HEPATIC LIPASE PROMOTES THE UPTAKE OF HDL ESTERIFIED CHOLESTEROL BY THE PERFUSED-RAT-LIVER - A STUDY USING RECONSTITUTED HDL PARTICLES OF DEFINED PHOSPHOLIPID-COMPOSITION

The role of hepatic triacylglycerol lipase (H-TGL) in promoting the li ver uptake of high density lipoprotein (HDL) free and esterified chole sterol was studied in a recirculating rat liver perfusion, a situation where the enzyme is physiologically expressed and is active at the va scular bed. For this purpose, reconstituted HDL of defined phospholipi d composition were prepared, containing either 1-palmitoyl-2-oleoyl-sn -glycero-3-phosphocholine, a substrate for H-TGL, or -O-hexadecyl-2-ol eoyl-sn-glycero-3-phosphocholine, a non-hydrolyzable analog. Reconstit uted HDL were then used in the perfused rat liver system. The main res ults are the following. 1) Reconstituted HDL were obtained by sonicati on of lipids and apolipoproteins and isolated by ultracentrifugation i n the 1.07-1.21 g/ml density interval. Reconstituted HDL containing- e ither diacylphosphatidylcholine or alkyl-acyl-phosphatidylcholine were similar in terms of chemical composition, apparent size, and apolipop rotein A-I immunoreactivity, and were comparable to native HDL(3). 2) Reconstituted HDL were labeled with free [C-14]cholesterol and [3H]cho lesteryl ether, a non-hydrolyzable tracer of esterified cholesterol, a nd were perfused through the rat liver. Liver uptake of [H-3]cholester yl ether, was 2.5-fold higher from reconstituted HDL containing diacyl phospholipid than from HDL reconstituted with alkyl-acyl-phospholipids . Liver uptake of free [C-14]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. Th e residual activity in the perfusate was inhibited by a specific antib ody 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 e sterified cholesterol from HDL reconstituted with alkyl-acyl-phospholi pids. The above findings support a role for the phospholipase Al activ ity of H-TGL in stimulating the delivery of HDL esterified cholesterol to liver cells.