Chromosome end-to-end associations and telomerase activity during cancer progression in human cells after treatment with alpha-particles simulating radon progeny

Chromosome end-to-end associations seen at metaphase involve telomeres and are commonly observed in cells derived from individuals with ataxia telangiectasia and most types of human tumors. The associations may arise because of short telomeres and/or alterations of chromatin structure. There is a growing consensus that telomere length is stabilized by the activity of telomerase in immortal cells; however, it is not clear why some immortal cells display chromosome end-to-end associations. In the present study we evaluated chromosome end-to-end associations, telomere length and telomerase activity with the tumorigenic status of human bronchial epithelial cells immortalized with human papillomavirus. Oncogenic transformation was initiated using radon simulated alpha-particles and cells evaluated as primary, secondary and metastatic transformants. The fewest chromosome end associations and lowest telomerase activity were observed in the parental immortalized cells. However, increased levels of telomerase activity were detected in alpha-particle survivors while robust telomerase activity was seen in the tumorigenic cell lines. The tumorigenic cells that were telomerase positive and had the highest frequency of cells with chromosome end-to-end associations were also metastatic. No correlation was found between telomere length and the different stages of carcinogenicity.

Oncogenic potential of bifunctional bioreductive drugs

Potential oncogenicity must be a factor of concern in the design and development of novel bioreductive drugs. In the present studies, the cytotoxicity and oncogenic transforming potential of a series of heterocyclic mono-N-oxides, designed to be used as bioreductive drugs, were examined using the mouse C3H 10T1/2 cell system. Exponential phase cultures of 10T1/2 cells were treated with graded doses of the bioreductive drugs for a 4 h period, either in air or hypoxia, at 37 degrees C. After treatment, cultures were replated for both survival and transformation assays. The fused pyrazine mono-N-oxide RB 90740 and its N-deoxy analogue, RB 92816, demonstrated a dose-dependent cytotoxicity and oncogenic transforming potency under aerobic conditions. Similarly, the indoloquinone E09 and the structurally related mitomycin C demonstrated dose dependence in both toxicity and oncogenic transforming potential. The most cytotoxic aromatic-N-oxides tested, RB 92816, also demonstrated the highest oncogenic transformation incidence. In hypoxia, the bioreductive metabolites of RB 90740 were substantially more cytotoxic and induced a higher oncogenic transformation yield than the drug in air. These data are consistent with the structure-activity relationship for bioreductive drugs in that heterocyclic-N-oxides with reactive side chains such as RB 92816 are cytotoxic and potentially carcinogenic.

Induction of heme oxygenase in mammalian cells by mineral fibers: distinctive effect of reactive oxygen species

Exponentially growing human-hamster hybrid [AL] cells treated with a 40 micrograms/ml (8 micrograms/cm2) dose of UICC standard reference chrysotile fibers induced heme oxygenase (HO) protein with a maximum expression level at 8 h post-treatment. While the constitutive HO expression was detectable in non-treated AL cells, the protein level was increased approximately 4.5-fold in fiber-treated cells. The induction was dose-dependent at fiber concentration between 2.5 micrograms/ml (0.5 microgram/cm2) and 40 micrograms/ml (8 micrograms/cm2) with the induced HO concentrated mostly in the cytoplasm as shown by immunostaining. Several other types of mineral fibers examined including crocidolites, tremolites, and erionites also induced HO synthesis with varying degree of efficiency. In general, chrysotile and crocidolite were more efficient inducers of HO than tremolite and erionite when compared at fiber doses that resulted in approximately 50% survival (LD50) level. The effects of antioxidant enzymes on HO induction were examined by concurrent treatment of fiber-exposed cultures with SOD and catalase. Although addition of superoxide dismutase (SOD) and catalase inhibited HO induction in a dose-dependent manner, they offered no protection on fiber-mediated clonogenic toxicity in the same population of treated cells. These results suggest that reactive oxygen species (ROS) produced by asbestos fibers play an essential role in the induction of HO and that different mineral fibers, when applied at equitoxic doses, often result in different oxidative stress status as determined by the induction of HO proteins.

Cellular and molecular analysis of mutagenesis induced by charged particles of defined linear energy transfer

Mutation induction by charged particles of defined linear energy transfer (LET) and gamma rays was scored using human-hamster hybrid AL cells. The LET values for charged particles accelerated at the Radiological Research Accelerator Facility ranged from 10 keV/microm protons to 150 keV/microm 4He ions. The induced mutant fractions at both the S1 and HGPRT loci were dependent on the dose and LET. In addition, for each dose examined, the mutant yield at the S1 locus was 30-60 fold higher than at the corresponding HGPRT locus. To determine whether the mutation spectrum was comparably dependent on dose and LET, independent S1- and HGPRT- mutants induced by 150 keV/microm 4He ions and gamma rays were isolated, and their DNA was analyzed by both Southern blotting and multiplex PCR methods. While the majority of radiation-induced mutants showed deletions of varying sizes, the relative percentage of large deletions was found to be related to both the dose and LET of the radiation examined. Using a mutation system that can detect multilocus changes, results of the present study show that radiation-induced chromosomal loss can be in the millions of base pairs.

