The toxicological relevance of paracetamol-induced inhibition of hepatic respiration and ATP depletion

Non-canonical BIM-regulated energy metabolism determines drug-induced liver necrosis

Paracetamol (acetaminophen, APAP) overdose severely damages mitochondria and triggers several apoptotic processes in hepatocytes, but the final outcome is fulminant necrotic cell death, resulting in acute liver failure and mortality. Here, we studied this switch of cell death modes and demonstrate a non-canonical role of the apoptosis-regulating BCL-2 homolog BIM/Bcl2l11 in promoting necrosis by regulating cellular bioenergetics. BIM deficiency enhanced total ATP production and shifted the bioenergetic profile towards glycolysis, resulting in persistent protection from APAP-induced liver injury. Modulation of glucose levels and deletion of Mitofusins confirmed that severe APAP toxicity occurs only in cells dependent on oxidative phosphorylation. Glycolytic hepatocytes maintained elevated ATP levels and reduced ROS, which enabled lysosomal recycling of damaged mitochondria by mitophagy. The present study highlights how metabolism and bioenergetics affect drug-induced liver toxicity, and identifies BIM as important regulator of glycolysis, mitochondrial respiration, and oxidative stress signaling.

Preventive Effects of Mandarin Fruit Peel Hydroethanolic Extract, Hesperidin, and Quercetin on Acetaminophen-Induced Hepatonephrotoxicity in Wistar Rats

Acetaminophen, also known as N-acetyl-para-aminophenol (NAPAP), is a traditional antipyretic and analgesic that is used extensively around the world to treat colds and fevers. However, a NAPAP excess causes rapid, severe liver and kidney damage. The goal of the study was to examine the protective effects and determine the mechanisms of action of MPHE, hesperidin, and quercetin in NAPAP-induced hepatorenal damage in Wistar rats. Male Wistar rats received a 0.5 g/kg oral supplement of NAPAP every other day for a period of four weeks. During the same period of NAPAP supplementation, MPHE (50 mg/kg), quercetin (20 mg/kg), and hesperidin (20 mg/kg) were administered to rats receiving NAPAP. MPHE, quercetin, and hesperidin treatments significantly improved liver function in NAPAP-supplemented rats. The high serum levels of aminotransferases, alkaline phosphatase, lactate dehydrogenase, and γ-glutamyl transferase as well as total bilirubin were significantly reduced, while the levels of suppressed serum albumin were significantly increased, demonstrating this improvement. Treatments utilizing these natural substances significantly enhanced kidney function as seen by a considerable decline in the increased blood levels of urea, uric acid, and creatinine. Additionally, the injection of MPHE, hesperidin, and quercetin resulted in a decrease in the quantity of lipid peroxides while increasing the activities of superoxide dismutase, glutathione peroxidase, and glutathione-S-transferase in the liver and kidneys. The treatments markedly abated the NAPAP-induced liver and kidney histological perturbations and reduced the NAPAP-induced serum tumor necrosis factor-α level and liver and kidney proapoptotic protein 53 and caspase 3 expressions. Otherwise, serum interleukin-4 level significantly increased by treatments. The MPHE, hesperidin, and quercetin treatments resulted in marked decrease in liver and kidney histopathological scores including inflammation, necrosis, apoptosis, and congestion. In conclusion, the MPHE, quercetin, and hesperidin may induce hepatonephropreventive impacts in NAPAP-supplemented rats via enhancing the antioxidant defense system, anti-inflammatory activity, and antiapoptotic action.

