Effect of tumor promoting agents on mutation frequencies in cultured V79 Chinese hamster cells

Cell-cell contact interactions conditionally determine suppression and selection of the neoplastic phenotype

Separation of chemical and physical carcinogenesis into the stages of initiation (mutation) and promotion (selection) established that incipient neoplastic cells could persist in the organism indefinitely without expression. Spontaneous mutations associated with cancer also lie dormant in untreated normal tissue. Without selection, there is no tumor development. Experiments in cell culture showed that confluent normal fibroblasts suppress growth of contacting transformed fibroblasts, and that normal keratinocytes similarly suppress tumor formation by adjacent papilloma cells. With cells that are generally more susceptible to transformation, however, prolonged contact inhibition progressively selects mutants that favor neoplastic growth. Selection of individual mutant cells allows them to become a significant fraction of the population and creates an enlarged target for additional genetic hits. Crucially, this enrichment step, not the initial mutation step, is the numerically limiting factor in tumor development. Unexpectedly, variants that are resistant to spontaneous transformation are selected in vitro by growing cells for many low density passages at maximal exponential rate. Confluent cultures of resistant variants suppress the growth and normalize the morphology of contacting transformed cells. Varying the conditions for selection shows that tumorigenic transformation is preceded by intermediate steps of progressively higher saturation density that are increasingly permissive for the expression of the more neoplastic cells in the population. There is also evidence of increasing permissiveness with age of normal tissues in vivo for solitary cancer cells transplanted in their midst. Spontaneous transformation in culture can be used to identify dietary components that are required for promotion and may therefore be applicable in prevention of human cancer.

Microenvironmental Regulation of the Initiated Cell

In the classical skin model of tumor initiation, keratinocytes treated once with carcinogen retain their normal appearance and growth behavior indefinitely unless promoted to growth into papillomas. Because many of the papillomas regress and may recur with further promotion, their cells can also be considered as initiated. The growth of initiated keratinocytes can be inhibited either in vitro or in vivo by close association with an excess of normal keratinocytes, but it is enhanced by dermal fibroblasts. Chick embryo fibroblasts (CEF) in culture produce transformed foci after infection with Rous sarcoma virus (RSV) on a background of normal CEF in a medium containing 10% or less calf serum (CS), but they retain normal appearance and growth regulation in 10% fetal bovine serum (FBS) or 20% CS. Transformation of a carcinogen-treated line of mouse embryo fibroblasts is prevented, and can be reversed, in high concentrations of FBS in the presence of an excess of normal cells. FBS has high, broad-spectrum antiprotease activity. Increased protease production occurs in a variety of transformed cells and is correlated with progression in tumors. Protease treatment stimulates DNA synthesis and mitosis in confluent, contact-inhibited normal cell cultures. Synthetic inhibitors of proteases suppress transformation in carcinogen-treated cultures and inhibit tumor formation in animals. Several different classes of protease may be overexpressed in the same transformed cells. It is proposed that excessive protease production accounts for major features of neoplastic transformation of initiated cells, but that transformation can be held in check by protease inhibitors present in serum and released from surrounding cells. It would be informative to determine whether high concentrations of FBS would inhibit the neoplastic development of initiated keratinocytes.

Selective clonal expansion and microenvironmental permissiveness in tobacco carcinogenesis

