Tumor Treating Fields (TTFields) significantly prolongs the survival of patients with glioblastoma, but glioma cells can still escape this therapy creating significant problems. Recent studies suggest glioma cells resist TTFields exposure by triggering AMP activated protein kinase (AMPK)-dependent autophagy pathway activation. Our group has been studying primary cilia, microtubule-based ‘antenna' whose growth is stimulated by autophagy-activated removal of Oral-Facial-Digital Syndrome 1 protein (OFD1) from the cilia base. Here we asked whether TTFields affect glioma primary cilia and/or play any role in the prosurvival response to TTFields. Using a low grade (S7) glioma cell line, we found that 1 or 3 intermittent cycles of 72hr 200kHz TTFields treatment in vitro caused a significant lengthening of primary cilia after cells were allowed to recover 3-4 days after TTFields. At the cilia base, TTFields led to recruitment of microtubule-associated protein 1A/1B light chain 3B (LC3B) and phospho-AMPK, and removal of OFD1, changes associated with autophagy pathway activation. Acutely after TTFields, we observed fewer cilia and apparent ‘ciliophagy' because the cilia appeared to detach or ‘dissolve' from the basal body. In U87 cells, TTFields induced a significant reduction of extranuclear clusters of OFD1 while increasing a dramatic formation of nuclear islands of OFD1, two phenomena that were disrupted by addition of the autophagy inhibitor chloroquine (CQ). TTFields also stimulated surprising growth of primary cilia in U87 cells which normally rarely elaborate cilia. Further, we found that TTFields was less effective at killing cilia-depleted S7 cells. Finally, TTFields plus inhibition of autophagy using CQ or a histone deacetylase 6 (HDAC6) inhibitor led to dramatically fewer cells and cilia, thus promoting TTFields toxicity. Altogether, our results suggest that primary cilia are potential membrane ‘hotspots' triggered by TTFields to promote glioma cell survival via OFD1.

Citation Format: Ping Shi, Jia Tian, Matthew R. Sarkisian. Tumor treating fields triggers OFD1 removal from primary cilia to promote glioma cell survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1975.