The caspase 8 inhibitor c-FLIP(L) modulates T-cell receptor-induced proliferation but not activation-induced cell death of lymphocytes

c-FLIP regulates autophagy by interacting with Beclin-1 and influencing its stability

c-FLIP (cellular FLICE-like inhibitory protein) protein is mostly known as an apoptosis modulator. However, increasing data underline that c-FLIP plays multiple roles in cellular homoeostasis, influencing differently the same pathways depending on its expression level and isoform predominance. Few and controversial data are available regarding c-FLIP function in autophagy. Here we show that autophagic flux is less effective in c-FLIP−/− than in WT MEFs (mouse embryonic fibroblasts). Indeed, we show that the absence of c-FLIP compromises the expression levels of pivotal factors in the generation of autophagosomes. In line with the role of c-FLIP as a scaffold protein, we found that c-FLIP_L interacts with Beclin-1 (BECN1: coiled-coil, moesin-like BCL2-interacting protein), which is required for autophagosome nucleation. By a combination of bioinformatics tools and biochemistry assays, we demonstrate that c-FLIP_L interaction with Beclin-1 is important to prevent Beclin-1 ubiquitination and degradation through the proteasomal pathway. Taken together, our data describe a novel molecular mechanism through which c-FLIP_L positively regulates autophagy, by enhancing Beclin-1 protein stability.

Moonlighting Proteins Are Important Players in Cancer Immunology

Plasticity and adaptation to environmental stress are the main features that tumor and immune system share. Except for intrinsic and high-defined properties, cancer and immune cells need to overcome the opponent's defenses by activating more effective signaling networks, based on common elements such as transcriptional factors, protein-based complexes and receptors. Interestingly, growing evidence point to an increasing number of proteins capable of performing diverse and unpredictable functions. These multifunctional proteins are defined as moonlighting proteins. During cancer progression, several moonlighting proteins are involved in promoting an immunosuppressive microenvironment by reprogramming immune cells to support tumor growth and metastatic spread. Conversely, other moonlighting proteins support tumor antigen presentation and lymphocytes activation, leading to several anti-cancer immunological responses. In this light, moonlighting proteins could be used as promising new potential targets for improving current cancer therapies. In this review, we describe in details 12 unprecedented moonlighting proteins that during cancer progression play a decisive role in guiding cancer-associated immunomodulation by shaping innate or adaptive immune response.

c-FLIP, a Novel Biomarker for Cancer Prognosis, Immunosuppression, Alzheimer's Disease, Chronic Obstructive Pulmonary Disease (COPD), and a Rationale Therapeutic Target

Dysregulation of c-FLIP (cellular FADD-like IL-1β-converting enzyme inhibitory protein) has been shown in several diseases including cancer, Alzheimer's disease, and chronic obstructive pulmonary disease (COPD). c-FLIP is a critical anti-cell death protein often overexpressed in tumors and hematological malignancies and its increased expression is often associated with a poor prognosis. c-FLIP frequently exists as long (c-FLIPL) and short (c-FLIPS) isoforms, regulates its anti-cell death functions through binding to FADD (FAS associated death domain protein), an adaptor protein known to activate caspases-8 and -10 and links c-FLIP to several cell death regulating complexes including the death-inducing signaling complex (DISC) formed by various death receptors. c-FLIP also plays a critical role in necroptosis and autophagy. Furthermore, c-FLIP is able to activate several pathways involved in cytoprotection, proliferation, and survival of cancer cells through various critical signaling proteins. Additionally, c-FLIP can inhibit cell death induced by several chemotherapeutics, anti-cancer small molecule inhibitors, and ionizing radiation. Moreover, c-FLIP plays major roles in aiding the survival of immunosuppressive tumor-promoting immune cells and functions in inflammation, Alzheimer's disease (AD), and chronic obstructive pulmonary disease (COPD). Therefore, c-FLIP can serve as a versatile biomarker for cancer prognosis, a diagnostic marker for several diseases, and an effective therapeutic target. In this article, we review the functions of c-FLIP as an anti-apoptotic protein and negative prognostic factor in human cancers, and its roles in resistance to anticancer drugs, necroptosis and autophagy, immunosuppression, Alzheimer's disease, and COPD.

Life and Death of Activated T Cells: How Are They Different from Naïve T Cells?

T cells are pivotal in immunity and immunopathology. After activation, T cells undergo a clonal expansion and differentiation followed by a contraction phase, once the pathogen has been cleared. Cell survival and cell death are critical for controlling the numbers of naïve T cells, effector, and memory T cells. While naïve T cell survival has been studied for a long time, more effort has gone into understanding the survival and death of activated T cells. Despite this effort, there is still much to be learnt about T cell survival, as T cells transition from naïve to effector to memory. One key advance is the development of inhibitors that may allow the temporal study of survival mechanisms operating in these distinct cell states. Naïve T cells were highly reliant on BCL-2 and sensitive to BCL-2 inhibition. Activated T cells are remarkably different in their regulation of apoptosis by pro- and antiapoptotic members of the BCL-2 family, rendering them differentially sensitive to antagonists blocking the function of one or more members of this family. Recent progress in understanding other programmed cell death mechanisms, especially necroptosis, suggests a unique role for alternative pathways in regulating death of activated T cells. Furthermore, we highlight a mechanism of epigenetic regulation of cell survival unique to activated T cells. Together, we present an update of our current understanding of the survival requirement of activated T cells.

Arecoline N-Oxide Upregulates Caspase-8 Expression in Oral Hyperplastic Lesions of Mice

Areca nut is strongly associated with oral squamous cell carcinoma (OSCC) occurrence. Arecoline N-oxide (ANO), a metabolite of the areca alkaloid arecoline, exhibits an oral fibrotic effect in NOD/SCID mice. Caspase-8, a cysteine protease encoded by the CASP8 gene, is a central mediator in the extrinsic apoptotic pathway via death receptors. Deregulation of caspase-8 in OSCC has been reported. This study investigates the regulation of caspase-8 in ANO-induced oral squamous epithelial hyperplasia that represents the initial highly proliferative stage of oral carcinogenesis. CASP8 somatic mutations were identified from whole-exome sequencing of OSCC samples. Immunohistochemical staining showed upregulation of caspase-8 in ANO-induced hyperplasia of both NOD-SCID and C57BL/6 mice. Levels of expression of CASP8, APAF-1, BAX, and BAD increased in ANO-treated DOK cells. Co-localization of increased caspase-8 and PCNA levels was detected in ANO-induced hyperplastic lesions, whereas no co-localization among γ-H2A.X, caspase-3, and upregulated caspase-8 was observed. The findings indicate that upregulation of caspase-8 is involved in cell proliferation rather than apoptosis during the initial stage of ANO-mediated oral tumorigenesis.

