Endothelial cell proliferation in the rat heart following local heart irradiation

Radiation-induced heart disease: a review of classification, mechanism and prevention

With the increasing incidence of thoracic tumors, radiation therapy (RT) has become an important component of comprehensive treatment. RT improves survival in many cancers, but it involves some inevitable complications. Radiation-induced heart disease (RIHD) is one of the most serious complications. RIHD comprises a spectrum of heart disease including cardiomyopathy, pericarditis, coronary artery disease, valvular heart disease and conduction system abnormalities. There are numerous clinical manifestations of RIHD, such as chest pain, palpitation, and dyspnea, even without obvious symptoms. Based on previous studies, the pathogenesis of RIHD is related to the production and effects of various cytokines caused by endothelial injury, inflammatory response, and oxidative stress (OS). Therefore, it is of great importance for clinicians to identify the mechanism and propose interventions for the prevention of RIHD.

Potential markers and metabolic processes involved in the mechanism of radiation-induced heart injury

Irradiation of normal tissues leads to acute increase in reactive oxygen/nitrogen species that serve as intra- and inter-cellular signaling to alter cell and tissue function. In the case of chest irradiation, it can affect the heart, blood vessels, and lungs, with consequent tissue remodelation and adverse side effects and symptoms. This complex process is orchestrated by a large number of interacting molecular signals, including cytokines, chemokines, and growth factors. Inflammation, endothelial cell dysfunction, thrombogenesis, organ dysfunction, and ultimate failing of the heart occur as a pathological entity - "radiation-induced heart disease" (RIHD) that is major source of morbidity and mortality. The purpose of this review is to bring insights into the basic mechanisms of RIHD that may lead to the identification of targets for intervention in the radiotherapy side effect. Studies of authors also provide knowledge about how to select targeted drugs or biological molecules to modify the progression of radiation damage in the heart. New prospective studies are needed to validate that assessed factors and changes are useful as early markers of cardiac damage.

Pathology and biology of radiation-induced cardiac disease

Heart disease is the leading global cause of death. The risk for this disease is significantly increased in populations exposed to ionizing radiation, but the mechanisms are not fully elucidated yet. This review aims to gather and discuss the latest data about pathological and biological consequences in the radiation-exposed heart in a comprehensive manner. A better understanding of the molecular and cellular mechanisms underlying radiation-induced damage in heart tissue and cardiac vasculature will provide novel targets for therapeutic interventions. These may be valuable for individuals clinically or occupationally exposed to varying doses of ionizing radiation.

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.

Detection of Myocardial Metabolic Abnormalities by 18F-FDG PET/CT and Corresponding Pathological Changes in Beagles with Local Heart Irradiation

Objective

To determine the efficacy of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) in the detection of radiation-induced myocardial damage in beagles by comparing two pre-scan preparation protocols as well as to determine the correlation between abnormal myocardial FDG uptake and pathological findings.

Materials and Methods

The anterior myocardium of 12 beagles received radiotherapy locally with a single X-ray dose of 20 Gy. 18F-FDG cardiac PET/CT was performed at baseline and 3 months after radiation. Twelve beagles underwent two protocols before PET/CT: 12 hours of fasting (12H-F), 12H-F followed by a high-fat diet (F-HFD). Regions of interest were drawn on the irradiation and the non-irradiation fields to obtain their maximal standardized uptake values (SUVmax). Then the ratio of the SUV of the irradiation to the non-irradiation fields (INR) was computed. Histopathological changes were identified by light and electron microscopy.

Results

Using the 12H-F protocol, the average INRs were 1.18 ± 0.10 and 1.41 ± 0.18 before and after irradiation, respectively (p = 0.021). Using the F-HFD protocol, the average INRs were 0.99 ± 0.15 and 2.54 ± 0.43, respectively (p < 0.001). High FDG uptake in irradiation field was detected in 33.3% (4/12) of 12H-F protocol and 83.3% (10/12) of F-HFD protocol in visual analysis, respectively (p = 0.031). The pathology of the irradiated myocardium showed obvious perivascular fibrosis and changes in mitochondrial vacuoles.

