VOL.: 82 (2002) (p. 171)
5. Summary of Data Reported
5.1 Exposure data
Aflatoxins are a family of fungal toxins produced mainly by two Aspergillus species which are especially abundant in areas of the world with hot, humid climates. Aspergillus flavus, which is ubiquitous, produces B aflatoxins. A. parasiticus, which produces both B and G aflatoxins, has more limited distribution. Major crops in which aflatoxins are produced are peanuts, maize and cottonseed, crops with which A. flavus has a close association. Human exposure to aflatoxins at levels of nanograms to micrograms per day occurs mainly through consumption of maize and peanuts, which are dietary staples in some tropical countries. Maize is also frequently contaminated with fumonisins. Aflatoxin M1 is a metabolite of aflatoxin B1 in humans and animals. Human exposure to aflatoxin M1 at levels of nanograms per day occurs mainly through consumption of aflatoxin-contaminated milk, including mothers’ milk. Measurement of biomarkers is being used increasingly to confirm and quantify exposure to aflatoxins.
5.2 Human carcinogenicity data
Studies evaluated in Volume 56 of the IARC Monographs led to the classification of naturally occurring aflatoxins as carcinogenic to humans (Group 1). Recent studies have incorporated improvements in study design, study size and accuracy of measurement of markers of exposure to aflatoxin and hepatitis viruses.
In a large cohort study in Shanghai, China, risk for hepatocellular carcinoma was elevated among people with aflatoxin metabolites in urine, after adjustment for cigarette smoking and hepatitis B surface antigen positivity. No association was observed between dietary aflatoxin levels, as ascertained by a diet frequency questionnaire, and risk for hepatocellular carcinoma.
There were four reports from cohort studies in Taiwan, China, although three of them partly overlapped. Selected subjects in the three overlapping studies were enrolled, were interviewed, had biological specimens taken, and were followed up intensively for liver cancer. In nested case–control studies, including some prevalent cases, subjects with exposure to aflatoxin, as assessed by biomarker measurements, had elevated risks for liver cancer, after adjustment for hepatitis B surface antigen positivity. The effect due to aflatoxin exposure was especially high among those who were positive for hepatitis B surface antigen, but there were few liver cancer cases negative for hepatitis B surface antigen. The other Taiwan study was carried out in a large cohort of chronic carriers of hepatitis B virus. These subjects were interviewed at baseline, had biological specimens taken, and were followed up intensively for liver cancer. Several aflatoxin metabolites and albumin adducts were measured in a nested case–control series. Subjects with quantified levels of most of the biomarkers of exposure to aflatoxin showed elevated risk for liver cancer.
In two studies in Qidong, China, cohorts of hepatitis B carriers were tested for biomarkers of aflatoxin and followed up for liver cancer. In both studies, subjects with aflatoxin biomarkers had excess risks for liver cancer.
In a Sudanese case–control study of liver cancer, a relationship was found between reported ingestion of peanut butter and liver cancer in a region with high aflatoxin contamination of peanuts, but no such relationship in a region with low contamination of peanuts.
In a hybrid ecological cross-sectional study in Taiwan, China, a number of subjects were selected from eight regions; for each subject several biomarkers of aflatoxin and hepatitis B viral infection were assessed in relation to the liver cancer rates in the region of residence. There were correlations between aflatoxin metabolites and liver cancer rates after adjustment for hepatitis B status.
The overall body of evidence supports a role of aflatoxins in liver cancer etiology, notably among subjects who are carriers of hepatitis B surface antigen. Nevertheless, the interpretation of human studies is hampered by the difficulties in properly assessing an individual’s lifetime exposure to aflatoxins and the difficulties in disentangling the effects of aflatoxins from those of hepatitis infections. Novel biomarkers, some still under development and validation, should bring greater clarity to the issue.
5.3 Animal carcinogenicity data
Extensive experimental studies on the carcinogenicity of aflatoxins led to a previous IARC Monographs evaluation of the evidence as follows: sufficient evidence for carcinogenicity of naturally occurring mixtures of aflatoxins and of aflatoxins B1, G1 and M1, limited evidence for aflatoxin B2 and inadequate evidence for aflatoxin G2. The principal tumours induced were liver tumours.
Carcinogenicity studies in experimental animals since 1993 were limited to a few experiments in rats, trout, mice, tree shrews and woodchucks. Under certain conditions, including increased pressure, decontamination of feed containing aflatoxins by ammoniation almost completely eliminated the induction of hepatic tumours in rats. Studies in trout showed that ammoniation of aflatoxin-contaminated maize significantly reduced the incidence of liver tumours. In trout fed non-fat dried milk from cows fed ammoniated or non-ammoniated aflatoxin-contaminated whole cottonseed, ammoniation almost eliminated the liver tumour response. Less hepatic tumours were induced in trout after exposure to aflatoxin M1 than with aflatoxin B1. One aflatoxin metabolite, aflatoxicol, elicited a slightly higher hepatic tumour response in fry and fish embryos than aflatoxin B1.
