Effect of macrophage-derived apolipoprotein E on hyperlipidemia and atherosclerosis of LDLR-deficient mice

LDL receptor-deficient (LDLR^-/-) mice fed a Western diet exhibit severe hyperlipidemia and develop significant atherosclerosis. Apolipoprotein E (apoE) is a multifunctional protein synthesized by hepatocytes and macrophages. We sought to determine effect of macrophage apoE deficiency on severe hyperlipidemia and atherosclerosis. Female LDLR^-/- mice were lethally irradiated and reconstituted with bone marrow from either apoE ^-/- or apoE^+/+ mice. Four weeks after transplantation, recipient mice were fed a Western diet for 8 weeks. Reconstitution of LDLR ^-/- mice with apoE^-/- bone marrow resulted in a slight reduction in plasma apoE levels and a dramatic reduction in accumulation of apoE and apoB in the aortic wall. Plasma lipid levels were unaffected when mice had mild hyperlipidemia on a chow diet, whereas IDL/LDL cholesterol levels were significantly reduced when mice developed severe hyperlipidemia on the Western diet. The hepatic VLDL production rate of mice on the Western diet was decreased by 46% as determined by injection of Triton WR1339 to block VLDL clearance. Atherosclerotic lesions in the proximal aorta were significantly reduced, partially due to reduction in plasma total cholesterol levels (r=0.56;P<0.0001). Thus, macrophage apoE-deficiency alleviates severe hyperlipidemia by slowing hepatic VLDL production and consequently reduces atherosclerosis in LDLR^-/- mice.