Current Molecular Pharmacology - Volume 4, Issue 2, 2011

Ionizing radiation has been indispensable to medical diagnosis and cancer therapy. Humans are also daily exposed to natural background low-level radiation. Despite a century having passed since the discovery of X-rays, the biological mechanism of radiation action remains incompletely understood. Over the last half-century, a central tenet in radiation biology has been that the nuclear DNA is the quintessential ‘target’ for the biological action of radiation. Accordingly, it has long been presumed that no effect would be expected in cells whose nucleus receives no radiation traversal; however, this dogma has been challenged by the recent observation of induced biological effects that are independent of nuclear irradiation. This phenomenon has been termed ‘non-targeted’ effects, which include radiation- induced bystander effects and genomic instability. On one hand, the former can be defined as the occurrence of biological effects in non-irradiated ‘bystander’ cells resulting from radiation exposure of other cells. On the other hand, the latter can be described as the delayed biological effects arising in the progeny cells surviving irradiation at many generations after the initial insult of the parental cell. Intriguingly, recent studies have documented that whereas the progeny of bystander cells manifest genomic instability, the progeny of irradiated cells cause the bystander effects in non-irradiated cells. Such interrelation between the bystander effects and genomic instability highlights that radiation signals can be spatiotemporally propagated over time through persistent non-targeted responses. Though there is mounting in vitro and in vivo evidence for non-targeted effects, its significance has been the subject of debate. This ‘Hot-Topic’ issue provides a forum to grasp the current knowledge on the non-targeted effects, and propose its significance. Nobuyuki Hamada et al. overview the manifestations of the bystander effects in vitro, in vivo and in humans. Their cellular and molecular underpinnings are then mentioned, especially focusing on the intercellular signaling from irradiated to bystander cells, and its downstream intracellular signaling in bystander cells. They also discuss the potential contribution of the bystander effects to cancer radiotherapy. Tom K. Hei et al. describe the molecular mechanisms of non-targeted effects with particular emphasis on nuclear factor κB dependent gene expression, and discuss the potential clinical implication of non-targeted effects in the induction of secondary cancer arising from radiation therapy of primary tumors. Astronauts are exposed to energetic heavy-ion radiation at low fluence during long-term space missions, but there is a lack of clear knowledge about space radiation-induced biological effects. Here, Edouard I. Azzam and his colleagues conducted the localized cranial heavy-ion irradiation of rats with the rest of the body shielded. They demonstrate that the non-targeted liver exhibits the elevated levels of key proteins involved in cellular defense mechanisms (e.g., antioxidation), but the decreased levels of those in fatty acid metabolism. Olga Kovalchuk and her colleagues have carried out a series of studies that address the in vivo epigenetic bystander effects and transgenetrational genomic instability in mouse or rat models. They here report the induction of DNA damage and altered DNA methylation in the non-targeted testis tissue of mice whose head was irradiated. A prior low-dose or low-dose-rate irradiation is known to mitigate the stressful biological effects of high-dose ‘challenging’ irradiation. Hideki Matsumoto et al. stress that such adaptive responses are interrelated with the bystander effects where reactive nitrogen species act as likely common initiators. Finally, William F. Morgan argues that non-targeted effects are a tissue-level response to restore equilibrium within an organ system, and discusses its potential implications for radiation carcinogenesis. I believe that this issue gives timely in-depth overviews to the readers and will contribute to progress in this important research area. A mechanistic understanding of the non-targeted effects should provide insights into cancer radiotherapy, and the more general biological response to external stress other than radiation. Last but not least, I am grateful to Dr. Nouri Neamati, an Editor-in-Chief of Current Molecular Pharmacology for the invitation to edit this issue, to the distinguished authors for their invaluable contributions, and to the expert referees for their cooperation and dedication.

