Variation in radiation sensitivity during the cell cycle of two human squamous cell carcinomas

Radiation Responses of Human Mesenchymal Stem Cells Derived From Different Sources

Mesenchymal stem cells (MSCs) derived from different tissues may aid in the regeneration of radiation-induced organ lesions; however, the radiation responses of human MSCs from different sources are unknown. In our study, a comparison of the results from cell proliferation, apoptosis, cell cycle, DNA damage, and DNA repair assays consistently showed that MSCs derived from adipose tissue possess a significantly stronger radiation resistance capacity than MSCs derived from umbilical cord and gingival, which is accompanied by a higher level of phosphorylated signal transducer and activator of transcription 3 (Stat3) expression. This reminds us Stat3 could be a potential biomarker of radiation resistance. These findings provide a better understanding of radiation-induced biologic responses in MSCs and may lead to the development of better strategies for stem cell treatment and cancer therapy.

Dual effects of radiation bystander signaling in urothelial cancer: purinergic-activation of apoptosis attenuates survival of urothelial cancer and normal urothelial cells

Radiation therapy (RT) delivers tumour kill, directly and often via bystander mechanisms. Bladder toxicity is a dose limiting constraint in pelvic RT, manifested as radiation cystitis and urinary symptoms. We aimed to investigate the impact of radiation-induced bystander signaling on normal/cancer urothelial cells. Human urothelial cancer cells T24, HT1376 and normal urothelial cells HUC, SV-HUC were used. Cells were irradiated and studied directly, or conditioned medium from irradiated cells (CM) was transferred to naïve, cells. T24 or SV-HUC cells in the shielded half of irradiated flasks had increased numbers of DNA damage foci vs non-irradiated cells. A physical barrier blocked this response, indicating release of transmitters from irradiated cells. Clonogenic survival of shielded T24 or SV-HUC was also reduced; a physical barrier prevented this phenomenon. CM-transfer increased pro-apoptotic caspase-3 activity, increased cleaved caspase-3 and cleaved PARP expression and reduced survival protein XIAP expression. This effect was mimicked by ATP. ATP or CM evoked suramin-sensitive Ca²⁺-signals. Irradiation increased [ATP] in CM from T24. The CM-inhibitory effect on T24 clonogenic survival was blocked by apyrase, or mimicked by ATP. We conclude that radiation-induced bystander signaling enhances urothelial cancer cell killing via activation of purinergic pro-apoptotic pathways. This benefit is accompanied by normal urothelial damage indicating RT bladder toxicity is also bystander-mediated.

Modeling the Cellular Response of Lung Cancer to Radiation Therapy for a Broad Range of Fractionation Schedules

Purpose: To demonstrate that a mathematical model can be used to quantitatively understand tumor cellular dynamics during a course of radiotherapy and to predict the likelihood of local control as a function of dose and treatment fractions.Experimental Design: We model outcomes for early-stage, localized non-small cell lung cancer (NSCLC), by fitting a mechanistic, cellular dynamics-based tumor control probability that assumes a constant local supply of oxygen and glucose. In addition to standard radiobiological effects such as repair of sub-lethal damage and the impact of hypoxia, we also accounted for proliferation as well as radiosensitivity variability within the cell cycle. We applied the model to 36 published and two unpublished early-stage patient cohorts, totaling 2,701 patients.Results: Precise likelihood best-fit values were derived for the radiobiological parameters: α [0.305 Gy^-1; 95% confidence interval (CI), 0.120-0.365], the α/β ratio (2.80 Gy; 95% CI, 0.40-4.40), and the oxygen enhancement ratio (OER) value for intermediately hypoxic cells receiving glucose but not oxygen (1.70; 95% CI, 1.55-2.25). All fractionation groups are well fitted by a single dose-response curve with a high χ² P value, indicating consistency with the fitted model. The analysis was further validated with an additional 23 patient cohorts (n = 1,628). The model indicates that hypofractionation regimens overcome hypoxia (and cell-cycle radiosensitivity variations) by the sheer impact of high doses per fraction, whereas lower dose-per-fraction regimens allow for reoxygenation and corresponding sensitization, but lose effectiveness for prolonged treatments due to proliferation.Conclusions: This proposed mechanistic tumor-response model can accurately predict overtreatment or undertreatment for various treatment regimens. Clin Cancer Res; 23(18); 5469-79. ©2017 AACR.

