Expression of human apolipoprotein A-II in apolipoprotein E-deficient mice induces features of familial combined hyperlipidemia

Apolipoprotein A-II, a Player in Multiple Processes and Diseases

Apolipoprotein A-II (apoA-II) is the second most abundant apolipoprotein in high-density lipoprotein (HDL) particles, playing an important role in lipid metabolism. Human and murine apoA-II proteins have dissimilar properties, partially because human apoA-II is dimeric whereas the murine homolog is a monomer, suggesting that the role of apoA-II may be quite different in humans and mice. As a component of HDL, apoA-II influences lipid metabolism, being directly or indirectly involved in vascular diseases. Clinical and epidemiological studies resulted in conflicting findings regarding the proatherogenic or atheroprotective role of apoA-II. Human apoA-II deficiency has little influence on lipoprotein levels with no obvious clinical consequences, while murine apoA-II deficiency causes HDL deficit in mice. In humans, an increased plasma apoA-II concentration causes hypertriglyceridemia and lowers HDL levels. This dyslipidemia leads to glucose intolerance, and the ensuing high blood glucose enhances apoA-II transcription, generating a vicious circle that may cause type 2 diabetes (T2D). ApoA-II is also used as a biomarker in various diseases, such as pancreatic cancer. Herein, we provide a review of the most recent findings regarding the roles of apoA-II and its functions in various physiological processes and disease states, such as cardiovascular disease, cancer, amyloidosis, hepatitis, insulin resistance, obesity, and T2D.

iTRAQ-based quantitative proteomic analysis of thoracic aortas from adult rats born to preeclamptic dams

Background

Preeclampsia and gestational hypertension can cause vascular function impairment in offspring. In our previous work, we described the protein expression profiles of umbilical artery tissues from patients with preeclampsia.

Methods

To gain insights into the mechanisms of vascular dysfunction in adult rats born to preeclamptic dams, we analyzed thoracic aorta tissues by using iTRAQ isobaric tags and 2D nano LC-MS/MS.

Results

By using the iTRAQ method, we analyzed 1825 proteins, of which 106 showed significantly different expression in the thoracic aortic. Ingenuity pathway analysis (IPA) showed that the majority of differentially expressed proteins (DEPs) were associated with cardiovascular function. Further analysis indicated that glucose-6-phosphate dehydrogenase (G6PD), which is inhibited by miR-423-5p and activated by TP53, had the strongest effect on cardiovascular function. The expression of G6PD was upregulated in thoracic aorta tissues, as confirmed by Western blotting. The expression of two other vascular function-related proteins, cysteine- and glycine-rich protein 2 (CSRP2) and tubulin alpha-4 A (TUBA4A), was upregulated, as demonstrated by mass spectrometry (MS).

Conclusions

Although the results require further functional validation, these data provide novel findings related to vascular function impairment in the adult offspring of preeclamptic mothers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12014-021-09327-9.

Interaction between Apo A-II -265T>C polymorphism and dietary total antioxidant capacity on some anthropometric indices and serum lipid profile in patients with type 2 diabetes mellitus

The present study aimed to investigate the interaction of Apo A-II polymorphism and dietary total antioxidant capacity (DTAC) with lipid profile and anthropometric markers in patients with type 2 diabetes (T2DM) that are at risk for atherosclerosis. This cross-sectional study was conducted on 778 patients with T2DM (35–65 years). Dietary intakes were assessed by a 147-item food frequency questionnaire. DTAC was computed using international databases. Participants were categorised into two groups based on rs5082 genotypes. The gene–diet interaction was analysed by an ANCOVA multivariate interaction model. Total cholesterol, TC; triacylglycerol, TG; high- and low-density lipoprotein, HDL and LDL; TC–HDL ratio; waist circumference, WC and body mass index, BMI were obtained according to standard protocols. Overall, the frequency of CC homozygous was 12⋅1 % among study participants. We found that a significant interaction between rs5082 variants and DTAC on mean WC (P_TEAC = 0⋅044), TC concentration (P_FRAP = 0⋅049 and P_TEAC = 0⋅031) and TC/HDL (P_FRAP = 0⋅031 and P_TRAP = 0⋅040). Among patients whose DTAC was higher than the median intake, the mean of weight, WC and TC/HDL were significantly higher only in individuals with CC genotype. Also, the high DTAC was associated with a lower TC concentration only in T-allele carriers (P_FRAP = 0⋅042). We found that adherence to a diet with high total antioxidant capacity can improve the complications of diabetes and atherosclerosis in the T carrier genotype more effectively than the CC genotype. These results could indicate the anti-atherogenic properties of Apo A-II. However, further studies are needed to shed light on this issue.

Human hepatic lipase overexpression in mice induces hepatic steatosis and obesity through promoting hepatic lipogenesis and white adipose tissue lipolysis and fatty acid uptake

Human hepatic lipase (hHL) is mainly localized on the hepatocyte cell surface where it hydrolyzes lipids from remnant lipoproteins and high density lipoproteins and promotes their hepatic selective uptake. Furthermore, hepatic lipase (HL) is closely associated with obesity in multiple studies. Therefore, HL may play a key role on lipid homeostasis in liver and white adipose tissue (WAT). In the present study, we aimed to evaluate the effects of hHL expression on hepatic and white adipose triglyceride metabolism in vivo. Experiments were carried out in hHL transgenic and wild-type mice fed a Western-type diet. Triglyceride metabolism studies included β-oxidation and de novo lipogenesis in liver and WAT, hepatic triglyceride secretion, and adipose lipoprotein lipase (LPL)-mediated free fatty acid (FFA) lipolysis and influx. The expression of hHL promoted hepatic triglyceride accumulation and de novo lipogenesis without affecting triglyceride secretion, and this was associated with an upregulation of Srebf1 as well as the main genes controlling the synthesis of fatty acids. Transgenic mice also exhibited more adiposity and an increased LPL-mediated FFA influx into the WAT without affecting glucose tolerance. Our results demonstrate that hHL promoted hepatic steatosis in mice mainly by upregulating de novo lipogenesis. HL also upregulated WAT LPL and promoted triglyceride-rich lipoprotein hydrolysis and adipose FFA uptake. These data support the important role of hHL in regulating hepatic lipid homeostasis and confirm the broad cardiometabolic role of HL.

ApoA-II HDL Catabolism and Its Relationships With the Kinetics of ApoA-I HDL and of VLDL1, in Abdominal Obesity

We study the associations between apoA-II fractional catabolic rate (FCR) and the kinetics of VLDL subspecies and apoA-I and show that, in abdominally obese individuals, apoA-II FCR is positively and independently associated with both apoA-I FCR and VLDL1-TG indirect FCR.

Metabolism of apolipoprotein A-II containing triglyceride rich ApoB lipoproteins in humans

OBJECTIVE

To characterize human triglyceride-rich lipoproteins (TRL) with and without apoA-II and to study their metabolism in vivo.

METHODS

Plasma from 11 participants on a controlled diet given a bolus infusion of [D5]l-phenylalanine to label apoB was combined into four pools and applied to anti-apoA-II immunoaffinity columns. Fractions with and without apoA-II were separated into VLDL and IDL by ultracentrifugation; lipids and apolipoproteins were measured. For kinetic measurements, apoB was isolated and hydrolyzed to the constituent amino acids. Tracer enrichment was measured by GCMS. Metabolic rates were determined by SAAM-II.

RESULTS

VLDL and IDL with apoA-II comprised 7% and 9% of total VLDL and IDL apoB respectively. VLDL with apoA-II was enriched in apoC-III, apoE, and cholesterol compared to VLDL without apoA-II. Mean apoB FCR of VLDL with apoA-II was significantly lower than for VLDL without apoA-II (2.80 ± 0.96 pools/day v.s. 5.09 ± 1.69 pools/day, P = 0.009). A higher percentage of VLDL with apoA-II was converted to IDL than was cleared from circulation, compared to VLDL without apoA-II (96 ± 8% vs. 45 ± 22%; P = 0.007). The rate constants for conversion of VLDL to IDL were similar for VLDL with and without apoA-II. Thus, a very low rate constant for clearance accounted for the lower FCR of VLDL with apoA-II.

