DDRE-13. PROSTAGLANDIN E RECEPTOR 3 (PTGER3) REGULATES RESISTANCE TO TUMOR TREATING FIELDS (TTFields) IN GLIOBLASTOMA CELLS

INTRODUCTION TTFields, a novel approved therapy for GBM, employ alternating intermediate-frequency electric fields to disrupt mitotic macromolecules leading to chromosome mis-segregation and apoptosis. The addition of TTFields significantly improves survival. However, most patients eventually develop resistance to TTFields through an unknown mechanism. METHODS Multiple human GBM cell lines were treated with TTFields continuously using Inovitro, an in vitro TTFields system, until cells with relative resistance to killing by TTFields emerged. Temporal gene expression profiles were analyzed using NETZEN, an innovative deep-learning and gene network-based ranking computational algorithm, to identify resistance pathways, followed by experimental validation. RESULTS PTGER3, a Gαi-protein-coupled cell surface receptor, is the top ranked master regulator in the predicted resistance program, which is upregulated in GBM cells within 24 hrs of exposure to TTFields and further reinforced as resistance sets in. Forced expression of PTGER3 in sensitive GBM cells confers relative resistance to TTFields, while PTGER3 depletion in resistant cells re-sensitizes them to TTFields. Most importantly, pharmacological inhibition of PTGER3 using either aspirin to reduce prostaglandin E production or PTGER3-specific inhibitors effectively prevent resistance from developing. Mechanistically, PTGER3 is rapidly translocated from the plasma membrane to the nucleus after TTFields exposure, where it interacts with ZNF488, a stemness transcription factor tightly linked to PTGER3 in our predicted network to initiate and maintain the resistance program. Indeed, TTFields resistance is associated with a transition to glioma stem cells (GSCs) as determined by increased neurosphere formation and orthotopic tumorigenesis in immunocompromised mice, and PTGER3 inhibition alone reverses the GSC transition leading to improved tumor control and survival. CONCLUSIONS PTGER3 is at the apex of a novel pathway that indispensably regulates TTFields resistance through a unique mechanism involving the physical nuclear translocation of this 7-transmembrane receptor. PTGER3 and its pathway are thus potential therapeutic targets to enhance therapeutic efficacy of TTFields.