Mitogen-Activated Protein Kinase and Phosphatidylinositol 3-Kinase Pathways Are Not Sufficient for Insulin-Like Growth Factor I-Induced Mitogenesis and Tumorigenesis

Abstract

Insulin-like growth factor-I (IGF-I) and insulin are known to activate a signaling cascade involving ras --> kappa raf-1 --> mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (Erk-1 and -2). Recent reports suggest that activation of this ras/MAP kinase pathway is involved in mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and GLUT-4-mediated glucose transport. Previously we and others have demonstrated that substitution of both tyrosines at positions 1250 and 1251 in the carboxy-terminal region of the human IGF-I receptor has relatively small effects on receptor and endogenous substrate phosphorylation but completely abrogated the ability of these cells to form tumors in nude mice or proliferate in response to IGF-I in culture. Replacement of the tyrosine at position 1316 also did not affect the kinase activity of the receptor with respect to autophosphorylation or phosphorylation of endogenous substrates but did reduce the ability of the receptor to mediate mitogenic or tumorigenic signals. To further characterize the role of these tyrosines in IGF-I receptor function, we have used three distinct approaches to examine the ras/MAP kinase pathway in IGF-I-induced mitogenesis and tumorigenesis in NIH-3T3 cells overexpressing wild-type and mutated IGF-I receptors: 1) tyrosine phosphorylation of the MAP kinases Erk-1 and -2; 2), mobility shifts indicative of MAP kinase phosphorylation; and 3) in vitro MAP kinase activation. We have also examined IGF-I-induced phosphatidylinositol (PI) 3-kinase activation in the same cell lines. By each method we show that the IGF-I-induced MAP kinase phosphorylation/activation and PI 3-kinase activation, are not different between cells overexpressing wild-type IGF-I receptors and cells carrying IGF-I receptors having tyrosine motifs replaced at positions 1250 and 1251. We conclude that mitogenic and tumorigenic signals involving tyrosine residues in the C-terminal domain of the IGF-I-receptor include pathways other than the MAP kinase and PI 3-kinase pathways.