Pathway of cholesterol efflux from human hepatoma cells

Studies have been carried out with HepG2 cells, as a model for human hepatocytes, to explore a novel proposition that the liver contributes free cholesterol to the plasma lipoproteins which participate in the process of reverse-cholesterol transport. Specifically, we compared efflux of cholesterol from HepG2 cells and human fibroblasts (a model for extrahepatic tissues) after labeling cells with [14C]cholesterol. Incubation of both types of cells with human serum resulted in the efflux of [14C]cholesterol and net cholesterol flux from the cells to the medium and its subsequent esterification. Rates of cholesterol efflux from HepG2 cells and fibroblasts were similar. Nondenaturing two-dimensional gel electrophoresis showed that about 10% of the cell-derived [14C]cholesterol moves rapidly through pre beta 1-. pre beta 3- and pre beta 2-HDL particles into alpha HDL and LDL, although the majority moves directly to alpha HDL and LDL, with most of [14C]cholesterol and cholesterol mass accumulating in LDL. When cells were incubated with equivalent concentrations of isolated lipoproteins, HDL was much more effective in promoting [14C]cholesterol efflux than LDL, suggesting that unesterified cholesterol is initially transferred to HDL and then to LDL. Incubation with whole serum in contrast to isolated lipoproteins did not enhance cholesterol efflux despite a 3-fold higher esterification rate. We also investigated the identity of newly secreted lipoproteins following the labeling of HepG2 cells with [14C]cholesterol: 72% of labeled cholesterol was released as LDL, 20% was released as pre beta 2-HDL and 8% as small alpha HDL particles. Novel apo A-I rich but [14C]cholesterol-deficient pre beta 1-HDL particles were also secreted by HepG2 cells.