Positive regulation of apoptosis signal-regulating kinase 1 by hD53L1

Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase family member that plays a central role in cytokine- and stress-induced apoptosis by activating c-Jun N-terminal kinase and p38 signaling cascades. ASK1-induced apoptotic activity is up-regulated by two cellular factors, Daxx and TRAF2, through direct protein-protein interactions. Daxx and TRAF2 are death receptor-associated proteins in Fas and tumor necrosis factor-alpha pathways, respectively. Recent studies suggest that calcium signaling may regulate ASK1 pathway. Here we report that human D53L1, a member of the tumor protein D52 family involved in cell proliferation and calcium signaling, up-regulates the ASK1-induced apoptosis. The human D53L1 physically interacts with the C-terminal regulatory domain of ASK1 and promotes ASK1-induced apoptotic activity by activating caspase signaling in mammalian cells. In luciferase reporter assays, hD53L1 activates c-Jun N-terminal kinase-mediated transactivation in the presence of ASK1. Expression of hD53L1 enhances autophosphorylation and kinase activity of ASK1 but has no effect on ASK1 oligomerization that is necessary for kinase activity and on binding of ASK1 to MKK6, a downstream factor of ASK1. Taken together, these results suggest that activation of ASK1 by hD53L1 may provide a novel mechanism for ASK1 regulation.     

Role of 4-hydroxynonenal and its metabolites in signaling

Available evidence from a multitude of studies on the effects of 4-hydroxynonenal (HNE) on cellular processes seem to converge on some common themes: (i) concentration-dependent opposing effects of HNE on key signaling components (e.g. protein kinase C, adenylate cyclase) predict that certain constitutive levels of HNE may be needed for normal cell functions - lowering of this constitutive HNE level in cells promotes proliferative machinery while an increase in this level promotes apoptotic signaling; (ii) HNE is a common denominator in stress-induced apoptosis caused by H(2)O(2), superoxide, UV, heat or oxidant chemicals such as doxorubicin; and (iii) HNE can modulate ligand-independent signaling by membrane receptors such as EGFR or Fas (CD95) and may act as a sensor of external stimuli for eliciting stress-response. Against a backdrop of various reported effects of HNE, in vitro and in vivo, we have critically evaluated the above mentioned hypotheses suggesting a key role of HNE in signaling.     

TGF-beta-induced apoptosis is mediated by the adapter protein Daxx that facilitates JNK activation

Transforming growth factor-beta (TGF-beta) is a multifunctional growth factor that has a principal role in growth control through both its cytostatic effect on many different epithelial cell types and its ability to induce programmed cell death in a variety of other cell types. Here we have used a screen for proteins that interact physically with the cytoplasmic domain of the type II TGF-beta receptor to isolate the gene encoding Daxx - a protein associated with the Fas receptor that mediates activation of Jun amino-terminal kinase (JNK) and programmed cell death induced by Fas. The carboxy-terminal portion of Daxx functions as a dominant-negative inhibitor of TGF-beta-induced apoptosis in B-cell lymphomas, and antisense oligonucleotides to Daxx inhibit TGF-beta-induced apoptosis in mouse hepatocytes. Furthermore, Daxx is involved in mediating JNK activation by TGF-beta. Our findings associate Daxx directly with the TGF-beta apoptotic-signalling pathway, and make a biochemical connection between the receptors for TGF-beta and the apoptotic machinery.     

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.     

Inhibition of Daxx-mediated apoptosis by heat shock protein 27

Heat shock protein 27 (HSP27) confers cellular protection against a variety of cytotoxic stresses and also against physiological stresses associated with growth arrest or receptor-mediated apoptosis. Phosphorylation modulates the activity of HSP27 by causing a major change in the supramolecular organization of the protein, which shifts from oligomers to dimers. Here we show that phosphorylated dimers of HSP27 interact with Daxx, a mediator of Fas-induced apoptosis, preventing the interaction of Daxx with both Ask1 and Fas and blocking Daxx-mediated apoptosis. No such inhibition was observed with an HSP27 phosphorylation mutant that is only expressed as oligomers or when apoptosis was induced by transfection of a Daxx mutant lacking its HSP27 binding domain. HSP27 expression had no effect on Fas-induced FADD- and caspase-dependent apoptosis. However, HSP27 blocked Fas-induced translocation of Daxx from the nucleus to the cytoplasm and Fas-induced Daxx- and Ask1-dependent apoptosis. The observations revealed a new level of regulation of the Fas pathway and suggest a mechanism for the phosphorylation-dependent protective function of HSP27 during stress and differentiation.     

