| Human HDL was incubated with rat hepatic lipase and subjected to compositional analysis. 30% of the HDL phosphatidylcholine was hydrolyzed by hepatic lipase but no change was detected in apoprotein content of the HDL compared to HDL incubated with heat-inactivated hepatic lipase. Cultured Fu5AH rat hepatoma cells exposed to hepatic lipase-modified HDL showed an increased uptake of HDL free cholesterol relative to cells exposed to control HDL. The increased delivery of HDL was dose-dependent, and could be demonstrated by both isotopic and mass determinations. The enhanced cellular accumulation of HDL free cholesterol observed with hepatic lipase was directly related to the extent of phosphatidylcholine hydrolysis, and appears to be due to the phospholipase activity of this enzyme, since similar results were obtained with HDL that had been modified with snake venom phospholipase A(,2). To investigate the mechanism of this response, the influence of hapatic lipase on the metabolism of other HDL components relative to free cholesterol was determined. Experiments utilizing doubly-labeled HDL ({('14)C}free cholesterol. ('125)I-labeled HDL) demonstrated that the mechanism does not involve an enhanced uptake and degradation of the entire lipoprotein molecule. The delivery of HDL phospholipid, as sphingomyelin, was also unaffected by the action of hepatic lipase. Hepatic lipase-modification does stimulate the incorporation of HDL cholesteryl ester, as measured by the uptake of either {('3)H}cholesteryl ester or the nondegradable marker, {('3)H}cholesteryl ether; however, this response is minor when compared to the amount of HDL free cholesterol that was transported into the cell. Only one-tenth of the HDL free cholesterol incorporated by the cell as a result of the modification of HDL by hepatic lipase could be attributed to the stimulated delivery of cholesteryl ester. In an attempt to correlate the events observed at the cellular level with lipoprotein structure, nuclear magnetic resonance spectroscopy was employed. Modification of HDL by either hepatic lipase or phospholipase A(,2) generated a movement of free cholesterol molecules from the core of the lipoprotein to the surface. These results indicate that HDL which has been modified by hepatic lipase preferentially delivers free cholesterol to cells by a surface transfer process and support the hypothesis that hepatic lipase may modulate the delivery of cholesterol to the liver. |
