Distinction in genetic determinants for injury-induced neointimal hyperplasia and diet-induced atherosclerosis in inbred mice

Mouse models of arteriosclerosis: from arterial injuries to vascular grafts

Animal models are designed to be preliminary tools for better understanding of the pathogenesis, improvement in diagnosis, prevention, and therapy of arteriosclerosis in humans. Attracted by the well-defined genetic systems, a number of investigators have begun to use the mouse as an experimental system for arteriosclerosis research. Hundreds of inbred lines have been established, and the genetic map is relatively well defined, and both congenic strains and recombinant strains are available to facilitate genetic experimentation. Because arteriosclerosis is a complicated disease, which includes spontaneous (native) atherosclerosis, transplant arteriosclerosis, vein graft atherosclerosis, and angioplasty-induced restenosis, several mouse models for studying all types of arteriosclerosis have recently been established. Using these mouse models, much knowledge concerning the pathogenesis of the disease and therapeutic intervention has been gained, eg, origins of endothelial and smooth muscle cells in lesions of transplant and vein graft atherosclerosis. This review will not attempt to cover all aspects of mouse models, rather focus on models of arterial injuries, vein grafts, and transplant arteriosclerosis, by which the major progress in understanding the mechanisms of the disease has been made. This article will also point out (dis)advantages of a variety of models, and how the models can be appropriately chosen for different purposes of study.

The 5-lipoxygenase pathway promotes pathogenesis of hyperlipidemia-dependent aortic aneurysm

Activation of the 5-lipoxygenase (5-LO) pathway leads to the biosynthesis of proinflammatory leukotriene lipid mediators. Genetic studies have associated 5-LO and its accessory protein, 5-LO-activating protein, with cardiovascular disease, myocardial infarction and stroke. Here we show that 5-LO-positive macrophages localize to the adventitia of diseased mouse and human arteries in areas of neoangiogenesis and that these cells constitute a main component of aortic aneurysms induced by an atherogenic diet containing cholate in mice deficient in apolipoprotein E. 5-LO deficiency markedly attenuates the formation of these aneurysms and is associated with reduced matrix metalloproteinase-2 activity and diminished plasma macrophage inflammatory protein-1alpha (MIP-1alpha; also called CCL3), but only minimally affects the formation of lipid-rich lesions. The leukotriene LTD(4) strongly stimulates expression of MIP-1alpha in macrophages and MIP-2 (also called CXCL2) in endothelial cells. These data link the 5-LO pathway to hyperlipidemia-dependent inflammation of the arterial wall and to pathogenesis of aortic aneurysms through a potential chemokine intermediary route.

Vascular apolipoprotein e expression and recruitment from circulation to modulate smooth muscle cell response to endothelial denudation

Apolipoprotein E (apoE) has been shown previously to have anti-proliferative and anti-migratory effects on smooth muscle cells in culture. In addition, overexpression of the apoE gene also reduces neointimal hyperplasia in mice after endothelial denudation. In this investigation, immunohistochemical techniques were used to demonstrate that apoE was present in the medial smooth muscle layers of the carotid artery between 1 and 28 days after endothelial cell denudation. Analysis of transgenic mice overexpressing human apoE in the liver revealed that apoE was recruited from the circulation to the injured site of the vessel wall. In situ hybridization using a mouse-specific apoE mRNA probe confirmed that apoE was also synthesized in the carotid artery after endothelial denudation. Interestingly, apoE accumulation in apoE transgenic mice followed a layer-specific pattern, and was inversely associated with smooth muscle alpha-actin expression. The vascular accumulation of apoE after endothelial denudation, and its association with alpha-actin-depleted smooth muscle cells, suggest that apoE inhibition of injury-induced neointimal hyperplasia is not due to the inhibition of injury-induced smooth muscle cell de-differentiation, but is likely a direct effect of apoE on smooth muscle cell migration and proliferation.

Rosiglitazone Reduces the Accelerated Neointima Formation After Arterial Injury in a Mouse Injury Model of Type 2 Diabetes

Background— Hyperglycemia (HG) and hyperinsulinemia (HI) may be factors enhancing the atherosclerotic complications of diabetes. We hypothesized that specific feeding of C57BL/6 apolipoprotein (apo) E −/− mice would alter their metabolic profiles and result in different degrees of neointima (NI) formation. We additionally hypothesized that an insulin-sensitizing agent (rosiglitazone) would prevent the development of type 2 diabetes and reduce neointima formation after carotid wire injury measured at 28 days.