Understanding lactic acidosis in paracetamol (acetaminophen) poisoning

Paracetamol (acetaminophen) is one of the most commonly taken drugs in overdose in many areas of the world, and the most common cause of acute liver failure in both the UK and USA. Paracetamol poisoning can result in lactic acidosis in two different scenarios. First, early in the course of poisoning and before the onset of hepatotoxicity in patients with massive ingestion; a lactic acidosis is usually associated with coma. Experimental evidence from studies in whole animals, perfused liver slices and cell cultures has shown that the toxic metabolite of paracetamol, N-acetyl-p-benzo-quinone imine, inhibits electron transfer in the mitochondrial respiratory chain and thus inhibits aerobic respiration. This occurs only at very high concentrations of paracetamol, and precedes cellular injury by several hours. The second scenario in which lactic acidosis can occur is later in the course of paracetamol poisoning as a consequence of established liver failure. In these patients lactate is elevated primarily because of reduced hepatic clearance, but in shocked patients there may also be a contribution of peripheral anaerobic respiration because of tissue hypoperfusion. In patients admitted to a liver unit with paracetamol hepatotoxicity, the post-resuscitation arterial lactate concentration has been shown to be a strong predictor of mortality, and is included in the modified King's College criteria for consideration of liver transplantation. We would therefore recommend that post-resuscitation lactate is measured in all patients with a severe paracetamol overdose resulting in either reduced conscious level or hepatic failure.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside and metformin inhibit hepatic glucose phosphorylation by an AMP-activated protein kinase-independent effect on glucokinase translocation

AMP-activated protein kinase (AMPK) controls glucose uptake and glycolysis in muscle. Little is known about its role in liver glucose uptake, which is controlled by glucokinase. We report here that 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), metformin, and oligomycin activated AMPK and inhibited glucose phosphorylation and glycolysis in rat hepatocytes. In vitro experiments demonstrated that this inhibition was not due to direct phosphorylation of glucokinase or its regulatory protein by AMPK. By contrast, AMPK phosphorylated liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase without affecting activity. Inhibitors of the endothelial nitric oxide synthase, stress kinases, and phosphatidylinositol 3-kinase pathways did not counteract the effects of AICAR, metformin, or oligomycin, suggesting that these signaling pathways were not involved. Interestingly, the inhibitory effect on glucose phosphorylation of these well-known AMPK activators persisted in primary cultured hepatocytes from newly engineered mice lacking both liver alpha1 and alpha2 AMPK catalytic subunits, demonstrating that this effect was clearly not mediated by AMPK. Finally, AICAR, metformin, and oligomycin were found to inhibit the glucose-induced translocation of glucokinase from the nucleus to the cytosol by a mechanism that could be related to the decrease in intracellular ATP concentrations observed in these conditions.

Hepatotoxicity of acetaminophen and N-acetyl-p-benzoquinone imine and enhancement by fructose

1. Although oral administration of 400 mg/kg acetaminophen (APAP) or 1.8-3.4 g/kg sucrose had no effect on serum levels of alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH), their co-administration resulted in 20-fold increases in ALT/SDH activities. APAP alone (1250 mg/kg, p.o.) caused the elevation hepatotoxicity parameters, but the levels were lower than observed with co-administration of APAP (400 mg/kg) and sucrose (2.6 or 3.4 g/kg). 2. Sucrose-associated increase in serum ALT/SDH activities was selective with APAP and not detected with carbon tetrachloride (160 mg/kg, i.p.), D-galactosamine (400 mg/kg, i.p.) or alpha-naphthyl isothiocyanate (100 mg/kg, p.o.). 3. To verify the synergistic mechanism of sucrose, a major reactive intermediate of APAP, N-acetyl-p-benzoquinone imine (NAPQI), was given via the portal vein to rat pretreated with sucrose. Clear elevation of ALT/SDH activities was detected in the co-treated group. These results, together with an allopurinol-inhibition experiment, suggest the involvement of high-dose sucrose at a step(s) occurring after the metabolic activation of APAP. 4. Co-administration of glucose or fructose as well as sucrose elevated APAP-induced hepatotoxicity parameters in rat. Fructose but not glucose elevated APAP- or NAPQI-induced LDH leakage in a primary hepatocyte system. The results suggest the primary role of fructose is on the sucrose enhancement of APAP toxicity in rat.

