Oxidized LDL at the crossroads of immunity in non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is viewed as the hepatic manifestation of the metabolic syndrome and is a condition hallmarked by lipid accumulation in the liver (steatosis) along with inflammation (hepatitis). Currently, the etiology and mechanisms leading to obesity-induced hepatic inflammation are not clear and, as a consequence, strategies to diagnose or treat NASH in an accurate manner do not exist. In the current review, we put forward the concept of oxidized lipids as a significant risk factor for NASH. We will focus on the contribution of the different types of oxidized lipids as part of the oxidized low-density lipoprotein (oxLDL) to the hepatic inflammatory response. Furthermore, we will elaborate on the underlying mechanisms linking oxLDL to inflammatory responses in the liver and on how these cascades can be used as therapeutic targets to combat NASH. This article is part of a Special Issue entitled: Lipid modification and lipid peroxidation products in innate immunity and inflammation edited by Christoph J. Binder.

Determining the association between adipokine expression in multiple tissues and phenotypic features of non-alcoholic fatty liver disease in obesity

Objectives:

Non-alcoholic fatty liver disease (NAFLD) is an obesity-associated disease, and in obesity adipokines are believed to be involved in the development of NAFLD. However, it is still not clear whether adipokines in the liver and/or adipose tissues can be related to the development of specific characteristics of NAFLD, such as steatosis and inflammation. We aimed to address this question by simultaneously examining the adipokine expression in three tissue types in obese individuals.

Methods:

We enrolled 93 severely obese individuals with NAFLD, varying from simple steatosis to severe non-alcoholic steatohepatitis. Their expression of 48 adipokines in the liver, visceral and subcutaneous adipose tissue (SAT) was correlated to their phenotypic features of NAFLD. We further determined whether the correlations were tissue specific and/or independent of covariates, including age, sex, obesity, insulin resistance and type 2 diabetes (T2D).

Results:

The expression of adipokines showed a liver- and adipose tissue-specific pattern. We identified that the expression of leptin, angiopoietin 2 (ANGPT2) and chemerin in visceral adipose tissue (VAT) was associated with different NAFLD features, including steatosis, ballooning, portal and lobular inflammation. In addition, the expression of tumor necrosis factor (TNF), plasminogen activator inhibitor type 1 (PAI-1), insulin-like growth factor 1 (somatomedin C) (IGF1) and chemokine (C-X-C motif) ligand 10 (CXCL10) in the liver tissue and the expression of interleukin 1 receptor antagonist (IL1RN) in both the liver and SAT were associated with NAFLD features. The correlations between ANGPT2 and CXCL10, and NAFLD features were dependent on insulin resistance and T2D, but for the other genes the correlation with at least one NAFLD feature remained significant after correcting for the covariates.

Conclusions:

Our results suggest that in obese individuals, VAT-derived leptin and chemerin, and hepatic expression of TNF, IGF1, IL1RN and PAI-1 are involved in the development of NAFLD features. Further, functional studies are warranted to establish a causal relationship.

Lysosomal cholesterol accumulation: driver on the road to inflammation during atherosclerosis and non-alcoholic steatohepatitis

Many studies show an association between the accumulation of cholesterol inside lysosomes and the progression towards inflammatory disease states that are closely related to obesity. While in the past, the knowledge regarding lysosomal cholesterol accumulation was limited to its association with plaque severity during atherosclerosis, recently, a growing body of evidence indicates a causal link between lysosomal cholesterol accumulation and inflammation. These findings make lysosomal cholesterol accumulation an important target for intervention in metabolic diseases that are characterized by the presence of an inflammatory response. In this review, we aim to show the importance of cholesterol trapping inside lysosomes to the development of inflammation by focusing upon cardiovascular disease and non-alcoholic steatohepatitis (NASH) in particular. We summarize current data supporting the hypothesis that lysosomal cholesterol accumulation plays a key role in the development of inflammation during atherosclerosis and NASH. In addition, potential mechanisms by which disturbed lysosomal function can trigger the inflammatory response, the challenges in improving cholesterol trafficking in macrophages and recent successful research directions will be discussed.

Genetic association analysis of LARS2 with type 2 diabetes

AIMS/HYPOTHESIS

LARS2 has been previously identified as a potential type 2 diabetes susceptibility gene through the low-frequency H324Q (rs71645922) variant (minor allele frequency [MAF] 3.0%). However, this association did not achieve genome-wide levels of significance. The aim of this study was to establish the true contribution of this variant and common variants in LARS2 (MAF > 5%) to type 2 diabetes risk.

METHODS

We combined genome-wide association data (n = 10,128) from the DIAGRAM consortium with independent data derived from a tagging single nucleotide polymorphism (SNP) approach in Dutch individuals (n = 999) and took forward two SNPs of interest to replication in up to 11,163 Dutch participants (rs17637703 and rs952621). In addition, because inspection of genome-wide association study data identified a cluster of low-frequency variants with evidence of type 2 diabetes association, we attempted replication of rs9825041 (a proxy for this group) and the previously identified H324Q variant in up to 35,715 participants of European descent.

RESULTS

No association between the common SNPs in LARS2 and type 2 diabetes was found. Our replication studies for the two low-frequency variants, rs9825041 and H324Q, failed to confirm an association with type 2 diabetes in Dutch, Scandinavian and UK samples (OR 1.03 [95% CI 0.95-1.12], p = 0.45, n = 31,962 and OR 0.99 [0.90-1.08], p = 0.78, n = 35,715 respectively).

CONCLUSIONS/INTERPRETATION

In this study, the largest study examining the role of sequence variants in LARS2 in type 2 diabetes susceptibility, we found no evidence to support previous data indicating a role in type 2 diabetes susceptibility.

Genetic variation in the hypothalamic pathways and its role on obesity

Over recent decades, the prevalence of obesity has increased dramatically worldwide. Although this epidemic is mainly attributable to modern (western) lifestyle, multiple twin and adoption studies indicate the significant role of genes in the individual's predisposition to becoming obese. As the hypothalamus plays a central role in controlling body weight, its regulatory circuits may represent a crucial system in the pathogenesis of the disorder. Genetic variations in genes in the hypothalamic pathways may therefore contribute to the susceptibility for obesity in humans and animals. We summarize current knowledge on the physiological role of the hypothalamus in body-weight regulation and review genetic studies on the hypothalamic candidate genes in relation to obesity. Together, data from functional and genetic studies as well as the new, common, obesity loci identified in genome-wide association scans support an important role for the hypothalamic genes in predisposing to obesity. However, findings are still inconclusive for many candidate genes. To improve our understanding of the genetic architecture of common obesity, we suggest that specific obesity phenotypes should be considered and different analytical approaches used. Such studies should consider multiple genes from the same physiological pathways, together with environmental risk factors.

TUB is a candidate gene for late-onset obesity in women

AIMS/HYPOTHESES

We recently reported significant associations between BMI and three TUB single nucleotide polymorphisms (SNPs) in two Dutch cohorts enriched for type 2 diabetes. Here, we attempted a replication of these associations in a large population-based cohort of female twins comprehensively phenotyped for measures of general and central obesity.

METHODS

Two TUB SNPs (rs2272382, rs2272383) and a third (rs1528133), 22 kb distal to RIC3, were genotyped in 2694 Europid women from the St Thomas' UK Adult Twin Registry (Twins UK) (mean age +/- SD: 47.6 +/- 12.7 years; 42.8% postmenopausal). We explored the hypothesis that TUB is a candidate gene for late-onset obesity in humans through testing the interaction of the SNPs by menopausal status.

RESULTS

In the whole cohort, none of the three SNPs showed a significant main effect on measures of general or central obesity. However, for central obesity the rs2272382 SNP showed a significant interaction with menopausal status (p = 0.036). Postmenopausal women homozygous for the minor allele of rs2272382 showed significantly more general obesity (p = 0.022) and central obesity (p = 0.009) than carriers of the major allele. Differences (beta [95% CI]) between the two genotype groups were 0.92 kg/m2 (0.03-1.81) for BMI (p = 0.036), 2.73 cm (0.62-4.84) for waist circumference (p = 0.013) and 2.43% (0.27-4.60) for per cent central fat (p = 0.027). These associations were confirmed by a sibling transmission disequilibrium test for central obesity, waist circumference and per cent central fat.

CONCLUSIONS/INTERPRETATION

We have replicated associations of TUB SNP rs2272382 with measures of general and central obesity in normal postmenopausal women. These findings confirm TUB as a candidate gene for late-onset obesity in humans.

Association of variants of transcription factor 7-like 2 (TCF7L2) with susceptibility to type 2 diabetes in the Dutch Breda cohort

AIM/HYPOTHESIS

A strong association between susceptibility to type 2 diabetes and common variants of transcription factor 7-like 2 (TCF7L2), encoding an enteroendocrine transcription factor involved in glucose homeostasis, has been reported in three different populations (Iceland, Denmark and USA) by Grant et al. We aimed to replicate these findings in a Dutch cohort.

METHODS

We analysed the genotypes of two intronic single nucleotide polymorphisms (SNPs) in TCF7L2 gene in 502 unrelated type 2 diabetes patients and in a set of healthy controls (n = 920). The two SNPs showed almost complete linkage disequilibrium (D' = 0.91).