Fas/CD95 prevents autoimmunity independently of lipid raft localization and efficient apoptosis induction

Mutations affecting the apoptosis-inducing function of the Fas/CD95 TNF-family receptor result in autoimmune and lymphoproliferative disease. However, Fas can also costimulate T-cell activation and promote tumour cell growth and metastasis. Palmitoylation at a membrane proximal cysteine residue enables Fas to localize to lipid raft microdomains and induce apoptosis in cell lines. Here, we show that a palmitoylation-defective Fas C194V mutant is defective in inducing apoptosis in primary mouse T cells, B cells and dendritic cells, while retaining the ability to enhance naive T-cell differentiation. Despite inability to efficiently induce cell death, the Fas C194V receptor prevents the lymphoaccumulation and autoimmunity that develops in Fas-deficient mice. These findings indicate that induction of apoptosis through Fas is dependent on receptor palmitoylation in primary immune cells, and Fas may prevent autoimmunity by mechanisms other than inducing apoptosis.

Fas drives apoptosis and mutations in this receptor can cause autoimmunity through failure of cell death. Here, the authors use lpr/lpr mice with palmitoylation-defective mutant Fas to provide evidence that Fas might limit spontaneous autoimmunity through a non-apoptotic mechanism.

The Janus Face of Death Receptor Signaling during Tumor Immunoediting

Cancer immune surveillance is essential for the inhibition of carcinogenesis. Malignantly transformed cells can be recognized by both the innate and adaptive immune systems through different mechanisms. Immune effector cells induce extrinsic cell death in the identified tumor cells by expressing death ligand cytokines of the tumor necrosis factor ligand family. However, some tumor cells can escape immune elimination and progress. Acquisition of resistance to the death ligand-induced apoptotic pathway can be obtained through cleavage of effector cell expressed death ligands into a poorly active form, mutations or silencing of the death receptors, or overexpression of decoy receptors and pro-survival proteins. Although the immune system is highly effective in the elimination of malignantly transformed cells, abnormal/dysfunctional death ligand signaling curbs its cytotoxicity. Moreover, DRs can also transmit pro-survival and pro-migratory signals. Consequently, dysfunctional death receptor-mediated apoptosis/necroptosis signaling does not only give a passive resistance against cell death but actively drives tumor cell motility, invasion, and contributes to consequent metastasis. This dual contribution of the death receptor signaling in both the early, elimination phase, and then in the late, escape phase of the tumor immunoediting process is discussed in this review. Death receptor agonists still hold potential for cancer therapy since they can execute the tumor-eliminating immune effector function even in the absence of activation of the immune system against the tumor. The opportunities and challenges of developing death receptor agonists into effective cancer therapeutics are also discussed.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Cellular FLIP Inhibits Myeloid Cell Activation by Suppressing Selective Innate Signaling

Cellular FLIP (c-FLIP) specifically inhibits caspase-8 and suppresses death receptor-induced apoptosis. c-FLIP has also been reported to transmit activation signals. In this study, we report a novel function of c-FLIP involving inhibition of myeloid cell activation through antagonizing the selective innate signaling pathway. We found that conditional knockout of c-FLIP in dendritic cells (DCs) led to neutrophilia and splenomegaly. Peripheral DC populations, including CD11b(+) conventional DCs (cDCs), CD8(+) cDCs, and plasmacytoid DCs, were not affected by c-FLIP deficiency. We also found that c-FLIP knockout cDCs, plasmacytoid DCs, and bone marrow-derived DCs (BMDCs) displayed enhanced production of TNF-α, IL-2, or G-CSF in response to stimulation of TLR4, TLR2, and dectin-1. Consistent with the ability of c-FLIP to inhibit the activation of p38 MAPK, the enhanced activation of c-FLIP-deficient BMDCs could be partly linked to an elevated activation of p38 MAPK after engagement of innate receptors. Increased activation was also found in c-FLIP(+/-) macrophages. Additionally, the increased activation in c-FLIP-deficient DCs was independent of caspase-8. Our results reveal a novel inhibitory role of c-FLIP in myeloid cell activation and demonstrate the unexpected anti-inflammatory activity of c-FLIP. Additionally, our observations suggest that cancer therapy targeting c-FLIP downregulation may facilitate DC activation and increase T cell immunity.

Fas and TRAIL 'death receptors' as initiators of inflammation: Implications for cancer

Fas (CD95/APO-1) and TRAIL (CD253, TNFSF10, APO2) are members of a subset of the TNF receptor superfamily known as 'death receptors'. To date, the overwhelming majority of studies on Fas and TRAIL (TNF-related apoptosis-inducing ligand) have explored the role of these receptors as initiators of apoptosis. However, sporadic reports also suggest that engagement of the Fas and TRAIL receptors can lead to other outcomes such as cytokine and chemokine production, cell proliferation, cell migration and differentiation. Indeed, although transformed cells frequently express Fas and TRAIL, most do not undergo apoptosis upon engagement of these receptors and significant effort has been devoted toward exploring how to sensitize such cells to the pro-apoptotic effects of 'death receptor' stimulation. Moreover, the expression of Fas and TRAIL receptors is greatly elevated in many cancer types such as hepatocellular carcinoma, renal carcinoma and ovarian cancer, suggesting that such tumors benefit from the expression of these receptors. Furthermore, several studies have shown that tumor proliferation, progression and invasion can be impaired through blocking or downregulation of Fas expression, but the mechanistic basis for these effects is largely unknown. Thus, the characterization of Fas and TRAIL as 'death receptors' is a gross oversimplification, especially in the context of cancer. It is becoming increasingly clear that 'death receptor' engagement can lead to outcomes, other than apoptosis, that become subverted by certain tumors to their benefit. Here we will discuss death-independent outcomes of Fas and TRAIL signaling and their implications for cancer.

