FAP-1: a protein tyrosine phosphatase that associates with Fas

Life, death, and the pursuit of apoptosis

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A novel function of CD40: induction of cell death in transformed cells

CD40 is known as an important T-B cell interaction molecule which rescues B lymphocytes from undergoing apoptosis. Like other receptors of the tumor necrosis factor (TNF) receptor gene family, CD40 is expressed on cells of different tissue origins including some transformed cells. In contrast to its well-studied effects on B cells, the biological functions of CD40 in non-immune cells remain largely unknown. Here we show that CD40 ligation induces apoptotic cell death in transformed cells of mesenchymal and epithelial origin. This CD40-mediated cell death seems to use a preformed signaling pathway since it occurs even when protein synthesis is blocked. Notably, the CD40 cytoplasmic domain shares a structural homology with the recently defined "death domains" of the 55-kD TNF receptor (p55TNFR) and Fas. Despite these structural similarities, differences are seen in the way phorbol myristate acetate, interleukin 1, TNF, and various metabolic inhibitors influence the cellular responsiveness to CD40, p55TNFR, and Fas-mediated killing. Our study indicates that CD40 induces cell death by a distinct mechanism.

Induction of cell death via Fas (CD95, Apo-1) may be associated with but is not dependent on Fas-induced tyrosine phosphorylation

Cross-linking of the Fas-antigen (CD95, Apo-1) triggers apoptosis in activated T cells and transformed T cell lines. Fas-induced apoptosis has been previously reported to require Fas-triggered tyrosine phosphorylation of various proteins. In the present study, we have compared the protein tyrosine phosphorylation pattern and the apoptosis sensitivity in a set of Jurkat variants selected for the absence or presence of T cell receptor (TCR)/CD3 expression and resistance or sensitivity to Fas-mediated apoptosis. While tyrosine phosphorylation upon Fas-ligation was readily apparent in wild-type Jurkat cells (which are sensitive to anti-Fas-induced apoptosis), drastically reduced tyrosine phosphorylation was observed in Fas-resistant Jurkat subclones (which still express CD95 on their surface). More importantly, TCR/CD3-negative Jurkat variants which expressed normal levels of CD95 and were fully susceptible to Fas-triggered cell death, did not show any protein tyrosine phosphorylation upon Fas-ligation. Taken together, our data demonstrate that Fas-induced cell death can be associated with but is not dependent on protein tyrosine phosphorylation.

The TNFR2-TRAF signaling complex contains two novel proteins related to baculoviral inhibitor of apoptosis proteins

The 75 kDa tumor necrosis factor receptor (TNFR2) transduces extracellular signals via receptor-associated cytoplasmic proteins. Two of these signal transducers, TRAF1 and TRAF2, were isolated and characterized previously. We report here the biochemical purification and subsequent molecular cloning of two novel TNFR2-associated proteins, designated c-IAP1 and c-IAP2, that are closely related mammalian members of the inhibitor of apoptosis protein (IAP) family originally identified in baculoviruses. The viral and cellular IAPs contain N-terminal baculovirus IAP repeat (BIR) motifs and a C-terminal RING finger. The c-IAPs do not directly contact TNFR2, but rather associate with TRAF1 and TRAF2 through their N-terminal BIR motif-comprising domain. The recruitment of c-IAP1 or c-IAP2 to the TNFR2 signaling complex requires a TRAF2-TRAF1 heterocomplex.

Neural apoptosis

Apoptosis is a mode of cell death in which the cell plays an active role in its own demise. The study of neural apoptosis, the identification of genes controlling apoptosis, and the examination of the mechanisms by which these genes achieve their effects have assumed increasing importance over the past few years. This is because (1) neural apoptosis occurs not only in development, but also in pathophysiological states such as stroke, glutamate toxicity, and beta-amyloid peptide toxicity; (2) genes that control apoptotic cell death, such as bcl-2, p35, p53, and p75NTR, also modulate necrotic neural death in some cases; (3) the emerging mechanisms by which these genes control apoptosis may be relevant for understanding neurodegenerative processes, and for the design of therapeutic agents; and (4) the findings that the cell plays an active role in its own demise, and that specific gene products are involved, suggest that therapeutic intervention may be feasible.