Historically our knowledge about the direct carcinogenic activity of cigarette smoke and its constituents grew from painting experiments on the skin of mice to produce papillomas and carcinomas. The neutral fraction of cigarette smoke condensate had most of the carcinogenic activity in this test and was rich in carcinogenic polycyclic aromatic hydrocarbons (PAHs), the most abundant by far being BP. However, the concentration of BP in the condensate was only about 2% the amount of pure BP required to cause skin tumors. In other fractions there were non-carcinogenic constituents that promoted tumor formation when applied repeatedly to mouse skin that had been initiated by a single subcarcinogenic application of BP. There were also constituents of cigarette smoke that acted as co-carcinogens when applied simultaneously with repeated applications of BP. BP was effective as an initiator at lower concentrations than as a complete carcinogen, and some non-carcinogenic PAHs in the condensate were also active initiators. It was concluded from these studies that cigarette smoke condensate is primarily a tumor-promoting and co-carcinogenic agent with weak activity as a complete carcinogen. A major effect of promoters, and possibly of co-carcinogens, is a diffuse hyperplasia which includes selective expansion of clones carrying endogenous mutations and/or mutations induced by PAHs and other carcinogens such as NNK. The induced mutations as well as damaged cells would occur throughout the exposed region and, along with the hyperplasia, increase the permissiveness of the cellular microenvironment for neoplastic expression of any potential tumor cell in its midst. Since neither the promoters nor co-carcinogens in tobacco smoke are known to interact directly with DNA, their effects can be considered epigenetic processes that act upon genetically altered cells. Examples are cited from studies of experimental skin carcinogenesis, smoking-induced histopathological changes in human lung and spontaneous transformation in cell culture to illustrate the genetic and epigenetic interactions of neoplastic development in general and their significance for smoking-induced lung cancer in particular. Certain dietary modifications that appear to be effective in moderating the promotional phase of animal and human carcinogenesis are suggested for trial in managing lung cancer.

Synergistic mechanisms in carcinogenesis by polycyclic aromatic hydrocarbons and by tobacco smoke: a bio-historical perspective with updates

B[a]P (benzo[a]pyrene) has been used as a prototype carcinogenic PAH since its isolation from coal tar in the 1930's. One of its diol epoxides, BPDE-2, is considered its ultimate carcinogen on the basis of its binding to DNA, mutagenicity and extreme pulmonary carcinogenicity in newborn mice. However, BPDE-1 has a similar binding to DNA and mutagenicity but it is not carcinogenic. In addition, BPDE-2 is a weak carcinogen relative to B[a]P when repeatedly applied to mouse skin, the conventional assay site. Its carcinogenicity is increased when applied once as an initiator followed repeatedly by a promoter. This indicates a major role for promotion in carcinogenesis by PAHs. Promotion itself is a 2-stage process, the second of which is selective propagation of the initiated cells. Persistent hyperplasia underlies selection by promoters. The non-carcinogenicity of BPDE-1 has yet to be resolved. PAHs have long been considered the main carcinogens of cigarette smoke but their concentration in the condensate is far too low to account by themselves for the production of skin tumors. The phenolic fraction does however have strong promotional activity when repeatedly applied to initiated mouse skin. Several constituents of cigarette smoke are co-carcinogenic when applied simultaneously with repeated applications of PAHs. Catechol is co-carcinogenic at concentrations found in the condensate. Since cigarette smoking involves protracted exposure to all the smoke constituents, co-carcinogenesis simulates its effects. Both procedures, however, indicate a major role for selection in carcinogenesis by cigarette smoke. That selection may operate on endogenous mutations as well as those induced by PAHs. There are indications that the nicotine-derived NNK which is a specific pulmonary carcinogen in animals contributes to smoking-induced lung cancer in man. Lung adenoma development by inhalation has been induced in mice by the gas phase of cigarette smoke. The role of selection has not been evaluated in either of these cases.

Avaliação do potencial recombinogênico do antibiótico danofloxacina em células diplóides de Apergillus nidulans

Estudou-se o potencial recombinogênico da danofloxacina, novo antimicrobiano pertencente ao grupo das 4-fluoroquinolonas e de uso exclusivo em medicina veterinária, no fungo filamentoso Aspergillus nidulans. A linhagem mestra UT196 e o mutante Z1 foram utilizados para formar o diplóide Z1//UT196. Conídios desse diplóide foram inoculados em placas de Petri contendo meio mínimo suplementado com 2,5, 5,0 e 10,0 mig/ml de danofloxacina. As placas foram incubadas por cinco dias a 37 ºC. Segregantes mitóticos foram isolados das colônias tratadas com o antimicrobiano e as análises de seus fenótipos evidenciaram o efeito recombinogênico da danofloxacin na dose de 10,0 mig/ml. Recombinantes para vários intervalos dos cromossomos I e II foram identificados entre os segregantes analisados.

