Radiation-Induced Atherosclerotic Plaque Progression in a Hypercholesterolemic Rabbit

Carotid Artery Stenosis and Ischemic Stroke in Patients With Head and Neck Cancer Treated With Radiation Therapy: A Critical Review

The purpose of this review is to summarize the literature on carotid artery stenosis (CAS) and ischemic stroke (IS) in patients with head and neck cancer (HNC) treated with radiation therapy (RT) to guide assessment, screening, and management strategies. Patients treated with RT for HNC are at an elevated risk of developing CAS, with published meta-analyses demonstrating that CAS >50% occurs in approximately 25% of patients. Previous research suggests a 10-year cumulative incidence of stroke between 5.7% and 12.5%. Cardiovascular disease (CVD) risk prediction tools such as Qstroke, QRISK-2, and Framingham risk score perform poorly for predicting IS for patients with HNC who received RT. Duplex ultrasound is the most common imaging modality to assess CAS, but controversy remains as to the utility of screening asymptomatic individuals. Only 3 of the 5 major HNC survivorship guidelines acknowledge RT as a risk factor for CAS or IS, while only 1 makes a specific recommendation on screening for CAS (American Head and Neck Society). Within the general population, only 1 CVD guideline discusses RT as a risk factor for CAS (Society for Vascular Surgery). Clinicians involved in the care of patients with HNC treated with RT should be aware of the increased risk of CAS and IS and the challenges in risk prediction. Although there is a lack of evidence to make firm recommendations, HNC survivorship recommendations should ensure HNC survivors and primary care providers are informed of these risks and the importance of assessment and management of CVD risk factors. Future studies are required to refine risk prediction models in patients with HNC and to determine those most likely to benefit from targeted screening and initiation of early preventative strategies.

Radiation-Induced Vascular Disease-A State-of-the-Art Review

Since the 1990s, there has been a steady increase in the number of cancer survivors to an estimated 17 million in 2019 in the US alone. Radiation therapy today is applied to a variety of malignancies and over 50% of cancer patients. The effects of ionizing radiation on cardiac structure and function, so-called radiation-induced heart disease (RIHD), have been extensively studied. We review the available published data on the mechanisms and manifestations of RIHD, with a focus on vascular disease, as well as proposed strategies for its prevention, screening, diagnosis, and management.

Ionizing radiation-induced circulatory and metabolic diseases

Risks to health are the prime consideration in all human situations of ionizing radiation exposure and therefore of relevance to radiation protection in all occupational, medical, and public exposure situations. Over the past few decades, advances in therapeutic strategies have led to significant improvements in cancer survival rates. However, a wide range of long-term complications have been reported in cancer survivors, in particular circulatory diseases and their major risk factors, metabolic diseases. However, at lower levels of exposure, the evidence is less clear. Under real-life exposure scenarios, including radiotherapy, radiation effects in the whole organism will be determined mainly by the response of normal tissues receiving relatively low doses, and will be mediated and moderated by systemic effects. Therefore, there is an urgent need for further research on the impact of low-dose radiation. In this article, we review radiation-associated risks of circulatory and metabolic diseases in clinical, occupational or environmental exposure situations, addressing epidemiological, biological, risk modelling, and systems biology aspects, highlight the gaps in knowledge and discuss future directions to address these gaps.

A Novel Anticancer Therapeutic Strategy to Target Autophagy Accelerates Radiation-Associated Atherosclerosis

PURPOSE

Autophagy inhibition is a novel therapeutic strategy suggested for patients with advanced cancer, especially those who have undergone radiation therapy. In the present study, we investigated whether autophagy inhibitors accelerate the progression of radiation-associated atherosclerosis (RAA).

METHODS AND MATERIALS

Eight-week-old apolipoprotein (ApoE^-/-) mice were fed a Western diet, and their left common carotid arteries were partially ligated to induce atherogenesis. Four weeks later, local ionizing radiation (IR) at a dose of 5 or 10 Gy was used to induce RAA in the left common carotid artery. After another 4 weeks, severe plaque burden associated with increased macrophage infiltration and lipid deposition, reduced smooth muscle cells, and decreased collagen expression was observed. In addition, these changes occurred in a dose-dependent manner. Improved autophagic flux caused by IR was observed in both macrophages of the atherosclerotic plaque and peritoneal macrophages in vitro. The inhibition of autophagic flux by chloroquine (50 mg/kg/d) further accelerated the progression of RAA in the left common carotid arteries of ApoE^-/- mice. Furthermore, chloroquine treatment exacerbated IR-induced p65 nuclear translocation, IκBα degradation, and transcription of nuclear factor-κB (NF-κB) target genes in peritoneal macrophages.

