Cell-cycle radiation response: role of intracellular factors

Environmental and industrial developments in radiation cataractogenesis

Purpose: This review discusses recent developments in our understanding of biological and physiological mechanisms underlying radiation cataractogenesis. The areas discussed include effects of low-dose exposures to the lens including potential relevance of non-targeted effects, the development of new personal-protective equipment (PPE) and standards in clinical and nuclear settings motivated by the updated ICRP recommendations to mitigate exposures to the lens of the eye. The review also looks at evidence from the field linking cataracts in birds and mammals to low dose exposures.Conclusions: The review suggests that there is evidence that cataractogenesis is not a tissue reaction (deterministic effect) but rather is a low dose effect which shows a saturable dose response relationship similar to that seen for non-targeted effects in general. The review concludes that new research is needed to determine the dose response relationship in environmental studies where field data are contradictory and lab studies confined to rodent models for human exposure studies.

Clair WH, St. Clair DK. Redox Paradox: A Novel Approach to Therapeutics-Resistant Cancer

SIGNIFICANCE

Cancer cells that are resistant to radiation and chemotherapy are a major problem limiting the success of cancer therapy. Aggressive cancer cells depend on elevated intracellular levels of reactive oxygen species (ROS) to proliferate, self-renew, and metastasize. As a result, these aggressive cancers maintain high basal levels of ROS compared with normal cells. The prominence of the redox state in cancer cells led us to consider whether increasing the redox state to the condition of oxidative stress could be used as a successful adjuvant therapy for aggressive cancers. Recent Advances: Past attempts using antioxidant compounds to inhibit ROS levels in cancers as redox-based therapy have met with very limited success. However, recent clinical trials using pro-oxidant compounds reveal noteworthy results, which could have a significant impact on the development of strategies for redox-based therapies.

CRITICAL ISSUES

The major objective of this review is to discuss the role of the redox state in aggressive cancers and how to utilize the shift in redox state to improve cancer therapy. We also discuss the paradox of redox state parameters; that is, hydrogen peroxide (H2O2) as the driver molecule for cancer progression as well as a target for cancer treatment.

FUTURE DIRECTIONS

Based on the biological significance of the redox state, we postulate that this system could potentially be used to create a new avenue for targeted therapy, including the potential to incorporate personalized redox therapy for cancer treatment.

RBE and related modeling in carbon-ion therapy

Carbon ion therapy is a promising evolving modality in radiotherapy to treat tumors that are radioresistant against photon treatments. As carbon ions are more effective in normal and tumor tissue, the relative biological effectiveness (RBE) has to be calculated by bio-mathematical models and has to be considered in the dose prescription. This review (i) introduces the concept of the RBE and its most important determinants, (ii) describes the physical and biological causes of the increased RBE for carbon ions, (iii) summarizes available RBE measurements in vitro and in vivo, and (iv) describes the concepts of the clinically applied RBE models (mixed beam model, local effect model, and microdosimetric-kinetic model), and (v) the way they are introduced into clinical application as well as (vi) their status of experimental and clinical validation, and finally (vii) summarizes the current status of the use of the RBE concept in carbon ion therapy and points out clinically relevant conclusions as well as open questions. The RBE concept has proven to be a valuable concept for dose prescription in carbon ion radiotherapy, however, different centers use different RBE models and therefore care has to be taken when transferring results from one center to another. Experimental studies significantly improve the understanding of the dependencies and limitations of RBE models in clinical application. For the future, further studies investigating quantitatively the differential effects between normal tissues and tumors are needed accompanied by clinical studies on effectiveness and toxicity.

Manganese superoxide dismutase regulates a redox cycle within the cell cycle

SIGNIFICANCE

Manganese superoxide dismutase (MnSOD) is a nuclear-encoded and mitochondria-matrix-localized oxidation-reduction (redox) enzyme that regulates cellular redox homeostasis. Cellular redox processes are known to regulate proliferative and quiescent growth states. Therefore, MnSOD and mitochondria-generated reactive oxygen species (ROS) are believed to be critical regulators of quiescent cells' entry into the cell cycle and exit from the proliferative cycle back to the quiescent state.

RECENT ADVANCES/CRITICAL ISSUES

Recent evidence suggests that the intracellular redox environment fluctuates during the cell cycle, shifting toward a more oxidized status during mitosis. MnSOD activity is higher in G0/G1 cells compared with S, G2 and M phases. After cell division, MnSOD activity increases in the G1 phase of the daughter generation. The periodic fluctuation in MnSOD activity during the cell cycle inversely correlates with cellular superoxide levels as well as glucose and oxygen consumption. Based on an inverse correlation between MnSOD activity and glucose consumption during the cell cycle, it is proposed that MnSOD is a central molecular player for the "Warburg effect."

