Fluoxetine Acts as a Calcium-Dependent Sensitizer of TRAIL-Induced Apoptosis in Glioblastoma Multiforme

Glioblastoma multiforme (GBM) is a highly aggressive brain tumor with limited therapeutic options and resistance to apoptosis. TNF-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis in malignant cells through extrinsic death receptor activation and downstream engagement of the intrinsic mitochondrial pathway, yet its therapeutic efficacy is hindered by intrinsic resistance mechanisms. Here, we demonstrate that the widely used antidepressant fluoxetine (FLX) acts as a potent TRAIL sensitizer by driving Ca2+ influx and mitochondrial priming. In U87MG and LN18 GBM cells, FLX and TRAIL combination therapy significantly reduced cell viability and elicited robust mitochondrial permeabilization compared to monotherapy. Dose–response profiling of TRAIL and FLX revealed strong synergy (Jin’s Q-values >2.5) across multiple dose pairs. Mechanistic studies identified GluR1+ AMPA receptor-mediated Ca2+ influx and Bax-dependent mitochondrial permeabilization as central effectors of the combination, while calpain activity served as an enhancer but was not a requirement for apoptosis. Pharmacological inhibition of AMPA receptors or Bax substantially attenuated the apoptotic response, confirming that FLX increased the TRAIL efficacy through Ca2+-mediated amplification of the intrinsic apoptosis pathway. These results identify fluoxetine as a clinically accessible, blood–brain barrier permeable agent that leverages Ca2+ signaling to overcome TRAIL resistance and highlight a novel therapeutic strategy that repurposes SSRIs to potentiate death receptor agonists in GBM.