Cycle-related toxicity and transformation in 10T1/2 cells treated with N-methyl-N'-nitro-N-nitrosoguanidine

Magnetically-responsive DDS

Magnetic-responsive drug delivery systems have received great attention due to the possibility of building theranostic systems. The application of a non-invasive external stimuli as a magnetic field that also allows the imaging and localization of the devices and the release of therapeutic drugs means a great opportunity for the development of new treatments to prevent diseases such as cancer. This chapter will focus on smart materials based on magnetic nanoparticles that have been studied for the formulation of such delivery systems and their synergic effect in combination with drugs for potential applications in the biomedical field. In addition, the possibility of applying hyperthermia at the macro and nanoscale levels and their implications will be discussed.

Temporal and functional analysis of DNA replicated in early S phase

In summary, recently developed technologies have begun to draw back the curtain of mystery that obscures some of the basic mechanisms of DNA replication at multiple levels. Studies using extended DNA and chromatin fiber techniques have proven valuable for identifying the location of origins of replication at specific genomic sites and determining their temporal order of replication, for identifying and quantifying sites of DNA damage and localizing chromatin proteins in relation to sites of DNA replication. The future potential of these methods include further discoveries in functional genomics and contributions to the elucidation of the histone code. Such studies could prove very valuable in studies of the mechanisms of cancer development, aging, and other processes of disordered genomic functioning.

Hyperthermia adds to chemotherapy

The hallmarks of hyperthermia and its pleotropic effects are in favour of its combined use with chemotherapy. Preclinical research reveals that for heat killing and synergistic effects the thermal dose is most critical. Thermal enhancement of drug cytotoxicity is accompanied by cellular death and necrosis without increasing its oncogenic potential. The induction of genetically defined stress responses can deliver danger signals to activate the host's immune system. The positive results of randomised trials have definitely established hyperthermia in combination with chemotherapy as a novel clinical modality for the treatment of cancer. Hyperthermia targets the action of chemotherapy within the heated tumour region without affecting systemic toxicity. In specific clinical settings regional hyperthermia (RHT) or hyperthermic perfusion has proved its value and deserve a greater focus and investigation in other malignancies. In Europe, more specialised centres should be created and maintained as network of excellence for hyperthermia in the field of oncology.

DNA replication in early S phase pauses near newly activated origins

During the S phase of the cell cycle, the entire genome is replicated. There is a high level of orderliness to this process through the temporally and topologically coordinated activation of many replication origins situated along chromosomes. We investigated the program of replication from origins initiating in early S phase by labeling synchronized normal human fibroblasts (NHF1) with nucleotide analogs for various pulse times and measuring labeled tracks in combed DNA fibers. Our analysis showed that replication forks progress 9-35 kilobases from newly initiated origins, followed by a pause in synthesis before replication resumes. Pausing was not observed near origins that initiated in the middle of S phase. No evidence for pausing near origins was found at the beginning of the S phase in glioblastoma T98G cells. Treatment with the S phase checkpoint inhibitor caffeine abrogated pausing in NHF1 cells in early S phase. This suggests that pausing may comprise a novel aspect of the intra-S phase checkpoint pathway or a related new early S checkpoint. Further, it is possible that the loss of this regulatory process in cancer cells such as T98G could be a contributing factor in the genetic instability that typifies cancers.

Early replication and the apoptotic pathway

In higher eukaryotes there is a link between time of replication and transcription. It is generally accepted that genes that are actively transcribed are replicated in the first half of S phase while inactive genes replicate in the second half of S phase. We have recently reported that in normal human fibroblasts there are some functionally related genes that replicate at the same time in S phase. This had been previously reported for functionally related genes that are located in clusters, for example the alpha- and beta-globin complexes. We have shown, however, that this also occurs with some functionally related genes that are not organized in a cluster, but rather are distributed throughout the genome. For example, using GOstat analysis of data from our and other groups, we found an overrepresentation of genes involved in the apoptotic process among sequences that are replicated very early (approximately in the first hour of S phase) in both fibroblasts and lymphoblastoid cells. This finding leads us to question how and why the replication of genes in the apoptotic pathway is temporally organized in this manner. Here we discuss the possible explanations and implications of this observation.

Genome-wide sequence and functional analysis of early replicating DNA in normal human fibroblasts

BACKGROUND

The replication of mammalian genomic DNA during the S phase is a highly coordinated process that occurs in a programmed manner. Recent studies have begun to elucidate the pattern of replication timing on a genomic scale. Using a combination of experimental and computational techniques, we identified a genome-wide set of the earliest replicating sequences. This was accomplished by first creating a cosmid library containing DNA enriched in sequences that replicate early in the S phase of normal human fibroblasts. Clone ends were then sequenced and aligned to the human genome.

