High density lipoprotein mediated lipid efflux from retinal pigment epithelial cells in culture.

BACKGROUND/AIM: [corrected] The transport of radiolabelled photoreceptor outer segments (POS) lipids was investigated by cultured retinal pigment epithelial cells (RPE). Phagocytosis of POS by the RPE is essential to maintain the health and function of the photoreceptors in vivo. POS are phagocytised at the apical cell surface of RPE cells. Phagocytised POS lipids may be either recycled to the photoreceptors for reincorporation into new POS or they may be transported to the basolateral surface for efflux into the circulation. RESULTS: The authors have demonstrated that high density lipoprotein (HDL) stimulates efflux of radiolabelled lipids, of POS origin, from the basal surface of RPE cells in culture. Effluxed lipids bind preferentially to HDL species of low and high molecular weight. Effluxed radiolabelled phosphotidyl choline was the major phospholipid bound to HDL, with lesser amounts of phosphatidyl ethanolamine, phosphatidyl inosotol. Effluxed radiolabelled triglycerides, cholesterol, and cholesterol esters also bound to HDL. Lipid free apolipoprotein A-I (apoA-I) and apoA-I containing vesicles also stimulate lipid efflux. CONCLUSION: The findings suggest a role for HDL and apoA-I in regulating lipid and cholesterol transport from RPE cells that may influence the pathological lipid accumulation associated with age related macular degeneration.

Plasma apolipoprotein L concentrations correlate with plasma triglycerides and cholesterol levels in normolipidemic, hyperlipidemic, and diabetic subjects.

Apolipoprotein L is a newly recognized component of human plasma lipoproteins. Mainly associated with apoA-I-containing lipoproteins, it is a marker of distinct HDL subpopulations. In an effort to gain inference as to its as yet unknown function, we studied biological determinants of apoL levels in human plasma. The distribution of apoL in normal subjects is asymmetric, with marked skewing toward higher values. No difference was found in apoL concentrations between males and females, but we observed an elevation of apoL in primary hypercholesterolemia (10.1 vs. 8.5 microgram/mL in control), in endogenous hypertriglyceridemia (13.8 microgram/mL, P < 0.001), combined hyperlipidemia phenotype (18.7 g/mL, P < 0.0001), and in patients with type II diabetes (16.2 microgram/mL, P < 0.02) who were hyperlipidemic. Significant positive correlations were observed between apoL and the log of plasma triglycerides in normolipidemia (0.446, P < 0.0001), endogenous hypertriglyceridemia (0.435, P < 0.01), primary hypercholesterolemia (0.66, P < 0.02), combined hyperlipidemia (0.396, P < 0.04), hypo-alphalipoproteinemia (0.701, P < 0.005), and type II diabetes with hyperlipidemia (0.602, P < 0. 01). Apolipoprotein L levels were also correlated with total cholesterol in normolipidemia (0.257, P < 0.004), endogenous hypertriglyceridemia (0.446, P = 0.001), and non-insulin-dependent diabetes mellitus (NIDDM) (0.548, P < 0.02). No significant correlation was found between apoL and body mass index, age, sex, HDL-cholesterol or fasting glucose and glycohemoglobin levels. ApoL levels in plasma of patients with primary cholesteryl ester transfer protein deficiency significantly increased (7.1 +/- 0.5 vs. 5.47 +/- 0.27, P < 0.006).

Binding of high density lipoprotein (HDL) and discoidal reconstituted HDL to the HDL receptor scavenger receptor class B type I. Effect of lipid association and APOA-I mutations on receptor binding.

