Secondary literature sources for the SEP domain

For each of the hand selected primary papers associated with the SEP domain, 100 neighbouring papers are retrieved from PubMed. If one of these neighbouring papers is referenced by more than two primary papers, it is shown in the table below.

Fas-associated factor 1 antagonizes Wnt signaling by promoting beta-catenin degradation.

Zhang L, Zhou F, van Laar T, Zhang J, van Dam H, Ten Dijke P
Mol Biol Cell. 2011; 22: 1617-24

The canonical Wnt pathway plays an important role in the regulation of cell proliferation and differentiation. Activation of this signaling pathway causes disruption of the Axin/adenomatous polyposis coli/glycogen synthase kinase 3beta complex, resulting in stabilization of beta-catenin and its association with lymphoid enhancer factor/T-cell factor in the nucleus. Here, we identify Fas-associated factor 1 (FAF1) as a negative regulator of Wnt/beta-catenin signaling. We found overexpression of FAF1 to strongly inhibit Wnt-induced transcriptional reporter activity and to counteract Wnt-induced beta-catenin accumulation. Moreover, knockdown of FAF1 resulted in an increase in beta-catenin levels and in activation of Wnt/beta-catenin-induced transcription. FAF1 was found to interact with beta-catenin upon inhibition of proteasome. Ectopic expression of FAF1 promoted beta-catenin degradation by enhancing its polyubiquitination. Functional studies in C2C12 myoblasts and KS483 preosteoblastic cells showed that FAF1 depletion resulted in activation of endogenous Wnt-induced genes and enhanced osteoblast differentiation, whereas FAF1 overexpression had the opposite effect. These results identify FAF1 as a novel inhibitory factor of canonical Wnt signaling pathway.

Hepatitis B virus X protein induces expression of Fas ligand gene through enhancing transcriptional activity of early growth response factor.

Yoo YG, Lee MO
J Biol Chem. 2004; 279: 36242-9

FasL expressed in tumor cells plays an important role in the escape from immune surveillance by inducing apoptosis in T-cells bearing Fas. Since the Fas/FasL signaling pathway requires transcriptional induction of the FasL gene, elucidation of the precise mechanisms underlying regulation of FasL gene expression may provide useful molecular insights on tumor progression. We and others (Shin, E. C., Shin, J. S., Park, J. H., Kim, H., and Kim, S. J. (1999) Int. J. Cancer 82, 587-591; Lee, M. O., Kang, H. J., Cho, H., Shin, E. C., Park, J. H., and Kim, S. J. (2001) Biochem. Biophys. Res. Commun. 288, 1162-1168) have previously reported that hepatitis B virus X protein (HBx) plays a role in the induction of FasL expression in hepatitis B virus-associated hepatoma. In the present study, we analyzed the potential cis- and trans-acting factors that regulate FasL promoter. We found that HBx induced activity of the reporter containing FasL promoter through binding site for Egr but not through NFAT or SP-1, which are known as strong activators of the FasL promoter in T-cells. Transient expression of antisense Egr-2 and antisense Egr-3 abolished expression of FasL, which further confirmed the role of Egr in the HBx-mediated FasL expression. Also we observed that HBx increased the transcriptional activity of Egr-2 and Egr-3 by enhancing expression as well as the transactivation function of these proteins. HBx interacted with Egr-2 and Egr-3 in vivo and enhanced binding of Egr to the co-activator, cAMP-response element-binding protein-binding protein, which may explain the molecular mechanism by which HBx induced the transactivation function of Egr. Finally, we found that the carboxyl terminus of HBx was necessary and sufficient for FasL induction as well as activation of Egr. Taken together, our results show a novel mechanism by which HBx induces FasL gene expression that is mediated by enhancing transcriptional activity of Egr-2 and Egr-3.

The molecular interaction of Fas and FAP-1. A tripeptide blocker of human Fas interaction with FAP-1 promotes Fas-induced apoptosis.

Yanagisawa J, Takahashi M, Kanki H, Yano-Yanagisawa H, Tazunoki T, Sawa E, Nishitoba T, Kamishohara M, Kobayashi E, Kataoka S, Sato T
J Biol Chem. 1997; 272: 8539-45

Fas (APO-1/CD95), which is a member of the tumor necrosis factor receptor superfamily, is a cell surface receptor that induces apoptosis. A protein tyrosine phosphatase, Fas-associated phosphatase-1 (FAP-1), that was previously identified as a Fas binding protein interacts with the C-terminal 15 amino acids of the regulatory domain of the Fas receptor. To identify the minimal region of the Fas C-terminal necessary for binding to FAP-1, we employed an in vitro inhibition assay of Fas/FAP-1 binding using a series of synthetic peptides as well as a screen of random peptide libraries by the yeast two-hybrid system. The results showed that the C-terminal three amino acids (SLV) of human Fas were necessary and sufficient for its interaction with the third PDZ (GLGF) domain of FAP-1. Furthermore, the direct cytoplasmic microinjection of this tripeptide (Ac-SLV) resulted in the induction of Fas-mediated apoptosis in a colon cancer cell line that expresses both Fas and FAP-1. Since t(S/T)X(V/L/I) motifs in the C termini of several other receptors have been shown to interact with PDZ domain in signal transducing molecules, this may represent a general motif for protein-protein interactions with important biological functions.

Dominant-negative FADD inhibits TNFR60-, Fas/Apo1- and TRAIL-R/Apo2-mediated cell death but not gene induction.

Wajant H, Johannes FJ, Haas E, Siemienski K, Schwenzer R, Schubert G, Weiss T, Grell M, Scheurich P
Curr Biol. 1998; 8: 113-6

Fas/Apo1 and other cytotoxic receptors of the tumor necrosis factor receptor (TNFR) family contain a cytoplasmic death domain (DD) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] that activates the apoptotic process by interacting with the DD-containing adaptor proteins TNFR-associated DD protein (TRADD) [12] [13] and Fas-associated DD protein (FADD/MORT1) [14] [15], leading to the activation of cysteine proteases of the caspase family [16]. Stimulation of Fas/Apo1 leads to the formation of a receptor-bound death-inducing signaling complex (DISC), consisting of FADD and two different forms of caspase-8 [17] [18] [19]. Transient expression of a dominant-negative mutant of FADD impairs TNFR60-mediated and Fas/Apo1-mediated apoptosis [13] [20], but has no effect on TNF-related apoptosis-inducing ligand (TRAIL/Apo2L)-induced cell death [7] [8] [9] [10] [21]. To study the function of FADD in DD-receptor signaling in more detail, we established HeLa cells that stably expressed a green fluorescent protein (GFP)-tagged dominant-negative mutant of FADD, GFP-DeltaFADD. Interestingly, expression of this mutant inhibited cell death induced by TNFR60, Fas/Apo1 and TRAIL-R/Apo2. In addition, GFP-DeltaFADD did not interfere with TNF-mediated gene induction or with activation of NF-kappaB or Jun N-terminal kinase (JNK), demonstrating that FADD is part of the TNFR60-initiated apoptotic pathway but does not play a role in TNFR60-mediated gene induction. Fas/Apo1-mediated activation of JNK was unaffected by the expression of GFP-DeltaFADD, suggesting that in Fas/Apo1 signaling the apoptotic pathway and the activation of JNK diverge at a level proximal to the receptor, upstream of or parallel to FADD.