γδ T Cells and dendritic cells in refractory Lyme arthritis

Lyme disease is a multisystem infection transmitted by tick vectors with an incidence of up to 300,000 individuals/yr in the United States. The primary treatments are oral or i.v. antibiotics. Despite treatment, some individuals do not recover and have prolonged symptoms affecting multiple organs, including the nervous system and connective tissues. Inflammatory arthritis is a common symptom associated with Lyme pathology. In the past decades, γδ T cells have emerged as candidates that contribute to the transition from innate to adaptive responses. These cells are also differentially regulated within the synovia of patients affected by RLA. Here, we review and discuss potential cellular mechanisms involving γδ T cells and DCs in RLA. TLR signaling and antigen processing and presentation will be the key concepts that we review in aid of understanding the impact of γδ T cells in RLA.

Old, new and emerging functions of caspases

Caspases are proteases with a well-defined role in apoptosis. However, increasing evidence indicates multiple functions of caspases outside apoptosis. Caspase-1 and caspase-11 have roles in inflammation and mediating inflammatory cell death by pyroptosis. Similarly, caspase-8 has dual role in cell death, mediating both receptor-mediated apoptosis and in its absence, necroptosis. Caspase-8 also functions in maintenance and homeostasis of the adult T-cell population. Caspase-3 has important roles in tissue differentiation, regeneration and neural development in ways that are distinct and do not involve any apoptotic activity. Several other caspases have demonstrated anti-tumor roles. Notable among them are caspase-2, -8 and -14. However, increased caspase-2 and -8 expression in certain types of tumor has also been linked to promoting tumorigenesis. Increased levels of caspase-3 in tumor cells causes apoptosis and secretion of paracrine factors that promotes compensatory proliferation in surrounding normal tissues, tumor cell repopulation and presents a barrier for effective therapeutic strategies. Besides this caspase-2 has emerged as a unique caspase with potential roles in maintaining genomic stability, metabolism, autophagy and aging. The present review focuses on some of these less studied and emerging functions of mammalian caspases.

Regulated cell death: signaling and mechanisms

Cell turnover is a fundamental feature in metazoans. Cells can die passively, as a consequence of severe damage to their structural integrity, or actively, owing to a more confined biological disruption such as DNA damage. Passive cell death is uncontrolled and often harmful to the organism. In contrast, active cell death is tightly regulated and serves to support the organism's life. Apoptosis-the primary form of regulated cell death-is relatively well defined. Necroptosis-an alternative, distinct kind of regulated cell death discovered more recently-is less well understood. Apoptosis and necroptosis can be triggered either from within the cell or by extracellular stimuli. Certain signaling components, including several death ligands and receptors, can regulate both processes. Whereas apoptosis is triggered and executed via intracellular proteases called caspases, necroptosis is suppressed by caspase activity. Here we highlight current understanding of the key signaling mechanisms that control regulated cell death.

The regulation and role of c-FLIP in human Th cell differentiation

The early differentiation of T helper (Th) cells is a tightly controlled and finely balanced process, which involves several factors including cytokines, transcription factors and co-stimulatory molecules. Recent studies have shown that in addition to the regulation of apoptosis, caspase activity is also needed for Th cell proliferation and activation and it might play a role in Th cell differentiation. The isoforms of the cellular FLICE inhibitory protein (c-FLIP) are regulators of CASPASE-8 activity and the short isoform, c-FLIP_S, has been shown to be up-regulated by IL-4, the Th2 driving cytokine. In this work, we have studied the expression and functional role of three c-FLIP isoforms during the early Th cell differentiation. Only two of the isoforms, c-FLIP_S and c-FLIP_L, were detected at the protein level although c-FLIP_R was expressed at the mRNA level. The knockdown of c-FLIP_L led to enhanced Th1 differentiation and elevated IL-4 production by Th2 cells, whereas the knockdown of c-FLIP_S diminished GATA3 expression and IL-4 production by Th2 cells. In summary, our results provide new insight into the role of c-FLIP proteins in the early differentiation of human Th cells.

c-FLIP-Short reduces type I interferon production and increases viremia with coxsackievirus B3

Cellular FLIP (c-FLIP) is an enzymatically inactive paralogue of caspase-8 and as such can block death receptor-induced apoptosis. However, independent of death receptors, c-FLIP-Long (c-FLIP_L) can heterodimerize with and activate caspase-8. This is critical for promoting the growth and survival of T lymphocytes as well as the regulation of the RIG-I helicase pathway for type I interferon production in response to viral infections. Truncated forms of FLIP also exist in mammalian cells (c-FLIP_S) and certain viruses (v-FLIP), which lack the C-terminal domain that activates caspase-8. Thus, the ratio of c-FLIP_L to these short forms of FLIP may greatly influence the outcome of an immune response. We examined this model in mice transgenically expressing c-FLIP_S in T cells during infection with Coxsackievirus B3 (CVB3). In contrast to our earlier findings of reduced myocarditis and mortality with CVB3 infection of c-FLIP_L-transgenic mice, c-FLIP_S-transgenic mice were highly sensitive to CVB3 infection as manifested by increased cardiac virus titers, myocarditis score, and mortality compared to wild-type C57BL/6 mice. This observation was paralleled by a reduction in serum levels of IL-10 and IFN-α in CVB3-infected c-FLIP_S mice. In vitro infection of c-FLIP_S T cells with CVB3 confirmed these results. Furthermore, molecular studies revealed that following infection of cells with CVB3, c-FLIP_L associates with mitochondrial antiviral signaling protein (MAVS), increases caspase-8 activity and type I IFN production, and reduces viral replication, whereas c-FLIP_S promotes the opposite phenotype.

