Characterizing novel functions of the ShcD phosphotyrosine adaptor protein in receptor tyrosine kinase signaling
Cell signaling involves a highly orchestrated cascade of molecular events that convert external stimuli into a biological response. Protein-protein interactions form the foundation for all intracellular signaling networks and are largely regulated by posttranslational modifications such as phosphorylation. Central to these pathways are surface receptor tyrosine kinases (RTKs), including the prototypical epidermal growth factor receptor (EGFR) and Trk neurotrophin receptors, which are phosphorylated and activated upon ligand binding. Once activated, RTKs recruit intracellular signaling molecules, including Shc (Src homology and collagen) adaptors, to the developing signaling complex. The Shc family (ShcA, B, C, D) of phosphotyrosine adaptor proteins consists of four structurally similar paralogs. ShcD is the fourth and most recently discovered member and shares the greatest homology with the well-characterized ShcA. ShcD contributes to melanoma and glioma metastasis and has been recently implicated in breast cancer signaling. ShcD also uniquely promotes ligand-independent EGFR hyperphosphorylation; however, the biological consequences of this phenomenon remain elusive, and warrant further investigation. Using a combination of biochemical approaches and cell-based assays, we show that the event of ShcD-induced EGFR hyperphosphorylation correlates with enhanced cell invasion and repressed PI3K/Akt signaling in metastatic breast cancer cells. Targeted disruption of EGFR-ShcD complexes reduces receptor phosphorylation and cell invasion, suggesting the interaction between EGFR and ShcD may contribute to breast cancer metastasis. We also demonstrate that ShcD uniquely regulates Ras/MAPK pathways distal to neural specific Trk RTKs by suppressing Erk phosphorylation. Notably, we observe enhanced pErk levels in the brains of ShcD knockout mice, indicating that this mechanism may have physiological relevance. Lastly, we describe another ShcD-mediated event of EGFR hyperubiquitination. ShcD disrupts normal EGFR internalization and trafficking, and here we demonstrate that this may be attributed to novel associations between ShcD and the Cbl ubiquitin ligase. Collectively, this work further demonstrates the ability of ShcD to uniquely regulate RTK dynamics and repress Ras/MAPK and PI3K/Akt signaling outcomes. Our findings also reveal a novel role for ShcD in breast cancer cell signaling which will help uncover deregulated signaling pathways in metastasis that may be targeted in the development of new therapeutics.