Mammary tumors induced by carcinogens such as N-nitroso-N-methylurea (NMU) are thought to be estrogen dependent since ovariectomy (OVX) or treatment with tamoxifen or aromatase inhibitors inhibits mammary tumor development. However, whether NMU-induced tumor formation can be restored in OVX animals with estradiol (E2) and/or progesterone (P4) administration is unclear. Common methods of hormone administration such as pellet implants, injections or gavage feeding can lead to supraphysiological levels of hormone or unwarranted stress for use in long term tumor studies. To circumvent these problems, we administered hormones daily in peanut butter to OVX Sprague-Dawley rats. Short-term (10 day) pilot studies were conducted to determine the dose of E2 that would decrease body weights and increase uterine weights of OVX animals relative to sham controls. An initial pilot using 150 μg/kg E2 for 10 days led to decreased body weight gain in OVX animals but was unable to restore uterine weights. A second pilot analyzed a dose of 300 μg/kg or 600 μg/kg E2. Body weights were repressed in both groups relative to OVX, but only the 600 μg/kg dose was able to restore uterine weights. Therefore the 600 μg/kg dose was used to test whether hormones fed in peanut butter could drive tumor development. Following surgery on PND 40, rats received daily treatments in peanut butter throughout the study. Rats that were OVX were divided into four treatment groups (n = 8): 1) E2; 2) P4 (15 mg/kg); 3) E2 + P4; and 4) vehicle, with a fifth group given vehicle after sham surgery (n = 8) as a positive control. On PND 50, all animals received a single I.P. injection of 50 mg/kg NMU to induce mammary tumors. Rats were weighed and palpated biweekly. Blood was collected via tail vein at 4, 8 and 12 weeks following NMU administration 2 to 4 hours after hormone feeding. Analysis of serum E2 indicated that levels were elevated and not supraphysiological. Body weights of E2 groups with or without P4 were similar to sham body weights while the P4 group had body weights similar to those of untreated OVX rats. Mammary tumors were first observed between 6 and 7 weeks post NMU-injection in sham, E2 + P4 and E2 animals. At 18 weeks, 62.5, 37.5, and 25% of rats presented with tumors and total tumors per group were 8, 3 and 3 in sham, E2 + P4 and E2 groups, respectively. One tumor was observed in an OVX + vehicle rat at 8.5 weeks while no tumors were observed in the P4 group. Collectively, these studies indicate that administering hormones perorally in peanut butter restores body weights and uterine weights of OVX animals and allows for tumor formation in response to NMU. The lower tumor incidence in OVX animals receiving hormones suggests that additional factors may be necessary to fully restore NMU-induced tumors.

Citation Format: Hillary Stires, Mariana D. Saboya, Samantha P. Globerman, Wendie S. Cohick. NMU induction of mammary tumors in ovariectomized rats administered peroral estrogen and progesterone. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2724. doi:10.1158/1538-7445.AM2015-2724