Summary Inherent plasticity and various survival cues allow glioblastoma stem-like cells (GSCs) to survive and proliferate under intrinsic and extrinsic stress conditions. Here, we report that GSCs depend on the adaptive activation of ER stress and subsequent activation of lipogenesis and particularly stearoyl CoA desaturase ( SCD1 ), which promotes ER homeostasis, cytoprotection, and tumor initiation. Pharmacological targeting of SCD1 is particularly toxic due to the accumulation of saturated fatty acids, which exacerbates ER stress, triggers apoptosis, impairs RAD51-mediated DNA repair, and achieves a remarkable therapeutic outcome with 25%–100% cure rate in xenograft mouse models. Mechanistically, divergent cell fates under varying levels of ER stress are primarily controlled by the ER sensor IRE1, which either promotes SCD1 transcriptional activation or converts to apoptotic signaling when SCD1 activity is impaired. Taken together, the dependence of GSCs on fatty acid desaturation presents an exploitable vulnerability to target glioblastoma.

The malignant primary brain tumor, glioblastoma (GBM) is generally incurable. New approaches are desperately needed. Adeno-associated virus (AAV) vector-mediated delivery of anti-tumor transgenes is a promising strategy, however direct injection leads to focal transgene spread in tumor and rapid tumor division dilutes out the extra-chromosomal AAV genome, limiting duration of transgene expression. Intravenous (IV) injection gives widespread distribution of AAV in normal brain, however poor transgene expression in tumor, and high expression in non-target cells which may lead to ineffective therapy and high toxicity, respectively. Delivery of transgenes encoding secreted, anti-tumor proteins to tumor stromal cells may provide a more stable and localized reservoir of therapy as they are more differentiated than fast-dividing tumor cells. Reactive astrocytes and tumor-associated macrophage/microglia (TAMs) are stromal cells that comprise a large portion of the tumor mass and are associated with tumorigenesis. In mouse models of GBM, we used IV delivery of exosome-associated AAV vectors driving green fluorescent protein expression by specific promoters (NF-κB-responsive promoter and a truncated glial fibrillary acidic protein promoter), to obtain targeted transduction of TAMs and reactive astrocytes, respectively, while avoiding transgene expression in the periphery. We used our approach to express the potent, yet toxic anti-tumor cytokine, interferon beta, in tumor stroma of a mouse model of GBM, and achieved a modest, yet significant enhancement in survival compared to controls. Noninvasive genetic modification of tumor microenvironment represents a promising approach for therapy against cancers. Additionally, the vectors described here may facilitate basic research in the study of tumor stromal cells in situ.

Glioblastoma is an incurable and highly aggressive brain tumor. Understanding therapeutic resistance and survival mechanisms driving this tumor type is key to finding effective therapies. Smac mimetics (SM) emerged as attractive cancer therapeutics particularly for tumor populations that are highly resistant to conventional apoptosis-inducing therapies. We evaluated the therapeutic efficacy of SM on Glioma stem-like cells (GSCs) and showed that this family of compounds stimulates an adaptive response triggered by TNFα. Increased expression of TNFα results in a prolonged and sustained activation of NF-κB and STAT3 signaling thus activating several tumor cell resistance mechanisms in GSCs. We show that STAT3 activation is contingent on EZH2 activation and uncover a synergistic lethality between SM and EZH2 inhibitors. Therapeutic inhibition of EZH2 impaired the viability of SM-treated GSCs. Our study outlines the molecular underpinnings of SM resistance in glioblastoma and provides mechanistic insight to overcome this resistance and increase therapeutic efficacy.