Redox signaling by ionizing radiation in mouse liver

Radiation-induced liver disease: beyond DNA damage

Radiation-induced liver disease (RILD), also known as radiation hepatitis, is a serious side effect of radiotherapy (RT) for hepatocellular carcinoma. The therapeutic dose of RT can damage normal liver tissue, and the toxicity that accumulates around the irradiated liver tissue is related to numerous physiological and pathological processes. RILD may restrict treatment use or eventually deteriorate into liver fibrosis. However, the research on the mechanism of radiation-induced liver injury has seen little progress compared with that on radiation injury in other tissues, and no targeted clinical pharmacological treatment for RILD exists. The DNA damage response caused by ionizing radiation plays an important role in the pathogenesis and development of RILD. Therefore, in this review, we systematically summarize the molecular and cellular mechanisms involved in RILD. Such an analysis is essential for preventing the occurrence and development of RILD and further exploring the potential treatment of this disease.

Brain Damage and Patterns of Neurovascular Disorder after Ionizing Irradiation. Complications in Radiotherapy and Radiation Combined Injury

Exposure to ionizing radiation, mechanical trauma, toxic chemicals or infections, or combinations thereof (i.e., combined injury) can induce organic injury to brain tissues, the structural disarrangement of interactive networks of neurovascular and glial cells, as well as on arrays of the paracrine and systemic destruction. This leads to subsequent decline in cognitive capacity and decompensation of mental health. There is an ongoing need for improvement in mitigating and treating radiation- or combined injury-induced brain injury. Cranial irradiation per se can cause a multifactorial encephalopathy that occurs in a radiation dose- and time-dependent manner due to differences in radiosensitivity among the various constituents of brain parenchyma and vasculature. Of particular concern are the radiosensitivity and inflammation susceptibility of: 1. the neurogenic and oligodendrogenic niches in the subependymal and hippocampal domains; and 2. the microvascular endothelium. Thus, cranial or total-body irradiation can cause a plethora of biochemical and cellular disorders in brain tissues, including: 1. decline in neurogenesis and oligodendrogenesis; 2. impairment of the blood-brain barrier; and 3. ablation of vascular capillary. These changes, along with cerebrovascular inflammation, underlie different stages of encephalopathy, from the early protracted stage to the late delayed stage. It is evident that ionizing radiation combined with other traumatic insults such as penetrating wound, burn, blast, systemic infection and chemotherapy, among others, can exacerbate the radiation sequelae (and vice versa) with increasing severity of neurogenic and microvascular patterns of radiation brain damage.

An improved pseudotargeted GC-MS/MS-based metabolomics method and its application in radiation-induced hepatic injury in a rat model

The liver is the pivotal metabolic organ primarily responsible for metabolic activities, detoxification and regulation of carbohydrate, protein, amino acid, and lipid metabolism. However, very little is known about the complicated pathophysiologic mechanisms of liver injury result from ionizing radiation exposure. Therefore, a pseudotargeted metabolomics approach based on gas chromatography-tandem mass spectrometry with selected reaction monitoring (GC-MS-SRM) was developed to study metabolic alterations of liver tissues in radiation-induced hepatic injury. The pseudotargeted GC-MS-SRM method was validated with satisfactory analytical characteristics in terms of precision, linearity, sensitivity and recovery. Compared to the SIM-based approach, the SRM scanning method had mildly better precision, higher sensitivity, and wider linear ranges. A total of 37 differential metabolites associated with radiation-induced hepatic injury were identified using the GC-MS-SRM metabolomics method. Global metabolic clustering analysis showed that amino acids, carbohydrates, unsaturated fatty acids, organic acids, metabolites associated with pyrimidine metabolism, ubiquinone biosynthesis and oxidative phosphorylation appeared significantly declined after high dose irradiation exposure, whereas metabolites related to lysine catabolism, glycerolipid metabolism and glutathione metabolism presented the opposite behavior. These changes indicate energy deficiency, antioxidant defense damage, accumulation of ammonia and lipid oxidation of liver tissues in response to radiation exposure. It is shown that the developed pseudotargeted method based on GC-MS-SRM is a useful tool for metabolomics study.

