Differential regulation of endobiotic-oxidizing cytochromes P450 in vitamin A-deficient male rat liver

1. The hepatic CYP4A-dependent omega-hydroxylation of arachidonic acid and CYP2C11-dependent 2alpha-/16alpha-hydroxylations of testosterone were decreased to 74 and 60% of respective control in microsomal fractions from vitamin A-deficient rats. Decreases in the rates of arachidonic acid omega-1-hydroxylation and testosterone 6beta-, 7alpha- and 17alpha-hydroxylations were less pronounced. 2. Corresponding decreases in microsomal CYP4A and CYP2C11 immunoreactive protein expression to 64 and 68% of respective control were observed in vitamin A-deficient rat liver. Expression of CYP3A proteins was unchanged from vitamin A-adequate control. 3. Northern analysis revealed a selective decrease in CYP4A2 mRNA expression in vitamin A-deficient rat liver to approximately 5% of control; expression of the related CYP4A1/4A3 mRNAs was not decreased. CYP2C11 mRNA expression was also decreased in vitamin A-deficient male rat liver to 39% of control levels. 4. Intake of the deficient diet containing all-trans-retinoic acid (ATRA) during the final week of the experiment restored CYP4A2 mRNA and CYP4A protein. Administration of exogenous androgen or episodic growth hormone was ineffective. In contrast, CYP2C11 expression was restored by ATRA and androgen, but not by growth hormone. 5. From these studies it emerges that CYP4A2, a fatty acid omega-hydroxylase in rat liver, is highly dependent on vitamin A for optimal expression, whereas CYP2C11 is indirectly down regulated by androgen deficiency resulting from vitamin A-deficiency. Altered CYP expression in vitamin A-deficiency provides insights into the relationship between dietary constituents and the intracellular formation of vasoactive eicosanoids as well as the clearance of androgenic steroids.

Down-regulation of rat hepatic microsomal cytochromes P-450 in microvesicular steatosis induced by orotic acid

Microvesicular steatosis is an important component of the overall pathogenesis of drug-mediated liver injury. Although mitochondrial damage has a role in the development of microvesicular steatosis, the consequences of fatty change for hepatic gene function are unclear. The present study was undertaken to evaluate hepatic cytochrome P-450 (CYP) function in a rat model of microvesicular steatosis produced by the intake of diets containing 1% orotic acid (OA) that were administered for 5, 10, or 21 days. Hepatic triglyceride levels were increased to 3-fold of control after 5 days and were elevated further at 10 and 21 days. Cholesterol and phospholipid contents were increased after 10 and 21 days but not by 5 days of feeding. Microsomal androst-4-ene-3,17-dione hydroxylation activities mediated by CYP2C11 (16alpha-hydroxylation) and CYP3A2 (6beta-hydroxylation) were decreased in liver from OA-fed rats for only 5 days, whereas CYP2A1/2-mediated steroid 7alpha-hydroxylation was decreased after 10 days; these observations were complemented by immunoblot analysis that demonstrated the impaired expression of the corresponding CYP proteins. CYP2C11 mRNA, the major CYP in male rat liver, was down-regulated in steatotic liver to 52 +/- 4% of control. Thus, microvesicular steatosis induced by short-term intake of OA-containing diets is histologically similar to that produced by hepatotoxic drugs and produces the rapid down-regulation of constitutive CYPs in rat liver. Analogous processes of lipid deposition in human liver after drug- or disease-related injury could precipitate adverse effects during subsequent drug therapy.

Comparative induction of CYP3A and CYP2B in rat liver by 3-benzoylpyridine and metyrapone

3-Benzoylpyridine (3BP) is a major metabolite of HGG-12, and oxime that has been synthesized as a potential antidote to the toxic effects of soman and other anticholinesterases. Structural similarities exist between 3BP, the cytochrome P450 (CYP)-inducer metyrapone (MET) and other 3-substituted pyridines that interact with CYPs. The present study evaluated the regulatory effects of 3BP on CYP expression in rat liver. Both 3BP and MET (100 mg/kg) increased total hepatic microsomal holo-CYP content significantly 24 h after administration to male rats. Pronounced increases in activities mediated by CYP2B (androstenedione 16 beta-hydroxylation and 7-pentylresorufin O-depentylation) were produced by 3BP and MET, which correlated with respective 9- and 14-fold increases in CYP2B immunoreactive protein. In addition, both agents slightly increased rates of microsomal CYP3A-dependent steroid 6 beta-hydroxylation, troleandomycin metabolite complex formation and total CYP3A immunoreactive protein. Induction of the dexamethasone-inducible CYP3A23 mRNA to 4.5- and 2.5-fold of control was detected in liver of MET- and 3BP-induced rats; CYP3A2 mRNA levels were unchanged. Analogous in vitro studies revealed that MET was a preferential inhibitor of CYP3A-mediated steroid 6 beta-hydroxylation activity, but 3BP was inactive against constitutive steroid hydroxylase CYPs. These findings indicate that the structurally related 3BP and MET elicit similar induction effects on CYPs 2B and 3A23 in rat liver after in vivo administration, but differential inhibitory effects of the chemicals on CYP activity in vitro. Recent reports have implicated a microsomal binding site in the induction of CYP3A1/3A23 in rat liver. In light of the present findings, substituted pyridines like 3BP may be useful tools in structure-activity studies to evaluate the physicochemical requirements for binding to this protein.

Induction of cytochromes P450 2B and 2E1 in rat liver by isomeric picoline N-oxides

Pyridine derivatives are widely used solvents and precursors for the synthesis of chemicals of industrial importance. Oxidized metabolites have been implicated in the observed toxicity of pyridines and are known to induce drug-metabolizing enzymes in rat liver. In this study the three isomeric picoline (methylpyridine) N-oxides, as major oxidized metabolites of 2-, 3- and 4-picoline, were evaluated as inducers of cytochrome P450 (CYP) enzymes in rat liver. After a single dose of 100 mg/kg 24 h before sacrifice the 3- and 4-isomers were effective inducers of microsomal substrate oxidations associated with the phenobarbital-inducible CYPs 2B; upregulation of CYP2B protein was confirmed by immunoblotting. In contrast, the 2-isomer did not increase CYP2B protein or activity in rat liver but CYP2E1 protein expression was upregulated by the isomers to 160-200% of control. The three chemicals increased aniline 4-hydroxylation activity in rat liver, which is consistent with induction of CYPs 2B or 2E1 and 4-nitrophenol 2-hydroxylation activity was increased in microsomal fractions from 3- and 4-picoline N-oxide-treated rats. The activities of several other CYPs were also determined and CYP1A-dependent 7-ethylresorufin O-deethylation was increased (to approximately 6- and 2-fold of control) by the 3- and 4-isomer, respectively, whereas the activity of CYP3A-mediated androstenedione 6beta-hydroxylation was decreased by the agents--most notably by the 2-isomer. During NADPH-supported oxidation of CCl4, lipid peroxidation was increased in microsomes from 3- and 4-picoline N-oxide-pretreated rats and was modulated in vitro by the CYP2B inhibitor orphenadrine, but not by the CYP2E1 inhibitor 4-methylpyrazole. These findings establish that particular isomers of picoline N-oxide rapidly upregulate CYP2B or, to a lesser extent, CYP2E1 and implicate CYP2B in the enhanced lipid peroxidation observed in microsomes from rats treated with 3- and 4-picoline N-oxides. Such induction process may contribute to the hepatotoxicity of pyridines by enhancing the capacity for microsomal lipid peroxidation.