Diminished or inversed dose-rate effect on clonogenic ability in Ku-deficient rodent cells

The biological effects of ionizing radiation, especially those of sparsely ionizing radiations like X-ray and γ-ray, are generally reduced as the dose rate is reduced. This phenomenon is known as 'the dose-rate effect'. The dose-rate effect is considered to be due to the repair of DNA damage during irradiation but the precise mechanisms for the dose-rate effect remain to be clarified. Ku70, Ku86 and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) are thought to comprise the sensor for DNA double-strand break (DSB) repair through non-homologous end joining (NHEJ). In this study, we measured the clonogenic ability of Ku70-, Ku86- or DNA-PKcs-deficient rodent cells, in parallel with respective control cells, in response to high dose-rate (HDR) and low dose-rate (LDR) γ-ray radiation (~0.9 and ~1 mGy/min, respectively). Control cells and murine embryonic fibroblasts (MEF) from a severe combined immunodeficiency (scid) mouse, which is DNA-PKcs-deficient, showed higher cell survival after LDR irradiation than after HDR irradiation at the same dose. On the other hand, MEF from Ku70-/- mice exhibited lower clonogenic cell survival after LDR irradiation than after HDR irradiation. XR-V15B and xrs-5 cells, which are Ku86-deficient, exhibited mostly identical clonogenic cell survival after LDR and HDR irradiation. Thus, the dose-rate effect in terms of clonogenic cell survival is diminished or even inversed in Ku-deficient rodent cells. These observations indicate the involvement of Ku in the dose-rate effect.

Radiation-induced alterations in immunogenicity of a murine pancreatic ductal adenocarcinoma cell line

Pancreatic ductal adenocarcinoma (PDA) is highlighted by resistance to radiotherapy with the possible exception of hypofractionated irradiation. As single photon doses were reported to increase immunogenicity, we investigated dose-dependent irradiation effects on clonogenic survival, expression of immunologically relevant cell surface molecules and susceptibility to cytotoxic T cell (CTL) mediated killing using a murine PDA cell line. Clonogenicity decreased in a dose-responsive manner showing enhanced radioresistance at single photon doses below 5 Gy. Cell cycle analysis revealed a predominant G2/M arrest, being most pronounced 12 h after irradiation. Polyploidy increased in a dose- and time-dependent manner reaching a maximum frequency 60 h following irradiation with 10 Gy. Irradiation increased surface expression of MHC class I molecules and of immunological checkpoint molecules PDL-1 and CD73, especially at doses ≥ 5 Gy, but not of MHC class II molecules and CXCR4 receptors. Cytotoxicity assays revealed increased CTL lysis of PDA cells at doses ≥ 5 Gy. For the PDA cell line investigated, our data show for the first time that single photon doses ≥ 5 Gy effectively inhibit colony formation and induce a G2/M cell cycle arrest. Furthermore, expression levels of immunomodulatory cell surface molecules became altered possibly enhancing the susceptibility of tumour cells to CTL lysis.

The Impact of Environmental and Endogenous Damage on Somatic Mutation Load in Human Skin Fibroblasts

Accumulation of somatic changes, due to environmental and endogenous lesions, in the human genome is associated with aging and cancer. Understanding the impacts of these processes on mutagenesis is fundamental to understanding the etiology, and improving the prognosis and prevention of cancers and other genetic diseases. Previous methods relying on either the generation of induced pluripotent stem cells, or sequencing of single-cell genomes were inherently error-prone and did not allow independent validation of the mutations. In the current study we eliminated these potential sources of error by high coverage genome sequencing of single-cell derived clonal fibroblast lineages, obtained after minimal propagation in culture, prepared from skin biopsies of two healthy adult humans. We report here accurate measurement of genome-wide magnitude and spectra of mutations accrued in skin fibroblasts of healthy adult humans. We found that every cell contains at least one chromosomal rearrangement and 600–13,000 base substitutions. The spectra and correlation of base substitutions with epigenomic features resemble many cancers. Moreover, because biopsies were taken from body parts differing by sun exposure, we can delineate the precise contributions of environmental and endogenous factors to the accrual of genetic changes within the same individual. We show here that UV-induced and endogenous DNA damage can have a comparable impact on the somatic mutation loads in skin fibroblasts.

Somatic genomes are constantly accumulating changes caused by endogenous lesions, errors in DNA replication and repair, as well as environmental insults. Despite the importance of somatic genome instability in aging and age-related pathologies, including cancers, accurate measurements of mutation loads in healthy cells is still missing. In this study, we developed an experimental approach to accurately determine the somatic genome changes accrued in cell lineages over the lifetime of healthy humans. We show that the amounts and types of mutations in skin cells resemble many cancers, thus indicating that the mechanisms that lead to carcinogenesis are also functional in healthy cells. Moreover, sun-exposed skin cells have a higher mutation load attributable to ultraviolet radiation (UV) unlike cells from hips that were protected by clothing. Our work provides precise measurements of the mutation loads in single cells in human skin. Furthermore our data allowed defining the mutagenic impacts of environmental and endogenous processes within the same individual and led to conclusion that these processes have a comparable impact on the somatic mutation load.

Functional genomics screening utilizing mutant mouse embryonic stem cells identifies novel radiation-response genes

Elucidating the genetic determinants of radiation response is crucial to optimizing and individualizing radiotherapy for cancer patients. In order to identify genes that are involved in enhanced sensitivity or resistance to radiation, a library of stable mutant murine embryonic stem cells (ESCs), each with a defined mutation, was screened for cell viability and gene expression in response to radiation exposure. We focused on a cancer-relevant subset of over 500 mutant ESC lines. We identified 13 genes; 7 genes that have been previously implicated in radiation response and 6 other genes that have never been implicated in radiation response. After screening, proteomic analysis showed enrichment for genes involved in cellular component disassembly (e.g. Dstn and Pex14) and regulation of growth (e.g. Adnp2, Epc1, and Ing4). Overall, the best targets with the highest potential for sensitizing cancer cells to radiation were Dstn and Map2k6, and the best targets for enhancing resistance to radiation were Iqgap and Vcan. Hence, we provide compelling evidence that screening mutant ESCs is a powerful approach to identify genes that alter radiation response. Ultimately, this knowledge can be used to define genetic variants or therapeutic targets that will enhance clinical therapy.

Genetic and epigenetic changes in chromosomally stable and unstable progeny of irradiated cells

Radiation induced genomic instability is a well-studied phenomenon, the underlying mechanisms of which are poorly understood. Persistent oxidative stress, mitochondrial dysfunction, elevated cytokine levels and epigenetic changes are among the mechanisms invoked in the perpetuation of the phenotype. To determine whether epigenetic aberrations affect genomic instability we measured DNA methylation, mRNA and microRNA (miR) levels in well characterized chromosomally stable and unstable clonally expanded single cell survivors of irradiation. While no changes in DNA methylation were observed for the gene promoters evaluated, increased LINE-1 methylation was observed for two unstable clones (LS12 and CS9) and decreased Alu element methylation was observed for the other two unstable clones (115 and Fe5.0–8). These relationships also manifested for mRNA and miR expression. mRNA identified for the LS12 and CS9 clones were most similar to each other (261 mRNA), while the 115 and Fe5.0–8 clones were more similar to each other, and surprisingly also similar to the two stable clones, 114 and 118 (286 mRNA among these four clones). Pathway analysis showed enrichment for pathways involved in mitochondrial function and cellular redox, themes routinely invoked in genomic instability. The commonalities between the two subgroups of clones were also observed for miR. The number of miR for which anti-correlated mRNA were identified suggests that these miR exert functional effects in each clone. The results demonstrate significant genetic and epigenetic changes in unstable cells, but similar changes are almost as equally common in chromosomally stable cells. Possible conclusions might be that the chromosomally stable clones have some other form of instability, or that some of the observed changes represent a sort of radiation signature and that other changes are related to genomic instability. Irrespective, these findings again suggest that a spectrum of changes both drive genomic instability and permit unstable cells to persist and proliferate.

Intraclonal recovery of 'slow clones'-a manifestation of genomic instability: are mitochondria the key to an explanation?

