Phosphorylation/Dephosphorylation steps are crucial for the induction of CYP2B1 and CYP2B2 gene expression by phenobarbital

Effects of marine biotoxins on drug-metabolizing cytochrome P450 enzymes and their regulation in mammalian cells

Marine biotoxins are a heterogenous group of natural toxins, which are able to trigger different types of toxicological responses in animals and humans. Health effects arising from exposure to marine biotoxins are ranging, for example, from gastrointestinal symptoms to neurological effects, depending on the individual toxin(s) ingested. Recent research has shown that the marine biotoxin okadaic acid (OA) can strongly diminish the expression of drug-metabolizing cytochrome P450 (CYP) enzymes in human liver cells by a mechanism involving proinflammatory signaling. By doing so, OA may interfere with the metabolic barrier function of liver and intestine, and thus alter the toxico- or pharmacokinetic properties of other compounds. Such effects of marine biotoxins on drug and xenobiotic metabolism have, however, not been much in the focus of research yet. In this review, we present the current knowledge on the effects of marine biotoxins on CYP enzymes in mammalian cells. In addition, the role of CYP-regulating nuclear receptors as well as inflammatory signaling in the regulation of CYPs by marine biotoxins is discussed. Strong evidence is available for effects of OA on CYP enzymes, along with information about possible molecular mechanisms. For other marine biotoxins, knowledge on effects on drug metabolism, however, is scarce.

Nuclear receptor phosphorylation in xenobiotic signal transduction

Nuclear pregnane X receptor (PXR, NR1I2) and constitutive active/androstane receptor (CAR, NR1I3) are nuclear receptors characterized in 1998 by their capability to respond to xenobiotics and activate cytochrome P450 (CYP) genes. An anti-epileptic drug, phenobarbital (PB), activates CAR and its target CYP2B genes, whereas PXR is activated by drugs such as rifampicin and statins for the CYP3A genes. Inevitably, both nuclear receptors have been investigated as ligand-activated nuclear receptors by identifying and characterizing xenobiotics and therapeutics that directly bind CAR and/or PXR to activate them. However, PB, which does not bind CAR directly, presented an alternative research avenue for an indirect ligand-mediated nuclear receptor activation mechanism: phosphorylation-mediated signal regulation. This review summarizes phosphorylation-based mechanisms utilized by xenobiotics to elicit cell signaling. First, the review presents how PB activates CAR (and other nuclear receptors) through a conserved phosphorylation motif located between two zinc fingers within its DNA-binding domain. PB-regulated phosphorylation at this motif enables nuclear receptors to form communication networks, integrating their functions. Next, the review discusses xenobiotic-induced PXR activation in the absence of the conserved DNA-binding domain phosphorylation motif. In this case, phosphorylation occurs at a motif located within the ligand-binding domain to transduce cell signaling that regulates hepatic energy metabolism. Finally, the review delves into the implications of xenobiotic-induced signaling through phosphorylation in disease development and progression.

Epidermal Growth Factor Represses Constitutive Androstane Receptor Expression in Primary Human Hepatocytes and Favors Regulation by Pregnane X Receptor

Growth factors have key roles in liver physiology and pathology, particularly by promoting cell proliferation and growth. Recently, it has been shown that in mouse hepatocytes, epidermal growth factor receptor (EGFR) plays a crucial role in the activation of the xenosensor constitutive androstane receptor (CAR) by the antiepileptic drug phenobarbital. Due to the species selectivity of CAR signaling, here we investigated epidermal growth factor (EGF) role in CAR signaling in primary human hepatocytes. Primary human hepatocytes were incubated with CITCO, a human CAR agonist, or with phenobarbital, an indirect CAR activator, in the presence or absence of EGF. CAR-dependent gene expression modulation and PXR involvement in these responses were assessed upon siRNA-based silencing of the genes that encode CAR and PXR. EGF significantly reduced CAR expression and prevented gene induction by CITCO and, to a lower extent, by phenobarbital. In the absence of EGF, phenobarbital and CITCO modulated the expression of 144 and 111 genes, respectively, in primary human hepatocytes. Among these genes, only 15 were regulated by CITCO and one by phenobarbital in a CAR-dependent manner. Conversely, in the presence of EGF, CITCO and phenobarbital modulated gene expression only in a CAR-independent and PXR-dependent manner. Overall, our findings suggest that in primary human hepatocytes, EGF suppresses specifically CAR signaling mainly through transcriptional regulation and drives the xenobiotic response toward a pregnane X receptor (PXR)-mediated mechanism.

Transdifferentiated rat pancreatic progenitor cells (AR42J-B13/H) respond to phenobarbital in a rat hepatocyte-specific manner

Phenobarbital (PB) is known to produce species-specific effects in the rat and mouse, being carcinogenic in certain mouse strains, but only in rats if treated after a DNA damaging event. PB treatment in the rat and mouse also produces disparate effects on cell signalling and miRNA expression profiles. These responses are induced by short term and prolonged PB exposure, respectively, with the latter treatments being difficult to examine mechanistically in primary hepatocytes due to rapid loss of the original hepatic phenotype and limited sustainability in culture. Here we explore the rat hepatocyte-like B13/H cell line as a model for hepatic response to PB exposure in both short-term and longer duration treatments. We demonstrate that PB with Egf treatment in the B13/H cells resulted in a significant increase in Erk activation, as determined by the ratio of phospho-Erk to total Erk, compared to Egf alone. We also show that an extended treatment with PB in the B13/H cells produces a miRNA response similar to that seen in the rat in vivo, via the time-dependent induction of miR-182/96. Additionally, we confirm that B13/H cells respond to Car activators in a typical rat-specific manner. These data suggest that the B13/H cells produce temporal responses to PB that are comparable to those reported in short-term primary rat hepatocyte cultures and in the longer term are similar to those in the rat in vivo. Finally, we also show that Car-associated miR-122 expression is decreased by PB treatment in B13/H cells, a PB-induced response that is common to the rat, mouse and human. We conclude that the B13/H cell system produces a qualitative response comparable to the rat, which is different to the response in the mouse, and that this model could be a useful tool for exploring the functional consequences of PB-sensitive miRNA changes and resistance to PB-mediated tumours in the rat.

