Participation of gap-junctional cell communication on the adaptive response in human cells induced by low dose of X-rays

Re-evaluation of the linear no-threshold (LNT) model using new paradigms and modern molecular studies

The linear no-threshold (LNT) model is currently used to estimate low dose radiation (LDR) induced health risks. This model lacks safety thresholds and postulates that health risks caused by ionizing radiation is directly proportional to dose. Therefore even the smallest radiation dose has the potential to cause an increase in cancer risk. Advances in LDR biology and cell molecular techniques demonstrate that the LNT model does not appropriately reflect the biology or the health effects at the low dose range. The main pitfall of the LNT model is due to the extrapolation of mutation and DNA damage studies that were conducted at high radiation doses delivered at a high dose-rate. These studies formed the basis of several outdated paradigms that are either incorrect or do not hold for LDR doses. Thus, the goal of this review is to summarize the modern cellular and molecular literature in LDR biology and provide new paradigms that better represent the biological effects in the low dose range. We demonstrate that LDR activates a variety of cellular defense mechanisms including DNA repair systems, programmed cell death (apoptosis), cell cycle arrest, senescence, adaptive memory, bystander effects, epigenetics, immune stimulation, and tumor suppression. The evidence presented in this review reveals that there are minimal health risks (cancer) with LDR exposure, and that a dose higher than some threshold value is necessary to achieve the harmful effects classically observed with high doses of radiation. Knowledge gained from this review can help the radiation protection community in making informed decisions regarding radiation policy and limits.

Unstable chromosome aberrations do not accumulate in normal human fibroblast after fractionated x-irradiation

We determined the frequencies of dicentric chromosomes per cell in non-dividing confluent normal human fibroblasts (MRC-5) irradiated with a single 1 Gy dose or a fractionated 1 Gy dose (10X0.1 Gy, 5X0.2 Gy, and 2X0.5 Gy). The interval between fractions was between 1 min to 1440 min. After the completion of X-irradiation, the cells were incubated for 24 hours before re-plating at a low density. Then, demecolcine was administrated at 6 hours, and the first mitotic cells were collected for 42 hours. Our study demonstrated that frequencies of dicentric chromosomes in cells irradiated with a 1 Gy dose at different fractions were significantly reduced if the fraction interval was increased from 1 min to 5 min (p<0.05, χ2-test). Further increasing the fraction interval from 5 up to 1440 min did not significantly affect the frequency of dicentric chromosomes. Since misrejoining of two independent chromosome breaks introduced in close proximity gives rise to dicentric chromosome, our results indicated that such circumstances might be quite infrequent in cells exposed to fractionated X-irradiation with prolonged fraction intervals. Our findings should contribute to improve current estimation of cancer risk from chronic low-dose-rate exposure, or intermittent exposure of low-dose radiation by medical exposure.

Dynamics of cellular responses to radiation

Understanding the consequences of exposure to low dose ionizing radiation is an important public health concern. While the risk of low dose radiation has been estimated by extrapolation from data at higher doses according to the linear non-threshold model, it has become clear that cellular responses can be very different at low compared to high radiation doses. Important phenomena in this respect include radioadaptive responses as well as low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR). With radioadaptive responses, low dose exposure can protect against subsequent challenges, and two mechanisms have been suggested: an intracellular mechanism, inducing cellular changes as a result of the priming radiation, and induction of a protected state by inter-cellular communication. We use mathematical models to examine the effect of these mechanisms on cellular responses to low dose radiation. We find that the intracellular mechanism can account for the occurrence of radioadaptive responses. Interestingly, the same mechanism can also explain the existence of the HRS and IRR phenomena, and successfully describe experimentally observed dose-response relationships for a variety of cell types. This indicates that different, seemingly unrelated, low dose phenomena might be connected and driven by common core processes. With respect to the inter-cellular communication mechanism, we find that it can also account for the occurrence of radioadaptive responses, indicating redundancy in this respect. The model, however, also suggests that the communication mechanism can be vital for the long term survival of cell populations that are continuously exposed to relatively low levels of radiation, which cannot be achieved with the intracellular mechanism in our model. Experimental tests to address our model predictions are proposed.

