N-terminal domain (NTD) could overcome such resistance.
METHODS: Luciferase reporter assays after transient transfection of various DNA constructs were used to assess effects of E1A proteins on AR-mediated transcription. Immunofluorescence microscopy and subcellular fractionation were applied to assess intracellular protein localization. Immunoprecipitation and mammalian two-hybrid assays were used to detect protein-protein interactions. qRT-PCR was employed to determine RNA levels. Western blotting was used to detect protein expression in cells. Effects of adenoviruses on prostate cancer cell survival were evaluated with CellTiter-Glo assays.
RESULTS: Adenovirus 12 E1A (E1A12) binds specifically to the AR. Interestingly, the full-length E1A12 (266 aa) preferentially binds to full-length AR, while the small E1A12 variant (235 aa) interacts more strongly with AR-V7. E1A12 promotes AR nuclear translocation, likely through mediating intramolecular AR NTD-LBD interactions. In the nucleus, AR and E1A12 co-expression in AR-null PCa cells results in E1A12 redistribution from nuclear foci containing CBX4 (also known as Pc2), suggesting a preferential AR-E1A12 interaction over other E1A12 interactors. E1A12 represses AR-mediated transcription in reporter gene assays and endogenous AR target genes such as ATAD2 and MYC in AR-expressing PCa cells. AR-expressing PCa cells are more sensitive to death induced by a recombinant adenovirus expressing E1A12 (Ad-E1A12) than AR-deficient PCa cells, which could be attributed to the increased viral replication promoted by androgen stimulation. Targeting the AR by E1A12 promotes apoptosis in PCa cells that express the full-length AR or C-terminally truncated AR variants. Importantly, inhibition of mTOR signaling that blocks the expression of anti-apoptotic proteins markedly augments Ad-E1A12-induced apoptosis of AR-expressing cells. Mechanistically, Ad-E1A12 infection triggers apoptotic response while activating the PI3K-AKT-mTOR signaling axis; thus, mTOR inhibition enhances apoptosis in AR-expressing PCa cells infected by Ad-E1A12.
CONCLUSION: Ad12 E1A inhibits AR-mediated transcription and suppresses PCa cell survival, suggesting that targeting the AR by E1A12 might have therapeutic potential for treating advanced PCa with heightened AR signaling.