Microarray analysis of hepatic gene expression in pyrazole-mediated hepatotoxicity: identification of potential stimuli of Cyp2a5 induction

Brief Report: HIV Infection Does Not Explain Higher Nicotine Metabolism in People Living With HIV

BACKGROUND

Smoking contributes to significant morbidity and mortality in people with HIV. People with HIV have relatively high nicotine metabolism rates, as measured by the nicotine metabolite ratio (NMR, 3-hydroxycotinine/cotinine). A higher NMR is associated with difficulty quitting smoking. We hypothesized that HIV infection might upregulate nicotine metabolism.

SETTING

A retrospective study of male current smokers in the Multicenter AIDS Cohort Study who HIV seroconverted between 1985 and 1993.

METHODS

Eligibility included having plasma stored before and after confirmed HIV seroconversion and current tobacco use. Samples were selected from the closest available visits before (median 3.3 months) and after (median 9.4 months) seroconversion. Antiretroviral therapy use was exclusionary. Cotinine and 3-hydroxycotinine were measured using liquid chromatography-tandem mass spectrometry. We compared NMR from plasma pre-HIV and post-HIV infection using signed-rank tests. We targeted a sample size of 71 pairs to achieve 80% power to detect a 0.1 unit increase in NMR with P = 0.05.

RESULTS

We analyzed paired samples from 78 participants; the median age was 34.5 years [interquartile range (IQR 29-40 years)]. The median NMR pre-HIV and post-HIV was 0.45 (IQR 0.32-0.54) and 0.46 (IQR 0.34-0.56), respectively. The median change in NMR postseroconversion was +0.01 (IQR -0.05, +0.09), P = 0.25. Stratification of median change in NMR by timing between samples or time since HIV seroconversion did not alter this finding.

CONCLUSIONS

Acquiring HIV had no measurable effect on NMR. We postulate that upregulation of the NMR may be due to direct pharmacologic effects of HIV medications or metabolic changes in response to HIV infection.

Nrf2-mediated induction of Cyp2a5 partially protects against reductive endoplasmic reticulum stress in mouse hepatocytes

Cytochrome P450 2a5 (Cyp2a5) is distinct from other P450 enzymes in that it is induced in the endoplasmic reticulum (ER) of mouse hepatocytes in conditions that are injurious to the liver. These conditions cause ER stress eventually resulting in apoptosis if not rectified. We previously showed that mouse hepatic Cyp2a5 is induced during reductive ER stress caused by the intramolecular disulfide form of dithiothreitol, trans-4,5-dihydroxy-1,2-dithiane (DTTox), and that overexpression of Cyp2a5 provides partial protection against apoptosis due to bilirubin (BR), a compound known to cause ER stress. The purpose of this study was to investigate the mechanism of Cyp2a5 gene regulation by DTTox and to determine if Cyp2a5 plays a cytoprotective role during reductive ER stress. Exposure to DTTox (10 mM) and another reductive ER stressor, 2-mercaptoethanol (1 mM), for 48 h markedly increased Cyp2a5 protein levels in primary mouse hepatocytes. In addition, DTTox transactivated Cyp2a5 via a mechanism involving the transcription factor nuclear factor-(erythroid-derived 2)-like 2 (Nrf2). Expression of the BR-conjugating enzyme, UDP glucuronosyl transferase 1A1 (UGT1A1) was also increased after DTTox treatment, however, this was reduced by Cyp2a5 overexpression. Hemin, a porphyrin inducer of Cyp2a5, induced mRNA splicing of X-box binding protein 1 (XBP-1), a transcription factor involved in the ER stress response, however, this was also reduced by Cyp2a5 overexpression. Finally, overexpression of Cyp2a5 partially blocked DTTox-mediated caspase-3 cleavage in Hepa 1-6 cells suggesting a cytoprotective role during ER stress. These findings demonstrate that Nrf2-mediated induction of Cyp2a5 in a reducing ER environment provides partial protection against ER stress-induced apoptosis by decreasing XBP-1 mRNA splicing and caspase-3 cleavage.

