Relationship between somatic mutation and neoplastic transformation

Biochemical studies of transient intermediates in relation to chemical carcinogenesis

Many chemical carcinogens must be metabolized to chemically reactive transient species before they can exert their full toxic action on mammalian cells. In general, this metabolic activation is performed by NADPH-dependent enzymes in the endoplasmic reticulum; the NADPH-cytochrome P-450 electron-transport chain is very important in this respect. Biochemical studies on the chemical reactivities of such transient intermediates require the application of various fast-reaction and free-radical techniques: the use of such techniques is illustrated by reference to the metabolism of carbon tetrachloride. CCl4 is metabolized by liver endoplasmic reticulum in the presence of NADPH to a highly reactive product, probably CCl3; this activation of CCl4 results in covalent binding of CCl3 and lipid peroxidation. The steady-state concentration of CCl3 is too low to be measured directly by e.s.r. spectroscopy but radical species can be accumulated with spin-trap techniques. The CCl3 radical can be generated by pulse radiolysis and the ensuing reactions with biologically important neighbouring species can be followed in the microsecond range by kinetic spectroscopy. The results point to the high reactivity of CCl3 and its restriction to a microenvironment within the endoplasmic reticulum. Highly reactive electrophilic radicals (e.g. CCl3) can initiate lipid peroxidation in biomembranes and this is associated with changes in polyunsaturated fatty acids and in membrane fluidity. The results are discussed in relation to carcinogen activation, to free-radical-mediated reactions in biomembranes, and to the general thesis that the production of reactive aldehydes by lipid peroxidation may act as a 'coarse control' of cell division.

Ultraviolet radiation-induced DNA damage in promoter elements inhibits gene expression

Repair of DNA damage in gene promoters is slower than in actively transcribed genes. Persistent damage in gene promoters though transient can have significant biological effects on regulated gene expression. In this study we investigated the effect of ultraviolet radiation on gene promoter-associated functions when DNA damage is located within and outside transcription factor binding sites. Our results show that both cyclobutane pyrimidine dimers and (6-4) photoproducts inhibit DNA-protein interaction, in vitro transcript production and transactivation of reporter genes. The biological significance of transient DNA damage as a mechanism in carcinogenesis is discussed.

Mechanisms of radiation-induced neoplastic transformation of human bronchial epithelial cells

Carcinogenesis is a multistage process with sequences of genetic events that govern the phenotypic expression of a series of transformation steps that lead to the development of metastatic cancer. To better understand the mechanisms involved in human bronchial carcinogenesis induced by alpha particles from radon, we have developed a model of neoplastic transformation based on human papillomavirus-immortalized human bronchial epithelial (BEP2D) cells. Cells exposed to alpha particles become tumorigenic after progressing through a series of sequential stages including altered growth pattern, resistance to serum-induced terminal differentiation, agar-positive growth, tumorigenicity, and metastasis, with each step representing a necessary yet insufficient step toward the later, more malignant phase. Cell fusion studies indicated that the radiation-induced tumorigenic phenotype in BEP2D cells can be completely suppressed by fusion with nontumorigenic BEP2D cells. Several cellular differentiation and growth regulation genes such as DCC (deleted in colorectal cancer), CDKN1A (also known as p21(C1P1)) and the gene that encodes DNA-PK were frequently found to be modulated in tumorigenic BEP2D cells and may be related to the process of carcinogenesis.

Effect of oxidative DNA damage in promoter elements on transcription factor binding

Reactive oxygen species produced by endogenous metabolic activity and exposure to a multitude of exogenous agents impact cells in a variety of ways. The DNA base damage 8-oxodeoxyguanosine (8-oxodG) is a prominent indicator of oxidative stress and has been well-characterized as a premutagenic lesion in mammalian cells and putative initiator of the carcinogenic process. Commensurate with the recent interest in epigenetic pathways of cancer causation we investigated how 8-oxodG alters the interaction between cis elements located on gene promoters and sequence-specific DNA binding proteins associated with these promoters. Consensus binding sequences for the transcription factors AP-1, NF-kappaB and Sp1 were modified site-specifically at guanine residues and electrophoretic mobility shift assays were performed to assess DNA-protein interactions. Our results indicate that whereas a single 8-oxodG was sufficient to inhibit transcription factor binding to AP-1 and Sp1 sequences it had no effect on binding to NF-kappaB, regardless of its position. We conclude from these data that minor alterations in base composition at a crucial position within some, but not all, promoter elements have the ability to disrupt transcription factor binding. The lack of inhibition by damaged NF-kappaB sequences suggests that DNA-protein contact sites may not be as determinative for stable p50 binding to this promoter as other, as yet undefined, structural parameters.

