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PubMed2006

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.

Apoptosis↗

PubMed2006

Effects of hypoxia and limited diffusion in tumor cell microenvironment on bystander effect of P450 prodrug therapy.

Cytochrome P450 (CYP) enzyme 2B1 metabolizes the anticancer prodrug cyclophosphamide (CPA) to 4-hydroxy-CPA, which decomposes to the cytotoxic metabolites acrolein and phosphoramide mustard. We have evaluated the bystander cytotoxicity of CPA in combination with CYP2B1 gene-directed enzyme prodrug therapy using a cell culture-based agarose overlay technique. This method mimics the tumor microenvironment by limiting the diffusion of metabolites and by reducing the oxygen concentration to levels similar to those found in solid tumors. Under these conditions, the CYP activity of CYP2B1-expressing tumor cells was decreased by 80% compared to standard aerobic conditions. Despite this decrease in metabolic activity, a potent bystander effect was observed, resulting in up to 90% killing by CPA of a tumor cell population comprised of only approximately 20% CYP-expressing tumor cells. Similarly, transient transfection of a small fraction ( approximately 14%) of a human hepatoma Huh7 cell population with a CYP2B1 expression plasmid followed by short-term treatment with CPA (5 h) led to an eradication of 95% of the cells. No such bystander effect was observed without the agarose overlay. These findings suggest that the agarose overlay technique is very useful as an in vitro test system for investigation of the bystander effect of CYP/CPA and other enzyme/prodrug combinations under conditions that mimic the hypoxic conditions present in solid tumors in vivo.

Animals↗

PubMed1995

Effect of herpes simplex virus thymidine kinase expression levels on ganciclovir-mediated cytotoxicity and the "bystander effect".

Transfer of the herpes simplex virus type-1 thymidine kinase (HSV-tk) gene into tumor cells followed by ganciclovir (GCV) administration, will provide selective tumor cell killing. We studied the effect of herpes simplex virus thymidine kinase (HSV-tk) expression level on the HSV-tk/GCV-mediated "bystander effect." Clones of HSV-tk-transduced rat glioma cells (9L) were isolated that stably expressed with different levels of HSV-tk. All clones studied had similar sensitivity to ganciclovir with IC50 values ranging from 0.45 to 1.3 microM. Within certain enzyme level thresholds, in vitro evaluation of the bystander effect has shown that clones with higher level of HSV-tk expression exhibited a better bystander effect. Interestingly, the bystander effect was observed between different cell types. Both the transduction efficiency and bystander effect are essential factors for the success of the antitumor effect by the HSV-tk/prodrug GCV suicide gene system.

3T3 Cells↗

PubMed2006

Non-targeted bystander effects induced by ionizing radiation.

Radiation-induced bystander effects refer to those responses occurring in cells that were not subject to energy deposition events following ionizing radiation. These bystander cells may have been neighbors of irradiated cells, or physically separated but subject to soluble secreted signals from irradiated cells. Bystander effects have been observed in vitro and in vivo and for various radiation qualities. In tribute to an old friend and colleague, Anthony V. Carrano, who would have said "well what are the critical questions that should be addressed, and so what?", we review the evidence for non-targeted radiation-induced bystander effects with emphasis on prevailing questions in this rapidly developing research field, and the potential significance of bystander effects in evaluating the detrimental health effects of radiation exposure.

Animals↗

PubMed1996

HSV-tk gene therapy in head and neck squamous cell carcinoma. Enhancement by the local and distant bystander effect.

OBJECTIVES: To determine whether the bystander effect demonstrated in vitro for ganciclovir-mediated killing of a herpes simplex virus thymidine kinase (HSV-tk) gene-infected human squamous cell carcinoma is operative in vivo in a nude mouse model. DESIGN: Prospective study in a murine model. INTERVENTION: Human head and neck squamous cell carcinoma tumors were grown as xenografts on the flanks of 20 nude mice. The tumors in the left flank were then infected with the HSV-tk gene. Then, after 48 hours, the animals were treated with intraperitoneal ganciclovir twice daily. Assessment of the tumors on both flanks was performed over a 31-day period. MAIN OUTCOME MEASURES: Resolution of tumors infected with HSV-tk gene in animals treated with ganciclovir; resolution of tumors uninfected with HSV-tk gene on the contralateral flank in animals treated with ganciclovir. RESULTS: Following HSV-tk gene therapy in nude mice, complete resolution of HSV-tk-gene-infected human head and neck squamous cell carcinoma tumors was observed following ganciclovir treatment. Uninfected tumors were also noted to regress, but not completely resolve, in response to intraperitoneal ganciclovir (distant bystander effect). CONCLUSIONS: This study confirms that the local and distant bystander effects exist in this murine model, enhancing the possibility of its role for treatment of human squamous cell carcinoma of the head and neck.

