Comparison of oxidative stress response parameters in newborn mouse liver versus simian virus 40 (SV40)-transformed hepatocyte cell lines

Emerging Roles of Aryl Hydrocarbon Receptors in the Altered Clearance of Drugs during Chronic Kidney Disease

Chronic kidney disease (CKD) is a major public health problem, since 300,000,000 people in the world display a glomerular filtration rate (GFR) below 60 mL/min/1.73m². Patients with CKD have high rates of complications and comorbidities. Thus, they require the prescription of numerous medications, making the management of patients very complex. The prescription of numerous drugs associated with an altered renal- and non-renal clearance makes dose adjustment challenging in these patients, with frequent drug-related adverse events. However, the mechanisms involved in this abnormal drug clearance during CKD are not still well identified. We propose here that the transcription factor, aryl hydrocarbon receptor, which is the cellular receptor for indolic uremic toxins, could worsen the metabolism and the excretion of drugs in CKD patients.

Regulation of NAD(P)H:Quinone Oxidoreductase 1 Gene Expression by CYP1A1 Activity

The dioxin 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD) induces phase I and II xenobiotic metabolizing enzymes (XME) which act sequentially to eliminate different classes of xenobiotics. The transcriptional effects of TCDD are generally mediated by the arylhydrocarbon receptor (AhR). We hypothesized that TCDD could also act indirectly, by increasing the activity of cytochrome P450 1A1 (CYP1A1), a phase I gene, which could then mediate the induction of other XME genes, such as the NAD(P)H:quinone oxidoreductase 1 (NQO1). To test this hypothesis, NQO1 gene expression was monitored after either overexpression of CYP1A1 or siRNA-mediated knock-down of CYP1A1 activity in the hepatoma cell line HepG2. Overexpression of CYP1A1 in the absence of TCDD was carried out using either adenoviral infection or the "Tet-off" system. Recombinant adenoviruses were produced encoding no protein, CYP1A1 (Ad1A1), or a mutated inactive CYP1A1 (Ad1A1mut). In the HepG2 Tet-off cell line, CYP1A1 expression was induced by the removal of doxycycline (dox) from the cell medium. Ad1A1 infection or dox removal induced CYP1A1 activity and H(2)O(2) production similarly to TCDD treatment. Moreover, in both systems, the amount of NQO1 mRNA increased to the same level as after TCDD treatment (approximately 2-fold). The UDP-glucuronosyl transferase 1A6 (UGT1A6) gene is also similarly regulated. NQO1 gene expression was not induced when mutant, inactive CYP1A1 was overexpressed or when the antioxidant N-acetyl cysteine (NAC) was added to Ad1A1. Finally, either NAC or siRNA directed against CYP1A1 mRNA decreased the induction of NQO1 gene expression by TCDD. We conclude that, after exposure to TCDD, the NQO1 gene expression can be controlled by CYP1A1 activity through an oxidative stress mediated pathway.

Extensive changes in liver gene expression induced by hereditary tyrosinemia type I are not normalized by treatment with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC)

BACKGROUND

Hereditary Tyrosinemia type I, caused by deficiency of fumarylacetoacetate hydrolase (FAH), is characterized by liver and kidney damage. Administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) corrects the tyrosinemia phenotype, but does not prevent development of hepatocellular carcinoma.

AIM

To gain insight into the pathophysiological changes associated with liver damage induced by tyrosinemia and the preventive action of NTBC on these changes.

METHODS

Differential gene expression patterns in livers of tyrosinemia-affected and healthy mice, and of tyrosinemia-affected and NTBC-treated Fah-/- mice were investigated by suppression subtractive hybridization.

RESULTS

Transcripts encoding proteins playing a role in protein turnover, growth and proliferation, RNA processing, and signal transduction were primarily induced in tyrosinemia-affected livers. Transcripts mainly contributing to the profile of suppressed genes encode proteins that are secreted by the liver, or are necessary for intermediate metabolism. NTBC treatment fails to normalize the tyrosinemia-induced alterations in expression of transcripts encoding proteins involved in protein turnover, signal transduction, and cell growth and proliferation.

CONCLUSIONS

The failure of NTBC to normalize liver gene expression of Fah-/- mice may play a role in rendering the tyrosinemia-affected liver susceptible to development of hepatocellular carcinoma under NTBC treatment.