The binding of apoA-I-containing ligands to the HDL receptor scavenger receptor class B type I (SR-BI) was characterized using two different assays. The first employed conventional binding or competition assays with (125)I-labeled ligands. The second is a new nonradioactive ligand binding assay, in which the receptor-associated ligand is detected by quantitative immunoblotting ("immunoreceptor assay"). Using both methods, we observed that the K(d) value for spherical HDL (density = 1.1-1.13 g/ml) was approximately 16 microgram of protein/ml, while the values for discoidal reconstituted HDL (rHDL) containing proapoA-I or plasma apoA-I were substantially lower (approximately 0.4-5 microgram of protein/ml). We also observed reduced affinity and/or competition for spherical (125)I-HDL cell association by higher relative to lower density HDL and very poor competition by lipid-free apoA-I and pre-beta-1 HDL. Deletion of either 58 carboxyl-terminal or 59 amino-terminal residues from apoA-I, relative to full-length control apoA-I, resulted in little or no change in the affinity of corresponding rHDL particles. However, rHDL particles containing a double mutant lacking both terminal domains competed poorly with spherical (125)I-HDL for binding to SR-BI. These findings suggest an important role for apoA-I and its conformation/organization within particles in mediating HDL binding to SR-BI and indicate that the NH(2) and COOH termini of apoA-I directly or indirectly contribute independently to binding to SR-BI.

The db/db mouse, a model for diabetic dyslipidemia: molecular characterization and effects of Western diet feeding.

Diabetic dyslipidemia is a major factor contributing to the accelerated atherosclerosis in type 2 diabetes mellitus. Although several mouse models are available, the plasma lipoproteins in response to diet have not been fully characterized in these animals. In this study, we have characterized the plasma lipoproteins and related apolipoproteins, as well as the vascular lipases, in diabetes (db/db) mice and their nondiabetic controls (+/?) in the C57BL/KsJ strain. Within 6 weeks of age, db/db mice developed significant obesity, fasting hyperglycemia, and hyperinsulinemia. By FPLC analysis, db/db mice showed a prominent peak in the low-density lipoprotein (LDL) range that was absent in +/? mice, although high-density lipoprotein (HDL) was the predominant species in both groups of animals. Postheparin lipoprotein lipase (LPL) activity in db/db mice was 28% of the level in +/? mice. Upon feeding a human-like 0.15% (wt/wt) cholesterol and 21% (wt/wt) fat "Western" diet, db/db mice developed elevated plasma cholesterol, accompanied by an exaggerated apolipoprotein E (apoE) response compared with +/? mice. FPLC analysis showed that the marked hypercholesterolemic response in db/db mice was the result of a massive increase in the LDL region, which overshadowed a moderate increase in HDL. We next isolated lipoproteins by ultracentrifugation and characterized them by nondenaturing gradient gel electrophoresis. With regular chow, db/db mice had almost exclusively small dense LDL with a peak size at 21.4 nm, as compared with 26.6 nm in nondiabetic controls. On the Western diet, the small dense LDLs persisted but larger particles also appeared in db/db mice, whereas the size distribution in +/? mice was unchanged by the diet. Our results suggest that db/db mice fed a Western diet have a plasma lipoprotein phenotype that shows some similarities to that in patients with type 2 diabetes mellitus, and that db/db mice are a useful model to study the pathogenesis and treatment of diabetic dyslipidemia.

Phenotype interaction of apobec-1 and CETP, LDLR, and apoE gene expression in mice: role of apoB mRNA editing in lipoprotein phenotype expression.