Molecular cloning and expression of the Fas ligand, a novel member of the tumor necrosis factor family.

Suda T, Takahashi T, Golstein P, Nagata S
Cell. 1993; 75: 1169-78

The Fas antigen (Fas) belongs to the tumor necrosis factor (TNF)/nerve growth factor receptor family, and it mediates apoptosis. Using a soluble form of mouse Fas, prepared by fusion with human immunoglobulin Fc, Fas ligand was detected on the cell surface of a cytotoxic T cell hybridoma, PC60-d10S. A cell population that highly expresses Fas ligand was sorted using a fluorescence-activated cell sorter, and its cDNA was isolated from the sorted cells by expression cloning. The amino acid sequence indicated that Fas ligand is a type II transmembrane protein that belongs to the TNF family. The recombinant Fas ligand expressed in COS cells induced apoptosis in Fas-expressing target cells. Northern hybridization revealed that Fas ligand is expressed in activated splenocytes and thymocytes, consistent with its involvement in T cell-mediated cytotoxicity and in several nonlymphoid tissues, such as testis.

Purification and characterization of the Fas-ligand that induces apoptosis.

Suda T, Nagata S
J Exp Med. 1994; 179: 873-9

Fas is a 45-kD cell surface protein belonging to the tumor necrosis factor/nerve growth factor receptor family, and transduces the signal for apoptosis. The cytotoxic T lymphocyte (CTL) hybridoma, PC60-d10S requires the presence of Fas on target cells to induce cytolysis in target cells. This CTL cell line was weakly but specifically stained by a chimeric protein that consisted of the extracellular domain of mouse Fas and the Fc portion of human immunoglobulin G1 (mFas-Fc). Moreover, mFas-Fc inhibited the cytotoxic activity of PC60-d10S. Sublines of d10S that were stained intensively by mFas-Fc were isolated by repetitive fluorescence-activated cell sorter sorting. A cell-surface protein of about 40 kD was specifically precipitated by mFas-Fc from the lysates of these sublines. This protein was homogeneously purified by sequential affinity chromatographies using mFas-Fc and concanavalin A beads. The purified protein exhibited cytotoxic activity against cells expressing Fas but not to the cells which do not express Fas. These results indicated that the 40-kD membrane glycoprotein expressed on PC60-d10S cells is the Fas-ligand that induces the apoptotic signal by binding to Fas.

Human Fas-associated factor 1, interacting with ubiquitinated proteins and valosin-containing protein, is involved in the ubiquitin-proteasome pathway.

Song EJ, Yim SH, Kim E, Kim NS, Lee KJ
Mol Cell Biol. 2005; 25: 2511-24

Human Fas-associated factor 1 (hFAF1) is a novel protein having multiubiquitin-related domains. We investigated the cellular functions of hFAF1 and found that valosin-containing protein (VCP), the multiubiquitin chain-targeting factor in the degradation of the ubiquitin-proteasome pathway, is a binding partner of hFAF1. hFAF1 is associated with the ubiquitinated proteins via the newly identified N-terminal UBA domain and with VCP via the C-terminal UBX domain. The overexpression of hFAF1 and a truncated UBA domain inhibited the degradation of ubiquitinated proteins and increased cell death. These results suggest that hFAF1 binding to ubiquitinated protein and VCP is involved in the ubiquitin-proteasome pathway. We hypothesize that hFAF1 may serve as a scaffolding protein that regulates protein degradation in the ubiquitin-proteasome pathway.

Identification of p18 INK4c as a tumor suppressor gene in glioblastoma multiforme.

Solomon DA, Kim JS, Jenkins S, Ressom H, Huang M, Coppa N, Mabanta L, Bigner D, Yan H, Jean W, Waldman T
Cancer Res. 2008; 68: 2564-9

Genomic alterations leading to aberrant activation of cyclin/cyclin-dependent kinase (cdk) complexes drive the pathogenesis of many common human tumor types. In the case of glioblastoma multiforme (GBM), these alterations are most commonly due to homozygous deletion of p16(INK4a) and less commonly due to genomic amplifications of individual genes encoding cyclins or cdks. Here, we describe deletion of the p18(INK4c) cdk inhibitor as a novel genetic alteration driving the pathogenesis of GBM. Deletions of p18(INK4c) often occurred in tumors also harboring homozygous deletions of p16(INK4a). Expression of p18(INK4c) was completely absent in 43% of GBM primary tumors studied by immunohistochemistry. Lentiviral reconstitution of p18(INK4c) expression at physiologic levels in p18(INK4c)-deficient but not p18(INK4c)-proficient GBM cells led to senescence-like G(1) cell cycle arrest. These studies identify p18(INK4c) as a GBM tumor suppressor gene, revealing an additional mechanism leading to aberrant activation of cyclin/cdk complexes in this terrible malignancy.

Fas-associated death domain protein (FADD) and caspase-8 mediate up-regulation of c-Fos by Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) via a FLICE inhibitory protein (FLIP)-regulated pathway.

Siegmund D, Mauri D, Peters N, Juo P, Thome M, Reichwein M, Blenis J, Scheurich P, Tschopp J, Wajant H
J Biol Chem. 2001; 276: 32585-90

Fas, a death domain-containing member of the tumor necrosis factor receptor family and its ligand FasL have been predominantly studied with respect to their capability to induce cell death. However, a few studies indicate a proliferation-inducing signaling activity of these molecules too. We describe here a novel signaling pathway of FasL and the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) that triggers transcriptional activation of the proto-oncogene c-fos, a typical target gene of mitogenic pathways. FasL- and TRAIL-mediated up-regulation of c-Fos was completely dependent on the presence of Fas-associated death domain protein (FADD) and caspase-8, but caspase activity seemed to be dispensable as a pan inhibitor of caspases had no inhibitory effect. Upon overexpression of the long splice form of cellular FADD-like interleukin-1-converting enzyme (FLICE) inhibitory protein (cFLIP) in Jurkat cells, FasL- and TRAIL-induced up-regulation of c-Fos was almost completely blocked. The short splice form of FLIP, however, showed a rather stimulatory effect on c-Fos induction. Together these data demonstrate the existence of a death receptor-induced, FADD- and caspase-8-dependent pathway leading to c-Fos induction that is inhibited by the long splice form FLIP-L.

Crystallization and preliminary X-ray crystallographic analysis of the N domain of p97/VCP in complex with the UBX domain of FAF1.