Differences in the rate of nicotine metabolism among smokers with and without HIV

OBJECTIVE

HIV-infected smokers lose more life years to tobacco use than to HIV infection. The nicotine metabolite ratio (NMR), a biomarker of CYP2A6, represents individual variation in the rate at which nicotine is metabolized and is associated with response to smoking cessation treatments. We evaluated whether HIV-infected smokers metabolize nicotine faster than HIV-uninfected smokers, which may contribute to the disproportionate smoking burden and may have important treatment implications.

DESIGN

We analysed baseline data from two clinical trials (NCT01710137; NCT01314001) to compare the NMR in HIV-infected smokers (N = 131) to HIV-uninfected smokers (N = 199).

METHODS

Propensity scores were used to match the groups 2 : 1 on characteristics that influence NMR: sex, race, BMI and smoking rate. Nicotine metabolites were assessed via liquid chromatography-tandem mass spectrometry methods and the ratio of 3-hydroxycotinine:cotinine was used to compute the NMR.

RESULTS

HIV-infected smokers had significantly higher NMR (mean = 0.47, SEM = 0.02) and were more likely to be in the highest NMR quartile compared with HIV-uninfected smokers (mean = 0.34, SEM = 0.02; Ps < 0.001).

CONCLUSION

The higher NMR observed among HIV-infected smokers may partially explain higher smoking rates and lower response to transdermal nicotine therapy. Understanding the mechanisms by which HIV and/or ART contribute to faster nicotine metabolism may guide the use of the NMR to personalize tobacco cessation strategies in this underserved population.

Interstrain differences in the expression and activity of Cyp2a5 in the mouse liver

Background

Cytochrome P450 2A5 (Cyp2a5), a mouse enzyme orthologous of human CYP2A6, catalyzes a number of toxicologically important reactions, including the metabolism of nicotine, aflatoxin B1, and several other xeno- and endobiotics. Cyp2a5 expression is complex and not yet fully understood. We investigated inter-strain differences in the activity and mRNA expression of hepatic Cyp2a5. Cyp1a1/2 and Cyp2b9/10 activities were evaluated for comparative purposes. Data on the interstrain differences in the expression and activity of Cyp2a5 are important to select a suitable mouse model for studying CYP2A6-mediated metabolism.

Results

Activity of Cyp2a5 (coumarin 7-hydroxylase) was highest in DBA-2 and DBA-1, intermediate in B6D2F1 (hybrid) and low in the remaining strains (C57BL/6, C57BL/10, CBA, BALB/cAn, SW). Contrasting with the activity, background levels of Cyp2a4/5 mRNA did not differ between high- and low-activity murine strains. Phenobarbital (PB, 80 mg/kg body weight/day × 3 days, i.p.) increased Cyp2a5, Cyp1a1/2 (ethoxyresorufin-O-deethylase) and Cyp2b9/10 (bezyloxyresorufin-O-debenzylase) activities while only Cyp2a5 was enhanced by pyrazole (PYR, 100 mg/kg body weight/day × 3 days, i.p.). Inductions of Cyp2a5 activity by PYR and PB were accompanied by increases of Cyp2a4/5 mRNA. PYR and PB did not upregulate heme oxygenase-1 (hmox-1) mRNA expression in any strain, a finding that is apparently at odds with the notion that Cyp2a5 and hmox-1 inductions are coordinated events.

Conclusions

Since background levels of Cyp2a4/5 gene transcripts of high-activity strains did not differ from those of low-activity mice, distinct constitutive activities did not result from different transcription rates and/or mRNA half-lives. Results therefore suggested that interstrain differences in constitutive activity of Cyp2a5 possibly arise from distinct translation efficiencies, protein half-lives and/or enzyme kinetics toward the substrate. Data from this study indicated that all tested strains are suitable models for studying toxicants that are substrates for human CYP2A6; DBA-2, DBA-1 and the hybrid B62DF1, however, have the advantage of presenting high constitutive activities of Cyp2a5.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-017-2435-x) contains supplementary material, which is available to authorized users.