The effect of low-dose radiation on cells and tissues is a public health concern, because the human population is exposed to low-dose ionizing radiation coming from a variety of sources, such as cosmic rays, soil radioactivity, environmental contaminations, and various medical procedures. At low doses of radiation, phenomena are observed that do not occur at higher doses, such as radioadaptive responses as well as low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR), which are so far not fully understood. Each of these phenomena have been investigated separately, and specific mechanisms have been suggested to explain them. Using mathematical models that are successfully fitted to experimental data under a variety of conditions, we show that a set of basic and documented assumptions about cellular responses to low-dose radiation can explain all three low-dose phenomena, indicating that they are inter-related. According to the model, these phenomena are brought about by the multi-factorial interactions that underlie the population dynamics of the cells involved, and this provides a new framework to understand these responses, and to evaluate the risk to human health posed by exposure to low-dose radiation.

Adaptive response to hydrogen peroxide in yeast: induction, time course, and relationship to dose-response models

The assay for trp5 gene conversion and ilv1-92 reversion in Saccharomyces cerevisiae strain D7 was used to characterize the induction of an adaptive response by hydrogen peroxide (H(2)O(2)). Effects of a small priming dose on the genotoxic effects of a larger challenge dose were measured in exponential cultures and in early stationary phase. An adaptive response, indicated by smaller convertant and revertant frequencies after the priming dose, occurred at lower priming and challenge doses in young, well-aerated cultures. Closely spaced priming doses from 0.000975 to 2 mM, followed by a 1 mM challenge, showed that the induction of the adaptive response is biphasic. In exponential cultures it was maximal with a priming dose of 0.125-0.25 mM. Very small priming doses were insufficient to induce the adaptive response, whereas higher doses contributed to damage. A significant adaptive response was detected when the challenge dose was administered 10-20 min after the priming exposure. It was fully expressed within 45 min, and the yeast began to return to the nonadapted state after 4-6 hr. Because of the similarity of the biphasic induction to hormetic curves and the proposal that adaptive responses are a manifestation of hormesis, we evaluated whether the low doses of H(2)O(2) that induce the adaptive response show a clear hormetic response without a subsequent challenge dose. Hormesis was not evident, but there was an apparent threshold for genotoxicity at or slightly below 0.125 mM. The results are discussed with respect to linear, threshold, and hormesis dose-response models.

Phenolphthalein induces centrosome amplification and tubulin depolymerization in vitro

Aneuploidy is a major cause of human reproductive failure and plays a large role in cancer. Phenolphthalein (PHT) induces tumors in rodents but its primary mechanism does not seem to be DNA damage. In heterozygous TSG-p53(®) mice, PHT induces lymphomas and also micronuclei (MN), many containing kinetochores (K), implying chromosome loss (aneuploidy). The induction of aneuploidy would be compatible with the loss of the normal p53 gene seen in the lymphomas. In this study, we confirm PHT's aneugenicity and determine the aneugenic mechanism of PHT by combining traditional genetic toxicology assays with image and flow cytometry methods. The data revealed that PHT induces tubulin polymerization abnormalities and deregulates the centrosome duplication cycle causing centrosome amplification. We also show that one of the consequences of these events is apoptosis.

Human cell responses to ionizing radiation are differentially affected by the expressed connexins

In multicellular organisms, intercellular communication is essential for homeostatic functions and has a major role in tissue responses to stress. Here, we describe the effects of expression of different connexins, which form gap junction channels with different permeabilities, on the responses of human cells to ionizing radiation. Exposure of confluent HeLa cell cultures to (137)Cs γ rays, 3.7 MeV α particles, 1000 MeV protons or 1000 MeV/u iron ions resulted in distinct effects when the cells expressed gap junction channels composed of either connexin26 (Cx26) or connexin32 (Cx32). Irradiated HeLa cells expressing Cx26 generally showed decreased clonogenic survival and reduced metabolic activity relative to parental cells lacking gap junction communication. In contrast, irradiated HeLa cells expressing Cx32 generally showed enhanced survival and greater metabolic activity relative to the control cells. The effects on clonogenic survival correlated more strongly with effects on metabolic activity than with DNA damage as assessed by micronucleus formation. The data also showed that the ability of a connexin to affect clonogenic survival following ionizing radiation can depend on the specific type of radiation. Together, these findings show that specific types of connexin channels are targets that may be exploited to enhance radiotherapeutic efficacy and to formulate countermeasures to the harmful effects of specific types of ionizing radiation.

