Induction of hepatic microsomal cytochrome P450 and drug-metabolizing enzymes by 4-benzylpyridine and its structurally related compounds in rats. Dose- and sex-related differential induction of cytochrome P450 species

Chemical-induced coordinated and reciprocal changes in heme metabolism, cytochrome P450 synthesis and others in the liver of humans and rodents

A wide variety of drugs and chemicals have been shown to produce induction and inhibition of heme-metabolizing enzymes, and of drug-metabolizing enzymes, including cytochrome P450s (P450s, CYPs), which consist of many molecular species with lower substrate specificity. Such chemically induced enzyme alterations are coordinately or reciprocally regulated through the same and/or different signal transductions. From the toxicological point of view, these enzymatic changes sometimes exacerbate inherited diseases, such as precipitation of porphyrogenic attacks, although the induction of these enzymes is dependent on the animal species in response to the differences in the stimuli of the liver, where they are also metabolized by P450s. Since P450s are hemoproteins, their induction and/or inhibition by chemical compounds could be coordinately accompanied by heme synthesis and/or inhibition. This review will take a retrospective view of research works carried out in our department and current findings on chemical-induced changes in hepatic heme metabolism in many places, together with current knowledge. Specifically, current beneficial aspects of induction of heme oxygenase-1, a rate-limiting heme degradation enzyme, and its relation to reciprocal and coordinated changes in P450s, with special reference to CYP2A5, in the liver are discussed. Mechanistic studies are also summarized in relation to current understanding on these aspects. Emphasis is also paid to an example of a single chemical compound that could cause various changes by mediating multiple signal transduction systems. Current toxicological studies have been developing by utilizing a sophisticated "omics" technology and survey integrated changes in the tissues produced by the administration of a chemical, even in time- and dose-dependent manners. Toxicological studies are generally carried out step by step to determine and elucidate mechanisms produced by drugs and chemicals. Such approaches are correct; however, current "omics" technology can clarify overall changes occurring in the cells and tissues after treating animals with drugs and chemicals, integrate them and discuss the results. In the present review, we will discuss chemical-induced similar changes of heme synthesis and degradation, and of P450s and finally convergence to similar or different directions.

Induction and inhibition of cytochrome P450 and drug-metabolizing enzymes by climbazole

To determine the effect of climbazole on hepatic microsomal cytochrome P450 (P450) and drug-metabolizing enzymes, four different P450 isoforms (CYP2B1, 3A2, 2E1, and 2C12) were examined in female Long-Evans rats. Treatment of rats with climbazole resulted in the induction of P450 content. Climbazole both induced and inhibited aminopyrine N-demethylase activity, but not erythromycin N-demethylase activity. Uridine 5'-phosphate (UDP)-glucuronosyl transferase and glutathione S-transferase activities were also increased with climbazole treatment. Immunoblot analyses revealed that climbazole induces CYP2B1 and CYP3A2 at the lower dose examined, but it failed to increase CYP2B1 at the higher dose. Northern blot analysis revealed that climbazole markedly increases P450 2B1 mRNA. These results indicate that climbazole induces and inhibits P450-dependent drug-metabolizing enzymes in vivo and may have the dose-differential effect on CYP2B1 in rat liver.

Climbazole is a new potent inducer of rat hepatic cytochrome P450

We examined the effect of climbazole on the induction of rat hepatic microsomal cytochrome P450 (P450), and compared the induction potency with other N-substituted azole drugs such as clorimazole. We found that climbazole is found to be a potent inducer of rat hepatic microsomal P450 as clorimazole. Induced level of P450 by climbazole was almost similar in extent to clorimazole when compared with other imidazole drugs in a dose- and time-dependent manner. Parallel to the increase in P450, climbazole increased aminopyrine and erythromycin N-demethylase, ethoxycoumarin O-deethylase, and androstenedione 16 beta- and 15 alpha/6 beta hydroxylase activities; however, clorimazole did not induce aminopyrine N-demethylase activity irrespective of its marked increase in P450 content. Immunoblot analyses revealed that climbazole induced CYP2B1, 3A2 and 4A1. The present findings indicate that climbazole is a new potent inducer of hepatic microsomal P450 and drug-metabolizing enzymes like clorimazole, but it may have some differential mechanism(s) for these enzymes' induction in rat liver.

