Dissociation of xanthine oxidase induction and cytochrome P450 depression during interferon induction in the rat

The effect of interferon-α on the expression of cytochrome P450 3A4 in human hepatoma cells

Interferon α (IFNα) is used to treat malignancies and chronic viral infections. It has been found to decrease the rate of drug metabolism by acting on cytochrome P450 enzymes, but no studies have investigated the consequences of IFNα treatment on the CYP3A4 isoform, responsible for the metabolism of a majority of drugs. In this study, we have examined the effect of IFNα on CYP3A4 catalytic activity and expression in human hepatoma cells. We found that IFNα inhibits CYP3A4 activity and rapidly down-regulates the expression of CYP3A4, independent of de novo protein synthesis. Pharmacologic inhibitors and a dominant-negative mutant expression plasmid were used to dissect the molecular pathway required for CYP3A4 suppression, revealing roles for Jak1 and Stat1 and eliminating the involvement of the p38 mitogen-activated and extracellular regulated kinases. Treatment of hepatoma cells with IFNα did not affect the nuclear localization or relative abundance of Sp1 and Sp3 transcription factors, suggesting that the suppression of CYP3A4 by IFNα does not result from inhibitory Sp3 out-competing Sp1. To our knowledge, this is the first report that IFNα down-regulates CYP3A4 expression largely through the JAK-STAT pathway. Since IFNα suppresses CYP3A4 expression, caution is warranted when IFNα is administered in combination with CYP3A4 substrates to avoid the occurrence of adverse drug interactions.

Variability in drug metabolizing enzyme activity in HIV-infected patients

AIMS

To evaluate variability in cytochrome P450 (CYP) 1A2, CYP2D6, CYP3A, N-acetyltransferase 2 (NAT2), and xanthine oxidase (XO) activity in HIV-infected patients and compare this with data from uninfected, healthy volunteers.

METHODS

Ten HIV-infected men and seven women on medication affecting CYP enzyme activity were phenotyped four times over 2 months using caffeine, dextromethorphan, and midazolam. Urinary caffeine and dextromethorphan metabolite ratios were used to phenotype CYP1A2, NAT2, XO, and CYP2D6 activity and midazolam plasma clearance was used to phenotype CYP3A activity. Plasma and urine samples were analyzed by validated LC/UV or LC/MS methods for midazolam, caffeine, and dextromethorphan. Noncompartmental pharmacokinetics and nonparametric statistical analyses were performed, and the data compared with those of healthy volunteer historic controls.

RESULTS

Compared with age and sex-matched healthy volunteers, HIV-infected subjects had 18% lower hepatic CYP3A4 activity, 90% lower CYP2D6 activity, 53% lower NAT2 activity, and 22% higher XO activity. No significant difference was found in CYP1A2 activity. Additionally, 25% genotype-phenotype discordance in CYP2D6 activity was noted in HIV-infected subjects. Intraindividual variability in enzyme activity increased by 42-62% in HIV-infected patients for CYP1A2, NAT2, and XO, and decreased by 33% for CYP2D6. Interindividual variability in enzyme activity increased by 27-63% in HIV-infected subjects for CYP2D6, CYP1A2, and XO, and decreased by 38% for NAT2. Higher plasma TNFalpha concentrations correlated with lower CYP2D6 and CYP3A4 activity.

CONCLUSIONS

Infection with HIV or stage of HIV infection may alter Phase I and II drug metabolizing enzyme activity. HIV infection was related to an increase in variability of these drug-metabolizing enzymes. Altered metabolism may be a consequence of immune activation and cytokine exposure.

Alteration of drug biotransformation and elimination during infection and inflammation

During infection or inflammation, the expression of cytochrome P450 and its dependent biotransformation pathways are modified. This results in a change in the capacity of the liver to handle drugs and in alterations in the production and elimination of endogenous substances throughout the body. The majority of the CYP isoforms are modified at pre-translational steps in protein synthesis, and, in most cases, cytokines are involved as mediators of the response. Recent information suggests that inflammatory responses that are localized to the CNS cause a loss of CYP within the brain. This is accompanied by a parallel down-regulation of CYP in peripheral organs that is mediated by a signaling pathway between the brain and periphery. This review covers the loss that occurs in the major mammalian CYP families in response to infection/inflammation and the mediator pathways that are key to this response.