Mutation induction in gamma-irradiated primary human bronchial epithelial cells and molecular analysis of the HPRT- mutants

We have examined various radiobiological parameters using commercially-available primary normal human bronchial epithelial (NHBE) cells, which can be subcultured more than 20 population doublings, and have established the mutation system in order to characterize the molecular changes in gamma-irradiated primary cells. The survival curve, obtained after irradiation of cells with 137Cs gamma-rays, indicates that the D0, Dq, and n values are 1.34 Gy, 1.12 Gy, and 2.3, respectively. The induction of HPRT- mutation was dose-dependent and the mutant fraction increased in a non-linear fashion. Since the doubling number of NHBE cells is limited, DNA was extracted directly from the single mutant colonies and alteration in the HPRT gene locus was analyzed using multiplex PCR technique. Among spontaneous mutants, the proportion with total and partial deletions of the gene was 10.0% (2/20) and 60.0% (12/20), respectively, while 30.0% (6/20) did not have any detectable changes in the nine exons examined. On the other hand, the fraction of total deletion increased by more than 2-fold among mutants induced by gamma-rays in that 26.3% (10/38) of them showed the total gene deletions. Twenty-five out of 38 gamma-induced mutants (65.8%) had partial deletions and 3 mutants (7.9%) had no detectable alteration. The present results showed that gamma-irradiation efficiently induced HPRT gene mutation in primary human epithelial cells and that most of the induced mutants suffered larger deletions compared to that observed in spontaneous mutants. This system provides an useful tool for determination of mutagenicity and understanding the molecular mechanisms of environmental carcinogens in primary human bronchial cells.

Fused pyrazine mono-n-oxides as bioreductive drugs. II Cytotoxicity in human cells and oncogenicity in a rodent transformation assay

PURPOSE

To determine what structural moieties of the fused pyrazine mono-N-oxides are determining factors in their in vitro cytotoxicity and oncogenicity.

METHODS AND MATERIALS

A new series of experimental bioreductive drugs, fused pyrazine mono-N-oxides, was evaluated in vitro for aerobic and hypoxic cytotoxicity in the HT29 human colon adenocarcinoma cell line by using clonogenic assays. The relative oncogenicities of these compounds were also determined in aerobic cultures of C3H 10T1/2 mouse embryo fibroblasts by using a standard transformation assay.

RESULTS

Removal of the 4-methyl piperazine side chain from the parent compound, RB 90740, reduced the potency of the hypoxic cytotoxin. Reduction of the N-oxide function increased the aerobic cytotoxicity and eliminated most of the hypoxic/aerobic cytotoxic differential. The reduced N-oxide also had significant oncogenicity, consistent with a mechanism of genotoxicity following bioreduction of RB 90740.

CONCLUSION

This new series of bioreductive compounds may be effective in cancer therapy, particularly the lead compound RB 90740. The oncogenic potential of these compounds is similar to that for other cancer therapies. Further studies should include evaluation of these compounds in vivo and the development of analogs with reduced oncogenic potential and retention of the hypoxic/aerobic cytotoxicity differential.

Cellular and molecular alterations in human epithelial cells transformed by high LET radiation

An understanding of the radiobiological effects of high LET radiation is essential for human risk estimation and radiation protection. In the present study, we show that a single, 30 cGy dose of 150 keV/micrometer 4He ions can malignantly transform human papillomavirus immortalized human bronchial epithelial [BEP2D] cells. Transformed cells produce progressively growing tumors in nude mice. The transformation frequency by the single dose of alpha particles is estimated to be approximately 4 X 10(-7). Based on the average cross-sectional area of BEP2D cells, it can be calculated that a mean traversal of 1.4 particles per cell is sufficient to induce tumorigenic conversion of these cells 3 to 4 months post-irradiation. Tumorigenic BEP2D cells overexpress mutated p53 tumor suppressor oncoproteins in addition to the cell cycle control gene cyclin D1 and D2. This model provides an opportunity to study the cellular and molecular changes at the various stages in radiation carcinogenesis involving human cells.

Photoneutrons from medical linear accelerators--radiobiological measurements and risk estimates

PURPOSE

To assess the oncogenic potential of the photoneutrons produced by high energy medical linear accelerators.

METHODS AND MATERIALS

An established line of cells of rodent origin (C3H 10T1/2) was used to assess the oncogenic potential of the radiation dose received in the breast of an anthropomorphic "randoman" phanton, while the cervix received a dose of 70 Gy. Experiments were performed at 6 MV, below the threshold for the production of photoneutrons, and at 20 MV where the dose includes about 0.01 Gy of photoneutrons as well as scattered x-rays.

RESULTS

A significantly higher transformation incidence was observed for the 20-MV machine, consistent with the measured neutron dose of about 0.01 Gy and a quality factor of 20.

CONCLUSION

An estimate can be made of the additional deaths from second malignancies that might result from the photoneutrons generated by higher energy linear accelerators (Linacs), which must be offset against the possible improvements in survival that might result from the higher tumor doses made possible by the increased percentage depth doses.

Effects of antioxidants on fiber mutagenesis

Recent studies from this laboratory have shown that asbestos fibers are mutagenic in cultured mammalian cells when assayed using a system that can detect multilocus deletions. Southern analysis of the induced mutants shows that the majority contain large deletions ranging in size from a few thousand to several million basepairs. In the present study, the effects of free radical scavenging enzymes on the cytotoxic and mutagenic potential of chrysotile fibers were examined using the human-hamster hybrid (AL) cells. Exponentially growing cells were treated with graded doses of fibers for a 24 h period either in the presence or absence of catalase, superoxide dismutase (SOD) or Tempol. Fiber-exposed cells were treated with the various enzymes either concurrently with the fiber or extended through the entire expression period. While the survival of AL cells treated with graded doses of chrysotile fibers with or without a concurrent treatment with SOD and catalase was not significantly different, the mutation yield at the S1 locus was significantly reduced in cells treated with these antioxidant enzymes. Furthermore, cells treated with the enzymes for a prolonged period were not better protected than those treated only during fiber treatment. The SOD mimic nitroxide, Tempol, had no effect on either the survival or mutagenic yield of chrysotile fibers. While SOD and catalase reduced the mutagenic potency of asbestos fibers in AL cells, they did not alter the molecular spectrum of fiber-induced mutagenesis. Our results indicate that antioxidant enzymes can protect cells against the genotoxic damages induced by chrysotile fibers, and are highly suggestive of the roles of oxyradicals in the fibrogenic and carcinogenic mechanisms of asbestos fibers.

