P02.26.b metabolic plasticity in glioblastoma tumor-initiating cell and resistance to lsd1-directed therapy

Abstract BACKGROUND Glioblastoma (GBM), the most common primary brain tumor in adults, remains incurable despite multimodal therapy. There is a pressing unmet medical need for new therapeutic strategies, especially because patients experience only short-term benefit after primary treatment and the vast majority inevitably relapse. Adaptation to harsh microenvironmental conditions, including nutrient deprivation, proteostasis perturbation, hypoxia and drug treatment, induce molecular and metabolic changes in glioblastoma that lead to the acquisition of a tolerance to stress and resistance to drug. In this context, we demonstrated that Lysine-specific histone demethylase 1 (LSD1) is a regulator of tumor-initiating cell (TICs) survival, adaptation and recovery from stress. LSD1 pharmacological inhibition with a specific and brain penetrant inhibitor (LSD1i) induces a maladaptive integrated stress response (ISR) which leads to the death of TICs exposed to ER stress or nutrient deficiency. MATERIAL AND METHODS Given the high GBM heterogeneity, we analyzed different patient-derived TICs grown in vitro as sphere and characterized for their genetic landscape and in vitro/in vivo stem cell-like features. We applied mechanistically different strategies, including biological, metabolic and pharmacological approaches, as well as transcriptomic and proteomic profiling. Zebrafish and GBM xenografts have been exploited as in vivo models. RESULTS LSD1i treatment induces physical and functional rearrangements of mitochondria, culminating into their enhanced bioenergetic activity and ATP production that predispose to cell death. Interestingly, only a subset of patient-derived TICs benefits from LSD1i therapy. These samples are largely dependent on glycolysis and, upon LSD1i administration, they are unable to further enhance it when mitochondrial energy production is inhibited. The samples resistant to LSD1i maintain their cellular behavior unaltered even upon prolonged treatment. They display reduced susceptibility to the stressful cues represented by either LSD1i and the known ER stressors Thapsigargin and Tunicamycin, by efficiently restoring energy homeostasis and properly activating the Activating Transcription Factor 4 (ATF4)-dependent ISR. Remarkably, LSD1i resistant samples display metabolic flexibility and switch between glycolytic and oxidative metabolism to ensure their survival. CONCLUSION The identification of specific metabolic vulnerabilities could offer unique opportunities for therapeutic intervention: the selective LSD1i vulnerability relies on the preferential dependence on glycolysis, while the great metabolic plasticity of LSD1i resistant TICs coordinates changes that favor their adaptation to stressful microenvironments and therapeutic insults.