Characterization of Insulin-Like Growth Factor-Binding Proteins in the Uterus and Conceptus during Early Conceptus Elongation in Cattle1

As a first step in determining the role that insulin-like growth factor (IGF)-binding proteins (BPs) may have in regulating initial stages of conceptus elongation in cattle, the type and relative abundance of IGFBPs in serum, uterine tissues, and uterine fluid from pregnant and noninseminated cows on Days 13 and 15 postestrus and in Day 15 conceptuses was evaluated. Uterine and serum samples contained IGFBPs 2, 3, 4, and 5 as determined by immunoprecipitation followed by Western ligand blots of precipitates. Compared with those in uterine and serum samples, IGFBPs in conceptuses and conceptus-conditioned culture media were only faintly detectable. The percentage of the total IGF-I binding activity attributed to IGFBP-3 was greater (p < 0.05) in myometrium, serum, and uterine fluid (> 50%) than in inter- (40%) and intracaruncular (37%) endometrium. Percentage of total binding attributed to IGFBP-2 was greater (p < 0.05) in endometrium and serum ( approximately 30%) than in myometrium (16%) and uterine fluid (9%). Binding activity of certain IGFBPs varied due to day of the estrous cycle or due to pregnancy status. Concentrations of IGF-I in serum were greater (p < 0.05) in nonpregnant (52 +/- 2 ng/ml) than in pregnant (40 +/- 4 ng/ml) cows. Concentration of IGF-I in uterine fluid did not differ due to pregnancy status or stage of cycle (4.4 pg IGF-I/ microg uterine protein). Northern blots revealed mRNAs for IGFBPs 1, 2, 3, 4, and 5 in uterine tissues but not in conceptuses. In situ hybridization indicated that IGFBP-1 mRNA was primarily localized in luminal epithelium of endometrium; IGFBP-2 mRNA was in luminal epithelium and dense stromal cells adjacent to endometrial epithelium; and IGFBP-3 mRNA was in vascular endothelial cells and was more prevalent in myometrium than in endometrium. Tissue specificity and changes in abundance of IGFBPs in the uterus during early conceptus elongation indicate the potential importance of IGFBP regulation of uterine IGFs during this time period.