RESULTS

We were able to replicate the previously reported association in our Breda cohort. The minor alleles of both variants were significantly over-represented in cases (odds ratio [OR] 1.29, 95% CI 1.09-1.52, [Formula: see text] for rs12255372; OR 1.41, 95% CI 1.19-1.66, [Formula: see text] for rs7903146). In addition, TCF7L2 haplotypes were analysed for association with the disease. The analysis of haplotypes did not reveal any strong association beyond that expected from analysing individual SNPs. The TT haplotype carrying the minor alleles was more frequent among cases (OR 1.38, [Formula: see text]).

CONCLUSIONS/INTERPRETATION

Our data strongly confirm that variants of the TCF7L2 gene contribute to the risk of type 2 diabetes. The population-attributable risk from this factor in the Dutch type 2 diabetes population is 10%.

Identification of differentially regulated genes in mildly hyperlipidemic ApoE3-Leiden mice by use of serial analysis of gene expression

Although genes determining lipoprotein homeostasis and atherosclerosis are the subject of intensive investigation, only a subset of these genes is known at present. Hence, we do not have sufficient knowledge to explain the genetic basis of hyperlipidemia in the majority of subjects. Our aim was to identify novel genes and pathways underlying lipoprotein homeostasis by using serial analysis of gene expression. The liver expression profile of mild hyperlipidemic apolipoprotein E3-Leiden (E3L) transgenic mice was compared with that of the wild-type C57BL/6JIco (B6) mice. Over 18 000 liver transcripts of B6 as well as E3L mice were analyzed, representing >9400 unique genes. One hundred seventy-five genes showed altered expression between the strains (P<0.05). Although several of these genes belonged to known metabolic pathways, such as lipoprotein metabolism, detoxification processes, glycolysis, and the acute-phase response, most were novel. Differential gene expression of 8 of 10 genes tested could be confirmed by Northern blot analysis. This inventory of differentially expressed genes will provide a unique basis for detailed studies to gain more insight into their role in lipoprotein homeostasis and atherosclerosis.

Living up to a name: the role of the VLDL receptor in lipid metabolism

The VLDL receptor (VLDLR) is a member of the LDL receptor family. The VLDLR was hypothesized to mediate fatty acid entry into peripheral tissues, on the basis of its expression in tissues that are active in fatty acid metabolism and its capacity to bind apolipoprotein-E-rich VLDL in vitro. This hypothesis initially proved difficult to confirm, because VLDLR-knockout mice were reported to display normal plasma lipid levels. Moreover, studies in VLDLR-knockout mice that were also deficient in a second LDL receptor family member, the apolipoprotein E receptor 2, indicated a role for the VLDLR in neuronal migration during brain development. However, in accordance with what the term VLDLR suggests, recent studies using VLDLR-deficient and transgenic mice have provided compelling evidence that the VLDLR does indeed play a role in VLDL-triglyceride metabolism, and that it is important for triglyceride storage in the adipocyte.

Effective generation of very low density lipoprotein receptor transgenic mice by overlapping genomic DNA fragments: high testis expression and disturbed spermatogenesis

The generation of functional transgenes via microinjection of overlapping DNA fragments has previously been reported to be successful, but it is still not a widely applied approach. Here we show that the method is very reliable, and should be considered, in case a single large insert clone of the desired gene is not available. In the present study, two large DNA fragments consisting of overlapping cosmids, together constituting the human very low density lipoprotein receptor (VLDLR) gene (35 kb), were used to generate VLDLR transgenic (VLDLR-Tg) mice. Three transgenic founders were born, of which two (strain #2 and #3) generated transgenic offspring. Using Fiber-FISH analysis, the integration site was shown to contain at least 44 and 64 DNA fragments in mouse strains #2 and #3, respectively. This copy number resulted in integration sites of 1.5 and 2.5 megabase in size. Notably, over 90% of the fragments in both mouse strains #2 and #3 were flanked by their complementary fragment. In line with this observation, Southern blot analysis demonstrated that the correct recombination between fragments predominated in the transgenic insertion. Human VLDLR expression was detected in testis, kidney and brain of both mouse strains. Since this pattern did not parallel the endogenous VLDLR expression, some crucial regulatory elements were probably not present in the cosmid clones. Human VLDLR expression in testis was detected in germ cells up to the meiotic stage by in situ mRNA analysis. Remarkably, in the F1 generation of both VLDLR-Tg mouse strains the testis was atrophic and giant cells were detected in the semineferous tubuli. Furthermore, male VLDLR-Tg mice transmitted the transgene to their progeny with low frequencies. This could imply that VLDLR overexpression in the germ cells disturbed spermatogenesis.

Transgenic mouse models to study the role of the macrophage scavenger receptor class A in atherosclerosis

Several in vivo studies have been performed on the role of the macrophage scavenger receptor class A (SR-A) in atherosclerosis using SR-A knockout mice. The results indicate both an antiatherogenic and a proatherogenic role of SR-A, depending on the nature of the animal model serving as the athero-susceptible background. To study the role of SR-A in a different model, we generated a transgenic mouse model with high level expression of the human SR-A gene using a 180 Kb yeast artificial chromosome (MSR1 transgenic mice). These mice show increased expression of SR-A according to the natural expression pattern. The MSR1 transgenic mice were crossed onto a low-density lipoprotein receptor deficient background and were fed a high fat diet for 10 weeks. After this period, the size of the atherosclerotic lesions in the proximal aorta was measured. Surprisingly, atherosclerosis was significantly reduced in the MSR1 transgenic mice. In a second study, the effect of SR-A was examined in APOE-3 Leiden mice providing a different athero-susceptible background. To exclude nonmacrophage effects, bone marrow was transplanted from MSR1 mice and wild-type littermates to APOE-3 Leiden transgenic mice. After 8 weeks on a high fat diet, atherosclerosis in the mice that had received MSR1 bone marrow was reduced compared with mice that had received wild-type bone marrow. This difference reached statistical significance when individual cholesterol exposure of the mice was taken into account. Both experiments indicated an antiatherogenic role of the SR-A. This observation cannot be explained easily by SR-A function in foam cell formation because in MSR1 macrophages in vitro foam cell formation is increased. Alternatively, however, SR-A may affect the activation of macrophages. Hence the response to lipopolysaccharide was measured in MSR1-transgenic macrophages. These macrophages showed a reduction in their activation in response to lipopolysaccharide, as measured by nitric oxide production. These data show that an elevated level of SR-A expression reduces atherosclerosis, potentially by modifying the response of macrophages to activation signals in the plaque.

Use of transgenic mice to study the role of apolipoprotein E in lipid metabolism and atherosclerosis

Insight into the role of apolipoprotein (apo) E in lipoprotein metabolism and atherosclerosis has increased dramatically with the generation and analysis of novel transgenic, knockout and knockin mouse models. Moreover, the recent development and application of somatic gene and cell transfer technologies which can express (or delete) apoE in specific tissues of virtually any mouse model have further added to this increase in knowledge. It is now well established that apoE plays a role in virtually every step in the metabolism of very low-density lipoproteins and in the efflux of cholesterol from macrophages. In this review we will discuss recent insights into the role of apoE in these processes with particular emphasis on the specific effects of variation in apoE structure and quantity.

Essential role for the (hepatic) LDL receptor in macrophage apolipoprotein E-induced reduction in serum cholesterol levels and atherosclerosis

Apolipoprotein E (apoE) is a high affinity ligand for several receptor systems in the liver, including the low-density lipoprotein (LDL) receptor, and non-LDL receptor sites, like the LDL receptor-related protein (LRP), the putative remnant receptor and/or proteoglycans. Although the liver is the major source of apoE synthesis, apoE is also produced by a wide variety of other cell types, including macrophages. In the present study, the role of the LDL receptor in the removal of lipoprotein remnants, enriched with macrophage-derived apoE from the circulation, was determined using the technique of bone marrow transplantation (BMT). Reconstitution of macrophage apoE production in apoE-deficient mice resulted in a serum apoE concentration of only 2% of the concentration in wild-type C57Bl/6 mice. This low level of apoE nevertheless reduced VLDL and LDL cholesterol 12-fold (P<0.001) and fourfold (P<0.001), respectively, thereby reducing serum cholesterol levels and the susceptibility to atherosclerosis. In contrast, reconstitution of macrophage apoE synthesis in mice lacking both apoE and the LDL receptor induced only a twofold (P<0.001) reduction in VLDL cholesterol and had no significant effect on atherosclerotic lesion development, although serum apoE levels were 93% of the concentration in normal C57Bl/6 mice. In conclusion, a functional (hepatic) LDL receptor is essential for the efficient removal of macrophage apoE-enriched lipoprotein remnants from the circulation and thus for normalization of serum cholesterol levels and protection against atherosclerotic lesion development in apoE-deficient mice.

