Modulation of PDGF receptors function by posttranslational modifications

The platelet-derived growth factor receptors (PDGFRs) play important roles in multiple cellular processes including cell survival, cell growth and cell migration. Dysregulation of PDGFRs causes aberrant PDGF signaling, which leads to various diseases. Insights from the research about PDGF/PDGFR signaling have enlightened new opportunities to understand the molecular mechanisms of cancer and other diseases. The goal of this thesis is to further investigate the mechanisms of the modulation of PDGF/PDGFR signaling to identify new ways of controlling aberrant signaling of these RTKs in various diseases, including cancer. Ubiquitination is an important post-translational modification related to protein degradation, receptor internalization, intracellular trafficking, cell proliferation, and other cellular processes. It can be reversed by DUBs, and the overexpression of DUBs is involved in various diseases including cancers. In paper I, we identified two main DUBs working on PDGFRβ, USP17 and USP4. They affected the timing of STAT3 activation and trafficking via different mechanisms, thus fine-tuning its transcriptional activity, which further regulated the proliferative response induced by PDGF-BB. SUMOylation is another post-translational modification that is important for the regulation of protein subcellular localization, protein stability, protein-DNA interactions, protein-protein interactions, genome organization, DNA repair and transcriptional regulation. In paper II, we have identified PDGFRα as a SUMOylation substrate and performed a characterization of the functional role of SUMOylation in PDGFRα signaling and cell proliferation. Proteolytic cleavage of RTKs regulates their downstream signaling pathways by affecting their structure, stability, subcellular localization and interaction with other proteins. In paper III, we have identified that PDGFRβ is cleaved in the region Y579-Y857 upon ligand stimulation by a Ca2+-dependent protease, which is dependent on its internalization. The proteasomal inhibitor bortezomib blocked the internalization, as well as the cleavage of PDGFRβ, and also affected its downstream signaling. Apart from the classic degradation in lysosomes and proteasomes, several RTKs have also been found to undergo autophagy and to be targeted in autophagosomes which further fuse with lysosomes. In liver hepatocytes (LX2) cells, it has been reported that PDGFRα, but not PDGFRβ, undergo selective autophagy. In paper IV, we identified that in certain types of cells, PDGFRβ may be involved in the autophagy pathway, which may affect the synthesis of new PDGFRβ. To conclude, in this study, we investigated how the turnover and signaling of the RTKs PDGFR isoforms α and β are modulated by ubiquitination, SUMOylation, proteolytic cleavage and degradation.