Mode of Action Analysis for the Synthetic Pyrethroid Metofluthrin-Induced Rat Liver Tumors: Evidence for Hepatic CYP2B Induction and Hepatocyte Proliferation

Abstract

Two-year treatment with high doses of Metofluthrin produced hepatocellular tumors in both sexes of Wistar rats. To understand the mode of action (MOA) by which the tumors are produced, a series of studies examined the effects of Metofluthrin on hepatic microsomal cytochrome P450 (CYP) content, hepatocellular proliferation, hepatic gap junctional intercellular communication (GJIC), oxidative stress and apoptosis was conducted after one or two weeks of treatment. The global gene expression profile indicated that most genes with upregulated expression with Metofluthrin were metabolic enzymes that were also upregulated with phenobarbital. Metofluthrin induced CYP2B and increased liver weights associated with centrilobular hepatocyte hypertrophy (increased smooth endoplasmic reticulum [SER]), and induction of increased hepatocellular DNA replication. CYP2B1 mRNA induction by Metofluthrin was not observed in CAR knockdown rat hepatocytes using the RNA interference technique, demonstrating that Metofluthrin induces CYP2B1 through CAR activation. Metofluthrin also suppressed hepatic GJIC and induced oxidative stress and increased antioxidant enzymes, but showed no alteration in apoptosis. The above parameters related to the key events in Metofluthrin-induced liver tumors were observed at or below tumorigenic dose levels. All of these effects were reversible upon cessation of treatment. Metofluthrin did not cause cytotoxicity or peroxisome proliferation. Thus, it is highly likely that the MOA for Metofluthrin-induced liver tumors in rats is through CYP induction and increased hepatocyte proliferation, similar to that seen for phenobarbital. Based on analysis with the International Life Sciences Institute/Risk Science Institute MOA framework, it is reasonable to conclude that Metofluthrin will not have any hepatocarcinogenic activity in humans, at least at expected levels of exposure.

Liver tumor promoting effect of orphenadrine in rats and its possible mechanism of action including CAR activation and oxidative stress

“…These results suggest that patients suffering from Parkinson's disease given clinical doses of ORPH may not promote hepatocarcinogenesis, while the contribution of genotoxic potential of this compound should be addressed on the hepatocarcinogenicity. It has been shown that the chemicals that cause nuclear translocation of CAR and induce CYP2B have a liver tumor-promoting activity in rats (Deguchi et al, 2009). Chang (2009 described that the CAR protein is a receptor for xenobiotic ligands and an inducer of CYP2B1 in rats.…”

“…Furthermore, CAR regulates not only the expression of various genes of Cyp2b but also of genes related to bioactivation, detoxification, and cell proliferation/ tumorigenesis (Chang, 2009), such as Cyclin D1, one of the genes activated by CAR and promoting hepatocellular growth (Ledda-Columbano et al, 2000). In addition, CAR is essential in the hepatic tumor-promotion mechanism (Deguchi et al, 2009).…”

“…It has been reported that treatment with PB increased mRNA expression and protein levels of CYP2B in a twostage liver carcinogenesis bioassay in rats (Kinoshita et al, 2003;Waxman and Azaroff, 1992). The nuclear translocation of CAR induced by PB stimulates the expression of various genes that enhance hepatic tumor promotion (Deguchi et al, 2009;Kawamoto et al, 1999). In addition, it has been shown that microsomal ROS production and production of thiobarbituric acid-reactive substances (TBARS) and are induced by PB with increasing the Cyp2b (Morita et al, 2011).…”

Effects of arecoline on hepatic cytochrome P450 activity and oxidative stress

“…On the other hand, AH (4 and 20 mg/kg/d) also significantly increased the hepatic CYP2B activity. CYP2B induction is asso- ciated with liver tumor formation in rodents by activating nuclear receptors, particularly constitutive androstane receptor (CAR) (Deguchi et al, 2009;Lo et al, 2012). Therefore, betel-quid-induced liver cirrhosis and hepatocellular carcinoma might be associated with the induction of AH on rat hepatic 2B.…”

Role of CYP2B in Phenobarbital-Induced Hepatocyte Proliferation in Mice

“…Thus, it can be deduced that Cyp2b10 was involved in PB-induced hepatocyte proliferation in male mice. Notably, the association of hepatocyte proliferation with CYP2B induction has also been observed for other CYP2B inducers, such as the environmental pollutant potassium perfluorooctanesulfonate and the synthetic pyrethroid metofluthrin (Deguchi et al, 2009;Elcombe et al, 2012). It remains to be determined whether CYP2B induction contributes to the hepatic hypertrophy induced by these other compounds.…”