Natural killer T cells in lipoprotein metabolism and atherosclerosis

Cells of both the innate and adaptive immune system participate in the development of atherosclerosis, a chronic inflammatory disorder of medium and large arteries. Natural killer T (NKT) cells express surface markers characteristic of natural killer cells and conventional T cells and bridge the innate and adaptive immune systems. The development and activation of NKT cells is dependent upon CD1d, a MHC-class I-type molecule that presents lipids, especially glycolipids to the T cell receptors on NKT cells. There are two classes of NKT cells; invariant NKT cells that express a semi-invariant T cell receptor and variant NKT cells. This review summarises studies in murine models in which the effect of the activation, overexpression or deletion of NKT cells or only invariant NKT cells on atherosclerosis has been examined.

HDL apolipoprotein-related peptides in the treatment of atherosclerosis and other inflammatory disorders

Elevations of HDL levels or modifying the inflammatory properties of HDL are being evaluated as possible treatment of atherosclerosis, the underlying mechanism responsible for most cardiovascular diseases. A promising approach is the use of small HDL apoprotein-related mimetic peptides. A number of peptides mimicking the repeating amphipathic α-helical structure in apoA-I, the major apoprotein in HDL, have been examined in vitro and in animal models. Several peptides have been shown to reduce early atherosclerotic lesions, but not more mature lesions unless coadministered with statins. These peptides also influence the vascular biology of the vessel wall and protect against other acute and chronic inflammatory diseases. The biologically active peptides are capable of reducing the pro-inflammatory properties of LDL and HDL, likely due to their high affinity for oxidized lipids. They are also capable of influencing other processes, including ABCA1 mediated activation of JAK-2 in macrophages, which may contribute to their anti-atherogenic function. The initial studies involved monomeric 18 amino acid peptides, but tandem peptides are being investigated for their anti-atherogenic and anti-inflammatory properties as they more closely resemble the repeating structure of apoA-I. Peptides based on other HDL associated proteins such as apoE, apoJ and SAA have also been studied. Their mechanism of action appears to be distinct from the apoA-I based mimetics.

The influence of acute phase proteins on murine atherosclerosis

Atherosclerosis is a chronic inflammatory reaction that is initiated in response to hyperlipidemia and the retention and modification of lipids within the vascular wall. Chronic inflammatory states lead to steady low-level induction of the acute phase reaction and chronic inflammation is associated with elevated cardiovascular disease and atherosclerosis. The acute phase reaction is mediated by cytokines and results in significant changes in the plasma level of several proteins referred to as acute phase proteins. The liver is a major source of these proteins. Several recent studies in humans have shown that levels of acute phase proteins are modified in patients with established cardiovascular disease or are predictors of future disease. Whether these acute phase proteins are a biomarker of inflammation or have a direct role in the development of atherosclerosis is not clear. Murine models of atherosclerosis have been used to address the role of acute phase proteins in atherosclerosis. Modification of the expression level of these proteins has shown that the individual acute phase proteins are either pro-atherogenic or anti-atherogenic. The absence of an overall trend is perhaps not surprising given the complex nature of the acute phase response.

In vivo studies of HDL assembly and metabolism using adenovirus-mediated transfer of ApoA-I mutants in ApoA-I-deficient mice

We have used adenovirus-mediated gene transfer in apoA-I-deficient (A-I-/-) mice to probe the in vivo assembly and metabolism of HDL using apoA-I variants, focusing primarily on the role of the C-terminal 32 amino acids (helices 9-10). Lipid, lipoprotein, and apoA-I analyses showed that plasma levels of apoA-I and HDL of the mutants were 40-88% lower than that of wild type (WT) human apoA-I despite comparable levels of expression in the liver. WT apoA-I and mutant 1 (P165A, E172A) formed spherical particles with the size and density of HDL2 and HDL3. Mutant 2 (E234A, E235A, K238A, K239A) generated spherical particles with density between HDL2 and HDL3. Mutant 3 (L211V, L214V, L218V, L219V) and mutant 4 (L222K, F225K, F229K), which have substitutions of hydrophobic residues in the C-terminus, generated discoidal HDL particles indicating a defect in their conversion to mature spherical HDL. Significant amounts of mutant 4 and mutant 5 (truncated at residue 219) were found in the lipid poor fractions after ultracentrifugation of the plasma (18 and 35%, respectively, of total apoA-I). These findings suggest that hydrophobic residues in and/or between helices 9 and 10 are important for the maturation of HDL in vivo.

Apolipoprotein E and apolipoprotein E receptors modulate A beta-induced glial neuroinflammatory responses

Large numbers of activated glia are a common pathological feature of many neurodegenerative disorders, including Alzheimer's disease (AD). Several different stimuli, including lipopolysaccharide (LPS), dibutyryl (db)cAMP, and aged amyloid-beta 1-42 (A beta), can induce glial activation in vitro, as measured by morphological changes and the production of pro-inflammatory cytokines and oxidative stress molecules. Only A beta-induced activation is attenuated by the addition of exogenous apolipoprotein E (apoE)-containing particles. In addition, only A beta also induces an increase in the amount of endogenous apoE, the primary apolipoprotein expressed by astrocytes in the brain. The functional significance of the increase in apoE appears to be to limit the inflammatory response. Indeed, compared to wild type mice, glial cells cultured from apoE knockout mice exhibit an enhanced production of several pro-inflammatory markers in response to treatment with A beta and other activating stimuli. The mechanism for both the A beta-induced glial activation and the increase in apoE appears to involve apoE receptors, a variety of which are expressed by both neurons and glia. Experiments using receptor associated protein (RAP), an inhibitor of apoE receptors with a differential affinity for the low-density lipoprotein receptor (LDLR) and the LDLR-related protein (LRP), revealed that LRP mediates A beta-induced glial activation, while LDLR mediates the A beta-induced changes in apoE levels. In summary, both an apoE receptor agonist (apoE) and an antagonist (RAP) inhibit A beta-induced glial cell activation. Thus, apoE receptors appear to translate the presence of extracellular A beta into cellular responses, both initiating glial cell activation and limiting its scope by inducing apoE, an anti-inflammatory agent.

