Non-Specific Blocking of miR-17-5p Guide Strand in Triple Negative Breast Cancer Cells by Amplifying Passenger Strand Activity

Conventional wisdom holds that only one of the two strands in a micro ribonucleic acid (miRNA) precursor duplex is selected as the active miRNA guide strand. The complementary miRNA passenger strand, however, is thought to be inactive. High levels of the oncogenic miRNA (oncomiR) guide strand called miR-17-5p is overexpressed in triple negative breast cancer (TNBC) and can inhibit ribosomal translation of tumor suppressor gene mRNAs, such as programmed cell death 4 (PDCD4) or phosphatase and tensin homolog (PTEN). We hypothesized that knocking down the oncogenic microRNA (oncomiR) miR-17-5p might restore the expression levels of PDCD4 and PTEN tumor suppressor proteins, illustrating a route to oligonucleotide therapy of TNBC. Contrary to conventional wisdom, antisense knockdown of oncomiR miR-17-5p guide strand reduced PDCD4 and PTEN proteins by 1.8 +/- 0.3 fold in human TNBC cells instead of raising them. Bioinformatics analysis and folding energy calculations revealed that mRNA targets of miR-17-5p guide strand, such as PDCD4 and PTEN, could also be regulated by miR-17-3p passenger strand. Due to high sequence homology between the antisense molecules and miR-17-3p passenger strand, as well as the excess binding sites for the passenger strand on the 3'UTR of PDCD4 and PTEN mRNAs, introducing a miR-17-3p DNA-LNA mimic to knock down miR-17-5p reduced PDCD4 and PTEN protein expression instead of raising them. Our results imply that therapeutic antisense sequences against miRNAs should be designed to target the miRNA strand with the greatest number of putative binding sites in the target mRNAs, while minimizing affinity for the minor strand.