Effect of aging on fatty streak formation in a diet-induced mouse model of atherosclerosis

Advancing age increases the size and severity of spontaneous atheromas in mouse models of atherosclerosis

Using multiple mouse models, we explored the impact of aging on the size and severity of atherosclerotic lesions. In young, middle-aged and old apolipoprotein E knockout mice (ApoE^-/-) fed an atherogenic diet (AD) for 3-8 weeks, plaque/atheroma formation in the descending aorta and aortic root, and atheroma development in the carotid in response to partial carotid ligation (PCL) were assessed. Total and LDL cholesterol, and triglycerides were higher in old compared to both other age groups, regardless of AD duration. Aortic plaque burden increased with AD duration in all ages. The size and plaque morphology grade of aortic root atheromas was higher with age; however, there was no effect of age on the size or severity of carotid atheromas after PCL. We additionally induced hyperlipidemia in young and old C57BL/6 mice by adeno-associated virus mediated upregulation of LDL receptor regulator, Pcsk9, and 5 weeks of AD. Despite lower cholesterol in old compared to young Pcsk9 mice, there was a greater size and severity of aortic root atheromas in old mice. However, like the ApoE^-/- mice, there was no effect of age on size or severity of PCL-induced carotid artery atheromas in Pcsk9 mice. Together, these results suggest that aging increases the size and severity of spontaneous aortic atheromas.

A Unified Model of Age-Related Cardiovascular Disease

Despite progress in biomedical technologies, cardiovascular disease remains the main cause of mortality. This is at least in part because current clinical interventions do not adequately take into account aging as a driver and are hence aimed at suboptimal targets. To achieve progress, consideration needs to be given to the role of cell aging in disease pathogenesis. We propose a model unifying the fundamental processes underlying most age-associated cardiovascular pathologies. According to this model, cell aging, leading to cell senescence, is responsible for tissue changes leading to age-related cardiovascular disease. This process, occurring due to telomerase inactivation and telomere attrition, affects all components of the cardiovascular system, including cardiomyocytes, vascular endothelial cells, smooth muscle cells, cardiac fibroblasts, and immune cells. The unified model offers insights into the relationship between upstream risk factors and downstream clinical outcomes and explains why interventions aimed at either of these components have limited success. Potential therapeutic approaches are considered based on this model. Because telomerase activity can prevent and reverse cell senescence, telomerase gene therapy is discussed as a promising intervention. Telomerase gene therapy and similar systems interventions based on the unified model are expected to be transformational in cardiovascular medicine.

Endothelial reactive oxygen-forming NADPH oxidase 5 is a possible player in diabetic aortic aneurysm but not atherosclerosis

Atherosclerosis and its complications are major causes of cardiovascular morbidity and death. Apart from risk factors such as hypercholesterolemia and inflammation, the causal molecular mechanisms are unknown. One proposed causal mechanism involves elevated levels of reactive oxygen species (ROS). Indeed, early expression of the ROS forming NADPH oxidase type 5 (Nox5) in vascular endothelial cells correlates with atherosclerosis and aortic aneurysm. Here we test the pro-atherogenic Nox5 hypothesis using mouse models. Because Nox5 is missing from the mouse genome, a knock-in mouse model expressing human Nox5 in its physiological location of endothelial cells (eNOX5^ki/ki) was tested as a possible new humanised mouse atherosclerosis model. However, whether just on a high cholesterol diet or by crossing in aortic atherosclerosis-prone ApoE^−/− mice with and without induction of diabetes, Nox5 neither induced on its own nor aggravated aortic atherosclerosis. Surprisingly, however, diabetic ApoE^−/− x eNOX5^ki/ki mice developed aortic aneurysms more than twice as often correlating with lower vascular collagens, as assessed by trichrome staining, without changes in inflammatory gene expression, suggesting that endothelial Nox5 directly affects extracellular matrix remodelling associated with aneurysm formation in diabetes. Thus Nox5-derived reactive oxygen species are not a new independent mechanism of atherosclerosis but may enhance the frequency of abdominal aortic aneurysms in the context of diabetes. Together with similar clinical findings, our preclinical target validation opens up a first-in-class mechanism-based approach to treat or even prevent abdominal aortic aneurysms.