Hepatic drug metabolism and immunostimulation

When host defence mechanisms are stimulated there is a concomitant decrease in cytochrome P450 based drug biotransformation and elimination. This has resulted in a number of clinically important unwanted drug responses in patients with infections or inflammatory responses. The loss in cytochrome P450 is predominantly an effect at the level of the gene expression and the majority of enzyme forms examined to date are involved. Although the effect occurs predominantly in the liver it has been recently shown that inflammatory responses in the brain also cause a loss of the same enzyme forms in that organ. The loss of cytochrome P450 in the brain in response to localised inflammation is accompanied by a similar loss in the liver. The decrease of cytochrome P450 and its dependent drug biotransformation is of concern whenever drugs are used in patients with infections or disease states with an inflammatory component.

Effects of Plasmodium berghei infection on cytochromes P-450 2E1 and 3A2

Metabolism and disposition of most drugs used to treat malaria are substantially altered in malaria infection. Few data are available that specify effects of malaria infection on drug metabolism pathways in humans or animal model systems. In this report, studies were undertaken to determine the effect of Plasmodium berghei infection on cytochrome P-450 (CYP450) 2E1 and 3A2-mediated metabolism and enzyme expression in rat liver microsomes. Malaria infection (MAL) resulted in significant decreases in total cytochrome P-450 content (56%, P < 0.05) and NADPH cytochrome P-450 reductase activity (32%, P < 0.05) as compared to control (CON) rats. Chlorzoxazone 4-hydroxylase activity (CYP2E1-mediated) showed no significant difference between CON and MAL microsomes while testosterone 6-beta-hydroxylase activity (CYP3A2-mediated) was reduced by 41% (P < 0.05) in MAL. Enzyme kinetic studies and immunoblot analysis indicate that the loss of activity for CYP3A2 in malaria infection is due to significantly decreased CYP3A2 protein expression. The altered expression of CYP450s in malaria infection should be taken into account when treating patients with malaria in order to minimize drug-drug interactions or toxicity.

Depression of the hepatic cytochrome P450 by an acute inflammatory reaction: characterization of the nature of mediators in human and rabbit serum, and in the liver

There is increasing evidence suggesting that several mediators are involved in the cascade of events leading to the depression of the cytochrome P450 (P450) by an inflammatory reaction. The present study aimed to confirm the presence of mediators in the serum (RS(INFLA)) and hepatocytes (H(INFLA)) of rabbits with an acute inflammatory reaction, and in the serum of humans with an acute upper respiratory tract viral infection (HS(URTVI)). The inflammatory reaction was induced by the s.c. injection of 5 ml of turpentine. Incubation of RS(INFLA) or HS(URTVI) with H(INFLA) depressed the P450, diminished the formation of theophylline metabolites (3-methylxanthine, 1-methyluric acid, and 1,3-dimethyluric acid), and increased lipid peroxidation. The addition of preheated RS(INFLA) or HS(URTVI) to H(INFLA) did not diminish the amount of P450 or theophylline metabolites, and prevented the increase in lipid peroxidation. Incubating the filtrate of RS(INFLA) or HS(URTVI) dialyzed through membranes with cut-off of 10, 30, 50 and 100 kd, with H(INFLA) showed that rabbit and human mediators have molecular weights ranging from 10 to 30 kd. Incubation of H(INFLA) with hepatocytes from control rabbits (H(CONT)) did not decrease further the P450. However, when RS(INFLA) was added to co-cultured H(CONT) + H(INFLA), the depression of P450 was 37% greater (p<0.05), and the amount of theophylline metabolites generated was around 30% (p<0.05) smaller than that observed when H(CONT) or H(INFLA) were incubated with RS(INFLA). Based on the present results we may speculate that human and rabbit serum mediators are proteins of molecular weights ranging from 10 to 30 kd, and in addition, primed hepatocytes once exposed to the serum mediators release mediators able to depress the P450 in H(CONT).

