Inhibition of Fas-mediated apoptosis by the B cell antigen receptor through c-FLIP

Non-apoptotic FAS signaling controls mTOR activation and extrafollicular maturation in human B cells

Defective FAS (CD95/Apo-1/TNFRSF6) signaling causes autoimmune lymphoproliferative syndrome (ALPS). Hypergammaglobulinemia is a common feature in ALPS with FAS mutations (ALPS-FAS), but paradoxically, fewer conventional memory cells differentiate from FAS-expressing germinal center (GC) B cells. Resistance to FAS-induced apoptosis does not explain this phenotype. We tested the hypothesis that defective non-apoptotic FAS signaling may contribute to impaired B cell differentiation in ALPS. We analyzed secondary lymphoid organs of patients with ALPS-FAS and found low numbers of memory B cells, fewer GC B cells, and an expanded extrafollicular (EF) B cell response. Enhanced mTOR activity has been shown to favor EF versus GC fate decision, and we found enhanced PI3K/mTOR and BCR signaling in ALPS-FAS splenic B cells. Modeling initial T-dependent B cell activation with CD40L in vitro, we showed that FAS competent cells with transient FAS ligation showed specifically decreased mTOR axis activation without apoptosis. Mechanistically, transient FAS engagement with involvement of caspase-8 induced nuclear exclusion of PTEN, leading to mTOR inhibition. In addition, FASL-dependent PTEN nuclear exclusion and mTOR modulation were defective in patients with ALPS-FAS. In the early phase of activation, FAS stimulation promoted expression of genes related to GC initiation at the expense of processes related to the EF response. Hence, our data suggest that non-apoptotic FAS signaling acts as molecular switch between EF versus GC fate decisions via regulation of the mTOR axis and transcription. The defect of this modulatory circuit may explain the observed hypergammaglobulinemia and low memory B cell numbers in ALPS.

CRISPR-mediated upregulation of DR5 and downregulation of cFLIP synergistically sensitize HeLa cells to TRAIL-mediated apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has received attention as an anticancer therapy because it mediates apoptosis of several cancer cell types but not normal human cell types. In this study, we implemented genome editing techniques to upregulate DR5 and downregulate cFLIP in HeLa cells to stimulate TRAIL-induced apoptosis. We designed and validated sgRNAs to enrich the endogenous level of DR5 by dead Cas9 (dCas9). Similarly, we designed two sgRNAs to disrupt the cFLIP gene by CRISPR/Cas9. We analyzed the effect of TRAIL on tumor cells by co-transfecting HeLa cells with the best combinations of sgRNAs regulating DR5 and cFLIP genes. TRAIL-induced apoptosis in HeLa cells was evaluated by the γH2AX foci formation assay to check for double-strand break and propidium iodide and Annexin V staining to quantify apoptotic cells. Viable cells were identified by CCK-8 assay, and cleaved-PARP level was evaluated by Western blot. This is the first study to demonstrate that genome editing techniques can be used as an effective combinatorial treatment strategy to induce apoptosis of cancer cells. In particular, enhancement of DR5 expression and inhibition of cFLIP expression by genome editing had a synergistic effect of inhibiting proliferation and inducing apoptosis in TRAIL-resistant HeLa cells. These results suggest that combinatorial treatment strategies mediated by the CRISPR/Cas9 system may be effective for design of other human TRAIL-resistant cell types.

Inhibition of Apoptosis by Expression of Antiapoptotic Proteins in Recombinant Human Keratinocytes

The Fas ligand/Fas interaction plays an important role in the regulation of immune responses. Allografted cells undergo Fas-mediated apoptosis induced by CD8+ T cells. Our objective was to prevent human keratinocytes from immunologically induced apoptosis. We focused on three proteins with inhibitory function on Fas-mediated apoptosis. Human keratinocytes were transfected with either Flip, Faim, or Lifeguard (LFG). The treatment proved to be practicable and efficient. The recombinant keratinocytes with expression of our target proteins were cocultured with CD8+ T cells and the apoptotic activity was then evaluated. Activation of caspase-8 was detectable in control but not in the recombinant cells. Quantitative analysis revealed significant induction of T-cell-induced apoptosis in nontransfected keratinocytes (p = 0.04, n = 12) but not in Flip (p = 0.66), Faim (p = 0.42), or LFG (p = 0.44) expressing cells. Our results suggest that heterotopic expression of antiapoptotic proteins can induce the resistance of keratinocytes to a major mechanism of rejection.

Cyclopamine sensitizes TRAIL-resistant gastric cancer cells to TRAIL-induced apoptosis via endoplasmic reticulum stress-mediated increase of death receptor 5 and survivin degradation

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is one of the most effective cancer treatments owing to its ability to selectively kill cancer cells, without affecting normal cells. However, it has been reported that several gastric cancer cells show resistance to TRAIL because of a scarcity of death receptor 5 (DR5) expressed on the cell surface. In this study, we show that cyclopamine sensitizes gastric cancer cells to TRAIL-induced apoptosis by elevating the expression of DR5. Interestingly, survivin hampers the existence of DR5 protein under normal conditions and cyclopamine decreases the expression of survivin, thus acting as a TRAIL sensitizer. Mechanistically, cyclopamine induces endoplasmic reticulum (ER) stress via reactive oxygen species (ROS) and CHOP, the last protein of the ER stress pathway and it regulates the proteasome degradation of survivin. Taken together, our results indicate that cyclopamine can be used for combination therapy in TRAIL-resistant gastric cancer cells.

Endotoxemia contributes to CD27+ memory B-cell apoptosis via enhanced sensitivity to Fas ligation in patients with Cirrhosis

Peripheral CD27⁺ memory B-cells become quantitatively reduced and dysfunctional in patients with cirrhosis through poorly characterized mechanisms. We hypothesized that the disappearance of CD27⁺ memory B-cells results from enhanced sensitivity to apoptosis caused by exposure to gut microbial translocation products. Using isolated naïve and memory B-cells from patients with cirrhosis and age-matched controls, ex vivo and activation-induced sensitivity to Fas-mediated apoptosis was assessed under relevant experimental conditions. We observed differential expression of CD95(Fas) in CD27⁺ B-cells from cirrhotic patients that was inversely correlated with peripheral CD27⁺ B-cell frequency. While memory B-cells from cirrhotic patients were resistant to Fas-mediated apoptosis ex vivo, Toll-like receptor 4(TLR4)-ligation restored Fas-sensitivity. Sensitivity to Fas-mediated apoptosis could be transferred to healthy donor memory B-cells by co-culturing these cells with plasma from cirrhotic patients, a sensitivity partially mediated by Fas and TLR4 signaling, and partially rescued via B-cell receptor crosslinking. We conclude that peripheral CD27⁺ memory B-cells in cirrhosis exhibit increased sensitivity to Fas-induced apoptosis in an activation-dependent manner to which endotoxin contributes, associated with reduced frequency of circulating memory B-cells. Destruction of this critical cell subset may contribute to the cirrhotic immunodeficiency state and heightened risk of systemic infections in advanced liver disease.

