Identification of two new nonclassical members of the rat prolactin family

The prolactin (PRL) family is comprised of a group of hormones/cytokines that are expressed in the anterior pituitary, uterus, and placenta. These proteins participate in the control of maternal and fetal adaptations to pregnancy. In this report, we have identified two new nonclassical members of the rat PRL family through a search of the National Center for Biotechnology Information dbEST database. The cDNAs were sequenced and their corresponding mRNAs characterized. Overall, the rat cDNAs showed considerable structural similarities with mouse proliferin-related protein (PLF-RP) and prolactin-like protein-F (PLP-F), consistent with their classification as rat homologs for PLF-RP and PLP-F. The expression of both cytokines/hormones was restricted to the placenta. The intraplacental sites of PLF-RP and PLP-F synthesis differed in the rat and the mouse. In the mouse, PLF-RP was expressed in the trophoblast giant cell layer of the midgestation chorioallantoic and choriovitelline placentas and, during later gestation, in the trophoblast giant cell and spongiotrophoblast layers within the junctional zone of the mouse chorioallantoic placenta. In contrast, in the rat, PLF-RP was first expressed in the primordium of the chorioallantoic placenta (ectoplacental cone region) and, later, exclusively within the labyrinth zone of the chorioallantoic placenta. In the mouse, PLP-F is an exclusive product of the spongiotrophoblast layer, whereas in the rat, trophoblast giant cells were found to be the major source of PLP-F, with a lesser contribution from spongiotrophoblast cells late in gestation. In summary, we have established the presence of PLF-RP and PLP-F in the rat.