STAT5/OLIG2 regulation of GBM therapeutic resistance and recurrence - Resubmit 3.22

Project Summary Glioblastoma (GBM) is among the most lethal human cancers with extensive genetic and cellular heterogeneity. Genetic heterogeneity in GBM is seen through activation of multiple receptor tyrosine kinases (RTKs) including EGFR. Approximately ~50-60% of GBM tumors harbor amplification of the EGFR gene, including the constitutively active variant EGFRvIII, and point mutations within the extracellular domain of EGFR. Clinical effort at targeting EGFR itself has failed to increase survival, in part due to redundancy in the downstream signaling pathways triggered through EGFR or other oncogenic RTKs. Our data shows that EGFRvIII and possibly the hot spot mutations promote therapeutic resistance through downstream activation of oncogenic STAT5 signaling. Thus, understanding how STAT5 can serve as a point of convergence for oncogenic RTKs, such as EGFR will provide new opportunities for therapeutic interventions in GBM patients. One of the reasons why role of STAT5 signaling has been underappreciated in GBM is because long-term cultured GBM cells often lose EGFRvIII expression and do not show STAT5 activation. However, primary GBMs that retain EGFRvIII expression exhibit active STAT5 signaling. Our data indicate that EGFRvIII signaling activates STAT5 through JAK1/2- independent mechanisms and represents an attractive RTK model for understanding the role of STAT5 in GBM. We also find that therapy-resistant glioma stem cells (GSCs) that retain EGFRvIII expression also have increased STAT5 activation. In GSCs, EGFR participates in a feed-forward loop with a key cell fate and pro-mitogenic transcription factor, OLIG2. GSCs depleted of OLIG2 show decreased levels of phospho- STAT5 and expression of its downstream targets, (e.g. Fn14). Both STAT5 and OLIG2 function as dimers and in GSCs we find that STAT5 interacts with OLIG2. Furthermore, STAT5 inhibition decreases OLIG2- dependent GSC invasion. However, the upstream molecular mechanisms involved in OLIG2-dependent activation of STAT5 and signaling pathway(s) regulated by the OLIG2-STAT5 complex must be identified to effectively target the infiltrative and resistant glioma stem cells. We hypothesize that activation of STAT5-OLIG2 signaling mediated by oncogenic RTKs, such as EGFRvIII, promote increased invasion, therapy resistance, and stemness properties in GBM. Aim 1 will determine the mechanistic role of STAT5 isoform signaling in the context of EGFR variants in GBM cells. Aim 2 will determine the molecular mechanism(s) involved in OLIG2-mediated regulation of STAT5 activation in GSCs. Aim 3 will assess the functional and clinical significance of STAT5-OLIG2 signaling axis in GBM. Success of the proposal will identify, validate, and place into a clinically meaningful context the STAT5/OLIG2 signaling pathway as a therapeutic target for infiltrating cells that commonly underlie GBM fatality.