Tissue sites of degradation of high density lipoprotein apolipoprotein A-IV in rats

Abstract

The in vivo metabolism of high density lipoprotein (HDL), labeled by incorporation of 125I-apolipoprotein (apo) A-IV, was studied in the rat and compared with the metabolism of HDL labeled with 131I-apo A-I. The 125I-apo A-IV labeled HDL was obtained by adding small amounts of radioiodinated apo A-IV to rat serum, followed by separation of the different lipoprotein fractions by chromatography on 6% agarose columns in order to avoid 'stripping' of apolipoproteins by ultracentrifugation. Under both in vitro and in vivo conditions, the 125I-apo A-IV remained an integral component of HDL and was not exchanged to other lipoproteins, including the 'free' apo A-IV fraction. The serum half-life, measured at between 8 and 28 hours after intravenous injection of labeled HDL, was 8.5 ± 0.5 hours for HDL apo A-IV and 10.2 ± 0.7 hours for HDL apo A-I. The tissue sites of catabolism of HDL apo A-IV and HDL apo A-I were analyzed in the 'leupeptin-model'. Only the kidneys and liver showed a significant leupeptin-dependent accumulation of radioactivity. At 4 hours after infection of 125I-apo A-IV/131I-apo A-I labeled HDL, 3.5% ± 1.0% and 8.4% ± 2.0% of HDL apo A-IV and 4.6% ± 1.3% and 2.6% ± 0.6% of the HDL apo A-I were accumulated in a leupeptin-dependent process in the kidneys and liver, respectively. Immunocytochemical studies revealed that the renal localization of apo A-IV was intracellular and confined to the epithelial cells of the proximal tubuli. The amount of intracellular apo A-IV in rat kidneys was increased in leupeptin-treated animals. The data suggest that the leupeptin-dependent degradation of HDL apo A-IV is more active in the liver than in the kidneys, while the opposite was observed for HDL apo A-I. These results, as well as the short half-life of HDL apo A-IV as compared to HDL apo A-I, are compatible with the existence of an apo A-IV-containing HDL subfraction with a relatively fast turnover for which the liver is the major catabolic site.