Immunohistochemical localization of low density lipoprotein (LDL) receptor-related protein-1 (LRP1) in the endometrium of cyclic and pregnant pigs

The low density lipoprotein (LDL) receptor-related protein-1 (LRP1), also known as α2macroglobulin receptor or CD 91, is a multifunctional cell surface receptor that plays an important role in the endocytosis of several ligands and regulation of signalling pathways. In human endometrium, LRP1 was shown to be involved in the endocytic clearance of specific matrix metalloproteinases (MMPs) from the stroma during different phases of the cycle. However, in the pig, it is currently not known whether LRP1 is actually expressed in the endometrium and functions in a similar manner, respectively. For that reason, we examined the localization of LRP1 in the porcine endometrium at different stages of the estrous cycle and pregnancy by immunohistochemistry. Our results showed that LRP1 immunostaining is found in all endometrial specimens examined of both cyclic and pregnant animals. Especially in metestrus and estrus, immunoreactivity (IR) of LRP1 was strongly detected in stromal cells underlying the luminal epithelium (LE). Endometrial glands were mostly surrounded by LRP1-positive cells, which showed some concomitant staining with an antibody against porcine macrophages. In pregnant animals, the number of LRP1-positively stained cells was comparable high within the subepithelial stroma of early pregnant pigs. During apposition and implantation, IR of LRP1 remained high in stromal cells of the endometrium and declined markedly during the ongoing pregnancy stages examined. Our data show, that endometrial LRP1 protein expression was specifically high in such cyclic and pregnancy stages which have a high tissue remodelling activity in dependence of differing steroid hormone concentrations.