Expression of the insulin-like growth factor I receptor C terminus as a myristylated protein leads to induction of apoptosis in tumor cells

The insulin-like growth factor I receptor (IGF-IR) has been shown to mediate mitogenesis and suppression of apoptosis, Certain mutations in the COOH terminus of the receptor abrogate the antiapoptotic activity but not the mitogenic activity, However, truncation of the receptor by deletion of the COOH-terminal 108 amino acids enhances suppression of apoptosis by the IGF-IR, which suggests that the COOH terminus has a negative regulatory role. To investigate this further, a series of mammalian expression vectors were generated that encoded either the COOH terminus of the receptor or the COOH terminus plus the kinase domain, In some cases, the first 16 amino acids of SRC were included at the NH2 terminus to provide a site for myristylation, In transient transfection assays, the membrane-targeted COOH-terminal construct, MyCF, was found to induce apoptosis in MCF-7 breast carcinoma cells and C6 glioblastoma cells, whereas the COOH-terminal construct without the myristylation signal, CF, was poorly cytotoxic, MyKCF, which encodes the kinase domain as well as the COOH terminus, had intermediate cytotoxicity. The cytotoxicity of MyCF was diminished by point mutations that were previously shown to abrogate suppression of apoptosis in the context of the full-length receptor, MCF-7 cells stably expressing the CF or the MyCF proteins exhibited decreased clonogenicity in soft agar and increased sensitivity to UV irradiation. These results indicate that expression of the IGF-IR COOH terminus promotes apoptosis of tumor cells, possibly by interfering with signals necessary for cell survival.