Chemical induction of interleukin-8, a proinflammatory chemokine, in human epidermal keratinocyte cultures and its relation to cytogenetic toxicity

Tumor promoters, proinflammatory cytokines, endotoxins, and protein synthesis inhibitors can modulate cell cycle kinetics of various cell types, stimulate production of reactive oxygen species, and induce keratinocytes to produce interleukin-8 (IL-8), a potent chemotactant for polymorphonuclear neutrophils and T lymphocytes. The aim of this study was to determine whether perturbations of cytogenetic responses correlated with the induction of IL-8 expression. Cultures of primary human keratinocytes were grown in serum-free medium with 5 mumol/L bromodeoxyuridine to label DNA and exposed either to phorbol-13-myristate-12-acetate (PMA) (0.0001-100 ng/ml), cycloheximide (CHX) (0.01-50 micrograms), lipopolysaccharide (0.1-100 micrograms/ml), tumor necrosis factor-alpha (TNF alpha) (3.13-50 ng/ml), or interleukin-1 alpha (IL-1 alpha) (1-182 pg/ml). Metaphase chromosome preparations were stained by a fluorescence-plus-Giemsa technique to differentiate sister chromatids. For IL-8 production, keratinocytes were grown to 70% confluency and then exposed to chemicals for 24 h. Immunoreactive IL-8 was quantitated from the supernatants by ELISA. With the exception of benzo(a)pyrene used as a positive control, none of the agents induced sister chromatid exchanges. However, PMA and TNF alpha induced IL-8 production that coincided with significant cell cycle inhibition. IL-1 alpha had no effect on cytogenetic endpoints, yet stimulated a 6.3-fold increase in IL-8. CHX inhibited cell cycle progression and mitotic activity at concentrations that were 200 times lower than required for IL-8 induction; however, puromycin (0.31-10 micrograms/ml), another protein synthesis inhibitor, did not induce IL-8. At all concentrations tested, TNF alpha reduced the mitotic index by approximately 45%, slowed cell cycle progression by approximately 3.5 h, and induced a flat, albeit large, IL-8 response at concentrations > or = 12.5 ng/ml. These agent-specific response patterns suggest that induction of IL-8 production is not always the inevitable result of cell cycle perturbations or genetic damage.

Multistage carcinogenesis in mouse skin

The mouse skin model of multistage carcinogenesis has for many years provided a conceptual framework for studying carcinogenesis mechanisms and potential means for inhibiting specific stages of carcinogenesis. The process of skin carcinogenesis involves the stepwise accumulation of genetic change ultimately leading to malignancy. Initiation, the first step in multistage skin carcinogenesis involves carcinogen-induced genetic changes. A target gene identified for some skin tumor initiators is c-Ha-ras. The second step, the promotion stage, involves processes whereby initiated cells undergo selective clonal expansion to form visible premalignant lesions termed papillomas. The process of tumor promotion involves the production and maintenance of a specific and chronic hyperplasia characterized by a sustained cellular proliferation of epidermal cells. These changes are believed to result from epigenetic mechanisms such as activation of the cellular receptor, protein kinase C, by some classes of tumor promoters. The progression stage involves the conversion of papillomas to malignant tumors, squamous cell carcinomas. The accumulation of additional genetic changes in cells comprising papillomas has been correlated with tumor progression, including trisomies of chromosomes 6 and 7 and loss of heterozygosity. The current review focuses on the mechanisms involved in multistage skin carcinogenesis, a summary of known inhibitors of specific stages and their proposed mechanisms of action, and the relevance of this model system to human cancer.

