Investigation of dose-rate effects and cell-cycle distribution under protracted exposure to ionizing radiation for various dose-rates

The impact of dose rate on responses of human lens epithelial cells to ionizing irradiation

The knowledge on responses of human lens epithelial cells (HLECs) to ionizing radiation exposure is important to understand mechanisms of radiation cataracts that are of concern in the field of radiation protection and radiation therapy. However, biological effects in HLECs following protracted exposure have not yet fully been explored. Here, we investigated the temporal kinetics of γ-H2AX foci as a marker for DNA double-strand breaks (DSBs) and cell survival in HLECs after exposure to photon beams at various dose rates (i.e., 150 kVp X-rays at 1.82, 0.1, and 0.033 Gy/min, and 137Cs γ-rays at 0.00461 Gy/min (27.7 cGy/h) and 0.00081 Gy/min (4.9 cGy/h)), compared to those in human lung fibroblasts (WI-38). In parallel, we quantified the recovery for DSBs and cell survival using a biophysical model. The study revealed that HLECs have a lower DSB repair rate than WI-38 cells. There is no significant impact of dose rate on cell survival in both cell lines in the dose-rate range of 0.033-1.82 Gy/min. In contrast, the experimental residual γ-H2AX foci showed inverse dose rate effects (IDREs) compared to the model prediction, highlighting the importance of the IDREs in evaluating radiation effects on the ocular lens.

Modeling for predicting survival fraction of cells after ultra-high dose rate irradiation

Objective. FLASH radiotherapy (FLASH-RT) with ultra-high dose rate (UHDR) irradiation (i.e. > 40 Gy s-1) spares the function of normal tissues while preserving antitumor efficacy, known as the FLASH effect. The biological effects after conventional dose rate-radiotherapy (CONV-RT) with ≤0.1 Gy s-1have been well modeled by considering microdosimetry and DNA repair processes, meanwhile modeling of radiosensitivities under UHDR irradiation is insufficient. Here, we developed anintegrated microdosimetric-kinetic(IMK)model for UHDR-irradiationenabling the prediction of surviving fraction after UHDR irradiation.Approach.TheIMK model for UHDR-irradiationconsiders the initial DNA damage yields by the modification of indirect effects under UHDR compared to CONV dose rate. The developed model is based on the linear-quadratic (LQ) nature with the dose and dose square coefficients, considering the reduction of DNA damage yields as a function of dose rate.Main results.The estimate by the developed model could successfully reproduce thein vitroexperimental dose-response curve for various cell line types and dose rates.Significance.The developed model would be useful for predicting the biological effects under the UHDR irradiation.

Catalase-gold nanoaggregates manipulate the tumor microenvironment and enhance the effect of low-dose radiation therapy by reducing hypoxia

Radiotherapy as a standard method for cancer treatment faces tumor recurrence and antitumoral unresponsiveness. Suppressive tumor microenvironment (TME) and hypoxia are significant challenges affecting efficacy of radiotherapy. Herein, a versatile method is introduced for the preparation of pH-sensitive catalase-gold cross-linked nanoaggregate (Au@CAT) having acceptable stability and selective activity in tumor microenvironment. Combining Au@CAT with low-dose radiotherapy enhanced radiotherapy effects via polarizing protumoral immune cells to the antitumoral landscape. This therapeutic approach also attenuated hypoxia, confirmed by downregulating hypoxia hallmarks, such as hypoxia-inducible factor α-subunits (HIF-α), vascular endothelial growth factor (VEGF), and EGF. Catalase stability against protease digestion was improved significantly in Au@CAT compared to the free catalase. Moreover, minimal toxicity of Au@CAT on normal cells and increased reactive oxygen species (ROS) were confirmed in vitro compared with radiotherapy. Using the nanoaggregates combined with radiotherapy led to a significant reduction of immunosuppressive infiltrating cells such as myeloid-derived suppressor cells (MDSCs) and regulatory T cells (T-regs) compared to the other groups. While, this combined therapy could significantly increase the frequency of CD8+ cells as well as M1 to M2 macrophages (MQs) ratio. The combination therapy also reduced the tumor size and increased survival rate in mice models of colorectal cancer (CRC). Our results indicate that this innovative nanocomposite could be an excellent system for catalase delivery, manipulating the TME and providing a potential therapeutic strategy for treating CRC.

Dose Rate Effect on Cell Survival in BNCT

The output constancy of the accelerator used for boron neutron capture therapy (BNCT) is essential to ensuring anti-tumor efficacy and safety. BNCT as currently practiced requires a wide variety of beam quality assessments to ensure that RBE dose errors are maintained within 5%. However, the necessity of maintaining a constant beam dose rate has not been fully discussed. We therefore clarified the effect of different physical dose rates of the accelerator BNCT on biological effects. SAS and A172 cells exposed to 10B-boronophenylalanine were irradiated using a neutron beam (normal operating current, 100 μA) at the Aomori Quantum Science Center. Thermal neutron flux was attenuated to 50.0 ± 0.96% under 50 μA irradiation compared to that under 100 μA irradiation. Cells were given physical doses of 1.67 and 3.36 Gy at 30 and 60 mC, respectively, and survival was significantly increased after 50 μA irradiation for both cell types (p = 0.0052 for SAS; p = 0.046 for A172, for 60 mC). Differences in accelerator BNCT beam dose rates have non-negligible effects on biological effects. Dose rate fluctuations and differences should not be easily permitted to obtain consistent biological effects.

Formulation of Time-Dependent Cell Survival with Saturable Repairability of Radiation Damage

This study aims to provide a model that compounds historically proposed ideas regarding cell survival irradiated with X rays or particles. The parameters used in this model have simple meanings and are closely related to cell death-related phenomena. The model is adaptable to a wide range of doses and dose rates and thus can consistently explain previously published cell survival data. The formulas of the model were derived by using five basic ideas: 1. "Poisson's law"; 2. "DNA affected damage"; 3. "repair"; 4. "clustered affected damage"; and 5. "saturation of reparability". The concept of affected damage is close to but not the same as the effect caused by the double-strand break (DSB). The parameters used in the formula are related to seven phenomena: 1. "linear coefficient of radiation dose"; 2. "probability of making affected damage"; 3. "cell-specific repairability", 4. "irreparable damage by adjacent affected damage"; 5. "recovery of temporally changed repairability"; 6. "recovery of simple damage which will make the affected damage"; 7. "cell division". By using the second parameter, this model includes cases where a single hit results in repairable-lethal and double-hit results in repairable-lethal. The fitting performance of the model for the experimental data was evaluated based on the Akaike information criterion, and practical results were obtained for the published experiments irradiated with a wide range of doses (up to several 10 Gy) and dose rates (0.17 Gy/h to 55.8 Gy/h). The direct association of parameters with cell death-related phenomena has made it possible to systematically fit survival data of different cell types and different radiation types by using crossover parameters.