Constitutive expression of murine c-FLIPR causes autoimmunity in aged mice

Death receptor-mediated apoptosis is a key mechanism for the control of immune responses and dysregulation of this pathway may lead to autoimmunity. Cellular FLICE-inhibitory proteins (c-FLIPs) are known as inhibitors of death receptor-mediated apoptosis. The only short murine c-FLIP splice variant is c-FLIP_Raji (c-FLIP_R). To investigate the functional role of c-FLIP_R in the immune system, we used the vavFLIP_R mouse model constitutively expressing murine c-FLIP_R in all hematopoietic compartments. Lymphocytes from these mice are protected against CD95-mediated apoptosis and activation-induced cell death. Young vavFLIP_R mice display normal lymphocyte compartments, but the lymphocyte populations alter with age. We identified reduced levels of T cells and slightly higher levels of B cells in 1-year-old vavFLIP_R mice compared with wild-type (WT) littermates. Moreover, both B and T cells from aged vavFLIP_R animals show activated phenotypes. Sera from 1-year-old WT and transgenic animals were analysed for anti-nuclear antibodies. Notably, elevated titres of these autoantibodies were detected in vavFLIP_R sera. Furthermore, tissue damage in kidneys and lungs from aged vavFLIP_R animals was observed, indicating that vavFLIP_R mice develop a systemic lupus erythematosus-like phenotype with age. Taken together, these data suggest that c-FLIP_R is an important modulator of apoptosis and enforced expression leads to autoimmunity.

Life in the Fas lane: differential outcomes of Fas signaling

Fas, also known as CD95 or APO-1, is a member of the tumor necrosis factor/nerve growth factor superfamily. Although best characterized in terms of its apoptotic function, recent studies have identified several other cellular responses emanating from Fas. These responses include migration, invasion, inflammation, and proliferation. In this review, we focus on the diverse cellular outcomes of Fas signaling and the molecular switches identified to date that regulate its pro- and anti-apoptotic functions. Such switches occur at different levels of signal transduction, ranging from the receptor through to cross-talk with other signaling pathways. Factors identified to date including other extracellular signals, proteins recruited to the death-inducing signaling complex, and the availability of different intracellular components of signal transduction pathways. The success of therapeutically targeting Fas will require a better understanding of these pathways, as well as the regulatory mechanisms that determine cellular outcome following receptor activation.

Cellular-FLIP, Raji isoform (c-FLIP R) modulates cell death induction upon T-cell activation and infection

Dysregulation of apoptosis caused by an imbalance of pro- and anti-apoptotic protein expression can lead to cancer, neurodegenerative, and autoimmune diseases. Cellular-FLIP (c-FLIP) proteins inhibit apoptosis directly at the death-inducing signaling complex of death receptors, such as CD95, and have been linked to apoptosis regulation during immune responses. While the isoforms c-FLIPL and c-FLIPS are well characterized, the function of c-FLIPR remains poorly understood. Here, we demonstrate the induction of endogenous murine c-FLIPR in activated lymphocytes for the first time. To analyze c-FLIPR function in vivo, we generated transgenic mice expressing murine c-FLIPR specifically in hematopoietic cells. As expected, lymphocytes from c-FLIPR transgenic mice were protected against CD95-induced apoptosis in vitro. In the steady state, transgenic mice had normal cell numbers and unaltered frequencies of B cells and T-cell subsets in lymphoid organs. However, when challenged with Listeria monocytogenes, c-FLIPR transgenic mice showed less liver necrosis and better bacterial clearance compared with infected wild-type mice. We conclude that c-FLIPR expression in hematopoietic cells supports an efficient immune response against bacterial infections.

Caspase functions in cell death and disease

Caspases are a family of endoproteases that provide critical links in cell regulatory networks controlling inflammation and cell death. The activation of these enzymes is tightly controlled by their production as inactive zymogens that gain catalytic activity following signaling events promoting their aggregation into dimers or macromolecular complexes. Activation of apoptotic caspases results in inactivation or activation of substrates, and the generation of a cascade of signaling events permitting the controlled demolition of cellular components. Activation of inflammatory caspases results in the production of active proinflammatory cytokines and the promotion of innate immune responses to various internal and external insults. Dysregulation of caspases underlies human diseases including cancer and inflammatory disorders, and major efforts to design better therapies for these diseases seek to understand how these enzymes work and how they can be controlled.

ATM kinase activity modulates ITCH E3-ubiquitin ligase activity

Ataxia Telangiectasia Mutated (ATM) kinase, a central regulator of the DNA damage response regulates the activity of several E3-ubiquitin ligases and the ubiquitination-proteasome system is a consistent target of ATM. ITCH is an E3-ubiquitin ligase that modulates the ubiquitination of several targets, therefore participating to the regulation of several cellular responses, among which the DNA damage response, TNFα, Notch and Hedgehog signalling and T cell development.

Here we uncover ATM as a novel positive modulator of ITCH E3-ubiquitin ligase activity. A single residue on ITCH protein, S161, which is part of an ATM SQ consensus motif, is required for ATM-dependent activation of ITCH. ATM activity enhances ITCH enzymatic activity, which in turn drives the ubiquitination and degradation of c-FLIP-L and c-Jun, previously identified as ITCH substrates. Importantly, Atm deficient mice show resistance to hepatocyte cell death, similarly to Itch deficient animals, providing in vivo genetic evidence for this circuit.

Our data identify ITCH as a novel component of the ATM-dependent signaling pathway and suggest that the impairment of the correct functionality of ITCH caused by Atm deficiency may contribute to the complex clinical features linked to Ataxia Telangiectasia.

Regulation of chemoresistance via alternative messenger RNA splicing

The acquisition of resistance to chemotherapy is a significant problem in the treatment of cancer, greatly increasing patient morbidity and mortality. Tumors are often sensitive to chemotherapy upon initial treatment, but repeated treatments can select for those cells that were able to survive initial therapy and have acquired cellular mechanisms to enhance their resistance to subsequent chemotherapy treatment. Many cellular mechanisms of drug resistance have been identified, most of which result from changes in gene and protein expression. While changes at the transcriptional level have been duly noted, it is primarily the post-transcriptional processing of pre-mRNA into mature mRNA that regulates the composition of the proteome and it is the proteome that actually regulates the cell's response to chemotherapeutic insult, inducing cell survival or death. During pre-mRNA processing, intronic non-protein-coding sequences are removed and protein-coding exons are spliced to form a continuous template for protein translation. Alternative splicing involves the differential inclusion or exclusion of exonic sequences into the mature transcript, generating different mRNA templates for protein production. This regulatory mechanism enables the potential to produce many different protein isoforms from the same gene. In this review I will explain the mechanism of alternative pre-mRNA splicing and look at some specific examples of how splicing factors, splicing factor kinases and alternative splicing of specific pre-mRNAs from genes have been shown to contribute to acquisition of the drug resistant phenotype.