Molecular analysis of mutagenesis by high LET radiation

Mutation induction by high linear energy transfer [LET] alpha particles and gamma-rays was scored in the human hamster hybrid [AL] cells. Southern blotting technique was used to analyse the molecular changes in the DNA from both the HGPRT- and S1- mutants. Dose dependent mutagenesis in the AL cells irradiated with the charged particles was higher by almost 20 fold at the S1 than the corresponding HGPRT locus. Southern analysis of the mutants induced by the high LET particles showed mostly multilocus deletion at both the HGPRT and S1 genes.

Taxol and ionizing radiation: interaction and mechanisms

PURPOSE

Taxol has been shown to be clinically active against several types of human tumors. To assess the potential oncogenic effect of taxol, the in vitro cytotoxic and oncogenic transforming effects of taxol, either alone or in combination with gamma-irradiation, were examined.

METHODS AND MATERIALS

Exponentially growing mouse C3H 10T1/2 cells were treated with taxol with or without concurrent gamma-irradiation. After treatment, cultures were replated for both clonogenic survival and transformation assays. To determine the effects of taxol on cell cycle kinetics, treated cells were concurrently labelled with bromodeoxyuridine coupled with fluorescein. Accumulated mitotic cells were isolated by the shake-off technique and their plating efficiency and radiosensitivity were determined.

RESULTS

Taxol induced a dose dependent toxicity in 10T1/2 cells. In contrast to human tumor cells in culture, the mitotic block induced by a 100 nM dose of taxol in 10T1/2 cells was only partial. While taxol was ineffective in transformant induction, it enhanced the oncogenic transforming potential of gamma-rays in a supra-additive manner. The fact that approximately 15% of taxol-induced mitotic cells were clonogenically viable and at a cell cycle stage that was most radiosensitive suggests a mechanistic basis for the observed enhancement in transformation incidence by ionizing radiation.

CONCLUSION

Taxol enhances the oncogenicity of radiation by partially blocking the 10T1/2 cells in G2/M phases of the cell cycle, phases that are most sensitive to radiation induced oncogenic transformation.

Malignant transformation of human bronchial epithelial cells by radon-simulated alpha-particles

Epidemiological studies have shown that inhalation of radon is associated with an increased risk for bronchogenic carcinoma in uranium miners. These alpha-emitting radon daughters also represent the largest component of background radiation to the general public. In the present study, the oncogenic transforming effects of single versus multiple doses of radon-simulated alpha-particles were examined using human papillomavirus-immortalized human bronchial epithelial cells. Endpoints such as growth kinetics, resistance to serum and 12-O-tetradecanoylphorbol-13-acetate-induced terminal differentiation, anchorage-independent growth and tumorigenicity in nude mice were used to assess the various stages of transformation in the bronchial epithelial cells. We show here, for the first time, that immortalized human cells in culture can be malignantly transformed by a single 30 cGy dose of alpha-particles. Transformed cells produced progressively growing subcutaneous tumors upon inoculation into athymic nude mice. Immunofluorescent staining of keratin and isozyme analysis of the cell lines subsequently generated from these tumors indicated that the cells were of human epithelial origin. Analysis of genomic DNA from the tumorigenic cell lines using PCR amplification and restriction enzyme analysis demonstrated no point mutation at either codon 12/13 or 61 in any of the ras oncogenes examined (K-, N- and H-ras). This system provides an opportunity to study the cellular and molecular changes at the various stages in radiation carcinogenesis involving human cells.

Effects of increased expression of protein kinase C on radiation-induced cell transformation

The in vitro oncogenic transformation of C3H 10T1/2 cells by ionizing radiation is known to be enhanced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). It is also known that the activation of protein kinase C (PKC) by TPA is an important step in its tumor-promoting effect. In the present study, we examined the effects of overexpression of a specific isoform of PKC, PKC beta 1 on gamma-ray-induced transformation of 10T1/2 cells. In addition, the effects of overexpression of PKC beta 1 on the malignant phenotype of a previously transformed 10T1/2 cell line were also evaluated. Derivatives of 10T1/2 cells that stably overexpress PKC beta 1 were obtained by transduction with the retroviral expression vector pMV7 carrying the rat PKC beta 1 cDNA sequence. We found that the parental 10T1/2 cells and a control cell line 10T1/2 MV7, which carried only the pMV7 vector without the cDNA insert, expressed dose-dependent transformation frequencies when exposed to gamma-rays. On the other hand, concurrently treated PKC-overexpressing cells that had an 11-fold increase in enzyme activity (PKC-4 cells) failed to yield any morphologically identifiable foci. Cell lines that expressed lower levels of PKC beta 1 were partially resistant to transformation by gamma-rays. Clonogenic survival data indicated that this observation was not due to radioresistance per se. Thus, overexpression of PKC beta 1 did not appear to function as an endogenous substitute for TPA in promoting radiation-induced transformation. Furthermore, overexpression of PKC did not reverse the transformation phenotypes in tumorigenic 10T1/2 cells once it was established. These findings are discussed with respect to the specific roles of individual isoforms of PKC in growth control.

