Radiation oncogenesis in cell culture

Hyperradiosensibilité aux très faibles doses: impact en radiothérapie des micrométastases

Radiobiologists have pointed out a novel radiobiological phenomenon observed in many tumor and normal cell lines: hyper-radiosensitivity to very low-dose (HRS) followed by induced radioresistance (IRR) after a threshold dose of 0.1-0.3 Gy that depends on the cell line. Radioresistance at high dose (i.e. higher than 0.5 Gy) and metastatic potential of tumor cells are likely major factors of failure in radiotherapy. A careful review of literature suggests that: 1) radiotherapy does not increase the metastatic potential of tumor cells; 2) radioresistance at high dose and metastatic potential are not related. However, inside a given tumor cell line, highly metastatic clones may elicit more cells showing HRS or are more radiosensitive at high dose than poorly metastatic ones. Recent data obtained from molecular techniques (comet and immunofluorescence assays) applied to single cells irradiated at very low radiation doses (1-100 mGy) suggest that DNA single-strand breaks (SSB) and double-strand breaks (DSB) may be the key-lesions responsible for the HRS phenomenon. These data suggest that the HRS phenomenon may find application in radiotherapy for micrometastasis. These early disseminated and probably unvascularised cells may escape the influence of high-dose chemotherapy after excision of the primary tumor. Considering the link between metastatic potential and HRS, we have previously proposed to apply very low-dose total body irradiation (TBI) at M(0) stage that may prevent the development of micrometastases. Literature data suggest that the smallest radiation dose that can produce HRS without increasing the risk of cancer may be in the milliGrays range.

Uranium in the environment: occurrence, transfer, and biological effects

The assessment of uranium chemical and radiological consequences depends on the physicochemical properties of these radioelements and the knowledge of their environmental fate. Although uranium is the source of all these fissionable isotopes, its fate in ecosystems has been poorly investigated. In this review, we have updated information concerning the fate of uranium in the different compartments of the environment, the possibility of transfer to man through the food chain, and the biological and toxicological effects of this metal at cellular, tissular, or organism levels. The physicochemical characteristics of uranium, as well as its regulatory statutes, were reviewed. The fate of uranium in the environment was presented by indicating sources of uranium emission and the possible routes of transfer to man. The biological alterations caused by uranium exposure were discussed, and finally, we presented results collected during our recent study. Some propositions on research to be done to advance the understanding of uranium occurrence in the environment were also given.

Carcinogenic risk in diagnostic nuclear medicine: biological and epidemiological considerations

During the last decade new data have become available on the mechanism of carcinogenesis and on cancer induction by ionizing radiation. This review concentrates on these two items in relation to the use of radiopharmaceuticals in diagnostic nuclear medicine. On the basis of reports of expert committees, the concept of radiation risk is elucidated for high and low doses. Mortality risk factors due to ionizing radiation are put in perspective to other risks. The extra risk for patients who undergo a scintigraphic examination for fatal cancer is very small and is of the order of 1.4 x 10(-4). It is most unlikely that this figure can even be verified by actual measurement since the majority of nuclear medicine patients will die of other causes before the radiogenic cancer manifests itself.

Development of human epithelial cell systems for radiation risk assessment

The most important health effect of space radiation for astronauts is cancer induction. For radiation risk assessment, an understanding of carcinogenic effect of heavy ions in human cells is most essential. In our laboratory, we have successfully developed a human mammary epithelial cell system for studying the neoplastic transformation in vitro. Growth variants were obtained from heavy ion irradiated immortal mammary cell line. These cloned growth variants can grow in regular tissue culture media and maintain anchorage dependent growth and density inhibition property. Upon further irradiation with high-LET radiation, transformed foci were found. Experimental results from these studies suggest that multiexposure of radiation is required to induce neoplastic transformation of human epithelial cells. This multihits requirement may be due to high genomic stability of human cells. These growth variants can be useful model systems for space flight experiments to determine the carcinogenic effect of space radiation in human epithelial cells.

