The insulin-like growth factor 1 (IGF-1) receptor is responsible for mediating the effects of insulin, IGF-1, and IGF-2 in Xenopus laevis oocytes.
Competitive hormone binding studies with membrane and partially purified receptors from Xenopus laevis oocytes revealed that the oocyte possesses high affinity (KD = 1-3 nM) binding sites for both insulin growth factors 1 and 2 (IGF-1 and IGF-2), but not for insulin. Consistent with these findings, IGF-1 activates hexose uptake by Xenopus oocytes with a KA (3 nM) identical with its KD, while IGF-2 and insulin activate hexose uptake with KA values of 50 nM and 200-250 nM, respectively, suggesting activation mediated through an IGF-1 receptor. Both IGF-1 and insulin activate receptor beta-subunit autophosphorylation and, thereby, protein substrate (reduced and carboxyamidomethylated lysozyme, i.e. RCAM-lysozyme) phosphorylation with KA values comparable to their respective KD values for ligand binding and KA values for activation of hexose uptake. The autophosphorylated beta-subunit(s) of the receptor were resolved into two discrete components, beta 1 and beta 2 (108 kDa and 94 kDa, respectively), which were phosphorylated exclusively on tyrosine and which exhibited similar extents of IGF-1-activated autophosphorylation. When added prior to autophosphorylation, RCAM-lysozyme blocks IGF-1-activated autophosphorylation and, thereby, IGF-1-activated protein substrate (RCAM-lysozyme) phosphorylation. Based on these findings, we conclude that IGF-1-stimulated autophosphorylation of its receptor is a prerequisite for catalysis of protein substrate phosphorylation by the receptor's tyrosine-specific protein kinase. The IGF-1 receptor kinase is implicated in signal transmission from the receptor, since anti-tyrosine kinase domain antibody blocks IGF-1-stimulated kinase activity in vitro and, when microinjected into intact oocytes, prevents IGF-1-stimulated hexose uptake.
Janicot
M
M
Department of Biological Chemistry, Johns Hopkins Univesity School of Medicine, Baltimore, Maryland 21205.
Flores-Riveros
J R
JR
Lane
M D
MD
D000818:Animals; D001667:Binding, Competitive; D002851:Chromatography, High Pressure Liquid; D003847:Deoxyglucose / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006601:Hexoses / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism*; D010449:Peptide Mapping; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin; D014357:Trypsin; D014982:Xenopus laevis
D006601:Hexoses; D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D003847:Deoxyglucose; D011505:Protein-Tyrosine Kinases; D014357:Trypsin