Mutagenic effects of TPA-induced clastogenic factor in Chinese hamster cells

Previous work in our laboratory has shown that the clastogenic and SCE-inducing effects of 12-O-tetradecanoylphorbol 12-acetate (TPA) are mediated by secondary products formed by the cell in response to the tumor promoter. A low-molecular-weight clastogenic factor (CF) was isolated from the medium of TPA-treated human leukocytes and caused chromosome aberrations and sister-chromatid exchanges (SCE) in fresh cultures not exposed to TPA itself. In the present study, we show that Chinese hamster fibroblasts (V79 cells) also produce CF when exposed to TPA. CF from V79 cells induced SCE not only in hamster cells, but also in human lymphocytes. Vice versa, CF from human leukocyte cultures induced SCE in hamster cells. It also increased the frequency of 6-thioguanine-resistant mutants in this cell system. All cyto- and geno-toxic effects of TPA-induced CF were prevented if the cells were treated with superoxide dismutase before exposure. The lipophilic CF seems to be derived from arachidonic acid of cell membranes released as a consequence of oxidative damage and subsequently degraded to genotoxic aldehydes in an autoxidative process. CF is formed only under culture conditions with low antioxidant content in culture media and sera. This may explain the discordant results obtained by different laboratories with regard to the genotoxic effects of TPA.

Enhancement of carcinogen-induced mutations or recombinations by 12-O-tetradecanoyl-phorbol-13-acetate in the mammalian spot test

The mammalian spot test is suitable for detecting gene mutations and reciprocal recombinations. When given alone 12-O-tetradecanoyl-phorbol-13-acetate (TPA) did not have any statistically significant effect upon these genetic alterations. In combination with ethylnitrosourea (ENU) however TPA enhanced the effect of the mutagen/carcinogen. The effective dose range of TPA + ENU was very small, i.e., between 2 X 0.2 and 2 X 0.33 mg/kg, but the effect was very strong as shown by the steep slope of the dose-effect curve. The results agree with the hypothesis that the mode of action of cocarcinogens and tumor promoters is a genetic one and that recombination plays an important role in this process.

A mutagenesis-enhancing activity of 12-O-tetradecanoylphorbol 13-acetate detected in Chinese hamster ovary cells

Tumour-promoting agents may bring about the completion of multi-step carcinogenesis by acting as enhancers of mutagenesis, recombinogens or clastogens. We report here that the classical mouse skin tumour promoter TPA, although non-mutagenic per se, can enhance the induction of OuaR CHO-K1 cell mutants by MNNG approximately 2-fold. This observation was made at a concentration approaching the compounds aqueous solubility limit which was non-cytotoxic. Mutagenesis enhancement was dependent on TPA being present throughout mutation expression and mutant selection. It was not accompanied by any modification of cell sensitivity to mutagen killing. In the same treatment protocol TPA did not enhance either EMS- or UV-induced mutagenesis. TPA exposure over 2 rounds of cell replication failed to produce an increase in the frequency of SCE in control or mutagen-treated CHO-K1 cultures. Likewise TPA exposure over 1 round of cell replication failed to produce an increase in the frequency of chromosomal aberrations. Apparently TPA is not a recombinogen or clastogen but in the right exposure regime is capable of acting to enhance mutagenesis by certain genotoxic agents, an action which may contribute to tumour promotion.

Genetic mode of action of cocarcinogens and tumor promoters in yeast and mice

In experiments with yeast, cocarcinogens were found to be comutagenic and antirecombinogenic , tumor promoters to be corecombinogenic and antimutagenic. Substances that were cocarcinogens as well as tumor promoters had an intermediary effect. These results were confirmed in the mammalian spot test: By in vivo treatment of mice with the cocarcinogen catechol and the tumor promoter limonene carcinogen-induced recombination due to mitotic crossing over and gene mutations was reduced and enhanced, respectively. Our results support the hypothesis that mutagenesis is the mechanism by which chemicals induce malignancy, and that cocarcinogens modify the process by enhancement of mutagenicity whereas tumor promoters effect carcinogenesis by increase of the spontaneous frequency of recombination. In addition, induced mitotic recombination in mammals in vivo has been demonstrated for the first time.

Tumor-promoting phorbol esters inhibit procollagen synthesis at a pretranslational level in JB-6 mouse epidermal cells

Collagen synthesis was inhibited in JB-6 mouse epidermal cells after exposure to 12-O-tetradecanoylphorbol-13-acetate under conditions leading to irreversible neoplastic transformation. In vitro translation and hybridization studies demonstrated a dramatic decrease in collagen mRNA in 12-O-tetradecanoylphorbol-13-acetate-treated cells, suggesting that the inhibition of collagen synthesis in response to 12-O-tetradecanoylphorbol-13-acetate is due to regulation at a pretranslational level.

