Hyperlipidemia of ApoE2(Arg ₁₅₈ -Cys) and ApoE3-Leiden Transgenic Mice Is Modulated Predominantly by LDL Receptor Expression

Abstract

Abstract-To investigate the relative roles of the LDL receptor-and non-LDL receptor-mediated pathways in the clearance of apolipoprotein E (apoE) variants in vivo, we have generated apoE2(Arg 158 -Cys) (apoE2) and apoE3-Leiden transgenic mice deficient for the endogenous mouse Apoe and Ldl receptor genes (ApoeϪ/Ϫ.LdlrϪ/Ϫ mice). Unexpectedly, on the ApoeϪ/Ϫ.LdlrϪ/Ϫ background, expression of neither apoE2 nor apoE3-Leiden results in a decrease of the hyperlipidemia. In contrast, serum cholesterol levels are increased by the introduction of apoE2 and apoE3-Leiden in ApoeϪ/Ϫ.LdlrϪ/Ϫ mice (to 39.1Ϯ7.1 and 37.6Ϯ7.6 mmol/L, respectively, from 25.9Ϯ6.5 mmol/L). In addition, in these transgenic mice, the serum triglyceride levels are substantially increased (to 9.6Ϯ7.0 and 5.8Ϯ2.8 mmol/L, respectively, from 0.7Ϯ0.5 mmol/L), which is associated with a decreased efficiency of in vitro LPL-mediated lipolysis of circulating VLDL. The VLDL-triglyceride secretion rate is not affected by the expression of apoE2 or apoE3-Leiden on the ApoeϪ/Ϫ.LdlrϪ/Ϫ background. These results indicate that in the absence of the LDL receptor, clearance of triglyceride-rich apoE2 and apoE3-Leiden-containing lipoproteins via alternative hepatic receptors, such as the LDL receptor-related protein (LRP) is inefficient. Although apoE2 and apoE3-Leiden are disturbed in binding to the LDL receptor in vitro, expression of 1 or 2 mouse Ldlr alleles in an apoE2.ApoeϪ/Ϫ or apoE3-Leiden.ApoeϪ/Ϫ background results in a gene dose-dependent decrease of the hyperlipidemia. Furthermore, overexpression of the LDL receptor via adenovirus-mediated gene transfer rescues the hyperlipidemia associated with apoE2 and apoE3-Leiden expression. These data indicate that in apoE2 and apoE3-Leiden transgenic mice, the LDL receptor constitutes the predominant route for clearance of VLDL remnants, carrying even poorly binding apoE variants, and that this pathway is functional despite an apoE-mediated disturbance in VLDL triglyceride lipolysis. Key Words: apolipoprotein E Ⅲ LDL receptor Ⅲ LDL receptor-related protein Ⅲ hypertriglyceridemia Ⅲ VLDL triglyceride lipolysis A polipoprotein (apo) E plays a key role in the lipoprotein metabolism by functioning as a ligand for receptormediated uptake of chylomicron and VLDL remnants by the liver (for review, see Reference 1). Mutations in apoE are associated with familial dysbetalipoproteinemia (FD), which is characterized by elevated levels of chylomicron and VLDL remnants in the serum and premature atherosclerosis (for reviews, see References 2 and 3). Several variant forms of apoE leading to FD have been described, such as the common apoE2(Arg 158 -Cys) (apoE2) variant, which is inherited as a recessive trait, and the rare apoE3-Leiden variant, which is inherited as a dominant trait. 4 The penetrance and severity of FD associated with the apoE2 and apoE3-Leiden variants is variable, 1,4 most likely because of the genetic and environmental heterogeneity of the human population. To investigate the role of these apoE variants in FD in a...

Chylomicron remnant uptake in the livers of mice expressing human apolipoproteins E3, E2 (Arg158→Cys), and E3-Leiden

“…As previously reported (30), for mice on a normal diet, fasting plasma triglyceride and cholesterol levels were significantly higher in A mApoe Ϫ/Ϫ /htgApoe3L mice than in mApoe Ϫ/Ϫ /htgApoe3 mice and levels in mApoe Ϫ/Ϫ / htgApoe2 mice were significantly elevated as compared with either mApoe Ϫ/Ϫ /htgApoe3 or mApoe Ϫ/Ϫ /htgApoe3L mice (Fig. 1A).…”

“…Transgenic mice expressing apoE2 and apoE3-Leiden in the absence of endogenous mouse apoE ( mApoe Ϫ / Ϫ /htgApoe2 and mApoe Ϫ / Ϫ /htgApoe3L ), which were previously described (30), were kindly provided by Dr. Havekes and Ko Willems van Dijk. LDLR-deficient mice ( mldlr Ϫ / Ϫ ) and double knockout mice homozygous for the absence of LDLR and apoE ( mApoe Ϫ / Ϫ /mldlr Ϫ / Ϫ ) were purchased from the Jackson Laboratory (Bar Harbor, ME).…”

Understanding hyperlipidemia and atherosclerosis: lessons from genetically modified apoe and ldlr mice

“…This showed that LpL enhances this process and that ApoE is not an absolute requirement for binding, but results in isoform-dependent increased binding (72). Crossing APOE2TG and APOE3L mice on an apoe y/y ldlr y/y background showed that clearance of ApoE2-and ApoE3L-containing lipoproteins in the absence of the LDLR, and thus via alternative pathways such as LRP, is inefficient (73). This shows the pivotal role of the LDLR for clearance of VLDL remnants.…”

Human LDL Receptor Enhances Sequestration of ApoE4 and VLDL Remnants on the Surface of Hepatocytes but Not Their Internalization in Mice

“…In contrast, limited interaction of apoE2 with the LDLR enhances VLDL clearance and reduces remnant production in apoE2 livers. 21,40 Consequently, the overall degrees of reduction in LDL-cholesterol resulting from the increase in LDLR expression in humans is likely to be apoE-isoform-dependent; with more pronounced reduction in individuals with apoE2 but lesser reduction with apoE4.…”

“…[17][18][19][20] Both adenovirus-mediated or global overexpression of the human LDLR in mice with apoE2 results in reduction of plasma cholesterol and TG and the absence of atherosclerosis. 17,21 Mice with human apoE3 and the Ldlr*h allele (Apoe 3/3 Ldlr h/ϩ ) have significantly decreased HDL-cholesterol but only a small increase in remnants and do not develop atherosclerosis. 18 In contrast, on a high-fat Western type diet, mice with apoE4 overexpressing the LDLR (Apoe 4/4 Ldlr h/ϩ ) have increased plasma very low density lipoproteins (VLDL)/ chylomicron remnants, decreased HDL-cholesterol levels, and develop atherosclerosis.…”