Effect of immune deficiency on lipoproteins and atherosclerosis in male apolipoprotein E-deficient mice

To determine whether T cells and B cells influence lipid metabolism and atherosclerosis, we crossed apolipoprotein E-deficient (apoE degrees ) mice with recombination activating gene 2-deficient (RAG2 degrees ) mice. Total plasma cholesterol levels were approximately 20% higher in male apoE degrees mice compared with the apoE degrees RAG2 degrees mice at 8 weeks of age, and plasma triglyceride levels were 2.5-fold higher in the apoE degrees mice even when plasma cholesterol levels were similar. Male mice with plasma cholesterol levels between 400 and 600 mg/dL at 8 weeks of age were euthanized at 27 and 40 weeks of age. The aortic root lesion area in the apoE degrees RAG2 degrees mice, compared with that in the immune-competent apoE degrees mice, was 81% and 57% smaller at 27 and 40 weeks of age, respectively. In contrast, there was no difference in the size of the brachiocephalic trunk lesions. Similar results were obtained with mice euthanized at 40 weeks of age that had 8-week cholesterol levels between 300 and 399 mg/dL. In apoE degrees RAG2 degrees mice, aortic root atherosclerosis was more profoundly suppressed at lower cholesterol levels. Thus, T and B cells and their products differentially influence the development of atherosclerosis at different sites. We also demonstrate a profound effect of the immune system on plasma lipid homeostasis.

Mouse models of atherosclerosis

Atherosclerosis bears many features of a chronic inflammation that affects the intima of large and medium-sized arteries. In recent years apolipoprotein E-deficient and LDL receptor-deficient mice have been used to examine the effects of various gene products on the development of atherosclerosis. In the present review the effects of genetics, apolipoprotein E, inflammatory gene modifiers, lipoprotein modifications, lipoprotein receptors, vessel wall expression of lipoprotein-metabolizing enzymes, and the atheroprotective role of HDL on atherosclerosis in these mice are discussed. The importance of examining lesions that are more advanced than fatty streaks and careful histologic and immunologic examination of lesion composition is emphasized.

SDS-stable complex formation between native apolipoprotein E3 and beta-amyloid peptides

Extracellular senile plaques composed predominantly of fibrillar amyloid-beta (Abeta) are a major neuropathological feature of Alzheimer's disease (AD). Genetic evidence and in vivo studies suggest that apolipoprotein E (apoE) may contribute to amyloid clearance and/or deposition. In vitro studies demonstrate that native apoE2 and E3 form an SDS-stable complex with Abeta(1-40), while apoE4 forms little such complex. Our current work extends these observations by presenting evidence that apoE3 also binds to Abeta(1-42) and with less avidity to modified species of the peptide found in senile plaque cores. These modified peptides include a form that originates at residue 3-Glu as pyroglutamyl and another with isomerization at the 1-Asp and 7-Asp positions. In addition, we used binding reactions between apoE3 and various Abeta fragments, as well as binding reactions with apoE3 and Abeta(1-40) plus Abeta fragments as competitors, to identify the domain(s) of Abeta involved in the formation of an SDS-stable complex with apoE3. Residues 13-28 of Abeta appear to be necessary, while complex formation is further enhanced by the presence of residues at the C-terminus of the peptide. These results contribute to our understanding of the biochemical basis for the SDS-stable apoE3/Abeta complex and support the hypothesis that Abeta can be transported in vivo complexed with apoE. This complex may then be cleared from the interstitial space by apoE receptors in the brain or become part of an extracellular amyloid deposit.

Apolipoprotein E receptors mediate the effects of beta-amyloid on astrocyte cultures

We have previously shown that beta-amyloid (Abeta) induces astrocyte activation in vitro and that this reaction is attenuated by the addition of exogenous apolipoprotein E (apoE)-containing particles. However, the effects of Abeta on endogenous apoE and apoJ levels and the potential role of apoE receptors in astrocyte activation have not been addressed. Three activating stimuli (lipopolysaccharide, dibutyryl cAMP, and aged Abeta 1-42) were used to induce activation of rat astrocyte cultures, as assessed by changes in morphology and an increase in interleukin-1beta. However, only Abeta also induced approximately 50% reduction in the amount of released apoE and apoJ and an 8-fold increase in the levels of cell-associated apoE and apoJ. Experiments using two concentrations of receptor-associated protein, an inhibitor of apoE receptors with a differential affinity for the low density lipoprotein receptor (LDLR) and the LDLR-related protein (LRP), suggest that LRP mediates Abeta-induced astrocyte activation, whereas LDLR mediates the Abeta-induced changes in apoE levels. Receptor-associated protein had no effect on apoJ levels or on activation by either dibutyryl cAMP or lipopolysaccharide. These data suggest that apoE receptors translate the presence of extracellular Abeta into cellular responses, both initiating and modulating the inflammatory response induced by Abeta.

Unique lipoproteins secreted by primary astrocytes from wild type, apoE (-/-), and human apoE transgenic mice

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease via an unknown mechanism(s). As glia are the primary central nervous system cell type that synthesize apoE, we characterized lipoproteins secreted by astrocytes from wild type (WT), apoE (-/-), and apoE transgenic mice expressing human apoE3 or apoE4 in a mouse apoE (-/-) background. Nondenaturing size exclusion chromatography demonstrates that WT, apoE3, and apoE4 astrocytes secrete particles the size of plasma high density lipoprotein (HDL) composed of phospholipid, free cholesterol, and protein, primarily apoE and apoJ. However, the lipid:apoE ratio of particles containing human apoE is significantly lower than WT. ApoE localizes across HDL-like particle sizes. ApoJ localizes to the smallest HDL-like particles. ApoE (-/-) astrocytes secrete little phospholipid or free cholesterol despite comparable apoJ expression, suggesting that apoE is required for normal secretion of astrocyte lipoproteins. Further, particles were not detected in apoE (-/-) samples by electron microscopy. Nondenaturing immunoprecipitation experiments indicate that apoE and apoJ reside predominantly on distinct particles. These studies suggest that apoE expression influences the unique structure of astrocyte lipoproteins, a process further modified by apoE species.

