[Evaluation of the genotoxicity of vincristine and colchicine using mouse lymphoma tk mutation assay]

OBJECTIVE

To evaluate the genotoxicity of two mitotic inhibitors vincristine and colchicine, and to compare the detecting sensitivity of tk gene mutation assay with short-and long-term treatment by them.

METHODS

Tk gene mutation assay was performed by microwell method with L5178Y mouse lymphoma cell line after exposing to vincristine and colchicine for 3 h and 24 h respectively, and the results of two different treating time were compared.

RESULTS

The two chemicals didn't show positive responses in mouse lymphoma assay (MLA) with 3h treatment, but they could induce tk mutation frequency increase 1.6 to 4.8 and 2.1 to 6.2 times higher to spontaneous MF respectively after continuous treatment for 24 h.

CONCLUSION

Vincristine and colchicine had mutagenic effect on tk gene with long-term treatment, which indicated that the detecting sensitivity of MLA could be enhanced by long-term treatment.

Genetic toxicity studies with genistein

Genistein is a phytoestrogen that occurs naturally in the diet especially in soybeans and soy-based foods. Genistein and related phytoestrogens are of interest as chemopreventive agents for a variety of diseases and cancers based on epidemiologic evidence of reduced cancer rates in populations with a high intake of soy. Although soy and its constituents have been consumed at high levels in Asian populations without apparent adverse effects, concern has been raised of potential adverse effects due to estrogenic and other activities of the isoflavones. In these studies, genistein was evaluated for mutagenicity and clastogenicity in vitro in the S. typhimurium assay (Ames Test), the mouse lymphoma assay and in vivo in the micronucleus test in mice and rats. There was no evidence for a mutagenic effect in the in vitro S. typhimurium assay with and without metabolic activation (S9). In the in vitro mouse lymphoma assay, genistein increased resistant mutants with and without metabolic activation (S9), which were predominantly small colonies indicating that genistein acts as a clastogen. Three independent in vivo micronucleus tests were performed in Moro mice, RAIf rats and Wistar rats. MORO male and female mice were treated orally for 14 days at doses up to 20 mg/kg/day. RAIf and Wistar male and female rats were treated orally at doses up to 2000 mg/kg without an increase in micronuclei in treated mice or rats. It is concluded that genistein was not mutagenic in the S. typhimurium assay or mutagenic or clastogenic in vivo in the mouse and rat micronucleus test. In the mouse lymphoma assay, genistein induced an increase of predominantly small colonies indicating that genistein acts as a clastogen. This observation is in agreement with published data on the inhibitory action of genistein on topoisomerase II, which is known to lead to chromosomal damage with a threshold dose response.

Mutagenic effects of 4-hydroxynonenal triacetate, a chemically protected form of the lipid peroxidation product 4-hydroxynonenal, as assayed in L5178Y/Tk+/- mouse lymphoma cells

The lipid peroxidation product 4-hydroxynon-2-enal (4-HNE) is cytotoxic and genotoxic at superphysiological concentrations. To characterize the mechanism of action of 4-HNE, we assessed genotoxic damage by 4-HNE and by 4-HNE triacetate [4-HNE(Ac)(3)] using the mouse lymphoma assay that measures the mutant frequency in the Tk gene. As a strong electrophile, 4-HNE reacts readily with nucleophilic centers on cellular components. When added extracellularly, it may react preferentially with proteins in culture medium or on the cell surface and not reach deeper cellular targets such as nuclear DNA. Therefore, 4-HNE(Ac)(3), a protected form of 4-HNE that is metabolically converted to 4-HNE in cells (Neely MD, Amarnath V, Weitlauf C, and Montine TJ, Chem Res Toxicol 15:40-47, 2002), was assayed in addition to 4-HNE. When added in serum-containing medium, 4-HNE was not mutagenic in the mouse lymphoma assay up to 38 muM (cytotoxicity = 13%). In contrast, exposure to 4-HNE(Ac)(3), which mimics intracellular formation of 4-HNE, resulted in dose-dependent induction of mutations. At 17 muM 4-HNE(Ac)(3) (cytotoxicity = 33%), the mutant frequency was 719 x 10(-6) (>7-fold higher than the spontaneous mutant frequency). Loss of heterozygosity analysis in the Tk mutants revealed that the majority of mutations induced by 4-HNE(Ac)(3) resulted from clastogenic events affecting a large segment of the chromosome. The results indicate that, in the presence of serum that approximates physiological conditions, 4-HNE generated intracellularly but not extracellularly is a strong mutagen via a clastogenic action at concentrations that may occur during oxidative stress.

Loss of P53 heterozygosity is not responsible for the small colony thymidine kinase mutant phenotype in L5178Y mouse lymphoma cells

The mouse lymphoma L5178Y Tk+/- 3.7.2C assay is a well-characterized in vitro system used for the study of somatic cell mutation. It was determined that this cell line has a heterozygous mutation in exon 5 of Trp53. Based on this assumption that the cell line is heterozygous for the Trp53 gene, it was postulated that the small colony thymidine kinase (Tk) mutant phenotype may be due to a newly induced mutation/deletion in both the Trp53 and Tk1 alleles. The resultant Tk-/- mutants would also be Trp53+/0 or Trp53+/+ and would lose their ability to grow at normal rates. Subsequently, we published our evaluation of the Trp53 status in L5178Y cells. This analysis included sequencing of Trp53 exon 4 and determined that the mouse lymphoma cell line has a mutation in both of the Trp53 alleles and, therefore, no wild-type Trp53 allele in either Tk+/- cells or Tk-/- mutants. Because the cells have no wild-type Trp53, it is not possible that the small colony phenotype results from a newly induced loss of both functional Trp53 and Tk. To determine whether small colonies might, however, include the deletion of both Trp53 and Tk we evaluated, using microsatellite marker analysis, a series of small colony mutants. We also utilized in situ hybridization to determine that the Trp53 alleles are, in fact, in their normal chromosome 11 location in Tk+/- 3.7.2C mouse lymphoma cells. From all of these analyses we can conclude that the small colony mutant phenotype is not caused by deletion of both Trp53 and Tk1.

[TK locus mutation assay: comparison of L5178Y and TK6 cell lines]

OBJECTIVE

To compare the advantages and disadvantages of utilizing L5178Y and TK6 cell lines in the TK locus mutation assay.

METHODS

The two cell lines were used for detecting and assessing the mutability of four chemicals (MMS, EMS, MMC and KCl); the microwell method of TK locus mutation assay was adopted.

RESULTS

The two cell lines brought about similar results in the study. The tolerance of TK6 to all four chemicals was lower than that of L5178Y, and all the relative mutation indices of TK6 were higher than those of L5178Y.

CONCLUSION

Each of the two cell lines has its own strong points; nevertheless, the authors recommend the applications of TK6 cell in the assay system since this cell line comes from human.

[Thymidine kinase gene mutation assay--comparison of microwell method and soft agar method]

OBJECTIVE

TK gene mutation assay was developed for a long time. Two methods are used in TK locus mutation assay, i.e. microwell method and soft agar method. Two methods were compared in this study so that to find out the better one to be used in China.

METHODS

In this study, two methods (microwell method and soft agar method) were used for detecting and evaluating the mutagenesis of three positive mutagens, i.e. MMS, EMS, and MMC, and a non-mutagen KCl.

RESULTS

Two methods was similar in results for three mutagens. For KCl, however, a negative result was showed in soft agar method while a positive result was showed in microwell method.

CONCLUSION

According to the present study, it is recommended that the soft agar method should be popularized in China.

[Effects of total alkaloids of Tripterygium hypoglaucum Hutch on tk gene of mouse lymphoma cells]

OBJECTIVE

To investigate the effects of total alkaloids of Tripterygium hypoglaucum Hutch (THH) on the tk gene of mouse lymphoma cells.