Hyperthermia restores apoptosis induced by death receptors through aggregation-induced c-FLIP cytosolic depletion

TRAIL is involved in immune tumor surveillance and is considered a promising anti-cancer agent owing to its limited side effects on healthy cells. However, some cancer cells display resistance, or become resistant to TRAIL-induced cell death. Hyperthermia can enhance sensitivity to TRAIL-induced cell death in various resistant cancer cell lines, including lung, breast, colon or prostate carcinomas. Mild heat shock treatment has been proposed to restore Fas ligand or TRAIL-induced apoptosis through c-FLIP degradation or the mitochondrial pathway. We demonstrate here that neither the mitochondria nor c-FLIP degradation are required for TRAIL-induced cell death restoration during hyperthermia. Our data provide evidence that insolubilization of c-FLIP, alone, is sufficient to enhance apoptosis induced by death receptors. Hyperthermia induced c-FLIP depletion from the cytosolic fraction, without apparent degradation, thereby preventing c-FLIP recruitment to the TRAIL DISC and allowing efficient caspase-8 cleavage and apoptosis. Hyperthermia-induced c-FLIP depletion was independent of c-FLIP DED2 FL chain assembly motif or ubiquitination-mediated c-FLIP degradation, as assessed using c-FLIP point mutants on lysine 167 and 195 or threonine 166, a phosphorylation site known to regulate ubiquitination of c-FLIP. Rather, c-FLIP depletion was associated with aggregation, because addition of glycerol not only prevented the loss of c-FLIP from the cytosol but also enabled c-FLIP recruitment within the TRAIL DISC, thus inhibiting TRAIL-induced apoptosis during hyperthermia. Altogether our results demonstrate that c-FLIP is a thermosensitive protein whose targeting by hyperthermia allows restoration of apoptosis induced by TNF ligands, including TRAIL. Our findings suggest that combining TRAIL agonists with whole-body or localized hyperthermia may be an interesting approach in cancer therapy.

TLR9-mediated signals rescue B-cells from Fas-induced apoptosis via inactivation of caspases

The death receptor, CD95/Fas, serves to eliminate potentially dangerous, self-reactive B cells. Engagement of B-cell receptors (BCR) on mature B-cells mediates the escape from cell death resulting in the activation and expansion of antigen specific clones. In addition to the antigen receptors, the receptors of B-cell activating factor belong to the tumor necrosis factor (TNF) family (BAFFR); moreover, the pattern recognition receptor, TLR9 may also deliver survival signals inhibiting Fas-mediated death of B-cells. Our aim was to compare the mechanism of BCR-induced and the BAFFR- or TLR9-stimulated rescue of B-cells from CD95/Fas-mediated apoptosis. We have found that BAFFR and TLR9 collaborate with BCR to protect B-cells from Fas-induced elimination and the rescue is independent of protein synthesis. The results revealed that the TLR9- and BCR-triggered rescue signals are transmitted through partially overlapping pathways; the protein kinase C (PKC) and the abl kinase induced phosphorylation may inactivate caspases in both CpG and anti-IgG stimulated cells. However, PI3-K activation is crucial upon the BCR driven anti-apoptotic effect, while p38 MAPK-mediated inactivation of caspases seems to play essential role in TLR9-mediated protection against Fas-induced programmed cell death.

The Fas/CD95 Receptor Regulates the Death of Autoreactive B Cells and the Selection of Antigen-Specific B Cells

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

Simultaneously targeting bcl-2 and Akt pathways sensitizes nasopharyngeal carcinoma to tumor necrosis factor-related apoptosis-inducing ligand

The 5-year survival rate of nasopharyngeal carcinoma (NPC) is disappointing despite the much improved technologies in its treatment. Thus, finding more effective treatment for NPC has become an urgent priority. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can selectively induce apoptosis in most tumor cells while sparing normal cells. However, its potential in the treatment of NPC has been limited by the eventual emergence of drug resistance. Bcl-2 and Akt contribute to TRAIL resistance in some cancer cells. In this study, CNE-2 was found to be the most resistant NPC cell line to TRAIL, and whether Bcl-2 small-interfering RNA (siRNA) and phosphatidylinositol 3-kinase (PI3-K) inhibitors (LY294002 and Wortmannin) could prevent TRAIL resistance in CNE-2 was also investigated. Results showed that both Bcl-2 siRNA and PI3-K inhibitors could prevent TRAIL resistance in CNE-2. Bcl-2 siRNA sensitized CNE-2 by activating the intrinsic apoptotic pathway and PI3-K inhibitors sensitized CNE-2 by activating both intrinsic and extrinsic pathways. Further, simultaneously targeting Bcl-2 and Akt was found to be a more efficient approach to prevent TRAIL resistance in CNE-2 and this synergistic effect happened at the level of Bid downstream. At last, the combinative treatments did not enhance toxicity of TRAIL in MRC5, a human benign fibroblast cell line. This study suggests that simultaneously targeting Bcl-2 and Akt pathway might be effective in preventing TRAIL resistance of NPC cells.

Regulating TRAIL receptor-induced cell death at the membrane : a deadly discussion

The use of TRAIL/APO2L and monoclonal antibodies targeting TRAIL receptors for cancer therapy holds great promise, due to their ability to restore cancer cell sensitivity to apoptosis in association with conventional chemotherapeutic drugs in a large variety of tumors. TRAIL-induced cell death is tightly regulated right from the membrane and at the DISC (Death-Inducing Signaling Complex) level. The following patent and literature review aims to present and highlight recent findings of the deadly discussion that determines tumor cell fate upon TRAIL engagement.

B cell receptor triggering sensitizes human B cells to TRAIL-induced apoptosis

TRAIL is known to cause death in tumor cells, but physiological regulation of its activity remains poorly characterized. We demonstrate that BCR triggering sensitizes transformed centroblast-like BL cells and peripheral blood memory B cells to TRAIL-mediated apoptosis. The sensitization correlated with surface down-regulation and intracellular retention of TRAIL-R4, along with changes in the expression of several Bcl-2 protein family members. Although enhancing FAS-mediated cell death, CD40 activation protected B cells from TRAIL-induced apoptosis. Combination of Ig cross-linking with CD40 ligation did not prevent TRAIL-R4 down-regulation but induced changes in the mitochondria-regulated pathway of apoptosis that are known to be associated with resistance to TRAIL. Human CD5(+) B cells, presumably stimulated by reactivity to self without immunological help, exhibited very high ex vivo sensitivity to TRAIL. Our results define the first B-lymphocyte-specific physiological signal that increases cellular sensitivity to TRAIL. This may be important for our understanding of TRAIL involvement in the control of B cell responses and aid in designing TRAIL-based therapies for B cell lymphomas.

Expression of cellular FLIP by B cells is required for their participation in an immune response

High levels of the Fas-signaling antagonist cellular FLIP (cFLIP) in germinal center (GC) B cells suggests an important role for this factor during this stage of the T cell-dependent B cell immune response. To test this idea, we used mice with B cell-specific deletion of a floxed cFLIP allele. Although deletion of cFLIP did not alter their primary development, participation of cFLIP-deficient B cells in the immune response was severely perturbed. Using previously characterized IgH locus-targeted BCR transgenic mice, we showed that adoptively transferred cFLIP-deficient follicular B cells do not effectively participate in the GC response in wild-type hosts. However, this failure was accompanied by severe defects in the initial activation and proliferation of these B cells in vivo. In addition, immunization of mice with B cell-specific cFLIP deletion resulted in selective recruitment into GCs and Ab-forming cell responses of B cells that had not deleted the floxed cFLIP allele. Together, these findings demonstrate that expression of cFLIP is a prerequisite for participation of B cells in all stages of Ag-driven immune responses.

