Upstream eventFormation, Pro-mutagenic DNA Adducts
Tumorigenesis, Hepatocellular carcinoma
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Directness||Weight of Evidence||Quantitative Understanding|
|AFB1: Mutagenic Mode-of-Action leading to Hepatocellular Carcinoma (HCC)||indirectly leads to||Moderate||Moderate|
Life Stage Applicability
How Does This Key Event Relationship Work
Formation of the pro-mutagenic DNA adduct, N7-AFB1-G (or its conversion product, N7-AFB1-FAPy) is the first step in the initiation of a process that may finish in development of hepatocellular carcinoma (HCC). These steps (pro-mutagenic adduct formation and HCC) are indirectly linked through insufficient/mis-repair of DNA and induction of a mutation in a critical gene and clonal expansion/cell proliferation with formation of altered hepatic foci (AHF).
Weight of Evidence
While there is no specific information for AFB1, it is widely recognized that pro-mutagenic adducts formed by AFB1 metabolites may be repaired/removed or may result in mutations. The fidelity of the repair processes and probability of mis-repair determine whether mutations arise in tumor-critical genes. The altered hepatic foci (AHF) are believed to result from mutations expressed in cells that demonstrate reduced apoptosis and increased proliferation, likely linked to the mutations. (Alekseyev et al., 2004; Zhang et al., 2003; Giri et al., 2002; Bailey et al., 1996; Lin et al., 2014). The further development of AHF to HCC is believed to be a continuum of these same processes over time. These are discussed in a previous section and include effects on apoptosis, inflammation, the development of a tumor microenvironment, interference with the anti-oxidant response, and likely others.
Empirical Support for Linkage
Formation of AFB1-induced DNA adducts clearly precede tumor formation, thus the necessary temporal pattern exists. The plethora of published chemoprevention studies support a relationship between AFB1-induced DNA adducts and hepatocellular carcinoma (HCC). There are some evaluations that claim (Bechtel et al. 1989) linearity in the dose-response for adduct formation as a function of dose in both rats and trout; but linearity does not seem to apply, however, for adducts and HCC. Recent work of Johnson et al. (2014) showed that HCC incidence was reduced to zero by the chemoprotectant CDDO-Im, while around 30% of the original level of N7-AFB1-G adducts were still present. These authors note that there remained ~15,000 adducts per cell after application of the chemoprotectant, while no HCC tumors were induced. Other chemoprevention data also support the reduction or elimination of HCC, accompanied by a reduction in AFB1-induced DNA adducts, e.g., oltipraz (Kensler et al.,1987 ; Roebuck et al., 1991; Maxuitenko et al., 1993). There are also chemoprevention data from humans demonstrating administration of chlorophyllin results in an increased elimination of urinary AFB1-induced adducts, presumably stemming from a reduced burden of AFB1-induced adducts, and an assumed concomitant reduction in tumor burden (Jubert et al., 2009) (see Chemoprevention section).
Uncertainties or Inconsistencies
The direct KER relationships between adducts and mutations (MIE→KE#2) and from mutations to AHF (KE#2→KE#3) and from AHF to HCC (KE#3→AO) determine this indirect relationship. Unfortunately, there is a paucity of data to support quantification of a relationship between adducts and HCC; neither are there data to address an AFB1-related dose-response for both KEs.
Quantitative Understanding of the Linkage
One might obtain a quantitative understanding of this linkage from studies with AFB1 that reported dose-response data for both AFB1-induced adduct levels and HCC tumor incidence. However, no such data were identified. As mentioned above, some chemoprevention studies report reduced levels of AFB1-induced adducts (or increased urinary elimination of AFB1-induced adducts) but they only used single dose levels, so there are no dose-response data. Most initiation-promotion studies used a highly artificial system with chemical initiation with an alkylating agent and promotion with phenobarbital, TCDD, or some other compound, coupled with partial hepatectomy to further stimulate rapid cell proliferation (Xu et al., 1990a, 1990b). Chemoprotection studies such as Johnson et al. (2014) indicate that a strong relationship likely exists between AFB1-induced adducts and tumors, but insufficient data exist for quantification or definitive dose-response determination.
Evidence Supporting Taxonomic Applicability
HCC has been observed essentially universally in AFB1-treated mammals, birds, and fish examined (Pottenger et al., 2014; Kensler et al., 2011; Kimura et al., 2004; Cullen et al., 1990; Kirby et al., 1990).
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