Modulation of radiation-induced strand break repair by cisplatin in mammalian cells

Assay conditions for estimating differences in base excision repair activity with Fpg-modified comet assay

DNA repair is an essential agent in cancer development, progression, prognosis, and response to therapy. We have adapted a cellular repair assay based on the formamidopyrimidine DNA glycosylase (Fpg)-modified comet assay to assess DNA repair kinetics. The removal of oxidized nucleobases over time (0-480 min) was analyzed in peripheral blood mononuclear cells (PBMCs) and 8 cell lines. DNA damage was induced by exposure to either Ro19-8022 plus visible light or potassium bromate (KBrO3). The initial amount of damage induced by Ro 19-8022 plus light varied between cell lines, and this was apparently associated with the rate of repair. However, the amount of DNA damage induced by KBrO3 varied less between cell types, so we used this agent to study the kinetics of DNA repair. We found an early phase of ca. 60 min with fast removal of Fpg-sensitive sites, followed by slower removal over the following 7 h. In conclusion, adjusting the initial damage at T0 to an equal level can be achieved by the use of KBrO3, which allows for accurate analysis of subsequent cellular DNA repair kinetics in the first hour after exposure.

Impact of fractionated cisplatin and radiation treatment on cell growth and accumulation of DNA damage in two normal cell types differing in origin

Evidence on the impact of chemotherapy on radiotherapy-induced second malignant neoplasms is controversial. We estimated how cisplatin modulates the in vitro response of two normal cell types to fractionated radiation. AHH-1 lymphoblasts and VH10 fibroblasts were irradiated at 1 Gy/fraction 5 and 3 times per week during 12 and 19 days, respectively, and simultaneously treated with 0.1, 0.2, 0.4, 0.8, 1.7 and 3.3 µM of cisplatin twice a week. Cell growth during treatment was monitored. Cell growth/cell death and endpoints related to accumulation of DNA damage and, thus, carcinogenesis, were studied up to 21 days post treatment in cells exposed to radiation and the lowest cisplatin doses. Radiation alone significantly reduced cell growth. The impact of cisplatin alone below 3.3 µM was minimal. Except the lowest dose of cisplatin in VH10 cells, cisplatin reduced the inhibitory effect of radiation on cell growth. Delayed cell death was highest in the combination groups while the accumulation of DNA damage did not reveal a clear pattern. In conclusion, fractionated, concomitant exposure to radiation and cisplatin reduces the inhibitory effect of radiation on cell proliferation of normal cells and does not potentiate delayed effects resulting from accumulation of DNA damage.

Neoadjuvant chemotherapy with weekly cisplatin and paclitaxel followed by chemoradiation for locally advanced cervical cancer

BACKGROUND

Currently, the standard treatment for locally advanced cervical cancer is concurrent chemoradiation (CCRT). Forty percent of patients present with disease recurrence. This study aims to investigate the feasibility, safety and efficacy of neoadjuvant chemotherapy (NACT) with weekly cisplatin and paclitaxel (TP) followed by CCRT.

METHODS

We are conducting a phase III trial comparing the efficacy and side effects of patients with cervical cancer (FIGO 2018 stage IIB to IVA) who were assigned to four cycles of NACT with cisplatin (40 mg/m²) and paclitaxel (60 mg/m²) weekly followed by CCRT or CCRT alone. In this report, we studied the medium-term effect of 50 patients enrolled in the NACT + CCRT arm. The primary endpoints were the response rate post-NACT and 12 weeks post-CCRT evaluated by MR/CT based on RECIST v 1.1. The secondary endpoints were 3-year OS (overall survival) and PFS (progression-free survival) measured by the Kaplan-Meier method.

RESULTS

Among 50 patients enrolled in the NACT + CCRT arm, the complete and partial response rates were 10.4% and 68.8%, post-NACT. Twelve weeks after treatment completion, the complete response rate was 72.0%, whereas the total response rate (complete and partial response) was 90.0%. After a median follow-up of 28 months, the 3-year OS rate was 83.9%, and the 3-year PFS rate was 73.6%. NACT response was related to superior PFS and OS compared with NACT nonresponse (P < 0.01). Late AEs were exiguous, while early AEs mainly included myelosuppression and gastrointestinal AEs.

CONCLUSIONS

This study showed a good response rate achieved by dose-dense weekly cisplatin and paclitaxel followed by standard CCRT. The treatment regimen is feasible, as evidenced by the acceptable toxicity of NACT and by the high compliance with radiotherapy.

TRIAL REGISTRATION

Protocol version number and date. Chinese clinical trial registry, ChiCTR1900025327; http://www.chictr.org.cn . Registered 24 August 2019. Retrospectively registered, medresman.org.cn/ChiCTR1900025326. The date recruitment began 01-01-2019.