DNA‑PKcs phosphorylation specific inhibitor, NU7441, enhances the radiosensitivity of clinically relevant radioresistant oral squamous cell carcinoma cells

Radioresistant cancer cells lead to poor prognosis after radiotherapy. However, the mechanisms underlying cancer cell radioresistance have not been fully elucidated. Thus, the DNA damage response of clinically relevant radioresistant oral squamous cell carcinoma HSC2-R cells, established by long-term exposure of parental HSC2 cells to fractionated radiation, was investigated. The DNA double-strand break (DSB) repair protein-specific inhibitor, NU7441, which targets DNA-dependent protein kinase catalytic subunit (DNA-PKcs) phosphorylation, and IBR2, which targets Rad51, were administered to HSC2 and HSC2-R cells. NU7441 administration eliminated colony formation in both cell lines under 6 Gy X-ray irradiation, whereas IBR2 did not affect colony formation. NU7441 and IBR2 significantly enhanced 6 Gy X-ray irradiation-induced apoptosis in HSC2-R cells. In HSC2-R cells, cell cycle arrest released earlier than in HSC2 cells, and phosphorylated-H2A histone family member X (γH2AX) expression rapidly decreased. Following NU7441 administration, γH2AX expression and the cell percentages of the G2/M phase were not decreased at 48 h after treatment in HSC2-R cells. DNA-PKcs has been demonstrated to regulate non-homologous end-joining (NHEJ) and homologous recombination (HR) repair, and the later phase of DSB repair is dominated by HR. Therefore, the results of the present study indicated that the DSB repair mechanism in HSC2-R cells strongly depends on NHEJ and loss of HR repair function. The present study revealed a potential mechanism underlying the acquired radioresistance and therapeutic targets in radioresistant cancer cells.

Translational study for stereotactic body radiotherapy against non-small cell lung cancer, including oligometastases, considering cancer stem-like cells enable predicting clinical outcome from in vitro data

BACKGROUND

Curative effects of stereotactic body radiotherapy (SBRT) for non-small cell lung cancer (NSCLC) have been evaluated using various biophysical models. Because such model parameters are empirically determined based on clinical experience, there is a large gap between in vitro and clinical studies. In this study, considering the heterogeneous cell population, we performed a translational study to realize the possible linkage based on a modeling approach.

METHODS

We modeled cell-killing and tumor control probability (TCP) considering two populations: progeny and cancer stem-like cells. The model parameters were determined from in vitro survival data of A549 and EBC-1 cells. Based on the cellular parameters, we predicted TCP and compared it with the corresponding clinical data from 553 patients collected at Hirosaki University Hospital.

RESULTS

Using an all-in-one developed model, the so-called integrated microdosimetric-kinetic (IMK) model, we successfully reproduced both in vitro survival after acute irradiation and the 3-year TCP with various fractionation schemes (6-10 Gy per fraction). From the conventional prediction without considering cancer stem cells (CSCs), this study revealed that radioresistant CSCs play a key role in the linkage between in vitro and clinical outcomes.

CONCLUSIONS

This modeling study provides a possible generalized biophysical model that enables precise estimation of SBRT worldwide.

Synthesis and characterization of actively HER-2 Targeted Fe3O4@Au nanoparticles for molecular radiosensitization of breast cancer

Introduction: The present study was done to assess the effect of molecularly-targeted core/shell of iron oxide/gold nanoparticles (Fe₃O₄@AuNPs) on tumor radiosensitization of SKBr-3 breast cancer cells. Methods: Human epidermal growth factor receptor-2 (HER-2)-targeted Fe₃O₄@AuNPs were synthesized by conjugating trastuzumab (TZ, Herceptin) to PEGylated (PEG)-Fe₃O₄@AuNPs (41.5 nm). First, the Fe₃O₄@Au core-shell NPs were decorated with PEG-SH to synthesize PEG-Fe₃O₄@AuNPs. Then, the TZ was reacted to OPSS-PEG-SVA to conjugate with the PEG-Fe₃O₄@AuNPs. As a result, structure, size and morphology of the developed NPs were assessed using Fourier-transform infrared (FT-IR) spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM), and ultraviolet-visible spectroscopy. The SKBr-3 cells were treated with different concentrations of TZ, Fe₃O₄@Au_, and TZ-PEG-Fe₃O₄@AuNPs for irradiation at doses of 2, 4, and 8 Gy (from X-ray energy of 6 and 18 MV). Cytotoxicity was assessed by MTT assay, BrdU assay, and flow cytometry. Results: Results showed that the targeted TZ-PEG-Fe₃O₄@AuNPs significantly improved cell uptake. The cytotoxic effects of all the studied groups were increased in a higher concentration, radiation dose and energy-dependent manner. A combination of TZ, Fe₃O₄@Au, and TZ-PEG-Fe₃O₄@AuNPs with radiation reduced cell viability by 1.35 (P=0.021), 1.95 (P=0.024), and 1.15 (P=0.013) in comparison with 8 Gy dose of 18 MV radiation alone, respectively. These amounts were obtained as 1.27, 1.58, and 1.10 for 8 Gy dose of 6 MV irradiation, respectively. Conclusion: Radiosensitization of breast cancer to mega-voltage radiation therapy with TZ-PEG-Fe₃O₄@AuNPs was successfully obtained through an optimized therapeutic approach for molecular targeting of HER-2.

Radiation dose rate effects: what is new and what is needed?

Despite decades of research to understand the biological effects of ionising radiation, there is still much uncertainty over the role of dose rate. Motivated by a virtual workshop on the "Effects of spatial and temporal variation in dose delivery" organised in November 2020 by the Multidisciplinary Low Dose Initiative (MELODI), here, we review studies to date exploring dose rate effects, highlighting significant findings, recent advances and to provide perspective and recommendations for requirements and direction of future work. A comprehensive range of studies is considered, including molecular, cellular, animal, and human studies, with a focus on low linear-energy-transfer radiation exposure. Limits and advantages of each type of study are discussed, and a focus is made on future research needs.

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation

Exposure to ionizing radiation can occur during medical treatments, from naturally occurring sources in the environment, or as the result of a nuclear accident or thermonuclear war. The severity of cellular damage from ionizing radiation exposure is dependent upon a number of factors including the absorbed radiation dose of the exposure (energy absorbed per unit mass of the exposure), dose rate, area and volume of tissue exposed, type of radiation (e.g., X-rays, high-energy gamma rays, protons, or neutrons) and linear energy transfer. While the dose, the dose rate, and dose distribution in tissue are aspects of a radiation exposure that can be varied experimentally or in medical treatments, the LET and eV are inherent characteristics of the type of radiation. High-LET radiation deposits a higher concentration of energy in a shorter distance when traversing tissue compared with low-LET radiation. The different biological effects of high and low LET with similar energies have been documented in vivo in animal models and in cultured cells. High-LET results in intense macromolecular damage and more cell death. Findings indicate that while both low- and high-LET radiation activate non-homologous end-joining DNA repair activity, efficient repair of high-LET radiation requires the homologous recombination repair pathway. Low- and high-LET radiation activate p53 transcription factor activity in most cells, but high LET activates NF-kB transcription factor at lower radiation doses than low-LET radiation. Here we review the development, uses, and current understanding of the cellular effects of low- and high-LET radiation exposure.

