INTRACELLULAR-LOCALIZATION AND METABOLISM OF CHYLOMICRON REMNANTS IN THE LIVERS OF LOW-DENSITY-LIPOPROTEIN RECEPTOR-DEFICIENT MICE AND APOE-DEFICIENT MICE - EVIDENCE FOR SLOW METABOLISM VIA AN ALTERNATIVE APOE-DEPENDENT PATHWAY

The metabolism of chylomicron remnants in mice deficient in low densit y lipoprotein receptor (LDLr) or apolipoprotein E (apoE) was compared with that of control C57BL/6J mice. Mice were injected intravenously w ith chylomicron-like emulsions labeled with radioactive lipids. Blood samples were taken at fixed time intervals from the retro-orbital sinu s, and clearance rates of the lipoproteins were assessed from the decl ine in plasma radioactivities. To follow the intracellular pathway of remnants in the liver, emulsions labeled with a fluorescent cholestery l ester (BODIPY) were injected, and liver sections were processed and assayed by laser confocal microscopy, Catabolism of remnant cholestery l esters was assessed by injecting emulsions labeled with cholesteryl[ 1-C-14]oleate and measuring the expired CO2 from each animal. In apoE- deficient mice, remnant removal from plasma was totally impeded, while the clearance of remnants in LDLr-deficient mice was similar to that in C57BL/6J control mice, The confocal micrographs of livers 20 min af ter injection of fluorescent chylomicron-like emulsions showed evenly distributed fluorescent particles in the hepatocytes from control mice . In contrast, the fluorescent particles were mainly located in sinuso idal spaces in LDLr deficient mice, Three hours after injection the li vers from control mice showed few fluorescent particles, indicating th at remnants have been catabolized, while the sections from LDLr-defici ent mice were still highly fluorescent, Micrographs from apoE deficien t mice showed no fluorescent particles in the liver at any time after injection, Measurement of expired radioactive CO2 after injection of e mulsions labeled in the fatty acid moiety of cholesteryl oleate indica ted that remnant metabolism was slower in the LDLr-deficient mice and essentially nil in the apoE-deficient mice, Control mice had expired 5 0% of the injected label by 3 h after injection. We conclude that unde r normal circumstances, chylomicron remnants are rapidly internalized by LDLr and catabolized in hepatocytes, with a critical requirement fo r apoE, When LDLr is absent, remnants are taken up by a second apoE-de pendent pathway, first to the sinusoidal space of the liver, with subs equent slow endocytosis and slow catabolism. Hepatic clearance via thi s second pathway is increased by heparin, inhibited by lactoferrin, he parinase, and suramin, and down-regulated by feeding a high fat diet.