METHOD

L5178Y cells were infected with total alkaloids of THH with different concentrations and put into single-cell wells at different time phases. Then the numbers of positive wells were counted and the cell plating efficiency, relative suspension growth and mutation frequency were determined.

RESULT

Total alkaloids of THH (0.1-2.0 g x L(-1)) induced tk locus mutation with mutation frequency 2-9 times higher than that of spontaneous mutation frequency of L5178Y cells. There were two different phenotypes of mutation colonies, large colony and small colony, but the main colony was large colony. This phenomenon might be related with the mutagenesis of THH.

CONCLUSION

Total alkaloids of THH can exert toxicity and mutagenic effects on tk gene in L5178Y cells, and there may be range limit in gene mutation.

Fibroblast growth factor-inducible 14 mediates multiple pathways of TWEAK-induced cell death

TWEAK, a TNF family member, is produced by IFN-gamma-stimulated monocytes and induces multiple pathways of cell death, including caspase-dependent apoptosis, cathepsin B-dependent necrosis, and endogenous TNF-alpha-mediated cell death, in a cell type-specific manner. However, the TWEAK receptor(s) that mediates these multiple death pathways remains to be identified. Recently, fibroblast growth factor-inducible 14 (Fn14) has been identified to be a TWEAK receptor, which was responsible for TWEAK-induced proliferation of endothelial cells and angiogenesis. Because Fn14 lacks the cytoplasmic death domain, it remains unclear whether Fn14 can also mediate the TWEAK-induced cell death. In this study, we demonstrated that TWEAK could induce apoptotic cell death in Fn14 transfectants. A pan-caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, rather sensitized the Fn14 transfectants to TWEAK-induced cell death by necrosis via reactive oxygen intermediates and cathepsin B-dependent pathway. By using newly generated agonistic anti-Fn14 mAbs, we also observed that Fn14 is constitutively expressed on the cell surface of all TWEAK-sensitive tumor cell lines, and can transmit the multiple death signals. Moreover, an anti-Fn14 mAb that blocks TWEAK-Fn14 interaction could totally abrogate TWEAK binding and TWEAK-induced cell death in all TWEAK-sensitive tumor cell lines. These results revealed that the multiple pathways of TWEAK-induced cell death are solely mediated by Fn14.

The inhibitory effect of the fungicides captan and captafol on eukaryotic topoisomerases in vitro and lack of recombinagenic activity in the wing spot test of Drosophila melanogaster

In studies on the mechanisms of mutagenic and carcinogenic action of captan and captafol-related chloroalkylthiocarboximide fungicides, two effects were tested: (i) the effect of both compounds on the activity of eukaryotic topoisomerases I and II in vitro, and (ii) their mutagenic and recombinagenic activity in the somatic mutation and recombination test (SMART) in wing cells of Drosophila melanogaster. Only captafol inhibited the activity of topoisomerase I (10-20% inhibition of activity in the range of 10-100microM). In contrast, both chemicals decreased the activity of topoisomerase II already at 1microM concentration (50 and 20% inhibition of activity by captafol and captan, respectively).Genotoxicity was tested in vivo by administrating both compounds by acute (3h) and chronic feeding (48h) of 3-day-old larvae. In acute feeding, captan and captafol demonstrated positive results only for small single and total spots in 10-100mM exposure concentration range. Both chemicals were inconclusive for large single spots, as well as for twin spots. In chronic treatment, captan showed positive results only for small single and total spots at 2.5 and 5mM concentrations. Captafol gave inconclusive results over all concentrations tested. The results of the acute treatment experiments which have been performed at very high doses (50% toxicity at higher doses) indicate very weak overall mutagenic activity of both test fungicides.

Genotoxicity assessment of the antiepileptic drug AMP397, an Ames-positive aromatic nitro compound

AMP397 is a novel antiepileptic agent and the first competitive AMPA antagonist with high receptor affinity, good in vivo potency, and oral activity. AMP397 has a structural alert (aromatic nitro group) and was mutagenic in Salmonella typhimurium strains TA97a, TA98 and TA100 without S9, but negative in the nitroreductase-deficient strains TA98NR and TA100NR. The amino derivative of AMP397 was negative in wild-type strains TA98 and TA100. AMP397 was negative in a mouse lymphoma tk assay, which included a 24h treatment without S9. A weak micronucleus induction in vitro was found at the highest concentrations tested in V79 cells with S9. AMP397 was negative in the following in vivo studies, which included the maximum tolerated doses of 320mg/kg in mice and 2000mg/kg in rats: MutaMouse assay in colon and liver (5x320mg/kg) at three sampling times (3, 7 and 31 days after the last administration); DNA binding study in the liver of mice and rats after a single treatment with [14C]-AMP397; comet assay (1x2000mg/kg) in jejunum and liver of rats, sampling times 3 and 24h after administration; micronucleus test (2x320mg/kg) in the bone marrow of mice, sampling 24h after the second administration. Based on these results, it was concluded that AMP397 has no genotoxic potential in vivo. In particular, no genotoxic metabolite is formed in mammalian cells, and, if formed by intestinal bacteria, is unable to exert any genotoxic activity in the adjacent intestinal tissue. These data were considered to provide sufficient safety to initiate clinical development of the compound.

Mutant frequencies and loss of heterozygosity induced by N-ethyl-N-nitrosourea in the thymidine kinase gene of L5178Y/TK(+/-)-3.7.2C mouse lymphoma cells

N-ethyl-N-nitrosourea (ENU) is a potent monofunctional ethylating agent that has been found to be mutagenic in a wide variety of organisms from viruses to mammalian germ cells. To elucidate the mutagenicity of ENU at the Tk(+/-) locus of mouse lymphoma cells and to confirm the ability of the mouse lymphoma assay (MLA) to detect both point mutations and large DNA alterations, Tk(+/-) L5178Y cells were exposed to different doses of ENU. Treatment of the cells with ENU resulted in a linear dose response with mutant frequencies of up to 16-fold over control. Evaluation of mutant clone size showed that 36% of the 100 microg/ml ENU-induced clones (66% in control) were small colony mutants and 64% (34% in control) were large colony mutants. DNA isolated from mutants in the control culture and the 100 microg/ml ENU treatment group was analyzed for loss of heterozygosity (LOH) using allele-specific PCR. The majority of the small colony mutants, both ENU-treated (97%) and spontaneous (91%), lost the Tk1b allele. The percentage of allele loss in ENU-induced large colony mutants was distinctly different from that of the control. Twenty-three percent of ENU-induced large colony mutants lost their Tk1b alleles, whereas 73% of the large colony mutants from the control culture lost the allele (P < 0.001). Overall, 50% of the Tk mutants from the 100 microg/ml ENU-treated cultures (86% in control) showed LOH. Our data indicate that ENU is a potent mutagen in mouse lymphoma cells and that 100 microg/ml ENU induces equal numbers of point mutations and chromosomal mutations. This study serves to verify that the MLA detects both point mutations and chromosomal mutations.