CONCLUSIONS

IR promotes atherogenesis and increases autophagic flux. In addition, autophagy inhibition by chloroquine accelerates the progression of RAA lesions by stimulating NF-κB-mediated inflammatory responses in macrophages.

Mechanisms of vascular dysfunction evoked by ionizing radiation and possible targets for its pharmacological correction

Ionizing radiation (IR) leads to a variety of the cardiovascular diseases, including the arterial hypertension. A number of studies have demonstrated that blood vessels represent important target for IR, and the endothelium is one of the most vulnerable components of the vascular wall. IR causes an inhibition of nitric oxide (NO)-mediated endothelium-dependent vasodilatation and generation of reactive oxygen (ROS) and nitrogen (RNS) species trigger this process. Inhibition of NO-mediated vasodilatation could be due to endothelial NO synthase (eNOS) down-regulation, inactivation of endothelium-derived NO, and abnormalities in diffusion of NO from the endothelial cells (ECs) leading to a decrease in NO bioavailability. Beside this, IR suppresses endothelial large conductance Ca²⁺-activated K⁺ channels (BK_Ca) activity, which control NO synthesis. IR also leads to inhibition of the BK_Ca current in vascular smooth muscle cells (SMCs) which is mediated by protein kinase C (PKC). On the other hand, IR-evoked enhanced vascular contractility may result from PKC-mediated increase in SMCs myofilament Ca²⁺ sensitivity. Also, IR evokes vascular wall inflammation and atherosclerosis development. Vascular function damaged by IR can be effectively restored by quercetin-filled phosphatidylcholine liposomes and mesenchymal stem cells injection. Using RNA-interference technique targeted to different PKC isoforms can also be a perspective approach for pharmacological treatment of IR-induced vascular dysfunction.

Chronic Exposure to External Low-Dose Gamma Radiation Induces an Increase in Anti-inflammatory and Anti-oxidative Parameters Resulting in Atherosclerotic Plaque Size Reduction in ApoE-/- Mice

Populations living in radiation-contaminated territories, such as Chernobyl and Fukushima, are chronically exposed to external gamma radiation and internal radionuclide contamination due to the large amount of ¹³⁷Cs released in the environment. The effect of chronic low-dose exposure on the development of cardiovascular diseases remains unclear. Previously reported studies have shown that low-dose radiation exposure could lead to discrepancies according to dose rate. In this study, we examined the effect of very low-dose and dose-rate chronic external exposure on atherosclerosis development. ApoE^-/- mice were chronically irradiated with a gamma source for 8 months at two different dose rates, 12 and 28 μGy/h, equivalent to dose rates measured in contaminated territories, with a cumulative dose of 67 and 157 mGy, respectively. We evaluated plaque size and phenotype, inflammatory profile and oxidative stress status. The results of this study showed a decrease in plaque sizes and an increase in collagen content in ApoE^-/- mice exposed to 28 μGy/h for 8 months compared to nonexposed animals. The plaque phenotype was associated with an increase in anti-inflammatory and anti-oxidative gene expression. These results suggest that chronic low-dose gamma irradiation induces an upregulation of organism defenses leading to a decrease in inflammation and plaque size. To our knowledge, this is the first study to describe the possible effect of chronic external very low-dose ionizing radiation exposure for 8 months. This work could help to identify the potential existence of a dose threshold, below that which harmful effects are not exhibited and beneficial effects are potentially observed. Furthermore, these findings permit consideration of the importance of dose rate in radiation protection.

