Long form of cellular FLICE-inhibitory protein interacts with Daxx and prevents Fas-induced JNK activation

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

Regulation of extrinsic apoptotic signaling by c-FLIP: towards targeting cancer networks

The extrinsic pathway is mediated by death receptors (DRs), including CD95 (APO-1/Fas) or TRAILR-1/2. Defects in apoptosis regulation lead to cancer and other malignancies. The master regulator of the DR networks is the cellular FLICE inhibitory protein (c-FLIP). In addition to its key role in apoptosis, c-FLIP may exert other cellular functions, including control of necroptosis, pyroptosis, nuclear factor κB (NF-κB) activation, and tumorigenesis. To gain further insight into the molecular mechanisms of c-FLIP action in cancer networks, we focus on the structure, isoforms, interactions, and post-translational modifications of c-FLIP. We also discuss various avenues to target c-FLIP in cancer cells for therapeutic benefit.

Protein Kinase-Mediated Decision Between the Life and Death

Protein kinases are intracellular signaling enzymes that catalyze the phosphorylation of specific residues in their target substrate proteins. They play important role for regulation of life and death decisions. The complexity of the relationship between death receptors and protein kinases' cell death decision-making mechanisms create many difficulties in the treatment of various diseases. The most of fifteen different cell death pathways, which are reported by Nomenclature Committee on Cell Death (NCCD) are protein kinase signal transduction-mediated negative or positive selections. Tumor necrosis factor (TNF) as a main player of death pathways is a dual-functioning molecule in that it can promote both cell survival or cell death. All apoptotic and necrotic signal transductions are conveyed through death domain-containing death receptors, which are expressed on the surface of nearly all human cells. In humans, eight members of the death receptor family have been identified. While the interaction of TNF with TNF Receptor 1 (TNFR1) activates various signal transduction pathways, different death receptors activate three main signal transduction pathways: nuclear factor kappa B (NF-ĸB)-mediated differentiation or pro-inflammatory cytokine synthesis, mitogen-activated protein kinase (MAPK)-mediated stress response and caspase-mediated apoptosis. The link between the NF-ĸB and the c-Jun NH₂-terminal kinase (JNK) pathways comprise another check-point to regulate cell death. TNF-α also promotes the "receptor-interacting serine/threonine protein kinase 1" (RIPK1)/RIPK3/ mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necrosis. Thus, necrosome is mainly comprised of MLKL, RIPK3 and, in some cases, RIPK1. In fact, RIPK1 is at the crossroad between life and death, downstream of various receptors as a regulator of endoplasmic reticulum stress-induced death. TNFR1 signaling complex (TNF-RSC), which contains multiple kinase activities, promotes phosphorylation of transforming growth factor β-activated kinase 1 (TAK1), inhibitor of nuclear transcription factor κB (IκB) kinase (IKK) α/IKKβ, IκBα, and NF-κB. IKKs affect cell-survival pathways in NF-κB-independent manner. Toll-like receptor (TLR) stimulation triggers various signaling pathways dependent on myeloid differentiation factor-88 (MyD88), Interleukin-1 receptor (IL-1R)-associated kinase (IRAK1), IRAK2 and IRAK4, lead to post-translational activation of nucleotide and oligomerization domain (NLRP3). Thereby, cell fate decisions following TLR signaling is parallel with death receptor signaling. Inhibition of IKKα/IKKβ or its upstream activators sensitize cells to death by inducing RIPK1-dependent apoptosis or necroptosis. During apoptosis, several kinases of the NF-κB pathway, including IKK1 and NF-κB essential modulator (NEMO), are cleaved by cellular caspases. This event can terminate the NF-κB-derived survival signals. In both canonical and non-canonical pathways, IKK is key to NF-κB activation. Whereas, the activation process of IKK, the functions of NEMO ubiquitination, IKK-related non-canonical pathway and the nuclear transportation of NEMO and functions of IKKα are still debated in cell death. In addition, cluster of differentiation 95 (CD95)-mediated non-apoptotic signaling and CD95- death-inducing signaling complex (DISC) interactions are waiting for clarification.