Increased Radioresistance to Lethal Doses of Gamma Rays in Mice and Rats after Exposure to Microwave Radiation Emitted by a GSM Mobile Phone Simulator

The aim of this study was to investigate the effect of pre-irradiation with microwaves on the induction of radioadaptive response. In the 1(st) phase of the study, 110 male mice were divided into 8 groups. The animals in these groups were exposed/sham-exposed to microwave, low dose rate gamma or both for 5 days. On day six, the animals were exposed to a lethal dose (LD). In the 2(nd) phase, 30 male rats were divided into 2 groups of 15 animals. The 1(st) group received microwave exposure. The 2(nd) group (controls) received the same LD but there was no treatment before the LD. On day 5, all animals were whole-body irradiated with the LD. Statistically significant differences between the survival rate of the mice only exposed to lethal dose of gamma radiation before irradiation with a lethal dose of gamma radiation with those of the animals pre-exposed to either microwave (p=0.02), low dose rate gamma (p=0.001) or both of these physical adapting doses (p=0.003) were observed. Likewise, a statistically significant difference between survival rates of the rats in control and test groups was observed. Altogether, these experiments showed that exposure to microwave radiation may induce a significant survival adaptive response.

Low dose ionizing radiation‐induced activation of connexin 43 expression

PURPOSE

Connexin 43 has been implicated in the cellular response to ionizing radiation by enabling cell-to-cell communication. It is established here that the expression of connexin 43 is affected by ionizing radiation and the mechanism involved is investigated.

MATERIALS AND METHODS

The human connexin 43 promoter was cloned into a Luciferase reporter plasmid and activation by ionizing radiation was measured in normal human fibroblasts as well as HeLa cells. The regions responsible for the radiation inducibility were defined using deletion and point mutations of the construct. The results were confirmed by Northern and Western blotting.

RESULTS

Ionizing radiation activates the human connexin 43 promoter in a time- and dose-dependent manner with a maximal induction (4.2-fold +/-0.58) after 6 h and a dose of 0.5 Gy. Higher doses up to 5 Gy led to a less marked increase (2-fold) over the same period. This promoter activation was associated with comparable increases in both connexin 43 mRNA and protein levels. The low dose radiation response of the promoter is mainly dependent on consensus binding sites for nuclear factor of activated T-cells (NFAT) and activator protein (AP1) in a region -2537 and -2110 bp from the transcriptional start site as determined by mutation analysis.

CONCLUSIONS

Low doses of ionizing radiation induce the transcriptional upregulation of connexin 43 expression employing NFAT and AP1 sites.

A perspective on the scientific, philosophical, and policy dimensions of hormesis

The hormesis concept has broad implications for biology and the biomedical sciences. This perspective on hormesis concentrates on toxicology and toxicological risk assessment and secondarily explores observations from other fields. It considers the varied manifestations of hormesis in the context of a broad family of biological stress responses. Evidence for hormesis is reviewed, and the hormesis model is contrasted with more widely accepted dose-response models in toxicology: a linear nonthreshold (LNT) model for mutagenesis and carcinogenesis, and a threshold model for most other toxicologic effects. Scientific, philosophical, and political objections to the hormesis concept are explored, and complications in the hormesis concept are analyzed. The review concludes with a perspective on the current state of hormesis and challenges that the hormesis model poses for risk assessment.