Methods and Results— Fasting glucose and insulin levels were elevated in the Western diet (WD) group, with a trend toward higher insulin levels and euglycemia in the fructose diet (FD)–fed mice. NI formation was exaggerated in the WD group compared with the FD or chow control groups. In the WD mice given rosiglitazone, glucose and insulin levels remained normal and NI formation was significantly reduced, as was NI macrophage content.

Conclusions— These findings demonstrate that apoE −/− mice fed a WD develop type 2 diabetes with an exaggerated NI response to injury. FD mice maintain euglycemia but develop insulin resistance, with an intermediate degree of NI growth compared with chow diet controls. Rosiglitazone prevents the development of hyperglycemia and hyperinsulinemia and normalizes the insulin release profile in the apoE −/− , WD-fed mouse and significantly reduces NI formation by 65% after carotid wire injury while reducing macrophage infiltration. These data support the hypothesis that type 2 diabetes in the setting of elevated cholesterol accelerates the response to vascular injury and suggest that agents that improve insulin sensitivity may have therapeutic value in reducing restenosis in type 2 diabetes.

Grb2 is required for the development of neointima in response to vascular injury

OBJECTIVE

Neointima formation occurs in arteries in response to mechanical or chemical injury and is responsible for substantial morbidity. In this work, the role of the intracellular linker protein Grb2 in the pathogenesis of neointima formation was examined. Grb2 is a critical signaling protein that facilitates the activation of the small GTPase ras by receptor tyrosine kinases.

METHODS AND RESULTS

Cultured rat aortic smooth muscle cells were treated with an antisense morpholino to Grb2 and these cells showed a reduced proliferative response to platelet-derived growth factor stimulation. Grb2-/- mice do not survive embryonic development. Grb2+/- mice appear normal at birth and are fertile but have defective signaling in several tissues. Cultured smooth muscle cells derived from Grb2+/- mice grew at a much slower rate than cells derived from Grb2+/+ mice. Grb2+/- and Grb2+/+ mice were subjected to carotid injury. After 21 days, Grb2+/+ mice developed robust neointima formation that, in some cases, resulted in an occlusive lesion. In contrast, Grb2+/- mice were resistant to the development of neointima

CONCLUSIONS

Grb2 is an essential component of the signaling cascade resulting in neointima formation after arterial injury.

Model of robust induction of glomerulosclerosis in mice: importance of genetic background

BACKGROUND

Increasing evidence suggests that genetic background plays an important role in the development of progressive glomerulosclerosis. The remnant kidney model (RKM) of progressive renal disease has been used extensively in rats. However, C57BL/6 mice are resistant to glomerulosclerosis with RKM induced by either pole amputation or renal artery ligation. A pole resection protocol, applied in 129/Sv mice, induced only mild glomerulosclerosis. We present here a highly reproducible, modified RKM approach to successfully establish a glomerulosclerosis model in mice.

METHODS

Male C57BL/6 (N = 17), 129/Sv (N = 20) and Swiss-Webster (N = 3) mice underwent RKM as follows: the lower branch of the left renal artery was ligated to produce about one third infarct; the upper pole of the left kidney (about one third kidney size) was removed by cautery and the right kidney was nephrectomized to induce a total 5/6 nephrectomy (Nx). In some C57BL/6 mice, 7/8 nephrectomy was induced by removing additional renal mass from the upper pole of the left kidney by cautery. Systolic blood pressure (BP) was measured in conscious mice using a tail-cuff blood pressure monitor and animals were sacrificed at 9, 12, 18, and 24 weeks after nephrectomy. Kidneys were harvested for morphologic analysis.