Pathogenesis and Prevention of Radiation-induced Myocardial Fibrosis

Radiation therapy is one of the most important methods for the treatment of malignant tumors. However, in radiotherapy for thoracic tumors such as breast cancer, lung cancer, esophageal cancer, and mediastinal lymphoma, the heart, located in the mediastinum, is inevitably affected by the irradiation, leading to pericardial disease, myocardial fibrosis, coronary artery disease, valvular lesions, and cardiac conduction system injury, which are considered radiation-induced heart diseases. Delayed cardiac injury especially myocardial fibrosis is more prominent, and its incidence is as high as 20–80%. Myocardial fibrosis is the final stage of radiation-induced heart diseases, and it increases the stiffness of the myocardium and decreases myocardial systolic and diastolic function, resulting in myocardial electrical physiological disorder, arrhythmia, incomplete heart function, or even sudden death. This article reviews the pathogenesis and prevention of radiation-induced myocardial fibrosis for providing references for the prevention and treatment of radiation-induced myocardial fibrosis.

Ionizing radiation and heart risks

Cardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. As advancements in radiation therapy (RT) have significantly increased the number of cancer survivors, the risk of radiation-induced cardiovascular disease (RICD) in this group is a growing concern. Recent epidemiological data suggest that accidental or occupational exposure to low dose radiation, in addition to therapeutic ionizing radiation, can result in cardiovascular complications. The progression of radiation-induced cardiotoxicity often takes years to manifest but is also multifaceted, as the heart may be affected by a variety of pathologies. The risk of cardiovascular disease development in RT cancer survivors has been known for 40 years and several risk factors have been identified in the last two decades. However, most of the early work focused on clinical symptoms and manifestations, rather than understanding cellular processes regulating homeostatic processes of the cardiovascular system in response to radiation. Recent studies have suggested that a different approach may be needed to refute the risk of cardiovascular disease following radiation exposure. In this review, we will focus on how different radiation types and doses may induce cardiovascular complications, highlighting clinical manifestations and the mechanisms involved in the pathophysiology of radiation-induced cardiotoxicity. We will finally discuss how current and future research on heart development and homeostasis can help reduce the incidence of RICD.

Chronic Internal Exposure to Low Dose 137Cs Induces Positive Impact on the Stability of Atherosclerotic Plaques by Reducing Inflammation in ApoE-/- Mice

After Chernobyl and Fukushima Daï Chi, two major nuclear accidents, large amounts of radionuclides were released in the environment, mostly caesium 137 (137Cs). Populations living in contaminated territories are chronically exposed to radionuclides by ingestion of contaminated food. However, questions still remain regarding the effects of low dose ionizing radiation exposure on the development and progression of cardiovascular diseases. We therefore investigated the effects of a chronic internal exposure to 137Cs on atherosclerosis in predisposed ApoE-/- mice. Mice were exposed daily to 0, 4, 20 or 100 kBq/l 137Cs in drinking water, corresponding to range of concentrations found in contaminated territories, for 6 or 9 months. We evaluated plaque size and phenotype, inflammatory profile, and oxidative stress status in different experimental groups. Results did not show any differences in atherosclerosis progression between mice exposed to 137Cs and unexposed controls. However, 137Cs exposed mice developed more stable plaques with decreased macrophage content, associated with reduced aortic expression of pro-inflammatory factors (CRP, TNFα, MCP-1, IFNγ) and adhesion molecules (ICAM-1, VCAM-1 and E-selectin). Lesions of mice exposed to 137Cs were also characterized by enhanced collagen and smooth muscle cell content, concurrent with reduced matrix metalloproteinase MMP8 and MMP13 expression. These results suggest that low dose chronic exposure of 137Cs in ApoE-/- mice enhances atherosclerotic lesion stability by inhibiting pro-inflammatory cytokine and MMP production, resulting in collagen-rich plaques with greater smooth muscle cell and less macrophage content.

Stem cell therapies for the treatment of radiation-induced normal tissue side effects

SIGNIFICANCE

Targeted irradiation is an effective cancer therapy but damage inflicted to normal tissues surrounding the tumor may cause severe complications. While certain pharmacologic strategies can temper the adverse effects of irradiation, stem cell therapies provide unique opportunities for restoring functionality to the irradiated tissue bed.

RECENT ADVANCES

Preclinical studies presented in this review provide encouraging proof of concept regarding the therapeutic potential of stem cells for treating the adverse side effects associated with radiotherapy in different organs. Early-stage clinical data for radiation-induced lung, bone, and skin complications are promising and highlight the importance of selecting the appropriate stem cell type to stimulate tissue regeneration.

