Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

Abstract

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are important nuclear receptors involved in the regulation of cellular responses from exposure to many xenobiotics and various physiological processes. Phenobarbital (PB) is a non-genotoxic indirect CAR activator, which induces cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes and is known to produce liver foci/tumors in mice and rats. From literature data, a mode of action (MOA) for PB-induced rodent liver tumor formation was developed. A MOA for PXR activators was not established owing to a lack of suitable data. The key events in the PB-induced liver tumor MOA comprise activation of CAR followed by altered gene expression specific to CAR activation, increased cell proliferation, formation of altered hepatic foci and ultimately the development of liver tumors. Associative events in the MOA include altered epigenetic changes, induction of hepatic CYP2B enzymes, liver hypertrophy and decreased apoptosis; with inhibition of gap junctional intercellular communication being an associative event or modulating factor. The MOA was evaluated using the modified Bradford Hill criteria for causality and other possible MOAs were excluded. While PB produces liver tumors in rodents, important species differences were identified including a lack of cell proliferation in cultured human hepatocytes. The MOA for PB-induced rodent liver tumor formation was considered to be qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies conducted in human populations chronically exposed to PB in which there is no clear evidence for increased liver tumor risk.

• Constitutive androstane receptor
• dose–response assessment
• human relevance framework
• key and associative events
• liver cancer
• mode of action
• modulating factors
• phenobarbital
• pregnane X receptor

Acknowledgements

The authors would like to thank the Nuclear Receptor Workshop Steering Committee members (Dr Melvin Andersen, Dr Richard Becker, Dr Michael Cunningham, Dr Vicki Dellarco, Dr Michael Dourson, Dr Michael Honeycutt and Dr Julian Preston) for their input in the workshop development, along with the Case Study Leaders (Dr Chris Corton, Dr Cliff Elcombe, Dr James Klaunig, Dr Richard Peffer and Dr Douglas Wolf). The authors would also like to thank Ms. Alison Willis and Dr Andrew Maier for their contribution and review of this manuscript and for their help in coordination and compiling drafts prior to submission.