Analysis of cytochrome P450 and phase II conjugating enzyme expression in adult male rat hepatocytes

An update on T-2 toxin and its modified forms: metabolism, immunotoxicity mechanism, and human exposure assessment

T-2 toxin is the most toxic trichothecene mycotoxin, and it exerts potent toxic effects, including immunotoxicity, neurotoxicity, and reproductive toxicity. Recently, several novel metabolites, including 3',4'-dihydroxy-T-2 toxin and 4',4'-dihydroxy-T-2 toxin, have been uncovered. The enzymes CYP3A4 and carboxylesterase contribute to T-2 toxin metabolism, with 3'-hydroxy-T-2 toxin and HT-2 toxin as the corresponding primary products. Modified forms of T-2 toxin, including T-2-3-glucoside, exert their immunotoxic effects by signaling through JAK/STAT but not MAPK. T-2-3-glucoside results from hydrolyzation of the corresponding parent mycotoxin and other metabolites by the intestinal microbiota, which leads to enhanced toxicity. Increasing evidence has shown that autophagy, hypoxia-inducible factors, and exosomes are involved in T-2 toxin-induced immunotoxicity. Autophagy promotes the immunosuppression induced by T-2 toxin, and a complex crosstalk between apoptosis and autophagy exists. Very recently, "immune evasion" activity was reported to be associated with this toxin; this activity is initiated inside cells and allows pathogens to escape the host immune response. Moreover, T-2 toxin has the potential to trigger hypoxia in cells, which is related to activation of hypoxia-inducible factor and the release of exosomes, leading to immunotoxicity. Based on the data from a series of human exposure studies, free T-2 toxin, HT-2 toxin, and HT-2-4-glucuronide should be considered human T-2 toxin biomarkers in the urine. The present review focuses on novel findings related to the metabolism, immunotoxicity, and human exposure assessment of T-2 toxin and its modified forms. In particular, the immunotoxicity mechanisms of T-2 toxin and the toxicity mechanism of its modified form, as well as human T-2 toxin biomarkers, are discussed. This work will contribute to an improved understanding of the immunotoxicity mechanism of T-2 toxin and its modified forms.

Ginsenoside Rb1 Inhibits Doxorubicin-Triggered H9C2 Cell Apoptosis via Aryl Hydrocarbon Receptor

Doxorubicin (DOX) is a highly effective chemotherapeutic agent; however, the dose-dependent cardiotoxicity associated with DOX significantly limits its clinical application. In the present study, we investigated whether Rb1 could prevent DOX-induced apoptosis in H9C2 cells via aryl hydrocarbon receptor (AhR). H9C2 cells were treated with various concentrations (− μM) of Rb1. AhR, CYP1A protein and mRNA expression were quantified with Western blot and real-time PCR analyses. We also evaluated the expression levels of caspase-3 to assess the anti-apoptotic effects of Rb1. Our results showed that Rb1 attenuated DOX-induced cardiomyocytes injury and apoptosis and reduced caspase-3 and caspase-8, but not caspase-9 activity in DOX-treated H9C2 cells. Meanwhile, pre-treatment with Rb1 decreased the expression of caspase-3 and PARP in the protein levels, with no effects on cytochrome c, Bax, and Bcl-2 in DOX-stimulated cells. Rb1 markedly decreased the CYP1A1 and CYP1A2 expression induced by DOX. Furthermore, transfection with AhR siRNA or pre-treatment with AhR antagonist CH-223191 significantly inhibited the ability of Rb1 to decrease the induction of CYP1A, as well as caspase-3 protein levels following stimulation with DOX. In conclusion, these findings indicate that AhR plays an important role in the protection of Ginsenoside Rb1 against DOX-triggered apoptosis of H9C2 cells.