Apoptotic Cell Death Induced by Low-Dose Radiation in Male Germ Cells: Hormesis and Adaptation

Biological effects of low-dose radiation (LDR) in somatic cells have captured the interest of radiobiologists for the last two decades. Apoptosis of germ cells is required for normal spermatogenesis and often occurs through highly conserved events, including the transfer of vital cellular materials to the growing gametes following death of neighboring cells. Apoptosis of germ cells also functions in diverse processes, including removal of abnormal or superfluous cells at specific checkpoints, establishment of caste differentiation, and individualization of gametes. Moreover, germ cells are very sensitive to radiation-induced genomic and cytological effects. Therefore, induction of germ-cell apoptosis has been observed in the testis of animals exposed to both high-dose radiation (HDR) and LDR. Exposure of male germ cells to LDR induces a stimulating effect, while exposure to HDR causes an inhibitory effect on the metabolism, antioxidant capacity, and proliferation and maturation of cells, a phenomenon termed hormesis. Preexposure to LDR also protects cells from subsequently HDR-induced genomic and cytological effects, a phenomenon termed adaptive response. This review describes the features of male germ-cell apoptosis. It reviews the evidence that LDR induces the hormesis and adaptive responses in the male germ cells in terms of apoptosis. This review also discusses the possible effects of LDR-induced apoptotic hormesis and adaptive response on the modulation of inheritable genomic damage caused by subsequent radiation exposure to male germ cells.

Hdm2 and nitric oxide radicals contribute to the p53-dependent radioadaptive response

PURPOSE

The aim of this work was to characterize the radioadaptive response at the molecular level.

METHODS AND MATERIALS

We used wild-type (wt) p53 and mutated (m) p53-containing cells derived from the human lung cancer H1299 cell line, which is p53-null. Cellular radiation sensitivities were determined with a colony-forming assay. The accumulations of p53, the human homolog of endogenous murine double minute 2 (Hdm2), and inducible nitric oxide synthase were analyzed with Western blotting. Quantification of chromosomal aberrations was estimated by scoring dicentrics per cell.

RESULTS

In wtp53 cells, it was demonstrated that the lack of p53 accumulation was coupled with the activation of Hdm2 after low-dose irradiation (0.02 Gy). Although NO radicals were only minimally induced in wtp53 cells irradiated with a challenging irradiation (6 Gy) alone, NO radicals were seen to increase about two- to fourfold after challenging irradiation subsequent to a priming irradiation (0.02 Gy). Under similar irradiation conditions with a priming and challenging irradiation in wtp53 cells, induction of radioresistance and a depression of chromosomal aberrations were observed only in the absence of 5, 5'-(2, 5-Furanidiyl)bis-2-thiophenemethanol (RITA) or Nutlin-3 (p53-Hdm2 interaction inhibitors), aminoguanidine (an inducible nitric oxide synthase inhibitor), and c-PTIO (an NO radical scavenger). On the other hand, in p53 dysfunctional cells, a radioadaptive response was not observed in the presence or absence of those inhibitors. Moreover radioresistance developed when wtp53 cells were treated with isosorbide dinitrate (an NO-generating agent) alone.

CONCLUSIONS

These findings suggest that NO radicals are initiators of the radioadaptive response, acting through the activation of Hdm2 and the depression of p53 accumulations.

Modification of low dose radiation induced radioresistance by 2-deoxy-D-glucose in Saccharomyces cerevisiae: mechanistic aspects

Use of 2-deoxy-D-glucose (2-DG) in combination with radiotherapy to radio-sensitize the tumor tissue is undergoing clinical trials. The present study was designed to investigate the effect of 2-DG on radiation induced radioresistance (RIR) in normal cells. The sub-lethal radiation dose to the normal cells at the periphery of target tumor tissue is likely to induce radioresistance and protect the cells from lethal radiation dose. 2-DG, since, enters both normal and tumor cells, this study have clinical relevance. A diploid respiratory proficient strain D7 of S. cerevisiae was chosen as the model system. In comparison to non-pre-irradiated cultures, the cultures that were pre-exposed to low doses of UVC (254 nm) or (60)Co-gamma-radiation, then maintained in phosphate buffer (pH 6.0, 67 mM), containing 10 mM glucose (PBG), for 2-5 h, showed 18-35% higher survivors (CFUs) after subsequent exposure to corresponding radiation at lethal doses suggesting the radiation induced radioresistance (RIR). The RIR, in the absence of 2-DG, was associated with reduced mutagenesis, decreased DNA damage, and enhanced recombinogenesis. Presence of 2-DG in PBG countered the low dose induced increase in survivors and protection to DNA damage. It also increased mutagenesis, altered the recombinogenesis and the expression of rad50 gene. The changes differed quantitatively with the type of radiation and the absorbed dose. These results, since, imply the side effects of 2-DG, it is suggested that new approaches are needed to minimize the retention of 2-DG in normal cells at the time of radiation exposure.