3T3 Cells↗

PubMed2006

Inhibitory and stimulatory bystander effects are differentially induced by Iodine-125 and Iodine-123.

The bystander effect, originating from cells irradiated in vitro, describes responses of surrounding cells not targeted by the radiation. Previously we demonstrated that the subcutaneous injection into nude mice of human adenocarcinoma LS174T cells lethally irradiated by Auger electrons from the decay of DNA-incorporated (125)I inhibits growth of co-injected LS174T cells (inhibitory bystander effect; Proc. Natl. Acad. Sci. USA 99, 13765-13770, 2002). We have repeated these studies using cells exposed to lethal doses of (123)I, an Auger electron emitter whose emission spectrum is identical to that of (125)I, and report herein that the decay of (123)I within tumor cell DNA stimulates the proliferation of neighboring unlabeled tumor cells growing subcutaneously in nude mice (stimulatory bystander effect). Similar inhibitory bystander effects ((125)I) and stimulatory bystander effects ((123)I) are obtained in vitro. Moreover, supernatants from cultures with (125)I-labeled cells are positive for tissue inhibitors of metalloproteinases (TIMP1 and TIMP2), and those from cultures with (123)I-labeled cells are positive for angiogenin. These findings call for the re-evaluation of current dosimetric approaches for the estimation of dose-response relationships in individuals after radiopharmaceutical administration or radiocontamination and demonstrate a need to adjust all "calculated" dose estimates by a dose modification factor (DMF), a radionuclide-specific constant that factors in hitherto not-so-well recognized biophysical processes.

Adenocarcinoma↗

PubMed2003

Biophysical model of the radiation-induced bystander effect.

PURPOSE: To construct a quantitative model of the radiation-induced bystander effect based on diffusion-type spreading of bystander signal communication between the hit and non-hit cells. Cell inactivation and induced oncogenic transformation by broad- and microbeam irradiation systems are considered. MATERIALS AND METHODS: The biophysical model ByStander Diffusion Modelling (BSDM) postulates that the oncogenic bystander response observed in non-hit cells originates from specific signals received from inactivated cells. The bystander signals are assumed to be protein-like molecules spreading in the culture media by Brownian motion. The bystander signals are assumed to switch cells into a state of cell death (apoptotic/mitotic/necrosis) or induced oncogenic transformation modes. RESULTS: The bystander cell survival observed after treatment with the irradiated conditioned medium (ICM) using the broad-beam and the microbeam irradiation modalities were analysed and interpreted in the framework of the BSDM model. The model predictions for cell inactivation and induced oncogenic transformation frequencies agree well with observed data from micro and broad-beam experiments. In the case of irradiation with constant fraction of cells, transformation frequency for the bystander effect increases with increasing radiation dose. CONCLUSIONS: Bystander modelling based on diffusion of signals is in good agreement with experimental cell survival data and induced oncogenic transformation frequencies. The data confirm the protein-like nature of the bystander signal. Linear extrapolation of the cell response to low doses of radiation might underestimate carcinogenic risk, for example for domestic radon hazards, if the contribution from the bystander effect is neglected. The BSDM predicts that the bystander effect cannot be interpreted solely as a low-dose effect phenomenon. It is shown that the bystander component of radiation response can increase with dose and be observed at high doses as well as at low doses. The validity of this conclusion is supported by analysis of experimental results from high-linear energy transfer microbeam experiments.

Alpha Particles↗

PubMed2004

Radiation-induced bystander effect in immune response.

