Chromosomal instability in mutagen-sensitive mutants isolated from mouse lymphoma L5178Y cells. I. Five different genes participate in the formation of baseline sister-chromatid exchanges and spontaneous chromosomal aberrations

In a search for cell mutants that show an increase or a decrease in the frequency of baseline sister-chromatid exchanges (SCEs) or spontaneous chromosomal aberrations (CAs), large numbers of mutagen-sensitive clones previously isolated from mouse lymphoma L5178Y cells were analyzed. In addition to two SCE mutants (ES 4 and AC 12) previously reported, three other mutants were identified as an SCE mutant. An ethyl methanesulfonate-sensitive mutant ES 2 and an alkylating agent-sensitive mutant MS 1 exhibited, respectively, 1.4-fold and 1.8-fold higher baseline SCE frequencies than did the parental L5178Y. In contrast, M10, which is sensitive to X-ray and 4-nitroquinoline 1-oxide, showed a reduced frequency of baseline SCEs (0.65-fold). These 5 mutants including ES 4 and AC 12 had 3--9-fold increases in spontaneous CA frequencies. Measurement of baseline SCE formation in inter-mutant hybrids revealed that M10 mutation is dominant, MS 1 and ES 4 mutations are semidominant, and ES 2 and AC 12 mutations are recessive. Because SCE frequencies in hybrids formed between pairs of 4 mutants (ES 2, MS 1, ES 4 and AC 12) were significantly lower than those in the tetraploid mutant cells, these 4 mutants probably belong to different complementation groups. Since M10 behaved dominantly with respect to SCE phenotype, it was not possible to determine by complementation test whether it belongs to a different group from the other mutants. However, the finding that M10 is complemented by other mutants for EMS sensitivity indicates that the M10 mutation is different from the other mutations. From these results, it is concluded that at least 4 different genes participate in the formation of high levels of baseline SCEs. The defects in ES 2, MS 1, ES 4, and AC 12 produce common lesions responsible for the formation of both SCEs and CAs. In contrast, the defect in M10 is associated with a high increase in spontaneous CA frequency, but conversely associated with a decrease in baseline SCE frequency. This suggests that M10 is defective in the process involved in the formation of baseline SCEs.     

Chromosomal hypersensitivity in mutant M10 and Q31 mouse cells exposed to ultraviolet radiation (UV) and 4-nitroquinoline-1-oxide (4NQO)

2 mutant mouse cells M10 and Q31 were examined for chromosomal aberrations induced by ultraviolet radiation (UV) and 4-nitroquinoline-1-oxide (4NQO), as compared with mouse lymphoma L5178Y cells. Q31 cells are UV- and 4NQO-sensitive cells isolated from L5178Y cells. M10 cells are similar but are sensitive to ionizing radiation and 4NQO. After treatment with UV or 4NQO, chromatid-type aberrations in these cell strains were induced more frequently in the first mitotic cells, at late fixation times. After UV exposure (2.4 J/m2), the maximal frequencies of chromatid-type breaks in Q31 cells were about 5 times higher than in L5178Y cells. In M10 cells such breaks were only as frequent as in L5178Y cells. After 4NQO treatment (50 ng/ml) the frequencies of chromatid-type breaks in M10 and Q31 cells were significantly higher than in L5178Y cells. From these results and those of previous studies (Takahashi et al., 1982), M10 cells may be considered hypersensitive to gamma-rays and 4NQO, but not to UV, and thus react similarly to L5178Y cells. The hypersensitivity of M10 cells to 4NQO may result from a defect in the ionizing-radiation repair mechanism as has been suggested to occur in ataxia telangiectasia (AT) cells. Q31 cells are hypersensitive to UV and 4NQO, but not to gamma-rays. Q31 cells may be considered to be deficient in a UV-like repair pathway. In conclusion, characteristics of murine M10 and Q31 cells are compared with those of human AT and xeroderma pigmentosum (XP) cells.     