Daxx overexpression in T-lymphoblastic Jurkat cells enhances caspase-dependent death receptor- and drug-induced apoptosis in distinct ways

The role of Daxx, in particular, its ability to promote or hinder apoptosis, still remains controversial. In order to elucidate the functional relevance of Daxx in apoptosis signaling of malignant lymphocytes, Jurkat T-cells were stably transfected with a Daxx-expressing vector or with the respective Daxx-negative control vector. We thus demonstrate that ectopic expression of Daxx substantially increases the rate of apoptosis upon incubation with death receptor agonists such as Fas and TRAIL as well as upon incubation with the cytotoxic drug doxorubicin (DOX). Analysis of the molecular changes induced in the extrinsic and intrinsic apoptosis pathways reveals that augmentation of apoptosis by Daxx overexpression is conveyed by distinctly different mechanisms. Although enforced apoptosis caused by ectopic Daxx expression is caspase-dependent in both cases, major differences between Fas/TRAIL-induced apoptosis and doxorubicin-induced apoptosis are observed in expression patterns of X-linked inhibitor of apoptosis (XIAP), p53, Bid, ZIP kinase, and prostate apoptosis response gene 4 (Par-4). Moreover, we could show that addition of a CD95 blocking antibody to the clones treated with doxorubicin was able to increase apoptosis as compared to doxorubicin treatment alone and was accompanied by an enhancement of the mitochondrial branch of apoptosis. In conclusion, we here outline the major molecular mechanisms underlying the apoptosis-promoting effect of Daxx in neoplastic lymphocytes and demonstrate fundamental molecular differences elicited by the overexpression of Daxx in the extrinsic and intrinsic signaling pathways.     

Negative regulation of p53 functions by Daxx and the involvement of MDM2

In normal cells p53 activity is tightly controlled and MDM2 is a known negative regulator. Here we show that via its acidic domain, Daxx binds to the COOH-terminal domain of p53, whose positive charges are critical for this interaction, as Lys to Arg mutations preserved, but Lys to Ala or Ser to Glu mutations abolished Daxx-p53 interaction. These results thus implicate acetylation and phosphorylation of p53 in regulating its binding to Daxx. Interestingly, whereas Daxx did not bind to p53 in cells as assessed by immunoprecipitation, MDM2 expression restored p53-Daxx interaction, and this correlated with deacetylation of p53. In p53/MDM2-null mouse embryonic fibroblasts (DKO MEF), Daxx repressed p53 target promoters whose p53-binding elements were required for the repression. Coexpression of Daxx and MDM2 led to further repression. p53 expression in DKO MEF induced apoptosis and Daxx expression relieved this effect. Similarly, in HCT116 cells, Daxx conferred striking resistance to 5-fluorouracil-induced apoptosis. As p53 is required for 5-fluorouracil-induced cell death, our data show that Daxx can suppress cell death induced by p53 overexpression and p53-dependent stress response. Collectively, our data reveal Daxx as a novel negative regulator of p53. Importantly, posttranslational modifications of p53 inhibit Daxx-p53 interaction, thereby relieving negative regulation of p53 by Daxx.     

Critical role for Daxx in regulating Mdm2

The tumour suppressor p53 induces apoptosis or cell-cycle arrest in response to genotoxic and other stresses. In unstressed cells, the anti-proliferative effects of p53 are restrained by mouse double minute 2 (Mdm2), a ubiquitin ligase (E3) that promotes p53 ubiquitination and degradation. Mdm2 also mediates its own degradation through auto-ubiquitination. It is unclear how the cis- and trans-E3 activities of Mdm2, which have opposing effects on cell fate, are differentially regulated. Here, we show that death domain-associated protein (Daxx) is required for Mdm2 stability. Downregulation of Daxx decreases Mdm2 levels, whereas overexpression of Daxx strongly stabilizes Mdm2. Daxx simultaneously binds to Mdm2 and the deubiquitinase Hausp, and it mediates the stabilizing effect of Hausp on Mdm2. In addition, Daxx enhances the intrinsic E3 activity of Mdm2 towards p53. On DNA damage, Daxx dissociates from Mdm2, which correlates with Mdm2 self-degradation. These findings reveal that Daxx modulates the function of Mdm2 at multiple levels and suggest that the disruption of the Mdm2-Daxx interaction may be important for p53 activation in response to DNA damage.     

