The role of microRNAs in cell death pathways

MicroRNAs (miRNAs or miRs) are small (20-23 nucleotides) noncoding RNAs that negatively regulate messenger RNAs (mRNA) expression by blocking translation or by directly promoting degradation of the target mRNA [1].It is estimated that miR-NAs regulate approximately 30% of the protein-coding genes in humans.Individual miRNAs can regulate the expression of multiple genes, and conversely, a single target gene can be regulated by many miRNAs [1].More than half of the miRNA genes are commonly located in introns.miRNAs are produced by endogenously transcribed long primary miRNA (pri-miRNA) by RNA polymerase (Pol) II or Pol III and show a typical mRNA form with a 5'-cap structure and a 3'poly (A) tail [2,3].The pri-miRNAs are further processed into a hairpin-shaped structure, 60 to 100 nucleotides in length, known as precursor miRNA (pre-miRNA) by the nuclear RNase III Drosha.This nuclear processing event is site-specific and critical for determining the mature miRNA sequence.After being exported to the cytoplasm by double-stranded RNA-binding proteins (exportin-5), pre-miRNAs undergo a final processing event by RNase III family enzymes (Dicer) to form a 20-nucleotides miRNA duplex.