TUMORIGENIC AND MITOGENIC CAPACITIES ARE REDUCED IN TRANSFECTED FIBROBLASTS EXPRESSING MUTANT INSULIN-LIKE GROWTH-FACTOR (IGF)-I RECEPTORS - THE ROLE OF TYROSINE RESIDUE-1250, RESIDUE-1251, AND RESIDUE-1316 INTHE CARBOXY-TERMINUS OF THE IGF-I RECEPTOR

Regulation of ligand-mediated signal transduction through transmembran e tyrosine kinase growth factor receptors involves phosphorylation of tyrosine residues in the intracellular domain of the receptor. The ins ulin-like growth factor-I (IGF-I) receptor contains three tyrosine res idues in the carboxy-terminal domain at positions 1250, 1251, and 1316 . Of these, only the tyrosine at position 1316 is conserved in the hom ologous position of the insulin receptor. Mutational analysis was used to study the role of these tyrosines in specific outcomes of IGF-I-me diated signal transduction. Mutations in the human IGF-I receptor were either replacement of tyrosines 1250 and 1251 with phenylalanine and histidine (yyFH), respectively, or replacement of the conserved distal tyrosine (position 1316) with phenylalanine (yCF). The yyFH mutation results in an IGF-I receptor with the amino acids found in the homolog ous position of the human insulin receptor. Cells overexpressing mutat ed IGF-I receptors were compared with cells expressing only endogenous IGF-I receptors or overexpressing wild-type IGF-I receptors. The abil ity of yyFH mutant IGF-I receptors to autophosphorylate the beta-subun it or phosphorylate insulin receptor substrate-1 was not significantly different from wild-type IGF-I receptors. However, one or both of the proximal tyrosine residues (positions 1250 and 1251) in the carboxy-t erminus of the IGF-I receptor are essential for IGF-I-stimulation of m itogenic and tumorigenic pathways. IGF-I-induced mitog enesis, measure d as thymidine incorporation and cellular proliferation, was abrogated in cells overexpressing mutant IGF-I receptors with replacement of th e proximal double tyrosines (positions 1250 and 1251). Fibroblasts exp ressing this mutant IGF-I receptor formed fewer tumors than the negati ve control cells, whereas cells expressing wild-type IGF-I receptors f ormed large tumors in all recipient mice injected. Conversely, cells e xpressing mutant IGF-I receptors viith only the conserved distal tyros ine (position 1316) replaced had slightly reduced IGF-I-stimulated bet a-subunit autophosphorylation, thymidine incorporation, and cellular p roliferation when compared with cells expressing wild-type receptors. Phosphorylation of insulin receptor substrate-1 by the yCF mutant rece ptors was not impaired. Despite the ability of these mutant receptors to stimulate mitogenic growth, fibroblasts expressing this mutant rece ptor were also incapable of forming tumors in recipient nude mice. The distal tyrosine (position 1316) of the IGF-I receptor is crucial for tumor formation but is not essential for IGF-I stimulated mitogenesis. Thus, the tyrosine moieties in the carboxy-terminus of the IGF-I rece ptor participate in the signal transduction pathways that affect the m itogenic and tumorigenic potentials of cells expressing mutant IGF-I r eceptors.