Liver Fatty Acid Binding Protein Deficiency Provokes Oxidative Stress, Inflammation, and Apoptosis-Mediated Hepatotoxicity Induced by Pyrazinamide in Zebrafish Larvae

Pyrazinamide (PZA) is an essential antitubercular drug, but little is still known about its hepatotoxicity potential. This study examined the effects of PZA exposure on zebrafish (Danio rerio) larvae and the mechanisms underlying its hepatotoxicity. A transgenic line of zebrafish larvae that expressed enhanced green fluorescent protein (EGFP) in the liver was incubated with 1, 2.5, and 5 mM PZA from 72 h postfertilization (hpf). Different endpoints such as mortality, morphology changes in the size and shape of the liver, histological changes, transaminase analysis and apoptosis, markers of oxidative and genetic damage, as well as the expression of certain genes were selected to evaluate PZA-induced hepatotoxicity. Our results confirm the manner of PZA dose-dependent hepatotoxicity. PZA was found to induce marked injury in zebrafish larvae, such as liver atrophy, elevations of transaminase levels, oxidative stress, and hepatocyte apoptosis. To further understand the mechanism behind PZA-induced hepatotoxicity, changes in gene expression levels in zebrafish larvae exposed to PZA for 72 h postexposure (hpe) were determined. The results of this study demonstrated that PZA decreased the expression levels of liver fatty acid binding protein (L-FABP) and its target gene, peroxisome proliferator-activated receptor α (PPAR-α), and provoked more severe oxidative stress and hepatitis via the upregulation of inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and transforming growth factor β (TGF-β). These findings suggest that L-FABP-mediated PPAR-α downregulation appears to be a hepatotoxic response resulting from zebrafish larva liver cell apoptosis, and L-FABP can be used as a biomarker for the early detection of PZA-induced liver damage in zebrafish larvae.

Modulation of cytochrome P450 2A5 activity by lipopolysaccharide: low-dose effects and non-monotonic dose-response relationship

Mouse cytochrome P450 (CYP) 2A5 is induced by inflammatory conditions and infectious diseases that down-regulate the expression and activity of most other CYP isoforms. Enhanced oxidative stress and nuclear factor (erythroid 2-related factor) 2 (Nrf2) transcription factor activation have been hypothesised to mediate up-regulation of CYP2A5 expression in the murine liver. The unique and complex regulation of CYP2A5, however, is far from being thoroughly elucidated. Sepsis and high doses of bacterial lipopolysaccharide (LPS) elicit oxidative stress in the liver, but depression, not induction, of CYP2A5 has been observed in studies of mice treated with LPS. The foregoing facts prompted us to evaluate the response of CYP2A5 liver activity in female DBA-2 mice over a broad range of LPS doses (0, 0.025, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 mg/kg). Cytokine levels (interleukin [IL]-2, IL-4, IL-6, IL-10, IL-17A, interferon gamma, tumour necrosis factor alpha) and nitric oxide (NO) were measured in the blood serum. Activities of CYP1A (EROD) and CYP2B (BROD) in the liver were also determined for comparative purposes. LPS depressed CYP2A5 at low doses (0.025–2.0 mg/kg) but not at doses (>2 mg/kg) that increased pro-inflammatory cytokines and NO serum levels, and depressed CYP1A and CYP2B activities. Blockade of pro-inflammatory cytokines and the overproduction of NO induced by co-treatment with pentoxifylline and LPS and iNOS inhibition with aminoguanidine both extended down-regulation of CYP2A5 to the high dose range while not affecting LPS-induced depression of CYP1A and CYP2B. Overall, the results suggested that NO plays a role in the reversal of the low-dose LPS-induced depression of CYP2A5 observed when mice were challenged with higher doses of LPS.