Pathological effects of ionizing radiation: endothelial activation and dysfunction

The endothelium, a tissue that forms a single layer of cells lining various organs and cavities of the body, especially the heart and blood as well as lymphatic vessels, plays a complex role in vascular biology. It contributes to key aspects of vascular homeostasis and is also involved in pathophysiological processes, such as thrombosis, inflammation, and hypertension. Epidemiological data show that high doses of ionizing radiation lead to cardiovascular disease over time. The aim of this review is to summarize the current knowledge on endothelial cell activation and dysfunction after ionizing radiation exposure as a central feature preceding the development of cardiovascular diseases.

Radioprotective Role of Peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a member of an evolutionary ancient family of peroxidase enzymes with diverse functions in the cell. Prdx6 is an important enzymatic antioxidant. It reduces a wide range of peroxide substrates in the cell, thus playing a leading role in the maintenance of the redox homeostasis in mammalian cells. Beside peroxidase activity, Prdx6 has been shown to possess an activity of phospholipase A2, an enzyme playing an important role in membrane phospholipid metabolism. Moreover, Prdx6 takes part in intercellular and intracellular signal transduction due to its peroxidase and phospholipase activity, thus facilitating the initiation of regenerative processes in the cell, suppression of apoptosis, and activation of cell proliferation. Being an effective and important antioxidant enzyme, Prdx6 plays an essential role in neutralizing oxidative stress caused by various factors, including action of ionizing radiation. Endogenous Prdx6 has been shown to possess a significant radioprotective potential in cellular and animal models. Moreover, intravenous infusion of recombinant Prdx6 to animals before irradiation at lethal or sublethal doses has shown its high radioprotective effect. Exogenous Prdx6 effectively alleviates the severeness of radiation lesions, providing normalization of the functional state of radiosensitive organs and tissues, and leads to a significant elevation of the survival rate of animals. Prdx6 can be considered as a potent and promising radioprotective agent for reducing the pathological effect of ionizing radiation on mammalian organisms. The radioprotective properties and mechanisms of radioprotective action of Prdx6 are discussed in the current review.

Peroxiredoxins in Colorectal Cancer: Predictive Biomarkers of Radiation Response and Therapeutic Targets to Increase Radiation Sensitivity?

Colorectal cancer (CRC) is the third most common cancer in the Western world, with one-third of cases located in the rectum. Preoperative radiotherapy is the standard of care for many patients with rectal cancer but has a highly variable response rate. The ability to predict response would be of great clinical utility. The response of cells to ionizing radiation is known to involve immediate damage to biomolecules and more sustained disruption of redox homeostasis leading to cell death. The peroxiredoxins are an important group of thiol-dependent antioxidants involved in protecting cells from oxidative stress and regulating signaling pathways involved in cellular responses to oxidative stress. All six human peroxiredoxins have shown increased expression in CRC and may be associated with clinicopathological features and tumor response to ionizing radiation. Peroxiredoxins can act as markers of oxidative stress in various biological systems but they have not been investigated in this capacity in CRC. As such, there is currently insufficient evidence to support the role of peroxiredoxins as clinical biomarkers, but it is an area worthy of investigation. Future research should focus on the in vivo response of rectal cancer to radiotherapy and the redox status of peroxiredoxins in rectal cancer cells, in order to predict response to radiotherapy. The peroxiredoxin system is also a potential therapeutic target for CRC.