Intraclonal recovery following X-irradiation in an in vitro study of L5178Y-S murine leukaemic cells is reviewed. This phenomenon was first described in 1994 occurring in the slowly growing clones ('slow clones') present among the survivors in irradiated cell populations. An attempt to explain these experimental data is given in terms of the present knowledge of the role of mitochondria in nontargeted radiation effects, with the focus on genomic instability and mtDNA-related epigenetic modifications of the nuclear genome. An understanding of this intraclonal recovery may be important in preventing tumour regrowth following radiotherapy, as well as in decreasing the risk of secondary radiation-induced malignancies.

Mutations and chromosomal aberrations in hMTH1-transfected and non-transfected TK6 cells after exposure to low dose rates of gamma radiation

The aim of the present study was to analyse the dose rate effect of gamma radiation at the level of mutations, chromosomal aberrations, and cell growth in TK6 cells with normal as well as reduced levels of hMTH1 protein. TK6 cells were exposed to gamma radiation at dose rates ranging from 1.4 to 30.0 mGy/h (chronic exposure) as well as 24 Gy/h (acute exposure). Cell growth, frequency of thymidine kinase mutants, and of chromosomal aberrations in painted chromosomes 2, 8, and 14 were analysed. A decline in cell growth and an increase in unstable-type chromosomal aberrations with increasing dose rate were observed in both cell lines. A dose rate effect was not seen on mutations or stable-type chromosomal aberrations in any of the two cell lines. Reduction in the hMTH1 protein does not influence the sensitivity of TK6 cells to gamma radiation. This result fits well with data of others generated with the same cell line.

Novel synthetic monoketone transmute radiation-triggered NFκB-dependent TNFα cross-signaling feedback maintained NFκB and favors neuroblastoma regression

Recently, we demonstrated that radiation (IR) instigates the occurrence of a NFκB-TNFα feedback cycle which sustains persistent NFκB activation in neuroblastoma (NB) cells and favors survival advantage and clonal expansion. Further, we reported that curcumin targets IR-induced survival signaling and NFκB dependent hTERT mediated clonal expansion in human NB cells. Herein, we investigated the efficacy of a novel synthetic monoketone, EF24, a curcumin analog in inhibiting persistent NFκB activation by disrupting the IR-induced NFκB-TNFα-NFκB feedback signaling in NB and subsequent mitigation of survival advantage and clonal expansion. EF24 profoundly suppressed the IR-induced NFκB-DNA binding activity/promoter activation and, maintained the NFκB repression by deterring NFκB-dependent TNFα transactivation/intercellular secretion in genetically varied human NB (SH-SY5Y, IMR-32, SK-PN-DW, MC-IXC and SK-N-MC) cell types. Further, EF24 completely suppressed IR-induced NFκB-TNFα cross-signaling dependent transactivation/translation of pro-survival IAP1, IAP2 and Survivin and subsequent cell survival. In corroboration, EF24 treatment maximally blocked IR-induced NFκB dependent hTERT transactivation/promoter activation, telomerase activation and consequent clonal expansion. EF24 displayed significant regulation of IR-induced feedback dependent NFκB and NFκB mediated survival signaling and complete regression of NB xenograft. Together, the results demonstrate for the first time that, novel synthetic monoketone EF24 potentiates radiotherapy and mitigates NB progression by selectively targeting IR-triggered NFκB-dependent TNFα-NFκB cross-signaling maintained NFκB mediated survival advantage and clonal expansion.

Clonal and lineage analysis of melanocyte stem cells and their progeny in the zebrafish

The study of melanocyte biology in the zebrafish presents a highly tractable system for understanding fundamental principles of developmental biology. Melanocytes are visible in the transparent embryo and in the mature fish following metamorphosis, a physical transformation from the larval to adult form. While early developing larval melanocytes are direct derivatives of the neural crest, the remainder of melanocytes develop from unpigmented precursors, or melanocyte stem cells (MSCs). The Tol2 transposable element has facilitated the construction of stable transgenic lines that label melanocytes. In another application, integration of Tol2 constructs makes possible clonal analysis of melanocyte and MSC lineages. Drugs that block melanin synthesis, ablate melanocytes, and block establishment of MSC populations allow the interrogation of this model system for mechanisms of adult stem cell development and regulation.

[High frequency induction of small mosaic cell clones in the Drosophila wings under the influence of gamma-irradiation]

As it was shown earlier in Gonzalez-Gaitan et al., one-cell and two-cells clones (tailing clones) are induced in the Drosophila wings after irradiation and represent a significant portion of clones detected with the use of mwh genetic marker. Our experiments shown that gamma-irradiation occur to be more efficient inductor of such small clones. Earlier small clones were considered as a result of the induced chromosomal aneuploidy of those low proliferating cells. Our data suggest that the small clones descend from the low proliferative cells of non-imaginal disc origin that migrate to the wing imaginal disc at some developmental point.

Temporally distinct response of irradiated normal human fibroblasts and their bystander cells to energetic heavy ions

Ionizing radiation-induced bystander effects have been documented for a multitude of endpoints such as mutations, chromosome aberrations and cell death, which arise in nonirradiated bystander cells having received signals from directly irradiated cells; however, energetic heavy ion-induced bystander response is incompletely characterized. To address this, we employed precise microbeams of carbon and neon ions for targeting only a very small fraction of cells in confluent fibroblast cultures. Conventional broadfield irradiation was conducted in parallel to see the effects in irradiated cells. Exposure of 0.00026% of cells led to nearly 10% reductions in the clonogenic survival and twofold rises in the apoptotic incidence regardless of ion species. Whilst apoptotic frequency increased with time up to 72 h postirradiation in irradiated cells, its frequency escalated up to 24h postirradiation but declined at 48 h postirradiation in bystander cells, indicating that bystander cells exhibit transient commitment to apoptosis. Carbon- and neon-ion microbeam irradiation similarly caused almost twofold increments in the levels of serine 15-phosphorylated p53 proteins, irrespective of whether 0.00026, 0.0013 or 0.0066% of cells were targeted. Whereas the levels of phosphorylated p53 were elevated and remained unchanged at 2h and 6h postirradiation in irradiated cells, its levels rose at 6h postirradiation but not at 2h postirradiation in bystander cells, suggesting that bystander cells manifest delayed p53 phosphorylation. Collectively, our results indicate that heavy ions inactivate clonogenic potential of bystander cells, and that the time course of the response to heavy ions differs between irradiated and bystander cells. These induced bystander responses could be a defensive mechanism that minimizes further expansion of aberrant cells.

Pre-treatment number of clonogenic cells and their radiosensitivity are major determinants of local tumour control after fractionated irradiation

OBJECTIVE

The response of tumours to fractionated radiotherapy is determined by many factors including repopulation, reoxygenation, the number of clonogenic cells, and their intrinsic radiosensitivity. However, after single radiation doses given under conditions of clamp hypoxia, the dose to control a tumour locally is dependent only on the number of clonogenic cells and their cellular radiosensitivity. Therefore, these parameters were investigated using local control after single doses given under hypoxia, to predict the outcome of fractionated irradiation.

MATERIALS AND METHODS

Ten hSCC cell lines (FaDu, UT-SCC-15, UT-SCC-14, XF354, UT-SCC-5, UT-SCC-45, SAS, CAL-33, UT-SCC-8, and HSC-4) were transplanted subcutaneously into the right hind-leg of NMRI nude mice. At 7mm in diameter, tumours were irradiated either with graded single doses under clamp blood flow conditions (n=873) or with 30 graded fractions within 6 weeks (n=905) under ambient conditions. Local tumour control was determined 120 days after irradiation. Radiation response was quantified in terms of TCD(50), i.e. the dose required to control 50% of tumours locally.

RESULTS

Ten tumour lines investigated showed a pronounced heterogeneity in both TCD(50(30fx/6w)) after fractionated irradiation and TCD(50(SDclamp)) after single dose irradiation. TCD(50(30fx/6w)) varied between 45Gy for UT-SCC-45 and 127Gy for SAS; TCD(50(SDclamp)) varied between 42Gy for UT-SCC-14 and 66Gy for CAL-33. Two tumours were excluded from further analysis due to immunogenicity or non-defined TCD(50). Linear regression analysis revealed a significant positive correlation between TCD(50(SDclamp)) and TCD(50(30fx/6w)) (R(2)=0.82, p=0.002).