RESULTS

BP in C57BL/6 mice increased slightly after 5/6 nephrectomy over time without significant difference compared to baseline blood pressure except at 8 weeks (blood pressure at week 0, 98 +/- 1 mm Hg; week 4, 105 +/- 2 mm Hg; week 8, 113 +/- 4 mm Hg; and week 12, 110 +/- 3 mm Hg). Blood presssure remained normal in C57BL/6 mice at 18 weeks after 7/8 nephrectomy (103 +/- 2 mm Hg). Blood pressure in 129/Sv mice increased significantly after 5/6 nephrectomy from 4 to 12 weeks (week 0, 112 +/- 3 mm Hg; week 4, 161 +/- 9 mm Hg; week 8, 166 +/- 5 mm Hg; and week 12, 176 +/- 5 mm Hg; P < 0.01 weeks 4, 8, and 12 vs. week 0 blood pressure). Urine protein excretion in C57BL/6 mice increased only at 4 weeks after 5/6 nephrectomy, and was back to normal at 8 and 12 weeks (week 0, 13.2 +/- 1.4 mg/24 hours; week 4, 20.5 +/- 1.8 mg/24 hours; week 8, 18.8 +/- 1.6 mg/24 hours; and week 12, 17.2 +/- 1.2 mg/24 hours, P < 0.05 week 4 vs. week 0). 129/Sv mice developed significant proteinuria 12 weeks after 5/6 nephrectomy compared to their baseline and to levels achieved in C57BL/6 mice (week 0, 17.2 +/- 1 mg/24 hours; week 4, 14.9 +/- 1.8 mg/24 hours; week 8, 23.8 +/- 6.7 mg/24 hours; and week 12, 36.3 +/- 6.6 mg/24 hours, P < 0.01 week 12 vs. week 0; P < 0.01 129/Sv vs. C57BL/6 at week 12). Mortality varied in response to nephrectomy injury in the different strains. Ten percent of C57BL/6 and 43% of 129/Sv died within 12 weeks after 5/6 nephrectomy. Although 50% of C57BL/6 mice died by 12 weeks after 7/8 nephrectomy, there was only mild glomerulosclerosis (<5%) in C57BL/6 mice even at 24 weeks after 5/6 nephrectomy or 18 weeks after 7/8 nephrectomy. In contrast, glomerulosclerosis was marked in both 129/Sv mice and Swiss-Webster mice as early as 9 weeks after 5/6 nephrectomy: 42% of glomeruli showed sclerosis in 129/Sv mice [average sclerosis index (SI), 0 to 4+ scale, 1.08] vs. 24% in Swiss-Webster mice (average SI, 0.57). Tubulointerstitial fibrosis developed in parallel with glomerulosclerosis in both 129/Sv and Swiss-Webster mice.

CONCLUSION

We conclude that genetic background is one of the important factors determining the susceptibility to the development of glomerulosclerosis in mice. We speculate that the superior effects of renal artery ligation plus cautery to produce glomerulosclerosis may result from higher blood pressure responses due to local ischemia activating the renin-angiotensin system.

Androgens and cardiovascular disease

Globally, cardiovascular disease will continue causing most human deaths for the foreseeable future. The consistent gender gap in life span of approximately 5.6 yr in all advanced economies must derive from gender differences in age-specific cardiovascular death rates, which rise steeply in parallel for both genders but 5-10 yr earlier in men. The lack of inflection point at modal age of menopause, contrasting with unequivocally estrogen-dependent biological markers like breast cancer or bone density, makes estrogen protection of premenopausal women an unlikely explanation. Limited human data suggest that testosterone exposure does not shorten life span in either gender, and oral estrogen treatment increases risk of cardiovascular death in men as it does in women. Alternatively, androgen exposure in early life (perinatal androgen imprinting) may predispose males to earlier onset of atherosclerosis. Following the recent reevaluation of the estrogen-protection orthodoxy, empirical research has flourished into the role of androgens in the progression of cardiovascular disease, highlighting the need to better understand androgen receptor (AR) coregulators, nongenomic androgen effects, tissue-specific metabolic activation of androgens, and androgen sensitivity. Novel therapeutic targets may arise from understanding how androgens enhance early plaque formation and cause vasodilatation via nongenomic androgen effects on vascular smooth muscle, and how tissue-specific variations in androgen effects are modulated by AR coregulators as well as metabolic activation of testosterone to amplify (via 5alpha-reductase to form dihydrotestosterone acting on AR) or diversify (via aromatization to estradiol acting upon estrogen receptor alpha/beta) the biological effects of testosterone on the vasculature. Observational studies show that blood testosterone concentrations are consistently lower among men with cardiovascular disease, suggesting a possible preventive role for testosterone therapy, which requires critical evaluation by further prospective studies. Short-term interventional studies show that testosterone produces a modest but consistent improvement in cardiac ischemia over placebo, comparable to the effects of existing antianginal drugs. By contrast, testosterone therapy has no beneficial effects in peripheral arterial disease but has not been evaluated in cerebrovascular disease. Erectile dysfunction is most frequently caused by pelvic arterial insufficiency due to atherosclerosis, and its sentinel relationship to generalized atherosclerosis is insufficiently appreciated. The commonality of risk factor patterns and mechanisms (including endothelial dysfunction) suggests that the efficacy of antiatherogenic therapy is an important challenge with the potential to enhance men's motivation for prevention and treatment of cardiovascular diseases.