Loxin Reduced the Inflammatory Response in the Liver and the Aortic Fatty Streak Formation in Mice Fed with a High-Fat Diet

Oxidized low-density lipoprotein (ox-LDL) is the most harmful form of cholesterol associated with vascular atherosclerosis and hepatic injury, mainly due to inflammatory cell infiltration and subsequent severe tissue injury. Lox-1 is the central ox-LDL receptor expressed in endothelial and immune cells, its activation regulating inflammatory cytokines and chemotactic factor secretion. Recently, a Lox-1 truncated protein isoform lacking the ox-LDL binding domain named LOXIN has been described. We have previously shown that LOXIN overexpression blocked Lox-1-mediated ox-LDL internalization in human endothelial progenitor cells in vitro. However, the functional role of LOXIN in targeting inflammation or tissue injury in vivo remains unknown. In this study, we investigate whether LOXIN modulated the expression of Lox-1 and reduced the inflammatory response in a high-fat-diet mice model. Results indicate that human LOXIN blocks Lox-1 mediated uptake of ox-LDL in H4-II-E-C3 cells. Furthermore, in vivo experiments showed that overexpression of LOXIN reduced both fatty streak lesions in the aorta and inflammation and fibrosis in the liver. These findings were associated with the down-regulation of Lox-1 in endothelial cells. Then, LOXIN prevents hepatic and aortic tissue damage in vivo associated with reduced Lox-1 expression in endothelial cells. We encourage future research to understand better the underlying molecular mechanisms and potential therapeutic use of LOXIN.

Hyperlipidemia Influences the Accuracy of Glucometer-Measured Blood Glucose Concentrations in Genetically Diverse Mice

BACKGROUND

Glucometers are widely used in animal research due to simplicity and ease of utilization, but their accuracy in blood glucose assessment for hyperlipidemic mice is unknown.

METHODS

Here, we compared blood glucose levels measured by a glucometer with plasma glucose levels measured by a standard enzymatic assay for 325 genetically diverse F2 mice derived from LP and BALB/c (BALB) Apoe^-/- mice. Non-fasting glucose levels were measured before initiation of a Western diet and after 11 weeks on the diet.

RESULTS

On chow diet, lab-measured plasma glucose levels were 279.5 ± 42.6 mg/dl (mean ± SD), while blood glucose values measured by glucometer were 138.7 ± 16.6 mg/dl. The two measures had no correlation (R² = 0.006, p = 0.167). On the Western diet, plasma glucose levels rose to 351.1 ± 121.6 mg/dl, while glucometer-measured blood glucose fell to 128.7 ± 27.9 mg/dl. The two measures showed a moderate correlation (R² = 0.111, p = 3.1E-9). Lab-measured plasma glucose showed strong positive correlations with plasma triglyceride and non-high-density lipoprotein cholesterol levels, while glucometer-measured blood glucose showed an inverse correlation with non-high-density lipoprotein levels on the chow diet.

CONCLUSIONS

Our results indicate that hyperlipidemia affects the accuracy of glucometers in measuring blood glucose levels of mice.

Inflammation and enhanced atherogenesis in the carotid artery with altered blood flow in an atherosclerosis-resistant mouse strain