Phenobarbital and Insulin Reciprocate Activation of the Nuclear Receptor Constitutive Androstane Receptor through the Insulin Receptor

Phenobarbital (PB) antagonized insulin to inactivate the insulin receptor and attenuated the insulin receptor downstream protein kinase B (AKT)-forkhead box protein O1 and extracellular signal-regulated kinase 1/2 signals in mouse primary hepatocytes and HepG2 cells. Hepatic AKT began dephosphorylation in an early stage of PB treatment, and blood glucose levels transiently increased in both wild-type and constitutive androstane receptor (CAR) knockout (KO) mice. On the other hand, blood glucose levels increased in wild-type mice, but not KO mice, in later stages of PB treatment. As a result, PB, acting as an insulin receptor antagonist, elicited CAR-independent increases and CAR-dependent decreases of blood glucose levels at these different stages of treatment, respectively. Reciprocally, insulin activation of the insulin receptor repressed CAR activation and induction of its target CYP2B6 gene in HepG2 cells. Thus, PB and insulin cross-talk through the insulin receptor to regulate glucose and drug metabolism reciprocally.

Phenobarbital reduces blood glucose and gluconeogenesis through down-regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression in rats

The regulatory mechanism of phosphoenolpyruvate carboykinase (GTP) (EC 4.1.1.32) (PEPCK) gene expression and gluconeogenesis by phenobarbital (PB), which is known to induce drug-metabolizing enzymes, was investigated. Higher level of PEPCK mRNA was observed in spherical rat primary hepatocytes on EHS-gel than monolayer hepatocytes on TIC (type I collagen). We found that PB directly suppressed PEPCK gene expression in spherical hepatocytes on EHS-gel, but not in those on TIC. PB strongly suppressed cAMP-dependent induction of PEPCK gene expression. Tyrosine aminotransferase (TAT), another gluconeogenic enzyme, was induced by cAMP, but not suppressed by PB. Chronic administration of PB reduced hepatic PEPCK mRNA in streptozotocin-induced diabetic and nondiabetic rats, and PB reduced blood glucose level in diabetic rats. Increased TAT mRNA in diabetic rats was not suppressed by PB. These results indicated that PB-dependent reduction is specific to PEPCK. From pyrvate challenge test, PB suppressed the increased gluconeogenesis in diabetic rats. PEPCK gene promoter activity was suppressed by PB in HepG2 cells. In conclusion, we found that spherical hepatocytes cultured on EHS-gel are capable to respond to PB to suppress PEPCK gene expression. Moreover, our results indicate that hypoglycemic action of PB result from transcriptional repression of PEPCK gene and subsequent suppression of gluconeogenesis.

Effect of enzyme inducers and inhibitors on the pharmacokinetics of oltipraz in rats

A series of in-vitro and in-vivo experiments, using various inducers and inhibitors of hepatic microsomal cytochrome P450 (CYP) isozymes, was conducted to study oltipraz pharmacokinetics in rats. In in-vivo studies, oltipraz at a dose of 10 mg kg−1 was administered intravenously to rats. In rats pretreated with SKF 525-A (a nonspecific CYP isozyme inhibitor in rats; n = 9), the time-averaged total body clearance (CL) of oltipraz was significantly slower (56.6% decrease) than that in untreated rats (n = 9). This indicated that oltipraz is metabolized via CYP isozymes in rats. Hence, various enzyme inducers or inhibitors were used in in-vitro and in-vivo studies in rats. In rats pretreated with 3-methylcholanthrene (n = 9 and 8 for untreated and treated groups, respectively), phenobarbital (n = 7 and 10 for untreated and treated groups, respectively) or dexamethasone (n = 7 and 12 for untreated and treated groups, respectively) (main inducers of CYP1A1/2, 2B1/2 and 3A1/2 in rats, respectively), the CL values were significantly faster (38.4, 94.4 and 33.6% increase, respectively). In rats pretreated with sulfaphenazole (n = 8 and 9 for untreated and treated groups, respectively), quinine (n = 7 and 9 for untreated and treated groups, respectively) or troleandomycin (n = 8 and 9 for untreated and treated groups, respectively) (main inhibitors of CYP2C11, 2D1 and 3A1/2 in rats, respectively), the CL values were significantly slower (31.0, 27.6 and 36.3% decrease, respectively). The in-vivo results with various enzyme inhibitors correlated well with the in-vitro intrinsic clearance for disappearance of oltipraz (CLint) (n = 5, each). The above data suggested that oltipraz could be metabolized in male rats mainly via CYP1A1/2, 2B1/2, 2C11, 3A1/2 and 2D1.

Effects of enzyme inducers and inhibitors on the pharmacokinetics of intravenous ipriflavone in rats

In order to find out what types of the hepatic microsomal cytochrome P450 (CYP) isozymes are involved in the metabolism of ipriflavone, ipriflavone at a dose of 20 mg kg−1 (or 15 mg kg−1) was infused in male Sprague—Dawley rats. In rats pretreated with SKF 525-A (a non-specific CYP isozyme inhibitor in rats), the total body clearance (CL) of ipriflavone was significantly slower (29.9% decrease) than that in control rats. This indicates that ipriflavone is metabolized via CYP isozymes in rats, hence various enzyme inducers and inhibitors were used in in-vitro or in-vivo studies in rats. In rats pretreated with 3-methylcholanthrene and phenobarbital (main inducers of CYP1A1/2 and 2B1/2 in rats, respectively), the CL values were significantly higher (153 and 67.2% increases, respectively). In rats pretreated with sulfaphenazole (a main inhibitor of CYP2C11 in rats), the CL was significantly slower (22.5% decrease) than that in control rats. On addition of furafylline (a main inhibitor of CYP1A2 in rats), the in-vitro intrinsic clearance for the disappearance of ipriflavone was significantly slower (50.8% decrease) than that without furafylline. However, the CL values were not significantly different in rats pretreated with orphenadrine and isoniazid (a main inducer of CYP2E1 in rats), and quinine and troleandomycin (main inhibitors of CYP2D1 and 3A23/2 in rats, respectively) compared to controls. These data suggest that ipriflavone could be metabolized mainly via CYP1A1/2, 2B1/2 and 2C11 in rats.