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Sterility of gamma-irradiated pathogens: a new mathematical formula to calculate sterilizing doses

In recent years there has been increasing advocacy for highly immunogenic gamma-irradiated vaccines, several of which are currently in clinical or pre-clinical trials. Importantly, various methods of mathematical modelling and sterility testing are employed to ensure sterility. However, these methods are designed for materials with a low bioburden, such as food and pharmaceuticals. Consequently, current methods may not be reliable or applicable to estimate the irradiation dose required to sterilize microbiological preparations for vaccine purposes, where bioburden is deliberately high. In this study we investigated the applicability of current methods to calculate the sterilizing doses for different microbes. We generated inactivation curves that demonstrate single-hit and multiple-hit kinetics under different irradiation temperatures for high-titre preparations of pathogens with different genomic structures. Our data demonstrate that inactivation of viruses such as Influenza A virus, Zika virus, Semliki Forest virus and Newcastle Disease virus show single-hit kinetics following exposure to gamma-irradiation. In contrast, rotavirus inactivation shows multiple-hit kinetics and the sterilizing dose could not be calculated using current mathematical methods. Similarly, Streptococcus pneumoniae demonstrates multiple-hit kinetics. These variations in killing curves reveal an important gap in current mathematical formulae to determine sterility assurance levels. Here we propose a simple method to calculate the irradiation dose required for a single log10 reduction in bioburden (D10) value and sterilizing doses, incorporating both single- and multiple-hit kinetics, and taking into account the possible existence of a resistance shoulder for some pathogens following exposure to gamma-irradiation.

Cisplatin plus capecitabine concomitant with intensity-modulated radiation therapy in non-metastatic anal squamous cell carcinoma: the experience of a single research cancer center

Background and Aims:

The standard treatment of non-metastatic anal squamous cell carcinoma (ASCC) consists of chemotherapy with mitomycin (MMC) plus 5-fluorouracil (5FU) for 1–2 cycles concomitant with pelvic radiotherapy. Subsequent studies introduced cisplatin (CDDP) combined with 5FU, with unclear results. We evaluated the doublet capecitabine (C) and CDDP as a possible alternative to MMC-5FU regimen concomitant with intensity-modulated radiation therapy (IMRT).

Patients and Methods:

We carried out a retrospective study on 67 patients affected by stage I–III ASCC, treated with CDDP (60–70 mg/m² every 21 days for two courses) plus C (825 mg/m² twice daily for 5 days/week) chemotherapy concomitant with IMRT for curative intent.

Results:

At a median follow up of 41 months, the clinical complete response calculated at the 6-month time-point (6-moCR), the 6-month objective response rate and the 6-month disease control rate were 93%, 94%, and 99%, respectively.

Disease-free survival rates at 1, 2, and 3 years were 89%, 87%, and 85%, while the overall survival rates at 1 and 2 years were 100% and 95%. The colostomy-free survival rates were 90% at 1 year and 88% at 2 years. Grade 3–4 acute adverse events were reported in 61% of patients; predominantly skin toxicity (46%) and limited hematological toxicity (12%).

Conclusion:

In this retrospective study, chemotherapy with C plus CDDP concomitant with IMRT proved safe and effective, and may represent a possible alternative option to standard MMC-containing regimen for curative intent.

Neoadjuvant Chemotherapy With Cisplatin and Gemcitabine Followed by Chemoradiation Versus Chemoradiation for Locally Advanced Cervical Cancer: A Randomized Phase II Trial

PURPOSE

Although chemoradiation therapy (CRT) with cisplatin remains the standard treatment of patients with locally advanced cervical cancer (LACC), 40% of patients present with disease recurrence. Additional treatment strategies are required to improve outcomes. We conducted a trial to evaluate the efficacy and safety of neoadjuvant chemotherapy (NAC) with cisplatin and gemcitabine followed by CRT.

METHODS

In this phase II trial, patients with LACC (International Federation of Gynecology and Obstetrics stage IIB to IVA or with positive lymph nodes) were randomly assigned to three cycles of NAC with cisplatin and gemcitabine followed by standard CRT with weekly cisplatin plus pelvic radiotherapy or to standard CRT alone. The primary end point was 3-year progression-free survival (PFS). Secondary end points were response rate, 3-year locoregional control, 3-year overall survival (OS), safety, and quality of life.

RESULTS

From 107 patients enrolled in the trial, 55 were randomly assigned to the NAC arm and 52 to the CRT-alone arm. The majority of patients had squamous cell carcinoma (87.8%). After a median follow-up of 31.7 months, NAC was associated with an inferior PFS, with 3-year PFS rates of 40.9% v 60.4% in the CRT arm (hazard ratio, 1.84; 95% CI, 1.04 to 3.26; P = .033). NAC also was associated with a lower OS (3-year OS rate, 60.7% v 86.8%; hazard ratio, 2.79; 95% CI, 1.29 to 6.01; P = .006). After treatment completion, complete response rates were 56.3% in the NAC arm and 80.3% in the CRT arm (P = .008). Toxicities were similar in both arms, with the exception of hypomagnesemia and neuropathy being more common with NAC.

CONCLUSION

This study shows that the addition of NAC consisting of cisplatin and gemcitabine to standard CRT is not superior and is possibly inferior to CRT alone for the treatment of LACC.

