Structure-function of the human insulin-like growth factor-I receptor: a discordance of somatotroph internalization and signaling.
Insulin-like growth factor-I (IGF-I), a GH-dependent growth factor, suppresses GH secretion by pituitary cells. To clarify the role of ligand-mediated receptor internalization for IGF-I signaling to GH, human IGF-I receptor (IGF-IR) cDNAs mutated in the beta-subunit were stably transfected into GC rat pituitary cells. Overexpression of wild-type IGF-IR markedly enhanced IGF-I suppression of GH 4-fold (P < 0.005) compared to that of untransfected cells. A mutant IGF-IR with a 943Tyr-->Ala substitution in the IGF-IR submembrane domain only partially suppressed GH (73% of IGF-IR wild type), while replacement of 957Tyr-->Ala or 940Gly-->Ala produced IGF-IRs that retained enhanced IGF-I signaling to GH. Substitution of 950Tyr-->Ala or 1003Lys-->Ala in the human IGF-IR beta-subunit failed to enhance IGF-I signaling to GH above that of untransfected cells. Intracellular phosphorylation of insulin-responsive substrate-I by these mutant IGF-IRs paralleled the observed IGF-I suppression of GH, with no phosphorylation of IRS-I by 950Tyr-->Ala. Ligand-mediated receptor internalization, however, was not reduced by substitution of either 943Tyr-->Ala or 950Tyr-->Ala. In contrast, substitution of 957Tyr-->Ala reduced the internalization of labeled IGF-I to 35% that of wild-type IGF-IR. Substitution of 1003Lys-->Ala abolished IGF-IR internalization, as expected. These results demonstrate that both 950Tyr and 943Tyr are important for IGF-I signaling to GH and that IGF-IR internalization is discordant for IGF-I signaling to the GH gene.
Yamasaki
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Department of Medicine, Cedars-Sinai Medical Center, University of California Los Angeles, School of Medicine 90048.
Prager
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Melmed
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D000818:Animals; D001483:Base Sequence; D002460:Cell Line; D004247:DNA / Q000235:genetics; D013006:Growth Hormone / Q000378:metabolism*; D006801:Humans; D015151:Immunoblotting; D007334:Insulin-Like Growth Factor I / Q000502:physiology; D008969:Molecular Sequence Data; D016296:Mutagenesis; D010766:Phosphorylation; D010902:Pituitary Gland / Q000378:metabolism; D051381:Rats; D017526:Receptor, IGF Type 1 / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D013329:Structure-Activity Relationship; D014162:Transfection
D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone; D004247:DNA; D017526:Receptor, IGF Type 1