Insulin-like growth factor-binding proteins in hypophysectomized rat liver: characterization and subcellular localization

Abstract

We have characterized binding proteins for insulin-like growth factors (IGFs) in hepatic subcellular fractions and in the washed supernatants of these fractions in normal and hypophysectomized (hypox) rats. In the course of assessing IGF-II-binding sites on rat liver microsomes, we observed that [125I] IGF-II binding to the microsomal membranes of hypox rats was much lower than that in normal rats. Paradoxically, binding increased in hypox animals at low concentrations (0.5-5 ng/ml) of unlabeled IGF-II. After resuspension and centrifugation (washing) of the microsomes, no difference was found in [125I]IGF-II binding to hypox vs. normal microsomes. However, the binding of [125I]IGF-II to the washing supernatant (SN) from hypox rat microsomes was greater than binding to that from normal animals. Binding to SN was inhibited by unlabeled IGF-II in a dose-dependent manner. Scatchard analyses indicated that the affinity constant for binding by hypox SN was higher than that of normal SN and the microsomal fractions of both hypox and normal rats. After further subfractionation of the liver, no binding activity was found in SN from plasmalemma, whereas it was about 20% of input counts per min of [125I]IGF-II in SN from combined Golgi-endosome fractions of hypox rat liver. We next compared IGF-binding moieties in microsomal SN with those in plasma using cross-linking of [125I]IGF-II followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In normal rat plasma, we observed the presence of 42K, 39K, 31K, and 27K binding complexes. In hypox rat plasma only a 42-39K doublet was found. In the SN of normal rat microsomes, the predominant complex migrated at 39K and was distinguishable only after acidification. In the SN of hypox rat microsomes, the 42K complex was predominant, with a minor 34K complex. These studies have identified IGF-binding moieties in hepatic tissues, particularly in hepatic vesicular elements, which interfere in the binding of IGF-II to membrane receptors. Their presence in these receptor-rich elements may influence IGF binding to intracellular receptors and, hence, the biological activity of the peptide.