For nearly a century, ionizing radiation has been indispensable to medical diagnosis. Furthermore, various types of electromagnetic and particulate radiation have also been used in cancer therapy. However, the biological mechanism of radiation action remains incompletely understood. In this regard, a rapidly growing body of experimental evidence indicates that radiation exposure induces biological effects in cells whose nucleus has not been irradiated. This phenomenon termed the ‘non-targeted effects’ challenges the long-held tenet that radiation traversal through the cell nucleus is a prerequisite to elicit genetic damage and biological responses. The non-targeted effects include biological effects in cytoplasm-irradiated cells, bystander effects that arise in non-irradiated cells having received signals from irradiated cells, and genomic instability occurring in the progeny of irradiated cells. Such non-targeted responses are interrelated, and the bystander effect is further related with an adaptive response that manifests itself as the attenuated stressful biological effects of acute high-dose irradiation in cells that have been pre-exposed to low-dose or low-dose-rate radiation. This paper reviews the current body of knowledge about the bystander effect with emphasis on experimental approaches, in vitro and in vivo manifestations, radiation quality dependence, temporal and spatial dependence, proposed mechanisms, and clinical implications. Relations of bystander responses with the effects in cytoplasm-irradiated cells, genomic instability and adaptive response will also be briefly discussed.

Generations of students in radiation biology have been taught that heritable biological effects require direct damage to DNA. Radiation-induced non-targeted/bystander effects represent a paradigm shift in our understanding of the radiobiological effects of ionizing radiation in that extranuclear and extracellular effects may also contribute to the biological consequences of exposure to low doses of radiation. Although radiation induced bystander effects have been well documented in a variety of biological systems, including 3D human tissue samples and whole organisms, the mechanism is not known. There is recent evidence that the NF-κB-dependent gene expression of interleukin 8, interleukin 6, cyclooxygenase- 2, tumor necrosis factor and interleukin 33 in directly irradiated cells produced the cytokines and prostaglandin E2 with autocrine/paracrine functions, which further activated signaling pathways and induced NF-κB-dependent gene expression in bystander cells. The observations that heritable DNA alterations can be propagated to cells many generations after radiation exposure and that bystander cells exhibit genomic instability in ways similar to directly hit cells indicate that the low dose radiation response is a complex interplay of various modulating factors. The potential implication of the non-targeted response in radiation induced secondary cancer is discussed. A better understanding of the mechanism of the non-targeted effects will be invaluable to assess its clinical relevance and ways in which the bystander phenomenon can be manipulated to increase therapeutic gain in radiotherapy.

The lack of clear knowledge about space radiation-induced biological effects has been singled out as the most important factor limiting the prediction of radiation risk associated with human space exploration. The expression of space radiation-induced non-targeted effects is thought to impact our understanding of the health risks associated with exposure to low fluences of particulate radiation encountered by astronauts during prolonged space travel. Following a brief review of radiation-induced bystander effects and the growing literature for the involvement of oxidative metabolism in their expression, we show novel data on the induction of in vivo non-targeted effects following exposure to 1100 MeV/nucleon titanium ions. Analyses of proteins by two-dimensional gel electrophoresis in non-targeted liver of cranially-irradiated Sprague Dawley rats revealed that the levels of key proteins involved in mitochondrial fatty acid metabolism are decreased. In contrast, those of proteins involved in various cellular defense mechanisms, including antioxidation, were increased. These data contribute to our understanding of the mechanisms underlying the biological responses to space radiation, and support the involvement of mitochondrial processes in the expression of radiation induced non-targeted effects. Significantly, they reveal the cross-talk between propagated stressful effects and induced adaptive responses. Together, with the accumulating data in the field, our results may help reduce the uncertainty in the assessment of the health risks to astronauts. They further demonstrate that ‘network analyses’ is an effective tool towards characterizing the signaling pathways that mediate the long-term biological effects of space radiation.