Radiation-induced deletion of chromosomal regions containing tumor suppressor genes in human bronchial epithelial cells

While several specific genetic alterations have been associated with malignant transformation of human bronchial epithelial cells, they are not all present in every tumor and there is reason to believe that additional genes important for loss of replication control in these cells remain to be identified. In an effort to develop a human bronchial epithelial cell model suitable for identification and functional analysis of genes involved in loss of replication control, and for studying the genetic basis of the multi-stage phenotypic changes associated with tumorigenesis, we treated multiple independent populations of the human papillomavirus (HPV)-immortalized human bronchial epithelial cell line BEP2D with ionizing radiation. Following irradiation, cell lines representing the radiated populations were established from single soft agar-selected colonies. These lines--R2B5SA, R3B5DSA, R2H9S, R2H16S, R2H18S and R3D7S--were compared cytogenetically to the parent cell line and found to have new chromosomal deletions involving putative or confirmed tumor suppressor genes, including chromosome 13 in most R2B5SA cells and all R3B5SA cells, chromosomes 11p and 22 in R216S, and 3p, 11p and 22 in R2H18S. The R2B5SA cells that have lost one copy of chromosome 13 overgrow the culture but are not tumorigenic, suggesting that loss of a single copy of chromosome 13 confers growth advantage but not tumorigenicity. The data confirm that ionizing radiation induces many large chromosomal alterations including chromosomal loss, translocation and deletion and that following radiation it is possible to isolate immortalized nontumorigenic cell lines monosomic for regions known or suspected to contain tumor suppressor genes. The cell lines described here provide powerful models for further investigation of putative tumor suppressor genes including identification, functional analysis and stage of transformation at which they are operative.

Taxol, radiation, and oncogenic transformation

The novel antineoplastic drug taxol has been shown to be active clinically against several types of human tumors. With improvement in treatment strategy and the number of long-term survivors increasing, a question that needs to be addressed is the potential carcinogenic effect of the treatment in the induction of second malignancies. We show here that when tested using an in vitro assay for oncogenic transforming potential, taxol is ineffective in focus induction at doses significantly higher than those used in the clinic. However, taxol enhances the oncogenic potential of gamma-rays in a synergistic fashion. The fact that taxol blocks cells at the G2/M phases of the cell cycle may account for this interaction since G2/M are relatively radiosensitive phases of the cell cycle.

The biological effectiveness of radon daughter alpha particles. I. Radon, cigarette smoke and oncogenic transformation

Epidemiological data from uranium miners have not been able to provide a definitive model depicting the interaction between radiation exposure and smoking in the etiology of lung cancers among miners who also smoke. Using the well-characterized C3H 10T1/2 cell system, the toxicity and oncogenic transforming incidence by either gamma-rays or He4 ions, with or without concurrent exposure to cigarette smoke condensate (CSC), were examined. He4 ions had the same energy spectrum as radon. CSC, extracted from 2R1 cigarettes, induced a dose-dependent toxicity and oncogenic transformation incidence in 10T1/2 cells. CSC, in combination with a 2 Gy dose of gamma-rays, induced a toxicity and transforming response that was largely additive in nature. Similar additive modes of interaction were also observed when CSC was combined with He4 ions at a dose that was equivalent in cell killing to that used for gamma-rays. These results provide evidence that radiation and smoking have an additive effect in oncogenic transforming potential and may provide an experimental handle towards an epidemiological uncertainty.

Chrysotile fiber is a strong mutagen in mammalian cells

Although chrysotile asbestos is a proven human carcinogen, several studies have concluded that these fibers are not mutagenic to cultured mammalian cells. We show here, on the other hand, that when tested using the AL cell system that detects both intragenic and multilocus mutations, chrysotile is indeed mutagenic and comparable in strength to that of gamma-rays. Southern analysis of the induced mutants shows that the majority contains large deletions ranging in size from a few thousand to several million base pairs. Results of our study demonstrate that, while chrysotile may be less durable in vivo than the amphibole fibers such as crocidolites and amosites, it can effectively create genetic damage involved in the cancer process.

Television viewing and pediatric hypercholesterolemia

Cholesterol screening for children is recommended currently only for those with a family history of premature coronary heart disease or hyperlipidemia. The authors report on a pediatric-office-based cholesterol screening program where the predictive values of family history indicators were evaluated along with reported television viewing, physical activity, and dietary habits in 1081 children (aged 2 to 20 years, mean 7.4 +/- 3.6 [SD] years). Eight percent of these children had a total cholesterol value of 200 mg/dL or higher; 53% of such children reported watching 2 or more hours of television daily compared with 34% of children with lower cholesterol levels. Multivariate analyses revealed that excessive television viewing was the strongest predictor for a child to have a cholesterol value of 200 mg/dL or higher, with relative risks of 2.2 for 2 to 4 hours of television viewing per day (P less than .01) and 4.8 for children watching more than 4 hours/day, when compared to those watching less than 2 hours/day (P less than .01). In contrast, a positive family history of a high cholesterol level was only modestly associated with an increased probability of having a high cholesterol level (relative risk = 1.6, P less than .05), and a history of premature myocardial infarction in a parent or grandparent was not associated with a child's cholesterol level. Excessive television viewing was found to be associated with certain dietary and physical activity habits and may prove to be a useful, global marker for several life-style factors predisposing children to hypercholesterolemia.

Mechanism of oncogenicity for bioreductive drugs

The oncogenic transforming potential of a series of bifunctional bioreductive drugs were examined under either aerated or hypoxic conditions to assess the contribution of side chains or nitroreduced products toward their carcinogenic mechanisms. Both the cytotoxicity and transforming effects of these drugs increased as a function of dose under hypoxia. In air and at doses that resulted in comparable cell killing, RSU-1069 and RB-88716 were substantially more oncogenic than RSU-1164 or SR-4233. In nitrogen, the oncogenicity of SR-4233 as a function of survival increased, whereas the transforming effect for the aziridine-containing drugs, RSU-1969 and RB-88716, decreased. These data suggest that, among the drugs examined, the transforming moiety in air is largely a function of the alkylating aziridine group. In hypoxia, the reduction of the nitro-moiety to the corresponding active metabolites may be responsible for much of the transformation observed.