Review: on TRAIL for malignant glioma therapy?

Glioblastoma (GBM) is a devastating cancer with a median survival of around 15 months. Significant advances in treatment have not been achieved yet, even with a host of new therapeutics under investigation. Therefore, the quest for a cure for GBM remains as intense as ever. Of particular interest for GBM therapy is the selective induction of apoptosis using the pro-apoptotic tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL signals apoptosis via its two agonistic receptors TRAIL-R1 and TRAIL-R2. TRAIL is normally present as homotrimeric transmembrane protein, but can also be processed into a soluble trimeric form (sTRAIL). Recombinant sTRAIL has strong tumouricidal activity towards GBM cells, with no or minimal toxicity towards normal human cells. Unfortunately, GBM is a very heterogeneous tumour, with multiple genetically aberrant clones within one tumour. Consequently, any single agent therapy is likely to be not effective enough. However, the anti-GBM activity of TRAIL can be synergistically enhanced by a variety of conventional and novel targeted therapies, making TRAIL an ideal candidate for combinatorial strategies. Here we will, after briefly detailing the biology of TRAIL/TRAIL receptor signalling, focus on the promises and pitfalls of recombinant TRAIL as a therapeutic agent alone and in combinatorial therapeutic approaches for GBM.

Antisense targeting of cFLIP sensitizes activated T cells to undergo apoptosis and desensitizes responses to contact dermatitis

Contact dermatitis is the result of inflammatory responses mediated by hapten-specific activated CD8+ and CD4+ T cells. Activation-induced cell death (AICD) is a naturally occurring process regulating the resolution of T-cell responses through decreased expression of the antiapoptotic molecule cellular FLICE inhibitory protein (cFLIP). We show that targeting cFLIP expression in vitro and in vivo, with morpholino antisense applied systemically or topically in conjunction with antigen, sensitizes T cells to undergo "early" AICD resulting in tolerance. Analysis of antisense-treated CD8+ OT-1 splenocytes after co-culture with SIINFEKL-pulsed DCs showed apoptosis occurring in a dose-dependent manner with respect to cFLIP peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) concentration. A transplant acceptance model using male DO.11 donor cells and female BALB/c recipient mice showed that cFLIP antisense treatment could promote antigen tolerance. Hypersensitivity responses induced in mice by the epicutaneous application of the haptens FITC and oxazolone confirmed that topically applied cFLIP antisense could reduce inflammation. Treatment of the skin produced significant reduction in dermatitis and localized infiltration of lymphocytes. Moreover, the treatment was target- and antigen-specific, dose-dependent, and capable of inducing long-lived tolerance. These data suggest that the targeted expression of immune-regulating molecules is possible through the application of antisense to the skin.

A role for cFLIP in B cell proliferation and stress MAPK regulation

Fas/Apo-1 signals through the FADD (Fas-associated death domain) adaptor protein, which recruits and activates the apical caspase 8 and leads to apoptosis. Cellular FLIP (cFLIP) is a homolog of caspase 8 and is also capable of binding to FADD. Previous studies suggest that cFLIP could either enhance or inhibit apoptosis and lead to NF-kappaB and Erk1/2 activation. Like FADD or caspase 8 deficiency, a lack of cFLIP disrupts embryogenesis and T cell proliferation. It has been demonstrated that B cells lacking either FADD or caspase 8 were defective in both Fas-induced apoptosis and TLR-induced proliferation, which indicates that these death-inducing proteins have an additional role in regulating innate immunity. To analyze the function of cFLIP in B cells, conditional deletion of cFLIP was induced by using CD19(Cre). The resulting B cell-specific cFLIP-deficient mice were found to have reduced numbers of peripheral B cells that were hypersensitive to Fas-induced apoptosis and impaired in proliferation induced by TLRs and the BCR. Furthermore, there was aberrant expression of costimulatory proteins and activation markers in cFLIP-deficient B cells. Whereas LPS-induced activation of NF-kappaB and Erk1/2 appears to be unaffected, p38 and Jnk were spontaneously activated and hyperinduced in cFLIP-deficient B cells. Therefore, these data revealed novel functions of cFLIP in B cells.

Capturing cell-fate decisions from the molecular signatures of a receptor-dependent signaling response

We examined responses of the B-cell antigen receptor-dependent intracellular signaling network to targeted perturbations induced through siRNA-mediated depletion of select signaling intermediates. The constituent nodes displayed graded sensitivities, which resulted from the differential effects of perturbations on the kinetic and quantitative aspects of phosphorylation at each node. By taking the rate of initial phosphorylation, rate of subsequent dephosphorylation, and the total intensity of phosphorylation at each node as separate signaling parameters, we generated data-driven models that accurately predicted the cellular responses of apoptosis, proliferation, and cytokine secretion. Importantly, the effects of perturbation on the primary target alone did not yield successful models. Rather, it also required incorporation of secondary effects on many other nodes. A significant feature of these models was that the three signaling parameters derived from each node functioned largely as independent entities, making distinctive contributions to the cellular response. Thus, the kinetic and quantitative features of phosphorylation at a node appear to play discrete roles during signal processing.

N-acetyl cysteine and caffeic acid phenethyl ester sensitize astrocytoma cells to Fas-mediated cell death in a redox-dependent manner

In this study, we investigated the role of reactive oxygen species (ROS) in Fas-induced cell death in human astrocytoma cells. Fas activation increased intracellular ROS levels in a NADPH oxidase- and caspase-dependent manner. ROS inhibitors such as N-acetyl cysteine (NAC) and caffeic acid phenethyl ester (CAPE) dramatically sensitized astocytoma cells to Fas-induced loss of mitochondrial transmembrane potential and subsequent cell death, which were abrogated by pretreatment with z-VAD-fmk, a broad-spectrum caspase inhibitor. These results collectively indicate that NAC and CAPE sensitize astrocytoma cells to Fas-induced apoptosis in a redox-dependent manner, suggesting a potential use in the treatment of malignant brain tumors.

TORC2 regulates germinal center repression of the TCL1 oncoprotein to promote B cell development and inhibit transformation

Aberrant expression of the TCL1 oncoprotein promotes malignant transformation of germinal center (GC) B cells. Repression of TCL1 in GC B cells facilitates FAS-mediated apoptosis and prevents lymphoma formation. However, the mechanism for this repression is unknown. Here we show that the CREB coactivator TORC2 directly regulates TCL1 expression independent of CREB Ser-133 phosphorylation and CBP/p300 recruitment. GC signaling through CD40 or the BCR, which activates pCREB-dependent genes, caused TORC2 phosphorylation, cytosolic emigration, and TCL1 repression. Signaling via cAMP-inducible pathways inhibited TCL1 repression and reduced apoptosis, consistent with a prosurvival role for TCL1 before GC selection and supporting an initiating role for aberrant TCL1 expression during GC lymphomagenesis. Our data indicate that a novel CREB/TORC2 regulatory mode controls the normal program of GC gene activation and repression that promotes B cell development and circumvents oncogenic progression. Our results also reconcile a paradox in which signals that activate pCREB/CBP/p300 genes concurrently repress TCL1 to initiate its silencing.