Adaptive responses to low-dose/low-dose-rate gamma rays in normal human fibroblasts: the role of growth architecture and oxidative metabolism

To investigate low-dose/low-dose-rate effects of low-linear energy transfer (LET) ionizing radiation, we used gamma-irradiated cells adapted to grow in a three-dimensional architecture that mimics cell growth in vivo. We determined the cellular, molecular and biochemical changes in these cells. Quiescent normal human fibroblasts were irradiated with single acute or chronic doses (1-10 cGy) of (137)Cs gamma rays. Whereas exposure to an acute dose of 10 cGy increased micronucleus formation, protraction of the dose over 48 h reduced micronucleus frequency to a level similar to or lower than what occurs spontaneously. The protracted treatment also up-regulated the cellular content of the antioxidant glutathione. These changes correlated with modulation of phospho-TP53 (serine 15), a stress marker that was regulated by doses as low as 1 cGy. The DNA damage that occurred after exposure to an acute dose of 10 cGy was protected against in two ways: (1) up-regulation of cellular antioxidant enzyme activity by ectopic overexpression of MnSOD, catalase or glutathione peroxidase, and (2) inhibition of superoxide anion generation by flavin-containing oxidases. These results support a significant role for oxidative metabolism in mediating low-dose radiation effects and demonstrate that cell culture in three dimensions is ideal to investigate radiation-induced adaptive responses. Expression of connexin 43, a constitutive protein of gap junctions, and the G(1) checkpoint were more sensitive to regulation by gamma rays in cells maintained in a three-dimensional than in a two-dimensional configuration.

Cell-cell interactions in spheroids maintained in suspension

We have developed a system of mixed aggregates of cultured cells, to model in situ cell interactions. This three-dimensional (3D) system of floating cell aggregates, termed spheroids for their round shape, enables one to monitor their growth in both size and number of constituent clonogens and to measure survival curves for cells having 3D cell-cell interactions. This system was used to measure the three-dimensional cell-cell interactions on growth, and clonogenicity of either AG1522 fibroblasts, or HeLa cervical cancer cells (pure spheroids, or if both feeder and test cells are the same type, pseudohybrid spheroids), and/or of mixtures of both (hybrid spheroids). By following the increase or decrease in size of, or number of clonogens per, spheroid over time, one obtains growth or inhibition curves. By relating these clonogen numbers, one obtains, after a suitable growth period, relative survival. The system allows one to score the effects of irradiation and of other treatments, as well as the effect of interaction of the constituent cells on their survival. Floating pure, or pseudohybrid (composed of 10% live fibroblasts and 90% supralethally irradiated fibroblast feeder cells) spheroids, shrank to about 10-20% of their volume in three days and then remained at that size for up to six days. In contrast, pure spheroids composed of live HeLa cells increased their volume by an order of magnitude over the same period. Survival of cells in spheroids was measured by the ability of individual spheroids to grow beyond a size implying a ten-fold increase. A caveat to be observed is to correct survival for cellular multiplicity, i.e. reduce survival values to compensate for more than one colony former at the time of irradiation. The system of spheroids floating and growing in nutrient medium provides a selective system for evaluating growth of HeLa, and by implication, other neoplastic cells, without interference from (overgrowth by) normal fibroblasts. Thus it is possible to discriminate between normal and neoplastic cells by virtue of whether or not cells grow in suspension. Such a system seems ideal for testing novel strategies (radiation in combination with chemicals), in an in vivo-like environment.