CYP450 Enzyme-Mediated Metabolism of TCAS and Its Inhibitory and Induced Effects on Metabolized Enzymes in Vitro

In this study, we investigated the enzymes catalyzing the phaseⅠmetabolism of thiacalixarene (TCAS) based on in vitro system including cDNA-expressed P450 enzymes, human liver microsomes plus inhibitors and monoclonal antibodies. In addition, the inhibitory potential of TCAS on major CYP450 drug metabolizing enzymes (CYP1A2, CYP2C9, CYP2B6, CYP2D6 and CYP3A4) was assessed. The results showed that CYP1A2 and CYP2C9 mediated TCAS hydroxylation. IC₅₀ values for TCAS in rat and human liver microsomes were greater than 50 µM, and it demonstrated a weak inhibition of rat and human CYP450 enzymes. Finally, sandwiched hepatocytes were used to evaluate the induction of CYP1A and CYP3A to define the function of TCAS in vivo. The results showed that incubation of TCAS at different concentrations for 72 h failed to induce CYP1A and CYP3A. However, incubation of the cells with 50 and 100 µM TCAS caused a profound decrease in the activities of CYP1A and CYP3A, which was probably due to cytotoxic effects, suggesting that exposure to TCAS might be a health concern.

High-throughput gene silencing and mRNA expression analysis in hepatocyte sandwich cultures

Primary hepatocyte sandwich cultures are useful for a variety of research applications where maintenance of metabolic competency is essential. To ensure an optimal hepatocellular phenotype, cells are seeded on collagen-coated dishes and embedded with an overlay of Matrigel. This culturing condition makes gene silencing by traditional reagent-mediated transfection methods challenging. Here, an siRNA delivery method in primary mouse hepatocytes that allows cells to be cultured with Matrigel overlay is described. This method delivers >80% mRNA silencing with minimal alterations in cell viability. A 96-well format allows for high-throughput RNA processing and downstream quantitative PCR applications and reduces time and resources. This format is particularly useful when experiments requiring many different sampling conditions (such as pharmacologic dose-response curves) are required.

The use of a high-throughput luminescent method to assess CYP3A enzyme induction in cultured rat hepatocytes

Assessment of a new chemical entity for cytochrome P450 (CYP) enzyme induction at an early stage in discovery is crucial to prevent potential drug-drug interactions. CYP3A, the most abundant CYP isoform in the liver, metabolizes approximately 50% of drugs currently on the market and is also a highly inducible enzyme. The use of both rat and human hepatocyte culture for the prediction of in vivo CYP3A induction has become refined and validated and is considered a standard in vitro model. The current evaluation of CYP3A enzyme induction involves the use of substrates requiring subsequent analysis of metabolites by high-performance liquid chromatography/mass spectrometry, which adds considerable time and cost. In the present study, we describe the use of a novel luminogenic substrate, luciferin-6'-pentafluoro-benzyl ether (PFBE), which allows for a fast and selective measurement of CYP3A enzyme induction in cultured rat hepatocytes. The extent of induction was evaluated using cells treated for 3 d with the prototypical inducers, dexamethasone, phenobarbital, and pregnenolone 16 alpha-carbonitrile (PCN). Enzyme activity was measured in the treated cells either by the depentafluorobenzylation of luciferin-PFBE or the testosterone 6-beta-hydroxylation. Using both methods, dexamethasone and PCN-treated cells exhibited strong CYP3A activity, whereas phenobarbital treatment resulted in a weak response. The fold induction varied between both methods, but this variability can be controlled by normalizing data from each treatment to a positive control. The results indicate that luciferin-PFBE is an attractive alternative to the use of conventional substrate, testosterone, providing a sensitive, robust, and rapid method compatible with the multiwell plate format for the assessment of CYP3A induction.

Role of NAD(P)H:quinone oxidoreductase 1 in clofibrate-mediated hepatoprotection from acetaminophen

Mice pretreated with the peroxisome proliferator clofibrate (CFB) are resistant to acetaminophen (APAP) hepatotoxicity. Whereas the mechanism of protection is not entirely known, CFB decreases protein adducts formed by the reactive metabolite of APAP, N-acetyl-p-benzoquinone imine (NAPQI). NAD(P)H:quinone oxidoreductase 1 (NQO1) is an enzyme with antioxidant properties that is responsible for the reduction of cellular quinones. We hypothesized that CFB increases NQO1 activity, which in turn enhances the conversion of NAPQI back to the parent APAP. This could explain the decreases in APAP covalent binding and glutathione depletion produced by CFB without affecting APAP bioactivation to NAPQI. Administration of CFB (500mg/kg, i.p.) to male CD-1 mice for 5 or 10 days increased NQO1 protein and activity levels. To evaluate the capacity of NQO1 to reduce NAPQI back to APAP, we utilized a microsomal activating system. Cytochrome P450 enzymes present in microsomes bioactivate APAP to NAPQI, which binds the electrophile trapping agent, N-acetyl cysteine (NAC). We analyzed the formation of APAP-NAC metabolite in the presence of human recombinant NQO1. Results indicate that NQO1 is capable of reducing NAPQI. The capacity of NQO1 to amelioriate APAP toxicity was then evaluated in primary hepatocytes. Primary hepatocytes isolated from mice dosed with CFB are resistant to APAP toxicity. These hepatocytes were also exposed to ES936, a high affinity, and irreversible inhibitor of NQO1 in the presence of APAP. Concentrations of ES936 that resulted in over 94% inhibition of NQO1 activity did not increase the susceptibility of hepatocytes from CFB treated mice to APAP. Whereas NQO1 is mechanistically capable of reducing NAPQI, CFB-mediated hepatoprotection does not appear to be dependent upon enhanced expression of NQO1.

