Isolation and characterization of a full-length rabbit apolipoprotein E cDNA

Scavenger receptor activity is increased in macrophages from rabbits with low atherosclerotic response: studies in normocholesterolemic high and low atherosclerotic response rabbits

We have previously described 2 strains of New Zealand White rabbits with a high (HAR) or low (LAR) atherosclerotic response to hypercholesterolemia. In the present study, we focused on class A scavenger receptor (SR-A) activity and ApoE expression in macrophages from both rabbit strains. These parameters play a crucial role in maintaining cholesterol homeostasis in the arterial wall and may be involved in the development of atherosclerosis. SR activity, as measured by uptake of DiI-labeled acetylated LDL, was significantly higher in macrophages from LAR rabbits (2177+/-253 ng/mg cell protein) than in macrophages from HAR rabbits (1153+/-200 ng/mg cell protein). The higher SR activity was caused by a greater number of SRs (apparent Vmax, 4100 ng/mg in LAR and 1980 ng/mg in HAR rabbits). The high SR activity in macrophages from LAR rabbits was associated with a significantly higher expression of SR-A mRNA compared with macrophages from HAR rabbits. However, the latter finding could not be explained by differences in the activity of transcription factor-activating protein 1 (AP-1), which was comparable in macrophages from both strains of rabbits. Because under certain circumstances SR-A mRNA expression is regulated in parallel with ApoE expression, we also evaluated this parameter. Although ApoE mRNA was 74% higher in macrophages from LAR rabbits, the difference did not reach statistical significance. In conclusion, the increased expression of SR-A in macrophages in the presence of adequate amounts of ApoE may play a role in attenuating atherosclerosis in LAR rabbits.

Cerebrovascular amyloidosis in squirrel monkeys and rhesus monkeys: apolipoprotein E genotype

Some neuropathological changes characteristic of aging and Alzheimer's disease (AD) in humans are present also in senescent non-human primates. The human apoE4 allele is associated with an increased risk of developing late-onset familial and sporadic AD. We found that rhesus monkeys and three subspecies of squirrel monkeys are homozygous for apoE phenotype with arginine at positions 112 and 158 as in human apoE4. However, in both species threonine replaces arginine at position 61 of human apoE. It was previously shown that arginine 61 was critical in determining apoE4 lipoprotein distribution in humans.

Expression of apolipoprotein E in cholesterol-loaded macrophages of extrahepatic tissues during experimental hypercholesterolemia

To study the expression of extrahepatic apolipoprotein E (apoE) under hypercholesterolemic conditions, apoE mRNA levels were evaluated in 14 tissues of rabbits fed on a cholesterol rich diet and compared to age-matched control animals. In hypercholesterolemic rabbits apoE expression was significantly induced in adipose tissue, adrenals, aorta, lung and spleen. The increase in apoE mRNA levels in lung and spleen was associated with the presence of cholesterol-loaded macrophages. These cells were found to express high levels of apoE mRNA as demonstrated by in situ mRNA hybridization. Our results suggest that extrahepatic tissues would be partially responsible for the rise in serum apoE levels detected under hypercholesterolemic conditions.

Arginine residues at codons 112 and 158 in the apolipoprotein E gene correspond to the ancestral state in humans

Apolipoprotein E is a secretory glycoprotein that associates with lipoprotein particles and is coded for by a single locus on chromosome 19. The three common allelic isoforms of this protein (apo E2, apo E3 and apo E4) are associated with distinct patterns of lipoprotein metabolism and variable risks for coronary artery disease. In addition, recent work has shown that the presence of the apo E4 isoform constitutes a major risk for developing late-onset Alzheimer's disease as well as hypercholesterolaemia. The only differences between these isoforms result from cysteine-arginine interchanges at codons 112 and 158. There is considerable disagreement in the literature concerning the identity of the ancestral allele. In order to resolve this, 24 chimpanzees and individuals from a number of other primate species were analysed. All were similar to apo E4. This suggests that apo E4 is the ancestral allele and that apo E2 and apo E3 arose after the split of the human and chimpanzee lineages.

Neuropathology and apolipoprotein E profile of aged chimpanzees: implications for Alzheimer disease

Neuropathological findings in three aged chimpanzees were compared with those in rhesus monkeys and individuals with Alzheimer disease. Senile plaques and blood vessels were immunoreactive for amyloid beta-protein and apolipoprotein E (apoE) in the nonhuman primates, recapitulating findings in human aging and Alzheimer disease. Neurofibrillary tangles, another hallmark of Alzheimer disease, were absent. PCR/restriction-enzyme analysis in chimpanzees revealed an APOE profile similar to the human APOE type 4 allele associated with an increased risk of Alzheimer disease. These findings militate against the hypothesis that the absence of APOE type 3 allele predisposes to neurofibrillary tangle formation and support the value of aged primates for exploring mechanisms of amyloid processing and the role of apoE.

