Induction of cytochrome P450 activities by polychlorinated biphenyls in isolated mouse hepatocytes: Influence of Ah-phenotype and iron

Abstract

Exposure of cultured primary hepatocytes from Ah-responsive male C57BL/10ScSn mice to a polychlorinated biphenyl (PCB) mixture (Aroclor 1254) at 0.1–20 μg/mL for up to 96 hr induced cytochrome P4501AI-mediated activity (ethoxyresorufin 0-deethylase, EROD) up to 50-fold. In contrast, pentoxyresorufin O-dealkylase (PROD), which in some circumstances is a measure of phenobarbitone induced cytochrome P450 isoenzymes, was induced only 5-fold. There were similar findings on EROD activities with the pure compounds 3,3′,4,4′,5,5′-hexachlorobiphenyl, 3,3′,4,4′,5,5′-hexabromobiphenyl and 3,3′,4,4′-tetrachlorobiphenyl(TCB) and also β-naphthoflavone but not with 2,2′,4,4′-TCB or phenobarbitone. The higher concentrations of Aroclor 1254 were also associated with cytotoxicity as estimated by release of alanine aminotransferase (ALT) into the medium. Unlike in C57BL/10ScSn hepatocytes induction of EROD and cytotoxicity was minimal in hepatocytes from the Ah-non-responsive strain DBA/2. Although in vivo the hepatic toxicity and carcinogenicity of polyhalogenated aromatics are markedly potentiated by iron, no enhancement of the cytotoxicity of Aroclor 1254 towards C57BL/10ScSn hepatocytes by iron was observed in vitro. However, iron caused decreased EROD activities and possibly cytochrome P4501AI (as judged by Western blotting) as in vivo. Even in the presence of iron and the haem precursor 5-aminolaevulinic acid (5-ALA) there was no development of uroporphyria in this system although this occurs with Aroclor in vivo and is enhanced by iron. Accumulation of uroporphyrin did occur after extended culture of C57BL/10ScSn hepatocytes on matrigel for 8 days in the presence of 5-ALA and Aroclor 1254 but again no potentiation by iron was observed. Thus, although culture of Ah-responsive and -non-responsive hepatocytes mimics some aspects of the mechanisms of in vivo toxicity of PCBs, there is some unknown associated influence of iron metabolism which cannot, as yet, be produced vitro but which is of importance in vivo.