ARTS (Sept4_i2) is a pro-apoptotic tumor suppressor protein that functions as an antagonist of X-linked IAP (XIAP) to promote apoptosis. ARTS-induced caspase activation leads to cleavage of the pro-apoptotic Bcl-2 family protein Bid known to Opicapone (BIA 9-1067) promote MOMP. We propose that translocation of ARTS initiates a first wave of caspase activation that can promote MOMP. This leads to the subsequent release of additional mitochondrial factors including cytochrome C and SMAC/Diablo which then amplifies the caspase cascade and causes apoptosis. gene does not contain an IBM instead it uses unique sequences to bind XIAP.18 ARTS expression is frequently lost in acute lymphoblastic leukemia patients indicating that it functions as a tumor suppressor protein.21 ARTS KD HeLa cells we have performed several assays including XTT assay counting of DAPI-stained nuclei clonogenic survival MSH4 assay and counting of terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL)-positive cells (Figure 7). All four different assays showed reduced cell death in ARTS KD cells following treatment with STS when compared with wt HeLa (Figures 7a-d). These data support our hypothesis that early activation of caspases induced by ARTS promotes cell death. Figure 7 Inactivation of ARTS in HeLa cells leads to resistance toward apoptosis. (a) Knockdown of ARTS in HeLa cells results in increased viability of cells following STS induction-XTT results. Cell viability was quantified using XTT-based assay (for details … ARTS promotes rapid and specific degradation of XIAP but not cIAP1 protein upon induction of apoptosis Upon induction of apoptosis XIAP protein is degraded by the ubiquitin-proteasome system (UPS).4 SMAC and/or small-molecule derivatives (‘SMAC-mimetics’) selectively reduce the levels of cIAP1 and cIAP2 but not that of XIAP.33 ARTS can bind to multiple IAP family members; cIAP1 (Figure 8a) ML-IAP (Livin) (data not shown) and XIAP (Figures 5a b and ?and8a;8a; Gottfried ARTS KD HeLa cells treated with STS (Figure 8cII). We found that knockdown of ARTS blocked the decrease of XIAP protein almost as well as MG132 (Figure 8cII). Taken together our results suggest that ARTS is required for the rapid early reduction of XIAP in response to STS treatment. Figure 8 ARTS promotes rapid and specific degradation of XIAP but not of cIAP1 protein upon induction of apoptosis. (a) ARTS binds to both XIAP Opicapone (BIA 9-1067) and cIAP1. COS-7 cells were transiently transfected with pSC2-6myc-ARTS construct together with mammalian GST-XIAP … Discussion The release of pro-apoptotic mitochondrial factors such as cytoC and SMAC has been traditionally viewed as the initiation stage of the mitochondrial pathway promoting caspase activation. This redistribution of cytoC and SMAC from mitochondria to the cytosol requires MOMP.24 Although several studies indicate that the release of SMAC and cytoC can occur independently of caspases 34 others suggest that caspase activity is required for this.35 36 Here we provide evidence that the mitochondrial IAP-antagonist ARTS may be the ‘missing link’ enabling MOMP and the translocation of cytoC and SMAC in paradigms where this release depends on caspase activation. In particular we suggest that ARTS has a critical role in initiating the mitochondrial apoptotic pathway upstream of MOMP and that it acts by a different mechanism than other known IAP antagonists. The first phase of ARTS translocation from mitochondria as well as binding of ARTS to XIAP occurs in a caspase-independent manner (Figure 2d; Gottfried for 5?min at 4°C and the supernatant was centrifuged at 10?000 × for 20?min to obtain mitochondria. Mitochondria were washed with homogenization buffer. PK treatment of mitochondrial fractions Purified mitochondria were resuspended in Tris buffer (30?mM Tris pH 7.6 1 CaCl2). The suspension Opicapone (BIA 9-1067) was divided Opicapone (BIA 9-1067) into equal aliquots for PK treatment. Aliquots of mitochondria were treated with 50 and 400?for 20?min washed in homogenization buffer and resuspended in SDS-PAGE sample buffer. To confirm the activity of the PK the mitochondrial fraction was solubilized.