Potentiation of the toxicity of model hepatotoxicants by acetaminophen

Acetaminophen increases the risk of arsenic-mediated development of hepatic damage in rats by enhancing redox-signaling mechanism

We evaluated whether the commonly used analgesic-antipyretic drug acetaminophen can modify the arsenic-induced hepatic oxidative stress and also whether withdrawal of acetaminophen administration during the course of long-term arsenic exposure can increase susceptibility of liver to arsenic toxicity. Acetaminophen was co-administered orally to rats for 3 days following 28 days of arsenic pre-exposure (Phase-I) and thereafter, acetaminophen was withdrawn, but arsenic exposure was continued for another 28 days (Phase-II). Arsenic increased lipid peroxidation and reactive oxygen species (ROS) generation, depleted glutathione (GSH), and decreased superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), and glutathione reductase (GR) activities. Acetaminophen caused exacerbation of arsenic-mediated lipid peroxidation and ROS generation and further enhancement of serum alanine aminotransferase and aspartate aminotransferase activities. In Phase-I, acetaminophen caused further GSH depletion and reduction in SOD, catalase, GPx and GR activities, but in Phase-II, only GPx and GR activities were more affected. Arsenic did not alter basal and inducible nitric oxide synthase (iNOS)-mediated NO production, but decreased constitutive NOS (cNOS)-mediated NO release. Arsenic reduced expression of endothelial NOS (eNOS) and iNOS genes. Acetaminophen up-regulated eNOS and iNOS expression and NO production in Phase-I, but reversed these effects in Phase-II. Results reveal that acetaminophen increased the risk of arsenic-mediated hepatic oxidative damage. Withdrawal of acetaminophen administration also increased susceptibility of liver to hepatotoxicity. Both ROS and NO appeared to mediate lipid peroxidation in Phase-I, whereas only ROS appeared responsible for peroxidative damage in Phase-II.

Alteration in metabolism and toxicity of acetaminophen upon repeated administration in rats

Our previous studies showed that administration of a subtoxic dose of acetaminophen (APAP) to female rats increased generation of carbon monoxide from dichloromethane, a metabolic reaction catalyzed mainly by cytochrome P450 (CYP) 2E1. In this study we examined the changes in metabolism and toxicity of APAP upon repeated administration. An intraperitoneal dose of APAP (500 mg/kg) alone did not increase aspartate aminotransferase, alanine aminotransferase, or sorbitol dehydrogenase activity in serum, but was significantly hepatotoxic when the rats had been pretreated with an identical dose of APAP 18 h earlier. The concentrations and disappearance of APAP and its metabolites in plasma were monitored for 8 h after the treatment. APAP pretreatment reduced the elevation of APAP-sulfate, but increased APAP-cysteine concentrations in plasma. APAP or APAP-glucuronide concentrations were not altered. Administration of a single dose of APAP 18 h before sacrifice increased microsomal CYP activities measured with p-nitrophenol, p-nitroanisole, and aminopyrine as probes. Expression of CYP2E1, CYP3A, and CYP1A proteins in the liver was also elevated significantly. The results suggest that administration of APAP at a subtoxic dose may result in an induction of hepatic CYP enzymes, thereby altering metabolism and toxicological consequences of various chemical substances that are substrates for the same enzyme system.

Induction of Hepatic CYP2E1 by a Subtoxic Dose of Acetaminophen in Rats: Increase in Dichloromethane Metabolism and Carboxyhemoglobin Elevation

Dichloromethane (DCM) is metabolically converted to carbon monoxide mostly by CYP2E1 in liver, resulting in elevation of blood carboxyhemoglobin (COHb) levels. We investigated the effects of a subtoxic dose of acetaminophen (APAP) on the metabolic elimination of DCM and COHb elevation in adult female rats. APAP, at 500 mg/kg i.p., was not hepatotoxic as measured by a lack of change in serum aspartate aminotransferase, alanine aminotransferase, and sorbitol dehydrogenase activities. In rats pretreated with APAP at this dose, the COHb elevation resulting from administration of DCM (3 mmol/kg i.p.) was enhanced significantly. Also blood DCM levels were reduced, and its disappearance from blood appeared to be increased. Hepatic CYP2E1-mediated activities measured with chlorzoxazone, p-nitrophenol, and p-nitroanisole as substrates were all induced markedly in microsomes of rats treated with APAP. Aminopyrine N-demethylase activity was also increased slightly, but significantly. Western blot analysis showed that APAP treatment induced the expression of CYP2E1 and CYP3A proteins. Neither hepatic glutathione contents nor glutathione S-transferase activity was changed by the dose of APAP used. The results indicate that, contrary to the well known hepatotoxic effects of this drug at large doses, a subtoxic dose of APAP may induce CYP2E1, and to a lesser degree, CYP3A expression. This is the first report that APAP can increase cytochrome P450 (P450)-mediated hepatic metabolism and the resulting toxicity of a xenobiotic in the whole animal. The pharmacological/toxicological significance of induction of P450s by a subtoxic dose of APAP is discussed.

Expression of liver functions following sub-lethal and non-lethal doses of allyl alcohol and acetaminophen in the rat

BACKGROUND/AIMS

To relate severity of intoxication with allyl alcohol and acetaminophen to modulated hepatic gene expression of liver functions and regeneration.

METHODS

Rats fasted for 12 h received acetaminophen 3.5 or 5.6 g per kg body weight, or allyl alcohol 100 or 125 microl by gastric tube, doses producing no and about 30% mortality, respectively, within 2 days. In the morning 2, 6, 12, 24, and 36 h after intoxication, RNA was extracted from liver tissue. By slot blot hybridization mRNA levels were determined for acute phase proteins, enzymes involved in ammonia elimination and urea synthesis, and for proteins related to liver regeneration.

RESULTS

After allyl alcohol, mRNA of "positive" acute phase proteins was higher than after acetaminophen and increased with the dose, whereas after acetaminophen it decreased with the dose. The mRNA of the urea cycle enzymes and glutamine synthetase was uniformly reduced by allyl alcohol, whereas that of most urea cycle enzymes was above the controls after the non-lethal, but not after the sub-lethal, dose of acetaminophen. The mRNA of glutamine synthetase was significantly more reduced by acetaminophen than by allyl alcohol. The mRNA of cell-cycle dependent proteins was greatly reduced after both toxins, more after the higher dose.

CONCLUSIONS

The study shows that acetaminophen intoxication inhibits or fails to induce the expression of acute phase proteins in contrast to allyl alcohol intoxication. Allyl alcohol suppressed the expression of urea cycle enzymes, whereas that of the rate limiting enzymes carbamoylphosphate synthase and argininosuccinate synthetase was increased by the non-lethal but not by the sub-lethal dose of acetaminophen. The expression of the cell-cycle dependent proteins was more suppressed after the sub-lethal than after the non-lethal dose of both toxins. The data support the view that a fatal outcome of the intoxications depends more on the ability to regenerate than on the maintenance of liver-specific functions.