CRITICAL ISSUES

While therapeutic efficacy has been demonstrated in a variety of animal models and human trials, a range of additional concerns regarding stem cell transplantation for ameliorating radiation-induced normal tissue sequelae remain. Safety issues regarding teratoma formation, disease progression, and genomic stability along with technical issues impacting disease targeting, immunorejection, and clinical scale-up are factors bearing on the eventual translation of stem cell therapies into routine clinical practice.

FUTURE DIRECTIONS

Follow-up studies will need to identify the best possible stem cell types for the treatment of early and late radiation-induced normal tissue injury. Additional work should seek to optimize cellular dosing regimes, identify the best routes of administration, elucidate optimal transplantation windows for introducing cells into more receptive host tissues, and improve immune tolerance for longer-term engrafted cell survival into the irradiated microenvironment.

Animal models of atherosclerosis and magnetic resonance imaging for monitoring plaque progression

Atherosclerosis, the main cause of heart attack and stroke, is the leading cause of death in most modern countries. Preventing clinical events depends on a better understanding of the mechanism of atherosclerotic plaque destabilization. Our knowledge on the characteristics of vulnerable plaques in humans has grown past decades. Histological studies have provided a precise definition of high-risk lesions and novel imaging methods for human atherosclerotic plaque characterization have made significant progress. However the pathological mechanisms leading from stable lesions to the formation of vulnerable plaques remain uncertain and the related clinical events are unpredictable. An animal model mimicking human plaque destablization is required as well as an in vivo imaging method to assess and monitor atherosclerosis progression. Magnetic resonance imaging (MRI) is increasingly used for in vivo assessment of atherosclerotic plaques in the human carotids. MRI provides well-characterized morphological and functional features of human atherosclerotic plaque which can be also assessed in animal models. This review summarizes the most common species used as animal models for experimental atherosclerosis, the techniques to induce atherosclerosis and to obtain vulnerable plaques, together with the role of MRI for monitoring atherosclerotic plaques in animals.

Understanding radiation-induced cardiovascular damage and strategies for intervention

There is a clear association between therapeutic doses of thoracic irradiation and an increased risk of cardiovascular disease (CVD) in cancer survivors, although these effects may take decades to become symptomatic. Long-term survivors of Hodgkin's lymphoma and childhood cancers have two-fold to more than seven-fold increased risks for late cardiac deaths after total tumour doses of 30-40 Gy, given in 2 Gy fractions, where large volumes of heart were included in the field. Increased cardiac mortality is also seen in women irradiated for breast cancer. Breast doses are generally 40-50 Gy in 2 Gy fractions, but only a small part of the heart is included in the treatment fields and mean heart doses rarely exceeded 10-15 Gy, even with older techniques. The relative risks of cardiac mortality (1.1-1.4) are consequently lower than for Hodgkin's lymphoma survivors. Some epidemiological studies show increased risks of cardiac death after accidental or environmental total body exposures to much lower radiation doses. The mechanisms whereby these cardiac effects occur are not fully understood and different mechanisms are probably involved after high therapeutic doses to the heart, or part of the heart, than after low total body exposures. These various mechanisms probably result in different cardiac pathologies, e.g. coronary artery atherosclerosis leading to myocardial infarct, versus microvascular damage and fibrosis leading to congestive heart failure. Experimental studies can help to unravel some of these mechanisms and may identify suitable strategies for managing or inhibiting CVD. In this overview, the main epidemiological and clinical evidence for radiation-induced CVD is summarised. Experimental data shedding light on some of the underlying pathologies and possible targets for intervention are also discussed.

A new murine model of stress-induced complex atherosclerotic lesions

The primary purpose of this investigation was to determine whether ApoE^−/− mice, when subjected to chronic stress, exhibit lesions characteristic of human vulnerable plaque and, if so, to determine the time course of such changes. We found that the lesions were remarkably similar to human vulnerable plaque, and that the time course of lesion progression raised interesting insights into the process of plaque development. Lard-fed mixed-background ApoE^−/− mice exposed to chronic stress develop lesions with large necrotic core, thin fibrous cap and a high degree of inflammation. Neovascularization and intraplaque hemorrhage are observed in over 80% of stressed animals at 20 weeks of age. Previously described models report a prevalence of only 13% for neovascularization observed at a much later time point, between 36 and 60 weeks of age. Thus, our new stress-induced model of advanced atherosclerotic plaque provides an improvement over what is currently available. This model offers a tool to further investigate progression of plaque phenotype to a more vulnerable phenotype in humans. Our findings also suggest a possible use of this stress-induced model to determine whether therapeutic interventions have effects not only on plaque burden, but also, and importantly, on plaque vulnerability.

