The oncogenic lncRNA MIR503HG suppresses cellular senescence counteracting supraphysiological androgen treatment in prostate cancer

Background: The androgen receptor (AR), a ligand-dependent transcription factor, plays a key role in regulating prostate cancer (PCa) growth. The novel bipolar androgen therapy (BAT) uses supraphysiological androgen levels (SAL) that suppresses growth of PCa cells and induces cellular senescence functioning as a tumor suppressive mechanism. The role of long non-coding RNAs (lncRNAs) in the regulation of SAL-mediated senescence remains unclear. This study focuses on the SAL-repressed lncRNA MIR503HG, examining its involvement in androgen-controlled cellular senescence in PCa.

Methods: Transcriptome and ChIP-Seq analyses of PCa cells treated with SAL were conducted to identify SAL-downregulated lncRNAs. Expression levels of MIR503HG were analyzed in 691 PCa patient tumor samples, mouse xenograft tumors and treated patient-derived xenografts. Knockdown and overexpression experiments were performed to assess the role of MIR503HG in cellular senescence and proliferation using senescence-associated β-Gal assays, qRT-PCRs, and Western blotting. The activity of MIR503HG was confirmed in PCa tumor spheroids.

Results: A large patient cohort analysis shows that MIR503HG is overexpressed in metastatic PCa and is associated with reduced patient survival, indicating its potential oncogenic role. Notably, SAL treatment suppresses MIR503HG expression across four different PCa cell lines and patient-derived xenografts but interestingly not in the senescence-resistant LNCaP Abl EnzaR cells. Functional assays reveal that MIR503HG promotes PCa cell proliferation and inhibits SAL-mediated cellular senescence, partly through miR-424-5p. Mechanistic analyses and rescue experiments indicate that MIR503HG regulates the AKT-p70S6K and the p15^INK4b-pRb pathway. Reduced expression of MIR503HG by SAL or knockdown resulted in decreased BRCA2 levels suggesting a role in DNA repair mechanisms and potential implications for PARP inhibitor sensitivity by SAL used in BAT clinical trial.

Conclusions: The lncRNA MIR503HG acts as an oncogenic regulator in PCa by repressing cellular senescence. SAL-induced suppression of MIR503HG enhances the tumor-suppressive effects of AR signaling, suggesting that MIR503HG could serve as a biomarker for BAT responsiveness and as a target for combination therapies with PARP inhibitors.