Let-7b overexpression leads to increased radiosensitivity of uveal melanoma cells

Uveal melanoma (UM) is an intraocular malignant tumor in adults that is characterized by rapid progression and recurrence. Irradiation has become the primary therapy for UM patients who are not candidates for surgery. However, after large-dose fraction irradiation treatment, some patients undergo subsequent enucleation because of radiotherapy-related complications. This situation has raised concerns on how to optimize the effectiveness of radiation treatment. Recent investigations of microRNAs are changing our understanding of UM tumor biology and are helping to identify novel targets for radiotherapy. The radioresistant UM cell lines OM431 and OCM1 were selected and exposed to irradiation, and let-7b was found to be downregulated after exposure. We then confirmed that let-7b mimics could inhibit UM growth both in vitro and in vivo. More specifically, transfection with let-7b mimics markedly resensitized OCM1 and OM431 cells to irradiation by reducing the population of S-phase cells. Cyclin D1 plays a vital role in cell cycle arrest, which is induced by let-7b overexpression. Cyclin D1 is also a target of let-7b and its expression is suppressed by upregulation of let-7b. Collectively, our results indicate that let-7b overexpression can in turn downregulate cyclin D1 expression and enhance the radiosensitivity of UM through cell cycle arrest. Let-7b could serve as a marker for radiosensitivity and could enhance the therapeutic benefit of UM cell irradiation.

Radiation response of mesenchymal stem cells derived from bone marrow and human pluripotent stem cells

Mesenchymal stem cells (MSCs) isolated from human pluripotent stem cells are comparable with bone marrow-derived MSCs in their function and immunophenotype. The purpose of this exploratory study was comparative evaluation of the radiation responses of mesenchymal stem cells derived from bone marrow- (BMMSCs) and from human embryonic stem cells (hESMSCs). BMMSCs and hESMSCs were irradiated at 0 Gy (control) to 16 Gy using a linear accelerator commonly used for cancer treatment. Cells were harvested immediately after irradiation, and at 1 and 5 days after irradiation. Cell cycle analysis, colony forming ability (CFU-F), differentiation ability, and expression of osteogenic-specific runt-related transcription factor 2 (RUNX2), adipogenic peroxisome proliferator-activated receptor gamma (PPARγ), oxidative stress-specific dismutase-1 (SOD1) and Glutathione peroxidase (GPX1) were analyzed. Irradiation arrested cell cycle progression in BMMSCs and hESMSCs. Colony formation ability of irradiated MSCs decreased in a dose-dependent manner. Irradiated hESMSCs showed higher adipogenic differentiation compared with BMMSCs, together with an increase in the adipogenic PPARγ expression. PPARγ expression was upregulated as early as 4 h after irradiation, along with the expression of SOD1. More than 70% downregulation was found in Wnt3A, Wnt4, Wnt 7A, Wnt10A and Wnt11 in BMMSCs, but not in hESMSCs. hESMSCs are highly proliferative but radiosensitive compared with BMMSCs. Increased PPARγ expression relative to RUNX2 and downregulation of Wnt ligands in irradiated MSCs suggest Wnt mediated the fate determination of irradiated MSCs.

Modeling the Interplay Between Tumor Volume Regression and Oxygenation in Uterine Cervical Cancer During Radiotherapy Treatment

This paper describes a patient-specific mathematical model to predict the evolution of uterine cervical tumors at a macroscopic scale, during fractionated external radiotherapy. The model provides estimates of tumor regrowth and dead-cell reabsorption, incorporating the interplay between tumor regression rate and radiosensitivity, as a function of the tumor oxygenation level. Model parameters were estimated by minimizing the difference between predicted and measured tumor volumes, these latter being obtained from a set of 154 serial cone-beam computed tomography scans acquired on 16 patients along the course of the therapy. The model stratified patients according to two different estimated dynamics of dead-cell removal and to the predicted initial value of the tumor oxygenation. The comparison with a simpler model demonstrated an improvement in fitting properties of this approach (fitting error average value <5%, p < 0.01), especially in case of tumor late responses, which can hardly be handled by models entailing a constant radiosensitivity, failing to model changes from initial severe hypoxia to aerobic conditions during the treatment course. The model predictive capabilities suggest the need of clustering patients accounting for cancer cell line, tumor staging, as well as microenvironment conditions (e.g., oxygenation level).

Radiotherapy scheduling using prime numbers

Background

The optimal delivery of radiation therapy to achieve maximum tumour cell kill while limiting damage to normal tissues underlies any radiation therapy treatment protocol. The biological effectiveness of radiation therapy is closely related to cellular reproductive activity. The scheduling of dose fraction to a time where actively dividing cells are at their most radiosensitive stage (RS) has potential to enhance therapeutic efficacy.

Materials and methods

A prime number is a natural number >1 whose only divisors are 1 and the number itself.

Purpose

We propose that the use of prime numbers in the scheduling of radiotherapy treatments could maximise biological effectiveness by facilitating the irradiation of the greatest number of cells at their most RS stage, and ultimately improve the therapeutic ratio of radiation therapy.