Expression of cytochrome P450 2A5 in a C57BL/6J mouse model of nonalcoholic fatty liver disease

Cytochrome P450 2A5 (CYP2A5) expression is induced during the hepatotoxicity caused by various hepatotoxins and hepatitis. However, CYP2A5 expression during nonalcoholic fatty liver disease (NAFLD) is still unknown. In this study, serum biochemical parameters and liver histopathological analyses found that NAFLD had developed in C57BL/6J mice via administration of a high-fat diet continuously for 8 weeks. Subsequently, CYP2A5 expression was probed in the mice diagnosed with NAFLD that were treated with or without pyrazole, the inducer of chemical liver injury. It is shown that hepatic CYP2A5 mRNA, protein expression and coumarin 7-hydroxylase activity are enhanced with high-fat feed, and that pyrazole is able to further increase CYP2A5 expression and activity in mice with NAFLD. These results revealed that CYP2A5 is elevated during NAFLD and suggested that pyrazole and NAFLD act synergistically to induce the expression of CYP2A5 via an unclear mechanism.

Toxicogenomic analysis of the gene expression changes in rat liver after a 28-day oral Tripterygium wilfordii multiglycoside exposure

ETHNOPHARMACOLOGICAL RELEVANCE

Tripterygium wilfordii multiglycoside (GTW), which is an extract derived from Tripterygium wilfordii Hook.f., has been used for the treatment of rheumatoid arthritis and other immune diseases in China. However, its potential hepatotoxicity has not been completely investigated.

THE AIM OF THE STUDY

The aim of the study was to determine the hepatotoxicity of GTW in Wistar rats and to investigate the underlying cellular mechanism further by microarray analysis.

MATERIALS AND METHODS

Doses of GTW at 60, 100 and 120mg/kg/day were administered by oral gavage for subchronic toxicity in Wistar rats. Changes in the hepatic gene expression were identified with oligonucleotide microarrays at the 100-mg/kg/day dose level to study the hepatotoxic mechanism of GTW.

RESULTS AND CONCLUSIONS

A number of changes in the body weight and food consumption, absolute and relative liver weight, biochemical analysis and histopathology were observed after the subacute exposure to GTW, and a dose-dependent hepatotoxicity was observed. A total of 1312 genes were found to be significantly altered (2-fold, P<0.05), including 582 up-regulated genes and 730 down-regulated genes. According to our biological pathway analysis, the GTW resulted in aberrant gene expression in metabolic pathways and the peroxisome proliferator-activated receptor (PPAR) signaling pathway and cellular stress. Real-time PCR analyses of several genes verified these results. Consequently, our gene expression microarray study will be useful for future GTW hepatotoxicity studies.

In vivo study of hepatitis B vaccine effects on inflammation and metabolism gene expression

Pharmaceutical companies usually perform safety testing of vaccines, but all requirements of the World Health Organization and drug pharmacopoeias depend on general toxicity testing, and the gene expression study of hepatitis B vaccine is not done routinely to test vaccine quality. In this study, we applied a new technique of gene expression analysis to detect the inflammation and metabolism genes that might be affected by hepatitis B vaccine in mouse liver. Mice were used and divided into three groups: the first and second groups were treated with one or two human doses of vaccine, respectively, and the third group was used as a control. A microarray test showed that expression of 144 genes in the liver was significantly changed after 1 day of vaccination. Seven of these genes, which were related to inflammation and metabolism, were chosen and confirmed by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) at 1, 4 and 7 days. The expression level of these genes can be considered as a biomarker for the effects of the vaccine.

Inducible bilirubin oxidase: a novel function for the mouse cytochrome P450 2A5

We have previously shown that bilirubin (BR), a breakdown product of haem, is a strong inhibitor and a high affinity substrate of the mouse cytochrome P450 2A5 (CYP2A5). The antioxidant BR, which is cytotoxic at high concentrations, is potentially useful in cellular protection against oxygen radicals if its intracellular levels can be strictly controlled. The mechanisms that regulate cellular BR levels are still obscure. In this paper we provide preliminary evidence for a novel function of CYP2A5 as hepatic "BR oxidase". A high-performance liquid chromatography/electrospray ionisation mass spectrometry screening showed that recombinant yeast microsomes expressing the CYP2A5 oxidise BR to biliverdin, as the main metabolite, and to three other smaller products with m/z values of 301, 315 and 333. The metabolic profile is significantly different from that of chemical oxidation of BR. In chemical oxidation the smaller products were the main metabolites. This suggests that the enzymatic reaction is selective, towards biliverdin production. Bilirubin treatment of primary hepatocytes increased the CYP2A5 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A5 compared to cells treated only with CHX. Collectively, the observations suggest that the CYP2A5 is potentially an inducible "BR oxidase" where BR may accelerate its own metabolism through stabilization of the CYP2A5 protein. It is possible that this metabolic pathway is potentially part of the machinery controlling intracellular BR levels in transient oxidative stress situations, in which high amounts of BR are produced.

