Protective effects of exogenous glutathione and related thiol compounds against drug-induced liver injury

An overdose of acetaminophen (APAP) causes liver injury both in experimental animals and humans. N-acetylcysteine (NAC) is clinically used as an antidote for APAP intoxication, and it is thought to act by providing cysteine as a precursor of glutathione, which traps a reactive metabolite of APAP. Other hepatoprotective mechanisms of NAC have also been suggested. Here, we examined the effects of thiol compounds with different abilities to restore hepatic glutathione, on hepatotoxicity of APAP and furosemide in mice. Overnight-fasted male CD-1 mice were given APAP or furosemide intraperitoneally. NAC, cysteine, glutathione, or glutathione-monoethyl ester was administered concomitantly with APAP or furosemide. All thiol compounds used in this study effectively protected mice against APAP-induced liver injury. Only glutathione-monoethyl ester completely prevented APAP-induced early hepatic glutathione depletion. Cysteine also significantly restored hepatic glutathione levels. NAC partially restored glutathione levels. Exogenous glutathione had no effect on hepatic glutathione loss. NAC and glutathione highly stimulated the hepatic expression of cytokines, particularly interleukin-6, which might be involved in the alleviation of APAP hepatotoxicity. Furosemide-induced liver injury, which does not accompany hepatic glutathione depletion, was also attenuated by NAC and exogenous glutathione, supporting their protective mechanisms other than replenishment of glutathione. In conclusion, exogenous thiols could alleviate drug-induced liver injury. NAC and glutathione might exert their effects, at least partially, via mechanisms that are independent of increasing hepatic glutathione, but probably act through cytokine-mediated and anti-inflammatory mechanisms.     

Acetaminophen hepatotoxicity: Is there a role for prostaglandin synthesis?

The hepatotoxicity of acetaminophen (APAP) overdose depends on metabolic activation to a toxic reactive metabolite via hepatic mixed function oxidase. In vitro studies have indicated that APAP may also be cooxidized by prostaglandin H synthetase. The present experiments were designed to assess the possible contribution of hepatic prostaglandin synthesis to APAP toxicity. Adult fed male mice were overdosed with 400 mg APAP/kg. Liver toxicity was estimated by measurement of serum transaminases. Hypertonic xylitol or sodium chloride (2250 mOsm/l), administered intragastrically to stimulate prostaglandin synthesis, increased APAP toxicity. By contrast, the cyclooxygenase inhibiting drugs aspirin (at 25 mg/kg) and indomethacin (at 10 mg/kg) protected against APAP-induced toxicity. APAP kinetics were not affected by hypertonic xylitol or indomethacin, nor were hepatic glutathione levels in overdosed mice. Imidazole, a nonspecific thromboxane synthetase inhibitor, also protected overdosed mice. This drug prolonged hexobarbital sleeping time and prevented the depletion of hepatic glutathione that followed APAP intoxication. Thus, the data support the conclusion that APAP-induced hepatotoxicity may be modulated not only by inhibition of cytochrome P450 mediated oxidation, but also by controlling hepatic cyclooxygenase activity.The Eugene Hecht Chair in Clinical Pharmacology     

Role of tumor necrosis factor receptor 1 (p55) in hepatocyte proliferation during acetaminophen-induced toxicity in mice