RESULTS

By clustering adjacent or overlapping early replicating clones, we identified 1759 "islands" averaging 100 kb in length, allowing us to perform the most detailed analysis to date of DNA characteristics and genes contained within early replicating DNA. Islands are enriched in open chromatin, transcription related elements, and Alu repetitive elements, with an underrepresentation of LINE elements. In addition, we found a paucity of LTR retroposons, DNA transposon sequences, and an enrichment in all classes of tandem repeats, except for dinucleotides.

CONCLUSION

An analysis of genes associated with islands revealed that nearly half of all genes in the WNT family, and a number of genes in the base excision repair pathway, including four of ten DNA glycosylases, were associated with island sequences. Also, we found an overrepresentation of members of apoptosis-associated genes in very early replicating sequences from both fibroblast and lymphoblastoid cells. These data suggest that there is a temporal pattern of replication for some functionally related genes.

Reduction of Genotoxic Effects of the Carcinogen N-Methyl-N'-Nitro-N-Nitrosoguanidine by Dietary Lignin in Mammalian Cells Cultured In Vitro

In the present study the protective effect of several lignin polymers against the genotoxic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) was tested in hamster lung V79 cells and human colon Caco-2 cells. Preculturing of cells with sublethal, nongenotoxic concentrations of the lignins A, B, and C (50 microg/ml) was found to decrease significantly the level of DNA strand breaks in both hamster and human cells treated with MNNG. Lignin A also reduced MNNG-induced gene mutations in V79 cells. As in addition to alkyl lesions MNNG gives rise to hydroxyl free radicals (OH) and nitrogen-centered free radicals (NR), we tried to determine whether antimutagenicity of lignin A was connected only with the well-known ability of lignin to bind MNNG molecules or also with its antioxidative effects. The use of the modified comet assay technique proved that preculturing of hamster V79 cells with lignin A resulted in a significant decrease of the level of DNA strand breaks originating from oxidized DNA bases. Therefore, we suggest that the antimutagenic effect of lignin A against MNNG is associated with both adsorptive and antioxidative action. This study also showed that the presence of lignin A neither helped to renew DNA replication nor influenced the kinetics of DNA rejoining in MNNG-treated V79 cells.

Roadblocks and detours during DNA replication: mechanisms of mutagenesis in mammalian cells

Mutations in specific genes result in birth defects, cancer, inherited diseases or lethality. The frequency with which DNA damage is converted to mutations increases dramatically when the cellular genome is replicated. Although DNA damage poses special problems to the fidelity of DNA replication, efficient mechanisms exist in mammalian cells which function to replicate their genome despite the presence of many damaged sites. These mechanisms operate in either error-prone or error-free modes of DNA synthesis, and frequently involve DNA strand-pairing reactions. Genetic studies in yeast and other eukaryotes suggest that replication through DNA damage is highly regulated and catalysed by complex biochemical machineries composed of many specialized gene products. Knowledge of the molecular details by which such factors facilitate the replication of damaged DNA in mammalian cells should reveal basic rules about how DNA damage induces mutagenesis and carcinogenesis.

Interaction of hyperthermia and chemotherapy agents; cell lethality and oncogenic potential

Hyperthermia was combined with bleomycin, melphalan and cis-platinum in order to examine cell lethality and oncogenic transformation in C3H10T1/2 cells from the adjuvant use of heat with chemotherapy agents. When cells were exposed concurrently to 42.5 degrees C and each of the three chemotherapy agents, heat enhanced both the cytotoxic and oncogenic potential of the drugs. Hyperthermia-enhanced ratios were largest for bleomycin-treated cells. Examination of transformation incidences expressed as a function of surviving fraction, i.e. the cytotoxicity of treatment and therefore drug-heat efficacy, showed that for a given level of cell killing the combination of heat and cis-platinum resulted in fewer transformants per surviving cell than for cis-platinum alone. Hyperthermia appears to reduce the oncogenic potential of low concentrations of melphalan but has no effect on bleomycin-induced oncogenic transformation.

Neoplastic transformation of C3H mouse embryo cells, 10T1/2: cell-cycle dependence for 50 kV X-rays and UV-B light

The variation of neoplastic transformation induced by 50 kV X-rays, and by solar-simulating UV-B light, was studied through the cell cycle of C3H mouse embryo cells designated 10T1/2. A mitotic shake-off method was used to harvest mitotic cells. The progression through the cell cycle of initially mitotic cells was followed as a function of time by flow cytometry, DNA labelling for passage through S-phase, and growth curves for cell number. At 2-3 h after shake-off, about 90% of the cells were in early G1-phase and by 15 h 60-70% of cells had reached S-phase. For 2.5 Gy, the transformation frequency per viable cell in M-phase was some five times higher than in S-phase. In contrast, at similar survival levels, UV-B light is less efficient in transforming mitotic cells. For both types of radiation, the frequency of neoplastic transformation per viable cell was roughly inversely proportional to survival.