The Exploitation of Low-Energy Electrons in Cancer Treatment

Given the distinct characteristics of low-energy electrons (LEEs), particularly at energies less than 30 eV, they can be applied to a wide range of therapeutic modalities to improve cancer treatment. LEEs have been shown to efficiently produce complex molecular damage resulting in substantial cellular toxicities. Since LEEs are produced in copious amounts from high-energy radiation beam, including photons, protons and ions; the control of LEE distribution can potentially enhance the therapeutic radio of such beams. LEEs can play a substantial role in the synergistic effect between radiation and chemotherapy, particularly halogenated and platinum-based anticancer drugs. Radiosensitizing entities containing atoms of high atomic number such as gold nanoparticles can be a source of LEE production if high-energy radiation interacts with them. This can provide a high local density of LEEs in a cell and produce cellular toxicity. Auger-electron-emitting radionuclides also create a high number of LEEs in each decay, which can induce lethal damage in a cell. Exploitation of LEEs in cancer treatment, however, faces a few challenges, such as dosimetry of LEEs and selective delivery of radiosensitizing and chemotherapeutic molecules close to cellular targets. This review first discusses the rationale for utilizing LEEs in cancer treatment by explaining their mechanism of action, describes theoretical and experimental studies at the molecular and cellular levels, then discusses strategies for achieving modification of the distribution and effectiveness of LEEs in cancerous tissue and their associated clinical benefit.

Adaption By Low Dose Radiation Exposure: A Look at Scope and Limitations for Radioprotection

The procedures and dose limitations used for radiation protection in the nuclear industry are founded on the assumption that risk is directly proportional to dose, without a threshold. Based on this idea that any dose, no matter how small, will increase risk, radiation protection regulations generally attempt to reduce any exposure to “as low as reasonably achievable” (ALARA). We know however, that these regulatory assumptions are inconsistent with the known biological effects of low doses. Low doses induce protective effects, and these adaptive responses are part of a general response to low stress. Adaptive responses have been tightly conserved during evolution, from single celled organisms up to humans, indicating their importance. Here we examine cellular and animal studies that show the influence of radiation induced protective effects on diverse diseases, and examine the radiation dose range that is effective for different tissues in the same animal. The concept of a dose window, with upper and lower effective doses, as well as the effect of multiple stressors and the influence of genetics will also be examined. The effect of the biological variables on low dose responses will be considered from the point of view of the limitations they may impose on any revised radiation protection regulations.

Non-DSB clustered DNA lesions induced by ionizing radiation are largely responsible for the loss of plasmid DNA functionality in the presence of cisplatin

The combination of cisplatin and ionizing radiation (IR) increases cell toxicity by both enhancing DNA damage and inhibiting repair mechanisms. Although the formation of cluster DNA lesions, particularly double-strand breaks (DSB) at the site of cisplatin-DNA-adducts has been reported to induce cell death, the contribution of DSB and non-DSB cluster lesions to the cellular toxicity is still unknown. Although both lesions are toxic, it is not always possible to measure their frequency and cell survival in the same model system. To overcome this problem, here, we investigate the effect of cisplatin-adducts on the induction of DSB and non-DSB cluster DNA lesions by IR and determine the impact of such lesions on plasmid functionality. Cluster lesions are two or more lesions on opposite DNA strands with a short distance such that error free repair is difficult or impossible. At a ratio of two cisplatin per plasmid, irradiation of platinated DNA in solution with (137)Cs γ-rays shows enhancements in the formation of DNA DSB and non-DSB cluster lesions by factors of 2.6 and 2.1, respectively, compared to unmodified DNA. However, in absolute terms, the yield for non-DSB cluster lesions is far larger than that for DSB, by a factor of 26. Unmodified and cisplatin-modified DNA were irradiated and subsequently transformed into Escherichia coli to give survival curves representing the functionality of the plasmid DNA as a function of radiation dose. Our results demonstrate that non-DSB cluster lesions are the only toxic lesions present at a sufficient frequency to account for the loss of DNA functionality. Our data also show that Frank-DSB lesions are simply too infrequent to account for the loss of DNA functionality. In conclusion, non-DSB cluster DNA damage is known to be difficult to repair and is probably the lesion responsible for the loss of functionality of DNA modified by cisplatin.

Different responses of tumor and normal cells to low-dose radiation

AIM OF THE STUDY

We demonstrated stimulation of both erythrocyte immune function and superoxide dismutase activity in tumor-bearing mice in response to whole-body 75 mGy X-rays. In addition, we enhanced the chemotherapeutic effect by exposing tumor-bearing mice to low-dose radiation (LDR). This study aims to investigate the different responses of tumor cells and normal cells to LDR.

MATERIAL AND METHODS

Survival fraction, micronucleus frequency, and cell cycle of Lewis cells and primary human fibroblast AG01522 cells were measured. S180 sarcoma cells were implanted in mice, and tumor sizes were measured in vivo.

RESULTS

In response to LDR exposure in vitro, a stimulating effect was observed in AG01522 cells but not in Lewis cells. Low-dose radiation did not cause an adaptive response in the Lewis cell cycle. Lack of an LDR-induced radioadaptive response in tumor cells was observed in tumor-bearing mouse models. Furthermore, a higher apoptotic effect and lower expression of the anti-apoptosis gene Bcl-2 were found in tumor cells of tumor-bearing mice exposed to D1 + D2 than those in tumor cells of tumor-bearing mice exposed to D2 alone.

