The human prolactin receptor in the fetal membranes, decidua, and placenta

Human PRL is synthesized and secreted by the maternal decidua, but not by the chorionic cytotrophoblast of the chorion laeve or the placenta. The sites of action for decidual PRL are currently unknown. Accordingly, Northern analysis and in situ hybridization histochemistry have been used respectively to quantitate and localize the expression of the PRL receptor (PRL-R) gene within the uterus during the peripartal period. Immunocytochemistry and Western blot analysis using an anti-PRL-R antibody (U5) localized the translated protein at the cellular level in the same tissues. As judged by the level of expression of the PRL-R gene and its translated products, the chorionic cytotrophoblast has been shown to be a primary site of action. Novel sites were also shown in the decidua, placental trophoblast, and amniotic epithelium. In situ hybridization was not obtained in the latter despite positive Northern analysis and immunostaining. Western analyses with an antibody (U5) to the extracellular domain of the rat PRL-R detected six major molecular species of 95, 85, 63, less than 63, more than 30, and 30 kDa in cytosol from separated amnion, chorion, and decidua. The two bands at 95 and 85 kDa were approximate values only and represent the mature glycosylated forms of the human PRL-R. The other four major bands were partial degradation products from the PRL-R, showing tissue-specific processing and patient to patient variation related to the spectrum of proteases present in these tissues. The 63- and 30-kDa PRL-R-related proteins were detected in both the cytosol and medium from amnion, chorion, and decidua and were also present in amniotic fluid. The 30-kDa species was equal in size to a recently reported PRL-binding protein in human milk. The release of these two PRL-R-related proteins into amniotic fluid suggests possible functions as binding and or/PRL transport proteins in these tissues. The more than 30-kDa species was detected in high amounts in both cytosol and medium from the decidua, but was absent from amniotic fluid. Further work is required to clarify the structural relationships and potential functions of these immunologically PRL-R-related proteins. This study shows that the PRL-R is widely expressed by both fetal and maternal tissues in late pregnancy. Its increased expression during labor and delivery in the chorion, decidua, and placenta supports an autocrine/paracrine role for decidual PRL in the peripartum.