Cytochrome P450-mediated metabolism of tumour promoters modifies the inhibition of intercellular communication: a modified assay for tumour promotion.

Abstract

The role of metabolism of tumour promoters on the inhibition of intercellular communication was investigated in a modified V79 metabolic cooperation system. V79 cells, which stably express different rat cytochrome P450 enzymes (CYP1A1, CYP1A2 or CYP2B1), were used in the metabolic cooperation assay. The inhibitory effect on intercellular communication of four compounds was changed in cells expressing cytochrome P450 enzymes, compared to cells without. The phorbol ester TPA and di(2-ethylhexyl)phthalate blocked intercellular communication in all the cell lines tested, but expression of CYP1A1 enzyme reduced the inhibitory activity in these cells. Diethylstilbestrol caused inhibition only with cells containing cytochrome P450 enzymes. In contrast, the benzene metabolite hydroquinone inhibited metabolic cooperation preferentially in cells without cytochrome P450 enzymes. The inhibition of metabolic cooperation by another benzene metabolite, phenol, was not affected by the cytochrome P450 enzymes. The inhibitory activity of several chemicals that have not been tested previously was analysed in the new metabolic cooperation assay. The inhibitory activity of none of these chemicals was affected by cytochrome P450-associated metabolism. 7-Octylindolactam V was as potent as TPA, whereas the related indolactam V was 100-fold less active. The carcinogenic aromatic amine 4-aminobiphenyl, but not its primary metabolite 4-hydroxyaminobiphenyl, inhibited metabolic cooperation. Other known carcinogens, ochratoxin A, aflatoxin B1 and 4-nitrobiphenyl, did not inhibit metabolic cooperation in either V79 cells expressing or cells not expressing cytochrome P450. We conclude that cytochrome P450-associated metabolism plays an important role in the inhibition of gap junctional intercellular communication of some tumour promoters. The modified metabolic cooperation assay presented here is valuable for detecting some inhibitory chemicals which have been 'false negative' in previous assays for gap junctional intercellular communication. The assay also discloses that cytochrome P450 metabolism alters intercellular communication by a mechanism other than metabolism of the exogenous inhibitor.