The role of aryl hydrocarbon receptor in the pathogenesis of cardiovascular diseases

Numerous experimental and epidemiological studies have demonstrated that polycyclic aromatic hydrocarbons (PAHs), which are major constituents of cigarette tobacco tar, are strongly involved in the pathogenesis of the cardiovascular diseases (CVDs). Knowing that PAH-induced toxicities are mediated by the activation of a cytosolic receptor, aryl hydrocarbon receptor (AhR), which regulates the expression of a group of xenobiotic metabolizing enzymes (XMEs) such as CYP1A1, CYP1A2, CYP1B1, NQO1, and GSTA1, suggests a direct link between AhR-regulated XMEs and CVDs. Therefore, identifying the localization and expression of the AhR and its regulated XMEs in the cardiovascular system (CVS) is of major importance in understanding their physiological and pathological roles. Generally, it was believed that the levels of AhR-regulated XMEs are lower in the CVS than in the liver; however, it has been shown that similar or even higher levels of expression are demonstrated in the CVS in a tissue- and species-specific manner. Moreover, most, if not all, AhR-regulated XMEs are differentially expressed in most of the CVS, particularly in the endothelium cells, aorta, coronary arteries, and ventricles. Although the exact mechanisms of PAH-mediated cardiotoxicity are not fully understood, several mechanisms are proposed. Generally, induction of CYP1A1, CYP1A2, and CYP1B1 is considered cardiotoxic through generating reactive oxygen species (ROS), DNA adducts, and endogenous arachidonic acid metabolites. However the cardioprotective properties of NQO1 and GSTA1 are mainly attributed to the antioxidant effect by decreasing ROS and increasing the levels of endogenous antioxidants. This review provides a clear understanding of the role of AhR and its regulated XMEs in the pathogenesis of CVDs, in which imbalance in the expression of cardioprotective and cardiotoxic XMEs is the main determinant of PAH-mediated cardiotoxicity.

Induction of CYP2B and CYP2E1 in precision-cut rat liver slices cultured in defined medium

Many drugs and endogenous substances undergo biotransformation by cytochrome P450s (CYPs), and some drugs are also capable of modulating the expression of various CYPs. Knowledge of the potential of a drug to modulate CYPs is useful to help predict potential drug interactions. This study utilized precision-cut rat liver slices in dynamic organ culture to assess the effects of various media on the viability of rat liver slices and the expression of CYP2B and CYP2E1 when the slices are exposed to phenobarbital and isoniazid, which are drugs capable of inducing these respective CYPs. Liver slices were maintained in serum supplemented Waymouths medium and two different serum-free media, Hepatozyme (Life Technologies) and a new defined medium, which is named BPM. While Hepatozyme is considered a suitable medium to support primary hepatocyte cultures, this product did not maintain viable liver slices, even for 24 h. The serum containing and new defined media maintained viable liver slices for up to 96 h in culture. Phenobarbital (0.5 mM) and isoniazid (0.1 or 0.6 mM) did not affect viability in this model. In the absence of phenobarbital or isoniazid, liver slices maintained for 96 h in the new BPM medium maintained the respective levels of CYP2B and 2E1 protein at 1.8 and 1.9-fold higher than in slices maintained in the serum-containing medium. Phenobarbital exposure (0.5 mM) for 96 h induced CYP2B protein 5.2-fold in the BPM medium and 2.5-fold in the serum-containing medium. Isoniazid exposure (0.1 and 0.5 mM) for 96 h induced CYP2E1 protein 1.9 and 2.1-fold (respectively) in the BPM medium and 2.1 and 2.0-fold in the serum-containing medium. The respective CYP enzymatic activities were also increased by these drugs in a similar manner. Thus, the new defined BPM medium provides suitable conditions for maintaining CYP2B and 2E1 in liver slices and supports the investigation of drug-induced modulation of these enzymes.

