Human insulin-like growth factor I receptor 950tyrosine is required for somatotroph growth factor signal transduction

Insulin-like growth factor I (IGF-I), a growth hormone (GH)-dependent growth factor exerts feedback regulation of GH by inhibiting GH gene expression. IGF-I inhibition of GH secretion is enhanced 3-5-fold in GC rat pituitary cells overexpressing the wild type 950Tyr human IGF-I receptor which autophosphorylates appropriately. To determine the critical amino acid sequence responsible for IGF-I signaling, insertion, deletion, and site-directed mutants were constructed to substitute for 950Tyr in exon 16 of the human IGF-I receptor beta-subunit transmembrane domain. All mutant transfectants bound IGF-I with a similar Kd to untransfected cells but had markedly increased (7-34-fold) IGF-I-binding sites. GH responsiveness to IGF-I was tested in mutant transfectants. Overexpressed site-directed and insertion mutant IGF-I receptors exhibited a modest suppressive effect on GH in response to the IGF-I ligand, similar to that observed in untransfected cells. Deletion mutant (IG-FIR delta 22) (amino acid 944-965) did not transduce the IGF-I signal to the GH gene. Site-directed and insertion mutants therefore did not enhance the IGF-I response of the endogenous rat receptor, unlike the 950Tyr wild type transfectants which enhanced the IGF-I signal. All mutant transfectants, except the deletion mutant, internalized radioactive ligand similarly to 950Tyr wild type transfectants. 950Tyr of the human IGF-I receptor is therefore required for IGF-I signal transduction in the pituitary somatotroph, but not for IGF-I-mediated internalization.