Fas and FasL in the homeostatic regulation of immune responses

Studies of the biological effects of Fas signaling, using transformed cell lines as targets, indicate that ligation of the Fas receptor induces an apoptotic death signal. Chronically activated normal human T cells are also susceptible to Fas-mediated apoptosis. However, interactions between Fas and Fas ligand can also yield a costimulatory signal. Here, David Lynch, Fred Ramsdell and Mark Alderson present a model for the role of As and FasL in the homeostatic regulation of normal immune responses. They discuss how dysregulation of the Fas apoptotic pathway may contribute to certain disease states, including autoimmune disease and human immunodeficiency virus (HIV)-induced depletion of CD4+ T cells.

T helper type 1/T helper type 2 cytokines and T cell death: preventive effect of interleukin 12 on activation-induced and CD95 (FAS/APO-1)-mediated apoptosis of CD4+ T cells from human immunodeficiency virus-infected persons

Human immunodeficiency virus (HIV) infection leads to a progressive loss of CD4+ T helper (Th) type 1 cell-mediated immunity that is associated with defective in vitro CD4+ T cell proliferation and abnormal T cell death by apoptosis in response to T cell receptor (TCR) stimulation. Quantification of interleukin (IL)-2, interferon gamma, IL-4, IL-5, and IL-10 secretion by immunoassays, and of interferon gamma, IL-4 and IL-10 messenger RNA expression by competitive reverse transcriptase polymerase chain reaction after in vitro stimulation of the TCR revealed a similar Th1 cytokine profile in T cells from HIV-infected persons and from controls. These data indicated that the loss of CD4+ Th1 cell function in HIV-infected persons is not related to a Th1 to Th2 cytokine switch as previously proposed, but to a process of activation-induced death of CD4+ Th1 cells. Despite the absence of elevated levels of Th2 cytokines, apoptosis of CD4+ T cells, but not of CD8+ T cells, was prevented in vitro by antibodies to IL-10 or IL-4, two Th2 cytokines that downregulate Th1 cell responses, or by the addition of recombinant IL-12, a cytokine that upregulates Th1 functions. TCR-induced apoptosis of T cell hybridomas and preactivated T cells has been shown to involve the CD95 (Fas/Apo-1) molecule. CD4+ and CD8+ T cells from HIV-infected persons expressed high levels of the CD95 molecule, and, in contrast to T cells from controls, were highly sensitive to antibody-mediated CD95 ligation, which induced apoptosis in a percentage of T cells similar to that induced by TCR stimulation. As TCR-induced apoptosis, CD95-mediated apoptosis of CD4+ T cells, but not of CD8+ T cells, was prevented by the addition of recombinant IL-12. Together, these findings suggest that apoptosis of CD4+ T cells from HIV-infected persons involves an abnormal sensitivity to CD95 ligation, and to TCR stimulation in the presence of normal levels of Th2 cytokines. The preventive effect of IL-12 on both mechanisms has potential implications for the design of immunotherapy strategies aimed at the upregulation of CD4+ Th1 cell functions in AIDS.

Th1 CD4+ lymphocytes delete activated macrophages through the Fas/APO-1 antigen pathway

The Fas/APO-1 cytotoxic pathway plays an important role in the regulation of peripheral immunity. Recent evidence indicates that this regulatory function operates through deletion of activated T and B lymphocytes by CD4+ T cells expressing the Fas ligand. Because macrophages play a key role in peripheral immunity, we asked whether Fas was involved in T-cell-macrophage interactions. Two-color flow cytometry revealed that Fas receptor (FasR) was expressed on resting murine peritoneal macrophages. FasR expression was upregulated after activation of macrophages with cytokines or lipopolysaccharide, although only tumor necrosis factor-alpha rendered macrophages sensitive to anti-FasR antibody-mediated death. To determine the consequence of antigen presentation by macrophages to CD4+ T cells, macrophages were pulsed with antigen and then incubated with either Th1 or Th2 cell lines or clones. Th1, but not Th2, T cells induced lysis of 60-80% of normal macrophages, whereas macrophages obtained from mice with mutations in the FasR were totally resistant to Th1-mediated cytotoxicity. Macrophage cytotoxicity depended upon specific antigen recognition by T cells and was major histocompatibility complex restricted. These findings indicate that, in addition to deletion of activated lymphocytes, Fas plays an important role in deletion of activated macrophages after antigen presentation to Th1 CD4+ T cells. Failure to delete macrophages that constitutively present self-antigens may contribute to the expression of autoimmunity in mice deficient in FasR (lpr) or Fas ligand (gld).

