Search

Summary Fas (CD95, Apo-1, or TNFRSF6) is a prototypical apoptosis-inducing death receptor in the tumor necrosis factor receptor (TNFR) superfamily. While the extracellular domains of TNFRs form trimeric complexes with their ligands and the intracellular domains engage in higher-order oligomerization, the role of the transmembrane (TM) domains is unknown. We determined the NMR structures of mouse and human Fas TM domains in bicelles that mimic lipid bilayers. Surprisingly, these domains use proline motifs to create optimal packing in homotrimer assembly distinct from classical trimeric coiled-coils in solution. Cancer-associated and structure-based mutations in Fas TM disrupt trimerization in vitro and reduce apoptosis induction in vivo, indicating the essential role of intramembrane trimerization in receptor activity. Our data suggest that the structures represent the signaling-active conformation of Fas TM, which appears to be different from the pre-ligand conformation. Analysis of other TNFR sequences suggests proline-containing sequences as common motifs for receptor TM trimerization. [ABSTRACT FROM AUTHOR]

CD95/Fas/APO-1 is a member of the death receptor (DR) family. Stimulation of CD95 leads to the induction of apoptosis as well as to NF-κB signaling. Crosstalk between these two pathways plays a central role in cell fate. Defects in the regulation of apoptosis and of NF-κB are connected to a number of chronic inflammatory diseases and cancer. For a better understanding of the life/death decisions in the cell and their contribution to disease progression, the development of new technologies is required. Using imaging flow cytometry we developed a method that enables a quantitative detection of different CD95 signaling pathways in the single cell. The important advantage of this method compared to other approaches is that it allows quantifying a large number of single cells undergoing apoptosis and NF-κB activation. This technology could provide new insights into the quantitative characterization of apoptosis and NF-κB at the single cell level and could be used for the quantitative network analysis in systems biology studies. [ABSTRACT FROM AUTHOR]

CD95 (Fas/APO-1) and its ligand, CD95L, have long been viewed as a death receptor/death ligand system that mediates apoptosis induction to maintain immune homeostasis. In addition, these molecules are important in the immune elimination of virus-infected cells and cancer cells. CD95L was, therefore, considered to be useful for cancer therapy. However, major side effects have precluded its systemic use. During the last 10 years, it has been recognized that CD95 and CD95L have multiple cancer-relevant nonapoptotic and tumor-promoting activities. CD95 and CD95L were discovered to be critical survival factors for cancer cells, and were found to protect and promote cancer stem cells. We now discuss five different ways in which inhibiting or eliminating CD95L, rather than augmenting, may be beneficial for cancer therapy alone or in combination with standard chemotherapy or immune therapy. [ABSTRACT FROM AUTHOR]

The present studies were designed to compare and contrast the abilities of TRAIL (death receptor agonist) and obatoclax (BCL-2 family inhibitor) to enhance sorafenib + HDAC inhibitor toxicity in GI tumor cells. Sorafenib and HDAC inhibitor treatment required expression of CD95 to kill GI tumor cells in vitro and in vivo. In cells lacking CD95 expression, TRAIL treatment, and to a lesser extent obatoclax, enhanced the lethal effects of sorafenib + HDAC inhibitor exposure. In hepatoma cells expressing CD95 a similar data pattern emerged with respect to the actions of TRAIL. Downstream of the death receptor the ability of TRAIL to enhance cell killing correlated with reduced AKT, ERK1/2, p70 S6K, and mTOR activity and enhanced cleavage of pro-caspase 3 and reduced expression of MCL-1 and BCL-XL. Over-expression of BCL-XL or MCL-1 or expression of dominant negative pro-caspase 9 protected cells from drug toxicity. Expression of activated AKT, p70 S6K, mTOR, and to a lesser extent MEK1EE also protected cells that correlated with maintained c-FLIP-s expression, reduced BIM expression, and increased BAD phosphorylation. In vivo sorafenib + HDAC inhibitor toxicity against tumors was increased in a greater than additive fashion by TRAIL. Collectively, our data argue that TRAIL, rather than obatoclax, is the most efficacious agent at promoting sorafenib + HDAC inhibitor lethality. J. Cell. Physiol. 228: 1996-2005, 2013. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Background: Fas/CD95 is the best-studied member of the death receptor (DR) superfamily in the central nervous system where it can trigger cellular responses other than apoptosis, including the promotion of neurogenesis and neuritogenesis, stimulation of the progression of gliomas, and regulation of the immune response of astrocytes. Methods: We have investigated the role of Fas/CD95 in the regulation of the proliferation of fetal astrocytes in vitro, as well as the signalling pathways involved. Results: Fas/CD95 ligation stimulated the proliferation of primary fetal astrocytes, through a mechanism that depends on the activation of caspase 8 and subsequent phosphorylation of extracellular signal regulated kinase (ERK). Interestingly this proliferative effect is only observed with a low dose of the Fas/CD95 agonist. In contrast, when primary astrocytes are challenged with a high dose of the Fas/CD95 agonist significant cell death occurs. Conclusions: Our findings support that, besides its effects on cell survival, Fas/CD95 may play a complex and prominent role in the regulation of astrocyte proliferation during development. Copyright © 2013 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The potential role of cysteine cathepsins in tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L)- and CD95 (Fas/APO-1)-induced apoptosis was investigated using four different cell lines (HeLa, HuH-7, Jurkat, and U-937). All four cell lines exhibited different levels of cathepsins and responded differently to apoptosis triggering, with Jurkat cells being the most sensitive and the only ones that were sensitive to the agonistic anti-APO-1 antibody. Apoptosis was accompanied by caspase activation, loss of the mitochondria and lysosome integrity, and the release of cysteine cathepsins into the cytosol, as judged based on the hydrolysis of the cysteine cathepsin substrate benzyloxycarbonyl-Phe-Arg-7-amino-4-methylcoumarin and by the immunological detection of cathepsin B. The inhibition of caspases by the broad-spectrum inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone prevented apoptosis, including the mitochondrial and lysosomal membrane permeabilization, as well as cathepsin release into the cytosol, consistent with caspases playing a crucial role in the process. Conversely, however, although the broad-spectrum cysteine cathepsin inhibitor (2 S,3 S)- trans-epoxysuccinyl-leucylamido-3-methyl-butane ethyl ester and the more cathepsin B-selective inhibitor [(2 S,3 S)-3-propylcarbamoyloxirane-2-carbonyl]-l-isoleucyl-l-proline methyl ester completely blocked cathepsin activity, these inhibitors neither prevented apoptosis and its progression nor the mitochondrial and lysosomal membrane permeabilization associated with this type of cell death. Consequently, cathepsin release into the cytosol was also not prevented. Together, these data indicate that cysteine cathepsins are not required for the TRAIL- and CD95-mediated apoptosis in various human cancer cell lines. This does not, however, rule out that lysosomes and cysteine cathepsins are involved in the amplification, but not in the initiation, of death receptor-mediated apoptosis in certain cell lines or under different stimulation conditions than the ones employed here. [ABSTRACT FROM AUTHOR]