Conclusions

The theoretical clinical implementation of this concept into the scheduling of radiation therapy is discussed.

Broad phenotypic changes associated with gain of radiation resistance in head and neck squamous cell cancer

AIMS

The central issue of resistance to radiation remains a significant challenge in the treatment of cancer despite improvements in treatment modality and emergence of new therapies. To facilitate the identification of molecular factors that elicit protection against ionizing radiation, we developed a matched model of radiation resistance for head and neck squamous cell cancer (HNSCC) and characterized its properties using quantitative mass spectrometry and complementary assays.

RESULTS

Functional network analysis of proteomics data identified DNA replication and base excision repair, extracellular matrix-receptor interaction, cell cycle, focal adhesion, and regulation of actin cytoskeleton as significantly up- or downregulated networks in resistant (rSCC-61) HNSCC cells. Upregulated proteins in rSCC-61 included a number of cytokeratins, fatty acid synthase, and antioxidant proteins. In addition, the rSCC-61 cells displayed two unexpected features compared with parental radiation-sensitive SCC-61 cells: (i) rSCC-61 had increased sensitivity to Erlotinib, a small-molecule inhibitor of epidermal growth factor receptor; and (ii) there was evidence of mesenchymal-to-epithelial transition in rSCC-61, confirmed by the expression of protein markers and functional assays (e.g., Vimentin, migration).

INNOVATION

The matched model of radiation resistance presented here shows that multiple signaling and metabolic pathways converge to produce the rSCC-61 phenotype, and this points to the function of the antioxidant system as a major regulator of resistance to ionizing radiation in rSCC-61, a phenomenon further confirmed by analysis of HNSCC tumor samples.

CONCLUSION

The rSCC-61/SCC-61 model provides the opportunity for future investigations of the redox-regulated mechanisms of response to combined radiation and Erlotinib in a preclinical setting.

MicroRNA-449a enhances radiosensitivity in CL1-0 lung adenocarcinoma cells

Lung cancer is the leading cause of cancer-related mortality worldwide. Radiotherapy is often applied for treating lung cancer, but it often fails because of the relative non-susceptibility of lung cancer cells to radiation. MicroRNAs (miRNAs) have been reported to modulate the radiosensitivity of lung cancer cells and have the potential to improve the efficacy of radiotherapy. The purpose of this study was to identify a miRNA that can adjust radiosensitivity in lung adenocarcinoma cells. Two lung adenocarcinoma cell lines (CL1-0 and CL1-5) with different metastatic ability and radiosensitivity were used. In order to understand the regulatory mechanisms of differential radiosensitivity in these isogenic tumor cells, both CL1-0 and CL1-5 were treated with 10 Gy radiation, and were harvested respectively at 0, 1, 4, and 24 h after radiation exposure. The changes in expression of miRNA upon irradiation were examined using Illumina Human microRNA BeadChips. Twenty-six miRNAs were identified as having differential expression post-irradiation in CL1-0 or CL1-5 cells. Among these miRNAs, miR-449a, which was down-regulated in CL1-0 cells at 24 h after irradiation, was chosen for further investigation. Overexpression of miR-449a in CL1-0 cells effectively increased irradiation-induced DNA damage and apoptosis, altered the cell cycle distribution and eventually led to sensitization of CL1-0 to irradiation.

The NEDD8 conjugation pathway regulates p53 transcriptional activity and head and neck cancer cell sensitivity to ionizing radiation

Human papillomavirus (HPV)-related oropharyngeal cancer represents a distinct head and neck squamous cell carcinoma (HNSCC) subpopulation, with improved disease-free and overall survival. In general, HPV-positive HNSCCs express wild-type TP53, which could explain its increased radiosensitivity. However, the molecular mechanisms underlying this higher sensitivity remain elusive. We have previously shown that HPV-related oropharyngeal carcinomas express decreased levels of the NEDD8-activating enzyme 1/amyloid β precursor protein-binding protein 1 (NAE1/APP-BP1) gene. NAE1/APP-BP1 function is required for the NEDDylation of target proteins, and has been shown to be a negative regulator of p53 transcriptional activity. In this study, we addressed the hypothesis that NAE1/APP-BP1 expression levels regulate p53 activity and cell survival upon ionizing irradiation. We used the radiosensitive and naturally HPV16-infected UPCI:SCC90 cell line and the radioresistant and HPV-negative SQ20B cell line as the control. NAE1/APP-BP1 expression levels were modulated with expression constructs and siRNAs. Radiosensitivity was evaluated with clonogenic survival assays. p53 transcriptional activity was measured with a luciferase assay. The overexpression of NAE1/APP-BP1 in UPCI:SCC90 cells resulted in the increased NEDDylation of p53, inhibition of p53 activity and increased cell resistance to ionizing radiation. Conversely, the inhibition of NAE1/APP-BP1 expression in SQ20B cells induced p53-dependent cell death after treatment with X-rays. Taken together, these results indicate that NAE1/APP-BP1 and NEDDylation are invovled in modulating p53 activity and regulating its role in the response of cells to ionizing radiation. Our findings bring new insights in the molecular mechanisms underlying the increased radiosensitivity of HPV-related oropharyngeal tumors. This is of importance, as no reliable and robust predictive biomarkers for tumor response to radiotherapy are currently available. These results also have potential clinical significance, as drugs targeting NAE1/APP-BP1 have recently emerged as a novel therapeutic modality in cancer treatment.

