Chronic and Acute Toxicities of Aflatoxins: Mechanisms of Action

Abstract

There are presently more than 18 known aflatoxins most of which have been insufficiently studied for their incidence, health-risk, and mechanisms of toxicity to allow effective intervention and control means that would significantly and sustainably reduce their incidence and adverse effects on health and economy. Among these, aflatoxin B1 (AFB1) has been by far the most studied; yet, many aspects of the range and mechanisms of the diseases it causes remain to be elucidated. Its mutagenicity, tumorigenicity, and carcinogenicity-which are the best known-still suffer from limitations regarding the relative contribution of the oxidative stress and the reactive epoxide derivative (Aflatoxin-exo 8,9-epoxide) in the induction of the diseases, as well as its metabolic and synthesis pathways. Additionally, despite the well-established additive effects for carcinogenicity between AFB1 and other risk factors, e.g., hepatitis viruses B and C, and the hepatotoxic algal microcystins, the mechanisms of this synergy remain unclear. This study reviews the most recent advances in the field of the mechanisms of toxicity of aflatoxins and the adverse health effects that they cause in humans and animals.

Keywords: acute toxicity; aflatoxins; carcinogenicity; genotoxicity; immunogenicity; tumorigenicity.

Figure 1

Main aflatoxin B1 toxicity mechanisms mediated by the oxidative stress and AFB1-exo-8,9 epoxide (AFBO; see text for explanations). NB: ROS also affect proteins, RNA molecules, and immunity as does AFBO (not shown in the figure. For details, see [20]). Abbreviations: AFBO: Aflatoxin B1-exo-8,9-epoxide; NA: Nucleic Acids; ROS: Reactive Oxygen Species; LPO: Lipid Peroxidation; ODD: Oxidative DNA Damage; Acr: Acrolein; Cro: Crotonaldehyde; Acet: Acetaldehyde; HNE: 4-Hydroxy-2-Nonenal; uFA: Unsaturated Fatty Acids; IL1β: Interleukin 1β, IL6: Interleukin 6; TNFα: Tumour Necrotizing Factor α; P-dG: Cyclic Propano-Deoxyguanosine; Igs: Immunoglobulins. See text for the other abbreviations.

Figure 2

Activation of aflatoxin B1 and its interaction with the DNA leading to the formation of aflatoxin DNA adducts which cause three main DNA lesions, AFB1-N⁷-guanine (1), apurinic DNA (2), and AFB1-FAPy (3,4), involved in mutagenicity and carcinogenicity. Upon furan ring opening to stabilize the AFB1-N⁷-gua DNA adduct, the “cis” (minor) rotamer (3) of AFB1-FAPy is formed first and is then transformed into the “trans” (major) rotamer (4) to an equilibrium where the major rotamer is predominating (2:1; major to minor ratio) [52].

Figure 3

Main mechanisms used in normally functioning cells to induce cell cycle arrest or apoptosis as a response to DNA damage affecting p53 gene to inhibit cell cycle progression in the nucleus (A,B), or apoptosis in the cytoplasm (C,D). (A) p21, as a potent inhibitor of CDKs, inhibits the phosphorylation of p107 and p130 proteins, which in their hypo-phosphorylated states can bind to MuvB core complex, E2F4-5, and DP and form an active DREAM complex. Once formed, DREAM binds to E2F and CHR promoters and represses the transcription of many genes, e.g., polo-like kinases (PLK1), cyclins A, B1, and B2, CDK1, CDCs 20, 25A, and 25C, MCM5, BIRC5, etc., involved in the progress of the cell cycle at different stages and checkpoints, thereby arresting the cell cycle at any stage of the progression depending on the gene(s) inhibited [55]. In the absence of p21, CDKs remain active and hyper-phosphorylate p107 and p130 preventing them from binding to the other DREAM components, thereby leaving E2F and CHR promoter sites free to bind transcriptional activators that, on the contrary, promote the cell cycle progression [54,56]. (B) p21 interacts with PCNA in the nucleus and prevents it from binding to the δ subunit of DNA-polymerase, which blocks DNA replication as well as DNA repair, among other functions ensuring the fidelity of DNA duplication [57]. (C) p21 can be phosphorylated by the serine threonine kinase AKT1 and prevented from translocating into the nucleus; in the cytoplasm, it acts as an anti-apoptotic factor that inhibits pro-apoptotic enzymes, such as ASK, SAPK, and different caspases. It also inhibits transcriptional factors, such as E2F1, STAT3, and MYC preventing the transactivation of pro-apoptotic genes [54,58,59]. (D) p53 transactivates PUMA gene as the major p-53-dependent mechanism for intrinsic apoptosis induction. Under normal conditions and in the absence of stimuli, apoptosis is restricted by five pro-survival proteins of the Bcl-2 family; Bcl-2, Bcl-X_L, Mcl-1, Bcl-W, and Blf-1. Upon exposure to genotoxic stimuli, such as aflatoxins, p53 upregulates the expression of PUMA, a member of the Bcl-2 homology 3 (BH3)-only family, which inhibits all of the five pro-survival Bcl-2 proteins, thereby de-repressing the pro-apoptotic proteins BAX and/or BAK. This initiates mitochondrial damage allowing leakage of pro-apoptotic proteins through MOMP formation upon oligomerization of BAX/BAK, namely cytochrome C, HtrA2/OMI, and SMAC, which cooperatively induce apoptosis; cytochrome C binds APAF-1 and procaspase 9 to form an apoptosome and activate caspase 9 triggering the caspase cascade directly involved in apoptosis. Yet, caspase cascade can still be blocked by the pro-survival protein XIAP inhibitory to caspases 9 and 3. To proceed with apoptosis, SMAC and HtrA2/OMI combine to inhibit XIAP and relieve the caspases [60]?: In the absence or saturation of pro-survival Bcl-2 proteins, PUMA can directly activate BAX/BAK to resume the apoptosis process starting from MOMP formation, but this needs further studies to be ascertained [61]. Abbreviations: Bcl-2: B cell lymphoma-2; BH3-only: Bcl-2 homologue 3-only; Bcl-X_L: B cell lymphoma extra-large; MuvB: Multivulval class B; DP: Dimerization partner; DREAM: Dimerization partner, RB-like, E2F and multivulval class B; CHR: Cell cycle gene homology region; PLK: Polo-like kinase; CDK: Cyclin dependent kinase; CDC: Cell division cycle; MCM: Minichromosome maintenance; BIRC: Baculoviral inhibitor of apoptosis repeat-containing 5; PCNA: Proliferating cell nuclear antigen; ASK: Apoptosis signal-regulating kinase; SAPK: Stress-activated protein kinase; STAT3: Signal transducer and activator of transcription; MYC: Myelocytomatosis; PUMA: p53-upregulated modulatory apoptosis; Mcl-1: Myeloid cell leukaemia-1; Blf: BCL-2-related protein isolated from foetal liver; BAX: Bcl-2-associated X protein; BAK: Bcl-2 antagonist/killer; MOMP: Mitochondrial outer membrane permeabilization; Cyt C: Cytochrome C; APAF-1: Apoptotic protease-activating factor 1; SMAC: Second mitochondria-derived activator of caspases; XIAP: X-linked inhibitor of apoptosis protein; HtrA2/OMI: High-temperature requirement protein A2; Hypo-P: Hypo-phosphorylated; Hyper-P: Hyper-phosphorylated.