Periodontal Pathogens Promote Foam Cell Formation by Blocking Lipid Efflux

Abstract

Foam cells are one of the major cellular components of atherosclerotic plaques, within which the trace of periodontal pathogens has also been identified in recent studies. In line with these findings, the correlation between periodontitis and atherosclerotic cardiovascular incidences has been repetitively supported by evidence from a number of experimental studies. However, the direct role of periodontal pathogens in altered cellular signaling underlying such cardiovascular events has not been clearly defined. To determine the role of periodontal pathogens in the pathogenesis of atherosclerosis, especially in the evolution of macrophages into foam cells, we monitored the pattern of lipid accumulation within macrophages in the presence of periodontal pathogens, followed by characterization of these lipids and investigation of major molecules involved in lipid homeostasis. The cells were stained with the lipophilic fluorescent dye BODIPY 493/503 and Oil Red O to characterize the lipid profile. The amounts of Oil Red O–positive droplets, representing neutral lipids, as well as fluorescent lipid aggregates were prominently increased in periodontal pathogen–infected macrophages. Subsequent analysis allowed us to locate the accumulated lipids in the endoplasmic reticulum. In addition, the levels of cholesteryl ester in periodontal pathogen–infected macrophages were increased, implying disrupted lipid homeostasis. Further investigations to delineate the key messengers and regulatory factors involved in the altered lipid homeostasis have revealed alterations in cholesterol efflux–related enzymes, such as ABCG1 and CYP46A1, as contributors to foam cell formation, and increased Ca2+ signaling and reactive oxygen species (ROS) production as key events underlying disrupted lipid homeostasis. Consistently, a treatment of periodontal pathogen–infected macrophages with ROS inhibitors and nifedipine attenuated the accumulation of lipid droplets, further confirming periodontal pathogen–induced alterations in Ca2+ and ROS signaling and the subsequent dysregulation of lipid homeostasis as key regulatory events underlying the evolution of macrophages into foam cells.