The bystander effect: recent developments and implications for understanding the dose response

Biotransformation of Doxorubicin Promotes Resilience in Simplified Intestinal Microbial Communities

Chemotherapeutic drugs can cause harmful gastrointestinal side effects, which may be modulated by naturally occurring members of our microbiome. We constructed simplified gut-associated microbial communities to test the hypothesis that bacteria-mediated detoxification of doxorubicin (i.e., a widely used chemotherapeutic) confers protective effects on the human microbiota. Mock communities composed of up to five specific members predicted by genomic analysis to be sensitive to the drug or resistant via biotransformation and/or efflux were grown in vitro over three generational stages to characterize community assembly, response to perturbation (doxorubicin exposure), and resilience. Bacterial growth and drug concentrations were monitored with spectrophotometric assays, and strain relative abundances were evaluated with 16S rRNA gene sequencing. Bacteria with predicted resistance involving biotransformation significantly lowered concentrations of doxorubicin in culture media, permitting growth of drug-sensitive strains in monoculture. Such protective effects were not produced by strains with drug resistance conferred solely by efflux. In the mixed communities, resilience of drug-sensitive members depended on the presence and efficiency of transformers, as well as drug exposure concentration. Fitness of bacteria that were resistant to doxorubicin via efflux, though not transformation, also improved when the transformers were present. Our simplified community uncovered ecological relationships among a dynamic consortium and highlighted drug detoxification by a keystone species. This work may be extended to advance probiotic development that may provide gut-specific protection to patients undergoing cancer treatment. IMPORTANCE While chemotherapy is an essential intervention for treating many forms of cancer, gastrointestinal side effects may precede infections and risks for additional health complications. We developed an in vitro model to characterize key changes in bacterial community dynamics under chemotherapeutic stress and the role of bacterial interactions in drug detoxification to promote microbiota resilience. Our findings have implications for developing bio-based strategies to promote gut health during cancer treatment.

Low-dose or low-dose-rate ionizing radiation-induced bioeffects in animal models

Animal experimental studies indicate that acute or chronic low-dose ionizing radiation (LDIR) (≤100 mSv) or low-dose-rate ionizing radiation (LDRIR) (<6 mSv/h) exposures may be harmful. It induces genetic and epigenetic changes and is associated with a range of physiological disturbances that includes altered immune system, abnormal brain development with resultant cognitive impairment, cataractogenesis, abnormal embryonic development, circulatory diseases, weight gain, premature menopause in female animals, tumorigenesis and shortened lifespan. Paternal or prenatal LDIR/LDRIR exposure is associated with reduced fertility and number of live fetuses, and transgenerational genomic aberrations. On the other hand, in some experimental studies, LDIR/LDRIR exposure has also been reported to bring about beneficial effects such as reduction in tumorigenesis, prolonged lifespan and enhanced fertility. The differences in reported effects of LDIR/LDRIR exposure are dependent on animal genetic background (susceptibility), age (prenatal or postnatal days), sex, nature of radiation exposure (i.e. acute, fractionated or chronic radiation exposure), type of radiation, combination of radiation with other toxic agents (such as smoking, pesticides or other chemical toxins) or animal experimental designs. In this review paper, we aimed to update radiation researchers and radiologists on the current progress achieved in understanding the LDIR/LDRIR-induced bionegative and biopositive effects reported in the various animal models. The roles played by a variety of molecules that are implicated in LDIR/LDRIR-induced health effects will be elaborated. The review will help in future investigations of LDIR/LDRIR-induced health effects by providing clues for designing improved animal research models in order to clarify the current controversial/contradictory findings from existing studies.

