Molecular mechanisms for bromotrichloromethane cytotoxicity in isolated rat hepatocytes

Comparison of Xenobiotic-mediated Cytotoxicity in Rat Cultured Hepatocytes and the V79 Chinese Hamster Lung Fibroblast Cell Line: Can Metabolically-activated Hepatotoxins be Identified by Selective Cytotoxicity to Hepatocytes?

The effects of 17 xenobiotics on rat hepatocytes and V79 cells were evaluated under identical exposure conditions (confluent monolayer for 24 hours) and endpoint measurement (MTT reduction). The data indicated that the majority of metabolically-activated rat hepatotoxins could be identified by greater cytotoxicity to rat hepatocytes relative to V79 cells, but that direct-acting hepatotoxins (galactosamine and ethionine) and a group of eight compounds likely to act through interference of basal functions in non-dividing and dividing cells produced similar toxicity in each cell type. It is possible that the greater water-solubility of the direct-acting hepatotoxins relative to the indirect-acting hepatotoxins may contribute to the lack of effect seen with the former group under the conditions of the assay (top concentration of 1mM).

Formation of 4-hydroxynonenal and further aldehydic mediators of inflammation during bromotrichlorornethane treatment of rat liver cells

Bromotrichloromethane (CBrCl₃) treatment is a model for studies on molecular mechanisms of haloalkane toxicity with some advantages compared with CCl₄ treatment. The formation of 4-hydroxynonenal and similar aldehydic products of lipid peroxidation, which play a role as mediators of inflammatory processes, was clearly demonstrated in rat hepatocytes treated with CBrCl₃. It may be assumed that haloalkane toxicity is connected with the biological effects of those inflammation mediatory aldehydic compounds.

Coagulation-Mediated Hypoxia and Neutrophil-Dependent Hepatic Injury in Rats Given Lipopolysaccharide and Ranitidine

Idiosyncrasy-like liver injury occurs in rats cotreated with nonhepatotoxic doses of ranitidine (RAN) and bacterial lipopolysaccharide (LPS). Hepatocellular oncotic necrosis is accompanied by neutrophil (PMN) accumulation and fibrin deposition in LPS/RAN-treated rats, but the contribution of PMNs to injury has not been shown. We tested the hypothesis that PMNs are critical mediators of LPS/RAN-induced liver injury and explored the potential for interaction between PMNs and hemostasis-induced hypoxia. Rats were given either LPS (44.4 x 10(6) endotoxin units/kg) or its vehicle and then RAN (30 mg/kg) or its vehicle 2 h later. They were killed 3 or 6 h after RAN treatment, and hepatocellular injury was estimated from serum alanine aminotransferase activity and liver histopathology. Plasma PMN chemokine concentration and the number of PMNs in liver increased after LPS treatment at 3 h and were not markedly altered by RAN cotreatment. Depletion of circulating PMNs attenuated hepatic PMN accumulation and liver injury and had no effect on coagulation system activation. Anticoagulation with heparin attenuated liver fibrin deposition and injury in LPS/RAN-treated rats; however, heparin had little effect on liver PMN accumulation or plasma chemokine concentration. Liver hypoxia occurred in LPS/RAN-cotreated rats and was significantly reduced by heparin. In vitro, hypoxia enhanced the killing of rat hepatocytes by PMN elastase and shortened its onset, indicating a synergistic interaction between PMNs and hypoxia. The results suggest that PMNs are involved in the hepatocellular injury caused by LPS/RAN-cotreatment and that hemostasis increases sensitivity to PMN-induced hepatocellular injury by causing liver hypoxia.

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.

