Cosegregation of aortic root atherosclerosis and intermediate lipid phenotypes on chromosomes 2 and 8 in an intercross of C57BL/6 and BALBc/ByJ low-density lipoprotein receptor-/- mice

Trib1 Deficiency Promotes Hyperlipidemia, Inflammation, and Atherosclerosis in LDL Receptor Knockout Mice

BACKGROUND

Genetic variants at the TRIB1 gene locus are strongly associated with plasma lipid traits and the risk of coronary artery disease in humans. Here, we analyzed the consequences of Trib1 deficiency on lipid metabolism and atherosclerotic lesion formation in atherosclerosis-susceptible Ldlr-/- mice.

METHODS

Trib1-/- mice were crossed onto the Ldlr-/- background to generate double-knockout mice (Trib1-/-Ldlr-/-) and fed a semisynthetic, modified AIN76 diet (0.02% cholesterol and 4.3% fat) until 20 weeks of age.

RESULTS

Trib1-/-Ldlr-/- mice had profoundly larger (5.8-fold) and more advanced atherosclerotic lesions at the aortic root as compared with Trib1+/+Ldlr-/- controls. Further, we observed significantly elevated plasma total cholesterol and triglyceride levels in Trib1-/-Ldlr-/- mice, resulting from higher VLDL (very-low-density lipoprotein) secretion. Lipidomics analysis revealed that loss of Trib1 altered hepatic lipid composition, including the accumulation of cholesterol and proinflammatory ceramide species, which was accompanied by signs of hepatic inflammation and injury. Concomitantly, we detected higher plasma levels of IL (interleukin)-6 and LCN2 (lipocalin 2), suggesting increased systemic inflammation in Trib1-/-Ldlr-/- mice. Hepatic transcriptome analysis demonstrated significant upregulation of key genes controlling lipid metabolism and inflammation in Trib1-/-Ldlr-/- mice. Further experiments suggested that these effects may be mediated through pathways involving a C/EPB (CCAAT/enhancer binding protein)-PPARγ (peroxisome proliferator-activated receptor γ) axis and JNK (c-Jun N-terminal kinase) signaling.

CONCLUSIONS

We provide experimental evidence that Trib1 deficiency promotes atherosclerotic lesion formation in a complex manner that includes the modulation of lipid metabolism and inflammation.

Genetic Evidence for a Causal Relationship between Hyperlipidemia and Type 2 Diabetes in Mice

Dyslipidemia is considered a risk factor for type 2 diabetes (T2D), yet studies with statins and candidate genes suggest that circulating lipids may protect against T2D development. Apoe-null (Apoe^-/-) mouse strains develop spontaneous dyslipidemia and exhibit a wide variation in susceptibility to diet-induced T2D. We thus used Apoe^-/- mice to elucidate phenotypic and genetic relationships of circulating lipids with T2D. A male F2 cohort was generated from an intercross between LP/J and BALB/cJ Apoe^-/- mice and fed 12 weeks of a Western diet. Fasting, non-fasting plasma glucose, and lipid levels were measured and genotyping was performed using miniMUGA arrays. We uncovered a major QTL near 60 Mb on chromosome 15, Nhdlq18, which affected non-HDL cholesterol and triglyceride levels under both fasting and non-fasting states. This QTL was coincident with Bglu20, a QTL that modulates fasting and non-fasting glucose levels. The plasma levels of non-HDL cholesterol and triglycerides were closely correlated with the plasma glucose levels in F2 mice. Bglu20 disappeared after adjustment for non-HDL cholesterol or triglycerides. These results demonstrate a causative role for dyslipidemia in T2D development in mice.

Immune-Deficient Pfp/Rag2-/- Mice Featured Higher Adipose Tissue Mass and Liver Lipid Accumulation with Growing Age than Wildtype C57BL/6N Mice

Aging is a risk factor for adipose tissue dysfunction, which is associated with inflammatory innate immune mechanisms. Since the adipose tissue/liver axis contributes to hepatosteatosis, we sought to determine age-related adipose tissue dysfunction in the context of the activation of the innate immune system fostering fatty liver phenotypes. Using wildtype and immune-deficient mice, we compared visceral adipose tissue and liver mass as well as hepatic lipid storage in young (ca. 14 weeks) and adult (ca. 30 weeks) mice. Adipocyte size was determined as an indicator of adipocyte function and liver steatosis was quantified by hepatic lipid content. Further, lipid storage was investigated under normal and steatosis-inducing culture conditions in isolated hepatocytes. The physiological age-related increase in body weight was associated with a disproportionate increase in adipose tissue mass in immune-deficient mice, which coincided with higher triglyceride storage in the liver. Lipid storage was similar in isolated hepatocytes from wildtype and immune-deficient mice under normal culture conditions but was significantly higher in immune-deficient than in wildtype hepatocytes under steatosis-inducing culture conditions. Immune-deficient mice also displayed increased inflammatory, adipogenic, and lipogenic markers in serum and adipose tissue. Thus, the age-related increase in body weight coincided with an increase in adipose tissue mass and hepatic steatosis. In association with a (pro-)inflammatory milieu, aging thus promotes hepatosteatosis, especially in immune-deficient mice.