Effect of human scavenger receptor class A overexpression in bone marrow-derived cells on cholesterol levels and atherosclerosis in ApoE-deficient mice

In the arterial wall, scavenger receptor class A (SRA) is implicated in pathological lipid deposition. In contrast, in the liver, SRA is suggested to remove modified lipoproteins from the circulation, thereby protecting the body from their pathological action. The role of SRA on bone marrow-derived cells in lipid metabolism and atherogenesis was assessed in vivo by transplantation of bone marrow cells overexpressing human SRA (MSR1) to apoE-deficient mice. In vitro studies with peritoneal macrophages from the transplanted mice showed that macrophage scavenger receptor function, as measured by cell association and degradation studies with acetylated LDL, was approximately 3-fold increased on overexpression of MSR1 in bone marrow-derived cells as compared with control mice. Despite the increased macrophage scavenger receptor function in vitro, no significant effect of MSR1 overexpression in bone marrow-derived cells on the in vivo atherosclerotic lesion development was found. In addition to arterial wall macrophages, liver sinusoidal Kupffer cells also overexpress MSR1 after bone marrow transplantation, which may scavenge atherogenic particles more efficiently from the blood compartment. Introduction of bone marrow cells overexpressing human MSR1 in apoE-deficient mice induced a significant reduction in serum cholesterol levels of approximately 20% (P:<0.001, 2-way ANOVA) as the result of a decrease in VLDL cholesterol. It is suggested that the reduction in VLDL cholesterol levels is due to increased clearance of modified lipoproteins by the overexpressed MSR1 in Kupffer cells of the liver, thereby protecting the arterial wall against the proatherogenic action of modified lipoproteins.

LDL receptor deficiency unmasks altered VLDL triglyceride metabolism in VLDL receptor transgenic and knockout mice

The very low density lipoprotein receptor (VLDLR) has been proposed to play a role in the delivery of fatty acids to peripheral tissues. However, despite reduced adipose tissue mass in VLDLR-deficient (VLDLR(-)(/-)) mice, this has been difficult to substantiate. In the present study, VLDLR-deficient and VLDLR-overexpressing (PVL) mice were cross-bred onto a low density lipoprotein receptor knockout (LDLR(-)(/-)) background to study the VLDLR under conditions of relatively high serum VLDL and triglyceride levels. Absence of the VLDLR resulted in a significant increase in serum triglyceride levels (1.9-fold) when mice were fed a high fat diet. In contrast, overexpression of the VLDLR resulted in a significant decrease in serum triglyceride levels (2.0-fold) under similar conditions. When kept on a chow diet, a period of prolonged fasting revealed a significant increase in serum triglyceride levels in VLDLR(-)(/-); LDLR(-)(/-) mice (2.3-fold) as compared with LDLR(-)(/-) controls. This could not be attributed to altered apolipoprotein B and VLDL triglyceride production rates. Furthermore, no major differences in nascent VLDL triglyceride content were found between VLDLR(-)(/-); LDLR(-)(/-) mice and LDLR(-)(/-) controls. However, the triglyceride content of circulating VLDL of VLDLR(-)(/-); LDLR(-)(/-) mice (63%) was relatively high as compared with LDLR(-)(/-) controls (49%). These observations suggest that the VLDLR affects peripheral uptake of VLDL triglycerides. In conclusion, under conditions of LDLR deficiency in combination with high fat feeding or prolonged fasting, the effect of the VLDLR on VLDL triglyceride metabolism was revealed.

No evidence for cholinergic problems in apolipoprotein E knockout and apolipoprotein E4 transgenic mice

The varepsilon4 allele of the apolipoprotein E gene constitutes the major genetic risk factor to develop Alzheimer's disease. If and how this protein contributes to the pathological cascade of Alzheimer's disease is not known. The varepsilon4 allele particularly affects the cholinergic defect, which is one of the most consistent neurotransmitter problems in an Alzheimer's disease brain. We have analysed several parameters of the cholinergic system in brain of apolipoprotein E knockout mice as well as in transgenic mice overexpressing human apolipoprotein E4. We analysed the distribution of cholinergic fibers, the number and morphology of cholinergic neurons and the enzymatic activity of acetylcholinesterase and choline acetyltransferase in different brain regions. Finally, we analysed the distribution and the binding parameters of [3H]hemicholinium-3, a specific marker for the high affinity choline transporter in different brain sections and regions. This extensive effort failed to show any consistent difference in the cholinergic parameters studied, in either the apolipoprotein E4 transgenic mice or in the apolipoprotein E knockout mice, compared to age-matched non-transgenic mice. We conclude that the apolipoprotein E4 is not deleterious per se for the cholinergic system in mouse brain.

Effect of human scavenger receptor class A overexpression in bone marrow-derived cells on lipoprotein metabolism and atherosclerosis in low density lipoprotein receptor knockout mice

Scavenger receptors, which include various classes, play an important role in atherogenesis by mediating the unrestricted uptake of modified lipoproteins, resulting in the massive accumulation of cholesteryl esters. Because macrophage-derived foam cells are considered to be an important feature in early atherogenesis, we investigated the role of scavenger receptor class A (SR-A) overexpression, especially on macrophages in lipoprotein metabolism and atherosclerosis. Bone marrow from human SR-A (MSR1)-overexpressing mice was transplanted into irradiated low density lipoprotein receptor knockout [LDLR(-/-)] mice. The transplantation resulted in an increase in total serum cholesterol (approximately 15 to 25%), especially in the VLDL fraction, when compared with LDLR(-/-) mice that were transplanted with bone marrow of wild-type littermates. Quantification of atherosclerotic lesions in the mice that were fed a "Western-type" diet for 3 months revealed that there were no differences in mean lesion area between LDLR(-/-) mice transplanted with MSR1 overexpressing and wild-type littermate bone marrow, despite increased scavenger receptor activity in vitro. The presence or absence of the LDLR in the transplanted bone marrow did not influence these results.In conclusion, introduction of MSR1-overexpressing bone marrow in LDLR(-/-) mice via bone marrow transplantation resulted in a slight increase in lipoprotein levels, but had no effect on the atherosclerotic lesion area, despite increased scavenger receptor activity in vitro.

Hepatic lipid accumulation, altered very low density lipoprotein formation and apolipoprotein E deposition in apolipoprotein E3-Leiden transgenic mice

BACKGROUND/AIM

Apolipoprotein (apo) E-deficiency leads to hepatic steatosis and impaired Very Low Density Lipoprotein (VLDL)-triglyceride production rates in mice. A mutant apoE isoform, apoE3-Leiden, is associated with a dominantly inherited form of dysbetalipoproteinemia in humans. The aim of this study was to evaluate the effects of APOE*3-Leiden expression on hepatic lipid content, VLDL formation and liver morphology in mice.

METHODS

Comparison of lipid parameters and liver morphology in mouse strains with different expression of the APOE*3-Leiden transgene with and without co-expression of human APOCI.

RESULTS

Hepatic triglyceride content was increased to maximally 233% of control values, depending on hepatic APOE*3-Leiden expression. Hepatic secretion of VLDL-associated triglycerides was impaired (-20%) in high-expressing transgenics, with a concomitant increase from 1.6 to 8.1 of the apoB48/ apoB100 ratio in newly-formed VLDL. Hepatocytes of the transgenic mice contained characteristic inclusions, up to 20 microm in diameter, in numbers dependent on APOE*3-Leiden expression and independent of APOCI expression. These inclusions contained material positively reacting with antihuman apoE antibodies. Immunogold-labeling confirmed the presence of apoE3-Leiden within these inclusions and also revealed the presence of the mutant protein on sinusoidal membranes, in multivesicular bodies and in peroxisomes, i.e., a distribution pattern similar to that of endogenous apoE in rodents. Nascent VLDL particles associated with the Golgi apparatus were also labeled.

CONCLUSION

This study has demonstrated that introduction of human apoE3-Leiden in mice, in addition to its reported effects on lipolysis and lipoprotein clearance, leads to hepatic deposition of the mutant apolipoprotein, development of fatty liver and to altered hepatic VLDL secretion. The latter findings are consistent with a role of apoE in the regulation of intrahepatic lipid metabolism.

Identification of a novel beta-tubulin subfamily with one member (TUBB4Q) located near the telomere of chromosome region 4q35

The human beta-tubulin supergene family consists of several isotypes with many associated pseudogenes. Here we report the identification of yet another beta-tubulin sequence designated TUBB4Q. This tubulin maps 80 kb proximal to the facioscapulohumeral muscular dystrophy (FSHD1) associated D4Z4 repeats on chromosome 4q35. The genomic structure contains four exons encoding a putative protein of 434 amino acids. The TUBB4Q nucleotide and protein sequence show 87% and 86% homology to beta2-tubulin, respectively. Although the genomic structure shows all functional aspects of a genuine gene, no transcript could be detected. TUBB4Q-related sequences were identified on multiple chromosomes. Since these sequences mutually exhibit a high nucleotide sequence homology, they presumably belong to a novel subfamily of beta-tubulin genes. Although the chromosome 4q35 tubulin-member probably represents a pseudogene, ectopic expression due to a postulated position effect variegation (PEV), makes TUBB4Q an ideal dominant-negative candidate gene for FSHD1.

