Pyrazole treatment of rats potentiates CCL4-but not CHCL3-hepatotoxicity

Age dependent different influence of carbon tetrachloride on biotransformation of xenobiotics, glutathione content, lipid peroxidation and histopathology of rat liver

15- and 60-day-old male rats were treated with different doses of CCl4 orally. 24 h later cytochrome P-450 (P450) concentration, 7-ethoxyresorufin O-deethylation (EROD) and 7-pentoxy-resorufin O-deethylation (PEROD) activities were determined. Whereas P450 and EROD are lowered to the same extent in both ages, PEROD shows a more pronounced inhibition in the livers of younger rats. The formation of endogenous lipid peroxides (measured as thiobarbituric acid reactive substances) is drastically increased only in the livers of young rats. The hepatic glutathione (GSH) content was unaffected by CCl4 treatment whereas oxidized glutathione is more increased in the livers of adult rats. This can be caused by a higher activity of GSH-peroxidase in the livers of adult rats. The changes in NADPH-induced lipid peroxidation and chemiluminescence correlate partially with the changes in P450 and biotransformation reactions. Histopathologically the liver damage is more extensive in suckling rats. The necrosis is localized predominantly in the perivenous tissue, which has normally the highest activities of toxification and detoxification enzymes.

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.

Inducibility of P450Coh by pyrazole and its derivatives

Pyrazole and several of its derivatives increase the hepatic microsomal coumarin 7-hydroxylase to a variable extent. The strongest inducers are pyrazole itself and those derivatives which have a hydroxy group or a halogen at the 4-position of the molecule. The increase in coumarin 7-hydroxylase is due to an increase in the microsomal P450Coh and the corresponding mRNA. The increase of P450Coh by pyrazole and 4-hydroxypyrazole is selective because several other mono-oxygenase enzymes and the total P450 content are either not affected or even decreased. These include the testosterone 15 alpha-hydroxylase (P45015 alpha), a close structural analogue of P450Coh, which is induced only marginally by pyrazole and even decreased by 4-iodopyrazole, and P450ac which is decreased by pyrazole and 4-hydroxypyrazole. Introducing a methyl residue at the 4-position will alter the induction properties of the compound essentially bymaking it less selective for P450Coh. These results demonstrate the special selective action of pyrazole and some of its derivatives on the hepatic microsomal mono-oxygenase complex and the unique mode of regulation of the cytochrome P450Coh even within the same subfamily of cytochromes P450.

Potentiating effects of chlorinated hydrocarbons on carbon tetrachloride toxicity in isolated rat hepatocytes and plasma membranes

A number of chemicals are known to potentiate the hepatotoxicity of carbon tetrachloride. The halocarbon trichloroethylene was shown in a previous study to enhance both carbon tetrachloride-induced toxicity and lipid peroxidation in isolated hepatocytes. In this study three other chlorocarbons have been investigated in order to determine whether this interaction was peculiar to trichloroethylene or common to chlorinated solvents. Hepatocyte suspensions were exposed to carbon tetrachloride at subthreshold levels of toxicity and various concentrations of 1,1,1-trichloroethane, tetrachloroethylene, and chloroform over an eightfold concentration range. Plasma membrane preparations were exposed to tetrachloroethylene and carbon tetrachloride and effects on Mg(2+)- and Na(+)-K(+)-ATPase activities determined. None of the treatments alone caused statistically significant toxicity. Combined treatments resulted in toxicity as demonstrated by potassium ion, alanine aminotransferase, and lactate dehydrogenase leakage from the cells on coincubation of carbon tetrachloride with each of the other halocarbons studied. Only tetrachloroethylene and chloroform were found to potentiate lipid peroxidation, however. In liver plasma membranes no changes in Na(+)-K(+)-ATPase were observed with any of the treatments and only the highest dose of tetrachloroethylene was able to inhibit Mg(2+)-ATPase activity. There was no increase in this inhibition on coincubation with carbon tetrachloride, which does not support involvement of ATPases in combined halocarbon toxicity. In conclusion, the data suggest a mechanism of action common to this class of chemical although its specific nature remains to be established.

Alteration of rat liver microsomal monooxygenase activities by gasoline treatment

Previous work has shown an increase in rat liver enzyme activities after chronic exposure to gasoline vapor. In the present study, male Sprague-Dawley rats were pretreated with unleaded gasoline at 1 and 5 ml/kg, i.p., and selected hepatic microsomal monooxygenase activities were determined at 18, 48, and 72 h. At 18 h, moderate increases were observed in P450 content (1.3-fold), cytochrome c-reductase activity (1.25-fold), and in N-nitrosodimethylamine demethylation rate (1.25- to 1.6-fold). Pentoxyresorufin dealkylase activity (an activity displayed primarily by P450IIB1) was significantly elevated at 18 and 48 h (30- to 60-fold), and ethoxyresorufin dealkylase activity (an activity displayed by P450 IA1) was elevated (2- to 4-fold). Immunoblot analysis revealed no change in P450IIE1 at these time points, but an elevation in P450IIB1 in agreement with the pentoxyresorufin dealkylase activity measurements.