The role of stem cells and gap junctions as targets for cancer chemoprevention and chemotherapy

Since carcinogenesis is a multi-stage, multi-mechanism process, involving mutagenic, cell death and epigenetic mechanisms, during the "initiation/promotion/and progression" phases, chemoprevention must be based on understanding the mechanism(s) of each phase. Prevention of each phase could reduce the risk to cancer. Because reducing the initiation phase to a zero level is impossible, the most effective intervention would be at the promotion phase. Assuming the "target" cells for carcinogenesis are the pluri-potent stem cells and their early progenitor or transit cells, chemoprevention strategies for inhibiting the promotion of these two types of pre-malignant "initiated" cells will require different agents. A hypothesis will be proposed that involves stem cells, which lack gap junctional intercellular communication (GJIC-) or their early progenitor daughter cells, which express GJIC+ and are partially-differentiated, if initiated, will be promoted by agents that either inhibit secreted negative growth regulators or by inhibitors of GJIC. Chemopreventing agents to each of these two types of initiated cells must have different mechanisms of action. Assuming stem cells are target cells for carcinogenesis, an alternative method of chemoprevention would be to reduce the stem cell pool. Anti-tumor promoter chemopreventive agents, such as green tea components, resveratrol, caffeic acid phenethylene ester, that either up-regulate GJIC in stem cells or prevent the down regulation of GJIC by tumor promoters in early progenitor cells, will be provided. Human pluri-potent stem cell systems, that can be induced to form 3-dimensional "organoid" structures, will be discussed as a more realistic model system to screen for relevant chemopreventive agents.

Effect of Ionizing Radiation on Rat Tissue: Proteomic and Biochemical Analysis

Reactive oxygen species (ROS), generated by ionizing radiation, has been implicated in its effect on living tissues. We confirmed the changes in the oxidative stress markers upon irradiation. We characterized the changes in the proteome profile in rat liver after administering irradiation, and the affected proteins were identified by MALDI-TOF-MS and ESI-MS/MS. The identified proteins represent diverse sets of proteins participating in the cellular metabolism. Our results demonstrated that proteomics analysis is a useful method for characterization of a global proteome change caused by ionizing radiation to unravel the molecular mechanisms involved in the cellular responses to ionizing radiation.

Oxidative stress, radiation-adaptive responses, and aging

Organisms living in an aerobic environment were forced to evolve effective cellular strategies to detoxify reactive oxygen species. Besides diverse antioxidant enzymes and compounds, DNA repair enzymes, and disassembly systems, which remove damaged proteins, regulation systems that control transcription, translation, and activation have also been developed. The adaptive responses, especially those to radiation, are defensive regulation mechanisms by which oxidative stress (conditioning irradiation) elicits a response against damage because of subsequent stress (challenging irradiation). Although many researchers have investigated these molecular mechanisms, they remain obscure because of their complex signaling pathways and the involvement of various proteins. This article reviews the factors concerned with radiation-adaptive response, the signaling pathways activated by conditioning irradiation, and the effects of aging on radiation-adaptive response. The proteomics approach is also introduced, which is a useful method for studying stress response in cells.

Adaptive Response in Embryogenesis: IV. Protective and Detrimental Bystander Effects Induced by X Radiation in Cultured Limb Bud Cells of Fetal Mice

