Transgenic rabbits were produced that expressed high plasma levels (30-70 mg/dl) of human
apolipoprotein (apo) E2(Cys-158), an
apoE variant associated with the human
genetic disorder type III hyperlipoproteinemia (HLP). Male transgenic rabbits fed normal chow had up to 8-fold (289 +/- 148 mg/dl) and 15-fold (697 +/- 452 mg/dl) increases in plasma total
cholesterol and
triglycerides, respectively, compared with nontransgenic males. Female transgenic rabbits had only a modest
hyperlipidemia (total
cholesterol, 140 +/- 46 mg/dl; total
triglycerides, 174 +/- 66 mg/dl). Both sexes displayed the hallmarks fo type III HLP: beta-migrating
very low density lipoproteins (
beta-VLDL) (intestinal and hepatic remnant
lipoproteins) and significantly increased VLDL and
intermediate density lipoproteins.
Apolipoprotein E2-containing VLDL particles were cleared from teh circulation more slowly and were more resistant to
lipoprotein lipase-mediated lipolysis than normal VLDL. Only females had increased
high density lipoproteins (HDL) (40%), which were shifted from typical small HDL to larger HDL1. Plasma
apoE2 was predominantly associated with
beta-VLDL in males and with HDL in females. To ascertain reasons for the phenotypic gender difference, we treated male transgenic rabbits with 17alpha-ethinyl
estradiol.
Estrogen treatment for 10 days dramatically decreased total
cholesterol (73%) and
triglycerides (89%) and converted
beta-VLDL to pre-beta-migrating VLDL. Concomitantly,
lipoprotein lipase and hepatic
lipase activities increased by 90%,
low density lipoprotein receptor activity was stimulated significantly,
apoE2 was redistributed to HDL, and HDL were converted to HDL1. Conversely,
ovariectomy in female transgenic rabbits significantly increased total
cholesterol (75%),
triglycerides (117%), and
beta-VLDL, while decreasing
lipoprotein lipase and hepatic
lipase activities by 35% and redistributing
apoE2 to the
beta-VLDL. Thus,
estrogen status appears to be responsible for much of the gender difference of the
lipoprotein phenotype, mainly by modulating both
lipase and
low density lipoprotein receptor activities. Furthermore, transgenic rabbits fed normal chow for 11 months developed fatty streaks, and some had more advanced atherosclerotic lesions, especially around the aortic arch and proximal abdominal aorta. The lesions were more extensive in males, roughly correlating with the magnitude of the
hyperlipidemia. Therefore, high plasma levels of human
apoE2 in transgenic rabbits result in a type III HLP phenotype, in which males have both more severe
hyperlipidemia and more extensive
atherosclerosis than females.