Adding fuel to fire: microRNAs as a new class of mediators of inflammation.

MicroRNAs (miRNAs) are recently discovered regulators of gene expression, and early studies have indicated that they have a role in the regulation of haematopoiesis, the immune response and inflammation. They bind the 3'UTR of target mRNAs and mainly prevent translation of the protein product. Dysregulation of these molecules has been shown to be a hallmark of cancer and now investigators are examining their role in the pathogenesis of inflammatory diseases. miR-146 and miR-155 have been a particular focus for investigators, and these two miRNAs have been shown to be induced by proinflammatory stimuli such as interleukin 1, tumour necrosis factor alpha (TNFalpha) and Toll-like receptors (TLRs). They have also been detected in synovial fibroblasts and rheumatoid synovial tissue. Both have multiple targets, with miR-146 inhibiting TLR signalling and miR-155 regulating Th1 cells and also, interestingly, positively regulating mRNA for TNFalpha. The potential of miRNAs for improving our understanding of the pathogenesis of diseases such as rheumatoid arthritis, and for developing potentially new treatments for these diseases, is substantial.