Apoptotic cell death in atherosclerosis

PURPOSE OF REVIEW: Apoptosis is a critical regulator of homeostasis in many tissues, including the vasculature. Apoptosis in atherosclerotic lesions is triggered by inflammatory processes, both via cell-cell contact and by cytokines and oxidized lipids. Apoptosis of vascular smooth muscle cells, endothelial cells and macrophages may promote plaque growth and pro-coagulation and may induce rupture, the major consequence of atherosclerosis in humans. RECENT FINDINGS: Studies over the past year have clearly demonstrated the significance of cell death in atherosclerosis. Some of the key cellular, cytokine and molecular regulators that contribute to the apoptosis of cells within the atherosclerotic lesion have been identified and their mechanism of action elucidated. Other studies have shed some light on the identity of cells whose loss by apoptosis contributes to plaque instability. SUMMARY: The identification of which cell types undergo apoptosis within the atherosclerotic lesion, the extracellular factors that impinge on these cells, and the intracellular mechanisms that govern their demise have begun to be elucidated. This information is critical in the design of further in-vivo experiments such as the exploitation of animal models, and ultimately, in applying this knowledge to clinical practice.     

Sphingomyelin metabolites in vascular cell signaling and atherogenesis

The atherosclerotic lesion most probably develops through a number of cellular events which implicate all vascular cell types and include synthesis of extracellular proteins, cell proliferation, differentiation and death. Sphingolipids and sphingolipid metabolizing enzymes may play important roles in atherogenesis, not only because of lipoprotein alterations but also by mediating a number of cellular events which are believed to be crucial in the development of the vascular lesions such as proliferation or cell death. Exogenous sphingolipids may mediate various biological effects such as apoptosis, mitogenesis or differentiation depending on the cell type. Moreover, several molecules present in the atherogenic lesion, such as oxidized LDL, growth factors or cytokines, which activate intracellular signaling pathways leading to vascular cell modifications, can stimulate sphingomyelin hydrolysis and generation of ceramide (and other metabolites as sphingosine-1-phosphate). Here we review the potential implication of the sphingomyelin/ceramide cycle in vascular cell signaling related to atherosclerosis, and more generally the role of sphingolipids in the events observed during the atherosclerotic process as cell differentiation, migration, adhesion, retraction, proliferation and death.     

Oxidized low-density lipoprotein-induced apoptosis

Cultured cells are able to oxidize low-density lipoproteins (LDL) and oxidized LDL (oxLDL), which are present in atherosclerosis areas, exhibit a variety of biological properties potentially involved in atherogenesis. This review is focused on the toxicity of oxLDL, more precisely on the toxic compounds generated during LDL oxidation, the features and the mechanisms of cell death (apoptosis or necrosis) induced by oxLDL. After internalization, toxic oxidized lipids, namely lipid peroxides, oxysterols and aldehydes, induce modifications of cell proteins, elicit oxidative stress, lipid peroxidation and alter various signaling pathways and gene expression. These events may participate in the toxic effect, and converge to trigger an intense, delayed and sustained calcium peak which elicits either apoptosis or necrosis processes. OxLDL-induced apoptosis involves both mitochondrial and death-receptor (Fas/FasL) apoptotic pathways, thereby activating the classical caspase cascade and subsequent biochemical and morphological apoptotic features. When apoptosis is blocked by overexpression of Bcl-2, oxLDL trigger necrosis through a calcium-dependent pathway. Apoptosis occurring in atherosclerotic areas is potentially involved in endothelial cell lining defects, necrotic core formation and plaque rupture or erosion which may trigger atherothrombotic events. However, the precise role of oxLDL in apoptosis/necrosis occurring in vivo in atherosclerotic plaques remains to be clarified.     

辛伐他汀对动脉粥样硬化大鼠凋亡相关基因Fas及FasL蛋白表达的影响

目的通过研究辛伐他汀对动脉粥样硬化大鼠血管壁中细胞凋亡相关基因Fas及FasL蛋白表达产物的影响,探讨其在预防动脉粥样硬化发生中的可能机制。方法复制动脉粥样硬化大鼠模型,以辛伐他汀干预,取胸主动脉,观察其斑块变化,采用免疫组化Elivision法测定动脉粥样硬化血管壁中Fas、FasL蛋白表达。结果Fas蛋白表达在实验组明显高于对照组及干预组(P<0.01,P<0.05),实验组FasL蛋白表达也明显高于对照组及干预组(P<0.05)。结论Fas及FasL基因通过促进细胞凋亡作用而诱发动脉粥样硬化过程,辛伐他汀可通过调节细胞凋亡过程发挥抗动脉粥样硬化作用。     

Induction of antioxidant stress proteins in vascular endothelial and smooth muscle cells: protective action of vitamin C against atherogenic lipoproteins.