Conclusion

High FDG uptake in an irradiated field may be related with radiation-induced myocardial damage resulting from microvascular damage and mitochondrial injury. An F-HFD preparation protocol used before obtaining PET/CT can improve the sensitivity of the detection of cardiotoxicity associated with radiotherapy.

Low-dose irradiation affects expression of inflammatory markers in the heart of ApoE -/- mice

Epidemiological studies indicate long-term risks of ionizing radiation on the heart, even at moderate doses. In this study, we investigated the inflammatory, thrombotic and fibrotic late responses of the heart after low-dose irradiation (IR) with specific emphasize on the dose rate. Hypercholesterolemic ApoE-deficient mice were sacrificed 3 and 6 months after total body irradiation (TBI) with 0.025, 0.05, 0.1, 0.5 or 2 Gy at low (1 mGy/min) or high dose rate (150 mGy/min). The expression of inflammatory and thrombotic markers was quantified in frozen heart sections (CD31, E-selectin, thrombomodulin, ICAM-1, VCAM-1, collagen IV, Thy-1, and CD45) and in plasma samples (IL6, KC, MCP-1, TNFα, INFγ, IL-1β, TGFβ, INFγ, IL-10, sICAM-1, sE-selectin, sVCAM-1 and fibrinogen) by fluorescence analysis and ELISA. We found that even very low irradiation doses induced adaptive late responses, such as increases of capillary density and changes in collagen IV and Thy-1 levels indicating compensatory regulation. Slight decreases of ICAM-1 levels and reduction of Thy 1 expression at 0.025–0.5 Gy indicate anti-inflammatory effects, whereas at the highest dose (2 Gy) increased VCAM-1 levels on the endocardium may represent a switch to a pro-inflammatory response. Plasma samples partially confirmed this pattern, showing a decrease of proinflammatory markers (sVCAM, sICAM) at 0.025–2.0 Gy. In contrast, an enhancement of MCP-1, TNFα and fibrinogen at 0.05–2.0 Gy indicated a proinflammatory and prothrombotic systemic response. Multivariate analysis also revealed significant age-dependent increases (KC, MCP-1, fibrinogen) and decreases (sICAM, sVCAM, sE-selectin) of plasma markers. This paper represents local and systemic effects of low-dose irradiation, including also age- and dose rate-dependent responses in the ApoE^-/- mouse model. These insights in the multiple inflammatory/thrombotic effects caused by low-dose irradiation might facilitate an individual evaluation and intervention of radiation related, long-term side effects but also give important implications for low dose anti-inflammatory radiotherapy.

Mechanisms of cardiac radiation injury and potential preventive approaches

In addition to cytostatic treatment and surgery, the most common cancer treatment is gamma radiation. Despite sophisticated radiological techniques however, in addition to irradiation of the tumor, irradiation of the surrounding healthy tissue also takes place, which results in various side-effects, depending on the absorbed dose of radiation. Radiation either damages the cell DNA directly, or indirectly via the formation of oxygen radicals that in addition to the DNA damage, react with all cell organelles and interfere with their molecular mechanisms. The main features of radiation injury besides DNA damage is inflammation and increased expression of pro-inflammatory genes and cytokines. Endothelial damage and dysfunction of capillaries and small blood vessels plays a particularly important role in radiation injury. This review is focused on summarizing the currently available data concerning the mechanisms of radiation injury, as well as the effectiveness of various antioxidants, anti-inflammatory cytokines, and cytoprotective substances that may be utilized in preventing, mitigating, or treating the toxic effects of ionizing radiation on the heart.