Effect of transgenic overexpression of FLIP on lymphocytes on development and resolution of experimental autoimmune thyroiditis

In our previous studies, resolution of granulomatous experimental autoimmune thyroiditis (G-EAT) was promoted when thyroid epithelial cells were protected from Fas-mediated apoptosis due to transgenic overexpression of FLIP. We hypothesized that if FLIP were overexpressed on lymphocytes, CD4(+) effector cells would be protected from Fas-mediated apoptosis, and resolution would be delayed. To test this hypothesis, we generated transgenic (Tg) mice overexpressing FLIP under the CD2 promoter. Transgenic FLIP was expressed on CD4(+) and CD8(+) T cells and B cells. Transgenic overexpression of FLIP protected cultured splenocytes from Fas-mediated, but not irradiation-induced, apoptosis in vitro. Unexpectedly, Tg(+) donor cells transferred minimal G-EAT, which was partially overcome by depleting donor CD8(+) T cells. When Tg(+) and Tg(-) donors transferred equivalent disease, G-EAT resolution was delayed in FLIP transgenic mice. However, CD2-FLIP Tg(+) donors often transferred less severe G-EAT, even after depletion of CD8(+) T cells. This influenced the rate of G-EAT resolution, resulting in little difference in G-EAT resolution between groups. Tg(+) mice always had reduced anti-mouse thyroglobulin autoantibody responses, compared with Tg(-) littermates, presumably because of FLIP overexpression on B cells. These results suggest that effects of transgenic FLIP on a particular autoimmune disease vary, depending on what cells express the transgene and whether those cells are effector cells or if they function to modulate disease.

Proliferative versus apoptotic functions of caspase-8 Hetero or homo: the caspase-8 dimer controls cell fate

Caspase-8, the initiator of extrinsically-triggered apoptosis, also has important functions in cellular activation and differentiation downstream of a variety of cell surface receptors. It has become increasingly clear that the heterodimer of caspase-8 with the long isoform of cellular FLIP (FLIP(L)) fulfills these pro-survival functions of caspase-8. FLIP(L), a catalytically defective caspase-8 paralog, can interact with caspase-8 to activate its catalytic function. The caspase-8/FLIP(L) heterodimer has a restricted substrate repertoire and does not induce apoptosis. In essence, caspase-8 heterodimerized with FLIP(L) prevents the receptor interacting kinases RIPK1 and -3 from executing the form of cell death known as necroptosis. This review discusses the latest insights in caspase-8 homo- versus heterodimerization and the implication this has for cellular death or survival. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Cellular FLIP(L) plays a survival role and regulates morphogenesis in breast epithelial cells

Strong evidences support the inhibitory activity of cellular FLICE-inhibitory protein (FLIP) in the apoptotic signalling by death receptors in tumor cells. However, little is known about the role of FLIP in the regulation of apoptosis in non-transformed cells. In this report, we demonstrate that FLIP(L) plays an important role as a survival protein in non-transformed breast epithelial cells. Silencing of FLIP(L) by siRNA methodology enhances TRAIL-R2 expression and activates a caspase-dependent cell death process in breast epithelial cells. This cell death requires the expression of TRAIL, TRAIL-R2, FADD and procaspase-8 proteins. A mitochondria-operated apoptotic pathway is partially required for FLIP(L) siRNA-induced apoptosis. Interestingly, FLIP(L) silencing markedly abrogates formation of acinus-like structures in a three-dimensional basement membrane culture model (3D) of the human mammary MCF-10A cell line through a caspase-8 dependent process. Furthermore, over-expression of FLIP(L) in MCF-10A cells delayed lumen formation in 3D cultures. Our results highlight the central role of FLIP in maintaining breast epithelial cell viability and suggest that the mechanisms regulating FLIP levels should be finely controlled to prevent unwanted cell demise.

Viral cell death inhibitor MC159 enhances innate immunity against vaccinia virus infection

Viral inhibitors of host programmed cell death (PCD) are widely believed to promote viral replication by preventing or delaying host cell death. Viral FLIPs (Fas-linked ICE-like protease [FLICE; caspase-8]-like inhibitor proteins) are potent inhibitors of death receptor-induced apoptosis and programmed necrosis. Surprisingly, transgenic expression of the viral FLIP MC159 from molluscum contagiosum virus (MCV) in mice enhanced rather than inhibited the innate immune control of vaccinia virus (VV) replication. This effect of MC159 was specifically manifested in peripheral tissues such as the visceral fat pad, but not in the spleen. VV-infected MC159 transgenic mice mounted an enhanced innate inflammatory reaction characterized by increased expression of the chemokine CCL-2/MCP-1 and infiltration of γδ T cells into peripheral tissues. Radiation chimeras revealed that MC159 expression in the parenchyma, but not in the hematopoietic compartment, is responsible for the enhanced innate inflammatory responses. The increased inflammation in peripheral tissues was not due to resistance of lymphocytes to cell death. Rather, we found that MC159 facilitated Toll-like receptor 4 (TLR4)- and tumor necrosis factor (TNF)-induced NF-κB activation. The increased NF-κB responses were mediated in part through increased binding of RIP1 to TNFRSF1A-associated via death domain (TRADD), two crucial signal adaptors for NF-κB activation. These results show that MC159 is a dual-function immune modulator that regulates host cell death as well as NF-κB responses by innate immune signaling receptors.

c-FLIPL enhances anti-apoptotic Akt functions by modulation of Gsk3β activity

Akt is a serine-threonine kinase that has an important role in transducing survival signals. Akt also regulates a number of proteins involved in the apoptotic process. To find new Akt interactors, we performed a two-hybrid screening in yeast using full-length Akt cDNA as bait and a human cDNA heart library as prey. Among 200 clones obtained, two of them were identified as coding for the c-FLIP(L) protein. c-FLIP(L) is an endogenous inhibitor of death receptor-induced apoptosis through the caspase-8 pathway. Using co-immunoprecipitation experiments of either transfected or endogenous proteins, we confirmed the interaction between Akt and c-FLIP(L). Furthermore, we observed that c-FLIP(L) overexpression interferes with Gsk3-β phosphorylation levels. Moreover, through its effects on Gsk3β, c-FLIP(L) overexpression in cancer cells induced resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). This effect was mediated by the regulation of p27(Kip1) and caspase-3 expression. These results indicate the existence of a new mechanism of resistance to TRAIL in cancer cells, and unexpected functions of c-FLIP(L).