Evaluation of mutagenic effects of hyperbaric oxygen (HBO) in vitro. II. Induction of oxidative DNA damage and mutations in the mouse lymphoma assay

We recently showed that treatment of V79 cells with hyperbaric oxygen (HBO) efficiently induced DNA effects in the comet assay and chromosomal damage in the micronucleus test (MNT), but did not lead to gene mutations at the hprt locus. Using the comet assay in conjunction with bacterial formamidopyrimidine DNA glycosylase (FPG protein), we now provide indirect evidence that the same treatment leads to the induction of 8-oxoguanine, a premutagenic oxidative DNA base modification in V79 and mouse lymphoma (L5178Y) cells. We also demonstrate that HBO efficiently induces mutations in the mouse lymphoma assay (MLA). Exposure of L5178Y cells to HBO (98% O(2); 3bar) for 2h caused a clear mutagenic effect in the MLA, which was further enhanced after a 3h exposure. As this mutagenic effect was solely due to the strong increase of small colony (SC) mutants, we suggest that HBO causes mutations by induction of chromosomal alterations. Molecular characterization of induced SC mutants by loss of heterozygosity (LOH) analysis showed an extensive loss of functional tk sequences similar to the pattern found in spontaneous SC mutants. This finding confirmed that the majority of HBO-induced mutants is actually produced by a clastogenic mechanism. The induction of point mutations as a consequence of induced oxidative DNA base damage seems to be of minor importance.

In vitro genotoxicity testing of ARASCO and DHASCO oils

ARASCO and DHASCO oils are microbially-derived triglycerides rich in arachidonic (20:4n-6) and docosahexaenoic (22:6n-3) acids, respectively. Both oils were tested for mutagenic activity in three different in vitro mutagenesis assays. All assays were conducted with and without metabolic activation. Neither ARASCO nor DHASCO oil was mutagenic in the Ames reverse mutation assay using five different Salmonella histidine auxotroph tester strains, nor were the oils mutagenic in the mouse lymphoma TK(+/-) forward mutation assay. The oils showed no clastogenic activity in chromosomal aberration assays performed with Chinese hamster ovary cells. Based on these assays, neither ARASCO nor DHASCO oils appear to have any genotoxic potential.

Inverse radiation dose-rate effects on somatic and germ-line mutations and DNA damage rates

The mutagenic effect of low linear energy transfer ionizing radiation is reduced for a given dose as the dose rate (DR) is reduced to a low level, a phenomenon known as the direct DR effect. Our reanalysis of published data shows that for both somatic and germ-line mutations there is an opposite, inverse DR effect, with reduction from low to very low DR, the overall dependence of induced mutations being parabolically related to DR, with a minimum in the range of 0.1 to 1.0 cGy/min (rule 1). This general pattern can be attributed to an optimal induction of error-free DNA repair in a DR region of minimal mutability (MMDR region). The diminished activation of repair at very low DRs may reflect a low ratio of induced ("signal") to spontaneous background DNA damage ("noise"). Because two common DNA lesions, 8-oxoguanine and thymine glycol, were already known to activate repair in irradiated mammalian cells, we estimated how their rates of production are altered upon radiation exposure in the MMDR region. For these and other abundant lesions (abasic sites and single-strand breaks), the DNA damage rate increment in the MMDR region is in the range of 10% to 100% (rule 2). These estimates suggest a genetically programmed optimatization of response to radiation in the MMDR region.

Acute toxicity, primary irritancy, and genetic toxicity studies with 3-(methylthio)propionaldehyde

Basic acute toxicity, primary irritancy, and genetic toxicity studies were conducted with 3-(methylthio)propionaldehyde (3-MTP). The acute rat peroral LD50 (with 95% confidence limits) for 3-MTP as a 25% (v/v) dilution in corn oil was 1.00 (0.59-1.70) ml/kg (males) and 1.68 (0.95-2.99) ml/kg (females); most deaths occurred 1.5 to 4 h postdosing. By 24-h occluded contact with undiluted 3-MTP, the rabbit acute percutaneous LD50 was 0.71 (0.43-1.15) ml/kg (males) and 0.79 (0.49-1.30) ml/kg (females): times to death ranged from 2 h to 2 d after the start of dosing. Exposure of rats to a statically generated saturated atmosphere killed all 5 males with a 40 min exposure and all 5 females with a 24 min exposure. In contrast, a 4-h exposure of rats to a dynamically generated saturated vapor atmosphere of 3-MTP did not produce any mortalities or signs of toxicity. A 4-hr occluded contact with 0.5 ml undiluted 3-MTP caused moderate to severe erythema and severe edema resolving by 7 to 17 d. Five/6 animals had necrosis apparent on removal of the occlusive dressing and persisting 10 to 17 d. On the rabbit eye, 0.1 ml undiluted 3-MTP produced moderate to severe corneal injury with iritis and moderate conjunctival inflammation which persisted 21 d in 3/6 animals; 0.01 ml caused moderate diffuse corneal injury and moderate conjunctival inflammation with healing by 7 d. 3-MTP did not produce mutagenic activity either in the absence or presence of metabolic activation with a Salmonella typhimurium reverse mutation assay using strains TA98, TA100, TA1535, TA1537 and TA1538. In a mouse lymphoma cell (L5178Y/tk +/-) assay, 3-MTP produced concentration-related increases in mutant colonies, both in the absence and presence of metabolic activation. Increases were mainly in the sigma (chromosomal damaging) colonies. In a mouse bone marrow micronucleus study, with vapor exposures to 37.4, 88.5 and 155.6 ppm for 1 h/d for 2 consecutive d, there were exposure concentration-related increases in micronucleated erythrocytes which were statically significant for male mice.

Modeling the mouse lymphoma forward mutational assay: the Gene-Tox program database

An SAR model of the induction of mutations at the tk(+/-) locus of L5178Y mouse lymphoma cells (MLA, for mouse lymphoma assay) was derived based upon a re-evaluation of experimental results reported by a Gene-Tox (GT) working group [A.D. Mitchell, A.E. Auletta, D. Clive, P.E. Kirby, M.M. Moore, B.C. Myhr, The L5178Y/tk(+/-) mouse lymphoma specific gene and chromosomal mutation assay. A phase III report of the U.S. Environmental Protection Agency Gene-Tox Program, Mutation Res. 394 (1997) 177-303.]. The predictive performance of the GT MLA SAR model was similar to that of a Salmonella mutagenicity model containing the same number of chemicals. However, the structural determinants (biophores) derived from the GT MLA SAR model include both electrophilic as well as non-electrophilic moieties, suggesting that the induction of mutations in the MLA may occur by both direct interaction with DNA and by non-DNA-related mechanisms. This was confirmed by the observation that the set of biophores associated with MLA overlapped significantly with those associated with phenomena related to loss of heterozygosity, chromosomal rearrangements and aneuploidy. The MLA SAR model derived from the GT data evaluation was significantly more predictive than an SAR model previously derived from MLA data reported by the US National Toxicology Program [B. Henry, S.G. Grant, G. Klopman, H.S. Rosenkranz, Induction of forward mutations at the thymidine kinase locus of mouse lymphoma cells: evidence for electrophilic and non-electrophilic mechanisms, Mutation Res. 397 (1998) 331-335.]. Moreover, the latter model appeared to be more complex than the former, suggesting that the GT induction data was both simpler mechanistically and more homogeneous than that of the NTP.

Evaluation of the mouse lymphoma tk assay (microwell method) as an alternative to the in vitro chromosomal aberration test

In order to evaluate the utility of the mouse lymphoma assay (MLA) for detecting in vitro clastogens and spindle poisons and to compare it with the in vitro chromosomal aberration test (CA), we conducted an international collaborative study of the MLA that included 45 Japanese laboratories and seven overseas laboratories under the cooperation of the Ministry of Health and Welfare of Japan and the Japanese Pharmaceutical Manufacturer's Association. We examined 40 chemicals; 33 were reportedly positive in the CA but negative in the bacterial reverse mutation assay, six were negative in both assays and one was positive in both. We assayed mutations of the thymidine kinase (TK) locus (tk) of L5178Y tk +/- mouse lymphoma cells using the microwell method. According to our standard protocol, cells were exposed to the chemical for 3 h, cultured for 2 days and TK-deficient mutants were expressed in 96-well plates under trifluorothymidine. Each chemical was coded and tested by two or three laboratories. Among the 34 CA-positive chemicals, positive MLA results were obtained for 20 and negative results were obtained for nine. The remaining five chemicals were inconclusive or equivocal because of discrepant inter-laboratory results or reproduced discrepant results, respectively. Among the six CA-negative chemicals, one was negative in the MLA, two were positive and three were inconclusive. Thus, the MLA could detect only 59% (20/34) of CA-positive chemicals. We concluded that the MLA was not as sensitive as the CA. Some MLA-negative chemicals evoked positive responses in the CA only after long continuous treatment. These might also be genotoxic in the MLA with long continuous treatment. Improvement of the MLA protocol, including alteration of the duration of the treatment, might render the MLA as sensitive as the CA.

