Year : 2021 | Volume
: 12 | Issue : 3 | Page : 91--95
Cellular sensitivity to low dose ionizing radiation
Kaushala Prasad Mishra
Ex Bhabha Atomic Research Center; Foundation for Education and Research (FERI),504, Neelyog Residency, K-1, Mumbai, Maharashtra, India
Dr. Kaushala Prasad Mishra
Ex Bhabha Atomic Research Center; Foundation for Education and Research (FERI),504, Neelyog Residency, K-1, Mumbai, Maharashtra
Cellular sensitivity to ionizing radiation is largely understood in terms of their DNA damage repair capacity. Efficient repair of DNA damage leaves irradiated cells unharmed. The observed differential responses of high doses (>500 mGy) and low doses (<200 mGy) of ionizing radiation are generally accounted by the differences in DNA damage repair processes. High dose radiation-induced cellular toxicity is gainfully employed in cancer radiotherapy. However, effects of low dose radiation (LDR) on cells and organisms have remained controversial. Some studies have reported LDR suppressive effects to high dose radiation-induced cancer. The purpose of this article is to briefly discuss the current understanding of low dose-induced DNA damage in cell survival responses. Evidence is accumulating to suggest that low dose irradiated cells remain insensitive to a window of LDR. Clearly, these findings give support to negate the linear dose effect assumptions followed in radioprotection regulation and also address the question of safety issues in utilizing LDR therapies to treat cancer and noncancer diseases. A range of new LDR technologies seem to be in waiting for improving human health.
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Mishra KP. Cellular sensitivity to low dose ionizing radiation.J Radiat Cancer Res 2021;12:91-95
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Mishra KP. Cellular sensitivity to low dose ionizing radiation. J Radiat Cancer Res [serial online] 2021 [cited 2022 Jan 21 ];12:91-95
Available from: https://www.journalrcr.org/text.asp?2021/12/3/91/323843
The effects of ionizing radiation such as γ-or X-ray on biological cells and organisms have actively been investigated since the discovery of X-ray and radioactivity at the end of 19th century. In view of increasing applications of ionizing radiation in research, industry and medical diagnosis and therapy, continued research is highly warranted to unravel deeper insights of the molecular mechanisms involved in the radiation action on cells and organisms. It is well-known that radiation affects cell function by causing alterations in molecular machinery, especially DNA damage leading to diseases including threat to their survival. Radiobiological studies on living cells have enabled us to understand why some cells are radiosensitive while some others ignore radiation. However, mechanism of cellular responses to low dose radiation (LDR) has remained elusive. For example, responses of cells to low (<500 mGy) and high doses (>1 Gy) of radiation are found to be different. The results do not fit in the linear no threshold (LNT) dose-response plot in the wide range of doses. Low doses show random effects (stochastic) while high doses follow deterministic pattern. Health effects of LDR have attracted greater attention of the researchers due to their relevance to environmental exposures, high background radiation populations, occupational workers, and nuclear accidents. In fact, many recent radiobiological studies have demonstrated that LDR effects are far from linear and the low level dose radiation effects cannot be extrapolated from high-level dose effects to the lowest doses, say 10 mGy. Notably, research in the last decades has shown that low dose-ionizing radiations have many beneficial effects on living organisms, including immune enhancement, anti-inflammatory, radiation hormesis, cell growth stimulation, lower mortality rate, and cancer occurrence which may turn into technologies useful to public health. The major obstacle is assumed LNT model of radioprotection which dictates the harmful effects even at smallest dose exposure. Therefore, it is important to examine whether or not LDR affect cellular radiosensitivity.
Ionizing radiations are known to damage cellular molecules or their assemblies such as DNA, protein, and membrane making cells malfunctional or nonfunctional including death. The type and extent of radiation damage determines the survival or death of the exposed cells. Cellular molecules are altered by direct as well as indirect effects of radiations. The most common radiation damage to DNA occurs as breakage of the strands resulting in single-strand breaks (SSB) or double-strand breaks (DSB) breaks. Oxidative damage and changes in the membrane permeability of irradiated cells have been reported. It is important to note that cells/organisms are equipped with tools to repair the DNA damage. In fact, DNA damage occurs spontaneously during normal cellular functions which are mostly self-repaired. Consequent to radiation damage of DNA, cells recruit specific molecules to repair the DNA damage. The major DNA damage repair mechanisms involve homologous and nonhomologous end joining which have yielded plethora of new knowledge of cellular capability to repair radiation DNA damages.
The present paper is an attempt to briefly highlight the current state of knowledge on low and very LDR induced mechanisms in alterations in membrane, DNA damage, and repair with regard to sensitivity responses of cells to low doses in terms of their viability and growth. Evidently, it is crucial to examine whether or not cells responding to very low and LDR exposures such as those from background radiation exposures, X-ray diagnostic procedures, and medical imaging examinations.