Interference of benzo[a]pyrene diol epoxide-deoxyguanosine adducts in a GC box with binding of the transcription factor Sp1

Previous studies indicated that DNA adducts formed by the carcinogenic diol epoxide 7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) can increase the affinity of the transcription factor Sp1 for DNA sequences that are not normally specific binding sites. Whether adducts that form in the normal binding site, the GC box sequence, increase the affinity of Sp1 for the modified GC-box was not determined. Starting with a 23-nt sequence that contains two natural GC box sequences, site-specifically modified oligonucleotides were prepared with a single(+)-BPDE-deoxyguanosine adduct at one of three positions: the center of each GC-box or in between the two boxes. Four modified oligonucleotides were studied, two derived from cis addition of BPDE to the exocyclic amino group and two from trans addition. For three of these site-specifically modified oligonucleotides, there was a diminution in Sp1 affinity, whereas Sp1 binding to the fourth modified oligonucleotide was abolished. Furthermore, random modification of the oligonucleotide to a level of about 1 BPDE adduct per fragment slightly decreased the affinity for Sp1, and no evidence was found for a subpopulation of molecules with high affinity. These findings suggest that BPDE modification of the GC box does not lead to an increased affinity for Sp1. This is consistent with a model in which a BPDE-induced bend in the DNA mimics the conformation of the normal GC box:Sp1 complex, leading to high-affinity binding of Sp1 to non-Gc box sites.

A possible role in chemical carcinogenesis for epigenetic, heritable changes in gene expression

Although genetic changes are clearly important in the initiation of carcinogenesis, there is reason to think that epigenetic changes may also play a role in the process. A key feature of carcinogenesis is the long latency between exposure to carcinogenic insults and the appearance of malignancy. Thus, if epigenetic changes are to be involved, they must somehow be inherited at each cell division without the continued presence of the carcinogen. I propose that self-perpetuating changes in patterns of gene expression are a plausible mechanism for an epigenetic component of carcinogenesis. Networks of transcription factors that regulate each other's and their own expression are known to control important developmental processes, particularly the determination of entire cell lineages. An inherent property of many such autoregulatory networks is the existence of two very distinct, stable steady-states, defined in terms of the concentration of each transcription factor in the network. In this report, I present a model in which an acute carcinogen exposure is postulated to shift such a network from one steady-state to the other, effectively turning on or off the expression of at least one of the genes. Because of the autoregulatory nature of the network, this new steady-state is stably inherited at each cell division. Such changes in gene expression may ultimately contribute to the malignant phenotype if the regulatory network affects genes important in cell-cycle checkpoints, maintenance of genome stability, signal transduction, or other processes that are altered in tumor cells.

Two-dimensional gel electrophoresis analysis of Syrian hamster embryo cells: morphological transformation is not cell type specific

Studies were conducted to investigate the protein phenotype of normal and morphologically transformed Syrian hamster embryo (SHE) cells. Based upon two-dimensional gel protein phenotype analysis, we conclude that (i) SHE cells are a mixture of multiple cell types including mesenchymal and epithelial cells and (ii) several cell types present in the SHE cell population can be morphologically transformed by a variety of genotoxic and non-genotoxic carcinogens.

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.

Characterization of activated proto-oncogenes in chemically transformed Syrian hamster embryo cells

The Syrian hamster embryo (SHE) cell transformation model has been used by many investigators to study the multistep process of neoplastic transformation induced by chemical carcinogens. In this study we have attempted to determine if activated proto-oncogenes are present in the transformed cells induced by a variety of chemical carcinogens. Twelve carcinogen-induced hamster cell lines, established by treatment of normal SHE cells with benzo[a]pyrene, diethylstilbestrol, or asbestos, were examined. One spontaneously transformed cell line (BHK-A) was also studied. Some of the cell lines were also tested for oncogene activation at the preneoplastic stage, before they acquired tumorigenic potential. DNAs from normal, preneoplastic, and neoplastic cells were tested by transfection into mouse NIH 3T3 cells, and morphologically transformed foci were scored on the contact-inhibited monolayer of 3T3 cells. The frequency of focus formation for normal SHE cell DNA was less than 0.0008 foci/microgram DNA, while approximately 40% (5 of 12) of the DNAs from carcinogen-induced, tumorigenic hamster cell lines induced foci at a frequency of greater than or equal to 0.012 foci/microgram DNA. The other seven carcinogen-induced cell lines and the BHK-A cells were negative (less than 0.002 foci/microgram DNA). When the DNAs from transformed foci induced by the five positive cell lines were retransfected into NIH 3T3 cells, the frequency of secondary foci of 3T3 cells was as much as 50-fold higher (1.34 foci/microgram DNA) than with the primary transfectants. DNAs from transformed foci or tumors derived from transformed foci were screened by Southern blot analyses with known oncogenes and with a hamster repetitive DNA probe for the presence of transfected hamster oncogenes.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