Shin HY, Kang W, Lee SY, Yang JK
Acta Crystallogr Sect F Struct Biol Cryst Commun. 2010; 66: 41-3

p97/VCP is a multifunctional AAA(+)-family ATPase that is involved in diverse cellular processes. p97/VCP directly interacts with various adaptors for activity in different biochemical contexts. Among these adaptors are p47 and Fas-associated factor 1 (FAF1), which contain a common UBX domain through which they bind to the N domain of p97/VCP. In the ubiquitin-proteasome pathway, p97/VCP acts as a chaperone that presents client proteins to the proteasome for degradation, while FAF1 modulates the process by interacting with ubiquitinated client proteins and also with p97/VCP. In an effort to elucidate the structural details of the interaction between p97/VCP and FAF1, the p97/VCP N domain was crystallized in complex with the FAF1 UBX domain. X-ray data were collected to 2.60 A resolution and the crystals belonged to space group C222(1), with unit-cell parameters a = 58.24, b = 72.81, c = 132.93 A. The Matthews coefficient and solvent content were estimated to be 2.39 A(3) Da(-1) and 48.4%, respectively, assuming that the asymmetric unit contained p97/VCP N domain and FAF1 molecules in a 1:1 ratio, which was subsequently confirmed by molecular-replacement calculations.

Soluble HLA class I molecules/CD8 ligation trigger apoptosis of CD8+ cells by Fas/Fas-ligand interaction.

Puppo F, Contini P, Ghio M, Indiveri F
ScientificWorldJournal. 2002; 2: 421-3

The human major histocompatibility complex (HLA) encodes two sets of HLA class I molecules, which have been termed class Ia (or classical) and class Ib (or nonclassical) molecules. The class Ia molecules include the gene products of HLA-A, HLA-B, and HLA-C loci and are characterized by broad tissue expression and by a high degree of polymorphism. The class Ib molecules include the gene products of HLA-E, HLA-F, and HLA-G loci and are characterized by a restricted tissue distribution and by limited polymorphism. Besides being expressed on nucleated cells, classical and nonclassical HLA class I molecules are present in serum in soluble form (sHLA-I). The serum level of sHLA-I molecules is significantly increased in a variety of physiological and pathological conditions such as pregnancy, acute rejection episodes following organ allografts, acute graft-versus-host-disease (GVHD) following bone marrow transplantation, autoimmune diseases, viral infections, and malignant melanoma. Because of the statistically significant association with clinical parameters, the level of sHLA-I antigens has been suggested to represent a useful marker to predict the evolution of viral infections and to monitor the clinical course of allografts. Moreover, elevated levels of functional sHLA-I and soluble Fas-ligand molecules have been detected by our group in blood components and might play a role in the immunomodulatory effect of autologous and allogeneic transfusions. Several lines of evidence suggest that sHLA-I molecules are immunologically functional and may play an immunoregulatory role. In fact, they have been shown to elicit antibodies in both allogeneic and xenogeneic combinations, to inhibit the activity of alloreactive cytotoxic T lymphocytes (CTL), and to induce apoptosis in alloreactive and virus-specific CTL, in activated autologous and allogeneic CD8+ T cells, and in CD8+ NK cells. There is general agreement about the mechanism underlying the inhibition of CTL activity by sHLA antigens. This inhibition appears to be mediated by interactions of sHLA-I antigens a1 and a2 domains with T cell receptor (TCR). By contrast, there is conflicting information about the mechanism underlying induction of apoptosis of activated T cells by sHLA-I antigens. Several authors reported that sHLA-I molecules induced apoptosis of alloreactive CD8+ cytotoxic T lymphocytes through interaction with their TCR. However, our own data and those other groups indicate that classical and nonclassical sHLA-I molecules trigger Fas/Fas-ligand mediated apoptosis of phytohemoagglutinin (PHA)-activated and virus-specific CD8+ T lymphocytes as well as of CD8+ NK cells by interacting with CD8 coreceptor. Recently, we performed a series of experiments in our laboratory to clarify the intracellular mechanism(s) leading to Fas-ligand upregulation and secretion. These unpublished data indicate that sHLA-I/CD8 ligation elicits the phosphorylation of p56lck protein thyrosin kinase (PTK) associated with CD8 cytoplasmic domain in the absence of any other TCR-derived signal, the activation of syk-like ZAP-70 PTK and protein kinase C, and extracellular calcium influx. Then, activation and nuclear translocation of NF-kB and NF-AT occurs, leading to Fas-ligand mRNA transcription and soluble Fas-ligand secretion, which delivers the death signal. Interestingly, soluble Fas-ligand secretion and CD8+ cell apoptosis, but not CD8+ cell cytolitic activity, are completely inhibited by Cyclosporin A, which specifically blocks the activation of the calcineurin/calmodulin pathway. Taken together, these data suggest that sHLA-I molecules are involved in a signal-transduction pathway leading to Fas-ligand expression, soluble Fas-ligand secretion, and CD8+ cells apoptosis.

Fas-associated factor-1 inhibits nuclear factor-kappaB (NF-kappaB) activity by interfering with nuclear translocation of the RelA (p65) subunit of NF-kappaB.

Park MY, Jang HD, Lee SY, Lee KJ, Kim E
J Biol Chem. 2004; 279: 2544-9

Fas-associated factor-1 (FAF1) is a Fas-binding pro-apoptotic protein that is a component of the death-inducing signaling complex in Fas-mediated apoptosis. Here, we show that FAF1 is involved in negative regulation of NF-kappaB activation. Overexpression of FAF1 decreased the basal level of NF-kappaB activity in 293 cells. NF-kappaB activation induced by tumor necrosis factor (TNF)-alpha, interleukin-1beta, and lipopolysaccharide was also inhibited by FAF1 overexpression. Moreover, FAF1 suppressed NF-kappaB activation induced by transducers of diverse NF-kappaB-activating signals such as TNF receptor-associated factor-2 and -6, MEKK1, and IkappaB kinase-beta as well as NF-kappaB p65, one of the end point molecules in the NF-kappaB activation pathway, suggesting that NF-kappaB p65 might be a target molecule upon which FAF1 acts. Subsequent study disclosed that FAF1 physically interacts with NF-kappaB p65 and that the binding domain of FAF1 is the death effector domain (DED)-interacting domain (amino acids 181-381), where DEDs of the Fas-associated death domain protein and caspase-8 interact. The NF-kappaB activity-modulating potential of FAF1 was also mapped to the DED-interacting domain. Finally, overexpression of FAF1 prevented translocation of NF-kappaB p65 into the nucleus and decreased its DNA-binding activity upon TNFalpha treatment. This study presents a novel function of FAF1, in addition to the previously known function as a component of the Fas death-inducing signaling complex, i.e. NF-kappaB activity suppressor by cytoplasmic retention of NF-kappaB p65 via physical interaction.

Protection against Fas/APO-1- and tumor necrosis factor-mediated cell death by a novel protein, sentrin.

