Induction of cytochrome P-450 by dimethyl sulfoxide in primary cultures of adult rat hepatocytes

Comparison of the cytotoxicity of the nitroaromatic drug flutamide to its cyano analogue in the hepatocyte cell line TAMH: evidence for complex I inhibition and mitochondrial dysfunction using toxicogenomic screening

Flutamide (FLU) is an antiandrogen primarily used in the treatment of metastatic prostate cancer. It is an idiosyncratic hepatotoxicant that sometimes results in severe liver toxicity. FLU possesses a nitroaromatic group, which may be a contributor to its mechanism of toxicity. A nitro to cyano analogue of FLU (CYA) was synthesized and used to test this hypothesis in the TGFalpha-transfected mouse hepatocyte cell line (TAMH). MTT cell viability assays and confocal microscopy showed that hepatocytes are more sensitive to cytotoxicity caused by FLU than CYA (LD 50 75 vs 150 microM, respectively). Despite the structural modification, the antiandrogen activity of CYA is comparable to that of FLU. Comparisons of transcriptomic changes caused by FLU with those caused by a panel of known cytotoxicants [acetaminophen, tetrafluoroethylcysteine, diquat, and rotenone (ROT)] indicated that FLU results in a temporal gene expression pattern similar to ROT, a known inhibitor of complex I of the electron transport chain. A subsequent microarray analysis comparing FLU to CYA and ROT revealed many similarities among these three compounds; however, FLU and ROT result in more substantial changes than CYA in the expression of genes associated with oxidative phosphorylation, fatty acid beta-oxidation, antioxidant defense, and cell death pathways. Electron microscopy confirmed that FLU leads to mitochondrial toxicity that has some similarities to the mitochondrial effects of ROT, but the morphologic changes caused by FLU were greater in scope with both intra- and intercellular manifestations. Biochemical studies confirmed that both ROT and FLU deplete cellular ATP levels and inhibit complex I of the electron transport chain to a greater extent than CYA. Thus, as compared to CYA, the nitroaromatic group of FLU enhances cytotoxicity to hepatocytes, likely through mechanisms involving mitochondrial dysfunction and ATP depletion that include complex I inhibition.

Expression of E-cadherin and other paracellular junction genes is decreased in iron-loaded hepatocytes

Iron overload in the liver may occur in the clinical conditions hemochromatosis and transfusion-dependent thalassemia or by long-term consumption of large amounts of dietary iron. As iron concentrations increase in the liver, cirrhosis develops, and subsequently the normal architecture of the liver deteriorates. The underlying mechanisms whereby iron loading of hepatocytes leads to the pathology of the liver are not understood. Similarly, a direct relationship between the expression levels of paracellular junction genes and altered hepatocellular physiology has been reported; however, no relationship has been identified between iron loading and the expression of paracellular junction genes. Here, we report that the expression of numerous paracellular junction genes was decreased in iron-loaded hepatocytes, leading to increased cellular permeability, increased baculovirus-mediated gene transfer, and decreased gap junction communication. Iron loading of hepatocytes resulted in decreased E-cadherin promoter activity and subsequently decreased E-cadherin mRNA and protein expression. The data presented in this study describe a clear relationship between iron overload and decreased expression of paracellular junction genes in hepatic cells of rat and human origin.

The effect of different solvents on the ATP/ADP content and growth properties of HeLa cells

Testing of the effects of xenobiotics in cultured cells often requires the use of organic solvents to effect suspension of the test agents in cell culture media. However, the toxic effects of the solvents themselves may introduce artifacts, which obscure interpretation of the experimental results. In this article, the toxicity of different solvents commonly used for solvation of a variety of xenobiotic agents was studied. We show that ethanol, acetone, isooctane, methanol, and hexane were considerably less toxic than the more commonly used solvent, DMSO, when ATP content and growth rates of HeLa cells exposed to these solvents was measured.

Protective effects of diallyl sulfide on N-nitrosodimethylamine-induced immunosuppression in mice

Diallyl sulfide (DAS), a flavor component of garlic that has been used as a food additive, exerts chemopreventive effects at several organ sites in rodents after administration of chemical carcinogens possibly by inhibiting carcinogen activation via cytochrome P450-mediated oxidative metabolism. In this study, we investigated the protective effect of DAS on the N-nitrosodimethylamine (NDMA)-induced immunosuppression of humoral and cellular responses in BALB/c mice and the possible mechanisms involved in this protection. We observed that oral administration of DAS prior to NDMA treatment for 14 consecutive days blocked the NDMA-induced suppression of the antibody response to a T-cell-dependent antigen, sheep erythrocytes, and the lymphoproliferative response to the T-cell and the B-cell mitogens in dose-dependent manners. Treatment of mice with DAS resulted in a significant decrease of cytochrome P450 2E1-dependent p-nitrophenol hydroxylase and NDMA demethylase activities. The results show that the protective effects of DAS against the NDMA-induced immunotoxicity may, at least in part, be due to its ability to block bioactivation of NDMA mainly by the inhibition of cytochrome P450 2E1.