Hepatocyte proliferation represents an important part of tissue repair. In these studies, TNF receptor 1 (TNFR1) knockout mice were used to analyze the role of TNF-alpha in hepatocyte proliferation during acetaminophen-induced hepatotoxicity. Treatment of wild-type (WT) mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis. This was associated with proliferation of hepatocytes surrounding the damaged areas, which was evident at 24 h. The cell cycle regulatory proteins, cyclin D1 and cyclin A, were also up regulated in hepatocytes. In contrast, in TNFR1-/- mice, which exhibit exaggerated acetaminophen hepatotoxicity, hepatocyte proliferation, and expression of cyclin D1 and cyclin A, as well as the cyclin dependent kinases, Cdk4 and Cdk2, were reduced. The cyclin-dependent kinase inhibitor p21 was also induced in the liver following acetaminophen administration. This was greater in TNFR1-/- mice compared to WT mice. To investigate mechanisms mediating the reduced hepatic proliferative response of TNFR1-/- mice, we analyzed phosphatidyl inositol-3-kinase (PI-3K) signaling. In both WT and TNFR1-/- mice, acetaminophen caused a rapid increase in total PI-3K within 3 h. Acetaminophen also increased phosphorylated PI-3K, but this was delayed 6-12 h in TNFR1-/- mice. Expression of Akt, a downstream target of PI-3K, was increased in both WT and TNFR1-/- mice in response to acetaminophen. However, the increase was greater in WT mice. Acetaminophen-induced expression of phosphorylated STAT3, a key regulator of cytokine-induced hepatocyte proliferation, was also delayed in TNFR1-/- mice relative to WT. These data suggest that TNF-alpha signaling through TNFR1 is important in regulating hepatocyte proliferation following acetaminophen-induced tissue injury. Delayed cytokine signaling may account for reduced hepatocyte proliferation and contribute to exaggerated acetaminophen-induced hepatotoxicity in TNFR1-/- mice.     

Protective effects of salidroside against acetaminophen-induced toxicity in mice

The protective effect of salidroside (SDS) isolated from Rhodiola sachalinensis A. BOR. (Crassulaceae), was investigated in acetaminophen (APAP)-induced hepatic toxicity mouse model in comparison to N-acetylcysteine (NAC). Drug-induced hepatotoxicity was induced by an intraperitoneal (i.p.) injection of 300 mg/kg (sub-lethal dose) of APAP. SDS was given orally to mice at a dose of 50 or 100 mg/kg 2 h before the APAP administration in parallel with NAC. Mice were sacrificed 12 h after the APAP injection to determine aspartate aminotransferase (AST), alanine aminotransferase (ALT), and tumor necrosis factor-alpha (TNF-alpha) levels in serum and glutathione (GSH) depletion, malondialdehyde (MDA) accumulation, and caspase-3 expression in liver tissues. SDS significantly protected APAP-induced hepatotoxicity for SDS improved mouse survival rates better than NAC against a lethal dose of APAP and significantly blocked not only APAP-induced increases of AST, ALT, and TNF-alpha but also APAP-induced GSH depletion and MDA accumulation. Histopathological and immunohistochemical analyses also demonstrated that SDS could reduce the appearance of necrosis regions as well as caspase-3 and hypoxia inducible factor-1alpha (HIF-1alpha) expression in liver tissue. Our results indicated that SDS protected liver tissue from the APAP-induced oxidative damage via preventing or alleviating intracellular GSH depletion and oxidation damage, which suggested that SDS would be a potential antidote against APAP-induced hepatotoxicity.     

Enhanced acetaminophen hepatotoxicity in transgenic mice overexpressing BCL-2

Mitochondria play an important role in the cell death induced by many drugs, including hepatotoxicity from overdose of the popular analgesic, acetaminophen (APAP). To investigate mitochondrial alterations associated with APAP-induced hepatotoxicity, the subcellular distribution of proapoptotic BAX was determined. Based on the antiapoptotic characteristics of BCL-2, we further hypothesized that if a BAX component was evident then BCL-2 overexpression may be hepatoprotective. Mice, either with a human bcl-2 transgene (-/+) or wild-type mice (WT; -/-), were dosed with 500 or 600 mg/kg (i.p.) APAP or a nonhepatotoxic isomer, N-acetyl-m-aminophenol (AMAP). Immunoblot analyses indicated increased mitochondrial BAX-beta content very early after APAP or AMAP treatment. This was paralleled by disappearance of BAX-alpha from the cytosol of APAP treated animals and, to a lesser extent, with AMAP treatment. Early pathological evidence of APAP-induced zone 3 necrosis was seen in bcl-2 (-/+) mice, which progressed to massive panlobular necrosis with hemorrhage by 24 h. In contrast, WT mice dosed with APAP showed a more typical, and less severe, centrilobular necrosis. AMAP-treated bcl-2 (-/+) mice displayed only early microvesicular steatosis without progression to extensive necrosis. Decreased complex III activity, evident as early as 6 h after treatment, correlated well with plasma enzyme activities at 24 h (AST r(2) = 0.89, ALT r(2) = 0.87) thereby confirming a role for mitochondria in APAP-mediated hepatotoxicity. In conclusion, these data suggest for the first time that BAX may be an early determinant of APAP-mediated hepatotoxicity and that BCL-2 overexpression unexpectedly enhances APAP hepatotoxicity.     