OBJECTIVE: Since most reports on bystander effect have been only concerned with radiation-induced damage, the present paper aimed at disclosing whether low dose radiation could induce a stimulatory or beneficial bystander effect. METHODS: A co-culture system containing irradiated antigen presenting cells (J774A.1) and unirradiated T lymphocytes (EL-4) was established to observe the effect of J774A.1 cells exposed to both low and high doses of X-rays on the unirradiated EL-4 cells. Incorporation of 3H-TdR was used to assess the proliferation of the EL-4 cells, expression of CD80/86 and CD48 on J774A.1 cells was measured with immunohistochemistry and flow cytometry, respectively. NO release from J774A.1 cells was estimated with nitrate reduction method. RESULTS: Low dose-irradiated J774A.1 cells could stimulate the proliferation of the unirradiated EL-4 cells while the high dose-irradiated J774A.1 cells exerted an inhibitory effect on the proliferation of the unirradiated EL-4 cells. Preliminary mechanistic studies illustrated that the differential changes in CD48 expression and NO production by the irradiated J774A.1 cells after high and low dose radiation might be important factors underlying the differential bystander effect elicited by different doses of radiation. CONCLUSION: Stimulatory bystander effect can be induced in immune cells by low dose radiation.

Animals↗

PubMed2001

Immune-dependent distant bystander effect after adenovirus-mediated suicide gene transfer in a rat model of liver colorectal metastasis.

Gene transfer of the herpes simplex virus thymidine kinase (HSV-tk) gene sensitizes tumor cells to the toxic effect of ganciclovir (GCV). The toxic effect of GCV extends to nontransduced surrounding cells by a metabolic process known as the bystander effect. A distant bystander effect, which involves anatomically separated tumors, has been reported in vivo. Our aim was to evaluate and characterize such distant effect in a rat model of colorectal tumors implanted in the liver using adenovirus to carry the HSV-tk gene. Two colorectal tumors were implanted in two distinct liver lobes of the liver. One of the tumor was transduced with an adenoviral vector containing HSV-tk gene. The volumes of the tumors were monitored after GCV treatment. Implication of the immune system was studied histologically and after in vivo manipulations. After GCV administration, the nontransduced distant tumor regressed partially or completely in the experimental group. Immunohistochemical analysis revealed the presence of CD8+ lymphocytes in the distant lesion. HSV-tk/GCV-induced immune response against tumors was evidenced by an adoptive transfer assay (Winn assay) and the distant bystander effect was blunted after CD8+ lymphocytes depletion. However, the survival rates for treated animals were not improved. These findings demonstrate that an immune-mediated effective distant bystander effect can be obtained after limited adenoviral-mediated transfer of the HSV-tk gene.

Adenoviridae↗

PubMed2004

Relationship between the expression of connexin43 and bystander effect of suicide gene therapy in ovarian cancer.

The relationship of connexin43 (Cx43) and bystander effect in ovarian tumor cells in herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) gene therapy in vitro was explored and the effect of all-trans retinoic acid (RA) on the expression of Cx43 and bystander effect investigated. The Cx43 expression was detected by flowcytometry, Western blot, and immunofluorescence in two ovarian tumor cell lines OVCAR3, CaOV3 before and after RA treatment. Bystander effect was determined by the cells growth inhibitory rate with methyl thiazolyl tetrazolium. Following exposure to ganciclovir, there was much greater bystander killing in OVCAR3 than that in CaOV3 (P<0.05). The expression of Cx43 was detected in OVCAR3 by flowcytometry and Western blot, but it could not be detected in CaOV3. The expression of Cx43 in both cell lines could be induced by RA. Immunofluoresence staining showed that Cx43 protein of OVCAR3 was located on membrane surface, whereas CaOV3 in cytoplasm. RA could not change the location of Cx43 protein in both cell lines. There is relationship between Cx43 expression and HSV-TK/GCV bystander effect. HSV-TK/GCV bystander effect can be enhanced by RA in ovarian cancer.

Antiviral Agents↗

PubMed2003

Radiation-induced bystander effects. Mechanisms, biological implications, and current investigations at the Leipzig LIPSION facility.

