The proto-oncogene Pim-1 is a target of miR-33a

The constitutively active kinase Pim-1 is upregulated in different cancer types. So far, a regulation of oncogenic Pim-1 by miRNAs, which are small ncRNAs that regulate mainly all cellular processes at the post-transcriptional level, has not been reported. Tumor relevant miRNAs may either act as oncogenic miRNAs or exert tumor suppressor activity. Therefore, miRNA based strategies that uses miRNA mimics or antisense microRNAs (AntimiRs), generates the ability to regulate a whole network of co-interacting pathways.

We establish miR-33a as a miRNA with potential tumor suppressor activity due to its inhibitory effect on Pim-1. A screen for miRNA expression in several cancer cell lines revealed general low endogenous miR-33a levels relative to other miRNAs. Transfection of K562 and LS174T cells with miR-33a mimics reduced Pim-1 levels substantially, whereas the cell cycle regulator Cdk6, which is predicted to be a conserved miR-33a target, was not regulated by miR-33a mimics. Seed mutagenesis of the Pim-1 3'-UTR in a luciferase reporter construct and in a genuine Pim-1 cDNA demonstrated the specificity of the miR-33a-dependent downregulation. The persistence of this effect was comparable to that of a siRNA-mediated knockdown of Pim-1 and results in decelerated cell proliferation.

In conclusion, we demonstrate the potential of miR-33a to act as a tumor suppressor miRNA and identify an underlying mechanism based on deceleration of cell cycle progression upon downregulation of Pim-1, which suggests miR-33a replacement therapy through forced expression as a novel therapeutic strategy.