Modification of effects of radiation on thymidine kinase

No evidence of persisting unrepaired nuclear DNA single strand breaks in distinct types of cells in the brain, kidney, and liver of adult mice after continuous eight-week 50 Hz magnetic field exposure with flux density of 0.1 mT or 1.0 mT

BACKGROUND

It has been hypothesized in the literature that exposure to extremely low frequency electromagnetic fields (50 or 60 Hz) may lead to human health effects such as childhood leukemia or brain tumors. In a previous study investigating multiple types of cells from brain and kidney of the mouse (Acta Neuropathologica 2004; 107: 257-264), we found increased unrepaired nuclear DNA single strand breaks (nDNA SSB) only in epithelial cells of the choroid plexus in the brain using autoradiographic methods after a continuous eight-week 50 Hz magnetic field (MF) exposure of adult mice with flux density of 1.5 mT.

METHODS

In the present study we tested the hypothesis that MF exposure with lower flux densities (0.1 mT, i.e., the actual exposure limit for the population in most European countries, and 1.0 mT) shows similar results to those in the previous study. Experiments and data analysis were carried out in a similar way as in our previous study.

RESULTS

Continuous eight-week 50 Hz MF exposure with 0.1 mT or 1.0 mT did not result in increased persisting unrepaired nDNA SSB in distinct types of cells in the brain, kidney, and liver of adult mice. MF exposure with 1.0 mT led to reduced unscheduled DNA synthesis (UDS) in epithelial cells in the choroid plexus of the fourth ventricle in the brain (EC-CP) and epithelial cells of the cortical collecting duct in the kidney, as well as to reduced mtDNA synthesis in neurons of the caudate nucleus in the brain and in EC-CP.

CONCLUSION

No evidence was found for increased persisting unrepaired nDNA SSB in distinct types of cells in the brain, kidney, and liver of adult mice after continuous eight-week 50 Hz magnetic field exposure with flux density of 0.1 mT or 1.0 mT.

Biological consequences and health risks of low-level exposure to ionizing radiation: commentary on the workshop

This paper provides an integration and discussion of the information presented at the workshop held from 2-5 May 2010 in Richland, WA, adjacent to the Pacific Northwest National Laboratory (PNNL). Consequently, this is commentary and not necessarily a consensus document. This workshop was in honor of Dr. Victor P. Bond in celebration of his numerous contributions to the radiation sciences.

Low-dose cancer risk modeling must recognize up-regulation of protection

IONIZING RADIATION PRIMARILY PERTURBS THE BASIC MOLECULAR LEVEL PROPORTIONAL TO DOSE, WITH POTENTIAL DAMAGE PROPAGATION TO HIGHER LEVELS: cells, tissues, organs, and whole body. There are three types of defenses against damage propagation. These operate deterministically and below a certain impact threshold there is no propagation. Physical-static defenses precede metabolic-dynamic defenses acting immediately: scavenging of toxins; - molecular repair, especially of DNA; - removal of damaged cells either by apoptosis, necrosis, phagocytosis, cell differentiation-senescence, or by immune responses, - followed by replacement of lost elements. Another metabolic-dynamic defense arises delayed by up-regulating immediately operating defense mechanisms. Some of these adaptive protections may last beyond a year and all create temporary protection against renewed potentially toxic impacts also from non-radiogenic endogenous sources. Adaptive protections have a maximum after single tissue absorbed doses around 100 to 200 mSv and disappear with higher doses. Low dose rates initiate maximum protection likely at lower cell doses delivered repetitively at certain time intervals. Adaptive protection preventing only about 2 - 3 % of endogenous life-time cancer risk would fully balance a calculated induced cancer risk at about 100 mSv, in agreement with epidemiological data and concordant with an hormetic effect. Low-dose-risk modeling must recognize up-regulation of protection.

Monitoring response to radiotherapy in human squamous cell cancer bearing nude mice: comparison of 2'-deoxy-2'-[18F]fluoro-D-glucose (FDG) and 3'-[18F]fluoro-3'-deoxythymidine (FLT)

Objective

The uptake of 3′-[¹⁸F]fluoro-3′-deoxythymidine (FLT), a proliferation marker, was measured before and during fractionated radiotherapy to evaluate the potential of FLT-positron emission tomography (PET) imaging as an indicator of tumor response compared to 2′-deoxy-2′-[¹⁸F]fluoro-d-glucose (FDG).