Use of rat liver slices for the study of oxidative DNA damage in comparison with isolated rat liver nuclei and HepG2 human hepatoma cells

Tissue slices are a useful biological system for lipid peroxidation studies but their use for DNA damage studies is not well characterized. Hence, the present study investigates DNA damage in rat liver slices, in comparison with isolated rat liver nuclei and HepG2 human hepatoma cells, incubated with ferric nitrilotriacetate (Fe(III)-NTA), bromotrichloromethane (BrCCl(3)), bromobenzene (BrB) or 2-nitropropane (2-NP) at 37 degrees C for 2 hr. DNA damage was measured in slices, cells or nuclei after centrifugation as formation of as 8-hydroxy-2'-deoxyguanosine (8-OH-dGu) and loss of double-stranded (dsDNA) due to strand breakage using a fluorometric analysis of DNA unwinding (FADU). Lipid peroxidation was measured as thiobarbituric acid-reactive substances (TBARS) released into the medium. The results show that in liver slices and isolated nuclei, Fe/NTA (1 mM/4 mM) induced high levels of TBARS but low levels of 8-OH-dGu, whereas the oxidant induced low levels of TBARS and no formation of 8-OH-dGu in HepG2 cells. In all three systems, inclusion of ascorbate caused dose-dependent formation of 8-OH-dGu, and the levels were similar between liver slices and HepG2 cells but were far higher in isolated nuclei. In liver slices the FADU assay was not applicable due to limited solubilization of DNA from the slice, whereas the assay detected significant loss of dsDNA in HepG2 cells and slight loss in isolated nuclei induced by Fe/NTA with or without ascorbate. Liver slices incubated with 1 mm BrCCl(3), BrB or 2-NP had elevated TBARS but had little or no formation of 8-OH-dGu; none of these oxidants induced lipid peroxidation or DNA damage in HepG2 cells. When liver slices obtained from rats injected with diethylmaleate (to deplete GSH) were incubated with BrCCl(3), BrB or 2-NP, levels of TBARS and 8-OH-dGu increased markedly. Similarly, HepG2 cells with decreased GSH showed marked elevation of TBARS and loss of dsDNA induced by these oxidants, although no formation of 8-OH-dGu was detected. The present study demonstrates the usefulness and limitations of liver slices for DNA damage studies and the importance of cellular GSH in the protection of DNA against environmental toxicants.

Suicidal inactivation of haemoproteins by reductive metabolites of halomethanes: a structure-activity relationship study

Human haemoglobin (Hb), methaemalbumin (MHA) or rat liver microsomal cytochrome P-450 (P-450) were incubated anaerobically at microM concentrations with 1 mM carbon tetrachloride (CCl4), trichlorobromomethane (CCl3Br), chloroform (CHCl3) or methylene chloride (CH2Cl2) in presence of 1 mM sodium dithionite as the reducing agent. At the end of a 5-min incubation, haem was measured by various methods, i.e. binding spectrum with CO, pyridine-haemochromogen haem assay and porphyrin fluorescence, and compared for the four analogues. Statistically significant losses were observed, with all three haemo-protein systems, for CCi3Br, CCl4 and CHCl3, but not CH2Cl2. For Hb, the loss was greater with CCl3Br (haem assay, 63%; porphyrin fluorescence, 48%; CO binding, 24%) than with CCl4 (haem assay, 31%) or CHCl3 (haem assay, 13%). On the other hand, with MHA, CCl4 gave a dramatic loss (haem assay, 88%; porphyrin fluorescence, 83%; CO binding, 67%), which was greater than that observed with CCl3Br (haem assay, 49%; porphyrin fluorescence, 38%; CO binding, 25%). No loss was found with CHCl3. Finally, with microsomes, the inactivation was larger with CCl4 (CO binding, 58%; haem assay, 50%; porphyrin fluorescence, 33%) than with CCl3Br (CO binding, 33%; haem assay, 10%) or CHCl3 (haem assay, 9%; CO binding, 6%). In a separate set of similar experiments, an ion-pairing reverse phase HPLC method showed the formation of substrate-dependent hae-derived products during incubation of CCl3Br with Hb or microsomes, and of CCl4 with Hb. A correlation between potential for free radical formation (CCl3Br > CCl4 > CHCl3 > CH2Cl2) and extent of haem inactivation was observed with all methods for Hb, but not for microsomal P-450 or MHA. The results indicate that these halomethanes may be activated differently by different haemoproteins and suggest that their potential ability to undergo reductive metabolism may not be the only critical factor involved in P-450 haem inactivation by these chemicals.