Defective replication of psoralen adducts detected at the gene-specific level in xeroderma pigmentosum variant cells

Replication of damaged DNA is suspected to play an important role in cell cycle, genetic stability, and survival pathways. Using psoralen photoaddition as prototype DNA damage and the renaturing agarose gel electrophoresis technique to measure DNA cross-linking in individual genes, Vos and Hanawalt previously observed efficient bypass replication of psoralen monoadducts in human genes (J.-M. H. Vos and P. C. Hanawalt, Cell 50:789-799, 1987). To understand the mechanism of bypass replication in human cells, mutants affected in such a process would be useful. We now report that cells from individuals suffering from the hereditary recessive syndrome xeroderma pigmentosum variant (XPV) are hypersensitive to killing induced by photoactivated psoralen. In addition, analysis of psoralen-mediated DNA cross-linking in the rRNA genes indicated that although repair of psoralen adducts was similar to that of normal individuals, XPV cells were markedly deficient in the ability to bypass psoralen adducts during replication; in comparison with normal cells, approximately half as many monoadducts were bypassed during replication in XPV cells. Furthermore, in contrast to normal cells, replication of interstrand cross-links was not detected in XPV. This is the first demonstration of a deficiency in bypass replication detected at the gene-specific level in vivo. A model involving a strand-specific defect in recombinational bypass in XPV is proposed.

Defective replication of psoralen adducts detected at the gene-specific level in xeroderma pigmentosum variant cells

Replication of damaged DNA is suspected to play an important role in cell cycle, genetic stability, and survival pathways. Using psoralen photoaddition as prototype DNA damage and the renaturing agarose gel electrophoresis technique to measure DNA cross-linking in individual genes, Vos and Hanawalt previously observed efficient bypass replication of psoralen monoadducts in human genes (J.-M. H. Vos and P. C. Hanawalt, Cell 50:789-799, 1987). To understand the mechanism of bypass replication in human cells, mutants affected in such a process would be useful. We now report that cells from individuals suffering from the hereditary recessive syndrome xeroderma pigmentosum variant (XPV) are hypersensitive to killing induced by photoactivated psoralen. In addition, analysis of psoralen-mediated DNA cross-linking in the rRNA genes indicated that although repair of psoralen adducts was similar to that of normal individuals, XPV cells were markedly deficient in the ability to bypass psoralen adducts during replication; in comparison with normal cells, approximately half as many monoadducts were bypassed during replication in XPV cells. Furthermore, in contrast to normal cells, replication of interstrand cross-links was not detected in XPV. This is the first demonstration of a deficiency in bypass replication detected at the gene-specific level in vivo. A model involving a strand-specific defect in recombinational bypass in XPV is proposed.

Effect of excision repair by diploid human fibroblasts on the kinds and locations of mutations induced by (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene in the coding region of the HPRT gene

(+/-)-7 beta,8 alpha-Dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (BPDE) is a direct-acting carcinogen that forms DNA adducts only with purines, predominantly (greater than 95%) with guanine. To investigate the effect of nucleotide excision repair on the kinds and locations (spectra) of mutations induced in diploid human fibroblasts by BPDE, we synchronized cells and exposed them to BPDE either at the beginning of S phase just when the target gene hypoxanthine (guanine) phosphoribosyltransferase (HPRT) is replicated or 12 hr prior to the beginning of S phase (early G1 phase). Clones resistant to 6-thioguanine were isolated, and the mRNA in lysates of 100-500 cells from each mutant clone was used to synthesize cDNA. HPRT cDNA was amplified 10(11)-fold by the polymerase chain reaction and then sequenced directly. The mutants derived from the two populations did not differ in the kinds of mutations; 19/20 of the base substitutions in cells taken from S phase and 19/19 of those from G1 phase involved G.C base pairs, predominantly G.C----T.A. However, they differed significantly in the distribution of the mutations in the coding region of the gene. In the cells from G1 phase, 29% of the mutations were clustered within a unique run of six guanine bases; in the S-phase cells, only 4% were located there. Assuming that the premutagenic BPDE-induced lesions involved purines, in the cells treated at the beginning of S phase, 24% of these lesions were located in the transcribed strand, whereas in the G1-treated cells, none were. This suggests that in the HPRT gene of diploid human cells excision repair of BPDE adducts occurs preferentially on the transcribed strand.

Tumorigenic methylcholanthrene transformants of C3H/10T1/2 cells have a common nucleotide alteration in the c-Ki-ras gene

The polymerase chain reaction was used to amplify DNA surrounding the codon 12 region of the c-Ki-ras gene from C3H/10T1/2 cells and from a number of 3-methylcholanthrene (MCA)-transformed derivatives of these cells. Sequence analysis demonstrated that tumorigenic MCAC116/39 cells, known by DNA-mediated transfection to contain an activated c-Ki-ras oncogene, had a G----T transversion in the first position of codon 12 of this gene, resulting in a Gly12----Cys mutation. A combination of polymerase chain-reaction amplification and oligonucleotide hybridization demonstrated that three additional tumorigenic MCA transformants of C3H/10T1/2 cells had an identical mutation in the c-Ki-ras gene. In contrast, this mutation was not present in an MCA-induced C3H/10T 1/2 transformant that was not tumorigenic. The molecular specificity of this MCA-induced mutation resulting in C3H/10T1/2 tumorigenic transformants should provide an excellent system in which to study the roles of transcription, replication, repair, and exogenous factors in the establishment and expression of transformation and tumorigenicity.

