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Modulation of rat hepatic cytochromes P450 by chronic methapyrilene treatment
S A Wrighton1, M VandenBranden, T J Brown
1Department of Drug Metabolism and Disposition, Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285.
Abstract:
The antihistaminic compound methapyrilene (MP) when chronically administered has been shown to be a rat-specific hepatocarcinogen. To examine the effects of chronic MP treatment on the hepatic microsomal cytochromes P450. Fischer 344 rats were gavaged for 10 weeks (5 days on, 2 days off) with either vehicle or 50, 100, or 150 mg MP/kg body weight. Chronic MP treatment was found to have a significant effect on several microsomal enzymatic activities. Small (17-28%) but significant (P less than 0.05) decreases were observed for total P450 levels and the activities of erythromycin N-demethylase (catalyzed by P450IIIA), N-nitrosodimethylamine demethylase (catalyzed by P450IIE1) and pentoxyresorufin O-dealkylase (catalyzed by P450IIB1). In addition, a relatively large decrease (approximately 80%) was observed for the activity of benzphetamine N-demethylase (representative of P450IIC11) and an induction of about 40% was observed for ethoxyresorufin O-dealkylase (catalyzed by P450IA). The metabolism of testosterone by microsomes isolated from the rats chronically treated with MP indicated that several reactions were compromized. Specifically, testosterone 2 alpha-hydroxylase, indicative of P450IIC11, was reduced greatly (86%), whereas testosterone 6 beta-hydroxylase, reflecting P450IIIA, and testosterone 7 alpha-hydroxylase, indicative of P450IIIA1, were affected only slightly by MP treatment (approximately 25%). Immunoblot analyses of the various microsomal samples were performed to determine if chronic MP treatment had direct effects on the level of expression of the cytochromes P450. Decreases in the levels of P450IIIA, IIE1, and IIC11, determined by immunoblot analyses, closely paralleled those observed for their marker catalytic activities. Further studies will be required to determine the mechanism by which MP affects the levels of the cytochromes P450 (i.e. increased degradation or decreased synthesis).
Insights
Chronic methapyrilene (MP) administration significantly alters rat liver enzyme activity, decreasing key cytochrome P450 levels. These changes in metabolic enzymes may explain MP's rat-specific carcinogenicity.
Area of Science:
- Toxicology
- Biochemistry
- Pharmacology
Background:
- Methapyrilene (MP) is a known rat-specific hepatocarcinogen.
- Chronic MP exposure impacts hepatic microsomal enzymes, particularly cytochrome P450 (CYP) isoforms.
- Understanding these effects is crucial for elucidating MP's carcinogenic mechanism.
Purpose of the Study:
- To investigate the chronic effects of MP on hepatic microsomal cytochrome P450 enzyme activities and expression levels in Fischer 344 rats.
- To correlate changes in enzyme activity with alterations in CYP protein levels.
Main Methods:
- Fischer 344 rats were administered varying doses of MP for 10 weeks.
- Microsomal enzymatic activities (e.g., N-demethylase, O-dealkylase) were measured.
- Testosterone metabolism assays were performed.
- Immunoblot analyses were conducted to quantify CYP protein levels.
Main Results:
- Chronic MP treatment significantly decreased total P450 levels and activities of CYPIIIA, CYPIIE1, and CYPIIB1.
- A substantial decrease in CYP2C11 activity (benzphetamine N-demethylase and testosterone 2 alpha-hydroxylase) was observed.
- CYP1A activity (ethoxyresorufin O-dealkylase) was induced, while CYPIIIA-related testosterone hydroxylases showed slight decreases.
- Immunoblot results confirmed reduced protein levels for CYPIIIA, CYP2E1, and CYP2C11, mirroring activity changes.
Conclusions:
- Chronic methapyrilene administration significantly alters the activity and expression of multiple hepatic cytochrome P450 isoforms in rats.
- The observed changes in CYP enzyme profiles provide insights into the metabolic pathways affected by MP.
- Further research is needed to determine whether MP affects CYP P450 levels through increased degradation or decreased synthesis.