Recognition, signaling, and repair of DNA double-strand breaks produced by ionizing radiation in mammalian cells: the molecular choreography

The faithful maintenance of chromosome continuity in human cells during DNA replication and repair is critical for preventing the conversion of normal diploid cells to an oncogenic state. The evolution of higher eukaryotic cells endowed them with a large genetic investment in the molecular machinery that ensures chromosome stability. In mammalian and other vertebrate cells, the elimination of double-strand breaks with minimal nucleotide sequence change involves the spatiotemporal orchestration of a seemingly endless number of proteins ranging in their action from the nucleotide level to nucleosome organization and chromosome architecture. DNA DSBs trigger a myriad of post-translational modifications that alter catalytic activities and the specificity of protein interactions: phosphorylation, acetylation, methylation, ubiquitylation, and SUMOylation, followed by the reversal of these changes as repair is completed. "Superfluous" protein recruitment to damage sites, functional redundancy, and alternative pathways ensure that DSB repair is extremely efficient, both quantitatively and qualitatively. This review strives to integrate the information about the molecular mechanisms of DSB repair that has emerged over the last two decades with a focus on DSBs produced by the prototype agent ionizing radiation (IR). The exponential growth of molecular studies, heavily driven by RNA knockdown technology, now reveals an outline of how many key protein players in genome stability and cancer biology perform their interwoven tasks, e.g. ATM, ATR, DNA-PK, Chk1, Chk2, PARP1/2/3, 53BP1, BRCA1, BRCA2, BLM, RAD51, and the MRE11-RAD50-NBS1 complex. Thus, the nature of the intricate coordination of repair processes with cell cycle progression is becoming apparent. This review also links molecular abnormalities to cellular pathology as much a possible and provides a framework of temporal relationships.

Manganese superoxide dismutase is not protective in bovine pulmonary artery endothelial cells at systemic oxygen levels

Bovine pulmonary artery endothelial cells (BPAEC) are extremely sensitive to oxygen, mediated by superoxide production. Ionizing radiation is known to generate superoxide in oxygenated aqueous media; however, at systemic oxygen levels (3%), no oxygen enhancement is observed after irradiation. A number of markers (cell growth, alamarBlue, mitochondrial membrane polarization) for metabolic activity indicate that BPAEC maintained under 20% oxygen grow and metabolize more slowly than cells maintained under 3% oxygen. BPAEC cultured in 20% oxygen grow better when they are transiently transfected with either manganese superoxide dismutase (MnSOD) or copper zinc superoxide dismutase (CuZnSOD) and exhibit improved survival after irradiation (0.5-10 Gy). Furthermore, X irradiation of BPAEC grown in 20% oxygen results in very diffuse colony formation, which is completely ameliorated by either growth in 3% oxygen or overexpression of MnSOD. However, MnSOD overexpression in BPAEC grown in 3% oxygen provides no further radioprotection, as judged by clonogenic survival curves. Radiation does not increase apoptosis in BPAEC but inhibits cell growth and up-regulates p53 and p21 at either 3% or 20% oxygen.

Statins can modulate effectiveness of antitumor therapeutic modalities

Despite significant, frequently very strong, antiproliferative and tumoricidal effects of statins demonstrated in vitro, their antitumor effects in animal models are modest, and their efficacy in clinical trials has not been proven. As such, statins seem unlikely to be ever regarded as antitumor agents. However, statins are regularly taken by many elderly cancer patients for the prevention of cardiovascular events. Owing to their pleiotropic effects in normal and tumor cells, statins interact in various ways with many antitumor treatment modalities, either potentiating or diminishing their effectiveness. Elucidation of these interactions might affect the choice of treatment to be planned in cancer patients as some combinations might be contraindicated, whereas others might elicit potentiated antitumor effects but at a cost of increased general toxicity. Some other combinations might induce either comparable or even stronger antitumor effects, but with a beneficial concomitant reduction of specific side effects. Most of the studies reviewed in this article have been carried in vitro or in experimental tumor models, but clinical relevance of the findings is also discussed.