Acetaminophen-induced hepatotoxicity of gel entrapped rat hepatocytes in hollow fibers

An important application of primary hepatocyte cultures is for hepatotoxicity research. In this paper, gel entrapment culture of rat hepatocytes in miniaturized BAL system were evaluated as a potential in vitro model for hepatotoxicity studies in comparison to monolayer cultures. After exposure for 24 and 48 h to acetaminophen (2.5 mM), gel entrapped hepatocytes were more severely damaged than hepatocyte monolayer detected by methyl thiazolyl tetrazolium (MTT) reduction, intracellular glutathione (GSH) content, reactive oxygen species (ROS) levels, urea genesis and albumin synthesis. CYP 2E1 activities detected by 4-nitrocatechol (4-NC) formation were higher in gel entrapped hepatocytes than in hepatocyte monolayers while the addition of CYP 2E1 inhibitor, diethyl-dithiocarbamate (DDC), more significantly reduced acetaminophen-induced toxicity in gel entrapped hepatocytes. In addition, protective effects of GSH, liquorice extract and glycyrrhizic acid against acetaminophen hepatotoxicity were clearly observed in gel entrapped hepatocytes but not in hepatocyte monolayer at an incubation time of 48 h. Overall, gel entrapped hepatocytes showed higher sensitivities to acetaminophen-induced hepatotoxicity than hepatocyte monolayer by a mechanism that higher CYP 2E1 activities of gel entrapped hepatocytes could induce more severe acetaminophen toxicity. This indicates that gel entrapped hepatocytes in hollow fiber system could be a promising model for toxicological study in vitro.

Inducibility of male-specific isoforms of cytochrome p450 by sex-dependent growth hormone profiles in hepatocyte cultures from male but not female rats

Although in vivo expression levels of the male-specific hepatic isoforms of cytochrome P450 (P450) (CYP2C11, CYP2C13, CYP2A2, and CYP3A2) are determined by the episodic growth hormone profile secreted by male rats, these isoforms have been completely refractory to growth hormone regulation in hepatocyte culture. By using species-specific rat growth hormone, at subphysiologic in vivo concentrations administered in two daily episodic pulses, we successfully induced CYP2C11 and CYP2A2 to near normal concentrations. Whereas inductive levels of CYP2C13 were subnormal, CYP3A2 was unresponsive to all hormonal treatments, quickly declining to undetectable concentrations. In agreement with in vivo findings, we observed that induction levels of the isoforms were always greatest when the male hepatocytes were exposed to the masculine-like episodic growth hormone profile and least stimulated by the continuous feminine-like hormone profile. When administered alone, dexamethasone consistently increased isoform levels. However, when administered with growth hormone, the glucocorticoid was always antagonistic, suppressing growth hormone induction of CYP2C11, CYP2C13, and CYP2A2. Finally, the P450 isoforms were completely unresponsive to all treatments when the hepatocytes were derived from female rats, supporting earlier findings that expression levels of sexually dimorphic P450 isoforms are inherently irreversible between sexes.

Differential effects of histone deacetylase inhibitors in tumor and normal cells-what is the toxicological relevance?

Histone deacetylase (HDAC) inhibitors target key steps of tumor development: They inhibit proliferation, induce differentiation and/or apoptosis, and exhibit potent antimetastatic and antiangiogenic properties in transformed cells in vitro and in vivo. Preliminary studies in animal models have revealed a relatively high tumor selectivity of HDAC inhibitors, strenghtening their promising potential in cancer chemotherapy. Until now, preclinical in vitro research has almost exclusively been performed in cancer cell lines and oncogene-transformed cells. However, as cell proliferation and apoptosis are essential for normal tissue and organ homeostasis, it is important to investigate how HDAC inhibitors influence the regulation of and interplay between proliferation, differentiation, and apoptosis in primary cells as well. This review highlights the discrepancies in molecular events triggered by trichostatin A, the reference compound of hydroxamic acid-containing HDAC inhibitors, in hepatoma cells and primary hepatocytes (which are key targets for drug-induced toxicity). The implications of these differential outcomes in both cell types are discussed with respect to both toxicology and drug development. In view of the future use of HDAC inhibitors as cytostatic drugs, it is highly recommended to include both tumor cells and their healthy counterparts in preclinical developmental studies. Screening the toxicological properties of compounds early in their development process, using a battery of different cell types, will enable researchers to discard those compounds bearing undesirable adverse activity before entering into expensive clinical trials. This will not only reduce the risk for harmful exposure of patients but also save time and money.

