Transformation and Radiosensitivity of Human Diploid Skin Fibroblasts Transfected with SV40 T-antigen Mutants Defective in RB and P53 Binding Domains

Lack of sensitivity of primary Fanconi's anemia fibroblasts to UV and ionizing radiation

Clinical observations and theoretical considerations suggest some degree of radiosensitivity in Fanconi's anemia (FA), but experimental evidence remains controversial. We tested the sensitivity of primary skin fibroblast cultures from all known FA complementation groups to ionizing radiation and ultraviolet light using conventional cell growth and colony formation assays. In contrast to previous studies, and because FA fibroblasts grow and clone poorly at ambient oxygen, we performed our sensitivity tests under hypoxic cell culture conditions. Fibroblast strains from healthy donors served as negative controls and those from patients with ataxia telangiectasia (AT) and Cockayne syndrome (CS) as positive controls. We observed interstrain variation but no systematic difference in the response of FA and non-FA control fibroblasts to ionizing radiation. After exposure to UV radiation, only complementation group A, G and D2 strains displayed values for colony formation EC50 that were intermediate between those for the negative and positive controls. Because of considerable interstrain variation, minor alterations of the response of individual FA strains to ionizing and UV radiation should be interpreted with caution and should not be taken as evidence for genotype-specific sensitivities of primary FA fibroblasts. All together, our data indicate neither systematic nor major sensitivities of primary FA fibroblast cultures of any complementation group grown under hypoxic cell culture conditions to ionizing or UV radiation.

Influence of interferon-gamma on radiation-induced apoptosis in normal and ataxia-telangiectasia fibroblast cell lines

Combination of interferon-gamma (IFN-gamma) with radiation, or chemotherapeutic agents, produces different kind of modulatory effects, depending on the cell types and experimental conditions. The objective of the present study was verify the influence of IFN-gamma on the induction of apoptosis by gamma-radiation. Experiments were carried out on human fibroblast cell lines: VH-25 (primary), MRC-5, and AT-5BIVA (SV40-transformed). Exponentially growing cells were irradiated and exposed to IFN-gamma (1,000, 2,000, and 3,000 UI/mL) until in situ cell staining performed at 6, 24, and 48 h. Induction of apoptosis by ionising radiation was not verified in primary VH-25 cells. A significant increase in the frequencies of apoptotic cells was observed in SV-40-transformed cells lines, MRC-5, and AT-5BIVA fibroblasts, which were irradiated with 1.0 Gy, but the frequencies of necrotic cells were similar to the control levels. In MRC-5 cells, combined treatments with radiation and IFN-gamma induced a statistically significant reduction in the frequencies of apoptotic cells detected at 24 and 48 h after cell irradiation, while for AT cells the interaction effect (reduction of apoptosis frequency) was significant even at earlier time collection (6 h) after gamma-irradiation, and higher when compared to MRC-5 cells. The present study demonstrated that IFN-gamma showed an anti-apoptotic activity in SV40-transformed fibroblasts, normal and AT cells, which were irradiated with gamma-rays, thus indicating a mechanism dependent on the cellular type.

Prospects for viral-based strategies enhancing the anti-tumor effects of ionizing radiation

Ionizing radiation (IR) has been extensively used to treat a variety of solid tumors to improve local control and overall survival in patients. Gene therapy strategies represent one experimental direction to improve radiocurability. These gene therapy strategies include (1) replacement of mutated or deleted tumor-suppressor genes, (2) delivery of prodrugs, (3) transduction of genes under the control of radiation-inducible promoters, and (4) genetically engineered viruses that replicate preferentially in tumor cells after IR. Although any one of these viral-based gene therapy approaches is unlikely to succeed independently, experimental results suggest that clinically important antitumor can be achieved when these strategies are combined with IR. Several of these strategies are currently being or soon will be evaluated in clinical trials. This review focuses on molecular mechanisms and potential clinical application of these viral-based gene therapy strategies to improve the therapeutic index of IR.

p53, p21 and mdm2 expression vs the response to radiotherapy in transitional cell carcinoma of the bladder

OBJECTIVE

To identify, in a retrospective study, possible molecular markers predictive of radioresponsiveness in patients with transitional cell carcinoma (TCC) of the bladder.

