THE ROLE OF THE TYROSINE KINASE DOMAIN OF THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR IN INTRACELLULAR SIGNALING, CELLULAR PROLIFERATION AND TUMORIGENESIS

Insulin and insulin-like growth factor (IGF-I) receptors are heterotet rameric proteins consisting of two alpha- and two beta-subunits and me mbers of the transmembrane tyrosine kinase receptors. Specific ligand binding to the receptor triggers a cascade of intracellular events, wh ich begins with autophosphorylation of several tyrosine residues of th e beta-subunit of the receptor. The triple cluster in the tyrosine kin ase domain of the beta-subunit is the earliest and major autophosphory lation site. Previous studies have shown that substitutions of these t hree tyrosines by phenylalanines of both insulin and IGF-I receptors p ractically abolish any activation of cellular signaling pathways. We h ave studied the effect of double tyrosine mutations on IGF-I-induced r eceptor autophosphorylation, activation of Shc and IRS-1 pathways, and cell proliferation and tumorigenicity. Substitution of tyrosines 1131 /1135 blocks any detectable autophosphorylation, whereas substitution of tyrosines 1131/1136 or 1135/1136 only reduces autophosphorylation l evels in some clones by similar to 50%. Nevertheless, all the cells ex pressing IGF-I receptors with double tyrosine substitutions demonstrat ed markedly reduced signaling through Shc and IRS-1 pathways. In addit ion, they were unable to respond to IGF-I-stimulated cell growth in cu lture, and tumor formation in nude mice was abrogated. These data sugg est that the presence of tyrosine 1131 or 1135 essential for receptor autophosphorylation, whereas the presence of each of these tyrosines i s necessary for a fully functional receptor.