Ligation of the common carotid artery near its bifurcation in apolipoprotein E-deficient (Apoe^-/- ) mice leads to rapid atherosclerosis development, which is affected by genetic backgrounds. BALB/cJ (BALB) mice are resistant to atherosclerosis, developing much smaller aortic lesions than C57BL/6 (B6) mice. In this study, we examined cellular events leading to lesion formation in carotid arteries with or without blood flow restriction of B6 and BALB Apoe^-/- mice. Blood flow was obstructed by ligating the left common carotid artery near its bifurcation in one group of mice, and other group received no surgical intervention. Without blood flow interruption, BALB-Apoe^-/- mice formed much smaller atherosclerotic lesions than B6-Apoe^-/- mice after 12 weeks of Western diet (3,325 ± 1,086 vs. 81,549 ± 9,983 µm² /section; p = 2.1E-7). Lesions occurred at arterial bifurcations in both strains. When blood flow was obstructed, ligated carotid artery of both strains showed notable lipid deposition, inflammatory cell infiltration, and rapid plaque formation. Neutrophils and macrophages were observed in the arterial wall of BALB mice 3 days after ligation and 1 week after ligation in B6 mice. CD4 T cells were observed in intimal lesions of BALB but not B6 mice. By 4 weeks, both strains developed similar sizes of advanced lesions containing foam cells, smooth muscle cells, and neovessels. Atherosclerosis also occurred in straight regions of the contralateral common carotid artery where MCP-1 was abundantly expressed in the intima of BALB mice. These findings indicate that the disturbed blood flow is more prominent than high fat diet in promoting inflammation and atherosclerosis in hyperlipidemic BALB mice.

Hyperlipidemic mice as a model for a real-time in vivo detection of atherosclerosis by gold nanorods-based diffusion reflection technique

Atherosclerosis (AS), the leading cause of morbidity and mortality in cardiovascular disease, needs an early detection for treatment and prevention of fatal events. Here, for the first time, we applied gold nanorods (GNRs)-assisted diffusion reflection (DR), a noninvasive technique for in vivo detection of AS in a high-fat-diet-induced c57bl mouse model, which resembles the manifestation of AS in humans. DR simply detects the change in light reflection profile of tissue due to the accumulation of GNRs in the AS plaques and enables clear detection of AS lesions in carotid and femoral arteries of these hyperlipidemic mice. After 24 hours post-GNRs injection, DR showed the highest efficiency of AS detection. Moreover, the sensitivity of the DR method is much higher than computed tomography (CT) and is comparable to ex vivo high-resolution CT. Our results strongly suggest that the DR method can detect early atherosclerotic lesions in a sensitive and specific manner.

Three-Dimensional Highly Sensitive Diffusion Reflection-Based Imaging Method for the in Vivo Localization of Atherosclerosis Plaques Following Gold Nanorods Accumulation

In this work, we present a novel, simple, and highly accurate three-dimensional (3D) diffusion reflection (DR) imaging system and method for the detection of accumulation sites of gold nanorods (GNRs) within the tissue. GNRs are intensively used for diagnosis purposes of varied diseases, mainly because of their ability to well absorb visible light, which introduces them as terrific contrast agents in various imaging and theranostics methods. Lately, these GNRs unique absorption properties have served in DR intensity-based measurements, suggesting a novel diagnostic tool, DR-GNRs. In this paper, we show a new measurement system and method for DR, based on its radial collection from the tissue. These radial measurements enabled a unique 3D presentation of the DR-GNR, introducing the dimensions ρ for the radius, θ for the angle, and Γ for the reflected intensity. On the basis of the diffusion model, which enables to correlate between the sample's optical properties and its reflectance, a unique, radial map is presented. This map introduces the slopes of the DR curves in each measured angle, which are linearly correlated with the tissue's optical properties and with the GNRs concentrations within the tissue, thus enables the exact radial localization of the GNRs in the sample. We show the detection of macrophage accumulation in tissue-like phantoms, as well as the localization of unstable plaques in hyperlipidemic mice, in vivo. This highly accurate, powerful technology paves the way toward a real-time detection method that can be successfully integrated in the rapid increasing field of personalized medicine.