Xanthine oxidoreductase in human mammary epithelial cells: activation in response to inflammatory cytokines

Xanthine oxidoreductase (XOR) in human mammary epithelial cells was shown to have low true specific activity, similar to that in breast milk. Enzymic activity was increased in response to inflammatory cytokines; increases of 2-2.5-fold being seen with TNF-alpha and IL-1beta and of approximately 8-fold with IFN-gamma. No significant increase was seen with IL-6. A combination of IFN-gamma and TNF-alpha, or of these two cytokines plus IL-1beta, led to responses representing the sum of those obtained by using the individual cytokines. The 8-fold increase in enzymic activity, stimulated by IFN-gamma, corresponded to only a 2-3-fold increase in specific mRNA, suggesting the possibility of post-translational activation; a possibility strongly supported by the corresponding 2-3-fold rise in XOR protein, as determined by ELISA. In no case was cytokine-induced activation accompanied by changes in the oxidase-dehydrogenase ratio of XOR. These data strongly support a role for XOR in the inflammatory response of the human mammary epithelial cell, and provide further evidence of post-translational activation of a low activity form of human XOR, similar to that previously observed in vivo for the breast milk enzyme.

Effect of zidovudine therapy in patients with HIV infection on endogenous interferon plasma levels and the hepatic cytochrome P450 enzyme system

In this study, we wanted to investigate if there are differences in endogenous interferon (IFN) plasma levels in patients with different stages of HIV infections before and after therapy with zidovudine (ZDV) and determined the influence of ZDV therapy on the hepatic monooxygenase system by measuring the antipyrine pharmacokinetics. Therefore we investigated the endogenous IFN plasma levels in patients with asymptomatic HIV infection (CDC/WHO A1, n = 10) and patients with AIDS (CDC/WHO C3, n = 10). In AIDS plasma IFN-alpha and IFN-gamma levels are elevated (15.6 +/- 5.8 U/ml; 2.1 +/- 0.7 U/ml) compared to patients with an asymptomatic HIV infection (6.1 +/- 3.3 U/ml; 0.6 +/- 0.3 U/ml). The antipyrine clearance was significantly reduced in the group of AIDS patients (43.1 +/- 7.2 ml/min compared to 56.4 +/- 8.7 ml/min). In a second study with 11 patients in stage CDC/WHO A1/2 and CDC/ WHO B/C3 each, we studied the effect of a 14-day administration of ZDV on the endogenous plasma IFN levels and the CYP450 enzyme activity using the antipyrine pharmacokinetics as a parameter. We investigated the antipyrine clearance, clearance to metabolite and half-life by using HPLC. IFNs were measured by RIA or ELISA, respectively. In the first group no significant alterations of antipyrine kinetics or plasma IFN levels were observed after treatment with ZDV. In contrast to these results, we found a significant decrease in IFN-alpha and IFN-gamma (19.8 +/- 3.6 U/ml, 4.6 +/- 1.5 U/ml before; 7.9 +/- 2.6 U/ml, 1.9 +/- 1.3 U/ml after administration of ZDV), a decrease in antipyrine half-life, an elevation of the antipyrine clearance (49.8 +/- 15.7 ml/min, 57.3 +/- 13.7 ml/min) and an elevation of the clearances to metabolite.