PATIENTS AND METHODS

Patients with T2-T4a TCC treated with preoperative radiotherapy and cystectomy were included in the study if their cystectomy specimen was pT3b (in 42) or pT0 (in 17). Because treatment schedules changed over time, radiotherapy was given either as 2 Gy x 23 over 4-5 weeks with cystectomy 4-5 weeks later (in 23), or as 4 Gy x 5 during 1 week with cystectomy in the following week (in 36 patients). Protein expression of p53, mdm2 and p21 (CDKN1 A/WAF1/CIP1/SDI1) was assessed immunohistochemically in biopsies taken before radiotherapy.

RESULTS

There was no difference in protein expression when comparing all patients with pT0 and pT3b. However, for patients receiving 46 Gy, increased p53 expression (but not p21 or mdm2) predicted the absence of residual tumour (P = 0.005): six of seven patients with > 50% p53 expression had pT0 in the cystectomy specimen, whereas 10 of 12 patients with < or = 5% expression had pT3b. Over-expression of p53 correlated with longer overall (P = 0.045) and cancer-specific survival (P = 0.020).

CONCLUSION

The expression of mdm2 or p21 did not predict radioresponsiveness in patients with TCC of the bladder. The role of p53 remains unclear; the view that p53 over-expression confers radioresistance in bladder cancer is not supported.

Radiation-induced carcinogenesis: studies using human epithelial cell lines

It has proved difficult to develop suitable models to study radiation-induced carcinogenesis by using human epithelial cells. However, immortalised human epithelial cell lines have proved useful. Unirradiated cells from the human keratinocyte cell line (HPV-G) and the human embryonic lung cell line (L132) were found to be tumourigenic in T-cell-deficient mice; thus, they are not suitable for transformation studies. Human urothelial cell lines (SV-HUC-1, NT11, BC16) and the human thyroid epithelial cell line (HTori-3) were nontumourigenic. The urothelial cell lines were refractory to radiation-induced carcinogenesis, and only one small tumour was observed in 57 mice that received irradiated cells. Whereas tumours were not produced following irradiation of these urothelial cells, changes in anchorage-independent growth were observed after a single dose of 8 Gy gamma-irradiation but not after 2 or 4 Gy. Irradiation of the human thyroid epithelial cell line (HTori-3) in vitro resulted in tumour formation. Passaging of the cells in vitro before injection did not seem to be critical. Some of the cell lines derived from the primary thyroid tumours exhibited p53 mutations in exons 5, 6, 7, and 8, as detected by single-stranded conformational polymorphism (SSCP) analysis. Thus, the human thyroid epithelial cell line (HTori-3) looks promising as a model for investigating the molecular events in radiation-induced carcinogenesis.

Does p53 status influence tumor response to anticancer therapies?

Abnormalities in the tumor suppressor gene p53 have been identified in over 60% of human cancers. Since it plays such a pivotal role in cell growth regulation and apoptosis, the status of the p53 gene has been proposed as one of the major determinants of a tumor's response to anticancer therapies. In this review we examine the relationship between functional p53 and sensitivity/resistance to both chemotherapy and radiotherapy, and discuss the potential use of some of the current gene therapy approaches to restore functional p53 to tumors as a means of modulating the effects of radiation and chemotherapy.