CONCLUSION

VLDL with apoA-II represents a small pool of VLDL particles that has a slow FCR and is predominantly converted to IDL rather than cleared from the circulation.

Determinants of VLDL composition and apo B-containing particles in familial combined hyperlipidemia

BACKGROUND

In familial combined hyperlipidemia (FCHL) the severity of the dyslipidemia is determined by an overproduction of VLDL (very low density lipoprotein) particles and by its abnormal lipid composition. However, few are known regarding the metabolic factors that determine these abnormalities. We investigated the impact of metabolic factors on the number of atherogenic particles (apolipoprotein B level (apoB)) and the triglyceride content of very low-density lipoproteins (VLDLs-TG).

METHODS

A cross-sectional study done in FCHL subjects and gender and age-matched healthy subjects. A clinical assessment, lipid profile and plasma concentrations of insulin, apolipoprotein CIII (apo CIII), apolipoprotein AII (apo AII), high sensitive C-reactive protein (HS-CRP), adiponectin and leptin were documented in 147 FCHL patients and 147 age-matched healthy subjects. Multivariate regression models were performed to investigate the independent determinants of VLDL-TG and apo B levels adjusting for confounding factors.

RESULTS

The variables that determined the VLDL-triglyceride content as a surrogate of VLDL composition were apo CIII (β=0.365, p<0.001), insulin (β=0.281, p<0.001), Apo AII (β=0.145, p<0.035), and adiponectin levels (β=-0.255, p<0.001). This model explained 34% of VLDL composition (VLDL-TG) variability. However, none of these variables were independent contributors of apo B-containing particles.

CONCLUSIONS

In patients with FCHL apo CIII, apo AII and adiponectin are major novel factors determining the VLDL particle composition. However, such factors do not explain apo B-containing particles.

Setting the course for apoAII: a port in sight?

ApoAII, the second most abundant protein of the human plasma HDLs, was discovered nearly 50 years ago. Over the subsequent years, nearly 2000 studies - epidemiological, cell-based, biochemical, mouse and human - have attempted to unravel its role in human lipid metabolism. On the basis of these studies, apoAII has been described as an activator and inhibitor of various plasma activities, and as both pro- and anti-atherogenic. Here, we summarize the studies of apoAII, use the preponderance of evidence to propose that the apoAII compass can be reset towards an antiatherogenic course, and suggest ways to stay the course.

Expression and purification of recombinant human apolipoprotein A-II in Pichia pastoris

Apolipoprotein A-II (ApoA-II) is the second most abundant protein constituent of high-density lipoprotein (HDL). The physiologic role of ApoA-II is poorly defined. ApoA-II may inhibit lecithin:cholesterol acyltransferase and cholesteryl-ester-transfer protein activities, but may increase the hepatic lipase activity. ApoA-II may also inhibit the hepatic cholesteryl uptake from HDL probably through the scavenger receptor class B type I depending pathway. Interpretation of data from transgenic and knockout mice of genes involved in lipoprotein metabolism has been often complicated as clinical implications because of species difference. So it is important to obtain human ApoA-II for further studies about its functions. In our studies, Pichia pastoris expression system was first used to express a high-level secreted recombinant human ApoA-II (rhApoA-II). We have cloned the cDNA encoding human ApoA-II and achieved its high-level secreting expression with a yield of 65 mg/L of yeast culture and the purification process was effective and easy to handle. The purified rhApoA-II can be used to further study its biological activities.

Mouse models of atherosclerosis: explaining critical roles of lipid metabolism and inflammation

Atherosclerosis is the most common cause of death globally. It is a complex disease involving morphological and cellular changes in vascular walls. Studying molecular mechanism of the disease is hindered by disease complexity and lack of robust noninvasive diagnostics in human. Mouse models are the most popular animal models that allow researchers to study the mechanism of disease progression. In this review we discuss the advantage and development of mouse as a model for atherosclerotic research. Along with commonly used models, this review discusses strains that are used to study the role of two critical processes associated with the disease-lipid metabolism and inflammation.

Global proteomic profiling reveals altered proteomic signature in schizophrenia serum

Schizophrenia is one of the most severe psychiatric disorders affecting 1% of the world population. There is yet no empirical method to validate the diagnosis of the disease. The identification of an underlying molecular alteration could lead to an improved disease understanding and may yield an objective panel of biomarkers to aid in the diagnosis of this devastating disease. Presented is the largest reported liquid chromatography-mass spectrometry-based proteomic profiling study investigating serum samples taken from first-onset drug-naive patients compared with samples collected from healthy volunteers. The results of this large-scale study are presented along with enzyme-linked immunosorbent assay-based validation data.

Phytosterols do not change susceptibility to obesity, insulin resistance, and diabetes induced by a high-fat diet in mice

Most studies have focused on the cholesterol-lowering activity of phytosterols; however, other biological actions have also been attributed to these plant compounds. In this study, we investigated whether phytosterols could delay (progression phase) and/or reverse (regression phase) insulin resistance or type 2 diabetes mellitus in an experimental mouse model of diet-induced obesity, insulin resistance, and diabetes. Body mass, plasma lipid levels, insulin resistance, and hyperglycemia were determined. Phytosterol intake did not improve these metabolic parameters. Therefore, we were unable to substantiate any protective effect of phytosterol intake on diabetes development or regression in the mouse model used.

Apolipoprotein AII is a regulator of very low density lipoprotein metabolism and insulin resistance

Apolipoprotein AII (apoAII) transgenic (apoAIItg) mice exhibit several traits associated with the insulin resistance (IR) syndrome, including IR, obesity, and a marked hypertriglyceridemia. Because treatment of the apoAIItg mice with rosiglitazone ameliorated the IR and hypertriglyceridemia, we hypothesized that the hypertriglyceridemia was due largely to overproduction of very low density lipoprotein (VLDL) by the liver, a normal response to chronically elevated insulin and glucose. We now report in vivo and in vitro studies that indicate that hepatic fatty acid oxidation was reduced and lipogenesis increased, resulting in a 25% increase in triglyceride secretion in the apoAIItg mice. In addition, we observed that hydrolysis of triglycerides from both chylomicrons and VLDL was significantly reduced in the apoAIItg mice, further contributing to the hypertriglyceridemia. This is a direct, acute effect, because when mouse apoAII was injected into mice, plasma triglyceride concentrations were significantly increased within 4 h. VLDL from both control and apoAIItg mice contained significant amounts of apoAII, with approximately 4 times more apoAII on apoAIItg VLDL. ApoAII was shown to transfer spontaneously from high density lipoprotein (HDL) to VLDL in vitro, resulting in VLDL that was a poorer substrate for hydrolysis by lipoprotein lipase. These results indicate that one function of apoAII is to regulate the metabolism of triglyceride-rich lipoproteins, with HDL serving as a plasma reservoir of apoAII that is transferred to the triglyceride-rich lipoproteins in much the same way as VLDL and chylomicrons acquire most of their apoCs from HDL.

Apolipoprotein A-II Is Inversely Associated With Risk of Future Coronary Artery Disease

Background— Although the vasculoprotective effects of apolipoprotein A-I (apoA-I), the major protein associated with high-density lipoprotein, have been universally accepted, apoA-II has been suggested to have poor antiatherogenic or even proatherogenic properties. To study this suggestion more closely, we evaluated how serum levels of apoA-II and apoA-I relate to the risk of future coronary artery disease (CAD) in a large, prospective study.

