N-acetylcysteine-induced inhibition of gastric emptying: a mechanism affording protection to mice from the hepatotoxicity of concomitantly administered acetaminophen

Exogenous supplementation of N-acetylcysteine Can Reduce Hepatotoxicity Induced by Ascites Fluid (Cell-Free) Adsorbed Over Protein-A-Containing Staphylococcus aureus Cowan-I Without Compromising Its Antitumor Effect

Introduction:

Hepatotoxicity along with enhanced mortality has remained a major concern during the development of antitumor therapy with the use of cell-free ascites fluid adsorbed (ad-AF) over Protein-A-containing Staphylococcus aureus Cowan I (SAC). Major issue with ad-AF inoculation is the significant depletion of hepatic glutathione (GSH). Exogenous supplementation of –SH contents to the host has offered an encouraging hope to explore the possibilities to use ad-AF as a therapeutic material due to its antitumor effects. GSH and l-cysteine have shown a promise with the recovery of –SH contents as well as the recovery of phase I and phase II biotransformation enzymes. Aforementioned observations prompted us to try other –SH donors.

Materials and Methods:

Therefore, in this study, N-acetylcysteine (NAC) was used as an exogenous source to provide –SH contents to reduce hepatotoxicity and mortality induced by ad-AF treatment.

Results:

Exogenous supplementation of NAC along with ad-AF treatment to ascites tumor bearers has shown a significant protection against hepatotoxicity and mortality caused by ad-AF. NAC substitution along with ad-AF has significantly enhanced the mean survival time (MST), without altering the antitumor effect of ad-AF as evident from tumor cell counts and viability.

Discussion:

NAC supplementation has been successful to recover hepatic –SH contents along with the significant recovery of phase I and phase II biotransformation enzymes. Marker enzymes for liver injury have also given clear-cut indications for the recovery of tumor bearers from hepatotoxicity induced by ad-AF.

Conclusion:

This study has shown that exogenous supplementation of NAC protects the host from the enhanced mortality and hepatotoxicity induced by ad-AF. These observations offer a hope to develop ad-AF as one of the probable treatment strategies for ascites tumors at least at experimental levels.

Circulating acylcarnitines as biomarkers of mitochondrial dysfunction after acetaminophen overdose in mice and humans

Acetaminophen (APAP) is a widely used analgesic. However, APAP overdose is hepatotoxic and is the primary cause of acute liver failure in the developed world. The mechanism of APAP-induced liver injury begins with protein binding and involves mitochondrial dysfunction and oxidative stress. Recent efforts to discover blood biomarkers of mitochondrial damage have identified increased plasma glutamate dehydrogenase activity and mitochondrial DNA concentration in APAP overdose patients. However, a problem with these markers is that they are too large to be released from cells without cell death or loss of membrane integrity. Metabolomic studies are more likely to reveal biomarkers that are useful at early time points, before injury begins. Similar to earlier work, our metabolomic studies revealed that acylcarnitines are elevated in serum from mice after treatment with toxic doses of APAP. Importantly, a comparison with furosemide demonstrated that increased serum acylcarnitines are specific for mitochondrial dysfunction. However, when we measured these compounds in plasma from humans with liver injury after APAP overdose, we could not detect any significant differences from control groups. Further experiments with mice showed that N-acetylcysteine, the antidote for APAP overdose in humans, can reduce acylcarnitine levels in serum. Altogether, our data do not support the clinical measurement of acylcarnitines in blood after APAP overdose due to the standard N-acetylcysteine treatment in patients, but strongly suggest that acylcarnitines would be useful mechanistic biomarkers in other forms of liver injury involving mitochondrial dysfunction.