The need for long-term treatment in the mouse lymphoma assay

The L5178Y tk +/- mouse lymphoma assay (MLA) has been widely used as a genotoxicity test for the detection of mutagens and clastogens. The standard MLA, as well as other mammalian cell gene mutation assays, usually employs a short treatment period (3-6 h). Our previous report, however, suggested that such short treatments may be insufficient for detecting some clastogens and spindle poisons. For the present study, we introduced and evaluated a longer treatment (24 h) in the MLA. We examined 15 chemicals which were evaluated as negative or inconclusive in the short-term study. Cells were exposed to the chemical for 24 h without S9 mix, cultured for 2 days and then thymidine kinase-deficient mutants were selected in 96-well microtiter plates under trifluorothymidine. Eleven chemicals yielded positive responses in the 24 h treatment MLA. They included nucleoside analogs (2'-deoxycoformycin and dideoxycytidine), a base analog (1,3-dimethylxanthine) and spindle poisons (colchicine and vinblastine sulfate), all of which do not directly affect DNA, but bring about mutations and chromosome alterations through nucleoside metabolism and chromosome segregation. Because the mutagenicities of these non-DNA targeting chemicals appear to be cell cycle dependent, treatment extending over more than one cell cycle may be required for their effect. Combining results from the present and previous studies, 31 of 34 (91%) chromosome aberration-positive chemicals exhibited positive responses in the MLA, suggesting that the sensitivity of the MLA with 24 h treatment periods approaches that of the chromosome aberration test.

Identification of a heteromorphic microsatellite within the thymidine kinase gene in L5178Y mouse lymphoma cells

The objective of this work is to identify a heteromorphism within the thymidine kinase (Tk1) gene which can be used to assay for allele loss by means of PCR. Intron F of mouse Tk1 contains two (CA)n microsatellite sequences separated by 107 bp of non-repetitive sequence. We tested this region for heteromorphism in L5178Y mouse lymphoma cells. A PCR primer pair designated Agl1 yielded products of 396 and 194 bp from L5178Y tk+/- genomic DNA. The 194-bp product resulted from a secondary binding site between the two (CA)n repeats for the forward Ag11 primer and was not produced from tk-/- mutants that had lost the functional Tk1b allele. Agl2 primers produced two PCR products of 523 and approximately 440 bp and Agl3 primers produced products of 579 and approximately 500 bp. In both these cases, the difference in product size was approximately equal, indicating that Intron F is approximately 80 bp shorter in the non-functional Tk1a allele than in Tk1b. This heteromorphism forms the basis for an assay for allele loss by means of PCR. Agl1 and Agl3 primers yielded additional products of 91 and 274 bp, respectively, consistent with sizes expected from the mouse Tk1 pseudogenes (Tk1-ps). Our conclusions drawn from an analysis of 122 mutants for Tk1b loss using Agl2 primers agreed with previous analysis of the NcoI heteromorphism. Thus, a simple PCR-based analysis can identify Tk1b loss in the L5178Y mouse lymphoma cells.

Phosphorylation of topoisomerase I in L5178Y-S cells is associated with poly(ADP-ribose) metabolism

The reason for different phosphorylation of topoisomerase I in two sublines of L5178Y murine lymphoma (LY cells) was investigated. Camptothecin-resistant LY-S cells show increased poly(ADP-ribose) level and lowered topoisomerase I phosphorylation compared to camptothecin-sensitive LY-R cells. In this study diminished phosphorylation of LY-S topoisomerase I was observed for sites recognized by casein kinase 2 but not for those phosphorylated by protein kinase C. Tryptic digests of LY-S topoisomerase I labeled in vitro by casein kinase 2 indicated that phosphorylation was similarly lowered at different sites. Activity of casein kinase 2 measured in nuclear extracts was about 1.7 times lower for LY-S than LY-R cells. This difference was diminished or eliminated by increasing casein concentration, diluting the extract or increasing the ionic strength. Activity of poly(ADP-ribose) polymerase was 5.3 times higher in LY-S than in LY-R nuclei. When the activity of the polymerase was inhibited by treatment of LY-S cells with benzamide, casein kinase 2-catalyzed phosphorylation of topoisomerase I increased. This was accompanied by an increase in sensitivity to camptothecin as reflected in the diminished viability of LY-S cells.

Effects of topoisomerase I-targeted drugs on radiation response of L5178Y sublines differentially radiation and drug sensitive

The murine L5178Y (LY) lymphoma sublines, LY-R (radiation resistant) and LY-S (radiation sensitive) display a difference in susceptibility to camptothecin (CPT): LY-S cells are less sensitive to killing by this inhibitor of topoisomerase I than LY-R cells. Post-treatment (CPT present until 3 h after irradiation) sensitizes only LY-S cells. In agreement with this, only in LY-S cells is the relative number of DNA-protein cross-links formed after treatment with CPT + X higher than expected for additivity of X-ray and CPT-induced damage. The pattern of changes in the labelling indices and cell cycle distribution in cells that underwent combined treatment is essentially like that seen for single-agent treatment: for LY-S cells like that for radiation, for LY-R cells like that for CPT. We found no direct relation between the patterns of cell cycle distributions and the enhancement of the lethal effect of X-irradiation by CPT post-treatment. The sublines are not markedly differentially sensitive to beta-lapachone, which modifies topoisomerase I activity, and not sensitized to X-rays by post-irradiation treatment with the drug.

Enhancement of hyperthermic killing in L5178Y cells by protease inhibitors

We have investigated the effect of protease inhibitors on hyperthermic cell killing using cultured mammalian cells (L5178Y) and found that protease inhibitors were potent hyperthermia sensitizers. At 37 degrees C, phenylmethylsulfonyl fluoride (PMSF), a serine protease inhibitor, was not cytotoxic at the concentration of 400 micrograms/ml for up to 6 h. When cells were exposed to PMSF (200-400 micrograms/ml) during heating at 43 degrees C, significant potentiation of hyperthermic cell killing was observed. Other protease inhibitors, such as chymostatin and diisopropylfluorophosphate (both are serine protease inhibitors); (2S,3S)-trans-epoxy-succinyl-L-leucylamido-3-methylbutane ethyl ester (cysteine protease inhibitor) and pepstatin-A (aspartate protease inhibitor) showed similar effects. However, when cells were heated at 43 degrees C in the presence of cycloheximide (a protein synthesis inhibitor) together with PMSF, hyperthermic enhancement by PMSF decreased markedly. A decrease in potentiating the effect of PMSF was also noted with thermotolerant cells. These facts suggest that protease inhibitors may exert their hyperthermic cell killing by inhibiting proteases and ubiquitin, which are necessary to degrade denatured proteins induced by heat.

