Kinta Hatakeyama

BACKGROUND: The endoplasmic reticulum (ER) responds to various stresses by upregulation of ER chaperones, but prolonged ER stress eventually causes apoptosis. Although apoptosis is considered to be essential for the progression and rupture of atherosclerotic plaques, the influence of ER stress and apoptosis on rupture of unstable coronary plaques remains unclear. METHODS AND RESULTS: Coronary artery segments were obtained at autopsy from 71 patients, and atherectomy specimens were obtained from 40 patients. Smooth muscle cells and macrophages in the fibrous caps of thin-cap atheroma and ruptured plaques, but not in the fibrous caps of thick-cap atheroma and fibrous plaques, showed a marked increase of ER chaperone expression and apoptotic cells. ER chaperones also showed higher expression in atherectomy specimens from patients with unstable angina pectoris than in specimens from those with stable angina. Expression of 7-ketocholesterol was increased in the fibrous caps of thin-cap atheroma compared with thick-cap atheroma. Treatment of cultured coronary artery smooth muscle cells or THP-1 cells with 7-ketocholesterol induced upregulation of ER chaperones and apoptosis, whereas these changes were prevented by antioxidants. We also investigated possible signaling pathways for ER-initiated apoptosis and found that the CHOP (a transcription factor induced by ER stress)-dependent pathway was activated in unstable plaques. In addition, knockdown of CHOP expression by small interfering RNA decreased ER stress-dependent death of cultured coronary artery smooth muscle cells and THP-1 cells. CONCLUSIONS: Increased ER stress occurs in unstable plaques. Our findings suggest that ER stress-induced apoptosis of smooth muscle cells and macrophages may contribute to plaque vulnerability.

Abdominal aortic aneurysm (AAA) is histologically characterized by medial degeneration and various degrees of chronic adventitial inflammation, although the mechanisms for progression of aneurysm are poorly understood. In the present study, we carried out histological study of AAA tissues of patients, and interventional animal and cell culture experiments to investigate a role of mast cells in the pathogenesis of AAA. The number of mast cells was found to increase in the outer media or adventitia of human AAA, showing a positive correlation between the cell number and the AAA diameter. Aneurysmal dilatation of the aorta was seen in the control (+/+) rats following periaortic application of calcium chloride (CaCl2) treatment but not in the mast cell-deficient mutant Ws/Ws rats. The AAA formation was accompanied by accumulation of mast cells, T lymphocytes and by activated matrix metalloproteinase 9, reduced elastin levels and augmented angiogenesis in the aortic tissue, but these changes were much less in the Ws/Ws rats than in the controls. Similarly, mast cells were accumulated and activated at the adventitia of aneurysmal aorta in the apolipoprotein E-deficient mice. The pharmacological intervention with the tranilast, an inhibitor of mast cell degranulation, attenuated AAA development in these rodent models. In the cell culture experiment, a mast cell directly augmented matrix metalloproteinase 9 activity produced by the monocyte/macrophage. Collectively, these data suggest that adventitial mast cells play a critical role in the progression of AAA.

BACKGROUND: Macrophage metalloelastase (matrix metalloproteinase [MMP]-12) is upregulated in atherosclerotic lesions and aneurysm; thus, increased MMP-12 activity may play an important role in the pathogenesis of atherosclerosis. However, the pathological roles of MMP-12 in the initiation and progression of atherosclerosis have not been defined. METHODS AND RESULTS: We compared the susceptibility of MMP-12 transgenic (Tg) rabbits to cholesterol-rich diet-induced atherosclerosis with that of non-Tg littermate rabbits. The rabbits were maintained at either relatively lower levels of hypercholesterolemia for shorter periods or higher levels of hypercholesterolemia for longer periods through a diet containing different amounts of cholesterol. We found no significant difference in the aortic atherosclerotic lesion size or quality between Tg and non-Tg rabbits at lower hypercholesterolemia. At higher hypercholesterolemia for longer periods, however, Tg rabbits developed more extensive atherosclerosis in the aortas and coronary arteries than did non-Tg rabbits. Histological examinations revealed that atherosclerotic lesions of Tg rabbits contained prominent macrophage infiltration associated with marked disruption of the elastic lamina in the tunica media with occasional formation of aneurysm-like lesions. Furthermore, increased expression of MMP-12 derived from macrophages was associated with elevated expression of MMP-3, suggesting that MMP-12 may play a pivotal role in the cascade activation of other MMPs, thereby exacerbating extracellular matrix degradation during the progression of atherosclerosis. CONCLUSIONS: Overexpression of MMP-12 causes accelerated atherosclerosis in Tg rabbits. These results suggest that macrophage-derived MMP-12 participates in the progression of atherosclerosis.

Atherothrombosis is a leading cause of cardiovascular mortality and morbidity worldwide. The underlying mechanisms of atherothrombosis comprise plaque disruption and subsequent thrombus formation. Arterial thrombi are thought to mainly comprise aggregated platelets as a result of high blood velocity. However, thrombi that develop on disrupted plaques comprise not only aggregated platelets, but also large amounts of fibrin, because plaques contain large amount of tissue factor that activate the coagulation cascade. Since not all thrombi grow large enough to occlude the vascular lumen, the propagation of thrombi is also critical in the onset of adverse vascular events. Various factors such as vascular wall thrombogenicity, local hemorheology, systemic thrombogenicity and fibrinolytic activity modulate thrombus formation and propagation. Although the activation mechanisms of platelets and the coagulation cascade have been intensively investigated, the underlying mechanisms of occlusive thrombus formation on disrupted plaques remain obscure. Pathological findings derived from humans and animal models of human atherothrombosis have uncovered pathophysiological processes during thrombus formation and propagation after plaque disruption, and novel factors have been identified that modulate the activation of platelets and the coagulation cascade. These findings have also provided insights into the development of novel drugs for atherothrombosis.