Overcoming therapy resistance and treatment failure in glioblastoma
Authors:   , da Hora Cintia Carla 
Published: April 2023

The overall survival of GBM patients remains dreary. The infiltrative nature of this tumor and its ability to adapt and become resistant to therapy have limited the success of commonly used interventions. New insight into GBM’s molecular landscape is helping tailor novel and
personalized treatment paradigms.

Chapter 1 is an overview of strategies for generating glioblastoma models. We highlight different preclinical GBM models and pinpoint the advantages and shortcomings of each, with an emphasis on glioma stem cells (GSCs) as a clinically relevant model.

In Chapter 2, we cultured and characterized cells that escape the differentiation process. Our studies revealed that this subpopulation of GSCs ( called floating cells or Fcs) escapes serum-induced differentiation and retains their stem cell markers and properties. FCs demonstrate tumor cell plasticity and serve as a preclinical model to study critical factors involved in therapy resistance and response. We further show that this acquired or inherited plasticity results from the activation of key transcription factors and is the hallmark of the heterogeneity seen in GBM.

Another crucial feature of GBM is the migration and invasion ability of its tumor cells. Chapter 3 describes a new 3D migration assay created by our group that could be optimized to study the motility and migration of GBM cells and their stromal counterparts. This new assay could be a novel platform for studying glioma cancer cell migration since we can explore the dynamics of cell motility in real-time and evaluate new therapeutics targeting GBM migration.

In Chapter 4 we continue to investigate tumor plasticity, in particular, the genetic switches that
allow GSCs to adapt to therapeutic insults. This study describes a signaling mechanism that helps GBM stem cells escape cell death induced by a family of antineoplastic compounds called Smac mimetics (SM). These compounds emerged as attractive cancer therapeutics, particularly for tumor populations that are highly resistant to conventional apoptosis-inducing therapies such as GBM. In this study, we outline the molecular underpinnings of SM resistance in GSCs and provide mechanistic insight to overcome this resistance and increase therapeutic efficacy; mechanistically, we show that this family of compounds stimulates an adaptive response triggered by the cytokine TNFα and we uncover a synergistic lethality between SM and STAT3/EZH2 inhibitors.

Chapter 5 delves into the antineoplastic working mechanism of the natural compound Obtusaquinone (OBT). A multidisciplinary approach revealed that OBT binds to cysteine residues with a particular affinity to cysteine-rich Keap1, a member of the CUL3 ubiquitin ligase complex. This binding promotes an overall stress response and results in ubiquitination and proteasomal degradation of Keap1 and downstream activation of the Nrf2 pathway. We also used positron emission tomography (PET) imaging to confirm that OBT can penetrate the brain and functionally target brain tumors. Finally, we generated an OBT analog with improved pharmacological properties through medicinal chemistry.

In the final chapter, we discuss how OBT in combination with xCT inhibitors (cysteine-glutamate transporter inhibitor) improved the efficacy of OBT and prevented therapy resistance in GBM. The improved efficacy is due to an increase in oxidative stress by inhibiting the cells's natural antioxidant response through the activation of Nrf2.

In summary, this thesis discussed possible mechanisms of therapy resistance and treatment failure in GBM and identified new susceptible targets that could aid in the fight against GBM. It is critical to understand that GBM is not a single disease process but rather a tapestry of genetically diverse tumor cells working together in perfect synergy to support tumor growth and invasion, which would eventually open the door to rationally-designed combination therapies tailored toward eradicating this fatal brain cancer.