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Increase, Preneoplastic foci (hepatocytes) leads to Increase, hepatocellular adenomas and carcinomas
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Constitutive androstane receptor activation leading to hepatocellular adenomas and carcinomas in the mouse and the rat||adjacent||High||Not Specified||Kristin Lichti-Kaiser (send email)||Open for citation & comment||EAGMST Under Review|
|Androgen receptor activation leading to hepatocellular adenomas and carcinomas (in mouse and rat)||adjacent||Charles Wood (send email)||Open for adoption||Under Development|
|Chronic cytotoxicity leading to hepatocellular adenomas and carcinomas (in mouse and rat)||adjacent||Charles Wood (send email)||Under Development: Contributions and Comments Welcome|
Life Stage Applicability
Key Event Relationship Description
Clonally expanded cells (foci of cellular alteration – either eosinophilic, basophilic or clear cell) have been shown to be increased at tumorigenic dose levels of CAR activators such as phenobarbital, TCPOBOP and metofluthrin. As discussed for earlier key events, the CAR-mediated events that lead to an increase in altered foci lead to a greater abundance of cells with mutations in their DNA that are less responsive to normal cell-cell signaling and control mechanisms. As a result, these foci are considered preneoplastic lesions, and can progress with time into adenomas and carcinomas. The continued CAR-mediated stimulus for increased cell proliferation within these foci (e.g. as demonstrated in studies by Kolaja et al., 1996b) will also provide an environment where the mutant cells can survive and develop into tumors.
Evidence Supporting this KER
The development of liver tumors in rodents, whether spontaneously or induced by a non-genotoxic carcinogen, has consistently included the development of altered foci as a precursor step to hepatocellular adenomas and carcinomas (Goldsworthy and Fransson-Steen, 2002; Tamura et al., 2015). These foci are considered preneoplastic lesions, and their ability to progress to form adenomas and/or carcinomas in rodents has been previously recognized. In the case of CAR activators, an increased incidence of preneoplastic foci has been consistently shown to precede tumor development, and there is a high biological plausibility for this Key Event Relationship (Elcombe et al., 2014; Goldsworthy and Fransson-Steen, 2002; Jones et al., 2009; Lake, 2009).
Uncertainties and Inconsistencies
The incidence of altered foci, and their staining properties (e.g. eosinophilic, basophilic, clear cell, mixed) are not always reported in published studies of carcinogenicity with CAR activation compounds. However, the consistent findings with well-known CAR activating compounds and their absence in CAR knockout mouse studies provide a strong basis for their existence in the CAR AOP.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Phenobarbital and other CAR activators do not produce liver tumors in long term studies in hamsters (Diwan et al., 1986; Elcombe et al., 2014). Consistent with the lack of effects on proliferation and on tumor development, Diwan et al. (1986) also reported that phenobarbital treatment at 500 ppm in the drinking water did not produce any increases in preneoplastic foci of cellular alteration compared to groups that received an initiator alone. Further, treatment of CAR knockout mice lacking the CAR nuclear receptor with phenobarbital or TCPOBOP produced none of the early key events (e.g. altered expression of CAR-responsive cell cycle genes, increased cell proliferation) and no increases in altered foci or tumors (Huang et al., 2005; Yamamoto et al., 2004). Therefore, the development of increased foci in the liver in response to treatment with CAR activators has strong data indicating it is specific to mice and rats, the species which also develop hepatocellular tumors in response to known CAR activators.
[see reference list at end of this AOP; it includes all cited references]