Recycling of Apolipoprotein E and Lipoprotein Lipase through Endosomal Compartments in Vivo

We have recently described a novel recycling pathway of triglyceride-rich lipoprotein (TRL)-associated apolipoprotein (apo) E in human hepatoma cells. We now demonstrate that not only TRL-derived apoE but also lipoprotein lipase (LPL) is efficiently recycled in vitro and in vivo. Similar recycling kinetics of apoE and LPL in normal and low density lipoprotein receptor-negative human fibroblasts also indicate that the low density lipoprotein receptor-related protein seems to be involved. Intracellular sorting mechanisms are responsible for reduced lysosomal degradation of both ligands after receptor-mediated internalization. Immediately after internalization in rat liver, TRLs are disintegrated, and apoE and LPL are found in endosomal compartments, whereas TRL-derived phospholipids accumulate in the perinuclear region of hepatocytes. Subsequently, substantial amounts of both proteins can be found in purified recycling endosomes, indicating a potential resecretion of these TRL components. Pulse-chase experiments of perfused rat livers with radiolabeled TRLs demonstrated a serum-induced release of internalized apoE and LPL into the perfusate. Analysis of the secreted proteins identified approximately 80% of the recycled TRL-derived proteins in the high density lipoprotein fractions. These results provide the first evidence that recycling of TRL-derived apoE and LPL could play an important role in the modulation of lipoproteins in vivo.