CYTOCHROME-P4501A INDUCTION AND INHIBITION BY 3,3',4,4'-TETRACHLOROBIPHENYL IN AN AH RECEPTOR-CONTAINING FISH HEPATOMA-CELL LINE (PLHC-1)

The induction of cytochrome P4501A1 (CYP1A1) in rat hepatoma cells has been used by some investigators to determine 'dioxin equivalents' in-environmental samples, including extracts of fish tissues. However, the relative potency of inducing compounds may vary between species, suggesting the need for taxon-specific model systems. In this paper we present an initial characterization of CYP1A induction in one such system, a teleost liver cell line (PLHC-1) derived from a hepatocellular carcinoma of Poeciliopsis lucida (Hightower, L.E. and Renfro, J.L., 1988. J. Exp. Zool. 248, 290). Specific binding of the photoaffinity ligand 2-azido-3-[I-125]iodo-7,8-dibromodibenzo-p-dioxin ([I-125]N3Br2DD) to proteins in PLHC-1 cytosol indicated the presence of the Ah receptor, which is known to control CYP1A induction in mammals. 3,3',4,4'-Tetrachlorobiphenyl (TCB) induced a microsomal protein in PLHC-1 cells that was recognized by monoclonal antibody (MAb) 1-12-3 to scup CYPIA1 (P450E) on immunoblots. Immunohistochemical staining of whole cells with MAb 1-12-3 showed specific recognition of CYP1A induced by TCB. No staining was seen in untreated or vehicle-treated cells. There was an excellent quantitative correlation between amounts of CYPIA protein detected immunohistochemically and in immunoblots of cell homogenates. In a dose response experiment, maximal induction of ethoxyresorufin O-deethylase (EROD) activity occurred at 0.1 muM TCB; at higher concentrations (1 and 10 muM), EROD activity was reduced as compared to the activity at 0.1 muM TCB. In contrast, immunoreactive CYP1A protein increased with increasing TCB concentration up to 10 muM. The loss of EROD activity at high concentrations of TCB did not result from changes in cell number or viability. The apparent inhibition or inactivation of CYPIA catalytic activity by the higher concentrations of halogenated biphenyls has been seen, but not generally recognized, both in vivo and in cultured cells from diverse vertebrate species. PLHC-1 cells may be a good model system for studying Ah receptor-mediated regulation of gene expression, for determining the fish-specific toxic or inducing potency of halogenated aromatic hydrocarbon congeners, and for investigating the mechanism of CYP1A inhibition or inactivation by environmental contaminants such as TCB.