The death receptor antagonist FLIP-L interacts with Trk and is necessary for neurite outgrowth induced by neurotrophins

FLICE-inhibitory protein (FLIP) is an endogenous inhibitor of the signaling pathway triggered by the activation of death receptors. Here, we reveal a novel biological function for the long form of FLIP (FLIP-L) in neuronal differentiation, which can be dissociated from its antiapoptotic role. We show that FLIP-L is expressed in different regions of the mouse embryonic nervous system. Immunohistochemistry of mouse brain sections at different stages reveals that, in neurons, FLIP is expressed early during the embryonic neuronal development (embryonic day 16) and decreases at later stages (postnatal days 5-15), when its expression is essentially detected in glial cells. FLIP-L overexpression significantly enhances neurotrophin-induced neurite outgrowth in motoneurons, superior cervical ganglion neurons, and PC12 cells. Conversely, the downregulation of FLIP-L protein levels by specific RNA interference significantly reduces neurite outgrowth, even in the presence of the appropriate neurotrophin stimulus. Moreover, NGF-dependent activation of two main intracellular pathways involved in the regulation of neurite outgrowth, extracellular signal-regulated kinases (ERKs) and nuclear factor kappaB (NF-kappaB), is impaired when endogenous FLIP-L is downregulated, although TrkA remains activated. Finally, we demonstrate that FLIP-L interacts with TrkA, and not with p75(NTR), in an NGF-dependent manner, and endogenous FLIP-L interacts with TrkB in whole-brain lysates from embryonic day 15 mice embryos. Altogether, we uncover a new role for FLIP-L as an unexpected critical player in neurotrophin-induced mitogen-activated protein kinase/ERK- and NF-kappaB-mediated control of neurite growth in developing neurons.

Dynamics within the CD95 death-inducing signaling complex decide life and death of cells

This study explores the dilemma in cellular signaling that triggering of CD95 (Fas/APO-1) in some situations results in cell death and in others leads to the activation of NF-kappaB. We established an integrated kinetic mathematical model for CD95-mediated apoptotic and NF-kappaB signaling. Systematic model reduction resulted in a surprisingly simple model well approximating experimentally observed dynamics. The model postulates a new link between c-FLIP(L) cleavage in the death-inducing signaling complex (DISC) and the NF-kappaB pathway. We validated experimentally that CD95 stimulation resulted in an interaction of p43-FLIP with the IKK complex followed by its activation. Furthermore, we showed that the apoptotic and NF-kappaB pathways diverge already at the DISC. Model and experimental analysis of DISC formation showed that a subtle balance of c-FLIP(L) and procaspase-8 determines life/death decisions in a nonlinear manner. We present an integrated model describing the complex dynamics of CD95-mediated apoptosis and NF-kappaB signaling.

Altered thymic selection by overexpressing cellular FLICE inhibitory protein in T cells causes lupus-like syndrome in a BALB/c but not C57BL/6 strain

Cellular FLICE inhibitory protein (c-FLIP) is an endogenous inhibitor of the caspase-8 pro-apoptotic signaling pathway downstream of death receptors. Recent evidence indicates that the long form of c-FLIP (c-FLIP_L) is required for proliferation and effector T cell development. However, the role of c-FLIP_L in triggering autoimmunity has not been carefully investigated. We now report that c-FLIP_L transgenic (Tg) mice develop splenomegaly, lymphadenopathy, multi-organ infiltration, high titers of autoantibodies, and proliferative glomerulonephritis with immune complex deposition in a strain-dependent fashion. The development of autoimmunity requires CD4⁺ T cells and may result from impaired thymic selection. At the molecular level, c-FLIP_L over-expression inhibits the ZAP-70 activation, thus impairing the signaling pathway derived from ZAP-70 required for thymic selection. Therefore, we have identified c-FLIP_L as a susceptibility factor under the influence of epistatic modifiers for the development of autoimmunity.

Pregnancy alters the proliferation and apoptosis of mouse splenic erythroid lineage cells and leukocytes

Pregnancy induces dynamic changes in the maternal environment that include reversible modifications in response to systemic mediators and local signals. The spleen can be used to determine the effects of pregnancy on multiple cellular populations, including those of the erythroid lineage and the immune system. Current evidence suggests that the transient increase in the size of the spleen during pregnancy is due to the expansion of erythroid precursors. However, it is unclear what factors contribute to this increase. Moreover, the additional erythroid cells may compete with neighboring leukocytes for growth factors or space, and this may in turn alter the function of these populations. Therefore, we assessed proliferation and apoptosis throughout gestation using in vivo bromodeoxyuridine incorporation and the TUNEL assay, respectively. Here, we show that erythroid-lineage TER-119(+) cells expanded significantly in midgestation because of enhanced proliferation and diminished apoptosis. This correlated with increased expression of the erythropoietin receptor (Epor) and decreased expression of the death receptor Fas, respectively. Leukocytes demonstrated population-specific responses. Natural killer cells proliferated in early pregnancy. Both lymphocytes and CD11B(+) cells underwent enhanced proliferation during midgestation. In contrast, neutrophils exhibited augmented proliferation throughout pregnancy. These subset-specific alterations in proliferation and death in the spleen suggest that complex regulation of population dynamics exists during pregnancy.