Methods and Results— We performed a nested case-control study in the prospective EPIC-Norfolk (European Prospective Investigation into Cancer and Nutrition–Norfolk) cohort. Case subjects (n=912) were apparently healthy men and women aged 45 to 79 years who developed fatal or nonfatal CAD during a mean follow-up of 6 years. Control subjects (n=1635) were matched by age, gender, and enrollment time. Conditional logistic regression was used to quantify the relationship between serum apoA-II levels and risk of CAD. Serum apoA-II concentration was significantly lower in case subjects (34.5±6.3 mg/dL) than in control subjects (35.2±5.8 mg/dL) and was inversely associated with risk of CAD, such that patients in the upper quartile (>38.1 mg/dL) had an odds ratio of 0.59 (95% confidence interval 0.46 to 0.76) versus those in the lowest quartile (<31.1 mg/dL; P for linearity <0.0001). After adjustment for fasting time, alcohol use, and cardiovascular risk factors (systolic blood pressure, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, body mass index, smoking, diabetes mellitus, and C-reactive protein), the corresponding risk estimate was 0.48 (95% confidence interval 0.34 to 0.67, P for linearity <0.0001). Surprisingly, additional adjustment for serum apoA-I levels did not affect risk prediction of apoA-II for future CAD (odds ratio 0.49, 95% confidence interval 0.34 to 0.68, P for linearity <0.0001). Also, after adjustment for high-density lipoprotein particle number and size, apoA-II was still associated with the risk of future CAD (odds ratio 0.62, 95% confidence interval 0.43 to 0.90, P for linearity 0.02).

Conclusions— ApoA-II is associated with a decreased risk of future CAD in apparently healthy people. These findings imply that apoA-II itself exerts effects on specific antiatherogenic pathways. On the basis of these findings, discussion of the potential proatherogenic effects of apoA-II can cease.

Deficiency in monocyte chemoattractant protein-1 modifies lipid and glucose metabolism

We describe the effect of MCP-1 deficiency in mice rendered hyperlipemic by the concomitant ablation of the LDL receptor. The MCP-1(-/-)LDLr(-/-) mice in comparison with LDLr(-/-) mice showed a decreased lipoprotein clearance, derangements in free fatty acids delivery and less glucose tolerance when fed a regular chow, and they showed a partial resistance to alterations in glucose and lipid metabolism induced by dietary fat and cholesterol. They also were less prone to the development of diet-induced obesity. Our results suggest that the role of MCP-1 in metabolism is relevant and that, although new hidden complexities are evident, the function of MCP-1/CCL2 extends far beyond the monocyte chemoattractant effect. Therefore, the regulatory mechanisms influenced by MCP-1 should be fully ascertained to understand the metabolic consequences of inflammation and before considering MCP-1 as a therapeutic target.

Differential intestinal mucosal protein expression in hypercholesterolemic mice fed a phytosterol-enriched diet

The molecular mechanisms involved in the phytosterol-induced decrease in intestinal cholesterol absorption remain unclear. Further, other biological properties such as immunomodulatory activity and protection against cancer have also been ascribed to these plant compounds. To gain insight into the mechanisms underlying phytosterol actions, we conducted a proteomic study in the intestinal mucosa of phytosterol-fed apolipoprotein E-deficient hypercholesterolemic (apoE-/-) mice. With respect to control-fed apoE-/- mice, nine differentially expressed proteins were identified in whole-enterocyte homogenates using 2-D DIGE and MALDI-TOF MS. These proteins are involved in plasma membrane stabilization, cytoskeleton assembly network, and cholesterol metabolism. Four of these proteins were selected for further study since they showed the highest abundance change or had a potential functional relationship with known effects of phytosterols. Annexin A2 (ANXA2) and beta-actin decrease and annexin A4 (ANXA4) and annexin A5 (ANXA5) increase were confirmed by Western blot analysis. Intestinal gene expression of ANXA2 and A5 and beta-actin was reduced, whereas that of ANXA4 was unchanged. The main results were retested in normocholesterolemic C57BL/6J mice. ANXA4 and ANXA5 protein upregulation and ANXA2 and beta-actin downregulation were reproduced in these animals. However, no changes in gene expression were found in C57BL/6J mice in either of the four proteins selected. ANXA2, A4, and A5 and beta-actin are proteins of special interest given their pleiotropic functions that include cholesterol-ester transport from caveolae, apoptosis, and anti-inflammatory properties. Therefore, the protein expression changes identified in this study might be involved in the biological effects of phytosterols.

The results in rodent models of atherosclerosis are not interchangeable: the influence of diet and strain

The determinant factors for the development of atherosclerosis in response to dietary cholesterol were examined in two animal models to assess the comparability of results. We studied 128 male Apo E(-/-) and 128LDLr(-/-) mice randomly assigned to baseline (n=8) and 5 groups (n=24 each) that differed only in their dietary fat and cholesterol supplements. At 10, 16, 24 and 32 weeks of age, 8 animals from each group were sequentially sacrificed and the variables analyzed. The lesion sizes changed at different rates but they were predictable and did not differ in complexity. We observed, however, significant differences between strains, particularly in the constitutive expression of liver genes, their metabolic response to dietary cholesterol, their feeding behaviour, their glucose tolerance and the gain in body weight. Both strains presented characteristics that resemble steatohepatitis but manifestations were more severe in LDLr(-/-) mice. The divergent responses indicate that the choice of the diet and the model should be carefully considered in atherosclerosis studies and extrapolations interpreted with caution.

Phytosterol-mediated inhibition of intestinal cholesterol absorption is independent of ATP-binding cassette transporter A1

An increased activity of ATP-binding cassette transporter (ABC) A1 has been proposed as a mechanism underlying the hypocholesterolaemic effect of phytosterols. In the present study, ABCA1-deficient mice (ABCA1−/− mice) were used to examine the involvement of the ABCA1 in the reduction of intestinal cholesterol absorption in response to a phytosterol-enriched diet. A decrease in intestinal cholesterol absorption of 39 and 35% was observed after phytosterol treatment in ABCA1+/+ mice and in ABCA1−/− mice, respectively. No statistically significant changes in plasma lipoprotein profile or in intestinal ABCG5, ABCG8 and Niemann-Pick C1-Like 1 gene expression levels were found when phytosterol-treated ABCA1−/− mice and untreated ABCA1−/− mice were compared. We conclude that phytosterol inhibition of cholesterol absorption in mice is independent of ABCA1

Dietary phytosterols modulate T-helper immune response but do not induce apparent anti-inflammatory effects in a mouse model of acute, aseptic inflammation

Although most studies have focused on the cholesterol-lowering activity of phytosterols, other biological actions have been ascribed to these plant sterol compounds, one of which is a potential immune modulatory effect. To gain insight into this issue, we used a mouse model of acute, aseptic inflammation induced by a single subcutaneous turpentine injection. Hypercholesterolemic apolipoprotein E-deficient (apoE(-/-)) mice, fed with or without a 2% phytosterol supplement, were treated with turpentine or saline and euthanized 48 h later. No differences were observed in spleen lymphocyte subsets between phytosterol- and control-fed apoE(-/-) mice. However, cultured spleen lymphocytes of apoE(-/-) mice fed with phytosterols and treated with turpentine showed increased IL-2 and IFN-gamma secretion (T-helper type1, Th1 lymphocyte cytokines) compared with turpentine-treated, control-fed animals. In contrast, there was no change in Th2 cytokines IL-4 and IL-10. Phytosterols also inhibit intestinal cholesterol absorption in wild-type C57BL/6J mice but, in this case, without decreasing plasma cholesterol. Spleen lymphocytes of turpentine-treated C57BL/6J mice fed with phytosterols also showed increased IL-2 production, but IFN-gamma, IL-4 and IL-10 production was unchanged. The Th1/Th2 ratio was significantly increased both in phytosterol-fed apoE(-/-) and C57BL/6J mice. We conclude that phytosterols modulate the T-helper immune response in vivo, in part independently of their hypocholesterolemic effect in a setting of acute, aseptic inflammation. Further study of phytosterol effects on immune-based diseases characterized by an exacerbated Th2 response is thus of interest.

