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

Nitrapyrin, a nitrification inhibitor, produces liver tumors in B6C3F1 mice. In a 2-year oncogenicity study, increased incidence of mice with hepatocellular tumors was observed following exposure to 125 (females only) or 250 mg/kg/day (males and females) nitrapyrin in the diet. Previous data was generated in male mice to support a mode-of-action (MoA) characterized by constitutive androstane receptor (CAR) nuclear receptor (NR) activation, increased hepatocellular proliferation, and subsequent hepatocellular foci and tumor formation. Uncertainty as to the relevance of this MoA for females remained given the increased sensitivity to tumor formation in female mice. A targeted MoA study was conducted to evaluate CAR activation and hepatic responses in female mice treated with the female carcinogenic dose of nitrapyrin for 4 days. Nitrapyrin induced a treatment-related increase in hepatocellular hypertrophy and hepatocellular proliferation. Nitrapyrin also induced a dose-related increase in the Cyp2b10/CAR-associated transcript and liver weights. Nitrapyrin-induced liver weights and Cyp2b10 gene expression for both males and females were compared to data generated from three other established CAR activators; methyl isobutyl ketone, phenobarbital, and sulfoxaflor. The response observed in female mice following exposure to nitrapyrin was within range of the degree of change observed in mice following exposure to tumorigenic doses of other CAR activators. Consistent with the liver MoA in male mice, these data support a CAR-mediated mode of action for nitrapyrin-induced liver tumors in female mice, with the understanding that a focused approach minimizing animal use can bridge male and female datasets when sex-specific carcinogenic differences are observed.

Download Full-text

An Evaluation of the Human Relevance of the Liver Tumors Observed in Female Mice Treated With Permethrin Based on Mode of Action

Toxicological Sciences ◽

10.1093/toxsci/kfaa017 ◽

2020 ◽

Vol 175 (1) ◽

pp. 50-63 ◽

Cited By ~ 1

Author(s):

Miwa Kondo ◽

Hiroko Kikumoto ◽

Thomas G Osimitz ◽

Samuel M Cohen ◽

Brian G Lake ◽

...

Keyword(s):

Cell Proliferation ◽

Dna Synthesis ◽

Liver Tumor ◽

Mode Of Action ◽

Mouse Liver ◽

Liver Tumors ◽

Tumor Formation ◽

Human Hepatocytes ◽

Mrna Levels ◽

Mouse Hepatocytes

Abstract 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.

Download Full-text

Suppression of Liver Tumor Formation by the Liver Microenvironment of Female, but Not Male, Syngeneic Hosts

Hormonal Carcinogenesis III ◽

10.1007/978-1-4612-2092-3_53 ◽

2001 ◽

pp. 489-495

Author(s):

Gary J. Smith ◽

Sharon C. Presnell ◽

William B. Coleman ◽

Joe W. Grisham

Keyword(s):

Liver Tumor ◽

Tumor Formation

Download Full-text

Comparison of the Hepatic Effects of Phenobarbital in Chimeric Mice Containing Either Rat or Human Hepatocytes With Humanized Constitutive Androstane Receptor and Pregnane X Receptor Mice

Toxicological Sciences ◽

10.1093/toxsci/kfaa125 ◽

2020 ◽

Vol 177 (2) ◽

pp. 362-376

Author(s):

Tomoya Yamada ◽

Ayako Ohara ◽

Naoya Ozawa ◽

Keiko Maeda ◽

Miwa Kondo ◽

...

Keyword(s):

Gene Expression ◽

Liver Tumor ◽

Constitutive Androstane Receptor ◽

Pregnane X Receptor ◽

Rat Hepatocytes ◽

Global Gene Expression ◽

Human Hepatocytes ◽

Chimeric Mouse ◽

Chimeric Mice ◽

Wild Type

Abstract Using a chimeric mouse humanized liver model, we provided evidence that human hepatocytes are refractory to the mitogenic effects of rodent constitutive androstane receptor (CAR) activators. To evaluate the functional reliability of this model, the present study examined mitogenic responses to phenobarbital (PB) in chimeric mice transplanted with rat hepatocytes, because rats are responsive to CAR activators. Treatment with 1000 ppm PB for 7 days significantly increased replicative DNA synthesis (RDS) in rat hepatocytes of the chimeric mice, demonstrating that the transplanted hepatocyte model is functionally reliable for cell proliferation analysis. Treatment of humanized CAR and pregnane X receptor (PXR) mice (hCAR/hPXR mice) with 1000 ppm PB for 7 days significantly increased hepatocyte RDS together with increases in several mitogenic genes. Global gene expression analysis was performed with liver samples from this and from previous studies focusing on PB-induced Wnt/β-catenin signaling and showed that altered genes in hCAR/hPXR mice clustered most closely with liver tumor samples from a diethylnitrosamine/PB initiation/promotion study than with wild-type mice. However, different gene clusters were observed for chimeric mice with human hepatocytes for Wnt/β-catenin signaling when compared with those of hCAR/hPXR mice, wild-type mice, and liver tumor samples. The results of this study demonstrate clear differences in the effects of PB on hepatocyte RDS and global gene expression between human hepatocytes of chimeric mice and hCAR/hPXR mice, suggesting that the chimeric mouse model is relevant to humans for studies on the hepatic effects of rodent CAR activators whereas the hCAR/hPXR mouse is not.

Download Full-text