Elevated levels of lipid peroxidation and increased formation of reactive oxygen species within the vascular wall in atherosclerosis can overwhelm cellular antioxidant defence mechanisms. Accumulating evidence implicates oxidatively modified low density lipoproteins (LDL) in vascular dysfunction in atherosclerosis and oxidized LDL have been localized with in atherosclerotic lesions. We here report that human oxidatively modified LDL induce expression of 'antioxidant-like' stress proteins in vascular cells, involving increases in the activity of L-cystine transport, glutathione synthesis, heme oxygenase-1 and the murine stress protein MSP23. Moreover, treatment of human arterial smooth muscle cells with the dietary antioxidant vitamin C markedly attenuates adaptive increases in endogenous antioxidant gene expression and affords protection against smooth muscle cell apoptosis induced by moderately oxidized LDL. As vascular cell death is a key feature of atherosclerotic lesions and may contribute to the plaque 'necrotic' core, cap rupture and thrombosis, our findings suggest that the cytoprotective actions of vitamin C could limit plaque instability in advanced atherosclerosis.     

Induction of antioxidant stress proteins in vascular endothelial and smooth muscle cells: Protective action of vitamin C against atherogenic lipoproteins

Elevated levels of lipid peroxidation and increased formation of reactive oxygen species within the vascular wall in atherosclerosis can overwhelm cellular antioxidant defence mechanisms. Accumulating evidence implicates oxidatively modified low density lipoproteins (LDL) in vascular dysfunction in atherosclerosis and oxidized LDL have been localized with in atherosclerotic lesions. We here report that human oxidatively modified LDL induce expression of ‘antioxidant-like’ stress proteins in vascular cells, involving increases in the activity of l-cystine transport, glutathione synthesis, heme oxygenase-1 and the murine stress protein MSP23. Moreover, treatment of human arterial smooth muscle cells with the dietary antioxidant vitamin C markedly attenuates adaptive increases in endogenous antioxidant gene expression and affords protection against smooth muscle cell apoptosis induced by moderately oxidized LDL. As vascular cell death is a key feature of atherosclerotic lesions and may contribute to the plaque ‘necrotic’ core, cap rupture and thrombosis, our findings suggest that the cytoprotective actions of vitamin C could limit plaque instability in advanced atherosclerosis.     

Endothelial autophagic flux hampers atherosclerotic lesion development

Blood flowing in arteries generates shear forces at the surface of the vascular endothelium that control its anti-atherogenic properties. However, due to the architecture of the vascular tree, these shear forces are heterogeneous and atherosclerotic plaques develop preferentially in areas where shear is low or disturbed. Here we review our recent study showing that elevated shear forces stimulate endothelial autophagic flux and that inactivating the endothelial macroautophagy/autophagy pathway promotes a proinflammatory, prosenescent and proapoptotic cell phenotype despite the presence of atheroprotective shear forces. Specific deficiency in endothelial autophagy in a murine model of atherosclerosis stimulates the development of atherosclerotic lesions exclusively in areas of the vasculature that are normally resistant to atherosclerosis. Our findings demonstrate that adequate endothelial autophagic flux limits atherosclerotic plaque formation by preventing endothelial apoptosis, senescence and inflammation.     

Apoptosis of vascular smooth muscle cells induces features of plaque vulnerability in atherosclerosis

Vascular smooth muscle cell (VSMC) apoptosis occurs in many arterial diseases, including aneurysm formation, angioplasty restenosis and atherosclerosis. Although VSMC apoptosis promotes vessel remodeling, coagulation and inflammation, its precise contribution to these diseases is unknown, given that apoptosis frequently accompanies vessel injury or alterations to flow. To study the direct consequences of VSMC apoptosis, we generated transgenic mice expressing the human diphtheria toxin receptor (hDTR, encoded by HBEGF) from a minimal Tagln (also known as SM22alpha) promoter. Despite apoptosis inducing loss of 50-70% of VSMCs, normal arteries showed no inflammation, reactive proliferation, thrombosis, remodeling or aneurysm formation. In contrast, VSMC apoptosis in atherosclerotic plaques of SM22alpha-hDTR Apoe-/- mice induced marked thinning of fibrous cap, loss of collagen and matrix, accumulation of cell debris and intense intimal inflammation. We conclude that VSMC apoptosis is 'silent' in normal arteries, which have a large capacity to withstand cell loss. In contrast, VSMC apoptosis alone is sufficient to induce features of plaque vulnerability in atherosclerosis. SM22alpha-hDTR Apoe-/- mice may represent an important new model to test agents proposed to stabilize atherosclerotic plaques.     