DAXX in cancer: phenomena, processes, mechanisms and regulation

DAXX displays complex biological functions. Remarkably, DAXX overexpression is a common feature in diverse cancers, which correlates with tumorigenesis, disease progression and treatment resistance. Structurally, DAXX is modular with an N-terminal helical bundle, a docking site for many DAXX interactors (e.g. p53 and ATRX). DAXX's central region folds with the H3.3/H4 dimer, providing a H3.3-specific chaperoning function. DAXX has two functionally critical SUMO-interacting motifs. These modules are connected by disordered regions. DAXX's structural features provide a framework for deciphering how DAXX mechanistically imparts its functions and how its activity is regulated. DAXX modulates transcription through binding to transcription factors, epigenetic modifiers, and chromatin remodelers. DAXX's localization in the PML nuclear bodies also plays roles in transcriptional regulation. DAXX-regulated genes are likely important effectors of its biological functions. Deposition of H3.3 and its interactions with epigenetic modifiers are likely key events for DAXX to regulate transcription, DNA repair, and viral infection. Interactions between DAXX and its partners directly impact apoptosis and cell signaling. DAXX's activity is regulated by posttranslational modifications and ubiquitin-dependent degradation. Notably, the tumor suppressor SPOP promotes DAXX degradation in phase-separated droplets. We summarize here our current understanding of DAXX's complex functions with a focus on how it promotes oncogenesis.

Daxx plays a novel role in T cell survival but is dispensable in Fas-induced apoptosis

Daxx was originally isolated as a Fas-binding protein. However, the in vivo function of Daxx in Fas-induced apoptosis has remained enigmatic. Fas plays an important role in homeostasis in the immune system. Fas gene mutations lead to autoimmune-lymphoproliferation (lpr) diseases characterized by hyperplasia of secondary lymphoid organs. It is well established that the FADD adaptor binds to Fas, and recruits/activates caspase 8. However, additional proteins including Daxx have also been indicated to associate with Fas. It was proposed that Daxx mediates a parallel apoptotic pathway that is independent of FADD and caspase 8, but signals through ASK1-mediated apoptotic pathway. However, because the deletion of Daxx leads to embryonic lethality, the in vivo function of Daxx has not been properly analyzed. In the current study, analysis was performed using a conditional mutant mouse in which Daxx was deleted specifically in T cells. The data show that Daxx^-/- T cells were able to undergo normal Fas-induced apoptosis. While containing normal thymocyte populations, the T cell-specific Daxx^-/- mice have a reduced peripheral T cell pool. Importantly, Daxx-deficient T cells displayed increased death responses upon activation through TCR stimulation. These results unequivocally demonstrated that Daxx does not mediate Fas-induced apoptosis, but rather that it plays a critical role in survival responses in primary mature T cells.

Targeting the Anti-Apoptotic Protein c-FLIP for Cancer Therapy

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor and critical anti-apoptotic regulator that inhibits tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis as well as chemotherapy-triggered apoptosis in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 in a ligand-dependent and-independent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. Moreover, c-FLIP(L) and c-FLIP(S) are known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective signaling molecules. Upregulation of c-FLIP has been found in various tumor types, and its downregulation has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and efforts are underway to develop other c-FLIP-targeted cancer therapies. This review focuses on (1) the functional role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and drug resistance; (2) the molecular mechanisms that regulate c-FLIP expression; and (3) strategies to inhibit c-FLIP expression and function.

