An Insulin-Like Growth Factor-Binding Protein in the Baboon (Papio anubis) Endometrium: Synthesis, Immunocytochemical Localization, and Hormonal Regulation*

Abstract

The major secretory product of the baboon and human decidua during pregnancy is an insulin-like growth factor-binding protein (IGF-BP). This study was designed to determine the site and regulation of synthesis of this protein in the nonpregnant baboon based on our previous findings that this molecule is biochemically and immunologically similar in the two species during pregnancy. Endometrial tissue was obtained from cycling baboons and steroid-treated ovariectomized animals. Portions of the tissue were fixed for immunocytochemical analysis, cultured in the presence or absence of cyclohexamide and actinomycin-D, or snap-frozen in liquid nitrogen for RNA isolation. Immunostaining using monoclonal antibodies to IGF-BP indicated that the protein was localized predominantly in the epithelium of the deep glands and was most intense during the late luteal stage of the cycle. Immunoreactive product was also present in tissue from estrogen-primed progesterone-treated ovariectomized animals. However, the staining pattern was more variable and less intense than that in intact animals. Western blots of explant culture media showed the presence of an immunoreactive product only in those tissues that also demonstrated immunocytochemical localization. The absence of an immunoreactive band in medium obtained from tissue incubated in the presence of cyclohexamide suggested that this protein was synthesized de novo. The mRNA coding for IGF-BP appeared to be stable, as synthesis in explant cultures continued in the presence of actinomycin-D. The cDNA probe hybridized to a single message transcript of 1.65 kilobases. The presence of mRNA in tissues coding for this protein correlates with the immunochemical data relating to the site and hormonal regulation of its synthesis. The presence of this protein in the glandular epithelium of the baboon endometrium may have implications in the autocrine and/or paracrine regulation of trophoblast growth and penetration during implantation.