Overexpression of p73 causes apoptosis in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSM) cell proliferation contributes to the development of atherosclerosis and its associated disorders, including angioplasty restenosis. The tumor-suppressor protein p53 has been linked to the development of atherosclerotic lesions, and its homolog, p73, is proving to have contrasting functions in a variety of tissues. As an outgrowth of our previous finding that p73 is increased in serum-stimulated VSM cells and human atherosclerotic tissue, we examined p73 overexpression in VSM cells to elucidate causality of p73 expression with growth response. Overexpression of p73 results in decreased cell cycle transit and is accompanied by apoptosis. The apoptotic changes in p73 overexpressing VSM cells are independent of p53 and are associated with a decrease in levels of p21(waf1/cip1). In conjunction with our previous data finding that p73 is increased in serum-stimulated VSM cells, this work suggests a role for p73 in vascular proliferative diseases.     

Bcl-2 protein family: Implications in vascular apoptosis and atherosclerosis

Apoptosis has been recognized as a central component in the pathogenesis of atherosclerosis, in addition to the other human pathologies such as cancer and diabetes. The pathophysiology of atherosclerosis is complex, involving both apoptosis and proliferation at different phases of its progression. Oxidative modification of lipids and inflammation differentially regulate the apoptotic and proliferative responses of vascular cells during progression of the atherosclerotic lesion. Bcl-2 proteins act as the major regulators of extrinsic and intrinsic apoptosis signalling pathways and more recently it has become evident that they mediate the apoptotic response of vascular cells in response to oxidation and inflammation either in a provocative or an inhibitory mode of action. Here we address Bcl-2 proteins as major therapeutic targets for the treatment of atherosclerosis and underscore the need for the novel preventive and therapeutic interventions against atherosclerosis, which should be designed in the light of molecular mechanisms regulating apoptosis of vascular cells in atherosclerotic lesions.     

膜片钳技术在动脉粥样硬化研究中的应用

膜片钳技术是一种先进的电生理技术,在生命科学研究中已得到了广泛的应用.最近几年已把它运用于研究动脉粥样硬化血管平滑肌细胞离子通道电生理特性的改变.研究发现血管平滑肌细胞的凋亡与K+通道活动增加有关,在动脉粥样硬化发生与发展过程中大电导型钙激活钾通道起着重要的功能作用.某些药物影响动脉粥样硬化血管平滑肌细胞离子通道而发挥作用.膜片钳技术给动脉粥样硬化发病机理研究带来了新的亮点.     

The multiple actions of angiotensin II in atherosclerosis

Angiotensin II (Ang II), the effector peptide of the renin-angiotensin system, has been implied in the pathogenesis of atherosclerosis on various levels. There is abundant experimental evidence that pharmacological antagonism of Ang II formation by angiotensin converting enzyme inhibition or blockade of the cellular effects of Ang II by angiotensin type 1 receptor blockade inhibits formation and progression of atherosclerotic lesions. Angiotensin promotes generation of oxidative stress in the vasculature, which appears to be a key mediator of Ang II-induced endothelial dysfunction, endothelial cell apoptosis, and lipoprotein peroxidation. Ang II also induces cellular adhesion molecules, chemotactic and proinflammatory cytokines, all of which participate in the induction of an inflammatory response in the vessel wall. In addition, Ang II triggers responses in vascular smooth muscle cells that lead to proliferation, migration, and a phenotypic modulation resulting in production of growth factors and extracellular matrix. While all of these effects contribute to neointima formation and development of atherosclerotic lesions, Ang II may also be involved in acute complications of atherosclerosis by promoting plaque rupture and a hyperthrombotic state. Accordingly, Ang II appears to have a central role in the pathophysiology of atherosclerosis.     

