Novel insights into coronary lumen preservation during progression of coronary atherosclerosis in coronary atherosclerosis-prone rabbits

The History of the WHHL Rabbit, an Animal Model of Familial Hypercholesterolemia (I) - Contribution to the Elucidation of the Pathophysiology of Human Hypercholesterolemia and Coronary Heart Disease

Animal models that closely resemble both human disease findings and their onset mechanism have contributed to the advancement of biomedical science. The Watanabe heritable hyperlipidemic (WHHL) rabbit and its advanced strains (the coronary atherosclerosis-prone and the myocardial infarction-prone WHHL rabbits) developed at Kobe University (Kobe, Japan), an animal model of human familial hypercholesterolemia, have greatly contributed to the elucidation of the pathophysiology of human lipoprotein metabolism, hypercholesterolemia, atherosclerosis, and coronary heart disease, as described below. 1) The main part of human lipoprotein metabolism has been elucidated, and the low-density lipoprotein (LDL) receptor pathway hypothesis derived from studies using fibroblasts was proven in vivo. 2) Oxidized LDL accumulates in the arterial wall, monocyte adhesion molecules are expressed on arterial endothelial cells, and monocyte-derived macrophages infiltrate the arterial intima, resulting in the formation and progression of atherosclerosis. 3) Coronary lesions differ from aortic lesions in lesion composition. 4) Factors involved in the development of atherosclerosis differ between the coronary arteries and aorta. 5) The rupture of coronary lesions requires secondary mechanical forces, such as spasm, in addition to vulnerable plaques. 6) Specific lipid molecules in the blood have been identified as markers of the progression of coronary lesions. At the end of the breeding of the WHHL rabbit family at Kobe University, this review summarizes the history of the development of the WHHL rabbit family and their contribution to biomedical science.

Morphological features of coronary plaques in WHHLMI rabbits (Oryctolagus cuniculus), an animal model for familial hypercholesterolemia

In order to examine their suitability for studies on coronary atherosclerosis, we evaluated the features of coronary atherosclerotic plaques in myocardial infarction-prone Watanabe heritable hyperlipidemic (WHHLMI) rabbits, a spontaneous animal model for coronary atherosclerosis and myocardial infarction. Coronary segments of the hearts of 187 WHHLMI rabbits (10–29 months old) were sectioned serially and stained histopathologically and immunohistologically. Progression of coronary lesions was prominent in rabbits that had died suddenly. The degree of coronary lesions of females was higher than that of males. Various types of atherosclerotic lesions were observed in the coronary arteries, such as plaques with a large lipid core covered by a thin fibrous cap, fatty streaks, early and advanced fibroatheromas, fibrous lesions, and advanced lesions with calcium accumulation and the vasa vasorum. In rabbits that had died suddenly, the frequencies of fibroatheromas or advanced lesions were higher than those of rabbits euthanized. Matrix metalloproteinase (MMP)-positive macrophages were detected in gaps among endothelial cells at the plaque surface, beneath the fibrous cap of thin-capped fibroatheromas, and at the bottom of the intimal plaques in which the tunica media was attenuated. Immunohistological results suggest that MMP-positive macrophages are involved in the initiation, progression, and destabilization of coronary plaques, in addition to vascular remodeling, even in WHHLMI rabbits. In conclusion, coronary lesions in WHHLMI rabbits resemble human atherosclerotic lesions, and thus, the WHHLMI rabbit is a suitable animal model for studies on human coronary plaques.

Early prevention by L-Arginine attenuates coronary atherosclerosis in a model of hypercholesterolemic animals; no positive results for treatment

Background

Endothelial dysfunction (ED) is an independent predictor of cardiovascular events. ED is also a reversible disorder, and nitric oxide donors like L-arginine may promote this process. Despite the positive results from several studies, there are some studies that have shown that L-arginine administration did not improve endothelium-dependent dilation or the inflammatory state of patients. In this study the early and the late effects of L-arginine on coronary fatty streak formation and ED biomarkers were considered in hypercholesterolemic rabbits.

Methods

36 white male rabbits randomly assigned in 3 groups. Rabbits were fed 1% high-cholesterol diet (LP group, n = 15), or high-cholesterol diet with oral L-arginine (3% in drinking water) (EP group, n = 15) or standard diet (control group, n = 6) for 4 weeks (phase I). Afterward, all animals were fed normal diet for 4 weeks (phase II). In the second phase, L-arginine was discontinued for EP group and was begun for LP group. The plasma levels of lipids, von Willebrand factor (vWF), and nitrite were compared before and after 4 and 8 weeks of experiment. Coronary fatty streak formation was measure after 4 and 8 weeks of experiment.

