Vol. 304, Issue 1, 145-155, January 2003

Blockage of Multidrug Resistance-Associated Proteins Potentiates the Inhibitory Effects of Arsenic Trioxide on CYP1A1 Induction by Polycyclic Aromatic Hydrocarbons

Laurent Vernhet, Nathalie Allain, Marc Le Vée, Fabrice Morel, André Guillouzo and Olivier Fardel

Institut National de la Santé et de la Recherche Médicale U456, Détoxication et Réparation Tissulaire, Faculté des Sciences Pharmaceutiques et Biologiques, Université de Rennes I, Rennes, France

Arsenic is a toxic metalloid known to interact with drug-metabolizing enzymes. In the present study, we investigated the effects of arsenic trioxide (As₂O₃), recently used as an anticancer drug, on the expression of human cytochrome P450 (P450) 1A1, which bioactivates polycyclic aromatic hydrocarbons into mutagenic metabolites. Clinically relevant concentrations (0.25-5 µM) of As₂O₃ were demonstrated to inhibit CYP1A activity in primary human hepatocytes and hepatoma Hep3B and HepG2 cells coexposed to 3-methylcholanthrene (3MC), benzo(a)pyrene, or dioxin and the metalloid for 24 h. Inhibition reached 50 and 90% in Hep3B cells treated with 1 and 5 µM As₂O₃, respectively, and was not due to direct interaction of the metalloid with CYP1A1. As₂O₃ (2.5-5 µM) was demonstrated to markedly reduce induction of CYP1A1 mRNA and apoprotein levels and gene promotor activity in 3MC-treated Hep3B cells, whereas lower concentrations (0.25-1 µM) were ineffective. These effects of As₂O₃ were abrogated by N-acetylcysteine. Surprisingly, this agent was found 1) to block cellular arsenic uptake when coincubated with the metalloid and 2) to increase arsenic efflux through multidrug resistance-associated proteins. In addition, blockade of these transporters was shown to enhance intracellular amounts of metalloid and to potentiate its effects on CYP1A1 gene. Finally, our results have demonstrated that As₂O₃, at low concentrations routinely reached in As₂O₃-treated patients, prevents induction of human CYP1A1 gene expression and that such an effect is increased by blocking multidrug resistance-associated proteins.