Enzalutamide Enhanced Honokiol-induced Apoptotic Insults to Drug-resistant Glioblastoma Cells <i>via</i> an Intrinsic Bak-mitochondrion-caspase Cascade Mechanism

<h3>Background/Aim:</h3> Most glioblastoma multiforme (GBM) patients experience tumor recurrence due to their resistance to therapy, especially temozolomide (TMZ). Our previous studies showed the potential of honokiol, a small biphenolic molecule, for treatment of GBM. Recently, the roles of androgen receptor (AR) in GBM malignance were further identified. This study was aimed to further evaluate the effects of enzalutamide, an inhibitor of AR, on honokiol-induced killing to drug-resistant glioblastoma cells and the possible mechanisms. <h3>Materials and Methods:</h3> TMZ-resistant human and mouse glioblastoma cells, prepared from respective drug-sensitive cells, were pretreated with enzalutamide and then exposed to honokiol. Death events and apoptotic mechanisms were subsequently determined. <h3>Results:</h3> AR was immunodetected in TMZ-sensitive and -resistant glioblastoma cells. Pretreatment of human TMZ-resistant glioblastoma cells with enzalutamide enhanced honokiol-induced DNA fragmentation and cell apoptosis. As to the mechanisms, combined treatment of enzalutamide and honokiol selectively stimulated release of cytochrome <i>c</i> from mitochondria and succeeding activation of initiator caspase-9. Furthermore, the honokiol-induced augmentation of proapoptotic Bak production, disruption of the mitochondrial membrane potential, and activation of executioner caspase-3 were significantly enlarged by enzalutamide. Interestingly, suppressing caspase-9 activity concurrently attenuated the enhanced effects of enzalutamide on honokiol-triggered apoptotic insults. Moreover, the synergistic effects of enzalutamide and honokiol were further confirmed in mouse drug-resistant glioblastoma cells. <h3>Conclusion:</h3> Suppressing AR activity by enzalutamide improved honokiol-induced apoptosis of TMZ-resistant glioblastoma cells <i>via</i> an intrinsic Bak-mitochondrion-cytochrome c-dependent caspase cascade pathway. Combined treatment of enzalutamide and honokiol could represent a promising adjuvant strategy for treatment of drug-resistant glioblastomas.