Pharmacological induction of transforming growth factor-beta1 in rat models enhances radiation injury in the intestine and the heart

Radiation therapy in the treatment of cancer is dose limited by radiation injury in normal tissues such as the intestine and the heart. To identify the mechanistic involvement of transforming growth factor-beta 1 (TGF-β1) in intestinal and cardiac radiation injury, we studied the influence of pharmacological induction of TGF-β1 with xaliproden (SR 57746A) in rat models of radiation enteropathy and radiation-induced heart disease (RIHD). Because it was uncertain to what extent TGF-β induction may enhance radiation injury in heart and intestine, animals were exposed to irradiation schedules that cause mild to moderate (acute) radiation injury. In the radiation enteropathy model, male Sprague-Dawley rats received local irradiation of a 4-cm loop of rat ileum with 7 once-daily fractions of 5.6 Gy, and intestinal injury was assessed at 2 weeks and 12 weeks after irradiation. In the RIHD model, male Sprague-Dawley rats received local heart irradiation with a single dose of 18 Gy and were followed for 6 months after irradiation. Rats were treated orally with xaliproden starting 3 days before irradiation until the end of the experiments. Treatment with xaliproden increased circulating TGF-β1 levels by 300% and significantly induced expression of TGF-β1 and TGF-β1 target genes in the irradiated intestine and heart. Various radiation-induced structural changes in the intestine at 2 and 12 weeks were significantly enhanced with TGF-β1 induction. Similarly, in the RIHD model induction of TGF-β1 augmented radiation-induced changes in cardiac function and myocardial fibrosis. These results lend further support for the direct involvement of TGF-β1 in biological mechanisms of radiation-induced adverse remodeling in the intestine and the heart.

Endoglin haplo-insufficiency modifies the inflammatory response in irradiated mouse hearts without affecting structural and mircovascular changes

Background

It is now widely recognized that radiotherapy of thoracic and chest wall tumors increases the long-term risk of cardiovascular damage although the underlying mechanisms are not fully elucidated. There is increasing evidence that microvascular damage is involved. Endoglin, an accessory receptor for TGF-β1, is highly expressed in damaged endothelial cells and may play a crucial role in cell proliferation and revascularization of damaged heart tissue. We have therefore specifically examined the role of endoglin in microvascular damage and repair in the irradiated heart.

Materials & Methods

A single dose of 16 Gy was delivered to the heart of adult Eng^+/+ or Eng^+/− mice and damage was evaluated at 4, 20 and 40 weeks, relative to age-matched controls. Gated single photon emission computed tomography (gSPECT) was used to measure cardiac geometry and function, and related to histo-morphology, microvascular damage (detected using immuno- and enzyme-histochemistry) and gene expression (detected by microarray and real time PCR).

Results

Genes categorized according to known inflammatory and immunological related disease were less prominently regulated in irradiated Eng^+/− mice compared to Eng^+/+ littermates. Fibrosis related genes, TGF-β1, ALK 5 and PDGF, were only upregulated in Eng^+/+ mice during the early phase of radiation-induced cardiac damage (4 weeks). In addition, only the Eng^+/+ mice showed significant upregulation of collagen deposition in the early fibrotic phase (20 weeks) after irradiation. Despite these differences in gene expression, there was no reduction in inflammatory invasion (CD45+cells) of irradiated Eng^+/− hearts. Microvascular damage (microvascular density, alkaline phosphatase and von-Willebrand-Factor expression) was also similar in both strains.

Conclusion

Eng^+/− mice displayed impaired early inflammatory and fibrotic responses to high dose irradiation compared to Eng^+/+ littermates. This did not result in significant differences in microvascular damage or cardiac function between the strains.

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.

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).

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

Potential targets for intervention in radiation-induced heart disease

Radiotherapy of thoracic and chest wall tumors, if all or part of the heart was included in the radiation field, can lead to radiation-induced heart disease (RIHD), a late and potentially severe side effect. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. The pathogenesis of RIHD is largely unknown, and a treatment is not available. Hence, ongoing pre-clinical studies aim to elucidate molecular and cellular mechanisms of RIHD. Here, an overview of recent pre-clinical studies is given, and based on the results of these studies, potential targets for intervention in RIHD are discussed.