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.

The role of glutathione and protein thiols in CBrCl3-induced cytotoxicity in isolated rat hepatocytes

The role of glutathione (GSH) and protein thiols in the pathobiochemical process of CBrCl3 cytotoxicity was investigated in isolated hepatocytes. Administration of 0.5, 1.0 and 1.5 mmol/l CBrCl3 affected cellular viability as assessed by trypan blue exclusion, release of lactate dehydrogenase and loss of intracellular potassium in a dose-dependent manner. Intracellular glutathione and the capacity to reduce 3-(4,5-dimethylthiazolyl-2-)-2,5-diphenyltetrazolium bromide (MTT, thiazolyl blue) decreased almost independently of the CBrCl3 concentration. Protein thiols were not markedly oxidized in the presence of CBrCl3. However, compromising cellular defence mechanisms by either inhibition of glutathione regeneration or depletion of glutathione enhanced the cytotoxicity of CBrCl3 and induced a loss of protein thiols in the late phase of cellular injury. Under these conditions the thiol-dependent Na+,K+ATPase revealed high sensitivity towards CBrCl3. Thus, glutathione proved to exert effective cytoprotection, and sulfhydryl groups of particular proteins were supposed to be an important target of radical attack.

Hepatocyte cytotoxicity induced by various hepatotoxins mediated by cytochrome P-450IIE1: protection with diethyldithiocarbamate administration

The objective of this study was to determine whether the thiol drug, diethyldithiocarbamate (DEDC) and its two metabolites, disulfiram (DS) and carbon disulfide (CS2) could be used as inhibitors of cytochrome P-450IIE1 to protect hepatocytes from cytotoxic xenobiotics. (1) Hepatocytes isolated from rats following pyrazole administration to induce cytochrome P-450IIE1 were much more susceptible to carbon tetrachloride (CCl4) and dimethylnitrosamine (DMN) than hepatocytes from untreated rats. Microsomes isolated from P-450IIE1-induced liver were also much more effective at catalysing a NADPH-dependent metabolism of CCl4 and DMN. The activities of aniline hydroxylase and p-nitroanisole-O-demethylase increased whereas ethoxyresorufin-O-dealkylase activity was much less induced and pentoxyresorufin-O-dealkylase activity was decreased. The P-450IIE1 antibody markedly inhibited the NADPH-dependent metabolism of these compounds indicating that IIE1 is a major catalyst of the microsomal metabolism of CCl4 and DMN. (2) Hepatocytes isolated from rats treated with DEDC or its metabolites, DS and CS2, on the other hand, were resistant to CCl4 and DMN. Microsomes isolated from the liver of animals treated with DEDC or DS or CS2 were also much less effective at catalysing the NADPH-dependent metabolism of the above compounds. DEDC markedly decreased the activities of aniline hydroxylase, p-nitroanisole-O-demethylase and pentoxyresorufin-O-dealkylase but had no effect on ethoxyresorufin-O-dealkylase activity. (3) Hepatocytes isolated from pyrazole-treated rats were also more susceptible to bromobenzene (BB) and naphthalene-induced cytotoxicity than hepatocytes from untreated rats. Furthermore, DEDC or CS2 administration beforehand significantly protected hepatocytes against both xenobiotics. (4) By contrast, hepatocytes isolated from P-450IIE1 induced rats were not more susceptible to lactonitrile or cyclophosphamide. Instead, cyclophosphamide was activated by phenobarbital-induced P-450 isozymes whereas lactonitrile was activated by alcohol dehydrogenase. Hepatocytes isolated from DEDC-treated rats were also resistant to cyclophosphamide but not lactonitrile. (5) The above results suggest that P-450IIE1 catalyses the cytotoxic activation of CCl4, DMN, BB and naphthalene but not of lactonitrile or cyclophosphamide. Furthermore, the administration of DEDC and its metabolites, disulfiram or CS2, inactivates P-450IIE1 so that the hepatocytes become resistant to these hepatotoxins.