CONCLUSIONS

Significant association between TCD(50(SDclamp)) and TCD(50(30fx/6w)) suggests that the pre-treatment number of clonogenic tumour cells and their cellular radiosensitivity have a major impact on local control after fractionated radiotherapy.

The effects of ZD1839 (Iressa), a highly selective EGFR tyrosine kinase inhibitor, as a radiosensitiser in bile duct carcinoma cell lines

The signaling pathway that is initiated by binding of epidermal growth factor receptor (EGFR) and results in sustained signaling through PI3K plays an important role in a tumor's response to ionizing radiation. The current in vitro study explored both the effects of ZD1839 (Iressa), a highly selective EGFR tyrosine kinase inhibitor, as a radiosensitiser for bile duct carcinoma cell lines and ZD1839's general effects on cell growth in the same two lines. Secondly, we ensured suppression of radiation-induced phosphorylation of EGFR by ZD1839 using an immunoprecipitation technique. Furthermore, we examined radiation-induced phosphorylation of ERK, p38, JNK, and AKT with or without inhibitor with use of Western blot techniques and performed clonogenic assays to confirm radiosensitivity in the presence of a drug. ZD1839 inhibited cell growth of both cell lines and suppressed radiation-induced phosphorylation of EGFR. After exposure to radiation, there was an increase in phosphorylation of AKT as shown by Western blot. Treatment with either ZD1839 or LY294002 (the latter, a PI3K inhibitor) suppressed phosphorylation of AKT by Western blot. Both ZD1839 and LY294002 significantly suppressed colony formation by clonogenic assay; however, U0126 (a MEK1/2 inhibitor), SB203580 (a p38 inhibitor), and SP600125 (a JNK inhibitor) had no effect on colony formation. These results suggest that AKT may be a useful target molecule for enhancement of radiotherapy effect and that ZD1839 may have an important role in combination with radiotherapy for patients with bile duct carcinoma.

Optimal combination chemotherapy and chemoradiotherapy with etoposide for advanced cervical squamous cancer cells in vitro

Patients with unresectable advanced carcinoma of the uterine cervix are usually treated with chemotherapy or chemoradiotherapy. In the present study, the optimal administration protocol for etoposide in chemotherapy and chemoradiotherapy for advanced cervical cancer patients was investigated in vitro using the radio-sensitive and anticancer drug-sensitive human cervical squamous cell carcinoma cell line ME180. Therapeutic doses of concurrent irradiation reduced the cellular etoposide sensitivity in a dose-dependent manner, while postirradiation-surviving subclones established from repeatedly irradiated ME180 cells showed significantly higher etoposide sensitivities than the non-irradiated parent cells. Of the 6 monoclonal etoposide-resistant subclones established from ME180 cells, 5 were significantly radioresistant. Although the etoposide-resistant subclones were also significantly resistant to other anticancer drugs, such as cisplatin, carboplatin, nedaplatin, pirarubicin, paclitaxel and docetaxel, they were more sensitive to 5-fluorouracil, mitomycin C and SN38 than the parent cells. Flow cytometric analyses revealed that the etoposide-resistant subclones showed significantly increased cell surface expression of CD40 compared to the parent cells, which expressed undetectable levels of CD40. However, the expression of some integrin receptor subunits, such as CD29, CD49a and CD49f, was apparently reduced in the etoposide-resistant subclones. These results indicate that etoposide should be administered to advanced cervical squamous cancer patients after the completion of radiotherapy, rather than as a concurrent chemoradiotherapy. In order to kill surviving etoposide-resistant cancer cells more effectively, 5-fluorouracil, mitomycin C and irinotecan may be candidate combination drugs for use with etoposide. Differential expression of integrin receptors and CD40 may be involved in the acquisition of etoposide resistance by cervical squamous cancer cells.

Evasion of a Single-Step, Chemotherapy-Induced Senescence in Breast Cancer Cells: Implications for Treatment Response

PURPOSE

The purpose of this study is to define the mechanistic basis for recovery of proliferative capacity in breast tumor cells after chemotherapy. Here, we test the hypothesis that evasion of senescence confers resistance to chemotherapeutic drugs and ionizing radiation.

EXPERIMENTAL DESIGN

MCF-7 cells were treated with a single, clinically relevant dose (0.75-1.0 micromol/L) of Adriamycin. Two weeks following induction of senescence, clonal outgrowths were expanded and characterized in terms of senescence-associated beta-galactosidase activity, gene expression profiles (Affymetrix U95 probe sets, Affymetrix, Santa Clara, CA) with confirmatory Western analyses, and telomerase activity following a second drug treatment. Levels of intracellular Adriamycin, as well as cross-resistance to other therapeutic agents, were also determined to define the resistance phenotype.

RESULTS

A senescence-resistant (SR) clone (clone 2) was identified that was largely refractory to both Adriamycin-induced and gamma-irradiation-induced senescence. Clone 2 continued to proliferate and maintain high levels of telomerase activity following a second drug treatment, when treated parental cells expressed very low levels of telomerase and many positive cell cycle regulators. SR clone 2 also expressed substantially more cdc-2 than parental cells and undetectable levels of MDR1, showed an intact p53 checkpoint and only a modestly lower level of intracellular drug accumulation, while exhibiting cross-resistance to other topoisomerase inhibitors.

CONCLUSIONS

SR clone 2 is intrinsically resistant to DNA damage-induced senescence perhaps through an ability to prevent down-regulation of cdc-2. Telomerase is a marker of proliferative recovery for breast cancer cells after chemotherapy exposure. Evasion or escape from a single-step, drug-induced senescence may represent a unique and previously unrecognized drug-resistance phenotype.

Apoptosis and clonogenic survival in three tumour cell lines exposed to gamma rays or chemical genotoxic agents

We compared the extent to which apoptosis is induced and clonogenicity reduced in three tumour cell lines - the human melanoma Me45 and promyelocytic leukaemia HL-60, and the rat rhabdomyosarcoma R1 - after exposure to the anticancer drugs etoposide and cis-platinum or to gamma radiation; each induces different types of DNA damage. Cells which readily underwent apoptosis did not necessarily show a correlated loss of clonogenicity; for example, Me45 cells showed the highest sensitivity to all three agents in clonogenic assays but much lower levels of apoptotic cells than R1 or HL-60 cells. These results show that the efficiency of the eradication of clonogenic cells by genotoxic agents does not solely depend on the induction of apoptotic processes, and suggest that the induction of apoptosis and suppression of clonogenicity are independent processes.

Human fibroblasts expressing hTERT show remarkable karyotype stability even after exposure to ionizing radiation

Ectopic expression of telomerase results in an immortal phenotype in various types of normal cells, including primary human fibroblasts. In addition to its role in telomere lengthening, telomerase has now been found to have various functions, including the control of DNA repair, chromatin modification, and the control of expression of genes involved in cell cycle regulation. The investigations on the long-term effects of telomerase expression in normal human fibroblast highlighted that these cells show low frequencies of chromosomal aberrations. In this paper, we describe the karyotypic stability of human fibroblasts immortalized by expression of hTERT. The ectopic overexpression of telomerase is associated with unusual spontaneous as well as radiation-induced chromosome stability. In addition, we found that irradiation did not enhance plasmid integration in cells expressing hTERT, as has been reported for other cell types. Long-term studies illustrated that human fibroblasts immortalized by telomerase show an unusual stability for chromosomes and for plasmid integration sites, both with and without exposure to ionizing radiation. These results confirm a role for telomerase in genome stabilisation by a telomere-independent mechanism and point to the possibility for utilizing hTERT-immortalized normal human cells for the study of gene targeting.

