Smoking promotes pathogenesis of aortic aneurysm through the 5-lipoxygenase pathway

Study of the Biomechanical and Histological Properties of the Abdominal Aorta of Diabetic Rats Exposed to Cigarette Smoke

INTRODUCTION

In spite of the great relevance of abdominal aortic aneurysm, its etiopathogenesis is not fully understood. The biomechanical and histological study of the aortic wall may contribute to this elucidation.

METHODS

Seventy-five male Wistar rats were divided into 4 groups: control (CG), smoker (SG), diabetic (DG), and diabetic + smoker (DSG). The SG and DSG rats were exposed to cigarette smoke for 30 min/day, 5 days a week. Diabetes was induced by the intravenous injection of streptozotocin. After 16 weeks, the abdominal aorta was collected for biomechanical, histological, and matrix metalloproteinase 2 (MMP-2) activity analyses.

RESULTS

The valid biomechanical tests of 52 specimens were analyzed: 11 in the CG, 10 in the DG, 16 in the SG, and 15 in the DSG. The biomechanical analysis of the fragments showed no differences between the control, DG, SG, and DSG. Collagen deposition also did not present a significant difference between the studied groups. The total count of elastic fibers was higher in diabetic rats (DG and DSG) than in the SG. The inflammatory response observed in all experimental groups was significantly more intense than in the CG. Compared to the DSG, MMP-2 activity showed a significant decrease in the DG.

CONCLUSIONS

Resistance and elasticity did not present a difference between the CG and the DG, SG, and DSG. Compared to the CG, the total count of elastic fibers, fragmentation of the elastic lamina, pericellular matrix deposition, and cell loss/substitution in the tunica media showed significant alterations in the aortic walls of the DG, SG, and DSG. MMP-2 activity was lower in the DG aorta than in the DSG aorta.

Genetic and Epigenetic Regulation of Aortic Aneurysms

Aneurysms are characterized by structural deterioration of the vascular wall leading to progressive dilatation and, potentially, rupture of the aorta. While aortic aneurysms often remain clinically silent, the morbidity and mortality associated with aneurysm expansion and rupture are considerable. Over 13,000 deaths annually in the United States are attributable to aortic aneurysm rupture with less than 1 in 3 persons with aortic aneurysm rupture surviving to surgical intervention. Environmental and epidemiologic risk factors including smoking, male gender, hypertension, older age, dyslipidemia, atherosclerosis, and family history are highly associated with abdominal aortic aneurysms, while heritable genetic mutations are commonly associated with aneurysms of the thoracic aorta. Similar to other forms of cardiovascular disease, family history, genetic variation, and heritable mutations modify the risk of aortic aneurysm formation and provide mechanistic insight into the pathogenesis of human aortic aneurysms. This review will examine the relationship between heritable genetic and epigenetic influences on thoracic and abdominal aortic aneurysm formation and rupture.

The Apoe(-/-) mouse model: a suitable model to study cardiovascular and respiratory diseases in the context of cigarette smoke exposure and harm reduction

Atherosclerosis-prone apolipoprotein E-deficient (Apoe(-/-)) mice display poor lipoprotein clearance with subsequent accumulation of cholesterol ester-enriched particles in the blood, which promote the development of atherosclerotic plaques. Therefore, the Apoe(-/-) mouse model is well established for the study of human atherosclerosis. The systemic proinflammatory status of Apoe(-/-) mice also makes them good candidates for studying chronic obstructive pulmonary disease, characterized by pulmonary inflammation, airway obstruction, and emphysema, and which shares several risk factors with cardiovascular diseases, including smoking. Herein, we review the results from published studies using Apoe(-/-) mice, with a particular focus on work conducted in the context of cigarette smoke inhalation studies. The findings from these studies highlight the suitability of this animal model for researching the effects of cigarette smoking on atherosclerosis and emphysema.

