miR-221 activates the NF-κB pathway by targeting A20

MicroRNAs play an important role in regulating the inflammatory response, and are critically involved in the development of inflammatory disorders, including those affecting the lungs. While the microRNA miR-221 is involved in embryonic lung branching morphogenesis and epithelial cell development, its importance in lung inflammation has not been previously explored. In our current study, expression of miR-221 was selectively decreased by exposure to lipopolysaccharides (LPS) both in vitro and in vivo. Enforced expression of miR-221 significantly increased the production of proinflammatory cytokines TNF-α and IL-6, and enhanced the activation of NF-κB and MAPKs upon LPS stimulation. Accordingly, intratracheal stimulation of miR-221 was shown to aggravate endotoxin-induced acute lung injuries and inflammation in mice. Mechanistic studies showed that miR-221 directly targets A20, a master regulator of NF-κB and MAPK signaling, and thus represses inflammatory signaling. Restoration of A20 in macrophages abolished the stimulatory effect of miR-221 on production of proinflammatory cytokines. Together, these results indicate the presence of a novel miRNA-mediated feed-back mechanism that controls inflammation, and suggest involvement of aberrant miR-221 expression in the development of inflammatory lung disorders.