Elevated expression of genes assigned to NF-kappaB and apoptotic pathways in human periodontal ligament fibroblasts following mechanical stretch

There is growing evidence that apoptosis involves the nuclear transcription factor NF-kappaB in conjunction with related genes. However, in the context of mechanical orthodontic forces, force-sensing target genes assigned to pathways of NF-kappaB and apoptosis have not been fully characterised. To contribute to the identification of putative target genes, we used cDNA arrays specific for NF-kappaB and apoptotic pathways and analysed elevated gene expression in primary human periodontal ligament fibroblasts (PDL-F) after a 6 h application of mechanical force. Among several identified genes (including several caspases), interleukin-1 beta (IL-1 beta) and NF-kappaB displayed significantly higher expression on the NF-kappaB array, whereas higher expression was obtained for BCL2-antagonist of cell death (BAD), member 6 of the TNF-receptor superfamily (FAS) and CASP2 and RIPK1 domain-containing adaptor with death domain (CRADD) on the apoptosis array. Based on a defined cut-off level of a more than 1.5-fold higher expression, this significance in elevated gene expression was corroborated by reverse transcription/polymerase chain reaction (RT-PCR). Here, semi-quantitative (sq) PCR revealed a more pronounced elevation of mRNA gene expression in PDL-F after 6 h of stretch, when compared with 12 h. Moreover, the elevation after 6 h as observed by sq-PCR was convergent with quantitative PCR (q-PCR). q-PCR yielded levels of 5.8-fold higher relative gene expression for IL-1 beta and 1.7-fold for NF-kappaB, whereas that computed for BAD indicated a 5.2-fold, for CRADD a 2.1-fold and for FAS a 2.0-fold higher expression. The data obtained from the expression analysis thus indicate a stretch-induced transcriptional elevation of genes assigned to the NF-kappaB and apoptotic pathways. This elevation may render them target candidates for being addressed by mechanical orthodontic forces.

Control of B lymphocyte apoptosis by the transcription factor NF-kappaB

B cells maintain homeostasis by balancing cell viability and cell death. B lymphocytes are susceptible to mitochondria- and receptor-initiated cell death at various stages of peripheral differentiation and during immune responses. The inducible transcription factor NF-kappaB enhances cell viability by activating genes that counteract both cell-death pathways. This review uses characteristic features of NF-kappaB activation and downregulation to provide insight into the regulation of B cell apoptosis in the periphery. In particular, the temporal patterns of NF-kappaB induction, differences between Rel family members, and the intersection between canonical and noncanonical signaling pathways in keeping B cells alive are discussed.

Rapid up-regulation of c-FLIP expression by BCR signaling through the PI3K/Akt pathway inhibits simultaneously induced Fas-mediated apoptosis in murine B lymphocytes

Cross-linking of BCR rapidly induces protection of B cells from Fas-mediated apoptosis, which has been assumed one of the important survival mechanisms of B cells during antigen stimulation. In the mouse B cell line A20, which is sensitive to Fas-mediated apoptosis, stimulation of BCR inhibited apoptosis induced via Fas upstream of caspase-8 activation with an associated rapid increase in the expression of both short and long forms of cellular caspase-8/FLICE-inhibitory protein (c-FLIP). The c-FLIP competitively inhibited the recruitment of caspase-8 to the death-inducing signaling complex (DISC), which took as long as 3h to form after the stimulation of Fas in A20 cells. Knockdown of c-FLIP by a short hairpin RNA-expressing method rendered BCR-stimulated A20 cells sensitive to Fas-mediated apoptosis. The BCR-induced rapid expression of c-FLIP was not affected by inactivation of NF-kappaB, but was inhibited by either treatment with a PI3K inhibitor, LY294002, or expression of a dominant negative PI3K p85 subunit, both of which suppressed phosphorylation of Akt and sensitized BCR-stimulated A20 cells to Fas-mediated apoptosis. Overexpression of constitutively active Akt was shown not only to up-regulate c-FLIP expression but also to render A20 cells resistant to Fas-mediated apoptosis. Moreover, treatment with LY294002 also suppressed BCR-induced up-regulation of c-FLIP expression in spleen B cells. Taken together, BCR-stimulation was shown to rapidly trigger a survival signal against simultaneously or ongoingly stimulated Fas-mediated apoptosis by promoting a PI3K/Akt signaling pathway-mediated up-regulation of c-FLIP expression.

Profiling of apoptosis genes allows for clinical stratification of primary nodal diffuse large B-cell lymphomas

Intrinsic resistance of lymphoma cells to apoptosis is a probable mechanism causing chemotherapy resistance and eventual fatal outcome in patients with diffuse large B cell lymphomas (DLBCL). We investigated whether microarray expression profiling of apoptosis related genes predicts clinical outcome in 46 patients with primary nodal DLBCL. Unsupervised cluster analysis using genes involved in apoptosis (n = 246) resulted in three separate DLBCL groups partly overlapping with germinal centre B-lymphocytes versus activated B-cells like phenotype. One group with poor clinical outcome was characterised by high expression levels of pro-and anti-apoptotic genes involved in the intrinsic apoptosis pathway. A second group, also with poor clinical outcome, was characterised by high levels of apoptosis inducing cytotoxic effector genes, possibly reflecting a cellular cytotoxic immune response. The third group showing a favourable outcome was characterised by low expression levels of genes characteristic for both other groups. Our results suggest that chemotherapy refractory DLBCL are characterised either by an intense cellular cytotoxic immune response or by constitutive activation of the intrinsic mediated apoptosis pathway with concomitant downstream inhibition of this apoptosis pathway. Consequently, strategies neutralising the function of apoptosis-inhibiting proteins might be effective as alternative treatment modality in part of chemotherapy refractory DLBCL.

Rapid CD40-mediated rescue from CD95-induced apoptosis requires TNFR-associated factor-6 and PI3K

The activation molecule CD40 and the death receptor CD95/Fas play important roles in regulating B cells so that effective antimicrobial immunity occurs without autoimmunity. CD40 signaling increases CD95 expression, sensitizing cells to apoptosis, but sustained CD40 signals rescue B cells from CD95 killing. Here we describe a mechanism of early CD40-mediated rescue from CD95-induced apoptosis in B cells. Maximal rescue was achieved when CD40 signals were given within 1-2 h of initiating CD95 apoptosis. CD40 signaling did not block association of Fas-associated death domain-containing protein with CD95, but decreased CD95-induced activation of caspases 3 and 8. Rapid CD40 rescue did not require NF-kappaB activation and was independent of de novo protein synthesis, but was dependent upon active PI3 K. Signaling via a CD40 mutant that does not bind TNFR-associated factor (TRAF)1, TRAF2, and TRAF3 rescued B cells from CD95-induced apoptosis. TRAF1/2/3-independent rescue was confirmed in B cell lines made deficient in these TRAF molecules by gene targeting. In contrast, CD40 rescue was completely abrogated in TRAF6-deficient B cells, which showed reduced activation of Akt in response to CD40 engagement. These results reveal a new rapid mechanism to balance B cell activation and apoptosis.

