PDGF receptor tyrosine kinases play crucial physiological roles in cell proliferation, migration and differentiation during development, wound healing and tissue remodeling. Activation of these receptors is linked to cell proliferation; invasiveness and angiogenesis in human cancers, in particular glial tumors, and PDGFR fusion oncogenes are responsible for a subset of myelogenous leukemia. Indeed, inhibitors of PDGFR kinase activity are being evaluated as anti-cancer agents. Understanding the mechanisms that regulate the level of signaling downstream of PDGF receptors is therefore a major goal of research in cell and cancer biology. Ligand-induced downregulation, representing a balance between lysosomal degradation and recycling to cell surface, constitutes a major determinant of signaling potency of receptor tyrosine kinases. Our recent work has established the Cbl proto-oncoprotein as a crucial regulator of PDGF receptor down-regulation. Cbl, an ubiquitin ligase, targets activated PDGF receptors for ubiquitination that in turn facilitates their lysosomal sorting. How Cbl-dependent ubiquitination functions as a lysosomal sorting signal for PDGF receptors is unknown. Based on recent yeast genetic studies, we hypothesize the role of a novel endosomal sorting complex, ESCRT-1, which incorporates the TSG101 tumor suppressor protein, in Cbl- and ubiquitin-dependent lysosomal sorting of PDGF receptor. Here, we will test this hypothesis using a number of complementary approaches. We will use Cbl-deficient mouse embryonic fibroblasts, Cbl-insensitive PDGFR mutants expressed in PDGFR-null fibroblasts, cells with a conditional defect in ubiquitination, and PDGFR-ubiquitin fusion proteins to establish the essential role of Cbl-dependent ubiquitination in PDGFR down-regulation. We will use dual immunolabeling studies to identify the compartment(s) where Cbl-dependent lysosomal sorting of PDGFR occurs. We will use biochemical and co-localization analyses to establish that PDGFR and ESCRT-1 interact, and use over-expression and dominant-negative approaches to establish the requirement of this novel protein complex in lysosomal sorting of PDGFR. Functional analyses will assess if ESCRT-1 complex is critical in Cbl-mediated downregulation of PDGFR signaling. Insights gained through these studies should enhance the molecular understanding of a basic cellular process, downregulation of activated receptor tyrosine kinases. Given the role of PDGF receptors in maligant cell proliferation, invasiveness and angiogenesis, our studies are particularly relevant to cancer. Identification of novel regulatory mechanisms of receptor tyrosine kinase function may provide avenues to develop newer therapeutic agents and/or chemo- and radio-sensitizers.
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