(336d) Mitochondria Depolarizing Peptides Sensitize Prostate Cancer Cells to Death Receptor Mediated Apoptosis

Prostate cancer is the most common malignancy and the second-leading cause of cancer-related death in men in the US. Resistance to conventional treatments, such as radiotherapy, is the major cause of mortality in advanced prostate cancer disease and there is no chemotherapeutic regimen that significantly increases survival in these cases. Despite their intuitive promise, a wide variety of chemotherapeutic agents have met with limited success. Combination therapies that act at different locations within cellular suicide (apoptosis) pathways demonstrate enhanced therapeutic efficacies and are emerging alternatives to ‘single-agent' treatments. Mitochondria are the energy production centers of the cell and play a pivotal role in cellular survival. After cellular uptake, positively-charged amphipathic peptides preferentially localize in mitochondria due to the large negative potential across mitochondrial membranes. At sufficiently high concentrations, these peptides lead to mitochondrial membrane depolarization, loss of mitochondrial integrity, and ultimately, apoptosis.

Death receptors (DRs) belong to the Tumor Necrosis Factor (TNF) super-family of cell surface receptors and contain a cytoplasmic death domain that engages the cellular apoptosis machinery upon stimulation with their respective ligands (e.g., TNF-alpha). We asked whether mitochondrial depolarization induced by sub-toxic concentrations of a mitochondrial depolarizing peptide (MDP) could sensitize prostate cancer cells to death receptor mediated apoptosis. We first explored the kinetics of mitochondrial depolarization induced by different concentrations of the MDP in the LNCaP prostate cancer cell line. Next, we investigated changes in the cell-surface expression of death receptors 4 and 5 as a function of MDP concentration and incubation time. Finally, we induced mitochondrial depolarization in prostate cancer cells and followed this treatment with agonistic antibodies to DR4, DR5 or TRAIL (TNF-alpha Related Apoptosis Inducing Ligand) as a dual-agent combination therapy. The loss in cell viability following the combination treatment was compared to that observed in each of the individual treatments. MDP treatment led to differential cell-surface expression of DR4 and DR5; while the expression of DR4 increased following MDP treatment, the cell-surface expression of DR5 decreased compared to untreated cells. Combination treatment experiments indicated that mitochondrial depolarization followed by agonist antibody to DR4 acts in a synergistic manner to enhance death in LNCaP cells. Our results indicate that sub-toxic mitochondrial depolarization, an unexplored chemotherapeutic regimen, can be interfaced with other treatments, potentially leading to synergistic combination therapies for the ablation of advanced prostate cancer.