Prolactin receptor expression in the epithelia and stroma of the rat mammary gland

The importance of prolactin (PRL) in regulating growth and differentiation of the mammary gland is well known. However, it is not well established whe ther PRL acts solely on the mammary epithelia or if it can also directly af fect the mammary stroma, To determine where PRL could exert its effects wit hin the mammary, gland, we investigated the levels of expression and the lo calization of the PRL receptor (PRLR) in the epithelia and stroma of the ra t mammary gland at different physiological stages. For these studies, we is olated parenchymal-free 'cleared', glands from virgin, 18-day-fat pads and intact mammary glands from virgin, 18-day-pregnant and 6-day-lactating rats . In addition, intact mammary tissues were enzymatically, digested to obtai n epithelial cells, free of stroma. The mammary tissues, intact gland, stro ma and isolated epithelia, were their used for immunocytochemistry, protein extraction and isolation of total RNA. PRLR, protein was detected in tissu es using specific polyclonal antisera (PRLR-1) by immunocytochemistry and W estern blot analysis, Messenger RNA for PRLR, was measured by ribonuclease protection assay. Immunocytochemistry and Western blots with the PRLR-1 ant isera detected PRLR in wild-type rat and mouse tissues, whereas the recepto r protein was absent in tissues from PRLP, gene-deficient mice. PRLP, was f ound to be present both in the epithelia and stroma of mammary glands from virgin, pregnant and lactating rats, as determined by immunocytochemistry a nd Western blotting. Western blots revealed the predominance of three hands migrating at 88, 90 and 92 kDa in each of the rat mammary samples. These r epresent the long form of the PRLR. During pregnancy and lactation, PRLR pr otein increased in the epithelial compartment of the mammary gland but did not change within the stromal compartment at any physiological stage examin ed. We also found PRLR, mRNA in both the epithelia and stroma of the mammar y gland. Again, the stroma contained lower levels of PRLR, mRNA compared wi th the epithelia at all physiological stages examined. Also, the PRLR, mRNA levels within the stroma did not change significantly during pregnancy or lactation, whereas PRLR mRNA within the epithelia increased twofold during pregnancy and fourfold during lactation when compared with virgin rats. We conclude from this study that PRLR is expressed both in the stromal and epi thelial compartment of the mammary gland. This finding suggests PRL may hav e a direct affect on the mammary. stroma and by that route affect mammary g land development.