A Combination Therapy Of LXR Ligand And MEK1/2Inhibitor Against Atherosclerosis

Formation of macrophage/foam cells, endothelial injury and hypertriglyceridemia contribute to the development of atherosclerosis. Macrophage ATP-binding cassette transporter Al (ABCA1) mediates cholesterol efflux to apoAI thereby facilitating formation of nascent HDL and reverse cholesterol transport (RCT). Macrophage ABCA1has been demonstrated to inhibit the formation of foam cells and the development of atherosclerosis. Besides the efflux of free cholesterol mediated by ABCA1, inhibition of cholesterol uptake can also inhibit foam cell formation and lesion development. Interleukin5(IL-5) activates B-1cells to produce and secret T15/EO6IgM antibodies which have a high affinity for oxidized low-density lipoprotein (ox-LDL) in circulation, thereby blocks the uptake of ox-LDL by macrophages and attenuates atherosclerosis. In contrast, deficiency of macrophage IL-5expression accelerates the development of atherosclerosis. Endothelial injury is the initial event of atherogenesis. MiR-126inhibits expression of RGS16, the inhibitor of CXCL12/CXCR4signaling pathways, thereby promoting endothelial repair. MiR-126can also reduce expression of VCAM-1. Therefore, miR-126plays an atheroprotective role. Liver X receptor (LXR), a ligand-activated transcription factor, induces macrophage ABCA1expression. However, LXR also induces fatty acid biosynthesis that results in severe hepatic lipogenesis and hypertriglyceridemia. We previously reported that mitogen-activated protein kinase kinases1and2(MEK1/2) inhibitors synergize LXR-induced ABCA1expression and cholesterol efflux. Meanwhile, it has been reported that the mice lacking ERK1activity demonstrate decreased adiposity, reduced adipocytes and attenuated fat tissue inflammation, while enhanced basal metabolic levels. Therefore, we determined if the combined MEK1/2inhibitor and LXR ligand can reduce atherosclerosis in a synergistic manner while eliminating LXR ligand-induced side effects. We also attempted to disclose the involved mechanisms. Indeed, after completing this study, we observed that MEK1/2inhibitor, U0126, synergized LXR ligand, T0901317, inhibited the development of atherosclerosis but it totally blocked T0901317-induced hypertriglyceridemia and fatty liver. The advanced lesions were also regressed by the combined agents. Mechanistically, we determined that the combination can synergistically activate macrophage ABCA1expression and RCT in vivo, inhibit foam cell formation and macrophage accumulation in aortic root. It also induced miR-126expression in endothelial and smooth muscle cells thereby decreasing RGS16and VCAM-1expression while increasing CXCL12and CXCR4expression in aortic root.In addition, we observed that T0901317demonstrated immunological functions, such as induced macrophage IL-5protein expression and secretion, serum IL-5and total IgM levels, and IL-5expression in the aortic root area. U0126alone or in combination with T0901317inhibited expression of genes for triglyceride (TG) biosynthesis while activated expression of genes for TG hydrolysis and free fatty acid oxidation thereby blocking hypertriglyceridemia and hepatic lipogenesis. Taken together, our study demonstrates by increasing ABCA1expression/RCT, enhancing expression of genes for endothelial repair and blocking hypertriglyceridemia, the combined MEK1/2inhibitor and LXR ligand synergistically inhibits atherosclerosis. Our study also suggests the combined MEK1/2inhibitor and LXR ligand can function as a novel therapy in treatment of atherosclerosis.