Differences in membrane electrical properties between C3H 10T1/2 mouse embryo fibroblasts and their ionizing radiation and chemically transformed counterparts

Membrane electrical properties of mouse embryo fibroblasts and their ionizing radiation and chemically transformed counterparts were investigated using dielectric relaxation measurements in the radio frequency range. This determination is possible because, in the radio frequency range, suspensions of cells in an electrolyte buffer show a conductivity dispersion due to interfacial polarization. An analysis of the experimental data based on a "single-shell" model showed that conductivity and permittivity of the membranes of both radiation and chemically transformed fibroblasts were lower than in normal cells. In addition, the conductivity of the cytoplasm was higher in both transformed cell types than in the normal mouse fibroblasts. We discuss the significance of these findings in view of the possible structural and functional modifications brought about by the process of neoplastic transformation.

Chromosomal changes in cultured human epithelial cells transformed by low- and high-LET radiation

For a better assessment of radiation risk in space, an understanding of the responses of human cells, especially the epithelial cells, to low- and high-LET radiation is essential. In our laboratory, we have successfully developed techniques to study the neoplastic transformation of two human epithelial cell systems by ionizing radiation. These cell systems are human mammary epithelial cells (H184B5) and human epidermal keratinocytes (HEK). Both cell lines are immortal, anchorage dependent for growth, and nontumorigenic in athymic nude mice. Neoplastic transformation was achieved by irradiating cells successively. Our results showed that radiogenic cell transformation is a multistep process and that a single exposure of ionizing radiation can cause only one step of transformation. It requires, therefore, multihits to make human epithelial cells fully tumorigenic. Using a simple karyotyping method, we did chromosome analysis with cells cloned at various stages of transformation. We found no consistent large terminal deletion of chromosomes in radiation-induced transformants. Some changes of total number of chromosomes, however, were observed in the transformed cells. These transformants provide an unique opportunity for further genetic studies at a molecular level.

Dose-dependent induction of resistance to terminal differentiation in x-irradiated cultures of normal human keratinocytes

Epidermal cell clones able to proliferate under conditions that cause normal human foreskin keratinocytes (NHK) to terminally differentiate were obtained in a dose-dependent fashion after repeated x-irradiation. No terminal differentiation-resistant (TDR) clones were detected unless the total x-ray dose was split in several fractions given at protracted intervals. The x-ray-induced TDR cells were aneuploid and differed from growing NHK with regard to expression of specific protein markers of differentiation. One of the isolated TDR clones escaped senescence but failed to form tumors in nude mice. Altogether, these data indicate that NHK cultures can be used to quantitate phenotypic changes associated with neoplastic transformation of normal human epithelial cells upon exposure to defined regimens of physicochemical treatments.

Morphological transformation of 10T1/2 mouse embryo cells can be initiated by DNA double-strand breaks alone

Malignant transformation of mouse fibroblasts was produced by electroporation with restriction enzymes. Similar transformation frequencies were observed with Pstl, Pvull, and Xbal, which cut genomic DNA at similar overall frequencies but have different termini, i.e., a 3' overhang, a blunt end, and a 5' overhang, respectively. The dose-response curve for restriction enzyme transformation shows a marked plateau in frequencies of transformed foci per surviving cell, whereas x-irradiation of the same cells gives a linear dose-response curve. Evidently, transformation can be caused by DNA double-strand breaks alone at a limited number of sites, but the evidence from x rays suggests that other kinds of DNA damage can cause transformation independently.

Growth kinetics of C3H10T1/2 cells exposed to low-LET radiation

Growth curves and size of the colonies of C3H10T1/2 cells exposed to low-LET radiation (31 MeV protons) were determined after 0, 1, 3, 5, and 7 Gy. The data show that: cell density at confluence was 3.3 x 10(4) cells/cm2; the initial division delay was very small; in the first 15 h the increase in the cell number was essentially the same at all doses; at 100 h the colony size distribution was very large, ranging from 0 to 7 generations, even within the control population. The temporal dependence of the growth properties of surviving and non-surviving cells was represented by the equation N = N0(Fe(a(t - dD] + (1 - F)ea/bD(1 - e - bD(1 - e - bD(t - dD]) where F is the surviving fraction, t the time of sampling, a the growth rate, d the division delay per unit dose, b the rate per unit of dose at which the non-surviving cells lose their ability to divide. The resulting values were: a = 0.029 +/- 0.002 h-1; b = 0.0041 +/- 0.0009 h-1 Gy-1 and d = 1 +/- 0.8 h Gy-1. It was found that growth curves are affected by non-surviving progeny up to 150, 200 and 250 h after irradiation at 3, 5 and 7 Gy, whereas at longer times the population consists essentially of progeny of surviving cells.