Preclinical research into basic mechanisms of radiation-induced heart disease

Radiation-induced heart disease (RIHD) is a potentially severe side effect of radiotherapy of thoracic and chest wall tumors if all or part of the heart was included in the radiation field. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. There is no method to prevent or reverse these injuries when the heart is exposed to ionizing radiation. This paper presents an overview of recent studies that address the role of microvascular injury, endothelial dysfunction, mast cells, and the renin angiotensin system in animal models of cardiac radiation injury. These insights into the basic mechanisms of RIHD may lead to the identification of targets for intervention in this late radiotherapy side effect.

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.

Non-cancer diseases and non-targeted effects

It is well established that moderate to high doses of radiation can increase the occurrence also of a variety of non-cancer effects in exposed individuals, but for radiation protection purposes it has generally been assumed that there is a threshold of dose below which no significant non-cancer effects (apart from hereditary disease) arise. In recent years, there is growing epidemiological evidence of excess risk of late occurring cardiovascular disease at much lower radiation doses and occurring over much longer intervals after radiation exposure without a clear cut threshold. However, the epidemiological evidence available so far for non-cancer health effects after exposure to moderate or low radiation doses is suggestive rather than persuasive. The mechanisms of radiation-induced vascular disease induction are far away from being understood. However, it seems to be very likely that inflammatory responses are involved. Recent experimental studies by Stewart et al. [25] could demonstrate that high dose exposure to the cardiovascular system is associated with an earlier onset and accelerated development of macrophage-rich, inflammatory atherosclerotic lesions prone to intra-plaque hemorrhage and may also cause a decrease in myocardial perfusion. Both, macro-vascular and micro-vascular radiation effects involve the endothelium and pro-inflammatory signalling cascades. If modulation of inflammatory response is arguably also the most likely cause of radiation-induced cardiovascular disease after low dose exposure, this also implies a role for non-targeted radiation effects. In the absence of a convincing mechanistic explanation of the currently available epidemiological evidence for radiation-induced cardiovascular risk at low radiation doses, caution is required in the interpretation of the statistical associations. On the other hand, the possibility of such a causal explanation cannot be reliably excluded. Further epidemiological and biological evidence from currently ongoing research projects will allow a firmer conclusion to be drawn.

Radiation-induced cardiovascular diseases: is the epidemiologic evidence compatible with the radiobiologic data?

The Life Span Study of Japanese atomic bomb survivors demonstrates that radiation exposure significantly increased the risk of developing ischemic heart disease, in particular myocardial infarction. Similarly, epidemiologic investigations in very large populations of patients who had received postoperative radiotherapy for breast cancer or for peptic ulcer demonstrate that radiation exposure of the heart with an average equivalent single dose of approximately 2 Gy significantly increased the risk of developing ischemic heart disease more than 10 years after irradiation. These epidemiologic findings are compatible with radiobiologic data on the pathogenesis of radiation-induced heart disease in experimental animals. The critical target structure appears to be the endothelial lining of blood vessels, in particular arteries, leading to early functional alterations such as pro-inflammatory responses and other changes, which are slowly progressive. Research should concentrate on the interaction of these radiation-induced endothelial changes with the early stages of age-related atherosclerosis to develop criteria for optimizing treatment plans in radiotherapy and also potential interventional strategies.

Radiation-induced cardiomyopathy as a function of radiation beam gating to the cardiac cycle

Portions of the heart are often unavoidably included in the primary treatment volume during thoracic radiotherapy, and radiation-induced heart disease has been observed as a treatment-related complication. Such complications have been observed in humans following radiation therapy for Hodgkin's disease and treatment of the left breast for carcinoma. Recent attempts have been made to prevent re-stenosis following angioplasty procedures using external beam irradiation. These attempts were not successful, however, due to the large volume of heart included in the treatment field and subsequent cardiac morbidity. We suggest a mechanism for sparing the heart from radiation damage by synchronizing the radiation beam with the cardiac cycle and delivering radiation only when the heart is in a relatively hypoxic state. We present data from a rat model testing this hypothesis and show that radiation damage to the heart can be altered by synchronizing the radiation beam with the cardiac cycle. This technique may be useful in reducing radiation damage to the heart secondary to treatment for diseases such as Hodgkin's disease and breast cancer.