Ionizing radiation (IR) is a curative treatment for many human malignancies, an important diagnostic modality, and a pivotal preparative regimen for bone marrow transplantation. On the other hand, IR is a potent damaging agent that can affect a variety of processes in directly exposed cells, in their descendents, and in neighboring un-irradiated naïve ‘bystander’ cells. Accumulation of DNA damage caused by IR in conjunction with disrupted cellular regulation processes can lead to genome instability in the germline, and therefore to transgenerational genome instability in offspring of exposed males. The exact mechanisms of IR-induced genome instability in directly exposed and in naïve bystander germ cells remain obscure, yet accumulating evidence points to the role of DNA damage and DNA methylation changes in genome instability development. In the current study, we used a well-established murine model to define the role of DNA methylation, DNA damage, as well as two important germline regulators of DNA methylation, Brother of the Regulator of Imprinted Sites (BORIS) and CCCTC binding factor (CTCF), in IR and bystander responses of the male germline. Here we report that irradiation leads to a significant accumulation of DNA damage in the exposed and bystander testis tissue, and to altered DNA methylation and dysregulated BORIS expression in the exposed testis tissue. The possible molecular mechanisms and biological consequences of the observed changes are discussed.

A classical paradigm of radiation biology asserts “targeted effect” that all radiation effects on cells, tissues and organisms are due to the direct action of radiation. However, over the past two decades, a paradigm of radiation biology has undergone a shift away from “targeted effect” relationships and towards complex ongoing “intra- and inter-cellular responses”, which involve not only targeted but also non-targeted ones. These responses include now familiar, but still fully unknown, phenomena associated with low-dose/low-dose-rate radiation exposure such as adaptive responses, bystander responses, low-dose hypersensitivity, and genomic instability. The mechanisms underlying these responses often involve biochemical/molecular signals that respond to targeted and non-targeted events. Matsumoto et al. have previously found that nitric oxide functions as initiators of radiation-induced bystander and adaptive responses. These findings suggest correlations between the radioadaptive and bystander responses. The present review focuses on these two phenomena by summarizing observations supporting their existence, and discussing the linkage between them from the aspect of production of reactive nitrogen species.

Non-targeted effects, i.e., those responses in cells or tissues that were not subject to energy deposition events after localized exposure to ionizing radiation, are well established. While they are not a universal phenotype, when they do occur they can be associated with subsequent tissue or whole body responses. Here it is argued that non-targeted effects are a tissue level response to restore equilibrium within an organ system, and thus restore tissue homeostasis. This “adaptive homeostasis” has evolved in response to a variety of environmental and other such stresses an individual is exposed to in their lifetime. These non-targeted effects are not likely to impact significantly on estimates of potential risks associated with radiation exposure because they are presumably “built into” current risk estimates. However, they could have implications for radiation carcinogenesis, by driving processes in targeted and non-targeted cells that could eliminate transformed cells or transform cells from a normal phenotype to a phenotype associated with malignancy within a tissue.

Smad7 is an inhibitory Smad protein that blocks Transforming Growth Factor-beta (TGF-β) signaling through a negative feedback loop, also capable of mediating the crosstalk between TGF-β and other signaling pathways. Smad7 mRNA and protein levels are upregulated after TGF-β signaling; subsequently, Smad7 protein binds TGF-β type I receptor blocking R-Smad phosphorylation and eventually TGF-β signaling. Because of this inhibitory function, Smad7 can antagonize diverse cellular processes regulated by TGF-β such as cell proliferation, differentiation, apoptosis, adhesion and migration. Smad7 induction by different cytokines, besides TGF-β, is also critical for crosstalk/integration of a variety of signaling pathways, and relevant in the pathology of some diseases. Thus, Smad7 plays a key role in the control of various physiological events, and even in some pathological processes including fibrosis and cancer. This review highlights the main known functions of Smad7 with a particular focus on the relevance that alterations of Smad7 function may have in homeostasis, also describing some Smad7 emerging roles in the development of several human diseases that identify this protein as a potential therapeutic target.