Dysregulated TCL1 requires the germinal center and genome instability for mature B-cell transformation

Most lymphomas arise by transformation of germinal center (GC) B cells. TCL1, a proto-oncogene first recognized for its role in T-cell transformation, also induces GC B-cell malignancies when dysregulated in pEmu-B29-TCL1 transgenic (TCL1-tg) mice. Clonal B-cell lymphomas develop from polyclonal populations with latencies of 4 months or more, suggesting that secondary genetic events are required for full transformation. The goals of this study were to determine the GC-related effects of TCL1 dysregulation that contribute to tumor initiation and to identify companion genetic alterations in tumors that function in disease progression. We report that compared with wild-type (WT) cells, B cells from TCL1-tg mice activated in a manner resembling a T-dependent GC reaction show enhanced resistance to FAS-mediated apoptosis with CD40 stimulation, independent of a B-cell antigen receptor (BCR) rescue signal. Mitogenic stimulation of TCL1-tg B cells also resulted in increased expression of Aicda. These GC-related enhancements in survival and Aicda expression could underlie B-cell transformation. Supporting this notion, no B-cell lymphomas developed for 20 months when TCL1-tg mice were crossed onto an Oct coactivator from B cell (OCA-B)-deficient background to yield mice incapable of forming GCs. Spectral karyotype analyses showed that GC lymphomas from TCL1-tg mice exhibit recurrent chromosome translocations and trisomy 15, with corresponding MYC overexpression. We conclude that pEmu-B29-TCL1 transgenic B cells primed for transformation must experience the GC environment and, for at least some, develop genome instability to become fully malignant.

cFLIP regulation of lymphocyte activation and development

Cellular caspase-8 (FLICE)-like inhibitory protein (cFLIP) was originally identified as an inhibitor of death-receptor signalling through competition with caspase-8 for recruitment to FAS-associated via death domain (FADD). More recently, it has been determined that both cFLIP and caspase-8 are required for the survival and proliferation of T cells following T-cell-receptor stimulation. This paradoxical finding launched new investigations of how these molecules might connect with signalling pathways that link to cell survival and growth following antigen-receptor activation. As discussed in this Review, insight gained from these studies indicates that cFLIP and caspase-8 form a heterodimer that ultimately links T-cell-receptor signalling to activation of nuclear factor-kappaB through a complex that includes B-cell lymphoma 10 (BCL-10), mucosa-associated-lymphoid-tissue lymphoma-translocation gene 1 (MALT1) and receptor-interacting protein 1 (RIP1).

Immunohistochemical profiling based on Bcl-2, CD10 and MUM1 expression improves risk stratification in patients with primary nodal diffuse large B cell lymphoma

Clinical outcome in patients with diffuse large B cell lymphomas (DLBCL) is poorly predictable. Expression of proteins related to germinal centre B (GCB) cell or activated B cells (ABC) and expression of apoptosis-regulating proteins Bcl-2 and XIAP have been found previously to be strongly associated with clinical outcome. In this study we aimed to develop an algorithm based on expression of GCB/ABC-related proteins CD10, Bcl-6 and MUM1 and apoptosis-inhibiting proteins Bcl-2, XIAP and cFLIP for optimal stratification of DLBCL patients into prognostically favourable and unfavourable groups. Expression of CD10 and cFLIP was associated with better overall survival (both p = 0.03), whereas expression of MUM1, Bcl-2 and XIAP was associated with poor clinical outcome (p = 0.01, p = 0.0007 and p = 0.03, respectively). Multivariate analysis revealed that Bcl-2 was the strongest prognostic marker followed by CD10 and MUM1. Stratification of patients according to a new algorithm based on expression of these three markers improved patient risk stratification into low and particularly high clinical risk groups (p = 0.04 and p < 0.0001, respectively). We conclude that, in our group of primary nodal DLBCLs, a new algorithm, based on expression of the apoptosis-inhibiting protein Bcl-2 and the GCB/ABC-related proteins CD10 and MUM1, strongly predicts outcome in International Prognostic Index (IPI)-low and -high patients. Its predictive power is stronger than previously published algorithms based on only GCB/ABC- or apoptosis-regulating proteins.

Hydrogen peroxide predisposes neonatal rat ventricular myocytes to Fas-mediated apoptosis

Neonatal rat ventricular myocytes (NRVM) grown in normoxic environment are not susceptible to Fas-induced apoptosis. In the present work, we tested the hypothesis that free radical injury represented by transient exposure to H2O2 sensitizes NRVM to Fas-mediated apoptosis. NRVM were treated with H2O2 (0.5 mM) for 2-4 h and thereafter exposed for 7 h to recombinant Fas ligand (rFasL, 10 ng/ml) plus an enhancing antibody (1 microg/ml). Apoptotic cardiomyocytes were counted and apoptosis-related proteins were measured by Western blot. H2O2 alone induced apoptosis (9.4+/-1.0%) that was preceded by activation of caspases-8 and -3, and PARP degradation. Incubation of NRVM with H2O2, followed by exposure to rFasL, increased the apoptotic index to 13.8+/-2.0%, but did not change caspase-8 or PARP activation. To investigate the mechanism underlying the sensitizing affect of H2O2 towards Fas-induced apoptosis, we studied the effects of H2O2 on the expression of key apoptosis signaling proteins. Incubation with H2O2 for 2-4 h decreased Fas expression and the expression of the Fas-related antiapoptotic proteins FLIP(L) and ARC, and increased the expression of the antiapoptotic proteins bcl-2 and xIAP. FADD expression was unchanged. Next, we tested the effect of H2O2 on the apoptosis-inducing, Fas-dependent Daxx-ASK-1-JUN kinase pathway. H2O2 dramatically increased ASK-1 expression and JUN kinase activation, but did not effect Daxx expression. Based on these findings we concluded that H2O2 sensitizes NRVM to Fas-mediated apoptosis by activating the Daxx-ASK-1-JUN kinase pathway, and by shifting the balance between proapoptotic and antiapoptotic proteins towards the former.

Molecular and cellular mechanisms of neuronal cell death in HIV dementia

The deaths of neurons, astrocytes and endothelial cells have been described in patients with HIV (human immunodeficiency virus) dementia. HIV-1 does not infect neurons; instead, neurotoxic substances shed by infected glia and macrophages can induce a form of programmed cell death called apoptosis in neurons. These neurotoxins include the HIV-1 proteins Tat and gp120, as well as pro-inflammatory cytokines, chemokines, excitotoxins and proteases. In this article we review the evidence for apoptosis of various cell types within the brain of HIV-infected patients, and describe in vitro and in vivo experimental studies that have elucidated the mechanisms by which HIV causes apoptosis of brain cells.