Human Metabolism and Metabolic Interactions of Deployment-Related Chemicals

It has been suggested that chemicals and, more specifically, chemical interactions, are involved as causative agents in deployment-related illnesses. Unfortunately, this hypothesis has proven difficult to test, because toxicological investigations of deployment-related chemicals are usually carried out on surrogate animals and are difficult to extrapolate to humans. Other parts of the problem, such as the definition of variation within human populations and the development of methods for designating groups or individuals at significantly greater risk, cannot be carried out on surrogate animals, and the data must be derived from humans. The relatively recent availability of human cell.fractions, such as microsomes, cytosol, etc., human cells such as primary hepatocytes, recombinant human enzymes, and their isoforms and polymorphic variants has enabled a significant start to be made in developing the human data needed. These initial studies have examined the human metabolism by cytochrome P450, other phase I enzymes, and their isoforms and, in some cases, their polymorphic variants of compounds such as chlorpyrifos, carbaryl, DEET, permethrin, and pyridostigmine bromide, and, to a lesser extent, other chemicals from the same chemical and use classes, including solvents, jet fuel components, and sulfur mustard metabolites. A number of interactions at the metabolic level have been described both with respect to other xenobiotics and to endogenous metabolites. Probably the most dramatic have been seen in the ability of chlorpyrifos to inhibit not only the metabolism of other xenobiotics such as carbaryl and DEET but also to inhibit the metabolism of steroid hormones.

Cell shape, cell-cell contact, cell-extracellular matrix contact and cell polarity are all required for the maximum induction of CYP2B1 and CYP2B2 gene expression by phenobarbital in adult rat cultured hepatocytes

The effect of cell shape, cell density, contact with extracellular matrix and cell polarity on the phenobarbital (PB)-induced gene expression of CYP2B1 and CYP2B2 (CYP2B1/2B2) in adult rat hepatocytes was investigated. High cell density enhanced the induction of CYP2B1/2B2 gene expression by PB. Hepatocytes cultured on EHS gel showed a spherical cell shape and highly enhanced the induction of CYP2B1/2B2 gene expression by PB. Although monolayer hepatocytes cultured on type I collagen (TIC) and type IV collagen exhibited poor induction of CYP2B1/2B2 gene expression by PB, monolayer cells on laminin showed substantial induction. The addition of soluble laminin to media did not show any effect on induction in monolayer hepatocytes cultured on TIC. Dishes coated with different concentrations of immovable laminin demonstrated complicated effects. Coating with higher concentrations of laminin resulted in greater induction of CYP2B1/2B2 gene expression by PB. On the other hand, when hepatocytes were cultured on dishes coated with lower concentrations of laminin, they became round and greater induction of CYP2B1/2B2 gene expression by PB was observed. Spherical hepatocytes cultured on low concentrations of TIC also showed highly enhanced induction of CYP2B1/2B2 gene expression by PB. EHS gel overlay to hepatocytes cultured on TIC and collagen sandwich configurations that are known to induce cell polarity enhanced the induction by PB. The induction of CYP2B1/2B2 gene expression needed cytoskeleton organization, such as actin filament, microtubule filament and intermediate filament. These results demonstrate that cell shape, cell density, contact with extracellular matrix and cell polarity all play critical roles in the induction of CYP2B1/2B2 gene expression by PB.

A physiological role of AMP-activated protein kinase in phenobarbital-mediated constitutive androstane receptor activation and CYP2B induction

CAR (constitutive androstane receptor) is a nuclear receptor that regulates the transcription of target genes, including CYP (cytochrome P450) 2B and 3A. The transactivation by CAR is regulated by its subcellular localization; however, the mechanism that governs nuclear translocation has yet to be clarified. It has been reported recently that AMPK (AMP-activated protein kinase) is involved in phenobarbital-mediated CYP2B induction in a particular culture system. We therefore investigated in vivo whether AMPK is involved in the activation of CAR-dependent gene expression. Immunoblot analysis using an antibody which recognizes Thr-172-phosphorylated AMPKalpha1/2 revealed phenobarbital-induced AMPK activation in rat and mouse livers as well. Phenobarbital, however, failed to increase the liver phospho-AMPK level of tumour-bearing rats in which CAR nuclear translocation had been impaired. In in vivo reporter gene assays employing PBREM (phenobarbital-responsive enhancer module) from CYP2B1, an AMPK inhibitor 8-bromo-AMP abolished phenobarbital-induced transactivation. In addition, Cyp2b10 gene expression was attenuated by 8-bromo-AMP. Forced expression of a dominant-negative mutant and the wild-type of AMPKalpha2 in the mouse liver suppressed and further enhanced phenobarbital-induced PBREM-reporter activity respectively. Moreover, the AMPK activator AICAR (5-amino-4-imidazolecarboxamide riboside) induced PBREM transactivation and an accumulation of CAR in the nuclear fraction of the mouse liver. However, AICAR and metformin, another AMPK activator, failed to induce hepatic CYP2B in mice and rats. These observations suggest that AMPK is at least partly involved in phenobarbital-originated signalling, but the kinase activation by itself is not sufficient for CYP2B induction in vivo.

