Down-regulation of miR-301a suppresses pro-inflammatory cytokines in Toll-like receptor-triggered macrophages

In many types of tumours, especially pancreatic adenocarcinoma, miR-301a is over-expressed. This over-expression results in negative regulation of the target gene of miR-301a, the nuclear factor-κB (NF-κB) repressing factor (NKRF), increasing the activation of NF-κB and production of NF-κB-responsive pro-inflammatory cytokines such as interleukin-8, interferon-β, nitric oxide synthase 2A and cytochrome oxidase subunit 2 (COX-2). However, in immune cells, mechanisms that regulate miR-301a have not been reported. Similar to tumour cells, Toll-like receptor (TLR) -activated macrophages produce NF-κB-responsive pro-inflammatory cytokines. Therefore, it is of considerable interest to determine whether miR-301a regulates the secretion of cytokines by immune cells. In the present study, we demonstrate that the expression of miR-301a was decreased in TLR-triggered macrophages. Through targeting NKRF, miR-301a affected the activity of NF-κB and the expression of pro-inflammatory genes downstream of NF-κB such as COX-2, prostaglandin E2 and interleukin-6. In addition, when lipopolysaccharide-treated macrophages were simultaneously stimulated with trichostatin A, an inhibitor of histone deacetylases, the expression of miR-301a increased, whereas NKRF and pro-inflammatory cytokine expression decreased. However, further investigation revealed that there was no correlation between the induction of miR-301a and the inhibitory effect of trichostatin A on lipopolysaccharide-induced gene expression in macrophages. In summary, our study indicates a new mechanism by which miR-301a regulates inflammatory cytokine expression in macrophages, which may clarify the regulatory role of microRNAs in immune-mediated inflammatory responses.