INSULIN-LIKE GROWTH-FACTORS (IGFS), IGF-BINDING PROTEINS (IGFBPS), AND PROTEOLYZED IGFBP-3 IN EMBRYONIC CAVITIES IN EARLY HUMAN-PREGNANCY - THEIR POTENTIAL RELEVANCE TO MATERNAL-EMBRYONIC AND FETAL INTERACTIONS

insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) are believed to be important in fetal growth and development. In the current study, the developmental changes in the IGF and IGFBP axis were examined in 23 paired samples of human amniotic fluid (AF), extraembryonic coelomic (EEC) fluid, and maternal serum (MS) between 9 and 12 weeks gestation. Levels of IGF-I were very low in AF (7 +/- 3 ng/mL) and EEC (10 +/- 3 ng/mL) compared to those in MS (237 +/- 42 ng/mL). In contrast, IGF-II concentrations were 210 +/- 36 and 174 +/- 22 ng/mL in AF and EEC, respectively, and were approximately 25% of MS serum levels (884 +/- 122 ng/mL). There was no dependence on gestational age for either peptide in AF or EEC during the period of gestation examined. IGFBP-1 levels in AF increased about 20-fold (1.6 +/- 0.3 to 33.0 +/- 0.1 ng/mL) between 9 and 12 weeks of pregnancy, and IGFBP-1 levels were nearly 2 orders of magnitude higher in EEC, increasing about 100-fold (365 +/- 119 to 3014 +/- 100.0 ng/mL) by the end of the first trimester. In contrast, IGFBP-1 levels were low in MS (24.9 +/- 3.5 ng/mL) and showed no gestational age dependence. Using RIA, high levels of IGFBP-3 were found in EEC (2062 +/- 177 ng/mL) and MS (6590 +/- 357 ng/mL) compared to those in AF (152 +/- 24 ng/mL). Levels of IGFBP-3 in MS and EEC did not change significantly with gestational age, whereas an increase in IGFBP-1 was observed in AF after the tenth week of pregnancy. In contrast to high levels of IGFBP-3 in MS and EEC, determined by RIA, the 37- to 43-kilodalton IGFBP-3 doublet was barely detectable by Western ligand blot analysis. This discrepancy suggested the presence of an IGFBP-3 protease in EEC, as has been found in MS, that decreases the affinity of this BP for IGF peptides and, therefore, renders it less readily detectable by Western ligand blot analysis. Using [I-125]IGFBP-3 as substrate, lower levels of IGFBP-8 protease activity were detected in EEC compared to MS, and nearly undetectable levels were found in AF. By Western immunoblotting, a smaller (28-kilodalton) immunoreactive form of IGFBP-3 was detected only in MS and EEC, suggesting proteolyzed IGFBP-3 in MS and EEC, but not in AF, during this gestational period. These data show that IGF-II is the predominant IGF in extraembryonic cavities of the developing human embryo/fetus. In addition, the high levels of IGFBP-3 and IGFBP-1 present in EEC compared to AF suggest that the amnion serves as an effective barrier for transport of the higher mol wt IGFBPs from EEC to AF. Although the sites of synthesis of IGF-II, the IGFBPs, and IGFBP-3 protease are uncertain, the maternal decidua and chorion laeve are likely sources. As the chorionic cavity (EEC) is compressed by the expanding amnion as the first trimester proceeds, and the chorion itself comes into contact with the maternal decidua parietalis at the end of this period, it is likely that components from the maternal decidua are transported into the EEC before 12 weeks gestation, and then directly into AF thereafter. Synthesis of some of the components of the IGF system by the extraembryonic membranes is also possible. We propose that the relative affinities of partially proteolyzed IGFBP-3 and IGFBP-1 for IGF-II in EEC regulate the amount of bioavailable IGF-II for action on target cells within the chorion and/or amnion. The presence of IGFs, IGFBPs, and lower mol wt forms of IGFBP-3 in human extraembryonic cavities that may be both maternally and fetally derived suggests that crosstalk between the maternal host and the early developing embryo/fetus is an important process in early fetal development.