Effects of enzyme inducers and inhibitors on the pharmacokinetics of intravenous torasemide in rats

In order to find whether torasemide is metabolized via CYP isozymes in rats, torasemide at a dose of 2mg/kg was infused in rats pretreated with SKF 525-A, a non-specific CYP isozyme inhibitor in male Sprague-Dawley rats. The total area under the plasma concentration-time curve from time zero to time infinity (AUC) of torasemide was significantly greater in rats pretreated with SKF 525-A (a non-specific CYP isozyme inhibitor in rats) than that in control rats (3570 versus 1350 microg min/ml). This indicated that torasemide is metabolized via CYP isozymes in rats. Hence, torasemide was infused in rats pretreated with various enzyme inducers and inhibitors to find what types of CYP isozymes are involved in the metabolism of torasemide in rats. The AUC values were not significantly different in rats pretreated with 3-methylcholanthrene, phenobarbital, isoniazid, quinine and troleandomycin (main inducers of CYP1A1/2, CYP2B1/2, and CYP2E1, and main inhibitors of CYP2D1 and CYP3A1/2 in rats, respectively) compared with those in respective control rats. However, in rats pretreated with dexamethasone (a main inducer of CYP3A1/2 in rats), the AUC was significantly smaller than that in control rats (1290 versus 1590 microg min/ml). Dexamethasone probably also induces rat CYP2C11; this could be due to an increase in CYP2C11 in rats pretreated with dexamethasone. It has been reported from our laboratories that in rats pretreated with sulfaphenazole (a main inhibitor of CYP2C11 in rats) the AUC was significantly greater than that in control rats (2970 versus 1610 microg min/ml). The above data suggested that torasemide could be metabolized in male rats mainly via CYP2C11.

Impaired nuclear translocation of CAR in hepatic preneoplastic lesions: Association with an attenuated CYP2B induction by phenobarbital

Phenobarbital (PB) induction of CYP2B, a representative target gene of constitutive androstane receptor (CAR), has been observed to be attenuated in preneoplastic lesions of rat liver; however, molecular basis for this attenuation is poorly understood. In this report, we provide evidence indicating that the CAR expressed in the hepatic preneoplastic lesions of rats and mice was resistant to nuclear translocation and transactivation of the PB-responsive enhancer module upon PB treatment. These observations suggest that the attenuation of the induction of CYP2B by PB in hepatic preneoplastic lesions is evidently a consequence of impaired nuclear translocation of CAR.

Effects of enzyme inducers and inhibitors on the pharmacokinetics of intravenous DA-8159, a new erectogenic, in rats

In order to find what types of hepatic microsomal cytochrome P450 (CYP) isozymes are involved in the metabolism of DA-8159 and in the formation of DA-8164 in rats, enzyme inducers, such as dexamethasone, phenobarbital, 3-methylcholanthrene and isoniazid, and enzyme inhibitors, such as troleandomycin and quinine, were pretreated in rats. After a 1 min intravenous administration of DA-8159 at a dose of 30 mg/kg to rats pretreated with dexamethasone (a main inducer of CYP3A1/2 in rats), the total areas under the plasma concentration-time curve from time zero to time infinity (AUC) values of DA-8159 (283 versus 349 microg min/ml) and DA-8164 (98.0 versus 79.8 microg min/ml) were significantly smaller and greater, respectively, than those in control rats. However, the AUC values of DA-8159 were not significantly different after pretreatment with phenobarbital, isoniazid and 3-methylcholanthrene (main inducers of CYP2B1/2, 2E1 and 1A1/2, respectively, in rats). In rats pretreated with troleandomycin (a main inhibitor of CYP3A1/2 in rats), the AUC values of DA-8159 (435 versus 370 microg min/ml) and DA-8164 (34.8 versus 76.5 microg min/ml) were significantly greater and smaller, respectively. However, in rats pretreated with quinine (a main inhibitor of CYP2D1 in rats), the AUC of DA-8159 was comparable to that in control rats. The above data indicate that DA-8159 was metabolized and DA-8164 was formed mainly via CYP3A1/2 in rats.

Nuclear receptors CAR and PXR cross talk with FOXO1 to regulate genes that encode drug-metabolizing and gluconeogenic enzymes

The nuclear receptors CAR and PXR activate hepatic genes in response to therapeutic drugs and xenobiotics, leading to the induction of drug-metabolizing enzymes, such as cytochrome P450. Insulin inhibits the ability of FOXO1 to express genes encoding gluconeogenic enzymes. Induction by drugs is known to be decreased by insulin, whereas gluconeogenic activity is often repressed by treatment with certain drugs, such as phenobarbital (PB). Performing cell-based transfection assays with drug-responsive and insulin-responsive enhancers, glutathione S-transferase pull down, RNA interference (RNAi), and mouse primary hepatocytes, we examined the molecular mechanism by which nuclear receptors and FOXO1 could coordinately regulate both enzyme pathways. FOXO1 was found to be a coactivator to CAR- and PXR-mediated transcription. In contrast, CAR and PXR, acting as corepressors, downregulated FOXO1-mediated transcription in the presence of their activators, such as 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) and pregnenolone 16alpha-carbonitrile, respectively. A constitutively active mutant of the insulin-responsive protein kinase Akt, but not the kinase-negative mutant, effectively blocked FOXO1 activity in cell-based assays. Thus, insulin could repress the receptors by activating the Akt-FOXO1 signal, whereas drugs could interfere with FOXO1-mediated transcription by activating CAR and/or PXR. Treatment with TCPOBOP or PB decreased the levels of phosphoenolpyruvate carboxykinase 1 mRNA in mice but not in Car(-/-) mice. We conclude that FOXO1 and the nuclear receptors reciprocally coregulate their target genes, modulating both drug metabolism and gluconeogenesis.