Toll-like receptor 4 and atherogenesis

Toll-like receptor 4 (TLR4) is a pattern recognition receptor involved in the innate immune response to various microorganisms and other exogenous and endogenous stress factors. Recently, evidence emerged that important inflammatory processes implicit in human atherogenesis are mediated in part via the TLR4/nuclear factor-kappaB pathway. Polymorphisms of TLR4, which attenuate receptor signalling, enhance the risk of acute severe infections but may have opposite effects on atherogenesis. The aim of this review is to critically discuss current experimental and epidemiological evidence for a role of TLR4 in atherogenesis and to highlight the main controversies and perspectives in this emerging field of vascular biology.

Minireview: estrogen and mouse models of atherosclerosis

The use of hormone replacement therapy for coronary heart disease prevention in humans has been an area of intense controversy. The atheroprotective qualities of estrogens have been challenged recently by several negative results of randomized clinical trials in postmenopausal women. However, the inhibitory effects of estrogens on atherogenesis are well documented in numerous animals, including atherosclerotic mouse models, but the detailed mechanisms of this protection are not understood. In this minireview, we will focus on the considerable success that has been achieved in demonstrating the atheroprotective effects of 17beta-estradiol in apolipoprotein E and low-density lipoprotein receptor-deficient mice and the use of these atherosclerotic mouse models in pharmacological and genetic study designs to investigate antiatherogenic mechanisms of estrogens. Mouse models of atherosclerosis should prove beneficial to understanding the cellular and molecular mechanisms of estrogen-mediated atheroprotection and aid the development of improved therapies to confer the benefits and reduce the risks associated with hormone replacement therapy.

Targeted disruption of the matrix metalloproteinase-9 gene impairs smooth muscle cell migration and geometrical arterial remodeling

Matrix remodeling plays an important role in the physiological and pathological remodeling of blood vessels. We specifically investigated the role of matrix metalloproteinase (MMP)-9, an MMP induced during arterial remodeling, by assessing the effects of genetic MMP-9 deficiency on major parameters of arterial remodeling using the mouse carotid artery flow cessation model. Compared with remodeling of matched wild-type (WT) arteries, MMP-9 deficiency decreased intimal hyperplasia, reduced the late lumen loss, eliminated the correlation between intimal hyperplasia and geometric remodeling, and led to significant accumulation of interstitial collagen. Biochemical analysis of MMP-9 knockout (KO) arterial tissue and isolated smooth muscle cells (SMCs) confirmed the lack of MMP-9 expression or compensation by other gelatinases. To investigate potential mechanisms for the in vivo observations, we analyzed in vitro effects of MMP-9 deficiency on the migration, proliferation, and collagen gel contracting capacity of aortic SMCs isolated from MMP-9 KO and WT mice. Although proliferation was comparable, we found that MMP-9-deficient cells had not only decreased migratory activity, but they also had decreased capacity to contract collagen compared with WT cells. Thus, MMP-9 appears to be involved not only in degradation, but also in reorganization of a collagenous matrix, both facets being essential for the outcome of arterial remodeling. Our results also establish MMP-9 as an attractive therapeutic target for limiting the effects of pathological arterial remodeling in restenosis and atherosclerosis.

Matrix metalloproteinase-9 is necessary for the regulation of smooth muscle cell replication and migration after arterial injury

Matrix metalloproteinases (MMPs) and, in particular, MMP-9 are important for smooth muscle cell (SMC) migration into the intima. In this study, we sought to determine whether MMP-9 is critical for SMC migration and for the formation of a neointima by using mice in which the gene was deleted (MMP-9(-/-) mice). A denuding injury to the arteries of wild-type mice promoted the migration of medial SMCs into the neointima at 6 days, and a large neointimal lesion was observed after 28 days. In wild-type arteries, medial SMC replication was approximately 8% at day 4, 6% at day 6, and 4% at day 8 and had further decreased to 1% at day 14. Intimal cell replication was 65% at 8 days and had decreased to approximately 10% at 14 days after injury. In MMP-9(-/-) arteries, SMC replication was significantly lower at day 8. In addition, SMC migration and arterial lesion growth were significantly impaired in MMP-9(-/-) arteries. SMCs, isolated from MMP-9(-/-) mouse arteries, showed an impairment of migration and replication in vitro. Thus, our present data indicate that MMP-9 is critical for the development of arterial lesions by regulating both SMC migration and proliferation.