Colin Seymour

Radiation-induced bystander effects: Past history and future directions

Abstract Mothersill, C. and Seymour, C. Radiation-Induced Bystander Effects: Past History and Future Directions. Radiat. Res. 155, 757–765 (2001). There has been a recent upsurge of interest in the phenomenon now known as radiation-induced bystander effects. This is largely due to the increased awareness of the contribution of indirect and delayed effects, such as genomic instability, to cellular outcomes after low-dose exposures. It is also due to the availability of tools such as the microbeam and advanced cell culture systems and to the ability to study end points such as gene or protein expression at low doses which were previously difficult to study. This review looks at the history of bystander effects in the earlier literature, in which the clastogenic effect of plasma from irradiated patients was well known. The effect was known to persist for several years and to cause transgenerational effects, making it similar to what we now call genomic instability. The review then examines the current data and controversies which are now beginning to resolve the questions concerning the mechanisms underlying the induction and transmission of both bystander effects and genomic instability. Finally, the possible impact of data concerning radiation-induced bystander effects on radiotherapy and radiation protection is discussed.

Cell-cell contact during gamma irradiation is not required to induce a bystander effect in normal human keratinocytes : Evidence for release during irradiation of a signal controlling survival into the medium

Killing of unirradiated cells by medium from cultures of irradiated cells implies the release of a cytotoxic substance by the irradiated cells. The finding of the gamma-ray-induced cytotoxic effect exclusively in epithelial cells and not in fibroblasts suggested that tissue architecture or cell communication might be important. Normal human keratinocytes and fibroblasts and radiosensitive carcinoma cells were irradiated as single cells, microcolonies of three or four cells, or confluent monolayers. The medium was removed and filtered, and cultures which had never been irradiated were seeded at cloning densities and treated with the medium from the irradiated cells. It was found that the degree of cell-cell contact had no effect on the ability of medium from irradiated epithelial cell cultures to reduce the clonogenic survival of unirradiated cells. Cell density was the only important factor. Inhibition of gap junction intercellular communication using the tumor promoter phorbol myristate acid (PMA), which closes gap junctions, increased killing by the bystander effect when the PMA was added to epithelial cells prior to irradiation. Rescue of epithelial cells exposed to the medium from the irradiated cells was not possible even after only 30 min exposure. This suggests that a signal transduction mechanism may control death or survival by the bystander effect rather than by release of a factor which is directly cytotoxic.

High yields of lethal mutations in somatic mammalian cells that survive ionizing radiation

When mammalian cells are irradiated in vitro, the component cells of a normal-appearing survivor colony or clone are commonly thought to have proliferative capacity equivalent to that of the unirradiated cells. We have found, however, that cells appearing in survivor colonies may carry heritable lethal defects which come to light, perhaps only after numerous successful divisions, in the form of plating efficiencies that are reduced below those of unirradiated cells in a dose-dependent manner. We regard these heritable defects as signs of the induction of lethal mutations, which, like non-lethal mutations, may require many generations before they are expressed. This effect has been noted in two very dissimilar mammalian cell lines, one a primary culture from adult tissue, the other an immortal cell line. We suggest that induction of lethal mutations may occur also in somatic cells in vivo; this would account for the well-known observation that previously irradiated but apparently healed tissue is subsequently proved to be extraordinarily sensitive to subsequent exposure to irradiation or cytotoxic drugs. The results of our experiments in vitro suggest that current methods of estimating mutation or transformation yields may yield underestimates. If lethal mutations are induced also in vivo, interpretations of the results of fractionation experiments on normal tissues may have to be reconsidered.

Relative Contribution of Bystander and Targeted Cell Killing to the Low-Dose Region of the Radiation Dose–Response Curve

Abstract Seymour, C. B. and Mothersill, C. Relative Contribution of Bystander and Targeted Cell Killing to the Low-Dose Region of the Radiation Dose–Response Curve. Human keratinocytes show a bystander effect when exposed to low doses of low-LET radiation. In this paper, data are presented showing a method of correcting the overall survival curve to enable analysis of the relative contributions of the bystander effect and the effect attributable to direct interaction of the radiation with the target cell. The technique used is to obtain a standard clonogenic survival curve using the assay of Puck and Marcus and, with a different set of flasks containing cloning densities of unirradiated cells, to assay the cell killing caused by medium harvested from 2 × 105 cells irradiated with the same doses. The data show that for this human epithelial cell line, doses of 0.01–0.5 Gy show clonogenic death by the bystander effect only, if maximum potential bystander killing is assumed. The magnitude of the effect is relatively constant, and it appears to saturate at doses in the range of 0.03–0.05 Gy. After doses greater than 0.5 Gy, the curves for clonogenic death are the result of a dose-dependent non-bystander effect and a dose-independent bystander effect. If these particular dose–response effects occur in epithelial cells in vivo, they may have important consequences for therapy and studies of low-dose risk.

