Sterigmatocystin-induced DNA damage triggers G2 arrest via an ATM/p53-related pathway in human gastric epithelium GES-1 cells in vitro

Sterigmatocystin (ST), which is commonly detected in food and feed commodities, is a mutagenic and carcinogenic mycotoxin that has been recognized as a possible human carcinogen. Our previous study showed that ST can induce G2 phase arrest in GES-1 cells in vitro and that the MAPK and PI3K signaling pathways are involved in the ST-induced G2 arrest. It is now widely accepted that DNA damage plays a critical role in the regulation of cell cycle arrest and apoptosis. In response to DNA damage, a complex signaling network is activated in eukaryotic cells to trigger cell cycle arrest and facilitate DNA repair. To further explore the molecular mechanism through which ST induces G2 arrest, the current study was designed to precisely dissect the role of DNA damage and the DNA damage sensor ataxia telangiectasia-mutated (ATM)/p53-dependent pathway in the ST-induced G2 arrest in GES-1 cells. Using the comet assay, we determined that ST induces DNA damage, as evidenced by the formation of DNA comet tails, in GES-1 cells. We also found that ST induces the activation of ATM and its downstream molecules, Chk2 and p53, in GES-1 cells. The ATM pharmacological inhibitor caffeine was found to effectively inhibit the activation of the ATM-dependent pathways and to rescue the ST-induced G2 arrest in GES-1 cells, which indicating its ATM-dependent characteristic. Moreover, the silencing of the p53 expression with siRNA effectively attenuated the ST-induced G2 arrest in GES-1 cells. We also found that ST induces apoptosis in GES-1 cells. Thus, our results show that the ST-induced DNA damage activates the ATM/53-dependent signaling pathway, which contributes to the induction of G2 arrest in GES-1 cells.

Larvicidal activity of metabolites from the endophytic Podospora sp. against the malaria vector Anopheles gambiae

In a screening for natural products with mosquito larvicidal activities, the endophytic fungus Podospora sp. isolated from the plant Laggera alata (Asteraceae) was conspicuous. Two xanthones, sterigmatocystin (1) and secosterigmatocystin (2), and an anthraquinone derivative (3) 13-hydroxyversicolorin B were isolated after fermentation on M(2) medium. These compounds were characterised using spectroscopic and X-ray analysis and examined against third instar larvae of Anopheles gambiae. The results demonstrated that compound 1 was the most potent one with LC(50) and LC(90) values of 13.3 and 73.5 ppm, respectively. Over 95% mortality was observed at a concentration 100 ppm after 24 h. These results compared farvorably with the commercial larvicide pylarvex® that showed 100% mortality at the same concentration. Compound 3 was less potent and had an LC(50) of 294.5 ppm and over 95% mortality was achieved at a concentration of 1,000 ppm. Secosterigmatocystin (2) revealed relatively weak activity and therefore LC values were not determined.

Selective inhibition of acyl-CoA:cholesterol acyltransferase 2 isozyme by flavasperone and sterigmatocystin from Aspergillus species

Five known fungal metabolites, aurasperone A, aurasperone D, averufanin, flavasperone and sterigmatocystin, were isolated from the culture broths of Aspergillus species as inhibitors of acyl-CoA:cholesterol acyltransferase (ACAT) in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. These compounds share a similar polycyclic skeleton. Among them, flavasperone and sterigmatocystin, having an angular skeleton, showed selective inhibition toward ACAT2 isozyme, while the others having a linear one had no selectivity in inhibition.

Kipukasins, nucleoside derivatives from Aspergillus versicolor

Seven new aroyl uridine derivatives (kipukasins A-G; 1-7) were isolated from solid-substrate fermentation cultures of two different Hawaiian isolates of Aspergillus versicolor. The structures of compounds 1-7 were determined by analysis of NMR and MS data. The nucleoside portion of lead compound 1 was assigned as uracil-1-beta-D-ribofuranoside by spectral comparison with an authentic standard. The bioactivity of the original A. versicolor extracts was accounted for mainly by the presence of the known metabolite sterigmatocystin, but kipukasins A and B showed modest activity in assays against Gram-positive bacteria.