The sensitivity to camptothecin of DNA topoisomerase I in L5178Y-S lymphoma cells

Sensitivity to camptothecin (CPT) of type I DNA topoisomerases isolated from two L5178Y (LY) sublines was examined in reaction media containing either aspartate or chloride. The enzyme from LY-S cells was sensitive to the drug in the presence of 120 mM K-aspartate, but the sensitivity was markedly reduced in the presence of 120 mM KCl. The enzyme from LY-R cells was similarly sensitive to camptothecin in the presence of either aspartate or chloride. The optimum ionic strength for the relaxation reaction catalyzed by both LY-R and LY-S type I DNA topoisomerases was similar. We suggest that sensitivity of the LY-S enzyme to CPT depends on the amount of cleavable complex formed, which in turn depends on the ionic conditions of the assay.

Is micronucleus induction by aneugens an early event leading to mutagenesis?

This study was designed to investigate a previously unidentified potential mechanism for mutation induction as well as to clarify a biological consequence of micronuclei with mutation induction as measured by trifluorothymidine (TFT) resistance in mouse L5178Y cells using four aneugens: colcemid, diethylstilbestrol, griseofulvin and vinblastine. All four compounds induced micronuclei which appeared in the first cell cycle after treatment. More than 85% of the micronuclei induced by each compound stained positive for the presence of kinetochores implying that the micronuclei contained whole chromosomes. However, these same compounds were unable to induce TFT resistance under three different treatment regimes. We concluded that these compounds, under conditions where they induce primarily kinetochore positive micronuclei, were not able to induce mutations. Thus, the induction of micronuclei containing whole chromosomes harboring a selectable gene is not an early event leading to mutations in these cells.

Use of microsatellite DNA polymorphisms on mouse chromosome 11 for in vitro analysis of thymidine kinase gene mutations

The mouse lymphoma (L5178Y tk+/- 3.7.2C) in vitro mutagenesis assay can measure the genotoxic effects of a wide variety of chemical agents by inactivation of a single functional thymidine kinase (tk-1) gene. We have previously demonstrated, using cytogenetic and molecular techniques, that the types of molecular lesions associated with tk-1 gene inactivation span a wide range similar to that seen in tumor cells at specific oncogene and tumor suppressor gene loci. We have identified, using polymerase chain reaction techniques, 21 microsatellite, or 'simple sequence repeat', polymorphisms between chromosomes 11a and 11b in 3.7.2C cells. These microsatellite polymorphisms span virtually the entire chromosome, from mapping positions of 3-78 centiMorgans (cM) from the centromere, thus providing landmarks to study loss of genetic material across the entire chromosome. Four of the microsatellite polymorphisms lie within 12 cM of tk-1, and provide a means of mapping loss of genetic material in the immediate vicinity of tk-1, a capability that we have not previously had in the mouse lymphoma assay. Loss of alleles (i.e. loss of heterozygosity) is an important feature of tumor development, having to do with tumor suppressor gene expression. Therefore, the ability to detect loss of heterozygosity in the mouse lymphoma assay will make the assay an extremely valuable tool in the detection of agents capable of inducing loss of heterozygosity.

Mammalian cell gene mutation assays working group report

As part of the International Workshop on Standardization of Genotoxicity Test Procedures, in Melbourne, 27-28 February 1993, various international guidelines were examined with respect to protocol issues in the area of mammalian cell gene mutation assays. The working group on mammalian cell gene mutation assays discussed a wide range of protocol issues related to study design; in most cases the recommendations are reasonably consistent with existing guidelines. Agreement was reached on several issues as follows. The upper limit of concentration for testing non-toxic substances should be 10 mM or 5 mg/ml, whichever is lower. For testing toxic substances the criteria of an acceptable upper limit of concentration should yield 10-20% survival. Any of several established mammalian cell mutation assays (L5178Y TK+/-, CHO/HPRT, AS52/XPRT, V79/HPRT) can be used to evaluate mutagenesis in mammalian cells; the ouabain (Na/K-ATPase) system is not an acceptable mutation assay for routine evaluation of mutagenesis in mammalian cells. Ability to recover small colonies must be convincingly demonstrated when using the L5178Y TK+/- mouse lymphoma assay. In the mouse lymphoma assay (L5178Y TK+/-), colonies in positive controls and at least two (if available) representative positive doses of the test compound should be sized if a positive response is seen; in the event of a negative response due to the test compound, colony sizing of the positive control is necessary to validate the conduct of the assay. Testing both in the presence and absence of S9 metabolic activation is necessary. It was not possible to come to a firm conclusion about the length of treatment. There was a general agreement that extended treatment times (> 2 cell cycles) often bear more disadvantages than advantages and should only be used with adequate justification. It is not necessary to repeat clear positive or clear negative tests when the assay has been adequately performed; this recommendation differs significantly from the UK guidelines. If treatment groups are not replicated, the numbers of doses tested should be increased; this recommendation differs significantly from the UK guidelines. Each laboratory should establish a historical database for the performance of a given assay in that laboratory.

PKA controls a level of topoisomerase I mRNA in mouse L5178Y lymphoma cells treated with db-cAMP

The level of topoisomerase I mRNA was measured in cells of two mouse lymphoma (LY) sublines treated with db-cAMP. A transient increase of the level was observed to be of about 60% of the basic level and to have maximum after the 3 h treatment of LY-S cells. The increase in LY-R subline was two-fold lower. The activity of PKA in a cytosol fraction of LY-S cells was 1.75 times higher than that in LY-R cells. The activity of PKA in membranes and nuclear fraction did not differ significantly in both cell types. When the activity of PKA in LY-S cells was inhibited with H8, no increase of the level of topoisomerase I mRNA was observed upon db-cAMP treatment of cells. We suggest that the activity of PKA in the cytosol controls the expression of topoisomerase I gene in LY cells at high concentration of cAMP.

The sensitivity to DNA topoisomerase inhibitors in L5178Y lymphoma strains is not related to a primary defect of DNA topoisomerases

DNA topoisomerase-targeting antitumor drugs are potent inducers of protein-concealed strand breaks in mammalian cells and act by trapping DNA topoisomerases on chromosomal DNA in the form of drug-enzyme-DNA cleavable complexes. It has been proposed that the cleavable complex is an unusual form of DNA damage that elicits cellular responses analogous to those caused by DNA damaging agents. The relationship between topoisomerase-targeting drug-induced damage and radiation-induced damage has been investigated by analyzing the properties of DNA topoisomerases in mouse L5178Y lymphoma strains that are cross-sensitive to topoisomerase I-II inhibitors and to UV light or X-ray irradiation. The strains are LY-R, isolated from L5178Y cells on the basis of increased resistance to ionizing radiation, and strain LY-S, isolated from LY-R cells following a spontaneous increase in the sensitivity to ionizing radiation. LY-S cells, deficient in the rejoining of DNA double-strand breaks, show enhanced sensitivity to topoisomerase II-targeting inhibitors, whereas LY-R cells have an increased sensitivity to UV radiation and to the topoisomerase I inhibitor, camptothecin. The cellular availability of DNA topoisomerase I and II and the sensitivity of the enzymes to their specific inhibitors have been measured in the two related strains. In the LY-R strain, we found a 30% decrease in topoisomerase I content but no difference in camptothecin sensitivity, while no quantitative or qualitative differences were observed for the topoisomerase II. The results indicate that variations in sensitivity of the L5178Y strains to topoisomerase inhibitors are unlikely to be related to primary defects of the target enzymes, and thus it is possible that common pathways exist for processing of topoisomerase- and radiation-induced damage.