Kaempferol protects against gamma radiation-induced mortality and damage via inhibiting oxidative stress and modulating apoptotic molecules in vivo and vitro

To investigate the potential protective effect of kaempferol, a representative flavonoid, against radiation induced mortality and injury in vivo and vitro.C57BL/6 male mice and human umbilical venous endothelial cells (HUVECs) were pretreated with kaempferol before radiation. We found that kaempferol can effectively increase 30-day survival rate after 8.5 Gy lethal total body irradiation (TBI). Mice were sacrificed at 7th day after 7 Gy TBI, we found kaempferol against radiation-induced tissues damage, by inhibiting the oxidative stress, and attenuating morphological changes and cell apoptosis. In vitro, kaempferol increased HUVECs cell viability and decrease apoptosis. It also mitigated oxidative stress and restored the abnormal expression of prx-5, Cyt-c, Caspase9 and Caspase3 in mRNA and protein level in HUVECs after radiation. Taken together, it suggests kaempferol can protect against gamma-radiation induced tissue damage and mortality. The present study is the first report of the radioprotective role of kaempferol in vivo and vitro.

Effect of ionizing radiation on liver protein oxidation and metabolic function in C57BL/6J mice

PURPOSE

Protein oxidation in response to radiation results in DNA damage, endoplasmic reticulum stress/unfolded protein response, cell cycle arrest, cell death and senescence. The liver, a relatively radiosensitive organ, undergoes measurable alterations in metabolic functions following irradiation. Accordingly, we investigated radiation-induced changes in liver metabolism and alterations in protein oxidation.

MATERIALS AND METHODS

C57BL/6 mice were sham irradiated or exposed to 8.5 Gy (60)Co (0.6 Gy/min) total body irradiation. Metabolites and metabolic enzymes in the blood and liver tissue were analyzed. Two-dimensional gel electrophoresis and OxyBlot™ were used to detect carbonylated proteins that were then identified by peptide mass fingerprinting.

RESULTS

Analysis of serum metabolites revealed elevated glucose, bilirubin, lactate dehydrogenase (LDH), high-density lipoprotein, and aspartate aminotransferase within 24-72 h post irradiation. Liver tissue LDH and alkaline phosphatase activities were elevated 24-72 h post irradiation. OxyBlotting revealed that the hepatic proteome contains baseline protein carbonylation. Radiation exposure increased carbonylation of specific liver proteins including carbonic anhydrase 1, α-enolase, and regucalcin.

CONCLUSIONS

8.5 Gy irradiation resulted in distinct metabolic alterations in hepatic functions. Coincident with these changes, radiation induced the carbonylation of specific liver enzymes. The oxidation of liver enzymes may underlie some radiation-induced alterations in hepatic function.

Effects of ionizing radiation on biological molecules--mechanisms of damage and emerging methods of detection

SIGNIFICANCE

The detrimental effects of ionizing radiation (IR) involve a highly orchestrated series of events that are amplified by endogenous signaling and culminating in oxidative damage to DNA, lipids, proteins, and many metabolites. Despite the global impact of IR, the molecular mechanisms underlying tissue damage reveal that many biomolecules are chemoselectively modified by IR.

RECENT ADVANCES

The development of high-throughput "omics" technologies for mapping DNA and protein modifications have revolutionized the study of IR effects on biological systems. Studies in cells, tissues, and biological fluids are used to identify molecular features or biomarkers of IR exposure and response and the molecular mechanisms that regulate their expression or synthesis.

CRITICAL ISSUES

In this review, chemical mechanisms are described for IR-induced modifications of biomolecules along with methods for their detection. Included with the detection methods are crucial experimental considerations and caveats for their use. Additional factors critical to the cellular response to radiation, including alterations in protein expression, metabolomics, and epigenetic factors, are also discussed.

FUTURE DIRECTIONS

Throughout the review, the synergy of combined "omics" technologies such as genomics and epigenomics, proteomics, and metabolomics is highlighted. These are anticipated to lead to new hypotheses to understand IR effects on biological systems and improve IR-based therapies.