Apolipoprotein E2 (Lys146-->Gln) causes hypertriglyceridemia due to an apolipoprotein E variant-specific inhibition of lipolysis of very low density lipoproteins-triglycerides

The apolipoprotein E2 (Lys146-->Gln) variant is associated with a dominant form of familial dysbetalipoproteinemia. Heterozygous carriers of this variant have elevated levels of plasma triglycerides, cholesterol, and apolipoprotein E (apoE). It was hypothesized that the high amounts of triglycerides in the very low density lipoprotein (VLDL) fraction are due to a disturbed lipolysis of VLDL. To test this hypothesis, apoE knockout mice were injected with an adenovirus containing the human APOE*2 (Lys146-->Gln) gene, Ad-E2(146), under the control of the cytomegalovirus promoter. ApoE knockout mice injected with an adenovirus vector encoding human apoE3 (Ad-E3) were used as controls. Five days after adenovirus injection, plasma cholesterol levels of mice injected with a high dose of Ad-E2(146) (2x10(9) plaque-forming units) were not changed compared with preinjection levels, whereas in the group who received a low dose of Ad-E2(146) (5x10(8) plaque-forming units) and in the groups injected with a low or a high dose of Ad-E3, plasma cholesterol levels were decreased 5-, 6-, and 12-fold, respectively. Plasma triglycerides were not affected in mice injected with Ad-E3. In contrast, a 7-fold increase in plasma triglycerides was observed in mice injected with the low dose of Ad-E2(146) compared with mice injected with Ad-E3. Injection with the high dose of Ad-E2(146) resulted in a dramatic increase of plasma triglycerides (50-fold compared with Ad-E3 injection). In vitro lipolysis experiments showed that the lipolysis rate of VLDLs containing normal amounts of apoE2 (Lys146-->Gln) was decreased by 54% compared with that of VLDLs containing comparable amounts of apoE3. The in vivo VLDL-triglyceride production rate of Ad-E2(146)-injected mice was not significantly different from that of Ad-E3-injected mice. These results demonstrate that expression of apoE2 (Lys146-->Gln) causes hypertriglyceridemia due to an apoE variant-specific inhibition of the hydrolysis of VLDL-triglycerides.

Very-low-density lipoprotein binding to the apolipoprotein E receptor 2 is enhanced by lipoprotein lipase, and does not require apolipoprotein E

The apolipoprotein (apo)E receptor 2 (apoER2) is a recently cloned member of the low-density lipoprotein (LDL) receptor (LDLR) family, showing a high homology with both the LDLR and the very-low-density lipoprotein (VLDL) receptor (VLDLR). In the present study, the binding characteristics of the apoER2 with respect to apoE and lipoprotein lipase (LPL) were investigated. VLDL was isolated from both apoE-deficient mice and mice expressing the human APOE2 (Arg(158)-->Cys) and APOE3-Leiden isoforms on an Apoe(-/-),Ldlr(-/-) double knock-out background. apoE-rich rabbit beta-VLDL was used as a positive control for binding. Binding experiments performed with Chinese hamster ovary cells expressing the human apoER2 showed that the receptor was able to bind VLDL containing either of the apoE isoforms, as well as the apoE-deficient VLDL. Hence, in contrast with the VLDLR, the apoER2 is not strictly dependent on apoE for VLDL binding. Since LPL has been shown to enhance the binding of lipoproteins to several members of the LDLR family, including the LDLR-related protein, VLDL receptor, gp330 and the LDLR itself, VLDL binding experiments were performed in the presence of LPL. Addition of LPL resulted in a significant increase in apoER2 binding for all VLDL fractions used in this study. In conclusion, lipoprotein binding of VLDL to the apoER2 is enhanced in the presence of LPL, and is not restricted to apoE-containing lipoproteins.

Macrophage scavenger receptor class A: A multifunctional receptor in atherosclerosis

In atherogenesis, elevated plasma levels of low density lipoprotein (LDL) lead to the chronic presence of LDL in the arterial wall. There, LDL is modified (eg, oxidized), and these modified lipoproteins activate endothelial cells, which attract circulating monocytes. These monocytes enter the vessel wall, differentiate into macrophages, and subject the modified lipoproteins to endocytosis through scavenger receptor pathways. This unrestricted uptake, which is not limited by intracellular cholesterol levels, eventually leads to the formation of lipid-filled foam cells, the initial step in atherosclerosis. Macrophage scavenger receptor class A (SRA) is thought to be one of the main receptors involved in foam cell formation, mediating the influx of lipids into the macrophages. In addition to this role in modified lipoprotein uptake by macrophages, the SRA has been shown to be important in the inflammatory response in host defense, cellular activation, adhesion, and cell-cell interaction. Given the importance of these processes in atherogenesis, these latter functions may prove to make the SRA a multifunctional player in the atherosclerotic process.

Effect of macrophage-derived mouse ApoE, human ApoE3-Leiden, and human ApoE2 (Arg158-->Cys) on cholesterol levels and atherosclerosis in ApoE-deficient mice

The effect of monocyte/macrophage-derived wild-type mouse apolipoprotein E (apoE), human apoE3-Leiden, and human apoE2 on serum cholesterol levels and the development of atherosclerosis in apoE-deficient (apoe-/-) mice was investigated by using bone marrow transplantation (BMT). At 4 weeks after BMT, murine apoe+/+ bone marrow reduced serum cholesterol levels by 87% in apoe-/- mice, whereas macrophage-derived human apoE3-Leiden and human apoE2 induced a maximal, transient reduction of 35% and 48%, respectively. At 4 months after BMT, atherosclerosis was 23-fold (P<0.001) reduced in apoe+/+-->apoe-/- mice, whereas no significant reduction in apoE3-Leiden.apoe-/--->apoe-/- and apoE2.apoe-/--->apoe-/- mice could be demonstrated. A highly significant decrease in serum cholesterol levels (78% reduction) and atherosclerosis (21-fold, P<0. 001) was found in apoE3-Leiden.apoe-/- animals expressing high levels of apoE in multiple tissues, whereas apoE2 was ineffective even at high concentrations. Furthermore, in contrast to apoE-deficient macrophages, cholesterol efflux from apoE2 or apoE3-Leiden macrophages was not impaired. In conclusion, apoE3-Leiden as well as apoE2 are less effective in reducing cholesterol levels and atherosclerosis in apoe-/- animals, compared with apoe+/+, with apoE2<apoE3-Leiden<apoe+/+, irrespective of the observed adequate efflux of cholesterol from macrophages expressing apoE2 and apoE3-Leiden, indicating that normalization of cholesterol efflux by macrophages is not accompanied by measurable effects on lesion growth.

Apolipoprotein E participates in the regulation of very low density lipoprotein-triglyceride secretion by the liver

ApoE-deficient mice on low fat diet show hepatic triglyceride accumulation and a reduced very low density lipoprotein (VLDL) triglyceride production rate. To establish the role of apoE in the regulation of hepatic VLDL production, the human APOE3 gene was introduced into apoE-deficient mice by cross-breeding with APOE3 transgenics (APOE3/apoe-/- mice) or by adenoviral transduction. APOE3 was expressed in the liver and, to a lesser extent, in brain, spleen, and lung of transgenic APOE3/apoe-/- mice similar to endogenous apoe. Plasma cholesterol levels in APOE/apoe-/- mice (3.4 +/- 0.5 mM) were reduced when compared with apoe-/- mice (12.6 +/- 1.4 mM) but still elevated when compared with wild type control values (1.9 +/- 0.1 mM). Hepatic triglyceride accumulation in apoE-deficient mice was completely reversed by introduction of the APOE3 transgene. The in vivo hepatic VLDL-triglyceride production rate was reduced to 36% of control values in apoE-deficient mice but normalized in APOE3/apoe-/- mice. Hepatic secretion of apoB was not affected in either of the strains. Secretion of (3)H-labeled triglycerides synthesized from [(3)H]glycerol by cultured hepatocytes from apoE-deficient mice was four times lower than by APOE3/apoe-/- or control hepatocytes. The average size of secreted VLDL particles produced by cultured apoE-deficient hepatocytes was significantly reduced when compared with those of APOE3/apoe-/- and wild type mice. Hepatic expression of human APOE3 cDNA via adenovirus-mediated gene transfer in apoE-deficient mice resulted in a reduction of plasma cholesterol depending on plasma apoE3 levels. The in vivo VLDL-triglyceride production rate in these mice was increased up to 500% compared with LacZ-injected controls and correlated with the amount of apoE3 per particle. These findings indicate a regulatory role of apoE in hepatic VLDL-triglyceride secretion, independent from its role in lipoprotein clearance.

