Margot Thome

Inhibition of death receptor signals by cellular FLIP

The widely expressed protein Fas is a member of the tumour necrosis factor receptor family which can trigger apoptosis. However, Fas surface expression does not necessarily render cells susceptible to Fas ligand-induced death signals,, indicating that inhibitors of the apoptosis-signalling pathway must exist. Here we report the characterization of an inhibitor of apoptosis, designated FLIP (for FLICE-inhibitory protein), which is predominantly expressed in muscle and lymphoid tissues. The short form, FLIPS, contains two death effector domains and is structurally related to the viral FLIP inhibitors of apoptosis, whereas the long form, FLIPL, contains in addition a caspase-like domain in which the active-centre cysteine residue is substituted by a tyrosine residue. FLIPS and FLIPL interact with the adaptor protein FADD, and the protease FLICE,, and potently inhibit apoptosis induced by all known human death receptors. FLIPL is expressed during the early stage of T-cell activation, but disappears when T cells become susceptible to Fas ligand-mediated apoptosis. High levels of FLIPL protein are also detectable in melanoma cell lines and malignant melanoma tumours. Thus FLIP may be implicated in tissue homeostasis as an important regulator of apoptosis.

Fas triggers an alternative, caspase-8–independent cell death pathway using the kinase RIP as effector molecule

Cell death is achieved by two fundamentally different mechanisms: apoptosis and necrosis. Apoptosis is dependent on caspase activation, whereas the caspase-independent necrotic signaling pathway remains largely uncharacterized. We show here that Fas kills activated primary T cells efficiently in the absence of active caspases, which results in necrotic morphological changes and late mitochondrial damage but no cytochrome c release. This Fas ligand–induced caspase-independent death is absent in T cells that are deficient in either Fas-associated death domain (FADD) or receptor-interacting protein (RIP). RIP is also required for necrotic death induced by tumor necrosis factor (TNF) and TNF-related apoptosis-inducing ligand (TRAIL). In contrast to its role in nuclear factor κB activation, RIP requires its own kinase activity for death signaling. Thus, Fas, TRAIL and TNF receptors can initiate cell death by two alternative pathways, one relying on caspase-8 and the other dependent on the kinase RIP.

TRAIL Receptors 1 (DR4) and 2 (DR5) Signal FADD-Dependent Apoptosis and Activate NF-κB

TRAIL induces apoptosis through two closely related receptors, TRAIL-R1 (DR4) and TRAIL-R2 (DR5). Here we show that TRAIL-R1 can associate with TRAIL-R2, suggesting that TRAIL may signal through heteroreceptor signaling complexes. Both TRAIL receptors bind the adaptor molecules FADD and TRADD, and both death signals are interrupted by a dominant negative form of FADD and by the FLICE-inhibitory protein FLIP. The recruitment of TRADD may explain the potent activation of NF-kappaB observed by TRAIL receptors. Thus, TRAIL receptors can signal both death and gene transcription, functions reminiscent of those of TNFR1 and TRAMP, two other members of the death receptor family.

The caspase-8 inhibitor FLIP promotes activation of NF-κB and Erk signaling pathways

BACKGROUND Activation of Fas (CD95) by its ligand (FasL) rapidly induces cell death through recruitment and activation of caspase-8 via the adaptor protein Fas-associated death domain protein (FADD). However, Fas signals do not always result in apoptosis but can also trigger a pathway that leads to proliferation. We investigated the level at which the two conflicting Fas signals diverge and the protein(s) that are implicated in switching the response. RESULTS Under conditions in which proliferation of CD3-activated human T lymphocytes is increased by recombinant FasL, there was activation of the transcription factors NF-kappaB and AP-1 and recruitment of the caspase-8 inhibitor and FADD-interacting protein FLIP (FLICE-like inhibitory protein). Fas-recruited FLIP interacts with TNF-receptor associated factors 1 and 2, as well as with the kinases RIP and Raf-1, resulting in the activation of the NF-kappaB and extracellular signal regulated kinase (Erk) signaling pathways. In T cells these two signal pathways are critical for interleukin-2 production. Increased expression of FLIP in T cells resulted in increased production of interleukin-2. CONCLUSIONS We provide evidence that FLIP is not simply an inhibitor of death-receptor-induced apoptosis but that it also mediates the activation of NF-kappaB and Erk by virtue of its capacity to recruit adaptor proteins involved in these signaling pathways.