Hormesis and radiation safety norms

Today's radiation safety norms are based on the linear no-threshold theory (LNT): extrapolation of the dose-response relationships down to the minimal doses, where such relationships are unproven and can be inverse due to hormesis. The most promising way to obtaining reliable data on the dose-effect relationships for low radiation doses would be large-scale animal experiments. Outstanding published data on carcinogenic effects of the doses e.g. below 100 mSv should be verified by experiments. Arguments against applicability of the LNT to the doses comparable to those from the natural radiation background are discussed. Furthermore it is stressed that medical consequences of the Chernobyl accident have been overestimated; and this theme has been exploited to strangle development of atomic energy and to elevate prices for fossil fuels. Worldwide introduction of nuclear energy will be possible only after a concentration of authority within a powerful international executive. It would enable the construction of nuclear reactors in optimally suitable places, considering all sociopolitical, geographical, and geological conditions, which would contribute to the prevention of accidents like in Japan in 2011. A concluding point is that radiation safety norms are exceedingly restrictive and should be revised to become more realistic and workable. Elevation of the limits must be accompanied by measures guaranteeing their strict observance. It is also concluded that there are no evidence-based contraindications to fivefold elevation of the total equivalent effective doses to individual members of the public (up to 5 mSv/year), and doubling of the limits for professional exposures.

The effect of non-targeted cellular mechanisms on lung cancer risk for chronic, low level radon exposures

PURPOSE

The goal of the present study was to investigate the effect of non-targeted mechanisms on the shape of the lung cancer risk function at chronic, low level radon exposures relative to direct cellular radiation effects. This includes detrimental and protective bystander effects, radio-adaptive bystander response, genomic instability and induction of apoptosis by surrounding cells.

METHODS

To quantify the dependence of these mechanisms on dose, analytical functions were derived from the experimental evidence presently available. Alpha particle intersections of bronchial target cells during a given exposure period were simulated by a Transformation Frequency-Tissue Response (TF-TR) model, formulated in terms of cellular hits within the cycle time of the cell and then integrated over the whole exposure period.

RESULTS

In general, non-targeted effects like genomic instability and bystander effects amplify the biological effectiveness of a given radiation dose, while induction of apoptosis and adaptive response will decrease the risk values. While these observations are related to the absolute number of lung cancer cases, normalization to the epidemiologically observed risk at 0.675 Gy suggests that the effect of such mechanisms on the shape of the dose-response relationship may be different. Indeed, genomic instability and adaptive response cause a substantial reduction of the risk at low doses, while induction of apoptosis and detrimental bystander effects slightly increase the risk.

CONCLUSIONS

Predictions of lung cancer risk, including these mechanisms, exhibit a distinct sublinear dose-response relationship at low exposures, particularly for very low exposure rates. However, the relatively large error bars of the epidemiological data do not currently allow the prediction of a statistically significant deviation from the Linear - No Threshold (LNT) assumption.

A brief review of radiation hormesis

This paper reviews physical, experimental and epidemiological evidence for and against radiation hormesis and discusses implications with regards to radiation protection. The scientific community is still divided on the premise of radiation hormesis, with new literature published on a regular basis. The International Commission on Radiological Protection (ICRP) recommends the use of the Linear No Threshold (LNT) model, for planning radiation protection. This model states that the probability of induced cancer and hereditary effects increases with dose in a linear fashion. As a consequence, all radiation exposures must be justified and have a sufficient protection standard in place so that exposures are kept below certain dose limitations. The LNT model has sufficient evidence at high doses but has been extrapolated in a linear fashion to low dose regions with much less scientific evidence. Much experimentation has suggested discrepancies of this extrapolation at low doses. The hypothesis of radiation hormesis suggests low dose radiation is beneficial to the irradiated cell and organism. There is definite standing ground for the hormesis hypothesis both evolutionarily and biophysically, but experimental evidence is yet to change official policies on this matter. Application of the LNT model has important radiation protection and general human health ramifications, and thus it is important that the matter be resolved.

