Colchicine down-regulates cytochrome P450 2B6, 2C8, 2C9, and 3A4 in human hepatocytes by affecting their glucocorticoid receptor-mediated regulation

Role of mitogen-activated protein kinases in aryl hydrocarbon receptor signaling

Human populations are increasingly exposed to a number of environmental pollutants such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and dioxins. These compounds are activators of the aryl hydrocarbon receptor (AhR) that controls the expression of many genes including those for detoxification enzymes. The regulatory mechanisms of AhR are multi-factorial and include phosphorylation by various protein kinases. Significant progress in the research of mitogen-activated protein kinases (MAPKs) has been achieved in the last decade. Isolated reports have been published on the role of MAPKs in AhR functions and vice versa, with activation of MAPKs by AhR ligands. This mini-review summarizes current knowledge on the mutual interactions between MAPKs and AhR. The majority of studies has been done on cancer-derived cell lines that have impaired cell cycle regulation and lacks the complete detoxification apparatus. We emphasize the importance of the future studies that should be done on non-transformed cells to distinguish the role of MAPKs in cancer and normal cells. Primary cultures of human or rodent hepatocytes that are equipped with a fully functional biotransformation battery or xenobiotics-metabolizing extra-hepatic tissues should be the models of choice, as the results in our experiments confirm.

Expression and transcriptional activities of nuclear receptors involved in regulation of drug-metabolizing enzymes are not altered by colchicine: focus on PXR, CAR, and GR in primary human hepatocytes

Recent findings show that colchicine (COL) in submicromolar concentrations downregulates the expression of major drug-metabolizing P450 enzymes in human hepatocytes. Concomitantly, the expression of pregnane X receptor (PXR) and constitutive androstane receptor (CAR) was diminished by COL, whereas expression of glucocorticoid receptor (GR) remained unaltered. A tentative mechanism is perturbation of the GR-PXR/CAR-CYP2/3 signaling cascade, resulting in restricted transcriptional activity of GR receptor by colchicine. In this work we focused on 10-demethylcolchicine (colchiceine; EIN), a structural analogue and a putative metabolite of COL that lacks tubulin-binding activity. We investigated the effects of EIN on the expression of PXR, CAR, and GR receptors in primary cultures of human hepatocytes. In contrast with the effects of COL, EIN does not alter the expression of PXR, CAR, and/or GR receptors mRNAs. In addition, EIN had no effects on transcriptional activities of PXR, CAR, and GR receptors in reporter gene assays using transfected cell lines. Considering that COL and EIN are structurally very close and differ only in their tubulin-binding activity, the data presented imply that the deleterious effects of COL on the GR-PXR/CAR-CYP2/3 cascade are primarily due to perturbation of the microtubule network. Our data support the idea of replacing COL by EIN, which is less toxic and does not interact with xenoreceptors.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

Ketoconazole and Miconazole Are Antagonists of the Human Glucocorticoid Receptor: Consequences on the Expression and Function of the Constitutive Androstane Receptor and the Pregnane X Receptor

The constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) play a major part in the control of drug metabolism and transport. We have previously shown that PXR and CAR expression is controlled by the glucocorticoid receptor (GR) and proposed the existence of a signal transmission cascade GR-(PXR/CAR)-drug metabolizing and transporter systems. In the current study, we investigated the effect of ketoconazole and other azole-derived drugs, miconazole and fluconazole, on the transcriptional activity of the human GR (hGR) in HeLa and HepG2 cells, and in primary human hepatocytes. The data show that ketoconazole inhibits GR transcriptional activity and competes with dexamethasone for hGR binding. In primary human hepatocytes, ketoconazole inhibits the expression of 1) GR-responsive genes tyrosine aminotransferase and both PXR and CAR; 2) CAR and PXR target genes, including cytochromes P450 (P450) CYP2B6, CYP2C9, and CYP3A4; UDP-glucuronosyltransferase 1A1, glutathione S-transferases A1 and A2; and transporter proteins (phase III) solute carrier family 21 form A6 and multidrug resistance protein 2. In parallel experiments, ketoconazole affected neither the expression of GR, the expression of glyceraldehyde-3-phosphate dehydrogenase, nor the inducible expression of CYP1A1 and 1A2. Miconazole behaved like ketoconazole, whereas fluconazole had no effect. We conclude that, in addition to their well known inhibitory effect on P450 enzyme activities, ketoconazole and miconazole are antagonists of hGR. These results provide a novel molecular mechanism by which these compounds may exert adverse and toxic effects on drug metabolism and other functions in human.

