the worldwide number of long - term cancer survivors is growing fast with ongoing improvements in cancer therapies [ 1 , 2 ] .
however , long - term cancer survivors may suffer from late side effects of cancer therapy .
one of these late side effects is radiation - induced heart disease ( rihd ) , which may occur after radiotherapy of thoracic and chest wall tumors whenever all or part of the heart is situated in the radiation field .
rihd has been described to occur , for instance , among survivors of hodgkin 's disease [ 3 , 4 ] and breast cancer [ 5 , 6 ] .
radiotherapy planning has undergone many improvements over the last decades , with modalities such as intensity - modulated radiation therapy ( imrt ) , image - guided radiation therapy , and proton therapy , leading to reduced exposures of the heart .
for instance , patients with hodgkin 's disease , lung cancer , and esophageal and proximal gastric cancer may still receive either a high dose of radiation to a small part of the heart or a lower dose to the whole heart [ 711 ] .
in addition , although there is increasing use of concomitant therapies , the extent to which these therapies affect radiotherapy side effects such as rihd is largely unknown .  
manifestations of rihd include accelerated atherosclerosis , pericardial and myocardial fibrosis , conduction abnormalities , and injury to cardiac valves [ 4 , 12 ] .
the disease is progressive and both incidence and severity increase with a higher radiation dose volume , younger age at the time of radiotherapy , a greater time elapsed since treatment , and concomitant use of cardiotoxic chemotherapeutic agents such as anthracyclines . although rihd is widely acknowledged as an impediment to quality of life for certain long - term cancer survivors , from a clinical perspective the only current way to reduce rihd is through efforts to improve radiotherapy treatment planning , as other methods to prevent or reverse rihd are not yet available .
hence , pre - clinical studies seek to unravel basic mechanisms of rihd , with the ultimate goal to identify potential targets for intervention .
pre - clinical animal models have long been used to study rihd [ 1318 ] . while transgenic mouse models are being used in investigations of radiation - accelerated atherosclerosis [ 19 , 20 ] , wild type rodents are usually not atherosclerosis prone .
hence , studies that use rodents to investigate radiation - induced coronary artery disease are limited in number [ 21 , 22 ] . on the other hand , many laboratory animals , including rodent , have been used successfully as models of radiation - induced cardiomyopathy [ 16 , 2327 ] .
common doses used in these pre - clinical models of localized heart irradiation are either a single dose between 5  gy and 25  gy , or fractionated schedules of , for instance , 5 daily fractions of 9  gy . some of the histopathological changes in pre - clinical models , such as myocardial degeneration and fibrosis , are also commonly described in human cases of rihd , mainly after exposure to doses of ~30 gy and above [ 3 , 4 , 2830 ] .
although clinical and pre - clinical data on the cardiovascular effects of lower radiation doses are growing [ 11 , 31 ] , the focus of this review will be on myocardial injury and cardiac function changes after exposure to higher doses of radiation .
previous paper indicate the important role of vascular injury and endothelial dysfunction ( loss of thromboresistance and increased expression of adhesion molecules and cytokines ) in the pathogenesis of normal tissue radiation injury [ 42 , 43 ] .
endothelial dysfunction may contribute to profibrotic and proinflammatory environments , which are common aspects of normal tissue radiation injury [ 42 , 44 ] .
although the role of endothelial dysfunction in rihd has not been studied in detail , experimental rihd is known to be associated with reduced myocardial capillary density [ 32 , 33 ] , focal loss of endothelial alkaline phosphatase [ 14 , 34 ] , and increased expression of von willebrand factor .
hence , microvascular injury and the resulting local ischemic injury are considered to be some of the underlying mechanisms of rihd .  
radiation - induced vascular injury and endothelial dysfunction are mediated in part by transforming growth factor- ( tgf- ) [ 45 , 46 ] , a pluripotent growth factor that is part of many normal tissue radiation responses [ 4749 ] .
previous studies have shown cardiac upregulation of tgf- in rat models of rihd after localized heart irradiation with 20  gy or 5 fractions of 9  gy   [ 3638 ] .
a tgf--inducing compound was used to investigate the role of tgf- in rihd in the rat .
cardiac radiation fibrosis was more severe in animals that had been administered the tgf--inducing compound during the 6-month followup time after irradiation ( unpublished data ) .
mast cells , cells that belong to the hematopoietic myeloid lineage , reside in many organs and tissues including the heart .
although best known for their role in hypersensitivity reactions , mast cells are also intimately involved in wound healing and tissue remodeling [ 5052 ] .
mast cells store and release a wide range of cellular mediators , both via degranulation and via constitutive pathways that do not involve degranulation .
increased mast cell numbers are commonly found in coronary atherosclerosis , myocardial fibrosis [ 54 , 55 ] , and also in animal models of rihd [ 40 , 56 ] , where mast cell numbers correlate with myocardial radiation injury .
the development and maturation of mast cells depend on the c - kit receptor , which is specific for stem cell factor .
several mast cell deficient animal models , based on a mutation in the c - kit receptor or stem cell factor , are available [ 5759 ] .
our laboratories have made use of a rat model that is homozygous for a 12-base deletion in the c - kit receptor gene [ 60 , 61 ] .
both mast cell - deficient rats and their mast cell - competent wild type litter mates were exposed to localized heart irradiation with a single dose of 18  gy . although mast cell - deficiency was associated with reduced radiation - induced myocardial inflammation and degeneration , other manifestations of cardiac radiation injury such as myocardial fibrosis and ex vivo measures of myocardial stiffness were exacerbated in the absence of mast cells .
these studies suggest that mast cells , in contrast to what had been the prevailing assumption but similar to what has been found in some other cardiac disease models [ 62 , 63 ] , play a predominantly protective role in rihd .
