Mode of action and dose–response framework analysis for receptor-mediated toxicity: The aryl hydrocarbon receptor as a case study

Abstract

Dioxins and dioxin-like compounds are tumor promoters that cause liver cancer in rats and mice. The aryl hydrocarbon receptor (AHR) has been implicated as a key component in this tumor promotion response. Despite extensive knowledge of the toxicology of dioxins, no mode of action (MOA) hypothesis for their tumorigenicity has been formally documented using the Human Relevance MOA framework developed by the International Programme on Chemical Safety (IPCS). To address this information gap, an expert panel was convened as part of a workshop on receptor-mediated liver tumorigenicity. Liver tumors induced by ligands of the AHR were assessed using data for dioxins and related chemicals as a case study. The panel proposed a MOA beginning with sustained AHR activation, eventually leading to liver tumors via a number of other processes, including increased cell proliferation of previously initiated altered hepatic foci, inhibition of intrafocal apoptosis and proliferation of oval cells. These processes have been identified and grouped as three key events within the hepatocarcinogenic MOA: (1) sustained AHR activation, (2) alterations in cellular growth and homeostasis and (3) pre-neoplastic tissue changes. These key events were identified through application of the Bradford-Hill considerations in terms of both their necessity for the apical event/adverse outcome and their human relevance. The panel identified data supporting the identification and dose–response behavior of key events, alteration of the dose–response by numerous modulating factors and data gaps that potentially impact the MOA. The current effort of applying the systematic frameworks for identifying key events and assessing human relevance to the AHR activation in the tumorigenicity of dioxins and related chemicals is novel at this time. The results should help direct future regulatory efforts and research activities aimed at better understanding the potential human cancer risks associated with dioxin exposure.

• Dioxin
• dose–response assessment
• human relevance framework
• key events
• liver cancer
• mode of action
• modulating factors
• TCDD

Acknowledgements

The authors would like to thank the Dr Melvin Andersen and Dr Russell Thomas for their valuable contribution to this manuscript and their participation on the case study panel. The authors would also like to thank the Nuclear Receptor Workshop Steering Committee members and Case Study Leadership Team for their input in workshop development (Dr Melvin Andersen, Dr Chris Corton, Dr Cliff Elcombe, Dr James Klaunig, Dr Richard Peffer, Dr Julian Preston and Dr Douglas Wolf). The authors would like to thank Dr Bruce Allen for his participation on the AHR panel. The authors would also like to thank Ms. Alison Willis, Dr Irene Abraham and Ms. Kiran Sharma for their contribution and review of this manuscript and for their help in coordination and compiling drafts prior to submission. The findings and conclusions in this report (presentation) are those of the authors and do not necessarily represent the views of their respective institutions.

Supplementary material available online Supplementary Summary Tables Supplementary Figure

Notes

¹A threshold is defined as a non-linear dose–response that is a range of exposures from zero to some finite value with no detectable expression of a toxic effect, and the threshold of toxicity is where the effects (or their precursors) become quantifiable.

²It is important to recognize that the nitrosamine initiation and partial hepatectomy design likely create somewhat spurious dose–response information by artificially stimulating the development and clonal expansion of altered hepatic foci before the promotion effect of sustained AHR activation was added. Hence, dose–response estimates derived from these initiation–promotion studies must account for this factor. Altered foci do occur in TCDD-treated non-initiated rats, but at a very low rate (Brix et al., 2005; Maronpot et al., 1993; McMartin et al., 1992; Newsholme & Fish, 1994).

³Sustained near-maximal activation is defined for tumor outcomes based on the dose–response relationship between CYP1A induction over time and the occurrence of tumors at several time intervals up to 1 year. CYP1A1 induction between 75% and 90% of the maximal induction corresponded to a 1% tumor response for both hepatic adenoma and cholangiocarcinoma (Simon et al., 2009).

⁴The rate of cell proliferation within any population of cells depends on three parameters: (a) the rate of cell division, (b) the fraction of cells within the population undergoing cell division (growth fraction) and (c) the rate of cell loss from the population due to terminal differentiation or cell death. If the rate of replication outpaces the rate of cell death, then cell proliferation occurs (http://www.ncbi.nlm.nih.gov/books/NBK20860/).

⁵Minimal (grade 1) toxic hepatopathy was diagnosed when additional changes indicative of a toxic effect, usually a slight degree of bile duct and/or oval cell hyperplasia or a few large prominent altered hepatocellular foci, and occasionally a small focus of cholangiofibrosis were present. Mild (grade 2) toxic hepatopathy was characterized by the presence of multiple toxic changes, all of which were of minimal to mild severity. In addition, multiple prominent altered hepatocellular foci (usually mixed cell foci) and an occasional focus of nodular hyperplasia were sometimes present. Moderate (grade 3) toxic hepatopathy was diagnosed when most or all the spectrum of toxic changes were present, with some degree of distortion of the normal liver structure caused by prominent altered hepatocellular foci, nodular hyperplasia and cholangiofibrosis. Marked (grade 4) toxic hepatopathy was diagnosed when severe toxic changes were present with pronounced distortion of the liver architecture. Livers with marked toxic hepatopathy often had a multinodular appearance due to the presence of numerous large foci of nodular hyperplasia that replaced much of the liver parenchyma (Hailey et al., 2005).