Radioresistance of Stat1 over-expressing tumour cells is associated with suppressed apoptotic response to cytotoxic agents and increased IL6-IL8 signalling

PURPOSE

To determine the mechanisms of Signal Transducer and Activator of Transcription 1 (Stat1)-associated radioresistance developed by nu61 tumour selected in vivo by fractionated irradiation of the parental radiosensitive tumour SCC61.

MATERIALS AND METHODS

Radioresistence of nu61 and SCC61 in vitro was measured by clonogenic assay. Apoptotic response of nu61 and SCC61 cells to genotoxic stress was examined using caspase-based apoptotic assays. Co-cultivation of carboxyfluorescein diacetate, succinimidyl ester (CFDE-SE)-labeled nu61 with un-labeled SCC61 was performed at 1:1 ratio. Production of interleukin-6, interleukin-8 and soluble receptor of interleukin 6 (IL6, IL8 and sIL6R) was measured using Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

Radioresistant nu61 was also resistant to interferon-gamma (IFNgamma) and the death ligands of tumour necrosis factor alpha receptor (TNFR) family when compared to SCC61. This combined resistance is due to an impaired apoptotic response in nu61. Relative to SCC61, nu61 produced more IL6, IL8 and sIL6R. Using Stat1 knock-downs we demonstrated that IL6 and IL8 production is Stat1-dependent. Treatment with neutralising antibodies to IL6 and IL8, but not to either cytokine alone sensitised nu61 to genotoxic stress induced apoptosis.

CONCLUSION

Nu61, which over-expresses Stat1 pathway, is deficient in apoptotic response to ionising radiation and cytotoxic ligands. This resistance to apoptosis is associated with Stat1-dependent production of IL6 and IL8 and suppression of caspases 8, 9 and 3.

Influence of double-strand-break repair pathways on radiosensitivity throughout the cell cycle in CHO cells

Unrepaired DNA double-strand breaks (DSBs) produced by ionizing radiation (IR) are a major determinant of cell killing. To determine the contribution of DNA repair pathways to the well-established cell cycle variation in IR sensitivity, we compared the radiosensitivity of wild-type CHO cells to mutant lines defective in nonhomologous end joining (NHEJ), homologous recombination repair (HRR), and the Fanconi anemia pathway. Cells were irradiated with IR doses that killed approximately 90% of each asynchronous population, separated into synchronous fractions by centrifugal elutriation, and assayed for survival (colony formation). Wild-type cells had lowest resistance in early G1 and highest resistance in S phase, followed by declining resistance as cells move into G2/M. In contrast, HR-defective cells (xrcc3 mutation) were most resistant in early G1 and became progressively less resistant in S and G2/M, indicating that the S-phase resistance in wild-type cells requires HRR. Cells defective in NHEJ (dna-pk(cs) mutation) were exquisitely sensitive in early G1, most resistant in S phase, and then somewhat less resistant in G2/M. Fancg mutant cells had almost normal IR sensitivity and normal cell cycle dependence, suggesting that Fancg contributes modestly to survival and in a manner that is independent of cell cycle position.

Enhancement of Radiation Sensitivity of Human Squamous Carcinoma Cells by Histone Deacetylase Inhibitors

Histone deacetylase (HDAC) inhibitors are emerging therapeutic agents with potential for disruption of critical cellular processes in cancer cells. Transcriptional regulation, differentiation, cell cycle arrest, radiation sensitization, and apoptosis have been observed in response to exposure to HDAC inhibitors. In the present study, we observed that several potent HDAC inhibitors, including trichostatin A, suberoylanilide hydroxamic acid, M344 (an analogue of hydroxamic acid), and the cyclic tetrapeptide, depsipeptide (FR90228), modulate cellular responses to ionizing radiation in cells of two human squamous carcinoma lines (SQ-20B and SCC-35), previously characterized as intrinsically resistant to radiation. Also exposure to IC(50) concentrations of these inhibitors, radiation sensitivities were enhanced in both cell lines. Depsipeptide exhibited the greatest effect on SQ-20B cells, decreasing D(0) values from 2.62 Gy to 1.64 Gy. M344 was the most active drug in sensitizing SCC-35 cells, decreasing D(0) values from 1.91 Gy to 1.21 Gy. The mechanisms underlying HDAC inhibitor-induced radiosensitization were further investigated by extending trichostatin A studies to assess cell cycle distributions and levels of apoptosis. Treatment of SQ-20B cells with radiosensitizing concentrations of trichostatin A resulted in cell cycle arrest in G(1) phase (>70%) and inhibition of DNA synthesis. Contrary to previous reports, induction of apoptosis was very low and caspase 3 and 9 were not activated. Taken together, these results implicate G(1) arrest and inhibition of DNA synthesis in the mechanisms underlying radiation sensitization by trichostatin A and support the use of HDAC inhibitors for targeting radioresistant cancers.

