Antiluteolytic mechanisms and the establishment of pregnancy in the pig

Extended exposure of progesterone and conceptus estrogen influences the vascular compartment of the uterus and expression of many factors, such as prostaglandins (PGs), growth factors, extracellular matrix and adhesion molecules, cytokines and transcription factors. One of the supportive mechanisms by which the conceptus inhibits luteolysis is by changing PG synthesis in favor of luteoprotective PGE2. Alteration in PG synthesis may result from increased PGE synthase (mPGES-1) expression in the trophoblast and endometrium on days 10-13 of pregnancy with simultaneous down-regulation of PGF synthase (PGFS) and prostaglandin 9-ketoreductase (CBR1). Conceptus and endometrial, rather than luteal, synthesis of PGE2, is involved in the process of maternal recognition of pregnancy. However, complex (direct and indirect) actions of estrogen on the CL, including decreased luteal VEGF soluble receptor on day 12 of pregnancy, are important for luteal maintenance. Moreover, conceptus signals affect another lipid signaling component - lysophosphatidic acid receptor (LPA3), as well as HoxA10 and Wnt in the endometrium, to create the appropriate uterine environment for establishment of pregnancy and implantation.