Abstract LB304: Mechanisms of cell death escape in glioblastoma

Abstract

Glioblastoma is the most common brain tumor in adults and among the deadliest malignancies per se with a highly invasive phenotype upon presentation. To achieve rapid colonialization throughout the central nervous system, glioblastoma cells have to be equipped with a high resistance to several forms of programmed cell death, such as apoptosis. All this occurs in the absence of any tumor-initiated signature mutations. Using a comprehensive comparative analysis combining expression profiles and functional analysis of normal brain glia cells, primary tumors and tumor-derived organoids with distinct differentiation subtypes we investigated the underlying features associated with the high resistance to cell death-induced by conventional treatment. To break this resistance, we looked into the possibility to add the small molecule inhibitor Venetoclax, that targets the Bcl-2 family, to conventional therapy. The Bcl-2 family are a number of evolutionarily-conserved proteins that share Bcl-2 homology (BH) domains and are most notable for their regulation of apoptosis at the mitochondrion. Interestingly, although stem cell-like cells (SCs) express more pro-apoptotic Bcl-2 family proteins than their differentiated progeny they remain more resistant to apoptosis, suggesting that the Bcl-2 family is not the main mediator of apoptosis resistance. In stark contrast to leukemia cells, inhibition of Bcl-2 alone has no effect on glioblastoma cells, but combining this with either Temozolomide (TMZ), the standard chemotherapeutic option, or serum starvation leads to synergistic effects. These are, however, weak hence we investigated whether compensatory mechanisms are activated. Indeed, in both SCs and DCs we found an upregulation of Mcl-1, a molecule known to compensate for inhibited Bcl-2. However, additionally blocking Mcl-1 with several different small molecule inhibitors did not further sensitize primary Glioblastoma cells. This suggests that while mechanical upregulation of Mcl-1 occurs in Glioblastoma, it is of little functional consequence. The reduced importance of the Bcl-2 family is also reflected in the non-transformed astrocytes which are precursor of glioblastoma. They already display an intrinsic high resistance to apoptotic cell death. Treatment with TMZ and modulators of apoptosis does not significantly affect their viability. Our findings suggest that the Bcl-2 family has a reduced role in mediating the survival of brain cells, at least when compared to haemopoietic cells. While a certain sensitizing effect can be achieved in Glioblastoma cells by combining cellular stressors with inhibitors of the Bcl-2 family, this is unlikely to be a sufficient to overcome therapy resistance. Citation Format: Helene von Bandemer, Felix Seyfried, Anne Fleischmann, Georg Karpel-Massler, Aurelia Peraud, Hannah Strobel, Marcin Lyszkiewicz, Klaus Michael Debatin, Mike Andrew Westhoff. Mechanisms of cell death escape in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB304.