Overexpression of lysosomal acid lipase and other proteins in atherosclerosis

Atherosclerosis is one of the major causes of morbidity and mortality in the western world. The existing data of elevated expression levels of proteins like DNA damage and DNA repair enzymes in human atherosclerotic plaques are reviewed. From the literature, the effect of overexpression of different proteins using adenoviral vectors or the model of transgenic mice on the development of atherosclerosis will be discussed. Special focus is placed on the lysosomal acid lipase (LAL), because LAL connects extra-cellular with intra-cellular lipid metabolism and is the only hydrolase for cleavage of cholesteryl esters delivered to the lysosomes. Patients with a deficiency of LAL show an accumulation of lipids in the cells and develop pre-mature atherosclerosis. To answer the question of the influence of LAL in atherosclerosis if overexpressed, we show for the first time data of transgenic mice overexpressing LAL and the effect on the lipid level.

Manipulation of inflammation modulates hyperlipidemia in apolipoprotein E-deficient mice: a possible role for interleukin-6

There are increasing evidences showing that inflammation participates in atherosclerosis. Therefore, the therapeutic use of anti-inflammatory agents should be considered. We have induced chronic, aseptic inflammation upon the injection of turpentine and tested the effect of dexamethasone on lipoprotein metabolism and, consequently, atherosclerosis in apolipoprotein E-deficient mice. Aseptic inflammation caused a significant decrease in hyperlipidemia. Treatment with dexamethasone elicited the opposite effect increasing hyperlipidemia through mechanisms related to the increase in the synthesis of triglyceride-rich lipoproteins. Changes in plasma lipids correlated with those observed in the size of atherosclerotic lesions. Our data suggest the presence of a common mechanism present in both observations and which is probably related to the cytokine secretion. Among the candidates, we chose to test the effect of interleukin-6 because it is involved in both processes, atherosclerosis and inflammation, and its expression is efficiently repressed by corticosteroids. The injection of recombinant interleukin-6 in our mice elicited the same effects observed in our model of inflammation. We conclude that manipulation of inflammation-related mechanisms modulates lipid homeostasis and development of atherosclerotic plaque in rodents.

Trans-10, cis-12- and cis-9, trans-11-conjugated linoleic acid isomers selectively modify HDL-apolipoprotein composition in apolipoprotein E knockout mice

Trans-10, cis-12-conjugated linoleic acid (CLA)-enriched diets promote atherosclerosis in mice despite increasing blood concentrations of HDL cholesterol. This suggests that under these conditions, the HDL apolipoproteins (apo) produced are abnormal. To test this hypothesis, apoE-deficient mice were fed a Western-type diet enriched with linoleic acid (control), cis-9, trans-11-CLA or trans-10, cis-12-CLA (1.0% wt/wt) for 12 wk, and the effects on HDL metabolism and apoC-III levels recorded. Compared with the control and cis-9, trans-11-CLA mice, those fed the trans-10, cis-12-CLA diet had significantly higher HDL cholesterol concentrations, and had a higher incidence of hypertriglyceridemia and hepatic steatosis. Plasma apoA-I and paraoxonase concentrations were significantly lower in the trans-10, cis-12-CLA group than in the cis-9, trans-11-CLA group. These reductions were associated with decreased hepatic expression of these proteins and a shift toward lipid-poor apolipoprotein particles. The plasma apoA-II concentration increased with its corresponding mRNA concentration in the liver, and was preferentially bound to HDL in the trans-10, cis-12-CLA mice, thus explaining the increased HDL cholesterol concentrations in this group. Significant, positive associations were found between apoA-II and C-III (r=0.883, P<0.001) and between apoA-II and atherosclerosis (r=0.68, P<0.001). These results indicate that trans-10, cis-12-CLA intake modifies HDL to form a proatherogenic apoA-II containing particle and promotes phenotypic changes compatible with metabolic syndrome. Cis-9, trans-11-CLA does not promote this detrimental effect.

Dietary cholesterol and differential monocyte chemoattractant protein-1 gene expression in aorta and liver of apo E-deficient mice

In humans, hypercholesterolemia, steatohepatitis, and risk for arteriosclerosis are associated. Apolipoprotein E-deficient mice, a widely used animal model, show both arteriosclerosis and steatohepatitis in response to high-fat and cholesterol diets. We have found a relationship between these conditions and a higher mRNA aortic and hepatic monocyte chemoattractant protein-1 (mcp-1) gene expression. Both tissues respond in a similar way when dietary cholesterol is provided for a few weeks but differently if the conditions persist for a protracted period of time. After 8 months of treatment, the mcp-1 gene expression in the aorta continues increasing but in the liver decreases. This coincides with a significant increase in hepatic ppar-delta anti-inflammatory gene expression. Apparently, the arterial wall cannot prevent the deleterious effects of higher mcp-1 expression by increasing ppar-delta gene expression and the lesion progress. However, in the liver, the activation of anti-inflammatory genes may reduce the hepatic mcp-1 expression which significantly decreases the inflammatory response. This differential inflammatory gene expression in aorta and liver may support the idea that anti-inflammatory transcription factors are involved in the response to diet and inflammation. Therefore, the use of cholesterol-enriched diets should be carefully considered in the apolipoprotein E-deficient mice because they may trigger different stimuli and seriously hinder the interpretation of possible findings.

Paradoxical exacerbation of combined hyperlipidemia in human apolipoprotein A-II transgenic mice treated with fenofibrate

Apolipoprotein (apo) A-II has been biochemically and genetically linked to familial combined hyperlipidemia. Human ApoA-II transgenic mice and peroxisome proliferator-activated receptor alpha (PPARalpha)-deficient mice share some similar phenotypic characteristics. The aim of this study was to determine whether a fibrate-induced PPARalpha activation corrects the combined hyperlipidemia present in human apoA-II transgenic mice. ApoA-II transgenic mice were treated with fenofibrate (250 mg/kg) for 13 days. After this period, they presented a remarkable 8-fold increase in plasma triglycerides. This was concomitant with a 4-fold increase in non-high-density lipoprotein (non-HDL) cholesterol, a quantitatively similar decrease in HDL cholesterol and a severe reduction in mouse plasma apoA-I and apoA-II. Fenofibrate stimulated liver fatty acid beta-oxidation, increased the transcriptional expression of carnitine palmitoyltransferase 1 and phospholipid transfer protein, and decreased expression of apoA-I and apoC-III. However, very-low-density lipoprotein (VLDL)-triglyceride production and lipoprotein lipase (LPL) activities and the expression of other PPARalpha target genes were similar in mice treated with vehicle and fenofibrate. Further, fenofibrate-treated mice presented decreased in vivo [3H]VLDL catabolism and decreased VLDL-triglyceride hydrolysis by exogenous LPL. Therefore, the paradoxical enhancement of hyperlipidemia in fenofibrate-treated apoA-II transgenic mice is mainly due to decreased VLDL catabolism and, also, to a partial impairment in PPARalpha-signaling.

Overexpression of Human Apolipoprotein A-II in Transgenic Mice Does Not Impair Macrophage-Specific Reverse Cholesterol Transport In Vivo

Background— Overexpression of human apolipoprotein (apo) A-II in transgenic mice induces high-density lipoprotein (HDL) deficiency, and increased atherosclerosis susceptibility only when fed an atherogenic diet. This may, in part, be caused by impairment in reverse cholesterol transport (RCT).