SAA-only HDL formed during the acute phase response in apoA-I+/+ and apoA-I-/- mice

Serum amyloid A (SAA) is an acute phase protein of unknown function that is involved in systemic amyloidosis and may also be involved in atherogenesis. The precise role of SAA in these processes has not been established. SAA circulates in plasma bound to high density lipoprotein-3 (HDL3). The pathway for the production of SAA-containing HDL is not known. To test whether apolipoprotein (apo)A-I-HDL is required in the production of SAA-HDL, we analyzed the lipopolysaccharide (LPS)-induced changes in apoA-I+/+ and apoA-I-/- mice. In apoA-I+/+ mice, after injection of LPS, remodeling of HDL occurred: total cholesterol increased and apoA-I decreased slightly and shifted to lighter density. Dense (density of HDL3) but large (size of HDL2 ) SAA-containing particles were formed. Upon fast phase liquid chromatography fractionation of plasma, >90% of SAA eluted with HDL that was enriched in cholesterol and phospholipid and shifted "leftward" to larger particles. Non-denaturing immunoprecipitation with anti-mouse apoA-I precipitated all of the apoA-I but not all of the SAA, confirming the presence of SAA-HDL devoid of apoA-I. In the apoA-I-/- mice, which normally have very low plasma lipid levels, LPS injection resulted in significantly increased total and HDL cholesterol. Greater than 90% of the SAA was lipid associated and was found on dense but large, spherical HDL particles essentially devoid of other apolipoproteins.We conclude that serum amyloid A (SAA) is able to sequester lipid, forming dense but large HDL particles with or without apoA-I or other apolipoproteins. The capacity to isolate lipoprotein particles containing SAA as the predominant or only apolipoprotein provides an important system to further explore the biological function of SAA.

Peptide model of a highly conserved, N-terminal domain of apolipoprotein E is able to modulate lipoprotein binding to a member of the class A scavenger receptor family

Apolipoprotein E plays a critical role in plasma lipoprotein clearance. Peptide models of a highly conserved, N-terminal domain of this protein have been shown to increase the binding of low density lipoprotein (LDL) to fibroblast cell surfaces independently of the low density lipoprotein receptor. Here we provide data to show that these peptides not only increase the binding of LDL, but also of high density lipoprotein, though not acetylated LDL. We also have data suggesting that this novel activity is mediated, at least in part, by a member of the scavenger receptor family, SR-AI. Furthermore, we show that this activity is also prominent in macrophages, a cell relevant to atherogenesis. In addition, this current paper provides evidence suggesting that this complex binding activity is initiated by a peptide-receptor interaction, and that our peptides are able to induce activity at physiologically relevant concentrations. This study provides evidence for a possible novel receptor interaction and further anti-atherogenic properties of apolipoprotein E and raises the possibility of a therapeutic potential of our peptide models.

Association of human apolipoprotein E with lipoproteins secreted by transfected McA RH7777 cells

To examine the association of apolipoprotein (apo) E with nascent hepatic lipoproteins we have prepared stable transfectants of the rat hepatoma cell line McA RH7777 expressing the human apoE3 cDNA. When the nascent lipoproteins secreted from control cells were separated on fast protein liquid chromatography (FPLC) columns, rat apoE was detected in the very low density (VLDL) and high density lipoprotein (HDL) fractions, while rat apoA-I was found in the HDL and lipoprotein free fractions. Human apoE was also associated with the VLDL and HDL particles secreted from the transfected McA RH7777 cells. Expression of human apoE resulted in a significant decrease in the amount of rat apoA-I associated with the lipoprotein particles. Rat apoE was also displaced, but to a lesser extent. Infection of McA RH7777 cells at different multiplicities of infection with recombinant adenoviral vector containing the human apoE cDNA indicated that rat apoA-I was decreased in the HDL fractions at lower levels of expression of human apoE than was rat apoE. The HDL particles were further examined by immunoblotting of nondenaturing gradient gels and by non-denaturing immunoprecipitation. The results indicate that the high density lipoprotein (HDL) particles are heterogeneous in size and apolipoprotein composition with the majority of the rat and human apolipoproteins being located on different particles. These results suggest that the profile and concentration of HDL apolipoproteins produced in hepatocytes influences the assembly of the various subsets of secreted HDL.

Nascent astrocyte particles differ from lipoproteins in CSF

Little is known about lipid transport and metabolism in the brain. As a further step toward understanding the origin and function of CNS lipoproteins, we have characterized by size and density fractionation lipoprotein particles from human CSF and primary cultures of rat astrocytes. The fractions were analyzed for esterified and free cholesterol, triglyceride, phospholipid, albumin, and apolipoproteins (apo) E, AI, AII, and J. As determined by lipid and apolipoprotein profiles, gel electrophoresis, and electron microscopy, nascent astrocyte particles contain little core lipid, are primarily discoidal in shape, and contain apoE and apoJ. In contrast, CSF lipoproteins are the size and density of plasma high-density lipoprotein, contain the core lipid, esterified cholesterol, and are spherical. CSF lipoproteins were heterogeneous in apolipoprotein content with apoE, the most abundant apolipoprotein, localized to the largest particles, apoAI and apoAII localized to progressively smaller particles, and apoJ distributed relatively evenly across particle size. There was substantial loss of protein from both CSF and astrocyte particles after density centrifugation compared with gel-filtration chromatography. The differences between lipoproteins secreted by astrocytes and present in CSF suggest that in addition to delivery of their constituents to cells, lipoprotein particles secreted within the brain by astrocytes may have the potential to participate in cholesterol clearance, developing a core of esterified cholesterol before reaching the CSF. Study of the functional properties of both astrocyte-secreted and CSF lipoproteins isolated by techniques that preserve native particle structure may also provide insight into the function of apoE in the pathophysiology of specific neurological diseases such as Alzheimer's disease.