Oncogenic transforming potential of etanidazole

The 2-nitroimidazole etanidazole is in Phase III randomized trials as an adjunct to radiotherapy in both the United States and Europe. Paradoxically, it is one of the more oncogenic radiosensitizers examined with the in vitro oncogenic transformation assay based on C3H 10T1/2 cells. To obtain an estimate of the possible number of second malignancies that might be induced in patients receiving the drug, the in vitro transformation assay was used to compare etanidazole with gamma-rays, for which cancer risk estimates are available from the Japanese survivors. For the clinical protocol used in RTw] studies, which results in a cumulative dose of 36 mM hr, the drug-induced cancer incidence was estimated to be 2.8 to 14%.

Neutron dose-rate experiments at the AFRRI nuclear reactor. Armed Forces Radiobiology Research Institute

Some in vitro and in vivo studies with neutrons have shown increased carcinogenic effectiveness following low-dose-rate or fractionated irradiation compared to acute exposure in the low-dose range. This would imply that the risk of cancer for persons exposed occupationally to low doses of neutrons is underestimated at present. The C3H 10T1/2 assay has played a major role in investigating neutron dose-rate effects. We describe three independent series of experiments addressing the question of the influence of dose rate using the AFRRI fission-neutron source with the C3H 10T1/2 cell transformation assay as well as with two mutation assays utilizing human-hamster hybrid AL cells. In the first two series, we focused on performing experiments with fission-neutron doses and dose rates similar to those for which enhancement of neoplastic transformation of C3H 10T1/2 cells was originally reported, and observed no discernible dose-rate effect. In the third series concurrent with the induction of neoplastic transformation in C3H 10T1/2 cells, we also measured mutagenesis at two loci in AL cells. Data for survival, neoplastic transformation, and mutation were obtained at two dose rates in the range of neutron doses 0.005 to 0.9 Gy. Dose-rate effectiveness factors expressed as ratios of the effect for low compared to high dose rate did not differ from one, indicating no influence of dose rate on these end points.

Aneuploidy and progression in promoted preneoplastic foci during chemical hepatocarcinogenesis in the rat

Since a number of rat hepatocarcinomas are aneuploid, the model DNA content of enzyme-altered foci was determined cytospectrophotometrically, to assess if ploidy changes occur before cancer is established. Male F344 rats treated with diethylnitrosamine and promoted with a choline-deficient, phenobarbital supplemented diet showed in most enzyme-altered foci a multimodal ploidy distribution with diploid, tetraploid and octoploid peaks. A minority of foci, however, exhibited an aneuploid pattern. This change in ploidy reflects irreversible genomic alterations, indicative of tumor progression. Thus, promotion and progression may coexist simultaneously in this model of carcinogenesis, long before hepatomas can be diagnosed.

Studies with bifunctional bioreductive drugs. I. In vitro oncogenic transforming potential

The oncogenic transforming potential of a series of bioreductive drugs including RSU-1069 and its various alkyl-substituted derivatives, RB-7040, RB-88716, RSU-1164, and RB-88712, has been compared using the C3H 10T1/2 cell system. While the aziridine moiety at the terminal end of the side chain confers greater cytotoxicity to both the 2-nitroimidazole (RSU-1069) and the 5-nitrofuran (RB-88716), it also increases the oncogenic transforming potential of the drugs correspondingly. By substituting the aziridine ring with methyl groups, the cytotoxicity and oncogenicity of these bioreductive drugs decrease in a way that is proportional to the degree of methylation. A clear structure-activity relationship can be demonstrated from these methyl-substituted derivatives such that a tetramethyl-substituent (RB-7040) is much less cytotoxic and oncogenic than a dimethyl-substituent (RSU-1164). RB-7040, which has in vitro and in vivo sensitizing efficiency comparable to the parental compound RSU-1069, is roughly tenfold less cytotoxic and, at concentrations that achieve an in vitro enhancement ratio of 2.9, induces a transforming frequency that is indistinguishable from the spontaneous rate.

Modulating factors in the expression of radiation-induced oncogenic transformation

Many assays for oncogenic transformation have been developed ranging from those in established rodent cell lines where morphological alteration is scored, to those in human cells growing in nude mice where tumor invasiveness is scored. In general, systems that are most quantitative are also the least relevant in terms of human carcinogenesis and human risk estimation. The development of cell culture systems has made it possible to assess at the cellular level the oncogenic potential of a variety of chemical, physical and viral agents. Cell culture systems afford the opportunity to identify factors and conditions that may prevent or enhance cellular transformation by radiation and chemicals. Permissive and protective factors in radiation-induced transformation include thyroid hormone and the tumor promoter TPA that increase the transformation incidence for a given dose of radiation, and retinoids, selenium, vitamin E, and 5-aminobenzamide that inhibit the expression of transformation. Densely ionizing alpha-particles, similar to those emitted by radon daughters, are highly effective in inducing transformations and appear to interact in a supra-additive fashion with asbestos fibers. The activation of a known dominant oncogene has not yet been demonstrated in radiation-induced oncogenic transformation. The most likely mechanism for radiation activation of an oncogene would be via the production of a chromosomal translocation. Radiation also efficiently induces deletions and may thus lead to the loss of a suppressor gene.