Sensitivity of different phases of the cell cycle to selected hepatotoxins in cultured liver-derived (BL9L) cells

1. Many toxins are active against dividing cells and cytofluorometric analysis of synchronized dividing liver-derived (BL9L) cells has been employed to study the relative sensitivity of the G1(G0), S and G2/M phases of the cell cycle to selected hepatotoxins. 2. The cytotoxic metal beryllium, which inhibits cell division, caused a specific block at the G1 phase of the cell cycle. 3. Dehydroretronecine, an antimitotic metabolite of the hepatotoxic plant pyrrolizidine alkaloids, retarded progression of cells through the cell cycle with a consistent accumulation at the late S to G2 phase. 4. Exposure of cells to aflatoxin B1-8,9-epoxide, the putative carcinogenic metabolite of the hepatocarcinogen aflatoxin B1, particularly during the early period of S phase, produced morphologically transformed cells.

Benzo[alpha]pyrene diol epoxide I binds to DNA at replication forks

The distribution of lesions in DNA caused by (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo [alpha]pyrene (B[alpha]P diol epoxide-I) was studied in synchronized C3H/10T1/2 cells treated in S phase. Sites of carcinogen modification of DNA were identified by polyclonal rabbit antibodies elicited against DNA modified with B[alpha]P diol epoxide-I in vitro. This antigenic DNA contained trans-(7R)-N2-[10-(7 beta,8 alpha,9 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene)-yl]- deoxyguanosine; other adducts were not detected by liquid chromatography. In this study, DNA replication forks with antibodies bound to B[alpha]P diol epoxide-I adducts were detected by electron microscopy. The frequency of replication forks containing carcinogen adducts associated with the fork junction was found to be 8-fold higher than expected for an average distribution. The proportion of replication forks that were apparently blocked at the site of the DNA damage increased when replication was allowed to occur after carcinogen exposure. These results support the conclusions that the fork junction is particularly vulnerable to adduction by B[alpha]P diol epoxide-I and that B[alpha]P diol epoxide-I adducts block the displacement of replication forks during DNA synthesis in intact cells.

Timing of proto-oncogene replication: a possible determinant of early S phase sensitivity of C3H 10T1/2 cells to transformation by chemical carcinogens

The temporal order of replication of several genes was studied in 10T1/2 cells synchronized by release from confluence-induced arrest of proliferation followed by treatment with 2 micrograms/mL aphidicolin for 24 h. DNA subjected to bromodeoxyuridine substitution for 1- or 2-h intervals spanning the S phase was separated from the remaining DNA in cesium chloride gradients, filtered onto nitrocellulose in a slot-blot apparatus, and hybridized with various 32P-labeled probes. Ha-ras was among the first genes replicated at the onset of the S phase. The myc proto-oncogene replicated later but within the first hour of the S phase. The replication of Ki-ras, raf, and mos was detected between hour 1 and 2 of the S phase. The dihydrofolate reductase gene replicated early (0-2 h) and the myb proto-oncogene replicated in mid-S phase (2-4 h). An immunoglobulin VH sequence and the beta-globin gene replicated late in 10T1/2 cells, 4-6 h after removal of aphidicolin. Replicating DNA is preferentially adducted by chemical carcinogens, and replication of damaged proto-oncogenes before they are repaired may activate their transforming potential. Therefore, the observed replication of proto-oncogenes during the early S phase may underlie the enhanced sensitivity of 10T1/2 cells to chemically induced transformation at this point in the cell cycle.

Mutagenic potency at the Na+/K+ ATPase locus correlates with cycle-dependent killing of 10T1/2 cells

Perturbation of DNA replication by chemical-DNA adducts produced by exposure to mutagenic/carcinogenic chemicals results in mutagenic or cytotoxic damage in the DNA. Demonstration of a correlation between cell cycle dependency of cytotoxicity and point mutation at the Na+/K+ ATPase gene could suggest that the two consequences of chemical exposure are caused by the same damage in the template DNA and that both are mediated through DNA replication-associated mechanisms. N-methyl-N'-nitro-N-nitrosoguanidine, N-ethyl-N'-nitro-N-nitrosoguanidine, 4-nitroquinoline-1-oxide, and benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxide demonstrated cell cycle-related patterns of cytotoxicity in 10T1/2 cells, with maximal cell killing produced by exposure in early S phase, and were highly efficient mutagens of the Na+/K+ ATPase gene relative to their cytotoxic potential. In contrast, methyl methanesulfonate and N-acetoxy-N-2-fluorenylacetamide were maximally cytotoxic in cell populations exposed in early G1 phase and were weak mutagens of the Na+/K+ ATPase gene at comparable levels of cytotoxicity. These data suggest that mutagenic/carcinogenic chemicals that are effective at producing mutations by misreplication kill cells by a related mechanism that may be associated with the perturbation of DNA replication.

