Uptake of apolipoprotein E-containing high density lipoproteins by hepatic parenchymal cells

Cholesterol-enriched, apolipoprotein E-containing high density lipoproteins (apo E HDLc), which were isolated from the plasma of cholesterol-fed dogs by using agarose column chromatography or ultracentrifugation, possessed essentially identical biochemical and metabolic characteristics. Radioiodinated (125I) apo E HDLc isolated by either method gave identical rates of clearance from the plasma, i.e., greater than 50% of the injected dose was cleared from the plasma within 5 to 10 minutes, principally by the liver. Detailed studies localizing apo E HDLc uptake to specific cell types within the liver were performed in both normal and cholesterol-fed rats. The validity of using the canine apo E HDLc in the rat was supported by observations of marked similarities in plasma clearance, i.e., a rapid acute phase of disappearance, and a near-quantitative hepatic uptake of lipoproteins in both species. Canine apo E HDLc (fluorescently labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine), which were injected into normal rats, appeared to be taken up primarily by parenchymal cells, as determined by fluorescence microscopy. Light microscopic autoradiography also revealed that the uptake of 125I apo E HDLc was principally carried out by parenchymal cells in rat liver. Likewise, the uptake of apo E HDLc by the liver of cholesterol-fed rats was extensively localized to parenchymal cells. An in situ, single-pass perfusion of a lobule of the liver of a normal dog with iodinated and fluorescently labeled apo E HDLc confirmed that the uptake of the lipoproteins was principally carried out by parenchymal cells. The plasma clearance of apo E HDLc by hepatic parenchymal cells, even in cholesterol-fed animals in which the apo B,E (LDL) receptors were markedly down-regulated (undetectable), suggests that the apo E receptor, presumably the remnant receptor, is localized in the parenchymal cells.