An interlaboratory comparison of transformation in Syrian hamster embryo cells with model and coded chemicals

Three independent laboratories tested eight "model" and five coded chemicals in the Syrian hamster embryo clonal transformation assay system to establish the intra- and interlaboratory reproducibility of the system and to identify sources of variability. When a common cell pool and the same lot of fetal calf serum were used, the three laboratories obtained consensus on the activity of eight model chemicals: five chemicals (benzo(a)pyrene, 7,12-dimethylbenz(a)anthracene, N-methyl-N'-nitro-N-nitrosoguanidine, nitroquinoline-N-oxide, and lead chromate) induced morphological transformation without exogenous metabolic activation and three (N-2-fluorenylacetamide, pyrene, and anthracene) produced no transformation response. Five coded chemicals (2,6-dichloro p-phenylenediamine, 4,4'-oxydianiline, cinnamyl anthranilate, dichlorvos, and reserpine), representative of environmental chemical classes, but not necessarily strong carcinogens, produced more equivocal responses in this interlaboratory study. Thus, while the assay can be used to distinguish between transforming and nontransforming chemicals in some cases, the intrinsic limitations in low transformation frequency and in achieving any dose-response results are major constraints to the use of this system in a routine testing program at the present time. Efforts to increase the transformation frequency or to amplify the expression of the transformed phenotype constitute some of the approaches which should be explored in order to overcome these limitations.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

A contact-insensitive subpopulation in Syrian hamster cell cultures with a greater susceptibility to chemically induced neoplastic transformation

We previously have identified a subpopulation of contact-insensitive (CS-) cells which lacks density-dependent inhibition of cell division in primary and low-passage cultures of Syrian hamster embryonic (SHE) fibroblastic cells. Further, we have shown that the proportion of these CS- cells declines as a result of the stable phenotypic conversion of the CS- cells to contact-sensitive (CS+) cells. To determine whether these transient CS- cells are more sensitive to carcinogenic/mutagenic perturbation, the susceptibility to neoplastic transformation and somatic mutation induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was examined in clonally isolated cell cultures containing various proportions of CS- cells (0.02-4%). The frequencies of morphological transformation, focus formation, and neoplastic transformation showed a positive correlation to the proportion of CS- cells in the treated cultures. In contrast, the frequency of MNNG-induced somatic mutation at the Na+,K+-ATPase locus was similar among cultures varying in their proportion of CS- cells. Thus, there is a transient subpopulation of CS- cells in primary SHE cell cultures that is more susceptible to neoplastic transformation although equally susceptible to induced point mutation. This dissociation between somatic point mutation and neoplastic transformation indicates a fundamental difference in the nature of these two phenomena. A possible relationship between the propensity of CS- cells (versus CS+ cells) to carcinogen-induced neoplastic transformation and the state of differentiation of the CS- cells is discussed.

Karyotype analysis of carcinogen-treated Chinese hamster cells in vitro evolving from a normal to a malignant phenotype

The relationship of cytogenetic changes with the acquisition of an indefinite life span in vitro, the ability of cells to grow in soft agar and their tumourigenicity in syngeneic animals has been studied in control, trans-7,8-dihydrodiolbenzo(a)pyrene and 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)-pyrene-treated secondary cultures derived from Chinese hamster embryonic lung. Karyotype analysis revealed a sequence of chromosome changes as the cells progressed through culture. Aneuploidy, namely trisomy of chromosome 4, the long arm in particular, was an early dominant change. The possible association of this trisomy with the acquisition of immortality in vitro is implicated, although the involvement of other nonrandom chromosome changes cannot be eliminated, implying that there may be several genomic sites in the Chinese hamster which may potentially be involved with the acquisition of unlimited growth potential. Neither the ability of cells to grow in soft agar nor as tumours could be associated with any specific chromosome(s). Double minutes were observed in metaphases from the cell lines, agar colonies and tumours; their possible relationship with growth advantage is discussed.