CONCLUSIONS

Different responses of tumor cells and normal cells to LDR were found. Low-dose radiation was found to stimulate the growth of normal cells but not of tumor cells in vitro and in vivo, which is a very important and clinically relevant phenomenon.

Cisplatin enhances the formation of DNA single- and double-strand breaks by hydrated electrons and hydroxyl radicals

The synergistic interaction of cisplatin with ionizing radiation is the clinical rationale for the treatment of several cancers including head and neck, cervical and lung cancer. The underlying molecular mechanism of the synergy has not yet been identified, although both DNA damage and repair processes are likely involved. Here, we investigate the indirect effect of γ rays on strand break formation in a supercoiled plasmid DNA (pGEM-3Zf-) covalently modified by cisplatin. The yields of single- and double-strand breaks were determined by irradiation of DNA and cisplatin/DNA samples with (60)Co γ rays under four different scavenging conditions to examine the involvement of hydrated electrons and hydroxyl radicals in inducing the DNA damage. At 5 mM tris in an N2 atmosphere, the presence of an average of two cisplatins per plasmid increased the yields of single- and double-strand breaks by factors of 1.9 and 2.2, respectively, relative to the irradiated unmodified DNA samples. Given that each plasmid of 3,200 base pairs contained an average of two cisplatins, this represents an increase in radiosensitivity of 3,200-fold on a per base pair basis. When hydrated electrons were scavenged by saturating the samples with N2O, these enhancement factors decreased to 1.5 and 1.2, respectively, for single- and double-strand breaks. When hydroxyl radicals were scavenged using 200 mM tris, the respective enhancement factors were 1.2 and 1.6 for single- and double-strand breaks, respectively. Furthermore, no enhancement in DNA damage by cisplatin was observed after scavenging both hydroxyl radicals and hydrated electrons. These findings show that hydrated electrons can induce both single- and double-strand breaks in the platinated DNA, but not in unmodified DNA. In addition, cisplatin modification is clearly an extremely efficient means of increasing the formation of both single- and double-strand breaks by the hydrated electrons and hydroxyl radicals created by ionizing radiation.

Long-term update of US GI intergroup RTOG 98-11 phase III trial for anal carcinoma: survival, relapse, and colostomy failure with concurrent chemoradiation involving fluorouracil/mitomycin versus fluorouracil/cisplatin

PURPOSE

On initial publication of GI Intergroup Radiation Therapy Oncology Group (RTOG) 98-11 [A Phase III Randomized Study of 5-Fluorouracil (5-FU), Mitomycin, and Radiotherapy Versus 5-Fluorouracil, Cisplatin and Radiotherapy in Carcinoma of the Anal Canal], concurrent chemoradiation (CCR) with fluorouracil (FU) plus mitomycin (MMC) decreased colostomy failure (CF) when compared with induction plus concurrent FU plus cisplatin (CDDP), but did not significantly impact disease-free survival (DFS) or overall survival (OS) for anal canal carcinoma. The intent of the updated analysis was to determine the long-term impact of treatment on survival (DFS, OS, colostomy-free survival [CFS]), CF, and relapse (locoregional failure [LRF], distant metastasis) in this patient group.

PATIENTS AND METHODS

Stratification factors included sex, clinical node status, and primary size. DFS and OS were estimated univariately by the Kaplan-Meier method, and treatment arms were compared by log-rank test. Time to relapse and CF were estimated by the cumulative incidence method and treatment arms were compared by using Gray's test. Multivariate analyses used Cox proportional hazard models to test for treatment differences after adjusting for stratification factors.

RESULTS

Of 682 patients accrued, 649 were analyzable for outcomes. DFS and OS were statistically better for RT + FU/MMC versus RT + FU/CDDP (5-year DFS, 67.8% v 57.8%; P = .006; 5-year OS, 78.3% v 70.7%; P = .026). There was a trend toward statistical significance for CFS (P = .05), LRF (P = .087), and CF (P = .074). Multivariate analysis was statistically significant for treatment and clinical node status for both DFS and OS, for tumor diameter for DFS, and for sex for OS.

CONCLUSION

CCR with FU/MMC has a statistically significant, clinically meaningful impact on DFS and OS versus induction plus concurrent FU/CDDP, and it has borderline significance for CFS, CF, and LRF. Therefore, RT + FU/MMC remains the preferred standard of care.

Complex cisplatin-double strand break (DSB) lesions directly impair cellular non-homologous end-joining (NHEJ) independent of downstream damage response (DDR) pathways

The treatment for advanced stage non-small cell lung cancer (NSCLC) often includes platinum-based chemotherapy and IR. Cisplatin and IR combination therapy display schedule and dose-dependent synergy, the mechanism of which is not completely understood. In a series of in vitro and cell culture assays in a NSCLC model, we investigated both the downstream and direct treatment and damage effects of cisplatin on NHEJ catalyzed repair of a DNA DSB. The results demonstrate that extracts prepared from cisplatin-treated cells are fully capable of NHEJ catalyzed repair of a DSB using a non-cisplatin-damaged DNA substrate in vitro. Similarly, using two different host cell reactivation assays, treatment of cells prior to transfection of a linear, undamaged reporter plasmid revealed no reduction in NHEJ compared with untreated cells. In contrast, transfection of a linear GFP-reporter plasmid containing site-specific, cisplatin lesions 6-bp from the termini revealed a significant impairment in DSB repair of the cisplatin-damaged DNA substrates in the absence of cellular treatment with cisplatin. Together, these data demonstrate that impaired NHEJ in combined cisplatin-IR treated cells is likely the result of a direct effect of cisplatin-DNA lesions near a DSB and that the indirect cellular effects of cisplatin treatment are not significant contributors to the synergistic cytotoxicity observed with combination cisplatin-IR treatment.