Ethanol induction of CYP2A5: permissive role for CYP2E1

CYP2A5 metabolizes xenobiotics and activates hepatocarcinogens, and induction occurs in response to hepatic damage and cellular stress. We evaluated whether ethanol can elevate CYP2A5 and whether CYP2E1 plays a role in the ethanol induction of CYP2A5. Wild-type (WT), CYP2E1 knockout (KO), and CYP2E1 knockin (KI) mice were fed ethanol for 3 weeks. Ethanol increased CYP2E1 and CYP2A5 protein and activity in WT mice but not in the KO mice. Induction of CYP2A5 (and CYP2E1) was restored in the KI mice. Ethanol induction of CYP2A5 occurred only after CYP2E1 was first induced. Immunohistochemical staining revealed that CYP2E1 and CYP2A5 colocalize to the same zones in the liver. Ethanol also elevated CYP2A5 mRNA levels in WT and KI mice but not in KO mice. Induction of CYP2A5 by cadmium was partially decreased in KO mice compared with WT or KI mice. Ethanol elevated CYP2A4 mRNA levels in all mice although the extent of induction was lowest in the KO mice. In summary, ethanol elevated mouse hepatic CYP2A5 levels, which may be of toxicological significance because CYP2A5 metabolizes nicotine and other drugs and activates hepatocarcinogens. Induction of CYP2A5 by ethanol is potentiated by the induction of CYP2E1. We speculate that ethanol induction of CYP2E1 followed by increases in reactive oxygen species and activation of Nrf2 are important steps in the mechanism by which ethanol induces CYP2A5. The possibility that induction of CYP2E1 is permissive for the induction of CYP2A5 may reflect a new contribution by CYP2E1 to the actions of ethanol.

Synthesis of new 2-naphthyl ethers and their protective activities against DNA damage induced by bleomycin-iron

The reaction of 2-naphthaloxyacetic acid with thiosemicarbazide in the presence of phosphoryl chloride, followed by treatment with phenacylbromides, led to the formation of imidazo[2,1-b][1,3,4]thiadiazoles 3a-c. 2-(Naphthalen-3-yloxy)acetohydrazide 4 on treatment with ethyl 2-(2-arylhydrazono)-3-oxobutanoates (5a-c), 2-methoxymethylene)malononitrile, or ethyl 2-cyano-3,3-bis(methylthio)acrylate led to the formation of substituted pyrazoles 6-8. The reaction of the hydrazide 4 with hydrazonoyl chlorides 9a-c and 1,2,4,5-benzene tetracarboxylic-1,2:4,5-dianhydride produced bis-diazo compounds 10a-c and dimide 11 respectively. All new compounds were tested for their protective activity against DNA damage induced by bleomycin-iron complex. Compound 2 showed the greatest protection against DNA damage, thus diminishing chromogen formation between the damaged DNA and thiobarbituric acid.

Hepatic cytochrome P450 enzyme alterations in humans with progressive stages of nonalcoholic fatty liver disease