Effects of N-acetylcysteine and dithiothreitol on glutathione and protein thiol replenishment during acetaminophen-induced toxicity in isolated mouse hepatocytes

Isolated mouse hepatocytes were incubated with 1.0 mM acetaminophen (AA) for 1.5 h to initiate glutathione (GSH) and protein thiol (PSH) depletion and cell injury. Cells were subsequently washed to remove non-covalently bound AA and resuspended in medium containing N-acetylcysteine (NAC, 2.0 mM) or dithiothreitol (DTT, 1.5 mM). The effects of these agents on the replenishment of GSH and total PSH content were related to the development of cytotoxicity. When cells exposed to AA were resuspended in medium containing NAC or DTT, both agents replenished GSH and total PSH content to levels observed in untreated cells but only DTT was able to attenuate cytotoxicity. Addition of the GSH synthesis inhibitor, buthionine sulfoximine (BSO, 1.0 mM, 1.5 h), to cells in incubation medium containing AA, enhanced GSH and total PSH depletion and potentiated cytotoxicity. Resuspension of these cells in medium containing NAC did not alter the potentiating effects of BSO; GSH and PSH levels were not replenished and no cytoprotective effects were observed. However, when cells exposed to AA and BSO were resuspended in medium containing DTT, PSH content was replenished but GSH levels were not restored. In addition, DTT was able to delay the development of cytotoxicity. It appears that DTT, unlike NAC, has a GSH-independent mechanism of PSH replenishment. These observations suggest that while replenishment of GSH and total PSH content does not result in cytoprotection, the regeneration of critical PSH by DTT may play an important role in the maintenance of proper cell structure and/or function.

The role of heme oxygenase and aryl hydrocarbon hydroxylase in the protection by cysteamine from acetaminophen hepatotoxicity

Administration of cysteamine to rats depressed hepatic aryl hydrocarbon hydroxylase (AHH) activity, cytochrome P-450, and total heme at 24 hr. Total heme remained decreased at 48 hr when all other parameters returned to control values. A significant 5-fold increase in heme oxygenase activity occurred in rat liver 5 hr after treatment, when AHH activity and total heme were unchanged. Histological examination of liver biopsies from rats treated with cysteamine revealed normal hepatic architecture. The observed effects of cysteamine on hepatic drug-metabolizing enzymes in vivo were not due to cysteamine-induced hepatotoxicity. Our results indicate that cysteamine increases heme oxygenase activity in rat liver, with a subsequent decrease in total heme, AHH activity, and cytochrome P-450 content. The depression of P-450 by cysteamine is likely to be an important mechanism for its protection in acetaminophen overdose. The protection studies illustrate this mechanism. Centrilobular hepatic necrosis and elevation in transaminase activity following a toxic dose of acetaminophen were prevented by treatment with cysteamine. The hepatoprotective effect of cysteamine was evident when acetaminophen was administered 24 hr after cysteamine but did not occur when acetaminophen was administered 5 hr after cysteamine or simultaneously. All groups of rats receiving cysteamine showed decreased mortality compared to the group receiving acetaminophen alone.

Efficacy of paracetamol-esterified methionine versus cysteine or methionine on paracetamol-induced hepatic GSH depletion and plasma ALAT level in mice

The effect of paracetamol-N-acetyl-DL-methionate (PAM) in preventing paracetamol-induced hepatic glutathione (GSH) depletion and hepatic cell damage assessed by plasma ALAT level, was compared to those of concomitantly administered paracetamol and N-acetyl-L-cysteine (NAC) or N-acetyl-DL-methionine (NAM) and paracetamol 400 mg/kg (P) alone. PAM, NAM and NAC reduced hepatic GSH depletion compared to P. The concomitant administration of GSH precursors in either form apparently maintained hepatic cell integrity as evaluated by plasma ALAT compared to predose and 16 hr control measurements. No statistically significant difference between PAM, NAM and NAC was observed. In group P a statistically significant, but transitory, rise in plasma ALAT level following dosage was seen. NAC was more effective than PAM and NAM in the prevention of GSH depletion 1 hr after dosing but was less effective in promoting de novo GSH synthesis towards 16 hr. There was no statistically significant difference between PAM and NAM with respect to effect on GSH depletion or hepatic cell integrity. PAM and NAM increased the GSH level significantly above control level 16 hr after dosing. PAM is rapidly cleaved to paracetamol and methionine following dosage as shown by the observed plasma paracetamol level. PAM compares favourably in hepatoprophylactic effect, to concomitant administration of equimolar doses of free N-acetyl-DL-methionine added to the paracetamol formulation.