A comparison of the CHO/HGPRT+ and the L5178Y/TK+/- mutation assays using suspension treatment and soft agar cloning: results for 10 chemicals

The mouse lymphoma assay (MLA) and Chinese hamster ovary (CHO) cell assay are sensitive indicators of mutagenicity. The CHO assay has been modified technically to permit treatment in suspension and soft agar cloning comparable to the MLA. This methodology eliminates the risk of metabolic cooperation and the trauma of trypsinization. In addition, a larger population of cells can be treated and cloned for mutant selection. In order to compare the effectiveness of the test systems, 10 chemicals were evaluated for the induction of forward mutations in the CHO and MLA. Several of these chemicals have been reported as clastogenic; therefore, abbreviated colony sizing was performed to gauge the extent of genetic damage to the MLA cells. Both test systems detected benzo[a]pyrene, mitomycin C, acridine orange, and proflavin, and, with the exception of proflavin, more large colonies were present than small colonies. The suspect clastogen, phenytoin, was not mutagenic in the MLA and produced inconclusive results in the CHO. Ethidium bromide, a clastogen and a bacterial mutagen, was not mutagenic in either the MLA or CHO. Four compounds (p-aminophenol, benzoin, methoxychlor, and pyrene) were positive in the MLA, generally inducing a large number of small colonies, while demonstrating no mutagenic activity in the CHO assay. They have also been shown to be generally nongenotoxic in other test systems. Overall, the modified CHO assay did not appear to be better than the MLA for the detection of mutagenic agents. However, the MLA does appear to have lower specificity.

Reduced sensitivity to camptothecin of topoisomerase I from a L5178Y mouse lymphoma subline sensitive to X-radiation

Murine L517BY (LY) lymphoma sublines, LY-R (X-radiation resistant) and LY-S (X-radiation sensitive) displayed a difference in susceptibility to camptothecin: susceptibility of LY-S cells to the alkaloid was shifted towards higher concentrations as compared to LY-R cells. A similar difference was observed at the level of genomic DNA when a number of DNA-protein cross-links was determined or single-strand breaks were revealed by the fluorescent nucleoid halo assay. Activities of topoisomerases I and II were the same in both sublines. In turn, a higher resistance to camptothecin was found for the isolated LY-S topoisomerase I in the DNA cleavage test, suggesting that an altered enzyme was responsible for the susceptibility difference observed at the cellular level. In the relaxation test the enzymes from the two sublines showed a different sensitivity to beta-lapachone, an activator of topoisomerase I, but were similarly sensitive to all inhibitors, except camptothecin.

Photofrin II photosensitization is mutagenic at the tk locus in mouse L5178Y cells

Photosensitization mediated by Photofrin II (PFII) was found to be mutagenic at the heterozygous thymidine kinase (tk) locus in mouse L5178Y lymphoma strains LY-S1 and LY-R16 but not in strain LY-R83 which is hemizygous at the tk locus. After treatments yielding 37% survival, the mutagenicity of photosensitization with PFII in strain LY-S1 was similar to that of other mutagenic agents including x-radiation, ethyl methanesulfonate, and photosensitization with chloroaluminum phthalocyanine (AlPcCl). Although both strain LY-S1 and strain LY-R16 were mutagenized by photosensitization with PFII, only strain LY-S1 was mutagenized by photosensitization with AlPcCl. The non-mutability of strain LY-R83 following photodynamic treatment with either sensitizer may be because of the poor recovery of mutants with intergenic mutations in this TK+/0 hemizygous strain, whereas the non-mutability of strain LY-R16 subjected to photodynamic treatment with AlPcCl may be because LY-R16 cells sustaining mutagenic damage do not survive for reasons other than the loss of an essential gene.

Comparison of chromosome aberration frequency and small-colony TK-deficient mutant frequency in L5178Y/TK(+/-)-3.7.2C mouse lymphoma cells

The L5178Y/TK(+/-)-3.7.2C mouse lymphoma assay is used to quantitate the induction of thymidine kinase (TK)-deficient mutants. The mutants detected in the assay form colonies that can be distinguished as large or small. The induction of small-colony mutants has been associated with the induction of chromosome mutations. In the present paper, we compare the analysis of induced small-colony TK mutants with gross aberration analysis (the more classical approach to analyzing chromosomal damage). Data are presented for 34 mutagens. As expected, we find that while the induction of gross aberrations and the induction of small-colony TK mutants is correlated, there is no simple mathematical relationship between the two endpoints. The two markers evaluate different subpopulations of chromosome mutations. While either endpoint can be used to detect chromosomal mutations, it should be remembered that the small-colony TK mutants represent genetic events which are compatible with cell viability. Only those alterations compatible with cell viability are a significant risk for human carcinogenicity or mutagenicity.

Molecular dissection of mutations at the heterozygous thymidine kinase locus in mouse lymphoma cells

The mouse lymphoma L5178Y TK+/- 3.7.2C cell line allows quantitation of induced TK(+/-)----TK-/- mutations at the heterozygous thymidine kinase (Tk) locus. TK-/- mutant colonies show a bimodal size distribution, reflecting a difference in the growth rates of the two size classes that is hypothesized to result from different degrees of genetic damage. The two homologous chromosomes 11 containing the alleles of the Tk gene in L5178Y 3.7.2C TK+/- cells are distinguishable at the cytogenetic level. We find, in addition, that the two alleles are distinguishable at the molecular level because of an Nco I restriction fragment length polymorphism at the 3' end of the gene. In a set of 51 large-colony and 48 small-colony TK-/- mutants induced by ionizing radiation or by chemical mutagens, we find that 78, including all except one of the small-colony mutants, have lost the Tk+ allele and that some of these have two to four copies of the remaining Tk- allele. Nineteen of the large-colony TK-/- mutants that do not show Tk+ allele loss show no other structural changes detectable at the level of Southern blot analysis. One shows a partial deletion. The variety of mutagenic lesions recorded at the heterozygous Tk locus may be representative of events observed in human malignancy and may include a wider range of mutagenic events than can be observed at hemizygous test loci.

The effect of ara-C-induced inhibition of DNA synthesis on its cellular pharmacology

The cytotoxicity of ara-C is believed to result from incorporation of ara-CTP into DNA and inhibition of DNA synthesis. Since complete inhibition of DNA synthesis would prevent further incorporation of ara-CTP, ara-C may have a self-limiting effect on its own cytotoxicity, particularly at the high concentrations typical of high-dose ara-C clinical protocols. In this study, the incorporation of [3H]-dThd and [3H]-ara-C into DNA were compared. Within 1 h of exposure of L5178Y cells to ara-C, the rate of [3H]-dThd incorporation into the acid-insoluble fraction was reduced by 98%. Despite this nearly complete block in [3H]-dThd incorporation, DNA synthesis was not completely inhibited since [3H]-ara-C continued to be incorporated for up to 6 h, although a plateau in ara-CDNA synthesis was observed between 2 and 3 h exposure when ara-CTP levels were maximal. The effect of ara-C on [3H]-dThd incorporation into DNA was due in part to an indirect effect of ara-C on the metabolism of intracellular [3H]-dThd to [3H]-dTTP. Within 30 min exposure to 10 microM ara-C, the rate of cellular [3H]-dTTP synthesis was slowed to only 15% of the control rate. This was not due to inhibition of [3H]-dThd transport, since the intracellular and extracellular concentrations of the nucleoside were equal. The effect of ara-C on [3H]-dTTP synthesis resulted from significant changes in deoxynucleoside 5'-triphosphate (dNTP) pools. dTTP, dATP, and dGTP levels were increased, whereas the dCTP concentration was decreased. When dThd kinase from L5178Y cells was assayed with increased dTTP levels induced by ara-C vs the dTTP level in control cells, its activity was reduced by 72%. Thus, the [3H]-dThd incorporation experiment overestimated the extent of inhibition of DNA synthesis by ara-C due to increased feedback inhibition of dThd kinase and increased competition for DNA polymerase between the elevated unlabeled dTTP pool and the decreased levels of [3H]-dTTP. In vitro assay of DNA polymerase in the presence of the ara-CTP concentration achieved after 0.5 or 3 h exposure to 10 microM ara-C (60 microM and 200 microM, respectively), plus the mixture of dNTPs found intracellularly at these times, resulted in 57% and 80% inhibition of the polymerase, respectively. This inhibition may account for the plateau in the accumulation of ara-CDNA that was observed at 3 h and suggests that ara-C incorporation may be self-limiting at high cellular concentrations of ara-CTP.(ABSTRACT TRUNCATED AT 400 WORDS)