Isoform-specific effect of apolipoprotein E on cell survival and beta-amyloid-induced toxicity in rat hippocampal pyramidal neuronal cultures

Although the genetic link between the epsilon4 allele of apolipoprotein E (apoE) and Alzheimer's disease is well established, the isoform-specific activity of apoE underlying this correlation remains unclear. To determine whether apoE influences the neurotoxic actions of beta-amyloid (Abeta), we examined the effect of native preparations of apoE3 and E4 on Abeta-induced toxicity in primary cultures of rat hippocampal pyramidal neurons. The source of apoE was conditioned medium from HEK-293 cells stably transfected with human apoE3 or E4 cDNA. ApoE4 (10 microg/ml) alone was toxic to the cultures, whereas apoE3 had no effect. ApoE3 treatment prevented the toxicity induced by 10 microM Abeta(1-40) or Abeta(25-35). The apoE3 protective effect appears to be specific to Abeta-induced toxicity, because apoE3 did not protect against the cytotoxicity produced by NMDA or staurosporine, nor did apoE3 affect the increase in intracellular calcium induced by either NMDA or KCl. ApoE3 had no effect on the toxicity produced by Abeta in the presence of receptor-associated protein, an inhibitor of apoE receptors, particularly the LDL-receptor-related protein. Interaction with apoE receptors may not mediate the toxic actions of apoE4, because receptor-associated protein did not affect apoE4-induced neurotoxicity. Consistent with our previous biochemical experiments, analysis of the culture medium revealed that SDS-stable apoE3:Abeta complex is present in greater abundance than apoE4:Abeta complex. Thus, the protection from Abeta-induced neurotoxicity afforded by apoE3 treatment may result from clearance of the peptide by apoE3:Abeta complex formation and uptake by apoE receptors.

Association of human, rat, and rabbit apolipoprotein E with beta-amyloid

In humans, apolipoprotein E (apoE) has three major isoforms, E2 (Cys112, Cys158), E3 (Cys112, Arg158), and E4 (Arg112, Arg158). While epsilon4 is a genetic risk factor for Alzheimer's disease (AD), epsilon2 may protect against late-onset AD. Using native preparations of apoE from conditioned tissue culture media or plasma lipoproteins, we have previously shown that when equivalent amounts of apoE3 or E4 were incubated with beta-amyloid (A beta), apoE3 formed 20 times as much SDS-stable complex with the peptide as apoE4. This preferential binding of A beta to apoE3 was abolished when apoE was purified by a process which includes delipidation and denaturation. Here we expand these observations to include A beta binding to lipoprotein-associated and purified apoE2. Lipoproteins isolated from the plasma of individuals homozygous for either epsilon2 or epsilon3 were incubated with A beta(1-40). SDS-stable complex formation was analyzed by a non-reducing gel shift assay, followed by immunoblotting with either A beta or apoE antibodies. ApoE2:A beta complex formation was comparable to apoE3:A beta in both native and purified preparations of apoE. In addition, lipoprotein-associated rat apoE (Arg112, Arg158), like human apoE4, did not form complex with A beta, while lipoprotein-associated rabbit apoE (Cys112, Arg158) did bind the peptide. These binding studies provide one possible explanation for protective effects of both apoE2 and E3 against the development of Alzheimer's disease.

Lipoprotein association of human apolipoprotein E/A-I chimeras. Expression in transfected hepatoma cells

Both apolipoprotein (apo) E and apoA-I are associated with lipoproteins, although with different particle classes. ApoE is associated with very low density lipoprotein (VLDL) and with the larger high density lipoprotein (HDL) subspecies, while apoA-I is found predominantly in association with most HDL subclasses. The genes encoding these proteins have a similar overall structure with the nucleotide sequences of the third and fourth exons coding for the mature protein. In an effort to understand the difference in lipoprotein association patterns of these two apoproteins, we have constructed and expressed chimeric apoproteins using cDNAs in which the third (n) and fourth (c) exons of human apoE and apoA-I are exchanged. McArdle rat hepatoma cells (McA-RH7777), which secrete VLDL- and HDL-like particles, were stably transfected with these cDNAs, and the cDNAs for human apoE and human apoA-I. Single spin NaBr gradient fractions of lipoprotein deficient serum-treated cell medium from transfected McA-RH7777 cells were analyzed. The distributions of transfected human apoE and apoA-I and endogenous rat apoE and apoA-I were compared with those of the chimeras. Among HDL subspecies, human apoE expressed by these cells is associated with particles of density 1.108 g/ml. Similarly, chimera apoA-InEc (exon 3 of apoA-I and exon 4 of apoE) is found in particles of density 1.111 g/ml. Human apoA-I, however, distributes in a broader range of particles with peak densities of 1.111 g/ml and 1.164 g/ml. The distribution of the complementary chimera, apoEnA-Ic, follows this same pattern, with peak particle densities of 1.098 and 1.137 g/ml. This is in contrast to the narrow distributions of endogenous rat apoE and apoA-I, which were found in particles of density 1.099 and 1.089 g/ml, respectively. When metabolically labeled medium was fractionated via gel filtration column chromatography, apoA-InEc was found to associate with the VLDL fractions; apoEnA-Ic was absent from these same fractions. These results suggest that the fourth exon largely determines the distinctive lipoprotein distribution patterns of these two human apoproteins and that the human apoA-I fourth exon sequence may account for the polydisperse HDL pattern as observed by others in transgenic mice expressing human apoA-I.