Okura T, Gong L, Kamitani T, Wada T, Okura I, Wei CF, Chang HM, Yeh ET
J Immunol. 1996; 157: 4277-81

Fas/APO-1 and TNF receptor 1 share a common signaling motif in their cytoplasmic tail called the "death domain." Using the death domain as bait in the yeast two-hybrid system, several death domain-containing proteins that participate in cell death signaling have been identified. Here we report the isolation of a novel protein, sentrin, which interacts with Fas/APO-1 and TNF receptor 1 but not with FADD/MORT1 or CD40. Two-hybrid interaction assays reveal that sentrin associates only with the signal-competent forms of Fas/APO-1 or TNF receptor 1 death domains. Sentrin is a novel protein of 101 amino acids with homology to ubiquitin, Nedd8, and a Saccharomyces cerevisiae protein, Smt3. When overexpressed, sentrin provides protection against both anti-Fas/APO-1 and TNF-induced cell death.

Apoptosis signaling pathways in normal T cells: differential activity of Bcl-2 and IL-1beta-converting enzyme family protease inhibitors on glucocorticoid- and Fas-mediated cytotoxicity.

Moreno MB, Memon SA, Zacharchuk CM
J Immunol. 1996; 157: 3845-9

Fas-mediated apoptosis plays an important role in regulating the immune response in peripheral T cells. Restimulation of T cell blasts up-regulates Fas and Fas ligand expression, with subsequent interaction leading to cell death. Overexpression of Bcl-2 in tumor cells blocks apoptosis induced by many stimuli, but inhibition of Fas-mediated killing has not been consistently observed. To examine the behavior of Bcl-2 in normal cells, T cell blasts were transiently transfected with Bcl-2 and related gene products to determine the effect on apoptotic signaling. Transient overexpression of Bcl-2 in mouse and human T cell blasts did not block Fas-mediated apoptosis, whereas etoposide- and glucocorticoid-induced cytotoxicity was potently inhibited. Expression of Bcl-xL and adenovirus E1B 19K did not interfere with anti-Fas killing. In contrast, interleukin-1beta-converting enzyme family protease inhibitors Ac-DEVD-CHO and CrmA blocked Fas-mediated apoptosis. These results suggest that peripheral T cells use distinct apoptosis signaling pathways with differential sensitivity to Bcl-2 and interleukin-1beta-converting enzyme family protease inhibitors. Since T cells normally express Bcl-2 and Bcl-xL following activation, their inability to block Fas-mediated apoptosis may allow for the elimination of self-reactive cells and the appropriate regulation of immune responses.

Thyroid carcinoma cells are resistant to FAS-mediated apoptosis but sensitive to tumor necrosis factor-related apoptosis-inducing ligand.

Mitsiades N, Poulaki V, Tseleni-Balafouta S, Koutras DA, Stamenkovic I
Cancer Res. 2000; 60: 4122-9

Fas (APO-1/CD95) is a transmembrane protein of the tumor necrosis factor (TNF)/nerve growth factor receptor superfamily that induces apoptosis in susceptible normal and neoplastic cells upon cross-linking by its ligand (FasL). TNF-related apoptosis-inducing ligand (TRAIL) is a more recently identified member of the TNF superfamily that has been shown to selectively kill neoplastic cells by engaging two cell-surface receptors, DR4 and DR5. Two additional TRAIL receptors (DcR1 and DcR2) do not transmit an apoptotic signal and have been proposed to confer protection from TRAIL-induced apoptosis. We addressed the expression of Fas, DR4, and DR5 in thyroid carcinoma cell lines and in 31 thyroid carcinoma specimens by Western blot analysis and immunohistochemistry, respectively, and tested the sensitivity of thyroid carcinoma cell lines to Fas- and TRAIL-induced apoptosis. Fas was found to be expressed in most thyroid carcinoma cell lines and tissue specimens. Although cross-linking of Fas did not induce apoptosis in thyroid carcinoma cell lines, Fas-mediated apoptosis did occur in the presence of the protein synthesis inhibitor cycloheximide, suggesting the presence of a short-lived inhibitor of the Fas pathway in these cells. Cross-linking of Fas failed to induce recruitment and activation of caspase 8, whereas transfection of a constitutively active caspase 8 construct effectively killed the SW579 papillary carcinoma cell line, arguing that the action of the putative inhibitor occurs upstream of caspase 8. By contrast, recombinant TRAIL induced apoptosis in 10 of 12 thyroid carcinoma cell lines tested, by activating caspase-10 at the receptor level and triggering a caspase-mediated apoptotic cascade. Resistance to TRAIL did not correlate with DcR1 or DcR2 protein expression and was overcome by protein synthesis inhibition in 50% of the resistant cell lines. One medullary carcinoma cell line was resistant to Fas-and TRAIL-induced apoptosis, even in the presence of cycloheximide, and to transfection of constitutively active caspase-8, suggesting a different regulation of the apoptotic pathway. Our observations indicate that TRAIL effectively kills carcinomas that originate from the follicular epithelium of the thyroid gland, by inducing caspase-mediated apoptosis, and may provide a potentially potent therapeutic reagent against thyroid cancer.

The long form of FLIP is an activator of caspase-8 at the Fas death-inducing signaling complex.

Micheau O, Thome M, Schneider P, Holler N, Tschopp J, Nicholson DW, Briand C, Grutter MG
J Biol Chem. 2002; 277: 45162-71

Death receptors, such as Fas and tumor necrosis factor-related apoptosis-inducing ligand receptors, recruit Fas-associated death domain and pro-caspase-8 homodimers, which are then autoproteolytically activated. Active caspase-8 is released into the cytoplasm, where it cleaves various proteins including pro-caspase-3, resulting in apoptosis. The cellular Fas-associated death domain-like interleukin-1-beta-converting enzyme-inhibitory protein long form (FLIP(L)), a structural homologue of caspase-8 lacking caspase activity because of several mutations in the active site, is a potent inhibitor of death receptor-induced apoptosis. FLIP(L) is proposed to block caspase-8 activity by forming a proteolytically inactive heterodimer with caspase-8. In contrast, we propose that FLIP(L)-bound caspase-8 is an active protease. Upon heterocomplex formation, a limited caspase-8 autoprocessing occurs resulting in the generation of the p43/41 and the p12 subunits. This partially processed form but also the non-cleaved FLIP(L)-caspase-8 heterocomplex are proteolytically active because they both bind synthetic substrates efficiently. Moreover, FLIP(L) expression favors receptor-interacting kinase (RIP) processing within the Fas-signaling complex. We propose that FLIP(L) inhibits caspase-8 release-dependent pro-apoptotic signals, whereas the single, membrane-restricted active site of the FLIP(L)-caspase-8 heterocomplex is proteolytically active and acts on local substrates such as RIP.

Apoptosis induced by a chimeric Fas/FLICE receptor: lack of requirement for Fas- or FADD-binding proteins.

