Evaluation of the human relevance of the constitutive androstane receptor-mediated mode of action for rat hepatocellular tumor formation by the synthetic pyrethroid momfluorothrin

Abstract

-High dietary levels of the non-genotoxic synthetic pyrethroid momfluorothrin increased the incidence of hepatocellular tumors in male and female Wistar rats. Mechanistic studies have demonstrated that the mode of action (MOA) for momfluorothrin-induced hepatocellular tumors is constitutive androstane receptor (CAR)-mediated. In the present study, to evaluate the potential human carcinogenic risk of momfluorothrin, the effects of momfluorothrin (1-1,000 μM) and a major metabolite Z-CM-CA (5-1,000 μM) on hepatocyte replicative DNA synthesis and CYP2B mRNA expression were examined in cultured rat and human hepatocyte preparations. The effect of sodium phenobarbital (NaPB), a prototypic rodent hepatocarcinogen with a CAR-mediated MOA, was also investigated. Human hepatocyte growth factor (hHGF) produced a concentration-dependent increase in replicative DNA synthesis in rat and human hepatocytes. However, while NaPB and momfluorothrin increased replicative DNA synthesis in rat hepatocytes, NaPB, momfluorothrin and Z-CMCA did not increase replicative DNA synthesis in human hepatocytes. NaPB, momfluorothrin and Z-CMCA increased CYP2B1/2 mRNA levels in rat hepatocytes. NaPB and momfluorothrin also increased CYP2B6 mRNA levels in human hepatocytes. Overall, while momfluorothrin and NaPB activated CAR in cultured human hepatocytes, neither chemical increased replicative DNA synthesis. Furthermore, to confirm whether the findings observed in vitro were also observed in vivo, a humanized chimeric mouse study was conducted. Replicative DNA synthesis was not increased in human hepatocytes of chimeric mice treated with momfluorothrin or its close structural analogue metofluthrin. As human hepatocytes are refractory to the mitogenic effects of momfluorothrin, in contrast to rat hepatocytes, the data support the hypothesis that the MOA for momfluorothrin-induced rat liver tumor formation is not relevant for humans.

Case examples of an evaluation of the human relevance of the pyrethroids/pyrethrins-induced liver tumours in rodents based on the mode of action

“…17 A number of investigative studies were performed to obtain data on the proposed key and associative events. [24][25][26][27] In some MOA studies, rats were also given sodium phenobarbital (NaPB), which is known to activate CAR in the rat and to produce a variety of hepatic effects including CYP2B enzyme induction and increased cell proliferation. 17 Phenobarbital (PB) (or NaPB) served as a positive control to confirm the potential responsiveness of the animals used in these MOA studies to a known CAR activator.…”

“…To evaluate the potential human carcinogenic risk of metofluthrin and momfluorothrin, the effects of these compounds on hepatocyte replicative DNA synthesis and CYP2B mRNA expression (an associative event, as a marker of CAR activation) were examined in cultured rat and human hepatocyte preparation. 26,27,40 The effect of PB was also investigated. 28 + indicates the effect is present at indicated time points of the treatment.…”

“…As human hepatocytes are refractory to the mitogenic effects of metofluthrin and momfluorothrin, in contrast to rat hepatocytes, these data support the hypothesis that the MOA for metofluthrin-and momfluorothrin-produced rat liver tumours is not relevant to humans. 26,27,40 Chimeric liver mouse study…”

Constitutive Androstane Receptor and Hepatitis B Virus X Protein Cooperatively Induce β-catenin-Activated Liver Tumors

“…In order to study specific types of HCC, we need models that align well with various tumor subtypes. Additionally, some questions have been raised about whether CAR activation in humans causes direct DNA replication, and by extension, whether CAR is relevant to human HCC (50). Although the answers to these questions are still being investigated, our model shows a conserved gene signature with three independent sets of HBVrelated HCC.…”

Role of Nuclear Receptors PXR and CAR in Xenobiotic-Induced Hepatocyte Proliferation and Chemical Carcinogenesis

“…74) Phenobarbital, a well-known liver tumor promoter in rodents, activates CAR in mice, rats, and humans, but induces DNA synthesis and hepatocyte proliferation only in rodents. For example, Yamada's group 75,76) demonstrated that hepatocyte growth factor increased DNA synthesis (BrdU labeling) in rat and human hepatocytes in vitro but phenobarbital did only in rat hepatocytes whereas it increased CYP2B6 and CYP2B1 mRNA levels in human and rat hepatocytes, respectively.…”