In the absence of endogenous mouse apolipoprotein E, apolipoprotein E*2(Arg-158 → Cys) transgenic mice develop more severe hyperlipoproteinemia than apolipoprotein E*3-Leiden transgenic mice

Abstract

Apolipoprotein E*2(Arg-155 → Cys) (APOE*2) transgenic mice were generated and compared to the previously generated apolipoprotein E*3- Leiden (APOE*3-Leiden) transgenic mice to study the variable expression of hyperlipoproteinemia associated with these two APOE variants. In the presence of the endogenous mouse Apoe gene, the expression of the APOE*3-Leiden gene resulted in slightly elevated levels of serum cholesterol as compared with control mice (2.7 ± 0.5 versus 2.1 ± 0.2 mmol/liter, respectively), whereas the expression of the APOE*2(Arg-158 → Cys) gene did not affect serum cholesterol levels, even after high/fat cholesterol feeding. The extreme cholesterol level usually found in apoE-deficient mice (Apoe(-/-) mice; 23.6 ± 5.0 mmol/liter) could be rescued by introducing the APOE*3-Leiden gene (APOE*3-Leiden·Apoe(-/-); 3.6 ± 1.5 mmol/liter), whereas the expression of the APOE*2(Arg-158 → Cys) gene in Apoe(-/-) mice minimally reduced serum cholesterol levels (APOE*2·Apoe(-/-); 16.6 ± 2.9 mmol/liter). In vivo very low density lipoprotein (VLDL) turnover studies revealed that APOE*2·Apoe(- /-) VLDL and APOE*3-Leiden·Apoe(-/-) VLDL display strongly reduced fractional catabolic rates as compared with control mouse VLDL (4.0 and 6.1 versus 22.1 pools/h). In vitro low density lipoprotein (LDL) receptor binding studies using HepG2 and J774 cells showed that APOE*2·Apoe(-/-) VLDL is completely defective in binding to the LDL receptor, whereas APOE*3- Leiden·Apoe(-/-) VLDL still displayed a considerable binding activity to the LDL receptor. After transfection of APOE*2·Apoe(-/-) and APOE*3- Leiden·Apoe(-/-) mice with adenovirus carrying the gene for the receptor- associated protein (AdCMV-RAP), serum lipid levels strongly increased (15.3 to 42.8 and 1.4 to 15.3 mmol/liter for cholesterol and 5.0 to 35.7 and 0.3 to 20.7 mmol/liter for triglycerides, respectively). This indicates that RAP- sensitive receptors, possibly the LDL receptor-related protein (LRP), mediate the plasma clearance of both APOE*2·Apoe(-/-) and APOE*3-Leiden·Apoe(-/- ) VLDL. We conclude that in vivo the APOE*2 variant is completely defective in LDL receptor binding but not in binding to LRP, whereas for the APOE*3- Leiden mutant both LRP and LDL receptor binding activity are only mildly affected. As a consequence of this difference, APOE*2·Apoe(-/-) develop more severe hypercholesterolemia than APOE*3-Leiden·Apoe(-/-) mice.