Cholesterol-dependent molecular mechanisms contribute to cationic amphiphilic drugs' prevention of silica-induced inflammation.

Silicosis is considered an irreversible chronic inflammatory disease caused by the inhalation of crystalline silica (cSiO). The cycle of inflammation that drives silicosis and other particle-caused respiratory diseases is mediated by NLRP3 inflammasome activity in macrophages resulting in the release of IL-1β. Lysosomal membrane permeability (LMP) initiated by inhaled particles is the key regulatory step in leading to NLRP3 activity. In addition to its role in LMP, the lysosome is crucial to cellular cholesterol trafficking. Lysosomal cholesterol has been demonstrated to regulate LMP while cationic amphiphilic drugs (CADs) reduce cholesterol trafficking from lysosomes and promote endolysosomal cholesterol accumulation as seen in Niemann Pick disease. Using a bone marrow derived macrophage (BMdM) model, four CADs were examined for their potential to reduce cSiO-induced inflammation. Here we found that FDA-approved CAD drugs imipramine, hydroxychloroquine, fluvoxamine, and fluoxetine contributed to reduced LMP and IL-1β release in cSiO treated BMdM. These drugs inhibited lysosomal enzymatic activity of acid sphingomyelinase, decreased lysosomal proteolytic function, and increased lysosomal pH. CADs also demonstrated a significant increase in lysosomal-associated free cholesterol. Increased lysosomal cholesterol was associated with a significant reduction in cSiO induced LMP and IL-1β release. In contrast, reduced lysosomal cholesterol significantly exacerbated cSiO-induced IL-1β release and reduced the protective effect of CADs on IL-1β release following cSiO exposure. Taken together, these results suggest that CAD modification of lysosomal cholesterol may be used to reduce LMP and cSiO-induced inflammation and could prove an effective therapeutic for silicosis and other particle-caused respiratory diseases.