Short Term Acid Sphingomyelinase Deficiency Exerts Proinflammatory and Antiapoptotic Effects During LPS-induced Lung Injury in Mice

Lysosomal acid sphingomyelinase (ASM; SMPD1) deficiency causes Niemann-Pick Disease (NPD) that, in Type B, manifests with interstitial lung disease and susceptibility to infections. Constitutional Smpd1 (Smpd1-/- mice) deletion causes lung inflammation with foamy dysfunctional macrophages, but is protective against acute lung injury. It is unknown whether these manifestations are from progressive accumulation of sphingomyelin, decreased ceramide, or compensatory alterations in sphingolipid metabolism. We developed a conditional knockout mouse, Smpd1fl/flxCAGG-CreERTM, induced by tamoxifen (5 weeks), with decreased Smpd1 expression (by 75%) and ASM activity (by up to 40%). We investigated how brief post-developmental ASM insufficiency affects lung sphingolipids and pathology including following lipopolysaccharide (LPS)-induced injury. Compared to controls, Smpd1fl/fl mice exhibited modest sphingomyelin elevation with lower palmitoyl/lignoceroyl ceramide (C16/C24) ratios, increased de novo sphingolipid synthesis and sphingosine-1 phosphate levels. At 3 days following LPS instillation (20μg) control mice had increased lung (neutrophilic and monocytic) inflammation and apoptosis; Smpd1fl/fl mice showed more exuberant inflammation with reduced apoptosis, particularly in endothelial cells. During repair (6-9 days), Smpd1fl/fl lungs had increased cell proliferation with reduced autophagosome-tagging p62/SQSTM1. These results indicate that prior to significant lysosomal lipid storage, ASM insufficiency inhibits stress-induced lung apoptosis and promotes compensatory sphingolipid changes that favor exuberant inflammatory responses to LPS. Overall, ASM inhibition limits lung vascular injury and stimulates repair following inflammatory insults. These results provide novel insights into the function of ASM in the lung, which are relevant to understanding the pathogenesis and complications of NPD and the role of distinct sphingolipid metabolites in lung injury and repair.