Strategies for enhancing viral-based gene therapy using ionizing radiation

Many gene-therapy strategies under investigation aim to increase the efficacy of current cancer-treatment regimens. Promising results have been obtained in the laboratory and early clinical trials using viral-based motifs specifically designed to enhance the efficacy of ionizing radiation or chemotherapy. These strategies fall into two general categories: replication-incompetent viral shuttle vectors for the delivery of specific genes encoding a chemo/radiation modulator and attenuated replication-competent viruses with proposed replicative advantages in tumor cells. In this review, we discuss the rational, molecular mechanisms, and clinical application of these strategies with particular focus on recent research applying these viral-based strategies to improve the therapeutic index of ionizing radiation.

p53 mutation decreased radiosensitivity in rat yolk sac tumor cell lines

PURPOSE

We reported that two established rat yolk sac tumor cell lines differ in their radiosensitivity by 1.7 fold, and the variation is most likely manifested by the differences seen in their apoptotic response. We investigated the relationship between radiosensitivity and p53 in these cell lines.

METHODS AND MATERIALS

We assessed the status of p53 in cell lines by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and sequence analysis, and also analyzed protein expression of p53, p21, and bax as a function of time after irradiation to determine the signal transduction for p53 by immunoblotting.

RESULTS

A band shift was observed only in exon 7 for the radioresistant NMT-1R cells and no band shift was detected for the radiosensitive NMT-1 cells. A band shift was confirmed also at the mRNA level. Exon 7 of p53 DNA showed a three base substitution of DNA at codon 267 to 268. Expression of p53, p21, and bax proteins in NMT-1R cells did not change after 10 Gy irradiation; however, in NMT-1 cells, the expression of these proteins was increased from 1-12 h after irradiation.

CONCLUSION

A loss of p53 function by radiation-induced mutation of p53 decreased the radiosensitivity in these cell lines.

Characterization of the response of human bone marrow endothelial cells to in vitro irradiation

Endothelial cell dysfunction is a classic consequence of radiation damage. Bone marrow endothelial cells (BMEC) are a critical component of the stroma in the regulation of haemopoiesis. In animal models, radiation-induced injury of BMEC has been described and a role for BMEC in haemopoietic regeneration after irradiation has been suggested. However, functions of BMEC involved in the haemopoietic regeneration have not been assessed. Therefore we studied the functional response of human BMEC to irradiation using the transformed human BMEC line (TrHBMEC) irradiated with 2. 5 or 10Gy. Our results showed a time- and a dose-dependent increase in damage to irradiated TrHBMEC measured by a decreased number of adherent cells which correlated with increased apoptosis and augmented release of soluble ICAM-1 and von Willebrand factor. 2 Gy irradiated TrHBMEC expressed more ICAM-1 on their surface than non-irradiated cells, whereas no change in VCAM-1, E-selectin and PECAM-1 expression was observed. An increased production of G-CSF, GM-CSF, IL-8, IL-6, IL-1alpha, IL-11, MIP-1alpha and SCF and no production of LIF, TNF-alpha, TPO and IL-3 by 2 Gy irradiated TrHBMEC was observed. The haemopoietic supportive function of TrHBMEC was not altered after a 2 Gy exposure. These results suggest that although radiation induces endothelial cell damage, irradiated cells still support the proliferation and the differentiation of CD34+ haemopoietic cells.

Dexamethasone-induced radioresistance occurring independent of human papilloma virus gene expression in cervical carcinoma cells

BACKGROUND

Inactivation of p53 by binding to simian virus 40-T antigen (SV40-T) and human papilloma virus type 16 protein E6 (HPV 16 E6) in transfected human diploid fibroblasts causes enhanced radioresistance. The aim of this study was to investigate the role of HPV 18 E6 and E7 gene products with respect to radiosensitivity of two cervical carcinoma cell lines.

MATERIALS AND METHODS

The two cervical carcinoma lines C4-1 and SW 756 were used in which treatment with dexamethasone allows to modulate expression levels of HPV 18 E6 and E7 genes: upregulation in C4-1, downregulation in SW 756. Effects of treatment with dexamethasone on plating efficiency and radiosensitivity were assessed using a clonogenic assay.

