Identification of a small-molecule, nonpeptide macrophage scavenger receptor antagonist

Class A scavenger receptor (SR-A) antagonists may prevent the initiation of atherosclerosis, because a recent report found that SR-A/apolipoprotein E (apoE) double-knockout mice had 60% smaller lesions than apoE single-knocko ut littermates. We transfected human embryonic kidney (HEK) 293 cells with SR-A type I or II receptors to find small-molecule antagonists. Uptake of 1 ,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-labeled a cetylated low-density lipoprotein (Dil-AcLDL) showed that among common poly anionic ligands, polyinosine was the most potent, with an IC50 of 0.74 mu g /ml, whereas the novel compound (E)-methyl 4-chloro-alpha-[4-(4-chloropheny l)1,5-dihydro-3-hydroxy-5-oxo-1-(2-thiazolyl)-2H-pyrrol-2-ylidene]benzeneac etate gave an IC50 of 6.1 mu g/ml (13 mu M). The novel antagonist also inhi bited Dil-AcLDL uptake in cultured human peripheral and rat peritoneal macr ophages with IC50 values of 21 mu M and 17 mu M, respectively, With [I-125] AcLDL as ligand for transfected HEK 293 cells, binding/uptake and degradati on at 37 degrees C for 5 h was saturable and selective. In a comparison of both types of receptor, we found no difference between the capacity of SR-A I or SR-AII for either binding or degradation. Polyinosine competed both [I -125]AcLDL binding and degradation with a K-i of 1 mu g/ml, whereas the nov el antagonist competed with a K-i of 19 mu g/ml (40 mu M) and 8.6 mu g/ml ( 18 mu M), respectively, for binding and degradation. Saturation binding in the presence of the ionophore monensin indicated that the novel compound be haved as a noncompetitive antagonist and perhaps as an allosteric effector. This is the first report to describe a small-molecule macrophage scavenger receptor antagonist. Utilization of this permanently transfected HEK 293 c ell line will allow the identification of more potent macrophage scavenger receptor antagonists, so that their utility as therapeutics for atheroscler osis can be determined.