SELECTIVE PROTEIN ADDUCT FORMATION OF DICLOFENAC GLUCURONIDE IS CRITICALLY DEPENDENT ON THE RAT CANALICULAR CONJUGATE EXPORT PUMP (MRP2)

Previous work demonstrates that the reactive acyl glucuronide of the n onsteroidal antiinflammatory drug diclofenac forms selective protein a dducts in the liver, which may play a causal role in the pathogenesis of diclofenac-associated liver toxicity. Because glucuronide conjugate s can be exported into the bile, we explored the role of diclofenac gl ucuronide hepatobiliary transport in the formation of site-specific pr otein adducts. Specifically, to analyze intracellular (hepatocytes) ve rsus extracellular (biliary tree) targeting of proteins, we have compa red the pattern of diclofenac binding in normal Wistar rats with that in mutant transport-deficient (TR-) rats which lack the functional can alicular isoform of the conjugate export pump, Mrp2. In bile duct-cann ulated normal rats, >50% of an iv injected dose of [C-14]diclofenac ap peared in the bile over a 90-min period. In contrast, in TR- rats virt ually no hepatobiliary excretion of diclofenac glucuronide was found. After administration of diclofenac (30 mg/kg/day, ip for 3 days) to ra ts of both genotypes, a major protein adduct of an apparent molecular mass of 118 kDa was selectively detected by immunoblotting in isolated canalicular, but not in basolateral, membrane subfractions of wild-ty pe rats, whereas no plasma membrane adducts could be found in the live rs of TR- rats. Furthermore, immunohistochemical analysis using an ant i-diclofenac antibody revealed the presence of distinct diclofenac-mod ified proteins on canalicular membranes of liver sections from diclofe nac-treated normal rats, whereas no adducts could be identified in liv ers of TR- rats. In Western blots, the major diclofenac-modified canal icular membrane protein did not comigrate with Mrp2, indicating that t he glucuronide carrier itself was unlikely to be a target. Collectivel y, the results demonstrate that the reactive diclofenac glucuronide is selectively transported into bile via Mrp2 and that hepatobiliary tra nsport is critical for diclofenac covalent binding to proteins in the biliary tree.