Hyperlipidemia of ApoE2(Arg(158)-Cys) and ApoE3-Leiden transgenic mice is modulated predominantly by LDL receptor expression

To investigate the relative roles of the LDL receptor- and non-LDL receptor-mediated pathways in the clearance of apolipoprotein E (apoE) variants in vivo, we have generated apoE2(Arg(158)-Cys) (apoE2) and apoE3-Leiden transgenic mice deficient for the endogenous mouse Apoe and Ldl receptor genes (Apoe-/-.Ldlr-/- mice). Unexpectedly, on the Apoe-/-.Ldlr-/- background, expression of neither apoE2 nor apoE3-Leiden results in a decrease of the hyperlipidemia. In contrast, serum cholesterol levels are increased by the introduction of apoE2 and apoE3-Leiden in Apoe-/-.Ldlr-/- mice (to 39.1+/-7.1 and 37.6+/-7.6 mmol/L, respectively, from 25. 9+/-6.5 mmol/L). In addition, in these transgenic mice, the serum triglyceride levels are substantially increased (to 9.6+/-7.0 and 5. 8+/-2.8 mmol/L, respectively, from 0.7+/-0.5 mmol/L), which is associated with a decreased efficiency of in vitro LPL-mediated lipolysis of circulating VLDL. The VLDL-triglyceride secretion rate is not affected by the expression of apoE2 or apoE3-Leiden on the Apoe-/-.Ldlr-/- background. These results indicate that in the absence of the LDL receptor, clearance of triglyceride-rich apoE2 and apoE3-Leiden-containing lipoproteins via alternative hepatic receptors, such as the LDL receptor-related protein (LRP) is inefficient. Although apoE2 and apoE3-Leiden are disturbed in binding to the LDL receptor in vitro, expression of 1 or 2 mouse Ldlr alleles in an apoE2.Apoe-/- or apoE3-Leiden.Apoe-/- background results in a gene dose-dependent decrease of the hyperlipidemia. Furthermore, overexpression of the LDL receptor via adenovirus-mediated gene transfer rescues the hyperlipidemia associated with apoE2 and apoE3-Leiden expression. These data indicate that in apoE2 and apoE3-Leiden transgenic mice, the LDL receptor constitutes the predominant route for clearance of VLDL remnants, carrying even poorly binding apoE variants, and that this pathway is functional despite an apoE-mediated disturbance in VLDL triglyceride lipolysis.

Macrophage specific overexpression of the human macrophage scavenger receptor in transgenic mice, using a 180-kb yeast artificial chromosome, leads to enhanced foam cell formation of isolated peritoneal macrophages

Macrophage scavenger receptors class A (MSR) are thought to play an important role in atherogenesis by mediating the unrestricted uptake of modified lipoproteins by macrophages in the vessel wall leading to foam cell formation. To investigate the in vivo role of the MSR in this process, a transgenic mouse model expressing both isoforms of the human MSR was generated. A 180-kb yeast artificial chromosome (YAC) containing the human MSR gene (MSR1) with 60- and 40-kb flanking sequence at the 5' and 3' end, respectively, was obtained by reducing the size of a 1050-kb YAC by homologous recombination. This 180-kb YAC was microinjected into mouse oocytes. In the resulting transgenic mice, high levels of mRNA for both type I and type II human MSR1 were detected in peritoneal macrophages and trace levels in other organs, known to contain macrophage-derived cells. Using an antibody against the human MSR, the Kupffer cells in the liver were shown to contain the MSR protein. In vivo clearance of acetyl-LDL was not changed in the MSR1-transgenic mice. However, in vitro studies using peritoneal macrophages from the transgenic mice showed a two-fold increased degradation of acetyl-LDL and cholesterolester accumulation concomitant with a four-fold increase in foam cell formation, as compared to wild-type macrophages. Thus, macrophage specific overexpression of the MSR may lead to increased foam cell formation, which is one of the initial and crucial steps in atherogenesis.

Dissection of the complex role of apolipoprotein E in lipoprotein metabolism and atherosclerosis using mouse models

Transgenic and knockout mice have been instrumental in delineating the role of apolipoprotein (apo) E in lipoprotein metabolism and atherosclerosis. The severe hypercholesterolemia and premature atherosclerosis of the apoE knockout mouse have been the starting point from which various physiologic processes have been identified in which apoE plays a critical role. These processes include 1) very low density lipoprotein (VLDL) triglyceride production; 2) lipoprotein lipase mediated triglyceride lipolysis; 3) VLDL remnant clearance and intracellular processing; and 4) the efflux of cellular cholesterol. In this review we will discuss the recent insight in the role of apoE in these processes, which has been obtained using a variety of in vivo and in vitro approaches to modify apoE expression and function.

Scavenger receptor deficiency leads to more complex atherosclerotic lesions in APOE3Leiden transgenic mice

Apolipoprotein (apo) E3Leiden is a dysfunctional apo E variant associated with familial dysbetalipoproteinemia in humans. Transgenic mice carrying the APOE3Leiden gene develop hyperlipidemia and are highly susceptible to diet-induced atherosclerosis. An early step in atherosclerosis is foam cell formation, which is thought to result from the unrestricted uptake of modified lipoproteins by macrophages. To investigate the role of the macrophage scavenger receptor type I and II (MSR-A) in this process, APOE3Leiden transgenic mice were crossed onto a MSR-A deficient background and the development of atherosclerosis was examined. In view of recent results with apo E deficient mice (Suzuki H et al., A role for the macrophage scavenger receptors in atherosclerosis. Nature 1997; 386(6622):292-296), absence of the MSR-A in APOE3Leiden mice was expected to lead to a reduction of atherosclerosis. In our study we compared APOE3Leiden/MSR-A deficient mice (E3L MSR-A -/-) to APOE3Leiden/MSR-A wild-type mice (E3L MSR-A +/+). These animals were fed an atherogenic diet for 10 weeks. Quantification of the lesion area showed no significant difference between E3L MSR-A -/- and E3L MSR-A +/+ mice although there was a trend towards the development of larger lesions in the E3L MSR-A -/- mice. All lesions were typed according to their cellular composition. In both male and female E3L MSR-A -/- mice, significantly more severe lesions developed as compared to E3L MSR-A +/+ mice. These results indicate that the effect of MSR-A deficiency on atherogenesis may depend on the presence or absence of apo E.

Reversal of hyperlipidaemia in apolipoprotein C1 transgenic mice by adenovirus-mediated gene delivery of the low-density-lipoprotein receptor, but not by the very-low-density-lipoprotein receptor

We have shown previously that human apolipoprotein (apo)C1 transgenic mice exhibit hyperlipidaemia, due primarily to an impaired clearance of very-low-density lipoprotein (VLDL) particles from the circulation. In the absence of at least the low-density-lipoprotein receptor (LDLR), it was shown that APOC1 overexpression in transgenic mice inhibited the hepatic uptake of VLDL via the LDLR-related protein. In the present study, we have now examined the effect of apoC1 on the binding of lipoproteins to both the VLDL receptor (VLDLR) and the LDLR. The binding specificity of the VLDLR and LDLR for apoC1-enriched lipoprotein particles was examined in vivo through adenovirus-mediated gene transfer of the VLDLR and the LDLR [giving rise to adenovirus-containing (Ad)-VLDLR and Ad-LDLR respectively] in APOC1 transgenic mice, LDLR-deficient (LDLR-/-) mice and wild-type mice. Remarkably, Ad-VLDLR treatment did not reduce hyperlipidaemia in transgenic mice overexpressing human APOC1, irrespective of both the level of transgenic expression and the presence of the LDLR, whereas Ad-VLDLR treatment did reverse hyperlipidaemia in LDLR-/- and wild-type mice. On the other hand, Ad-LDLR treatment strongly decreased plasma lipid levels in these APOC1 transgenic mice. These results suggest that apoC1 inhibits the clearance of lipoprotein particles via the VLDLR, but not via the LDLR. This hypothesis is corroborated by in vitro binding studies. Chinese hamster ovary (CHO) cells expressing the VLDLR (CHO-VLDLR) or LDLR (CHO-LDLR) bound less APOC1 transgenic VLDL than wild-type VLDL. Intriguingly, however, enrichment with apoE enhanced dose-dependently the binding of wild-type VLDL to CHO-VLDLR cells (up to 5-fold), whereas apoE did not enhance the binding of APOC1 transgenic VLDL to these cells. In contrast, for binding to CHO-LDLR cells, both wild-type and APOC1 transgenic VLDL were stimulated upon enrichment with apoE. From these studies, we conclude that apoC1 specifically inhibits the apoE-mediated binding of triacylglycerol-rich lipoprotein particles to the VLDLR, whereas apoC1-enriched lipoproteins can still bind to the LDLR. The variability in specificity of these lipoprotein receptors for apoC1-containing lipoprotein particles provides further evidence for a regulatory role of apoC1 in the delivery of lipoprotein constituents to different tissues on which these receptors are located.

Progression and regression of atherosclerosis in APOE3-Leiden transgenic mice: an immunohistochemical study

Apolipoprotein E3-Leiden (APOE3-Leiden) transgenic mice develop hyperlipidemia and are highly susceptible to diet-induced atherosclerosis. We have studied the progression and regression of atherosclerosis using immunohistochemistry. Female transgenic mice were fed a moderate fat diet to study atherosclerosis over a longer time period. Fatty streaks arose in the intima and consisted of lipid filled macrophages which differed in origin. All macrophages expressed the macrophage scavenger receptor while two thirds expressed sialoadhesin and were positive for an antibody recognizing marginal zone macrophages (MOMA-1). All macrophages were negative for the scavenger receptor MARCO and 50% were positive for CD4. Small fatty streaks contained CD-3 positive T-lymphocytes which were for more than 70% CD4-positive. ICAM-1 was positive both in atherosclerotic and control mice. In early plaques, fibrosis was observed on the luminal and medial site of the foam cells while smooth muscle cells were only observed in the fibrous cap. To study regression, we used a high fat, high cholesterol diet to rapidly induce atherosclerosis (14 weeks). The animals were then fed normal chow. Subsequently, atherosclerosis was assayed over time (4, 8, 16 weeks). Cholesterol levels dropped in 4 weeks to control levels. The animals did not show a significantly decrease in plaque size over time. but the percentage macrophages was significantly smaller in the animals after 4 weeks. In conclusion, the APOE3-Leiden mouse is a useful model to study the progression and regression of atherosclerosis.