Aphidicolin-Resistant Mutants of Mouse Lymphoma L5178y Cells with a High Incidence of Spontaneous Sister Chromatid Exchanges

Two aphidicolin-resistant cell mutants (AC 12 and AC 41) with a fourfold increase in spontaneous frequency of sister chromatid exchanges (SCEs) were obtained out of over 400 aphidicolin-resistant mutants isolated from mouse lymphoma L5178Y cells. They also exhibited three- to fourfold increases in spontaneous frequency of chromosome aberrations (CAs). To determine whether the high level of SCE frequency in AC 12 is caused by 5-bromodeoxyuridine (BrdUrd) used for visualizing SCEs, the effect of BrdUrd incorporated into DNA on SCE induction was analyzed. The SCE frequencies in AC 12 remained constant at BrdUrd incorporation levels corresponding to 2-90% substitution for thymidine in DNA. In addition, the small amount of BrdUrd incorporated into both daughter and parenteral DNA strands in AC 12 had minimal effect on SCE induction. Furthermore, AC 12 and AC 41 were slightly resistant to BrdUrd with respect to the induction of CAs, the inhibition of cell-cycle progression and the decrease in mitotic activity. These findings suggest that the high incidence of SCEs in AC 12 and AC 41 is formed by their intrinsic defects, not by the effects of BrdUrd used. The analysis of SCE frequencies in hybrid cells between these mutants and the parental L5178Y revealed that the genetic defects in AC 12 and AC 41 appear to be recessive, and that these two mutants belong to the same complementation group. Furthermore, AC 12 belonged to a different complementation group from ES 4, which was isolated previously from L5178Y as an SCE mutant with a twofold higher frequency of spontaneous SCEs. This finding indicates that at least two different genetic defects participate in the formation of the high incidence of spontaneous SCEs in mouse cells. These SCE mutants would provide valuable cell materials for studying the molecular mechanism of SCE formation.     

Antagonizing effect of 3-aminoharman on induction of sister-chromatid exchanges by mutagens

3-Aminoharman (3AH, 3-amino-1-methyl-9H-pyrido[3,4-b]indole), which has been reported as a novel substance with an antagonistic effect on induction of sister-chromatid exchange (SCE) by polycyclic mutagens in the presence of the metabolic activation system, was examined with a cultured human lymphoblastoid cell line, NL3, for its effect on SCE induction by direct-acting mutagens such as mitomycin C (MMC), nitrogen mustard N-oxide (NMO), methyl methanesulfonate (MMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 4-nitroquinoline 1-oxide (4NQO) and 3-hydroxyamino-1-methyl-5H-pyrido[4,3-b]indole (OH-Trp-P-2), and also by ultraviolet light (UV) irradiation. The results obtained on simultaneous treatment with 3AH and mutagens were as follows: (1) 3AH suppressed more than 50% of SCEs induced by MMC, NMO and OH-Trp-P-2; (2) 4NQO- and MNNG-induced SCEs were also suppressed by 3AH but to a lesser degree; (3) MMS-induced SCEs were not, however, altered by 3AH; and (4) the suppression of SCE by 3AH was dose-dependent. Treatment of cells with 3AH for 2 h immediately before MMC exposure suppressed SCE induction to a significant degree similar to the simultaneous treatment, but post-treatment with 3AH was much less effective. 3AH inhibited SCE induction by NMO when 3AH treatment was carried out either before or after NMO treatment, to an extent similar to the simultaneous treatment. Treatments with 3AH either before or after UV exposure did not change the UV-induced SCEs. Results with these direct-acting mutagens ruled out the relevance of metabolic activation as a necessary step for the antagonizing effect of 3AH.     

Studies on three mutagen-sensitive mutants of mouse L5178Y cells. I. Characterization of the hybrids between L5178Y and mutagen-sensitive mutants

Three mutagen-sensitive mutants, MS-1, M10 and Q31, have been isolated from mouse L5178Y cells. MS-1 cells are sensitive to methyl methanesulfonate (MMS), M10 cells are cross-sensitive to X-rays, MMS and 4-nitroquinoline 1-oxide (4NQO), and Q31 cells are cross-sensitive to UV and 4NQO. Lines resistant to 6-thioguanine (TGr) and 5-bromo-2'-deoxyuridine (BUr) were isolated from L5178Y and these three mutagen -sensitive mutants. All the TGr lines were sensitive to 5-bromo-2'-deoxyuridine and HAT medium and all the BUr lines were sensitive to 6-thioguanine and HAT medium. The hybrids homozygous for the mutagen-sensitive markers showed nearly the same sensitivity to UV, 4NQO, X-rays and MMS as their parental TGr and BUr lines. The hybrids constructed by fusing L5178Y BUr and TGr lines from each of MS-1, M10 and Q31 displayed the normal UV, X-ray and MMS resistancy of L5178Y cells. Thus the UV-, X-ray- and MMS-sensitive markers in MS-1, M10 and Q31 were recessive in somatic cell hybrids. The 4NQO-sensitive phenotype, however, behaved codominantly in somatic cell hybrids.     