Mechanisms and dose-response relationships for radiation-induced cardiovascular disease

Epidemiological studies have shown a clear association between therapeutic doses of thoracic irradiation and increased risk of cardiovascular disease in long-term cancer survivors. Survivors of Hodgkin's lymphoma and childhood cancers, for example, show 2- to >7-fold increases in risk of cardiac death after total tumour doses of 30-40 Gy, given in 2-Gy fractions. The risk of cardiac mortality increases linearly with dose, although there are large uncertainties for mean cardiac doses <5 Gy. Experimental studies show that doses of ≥ 2 Gy induce the expression of inflammatory and thrombotic molecules in endothelial cells. In the heart, this causes progressive loss of capillaries and eventually leads to reduced perfusion, myocardial cell death, and fibrosis. In large arteries, doses of ≥ 8 Gy, combined with elevated cholesterol, initiates atherosclerosis and predisposes to the formation of inflammatory, unstable lesions, which are prone to rupture and may cause a fatal heart attack or stroke. In contrast, doses <1 Gy inhibit inflammatory cell adhesion to endothelial cells and inhibit the development of atherosclerosis in mice. It seems likely that mechanisms other than accelerated atherosclerosis are responsible for cardiovascular effects after low total-body exposures of radiation (e.g. impaired T-cell immunity or persistent increase in systemic cytokines).

Low-dose radiation exposure and atherosclerosis in ApoE⁻/⁻ mice

The hypothesis that single low-dose exposures (0.025-0.5 Gy) to low-LET radiation given at either high (about 150 mGy/min) or low (1 mGy/min) dose rate would promote aortic atherosclerosis was tested in female C57BL/6J mice genetically predisposed to this disease (ApoE⁻/⁻). Mice were exposed either at an early stage of disease (2 months of age) and examined 3 or 6 months later or at a late stage of disease (8 months of age) and examined 2 or 4 months later. Changes in aortic lesion frequency, size and severity as well as total serum cholesterol levels and the uptake of lesion lipids by lesion-associated macrophages were assessed. Statistically significant changes in each of these measures were observed, depending on dose, dose rate and disease stage. In all cases, the results were distinctly non-linear with dose, with maximum effects tending to occur at 25 or 50 mGy. In general, low doses given at low dose rate during either early- or late-stage disease were protective, slowing the progression of the disease by one or more of these measures. Most effects appeared and persisted for months after the single exposures, but some were ultimately transitory. In contrast to exposure at low dose rate, high-dose-rate exposure during early-stage disease produced both protective and detrimental effects, suggesting that low doses may influence this disease by more than one mechanism and that dose rate is an important parameter. These results contrast with the known, generally detrimental effects of high doses on the progression of this disease in the same mice and in humans, suggesting that a linear extrapolation of the known increased risk from high doses to low doses is not appropriate.

Vascular damage as an underlying mechanism of cardiac and cerebral toxicity in irradiated cancer patients

Radiation is an independent risk factor for cardiovascular and cerebrovascular disease in cancer patients. Modern radiotherapy techniques reduce the volume of the heart and major coronary vessels exposed to high doses, but some exposure is often unavoidable. Radiation damage to the myocardium is caused primarily by inflammatory changes in the microvasculature, leading to microthrombi and occlusion of vessels, reduced vascular density, perfusion defects and focal ischemia. This is followed by progressive myocardial cell death and fibrosis. Clinical studies also demonstrate regional perfusion defects in non-symptomatic breast cancer patients after radiotherapy. The incidence and extent of perfusion defects are related to the volume of left ventricle included in the radiation field. Irradiation of endothelial cells lining large vessels also increases expression of inflammatory molecules, leading to adhesion and transmigration of circulating monocytes. In the presence of elevated cholesterol, invading monocytes transform into activated macrophages and form fatty streaks in the intima, thereby initiating the process of atherosclerosis. Experimental studies have shown that radiation predisposes to the formation of inflammatory plaque, which is more likely to rupture and cause a fatal heart attack or stroke. This paper presents a brief overview of the current knowledge on mechanisms for development of radiation-induced cardiovascular and cerebrovascular damage. It does not represent a comprehensive review of the literature, but reference is made to several excellent recent reviews on the topic.