STAT1 is overexpressed in tumors selected for radioresistance and confers protection from radiation in transduced sensitive cells

Nu61, a radiation-resistant human tumor xenograft, was selected from a parental radiosensitive tumor SCC-61 by eight serial cycles of passage in athymic nude mice and in vivo irradiation. Replicate DNA array experiments identified 52 genes differentially expressed in nu61 tumors compared with SCC-61 tumors. Of these, 19 genes were in the IFN-signaling pathway and moreover, 25 of the 52 genes were inducible by IFN in the nu61 cell line. Among the genes involved in IFN signaling, STAT1alpha and STAT1beta were the most highly overexpressed in nu61 compared to SCC-61. STAT1alpha and STAT1beta cDNAs were cloned and stably transfected into SCC-61 tumor cells. Clones of SCC-61 tumor cells transfected with vectors expressing STAT1alpha and STAT1beta demonstrated radioprotection after exposure to 3 Gy (P < 0.038). The results indicate that radioresistance acquired during radiotherapy treatment may account for some treatment failures and demonstrate an association of acquired tumor radioresistance with up-regulation of components of the IFN-related signaling pathway.

The contribution of DNA ploidy to radiation sensitivity in human tumour cell lines

The contribution of DNA ploidy to radiation sensitivity was investigated in a group of eight human tumour cell lines. As previous studies suggest, while more aneuploid tumours tend to be more radioresistant, there is no significant relationship between ploidy and radiation sensitivity (SF2). The failure to observe a significant effect of ploidy on radiation sensitivity is due to the complex and multifactorial basis of radiation sensitivity. When we determined the relationship between survival and radiation-induced chromosome aberration frequency, a measure independent of most other modifiers of sensitivity, we observed a direct relationship between ploidy and mean lethal aberration frequency. The mean lethal frequency of aberrations increased from about 1 for diploid cells to about 2 for tetraploid cells. The mean lethal frequency of aberrations was independent of DNA repair variations. These observations demonstrate that changes in DNA ploidy are an important contributor to radiation sensitivity variations in human tumour cell lines. Therefore, any battery of predictive assays should include DNA ploidy measurements.

A radioresistant variant cell line, NMT-1R, isolated from a radiosensitive rat yolk sac tumour cell line, NMT-1: differences of early radiation-induced morphological changes, especially apoptosis

A radioresistant variant cell line, NMT-1R, was isolated by repeated radiation exposure of a radiosensitive rat yolk sac tumour cell line, NMT-1, producing alpha-fetoprotein, with the potential for lymphatic metastasis in the inbred Wistar rat. Cultured NMT-1R cells showed more cobblestone-like appearances, although the morphological features were almost the same as radiosensitive NMT-1 cells reported previously. The doubling time of NMT-1R cells was 13.6 h, being shorter than that of NMT-1 cells (16.0 h). For NMT-1R cells, D0 for radiation sensitivity was 165 +/- 3 cGy, 1.7 times as large as for NMT-1 cells. The extrapolation number, n, was 1.48 +/- 0.17 for NMT-1R cells although that for NMT-1 cells was 1.08 +/- 0.15. The surviving fractions at 2 Gy (SF2) were 0.42 for NMT-1R cells and 0.28 for NMT-1 cells. The population of G2-M phase for NMT-1R cells was larger than for NMT-1 cells (32.5 versus 26.8%) in exponentially growing cells. Although a clear G2 delay was observed after irradiation with a dose of 182 cGy for both cell lines, NMT-1R cells had a shorter recovery time from G2 block than NMT-1 cells, G1 arrest was observed in NMT-1 cells. NMT-1 cells showed much higher incidence of early morphological changes, especially apoptosis, after irradiation with a dose > 500 cGy compared with NMT-1R cells.

Absence of correlation between chemo- and radioresistance in a range of human tumour cell lines

The correlation between cellular resistance to radiation and to chemotherapeutic drugs has been investigated in a number of solid tumour cell lines, and preliminary results indicate no direct relationship. The acquisition of a multidrug resistance (MDR) profile by adriamycin-selected variants of a human squamous lung carcinoma, an ovarian carcinoma, a cervical carcinoma and by a colchicine-selected variant of a Chinese hamster ovarian carcinoma resulted in alterations to their radiosensitivity. However, the degree of change in the radiosensitivity of the MDR cell lines could not be predicted from their level of resistance to adriamycin. Clonal populations derived from DLKP-A, an adriamycin-selected MDR variant of the human lung carcinoma cell line DLKP, exhibited individual radiosensitivity profiles, which did not correlate with their chemoresistance. Exposure of DLKP to consecutive increasing doses of radiation did not confer cross-resistance to chemotherapeutic drugs.