Studies on three mutagen-sensitive mutants of mouse L5178Y cells. II. Complementation analyses between two methyl methanesulfonate-sensitive mutants and between two 4-nitroquinoline-1-oxide-sensitive mutants

Three mutagen-sensitive mutants, MS-1, M10 and Q31, were isolated from mouse L5178Y cells. MS-1 cells are sensitive to methyl methanesulfonate (MMS), M10 cells are cross-sensitive to X-rays, MMS and 4-nitroquinoline-1-oxide (4NQO); and Q31 cells are cross-sensitive to UV and 4NQO. MMS-, X-ray- and UV-sensitive markers in these mutants behaved recessively in hybrids between pairs of these mutants as in hybrids between L5178Y and these mutants as reported before (Shiomi et al., 1982b). Complementation analyses were carried out by forming hybrids between two MMS-sensitive mutants (MS-1 and M10) and between two 4NQO-sensitive mutants (M10 and Q31). MMS and 4NQO survivals were measured in these hybrid cells. MS-1 and M10 were found to belong to different complementation groups for MMS-sensitive phenotypes. The hybrid clones between M10 and Q31 were as sensitive to 4NQO as each of the mutants, indicating codominance of 4NQO sensitivity in these mutants. The hybrids constructed with L5178Y and three mutants were stable as to their chromosome constitution for 100 days of cultivation without selective pressure. From the segregation studies on these hybrids, it is concluded that neither the X-ray-sensitive mutation in M10 nor the UV-sensitive mutation in Q31 is located on the X chromosome.     

UV- and X-ray-sensitive double mutants of mouse L5178Y cells are synergistically more sensitive to 4-nitroquinoline-1-oxide than is either of the single mutants

The X-ray-sensitive mutant M10 and the UV-sensitive mutant Q31 of mouse lymphoma L5178Y cells are both sensitive to killing by 4-nitroquinoline-1-oxide (4NQO). Since cell hybridization experiments showed that the 4NQO sensitivities in M10 and Q31 cells behaved as codominant traits (Shiomi et al., 1982c), it is not possible to determine by complementation test whether the M10 and the Q31 mutations responsible for 4NQO sensitivities are allelic. We have obviated this difficulty by selecting double mutants that are sensitive to both X-rays and UV. From X-ray-sensitive M10 cells, two UV-sensitive mutants (XU 1 and XU 2) were isolated by a cell-suspension spotting method. XU 1 and XU 2 were found to belong to the same complementation group as Q31 (group I). Double mutants XU 1 and XU 2 were 30-37-fold more sensitive to 4NQO than parental L5178Y cells, whereas the single mutants M10 and Q31 were only 6-8-fold more sensitive to 4NQO than L5178Y cells in terms of D10 values (dose required to reduce survival to 10%). These results show that the M10-Q31-double mutations enhance 4NQO sensitivity synergistically, indicating that the M10 and the Q31 mutations relevant to 4NQO sensitivities are non-allelic. The implications of this finding are discussed.     

Cytogenetic analysis of an immunogenic mutant of the L5178Y lymphoma

A mutant of the uniformly lethal L5178Y lymphoma, called the L5178Y/Manitoba (L5178Y/M), was rejected after subcutaneous challenge in syngeneic DBA/2 mice. Karyotypic analysis revealed that the parent L5178Y lymphoma had four chromosome markers, with the mutant L5178Y/M sharing one of them as well as possessing two distinguishing markers. One diploid and two hypotetraploid clones were isolated from the L5178Y/M; they contained all the marker chromosomes and were also rejected by the syngeneic host. In addition to the shared chromosome markers, the L5178Y/M possessed antigens in common with the parent L5278Y. DBA/2 mice made immune to the mutant by subcutaneous immunization were able to slow the growth of the parent tumor but not the unrelated P-815-X2 mastocytoma.     

