Identification of a rat oltipraz-inducible UDP-glucuronosyltransferase (UGT1A7) with activity towards benzo(a)pyrene-7,8-dihydrodiol.

Previous work has shown that polycyclic aromatic hydrocarbons and oltipraz both induce an unidentified rat liver UDP-glucuronosyltransferase with activity toward benzo(a)pyrene-7, 8-diol, the proximate carcinogenic form of benzo(a)pyrene. Here we report the isolation of a benzo(a)pyrene-7,8-diol transferase-encoding cDNA, LC14, from an adult rat hepatocyte-derived cell line (RALA255-10G LCS-3). The predicted amino acid sequence of LC14 is nearly identical (5 differences out of 531 residues) to that deduced from UGT1A7, recently cloned at the genomic DNA level (Emi, Y., Ikushiro, S., and Kyanagi, T. (1995) J. Biochem. (Tokyo) 117, 392-399). Northern analysis of RNA from female F344 rat liver and LCS-3 cells revealed over a 40-fold and 4.4-fold enhancement by oltipraz treatment, respectively. Benzo(a)pyrene-7, 8-diol glucuronidating activity was detected (0.4 nmol/10(6) cells/16 h) in AHH-1 cells transfected with the LC14 expression vector, pMF6-LC14-3. The LC14-encoded transferase exhibited even higher activity toward certain benzo(a)pyrene phenols, including the major 3- and 9-phenol metabolites (4.1 and 2.8 nmol/10(6) cells/16 h, respectively). The Km of the enzyme for (-)-trans benzo(a)pyrene-7, 8-diol and 3-OH-BP was 15.5 and 12.3 microM, respectively. Northern analyses of total RNA revealed expression of LC14 or LC14-like RNA in all extrahepatic tissues tested. Marked inducibility by oltipraz was observed only in liver and (to a lesser extent) intestine. The results suggest that induction of UGT1A7 may explain the increased glucuronidating activities toward benzo(a)pyrene-7,8-diol and other metabolites that occur following treatment with polycyclic aromatic hydrocarbon-type inducing agents and oltipraz. UGT1A7 appears to represent an important cellular chemoprotective enzyme which mediates conjugation and elimination of toxic benzo(a)pyrene metabolites.