BACKGROUND: The bystander effect is a relatively new area of radiobiological research, which is aimed at studying post-radiation changes in neighboring non-hit cells or tissues. The bystander effect of ionizing irradiation is important after low-dose irradiation in the range of up to 0.2 Gy, where a higher incidence of stochastic damage was observed than was expected from a linear-quadratic model. It is also important when the irradiation of a cell population is highly non-uniform. OBJECTIVE: This review summarizes most of the important results and proposed bystander effect mechanisms as well as their impact on theory and clinical practice. The literature, in parts contradictory, is collected, the main topics are outlined, and some basic papers are described in more detail. In order to illustrate the microbeam technique, which is considered relevant for the bystander effect research, the state of the Leipzig LIPSION nanoprobe facility is described. RESULTS: The resistance of a radiation-induced bystander effect is now generally accepted. The current state of knowledge on it is summarized here. Several groups worldwide are working on understanding its different aspects and its impact on radiobiology and radiation protection. CONCLUSION: The observation of a bystander effect has posed many questions, and answering them is a challenging topic for radiobiology in the future.

Animals↗

PubMed2003

Cell to cell contact required for bystander effect of the TNF-related apoptosis-inducing ligand (TRAIL) gene.

We have previously reported that direct transfer of the TNF-related apoptosis-inducing ligand (TRAIL) gene resulted in an apoptotic bystander effect, and that this bystander effect was not transferable with cell culture media. To further characterize its mechanism we tested the bystander effect of TRAIL in the human ovarian cancer cell line DOV13, human lung cancer cell line A549, human hepatoma cell line Hepa G2, human breast cancer cell line MDA-MB231 and human colon cancer cell lines Lovo and DLD1. The bystander target cells were transduced with an adenovector expressing the lacZ gene (Ad/CMV-LacZ), while the effector cells were transduced with an adenovector expressing the green fluorescent protein (GFP)/TRAIL fusion gene. Effector and target cells were then cocultured in the same well with or without effector and target cell contact. In all the cell lines tested, target cells were killed if effector and target cell contact was permitted. However, no bystander effect occurred if effector and target cell contact was prevented. Furthermore, the bystander effect and apoptosis induction of TRAIL was dramatically reduced if cells were seeded at a very low density. Moreover, in all the cell lines tested, no detectable soluble TRAIL was found in media from the TRAIL-expressing cell cultures. Together, our results demonstrated that release of soluble TRAIL from transfer of the wild-type TRAIL gene is minimal, and that the bystander effect of the TRAIL gene is mainly mediated by membrane-bound TRAIL on the surface of transduced cells.

Apoptosis↗

PubMed2002

[Bystander effect mediated by herpes simplex virus-thymidine kinase/ganciclovir approach on prostatic cancer cells and its regulation].

OBJECTIVE: To estimate the bystander effect mediated by herpes simplex virus-thymidine kanase/ganciclovir (HSV-TK/GCV) suicide gene therapy approach on PC-3m, a prostate cancer cell line, to explore the role of connexin (Cx) mediated gap junctional intercellular communication (GJIC) in the procedure of bystander effect of HSV-TK/GCV system and to investigate the modulation of apigenin, a Cx expression up-regulator on the connexin43 (Cx43) expression and GJIC of PC-3m cells. METHODS: PC-3m cells were cultured and PC-3m cells transfected with EBV-based expression vector containing HSV-TK gene (TK(+) PC-3m cells) and TK(-) PC-3m cells were mixed at the ratio of 1:9. GCV was added into the mixture. The bystander effect was evaluated by MTT assay. GJIC and HSV-TK/GCV induced bystander effect in several typical cell lines, such as NIH-3T3, Cos-7, and L-02 cells, were determined by crape loading dye tracing (SLDT) and MTT assay respectively. Cx43 mRNA expression and inherent GJIC capacity of PC-3m cells were examined by RT-PCR and SLDT. TK(+) PC-3m cells and TK(-) cells were mixed and divided into 4 groups and added with GCV, apigenin, apigenin + GCV, and apigenin + GCV + 18-alpha-glycyrrhetinic acid (AGA) respectively. Then the killing rate on PC-3m cells was examined by MTT. RESULTS: After 72 h treatment of 100 micro mol/L GCV on the mixture of wild-type PC-3m cells and HSV-TK gene modified PC-3m cells, only 23.5% +/- 3.2% cells were killed. The magnitude of HSV-TK/GCV bystander effect were more powerful in NIH-3T3, Cos-7, and L-02 cells which manifested excellent GJIC than in ACHN and HeLa cells (P < 0.001). Expression of Cx43 mRNA was shown by RT-PCR, however, it is weaker than that in ACHN cells and normal prostate tissue. With the administration of apigenin, the expression of Cx43 mRNA and the GJIC function of PC-3m cells were increased by 2.2 times (P < 0.01) The enhancing effect of apigenin on GJIC function of PC-3m cells lasted 48 hours and could be inhibited by addition of AGA. Apigenin of the concentration of 10 micro mol/L could obviously improve the bystander effect of TK system on PC-3m cells (P < 0.001). The killing rate of GCV on the mixed PC-3m cells was 59.86% +/- 2.44%, and was only 25.34% +/- 2.89% with the addition of AGA. CONCLUSION: There is a positive correlation between the magnitude of bystander effect mediated by HSV-TK/GCV approach and the potency of internal GJIC in the target cells. Down-regulated Cx43 expression and disrupted inherent GJIC potential of PC-3m cells result in the poor magnitude of HSV-TK/GCV bystander effect. Chemical agent like apigenin up-modulates Cx43 expression and invokes GJIC capacity of PC-3m cells, thus enhancing the bystander effect and augmenting the efficacy of TK suicide therapy.

