microRNA-26a modulates inflammatory response induced by toll-like receptor 4 stimulation in microglia.

MiRNAs, a family of small non-coding RNAs, have emerged as novel post-transcriptional regulators of numerous cellular responses. Although the involvement of miRNAs in the regulation of neuroinflammation in various neurological diseases has been previously studied, their role in the production of inflammatory mediators during microglia activation is poorly understood. In this study, the role of miR-26a has been investigated in the modulation of inflammatory response in cultured microglia. Using real-time PCR, the expression of miR-26a was studied in toll-like receptors 4 stimulated primary mouse microglia. miR-26a expression was found to be rapidly reduced after the stimulation of toll-like receptors 4 in microglia. Over-expression of miR-26a significantly decreased the production of inflammatory cytokines such as tumor necrosis factor α and IL-6, whereas knockdown of miR-26a increased the expression of these mediators. Furthermore, using in silico analysis, we identified that the activating transcription factor (ATF) 2 is directly targeted by miR-26a. This finding was confirmed by loss and gain of function studies. Similar to the effect of miR-26a over-expression, knockdown of activating transcription factor 2 inhibited the production of proinflammatory cytokines, particularly IL-6. Taken together, our results suggest the involvement of miR-26a in the regulation of the production of proinflammatory cytokines in microglia. We proposed that in microglia, activation of toll-like receptor 4 (TLR4) by lipopolysaccharide (LPS) down-regulates miR-26a. The down-regulation of this miR increases expression of activating transcription factor 2 (ATF2). This event, in addition to the activation of ATF2 by c-Jun N-terminal kinase (JNK), increases interleukin-6 (IL-6) production. On the other hand, miR-26a also increases the production of tumor necrosis factor α (TNFα) by a mechanism independent of ATF2.