Identification of the hepatic protein targets of reactive metabolites of acetaminophen in vivo in mice using two-dimensional gel electrophoresis and mass spectrometry

Liver toxicity following an overdose of acetaminophen is frequently considered a model for drug-induced hepatotoxicity. Extensive studies over many years have established that such toxicity is well correlated with liver protein arylation by acetaminophen metabolites. Identification of protein targets for covalent modifications is a challenging but necessary step in understanding how covalent binding could lead to liver toxicity. Previous approaches suffered from technical limitations, and thus over the last 10 years heroic efforts were required to determine the identity of only a few target proteins. We present a new mass spectrometry-based strategy for identification of all target proteins that now provides a comprehensive survey of the suite of liver proteins modified. After administration of radiolabeled acetaminophen to mice, the proteins in the liver tissue lysate were separated by two-dimensional polyacrylamide gel electrophoresis. In-gel digestion of the radiolabeled gel spots gave a set of tryptic peptides, which were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Interrogation of data bases based on experimentally determined molecular weights of peptides and product ion tags from postsource decay mass spectra was employed for the determination of the identities of modified liver proteins. Using this method, more than 20 new drug-labeled proteins have been identified.

Spontaneous chemiluminescence production, lipid peroxidation, and covalent binding in rat hepatocytes exposed to acetaminophen

Spontaneous chemiluminescence associated with the cell injury was observed in the isolated rat hepatocyte suspension during acetaminophen (APAP) metabolism, indicating the occurrence of oxidative stress. APAP apparently affected the hepatocytes in various manners. APAP, at low concentrations (1-2 mM), damaged the hepatocytes due to lipid peroxidation provoked during APAP metabolism, while at high concentrations (5-50 mM), APAP protected the hepatocytes due to a chemical antioxidant effect of the unmetabolized APAP that remained in the medium because of the saturation of APAP metabolism. The covalent binding of APAP to the hepatocytes increased with APAP concentration up to 50 mM without loss of cell viability. When an overdose of APAP was administered to rats, the APAP plasma concentration was around 1-3 mM, which corresponded to the concentration range where lipid peroxidation occurred in the isolated hepatocytes. Thus, it seems likely that lipid peroxidation contributes to the APAP-induced hepatotoxicity in the early stage of the toxic process.

Mitochondrial ATPase: a target for paracetamol-induced hepatotoxicity

We examined the effect of paracetamol treatment (650 mg/kg) on the function of ATPase from rat hepatic mitochondria. The drug treatment caused an overall 35% decrease in ATPase activity, with a complete loss of the high affinity component as determined by substrate kinetic studies. The Km for the intermediate and low affinity components decreased by about 30% without change in Vmax, which may represent a compensatory mechanism. The drug treatment also resulted in a dramatic decrease in the phase transition temperature by about 19 degrees C without affecting the energies of activation of the enzyme. Mitochondrial total phospholipid content increased significantly with a reciprocal decrease in the cholesterol content. The total phospholipid/cholesterol molar ration increased by 50% after paracetamol treatment. However, phospholipid composition (as % of total) of the mitochondria was unaltered.

Chemotactic factors released from hepatocytes exposed to acetaminophen

To clarify the mechanism of neutrophil infiltration in the liver of acetaminophen-induced hepatic injury, chemotactic factor released from hepatocytes exposed to acetaminophen has been investigated. Hepatocytes exposed to acetaminophen release nondialyzable chemotactic factor, although acetaminophen in itself inhibits chemotaxis of neutrophils. Chemotactic activity of the nondialyzable chemotactic factor was reduced after treatment with heat (56 degrees C, 30 min) or trypsin. Chemotactic activity was demonstrated at the molecular weights of around 25 and 55 kDa. Chemotactic activity of the conditioned medium was not significantly reduced in the presence of antibody against rat KC/gro protein (interleukin-8-related cytokine in rodent). Chemotactic activity of a 25-kDa factor was reduced by the antibody against the antibody against KC/gro protein, but that of a 55-kDa factor was not reduced. Immunoblot analysis revealed that the peptide reacted with antibody against rat KC/gro protein was demonstrated at a molecular weight of around 20-25 kDa, but not around 55kDa, when the conditioned medium of acetaminophen-treated hepatocytes was electrophoresed. These results suggest that hepatocytes exposed to acetaminophen release two types of chemotactic factors for neutrophils and that a major part of the chemotactic factor could be different from a member of interleukin-8 family.