Neutral cholesteryl ester hydrolase (NCEH) is an enzyme that mediates the intracellular hydrolysis of cholesteryl ester in cytoplasm. It has been shown to play an important role in the regulation of cholesterol metabolism in the liver. Recently, it has been reported that high-density lipoprotein (HDL)associated cholesterol is processed via a pathway different from that of low-density lipoprotein (LDL) metabolism and that degradation of HDL occurs in a non-lysosomal pathway. In the present study, we examined whether hepatic NCEH activity in H-35 rat hepatoma cells is affected by HDL.
NCEH activity was increased when the cells were incubated with increasing concentrations of HDL (from 25 to 100μg protein/ml) for 20 hours. Approximately two-fold increases of NCEH activity were obtained with 100μg protein/ml of HDL. NCEH activity was elevated significantly when the cells were incubated with HDL for more than 10 hours. Moreover, NCEH activity was enhanced by dimyristoyl phosphatidylcholine (DMPC)/cholesteryl oleate (CO)/apoHDL compex, as well as native HDL, but not by DMPC/apoHDL or DMPC/CO complexes. In contrast, neither LDL nor VLDL affected NCEH activity. In addition, HDL induced increases in NCEH activity were not inhibited by coincubation with 50μM of chloroquine, which is regarded as an inhibitor of lysosomal hydrolases.
These findings suggest that uptaken HDL-CE is delivered to a cytoplasmic compartment in a manner distinct from the lysosomal pathway, and is hydrolyzed by NCEH in H-35 rat hepatoma cells.