Pharmacogenetics of oral anticoagulants

The use of oral anticoagulants (OA) is problematic due to its association with hemorrhagic complications. OA metabolism relies on the CYP2C9 complex. Genetic variations compromising metabolic competence of this complex may explain the risk of excessive and hazardous anticoagulation. A pharmacogenetics-based approach to this issue could be beneficial for choosing adequate dose and duration of treatment, in addition to having a better understanding of pharmacological interactions to which OA are susceptible. However, evidence from several basic and clinical studies indicates that both a complicated system of regulation of expression of multiple genes and the influence of a wide variety of epigenetic factors could be responsible for adverse drug reactions associated with the use of OA. Emphasis on understanding the gene-environment interactions could attain new paths to facilitate the use of these important drugs in the quotidian clinical practice.

Differential effects of selected natural compounds with anti-inflammatory activity on the glucocorticoid receptor and NF-κB in HeLa cells

Natural compounds have been used in the treatment of various diseases for centuries. Herein, we investigated the effects of structurally diverse alkaloids with anti-inflammatory activity (berberine, sanguinarine, chelerythrine, and colchicine) on two important anti-inflammatory and pro-inflammatory players, i.e. glucocorticoid receptor (GR) and nuclear factor kappa B (NF-kappaB), respectively. Sanguinarine and chelerythrine elicited nuclear translocation of the p65 subunit of NF-kappaB. The nuclear import of p65 was strongly augmented by these akaloids in non-stimulated cells as well as in cells challenged with tumor necrosis factor alpha (TNFalpha). Colchicine and berberine had no effect on p65 nuclear translocation regardless of the presence or absence of TNFalpha. Colchicine caused rapid degradation of the GR protein, whereas berberine had no effect on GR content or cellular localization. Sanguinarine and chelerythrine induced accumulation of GR in the nucleus with concomitant diminution of cytosolic GR. Analyses on the transcriptional activity of GR and NF-kappaB monitored by reporter assays using HeLa cells transiently transfected with glucocorticoid response element (pGRE-LUC) and/or NF-kappaB elements fused to luciferase gene (pNF-kappaB-luc) showed that none of the compounds tested had the capability to trigger GR and/or NF-kappaB transcriptional activities, respectively. The ligand binding assay showed that colchicine and berberine are not GR ligands whereas sanguinarine and chelerythrine significantly decreased binding of (3)H-labelled dexamethasone to GR. In conclusion, structurally diverse natural antiflogistics displayed differential effects on GR and NF-kappaB in HeLa cells.

A Toxicologist's Guide to the Preclinical Assessment of Hepatic Microsomal Enzyme Induction

The assessment of hepatic microsomal enzyme induction at the completion of preclinical toxicology studies in rodents and large mammals provides a wealth of information to the toxicologist and pharmacokineticist regarding how the drug-metabolizing system of the hepatocyte endoplasmic reticulum responded to high-dose levels of a xenobiotic designed for a specific pharmacological target in any of several target organs. The interpretation of these data can be greatly enhanced by a clear understanding of how this system functions and what the immediate and long-term ramifications are to organs and organ systems. This review focuses on how drugs modify the hepatic cytochrome P450 system, how those modifications are detected, the various consequences of these modifications, and some differences in the induction response among species.

Activation of the steroid and xenobiotic receptor (human pregnane X receptor) by nontaxane microtubule-stabilizing agents

PURPOSE

Because induction of drug efflux transporters is one of the major underlying mechanisms of drug resistance in cancer chemotherapy, and human pregnane X receptor (hPXR) is one of the principal "xenobiotic" receptors whose activation induces transporter and drug-metabolizing enzyme gene transcription, it would be ideal to develop chemotherapy drugs that do not activate hPXR. This report describes studies undertaken to explore the characteristics of hPXR stimulation and mechanisms of drug-receptor interactions in vitro with new anti-tubulin drugs.

EXPERIMENTAL DESIGN

In vitro transient transcription, glutathione S-transferase pull-down assays, and mammalian one-hybrid and two-hybrid systems were used to explore drug-receptor interactions. Loss of righting reflex was used to assess effects of drugs on PXR activity in vivo.

RESULTS

The current study showed that paclitaxel, discodermolide, and an analogue of epothilone B, BMS-247550, induced CYP3A4 protein expression in HepG2 hepatoma cells. Transient transcription assays of a luciferase reporter in the presence and absence of a GAL4-steroid and xenobiotic receptor (SXR) plasmid in HepG2 cells showed that these drugs activate hPXR. This was not true for the inactive analogue of paclitaxel, baccatin III, or for an analogue of epothilone A, analogue 5, none of which stabilizes microtubules. To determine the mechanisms by which paclitaxel, discodermolide, and BMS-247550 activate hPXR, a mammalian two-hybrid assay was done using VP16SRC-1 (coactivator) and GAL4-SXR. SRC-1 preferentially augmented the effects of these drugs on hPXR. Expression of SMRT (corepressor) but not NCoR suppressed the drug-induced activation of SXR by approximately 50%, indicating a selectivity in corepressor interaction with hPXR. These drugs resulted in shortened duration of loss of righting reflex in vivo, indicating drug-induced activation of PXR in mice.

