High estrogen and low progestin receptor levels in outgrowths of hypophyseal isografts.

Markedly enlarged pituitary isografts in mice contain high concentrations of estrogen receptors and low concentrations, if any, of progestin receptor. This finding is consistent with previous observations that estrogen increases the isograft growth and prolactin secretion while progestin does not. Both types of evidence indicate that in this model system estrogens act indirectly on mammary tumorigenesis by enhancing prolactin secretion in the pituitary isograft and perhaps also the hypophysis in situ, whereas progesterone acts directly on the mammary gland itself.