Methods and Results— [ 3 H]cholesterol-labeled macrophages were injected intraperitoneally into mice maintained on a chow diet or an atherogenic diet. Plasma [ 3 H]cholesterol did not differ from human apoA-II transgenic and control mice at 24 or 48 hours after the label injection. On the chow diet, human apoA-II transgenic mice presented increased [ 3 H]cholesterol in liver (1.3-fold) and feces (6-fold) compared with control mice ( P <0.05). The magnitude of macrophage-specific RCT did not differ between transgenic and control mice fed the atherogenic diet.

Conclusions— Human apoA-II maintains effective RCT from macrophages to feces in vivo despite an HDL deficiency. These findings suggest that the increased atherosclerotic lesions observed in apoA-II transgenic mice fed an atherogenic diet are not caused by impairment in macrophage-specific RCT.

Turpentine-induced inflammation reduces the hepatic expression of the multiple drug resistance gene, the plasma cholesterol concentration and the development of atherosclerosis in apolipoprotein E deficient mice

We aimed to investigate the effect of turpentine-induced inflammation in an atherosclerosis-prone murine model. We have induced a chronic aseptic inflammation in apolipoprotein E-deficient mice, with or without a dietary supplement of aspirin (n = 10, each), by the injection of a mixture (1:1) of turpentine and olive oil in the hind limb twice weekly for a period of 12 weeks. Control animals were injected with olive oil alone (n = 10). The control mice did show any alteration neither in plasma nor at the site of injection. Turpentine-treated mice showed a significant increase in plasma TNF-alpha and SAA concentrations which indicated a systemic inflammatory response that was not substantially affected by aspirin. Also, turpentine injections significantly reduced the plasma cholesterol concentration, probably decreasing intestinal cholesterol re-absorption, and attenuated the size of atherosclerotic lesion. Both effects were minimally influenced by aspirin. The burden of atherosclerosis correlated with plasma lipid levels but not with plasma inflammatory markers. Finally, there was a concomitant decrease in the expression of the hepatic mdr1b gene that correlated with the decrease in plasma cholesterol concentration. Therefore, we conclude that mdr1 is an additional factor to consider in the complexity of alterations in cholesterol metabolism that occur in this model.

Changes in intestinal and liver global gene expression in response to a phytosterol-enriched diet

BACKGROUND

Dietary phytosterols are a recommended therapeutic option for decreasing plasma cholesterol. The increased activity of ATP-binding cassette (ABC) transporters ABCA1, ABCG5 and ABCG8, or, alternatively, a decrease in Niemann-Pick C1 Like 1 (NPC1L1) could mediate the reduction in intestinal cholesterol absorption caused by phytosterols. Other biological properties such as a direct immune modulatory activity have recently been ascribed to these plant compounds.

METHODS

To gain insight into the molecular effects of phytosterols, global genome-wide gene profiling and real-time RT-PCR studies were conducted in small intestines and livers of phytosterol-treated apolipoprotein E-deficient (apoE(-/-)) mice. Re-testing of the main results was performed in C57BL/6J and LDL receptor-deficient (LDLR(-/-)) mice.

RESULTS

Intestinal cholesterol absorption was decreased in all mouse models but plasma cholesterol was only decreased in apoE(-/-) and LDLR(-/-) mice. ABCA1, ABCG5, ABCG8 and NPC1L1 mRNA levels were slightly reduced in the intestine of phytosterol-treated apoE(-/-) and LDLR(-/-) mice, but increased in C57BL/6J-treated mice. Phytosterols changed genes involved in immune regulation in apoE(-/-) mice. However, these changes were less extensive in LDLR(-/-) mice and were not found in C57BL/6J mice.

CONCLUSIONS

Inhibition of intestinal cholesterol absorption by phytosterols is not mediated via transcriptional changes in ABCA1, ABCG5, ABCG8 or NPC1L1. Changes suggestive of immunomodulation are associated with the hypocholesterolemic effect of phytosterols and with apoE deficiency.

Human Apolipoprotein A-II Enrichment Displaces Paraoxonase From HDL and Impairs Its Antioxidant Properties

Apolipoprotein A-II (apoA-II), the second major high-density lipoprotein (HDL) apolipoprotein, has been linked to familial combined hyperlipidemia. Human apoA-II transgenic mice constitute an animal model for this proatherogenic disease. We studied the ability of human apoA-II transgenic mice HDL to protect against oxidative modification of apoB-containing lipoproteins. When challenged with an atherogenic diet, antigens related to low-density lipoprotein (LDL) oxidation were markedly increased in the aorta of 11.1 transgenic mice (high human apoA-II expressor). HDL from control mice and 11.1 transgenic mice were coincubated with autologous very LDL (VLDL) or LDL, or with human LDL under oxidative conditions. The degree of oxidative modification of apoB lipoproteins was then evaluated by measuring relative electrophoretic mobility, dichlorofluorescein fluorescence, 9- and 13-hydroxyoctadecadienoic acid content, and conjugated diene kinetics. In all these different approaches, and in contrast to control mice, HDL from 11.1 transgenic mice failed to protect LDL from oxidative modification. A decreased content of apoA-I, paraoxonase (PON1), and platelet-activated factor acetyl-hydrolase activities was found in HDL of 11.1 transgenic mice. Liver gene expression of these HDL-associated proteins did not differ from that of control mice. In contrast, incubation of isolated human apoA-II with control mouse plasma at 37°C decreased PON1 activity and displaced the enzyme from HDL. Thus, overexpression of human apoA-II in mice impairs the ability of HDL to protect apoB-containing lipoproteins from oxidation. Further, the displacement of PON1 by apoA-II could explain in part why PON1 is mostly found in HDL particles with apoA-I and without apoA-II, as well as the poor antiatherogenic properties of apoA-II–rich HDL.

Mechanisms mediating insulin resistance in transgenic mice overexpressing mouse apolipoprotein A-II

We previously demonstrated that transgenic mice overexpressing mouse apolipoprotein A-II (apoA-II) exhibit several traits associated with the insulin resistance (IR) syndrome, including increased atherosclerosis, hypertriglyceridemia, obesity, and IR. The skeletal muscle appeared to be the insulin-resistant tissue in the apoA-II transgenic mice. We now demonstrate a decrease in FA oxidation in skeletal muscle of apoA-II transgenic mice, consistent with reports that decreased skeletal muscle FA oxidation is associated with increased skeletal muscle triglyceride accumulation, skeletal muscle IR, and obesity. The decrease in FA oxidation is not due to decreased carnitine palmitoyltransferase 1 activity, because oxidation of palmitate and octanoate were similarly decreased. Quantitative RT-PCR analysis of gene expression demonstrated that the decrease in FA oxidation may be explained by a decrease in medium chain acyl-CoA dehydrogenase. We previously demonstrated that HDLs from apoA-II transgenic mice exhibit reduced binding to CD36, a scavenger receptor involved in FA metabolism. However, studies of combined apoA-II transgenic and CD36 knockout mice suggest that the major effects of apoA-II are independent of CD36. Rosiglitazone treatment significantly ameliorated IR in the apoA-II transgenic mice, suggesting that the underlying mechanisms of IR in this animal model may share common features with certain types of human IR.

Apolipoprotein A-II, genetic variation on chromosome 1q21-q24, and disease susceptibility

PURPOSE OF REVIEW

Apolipoprotein (apo) A-II is the second most abundant HDL apolipoprotein; however its function remains largely unknown. Owing to the lack of consequences of apoA-II deficiency in humans, it has long been considered an apolipoprotein of minor importance. Overexpression of apoA-II in transgenic mice, however, causes combined hyperlipidemia and, in some cases, insulin resistance. This, and the location of the apoA-II gene in chromosome 1q23, a hot region in the search for genes associated with familial combined hyperlipidemia, insulin resistance and type 2 diabetes mellitus, has greatly increased interest in this protein.