Effect of 4-(4-chlorobenzyl)pyridine on rat hepatic microsomal cytochrome P450 and drug-metabolizing enzymes in vivo and in vitro

The effect of 4-(4-chlorobenzyl)pyridine (4-CBP) on rat hepatic microsomal cytochrome P450 (P450) and its molecular species (CYP2B1, 2E1, 3A2, 2C11, and 2C12), and on drug-metabolizing enzyme activities were examined in vivo and in vitro. Treatment of rats with 4-CBP resulted in the induction of P450 and drug-metabolizing enzymes in a dose-dependent manner, but it was markedly inhibitory at higher dose levels. Immunoblot analyses revealed that 4-CBP induces both CYP2B1 and 2E1; however, both were decreased by increasing the dose of 4-CBP. The in vitro inhibitory experiment revealed that 4-CBP strongly inhibited benzphetamine N-demethylase activity, but not dimethylnitrosamine N-demethylase activity. The present findings provide information on the induction and inhibition effect of chlorinated benzylpyridine on hepatic microsomal P450s and drug-metabolizing enzymes in vivo and in vitro.

Concurrent induction of rat hepatic microsomal cytochrome P450 and haem oxygenase by 2,2'-dipyridyl ketone: comparison with the effect of 2,2'-dipyridyl amine

1. The effect of 2,2'-dipyridyl ketone and 2,2'-dipyridyl amine on the induction of hepatic microsomal cytochrome P450 (P450) and heme oxygenase was compared, and their effects on five different P450 isoforms (P4501A1, 3A2, 2B1, 2E1 and 2C11) in rat were examined. 2. Treatment of rat with 2,2'-dipyridyL amine resulted in the marked induction of haem oxygenase to about seven-fold of the controls with a decrease in p450 content. 2,2'-Dipyridyl ketone produced concomitant induction of both P450 and haem oxygenase activity in a dose- and time-dependent manner without showing any sex differences. 3. Immunoblot analysis revealed that 2,2'-dipyridyl ketone slightly increased CYP2E1 and CYP3A2 at low doses, but not at high dose levels. There was no effect on P4502C11. P4502B1 was induced by the treatment with 2,2'-dipyridyl ketone in a dose-dependent manner. 4. These results indicate that dipyridyl compounds having different bridges between two aromatic moieties act as differential inducers of hepatic microsomal P450s and haem oxygenase.

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.

Induction of hepatic microsomal P450 by 4-phenylalkylpyridines in rat: chain length-dependent and sex-related differential induction of P450s

The effects of 4-phenylalkylpyridines (chain length of 0-5, 7, 9 and 11 carbon atoms) on the induction of hepatic microsomal P450 and its multiple forms (1A1/2 (1A1, 1A2), 3A2, 2B1/2, 2C6 and 2E1) in the male and female rat have been investigated. P450 induction gradually declined with increasing chain length of the 4-phenylalkylpyridines. Immunoblot analysis revealed that three pyridine compounds having methylene units of 0, 1 and 2 only induced P4501A2, whereas those having 4, 5 or 7 methylene units only induced 1A1 in the male rat. In the female rat, however, we could not observe such a chain length-dependent differential induction of the P4501A subfamily. Induced levels of P4503A2 and 2E1 were dependent on total P450 specific content, but 2C6 was increased in a chain length-dependent manner in both sexes. These results provide new information on the differential effects of pyridine-containing compounds on P450 induction in the rat.

Differential effects of 3 dipyridyl isomers on hepatic microsomal cytochrome P450 and heme oxygenase in rats

