Salivary Lipocalin Is Uniquely Expressed in the Uterine Endometrial Glands at the Time of Conceptus Implantation and Induced by Interleukin 1Beta in Pigs

Uterine secretions are essential for the development of the conceptus during pregnancy. In pigs, various molecules, including transport proteins, growth factors, enzymes, and extracellular matrix proteins, are secreted into the uterine lumen. Our previous work identified salivary lipocalin (SAL1), a steroidal pheromone-binding protein, as present in the porcine uterus. To initiate studies on the role of SAL1 in the porcine uterus, we evaluated 1) the spatial and temporal expression of SAL1 in the uterine endometrium during the estrous cycle and pregnancy, and in the conceptus during early pregnancy; 2) secretion of SAL1 into the uterine lumen on Day (D) 12 of the estrous cycle and pregnancy; and 3) the effects of steroid hormones and cytokines on SAL1 mRNA levels. SAL1 was localized to glandular epithelial cells (GE) in the endometrium during the estrous cycle and pregnancy, with the highest level of SAL1 expression on D12 of pregnancy. In addition, SAL1 protein secretion into the uterine lumen was detected in uterine flushings on D12 of the estrous cycle and pregnancy, with higher levels on D12 of pregnancy. SAL1 protein, but not SAL1 transcript, was also detected in the conceptuses on D12 and D15. In explant culture experiments, SAL1 mRNA levels in the endometrium were increased by interleukin 1beta. The results of a GE- and implantation stage-specific expression and uterine secretion of SAL1 in the porcine uterus suggest that SAL1 present at the maternal-fetal interface may act as a histotroph and play an important role in the establishment of pregnancy.