RECENT FINDINGS

ApoA-II is biochemically and genetically linked to familial combined hyperlipidemia. Given that the chromosome 1q21-q24 region is associated with insulin resistance or type 2 diabetes, this region is a now a focus of interest in the study of these complex, often overlapping diseases. However, no polymorphisms that increase apoA-II levels have been identified to date in humans. Other nonstructural loci may regulate apoA-II plasma concentration. Further, plasma apoA-II concentration is increased by saturated fat intake. Several reports have added to our understanding of the relationship between apoA-II mutations and amyloidosis both in humans and mice.

SUMMARY

An increased plasma concentration of apoA-II might contribute to familial combined hyperlipidemia or type 2 diabetes mellitus expression, which emphasizes the need to understand its function and metabolism. Genetic studies in well characterized patients and genomic and proteomic approaches in cell and mouse models may help to achieve this understanding.

Phenytoin treatment reduces atherosclerosis in mice through mechanisms independent of plasma HDL-cholesterol concentration

Phenytoin (PHT) increases high density lipoprotein cholesterol (HDL-C) and reduces coronary artery disease mortality in humans. We report the results of PHT treatment on atherosclerosis susceptibility and lipid profile in four different types of mouse: control C57BL/6 mice and cholesteryl ester transfer protein transgenic mice as models of fatty streak, and LDL receptor-deficient mice and apolipoprotein E-deficient mice as models of mature atherosclerosis. Each mouse type was fed an appropriate diet to induce atherosclerosis and prevent liver toxicity. PHT treatment demonstrated a protective effect in all models. Reduction in aortic atherosclerotic area by PHT treatment was more evident in early atherosclerosis (2.3-fold) than in mature atherosclerosis (decreases of 40 and 23%, respectively, but only in mice in the upper 50% percentile of plasma PHT concentration). Atherosclerosis prevention was not concomitant with a consistent increase in HDL-C or any other protective change in the lipid profile. Different analyses of potential antiatherogenic HDL functions did not provide additional information. Microarray liver gene expression analyses identified a potential atheroprotective mechanism characterized by decreased expression of syndecan-4, RhoA2, double LIM protein-1, zeta-chain-associated protein kinase-70 and interleukin 6 receptor-alpha. However, to demonstrate that these changes are part of a PHT-antiatherogenic effect, they will need to be found also in arteries, maintained at protein level and proved to be causal rather than reactive.

ApoA-II modulates the association of HDL with class B scavenger receptors SR-BI and CD36

The class B scavenger receptors SR-BI and CD36 exhibit a broad ligand binding specificity. SR-BI is well characterized as a HDL receptor that mediates selective cholesteryl ester uptake from HDL. CD36, a receptor for oxidized LDL, also binds HDL and mediates selective cholesteryl ester uptake, although much less efficiently than SR-BI. Apolipoprotein A-II (apoA-II), the second most abundant HDL protein, is considered to be proatherogenic, but the underlying mechanisms are unclear. We previously showed that apoA-II modulates SR-BI-dependent binding and selective uptake of cholesteryl ester from reconstituted HDL. To investigate the effect of apoA-II in naturally occurring HDL on these processes, we compared HDL without apoA-II (from apoA-II null mice) with HDLs containing differing amounts of apoA-II (from C57BL/6 mice and transgenic mice expressing a mouse apoA-II transgene). The level of apoA-II in HDL was inversely correlated with HDL binding and selective cholesteryl ester uptake by both scavenger receptors, particularly CD36. Interestingly, for HDL lacking apoA-II, the efficiency with which CD36 mediated selective uptake reached a level similar to that of SR-BI. These results demonstrate that apoA-II exerts a marked effect on HDL binding and selective lipid uptake by the class B scavenger receptors and establishes a potentially important relationship between apoA-II and CD36.

Moderate beer consumption does not change early or mature atherosclerosis in mice

BACKGROUND

Although the consumption of wine in particular has been associated with a lower risk of atherothrombotic cardiovascular disease, systematic reviews differ as to the relative protective effect of beer, wine and spirits. Two previous studies showed that red wine reduces fatty streak formation (early atherosclerosis) but not mature atherosclerosis in apolipoprotein (apo) E-deficient (apoE-/-) mice.

AIM OF THE STUDY

To determine whether a moderate beer intake would affect early and mature atherosclerotic lesion formation using control C57BL/6 and apoE-/- mice, respectively, as models.

METHODS

Control C57BL/6 and apoE-/- mice were randomized to receive either water, ethanol, mild beer, dark beer or ethanol-free beer. The level of beer was designed to approximate the alcohol intake currently believed to be beneficial in reducing human vascular risk. Control C57BL/6 mice were fed a Western diet for 24 weeks, and apoE-/- mice a chow diet for 12 weeks. At the end of the trial period, mice were euthanized and atherosclerotic lesions quantified. Plasma lipid concentrations were also measured.

RESULTS

The amount of atherosclerosis and average number of lesions in the proximal aortic region did not differ among groups in control C57BL/6 mice (p = 0.32 and p = 0.29, respectively) and apoE-/- mice (p = 0.19 and p = 0.59, respectively). No consistent differences were observed in plasma lipid and lipoprotein concentrations among water, ethanol and beer groups.

CONCLUSIONS

Moderate beer consumption does not change the development of early or mature atherosclerosis in mice. Our findings do not support the hypothesis of an anti-atherogenic effect of beer. Other potential protective actions of moderate beer consumption such as plaque stabilization, a reduction in plaque intrinsic thrombogenicity, or a reduction in the systemic propensity to thrombosis, remain to be studied.

Fundamental image quality limits for microcomputed tomography in small animals

Small-animal imaging has become increasingly more important as transgenic and knockout mice are produced to model human diseases. One imaging technique that has emerged is microcomputed tomography (micro-CT). For live-animal imaging, the precision in the images will be determined by the x-ray dose given to the animal. As a result, we propose a simple method to predict the noise performance of an x-ray micro-CT system as a function of dose and image resolution. An ideal, quantum-noise limited micro-CT scanner, assumed to have perfect resolution and ideal efficiency, was modeled. Using a simplified model, the coefficient of variation (COV) of the linear attenuation coefficient was calculated for a range of entrance doses and isotropic voxel sizes. COV calculations were performed for the ideal case and with simulated imperfections in efficiency and resolution. Our model was validated in phantom studies and mouse images were acquired with a specimen scanner to illustrate the results. A simplified model of noise propagation in the case of isotropic resolution indicates that the COV in the linear attenuation coefficient is proportional to (dose)(-1/2) and to the (isotropic voxel size)(-2) in the reconstructed volume. Therefore an improvement in the precision can be achieved only by increasing the isotropic voxel size (thereby decreasing the resolution of the image) or by increasing the x-ray dose. For the ideal scanner, a COV of 1% in the linear attenuation coefficient for an image of a mouse exposed to 0.25 Gy is obtained with a minimum isotropic voxel size of 135 microm. However, the same COV is achieved at a dose of 5.0 Gy with a 65 microm isotropic voxel size. Conversely, for a 68 mm diameter rat, a COV of 1% obtained from an image at 5.0 Gy would require an isotropic voxel size of 100 microm. These results indicate that short-term, potentially lethal, effects of ionizing radiation will limit high-resolution live animal imaging. As improvements in detector technology allow the resolution to improve, by decreasing the detector element size to tens of microns or less, high quality images will be limited by the x-ray dose administered. For the highest quality images, these doses will approach the lethal dose or LD50 for the animals. Approaching the lethal dose will affect the way experiments are planned, and may reduce opportunities for experiments involving imaging the same animal over time. Dose considerations will become much more important for live small-animal imaging as the limits of resolution are tested.