A theory of carcinogenesis based on an analysis of the effects of carcinogens

Carcinogenic stimuli appear to act on target cells (and their daughters) by one or more of three mechanisms. The first is by oxidation of membrane component molecules on the extracellular surfaces of their plasma membranes. The second is by chronic and continuous impingement of electrons on the extracellular surfaces of their plasma membranes and the third is by relocation of predominantly basic molecules to the cytoplasmic surfaces of their plasma membranes. This latter effect in turn causes electrostatic attraction of image charged acidic molecules to the extracellular surfaces to balance the transmembrane charge of the target cells. Each of the above mechanisms results in a condition of increased electronegativity of the extracellular surfaces of plasma membranes of the target cells and their daughters. A theory of transformation is advanced based on the above related modes of action and it is used to explain some previously unexplainable properties of tumors.

Distinctive transforming genes in x-ray-transformed mammalian cells

DNAs from hamster embryo cells and mouse C3H/10T1/2 cells transformed in vitro by x-irradiation into malignant cells transmit the radiation transformation phenotype by producing transformed colonies (transfectants) in two mouse recipient lines, the NIH 3T3 and C3H/101/2 cells, and in a rat cell line, the Rat-2 cells. DNAs from unirradiated cells or irradiated and visibly untransformed cells do not produce transformed colonies. The transfectants grow in agar and form tumors in nude mice. Treatment of the DNAs with restriction endonucleases prior to transfection indicates that the same transforming gene (oncogene) is present in each of the transformed mouse cells and is the same in each of the transformed hamster cells. Southern blot analysis of 3T3 or Rat-2 transfectants carrying oncogenes from radiation-transformed C3H/10T1/2 or hamster cells indicates that the oncogenes responsible for the transformation of 3T3 cells are not the Ki-ras, Ha-ras, or N-ras genes, nor are they neu, trk, raf, abl, or fms, although quick blot analysis using 11 oncogene probes detected increased transcripts of c-abl and c-fms in the 3T3 transformants containing oncogenic sequences from the x-ray-transformed C3H/10T1/2 cells. The work demonstrates that DNAs from mammalian cells transformed into malignancy by direct exposure in vitro to radiation contain genetic sequences with detectable transforming activity in three recipient cell lines. The results provide evidence that DNA is the target of radiation carcinogenesis induced at a cellular level in vitro. The experiments indicate that malignant radiogenic transformation in vitro of hamster embryo and mouse C3H/10T1/2 cells involves the activation of unique non-ras transforming genes, which heretofore have not been described.

In vitro models for studying the molecular biology of carcinogenesis

Although carcinogens cause various similar deleterious effects on rodent and human cells, only rodent cells can convert to malignancy in a quantitative, predictable fashion. Therefore, the control mechanisms involving indefinite proliferation and tumorigenicity are different. Human cell lines may exhibit normal or aneuploid chromosome constitutions with numerical or structural alterations frequently involving proto-oncogene loci, but fail to produce progressively growing tumors in nude mice. A new approach for obtaining human cells susceptible to malignant transformation by chemical or physical carcinogens is to use DNA from a cancer associated virus. Transfection of human papilloma virus (HPV) DNA associated with genital cancer can extend life-span of human cells; post-X-irradiated cells grow in agar suspension. Southern blot analysis of extracted DNA indicates that HPV sequences persist. Similar results are obtained with human fibroblast and epithelial cells.