Caspase-8 association with the focal adhesion complex promotes tumor cell migration and metastasis

Caspase-8 is a proapoptotic protease that suppresses neuroblastoma metastasis by inducing programmed cell death. Paradoxically, caspase-8 can also promote cell migration among nonapoptotic cells; here, we show that caspase-8 can promote metastasis when apoptosis is compromised. Migration is enhanced by caspase-8 recruitment to the cellular migration machinery following integrin ligation. Caspase-8 catalytic activity is not required for caspase-8-enhanced cell migration; rather, caspase-8 interacts with a multiprotein complex that can include focal adhesion kinase and calpain 2 (CPN2), enhancing cleavage of focal adhesion substrates and cell migration. Caspase-8 association with CPN2/calpastatin disrupts calpastatin-mediated inhibition of CPN2. In vivo, knockdown of either caspase-8 or CPN2 disrupts metastasis among apoptosis-resistant tumors. This unexpected molecular collaboration provides an explanation for the continued or elevated expression of caspase-8 observed in many tumors.

A new function of the Fas-FasL pathway in macrophage activation

Upon infection with the protozoan parasite Leishmania major, susceptible BALB/c mice develop unhealing lesions associated with the maturation of CD4(+)Th2 cells secreting IL-4. In contrast, resistant C57BL/6 mice heal their lesions, because of expansion and secretion of IFN-gamma of CD4(+) Th1 cells. The Fas-FasL pathway, although not involved in Th cell differentiation, was reported to be necessary for complete resolution of lesions. We investigate here the role of IFN-gamma and IL-4 on Fas-FasL nonapoptotic signaling events leading to the modulation of macrophage activation. We show that addition of FasL and IFN-gamma to BMMø led to their increased activation, as reflected by enhanced secretion of TNF, IL-6, NO, and the induction of their microbicidal activity, resulting in the killing of intracellular L. major. In contrast, the presence of IL-4 decreased the synergy of IFN-gamma/FasL significantly on macrophage activation and the killing of intracellular L. major. These results show that FasL synergizes with IFN-gamma to activate macrophages and that the tight regulation by IFN-gamma and/or IL-4 of the nonapoptotic signaling events triggered by the Fas-FasL pathway affects significantly the activation of macrophages to a microbicidal state and may thus contribute to the pathogenesis of L. major infection.

Nuclear localization of c-FLIP-L and its regulation of AP-1 activity

Cellular FLICE-like inhibitory protein (c-FLIP-L), similar in structure to caspase-8, is capable of blocking Fas- or other death receptors (DR)-mediated apoptosis through association with FADD in the DISC. Recent studies have implicated the function of c-FLIP-L in T-cell proliferation, but the exact mechanism underlying this process remains to be elucidated. In this report, we showed for the first time that c-FLIP-L was present in both the cytoplasm and nucleus of cells, but was more abundantly distributed in the nucleus. The putative NLS signal locates within the p12 region of caspase-like domain. Furthermore, c-FLIP's export to cytoplasm membrane was dependent on apoptotic stimulation, while it rapidly translocated to the nucleus in response to proliferative stimuli. To gain insights into the possible function of c-FLIP-L in the nucleus, we found c-FLIP-L could activate the AP-1 transcriptional activity independent of MAPK activation. In sum, our findings describe a novel function of c-FLIP-L involved in AP-1 activation and cell proliferation.

FLIP and the death effector domain family

Death effector domains (DEDs) are protein interaction modules found in a number of proteins known to regulate apoptosis from death receptors. The core DED family members that orchestrate programmed cell death from death receptors include the adaptor protein FADD, the initiator caspases procaspases-8 and -10 and the regulatory protein c-FLIP. Through homotypic DED interactions, these proteins assemble into the death-inducing signaling complex (DISC) to regulate initiator caspase activation and launch the apoptotic proteolytic cascade. A considerable body of evidence, however, is revealing that the same core group of DED-containing proteins also paradoxically promotes survival and proliferation in lymphocytes and possibly other cell types. This review delves into recent findings regarding these two opposing functional aspects of the core DED proteins. We discuss the current effort expanding our structural and biochemical view of how DED proteins assemble into the DISC to fully activate initiator caspases and execute cell death, and finally we examine details linking the same proteins to proliferation and describe how this outcome might be achieved through restricted activation of initiator caspases.

Caspases as therapeutic targets

The development of small molecules to modulate caspase activity offers a novel therapeutic strategy in the treatment of apoptosis-related and inflammatory diseases. Caspases are key mediators of apoptosis and inflammation; deregulation of their activation or expression can lead to the development of conditions such as neurodegenerative and autoinflammatory disorders. This review details the different caspase-associated disorders while focusing on caspase-1 inhibition as a potential therapeutic strategy. Problems facing the development of effective and safe caspase therapeutics will also be addressed.

Death receptors and their ligands in atherosclerosis

Atherosclerosis is characterized by the accumulation of a fibro-fatty plaque consisting of immune cells, vascular smooth muscle cells (VSMCs), vascular endothelial cells (ECs), and extracellular matrix, surrounding a lipid-rich core. The complexity of atherosclerosis is highlighted by the multifaceted effects that apoptosis and proliferation of specific cell types can have on vessels at different stages of the disease. Death receptors are membrane-bound protein complexes that on binding their cognate ligand, activate an intracellular signaling cascade that results in apoptosis. More recently, signaling from these receptors has been shown to activate multiple other processes, including cell proliferation. This review summarizes our current understanding of signaling events after death receptor activation and the role of death receptors and their ligands in atherosclerosis.

The long isoform of cellular FLIP is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway

Although the long isoform of cellular FLIP (c-FLIP(L)) has been implicated in TCR-mediated signaling, its role in T cell proliferation remains controversial. Some studies have demonstrated that overexpression of c-FLIP(L) promotes T cell proliferation and NF-kappaB activation, whereas others have reported that c-FLIP(L) overexpression has no effect or even inhibits T cell proliferation. To establish the role of c-FLIP(L) in T lymphocyte proliferation, we have generated a conditional knockout mouse strain specifically lacking c-FLIP(L) in T lymphocytes. c-FLIP(L)(-/-) mice exhibit severely impaired effector T cell development after Listeria monocytogenes infection in vivo and c-FLIP(L)-deficient T cells display defective TCR-mediated proliferation in vitro. However, c-FLIP(L)(-/-) T cells exhibit normal NF-kappaB activity upon TCR stimulation. These results demonstrate that c-FLIP(L) is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway.