Cryo-electron tomography related radiation-damage parameters for individual-molecule 3D structure determination

To understand the dynamic structure-function relationship of soft- and biomolecules, the determination of the three-dimensional (3D) structure of each individual molecule (nonaveraged structure) in its native state is sought-after. Cryo-electron tomography (cryo-ET) is a unique tool for imaging an individual object from a series of tilted views. However, due to radiation damage from the incident electron beam, the tolerable electron dose limits image contrast and the signal-to-noise ratio (SNR) of the data, preventing the 3D structure determination of individual molecules, especially at high-resolution. Although recently developed technologies and techniques, such as the direct electron detector, phase plate, and computational algorithms, can partially improve image contrast/SNR at the same electron dose, the high-resolution structure, such as tertiary structure of individual molecules, has not yet been resolved. Here, we review the cryo-electron microscopy (cryo-EM) and cryo-ET experimental parameters to discuss how these parameters affect the extent of radiation damage. This discussion can guide us in optimizing the experimental strategy to increase the imaging dose or improve image SNR without increasing the radiation damage. With a higher dose, a higher image contrast/SNR can be achieved, which is crucial for individual-molecule 3D structure. With 3D structures determined from an ensemble of individual molecules in different conformations, the molecular mechanism through their biochemical reactions, such as self-folding or synthesis, can be elucidated in a straightforward manner.

On the calculation of the relative biological effectiveness of ion radiation therapy using a biological weighting function, the microdosimetric kinetic model (MKM) and subsequent corrections (non-Poisson MKM and modified MKM)

Objective. To investigate similarities and differences in the formalism, processing, and the results of relative biological effectiveness (RBE) calculations with a biological weighting function (BWF), the microdosimetric kinetic model (MKM) and subsequent modifications (non-Poisson MKM, modified MKM). This includes: (a) the extension of the V79-RBE10% BWF to model the RBE for other clonogenic survival levels; (b) a novel implementation of MKMs as weighting functions; (c) a benchmark against Chinese Hamster lung fibroblast (V79) in vitro data; (d) a study on the effect of pre- or post- processing the average biophysical quantities used for the RBE calculations; (e) a possible modification of the modified MKM parameters to improve the model accuracy at high linear energy transfer (LET). Methodology. Lineal energy spectra were simulated for two spherical targets (diameter = 0.464 or 1.0 μm) using PHITS for 1H, 4He, 12C, 20Ne, 40Ar, 56Fe and 132Xe ions. The results of the in silico calculations were compared with published in vitro data. Main results. All models appear to underestimate the RBE α of hydrogen ions. All MKMs generally overestimate the RBE50%, RBE10% and RBE1% for ions with an LET greater than ∼200 keV μm−1. This overestimation is greater for small surviving fractions and is likely due to the assumption of a radiation-independent quadratic term of clonogenic survival (ß). The overall RBE trends seem to be best described by the novel ‘post-processing average’ implementation of the non-Poisson MKM. In case of calculations with the non-Poisson MKM, pre- or post- processing the average biophysical quantities affects the computed RBE values significantly. Significance. This study presents a systematic analysis of the formalism and results of widely used microdosimetric models of clonogenic survival for ions relevant for cancer particle therapy and space radiation protection. Points for improvements were highlighted and will contribute to the development of upgraded biophysical models.

Pulsed low dose-rate radiotherapy: radiobiology and dosimetry

Pulsed low dose-rate radiotherapy (PLDR) relies on two radiobiological findings, the hyper-radiosensitivity of tumor cells at small doses and the reduced normal tissue toxicity at low dose rates. This is achieved by delivering the daily radiation dose of 2 Gy in 10 sub-fractions (pulses) with a 3 min time interval, resulting in an effective low dose rate of 0.067 Gy min−1. In vitro cell studies and in vivo animal experiments demonstrated the therapeutic potential of PLDR treatments and provided useful preclinical data. Various treatment optimization strategies and delivery techniques have been developed for PLDR on existing linear accelerators. Preliminary results from early clinical studies have shown favorable outcomes for various treatment sites especially for recurrent cancers. This paper reviews the experimental findings of PLDR and dosimetric requirements for PLDR treatment planning and delivery, and summarizes major clinical studies on PLDR cancer treatments.

Analysis of whole-blood antioxidant capacity after chronic and localized irradiation using the i-STrap method

Ionizing radiation exposure affects the redox state in vivo. Recently, whole-blood antioxidant capacity (WBAC) has been reported to decrease in a dose-dependent manner after acute total body irradiation (TBI). However, changes in WBAC after localized and chronic irradiations have not been reported. This study analyzed changes to WBAC in mice after either localized irradiation (irradiation of the left hind leg only) or chronic TBI using the i-STrap method. Leg-localized irradiation exerted limited effects on WBAC, while WBAC decreased in a dose rate-dependent manner after TBI. Further, the WBAC reached the minimum value in a shorter period at a smaller dose rate. Our results suggest that changes in WBAC do not directly reflect absorbed dose, but may reflect radiation-induced biological damage at the systemic level. This study will contribute to the understanding of radiation-induced injuries and diseases, and will facilitate the establishment of biomarkers for radiation exposure.

Tumor radioresistance caused by radiation-induced changes of stem-like cell content and sub-lethal damage repair capability

Cancer stem-like cells (CSCs) within solid tumors exhibit radioresistance, leading to recurrence and distant metastasis after radiotherapy. To experimentally study the characteristics of CSCs, radioresistant cell lines were successfully established using fractionated X-ray irradiation. The fundamental characteristics of CSCs in vitro have been previously reported; however, the relationship between CSC and acquired radioresistance remains uncertain. To efficiently study this relationship, we performed both in vitro experiments and theoretical analysis using a cell-killing model. Four types of human oral squamous carcinoma cell lines, non-radioresistant cell lines (SAS and HSC2), and radioresistant cell lines (SAS-R and HSC2-R), were used to measure the surviving fraction after single-dose irradiation, split-dose irradiation, and multi-fractionated irradiation. The SAS-R and HSC2-R cell lines were more positive for one of the CSC marker aldehyde dehydrogenase activity than the corresponding non-radioresistant cell lines. The theoretical model analysis showed that changes in both the experimental-based ALDH (+) fractions and DNA repair efficiency of ALDH (-) fractions (i.e., sub-lethal damage repair) are required to reproduce the measured cell survival data of non-radioresistant and radioresistant cell lines. These results suggest that the enhanced cell recovery in SAS-R and HSC2-R is important when predicting tumor control probability in radiotherapy to require a long dose-delivery time; in other words, intensity-modulated radiation therapy is ideal. This work provides a precise understanding of the mechanism of radioresistance, which is induced after irradiation of cancer cells.

