McBride, W. H., Chiang, C-S., Olson, J. L., Wang, C-C., Hong, J-H., Pajonk, F., Dougherty, G. J., Iwamoto, K. S., Pervan, M. and Liao, Y-P. A Sense of Danger from Radiation. Radiat. Res. 162, 1–19 (2004).

Tissue damage caused by exposure to pathogens, chemicals and physical agents such as ionizing radiation triggers production of generic “danger” signals that mobilize the innate and acquired immune system to deal with the intrusion and effect tissue repair with the goal of maintaining the integrity of the tissue and the body. Ionizing radiation appears to do the same, but less is known about the role of “danger” signals in tissue responses to this agent. This review deals with the nature of putative “danger” signals that may be generated by exposure to ionizing radiation and their significance. There are a number of potential consequences of “danger” signaling in response to radiation exposure. “Danger” signals could mediate the pathogenesis of, or recovery from, radiation damage. They could alter intrinsic cellular radiosensitivity or initiate radioadaptive responses to subsequent exposure. They may spread outside the locally damaged site and mediate bystander or “out-of-field” radiation effects. Finally, an important aspect of classical “danger” signals is that they link initial nonspecific immune responses in a pathological site to the development of specific adaptive immunity. Interestingly, in the case of radiation, there is little evidence that “danger” signals efficiently translate radiation-induced tumor cell death into the generation of tumor-specific immunity or normal tissue damage into autoimmunity. The suggestion is that radiation-induced “danger” signals may be inadequate in this respect or that radiation interferes with the generation of specific immunity. There are many issues that need to be resolved regarding “danger” signaling after exposure to ionizing radiation. Evidence of their importance is, in some areas, scant, but the issues are worthy of consideration, if for no other reason than that manipulation of these pathways has the potential to improve the therapeutic benefit of radiation therapy. This article focuses on how normal tissues and tumors sense and respond to danger from ionizing radiation, on the nature of the signals that are sent, and on the impact on the eventual consequences of exposure.

Mitchel, R. E. J., Jackson, J. S. and Carlisle, S. M. Upper Dose Thresholds for Radiation-Induced Adaptive Response against Cancer in High-Dose-Exposed, Cancer-Prone, Radiation-Sensitive Trp53 Heterozygous Mice. Radiat. Res. 162, 20–30 (2004).

Trp53 heterozygous mice are radiation-sensitive and cancer-prone. Groups of 7–8-week-old female Trp53 heterozygous mice were exposed to 4 Gy of ⁶⁰Co γ radiation at high (0.5 Gy/min) or low (0.5 mGy/min) dose rate. Other groups received 10 or 100 mGy at low dose rate 24 h prior to the 4-Gy dose. Tumor frequency and latency were measured over the animals' life span. Exposure to 10 mGy prior to 4 Gy resulted in a small (∼5%) but significant life-span regain and increased latency (∼9%) for all malignant tumors taken together, but 100 mGy further reduced life span slightly (∼7%). Latency responses were tumor type-specific. The prior 10-mGy exposure resulted in a small (∼7%) regain in latency for lymphomas but no change in latency for spinal osteosarcomas. Increasing the adapting dose to 100 mGy eliminated the increase in lymphoma latency and further reduced life span (∼8%). A 10-mGy dose prior to 4 Gy at low dose rate had no effects. Adapting exposures had no significant effect on tumor frequency. We conclude that a single low dose induced a small protective response in vivo in Trp53 ^/− mice, reducing the carcinogenic effects of a subsequent large, high-dose-rate exposure by increasing tumor latency. The upper dose threshold at which low-dose protective effects gave way to detrimental effects was tumor type-specific, as found previously for spontaneous tumors in these same cancer-prone mice (Radiat. Res. 159, 320–327, 2003). However, the upper dose thresholds appear to be lower (below 100 mGy) for radiation-induced tumors than for the same tumors appearing spontaneously.

Rodríguez, P., Montoro, A., Barquinero, J. F., Caballín, M. R., Villaescusa, I. and Barrios, L. Analysis of Translocations in Stable Cells and their Implications in Retrospective Biological Dosimetry. Radiat. Res. 162, 31–38 (2004).