Mutation induction in a mouse lymphoma cell mutant sensitive to 4-nitroquinoline 1-oxide and ultraviolet radiation

The mutant mouse lymphoma cell Q31, which is sensitive to 4-nitroquinoline 1-oxide and ultraviolet radiation (UV), was compared with the parental L5178Y cell for the effect of caffeine and mutation induction after UV irradiation. Caffeine potentiated the lethal effect of UV in both cell strains to a similar extent, indicating that the defective process in Q31 cells was caffeine-insensitive. UV-induced mutation to 6-thioguanine resistance was determined in L5178Y and Q31 cells. The maximal yield of mutants was obtained 7 days post-irradiation in L5178Y cells and 14 days in Q31 cells for higher UV doses. It appears that a much longer time is required for the mutant cells than for the parental cells for full expression of the resistance phenotype even at equitoxic UV doses. A substantially higher frequency in induced mutations was observed in Q31 cells than in L5178Y cells at a given dose of UV. A plot of induced mutation frequency as a function of logarithm of surviving fraction again indicates hypermutability of Q31 cells as compared with the parental strain. In contrast, X-rays induced a similar frequency of mutations to 6-thioguanine resistance in L5178Y and Q31 cells.     

Repair of sister-chromatid exchange-inducing lesions in mutagen-treated cultures of human whole blood and purified fresh or frozen lymphocytes

Repair of mutagen-induced lesions that result in sister-chromatid exchanges was evaluated in 10 normal individuals. The mutagens used were mitomycin C (MMC), 4-nitroquinoline 1-oxide (4NQO), and N-methyl-N'nitro-N-nitrosoguanidine (MNNG). Cultures of whole blood, freshly purified lymphocytes, or purified lymphocytes cryopreserved for 6 months were analyzed after the mutagen treatments. All 3 mutagens induced reparable damage as evaluated by comparison of sister-chromatid exchanges between cultures that were given time to repair induced damage before 5-bromo-2'-deoxyuridine (BrdUrd) was added to the culture medium with those where BrdUrd was added immediately after the administration of the mutagens (MMC or 4NQO) or at culture initiation (MNNG). Repair of mutagen-induced DNA damage was detected in all 3 culture types; thus cryopreservation did not appear to alter the capacity of lymphocytes to repair mutagen-induced lesions. Quantitative differences in apparent repair capabilities were observed among individuals. Variability also existed among the different culture types within an individual, suggesting that caution should be exercised in interpreting these apparent differences.     

In vivo fate of MMS-induced DNA lesions that elicit SCE.

A previously reported in vivo protocol, which uses three-way differential staining (TWD) of sister chromatids, allows the screening of mutagen-induced sister-chromatid exchange (SCE) in each of the two cell divisions after mutagen treatment and also those occurring at apparently the same locus in both divisions. In the present work the effect of methyl methanesulfonate (MMS) was studied by means of this protocol. The results showed that MMS-induced DNA lesions that cause SCE are persistent. Some lesions were induced in the second division, as was inferred from the analysis of the response in single cells. The data also indicate that bromodeoxyuridine reduces DNA sensitivity to SCE induction by MMS.     

Disparate effects of 5-bromodeoxyuridine on sister-chromatid exchanges and chromosomal aberrations in Bloom syndrome fibroblasts

The existence of a high frequency of spontaneous sister-chromatid exchanges (SCEs) in Bloom syndrome (BS) has thus far been supported by data on a small number of BS cell lines. To examine the cause of baseline SCEs more broadly, the frequencies of SCEs, as well as chromosomal aberrations (CAs) in 4 additional BS fibroblast strains were compared, under different assay and cell culture conditions, with those of normal cells in the range of approximately 0.9-90% 5-bromodeoxyuridine (BrdUrd) substitution into template DNA. SCEs at low levels of BrdUrd substitution were detected by an extremely sensitive immunofluorescent technique. From approximately 0.9% to 4.5% BrdUrd substitution, the SCE frequency in BS cells remained constant, at a level (40/cell) 8 times higher than that of normal cells. As BrdUrd substitution increased further, the SCE frequency in BS cells increased almost linearly, reaching 70-100 per cell at approximately 90% substitution, while the SCE increment in control fibroblasts was less than 5 per cell. Analysis of SCEs in 3 successive replication cycles similarly revealed that the SCE increment in BS cells depended on BrdUrd only at a high BrdUrd substitution level. In contrast to data on SCEs, CA induction by incorporated BrdUrd in BS cells was only slightly higher than that in normal cells. Thus, BS cells are extremely sensitive to BrdUrd for SCE induction, but much less so for CA induction.     