FUTURE DIRECTIONS

In general, loss of MnSOD activity results in aberrant proliferation. A better understanding of the MnSOD and mitochondrial ROS-dependent cell cycle processes may lead to novel approaches to overcome aberrant proliferation. Since ROS have both deleterious (pathological) and beneficial (physiological) effects, it is proposed that "eustress" should be used when discussing ROS processes that regulate normal physiological functions, while "oxidative stress" should be used to discuss the deleterious effects of ROS.

Single cell DNA damage/repair assay using HaloChip

The molecular level damage to DNA is important due to DNA's susceptibility to free radical attacks and crucial roles in maintaining cell functions. Although a panel of techniques can be used to detect DNA damages, most of them are limited due to low sensitivity, low throughput, incompatibility for automated data analysis, and labor-intensive operations. We have developed a cell array based DNA damage assay in which mammalian cells are attached on an array of microfabricated patterns through electrostatic interactions. After trapping patterned cells inside gels, damaged DNA fragment can diffuse out of the nucleus and form a halo around each cell inside gels. The halo array can be observed fluorescently after labeling DNA with ethidium bromide. DNA damages can be determined sensitively at the single cell level, accurately due to the symmetric shape of the halo, and automatically due to the spatial registry of each cell and the nonoverlapping halos surrounding cells. The HaloChip can be used to detect DNA damages caused by chemicals and ultraviolet and X-ray irradiations with high efficiency. A major advantage of HaloChip is the ability to increase throughout by spatially encoding multiple dosing conditions on the same chip. Most importantly, the method can be used to measure variations in response to DNA damaging agents within the same cell population. Compared with halo assay or comet assay alone, this method allows automated analysis of a million cells without an overlapping issue. Compared with the microwell array based comet assay, this method can selectively capture and analyze cells, and the results can be easily analyzed to provide precise information on DNA damage. This method can be used in a broad range of clinical, epidemiological, and experimental settings.

Repair of DNA damage induced by accelerated heavy ions--a mini review

Increasing use of heavy ions for cancer therapy and concerns from exposure to heavy charged particles in space necessitate the study of the basic biological mechanisms associated with exposure to heavy ions. As the most critical damage induced by ionizing radiation is DNA double strand break (DSB), this review focuses on DSBs induced by heavy ions and their repair processes. Compared with X- or gamma-rays, high-linear energy transfer (LET) heavy ion radiation induces more complex DNA damage, categorized into DSBs and non-DSB oxidative clustered DNA lesions (OCDL). This complexity makes the DNA repair process more difficult, partially due to retarded enzymatic activities, leading to increased chromosome aberrations and cell death. In general, the repair process following heavy ion exposure is LET-dependent, but with nonhomologous end joining defective cells, this trend is less emphasized. The variation in cell survival levels throughout the cell cycle is less prominent in cells exposed to high-LET heavy ions when compared with low LET, but this mechanism has not been well understood until recently. Involvement of several DSB repair proteins is suggested to underlie this interesting phenomenon. Recent improvements in radiation-induced foci studies combined with high-LET heavy ion exposure could provide a useful opportunity for more in depth study of DSB repair processes. Accelerated heavy ions have become valuable tools to investigate the molecular mechanisms underlying repair of DNA DSBs, the most crucial form of DNA damage induced by radiation and various chemotherapeutic agents.

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

Base excision repair of ionizing radiation-induced DNA damage in G1 and G2 cell cycle phases

BACKGROUND

Major genomic surveillance mechanisms regulated in response to DNA damage exist at the G1/S and G2/M checkpoints. It is presumed that these delays provide time for the repair of damaged DNA. Cells have developed multiple DNA repair pathways to protect themselves from different types of DNA damage. Oxidative DNA damage is processed by the base excision repair (BER) pathway. Little is known about the BER of ionizing radiation-induced DNA damage and putative heterogeneity of BER in the cell cycle context. We measured the activities of three BER enzymes throughout the cell cycle to investigate the cell cycle-specific repair of ionizing radiation-induced DNA damage. We further examined BER activities in G2 arrested human cells after exposure to ionizing radiation.