Posttranslational elevation of cytochrome P450 3A levels and activity by dimethyl sulfoxide

The molecular mechanisms by which dimethyl sulfoxide (DMSO) enhances CYP3A protein in phenobarbital-treated primary cultured rat hepatocytes were examined. DMSO treatment rapidly increased CYP3A protein levels in the absence of an increase in CYP3A mRNA levels or an increase in CYP2B protein or mRNA levels. CYP3A levels were increased approximately 3.7- and 9-fold following 0.1% DMSO treatment for 6 and 48 h, respectively. Analyses of the polysomal distribution of CYP3A mRNA suggested that DMSO treatment did not significantly alter the translational efficiency of the CYP3A mRNA. Comparative analyses of immunodetectable protein levels following treatment with cycloheximide showed that DMSO clearly decreased the rate of CYP3A protein turnover but not that of CYP2B. Examination of testosterone metabolism in hepatocyte cultures revealed that DMSO pretreatment increased CYP3A-catalyzed 2 beta- and 6 beta-testosterone hydroxylation. When DMSO was in the culture medium, no inhibitory affect on CYP3A-catalyzed testosterone metabolism was observed, although a slight (15-21%) inhibitory effect was noted for CYP2B-catalyzed 16 alpha- and 16 beta-testosterone hydroxylation. These data provide evidence that DMSO increased CYP3A protein levels as a result of decreased protein degradation. DMSO increased both immunodetectable CYP3A protein levels and catalytic activity, in contrast to compounds that have been reported to stabilize CYP3A protein and inhibit activity.

Murine Cyp1a-1 induction in mouse hepatoma Hepa-1C1C7 cells by myristicin

Mouse hepatoma Hepa-1c1c7 (Hepa-1) cells were treated with myristicin to assess the role of myristicin in the process of Cyp1a-1 induction. Treatment of Hepa-1 cells with myristicin increased Cyp1a-1 transcription in a dose-dependent manner as shown by analysis of 7-ethoxyresorufin O-deethylase activity, Cyp1a-1 protein level, and Cyp1a-1 mRNA. Myristicin, however, did not competitively displace [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin from the Hepa-1 cytosolic aryl hydrocarbon (Ah) receptor in a competitive Ah receptor binding analysis using sucrose density gradient sedimentation and did not affect formation of DNA-protein complexes between the Ah receptor and its DRE target in a gel mobility shift assay using oligonucleotides corresponding to DRE 3 of the Cyp1a-1. These results suggest that the induction of Cyp1a-1 gene expression by myristicin in Hepa-1 cells might occur through an Ah receptor-independent pathway.

Characterization of a primary hepatocyte culture system for toxicological studies

An hepatocyte culture system was developed for potential use in toxicological studies in vitro. Rat hepatocytes were isolated by two-step collagenase perfusion and cultured on Vitrogen-coated Permanox dishes in a modified Chee's medium containing 1 microM dexamethasone and 1% dimethylsulfoxide. The cells remained highly viable for at least 10 d as determined by lactate dehydrogenase release and total protein levels. Albumin secretion into the medium, as a measure of differentiated function, was maintained at elevated levels over the course of 10 d in culture. A number of CYP activities were determined by the analysis of testosterone metabolism in freeze-thawed cells, diazepam metabolism in live cells, and specific assays for CYP 1A1/2, 2B1/2, 2E1, and 3A. Results of these assays indicated that a wide range of CYP isozymes were maintained, some activities were enhanced under the conditions of culture and some activities were inducible. Activities of the phase II enzymes, glutathione S-transferase and UDP-glucuronosyltransferase, and glutathione levels were also maintained in the cultured hepatocytes for at least 6 d. These results strongly support the use of this hepatocyte culture system for in vitro toxicological studies.