Preclinical models of atherosclerosis. The future of Hybrid PET/MR technology for the early detection of vulnerable plaque

Cardiovascular diseases are the leading cause of death in developed countries. The aetiology is currently multifactorial, thus making them very difficult to prevent. Preclinical models of atherothrombotic diseases, including vulnerable plaque-associated complications, are now providing significant insights into pathologies like atherosclerosis, and in combination with the most recent advances in new non-invasive imaging technologies, they have become essential tools to evaluate new therapeutic strategies, with which can forecast and prevent plaque rupture. Positron emission tomography (PET)/computed tomography imaging is currently used for plaque visualisation in clinical and pre-clinical cardiovascular research, albeit with significant limitations. However, the combination of PET and magnetic resonance imaging (MRI) technologies is still the best option available today, as combined PET/MRI scans provide simultaneous data acquisition together with high quality anatomical information, sensitivity and lower radiation exposure for the patient. The coming years may represent a new era for the implementation of PET/MRI in clinical practice, but first, clinically efficient attenuation correction algorithms and research towards multimodal reagents and safety issues should be validated at the preclinical level.

Graded Maximal Exercise Testing to Assess Mouse Cardio-Metabolic Phenotypes

Functional assessments of cardiovascular fitness (CVF) are needed to establish animal models of dysfunction, test the effects of novel therapeutics, and establish the cardio-metabolic phenotype of mice. In humans, the graded maximal exercise test (GXT) is a standardized diagnostic for assessing CVF and mortality risk. These tests, which consist of concurrent staged increases in running speed and inclination, provide diagnostic cardio-metabolic parameters, such as, VO_2max, anaerobic threshold, and metabolic crossover. Unlike the human-GXT, published mouse treadmill tests have set, not staged, increases in inclination as speed progress until exhaustion (PXT). Additionally, they often lack multiple cardio-metabolic parameters. Here, we developed a mouse-GXT with the intent of improving mouse-exercise testing sensitivity and developing translatable parameters to assess CVF in healthy and dysfunctional mice. The mouse-GXT, like the human-GXT, incorporated staged increases in inclination, speed, and intensity; and, was designed by considering imitations of the PXT and differences between human and mouse physiology. The mouse-GXT and PXTs were both tested in healthy mice (C57BL/6J, FVBN/J) to determine their ability to identify cardio-metabolic parameters (anaerobic threshold, VO_2max, metabolic crossover) observed in human-GXTs. Next, theses assays were tested on established diet-induced (obese-C57BL/6J) and genetic (cardiac isoform Casq2^-/-) models of cardiovascular dysfunction. Results showed that both tests reported VO_2max and provided reproducible data about performance. Only the mouse-GXT reproducibly identified anaerobic threshold, metabolic crossover, and detected impaired CVF in dysfunctional models. Our findings demonstrated that the mouse-GXT is a sensitive, non-invasive, and cost-effective method for assessing CVF in mice. This new test can be used as a functional assessment to determine the cardio-metabolic phenotype of various animal models or the effects of novel therapeutics.

PET imaging detection of macrophages with a formyl peptide receptor antagonist

Macrophages are a major inflammatory cell type involved in the development and progression of many important chronic inflammatory diseases. We previously found that apolipoprotein E-deficient (Apoe(-/-)) mice with the C57BL/6 (B6) background develop type 2 diabetes mellitus (T2DM) and accelerated atherosclerosis when fed a Western diet and that there are increased macrophage infiltrations in pancreatic islets and aorta. The formyl peptide receptor 1 (FPR1) is abundantly expressed on the surface of macrophages. The purpose of this study was to evaluate the applicability of cinnamoyl-F-(D)L-F-(D)L-F (cFLFLF), a natural FPR1 antagonist, to detection of macrophages in the pancreatic islets and aorta. (64)Cu labeled cFLFLF and (18)F-fluorodeoxyglucose (FDG) were administered to mice with or without T2DM. Diabetic mice showed an increased (18)FDG uptake in the subcutaneous fat compared with control mice, but pancreatic uptake was minimal for either group. In contrast, diabetic mice exhibited visually noticeable more cFLFLF-(64)Cu retention in pancreas and liver than control mice. The heart and pancreas isolated from diabetic mice contained more macrophages and showed stronger PET signals than those of control mice. Flow cytometry analysis revealed the presence of macrophages but not neutrophils in pancreatic islets. Real-time PCR analysis revealed much higher FPR1 expression in pancreatic islets of diabetic over control mice. Autoradiography and immunohistochemical analysis confirmed abundant FPR1 expression in atherosclerotic lesions. Thus, (64)Cu-labeled cFLFLF peptide is a more effective PET agent for detecting macrophages compared to FDG.