Memon SA, Hou J, Moreno MB, Zacharchuk CM
J Immunol. 1998; 160: 2046-9

Current models for Fas (CD95)-mediated apoptosis suggest that FLICE/caspase-8 is recruited and activated, which results in cell death. However, the role of additional molecules in Fas signaling and FLICE activation is not clear. A chimeric Fas/FLICE (F/F) receptor, containing the extracellular/transmembrane portion of Fas and the caspase region of FLICE, mediated anti-Fas apoptosis. FLICE protease subunits were generated from the F/F precursor. Killing induced by Fas, but not F/F, was blocked by a dominant negative FADD. Apoptosis triggered through Fas and F/F was inhibited by coexpression of CrmA and p35, but not Bcl-xL. F/F bypassed Fas resistance in COS-7 cells and blocking by the death effector domain (DED)-containing viral protein MC159. These results show that: 1) F/F induces cell death, indicating that FLICE activation is sufficient for apoptosis and does not require additional Fas- or FADD-binding proteins; and 2) F/F bypasses proximal defects in Fas signaling that prevent FLICE recruitment or activation.

The functional genomic response of developing embryonic submandibular glands to NF-kappa B inhibition.

Melnick M, Chen H, Min Zhou Y, Jaskoll T
BMC Dev Biol. 2001; 1: 15-15

BACKGROUND: The proper balance between epithelial cell proliferation, quiescence, and apoptosis during development is mediated by the specific temporal and spatial appearance of transcription factors, growth factors, cytokines, caspases, etc. Since our prior studies suggest the importance of transcription factor NF-kappaB during embryonic submandibular salivary gland (SMG) development, we attempted to delineate the emergent dynamics of a cognate signaling network by studying the molecular patterns and phenotypic outcomes of interrupted NF-kappaB signaling in embryonic SMG explants. RESULTS: SN50-mediated inhibition of NF-kappaB nuclear translocation in E15 SMG explants cultured for 2 days results in a highly significant increase in apoptosis and decrease in cell proliferation. Probabilistic Neural Network (PNN) analyses of transcriptomic and proteomic assays identify specific transcripts and proteins with altered expression that best discriminate control from SN50-treated SMGs. These include PCNA, GR, BMP1, BMP3b, Chk1, Caspase 6, E2F1, c-Raf, ERK1/2 and JNK-1, as well as several others of lesser importance. Increased expression of signaling pathway components is not necessarily probative of pathway activity; however, as confirmation we found a significant increase in activated (phosphorylated/cleaved) ERK 1/2, Caspase 3, and PARP in SN50-treated explants. This increased activity of proapoptotic (caspase3/PARP) and compensatory antiapoptotic (ERK1/2) pathways is consistent with the dramatic cell death seen in SN50-treated SMGs. CONCLUSIONS: Our morphological and functional genomic analyses indicate that the primary and secondary effects of NF-kappaB-mediated transcription are critical to embryonic SMG developmental homeostasis. Relative to understanding complex genetic networks and organogenesis, our results illustrate the importance of evaluating the gene, protein, and activated protein expression of multiple components from multiple pathways within broad functional categories.

Cytolytic T-cell cytotoxicity is mediated through perforin and Fas lytic pathways.

Lowin B, Hahne M, Mattmann C, Tschopp J
Nature. 1994; 370: 650-2

The recent generation of perforin knock-out mice has demonstrated a crucial role for the pore-forming perforin in cytolytic T-lymphocyte (CTL)-mediated cytolysis. Perforin-deficient mice failed to clear lymphocytic choriomeningitis virus in vivo, yet substantial killing activity still remained in perforin-free CTLs in vitro, indicating the presence of (a) further lytic pathway(s). Fas is an apoptosis-signalling receptor molecule on the surface of a number of different cells. Here we report that both perforin-deficient and Fas-ligand-deficient CTLs show impaired lytic activity on all target cells tested. The killing activity was completely abolished when both pathways were inactivated by using target cells from Fas-receptor-deficient lpr mice and perforin-free CTL effector cells. Fas-ligand-based killing activity was triggered upon T-cell receptor occupancy and was directed to the cognate target cell. Thus, two complementary, specific cytotoxic mechanisms are functional in CTLs, one based on the secretion of lytic proteins and one which depends on cell-surface ligand-receptor interaction.

Fas-mediated apoptosis is dependent on wild-type p53 status in human cancer cells expressing a temperature-sensitive p53 mutant alanine-143.

Li Y, Raffo AJ, Drew L, Mao Y, Tran A, Petrylak DP, Fine RL
Cancer Res. 2003; 63: 1527-33

The p53 mutant 143Ala is a human temperature-sensitive mutant with two conformational states. To definitively determine whether the Fas signal transduction pathway and the function of the pathway are dependent on p53 status, we have established stable transfectants of p53 mutant 143Ala in two human cancer cell lines: H1299 (lung cancer line) and PC-3 (prostate cancer line), the native state of which contains null p53 status and can grow at 37 degrees C and 32.5 degrees C. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and cell cycle analysis showed inhibition of the growth of cells overexpressing p53 mutant 143Ala in the wild-type p53 form at 32.5 degrees C because of induction of G0/G1 arrest. Transfected cells had increased protein expression of p21, Fas, and MDM2 at the wild-type p53 conformation at 32.5 degrees C, but not in the mutant p53 form at 37 degrees C. However, there was no change in protein expression of FADD, FAP-1, Bcl-2, or Bax at 32.5 or 37 degrees C. Assays for apoptosis demonstrated that anti-Fas antibody CH-11 and FasL induced apoptosis only in cells that overexpress p53 mutant 143Ala at 32.5 degrees C with the wild-type p53 form. Both caspase-3 and caspase-8 activities were increased by anti-Fas antibody CH-11 only in cells at 32.5 degrees C with wild-type p53. Our results demonstrated that Fas-mediated apoptosis in H1299 and PC-3 cells expressing p53 mutant 143Ala occurred only with the wild-type p53 phenotype. These results support the hypothesis that Fas-mediated apoptosis is dependent, at least partially, on the presence of a functional wild-type p53 state. This model may be a useful tool for dissecting the specific interactions between wild-type p53 and the Fas signal transduction pathway in human cancer cells.

TRAF-interacting protein (TRIP): a novel component of the tumor necrosis factor receptor (TNFR)- and CD30-TRAF signaling complexes that inhibits TRAF2-mediated NF-kappaB activation.

Lee SY, Lee SY, Choi Y
J Exp Med. 1997; 185: 1275-85

Through their interaction with the TNF receptor-associated factor (TRAF) family, members of the tumor necrosis factor receptor (TNFR) superfamily elicit a wide range of biological effects including differentiation, proliferation, activation, or cell death. We have identified and characterized a novel component of the receptor-TRAF signaling complex, designated TRIP (TRAF-interacting protein), which contains a RING finger motif and an extended coiled-coil domain. TRIP associates with the TNFR2 or CD30 signaling complex through its interaction with TRAF proteins. When associated, TRIP inhibits the TRAF2-mediated NF-kappaB activation that is required for cell activation and also for protection against apoptosis. Thus, TRIP acts as a receptor-proximal regulator that may influence signals responsible for cell activation/proliferation and cell death induced by members of the TNFR superfamily.

MST, a physiological caspase substrate, highly sensitizes apoptosis both upstream and downstream of caspase activation.

