TRAIL is a cytokine that is produced and secreted by most normal tissue cells. It causes apoptosis primarily in tumor cells, by binding to certain death receptors. It has been used as a target of several anti-cancer therapeutics since the mid-1990s. These however have not shown significant survival benefit.
In humans, the gene that encodes TRAIL is located at chromosome 3q26, which is not close to other TNF family members. The genomic structure of the TRAIL gene spans approximately 20 kb and is composed of five exonic segments 222, 138, 42, 106, and 1245 nucleotides and four introns of approximately 8.2, 3.2, 2.3 and 2.3 kb. The TRAIL gene lacks TATA and CAAT boxes and the promotor region contains putative response elements for GATA, AP-1, C/EBP, SP-1, OCT-1, AP3, PEA3, CF-1, and ISRE.
TRAIL shows homology to other members of the tumor necrosis factor superfamily. It is composed of 281 amino acids and has characteristics of a type II transmembrane protein (i.e. no leader sequence and an internal transmembrane domain). The N-terminal cytoplasmic domain is not conserved across family members, however, the C-terminal extracellular domain is conserved and can be proteolytically cleaved from the cell surface. TRAIL forms a homotrimer that binds three receptor molecules.
TRAIL binds to the death receptors DR4 (TRAIL-RI) and DR5 (TRAIL-RII). The process of apoptosis is caspase-8-dependent. Caspase-8 activates downstream effector caspases including procaspase-3, -6, and -7, leading to activation of specific kinases. TRAIL also binds the receptors DcR1 and DcR2, which do not contain a cytoplasmic domain (DcR1) or contain a truncated death domain (DcR2). DcR1 functions as a TRAIL-neutralizing decoy-receptor. The cytoplasmic domain of DcR2 is functional and activates NFkappaB. In cells expressing DcR2, TRAIL binding therefore activates NFkappaB, leading to transcription of genes known to antagonize the death signaling pathway and/or to promote inflammation.
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