A system for transformation of rat liver cells in vitro by acute treatment with aflatoxin

Aflatoxin B1 (AFB1) induced rat liver cancer is a well studied system of hepatocarcinogenesis. AFB1 has also been used to transform cultured rat liver derived cells in vitro. Cells in culture often have a reduced capacity to metabolise the AFB1 to its active metabolite, and often prolonged periods of exposure to the toxin have to be employed, with a long latency in the appearance of transformed cells in culture. We report here the transformation of a rat liver derived cell line by acute treatment with AFB1. An extrinsic metabolising system of quail microsomes, which convert AFB1 to its epoxide form with high efficiency, was used to activate the AFB1. A dose dependent cytotoxicity was obtained and neoplastic transformation was seen in the higher doses used. The enzyme GGT which has strong association with liver cell transformation both in vivo and in vitro was also elevated in the treated cells.

Transformation sensitivity in early S-phase and clonogenic potential are target-cell characteristics in liver carcinogenesis by N-methyl-N-nitrosourea

Changes in the transformation sensitivity of hepatocytes during the cell cycle were in relation to clonogenicity investigated in partially resected rat liver. The hourly rate of the G1-S transit was measured in a control group by means of tritium-labelled thymidine (3H-TdR). At defined periods after partial hepatectomy the animals were injected with a single dose of N-methyl-N-nitrosourea (MNU) (25 mg/kg) and subsequently exposed to phenobarbital (0.05% in the diet) for 80 days. ATPase-deficient cell populations which arise by clonal growth (Rabes et al., 1982) were determined in the liver 90 days after MNU treatment and served as a marker for the initiating action of the carcinogen. The number of foci appeared to be related to the hourly rate of influx of hepatocytes into DNA synthesis at the time of MNU administration. Few foci were observed after MNU exposure in G1 and their frequency increased steeply when MNU treatment coincided with the first peak of cellular G1-S transit. As the rate of influx into S-phase decreased, so did the number of foci, despite the proportion of cells in S-phase still being at a maximum. The possibility of a gradient of clonogenicity existing within the cells of the liver lobule is envisaged. It is suggested that proliferative cells of the peri-portal region, which begin DNA synthesis early after partial hepatectomy, have the greatest ability to form clones of ATPase-deficient putative pre-neoplastic foci after initiation by a carcinogen and might thus represent, if exposed to carcinogen in early S-phase, the effective target cell population for the induction of hepatocellular carcinomas.

Replication of proto-oncogenes early during the S phase in mammalian cell lines

Members of several classes of proto-oncogenes replicate during the first third of S-phase in two human (K562 erythroleukemia and HeLa), one Chinese hamster (CHO) and eight mouse cell lines. These cell lines exhibit a variety of specialized functions characteristic of pre-B and B cells, T cells and erythroid cells. The proto-oncogenes studied include fos, myc, myb, abl, fes, fms, mos, raf, rel, sis, Ha-ras, Ki-ras, and N-ras. In K562 cells, amplified and rearranged c-abl genes show a pattern of temporal replication during S that is similar to the pattern observed for the 5' breakpoint cluster region (bcr) and the amplified C lambda light chain immunoglobulin genes. The c-Ki-ras related sequences in CHO cells provide one example of late replicating proto-oncogene sequences that are present in multiple copies. The cellular gene N-myc replicates late during S in some of these cell lines. In three pre-B cell lines in which N-myc specific transcripts have been detected, N-myc replicates earlier in the S phase than in the other cell lines studied here.

DNA adducts and cell cycle

Cell cycle-dependent differences of transformation sensitivity may be due to alterations in the formation of ultimate electrophilic carcinogens during the cell cycle, preferential primary adduct formation during specific phases of the cell cycle, e.g. binding to single stranded DNA at the replication fork, base-mispairing and mutation of transformation-related genes replicating at critical phases of DNA synthesis, or cell cycle-related differences in the repair of DNA adducts. Some recent data on these subjects are summarized, mainly in context of cell cycle-dependent transformation sensitivity of regenerating rat liver.

Kinetics of cell death induced in 10T1/2 cells by methyl methanesulfonate and the effects of extracellular calcium on cell death

The cytotoxic effect of methyl methanesulfonate (MMS) on C3H 10T1/2 cells is characterized by a complex pattern of changes in the permeability of the cell membrane to trypan blue and, therefore, presumably to extracellular calcium. 10T1/2 cells are temporarily, and reversibly, permeable to trypan blue during the initial 30 minutes following exposure to MMS when incubated in the presence of extracellular calcium. By 90-120 minutes after the exposure, the MMS treated cells have restored control of the membrane permeability, and for the next 6-7 hours they exhibit a level of trypan blue uptake comparable to that observed in the untreated cohort population. Between 9 and 15 hours after exposure to MMS the fraction of the population permeable to trypan blue increases rapidly, ultimately approximating the level of cell killing measured concurrently in a colony formation assay. Transient culture in calcium-free medium immediately after exposure to MMS does not protect 10T1/2 cells from cytotoxicity, but incubation in the calcium-free medium does prevent the initial transient episode of permeability to trypan blue observed when the 10T1/2 cells are incubated in calcium-containing medium. These observations suggest that MMS-induced cytotoxicity results from a complex course of events, possibly from damage to an intracellular target rather than from damage to the plasma membrane.

