Triacylglycerol Accumulation Activates the Mitochondrial Apoptosis Pathway in Macrophages

Abstract

Programmed cell death of lipid-laden macrophages is a prominent feature of atherosclerotic lesions and mostly ascribed to accumulation of excess intracellular cholesterol. The present in vitro study investigated whether intracellular triacylglycerol (TG) accumulation could activate a similar apoptotic response in macrophages. To address this question, we utilized peritoneal macrophages isolated from mice lacking adipose triglyceride lipase (ATGL), the major enzyme responsible for TG hydrolysis in multiple tissues. In Atgl−/− macrophages, we observed elevated levels of cytosolic Ca2+ and reactive oxygen species, stimulated cytochrome c release, and nuclear localization of apoptosis-inducing factor. Fragmented mitochondria prior to cell death were indicative of the mitochondrial apoptosis pathway being triggered as a consequence of defective lipolysis. Other typical markers of apoptosis, such as externalization of phosphatidylserine in the plasma membrane, caspase 3 and poly(ADP-ribose) polymerase cleavage, were increased in Atgl−/− macrophages. An artificial increase of cellular TG levels by incubating wild-type macrophages with very low density lipoprotein closely mimicked the apoptotic phenotype observed in Atgl−/− macrophages. Results obtained during the present study define a novel pathway linking intracellular TG accumulation to mitochondrial dysfunction and programmed cell death in macrophages.