Ionizing radiation activates the Nrf2 antioxidant response

The transcription factor NF-E2-related factor 2 (Nrf2) binds the antioxidant DNA response element (ARE) to activate important cellular cytoprotective defense systems. Recently several types of cancers have been shown to overexpress Nrf2, but its role in the cellular response to radiation therapy has yet to be fully determined. In this study, we report that single doses of ionizing radiation from 2 to 8 Gy activate ARE-dependent transcription in breast cancer cells in a dose-dependent manner, but only after a delay of five days. Clinically relevant daily dose fractions of radiation also increased ARE-dependent transcription, but again only after five days. Downstream activation of Nrf2-ARE-dependent gene and protein markers, such as heme oxygenase-1, occurred, whereas Nrf2-deficient fibroblasts were incapable of these responses. Compared with wild-type fibroblasts, Nrf2-deficient fibroblasts had relatively high basal levels of reactive oxygen species that increased greatly five days after radiation exposure. Further, in vitro clonogenic survival assays and in vivo sublethal whole body irradiation tests showed that Nrf2 deletion increased radiation sensitivity, whereas Nrf2-inducing drugs did not increase radioresistance. Our results indicate that the Nrf2-ARE pathway is important to maintain resistance to irradiation, but that it operates as a second-tier antioxidant adaptive response system activated by radiation only under specific circumstances, including those that may be highly relevant to tumor response during standard clinical dose-fractionated radiation therapy.

Proteomic analysis of the protective effects of Platycodi Radix in liver of chronically alcoholic rats

In this study, we examined the effect of Platycodi Radix (PR) supplementation in chronically alcoholic rats. Sprague-Dawley rats were divided into three groups: control group (no alcohol), alcohol group (36.8% of total calories), and 0.3% PR group. The levels of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were increased by alcohol treatment, and PR supplementation normalized the AST level. Moreover, alcohol-induced cytochrome P450 2E1 was decreased by PR treatment. Proteomic analysis of liver tissues of alcohol-exposed rats and PR-supplemented rats revealed that 50 different proteins functionally characterized as involved with cytoskeleton regulation, signal transduction, cytokine, apoptosis, and reactive oxygen species metabolism showed significant quantitative changes. The expression levels of glutathione S-transferase mu, Bcl-2-like protein, and peroxiredoxin IV were decreased in the alcoholic group, whereas the levels of these proteins were increased more than threefold in the PR group. However, the expression levels of smooth muscle actin, cytochrome P450 2D, mitogen-activated protein kinase 8, and 3alpha-hydroxysteroid dehydrogenase were increased in the alcohol group and were decreased in the PR group. These data suggest that the antioxidant enzymes may play a protective role against alcohol-induced damage via oxidative stress defense mechanisms induced by PR supplementation.

Single-dose gamma-irradiation induces up-regulation of chemokine gene expression and recruitment of granulocytes into the portal area but not into other regions of rat hepatic tissue

Liver damage is a serious clinical complication of gamma-irradiation. We therefore exposed rats to single-dose gamma-irradiation (25 Gy) that was focused on the liver. Three to six hours after irradiation, an increased number of neutrophils (but not mononuclear phagocytes) was observed by immunohistochemistry to be attached to portal vessels between and around the portal (myo)fibroblasts (smooth muscle actin and Thy-1(+) cells). MCP-1/CCL2 staining was also detected in the portal vessel walls, including some cells of the portal area. CC-chemokine (MCP-1/CCL2 and MCP-3/CCL7) and CXC-chemokine (KC/CXCL1, MIP-2/CXCL2, and LIX/CXCL5) gene expression was significantly induced in total RNA from irradiated livers. In laser capture microdissected samples, an early (1 to 3 hours) up-regulation of CCL2, CXCL1, CXCL8, and CXCR2 gene expression was detected in the portal area but not in the parenchyma; with the exception of CXCL1 gene expression. In addition, treatment with an antibody against MCP-1/CCL2 before irradiation led to an increase in gene expression of interferon-gamma and IP-10/CXCL10 in liver tissue without influencing the recruitment of granulocytes. Indeed, the CCL2, CXCL1, CXCL2, and CXCL5 genes were strongly expressed and further up-regulated in liver (myo)fibroblasts after irradiation (8 Gy). Taken together, these results suggest that gamma-irradiation of the liver induces a transient accumulation of granulocytes within the portal area and that (myo)fibroblasts of the portal vessels may be one of the major sources of the chemokines involved in neutrophil recruitment. Moreover, inhibition of more than one chemokine (eg, CXCL1 and CXCL8) may be necessary to reduce leukocytes recruitment.