Materials and Methods

Nude mice bearing established human head and neck xenografts (HNX-OE; nu/nu mice) were locally irradiated (three fractions/week; 22 Gy) using a 150-kV_p unit. Multiple FDG- and FLT-PET scans were acquired during treatment. Tumor volume was determined regularly, and tissue was analyzed for biomarkers involved in tracer uptake.

Results

Both groups revealed a significant decline in tumor volume (P < 0.01) compared to untreated tumors. For FDG as well as for FLT, a significant decline in retention was observed at day 4. For FLT, most significant decline in retention was observed at day 12; whereas, for FDG, this was already noted at day 4. Maximum decline in tumor-to-nontumor ratios (T/NT) for FDG and FLT was 42 ± 18% and 49 ± 16% (mean ± SD), respectively. FLT uptake was higher then that of FDG. For FLT, statistical significant correlations were found for both tumor volume at baseline and at day 29 with T/NT and ΔT/NT. All tumors demonstrated expression of glucose transporter-1, thymidine kinase-1, and hexokinase II. No differences were found for amount of tumor cells and necrosis at the end of treatment.

Conclusion

This new experimental in vivo model supports the promise of using FLT-PET, as with FDG-PET, to monitor response to external radiotherapy. This warrants further clinical studies to compare these two tracers especially in cancers treated with radiotherapy.

Radiation-induced versus endogenous DNA damage: possible effect of inducible protective responses in mitigating endogenous damage

Ionizing radiation (IR) causes damage to DNA that is apparently proportional to absorbed dose. The incidence of radiation-induced cancer in humans unequivocally rises with the value of absorbed doses above about 300 mGy, in a seemingly linear fashion. Extrapolation of this linear correlation down to zero-dose constitutes the linear-no-threshold (LNT) hypothesis of radiation-induced cancer incidence. The corresponding dose-risk correlation, however, is questionable at doses lower than 300 mGy. Non-radiation induced DNA damage and, in consequence, oncogenic transformation in non-irradiated cells arises from a variety of sources, mainly from weak endogenous carcinogens such as reactive oxygen species (ROS) as well as from micronutrient deficiencies and environmental toxins. In order to relate the low probability of radiation-induced cancer to the relatively high incidence of non-radiation carcinogenesis, especially at low-dose irradiation, the quantitative and qualitative differences between the DNA damages from non-radiation and radiation sources need to be addressed and put into context of physiological mechanisms of cellular protection. This paper summarizes a co-operative approach by the authors to answer the questions on the quantitative and qualitative DNA damages from non-radiation sources, largely endogenous ROS, and following exposure to low doses of IR. The analysis relies on published data and justified assumptions and considers the physiological capacity of mammalian cells to protect themselves constantly by preventing and repairing DNA damage. Furthermore, damaged cells are susceptible to removal by apoptosis or the immune system. The results suggest that the various forms of non-radiation DNA damage in tissues far outweigh corresponding DNA damage from low-dose radiation exposure at the level of, and well above, background radiation. These data are examined within the context of low-dose radiation induction of cellular signaling that may stimulate cellular protection systems over hours to weeks against accumulation of DNA damage. The particular focus is the hypothesis that these enhanced and persisting protective responses reduce the steady state level of non-radiation DNA damage, thereby reducing deleterious outcomes such as cancer and aging. The emerging model urgently needs rigorous experimental testing, since it suggests, importantly, that the LNT hypothesis is invalid for complex adaptive systems such as mammalian organisms.

X-irradiation effects on thymidine kinase (TK): II. The significance of deoxythymidine triphosphate for inhibition of TK1 activity

The purpose of this study was to investigate the mechanism behind the high sensitivity of thymidine kinase 1 (TK1) to X-irradiation. The deoxythymidine triphosphate (dTTP) pool was studied in mouse ascites tumour cells 1-24 h after X-irradiation with 5 Gy. Irradiation changed the Michaelis-Menten kinetics of TK1 from linear to biphasic, showing a negative co-operativity. These changes were closely related to changes in the dTTP pool. Addition of dTTP to the cell extract of non-irradiated cells, or thymidine (dTdR) to the culture medium, resulted in changes very similar to the kinetics found in the irradiated cells. Addition of 5 cent-amino-5 cent-deoxythymidine (5 cent-AdTdR), a thymidine analogue that eliminated the inhibitory effect of dTTP on TK1 activity, completely abolished the irradiation-induced inhibition of TK1 activity. We suggest that the reduced TK1 activity is mainly due to an elevated intracellular concentration of dTTP.

