Harmful Effects of Increased LDLR Expression in Mice With Human APOE*4 But Not APOE*3

Abstract

Objective-Increased expression of the low-density lipoprotein receptor (LDLR) is generally considered beneficial for reducing plasma cholesterol and atherosclerosis, and its downregulation has been thought to explain the association between apolipoprotein (apo) E4 and increased risk of coronary heart disease in humans. Methods and Results-Contrary to this hypothesis, doubling Ldlr expression caused severe atherosclerosis with marked accumulation of cholesterol-rich, apoE-poor remnants in mice with human apoE4, but not apoE3, when the animals were fed a Western-type diet. The increased Ldlr expression enhanced in vivo clearance of exogenously introduced remnants in mice with apoE4 only when the remnants were already enriched with apoE4. The rates of nascent lipoprotein production were the same. The adverse effects of increased LDLR suggest a possibility that the receptor can trap apoE4, reducing its availability for the transfer to nascent lipoproteins needed for their rapid clearance, thereby increasing the production of apoE-poor remnants that are slowly cleared. The lower affinity for the LDLR of apoE3 compared with apoE4 could then explain why increased receptor expression had no adverse effects with apoE3. A polipoprotein E (apoE) plays a central role in the clearance of atherogenic lipoprotein particles from the circulation.1 The APOE gene in humans is polymorphic with 3 common alleles, APOE*2, APOE*3, and APOE*4, which code for apoE2, apoE3, and apoE4. These isoforms differ by the amino acids at positions 112 and 158, where apoE2 has Cys at both sites, apoE4 has Arg at both sites, and apoE3 has Cys-112 and Arg-158. There is a well-established association between the APOE polymorphism and the risk for vascular diseases; individuals with APOE*4 allele have increased plasma cholesterol and an increased risk of atherosclerosis.2,3 Although these increases are modest, they have a large impact on the overall human population, because 25% carry 1 or 2 APOE*4 alleles. However, this association is paradoxical when one considers that apoE4 binds to the LDL receptor (LDLR) with equal or a slightly greater affinity than apoE3, the most common isoform. 4 -6 Additionally, individuals homozygous for apoE2, which binds to the LDLR with much less affinity than either apoE3 or apoE4, have low plasma cholesterol and are generally protected from atherosclerosis, except for the 5% to 10% of apoE2 homozygotes who develop type III hyperlipoproteinemia. 1 The present explanation of this paradox is that the high affinity of apoE4 for the receptor leads to increased apoE-mediated cholesterol uptake followed subsequently by downregulation of the LDLR gene. This then leads to reduced apoB100-mediated uptake of LDL, accumulation of LDL cholesterol, and the vascular problems. 2,7,8 Conversely, the low affinity of apoE2 is thought to lead to upregulation of LDLR. Although this explanation seems reasonable given that the loss of function of even one LDLR allele leads to markedly elevated plasma LDL and premature atherosclerosis, 9,10 it...

Expression of the human apoE2 isoform in adipocytes: altered cellular processing and impaired adipocyte lipogenesis

“…Regulation of apoE genes in these knock-in mice remains under the control of endogenous apoE gene regulatory elements (29)(30)(31)(32)(33). We, therefore, fi rst evaluated level of larger size adipocytes; this shift was refl ected in a higher mean adipocyte diameter ( Fig.…”

“…These models have been described in detail in previous publications (29)(30)(31)(32)(33). In these mice, expression of the apoE isoform remains under the control of the endogenous apoE gene control elements.…”

Regulation of macrophage apoE secretion and sterol efflux by the LDL receptor

“…Mice homozygous for these apoE isoforms were bred with mice heterozygous for targeted replacement of the mouse LDL receptor gene with the human LDL receptor minigene (17). This LDL receptor minigene produces an mRNA with an increased half-life, leading to a 2-to 3-fold increase in LDL receptor transcript expression and increased LDL receptor activity (17,20). Cells isolated from each of the human genotypes were named according to the apoE isoform expressed (E2, E3, and E4) and designated WT cells if they had basal, or Ldlr cells if they had increased, LDL receptor expression.…”

“…LDL receptor protein level was measured by immunoblot analysis as described previously (22). mRNA levels for LDL receptor were quantitated by RT-PCR using a probe and primer set for murine exon 1, which measures both human and murine mRNA species by a method described previously (17,23).…”

“…The aim of the current studies was to address the role of macrophage LDL receptor expression for modulating apoE secretion from macrophages. Because of the wellknown differences in human apoE isoform interaction with the LDL receptor (17,18), we also addressed apoE isoform-specific effects. We used two models to investigate the role of the LDL receptor.…”