Understanding radiation-induced vascular disease.

Most cardiovascular events occur >10 years after completing radiotherapy, so demonstrating causality has proven difficult (5). An estimated 50 million cancer survivors worldwide have been treated with radiation therapy; accordingly, clinicians must be aware of the potential cardiovascular risk and manage risk factors appropriately. Moreover, research into the mechanisms of radiation-induced vascular disease is paramount to understanding and potentially modifying the disease process. The study by Martin et al. (6) in this issue of the Journal is welcome, because it sheds new light on the pathogenesis of radiation-induced vascular disease in humans. Experimental studies in animals have firmly established a causal relationship between irradiation and vascular disease. Lethal total-body irradiation of atherosclerosis-prone mice followed by bone marrow transplantation noticeably altered lesion composition and stability (7,8). Nonlethal irradiation of atherosclerosis-prone mice did not change systemic indicators of inflammation or cholesterol levels but dramatically altered lesion composition long after treatment (9). There were no changes in the atherosclerotic lesions of “out-of-field” arteries, consistent with a local rather than systemic effect of radiation. Irradiated arteries 22 to 34 weeks after treatment were highly enriched with macrophages, which accounted for the majority of the lesion area. Also, intraplaque hemorrhage was restricted to and commonly observed in irradiated arteries. These studies, however, did not identify a molecular mechanism to explain the observations. Studies to address radiation-induced vascular disease in humans have largely been descriptive in nature. From the histological perspective, lesions in medium-sized to large vessels (>100 μm in diameter) exhibit typical features of atherosclerosis, including lipid accumulation, inflammation, and thrombosis (3). Increases in intimal thickness and connective tissue content are also prominent features (2). From the angiographic perspective, the lesions are longer than traditional atherosclerotic lesions, and the regions of maximal stenosis tend to be at the ends of the lesions (10). Treating these lesions via open surgical procedures is often problematic, due to extensive soft tissue scarring; hence, percutaneous approaches are usually preferred (5). How does a course of radiotherapy initiate a chronic vascular process that eventually leads to clinical events many years after treatment? Experimental studies in vitro and in vivo indicate that radiation therapy causes acute up-regulation of pro-inflammatory cytokines and adhesion molecules in endothelium that recruits inflammatory cells to sites of vascular injury (11). It is unlikely, however, that this acute insult per se is sufficient to produce long-term occlusive atherosclerotic disease. Thus, late effects of radiation therapy are more likely responsible. In this regard, induction of chronic oxidative stress is increasingly being implicated in radiation-induced late tissue injury (12). In addition to the rapid burst of free radicals produced acutely by ionization of water molecules, radiation increases chronic free radical production and oxidative stress in the affected tissues. Oxidative stress up-regulates numerous pathways pertinent to vascular disease, including matrix metalloproteinases, adhesion molecules, pro-inflammatory cytokines, and smooth muscle cell proliferation and apoptosis, while inactivating vasculoprotective nitric oxide. Considerable evidence suggests that the nuclear transcription factor NF-κB serves as a molecular link between oxidative stress and chronic inflammation (13). The nuclear factor-kappa B (NF-κB) family of transcription factors includes 5 members: p50, p52, p65, RelB, and c-Rel. The NF-κB is involved in numerous pathological and physiological conditions, including cellular function (i.e., proliferation, differentiation, and survival), tumorigenesis, and inflammation. Upon activation, NF-κB is released from its inhibitory association with the IκB proteins in the cytoplasm and translocates to the nucleus. In the nucleus, NF-κB binding to specific deoxyribonucleic acid response elements initiates robust transcriptional responses and reprograms cellular function. Depending upon the stimuli, NF-κB can activate distinct sets of downstream genes that mediate different outcomes (14). In the context of vascular biology, NF-κB is a master regulator of inflammation and leukocyte adhesion and synchronizes the expression of adhesion molecules, cytokines, and chemokines in endothelial cells. Importantly, NF-κB is controlled by redox regulation, making it a prime candidate to link chronic oxidative stress to activation of downstream inflammatory pathways in radiation injury. The study by Martin et al. (6) provides the first direct evidence that NF-κB is chronically up-regulated in human arteries after radiation exposure. The investigators examined paired arterial specimens from skin flaps of patients who had undergone irradiation therapy for head-and-neck cancer between 4 and 500 weeks previously. They directly compared irradiated versus nonirradiated arteries from the same patients, thereby avoiding confounding inter-patient variables. Differentially expressed genes in the irradiated arteries were detected with oligonucleotide microarrays, and the data were validated by real-time polymerase chain reaction. These data were used to detect clusters of altered gene expression, which demonstrated patterns consistent with up-regulated inflammation, coagulation, and angiogenesis. Importantly, the pattern of altered gene expression in the irradiated arteries suggested transcriptional regulation by NF-κB. Indeed, immunohistological studies demonstrated up-regulated NF-κB in vascular wall cells, with specific localization to macrophages. Interestingly, a group of putative NF-κB– dependent genes were found to be similarly dysregulated, regardless of the amount of time since irradiation (4 to 7 weeks vs. 20 to 500 weeks). A similar study with larger number of patients would be needed to separate early-stage from late-stage gene expression programs, which might be highly informative. The study by Martin et al. (6) provides the foundation for a molecular mechanism to explain the effects of radiation on vascular biology (Fig. 1). As with many studies that address a poorly understood phenomenon, the data generate more questions than answers. Are the results observed in the small conduit arteries in this study translatable to large arteries that produce most cardiovascular events? Does oxidative stress cause the persistent NF-κB up-regulation and reprogramming of gene expression in irradiated blood vessels? Lastly and most importantly, could modulation of NF-κB ameliorate the disease process? Several commonly used agents, such as aspirin, omega-3 fatty acids, and statins, directly or indirectly modulate NF-κB activity; whether these medications could ameliorate radiation-induced vascular disease remains to be determined. Also, inhibitors of NF-κB are being tested for a variety of inflammatory states and might eventually make their way into clinical medicine (13,14). Perhaps such therapy could be employed to treat radiation-induced vascular disease. Alternatively, the pathways responsible for up-regulated oxidative stress might be targeted. In this regard, activation of the angiotensin II-aldosterone system has been hypothesized to play a key role in propagating oxidative stress after radiation therapy (12,15). Thus, pharmacotherapy directed against this pathway could potentially be efficacious against radiation-induced vascular disease. Figure 1 Proposed Mechanism of Involvement of NF-κB in Radiation-Induced Vascular Disease In conclusion, Martin et al. (6) have made an important contribution to the field of radiation-induced vascular disease by demonstrating local and sustained up-regulation of NF-κB in irradiated human blood vessels. The expression profiles suggest that NF-κB contributes to the pathology by inducing pro-inflammatory genes. Further research is needed to determine the clinical significance of their findings and to investigate whether currently available and/or emerging therapies can modulate the disease process.