Selenium and vitamin E inhibit radiogenic and chemically induced transformation in vitro via different mechanisms

Results from in vivo and in vitro studies showing that antioxidants may act as anticarcinogens support the role of active oxygen in carcinogenesis and provide impetus for exploring the functions of dietary antioxidants in cancer prevention by using in vitro models. We examined the single and combined effects of selenium, a component of glutathione peroxidase, and vitamin E, a known antioxidant, on cell transformation induced in C3H/10T-1/2 cells by x-rays, benzo[a]pyrene, or tryptophan pyrolysate and on the levels of cellular scavenging systems and peroxide destruction. Incubation of C3H/10T-1/2 cells with 2.5 microM Na2SeO3 (selenium) or with 7 microM alpha-tocopherol succinate (vitamin E) 24 hr prior to exposure to x-rays or the chemical carcinogens resulted in an inhibition of transformation by each of the antioxidants with an additive-inhibitory action when the two nutrients were combined. Cellular pretreatment with selenium resulted in increased levels of cellular glutathione peroxidase, catalase, and nonprotein thiols (glutathione) and in an enhanced destruction of peroxide. Cells pretreated with vitamin E did not show these biochemical effects, and the combined pretreatment with vitamin E and selenium did not augment the effect of selenium on these parameters. The results support our earlier studies showing that free radical-mediated events play a role in radiation and chemically induced transformation. They indicate that selenium and vitamin E act alone and in additive fashion as radioprotecting and chemopreventing agents. The results further suggest that selenium confers protection in part by inducing or activating cellular free-radical scavenging systems and by enhancing peroxide breakdown while vitamin E appears to confer its protection by an alternate complementary mechanism.

Antagonistic action of a tumor promoter and a poly(adenosine diphosphoribose) synthesis inhibitor in radiation-induced transformation in vitro

Transformation of mouse C3H 10T1/2 cells by X-irradiation in vitro was blocked by the addition of 1 mM 3-aminobenzamide, an inhibitor of polyadenosine diphosphoribose (poly[ADP-ribose]) synthesis immediately after irradiation. 3-Aminobenzamide also inhibited an increase in the frequency of transformants caused by the addition of the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate, 7 days after irradiation. These results demonstrate a role for poly(ADP-ribose) synthesis during the initiation and promotion stages of transformation. From previous studies it is known that poly(ADP-ribose) synthesis is stimulated by the DNA damage caused by X rays during initiation. During promotion, however, 12-O-tetradecanoylphorbol-13-acetate acted as a mitogen but did not induce detectable DNA damage, and we could detect no stimulation of poly(ADP-ribose) synthetase. The roles of poly(ADP-ribose) during initiation and during promotion must, therefore, be significantly different.

Dose protraction studies with low- and high-LET radiations on neoplastic cell transformation in vitro

A major objective of our heavy-ion research is to understand the potential carcinogenic effects of cosmic rays and the mechanisms of radiation-induced cell transformation. During the past several years, we have studied the relative biological effectiveness of heavy ions with various atomic numbers and linear energy transfer on neoplastic cell transformation and the repair of transformation lesions induced by heavy ions in mammalian cells. All of these studies, however, were done with a high dose rate. For risk assessment, it is extremely important to have data on the low-dose-rate effect of heavy ions. Recently, with confluent cultures of the C3H10T1/2 cell line, we have initiated some studies on the low-dose-rate effect of low- and high-LET radiation on cell transformation. For low-LET photons, there was a decrease in cell killing and cell transformation frequency when cells were irradiated with fractionated doses and at low dose rate. Cultured mammalian cells can repair both subtransformation and potential transformation lesions induced by X rays. The kinetics of potential transformation damage repair is a slow one. No sparing effect, however, was found for high-LET radiation. There was an enhancement of cell transformation for low-dose-rate argon (400 MeV/u; 120 keV/micrometer) and iron particles (600 MeV/u; 200 keV/micrometer). The molecular mechanisms for the enhancement effect is unknown at present.