Apolipoprotein (apo) B mRNA editing determines the amount of apoB-100 and apoB-48 produced. Surprisingly, apobec-1 knockout mice, which do not edit apoB, have an essentially normal lipoprotein phenotype. By selected cross-breeding of mice of different genotypes, we show in this report that inactivation of editing produces profound phenotypic effects in cholesteryl ester transfer protein (CETP) transgenic mice and in apoE and low density lipoprotein receptor (LDLR) knockout mice. Compared with mice with an apobec-1+/+ background, CETP expression in apobec-1-/- mice caused a doubling of the plasma apoB-100 concentration (from 3.5+/-0.6 to 8.8+/-1.9 mg/dL, P<.01) and a much greater shift of plasma cholesterol from HDL to IDL/LDL as assayed by fast protein liquid chromatography analysis; the ratio of non-HDL to HDL cholesterol was 0.47, 0.46, 0.76, and 1.43 in apobec-1(+/+)/CETP-/-, apobec-1(-/-)/CETP-/-, apobec-1(+/+)/CETP+/-, and apobec-1(-/-)/CETP+/- animals, respectively. Feeding of a Western-type diet further exaggerated the shift in this ratio. In LDLR-/- mice, inactivation of apobec-1 caused an approximately 200% rise in plasma apoB-100 concentration, an approximately 60% increase in apoE concentration, and a 70% increase in total plasma cholesterol, which resulted exclusively from an increase in non-HDL cholesterol. The exaggerated hypercholesterolemia involving the VLDL+LDL fractions was further enhanced by a Western-type diet. In contrast, in apoE-/- mice, inactivation of apobec-1 caused a massive increase (from <0.5 to 55.5+/-16.4 mg/dL) in plasma apoB-100 concentration but an approximately 55% reduction in hypercholesterolemia due to partial amelioration of the marked VLDL+IDL elevation. However, the difference in lipid profiles between apobec-1(+/+)/apoE-/- and apobec-1(-/-)/apoE-/- mice was abolished in a time-dependent manner as further increases in total plasma cholesterol were induced by a Western-type diet. Whereas apobec-1 inactivation in wild-type mice produced little or no change in lipoprotein phenotype, giving rise to speculation that apoB mRNA editing does not have significant effect on lipoprotein dynamics, we show herein that there is important gene-gene interaction between apobec-1 and the CETP, LDLR, and apoE loci, which is subject to further substantial modulation by environmental factors such as a Western-type diet in mice.

Human apolipoprotein A-II is a pro-atherogenic molecule when it is expressed in transgenic mice at a level similar to that in humans: evidence of a potentially relevant species-specific interaction with diet.

We report on the effect of human apolipoprotein (apo) A-II transgene expression on atherosclerosis susceptibility in two transgenic lines (25.3 and 11.1) whose plasma human apoA-II concentrations (approximately 23 and 96 mg/dl, respectively) span the normal range in humans. After 9 months of an atherogenic diet, 25.3 and 11.1 transgenic mice developed aortic atherosclerotic lesions that were approximately 1.7- and 7-fold, respectively, more extensive than those of non-transgenic control mice. However, there was no difference in the area of atherosclerosis of transgenic and control mice when fed a regular chow diet This contrasts with the findings in murine apoA-II transgenic mice and provides evidence of a species-specific characteristic that could be of relevance with respect to the high fat intake diets common in most industrialized countries. A possible mechanism of the pro-atherogenic action of human apoA-II could be the inhibition of reverse cholesterol transport and, in support of this, we observed an impairment of apoA-I-HDL particle interconversion in the plasma of 11.1 transgenic mice caused, at least in part, by a marked decrease in the endogenous lecithin:cholesterol acyltransferase activity.

Prebeta-1 HDL in plasma of normolipidemic individuals: influences of plasma lipoproteins, age, and gender.

Prebeta-1 HDL is a molecular species of plasma HDL of approximately 67 kDa mass that contains apolipoprotein A-I, phospholipids, and unesterified cholesterol. It participates in a cyclic process involved in the retrieval of cholesterol from peripheral tissues. In this cycle, unesterified cholesterol from cells is incorporated into prebeta-1 HDL, providing a substrate for esterification of cholesterol by lecithin:cholesterol acyltransferase. Prebeta-1 HDL then becomes incorporated into larger HDL species of alpha mobility as esterification proceeds and is regenerated during the transfer of cholesteryl esters from alpha HDL particles to acceptor lipoproteins. Thus the steady state level of prebeta-1 HDL in plasma reflects the relative efficiencies of the major metabolic processes involved in its generation and removal. We have used an isotope dilution technique to measure prebeta-1 HDL levels in the plasmas of 136 normolipidemic individuals (46 M, 90 F). The mean absolute concentration of prebeta-1 HDL as apolipoprotein A-I was 68 +/- 40 microg/ml for women, and 84 +/- 49 m/ml for men. Prebeta-1 HDL represented 5.5 +/- 3.3% of total apolipoprotein A-I in women, and 7.2 +/- 4.0% in men. The distributions of both absolute and percent prebeta-1 HDL are highly asymmetric, with skew toward higher values. However, the skew appears not to be attributable to either plasma cholesterol or triglyceride levels which are also skewed in population samples. The percent prebeta-1 HDL was negatively correlated with HDL cholesterol levels (P < 0.0001), whereas absolute levels of prebeta-1 HDL were positively correlated with apolipoprotein A-I and negatively correlated with HDL cholesterol (P, for both, < 0.0001). Multiple linear regression analysis revealed effects of age and gender, but no association with lipoprotein fractions other than HDL. Lower levels of prebeta-1 HDL were associated with female gender in all models.