Role of vitamin B12 and folate deficiencies in carcinogenesis

A significant body of experimental evidence supports the notion that a deficiency of either vitamin B12 or folic acid enhances the activity of various carcinogens. Unifying mechanisms are proposed to explain this cocarcinogenic role.

Kinetics of DNA replication in C3H 10T1/2 cells synchronized by aphidicolin

Aphidicolin is an inhibitor of DNA polymerase alpha and blocks nuclear DNA replication without interfering with mitochondrial DNA synthesis. The efficacy of this mycotoxin as a tool in cell synchronization was evaluated in C3H 10T1/2 clone 8 cells. At concentrations of 1-2 micrograms/mL, aphidicolin quickly reduced the [3H]thymidine uptake to less than 5% of control levels in the first 5 min of incubation. This inhibition was easily reversed by washing and refeeding cells with fresh medium. The synchronization protocol consisted of first blocking cells by confluence arrest, replating them at lower density, and then treating the cells with aphidicolin for 24 h. Once the inhibitor was removed, DNA replication started without any delay. The cell population traversed the S phase in about 8 h and synchronously doubled in cell number. Autoradiography studies revealed a labeling index of 89-93% during the S phase. However, it was also observed that 10T1/2 cells were able to enter S phase in the presence of aphidicolin. The extent of the ensuing replication in the nucleus was dependent on the time that cells remained arrested in early S phase. Analyses of the newly replicated DNA in alkaline sucrose gradients revealed a fairly homogeneous distribution of sizes of nascent DNA in synchronized cells pulse-labeled at the beginning of the S phase. Upon chase in nonradioactive medium, the average molecular weight of the nascent DNA increased linearly with time of DNA synthesis for 2 h. The apparent rate of DNA chain growth determined from pulse and chase experiments was 1.2 micron/min. This rate was strongly inhibited (93%) by aphidicolin at a concentration of 2 micrograms/mL.

Mutagenesis by incorporation of alkylated nucleotides

The cellular DNA precursor pool was shown to be a target for N-methyl-N-nitrosourea, a potent mutagen and carcinogen. O6medGTP, a product of this interaction, was chemically synthesized and shown to be incorporated into DNA in vitro by Klenow E. coli pol I and phage T4 DNA polymerases. O6medGTP incorporated predominantly opposite T template residues and to a lower extent opposite C. At some loci incorporation of O6medGTP caused DNA synthesis arrest. The significance of the behavior of O6medGTP for mutagenesis in vivo is discussed.

O6-methylguanine repair in liver cells in vivo: comparison between G1- and S-phase of the cell cycle

To compare the formation and persistence of alkylated DNA bases in the G1- and S-phase compartments in liver in vivo, regenerating rat liver was exposed to [14C]dimethylnitrosamine (0.57 mg/kg, IP injection) or N-[methyl14C]-N-nitrosourea (3.3 mg/kg, intraportal injection) during the G1 phase of the cell cycle (12 h after partial hepatectomy), or at 24 h after partial hepatectomy with 30% hepatocytes in DNA synthesis, or at 43 h after partial hepatectomy, 4 h after an hydroxyurea block from 14 to 39 h after operation with 80% hepatocytes in DNA synthesis. At 120 min after dimethylnitrosamine and 90 s, 5, 10, or 60 min after the intraportal pulse of N-methyl-N-nitrosourea the molar fractions of 7-methylguanine (7megua), O6-methylguanine (O6megua), and 3-methyladenine (3mead) and of metabolically labeled guanine were determined from DNA hydrolysates by Sephadex-G10 radiochromatography. After dimethylnitrosamine only minor differences were observed for 7megua formation in the three groups; the 3mead/7megua ratio remained constant irrespective of the number of cells in S phase. In contrast, the O6megua/7megua ratio revealed a loss of O6megua, the extent of which appeared proportional to the fraction of DNA-synthesizing cells in the liver. The rapid loss of O6megua in S-phase cells was confirmed after intraportal administration of N-methyl-N-nitrosourea. During the first 10 min after the methylnitrosourea pulse the O6megua/7megua ratio was constant in G1 cells and dropped from 90 s to 10 min by about 15% in liver containing 30% S-phase cells and by about 40% with 80% cells in DNA synthesis. DNA-synthesizing hepatocytes are apparently endowed with a higher O6megua DNA transferase activity than nonproliferating liver cells. The rapid, though exhaustible elimination of O6megua during S-phase might result in partial protection of DNA-synthesizing cells from base-mispairing and/or from hypomethylation at G-C sites.