Pathogenesis of abdominal aortic aneurysms: role of nicotine and nicotinic acetylcholine receptors

Inflammation, proteolysis, smooth muscle cell apoptosis, and angiogenesis have been implicated in the pathogenesis of abdominal aortic aneurysms (AAAs), although the well-defined initiating mechanism is not fully understood. Matrix metalloproteinases (MMPs) such as MMP-2 and -9 and other proteinases degrading elastin and extracellular matrix are the critical pathogenesis of AAAs. Among the risk factors of AAAs, cigarette smoking is an irrefutable one. Cigarette smoke is practically involved in various aspects of the AAA pathogenesis. Nicotine, a major alkaloid in tobacco leaves and a primary component in cigarette smoke, can stimulate the MMPs expression by vascular SMCs, endothelial cells, and inflammatory cells in vascular wall and induce angiogenesis in the aneurysmal tissues. However, for the inflammatory and apoptotic processes in the pathogenesis of AAAs, nicotine seems to be moving in just the opposite direction. Additionally, the effects of nicotine are probably dose dependent or associated with the exposure duration and may be partly exerted by its receptors—nicotinic acetylcholine receptors (nAChRs). In this paper, we will mainly discuss the pathogenesis of AAAs involving inflammation, proteolysis, smooth muscle cell apoptosis and angiogenesis, and the roles of nicotine and nAChRs.

Cigarette smoke enhances abdominal aortic aneurysm formation in angiotensin II-treated apolipoprotein E-deficient mice

Cigarette smoke, hyperlipidemia, and hypertension with the risk of development and progression of atherosclerosis and associated pathologies such as abdominal aortic aneurysm (AAA) are correlated. We examined the interaction of cigarette mainstream smoke (MS) and angiotensin-II (Ang II)-induced hypertension in the atherosclerotic process using hyperlipidemic apolipoprotein E-knockout (ApoE(-/-)) mice. ApoE(-/-) mice were treated with Ang II for 4 weeks and then further exposed to MS or to fresh air for 4 weeks. AAA formation was observed in all mice treated with Ang II, regardless of smoke exposure; however, smoke exposure increased the incidence of AAA in these mice. Ang II treatment resulted in higher gene expression of matrix metalloproteinases (MMP)-2, -3, -8, -9, and -12 in the abdominal aortas, which was further increased by MS exposure. The proteolytic activity of MMP-2 and MMP-9 was also enhanced in Ang II-treated mice exposed to MS, but only minor changes were seen with either smoke exposure or Ang II treatment alone. This study shows for the first time that both formation and severity of AAA in hypertensive ApoE(-/-) mice are accelerated by exposure to MS and that the proteolytic activity of MMPs is enhanced by the combination of Ang II and MS.

Genetic and epigenetic mechanisms and their possible role in abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a common disease associated with significant cardiovascular morbidity and mortality. The pathogenesis of AAA is poorly defined, making targeting of new therapies problematic. Current evidence favours an interaction of multiple environmental and genetic factors in the initiation and progression of AAA. Epigenetics is the term used to define the properties of the genome that are not explained by the primary sequence, but are due to the modifications of DNA and/or associated proteins. Previous research indicates the association of gene specific promoter DNA hyper-methylation and global DNA hypo-methylation with atherosclerosis. Evidence also suggests an important role for epigenetic processes such as histone acetylation in cardiovascular diseases including atherosclerosis and restenosis. Altered DNA methylation or histone acetylation occur in inflammation, cellular proliferation and remodelling processes and therefore maybe relevant to the pathology of AAA. Important risk factors for AAA, including cigarette smoking, older age, male gender and hypertension, have been linked with epigenetic effects and thus could act in this way to promote AAA. In this review, we discuss the potential role of epigenetic mechanisms in AAA. Since epigenetic alterations are to some extent reversible, further study of this area may identify new treatment targets for AAA.

Adventitial mast cells contribute to pathogenesis in the progression of abdominal aortic aneurysm

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.

Do Different Vascular Risk Factors Affect All Arteries Equally?

Established vascular risk factors (ie, smoking, hypertension, diabetes mellitus, and dyslipidemia) play an important role in the development of vascular disease. Emerging evidence suggests that some of these risk factors may have a more intense effect on specific arterial beds, a finding that holds implications for a prognostic role for certain types of vascular disease.