Aorta of ApoE-deficient mice responds to atherogenic stimuli by a prelesional increase and subsequent decrease in the expression of antioxidant enzymes

Oxidative stress has been implicated in the development of atherosclerotic lesions. We evaluated the relationship between extent of atherosclerotic lesion formation and vascular expression of pro- and antioxidant enzymes in apoE-deficient mice. On normal chow, these mice showed elevated serum cholesterol levels (7.5- to 9.5-fold), and age-dependent, spontaneous development of all stages of atherosclerotic lesions, starting at the age of 12 weeks. RNA was extracted from the aortic arch and descending aorta, and mRNA expression of pro- and antioxidant enzymes was measured with real-time PCR. Local infiltration of monocytes/macrophages, reflected by increased vascular expression of CD68 mRNA (>10-fold), indicated that the arch was more susceptible than the descending aorta. The expression of catalase-1 and various isoforms of superoxide dismutase, glutathione peroxidase, and glutathione S-transferase alpha was significantly increased in the aortic arch, but not in the descending aorta, in the period preceding lesion formation (age 6 to 12 weeks). These expression levels were 1.5 to 5 times higher than in age-matched wild-type animals. Remarkably, there was an inverse relationship between extent of lesion formation and the mRNA levels of antioxidant enzymes, most of which started to decline after 12 weeks, as lesions developed. In contrast, inducible nitric oxide synthase expression increased 4-fold in the aortic arch over the course of the disease. Our results suggest that the arterial wall responds to increased serum levels of atherogenic lipoproteins by stimulating expression of antioxidant enzymes. The observed co-ordinate decline in expression of many of these protective systems may greatly accelerate the development of atherosclerosis.

Increased constitutive c-Jun N-terminal kinase signaling in mice lacking glutathione S-transferase Pi

Glutathione S-transferase Pi (GSTP) detoxifies electrophiles by catalyzing their conjugation with reduced glutathione. A second function of this protein in cell defense has recently been proposed that is related to its ability to interact with c-Jun N-terminal kinase (JNK). The present study aimed to determine whether this interaction results in increased constitutive JNK activity in the absence of GSTP in GstP1/P2(-/-) mice and whether such a phenomenon leads to the up-regulation of genes that are relevant to cell defense. We found a significant increase in constitutive JNK activity in the liver and lung of GstP1/P2-/- compared with GstP1/P2(+/+) mice. The greatest increase in constitutive JNK activity was observed in null liver and was accompanied by a significant increase in activator protein-1 DNA binding activity (8-fold) and in the mRNA levels for the antioxidant protein heme oxygenase-1 compared with wild type. Furthermore UDP-glucuronosyltransferase 1A6 mRNA levels were significantly higher in the livers of GstP1/P2(-/-) compared with GstP1/P2(+/+) mice, which correlated to a 2-fold increase in constitutive activity both in vitro and in vivo. There was no difference in the gene expression of other UDP-glucuronosyltransferase isoforms, manganese superoxide dismutase, microsomal epoxide hydrolase, or GSTA1 between GstP1/P2(-/-) and GstP1/P2(+/+) mice. Additionally there was no phenotypic difference in the induction of heme oxygenase-1 mRNA after acetaminophen administration. This study not only demonstrates the role of GSTP as a direct inhibitor of JNK in vivo but also its role in regulating the constitutive expression of specific downstream molecular targets of the JNK signaling pathway.

Response of anti-oxidant enzymes mRNA in the neonatal rat liver exposed to 1,2,3,4-tetrachlorodibenzo-p-dioxin via lactation

BACKGROUND

The aim of this study was to assess the response to dioxin-induced oxidative stress in neonates via lactation in the model we have described previously.

METHODS

Maternal rats were treated with a single dose of 50 or 100 micro mol/kg 1,2,3,4-tetrachlorodibenzo-p-dioxin (TCDD) on the first day postpartum (day 1). Messenger RNA levels of the key anti-oxidant enzymes (AOE), phospholipid hydroperoxide-glutathione peroxidase (PH-GPx), cellular-glutathione peroxidase (cell-GPx), copper-zinc superoxide dismutase (CuZn SOD), manganese superoxide dismutase (Mn SOD) and catalase (CAT) in the neonatal and maternal livers were determined by a competitive reverse transcription- polymerase chain reaction method.

RESULTS

Lactational transfer of 1,2,3,4-TCDD induced an inhibition of PH-GPx and cell-GPx mRNA in the neonatal liver on day 2 to 68 (P < 0.01) and 62% (P < 0.05) of the control at 100 micro mol/kg, respectively. Both GPx mRNA returned to control levels on day 6 and thereafter increased to levels higher than the controls on day 10. In the dam rat, 10 days after the treatment, no remarkable change of PH-GPx or cell-GPx mRNA was observed. Copper-zinc superoxide dismutase and CAT mRNA of neonates on day 2 were also suppressed at 100 micro mol/kg and then slightly increased on day 10. However, Mn SOD mRNA was not suppressed, but increased to a 2.1-fold level of the control (P < 0.05) on day 10 with 100 micro mol/kg 1,2,3,4-TCDD.