Oncogenic transforming potential of nitroimidazole radiosensitizers

The oncogenic transforming potentials of a number of hypoxic cell radiosensitizers have been compared including misonidazole, SR-2508, Ro-03-8799, and RSU-1069, using the C3H 10T1/2 cell system. While the dual function bioreductive 2-nitroimidazole RSU-1069 is a particularly potent hypoxic cell radiosensitizer, both in vitro and in vivo, it is also more cytotoxic and has a considerably higher oncogenic transforming potential than the other nitroimidazoles tested. In combination with radiation, RSU-1069 produces a supra-additive transforming response in C3H 10T1/2 cells. For all the other sensitizers tested, the number of transformants produced by drug and radiation combined are simply additive. RB-6145, an analog of RSU-1069 with an equivalent sensitizing efficiency, is substantially less toxic and much less oncogenic than RSU-1069. At doses that produce similar in vitro enhancement ratios, RB-6145 is no more oncogenic than misonidazole and produces an additive transforming incidence in combination with radiation. RB-6145 appears to be a promising substitute for RSU-1069 for future clinical application(s) as a hypoxic cell sensitizer.

Mutation induction and relative biological effectiveness of neutrons in mammalian cells. Experimental observations

The induction of mutation by graded doses of monoenergetic neutrons was examined using the human-hamster hybrid cell system. The AL cells, formed by fusion of human fibroblasts with the gly- A mutant of the Chinese hamster ovary cells, contain the standard set of hamster chromosomes plus a single human chromosome, number 11. These cells contain specific human cell surface antigens that render them sensitive to killing by specific antisera in the presence of complement. Mutant AL cells that have lost the surface markers, however, would survive and give rise to scorable colonies. The cells were irradiated with neutrons produced at the Radiological Research Accelerator Facility of Columbia University. Doses corresponding to low, moderate, and high cytotoxicities and in energies ranging from 0.33 to 14 MeV were used. Neutrons induced a dose-dependent cytotoxicity and mutation frequency in the AL cells. Over the range of doses examined, it was found that the mutagenesis induced by neutrons was energy-dependent and the frequencies were a curvilinear function of dose for both the a1 and a2 antigenic loci examined. In comparison to gamma rays, the relative biological effectiveness (RBE) for cell lethality at the 10% survival level ranged from 5.2 for 0.33 MeV to 1.8 for 14 MeV neutrons. The RBE for mutation induction at the a1 locus, however, ranged from 30 for 0.33 MeV to 4.2 for 14 MeV neutrons at or around the lowest levels of effect examined. Results of the present study demonstrated that neutrons, when measured under conditions which permit detection of a spectrum of gene and chromosomal mutations, in fact, are more efficient mutagens than previously thought.

Mutation induction by charged particles of defined linear energy transfer

The mutagenic potential of charged particles of defined linear energy transfer (LET) was assessed using the hypoxanthine-guanine phosphoribosyl transferase locus (HGPRT) in primary human fibroblasts. Exponentially growing cultures of early passaged fibroblasts were grown as monolayers on thin mylar sheets and were irradiated with accelerated protons, deuterons or helium-3 ions. The mutation rates were compared with those generated by 137Cs gamma-rays. LET values for charged particles accelerated at the Radiological Research Accelerator Facility, using the track segment mode, ranged from 10 to 150 keV/micron. After irradiation, cells were trypsinized, subcultured and assayed for both cytotoxicity and 6-thioguanine resistance. For gamma-rays, and for the charged particles of lower LET, the dose-response curves for cell survival were characterized by a marked initial shoulder, but approximated to an exponential function of dose for higher LETs. Mutation frequencies, likewise, showed a direct correlation to LET over the dose range examined. Relative biological effectiveness (RBE) for mutagenesis, based on the initial slopes of the dose-response curves, ranged from 1.30 for 10 keV/micron protons to 9.40 for 150 keV/micron helium-3 ions. Results of the present studies indicate that high-LET radiations, apart from being efficient inducers of cell lethality, are even more efficient in mutation induction as compared to low-LET ionizing radiation. These data are consistent with results previously obtained with both rodent and human fibroblast cell lines.

Radiation response characteristics of human cells in vitro

Improvements in tissue culture techniques and growth media have made it possible to culture a range of cells of human origin, both normal and malignant. The most recent addition to the list are endothelial cells from umbilical cord veins. Interesting results in radiosensitivity studies of these human cells have been obtained, some of which may have implications in radiation therapy. (i) Repair of potentially lethal damage (PLDR) has been observed in all cell lines investigated; cells of normal origin repair PLD at least as well as malignant cells, which makes clinical trials of PLDR inhibitors of doubtful usefulness. (ii) No apparent correlation can be made between the extent of PLDR and the traditional radioresponsiveness of a particular tumor type. Indeed, if anything, it could appear to have an inverse correlation since the most resistant tumor cells show the smallest amount of PLD repair. (iii) Dose-rate effects appear to be better predictors of radiosensitivity than PLDR capacity. (iv) Sublethal damage repair, manifest by a dose-rate effect, has also been observed in all human cell lines tested. Cells of normal tissue origin, including fibroblasts and endothelial cells, exhibit a dose-rate effect that is intermediate between that for cells from traditionally resistant tumors (melanoma and osteosarcoma) and cells from more sensitive tumors (neuroblastoma and breast).

Analysis of mutant frequency curves and survival curves applied to the AL hybrid cell system

A model is presented for the statistical analysis of survival curves and mutant frequency curves for a hybrid cell system. The derivation of the model is given in the Appendix, and depends on simple assumptions about the distribution of insults, their repair, and the loss of a marker that is not rescued. A single formula (5) is found which relates a survival curve to the mutant frequency curve, i. e., the response curve for production of mutants per 10(5) survivors induced by a mutagen. The analysis is applied to loss of the a1 gene in AL-J1 hybrid cells submitted to Cesium gamma-rays. Previous experimental data using X-rays was reported by Waldren et al. (1986: Proc. Natl. Acad. Sci, USA 83, 4839.) Also, a derived formula (10), which predicts the probability that in a surviving cell a marker is lost and not rescued, will form the basis for testing the validity of the model in the future using new experimental data.