Production of a signal by irradiated cells which leads to a response in unirradiated cells characteristic of initiation of apoptosis.

This study investigated the ability of medium from irradiated cells to induce early events in the apoptotic cascade, such as mobilization of intracellular calcium, loss of mitochondrial membrane potential and increase in reactive oxygen species, in cells which were never exposed to radiation. Medium from irradiated human keratinocytes was harvested and transferred to unirradiated keratinocytes. Endpoints characteristic of the initiation of apoptosis were monitored for a period of 24 h following medium transfer. Clonogenic survival was also measured. Rapid calcium fluxes (within 30 s), loss of mitochondrial membrane potential, increases in reactive oxygen species (from 6 h after medium transfer), an increase in the number of apoptotic cells (48 hours after medium transfer) and a marked reduction in clonogenic survival (after 9 days) were observed. There was no significant difference between medium generated by cells irradiated at 0.5 Gy or 5 Gy. The data suggest that initiating events in the apoptotic cascade were induced in unexposed cells by a signal produced by irradiated cells.

Initiation of Apoptosis in Cells Exposed to Medium from the Progeny of Irradiated Cells: A Possible Mechanism for Bystander-Induced Genomic Instability?

Abstract Lyng, F. M., Seymour, C. B. and Mothersill, C. Initiation of Apoptosis in Cells Exposed to Medium from the Progeny of Irradiated Cells: A Possible Mechanism for Bystander-Induced Genomic Instability? Radiat. Res. 157, 365–370 (2002). Genomic instability and bystander effects have recently been linked experimentally both in vivo and in vitro. The aim of the present study was to determine if medium from irradiated cells several passages distant from the original exposure could initiate apoptosis in unirradiated cells. Human keratinocytes (from the HPV-G cell line) were irradiated with 0.5 Gy or 5 Gy γ rays. Medium was harvested at each passage up to the 7th passage (approximately 35 population doublings) postirradiation and transferred to unirradiated keratinocytes. Intracellular calcium levels, mitochondrial membrane potential, and the level of reactive oxygen species were all monitored for 24 h after medium transfer. Rapid calcium fluxes (within 30 s), loss of mitochondrial membrane potential, and increases in reactive oxygen species (from 6 h after medium transfer) were observed in the recipient cells. There was no significant difference between medium conditioned by cells irradiated with 0.5 or 5 Gy. The effect of medium from progeny was the same as the initial effect reported previously and did not diminish with increasing passage number. The data suggest that initiating events in the cascade that leads to apoptosis are induced in unirradiated cells by a signal produced by irradiated cells and that this signal can still be produced by the progeny of irradiated cells for several generations.

The Involvement of Calcium and MAP Kinase Signaling Pathways in the Production of Radiation-Induced Bystander Effects

Abstract Lyng, F. M., Maguire, P., McClean, B., Seymour, C. and Mothersill, C. The Involvement of Calcium and MAP Kinase Signaling Pathways in the Production of Radiation-Induced Bystander Effects. Radiat. Res. 165, 400–409 (2006). Much evidence now exists regarding radiation-induced bystander effects, but the mechanisms involved in the transduction of the signal are still unclear. The mitogen-activated protein kinase (MAPK) pathways have been linked to growth factor-mediated regulation of cellular events such as proliferation, senescence, differentiation and apoptosis. Activation of multiple MAPK pathways such as the ERK, JNK and p38 pathways have been shown to occur after exposure of cells to radiation and a variety of other toxic stresses. Previous studies have shown oxidative stress and calcium signaling to be important in radiation-induced bystander effects. The aim of the present study was to investigate MAPK signaling pathways in bystander cells exposed to irradiated cell conditioned medium (ICCM) and the role of oxidative metabolism and calcium signaling in the induction of bystander responses. Human keratinocytes (HPV-G cell line) were irradiated (0.005–5 Gy) using a cobalt-60 teletherapy unit. The medium was harvested 1 h postirradiation and transferred to recipient HPV-G cells. Phosphorylated forms of p38, JNK and ERK were studied by immunofluorescence 30 min–24 h after exposure to ICCM. Inhibitors of the ERK pathway (PD98059 and U0126), the JNK pathway (SP600125), and the p38 pathway (SB203580) were used to investigate whether bystander-induced cell death could be blocked. Cells were also incubated with ICCM in the presence of superoxide dismutase, catalase, EGTA, verapamil, nifedipine and thapsigargin to investigate whether bystander effects could be inhibited because of the known effects on calcium homeostasis. Activated forms of JNK and ERK proteins were observed after exposure to ICCM. Inhibition of the ERK pathway appeared to increase bystander-induced apoptosis, while inhibition of the JNK pathway appeared to decrease apoptosis. In addition, reactive oxygen species, such as superoxide and hydrogen peroxide, and calcium signaling were found to be important modulators of bystander responses. Further investigations of these signaling pathways may aid in the identification of novel therapeutic targets.