Identification of proteins indicating radiation-induced hepatic toxicity in cirrhotic rats

Radiation therapy (RT) has been emerging as one of the palliative treatments for locally advanced hepatocellular carcinoma (HCC). However, hepatic toxicity is a major obstacle in radiotherapy for HCC. The purpose of this study is to identify proteins indicating radiation-induced hepatic toxicity in cirrhotic rats, which can be used as possible biomarkers. Liver cirrhosis was induced in Wistar rats with thioacetamide (TAA) 0.3 g/L in drinking water for 9 weeks. The development of liver cirrhosis was observed histologically. Radiation hepatic injury was induced by treating 1/3 of the liver with 10 Gy single dose radiation. To find out commonly expressed proteins, liver tissue and serum were analyzed using two-dimensional electrophoresis and quadrupole time of flight mass spectrometry. Identified proteins were validated using western blotting. Histological examination showed that the degree of hepatic fibrosis increased by radiation in liver cirrhosis. It was associated with a decrease in the proliferation of cell nuclear antigen and an increase of apoptosis. The proteomic analysis of liver tissue and serum identified 60 proteins which showed significant change in expression between the TAA-alone and TAA-plus-radiation groups. Among these, an increase of heparanase precursor and decrease of hepatocyte growth factor were shown commonly in liver tissue and serum following radiation. Hepatic fibrosis increased following radiation in cirrhotic rats. These proteins might be useful in detecting and monitoring radiation-induced hepatic injury.

Identification of gliotropic factors that induce human stem cell migration to malignant tumor

Neural stem cells are mobile, are attracted to regions of brain damage, and can migrate a considerable distance to reach a glioma site. However, the molecular basis of the progression of gliotropism to malignant gliomas remains poorly understood. With the use of clinically and histologically assessed glioma cells, we have assessed their protein and gene profiles via proteomics and microarray approaches, and have identified candidate genes from human glioma tissues. This research is expected to provide clues to the molecular mechanisms underlying the migration of neural stem cells (F3 cell) to glioma sites. The expression of 16 proteins was shown to have increased commonly in human glioma tissues. Among them, the expression of annexin A2, TIMP-1, COL11A1, bax, CD74, TNFSF8, and SPTLC2 were all increased in human glioma cells, as confirmed by Western blotting and immunohistochemical staining. In particular, annexin A2 effects an increase in migration toward F3 and glioblastoma cells (U87 cell) in a Boyden chamber migration assay. An ERK inhibitor (PD98057) and a CDK5 inhibitor (rescovitine) inhibited 50% and 90% of annexin A2-induced migration in F3 cells, respectively. A similar chemotactic migration was noted in F3 and U87 cells. These results demonstrated that 7 candidate proteins may harbor a potential glioma tropism factor relevant to the pathology of malignant glioma. These results reveal that this novel molecular approach to the monitoring of glioma may provide clinically relevant information regarding tumor malignancy, and should also prove appropriate for high-throughput clinical screening applications.