CD21/CD19 Coreceptor Signaling Promotes B Cell Survival during Primary Immune Responses

The adaptive immune response is tightly regulated to limit responding cells in an Ag-specific manner. On B cells, coreceptors CD21/CD19 modulate the strength of BCR signals, potentially influencing cell fate. The importance of the CD95 pathway was examined in response of B cells to moderate affinity Ag using an adoptive transfer model of lysozyme-specific Ig transgenic (HEL immunoglobulin transgene (MD4) strain) B cells. Although adoptively transferred Cr2+/+ MD4 B cells are activated and persist within splenic follicles of duck egg lysozyme-immunized mice, Cr2-/- MD4 B cells do not. In contrast, Cr2-/- MD4 lpr B cells persist after transfer, suggesting that lack of CD21/CD35 signaling results in CD95-mediated elimination. Cr2 deficiency did not affect CD95 levels, but cellular FLIP (c-FLIP) protein and mRNA levels were reduced 2-fold compared with levels in Cr2+/+ MD4 B cells. In vitro culture with Cr2+/+ MD4 B cells demonstrated that equimolar amounts of rHEL-C3d3 were more effective than hen egg lysozyme alone in up-regulating c-FLIP levels and for protection against CD95-mediated apoptosis. Collectively, this study implies a mechanism for regulating B cell survival in vivo whereby the strength of BCR signaling (including coreceptor) determines c-FLIP levels and protection from CD95-induced death.

Death to the bad guys: targeting cancer via Apo2L/TRAIL

All higher organisms consist of an ordered society of individual cells that must communicate to maintain and regulate their functions. This is achieved through a complex but highly regulated network of hormones, chemical mediators, chemokines and other cytokines, acting as ligands for intra or extra-cellular receptors. Ligands and receptors of the tumor necrosis factor (TNF) superfamilies are examples of signal transducers, whose integrated actions influence the development, homeostasis and adaptive responses of many cells and tissue types. Apo2L/TRAIL is one of several members of the tumour necrosis factor superfamily that induce apoptosis through the engagement of death receptors. Apo2L/TRAIL interacts with an unusually complex receptor system, which in humans comprises two death receptors and three decoy receptors. This molecule has received considerable attention recently because of the finding that many cancer cell types are sensitive to Apo2L/TRAIL-induced apoptosis, while most normal cells appear to be resistant to this action of Apo2L/TRAIL. In this review, we specifically emphasise on the actions of Apo2L/TRAIL with respect to its apoptotic signaling pathways and summarise what is known about its physiological role. The potential therapeutic usefulness of Apo2L/TRAIL, especially in combination with chemotherapeutic agents, is also discussed in some detail.

Immunohistochemical profiling of caspase signaling pathways predicts clinical response to chemotherapy in primary nodal diffuse large B-cell lymphomas

We used biopsy specimens of primary nodal diffuse large B-cell lymphoma (DLBCL) to investigate whether the inhibition of caspase 8 and/or 9 apoptosis signaling pathways predicts clinical outcome. Expression levels of cellular FLICE inhibitory protein (c-Flip) and numbers of active caspase 3-positive lymphoma cells were used to determine the status of the caspase 8-mediated pathway. Expression levels of Bcl-2 and X-linked inhibitor of apoptosis (XIAP) were used to determine the status of the caspase 9-mediated pathway. Expression of c-Flip, XIAP, Bcl-2, and caspase 3 activity all provided prognostic information. According to these immunohistochemical parameters, inhibition of either or both caspase signaling pathways was detected in all patients. Three groups of patients were identified, one with a caspase 8 inhibition profile, one with caspase 8 and 9 inhibition profiles, and one with a caspase 9 inhibition profile. Caspase 9 inhibition was strongly associated with poor response to chemotherapy and usually with fatal outcome, whereas caspase 8 inhibition was associated with excellent clinical outcome. Thus, our data strongly suggest that inhibition of the caspase 9-mediated pathway, but not the caspase 8-mediated pathway, is a major cause for therapy resistance in patients with nodal DLBCL.

Possible role of FLICE-like inhibitory protein (FLIP) in chemoresistant ovarian cancer cells in vitro

Chemoresistance is a major therapeutic problem and the current knowledge on cellular mechanisms involved is incomplete. In the present study, we have investigated the possible involvement of Fas-associated death domain-like interleukin-1beta-converting enzyme (FLICE)-like inhibitory protein (FLIP) in ovarian cancer resistance by comparing chemosensitive (OV2008) and chemoresistant (C13*) ovarian cancer cells treated with cisplatin in vitro, and/or transfected with FLIP sense cDNA or FLIP small interfering RNA (siRNA) and determining FLIP protein content, cleavage of caspase-8 and caspase-3 and apoptosis. Cisplatin significantly decreased FLIP protein level, induced cleavage of caspase-8 and caspase-3 and apoptosis in a concentration-dependent manner in cisplatin-sensitive but not -resistant cells. While overexpression of FLIP-attenuated cisplatin-induced cleavage of caspase-8 and caspase-3 and apoptosis in chemosensitive cells, downregulation of FLIP in chemoresistant cells by siRNA increased apoptosis induced by cisplatin. These results suggest that FLIP plays a significant role in the regulation of apoptosis in human ovarian cancer cells and their sensitivity to cisplatin. This cell survival factor may be an important determinant in chemoresistance in ovarian cancer and may serve as a molecular target for the development of novel therapy for chemoresistant ovarian cancer.

Fas Ligand as a Tool for Immunosuppression and Generation of Immune Tolerance

The role of Fas ligand (FasL) in physiologically limiting immune responses and maintaining immune-privileged sites has led to a body of research aiming to confer protection to allogeneic grafts by expressing FasL on the allogeneic tissue or by administrating FasL-transduced donor dendritic cells. In addition, several studies have used FasL to abrogate autoimmune responses. This review presents the results of these studies and discusses the problems associated with FasL usage.

Mcl-1 mediates tumor necrosis factor-related apoptosis-inducing ligand resistance in human cholangiocarcinoma cells

Cholangiocarcinomas are usually fatal neoplasms originating from bile duct epithelia. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising agent for cancer therapy, including cholangiocarcinoma. However, many cholangiocarcinoma cells are resistant to TRAIL-mediated apoptosis. Thus, our aim was to examine the intracellular mechanisms responsible for TRAIL resistance in human cholangiocarcinoma cell lines. Three TRAIL-resistant human cholangiocarcinoma cell lines were identified. All of the cell lines expressed TRAIL receptor 1/death receptor 4 (TRAIL-R1/DR4) and TRAIL-R2/DR5. Expression of TRAIL decoy receptors and the antiapoptotic cellular FLICE-inhibitory protein (cFLIP) was inconsistent across the cell lines. Of the antiapoptotic Bcl-2 family of proteins profiled (Bcl-2, Bcl-x(L), and Mcl-1), Mcl-1 was uniquely overexpressed by the cell lines. When small-interfering-RNA (siRNA) technology was used to knock down expression of Bcl-2, Bcl-x(L), and Mcl-1, only the Mcl-1-siRNA sensitized the cells to TRAIL-mediated apoptosis. In a cell line stably transfected with Mcl-1-small-hairpin-RNA (Mcl-1-shRNA), Mcl-1 depletion sensitized cells to TRAIL-mediated apoptosis despite Bcl-2 expression. TRAIL-mediated apoptosis in the stably transfected cells was associated with mitochondrial depolarization, Bax activation, cytochrome c release from mitochondria, and caspase activation. Finally, flavopiridol, an anticancer drug that rapidly down-regulates Mcl-1, also sensitized cells to TRAIL cytotoxicity. In conclusion, these studies not only demonstrate that Mcl-1 mediates TRAIL resistance in cholangiocarcinoma cells by blocking the mitochondrial pathway of cell death but also identify two strategies for circumventing this resistance.