Dissociation of increased sulfation from sulfate replenishment and hepatoprotection in acetaminophen-poisoned mice by N-acetylcysteine stereoisomers

N-Acetylcysteine stereoisomers were compared for their ability to alter the sulfation and hepatotoxicity of acetaminophen. The clinically used L-isomer increased urinary excretion of inorganic sulfate 2-3 fold and prevented liver injury, but failed to increase acetaminophen sulfation in mice. Conversely, the nonphysiologic D-isomer failed to increase urinary excretion of inorganic sulfate or prevent hepatotoxicity, but increased acetaminophen sulfation appreciably (by 39%). The basis of the incongruence between changes in the availability of inorganic sulfate and the sulfation of acetaminophen is not known. These data indicate that a modest increase in acetaminophen sulfation, occurring alone following N-acetylcysteine treatment, is insufficient to explain the profound efficacy of the antidote in mice, and further suggest that this holds true for other species, such as humans, that are comparatively poor in the sulfoconjugation of acetaminophen.

Mechanism of action of paracetamol protective agents in mice in vivo

The mechanism of action of cysteine, methionine, N-acetylcysteine (NAC) and cysteamine in protecting against paracetamol (APAP) induced hepatotoxicity in male C3H mice in vivo has been investigated by, characterising the effect of the individual protective agents on the metabolism of an hepatotoxic dose of APAP, and determining the efficacy of the protective agents in animals treated with buthionine sulphoximine (BSO), a specific inhibitor of glutathione (GSH) synthesis. Co-administration of cysteine, methionine or NAC increased, while co-administration of cysteamine decreased, the proportion of GSH-derived conjugates of APAP excreted in the urine of mice administered APAP, 300 mg/kg. Pretreatment of animals with BSO abolished the protective effect of cysteine, methionine and NAC, whereas cysteamine still afforded protection against APAP after BSO treatment. In conjunction with other data, these results suggest the most likely mechanism for the protective effect of cysteine, methionine and NAC is by facilitating GSH synthesis, while the most likely mechanism for the protective effect of cysteamine is inhibition of cytochrome P-450 mediated formation of the reactive metabolite of APAP.

The effectiveness of N-acetylcysteine in isolated hepatocytes, against the toxicity of paracetamol, acrolein, and paraquat

The protective effect of N-acetylcysteine against the toxicity of paracetamol, acrolein, and paraquat was investigated using isolated hepatocytes as the experimental system. N-acetylcysteine protects against paracetamol toxicity by acting as a precursor for intracellular glutathione. N-acetylcysteine protects against acrolein toxicity by providing a source of sulfhydryl groups, and is effective without prior conversion. Paraquat toxicity can be decreased by coincubating the cells with N-acetylcysteine, but the mechanism for the protective effect is not as clear in this instance. It is probable that N-acetylcysteine protects against paraquat toxicity by helping to maintain intracellular glutathione levels.

Acetaminophen and its toxicity

Acetaminophen (APAP) is considered one of the safest of all minor analgesics, but when taken in large doses (greater than 10 g) toxicity occurs. Severely poisoned patients experience hepatic and/or renal failure. The major metabolic pathway of APAP is formation of glucuronide and sulfate conjugates. A minor pathway is formation of a reactive metabolite that conjugates with glutathione (GSH). When GSH is depleted, the reactive metabolite causes necrosis of hepatic and other tissues. Treatment of APAP toxicity involves supplying alternate sulfhydryl donors or inhibiting oxidative formation of the reactive metabolite. Estimation of plasma APAP levels is necessary for effective treatment.