Inhibition of vacuolation toxin activity of Helicobacter pylori by iodine, nitrite and potentiation by sodium chloride, sterigmatocystin and fluoride

The toxin VacA produced by Helicobacter pylori is an important determinant of virulence. VacA causes vacuolation of cultured cells such as HeLa cells. Iodine, nitrite, sodium chloride, thiocyanate and fungus toxin sterigmatocystin are universally present in nature and could possibly be related to carcinogenesis of the stomach. The present study was designed to examine the effects of the above-mentioned compound on VacA-induced vacuolation of HeLa cells, which was quantitated using the neutral red uptake assay. VacA-induced vacuolation was inhibited by BafA1 and NPPB. Formation of large vacuoles was inhibited in the presence of iodine, nitrite, but enhanced by sodium chloride, thiocyanate, fluoride and sterigmatocystin. Our results indicate that VacA toxin may interact with other gastric cancer risk factors present naturally in the environment, and suggest that those compounds may modulate the development of gastric cancer induced by H. pylori.

Effects of sterigmatocystin, deoxynivalenol and aflatoxin G1 on apoptosis of human peripheral blood lymphocytes in vitro

OBJECTIVE

To explore the effects of Sterigmatocystin (ST), Deoxynivalenol (DON) and Aflatoxin G1 (AFG1) on apoptosis of human peripheral blood lymphocytes (HPBLs) in vitro and thus to further elucidate the putative roles of these three mycotoxins on human immunosystem.

METHODS

The effects of ST, DON and AFG1 on apoptosis of HPBLs were studied with cell culture, flow cytometric (FCM) DNA analysis and DNA agarose gel electrophoresis.

RESULTS

DNA agarose gel electrophoresis results showed the characteristic "ladder" pattern of apoptosis in HPBLs treated with ST, DON and AFG1. Flow cytometric DNA analysis revealed that typical subdiploid peaks of apoptosis in DNA histogram could be seen in all groups treated with the three mycotoxins. Significant time-effect and dose-effect relationships were found between the apoptosis rates and treatment time as well as concentrations of the three mycotoxins.

CONCLUSION

ST, DON and AFG1 can induce apoptosis of HPBLs in vitro and may have some negative effects on human immunosystem.

Absence of p53-mediated G1 arrest with induction of MDM2 in sterigmatocystin-treated cells

P53 plays a critical role in G1 checkpoint after DNA damage. MDM2 gene is a p53 target gene and its protein forms a feedback loop with p53 and inhibits p53-mediated G1 arrest. Sterigmatocystin (ST) is a mycotoxin and carcinogen. In this study we show that exposure of cells to ST for 12 or 24 h resulted in failure of G1 arrest at both time points. Accordingly, p53 protein was not increased and p21WAF1 expression was inhibited at 12 h, and both proteins were weakly induced at 24 h after treatment with ST. Meanwhile, MDM2 protein was induced in a p53-dependent fashion by ST at both 12 and 24 h. The induction of MDM2 was coincident with the cellular responses of p53 and p21WAF1, and might contribute to the failure of G1 arrest in ST-treated cells. In addition, ST-treated cells exhibited G2M arrest, regardless of p53 status. Our results indicate that the carcinogenic effects of ST seem to be mediated by failure of p53-mediated G1 checkpoint.

Rat pulmonary microsomal cytochrome P-450 enzymes involved in the activation of procarcinogens

Rat lung microsomal cytochrome P-450 (P-450) enzymes have been characterized with regard to their catalytic specificities towards activation of several procarcinogens to genotoxic metabolites in Salmonella typhimurium TA1535/pSK1002. We first examined the roles of rat liver microsomal P-450 enzymes in the activation of benzo[a]pyrene and its 7,8-diol enantiomers to genotoxic products, and found that P-450 1A1 is a major catalyst for the activation of these potential procarcinogens in rat livers. Using lung microsomes isolated from rats treated with various P-450 inducers we obtained evidence that at least three P-450 enzymes are involved in the activation of several procarcinogens. Immunoinhibition studies support the view that benzo[a]pyrene and its 7,8-diol derivatives, other dihydrodiol derivatives of polycyclic aromatic hydrocarbons, and 3-amino-1-methyl-5H-pyrido[4,3-b]indole are activated to genotoxins mainly by rat P-450 1A1, which is inducible in rat lungs by 5,6-benzoflavone and the polychlorinated biphenyl mixture Aroclor 1254. Activation of 2-amino-3,5-dimethylimidazo[4,5-f]quinoline and 2-amino-3-methylimidazo[4,5-f]quinoline may be catalyzed by another P-450 enzyme because the activities were not induced by treatment with 5,6-benzoflavone or Aroclor 1254. The observation that both activities were inhibited by antibodies raised against P-450 1A2 and by 7,8-benzoflavone suggests a role for an enzyme of P-450 1A family, probably P-450 1A2, in rat lung microsomes. The activation of aflatoxin B1 and sterigmatocystin appears to be catalyzed by other P-450 enzyme(s) rather than the P-450 1A family as judged by the different responses of activities to the P-450 inducers and the specific antibodies in rat lung microsomes. Interestingly, lung microsomal activation of several procarcinogens was found to be suppressed in rats treated with isosafrole and pregnenolone 16 alpha-carbonitrile. Thus, the results support the roles of different P-450 enzymes in the activation of procarcinogens in rat lung microsomes.