Arterial wall dosimetry for non-Hodgkin lymphoma patients treated with radioimmunotherapy

Tumors in non-Hodgkin lymphoma (NHL) patients are often proximal to the major blood vessels in the abdomen or neck. In external-beam radiotherapy, these tumors present a challenge because imaging resolution prevents the beam from being targeted to the tumor lesion without also irradiating the artery wall. This problem has led to potentially life-threatening delayed toxicity. Because radioimmunotherapy has resulted in long-term survival of NHL patients, we investigated whether the absorbed dose (AD) to the artery wall in radioimmunotherapy of NHL is of potential concern for delayed toxicity. SPECT resolution is not sufficient to enable dosimetric analysis of anatomic features of the thickness of the aortic wall. Therefore, we present a model of aortic wall toxicity based on data from 4 patients treated with (131)I-tositumomab.

METHODS

Four NHL patients with periaortic tumors were administered pretherapeutic (131)I-tositumomab. Abdominal SPECT and whole-body planar images were obtained at 48, 72, and 144 h after tracer administration. Blood-pool activity concentrations were obtained from regions of interest drawn on the heart on the planar images. Tumor and blood activity concentrations, scaled to therapeutic administered activities-both standard and myeloablative-were input into a geometry and tracking model (GEANT, version 4) of the aorta. The simulated energy deposited in the arterial walls was collected and fitted, and the AD and biologic effective dose values to the aortic wall and tumors were obtained for standard therapeutic and hypothetical myeloablative administered activities.

RESULTS

Arterial wall ADs from standard therapy were lower (0.6-3.7 Gy) than those typical from external-beam therapy, as were the tumor ADs (1.4-10.5 Gy). The ratios of tumor AD to arterial wall AD were greater for radioimmunotherapy by a factor of 1.9-4.0. For myeloablative therapy, artery wall ADs were in general less than those typical for external-beam therapy (9.4-11.4 Gy for 3 of 4 patients) but comparable for 1 patient (32.6 Gy).

CONCLUSION

Blood vessel radiation dose can be estimated using the software package 3D-RD combined with GEANT modeling. The dosimetry analysis suggested that arterial wall toxicity is highly unlikely in standard dose radioimmunotherapy but should be considered a potential concern and limiting factor in myeloablative therapy.

Effects of exogenous peripheral-blood-derived endothelial progenitor cells or unfractionated bone-marrow-derived cells on neointimal formation and inflammation in cholesterol-fed, balloon-denuded, and radiated iliac arteries of inbred rabbits

BACKGROUND

Injection of bone marrow cells (BMC) and endothelial progenitor cells (EPC) or application of stem-cell-mobilizing factors has been associated both with reduction or exacerbation of atherosclerosis and with unstable plaque phenotype. The discrepancies may reflect the cell type, dosing, duration, and route of administration of cells in these studies. The aim of this study was to determine the effects of peripheral-blood-derived endothelial progenitor cells (PBEPC) or unfractionated BMC obtained from inbred siblings on neointimal formation and inflammation in cholesterol-fed, balloon-denuded, and radiated rabbit iliac arteries.

METHODS

Rabbits were fed a 1.0% cholesterol diet for 14 days, followed by endothelial denudation in both iliac arteries, and continued on a 0.15% cholesterol diet. On day 42, denuded areas were radiated, and animals were randomized. The first group received PBEPC (n=5), the second group received BMC (n=4), and the third group received heparinized (20 IU) saline (Control; n=3). PBEPC were characterized by flow cytometry. Cells (5x10(6)) or saline was administered twice through the ear vein: the first time at 1 h after radiation and the second time at 2 weeks after radiation. Four weeks after radiation, the animals were sacrificed, and arterial segments were processed for morphometry.

RESULTS

Administration of BMC or PBEPC from inbred siblings had no adverse effect. Lumen area (0.93+/-0.53 mm(2)), neointimal area (0.65+/-0.29 mm(2)), percent stenosis (44+/-21), and macrophage score (0.6+/-0.3) in controls were similar to those in cell-treated groups.

