Transfer of esterified cholesterol from serum lipoproteins to the liver

Cooperation between hepatic cholesteryl ester hydrolase and scavenger receptor BI for hydrolysis of HDL-CE

Liver is the sole organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or bile acids. High density lipoprotein (HDL)-derived cholesterol is the major source of biliary sterols and represents a mechanism for the removal of cholesterol from peripheral tissues including artery wall-associated macrophage foam cells. Via selective uptake through scavenger receptor BI (SR-BI), HDL-cholesterol is thought to be directly secreted into bile, and HDL cholesteryl esters (HDL-CEs) enter the hepatic metabolic pool and need to be hydrolyzed prior to conversion to bile acids. However, the identity of hepatic CE hydrolase (CEH) as well as the role of SR-BI in bile acid synthesis remains elusive. In this study we examined the role of human hepatic CEH (CES1) in facilitating hydrolysis of SR-BI-delivered HDL-CEs. Over-expression of CEH led to increased hydrolysis of HDL-[³H]CE in primary hepatocytes and SR-BI expression was required for this process. Intracellular CEH associated with BODIPY-CE delivered by selective uptake via SR-BI. CEH and SR-BI expression enhanced the movement of [³H]label from HDL-[³H]CE to bile acids in vitro and in vivo. Taken together, these studies demonstrate that SR-BI-delivered HDL-CEs are hydrolyzed by hepatic CEH and utilized for bile acid synthesis.

Hepatic overexpression of cholesteryl ester hydrolase enhances cholesterol elimination and in vivo reverse cholesterol transport

Neutral cholesteryl ester hydrolase (CEH)-mediated hydrolysis of cellular cholesteryl esters (CEs) is required not only to generate free cholesterol (FC) for efflux from macrophages but also to release FC from lipoprotein-delivered CE in the liver for bile acid synthesis or direct secretion into the bile. We hypothesized that hepatic expression of CEH would regulate the hydrolysis of lipoprotein-derived CE and enhance reverse cholesterol transport (RCT). Adenoviral-mediated CEH overexpression led to a significant increase in bile acid output. To assess the role of hepatic CEH in promoting flux of cholesterol from macrophages to feces, cholesterol-loaded and [3H]cholesterol-labeled J774 macrophages were injected intraperitoneally into mice and the appearance of [3H]cholesterol in gallbladder bile and feces over 48 h was quantified. Mice overexpressing CEH had significantly higher [3H]cholesterol radiolabel in bile and feces, and it was associated with bile acids. This CEH-mediated increased movement of [3H]cholesterol from macrophages to bile acids and feces was significantly attenuated in SR-BI(-/-) mice. These studies demonstrate that similar to macrophage CEH that rate-limits the first step, hepatic CEH regulates the last step of RCT by promoting the flux of cholesterol entering the liver via SR-BI and increasing hepatic bile acid output.

Transfer of plasma lipoprotein components and of plasma proteins into aortas of cholesterol-fed rabbits. Molecular size as a determinant of plasma lipoprotein influx

The arterial influx of esterified and free cholesterol from low density lipoproteins and very low density lipoproteins in 20 hypercholesterolemic rabbits was measured simultaneously by the use of lipoproteins labeled in vivo with [3H]- and [14C]-cholesterol. The simultaneous arterial influx of either [3H]-leucine-labeled very low density lipoproteins, low density lipoproteins, high density lipoproteins, or plasma proteins was also measured in each rabbit. The arterial influx was calculated as intimal clearance, i.e., the influx of a given fraction divided by its plasma concentration. The intimal clearance of low density lipoprotein esterified cholesterol was equal to that for the apolipoproteins of that fraction, which is compatible with an arterial influx of intact low density lipoprotein molecules. The intimal clearance of very low density apolipoprotein or cholesteryl ester was less than that for low density lipoprotein, whereas high density lipoprotein and albumin clearances exceeded low density lipoprotein clearance by 1.5- to 3-fold. The intimal clearances of plasma proteins, high density, low density, and very low density lipoproteins decreased linearly with the logarithm of the macromolecular diameter. This indicates that the arterial influx of three plasma lipoprotein fractions and of plasma proteins proceeds by similar mechanisms. Apparently the relative intimal clearances of lipoproteins are more dependent on their size relative to pores or vesicular diameters at the plasma-artery interface than on specific interactions between lipoproteins and the arterial intimal surface.

Characteristics of acid lipase and acid cholesteryl esterase activity in parenchymal and non-parenchymal rat liver cells

(1) Parenchymal and non-parenchymal cells were isolated from rat liver. The characteristics of acid lipase activity with 4-methylumbelliferyl oleate as substrate and acid cholesteryl esterase activity with cholesteryl[1-14C]oleate as substrate were investigated. The substrates were incorporated in egg yolk lecithin vesicles and assays for total cell homogenates were developed, which were linear with the amount of protein and time. With 4-methylumbelliferyl oleate as substrate, both parenchymal and non-parechymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 2.5 times higher than for parenchymal cells. It is concluded that 4-methylumbelliferyl oleate hydrolysis is catalyzed by similar enzyme(s) in both cell types. (2) With cholesteryl[1-14C]oleate as substrate both parenchymal and non-parenchymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 11.4 times higher than for parenchymal cells. It is further shown that the cholesteryl ester hydrolysis in both cell types show different properties. (3) The high activity and high affinity of acid cholesteryl esterase from non-parenchymal cells for cholesterol oleate hydrolysis as compared to parenchymal cells indicate a relative specialization of non-parenchymal cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells possess the enzymic equipment to hydrolyze very efficiently internalized cholesterol esters, which supports the suggestion that these cell types are an important site for lipoprotein catabolism in liver.

Uptake and degradation of cholesterol ester-labelled rat plasma lipoproteins in purified rat hepatocytes and nonparenchymal liver cells

1. A new method for isolation and purification of rat liver hepatocytes and nonparenchymal cells by differential centrifugation is described. 2. Cholesterol ester-labelled lipoproteins (prepared by the action of lecithin: cholesterol acyltransferase) intravenously injected were taken up by hepatocytes and nonparenchymal cells. 3. Hepatocytes and nonparenchymal cells in suspension were able to take up and hydrolyse the cholesterol ester portion of lipoproteins. 4. Uptake of cholesterol ester labelled whole rat plasma and high density lipoproteins (HDL) increased with increasing concentrations until a distinct saturation level was reached in hepatocytes. In nonparenchymal cells there was no saturation of lipoprotein uptake. 5. Concanavalin A inhibited cholesterol ester-labelled lipoprotein uptake in hepatocytes, indicating that the uptake at least partially depends on carbohydrate sites on the cell surface. The uptake in nonparenchymal cells was unaffected of concanavalin A. 6. The specific activity of the acid cholesterol ester hydrolase was the same in homogenates from hepatocytes and nonparenchymal cells while acyl-CoA: cholesterol acyltransferase was found almost exclusively in hepatocytes.