Studies on chemically induced neoplastic transformation and mutation in the BALB/3T3 Cl A31-1-1 cell line in relation to the quantitative evaluation of carcinogens

Mutagenesis and neoplastic transformation assays on mammalian cells in culture have been extensively used for quantitative estimates of the activity of carcinogens, in spite of the limitations that such in vitro systems have when compared with in vivo systems for tumor induction. In order to assess the validity of these correlations, a series of studies was undertaken in our laboratory with the BALB/3T3 Cl A31-1-1 mouse embryo cell line. Different carcinogens were found to induce dose-dependent frequencies of transformation, including the direct-acting alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and carcinogens that were metabolically activated by these cells through different pathways (benzo[a]pyrene, 3-methylcholanthrene, aflatoxin B1, and benzidine). Their respective level of activity on a molar basis was different from that obtained in standard Salmonella + S9 mutagenesis tests. Studies currently underway indicate the possibility of lowering the serum content in the medium considerably, thereby reducing a major variable in the assay. Methods were established for the induction of ouabain-resistant (ouar) mutants in these cells. Studies were conducted by applying 30-min MNNG exposures to cells that were synchronized by serum deprivation followed by serum-induced release from growth block. While maximal induction of mutants occurred in the S phase, the transformation frequency remained constant for treatments in G1 and early or late S.(ABSTRACT TRUNCATED AT 250 WORDS)

Repair of DNA adducts in asynchronous CHO cells and the role of repair in cell killing and mutation induction in synchronous cells treated with 7-bromomethylbenz[a]anthracene

CHO cells of normal or UV-sensitive phenotypes were analyzed for their ability to remove DNA adducts produced by the carcinogen 7-BrMeBA. At a dose of 0.1 microM, which reduced the survival of the normal AA8 cells to approximately 90% and the mutant UV5 cells to approximately 20%, the frequency of adducts was 5-6 per 10(6) nucleotides for both cell types, and AA8 cells removed approximately 30% of the adducts in 8 h and approximately 55% in 24 h. In contrast, UV5 and mutants from four other genetic complementation groups had no significant removal. Binding of 7-BrMeBA did not vary through the cell cycle in synchronous cultures. At a dose of mutagen (0.07 microM) resulting in approximately 25% survival of asynchronous UV5, the survival of synchronous cultures rose about threefold from early G1 to early S phase and then decreased somewhat in late S/G2. At a dose (0.28 microM) producing similar survival of asynchronous cultures, AA8 cells differed qualitatively in that survival decreased progressively by 5- to 10-fold between early G1 and the early part of S, and rose steeply through late S/G2 to give a 10- to 20-fold increase. We conclude that DNA repair is the major determinant of variations in survival through the cycle in normal cells. The patterns observed are consistent with a mechanism of killing in AA8 cells in which adducts disrupt DNA replication, while in UV5 cells transcriptional blocks or other effects may govern lethality. Induced mutations at the aprt and hprt loci showed changes through the cycle in both AA8 and UV5 cells, and the patterns were not readily explainable by the action of repair.

Adaptive response in mammalian cells: crossreactivity of different pretreatments on cytotoxicity as contrasted to mutagenicity

Pretreatment of H4 (rat hepatoma) cells for 48 hr with low nontoxic doses of alkylating agents [methyl methanesulfonate (MMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), and N-methyl-N-nitrosourea] renders the cells more resistant to the toxic effect of these compounds. Crossreactivity for survival is also observed with the different alkylating agents tested. Pretreatment with MNNG enables the cells to be less mutated than control cultures during a subsequent challenge with high doses of this compound. However, pretreatment with MMS does not modify the mutation frequency of cells challenged with either MMS or MNNG. The adaptive response to mutagenesis is correlated with a faster and more efficient removal of O6-methylguanine in MNNG-pretreated cells as compared to control cultures, whereas the disappearance of this lesion is not modified in MMS-pretreated cells. As MMS produces less methylation at the O6 position of guanine and more methylation at the N7 position in comparison to MNNG, the results suggest that: (i) N7-methylguanine is not implicated in the adaptive response and (ii) adaptation to mutagenesis can be correlated with the amount of O6-methylguanine induced during the pretreatment. The effect of pretreatment on other O-alkylated derivatives is not known.

Enhanced sensitivity of the C3H/10T1/2 cell transformation system to alkylating and chemotherapeutic agents by treatment with 12-O-tetradecanoylphorbol-13-acetate

The failure of the C3H/10T1/2 cell transformation system to respond to numerous known carcinogens has limited its applications for the detection and study of cancer-causing substances. Recent studies have found, however, that some carcinogens function as initiating agents for the process of transformation in these cells. Treatment with such agents is generally not sufficient to transform low-density asynchronous cultures of C3H/10T1/2 cells, but morphologic transformation will occur if such cultures are subsequently exposed to the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). In the present study, the ability of TPA to enhance transformation was examined in cultures treated with a variety of chemical agents. The addition of TPA after chemical treatment enhanced the transformation of these cells by methylmethanesulfonate, ethylmethanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, N-nitrosomethylurea, N-nitrosoethylurea, mitomycin C, 5-fluorodeoxyuridine, and 5-azacytidine. Treatment with amethopterin or benzo(e)pyrene did not produce significant numbers of foci in the presence or absence of TPA. TPA inhibited transformation by high concentrations of 3-methylcholanthrene and benzo(a)pyrene. Thus, numerous carcinogens function as initiating agents for these cells and the presence of TPA can dramatically increase the sensitivity of this cell transformation system.