X-irradiation effects on thymidine kinase (TK): I. TK1 and 2 in normal and malignant cells

The effect of radiation on TK is more complicated than would be expected from earlier results on bone marrow cells (Feinendegen et al. 1984, Int. J. Radiat. Biol. 45, 205). TK activity increased at 0.01 Gy and then decreased up to 1 Gy in mouse spleen. In contrast to the results for the spleen, an increase in activity at 0.1 Gy was seen in mouse thymus. The activity of dephosphorylated TK1 (TK1a) in both spleen and thymus was reduced to 50% after irradiation at 0.5-1 Gy. The degree of phosphorylation (TK1b/TK1a ratio) changed in spleen, but not in thymus. The activity of TK2 in mouse liver increased at 3 h after 5 Gy by about 60%. In mouse ascites tumour, a dose-independent (1-5 Gy) oscillating TK1 activity was found up to 24 h, especially for TK1a and TK1b. The amount of TK1 was unchanged up to 12 h, but decreased at 24 h. This suggests that the differences in the changes in the degree of phosphorylation of TK1 after irradiation among spleen, thymus and ascites tumour further underline the complexity of the response of TK1 activity to irradiation. The dramatic change in the activities of TK1a and TK1b may illustrate that both of them are more radiosensitive than TK-h, a variant with mixed TK1 and TK2 properties.

Cellular signal adaptation with damage control at low doses versus the predominance of DNA damage at high doses

Ionizing radiation is known to potentially interfere with cellular functions at all levels of cell organization and induces DNA lesions apparently with an incidence linearly related to D, also at low doses. On the other hand, low doses have also been observed to initiate a slowly appearing temporary protection against causation and accumulation of DNA lesions, involving the radical detoxification system, DNA repair and removal of DNA damage. This protection apparently does not operate at high doses; it has been described to be nonlinear, increasing initially with D, beginning to decrease when D exceeds approximately 0.1-0.2 Gy, and eventually disappearing at higher D. The various adaptive responses have been shown to last individually from hours to weeks in different cell types and resemble responses to oxidative stress. Damage to DNA is continuously and endogenously produced mainly by reactive oxygen species (ROS) generated in a normal oxidative metabolism. This endogenous DNA damage quantitatively exceeds DNA damage from low-dose irradiation, by several orders of magnitude. Thus, the protective responses following acute low-dose irradiation may be presumed to mainly counteract the endogenous DNA damage. Accordingly, the model described here uses two dose-effect functions, a linear one for causing and a nonlinear one for protecting against DNA damage from whatever cause in the irradiated cells and tissues. The resulting net dose-risk function strongly suggests that the incidence of cancer versus dose in the irradiated tissues is much less likely to be linear than to exhibit a threshold. The observed cancer incidence may even fall below the spontaneous incidence, when D to cells is below approximately 0.2 Gy. However incomplete, these data support a reexamination of the LNT hypothesis.

Inhibition of drug-naive and -resistant leukemia cell proliferation by low molecular weight thiols

The aim of these studies was to investigate the ability of cysteamine and its congeners to arrest the proliferation of leukemic cells and to determine the physico-chemical properties responsible for this ability. Fifteen low molecular weight thiol-bearing compounds were shown to arrest the proliferation of CCRF-CEM cells and a methotrexate-resistant subline, with IC50 values between 10(-5) and 10(-4) M. Cysteamine arrested proliferation by slowing the passage of cells through S phase. These cells subsequently resumed cycling, although a proportion went on to die by apoptosis. The antiproliferative action of cysteamine was shown to depend, in part, on H2O2 production. This ability to generate peroxide is shared by many thiol compounds, and molecular modeling indicated that thiol groups were required for the antiproliferative actions of the congeners of cysteamine. Molecular modeling also revealed that the most efficacious antiproliferative agents were those that had their amino acid and thiol moieties separated by an intramolecular distance of 3.17 to 5.9 A, as exemplified by WR 1065 and the aminothiophenols. These findings indicate that thiol-bearing compounds may have some efficacy in the treatment of drug-naive and -resistant leukemia cells.