MutS homologue hMSH5: role in cisplatin-induced DNA damage response

Background

Cisplatin (cis-diamminedichloroplatinum (II), CDDP) and its analogues constitute an important class of anticancer drugs in the treatment of various malignancies; however, its effectiveness is frequently affected by mutations in genes involved in the repair and signaling of cisplatin-induced DNA damage. These observations necessitate a need for a better understanding of the molecular events governing cellular sensitivity to cisplatin.

Results

Here, we show that hMSH5 mediates sensitization to cisplatin-induced DNA damage in human cells. Our study indicates that hMSH5 undergoes cisplatin-elicited protein induction and tyrosine phosphorylation. Silencing of hMSH5 by RNAi or expression of hMSH5 phosphorylation-resistant mutant hMSH5^Y742F elevates cisplatin-induced G2 arrest and renders cells susceptible to cisplatin toxicity at clinically relevant doses. In addition, our data show that cisplatin promotes hMSH5 chromatin association and hMSH5 deficiency increases cisplatin-triggered γ-H2AX foci. Consistent with a possible role for hMSH5 in recombinational repair of cisplatin-triggered double-strand breaks (DSBs), the formation of cisplatin-induced hMSH5 nuclear foci is hRad51-dependent.

Conclusion

Collectively, our current study has suggested a role for hMSH5 in the processing of cisplatin-induced DSBs, and silencing of hMSH5 may provide a new means to improve the therapeutic efficacy of cisplatin.

Phase I study of irinotecan and cisplatin in combination with pelvic radiotherapy in the treatment of locally advanced cervical cancer: A GINECO trial

PURPOSE

To define the recommended dose of the association of weekly irinotecan (Iri) and cisplatin (CP) with pelvic radiotherapy in Locally Advanced Cervical Cancer.

PATIENTS AND METHODS

Stage IB2-IV cervix cancer patients were treated with escalating doses of Iri starting from 30 mg/m(2) and a fixed dose of CP at 20 mg/m(2), both weekly concomitantly with a 45-Gy pelvic irradiation.

RESULTS

Fifteen patients entered the study, 6 at level 1 (Iri 30 mg/m(2)), 3 at level 2 (Iri 40 mg/m(2)) and 6 at intermediate dose (Iri 35 mg/m(2)). Median age was 47 years (34-72), FIGO stage IB (n=1), IIB (n=7), III (n=6), IVA (n=1). The recommended dose was weekly Iri 35 mg/m(2) and CP 20 mg/m(2). Dose limiting toxicities (grades 3-4) were diarrhea, abdominal pain, febrile neutropenia and fatigue.

CONCLUSION

In cervix cancer patients, radiosensitization with weekly cisplatin and irinotecan is feasible, and the recommended doses are cisplatin 20 mg/m(2)/week and irinotecan 35 mg/m(2)/week for future phase II studies.

Combined gamma‐irradiation and subsequent cisplatin treatment in human squamous carcinoma cell lines sensitive and resistant to cisplatin

PURPOSE

To investigate the effects of combined radiation and subsequent cisplatin treatment on the human squamous carcinoma cell line SCC-25 and its cisplatin-resistant derivative SCC-25/CP.

MATERIALS AND METHODS

SCC-25 and SCC-25/CP cells were treated with various gamma-ray doses (5 cGy-7 Gy) followed 60 min later by cisplatin treatment and subsequently assayed for survival using a conventional colony assay. For SCC-25, the subsequent cisplatin treatment was 0.1, 1, 10 and 20 microM for 1 h. For the more cisplatin-resistant SCC-25/CP cells, the subsequent cisplatin treatment was 10 and 50 microM for 1 h.

RESULTS

The cisplatin-resistant SCC-25/CP cells were not cross-resistant to gamma-irradiation. Subsequent treatment with an LD50 concentration of cisplatin (10 and 50 microM for SCC-25 and SCC-25/CP, respectively) resulted in radiosensitization for SCC-25/CP but not for SCC-25 cells. Gamma-irradiation of SCC-25/CP cells followed by treatment with 10 and 50 microM cisplatin for 1 h resulted in radiation survival curves displaying a significant low-dose hypersensitive region followed by increased radioresistance at higher doses. A total of 10 microM cisplatin resulted in radiosensitization confined to the low-dose region (0.05 and 0.25 Gy), whereas the higher cisplatin treatment of 50 microM resulted in the appearance of a hypersensitive region together with a reduction of the increased radioresistance region. In contrast, cisplatin treatment (0.1, 1, 10 and 20 microM for 1 h) of SCC-25 cells had no significant effect on survival following 2.5 or 7.0 Gy and actually resulted in an increased low-dose radiation survival (0.05, 0.25 and 1 Gy) when survival was corrected for cisplatin treatment (p<0.01 for all cisplatin concentrations tested).