CONCLUSION

This study demonstrates that, in the current animal model, either PBEPC or BMC failed to affect neointimal formation or inflammation.

Combining sirolimus-eluting stents and external irradiation in cholesterol-fed rabbits increased incomplete stent apposition and decreased re-endothelialization

Restenosis after the implantation of a drug-eluting stent or after vascular irradiation therapy shares similar physiopathological mechanisms. No experimental data are currently available on vascular wall behavior after external irradiation on arteries stented with sirolimus-eluting stents (SES). Ten New Zealand white rabbits received a 0.5% cholesterol-enriched chow for 1 month. Bilateral iliac artery stent implantation was then performed with an SES (Cypher; Cordis Corp). The animals were randomized into either an irradiated group (I, 2 Gy external x-ray irradiation, n = 5) or a control group (C, n = 5). The cholesterol-enriched chow was continued for 1 additional month after stent implantation. The stented arteries were harvested for histological analyses. The number and the percentage of incompletely apposed stents struts (IASS) were significantly higher in irradiated versus control group (3.05 +/- 0.46 vs. 1.57 +/- 0.27 IASS, P < 0.01, and 28.44% +/- 3.97% vs. 15.2% +/- 2.46% of IASS, P < 0.01, respectively). The mean neointimal thickness behind the IASS was also higher in the irradiated group (I: 28.3 +/- 2.5 microm vs. C: 18.2 +/- 2.3 microm, P < 0.01). Re-endothelialization was lower in irradiated group (I: 44.6% +/- 17.5% vs. C: 75.2% +/- 5.7%, P < 0.01). The present study revealed that low-dose external irradiation increased incomplete stent apposition and reduced re-endothelialization of SES. These results underscore the potential deleterious cumulative side effects of these 2 procedures to prevent restenosis.

Carvedilol, a pharmacological antioxidant, inhibits neointimal matrix metalloproteinase-2 and -9 in experimental atherosclerosis

Matrix metalloproteinase (MMP) is critical to the progression of atherosclerosis and neointima hyperplasia after vascular injury. We investigated the effects of carvedilol, a pharmacological antioxidant with alpha- and beta-adrenergic blocking activity, on MMP-2 and MMP-9 expression. Vascular injury was induced with the balloon catheters on abdominal aortas of high-cholesterol-fed rabbits. On Day 21, there was significant aortic neointima formation with increased oxidative DNA damage by immunostaining with 8-hydroxy-2'-deoxyguanosine and enhanced MMP-2 and MMP-9 expressions by Western blotting, which were significantly reduced by oral administration of carvedilol (20 mg/kg/day) or probucol (100 mg/kg/day). Vascular expression (by Western blot), activity (by gelatin zymography), and mRNA levels of MMP-2 and MMP-9 were also reduced by carvedilol or probucol. Besides, pretreatment with carvedilol or probucol but not propranolol, a beta-blocker, or prazocin, an alpha-blocker, inhibited tumor necrosis factor-alpha-stimulated expressions and activities of MMP-2 and MMP-9 in human aortic smooth muscle cells. On electrophoretic mobility-shift assay, carvedilol inhibited the binding activities of activator protein-1 and specific protein-1, two major transcription factors for MMP promoter regions. Accordingly, carvedilol, a pharmacological antioxidant, inhibited in vivo and in vitro expression of MMP-2 and MMP-9 properly by modulating the redox-related pathways, suggesting its potential clinical implications.

Peroxisome Proliferator-Activated Receptor γ Ligand Pioglitazone Alters Neointimal Composition in a Balloon-Denuded and Radiated Hypercholesterolemic Rabbit

