Liver X Receptor Agonist Methyl-3β-Hydroxy-5α,6α-Epoxycholanate Attenuates Atherosclerosis in Apolipoprotein E Knockout Mice without Increasing Plasma Triglyceride

Abstract

Background: Liver X receptors (LXRs) promote macrophage reverse cholesterol transport and cholesterol excretion from the body. The synthetic LXR ligands T0901317 and GW3965 were shown to significantly inhibit atherosclerosis in mice and to increase the expression of ATP-binding cassette transporter A1 (ABCA1) in the atherosclerotic lesions. However, these compounds increase plasma and hepatic triglyceride (TG) levels in mice. Methyl-3β-hydroxy-5α,6α-epoxycholanate (MHEC), synthesized from hyodeoxycholic acid, functions as an LXR agonist, but its role in atherogenesis and lipid metabolism remained to be elucidated. Methods: THP-1-derived macrophages were cultured in the medium con- taining various concentrations of MHEC or T0901317 (0–10 µmol/l) for 24 h. Reverse transcription polymerase chain reaction was used to quantify LXRα, LXRβ and ABCA1 mRNA levels in macrophages. Additionally, MHEC or T0901317 was orally administered at 10 mg/kg daily for 6 weeks in apolipoprotein E knockout (apoE^–/–) mice fed a high-cholesterol diet. Plasma lipids were determined enzymatically. The area of and ABCA1 expression in the aortic atherosclerotic lesions were measured by oil red O staining and immunohistochemistry, respectively. Results: Both MHEC and T0901317 equally stimulated LXRα and ABCA1 mRNA expression in a dose-dependent manner in THP-1-derived macrophages, but they did not induce LXRβ mRNA expression significantly. The plasma levels of total cholesterol, TG and high-density lipoprotein cholesterol were significantly higher in T0901317-treated mice than in the vehicle-treated control group. Interestingly, MHEC treatment dramatically increased plasma high-density lipoprotein cholesterol without altering plasma levels of total cholesterol and TG. Both MHEC and T0901317 equally inhibited the development of atherosclerotic lesions in apoE^–/– mice. The expression of ABCA1, a cholesterol efflux transporter, was greatly induced by the two LXR agonists in the artery wall. Conclusions: MHEC is a novel LXR agonist and it inhibits atherosclerosis in apoE^–/– mice without raising blood TG. Thus, MHEC relative to T0901317 may be a better therapeutic LXR agonist for the treatment of atherosclerosis.