Pre-association of CD95 at the plasma membrane is mandatory for efficient death receptor signaling. This homotrimerization occurs through self-association of an extracellular domain called the pre-ligand assembly domain (PLAD). Using novel molecular and cellular tools, we confirmed that CD95-PLAD is necessary to promote CD95 multimerization and plays a pivotal role in the transmission of apoptotic signals. However, while a human CD95 mutant deleted of the previously described PLAD domain (amino acids 1 to 66) fails to interact with its wild-type counterpart and trigger autonomous cell death, deletion of amino acids 1 to 42 does not prevent homo- or hetero (human/mouse)-oligomerization of CD95, and thus does not alter transmission of the apoptotic signal. Overall, these findings indicate that the region between amino acids 43 to 66 corresponds to the minimal motif involved in CD95 homotypic interaction and is necessary to convey an efficient apoptotic signal. Interfering with this PLAD may represent a new therapeutic strategy for altering CD95-induced apoptotic and non-apoptotic signals. [ABSTRACT FROM AUTHOR]

CD95 (APO-1/Fas) is a member of the death receptor (DR) family. Stimulation of CD95 leads to induction of apoptotic and non-apoptotic signaling pathways. The formation of the CD95 death-inducing signaling complex (DISC) is the initial step of CD95 signaling. Activation of procaspase-8 at the DISC leads to the induction of DR-mediated apoptosis. The activation of procaspase-8 is blocked by cellular FLICE-inhibitory proteins (c-FLIP). This review is focused on the role in the CD95-mediated signaling of the death effector domain-containing proteins procaspase-8 and c-FLIP. We discuss how dynamic cross-talk between procaspase-8 and c-FLIP at the DISC regulates life/death decisions at CD95. [ABSTRACT FROM AUTHOR]

Background: Structural homology modeling supported by bioinformatics analysis plays a key role in uncovering new molecular interactions within gene regulatory networks. Here, we have applied this powerful approach to analyze the molecular interactions orchestrating death receptor signaling networks. In particular, we focused on the molecular mechanisms of CD95-mediated NF-κB activation and the role of c-FLIP/NEMO interaction in the induction of this pathway. Results: To this end, we have created the homology model of the c-FLIP/NEMO complex using the reported structure of the v-FLIP/NEMO complex, and rationally designed peptides targeting this complex. The designed peptides were based on the NEMO structure. Strikingly, the experimental in vitro validation demonstrated that the best inhibitory effects on CD95-mediated NF-κB activation are exhibited by the NEMO-derived peptides with the substitution D242Y of NEMO. Furthermore, we have assumed that the c-FLIP/NEMO complex is recruited to the DED filaments formed upon CD95 activation and validated this assumption in silico. Further insight into the function of c-FLIP/NEMO complex was provided by the analysis of evolutionary conservation of interacting regions which demonstrated that this interaction is common in distinct mammalian species. Conclusions: Taken together, using a combination of bioinformatics and experimental approaches we obtained new insights into CD95-mediated NF-κB activation, providing manifold possibilities for targeting the death receptor network. [ABSTRACT FROM AUTHOR]