Regulation of cytochromes P450 during inflammation and infection

Hepatic P450 activities are profoundly affected by various infectious and inflammatory stimuli, and this has clinical and toxicological consequences. Whereas the expression of most P450s in the liver is suppressed, some are induced. Many of the effects observed in vivo can be mimicked by pro-inflammatory cytokines and IFNs, and P450s are differentially regulated by these agents. Therefore, different cytokine profiles and concentrations in the vicinity of the hepatocyte in different models of inflammation may result in qualitatively and quantitatively different effects on populations of P450s. In addition to cytokines, glucocorticoids may have an important role in P450 regulation in stress conditions, including that caused by inflammatory stimuli. Although in many cases the decreases in activity are due primarily to a down-regulation of P450 gene transcription, it is likely that modulation of RNA and protein turnover, as well as enzyme inhibition, contributes to some of the observed effects. The mechanisms whereby these effects are produced may also vary with both the P450 under study and the time course of the effect. The complexity of the P450 response to inflammation and infection means that all of the above factors must be considered when trying to predict the effect of a given infectious or inflammatory condition on the clinical or toxic response of humans or animals to an administered drug or toxin. The question of whether the down-regulation of the hepatic P450 system to inflammation or infection is a homeostatic or pathological response cannot be answered at present. It is difficult to discern the physiological benefit of reducing hepatic P450 activities, unless it is to prevent the generation of reactive oxygen species generated by uncoupled catalytic turnover of the enzymes. On the other hand, as we proposed some years ago [64], the suppression of P450 may be due to the liver's need to utilize its transcriptional machinery and energy for the synthesis of APPs involved in the inflammatory response. In that case, one could ask why the organism has gone to the trouble of employing differential mechanisms for suppression of P450. One answer could be that the response evolved after the divergence of many of the P450 genes, necessitating the evolution of multiple redundant mechanisms for P450 suppression. In contrast to the down-regulation of P450s in the liver, the induction of several forms in this and other tissues suggests a more specific homeostatic role of these effects, e.g., in generation or catabolism of bioactive metabolites.

Regulation of CYP4A1 and peroxisome proliferator-activated receptor alpha expression by interleukin-1beta, interleukin-6, and dexamethasone in cultured fetal rat hepatocytes

The CYP4A1 isoenzyme induced in rodents by peroxisome proliferators is known to be repressed at a pretranslational level by interferon. Interleukin-1beta (IL-1beta) also reduces CYP4A1-related 12-laurate hydroxylase activity in cultured fetal rat hepatocytes after induction by clofibric acid. In this fetal hepatocyte model, IL-1beta and interleukin-6 (IL-6) were tested for their ability to reduce 12-laurate hydroxylase activity, CYP4A1 apoprotein content, and the CYP4A1 mRNA level. IL-1beta and IL-6 strongly diminished CYP4A1 activity and apoprotein and mRNA levels in a dose- and time-dependent manner. CYP4A1 expression is thus down-regulated at a pretranslational level by these cytokines. As it has been shown that the peroxisome proliferator-activated receptor alpha (PPAR alpha) mediates the induction of the CYP4A1 gene by a peroxisome proliferator, the capacity of IL-1beta or IL-6 to modulate the PPAR alpha mRNA level was tested. It was found that IL-1beta and IL-6 both repress the induction of PPAR alpha expression exerted by the combined action of clofibric acid and dexamethasone. However, even at the highest concentration (10 ng/mL) tested for both cytokines, IL-1beta as well as IL-6 failed to abolish the induction of CYP4A1 by dexamethasone. The mechanism of the protective effect of the synthetic glucocorticoid on CYP4A1 repression by interleukins is discussed.

Poly ICLC enhances the antimalarial activity of chloroquine against multidrug-resistant Plasmodium yoelii nigeriensis in mice

Swiss mice infected with multidrug-resistant Plasmodium yoelii nigeriensis were treated with polyinosinic-polycytidylic acid stabilized with polylysine and carboxymethyl cellulose (Poly ICLC), a potent interferon (IFN) inducer and immune enhancer, in combination with chloroquine (CQ), which completely eliminated the malaria parasite from these animals. The enhancement of the antimalarial activity of poly ICLC was found to be completely reversed by the cytochrome P-450 inducer, phenobarbitone. No effect of Nw nitro-L-arginine (NLA), an inhibitor of nitric oxide, was seen on the enhancement of the antimalarial activity of CQ by Poly ICLC. These results suggest the possible involvement of cytochrome P-450 enzyme-mediated mechanism in the enhancement of the antimalarial activity of CQ by Poly ICLC.