Heterogeneity in the fractionation sensitivities of human tumor cell lines: studies in a three-dimensional model system

PURPOSE

Current concepts to optimize the therapeutic gain of radiotherapy by hyperfractionation assume that human tumors are less sensitive to fractionation than late reacting normal tissues. The aim of this study was to investigate the extent of the intercell line heterogeneity of fractionation sensitivity of a wide variety of human tumor cell lines in a three-dimensional model system under fully oxic conditions using schedules with one to eight fractions. Biological characteristics of the tumors that correlate with fractionation sensitivity should be identified.

METHODS AND MATERIALS

A total of 21 cell lines from human tumors maintained as multicellular spheroids consisting of 1000-1500 cells were given fractionated irradiation within a total treatment time of maximally 50 h. Complete dose-spheroid control curves were determined for each fractionation scheme. The spheroid control data were adequately described by the linear quadratic model assuming Poisson statistics. In addition, the induction of a G2 block by a fractionated test dose of seven 3 Gy fractions given at 6-h intervals was determined in spheroid cells using flow cytometry of propidium bromide stained cell nuclei.

RESULTS

The fractionation sensitivities of human tumor cells in multicellular spheroids could be characterized by alpha/beta values, ranging from 2.8-37 Gy in dependence on the cell line. The log normally distributed alpha/beta values were positively correlated with the percentage increase in G2/M phase after the fractionated test dose compared to the controls (r = 0.72, p < 0.01), and were associated with the degree of tumor differentiation (p = 0.01, ANOVA F-test). No significant correlation between the log (alpha/beta) values and the surviving fractions at 2 Gy (SF2) or the total doses with 2 Gy per fraction necessary to control 50% of the spheroids (SCD50) was observed. Despite the intercell line variability of the alpha/beta values, the SCD50 values of the different cell lines, given with one and eight fractions or one fraction and 2 Gy per fraction, were closely associated (Spearman rank correlation coefficients: r = 0.89 or r = 0.90, p < 0.0001).

CONCLUSION

Human tumor cell lines showed a marked heterogeneity in the fractionation sensitivity when irradiated as multicellular spheroids and assayed in situ using the spheroid control end point. Therefore, the therapeutic gain of altered fractionation also depends on those biological characteristics of each individual tumor that affects its fractionation sensitivity. Parameters that correlate with fractionation sensitivity of the tumor lines in the spheroid system were identified as grade of tumor differentiation and percentage increase in G2/M cells at the end of an eight-fraction schedule.

The role of cell cycle redistribution in radiosensitization: implications regarding the mechanism of fluorodeoxyuridine radiosensitization

PURPOSE

Radiosensitization has previously been demonstrated in a human colon cancer cell line (HT-29) following a 2 h exposure to low, clinically relevant concentrations (0.05-0.5 microM) of fluorodeoxyuridine (FdUrd) (15). The sensitizer enhancement ratio value (measured at 10% survival) plateaued at approximately 1.7 between 16 and 32 h following removal of drug. Parallel studies investigating the effect of FdUrd on the distribution of cells throughout the cell cycle found that the percentage of cells in early S-phase increased to approximately 70% during the same period that maximal radiosensitization was noted. As a follow-up to these findings, experiments have been designed to investigate the contribution of this early S-phase delay to radiosensitization.

METHODS AND MATERIALS

Synchronized populations of HT-29 cells have been obtained with three separate techniques. Two involve the induction of a reversible metaphase arrest (with high pressure N2O or colcemid) followed by a shakeoff of mitotic cells. The third uses a plant amino acid, mimosine, to induce a reversible block at the G1/S boundary. Flow cytometry was used to analyze the degree of synchrony based on bromodeoxyuridine (BrdUrd) uptake and propidium iodide (PI) staining. Radiation survival curves were obtained on these synchronized populations to investigate changes in radiosensitivity through the cell cycle. Additionally, levels of thymidylate synthase (TS), the primary target of FdUrd cytotoxicity, were measured in each phase of the cell cycle using the TS 106 monoclonal antibody against human TS.

