Apoptosis-based treatment of glioblastomas with ABT-737, a novel small molecule inhibitor of Bcl-2 family proteins

Abstract

Defects in the apoptotic signaling cascades contribute to the poor therapeutic response of malignant gliomas. As glioblastomas are characterized by high expression levels of anti-apoptotic Bcl-2 family proteins, we studied the effects of the novel Bcl-2 inhibitor, ABT-737, on malignant glioma cells. ABT-737 treatment released the pro-apoptotic Bax protein from its binding partner Bcl-2 and potently induced apoptotic cell death in glioblastoma cells in vitro and in vivo. The local administration of ABT-737 prolonged the survival in an intracranial glioma xenograft model. Downregulation of Mcl-1 and overexpression of Bcl-2 sensitized the cells to ABT-737-mediated apoptosis. Moreover, ABT-737 potentiated the cytotoxicity of the chemotherapeutic drugs vincristine and etoposide, and of the death ligand TRAIL. As glioma stem cells may play a crucial role for the tumor progression and the resistance to treatment in glioblastomas, we investigated the effects of ABT-737 on the subpopulation of glioma cells exhibiting stem cell characteristics. Inhibition of proliferation and induction of apoptosis by ABT-737 were less efficient in glioma stem cells than in non-stem cell-like glioma cells. As the resistance of glioma stem cells was associated with high Mcl-1 expression levels, ABT-737 treatment combined with downregulation of Mcl-1 could represent a promising novel approach in glioblastoma treatment.

Notch1 signaling promotes survival of glioblastoma cells via EGFR-mediated induction of anti-apoptotic Mcl-1

“…35,36 We previously reported that siRNA-mediated downregulation of Mcl-1 in glioblastoma cells results in a potent sensitization to ABT-737-induced apoptosis. 37 Similarly, the knockdown of Mcl-1 significantly increased the sensitivity of glioblastoma cells to the death ligand TRAIL (Figure 6c). Accordingly, overexpression of Mcl-1 rescued Notch1-mediated sensitization to ABT-737 and TRAILinduced apoptosis (Supplementary Figure S11) and inhibited cell death caused by Notch1 knockdown (Supplementary Figure S12).…”

“…Synergy was evaluated by the fractional product method that allows an evaluation of synergy at a defined level of effect. 37 Statistical analysis of in vivo survival experiments was performed with the log rank test.…”

“…In all, 1 Â 10 5 NCH441 or NCH421k glioma cells were intracranially injected into the basal ganglia of 6-week-old male athymic CD1-nude mice (Charles River, Sulzfeld, Germany) as previously described in detail. 37,50 Alternatively, glioma cells (7.5 Â 10 6 U251MG or U87MG) were subcutaneously injected into the flanks of 6-week-old male athymic CD1-nude mice using a 30-G needle. The tumor was removed and used for tissue slice culture when reaching a diameter of approximately 0.6 cm.…”

pRb/E2F-1-mediated caspase-dependent induction of Noxa amplifies the apoptotic effects of the Bcl-2/Bcl-xL inhibitor ABT-737

“…24,42 As Noxa potently binds to Mcl-1, 6 its ability to promote ABT-737 induction of cell death generally relies on its capacity to inhibit Mcl-1 that is not targeted by ABT-737. [13][14][15][16][17] Silencing of Mcl-1 increased cell death rates in ABT-737-treated U251 cells (without affecting Noxa expression in untreated or ABT-737-treated cells, Supplementary Figures S3A and S3B). This is consistent with the notion that, in these cells, Mcl-1 represents a major restrain to ABT-737 efficiency that Noxa induction contributes to overcome.…”

“…Thus, sensitivity to ABT-737 is enhanced by combined treatments that decrease Mcl-1 expression and/or induce Noxa, a BH3-only protein that essentially functions as an inhibitor of Mcl-1. [13][14][15][16][17][18] ABT-737 is a powerful tool to investigate how death signals induced by direct inhibition of subsets of anti-apoptotic Bcl-2 family members lead to cell demise. Caspase activity contributes to the final stages of cell death induced by inhibition of Bcl-2 homologs.…”