Binding of beta-VLDL to heparan sulfate proteoglycans requires lipoprotein lipase, whereas ApoE only modulates binding affinity

The binding of beta-VLDL to heparan sulfate proteoglycans (HSPG) has been reported to be stimulated by both apoE and lipoprotein lipase (LPL). In the present study we investigated the effect of the isoform and the amount of apoE per particle, as well as the role of LPL on the binding of beta-VLDL to HSPG. Therefore, we isolated beta-VLDL from transgenic mice, expressing either APOE*2(Arg158-->Cys) or APOE*3-Leiden (E2-VLDL and E3Leiden-VLDL, respectively), as well as from apoE-deficient mice containing no apoE at all (Enull-VLDL). In the absence of LPL, the binding affinity and maximal binding capacity of all beta-VLDL samples for HSPG-coated microtiter plates was very low. Addition of LPL to this cell-free system resulted in a 12- to 55-fold increase in the binding affinity and a 7- to 15-fold increase in the maximal binding capacity (Bmax). In the presence of LPL, the association constant (Ka) tended to increase in the order Enull-VLDL<E2-VLDL<E3Leiden-VLDL, whereas Bmax increased in the reverse order: E3Leiden-VLDL approximately E2-VLDL<Enull-VLDL. Addition of LPL resulted in a marked stimulation of both Ka and Bmax for binding of beta-VLDL samples to J774 cells similar to that found for the binding to HSPG-LPL complexes. Our results indicate that both Ka and Bmax for binding of beta-VLDL to HSPG are increased more than 1 order of magnitude on addition of LPL. In addition, for the binding of beta-VLDL to HSPG-LPL complexes, the presence of apoE is not a prerequisite, but results in an increased binding affinity, depending on the apoE isoform used.

In LDL receptor-deficient mice, catabolism of remnant lipoproteins requires a high level of apoE but is inhibited by excess apoE

To investigate the quantitative requirement for apolipoprotein (apo) E in the clearance of lipoproteins via the non-low density lipoprotein (LDL) receptor mediated pathway, human APOE was overexpressed at various levels in the livers of mice deficient for both the endogenous Apoe and Ldlr genes (Apoe -/-. Ldlr -/-) using adenovirus-mediated gene transfer. We found that a low level of APOE expression, that was capable of reducing the hyperlipidemia in Apoe -/- mice, did not result in a reduction of the hyperlipidemia in Apoe -/-. Ldlr -/- mice. Surpisingly, a very high level of APOE expression also did not result in a reduction of hypercholesterolemia in Apoe -/-. Ldlr -/- mice, despite very high levels of circulating apoE (>160 mg/dl). Only a moderately high level of APOE expression resulted in a reduction of serum cholesterol level (from 35.2 +/- 6.7 to 14.6 +/- 2.3 mmol/l) and the disappearance of VLDL from the serum. Moreover, the very high level of APOE expression resulted in a severe hypertriglyceridemia in Apoe -/-. Ldlr -/- mice and not Apoe -/- mice (25.7 +/- 8.9 and 2.2 +/- 1.8 mmol/l, respectively). This hypertriglyceridemia was associated with an APOE-induced increase in the VLDL triglyceride production rate and an inhibition of VLDL-triglyceride lipolysis. We conclude from these data that, for efficient clearance, the non-LDL receptor-mediated pathway requires a higher level of APOE expression as compared to the LDL receptor, but is more sensitive to an APOE-induced increase in VLDL production and inhibition of VLDL-triglyceride lipolysis.-van Dijk, K. W., B. J. M. Van Vlijmen, H. B. van't Hof, A. van der Zee, S. Santamarina-Fojo, T. J. C. van Berkel, L. M. Havekes, and M. H. Hofker. In LDL receptor-deficient mice, catabolism of remnant lipoproteins requires a high level of apoE but is inhibited by excess APOE.

Effects of fenofibrate on hyperlipidemia and postprandial triglyceride metabolism in human apolipoprotein C1 transgenic mice

To study the in vivo role of apolipoprotein (apo) C1 in lipoprotein metabolism, we have generated transgenic mice expressing the human apo C1 gene. Apo C1 is a small 6.6 kDa protein that is primarily synthesized by the liver and is present on chylomicrons, very low density lipoproteins (VLDL) and high density lipoproteins (HDL). In recent years, studies by our group have shown that apo C1 transgenic mice develop hyperlipidemia due to an accumulation of VLDL-sized lipoprotein particles. The underlying metabolic defect in apo C1 transgenic mice is an impaired uptake of VLDL particles by the liver. Although a role for apo C1 in human disease remains to be established, data presented in the current paper show that apo C1 transgenic mice are an instructive model of hyperlipidemia to (i) elucidate possible mechanisms underlying this disorder and (ii) test the activity and mode of action of hypolipidemic drugs.

Effects of dietary fish oil on serum lipids and VLDL kinetics in hyperlipidemic apolipoprotein E*3-Leiden transgenic mice

Studying the effects of dietary fish oil on VLDL metabolism in humans is subject to both large intra- and interindividual variability. In the present study we therefore used hyperlipidemic apolipoprotein (APO) E*3-Leiden mice, which have impaired chylomicron and very low density lipoprotein (VLDL) remnant metabolism, to study the effects of dietary fish oil on serum lipids and VLDL kinetics under highly standardized conditions. For this, female APOE*3-Leiden mice were fed a fat- and cholesterol-containing diet supplemented with either 0, 3 or 6% w/w (i.e. 0, 6, or 12% of total energy) of fish oil. Fish oil-fed mice showed a significant dose-dependent decrease in serum cholesterol (up to -43%) and triglyceride levels (up to -60%), mainly due to a reduction of VLDL (-80%). LDL and HDL cholesterol levels were not affected by fish oil feeding. VLDL-apoB kinetic studies showed that fish oil feeding resulted in a significant 2-fold increase in VLDL-apoB fractional catabolic rate (FCR). Hepatic VLDL-apoB production was, however, not affected by fish oil feeding. VLDL-triglyceride turnover studies revealed that fish oil significantly decreased hepatic VLDL-triglyceride production rate (-60%). A significant increase in VLDL-triglyceride FCR was observed (+70%), which was not related to increased lipolytic activity. We conclude that APOE*3-Leiden mice are highly responsive to dietary fish oil. The observed strong reduction in serum very low density lipoprotein (VLDL) is primarily due to an effect of fish oil to decrease hepatic VLDL triglyceride production rate and to increase VLDL-apoB fractional catabolic rate.

Apolipoprotein E*3-Leiden transgenic mice as a test model for hypolipidaemic drugs

Apolipoprotein (APO) E*3-Leiden mice with impaired chylomicron and VLDL (very low density lipoprotein) remnant metabolism display hyperlipidaemia and atherosclerosis. In the present study, these mice were used for testing the hypolipidaemic effect of two marketed agents, lovastatin (CAS 75330-75-5) and gemfibrozil (CAS 25812-30-0) as well as a novel compound, SB 204990 (the 5-ring lactone of +/-(3R*,5S*) 3-carboxy-11-(2,4-dichlorophenyl)-3,5-dihydroxyundecanoic acid, CAS 154566-12-8), a potent inhibitor of cholesterol and fatty acid synthesis at the level of ATP-citrate lyase. APOE*3-Leiden mice were fed a saturated fat and cholesterol-rich diet supplemented with either 0.05 or 0.1% w/w of lovastatin, 0.1 or 0.2% w/w of gemfibrozil or 0.1 or 0.2% w/w of SB 204990. Lovastatin showed a dose-related decrease in plasma cholesterol levels (up to -20%) due to a lowering of LDL and HDL (low density resp. high density lipoprotein)-cholesterol (-20 and -18%, respectively), while plasma triglyceride levels were unaffected. Gemfibrozil had no effect on plasma total cholesterol levels but gave significant dose-dependent decreases in plasma (VLDL) triglyceride levels (up to -53%). SB 204990 resulted in a dose-dependent reduction of plasma cholesterol (up to -29%) by lowering VLDL, LDL and HDL-cholesterol (-50, -20 and -20%, respectively). In addition, a strong dose dependent reduction of plasma (VLDL) triglycerides up to -43% was observed with this compound. Although the effects of gemfibrozil and SB 204990 were not simply explained by changes in a single determinant of VLDL metabolism--no effects of these drugs were seen on post-heparin plasma lipoprotein lipase activity, in vivo rate of VLDL synthesis or hepatic apoC-III mRNA levels--APOE*3-Leiden mice were found to give robust hypolipidaemic responses to these test compounds. The responsiveness to hypolipidaemic therapy combined with a clear relationship between aortic lesion size and plasma cholesterol exposure, as demonstrated previously, makes this mouse an attractive model for the testing of anti-atherosclerotic properties of hypolipidaemic drugs.

