Hepatic lipase promotes the uptake of HDL esterified cholesterol by the perfused rat liver: a study using reconstituted HDL particles of defined phospholipid composition

Abstract

The role of hepatic triacylglycerol lipase (H-TGL) in promoting the liver uptake of high density lipoprotein (HDL) free and esterified cholesterol was studied in a recirculating rat liver perfusion, a situation where the enzyme is physiologically expressed and is active at the vascular bed. For this purpose, reconstituted HDL of defined phospholipid composition were prepared, containing either 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, a substrate for H-TGL, or 1-O-hexadecyl-2-oleoyl-sn-glycero-3-phosphocholine, a non-hydrolyzable analog. Reconstituted HDL were then used in the perfused rat liver system. The main results are the following. 1) Reconstituted HDL were obtained by sonication of lipids and apolipoproteins and isolated by ultracentrifugation in the 1.07-1.21 g/ml density interval. Reconstituted HDL containing either diacylphosphatidylcholine or alkyl-acyl-phosphatidylcholine were similar in terms of chemical composition, apparent size, and apolipoprotein A-I immunoreactivity, and were comparable to native HDL3. 2) Reconstituted HDL were labeled with free [14C]cholesterol and [3H]cholesteryl ether, a non-hydrolyzable tracer of esterified cholesterol, and were perfused through the rat liver. Liver uptake of [3H]cholesteryl ether was 2.5-fold higher from reconstituted HDL containing diacylphospholipid than from HDL reconstituted with alkyl-acyl-phospholipids. Liver uptake of free [14C]cholesterol was identical in both cases. 3) H-TGL-depleted rat livers were obtained by a 12-min preperfusion in the presence of heparin, displacing 90% of the enzymatic activity. The residual activity in the perfusate was inhibited by a specific antibody directed against rat H-TGL. Liver uptake of [3H]cholesteryl ether from reconstituted HDL containing diacylphospholipid was reduced by 35% in hepatic lipase-depleted livers compared to controls. On the other hand, hepatic lipase depletion had no effect on the liver uptake of esterified cholesterol from HDL reconstituted with alkyl-acyl-phospholipids. The above findings support a role for the phospholipase A1 activity of H-TGL in stimulating the delivery of HDL esterified cholesterol to liver cells.