No photoperiodoc control of the formation of turions in eight clones of Spirodela polyrhiza

The influence of daily photoperiod (8, 16, 24 h) on eight clones of Spirodela polyrhiza was tested in two different nutrient media. The number of vegetative fronds and resting turions formed after 50 days of cultivation were scored. The specific turion yield (STY; number of turions formed per vegetative frond) was used to evaluate the effectiveness of turion formation of the tested clones. All clones formed turions in both nutrient media. The STY varied substantially between the different clones, ranging from 0.22 +/- 0.03 (clone SC from Cuba) to 3.9 +/- 0.3 (clone 9256 from Finland) in continuous light. The STY increased with increasing duration of the photoperiod. This increase may have been due to the extended period of photosynthesis rather than that of a photoperiodic long-day response. Shorter photoperiods did not stimulate turion formation in any of the clones. S. polyrhiza is a day-neutral plant with respect to turion formation, as noted previously (Appenroth et al. 1990. Annals of Botany 66: 163-168). In accordance with this conclusion, no correlation was detected between the STY and the latitude at which the clones occur naturally. Environmental factors other than shortening of photoperiods seem to be effective in signalling seasonal changes of growth conditions in advance to S. polyrhiza.

Radiation dose response for supratentorial low-grade glioma--institutional experience and literature review

PURPOSE

To examine radiation dose response for low-grade glioma (LGG) based on our institutional experience and to review the literature on this topic.

METHODS AND MATERIALS

Sixty-seven patients with supratentorial low-grade nonpilocytic astrocytomas (n=36) or oligodendrogliomas (n=31) were treated with postoperative radiation therapy (RT). Twenty-seven patients (group A) received 5520 cGy; 24 patients (group B) received 5940 cGy; and 16 patients (group C) received 6375 cGy. The corresponding median follow-up was 60, 35 and 91 months, respectively.

RESULTS

The disease-specific survival (DSS) at 5 and 10 years were 90.2% and 56.2%, 67.6% and 47.3%, and 62.5% and 50% for groups A, B and C, respectively (P=0.40). Only a greater extent of surgical resection and absence of contrast enhancement predicted DSS on multivariate analyses. Patients receiving higher doses of RT had higher complication rates.

CONCLUSION

Our data confirmed the lack of radiation dose response for supratentorial LGG as demonstrated in the previous randomized trials. The radiation dose should not exceed 5520 cGy because dose escalation did not result in an improvement of DSS and it also increased the complication rates. Future research should focus on the eradication of radioresistant clones either by the improvement of surgical resection or the use of cytotoxic agents that can target on the radioresistant tumor cells.

Biological response to radiation therapy

In an investigation by the Swedish Cancer Society, the present status, critical issues and future aspects and potentials were described by an expert group for each of nine major areas of radiation therapy research. This article deals with biological response to radiation. Separate sections deal with molecular responses to radiation, the stem cell and clonogenic cell concepts and the importance of cell proliferation, cell and tissue responses to doses above and below 1 Gy, respectively, the potential role of intercellular signalling pathways, the so-called bystander effect and radiation biology-based therapy planning and treatment optimization.

Persistent oxidative stress in chromosomally unstable cells

Past work using the human-hamster hybrid line GM10115 has demonstrated that exposure to a variety of DNA damaging agents can lead to the persistent destabilization of chromosomes. To gain insight into the potential biochemical mechanisms involved in perpetuating the unstable phenotype, groups of clones characterized as stable or unstable were analyzed for indications of oxidative stress. All of the clones were derived from single progenitor cells surviving exposure to ionizing radiation or chemicals. Compared with their stable counterparts, unstable clones possessed elevated levels of reactive oxygen species (ROS) as measured by their enhanced ability to oxidize fluorogenic dyes. Fluorescence automated cell sorting analysis indicated that unstable clones had significantly higher mean fluorescence signals of approximately 2-fold and approximately 1.25-fold, respectively, as derived from the dyes 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate and dihydrorhodamine 123, respectively. To determine whether mitochondria might constitute a potential source of ROS, stable and unstable clones of cells were analyzed for mitochondrial content using nonyl acridine orange and function using rhodamine 123. Fluorescence automated cell sorting data indicated that compared with stable clones, unstable clones possessed an elevated number (15% increase in mean nonyl acridine orange fluorescence) of dysfunctional mitochondria (27% decrease in mean rhodamine 123 fluorescence). Interestingly, the consequences of elevated ROS did not translate to an increase in oxidative base damage in nuclear DNA. Analysis of nine different base damage adducts by gas chromatography/mass spectrometry did not reveal significant differences between stable and unstable clones. The data suggest that the perpetuation of many of the abnormal phenotypes associated with genomic instability may be linked to a state of chronic oxidative stress derived in part from dysfunctional mitochondria.

Ex vivo drug and irradiation sensitivities in hypermutated and unmutated forms of chronic lymphocytic leukemia cells

Several investigators have now established that chronic lymphocytic leukemia (CLL) is not a uniform disease entity, since approximately half of the cases of CLL have undergone immunoglobulin V region (IgV) hypermutation, whereas the other half display unmutated Ig genes. The median survival time of mutated CLL (M-CLL) cases has been shown to be approximately twice as long as that for unmutated CLL (UM-CLL), but no clear explanation for this difference is currently available. In this work, we have investigated a cohort of previously untreated CLL patients, to see whether the ex vivo sensitivities of leukemic cells of 16 UM-CLL patients differ from those of 8 M-CLL patients, using nine different drugs and two types of irradiation. Our results demonstrated very similar ex vivo sensitivities and tumor cell heterogeneity of sensitivity of UM-CLL and M-CLL cells when tested against chlorambucil, 2-chloro-2'-deoxyadenosine, cyclosporin A, cis-platinum(II)diammine-dichloride, doxorubicin hydrochloride, 2-fluoroadenine-9-beta-D-arabinofuranoside, prednisolone sodium succinate, verapamil, vincristine, gamma-irradiation, and UV-irradiation. This indicates that de novo chemo/radiosensitivity cannot explain the survival difference observed between UM-CLL and M-CLL.

A prospective study on the evolution of the T-cell repertoire in patients with Sézary syndrome treated by extracorporeal photopheresis

Sézary syndrome is a leukemic form of epidermotropic cutaneous T-cell lymphoma related to the malignant proliferation of clonal CD4(+) T cells. Extracorporeal photochemotherapy may induce a transient improvement of the clinical signs, but its efficiency is discussed. To investigate the frequency of the T-cell clone in the peripheral blood of patients with Sézary syndrome and to monitor its evolution in patients treated using extracorporeal photopheresis or chemotherapy, we used the immunoscope technique. In one patient, we observed a decrease of the relative frequency of the clone from 15.6% to 0%, paralleling a complete remission of the clinical disease and a disappearance of the circulating Sézary cells. In the other cases, the evolution of the relative frequency paralleled the initial improvement of the clinical status and the absence of long-term efficiency in patients treated with extracorporeal photopheresis or chemotherapy. We observed a quick-acting direct cytotoxicity of the association 8MOP + UVA on the T-cell clone. The immunoscope technique appears to be an efficient tool to appreciate the amount of tumoral cells and to monitor the evolution of the clonal component in the Sézary syndrome.

The phosphatidylinositol-3 kinase pathway is not essential for insulin-like growth factor I receptor-mediated clonogenic radioresistance

The insulin-like growth factor I receptor (IGF-IR) is known to induce clonogenic radioresistance in cells following ionizing irradiation. To explore the downstream signaling pathways, we focused on the phosphatidylinositol-3 kinase (PI3-K) pathway, which is thought to be the primary cell survival signal originating from the receptor. For this purpose, R- cells deficient in the endogenous IGF-IR were used as a recipient of the human IGF-IR with or without mutations at potential PI3-K activation sites: NPXY950 and Y1316XXM. Mutants with double mutation at Y950/Y1316 exhibited not abrogated, but reduced activation of IRS-1, PI3-K, and Akt upon IGF-I stimulation. However, the mutants had the same clonogenic radioresistance as cells with wild type (WT) receptors. Neither wortmannin nor LY294002, specific inhibitors of PI3-K, affected the radioresistance of cells with WT receptors at concentrations specific for PI3-K. Collectively, these results indicate that the PI3-K pathway is not essential for IGF-IR-mediated clonogenic radioresistance.

