Loss of tumor-promoting activity of unleaded gasoline in N- nitrosodiethylamine-initiated ovariectomized B6C3F1 mouse liver

Unleaded gasoline (UG) vapor (2056 ppm) increased the incidence of livertumors in a chronic bioassay and exhibited tumor-promoting activity inN-nitrosodiethylamine (DEN)-initiated female mouse liver. Estrogeninhibited mouse liver tumor development and the hepatocarcinogenic andtumor-promoting dose of UG produced uterine changes suggestive of estrogenantagonism. To directly test the hypothesis that UG-induced tumor-promotingability is secondary to its interaction with the mouse liver tumorinhibitor, estrogen, we compared the tumor-promoting ability of UG inovariectomized (Ovex) mice with the hepatic tumor-promoting ability of UGin intact mice. Ovaries were surgically removed at 4 weeks of age. Exposureto wholly vaporized UG (2018 ppm) under bioassay and tumor-promotingconditions began at 8 weeks of age. After 4 months of exposure, UGincreased relative liver weight and hepatic microsomal cytochrome P450pentoxyresourfin-O- dealkylase and ethoxyresorufin-O-deethylase activity toa similar extent in intact and Ovex mice. Non-focal hepatocyteproliferation, as measured by the incorporation of bromo-deoxyuridine, wasnot changed by UG exposure and was similar in all treatment groups. After 4months of exposure to DEN-initiated mice, UG significantly increased thevolume fraction of liver occupied by foci (three-fold) as compared tocontrol intact mice. As expected, volume of foci was elevated inDEN/Ovex/control mice as compared to DEN/intact/control mice. In DEN/Ovexmice UG did not significantly increase the focal volume fraction. Thus, thetumor promoting activity of UG, as demonstrated by increased volumefraction of liver occupied by hepatic foci in intact mice, is greatlyattenuated in Ovex mice. The volume fraction data in Ovex mice support thehypothesis that the tumor promoting activity of UG is dependent upon theinteraction of UG with ovarian hormones. These data also indicate thathepatic microsomal cytochrome P450 PROD and EROD induction, hepatomegalyand non-focal hepatic LI are not specific markers of hepatic tumorpromoting activity of UG.