High-resolution cytogenetic analysis of L5178Y TK+/- 3.7.2C cells: variation in chromosome 11 breakpoints among small-colony TK-/- mutants

Since the finding that the mouse lymphoma L5178Y TK+/- ----TK-/- forward mutational assay system can detect and distinguish a range of genetic lesions, including large chromosomal aberrations and smaller, perhaps point mutational events, the chromosomal analysis of these lesions at the highest possible level of band resolution has become increasingly important. We have developed an acridine orange/colcemid/hypotonic treatment for TK-/- mutants to provide high-resolution chromosomes with over 500 G-bands for breakpoint analysis. Using such high-resolution procedures, we find that independently induced small-colony mutants show rearrangements in the distal portion of chromosome 11, with breakpoints occurring between bands B3 and E1.2. This finding of a range of chromosomal breakpoints in different TK-/- mutants complements recent molecular genetic analysis of mutants and is consistent with the hypothesis that chromosomal lesions in small-colony mutants may affect a large portion of the genome in the vicinity of the tk-1 gene.

Dual inhibitory effect of bredinin

Bredinin inhibition of cell growth was investigated in the mouse lymphoma cell line L5178Y. Bredinin caused the accumulation of IMP and the reduction of XMP. It was converted to the 5'-phosphate within the cells. Bredinin 5'-phosphate but not bredinin competitively inhibited both IMP dehydrogenase and GMP synthetase. Thus the inhibition of cell growth is probably due to bredinin 5'-phosphate, which inhibits the consecutive enzyme reactions IMP dehydrogenase and GMP synthetase. These inhibitions result in the accumulation of IMP and the reduction of XMP.

Mammalian glycinamide ribonucleotide transformylase. Kinetic mechanism and associated de novo purine biosynthetic activities

Glycinamide ribonucleotide transformylase catalyzes the conversion of glycinamide ribonucleotide and 10-formyltetrahydrofolate to formylglycinamide ribonucleotide and tetrahydrofolate. The enzyme purified from the murine lymphoma cell line L5178Y also catalyzes two other de novo purine biosynthetic activities, glycinamide ribonucleotide synthetase and aminoimidazole ribonucleotide synthetase. The transformylase reaction shows a 1:1 stoichiometry for substrate utilization and an optimum rate between pH 7.9 and 8.3. Initial velocity and dead-end inhibition patterns indicate that the kinetic mechanism of the transformylation reaction is ordered-sequential, with 10-formyltetrahydrofolate binding first. alpha, beta-Hydroxyacetamide ribonucleotide (alpha, beta-N-(hydroxyacetyl)-D-ribofuranosylamine) is shown to be an inhibitor of the transformylase, competitive against glycinamide ribonucleotide.

Micronucleus, chromosome aberration, and small-colony TK mutant analysis to quantitate chromosomal damage in L5178Y mouse lymphoma cells

In testing the hypothesis that the small-colony thymidine kinase-deficient mutants of L5178Y/TK+/- -3.7.2C mouse lymphoma cells represent an estimate of the clastogenicity of test chemicals, we have been performing gross aberration analysis. The present study was initiated to determine if the cytokinesis block method of micronucleus analysis could be performed in mouse lymphoma cells and to compare 3 different endpoints of clastogenicity: the number of metaphases with aberrations, number of binucleates with micronuclei, and small-colony TK mutant frequency. In this study, 12 compounds having varying clastogenic potencies were evaluated. As would be expected, the 3 endpoints vary in the relative magnitude of the quantitated response. This difference likely results from the types of clastogenic damage detected by each endpoint. Of the 3 endpoints tested, only the small-colony TK mutant frequency measures events compatible with long-term cell survival.

Studies of an S9-based metabolic activation system used in the mouse lymphoma L5178Y cell mutation assay

The purpose of this work was to define, in this laboratory, the conditions of most general utility for the metabolic activation of chemicals to mutagens in the mouse lymphoma L5178Y cell tk+tk- ----tk-tk- assay. The approach used was to optimize the generation of non-toxic concentrations of NADPH from NADP glucose-6-phosphate (G6P) and G6P dehydrogenase, then use that system to examine the effects of increasing concentrations of Aroclor-1254-induced Fischer 344 male rat liver post-mitochondrial supernatant fluid (S9) upon the mutagenicity of chemicals. The promutagens were 2-acetylaminofluorene, cyclophosphamide, dimethylnitrosamine, methanol, 3-methylcholanthrene and procarbazine. It was determined that: (i) cell population growth was reduced at NADPH concentrations of greater than or equal to 1 mM; (ii) to ensure a maximum conversion of NADP to NADPH the G6P/NADP ratio should be two or greater; (iii) excess G6P (2.5 mM) is not harmful to the cells; (iv) toxicity due to S9 was observed at 12.5 mg whole liver equivalents (WLE)/ml (i.e. 50 microliters of a 25% liver S9 preparation per ml); (v) concentrations of S9 between 2.5 and 7.5 mg WLE/ml appeared to be adequate for the activation of the six promutagens to demonstrably mutagenic products.

Mouse lymphoma L5178Y thymidine kinase locus assay of 50 compounds

Mutagenicity results are presented for 50 compounds tested in the mouse lymphoma TK+/(-)----TK-/- forward mutation assay. Test compounds were mostly from chemical classes not previously tested, to provide new information on the sensitivity of the assay; chemicals of low toxicity or thought to be non-carcinogenic and metabolic inhibitors, to indicate whether and under what conditions the assay can generate so-called false positive results. Twelve compounds that have been tested previously were included in this study to provide an indication of the reproducibility of the assay. Concordant results were obtained for nine of these, while disagreeing, positive results were seen with aniline, fluorene and pyrene. The following compounds belonging to the noncarcinogen category were positive at concentrations in the range 0.02-1 mol/l: dimethyl sulphoxide, EDTA, glucose, polyethyleneglycol, sodium chloride, sodium nitrite and urea. Measurements of the osmotic pressure indicated a lack of a simple relationship to mutagenic effects for these compounds. While the potent mutagenic/carcinogenic compounds tested gave greater than 4-fold increases in the mutation frequency, weak carcinogens or compounds not known to be carcinogenic that were positive in the assay gave increases of between 2- and 4-fold. Exceptions were aldehyde derivatives and chemicals that can lead to oxidative stress, which were detected with exaggerated sensitivity by the assay. Among the metabolic inhibitors tested, positive results were obtained with actinomycin D, cycloheximide, diethyl maleate, hydroxyurea and ouabain. Negative results were found with antimycin A. On the basis of the present results and previously published data it is concluded that a maximum limit for the test compound concentration can be set at 20 mmol/l and that testing to 20% total growth is adequate, with certain stipulations, to detect the mutagenic activity of test compounds. A similar analysis of the available test data shows that less than 4-fold increases in the mutation frequency have a lower predictivity for carcinogenicity.

