Abstract

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-knockout 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 acetylated low-density lipoprotein (DiI-AcLDL) showed that among common polyanionic ligands, polyinosine was the most potent, with an IC₅₀ of 0.74 μg/ml, whereas the novel compound (E)-methyl 4-chloro-α-[4-(4-chlorophenyl)-1,5-dihydro-3-hydroxy-5-oxo-1-(2-thiazolyl)-2H-pyrrol-2-ylidene]benzeneacetate gave an IC₅₀ of 6.1 μg/ml (13 μM). The novel antagonist also inhibited DiI-AcLDL uptake in cultured human peripheral and rat peritoneal macrophages with IC₅₀ values of 21 μM and 17 μM, respectively. With [¹²⁵I]AcLDL as ligand for transfected HEK 293 cells, binding/uptake and degradation at 37°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-AI or SR-AII for either binding or degradation. Polyinosine competed both [¹²⁵I]AcLDL binding and degradation with aK_i of 1 μg/ml, whereas the novel antagonist competed with a K_i of 19 μg/ml (40 μM) and 8.6 μg/ml (18 μM), respectively, for binding and degradation. Saturation binding in the presence of the ionophore monensin indicated that the novel compound behaved 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 cell line will allow the identification of more potent macrophage scavenger receptor antagonists, so that their utility as therapeutics for atherosclerosis can be determined.