Effects of targeted phosphorylation site mutations in the DNA-PKcs phosphorylation domain on low and high LET radiation sensitivity

3D high resolution clonogenic survival measurement of xrs-5 cells in low-dose region of carbon ion plans

PURPOSE

In this study, we performed biological verification measurements of cell survival of a ¹²C ion irradiation plan employing a high-resolution 3D culture setup. This allowed, in particular, to access the cell inactivation in the low-dose regions close to the target area.

MATERIALS AND METHODS

We established the protocol for a 3D culture setup where xrs-5 cells were grown inside a layered matrigel structure in 384-well plates. Their radiosensitivity to conventional and ¹²C ion radiation was evaluated by irradiating them either with 250 kV X-rays at GSI or with monoenergetic ¹²C beams of 110 MeV/u at MIT, and compared with those of monolayers. A treatment plan for a rectangular target was prepared using the GSI research treatment planning system TRiP98. xrs-5 cells were seeded in the matrigel-based setup and irradiated in dose fall-off regions using active scanning ¹²C ion beams. In addition, film dosimetry utilizing radiochromic EBT3 film has been performed to assess the field homogeneity downstream of 384-well V-bottom plates with or without additional agarose coating of the well plate bottom.

RESULTS

Dose response curves following X-ray and ¹²C ion irradiation had linear shape and showed a significant decrease in survival fraction at even moderate doses. Survival measurements in the low-dose regions of the plan for the extended target showed good agreement to the predicted survival fraction. The irradiated film profiles yielded a flat dose distribution without apparent artifacts or inhomogeneities for well plates both with and without agarose coating, confirming the suitability of the experimental setup.

CONCLUSIONS

We conclude that the V-bottom 384-well plates in combination with the radiation-sensitive xrs-5 cell line constitute a suitable radiobiological verification tool which can be used especially for low doses. Furthermore, the measured survival of xrs-5 cells show a good agreement with the expected survival in the low-dose out-of-field regions, both laterally and downstream of the target.

The Biological Basis for Enhanced Effects of Proton Radiation Therapy Relative to Photon Radiation Therapy for Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinomas (HNSCCs) often present as local-regionally advanced disease at diagnosis, for which a current standard of care is x-ray-based radiation therapy, with or without chemotherapy. This approach provides effective local regional tumor control, but at the cost of acute and late toxicity that can worsen quality of life and contribute to mortality. For patients with human papillomavirus (HPV)-associated oropharyngeal squamous cell carcinoma (SCC) in particular, for whom the prognosis is generally favorable, de-escalation of the radiation dose to surrounding normal tissues without diminishing the radiation dose to tumors is desired to mitigate radiation-related toxic effects. Proton radiation therapy (PRT) may be an excellent de-escalation strategy because of its physical properties (that eliminate unnecessary radiation to surrounding tissues) and because of its biological properties (including tumor-specific variations in relative biological effectiveness [RBE] and linear energy transfer [LET]), in combination with concurrent systemic therapy. Early clinical evidence has shown that compared with x-ray-based radiation therapy, PRT offers comparable disease control with fewer and less severe treatment-related toxicities that can worsen the quality of life for patients with HNSCC. Herein, we review aspects of the biological basis of enhanced HNSCC cell response to proton versus x-ray irradiation in terms of radiation-induced gene and protein expression, DNA damage and repair, cell death, tumor immune responses, and radiosensitization of tumors.

Patterns of protein expression in human head and neck cancer cell lines differ after proton vs photon radiotherapy

BACKGROUND

Proton radiotherapy (PRT) may be a less toxic alternative to photon radiotherapy (XRT) for patients with head and neck squamous cell carcinoma (HNSCC). However, the molecular responses of HNSCC cells to PRT vs XRT are unclear.

METHODS

Proteomics analyses of protein expression profiles by reverse-phase protein arrays were done for two human papillomavirus [HPV]-negative and two HPV+ cell lines. Expression patterns of 175 proteins involved in several signaling pathways were tested.

RESULTS

Compared with PRT, XRT tended to induce lower expression of DNA damage repair-and cell cycle arrest-related proteins and higher expression of cell survival- and proliferation-related proteins.