CONCLUSION

These findings suggest that activation of hPXR with selective displacement of corepressors is an important mechanism by which microtubule-stabilizing drugs induce drug-metabolizing enzymes both in vitro and in vivo.

Influence of Matrigel-overlay on constitutive and inducible expression of nine genes encoding drug-metabolizing enzymes in primary human hepatocytes

1. Previous studies reported that rat hepatocytes overlaid with extracellular matrix components (Matrigel) maintain the expression and responsiveness of drug-metabolizing enzymes. However, whether Matrigel provides similar advantages in human hepatocytes remains largely uncertain.2. The influence of Matrigel-overlay on the constitutive and phenobarbital- and oltipraz-inducible expression of nine biotransformation enzymes, cytochrome P450s 1A1, 1A2, 2B6, 3A4, and glutathione S-transferases A1, A2, M1, T1, P1, in primary human hepatocytes was evaluated.3. Hepatocytes from five livers were maintained on a rigid collagen substratum with or without Matrigel overlay and treated for 48?h with two doses of each inducer. Quantitative RT-PCR, and for selected genes, immunoblot and enzyme activity analyses, demonstrated that human hepatocytes overlaid with Matrigel showed consistently higher constitutive and inducible expression of biotransformation genes. 4. Phenobarbital-mediated responsiveness of cytochrome P450 2B6, a potential indicator of hepatocyte differentiation status, was markedly higher in overlaid relative to non-overlaid hepatocytes. 5. It is concluded that an Matrigel overlay facilitates the maintenance and induction of xenobiotic metabolizing enzymes in primary cultures of human hepatocytes.

Novel genetic polymorphisms in the NR3C1 (glucocorticoid receptor) gene in a Japanese population

Glucocorticoid receptor, encoded by NR3C1, is a transcriptional regulator of many drug metabolizing enzymes and anti-inflammatory molecules. In order to identify genetic variations of the NR3C1 gene, genomic DNA from 265 Japanese individuals was sequenced. Fifty genetic polymorphisms were identified, including 32 novel ones [3 were in coding exons, 17 in the introns, 4 in the 5'-untranslated region (UTR), and 8 in the 5'-flanking region]. The novel nonsynonymous variation was 420G>T (Lys140Asn), and the allele frequency was 0.004. We did not detect any nonsynonymous polymorphism reported previously in other races, including a relatively frequent SNP Asn363Ser found in Caucasians and African-Americans. Thus, ethnic differences between Japanese and other races are suggested to exist in NR3C1.

Mitochondria in homeostasis of reactive oxygen species in cell, tissues, and organism

The recent knowledge on mitochondria as the substantial source of reactive oxygen species, namely superoxide and hydrogen peroxide efflux from mitochondria, is reviewed, as well as nitric oxide and subsequent peroxynitrite generation in mitochondria and their effects. The reactive oxygen species formation in extramitochondrial locations, in peroxisomes, by cytochrome P450, and NADPH oxidase reaction, is also briefly discussed. Conditions are pointed out under which mitochondria represent the major ROS source for the cell: higher percentage of non-phosphorylating and coupled mitochondria, in vivo oxygen levels leading to increased intensity of the reverse electron transport in the respiratory chain, and nitric oxide effects on the redox state of cytochromes. We formulate hypotheses on the crucial role of ROS generated in mitochondria for the whole cell and organism, in concert with extramitochondrial ROS and antioxidant defense. We hypothesize that a sudden decline of mitochondrial ROS production converts cells or their microenvironment into a "ROS sink" represented by the instantly released excessive capacity of ROS-detoxification mechanisms. A partial but immediate decline of mitochondrial ROS production may be triggered by activation of mitochondrial uncoupling, specifically by activation of recruited or constitutively present uncoupling proteins such as UCP2, which may counterbalance the mild oxidative stress.

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.

Disruption of microtubules leads to glucocorticoid receptor degradation in HeLa cell line

The role of microtubules (MTs) in steroid hormone-dependent human glucocorticoid receptor (hGR) activation/translocation is controversial. It was demonstrated recently that colchicine (COL) down-regulates hGR-driven genes in primary human hepatocytes by a mechanism involving inhibition of hGR translocation to the nucleus. To investigate whether inhibition of hGR translocation is the sole reason for its inactivation, we used human cervical carcinoma cells (HeLa) as a model. Herein we present evidence that perturbation of microtubules in HeLa cells leads to rapid time- and dose-dependent degradation of hGR protein. Degradation is proteasome mediated as revealed by its reversibility by proteasome inhibitor MG132. Moreover, degradation was observed for neither wt-hGR nor hGR mutants S226A and K419A in transiently transfected COS-1 cells. On the other hand, c-jun N-terminal kinase (JNK) seems not to be involved in the process because JNK inhibitor 1,9-Pyrazoloanthrone (SP600125) does not reverse hGR degradation. Similarly, another hGR functional antagonist, nuclear factor kappa beta (NFkappaB), did not play any role in the degradation process.