New in vitro model to study high glucose-dependent endothelial dysfunctions

Several thrombogenic abnormalities are associated with diabetes. Since endothelial dysfunction occurs at early stages of disease, it may reflect pathophysiological changes that are responsible for alterations in vascular structure, growth and modifications of adhesivity to platelets and leukocytes, leading to atherosclerosis and thrombosis. Predisposing factors of vascular diseases, such as diabetes, are also associated with endothelial dysfunction. Restoration or replacement of endothelium-related factors like nitric oxide impede the progression of vascular thrombogenic diseases, and prevent the action of vasoconstrictor factors such as endothelin or other prothrombotic factors such as plasminogen-activator inhibitor-1. Since high glucose concentration in blood is the hallmark of diabetes and because the vascular lesions of atherosclerosis are localized in large artheries, we have cultured endothelial cells from the human aorta. Two endothelial cell strains from the same aortic tract that show different characteristics and behavior in high glucose were isolated. Such findings reflect the importance to have well characterized and standardized cell culture systems to carry out experiments to study the glucose-dependent atherosclerotic process in vitro. Our cell strains may represent a useful in vitro model to study the complex pathophysiology of diabetes-related atherosclerosis.     

Cholesterol oxidation products in the vascular remodeling due to atherosclerosis

Like the other oxidation products of the lipid moiety of plasma low density lipoproteins (LDL), cholesterol oxidation products are consistently found within the characteristic lesions of atherosclerosis, both in experimental animals and in man. A growing bulk of evidence suggests that oxysterols make a significant contribution to the vascular remodeling that occurs in atherosclerosis, being involved in various key steps of this complex process: endothelial cell dysfunction, adhesion of circulating blood cells, foam cell and fibrous cap formation, modulation of the extracellular matrix (ECM), vascular cell apoptosis and plaque’s instability. Moreover, oxysterols have been demonstrated to be at least one or two orders of magnitude more reactive than unoxidized cholesterol in exerting pro-inflammatory, pro-apoptotic, and pro-fibrogenic effects. Thus, a pathological level of cholesterol oxidation in the vasculature may be the missing molecular link between hypercholesterolemia and the formation of atherosclerotic lesions.     

ApoE-/-Fas-/- C57BL/6 mice: a novel murine model simultaneously exhibits lupus nephritis, atherosclerosis, and osteopenia

To establish a mouse model of accelerated atherosclerosis in lupus, we generated apolipoprotein E-deficient (apoE(-/-)) and Fas(lpr/lpr) (Fas(-/-)) C57BL/6 mice. On a normal chow diet, 5 month old apoE(-/-)Fas(-/-) mice had enlarged glomerular tuft areas, severe proteinuria, increased circulating autoantibody levels, and increased apoptotic cells in renal and vascular lesions compared with either single knockout mice. Also, double knockout mice developed increased atherosclerotic lesions but decreased serum levels of total and non-HDL cholesterol compared with apoE(-/-)Fas(+/+) littermates. Moreover, female apoE(-/-)Fas(-/-) mice had lower vertebral bone mineral density (BMD) and bone volume density (BV/TV) than age-matched female apoE(-/-)Fas(+/+) mice. Compared with apoE(-/-)Fas(+/+) and apoE(+/+)Fas(-/-) mice, apoE(-/-)Fas(-/-) mice had decreased circulating oxidized phospholipid (OxPL) content on apoB-100 containing lipoprotein particles and increased serum IgG antibodies to OxPL, which were significantly correlated with aortic lesion areas (r = 0.58), glomerular tuft areas (r = 0.87), BMD (r = -0.57), and BV/TV (r = -0.72). These results suggest that the apoE(-/-)Fas(-/-) mouse model might be used to study atherosclerosis and osteopenia in lupus. Correlations of IgG anti-OxPL with lupus-like disease, atherosclerosis, and bone loss suggested a shared pathway of these disease processes.     

Experimental atherosclerosis: a historical overview

Almost one-hundred years ago the first evidence of experimental atherosclerosis was reported. Over the past century, significant advances have been made in the development of animal models of human coronary artery disease. In this minireview, induction of atherosclerotic lesions in several animal models including rodents (mice, rabbits, rats, hamsters, guinea pigs), avian (pigeons, chickens, quail), swine, carnivora (dogs, cats), and non-human primates is discussed. The limitations and advantages of the animal models of atherosclerosis have been summarized. The transgenic/knockout animal models have greatly enhanced our understanding of atherosclerosis. Compared to wild-type counterparts, the knockout/transgenic animals develop atherogenesis faster without a need for a highly atherogenic diet. Although almost all investigations support a causal role for increased plasma cholesterol levels in the development of atherosclerotic vascular disease, an increasing body of evidence indicates serious invqlvement of other factors including oxidative stress, inflammation, infection and other emerging risk factors.