CONCLUSION

Quantitative analysis of AOE mRNA showed that PH-GPx and cell-GPx mRNA, as well as CuZn SOD and CAT mRNA in the neonatal liver were suppressed for a short period of time by 1,2,3,4-TCDD exposure via lactation. Dioxin induced oxidative stress by lactational transfer may alter pretranslation regulation of protective AOE in neonates.

Glutamate-cysteine ligase modifier subunit: mouse Gclm gene structure and regulation by agents that cause oxidative stress

Glutamate-cysteine ligase is a heterodimer comprising a modifier (GCLM) and a catalytic (GCLC) subunit. In mouse Hepa-1c1c7 hepatoma cell cultures, we found that tert-butylhydroquinone (tBHQ; 50 microM) induces the GCLM and GCLC mRNAs approximately 10- and approximately 2-fold, respectively, and that these increases primarily reflect de novo transcription. We determined that the mouse Gclm gene has seven exons, spanning 22.3 kb; all exons, intron-exon junctions, and 4.7 kb of 5'-flanking region were sequenced. By RNase protection analysis, we identified two major and several minor transcription start-site clusters over a 300-bp region. The Gclm 5'-flanking region is GC-rich and lacks a canonical TATA box. Transient and stable transfection studies, using luciferase reporter constructs containing incremental Gclm 5'-flanking deletions (4.7-0.5 kb), showed high basal activity but only modest ( approximately 2-fold) inducibility by tBHQ. The only candidate motif for oxidative stress regulation (in the 4.7-kb region we sequenced) is a putative inverted electrophile response element (EPRE) 9 bp upstream from the 5'-most transcription start-site. Site-directed mutagenesis of this -9 EPRE demonstrated minimal (30-40%) decreases in tBHQ induction and no effect on basal activity-suggesting that this EPRE might be necessary but not sufficient. The nuclear erythroid factor-2 (NEF2)-related factor-2 (NRF2) is known to transactivate via EPRE motifs. In the presence of co-transfected NRF cDNA expression vector, however, no increase in Gclm promoter activity was observed. Thus, the endogenous Gclm gene shows robust transcriptional activation by tBHQ in the intact Hepa-1 cell, but reporter constructs containing up to 4.7 kb of promoter (having only the one EPRE at -9) demonstrate a disappointing response, indicating that the major tBHQ-responsive regulatory element of the mouse Gclm gene must exist either further 5'- or 3'-ward of the 4.7-kb region studied.

Role of the aromatic hydrocarbon receptor and [Ah] gene battery in the oxidative stress response, cell cycle control, and apoptosis

The chronology and history of characterizing the aromatic hydrocarbon [Ah] battery is reviewed. This battery represents the Ah receptor (AHR)-mediated control of at least six, and probably many more, dioxin-inducible genes; two cytochrome P450 genes-P450 1A1 and 1A2 (Cypla1, Cypla2-and four non-P450 genes, have experimentally been documented to be members of this battery. Metabolism of endogenous and exogenous substrates by perhaps every P450 enzyme, but certainly CYP1A1 and CYP1A2 (which are located, in part, in the mitochondrion), have been shown to cause reactive oxygenated metabolite (ROM)-mediated oxidative stress. Oxidative stress activates genes via the electrophile response element (EPRE) DNA motif, whereas dioxin (acutely) activates genes via the AHR-mediated aromatic hydrocarbon response element (AHRE) DNA motif. In contrast to dioxin, AHR ligands that are readily metabolized to ROMs (e.g. benzo[a]pyrene, beta-naphthoflavone) activate genes via both AHREs and the EPRE. The importance of the AHR in cell cycle regulation and apoptosis has just begun to be realized. Current evidence suggests that the CYP1A1 and CYP1A2 enzymes might control the level of the putative endogenous ligand of the AHR, but that CYPA1/1A2 metabolism generates ROM-mediated oxidative stress which can be ameliorated by the four non-P450 EPRE-driven genes in the [Ah] battery. Oxidative stress is a major signal in precipitating apoptosis; however, the precise mechanism, or molecule, which determines the cell's decision between apoptosis and continuation with the cell cycle, remains to be elucidated. The total action of AHR and the [Ah] battery genes therefore represents a pivotal upstream event in the apoptosis cascade, providing an intricate balance between promoting and preventing ROM-mediated oxidative stress. These proposed endogenous functions of the AHR and [Ah] enzymes are, of course, in addition to the frequently described functions of "metabolic potentiation" and "detoxification" of various foreign chemicals.