Fas receptor-deficient lpr mice are protected against acetaminophen hepatotoxicity due to higher glutathione synthesis and enhanced detoxification of oxidant stress

Acetaminophen (APAP) overdose is a classical model of hepatocellular necrosis; however, the involvement of the Fas receptor in the pathophysiology remains controversial. Fas receptor-deficient (lpr) and C57BL/6 mice were treated with APAP to compare the mechanisms of hepatotoxicity. Lpr mice were partially protected against APAP hepatotoxicity as indicated by reduced plasma ALT and GDH levels and liver necrosis. Hepatic Cyp2e1 protein, adduct formation and hepatic glutathione (GSH) depletion were similar, demonstrating equivalent reactive metabolite generation. There was no difference in cytokine formation or hepatic neutrophil recruitment. Interestingly, hepatic GSH recovered faster in lpr mice than in wild type animals resulting in enhanced detoxification of reactive oxygen species. Driving the increased GSH levels, mRNA induction and protein expression of glutamate–cysteine ligase (gclc) were higher in lpr mice. Inducible nitric oxide synthase (iNOS) mRNA and protein levels at 6 h were significantly lower in lpr mice, which correlated with reduced nitrotyrosine staining. Heat shock protein 70 (Hsp70) mRNA levels were substantially higher in lpr mice after APAP. Conclusion: Our data suggest that the faster recovery of hepatic GSH levels during oxidant stress and peroxynitrite formation, reduced iNOS expression and enhanced induction of Hsp70 attenuated the susceptibility to APAP-induced cell death in lpr mice.     

Susceptibility to acetaminophen (APAP) toxicity unexpectedly is decreased during acute viral hepatitis in mice

Acetaminophen (APAP) hepatotoxicity results from cytochrome P450 metabolism of APAP to the toxic metabolite, n-acetyl-benzoquinone imine (NAPQI), which reacts with cysteinyl residues to form APAP adducts and initiates cell injury. As APAP is commonly used during viral illnesses there has been concern that APAP injury may be additive to that of viral hepatitis, leading physicians to advise against its use in such patients; this has not been investigated experimentally. We infected C57BL/6 male mice with replication-deficient adenovirus to produce moderately severe acute viral hepatitis and observed that APAP doses that were hepatotoxic or lethal in control mice produced neither death nor additional increase in serum ALT when administered to infected mice at the peak of virus-induced liver injury. Moreover, the concentration of hepatic APAP-protein adducts formed in these mice was only 10% that in control mice. Protection from APAP hepatotoxicity also was observed earlier in the course of infection, prior to the peak virus-induced ALT rise. Hepatic glutathione limits APAP-protein adduct formation but glutathione levels were similar in control and infected mice. Cyp1a2 (E.C. 1.14.14.1) and Cyp2e1 (E.C. 1.14.13.n7) mRNA expression decreased by 3 days post-infection and hepatic Cyp2e1 protein levels were reduced almost 90% at 7 days, when adduct formation was maximally inhibited. In vitro, hepatocytes from virally infected mice also were resistant to APAP-induced injury but sensitive to NAPQI. Rather than potentiating APAP-induced liver injury, acute viral hepatitis in this model resulted in selective down-regulation of APAP metabolizing P450s in liver and decreased the risk of APAP hepatotoxicity.     