Tumor-promoting phorbol esters inhibit procollagen synthesis at a pretranslational level in JB-6 mouse epidermal cells

Collagen synthesis was inhibited in JB-6 mouse epidermal cells after exposure to 12-O-tetradecanoylphorbol-13-acetate under conditions leading to irreversible neoplastic transformation. In vitro translation and hybridization studies demonstrated a dramatic decrease in collagen mRNA in 12-O-tetradecanoylphorbol-13-acetate-treated cells, suggesting that the inhibition of collagen synthesis in response to 12-O-tetradecanoylphorbol-13-acetate is due to regulation at a pretranslational level.

Measurement of the response of HELA cells towards the radiomimetic activity of 12-O-tetradecanoylphorbol-13-acetate

The sensitivity of HeLa cells in the G2 phase of the cell cycle towards the radiomimetic activity of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) (Kinzel et al. (1980) Science, 210, 429-431) has been utilized to establish dose response relationships. This was accomplished by analysis of mitotic curves and determination of cells not affected, a measurement known to be dose dependent in the case of very low X-ray doses. Half maximal activity was exerted by TPA at approximately 5 X 10(-9) M concentration. Mechanistically, however, TPA seems to exert its activity through a route different from that of X-rays or other radiomimetic drugs for 2 reasons: (1) the cellular response does not increase proportionally with dose; (2) cells recover from G2 blockage even in the presence of TPA. Therefore it appears as if the tumor promoter acts indirectly by triggering an exhaustable cellular activity which leads to the radiomimetic response.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Induction of supermelanin synthesis and morphological changes in interspecific reconstituted cells and its reversal by tumor promoter

Chloramphenicol-resistant (CAPr) reconstituted cells and cybrids were isolated by fusion of karyoplasts (or intact cells) of mouse amelanotic melanoma B16 cells with cytoplasts of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) -deficient, CAPr rat myoblastic cells, L6TG.CAPr, and double selection in HAT medium containing CAP. Reconstituted cells or cybrids exhibited unique cellular arrangement, and about one third of the isolated clones expressed high tyrosinase activity and marked melanin synthesis, although the parental mouse cells expressed low tyrosinase activity and the parental rat cells did not express tyrosinase activity. These phenotypic changes have been stable for more than a year. The phenotypic reversions of these clonal cells were induced by treatment with a tumor promoter. There were changes in the morphology of the treated cells to that of the mouse B16 cells and extinction of tyrosinase activity and melanin synthesis in pigmented clonal cells. These phenotypic changes and reversions induced by a promoter were repeatedly reversible.

Tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate enhances sister chromatid exchanges and numerical and structural chromosome aberrations in primary mouse epidermal cell cultures

The tumor 12-O-tetradecanoyl-phorbol-13-acetate (TPA) moderately stimulated sister chromatid exchanges in primary epidermal cultures (PEC) from C3H mice, and strongly enhanced structural chromosome aberrations. In G-banded metaphases for TPA (10-(8) and 10-(6) M for 54 h) treated PEC aneuploidy (hypo- and hyperdiploidy) increased and structural aberrations were enhanced 8- to 10-fold. Breaks, fragments and metacentric chromosomes had raised 7- to 11-fold. Chromatid interchanges (tri- and quadri-radials) and centromeric splitting, virtually absent in controls, appeared in 4--8% of metaphases. The non-promoting 4-O-methyl-TPA did not induce chromosomal alterations. These substantial effects on the genetic material of target cells represent a new aspect of the mechanism of action of tumor-promoting phorbol esters.