We compared the effects of 3 dipyridyl isomers, 2,2'-dipyridyl, 2,4'-dipyridyl and 4,4'-dipyridyl, on hepatic microsomal heme oxygenase and drug-metabolizing enzyme activities in male rats. 2,2'-Dipyridyl increased cytochrome P450 (P450) content at lower doses, but decreased with increasing dose levels. Immunoblot analysis revealed that 2,2'-dipyridyl did not induce both P450 1A1/2 and P450 2B1/2, in contrast to 2,4'- and 4,4'-dipyridyls, both of which were inducers of either P450 1A1/2 and/or P450 2B1/2. Some drug-metabolizing enzyme activities gradually declined with the increasing dose level of 2,2'-dipyridyl. 2,2'-Dipyridyl was able to induce hepatic microsomal heme oxygenase in a dose-dependent manner, but 2,4'- and 4,4'-dipyridyls did not, even at the highest dose (0.80 mmol/kg) examined. Treatment of rats with 2,2'-dipyridyl resulted in the increase of glutathione (GSH) content in a dose-dependent manner, but not 4-substituted isomers. A time course study with 2,2'-dipyridyl revealed that it produced a significant decrease in hepatic GSH content at early time periods (2-6 h) after its administration with an inverse increase in heme oxygenase activity. The present investigation has revealed that in contrast to the induction of P450 by 4-substituted dipyridyl compounds, 2,2'-dipyridyl is a novel inducer of hepatic microsomal heme oxygenase, together with the change in hepatic GSH content. This study would provide information on the differential effects of simple dipyridyl isomers on hepatic enzymes involved in heme and drug metabolism.

Involvement of testosterone in the induction of hepatic microsomal cytochrome P-450 2B1/2 (P-450 2B1/2) by 1-benzylimidazole in male and female rats: sex-differentiated induction of P-450 2B1/2 species

We examined the effect of 1-benzylimidazole on the induction of cytochrome P-450 1A1/2 (P-450 1A1/2) and cytochrome P-450 2B1/2 (P-450 2B1/2) in normal, castrated, ovariectomized and hypophysectomized male rats, and in castrated rats treated with testosterone. 1-Benzylimidazole markedly increased P-450 content in male and female rats. Parallel to the dose-dependent increase in P-450 content, 1-benzylimidazole produced a significant increase in P-450 2B1/2 in male rats, but not in female rats. 1-Benzylimidazole failed to induce P-450 2B1/2 in castrated male and ovariectomized female rats. Treatment of castrated male rats with testosterone restored the induction of P-450 2B1/2 by 1-benzylimidazole. Treatment of ovariectomized female rats with 1-benzylimidazole or phenobarbital led to the increase in P-450 content, accompanying by the induction of P-450 2B1/2 by the latter treatment, but not the former. In hypophysectomized male rats, 1-benzylimidazole was able to induce P-450 2B1/2 in contrast to castrated male rats. Neonatal male and female rats responded well to the induction of P-450 2B1/2 by 1-benzylimidazole. The present findings suggest that P-450 2B1/2 induction by 1-benzylimidazole would be coupled with circulating testosterone regulated by hypophysis-testis axis. 1-Benzylimidazole produced sex-differentiated induction of P-450 2B1/2 in pubertal rats, but not in neonatal animals. The present findings would be provide information on a unique effect of 1-benzylimidazole on P-450 2B1/2 induction in rats.

Acute N-(3,5-dichlorophenyl)succinimide nephrotoxicity in female Fischer 344 rats

The agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is an established nephrotoxicant in male Fischer 344 rats at i.p. doses of > or = mmol/kg. Since gender differences often exist in the susceptibility to toxicants, the nephrotoxic potential of NDPS was examined in female Fischer 344 rats. Rats (4-5/group) were administered NDPS (0.1, 0.2, 0.4, or 1.0 mmol/kg, i.p.) or vehicle (sesame oil, 2.5 ml/kg) and renal function monitored for 48 h. At a dose of 0.1 mmol/kg, NDPS had no effect on renal function. However, administration of NDPS at a dose of 0.2 or 0.4 mmol/kg resulted in marked nephrotoxicity characterized by diuresis, increased proteinuria, glucosuria, hematuria, elevated blood urea nitrogen (BUN) concentration and kidney weight, decreased organic ion accumulation and proximal tubular necrosis. NDPS treatment of 1.0 mmol/kg resulted in oliguric renal failure rather than polyuric renal failure in 3 of 4 rats. Proximal tubular damage was observed primarily in the S3 segment of the proximal tubule in NDPS-treated female rats, while in male rats the S1 and S2 segments are the initial renal targets. These results demonstrate that female Fischer 344 rats are more susceptible to NDPS nephrotoxicity than male Fischer 344 rats and that the site of the renal lesion is gender dependent.