Acetaminophen metabolism does not contribute to gender difference in its hepatotoxicity in mouse.

Gender is an important factor in pharmacokinetics and pharmacodynamics. In the current study, gender difference in acetaminophen (APAP)-induced hepatotoxicity has been examined. Male and female mice were injected with a toxic dose of APAP (500 mg/kg, ip). Female mice were resistant to the hepatotoxic effects of APAP, depicted by serum alanine aminotransferase and sorbital dehydrogenase activities and histological analysis. Basal hepatic reduced glutathione (GSH) levels were lower in females than in males, suggesting that basal GSH level may not be a factor in determining the gender difference of APAP hepatotoxicity. APAP metabolism was slower in females than males, revealed by lower levels of glucuronidation and sulfation and higher amounts of free APAP in the livers of female mice. Lower basal Cyp1a2 mRNA levels and lower expression of Cyp1a2 and Cyp3a11 mRNAs after APAP dosing were also observed in females compared with males. However, there was no gender difference in N-acetyl-p-benzoquinone imine covalent binding 2 h after APAP administration, suggesting similar APAP bioactivation between genders. Moreover, liver Gst pi mRNA levels were significantly lower in females than males. This finding is consistent with a previous report, which showed that Gst pi knockout mice are protected from APAP-induced liver toxicity. In conclusion, gender difference of APAP-induced hepatotoxicity is not likely due to APAP metabolism. Perhaps, it is in part due to gender-dependent Gst pi expression. However, the mechanism underlying the association between reduction in Gst pi expression and hepatoprotective effect against APAP toxicity remains to be further explored.     

Resistance to acetaminophen-induced hepatotoxicity in glutathione S-transferase Mu 1-null mice

We investigated the role of glutathione S-transferases Mu 1 (GSTM1) in acetaminophen (APAP)-induced hepatotoxicity using Gstm1-null mice. A single oral administration of APAP resulted in a marked increase in plasma alanine aminotransferase accompanied by hepatocyte necrosis 24 hr after administration in wild-type mice, but its magnitude was unexpectedly attenuated in Gstm1-null mice. Therefore, it is suggested that Gstm1-null mice are resistant to APAP-induced hepatotoxicity. To examine the mechanism of this resistance in Gstm1-null mice, we measured phosphorylation of c-jun N-terminal kinase (JNK), which mediates the signal of APAP-induced hepatocyte necrosis, by Western blot analysis 2 and 6 hr after APAP administration. A marked increase in phosphorylated JNK was observed in wild-type mice, but the increase was markedly suppressed in Gstm1-null mice. Therefore, it is suggested that suppressed phosphorylation of JNK may be a main mechanism of the resistance to APAP-induced hepatotoxicity in Gstm1-null mice, although other possibilities of the mechanism cannot be eliminated. Additionally, phosphorylation of glycogen synthase kinase-3β and mitogen-activated protein kinase kinase 4, which are upstream kinases of JNK in APAP-induced hepatotoxicity, were also suppressed in Gstm1-null mice. A decrease in liver total glutathione 2 hr after APAP administration, which is an indicator for exposure to N-acetyl-p-benzoquinoneimine, the reactive metabolite of APAP, were similar in wild-type and Gstm1-null mice. In conclusion, Gstm1-null mice are considered to be resistant to APAP-induced hepatotoxicity perhaps by the suppression of JNK phosphorylation. This study indicates the novel role of GSTM1 as a factor mediating the cellular signal for APAP-induced hepatotoxicity.     

