A Baboon (Papio anubis) simulated-pregnant model: cell specific expression of insulin-like growth factor binding protein-1 (IGFBP-1), type I IGF receptor (IGF-1 R) and retinol binding protein (RBP) in the uterus

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Abstract
In order to test the hypothesis that the baboon conceptus/placenta regulates the synthesis of specific proteins in the endometrium, we developed a simulated-pregnant baboon model. Baboons (n=2–6/group) were treated with increasing amounts of human chorionic gonadotrophin (hCG) for 10 or 12 days beginning on day 6 or 7 PO. Uterine tissues were obtained at day 18 PO following 12 days of hCG treatment. Animals in the day 25 and 32 PO group were treated for 10 days with hCG. Following the hCG treatment, estradiol (E) and progesterone (P) implants were inserted subcutaneously. Control groups consisted of E and P treatment only (day 25 PO), or ovariectomy on day 6 or 7 PO followed by hCG plus E and P treatment (days 18 and 25 PO). Serum samples were obtained daily or once every 2 days and analysed for E and P by radioimmunoassay. hCG activity in serum was determined by a Leydig cell bioassay. Portions of the endometrial tissue were either subjected to organ explant culture, analysed by immunocytochemistry or extracted for RNA. Peripheral serum levels of hCG, E and P in the experimental groups fell within the 95% confidence interval limits of hormone concentrations achieved during pregnancy. The morphology of the endometrium in the hCG treated baboons and pregnant baboons was similar i.e., distended convoluted glands, many spiral artery beds, a loose vacuolized stroma, and increased collagen staining. However, in the absence of hCG (E+P treatment only) the glands tended to be straight rather than corkscrew-shaped, and decreased stromal vacuolization and collagen staining was evident.35S-methionine labeled proteins in explant culture conditioned media (TCM) were analysed by two-dimensional SDS-PAGE and fluorography. A comparable pattern of protein synthesis was apparent in all treatment groups except for a low molecular weight (27 000–30 000 daltons) group of polypetides which only was evident in TCM from the hCG treated baboons. A similar group of proteins are also secreted by the baboon endometrium during pregnancy. The immunocytochemical localization of estrogen (ER) and progesterone receptors (PR) was comparable to that observed in pregnant baboons. IGFBP-1 localization was confined to the glandular epithelium in the hCG treated groups (intact and ovariectomized) and was virtually undetectable in the E and P treated group. The intensity of IGFBP-1 staining was variable within each of the hCG treatment groups on days 18, 25 and 32 PO. This variability was also apparent by Western blot analysis, immunoassay of proteins in TCM and on Northern blots of total RNA from the same animals. In contrast, IGF-I R immunostaining was evident in both glandular and surface epithelium of all treatment groups. Expression of RBP was confined to the basal glands. The characteristic upregulation of RBP synthesis in the functionalis observed during early pregnancy was not apparent in any of the treatment groups. In summary, these studies indicate that exogenous hCG in conjunction with E and P, can induce the general morphological and biosynthetic changes the baboon endometrium undergoes during early pregnancy. In addition, this hormonal treatment is also capable of maintaining the epithelial expression of IGFBP-1, IGF-1 and RBP. However, other factors from the conceptus appear to be necessary to induce the cell specific changes in the expression of these three proteins that are observed during pregnancy.