Differential requirement for SV40 early genes in immortalization and transformation of primary rat and human embryonic cells

A series of recombinant plasmids carrying various DNA fragments of SV40 early region were used to test for their ability to immortalize primary cultures of rat embryo (RE) and human embryonic kidney (HEK) cells. When primary RE cells were transfected with plasmids containing an entire early region of wild-type SV40- or a deletion mutant in the small tumor (t) antigen, dl 1410-DNA, they were all immortalized. The immortalized cells could grow in soft-agar medium and produced large tumor (T)-antigen. Cultured RE cells transfected with pW2-t, which contains a deletion in the large-T-specific coding region, also gave rise to continuous cell lines. Interestingly, two of nine RE lines immortalized by pW2-t could also grow in soft-agar medium. The plasmid pW-t8 carrying a similar fragment of SV40 DNA as pW2-t, but lacking the processing and polyadenylation signal sequences, also immortalized RE cells. Surprisingly, the plasmid pD-t1 which contains neither the intact large-T nor the small-t function also immortalized RE cells. However, the RE lines immortalized by pW-t8 or pD-t1 were unable to grow in soft-agar medium and displayed a wide range of growth phenotypes. On the contrary, when primary HEK cells were used for immortalization experiments, only those SV40 plasmids carrying the intact large-T function were able to generate immortalized lines. The growth properties of these immortalized HEK lines can be categorized into two groups. Those HEK lines immortalized by the large T alone grew slightly denser and rounder than their parental normal HEK cells, while those immortalized by both the large-T and small-t antigens grew extremely fast, reached higher density, piled up on each other, and were anchorage independent. In addition, when these SV40 plasmids were used to directly transform primary HEK cells by the focus assay, the large-T clone, pD3-05, only transformed HEK cells to form light foci. Transfection by the large-T plus the small-t sequences either in cis or in trans, did increase the frequency of focus formation, and gave rise to dense foci which could grow in soft-agar medium.

Transformation of human bronchial epithelial cells transfected by Harvey ras oncogene

Transfection of normal human bronchial epithelial (NHBE) cells with a plasmid carrying the ras oncogene of Harvey murine sarcoma virus (v-Ha ras) changed the growth requirements, terminal differentiation, and tumorigenicity of the recipient cells. One of the cell lines isolated after transfection (TBE-1) was studied extensively and shown to contain v-Ha ras DNA. Total cellular RNA from TBE-1 cells hybridized to v-Ha ras structural gene fragment probes five to eight times more than RNA from parental NHBE cells. The TBE-1 cells expressed phosphorylated v-Ha ras polypeptide p21, showed a reduced requirement for growth-factor supplements, and became aneuploid as an early cellular response to v-Ha ras expression. As the transfectants acquire an indefinite life-span and anchorage independence they became transplantable tumor cells and showed many phenotypic changes suggesting a pleiotropic mechanism for the role of Ha ras in human carcinogenesis.

Neoplastic cell transformation by energetic heavy ions and its modification with chemical agents

For many years we have been interested in understanding the potential carcinogenic effects of cosmic rays. We have studied the oncogenic effects of cosmic rays with accelerator-produced heavy particle radiation and with a cultured mammalian cell system--C3H10T1/2 cells. Our quantitative data obtained with carbon, neon, silicon, and iron particles showed that RBE is both dose and LET dependent for neoplastic cell transformation. RBE is higher at lower dose, and RBE increases with LET up to about 200 keV/micrometer. In nonproliferation confluent cells, heavy-ion induced transformation damage may not be repairable, although a dose modifying factor of about 1.7 was observed for X-ray radiation. Our recent studies with super-heavy high-energy particles, e.g., 960 MeV/U U235 ions (LET = 1900 keV/micrometer), indicate that these ions with a high inactivation cross-section can cause neoplastic cell transformation. The induction of cell transformation by radiation can be modified with various chemicals. We have found that the presence of DMSO (either during or many days after irradiation) decreased the transformation frequency significantly. It is, therefore, potentially possible to reduce the oncogenic effect of cosmic rays in space through some chemical protection.