Simultaneous LC/MS/MS quantification of eight apolipoproteins in normal and hypercholesterolemic mouse plasma

Apolipoproteins are major structural and functional constituents of lipoprotein particles. As modulators of lipid metabolism, adipose tissue biology, and energy homeostasis, apolipoproteins may serve as biomarkers or potential therapeutic targets for cardiometabolic diseases. Mice are the preferred model to study metabolic disease and CVD, but a comprehensive method to quantify circulating apolipoproteins in mice is lacking. We developed and validated a targeted proteomics assay to quantify eight apolipoproteins in mice via proteotypic signature peptides and corresponding stable isotope-labeled analogs. The LC/MS/MS method requires only a 3 µl sample volume to simultaneously determine mouse apoA-I, apoA-II, apoA-IV, apoB-100, total apoB, apoC-I, apoE, and apoJ concentrations. ApoB-48 concentrations can be calculated by subtracting apoB-100 from total apoB. After we established the analytic performance (sensitivity, linearity, and imprecision) and compared results for selected apolipoproteins against immunoassays, we applied the method to profile apolipoprotein levels in plasma and isolated HDL from normocholesterolemic C57BL/6 mice and from hypercholesterolemic Ldl-receptor- and Apoe-deficient mice. In conclusion, we present a robust, quantitative LC/MS/MS method for the multiplexed analysis of eight apolipoproteins in mice. This assay can be applied to investigate the effects of genetic manipulation or dietary interventions on apolipoprotein levels in plasma and isolated lipoprotein fractions.

Leptin decreases circulating inflammatory IL-6 and MCP-1 in mice

Leptin influences inflammation and immune response. Dose-dependent effects of leptin on biomarkers of inflammation have not been studied in vivo, so far. Leptin-deficient low-density lipoprotein receptor (LDLR) knockout (LDLR^-/- ;ob/ob) female mice were treated with three different leptin doses or saline for 12 weeks. The effect of leptin on plasma interleukin (IL)-6 and monocyte chemoattractant protein (MCP)-1 concentrations and Il-6 and Mcp-1 mRNA expression in vivo were assessed. Macrophage infiltration in epididymal adipose tissue (epiAT) after leptin treatment was determined by quantitative immunohistochemical analysis. Aortic root atherosclerotic lesions were analyzed by oil red O staining. Mean plasma IL-6 and MCP-1 decreased significantly in the 3.0 mg/kg BW/day group as compared to control mice (both P < 0.01). Messenger RNA expression of Il-6 and Mcp-1 was significantly down-regulated by leptin treatment in different adipose tissues in vivo. Characteristic crown-like structures formed by adipose tissue macrophages were significantly reduced by leptin treatment in epiAT. Recombinant leptin dose-dependently diminished plaque area in the aortic root. Leptin administration within the subphysiological to physiological range diminishes circulating pro-inflammatory IL-6 and MCP-1. Reduction of Il-6 and Mcp-1 gene expression in adipose tissue, as well as decreased adipose tissue macrophage infiltration might contribute. © 2018 BioFactors, 45(1):43-48, 2019.

Control of hepatic gluconeogenesis by Argonaute2

Objective

The liver performs a central role in regulating energy homeostasis by increasing glucose output during fasting. Recent studies on Argonaute2 (Ago2), a key RNA-binding protein mediating the microRNA pathway, have illustrated its role in adaptive mechanisms according to changes in metabolic demand. Here we sought to characterize the functional role of Ago2 in the liver in the maintenance of systemic glucose homeostasis.

Methods

We first analyzed Ago2 expression in mouse primary hepatocyte cultures after modulating extracellular glucose concentrations and in the presence of activators or inhibitors of glucokinase activity. We then characterized a conditional loss-of-function mouse model of Ago2 in liver for alterations in systemic energy metabolism.

