Different expression of modified low density lipoprotein receptors in rabbit peritoneal macrophages and Kupffer cells

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We have previously reported that mouse peritoneal macrophages have three types of modified low density lipoprotein (LDL) receptors. One is specific for acetylated LDL (Ac-LDL), the second is for oxidized LDL (Ox-LDL), and the third recognizes both (Arai, H. et al. (1989) Biochem. Biophys. Res. Commun. 159, 1375–1382). In the current study, the characteristics of modified LDL receptors in rabbit peritoneal macrophages and Kupffer cells from rabbits were investigated. Cross-competition studies of the degradation assay between Ox-LDL and Ac-LDL in rabbit peritoneal macrophages showed that the degradation of 125I-labeled Ox-LDL was almost completely inhibited by an excess amount of unlabeled Ac-LDL. On the other hand, an excess amount of unlabeled Ox-LDL suppressed 125I-labeled Ac-LDL degradation only partially. In contrast, in Kupffer cells an excess amount of unlabeled Ox-LDL inhibited the degradation of 125I-labeled Ac-LDL almost completely, whereas the degradation of 125I-labeled Ox-LDL was inhibited only partially by Ac-LDL. Scatchard analysis of binding assay showed that rabbit peritoneal macrophages have a single class of receptor for Ox-LDL, which binds maximally 0.31 μg/mg cellular protein ( B max ) with an apparent dissociation constant ( K d ) of 19.3 μg/ml, and two classes of receptors for Ac-LDL; one with high affinity ( B max 0.025 μg/mg cellular protein, K d 0.040 μg/ml) and the other with low affinity ( B max 0.08 μg/mg cellular protein, K d 11.31 μg/ml, On the other hand, Kupffer cells have two classes for Ox-LDL; one is a high affinity receptor ( B max 0.53 μg/mg cellular protein, K d 0.99 μg/ml) and the other is a low affinity receptor ( B max 3.71 μg/mg cellular protein, K d 16.2 μg/ml) and a single class for Ac-LDL ( B max 0.60 μg/mg cellular protein, K d 7.24 μg/ml). These results indicate that rabbit peritoneal macrophages have two kinds of modified LDL receptors; one is specific for Ac-LDL, and the other recognizes both Ox-LDL and Ac-LDL. These cells, thereby, have no specific receptor for Ox-LDL as was seen in mouse peritoneal macrophages. On the other hand, rabbit Kupffer cells have a receptor which recognizes Ox-LDL specifically. However, a specific receptor for Ac-LDL does not exist in rabbit Kupffer cells.