Genomic instability in human lymphoid cells exposed to 1 GeV/amu Fe ions

The goal of this study was to assess whether charged particle radiations of importance to spaceflight elicit genomic instability in human TK6 lymphoblasts. The incidence of genomic instability in TK6 cells was assessed ~21 days after exposure to 2, 4, or 6 Fe ions (1 GeV/amu, LET= 146 keV/micrometers). Three indices of instability were used: intraclonal karyotypic heterogeneity, mutation rate analysis at the thymidine kinase (TK1) locus, and re-cloning efficiency. Fifteen of sixty clones demonstrated karyotypic heterogeneity. Five clones had multiple indicators of karyotypic change. One clone was markedly hypomutable and polyploid. Six clones were hypomutable, while 21 clones were mutators. Of these, seven were karyotypically unstable. Six clones had low re-cloning efficiencies, one of which was a mutator. All had normal karyotypes. In summary, many clones that survived exposure to a low fluence of Fe ions manifested one or more forms of genomic instability that may hasten the development of neoplasia through deletion or by recombination.

Unexpected sensitivity to radiation of fibroblasts from unaffected parents of children with hereditary retinoblastoma

The response to ionizing radiation was examined in diploid skin fibroblasts derived from 5 patients with hereditary type retinoblastoma as well as their parents. Unexpected sensitivity to cell killing, as measured by clonogenic survival, as well as enhanced radiation-induced G(1) arrest were observed in at least 1 parental fibroblast strain in all 5 families. In all cases, parental strains were equally or more radiosensitive than the probands. The mutation of the retinoblastoma gene (RB) determined in 4 of 5 probands was either absent from the parental cells, as expected from the negative family histories, or identical, in 1 father who was a known carrier. In the fifth family, the family history was negative for retinoblastoma. We hypothesize that the increased parental cell sensitivity to radiation suggests the presence of an as yet unrecognized genetic event occurring in 1 or both parents of children with retinoblastoma. Whether it increases mutability of the RB locus or other loci or interacts with RB is conjectural.

Myogenic cell proliferation and generation of a reversible tumorigenic phenotype are triggered by preirradiation of the recipient site

Environmental influences have profound yet reversible effects on the behavior of resident cells. Earlier data have indicated that the amount of muscle formed from implanted myogenic cells is greatly augmented by prior irradiation (18 Gy) of the host mouse muscle. Here we confirm this phenomenon, showing that it varies between host mouse strains. However, it is unclear whether it is due to secretion of proliferative factors or reduction of antiproliferative agents. To investigate this further, we have exploited the observation that the immortal myogenic C2 C12 cell line forms tumors far more rapidly in irradiated than in nonirradiated host muscle. We show that the effect of preirradiation on tumor formation is persistent and dose dependent. However, C2 C12 cells are not irreversibly compelled to form undifferentiated tumor cells by the irradiated muscle environment and are still capable of forming large amounts of muscle when reimplanted into a nonirradiated muscle. In a clonal analysis of this effect, we discovered that C2 C12 cells have a bimodal propensity to form tumors; some clones form no tumors even after extensive periods in irradiated graft sites, whereas others rapidly form extensive tumors. This illustrates the subtle interplay between the phenotype of implanted cells and the factors in the muscle environment.

The radiation-induced bystander effect for clonogenic survival

It has long been accepted that the radiation-induced heritable effects in mammalian cells are the result of direct DNA damage. Recent evidence, however, suggests that when a cell population is exposed to a low dose of alpha particles, biological effects occur in a larger proportion of cells than are estimated to have been traversed by alpha particles. Experiments involving the Columbia University microbeam, which allows a known fraction of cells to be traversed by a defined number of alpha particles, have demonstrated a bystander effect for clonogenic survival and oncogenic transformation in C3H 10T(1/2) cells. When 1 to 16 alpha particles were passed through the nuclei of 10% of a C3H 10T(1/2) cell population, more cells were unable to form colonies than were actually traversed by alpha particles. Both hit and non-hit cells contributed to the outcome of the experiments. The present work was undertaken to assess the bystander effect of radiation in only non-hit cells. For this purpose, Chinese hamster V79 cells transfected with hygromycin- or neomycin-resistance genes were used. V79 cells stably transfected with a hygromycin resistance gene and stained with a nuclear dye were irradiated with the charged-particle microbeam in the presence of neomycin-resistant cells. The biological effect was studied in the neomycin-resistant V79 cells after selective removal of the hit cells with geneticin treatment.

Alpha-particle-induced increases in the radioresistance of normal human bystander cells

Numerous investigators have reported that direct exposure of cells to a low dose of ionizing radiation can induce a condition of enhanced radioresistance, i.e. a "radioadaptive" response. In this report, we investigated the hypothesis that a radioadaptive bystander effect may be induced in unirradiated cells by a transmissible factor(s) present in the supernatants of cells exposed to a low dose of alpha particles. Normal human lung fibroblasts (HFL-1) were irradiated with 1 cGy of alpha particles and their supernatants were transferred to unirradiated HFL-1 cells as a bystander cell model. Compared to directly irradiated cells that were not treated with supernatants from HFL-1 cells exposed to low-dose radiation, such treatment resulted in increased clonogenic survival after subsequent exposure to 10 and 19 cGy of alpha particles. Increases in protein levels of AP-endonuclease, a redox and DNA base excision repair protein, were found in the bystander cells, but not in directly irradiated cells. Supernatants from alpha-particle-irradiated cells were also found to increase the clonogenicity of unirradiated cells. These results, in conjunction with our earlier findings that supernatants from cells exposed to a low dose of alpha particles contain growth-promoting activity, suggest that this new bystander effect may be related to an increase in DNA repair and cell growth/cell cycle regulation.

Identification of radiation-responsive genes in vitro using a gene trap strategy predicts for modulation of expression by radiation in vivo

A large number of genes are known to be responsive to ionizing radiation, and there is strong evidence for the existence of inducible radiation resistance in mammalian cells. We have developed a gene trap insertional mutagenesis strategy to identify novel genes involved in responses to radiation. Using this approach, we have isolated four gene-trap integrations in embryonic stem cells. In three cases (9A, 3E and 9H) the trapped genes are radiation-inducible, and in one (7D) the gene is down-regulated. Sequence analysis of fusion transcripts from three of the integrations indicate one novel gene (3E), the mouse homologue (9A) of a known but uncharacterized human gene that encodes a protein with significant homology to several GTPase-activating proteins and a murine locus, Mym (9H). The embryonic stem cell clone with the 9A insertion was introduced into the mouse germline, and the in vivo expression pattern of 9A was studied in detail. A unique, spatially restricted pattern of expression in embryos and adult animals was observed. There is tissue-specific in vivo induction of the 9A gene in adult mice by radiation. This study demonstrates the potential of the gene trap approach for the identification and functional analysis of novel radiation-regulated genes. Similar strategies may facilitate the discovery and characterization of genes involved in other cellular stress responses.

Chromosome instability induced in the cell progeny of human T lymphocytes irradiated in G(0) with gamma-rays

We report the occurrence of chromosome instability in human T lymphocytes irradiated in vitro with gamma-rays and cultured for several generations before analysis. The delayed effects of gamma-radiation have been evaluated by conventional and molecular (chromosome painting) cytogenetics in preparations obtained from long-term bulk cultures or clonal cultures. The results indicate that the cell progeny of gamma irradiated human T lymphocytes can be characterized by a higher rate of chromosome damage, but this effect depends on the individual donor response to ionizing radiation. Evidence has been collected about a differential involvement of chromosomes 7, 9 and 19 in the induced chromosome rearrangements, and this effect is equally visible as an immediate or delayed response of human T lymphocytes to ionizing radiation.

Antiapoptotic activity is dispensable for insulin-like growth factor I receptor-mediated clonogenic radioresistance after gamma-irradiation

PURPOSE

The purpose of this study was to evaluate the relationship between apoptotic activity and clonogenic radiosensitivity in vitro using an insulin-like growth factor I receptor (IGF-IR) signaling model, which is known to exert tumorigenic and antiapoptotic effects.