RESULTS

Treatment with dexamethasone increased plating efficiency of the C4-1 cells, but did not affect plating efficiency of SW 756 cells. Treatment with dexamethasone induced enhanced radioresistance in both cell lines. Thus in C4-1 cells the observed changes in radioresistance correlate to the enhancement in expression of HPV 18 genes E6/E7, whereas in SW 756, a reduced expression correlates negatively with the enhanced radioresistance.

CONCLUSIONS

In C4-1 and SW 756 cells, treatment with dexamethasone induces radioresistance, and changes in expression levels of HPV 18 genes E6 and E7 do not correlate with the changes in radiosensitivity. Dexamethasone-induced radioresistance has previously been observed in HeLa cells, another human cervical carcinoma cell line. This leads us to speculate that dexamethasone-induced radioresistance may be important in certain clinical situations, and that therefore, the phenomenon deserves further study.

Radiation-induced transformation of SV40-immortalized human thyroid epithelial cells by single exposure to plutonium alpha-particles in vitro

Human thyroid carcinomas have been induced following exposure of SV40-immortalized human thyroid epithelial cells in vitro to single doses (0.14 Gy to 1.57 Gy) of 3.26 MeV alpha-particles from a plutonium 238 source. Tumours were detected between 50 and 160 days following subcutaneous transplantation of the irradiated cells in athymic mice. No tumours were observed following transplantation of unirradiated cells. The relative biological effectiveness (RBE) of the alpha-particles, estimated from cell survival curves, was 4.8 at 50% survival and 3.3 at 5% survival. A first estimate of the RBE at peak tumour induction was 3.8. This system provides a means of studying the mechanisms of tumourigenesis in human thyroid epithelial cells induced by ionizing radiations, including tumours induced by single alpha particles such as from environmental natural radon and polonium and artificial plutonium and americium, and those induced by beta- or Auger-emissions from particular iodine isotopes.

A role for p53 in DNA end rejoining by human cell extracts

The tumor suppressor p53 is a major regulator in the response of human cells to DNA damage. In this study we assessed the role of p53 in the repair of DNA double-strand breaks in plasmid DNA using cell extracts from three human lymphoblastoid cell lines derived from the same donor. TK6, WI-L2-NS and TK6-E6-5e cells express wild-type, mutated and essentially no p53 protein, respectively. Total cellular extracts from TK6, WI-L2-NS and TK6-E6-5e cells were incubated with EcoRI linearized pUC19 DNA. Southern blot analysis of end-rejoined DNA indicated that the major products formed were linear multimers. There was approximately 2-fold greater end rejoining in WI-L2-NS and TK6-E6-5e extracts compared with TK6 extracts. Total DNA from end-rejoining reactions was purified and used to transform bacteria. Using the lacZ reporter gene as a measure of repair fidelity we found that misrepair, as indicated by white colonies, occurred at 4.1% to 6.5% of transformants, with no significant difference between the three cell lines. Gel analysis revealed that misrepair involved only deletions. Sequence analysis of 11 misrepaired products from each cell line showed 12 different deletions from 4 to 48 bp in length, but each cell line yielded similar product types. These results indicate that total cellular extracts from human lymphoblastoid cells lacking p53 or expressing mutated p53 have increased end-rejoining activity as compared with extracts from cells expressing wild-type p53. However, the p53 status does not influence the ratio of misrepair:correct repair, or the type of misrepair events.