Vascularization in the transition area between free grafted soft tissues and pre-irradiated graft bed tissues following preoperative radiotherapy in the head and neck region

BACKGROUND

The healing of free vascular grafts in a pre-irradiated graft bed is characterized by an increased risk of wound healing disorders. For that reason, the aim of this study was to examine quantitative vascularization pattern between free vascular grafts and the pre-irradiated graft bed as a function of the preoperative irradiation dose.

METHODS

A total of 217 free microvascular hard and soft tissue grafts were used within 199 patients in the head and neck region to cover defects after ablative tumor surgery. Seventy-six patients (group 1) had no prior radiation (RT), 50 patients (group 2) were treated with preoperative radiochemotherapy using 40 to 50 Gy and 5-FU/cisplatin, and 73 patients (group 3) had prior RT (60-70 Gy) between 1 and 7 years before surgery. After healing of the grafts, samples were taken from 42 patients from the graft, the irradiated graft bed, and the transition area between graft and irradiated graft bed. Samples were analyzed as follows: capillary sprouting, structural changes, and distribution patterns were analyzed by immunohistochemical staining (CD34 labeling of capillary endothelium). Three histological sections (2-4 microm) per sample were examined histomorphometrically (ratio capillary area/total area, capillary lumen, and the number of capillaries) by (National Institute of Health) NIH-image-digitized measurements. A statistical analysis was performed using the Kruskal-Wallis and Mann-Whitney test (two-tailed p <.05).

RESULTS

The success rate of vascular grafts in group 1 (0 Gy) was 94%, in group 2 (40-50 Gy/5-FU/cisplatin) 90%, in group 3 (60-70 Gy) 84%. In contrast to groups 1 and 2, group 3 showed qualitatively reduced and more irregular capillary distribution with more marked pericapillary fibrosis in the irradiated tissue. Quantitatively, the ratio capillary area/total area, as a marker of improved capillarization, was significantly reduced in group 3 (median 0.01; IQR 0.02) compared with group 1 (median 0.53; IQR 0.32) and group 2 (median 0.44; IQR 1.40) (p <.001).

CONCLUSION

After preoperative RT, vascularization of the graft bed decreased continuously as a function of the total dose and time after RT. The results strongly advocate the use of a primary reconstruction after a time interval between 4 and 6 weeks following preoperative RT and suggest the use of a total radiation dose of 40 to 50 Gy.

Late effects of ionizing radiation on the microvascular networks in normal tissue

Damage to the microvascular networks constitutes one of the most important components of ionizing radiation damage to normal tissue. Previously, we have reported the early (3, 7 and 30 days postirradiation) effects of ionizing radiation on the structure and function of normal tissue microvascular networks. Here we report on the late effects of ionizing radiation on the structural and functional changes in microvascular networks in locally irradiated (single 10-Gy dose) hamster cremaster muscles observed 60, 120 and 180 days postirradiation; age-matched animals were used as controls. As in the previous study, intravital microscopy was used to measure structural and functional parameters in complete microvascular networks in vivo. A factorial design was used to examine the effects of radiation status, time postirradiation, and network vessel type on the structure and function of microvascular networks. Our results indicate that the progression of radiation-induced microvascular damage continues during the late times but that there is partial recovery from radiation damage within 6 months postirradiation. Red blood cell flux, red blood cell velocity, and capillary blood flow in irradiated networks at 180 days postirradiation were significantly greater than control levels. As at the early times, all vessel types were not damaged equally by radiation at every time.