Comparison and Evaluation of Different Radiotherapy Techniques Using Biodosimetry Based on Cytogenetics

Simple Summary

Cell killing and tumor response in cancer patients depends not only on the absorbed radiation dose but also on the dose rate and delivery time. In this study, a biodosimetry assay based on the frequency of dicentrics chromosomes scored in peripheral blood lymphocytes from prostate cancer patients and PC3 human prostate cancer cell line was used to investigate the radiobiological impact of the relative prolonged dose delivery time and/or decreased dose rate met in advanced modulated radiotherapy techniques (VMAT and IMRT) compared to conventional non-modulated (3D-CRT) in prostate patient plan irradiations. The results showed a small but statistically significant decrease in the number of dicentrics following radiation with the modulated techniques, suggesting a corresponding decrease on the radiation dose efficiency. The biodosimetry assay could be used as an alternative to the laborious conventional clonogenic assay, while both lymphocytes and cancer cell line could effectively be used for estimation of the biological absorbed dose.

Abstract

While rapid technological advances in radiotherapy techniques have led to a more precise delivery of radiation dose and to a decreased risk of side effects, there is still a need to evaluate the efficacy of the new techniques estimating the biological dose and to investigate the radiobiological impact of the protracted radiotherapy treatment duration. The aim of this study is to compare, at a cytogenetic level, advanced radiotherapy techniques VMAT and IMRT with the conventional 3D-CRT, using biological dosimetry. A dicentric biodosimetry assay based on the frequency of dicentrics chromosomes scored in peripheral blood lymphocytes from prostate cancer patients and PC3 human prostate cancer cell line was used. For each patient blood sample and each subpopulation of the cultured cell line, three different irradiations were performed using the 3D-CRT, IMRT, and VMAT technique. The absorbed dose was estimated with the biodosimetry method based on the induced dicentric chromosomes. The results showed a statistically significant underestimation of the biological absorbed dose of ~6% for the IMRT and VMAT compared to 3D-CRT irradiations for peripheral blood lymphocytes, whereas IMRT and VMAT results were comparable without a statistically significant difference, although slightly lower values were observed for VMAT compared to IMRT irradiation. Similar results were obtained using the PC3 cell line. The observed biological dose underestimation could be associated with the relative decreased dose rate and increase irradiation time met in modulated techniques compared to the conventional 3D-CRT irradiations.

FLASH ultra-high dose rates in radiotherapy: preclinical and radiobiological evidence

PURPOSE

Flash radiotherapy (FLASH-RT) is currently being regarded as the next breakthrough in radiation treatment of cancer, delivering ultrahigh radiation doses in a very short time, and sparing normal tissues from detrimental injury. Here we review the current evidence on the preclinical findings as well as the radiobiological mechanisms underlying the FLASH effect. We also briefly examine the scenario of available technologies for delivering FLASH dose-rates for research and their implications for future clinical use.

CONCLUSIONS

Preclinical studies report that the FLASH-RT reduces radiation-induced toxicity whilst maintaining an equivalent tumor response across different animal models. However, the molecular radiobiology underlying FLASH effect is not fully understood and further experiments are necessary to understand the biological response. Future studies also includes the design of a FLASH delivery system able to produce beams appropriate for treatment of tumors with ultra-high dose rates. All these research activities will greatly benefit from a multidisciplinary collaboration across biology, physics and clinical oncology, increasing the potential of a rapid clinical translation of FLASH-RT.

Chromosomal Location of Genes Differentially Expressed in Tumor Cells Surviving High-Dose X-ray Irradiation: A Preliminary Study on Radio-Fragile Sites

Altered gene expression is a common feature of tumor cells after irradiation. Our previous study showed that this phenomenon is not only an acute response to cytotoxic stress, instead, it was persistently detected in tumor cells that survived 10 Gy irradiation (IR cells). The current understanding is that DNA double-strand breaks (DSBs) are recognized by the phosphorylation of histone H2AX (H2AX) and triggers the ataxia-telangiectasia mutated (ATM) protein or the ATM- and Rad3-related (ATR) pathway, which activate or inactivate the DNA repair or apoptotic or senescence related molecules and causes the expression of genes in many instances. However, because changes in gene expression persist after passaging in IR cells, it may be due to the different pathways from these transient intracellular signaling pathways caused by DSBs. We performed microarray analysis of 30,000 genes in radiation-surviving cells (H1299-IR and MCF7-IR) and found an interesting relation between altered genes and their chromosomal loci. These loci formed a cluster on the chromosome, especially on 1q21 and 6p21-p22 in both irradiated cell lines. These chromosome sites might be regarded as “radio-fragile” sites.

[Comparison of 186Re to 662 keV photon radiation concerning biological radiation effect on the human B-cell line BV-173]

ZIEL: Ziel der Untersuchung ist es, die Strahlenwirkung des β--Emitters 186Re und von 662keV-Photonenstrahlung zu ermitteln, um die biologische Wirkung von Strahlung niedriger Dosisleistung (186Re) mit der hoher Dosisleistung zu vergleichen.

MATERIAL UND METHODEN

Zellen der humanen Leukämie-Zelllinie BV-173 wurden mit 662keV-Photonenstrahlung respektive 186Re bestrahlt. In einem Inkubationszeitraum von 7 Tagen wurden Zahl und Vitalität der Zellen täglich bestimmt und als Dosiseffektkurven basierend auf der Vitalität dargestellt. Hierfür wurde der Zeitpunkt mit minimalem Überleben verwendet (72h 186Re und 24h Photonenstrahlung).

ERGEBNISSE

Beide Strahlenarten zeigen am Auswertezeitpunkt (72h nach Versuchsbeginn für 186Re und 24h nach Versuchsbeginn für Photonenstrahlung) eine Überlebenskurve mit biexponentiellem Verlauf. Für Photonenstrahlung ist dies erklärbar durch eine Hypersensitivität im niedrigen Dosisbereich bis 1Gy, für die sich eine D0 von 3,3Gy ergibt, für Dosen über 1,0Gy liegt die D0 bei 10Gy. Für die 186Re-Inkubation ergibt sich eine D0 von 11,1Gy bei niedrigen Dosen verursacht durch die Reparatur subletaler Schäden, durch welche die biologische Wirkung abgeschwächt wird. Ab einer akkumulierten Dosis von etwa 1,6Gy zeichnet sich für 186Re ein wesentlich steilerer Kurvenverlauf mit einer D0 von 4,0Gy ab, der eine in diesem Bereich 2,5-fach stärkere biologische Wirkung als akute Photonenstrahlung wiedergibt (D0 4Gy für 186Re bzw. 10Gy für Photonen).

SCHLUSSFOLGERUNG

Strahlung niedriger Dosisleistung zeigt eine geringere biologische Wirkung als eine akute Bestrahlung. Es existiert aber ein Grenzwert der akkumulierten Dosis, ab dem die biologische Wirkung von β-Strahlung die der Photonenstrahlung sogar übertrifft.

