Alix Facilitates the Interaction between c-Cbl and Platelet-derived Growth Factor β-Receptor and Thereby Modulates Receptor Down-regulation

Alix (ALG-2-interacting protein X) is an adaptor protein involved in down-regulation and sorting of cell surface receptors through the endosomal compartments toward the lysosome. In this study, we show that Alix interacts with the C-terminal region of the platelet-derived growth factor (PDGF) β-receptor (PDGFRβ) and becomes transiently tyrosine-phosphorylated in response to PDGF-BB stimulation. Increased expression levels of Alix resulted in a reduced rate of PDGFRβ removal from the cell surface following receptor activation, and this was associated with decreased receptor degradation. Furthermore, Alix was found to co-immunoprecipitate with the ubiquitin ligase c-Cbl, and elevated Alix levels increased the interaction between c-Cbl and PDGFRβ. Interestingly, Alix interacted constitutively with both c-Cbl and PDGFRβ. Moreover, c-Cbl was found to be hyperphosphorylated in cells engineered to overexpress Alix compared with control cells. The increased c-Cbl phosphorylation correlated with enhanced proteasomal degradation of c-Cbl, which in turn correlated with a decreased ubiquitination of PDGFRβ. Our data suggest that Alix inhibits down-regulation of PDGFRβ by modulating the interaction between c-Cbl and the receptor, thereby affecting the ubiquitination of the receptor.