Induction of growth factors in rat cardiac tissue by X irradiation

To investigate the relationship between angiogenic growth factors and endothelial enzyme activity in capillaries after injury of rat cardiomyocytes caused by X irradiation, 7-week-old male Wistar rats were anesthetized with pentobarbitone and their hearts irradiated (X rays, 20 Gy) through a hole in the lead casing in which they were enclosed. The hearts were excised at 1 h, 1 week and 3 weeks after irradiation. Left ventricular cross sections were stained for capillary enzymes by double staining for two endothelial enzymes, alkaline phosphatase (AP) and dipeptidylpeptidase IV (DPP), immunohistochemically stained for basic fibroblast growth factor (Fgf, also known as bFgf) and vascular endothelial growth factor (Vegf), and stained for nick end-labeling of DNA by the TUNEL method. Staining for distribution of AP in the arteriolar portion was reduced at both 1 and 3 weeks after irradiation with 20 Gy, but staining for DPP in the venular portion was unchanged, suggesting a close relationship between growth factors and injury of the arteriolar capillary portion. Fgf and Vegf proteins were present within the cytoplasm of the cardiomyocytes, or around capillaries, 1 h, 1 week and 3 weeks after irradiation. Many TUNEL-stained cardiomyocyte nuclei were observed at 1 h, but they had decreased markedly at 1 week and had almost disappeared by 3 weeks after irradiation. Thus Fgf and Vegf were induced concomitantly with the decrease in the staining for endothelial AP by 20 Gy X irradiation, which also caused microeffects as indicated by TUNEL staining of many nuclei at 1 h postirradiation.

Reversal of malignant phenotype in human osteosarcoma cells transduced with the alkaline phosphatase gene

Alkaline phosphatases are a family of glycoproteins that are able to hydrolize various monophosphate esters at a high pH optimum. Liver/bone/kidney (L/B/K) alkaline phosphatase (ALP) is one of the four major isoenzymes that belong to this family. Apart from its role in normal bone mineralization, other functions of L/B/K ALP remain obscure, both in physiological and in neoplastic conditions, including the bone-forming tumor osteosarcoma. In this study, we transfected the U-2 OS osteosarcoma cell line, which does not show any basal expression of this enzyme, with the full-length gene of L/B/K ALP, and analyzed the in vitro and in vivo features of four transfectants showing different expression of L/B/K ALP. A reduced in vitro ability to invade Matrigel and to grow in a semi-solid medium, together with a lower tumorigenic and metastatic ability in athymic mice, was found to be associated with a high level of cell surface L/B/K ALP activity. Moreover, L/B/K ALP transfectants showed a reduced secretion of matrix metalloproteinase-9 enzyme. These findings indicate a loss of aggressiveness of osteosarcoma cells after the expression of L/B/K ALP on their surface and suggest a new role for this enzyme.

Adaptive changes in the capillary network in the left ventricle of rat heart

Capillaries are nonuniform thin tubes: The arteriolar and venular capillary portions express alkaline phosphatase (AP) and dipeptidyl peptidase IV (DPPIV), respectively. Differences in enzyme activities between arteriolar and venular capillary portions could be shown by staining sections of cardiac tissues for AP and DPPIV after coronary infusion of microspheres and by staining cultured endothelial cells that had been collected from coronary microvessels. Through use of a double staining method for AP and DPPIV, adaptive changes in the capillary network were studied in rat hearts exposed to cold, exercise, hypertension, chronic coronary occlusion, and transient coronary occlusion followed by reperfusion. Two patterns could be seen in the adaptations of the ventricular capillary network. The increase in the venular capillary portions is accompanied by remarkable increases in capillary density and capillary-to-myocyte ratio. The increase in the arteriolar capillary portion seemed to be accompanied by a decrease or only a limited increase in capillary density in stressed hearts. The increase in the total capillary density improves the capacity for oxygen transport to tissues with a high tissue perfusion and a short diffusion distance for oxygen. The increase in the arteriolar capillaries may also improve oxygen transport by increasing the arterial blood perfusing the tissue. This seems, however, a compensation for the limited angiogenesis: The alleviation of stresses, such as pharmacological treatment of the hypertrophied heart and reperfusion after transient ischemia, increases venular capillary portions and capillary density. These changes are discussed with immunohistochemical observations of rapid and prolonged expressions of angiogenic growth factors.

