PROLACTIN RECEPTOR GENE-EXPRESSION IN RAT MAMMARY-GLAND AND LIVER DURING PREGNANCY AND LACTATION

The expression of two forms of PRL receptor messenger RNA was measured at different stages of pregnancy and lactation in mammary gland and liver from Sprague-Dawley rats, using P-32-labeled complementary DNA probes encoding the extracellular part of the receptor (E probe), common to the two forms and a probe encoding the intracellular part of the long form of the receptor (I probe), that only recognizes sequences specific to the long form of the receptor. Hybridizations were performed in Northern blots obtained from electrophoreses of poly (A+) enriched RNA preparations from mammary glands and livers of rats on days 0, 6, 12, 19, and 21 of pregnancy and 5, 10, 15, and 20 of lactation. The Northern blots were also hybridized with a chicken beta-actin probe, to correct for the amount of mRNA added and the different metabolic states of the tissues. Both tissues expressed the same forms of PRL receptor mRNAs, namely bands at 2.5, 3, and 5.5 kilobases encoding the long form of the receptor and a major band at 1.8 kilobases encoding the short form. The liver expressed all the receptor mRNA forms in much higher quantity than the mammary gland, independent of the reproductive state. In liver there was an increase of all the transcripts on day 19 of pregnancy, followed by an abrupt decline at the onset of lactation, to levels lower than those of virgin rats. In contrast, mammary gland PRL receptor mRNAs were low in virgin and pregnant animals, increased significantly at day 21 of pregnancy, and continued to increase throughout lactation. Treatment of day 19 pregnant rats with the antiprogesterone RU 486 induced, 24 h later, PRL receptor mRNAs in mammary gland but not in liver. There were no significant differences in the relative proportions of long to short forms of PRL receptor mRNAs at the different reproductive states, but the proportion of the long form was slightly greater in mammary gland than in liver. Membrane PRL receptor concentrations were also measured in the same tissues used for the mRNA study by binding to a I-125-labeled monoclonal antibody (U5), which specifically recognizes the PRL receptor at a site different from the hormone binding site. The quantity of receptor measured by U5 binding was approximately 3 times higher than that measured with I-125-labeled ovine PRL. The pattern of U5 binding to membranes observed during pregnancy and lactation in liver and mammary gland was parallel to that observed for PRL receptor mRNAs, with the exception that the increases observed in mammary gland on day 21 of pregnancy or after RU 486 treatment were not seen. The present results indicate that PRL receptors are differentially regulated in liver and mammary gland during pregnancy and lactation in the rat and that the decline of progesterone at the end of pregnancy induces PRL receptor mRNA in mammary gland but not in liver.