EXPERIMENTAL DESIGN

We used mouse embryo fibroblast cell lines expressing either human IGF-IR [R+(Wt) and R+] or the marker gene alone [R-(puro)]; these cell lines were derived from R- cells, which are deficient in IGF-IR. After gamma-irradiation, apoptotic activity was determined by the presence of DNA fragmentation and caspase-3-, -8-, and -9-like activities. Clonogenic radiosensitivity was determined by a colony-forming assay.

RESULTS

R+(Wt) and R+ cells expressed similar levels of IGF-IR, transducing phosphorylation signals to major downstream substrates on insulin-like growth factor I stimulation. R+ cells were resistant to the induction of apoptosis after gamma-irradiation; however, both R+(Wt) and R-(puro) cells demonstrated significant DNA fragmentation and increase in caspase-3-, -8-, and -9-like activities. Both R+(Wt) and R+ cells were radioresistant (to a similar extent) compared with R-(puro) cells as measured by a colony-forming assay. Clonogenic radioresistance was not influenced by the inhibition of Akt/protein kinase B through treatment with wortmannin at low concentrations specifically inhibiting phosphatidylinositol 3'-kinase.

CONCLUSIONS

Our findings indicate that apoptotic activity does not necessarily predict clonogenic survival after exposure to ionizing radiation. This study provides clinical implications in the evaluation of apoptotic activities observed during the course of radiotherapy to predict accurate tumor response or local control.

[Radiation sensitivity for delayed reproductive death (DRD) following single or split-dose irradiation]

BACKGROUND

The growth kinetics of tumors after irradiation (Figure 1) is defined by cells surviving with delayed reproductive death (DRD). The prediction of radiation sensitivity of locally recurrent tumor growth is among other factors dependent on the knowledge of the impact of fractionated irradiation on these surviving cells with DRD. Short recovery effects can be estimated in vitro by comparing the difference of the medians of the distributions of clone sizes, the median clone sizes difference (MCD) after single and split exposure irradiation of the progenitor cells.

MATERIALS AND METHODS

CHO-cells of a sub clone of the line T71 have a spontaneous cell loss rate of < 5%. The cells were irradiated a) by a single exposure 3 hours after synchronization and b) by a fractionated irradiation of half the exposure of a 200-kVp radiation exposure 3 hours and 6 hours after synchronization, respectively. Clone survival was determined (Figure 2). As function of dose and incubation time the distributions of clone sizes and the MCD were determined by tapping the clone cells in microscopic projections.

RESULTS

The radiation sensitivity El of the DRD can be defined as the proportional factor of the linear relationship between the MCD on one side and the dose K x the cell division factor m on the other side. EI is dependent on the age of the cells during irradiation (Figure 3) and the cell line (Table 1). The slope of the dually logarithmic growth curve of the cell population is: s = 1 - El.K. Experimentally El was found to be equal for single and split dose irradiation (Figures 4 and 5) and amounted to El = 0.065 with Sd = +/- 0.004.--Literature analysis for the mathematical estimation of El.K (Figure 6) was based on reports of measurements of the local tumor recurrence growth of carcinomas and sarcomas of rodents and pulmonary metastases of sarcomas in humans, respectively, after fractional irradiation. We obtained values of 0 < or = El.K < or = 0.77 (Table 2). Values for El are independent of the dose and lie considerably below data derived from in-vitro measurements of different cell cultures.

CONCLUSIONS

Since recurrence kinetics of tumors are determined by the radiation sensitivity El of the DRD, El can be used for estimating the kinetics of tumor recurrence. As lately described, MCD is linearly proportional to the micro-nucleus frequency [12]. Determinations of the micro-nucleus frequencies in tumor cell biopsies pre and post radiation onset offer the option for developing a fast predictive assay. Organ malformations of embryos after exposition to ionizing radiation can be mathematically deduced by DRD to the partial cell mortality.

Characteristics of chromosome instability in the human lymphoblast cell line WTK1

The characteristics of spontaneous and radiation-induced chromosome instability were determined in each of 50 individual clones isolated from control populations of human lymphoblasts (WTK1), as well as from populations of these cells previously exposed to two different types of ionizing radiation, Fe-56 and Cs-137. The types of chromosome instability did not appear to change in clones surviving radiation exposure. Aneuploidy, polyploidy, chromosome dicentrics and translocations, and chromatid breaks and gaps were found in both control and irradiated clones. The primary effect of radiation exposure was to increase the number of cells within any one clone that had chromosome alterations. Chromosome instability was associated with telomere shortening and elevated levels of apoptosis. The results suggest that the proximal cause of chromosome instability is telomere shortening.

Repetitious appearance and disappearance of different kinds of clonal cytogenetic abnormalities after allogeneic bone marrow transplantation

We report a childhood case that showed the repeated appearance and disappearance of various kinds of cytogenetic abnormalities (CA) for 5.5 years after allogeneic bone marrow transplantation (BMT). The patient underwent allogeneic BMT from an HLA-matched unrelated donor during the second complete remission of acute lymphoblastic leukemia. The conditioning regimen for BMT consisted of etoposide, cyclophosphamide, anti-human thymocyte immunoglobulin, and total body irradiation. There were no leukemic relapses or secondary acute myeloid leukemia/myelodysplastic syndrome (AML/MDS) since the BMT. The CA occurred from residual recipient cells, which were damaged by chemotherapy or radiation prior to BMT. Although previous studies about post-BMT CA had reported the continuous emergence of identical clones, the present case showed the appearance of one different type of clone after another. Although the appearance of different types of CA may mean that these clones did not obtain any growth advantages, it may be a sign of genomic instability, which is probably a risk factor for the development of secondary AML/MDS.

Propagation of rat parvovirus in thymic lymphoma cell line C58(NT)d and subsequent appearance of a resistant cell clone after lytic infection

Rat parvovirus (RPV) is nonpathogenic in rats but causes persistent lymphocytotropic infection. We found that RPV was propagated in rat thymic lymphoma cell line C58(NT)D and induced apoptosis. Interestingly, a resistant subclone, C58(NT)D/R, from surviving cells after lytic infection had differentiated phenotypic modifications, such as increased cell adherence, resistance to apoptosis, and suppressed tumorigenicity.

Long-term efficacy of intracoronary irradiation in inhibiting in-stent restenosis

BACKGROUND

Intracoronary irradiation is a promising modality for inhibition of in-stent restenosis. Results of randomized clinical trials at 6 months after gamma ray irradiation are highly encouraging. The first results at 3 years after irradiation, while still showing benefit, have shown significant late loss. The probable mechanism of the radiation is to inactivate (prevent from dividing) most cells that otherwise could proliferate to produce neointimal formation. We measured the proportion of cells that survive with their clonogenic potential intact after the doses and dose rates used in the randomized trials, and we then modeled the subsequent repopulation of the surviving cells that might cause late restenosis.

METHODS AND RESULTS

Human aortic smooth muscle cells were irradiated with gamma rays, including the doses and dose rates used in current trials, and clonogenic surviving fractions were measured. The subsequent repopulation of the surviving cells was modeled with the assumption that the repopulation kinetics were similar to those in unirradiated cells. The radiation is expected to delay the time to restenosis by factors of approximately 6 to 8, depending on the dose, shifting the delay from a median of 6 months (for no irradiation) to median values from 36 months (for a nominal 13 Gy) to 43 months (for a nominal 15 Gy).

CONCLUSIONS

These results and predictions are quantitatively consistent with clinical results and suggest that clonogenic inactivation (prevention of cellular division) is the dominant mechanism of radiation action in the delay of restenosis. Intracoronary radiotherapy is a very promising modality for significantly delaying, although probably not preventing, in-stent restenosis.

Adenoviral p53 gene therapy promotes heat-induced apoptosis in a nasopharyngeal carcinoma cell line

BACKGROUND

It has previously been demonstrated that Ad5CMV-p53 gene transfer, either used alone or delivered concomitantly with ionizing radiation, resulted in cytotoxicity mediated by apoptosis in nasopharyngeal carcinoma (NPC) cell lines. In this study, a novel approach was evaluated of combining Ad5CMV-p53 gene therapy with hyperthermia (HT), in the CNE-1 NPC cell line, which harbours a mutation in codon 249 of the p53 gene.