Genotoxic effects induced by fotemustine and vinorelbine in human lymphocytes

The aim of this study was to investigate the in vitro genotoxic effects of the anticancer drugs fotemustine and vinorelbine on human lymphocytes and to determine individual and sex-related responses to these drugs. Fotemustine is a DNA-alkylating drug while vinorelbine is a semi-synthetic Vinca alkaloid. The study was carried out with twenty independent healthy donors for each drug. We have tested the ability of these drugs to induce chromosome aberrations (CAs) and sister chromatid exchanges (SCEs) as well as effect on the mitotic index (MI) in cultured human lymphocytes. Fotemustine was shown to induce CAs and SCEs at all concentrations tested (2, 4 and 8 microg/ml) in a dose-dependent manner. Additionally it also decreased the mitotic index in a similar dose-dependent manner. Vinorelbine had no effect on structural CAs, but it significantly increased the numerical CAs at all doses tested (0.5, 1 and 2 microg/ml). Vinorelbine also induced SCE events and increased the MI values. Two-way analyses of variance were used to compare the individual and gender-related susceptibilities to fotemustine and vinorelbine with respect to the CA, SCE and MI values. The results indicated that individuals in fotemustine treatment groups showed different genotoxic responses with respect to CA and SCE induction and additional findings indicated a gender-specific response in this group. Individuals in the vinorelbine test group also exhibited statistically significant numerical CA, SCE and MI responses to vinorelbine. A statistically significant gender-related SCE response to this drug was also evident. This study indicates that these drugs have potentially harmful effects on human health.     

Developmental and cytogenetic effects of caffeine on mouse blastocysts, alone or in combination with benzo(a)pyrene

Mouse blastocysts were treated with caffeine and/or benzo(a)pyrene (BP), and the effects on development and on induction of sister chromatid exchanges (SCEs) were examined. Caffeine interfered with blastocyst development in a dose-related manner. At 4 mM, the highest concentration tested, caffeine interfered with development of blastocysts to all four endpoints: hatching, trophoblast outgrowth, inner cell mass (ICM) growth, and two-layer (primary endoderm and ectoderm) differentiation of ICMs. At 2 mM, caffeine reduced the incidence of both ICM growth and differentiation but did not affect hatching or formation of trophoblast outgrowths. At 1 mM, caffeine interfered only with ICM differentiation. Cell proliferation was least sensitive to caffeine and was reduced at concentrations of greater than or equal to 2 mM. Induction of SCEs was most sensitive to caffeine exposure; an increase in SCE frequency was observed at 0.1 and 0.5 mM. When caffeine was added to cultures with BP (1 microM, a concentration that was not embryotoxic and did not induce SCEs), both embryotoxic effects and SCE frequency were increased. The enhancing effect on SCE induction was particularly marked; as little as 0.1 mM caffeine was sufficient to cause doubling of induced SCE frequencies when added to cultures with BP.     

Different contributions of the indirect effects of γ-rays on the cytotoxicity in M10 and XRCC4 transfected M10 cells

The protective effects of dimethyl sulfoxide (DMSO) against cell killing by ¹³⁷Cs γ-rays were investigated in XRCC4-deficient cell line M10, XRCC4-complemented M10 and the parental mouse leukemia cell line L5178Y. Cell survival was determined by the colony-forming ability. M10 cells were more sensitive to γ-ray-induced cell death than L5178Y and complemented M10 cells. Cell survival was increased in both M10 and L5178Y in the presence of DMSO. However, estimation of the DMSO-protectable fraction revealed a smaller protectable fraction for M10 cells than for L5178Y cells, indicating that indirect effects contributed in a smaller extent to the cytotoxicity in M10 than that in L5178Y. This effect is due to XRCC4 deficiency, since transfection of XRCC4 cDNA into M10 cells restored the radioprotective effects of DMSO to the level seen in L5178Y. In M10 cells, the killing effects of high LET radiation (Auger electrons from ¹²⁵I-antipyrine, carbon ions with an LET of 166 keV μm⁻¹) were similar to those of low LET radiation (¹³⁷Cs γ-rays, characteristic X-rays from ¹²⁵I-bovine serum albumin). We discuss that lethal lesions produced by indirect actions in L5178Y and XRCC4-complemented M10 cells may differ, at least in part, from DNA double-strand breaks repairable by non-homologous end joining.     

Resveratrol, a naturally occurring polyphenol, induces sister chromatid exchanges in a Chinese hamster lung (CHL) cell line.

