Protostemonine (PSN) is the main anti-inflammatory alkaloid extracted from the roots of Stemona sessilifolia (known as "Baibu" in traditional Chinese medicine).Here,we reported the inhibitory effects of PSN on lipopolysaccharide (LPS)-induced macrophage activation in vitro and LPS-induced acute lung injury in mice.Macrophage cell line RAW264.7 cells and mouse bone marrow-derived macrophages (BMDMs) were treated with PSN (1,3,10,30 and 100 μmol/L) for 0.5 h and then challenged with LPS (0.1 μg/mL) for 24 h.Pretreatment with PSN significantly inhibited LPS-induced phosphorylation of MAPKs and AKT,iNOS expression and NO production in the macrophages.C57BL/6 mice were intratracheally injected with LPS (5 mg/kg) to induce acute lung injury (ALl).The mice were subsequently treated with PSN (10 mg/kg,ip) at 4 and 24 h after LPS challenge.PSN administration significantly attenuated LPS-induced inflammatory cell infiltration,reduced pro-inflammatory cytokine (TNF-α,IL-1β and IL-6) production and eliminated LPS-mediated lung edema.Furthermore,PSN administration significantly inhibited LPS-induced pulmonary MPO activity.Meanwhile,LPS-induced phosphorylation of p38 MAPK,iNOS expression and NO production in the lungs were also suppressed.The results demonstrate that PSN effectively attenuates LPS-induced inflammatory responses in vitro and in vivo;the beneficial effects are associated with the decreased phosphorylation of MAPK and AKT and the reduced expression of pro-inflammatory mediators,such as iNOS,NO and cytokines.These data suggest that PSN may be a potential therapeutic agent in the treatment of ALl.