Endocrine Abstracts

Androgens initially drive prostate tumour growth. Although in advanced disease there is no longer dependence on circulating androgens, the androgen receptor (AR) remains a key driver of this lethal stage thus new ways to inhibit its activity are required. MicroRNAs play vital roles in prostate cancer (PCa) development, progression and metastasis. Previous studies have examined microRNAs dysregulated in PCa, and also identified androgen-regulated microRNAs. We approached microRNAs in PCa from the other angle. Having previously identified microRNA-27a as an androgen-regulated microRNA that in turn affects AR signalling, inhibition of which inhibits PCa cell proliferation, we hypothesised that microRNAs modulating AR activity in lethal castration-resistant PCa represent novel therapeutic targets.

Such microRNAs were systematically identified using high-throughput screens to examine effects of ~1000 microRNA inhibitors on AR activity in hormone-responsive and -resistant PCa cell lines. Results were cross-referenced with a database of microRNAs impacting PCa cell growth. Eighty significantly altered AR activity, eight in both cell lines. Upon validation, inhibition of selected identified microRNAs significantly reduced AR activity up to 90%, accompanied by reduced AR mRNA/protein and AR target gene expression. At the cellular level it also increased apoptosis (up to 800%), and reduced cell growth, migration and invasion. Opposing effects were observed upon microRNA overexpression. Inhibition of AR-modulatory microRNAs showed additive effects with AR silencing or anti-androgen treatment, suggesting potential combinatorial applications for PCa treatment.

Pathway analysis of AGO-PAR-CLIP-identified mRNA targets of these microRNAs identified roles in DNA replication and repair, cell cycle, signal transduction and immune function. In addition, inhibition of AR-modulatory microRNAs induced epithelial markers, while reducing levels of mesenchymal markers. Other targets include purported tumour suppressors: silencing of these phenocopies effects of the microRNAs, confirming their physiological relevance. Interestingly, the microRNAs appear to upregulate certain oncogenes as well as AR itself, suggesting novel regulation of the AR 3’UTR and contrary to the accepted dogma that microRNAs reduce expression of their targets via translational repression and/or transcript degradation.

In summary, we have identified microRNAs that significantly modulate AR activity in models of hormone-responsive and castration-resistant PCa. Inhibitors of these dramatically reduce AR activity and growth, migration and invasion of PCa cells, thus represent potential novel PCa therapeutics. Given androgen regulation of microRNA expression, our previous findings that androgen signalling can regulate microRNA biogenesis, and the ‘hormomiR’ hypothesis that microRNAs themselves can mediate long-range effects, this supports two-way crosstalk between these highly influential systems.

DOI: 10.1530/endoabs.54.IS1