Interactive effects of unleaded gasoline and estrogen on liver tumor promotion in female B6C3F1 mice.

We tested whether a concentration of unleaded gasoline (UG) vapor that was selectively hepatocarcinogenic in female mice in a chronic bioassay is antiestrogenic and whether liver tumor promotion by UG is secondary to antiestrogenicity. Twelve-day-old female C57BL/6 x C3H F1 mice (hereafter called B6C3F1) received i.p. injections of N-nitrosodiethylamine (5 mg/kg) or vehicle. Beginning at 5-7 weeks of age, mice were exposed to 0, 292, or 2056 ppm of PS-6 blend UG vapor for 6 h/day, 5 days/week for 16 weeks, 1 ppm ethinyl estradiol (EE2) in the diet, or 2056 ppm UG vapor and 1 ppm EE2 in the diet. Treatment with 2026 ppm UG but not 292 ppm UG increased relative liver weight, the number of macroscopic hepatic neoplasms, and the size and volume fraction of altered hepatic foci in N-nitrosodiethylamine-initiated mice. Treatment with 2056 ppm UG reduced relative uterus, ovary, and pituitary weights but did not change serum 17 beta-estradiol levels, uterine peroxidase activity, or uterine cytosolic estrogen receptor levels. EE2 treatment reduced the number and size of altered hepatic foci in N-nitrosodiethylamine-initiated mice, caused weight loss, anestrus, vaginal keratinization, decreased uterine peroxidase activity, and decreased uterine cytosolic estrogen receptor levels. UG/EE2 co-treatment attenuated the weight loss, anestrus, and vaginal keratinization caused by EE2 treatment alone but dramatically increased the number of macroscopic hepatic neoplasms and the size and volume fraction of altered hepatic foci as compared to UG treatment alone. Thus, in this two-stage model of carcinogenesis (a) 2056 ppm UG had antiestrogenic effects, particularly with respect to pharmacological actions of EE2; (b) 2056 ppm UG but not 292 ppm UG acted as a liver tumor promoter; (c) EE2 inhibited liver tumor promotion; and (d) EE2 strongly potentiated liver tumor promotion by UG. These data demonstrate significant individual and interactive effects of UG vapor and estrogens in liver tumor promotion in female mice.