Comparative Effects of Fibrates on Drug Metabolizing Enzymes in Human Hepatocytes

PURPOSE

The induction potential of different fibric acid derivatives on human drug metabolizing enzymes was evaluated to help assess the role of enzyme induction on pharmacokinetic drug interactions.

METHODS

Effects of gemfibrozil, fenofibric acid, and clofibric acid on expression levels of cytochromes P450 (CYPs) 3A4 and 2C8 and UDP-glucuronyltransferase (UGT) 1A1 were evaluated in primary human hepatocyte cultures. The potential for these fibrates to activate human pregnane X receptor (PXR) also was studied in a cell-based PXR reporter gene assay.

RESULTS

All three fibrates caused increases in mRNA levels of CYP3A4 (2- to 5-fold), CYP2C8 (2- to 6-fold), and UGT1A1 (2- to 3-fold). On average, the effects on CYP3A4 were less than (< or =30% of rifampin), while those on CYP2C8 and UGT1A1 were comparable to or slightly higher than (up to 200% of rifampin) the corresponding effects observed with rifampin (10 microM). Consistent with the mRNA results, all fibrates caused moderate (approximately 2- to 3-fold) increases in CYP3A4 activity (measured by testosterone 6beta hydroxylase), as compared to about a 10-fold increase by rifampin. Significant increases (3- to 6-fold) in amodiaquine N-deethylase (a functional probe for CYP2C8 activity) also were observed with clofibric acid, fenofibric acid, and rifampin, in agreement with the mRNA finding. However, in contrast to the mRNA induction, marked decreases (>60%) in CYP2C8 activity were obtained with gemfibrozil treatment. Consistent with this finding, co-incubation of amodiaquine with gemfibrozil, but not with fenofibric acid, clofibric acid, or rifampin, in human liver microsomes or hepatocytes resulted in significantly decreased amodiaquine N-deethylase activity (IC50 = 80 microM for gemfibrozil, >500 microM for fenofibric or clofibric acid, and >50 microM for rifampin). Similar to rifampin, all three fibrates caused a modest change in the glucuronidation of chrysin, a nonspecific substrate of UGTs. No significant activation on human pregnane X receptor (PXR) was observed with the three fibrates in a PXR reporter gene assay.

CONCLUSIONS

In human hepatocytes, both fenofibric acid and clofibric acid are inducers of CYP3A4 and CYP2C8. Gemfibrozil is also an inducer of CYP3A4, but acts as both an inducer and an inhibitor of CYP2C8. In this system, all fibrates are weak inducers of UGT1A1. The enzyme inducing effects of fibrates appear to be mediated via a mechanism(s) other than PXR activation. These results suggest that fibrates may have potential to cause various pharmacokinetic drug interactions via their differential effects on enzyme induction and/or inhibition.

Pathophysiological Factors Affecting CAR Gene Expression

The body defends itself against potentially harmful compounds, such as drugs and toxic endogenous compounds and their metabolites, by inducing the expression of enzymes and transporters involved in their metabolism and elimination. The orphan nuclear receptor CAR (NR1I3 controls phase I (CYP2B, CYP2C, CYP3A), phase II (UGT1A1), and transporter (SLC21A6, MRP2) genes involved in drug metabolism and bilirubin clearance. Constitutive androstane receptor (CAR) is activated by xenobiotics, such as phenobarbital, but also by toxic endogenous compounds such as bilirubin metabolite(s). To better understand the inter- and intravariability in drug detoxification, we studied the molecular mechanisms involved in CAR gene expression in human hepatocytes. We clearly identified CAR as a glucocorticoid receptor (GR) target gene, and we proposed the hypothesis of a signal transduction where the activation of GR plays a critical function in CAR-mediated cellular response. According to our model, chemicals or pathophysiological factors that affect GR function should decrease CAR function. To test this hypothesis, we recently investigated the effect of microtubule disrupting agents (MIAs) or proinflammatory cytokines. These compounds are well-known inhibitors of GR transactivation property. MIAs activate c-Jun N-terminal kinase (JNK), which phosphorylates and inactivates GR, whereas proinflammatory cytokines, such as IL-6 or IL1beta, induce AP-1 or NF-kB activation, respectively, leading to GR inhibition. As expected, we observed that these molecules inhibit both CAR gene expression and phenobarbital-mediated CYP gene expression in human hepatocytes.