Lactoferrin specifically inhibits endocytosis of chylomicron remnants but not alpha-macroglobulin

Our recently found nonlipoprotein inhibitor of chylomicron remnant uptake, lactoferrin, has been investigated in vivo and in vitro. Lipoprotein lipase extracted triglycerides from chylomicrons, doubly labeled with [3H]retinol/[14C]oleate, in the presence of lactoferrin normally. The subsequent uptake of remnants into liver was retarded considerably. In the intact rat, chylomicron remnants (CRs), predominantly labeled in the apoB48 moiety by 125I, were excluded from the hepatic endosomal compartment in the presence of lactoferrin as shown in subcellular fractionation studies of rat livers. In tissue culture, internalization of [125I]chylomicron remnants was inhibited in the presence of 14 pM lactoferrin by 70%. Upon removal of lactoferrin, internalization was rapidly restored. Protease digestion eliminated the inhibitory effect completely. Modification of arginine residues with cyclohexanedione reversibly removed the inhibitory potency of lactoferrin. We located by molecular modeling an alpha-helical segment in lactoferrin on the exposed surface of the molecule containing the sequence Arg-X-X-Arg-Lys-X-Arg, which resembles the receptor recognition structure in apolipoprotein E (apoE). This firmly established ligand correspondence with apoE, the candidate ligand for CR recognition by the receptor. Finally, the postulated second function of low density lipoprotein receptor-related protein, uptake of alpha-2-macroglobulin (alpha 2M) was found to be distinct from lipoprotein binding, since lactoferrin inhibited CR but not alpha 2M internalization. In addition, CR uptake was not affected by alpha 2M. We conclude that if a bifunctional receptor were to operate, its diverse functions were exerted by independently operating substructures. The results of our in vivo and cell culture experiments are, however, entirely compatible with the existence of two receptors as well.