The influence of contamination with separate mycotoxins (aflatoxins, ochratoxin A, citrinin, patulin, penicillic acid or sterigmatocystin) on the in vitro dry matter and organic matter digestibilities of some roughages (berseem hay and wheat straw)

In vitro study on berseem hay and wheat straw was undertaken to investigate the the effect of mycotoxin contamination on dry matter and organic matter digestibilities. The data revealed a negative effect of most studied mycotoxins on the materials digestibility. Among the investigated mycotoxins, penicillic acid with its two concentrations (5 and 10 nmol) was the most negative, affecting digestibilities of both feed materials. Wheat straw digestibility was more influenced than berseem hay by the ochratoxin A, citrinin and sterigmatocystin (besides the penicillic acid) particularly with their high level (10 nmol). Yet, some mycotoxins act as antibiotics which may affect only the harmful flora but encourage the rumen microflora resulting in slight improvement of digestibility. The rumen conditions were able to metabolize or deform the used levels of all mycotoxins studied. Thus, there were no detectable residues of these mycotoxins in the digestion media after the in vitro fermentation.

Modulation of hormonal induction of tyrosine aminotransferase and glucocorticoid receptors by aflatoxin B1 and sterigmatocystin in Reuber hepatoma cells

Employing Reuber rat hepatoma cells, H4-II-E, the effects of aflatoxin B1 (AFB1) and sterigmatocystin (STC), which exhibit a similar cytotoxicity but a marked difference in hepatocarcinogenicity, on the hormonal induction of tyrosine aminotransferase (TAT), on glucocorticoid receptors, and on their nuclear acceptor sites were investigated. AFB1 strongly inhibited hydrocortisone-inducible TAT activity. The IC50 value was 0.2 micrograms/ml. AFB1 also showed weak inhibitory effects on insulin- and dibutyryl cyclic AMP-inducible TAT activities. In contrast, the IC50 of STC on hydrocortisone-inducible TAT activity was 3.5 micrograms/ml, about 10 times higher than that of AFB1. Dibutyryl cyclic AMP- and insulin-inductions were not depressed by STC. AFB1 inhibited the formation of cytosolic glucocorticoid receptor-hormone complexes (GRCs) but STC did not. Moreover, AFB1, activated in vitro by the microsomal cytochrome P-450 system, interfered more markedly in the formation of cytosolic GRCs than STC did. Sucrose density gradient analysis of GRCs and Scatchard analysis revealed that AFB1 and STC mainly impaired glucocorticoid receptors and GRC-acceptor sites, respectively. The present data suggest a marked difference between AFB1 and STC with regard to the inhibition of hormonal induction of liver specific enzymes.

Effects of repeated intra-abdominal injections of sterigmatocystin on relative organ weights, concentration of serum and liver constituents, and histopathology of certain organs of the chick

The objectives of the study were to establish the effects of five successive intra-abdominal injections of sterigmatocystin (STG), administered at 11, 13, 15, 17, and 19 days of age, on the growth pattern of chicks and their organs, and on the concentration of certain blood and liver constituents. The STG, when administered at dosages of .5 and .7 mg per injection, markedly reduced chick growth and affected organ weights. In general, there was an increase in the relative size of the crop, proventriculus, gizzard, large intestine, kidney and pancreas and a decrease in relative size of the bursa of Fabricius. Liver, heart, and spleen size was not affected. Peritonitis was observed in chicks given the high dosage STG. The STG elevated the activity of serum aspartate aminotransferase and the number of circulating granulocytes and depressed concentrations of total serum proteins, albumin, potassium, and the total number of circulating white blood cells and granulocytes. The STG treatment decreased the concentration of dry matter, DNA, RNA, and protein in the liver, affected glycogen concentration differentially, and had no effect on lipid concentration. Liver and kidneys also showed degenerative changes as detected histopathologically. The results of these studies suggest that STG affects several tissues including the digestive system, liver, kidney, pancreas, and the immunological system.

