OVEREXPRESSION OF A CYTOCHROME-P-450 OF THE 2A FAMILY (CYP2A-5) IN CHEMICALLY-INDUCED HEPATOMAS FROM FEMALE MICE

Chemical hepatocarcinogenesis in female mice, induced by 5,9-dimethyl( 7H)dibenzo[c,g] carbazole, leads to the overexpression of a cytochrome P-450 of the 2a family. This protein was identified as Cyp2a-5, by th e use of immunoblots obtained from isoelectric focusing gels. This met hod allowed the distinction of Cyp2a-5 from Cyp2a-4, another mouse liv er cytochrome P-450, by taking advantage of their slightly different p i values. The theoretical pi values, determined from the amino acid se quence, were pI 9.91 for Cyp2a-4 and pI 10.01 for Cyp2a-5. Other struc turally related forms were not detected. In hepatomas from female mice , only the Cyp2a-5 form was overexpressed (2-3 fold). Male mice showed a weak expression of Cyp2a-4 and Cyp2a-5 in control liver samples and in hepatomas. The expression of both forms was increased more than fi vefold upon castration. Pyrazole induces specifically the Cyp2a-5 form . The Cyp2a-5 overexpression was correlated with enhanced microsomal c oumarin-7-hydroxylase and testosterone-15 alpha-hydroxylase activities . An immunohistochemical study showed that Cyp2a-4and Cyp2a-5 are expr essed uniformly in female livers, but centrilobularly in male livers. In hepatomas, this localisation is perturbed; in females we observed a focal cell localisation, and the Cyp2a-containing cells were often hy pertrophic and polyploid. In hepatomas from male mice, the Cyp2a-conta ining cells became dispersed. From a comparison with other studies, th e Cyp2a-5 overexpression appears to be a general feature of hepatocarc inogenesis in mice.