Influence of apoE content on receptor binding of large, bouyant LDL in subjects with different LDL subclass phenotypes.

We investigated the influence of apolipoprotein (apo) E-containing particles on LDL receptor binding of large, buoyant LDL subfractions (LDL I) from subjects with predominantly large (phenotype A) and small (phenotype B) LDL particles. Direct binding by human fibroblast LDL receptors was tested at 4 degrees C before and after removal of apoE-containing particles by immunoaffinity chromatography. The binding affinity of total LDL I in phenotype B was greater than that in phenotype A (Kd of 1.83+/-0.3 and 3.43+/-0.9 nmol/L, respectively, P<.05). LDL I from phenotype B subjects had a higher apoE to apoB molar ratio than did that from phenotype A (0.16+/-0.04 versus 0.06+/-0.02, P<.05). Nondenaturing gradient gel electrophoresis of apoE-containing LDL I isolated by immunoaffinity chromatography revealed a substantially larger peak particle diameter than in apoE-free LDL I, and comparison of LDL I composition before and after immunoaffinity chromatography suggested an increase in triglyceride content of apoE-containing particles. After removal of these particles, there was a greater than twofold reduction in LDL receptor affinity of phenotype B LDL (Kd of 1.83+/-0.3 to 3.76+/-0.6, P<.01), whereas in phenotype A no change was observed (Kd of 3.43+/-0.9 to 3.57+/-0.4, respectively). The receptor affinity of apoE-free LDL I from phenotype A and B subjects did not differ. These findings confirm that large, buoyant LDL particles from phenotype B subjects have a higher LDL receptor affinity than does LDL I from phenotype A subjects and suggest that this difference is due to an increased content of large, triglyceride-enriched, apoE-containing lipoproteins. It is possible that the accumulation of these particles reflects abnormalities in the metabolism of remnant lipoproteins that contribute to atherosclerosis risk in phenotype B subjects.

Measurement of prebeta-1 HDL in human plasma by an ultrafiltration-isotope dilution technique.

Prebeta-1 HDL is a 67-kDa species of plasma high-density lipoproteins (HDL) that contains two copies of apolipoprotein A-I. It functions in a metabolic cycle of cholesterol retrieval and may be formed during lipolysis in plasma. We have found that centrifugal ultrafiltration using a membrane with a permeability limit of 100 kDa discriminates categorically between the 67-kDa species and larger HDL particle species. Thus, the ultrafiltrate samples the pool of prebeta-1 HDL in plasma. We have developed a technique using the dispersal of purified prebeta-1 HDL, labeled covalently with tritium, in plasma samples, to label the prebeta-1 HDL pool. Subsequent determination of the specific activity of prebeta-1 HDL in the ultrafiltrate provides a means of calculating the content of prebeta-1 HDL in plasma by the isotope dilution principle. We employ a modification of an enzyme-linked immunosorbent assay technique for apolipoprotein A-I that allows the equal detection of that protein in prebeta-1 HDL and in other HDL particle species for determination of the fraction of total apolipoprotein A-I that is present in the prebeta-1 HDL particle species. The mean level of prebeta-1 HDL-associated apolipoprotein A-I in plasma samples from 86 normolipidemic adults was 74 +/- 43 microg/ml (+/-SD), representing an average of 6.6% of the total apolipoprotein A-I in plasma.

Complete phenotypic characterization of apobec-1 knockout mice with a wild-type genetic background and a human apolipoprotein B transgenic background, and restoration of apolipoprotein B mRNA editing by somatic gene transfer of Apobec-1.