Characterization of Bglu3, a mouse fasting glucose locus, and identification of Apcs as an underlying candidate gene

Bglu3 is a quantitative trait locus for fasting glucose on distal chromosome 1 identified in an intercross between C57BL/6 (B6) and C3H/HeJ (C3H) apolipoprotein E-deficient (apoE(-/-)) mice. This locus was subsequently replicated in two separate mouse intercrosses. The objective of this study was to characterize Bglu3 through construction and analysis of a congenic strain and identify underlying candidate genes. Congenic mice were constructed by introgressing a genomic region harboring Bglu3 from C3H.apoE(-/-) into B6.apoE(-/-) mice. Mice were started with a Western diet at 6 wk of age and maintained on the diet for 12 wk. Gene expression in the liver was analyzed by microarrays. Congenic mice had significantly higher fasting glucose levels and developed more significant glucose intolerance compared with B6.apoE(-/-) mice on the Western diet. Microarray analysis revealed 336 genes to be differentially expressed in the liver of congenic mice. Further pathway analysis suggested a role for acute phase response signaling in regulating glucose intolerance. Apcs, encoding an acute phase response protein serum amyloid P (SAP), is located underneath the linkage peak of Bglu3. Multiple single nucleotide polymorphisms between B6 and C3H mice were detected within and surrounding Apcs. Apcs expression in the liver was significantly higher in congenic and C3H mice compared with B6 mice. The Western diet consumption led to a gradual rise in plasma SAP levels, which was accompanied by rising fasting glucose in both B6 and C3H apoE(-/-) mice. Expression of C3H Apcs in B6.apoE(-/-) mice aggravated glucose intolerance. Bglu3 is confirmed to be a locus affecting diabetes susceptibility, and Apcs is a probable candidate gene.

Potential involvement of LOX-1 in functional consequences of endothelial senescence

Numerous studies have described the process of senescence associated with accumulation of oxidative damage, mutations and decline in proliferative potential. Although the changes observed in senescent cells are likely to result in significant phenotypic alterations, the studies on consequences of endothelial senescence, especially in relation to aging-associated diseases, are scarce. We have analyzed effects of senescence on the functions of endothelial cells relevant to the development of atherosclerosis including angiogenesis, adhesion, apoptosis and inflammation. In the course of progressing through the passages, human umbilical vein endothelial cells (HUVECs) displayed significant increase in size (+36% passage 12 vs. passage 4 , p<0.001) and reduction in both basal and VEGF-stimulated tube formation. The analysis of a scavenger receptor LOX-1, a key molecule implicated in atherogenesis, revealed a significant decline of its message (mRNA) and protein content in senescent endothelial cells (−33%) and in aortas of 50 wk (vs. 5 wk) old mice (all p<0.01). These effects were accompanied by a marked reduction of the basal expression of VCAM-1 and ICAM-1. Compared to early cultures, late passage HUVECs also exhibited nuclear translocation of NF-κB (p65) and reciprocal shifts in BAX and BCL2 protein content resulting in almost 2-fold increase in BAX/BCL2 ratio and 3-fold increase in apoptotic response to TNFα exposure (p<0.04). These changes in senescent endothelial cells are suggestive of aberrant responses to physiological stimuli resulting in a less permissive environment for tissue remodeling and progression of diseases requiring angiogenesis and cell adhesion in elderly, possibly, mediated by LOX-1.

A high cholesterol diet elevates hippocampal cytokine expression in an age and estrogen-dependent manner in female rats

BACKGROUND

While the effects of a proatherogenic diet have been widely studied in the context of systemic inflammation, much less is known about its effects on central or brain inflammation and its modulation with age. In this study, we examined the effect of a high cholesterol/choline diet in adult and older acyclic females to assess its impact on systemic and central inflammatory markers. Moreover, since the loss of ovarian hormones at menopause may predispose women to increased production of pro-inflammatory cytokines, we also tested the impact of estrogen replacement to adult and older females in diet-induced inflammation.