Effects of cysteine on the pharmacokinetics of intravenous torasemide in rats with protein−calorie malnutrition

Effects of cysteine on the pharmacokinetics of torasemide were investigated after intravenous administration at a dose of 2 mg/kg to control rats and rats with PCM and PCMC. Torasemide was reported to be mainly metabolized via hepatic CYP2C9 in humans, and human CYP2C9 and male rat CYP2C11 proteins have 77% homology. It has also been reported that in male rats with PCM, the CYP2C11 level decreased to approximately 20% of the control level, but the decreased CYP2C11 level in rats with PCM partially returned to the control level by oral cysteine supplementation (rats with PCMC). Hence, it could be expected that in rats with PCM, some pharmacokinetic parameters of torasemide could be significantly different compared with those in control rats and rats with PCMC; however, they could be not significantly different between control rats and rats with PCMC. This was proven by the following parameters; the AUC (1880, 4080, and 2290 microg x min/mL for control rats and rats with PCM and PCMC, respectively), terminal half-life (188, 277, and 139 min), MRT (154, 323, and 155 min), CL (1.06, 0.491, and 0.943 mL/min/kg), CL(NR) (0.992, 0.430, and 0.874 mL/min/kg), and in vitro intrinsic torasemide disappearance clearance, CL(int) (0.102, 0.0842, and 0.0997 mL/min/mg protein).

Pharmacokinetic changes in drugs during protein-calorie malnutrition: correlation between drug metabolism and hepatic microsomal cytochrome p450 isozymes

The rats with protein-calorie malnutrition (PCM, 5% casein diet for a period of 4-week) were reported to exhibit 60 and 80% suppression in the hepatic microsomal cytochrome P450 (CYP) 1A2 and CYP2C11 levels, respectively, and 40-50% decreases in CYP2E1 and CYP3A1/2 levels compared to control (23% casein diet for a period of 4-week) based on Western blot analysis. In addition, Northern blot analysis showed that CYP1A2, CYP2E1, CYP2C11, and CYP3A1/2 mRNAs decreased in the state of PCM as well. Hence, pharmacokinetic changes of the drugs in rats with PCM [especially the area under the plasma concentration-time curve from time zero to time infinity (AUC) changes of metabolite(s)] reported from literatures were tried to explain in terms of CYP isozyme changes in the rats. Otherwise, the time-averaged nonrenal clearance (CL NR) of parent drug was compared. Pharmacokinetic changes of the drugs in other types of malnutritional state, such as kwashiorkor and marasmus, in both human and animal models were also compared. The drugs reviewed are as follows: diuretics, antibiotics, anticancer agents, antiepileptics, antiarrythmics, analgesics, xanthines, antimalarials, and miscellaneous.

Induction of Drug Metabolism: The Role of Nuclear Receptors

Induction of drug metabolism was described more than 40 years ago. Progress in understanding the molecular mechanism of induction of drug-metabolizing enzymes was made recently when the important roles of the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of transcription factors, were discovered to act as sensors for lipophilic xenobiotics, including drugs. CAR and PXR bind as heterodimeric complexes with the retinoid X receptor to response elements in the regulatory regions of the induced genes. PXR is directly activated by xenobiotic ligands, whereas CAR is involved in a more complex and less well understood mechanism of signal transduction triggered by drugs. Most recently, analysis of these xenobiotic-sensing nuclear receptors and their nonmammalian precursors such as the chicken xenobiotic receptor suggests an important role of PXR and CAR also in endogenous pathways, such as cholesterol and bile acid biosynthesis and metabolism. In this review, recent findings regarding xenosensors and their target genes are summarized and are put into an evolutionary perspective in regard to how a living organism has derived a system that is able to deal with potentially toxic compounds it has not encountered before.

Effects of cysteine on the pharmacokinetics of intravenous 2-(allylthio)pyrazine, a new chemoprotective agent, in rats with protein-calorie malnutrition

The effects of cysteine on the pharmacokinetics of 2-(allylthio)pyrazine (2-AP) were investigated after intravenous administration of the drug (50 mg/kg) to control (Sprague-Dawley) rats (4-week fed on 23% casein diet), and rats with protein-calorie malnutrition (PCM, 4-week fed on 5% casein diet) and PCMC (PCM with 250 mg/kg of oral cysteine, twice daily starting from the fourth week). In rats with PCM, the area under the plasma concentration-time curve from time zero to time infinity (AUC) of 2-AP was significantly smaller than that in control rats. However, in rats with PCMC, the AUC of 2-AP was significantly greater than that in control rats and rats with PCM. This could be due to significantly greater formation of M4 in rats with PCM and significantly smaller formation of M4 in rats with PCMC than that in control rats. In rats with PCMC, some pharmacokinetic parameters of 2-AP restored fully or more than the levels of control rats. For example, in rats with PCMC, the apparent volume of distribution at steady state of 2-AP (7290, 16,600, and 7050 ml/kg for control rats, and rats with PCM and PCMC, respectively), the percentage of dose excreted in 24-h urine as unchanged 2-AP (0.242, 0.727, and 0.130%), and 'the amount' excreted in 24-h urine as M4 (100, 228, and 51%) were comparable to those in control rats. However, the AUC (739, 434, and 1240 microg/min/ml) and total body clearance (67.7, 115, and 40.2 ml/min/kg) of 2-AP were significantly greater and slower, respectively, than those in control rats. This could be at least partly due to increase in S-methyltransferase activity (to form M4) in rats with PCM and greater restoration of its activity (decrease in its activity) in rats with PCMC.

