Radiation hormesis: an evolutionary expectation and the evidence

The challenges of defining hormesis in epidemiological studies: The case of radiation hormesis

In the current radiation protection system, preventive measures and occupational exposure limits for controlling occupational exposure to ionizing radiation are based on the linear no-threshold extrapolation model. However, currently an increasing body of evidence indicates that this paradigm predicts very poorly biological responses in the low-dose exposure region. In addition, several in vitro and in vivo studies demonstrated the presence of hormetic dose response curves correlated to ionizing radiation low exposure. In this regard, it is noteworthy that also the findings of different epidemiological studies, conducted in different categories of occupationally exposed workers (e.g., healthcare, nuclear industrial and aircrew workers), observed lower rates of mortality and/or morbidity from cancer and/or other diseases in exposed workers than in unexposed ones or in the general population, then suggesting the possible occurrence of hormesis. Nevertheless, these results should be considered with caution since the identification of hormetic response in epidemiological studies is rather challenging because of a number of major limitations. In this regard, some of the most remarkable shortcomings found in epidemiological studies performed in workers exposed to ionizing radiation are represented by lack or inadequate definition of exposure doses, use of surrogates of exposure, narrow dose ranges, lack of proper control groups and poor evaluation of confounding factors. Therefore, considering the valuable role and contribution that epidemiological studies might provide to the complex risk assessment and management process, there is a clear and urgent need to overcome the aforementioned limits in order to achieve an adequate, useful and more real-life risk assessment that should also include the key concept of hormesis. Thus, in the present conceptual article we also discuss and provide possible approaches to improve the capacity of epidemiological studies to identify/define the hormetic response and consequently improve the complex process of risk assessment of ionizing radiation at low exposure doses.

Hormesis is an evolutionary expectation: implications for aging

This article argues that evolution and the concept of hormesis are biologically inseparable. It proposes that evolutionary processes led to the selection of inducible adaptive hormetic strategies that are necessary for wellbeing and survival. Hormesis has been demonstrated in essentially all organisms in which it has been studied from bacteria to humans, showing its highly conserved features. This evolution-hormesis integration should be a central feature in both understanding the biology of aging but also in ways to enhance improved health-based aging strategies.

The role of hormesis in the functional performance and protection of neural systems

This paper addresses how hormesis, a biphasic dose response, can protect and affect performance of neural systems. Particular attention is directed to the potential role of hormesis in mitigating age-related neurodegenerative diseases, genetically based neurological diseases, as well as stroke, traumatic brain injury, seizure, and stress-related conditions. The hormetic dose response is of particular significance since it mediates the magnitude and range of neuroprotective processes. Consideration of hormetic dose-response concepts can also enhance the quality of study designs, including sample size/statistical power strategies, selection of treatment groups, dose spacing, and temporal/repeat measures’ features.

Radiation Hormesis: Historical and Current Perspectives

The purpose of this article is to provide the reader with a better understanding of radiation hormesis, the investigational research that supports or does not support the theory, and the relationship between the theory and current radiation safety guidelines and practices. The concept of radiation hormesis is known to nuclear medicine technologists, but understanding its complexities and the historical development of the theory may bring about a better understanding of radiation safety and regulations.

Insecticide-induced hormesis and arthropod pest management

Ecological backlashes such as insecticide resistance, resurgence and secondary pest outbreaks are frequent problems associated with insecticide use against arthropod pest species. The last two have been particularly important in sparking interest in the phenomenon of insecticide-induced hormesis within entomology and acarology. Hormesis describes a biphasic dose-response relationship that is characterized by a reversal of response between low and high doses of a stressor (e.g. insecticides). Although the concept of insecticide-induced hormesis often does not receive sufficient attention, or has been subject to semantic confusion, it has been reported in many arthropod pest species and natural enemies, and has been linked to pest outbreaks and potential problems with insecticide resistance. The study of hormesis remains largely neglected in entomology and acarology. Here, we examined the concept of insecticide-induced hormesis in arthropods, its functional basis and potential fitness consequences, and its importance in arthropod pest management and other areas.