A study in transgenic mice overexpressing transforming growth factor b showed no increased susceptibility to induction of hepatocellular adenomas and carcinomas after intraperitoneal administration of aflatoxin B1. In another study, induction of hepatocellular tumours by aflatoxin B1 was significantly enhanced in transgenic mice heterozygous for the TP53 gene and expressing hepatitis B surface antigen. The tumour response for aflatoxin B1 was reduced in the absence of either one of these risk factors. The presence of the TP53 246ser mutant not only enhanced the synergistic effect of hepatitis B virus and aflatoxin B1 but also increased tumorigenesis due to aflatoxin B1 in the absence of hepatitis B virus.
In tree shrews, the incidence of hepatocellular carcinomas was significantly increased and the time of occurrence was shortened in animals treated with aflatoxin B1 and infected with (human) hepatitis B virus compared with aflatoxin B1-treated animals. Woodchucks infected with woodchuck hepatitis virus were more sensitive to the carcinogenic effects of aflatoxin B1 than uninfected woodchucks. The combined woodchuck hepatitis virus/aflatoxin B1 treatment not only reduced the time of appearance but also resulted in a higher incidence of liver tumours.
In conclusion, recent studies continue to confirm the carcinogenicity of aflatoxins in experimental animals.
5.4 Other relevant data
Metabolism of aflatoxin B1 in humans has been well characterized, with activation to aflatoxin B1 8,9-exo-epoxide resulting in DNA adduct formation. CYP1A2, 3A4, 3A5, 3A7 and GSTM1 enzymes among others mediate metabolism in humans. The expression of these enzymes can be modulated with chemopreventive agents, resulting in inhibition of DNA-adduct formation and hepatocarcinogenesis in rats. Oltipraz is a chemopreventive agent that increases glutathione conjugation and inhibits some cytochrome P450 enzymes. Results from clinical trials in China using oltipraz are consistent with experimental data in showing that, following dietary exposure to aflatoxins, modulation of the metabolism of aflatoxins can lead to reduced levels of DNA adducts.
Aflatoxin B1 is immunosuppressive in animals, with particularly strong effects on cell-mediated immunity. Exposure to aflatoxin results in increased susceptibility to bacterial and parasitic infections. Human monocytes treated with aflatoxin B1 had impaired phagocytic and microbicidal activity and decreases in specific cytokine secretion. Studies have linked human exposure to aflatoxins to increased prevalence of infection.
Aflatoxins cross the human placenta. Aflatoxin exposure has been associated with growth impairment in young children. Malformations and reduced fetal weight have been seen after mice were treated intraperitoneally with high doses of aflatoxin. In rats, decreased pup weight and behavioural changes have been found at low doses. Effects suggesting impairment of fertility have been reported in female and male rats and in male rabbits.
Aflatoxin B1 is genotoxic in prokaryotic and eukaryotic systems in vitro, including human cells, and in vivo in humans and in a variety of animal species. It forms DNA and albumin adducts and induces gene mutations and chromosomal alterations including micronuclei, sister chromatid exchange and mitotic recombination.
In geographical correlation studies, exposure to aflatoxin is associated with a specific G to T transversion in codon 249 of the TP53 gene in human hepatocellular carcinoma. This alteration is consistent with the formation of the major aflatoxin B1–N7-guanine adduct and the observation that G to T mutations are predominant in cell and animal model systems. The high prevalence of the codon 249 mutation in human hepatocellular carcinoma, however, is not fully explained in experimental studies either by the sequence-specific binding and mutation induced by aflatoxin B1 or by altered function of the p53 protein in studies of hepatocyte growth and transformation.
Current knowledge of the molecular mechanisms contributes to the understanding of the nature of the interaction between hepatitis B virus and aflatoxins in determining risk for hepatocellular carcinoma. Infection with hepatitis B virus may increase aflatoxin metabolism; in hepatitis B virus-transgenic mice, liver injury is associated with increased expression of cytochrome P450 (CYP) enzymes. Glutathione S-transferase activity is also reduced in human liver in the presence of hepatitis B virus infection. Other molecular mechanisms are, however, also likely to be relevant to aflatoxin-induced carcinogenesis.
On the basis of the data described above, the existing Group 1 evaluation of naturally occurring aflatoxins was reaffirmed.
5.5 Further research needs
Some research areas are identified here for the purpose of assisting in any future update by an IARC Monographs Working Group. It is not implied that these areas listed override the importance of other research areas or needs, nor should this be construed as endorsement for any specific studies planned or in progress.
Previous evaluations: Vol. 56 (1993); Suppl. 7 (1987) (p. 83)
See Also: Toxicological Abbreviations Aflatoxins (WHO Food Additives Series 40) Aflatoxins (IARC Summary & Evaluation, Volume 56, 1993) Aflatoxins (IARC Summary & Evaluation, Volume 56, 1993)