Oncogenic transformation by charged particles of defined LET

Neoplastic transformation incidence and cytotoxicity were scored in the C3H/10T1/2 cell system following irradiation with charged particles of defined linear energy transfer (LET) produced at the Radiological Research Accelerator Facility of Columbia University. Cells cultured in monolayers, attached to thin Mylar sheets, were irradiated with accelerated protons, deuterons or helium-3 ions, and the results compared with X-rays. Defined LET values obtained by using the track segment mode, ranged from 10 to 120 keV/microns. For X-rays, and for the charged particles of lower LET, the dose-response curves for cell survival have a marked initial shoulder. With increasing LET, there is a progressive decrease in the size of the shoulder and for the highest LETs, survival curves approximate an exponential function of dose. The transformation incidence, likewise, showed a direct correlation to LET over the dose range examined. The efficiencies of transformation, however, appeared to approach a plateau between 80 and 120 keV/micron. Transformation data were analyzed using a linear-quadratic function of dose for the transformation probability. This latter quantity, when analyzed using microdosimetric consideration, suggests that the target volume for this end point is of the order of micrometer.

Radiation repair in human endothelial cells

Damage to blood vessels caused by ionizing radiation is considered to be an important dose limiting factor for late effects in many organs. However, the radiation response of the endothelial cells which line the vasculature has not been well-documented, particularly for human endothelial cells. In the present study, human endothelial cells were obtained from fresh umbilical cords and cultured in monolayer. Immunofluorescent staining of factor VIII antigen was used to verify the endothelial nature of the cultured cells. The cells were irradiated with Cs-137 rays (1.35 Gy/min) in plateau phase to determine radiation sensitivity and ability to repair potentially lethal damage (PLD) and sub-lethal damage (SLD). The endothelial cells demonstrated moderate radiosensitivity that varied slightly between different cords. As demonstrated by delayed plating experiments, PLD repair ability was substantial, with repair factors of 0.70-0.80. SLD repair capability, as demonstrated by split dose experiments, was relatively modest. A survival enhancement of 2.0-2.2, for example, was observed when 8 Gy single dose survival was compared to two 4 Gy doses. In terms of PLD and SLD repair as well as initial slope (alpha) the endothelial cells were similar to normal human lung and skin fibroblasts previously studied. Compared to malignant cell lines, PLD repair was generally larger whereas the initial slope (alpha) was intermediate-steeper than the radioresistant tumor types but shallower than the more sensitive tumor derived cells.

Prestaining of membrane markers to identify specific areas for Feulgen cytophotometric determinations in a single section

In cytospectrophotometric determinations of the nuclear DNA content in tissues, two consecutive sections are commonly employed: one stoichiometrically stained (as with the Feulgen reaction) for the actual measurements and a second routinely stained (as with hematoxylin and eosin) to define the limits of abnormal areas. This paper proposes the use of stainable cell membrane markers to identify the boundaries of such areas in only the one section in which DNA measurements are to be performed. The use of this procedure for the analysis of enzyme-altered foci and preneoplastic nodules in the rat liver is described. The membrane marker staining, which does not affect the nucleus or cytoplasm, does not interfere with the nuclear DNA determinations.

Radiosensitivity and sublethal damage repair in human umbilical cord vein endothelial cells

The assessment of the comparative radiosensitivity of different parenchymal cells is potentially of great importance in clinical radiotherapy. Previous studies from various laboratories using vessel regeneration techniques have demonstrated that endothelial cells are quite radioresistant, having a Do in excess of 250 rads. The present studies were undertaken to investigate the radiosensitivity of human endothelial cells obtained from umbilical cord veins (HEC) using "colony formation" as an endpoint, and to evaluate their ability to repair sublethal radiation damage (SLD). The endothelial origin of the cells in the surviving colonies was demonstrated by their positive staining for factor VIII antigen. The survival curve for HEC has a Do of about 165 rads, with a moderate initial shoulder. Dose fractionation studies demonstrate a moderate capability to repair SLD, comparable to other human cells studied including fibroblasts and other normal diploid cells in vitro. These studies indicate that HEC, being a terminally differentiated cell type with various differentiated markers, may prove useful in radiobiological and radiocarcinogenesis studies.

Hyperthermia, chemotherapeutic agents and oncogenic transformation

The modulating effects of hyperthermia on cytotoxity and oncogenicity of several chemotherapeutic agents were investigated using the C3H 10T1/2 cell system. Logarithmic phase cultures of 10T1/2 cells were treated with various doses of cis-platinum or bleomycin sulphate for 2 h, either alone or with simultaneous hyperthermia (42 degrees C for 2 h). In a second set of experiments, cells were treated for 24 h with either melphalan or cis-platinum followed by a 2 h heat treatment. While hyperthermia alone was found to be ineffective in inducing oncogenic transformation and was only moderately cytotoxic, concurrent hyperthermia with drug treatment enhanced both the toxicity and oncogenic transforming potential of drugs 3-4-fold in C3H 10T1/2 cells. Sequential heat after drug treatment, however, was found to increase toxicity slightly but had no effect on the oncogenicity of the drugs.

Correlation of in vitro genotoxicity and oncogenicity induced by radiation and asbestos fibres

The in vitro cytotoxicity and oncogenic potential of both native and acid leached asbestos fibres were studied using the C3H 10T1/2 cell model. Both native and leached fibres induced a dose-dependent toxicity. At high fibre concentrations, acid leached fibres were significantly less toxic than their untreated counterparts. While asbestos fibres alone do not induce oncogenic transformation at the concentration examined, it was found that both leached and native fibres substantially enhanced the oncogenicity of gamma-irradiation in a more than additive fashion. Although no significant chromosomal aberrations or sister chromatid exchanges (SCE) were found in asbestos treated cultures, a significantly higher number of SCEs was observed in cells treated with both asbestos and radiation compared to cells receiving radiation alone. The results suggest that the enhancement in radiation induced oncogenicity by asbestos fibres may be attributed to the mere physical presence of the fibres rather than any chemical contaminants the fibres may contain. Furthermore, the carcinogenicity of asbestos may be unrelated to genotoxicity.