Daxx upregulation within the cytoplasm of reovirus-infected cells is mediated by interferon and contributes to apoptosis

Reovirus infection is a well-characterized experimental system for the study of viral pathogenesis and antiviral immunity within the central nervous system (CNS). We have previously shown that c-Jun N-terminal kinase (JNK) and the Fas death receptor each play a role in neuronal apoptosis occurring in reovirus-infected brains. Death-associated protein 6 (Daxx) is a cellular protein that mechanistically links Fas signaling to JNK signaling in several models of apoptosis. In the present study, we demonstrate that Daxx is upregulated in reovirus-infected brain tissue through a type I interferon-mediated mechanism. Daxx upregulation is limited to brain regions that undergo reovirus-induced apoptosis and occurs in the cytoplasm and nucleus of neurons. Cytoplasmic Daxx is present in Fas-expressing cells during reovirus encephalitis, suggesting a role for Daxx in Fas-mediated apoptosis following reovirus infection. Further, in vitro expression of a dominant negative form of Daxx (DN-Daxx), which binds to Fas but which does not transmit downstream signaling, inhibits apoptosis of reovirus-infected cells. In contrast, in vitro depletion of Daxx results in increased expression of caspase 3 and apoptosis, suggesting that Daxx plays an antiapoptotic role in the nucleus. Overall, these data imply a regulatory role for Daxx in reovirus-induced apoptosis, depending on its location in the nucleus or cytoplasm.

FLIP: a flop for execution signals

Resistance to apoptosis is one of the established hallmarks of cancer cells. This is a function of an imbalance between the proteins that facilitate death execution and those that inhibit apoptosis or promote cell proliferation. The anti-apoptotic protein, FLICE inhibitory protein (FLIP), first identified as a viral protein, is over-expressed in a variety of human pathologies. Initial observations linked FLIP expression to inhibition of death receptor induced apoptosis, due to its structural homology to the cysteine protease, caspase-8. FLIP impedes full processing of pro-caspase-8 to its active form and its release to the cytosol, and by doing so blocks apoptotic signaling downstream of the membrane death initiating signaling complex (DISC). Recent observations have highlighted the complex regulation of this protein and its cross talk with diverse signaling networks and metabolic processes. As FLIP expression is directly associated with chemotherapy resistance, a better understanding of its genomic organization, gene transcription, as well as post-transcriptional regulation could yield novel targets with potential therapeutic implications against drug refractory cancers. In this short review, we provide a brief overview of the structural and functional biology of this somewhat complex protein with direct relevance to carcinogenesis.

Daxx-beta and Daxx-gamma, two novel splice variants of the transcriptional co-repressor Daxx

Daxx is involved in transcriptional control and apoptosis. It comprises several domains, including a regulatory C terminus that is responsible for the interaction with numerous proteins such as p53, promyelocytic leukemia protein (PML), and Hsp27. Here, we describe the identification and characterization of two novel variants of Daxx termed Daxx-β and Daxx-γ, which are generated by alternative splicing. Alternative splicing results in a truncated regulatory C terminus in both proteins. As a consequence, Daxx-β and Daxx-γ show a markedly decreased affinity to PML, which in turn is associated with a different subnuclear localization of these proteins compared with Daxx. Although Daxx is localized mainly in PML-oncogenic domains (PODs) Daxx-β and Daxx-γ display a distinct distribution pattern. Furthermore, Daxx-β and Daxx-γ show a decreased affinity to p53 also due to the truncated C terminus. We provide evidence that the p53 recruitment into PODs is Daxx isoform-dependent. The decreased affinity of Daxx-β/-γ to p53 and PML results in a diffuse localization of p53 throughout the nucleus. In contrast to Daxx, Daxx-β and Daxx-γ are unable to repress p53-mediated transcription. Therefore, alternative splicing of Daxx might indicate an additional level in the cellular apoptosis network.

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.

