Antiatherogenic effects of oleanolic acid in apolipoprotein E knockout mice

Oleanolic acid (OA) is a plant triterpenoid steroid with potentially antiatherogenic properties. We investigated whether OA affected atherosclerosis development and vascular function in apolipoprotein E knockout (ApoE(-/-)) mice. ApoE(-/-) mice were fed a high cholesterol Western-type diet in combination with OA (100 mg/kg/day), fluvastatin (5 mg/kg/day) or vehicle, with wild type (WT) mice serving as controls. After 8 weeks of treatment atherosclerotic plaque areas in the aortic arch and plasma lipid concentrations were determined. Vasoconstriction and relaxation of the proximal part of aorta were investigated in vitro. Inducible nitric oxide synthase (iNOS) was visualized using immunoblotting. As opposed to WT and fluvastatin- and vehicle-treated mice, OA-fed ApoE(-/-) mice gained no weight during the treatment period. Plasma concentrations of total-cholesterol and triglyceride were not significantly reduced by OA- or fluvastatin treatment. Plaque area of vehicle-treated mice was 25%, but only 14% in OA- and 19% in fluvastatin-treated mice. As compared to WT, vasoconstriction to phenylephrine was attenuated in ApoE(-/-) mice. The NOS inhibitor asymmetric dimethylarginine (ADMA) enhanced phenylephrine constriction, but significantly more so in vehicle- and fluvastatin-treated than in OA-treated and WT mice. Relaxation to acetylcholine was only slightly attenuated in ApoE(-/-) mice and not affected by OA or fluvastatin treatment. ADMA abolished acetylcholine relaxation almost completely. In ApoE(-/-) mice iNOS expression was reduced by OA treatment. In conclusion OA exerts potent antiatherogenic effects independent of plasma lipid levels and without major changes in eNOS-mediated acetylcholine relaxation. However, OA reduced iNOS expression possibly altering vascular reactivity to phenylephrine.