We have produced gene knockout mice by targeted disruption of the apobec-1 gene. As recently reported by Hirano et al. (Hirano, K.-I., Young, S. G., Farese, R. V., Jr., Ng, J., Sande, E., Warburton, C., Powell-Braxton, L. M., and Davidson, N. O. (1996) J. Biol. Chem. 271, 9887-9890), these animals do not edit apolipoprotein (apo) B mRNA or produce apoB-48. In this study we have performed a detailed analysis of the lipoprotein phenotypic effects of apobec-1 gene disruption that were not examined in the previous study. We first analyzed the plasma lipoproteins in knockout animals with a wild-type genetic background. Although there was no difference in plasma cholesterol between apobec-1(-/-), +/-, or +/+ mice, there was a marked (176%) increase in plasma apoB-100, from 1.8 +/- 1.2 mg/dl in apobec-1(+/+) mice to 2.7 +/- 0.6 mg/dl in apobec-1(+/-) and 5.0 +/- 1.4 mg/dl in apobec-1(-/-) mice. Plasma apoE was similar in these animals. By fast protein liquid chromatography (FPLC) analysis, there was a significant decrease in plasma high density lipoprotein (HDL) cholesterol in apobec-1(-/-) mice. We further fractionated the plasma lipoproteins into d < 1.006, 1.006-1.02, 1.02-1.05, 1.05-1.08, 1.08-1.10, and 1.10-1.21 g/ml classes, and found a marked (30-40%) reduction in the cholesterol and protein content in the (d 1.08-1.10 and 1.10-1.21) HDL fractions, corroborating the FPLC data. SDS-gel analysis revealed an absence of apoB-48, an increase in apoB-100 in the very low density lipoprotein (VLDL) and low density lipoprotein (LDL) fractions, and a small decrease in apoA-I in the HDL fractions in the apobec-1(-/-) samples. We next raised the basal plasma apoB levels in the apobec-1(-/-) animals by cross-breeding them with human apoB transgenic (TgB) mice. The plasma apoB-100 was 3-fold higher in apobec-1(-/-)/TgB+/- mice (26.6 +/- 18.3 mg/dl) than in apobec-1(+/+)/TgB+/- mice (9.8 +/- 3.9 mg/dl, p < 0.05). The apobec-1(-/-)/TgB+/- mice had a plasma cholesterol levels of 170 +/- 28 mg/dl and triglyceride levels of 106 +/- 31 mg/dl, which are 80% and 58% higher, respectively, than the corresponding values of 94 +/- 21 mg/dl and 67 +/- 11 mg/dl in apobec+/+/TgB+/- mice. By FPLC, the apobec-1(-/-)/TgB+/- animals developed markedly elevated plasma LDL cholesterol (518.5 +/- 329.5 microg/ml) that is 373% that of apobec1(+/+)/TgB+/- mice (139.0 +/- 87.0 microg/ml) (p < 0.05). The elevated plasma triglyceride was accounted for mainly by a 97% increase in VLDL triglyceride in the apobec1(-/-)/TgB+/- mice. We conclude that apobec-1(-/-) animals have a distinctive lipoprotein phenotype characterized by significant hyperapoB-100 and HDL deficiency in mice with a wild-type genetic background. Furthermore, the abolition of apoB mRNA editing elevates plasma total cholesterol and LDL cholesterol in apobec-1(-/-) animals with a TgB background. Finally, to exclude the possibility that absence of apoB mRNA editing was a secondary effect of chronic Apobec-1 deficiency, we treated apobec-1(-/-) mice with a replication-defective mouse Apobec-1 adenoviral vector and found that we could acutely restore apoB mRNA editing in the liver. These experiments indicate that Apobec-1 is an essential component of the apoB mRNA editing machinery and absence of editing in the knockout animals is a direct consequence of the absence of functional Apobec-1.

Functional lecithin:cholesterol acyltransferase deficiency and high density lipoprotein deficiency in transgenic mice overexpressing human apolipoprotein A-II.

