SUMO and ubiquitin : the yin and yang of IGF-1R function

Abstract

Tumorigenesis is a multistep process involving genetic and epigenetic alterations that drive the progressive transformation of normal human cells into highly malignant derivatives. The insulin-like growth factor 1 receptor (IGF-1R) plays pivotal roles in cancer. Based on the traditional view, IGF-1R mediates its biological responses primarily through receptor phosphorylation and signaling. The subsequent termination of signaling occurs via receptor endocytosis and degradation. Over the last years we have begun to appreciate the biological roles of IGF-1R ubiquitination.

In this thesis we established that IGF-1R ubiquitination is dependent on receptor phosphorylation and integrity of the C-terminal domain. We identified β-arrestin as an adapter protein essential for congregating IGF-1R and its ligase MDM2. The β-arrestin/MDM2 mediated IGF-1R ubiquitination was crucial for receptor degradation and also for IGF-1 mediated ERK activation and cell cycle progression. Further, Cbl was recognized as a new ligase for IGF-1R inducing Lys48 poly-ubiquitination of the receptor, whereas MDM2 modified the receptor with Lys63 ubiquitin chain. Cbl-mediated ubiquitination caused caveolin lipid-raft internalization of the receptor, while MDM2- mediated ubiquitination caused clathrin dependent internalization.

Moreover, we identified IGF-1R as a novel target for SUMOylation. Ligand dependent SUMO-modification of IGF-1R promoted nuclear translocation of the receptor. Electrophoretic mobility shift assay and chromatin immunoprecipitation-based cloning studies demonstrated binding of IGF-1R to DNA and identified DNA binding regions. By cloning the obtained sequences upstream of a luciferase reporter, we found an IGF-1R induced enhancement of gene transcription. Altogether, Our data suggest that nuclear sequestered IGF-1R activates transcription by binding to enhancer regions. In conclusion, this thesis emphasizes the mechanisms by which ubiquitination of IGF-1R affects signaling and degradation. Furthermore, this thesis shows the importance of various ligases, orchestrating trafficking of the receptor. We also describe the novel involvement of IGF-1R SUMOylation in promoting the translocation of cell surface IGF-1R to the nucleus. Detailed understanding of how these processes are controlled under physiological and pathological conditions may be important for future therapeutic approaches.