RESULTS

Using an in vitro incision assay involving radiolabeled oligonucleotides with specific DNA lesions, we examined the activities of several BER enzymes in the whole cell extracts prepared from synchronized human HeLa cells irradiated in G1 and G2 phase of the cell cycle. The activities of human endonuclease III (hNTH1), a glycosylase/lyase that removes several damaged bases from DNA including dihydrouracil (DHU), 8-oxoguanine-DNA glycosylase (hOGG1) that recognizes 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxoG) lesion and apurinic/apyrimidinic endonuclease (hAPE1) that acts on abasic sites including synthetic analog furan were examined.

CONCLUSION

Overall the repair activities of hNTH1 and hAPE1 were higher in the G1 compared to G2 phase of the cell cycle. The percent cleavages of oligonucleotide substrate with furan were greater than substrate with DHU in both G1 and G2 phases. The irradiation of cells enhanced the cleavage of substrates with furan and DHU only in G1 phase. The activity of hOGG1 was much lower and did not vary within the cell cycle. These results demonstrate the cell cycle phase dependence on the BER of ionizing radiation-induced DNA damage. Interestingly no evidence of enhanced BER activities was found in irradiated cells arrested in G2 phase.

Comparison of clonogenic assay with premature chromosome condensation assay in prediction of human cell radiosensitivity

AIM: To determine whether the number of non-rejoining G2-chromatid breaks can predict the radiosensitivity of human cell lines.

METHODS: Cell lines of human ovary carcinoma cells (HO8910), human hepatoma cells (HepG2) and liver cells (L02) were irradiated with a range of doses and assessed both of cell survival and non-rejoining G2-chromatid breaks at 24 h after irradiation. Cell survival was documented by a colony assay. Non-rejoining G2-chromatid breaks were measured by counting the number of non-rejoining G2 chromatid breaks at 24 h after irradiation, detected by the prematurely chromosome condensed (PCC) technique.

RESULTS: A linear-quadratic survival curve was observed in three cell lines, and HepG2 was the most sensitive to γ-radiation. A dose-dependent linear increase was observed in radiation-induced non-rejoining G2-PCC breaks measured at 24 h after irradiation in all cell lines, and HepG2 was the most susceptible to induction of non-rejoining G2-PCC breaks. A close correlation was found between the clonogenic radiosensitivity and the radiation-induced non-rejoining G2-PCC breaks (r = 0.923). Furthermore, survival-aberration correlations for two or more than two doses lever were also significant.

CONCLUSION: The number of non-rejoining G2 PCC breaks holds considerable promise for predicting the radiosensitivity of normal and tumor cells when two or more than two doses lever is tested.

Hyperthermic enhancement of tumour growth inhibition by accelerated carbon-ions in transplantable human esophageal cancer

The study examined the effects of combination of hyperthermia (42 degrees C) and 290 MeV/u carbon-ion (C-) beams or 200 kVp X-rays on tumour regrowth delay of transplantable human esophageal cancer as an in vivo model for radiotherapy of cancer. The C-beams were more effective in the tumour growth inhibition than X-rays. The relative biological effectiveness (RBE) of C-beams against X-rays was 2.00. It was observed that the interactive hyperthermic (42 degrees C, for 30 min) enhancement of tumour regrowth delay by high-linear energy transfer (LET) C-beams was similar to that of combination of low-LET X-rays with hyperthermia. The thermal enhancement ratios (TER) were 6.10 and 5.57 for X-rays and C-beams, respectively. These results suggest that hyperthermic treatment is effective in radiotherapy not only by low-LET radiation but also by high-LET radiation such as C-beams. In conclusion, the depression of the tumour growth by the combined treatment of hyperthermia (42 degrees C) and the C-beams strongly suggests the available possible application of interdisciplinary cancer therapy for refractory tumours.

Effects of accelerated carbon-ions on growth inhibition of transplantable human esophageal cancer in nude mice

We have studied the effectiveness of 290 MeV/u carbon-ion (C-) beams (linear energy transfer (LET) of 70 keV/mm) and 200 kVp X-rays on tumor growth inhibition as an in vivo model for radiotherapy of cancer. We measured the size of tumor growth of transplantable human esophageal cancer in nude mice after radiation with C-beams and compared this with X-rays as the control. A significant inhibition of tumor growth was observed by C-beams as compared with X-rays. The relative biological effectiveness (RBE) of C-beams against X-rays was 2.02. Histopathological studies showed that C-beams at 20 Gy induced prominent necrosis in the central region and multinucleate giant cells and inflammatory cells in peripheral regions of the tumor, whereas X-rays at 20 Gy induced only mild necrosis. The high RBE of C-beams obtained in this study provides in vivo evidence that C-beams are more effective than conventional X-rays for radiotherapy of cancer.