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RADIOBIOLOGY Direct Damage from Radiation May Be Passed to Neighboring CellsThe poet John Donne wrote that no man is an island, and the same could be said of eukaryotic cells. Though separate, cells communicate, organize, share resources, and form direct connections with one another. Should it surprise us then that just as Donne felt diminished by any man's death, cells are affected by damage to their neighbors? Research led by John Little, the James Stevens Simmons professor of radiobiology and chair of the Department of Cancer Cell Biology at HSPH, shows that cells hit by radiation can send signals to neighboring cells that result in DNA damage. These findings challenge the long-held assumption that radiation harms cells only by direct contact, suggesting that radiation's biological effects are more complex than previously thought.
Immunofluorescence detection of p21 shows that irradiated cells transmit damage signals through gap junctions. Compared to nonirradiated cells (left), cells exposed to a low level of radiation express cellular stress protein p21 in larger numbers than expected (center). After incubation with lindane, which inhibits intercellular communication through gap junctions, only cells directly hit by radiation express p21 (right). Courtesy of Edouard Azzam "It's always been the theory that radiation directly produces DNA damage in cells," said Little. Ionizing radiation can damage DNA by directly knocking electrons out of DNA molecules, or by ionizing cellular water and creating free radicals that attack DNA. This damage can lead to DNA mutations and eventually cancer. The assumption is that radiation's biological effects follow a clear causal progression: a mutation arises at or near the site of DNA damage, and cancer, because it usually originates from a single cell, arises from cells directly hit by radiation. So models for predicting the biological effects of different levels of ionizing radiation take into account how many cells are directly hit. In 1992, Little's lab first published evidence that given a population of cells in which only an occasional cell is irradiated, biological effects associated with radiation damage can occur in nonirradiated, or bystander, cells. "This is really quite a new concept," said Little. "In fact, at first no one really believed it." The research languished until another group reproduced the results, and then the idea slowly began to gain support from other researchers. Sharing Poison PillsLittle's team has been able to study this bystander effect by irradiating a population of cells using very low levels of alpha radiation, so that only one percent of cells are actually traversed by an alpha particle. They then measure characteristics of radiation damage, such as mutations or the enhanced expression of tumor suppressor protein p53 and its downstream counterpart p21, both of which respond to stress in the cell. If radiation were only damaging the cells it hit, one would expect about one cell in a hundred to express these proteins. In fact, the proteins were detected at higher levels in clusters of cells. The team also looked for the formation of micronuclei, a sign that DNA has been damaged, and found that they occurred at higher levels in bystander cells.
John Little, shown using an alpha irradiator on a cell culture, has found that cells respond to radiation as a whole tissue not as isolated individuals. Photo by Steve Gilbert All of these experiments suggest that damage to one cell can affect its neighbors. If the radiation is not directly hitting each damaged cell, Little's team reasoned, the damage must be conveyed indirectly, through some kind of intercellular signal. "It's not new, the idea that cells are talking back and forth to each other," Little said. "What's new is that you could actually get a damage signal transmitted that would lead to cell death or lead to mutations. It means that the tissue is responding as a whole." The current paper, published in the Jan. 16 Proceedings of the National Academy of Sciences, focuses on the mechanism through which cells send these alarms to one another after being irradiated. The team, led by Little and Edouard Azzam, then a postdoc in Little's lab and now an assistant professor of radiology at New Jersey Medical School, has found that the signals are most likely transmitted through gap junctions, small channels that form between adjacent cells and enable them to share small molecules. The team found that treating the cells with a chemical inhibitor of gap junction communication blocked the effect of radiation on bystander cells. Similar results were obtained using a knockout cell line deficient in its ability to communicate through gap junctions. Although these experiments have helped track the path of damage signals, what these signals are and how they inflict damage is still unknown. Only a selection of small molecules can pass through gap junctions, limiting the kinds of signals that could be transmitted. Little has preliminary evidence that suggests there may be a role for oxidative stress in the damage to neighboring cells. Reactive oxygen species like hydrogen peroxide can be highly toxic to a cell, and such oxidative molecules could be passed directly through gap junctions or could be induced through a reaction within the bystander cell by some other signaling molecule. Reconsidering RiskOne of the most immediate implications for these findings is how risk is calculated for low levels of exposure to densely ionizing radiation such as residential radon. "The kinds of doses that come from radon in our houses and in our basements are the kind where only a small fraction of the bronchial epithelial cells are actually traversed by an alpha particle in a period of a week, a month, or even a year," said Little. Consequently, models that predict the relationship between the dose of radon and its effects on people take into account only the number of cells hit by radiation. "But if, down at these low doses, you're inducing mutations not only in the cells that are hit but in 10 cells around them, you may not be able to predict their effects just by the number of cells that are actually irradiated."The idea that radiation's effects can be more widespread than previously thought raises a host of questions. For one, this research has only looked at alpha radiation, which can be targeted easily to a small number of cells. Whether similar effects result from more sparsely ionizing radiation such as gamma or x-rays is unknown, and it is much more difficult to test these kinds of radiation on only a few cells in a population. It is also unclear if and how these effects relate to higher doses of radiation, or whether the bystander effect accounts for observed biological damage on a larger scale. It raises the possibility that damage signaling and the stress response of tissues may be an important consideration in treatment. Now that the path of the signals has been tracked, deciphering the language of these intercellular messages is the next step in understanding their effects. "If you can find out what the signals are," said Little, "you have the possibility of manipulating or modulating them." —Courtney Humphries |