Inhibition of Fas death signals by FLIPs

The death receptor Fas is a member of the tumor necrosis factor receptor family; upon interaction with its ligand it efficiently activates caspases and induces apoptosis. Despite abundant Fas surface expression, however, Fas death-signals are frequently interrupted. Many viruses express antiapoptotic proteins, including caspase inhibitors, Bcl-2 homologues and death-effector-domain-containing proteins that are termed FLIPs (FLICE [Fas-associated death-domain-like IL-1beta-converting enzyme]-inhibitory proteins). Cellular homologues of these inhibitors have been identified. Cellular FLIPs structurally resemble caspase-8 except that they lack proteolytic activity. FLIPs are highly expressed in tumor cells, T lymphocytes and healthy, but not injured, myocytes; this suggests a critical role of FLIPs as endogenous modulators of apoptosis.

Regulation of lymphocyte proliferation and death by FLIP.

Lymphocyte homeostasis is a balance between lymphocyte proliferation and lymphocyte death. Tight control of apoptosis is essential for immune function, because its altered regulation can result in cancer and autoimmunity. Signals from members of the tumour-necrosis-factor receptor (TNF-R) family, such as Fas and TNF-R1, activate the caspase cascade and result in lymphocyte death by apoptosis. Anti-apoptotic proteins, such as FLIP (also known as FLICE/caspase-8 inhibitory protein) have recently been identified. FLIP expression is tightly regulated in T cells and might be involved in the control of both T-cell activation and death. Abnormal expression of FLIP might have a role not only in autoimmune diseases, but also in tumour development and cardiovascular disorders.

CARMA1, BCL-10 and MALT1 in lymphocyte development and activation

CARMA1, BCL-10 and MALT1 are signalling proteins that have a key role in antigen-receptor-mediated lymphocyte activation through the nuclear factor-κB pathway. Recent genetic studies have revealed additional, previously unexpected roles for these proteins in the development of B and T cells, and in the CD40- and lipopolysaccharide-dependent activation of B cells. Here, I discuss recent advances in the understanding of the molecular and biological functions of these proteins.

CARMA1 is a critical lipid raft–associated regulator of TCR-induced NF-κB activation

CARMA1 is a lymphocyte-specific member of the membrane-associated guanylate kinase (MAGUK) family of scaffolding proteins, which coordinate signaling pathways emanating from the plasma membrane. CARMA1 interacts with Bcl10 via its caspase-recruitment domain (CARD). Here we investigated the role of CARMA1 in T cell activation and found that T cell receptor (TCR) stimulation induced a physical association of CARMA1 with the TCR and Bcl10. We found that CARMA1 was constitutively associated with lipid rafts, whereas cytoplasmic Bcl10 translocated into lipid rafts upon TCR engagement. A CARMA1 mutant, defective for Bcl10 binding, had a dominant-negative (DN) effect on TCR-induced NF-κB activation and IL-2 production and on the c-Jun NH2-terminal kinase (Jnk) pathway when the TCR was coengaged with CD28. Together, our data show that CARMA1 is a critical lipid raft–associated regulator of TCR-induced NF-κB activation and CD28 costimulation–dependent Jnk activation.

Characterization of two receptors for TRAIL1

Two receptors for TRAIL, designated TRAIL‐R2 and TRAIL‐R3, have been identified. Both are members of the tumor necrosis factor receptor family. TRAIL‐R2 is structurally similar to the death‐domain‐containing receptor TRAIL‐R1 (DR‐4), and is capable of inducing apoptosis. In contrast, TRAIL‐R3 does not promote cell death. TRAIL‐R3 is highly glycosylated and is membrane bound via a putative phosphatidylinositol anchor. The extended structure of TRAIL‐R3 is due to the presence of multiple threonine‐, alanine‐, proline‐ and glutamine‐rich repeats (TAPE repeats). TRAIL‐R2 shows a broad tissue distribution, whereas the expression of TRAIL‐R3 is restricted to peripheral blood lymphocytes (PBLs) and skeletal muscle. All three TRAIL receptors bind TRAIL with similar affinity, suggesting a complex regulation of TRAIL‐mediated signals.

TRAMP, a Novel Apoptosis-Mediating Receptor with Sequence Homology to Tumor Necrosis Factor Receptor 1 and Fas(Apo-1/CD95)

A novel member of the tumor necrosis factor (TNF) receptor family, designated TRAMP, has been identified. The structural organization of the 393 amino acid long human TRAMP is most homologous to TNF receptor 1. TRAMP is abundantly expressed on thymocytes and lymphocytes. Its extracellular domain is composed of four cysteine-rich domains, and the cytoplasmic region contains a death domain known to signal apoptosis. Overexpression of TRAMP leads to two major responses, NF-kappaB activation and apoptosis. TRAMP-induced cell death is inhibited by an inhibitor of ICE-like proteases, but not by Bcl-2. In addition, TRAMP does not appear to interact with any of the known apoptosis-inducing ligands of the TNF family.