A different TIPE of immune homeostasis

Proteins with death effector domains (DED) are key signal transducers involved in cell death and inflammation. In this issue of Cell, Sun et al. (2008) describe TIPE2, a DED protein that negatively regulates both T cell receptor and Toll-like receptor signaling. These findings reveal a new element critical to the maintenance of homeostasis in both the adaptive and innate immune systems.

Catalytically active Yersinia outer protein P induces cleavage of RIP and caspase-8 at the level of the DISC independently of death receptors in dendritic cells

Yersinia outer protein P (YopP) is injected by Y. enterocolitica into host cells thereby inducing apoptotic and necrosis-like cell death in dendritic cells (DC). Here we show the pathways involved in DC death caused by the catalytic activity of YopP. Infection with Yersinia enterocolitica, translocating catalytically active YopP into DC, triggered procaspase-8 cleavage and c-FLIPL degradation. YopP-dependent caspase-8 activation was, however, not mediated by tumor necrosis factor (TNF) receptor family members since the expression of both CD95/Fas/APO-1 and TRAIL-R2 on DC was low, and DC were resistant to apoptosis induced by agonistic anti-CD95 antibodies or TNF-related apoptosis-inducing ligand (TRAIL). Moreover, DC from TNF-Rp55-/- mice were not protected against YopP-induced cell death demonstrating that TNF-R1 is also not involved in this process. Activation of caspase-8 was further investigated by coimmunoprecitation of FADD from Yersinia-infected DC. We found that both cleaved caspase-8 and receptor interacting protein 1 (RIP1) were associated with the Fas-associated death domain (FADD) indicating the formation of an atypical death-inducing signaling complex (DISC). Furthermore, degradation of RIP mediated by the Hsp90 inhibitor geldanamycin significantly impaired YopP-induced cell death. Altogether our findings indicate that Yersinia-induced DC death is independent of death domain containing receptors, but mediated by RIP and caspase-8 at the level of DISC.

c-FLIP(S) reduces activation of caspase and NF-kappaB pathways and decreases T cell survival

Effective stimulation of NF-kappaB in T cells following TCR ligation requires the activity of caspase-8. The active caspase-8 complex includes the paracaspase, MALT1, and Bcl-10, which connect to the NF-kappaB pathway. It has been less clear what regulates the level of caspase-8 activity during T cell activation. A likely candidate is cellular FLIP (c-FLIP), an enzymatically inert caspase-8 homologue. Two alternatively spliced forms of c-FLIP exist, a long form (c-FLIP(L)) and a short-form (c-FLIP(S)). The latter lacks the C-terminal caspase-like domain. c-FLIP(L) can heterodimerize with and activate caspase-8 through an activation loop in the C terminus of c-FLIP(L). Here we show that, in contrast to c-FLIP(L), c-FLIP(S) inhibits activation of caspase-8 in T cells, and consequently reduces recruitment of MALT1 and Bcl-10 to the active caspase complex. This results in reduced activity of NF-kappaB. Consequently, T cells from c-FLIP(S)-transgenic mice undergo more rapid cell death both spontaneously and after activation. The findings suggest that c-FLIP(S) functions to reduce the expansion of T cells during an immune response.

Dendritic cell-induced apoptosis of human cytomegalovirus-infected fibroblasts promotes cross-presentation of pp65 to CD8+ T cells

An efficient host response to human cytomegalovirus (HCMV) infection may depend on rapid sensing of the infection by the innate immune response prior to deployment of viral immunosubversive functions. Control of HCMV dissemination could be ensured by apoptosis of cells immediately following infection. In the present report, it is demonstrated that changes in the ratio of c-FLIP to FLICE contributed to early sensitivity of HCMV-infected MRC5 fibroblasts to tumour necrosis factor alpha (TNF-alpha), providing an innate response to infection. Dendritic cells (DCs) co-cultured with HCMV-infected MRC5 cells acquired the ability to secrete TNF-alpha in an amount sufficient to kill infected fibroblasts. Blockage of TNF-alpha binding to its receptor on MRC5 cells with soluble TNF-R reduced the number of dead, HCMV-infected fibroblasts ingested by DCs, thus highlighting the impact of the apoptotic state of infected cells for efficient loading of DCs. Those DCs loaded with antigens available early in infection, such as input virion-associated pp65, could then engage antigen processing for cross-presentation to specific CD8(+) T cells. Cross-presentation was impaired when MRC5 cells were treated with the pan-caspase inhibitor ZVAD before co-culture with DCs. Altogether, our data suggest that the innate killing capacity of DCs at the early stage of infection plays a role in the activation of anti-HCMV CD8(+) T cells.

Expression of transgenic FLIP on thyroid epithelial cells inhibits induction and promotes resolution of granulomatous experimental autoimmune thyroiditis in CBA/J mice

Granulomatous experimental autoimmune thyroiditis (G-EAT) is induced by transfer of thyroglobulin-primed in vitro activated splenocytes. Thyroid lesions reach maximal severity 20 d later, and inflammation resolves or progresses to fibrosis by d 60, depending on the extent of thyroid damage at d 20. Depletion of CD8+ T cells inhibits G-EAT resolution. We showed that expression of Fas-associated death domain-like IL-1beta-converting enzyme inhibitory protein (FLIP) transgene (Tg) on thyroid epithelial cells (TECs) of DBA/1 mice had no effect on G-EAT induction but promoted earlier resolution of G-EAT. However, when CBA/J wild-type donor cells were transferred to transgenic CBA/J mice expressing FLIP on TECs, they developed less severe G-EAT than FLIP Tg- littermates. Both strains expressed similar levels of the FLIP Tg, but endogenous FLIP was up-regulated to a greater extent on infiltrating T cells during G-EAT development in DBA/1 compared with CBA/J mice. After transient depletion of CD8+ T cells, FLIP Tg+ and Tg- CBA/J recipients both developed severe G-EAT at d 20. Thyroid lesions in CD8-depleted Tg+ recipients were resolving by d 60, whereas lesions in Tg- littermates did not resolve, and most were fibrotic. FLIP Tg+ recipients had increased apoptosis of CD3+ T cells compared with Tg- recipients. The results indicate that transgenic FLIP expressed on TECs in CBA/J mice promotes G-EAT resolution, but induction of G-EAT is inhibited unless CD8+ T cells are transiently depleted.