Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Abstract

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARa), with PPARa activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor forma-

Increased Cell Proliferation as a Key Event in Chemical Carcinogenesis: Application in an Integrated Approach for the Testing and Assessment of Non-Genotoxic Carcinogenesis

“…In the mammalian liver, numerous chemicals activate nuclear receptors such as the CAR, pregnane X receptor (PXR) [14,15,46,47], or PPARα [16,17,48]. Activation triggers a proliferative stimulus in rodents to the hepatocyte, leading to the production of more hepatocytes.…”

“…This leads to an accumulation of cells, and (as the total number is higher at the start of the next cell replication cycle) providing more opportunities for spontaneous errors even if the rate of cell proliferation is the same as that of controls. There are specific examples of decreased cell death possibly contributing to increases in cell number, for example, some evidence in animal models exposed to nuclear receptor activators [14][15][16][17].…”

“…Enhanced rate is frequently associated with an increase in the number of cells that are present (hyperplasia), but the two do not necessarily occur together. An example is liver carcinogenesis in rodents caused by constitutive androstane receptor (CAR) [14,15] or peroxisome proliferator-activated receptor alpha (PPARα) [16,17] activators. In both cases, there is an initial increase in the rate of cell proliferation; then, the rate returns to control levels within 1-4 weeks of continued exposure.…”

Translational Relevance of Rodent Models to Predict Human Liver Disease

“…Huang et al 14 also demonstrated the phenobarbital mediated activation of CAR and murine CAR ligand 1,4-bis-[2-(3,5,-dichloropyridyloxy)] benzene (TCPOBOP) for murine liver tumorigenesis. 9,36,41 Several studies in the recent years have indicated the involvement of key signaling pathways Wnt/B catenin and Hippo/YAP in CAR-mediated hepatocellular carcinogenesis. CAR induction likely activates both signaling pathways synergistically leading to CAR-mediated liver cancer development.…”

Proteomic analysis of hepatic effects of phenobarbital in mice with humanized liver

“…Human relevance of liver tumorigenesis by PB and other activators of CAR is controversially discussed (Braeuning et al 2015;Braeuning and Schwarz 2016;Elcombe et al 2014;Yamada et al 2015). While the activation of CAR in humans by PB is generally accepted, the fact that PB induces hepatocellular proliferation in mouse liver but not in cultured human hepatocytes in vitro (Haines et al 2018;Parzefall et al 1991;Plummer et al 2019) has often been put forward as a key argument against human relevance of CAR-mediated nongenotoxic carcinogenesis in humans (Elcombe et al 2014;Lake 2018;Yamada et al 2021).…”