Splicing factor ASF/SF2 and transcription factor PPAR-gamma cooperate to directly regulate transcription of uncoupling protein-3

The different isoforms of the uncoupling protein-3 (UCP3) are expressed in skeletal muscle and are up-regulated by splicing factors. Here, we report that UCP3 alternative splicing (alternative polyadenylation) is regulated by cooperation between the splicing factor ASF/SF2 and the transcription factor PPAR-gamma. We found that ASF/SF2 activates formation of long-form UCP3 (UCP3(L)) by inhibiting a cleavage and polyadenylation signal (AATAAA) located in its final intron that prematurely terminates message elongation. PPAR-gamma activates this process by directly interacting with ASF/SF2, providing the first example of a direct link between a transcription factor and alternative splicing. Activation of ASF/SF2 promotes formation of UCP3(L), whereas loss of ASF/SF2 decreases production of both UCP3(L) and short-form UCP3 (UCP3(S)). We suggest that the relative abundance of ASF/SF2 and PPAR-gamma determines the ratio of UCP3 isoforms.

Cellular FLICE-like inhibitory protein (C-FLIP): a novel target for cancer therapy

Cellular FLICE-like inhibitory protein (c-FLIP) has been identified as a protease-dead, procaspase-8-like regulator of death ligand-induced apoptosis, based on observations that c-FLIP impedes tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis by binding to FADD and/or caspase-8 or -10 in a ligand-dependent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIP is a family of alternatively spliced variants, and primarily exists as long (c-FLIP(L)) and short (c-FLIP(S)) splice variants in human cells. Although c-FLIP has apoptogenic activity in some cell contexts, which is currently attributed to heterodimerization with caspase-8 at the DISC, accumulating evidence indicates an anti-apoptotic role for c-FLIP in various types of human cancers. For example, small interfering RNAs (siRNAs) that specifically knocked down expression of c-FLIP(L) in diverse human cancer cell lines, e.g., lung and cervical cancer cells, augmented TRAIL-induced DISC recruitment, and thereby enhanced effector caspase stimulation and apoptosis. Therefore, the outlook for the therapeutic index of c-FLIP-targeted drugs appears excellent, not only from the efficacy observed in experimental models of cancer therapy, but also because the current understanding of dual c-FLIP action in normal tissues supports the notion that c-FLIP-targeted cancer therapy will be well tolerated. Interestingly, Taxol, TRAIL, as well as several classes of small molecules induce c-FLIP downregulation in neoplastic cells. Efforts are underway to develop small-molecule drugs that induce c-FLIP downregulation and other c-FLIP-targeted cancer therapies. In this review, we assess the outlook for improving cancer therapy through c-FLIP-targeted therapeutics.

Association of TRAF2 with the short form of cellular FLICE-like inhibitory protein prevents TNFR1-mediated apoptosis

Background

We have previously shown that c-FLIP_L is a more potent inhibitor than c-FLIP_S of Fas ligand-induced apoptosis and that c-FLIP_L physically binds to Daxx, an alternative Fas-signaling adaptor. Here we examined whether c-FLIP_S effectively inhibits TNFR1-mediated apoptosis and triggers JNK activation through its interaction with TRAF2.

Results

Some cancer cell lines, such as DU145, AGS, and PC3, have higher levels of c-FLIP_S than other cell lines, such as SNU-719 and T24. The expression of c-FLIP_S correlated with the susceptibility to TNFR1-mediated apoptosis. In contrast to DU145 and PC3, which are resistant to TNFR1-mediated apoptosis, T24 and SNU719 were sensitive to TNF-α treatment. To address the role of c-FLIP_S in TNFR1-mediated apoptosis, we examined the molecular interaction between c-FLIP_S and TRAF2. As expected, western blot analysis revealed that TRAF2 antibody immunoprecipitated a greater amount of c-FLIP_S than c-FLIP_L. Also, we measured the involvement of c-FLIP_S in TNF-α-induced JNK activation and apoptosis by comparing these in TNF-α-resistant and TNF-α-sensitive cell lines. Treatment with TNF-α increased the phosphorylated JNK level in SNU719 and T24 cells, whereas DU145 and AGS cells were resistant to TNF-α-mediated apoptosis.

Conclusion

We now report that the short form of c-FLIP_S is a more efficient inhibitor of TNF-receptor 1-mediated apoptosis signaling than the long form of the protein.