Human papillomavirus E6 and E7 oncoproteins alter cell cycle progression but not radiosensitivity of carcinoma cells treated with low-dose-rate radiation

PURPOSE

Low-dose-rate radiation therapy has been widely used in the treatment of urogenital malignancies. When continuously exposed to low-dose-rate ionizing radiation, target cancer cells typically exhibit abnormalities in replicative cell-cycle progression. Cancer cells that arrest in the G2 phase of the cell cycle when irradiated may become exquisitely sensitive to killing by further low-dose-rate radiation treatment. Oncogenic human papillomaviruses (HPVs), which play a major role in the pathogenesis of uterine cervix cancers and other urogenital cancers, encode E6 and E7 transforming proteins known to abrogate a p53-dependent G1 cell-cycle checkpoint activated by conventional acute-dose radiation exposure. This study examined whether expression of HPV E6 and E7 oncoproteins by cancer cells alters the cell-cycle redistribution patterns accompanying low-dose-rate radiation treatment, and whether such alterations in cell-cycle redistribution affect cancer cell killing.

METHODS AND MATERIALS

RKO carcinoma cells, which contain wild-type P53 alleles, and RKO cell sublines genetically engineered to express HPV E6 and E7 oncoproteins, were treated with low-dose-rate (0.25-Gy/h) radiation and then assessed for p53 and p21WAF1/CIP1 polypeptide induction by immunoblot analysis, for cell-cycle redistribution by flow cytometry, and for cytotoxicity by clonogenic survival assay.

RESULTS

Low-dose-rate radiation of RKO carcinoma cells triggered p53 polypeptide elevations, p21WAF1/CIP1 induction, and arrest in the G1 and G2 phases of the cell cycle. In contrast, RKO cells expressing E6 and E7 transforming proteins from high-risk oncogenic HPVs (HPV 16) arrested in G2, but failed to arrest in G1, when treated with low-dose-rate ionizing radiation. Abrogation of the G1 cell-cycle checkpoint activated by low-dose-rate radiation exposure appeared to be a characteristic feature of transforming proteins from high-risk oncogenic HPVs: RKO cells expressing E6 from a low-risk nononcogenic HPV (HPV 11) exposed to low-dose-rate radiation arrested in both G1 and G2. Surprisingly, despite differences in cell-cycle redistribution accompanying low-dose-rate radiation treatment associated with high-risk HPV transforming protein expression, no consistent differences in clonogenic survival following low-dose-rate radiation treatment were found for RKO cell sublines expressing high-risk HPV oncoproteins and arresting only in G2 during low-dose-rate radiation exposure vs. RKO cell sublines exhibiting both G1 and G2 cell-cycle arrest when irradiated.

CONCLUSION

The results of this study demonstrate that neither HPV oncoprotein expression nor loss of the radiation-activated G1 cell-cycle checkpoint alter the sensitivity of RKO carcinoma cell lines to low-dose-rate radiation exposure in vitro. Perhaps for urogenital malignancies associated with oncogenic HPVs in vivo, HPV oncoprotein-mediated abrogation of the G1 cell-cycle checkpoint may not limit the potential efficacy of low-dose-rate radiation therapy.

Increased sensitivity of EBNA2-transformed rat fibroblasts to ionizing radiation

To clarify whether expression of Epstein-Barr virus(EBV) nuclear antigen 2 (EBNA2) correlates with sensitivity to ionizing radiation, we tested EBNA2-transformed rat fibroblast clones for their radiosensitivity to X-rays. These transformed clones reproducibly generated tumors in mice. X-irradiation suppressed the growth of the tumors, and irradiated mice survived longer than non-irradiated ones. In contrast, tumors formed by activated H-ras or E6-E7 genes of human papillomavirus type 16 (HPV16) were strongly resistant to the same dose of X-irradiation. In in vitro culture, these EBNA2-expressing clones also showed higher radiosensitivity than cell lines transformed by activated H-ras and E6E7 genes. The averaged D(o) of EBNA2-expressing clones was 2.3 times lower than that of non-expressing and control clones. These results suggest that expression of EBNA2 is responsible for the radiosensitivity.