Action of the antileukemic and anti-HTLV-III (anti-HIV) agent avarol on the levels of superoxide dismutases and glutathione peroxidase activities in L5178y mouse lymphoma cells

The antileukemic and anti-HTLV-III (anti-HIV) agent avarol, a sesquiterpenoid hydroquinone, was determined to be converted into its corresponding quinone derivative avarone via the semiquinone free radical. Its g-value was 2.0047; after hyperfine splitting the energy levels revealed 16 isotropic Hfs. The redox reaction products were identified at the pH values 4.0, 7.0 and 12.0 and the overall reaction pathways were formulated. In vivo experiments with L5178y mouse lymphoma cells in the ascites of mice revealed that the cytostatic potencies of avarol and avarone cannot be augmented by lowering the pH value. Incubation studies with L5178y cells in vitro showed that the intracellular levels of superoxide dismutases (SODases) and of glutathione (GSH) peroxidase activities significantly change after avarol administration. While both the Mn-SODase and the Cu/Zn-SODase activities dropped significantly, the GSH peroxidase activity increased inversely. From these experiments we assume that the anti-tumour and the antiviral effects of avarol/avarone may be due to an increase, induced by the drug, of the intracellular concentrations of superoxide radicals.

Procarbazine is a potent mutagen at the heterozygous thymidine kinase (tk +/-) locus of mouse lymphoma assay

Procarbazine (Natulan) is a potent inducer of gene mutations at the heterozygous tk +/- locus in L5178Y mouse lymphoma cells in the presence of Aroclor-induced rat liver S9 metabolic activation (approximately 10(-3) mutant frequency at 10 micrograms/ml) while exerting a far weaker effect in the absence of S9. This mutagenicity is fairly robust with respect to the quantitative composition of the S9 mix and to variations in mouse lymphoma assay protocols (soft agar cloning versus 'microwell' assays). The high proportion of small colony tk -/- mutants induced by procarbazine together with the far weaker mutagenic response at the hemizygous hgprt locus in these same cells is interpreted in terms of a chromosomal or multi-gene mutational mechanism. Although procarbazine is clastogenic in vivo, it does not appear to be so under standard protocols using cultured human lymphocytes (+/- S9). It is not yet clear why this should be so, especially in light of its apparent clastogenicity in mouse lymphoma cells.

Do trifluorothymidine-resistant mutants of L5178Y mouse lymphoma cells re-express thymidine kinase activity following 5-azacytidine treatment?

TFT is an effective selective agent for TK-deficient mutants of L5178Y TK+/- -3.7.2C mouse lymphoma cells. Mutants can be classified by colony size into small colonies (many of which show readily observable chromosome abnormalities associated with chromosome 11--the location of the TK gene) and large colonies (which may represent events affecting only the expression of the TK gene). The precise nature of the induced damage causing the loss of the TK-enzyme activity for both mutant type is not known and is currently under investigation. The hypomethylating agent 5-azacytidine can be utilized to investigate the possibility that mutants might be the result of a suppressed rather than an altered TK gene. Mutant cell lines are treated with 5-azacytidine and then evaluated for re-expression of the TK enzyme as measured by resistance to THMG. In these studies, 11 mutants have been evaluated. None of the 11, including 10 small-colony mutants (6 with chromosome 11 translocations) and 1 large-colony mutant, show a high conversion to TK competency following 5-azacytidine treatment.

Methapyrilene is a genotoxic carcinogen: studies on methapyrilene and pyrilamine in the L5178Y/TK +/- mouse lymphoma assay

Methapyrilene (MP), a sedating antihistamine, is a potent rat hepatocarcinogen which has been thought to be non-genotoxic on the basis of the negative results in a small number of short-term mutagenicity tests. The present studies show that MP is a moderately active mutagen in the L5178Y/TK +/-----TK-/- mouse lymphoma assay (MLA) in the presence of aroclor-induced rat-liver S9, and that it induces predominantly small-colony thymidine kinase-deficient (TK-/-) mutants of demonstrated chromosomal origin. 10 of 12 small colony TK-/- mutants analyzed by banded karyotype (230-band level of resolution) show aberrations to chromosome 11b, the known location of the single functional TK gene in these cells. The observed aberrations from nine of the mutants included insertions, deletions and translocations while the tenth mutant had highly rearranged, multiple copies of chromosome 11 segments. By varying the concentrations of the S9 protein and cofactors it was shown that our standard S9 composition was close to optimum for activating MP to a mutagen. The activity and stability of various lots of S9 prepared in-house or purchased from a contract laboratory revealed significant differences. The ability of 2 lots of in-house S9 to activate a standard concentration of MP increased rapidly over the first 4 weeks of liquid nitrogen storage then declined slowly over the next 16 weeks. Three separate lots of purchased S9 were essentially inactive for the first 2 weeks of liquid nitrogen storage then increased in activity thereafter; these were the only occasions in which MP was not mutagenic in our hands. The mutagenic activity of pyrilamine (PYR), a structurally related antihistamine which is far less carcinogenic in rats, but easily detected in short-term tests as being genotoxic, was also investigated in the MLA. PYR was slightly less mutagenic than MP over a comparable range of concentrations, and also induced predominantly small-colony mutants. These studies fail to adequately explain the great carcinogenic differences between these two compounds, but are consistent with the potent hepatocarcinogenicity of MP resulting through a mutagenic mechanism.

Mutagenic activity of 27 dyes and related chemicals in the salmonella/microsome and mouse lymphoma TK+/- assays

A total of 27 dyes and related chemicals were tested for mutagenicity in both the Salmonella typhimurium plate-incorporation and FMN-modified assays as well as the mouse lymphoma TK+/- assay. Half of the compounds tested were monoazo dyes (14); the remainder consisted of disazo (3), aminotriphenylmethane derivatives (4), and other miscellaneous (6) color compounds. The results obtained in this study are compared with data from dyes of the same batch tested in other laboratories in the Salmonella plate-incorporation assay and in both in vitro and in vivo/in vitro UDS assays. Agreement of results from the various assays that could be compared (excluding results that were equivocal or indeterminate) ranged from 80 to 91%. Sufficient data were available to provide an overall index of in vitro activity for 15 chemicals; of these, 14 compounds could be compared to and agreed with reports of their carcinogenic potential in the literature.

Mutagenicity of m-AMSA and o-AMSA in mammalian cells due to clastogenic mechanism: possible role of topoisomerase

We evaluated the ability of the antitumor agent 4-(9-acridinylamino)-methanesulfon-m-anisidide (amsacrine or m-AMSA) and its congener, o-AMSA, to induce specific-locus mutations at the heterozygous thymidine kinase (tk) locus of L5178Y/TK+/- -3.7.2C mouse lymphoma cells. These cells permit the recovery of mutants due to single-gene or chromosomal mutation. m-AMSA was highly mutagenic at the tk locus, producing approximately 3000 mutants/10(6) survivors at 10% survival; positive dose range 1-10 ng/ml; o-AMSA produced approximately 1500 mutants/10(6) survivors at 10% survival; positive dose range 0.1-2.5 micrograms/ml. Most of the TK mutants were small colonies, which suggests that m-AMSA and o-AMSA induce primarily chromosomal mutations as opposed to single-gene mutations. The potent clastogenicity of these agents was confirmed by cytogenetic analysis for chromosomal aberrations, which showed that m-AMSA (9 ng/ml, 10% survival) and o-AMSA (1 microgram/ml, 10% survival) produced 383 and 179 aberrations, respectively, per 100 metaphases (background = 3-4/100). The large-colony TK mutant frequencies produced by m-AMSA (67 - 112/10(6) survivors; background = 7/10(6); survival = 63 - 16%) were comparable to the published HPRT mutant frequencies produced by m-AMSA in V79 cells. Novobiocin (50 micrograms/ml), an inhibitor of mammalian DNA topoisomerase II and other enzymes, inhibited the mutagenic effects of m-AMSA, suggesting that DNA topoisomerase II (or another enzyme) may play a role in the mutagenic/clastogenic activity of m-AMSA.