Relationships of plasma and hepatic variables with rates of plasma low-density lipoprotein apolipoprotein B metabolism in baboons fed low- and high-fat diets

These studies were conducted to determine relationships of plasma low-density lipoprotein (LDL) cholesterol concentrations and hepatic mRNA levels for apolipoprotein (apo) B, LDL receptor, and hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) synthase with plasma LDL apo B production and catabolic rates in baboons maintained on a low-cholesterol, low-fat chow diet and on a high-cholesterol, high-fat (HCHF) diet. Twelve baboons with LDL cholesterol levels ranging from low to high on the HCHF diet but with similar high-density lipoprotein (HDL) cholesterol levels were selected from a colony of selectively bred pedigreed baboons. LDL apo B turnover and hepatic mRNA concentrations for apo B, LDL receptor, and HMG CoA synthase were measured on a chow diet and again on a HCHF diet fed for 14 weeks. LDL apo B fractional catabolic rates decreased and production rates increased on the HCHF diet. Hepatic mRNA concentrations for apo B were not affected by the HCHF diet. Hepatic LDL receptor and HMG CoA synthase mRNA concentrations decreased on the HCHF diet as compared with the chow diet. LDL apo B fractional catabolic rate was negatively correlated with plasma cholesterol, LDL cholesterol, LDL apo B, and LDL apo B production and positively correlated with hepatic LDL receptor and HMG CoA synthase mRNA concentrations and with plasma LDL triglyceride to cholesterol ratio on the chow diet but not on the HCHF diet. LDL apo B production was positively correlated with plasma cholesterol, LDL cholesterol, and LDL apo B on the HCHF diet and negatively correlated with LDL triglyceride to cholesterol ratio on both chow and HCHF diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of an upstream regulatory sequence and its binding protein in the mouse apolipoprotein E gene

The mouse apolipoprotein (apo) E gene from strain C57BL/6 was isolated from a genomic DNA library and its complete nucleotide sequence, together with 1.3 kilobase of 5' flanking DNA and 300 base pairs of the 3' flanking DNA, was determined. Regulatory sequences in the proximal 5' flanking region of the gene were identified. Using a chloramphenicol acetyltransferase transient assay system, positive and negative cis-acting sequences were mapped within 380 base pairs of the 5' flanking region of the mouse apoE gene. Two nuclear protein binding sites were identified within this region by DNase I footprinting. We have characterized one of these regions, termed mouse apoE regulatory sequence (MARS-2), which spans nucleotides -151 to -133. Gel mobility shift assays using oligonucleotides of the MARS-2 sequence having specific deletions or substitutions as probes or competitors showed that the essential sequence of MARS-2 required for nuclear protein binding consists of 16 nucleotides encompassing -151 to -136. When nuclear extracts from different cells were examined, L cells and mouse liver nuclear protein contained the highest levels of binding protein for the MARS-2 probe. This protein, termed MARS-2 binding protein, was purified from mouse liver nuclear extracts to homogeneity using gel filtration and MARS-2 oligonucleotide-specific column chromatographic procedures. The Mr = 66,000 binding protein showed a gel mobility shift band that was identical to that of crude nuclear extracts.

Purification of apolipoprotein E attenuates isoform-specific binding to beta-amyloid

Apolipoprotein E (apoE), particularly the e4 allele, is genetically linked to the incidence of Alzheimer's disease. In vitro, apoE has been shown to bind beta-amyloid (A beta), an amyloidogenic peptide that aggregates to form the primary component of senile plaques. In previous work, we demonstrated that apoE3 from tissue culture medium binds to A beta with greater avidity than apoE4 (LaDu, M. J., Falduto, M. T., Manelli, A. M., Reardon, C. A., Getz, G. S., and Frail, D. E. (1994) J. Biol. Chem. 269, 23403-23406). This is in contrast to data using purified apoE isoforms as substrate for A beta (Strittmatter, W. J., Weisgraber, K. H., Huang, D. Y., Dong, L.-M., Salvesen, G. S., Pericak-Vance, M., Schmechel, D., Saunders, A. M., Goldgaber, D., and Roses, A. D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8098-8102). Here we resolve this apparent discrepancy by demonstrating that the preferential binding of A beta to apoE3 is attenuated and even abolished with purification, a process that includes delipidation and denaturation. We compared the A beta binding capacity of unpurified apoE isoforms from both tissue culture medium and intact human very low density lipoproteins with that of apoE purified from these two sources. The interaction of human A beta-(1-40)-peptide and apoE was analyzed by nonreducing SDS-polyacrylamide gel electrophoresis followed by Western immunoblotting for either A beta or apoE immunoreactivity. While the level of the apoE3.A beta complex was approximately 20-fold greater compared with the apoE4.A beta complex in unpurified conditioned medium, apoE3 and apoE4 purified from this medium bound to A beta with comparable avidity. Moreover, using endogenous apoE on very low density lipoproteins from plasma of apoE3/3 and apoE4/4 homozygotes, apoE3 was again a better substrate for A beta than apoE4. However, apoE purified from these plasma lipoproteins exhibited little isoform specificity in binding to A beta. These results suggest that native preparations of apoE may be a more physiologically relevant substrate for A beta binding than purified apoE and further underscore the importance of subtle differences in apoE conformation to its biological activity.

Isoform-specific binding of apolipoprotein E to beta-amyloid

Apolipoprotein E (apoE), particularly the e4 allele, is genetically linked to the incidence of Alzheimer's disease. ApoE is present in the extracellular senile plaques and intracellular neurofibrillary tangles associated with Alzheimer's disease. In vitro, apoE has been shown to bind beta-amyloid (A beta), an amyloidogenic proteolytic product of amyloid precursor protein. To analyze the interaction of A beta and apoE, we used Western immunoblotting of human A beta-(1-40)-peptide incubated with conditioned medium from HEK-293 cells transfected with either human apoE3 or apoE4 (products of the e3 and e4 alleles, respectively) cDNA. Nonreducing SDS-polyacrylamide gel electrophoresis revealed the presence of an approximately 45-kDa complex with both A beta and apoE immunoreactivity. The level of the apoE3.A beta complex was approximately 20-fold greater than that of the apoE4.A beta complex. This apoE isoform-specific binding pattern was maintained from pH 5.0 to 9.0, from 2 min to 24 h of peptide incubation, and at concentrations of apoE from 5 to 100 micrograms/ml and of A beta from 10 microM to 1 mM. The higher level of apoE3 binding to A beta is in contrast to previously published data using purified apoE (Strittmatter, W. J., Weisgraber, K.H., Huang, D. Y., Dong, L.-M., Salvesen, G. S., Pericak-Vance, M., Schmechel, D., Saunders, A. M., Goldgaber, D., and Roses, A.D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8098-8102). Factors responsible for the isoform-specific interactions between apoE and A beta will require further study before the apparent discrepancy between these data can be reconciled.