Peroxisome proliferator-activated receptor (PPAR)-gamma activation suppresses inflammatory response, monocyte recruitment, and vascular cell proliferation. Because inflammation, deregulated growth, and migration of monocytes and vascular smooth muscle cells (VSMC) play important roles in the development of neointima, we tested the effect of pioglitazone, a high-affinity ligand, for PPAR-gamma on neointima formation in the iliac arteries of a balloon-denuded and radiated hypercholesterolemic rabbit. Rabbits were fed a 1.0% cholesterol diet for 7 days followed by denudation of endothelial layer and continued on a 0.15% cholesterol diet. On day 32, animals were divided into 2 groups. One group received a 0.15% cholesterol diet (n = 7) and the other group received a 0.15% cholesterol diet supplemented with 400 mg of pioglitazone per kilogram. On day 35, the balloon-denuded area was radiated. Four weeks after radiation, animals were sacrificed and arterial segments were processed for morphometry and immunohistochemistry. Data analysis showed that the pioglitazone group had smaller neointima (0.85 +/- 0.36 vs. 1.41 +/- 0.56, P < 0.05), with more cells positive for VSMC (23.07 +/- 6.16 vs. 18.33 +/- 5.19, P = 0.04), less for monocytes (16.01 +/- 5.33 vs. 21.29 +/- 4.33, P < 0.05), and fewer cells expressing metalloproteinase (MMP)-1 and MMP-9 (3.69 +/- 0.47 vs. 4.82 +/- 0.93, P < 0.05 and 3.24 +/- 0.71 vs. 4.29 +/- 0.74, P < 0.05, respectively). Pioglitazone reduced neointimal area and modified its composition in a balloon-denuded and radiated hypercholesterolemic rabbit model.

Ionizing radiation accelerates the development of atherosclerotic lesions in ApoE-/- mice and predisposes to an inflammatory plaque phenotype prone to hemorrhage

After radiotherapy treatment, there is an increased incidence of localized atherosclerosis in patients with Hodgkin's disease, breast cancer, and head and neck cancer. Here, we established a mouse model to study the development and progression of radiation-induced atherosclerosis and to compare the phenotype of these lesions with age-related atherosclerosis. Atherosclerosis-prone ApoE-/- mice fed a regular chow diet received single radiation doses of 14 Gy or sham treatments (0 Gy) to the neck, including both carotid arteries. At 22, 28, and 34 weeks after irradiation, blood samples were taken, and the arterial tree was removed for histological examination. Cholesterol levels in irradiated mice were not significantly different from age-matched controls, and markers of systemic inflammation (soluble intercellular adhesion molecule-1, soluble vascular cell adhesion molecule-1, and C-reactive protein) were not elevated. The lesions in irradiated arteries were macrophage rich, with a remarkable influx of inflammatory cells, predominantly granulocytes. Intraplaque hemorrhage and erythrocyte-containing macrophages were seen only in lesions of irradiated arteries. Based on these data, we propose that irradiation accelerates the development of macrophage-rich, inflammatory atherosclerotic lesions prone to intraplaque hemorrhage.

Effect of Antioxidants on Atherosclerotic Plaque Formation in Balloon-Denuded and Irradiated Hypercholesterolemic Rabbits

The oxidative modification of low-density lipoprotein (LDL) hypothesis implies that antioxidants should be effective in suppressing atherosclerosis. This study is designed to test the potential of antioxidants to inhibit atherosclerotic plaque progression in balloon-denuded and irradiated hypercholesterolemic rabbits. Rabbits were fed with a 1% cholesterol diet supplemented with or without a mixture of antioxidants (vitamin E, vitamin C, selenium, zinc, copper, manganese, N-acetylcysteine, glutamine). At 7 days both iliac arteries were balloon denuded, and 4 weeks later, 1 iliac artery underwent endovascular irradiation (n=12), while the contralateral was sham treated (n=12). Four weeks after irradiation, animals were euthanized, and arteries were fixed and processed for histo- or immunohistochemistry for determining the plaque area, macrophage count, and oxidized LDL-positive areas. Plasma antioxidant levels were significantly higher in the animals fed with antioxidant diet. Plasma (thiobarbituric acid-reactive substances) and arterial tissue oxidized LDL (immunoreactive to specific oxidized LDL antibody) levels were significantly higher in the irradiated as compared with nonirradiated animals (0.69+/-0.09 and 31.05+/-4.21 versus 0.24+/-0.04 and 18.42+/-4.62, P<0.001 and 0.05), and antioxidants partially lowered the oxidized LDL levels (0.35+/-0.14 and 25.41+/-4.82, P<0.001 and 0.01). Plaque area in the irradiated animals was 175% greater than in nonirradiated animals (P<0.05). Antioxidant supplementation resulted in a 50% decrease in plaque area of both control and irradiated animals. Antioxidants reduced both the cholesterol-induced and radiation-enhanced circulating and tissue oxidized LDL levels, resulting in reduced plaque.