Single-nucleotide polymorphism g.1548G > A (E469K) in human ICAM-1 gene affects mRNA splicing pattern and TPA-induced apoptosis

The single-nucleotide polymorphism (SNP) g.1548G > A (E469K) in the human intercellular adhesion molecule-1 (ICAM-1) gene has been suggested to have an association with several types of inflammatory diseases. The polymorphism is located at the three-base position upstream of the splice donor site that produces an alternatively spliced short isoform (ICAM-1-S). To clarify its functional relevance, we studied RNA splicing patterns by comparing cells with different genotype (G/G cells and A/A cells). G/G cells expressed a lower amount of ICAM-1-S mRNA than A/A cells. Since ICAM-1-S has no transmembrane or intracellular domain, ICAM-1 signal transduction and cell-cell contact including Fas-FasL interaction may be influenced. In addition, we studied the effect of this change on FLIP-L mRNA and apoptosis. FLIP-L mRNA tended to decrease, while cell death induced by phorbol 12-myristate 13-acetate was increased. These results suggest that the g.1548 polymorphism modifies inflammatory immune responses by changing cell-cell interaction and then regulating apoptosis.

Expression of SCC-S2, an antiapoptotic molecule, correlates with enhanced proliferation and tumorigenicity of MDA-MB 435 cells

SCC-S2/GG2-1/NDED is a recently discovered antiapoptotic molecule induced by the activation of the transcription factor NF-kappaB. Here we have examined a role of SCC-S2 in cell growth regulation in vitro and in vivo. Western blotting using an antipeptide antibody revealed endogenous SCC-S2 as a approximately 21 kDa cytosolic protein in human breast cancer cells (MDA-MB 231) and renal carcinoma cells (RCC-RS). The immunofluorescence detection method showed the cytosolic localization of FLAG-tagged human SCC-S2 in COS-1 transfectants. MDA-MB 435 human cancer cells stably transfected with the FLAG-tagged SCC-S2 cDNA exhibited increased growth rate as compared to control vector transfectants, as measured by the cell viability (>twofold; n=3; P<0.005) and thymidine-labeling procedures ( approximately sixfold; n=3; P<0.0001). SCC-S2 transfectants also displayed an increase in cell migration in collagen I as compared to control transfectants ( approximately twofold; n=3; P<0.005). In athymic mice, SCC-S2 transfectants showed significantly enhanced tumor growth as compared to control transfectants (mean tumor volumes, day 16: control, 56.86+/-19.82 mm(3); SCC-S2, 127.54+/-18.78 mm(3); n=5; P<0.03). The examination of a limited number of clinical specimens revealed higher expression levels of SCC-S2 protein in certain human tumor tissues as compared to the matched normal adjacent tissues. Taken together, the present studies demonstrate SCC-S2 as a novel oncogenic factor in cancer cells.

Retinoids and TRAIL: two cooperating actors to fight against cancer

Multiple studies performed in in vitro and in vivo settings have confirmed the cancer therapeutic and cancer preventive capacity of retinoids and rexinoids. These compounds mediate their actions through the retinoid and rexinoid receptors, respectively, which exist in multiple isoforms and form a plethora of distinct heterodimers. Despite their apparent anticancer potential, with one exception the molecular basis of this activity has remained largely elusive. The exception concerns acute promyelocytic leukemia (APL), the prototype of retinoic acid-dependent differentiation therapy, for which both the molecular nature of the disease and the mechanism of action of retinoids are well understood. However, retinoids and rexinoids are active beyond the borderlines of the well-defined chromosomal translocation that gives rise to curable APL. In this context, particularly interesting is that retinoic acid induces a member of the tumor necrosis factor family, tumor necrosis factor-related apoptosis inducing ligand (TRAIL) or Apo2L. This ligand is exceptional in that it is capable of inducing apoptosis in cancer cells but not in normal cells. It is possible that this connection to the TRAIL signaling pathway contributes to the anti-tumor activity of retinoids and rexinoids. This review focuses on what is presently known about the regulation of cell life and death by the retinoid/rexinoid and TRAIL signaling pathways.

The death effector domain protein family

Apoptosis signaling is regulated and executed by specialized proteins that often carry protein/protein interaction domains. One of these domains is the death effector domain (DED) that is predominantly found in components of the death-inducing signaling complex, which forms at the members of the death receptor family following their ligation. Both proapoptotic- and antiapoptotic-DED-containing proteins have been identified, which makes these proteins exquisitely suited to the regulation of apoptosis. Aside from their pivotal role in the control of the apoptotic program, DED-containing proteins have recently been demonstrated to exert their influence on other cellular processes as well, including cell proliferation. These data highlight the multiple roles for the members of this family, suggesting that they are suited to control both life and death decisions of cells. Additionally, because they can act proapoptotically, antiapoptotically, or in the regulation of the cell cycle, this family of proteins may be excellent candidates for cancer therapy targets. Oncogene (2003) 22, 8634-8644. doi:10.1038/sj.onc.1207103

Role of Complement-Binding CD21/CD19/CD81 in Enhancing Human B Cell Protection from Fas-Mediated Apoptosis

Defective expression of Fas leads to B cell autoimmunity, indicating the importance of this apoptotic pathway in eliminating autoreactive B cells. However, B cells with anti-self specificities occasionally escape such regulation in individuals with intact Fas, suggesting ways of precluding this apoptosis. Here, we examine whether coligation of the B cell Ag receptor (BCR) with the complement (C3)-binding CD21/CD19/CD81 costimulatory complex can enhance the escape of human B cells from Fas-induced death. This was warranted given that BCR-initiated signals induce resistance to Fas apoptosis, some (albeit not all) BCR-triggered events are amplified by coligation of BCR and the co-stimulatory complex, and several self Ags targeted in autoimmune diseases effectively activate complement. Using a set of affinity-diverse surrogate Ags (receptor-specific mAb:dextran conjugates) with varying capacity to engage CD21, it was established that BCR:CD21 coligation lowers the BCR engagement necessary for inducing protection from Fas apoptosis. Enhanced protection was associated with altered expression of several molecules known to regulate Fas apoptosis, suggesting a unique molecular model for how BCR:CD21 coligation augments protection. BCR:CD21 coligation impairs the generation of active fragments of caspase-8 via dampened expression of membrane Fas and augmented expression of FLIP(L). This, in turn, diminishes the generation of cells that would be directly triggered to apoptosis via caspase-8 cleavage of caspase 3 (type I cells). Any attempt to use the mitochondrial apoptotic protease-activating factor 1 (Apaf-1)-dependent pathway for apoptosis (as type II cells) is further blocked because BCR:CD21 coligation promotes up-regulation of the mitochondrial antiapoptotic molecule, Bcl-2.

