title: Assessment of insulin proteolysis in rat liver endosomes: its relationship to intracellular insulin signaling.
abstract: Insulin binding to insulin receptor (IR) at the cell surface results in the activation of IR kinase and initiates the translocation of insulin-IR complexes to clathrin-coated pits and to early endosomes containing internalized but still active receptors. In liver parenchyma, several mechanisms are involved in the regulation of endosomal IR tyrosine kinase activity. Two of these regulatory mechanisms are at the level of intraendosomal ligand. First, a progressive decrease in endosomal pH mediated by the vacuolar H(+)-ATPase proton pump promotes dissociation of the insulin-IR complex. Second, free dissociated insulin is degraded by a soluble endosomal acidic insulinase, which has been identified as aspartic acid protease cathepsin D. This enzyme catalyzes the cleavage of insulin at the Phe(B24)-Phe(B25) bond, generating a major clipped molecule, A(1-21)-B(1-24) insulin, that can no longer bind to IR within endosomes. Concomitant with, or shortly after, the tyrosine-phosphorylated IR is deactivated by two independent processes: its rapid dephosphorylation by endosome-associated phosphotyrosine phosphatase(s) and its association with the molecular adaptor Grb14, with resulting inhibition of IR catalytic activity. By mediating the removal and degradation of circulating insulin, as well as the deactivation of the activated IR, internalization of the insulin-receptor complex into endosomes represents a major mechanism involved in the negative regulation of insulin signaling.
authors.affiliation: Service information scientifique et technique (IST) de l'Inserm, Délégation régionale Inserm Paris V, Paris, France. Electronic address: francois.authier@inserm.fr.
authors.forename: François
authors.initials: F
authors.lastname: Authier
authors.affiliation: Inserm U567, Institut Cochin, CNRS UMR 8104, Université Paris-Descartes, Paris, France.
authors.forename: Bernard
authors.initials: B
authors.lastname: Desbuquois
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D011232:Chemical Precipitation; D002851:Chromatography, High Pressure Liquid; D056148:Chromatography, Reverse-Phase; D011992:Endosomes / Q000201:enzymology* / Q000378:metabolism; D007328:Insulin / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008969:Molecular Sequence Data; D010447:Peptide Hydrolases / Q000378:metabolism*; D010766:Phosphorylation; D011480:Protease Inhibitors / Q000494:pharmacology; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D059748:Proteolysis; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D015398:Signal Transduction; D014238:Trichloroacetic Acid / Q000737:chemistry
chemical_list: D007328:Insulin; D011480:Protease Inhibitors; D014238:Trichloroacetic Acid; D011972:Receptor, Insulin; D010447:Peptide Hydrolases
keywords: Cathepsin; Endosomes; Growth factor receptor bound protein 14 (Grb14); Insulin; Insulinase; Liver; Phosphotyrosine; Protein phosphotyrosine phosphatase; Protein tyrosine kinase; Proteolysis