Lee KK, Ohyama T, Yajima N, Tsubuki S, Yonehara S
J Biol Chem. 2001; 276: 19276-85

The human serine/threonine kinase, mammalian STE20-like kinase (MST), is considerably homologous to the budding yeast kinases, SPS1 and STE20, throughout their kinase domains. The cellular function and physiological activation mechanism of MST is unknown except for the proteolytic cleavage-induced activation in apoptosis. In this study, we show that MST1 and MST2 are direct substrates of caspase-3 both in vivo and in vitro. cDNA cloning of MST homologues in mouse and nematode shows that caspase-cleaved sequences are evolutionarily conserved. Human MST1 has two caspase-cleavable sites, which generate biochemically distinct catalytic fragments. Staurosporine activates MST either caspase-dependently or independently, whereas Fas ligation activates it only caspase-dependently. Immunohistochemical analysis reveals that MST is localized in the cytoplasm. During Fas-mediated apoptosis, cleaved MST translocates into the nucleus before nuclear fragmentation is initiated, suggesting it functions in the nucleus. Transiently expressed MST1 induces striking morphological changes characteristic of apoptosis in both nucleus and cytoplasm, which is independent of caspase activation. Furthermore, when stably expressed in HeLa cells, MST highly sensitizes the cells to death receptor-mediated apoptosis by accelerating caspase-3 activation. These findings suggest that MST1 and MST2 play a role in apoptosis both upstream and downstream of caspase activation.

Caspar, a suppressor of antibacterial immunity in Drosophila.

Kim M, Lee JH, Lee SY, Kim E, Chung J
Proc Natl Acad Sci U S A. 2006; 103: 16358-63

Drosophila has a primitive yet highly effective innate immune system. Although the infection-dependent activation mechanisms of the Drosophila immune system are well understood, its inhibitory regulation remains elusive. To find novel suppressors of the immune system, we performed a genetic screening for Drosophila mutants with hyperactivated immune responses and isolated a loss-of-function mutant of caspar whose product is homologous to Fas-associating factor 1 in mammals. Interestingly, caspar mutant flies showed increased antibacterial immune responses including increased resistance to bacterial infection and a constitutive expression of diptericin, a representative antibacterial peptide gene. Conversely, ectopic expression of caspar strongly suppressed the infection-dependent gene expression of diptericin, which allowed bacterial outgrowth. Consistent with these physiological phenotypes, Caspar negatively regulated the immune deficiency (Imd)-mediated immune responses by blocking nuclear translocation of Relish, an NF-kappaB transcription factor. In addition, we further demonstrated that Dredd-dependent cleavage of Relish, a prerequisite event for the nuclear entry of Relish, is the target of the Caspar-mediated suppression of the Imd pathway. Remarkably, Caspar was highly specific for the Imd pathway and did not affect the Toll pathway, which is crucial for antifungal immunity. Collectively, our elucidation of an inhibitory mechanism of the Imd pathway by Caspar will provide a valuable insight into understanding complex regulatory mechanisms of the innate immune systems in both Drosophila and mammals.

The expression of FAS-associated factor 1 and heat shock protein 70 in ovarian cancer.

Kang HJ, Moon HS, Chung HW
Obstet Gynecol Sci. 2014; 57: 281-90

OBJECTIVE: In this study, we evaluated the expression of FAS-associated factor 1 (FAF1) and heat shock protein 70 (HSP70) in normal ovary and ovarian cancer, and also analyzed the correlation between FAF1 and HSP70 in ovarian cancer. METHODS: The patient group consisted of 29 unrelated Korean women diagnosed as ovarian cancers and control samples were obtained from 7 patients who underwent oophorectomy for benign disease of uterus, and normal ovary was confirmed histologically from biopsy. We examined FAF1 and HSP70 expression by western blot analysis and immunohistochemical staining in normal ovary and ovarian cancer. Furthermore, we examined a correlation between FAF1 and HSP70 in ovarian cancer. RESULTS: The expression of FAF1 was lower in ovarian cancer than that in normal ovary (P=0.02), and the expression of HSP70 was increased in ovarian cancer in comparison to that in normal ovary (P=0.03). The expression of FAF1 was decreased in advanced stages (stage III or stage IV) as compared with early stages (stage I or stage II) (P=0.01). The expression of HSP70 was not significantly related with ovarian cancer histology (P=0.10), but the expression of HSP70 was most increased with papillary serous carcinomas and undifferentiated ovarian cancer. The expression of FAF1 was inversely correlated with the expression of HSP70 in ovarian cancer (Spearman correlation coefficience=-0.47). CONCLUSION: We concluded that the expression of FAF1 or HSP70 each seems to have a meaning as a biomarker for early detection of ovarian cancer. The expressions of FAF1 and HSP70 seem to be more valuable in predicting ovarian cancer when used together because of their inverse correlation. This is the first study about the expression of FAF1 in ovarian cancer and the correlation between FAF1 and HSP70 expression in ovarian cancer.

The polypeptide encoded by the cDNA for human cell surface antigen Fas can mediate apoptosis.

Itoh N, Yonehara S, Ishii A, Yonehara M, Mizushima S, Sameshima M, Hase A, Seto Y, Nagata S
Cell. 1991; 66: 233-43

Mouse anti-Fas monoclonal antibody has a cytolytic activity on human cells that express the antigen. Complementary DNAs encoding the cell surface antigen Fas were isolated from a cDNA library of human T cell lymphoma KT-3 cells. The nucleotide sequence of the cDNAs revealed that the molecule coding for the Fas antigen determinant is a 319 amino acid polypeptide (Mr 36,000) with a single transmembrane domain. The extracellular domain is rich in cysteine residue, and shows a similarity to that of human tumor necrosis factor receptors, human nerve growth factor receptor, and human B cell antigen CD40. Murine WR19L cells or L929 cells transformed with the human Fas antigen cDNA were killed by the anti-Fas antibody in the process known as apoptosis.

CLARP, a death effector domain-containing protein interacts with caspase-8 and regulates apoptosis.

Inohara N, Koseki T, Hu Y, Chen S, Nunez G
Proc Natl Acad Sci U S A. 1997; 94: 10717-22

We have identified and characterized CLARP, a caspase-like apoptosis-regulatory protein. Sequence analysis revealed that human CLARP contains two amino-terminal death effector domains fused to a carboxyl-terminal caspase-like domain. The structure and amino acid sequence of CLARP resemble those of caspase-8, caspase-10, and DCP2, a Drosophila melanogaster protein identified in this study. Unlike caspase-8, caspase-10, and DCP2, however, two important residues predicted to be involved in catalysis were lost in the caspase-like domain of CLARP. Analysis with fluorogenic substrates for caspase activity confirmed that CLARP is catalytically inactive. CLARP was found to interact with caspase-8 but not with FADD/MORT-1, an upstream death effector domain-containing protein of the Fas and tumor necrosis factor receptor 1 signaling pathway. Expression of CLARP induced apoptosis, which was blocked by the viral caspase inhibitor p35, dominant negative mutant caspase-8, and the synthetic caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-(OMe)-fluoromethylketone (zVAD-fmk). Moreover, CLARP augmented the killing ability of caspase-8 and FADD/MORT-1 in mammalian cells. The human clarp gene maps to 2q33. Thus, CLARP represents a regulator of the upstream caspase-8, which may play a role in apoptosis during tissue development and homeostasis.