mast cell - derived proteinases , for instance , have been shown to contribute to both the formation and degradation of endothelin-1 ( et-1 ) [ 6468 ] .
et-1 is a 21-amino acid peptide that was first discovered as a potent vasoconstrictor but also has proinflammatory and pro - fibrotic properties [ 69 , 70 ] .
the role of et-1 in cardiovascular pathology has been studied extensively [ 71 , 72 ] .
both et-1 receptors , eta   and etb are expressed by a wide variety of cell types in the heart [ 70 , 73 ] .
short - term upregulation of et-1 and its receptors may serve as a mechanism to maintain cardiac function in certain cardiovascular diseases [ 74 , 75 ] .
long - term up - regulation of the endothelin system , on the other hand , may have detrimental effects due to the vasopressor , prohypertrophic , and pro - fibrotic properties of et-1 [ 73 , 76 ] .
mast cells express the receptor eta , which upon activation by et-1 induces mast cell degranulation , a pathway by which et-1 may enhance the activity of matrix metalloproteinases ( mmps ) in the heart [ 78 , 79 ] .
dual inhibition of eta   and etb prevented mast cell degranulation and the associated increase in cardiac mmp levels , interstitial collagen degradation , and ventricular dilatation in a rat model of chronic volume overload . on the other hand , in a preliminary study of a rat model of rihd , dual inhibition of eta   and etb did not alter radiation - induced functional or structural cardiac changes .
moreover , vascular injury seemed aggravated by selective eta   inhibition in a rat model of localized intestine irradiation .
dosing of receptor antagonists and opposing cardiovascular effects of the eta and etb , receptors [ 83 , 84 ] warrant further studies to clarify the role of et-1 and its two receptors in rihd .
they are found in close proximity to nerve terminals or axons in many organs , including the heart [ 86 , 87 ] , and interact with nerves on the molecular level in many ways [ 85 , 88 , 89 ] .
mast cells express - and -adrenergic receptors [ 90 , 91 ] . in normal rat myocardium
some sensory neuropeptides such as calcitonin gene- related peptide ( cgrp ) , substance p , and neuropeptide y are able to induce or enhance mast cell degranulation [ 93100 ] while others have been shown to inhibit mast cell degranulation [ 101 , 102 ] .
mast cells , in turn , affect neuronal growth and function by producing nerve growth factor   and by activating proteinase - activated receptor-2 on the surface of neurons [ 104 , 105 ] . cardiac sensory nerves play a protective role in the heart via the release of nitric oxide and cgrp [ 106 , 107 ] . for instance , cgrp plays a protective role in myocardial injury such as from ischemia reperfusion and the cardiotoxic chemotherapeutic agent doxorubicin [ 108 , 109 ] .
cgrp is a potent vasodilator but also has beneficial effects in the heart by local downregulation of tumor necrosis factor - alpha ( tnf- ) and upregulation of insulin - like growth factor-1 ( igf-1 ) [ 110 , 111 ] .
interestingly , both downregulation of tnf- and upregulation of igf-1 are associated with reduced normal tissue radiation injury [ 112 , 113 ] . in line with this evidence
the role of the renin angiotensin system ( ras ) in normal tissue radiation injury has been well defined [ 115 , 116 ] .
inhibitors of angiotensin - converting enzyme ( ace ) and antagonists of angiotensin type 1 receptors reduce injury in animal models of localized kidney , lung , and brain irradiation [ 117119 ] .
although the role of ras in rihd is less well defined , ras mediators may be upregulated in the heart after irradiation .
however , while the ace inhibitor captopril reduced radiation injury in kidney , lung , and skin of rats [ 119121 ] , captopril did not prevent cardiac function loss after localized heart irradiation with 20  gy in a rat model .
captopril , on the other hand , did reduce myocardial fibrosis and prevented left ventricular capillary density loss after local heart irradiation .
it is not known whether these effects were due to properties of captopril other than its inhibition of ace .
inhibition of ace is considered to be cardioprotective in part by suppressing the breakdown of bradykinin by ace .
bradykinin , a small peptide hormone that is sometimes considered to aggravate cardiac disease with a significant inflammatory component such as myocardial infarction , is also known to mediate cardioprotection via induction of nitric oxide and prostacyclin [ 124126 ] .
bradykinin is formed in the kallikrein - kinin system by proteolytic cleavage of both high- and low - molecular weight kininogen by kallikrein enzymes , but also by the mast cell - derived enzyme tryptase [ 127 , 128 ] .
interestingly , the mast cell proteinases chymases are one of the main converters of angiotensin i into angiotensin ii .
mast cells seem to hereby provide a particularly large contribution to local extravascular generation of ang ii .
the roles of ras and bradykinin in cardiac radiation injury and the potential influence of mast cells herein need further investigation .
radiotherapy planning has undergone many improvements over the last decades , leading to improved targeting and reduced normal tissue radiation exposure .
nonetheless , some patients with hodgkin 's disease , lung cancer , and esophageal and proximal gastric cancer may still receive either a high dose of radiation to a small part of the heart or a lower dose to the whole heart , which may lead to late manifestations of rihd .
some of the basic mechanisms of rihd have begun to emerge from recent pre - clinical studies and include the involvement of vascular injury , mast cells , and the ras .
future studies will elucidate the significance of these mechanisms for clinical rihd and their usefulness as targets for intervention in rihd .
 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 . 
1. Introduction
2. Pre-Clinical Models of Radiation-Induced Heart Disease
3. Vascular Injury and Endothelial Dysfunction
4. Mast Cells
5. Mast Cell Interactions
6. The Renin-Angiotensin System
7. Conclusions