Abundant expression of apoprotein E by macrophages in human and rabbit atherosclerotic lesions

Previous studies have demonstrated the presence of apoprotein (apo) E protein and message in arterial lesions. To determine the source of the synthesized apoE, we performed simultaneous in situ hybridization and immunocytochemistry on human and rabbit atherosclerotic tissue. Studies of serial sections of aortic atherosclerotic lesions from humans and hypercholesterolemic New Zealand White rabbits and Watanabe heritable hyperlipidemic rabbits revealed a similar pattern of macrophage-specific apoE expression in the rabbit and human lesions. In early lesions of rabbit atherosclerotic tissue, in which many macrophages were present, there was abundant expression of apoE mRNA. Northern blot analyses of total mRNA obtained from arterial macrophage-derived foam cells, freshly isolated from ballooned, cholesterol-fed New Zealand White rabbits, demonstrated positive hybridization with an apoE-specific riboprobe. Western blot analyses of conditioned media from the isolated foam cells placed in culture for up to 24 hours demonstrated the presence of secreted apoE. These studies demonstrated that in atherosclerotic lesions, arterial wall macrophages synthesize and secrete apoE and probably account for most of the apoE synthesized in the atherosclerotic artery.

Cloning and sequencing of bovine apolipoprotein E complementary DNA and molecular evolution of apolipoproteins E, C-I, and C-II

Apolipoprotein (apo) E, a major protein component of plasma lipoproteins, is a physiological ligand for the low density lipoprotein (LDL) receptor as well as for a specific apoE receptor; it is therefore an important modulator of lipoprotein metabolism. In this study we cloned and sequenced bovine apoE complementary DNA. Comparison of nucleotide substitution rates shows that apoE is less conservative than apoA-I and evolves about 30% faster than an average mammalian protein. Although apoE is not a conservative protein, several regions have been well conserved among all eight mammalian sequences now available. These include a 33-amino-acid block immediately upsteam from the third intron/exon junction and the LDL receptor binding region. We have also compared published apoC-I and apoC-II sequences. Both proteins are less conservative than apoE. In particular, apoC-I shows no well-conserved region except for a small region in the common 33-amino-acid block, suggesting that the function of apoC-I does not have stringent structural requirements. On the other hand, in apoC-II the region encoded by exon 4, which consists of the last 29 amino acids of the polypeptide, has been rather well conserved, probably because this region is important for the activation of lipoprotein lipase and chylomicron and very low density lipoprotein metabolism.

Molecular phylogeny of Rodentia, Lagomorpha, Primates, Artiodactyla, and Carnivora and molecular clocks

Phylogenetic analysis of DNA sequences from primates, rodents, lagomorphs, artiodactyls, carnivores, and birds strongly suggests that the order Rodentia is an outgroup to the other four mammalian orders and that Artiodactyla and Carnivora belong to a superordinal clade. Further, there is strong evidence against the Glires concept, which unites Lagomorpha and Rodentia. The radiation among Lagomorpha, Primates, and Artiodactyla--Carnivora is very bush-like, but there is some evidence that Lagomorpha has branched off first. Thus, the branching sequence for these five orders of mammals seems to be Rodentia, Lagomorpha, Primates, Artiodactyla, and Carnivora. The branching date for Rodentia could be as early as 100 million years ago. The rate of nucleotide substitution in the rodent lineage is shown to be at least 1.5 times higher than those in the other four mammalian lineages.

Induction of apolipoprotein E gene expression in human and experimental atherosclerotic lesions

Hypercholesterolemia and atherosclerosis were induced in New Zealand White rabbits by cholesterol feeding. Apolipoprotein E mRNA levels in livers were found to be slightly increased, as determined by Northern blots. Apolipoprotein E gene expression was dramatically induced in rabbit atherosclerotic aortas with respect to healthy aortas. However, apolipoprotein E mRNA levels in atherosclerotic aortas were low as compared with the hepatic mRNA levels of the same animals. Interestingly, we also found a significant increase in apolipoprotein E expression in human atheromata with respect to healthy aorta from the same individual. This is the first report on apo E gene induction in human atherosclerotic lesions.