The Pan-Bcl-2 Inhibitor (−)-Gossypol Triggers Autophagic Cell Death in Malignant Glioma

Abstract

Antiapoptotic Bcl-2 family members suppress both apoptosis and autophagy and are of major importance for therapy resistance of malignant gliomas. To target these molecules, we used BH3 mimetics and analyzed the molecular mechanisms of cell death induced thereby. Glioma cells displayed only limited sensitivity to single-agent treatment with the BH3 mimetics HA14-1, BH3I-2′, and ABT-737, whereas the pan-Bcl-2 inhibitor (−)-gossypol efficiently induced cell death. Furthermore, (−)-gossypol potentiated cell death induced by temozolomide (TMZ) in MGMT (O 6 -methylguanine-DNA methyltransferase)-negative U343 cells and, to a lesser extent, in MGMT-expressing U87 cells. (−)-Gossypol triggered translocation of light chain 3 to autophagosomes and lysosomes and cytochrome c release, but cell death occurred in the absence of lysosomal damage and effector caspase activation. Lentiviral knockdown of Beclin1 and Atg5 in U87, U343, and MZ-54 cells strongly diminished the extent of cell death induced by (−)-gossypol and combined treatment with TMZ, indicating that autophagy contributed to this type of cell death. In contrast, stable knockdown of the endogenous autophagy inhibitor mammalian target of rapamycin increased autophagic cell death. Our data suggest that pan-Bcl-2 inhibitors are promising drugs that induce caspase-independent, autophagic cell death in apoptosis-resistant malignant glioma cells and augment the action of TMZ. Furthermore, they indicate that efficient killing of glioma cells requires neutralization of Mcl-1.

The autophagic paradox in cancer therapy

“…Autophagy is induced by different forms of cancer therapy, including conventional chemotherapeutic drugs, novel targeted cancer therapeutics and ionizing radiation, in various types of solid and hematological malignancies (Table 2) (Han et al, 2007(Han et al, , 2008Kim et al, 2007Kim et al, , 2008bKim et al, , 2009aMaiuri et al, 2007b;Kamitsuji et al, 2008;Meschini et al, 2008;Qadir et al, 2008;Turcotte et al, 2008;Fu et al, 2009;Lorin et al, 2009;Tormo et al, 2009;Fan et al, 2010;Gupta et al, 2010;Huang and Sinicrope, 2010;Li and Fan, 2010;Voss et al, 2010;Yacoub et al, 2010;Wu et al, 2010c;Lian et al, 2011;O'Donovan et al, 2011). In some circumstances, autophagy mediates the cytotoxic or cytostatic effect of anticancer agents, in which blockade of autophagy abolishes the therapeutic actions.…”

“…Impairment of autophagy by dominant-negative Vps34, the class III PI3K, also induces apoptotic cell death in neuroblastoma cells expressing a C98X vasopressin mutant but not in the wild-type vasopressin-expressing counterpart (Castino et al, 2008). Moreover, the pan-Bcl-2 inhibitor (-)-gossypol paradoxically induces cytoprotective autophagy in MCF-7 breast cancer cells but autophagic cell death in glioma cells, in which the effects on cell viability can be blocked by RNA interference against Beclin 1 and Atg5 in both cases (Gao et al, 2010;Voss et al, 2010). The molecular mechanism underlying these disparities is presently unknown, but it is speculated that these cells may differentially express some proteins with cell deathor cell survival-specific function in autophagy.…”

Cucurbitacin-I inhibits Aurora kinase A, Aurora kinase B and survivin, induces defects in cell cycle progression and promotes ABT-737-induced cell death in a caspase-independent manner in malignant human glioma cells

“…The complexity of the interactions between cell surface receptors and downstream signaling targets has called into question the clinical utility of blocking any target in isolation, and the results of single agent-based strategies have to date been disappointing in patients with glioma. 2,4 Combination approaches can enhance ABT-737-induced glioma cell death, including use with the proteasomal inhibitor bortezomib, 34 the survivin inhibitor YM-155, 30 the PI3K/Akt inhibitor NVP-BKM120, 31 the histone deacetylase inhibitor vorinostat, 35 the alkylating agent temozolomide, 36 or the death receptor ligand TRAIL. 37 We performed a screen of ABT-737 in combination with other signaling inhibitors used at clinically achievable doses; combination of ABT-737 and cucurbitacin-I promoted apoptosis in malignant human glioma cell lines.…”

Knockdown of Nrf2 enhances autophagy induced by temozolomide in U251 human glioma cell line

“…As an effective cytotoxic drug, temozolomide (TMZ) induces significant autophagic cell death in many GBM cells, such as U251 human glioma cell line (18,21,22). Thus, autophagy has potential utility as a target for GBM therapy (23,24). Recently, several experts proposed the hypothesis that there may be cross-talk between Nrf2/Keap1 and autophagy pathways (25).…”