Lipopolysaccharide (LPS), one of the most prominent pathogen-associated molecular patterns (PAMPs), activates macrophages, causing release of toxic cytokines (i.e. tumor necrosis factor (TNF)-alpha) that may provoke inflammation and endotoxin shock. Here, we tested the potential role of trametinib, a novel and highly potent MAPK/ERK kinase (MEK) inhibitor, against LPS-induced TNF-alpha response in monocytes, and analyzed the underlying mechanisms. We showed that trametinib, at nM concentrations, dramatically inhibited LPS-induced TNF-alpha mRNA expression and protein secretion in transformed (RAW 264.7 cells) and primary murine macrophages. In ex-vivo cultured human peripheral blood mononuclear cells (PBMCs), this MEK inhibitor similarly suppressed INF-alpha production by LPS. For the mechanism study, we found that trametinib blocked LPS-induced MEK-ERK activation in above monocytes, which accounted for the defective TNF-alpha response. Macrophages or PBMCs treated with a traditional MEK inhibitor PD98059 or infected with MEK1/2-shRNA lentivirus exhibited a similar defect as trametinib, and nullified the activity of trametinib. On the other hand, introducing a constitutively-active (CA) ERK1 restored TNF-alpha production by LPS in the presence of trametinib. In vivo, mice administrated with trametinib produced low levels of TNF-alpha after LPS stimulation, and these mice were protected from LPS-induced endotoxin shock. Together, these results show that trametinib inhibits LPS-induced TNF-alpha expression and endotoxin shock probably through blocking MEK-ERK signaling. (C) 2015 Elsevier Inc. All rights reserved.