Effect of dietary lipids on plasma activity and hepatic mRNA levels of cholesteryl ester transfer protein in high- and low-responding baboons (Papio species)

We compared the effects of dietary cholesterol, type of fat (coconut oil v corn oil), and phenotype (low low-density lipoprotein [LDL] response v high LDL response) on the plasma activity and hepatic mRNA levels of cholesteryl ester transfer protein (CETP). In a crossover design, eight high- and eight low-LDL-responding baboons were fed a series of diets with increasing amounts of cholesterol (0.05, 0.15, 0.45, and 1.35 mg/kcal) with either coconut oil or corn oil. All diets were fed for 7 weeks each. plasma and lipoprotein cholesterol concentrations, plasma lecithin:cholesterol acyltransferase (LCAT) and CETP activity, and hepatic mRNA levels for CETP and apolipoprotein (apo) A-I were measured after 6 weeks on each diet. Data were analyzed in two steps, ie, the effect of the initial change from chow to 0.05 mg cholesterol with each fat and the effect of the stepwise increase in cholesterol from 0.05 to 1.35 mg/kcal with each fat. High-responding baboons, as expected, showed a more pronounced increment in plasma LDL cholesterol at all dietary cholesterol levels, particularly with coconut oil as the dietary fat. Plasma high-density lipoprotein 2 (HDL2) and HDL3 cholesterol increased as dietary cholesterol increased on both the coconut and corn oil diets, with a greater increase in high-responding baboons than in low-responding baboons. The stepwise increase in dietary cholesterol increased plasma LCAT activity in both high- and low-responding baboons fed the coconut oil diet, but not in those fed the corn oil diet. Dietary cholesterol, regardless of type of fat, increased plasma CETP activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic mechanisms for responses to dietary cholesterol and fat in high and low LDL responding baboons (Papio sp.)

These studies were conducted to determine how plasma low density lipoprotein (LDL) cholesterol levels respond to dietary cholesterol, fed in increasing amounts with either corn oil or coconut oil diets, in high as compared to low LDL responding baboons; and to determine how apolipoprotein (apo) B transcription levels are modulated in response to dietary lipids. Eight high and eight low LDL responding pedigreed adult baboons, balanced for sire, age, sex, and weight, were challenged for successive 7-week periods with increasing levels of dietary cholesterol combined with either coconut oil or corn oil. At the end of each dietary period, plasma and lipoprotein lipids, apoB, apoA-I, and hepatic mRNA levels for apolipoproteins were measured. As dietary cholesterol increased, plasma cholesterol concentrations (mostly LDL cholesterol) increased in both phenotypes and with both types of fat, but phenotypic differences were greater with coconut oil. There was not a consistent dose-response relationship of plasma or LDL cholesterol levels to increasing intakes of dietary cholesterol. Neither dietary cholesterol, type of dietary fat, nor LDL phenotype affected hepatic apoB or apoE mRNA levels. In a second experiment to resolve the inconsistent dose-response to dietary cholesterol, we fed the animals varying levels of dietary cholesterol combined with coconut oil, and separated the challenge periods with intervening 12-week chow periods. Plasma and LDL cholesterol and apoB concentrations rose consistently with increasing dietary cholesterol, and the slope of the increase diminished at the higher doses. The results suggest that genetic differences in the initial response of LDL cholesterol to dietary cholesterol and saturated fatty acids are not due to the differences in hepatic transcription of apoB, and that the preceding dietary intake of cholesterol and saturated fatty acids is a major determinant of the response of plasma lipids and the associated metabolic processes to a dietary challenge. The response of baboon plasma LDL cholesterol concentrations to dietary cholesterol, when fed with saturated fatty acids, is similar to that of humans.

Inhibition of apolipoprotein E degradation in a post-Golgi compartment by a cysteine protease inhibitor

In our prior studies on lipoprotein stimulation of apolipoprotein E (apoE) secretion in HepG2 cells, it became clear that a proportion of the newly synthesized apoE was degraded intracellularly (Ye, S. Q., Olson, L. M., Reardon, C. A., and Getz, G. S. (1992) J. Biol. Chem. 267, 21961-21966). The present study was designed to determine the nature of the proteases and the intracellular sites involved in newly synthesized apoE degradation. The effect of seven protease inhibitors on total apoE levels was examined. Only N-acetyl-leucyl-leucyl-norleucinal (ALLN), a cysteine protease inhibitor, significantly blocked apoE degradation in HepG2 cells. The amount of total apoE from cells chased with ALLN for 4 h was increased by 1.58 +/- 0.05-fold relative to the controls (n = 11, p < 0.01). ALLN extended the half-life of apoE from 2.61 h to 4.38 h (p < 0.01). This effect occurs in a post-Golgi compartment since in the presence of brefeldin A, ALLN had no effect on intracellular apoE levels. Chloroquine and NH4Cl significantly reduced apoE degradation; however, ALLN plus either of these reagents appear to have an additive effect. The amount of apoE in cells chased in Ca(2+)-free medium was significantly higher than that in cells chased in Ca(2+)-containing medium (1.70 +/- 0.07-fold, n = 6, p < 0.01). ALLN plus Ca(2+)-free medium had no additive effect. ALLN had no significant influence on the degradation of albumin but had a similar effect on transfected apoE in Chinese hamster ovary cells. Overall, these data suggest that apoE may be degraded in a post-Golgi compartment of HepG2 and Chinese hamster ovary cells by lysosomal enzymes and cytosolic Ca(2+)-dependent cysteine proteases. ALLN inhibits the latter.