Structural requirements for signal-induced target binding of FADD determined by functional reconstitution of FADD deficiency.

Imtiyaz HZ, Zhang Y, Zhang J
J Biol Chem. 2005; 280: 31360-7

FADD is a key adaptor modulating several signaling pathways such as apoptosis induced by Fas (CD95) and tumor necrosis factor receptor 1, and cell proliferation induced by mitogens. Whereas mutations in Fas disrupt its binding to FADD and cause autoimmune lymphoproliferative (lpr) syndromes, a FADD deficiency blocks embryonic development in mice. To delineate the multifunction of FADD in vivo, we performed functional reconstitution analysis by introducing wild type and mutant FADD into FADD-/- cells or FADD-/- mice lacking the endogenous FADD. An lpr-like FADD mutant, V121N, was reported previously as being defective in Fas binding in vitro. However, we found that in mice V121N can bind to Fas and is functional in signaling apoptosis. Unexpectedly, this lpr-like mutant FADD failed to support mouse development, indicating that the death domain of FADD has an additional function required for embryogenesis, which is independent of that required for receptor-induced apoptosis. Further mutagenesis was targeted at charged residues in the FADD death domain, presumably mediating electrostatic interactions with Fas. We showed that the target binding and apoptosis signaling functions of FADD were not affected when mutations were introduced to a majority of the charged residues. In one exception, replacing arginine 117 with an uncharged residue disrupted target binding and apoptosis signaling, but restoring the positive charge at position 117 failed to reconstitute the FADD function. Therefore, in vivo target binding of FADD involves an additional mechanism distinct from electrostatic interaction.

The death domain motif found in Fas (Apo-1) and TNF receptor is present in proteins involved in apoptosis and axonal guidance.

Hofmann K, Tschopp J
FEBS Lett. 1995; 371: 321-3

The interaction of Fas (Apo-1) and TNF receptor-1 with their respective ligands can lead to cell death. The so-called death domain, a sequence motif present in the cytoplasmic portion of the two receptors, has been identified as a critical structural element involved in signal transduction that leads to apoptosis. Here we describe several additional proteins which contain a death domain. Novel members of this family include proteins known to be implicated not only in apoptosis but also in neuron guidance.

Fas-associated factor 1 plays a negative regulatory role in the antibacterial immunity of Locusta migratoria.

He Z, Wang P, Shi H, Si F, Hao Y, Chen B
Insect Mol Biol. 2013; 22: 389-98

Insect immune responses are precisely regulated to maintain immune balance. In this study, the Fas-associated factor 1 (FAF1) gene of Locusta migratoria manilensis, a homologue of the caspar gene that functions as a specific negative regulator in the antibacterial immunity pathway, was cloned. Gene expression analysis showed that FAF1 was expressed throughout the developmental stages and in all tested tissues, but its transcription levels varied significantly. Thus, FAF1 appears to be tightly regulated and is probably involved in multiple physiological processes. In addition, the antimicrobial peptide gene prolixicin was cloned and characterized. After bacterial challenge, prolixicin was rapidly up-regulated, whereas FAF1 was markedly down-regulated. This result was consistent with the observation that prolixicin was hyperactivated when FAF1 was suppressed by RNA interference. Moreover, after bacterial infection, the survival rate of FAF1-knockdown locusts was much higher than that of the wild-type. Taken together, these findings strongly suggest that FAF1 shares a similar function as caspar in Drosophila and may be involved in the negative regulation of antibacterial immunity in locusts.

Metabolic inhibitors sensitize for CD95 (APO-1/Fas)-induced apoptosis by down-regulating Fas-associated death domain-like interleukin 1-converting enzyme inhibitory protein expression.

Fulda S, Meyer E, Debatin KM
Cancer Res. 2000; 60: 3947-56

Protein or RNA synthesis inhibitors are known to sensitize some resistant cells for death receptor-induced apoptosis. However, the molecular mechanism(s) involved in sensitization have not yet been defined exactly. Here, we report that metabolic inhibitors such as cycloheximide (CHX) or actinomycin D (ActD) sensitize for CD95-induced apoptosis by strongly down-regulating FLIP and RIP expression. Metabolic labeling studies revealed that CHX or ActD inhibited protein or RNA synthesis at concentrations required for sensitization. In contrast to Fas-associated death domain (FADD) or caspase-8, FADD-like interleukin 1-converting enzyme-inhibitory protein (FLIP) and RIP protein levels rapidly decreased upon treatment with CHX or ActD, indicating that both molecules have a high turnover rate. Selective down-regulation of FLIP expression by FLIP antisense oligonucleotides sensitized for CD95-induced apoptosis. Reduction of FLIP levels resulted in undetectable amounts of FLIP at the CD95 death-inducing signaling complex (DISC) upon CD95 stimulation, thereby enhancing the recruitment of caspase-8 to the DISC and caspase-8 activation. CHX- or ActD-mediated sensitization to CD95-induced apoptosis was predominantly found in type I cells in which FADD and caspase-8 are recruited to CD95 upon stimulation but not in type II cells in which no DISC formation is detected. Pretreatment with CHX or ActD sensitized for subsequent CD95 stimulation compared with cells without pretreatment. CHX or ActD also reduced XIAP expression and similarly sensitized for tumor necrosis factor-related apoptosis-inducing ligand- or tumor necrosis factor-alpha-induced apoptosis. Because blockade of death receptor triggering by FLIP overexpression has recently been implicated in tumorigenesis and treatment resistance in vivo, strategies to inhibit FLIP expression, e.g., by metabolic inhibitors, may prove to be a useful complementary tool for the treatment of cancer.

The p97-FAF1 protein complex reveals a common mode of p97 adaptor binding.

Ewens CA, Panico S, Kloppsteck P, McKeown C, Ebong IO, Robinson C, Zhang X, Freemont PS
J Biol Chem. 2014; 289: 12077-84

p97, also known as valosin-containing protein, is a versatile participant in the ubiquitin-proteasome system. p97 interacts with a large network of adaptor proteins to process ubiquitylated substrates in different cellular pathways, including endoplasmic reticulum-associated degradation and transcription factor activation. p97 and its adaptor Fas-associated factor-1 (FAF1) both have roles in the ubiquitin-proteasome system during NF-kappaB activation, although the mechanisms are unknown. FAF1 itself also has emerging roles in other cell-cycle pathways and displays altered expression levels in various cancer cell lines. We have performed a detailed study the p97-FAF1 interaction. We show that FAF1 binds p97 stably and in a stoichiometry of 3 to 6. Cryo-EM analysis of p97-FAF1 yielded a 17 A reconstruction of the complex with FAF1 above the p97 ring. Characteristics of p97-FAF1 uncovered in this study reveal common features in the interactions of p97, providing mechanistic insight into how p97 mediates diverse functionalities.

T cell receptor ligation triggers novel nonapoptotic cell death pathways that are Fas-independent or Fas-dependent.