Effects of NO-1886 (Ibrolipim), a lipoprotein lipase-promoting agent, on gene induction of cytochrome P450s, carboxylesterases, and sulfotransferases in primary cultures of human hepatocytes

In the present study, the effects on expression of cytochrome P450 (CYP1A1, CYP1A2, CYP3A4 and CYP3A5), carboxylesterase (CES1 and CES2) and sulfotransferase (CHST1, CHST3, CHST4, CST, SULT2A1 and TPST2) mRNA in primary cultures of cryopreserved human hepatocytes were evaluated after exposure to NO-1886 (diethyl 4-[(4-bromo-2-cyanophenyl) carbamoyl] benzylphosphonate) for 48 hr at 2, 10, and 50 microM. Analysis was performed by RT-PCR in the presence of TaqMan probe. CYP1A1 and CYP1A2 mRNA levels after exposure to 50 microM omeprazole (positive control for CYP1As) were increased by 162 (p<0.001) and 37 times (p<0.001), respectively, compared with untreated controls. However, these mRNA levels were increased by 2 times or less after exposure to NO-1886. CYP3A4 and CYP3A5 mRNA levels after exposure to 50 microM rifampicin (positive control for CYP3As) were significantly increased by 5.8 (p<0.01) and 2.0 times (p<0.01), respectively, compared with untreated controls. The CYP3A4 mRNA level after exposure to 10 microM NO-1886 was increased by 1.3 times (p<0.05). Further, the CYP3A4 mRNA level after exposure to 50 microM NO-1886 was significantly increased by 3.6 times (p<0.001). However, the CYP3A5 mRNA level after exposure to 50 microM NO-1886 was not significantly increased. CES1 and CES2 mRNA levels after exposure to 50 microM NO-1886 were significantly increased by 1.4 (p<0.05) and 2.6 times (p<0.01), respectively, compared with untreated controls. CHST1, CST and SULT2A1 mRNA levels after exposure to 50 microM NO-1886 were significantly increased by 3.8 (p<0.001), 1.8 (p<0.01) and 4.4 times (p<0.01), respectively. CHST3, CHST4 and TPST2 mRNA levels after exposure to 50 microM NO-1886 were not significantly increased. This in vitro technique using primary cultured human hepatocytes is expected to be very useful for the preclinical evaluation of the induction of drug-metabolizing enzymes in humans.

Studies on the induction of rat hepatic CYP1A, CYP2B, CYP3A and CYP4A subfamily form mRNAs in vivo and in vitro using precision-cut rat liver slices