Peroxiredoxins, a novel target in cancer radiotherapy

Reactive oxygen species (ROS) are toxic at high levels in the mammalian cells. Mammalian cells have developed many enzymatic and nonenzymatic antioxidative systems in various cellular compartments to maintain an appropriate level of ROS and regulate their action. Peroxiredoxins (Prxs), a family of peroxidase that reduced intracellular peroxides (one type of ROS) with the thioredoxin system as the electron donor, were highly expressed in various cellular compartments. In this minireview, we discussed the regulation of Prxs expression in cancer cell and its relationship with ionizing radiation. As Prxs could be induced by radiation and its expression status could determine the radiosensitivity of cancer cells, Prxs might be a potential target for radiotherapy in cancer.

Induction of rhodanese, a detoxification enzyme, in livers from mice after long-term irradiation with low-dose-rate gamma-rays

The health effects of low-dose radiation exposure are of public concern. Although molecular events in the cellular response to high-dose-rate radiation exposure have been fully investigated, effects of long-term exposure to extremely low-dose-rate radiation remain unclear. Protein expression was analyzed by two-dimensional electrophoresis in livers from mice irradiated for 485 days (22 hr/day) at low-dose-rates of 0.032 microGy/min, 0.65 microGy/min and 13 microGy/min (total doses of 21 mGy, 420 mGy and 8000 mGy, respectively). One of the proteins that showed marked changes in expression was identified as rhodanese (thiosulfate sulfurtransferase). Rhodanese expression was increased after irradiation at 0.65 microGy/min and 13 microGy/min, while its expression was not changed at 0.032 microGy/min. Rhodanese is a detoxification enzyme, probably related to the regulation of antioxidative function. However, antioxidative proteins, such as superoxide dismutase (SOD)1 (also known as Cu,Zn-SOD) and SOD2 (also known as Mn-SOD), which can be induced by high-dose-rate radiation, were not induced at any low-dose-rates tested. These findings indicate that rhodanese is a novel protein induced by low-dose-rate radiation, and further analysis could provide insight into the effects of extremely low-dose-rate radiation exposure.

Application of HiCEP to screening of radiation stress-responsive genes in the soil microarthropod Folsomia candida (Collembola)

The field of ecotoxicogenomics has received increasing attention for its potential to provide insight into pressing ecological issues. However, its applications are limited due to a lack of genetic sequence information for organisms used in ecotoxicological studies. We used high-coverage expression profiling (HiCEP), a method that requires no prior sequence knowledge, to examine stress-responsive genes and their dose dependence in the springtail Folsomia candida using gamma radiation as the stressor. Radiation-responsive genes and their dose dependency were detected at effective doses for reproduction, and 16 up-regulated transcript-derived fragments (TDFs) were sequenced. Quantitative PCR analysis also found that most of the TDFs were up-regulated. The sequences of the TDFs showed resemblance to known genes, such as glutathione S-transferase and poly(ADP-ribose) polymerase, but most showed no similarity to any genes in the gene databases. These results suggest that HiCEP is effective for discovering differently expressed genes and their dose dependence, even in organisms for which few sequence data are available. The limited length of the TDFs, however, may impede functional annotation of the genes. In conclusion, HiCEP is useful for ecotoxicogenomic studies in which various organisms with few available genomic resources are involved.

Germline glutathione S-transferase variants in breast cancer: relation to diagnosis and cutaneous long-term adverse effects after two fractionation patterns of radiotherapy

PURPOSE

To explore whether certain glutathione S-transferase (GST) polymorphisms are associated with an increased risk of breast cancer or the level of radiation-induced adverse effects after two fractionation patterns of adjuvant radiotherapy.

METHODS AND MATERIALS

The prevalence of germline polymorphic variants in GSTM1, GSTP1, and GSTT1 was determined in 272 breast cancer patients and compared with that in a control group of 270 women from the general population with no known history of breast cancer. The genetic variants were determined using multiplex polymerase chain reaction followed by restriction enzyme fragment analysis. In 253 of the patients surveyed for radiotherapy-induced side effects after a median observation time of 13.7 years (range, 7-22.8 years), the genotypes were related to the long-term effects observed after two fractionation patterns (treatment A, 4.3 Gy in 10 fractions for 156 patients; and treatment B, 2.5 Gy in 20 fractions for 97; both administered within a 5-week period).