Fas Ag-FasL coupling leads to ERK1/2-mediated proliferation of gastric mucosal cells

When cells within the gastric mucosa progress from metaplasia to dysplasia to cancer, they acquire a Fas Ag apoptosis-resistant phenotype. It is unusual to completely abolish the pathway, suggesting other forms of Fas Ag signaling may be important or even necessary for gastric cancer to progress. Little is known about alternate signaling of the Fas Ag pathway in gastric mucosal cells. Using a cell culture model of rat gastric mucosal cells, we show that gastric mucosal cells utilize a type II signaling pathway for apoptosis. Under conditions of low receptor stimulation or under conditions where apoptosis is blocked downstream of the death-inducing signal complex, Fas Ag signaling proceeds toward proliferative signaling. Under conditions favoring proliferative signaling, cFLIP is recruited to the Fas-associated death domain-like interleukin-1beta-converting enzyme at the death-inducing signal complex and activates ERK1/2. ERK1/2 in turn activates NF-kappaB. ERK1/2 stimulates proliferation, whereas NF-kappaB activation results in upregulation of the antiapoptotic protein survivin, further promoting proliferation over apoptosis. These results suggest that factors that inhibit apoptosis confer a growth advantage to the cells beyond the survival advantage of avoiding apoptosis and in effect convert the Fas Ag signaling pathway from a tumor suppressor to a tumor promoter.

Transcriptional networks in plasmacytoid dendritic cells stimulated with synthetic TLR 7 agonists

Background

Plasmacytoid Dendritic Cells (pDC) comprise approximately 0.2 to 0.8% of the blood mononuclear cells and are the primary type 1 interferon (IFN), producing cells, secreting high levels of IFN in response to viral infections. Plasmacytoid dendritic cells express predominantly TLRs 7 & 9, making them responsive to ssRNA and CpG DNA. The objective of this study was to evaluate the molecular and cellular processes altered upon stimulation of pDC with synthetic TLR 7 and TLR 7/8 agonists. To this end, we evaluated changes in global gene expression upon stimulation of 99.9% pure human pDC with the TLR7 selective agonists 3M-852A, and the TLR7/8 agonist 3M-011.

Results

Global gene expression was evaluated using the Affymetrix U133A GeneChip^® and selected genes were confirmed using real time TaqMan^® RTPCR. The gene expression profiles of the two agonists were similar indicating that changes in gene expression were solely due to stimulation through TLR7. Type 1 interferons were among the highest induced genes and included IFNB and multiple IFNα subtypes, IFNα2, α5, α6, α8, α1/13, α10, α14, α16, α17, α21. A large number of chemokines and co-stimulatory molecules as well as the chemokine receptor CCR7 were increased in expression indicating maturation and change in the migratory ability of pDC. Induction of an antiviral state was shown by the expression of several IFN-inducible genes with known anti-viral activity. Further analysis of the data using the pathway analysis tool MetaCore gave insight into molecular and cellular processes impacted. The analysis revealed transcription networks that show increased expression of signaling components in TLR7 and TLR3 pathways, and the cytosolic anti-viral pathway regulated by RIG1 and MDA5, suggestive of optimization of an antiviral state targeted towards RNA viruses. The analysis also revealed increased expression of a network of genes important for protein ISGylation as well as an anti-apoptotic and pro-survival gene expression program.

Conclusion

Thus this study demonstrates that as early as 4 hr post stimulation, synthetic TLR7 agonists induce a complex transcription network responsible for activating pDC for innate anti-viral immune responses with optimized responses towards RNA viruses, increased co-stimulatory capacity, and increased survival.

cFLIP expression correlates with tumour progression and patient outcome in non-Hodgkin lymphomas of low grade of malignancy

The present study investigated whether the expression of cellular Fas-associated death domain-like interleukin-1beta-converting enzyme (FLICE) inhibitory protein (cFLIP) conveys prognostic information in non-Hodgkin lymphomas (NHLs). cFLIP expression was quantified by immunohistochemistry and immunofluorescence in biopsy specimens from 86 NHL patients for whom clinical information was available. NHL malignancy was graded as high/intermediate or low according to the World Health Organization Classification of Lymphoid Neoplasms. cFLIP was positive in 23 of 45 high-/intermediate-grade NHLs and in 25 of 41 low-grade NHLs. Negative expression of cFLIP was associated with the presence of apoptotic cells in the tumour mass, regardless of the histotype and of the malignancy grade. In NHLs positive for cFLIP, 11 of 23 (48%) high-/intermediate-grade cases and 18 of 25 (72%) low-grade cases showed a bad outcome. In NHLs negative for cFLIP, only four of 22 (18%) high-/intermediate-grade patients and 12 of 16 (75%) low-grade patients achieved complete remission. All these correlations were statistically significant. The correlation of cFLIP expression with clinical outcome was independent of therapy, whether or not it included anti-CD20 antibody (Rituximab). The present findings strongly indicate that cFLIP is a reliable predictor of tumour progression and clinical prognosis in NHLs of low grade of malignancy.

Response of XIAP, ARC, and FLIP apoptotic suppressors to 8 wk of treadmill running in rat heart and skeletal muscle

Although it has been demonstrated that exercise training has an antiapoptotic effect on postmitotic myocytes, the mechanisms responsible for this effect are still largely unclear. Because the antiapoptotic effect of exercise training in postmitotic myocytes could be possibly mediated by the upregulation of apoptotic suppressors, this study examined the effect of endurance training on endogenous apoptotic suppressors including X-chromosome-linked inhibitor of apoptosis protein (XIAP), apoptosis repressor with caspases recruitment domain protein (ARC), and FADD-like inhibitor protein (FLIP) in skeletal and cardiac muscles. Eight adult Sprague-Dawley rats were trained 5 days weekly for 8 wk on treadmill, and eight sedentary rats served as controls. Soleus and ventricle muscles were dissected 2 days after the last training session. The mRNA content of XIAP, ARC, and FLIP was estimated by RT-PCR with ribosomal 18S RNA used as an internal control. The protein expression of XIAP, ARC, FLIP(S), and FLIP(alpha) was assessed by Western immunoblot. After training, mRNA content of ARC and FLIP was not different between the control and trained animals, whereas XIAP mRNA content was elevated by 22 and 14% in the trained soleus and cardiac muscles, respectively, relative to the control samples. No difference was found in the protein content of FLIP(S) and FLIP(alpha) between control and trained muscles, whereas XIAP and ARC protein content was increased by 18 and 38%, respectively, in the soleus muscle of trained animals. Furthermore, negative relationships were found between XIAP and apoptotic DNA fragmentation as well as ARC and caspase-3 activity. These findings are consistent with the hypothesis that the modulation of apoptotic suppressors is involved in training-induced attenuation of apoptosis in skeletal and cardiac muscles.

Expression of a dominant negative form of Daxxin vivo rescues motoneurons from Fas (CD95)-induced cell death

Fas-induced death of motoneurons in vitro has been shown to involve two signaling cascades that act together to execute the death program: a Fas-Daxx-ASK-1-p38 kinase-nNOS branch, which controls transcriptional and post-translational events, and the second classical Fas-FADD-caspase-8 branch. To analyze the role of Daxx in the developmental motoneuron cell death, we studied Fas-dependent cell death in motoneurons from transgenic mice that overexpress a dominant-negative form of Daxx. Motoneurons purified from these transgenic mice are resistant to Fas-induced death. This protective effect is specific to Fas because ultraviolet irradiation-triggered death is not affected by the transgene. The Daxx and the FADD pathways work in parallel because only Daxx, but not FADD, is involved in the transcriptional control of neuronal nitric oxide synthase and nitric oxide production. Nevertheless, we do not observe involvement of Daxx in developmental motoneuronal cell death, as the pattern of naturally occurring programmed cell death in vivo is normal in transgenic mice overexpressing the dominant negative form of Daxx, suggesting that Daxx-independent pathways are used during development.