3T3 Cells↗

PubMed2003

[All-trans retinoic acid augments the bystander effect of herpes simplex virus thymidine kinase/ganciclovir system in the treatment of tongue carcinoma cell line].

OBJECTIVE: To investigate the augmentation effect and mechanism of all-trans retinoic acid (ATRA) on the bystander effect of herpes simplex virus thymidine kinase (HSV-TK)/ganciclovir (GCV) system in the treatment of tongue carcinoma cell line. METHODS: Immunocytochemistry and flow cytometry (FCM) were used to analyze the expression of Cx43 protein in Tca8113 cells after treated with ATRA; The bystander effect of HSV-TK/GCV system on tongue carcinoma cells before and after treatment with ATRA was detected by MTT assays. The interaction of ATRA and bystander effect was analyzed by factorial experiment. RESULTS: After treated by ATRA (10(-7) mol/L - 10(-5) mol/L), the expression of Cx43 protein was up-regulated in Tca8113 cells and the positive rate of Cx43 protein increased from 5.17% (before treatment) to 30.53% (10(-5) mol/L ATRA). There was significant difference between ATRA treated cells and untreated cells (P < 0.01). The bystander effect of HSV-TK/GCV system on Tca8113 cells was poor, but improved after combined with ATRA. There was cooperation effect between ATRA and bystander effect of HSV-TK/GCV (P < 0.05). CONCLUSIONS: ATRA can augment the bystander effect of HSV-TK/GCV system in the treatment of tongue carcinoma cell line. The mechanism might be due to up-regulation of Cx43 gene and restore gap junction intercellular communication.

Antineoplastic Agents↗

PubMedDate not supplied

The evaluation of adenoviral p53-mediated bystander effect in gene therapy of cancer.

Because many tumors have mutated p53, one potential strategy proposed for cancer gene therapy is the introduction of the wild-type p53 gene into tumor cells. One puzzling aspect of this approach is that currently available gene transfer protocols result in a small percentage of tumor cells being transduced in vivo, thus implicating a "bystander effect" to achieve therapeutic efficacy. Because bystander effects in the context of p53-mediated gene therapy have not been well characterized, we evaluated the role of in vitro and in vivo bystander effects of adenovirally delivered p53 (AdWTp53). Using human tumor cell lines that did not express p53 protein but were infectible with adenovirus and showed sensitivity to p53-mediated apoptosis, we were unable to demonstrate an AdWTp53-mediated in vitro bystander effect, despite seeing strong bystander effects when cells were infected with an adenovirus containing the suicide gene herpes simplex virus thymidine kinase and treated with ganciclovir. In contrast, in vivo flank mixing studies using one of these cell lines showed a weak but significant p53-mediated bystander effect (a 40% inhibition of tumor growth). This bystander effect translated into a small survival advantage in an established intraperitoneal tumor model when tumor burden was low at the time of viral instillation. The survival advantage was lost, however, when tumor burden was increased. This study indicates that treatment of human tumors using AdWTp53 may be possible; however, because of the weak bystander effect in vivo, effective treatment will likely require a large percentage of tumor cells to be transduced.