Davidson WF, Haudenschild C, Kwon J, Williams MS
J Immunol. 2002; 169: 6218-30

Short-term culture of activated T cells with IL-2 renders them highly susceptible to apoptotic death triggered by TCR cross-linking. Activation-induced apoptosis is contingent upon caspase activation and this is mediated primarily by Fas/Fas ligand (FasL) interactions that, in turn, are optimized by p38 mitogen-activated protein kinase (MAPK)-regulated signals. Although T cells from mice bearing mutations in Fas (lpr) or FasL (gld) are more resistant to activation-induced cell death (AICD) than normal T cells, a significant proportion of CD8(+) T cells and to a lesser extent CD4(+) T cells from mutant mice die after TCR religation. Little is known about this Fas-independent death process. In this study, we demonstrate that AICD in lpr and gld CD4(+) and CD8(+) T cells occurs predominantly by a novel mechanism that is TNF-alpha-, caspase-, and p38 MAPK-independent and has morphologic features more consistent with oncosis/primary necrosis than apoptosis. A related Fas- and caspase-independent, nonapoptotic death process is revealed in wild-type (WT) CD8(+) T cell blasts following TCR ligation and treatment with caspase inhibitors, the p38 MAPK inhibitor, SB203580, or neutralizing anti-FasL mAb. In parallel studies with WT CD4(+) T cells, two minor pathways leading to nonapoptotic, caspase-independent AICD were identified, one contingent upon Fas ligation and p38 MAPK activation and the other Fas- and p38 MAPK-independent. These data indicate that TCR ligation can activate nonapoptotic death programs in WT CD8(+) and CD8(+) T blasts that normally are masked by Fas-mediated caspase activation. Selective use of potentially proinflammatory oncotic death programs by activated lpr and gld T cells may be an etiologic factor in autosensitization.

Redesigning an FKBP-ligand interface to generate chemical dimerizers with novel specificity.

Clackson T, Yang W, Rozamus LW, Hatada M, Amara JF, Rollins CT, Stevenson LF, Magari SR, Wood SA, Courage NL, Lu X, Cerasoli F Jr, Gilman M, Holt DA
Proc Natl Acad Sci U S A. 1998; 95: 10437-42

FKBP ligand homodimers can be used to activate signaling events inside cells and animals that have been engineered to express fusions between appropriate signaling domains and FKBP. However, use of these dimerizers in vivo is potentially limited by ligand binding to endogenous FKBP. We have designed ligands that bind specifically to a mutated FKBP over the wild-type protein by remodeling an FKBP-ligand interface to introduce a specificity binding pocket. A compound bearing an ethyl substituent in place of a carbonyl group exhibited sub-nanomolar affinity and 1,000-fold selectivity for a mutant FKBP with a compensating truncation of a phenylalanine residue. Structural and functional analysis of the new pocket showed that recognition is surprisingly relaxed, with the modified ligand only partially filling the engineered cavity. We incorporated the specificity pocket into a fusion protein containing FKBP and the intracellular domain of the Fas receptor. Cells expressing this modified chimeric protein potently underwent apoptosis in response to AP1903, a homodimer of the modified ligand, both in culture and when implanted into mice. Remodeled dimerizers such as AP1903 are ideal reagents for controlling the activities of cells that have been modified by gene therapy procedures, without interference from endogenous FKBP.

Fas-associated factor 1 as a regulator of olfactory axon guidance.

Cheng K, Bai L, Belluscio L
J Neurosci. 2011; 31: 11905-13

Axon guidance is a crucial part of neural circuit formation. While precise axonal targeting forms the basis of accurate information delivery, the mechanisms that regulate this process are still unclear. Apoptotic signaling molecules have been identified in the axon terminal, but their specific role in axon guidance is not well understood. Here we use the mouse olfactory system as an in vivo model to demonstrate that by modulating Fas-associated factor 1 (FAF1), an apoptosis regulatory molecule, we can rewire axonal projections. Interestingly, FAF1 is highly expressed in the developing mouse olfactory system, but its expression is downregulated postnatally. Using a tetracycline-inducible promoter Tet-Off system, we generated transgenic mice in which FAF1 is specifically expressed in immature olfactory sensory neurons (OSNs) and show that overexpression of FAF1 not only misroutes OSN axons to deep layers of the olfactory bulb but also leads to widespread disruption of the glomerular layer. In addition, we also demonstrate that the specific convergence of P2 receptor OSN axons is completely distorted in the FAF1 mice. Strikingly, all of the mutant phenotypes can be recovered by shutting down FAF1 expression through the administration of doxycycline. Together, our study provides clear in vivo evidence that an apoptotic molecule can indeed regulate axon targeting and that OSNs can restore their organization even after broad disruption.

Protection from Fas-mediated apoptosis by a soluble form of the Fas molecule.

Cheng J, Zhou T, Liu C, Shapiro JP, Brauer MJ, Kiefer MC, Barr PJ, Mountz JD
Science. 1994; 263: 1759-62

Fas is an apoptosis-signaling receptor molecule on the surface of a number of cell types. Molecular cloning and nucleotide sequence analysis revealed a human Fas messenger RNA variant capable of encoding a soluble Fas molecule lacking the transmembrane domain because of the deletion of an exon encoding this region. The expression of soluble Fas was confirmed by flow cytometry and immunocytochemical analysis. Supernatants from cells transfected with the variant messenger RNA blocked apoptosis induced by the antibody to Fas. Levels of soluble Fas were elevated in patients with systemic lupus erythematosus, and mice injected with soluble Fas displayed autoimmune features.

CD95/Fas in the brain--not just a killer.

Beier CP, Schulz JB
Cell Stem Cell. 2009; 5: 128-30

Although CD95 (Fas/Apo-1) has long been known to be broadly expressed in the brain, its function has remained enigmatic. In this issue of Cell Stem Cell, Corsini et al. (2009) now show that CD95 serves as a potent activator of neurogenesis in both the healthy and injured brain.

Interaction of Fas(Apo-1/CD95) with proteins implicated in the ubiquitination pathway.

Becker K, Schneider P, Hofmann K, Mattmann C, Tschopp J
FEBS Lett. 1997; 412: 102-6

Fas(Apo-1/CD95), a receptor belonging to the tumor necrosis factor receptor family, induces apoptosis when triggered by Fas ligand. Upon its activation, the cytoplasmic domain of Fas binds several proteins which transmit the death signal. We used the yeast two-hybrid screen to isolate Fas-associated proteins. Here we report that the ubiquitin-conjugating enzyme UBC9 binds to Fas at the interface between the death domain and the membrane-proximal region of Fas. This interaction is also seen in vivo. UBC9 transiently expressed in HeLa cells bound to the co-expressed cytoplasmic segment of Fas. FAF1, a Fas-associated protein that potentiates apoptosis (Chu et al. (1996) Proc. Natl. Acad. Sci. USA 92, 11894-11898), was found to contain sequences similar to ubiquitin. These results suggest that proteins related to the ubiquitination pathway may modulate the Fas signaling pathway.