Apolipoprotein A-IV. A determinant for binding and uptake of high density lipoproteins by rat hepatocytes

To identify the role of a specific apoprotein other than apoE which might be responsible for the receptor-mediated uptake of high density lipoprotein (HDL) by rat hepatocytes, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) was combined with rat apoE, apoA-I, or apoA-IV to form apoprotein-phospholipid complexes and the complexes were tested for their binding and uptake by primary rat hepatocytes. Apoprotein-POPC complexes were labeled with the specific fluorescent probe, 1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine to monitor their uptake by cultured rat hepatocytes at 37° C using digital fluorescence imaging microscopy or were labeled with ¹²⁵I to study their binding to hepatocytes at 4°C. POPC, either alone or with apoA-I, was not internalized by rat hepatocytes while complexes containing apoE or apoA-IV were taken up by the cells. Specific binding at 4°C was demonstrated for apoE-free HDL, apoA-IV·POPC, an apoE·POPC but not for apoA-I·POPC. The binding of apoE-free HDL was inhibited by apoA-IV·POPC, apoE-free HDL, and apoA-IV + apoA-I·POPC but not by apoA-I·POPC. Binding of apoA-IV·POPC was inhibited by apoE-free HDL, apoA-IV·POPC, and apoA-IV + apoA-I·POPC, but not by apoE·POPC or apoE-enriched HDL. These data indicate that apoA-IV is a ligand responsible for the rat HDL binding to primary rat hepatocytes and that apoA-IV binds to a receptor site distinct from apoE-dependent receptors such as the apoB,E or chylomicron-remnant receptor.