CD95 is the quintessential death receptor and, when it is bound by ligand, cells undergo apoptosis. Recent evidence suggests, however, that CD95 mediates not only apoptosis but also diverse nonapoptotic functions depending on the tissue and the conditions.

Induction of apoptosis by oncogenes like c-myc may be important in restraining the emergence of neoplasia. However, the mechanism by which c-myc induces apoptosis is unknown. CD95 (also termed Fas or APO-1) is a cell surface transmembrane receptor of the tumor necrosis factor receptor family that activates an intrinsic apoptotic suicide program in cells upon binding either its ligand CD95L or antibody. c-myc-induced apoptosis was shown to require interaction on the cell surface between CD95 and its ligand. c-Myc acts downstream of the CD95 receptor by sensitizing cells to the CD95 death signal. Moreover, IGF-I signaling and Bcl-2 suppress c-myc-induced apoptosis by also acting downstream of CD95. These findings link two apoptotic pathways previously thought to be independent and establish the dependency of Myc on CD95 signaling for its killing activity.

Disruption of epithelial polarity is a key event in the acquisition of neoplastic growth. JNK signalling is known to play an important part in driving the malignant progression of many epithelial tumours, although the link between loss of polarity and JNK signalling remains elusive. In a Drosophila genome-wide genetic screen designed to identify molecules implicated in neoplastic growth, we identified grindelwald (grnd), a gene encoding a transmembrane protein with homology to members of the tumour necrosis factor receptor (TNFR) superfamily. Here we show that Grnd mediates the pro-apoptotic functions of Eiger (Egr), the unique Drosophila TNF, and that overexpression of an active form of Grnd lacking the extracellular domain is sufficient to activate JNK signalling in vivo. Grnd also promotes the invasiveness of Ras(V12)/scrib(-/-) tumours through Egr-dependent Matrix metalloprotease-1 (Mmp1) expression. Grnd localizes to the subapical membrane domain with the cell polarity determinant Crumbs (Crb) and couples Crb-induced loss of polarity with JNK activation and neoplastic growth through physical interaction with Veli (also known as Lin-7). Therefore, Grnd represents the first example of a TNFR that integrates signals from both Egr and apical polarity determinants to induce JNK-dependent cell death or tumour growth.

Thy-1, a single variable-like immunoglobulin superfamily domain anchored in the plasma membrane by a glycosyl phosphaditylinositol tail [1], is a major surface glycoprotein in adult mammalian neurons and rodent thymocytes [2]; the function of Thy-1 has remained enigmatic since its discovery [3]. Studies in vitro have implicated Thy-1 in homotypic and heterotypic cell-cell interactions [2,4]. Ligation of Thy-1 initiates transmembrane signaling pathways that lead to diverse physiological outcomes in different cells [2,5-7]. In rodents, Thy-1 is highly expressed on the surface of CD4+CD8+ double-positive immature thymocytes and downregulated in mature T cells. Here, we report that thymocytes from Thy-1-/- mice [8] had altered cell-cell contacts, and hyperresponsiveness to T-cell receptor (TCR) triggering as demonstrated by the heightened activation of p56lck, phosphorylation of TCR subunits, Ca2+ fluxes and cell proliferation. Thy-1-/- thymocytes exhibited impaired maturation from the double positive to single positive stage of thymocyte development, possibly due to inappropriate negative selection, and were prone to T lymphomas in aged mice. These observations indicate that Thy-1 negatively regulates TCR-mediated signaling and controls activation thresholds during thymocyte differentiation.

Cell death receptors have crucial roles in the regulation of immune responses. Here we review recent in vivo data confirming that the Fas death receptor (TNFSR6) on B cells is important for the regulation of autoimmunity since the impairment of only Fas function on B cells results in uncontrolled autoantibody production and autoimmunity. Fas plays a role in the elimination of the non-specific and autoreactive B cells in germinal center, while during the selection of antigen-specific B cells different escape signals ensure the resistance to Fas-mediated apoptosis. Antigen-specific survival such as BCR or MHCII signal or coreceptors (CD19) cooperating with BCR inhibits the formation of death inducing signaling complex. Antigen-specific survival can be reinforced by antigen-independent signals of IL-4 or CD40 overproducing the anti-apoptotic members of the Bcl-2 family proteins.

Necroptosis is a regulated necrotic-like cell death modality which has come into the focus of attention since it is known to contribute to the pathogenesis of many inflammatory and degenerative diseases as well as to tumor regulation. Based on current data, necroptosis serves as a backup mechanism when death receptor-induced apoptosis is inhibited or absent. However, the necroptotic role of the proteins involved in mitochondrial apoptosis has not been investigated. Here, we demonstrated that the stimulation of several death and pattern recognition receptors induced necroptosis under caspase-compromised conditions in wild-type, but not in caspase-9-negative human Jurkat and murine MEF cells. Cerulein-induced pancreatitis was significantly reduced in mice with acinar cell-restricted caspase-9 gene knockout. The absence of caspase-9 led to impaired association of receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3 and resulted in decreased phosphorylation of RIP kinases, but the overexpression of RIPK1 or RIPK3 rescued the effect of caspase-9 deficiency. Inhibition of either Aurora kinase A (AURKA) or its known substrate, glycogen synthase kinase 3b (GSK3ß) restored necroptosis sensitivity of caspase-9-deficient cells, indicating an interplay between caspase-9 and AURKA-mediated pathways to regulate necroptosis. Our findings suggest that caspase-9 acts as a newly identified regulator of necroptosis, and thus, caspase-9 provides a promising therapeutic target to manipulate the immunological outcome of cell death.

Activated T cells secrete Fas ligand (FasL)-containing vesicles (secreted vesicles) that induce death of target cells. We provide evidence that secreted vesicles from culture supernatants (Csup) of various origins are able to generate both Fas-dependent apoptotic and Fas-independent, nonapoptotic cell death. In the absence of Fas, the nonapoptotic, Fas-independent pathway could still induce cell death. In contrast to RIP-independent classical Fas-induced cell death triggered by cross-linked or membrane-bound FasL, CSup-derived stimuli-induced apoptosis exhibited unique molecular and enzymatic characteristics. It could be partially inhibited by blocking cathepsin D enzyme activity and required the presence of RIP. Whereas stimulation with CSup, derived from both FasL-overexpressing Jurkat cells and PBMC, could induce cell death, the requirements for Fas-associated death domain protein and caspase-9 were different between the two systems. Our study highlights an important distinction between cell contact-mediated and secreted vesicle-generated activation-induced cell death and also demonstrates that the type of the secreted vesicles can also modify the cell death route. We propose that besides cell-to-cell interaction-mediated Fas triggering, stimuli induced by secreted vesicles can mediate important additional cell death signals regulating activation-induced cell death under physiological conditions.

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