Modulation of Endosomal Cholesteryl Ester Metabolism by Membrane Cholesterol

Abstract

Cells acquire cholesterol in part by endocytosis of cholesteryl ester containing lipoproteins. In endosomes and lysosomes cholesteryl ester is hydrolyzed by acidic cholesteryl ester hydrolase producing cholesterol and fatty acids. Under certain pathological conditions, however, such as in atherosclerosis, excessive levels of cholesteryl ester accumulate in lysosomes for reasons that are poorly understood. Here, we have studied endosomal and lysosomal cholesteryl ester metabolism in cultured mouse macrophages and with cell-free extracts. We show that net hydrolysis of cholesteryl ester is coupled to the transfer of cholesterol to membranes. When membrane cholesterol levels are low, absorption of cholesterol effectively drives cholesteryl ester hydrolysis. When cholesterol levels in acceptor membranes approach saturation or when cholesterol export is blocked, cholesterol is re-esterified in endosomes. These results reveal a new facet of cellular cholesterol homeostasis and provide a potential explanation for cholesteryl ester accumulation in lysosomes of atherosclerotic cells.One of the major hallmarks of atherosclerosis is the progressive accumulation of intracellular and extracellular cholesterol (1-3). In early stages of atherosclerosis excess cholesterol is mainly deposited as cholesteryl fatty acyl esters ("cholesteryl ester"). As atherosclerosis progresses, both cholesteryl ester and cholesterol accumulate (4). Advanced lesions are characterized by the formation of extracellular lipid cores that consist of cholesteryl ester, crystals of cholesterol monohydrate, and debris (5-7). Intracellularly, cholesterol builds up as a component of membranes and as oily droplets of cholesteryl ester that form in the cytoplasm and inside lysosomes (4, 5). At the late stages of disease progression, lysosomes become the major site of lipid accumulation (4, 8).The mechanisms that are responsible for the formation of cholesteryl esters in atheromatous cells have only partly been resolved. It is clear that cholesteryl ester in cytoplasmic lipid droplets is produced by ER 1 -localized acyl-CoA:cholesterol acyltransferases (ACATs) that are allosterically activated by cholesterol (9, 10). The aberrant accumulation of cholesteryl ester in lysosomes, however, remains to be explained (8).Cholesterol in atherosclerotic lesions is predominantly derived from serum lipoproteins such as low density lipoprotein (LDL) (11). LDL consists of a hydrophobic core containing mainly cholesteryl ester and triglycerides surrounded by a lipid monolayer and apolipoprotein B100 (12). LDL and its atherogenic derivatives are recognized by cell surface receptors, internalized, and transferred to the endosomal system. Cholesteryl ester and triglycerides are then hydrolyzed by acidic cholesteryl ester hydrolase (aCEH), generating cholesterol and fatty acids (11, 13,14).It has previously been proposed that lysosomal cholesteryl ester accumulation in atheromatous cells might result from a deficiency of aCEH (5, 15). Most subsequent studies on aCEH activity in...

The lipofuscin fluorophore A2E perturbs cholesterol metabolism in retinal pigment epithelial cells

“…It is conceivable that once cholesterol displaced from A2E-containing membranes reaches a critical concentration, it is excluded from the pool destined for nonvesicular transport to the limiting membrane and thus gets trapped within the LE/Ly. Increased LE/Ly cholesterol levels result in an indirect inhibition of RPE acid lipase, in agreement with previous studies showing that late endosomal cholesterol efflux is necessary for acid lipase activity (19). The subsequent accumulation of CE along with free cholesterol generates a feed-forward cycle of further cholesterol accumulation, inhibition of acid lipase, and increased CE in the cell.…”

“…Thus, CE accumulation in RPE cells is not because of a direct inhibition of acid lipase by A2E. It has been proposed that CE hydrolysis in LE/Ly is biphasic; initial hydrolysis occurs until endosomal membranes become saturated with free cholesterol and further hydrolysis depends on desorption of cholesterol onto other membranes/carriers (19). Indeed, we found that increasing the cholesterol content of TFMUO micelles caused a marked decrease in acid lipase activity (Fig.…”

Sorghum distillers dried grain lipid extract increases cholesterol excretion and decreases plasma and liver cholesterol concentration in hamsters

“…While the reduction of free cholesterol was significant, the magnitude of change among treatment groups was relatively small indicating that free cholesterol in cell membranes is tightly regulated due to its essential role in cell metabolism. The trend of increasing liver triacylglycerol possibly indicates that secretion of VLDL is decreased (Wang et al, 2005), although this was not measured in the current study. Liver phospholipid is an indicator of cell mass and was not affect by dietary treatment.…”

Lipid Homeostasis and Lipoprotein Secretion in Niemann-Pick C1-deficient Hepatocytes

“…Nohturfft and coworkers (47) have shown that when cholesterol levels of endosomal/lysosomal membranes are high, as is the case in NPC1-deficient hepatocytes, cholesterol can be esterified in endosomes/lysosomes. We, therefore, performed the cholesterol esterification assay with [ 3 H]oleate in the presence and absence of an inhibitor of acylCoA:cholesterol acyltransferase (Sandoz 50-035) (48).…”