Osteopontin and its receptor alphavbeta(3) integrin are coexpressed in the human endometrium during the menstrual cycle but regulated differentially.

Abstract

Osteopontin is an arginine-glycine-aspartic acid-containing acidic glycoprotein component of the extracellular matrix that is postulated to bind to integrin receptors at the cell surface to mediate cellular adhesion and migration during embryo implantation. The primary aim of this study was to examine the uterine expression of osteopontin throughout the menstrual cycle in normal fertile controls sampled prospectively based on urinary LH surge detection. Expression of osteopontin was documented using Northern blot analysis, in situ hybridization, and immunohistochemistry. Furthermore, the temporal pattern of osteopontin expression was compared with that of its receptor, the alphavbeta3 integrin. Using Ishikawa cells, a well differentiated endometrial adenocarcinoma cell line, the in vitro regulation of osteopontin and its receptor alphavbeta3 integrin was studied. By Northern blot analysis, osteopontin mRNA appears during the early secretory phase, with maximal expression occurring in mid to late secretory-phase endometrium. The in situ hybridization analyses showed that osteopontin mRNA specifically localized in epithelial cells within the endometrium. Immunostaining of osteopontin was detected in the glandular secretions and on the apical portions of surface (luminal) epithelium. The patterns of expression of osteopontin by Northern blotting, in situ hybridization, and immunohistochemistry are remarkably similar to the pattern for the alphavbeta3 integrin. Despite these similarities in distribution, in vitro studies demonstrate that osteopontin and beta3 integrin subunit expression are differentially regulated. The expression of osteopontin was primarily induced in response to progesterone, whereas the beta3 integrin subunit was up-regulated by epidermal growth factor or heparin-binding epidermal growth factor. The differential regulation of these two endometrial proteins suggests the existence of two separate pathways regulating epithelial gene expression in human endometrium during the window of implantation. In adhesion assays using Ishikawa cells, alphavbeta3 but not alphavbeta5 or beta1 integrins appear to be the primary receptors for osteopontin. These findings may better define the factors that favor the development of a receptive endometrium.