Hepatobiliary elimination of the peroxisome proliferator nafenopin by conjugation and subsequent atp-dependent transport across the canalicular membrane

Abstract

Amphiphilic carboxylates acting as peroxisome proliferators and hypolipidemic drugs induce enzymes of peroxisomal lipid β-oxidation, certain drug-metabolizing enzymes in the liver, and a number of additional proteins. The peroxisome proliferators represent a well-established class of non-genotoxic hepatocarcinogens. In this study we characterized the hepatic elimination of the peroxisome proliferator nafenopin. In the rat in vivo, 1 hr after intravenous administration of [ 3H]nafenopin, approx. 40% of injected radioactivity was recovered in bile. HPLC analysis of bile samples revealed that only about 10% of the radioactivity recovered in bile was associated with non-metabolized nafenopin and approx. 90% with more polar metabolites. One of the main metabolites formed in the liver and excreted into bile was identified as nafenopin glucuronide by β-glucuronidase-catalysed reconversion to nafenopin. In mutant rats deficient in the canalicular transport of leukotriene C 4 and related amphiphilic anion conjugates, recovery of [ 3H]nafenopin-derived radioactivity in bile was reduced to 4% of the injected dose. Although nafenopin glucuronide could not be detected in bile, it was a major metabolite in the liver from these mutant rats. Using membrane vesicles enriched in bile canalicular membranes from normal rats, transport of nafenopin glucuronide was shown to be a primary-active ATP-dependent process which was inhibited by leukotriene C 4 and S-dinitrophenyl glutathione with IC 50 values of 0.2 and 12 μM, respectively. ATP-dependent transport was not detectable for non-conjugated nafenopin. In canalicular membrane vesicles prepared from the mutant rats, the rate of ATP-dependent transport of nafenopin glucuronide was less than 10% of the transport observed in vesicles from normal rats. These data indicate that conjugation and subsequent transport by the ATP-dependent export carrier for leukotriene C 4 and related conjugates is a major pathway for the elimination of nafenopin and structurally-related peroxisome proliferators.