Transgenic mouse models to study the role of APOE in hyperlipidemia and atherosclerosis

Transgenic technologies have provided a series of very useful mouse models to study hyperlipidemia and atherosclerosis. Normally, mice carry cholesterol mainly in the high density lipoprotein (HDL) sized lipoproteins, and have low density lipoprotein (LDL) and very low density lipoprotein (VLDL) cholesterol levels. These low LDL and VLDL levels are due to the very rapid metabolism of remnant clearance in mice, which hamper metabolic studies. In addition, due to the lack of atherogenic lipoproteins, mice will not readily develop atherosclerosis. This situation has changed completely, because to date, most known genes in lipoprotein metabolism have been used in transgenesis to obtain mice in which genes have been silenced or overexpressed. These experiments have yielded many mouse strains with high plasma lipid levels and a greater susceptibility for developing atherosclerosis. One of the most widely used strains are knock-out mice deficient for apoE, which is one of the central players in VLDL metabolism. Subsequently, a wide variety of other transgenic studies involving APOE have been performed elucidating the role of apoE and apoE mutants in lipolysis, remnant clearance, cellular cholesterol efflux and atherogenesis. In addition, the APOE mouse models are excellent tools for the development of gene therapy for hyperlipidemias.

Reversal of hypercholesterolemia in apolipoprotein E2 and apolipoprotein E3-Leiden transgenic mice by adenovirus-mediated gene transfer of the VLDL receptor

We have investigated the interaction of apolipoprotein E2(Arg158-Cys) (apoE2) and apolipoprotein E3-Leiden (apoE3-Leiden) with the very low density lipoprotein (VLDL) receptor in vivo and in vitro to define the possible role of this receptor in lipoprotein metabolism and atherosclerosis. The in vivo binding specificity of the VLDL receptor for apoE2 and apoE3-Leiden was investigated by adenovirus-mediated gene transfer of the VLDL receptor in apoE2 and apoE3-Leiden transgenic mice lacking endogenous mouse apoE (Apoe-/-). Ectopic overexpression of the VLDL receptor gene in the liver resulted in a >50% decrease of plasma cholesterol levels in both apoE2 and apoE3-Leiden transgenic mice compared with liver expression of the beta-galactosidase gene. This reduction in plasma cholesterol was mainly due to a reduction in the VLDL level. Overexpression of the VLDL receptor did not affect the hepatic VLDL triglyceride production, indicating that the hypocholesterolemic effect is due to an increased level of plasma clearance mediated by the VLDL receptor. In vitro binding analysis showed that both apoE2 and apoE3-Leiden VLDL compete efficiently with rabbit beta-VLDL for binding to the VLDL receptor expressed on LDL receptor-deficient Chinese hamster ovary cells. We conclude from these data that both apoE2 and apoE3-Leiden function as proper ligands for the VLDL receptor in vitro and in vivo. This finding substantiates a possible role for the VLDL receptor in atherosclerosis in hyperlipidemic subjects homozygous for apoE2 or carrying apoE3-Leiden and indicates that the VLDL receptor expressed on the liver has therapeutic potential as an alternative route for clearance of binding-defective lipoproteins.

Characterization of atherosclerotic lesions in apo E3-leiden transgenic mice

Apo E3-leiden transgenic mice express human dysfunctional apo E variant and develop hyperlipidemia and atherosclerosis on a high fat/high cholesterol diet. We characterized diet-induced atherosclerotic lesions in apo E3-leiden transgenic mice using immunocytochemical methods in order to examine foam cell formation and determine whether advanced atherosclerotic lesions develop in these animals. Special attention was given to the presence of oxidized lipoproteins and expression of lipoprotein receptors. Plasma cholesterol levels in apo E3-leiden mice on an atherogenic diet increased from 2 to 36 mmol/l in 4 months. At this time apo E3-leiden mice had developed lesions, which ranged from early fatty streaks in thoracic and abdominal aorta to advanced lesions in aortic arch. Early fatty streaks were entirely composed of macrophages which also expressed scavenger receptors. Epitopes characteristic of oxidized LDL were present in macrophage-rich foam cells. Advanced atherosclerotic lesions also developed in apo E3-leiden mice including smooth muscle cell cap formation and erosion of the media. Macrophages and epitopes characteristic of oxidized LDL were present in core and shoulder regions. Scavenger receptors were expressed in macrophages in advanced lesions, whereas LDL-receptor-related protein (LRP) was mainly expressed in smooth muscle cells. It is concluded that: (1) macrophages are the major cell type in both early and advanced atherosclerotic lesions; (2) scavenger receptors and oxidized lipoproteins are present in lesion macrophages; and (3) LRP is mostly expressed in smooth muscle cells. Thus, lesions in apo E3-leiden transgenic mice have features in common with human atherosclerosis. Since lesion macrophages also retain their ability to synthesize endogenous apo E, apo E3-leiden transgenic mouse may be a useful model for studies on the development and genetics of atherosclerosis.

Hyperlipidemia and cutaneous abnormalities in transgenic mice overexpressing human apolipoprotein C1

Transgenic mice were generated with different levels of human apolipoprotein C1 (APOC1) expression in liver and skin. At 2 mo of age, serum levels of cholesterol, triglycerides (TG), and FFA were strongly elevated in APOC1 transgenic mice compared with wild-type mice. These elevated levels of serum cholesterol and TG were due mainly to an accumulation of VLDL particles in the circulation. In addition to hyperlipidemia, APOC1 transgenic mice developed dry and scaly skin with loss of hair, dependent on the amount of APOC1 expression in the skin. Since these skin abnormalities appeared in two independent founder lines, a mutation related to the specific insertion site of the human APOC1 gene as the cause for the phenotype can be excluded. Histopathological analysis of high expressor APOC1 transgenic mice revealed a disorder of the skin consisting of epidermal hyperplasia and hyperkeratosis, and atrophic sebaceous glands lacking sebum. In line with these results, epidermal lipid analysis showed that the relative amounts of the sebum components TG and wax diesters in the epidermis of high expressor APOC1 transgenic mice were reduced by 60 and 45%, respectively. In addition to atrophic sebaceous glands, the meibomian glands were also found to be severely atrophic in APOC1 transgenic mice. High expressor APOC1 transgenic mice also exhibited diminished abdominal adipose tissue stores (a 60% decrease compared with wild-type mice) and a complete deficiency of subcutaneous fat. These results indicate that, in addition to the previously reported inhibitory role of apoC1 on hepatic remnant uptake, overexpression of apoC1 affects lipid synthesis in the sebaceous gland and/or epidermis as well as adipose tissue formation. These APOC1 transgenic mice may serve as an interesting in vivo model for the investigation of lipid homeostasis in the skin.

Nascent very-low-density lipoprotein triacylglycerol hydrolysis by lipoprotein lipase is inhibited by apolipoprotein E in a dose-dependent manner

In the present study it was investigated whether apolipoprotein (apoE) can inhibit the lipoprotein lipase (LPL)-mediated hydrolysis of very-low-density-lipoprotein (VLDL) triacylglycerols (TAGs). Previous studies have suggested such an inhibitory role for apoE by using as a substrate for LPL either plasma VLDL or artificial TAG emulsions. To mimic the in vivo situation more fully, we decided to investigate the effect of apoE on the LPL-mediated TAG hydrolysis by using VLDL from apoE-deficient mice that had been enriched with increasing amounts of apoE. Furthermore, since plasma VLDL isolated from apoE-deficient mice was relatively poor in TAGs and strongly enriched in cholesterol as compared with VLDL from wild-type mice, we used nascent VLDL obtained by liver perfusions. Nascent VLDL (d<1. 006) isolated from the perfusate of the apoE-deficient mouse liver was rich in TAGs. Addition of increasing amounts of apoE to apoE-deficient nascent VLDL effectively decreased TAG lipolysis as compared with that of apoE-deficient nascent VLDL without the addition of apoE (63.1+/-6.3 and 20.8+/-1.8% of the control value at 2.7 microg and 29.6 microg of apoE/mg of TAG added respectively). Since, in vivo, LPL is attached to heparan sulphate proteoglycans (HSPG) at the endothelial matrix, we also performed lipolysis assays with LPL bound to HSPG in order to preserve the interaction of the lipoprotein particle with the HSPG-LPL complex. In this lipolysis system a concentration-dependent decrease in the TAG lipolysis was also observed with increasing amounts of apoE on nascent VLDL, although to a lesser extent than with LPL in solution (72.3+/-3.6% and 56.6+/-1.7% of control value at 2.7 microg and 29.6 microg of apoE/mg TAGs added respectively). In conclusion, the enrichment of the VLDL particle with apoE decreases its suitability as a substrate for LPL in a dose-dependent manner.

