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

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.