CONCLUSIONS

The significant radiosensitization for SCC-25/CP given subsequent treatment with 50 microM cisplatin indicates cisplatin can inhibit the increased radioresistance response in SCC-25/CP cells. In contrast, the subsequent cisplatin treatment of SCC-25 cells can enhance their survival following low radiation doses.

Influence of DNA double‐strand break rejoining on clonogenic survival and micronucleus yield in human cell lines

PURPOSE

To examine the role of DNA double-strand break (DSB) rejoining in cell survival and micronucleus yield after 60Co gamma-irradiation.

MATERIALS AND METHOD

Thirteen human cell lines (six glioblastoma, five prostate, one melanoma, one squamous cell carcinoma) were irradiated with 60Co gamma-rays to doses of 0-10Gy for cell survival and micronucleus measurements and 0-100Gy for DSB rejoining. Measurements were performed using standard clonogenic, micronucleus and constant-field gel electrophoresis assays.

RESULTS

Radioresistance and micronucleus yield were positively correlated (r=0.74, p=0.004). A significant cell type-dependent correlation was demonstrated between total (0-20 h) DSB rejoining and cell survival (r=0.86, p=0.03 for glioblastomas; r=0.79, p=0.04 for other cell lines), with more resistant cell lines showing higher levels of DSB rejoining. No relationship was apparent between fast (0-2 h) or slow (2-20 h) DSB rejoining and clonogenic survival. While there was no relationship between total or slow DSB rejoining and micronucleus yield, a significant and cell type-specific correlation emerged between fast rejoining and micronucleus yield for the glioblastomas (r=0.89, p=0.04) and other cell lines (r=0.76, p=0.04). Cell lines with higher levels of DSB rejoining within 2 h of irradiation showed higher yields of micronuclei.

CONCLUSION

Fast DSB rejoining, possibly through interaction with slow DSB rejoining, appears to play an important role in the formation of micronuclei. However, total DSB rejoining reflects intrinsic radiosensitivity. Consideration of differences in DSB rejoining kinetics might contribute to a better understanding of the significance of cell survival and micronucleus data in the clinical and radiation protection setting.

Effects of cisplatin and gamma-irradiation on cell survival, the induction of chromosomal aberrations and apoptosis in SW-1573 cells

PURPOSE

Cisplatin was found to radiosensitize SW-1573 cells by inhibition of PLDR. Therefore, it was investigated whether cisplatin combined with gamma-radiation leads to an increase in the number of chromosomal aberrations or apoptotic cells compared with radiation alone.

METHODS

Confluent cultures of the human lung carcinoma cell line SW-1573 were treated with 1 microM cisplatin for 1 h, 4 Gy gamma-radiation, or a combination of both. Cell survival was studied by the clonogenic assay. Aberrations were analysed by FISH in prematurely condensed chromosomes (PCC) and the induction of apoptosis by counting fragmented nuclei.

RESULTS

A radiosensitizing effect of cisplatin on cell survival was observed if time for PLDR was allowed. An increased number of chromosomal fragments were observed immediately after irradiation compared with 24 h after irradiation whereas color junctions are only formed 24 h after irradiation. No increase in chromosomal aberrations was found after combined treatment, but a significantly enhanced number of fragmented nuclei were observed when confluent cultures were replated after allowing PLDR.

CONCLUSION

The inhibition of PLDR by cisplatin in delayed plated SW-1573 cells did not increase chromosomal aberrations, but increased the induction of apoptosis.

Cisplatin-mediated DNA double-strand breaks in replicating but not in quiescent cells of the yeast Saccharomyces cerevisiae

DNA double-strand breaks (DSBs) are formed during the processing of DNA interstrand crosslinks in replicating yeast and Chinese hamster cells exposed to DNA crosslinkers such as psoralen plus UVA or nitrogen mustard. They were also detected in human cells after treatment with photoactivated psoralen or mitomycin C. In contrast, no DSBs were observed after exposure of Chinese hamster cells to cisplatin, another crosslinking agent widely used for the therapy of various cancers, challenging a common role for DSBs in the processing of DNA interstrand crosslinks. Here we report for the first time that cisplatin-mediated DSBs are induced in replicating but not quiescent cells of the yeast Saccharomyces cerevisiae. When the main pathway of repair of DSBs is inhibited, these breaks accumulate in replicating cells. Thus it appears that DNA interstrand crosslinks induced by different crosslinking agents, including cisplatin, are processed yielding DSBs as an intermediate lesion. In stationary cells, however, removal of DNA interstrand crosslinks after cisplatin treatment occurs without the formation of DSBs. These findings point to an altered mode of processing of cisplatin-DNA adducts in replicating versus quiescent cells.