Members of the cytochrome P450 (P450) enzyme families CYP1, CYP2, and CYP3 are responsible for the metabolism of approximately 75% of all clinically relevant drugs. With the increased prevalence of nonalcoholic fatty liver disease (NAFLD), it is likely that patients with this disease represent an emerging population at significant risk for alterations in these important drug-metabolizing enzymes. The purpose of this study was to determine whether three progressive stages of human NALFD alter hepatic P450 expression and activity. Microsomes isolated from human liver samples diagnosed as normal, n = 20; steatosis, n = 11; nonalcoholic steatohepatitis (NASH) (fatty liver), n = 10; and NASH (no longer fatty), n = 11 were analyzed for P450 mRNA, protein, and enzyme activity. Microsomal CYP1A2, CYP2D6, and CYP2E1 mRNA levels were decreased with NAFLD progression, whereas CYP2A6, CYP2B6, and CYP2C9 mRNA expression increased. Microsomal protein expression of CYP1A2, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 tended to decrease with NAFLD progression. Likewise, functional activity assays revealed decreasing trends in CYP1A2 (p = 0.001) and CYP2C19 (p = 0.05) enzymatic activity with increasing NAFLD severity. In contrast, activity of CYP2A6 (p = 0.001) and CYP2C9 (diclofenac, p = 0.0001; tolbutamide, p = 0.004) was significantly increased with NAFLD progression. Increased expression of proinflammatory cytokines tumor necrosis factor alpha and interleukin 1beta was observed and may be responsible for observed decreases in respective P450 activity. Furthermore, elevated CYP2C9 activity during NAFLD progression correlated with elevated hypoxia-induced factor 1alpha expression in the later stages of NAFLD. These results suggest that significant and novel changes occur in hepatic P450 activity during progressive stages of NAFLD.

Genomic consequences of cytochrome P450 2C9 overexpression in human hepatoma cells

Cytochrome P450 2C9 (P450 2C9) is one of the most important P450 isoforms in the human liver, as it metabolizes numerous exogenous and endogenous substrates. Moreover, it is inducible by several compounds, such as rifampicin, phenobarbital, and NSAIDs (nonsteroidal anti-inflammatories). The aim of this study was to investigate the global cellular consequences of P450 2C9 overexpression at the transcriptional level using an untargeted approach: pangenomic microarrays. Recombinant adenovirus was used to express P450 2C9 instead of an inducer to prevent a per se effect of inducer or its metabolites. P450 2C9 overexpression induced endoplasmic reticulum (ER) stress and regulated genes implicated in the unfolded protein response (UPR) as heat shock protein (HSP) (we studied particurlarly HSPA5 and HSPB1) and in the endoplasmic reticulum associated degradation (ERAD) system as Sec61 and ubiquitin and proteasome pathways. UPR and ERAD are two mechanisms of adaptative response to ER stress. Moreover, activation of Akt was observed in HepG2 cells that overexpress P450 2C9 and might participate in the cellular adaptive response to stress, thus leading to the activation of cell survival pathways. UPR and ERAD should be caused by accumulation of native and misfolded P450 2C9 protein. Our results indicated that P450 2C9 overexpression did not lead to toxicity but induced an ER stress due to protein overexpression rather than mono-oxygenase activity. The ER stress triggered activation of the adaptative response and of pathways leading to cell survival.

Glucocorticoid and adrenalectomy effects on the rat aryl hydrocarbon receptor pathway depend on the dosing regimen and post-surgical time

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the effects of aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene (MC); the prototypical response is induction of drug-metabolizing enzymes. Factors that regulate AHR levels in vivo are poorly understood and it is also not clear how AHR levels affect aromatic hydrocarbon responsiveness. Our interest in pituitary-dependent regulation of AHR levels was prompted by two findings from our laboratory: (1) hypophysectomized rats have reduced hepatic levels of AHR protein; and (2) glucocorticoids increase AHR expression and aromatic hydrocarbon responsiveness in rodent hepatoma cells. To study whether adrenalectomy and glucocorticoids contribute to hormone-dependent regulation of the hepatic AHR pathway, male adrenalectomized (ADX) or SHAM-ADX rats were treated with dexamethasone (DEX) or vehicle. AHR protein was depleted by 50-60% at 4 days after ADX, but was not altered by DEX treatment. To assess whether the observed AHR depletion affected aromatic hydrocarbon responsiveness, the induction of hepatic cytochrome P450 1B1 (CYP1B1) mRNA by MC was measured as an AHR-mediated adaptive response. MC-induced hepatic CYP1B1 mRNA was reduced by 50% in ADX rats relative to SHAM-ADX. Exogenous glucocorticoid treatment (DEX - 1.5mg/kg) induced hepatic AHR nuclear translocator (ARNT) mRNA by up to 9-fold at 3 and 6h after dosing, with no corresponding change in ARNT protein levels. These data demonstrate that: (1) adrenal-dependent factors contribute to the physiological maintenance of hepatic AHR protein levels; (2) the depletion of hepatic AHR protein in ADX rats coincided with a diminished adaptive response to MC; and (3) exogenous glucocorticoid treatment increases hepatic ARNT mRNA levels regardless of adrenal status. This model is useful for studying the mechanisms of AHR and ARNT regulation and for further characterization of the impact of AHR protein depletion on the response to aromatic hydrocarbons in vivo.

