Diflubenzuron, a benzoyl-urea insecticide, is a potent inhibitor of TCDD-induced CYP1A1 expression in HepG2 cells

Diflubenzuron (DFB) belongs to a group of compounds called benzoyphenyl ure as acting as chitin synthesis inhibitors, which also inhibit growth of B16 murine melanomas. The present study was designed to investigate the effect of this insecticide, on CYP1A1 expression and induction in human hepatoma c ells HepG2, Treatment of HepG2 cells over 72 h with noncytotoxic concentrat ions of DFB resulted in a strong dose-dependent decrease in constitutive et hoxyresorufin-0-deethylase activity. Moreover, DFB significantly decreased CYP1A1 induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) after 24 h e xposure, as demonstrated by ethoxyresorufin-O-deethylase (EROD) activity an d Northern blot analysis. Additional studies were performed both on parenta l HepG2 cells and HepG-2241c.1, which were stably transfected with the chlo ramphenicol acetyltransferase (CAT) reporter gene, cloned under the control of the human CYP1A1 promoter (-1140 to +59). Ribonuclease protection assay s (RPA) analysis clearly demonstrated an inhibition of CYP1A1 transcription in both cell lines. Surprisingly, in corresponding experiments using 3-met hylcholanthrene (3-MC) as a CYP1A1 inducer, DFB was less effective. Finally , in competitive binding studies using a 9S-enriched fraction of HepG2 cyto sol, DFB was capable of displacing [H-3]-2,3,7,8-tetrachlorodibenzo-p-dioxi n (TCDD) from its Ah receptor binding site. Taken together, these results s upport the involvement of a transcriptional mechanism in the inhibition of CYP1A1 expression in HepG2 cells by DFB, possibly via an Ah receptor antago nism. (C) 2000 Academic Press.