1. Real-time quantitative reverse transcription-polymerase chain reaction methodology (TaqMan(R)) was used to examine the induction of some selected rat hepatic cyto-chrome P450 (CYP) forms in vivo and in vitro using cultured precision-cut liver slices. 2. TaqMan primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1, CYP2B1/2, CYP3A1, CYP3A2 and CYP4A1 mRNAs. 3. To characterize the responsiveness of the rat CYP mRNA TaqMan primers and probe sets, rats were treated in vivo with a single intraperitoneal dose of 500 mg kg(-1) Aroclor 1254 (ARO) and with four daily oral doses of either 50 mg kg(-1) day(-1) dexamethasone (DEX) or 75 mg kg(-1) day(-1) methylclofenapate (MCP). Treatment with ARO produced 22 600-, 5480-, 648-, 52-, 47- and 9-fold increases in levels of CYP1A1, CYP2B1, CYP2B1/2, CYP1A2, CYP3A1 and CYP3A2 mRNA, respectively. DEX treatment produced 97-, 24-, 8- and 4-fold increases, respectively, in CYP3A1, CYP2B1, CYP2B1/2 and CYP3A2 mRNA levels, and MCP produced 339-, 126- and 25-fold increases, respectively, in CYP4A1, CYP2B1 and CYP2B1/2 mRNA levels. All three CYP inducers also increased microsomal CYP content and produced corresponding increases in CYP1A, CYP2B, CYP3A and CYP4A form marker enzyme activities. 4. Rat liver slices were cultured for 6 and 24 h in medium containing 0.1 micro M insulin and 0.1 micro M DEX, and also for 24 h in medium containing only 0.1 micro M insulin (DEX-free medium). Liver slices were cultured in control medium or in medium containing either 10 micro M beta-naphthoflavone (BNF), 10 micro g ml(-1) ARO, 500 micro M sodium phenobarbitone (NaPB), 20 micro M pregnenolone-16alpha -carbonitrile (PCN), 50 micro M Wy-14,643 (WY) or 50 micro M MCP. 5. With the exception of the effect of BNF on CYP1A1 mRNA levels, the induction of all the CYP mRNAs studied was greater after 24- than after 6-h treatment. Generally, the magnitude of induction of CYP mRNA levels was greater after 24 h in liver slices cultured in DEX-free than in DEX-supplemented medium. 6. Treatment of liver slices with BNF and ARO for 24 h in DEX-free medium produced 21- and 35-fold increases, respectively, and 38- and 37-fold increases, respectively, in CYP1A1 and CYP1A2 mRNA levels. NaPB, PCN, WY and MCP did not increase either CYP1A1 or CYP1A2 mRNA levels. 7. After 24 h, levels of CYP2B1/2 mRNA were increased 18-, 20-, 9-, 16- and 13-fold by treatment with ARO, NaPB, PCN, WY and MCP, respectively. PCN also produced 56- and 4-fold increases, respectively, in CYP3A1 and CYP3A2 mRNA levels. 8. Treatment with WY and MCP for 24 h produced 437- and 186-fold increases, respectively, in levels of CYP4A1 mRNA. None of the other CYP inducers studied had any effect on CYP4A1 mRNA levels. 9. The results demonstrate the utility of cultured precision-cut liver slices as an in vitro model system to evaluate the effects of xenobiotics on rat CYP1A, CYP2B, CYP3A and CYP4A form mRNA levels.

The role of investigative molecular toxicology in early stage drug development

Molecular toxicology, the application of molecular biology principles and technologies to preclinical safety assessment, represents a key tool for understanding mechanisms of toxicity and assessing the risks associated with specific toxicities. The application of gene expression markers to early stage preclinical safety assessment has the potential to impact pipelines in two main areas: lead optimisation and issue management. Lead optimisation focuses on deprioritising leads with significant, development-limiting toxicological liabilities while advancing those compounds with the greatest chance of successfully navigating the gauntlet of preclinical and clinical safety studies. Issue management utilises mechanistic toxicology studies to position non-development-limiting findings prior to the onset of Good Laboratory Practice studies in full development, and can help to identify and validate gene expression markers predictive of adverse events to avoid issues in second-generation projects. In this review, the authors describe the application of molecular toxicology to a standard pharmaceutical testing funnel, provide examples of the successful application of gene expression markers, and discuss the potential for future impact in several broad categories of clinically relevant toxicity.

Toxicogenomics: an opportunity to optimise drug development and safety evaluation

'Toxicogenomics', the use of complex populations of mRNA to understand toxicity, is a relatively new field which combines the wealth of gene sequence information with advances in miniaturisation technology. In a parallel evolutionary path that the broader field of toxicology has encountered, toxicogenomics is steadily changing from descriptive to mechanistic research and will ultimately progress to a predictive science. This review focuses on the application of microarray research tools to toxicology studies in preclinical development. In particular, the 'testing funnel' approach to candidate selection in drug development is used to discuss the strategic implementation of toxicogenomics to help develop drugs that harbour less toxicity. Examples of where toxicogenomics has increased mechanistic and descriptive understanding of cellular toxicity are discussed, as are early efforts to develop molecular biomarkers that are predictive of longer-term toxicity.