4-Methylumbelliferone administration enhances radiosensitivity of human fibrosarcoma by intercellular communication

Hyaluronan synthesis inhibitor 4-methylumbelliferone (4-MU) is a candidate of radiosensitizers which enables both anti-tumour and anti-metastasis effects in X-ray therapy. The curative effects under such 4-MU administration have been investigated in vitro; however, the radiosensitizing mechanisms remain unclear. Here, we investigated the radiosensitizing effects under 4-MU treatment from cell experiments and model estimations. We generated experimental surviving fractions of human fibrosarcoma cells (HT1080) after 4-MU treatment combined with X-ray irradiation. Meanwhilst, we also modelled the pharmacological effects of 4-MU treatment and theoretically analyzed the synergetic effects between 4-MU treatment and X-ray irradiation. The results show that the enhancement of cell killing by 4-MU treatment is the greatest in the intermediate dose range of around 4 Gy, which can be reproduced by considering intercellular communication (so called non-targeted effects) through the model analysis. As supposed to be the involvement of intercellular communication in radiosensitization, the oxidative stress level associated with reactive oxygen species (ROS), which leads to DNA damage induction, is significantly higher by the combination of 4-MU treatment and irradiation than only by X-ray irradiation, and the radiosensitization by 4-MU can be suppressed by the ROS inhibitors. These findings suggest that the synergetic effects between 4-MU treatment and irradiation are predominantly attributed to intercellular communication and provide more efficient tumour control than conventional X-ray therapy.

Combined cell and nanoparticle models for TOPAS to study radiation dose enhancement in cell organelles

Dose enhancement by gold nanoparticles (AuNP) increases the biological effectiveness of radiation damage in biomolecules and tissue. To apply them effectively during cancer therapy their influence on the locally delivered dose has to be determined. Hereby, the AuNP locations strongly influence the energy deposit in the nucleus, mitochondria, membrane and the cytosol of the targeted cells. To estimate these effects, particle scattering simulations are applied. In general, different approaches for modeling the AuNP and their distribution within the cell are possible. In this work, two newly developed continuous and discrete-geometric models for simulations of AuNP in cells are presented. These models are applicable to simulations of internal emitters and external radiation sources. Most of the current studies on AuNP focus on external beam therapy. In contrast, we apply the presented models in Monte-Carlo particle scattering simulations to characterize the energy deposit in cell organelles by radioactive ¹⁹⁸AuNP. They emit beta and gamma rays and are therefore considered for applications with solid tumors. Differences in local dose enhancement between randomly distributed and nucleus targeted nanoparticles are compared. Hereby nucleus targeted nanoparticels showed a strong local dose enhancement in the radio sensitive nucleus. These results are the foundation for future experimental work which aims to obtain a mechanistic understanding of cell death induced by radioactive ¹⁹⁸Au.

Autophagy and PTEN in DNA damage-induced senescence

The use of DNA-damaging agents such as radiotherapy and chemotherapy has been a mainstay treatment protocol for many cancers, including lung and prostate. Recently, FDA approval of inhibitors of DNA repair, and targeting innate immunity to enhance the efficacy of DNA-damaging agents have gained much attention. Yet, inherent or acquired resistance against DNA-damaging therapies persists as a fundamental drawback. While cancer eradication by causing cancer cell death through induction of apoptosis is the ultimate goal of anti-cancer treatments, autophagy and senescence are two major cellular responses induced by clinically tolerable doses of DNA-damaging therapies. Unlike apoptosis, autophagy and senescence can act as both pro-tumorigenic as well as tumor suppressive mechanisms. DNA damage-induced senescence is associated with a pro-inflammatory secretory phenotype, which contributes to reshaping the tumor- immune microenvironment. Moreover, PTEN (phosphatase and tensin homolog) is a tumor supressor deleted in many tumors, and has been implicated in both senescence and autophagy. This review presents an overview of the literature on the regulation and consequences of DNA damage- induced senescence in cancer cells, with a specific focus on autophagy and PTEN. Both autophagy and senescence occur concurrently in the same cells in response to DNA damaging agents. However, a deterministic relationship between these fundamental processes has been controversial. We present experimental evidence obtained with tumor cells, with a prime focus on two models of cancer, prostate and lung. A better understanding of mechanisms associated with DNA damage-induced cellular senescence is central to fully exploit the potential of DNA-damaging agents against cancer.

Individual dosimetry system for targeted alpha therapy based on PHITS coupled with microdosimetric kinetic model

BACKGROUND

An individual dosimetry system is essential for the evaluation of precise doses in nuclear medicine. The purpose of this study was to develop a system for calculating not only absorbed doses but also EQDX(α/β) from the PET-CT images of patients for targeted alpha therapy (TAT), considering the dose dependence of the relative biological effectiveness, the dose-rate effect, and the dose heterogeneity.

METHODS

A general-purpose Monte Carlo particle transport code PHITS was employed as the dose calculation engine in the system, while the microdosimetric kinetic model was used for converting the absorbed dose to EQDX(α/β). PHITS input files for describing the geometry and source distribution of a patient are automatically created from PET-CT images, using newly developed modules of the radiotherapy package based on PHITS (RT-PHITS). We examined the performance of the system by calculating several organ doses using the PET-CT images of four healthy volunteers after injecting 18F-NKO-035.

RESULTS

The deposition energy map obtained from our system seems to be a blurred image of the corresponding PET data because annihilation γ-rays deposit their energies rather far from the source location. The calculated organ doses agree with the corresponding data obtained from OLINDA 2.0 within 20%, indicating the reliability of our developed system. Test calculations by replacing the labeled radionuclide from 18F to 211At suggest that large dose heterogeneity in a target volume is expected in TAT, resulting in a significant decrease of EQDX(α/β) for higher-activity injection.

CONCLUSIONS

As an extension of RT-PHITS, an individual dosimetry system for nuclear medicine was developed based on PHITS coupled with the microdosimetric kinetic model. It enables us to predict the therapeutic and side effects of TAT based on the clinical data largely available from conventional external radiotherapy.

Radioprotective effect of melatonin against radiotherapy-induced cerebral cortex and cerebellum damage in rat

PURPOSE

The present study aims to investigate the radioprotective effect of melatonin (MEL) against early period brain damage caused by different dose rate beams in the experimental rat model.

MATERIALS AND METHODS

Forty-eight Sprague Dawley rats were randomly divided into six groups; the control, only melatonin, low dose rate-radiotherapy (LDR-RT), high dose rate-radiotherapy (HDR-RT) groups and (LDR-RT) + MEL and (HDR-RT) + MEL radiotherapy plus melatonin groups. Each rat administered melatonin was given a dose of 10 mg/kg through intraperitoneal injection, 15 minutes before radiation exposure. The head and neck region of each rat in only radiotherapy and radiotherapy plus melatonin groups was irradiated with a single dose of 16 Gy in LDR-RT and HDR-RT beams. Rats in all groups were examined for histopathology and biochemistry analysis 10 days after radiotherapy.