CONCLUSIONS

Under these experimental conditions, PRT and XRT induced different protein expression and activation profiles. Further preclinical verification is needed, as are studies of tumor pathway mutations as biomarkers for choice of treatment or as radiosensitization targets to improve the response of HNSCC to PRT or XRT.

Radiobiological Characterization of Canine Malignant Melanoma Cell Lines with Different Types of Ionizing Radiation and Efficacy Evaluation with Cytotoxic Agents

Canine malignant melanoma (CMM) is a locally and systemically aggressive cancer that shares many biological and clinical characteristics with human mucosal melanoma. Hypofractionated radiation protocols have been used to treat CMM but little is known about its radiation biology. This pilot study is designed to investigate response of CMM cell lines to various ionizing radiations and cytotoxic agents to better understand this canine cancer. Four CMM cell lines were evaluated by clonogenic survival assay under aerobic and hypoxic conditions and parameters such as alpha beta (α/β) ratio, oxygen enhancement ratio (OER), and relative biological effectiveness (RBE) were calculated after ¹³⁷Cs, 6 megavoltage (MV) photon, or carbon ion irradiation. Six cytotoxic agents (cisplatin, camptothecin, mitomycin C, bleomycin, methtyl methanesulfonate and etoposide) were also assessed for their efficacy. Under aerobic condition with 6 MV photon, the α/β ratio of the four cell lines ranged from 0.3 to >100, indicating a wide variation of cellular sensitivity. The ratio increased under hypoxic condition compared to aerobic condition and this was more dramatic in ¹³⁷Cs and 6 MV photon treatments. OER of carbon was lower than ¹³⁷Cs at D₁₀ in 3 of the 4 cell lines. The RBE values generally increased with the increase of LET. Different cell lines showed sensitivity/resistance to different cytotoxic agents. This study revealed that CMM has a wide range of radiosensitivity and that hypoxia can reduce it, indicating that widely used hypofractionated protocols may not be optimal for all CMM patients. Several cytotoxic agents that have never been clinically assessed can improve treatment outcome.

Palmitoyl ascorbic acid 2-glucoside has the potential to protect mammalian cells from high-LET carbon-ion radiation

DMSO, glycerol, and ascorbic acid (AA) are used in pharmaceuticals and known to display radioprotective effects. The present study investigates radioprotective properties of novel glyceryl glucoside, ascorbic acid 2-glucoside, glyceryl ascorbate, and palmitoyl ascorbic acid 2-glucoside (PA). Gamma-rays or high-LET carbon-ions were irradiated in the presence of tested chemicals. Lambda DNA damage, cell survival, and micronuclei formation of CHO cells were analyzed to evaluate radioprotective properties. Radiation-induced Lambda DNA damage was reduced with chemical pre-treatment in a concentration-dependent manner. This confirmed tested chemicals were radical scavengers. For gamma-irradiation, enhanced cell survival and reduction of micronuclei formation were observed for all chemicals. For carbon-ion irradiation, DMSO, glycerol, and PA displayed radioprotection for cell survival. Based on cell survival curves, protection levels by PA were confirmed and comparable between gamma-rays and high-LET carbon-ions. Micronuclei formation was only decreased with AA and a high concentration of glycerol treatment, and not decreased with PA treatment. This suggests that mechanisms of protection against high-LET carbon-ions by PA can differ from normal radical scavenging effects that protect DNA from damage.

DNA Repair Deficient Chinese Hamster Ovary Cells Exhibiting Differential Sensitivity to Charged Particle Radiation under Aerobic and Hypoxic Conditions

It has been well established that hypoxia significantly increases both cellular and tumor resistance to ionizing radiation. Hypoxia associated radiation resistance has been known for some time but there has been limited success in sensitizing cells to radiation under hypoxic conditions. These studies show that, when irradiated with low linear energy transfer (LET) gamma-rays, poly (ADP-ribose), polymerase (PARP), Fanconi Anemia (FANC), and mutant Chinese Hamster Ovary (CHO) cells respond similarly to the non-homologous end joining (NHEJ) and the homologous recombination (HR) repair mutant CHO cells. Comparable results were observed in cells exposed to 13 keV/μm carbon ions. However, when irradiated with higher LET spread out Bragg peak (SOBP) carbon ions, we observed a decrease in the oxygen enhancement ratio (OER) in all the DNA of repair mutant cell lines. Interestingly, PARP mutant cells were observed as having the largest decrease in OER. Finally, these studies show a significant increase in the relative biological effectiveness (RBE) of high LET SOBP carbon and iron ions in HR and PARP mutants. There was also an increase in the RBE of NHEJ mutants when irradiated to SOBP carbon and iron ions. However, this increase was lower than in other mutant cell lines. These findings indicate that high LET radiation produces unique types of DNA damage under hypoxic conditions and PARP and HR repair pathways play a role in repairing this damage.