Mammogram and diagnostic X-rays--evidence of protective Bystander, Adaptive Response (AR) radio-protection and AR retention at high dose levels

PURPOSE

The recently published dose response data by Dr Redpath's research group for low energy (30 kVp) mammography X-rays, displaying Adaptive Response (AR) radio-protective behavior, is significant for millions of American women that undergo annual breast cancer screening. We here, using the recently developed Microdose Model that encompasses the Bystander Effect (BE) and AR behavior, examine the data for BE, AR and high radiation domination by the priming radiations high dose Direct Damage.

RESULTS

The dose response is divided into three regions, Bystander Effect Region, Adaptive Response Region and Direct Damage Region (with possible retention of the AR protection). The Bystander Effect Region is below the microdose Specific Energy deposition for single photon induced charged particle traversals through the cell nucleus (the microdose Specific Energy Deposition per Traversal value = < z1 > = 0.638 cGy per Hit). Strong evidence is shown that a protective BE of about 50% occurs at a very low dose of 0.054 cGy, the BE is depleted reverting the response back to nearly the zero dose control value at 0.27 cGy, a 42% AR protection then is developed at 1.08 cGy and then the Direct Damage increasingly begins to dominate in the range from 5.4-21.6 cGy. Using the precise Method of Maximum Likelihood Estimator (MLE), the high dose Direct Damage Region is examined. We show that to the dose of 21.6 cGy the AR protection is retained in spite of the significant Direct Damage. We apply the same MLE analysis to the Redpath data for 137Cs gammas and find that the AR protection is completely dissipated at high Direct Damage inducing doses of 100 cGy.

CONCLUSIONS

The model shows that a protective BE of about 50% occurs at a low factor of 12 below single tracks traversals where less than 10% of the cell nuclei have been hit. Poisson distributed single tracks activates the 42% AR protection. The AR protection is retained at high dose but one needs to understand why 137Cs does not. Other Redpath group AR data sets for 137Cs, 232 MeV protons, and brachytherapy 125I photons did not reveal BE since the lowest data points were above the < z1 > for the radiations, but diagnostic X-rays do.

A review: Development of a microdose model for analysis of adaptive response and bystander dose response behavior

Prior work has provided incremental phases to a microdosimetry modeling program to describe the dose response behavior of the radio-protective adaptive response effect. We have here consolidated these prior works (Leonard 2000, 2005, 2007a, 2007b, 2007c) to provide a composite, comprehensive Microdose Model that is also herein modified to include the bystander effect. The nomenclature for the model is also standardized for the benefit of the experimental cellular radio-biologist. It extends the prior work to explicitly encompass separately the analysis of experimental data that is 1.) only dose dependent and reflecting only adaptive response radio-protection, 2.) both dose and dose-rate dependent data and reflecting only adaptive response radio-protection for spontaneous and challenge dose damage, 3.) only dose dependent data and reflecting both bystander deleterious damage and adaptive response radio-protection (AR-BE model). The Appendix cites the various applications of the model. Here we have used the Microdose Model to analyze the, much more human risk significant, Elmore et al (2006) data for the dose and dose rate influence on the adaptive response radio-protective behavior of HeLa x Skin cells for naturally occurring, spontaneous chromosome damage from a Brachytherapy type (125)I photon radiation source. We have also applied the AR-BE Microdose Model to the Chromosome inversion data of Hooker et al (2004) reflecting both low LET bystander and adaptive response effects. The micro-beam facility data of Miller et al (1999), Nagasawa and Little (1999) and Zhou et al (2003) is also examined. For the Zhou et al (2003) data, we use the AR-BE model to estimate the threshold for adaptive response reduction of the bystander effect. The mammogram and diagnostic X-ray induction of AR and protective BE are observed. We show that bystander damage is reduced in the similar manner as spontaneous and challenge dose damage as shown by the Azzam et al (1996) data. We cite primary unresolved questions regarding adaptive response behavior and bystander behavior. The five features of major significance provided by the Microdose Model so far are 1. Single Specific Energy Hits initiate Adaptive Response. 2. Mammogram and diagnostic X-rays induce a protective Bystander Effect as well as Adaptive Response radio-protection. 3. For mammogram X-rays the Adaptive Response protection is retained at high primer dose levels. 4. The dose range of the AR protection depends on the value of the Specific Energy per Hit, 1 >. 5. Alpha particle induced deleterious Bystander damage is modulated by low LET radiation.

