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ABSTRACT:The involvement of the canalicular multidrug resistance protein 2 (Mrp2) in the hepatobiliary excretion of acetaminophen (APAP)-glutathione (GSH) conjugate and its derivatives was investigated using transport-deficient (TR Acetaminophen (APAP 1 ) is biotransformed through phase I and phase II reactions into negatively charged, hydrophilic metabolites that are eliminated in urine and bile. Cytochrome P450 converts APAP to the highly reactive electrophile N-acetyl-p-benzoquinoneimine (Dahlin et al., 1984), which produces liver injury unless it is neutralized by conjugation with glutathione (GSH) (Mitchell et al., 1973). The resulting conjugate (APAP-GSH) can be sequentially metabolized to APAP-cysteinylglycine (APAP-CG), -cysteine (APAP-CYS), and -mercapturate (N-acetylated L-cysteine) (APAP-NAC). By contrast, conjugation with glucuronic acid and sulfate produces APAP-glucuronide (APAP-GLU) and APAP-sulfate (APAP-SUL), respectively. In several species, a large fraction of APAP-GSH undergoes biliary excretion in its original form and the rest is excreted in urine and bile as a mixture of thiol-containing derivatives, whereas the majority of APAP-GLU and APAP-SUL is excreted in urine (Gregus et al., 1988).Measuring the amount of APAP metabolites in urine and/or bile is considered a useful in vivo method for examining the effects of other chemicals on APAP metabolism, because changes in urinary and/or biliary excretion of APAP conjugates correlate well with changes in APAP biotransformation produced by xenobiotics (Jollow et al., 1974;Madhu et al., 1989;Liu et al., 1993). This approach assumes that membrane-associated transport systems for APAP metabolites in hepatocytes are not affected by xenobiotic treatment. With the recent identification and characterization of several apical and basolateral transporters (Muller and Jansen, 1997), it is now clear that the hepatic levels of some transport systems can be modulated by prototypical microsomal inducers such as pregnenolone 16␣-carbonitrile (Salphati and Benet, 1998), dexamethasone (Courtois et al., 1999), and phenobarbital (Ogawa et al., 2000). Therefore, these types of APAP disposition studies should be interpreted with respect to both xenobiotic inducibility of hepatic transporters and molecular mechanisms for the hepatic excretion of APAP and its metabolites.We recently showed that the biliary concentration of APAP-GSH, APAP-NAC, and APAP-GLU, but not that of APAP itself, APAP-SUL or APAP-CG/CYS, was significantly decreased by coadministration of the nonmetabolizable organic anion indocyanine green (ICG) in male CD-1 mice (Chen et al., 2000). These findings suggest that several APAP conjugates share hepatobiliary transport systems with ICG.A primary active transporter on the canalicular domain of hepatocytes, known as multidrug resistance protein 2 (Mrp2), mediates the hepatobiliary transport of a wide range of organic anions, including GSH-S-conjugates (e.g., leukotriene C4 and 2,4-dinitrophenyl-S-GSH), oxidized GSH (GSSG), glucuronide conjugates (e.g., ...

The purpose of this study was to investigate whether activation of the nuclear receptor PPARalpha is needed for protection from acetaminophen (APAP) hepatotoxicity produced by repeated administration of the peroxisome proliferator clofibrate (CFB). Female wild-type and PPARalpha-null mice received corn oil vehicle or 500 mg CFB/kg, ip, daily for 10 days. They were then fasted overnight (18 h) and either killed at 4 or 24 h after challenge with 400 mg APAP/kg. Controls received 50% propylene glycol vehicle only. In this model of CFB hepatoprotection, liver injury was assessed by measuring plasma sorbitol dehydrogenase activity and by histopathology at 24 h after APAP challenge. Significant hepatocellular necrosis was evident in both corn oil-pretreated PPARalpha-null and wild-type mice at 24 h after APAP challenge. In agreement with previous studies, CFB-pretreated wild-type mice showed marked protection against APAP toxicity. In contrast, CFB did not provide protection against APAP hepatotoxicity in the PPARalpha-null mice. Similarly, at 4 h after APAP challenge, hepatic glutathione depletion and selective arylation of cytosolic proteins were reduced significantly in CFB-pretreated wild-type mice, but not in PPARalpha-null mice. The lack of changes in APAP binding and NPSH depletion in CFB-pretreated, PPARalpha-null mice is consistent with the presence of significant liver injury at 24 h in this treatment group. These findings demonstrate that the protection against APAP hepatotoxicity by peroxisome proliferator treatment is mediated by the activation of PPARalpha.

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Ribose cysteine (RibCys) is a cysteine prodrug that increases both hepatic and renal glutathione with documented antagonism of acetaminophen (APAP)-induced hepatotoxicity. To determine if RibCys could also protect against APAP-induced kidney damage, mice were injected with APAP (600 mg/kg) or APAP and RibCys (1.0 g/kg) (APAP/RIB) followed by additional RibCys injections 1 and 2 hours later. Mice were euthanatized 10-12 hours after APAP administration, and liver and kidney toxicity were assessed by plasma sorbitol dehydrogenase (SDH) activity and blood urea nitrogen (BUN), respectively, and by histopathology. APAP treatment resulted in elevation of SDH activity and BUN to 2,490 U/ml and 47 mg/dl, respectively. By contrast, SDH and BUN values for APAP/RIB-treated mice were not different from controls, 0 U/ml and 31 mg/dl, respectively. Histopathologic examination revealed moderate to severe hepatic centrilobular necrosis in 9/11 and renal proximal tubular necrosis in 10/11 APAP-treated mice. However, no evidence of hepatic or renal toxicity was noted in any of the 12 APAP/RIB-treated mice. Utilizing the same treatment regimen, APAP covalent binding to hepatic and renal cytosolic proteins was assessed 4 hours after APAP challenge. RibCys cotreatment decreased covalent binding to the 58-kDa acetaminophen-binding protein in both liver and kidney. RibCys decreased both toxicity and covalent binding after APAP administration, and in addition to protecting the liver, this cysteine prodrug can also effectively protect the kidney from APAP-induced injury.