Mechanistic studies on the protective effect of the peroxisome proliferator clofibrate against acetaminophen-induced hepatotoxicity

Pretreatment with clofibrate (CFB) and other peroxisome proliferators (PPs) confers resistance to acetaminophen (APAP) hepatotoxicity in mice. Previous studies have excluded the involvement of changes in APAP biotransformation and hepatic glutathione (GSH) content in this hepatoprotection model. The work presented here further explored the mechanisms underlying this drug interaction. Many hepatic effects of PPs have been determined to be receptor-mediated. Initial studies showed that peroxisome proliferator-activated receptor α (PPARα)-null mice exposed to CFB are not resistant to APAP hepatotoxicity. This documents that hepatoprotection by CFB is PPARα-dependent and that increased expression of gene products regulated by this receptor may lead to this response. Generation of reactive oxygen species (ROS) and oxidative damage play important roles in APAP hepatotoxicity. Exposure to PPs moderately increases the H2O2-degrading activity of catalase. To determine whether elevated catalase activity contributes to the resistance to APAP toxicity, the activity of this enzyme was modulated with the irreversible inhibitor, 3-amino-1, 2, 4-triazole (3-AT). Although 3-AT also inhibits APAP-metabolizing CYP2E1, we established a dose of 3-AT that selectively inhibits catalase without affecting APAP bioactivation. This dose of 3-AT exacerbated APAP toxicity in control mice. This finding further supports the role of ROS in APAP toxicity. By contrast, 3-AT did not abolish the resistance to APAP toxicity in CFB-pretreated mice. Thus, the increase in hepatic catalase activity may not be a major contributor to the hepatoprotection. These results also suggest that other antioxidant defense mechanisms may be involved. The effect of CFB pretreatment on the hepatobiliary disposition of APAP was then investigated. The results showed that hepatoprotection by CFB is not associated with changes in hepatobiliary disposition of APAP. This provides additional evidence for the lack of changes in APAP biotransformation and disposition in mice exposed to CFB. Additional studies examining the hepatobiliary excretion of APAP and its metabolites indicated that several APAP conjugates and model cholephilic organic anions are secreted in bile through common canalicular transport systems. Furthermore, biliary excretion of these conjugates is severely impaired in Mrp2-deficient TR− rats. These findings provide insightful information on hepatic transport processes for APAP and its metabolites.