Mutagenesis by chemical agents in V79 chinese hamster cells: a review and analysis of the literature. A report of the Gene-Tox Program

The report reviews and evaluates the current literature (about 125 primary publications) on chemically induced specific locus mutations in the V79 Chinese hamster lung cell line. The V79 cell is convenient to use for mutagenesis studies since it has a rapid growth rate, high plating efficiency, and a stable karyotype. Mutation can be easily measured at either the hypoxanthine-guanine phosphoribosyl transferase or the Na+/K+ ATPase locus, both of which have been well characterized. Other less-studied markers are also described. We discuss the protocols for quantitative mutation studies including measurements of cytotoxicity, mutant expression times, mutant selection agents, cell densities during selection, and the stability and verification of mutant phenotypes. Mutations in the V79 cells by chemicals that require activation can be tested after their metabolism by cell homogenates or by intact cells, and the results with each type of activation are compared. For purposes of analysis, we classified a compound as mutagenic if it induced a mutation frequency that is at least 3 times higher than the spontaneous mutant frequency reported for that specific experiment. By this criterion two-thirds of the chemicals analyzed were mutagenic--; 11% with and 55% without metabolic activation. Of the 191 chemicals examined; 119 were polycyclic aromatic hydrocarbons; 25 were nitro or nitroso compounds, 9 were alkyl halides; 7 were purine or pyrimidine derivatives and the remaining 31 were from other chemical classes. We also defined mutagenic potency as the concentration of a compound that increases the mutant frequency by 10 times the spontaneous frequency. Mutagenic potencies of the compounds examined varied over a range of 5 X 10(6). We have also found large interlaboratory variations in the mutagenic potencies. Such variation in potency could be reduced by normalizing the results to a standard mutagen such as N-methyl-N'-nitro-N-nitrosoguanidine. The role of the V79 assay in mutagenicity and carcinogenicity testing is discussed and recommendations are suggested for future investigation.

On the possibility of metabolic control of replicon "misfiring": relationship to emergence of malignant phenotypes in mammalian cell lineages

Constraints of a multireplicon chromosomal organization and of the necessity to maintain constant gene dosages demand that each origin of replication in a eukaryotic cell "fire" (initiate replication) only once per cell cycle. The central idea of this work is that a low probability of an extra ("illegitimate") round of DNA replication (called below "replicon misfiring") within any given chromosomal domain could be increased by certain substances of either intra- or extracellular origin. The term " "firone" is proposed for such a substance. It is shown that existence of firones could greatly speed up evolution of cellular systems under selection pressure, a developing tumor being one example of such a system. Experimentally testable predictions of the firone hypothesis are discussed.

Effect of tumor promoter 12-O-tetradecanoyl-phorbol 13-acetate on induction of sister-chromatid exchanges in Chinese hamster V79 cells treated with mutagens

The effect of a tumor promoter, 12-O-tetradecanoyl-phorbol 13-acetate (TPA) alone and in combination with mitomycin C (MMC) or cyclophosphamide (CPP) on the induction of sister-chromatid exchanges (SCE) in Chinese hamster V79 cells was investigated. TPA alone at various doses and durations caused no increase of SCE frequency. MMC either at the dose of 0.03 or 0.003 microgram/ml alone or in combination with TPA (2 microgram/ml) all caused a significant increase of SCE frequencies. There was no difference in SCE frequencies between the cultures treated with MMC alone at 0.03 microgram/ml and those treated with MMC plus TPA. However, cultures treated with MMC at 0.003 microgram/ml plus TPA had significantly and consistently higher SCE frequencies than those treated with MMC alone at all durations. Treatment of CPP at 1 microgram/ml activated by S9 mix caused significant increase of SCE frequencies. Surprisingly, the cultures treated with CPP, S9 mix plus TPA (2 microgram/ml) caused a drastic reduction of SCE frequencies as compared to those treated with CPP and S9 mix only at all durations. These results indicate that TPA alone had no effect on SCE in V79 cells. TPA enhanced the SCE induction in V79 cells treated with MMC at a low dose, i.e. 0.003 microgram/ml, but it inhibited SCE induction in cultures treated with the indirect mutagen CPP. Thus, TPA has no direct effect on genetic materials but it may indirectly alter the effects of a mutagen.

