Regulation of hepatocyte growth factor receptor endocytic trafficking




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The Hepatocyte Growth Factor Receptor (HGFR/cMet) is a receptor tyrosine kinase that is essential for multiple cell responses, including cell proliferation, survival, motility and branching morphogenesis. Normal HGFR signaling regulates embryonic development, organ regeneration and wound healing, whereas deregulated HGFR signaling is linked to tumor progression, metastasis and angiogenesis. Ligand activation of HGFR leads to receptor downregulation via endocytosis and lysosomal degradation, the major mechanism for terminating HGFR signaling. Perturbation of HGFR trafficking, either at the level of internalization or during sorting at the early endosome for degradation, leads to oncogenic activation of HGFR. Impaired HGFR trafficking is emerging as a key mechanism for HGFR-induced cancer progression and metastasis. \r\nA major goal of my dissertation was to lay the foundation for future studies examining different mechanisms leading to altered HGFR trafficking in human cancers, by determining and characterizing the mechanisms which normally function to regulate HGFR internalization and degradation. Two ligands have been identified for HGFR, the physiological ligand HGF and the Listeria surface protein Internalin B (InlB). I characterized the mechanisms for InlB and HGF induced HGFR trafficking using a combination of imaging, molecular biological and biochemical approaches. First, I demonstrated that InlB and HGF were mechanistically equivalent in triggering HGFR internalization primarily through clathrin-coated pits. Then, I determined that the Y1349 and Y1356 docking sites and tyrosine kinase activity of HGFR were required for receptor internalization. Recruitment of the adaptor protein Grb2, but not Gab1, was essential for ligand induced HGFR internalization. I then showed Cbl, an E3 ubiquitin ligase recruited by Grb2 to HGFR, played an essential role in receptor internalization. Furthermore, the E3 ligase activity of Cbl and ubiquitination machinery was involved in HGFR internalization. Finally, I demonstrated that ligand induced HGFR degradation occurred through the lysosomal pathway, involving the function of Hrs and PI3K. My studies represent the first detailed characterization of the trafficking events that normally function to inactivate HGFR signaling. My findings contribute to better understanding of how HGFR-induced tumorigenesis and tumor metastasis may result from impaired HGFR trafficking, and identify novel mechanisms that may function as therapeutic targets for treatment of human cancers due to impaired HGFR trafficking.\r\n