Post-transcriptional stimulation of gene expression by microRNAs.

MicroRNAs are small noncoding RNA regulatory molecules that control gene expression by guiding associated effector complexes to other RNAs via sequence-specific recognition of target sites. Misregulation of microRNAs leads to a wide range of diseases including cancers, inflammatory and developmental disorders. MicroRNAs were found to mediate deadenylation-dependent decay and translational repression of messages through partially complementary microRNA target sites in the 3'-UTR (untranslated region). A growing series of studies has demonstrated that microRNAs and their associated complexes (microRNPs) elicit alternate functions that enable stimulation of gene expression in addition to their assigned repressive roles. These reports, discussed in this chapter, indicate that microRNA-mediated effects via natural 3' and 5'-UTRs can be selective and controlled, dictated by the RNA sequence context, associated complex, and cellular conditions. Similar to the effects of repression, upregulated gene expression by microRNAs varies from small refinements to significant amplifications in expression. An emerging theme from this literature is that microRNAs have a versatile range of abilities to manipulate post-transcriptional control mechanisms leading to controlled gene expression. These studies reveal new potentials for microRNPs in gene expression control that develop as responses to specific cellular conditions.