| Apolipoprotein E (apoE) is a constituent of all plasma lipoproteins except the smallest low density lipoproteins (LDL) and maintains cholesterol homeostasis by transporting cholesterol and other lipids between peripheral tissues and the liver (1–4). The working hypothesis for this dissertation is that a portion of internalized apoE escapes the degradative pathway and is reutilized by the liver. Primary murine hepatocytes were utilized to investigate the routing and physiologic significance of this process. These studies provide definitive evidence that a portion of internalized apoE is recycled. Intracellular retention of apoE for extended time periods suggested that apoE is routed away from the lysosomal pathway and retained in this non-degradative compartment. In addition, apoE recycled from triglyceride-rich lipoproteins as well as high-density lipoproteins (HDL). This finding suggests multiple entry pathways exist that lead to recycling and implicates recycling apoE in HDL metabolism. Recycling was augmented by addition of apoA-I to the media, demonstrating that the process can be modulated and suggesting a role for recycling apoE in cholesterol efflux. Approximately 50% of recycling apoE is internalized by the low-density lipoprotein receptor (LDLR); however, the relative mass of internalized apoE that was resecreted was unaffected by the absence of the LDLR or by reduced LDLR-related protein (LRP) expression. These data suggest apoE is targeted to a pool from which the proportion of apoE that recycles remains the same, regardless of entry point or the mass of apoE routed to this compartment. In addition, we provided evidence that apoE recycles in the absence of an intact Golgi apparatus. The discovery of efficient recycling of the 22 kDa fragment of apoE suggests a role for apoE recycling in neurological disease. This project demonstrated that apoE recycling is a complex pathway that involves multiple lipoprotein classes and internalization points. Furthermore, the results suggested that apoE recycling is a physiologically relevant process related to the functions of apoE in lipid and lipoprotein metabolism, as well as in disease. |
