Regulation of cell signaling by MIG6 and Sprouty2 in cancers with EGFR mutations

Epidermal growth factor receptor (EGFR) mutation and overexpression promote tumorigenesis in multiple cancers. Understanding the complex EGFR regulatory network is critical for developing effective therapeutic interventions. To this end, this work investigated the functions of two incompletely characterized regulators of EGFR trafficking and signaling, mitogen-inducible gene 6 (MIG6) and Sprouty2 (SPRY2), in two cancer settings where EGFR mutation is common, non-small cell lung cancer (NSCLC) and glioblastoma multiforme (GBM). In NSCLC cells, results indicate that MIG6, an endogenous inhibitor of EGFR activity and endocytic adaptor, is surprisingly responsible for at least half of EGFR endocytosis, suggesting that a substantial fraction of internalized EGFR may not be competent to drive signaling. Computational modeling further suggested that in cells expressing kinase-activated, endocytosis-impaired EGFR mutants, the importance of MIG6 relative to other endocytic pathways is increased, but that MIG6 internalization capacity is reduced compared to cells expressing wild-type EGFR. Additional data indicate that SPRY2 expression reduces EGFR endocytosis rate primarily by promoting EGFR expression, which overwhelms the saturable EGFR endocytic pathway, but that SPRY2 also promotes ERK phosphorylation and resistance to EGFR inhibition independent of EGFR expression level. In GBM cell lines, our data demonstrate that SPRY2 expression promotes proliferation, anchorage-independent growth, resistance to EGFR and c-MET co-inhibition, and growth as mouse tumor xenografts. Additional studies identified SPRY2-mediated regulation of the strength and effects of JNK and p38 MAP kinase pathways as important for controlling GBM cell behaviors. Through analysis of public datasets and a collaborative analysis of human and rat tumors, we further found that elevated SPRY2 expression is associated with reduced patient survival and expression of EGFR variant III, an EGFR mutant linked to aggressive GBM. Thus, while SPRY2 is a candidate tumor suppressor in other contexts, our results support a tumor promoter role for SPRY2 in GBM and identify SPRY2 and the pathways it regulates as potential therapeutic targets or biomarkers for therapeutic response. Overall, these findings add new qualitative and quantitative understanding of the complexities of EGFR trafficking and signaling regulation and the functions of SPRY2 and MIG6 that may be leveraged to develop improved cancer therapies