Sphingomyelin Metabolism Impacts on Crystalline Silica-Induced Outcomes

Abstract There are numerous examples of endogenous and exogenous particle-induced chronic inflammation. One model of chronic environmentally-induced inflammation is driven by exposure to crystalline silica particles. There are various occupational lung diseases that arise from exposure to exogenous particles such as asbestos fibers and crystalline silica (cSiO2) leading to chronic inflammation and lung fibrosis, however there are insufficient treatment options. A commonality of particle-induced inflammation is lysosome membrane permeabilization (LMP), in macrophages. Previous studies have shown that cSiO2-induced LMP allows lysosomal enzymes to leak out into the cytosol where they can trigger cell death pathways and activate the NLRP3 inflammasome, a multiprotein complex that is responsible for the processing and release of IL-1β from macrophages. IL-1β is a potent proinflammatory cytokine that is involved in nearly all models of chronic inflammation and subsequent diseases. Therefore, understanding the mechanisms of cSiO2-induced LMP and downstream IL-1β release could provide information for identifying novel therapies that target chronic inflammation. The goal of this proposal is to further elucidate the interactions between cSiO2 particles and the inner lysosomal membrane leading to LMP with the ultimate goal of blocking LMP and downstream inflammation. Current research suggests that sphingomyelin metabolism mediates numerous vital and pathological cell processes. My preliminary results suggest that disruptions in sphingomyelin metabolism can prevent cSiO2-induced IL-1β release in macrophages and change the lipid content of isolated lysosomes by increasing sphingomyelin and cholesterol levels while decreasing ceramide. However, critical details are still needed to fully characterize the involvement of sphingomyelin metabolism in cSiO2-induced inflammation. Furthermore, the biophysical changes induced by cSiO2 on model membranes have been described but not in isolated lysosomes. Therefore, the studies proposed in this work will elucidate the biophysical changes caused by cSiO2 in isolated lysosome membranes that result in LMP and determine the involvement of sphingomyelin metabolism by comprehensively analyzing changes to the lipid profile of isolated lysosomes using a lipidomics approach. Finally, by using multiple inhibitory techniques to disrupt sphingomyelin metabolism at specific steps, previously unexplained mechanistic details of cSiO2- induced inflammation and cell death will be determined. Taken together, these novel studies will provide new information on mechanisms of particle-induced activation of macrophages that will provide potential new therapeutic targets.