In vitro assessment of the oncogenic potential of nitroimidazole radiosensitizers

Two hypoxic cell radiosensitizers, RSU-1069 and Ro-03-8799 were investigated for their in vitro cytotoxicity and ability to induce oncogenic transformation and sister chromatid exchanges in the C3H 10T1/2 cell system. Their effects were then compared to those of the clinically used sensitizer misonidazole. Equitoxic doses of Ro-03-8799 and RSU-1069 were approximately 3-fold and 150-fold less than misonidazole, respectively, with both agents exhibiting dose and contact time dependence for cell killing. Both sensitizers appeared no more oncogenic than misonidazole when administered at equitoxic dosages. At doses of equivalent sensitizing efficiencies relative to misonidazole, RSU-1069, but not Ro-03-8799, induced significantly higher transformation incidence. In conjunction with gamma-irradiation, both Ro-03-8799 and misonidazole induced an additive transformation response. Preliminary studies also indicate that RSU-1069, at a concentration of 0.03 mM, induced significantly higher sister chromatid exchanges (SCE) per chromosome than either Ro-03-8799 or misonidazole at concentrations 30-fold higher. Although several earlier studies have indicated that RSU-1069 may be more efficient than misonidazole as an hypoxic cell sensitizer, the present findings suggest that it may also carry a higher risk of inducing tumors by itself at clinically relevant concentrations.

Oncogenic transformation with radiation and chemicals

Quantitative in vitro assay systems for oncogenic transformation are a powerful research tool. They may be based on short-term cultures of hamster embryo cells, or established cell lines of mouse origin. While X-ray-induced transformation of human cells has been demonstrated, it has proved difficult to develop quantitative assay systems based on cells of human origin. The presently available quantitative assays have two quite distinct basic uses. First, they may be useful to accumulate data which is essentially pragmatic in nature. For example, they may be used to compare and contrast the oncogenic potential of chemotherapeutic agents or hypoxic cell sensitizers used or proposed in the clinic. They may be used to identify compounds that inhibit or suppress the transformation incidence resulting from known oncogenic agents, or they may be used to demonstrate the interaction between two different agents, such as radiation and asbestos. Second, they may prove to be invaluable in the study of the basic mechanisms of carcinogenesis, inasmuch as they represent models of tumourigenesis in which the various steps can be manipulated and modified more readily and in a controlled way.

Effects of a high-sucrose diet on the development of enzyme-altered foci in chemical hepatocarcinogenesis in rats

Dietary factors can modify metabolic events involved in the initiation, promotion, or progression of tumors. To determine whether a high-sucrose diet has any effect on the development of enzyme-altered foci during the promotion step of chemical hepatocarcinogenesis in rats, 1-day-old Sprague-Dawley rats were given a single i.p. dose of diethylnitrosamine; controls received an equivalent i.p. volume of 0.9% NaCl solution. At 21 days of age, the rats were weaned, segregated by sex, separated in groups, and fed modified AIN76A diets containing either 65% glucose or 65% sucrose, with or without 0.05% phenobarbital. At the end of a 4-week treatment period, the sucrose-fed control rats of either sex had significantly heavier livers than did those on the glucose diet. Enlarged livers were found also in the sucrose-fed diethylnitrosamine-treated female rats, which developed twice as many gamma-glutamyltranspeptidase-positive foci per sq cm of liver section than did those on the glucose diet. Addition of phenobarbital augmented the number of foci 3-fold in the sucrose group and 5-fold in the glucose group. Focus count per sq cm was similar in animals on the two phenobarbital-supplemented diets. Despite the absence of statistically significant liver enlargement, results analogous to those in females were obtained in carcinogen-treated males. Differences between treatments, however, were smaller. In both female and male rats, the DNA-synthesizing activity of hepatocytes in enzyme-altered foci was significantly higher than in the surrounding normal parenchymal cells, as determined by autoradiography. These studies indicate that a high-sucrose diet has a promoting effect during hepatocarcinogenesis induced in the rat by diethylnitrosamine and that this effect is weaker than that of 0.05% phenobarbital.

Effects of cellular non-protein sulfhydryl depletion in radiation induced oncogenic transformation and genotoxicity in mouse C3H 10T1/2 cells

A study was made of the effects of cellular non-protein sulfhydryl (NPSH) depletion on cytotoxicity, cell cycle kinetics, oncogenic transformation and sister chromatid exchange (SCE) in C3H 10T1/2 cells. Using DL-Buthionine S-R-Sulfoximine (BSO) at a concentration of 0.05 mM to deplete thiols, it was found spectrophotometrically that less than 5% of control NPSH level remained in the cells after 24-hour treatment under aerated conditions. Such NPSH depleted cells, when subject to a 3 Gy gamma-ray treatment, were found to have no radiosensitizing response either in terms of cell survival or oncogenic transformation. In addition, decreased levels of NPSH had no effect on spontaneous or radiation-induced SCE nor were cell cycle kinetics additionally altered. Therefore, the inability of NPSH depletion to alter gamma-ray induced cellular transformation was unrelated to any possible effect of BSO on the cell cycle. These results suggest that, while endogenous NPSH depletion has been considered to play an important role for most radiosensitizers in clinical or preclinical use, such depletion may result in little or no additional oncogenic or genotoxic effects on aerated normal tissues.