Radiation Effects on Cells
It is fairly well-known that radiosensitivity of cells to low radiation doses (<500 mGy) and high radiation doses (>0.5 Gy) is significantly different. Radiobiological and epidemiological studies have shown that LDR damaged cells are more likely repaired while high dose radiation damages may produce cell death/cancer incidence in linear fashion with dose (deterministic effects). High dose-rate irradiation, for example, radiation created by atomic bomb results in the delivery of radiation doses in very short time to every irradiated individual but low doses from the created background affect health for longer period of time. When radiation is delivered protracted over long periods of time, for example, the dose rates considered in the evaluation of occupational and environmental radiation risks, the cell turnover determines the number of “hits” a cell will receive. Notably, LDR influences the cell's reaction regarding the repair of DNA damage since low-dose rates allow more time for damage to be repaired which makes it more favorable for cells than at high-dose rate radiation (stochastic effects).
In present time, patients with cancer undergo radiotherapy in which high doses of ionizing radiation are aimed to kill cancer cells. However, radiation therapy treatments are substantially limited since moderate (0.1–2.0 Gy) or high (>2 Gy) radiation doses used in the radiotherapy also cause damage to normal tissues, inhibit immune functions, and enhance the risk of secondary neoplasms. In contrast, these complications do not occur when LDR exposures (≤100 mGy for acute exposure or ≤0.1 mGy/min dose rate for chronic exposures) are applied. Low-dose pretreatment has also been proposed as a promising therapeutic approach in radiation therapy, but medical professionals hesitate due to restrictions imposed by LNT model of radioprotection. Low dose pretreatment to cells and organisms triggers an adaptive response which could provide improved protection when large therapeutic doses are subsequently applied, thereby reducing the resultant damage and inhibit probability of secondary cancer. There is also some preclinical experimental evidence that LDR can be used in the treatment of several noncancer diseases, such as autoimmune diseases, neurodegenerative diseases, as well as diabetes.
Radiobiological studies show that cellular response to radiation depends on whether the cells were actively dividing or nondividing. Majority of the cells in human body are just going about their business and are not actively dividing, for example, blood cells carrying oxygen to tissues, muscle cells perform contracting and relaxing actions. A radiation damaged cells undertake to repair the DNA damage and if completely repaired the cell divides in normal manner. If the damaged cell repair is defective (mutation), the cell will divide and cause diseases such as cancer. In a situation when injured cell is un-repairable, it decides to commit suicide (apoptosis) for the sake of surviving cell population. Unrepaired DNA breaks are not so important in nondividing cells. It is to be noted that nondividing injured cells usually can still go about their normal business but unrepaired or defectively repaired DNA breaks in cells that are dividing can pose a problem because those breaks allow cells to dividing uncontrollably and may develop into cancer.
Consequences of Radiation Damage to DNA
It is well-known that ionizing radiation can induce direct and indirect actions on biological cells. In the effects of direct ionization of cellular macromolecules, ionizing radiation can lead to a large number and different types of molecular damage in DNA by oxidation processes and breaking of strands. The prominent damages include SSB, DSB, base damage of various types and DNA-protein cross-links, and local combinations of all of these. It is to be noted that since the DNA molecules make up just a small mass of the cell, the probability of the incident radiation energy absorbed in the DNA molecules is very small and hence the damage. However, irradiation at high doses increases the interaction probability of low linear energy transfer radiation of X-ray photons causing greater damage. Apart from the direct action, the ionizing radiation can induce indirect action on the cell as it irradiates the cellular water at the same time. Since most of the cell's volume is made up of water, there is a much higher probability of radiation absorbed in it. During the process of the interaction, the reactive oxygen species hydroxyl radical (*OH) and ionized water (H2O+), as well as reductants hydrogen radical (H*) and hydrated electrons (eaq-) are generated, these species also cause some damages in DNA. The DNA damages induced by both the direct and indirect effects are the powerful inducers of cell death by apoptosis. However, if the DNA damage is not strong enough to induce direct cell death, the cell cycle progression would stop to repair the damaged DNA. The cells that successfully performed an effective DNA repair thus can re-enter the cell cycle and continues their normal growth. Radiation produces oxidative damage in cellular biomolecules, which is generally measured by the oxidative product of DNA.
Apoptotic Response to Radiation Damage
In the event of severely radiation damaged cells, they decide to give up repairing the damaged molecules and allow damaged cells to be removed from the cell population by apoptotic mechanism or so-called cellular suicide. It is commonly observed that cancer cells harbor mutations in crucial safe guard proteins such as p53, which stops apoptosis process and thereby allowing cells to proliferate. Therefore, one of the strategies adopted in developing anti-cancer drugs is to look for their pro-apoptotic properties.
It is reported that exposure of cells to low doses of radiation did not repair some DNA DSBs for a long time. Obviously, this does not present any risk as long as the cells aren't dividing. However, then when these cells were made to start dividing and in response, the cells carrying unrepaired DNA DSBs commit suicide (apoptosis). This sounds worrisome, but it is actually a defense mechanism that cells use very effectively. Dead cells don't go on to become cancerous and hence when the authors observed that cells with DNA DSBs committed suicide (became apoptotic), these potentially troublesome cells were deleted from the cell population.
