American Association for Cancer Research
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Supplementary Figure Legends from PI3K and Bcl-2 Inhibition Primes Glioblastoma Cells to Apoptosis through Downregulation of Mcl-1 and Phospho-BAD

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journal contribution
posted on 2023-04-03, 16:23 authored by Fresia Pareja, David Macleod, Chang Shu, John F. Crary, Peter D. Canoll, Alonzo H. Ross, Markus D. Siegelin

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Glioblastoma multiforme (GBM) is a highly malignant human brain neoplasm with limited therapeutic options. GBMs display a deregulated apoptotic pathway with high levels of the antiapoptotic Bcl-2 family of proteins and overt activity of the phosphatidylinositol 3-kinase (PI3K) signaling pathway. Therefore, combined interference of the PI3K pathway and the Bcl-2 family of proteins is a reasonable therapeutic strategy. ABT-263 (Navitoclax), an orally available small-molecule Bcl-2 inhibitor, and GDC-0941, a PI3K inhibitor, were used to treat established glioblastoma and glioblastoma neurosphere cells, alone or in combination. Although GDC-0941 alone had a modest effect on cell viability, treatment with ABT-263 displayed a marked reduction of cell viability and induction of apoptotic cell death. Moreover, combinatorial therapy using ABT-263 and GDC-0941 showed an enhanced effect, with a further decrease in cellular viability. Furthermore, combination treatment abrogated the ability of stem cell–like glioma cells to form neurospheres. ABT-263 and GDC-0941, in combination, resulted in a consistent and significant increase of Annexin V positive cells and loss of mitochondrial membrane potential compared with either monotherapy. The combination treatment led to enhanced cleavage of both initiator and effector caspases. Mechanistically, GDC-0941 depleted pAKT (Serine 473) levels and suppressed Mcl-1 protein levels, lowering the threshold for the cytotoxic actions of ABT-263. GDC-0941 decreased Mcl-1 in a posttranslational manner and significantly decreased the half-life of Mcl-1 protein. Ectopic expression of human Mcl-1 mitigated apoptotic cell death induced by the drug combination. Furthermore, GDC-0941 modulated the phosphorylation status of BAD, thereby further enhancing ABT-263–mediated cell death.Implications: Combination therapy with ABT-263 and GDC-0941 has novel therapeutic potential by specifically targeting aberrantly active, deregulated pathways in GBM, overcoming endogenous resistance to apoptosis. Mol Cancer Res; 12(7); 987–1001. ©2014 AACR.

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