The concentration of high density lipoproteins (HDL) is inversely related to the risk of atherosclerosis. The two major protein components of HDL are apolipoprotein (apo) A-I and apoA-II. To study the role of apoA-II in lipoprotein metabolism and atherosclerosis, we have developed three lines of C57BL/6 transgenic mice expressing human apoA-II (lines 25.3, 21.5, and 11.1). Northern blot experiments showed that human apoA-II mRNA was present only in the liver of transgenic mice. SDS-polyacrylamide gel electrophoresis and Western blot analysis demonstrated a 17.4-kDa human apoA-II in the HDL fraction of the plasma of transgenic mice. After 3 months on a regular chow, the plasma concentrations of human apoA-II were 21 +/- 4 mg/dl in the 25.3 line, 51 +/- 6 mg/dl in the 21.5 line, and 74 +/- 4 mg/dl in the 11.1 line. The concentration of cholesterol in plasma was significantly lower in transgenic mice than in control mice because of a decrease in HDL cholesterol that was greatest in the line that expressed the most apoA-II (23 mg/dl in the 11.1 line versus 63 mg/dl in control mice). There was also a reduction in the plasma concentration of mouse apoA-I (32 +/- 2, 56 +/- 9, 91 +/- 7, and 111 +/- 2 mg/dl for lines 11.1, 21.5, 25.3, and control mice, respectively) that was inversely correlated with the amount of human apoA-II expressed. Additional changes in plasma lipid/lipoprotein profile noted in line 11.1 that expressed the highest level of human apoA-II include elevated triglyceride, increased proportion of total plasma, and HDL free cholesterol and a marked (>10-fold) reduction in mouse apoA-II. Total endogenous plasma lecithin:cholesterol acyltransferase (LCAT) activity was reduced to a level directly correlated with the degree of increased plasma human apoA-II in the transgenic lines. LCAT activity toward exogenous substrate was, however, only slightly decreased. The biochemical changes in the 11.1 line, which is markedly deficient in plasma apoA-I, an activator for LCAT, are reminiscent of those in patients with partial LCAT deficiency. Feeding the transgenic mice a high fat, high cholesterol diet maintained the mouse apoA-I concentration at a normal level (69 +/- 14 mg/dl in line 11.1 compared with 71 +/- 6 mg/dl in nontransgenic controls) and prevented the appearance of HDL deficiency. All this happened in the presence of a persistently high plasma human apoA-II (96 +/- 14 mg/dl). Paradoxical HDL elevation by high fat diets has been observed in humans and is reproduced in human apoA-II overexpressing transgenic mice but not in control mice. Finally, HDL size and morphology varied substantially in the three transgenic lines, indicating the importance of apoA-II concentration in the modulation of HDL formation. The LCAT and HDL deficiencies observed in this study indicate that apoA-II plays a dynamic role in the regulation of plasma HDL metabolism.

Atherosclerosis and plasma and liver lipids in nine inbred strains of mice.

Nine inbred strains of mice, which are progenitors of recombinant inbred sets, were evaluated for aortic lesion formation and plasma and liver lipid levels. This survey was done to determine if a semi-synthetic high-fat diet could elicit the same extent of diet-induced atherosclerosis as that observed in mice fed a natural ingredient high-fat diet and to discover strain-specific plasma and liver lipid variants for future genetic characterization. Evaluation of aortic lesions after 18 wk of diet consumption showed that strains C57BL/6J, C57L/J, SWR/J and SM/J were susceptible to atherosclerosis and that A/J, AKR/J, C3H/HeJ, DBA/2J and SJL/J were relatively resistant. High-density lipoprotein cholesterol (HDL-C) levels were negatively correlated to lesion formation. Susceptible strains had decreased HDL-C levels when switched from chow to the semi-synthetic high-fat, high cholesterol diet, whereas resistant strains either showed no change or a slight increase in HDL-C levels. The exception to this pattern was found in SM mice, which were susceptible to aortic lesion formation but maintained the same HDL-C level on both chow and high-fat diets. HDL size differed among the strains, and levels of plasma apolipoprotein A-I and A-II correlated with HDL-C levels. Liver damage was not correlated to HDL-C levels or to susceptibility to atherosclerosis. Mice from strain A, which are resistant to atherosclerosis, had evidence of liver damage as observed by elevated levels of plasma alanine aminotransferase activity, by liver histology, by increased liver weight and by exceptionally high hepatic cholesterol content.(ABSTRACT TRUNCATED AT 250 WORDS)

Silver-enhanced radial immunodiffusion assay of plasma apolipoproteins.