Human embryonic cell growth assay for teratogens with or without metabolic activation system using microplate

In vitro microassay for the screening of teratogens was investigated on cancer chemotherapeutic agents sterigmatocystins and benzimidazoles using human embryonic palatal mesenchymal (HEPM) cells. Five thousand cells were inoculated into each well of 96-well microtiter plates, and cultivated for 24 hr, after which the media were changed with new ones that contained various amounts of chemicals; after cultivation for an additional 72 hr, the media were discarded, and cells attached to the tissue plate were fixed and stained with Giemsa's solution; the cell number then was counted by colony counter with three readings for each well. For the metabolic activation, the liver S9 obtained from rats pretreated with phenobarbital and 5,6-benzoflavone and cofactors (S9 mix) were added directly to the HEPM cell cultures along with chemicals. After 6 hr, the cultures were exchanged with a fresh medium and incubated for a further 72 hr. The final IC50 (the concentration that inhibits growth 50%) concentration-finding run had 7 to 11 concentration points (mean, three to four wells). Concentrations of the cancer chemotherapeutic agents that inhibited growth by 50% ranged from 0.001 to 10 micrograms/ml. Sterigmatocystins indicated strong inhibition; among three derivatives, O-acetyl sterigmatocystin was the most potent inhibitor. Benzimidazoles also exhibited an inhibitory action on HEPM cell growth; nitro and chloro groups at the 5 position in 2-(2-pyridyl)benzimidazole were found to be potent substituents. As for the activation of cyclophosphamide in the HEPM cell culture, IC50 was decreased to 1.0 ug/ml by the incubation with S9 mix for 6 hr under our experimental conditions, and sterigmatocystin was found to be activated by S9 mix.(ABSTRACT TRUNCATED AT 250 WORDS)

Genotoxic effects of a variety of sterigmatocystin-related compounds in the hepatocyte/DNA-repair test and the Salmonella microsome assay

The genotoxicity and mutagenicity of 11 fungal metabolites structurally related to sterigmatocystin were examined in the hepatocyte primary culture/DNA-repair test and the Salmonella microsome assay. 10 out of the mycotoxins, i.e. dihydrosterigmatocystin, 5-methoxysterigmatocystin, 5-methoxydihydrosterigmatocystin, 5,6-dimethoxysterigmatocystin, 5,6-dimethoxydihydrosterigmatocystin, sterigmatin, O-methylsterigmatocystin and O-acetylsterigmatocystin showed a positive response for DNA repair, suggesting their carcinogenic potency. 5-Methoxysterigmatocystin, 5,6-dimethoxysterigmatocystin demethylsterigmatocystin and O-acetylsterigmatocystin were mutagenic in TA100 of the bacterial mutagenicity assay with liver S9.

Biosynthetic relationship among aflatoxins B1, B2, M1, and M2

Aflatoxins are a family of toxic, acetate-derived decaketides that arise biosynthetically through polyhydroxyanthraquinone intermediates. Most studies have assumed that aflatoxin B1 is the biosynthetic precursor of the other aflatoxins. We used a strain of Aspergillus flavus which accumulates aflatoxin B2 to investigate the later stages of aflatoxin biosynthesis. This strain produced aflatoxins B2 and M2 but no detectable aflatoxin B1 when grown over 12 days in a low-salt, defined growth medium containing asparagine. Addition of dichlorvos to this growth medium inhibited aflatoxin production with concomitant accumulation of versiconal hemiacetal acetate. When mycelial pellets were grown for 24, 48, and 72 h in growth medium and then transferred to a replacement medium, only aflatoxin B2 and M2 were recovered after 96 h of incubation. Addition of sterigmatocystin to the replacement medium led to the recovery of higher levels of aflatoxins B2 and M2 than were detected in control cultures, as well as to the formation of aflatoxins B1 and M1 and O-methylsterigmatocystin. These results support the hypothesis that aflatoxins B1 and B2 can arise independently via a branched pathway.

