Relation of prolactin and estrogen to mammary tumorigenesis in the rat

SUMMARY-The 2 most important hormones for development and growth of mammary tumors in rats are prolactin and estrogen, both under control of the pituitary gland and the brain. Estrogen stimulates prolactin secretion and acts with prolactin directly on the mammary tissues to promote tumori-genesis. Large doses of estrogen can inhibit mammary tumor growth, not by suppressing prolactin secretion but by interfering with the peripheral action of prolactin on the mammary tissues. Estrogen can neither induce nor maintain growth of mammary tumors in the absence of the pituitary, but prolactin may have a limited capacity to induce and maintain mammary tumor growth in the absence of the ovaries. Although rats show no sig-nificant change in serum-prolactin levels during growth of carcinogen-induced mammary tumors, elevations of serum prolactin produced by hypothalamic lesions, pituitary grafts, or administration of small doses of estrogen or appropriate drugs, can increase growth of spontaneous and carcinogen-induced mammary tumors. Drugs that reduce serum-prolacti