There is growing evidence that CD36 has an important physiological function in the uptake of oxidized low density lipoprotein (OxLDL) by macrophages. However, the ligand specificity and the nature of the ligands on OxLDL that mediate the binding to CD36 remain ill defined. Results from recent studies suggested that some of the macrophage scavenger receptors involved in the uptake of OxLDL recognized both the lipid and the protein moieties of OxLDL, but there was no conclusive direct evidence for this. The present studies were undertaken to test whether a single, well characterized OxLDL receptor, CD36, could bind both the lipid and protein moieties of OxLDL. COS-7 cells transiently transfected with mouse CD36 cDNA bound intact OxLDL with high affinity. This binding was very effectively inhibited ( approximately 50%) both by the reconstituted apoB from OxLDL and by microemulsions prepared from OxLDL lipids. The specific binding of both moieties to CD36 was further confirmed by direct ligand binding analysis and by demonstrating reciprocal inhibition, i.e. apoB from OxLDL inhibited the binding of the OxLDL lipids and vice versa. Furthermore, a monoclonal mouse antibody that recognizes oxidation-specific epitopes in OxLDL inhibited the binding of intact OxLDL and also that of its purified protein and lipid moieties to CD36. This antibody recognizes the phospholipid 1-palmitoyl 2-(5'-oxovaleroyl) phosphatidylcholine. This model of an oxidized phospholipid was also an effective competitor for the CD36 binding of both the resolubilized apoB and the lipid microemulsions from OxLDL. Our results demonstrate that oxidized phospholipids in the lipid phase or covalently attached to apoB serve as ligands for recognition by CD36 and, at least in part, mediate the high affinity binding of OxLDL to macrophages.

We have previously shown that CD36 recognizes oxidation products of phospholipids on oxidized LDL (OxLDL) such as 1-palmitoyl-2-(5'-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC). The current study was designed to examine whether the phosphocholine (PC) headgroup in POVPC constitutes an obligatory binding target for CD36. To examine the contribution of PC in the binding of POVPC to CD36, we used well-defined synthetic oxidized phospholipids (OxPLs) cross-linked to BSA or to a hexapeptide. The OxPL adducts were then tested for their ability to bind to CD36-transfected cells and for their ability to inhibit OxLDL binding to CD36. Both POVPC-BSA and POVPC-peptide adducts were high-affinity ligands for CD36 and potent inhibitors of OxLDL binding. Enzymatic removal of the entire PC moiety of the POVPC-peptide, or of the choline headgroup alone, as well as substitution of the choline headgroup by ethanolamine abrogated the inhibitory activity of POVPC. Interestingly, PC by itself or cross-linked to BSA did not show any intrinsic competition activity. In conclusion, our data demonstrate that the PC headgroup of OxPL alone is sufficient for binding to CD36, but only if presented in the correct conformation as in OxPL of OxLDL or as in POVPC-peptide adducts.