Results

The plasma levels of lipids were increased significantly in both groups of LP and EP after phase I. The hypercholesterolemia induced significant increased vWF release in LP group. The L-arginine supplementation led to significant plasma nitrite increment in EP group. The vWF in LP group was higher than other groups (p < 0.05). By the end of phase II, despite of start of L-arginine supplementation for LP group and L-arginine discontinuation in EP group, there were significantly more fatty streaks lesions in LP group coronary arteries than EP group. Furthermore, L-arginine supplementation did not result in significant nitrite increment in LP group.

Conclusion

Early prevention by L-arginine may be helpful to prevent the ED, but our study did not suggest the treatment. It seems reasonable to consider ED-aside from control the cardiovascular risk factors in primary prevention of atherosclerosis and its clinical outcomes before development of irreversible vascular damage.

A robust rabbit model of human atherosclerosis and atherothrombosis

Disruption and thrombosis of atherosclerotic plaques cause most acute cardiovascular events, but their systematic study has been hampered by the lack of suitable animal models. To assess the value of a modified rabbit model of atherothrombosis, we performed detailed histology of rabbit aortic plaques. Atherosclerosis was induced with a high cholesterol diet fed 2 weeks prior to and 6 weeks after balloon injury of the aorta, followed by 4 weeks of normal diet. We found six out of eight types of plaques cataloged by the American Heart Association in the rabbit aorta. Vulnerable plaques were defined as those with attached platelet and fibrin-rich thrombi after pharmacological triggering with Russell's viper venom and histamine. Ruptured plaques had, as also described for human plaques: i) marked medial and adventitial changes, including neovascularization and inflammation; ii) cholesterol monohydrate crystals and liquid crystalline cholesterol esters in the intima and the fibrous cap; and iii) inflamed, thin fibrous caps. Increased cholesterol monohydrate area, internal elastic lamina area, positive remodeling, fibrous cap inflammation, adventitia breakdown, and inflammation were independent predictors of plaque disruption. Our findings reveal novel insights into plaque vulnerability and could guide the design of noninvasive imaging approaches for detecting and treating high-risk plaques.

Invasion of atheromatous plaques into tunica media causes coronary outward remodeling in WHHLMI rabbits

To clarify the mechanism of coronary outward remodeling, we examined atherosclerotic coronary arteries morphologically using WHHLMI rabbits that develop coronary atherosclerosis spontaneously. Perfusion-fixed coronary segments of WHHLMI rabbits were prepared at 500microm intervals. After immunohistochemical staining and histopathological staining, the areas and lengths of the arterial wall and the lesions were measured. Obvious outward remodeling was observed in coronary sections with greater than 40% cross-sectional narrowing. In coronary sections with greater than 40% cross-sectional narrowing, the tunica media was thick at the shoulder of atheromatous plaque and was thin beneath the atheromatous plaques. Macrophages infiltrated those attenuated tunica media expressed matrix metalloproteinases and oxidized LDL was accumulated in those areas. In those areas, collagen fibers and the internal elastic lamina had disappeared partly and apoptotic smooth muscle cells were observed. Proliferation of smooth muscle cells was observed at the attenuated tunica media and adjacent adventitia. The present results suggest that invasion of atheromatous plaques into the tunica media causes coronary outward remodeling.

Lp(a) enhances coronary atherosclerosis in transgenic Watanabe heritable hyperlipidemic rabbits

Elevated plasma levels of LDL and lipoprotein (a) [Lp(a)] are associated with an increased risk of atherosclerosis and coronary heart disease. However, it is not known whether Lp(a) would enhance the atherogenic effect of LDL on coronary atherosclerosis and myocardial infarction. To address this issue, we cross-bred human Lp(a) transgenic (Tg) rabbits with Watanabe heritable hyperlipidemic (WHHL) rabbits and evaluated the long-term (at the age of 2 years) effects of Lp(a) on the development of coronary atherosclerosis. Compared to non-Tg WHHL rabbits, Tg WHHL rabbits did not show significant changes in plasma total cholesterol, triglycerides, or HDL-C. However, Tg WHHL rabbits showed significantly larger lesions in the right coronary arteries (p<0.05). Immunohistochemical staining revealed that the lesions of Tg WHHL rabbits were enriched in the extracellular matrix contents whereas the cellular components were not different from those in non-Tg WHHL rabbits. Increased atherosclerosis in the coronary arteries in Tg WHHL rabbit hearts was also associated with a higher incidence of chronic ischemia and myocardial infarction. These results suggest that increased plasma levels of Lp(a) enhance coronary atherosclerosis and myocardial infarction in the setting of hypercholesterolemia.