Insulin-like growth factor binding protein concentration and post- translational modification in embryological fluid

Levels of proteolytic activity directed against insulin-like growth factor binding protein 3 (IGFBP-3) and the distribution of phosphorylated isoforms of IGFBP-1 were assessed in matched sample sets of maternal serum, coelomic fluid and amniotic fluid from 21 pregnancies at 6-12 weeks gestation. In addition, concentrations of immunoreactive IGFBP-1 to -3, insulin-like growth factor (IGF)-I and - II were determined in all three compartments in 21 pregnancies, and in coelomic fluid and maternal serum in 58 pregnancies. IGF-I concentrations were highest in maternal serum and similarly low in coelomic and amniotic fluid. IGF-II concentrations were also highest in maternal serum but easily detectable in coelomic fluid where concentrations showed a significant correlation with gestational age. IGFBP-1 concentrations were higher in coelomic fluid than in either maternal serum or amniotic fluid and showed a significant correlation with gestational age in this compartment. Analysis of IGFBP-1 phosphoforms showed clear differences in phosphorylation of IGFBP-1 between groups with maternal serum containing predominantly the phosphorylated forms and coelomic fluid almost exclusively the non- phosphorylated form. First trimester amniotic fluid IGFBP-1 was barely detectable and appeared non-phosphorylated. These findings suggest that the high IGF-II concentrations and lack of inhibitory phosphoforms of IGFBP-1 in coelomic fluid could potentially enhance mitogenic activity in the early human gestational sac. IGFBP-2 concentrations were high in coelomic fluid compared with maternal serum whereas coelomic fluid IGFBP-3 concentrations were intermediate, easily detectable and correlated strongly with gestational age. Protease activity was far less in coelomic fluid than in matched maternal serum samples. Marked differences in both concentrations and post-translational modification of IGFBPs in maternal serum compared with embryonic fluid suggest different regulatory pathways.