HDL(3)-MEDIATED CHOLESTEROL EFFLUX FROM CULTURED ENTEROCYTES - THE ROLE OF APOPROTEINS A-I AND A-II

High density lipoproteins (HDL) were recently demonstrated in an enter ocyte model (CaCo-2 cells) to mediate reverse cholesterol transport by retroendocytosis. The present study was carried out to define the rol e of the major HDL apoproteins (ape) A-I and ape A-II in this pathway HDL(3) was fractionated by heparin affinity chromatography into the tw o main fractions containing either apo A-I only (fraction A) or both a pe A-I and apo A-II (fraction B). In addition, liposomes were reconsti tuted from purified apo A-I or apo A-II and dimyristoyl phosphatidylch oline. The cell binding properties and cholesterol efflux potential we re studied in the lipoprotein fractions and the liposomes. Both fracti ons exhibited similar maximal binding capacities of 4427 (A) and 5041 (B) ng/mg cell protein, but their dissociation constants differed (40. 5 and 167.7 mu g/mL, respectively). Fraction A induced cholesterol eff lux and stimulated cholesterol synthesis more than did fraction B. Fra ction A mobilized both cellular free and esterified cholesterol, where as fraction B preferentially mobilized cholesteryl esters. Liposomes, containing either ape A-I or ape A-II, showed specific binding, endocy tosis and endosomal transport, and were released as intact particles. Apo A-I liposomes also mediated cholesterol efflux. In conclusion, the re is evidence that the HDL(3) subfractions A and B, as well as recons tituted liposomes containing either ape A-I or ape A-II, were specific ally bound and entered a retroendocytosis pathway which was directly l inked to cholesterol efflux. Quantitatively, the ape A-I subfraction a ppeared to play the dominant role in normal enterocytes. The ape A-II content of fraction B was related to the mobilization of cholesteryl e sters.