Fas ligand (FasL), a member of the TNF protein family, potently induces cell death by activating its matching receptor Fas. Fas-mediated killing plays a critical role in naturally and pathologically occurring cell death, including development and homeostasis of the immune system. In addition to its receptor-interacting and cell death-inducing extracellular domain, FasL has a well-conserved intracellular portion with a proline-rich SH3 domainbinding site probably involved in nonapoptotic functions. We report here that, as with the Fas receptor, a fraction of FasL is constitutively localized in rafts. These dynamic membrane microdomains, enriched in sphingolipids and cholesterol, are important for cell signaling and trafficking processes. We show that FasL is partially localized in rafts and that increased amounts of FasL are found in rafts after efficient FasL/Fas receptor interactions. Raft disorganization after cholesterol oxidase treatment and deletions within the intracellular FasL domain diminish raft partitioning and, most important, lead to decreased FasL killing. We conclude that FasL is recruited into lipid rafts for maximum Fas receptor contact and cell death-inducing potency. These findings raise the possibility that certain pathologic conditions may be treated by altering the cell death-inducing capability of FasL with drugs affecting its raft localization. (
IntroductionThe Fas ligand (FasL, CD95/Apo-1 ligand, CD178, TNFSF6) molecule belongs to the TNF family and potently induces cell death in Fas (CD95/Apo-1/TNFRSF6) receptor-expressing cells. 1 Fas and FasL interact as oligomers, 2 and crosslinking of Fas leads to the recruitment of the adaptor protein FADD and of to the cytoplasmic Fas domain and to the formation of an intracellular receptor complex called death-inducing signaling complex (DISC). 3 Within the DISC, Casp-8 is activated and triggers the downstream caspase cascade, which executes the apoptotic dismantling of the cell.FasL is a type 2 transmembrane protein consisting of a receptor-interacting extracellular domain and a well-conserved 80-amino acid (aa)-long N-terminal cytoplasmic portion. 1,4,5 Cell death initiated by the Fas/FasL system is important for, among others things, homeostasis of the immune system (eg, activationinduced T-cell death 6 ), cytotoxic T cell (CTL)-mediated killing of virally infected or transformed cells, 7 and immune privilege maintenance. [8][9][10][11][12][13][14] The corresponding experimental studies are supported by the pathology of model systems displaying functional inactivation or aberrant activation of Fas/FasL. Mice carrying loss-of-function mutations in the Fas (lpr) or the Fasl (gld) genes develop lymphadenopathy and splenomegaly with a massive accumulation of CD4 Ϫ CD8 Ϫ T cells. 15 In addition, depending on the genetic background, these animals spontaneously acquire various forms of autoimmune disease with high titers of autoantibodies. 16 In humans, mutations in Fas and Fasl are associated with autoimmune lymphoproliferative syndrome (ALPS). [...