Cytotoxicity-dependent APO-1 (Fas/CD95)-associated proteins form a death-inducing signaling complex (DISC) with the receptor

APO-1 (Fas/CD95), a member of the tumor necrosis factor receptor superfamily, induces apoptosis upon receptor oligomerization. In a search to identify intracellular signaling molecules coupling to oligomerized APO-1, several cytotoxicity-dependent APO-1-associated proteins (CAP) were immunoprecipitated from the apoptosis-sensitive human leukemic T cell line HUT78 and the lymphoblastoid B cell line SKW6.4. CAP1-3 (27-29 kDa) and CAP4 (55 kDa), instantly detectable after the crosslinking of APO-1, were associated only with aggregated (the signaling form of APO-1) and not with monomeric APO-1. CAP1 and CAP2 were identified as serine phosphorylated MORT1/FADD. The association of CAP1-4 with APO-1 was not observed with C-terminally truncated non-signaling APO-1. In addition, CAP1 and CAP2 did not associate with an APO-1 cytoplasmic tail carrying the lprcg amino acid replacement. Moreover, no APO-1-CAP association was found in the APO-1+, anti-APO-1-resistant pre-B cell line Boe. Our data suggest that in vivo CAP1-4 are the APO-1 apoptosis-transducing molecules.

Domain interaction between NMDA receptor subunits and the postsynaptic density protein PSD-95

The N-methyl-D-aspartate (NMDA) receptor subserves synaptic glutamate-induced transmission and plasticity in central neurons. The yeast two-hybrid system was used to show that the cytoplasmic tails of NMDA receptor subunits interact with a prominent postsynaptic density protein PSD-95. The second PDZ domain in PSD-95 binds to the seven-amino acid, COOH-terminal domain containing the terminal tSXV motif (where S is serine, X is any amino acid, and V is valine) common to NR2 subunits and certain NR1 splice forms. Transcripts encoding PSD-95 are expressed in a pattern similar to that of NMDA receptors, and the NR2B subunit co-localizes with PSD-95 in cultured rat hippocampal neurons. The interaction of these proteins may affect the plasticity of excitatory synapses.

Role of apoptosis in HIV disease pathogenesis

After approximately one and a half decades of intensive studies, the exact mechanisms to explain HIV-mediated cytopathicity are still enigmatic and need closer scrutiny. There has been a dichotomy between virological and immunological viewpoints in understanding HIV-mediated cytopathicity, the former emphasizing a killing of infected cells by HIV-1 and the latter emphasizing indirect mechanisms wherein HIV or its soluble component(s) alter CD4 T-cell function and induce susceptibility to apoptosis. Accumulating evidence points to the notion that apoptosis might be a major contributor to the depletion of CD4 T-cells in HIV infection. This review summarizes current information about the regulatory mechanisms of T-cell apoptosis and the role of apoptosis in HIV pathogenesis with the goal of providing an integrated view of HIV cytopathicity.

APO-1(CD95)-mediated apoptosis in Jurkat cells does not involve src kinases or CD45

Tyrosine phosphorylation has been reported to be an early event required for APO-1/Fas(CD95) signalling in lymphocytes [Eischen, C.M., Dick, C.J. and Leibson, P.J. (1994) J. Immunol. 153, 1947-1954]. We have compared two mutant Jurkat cells, one largely deficient in expression of CD-45 (J45.01) and a second one deficient in expression of p56lck (JCaM1.6) with wild type Jurkat cells for their ability to undergo APO-1-induced apoptosis. No significant difference was observed among the three cell lines. In the mutant Jurkat cells APO-1 triggering did not result in increased tyrosine phosphorylation of cytosolic proteins. Furthermore, herbimycin A did not inhibit but rather augmented apoptosis at concentrations which effectively degraded the src related kinases lck and fyn. The data suggest that APO-1-mediated signalling is independent from src kinases and CD45.