DNA precursors in chemical mutagenesis: a novel application of DNA sequencing

Recently, we have shown that deoxyribonucleoside residues in the cellular DNA precursor pool are generally more susceptible to methylation than are residues within the DNA duplex. The N-1 position of adenosine, for example, was found to be at least 13,000 times more susceptible to methylation by N-methyl-N-nitrosourea (MNU) than the same site in the DNA. These results suggest that potential sites for alkylation in the double-strand duplex are relatively inaccessible to direct alkylation in vivo. Many of these sites are probably protected from alkylation not only by their position in the interstices of the DNA helix, but also by further in vivo 'packaging' of the DNA in chromatin. We have now used DNA sequencing to demonstrate the incorporation properties of products of the reaction of MNU with dATP and of deoxy-N4-hydroxycytidine triphosphate during DNA replication in vitro by phage T4 DNA polymerase and the 'Klenow' fragment of Escherichia coli pol I. The results suggest that DNA precursor nucleotides due to their greater availability for alkylation, may offer routes for the introduction of alkylated residues into double-stranded DNA.

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.

DNA precursor pool: a significant target for N-methyl-N-nitrosourea in C3H/10T1/2 clone 8 cells

Synchronized C3H/10T1/2 clone 8 cells were treated in vitro with a nontoxic dose of N-methyl-N-nitrosourea during their S phase. Chromatographic isolation of the deoxyribonucleotide DNA precursor pool and measurement of the precursor content per cell showed that a nucleic acid residue in the precursor pool is 190-13,000 times more susceptible to methylation than a residue in the DNA duplex, depending on the site of methylation. This conclusion comes from measurements indicating that, for example, the N-1 position of adenine in dATP is 6.3 times more methylated than the same position in the DNA, even though the adenine content of the pool is only a fraction (0.0005) of the adenine content of the DNA helix. The comparative susceptibility between pool and DNA was found to vary with the site of methylation in the order the N-1 position of adenine greater than phosphate greater than the N-3 position of adenine greater than the O6 position of guanine greater than the N-7 position of guanine. The significance of these results for chemical mutagenesis and carcinogenesis is discussed.

Alkylation of DNA and tissue specificity in nitrosamine carcinogenesis

A peculiarity of nitrosamines is the high degree of cell and organ specificity in inducing tumors. There is substantial evidence that the initiation of the carcinogenesis process by carcinogens of this group is linked to the metabolic competence of the target tissue or cell to convert these carcinogens into mutagenic metabolites and to the binding of those metabolites to cellular DNA. Alkylation occurs in the DNA at the N-1, N-3, and N-7 positions of adenine; the N-3, N-7, and O6 of guanine; the N-3, and O2 of cytosine; and the N-3, O4, and O2 of thymine; and the phosphate groups. The initial proportion of each DNA adduct depends upon the alkylating agent used. The various DNA adducts are lost to a variable extent from DNA in vivo by spontaneous release of bases and/or by specific DNA repair processes. Studies conducted in vitro and vivo indicate that alkylation at the oxygen atoms of DNA bases is more critical than alkylation at other positions in the mutagenesis and carcinogenesis induced by N-nitroso compounds. In particular, tissues in which tumors occur more frequently after a pulse dose of nitrosamine are those in which O6-alkylguanine persists longest in DNA, presumably resulting in an increased probability that a miscoding event (mutation) will take place during DNA synthesis. The more rapid removal of O6-methylguanine from the DNA of liver (as compared with extrahepatic tissues) of rats has been associated with the absence of tumor production in this organ by a single dose of dimethylnitrosamine; however, a significant incidence of liver tumors is observed if the same dose is given 24 hr after partial hepatectomy, and tumors are induced by such a dose of dimethylnitrosamine in the liver of hamsters, which has a low capacity to remove O6-methylguanine from its DNA. These data also indicate that the rate of disappearance of 7-methylguanine from the liver or extrahepatic tissues is independent of the dose of dimethylnitrosamine; whereas O6-methylguanine is lost from DNA more rapidly after a low dose of this nitrosamine. It has been shown that in liver the removal of O6-methylguanine but not other DNA adducts, from DNA can be affected by pretreating the animals with N-nitroso compounds. The modulation of DNA repair processes observed after a single dose and after chronic treatment with nitrosamines is discussed in relation to the tissue-specific carcinogenic effect of this group of carcinogens.