Cisplatin sensitizes cancer cells to ionizing radiation via inhibition of nonhomologous end joining

The combination of cisplatin and ionizing radiation (IR) treatment represents a common modality for treating a variety of cancers. These two agents provide considerable synergy during treatment, although the mechanism of this synergy remains largely undefined. We have investigated the mechanism of cisplatin sensitization to IR using a combination of in vitro and in vivo experiments. A clear synergistic interaction between cisplatin and IR is observed in cells proficient in nonhomologous end joining (NHEJ) catalyzed repair of DNA double-strand breaks (DSB). In contrast, no interaction between cisplatin and IR is observed in NHEJ-deficient cells. Reconstituted in vitro NHEJ assays revealed that a site-specific cisplatin-DNA lesion near the terminus results in complete abrogation of NHEJ catalyzed repair of the DSB. These data show that the cisplatin-IR synergistic interaction requires the DNA-dependent protein kinase-dependent NHEJ pathway for joining of DNA DSBs, and the presence of a cisplatin lesion on the DNA blocks this pathway. In the absence of a functional NHEJ pathway, although the cells are hypersensitive to IR, there is no synergistic interaction with cisplatin.

Cytogenetic dose-response and adaptive response in cells of ungulate species exposed to ionizing radiation

In the studies reported here, the micronucleus assay, a common cytogenetic technique, was used to examine the dose-responses in fibroblasts from three ungulate species (white-tailed deer, woodland caribou, and Indian muntjac) exposed to high doses of ionizing radiation (1-4 Gy of (60)Co gamma radiation). This assay was also used to examine the effects of exposure to low doses (1-100 mGy) typical of what these species experience in a year from natural and anthropogenic environmental sources. An adaptive response, defined as the induction of resistance to a stressor by a prior exposure to a small "adapting" stress, was observed after exposure to low doses. This work indicates that very small doses are protective for the endpoint examined. The same level of protection was seen at all adapting doses, including 1 radiation track per cell, the lowest possible cellular dose. These results are consistent with other studies in a wide variety of organisms that demonstrate a protective effect of low doses at both cellular and whole-organism levels. This implies that environmental regulations predicated on the idea that even the smallest dose of radiation carries a quantifiable risk of direct adverse consequences to the exposed organism require further examination. Cytogenetic assays provide affordable and feasible biological effects-based alternatives that are more biologically relevant than traditional contaminant concentration-based radioecological risk assessment.

Quantitative Analysis of Biological Responses to Ionizing Radiation, Including Dose, Irradiation Time, and Dose Rate

Because biological responses to radiation are complex processes that depend on both irradiation time and total dose, consideration of both dose and dose rate is necessary to predict the risk from long-term irradiations at low dose rates. Here we mathematically and statistically analyzed the quantitative relationships between dose, dose rate and irradiation time using micronucleus formation and inhibition of proliferation of human osteosarcoma cells as indicators of biological response. While the dose-response curves did not change with exposure times of less than 20 h, at a given dose, both biological responses clearly were reduced as exposure time increased to more than 8 days. These responses became dependent on dose rate rather than on total dose when cells were irradiated for 20 to 27 days. Mathematical analysis demonstrates that the relationship between effective dose and dose rate is well described by an exponential function when the logarithm of effective dose is plotted as a function of the logarithm of dose rate. These results suggest that our model, the modified exponential (ME) model, can be applied to predict the risk from exposure to low-dose/low-dose-rate radiation.

Recognition of cisplatin adducts by cellular proteins

Cisplatin is a widely used chemotherapeutic agent. It reacts with nucleophilic bases in DNA and forms 1,2-d(ApG), 1,2-d(GpG) and 1,3-d(GpTpG) intrastrand crosslinks, interstrand crosslinks and monofunctional adducts. The presence of these adducts in DNA is through to be responsible for the therapeutic efficacy of cisplatin. The exact signal transduction pathway that leads to cell cycle arrest and cell death following treatment with the drug is not known but cell death is believed to be mediated by the recognition of the adducts by cellular proteins. Here we describe the structural information available for cisplatin and related platinum adducts, the interactions of the adducts with cellular proteins and the implications of these interactions for cell survival.

Physiological functions of protein kinase inhibitors

Protein kinases are key regulatory enzymes involved in a multitude of biochemical pathways. This chapter will describe the current research on targeting specific protein kinases with inhibitors in attempts to disrupt flux through specific pathways. Targeting specific kinases presents a distinct challenge as there are hundreds of individual kinase enzymes that use ATP as a substrate to phosphorylate specific target molecules. The challenge clearly lies in obtaining specificity for a given kinase, thus allowing inhibition or activation of a specific pathway. This chapter will focus on two areas of kinase inhibitors, those that target the MAP kinase pathway and those directed against the phosphatidylinositol-3 kinase (PI-3K) related kinase family. The cellular and physiological effects of inhibition of the various pathways controlled by these kinases will be reviewed.