Pharmacokinetics and pharmacodynamics of azosemide

Azosemide is used in the treatment of oedematous states and hypertension. The exact mechanism of action is not fully understood, but it mainly acts on both the medullary and cortical segments of the thick ascending limb of the loop of Henle. Delayed tolerance was demonstrated in humans by homeostatic mechanisms (principally an increase in aldosterone secretion and perhaps also an increase in the reabsorption of solute in the proximal tubule). After oral administration to healthy humans in the fasting state, the plasma concentration of azosemide reached its peak at 3-4 h with an absorption lag time of approximately 1 h and a terminal half-life of 2-3 h. The estimated extent of absolute oral bioavailability in humans was approximately 20.4%. After oral administration of the same dose of azosemide and furosemide, the diuretic effect was similar between the two drugs, but after intravenous administration, the effect of azosemide was 5.5-8 times greater than that in furosemide. This could be due to the considerable first-pass effect of azosemide. The protein binding to 4% human serum albumin was greater than 95% at azosemide concentrations ranging from 10 to 100 microg/ml using an equilibrium dialysis technique. The poor affinity of human tissues to azosemide was supported by the relatively small value of the apparent post-pseudodistribution volume of distribution (Vdbeta), 0.262 l/kg. Eleven metabolites (including degraded products) of azosemide including M1, glucuronide conjugates of both M1 and azosemide, thiophenemethanol, thiophencarboxylic acid and its glycine conjugate were obtained in rats. Only azosemide and its glucuronide were detected in humans. In humans, total body clearance, renal clearance and terminal half-life of azosemide were 112 ml/min, 41.6 ml/min and 2.03 h, respectively. Azosemide is actively secreted in the renal proximal tubule possibly via nonspecific organic acid secretory pathway in humans. Thus, the amount of azosemide that reaches its site of action could be significantly modified by changes in the capacity of this transport system. This capacity, in turn, could be predictably changed in disease states, resulting in decreased delivery of the diuretic to the transport site, as well as in the presence of other organic acids such as nonsteroidal anti-inflammatory drugs which could compete for active transport of azosemide. The urinary excretion rate of azosemide could be correlated well to its diuretic effects since the receptors are located in the loop of Henle. The diuretic effects of azosemide were dependent on the rate and composition of fluid replacement in rabbits; therefore, this factor should be considered in the evaluation of bioequivalence assessment.

Induction of the CYP2B genes by triphenyldioxane treatment in the rat liver

Triphenyldioxane (TPD) is a potent phenobarbital-type (PB) inducer of the CYP2B cytochrome isoforms, the inducing effect of which is one order of magnitude higher than PB. The fact that TPD is unable to induce CYP2B genes having the proximal promoter disrupted (mouse Cyp2b10) suggests an existence of the proximal promoter-dependent mechanism of the CYP2B induction. So a TPD-dependent activation of the nuclear proteins to the binding with Barbie-box sequence (the most conservative part of the proximal promoter) was studied. In the nuclear extracts from the intact rat liver there were detected five proteins that could be activated to the Barbie-box binding by the TPD treatment in vitro (II, III, NI, NII and NIII). The first three were effected also by another PB-like inducers tested (PB and TCPOBOP), when NII and NIII complexes were formed under the influence of TPD only. It is possible that a direct activation of the NII and NIII proteins by TPD exists as (3)H-labeled TPD was detected in the composition of NII and NIII complexes. However, both of them disappeared from the nuclear extracts after the long exposure time with TPD (6 h or more). A short induction by the direct intra-liver delivery of TPD (15-30 min) led to the stabile activation of one TPD-specific protein. Apart from the activation of the Barbie-box-binding protein, the short TPD treatment caused the activation of three nuclear proteins being able to interact with the NR1 sequence of the distal promoter PBREM element. These findings suggest that TPD is really the first member of the PB-like inducers family for which a special mechanism of CYP2B induction may exist.

Enhanced induction of cytochrome P450 enzymes and CAR binding in TNF (p55(-/-)/p75(-/-)) double receptor knockout mice following phenobarbital treatment

Phenobarbital (PB) is a well characterized inducer of cytochrome P450 (P450) 2B and 3A subfamilies. Several proinflammatory cytokines have been shown to negatively modulate the induction of P450 by PB. In addition, PB is known to elicit an inflammatory mitogenic effect on the liver. To date, no studies have evaluated the PB induction profile of hepatic P450 in the absence of an intact tumor necrosis factor-alpha (TNFalpha) response. To test the hypothesis that endogenous TNFalpha signaling modulates hepatic P450 induction by PB in vivo, PB induction was examined in TNF (p55(-/-)/p75(-/-)) double receptor knockout mice (ko-TNF) and wild-type mice (wt-TNF). CYP2B- and CYP3A-associated activities and protein content were induced to a significantly greater extent (p < 0.05) in ko-TNF mice compared with wt-TNF mice. In parallel with enhanced CYP2B induction, an apparent elevation in the nuclear accumulation of the principal regulatory protein for transcription of CYP2B genes, the constitutively activated receptor (CAR), was detected in ko-TNF nuclear extracts following PB treatment. Additionally, nuclear factor kappa-B binding was induced by PB in wt-TNF mice, but not in ko-TNF mice, indicating that the hepatic inflammatory response following PB treatment differed between wt-TNF and ko-TNF mice. These data demonstrate that endogenous TNFalpha signaling modulates PB induction of hepatic CYP2B and CYP3A isoforms in vivo. Further, the data presented herein suggest that endogenous TNFalpha signaling influences PB induction of CYP2B through inhibition of CAR nuclear accumulation.

Effects of cysteine on the pharmacokinetics of intravenous phenytoin in rats with protein-calorie malnutrition

The effects of cysteine on the pharmacokinetics of phenytoin and one of its metabolites, 5-(p-hydroxyphenyl)-5-phenylhydantoin (pHPPH) were investigated after intravenous administration of phenytoin, 25 mg/kg, to control rats (4-week fed on 23% casein diet) and rats with PCM (protein-calorie malnutrition, 4-week fed on 5% casein diet) and PCMC (PCM with oral cysteine supplementation, 250 mg/kg, twice daily starting from the fourth week). In rats with PCM and PCMC, the phenytoin hydroxylation (to form pHPPH) activities were significantly smaller (164, 103 and 95.3 pmol/min per mg protein for the control rats, and rats with PCM and PCMC, respectively) than that in control rats. In rats with PCMC, the intrinsic clearance of phenytoin, CL(int) was significantly slower than those in control rats and rats with PCM (0.175, 0.131 and 0.044 ml/min). The above data suggested that the formation of pHPPH could be reduced in rats with PCM and PCMC. This was supported by significantly smaller 24-h urinary excretion of pHPPH (54.7, 35.6 and 32.5% of intravenous dose of phenytoin) in rats with PCM and PCMC than that in control rats. In rats with PCM, the maximum velocity (0.344, 0.203 and 0.196 microg/min), apparent volume of distribution in central compartment (44.4, 65.4 and 72.2 ml/kg) of phenytoin, and total area under the plasma concentration-time curve from time zero to time infinity (609, 714 and 1210 microg min/ml), renal clearance (20.5, 13.4 and 4.67 ml/min per kg) and 24-h urinary excretion (54.7, 35.6 and 32.5% of intravenous dose of phenytoin) of pHPPH were not returned to control levels by cysteine supplementation (rats with PCMC). This could be mainly due to the fact that the phenytoin hydroxylation activity in rats with PCMC was not returned to control level.

Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

Phenobarbital (PB) response elements are composed of various nuclear receptor (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR constitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element called the xenobiotic responsive module (XREM) is found in human CYP3A4 genes, which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer could transactivate XREM. These response elements and NRs are functionally versatile, and capable of responding to distinct but overlapping groups of xenochemicals.

Repression of Phenobarbital-Dependent CYP2B1 mRNA Induction by Reactive Oxygen Species in Primary Rat Hepatocyte Cultures

Xenobiotic-metabolizing cytochrome P-450 (P-450) enzymes not only play a pivotal role in elimination of foreign compounds but also contribute to generation of toxic intermediates, including reactive oxygen species, that may elicit cellular damage if produced excessively. Expression of several xenobiotic-metabolizing P-450 enzymes is induced by phenobarbital (PB). Pronounced induction is observed for the rat CYP2B1 isoform. A primary rat hepatocyte culture system was used to investigate whether reactive oxygen species might modulate PB-dependent CYP2B1 induction. In cells cultivated for 3 days with 1.5 mM PB, substantial CYP2B1 mRNA induction was observed (100%). Addition of H(2)O(2) or of the catalase inhibitor 3-amino-1,2,4-triazole (AT) to the medium repressed induction to approximately 30% (at 1 mM H(2)O(2) and 2 mM AT, respectively). Accordingly, treatment of hepatocytes with PB and the glutathione precursor N-acetylcysteine (NAC) led to enhanced PB-dependent induction (to over 1000% at 10 mM NAC). In primary hepatocyte cultures transfected with a CYP2B1 promoter-luciferase construct containing approximately 2.7 kilobase pairs of the native CYP2B1 promoter sequence, PB-dependent reporter gene activation was repressed by AT and stimulated by N-acetylcysteine. Furthermore, a 263-base pair CYP2B1 promoter fragment encompassing the phenobarbital-responsive enhancer module conferred suppression of PB-dependent luciferase expression by AT and activation by NAC in a heterologous SV40-promoter construct. In summary, these data demonstrate a regulatory mechanism that is dependent on the cellular redox status, which modulates CYP2B1 mRNA induction by PB on the transcriptional level, thus representing a feedback mechanism preventing further P-450-dependent production of reactive oxygen intermediates under oxidative stress.

Dietary taurine potentiates polychlorinated biphenyl-induced hypercholesterolemia in rats*

The effect of dietary taurine on cholesterol metabolism and the distribution of lipoprotein-cholesterol in serum of rats fed a diet containing polychlorinated biphenyls (PCB) was examined. Young male Wistar rats (60 g) were fed diets containing 0.2 g/kg diet of PCB and/or 30 g/kg diet of taurine for 15 days. The experiment was performed as the 2 (PCB) x 2 (taurine) factorial design. The addition of PCB elevated serum levels of total- and HDL-cholesterol and apolipoprotein A-I, which is a major apolipoprotein of HDL. Simultaneous supplementation of taurine with PCB amplified the increase of the serum level of total- and HDL-cholesterol. Hepatic concentrations of cholesterol and total lipids were significantly elevated by the supplementation of PCB, and taurine significantly amplified these increases caused by PCB. PCB suppressed hepatic cholesterol 7alpha-hydroxylase (CYP7A1) gene expression, and taurine induced CYP7A1 gene expression. Taurine also enhanced PCB-induced elevation of malic enzyme mRNA in the liver. These results suggest that taurine enhanced PCB-induced hyper-alpha-cholesterolemia and that taurine has a role in increasing HDL-cholesterol.

Apolipoprotein A-I gene expression is upregulated by polychlorinated biphenyls in rat liver

Xenobiotics such as polychlorinated biphenyls (PCB) increase serum cholesterol level (especially high density lipoprotein cholesterol) and apolipoprotein A-I (apo A-I) level in rats. The effect of PCB on serum apo A-I and hepatic apo A-I gene expression and the relationship between apo A-I and drug-metabolizing enzymes in rats were investigated. Serum levels of cholesterol and apo A-I were increased by dietary addition of PCB in a dose-dependent manner (0-500 mg/kg diet). Hepatic apo A-I mRNA level was also elevated by PCB in a similar fashion. Serum level of cholesterol gradually increased during feeding period of PCB (200 mg/kg diet, 105 days) and reached a two-fold higher level in PCB group than in controls. The levels of serum apo A-I and hepatic apo A-I mRNA linearly elevated during feeding period of PCB and were increased 3- or 4-fold, respectively, compared to controls. Although acute administration (16 hr) of PCB, 3-methylcholanthrene, and phenobarbital induced cytochrome P-450 gene expression in the liver, hepatic apo A-I gene expression was not increased by these xenobiotics. These results indicated that the serum levels of cholesterol and apo A-I had positive correlation with hepatic level of apo A-I mRNA in rats fed PCB, and that hepatic apo A-I gene expression was dependent upon intake of PCB but was not directly related to the induction of drug-metabolizing enzymes. This study demonstrated that xenobiotic-induced hyper-alpha-cholesterolemia would be caused by the increased apo A-I gene expression and cholesterol synthesis in the liver, coordinately.