The aim of the present study was to evaluate the influence of the exclusion of cells with unstable aberrations in the elaboration of dose–effect curves for translocations and their implications in biological dosimetry of past exposures. To establish dose–effect curves, peripheral blood samples were irradiated with ⁶⁰Co γ rays at ten different doses and the yield of translocations analyzed by FISH was considered in all cells and in stable cells (those without dicentrics, acentrics or rings). To discriminate transmissible translocations, the dose– effect curve for total apparently simple translocations in stable cells was chosen as the reference. In stable cells, dose– effect curves for apparently simple translocations without pseudosimple and complex-derived one-way patterns, t(Ab)t(Ba) and total translocations were obtained. None of these curves differed from the reference curve. When all cells were considered, only the curve for total translocations was significantly different from the reference curve. From the results obtained it can be concluded that the use of dose–effect curves for apparently simple translocations in stable cells and in all cells will give similar dose estimates in retrospective biological dosimetry studies. However, the use of dose–effect curves for total translocations in all cells will lead to underestimations of the dose mainly at high doses.

Raber, J., Rola, R., LeFevour, A., Morhardt, D., Curley, J., Mizumatsu, S., VandenBerg, S. R. and Fike, J. R. Radiation-Induced Cognitive Impairments are Associated with Changes in Indicators of Hippocampal Neurogenesis. Radiat. Res. 162, 39–47 (2004).

During treatment of brain tumors, some head and neck tumors, and other diseases, like arteriovenous malformations, the normal brain is exposed to ionizing radiation. While high radiation doses can cause severe tissue destruction, lower doses can induce cognitive impairments without signs of overt tissue damage. The underlying pathogenesis of these impairments is not well understood but may involve the neural precursor cells in the dentate gyrus of the hippocampus. To assess the effects of radiation on cognitive function, 2-month-old mice received either sham treatment (controls) or localized X irradiation (10 Gy) to the hippocampus/cortex and were tested behaviorally 3 months later. Compared to controls, X-irradiated mice showed hippocampal-dependent spatial learning and memory impairments in the Barnes maze but not the Morris water maze. No nonspatial learning and memory impairments were detected. The cognitive impairments were associated with reductions in proliferating Ki-67-positive cells and Doublecortin-positive immature neurons in the subgranular zone (SGZ) of the dentate gyrus. This study shows significant cognitive impairments after a modest dose of radiation and demonstrates that the Barnes maze is particularly sensitive for the detection of radiation-induced cognitive deficits in young adult mice. The significant loss of proliferating SGZ cells and their progeny suggests a contributory role of reduced neurogenesis in the pathogenesis of radiation-induced cognitive impairments.

Collis, S. J., Neutzel, S., Thompson, T. L., Swartz, M. J., Dillehay, L. E., Collector, M. I., Sharkis, S. J. and DeWeese, T. L. Hematopoietic Progenitor Stem Cell Homing in Mice Lethally Irradiated with Ionizing Radiation at Differing Dose Rates. Radiat. Res. 162, 48–55 (2004).

It has recently been shown that specific lineage-depleted murine hematopoietic stem cells that home to the bone marrow 2 days after transplantation of ablated primary recipients are capable of long-term engraftment and repopulation of secondary recipients. We were interested in determining whether the rate at which the ablating radiation dose was delivered to the mice affected the homing of lineage-depleted stem cells to the bone marrow and/or sites of tissue damage. Fractionated, lineage-depleted donor marrow cells were isolated and labeled with the membrane dye PKH26. Recipient mice were lethally irradiated with 11 Gy ionizing radiation using varying dose rates and were immediately injected with PKH26-labeled progenitor stem cells. With the exception of the lowest dose-rate group, all irradiated mice had an approximately fivefold (P = 0.014 to 0.025) reduction in stem cell homing to the bone marrow compared to unirradiated control animals. A fivefold reduction of stem cell homing to the spleen compared to unirradiated animals was also observed, though this was not statistically significant for any dose-rate group (P = 0.072 to 0.233). This difference in homing could not be explained by increased stem cell apoptosis/necrosis or non-marrow tissue homing to the intestine, lung or liver. We show that the dose rate at which a lethal dose of total-body radiation is delivered does not augment hematopoietic progenitor stem cell homing to the bone marrow, spleen or sites of early radiation-mediated tissue damage at either 2 or 5 days postirradiation/transplantation. The observation that greater homing was seen in unirradiated control mice calls into question the concept that adequate bone marrow stem cell homing requires radiation-induced “space” to be made in the marrow, certainly for the enriched early progenitor hematopoietic stem cells used for this set of experiments. Further experiments will be needed to determine whether these homed cells are as capable of giving rise to long-term engraftment/repopulation of the marrow of secondary recipients as they are in irradiated recipients.

