Systematic analysis of microRNAs targeting the androgen receptor in prostate cancer cells

Androgen receptor (AR) is expressed in all stages of prostate cancer progression and its levels increase in the castration-resistant tumors. However, the mechanisms regulating AR expression remain poorly understood. Here, our aim was to determine the role of miRNAs in regulating AR protein. To this end, we performed a systematic gain-of function screen for 1129 miRNA molecules in prostate cancer cell lines and quantified changes in AR protein content using protein lysate microarrays. We identified 71 unique miRNAs that influenced the level of AR in LNCaP, LAPC-4, 22Rv1, CWR-1R and MDA-PCa-2B cells. Our RNA-sequencing data showed that the 3'UTRs of many genes, including the AR, are much longer than currently used by target prediction programs. Our analyses predicted that the majority of the miRNA regulation of AR would occur through the extended 6 kb 3'UTR. 3'UTR-binding assays validated 13 miRNAs (miR-135b, miR-185, miR-297, miR-299-3p, miR-34a, miR-34c, miR-371-3p, miR-421, miR-449a, miR-449b, miR-634, miR-654-5p, and miR-9) as direct AR regulators. Fifteen AR down-regulating miRNAs decreased androgen-induced proliferation of prostate cancer cells in vitro. Negative correlation of miR-34a and miR-34c expression with AR levels was also found in clinical prostate cancers. To conclude, our results demonstrate that miRNAs interacting with the long 3'UTR of the AR gene are important regulators of AR protein levels. This could be applied for developing novel therapeutic strategies to inhibit AR function and androgen-dependent cell growth.