Tracheal epithelial cell transformation: a model system for studies on neoplastic progression

Most in vitro transformation studies have been conducted with fibroblast cultures of various origins. The phenotypic changes known to accompany transformation are therefore primarily those that are typical for transformed fibroblasts. Little information exists concerning phenotypic changes occurring during transformation of epithelial cells in vitro. However, recently a number of transformation studies have been reported with tracheal epithelium as a prototype for epithelium from the conducting airways. The initial studies were carried out with organ culture-cell culture systems. These studies reported the qualitative phenotypic changes developing in primary outgrowth cultures after exposure to the direct acting carcinogen N-methyl-N'-nitrosoguanidine. The phenotypic changes observed are all related to changes of in vitro growth characteristics. Several stages can be observed as the cell cultures progress from a "carcinogen altered" to the neoplastic state. While these studies laid the groundwork for the epithelial transformation field, they did not permit quantitation of transformants, since the size of the exposed cell population is unknown. More recently transformation systems with dispersed primary tracheal epithelial cells have been developed which allow quantitation of transformed phenotypes. These systems are being used for clonal analysis of the process of epithelial cell transformation and to study progression and promotion during development of neoplastic transformation.

Neoplastic conversion of preneoplastic Syrian hamster cells: rate estimation by fluctuation analysis

Analysis of the role of gene mutations in the multistep process of neoplastic transformation requires that the discrete steps in carcinogenesis first be dissected. Toward this end, we have isolated and characterized preneoplastic Syrian hamster cells which exhibit in vitro a trait highly correlated with neoplastic conversion in vivo. Previous findings (J. C. Barrett, Cancer Res. 40:91-94, 1980) indicate that spontaneous neoplastic transformation of Syrian hamster cells occurs in at least two steps. An intermediate stage, characterized by an aneuploid established cell line which has a propensity to become neoplastic spontaneously upon further growth in vitro, has been described. These preneoplastic cells differ from diploid early-passage Syrian hamster cells in becoming capable of anchorage-independent growth in semisolid agar, as well as becoming neoplastic in vivo when attached to a solid substrate. Evidence presented here demonstrates that anchorage-independent conversion in vitro is a reliable marker for neoplastic conversion in this cell system. Fluctuation analyses, patterned after those described by Luria and Delbruck for microbial genetics, demonstrate that anchorage-independent variants are generated randomly from clonally derived preneoplastic cells at the rate of 10(-8) to 10(-7) variants per cell per generation. These results establish a multistep stochastic process for transformation in vitro and indicate that conversion to anchorage independence may be necessary for Syrian hamster cells to become tumorigenic. The possible role of gene mutation in this step during neoplastic progression is discussed.

Role of gene and chromosomal mutations in cell transformation

To understand the role of mutagenesis in carcinogenesis fully, we must consider all types of mutations including gene, chromosomal, and gene-number mutations and all changes involved in the progressive development of neoplastic cells. We have found that certain known human carcinogens (i.e., DES and asbestos), which were classified as epigenetic carcinogens based on gene-mutation assays, have mutational activity at the chromosomal level that correlates with their ability to induce cell transformation. This should caution against classification of chemicals as genotoxic or epigenetic without a complete understanding of their mechanism of action. Furthermore, our studies indicate that more than gene-mutation assays is needed for carcinogen testing. In particular, chromosomal changes induced by chemicals, both aberrations and aneuploidy, need to be carefully assessed. In addition, the role of all types of mutation in the overall process of neoplastic transformation needs to be determined. This can only be examined by studying each individual change involved in neoplastic progression. Thus, any attempt to assess a chemical's carcinogenic potential should consider not only all types of mutational changes but both early and late changes involved in neoplastic transformation.

Neoplastic conversion of preneoplastic Syrian hamster cells: rate estimation by fluctuation analysis