Shi, C., Cheng, T., Su, Y., Mai, Y., Qu, J., Ran, X., Lou, S., Xu, H. and Luo, C. Transplantation of Dermal Multipotent Cells Promotes Survival and Wound Healing in Rats with Combined Radiation and Wound Injury. Radiat. Res. 162, 56–63 (2004).

Combined radiation and wound injury occurs after severe nuclear accidents that accompany explosions or nuclear attacks. High doses of ionizing radiation can cause bone marrow aplasia and delay wound healing. Combined radiation and wound injury is very complex and is more difficult to deal with than single injuries. Multipotent stem cells that have self-renewal potential and multilineage differentiation capacity are the relevant cells in regenerative medicine. To determine whether multipotent stem cells can have multiple therapeutic effects in vivo, systemic transplantation of cultured dermal multipotent cells was performed in rats with combined radiation and wound injury. The results showed that dermal multipotent cell transplantation promoted survival and accelerated both hematopoietic recovery and wound healing in rats with combined radiation and wound injury. FISH analysis using a Y-chromosome-specific probe indicated that donor dermal multipotent cells could engraft into recipient skin and bone marrow after transplantation. FACS analysis of the proportions of CD2- and CD25-positive peripheral lymphocytes indicated that dermal multipotent cell transplantation did not induce an obvious activation of allogeneic lymphocytes in vivo in 3 weeks. These data indicate that dermal multipotent cell transplantation may provide a new tool for the treatment of combined radiation and wound injuries.

Berrada, M., Yang, Z. and Lehnert, S. M. Sensitization to Radiation from an Implanted ¹²⁵I Source by Sustained Intratumoral Release of Chemotherapeutic Drugs. Radiat. Res. 162, 64–70 (2004).

We have investigated tumor response to low-dose-rate irradiation from an implanted ¹²⁵I source alone or in conjunction with intratumoral drug administration. The drug (cis-DDP or 5-FU) was incorporated homogeneously into the co-polymer CPP-SA, 20:80, and the polymer/drug rods were implanted in the RIF-1 fibrosarcomas growing subcutaneously in C3H mice. Twenty-four hours later, the tumor was implanted with an ¹²⁵I seed. Tumor growth time was the end point in these experiments. For implanted ¹²⁵I sources of different dose rates and implant times giving a range of total doses, a consistent dose–response relationship was shown between tumor growth time and total dose. In other experiments, ¹²⁵I sources of different specific activities were implanted for periods of time adjusted so that the total dose to the tumor was always the same. When the ¹²⁵I implant was combined with 5-FU, greater than additive responses were seen for both short (30 h) and long (96 h) ¹²⁵I treatment times. In contrast, a short-duration (30 h) ¹²⁵I implant combined with cis-DDP was the least effective treatment, giving a combined response that was no better than additive, whereas 96 h exposure to ¹²⁵I combined with cis-DDP was the most effective combined treatment. It is conjectured that this inverse dose-rate effect seen when cis-DDP is combined with low-dose rate radiation is related to a cell cycle effect and/or to inhibition of repair of radiation damage by cis-DDP.

Borkenstein, K., Levegrün, S. and Peschke, P. Modeling and Computer Simulations of Tumor Growth and Tumor Response to Radiotherapy. Radiat. Res. 162, 71–83 (2004).