Inhibitory effect of sterigmatocystin and 5,6-dimethoxysterigmatocystin on ATP synthesis in mitochondria

The inhibitory effects of sterigmatocystin, O-methylsterigmatocystin, and 5,6-dimethoxysterigmatocystin on the ATP synthesis system in mitochondria were compared with that of aflatoxin B1, which disturbs the respiratory chain in mitochondria. Sterigmatocystin and 5,6-dimethoxysterigmatocystin were found to uncouple the oxidative phosphorylation process without causing depression of state 3 respiration. O-Methylsterigmatocystin did not exhibit uncoupling activity at the limited concentrations tested (due to its low solubility in an aqueous system). These compounds, as well as aflatoxin B1, elicited neither pseudo-energized nor energized swelling of mitochondria and did not inhibit Ca2+-induced swelling of mitochondria.

The effects of naturally occurring bisdihydrofuran ring-containing mycotoxins on cultured chick embryonal hepatocytes

The cytotoxicity of 16 naturally occurring mycotoxins which contain the bisdihydrofuran ring was evaluated by electron-microscopy and radioautography in a primary culture of chick embryonal hepatocytes. A series of nucleolar changes occurred, including the appearance of microsegregation (stage I), concentration of nucleolar structure (stage II), formation of macrosegregation, fragmentation of nucleoli, and the appearance of homogeneously granular nucleoli (stage III). These nucleolar changes correlate well with the inhibition of RNA synthesis calculated from the results of radioautography. The compounds containing a 2,3-double bond in the bisdihydrofuran ring were most toxic. However, it was clear that there was a remarkable difference between mycotoxins of angular type (group A) and those of linear type (group C). Therefore, not only the active site in the molecule but also the shape of the molecule may play an important role in the cytotoxicity of these mycotoxins.

[Effect of sterigmatocystin on the toxinogenesis of the Aspergillus flavus group]

The aflatoxinogenesis of Aspergillus parasiticus is significantly enhanced by the presence, in the medium, of sterigmatocystin at a high level (35--50 microgram/ml); low concentrations, in the order of 175 microgram/ml, have no effect on the production of aflatoxins. During the period where the aflatoxinogenesis of the culture is high, no variation of the sterigmatocystin level is noted, Experiments with 14C-sterigmatocystin indicate that the mold does not utilize the metabolite itself as a precursor of aflatoxins.

Mutagenicity of 5,6-dimethoxysterigmatocystin, a metabolite from Aspergillus multicolor, in the Salmonella/microsome system

The natural sterigmatocystin derivative, 5,6-dimethoxysterigmatocystin, was found to be a mutagen for Salmonella typhimurium strains TA98 and TA100 after metabolic activation in a mammalian microsome system.

Lack of mutagenicity to Salmonella typhimurium of some Fusarium mycotoxins

The mutagenicity of eight Fusarium toxins (mono-, di-, and triacetoxyscirpenol, T-2 toxin, deoxynivalenol, 3-acetyl-deoxynivalenol, zearalenone, and moniliformin) and of two positive controls (aflatoxin B1 and sterigmatocystin) to histidine-requiring strains TA 98, 100, 1535, and 1537 of Salmonella typhimurium was tested both with and without metabolic activation. Both aflatoxin B1 and sterigmatocystin, but none of the eight Fusarium toxins, were mutagenic to S. typhimurium. The lack of mutagenic activity of T-2 toxin and diacetoxyscirpenol supports the negative results that have been obtained with in vivo carcinogenicity tests. The negative mutagenicity of the four other 12,13-epoxytrichothecenes tested, and of zearalenone and moniliformin, could not be correlated with in vivo tests because published accounts of their chronic toxicity were not available.

Mutagenicity and inducibility of DNA single-strand breaks and chromosome aberrations by various mycotoxins

Mutagenicity of various mycotoxins and the efficiency of mutagenic mycotoxins in producing DNA single-strand breaks and chromosome aberrations were examined using a mammalian cell line. It was found that aflatoxin-B1, mycophenolic acid, patulin, penicillic acid, and sterigmatocystin induced 8-azaguanine-resistant mutations. Aflatoxin-B1, mycophenolic acid, and sterigmatocystin had little effect on DNA single-strand at high concentrations. In the treatment with patulin and penicillic acid, severe breaks were found at higher concentration than at the concentration where the mutation was induced. Incidence of chromosome aberrations by the treatment with these mycotoxins correlated fairly well with their mutagenic activity. Chaetoglobosin-B, fusarenon-X, (--)luteoskyrin, and ochratoxin-A did not induce 8-azaguanine-resistant mutation.

