Prolactin and progesterone receptors in pregnancy-dependent mammary tumors in GR/A mice

In this study, cellular prolactin receptors and cytosolic progesterone receptors were examined and compared in pregnancy-dependent mammary tumors (PDMT) and in normal mammary glands of pregnant GR/A mice. PDMT and normal mammary glands were examined in the same animal, thus assuring an identical hormonal environment. The PDMT cells had a larger capacity to bind prolactin or the synthetic progesterone, R5020, than did the normal mammary gland. While the dissociation constant (Kd) value for prolactin binding to normal mammary epithelial cells was similar to that of PDMT cells, PDMT cells had 2.2 times more prolactin receptors than the normal cells. Progesterone binding activity was detected only in PDMT, but not in the normal mammary cells. The receptor concentration and the Kd value for progesterone binding of PDMT were 606 fmol/mg protein and 3.53 nM, respectively. It appears, therefore, that normal regulation of these receptors may be altered within the PDMT cells. The increased growth responsiveness of PDMT to the hormones of pregnancy, especially prolactin, progesterone, and placental lactogen, may be a function of a sharp increase in the level of cellular receptors for these mammotropic hormones.