BACKGROUND AND PURPOSEBetulinic acid (BA) is a naturally occurring triterpenoid widely distributed throughout the plant kingdom. We previously reported that BA inhibits lipopolysaccharide (LPS)-induced interleukin-6 production through modulation of nuclear factor kB (NF-kB) in human peripheral blood mononuclear cells (hPBMCs). This study attempted to identify other mechanisms through which BA modulates LPS signalling in mononuclear cells. The effects of BA on signalling pathways downstream were focused on in this study. EXPERIMENTAL APPROACHWe determined the ability of BA to interfere with p38 and extracellular regulated kinase (ERK) phosphorylation as well as Akt phosphorylation and nuclear factor-kB activation using LPS-activated hPBMCs as an in vitro model. LPS-induced endotoxin shock in mice was the in vivo model employed. KEY RESULTSBA inhibited LPS-induced COX-2 protein expression and prostaglandin E2 production and also attenuated LPS-induced ERK and Akt phosphorylation, but not p38 in hPBMCs. BA abolished LPS-induced IkBa phosphorylation and thus normalized the levels of IkBa in cytosol. BA also inhibited LPS-induced reactive oxygen species formation and lactate dehydrogenase release. Interestingly, BA improved the life span of mice in endotoxin shock and also inhibited PGE2 production and myeloperoxidase activity in vivo. CONCLUSIONS AND IMPLICATIONSBA modulates LPS-induced COX-2 expression in hPBMCs by inhibiting ERK and Akt pathways as well as by modulating IkBa phosphorylation. At the same time, no cell toxicity was observed. The effect of the drug was confirmed through in vivo experiments. The study gives an insight into the molecular mechanisms of BA. Abbreviations