Impaired secretion of very low density lipoprotein-triglycerides by apolipoprotein E- deficient mouse hepatocytes

To explore mechanisms underlying triglyceride (TG) accumulation in livers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Wolters, G. In't Veld, R.J. Vonk, H.M.G. Princen, and L.M. Havekes. 1996. Hepatology. 24:241-247), we investigated the effects of apo E deficiency on secretion of VLDL-associated TG (a) in vivo in mice, (b) in isolated perfused mouse livers, and (c) in cultured mouse hepatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined after Triton WR1339 injection, was reduced by 46% in apo E-deficient mice compared with controls. To eliminate the possibility that impaired VLDL secretion is caused by aspecific changes in hepatic function due to hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone marrow. Bone marrow- transplanted apo E-deficient mice, which do not express apo E in hepatocytes, showed normalized plasma cholesterol levels, but VLDL-TG production was reduced by 59%. (b) VLDL-TG production by isolated perfused livers from apo E-deficient mice was 50% lower than production by livers from control mice. Lipid composition of nascent VLDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured apo E-deficient hepatocytes was reduced by 23% compared with control values in serum-free medium, and by 61% in the presence of oleate in medium (0. 75 mM) to stimulate lipogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared with control cells in the presence of oleate (38 and 49 nm, respectively). In short-term labeling studies, apo E-deficient and control cells showed a similar time-dependent accumulation of [3H]TG formed from [3H]glycerol, yet secretion of newly synthesized VLDL-associated [3H]TG by apo E-deficient cells was reduced by 60 and 73% in the absence and presence of oleate, respectively. We conclude that apo E, in addition to its role in lipoprotein clearance, has a physiological function in the VLDL assembly-secretion cascade.

Use of transgenic mice in lipoprotein metabolism and atherosclerosis research

In APOE*3-Leiden transgenic mice the atherosclerotic lesion size is correlated with plasma cholesterol. In these mice the plasma lipid levels are positively correlated with the relative amount of APOE 3-Leiden protein on the VLDL particle. The plasma cholesterol levels are influenced by diet, age and gender, mainly due to an effect of these factors on VLDL production rate. Excess of APOC1 protein does inhibit the hepatic clearance of VLDL remnant particles, whereas excess of apoE leads to a hampered extra-hepatic lipolysis of VLDL triglyceride.

Uptake by J774 macrophages of very-low-density lipoproteins isolated from apoE-deficient mice is mediated by a distinct receptor and stimulated by lipoprotein lipase

Apolipoprotein (apo) E-deficient mice display marked accumulation in the plasma of VLDL deficient in both apoE and apoB100 but containing apoB48, apoA-I, apoCs, and apoA-IV. Since apoE-deficient mice develop severe atherosclerotic lesions with lipid-laden macrophages, we reasoned that the uptake of lipoproteins by intimal macrophages can take place in the absence of both apoE and apoB100. To get more insight into the mechanism of foam cell formation in apoE-deficient mice, we measured the interaction of VLDL from apoE-deficient mice (apoEnull VLDL) with the murine macrophage cell line J774. Scatchard analysis revealed that apoEnull VLDL is bound to J774 cells with a Kd value comparable to that of control VLDL (8.1 versus 4.7 micrograms/mL) and with a Bmax value about half that of control VLDL (40 versus 70 ng/mg cell protein, respectively). ApoEnull VLDL is also taken up and degraded by J774 macrophages via a high-affinity process less efficiently than control mouse VLDL (6-fold and 50-fold less efficiently, respectively). In line with this observation, incubation of J774 cells with 50 micrograms/mL apoEnull VLDL for 24 hours resulted in an increase in intracellular cholesteryl ester (CE) content, although 5-fold less pronounced than after incubation with 50 micrograms/mL control mouse VLDL. Under the conditions applied, simultaneous addition of 5 micrograms/mL lipoprotein lipase (LPL) stimulated the cellular uptake and degradation of apoEnull VLDL about 10-fold and resulted in a 5-fold stimulation of the intracellular CE accumulation, from 9 +/- 2 to 46 +/- 5 micrograms CE per milligram cell protein. In contrast to control mouse VLDL, apoEnull VLDL could not compete with 125I-labeled LDL for binding to the LDL receptor of J774 cells. Furthermore, neither LDL nor acetylated LDL could compete with 125I-labeled apoEnull VLDL for binding to these cells, whereas control mouse VLDL, VLDL from a hypertriglyceridemic patient, and apoEnull VLDL itself were efficient competitors. Thus, VLDL from apoE-deficient mice is taken up by J774 macrophages through recognition by a distinct receptor, which could be the triglyceride-rich lipoprotein receptor. We conclude that in apoE-deficient mice, foam cell formation occurs via a receptor-mediated uptake of apoEnull VLDL, which can be stimulated by the presence of LPL.

Reduced very-low-density lipoprotein fractional catabolic rate in apolipoprotein C1-deficient mice

The function of apolipoprotein (apo) C1 in vivo is not clearly defined. Because transgenic mice overexpressing human apoC1 show elevated triacylglycerol (TG) levels [Simonet, Bucay, Pitas, Lauer and Taylor (1991) J. Biol. Chem. 266, 8651-8654], an as yet unknown role for apoC1 in TG metabolism has been suggested. Here we investigated directly the effect of the complete absence of apoC1 on very-low-density lipoprotein (VLDL)-TG lipolysis, clearance and production, by performing studies with the previously generated apoC1-deficient mice. On a sucrose-rich, low fat/low cholesterol (LFC) diet, apoC1-deficient mice accumulate in their circulation VLDL particles, which contain relatively lower amounts of lipids when compared with VLDL isolated from control mice. Lipolysis assays in vitro on VLDL from apoC1-deficient and control mice showed no differences in apparent K(m) and Vmax values (0.27 +/- 0.06 versus 0.24 +/- 0.03 mmol of TG/litre and 0.40 +/- 0.03 versus 0.36 +/- 0.03 mmol of non-esterified fatty acid (NEFA)/min per litre respectively). To correct for potential differences in the size of the VLDL particles, the resulting K(m) values were also expressed relative to apoB concentration. Under these conditions apoC1-deficient VLDL displayed a lower, but not significant, K(m) value when compared with control VLDL (3.44 +/- 0.71 versus 4.44 +/- 0.52 mmol of TG2/g apoB per litre). VLDL turnover studies with autologous injections of [3H]TG-VLDL in vivo showed that the VLDL fractional catabolic rate (FCR) was decreased by up to 50% in the apoC1-deficient mice when compared with control mice (10.5 +/- 3.4 versus 21.0 +/- 1.2/h of pool TG). No significant differences between apoC1-deficient and control mice were observed in the hepatic VLDL production estimated by Triton WR139 injections (0.19 +/- 0.02 versus 0.21 +/- 0.05 mmol/h of TG per kg) and in the extra-hepatic lipolysis of VLDL-TG (4.99 +/- 1.62 versus 3.46 +/- 1.52/h of pool TG) in vivo. Furthermore, [125I]VLDL-apoB turnover experiments in vivo also showed a 50% decrease in the FCR of VLDL in apoC1-deficient mice when compared with control mice on the LFC diet (1.1 +/- 0.3 versus 2.1 +/- 0.1/h of pool apoB). When mice were fed a very high fat/high cholesterol (HFC) diet, the VLDL-apoB FCR was further decreased in apoC1-deficient mice (0.4 +/- 0.1 versus 1.4 +/- 0.4/h of pool apoB). We conclude that, in apoC1-deficient mice, the FCR of VLDL is reduced because of impaired uptake of VLDL remnants by hepatic receptors, whereas the production and lipolysis of VLDL-TG is not affected.

Exon mapping by fiber-FISH or LR-PCR

In this study we systematically assessed the sensitivity limits of fiber-FISH in model experiments. Exonic fragments and cDNAs with exon sizes of >/=200 bp could be mapped on their cognate cosmid. This positional fiber-FISH mapping was validated by long-range PCR. It is expected that these two independent mapping approaches will help to refine current available gene maps and show their applicability in fine mapping of sequence-tagged sites or expressed sequence tags. Also, they will be useful in resolving gene structures by mapping exon and intron locations.

Evidence for subtelomeric exchange of 3.3 kb tandemly repeated units between chromosomes 4q35 and 10q26: implications for genetic counselling and etiology of FSHD1

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant myopathy, clinically characterized by asymmetric weakness of muscles in the face, shoulder girdle and upper arm. Deletion of an integral number of 3.3 kb repeated units within a highly polymorphic EcoRI fragment at chromosome 4q35, generating a relatively short EcoRI fragment (< 35 kb), has been shown to cause FSHD1. Probe p13E-11 detects these short fragments in FSHD1 patients, and has therefore been used for diagnostic DNA analysis. However, the reliability of this analysis has been hampered by cross-hybridization of p13E-11 to chromosome 10q26-linked EcoRI fragments of comparable size, which also contain a variable number of 3.3 kb repeated units. Recently, a BinI restriction site was identified within each of the repeated units derived from chromosome 10q26, which enables differentiation of the two polymorphic p13E-11 loci in most cases without haplotype analysis. Remarkably, applying the differential analysis to screen DNA of 160 Dutch cases referred to us for FSHD1 diagnosis, we obtained evidence for subtelomeric exchange of 3.3 kb repeated units between chromosomes 4q35 and 10q26 in affected and unaffected individuals. Subsequently, analysis of 50 unrelated control samples indicated such exchange between chromosomes 4q35 and 10q26 in at least 20% of the population. These subtelomeric rearrangements have generated a novel interchromosomal polymorphism, which has implications for the specificity and sensitivity of the differential restriction analysis for diagnostic purposes. Moreover, the high frequency of the interchromosomal exchanges of 3.3 kb repeated units suggests that they probably do not contain (part of) the FSHD1 gene, and supports position effect variegation as the most likely mechanism for FSHD1.