We tested the genotoxicity of 3,5,4'-trihydroxystilbene (resveratrol), a polyphenolic phytoalexin found in grapes, in a bacterial reverse mutation assay, in vitro chromosome aberration (CA) test, in vitro micronucleus (MN) test, and sister chromatid exchange (SCE) test. Resveratrol was negative in the strains we used in the bacterial reverse mutation assay (S. typhimurium TA98 and TA100 and E. coli WP2uvrA) in the absence and presence of a microsomal metabolizing system. It induced structural CAs at 2.5-20 microg/ml and showed weak aneuploidy induction in a Chinese hamster lung (CHL) cell line. It induced MN cells and polynuclear and karyorrhectic cells after 48h treatments in the in vitro MN test. In the SCE test, resveratrol caused a clear cell-cycle delay; at 10 microg/ml, the cell cycle took twice as long as it did in the control. Resveratrol induced SCEs dose-dependently at up to 10 microg/ml, at which it increased SCE six-fold, and the number was almost as large as mitomycin C, a strong SCE inducer. No second mitoses were observed at 20 microg/ml even after 54h. Cell cycle analysis by FACScan indicated that resveratrol caused S phase arrest, and 48h treatment induced apoptosis. Our results suggest that resveratrol may preferentially induce SCE but not CA, that is, it may cause S phase arrest only when SCEs are induced.     

Lethal and mutagenic effects of radiation and chemicals on cultured fish cells derived from the erythrophoroma of goldfish (Carassius auratus)

GEM 199 cells derived from an erythrophoroma of goldfish (Carassius auratus), which had a high plating efficiency, were used to investigate the lethal and mutational effects of radiations (UV and gamma-rays) and chemicals (4NQO and MNNG). The cells were more resistant to gamma-rays than mammalian cells and CAF-MM1 cells derived from the normal fin tissue of goldfish. They were also more resistant to UV-irradiation than CAF-MM1 cells. Photoreactivation after UV-irradiation was present in GEM 199 cells for both survival and mutation. The initial shoulder of the survival curve of UV-irradiated cells was reduced greatly by caffeine, suggesting a high activity of the post-replication repair. The spontaneous mutation frequency to ouabain resistance was 1-5 X 10(-6) clones per viable cell. MNNG was effective in inducing ouabain-resistant mutation, while 4NQO and gamma-rays did not induce mutation.     

Response of two strains of L5178Y cells to cis-dichlorobis(cyclopentylamine)platinum(II). II. Differential effects of caffeine.

Two strains of L5178Y murine lymphoma, inversely cross-sensitive to X-rays and UV light, were shown previously to respond to treatment with an antitumour platinum complex, cis-dichlorobis(cyclopentylamine)-platinum(II) (cis-PAD), in a similar manner as to UV. Enhancement of chromosomal damage and potentiation of lethal effect of cis-PAD by 0.75 mM caffeine were found in cis-PAD and UV light-resistant L5178Y-S strain but not in cis-PAD and UV light-sensitive L5178Y-R strain. These results suggest that the extreme sensitivity of L5178Y-R strain to cis-PAD and UV light is caused to some extent by deficiency in a caffeine-sensitive post-replication repair system.     

Effects of temperature on chemically induced sister-chromatid exchange in human lymphocytes

Lymphocytes from healthy adults were studied for sister-chromatid exchanges (SCEs) when pulse-treated in G0 with mitomycin C (MMC), ethyl methanesulfonate (EMS), or 4-nitroquinoline N-oxide (4NQO) at various temperatures ranging from 0 degrees C to 41 degrees C and then cultured in medium containing 5-bromodeoxyuridine at 37 degrees C. The results showed that the frequencies of SCEs induced by MMC or EMS varied according to the treatment temperature. In MMC- or EMS-exposed cultures, the SCE frequency increased continuously with increasing treatment temperature; treatment at 37 degrees C resulted in a 3-4 times greater induction of SCEs than did that at room temperature (25 degrees C). On the other hand, SCE frequencies in cells exposed to 4NQO remained within normal deviation, showing no temperature-dependent changes. Baseline SCE frequencies remained almost constant within the temperature range tested. These data indicate that treatment temperature is a very critical factor in determining the sensitivity of cells to the chemical induction of SCEs.     

Effects of caffeine on sister-chromatid exchanges (SCE) in vivo.

Chinese hamsters were twice treated with caffeine via stomach tube. The single doses were either 20, 100, 200 or 400 mg per kg body weight. A dose-dependent increase was observed in the frequencies of SCE induced in vivo in bone-marrow cells. Two intraperitoneal injections of the chemical mutagens, cyclophosphamide or benzo[a]pyrene, led to a pronounced increase of the frequency of SCE. Simultaneous applications of the chemical mutagens and caffeine decreased the rate of SCE. The effect of caffeine per se to induce SCE, and the mechanisms by which caffeine reduces the level of SCE induced by chemical mutagens are discussed.