Peripheral Blood Mononuclear Cells Induce Programmed Cell Death in Human Endothelial Cells and May Prevent Repair: Role of Cytokines

Human umbilical vein endothelial cells (HUVECs) undergo programmed cell death (apoptosis) after coculture with peripheral blood mononuclear cells (PBMCs) preactivated by ionizing radiation (IR) or by bacterial endotoxin (lipopolysaccharide [LPS]). Cell-to-cell contact-mediated apoptosis could be blocked in both cases by anti–tumor necrosis factor-α (anti–TNF-α) monoclonal antibody MAK195 and also by the antagonistic cytokine interleukin-10 (IL-10). Cell-free PBMC supernatants from both preactivation treatments were sufficient to trigger endothelial apoptosis. In contrast, MAK195 and IL-10 were found to be ineffective in this system, suggesting a TNF-α–independent mechanism. However, N-Acetylcystein, an antioxidant, fully abrogated programmed cell death mediated by the supernatant of IR-treated PBMCs, but not of LPS-treated PBMCs. Additionally, we found that coculture and cell-free supernatants of preactivated as well as untreated PBMCs caused cell cycle arrest in proliferating EC in G0/1 , which could be relieved by IL-10, but not by anti–TNF-α. Further analysis showed that transforming growth factor-β, which was constitutively expressed in the supernatant of PBMCs, namely lymphocytes, was responsible for this. These data suggest a pathophysiologic model in which preactivated PBMCs cause EC damage and may prevent blood vessel repair by arresting the proliferation of ECs. This could contribute to the understanding of various clinical endothelial complications that occur after irradiation as well as in cases of endotoxemia or related inflammatory states.

Reirradiation tolerance of the rat heart

PURPOSE

To investigate the influence of reirradiation on the tolerance of the heart after a previous irradiation treatment.

METHODS AND MATERIALS

Female Wistar rats were locally irradiated to the thorax. Development of cardiac function loss was studied with the ex vivo working rat heart preparation (20). To compare the retreatment experiments, initial, and reirradiation doses were expressed as the percentage of the extrapolated tolerance dose (ETD) (1).

RESULTS

Local heart irradiation with a single dose led to a dose-dependent and progressive decrease in cardiac function. The progressive nature of irradiation-induced heart disease is shown to affect the outcome of the retreatment, depending on both the time interval between subsequent doses and the size of the initial dose. The present data demonstrate that hearts are capable of repairing a large part of the initial dose of 10 Gy within the first 24 h. However, once biological damage as a result of the first treatment is fixed, the heart does not show any long-term recovery. At intervals up to 6 months between an initial treatment with 10 Gy and subsequent reirradiation, the reirradiation tolerance dose slightly decreased from 74% of the ETDref (at 24-h interval) to 68% of the ETDref (at 6-month interval). Between 6 and 9 months, reirradiation tolerance dose dropped more even to 43% of the ETDref. Treatment of the heart with an initial dose of 17.5 Gy, instead of 10 Gy, 6 months prior to reirradiation, also led to a further decrease of the reirradiation tolerance dose (< 38 vs. 68% of the ETDref).

CONCLUSIONS

The outcome of the present study shows a decreased tolerance of the heart to reirradiation at long time intervals (interval > 6 months). This has clinical implications for the estimation of reirradiation tolerance in patients whose mediastinum has to be reirradiated a long time after a first irradiation course.

Cardiac doses in post-operative breast irradiation

Data were collected on radiation doses given to the heart and coronary arteries during primary breast irradiation in order to analyze factors which might be important in the aetiology of subsequent cardiac-related disease. Twenty eight patients with breast cancer were studied. Fourteen patients treated from 1957 to 1984 were studied retrospectively (group 1), and 14 treated from 1988 to 1989 were studied prospectively (group 2). All patients had stage I or II disease at presentation, and were under 70 years of age. None had chemotherapy as a primary form of treatment. Patients were given a computed tomography scan of the chest, and three-dimensional reconstruction was made of the heart, lung and body contour. Original dose distributions were super-imposed on these outlines, and doses to the total cardiac volume and three main coronary arteries were estimated using an alpha/beta ratio of 4 Gy. Nine out of 14 patients in group 1 had a mastectomy followed mainly by orthovoltage radiation with similar techniques used up until 1984. Thirteen out of 14 patients in group 2 had conservative surgery followed by a modern two- or four-field megavoltage technique. We found that for patients with left-sided tumours (n = 20), the heart volume irradiated to a minimum extrapolated target dose of 5 Gy is significantly decreased for patients treated with a modern technique (group 2) when compared with those treated with earlier techniques (group 1).(ABSTRACT TRUNCATED AT 250 WORDS)