Results

Here we show that Ago2 expression in liver is directly correlated to extracellular glucose concentrations and that modulating glucokinase activity is adequate to affect hepatic Ago2 levels. Conditional deletion of Ago2 in liver resulted in decreased fasting glucose levels in addition to reducing hepatic glucose production. Moreover, loss of Ago2 promoted hepatic expression of AMP-activated protein kinase α1 (AMPKα1) by de-repressing its targeting by miR-148a, an abundant microRNA in the liver. Deletion of Ago2 from hyperglycemic, obese, and insulin-resistant Lep^ob/ob mice reduced both random and fasted blood glucose levels and body weight and improved insulin sensitivity.

Conclusions

These data illustrate a central role for Ago2 in the adaptive response of the liver to fasting. Ago2 mediates the suppression of AMPKα1 by miR-148a, thereby identifying a regulatory link between non-coding RNAs and a key stress regulator in the hepatocyte.

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Hepatic Ago2 levels correlate with changes in extracellular glucose concentrations.

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Conditional deletion of Ago2 in liver decreased fasting glucose levels.

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Loss of Ago2 promoted AMPKα1 by de-repressing its targeting by miR-148a.

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Deletion of Ago2 from Lep^ob/ob mice improved glycemia and insulin sensitivity.

Genetic analysis of atherosclerosis identifies a major susceptibility locus in the major histocompatibility complex of mice

BACKGROUND AND AIMS

Recent genome-wide association studies (GWAS) have identified over 50 significant loci containing common variants associated with coronary artery disease. However, these variants explain only 26% of the genetic heritability of the disease, suggesting that many more variants remain to be discovered. Here, we examined the genetic basis underlying the marked difference between SM/J-Apoe^-/- and BALB/cJ-Apoe^-/- mice in atherosclerotic lesion formation.

METHODS

206 female F₂ mice generated from an intercross between the two Apoe^-/- strains were fed 12 weeks of western diet. Atherosclerotic lesion sizes in the aortic root were measured and 149 genetic markers genotyped across the entire genome.

RESULTS

A significant locus, named Ath49 (LOD score: 4.18), for atherosclerosis was mapped to the H2 complex [mouse major histocompatibility complex (MHC)] on chromosome 17. Bioinformatic analysis identified 12 probable candidate genes, including Tnfrsf21, Adgrf1, Adgrf5, Mep1a, and Pla2g7. Corresponding human genomic regions of Ath49 showed significant association with coronary heart disease. Five suggestive loci on chromosomes 1, 4, 5, and 8 for atherosclerosis were also identified. Atherosclerotic lesion sizes were significantly correlated with HDL but not with non-HDL cholesterol, triglyceride or glucose levels in the F₂ cohort.

CONCLUSIONS

We have identified the MHC as a major genetic determinant of atherosclerosis, highlighting the importance of inflammation in atherogenesis.

Leptin dose-dependently decreases atherosclerosis by attenuation of hypercholesterolemia and induction of adiponectin

OBJECTIVES

Conflicting evidence concerning leptin in atherosclerosis has been published. Furthermore, dose-dependent effects of leptin on atherogenesis have not been studied.

METHODS

Leptin-deficient low-density lipoprotein receptor (LDLR) knockout (LDLR(-/-);ob/ob) mice were treated with saline, 0.1, 0.5, or 3.0mg/kg body weight (BW)/d recombinant leptin over 12weeks starting at 8weeks of age. Aortic root and brachiocephalic artery (BCA) atherosclerotic lesions were analyzed by oil red O staining. Furthermore, glucose homeostasis, lipid metabolism, and liver function including tissue studies were assessed in all animals.

RESULTS

Leptin treatment dose-dependently decreased BW in LDLR(-/-);ob/ob mice as compared to saline. Mice in the 0.1 and 0.5mg/kgBW/d groups remained heavier (i.e. subphysiological leptin dose) and in the 3.0mg/kgBW/d group had similar weight (i.e. physiological leptin dose) as compared to non-leptin-deficient LDLR(-/-) animals. Recombinant leptin dose-dependently reduced plaque area in the aortic root and the BCA by 36% and 58%, respectively. Leptin-mediated reductions of plasma total and LDL-cholesterol (Chol) remained independent predictors for aortic root plaque area. Chol content in liver, as well as hepatic expression of key lipid and proinflammatory genes, were dose-dependently regulated by leptin. Furthermore, leptin treatment increased circulating levels and adipose tissue mRNA expression of the adipokine adiponectin.

CONCLUSIONS

Leptin administration within the subphysiological to physiological range diminishes atherosclerotic lesions. Leptin appears to mediate its antiatherogenic effects indirectly through reduction of hypercholesterolemia and liver steatosis, as well as upregulation of insulin-sensitizing and atheroprotective adiponectin.