The effect of non-phorbol promoters as compared with phorbol myristate acetate on sister chromatid exchange induction in cultured Chinese hamster cells

The effect of 5 unrelated chemicals with different promoting potencies on sister chromatid exchange (SCE) induction in V79 cells was investigated. Two powerful promoters--12-O-tetradecanoylphorbol-13-acetate (TPA) and anthralin; a moderate promoter--iodoacetic acid (IAA); two weak promoters--ethyl phenylpropiolate (EPP) and cantharidin--all induced similar and low numbers of SCE. These results do not support the hypothesis that enhanced mitotic recombination is responsible for tumour promotion.

Enhancement of recovery of chemical carcinogen-induced ouabain-resistant mutants in Chinese hamster cells by the tumor-promoting agent, 12-o-tetradecanoyl-phorbol-13-acetate

The effects of a tumor-promoting agent on the frequency of mutation to ouabain resistance and survival of Chinese hamster cells treated with a chemical carcinogen have been investigated. 12-O-Tetradecanoyl-phorbol-13-acetate (TPA) significantly enhanced the mutation frequency induced by the carcinogen, methylazoxymethanol acetate (MAM), without having similar effects on cytotoxicity, at concentrations of 2 micrograms/ml or less. The observed degree of enhancement of mutagenesis increased with promoter concentration up to the point where the latter exhibited frank toxicity. Exposure of the cells to the promoter for a period of 2 or 6 h was found ineffective, but subsequently a significant enhancement was found after a 27-h exposure time. The maximum effect occurred after a 5-day exposure, with a increase in the mutation frequency of 250%. Treatment of cells with TPA alone resulted in no enhancement of spontaneous mutation rates, nor did treatment of cells prior to addition of carcinogen-induced mutagenesis. In contrast, TPA was found to be effective when applied as late as 6 weeks following carcinogen treatment. These results are consistent with the hypothesis that TPA owes its promoting activity toward chemically-induced mutagenesis and carcinogenesis to its ability to enhance expression of latent somatic genetic modifications by epigenetic mechanisms. They do not support mechanisms involving TPA-induced inhibition of DNA-repair replication, or mutagenic activity of TPA per se. The notably similar qualitative response to TPA of several parameters in mouse-skin tumorigenesis and Chinese hamster cell mutagenesis suggest that the mechanism of action of the promoter is similar in the 2 diverse biological systems.

Tumor promoter TPA mimics irradiation effects on the cell cycle of HeLa cells

When asynchronous and synchronous HeLa cells were incubated with small doses (10(-7) M) of tumor promoter 12 O-tetradecanoylphorbol-13-acetate (TPA), a variety of transient alterations in the replication cycle were detected within 24 hours by the use of independent methods. Especially, a delayed passage through the S phase and influences on the G2 phase resemble x-ray irradiation effects on cell cultures. None of these alterations was observed with the hyperplasiogenic but nonpromoting 4-O-methyl-TPA.

Mutagenicity of airborne particles

The physical and chemical properties of airborne particles are important for the interpretation of their potential biologic significance as genotoxic hazards. For polydisperse particle size distributions, the smallest, most respirable particles are generally the most mutagenic. Particulate collection for testing purposes should be designed to reduce artifact formation and allow condensation of mutagenic compounds. Other critical factors such as UV irradiation, wind direction, chemical reactivity, humidity, sample storage, and temperature of combustion are important. Application of chemical extraction methods and subsequent class fractionation techniques influence the observed mutagenic activity. Particles from urban air, coal fly ash, automobile and diesel exhaust, agricultural burning and welding fumes contain primarily direct-acting mutagens. Cigarette smoke condensate, smoke from charred meat and protein pyrolysates, kerosene soot and cigarette smoke condensates contain primarily mutagens which require metabolic activation. Fractionation coupled with mutagenicity testing indicates that the most potent mutagens are found in the acidic fractions of urban air, coal fly ash, and automobile diesel exhaust, whereas mutagens in rice straw smoke and cigarette smoke condensate are found primarily in the basic fractions. The interaction of the many chemical compounds in complex mixtures from airborne particles is likely to be important in determining mutagenic or comutagenic potentials. Because the mode of exposure is generally frequent and prolonged, the presence of tumor-promoting agents in complex mixtures may be a major factor in evaluation of the carcinogenic potential of airborne particles.