Radiation-induced heart disease: review of experimental data on dose response and pathogenesis

Clinical and experimental heart irradiation can cause a variety of sequelae. A single dose of greater than or equal to 15 Gy leads to a reversible exudative pericarditis, occurring in dogs, rabbits or rats at around 100 days. Its time-course is very similar in all species investigated, but there are considerable species and strain differences in severity and incidence. After longer, dose-dependent latency times chronic congestive myocardial failure develops. At histological examination myocardial degeneration and necrosis is observed, which in some species is accompanied by a variable degree of interstitial fibrosis. In rabbits and rats, myocardial degeneration becomes apparent at around 70 days after 20 Gy and is preceded by a marked reduction in capillary density as well as ultrastructural endothelial cell degeneration. Simultaneously to structural capillary damage, a focal loss of the endothelial marker enzyme alkaline phosphatase was observed in rats in areas with subsequent myocardial degeneration. Cell kinetic studies in rabbits and rats revealed a radiation-induced wave of increased endothelial cell proliferation at 30-100 days postirradiation. In the rat it is exclusively seen in conjunction with alteration of endothelial cell marker enzymes. The temporal and spatial pattern of proliferative response exludes endothelial cell death in mitosis as the sole pathogenetic mechanism causing capillary loss and myocardial degeneration. Parallel to development of morphological damage, haemodynamic studies in various rats strains revealed a drop in cardiac output and left ventricular ejection fraction to about 64% of normal values after 20 Gy. In vivo, this slightly reduced cardiac function was then maintained in a steady state for many weeks, probably due to a compensatory up-regulation of cardiac beta-adrenergic receptors. In denervated working heart preparations in vitro, however, these compensatory mechanisms are not effective and stroke volume as well as cardiac contractility show a rapid and steady deterioration. In many respects radiation-induced heart disease conforms to radiobiological concepts of late-responding tissues, showing a chronic progressive time-course and a very pronounced fractionation effect. However, pathogenesis cannot be understood in terms of target cell depletion alone, and experimental evidence indicates the importance of alterations of regulatory mechanisms.

Fractionation response and repair kinetics of radiation-induced heart failure in the rat

Local heart irradiation with single or fractionated doses leads to heart failure after dose-dependent latency times. Clinical symptoms of heart failure are dyspnoea at rest, apathy and subcutaneous oedema. Animals autopsied when they presented with these symptoms, have a congested liver and occasional pleural effusions. The left ventricle is dilated, showing a reduction in wall thickness by 15-17% of control values. Histological examination reveals a focal degeneration and necrosis of about 23% of the total myocardial volume. Loss of alkaline phosphatase activity from myocardial capillaries, which is known to precede myocardial degeneration, involves 77% of the myocardium. These findings at the time of manifest heart failure are constant, independent on whether injury to the heart was inflicted by single-dose or fractionated irradiation or whether heart failure developed within a relatively short time after high total doses or within many months after low total doses. The latent time of heart failure therefore can be considered an appropriate endpoint for comparison of treatment groups. From experiments giving 1, 2, 4, or 10 dose fractions, a low alpha/beta ratio of 3.7 Gy (95% confidence interval 1.8-5.6 Gy) can be calculated. When the time interval between dose fractions is varied in a split-dose experiment, time intervals of up to 3 h do not increase the survival time significantly. This appears to indicate very slow repair of sublethal damage. On the other hand, it cannot be excluded that pathogenetic mechanisms independent of cell death in the renewing cell population contribute to this effect, making an interpretation of the alpha/beta ratio in terms of cell survival parameters of a defined target cell population difficult.