Inhibition of malignant transformation in vitro by inhibitors of poly(ADP-ribose) synthesis

Malignant transformation in vitro of hamster embryo cells and mouse C3H 10T 1/2 cells by x-rays, ultraviolet light, and chemical carcinogens was inhibited by benzamide and by 3-aminobenzamide at concentrations that are specific for inhibition of poly(ADP-ribose) formation. These compounds slow the ligation stage of repair of x-ray and alkylation damage but not of ultraviolet light damage. At high concentrations they also inhibited de novo synthesis of DNA purines and DNA methylation by S-adenosylmethionine. The suppression of transformation by the benzamides is in striking contrast to their reported effectiveness in enhancing sister chromatid exchange, mutagenesis, and killing in cells exposed to alkylating agents. Our results suggest that mechanisms regulating malignant transformation are different from those regulating DNA repair, sister chromatid exchange, and mutagenesis and may be associated with changes in gene regulation and expression caused by alterations in poly(ADP-ribosyl)ation.

Resistance of human cells to tumorigenesis induced by cloned transforming genes

The transformation of human cells was examined by transfection of cloned oncogenic DNAs derived from the tumor virus simian virus 40 and from the human bladder carcinoma cell line EJ into diploid fibroblasts derived from foreskin (FS-2 cells). The simian virus 40 DNA was found to induce a morphologically transformed phenotype, leading to easily detectable focus formation. Tumor antigen was produced, but the transformed cells were not tumorigenic in the nude mouse. The EJ gene, a mutant form of the cellular c-Ha-ras gene, actively transforms NIH/3T3 mouse cells and CHEF/18 hamster cells but is inactive in FS-2 cells. Morphological transformation, focus formation, and tumorigenicity in nude mice were not induced when EJ DNA was transfected into FS-2 cells by using the selectable vector pSVgptEJ. The intactness of the transfected EJ DNA was established by restriction fragment analysis. This result raises the question of what role, if any, the mutated gene derived from the EJ cells played in the origin of the EJ bladder carcinoma.

Critical role played by thyroid hormone in induction of neoplastic transformation by chemical carcinogens in tissue culture

Incubation of primary cultures of hamster embryo cells (HEC) or mouse fibroblasts (C3H/10T1/2 cells) in media depleted of thyroid hormones does not alter cell growth or survival but renders the cells resistant to neoplastic transformation by benzo[a]pyrene (B[a]P) or N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), carcinogens which yield transformation rates of 10(-4)-10(-2) in media supplemented with triiodothyronine (T3). In C3H/10T1/2 cells, the times of addition or removal of the hormone indicate that T3 exerts maximum effect when added 12 hr prior to treatment with B[a]P and that the progression of transformation from the time of initiation by the carcinogen to full expression and the appearance of transformed foci was independent of the presence or absence of the hormone in the medium. Dependence of transformation on T3 concentration in the medium was observed over the physiological range of 1 pM to 100 nM in C3H/10T1/2 cells treated with B[a]P. These results were similar to our previous findings on the T3 dose-related induction of radiogenic transformation and of Na+,K+-ATPase activity. The latter effect was used as a measure of T3 induction of protein synthesis. A further indication of the potential involvement of protein synthesis in T3 action is the suppression of T3- and B[a]P-dependent transformation by cycloheximide at concentrations that inhibit protein synthesis by approximately equal to 50% in the C3H/10T1/2 cells. We suggest that thyroid hormone induces the synthesis of a host protein that plays a key role in neoplastic transformation by direct-acting chemical carcinogens and by those requiring metabolic activation. In our previous studies, similar T3-dependent mechanisms were implicated in radiogenic transformations.

Modifiers of free radicals inhibit in vitro the oncogenic actions of x-rays, bleomycin, and the tumor promoter 12-O-tetradecanoylphorbol 13-acetate

Using short-term cultures of hamster embryo cells, we have examined the effects of the free-radical scavenger superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) and the enzyme catalase (hydrogen-peroxide:hydrogenperoxide oxidoreductase, EC 1.11.1.6) on x-ray- and bleomycin-induced transformation and on the enhancement of radiogenic transformation by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA). We find that superoxide dismutase inhibits (i) transformation induced by x-ray and bleomycin and (ii) promotional action of TPA in vitro. The results suggest that the oncogenic action of x-rays and bleomycin and the enhancement of oncogenic transformation by TPA are mediated in part by free radicals. The findings also suggest that superoxide dismutase can serve as an inhibitor of oncogenesis and that its actions, as seen in this in vitro system, are most predominantly on inhibiting late events in the progression of cellular transformation--those associated with promotion.