Human plasma lipoproteins regulate apolipoprotein E secretion from a post-Golgi compartment

The molecular regulation of apolipoprotein E (apoE) synthesis and secretion is incompletely understood. In this study, we have examined the effect of human low density lipoprotein (LDL) on apoE mRNA and protein levels in HepG2 and other eukaryotic cells. Exposing HepG2 cells to LDL for times up to 4 h resulted in an increase in 35S-labeled apoE accumulation in the medium by 2.2-fold, relative to serum free controls (n = 10, p < 0.001), with no changes in apoE mRNA levels. Similar observations have been made in JeG-3 cells and Chinese hamster ovary cells stably transfected with human apoE cDNA constructs. These results indicate that the LDL effect operates at a post-transcriptional level. In pulse-chase experiments, the LDL effect on apoE accumulation in the media was observed when it was added only during the chase even in the presence of cycloheximide, indicating that LDL is functioning at a post-translational level. The use of brefeldin A (BFA), an agent that impedes protein transport from the endoplasmic reticulum to the Golgi apparatus, suggests that the LDL effect occurs in a post-Golgi compartment. The addition of protease inhibitors could not duplicate the effects of LDL on the apoE accumulation in the medium. ApoA-I accumulation in the medium of HepG2 cells, but not albumin, was also significantly increased by 1.9-fold (n = 5, p < 0.001).

Post-translational regulation of macrophage apoprotein E production

We have transfected the murine macrophage cell line, J774, which does not express its endogenous apoE gene, with a constitutively expressed human apoE cDNA in order to study post-transcriptional and post-translational control of apoE production in macrophages. Loading cells with cholesterol using preincubations in acetylated low density lipoprotein, previously shown to enhance macrophage apoE gene transcription and apoE synthesis, did not increase apoE synthesis or secretion in constitutively expressing transfected cells, suggesting that sterol control of macrophage apoE production occurs predominantly at a transcriptional locus. However, incubation in human high density lipoprotein (HDL3) stimulated apoE secretion and appeared to inhibit degradation of newly synthesized apoE. This effect could be entirely reproduced by incubation with phosphatidylcholine vesicles which increased apoE accumulation in the medium by 2-6-fold. Pulse-chase experiments indicated that the effect of HDL3 or phospholipid vesicles was very rapid (occurring within 15 min) and was independent of changes in apoE synthesis. Furthermore, the increased apoE which accumulated in the medium in the presence of phospholipid vesicles or HDL3 was not due to altered rates of reuptake of labeled apoE since this difference was completely preserved in the absence of extracellular calcium. These results indicate that alteration of sterol content does not regulate macrophage apoE production at a translational or post-translational locus but that incubation with HDL3 or phospholipid vesicles can enhance apoprotein E production independent of changes in apoE gene transcription or apoE synthesis. The nature of the signal generated by the phospholipid vesicles which leads to inhibition of intracellular apoE degradation and enhanced apoE secretion will require further investigation.

Influence of dietary lipids on hepatic mRNA levels of proteins regulating plasma lipoproteins in baboons with high and low levels of large high density lipoproteins

Selective breeding of baboons has produced families with increased plasma levels of large high density lipoproteins (HDL1) and very low (VLDL) and low (LDL) density lipoproteins when the animals consume a diet enriched in cholesterol and saturated fat. High HDL1 baboons have a slower cholesteryl ester transfer, which may account for the accumulation of HDL1, but not of VLDL and LDL. To investigate the mechanism of accumulation of VLDL + LDL in plasma of the high HDL1 phenotype, we selected eight half-sib pairs of baboons, one member of each pair with high HDL1, the other member with little or no HDL1 on the same high cholesterol, saturated fat diet. Baboons were fed a chow diet and four experimental diets consisting of high and low cholesterol with corn oil, and high and low cholesterol with lard, each for 6 weeks, in a crossover design. Plasma lipids and lipoproteins and hepatic mRNA levels were measured on each diet. HDL1 phenotype, type of dietary fat, and dietary cholesterol affected plasma cholesterol and apolipoprotein (apo) B concentrations, whereas dietary fat alone affected plasma triglyceride and apoA-I concentrations. HDL1 phenotype and dietary cholesterol alone did not influence hepatic mRNA levels, whereas dietary lard, compared to corn oil, significantly increased hepatic apoE mRNA levels and decreased hepatic LDL receptor and HMG-CoA synthase mRNA levels. Hepatic apoA-I message was associated with cholesterol concentration in HDL fractions as well as with apoA-I concentrations in the plasma or HDL. However, hepatic apoB message level was not associated with plasma or LDL apoB levels. Total plasma cholesterol, including HDL, was negatively associated with hepatic LDL receptor and HMG-CoA synthase mRNA levels. However, compared with low HDL1 baboons, high HDL1 baboons had higher concentrations of LDL and HDL cholesterol at the same hepatic mRNA levels. These studies suggest that neither overproduction of apoB from the liver nor decreased hepatic LDL receptor levels cause the accumulation of VLDL and LDL in the plasma of high HDL1 baboons. These studies also show that, in spite of high levels of VLDL + LDL and HDL1, the high HDL1 baboons had higher levels of mRNA for LDL receptor and HMG-CoA synthase. This paradoxical relationship needs further study to understand the pathophysiology of VLDL and LDL accumulation in the plasma of animals with the high HDL1 phenotype.

Identification and characterization of transcriptional regulatory regions associated with expression of the human apolipoprotein E gene

Multiple cis-acting regulatory elements have been mapped within a 1-kilobase fragment spanning nucleotides -651 through +356 of the human apolipoprotein E gene using a transient expression system based on the chloramphenicol acetyltransferase gene as well as DNase I footprinting techniques. A 651-base pair 5'-flanking region of the human apolipoprotein E gene was capable of directing chloramphenicol acetyltransferase gene expression over a 48-fold range among the various cultured cell lines tested. Deletion analysis of this 651-base pair upstream region linked to either the chloramphenicol acetyltransferase gene or the intact apolipoprotein E structural sequences revealed at least three regulatory domains within the proximal 383 nucleotides. One of these domains contained a GC box transcriptional control element. Further analysis demonstrated that the other two domains contained enhancer-like activity. These enhancer-like elements were located within the nucleotides spanning -366 to -246 and -193 to -124. A third enhancer element was identified in the first intron, within nucleotides +44 to +262. Changing the distance of the three enhancer elements from the transcription start site and reversing their orientation did not significantly alter their effects on transcription rates. However, enhancer activity was influenced by the promoter and cell line that were used. DNase I footprinting assays showed that specific sequences within two of these elements (-193 to -124 and +44 to +262) bind proteins in nuclear extracts from HepG2 and Chinese hamster ovary cells. A protein footprint also was identified for a GC box element at nucleotides -59 to -45. Thus, control of apolipoprotein E gene expression is the result of a complex interaction of several different regulatory elements.