Mechanism of action of N-acetylcysteine in the protection against the hepatotoxicity of acetaminophen in rats in vivo

N-Acetylcysteine is the drug of choice for the treatment of an acetaminophen overdose. It is thought to provide cysteine for glutathione synthesis and possibly to form an adduct directly with the toxic metabolite of acetaminophen, N-acetyl-p-benzoquinoneimine. However, these hypothese have not been tested in vivo, and other mechanisms of action such as reduction of the quinoneimine might be responsible for the clinical efficacy of N-acetylcysteine. After the administration to rats of acetaminophen (1 g/kg) intraduodenally (i.d.) and of [(35)S]-N-acetylcysteine (1.2 g/kg i.d.), the specific activity of the N-acetylcysteine adduct of acetaminophen (mercapturic acid) isolated from urine and assayed by high pressure liquid chromatography averaged 76+/-6% of the specific activity of the glutathione-acetaminophen adduct excreted in bile, indicating that virtually all N-acetylcysteine-acetaminophen originated from the metabolism of the glutathione-acetaminophen adduct rather than from a direct reaction with the toxic metabolite. N-Acetylcysteine promptly reversed the acetaminophen-induced depletion of glutathione by increasing glutathione synthesis from 0.54 to 2.69 mumol/g per h. Exogenous N-acetylcysteine did not increase the formation of the N-acetylcysteine and glutathione adducts of acetaminophen in fed rats. However, when rats were fasted before the administration of acetaminophen, thereby increasing the stress on the glutathione pool, exogenous N-acetylcysteine significantly increased the formation of the acetaminophen-glutathione adduct from 57 to 105 nmol/min per 100 g. Although the excretion of acetaminophen sulfate increased from 85+/-15 to 211+/-17 mumol/100 g per 24 h after N-acetylcysteine, kinetic simulations showed that increased sulfation does not significantly decrease formation of the toxic metabolite. Reduction of the benzoquinoneimine by N-acetylcysteine should result in the formation of N-acetylcysteine disulfides and glutathione disulfide via thiol-disulfide exchange. Acetaminophen alone depleted intracellular glutathione, and led to a progressive decrease in the biliary excretion of glutathione and glutathione disulfide. N-Acetylcysteine alone did not affect the biliary excretion of glutathione disulfide. However, when administered after acetaminophen. N-acetylcysteine produced a marked increase in the biliary excretion of glutathione disulfide from 1.2+/-0.3 nmol/min per 100 g in control animals to 5.7+/-0.8 nmol/min per 100 g. Animals treated with acetaminophen and N-acetylcysteine excreted 2.7+/-0.8 nmol/min per 100 g of N-acetylcysteine disulfides (measured by high performance liquid chromatography) compared to 0.4+/-0.1 nmol/min per 100 g in rats treated with N-acetylcysteine alone. In conclusion, exogenous N-acetylcysteine does not form significant amounts of conjugate with the reactive metabolite of acetaminophen in the rat in vivo but increases glutathione synthesis, thus providing more substrate for the detoxification of the reactive metabolite in the early phase of an acetaminophen intoxication when the critical reaction with vital macromolecules occurs.

Acetaminophen-induced hypothermia in mice: evidence for a central action of the parent compound

Pretreatment of mice with phenobarbital, an inducer of oxidative drug metabolism, had no effect on the early hypothermic effect of a toxic dose of acetaminophen, while pretreatment with metyrapone, SKF-525A, or piperonyl butoxide (inhibitors of mixed-function oxidase) enhanced the hypothermia. In mice treated with acetaminophen alone, brain parent drug levels correlated with the degree of hypothermia, while liver drug levels did not. Also, intracerebroventricular injection of acetaminophen resulted in significant hypothermia within 20 min. These results indicate that the early hypothermia caused by acetaminophen in mice is due to the parent drug, not to its toxic reactive metabolite, and that the effect is mediated centrally. The observation that piperonyl butoxide and SKF-525A themselves caused significant hypothermia indicates that the use of these compounds should be avoided when body temperature is being followed in drug metabolism experiments.