The c-Met inhibitor capmatinib alleviates acetaminophen-induced hepatotoxicity

Acetaminophen (APAP)-induced hepatotoxicity comes among the most frequent humans’ toxicities caused by drugs. So far, therapeutic interventions for such type of drug-induced toxicity are still limited. In the current study, we examined the influence of capmatinib (Cap), a novel c-Met inhibitor, on APAP-induced hepatotoxicity in mice when administered 2 h prior, 2 h post and 4 h post APAP-challenge. The results revealed that Cap administration significantly attenuated APAP-induced liver injury when administered only 2 h prior and post APAP-administration. Cap hepatoprotective effect was mediated by lowering the excessive formation of lipid peroxidation and nitrosative stress products caused by APAP. Besides, Cap attenuated APAP-induced overproduction and release of proinflammatory mediators like TNF-α, IL-1β, IL-17A, IL-6, and MCP-1. Cap treatment also led to avoidance of APAP-subsequent repair by abating APAP-induced elevation of hepatic IL-22 and PCNA expressions. In conclusion, c-Met receptor inhibition may be a potential strategy for alleviating APAP-hepatotoxicity, especially when administered in the early phase of intoxication.     

Glycyrrhetinic acid prevents acetaminophen-induced acute liver injury via the inhibition of CYP2E1 expression and HMGB1-TLR4 signal activation in mice

Acetaminophen (APAP) is a widely used antipyretic and analgesic drug, which is safe and effective at the therapeutic dose. Unfortunately, excessive dosage of APAP could cause severe liver injury due to lack of effective therapy. Successful therapeutic strategies are urgently requested in clinic. Glycyrrhetinic acid (GA), derived from a traditional medicine licorice, has been shown to exert anti-inflammatory and antioxidant actions. In this study, the effect and the underlying mechanism of GA on APAP-induced hepatotoxicity were explored. Our results showed that pretreatment with GA significantly reduced serum ALT and AST activities, alleviated hepatic pathological damages with hepatocellular apoptosis, down-regulated expression of CYP2E1 mRNA and protein, increased GSH levels, and reduced reactive oxygen species (ROS) productions in the liver of APAP-exposed mice. Furthermore, GA obviously inhibited APAP-induced HMGB1-TLR4 signal activation, as evaluated by reduced hepatic HMGB1 release, p-IRAK1, p-MAPK and p-IκB expression as well as the productions of TNF-α and IL-1β. In addition, GA attenuated hepatic neutrophils recruitment and macrophages infiltration caused by APAP. These findings reflected that GA could alleviate APAP-induced hepatotoxicity, the possible mechanism is associated with down-regulation of CYP2E1 expression and deactivation of HMGB1-TLR4 signal pathway.     

Inhibition of matrix metalloproteinases minimizes hepatic microvascular injury in response to acetaminophen in mice.

The acetaminophen (APAP)-induced hepatic centrilobular necrosis is preceded by hepatic microcirculatory dysfunction including the infiltration of erythrocytes into the space of Disse. The purpose of this study was to examine the involvement of matrix metalloproteinases (MMPs) in the hepatic microvascular injury elicied by APAP. Male C57Bl/6 mice were pretreated with 2-[(4-biphenylsulfonyl) amino]-3-phenyl-propionic acid, an MMP-2/MMP-9 inhibitor (5 mg/kg, ip) 30 min before oral gavage with 600 mg/kg of APAP. The hepatic microvasculature in anesthetized mice was observed using established in vivo microscopic methods 2 and 6 h after APAP. The levels of mRNAs and activities of MMP-2 and MMP-9 in the liver were increased from 1 h through 6 h after APAP gavage. APAP increased alanine transferase (ALT) levels (41.1-fold) and resulted in centrilobular hemorrhagic necrosis at 6 h. Pretreatment with 2-[(4-biphenylsulfonyl) amino]-3-phenyl-propionic acid attenuated ALT values by 71% as well as the necrosis. APAP decreased the numbers of perfused sinusoids in centrilobular regions by 30% and increased the area occupied by infiltrated erythrocytes into Disse space. 2-[(4-Biphenylsulfonyl) amino]-3-phenyl-propionic acid restored the sinusoidal perfusion to 90% of control levels and minimized extrasinusoidal area occupied by erythrocytes. The present study showed that increased MMPs during APAP intoxication are associated with hepatocellular damage and with hepatic microcirculatory dysfunction including impaired sinusoidal perfusion and infiltration of erythrocytes in Disse space. 2-[(4-Biphenylsulfonyl) amino]-3-phenyl-propionic acid attenuated APAP-induced parenchymal and microvascular injury. These results suggest that MMPs participate in APAP hepatotoxicity mediated by sinusoidal endothelial cell injury, which results in impairment of microcirculation.     