MATERIALS AND METHODS

CNE-1 cells were infected using either Ad5CMV-p53 or Ad5CMV-B-gal, followed, 24 h later, by HT (43 degrees C x 0-2 h). Protein was extracted for Western blot analysis, and apoptosis was evaluated using acridine-orange ethidium bromide staining, followed immediately by fluorescent microscopy examination for the proportion of cells displaying morphologic features of apoptosis.

RESULTS

Ad5CMV-p53 gene therapy combined with HT resulted in a dose-dependent cytotoxicity with less than 1% clonogenic survival when 10 pfu/cell of Ad5CMV-p53 was combined with 2 h heating at 43 degrees C. Western blotting demonstrated that treatment with Ad5CMV-p53 resulted in the rapid expression of p53, which was minimally affected by HT. The inducible form of hsp70 was maximally expressed at 48 h post-HT, with minimal effect when cells were additionally treated with Ad5CMV-p53. Clonogenic cytotoxicity was associated with the development of apoptosis, with up to 70% of CNE-1 cells displaying morphologic features of apoptosis after the combination treatments.

CONCLUSION

Based on the shapes of the clonogenic survival curves, Ad5CMV-p53 gene therapy and HT appear to interact in an additive manner, suggesting the therapeutic potential of this combined treatment approach for patients with NPC.

Partial characterization of SUVi, a new mammalian gene induced by UV-C and expressed during the S phase of the cell cycle

By using a lacZ-based gene-trap approach, we identified a mammalian gene induced by UV-C in a Chinese hamster ovary cell clone (Menichini P et al. [1997]: Nucleic Acids Res 25:4803-4807). The activity of the encoded protein fused to a bacterial beta-galactosidase was followed through the hydrolysis of different beta-galactosidase substrates. In this study we describe how the expression of this gene is modulated during the cell cycle and in response to UV-irradiation. We show that the beta-galactosidase activity was virtually undetectable in quiescent cells (G[0]), started to increase when cells progressed in G(1), and reached a maximum in mid-S phase, indicating a possible role of the endogenous protein during DNA synthesis. Following UV-irradiation, besides a delay of the progression through the S phase, a twofold increase of the reporter protein activity in all phases of the cell cycle was observed. The partial sequence analysis showed that this gene, here named SUVi (for S phase UV-inducible), contains a domain that is highly conserved among different helicases. Together, these data suggest that the SUVi gene could be involved in DNA synthesis, a process that takes place both in the S phase and in the processing of UV-induced damage.

Hypoxia relieves X-ray-induced delayed effects in normal human embryo cells

We studied the effect of hypoxia on X-ray-induced delayed effects in normal human embryo cells to elucidate the role of oxidative stress in the susceptibility of cells to induction of genetic instability by radiation. We examined X-ray-induced delayed cell death, giant cell formation, and chromosome aberrations under normally oxygenated (20%) and hypoxic (2%) conditions at 28-38 population doublings postirradiation. The results revealed that hypoxia reduced the X-ray-induced delayed effects, suggesting that radiation enhances cellular oxidative stress, which plays a significant role in determining the susceptibility of irradiated cells to genetic instability. The present study emphasizes the biological significance of epigenetic effects, such as oxygen tension, as well as direct DNA damage in the induction of genetic instability by radiation.

Pretreatment of endometrial carcinoma cell lines with butyrate results in upregulation of Bax and correlates with potentiation of radiation induced cell kill

BACKGROUND

Endometrial carcinoma is the most common gynecologic malignancy. Surgery has been the standard therapy for stage I and II endometrial carcinoma and radiation therapy either before or after surgery has been used to improve local control especially for high grade lesions. We have used Sodium Butyrate (BA) in order to examine whether endometrial carcinoma cells can be rendered more sensitive to radiation therapy.

METHODS

Endometrial carcinoma cells in culture were pretreated with sodium butyrate and then irradiated. Clonogenic survival assay was used to determine percentage of surviving cells. Changes in Bax and Bcl-2 protein levels were determined by Western blot analyses. Effect of Bcl-2 overexpression on induction of Bax in response to butyrate pretreatment was studied in cells transfected with Bcl-2.

RESULTS

A 24 h pretreatment of SKUT2 and Hec-1A cells with BA has an additive effect with radiation. Analysis of pro and anti-apoptotic protein levels revealed that the 24 h pretreatment with BA resulted in increased expression of the proapoptotic protein Bax which correlated with potentiation of radiation induced cell kill. Treatment of cells over expressing Bcl-2 with BA did not show induction of Bax suggesting that higher levels of Bcl-2 can block butyrate induced increase in levels of Bax.

CONCLUSIONS

Use of BA at lower than toxic doses to upregulate the proapoptotic potential of cancer cells may be useful in an adjuvant or neoadjuvant setting but its success may depend upon the intrinsic Bcl-2 levels in the tumor.

Relationship between cellular radiosensitivity and non-repaired double-strand breaks studied for different growth states, dose rates and plating conditions in a normal human fibroblast line

PURPOSE

The aim of this study was to test under which conditions non-repaired DNA double-strand breaks (dsb) could be used as an indicator of cellular radiosensitivity of normal human fibroblasts.

MATERIALS AND METHODS

The experiments were performed with a primary normal skin fibroblast line (NFHH) derived from a healthy donor. Cells were X-irradiated either in exponential or confluent state with high (4 Gy/min) or low dose rate (0.04 Gy/min) and either plated immediately or delayed after irradiation. The fraction of clonogenic cells was determined after doses up to 12 Gy using colony forming assay and the number of non-repaired dsb were measured 24 h after X-irradiation with doses up to 180 Gy using constant-field gel electrophoresis.

RESULTS

Cellular radiosensitivity of NFHH cells was found to depend on all three conditions tested. In contrast, the number of non-repaired dsb was found to depend on dose rate and growth state only. There were, however, no differences for the plating conditions tested. This result was attributed to the almost complete inhibition of cell-cycle progression when cells were plated immediately after irradiation. For the two dose rates and growth conditions, differences in non-repaired dsb were found to correspond with the respective differences measured for the cellular radiosensitivity, and these data agreed fairly well with the correlation previously found for 11 fibroblast lines varying in dsb repair capacity.

CONCLUSIONS

For irradiation followed by delayed plating only, non-repaired dsb can be used to predict the cellular radiosensitivity.

Cellular communication in clone 9 cells exposed to magnetic fields

Magnetic-field exposure (45 Hz B(a.c.) over a flux density range of 7.7 to 49.9 microT r.m.s. with parallel B(d.c.) of 36.6 microT) has been reported by Blackman and coworkers to inhibit gap junction intercellular communication in Clone 9 cells treated with chloral hydrate for 24 h prior to field exposure in accord with predictions of the ion parametric resonance model. The study reported here is an attempt to reproduce this effect. Baseline experiments showed that growth in culture and state of confluence at time of addition of chloral hydrate were comparable in both laboratories. PMA inhibited cell-cell communication in a dose-dependent manner, similar to the results of Blackman and coworkers, whereas cells in the present study were somewhat more sensitive to chloral hydrate than reported by Blackman and coworkers. A total of 38 exposure experiments were undertaken using a 45 Hz magnetic field with a flux density of 23.8 microT r.m.s., in parallel with a 36.6-microT static magnetic field for 40 to 45 min, after pretreatment with 2.5 mM chloral hydrate for 24 h. In 14 unblinded experiments, a small but statistically significant effect of magnetic-field exposure was observed, but due to the subjective nature of the assay, it was deemed essential to carry out blinded experiments. The remaining 24 experiments were blinded. In 15 blinded experiments, cells purchased from the American Type Culture Collection and grown only in this laboratory were used, while in 9 experiments, the cells had originally been grown in Blackman's laboratory and were subsequently sent to this laboratory. There was no statistically significant effect of magnetic-field exposure on gap junction intercellular communication in these blinded experiments using either cell line.