Use of a quantitative real-time reverse transcription-polymerase chain reaction method to study the induction of CYP1A, CYP2B and CYP4A forms in precision-cut rat liver slices

1. The aim was to employ real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) technology (TaqMan to examine the induction of some selected cytochrome P450 (CYP) forms in precision-cut rat liver slices. 2. Taqman primers and probe sets were developed for rat CYP1A1, CYP1A2, CYP2B1 and CYP4A1 forms. 3. Rat liver slices were cultured in control medium or medium containing either 10 micro g ml(-1) Aroclor 1254 (ARO), 500 micro M sodium phenobarbitone (NaPB) or 50 micro M Wy-14643 (WY) for 3, 6 and 24 h. 4. Compared with control liver slices, treatment with ARO for 3 and 6 h produced 24- and 184-fold increases, respectively, in CYP1A1 mRNA levels, and after 24h produced an 85-fold increase in CYP1A2 mRNA levels. Levels of CYP1A1 and CYP1A2 mRNA were not markedly affected by NaPB and WY. 5. Treatment with ARO and PB for 24 h produced 10.6- and 23.8-fold increases, respectively, in CYP2B1 mRNA. Levels of CYP2B1 mRNA were not markedly affected by WY. 6. Treatment with WY, but not ARO and NaPB, for 24h produced a 20.4-fold increase in levels of CYP4A1 mRNA. 7. These results demonstrate that cultured liver slices may be used to evaluate the effect of xenobiotics on CYP form mRNA levels.

Species comparison in P450 induction: effects of dexamethasone, omeprazole, and rifampin on P450 isoforms 1A and 3A in primary cultured hepatocytes from man, Sprague-Dawley rat, minipig, and beagle dog

Induction of P450 isoforms 1A (CYP1A) and 3A (CYP3A) by model inducers dexamethasone, omeprazole and rifampin was evaluated in primary cultured hepatocytes from man and laboratory animals. Inducer-specific species-differences were observed. Results with human hepatocytes from six human donors consistently show that both rifampin and dexamethasone were inducers of CYP3A activity (measured as testosterone 6beta-hydroxylase activity), with rifampin being more potent. Conversely, in rat hepatocytes, dexamethasone was a potent CYP3A inducer while rifampin was not an inducer. Rifampin but not dexamethasone induced CYP3A in minipig and beagle dog hepatocytes. Omeprazole was a potent inducer of CYP1A activity (measured as ethoxyresorufin-O-deethylase activity) in human, beagle dog and minipig hepatocytes, and not an inducer in rat hepatocytes. The species-differences observed suggest that human hepatocytes represent the most appropriate preclinical experimental system for the evaluation of P450 induction in human.

Characterization of biotransformation enzyme activities in primary rat proximal tubular cells

The proximal tubule is a frequent target for nephrotoxic compounds due to it's ability to transport and accumulate xenobiotics and their metabolites, as well as by the presence of an organ-selective set of biotransformation enzymes. The aim of the present study was to characterize the activities of different biotransformation enzymes during primary culturing of rat proximal tubular cells (PT cells). Specific marker substrates for determining cytochrome P450 (CYP450) activity of primary cultured PT cells include 7-ethoxyresorufin (CYP1A1), caffeine (CYP1A), testosterone (CY2B/C, CYP3A), tolbutamide (CYP2C) and dextromethorphan (CYP2D1). Activities of the CYP450 isoenzymes decreased considerably during culture with the greatest loss in activity within 24 h of culture. In addition, expression of CYP450 apoprotein, including CYP1A, CYP2C, CYP2D, CYP2E and CYP4A, was detected in microsomes from freshly isolated PT cells by immunoblotting using specific antibodies. CYP2B and CYP3A apoprotein could not be detected. Activity of the phase II biotransformation enzymes GST, GGT, beta-lyase and UGT was determined with 1-chloro-2,4-dinitrobenzene, L-glutamic acid gamma-(7-amido-4-methyl-coumarin), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine and 1-naphthol, respectively, as marker substrates. Activity of the phase II enzymes remained more stable and, in contrast to CYP450 activity, significant activity was still expressed after 1 week of PT cell culture. Thus, despite the obvious advantages of PT cells as an in-vitro model for studies of biotransformation mediated toxicity, the strong time dependency of especially phase I and, to a lesser extent, phase II biotransformation activities confers limitations to their application.

