Dissection of compartments in rat hepatocytes involved in the intracellular trafficking of high-density lipoprotein particles or their selectively internalized cholesteryl esters

Abstract

The trafficking of apolipoprotein E-deficient high-density lipoprotein particles and of their component cholesteryl esters in rat hepatocytes was studied. Human high-density lipoprotein 3, labeled with two nondegradable, intracellularly trapped tracers in their apolipoprotein A-I and their cholesteryl esters, were injected into rats, and five subcellular hepatocytic fractions were isolated at various time intervals. In control experiments with homologous lipoproteins, doubly labeled rat high-density lipoproteins depleted of apolipoprotein E were used. In endosomes and lysosomes the two labels were recovered at near unity, indicating that high-density lipoproteins are endocytosed as particles, transported to early and late endosomes and finally subjected to lysosomal degradation. No significant amounts of label were found in receptor-recycling endosomes. In contrast to label of those of low-density lipoproteins, label of component protein and cholesteryl esters of high-density lipoproteins from isolated endosomes floated at different densities in gradient ultracentrifugation, indicating early disintegration of high-density lipoprotein particles. In contrast to the endocytic organelles, in the whole liver, label of high-density lipoprotein-associated cholesteryl esters exceeded the label of high-density lipoprotein-associated apolipoprotein A-I twofold to threefold. This finding is compatible with selective uptake of high-density lipoprotein cholesteryl esters in addition to uptake of high-density lipoprotein particles. The excess cholesteryl esters accumulated in a nonendosomal fraction, whose major proteins differed from the integral proteins of endosomes. These data suggest two distinct intracellular routes of hepatocytic high-density lipoprotein trafficking in vivo. High-density lipoproteins free of apolipoprotein E are internalized intact by hepatocytes, are predominantly transported to early and late endosomes and are finally subjected to lysosomal degradation. High-density lipoprotein particles do not undergo retroendocytosis in hepatocytes. In addition, high-density lipoprotein-associated cholesteryl esters can be taken up by hepatocytes selectively. They, however, accumulate in a nonendosomal, nonlysosomal compartment.

Cubilin, the endocytic receptor for intrinsic factor-vitamin B 12 complex, mediates high-density lipoprotein holoparticle endocytosis

“…The role of cubilin as a holoparticle HDL endocytosis receptor was confirmed through the use of cubilin antibodies to inhibit internalization of HDL labeled on either its apolipoprotein or lipid constituents. The endocytic activity of cubilin reported here is consistent with recent studies showing cubilin to be an endocytic receptor for intrinsic factor-vitamin B 12 and Ig light chain (33,36). Given the lack of a membrane spanning element, cubilin is considered to be a peripheral membrane protein (34).…”

“…As a result, it is known that the kidney and liver are major sites of HDL catabolism and that lipolytic modification of the particle and structural integrity of its apolipoprotein (apo) components are significant factors in determining the half life of HDL (5,(8)(9)(10)(11)(12). The mechanism by which HDL is catabolized in the kidney, liver, and other tissues such as the placenta and yolk sac is not fully understood.…”

Different transport routes for high density lipoprotein and its associated free sterol in polarized hepatic cells

“…However, it remains unclear how HDL-derived cholesterol becomes selectively targeted to the canalicular membrane for subsequent release into the bile. Earlier studies have suggested that HDL binds to the basolateral membrane of hepatic cells, resulting in the release of lipids from the lipoprotein without uptake of the particles (6,33). However, these studies might have missed a rapid internalization of HDL and resecretion into the medium.…”

“…Several studies have characterized a retroendocytic pathway for HDL in hepatocytes and hepatoma HepG2 cells by which internalized HDL has been proposed to recycle rapidly toward the basolateral membrane surface (6)(7)(8). By this process, HDL-associated lipids can be delivered to the hepatocyte (8).…”

Apolipoprotein AII Enrichment of HDL Enhances Their Affinity for Class B Type I Scavenger Receptor but Inhibits Specific Cholesteryl Ester Uptake

“…4 After uptake, CEs are directed intracellularly to a nonlysosomal destination for degradation. 5 In mice, SR-BI is largely expressed in liver, and overexpression of the SR-BI level in vivo on hepatocytes induces the disappearance of plasmatic HDL and the doubling of biliary cholesterol 6 ; inversely targeted disruption of the SR-BI gene induces a 2.2-fold increase in plasma cholesterol concentration. 7 All these results strongly suggest that SR-BI plays a key role in hepatic HDL metabolism in rodents.…”