Discernment of possible mechanisms of hepatotoxicity via biological processes over-represented by co-expressed genes

BACKGROUND

Hepatotoxicity is a form of liver injury caused by exposure to stressors. Genomic-based approaches have been used to detect changes in transcription in response to hepatotoxicants. However, there are no straightforward ways of using co-expressed genes anchored to a phenotype or constrained by the experimental design for discerning mechanisms of a biological response.

RESULTS

Through the analysis of a gene expression dataset containing 318 liver samples from rats exposed to hepatotoxicants and leveraging alanine aminotransferase (ALT), a serum enzyme indicative of liver injury as the phenotypic marker, we identified biological processes and molecular pathways that may be associated with mechanisms of hepatotoxicity. Our analysis used an approach called Coherent Co-expression Biclustering (cc-Biclustering) for clustering a subset of genes through a coherent (consistency) measure within each group of samples representing a subset of experimental conditions. Supervised biclustering identified 87 genes co-expressed and correlated with ALT in all the samples exposed to the chemicals. None of the over-represented pathways related to liver injury. However, biclusters with subsets of samples exposed to one of the 7 hepatotoxicants, but not to a non-toxic isomer, contained co-expressed genes that represented pathways related to a stress response. Unsupervised biclustering of the data resulted in 1) four to five times more genes within the bicluster containing all the samples exposed to the chemicals, 2) biclusters with co-expression of genes that discerned 1,4 dichlorobenzene (a non-toxic isomer at low and mid doses) from the other chemicals, pathways and biological processes that underlie liver injury and 3) a bicluster with genes up-regulated in an early response to toxic exposure.

CONCLUSION

We obtained clusters of co-expressed genes that over-represented biological processes and molecular pathways related to hepatotoxicity in the rat. The mechanisms involved in the response of the liver to the exposure to 1,4-dichlorobenzene suggest non-genotoxicity whereas the exposure to the hepatotoxicants could be DNA damaging leading to overall genomic instability and activation of cell cycle check point signaling. In addition, key pathways and biological processes representative of an inflammatory response, energy production and apoptosis were impacted by the hepatotoxicant exposures that manifested liver injury in the rat.

Role of nitric oxide and nuclear factor-kappaB in the CYP2E1 potentiation of tumor necrosis factor alpha hepatotoxicity in mice

Induction of CYP2E1 by pyrazole (PY) potentiated the hepatotoxicity induced by TNFalpha in mice. We evaluated the role of nitrosative and oxidative stress and the NF-kappaB activation pathway in this liver injury. The iNOS inhibitor N-(3-aminomethyl)benzylacetamindine (1400W) or the antioxidant N-acetyl-l-cysteine (NAC) prevented this liver injury. TNFalpha plus PY treatment triggered radical stress in the liver with increased lipid peroxidation and decreased glutathione and caused mitochondrial damage as reflected by elevated membrane swelling and cytochrome c release. The radical stress and mitochondrial damage were prevented by 1400W and NAC. TNFalpha plus PY treatment elevated 3-nitrotyrosine adduct formation and induced NOS2 in the liver; 1400W and NAC blocked these changes. A lower extent of liver injury and oxidative stress was found in NOS2(-/-) mice treated with TNFalpha plus PY compared with wild-type controls. Neither 1400W nor NAC modified CYP2E1 activity or protein. Activation of JNK and p38MAPK was weaker in TNFalpha plus PY-treated NOS2(-/-) mice and 1400W and NAC blocked the activation of JNK and p38MAPK in wild-type mice. IKKalpha/beta protein levels were decreased by TNFalpha plus PY treatment, whereas IkappaBalpha and IkappaBbeta protein levels were elevated compared with saline, PY, or TNFalpha alone. NF-kappaB DNA binding activity was increased by TNFalpha alone but lowered by TNFalpha plus PY. All these changes were blocked by 1400W and NAC. NF-kappaB activation products such as Bcl-2, Bcl-X(L), cFLIP(S), cFLIP(L), and Mn-SOD were reduced by TNFalpha plus PY and restored by 1400W or NAC. We conclude that TNFalpha plus CYP2E1 induces oxidative/nitrosative stress, which plays a role in the activation of JNK or p38MAPK and mitochondrial damage. These effects combine with the blunting of the NF-kappaB activation pathways and the synthesis of protective factors to cause liver injury.