Silver-staining of immunoprecipitates extends the sensitivity of the radial immunodiffusion assay by tenfold. This modification permits the quantification of apolipoproteins A-I, A-II, C, and E at levels of 0.2-1.0 mg/dl in plasma samples at a sensitivity threshold of 10 ng. The silver-enhanced radial immunodiffusion method is readily adapted from the standard method, simple and inexpensive to perform, and does not require costly instrumentation. These advantages make the modified RID assay an attractive alternative to other forms of immunoassay.

Effects of atherogenic diet consumption on lipoproteins in mouse strains C57BL/6 and C3H.

Inbred mouse strains C57BL/6J (B6) (susceptible) and C3H/HeJ (C3H) (resistant) differ in atherosclerosis susceptibility due to a single gene, Ath-1. Plasma lipoproteins from female mice fed chow or an atherogenic diet displayed strain differences in lipoprotein particle sizes and apolipoprotein (apo) composition. High density lipoprotein (HDL) particle sizes were 9.5 +/- 0.1 nm for B6 and 10.2 +/- 0.1 nm for C3H. No major HDL particle size subclasses were observed. Plasma HDL level in the B6 strain was reduced by the atherogenic diet consumption while the HDL level in the resistant C3H mice was unaffected. The reduction in HDL in the B6 strain was associated with decreases in HDL apolipoproteins A-I(-34%) and A-II(-60%). The HDL apoC content in mice fed chow was two-fold higher in C3H than B6. Lipoproteins containing apolipoprotein B (VLDL, IDL, LDL) shifted from a preponderance of the B-100 (chow diet) to a preponderance of the B-48 (atherogenic diet). The LDL-particle size distribution was strain-specific with the chow diet but not genetically associated with the Ath-1 gene. In both strains on each diet, apolipoprotein E was largely distributed in the VLDL, LDL, and HDL fractions. The B6 strain became sixfold elevated in total lipoprotein E content which in the C3H strain was not significantly affected by diet. However, the C3H LDL apoE content was reduced. On both diets, the C3H strain exhibited apolipoprotein E levels comparable to the atherogenic diet-induced levels of the B6 mice.

Expression of human apolipoprotein A-I in transgenic mice results in reduced plasma levels of murine apolipoprotein A-I and the appearance of two new high density lipoprotein size subclasses.

In Western societies high density lipoprotein (HDL) levels correlate inversely with the risk for coronary heart disease. The primary protein component of both human and mouse HDL is apolipoprotein A-I (apoAI), which comprises greater than 70% of HDL protein and 30% of HDL mass. Human HDLs include particles of several distinct size subpopulations, whereas HDLs from inbred C57BL/6 mice contain a single population of particles. To study the regulation of apoAI expression and its role in HDL assembly, we created transgenic C57BL/6 mice containing the human apoAI gene. Two independent lines of transgenic mice with approximately twice the normal plasma levels of total apoAI were studied. The level of mouse apoAI is reduced greater than 4-fold in both transgenic lines, comprising only 4% of total plasma apoAI levels in one transgenic line and 13% in the other. We demonstrate that the mechanism responsible for the decrease in mouse apoAI is posttranscriptional. Parallel to the replacement of mouse with human apoAI, the single HDL species normally present in the plasma of C57BL/6 is replaced by two HDL subclasses similar in size to human HDL2b and HDL3a. The changes in murine apolipoprotein levels and HDL subclass size are inherited by all transgenic offspring of the two founder animals. These results suggest a dominant role of apoAI in determining the HDL particle size distribution and a mechanism involving expression of human apoAI transgenes that alters the plasma levels of mouse apoAI.

Atherosclerosis susceptibility differences among progenitors of recombinant inbred strains of mice.