Cellular Membrane Damage Responses
The hypothesis of the membrane as a target for ionizing radiation action was put forward quite early in radiobiological studies. Mammalian cellular membranes are composed of lipids and proteins which are prone to radiation oxidative damages. Radiation-induced peroxidation processes are well documented in the literature and observed membrane permeability changes are found temporary and transient in nature. Studies from our laboratory on liposome model membrane and thymocytes, have shown that fluidity status of membrane determines membrane damage radiosensitivity. It has been shown that cholesterol enriched liposomes become rigid and exhibit reduced sensitivity to radiation in terms of lipid peroxidative damages. It was further found that cholesterol enriched thymocytes showed lower peroxidative damages and reduced apoptotic death.
Studies in the past decades were strongly in support of the membrane as sensitive and critical target in the mechanism of cellular radiation damage. Research from our laboratory has shown that radiation-induced cellular membrane damage, for example, signaling, apoptosis was inhibited by the antioxidant like eugenol., Reports from other laboratories have shown formation of ceramide in the membrane of irradiated cells.,, It has been shown that inhibition of ceramide resulted in significant reduction of apoptosis in cells.
Studies on membrane permeabilities have been reported by irradiation of cells by low and very low doses of X radiation. It has been shown that the activities of the Na+-K+-ATPase and Ca2+-ATPase were reduced as a function of irradiation dose., It is to be noted that damage to cell membranes by low doses of radiation are normally nonpermanent and cell survival is not threatened contrary to effects of high doses of radiation. Future research is needed to determine if radiation damage to membrane could be a prestage for nuclear DNA damage consequential to cell survival.
DNA Damage at Low Doses
Many researchers have reported the harmful effects of high-dose radiation exposure and there is no consensus in the scientific community on LDR effects on cells/organisms. Few studies have investigated the harmful effects of LDR exposure. Since the human population is not typically exposed to high doses of radiation, the effects of very low doses such as environmental radiation on the living body are need to be examined. Rothkamm and Löbrich examined DSBs in cultures of the nondividing primary human fibroblast cell line, medical research council (MRC)-5, in the G1 phase of the cell cycle after a very low dose of X-ray irradiation. They detected γ-H2AX foci using immunofluorescence to establish the lowest irradiation dose and background level of damage. The results of measurements of the background level of DSBs revealed that confluent MRC-5 cells had approximately 0.05 DSBs per cell. This group also assessed γ-H2AX foci in MRC-5 cells at an irradiation dose range between 1.2 mGy and 2 Gy and found a linear relationship between the number of foci induced per cell a few minutes after irradiation. They also showed that the number of foci did not change with a repair time up to 24 h. The initial number of foci were linearly dependent on the dose but not after a 24 h repair incubation. An examination of remaining foci 24 h after irradiation at doses of 1.2, 5, 20, and 200 mGy revealed the same level of approximately 0.1 foci per cell, which was significantly different from the background level. They concluded that DSBs after X-ray exposure at 1.2 mGy remained unrepaired.
The international commission on radiological protection recommended that the LNT hypothesis to be applied at radiation doses, which states that even very low doses of radiation need to be considered as being harmful in terms of cancer induction in order to achieve radiation protection. However, the effects of ultra-low doses on the living body were investigated and the findings obtained did not fit an LNT model. These authors concluded, “the results presented are in contrast to current models of risk assessment that assume that cellular responses are equally efficient at low and high doses.” Our current radiation protection regulations are based on the assumptions that (1) radiation risk is directly related to radiation dose, (2) the same happen at high and low doses, and (3) every dose of radiation, no matter how small, carries some risk. However, publications indicate that these assumptions are not accurate, and at least in this experiment, low doses of radiation did not increase risk. It, however, needs to be noted this paper experimented on cells in culture. There is some uncertainty on how to apply results in cells to risks of cancer in humans. However, this paper suggests that when we design studies that directly look at risks in human populations, we should consider and test for the possibility that low doses of radiation do not increase risk. The smallest radiation dose causing the changes in the levels of biomarkers appears to be between approximately 0.1 and 0.5 Gy. This dose may overlap with the induction of some adaptive responses.
Very low to low doses of radiation effects on mammalian cells in culture have been found unaffected/insensitive which may prompt clinical studies and therapeutic applications in view of safety assurances on LDR applications in hospital settings. These studies suggest that we should discuss whether our current regulations on low doses of radiation provide any benefits that outweigh the costs involved.
I extend my deep apology to a large number of esteemed authors whose references could not be included due to space limitations. Omissions to some genuine references are unintentional. This article was motivated by the talks and discussions from International School on Radiation Research (ISRR-2020) Theme: Radiation Induced DNA Damage Response: Mechanisms and Human Health Implications organized by the Society of Radiation Research (SRR, India) during September 2020.
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Conflicts of interest
There are no conflicts of interest.
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