TYROSINE RESIDUES IN THE C-TERMINAL DOMAIN OF THE INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR MEDIATE MITOGENIC AND TUMORIGENIC SIGNALS

We investigated cellular proliferation, the transforming activity, and activation of known signal transduction pathways in NIH-3T3 cells sta bly expressing insulin-like growth factor-I receptors (IGF-IRs) with a mino acid substitutions in the carboxy(C)-terminal domain. The mutant receptors contained substitutions of both tyrosines 1250 and 1251 with phenylalanine and histidine (amino acids present in the analogous pos itions in the insulin receptor), as well as phenylalanine 1310 replace d by tyrosine (IsY clones) to resemble the placement of tyrosine resid ues in the C-terminal domain of the insulin receptor. As a control for the IsY clones, a second mutant receptor was expressed with a substit ution of phenylalanine 1310 with tyrosine only (DBY clones). Clones ex pressing IGF-IRs with the IsY substitutions had a significantly slower rate of growth compared with cells expressing an equivalent number of wild-type IGF-IRs (NWT). In contrast, the DBY clones showed relativel y normal growth rates. Cells with wild-type IGF-IR demonstrated a tran sformed phenotype in softagar assays. The IsY clones lost the transfor ming ability of the wild type IGF-IR, whereas DBY clones formed coloni es. IGF-I-stimulated autophosphorylation of the IGF-IR and tyrosine ph osphorylation of IRS-1 and SHC, known substrates in the IGF-IR signal transduction pathway, were studied. Mutated IGF-IRs (IsY and DBY) did not alter the IGF-I-induced tyrosine phosphorylation of these proteins . Furthermore, the mutated IGF-IRs did not alter Grb2 association with phosphorylated IRS-I and SHC. IGF-I stimulation of Crk-II phosphoryla tion, a novel substrate of the IGF-LR, was similar in cells expressing mutated and wild-type IGF-IRs. IGF-I-induced activation of phosphatid ylinositol (PI) 3'-kinase was equivalent in cells expressing either mu tant or wild-type IGF-IRs. These data suggest that the IGF-IR mediates , at least in part, cellular proliferation and increased transforming ability through its C-terminal domain. The exact postreceptor signalin g pathway(s) involved have yet to be fully elucidated.