Cigarette smoke extract induces foam cell formation by impairing machinery involved in lipid droplet degradation.

The formation of foam cells, lipid-loaded macrophages, is the hallmark event of atherosclerosis. Since cigarette smoking is a risk factor for developing atherosclerosis, the current study investigated the effects of cigarette smoke extract (CSE) on different events like expressions of genes involved in lipid influx and efflux, lipophagy, etc., that play vital roles in foam cell formation. The accumulation of lipids after CSE treatment U937 macrophage cells was examined by staining lipids with specific dyes: Oil red O and BODIPY493/503. Results showed an accumulation of lipids in CSE-treated cells, confirming foam cell formation by CSE treatment. To decipher the mechanism, the levels of CD36, an ox-LDL receptor, and ABCA1, an exporter of lipids, were examined in CSE-treated and -untreated U937 cells by real-time PCR and immunofluorescence analysis. Consistent with lipid accumulation, an increased level of CD36 and a reduction in ABCA1 were observed in CSE-treated cells. Moreover, CSE treatment caused inhibition of lipophagy-mediated lipid degradation by blocking lipid droplets (LDs)-lysosome fusion and increasing the lysosomal pH. CSE also impaired mitochondrial lipid oxidation. Thus, the present study demonstrates that CSE treatment affects lipid homeostasis by altering its influx and efflux, lysosomal degradation, and mitochondrial utilization, leading to the formation of lipid-loaded foam cells. Moreover, the current study also showed that the leucine supplement caused a significant reduction of CSE-induced foam cell formation in vitro. Thus, the current study provides insight into CS-induced atherosclerosis and an agent to combat the disease.