Analysis of the role of gene mutations in the multistep process of neoplastic transformation requires that the discrete steps in carcinogenesis first be dissected. Toward this end, we have isolated and characterized preneoplastic Syrian hamster cells which exhibit in vitro a trait highly correlated with neoplastic conversion in vivo. Previous findings (J. C. Barrett, Cancer Res. 40:91-94, 1980) indicate that spontaneous neoplastic transformation of Syrian hamster cells occurs in at least two steps. An intermediate stage, characterized by an aneuploid established cell line which has a propensity to become neoplastic spontaneously upon further growth in vitro, has been described. These preneoplastic cells differ from diploid early-passage Syrian hamster cells in becoming capable of anchorage-independent growth in semisolid agar, as well as becoming neoplastic in vivo when attached to a solid substrate. Evidence presented here demonstrates that anchorage-independent conversion in vitro is a reliable marker for neoplastic conversion in this cell system. Fluctuation analyses, patterned after those described by Luria and Delbruck for microbial genetics, demonstrate that anchorage-independent variants are generated randomly from clonally derived preneoplastic cells at the rate of 10(-8) to 10(-7) variants per cell per generation. These results establish a multistep stochastic process for transformation in vitro and indicate that conversion to anchorage independence may be necessary for Syrian hamster cells to become tumorigenic. The possible role of gene mutation in this step during neoplastic progression is discussed.

The mutational origin of serum independence in Chinese hamster cells in vitro

The genetic mechanisms determining the ability of transformed cells to grow in a medium with a low serum content (ser+) were studied in a clone of Chinese hamster cells with normal serum requirements. The fluctuation test has shown that serum independence occurs as a random spontaneous event. Its rate of occurrence is about 10(-5). The concomitant study of a gene mutation (resistance to 6-mercaptopurine--6MP) revealed similar characteristics with respect to the distribution of the number of mutants in replicative cultures. N-methyl-N1-nitro-N-nitrosoguanidine (MNNG) and SV40 significantly increased the frequency of ser+ colonies. Induction was detected after an expression time of 3-4 days, which is typical of gene mutations. In 16 out of 18 ser+ clones of independent origin the ser+ character remained stable. The results suggest that the ser+ character originates in most cases from a mutation event.

Induction of ouabain-resistant mutants by chemical carcinogens in rat prostate epithelial cells

We determined optimal conditions to quantitatively select ouabain-resistant (Ouar) mutants induced by chemical carcinogens in a rat prostate epithelial cell line (RPYK). These conditions included selection of Ouar mutants in 3 mM ouabain, an expression time of two days following a two-day treatment with carcinogens, and a reseeding density of 2 X 10(5) mutagenized cells per 100 mm dish to select mutants in ouabain. Ouar mutants induced by N-methyl-N'-nitro-N'-nitrosoguanidine (MNNG) remained stably Ouar when passaged in nonselective medium. Hemicyst formation, a characteristic of epithelial cells, was reversibly inhibited by ouabain in wild-type cells and was resistant to ouabain in Ouar cells. The direct-acting carcinogens MNNG and methylazoxymethanol-acetate (MAMA) and the environmentally widespread procarcinogens aflatoxin B1 and benzo(a)pyrene increased the frequency of Ouar mutants in RPYK cells. The procedures developed here now make it possible to detect some environmental carcinogens likely to cause prostate cancer by virtue of their ability to mutate cultured prostate epithelial RPYK cells. The sensitivity of the RPYK cell line to aflatoxin-induced cytotoxicity and mutagenesis also makes it a useful cell system in which to study enzymes governing the conversion of aflatoxin to genotoxic metabolites.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

Use of a rapid DNA sequencing system to demonstrate the induction of frameshift mutations by bleomycin

The rapid DNA sequencing system based on the single-stranded bacteriophage M13 and the chain-terminator method has been used to look directly for mutational alterations. A small DNA fragment that primes DNA synthesis through the N-terminal 200 base pairs of the beta-galactosidase gene was prepared, and used to detect changes in base sequence among phages that give white plaques after treatment of the host cells with bleomycin. Bleomycin treatment of E. coli in which M13 mp2 was growing gave an increase in white plaque frequency. DNA sequence analysis of phage from 7 independent mutant plaques showed them all to have a frameshift mutation.

Differential cell cycle phase specificity for neoplastic transformation and mutation to ouabain resistance induced by N-methyl-N'-nitro-N-nitrosoguanidine in synchronized C3H10T 1/2 C18 cells

The transformable mouse embryo fibroblast cell line C3H10T 1/2 C18 has been employed to study the induction by the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) of morphological transformation and mutation to ouabain resistance throughout the cell cycle. Cells were synchronized by means of isoleucine deprivation for 24 hr and initiated DNA synthesis with a high degree of synchrony 7.5 hr after release of the isoleucine block. At various intervals throughout the cell cycle cultures were treated with MNNG at 1.0 microgram/ml and the induction of cytotoxicity, morphological transformation, and ouabain-resistant colonies was determined. All three phenomena exhibited marked cell-cycle phase dependency. Maximal induction of transformation occurred in cultured treated 7.5 hr after release from isoleucine deprivation, when the cells were at the G1/S boundary. In contrast, induction of ouabain-resistant colonies was at a minimum at the time of maximal induction of transformation, and peak induction of ouabain resistance did not occur until 16-18 hr after release from the isoleucine block, when cells were in late S phase. A close correlation was observed between the induction of cytotoxicity and of ouabain-resistant mutants. The results suggest that differences exist in the production or cellular processing of the various early lesions.