The p53 gene as a modifier of intrinsic radiosensitivity: implications for radiotherapy

BACKGROUND AND PURPOSE

Experimental studies have implicated the normal or "wild type' p53 protein (i.e. WTp53) in the cellular response to ionizing radiation and other DNA damaging agents. Whether altered WTp53 protein function can lead to changes in cellular radiosensitivity and/or clinical radiocurability remains an area of ongoing study. In this review, we describe the potential implications of altered WTp53 protein function in normal and tumour cells as it relates to clinical radiotherapy, and describe novel treatment strategies designed to re-institute WTp53 protein function as a means of sensitizing cells to ionizing radiation.

METHODS AND MATERIALS

A number of experimental and clinical studies are critically reviewed with respect to the role of the p53 protein as a determinant of cellular oncogenesis, genomic stability, apoptosis, DNA repair and radioresponse in normal and transformed mammalian cells.

RESULTS

In normal fibroblasts, exposure to ionizing radiation leads to a G1 cell cycle delay (i.e. a "G1 checkpoint') as a result of WTp53 mediated inhibition of G1-cyclin-kinase and retinoblastoma (pRb) protein function. The G1 checkpoint response is absent in tumour cells which express a mutant form of the p53 protein (i.e. MTp53), leading to acquired radioresistance in vitro. Depending on the cell type studied, this increase in cellular radiation survival can be mediated through decreased radiation-induced apoptosis, or altered kinetics of the radiation-induced G1 checkpoint. Recent biochemical studies support an indirect role for the p53 protein in both nucleotide excision and recombinational DNA repair pathways. However, based on clinicopathologic data, it remains unclear as to whether WTp53 protein function can predict for human tumour radiocurability and normal tissue radioresponse.

CONCLUSIONS

Alterations in cell cycle control secondary to aberrant WTp53 protein function may be clinically significant if they lead to the acquisition of mutant cellular phenotypes, including the radioresistant phenotype. Pre-clinical studies suggest that these phenotypes may be reversed using adenovirus-mediated gene therapy or pharmacologic strategies designed to re-institute WTp53 protein function. Our analysis of the published data strongly argues for the use of functional assays for the determination of WTp53 protein function in studies which attempt to correlate normal and tumour tissue radioresponse with p53 genotype, or p53 protein expression.

Reduced growth factor requirements and accelerated cell-cycle kinetics in adult human melanocytes transformed with SV40 large T antigen

Melanomas develop with high frequency in transgenic mice in which oncogenic sequences of the SV40 DNA tumor virus have been specifically targeted to melanocytes. To investigate the role of SV40 in melanomagenesis, cultured human melanocytes were transformed with a retroviral shuttle vector encoding the SV40 large T antigen and examined for changes in cell-cycle kinetics and growth-factor dependence. Colonies expressing the viral oncogene were morphologically indistinguishable from their non-T-antigen-transformed counterparts. Also like normal melanocytes, the infected cells remained anchorage dependent and non-tumorigenic in nude mice. However, T-antigen-positive cultures exhibited significantly accelerated population doubling times, increased saturation densities with highly confluent monolayers and a three- to fourfold extended life span. Most interestingly, cell-cycle analysis revealed a measurable shift from quiescent to cycling cells in T-antigen-expressing cultures and an acquired ability to progress more rapidly through G1. Moreover, T-antigen-positive melanocytes proliferated in the absence of PMA and required markedly reduced levels of exogenous bFGF. These studies indicate that the viral oncogen of simian virus 40 provides melanocytes with distinct growth advantages that may render these cells unusually susceptible to additional environmental challenges necessary for full expression of the malignant phenotype.