Dioscin, a natural steroid saponin, shows remarkable protective effect against acetaminophen-induced liver damage in vitro and in vivo

The aim of the study was to investigate the protective effect of dioscin against APAP-induced hepatotoxicity. In the in vitro tests, HepG2 cells were given APAP pretreatment with or without dioscin. In the in vivo experiments, mice were orally administrated dioscin for five days and then given APAP. Some biochemical and morphology parameters were assayed and the possible mechanism was investigated. Dioscin improved AST release, mitochondrial dysfunction, apoptosis and necrosis of HepG2 cells induced by APAP. Following administration of dioscin, APAP-induced hepatotoxicity in mice was significantly attenuated. Furthermore, the liver cell apoptosis and necrosis, and hepatic mitochondrial edema were also prevented. Fifteen differentially expressed proteins were found by using proteomics, and six of them, Suox, Krt18, Rgn, Prdx1, MDH and PNP were validated. These proteins may be involved in the hepatoprotective effect of dioscin and might cooperate with the levels of Ca(2+) in mitochondria, decreased expression of ATP2A2, and decreased mitochondrial cardiolipin. In addition, dioscin inhibited APAP-induced activation and expression of CYP2E1, up-regulated the expression of Bcl-2 and Bid, and inhibited the expression of Bax, Bak and p53. Dioscin showed a remarkable protective effect against APAP-induced hepatotoxicity by adjusting mitochondrial function. These results indicated that dioscin has the capability on the treatment of liver injury.     

Glutamate cysteine ligase modifier subunit deficiency and gender as determinants of acetaminophen-induced hepatotoxicity in mice.

The analgesic and antipyretic drug acetaminophen (APAP) is bioactivated to the reactive intermediate N-acetyl-p-benzoquinoneimine, which is scavenged by glutathione (GSH). APAP overdose can deplete GSH leading to the accumulation of APAP-protein adducts and centrilobular necrosis in the liver. N-acetylcysteine (NAC), a cysteine prodrug and GSH precursor, is often given as a treatment for APAP overdose. The rate-limiting step in GSH biosynthesis is catalyzed by glutamate cysteine ligase (GCL) a heterodimer composed of catalytic and modifier (GCLM) subunits. Previous studies have indicated that GCL activity is likely to be an important determinant of APAP toxicity. In this study, we investigated APAP toxicity, and NAC or GSH ethyl ester (GSHee)-mediated rescue in mice with normal or compromised GCLM expression. Gclm wild-type, heterozygous, and null mice were administered APAP (500 mg/kg) alone, or immediately following NAC (800 mg/kg) or GSHee (168 mg/kg), and assessed for hepatotoxicity 6 h later. APAP caused GSH depletion in all mice. Gclm null and heterozygous mice exhibited more extensive hepatic damage compared to wild-type mice as assessed by serum alanine aminotransferase activity and histopathology. Additionally, male Gclm wild-type mice demonstrated greater APAP-induced hepatotoxicity than female wild-type mice. Cotreatment with either NAC or GSHee mitigated the effects of APAP in Gclm wild-type and heterozygous mice, but not in Gclm null mice. Collectively, these data reassert the importance of GSH in protection against APAP-induced hepatotoxicity, and indicate critical roles for GCL activity and gender in APAP-induced liver damage in mice.