Oral Synthetic Phospholipid (DMPC) Raises High-Density Lipoprotein Cholesterol Levels, Improves High-Density Lipoprotein Function, and Markedly Reduces Atherosclerosis in Apolipoprotein E–Null Mice

BACKGROUND

Lecithin has been widely sold as a dietary supplement. 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) is a phospholipid that does not exist in nature and has been used in vitro to study lipid binding. We tested DMPC in vivo in apolipoprotein (apo) E-null mice.

METHODS AND RESULTS

DMPC or soy or egg lecithin at 1.0 mg/mL was added to the drinking water of 4-week-old apoE-null female mice. Eight weeks later, HDL cholesterol levels and apoA-I levels were markedly increased in the mice that received DMPC. HDL function was also dramatically improved in the mice receiving DMPC, and there was a significant reduction in aortic lesions (P=0.021) in the DMPC mice but not in those receiving lecithin. Adding 1.0 mg/mL of DMPC to the drinking water of 10-month-old apoE-null female mice for 5 weeks caused regression of aortic sinus lesions (P=0.003). Adding 1.0 mg/mL DMPC to the drinking water of 6-month-old apoE-null male mice for 8 weeks significantly reduced aortic sinus lesion area (P=0.0031) and en face whole aorta lesion area (P=0.001), whereas adding the same concentrations of soy or egg lecithin did not significantly alter lesion area. Jejunal apoA-I synthesis and plasma apoA-I levels were increased 2- to 3-fold in mice receiving DMPC but not soy or egg lecithin.

CONCLUSIONS

DMPC (but not lecithin) raises HDL cholesterol and apoA-I, improves HDL function, and prevents lesions or causes their regression in apoE-null mice.

Biochemical and genetic association of plasma apolipoprotein A-II levels with familial combined hyperlipidemia

Apolipoprotein A-II (apoA-II) is a major protein on high-density lipoprotein (HDL) particles, and in mice, its levels are associated with triglyceride and glucose metabolism. In particular, transgenic mice overexpressing apoA-II exhibit hypertriglyceridemia, increased body fat, and insulin resistance, whereas apoA-II-null mice have decreased triglycerides and increased insulin sensitivity. Given the phenotypic overlap between familial combined hyperlipidemia (FCH) and apoA-II transgenic mice, we investigated the relationship of apoA-II to this disorder. Despite having lower HDL-cholesterol (HDL-C), FCH subjects had higher apoA-II levels compared with unaffected relatives (P<0.00016). Triglyceride and HDL-C levels were significant predictors of apoA-II, demonstrating that apoA-II variation is associated with several FCH-related traits. After adjustment for multiple covariates, there was evidence for the heritability of apoA-II levels (h2=0.15; P<0.02) in this sample. A genome scan for apoA-II levels identified significant evidence (LOD=3.1) for linkage to a locus on chromosome 1q41, coincident with a suggestive linkage for triglycerides (LOD score=1.4). Thus, this locus may have pleiotropic effects on apoA-II and FCH traits. Our results demonstrate that apoA-II is biochemically and genetically associated with FCH and may serve as a useful marker for understanding the mechanism by which FCH develops.

Apolipoprotein A-II: beyond genetic associations with lipid disorders and insulin resistance

PURPOSE OF REVIEW

Apolipoprotein A-II, the second major HDL apolipoprotein, was often considered of minor importance relatively to apolipoprotein A-I and its role was controversial. This picture is now rapidly changing, due to novel polymorphisms and mutations, to the outcome of clinical trials, and to studies with transgenic mice.

RECENT FINDINGS

The -265 T/C polymorphism supports a role for apolipoprotein A-II in postprandial very-low-density lipoprotein metabolism. Fibrates, which increase apolipoprotein A-II synthesis, significantly decrease the incidence of major coronary artery disease events, particularly in subjects with low HDL cholesterol, high plasma triglyceride, and high body weight. The comparison of transgenic mice overexpressing human or murine apolipoprotein A-II has highlighted major structural differences between the two proteins; they have opposite effects on HDL size, apolipoprotein A-I content, plasma concentration, and protection from oxidation. Human apolipoprotein A-II is more hydrophobic, displaces apolipoprotein A-I from HDL, accelerates apolipoprotein A-I catabolism, and its plasma concentration is decreased by fasting. Apolipoprotein A-II stimulates ATP binding cassette transporter 1-mediated cholesterol efflux. Human and murine apolipoprotein A-II differently affect glucose metabolism and insulin resistance. A novel beneficial role for apolipoprotein A-II in the pathogenesis of hepatitis C virus has been shown.

SUMMARY

The hydrophobicity of human apolipoprotein A-II is a key regulatory factor of HDL metabolism. Due to the lower plasma apolipoprotein A-II concentration during fasting, measurements of apolipoprotein A-II in fed subjects are more relevant. More clinical studies are necessary to clarify the role of apolipoprotein A-II in well-characterized subsets of patients and in the insulin resistance syndrome.

Mechanisms of HDL deficiency in mice overexpressing human apoA-II

To ascertain the mechanisms underlying the hypoalphalipoproteinemia present in mice overexpressing human apolipoprotein A-II (apoA-II) (line 11.1), radiolabeled HDL or apoA-I were injected into mice. Fractional catabolic rate of [(3)H]cholesteryl oleoyl ether HDL ([(3)H]HDL) was 2-fold increased in 11.1 transgenic mice compared with control mice and this was concomitant with increased radioactivity in liver, gonads, and adrenals. However, scavenger receptor class B, type I (SR-BI) was increased only in adrenals. [(3)H]HDL of 11.1 transgenic mice presented greater binding but decreased uptake compared with control mice when Chinese hamster ovary cells transfected with SR-BI were used, thereby pointing to unknown but SR-BI-independent mechanisms as being responsible for the increased (3)H-radioactivity seen in liver and gonads. Synthesis rate (SR) of plasma [(3)H]HDL was 2-fold decreased in 11.1 transgenic mice. Mouse (125)I-apoA-I was 2-fold more rapidly catabolized (mainly by the kidney) in transgenic mice. Mouse apoA-I displacement from HDL by the addition of isolated human apoA-II was reproduced ex vivo; thus, this mechanism may be involved in the increased renal catabolism of apoA-I. ApoA-I SR was 2-fold decreased in 11.1 transgenic mice and this was concomitant with a 2.3-fold decrease in hepatic apoA-I mRNA abundance. Our findings show that multiple mechanisms are involved in the HDL deficiency presented by mice overexpressing human apoA-II.

