Lipophagy inhibits foam cell formation by reducing cholesterol content

AIM To observe the changes of lipophagy during foam cells formation， and to determine the effect of lipophagy on the lipid content and cholesterol outflow of foam cells. METHODS Human THP-1 monocytes were induced by phorbol-12-myristate-13-acetate for 48 h to differentiate into macrophages， and then were incubated with 50 mg/L oxidized low-density lipoprotein （oxLDL） to form foam cells. Lipids in foam cell were stained by oil red O， and the lipid content was determined. The total cholesterol （TC） and free cholesterol （FC） levels in foam cells were measured by cholesterol testing kit. Cholesteryl ester （CE） and CE/TC ratio were calculated. The cholesterol efflux rate was detected by cholesterol efflux assay kit. The expression of autophagy-related proteins， including autophagy-related protein 5 （Atg5）， microtubule-associated protein 1 light chain 3 （LC3） and P62， were detected by Western blot. The colocalization of lipid droplets （LD） and LC3 was detected by immunofluorescence staining. The autophagy inducer rapamycin （Rap） or blocker 3-methyladenine （3MA） was used to intervene foam cells， and the expression of Atg5， LC3 and P62， the co-expression of LD and LC3， the cholesterol content and the cholesterol efflux rate were determined. RESULTS Formation of foam cells was observed at 24 h after stimulation with oxLDL at 50 mg/L， as indicated by intracellular CE/TC ratio exceeding 50%.Cholesterol efflux assay revealed that the cholesterol efflux rate increased within 24 h during foam cell formation but decreased after 48 h （P<0.05）. Western blot results displayed that the expression of Atg5 and LC3-II/LC3-I ratio were increased within 24 h of foam cell formation， but was deceased after 48 h （P<0.05）. The expression of P62 was decreased within 24 h but was increased at 48 h （P<0.05）. The colocalization of LD and LC3 was increased at 24 h but was decreased at 48 h after oxLDL stimulation. Treatment with Rap up-regulated the expression of Atg5 and LC3-II/LC3-I ratio， reduced the level of P62， increased the colocalization of LD and LC3， promoted the cholesterol efflux， anf reduced cholesterol content in foam cells （P<0.05）. On the contrary， 3MA inhibited the expression of Atg5， reduced LC3-II/LC3-I ratio， elevated the level of P62， decreased the colocalization of LD and LC3， reduced the outflow of cholesterol， increased the content of TC and CE， and elevated CE/TC ratio in foam cells （P<0.05）. CONCLUSION Lipophagy is enhanced at 24 h but decreased at 48 h during foam cell formation. Lipophagy inhibited foam cell formation by reducing cholesterol content and increasing cholesterol efflux.