Antagonistic pleiotropy and the stress theory of aging

The ecological stress theory of aging incorporates the normally harsh environments of natural populations and hence restricted resources. Especially towards lethal extremes, positive associations are expected among fitness traits underlain by selection for energetic efficiency favoring genotypes for stress resistance. Positive pleiotropy is therefore expected for fitness traits across varying ages under these conditions. Furthermore, hormetic zones are regions of maximum energetic efficiency, also implying positive pleiotropy. Negative pleiotropy may therefore be mainly a phenomenon of benign environments.

Effect of low-level alpha-radiation on bioluminescent assay systems of various complexity

This study addresses the effects of low-level alpha-radiation on bioluminescent assay systems of different levels of organization: in vivo and in vitro. Three bioluminescent assay systems are used: intact bacteria, lyophilized bacteria, and bioluminescent system of coupled enzyme reactions. Solutions of 241Am(NO3)3 are used as a source of alpha-radiation. It has been shown that activation processes predominate in all the three bioluminescent assay systems subjected to short-term exposure (20-55 h) and inhibition processes in the systems subjected to longer-term exposure to radiation. It has been found that these effects are caused by the radiation component of 241Am3+ impact. The intensity of the 241Am3+ effect on the bioluminescent assay systems has been shown to depend on the 241Am3+ concentration, level of organization and integrity of the bioluminescent assay system. The bioluminescent assay systems in vivo have been found to be highly sensitive to 241Am3+ (up to 10(-17) M).

Radiation, ecology and the invalid LNT model: the evolutionary imperative

Metabolic and energetic efficiency, and hence fitness of organisms to survive, should be maximal in their habitats. This tenet of evolutionary biology invalidates the linear-no threshold (LNT) model for the risk consequences of environmental agents. Hormesis in response to selection for maximum metabolic and energetic efficiency, or minimum metabolic imbalance, to adapt to a stressed world dominated by oxidative stress should therefore be universal. Radiation hormetic zones extending substantially beyond common background levels, can be explained by metabolic interactions among multiple abiotic stresses. Demographic and experimental data are mainly in accord with this expectation. Therefore, non-linearity becomes the primary model for assessing risks from low-dose ionizing radiation. This is the evolutionary imperative upon which risk assessment for radiation should be based.

Metabolic efficiency in response to environmental agents predicts hormesis and invalidates the linear no-threshold premise: ionizing radiation as a case study

Hormesis derives from high metabolic efficiency and hence high fitness that evolve in response to single and multiple environmental agents in low to moderate stress habitats. Consequently, nonlinear fitness continua are an evolutionary expectation for all environmental agents, which invalidates the LNT premise. For ionizing radiation, hormesis is interpreted to be adaptation to background radiation exposures, combined with adaptation to higher radiation exposures dependent on metabolic protection from the array of other abiotic stresses in the environment. This model of radiation hormesis renders suggestions of therapeutic radiation supplementation redundant because of similar health effects from other environmental agents. Furthermore, the model is compatible with a return of exposure levels for radiation protection to higher doses than are presently permissible, a deduction with substantial economic benefits.

Radiation hormesis: challenging LNT theory via ecological and evolutionary considerations

Ecological and evolutionary considerations suggest that radiation hormesis is made up of two underlying components. The first (a) is background radiation hormesis based upon the background exposure to which all organisms are subjected throughout evolutionary time. The second and much larger component (b) is stress-derived radiation hormesis arising as a protective mechanism derived from metabolic adaptation to environmental stresses throughout evolutionary time especially from climate-based extremes. Since (b) > > (a), hormesis for ionizing radiation becomes an evolutionary expectation at exposures substantially exceeding background. This biological model renders linear no-threshold theory invalid. Accumulating evidence from experimental organisms ranging from protozoa to rodents, and from demographic studies on humans, is consistent with this interpretation. Although hormesis is not universally accepted, the model presented can be subjected to hypothesis-based empirical investigations in a range of organisms. At this stage, however, two consequences follow from this evolutionary model: (1) hormesis does not connote a value judgement usually expressed as a benefit; and (2) there is an emerging and increasingly convincing case for reviewing and relaxing some recommended radiation protection exposure levels in the low range.