Targeting DNA repair with PNKP inhibition sensitizes radioresistant prostate cancer cells to high LET radiation

High linear energy transfer (LET) radiation or heavy ion such as carbon ion radiation is used as a method for advanced radiotherapy in the treatment of cancer. It has many advantages over the conventional photon based radiotherapy using Co-60 gamma or high energy X-rays from a Linear Accelerator. However, charged particle therapy is very costly. One way to reduce the cost as well as irradiation effects on normal cells is to reduce the dose of radiation by enhancing the radiation sensitivity through the use of a radiomodulator. PNKP (polynucleotide kinase/phosphatase) is an enzyme which plays important role in the non-homologous end joining (NHEJ) DNA repair pathway. It is expected that inhibition of PNKP activity may enhance the efficacy of the charged particle irradiation in the radioresistant prostate cancer cell line PC-3. To test this hypothesis, we investigated cellular radiosensitivity by clonogenic cell survival assay in PC-3 cells.12Carbon ion beam of62 MeVenergy (equivalent 5.16 MeV/nucleon) and with an entrance LET of 287 kev/μm was used for the present study. Apoptotic parameters such as nuclear fragmentation and caspase-3 activity were measured by DAPI staining, nuclear ladder assay and colorimetric caspase-3method. Cell cycle arrest was determined by FACS analysis. Cell death was enhanced when carbon ion irradiation is combined with PNKPi (PNKP inhibitor) to treat cells as compared to that seen for PNKPi untreated cells. A low concentration (10μM) of PNKPi effectively radiosensitized the PC-3 cells in terms of reduction of dose in achieving the same survival fraction. PC-3 cells underwent significant apoptosis and cell cycle arrest too was enhanced at G2/M phase when carbon ion irradiation was combined with PNKPi treatment. Our findings suggest that combined treatment of carbon ion irradiation and PNKP inhibition could enhance cellular radiosensitivity in a radioresistant prostate cancer cell line PC-3. The synergistic effect of PNKPi and carbon ion irradiation could be used as a promising method for carbon-ion therapy in radioresistant cells.

Relative biological effectiveness in canine osteosarcoma cells irradiated with accelerated charged particles

Heavy ions, characterized by high linear energy transfer (LET) radiation, have advantages compared with low LET protons and photons in their biological effects. The application of heavy ions within veterinary clinics requires additional background information to determine heavy ion efficacy. In the present study, comparison of the cell-killing effects of photons, protons and heavy ions was investigated in canine osteosarcoma (OSA) cells in vitro. A total of four canine OSA cell lines with various radiosensitivities were irradiated with 137Cs gamma-rays, monoenergetic proton beams, 50 keV/µm carbon ion spread out Bragg peak beams and 200 keV/µm iron ion monoenergetic beams. Clonogenic survival was examined using colony-forming as says, and relative biological effectiveness (RBE) values were calculated relative to gamma-rays using the D₁₀ value, which is determined as the dose (Gy) resulting in 10% survival. For proton irradiation, the RBE values for all four cell lines were 1.0-1.1. For all four cell lines, exposure to carbon ions yielded a decreased cell survival compared with gamma-rays, with the RBE values ranging from 1.56-2.10. Iron ions yielded the lowest cell survival among tested radiation types, with RBE values ranging from 3.51-3.69 observed in the three radioresistant cell lines. The radiosensitive cell line investigated demonstrated similar cell survival for carbon and iron ion irradiation. The results of the present study suggest that heavy ions are more effective for killing radioresistant canine OSA cells when compared with gamma-rays and protons. This markedly increased efficiency of cell killing is an attractive reason for utilizing heavy ions for radioresistant canine OSA.