DAPK2 is a novel modulator of trail-induced apoptosis and it regulates oxidative stress in cancer cells by preserving mitochondrial function

Abstract

Targeting molecules involved in Tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) -mediated signalling has been hailed by many as a potential magic bullet to efficiently kill cancer cells, with little side effects on normal cells. Indeed, initial clinical trials showed that antibodies against TRAIL receptors, death receptor (DR) 4 and DR5 are well tolerated by cancer patients. Despite the initial efficacy issues in the clinical setting, novel approaches to trigger TRAIL-mediated apoptosis are being developed and its clinical potential is being reappraised. Unfortunately, many patients develop resistance to TRAIL-induced apoptosis and there is thus impetuous for identifying additional resistance mechanisms that may be targetable and usable in combination therapies. Here, we show that the death associated protein kinase (DAPK) 2 is a modulator of TRAIL signalling. DAPK2 is a serine/threonine kinase that belongs to the DAPK family. Like DAPK1, it has been implicated in programmed cell death, the regulation of autophagy and developmental processes. Ablation of DAPK2 using RNAi causes phosphorylation of NF-κB and its transcriptional activity in several cancer cell lines. This then leads to the induction of a variety of NF-κB target genes, which includes DR4 and DR5. DR4 and DR5 protein expression is correspondingly increased on the cell surface and this leads to the sensitisation of resistant cells to TRAIL-induced killing. As DAPK2 is a kinase, it is imminently druggable and our data thus offer a novel avenue to overcome TRAIL-resistance in the clinic. We have additionally identified a new role for DAPK2 in the regulation of mitochondrial integrity. RNAi-mediated depletion of DAPK2 leads to a number of metabolic changes, including a significantly decreased rate of oxidative phosphorylation in combination with an overall destabilised mitochondrial membrane potential. This phenotype is further corroborated by an increase in the production of mitochondrial superoxide anions and general oxidative stress. This role of DAPK2 is completely novel and could impact significantly on the understanding of DAPK2’s function in physiology and disease.