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