RESULTS

Comparing the findings for LDR-RT and HDR-RT only radiotherapy groups and the control group, there was a statistically significant difference in histopathological and biochemical parameters, however, melatonin administered in radiotherapy plus melatonin groups contributed improving these parameters (p < .05). There was no statistically significant difference between LDR-RT and HDR-RT beams (p > .05).

CONCLUSIONS

It was concluded that melatonin applied before LDR-RT and HDR-RT radiotherapy protected early period radiotherapy-induced brain damage. The effects of clinically low and high dose beams on the cerebral cortex and cerebellum were investigated histopathologically for the first time. HDR beams can be safely applied in brain radiotherapy. However, more experimental rat and clinical studies are needed to explain the radiobiological uncertainties about the clinic dose rate on different cancerous and healthy tissues.

Delivery of Radiation at the Lowest Dose Rate by a Modern Linear Accelerator is Most Effective in Inhibiting Prostate Cancer Growth

PURPOSE

External beam radiotherapy is one of the treatment options for organ-confined prostate cancer. A total dose of 70 to 81 Gray (Gy) is given daily (1.8-2.5 Gy/d), with a dose rate of 3 to 6 Gy/min over 28 to 45 treatments during 8 to 9 weeks. We applied the latest technological development in linear accelerators for enabling a wide range of dose rates (from 0.2-21 Gy/min) to test the effect of different delivery dose rates on prostate tumor growth in an animal xenograft model.

MATERIALS AND METHODS

A prostate cancer xenograft model was established in CD1/nude mice by means of PC-3 and CL-1 cells. The animals were radiated by a TrueBeam linear accelerator that delivered 4 dose rates ranging from 0.6 to 14 Gy/min, and reaching a total dose of 20 Gy. The mice were weighed and monitored for tumor development twice weekly. A 2-way analysis of variance was used to compare statistical differences between the groups.

RESULTS

Tumor growth was inhibited by radiation at all 4 dose rates in the 20 study mice compared to no radiation (n = 5, controls). The most significant reduction in tumor volumes was observed when the same dose of radiation was delivered at a rate of 0.6 Gy/min (P < .01). The animals' weights were not affected by any dose rate.

CONCLUSIONS

Delivery of radiation with a TrueBeam linear accelerator at the lowest possible rate was most effective in prostate cancer growth inhibition and might be considered a preferential treatment mode for localized prostate cancer.

A Model for Estimating Dose-Rate Effects on Cell-Killing of Human Melanoma after Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a type of radiation therapy for eradicating tumor cells through a ¹⁰B(n,α)⁷Li reaction in the presence of ¹⁰B in cancer cells. When delivering a high absorbed dose to cancer cells using BNCT, both the timeline of ¹⁰B concentrations and the relative long dose-delivery time compared to photon therapy must be considered. Changes in radiosensitivity during such a long dose-delivery time can reduce the probability of tumor control; however, such changes have not yet been evaluated. Here, we propose an improved integrated microdosimetric-kinetic model that accounts for changes in microdosimetric quantities and dose rates depending on the ¹⁰B concentration and investigate the cell recovery (dose-rate effects) of melanoma during BNCT irradiation. The integrated microdosimetric–kinetic model used in this study considers both sub-lethal damage repair and changes in microdosimetric quantities during irradiation. The model, coupled with the Monte Carlo track structure simulation code of the Particle and Heavy Ion Transport code System, shows good agreement with in vitro experimental data for acute exposure to ⁶⁰Co γ-rays, thermal neutrons, and BNCT with ¹⁰B concentrations of 10 ppm. This indicates that microdosimetric quantities are important parameters for predicting dose-response curves for cell survival under BNCT irradiations. Furthermore, the model estimation at the endpoint of the mean activation dose exhibits a reduced impact of cell recovery during BNCT irradiations with high linear energy transfer (LET) compared to ⁶⁰Co γ-rays irradiation with low LET. Throughout this study, we discuss the advantages of BNCT for enhancing the killing of cancer cells with a reduced dose-rate dependency. If the neutron spectrum and the timelines for drug and dose delivery are provided, the present model will make it possible to predict radiosensitivity for more realistic dose-delivery schemes in BNCT irradiations.

A theoretical cell-killing model to evaluate oxygen enhancement ratios at DNA damage and cell survival endpoints in radiation therapy

Radio-resistance induced under low oxygen pressure plays an important role in malignant progression in fractionated radiotherapy. For the general approach to predict cell killing under hypoxia, cell-killing models (e.g. the Linear-Quadratic model) have to be fitted to in vitro experimental survival data for both normoxia and hypoxia to obtain the oxygen enhancement ratio (OER). In such a case, model parameters for every oxygen condition needs to be considered by model-fitting approaches. This is inefficient for fractionated irradiation planning. Here, we present an efficient model for fractionated radiotherapy the integrated microdosimetric-kinetic model including cell-cycle distribution and the OER at DNA double-strand break endpoint (OER_DSB). The cell survival curves described by this model can reproduce the in vitro experimental survival data for both acute and chronic low oxygen concentrations. The OER_DSB used for calculating cell survival agrees well with experimental DSB ratio of normoxia to hypoxia. The important parameters of the model are oxygen pressure and cell-cycle distribution, which enables us to predict cell survival probabilities under chronic hypoxia and chronic anoxia. This work provides biological effective dose (BED) under various oxygen conditions including its uncertainty, which can contribute to creating fractionated regimens for multi-fractionated radiotherapy. If the oxygen concentration in a tumor can be quantified by medical imaging, the present model will make it possible to estimate the cell-killing and BED under hypoxia in more realistic intravital situations.

Pulsed low dose-rate irradiation response in isogenic HNSCC cell lines with different radiosensitivity

Background

Management of locoregionally recurrent head and neck squamous cell carcinomas (HNSCC) is challenging due to potential radioresistance. Pulsed low-dose rate (PLDR) irradiation exploits phenomena of increased radiosensitivity, low-dose hyperradiosensitivity (LDHRS), and inverse dose-rate effect. The purpose of this study was to evaluate LDHRS and the effect of PLDR irradiation in isogenic HNSCC cells with different radiosensitivity.

Materials and methods

Cell survival after different irradiation regimens in isogenic parental FaDu and radioresistant FaDu-RR cells was determined by clonogenic assay; post irradiation cell cycle distribution was studied by flow cytometry; the expression of DNA damage signalling genes was assesed by reverse transcription-quantitative PCR.

Results

Radioresistant Fadu-RR cells displayed LDHRS and were more sensitive to PLDR irradiation than parental FaDu cells. In both cell lines, cell cycle was arrested in G₂/M phase 5 hours after irradiation. It was restored 24 hours after irradiation in parental, but not in the radioresistant cells, which were arrested in G₁-phase. DNA damage signalling genes were under-expressed in radioresistant compared to parental cells. Irradiation increased DNA damage signalling gene expression in radioresistant cells, while in parental cells only few genes were under-expressed.