Inhibition of the mitochondrial permeability transition by cyclosporin A prevents pyrazole plus lipopolysaccharide-induced liver injury in mice

Previous results showed that pyrazole potentiates lipopolysaccharide (LPS)-induced liver injury in mice. Mechanisms involved the overexpression of cytochrome P450 2E1 (CYP2E1), oxidative stress, and activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). The current study was carried out to test the hypothesis that the mitochondria permeability transition (MPT) plays a role in this pyrazole plus LPS toxicity. Mice were injected intraperitoneally with pyrazole for 2 days, followed by a challenge with LPS with or without treatment with cyclosporin A (CsA), an inhibitor of the MPT. Serum alanine aminotransferase and aspartate aminotransferase were increased by pyrazole plus LPS treatment, and CsA treatment could attenuate these increases. CsA also prevented pyrazole plus LPS-induced hepatocyte necrosis. Formation of 4-hydroxynonenal protein adducts and 3-nitrotyrosine protein adducts in liver tissue was increased by the pyrazole plus LPS treatment, and CsA treatment blunted these increases. Swelling, cytochrome c release from mitochondria to the cytosol, and lipid peroxidation were increased in mitochondria isolated from the pyrazole plus LPS-treated mice, and CsA treatment prevented these changes. CsA did not prevent the increased levels of inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha (TNF-alpha), pp38 MAPK, and p-JNK2. In conclusion, although CsA does not prevent elevations in upstream mediators of the pyrazole plus LPS toxicity (iNOS, TNF-alpha, CYP2E1, MAPK), it does protect mice from the pyrazole plus LPS-induced liver toxicity by preventing the MPT and release of cytochrome c and decreasing mitochondrial oxidative stress. These results indicate that mitochondria are the critical targets of pyrazole plus LPS in mediating liver injury.

Expression of cytochrome P450 2A5 in a glucose-6-phosphate dehydrogenase-deficient mouse model of oxidative stress

Murine hepatic cytochrome P450 2A5 (CYP2A5), unlike most CYP enzymes, is upregulated during hepatitis and hepatotoxic conditions, but the common stimulus for its induction remains unknown. We investigated the involvement of oxidative stress in the regulation of CYP2A5 expression using an oxidative stress-sensitive glucose-6-phosphate dehydrogenase (G6PD)-deficient mouse model. Treatment of deficient and wild-type mice with the prototypical CYP2A5-inducer pyrazole for 72h led to a significantly greater degree of induction of CYP2A5 mRNA, protein and activity in deficient mice, with the greatest increase observed in animals homozygous for the deficiency. However, markers of oxidative stress including protein carbonyl, 8-hydroxydeoxyguanosine, malondiadehyde and 4-hydroxyalkenal levels were unaltered with pyrazole treatment. Furthermore, CYP2A5 expression was not altered in G6PD-deficient mice treated with the pro-oxidant menadione whereas DNA, lipid, and protein markers of oxidative stress were significantly increased. The antioxidant polyethylene glycol-conjugated catalase, while decreasing oxidative stress in menadione-treated mice, did not prevent the induction of CYP2A5 by pyrazole. Finally, the ER stress marker protein, GRP78, was increased following pyrazole treatment in G6PD-deficient compared to wild-type mice. These findings do not support a central role for generalized cellular oxidative stress in the regulation of CYP2A5 and suggest that additional factors related to G6PD-deficiency, such as ER stress, may be involved.