Chromosome aberration formation and sister chromatid exchange in relation to DNA repair in human cells

Apparent association between the ability to induce chromosome aberrations and sister chromatid exchanges and mutagenic-carcinogenic potential found in a variety of physical and chemical agents has led us to speculate that these cytogenetic changes might be reflection of DNA damage and repair and might provide induces of mutagenic changes. However, the mechanisms of their formation and their relation to DNA repair as well as the mechanism of their linking to mutation are by no means well understood. Studies in some human genetic mutant cells defective in their ability to repair DNA damage indicate, as a testable proposition, that sister chromatid exchanges and chromosome aberrations are cytological manifestations of replication-mediated dual-step repair pathways that are in operation to tolerate DNA damage when damage-bearing DNA enters and passes through semiconservative replication. The observations are also in line with idea that the majority of sister chromatid exchanges can arise when damage DNA attempts replication possibly by a process relating with the replicative bypass repair mechanisms such as those proposed by Fujiwara and Tatsumi [34] and Higgins et al. [54], while chromosome aberration formation and some fraction of sister chromatid exchanges are related with the post-replication repair processes which attempt to rescue damaged template post-replicationally by de novo synthesis or recombination type repair systems. The former sister chromatid exchange-relating process seems to link mutation to less extent, if any, than the latter process, which is caffeine sensitive and likely to be error-prone.

Effect of the co-carcinogen benzo[e]pyrene on microsome-mediated chemical mutagenesis in Salmonella typhimurium

Chemical agents that possess the ability to alter tumorigenicity of carcinogens (administered at subthreshold dose) constitute a major health hazard. We have employed the Ames Salmonella assay to examine the effect of co-carcinogenic benzo[e]pyrene (B[e]P) on microsome-mediated chemical mutagenesis. B[e]P enhanced the mutagenic activity induced by 2-acetylaminofluorene (2-AAF) and benzo[a]pyrene (B[a]P) in strains TA1538 and TA98. Enhancement was also noted with N-hydroxy-2-AAF (the proximal metabolite of 2-AAF) but not with an ultimate carcinogenic form (N-acetoxy-2-AAF). These results suggest the use of this approach to detect chemical agents that possess the ability to alter the activity of mutagenic or carcinogenic chemicals.

Genes, pollutants and human diseases

The use of traditional Western concepts of disease (i.e. the ‘germ’ theory) has guided the use of scientific knowledge and technologies in the Control of acute infectious diseases (Gori & Peters, 1975; Trosko & Chang, 1978c). The intervention of all sorts of technologies has prolonged the life expectancy, allowing more people to reach the limit of what appears to be a rather fixed life-span (Hayflick, 1976). At the same time, human beings have used technologies to alter out dependence on the natural enviroment We intervene on many levels to minimize threats to our dependence on natural forces for food (e.g. agricultural technology, food additives), for water (e.g. wells or dams), for health (e.g. instituting sanitary measures or using drugs), for protection against the elements (e.g. wearing clothes, building homes with heaters and air conditioners) and for pleasure (e.g. snowmobiles, etc.). It is now extremely clear that much of the ‘effluents of our technological affluence’ can be harmful to living systems.

Effect of alkane tumor-promoting agents on chemically induced mutagenesis in cultured V79 Chinese hamster cells

Linear alkanes of specific chain length between 6 and 16 carbon atoms, an aryl derivative of dodecane, and a phorbol diester were tested in a cell culture system for relative ability to enhance mutagenesis induced by a chemical carcinogen, methylazoxymethanol acetate (MAM). Mutation frequencies at the ouabain-resistance locus were measured. Results indicated an excellent correlation between the relative activities of the above compounds in enhancing mutagenesis in the in vitro culture system and their tumor-promoting activities in mouse skin. None of the compounds tested showed mutagenic activity per se, further lending support to the theory that promoters act via derepression of latent carcinogen-induced damage to the genome.