Recommendations for the performance of UDS tests in vitro and in vivo

The Working Group (WG) dealt with the harmonization of routine methodologies of tests for unscheduled DNA synthesis (UDS) both in vitro and in vivo. In contrast to the existing guidelines from OECD, EPA and EC on in vitro UDS tests (there is no Japanese UDS guideline), the Working Group recommends that in general in vitro UDS tests should be performed with primary hepatocytes. For routine applications any other cell types would need special justification. Hepatocytes from male rats are preferable, unless there are contra-indications on the basis of e.g. toxicokinetic data. According to the OECD, EPA and EC guidelines, UDS may be analysed by means of autoradiography (AR) or liquid scintillation counting (LSC). The WG recommends use of AR. LSC is less suitable due to the problem of differentiation between UDS activity and replicative DNA synthesis, and the disadvantage that cells cannot be analysed individually. Since a specific cell type was recommended by the WG, methodological aspects could be described in more detail than in the present guidelines. For in vitro tests, it was agreed that the initial viability of freshly isolated hepatocytes should be at least 70%. With regard to the need for confirmatory experiments in the event of a clear-cut negative result, the majority view was that confirmation by a second (normally not identical) experiment is still needed; this is in line with the present OECD and EC guidelines. Evaluation of results from UDS tests should be based primarily on net nuclear grain (NNG) values, although it is recognised that nuclear and cytoplasmic grains result from different biological processes. Since grain counts are influenced by a number of methodological parameters, no global threshold NNG value can be recommended for discrimination of positive and negative UDS results. For in vitro assays, the criteria for positive findings go beyond those of the present guidelines and two alternative approaches are given which are based on (1) dose-dependent increases in NNG values and (2) reproducibility, dose-effect relationship and cytotoxicity. At present there is no official guideline on the performance of in vivo UDS tests. Some fundamental recommendations given for in vitro methodology also apply to the in vivo assay. For routine testing with the in vivo UDS test, again the general use of hepatocytes from male rats is recommended. However, concerning the requirement to use one or two sexes, consistency with other in vivo genotoxicity assays (e.g. the micronucleus assay) would be preferable. As for the in vitro methodology, AR is preferred rather than LSC.(ABSTRACT TRUNCATED AT 400 WORDS)

Recovery of dimethylnitrosamine-induced immunosuppression by pargyline in the mixed cultures of murine hepatocytes and splenocytes

To investigate the role of monoamine oxidase (MAO) in dimethylnitrosamine (DMN)-induced suppression of the antibody response to sheep erythrocytes, the effect of an MAO inhibitor, pargyline, was studied in mixed cultures of murine hepatocytes and splenocytes. When pargyline was added simultaneously with DMN during the coculture, DMN-induced immunosuppression was clearly recovered dose-dependently. Cyclophosphamide was used as a comparative control in these studies. Surprisingly, pargyline also reversed cyclophosphamide-induced suppression of the antibody response in the coculture system. The results with cyclophosphamide were not consistent with a role by MAO, and suggested that pargyline may not be selective for MAO. To confirm our hypothesis, the ability of pargyline to inhibit three cytochrome P-450 (P-450) isozyme-specific monooxygenase activities in vitro was studied using mouse liver microsomes. Pargyline, under the same concentration ranges that we used in the coculture studies, clearly inhibited the P-450IIE1-specific p-nitrophenol hydroxylase activity and P-450IIB1-specific pentoxyresorufin O-dealkylase activity. Taken together, our present results indicate that pargyline inhibits P-450 activity and is not selective for MAO. Although further studies are required to confirm a possible role by MAO in DMN-induced immunosuppression, our results suggest that pargyline may recover DMN-induced immunosuppression by primarily inhibiting the ability of P-450 enzymes in hepatocytes to activate DMN to its immunosuppressive metabolite(s).

Alteration in the tissue retention of [14C]-caffeine in pregnant mice by dimethylsulfoxide

This study was undertaken to investigate if dimethylsulfoxide (DMSO)-induced potentiation of maternal toxicity and teratogenicity of caffeine (CA) is mediated by DMSO-induced alteration of CA levels in maternal and fetal tissues. Pregnant CD 1 mice were given single dose of [14C]-caffeine intraperitoneally (i.p.) on gestational day 12, in distilled water (DW) or 20% (v/v) solution of DMSO in DW. At 6, 12, and 24 hr following dosing, radioactivity in various maternal and fetal tissues was compared. Significantly (P < or = 0.05) higher CA-derived radioactivity was seen in maternal brain, carcass and heart as well as placenta and fetal heads and bodies 6 hr after dosing with CA in DMSO compared to CA in DW. A concomitant reduction in urinary excretion of CA-derived radioactivity in DMSO group or compared to CA in DW was observed at this time. At 12 hr following dosing, significantly (P < or = 0.05) higher CA-derived radioactivity was noted in maternal heart, brain, and carcass; placenta; and fetal bodies in mice receiving CA in DMSO compared to those receiving CA in DW. These results suggest that increased accumulation of CA in certain maternal and fetal tissues could contribute to the observed increase in maternal and fetal toxicity of CA by DMSO.