CYP450-Dependent Biotransformation of the Insecticide Fipronil into Fipronil Sulfone Can Mediate Fipronil-Induced Thyroid Disruption in Rats

Abstract

In rats, the widely used insecticide fipronil increases the clearance of thyroxine (T(4)). This effect is associated with a high plasma concentration of fipronil sulfone, the fipronil main metabolite in several species including rats and humans. In sheep, following fipronil treatment, fipronil sulfone plasma concentration and thyroid disruption are much lower than in rats. We postulated that fipronil biotransformation into fipronil sulfone by hepatic cytochromes P450 (CYP) could act as a potential thyroid disruptor. The aim of this study was to determine if fipronil sulfone treatment could reproduce the fipronil treatment effects on T(4) clearance and CYP induction in rats. Fipronil and fipronil sulfone treatments (3.4 μmol/kg/day per os, 14 days) increased total and free T(4) clearances to the same extent in THX + T(3), euthyroid-like rats. Both treatments induced a 2.5-fold increase in Ugt1a1 and Sult1b1 messenger RNA (mRNA) expressions and a twofold increase in UGT1A activity suggesting that T(4) elimination was mediated, at least in part, by hepatic uridine 5'-diphospho-glucuronosyltransferases (UGT) and/or sulfotransferases (SULT) induction. Both treatments induced a 10-fold increase in Cyp3a1 and Cyp2b2 mRNA expressions concomitant with a threefold increase in CYP3A immunoreactivity and a 1.7-fold increase in antipyrine clearance, a biomarker of CYP3A activity. All these results showed that fipronil sulfone treatment could reproduce the fipronil treatment effects on T(4) clearance and hepatic enzyme induction in rats. The potential of fipronil sulfone to act as a thyroid disruptor is all the more critical because it persists much longer in the organism than fipronil itself.

Fipronil induces CYP isoforms in rats

“…Also our results confirm the in vivo data obtained by Roques et al (2012) for CYP3A and 2B families. Roques et al (2012) demonstrated in rats that fipronil treatment [3.4 mmol/kg/day (equivalent to 1.5 mg/kg/day), per os, for 14 days] increased antipyrine clearance and induced CYP3A1 and CYP2B2 mRNA expressions; although no significant effect on CYP1A1, CYP1A2, and CYP3A2 mRNA expressions was noted. Our work is the first investigation that shows in vivo induction of CYP1A1/2 and 2A1 isoenzymes.…”

“…On the other hand, in rats, oral treatment with fipronil was also associated with high plasma concentration of fipronil sulfone concomitant with a marked increase in the clearance of tyroxine (Leghait et al, 2009). There is a dramatic increase in fipronil biotransformation into fipronil sulfone by hepatic CYP enzymes, and the potential of fipronil sulfone to act as a thyroid disruptor is more critical because it persists much longer in the organism (Roques et al, 2012). Fipronil is readily metabolized into fipronil sulfone in several mammalian species including humans (Mohamed et al, 2004) and this main metabolic pathway is clearly mediated by hepatic phase I CYP enzymes.…”

“…Such hypothesis is supported by in vitro data on human hepatocytes, both compounds resulted in a dose-dependent increase in CYP1A1 and CYP3A4 mRNA expressions, but fipronil sulfone was more cytotoxic to hepatocytes than fipronil itself (Das et al, 2006). It is noteworthy, however, that in recent in vivo study, both fipronil and fipronil sulfone treatments (3.4 mmol/kg/day per os for 14 days) in rats induced a significant increase in CYP3A1 mRNA expression, but not in CYP1A1 and CYP1A2 mRNA expressions (Roques et al, 2012).…”

“…Recently, Roques et al (2013) indicated that constitute androstane receptor (CAR) and pregnane X receptor (PXR) are key modulators of the hepatic gene expression profile following fipronil treatment which, in rats, may contribute to increase thyroid hormone clearance. In vivo data in rats assessing the antipyrine clearance, which is considered a relevant biomarker, but non specific, for phase I CYP enzymes involved in hepatic xenobiotic metabolism in different species (St Peter et al, 1991;Engel et al, 1996;Chan and Yeung, 2006;Anad on et al, 2013), demonstrated in thyroid-intact rats that fipronil treatment increased antipyrine clearance (Leghait et al, 2009;Roques et al, 2012). Fipronil treatment was also associated with an increase in hepatic microsomal 4-nitrophenol UDPglucuronosyltransferase activity, hepatic phase II enzyme involved in T4 glucuronidation, as well as with plasma concentrations of fipronil sulfone at least 20-fold higher than those of fipronil (Leghait et al, 2009).…”

“…To our knowledge, the in vivo and in vitro data concerning the ability of fipronil to induce hepatic phase I enzymes are conflicting respect to CYP1A1/2 induction. In vitro, fipronil increases CYP1A1 and 3A4 activities in human hepatocytes (Das et al, 2006) whereas no clear effect of fipronil was evidenced on the activities of microsomes obtained from fipronil-treated rats, rabbits or mice (AFSSA, 2005;Roques et al, 2012). Accordingly, in an attempt to contribute to the literature with new data of the role of fipronil to induce metabolic phase I CYP enzymes, the present study examined the effect of oral doses of fipronil on CYP1A1/2, CYP2B1/2, CYP2E1, CYP2C11, CYP2A1, CYP3A1/2 and CYP4A1/2 enzyme activities in liver microsomes from treated rats.…”