Chemical carcinogens transform BHK cells by inducing a recessive mutation

Treatment of BHK cells with mutagenic carcinogens induced neoplastic transformation in a single step. This transformation displayed the characteristics expected for a recessive mutation. Increasing doses of carcinogens induced transformants with kinetics similar to the kinetics with which they induced 6-thioguanine-resistant or ouabain-resistant mutants in the same population of cells. Transformants with temperature-restricted phenotypes were easily induced by carcinogens which cause mutations by base changes, but when ICR frameshift mutagens were used, the proportion of temperature-limited transformants was inversely related to the frequency with which a particular mutagen induced frameshift mutations. In hybrids between pseudodiploid isogenic strains of normal and transformed BHK cells, transformation was expressed as a dominant trait when the transformed parent was induced by a papovavirus, but was suppressed as a recessive trait when the transformed parent arose spontaneously or was chemically induced. Segregation of transformation was observed upon growth of suppressed normal hybrids, and the transformed phenotype which was reexpressed was in most cases characteristics of the original transformed parent.

The influence of cellular proliferative history on the susceptibility to oncogenic transformation

C3H mouse 10T1/2 clone 8 cells were serially transferred from passage 11 to 15 with 5 x 10(4) cells seeded per 60-mm dish at each passage. One group of cells was passaged as soon as confluence was reached. Two other groups were kept for 3 or 6 days in confluence at each passage before subculture. Cloning efficiency was found to increase progressively with passage of all three groups. At the 15th passage, cells from all three groups were harvested just prior to confluence, irradiated with ultraviolet light, and assayed for clonogenic survival and malignant transformation. Survival response was the same for all three groups, but cells which were kept constantly proliferating in previous passages were found to be much more susceptible to transformation. These results suggest that the susceptibility of these cells to transformation is influenced by their proliferative history; in particular, intermittent growth quiescence in previous passages decreased this susceptibility.

Presidential address. Recent concepts of initiation and promotion in carcinogenesis

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Induction of gene mutation in and cell transformation of mammalian cells by modified purines: 2-aminopurine and 6-N-hydroxylaminopurine

2-Aminopurine, a classical mutagen in prokaryotic systems, is inactive as a carcinogen in two animal species. To determine the basis for this discrepancy in the correlation between carcinogenesis and mutagenesis, the ability of 2-aminopurine to induce somatic mutation and neoplastic transformation concomitantly in the same cellular system was examined. 6-N-hydroxylaminopurine, a related modified purine that is a mutagen and a carcinogen, was also studied. 2-Aminopurine was a mutagen in Syrian hamster embryo cells, but its activity was very weak. The maximum induced mutation frequency with either of two mutational markers was only 7 X 10(-6) mutants per surviving cell. 2-Aminopurine also induced morphological transformation of the cells under the same conditions, but the frequency was only approximately 0.04% per surviving colony. Neoplastic transformation of the cells after 2-aminopurine treatment was not observed in these experiments. These results indicate that 2-aminopurine is, at best, a weak transforming agent. The lack of carcinogenic activity in vivo with 2-aminopurine is consistent with these observations. In contrast to the results with 2-aminopurine, 6-N-hydroxylaminopurine was a very effective mutagen in these cells (up to 10(-3) mutants per survivor) and induced morphological transformation of the cells in a dose-dependent manner. Furthermore, neoplastic transformation was induced by this nucleic acid base analog. The correlation of mutagenic activity with transforming ability of these two modified purines supports a relationship between mutagenesis and carcinogenesis. However, relative to other carcinogens, there is a quantitative difference in the ability of 6-N-hydroxylaminopurine to induce cell transformation and mutation. For example, in benzo[a]pyrene-treated cultures, the ratio of the frequency of induced morphological transformation to that of somatic mutation was approximately 100, whereas for 6-N-hydroxylaminopurine-treated cultures, the ratio of transformation to mutation was only 3-12.5. This indicates that 6-N-hydroxylaminopurine is less potent than benzo[a]pyrene in inducing transformation when compared at equal mutagenic potency. This is consistent with our hypothesis that cell transformation, and possibly cancer, occurs predominantly as the result of a mutation at the chromosome level rather than a gene mutation.

