Understanding EGFR Modification, Trafficking, and the Importance of its Juxtamembrane Domain in Cancer

Abstract

Much of what is known about the role of the ERBB family in cellular biology and in cancer has to do with canonical downstream signaling cascades and modifications associated with their trafficking and degradation. The focus on canonical activity, while important, ignores a rapidly expanding number of separate, arguably equally important functions for which there is emerging knowledge. These include the translocation of ERBB family members to non-canonical sub-cellular locations including the nucleus and mitochondria. Of current interest is the elucidation of fate determination mechanisms for these proteins. How is one ERBB receptor designated to traffic to the nucleus or mitochondria, versus degradative lysosomes? A portion of the work presented here addresses the potential role of ubiquitin in EGFR nuclear translocation, a role which ubiquitin has been shown to play in the context of other proteins. This work demonstrates that while ubiquitinated EGFR can be translocated from the plasma membrane to the nucleus in response to ligand, efficient ubiquitination is not essential for this process. This work also broadens the potential roles for ubiquitin to include those involving EGFR nuclear biology. Additional work described aimed to exploit what was already known regarding the diverse roles played by the EGFR juxtamembrane domain in the non-canonical activities of EGFR and the ERBBs. This work involves the creation and evaluation of an EGFR juxtamembrane domain derived peptide designed to competitively interact with endogenous ERBB domains and inhibit their function in cancer cells. Termed EJ1, treatment induces cell death and promotes the formation of inactive ERBB dimers and reduces ERBB activation. While inactivating CaMKII signaling, myosin light chain dependent cell blebbing occurs, coincident with the induction of cell death. EJ1 also directly translocates to mitochondria, potentially contributing to a loss of mitochondrial membrane potential and production of reactive oxygen species. Finally, treatment of mouse models of breast cancer with EJ1 results in the inhibition of tumor growth and metastasis, without observable side-effects/toxicities. These data demonstrate that a portion of the ERBB juxtamembrane domain, used as an intracellular decoy, can affect tumor growth and metastasis through ERBB-dependent and ERBB-independent mechanisms, representing a novel anti-cancer therapeutic.