Radiation: a dose of the bomb

Radiation-induced bystander effect: early process and rapid assessment

Radiation-induced bystander effect (RIBE) is a biological process that has received attention over the past two decades. RIBE refers to a plethora of biological effects in non-irradiated cells, including induction of genetic damages, gene expression, cell transformation, proliferation and cell death, which are initiated by receiving bystander signals released from irradiated cells. RIBE brings potential hazards to normal tissues in radiotherapy, and imparts a higher risk from low-dose radiation than we previously thought. Detection with proteins related to DNA damage and repair, cell cycle control, proliferation, etc. have enabled rapid assessment of RIBE in a number of research systems such as cultured cells, three-dimensional tissue models and animal models. Accumulated experimental data have suggested that RIBE may be initiated rapidly within a time frame as short as several minutes after radiation. These have led to the requirement of techniques capable of rapidly assessing RIBE itself as well as assessing the early processes involved.

Emerging role of radiation induced bystander effects: Cell communications and carcinogenesis

Ionizing radiation is an invaluable diagnostic and treatment tool used in various clinical applications. On the other hand, radiation is a known cytotoxic with a potential DNA damaging and carcinogenic effects. However, the biological effects of low and high linear energy transfer (LET) radiations are considerably more complex than previously thought. In the past decade, evidence has mounted for a novel biological phenomenon termed as "bystander effect" (BE), wherein directly irradiated cells transmit damaging signals to non-irradiated cells thereby inducing a response similar to that of irradiated cells. BE can also be induced in various cells irrespective of the type of radiation, and the BE may be more damaging in the longer term than direct radiation exposure. BE is mediated either through gap-junctions or via soluble factors released by irradiated cells. DNA damage response mechanisms represent a vital line of defense against exogenous and endogenous damage caused by radiation and promote two distinct outcomes: survival and the maintenance of genomic stability. The latter is critical for cancer avoidance. Therefore, efforts to understand and modulate the bystander responses will provide new approaches to cancer therapy and prevention. This review overviews the emerging role of BE of low and high LET radiations on the genomic instability of bystander cells and its possible implications for carcinogenesis.

A comparative study of protein kinase C activation in gamma-irradiated proliferating and confluent human lung fibroblast cells

Exposure to low doses of radiation has been recently proven to be much more mutagenic and carcinogenic than previously thought. Since radiation sensitivity varies with different phases of cell cycle, we have investigated the activation of protein kinase C (PKC) after low doses (0.10-1 Gy) of gamma-irradiation on proliferating (log) and non-proliferating (confluent/plateau) human normal lung fibroblast (MRC-5) cells. PKC isoforms have been shown to play key roles in the regulation of proliferation, differentiation, migration and survival. In this study, we have examined the activation of phosphorylated forms of PKC isoforms (PKC-betaII, PKC-alpha/beta, PKC-theta) and non-phosphorylated PKC-alpha in an attempt to understand its kinases in total and subcellular (cytosolic and nuclear) fractions. Cytosolic fraction of the log phase cells showed an increase in activity of PKC-betaII, PKC-alpha/beta and PKC-theta with the radiation dose. However, in the nuclear fraction, PKC-betaII and PKC-theta showed higher activity than the PKC-alpha/beta. In the plateau phase cells of the cytosolic fraction, PKC-betaII showed higher activity than the PKC-alpha/beta and PKC-theta isoforms. Furthermore, in the nuclear fraction PKC-betaII and PKC-alpha/beta isoforms showed higher activity than the PKC-theta. In total cellular protein of the log phase cells, a dose dependent increase in PKC-betaII activity followed by PKC-alpha/ beta was observed and in the plateau phase of cells, PKC-betaII showed higher activity than the PKC-alpha/ beta. The specific activation of PKC isoforms in the plateau phase cells, as demonstrated for the first time, may help us to understand the radiation induced initiation of cellular transformation like hyper-proliferative phenotype, if any.

Targeted and Nontargeted Effects of Low-Dose Ionizing Radiation on Delayed Genomic Instability in Human Cells

All humans receive some radiation exposure and the risk for radiation-induced cancer at low doses is based on the assumption that there is a linear non-threshold relationship between dose and subsequent effect. Consequently, risk is extrapolated linearly from high radiation doses to very low doses. However, adaptive responses, bystander effects, and death-inducing effect may influence health effects associated with low-dose radiation exposure. Adaptive response is the phenomenon by which cells irradiated with a sublethal radiation dose can become less susceptible to subsequent high-dose radiation exposure. Bystander effects are nontargeted effects observed in cells that were not irradiated but were either in contact with or received soluble signals from irradiated cells. These non-hit bystander cells can exhibit damage typically associated with direct radiation exposure. Death-inducing effect is a phenomenon whereby medium from human-hamster hybrid cells displaying radiation-induced chromosomal instability is toxic to unirradiated parental cells. In this study, we show that human RKO cells do not exhibit adaptive response, bystander effect, or death-inducing effect, as measured by cell killing, or delayed genomic instability in a stably transfected plasmid-based green fluorescent protein assay measuring homologous recombination and delayed mutation/deletion events. However, growth medium conditioned by some chromosomally unstable RKO derivatives induced genomic instability, indicating that these cells can secrete factor(s) that elicit responses in nonirradiated cells. Furthermore, low radiation doses suppressed the induction of delayed genomic instability by a subsequent high dose, indicative of an adaptive response for radiation-induced genomic instability. These results highlight the inherent variability in cellular responses to low-dose radiation exposure and add to the uncertainties associated with evaluating potential hazards at these low doses.

Age-at-exposure effects on risk estimates for non-cancer mortality in the Japanese atomic bomb survivors

Statistically significant increases in non-cancer disease mortality with radiation dose have been observed among survivors of the atomic bombings of Hiroshima and Nagasaki. The increasing trends arise particularly for diseases of the circulatory, digestive, and respiratory systems. Rates for survivors exposed to a dose of 1 Sv are elevated by about 10%, a smaller relative increase than that for cancer. The aetiology of this increased risk is not yet understood. Neither animal nor human studies have found clear evidence for excess non-cancer mortality at the lower range of doses received by A-bomb survivors. In this paper, we examine the age and time patterns of excess risks in the A-bomb survivors. The results suggest that the excess relative risk of non-cancer disease mortality might be highest for exposure at ages 30-49 years, and that those exposed at ages 0-29 years might have a very low excess relative risk compared with those exposed at older ages. The differences in excess relative risk for different age-at-exposure groups imply that the dose response relationships for non-cancer disease mortality need to be modelled with adjustment for age-at-exposure.