-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.

High doses of metofluthrin have been shown to produce hepatocellular tumours in rats. Previous in vivo and in vitro mode of action (MOA) studies have demonstrated that metofluthrin induces hepatic microsomal cytochrome P450 (CYP) 2B enzymes and hepatocellular replicative DNA synthesis in rats, suggesting that the MOA for liver tumour formation is similar to that of other rodent non-genotoxic hepatocarcinogens that are constitutive androstane receptor (CAR) activators. To evaluate the potential human carcinogenic risk of metofluthrin, in this study the effect of metofluthrin (7.7-770 µM) on replicative DNA synthesis (as determined by BrdU labeling) was examined in cultured human hepatocytes from four different donors. The effect of sodium phenobarbital (NaPB), a well-known rodent hepatocarcinogen with a CAR-mediated MOA, was also investigated. In addition, the effect of metofluthrin on the expression of Ki-67, CYP2B6 and CAR mRNAs in cultured human and rat hepatocytes was examined. Treatment with 10 and 100 ng mL −1 hepatocyte growth factor (HGF) produced a concentration-dependent increase in BrdU labeling in human hepatocyte preparations. However, no increase in BrdU labeling was observed after treatment with metofluthrin or NaPB. Treatment with HGF significantly increased Ki-67 mRNA expression in both human and rat hepatocytes. However, while metofluthrin increased Ki-67 mRNA expression in rat hepatocytes, treatment with metofluthrin and NaPB had no effect on Ki-67 mRNA expression in human hepatocytes. Treatment with NaPB produced an increase in CYP2B6 mRNA levels in human hepatocytes, with metofluthrin also producing a small effect. Neither metofluthrin nor NaPB significantly changed CAR mRNA expression levels in both cultured rat and human hepatocytes. Thus, while metofluthrin and NaPB could activate CAR in cultured human hepatocytes, neither compound increased BrdU labeling and Ki-67 mRNA expression. As human hepatocytes are refractory to the mitogenic effects of metofluthrin, in contrast to rat hepatocytes, the data suggest that the MOA for metofluthrin-induced rat liver tumour formation is not relevant for humans and hence that metofluthrin does not pose a carcinogenic hazard for humans. Fig. 2 Effect of NaPB and metofluthrin (MFT) on cell viability (MTT assay) in cultured human hepatocytes. Results are presented as mean ± SD of 12 replicate wells. No values were statistically significantly different (all p > 0.05) from the control (DMSO only treated).Paper Toxicology Research 906 | Toxicol. Res., 2015, 4, 901-913 This journal is

In 2-year studies, the nongenotoxic pyrethroid insecticide permethrin produced hepatocellular tumors in CD-1 mice but not in Wistar rats. Recently, we demonstrated that the mode of action (MOA) for mouse liver tumor formation by permethrin involves activation of the peroxisome proliferator-activated receptor alpha (PPARα), resulting in a mitogenic effect. In the present study, the effects of permethrin and 2 major permethrin metabolites, namely 3-phenoxybenzoic acid and trans-dichlorochrysanthemic acid, on cytochrome P450 mRNA levels and cell proliferation (determined as replicative DNA synthesis) were evaluated in cultured CD-1 mouse, Wistar rat, and human hepatocytes. Permethrin and 3-phenoxybenzoic acid induced CYP4A mRNA levels in both mouse and human hepatocytes, with trans-dichlorochrysanthemic acid also increasing CYP4A mRNA levels in mouse hepatocytes. 3-Phenoxybenzoic acid induced CYP4A mRNA levels in rat hepatocytes, with trans-dichlorochrysanthemic acid increasing both CYP4A mRNA levels and replicative DNA synthesis. Permethrin, 3-phenoxybenzoic acid, and trans-dichlorochrysanthemic acid stimulated replicative DNA synthesis in mouse hepatocytes but not in human hepatocytes, demonstrating that human hepatocytes are refractory to the mitogenic effects of permethrin and these 2 metabolites. Thus, although some of the key (eg, PPARα activation) and associative (eg, CYP4A induction) events in the established MOA for permethrin-induced mouse liver tumor formation could occur in human hepatocytes at high doses of permethrin, 3-phenoxybenzoic acid, and/or trans-dichlorochrysanthemic acid, increased cell proliferation (an essential step in carcinogenesis by nongenotoxic PPARα activators) was not observed. These results provide additional evidence that the established MOA for permethrin-induced mouse liver tumor formation is not plausible for humans.