(147c) Mitogen-Activated Protein Kinase-Driven Sprouty2 Expression Mediates Resistance to Receptor Tyrosine Kinase-Targeted Therapeutics in Glioblastoma Cells

The therapeutic efficacy of inhibitors of oncogenic kinases in glioblastoma multiforme (GBM) has been disappointing to date. In glioblastoma and other malignancies, resistance to targeted therapeutics can arise through dynamic rewiring of cell signaling pathways responsible for tumorigenesis and survival. Here, we identify one such signaling rewiring process in glioblastoma cells that promotes expression of Sprouty2 (SPRY2). SPRY2 expression is of interest because we recently identified SPRY2 as a driver of glioblastoma cell and tumor proliferation and resistance to kinase inhibitors. Specifically, we found that SPRY2 depletion reduced the ability of glioblastoma cells to form colonies in soft agar, to form subcutaneous tumors in mice, and to resist co-inhibition of the EGFR and MET receptor tyrosine kinases. In the present study we found that, in a panel of glioblastoma cell lines, SPRY2 expression was initially reduced in response to inhibition of EGFR and MET. However, at later time points (24-48 hr after receptor inhibition) resurgent SPRY2 expression was observed even as EGFR and MET phosphorylation remained suppressed. Further inspection of the experimental data revealed that phosphorylated ERK, which was initially suppressed in response to EGFR and MET inhibition, also displayed a resurgence that paralleled the trend in SPRY2 expression. This correlation between ERK phosphorylation and SPRY2 expression is consistent with the documented ability of ERK to regulate SPRY2 expression transcriptionally. The importance of resurgent ERK phosphorylation was confirmed in experiments showing that its suppression through the use of a MEK inhibitor prevented resurgent SPRY2 expression and augmented cell death in response to EGFR and MET inhibitors. We hypothesized that resurgent ERK phosphorylation and resultant SPRY2 expression occurred due to the ability of glioblastoma cells to activate receptor tyrosine kinases other than EGFR and MET in response to inhibition of those receptors. Antibody microarray experiments confirmed this hypothesis, and ongoing experiments demonstrate the efficacy of co-targeting the receptors that rescue ERK phosphorylation in augmenting response to EGFR and MET inhibition.