MicroRNA-146a regulates both transcription silencing and translation disruption of TNF-α during TLR4-induced gene reprogramming

Following the TLR-dependent initiation phase of acute systemic proinflammatory responses such as sepsis, an adaptive phase represses or activates a specific pattern of gene expression until the inflammation resolves. Here, we used the THP-1 sepsis cell model of bacterial LPS/endotoxin tolerance to show that TLR4-induced miR-146a supports the feed-forward adaptive processes that silence transcription and disrupt translation of acute proinflammatory genes. First, we found that miR-146a regulates a pathway that promotes the binding of transcription repressor RelB to the TNF-alpha promoter, a step known to precede histone and DNA modifications, which generate facultative heterochromatin to silence acute proinflammatory genes. However, once RelB binding occurred, miR-146a inhibition could not reverse compacted chromatin, and endotoxin tolerance persisted. Second, we observed that miR-146a regulates a pathway that supports assembly of the translation repressor complex of TNF-alpha by preventing the interaction of the RNA-binding protein effector Ago2 and RBM4. We also determined that once endotoxin tolerance is established, and specific genes have been reprogrammed, transcription and translation disruption can be reversed only by simultaneously depleting RelB and inhibiting miR-146a. Thus, miR-146a induction supports the TLR4-dependent shift from initiation to gene-specific repression at two levels. Our results also imply that therapies designed to reverse endotoxin tolerance as potential therapies for sepsis should be directed at the transcription and translation pathways of reprogramming. J. Leukoc. Biol. 90: 509-519; 2011.