SHORT COMMUNICATION Identification of a novel pro-apoptotic role of NF-jB in the regulation of TRAIL- and CD95-mediated apoptosis of glioblastoma cells C Jennewein 1 , S Karl 1 , B Baumann 2 , O Micheau 3 , K-M Debatin 1 and S Fulda 1,4 1 University Children’s Hospital, Ulm, Germany; 2 Institute for Physiological Chemistry, Ulm University, Ulm, Germany; 3 INSERM, U866, Dijon, France and 4 Institute for Experimental Cancer Research in Pediatrics, Goethe-University, Frankfurt, Germany We recently reported that nuclear factor-kappa B (NF-jB) promotes DNA damage-triggered apoptosis in glioblastoma, the most common brain tumor. In the present study, we investigated the role of NF-jB in death receptor-mediated apoptosis. Here, we identify a novel pro-apopotic function of NF-jB in TRAIL- and CD95- induced apoptosis. Inhibition of NF-jB by overexpression of the dominant-negative IjBa-superrepressor (IjBa-SR) significantly decreases tumor necrosis factor (TNF)- related apoptosis-inducing ligand (TRAIL)- or CD95- induced apoptosis. Vice versa, activation of NF-jB via overexpression of constitutively active IjB kinase complex (IKK)b (IKK-EE) significantly increases TRAIL-mediated apoptosis. Intriguingly, NF-jB inhibi- tion reduces the recruitment of Fas-associated death domain and caspase-8 and formation of the death- inducing signaling complex (DISC) upon stimulation of TRAIL receptors or CD95. This results in reduced TRAIL-mediated activation of caspases, loss of mitochon- drial potential and cytochrome c release in IjBa-SR- expressing cells. In comparison, NF-jB inhibition strongly enhances TNF-a-mediated apoptosis. Comparative studies revealed that TNF-a rapidly stimulates transcriptional activation and upregulation of anti-apoptotic proteins, whereas TRAIL causes apoptosis before transcriptional activation. Thus, this study demonstrates for the first time that NF-jB exerts a pro-apoptotic role in TRAIL- and CD95-induced apoptosis in glioblastoma cells by facil- itating DISC formation. Oncogene (2012) 31, 1468–1474; doi:10.1038/onc.2011.333; published online 8 August 2011 Keywords: TRAIL; apoptosis; NF-kB; glioblastoma; CD95 Glioblastoma is the most aggressive brain tumor, bearing a very poor outcome (DeAngelis, 2001). The failure of conventional therapy is mainly due to an aberrant regulation of multiple signaling pathways, leading to diffuse infiltration, genomic instability and resistance to apoptosis (Furnari et al., 2007). Apoptosis can be initiated via two pathways, the extrinsic (death receptor) and the intrinsic (mitochondrial) pathway (Fulda and Debatin, 2006). Binding of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) or CD95 ligand to their cognate receptors leads to formation of the death-inducing signaling complex (DISC) via the recruitment of the adapter protein Fas-associated death domain (FADD) and caspase-8, causing activation of caspase-8 and direct cleavage of downstream effector caspases (Fulda and Debatin, 2006). The extrinsic pathway can be linked to the intrinsic pathway via caspase-8-mediated cleavage of Bid, which translocates to the mitochondria, resulting in mitochondrial outer membrane permeabilization and an amplification of the apoptotic signal (Fulda and Debatin, 2006). TRAIL is considered as a promising agent for anticancer treatment, as it induces apoptosis in various cancers including glioblastoma (Humphreys and Halpern, 2008). In addition to apoptosis, TRAIL is implicated in the activation of several other pathways such as the NF-kB cascade, thereby modulating the apoptotic signal (Humphreys and Halpern, 2008). The transcription factor NF-kB is a dimer composed of proteins belonging to the NF-kB/Rel family (Karin et al., 2002). In resting cells, NF-kB is sequestered in the cytoplasm by an inhibitory IkB protein, predominantly IkBa (Karin et al., 2002). Upon stimulation, the IkB kinase complex (IKK) becomes activated, leading to proteasomal degradation of IkBa and the translocation of NF-kB to the nucleus (Karin et al., 2002). Generally, NF-kB is considered as anti-apoptotic, for example, in TNF-a signaling, as it is implicated in the transcriptional activation of proteins promoting cell survival (Karin et al., 2002). In line with this notion, several publications suggest NF-kB to be responsible for mediating resistance to TRAIL-induced apoptosis (Plantivaux et al., 2009). More recently, NF-kB has also been described to promote apoptosis in a cell type- and stimulus-dependent manner (Radhakrishnan and Kamalakaran, 2006). We previously reported that inhibition of NF-kB decreases anticancer drug-induced apoptosis in glioblas- toma cells, demonstrating a pro-apoptotic role of NF-kB in DNA damage-triggered apoptosis in glioblastoma (Karl et al., 2009). In the present study, we investigated the role of NF-kB in death receptor-induced apoptosis. Received 18 December 2010; revised 16 June 2011; accepted 25 June 2011; published online 8 August 2011 Correspondence: Professor S Fulda, Institute for Experimental Cancer Research in Pediatrics, Goethe-University, Komturstrasse 3a, Frankfurt, Hesse 60528, Germany. E-mail: simone.fulda@kgu.de Oncogene (2012) 31, 1468–1474 & 2012 Macmillan Publishers Limited All rights reserved 0950-9232/12 www.nature.com/onc