Mechanisms of tolerance to DNA lesions in mammalian cells

In recent years it has become clear that different pathways are involved in the process of removing lesions from DNA. In spite of a continuous surveillance of the genetic integrity by repair enzymes, quite often lesions are not eliminated before the portion of the genome where they have been inserted is used for DNA replication or transcription. Actually, the number of unexcised lesions a cell can tolerate without significantly losing its capacity to reproduce is surprising. As an example, human fibroblasts from certain patients with the genetic disease xeroderma pigmentosum (XP)† are virtually unable to excise pyrimidine dimers, the major DNA lesion produced by short-wavelength UV light.

Cellular differentiation and neoplasia: characterization of subpopulations of cells that have neoplasia-related growth properties in Syrian hamster embryo cell cultures

Cellular subpopulations having two of the growth properties of neoplastically transformed cells--lack of postconfluence inhibition of cell division (CI-) and anchorage independence of growth (AD-)--were found in cell cultures established from 10- to 13-day-old Syrian hamster embryos. The subpopulations having these properties decrease with increasing gestation period of the embryo as well as with continuing passage in vitro. The decrease in these subpopulations was also observed when they were cultured on a lethally irradiated confluent monolayer of contact-inhibited cells (cell mat), a selection condition for CI- cells. Therefore, negative selection cannot be the explanation for the loss of CI- cells in the population, leaving two other possibilities: either the loss of proliferative capacity of the CI- cells or the acquisition of sensitivity to postconfluence inhibition of cell division (CI+) of this subpopulation on in vitro culture or in vivo growth. The CI- subpopulations were isolated clonally from cell mats and were cultured continuously on both cell mats or plastic dishes. The results indicate that these cells did not lose proliferative capacity but acquired the contact-inhibited phenotype. This result, together with the fact that embryonic development in vivo also decreases CI/AD-subpopulations, suggests that the disappearance of these subpopulations is due to cellular differentiation of the CI-/AD- cells to become CI+/AD+ cells.

Diethylstilbestrol induces neoplastic transformation without measurable gene mutation at two loci

The frequency with which diethylstilbestrol induces neoplastic transformation and somatic mutation was measured concomitantly in Syrian hamster embryo cells. While diethylstilbestrol was as active as benzo[a]pyrene in inducing transformation, it failed to induce mutations at two conventionally studied loci. These results suggest that diethylstilbestrol may transform cells in the absence of gene mutations.

Mutagen-induced resistance to mycophenolic acid in hamster cells can be associated with increased inosine 5'-phosphate dehydrogenase activity

Cell variants resistant to the cytotoxic effect of mycophenolic acid, an inhibitor of IMP dehydrogenase (IMP:NAD+ oxidoreductase, EC 1.2.1.14), were selected by a one-step procedure from Chinese hamster V79 cells. The frequency of these variants was increased in a dose-dependent manner after treatment with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine and after an expression time of 8 days. The degree of resistance in five of the six isolated cell variants was associated with a comparable increase in the specific activity of IMP dehydrogenase, which was 3- to 6-fold higher than that of the parent V79 cells. The IMP dehydrogenase activity from both the variants and the V79 cells had a similar affinity for the substrate IMP with a Km of about 20 microM and a similar response to mycophenolic acid with a Ki of 12-16 nM. It is suggested that cell variants with an altered regulation of IMP dehydrogenase activity may be helpful in studying the control of nucleic acid biosynthesis, cell growth, and carcinogenesis. Mycophenolic acid resistance also may be useful as a marker in short-term assays for the identification of potential chemical carcinogens.

Interactions between initiating chemical carcinogens, tumor promoters, and adenovirus in cell transformation

Cell culture systems that respond to the combined action of initiating chemical carcinogens, tumor promoters, and transforming viruses represent useful model systems for studying the complex multifactor nature of the carcinogenic process. We have utilized both secondary rat embryo (2 degrees RE) and a clonal population of established Fischer rat embryo (CREF) cells to study the effect of multiple agents on the process of adenovirus transformation. In the present review we summarize our investigations on the effects of polycyclic aromatic hydrocarbons, tumor promoters, a bee venom polypeptide-melittin (MEL), and the polypeptide hormone epidermal growth factor (EGF) on transformation of rat embryo cells by a temperature-sensitive mutant of human adenovirus type 5 (H5ts125). We also describe the effect of tumor promoters on the progression of the transformed phenotype, ie the temporal acquisition of anchorage-independent growth in adenovirus-transformed clones and the ability of MEL and EGF to elicit effects similar to those of tumor promoters in adenovirus-transformed cells.