Pharmacological nature of nicotine-induced contraction in the rat basilar artery: Involvement of arachidonic acid metabolites

The pharmacological nature of nicotine-induced contraction in the rat basilar artery is poorly understood. The purpose of this study was to investigate the endothelium dependency and involvement of arachidonic acid metabolites in nicotine-induced contraction in the rat basilar artery. The rat basilar artery was removed from the brain and cut into a spiral preparation. Nicotine (3x10(-5) to 10(-2) M) induced the concentration-dependent contraction in the rat basilar artery, and the maximal contraction was obtained at 3x10(-3) M. The contraction induced by nicotine (3x10(-3) M) was significantly attenuated by the presence of saponin (0.05 mg/ml, 15 min). Phospholipase C (PLC) inhibitors (NCDC and U-73122), calcium-independent phospholipase A(2) (iPLA(2)) inhibitor (BEL), cyclooxygenase-2 (COX-2) inhibitors (nimesulide, L-745,337 and celecoxib), and a 5-lipoxygenase (5-LOX) inhibitor (ZM-230487) concentration-dependently attenuated the nicotine-induced contraction. A cytosolic phospholipase A(2) (cPLA(2)) inhibitor (AACOCF3), secretory phospholipase A(2) (sPLA(2)) inhibitor (indoxam), and cyclooxygenase-1 (COX-1) inhibitors (flurbiprofen and ketoprofen) did not affect the nicotine-induced contraction. From these results, it was suggested that nicotine-induced contraction in the rat basilar artery is endothelium-dependent and is due to arachidonic acid metabolites.

Effect of smoking on abdominal aortic aneurysms: novel insights through murine models

Abdominal aortic aneurysm is a poorly understood and fatal disease. The etiology of the disease is believed to be multifactorial. Of all the recognized clinical associations, none has a greater impact on the incidence and progression of the disease than exposure to tobacco smoke. Novel murine models developed over the past several years present the opportunity to investigate the mechanism of this critical clinical relationship.

Smoking and Aortic Diseases

Cigarette smoking is a major vascular risk factor and in this context, it is an independent risk factor for the development of aortic disease, especially the formation and growth of abdominal aortic aneurysms (AAA). Medline was searched up to January 31, 2007 for the relevant literature for this review of the mechanisms by which smoking causes aortic wall damage and its subsequent impact on the clinical manifestation of this process. Idiopathic AAAs and aortic dissection are considered, as well as other aortic diseases (eg, Takayasu, Kawasaki, Behcet and Buerger). There is evidence suggesting an abnormal homeostasis between proteolytic and antiproteolytic activity in the vascular wall during the development of AAAs, and these mechanisms can be influenced by smoking. Smoking cessation plays an important role in the management of aortic disease.

Abdominal aortic aneurysms: an underestimated type of immune-mediated large vessel arteritis?

PURPOSE OF REVIEW

To review recent studies on the immune-mediated pathogenesis of abdominal aortic aneurysms, opening a wide field for possible new therapeutic approaches.

RECENT FINDINGS

Immune-mediated processes including involvement of neutrophils, interferon-gamma producing T cells and proinflammatory cytokines play an important role especially in the initiation of abdominal aortic aneurysm disease. C-reactive protein was associated with aneurysm size and is possibly produced by the aneurysmal tissue itself. From the clinical perspective, both inflammatory and noninflammatory abdominal aortic aneurysms are associated with various autoimmune diseases. Preliminary data of F-FDG positron emission tomography imaging of abdominal aortic aneurysms suggest focal uptake of F-FDG within the aneurysm wall in patients with either large, rapidly expanding or symptomatic aneurysms that are prone to rupture. Thus basic research findings and clinical research focusing on the underlying immune-mediated mechanisms of abdominal aortic aneurysms will likely pave the way for new medical therapies in the future. In animal models the effects of rapamycin as an immunosuppressive agent, modulation of estrogen receptors by tamoxifen as well as gene therapy using decoy oligonucleotides binding to the transcription factor ets has already proved helpful in decreasing aneurysm expansion rates.

SUMMARY

Pathophysiological, immunogenetical and interventional studies support the concept of abdominal aortic aneurysm as an immune-mediated process, which will help to identify more laboratory and imaging signs of development in the future. Further research will now assess the possible benefit of antiinflammatory therapeutic approaches, especially in patients with small abdominal aortic aneurysms.