Suppression of the expression of the CYP2B1/2 gene by retinoic acids

The effects of 5alpha-androsten-3alpha-ol (ASE), and retinoic acids (RAs) and their precursors on the phenobarbital (PB)-mediated induction of CYP2B1 and 2B2 were examined in cultured rat hepatocytes. Two isomers of RA, 9-cis- and all-trans-RA, suppressed markedly the effect of PB on CYP2B1/2 expression, while ASE had no suppressive effect. The effect of 9-cis-RA appeared at a lower concentration than the all-trans-isomer, indicating the dominant action of the former isomer. Suppression with 9-cis-retinal was also observed, but all-trans-retinol and -retinal were without effect. These results suggest that: (1) ASE, an inverse agonist for the constitutive androstane receptor (CAR), does not play a major role in the suppression of the CYP2B; (2) 9-cis-RA suppresses CYP2B induction by reducing ligand-free retinoid X-receptors (RXR) available for dimerization with the CAR; and (3) enzymes responsible for RA formation play an important role in the mechanism governing CYP2B regulation.

Effects of enzyme inducers or inhibitors on the pharmacokinetics of intravenous parathion in rats

In order to find what form of hepatic cytochrome P450 (CYP) is involved in the metabolism of parathion to form paraoxon, rats were pretreated with the enzyme inhibitors, such as SKF 525-A and ketoconazole or enzyme inducers, such as dexamethasone, isoniazid, phenobarbital, and 3-methylcholanthrene. Parathion, 3 mg/kg, was infused in 1 min via the jugular vein. In rats pretreated with SKF 525-A or ketoconazole, nonspecific CYP inhibitors, the area under the plasma concentration-time curve from time zero to time infinity (AUC) and total body clearance (Cl) of parathion were significantly greater and slower, respectively, than those in respective control rats, suggesting that parathion was metabolized by CYPs. In rats pretreated with dexamethasone (CYP3A23 inducer), the AUC was significantly smaller (41.5 compared with 52.5 microg min/mL), Cl was significantly faster (72.2 compared with 57.1 mL/min/kg), and the amounts and/or tissue-to-plasma ratios of parathion was significantly (or tended to be) smaller than those in control rats. However, the pharmacokinetic parameters of parathion were not significantly different after pretreatment with other enzyme inducers compared with respective control rats. The above data suggested that parathion was metabolized to paraoxon by dexamethasone-inducible CYP3A23, the induction of which was confirmed by Western blot analysis. This was supported by in vitro intrinsic clearance (Cl(int)) of parathion to form paraoxon in hepatic microsomal fraction; the Cl(int) in rats pretreated with dexamethasone was significantly faster (0.0900 compared with 0.0290 mL/min/mg protein) than that in control rats.

A conserved nuclear receptor consensus sequence (DR-4) mediates transcriptional activation of the chicken CYP2H1 gene by phenobarbital in a hepatoma cell line

Phenobarbital-responsive DNA elements were identified in the 5'-flanking region of the chicken CYP2H1 gene by in reporter gene assays in a chicken hepatoma cell line (leghorn male hepatoma (LMH)). A 264-base pair (bp) enhancer sequence (phenobarbital-responsive unit (PBRU)) responded to phenobarbital and a variety of phenobarbital-type inducers. Analysis of putative transcription factor binding sites within the 264-bp element revealed a nuclear receptor half-site repeat (DR-4) neighboring a putative nuclear factor-1 site. This motif resembles phenobarbital response elements in the flanking regions of three phenobarbital-inducible genes, rat CYP2B2, mouse Cyp2b10, and human CYP2B6. Activation of the 264-bp element was eliminated after site-directed mutagenesis of the DR-4 hexamer half-sites. Evidence for evolutionary conservation of this recognition site was indicated by activation in LMH cells of a mouse Cyp2b10 phenobarbital-responsive enhancer by the same spectrum of inducers that activate the CYP2H1 264-bp PBRU. Inhibition of this activation by okadaic acid may explain the reported inhibitory effects on induction of CYP2B1/2 and Cyp2b10 by this phosphatase inhibitor. We show that this inhibition occurs directly on the 264-bp PBRU, whereas the proximal promoter of CYP2H1 is induced by okadaic acid in reporter gene assays. These experiments exploit the unique phenobarbital inducibility of the hepatoma-derived cell line LMH.

Effects of enzyme inducers and inhibitor on the pharmacokinetics of intravenous 2-(allylthio)pyrazine, a new chemoprotective agent, in rats

In order to find what types of hepatic cytochrome P450 (CYP) isozymes are involved in the metabolism of 2-(allylthio)pyrazine (2-AP) in rats, enzyme inducers, such as phenobarbital, 3-methylcholanthrene, dexamethasone, or isoniazid, and an enzyme inhibitor, such as SKF 525-A were pretreated. After 1-min intravenous administration of 2-AP, 50 mg/kg, to rats pretreated with SKF 525-A (a non-specific CYP inhibitor in rats), the plasma concentrations were significantly higher, and the area under plasma concentration-time curve from time zero to time infinity (AUC) was significantly greater (1365 compared with 1034 microg min/mL) as a result of significantly slower total body clearance (Cl) (36.6 compared with 48.3 mL/min/kg) than those in control rats, indicating that 2-AP was metabolized by CYP isozymes. After 1-min intravenous administration of 2-AP, 50 mg/kg, to rats pretreated with dexamethasone (an inducer of CYP3A in rats), phenobarbital (an inducer of CYP2B1/2, 2C6, 2C7, and 3A1/2 in rats), and 3-methylcholanthrene (an inducer of CYP1A1/2 and 2A1 in rats), the plasma concentrations were significantly lower, and AUC was significantly smaller (27, 41 and 60% decrease, respectively, compared with respective control rats) owing to faster Cl [37 (p>0.05), 70 (p<0.001), and 150% (p<0.001) increase, respectively, compared with respective control rats].