RESULTS

Synchronization with mitotic shakeoff produced relatively pure populations of cells in G1; however, the degree of synchrony in early S-phase was limited both by cells remaining in G1 and by cells progressing into late S-phase. These techniques failed to reveal increased radiosensitivity in early S-phase at 10% survival. An 18 h exposure to mimosine resulted in populations that more closely resembled the early S-phase enrichment following FdUrd exposure and revealed increased radiosensitivity during early S-phase. TS levels were noted to be only 1.3 times higher in S phase than in G0/G1.

CONCLUSION

Radiation survival data from cells synchronized with mitotic shakeoff techniques suggest that early S-phase delay is unlikely to be the primary mechanism of FdUrd radiosensitization. In contrast, the increased sensitivity seen in early S-phase with mimosine synchronized cells is similar to that seen with FdUrd. Although confounding biochemical pertubations cannot be ruled out, these data continue to suggest an association between early S-phase enrichment and radiosensitization. The significance of TS inhibition as a mechanism of FdUrd radiosensitization remains unclear.

Radiosurgery dose distributions: theoretical impact of inhomogeneities on lesion control

OBJECTIVE

To develop a mathematical model to predict the impact of dose heterogeneities on tumor/lesion control during radiosurgery. It is necessary to be able to estimate these effects in order to quantitatively and objectively assess competing treatment plans.

METHODS

Target cells are assumed to be uniformly distributed throughout the lesion. The control rate for the entire lesion is assumed to be the product of the control probabilities for each subregion within the target volume. The lesion control probability (LCP) for each region is assumed to equal EXP (the number of surviving target cells within the subregion), as predicted by Poisson statistics. Subregions of variable size are assumed to receive variable doses, and the impact of this dose heterogeneity on the LCP is calculated based on the single-fraction radiation cell survival curve predicted by the single-hit multitarget model.

RESULTS

The impact of a dose heterogeneity on LCP is related to three variables: the LCP predicted with uniform irradiation, the volume of the lesion that is irradiated to a new dose, and the magnitude of the dose change relative to the slope of the single-fraction radiation cell survival curve of (delta D/Dzero). The calculations predict that the detrimental effect of underdosing regions of the lesion can, in some instances, be offset by escalating the dose to other subregions within the target volume. In this regard, the "average" dose delivered to the lesion rather than the minimum dose may be most predictive of the lesion control probability. In some situations, escalating the dose to part of the lesion may improve the lesion control rate.

CONCLUSION

These calculations quantify the theoretical impact of dose heterogeneities on lesion control rate and may be very useful when comparing competing treatment plans.

Cytotoxicity of restriction enzyme-induced DNA strand breaks in radiosensitive and radioresistant human tumor cell lines

PURPOSE

To examine the role of sensitivity to specific types of DNA double strand breaks in human tumor cell response.

METHODS AND MATERIALS

The X ray-sensitive human squamous carcinoma cell line SCC-61 and the X ray-resistant line SQ-20B were exposed to the restriction enzymes HaeIII, HinfI, PvuII, BamHI by electroporation. Cytotoxicity of these restriction endonucleases was measured by a colony formation assay.

RESULTS

Cell killing by each enzyme occurred in a concentration-dependent manner. The radiosensitive cell line was more sensitive to all four restriction enzymes than the radioresistant line, paralleling the response to ionizing radiation. However, the magnitude of the difference was smaller than for radiation. The 5-base sticky ended cutter HinfI and 6-base blunt ended cutter PvuII were much more effective in killing cells from both lines than BamHI, a 6-base sticky ended cutter, whereas the 4-base blunt ended cutter HaeIII was intermediate in its effectiveness. Thus, enzyme sensitivity could not be related to the type of cutter or the distance between cutting sites.

The isoquinoline sulfonamide H7 attenuates radiation-mediated protein kinase C activation and delays the onset of x-ray-induced G2 arrest

Protein kinase C activation by ionizing radiation in human tumor cell lines participates in the transcriptional activation of genes which may be associated with the phenotypic response of cells to x-rays. We gamma-irradiated cell line RIT-3 (radiation-induced human sarcoma) and quantified the phosphorylating capacity of protein kinase C. Protein kinase C activity increased rapidly and transiently in these cells. The selective protein kinase C inhibitor H7 attenuated radiation-mediated protein kinase C activation when added to cells prior to irradiation. To determine whether protein kinase C activation is associated with radiation-induced G2 arrest, we analyzed the cell cycle distribution of cells following gamma-irradiation. Following irradiation, RIT-3 cells rapidly progressed through G1 and S and subsequently underwent a dose dependent G2 arrest. At concentrations which are selective for protein kinase C inhibition, H7 delayed the onset of radiation-induced G2 arrest. However, there was no difference in the duration of G2 arrest following the addition of inhibitor as compared to cells irradiated without inhibitor. We propose that protein kinase C activation by ionizing radiation is associated with radiation-mediated cell cycle regulation.