Adenoviridae↗

PubMed2002

Computer modelling of radiation-induced bystander effect.

Radiation-induced genomic instability and bystander effects are now well established consequences of exposure of living cells to ionising radiation. It has been observed that cells not directly hit by radiation tracks may still exhibit radiation effects. We present a quantitative modelling of the radiation-induced bystander effect based on a diffusion model of spreading the bystander signal. The model assumes the bystander factor to be aprotein of low molecular weight, given out by the hit cell, diffusing in the medium and reacting with non-hit cells. The model calculations successfully predict the results of cell survival in an irradiated conditioned medium. The model predicts the shape of dose-effect relationship for cell survival and oncogenic transformation induced by broad-beam and micro-beam irradiation by alpha-particles.

Bystander Effect↗

PubMed2006

A stochastic model of cancer initiation including a bystander effect.

A stochastic model of cancer initiation is considered. The model is used to evaluate whether a bystander effect may be important in the pre-malignant and malignant stages of carcinogenesis, and furthermore, on the basis of epidemiological data, to estimate the mutation rates of genes involved in the development of oral leukoplakias. The bystander effect is defined here as the capability of oncogenic mutations to increase the mutation probability of neighbouring (bystander) cells, thus leading potentially to a cascade of neighbouring mutated and neoplastic cells as a pre-stage in the development to leukoplakias and cancer. We find that incidence data for oral cancer are indeed in accordance with a significant bystander effect, operating either alone or in combination with genomic instability in the early stages of carcinogenesis, i.e. the development of neoplasia. Simulations performed gave a picture of how mutations and neoplasia may spread in a tissue, to form characteristic leukoplakias with a core of neoplastic cells. The model also showed that the probability of finding at least one neoplastic cell in the tissue after a given number of years is more sensitive to changes in genomic instability within the cell itself than to changes in a bystander effect. Based on epidemiological data we also calculate the maximum number of oncogenic genes that may be involved in the bystander effect and development of genomic instability. Even if capable of explaining the initial development of oncogenic mutations towards neoplastic cells, the bystander model could not reproduce the observed incidence rates of leukoplakia without assuming a carcinogen mutation probability per cell per year of neoplastic cells practically equal to one. This means that the bystander effect, to be of substantial importance in the final development of neoplastic cells towards leukoplakias, requires a very significant increase in mutation probabilities for bystanders to neoplastic cells. Alternatively, additional mechanisms such as abnormal cell differentiation and uncontrolled proliferation and apoptotis in the neoplastic stage may be of major importance during the development to cancerization.

Bystander Effect↗

PubMed2003

Domestic radon risks may be dominated by bystander effects--but the risks are unlikely to be greater than we thought.

Radon risks derive from exposure of bronchio-epithelial cells to alpha particles. Alpha-particle exposure can result in bystander effects when irradiated cells emit signals resulting in damage to nearby unirradiated bystander cells. Bystander effects can cause downwardly-curving dose-response relations and inverse dose-rate effects. We have extended a quantitative mechanistic model of bystander effects to include protracted exposure, with inverse dose-rate effects attributed to replenishment, during exposure, of a subpopulation of cells which are hypersensitive to bystander signals. In this approach, bystander effects and the inverse dose-rate effect are manifestations of the same basic phenomenon. The model was fitted to dose- and dose-rate dependent radon-exposed miner data; the results suggest that one directly-hit target cell can send bystander signals to about 50 neighboring cells and that, in the case of domestic radon exposures, the risk could be dominated by bystander effects. The analysis concludes that a naive linear extrapolation of radon miner data to low doses, without accounting for dose rate/bystander effects, would result in an underestimation of domestic radon risks by about a factor of approximately 4. However, recent domestic radon risk estimates (BEIR VI) have already applied a phenomenological correction factor of approximately 4 for inverse dose-rate effects, and have thus already implicitly taken into account corrections which we here suggest are due to bystander effects. Thus current domestic radon risk estimates are unlikely to be underestimates as a result of bystander effects.

Air Pollution, Indoor↗