Female mice of 16 inbred mouse strains were fed an atherogenic diet for 14 weeks and were then evaluated for atherosclerotic lesions in the aorta. Strains C57BL/6, C57BR/cd, C57L, and SM were very susceptible to atherosclerosis, with lesion area/aortic cross-sections in the range of 4500 to 8000 microns 2. Strains C58 and SWR were intermediate in susceptibility, with lesion area/sections in the range of 1670 to 1690 microns 2. Strains 129, AKR, DBA/2, and BALB/c had only small lesions in the range of 20 to 350 microns 2/section; strains C3H, NZB, CBA, HRS, SJL, and A had no lesions after 14 weeks. Lesion formation in five strains was compared at several time points. Strain C57BL/6 mice developed lesions by 7 weeks, and these continued to grow until all mice had large atheromatous plaques in the aorta and coronary arteries. Strains AKR and DBA/2 also had fatty streak lesions as early as 7 or 8 weeks, but these lesions had not progressed in size by 14 weeks. Strains BALB/c and C3H, which were both resistant to lesion formation at 14 weeks, diverged from each other as time progressed. By 1 year, BALB/c mice had large lesions, but C3H mice had none. Most of the inbred strains chosen for evaluation are the progenitors of recombinant inbred sets of strains, a genetic tool that greatly facilitates the analysis of strain differences. This survey indicates seven additional recombinant inbred sets of strains whose progenitors differ in atherosclerosis susceptibility: BXD, AKXL, SWXJ, NX8, 129XB, NXSM, and B6NXAKRN. An analysis of these recombinant inbred strains may reveal additional mouse genes affecting atherosclerosis susceptibility.

Interconversion of prebeta-migrating lipoproteins containing apolipoprotein A-I and HDL.

Mouse plasma from strains C57BL/6J and C3H/HeJ includes a high density lipoprotein (HDL) fraction containing apolipoprotein A-I which migrates in the prebeta region upon agarose gel electrophoresis, similar to the prebeta HDL previously reported in humans. This prebeta A-I lipoprotein species has a buoyant density of 1.080-1.210 g/ml and has two molecular weight species, 65,000 and 71,000. It is lipid-poor and deficient in apolipoprotein E. When mice are fed a high fat and high cholesterol diet, the quantity of prebeta A-I increases in both strains as determined by quantitative densitometry of agarose gel immunoblots. Prebeta A-I species are highly unstable in plasma at 37 degrees C. Initially (0-1 h) levels decreased and with further incubation (1-8 h) levels increased. Nondenaturing polyacrylamide gel electrophoresis (PAGE) demonstrated that the prebeta HDL formed during prolonged incubation (1-8 h) was identical in size to HDL in unincubated samples. The initial decrease of prebeta HDL observed during the first hour of incubation, phase I, was inhibited by DTNB, suggesting that phase I is dependent on lecithin:cholesterol acyltransferase (LCAT); however, the subsequent increase, phase II, was unaffected by DTNB and appears LCAT-independent. The prebeta A-I species formed in plasma containing DTNB after a 4-h incubation resulted in a polydisperse particle size distribution. The two strains, the atherosclerosis-susceptible C57BL/6 and -resistant C3H, displayed a similar elevation and induction of prebeta HDL during a dietary switch from laboratory chow to an atherogenic diet with a transient peak occurring at 7 days even when total HDL in the susceptible strain was greatly reduced.

Prebeta-migrating high density lipoprotein: quantitation in normal and hyperlipidemic plasma by solid phase radioimmunoassay following electrophoretic transfer.

A quantitative solid phase immunoassay has been developed for the determination of the mass of electrophoretically separated prebeta apolipoprotein A-I (apoA-I) in human plasma. Conditions have been identified for the quantitative transfer and immunoblotting of the apolipoprotein in the absence of organic solvents or detergents. In normolipidemic plasma, the prebeta-migrating fraction of apoA-I represented 4.2 +/- 1.8% of total apoA-I (61 +/- 26 micrograms of apoA-I per ml of plasma). Significantly higher levels were found in hypercholesterolemia of genetic origin, in primary and secondary hypertriglyceridemia, and in congenital lecithin:cholesterol acyltransferase deficiency. In all cases prebeta-migrating apoA-I consisted in large part of low molecular weight lipoprotein species, compared to the size of the major, alpha-migrating apoA-I fraction.