Conclusions

We demonstrated LDHRS in isogenic radioresistant cells, but not in the parental cells. Survival of LDHRS-positive radioresistant cells after PLDR was significantly reduced. This reduction in cell survival is associated with variations in DNA damage signalling gene expression observed in response to PLDR most likely through different regulation of cell cycle checkpoints.

Modulating the dose-rate differently affects the responsiveness of human epithelial prostate- and mesenchymal rhabdomyosarcoma-cancer cell line to radiation

Purpose: Radiation therapy (RT), by using ionizing radiation (IR), destroys cancer cells inducing DNA damage. Despite several studies are continuously performed to identify the best curative dose of IR, the role of dose-rate, IR delivered per unit of time, on tumor control is still largely unknown.Materials and methods: Rhabdomyosarcoma (RMS) and prostate cancer (PCa) cell lines were irradiated with 2 or 10 Gy delivered at dose-rates of 1.5, 2.5, 5.5 and 10.1 Gy/min. Cell-survival rate and cell cycle distribution were evaluated by clonogenic assays and flow cytometry, respectively. The production of reactive oxygen species (ROS) was detected by cytometry. Quantitative polymerase chain reaction assessed the expression of anti-oxidant-related factors including NRF2, SODs, CAT and GPx4 and miRNAs (miR-22, -126, -210, -375, -146a, -34a). Annexin V and caspase-8, -9 and -3 activity were assessed to characterize cell death. Senescence was determined by assessing β-galactosidase (SA-β-gal) activity. Immunoblotting was performed to assess the expression/activation of: i) phosphorylated H2AX (γ-H2AX), markers of DNA double strand breaks (DSBs); ii) p19^Kip1/Cip1, p21^Waf1/Cip1 and p27^Kip1/Cip1, senescence-related-markers; iii) p62, LC3-I and LC3-II, regulators of autophagy; iv) ATM, RAD51, DNA-PKcs, Ku70 and Ku80, mediators of DSBs repair.Results: Low dose-rate (LDR) more efficiently induced apoptosis and senescence in RMS while high dose-rate (HDR) necrosis in PCa. This paralleled with a lower ability of LDR-RMS and HDR-PCa irradiated cells to activate DSBs repair. Modulating the dose rate did not differently affect the anti-oxidant ability of cancer cells.Conclusion: The present results indicate that a stronger cytotoxic effect was induced by modulating the dose-rate in a cancer cell-dependent manner, this suggesting that choose the dose-rate based on the individual patient's tumor characteristics could be strategic for effective RT exposures.

New quercetin-coated titanate nanotubes and their radiosensitization effect on human bladder cancer

Interest in nanostructures such as titanate nanotubes (TNT) has grown notably in recent years due to their biocompatibility and economic viability, making them promising for application in the biomedical field. Quercetin (Qc) has shown great potential as a chemopreventive agent and has been widely studied for the treatment of diseases such as bladder cancer. Motivated by the possibilities of developing a new hybrid nanostructure with potential in biomedical applications, this study aimed to investigate the incorporation of quercetin in sodium (NaTNT) and zinc (ZnTNT) titanate nanotubes, and characterize the nanostructures formed. Qc release testing was also performed and cytotoxicity in Vero and T24 cell lines evaluated by the MTT assay. The effect of TNTs on T24 bladder cancer cell radiosensitivity was also assessed, using cell proliferation and a clonogenic assay. The TNT nanostructures were synthesized and characterized by FESEM, EDS, TEM, FTIR, XRD and TGA. The results showed that the nanostructures have a tubular structure and that the exchange of Na⁺ ions for Zn²⁺ and incorporation of quercetin did not alter this morphology. In addition, interaction between Zn and Qc increased the thermal stability of the nanostructures. The release test showed that maximum Qc delivery occurred after 24 h and the presence of Zn controlled its release. Biological assays indicated that the NaTNTQc and ZnTNTQc nanostructures decreased the viability of T24 cells after 48 h at high concentrations. Furthermore, the clonogenic assay showed that NaTNT, NaTNTQc, ZnTNT and ZnTNTQc combined with 5 Gy reduced the formation of polyclonal colonies of T24 cells after 48 h. The results suggest that the nanostructures synthesized in this study interfere in cell proliferation and can therefore be a powerful tool in the treatment of bladder cancer.

Track Structure Study for Energy Dependency of Electrons and X-rays on DNA Double-Strand Break Induction

Radiation weighting factor w_R for photons and electrons has been defined as unity independently of the energy of the particles. However, the biological effects depend on the incident energies according to in vitro experimental data. In this study, we have quantified the energy concentration along electron tracks in terms of dose-mean lineal energy (y_D) on chromosome (micro-meter) and DNA (nano-meter) order scales by Monte Carlo simulations, and evaluated the impact of photon energies on DNA double-strand break (DNA-DSB) induction from an experimental study of irradiated cells. Our simulation result shows that the y_D values for diagnostic X-rays (60-250 kVp) are higher than that for therapeutic X-rays (linac 6 MV), which agrees well with the tissue equivalent proportional counter (TEPC) measurements. The relation between the y_D values and the numbers of γ-H2AX foci for various photon energy spectra suggests that low energy X-rays induce DNA-DSB more efficiently than higher energy X-rays even at the same absorbed dose (e.g., 1.0 Gy). The relative biological effectiveness based on DNA-DSBs number (RBE_DSB) is proportionally enhanced as the y_D value increases, demonstrating that the biological impact of the photon irradiation depends on energy concentration along radiation tracks of electrons produced in the bio-tissues. Ultimately, our study implies that the value of w_R for photons varies depending on their energies.

The Impact of Dose Rate on DNA Double-Strand Break Formation and Repair in Human Lymphocytes Exposed to Fast Neutron Irradiation

The lack of information on how biological systems respond to low-dose and low dose-rate exposures makes it difficult to accurately assess the carcinogenic risks. This is of critical importance to space radiation, which remains a serious concern for long-term manned space exploration. In this study, the γ-H2AX foci assay was used to follow DNA double-strand break (DSB) induction and repair following exposure to neutron irradiation, which is produced as secondary radiation in the space environment. Human lymphocytes were exposed to high dose-rate (HDR: 0.400 Gy/min) and low dose-rate (LDR: 0.015 Gy/min) p(66)/Be(40) neutrons. DNA DSB induction was investigated 30 min post exposure to neutron doses ranging from 0.125 to 2 Gy. Repair kinetics was studied at different time points after a 1 Gy neutron dose. Our results indicated that γ-H2AX foci formation was 40% higher at HDR exposure compared to LDR exposure. The maximum γ-H2AX foci levels decreased gradually to 1.65 ± 0.64 foci/cell (LDR) and 1.29 ± 0.45 (HDR) at 24 h postirradiation, remaining significantly higher than background levels. This illustrates a significant effect of dose rate on neutron-induced DNA damage. While no significant difference was observed in residual DNA damage after 24 h, the DSB repair half-life of LDR exposure was slower than that of HDR exposure. The results give a first indication that the dose rate should be taken into account for cancer risk estimations related to neutrons.