Gadd45 beta mediates the protective effects of CD40 costimulation against Fas-induced apoptosis

In B lymphocytes, induction of apoptosis or programmed cell death (PCD) by Fas (CD95/APO-1) is suppressed by the triggering of CD40. This suppression controls various aspects of the humoral immune response, including antibody affinity maturation. The opposing effects of these receptors are also crucial to B-cell homeostasis, autoimmune disease, and cancer. Cytoprotection by CD40 involves activation of protective genes mediated by NF-kappa B transcription factors; however, its basis remains poorly understood. Here, we report that, in B cells, Gadd45 beta is induced by CD40 through a mechanism that requires NF-kappa B and that this induction suppresses Fas-mediated killing. Importantly, up-regulation of Gadd45 beta by CD40 precedes Fas-induced caspase activation, as well as up-regulation of other NF-kappa B-controlled inhibitors of apoptosis such as Bcl-xL and c-FLIPL. In the presence of Gadd45 beta, the Fas-induced apoptotic cascade is halted at mitochondria. However, in contrast to Bcl-xL, Gadd45 beta is unable to hamper the "intrinsic" pathway for apoptosis and in fact appears to block Fas cytotoxicity herein by suppressing a mitochondria-targeting mechanism activated by this receptor. These findings identify Gadd45 beta as a critical mediator of the prosurvival response to CD40 stimulation and provide important new insights into the apoptotic mechanism that is triggered by Fas in B cells.

Fas-stimulated generation of reactive oxygen species or exogenous oxidative stress sensitize cells to Fas-mediated apoptosis

Inhibition of Fas-mediated apoptosis in B cell lymphomas by thiol antioxidants (glutathione and N-acetylcysteine) supported previous studies, suggesting that Fas-stimulated ROS generation may play a role in Fas-mediated apoptosis. Thus, the goal of the current study was to determine if Fas stimulation could induce ROS generation and what role, if any, it played in apoptosis. Fas crosslinking induced rapid generation of ROS (within 15 min) well before the appearance of characteristic apoptotic changes. Overexpression of catalase or superoxide dismutase suggested that Fas induced production of both superoxide anion and hydrogen peroxide. ROS generation was only observed, however, in cells that were sensitive to apoptosis and not in B cells inherently resistant to anti-Fas or in those in which resistance was induced by B cell receptor crosslinking. The exogenous addition of 250 microM hydrogen peroxide could reverse the resistant phenotype and sensitize cells to Fas-induced apoptosis. In Fas-sensitive cells, depletion of endogenous antioxidant defenses with buthionine sulfoximine increased the sensitivity to Fas-induced apoptosis, while overexpression of antioxidant enzymes and antiapoptotic proteins suggested a role for Fas-induced production of hydrogen peroxide in apoptosis. Further analysis suggested a redox-sensitive step early in Fas signaling at the level of initiator caspase (caspase-8) activation. Thus, the data suggest that the level of oxidative stress, either from exogenous sources or generated endogenously upon receptor stimulation, regulates the sensitivity to Fas-mediated apoptosis.

An IL-2-dependent switch between CD95 signaling pathways sensitizes primary human T cells toward CD95-mediated activation-induced cell death

The CD95 (APO-1/Fas) system plays a critical role in activation-induced cell death (AICD) of T cells. We previously described two distinct CD95 (APO-1/Fas) signaling pathways: 1) type I cells show strong death-inducing signaling complex (DISC) formation and mitochondria-independent apoptosis and 2) DISC formation is reduced in type II cells, leading to mitochondria-dependent apoptosis. To investigate the relevance of these pathways, we set up an in vitro model that mimics the initiation and the down phase of an immune response, respectively. Freshly activated human T cells (initiation) are resistant toward CD95-mediated AICD despite high expression of CD95. We previously reported that these T cells show reduced DISC formation. In this study, we show that freshly activated T cells are CD95-type II cells that show high expression levels of Bcl-x(L) and display a block in the mitochondrial apoptosis pathway. Furthermore, we show that, upon prolonged culture (down phase), human T cells undergo a switch from type II to type I cells that renders T cells sensitive to CD95-mediated AICD. Finally, we demonstrate that this switch is dependent on the presence of IL-2. Our observations reveal for the first time that the existence of coexisting CD95 signaling pathways is of physiological relevance.

FLIP and FasL expression by inflammatory cells vs thyrocytes can be predictive of chronic inflammation or resolution of autoimmune thyroiditis

Spontaneous autoimmune thyroiditis (SAT) in NOD.H-2h4 mice is a model of chronic inflammation of the thyroid, while granulomatous experimental autoimmune thyroiditis (G-EAT) is a model with spontaneous resolution of inflammation. In chronic inflammation (SAT), Fas, FasL, and FLIP were upregulated and predominant in inflammatory cells. There were few apoptotic cells, and low expression of active caspase-8 and -3. In resolving G-EAT in CBA/J and NOD.H-2h4 mice, FasL and FLIP were predominantly expressed by thyrocytes. There were many apoptotic inflammatory cells, and increased expression of active caspase-8 and -3. Depletion of CD8+ T cells inhibited G-EAT resolution and resulted in chronic inflammation. FLIP was expressed predominantly by inflammatory cells, and apoptosis of inflammatory cells and expression of active caspase-3 was reduced as in chronic SAT. Thus, differences in expression of pro- or antiapoptotic molecules in SAT or G-EAT were apparently related to the acute vs chronic nature of the inflammatory response rather than the method of disease induction. Upregulation of FLIP by inflammatory cells may block Fas-mediated apoptosis, contributing to chronic inflammation, whereas increased FLIP expression by thyrocytes in resolving G-EAT may protect thyrocytes from apoptosis, and FasL expression by thyrocytes may induce apoptosis of inflammatory cells, contributing to resolution.

TNF-alpha-induced cell death in ethanol-exposed cells depends on p38 MAPK signaling but is independent of Bid and caspase-8

Alcoholic liver disease is associated with an increase in the number of necrotic and apoptotic liver parenchymal cells. Part of this injury is mediated by TNF-alpha. Ethanol exposure sensitizes cells to the cytotoxic effects of TNF-alpha. This may be due, in part, to the increased propensity of the mitochondria in ethanol-exposed cells to induction of mitochondrial permeability transition (MPT) by various agents, including the proapoptotic protein Bax. This idea is supported by the observation that increased cell death induced by TNF-alpha in ethanol-exposed cells was dependent on development of the MPT. In the present study, we elucidate the pathways through which ethanol exposure enhances TNF-alpha induction of the MPT and the resulting cytotoxicity. Specifically, ethanol-exposed cells display caspase-8- and Bid-independent cell killing during TNF-alpha treatment. Moreover, the ethanol-enhanced pathway is dependent on p38 MAPK signaling, which brings about caspase-3 activation, mitochondrial depolarization, accumulation of cytochrome c in the cytosol, and the translocation of Bax to the mitochondria. Additionally, ethanol-exposed cells display a blunting of TNF-alpha-induced Akt activation and Bcl-2 antagonist of cell death phosphorylation that may account, in part, for the increased sensitivity of the mitochondria to Bax-mediated damage.