Radiation hormesis: an ecological and energetic perspective

Organisms in natural habitats are exposed to an array of environmental stresses, which all have energetic costs. Under this ecological scenario, hormesis for ionizing radiation becomes an evolutionary expectation at exposures substantially exceeding background. This conclusion implies that some relaxation of radiation protection criteria is worthy of serious consideration.

Hormesis: an adaptive expectation with emphasis on ionizing radiation

Non-linear fitness gradients with maxima between extremes are expected for any environmental variable to which free-living populations are exposed. For exceedingly toxic agents, including ionizing radiation, such deviations from linearity are close to zero exposure and are conventionally called hormesis. Accordingly, hormesis is an extreme version of the non-linear fitness gradients for general environmental stresses such as temperature fluctuations, for which maximum fitness occurs at the moderate temperature fluctuations to which free-living populations are most commonly exposed. Some metabolic reserves should occur under moderate temperature stresses because of the need for pre-adaptation enabling survival during exposure to occasional periods of more extreme stress, especially at species borders where selection for stress resistance is likely to be most intense. Because heat shock proteins are induced by all stresses, adaptation to extreme temperatures should translate into adaptation to other stresses. Consequently, metabolic reserves from adaptation to extreme temperatures in the past should translate into protection from correlated abiotic stresses, especially in human populations where modern cultural processes are now ameliorating exposure to extreme stresses. Ambient and man-made radiations of non-catastrophic dimensions should therefore lead to stress-derived radiation hormesis. Other stresses can, in principle, be incorporated into this model. Accordingly, evolutionary and ecological considerations suggest two components of hormesis in relation to ionizing radiation: background radiation hormesis based upon the background exposure to which all organisms on earth are subjected; and stress-derived radiation hormesis. Exposure under stress-derived radiation hormesis is considerably larger than under background radiation hormesis, so significant deleterious effects from non-catastrophic radiation normally may be impossible to detect. Suggestions are provided for testing such postulated deviations from the commonly assumed linear no-threshold (LNT) hypothesis for the biological consequences of exposure to radiation.

Modeling radioprotective mechanisms in the dose effect relation at low doses and low dose rates of ionizing radiation

A new model (Random Coincidence Model--Radiation Adapted (RCM-RA)) is proposed which explains a possible pseudo threshold for stochastic radiation effects. It describes the formation of cancer in the case of multistep fixation of lesions in the critical regions of tumor associated genes such as proto-oncogenes or tumor-suppressor genes. The RCM-RA contains two different possibilities of DNA damage to complementary nucleotides. The damage may be caused either by radiation or by natural processes such as cellular radicals or thermal damage or by chemical cytotoxins. The model is based on the premise that radiation initially is bionegative, damaging organisms at their different levels of organization. The radiation, however, also induces various cellular radioprotective mechanisms which decrease the damage by natural processes. Considering both effects together, the theory explains apparent thresholds in the dose-response relation for radiation carcinogenesis without contradiction to the classical assumption that radiation is predominantly bionegative at doses typically found in occupational exposures.

Radiation hormesis - fact or fiction?

Deriving from the Greek verbhormein, which means to stimulate and excite, hormesis literally refers to any kind of stimulation and excitation. As a medical and geomedical term (though of unsettled status) it has a more restricted meaning however, indicating merely the putative or real stimulatory and beneficial effects observed when a biological system is exposed to a low dose of an agent known to be toxic or hazardous at a significantly larger dose. Depending on the type of stimulatory agent, one can speak of chemical or physical hormesis, radiation hormesis being a member of the latter group. The present paper reviews and evaluates the history and origins of the concept of radiation hormesis and its present status - fact or fiction. It is concluded that despite the numerous, sometimes undeniably strong, individual pieces of evidence that have been presented in favour of this phenomenon, the bulk of the evidence is so far not strong enough to establish it as a scientifically proven fact. It is also evident that, instead of speaking of radiation hormesis as an entity, one should pay attention separately to the effects of alpha, beta and gamma radiation, the deleterious and possible beneficial hormetic effects being different in each case.