DNA damage induction during localized chronic exposure to an insoluble radioactive microparticle

Insoluble radioactive microparticles emitted by the incident at the Fukushima nuclear power plant have drawn keen interests from the viewpoint of radiation protection. Cs-bearing particles have been assumed to adhere in the long term to trachea after aspirated into respiratory system, leading to heterogeneous dose distribution within healthy tissue around the particles. However, the biological effects posed by an insoluble radioactive particle remain unclear. Here, we show cumulative DNA damage in normal human lung cells proximal and distal to the particle (β-ray and γ-ray-dominant areas, respectively) under localized chronic exposure in comparison with uniform exposure. We put a Cs-bearing particle into a microcapillary tip and placed it onto a glass-base dish containing fibroblast or epithelial cells cultured in vitro. A Monte Carlo simulation with PHITS code provides the radial distribution of absorbed dose-rate around the particle, and subsequently we observed a significant change in nuclear γ-H2AX foci after 24 h or 48 h exposure to the particle. The nuclear foci in the cells distal to the particle increased even under low-dose-rate exposure compared with uniform exposure to ¹³⁷Cs γ-rays, which was suppressed by a treatment with a scavenger of reactive oxygen species. In contrast, such focus formation was less manifested in the exposed cells proximal to the particle compared with uniform exposure. These data suggest that the localized exposure to a Cs-bearing particle leads to not only disadvantage to distal cells but also advantage to proximal cells. This study is the first to provide quantitative evaluation for the spatial distribution of DNA double strand breaks after the heterogeneous chronic exposure to a Cs-bearing particle in comparison with uniform Cs exposure.

Intensity Modulated Radiation Fields Induce Protective Effects and Reduce Importance of Dose-Rate Effects

In advanced radiotherapy, intensity modulated radiation fields and complex dose-delivery are utilized to prescribe higher doses to tumours. Here, we investigated the impact of modulated radiation fields on radio-sensitivity and cell recovery during dose delivery. We generated experimental survival data after single-dose, split-dose and fractionated irradiation in normal human skin fibroblast cells (AGO1522) and human prostate cancer cells (DU145). The dose was delivered to either 50% of the area of a T25 flask containing the cells (half-field) or 100% of the flask (uniform-field). We also modelled the impact of dose-rate effects and intercellular signalling on cell-killing. Applying the model to the survival data, it is found that (i) in-field cell survival under half-field exposure is higher than uniform-field exposure for the same delivered dose; (ii) the importance of sub-lethal damage repair (SLDR) in AGO1522 cells is reduced under half-field exposure; (iii) the yield of initial DNA lesions measured with half-field exposure is smaller than that with uniform-field exposure. These results suggest that increased cell survival under half-field exposure is predominantly attributed not to rescue effects (increased SLDR) but protective effects (reduced induction of initial DNA lesions). In support of these protective effects, the reduced DNA damage leads to modulation of cell-cycle dynamics, i.e., less G₁ arrest 6 h after irradiation. These findings provide a new understanding of the impact of dose-rate effects and protective effects measured after modulated field irradiation.

Scaling laws of cell-fate responses to transient stress

Analysis and modelling of dose-survival curves of cells and tissues are often used to assess therapeutic efficacy or environmental risks, much less to infer the intracellular regulatory mechanisms of cellular stress response. However, systematic measurements of how cell survival depends on the time profile of stress, such as exposure duration, provide practical means to decipher the homeostatic dynamics of stress-response regulatory networks. In this paper, we propose a dynamical framework to theoretically address the relationship between cell fate response to a transient stress and the underlying regulatory feedback mechanisms. A simple network topology that couples a homeostatic negative feedback and a death-triggering positive feedback is shown to display four response regimes for which the iso-effect relationships between duration and intensity are captured by specific power laws. These distinct response regimes define several windows of stress duration for which lethality is not merely proportional to the product of intensity and duration, and, thus, for which cells are either more tolerant or more vulnerable to a given dose. Overall, this study highlights the differential roles of feedback strength, timescale and nonlinearity in promoting survivability to particular stress profiles, providing a valuable framework for a comparative analysis of diverse stress-specific regulatory networks.

Analysis of the high-dose-range radioresistance of prostate cancer cells, including cancer stem cells, based on a stochastic model

In radiotherapy, cancer stem cells (CSCs) are well recognized as one of the radioresistant cell types. Even in a small subpopulation, CSCs may have an influence on tumor control probability, represented by cell killing after irradiation. However, the relationship between the percentage content of CSCs and the cell survival dose-response curve has not yet been quantitatively clarified. In this study, we developed a cell-killing model for two cell populations (CSCs and progeny cells) to predict the surviving fractions, and compared it with the conventional linear-quadratic (LQ) model. Three prostate cancer cell lines (DU145, PC3 and LNCaP) were exposed to X-rays at doses ranging from 0 to 10 Gy. After the irradiation, we performed clonogenic survival assays to generate the cell survival curves, and carried out flow-cytometric analyses to estimate the percentage content of CSCs for each cell line. The cell survival curves for DU145 cells and PC3 cells seemed not to follow the conventional LQ model in the high dose range (>8 Gy). However, the outputs of the developed model agreed better with the experimental cell survival curves than those of the LQ model. The percentage content of CSCs predicted by the developed model was almost coincident with the measured percentage content for both DU145 cells and PC3 cells. The experiments and model analyses indicate that a small subpopulation of radioresistant CSCs has lower radiosensitivity in the high-dose range, which may lessen the clinical outcome for patients with prostate cancer after high-dose radiation therapy.

A variant of the Escherichia coli anaerobic transcription factor FNR exhibiting diminished promoter activation function enhances ionizing radiation resistance

We have previously generated four replicate populations of ionizing radiation (IR)-resistant Escherichia coli though directed evolution. Sequencing of isolates from these populations revealed that mutations affecting DNA repair (through DNA double-strand break repair and replication restart), ROS amelioration, and cell wall metabolism were prominent. Three mutations involved in DNA repair explained the IR resistance phenotype in one population, and similar DNA repair mutations were prominent in two others. The remaining population, IR-3-20, had no mutations in the key DNA repair proteins, suggesting that it had taken a different evolutionary path to IR resistance. Here, we present evidence that a variant of the anaerobic metabolism transcription factor FNR, unique to and isolated from population IR-3-20, plays a role in IR resistance. The F186I allele of FNR exhibits a diminished ability to activate transcription from FNR-activatable promoters, and furthermore reduces levels of intracellular ROS. The FNR F186I variant is apparently capable of enhancing resistance to IR under chronic irradiation conditions, but does not increase cell survival when exposed to acute irradiation. Our results underline the importance of dose rate on cell survival of IR exposure.