title: Receptor-mediated endocytosis and degradation of insulin-like growth factor I and II in neonatal rat astrocytes.
abstract: Receptor-mediated internalization and degradation of insulin-like growth factors, IGF-I and IGF-II, were studied in primary cultures of neonatal rat astrocytes. Surface-bound IGF-II was rapidly internalized, and 80% of cell-associated radioactivity was located intracellularly after 30 min. IGF-I was internalized at a slower rate, and only 40% of cell-associated radioactivity was inside the cell after 30 min. A pulse-chase experiment demonstrated that 55% and 70% of internalized IGF-I and IGF-II, respectively, was degraded to free amino acids after a 3-hr chase. Lysosomal and protease inhibitors had different effects on the binding, internalization, and processing of IGF-I and IGF-II. Inhibition of lysosomal acidification by chloroquine increased the amounts of surface-bound IGF-II and intracellular IGF-I and reduced the degradation of IGF-I. The chelating agent phenanthroline increased the surface binding of IGF-I and IGF-II and internalization of IGF-II and reduced the degradation of IGF-I and IGF-II. Finally, receptor-bound IGF-II on the cell surface was decreased with increasing cell density, whereas IGF-I binding was unaltered. Our data suggest that cell-surface expression of IGF-I receptors and IGF-II receptors is regulated by different mechanisms and that receptor-bound IGF-I and IGF-II are trafficked and processed by different intracellular pathways in neonatal rat astrocytes.
authors.affiliation: Department of Clinical Chemistry, Bispebjerg Hospital, Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Auletta
authors.forename: F C
authors.initials: FC
authors.lastname: Nielsen
authors.forename: S
authors.initials: S
authors.lastname: Gammeltoft
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000378:metabolism*; D002452:Cell Count; D002478:Cells, Cultured; D002738:Chloroquine / Q000494:pharmacology; D004705:Endocytosis; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D007700:Kinetics; D008247:Lysosomes / Q000187:drug effects; D010618:Phenanthrolines / Q000494:pharmacology; D011480:Protease Inhibitors / Q000494:pharmacology; D051381:Rats; D017527:Receptor, IGF Type 2; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin
chemical_list: D010618:Phenanthrolines; D011480:Protease Inhibitors; D017527:Receptor, IGF Type 2; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D002738:Chloroquine