title: Insulin-like growth factor (IGF) binding protein from human decidua inhibits the binding and biological action of IGF-I in cultured choriocarcinoma cells.
abstract: The placenta expresses genes for insulin-like growth factors (IGFs) and possesses IGF-receptors, suggesting that placental growth is regulated by IGFs in an autocrine manner. We have previously shown that human decidua, but not placenta, synthesizes and secretes a 34 K IGF-binding protein (34 K IGF-BP) called placental protein 12. We now used human choriocarcinoma JEG-3 cell monolayer cultures and recombinant (Thr59)IGF-I as a model to study whether the decidual 34 K IGF-BP is able to modulate the receptor binding and biological activity of IGFs in trophoblasts. JEG-3 cells, which possess type I IGF receptors, were unable to produce IGF-BPs. Purified 34 K IGF-BP specifically bound [125I]iodo-(Thr59)IGF-I. Multiplication-stimulating activity had 2.5% the potency of (Thr59)IGF-I, and insulin had no effect on the binding of [125I] iodo-(Thr59)IGF-I. 34 K IGF-BP inhibited the binding of [125I] iodo-(Thr59)IGF-I to JEG-3 monolayers in a concentration-dependent manner by forming with the tracer a soluble complex that could not bind to the cell surface as demonstrated by competitive binding and cross-linking experiments. After incubating the cell monolayers with [125I]iodo-(Thr59)IGF-I in the presence of purified binding protein, followed by cross-linking, no affinity labeled bands were seen on autoradiography. In contrast, an intensely labeled band at 40 K was detected when the incubation medium was analyzed, suggesting that (Thr59)IGF-I and 34 K IGF-BP formed a complex in a 1:1 molar ratio. Also, 34 K IGF-BP inhibited both basal and IGF-I-stimulated uptake of alpha-[3H]aminoisobutyric acid in JEG-3 cells. RNA analysis revealed that IGF-II is expressed in JEG-3 cells. We conclude that decidual 34 K IGF-BP inhibits the cellular binding and biological action of IGFs in JEG-3 cells. Our data show that JEG-3 cells represent a cell type that can produce IGF, but not IGF-BPs. These cells may thus provide a useful model system for a better understanding of autocrine growth regulation mediated by the IGFs.
mesh_terms: D001667:Binding, Competitive; D002352:Carrier Proteins / Q000502:physiology*; D002460:Cell Line; D002822:Choriocarcinoma / Q000378:metabolism*; D003656:Decidua / Q000378:metabolism*; D005260:Female; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007700:Kinetics; D008970:Molecular Weight; D011247:Pregnancy; D011972:Receptor, Insulin / Q000378:metabolism*; D017451:Receptors, Somatomedin; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D013002:Somatomedins / Q000378:metabolism*; D014594:Uterine Neoplasms / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D018969:Insulin-Like Growth Factor Binding Proteins; D007457:Iodine Radioisotopes; D017451:Receptors, Somatomedin; D011994:Recombinant Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D011972:Receptor, Insulin