Potential indicators of radiosensitivity in squamous cell carcinoma of the head and neck

Recently, attention has focused on the potential of oncogenes, tumour suppressor genes, and assessment of cell proliferation as biological response indicators in human cancer. In this study, immunocytochemical analysis was used to evaluate the usefulness of Ki67, epidermal growth factor receptor (EGFr), and the protein products of c-Myc and Bcl-2 as indicators of radiosensitivity in primary cultures of head and neck tumours. Primary cultures established from tumours taken at surgery were divided into two groups; the control group remained untreated, and the treatment group received a single dose of 2 Gy. The cultures were incubated for 14 days, after which time they were fixed and examined immunocytochemically. The response to treatment of the cultures was measured as the percentage of growth inhibition (%GI) in the treated cultures relative to the untreated controls. Expression of Ki67 measured after a single dose of 2 Gy significantly differentiated the radioresistant and radiosensitive groups (P = 0.045); high percentages of Ki67+ cells correlated with radioresistance. A significant difference was found between the expression of EGFr in the resistant and sensitive groups, as measured in control cultures and after a dose of 2 Gy (P = 0.002 and P = 0.03, respectively); high levels of expression of EGFr correlated with radioresistance. The level of expression c-Myc+ cells, as measured in control cultures, significantly distinguished the radiosensitive group from the radioresistant group (P = 0.05). These results indicate a potential role for these proteins as indicators of radioresistance.

Delayed expression of lethal mutations and genomic instability in the progeny of human epithelial cells that survived in a bystander-killing environment

It has recently been shown that, when irradiated, human epithelial cells produce a factor or signal in the culture medium that can reduce the clonogenic survival of unirradiated cells. The mechanism is unknown, as is the nature of the signal or substance. In this paper, we show that the medium from these irradiated cells is able to induce delayed effects in the progeny of some cell types that survive the initial exposure to the medium. The initial clonogenic survival of normal human keratinocytes exposed to medium from irradiated parallel cultures is reduced by approximately 40%. If the surviving keratinocytes (60%) are grown to confluence and replated for clonogenic assay, than they still show a reduced plating efficiency, this time of approximately 20% less than the parent line. Similarly treated normal human fibroblasts showed no delayed effects either from a direct dose or from receipt of irradiated medium. The progeny of directly irradiated tumourigenic cell lines have previously been shown to have better clonogenic survival (by a factor of at least two) than unirradiated parallel cultures, and this effect was also found, although it varied depending on the cell line used, when the distant progeny of PC-3 cells or SW48 colon carcinoma cells that received medium only from irradiated progenitors were assayed for expression of delayed lethal mutations. These data suggest that the signal/factor produced in medium by irradiated cells is able to induce genomic instability-type effects in distant progeny.

Relationship between Radiation-Induced Low-Dose Hypersensitivity and the Bystander Effect

Abstract Mothersill, C., Seymour, C. B. and Joiner, M. C. Relationship between Radiation-Induced Low-Dose Hypersensitivity and the Bystander Effect. Radiat. Res. 157, 526–532 (2002). Recent advances in our knowledge of the biological effects of low doses of ionizing radiation have shown two unexpected phenomena: a “bystander effect” that can be demonstrated at low doses as a transferable factor(s) causing radiobiological effects in unexposed cells, and low-dose hyper-radiosensitivity and increased radioresistance that can be demonstrated collectively as a change in the dose–effect relationship, occurring around 0.5–1 Gy of low-LET radiation. In both cases, the effect of very low doses is greater than would be predicted by conventional DNA strand break/repair-based radiobiology. This paper addresses the question of whether the two phenomena have similar or exclusive mechanisms. Cells of 13 cell lines were tested using established protocols for expression of both hyper-radiosensitivity/increased radioresistance and a bystander response. Both were measured using clonogenicity as an end point. The results showed considerable variation in the expression of both phenomena and suggested that cell lines with a large bystander effect do not show hyper-radiosensitivity. The reverse was also true. This inverse relationship was not clearly related to the TP53 status or malignancy of the cell line. There was an indication that cell lines that have a radiation dose–response curve with a wide shoulder show hyper-radiosensitivity/increased radioresistance and no bystander effect. The results may suggest new approaches to understanding the factors that control cell death or the sectoring of survival at low radiation doses.