RESULTS

None of the GST polymorphisms conferred an increased risk of breast cancer, either alone or in combination. Compared with treatment B, treatment A was followed by an increased level of moderate to severe radiation-induced side effects for all the endpoints studied (i.e., degree of telangiectasia, subcutaneous fibrosis and atrophy, lung fibrosis, costal fractures, and pleural thickening; p <0.001 for all endpoints). A significant association was found between the level of pleural thickening and the GSTP1 Ile105Val variant.

CONCLUSION

The results of this study have illustrated the impact of hypofractionation on the level of adverse effects and indicated that the specific alleles of GSTP1, M1, and T1 studied here may be significant in determining the level of adverse effects after radiotherapy.

Proteomics analysis of apoptosis-regulating proteins in tissues with different radiosensitivity

The aim of this study was to identify of radiosusceptibility proteins in tissues with different radiosensitivity. C3H/HeJ mice were exposed to 10 Gy. The tissues were processed for proteins extraction and were analyzed by 2-dimensional electrophoresis. The proteins were identified by matrix-assisted laser desorption ionizing time-of-flight mass spectrometry and validated by immunohistochemical staining and Western blotting. The peaks of apoptosis levels were 35.3 +/- 1.7% and 0.6 +/- 0.2% in the spleen and the liver, respectively, after ionizing radiation. Analysis of liver tissue showed that the expression level of ROS related proteins such as cytochrome c, glutathione S transferase, NADH dehydrogenase and peroxiredoxin VI increased after radiation. The expression level of cytochrome c increased to 3-fold after ionizing radiation in both tissues. However in spleen tissue, the expression level of various kinds of apoptosis regulating proteins increased after radiation. These involved iodothyronine, CD 59A glycoprotein precursor, fas antigen and tumor necrosis factor -inducible protein TSG-6n precursor after radiation. The difference in the apoptosis index between the liver and spleen tissues is closely associated with the expression of various kinds of apoptosis-related proteins. The result suggests that the expression of apoptosis-related protein and redox proteins play important roles in this radiosusceptibility.

Bystander effect: biological endpoints and microarray analysis

In cell populations exposed to ionizing radiation, the biological effects occur in a much larger proportion of cells than are estimated to be traversed by radiation. It has been suggested that irradiated cells are capable of providing signals to the neighboring unirradiated cells resulting in damage to these cells. This phenomenon is termed the bystander effect. The bystander effect induces persistent, long-term, transmissible changes that result in delayed death and neoplastic transformation. Because the bystander effect is relevant to carcinogenesis, it could have significant implications for risk estimation for radiation exposure. The nature of the bystander effect signal and how it impacts the unirradiated cells remains to be elucidated. Examination of the changes in gene expression could provide clues to understanding the bystander effect and could define the signaling pathways involved in sustaining damage to these cells. The microarray technology serves as a tool to gain insight into the molecular pathways leading to bystander effect. Using medium from irradiated normal human diploid lung fibroblasts as a model system we examined gene expression alterations in bystander cells. The microarray data revealed that the radiation-induced gene expression profile in irradiated cells is different from unirradiated bystander cells suggesting that the pathways leading to biological effects in the bystander cells are different from the directly irradiated cells. The genes known to be responsive to ionizing radiation were observed in irradiated cells. Several genes were upregulated in cells receiving media from irradiated cells. Surprisingly no genes were found to be downregulated in these cells. A number of genes belonging to extracellular signaling, growth factors and several receptors were identified in bystander cells. Interestingly 15 genes involved in the cell communication processes were found to be upregulated. The induction of receptors and the cell communication processes in bystander cells receiving media from irradiated cells supports the active involvement of these processes in inducing bystander effect.