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Hormones Influencing Postpartum Growth of 7, 12-Dimethylbenz(a)anthracene-induced Rat Mammary Tumors¹

Department of Physiology and Biophysics, University of Tennessee Medical Units, Memphis, Tennessee 38103

The endocrine factors affecting postpartum growth of mammary tumors were studied in Sprague-Dawley female rats bearing 7,12-dimethylbenz(a)anthracene-induced mammary tumors. The majority of tumors were classified as mammary adenocarcinomas, but a few mammary fibroadenomas and "mixed" tumors were also observed. All rats developed tumors during pregnancy and all tumors grew rapidly until parturition. Ovariectomy performed on Day 2 of lactation abolished the stimulatory effect of suckling on tumor growth in rats nursing 6 pups. While the daily administration of 1 mg progesterone/day to ovariectomized lactators resulted in growth of only a few tumors, daily administration of 6 mg progesterone stimulated growth of nearly 50% of the tumors during the postpartum period. Administration of either bovine or ovine prolactin to non-nursed, tumor-bearing rats for 25 days postpartum stimulated tumor maintenance and growth to varying degrees, depending upon the dose. The stimulatory action of prolactin on tumor growth was abolished by ovariectomy performed on Day 2 postpartum. Tumor growth was dependent on the continuance of the hormone treatment, as most growing tumors regressed after cessation of hormone treatment. These data indicate that ovarian progesterone, acting either directly or in synergism with prolactin, is necessary for the growth of chemically-induced mammary tumors during lactation.

¹ This investigation was supported in part by USPHS Research Grant CA-05105 from the National Cancer Institute.

² Present address: Department of Pathology, University of Tennessee Medical Units, Medical Units, Memphis, Tennessee.

³ Lederle Medical Faculty Awardee.

Received 3/30/66. Accepted 11/15/66.