Overexpression of p21 protein in radiation-transformed mouse 10T(1/2) cell clones

In order to investigate the hypothesis that aberrant expression of cell-cycle regulatory proteins may represent early events in the process of carcinogenesis, levels of expression of the negative regulators p21(waf1/cip1) (p21), p27(kip1) (p27), and p16(ink4a) (p16) and/or the positive regulators cyclin D(1) and cyclin E were examined by western blot analysis in cells transformed in vitro by ionizing radiation. The levels of these proteins in 12 independently derived mouse 10T(1/2) cell clones transformed by 1.5 Gy of alpha radiation were compared with those in nine similarly derived nontransformed control clones. Constitutive levels of p21 were very low in all control clones, whereas p21 expression was significantly elevated in nine of 12 transformed clones. Two of the three transformed clones displaying low levels of p21 expressed increased levels of p53. p21 regulation was also altered in response to radiation in transformed clones as compared with controls, only minimal induction was observed 4 h following gamma irradiation. Western blot analysis indicated a constant expression of p27 protein but slightly decreased levels of p16 in these transformed clones. Cyclin D(1) was overexpressed in 11 of 12 transformed clones; in only two of these were the levels of cyclin E elevated. Overall, the results suggest that alterations in the expression of cell cycle regulatory proteins may represent important events in radiation-induced oncogenic transformation in vitro. Although the specific alterations vary among different transformed clones, overexpression and aberrant regulation of p21 appear to be the most frequent ones.

Transfer of Ku86 RNA antisense decreases the radioresistance of human fibroblasts

Ku86 has been shown to be involved in DNA double-strand break (DSB) repair and radiosensitivity in rodents, but its role in human cells is still under investigation. The purpose of this study was to evaluate the radiosensitivity and DSB repair after transfection of a Ku86-antisense in a human fibroblast cell line. Simian virus 40-transformed MRC5V1 human fibroblasts were transfected with a vector (pcDNA3) containing a Ku86-antisense cDNA. The main endpoints were Ku86 protein level, Ku DNA end-binding and DNA protein kinase activity, clonogenic survival, and DSB repair kinetics. After transfection of the Ku86-antisense, decreased Ku86 protein expression, Ku DNA end-binding activity, and DNA protein kinase activity were observed in the uncloned cellular population. The fibroblasts transfected with the Ku86-antisense showed also a radiosensitive phenotype, with a surviving fraction at 2 Gy of 0.29 compared with 0.75 for the control and 20% of unrepaired DSB observed at 24 hours after irradiation compared with 0% for the control. Several clones were also isolated with a decreased level of Ku86 protein, a surviving fraction at 2 Gy between 0.05 and 0.40, and 10-20% of unrepaired DSB at 24 hours. This study is the first to show the implication of Ku86 in DSB repair and in the radiosensitivity of human cells. This investigation strongly suggests that Ku86 could constitute an appealing target for combining gene therapy and radiation therapy.

Lifetime persistence and clonality of chromosome aberrations in the peripheral blood of mice acutely exposed to ionizing radiation

As the measurement of chromosomal translocations increases in popularity for quantifying prior radiation exposure, information on the possible decline of these "stable" aberrations over time is urgently needed. We report here information about the persistence of radiation-induced chromosome aberrations in vivo over the life span of a rodent. Female C57BL/6 mice were given a single whole-body acute exposure of 0, 1, 2, 3 or 4 Gy (137)Cs gamma rays at 8 weeks of age. Chromosome aberrations were analyzed from peripheral blood samples at various intervals between 1 day and 21 months after exposure. Aberrations were detected by painting chromosomes 2 and 8. Translocations decreased dramatically during the first 3 months after irradiation, beyond which time the frequencies remained relatively constant out to 1 year, when the effects of aging and clonal expansion became significant. Both reciprocal and nonreciprocal translocations increased with age in the unexposed control animals and were involved in clones. As expected of unstable aberrations, dicentrics decreased rapidly after exposure and reached baseline levels within 3 months. These results indicate that the persistence of translocations induced by ionizing radiation is complicated by aging and clonal expansion and that these factors must be considered when quantifying translocations at long times after exposure. These results have implications for biological dosimetry in human populations.

Kinetics of mammary clonogenic cells and rat mammary cancer induction by X-rays or fission neutrons

Following the hormonal treatment of rats with high prolactin levels and glucocorticoid deficiency (Prl+/Glc-) for 48 days (Day +48), total recoverable mammary DNA was increased by more than sevenfold, tritiated thymidine uptake by nearly fourfold, and total mammary clonogens by about fivefold. Irradiation with 4, 40, and 80 cGy X-rays on Day +48 increased total mammary carcinomas per rat-day-at-risk linearly with dose, and 40 and 80 cGy significantly decreased first carcinoma latency. A dose of 40 cGy X-rays before hormone treatment (Day -1) yielded tumor latencies and frequencies insignificantly different from unirradiated controls but significantly different from those when the dose was given on Day +48. Total carcinomas per rat-day-at-risk were fitted better by a function of dose to the power 0.4 than by a linear function after exposure to 1, 10. and 20 cGy fission neutrons, and 10 and 20 cGy significantly shortened the time to appearance of the first cancer. In contrast to results with X-rays, 10 cGy neutrons on Day -1 yielded tumor frequencies and latencies insignificantly different from 10 cGy neutrons on Day +48. The carcinogenic action of X-rays, but not of neutrons, was thus influenced by total clonogen numbers and/or proliferation rates.

Expression of BRC repeats in breast cancer cells disrupts the BRCA2-Rad51 complex and leads to radiation hypersensitivity and loss of G(2)/M checkpoint control

BRCA2 is a breast tumor suppressor with a potential function in the cellular response to DNA damage. BRCA2 binds to Rad51 through its BRC repeats. In support of the biological significance of this interaction, we found that the complex of BRCA2 and Rad51 in breast cancer MCF-7 cells was diminished upon conditional expression of a wild-type, but not a mutated, BRC4 repeat using the tetracycline-inducible system. Cells expressing a wild-type BRC4 repeat showed hypersensitivity to gamma-irradiation, an inability to form Rad51 radiation-induced foci, and a failure of radiation-induced G(2)/M, but not G(1)/S, checkpoint control. These results strongly suggest that the interaction between BRCA2 and Rad51 mediated by BRC repeats is critical for the cellular response to DNA damage.

The ATPase domain but not the acidic region of Cockayne syndrome group B gene product is essential for DNA repair

Cockayne syndrome (CS) is a human genetic disorder characterized by UV sensitivity, developmental abnormalities, and premature aging. Two of the genes involved, CSA and CSB, are required for transcription-coupled repair (TCR), a subpathway of nucleotide excision repair that removes certain lesions rapidly and efficiently from the transcribed strand of active genes. CS proteins have also been implicated in the recovery of transcription after certain types of DNA damage such as those lesions induced by UV light. In this study, site-directed mutations have been introduced to the human CSB gene to investigate the functional significance of the conserved ATPase domain and of a highly acidic region of the protein. The CSB mutant alleles were tested for genetic complementation of UV-sensitive phenotypes in the human CS-B homologue of hamster UV61. In addition, the CSB mutant alleles were tested for their ability to complement the sensitivity of UV61 cells to the carcinogen 4-nitroquinoline-1-oxide (4-NQO), which introduces bulky DNA adducts repaired by global genome repair. Point mutation of a highly conserved glutamic acid residue in ATPase motif II abolished the ability of CSB protein to complement the UV-sensitive phenotypes of survival, RNA synthesis recovery, and gene-specific repair. These data indicate that the integrity of the ATPase domain is critical for CSB function in vivo. Likewise, the CSB ATPase point mutant failed to confer cellular resistance to 4-NQO, suggesting that ATP hydrolysis is required for CSB function in a TCR-independent pathway. On the contrary, a large deletion of the acidic region of CSB protein did not impair the genetic function in the processing of either UV- or 4-NQO-induced DNA damage. Thus the acidic region of CSB is likely to be dispensable for DNA repair, whereas the ATPase domain is essential for CSB function in both TCR-dependent and -independent pathways.