Objective: Endosomal signature phospholipid bis(monoacylglycero)phosphate (BMP) has been involved in the regulation of cellular cholesterol homeostasis. Accumulation of BMP is a hallmark of lipid storage disorders and was recently reported as a noticeable feature of oxidized low-density lipoprotein-laden macrophages. This study was designed to delineate the consequences of macrophage BMP accumulation on intracellular cholesterol distribution, metabolism, and efflux and to unravel the underlying molecular mechanisms.
Approach and results: We have developed an experimental design to specifically increase BMP content in RAW 264.7 macrophages. After BMP accumulation, cell cholesterol distribution was markedly altered, despite no change in low-density lipoprotein uptake and hydrolysis, cholesterol esterification, or total cell cholesterol content. The expression of cholesterol-regulated genes sterol regulatory element-binding protein 2 and hydroxymethylglutaryl-coenzyme A reductase was decreased by 40%, indicative of an increase of endoplasmic reticulum-associated cholesterol. Cholesterol delivery to plasma membrane was reduced as evidenced by the 20% decrease of efflux by cyclodextrin. Functionally, BMP accumulation reduced cholesterol efflux to both apolipoprotein A1 and high-density lipoprotein by 40% and correlated with a 40% decrease in mRNA contents of ATP-binding cassette transporter A1, ATP-binding cassette transporter G1, and liver-X receptor α and β. Foam cell formation induced by oxidized low-density lipoprotein exposure was exacerbated in BMP-enriched cells.
Conclusions: The present work shows for the first time a strong functional link between BMP and cholesterol-regulating genes involved in both intracellular metabolism and efflux. We propose that accumulation of cellular BMP might contribute to the deregulation of cholesterol homeostasis in atheromatous macrophages.
Keywords: ATP-binding cassette transporters; atherosclerosis; endosomes; lipoproteins.