Two copies of the human apolipoprotein C-I gene are linked closely to the apolipoprotein E gene

The gene for human apolipoprotein (apo) C-I was selected from human genomic cosmid and lambda libraries. Restriction endonuclease analysis showed that the gene for apoC-I is located 5.5 kilobases downstream of the gene for apoE. A copy of the apoC-I gene, apoC-I', is located 7.5 kilobases downstream of the apoC-I gene. Both genes contain four exons and three introns; the apoC-I gene is 4653 base pairs long, the apoC-I' gene 4387 base pairs. In each gene, the first intron is located 20 nucleotides upstream from the translation start signal; the second intron, within the codon of Gly-7 of the signal peptide region; and the third intron, within the codon for Arg39 of the mature plasma protein coding region. The upstream apoC-I gene encodes the known apoC-I plasma protein and differs from the downstream apoC-I' gene in about 9% of the exon nucleotide positions. The most important difference between the exons results in a change in the codon for Gln-2 of the signal peptide region, which introduces a translation stop signal in the downstream gene. Major sequence differences are found in the second and third introns of the apoC-I and apoC-I' genes, which contain 9 and 7.5 copies, respectively, of Alu family sequences. The apoC-I gene is expressed primarily in the liver, and it is activated when monocytes differentiate into macrophages. In contrast, no mRNA product of the apoC-I' gene can be detected in any tissue, suggesting that it may be a pseudogene. The similar structures and the proximity of the apoE and apoC-I genes suggest that they are derived from a common ancestor. Furthermore, they may be considered to be constituents of a family of seven apolipoprotein genes (apoE, -C-I, -C-II, -C-III, -A-I, -A-II, and -A-IV) that have a common evolutionary origin.

Expression of the human apolipoprotein E gene in cultured mammalian cells

The gene for human apolipoprotein (apo-) E was isolated from a human genomic library constructed in the cosmid shuttle vector pCV108. The transient expression of the apo-E gene was examined in cultured mammalian cells 48 h following calcium phosphate-mediated gene transfer. The expression of the cloned human apo-E gene, which contained between 0.7 and 29 kilobases of 5'-flanking DNA, was not restricted to human cells or to cultured cells derived from tissues that have been shown to synthesize apo-E. Several independent mouse L cell stable transfectants with the human apo-E gene integrated into their genome were selected on the basis of G418 resistance, which is conferred by the selectable gene marker in the cosmid vector. The levels of human apo-E mRNA found in the stable transfected mouse L cells ranged from undetectable to a level comparable to that found in the human liver. The size of the apo-E mRNA observed in the stable transfectants was identical to that found in the liver, indicating that the mouse L cells were capable of correctly processing the human apo-E gene transcripts. The integrated human apo-E genes had not undergone major rearrangements or deletions during transfer, and the level of apo-E mRNA found in the different stable transfectants correlated directly with the number of integrated copies of the human apo-E gene. The stable transfected L cells secreted authentic human apo-E into the medium. The secreted protein interacted specifically with antibodies to human plasma apo-E and had an apparent Mr = 35,000 to 36,000, which is slightly larger than that of plasma apo-E. The secreted human apo-E was associated with lipid (presumably phospholipids), floated at d approximately 1.09 g/ml, and bound with high affinity to the apo-B,E(LDL) receptor on fibroblasts.

The charge polymorphism of rat apoprotein E

Rat apolipoprotein E (apo-E) exists in plasma as four unique isoelectric forms (designated E-1, E-2, E-3, or E-4 from acidic to basic, respectively). We have examined the processes accounting for this polymorphism using intact rats or cultured rat hepatocytes. Intrahepatic precursors of rat apo-E were isolated and analyzed on isoelectric focusing gels. The primary translation product of rat liver apo-E mRNA focused as two isoproteins with more basic pI values than the isoproteins of plasma apo-E. The microsome-processed translation product also focused as two isoproteins having pI values corresponding to apo-E-4 and apo-E-3 isoproteins of plasma apo-E. Following a bolus injection of [3H]leucine into the portal vein, intrahepatic isoproteins corresponding to plasma apo-E-2 and apo-E-1 isoproteins were first detected in the rough endoplasmic reticulum (RER) and Golgi fractions, respectively. The apparent molecular weight of intrahepatic apo-E increased as it passed from the RER to the Golgi. Only the most acidic isoform, apo-E-1, of plasma apo-E was sensitive to neuraminidase treatment indicating that sialic acid residues are responsible, in part, for the polymorphism of rat apo-E. Using cultured hepatocytes, tunicamycin (1 microgram/ml) inhibited the incorporation of [3H]glucosamine into both molecular weight forms of apolipoprotein B but did not influence the synthesis, glycosylation (as measured by [3H]glucosamine incorporation), or secretion of apo-E. Tunicamycin-inhibited hepatocytes secreted the normal complement of apo-E isoforms including apo-E-1, thus confirming that apo-E-1 is not an N-linked glycoprotein. These results suggest that post-translational modifications involving both RER and Golgi-specific reactions contribute to the polymorphism of rat apo-E.

Processing of rat liver apoprotein E primary translation product

The primary translation product of rat liver apoE mRNA was isolated from wheat germ cell-free translation systems. Plasma apoE and the primary translation product migrated similarly on SDS-polyacrylamide gels, had similar partial proteolytic peptide maps, and bound to and coeluted from heparin-Sepharose columns. Comparison of the partial amino acid sequence of the primary translation product with the amino-terminal sequence of plasma apoE indicated that rat apoE is initially synthesized with an 18 amino acid amino-terminal extension. This entire segment was removed cotranslationally by canine microsomes possessing signal peptidase activity. The microsome-processed translation product did not contain an endoglycosidase H-sensitive oligosaccharide, suggesting that rat apoE is O-glycosylated.