METHODS

Ovariectomized adult female rats and older (reproductive senescent) female rats were replaced with estrogen or a control pellet and maintained thereafter on a diet containing either 4% cholesterol/1% choline or control chow for 10 weeks. Interleukin 1beta (IL-1beta) expression in the liver was used as a marker of systemic inflammation, while a panel of cytokine/chemokines were used to examine the effects of diet on the hippocampus.

RESULTS

IL-1beta expression was elevated in the liver of adult and reproductive senescent females fed with the high cholesterol diet, although this was restricted to groups that were ovariectomized and not replaced with estrogen. Estrogen-treated animals of both ages did not have elevated IL-1beta levels when fed the high cholesterol diet. Diet-induced changes in cytokine/chemokine expression in the hippocampus however were critically age dependent and restricted to the reproductive senescent females. In this group, the high cholesterol diet led to an increase in interleukin (IL)-4, IL-6, IL-12p70, IL-13, RANTES (Regulated on Activation, Normal T Expressed and Secreted) and VEGF (vascular endothelial growth factor). Moreover, estrogen treatment to reproductive senescent females suppressed diet-induced expression of specific cytokines (RANTES, VEGF, IL-6) and attenuated the expression of others (IL-4, IL-12p70, and IL-13).

CONCLUSIONS

These data indicate that a proatherogenic diet presents a significant risk for central inflammation in older females that are deprived of estrogen treatment.

Comparative antilipidemic effect of N-acetylcysteine and sesame oil administration in diet-induced hypercholesterolemic mice

Background

There is an increasing number of novel antilipidemic therapies under consideration. The putative hypolipidemic effect of N-acetylcysteine (NAC) and sesame oil was studied in a mouse model of dietary-induced hypercholesterolemia.

Methods

Male C57bl/6 mice were assigned to the following groups: (NC) control group, (HC) group receiving test diet supplemented with 2% cholesterol and 0.5% cholic acid for 8 weeks, (HCN) group receiving the test diet with NAC supplementation (230 mg/kg p.o.) and (HCS) group fed the test diet enriched with 10% sesame oil. Total serum cholesterol, LDL-cholesterol, HDL-cholesterol and triglycerides were assayed at the beginning and at the end of the experiment. Total peroxides and nitric oxide (NO) levels were measured in the serum at the end of the experiment. Hepatic and aortic lesions were evaluated by haematoxylin-eosin staining.

Results

Higher serum levels of total and LDL-cholesterol were recorded in all groups fed the high cholesterol diet. The HCN group presented reduced lipid levels compared to HC and HCS groups. No differences were observed between HCS and HC groups. Peroxide content in serum was markedly increased in mice consuming high cholesterol diet. NAC and sesame oil administration led to a significant decrease of serum lipid peroxidation in the levels of control group, whereas only NAC restored NO bioavailability. In terms of liver histology, the lesions observed in HCN group were less severe than those seen in the other high cholesterol groups.

Conclusion

Co-administration of NAC, but not sesame oil, restored the disturbed lipid profile and improved hepatic steatosis in the studied diet-induced hypercholesterolemic mice. Both agents appear to ameliorate serum antioxidant defense.

Biomarkers of premature atherosclerosis

C-reactive protein (CRP) is an acute phase protein and a biochemical marker with important prognostic value for cardiovascular events. Interleukins IL-1 and IL-6 are implicated in the pathogenesis of atherosclerosis and are associated with CRP. Apolipoproteins ApoA-I and ApoB are the main lipid metabolic markers implicated in the development and progression of atherosclerosis. Fibrinogen has also been proposed to be a major independent risk factor for cardiovascular events. Because premature atherosclerosis precedes the development of cardiovascular disease, identification of the associated biomarkers is of great importance. However, further studies will be needed to determine whether or not these markers are useful predictors of future cardiovascular events. Here, we review the roles of specific biomarkers that have been implicated in premature atherosclerosis.