Apolipoprotein A-II, HDL metabolism and atherosclerosis

Apolipoprotein (Apo) A-I and apo A-II are the major apolipoproteins of HDL. It is clearly demonstrated that there are inverse relationships between HDL-cholesterol and apo A-I plasma levels and the risk of coronary heart disease (CHD) in the general population. On the other hand, it is still not clearly demonstrated whether apo A-II plasma levels are associated with CHD risk. A recent prospective epidemiological (PRIME) study suggests that Lp A-I (HDL containing apo A-I but not apo A-II) and Lp A-I:A-II (HDL containing apo A-I and apo A-II) were both reduced in survivors of myocardial infarction, suggesting that both particles are risk markers of CHD. Apo A-II and Lp A-I:A-II plasma levels should be rather related to apo A-II production rate than to apo A-II catabolism. Mice transgenic for both human apo A-I and apo A-II are less protected against atherosclerosis development than mice transgenic for human apo A-I only, but the results of the effects of trangenesis of human apo A-II (in the absence of a co-transgenesis of human apo A-I) are controversial. It is highly suggested that HDL reduce CHD risk by promoting the transfer of peripherical free cholesterol to the liver through the so-called 'reverse cholesterol transfer'. Apo A-II modulates different steps of HDL metabolism and therefore probably alters reverse cholesterol transport. Nevertheless, some effects of apo A-II on intermediate HDL metabolism might improve reverse cholesterol transport and might reduce atherosclerosis development while some other effects might be deleterious. In different in vitro models of cell cultures, Lp A-I:A-II induce either a lower or a similar cellular cholesterol efflux (the first step of reverse cholesterol transport) than Lp A-I. Results depend on numerous factors such as cultured cell types and experimental conditions. Furthermore, the effects of apo A-II on HDL metabolism, beyond cellular cholesterol efflux, are also complex and controversial: apo A-II may inhibit lecithin-cholesterol acyltransferase (LCAT) (potential deleterious effect) and cholesteryl-ester-transfer protein (CETP) (potential beneficial effect) activities, but may increase the hepatic lipase (HL) activity (potential beneficial effect). Apo A-II may also inhibit the hepatic cholesteryl uptake from HDL (potential deleterious effect) probably through the SR-BI depending pathway. Therefore, in terms of atherogenesis, apo A-II alters the intermediate HDL metabolism in opposing ways by increasing (LCAT, SR-BI) or decreasing (HL, CETP) the atherogenicity of lipid metabolism. Effects of apo A-II on atherogenesis are controversial in humans and in transgenic animals and probably depend on the complex effects of apo A-II on these different intermediate metabolic steps which are in weak equilibrium with each other and which can be modified by both endogenous and environmental factors. It can be suggested that apo A-II is not a strong determinant of lipid metabolism, but is rather a modulator of reverse cholesterol transport.

Significant impact of the highly informative (CA)n repeat polymorphism of the APOA-II gene on the plasma APOA-II concentrations and HDL subfractions: The ECTIM study

High density lipoproteins (HDL) are heterogeneous in their apolipoprotein composition and the role of apolipoprotein A-II (APOA-II) in HDL structure and metabolism is poorly understood. Yet, studies of naturally occurring variations of APOA-II in mice and experiments in transgenic mice overexpressing the APOA-II gene (APOA-II) have shown that APOA-II expression influences APOA-II plasma levels and HDL size and composition. In humans, two RFLPs (BstNI and MspI) have been described in the APOA-II gene. These RFLPs, however, have been inconstantly associated with variations in APOA-II plasma levels. In particular, the large multicentric ECTIM Study did not show any significant effect of the two RFLPs. Other polymorphisms consisting of repetitive sequences have been proposed as more informative markers than RFLPs. Thus, data from the ECTIM Study were reconsidered by integrating the additional information obtained from a highly informative multiallelic (CA)(n)-repeat polymorphism located in the second intron of the gene. The population study was composed of 763 non-treated male controls and 594 cases of myocardial infarction. In controls, the (CA)(19) allele was associated with significantly decreased APOA-II (P < 0.0009) and LpA-II:A-I (P < 0.02) plasma levels. Although the APOA-I plasma levels were not affected by the polymorphism, the (CA)(19) allele was associated with an increased LpA-I/LpA-II:A-I ratio (P < 0.004). No effect, however, could be detected on myocardial infarction. Study of the linkage disequilibrium and the estimation of haplotype frequencies indicated that the impact of the APOA-II locus could hardly be detected by using the BstNI and MspI RFLPs. These data revive interest in evaluating the role of the APOA-II locus in the control of APOA-II plasma levels and HDL composition.

Hepatitis C virus core protein inhibits microsomal triglyceride transfer protein activity and very low density lipoprotein secretion: a model of viral-related steatosis

Liver steatosis, which involves accumulation of intracytoplasmic lipid droplets, is characteristic of hepatitis C virus (HCV) infection. By use of an in vivo transgenic murine model, we demonstrate that hepatic overexpression of HCV core protein interferes with the hepatic assembly and secretion of triglyceride-rich very low density lipoproteins (VLDL). Core expression led to reduction in microsomal triglyceride transfer protein (MTP) activity and in the particle size of nascent hepatic VLDL without affecting accumulation of MTP and protein disulfide isomerase. Hepatic human apolipoprotein AII (apo AII) expression in double-core/apo AII transgenic mice diminished intrahepatic core protein accumulation and abrogated its effects on VLDL production. Apo AII and HCV core colocalized in human HCV-infected liver biopsies, thus testifying to the relevance of this interaction in productive HCV infection. Our results lead us to propose a new pathophysiological animal model for induction of viral-related steatosis whereby the core protein of HCV targets microsomal triglyceride transfer protein activity and modifies hepatic VLDL assembly and secretion.

Advances in experimental dyslipidemia and atherosclerosis

Among the models of dyslipidemia and atherosclerosis, a number of wild-type, naturally defective, and genetically modified animals (rabbits, mice, pigeons, dogs, pigs, and monkeys) have been characterized. In particular, their similarities to and differences from humans in respect to relevant biochemical, physiologic, and pathologic conditions have been evaluated. Features of atherosclerotic lesions and their specific relationship to plasma lipoprotein particles have been critically reviewed and summarized. All animal models studied have limitations: the most significant advantages and disadvantages of using a specific animal species are outlined here. New insights in lipid metabolism and genetic background with regard to variations in pathogenesis of dyslipidemia-associated atherogenesis have also been reviewed. Evidence suggests that among wild-type species, strains of White Carneau pigeons and Watanabe Heritable Hyperlipidemic and St. Thomas's Hospital rabbits are preferable to the cholesterol-fed wild-type animal species in dyslipidemia and atherosclerosis research. Evidence for the usefulness of both wild-type and transgenic animals in studying the involvement of inflammatory pathways and Chlamydia pneumoniae infection in pathogenesis of atherosclerosis has also been summarized. Transgenic mice and rabbits are excellent tools for studying specific gene-related disorders. However, despite these significant achievements in animal experimentation, there are no suitable animal models for several rare types of fatal dyslipidemia-associated disorders such as phytosterolemia and cerebrotendinous xanthomatosis. An excellent model of diabetic atherosclerosis is unavailable. The question of reversibility of atherosclerosis still remains unanswered. Further work is needed to overcome these deficiencies.

Increased production of very-low-density lipoproteins in transgenic mice overexpressing human apolipoprotein A-II and fed with a high-fat diet

We investigated the mechanisms that lead to combined hyperlipidemia in transgenic mice that overexpress human apolipoprotein (apo) A-II (line 11.1). The 11.1 transgenic mice develop pronounced hypertriglyceridemia, and a moderate increase in free fatty acid (FFA) and plasma cholesterol, especially when fed a high-fat/high-cholesterol diet. Post-heparin plasma lipoprotein lipase and hepatic lipase activities (using artificial or natural autologous substrates), the decay of plasma triglycerides with fasting, and the fractional catabolic rate of the radiolabeled VLDL-triglyceride (both fasting and postprandial) were similar in 11. 1 transgenic mice and in control mice. In contrast, a 2.5-fold increase in hepatic VLDL-triglyceride production was observed in 11. 1 transgenic mice in a period of 2 h in which blood lipolysis was inhibited. This increased synthesis of hepatic VLDL-triglyceride used preformed FFA rather than FFA of de novo hepatic synthesis. The 11.1 transgenic mice also presented reduced epididymal/parametrial white adipose tissue weight (1.5-fold), increased rate of epididymal/parametrial hormone-sensitive lipase-mediated lipolysis (1.2-fold) and an increase in cholesterol and, especially, in triglyceride liver content, suggesting an enhanced mobilization of fat as the source of preformed FFA reaching the liver. Increased plasma FFA was reverted by insulin, demonstrating that 11.1 transgenic mice are not insulin resistant. We conclude that the overexpression of human apoA-II in transgenic mice induces combined hyperlipidemia through an increase in VLDL production. These mice will be useful in the study of molecular mechanisms that regulate the overproduction of VLDL, a situation of major pathophysiological interest since it is the basic mechanism underlying familial combined hyperlipidemia.