[Antibacterial effect of various mycotoxins and fungal metabolites against Bacillus thuringiensis (Berliner) strains sensitive or resistant to aflatoxin B1]

Antimicrobial activity of pure preparations of mycotoxins and fungal metabolites was studied against strains of Bacillus thuringiensis (Berliner). Two resistant strains, called stable-variant, were isolated after treatment with high concentrations of aflatoxin B1. These strains were then resistant also towards compounds with a double furan system (aflatoxins B1, B2, G1, G2, and sterigmatocystin).

DNA-attacking ability of carcinogenic mycotoxins in recombination-deficient mutant cells of Bacillus subtilis

Thirty mycotoxins and 5 chemically modified toxins were tested for DNA-attacking ability in the rec assay using the recombination-deficient mutant of Bacillus subtilis M45 (rec-) and the parent strain H17 (rec+). Six Penicillium toxins (citrinin, penicillic acid, patulin, (-)-luteoskyrin, (+)-rugulosin, and PR-toxin), 5 Aspergillus toxins (aflatoxins B1 and G1, sterigmatocystin, O-acetylsterigmatocystin, and O-acetyldihydrosterigmatocystin), and 2 Fusarium toxins (zearalenone and zearalenol-b) were positive. Among these 13 compounds, the following 8 mycotoxins have been reported to be carcinogenic in animals: citrinin, penicillic acid, patulin, (-)-luteoskyrin, (+)-rugulosin, aflatoxins B1, and G1, sterigmatocystin. Correlation between the rec effect and in vivo carcinogenicity of mycotoxins is discussed.

The effects of O-acetylsterigmatocystin and related compounds on rat liver and cultured chicken embryonal liver cells

Fine structural nucleolar changes induced in rat liver and primary tissue culture cells from 10-day-old chicken embryonal liver by O-acetylsterigmatocystin (AcO-stg), related compounds and aflatoxin B1 were compared. (1) Male Wistar rats were given a single i.p. injection of sterigmatocystin (stg), AcO-stg, and aflatoxin B1. 3 days after the injection of 15 mg/kg of stg, sporadic single cell necrosis was observed in rat liver, whereas rats treated with 8 mg/kg AcO-stg or more, and 3 mg/kg of aflatoxin B1 showed massive liver necrosis. Acetylation resulted in a marked increase in solubility in polar organic solvents. This increased solubility could play an important role in determining toxicity. (II) Treatment with the compounds with an unsaturateddelta1,2-furobenzofuranring system, such as AcO-stg, demethyl-diacetyl-stg (deMe-diAc-stg), and aflatoxin B1, resulted in nucleolar segregation and fragmentation of primary culture cells. Both parenchymal and mesenchymal cells in culture were susceptible to AcO-stg and deMe-diAc-stg, while the mesenchymal cells were more resistant to aflatoxin B1 than the hepatocytes. The inhibition of RNA synthesis in both cell types as determined in radioautography was in accordance with the electron-microscopic observations. Acetyldihydrosterigmatocystin (AcO-dihyd-stg), a saturated delta1,2-furobenzofuranring compound, was less toxic to primary tissue culture cells.

The response of Xeroderma pigmentosum cells and controls to the activated mycotoxins, aflatoxins and sterigmatocystin

The activation of the mycotoxins aflatoxin B1, G1, B2, G2, aflatoxicol and sterigmatocystin by 9S fraction, microsomal preparation (105,000 times g) and supernatant (105,000 times g) of livers of several species was examined. DNA repair synthesis, chromosome aberrations and clone forming capacity were used as endpoints. Cultured fibroblasts of normal persons and DNA repair deficient Xeroderma pigmentosum patients were employed as test subjects. The activation mixtures significantly increase the chromosome breaking function, lethality and DNA damaging effect (measured as DNA repair synthesis) of aflatoxin B1, G1, aflatoxicol ans sterigmatocystin. The DNA repair-deficient XP cells respond to the activated mycotoxins with a low level of unscheduled 3HTdR incorporation as compared to that of control cells, but show a highly elevated sensitivity to the chromosome-damaging and lethal effect of aflatoxin B1 and sterigmatocystin.