Serum paraoxonase: effect of the apolipoprotein composition of HDL and the acute phase response

Genetic variations of paraoxonase (PON) correlate with HDL cholesterol and apolipoprotein A-I (apoA-I), suggesting antiatherogenic properties. Atherosclerosis occurs naturally in humans and rabbits but not in mice. We compared variations of PON arylesterase activity (PON AEase, phenylacetate substrate) in humans, rabbits, and mice. In humans and rabbits, >95% of PON AEase is HDL associated. In mice, about 30% of PON AEase is lipid poor. In the absence of apoA-I in mice, total PON AEase is reduced and >60% is lipid poor. PON AEase level and distribution is restored in apoA-I-/- mice injected with adenoviruses encoding human apoA-I and in transgenic mice expressing human apoA-I at a steady-state level. Thus, while apoA-I is not required for the HDL association of PON AEase, induced variations in apoA-I correlate with changes in HDL-associated, but not lipid-poor, PON AEase. PON AEase associates only with apoA-I- or apoE-containing HDL but not VLDL. In the absence of both apoA-I and apoE, PON AEase is all-lipid-poor. PON AEase is displaced from HDL by ultracentrifugation and following incubation with serum amyloid A. Variations in the PON distribution between HDL and lipid-poor fractions may have important consequences in its antioxidant activity and in atherogenesis.

Import of yeast mitochondrial transcription factor (Mtf1p) via a nonconventional pathway

The yeast mitochondrial (mt) transcription factor Mtf1p is imported into the mitochondria from the cytoplasm without a conventional mt-targeting presequence. To understand its import the mt translocation of wild type and mutant Mtf1p constructs was investigated in vitro under various assay conditions. We report here that Mtf1p, unlike most mt matrix proteins hitherto studied, is translocated into the mitochondria independent of membrane potential, ATP hydrolysis, and membrane receptor. This unusual import of Mtf1p was also observed on ice (3 degrees C). Sub-mitochondrial fractionation demonstrated that Mtf1p was translocated in vitro to one or more of the same mt sites as the endogenous protein that includes the matrix. To identify the mt-targeting sequence of Mtf1p, various N-terminal, C-terminal, or internally deleted Mtf1p derivatives were generated. The full-length and C-terminal deletions but not the N-terminal truncated Mtf1p were imported into mitochondria, indicating the importance of its N-terminal sequence for mt targeting. However, the internal deletion of Mtf1p revealed that the first 150-amino acid N-terminal sequence alone was not sufficient for mt targeting of Mtf1p, suggesting that an extended rather than a short N-terminal sequence is required for import. We favor a model in which Mtf1p adopts an import-competent conformation during translation. Consistent with this model are three findings: most of the protein sequence appears to be required for optimal import, urea denaturation eliminates its import competence, and the import-competent form of the protein is more resistant to tryptic hydrolysis than is the denatured protein. This represents a novel mechanism for mitochondrial protein import.

Apolipoprotein E structural requirements for the formation of SDS-stable complexes with beta-amyloid-(1-40): the role of salt bridges

Of the three major isoforms of human apolipoprotein E (apoE), apoE4 is a risk factor for the development of Alzheimer's disease. Among possible neurologically relevant differences in the properties of apoE3 and apoE4 is the fact that apoE3 forms an SDS-stable complex with beta-amyloid-(1-40) (Abeta40) with greater avidity than does apoE4. This interaction may sequester potentially toxic species of Abeta or facilitate clearance. To understand more about this difference, we examined whether differences in salt bridges between apoE domains influence the capacity of apoE isoforms to form complexes with Abeta. In apoE3 there is a salt bridge between Arg-61 and Asp-65, while in apoE4 there are salt bridges between Arg-61 and Glu-255, and Arg-112 and Glu-109. Mutation of position 112, which is Cys in apoE3 and Arg in apoE4, to Ala or Lys abolished complex formation, while mutant apoE with Ser at this position retained the capacity to form complex. Substituting Ala for Glu-109 had no effect on the ability of either apoE4 or apoE3 to form complexes. On the other hand, substitution of Thr for Arg-61 in apoE3 abolished, and truncation of apoE3 at position 201 substantially lowered, but did not abolish, complex formation. Neither of these mutations within apoE4 had any affect on its complex formation with Abeta. These results suggest that the nature of the cysteine residue in apoE3 and interactions between the N-terminal and C-terminal domains of human apoE are important for the ability of apoE3 to form an SDS-stable complex with Abeta40.

Apolipoprotein A-I alpha -helices 7 and 8 modulate high density lipoprotein subclass distribution

Mice have a monodisperse high density lipoprotein (HDL) profile, whereas humans have two major subfractions designated HDL(2) and HDL(3). Human apoA-I transgenic mice exhibit a human-like HDL profile, indicating that the amino acid sequence of apoA-I is a determinant of the HDL profile. Comparison of the primary sequence of mouse and human apoA-I and the previously designated "hinge" domain of apoA-I led us to hypothesize that alpha-helices 7 and 8 (7/8) are determinants of HDL subclass distribution. The following proteins were expressed in Escherichia coli: human apoA-I, T7-hAI; mouse apoA-I, T7-mAI; chimeric human apoA-I containing murine helices 7/8 in place of human helices 7/8, T7-hAI(m7/8); and the reciprocal chimera, T7-mAI(h7/8). The recombinant proteins were examined for their association with human plasma HDL subclasses. The results demonstrated that T7-hAI bound HDL(2) and HDL(3) equally well, whereas T7-mAI bound to HDL(2) preferentially. T7-hAI(m7/8) behaved like T7-mAI, and T7-mAI(h7/8) behaved like T7-hAI. Thus, alpha-helices 7/8 are strong contributors to the pattern of HDL subclass association. Self-association, alpha-helicity, cholesterol efflux, and lecithin-cholesterol acyltransferase activity of the recombinant proteins were also assessed. Human apoA-I self-associates more and activates human lecithin-cholesterol acyltransferase better than mouse apoA-I. These differential characteristics of human and mouse apoA-I are not dependent on helices 7/8.