Quantitative trait loci affecting atherosclerosis at the aortic root identified in an intercross between DBA2J and 129S6 apolipoprotein E-null mice

Apolipoprotein E-null mice on a DBA/2J genetic background (DBA-apoE) are highly susceptible to atherosclerosis in the aortic root area compared with those on a 129S6 background (129-apoE). To explore atherosclerosis-responsible genetic regions, we performed a quantitative trait locus (QTL) analysis using 172 male and 137 female F2 derived from an intercross between DBA-apoE and 129-apoE mice. A genome-wide scan identified two significant QTL for the size of lesions at the root: one is Ath44 on Chromosome (Chr) 1 at 158 Mb, and the other Ath45 on Chr 2 at 162 Mb. Ath44 co-localizes with but appears to be independent of a previously reported QTL, Ath1, while Ath45 is a novel QTL. DBA alleles of both Ath44 and Ath45 confer atherosclerosis-susceptibility. In addition, a QTL on Chr 14 at 73 Mb was found significant only in males, and 129 allele conferring susceptibility. Further analysis detected female-specific interactions between a second QTL on Chr 1 at 73 Mb and a QTL on Chr 3 at 21 Mb, and between Chr 7 at 84 Mb and Chr 12 at 77 Mb. These loci for the root atherosclerosis were independent of QTLs for plasma total cholesterol and QTLs for triglycerides, but a QTL for HDL (Chr 1 at 126 Mb) overlapped with the Ath44. Notably, haplotype analysis among 129S6, DBA/2J and C57BL/6 genomes and their gene expression data narrowed the candidate regions for Ath44 and Ath45 to less than 5 Mb intervals where multiple genome wide associations with cardiovascular phenotypes have also been reported in humans. SNPs in or near Fmo3, Sele and Selp for Ath44, and Lbp and Pkig for Ath45 were suggested for further investigation as potential candidates underlying the atherosclerosis susceptibility.

Altered expression of Raet1e, a major histocompatibility complex class 1-like molecule, underlies the atherosclerosis modifier locus Ath11 10b

RATIONALE

Quantitative trait locus mapping of an intercross between C57.Apoe⁻/⁻ and FVB.Apoe⁻/⁻ mice revealed an atherosclerosis locus controlling aortic root lesion area on proximal chromosome 10, Ath11. In a previous work, subcongenic analysis showed Ath11 to be complex with proximal (10a) and distal (10b) regions.

OBJECTIVE

To identify the causative genetic variation underlying the atherosclerosis modifier locus Ath11 10b.

METHODS AND RESULTS

We now report subcongenic J, which narrows the 10b region to 5 genes, Myb, Hbs1L, Aldh8a1, Sgk1, and Raet1e. Sequence analysis of these genes revealed no amino acid coding differences between the parental strains. However, comparing aortic expression of these genes between F1.Apoe⁻/⁻ Chr10SubJ((B/F)) and F1.Apoe⁻/⁻ Chr10SubJ((F/F)) uncovered a consistent difference only for Raet1e, with decreased, virtually background, expression associated with increased atherosclerosis in the latter. The key role of Raet1e was confirmed by showing that transgene-induced aortic overexpression of Raet1e in F1.Apoe⁻/⁻ Chr10SubJ((F/F)) mice decreased atherosclerosis. Promoter reporter constructs comparing C57 and FVB sequences identified an FVB mutation in the core of the major aortic transcription start site abrogating activity.

CONCLUSIONS

This nonbiased approach has revealed Raet1e, a major histocompatibility complex class 1-like molecule expressed in lesional aortic endothelial cells and macrophage-rich regions, as a novel atherosclerosis gene and represents one of the few successes of the quantitative trait locus strategy in complex diseases.

Genetics of atherosclerosis

Genome-wide association studies (GWAS) from the past several years have provided the first unbiased evidence of the genes contributing to common cardiovascular disease traits in European and some Asian populations. The results not only confirmed the importance of prior knowledge, such as the central role of lipoproteins, but also revealed that there is still much to learn about the underlying mechanisms of this disease, as most of the associated genes do not appear to be involved in pathways previously connected to atherosclerosis. In this review, I focus on the common forms of the disease and look at both human and animal model studies. I summarize what was known before GWAS, highlight how the field has been changed by GWAS, and discuss future considerations, such as the limitations of GWAS and strategies that may lead to a more complete, mechanistic understanding of atherosclerosis.