DIMINISHED EXPRESSION OF CYP1A1 IN URETHANE-INDUCED LUNG-TUMORS IN STRAIN A J MICE - ANALYSIS BY IN-SITU HYBRIDIZATION AND IMMUNOHISTOCHEMICAL METHODS/

We have investigated the regulation and expression of CYP1A1 in solid and papillary lung tumors induced by the carcinogen urethane. Female s train A/J mice were administered urethane (1 mg/g body wt) intraperito neally, and when lung tumors were established at 16 weeks, mice were t reated with 3-methylcholanthrene to induce CYP1A1. CYP1A1 mRNA was det ected by in situ hybridization with a H-3-labeled RNA probe and quanti tated by image analysis, whereas CYP1A1 protein was detected by immuno histochemical staining with an avidin-biotin complex procedure. Our re sults showed that in untreated control and tumor-bearing mice, the CYP 1A1 mRNA was present at low levels, but the CYP1A1 protein was not det ectable. Treatment with 3-methylcholanthrene induced increased levels of both CYP1A1 mRNA and protein in lung parenchyma and tumor foci. Thi s induction was markedly higher in the parenchyma of control and tumor -bearing lungs than in either solid or papillary tumors. Differences i n CYP1A1 mRNA and protein expression were not evident in solid and pap illary tumors. In the parenchyma, induced CYP1A1 mRNA and protein were localized in the endothelial and alveolar septal cells. Endothelial c ells in tumors also contained substantial CYP1A1 mRNA and protein, whe reas low levels of both were found in lung tumor cells. Our finding of significant reduction in inducibility of CYP1A1 protein in conjunctio n with reduced CYP1A1 mRNA in tumor foci suggests reduced transcriptio nal activation as a regulatory mechanism. The lack of association betw een diminished CYP1A1 expression and either the tumor type or the mech anism mediating metabolic activation supports the hypothesis that the persistence of lung tumors may be due, in part, to a restricted capabi lity for the formation of reactive intermediates.