Cell cycle regulation in response to DNA damage in mammalian cells: a historical perspective

Cell cycle delay has long been known to occur in mammalian cells after exposure to DNA-damaging agents. It has been hypothesized that the function of this delay is to provide additional time for repair of DNA before the cell enters critical periods of the cell cycle, such as DNA synthesis in S phase or chromosome condensation in G2 phase. Recent evidence that p53 protein is involved in the delay in G1 in response to ionizing radiation has heightened interest in the importance of cell cycle delay, because mutations in p53 are commonly found in human cancer cells. Because mammalian cells defective in p53 protein show increased genomic instability, it is tempting to speculate that the instability is due to increased chromosome damage resulting from the lack of a G1 delay. Although this appears at first glance to be a highly plausible explanation, a review of the research performed on cell cycle regulation and DNA damage in mammalian cells provides little evidence to support this hypothesis. Studies involving cells treated with caffeine, cells from humans with the genetic disease ataxia telangiectasia, and cells that are deficient in p53 show no correlation between G1 delay and increased cell killing or chromosome damage in response to ionizing radiation. Instead, G1 delay appears to be only one aspect of a complex cellular response to DNA damage that also includes delays in S phase and G2 phase, apoptosis and chromosome repair. The exact mechanism of the genomic instability associated with p53, and its relationship to the failure to repair DNA before progression through the cell cycle, remains to be determined.

DNA damage triggers a prolonged p53-dependent G1 arrest and long-term induction of Cip1 in normal human fibroblasts

The tumor suppressor p53 is a cell cycle checkpoint protein that contributes to the preservation of genetic stability by mediating either a G1 arrest or apoptosis in response to DNA damage. Recent reports suggest that p53 causes growth arrest through transcriptional activation of the cyclin-dependent kinase (Cdk)-inhibitor Cip1. Here, we characterize the p53-dependent G1 arrest in several normal human diploid fibroblast (NDF) strains and p53-deficient cell lines treated with 0.1-6 Gy gamma radiation. DNA damage and cell cycle progression analyses showed that NDF entered a prolonged arrest state resembling senescence, even at low doses of radiation. This contrasts with the view that p53 ensures genetic stability by inducing a transient arrest to enable repair of DNA damage, as reported for some myeloid leukemia lines. Gamma radiation administered in early to mid-, but not late, G1 induced the arrest, suggesting that the p53 checkpoint is only active in G1 until cells commit to enter S phase at the G1 restriction point. A log-linear plot of the fraction of irradiated G0 cells able to enter S phase as a function of dose is consistent with single-hit kinetics. Cytogenetic analyses combined with radiation dosage data indicate that only one or a small number of unrepaired DNA breaks may be sufficient to cause arrest. The arrest also correlated with long-term elevations of p53 protein, Cip1 mRNA, and Cip1 protein. We propose that p53 helps maintain genetic stability in NDF by mediating a permanent cell cycle arrest through long-term induction of Cip1 when low amounts of unrepaired DNA damage are present in G1 before the restriction point.

Alterations in oncogene expression and radiosensitivity in the most frequently used SV40-transformed human skin fibroblasts

In comparison with primary cell cultures, SV40-transformed human skin fibroblasts, either from healthy donors or from patients suffering from ataxia-telangiectasia (AT) or xeroderma pigmentosum, are more resistant to the cytotoxic action of low LET 60cobalt gamma-rays as well as to high LET alpha-particles. Resistance factors calculated from D10's lie between 1.4 and 2.0. Northern blot analysis reveals spontaneous overexpression of the oncogenes c-myc, Ki-ras and c-raf and of the tumour suppressor gene p53 as a consequence of SV40 transformation. For c-myc, the increased expression is due to gene amplification and gene rearrangement. An even further increase in the expression of c-myc has been found for AT cells (AT5BI-VA) after moderate doses of 60cobalt gamma-irradiation. A possible correlation between SV40-induced changes in gene expression and cellular radioresistance is discussed.

DNA damage, oncogenesis and the p53 tumour-suppressor gene

The p53 tumour-suppressor gene encodes a transcription factor which plays a central role in controlling oncogenic development in mouse and humans. Mice which over-express mutant p53 transgenes or have a homozygous deletion of the p53 gene show a high frequency of spontaneous tumour development. This review will focus on recent developments using these transgenic and null mice which suggest that p53 is important in maintaining genomic stability, and is a critical component in the cellular response to ionizing radiation.