The radioadaptive response and the bystander effect represent important phenomena in radiobiology that have an impact on novel biological response mechanisms and risk estimates. Micromass cultures of limb bud cells provide an in vitro cellular maturation system in which the progression of cell proliferation and differentiation parallels that in vivo. This paper presents for the first time evidence for the correlation and interaction in a micromass culture system between the radioadaptive response and the bystander effect. A radioadaptive response was induced in limb bud cells of embryonic day 11 ICR mice. Conditioning irradiation of the embryonic day 11 cells with 0.3 Gy resulted in a significant protective effect against the occurrence of apoptosis, inhibition of cell proliferation, and differentiation induced by a challenging dose of 5 Gy given the next day. Both protective and detrimental bystander effects were observed; namely, irradiating 50% of the embryonic day 11 cells with 0.3 Gy led to a successful induction of the protective effect, and irradiating 70% of the embryonic day 12 cells with 5 Gy produced a detrimental effect comparable to that seen when all the cells were irradiated. Further, the bystander effect was markedly decreased by pretreatment of the cells with an inhibitor to block the gap junction-mediated intercellular communication. These results indicate that the bystander effect plays an important role in both the induction of a protective effect by the conditioning dose and the detrimental effect of the challenge irradiation. Gap junction-mediated intercellular communication was suggested to be involved in the induction of the bystander effect.

Adaptive response studies may help choose astronauts for long-term space travel

Long-term manned exploratory missions are planned for the future. Exposure to high-energy neutrons, protons and high charge and energy particles during a deep space mission, needs protection against the detrimental effects of space radiation. It has been suggested that exposure to unpredictable extremely large solar particle events would kill the astronauts without massive shielding. To reduce this risk to astronauts and to minimize the need for shielding, astronauts with highest significant adaptive responses should be chosen. It has been demonstrated that some humans living in very high natural radiation areas have acquired high adaptive responses to external radiation. Therefore, we suggest that for a deep space mission the adaptive response of all potential crew members be measured and only those with high adaptive response be chosen. We also proclaim that chronic exposure to elevated levels of radiation can considerably decrease radiation susceptibility and better protect astronauts against the unpredictable exposure to sudden and dramatic increase in flux due to solar flares and coronal mass ejections.

Extracellular signaling through the microenvironment: a hypothesis relating carcinogenesis, bystander effects, and genomic instability

Cell growth, differentiation and death are directed in large part by extracellular signaling through the interactions of cells with other cells and with the extracellular matrix; these interactions are in turn modulated by cytokines and growth factors, i.e. the microenvironment. Here we discuss the idea that extracellular signaling integrates multicellular damage responses that are important deterrents to the development of cancer through mechanisms that eliminate abnormal cells and inhibit neoplastic behavior. As an example, we discuss the action of transforming growth factor beta (TGFB1) as an extracellular sensor of damage. We propose that radiation-induced bystander effects and genomic instability are, respectively, positive and negative manifestations of this homeostatic process. Bystander effects exhibited predominantly after a low-dose or a nonhomogeneous radiation exposure are extracellular signaling pathways that modulate cellular repair and death programs. Persistent disruption of extracellular signaling after exposure to relatively high doses of ionizing radiation may lead to the accumulation of aberrant cells that are genomically unstable. Understanding radiation effects in terms of coordinated multicellular responses that affect decisions regarding the fate of a cell may necessitate re-evaluation of radiation dose and risk concepts and provide avenues for intervention.

Direct evidence for the participation of gap junction-mediated intercellular communication in the transmission of damage signals from -particle irradiated to nonirradiated cells

It has generally been considered that important biological effects of ionizing radiation arise as a direct consequence of DNA damage occurring in irradiated cells. We have examined this hypothesis by exposing cells to very low fluences of alpha-particles, similar to those emitted by radon gas, such that as few as 1% of the cells in a population are traversed by a particle and thus receive any radiation exposure. By using the endpoints of changes in gene expression and induction of DNA damage, we show that nonirradiated "bystander" cells participate in the overall response of confluent density-inhibited populations of cultured fibroblast and epithelial cells. By in situ immunofluorescence techniques and the use of cells genetically compromised in their ability to perform gap junction intercellular communication, we present direct evidence for the involvement of connexin43-mediated intercellular communication in the transmission of damage signals to nonirradiated cells. Induction of the stress-inducible p21(Waf1) protein in aggregates of neighboring cells far exceeding the fraction of cells whose nucleus has been traversed occurred in gap junction-competent cells only. These changes in p21(Waf1) expression correlated with both the induction of DNA damage (as measured by micronucleus formation) as well as increased Ser-15 phosphorylation of p53.

