ABT-737 promotes tBid mitochondrial accumulation to enhance TRAIL-induced apoptosis in glioblastoma cells

S Cristofanon,S Fulda

doi:10.1038/cddis.2012.163

Abstract

To search for novel strategies to enhance the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis pathways in glioblastoma, we used the B-cell lymphoma 2/Bcl2-like 2-inhibitor ABT-737. Here we report that ABT-737 and TRAIL cooperate to induce apoptosis in several glioblastoma cell lines in a highly synergistic manner (combination index <0.1). Interestingly, the concerted action of ABT-737 and TRAIL to trigger the accumulation of truncated Bid (tBid) at mitochondrial membranes is identified as a key underlying mechanism. ABT-737 and TRAIL cooperate to cleave BH3-interacting domain death agonist (Bid) into its active fragment tBid, leading to increased accumulation of tBid at mitochondrial membranes. Coinciding with tBid accumulation, the activation of Bcl2-associated X protein (Bax), loss of mitochondrial membrane potential, release of cytochrome-c and second mitochondria-derived activator of caspase (Smac) into the cytosol and caspase activation are strongly increased in cotreated cells. Of note, knockdown of Bid significantly decreases ABT-737- and TRAIL-mediated Bax activation and apoptosis. Also, caspase-3 silencing reduces ABT-737- and TRAIL-induced Bid cleavage and apoptosis, indicating that a caspase-3-driven, mitochondrial feedback loop contributes to Bid processing. Importantly, ABT-737 profoundly enhances TRAIL-triggered apoptosis in primary cultured glioblastoma cells derived from tumor material, underlining the clinical relevance. Also, ABT-737 acts in concert with TRAIL to suppress tumor growth in an in vivo glioblastoma model. In conclusion, the rational combination of ABT-737 and TRAIL cooperates to trigger tBid mitochondrial accumulation and apoptosis. This approach presents a promising strategy for targeting the apoptosis pathways in glioblastoma, which warrants further investigation.

Highlights

Proapoptotic tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptor agonists represent promising therapeutics in oncology, given their ability to trigger the cell’s intrinsic death program, preferentially, in cancer versus normal cells, and are currently being evaluated in early clinical trials.[10,11] primary or acquired resistance of glioblastoma toward TRAIL calls for alternative approaches to restore TRAIL sensitivity
To examine whether the neutralization of antiapoptotic B-cell lymphoma 2 (Bcl-2) family proteins can enhance TRAIL-induced cell death, we selected five glioblastoma cell lines (i.e., U87MG, U118MG, T98G, U138MG and A172), which are heterogeneous for p53 and PTEN status (Supplementary Table 1), two key signaling components that are often altered in glioblastoma.[17]
As mitochondrial carrier 2 (Mtch2) has recently been reported to facilitate the recruitment of truncated Bid (tBid) to mitochondria,[24] we examined whether the combination of TRAIL and ABT-737 causes an increase in Mtch[2] expression

Summary

Introduction

Proapoptotic TRAIL receptor agonists represent promising therapeutics in oncology, given their ability to trigger the cell’s intrinsic death program, preferentially, in cancer versus normal cells, and are currently being evaluated in early clinical trials.[10,11] primary or acquired resistance of glioblastoma toward TRAIL calls for alternative approaches to restore TRAIL sensitivity. Resistance to TRAIL may be caused by increased expression of antiapoptotic molecules

Methods

Results

Conclusion