It's time for a new low-dose-radiation risk assessment paradigm--one that acknowledges hormesis

The current system of radiation protection for humans is based on the linear-no-threshold (LNT) risk-assessment paradigm. Perceived harm to irradiated nuclear workers and the public is mainly reflected through calculated hypothetical increased cancers. The LNT-based system of protection employs easy-to-implement measures of radiation exposure. Such measures include the equivalent dose (a biological-damage-potential-weighted measure) and the effective dose (equivalent dose multiplied by a tissue-specific relative sensitivity factor for stochastic effects). These weighted doses have special units such as the sievert (Sv) and millisievert (mSv, one thousandth of a sievert). Radiation-induced harm is controlled via enforcing exposure limits expressed as effective dose. Expected cancer cases can be easily computed based on the summed effective dose (person-sievert) for an irradiated group or population. Yet the current system of radiation protection needs revision because radiation-induced natural protection (hormesis) has been neglected. A novel, nonlinear, hormetic relative risk model for radiation-induced cancers is discussed in the context of establishing new radiation exposure limits for nuclear workers and the public.

Cancer and low dose responses in vivo: implications for radiation protection

The Linear No Threshold (LNT) hypothesis states that ionizing radiation risk is directly proportional to dose, without a threshold. This hypothesis, along with a number of additional derived or auxiliary concepts such as radiation and tissue type weighting factors, and dose rate reduction factors, are used to calculate radiation risk estimates for humans, and are therefore fundamental for radiation protection practices. This system is based mainly on epidemiological data of cancer risk in human populations exposed to relatively high doses (above 100 mSv), with the results linearly extrapolated back to the low doses typical of current exposures. The system therefore uses dose as a surrogate for risk. There is now a large body of information indicating that, at low doses, the LNT hypothesis, along with most of the derived and auxiliary concepts, is incorrect. The use of dose as a predictor of risk needs to be re-examined and the use of dose limits, as a means of limiting risk needs to be re-evaluated. This re-evaluation could lead to large changes in radiation protection practices.

Sparsely ionizing diagnostic and natural background radiations are likely preventing cancer and other genomic-instability-associated diseases

Routine diagnostic X-rays (e.g., chest X-rays, mammograms, computed tomography scans) and routine diagnostic nuclear medicine procedures using sparsely ionizing radiation forms (e.g., beta and gamma radiations) stimulate the removal of precancerous neo-plastically transformed and other genomically unstable cells from the body (medical radiation hormesis). The indicated radiation hormesis arises because radiation doses above an individual-specific stochastic threshold activate a system of cooperative protective processes that include high-fidelity DNA repair/apoptosis (presumed p53 related), an auxiliary apoptosis process (PAM process) that is presumed p53-independent, and stimulated immunity. These forms of induced protection are called adapted protection because they are associated with the radiation adaptive response. Diagnostic X-ray sources, other sources of sparsely ionizing radiation used in nuclear medicine diagnostic procedures, as well as radioisotope-labeled immunoglobulins could be used in conjunction with apoptosis-sensitizing agents (e.g., the natural phenolic compound resveratrol) in curing existing cancer via low-dose fractionated or low-dose, low-dose-rate therapy (therapeutic radiation hormesis). Evidence is provided to support the existence of both therapeutic (curing existing cancer) and medical (cancer prevention) radiation hormesis. Evidence is also provided demonstrating that exposure to environmental sparsely ionizing radiations, such as gamma rays, protect from cancer occurrence and the occurrence of other diseases via inducing adapted protection (environmental radiation hormesis).