Microarray analysis of hepatotoxins in vitro reveals a correlation between gene expression profiles and mechanisms of toxicity

A rate-limiting step that occurs in the drug discovery process is toxicological evaluation of new compounds. New techniques that use small amounts of the experimental compound and provide a high degree of predictivity would greatly improve this process. The field of microarray technology, which allows one to monitor thousands of gene expression changes simultaneously, is rapidly advancing and is already being applied to numerous areas in toxicology. However, it remains to be determined if compounds with similar toxic mechanisms produce similar changes in transcriptional expression. In addition, it must be determined if gene expression changes caused by an agent in vitro would reflect those produced in vivo. In order to address these questions, we treated rat hepatocytes with 15 known hepatoxins (carbon tetrachloride, allyl alcohol, aroclor 1254, methotrexate, diquat, carbamazepine, methapyrilene, arsenic, diethylnitrosamine, monocrotaline, dimethyl-formamide, amiodarone, indomethacin, etoposide, and 3-methylcholanthrene) and used microarray technology to characterize the compounds based on gene expression changes. Our results showed that gene expressional profiles for compounds with similar toxic mechanisms indeed formed clusters, suggesting a similar effect on transcription. There was not complete identity, however, indicating that each compound produced a unique signature. These results show that large-scale analysis of gene expression using microarray technology has promise as a diagnostic tool for toxicology.

Effects of model inducers on thyroxine UDP-glucuronosyl-transferase activity in vitro in rat and mouse hepatocyte cultures

Thyroxine (T(4))-UDP-glucuronosyltransferase (UGT) activity was measured directly in cultured male Sprague-Dawley rat and OF-1 mouse hepatocyte monolayers. The activity of T(4)-UGT (pmol/min/g liver) in vitro in hepatocyte cultures was, after 24 hr in culture, equivalent to that previously measured in vivo in rat and mouse liver microsomes (Viollon-Abadie et al., 1999). A progressive decline in T(4)-UGT activity occurred over time in both rat and mouse hepatocyte cultures. Treatment of cultures with various model inducers such as phenobarbital (PB), beta-naphthoflavone (NF) and clofibric acid (CLO) induced a strong increase in T(4)-UGT activity in rat hepatocyte monolayers. In addition, and as expected from available in vivo data, treatment of rat hepatocyte cultures with NF also increased p-nitrophenol (PNP)-UGT activity and treatment with PB or CLO increased bilirubin (Bili)-UGT activity. In contrast, T(4)-UGT activity in mouse hepatocyte monolayers was not affected by the treatments, neither were PNP- and Bili- UGT activities. These in vitro data confirm our previous in vivo observations that these inducers increase rat but not mouse liver T(4)-UGT activities (Viollon-Abadie et al., 1999). The present study thus demonstrates that hepatocyte monolayers are appropriated for the evaluation and inter-species comparison of the effects of xenobiotics on T(4)-UGT activities.

Induction of cytochrome P450 2B1-mRNA and pentoxyresorufin O-depentylation after exposure of precision-cut rat liver slices to phenobarbital

Precision-cut rat liver slices were prepared from male Wistar rats with a Krumdieck slicer and cultured in William's medium E for up to 24 h. In untreated control slices, CYP2B1-mRNA concentration, which was quantified by competitive RT-PCR, did not decrease during this time. After exposure of the slices to 100 microM phenobarbital, CYP2B1-mRNA increased by about 10- or 60-fold after 6 or 24 h, respectively. The extent of this in vitro induction was similar to that after in vivo administration of 60 mg/kg phenobarbital. Pentoxyresorufin O-depentylation (PROD) was also inducible in vitro after 24 h, but to a lesser extent than the corresponding CYP-mRNA. Precision-cut liver slices proved to be a simple and reliable in vitro system for the sensitive detection of an induction by phenobarbital.

The pathologist and toxicologist in pharmaceutical product discovery

Significant change is occurring in the drug discovery paradigm; many companies are utilizing dedicated groups from the toxicology/ pathology disciplines to support early stage activities. The goal is to improve the efficiency of the discovery process for selecting a successful clinical candidate. Toxicity can be predicted by leveraging molecular techniques via rapid high-throughput, low-resource in vitro and in vivo test systems. Several important activities help create a platform to support rapid development of a new molecular entity. The proceedings of this symposium provide excellent examples of these applied concepts in pharmaceutical research and development. Leading biopharmaceutical companies recognize that a competitive advantage can be maintained via rapid characterization of animal models, the cellular identification of therapeutic targets, and improved sensitivity of efficacy assessment. The participation of the molecular pathologist in this quest is evolving rapidly, as evidenced by the growing number of pathologists that interact with drug discovery organizations.