Macrophage receptors responsible for distinct recognition of low density lipoprotein containing pyrrole or pyridinium adducts: models of oxidized lowdensity lipoprotein

Oxidation of low density lipoproteins (LDL) induced by incubation with Cu2 ions results in the formation of a heterogeneous group of aldehydic adduct s on lysyl residues (Lys) of apolipoprotein B (apoB) that are thought to be responsible for the uptake of oxidized LDL (oxLDL) by macrophages. To defi ne the structural and chemical criteria governing such cell recognition, we induced two modifications of lysines in LDL that mimic prototypic adducts present in oxLDL; namely, E-amino charge-neutralizing pyrrolation by treatm ent with 2,5-hexanedione (hdLDL), and epsilon-amino charge-retaining pyridi nium formation via treatment with 2,4,6-trimethylpyrylium (tmpLDL). Both mo difications led to recognition by receptors on mouse peritoneal macrophages (MPM). To assess whether the murine scavenger receptor class A-I (mSR-A) w as responsible for recognition of hdLDL or tmpLDL in MPM, we measured bindi ng at 4 degrees C and degradation at 37 degrees C of these modified forms o f I-125-labeled LDL by mSR-A-transfected CHO cells. Although uptake and deg radation of hdLDL by mSR-A-transfected CHO cells was quantitatively similar to that of the positive control, acLDL, tmpLDL was not recognized by these cells. However, both tmpLDL and hdLDL were recognized by 293 cells that ha d been transfected with CD36. In the human monocytic cell line THP-1 that h ad been activated with PMA, uptake of tmpLDL was significantly inhibited by blocking monoclonal antibodies to CD36, further suggesting recognition of tmpLDL by this receptor. Macrophage uptake and degradation of LDL oxidized by brief exposure to Cu2+ was inhibited more effectively by excess tmpLDL a nd hdLDL than tvas more extensively oxidized LDL, consistent with the recog nition of the former by CD36 and the latter primarily by SR-A. Collectively , these studies suggest that formation of specific pyrrole adducts on LDL l eads to recognition by both the mSR-A and mouse homolog of CD36 expressed o n MPM, while formation of specific pyridinium adducts on LDL leads to recog nition by the mouse homolog of CD 36 but not by mSR-A. As such, these two m odifications of LDL may represent useful models for dissecting the relative contributions of specific modifications on LDL produced during oxidation, to the cellular uptake of this heterogeneous ligand.-Podrez, E. A., G. Hopp e, J. O'Neil, L. M. Sayre, N. Sheibani, and H. F. Hoff. Macrophage receptor s responsible for distinct recognition of low density lipoprotein containin g pyrrole or pyridinium adducts: models of oxidized lo tv density lipoprote in.