Cisplatin-DNA adducts inhibit translocation of the Ku subunits of DNA-PK

We have determined the effect of cisplatin-DNA damage on the ability of the DNA-dependent protein kinase (DNA-PK) to interact with duplex DNA molecules in vitro. The Ku DNA binding subunits of DNA-PK display a reduced ability to translocate on duplex DNA containing cisplatin-DNA adducts compared to control, undamaged duplex DNA. The decreased rates of translocation resulted in a decrease in the association of the p460 catalytic subunit of DNA-PK (DNA-PKcs) with the Ku-DNA complex. In addition to a decrease in DNA-PKcs association, the DNA-PKcs that is bound with Ku at a DNA end containing cisplatin-DNA adducts has a reduced catalytic rate compared to heterotrimeric DNA-PK assembled on undamaged DNA. The position of the cisplatin-DNA lesion from the terminus also effects kinase activation, with maximal inhibition occurring when the lesion is closer to the terminus. These results are consistent with a model for DNA-PK activation where the Ku dimer translocates away from the DNA terminus and facilitates the association of DNA-PKcs which interacts with both Ku and DNA resulting in kinase activation. The presence of cisplatin adducts decreases the ability to translocate away from the terminus and results in the formation of inactive kinase complexes at the DNA terminus. The results are discussed with respect to the ability of cisplatin to sensitize cells to DNA damage induced by ionizing radiation and the ability to repair DNA double-strand breaks.

Evidence for adaptive response and implication in pulse-simulated low-dose-rate radiotherapy

PURPOSE

Pulsed-dose-rate (PDR) brachytherapy as a substitute for continuous low-dose-rate (LDR) brachytherapy has a number of clinical advantages. However, early results show that some cells can exhibit an adaptive response to radiation and in PDR where many pulses are given such an adaptive response may play an important role in the outcome.

METHODS AND MATERIALS

Nine human cell lines (two normal fibroblast and seven tumor) were evaluated for an adaptive response. Cells were given either a single adapting dose before a challenge dose or given PDR sequences for which the average dose rate matched the LDR dose rate. Response was assessed using the colony survival assay.

RESULTS

Five of the nine cell lines showed an adapting response to single small doses of radiation. Three of these cell lines were further investigated for adapting response to PDR and two of the three lines (one ovarian carcinoma and one glioma) showed an adaptive response which was dependent on pulse size and interval.

CONCLUSION

The data show that an adaptive response can occur in human cells and that it can vary among cell lines. In addition, PDR sequences also produced an adaptive response which could have an affect on PDR therapy if such a response is found in tissues.

Dose-rate effects for apoptosis and micronucleus formation in gamma-irradiated human lymphocytes

We have compared dose-rate effects for gamma-radiation-induced apoptosis and micronucleus formation in human lymphocytes. Long-term assessment of individual radiation-induced apoptosis showed little intraindividual variation but significant interindividual variation. The effectiveness of radiation exposure to cause apoptosis or micronucleus formation was reduced by low-dose-rate exposures, but the reduction was apparent at different dose rates for these two end points. Micronucleus formation showed a dose-rate effect when the dose rate was lowered to 0.29 cGy/min, but there was no accompanying cell cycle delay. A further increase in the dose-rate effect was seen at 0.15 cGy/min, but was now accompanied by cell cycle delay. There was no dose-rate effect for the induction of apoptosis until the dose rate was reduced to 0.15 cGy/min, indicating that the mechanisms or signals for processing radiation-induced lesions for these two end points must be different at least in part. There appear to be two mechanisms that contribute to the dose-rate effect for micronucleus formation. One of these does not affect binucleate cell frequency and occurs at dose rates higher than that required to produce a dose-rate effect for apoptosis, and one affects binucleate cell frequency, induced only at the very low dose rate which coincidentally produces a dose-rate effect for apoptosis. Since the dose rate at which cells showed reduced apoptosis as well as a further reduction in micronucleus formation was very low, we conclude that the processing of the radiation-induced lesions that induce apoptosis, and some micronuclei, is very slow in quiescent and PHA-stimulated lymphocytes, respectively.

Interactions of mammalian proteins with cisplatin-damaged DNA

We have undertaken the systematic isolation and characterization of mammalian proteins which display an affinity for cisplatin-damaged DNA. Fractionation of human cell extracts has led to the identification of two classes of proteins. The first includes proteins that bind duplex DNA in the absence of cisplatin damage and retain their affinity for DNA in the presence of cisplatin-DNA adducts. The DNA-dependent protein kinase (DNA-PK) falls into this class. The inhibition of DNA-PK phosphorylation activity by cisplatin-damaged DNA has led to the hypothesis that cisplatin sensitization of mammalian cells to ionizing radiation may be mediated by DNA-PK. The second class of proteins identified are those which display a high relative affinity for cisplatin-damaged DNA and a low affinity for undamaged duplex DNA. Proteins that fall into this class include high mobility group 1 protein (HMG-1), replication protein A (RPA) and xeroderma pigmentosum group A protein (XPA). Each protein has been isolated and purified in the lab. The interaction of each protein with cisplatin-damaged DNA has been assessed in electrophoretic mobility shift assays. A series of DNA binding experiments suggests that RPA binds duplex DNA via denaturation and subsequent preferential binding to the undamaged DNA strand of the partial duplex. DNA substrates prepared with photo-reactive base analogs on either the damaged or undamaged DNA strand have also been employed to investigate the mechanism and specific protein-DNA interactions that occur as each protein binds to cisplatin-damaged DNA. Results suggest both damage and strand specificity for RPA and XPA binding cisplatin-damaged DNA.