Lipid peroxidation in microsomes of murine bone marrow after low-dose gamma-irradiation

The principal aim of the study was to investigate the effect of low-dose gamma-irradiation on lipid peroxidation (LPO) in murine bone marrow. To this end, the degree of LPO in suspensions of microsomes of murine bone marrow cells (BMC) was determined in terms of malondialdehyde (MDA) formation after whole-body or in vitro exposure to various doses of gamma-radiation. These effects were compared to some extent with similar effects in liver and spleen preparations. As to the effect of gamma-irradiation on LPO in BMC, the response depends on the dose level and on whether whole-body or in vitro exposures are involved. Whole-body irradiation did not result in an increase in LPO in BMC microsomes, even at such high doses as 15 Gy, although hepatic microsomes showed a marked increase. In contrast, in vitro irradiation of BMC microsomes with 0.1, 10 and 50 Gy brought about an increase in LPO. This increase was already significant (P < 0.05) at 0.1 Gy following a post-irradiation incubation and substantial at 50 Gy, even without subsequent incubation. The results show that low doses of gamma-irradiation are able to induce an elevation of LPO in murine BMC microsomes, but only after in vitro irradiation. In the case of whole-body irradiation cellular radical scavengers and other metabolic reactions may prevent a measurable increase in LPO. This is partly illustrated by the case of vitamin-E deficiency, where a substantial increase in LPO in BMC microsomes is observed even without gamma-irradiation in comparison with euvitaminotic mice because normally occurring radicals are not scavenged sufficiently.

Influence of a stationary magnetic field on acetylcholinesterase in murine bone marrow cells

A thirty-minute exposure of mice to a homogeneous stationary magnetic field (SMF) of 1.4 Tesla at either 27 degrees C or 37 degrees C body temperature causes an inhibition of about 20 per cent of acetylcholinesterase (AChE, E.C. 3.11.7) in murine bone marrow cells (BMC) after 3.5 and 2 h, respectively, at the two aforementioned body temperatures. The extent of enzyme inhibition is independent of ambient temperature, but dependent on the time after exposure. This initial inhibition of AChE activity is followed by a limited recovery which is dependent upon the temperature during exposure to the SMF and remains incomplete even 15 h afterwards. We describe here certain enzymologic properties of AChE in BMC as well as inhibition studies with diisopropylfluorophosphate (DFP) to differentiate between AChE and nonspecific cholinesterases.

Rubidium transport in irradiated vitamin-E-deficient bone marrow cells

We showed previously that the Rb+ transport rate in bone marrow cells (BMC) of vitamin-E-deficient mice is significantly lower than that in BMC of euvitaminotic mice. It is now evident that 4 h after whole-body, low-dose (0.01-1.0 Gy) gamma-irradiation of avitaminotic mice, there is an increase in the rate of Rb+ transport. This increase is quite pronounced, exceeding at all dose levels the rate of Rb+ transport in euvitaminotic mice exposed to the same radiation dose.

Radiation risk of tissue late effects, a net consequence of probabilities of various cellular responses

Late effects from the exposure to low doses of ionizing radiation are hardly or not at all observed in man probably due to the low values of risk coefficients that preclude statistical analyses of data from populations that are exposed to doses less than 0.2 Gy. In order to arrive at an assessment of potential risk from radiation exposure in the low dose range, the microdosimetry approach is essential. In the low dose range, ionizing radiation generates particle tracks, mainly electrons, which are distributed rather heterogenously within the exposed tissue. Taking the individual cell as the elemental unit of life, observations and calculations of cellular responses to being hit by energy deposition events from low LET type are analysed. It emerges that besides the probability of a hit cell to sustain a detrimental effect with the consequence of malignant transformation there are probabilities of various adaptive responses that equip the hit cell with a benefit. On the one hand, an improvement of cellular radical detoxification was observed in mouse bone marrow cells; another adaptive response pertaining to improved DNA repair, was reported for human lymphocytes. The improved radical detoxification in mouse bone marrow cells lasts for a period of 5-10 hours and improved DNA repair in human lymphocytes was seen for some 60 hours following acute irradiation. It is speculated that improved radical detoxification and improved DNA repair may reduce the probability of spontaneous carcinogenesis. Thus it is proposed to weigh the probability of detriment for a hit cell within a multicellular system against the probability of benefit through adaptive responses in other hit cells in the same system per radiation exposure. In doing this, the net effect of low doses of low LET radiation in tissue with individual cells being hit by energy deposition events could be zero or even beneficial. Since there was no simple additivity of equal effects from repeated exposures to equal doses and because of the potential effect of adaptive cell responses on the spontaneous evolution of malignancy in tissue, the extrapolation of risk with absorbed dose reaching down to zero, does not appear to be generally valid.