Apollon (also known as BRUCE or BIRC6) is a large protein containing baculoviral-IAP-repeat (BIR) and ubiquitin-conjugating enzyme (UBC) domains at the amino- and carboxy termini, respectively. Apollon inhibits apoptosis, but its molecular and physiological function remains unclear. Here we report that Apollon binds to, ubiquitinates and facilitates proteasomal degradation of SMAC and caspase-9, which both contain IAP-binding motifs. Targeted disruption of Apollon in mice caused embryonic and neonatal lethality. Notably, SMAC induced apoptosis in Apollon-deficient cells, but not in Apollon-expressing cells. Furthermore, the IAP-binding motif of SMAC was required to induce apoptosis in Apollon-deficient cells. These results suggest that Apollon has an essential function in preventing SMAC-induced apoptosis.

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Ligation of death receptors, such as Fas, triggers apoptosis in many types of cells (28), which is inhibited by cellular FLIP (cFLIP, also known as I-FLICE, FLAME-I, Casper, CASH, MRIT and Usurpin) (8,11,13,16,34,40,41,44). The long form of cFLIP (cFLIP-L) is highly homologous to caspase 8, containing two death effector domains (DED) and a caspaselike domain at the amino and carboxy termini, respectively. cFLIP-L, however, does not have caspase activity due to the lack of a conserved cysteine residue in the caspase-like domain. Upon death receptor ligation, cFLIP-L is recruited to the death receptor complex, together with FADD and caspase 8, and inhibits apoptosis signaling. cFLIP-L is expressed in various cancers (2,16,27,36,43,44), which suggests a role for cFLIP-L in protecting cancer cells from cellular immunity using the Fas system (5, 6, 25).cFLIP, however, does not always inhibit apoptosis signaling but also mediates growth signals in some cases. Under conditions in which the proliferation of CD3-activated human T lymphocytes is increased by recombinant Fas ligand, cFLIP-L interacts with tumor necrosis factor receptor-associated factors 1 and 2, as well as the kinases RIP and Raf-1, resulting in the activation of the NF-B and ERK signaling pathways (17). The ability of cFLIP-L to switch Fas-mediated glucose signaling from a death signal into a proliferation signal in human pancreatic ␤ cells was also reported (24).The Wnt signal transduction pathway plays a critical and evolutionarily conserved role during embryogenesis (3, 10, 46). The Wnt signal is mediated by ␤-catenin, a transcription factor that is normally degraded by the ubiquitin-proteasome system in cytosol. Phosphorylation of ␤-catenin by a large protein complex involving adenomatous polyposis coli (APC) protein, Axin, and glycogen synthase kinase 3␤ (GSK3␤) initiates the ubiquitylation and proteasomal degradation of ␤-catenin (15, 49). Upon Wnt signaling, phosphorylation of ␤-catenin is inhibited, which results in the accumulation and translocation of ␤-catenin into nuclei, thereby inducing the expression of several genes, such as c-myc and the cyclin D gene. Mutations in APC, Axin, and ␤-catenin genes resulting in abolished ␤-catenin ubiquitylation are found in many human cancers (4,9,20,26,30,32,35,37), indicating that inappropriate activation of Wnt signaling plays an important role in human cancers (31,33).In this paper, we report that cFLIP-L inhibits ␤-catenin ubiquitylation and enhances Wnt signaling, which suggests an additional mechanism involved in tumorgenesis, in addition to inhibiting apoptosis signaling. MATERIALS AND METHODSPlasmids. Human cFLIP-L and a short splicing variant of cFLIP (cFLIP-S) were amplified by PCR from a Jurkat cDNA library and subcloned into pcDNAbased mammalian expression vectors (Invitrogen). For deletion mutant constructs, DNA sequences corresponding to different regions of cFLIP-L were

The history of studies in biology regarding reactive oxygen species (ROS) is approximately 40 years. During the initial 30 years, it appeared that these studies were mainly focused on the toxicity or microbicidal-related agents of ROS. However, recent studies have identified another action regarding oxidative signaling, other than toxicity of ROS. Basically, it is suggested that ROS are reactive, and degenerate to biomacromolecules such as DNA and proteins, leading to deterioration of cellular functions as an oxidative stress. On the other hand, recent studies have shown that ROS act as oxidative signaling in cells, resulting in various gene expressions. For example, NADPH oxidase, a major source of superoxide radicals (O(2)(-)), expresses in various tissues such as leukocytes and cardiovascular systems, and ROS derived from the enzyme play important roles in cell proliferation, differentiation, and cell death. In this review, we have focused on and described the basic properties, toxicity, and roles of ROS.