Constitutive uncoupling of pathways of gene expression that control growth and differentiation in myeloid leukemia: a model for the origin and progression of malignancy

Chemical carcinogens and tumor promoters have pleiotropic effects. Tumor initiators can produce a variety of mutations and tumor promotres can regulate a variety of physiological molecles that control growth and differentiation. The appropriate mutation and the regulation of the appropriate molecules to induce cell growth can initiate and promote the sequence of changes required for transformation of normal cells into malignant cells. After this sequence of changes, some tumors can still be induced to revert with a high frequency from a malignant phenotype to a nonmalignant phenotype. Results obtained from analysis of regulation of growth and differentiation in normal and leukemic myeloid cells, the phenotypic reversion of malignancy by induction of normal differentiation in myeloid leukemia, and the blocks in differentiation-defective leukemic cell mutants have been used to propose a general model for the origin and progression of malignancy. The model states that malignancy originates by changing specific pathways of gene expresion required for growth from inducible to constitutive in cells that can still be induced to differentiate normally by the physiological inducer of differentiation. The malignant cells, unlike the normal cells, then no longer require the physiological inducer for growth. This changes the requirements for growth and uncouples growth from differentiation. Constitutive expression of other specific pathways can uncouple other controls, which then causes blocks in differentiation and the further progression of malignancy. The existence of specific constitutive pathways of gene expression that uncouple controls in malignant cells can also exlain the expresion of fetal proteins, hormones, and some other specialized products of normal development in various types of tumors.

The cell growth inhibitory and antiviral effects of interferon in cloned transformed mouse cells

Eight clones and two subclones of SV40- and seven clones and one subclone of 20-methylcholanthrene-transformed C3H mouse embryonic fibroblasts were compared in tests for sensitivity to the antiviral and cell-growth inhibitory activities of a partially purified mouse L-cell interferon. While the sensitivity of clones and subclones to the antiviral activity of interferon was comparable to that of parent lines, the cell-growth inhibitory activity of interferon in the SV40 clones showed more than 100-fold variation and the methylcholanthrene-transformed cells could be divided into two groups in this respect. No correlation of sensitivity to the cell-growth inhibitory effect of interferon with the chromosome number, interferon-producing capacity or tumorigenicity of the clones could be detected. However, the cells of the interferon-sensitive clones No. 36 of the methylcholanthrene-transformed line were destroyed by macrophages at higher percentage binding of 125I-labeled soybean lectin. These results suggest that (1) the cell-growth inhibitory effect of interferon might be mediated by a specific type of receptors, and (2) N-acetyl-galactosamine present on the surface of interferon-resistant cells in a higher concentration than on interferon-sensitive cells hinders the recognition of cells both by macrophages and by interferon.

Postreplication repair and the susceptibility of Chinese hamster cells to cytotoxic and mutagenic effects of alkylating agents

A cell variant (VR-43) resistant to the cytotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine and N-methyl-N'-nitro-N-nitrosourea was isolated from Chinese hamster V79 cells for use in studies of the relationship among cell survival, mutagenesis, and DNA repair by alkylating agents. Resistance to ouabain or 6-thioguanine was used as the genetic marker. After treatment with N-methyl-N'-nitro-N-nitrosoguanidine, the VR-43 cells exhibited mutation frequencies that were lower, on a dose basis, than those of the wild-type V79 cells. However, when analyzed at equicytotoxic doses, the VR-43 cells were more mutable than the V79 cells. No difference in cell survival or mutagenicity could be observed after treatment with other mutagens such as N-ethyl-N'-nitro-N-nitrosoguanidine, ethyl methanesulfonate, or x-rays. Postreplication repair was analyzed by determination of the molecular weight of the newly synthesized DNA by alkaline sucrose gradients. After treatment with N-methyl-N'-nitro-N-nitrosoguanidine, the VR-43 cells exhibited an enhanced postreplication repair relative to the V79 cells. No such enhancement was found after N-ethyl-N'-nitro-N-nitrosoguanidine or ethyl methanesulfonate treatment. Based on these results we propose that, after treatment of these and presumably other mammalian cells with some methylating mutagens, postreplication repair can cope with DNA lesions responsible for cytotoxicity and, to a lesser degree, with lesions responsible for mutagenicity.