A model of tumor growth and tumor response to radiation is introduced in which each tumor cell is taken into account individually. Each cell is assigned a set of radiobiological parameters, and the status of each cell is checked in discrete intervals. Tumor proliferation is governed by the cell cycle times of tumor cells, the growth fraction, the apoptotic capacity of the tumor, and the degree of tumor angiogenesis. The response of tumor cells to radiation is determined by the radiosensitivities and the oxygenation status. Computer simulation is performed on a 3D rigid cubic lattice, starting out from a single tumor cell. Random processes are simulated by Monte Carlo methods. Short cell cycle time, high growth fraction, and tumor angiogenesis all increase tumor proliferation rates. Accelerated time–dose patterns result in lower total doses needed for tumor control, but the extent of dose reduction depends on the kinetics and the radiosensitivities of tumor cells. Tumor angiogenesis alters fully oxygenated and hypoxic fractions within the tumor and subsequently affects the radiation response. It is demonstrated for selected radiobiological parameters that the simulation tools are suitable to quantitatively assess the total doses needed for tumor control. Using the simulation tools, it is feasible to simulate time–dependent effects during fractionated radiotherapy and to compare different time–dose patterns in terms of their tumor control.

Lobachevsky, P. N., Karagiannis, T. C. and Martin, R. F. Plasmid DNA Breakage by Decay of DNA-Associated Auger Electron Emitters: Approaches to Analysis of Experimental Data. Radiat. Res. 162, 84–95 (2004).

Plasmid DNA is a popular substrate for the assay of DNA strand breakage by a variety of agents. The use of the plasmid assay relies on the assumption that individual damaging events occur at random, which allows the application of Poisson statistics. This assumption is not valid in the case of damage arising from decay of DNA-associated Auger electron emitters, since a single decay event can generate a few breaks in the same DNA strand, which is indistinguishable from a single break in the assay. The consequent analytical difficulties are overcome by considering relaxation events rather than single-strand breaks, and linearization events rather than double-strand breaks. A further consideration is that apart from damage at the site of DNA-associated decay, which is the principal interest of the analysis, some DNA damage also arises from the radiation field created by all decay events. These two components of damage are referred to as internal and external breakage, respectively, and they can be separated in the analysis since their contribution depends on the experimental conditions. The DNA-binding ligand Hoechst 33258 labeled with ¹²⁵I was used in our experiments to study breakage in pBR322 plasmid DNA arising from the decay of this Auger electron emitter. The values obtained for the efficiency (per decay) of plasmid relaxation and linearization by the ¹²⁵I-labeled ligand were 0.090 ± 0.035 and 0.82 ± 0.04, respectively. When dimethylsulfoxide was included as a radical scavenger, the efficiency values for relaxation and linearization were 0.15 ± 0.02 and 0.65 ± 0.05, respectively.

Vanhaelewyn, G. C. A. M., Jansen, B., Callens, F. J. and Sagstuen, E. ENDOR-Assisted Study of the Stable EPR Spectrum of X-Irradiated α-l-Sorbose Single Crystals: MLCFA and Simulation Decomposition Analyses. Radiat. Res. 162, 96–104 (2004).

After X irradiation of single crystals of α-l-sorbose at 295 K, previous electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) results have indicated the formation of at least 10 different free radicals, and also that conceivably each carbon in the pyranose ring is a possible radical center. The radicals appear to be formed mostly by net H-abstraction reactions followed by standard elimination (e.g. β-OH elimination) reactions or proton shifts, in turn leading to ring opening and fragmentation. In the present work, EPR spectra were recorded at room temperature with the external magnetic field along each of the three crystallographic axes subsequent to careful annealing at different temperatures using a high-temperature cavity. Each of the three sets of spectra was subjected to a maximum likelihood common factor analysis (MLCFA) that contributed to a better understanding of the spectral decays. Furthermore, the most stable spectra were simulated by optimization of previous ENDOR and EI-EPR results. The optimized EPR parameters resulted in excellent simulations of the experimental stable sorbose spectra and hence provided an improved insight of their spectral compositions.