Date Published: December 31, 2009
Publisher: Public Library of Science
Author(s): Zhi-Xiang Xu, Tian Ding, Valsala Haridas, Fiona Connolly, Jordan U. Gutterman, Gordon Langsley. http://doi.org/10.1371/journal.pone.0008532
Abstract: Avicins, a family of triterpene electrophiles originally identified as potent inhibitors of tumor cell growth, have been shown to be pleiotropic compounds that also possess antioxidant, anti-mutagenic, and anti-inflammatory activities. We previously showed that Jurkat cells, which express a high level of Fas, are very sensitive to treatment with avicins. Thus, we hypothesized that avicins may induce cell apoptosis by activation of the Fas pathway. By using a series of cell lines deficient in cell death receptors, we demonstrated that upon avicin D treatment, Fas translocates to the cholesterol- and sphingolipid-enriched membrane microdomains known as lipid rafts. In the lipid rafts, Fas interacts with Fas-associated death domain (FADD) and Caspase-8 to form death-inducing signaling complex (DISC) and thus mediates cell apoptosis. Interfering with lipid raft organization by using a cholesterol-depleting compound, methyl-β-cyclodextrin, not only prevents the clustering of Fas and its DISC complex but also reduces the sensitivity of the cells to avicin D. Avicin D activates Fas pathways independent of the association between extracellular Fas ligands and Fas receptors. A deficiency in Fas and its downstream signaling molecules leads to the resistance of the cells to avicin D treatment. Taken together, our results demonstrate that avicin D triggers the redistribution of Fas in the membrane lipid rafts, where Fas activates receptor-mediated cell death.
Partial Text: The Fas death receptor (CD95 or APO-1) conveys apoptotic signals through binding to its cognate ligand, FasL (CD95L), making the interaction between FasL and Fas a potential tumor-fighting target. Unfortunately, the putative clinical antitumor action of FasL cannot be reached because of severe liver toxicity due to the high expression of Fas on the surface of hepatocyte . Recent evidence for a FasL-independent activation of Fas suggests that the death receptor can also be activated intracellularly, in the absence of its ligand . Unraveling the mechanisms could provide the basis for novel therapeutic strategies for various diseases and could aid in the development of new compounds able to exploit cytoplasmic triggers of apoptosis for clinical use. This is of importance in apoptosis-deficient disorders such as cancer  and autoimmune diseases .
Lipid rafts are enriched membrane microdomains that contain high concentrations of cholesterol and glycosphingolipids, which are resistant to solubilization by detergents and represent areas of reduced fluidity plane of the lipid bilayer , . A variety of proteins, especially those involved in cell signaling, have been shown to translocate into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction.