CYTOCHROME P450-MEDIATED METABOLISM OF TUMOR PROMOTERS MODIFIES THE INHIBITION OF INTERCELLULAR COMMUNICATION - A MODIFIED ASSAY FOR TUMOR PROMOTION

The role of metabolism of tumour promoters on the inhibition of interc ellular communication was investigated in a modified V79 metabolic coo peration system. V79 cells, which stably express different rat cytochr ome P450 enzymes (CYP1A1, CYP1A2 or CYP2B1), were used in the metaboli c cooperation assay. The inhibitory effect on intercellular communicat ion of four compounds was changed in cells expressing cytochrome P450 enzymes, compared to cells without. The phorbol ester TPA and di(2-eth ythexyl)phthalate blocked intercellular communication in all the cell lines tested, but expression of CYP1A1 enzyme reduced the inhibitory a ctivity in these cells. Diethylstilbestrol caused inhibition only with cells containing cytochrome P450 enzymes. In contrast, the benzene me tabolite hydroquinone inhibited metabolic cooperation preferentially i n cells without cytochrome P450 enzymes. The inhibition of metabolic c ooperation by another benzene metabolite, phenol, was not affected by the cytochrome P450 enzymes. The inhibitory activity of several chemic als that have not been tested previously was analysed in the new metab olic cooperation assay. The inhibitory activity of none of these chemi cals was affected by cytochrome P450-associated metabolism. 7-Octylind olactam V was as potent as TPA, whereas the related indolactam V was 1 00-fold less active. The carcinogenic aromatic amine 4-aminobiphenyl, but not its primary metabolite 4-hydroxyaminobiphenyl, inhibited metab olic cooperation. Other known carcinogens, ochratoxin A, aflatoxin B1 and 4-nitrobiphenyl, did not inhibit metabolic cooperation in either V 79 cells expressing or cells not expressing cytochrome P450. We conclu de that cytochrome P450-associated metabolism plays an important role in the inhibition of gap junctional intercellular communication of som e tumour promoters. The modified metabolic cooperation assay presented here is valuable for detecting some inhibitory chemicals which have b een 'false negative' in previous assays for gap junctional intercellul ar communication. The assay also discloses that cytochrome P450 metabo lism alters intercellular communication by a mechanism other than meta bolism of the exogenous inhibitor.