Role of stem cells and gap junctional intercellular communication in human carcinogenesis

Epidemiological data, experimental animal bioassays, studies of in vitro neoplastic transformation, and molecular oncology studies have implicated a multistage, multimechanism process in human carcinogenesis. From animal carcinogenesis studies, the operational concept of a single normal cell being irreversibly altered during the first step in carcinogenesis is called initiation. The subsequent interruptible or reversible clonal expansion of these initiated cells by non-cytotoxic mitogenic stimuli, compensatory hyperplasia due to cell death by necrosis, or inhibition of apoptosis is referred to as the promotion phase. Last, when one of these clonally expanded, initiated cells acquires sufficient genetic/epigenetic alterations to become neoplastically transformed and acquire the phenotypes of promoter independence, invasiveness and metastasis, it is referred to as the progression step of carcinogenesis. This report hypothesizes that the single normal cell that is initiated is a pluripotent stem cell. By assuming that the normal pluripotent stem cell is immortal and becomes mortal when induced to terminally differentiate, initiation would be viewed as the irreversible process by which a stable alteration in a finite number of proto-oncogenes and/or tumor suppressor genes could block terminal differentiation or "mortalization". Promotion would involve the reversible inhibition of gap junctional intercellular communication (GJIC) and while progression occurs with the stable down-regulation of GJIC.

Radiation response of cell organelles

The cellular responses to various form of radiation, including ionizing- and UV-irradiation or exposure to electromagnetic fields is manifested as irreversible and reversible structural and functional changes to cells and cell organelles. Moreover, beside the morphological signs related to cell death, there are several reversible alterations in the structure of different cell organelles. The radiation-induced changes in the supramolecular organization of the membranes, including plasma membrane, and different cell organelle membranes, play a significant role in the development of acute radiation injury. These signs of radiation-induced reversible perturbation biological membranes reflect changes in the organization and/or composition of the glycocalix, modified activity and/or distribution of different membrane domains, including enzymes and binding sites. The observed changes of the cell surface micromorphology and the alteration of intercellular connections are closely related to the reorganization of the cytoskeletal elements in the irradiated cells. The mitochondria, endoplasmic reticulum, Golgi-complex, the lysosomal system have long been considered to be direct intracellular targets of irradiation. The listed morphological alterations of nuclear chromatin (e.g. changes of fine structure, altered number of nucleolar organizing regions and micronuclei, development of chromosome aberrations) may originate from the radiation-induced damage to the supramolecular organization of DNA and/or nucleus specific proteins. These endpoints of radiation effects resulted as direct consequence(s) of absorbed radiation energy, and indirectly altered intra-, intercellular communication or modified signal transduction. Some complementary data suggest that all these effects are not strictly specific to radiation and may be best considered as general stress responses, similar to those observed after application of various injurious agents and treatments to cells. Moreover, they may be equally responsible for direct degradation of supramolecular component of cells, altered signal transduction, or changes in the amount or ratio of any extracellular mediators upon irradiation. Nevertheless, qualitative and/or quantitative evaluation of any changes of chromosomes by different techniques (morphological analysis of metaphase chromosomes, fluorescent in situ hybridization, development of micronuclei etc.) are useful biological indicators as well as "biological dosimeters" of radiation injury. It is suggested, that some modern methods such as immunohistochemical detection of different proteins, specific markers of cell organelles and cytoskeleton, inspection of distribution of cell surface charged sites and different membrane domains and application of tracer substances may all be included into protocols for evaluation of cell alterations induced by different types and intensities of radiation.

Bystander effects: a concept in need of clarification

An increasing body of evidence indicates that the response to genotoxic agents such as radiation or drugs is a group phenomenon, rather than the summed response of individual independent cells to injury. Thus, a complex contagion-like response may spread beyond the initial impact of an agent to enlarge its effect. This indirect effect, termed "Bystander Effect," is multifaceted and may play a significant role in the therapy of tumors and in carcinogenesis. A better understanding of this phenomenon is needed in order to modulate treatment protocols to therapeutic advantage and to provide more rational guidelines for the evaluation of environmental hazards.