Identification of two new nonclassical members of the rat prolactin family

The prolactin (PRL) family is comprised of a group of hormones/cytokines th at are expressed in the anterior pituitary, uterus, and placenta. These pro teins participate in the control of maternal and fetal adaptations to pregn ancy. In this report, we hate identified two new nonclassical members of th e rat PRL family through a search of the National Center far Biotechnology Information dbEST database. The cDNAs were sequenced and their correspondin g mRNAs characterized. Overall, the rat cDNAs showed considerable structura l similarities with mouse proliferin-related protein (PLF-RP) and prolactin -like protein-F (PLP-F), consistent with their classification as rat homolo gs for PLF-RP and PLP-F. The expression of both cytokines/hormones was rest ricted to the placenta. The intraplacental sites of PLF-RP and PLP-F synthe sis differed in the rat and the mouse. In the mouse, PLF-RP was expressed i n the trophoblast giant cell laver of the midgestation chorioallantoic and choriovitelline placentas and, during later gestation, in the trophoblast g iant cell and spongiotrophoblast layers within the junctional zone of the m ouse chorioallantoic placenta. In contrast, in the rat, PLF-RP was first ex pressed in the primordium of the chorioallantoic placenta (ectoplacental co ne region) and, later, exclusively within the labyrinth zone of the chorioa llantoic placenta. In the mouse, FLP-F is an exclusive product of the spong iotrophoblast layer, whereas in the rat, trophoblast giant cells were found to be the major source of PLP-F, with a lesser contribution from spongiotr ophoblast cells late in gestation. In summary, we have established the pres ence of PLF-RP and PLP-F in the rat.