SELECTIVE UPTAKE OF HIGH-DENSITY LIPOPROTEIN-ASSOCIATED CHOLESTERYL ESTERS BY HUMAN HEPATOCYTES IN PRIMARY CULTURE

High-density lipoprotein cholesteryl esters are taken up by many cells without simultaneous uptake of high-density lipoprotein apolipoprotei ns. This selective uptake was investigated in human hepatocytes in pri mary culture. Human high-density lipoprotein-3 (density, 1.125 to 1.21 gm/ml) was radiolabeled in both its apolipoprotein and in its cholest eryl ester moiety; uptake of these high-density lipoprotein(3) tracers by hepatocytes was investigated. Apparent high-density lipoprotein(3) particle uptake as measured with the cholesteryl ester tracer was in excess of that from the apolipoprotein tracer, indicating selective up take of high-density lipoprotein(3) cholesteryl esters by hepatocytes. This selective uptake is a regulated pathway in hepatocytes, as demon strated by an inverse relationship between cell cholesterol and the ra te of selective uptake. Studies on the mechanism of selective uptake h ave used inhibitors such as monensin, chloroquine, heparin, and a mono clonal antibody directed against low-density lipoprotein receptors. Th ese experiments provide no evidence for a role of cell-secreted apolip oprotein E, endocytosis or retroendocytosis in selective uptake. The i ntracellular fate of high-density lipoprotein(3)-associated cholestery l esters was investigated with [H-3]cholesteryl oleate-labeled high-de nsity lipoprotein(3). Hepatocytes hydrolyzed [H-3]cholesteryl oleate i nternalized from labeled high-density lipoprotein(3); this catabolism was not inhibited by the presence of chloroquine. In parallel hepatocy tes were incubated with [H-3]cholesteryl oleate-labeled low-density li poprotein. Cells hydrolyzed [H-3]cholesteryl oleate taken up with low- density lipoprotein; however, this hydrolysis was inhibited by chloroq uine, indicating lysosomal low-density lipoprotein cholesteryl ester c atabolism. These experiments show that high-density lipoprotein(3) cho lesteryl esters selectively taken up by hepatocytes are hydrolyzed ind ependently from the classical lysosomal catabolic pathway. The questio n was addressed if selective uptake mediates a net mass uptake of chol esterol rather than an isotope exchange phenomenon. Incubation of hepa tocytes with high-density lipoprotein-3 suppressed endogenous sterol s ynthesis from sodium [C-14]acetate. Hepatocytes were incubated in the presence of high-density lipoprotein(3); medium cholesteryl esters dec reased as a result of incubation with hepatocytes. These results show a net mass delivery of high-density lipoprotein cholesteryl esters to hepatocytes. In conclusion, the pathway for selective uptake of high-d ensity lipoprotein cholesteryl esters could be demonstrated in human h epatocytes in primary culture. A role for selective uptake in high-den sity lipoprotein-mediated cholesterol delivery to the liver in human b eings in vivo is proposed.