Both apolipoprotein E and immune deficiency exacerbate neointimal hyperplasia after vascular injury in mice

In this study, we investigated the role of T and B lymphocytes in neointimal hyperplasia after endothelial denudation. Catheter-induced endothelial denudation of wild-type mice resulted in rapid infiltration of lymphocytes to the site of injury. Mice defective in recombination-activating gene 2 (RAG2(-/-)) showed increased neointimal formation 14 days after vascular injury in comparison to their wild-type immune-competent littermates. Immunohistochemical studies revealed the preponderance of smooth muscle cells and a significantly higher number of proliferating cells in the neointima of the RAG2(-/-) mice. The neointima size and the number of proliferating smooth muscle cells in the injured vessel of RAG2(-/-) mice were similar to those observed in the injured arteries of apolipoprotein E (apoE)-deficient (apoE(-/-)) mice. Interestingly, mice with double apoE and RAG2 gene mutations (apoE(-/-) RAG2(-/-)) displayed similar neointimal characteristics as mice with a single gene defect, suggesting a similar mechanism for apoE and lymphocyte protection against injury-induced neointimal formation. The protective role of lymphocytes against neointimal formation after vascular injury directly contrasts to their reported role in the promotion of atherosclerosis, which was observed in both apoE(+/+) and apoE(-/-) mice. Thus, these results support the hypothesis of different etiology between hyperlipidemia-induced atherosclerosis and injury-induced vascular occlusion.

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.

Lipoproteins in the central nervous system

Although the synthesis and metabolism of plasma lipoproteins are well characterized, little is known about lipid delivery and clearance within the central nervous system (CNS). Our work has focused on characterizing the lipoprotein particles present in the cerebrospinal fluid (CSF) and the nascent particles secreted by astrocytes. In addition to carrying lipids, we have found that beta-amyloid (A beta) associates with lipoproteins, including the discoidal particles secreted by cultured astrocytes and the spherical lipoproteins found in CSF. We believe that association with lipoproteins provides a means of transport and clearance for A beta. This process may be further influenced by an interaction between A beta and apoprotein E (apoE), the primary protein component of CNS lipoproteins. Specifically, we have investigated the formation and physiologic relevance of a SDS-stable complex between apoE and A beta. In biochemical assays, native apoE2 and E3 (associated with lipid particles) form an SDS-stable complex with A beta that is 20-fold more abundant than the apoE4:A beta complex. In cell culture, native apoE3 but not E4 prevents A beta-induced neurotoxicity by a mechanism dependent on cell surface apoE receptors. In addition, apoE and the inhibition of apoE receptors prevent A beta-induced astrocyte activation. Therefore, we hypothesize that the protection from A beta-induced neurotoxicity afforded by apoE3 may result from clearance of the peptide by SDS-stable apoE3:A beta complex formation and uptake by apoE receptors.

Loss of atheroprotective effect of estradiol in immunodeficient mice

Estradiol significantly decreases fatty streak formation in the aortic root of chow-fed apolipoprotein E-deficient mice. In contrast, immunodeficient mice with homozygous disruption at the recombinase activating gene 2 loci present fatty streak development that is insensitive to estradiol. Lymphocytes thus appear to be required for development of the atheroprotective effect of estradiol in this mouse model.

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.

The single amino acid changes in the yeast mitochondrial S4 ribosomal protein cause temperature-sensitive defect in the accumulation of mitochondrial 15S rRNA

Four different mutant alleles of a nuclear gene (MNA6), which lose mt 15S rRNA at nonpermissive temperature (36 degrees C), were previously generated by EMS mutagenesis of Saccharomyces cerevisiae. To understand the biochemical basis for the loss of 15S rRNA in these mutants, the wild-type and mutant alleles of the MNA6 gene were isolated and characterized. The DNA sequencing of the cloned MNA6 gene revealed that it has an open reading frame specifying a 486 amino acid polypeptide, which appears to be a yeast mt homologue of the S4 r-protein family. The large size of this yeast S4 homologue is due to a nonhomologous long C-terminal extension. The MNA6 gene also appeared to be identical to the previously isolated yeast NAM9 gene. The in vitro expression under coupled transcription-translation reaction conditions followed by mt import demonstrated that MNA6 indeed encodes a approximately 56 kDa protein targeted to the mitochondria. We have also demonstrated by Western blot analysis using anti-Mna6p antibody that Mna6p is associated with the small subunit of mitoribosomes. The sequence analysis of the four mutant mna6 alleles revealed that Leu(109) --> Phe, Arg(111) --> Lys, Pro(424) --> Leu, or Pro(438) --> Leu amino acid substitution in Mna6p causes temperature-dependent loss of the 15S rRNA. These mutations do not affect the mitochondrial import or accumulation of Mna6p. Rather the evidence points to an inability of mutant Mna6p to be assembled into the mitoribosomes of cells grown at 36 degrees C.

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.

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.