Effects of interferon-gamma and streptolysin O on hepatic procainamide N-acetyltransferase and various microsomal cytochrome P450-dependent monooxygenases in rats

Immunostimulants known to initiate cytokine production were found to inhibit processes of microsomal drug oxidation but to activate arylamine N-acetylation. The present study investigated the effects of immunstimulating doses of rat interferon-gamma (IFN gamma, 670,000 units ip) and streptolysin O (SLO, 100 HU/kg iv for 5 days) on hepatic cytosolic N-acetyltransferase (NAT) and microsomal cytochrome P450 (CYP)-dependent monooxygenases in male Wistar rats. Both IFN gamma and SLO activated NAT to 120% (P < 0.05) and 135% (P < 0.05), respectively. As expected, monooxygenases were depressed by IFN gamma (P < 0.05) and SLO, the ethylresorufin O-deethylase being the most susceptible enzyme. The results suggested that not only the toxin of gram-positive streptococcal bacteria SLO, but also the cytokine IFN gamma can stimulate NAT activity in rat hepatic cytosol. While the enhancing SLO effect on NAT could not be neutralized by the inhibitor of transcription actinomycin D, NAT stimulation by IFN gamma was abolished by actinomycin D and by the inhibitor of translation, cycloheximide. Obviously, SLO activated NAT independent of protein synthesis and different from IFN gamma-mediated pathways. Posttranslational processes might be involved in NAT stimulation in the rats.

Effects of phenobarbital and interleukin-6 on cytochrome P4502B1 and 2B2 in cultured rat hepatocytes

The objectives of this study were to characterize further the effects of phenobarbital (PB) on cytochrome P4502B1 and 2B2 (P4502B1/2) enzyme activity and immunoreactivity in rat hepatocytes and to investigate the mechanism(s) mediating the ability of interleukin-6 (IL-6) to inhibit this induction. PB caused a concentration-dependent increase in benzyloxyresorufin O-deethylase (BROD) activity with maximal effects (a 25-fold increase) at concentrations of 0.3 to 1 mM. The induction of BROD activity was linear over 24 hr of exposure. Immunoblot profiles of P4502B1/2 agreed with measurements of enzyme activity. In addition to inducing P4502B1/2, PB (0.75 mM) also increased the levels of P450 reductase by approximately 2-fold following a 24-hr exposure to PB. When IL-6 was added concomitantly with or up to 12 hr after the addition of PB, the PB induction of BROD activity and immunoreactivity was inhibited significantly. When 18 hr elapsed between the time of addition of PB and IL-6, the inhibitory effects of IL-6 were no longer apparent, suggesting that the actions of IL-6 were mediated by early events in the induction process. IL-6 did not affect the PB induction of P450 reductase. To determine whether IL-6 altered the degradation of P4502B1/2, hepatocytes were exposed to PB for 24 hr, then washed, and the loss of BROD activity and immunoreactivity following incubation with a protein synthesis inhibitor was measured. IL-6 did not alter the rate of loss of either enzyme activity or immunoreactivity, indicating that the effects of IL-6 could not be attributed to the enhanced degradation of P4502B1/2. Results suggest that the inhibition of PB-induced BROD activity by IL-6 is due to an action on early cellular and molecular events in the induction process.

Modulation of hepatic mRNA levels after administration of lipopolysaccharide and diphtheria and tetanus toxoids and pertussis vaccine adsorbed (DTP vaccine) to mice

Administration of whole-cell diphtheria and tetanus toxoids and pertussis vaccine adsorbed (DTP vaccine) caused marked depression in the expression of mRNA for isozymes of cytochrome P-450 in the livers of endotoxin-responsive and nonresponsive mice. The levels of expression of mRNA for a polycyclic aromatic hydrocarbon-inducible (CYP1A2) and an ethanol-inducible (CYP2E1) form of P-450 were reduced by 70% to 80% 8 to 12 hr after vaccination or Bordetella pertussis endotoxin administration. These effects are preceded by marked increases (threefold to sixfold) in mRNA expression for interleukin-6, interleukin-1 and tumor necrosis factor in both strains of mice, with maximal increases 1 to 2 hr after injection. This is the first demonstration that levels of cytokine mRNA are altered in the liver in response to DTP vaccine administration. The finding of increased cytokine mRNA in the livers of mice injected with vaccine supports a role for cytokines as mediators of the decreased levels of cytochrome P-450. In addition, inducible nitric oxide synthase mRNA expression is also increased after vaccine administration, with a peak at 4 hr. The temporal relationship of the increased cytokine mRNA expression, increased nitric oxide synthase and decreased expression of P-450 mRNAs suggests a mechanism by which cytokines mediate the induction of nitric oxide synthase, which increases nitric oxide and decreases the activities of some cytochromes P-450.