Cell death and diseases related to oxidative stress:4-hydroxynonenal (HNE) in the balance

During the last three decades, 4-hydroxy-2-nonenal (HNE), a major α,β-unsaturated aldehyde product of n-6 fatty acid oxidation, has been shown to be involved in a great number of pathologies such as metabolic diseases, neurodegenerative diseases and cancers. These multiple pathologies can be explained by the fact that HNE is a potent modulator of numerous cell processes such as oxidative stress signaling, cell proliferation, transformation or cell death. The main objective of this review is to focus on the different aspects of HNE-induced cell death, with a particular emphasis on apoptosis. HNE is a special apoptotic inducer because of its abilities to form protein adducts and to propagate oxidative stress. It can stimulate intrinsic and extrinsic apoptotic pathways and interact with typical actors such as tumor protein 53, JNK, Fas or mitochondrial regulators. At the same time, due to its oxidant status, it can also induce some cellular defense mechanisms against oxidative stress, thus being involved in its own detoxification. These processes in turn limit the apoptotic potential of HNE. These dualities can imbalance cell fate, either toward cell death or toward survival, depending on the cell type, the metabolic state and the ability to detoxify.     

hDaxx与细胞肿瘤抑制子p53在体内外的相互作用

与急性早幼粒细胞性白血病蛋白(promyelocytic leukemia protein,PML)在体内相互作用共定位于细胞核PML致癌结构城(PML oncogenic domains,PODs)的人Daxx(human Daxx,hDaxx),能结合Fas死亡结构域诱导细胞凋亡.细胞肿瘤抑制子p53抑制细胞及病毒转录,提高细胞内Fas的表达并调节细胞凋亡.为了探索hDaxx与p53在诱导细胞凋亡中有无相互作用及其作用效果,利用酵母双杂交体系测定发现p53通过C端与hDaxx结合,共免疫沉淀反应及Western blot结果显示hDaxx与p53能在体内外直接结合.hDaxx与p53结合并发生相互作用可能对细胞周期有一定的调节作用.     

Upregulation of Daxx mediates apoptosis in response to oxidative stress

Oxidative stress induces apoptosis in a variety of cell types by as yet unclear signaling mechanisms. The Daxx protein is reportedly involved in apoptosis through its interactions with Fas, transforming growth factor-beta receptor, and promyelocytic leukemia protein (PML). Here, we explored the possible roles of Daxx in oxidative stress-induced apoptosis. We found that both the mRNA and protein levels of Daxx markedly increased when cells underwent apoptosis after H2O2 treatment. Pretreatment with the cell-permeable antioxidant, N-acetyl cysteine, prevented cells from H2O2-induced Daxx upregulation and subsequent apoptosis, indicating that the endogenous oxidant regulated Daxx expression. Furthermore, suppression of endogenous Daxx expression by antisense oligonucleotide technology inhibited oxidative stress-induced apoptosis in HeLa cells. Taken together, these results suggest that Daxx acts as an intermediary messenger of pro-apoptotic signals triggered by oxidative stress.     

Interaction of Daxx, a Fas binding protein, with sentrin and Ubc9

Sentrin is a ubiquitin-like protein that can covalently modify cellular proteins, and is a Fas binding protein that protects cells against anti-Fas induced cell death. However, the mechanism by which sentrin exerts its effect upon Fas-mediated apoptosis is not well known. Thus, this study examined the interaction of sentrin with Daxx. Sentrin interacted with Daxx but not with FADD when analyzed by yeast two-hybrid assay. In vitro translated Daxx bound to GST-sentrin fusion protein. FLAG-sentrin fusion protein was also coimmunoprecipitated with Daxx in BOSC23 cells. Also, Daxx interacted with Ubc9, an essential protein as a key conjugating enzyme. Amino acids 625-740 of Daxx, known as Fas binding region, was also mapped as sentrin and Ubc9 binding region. Colocalization of Fas, sentrin, and Ubc9 binding regions suggests the importance of that region upon the regulation of Daxx. Our data also demonstrated that sentrin could homooligomerize by protein-protein interaction.     

Daxx deletion mutant (amino acids 501-625)-induced apoptosis occurs through the JNK/p38-Bax-dependent mitochondrial pathway

Death-associated protein (Daxx) deletion mutant (aa 501-625) has been known to be an inducer of apoptosis. In this study, we observed that the Bax-dependent mitochondrial death signaling pathway plays an important role in Daxx501-625-induced apoptosis. Daxx fragment-induced activation of caspase-9 and -3 was mediated through the apoptosis signal-regulating kinase 1 (ASK1)-MEK-c-Jun-N-terminal kinase (JNK)/p38-Bax pathway. By overexpressing JNK-binding domain (JBD) of JIP1, a JNK-inhibitory protein, and treatment with SB203580, a specific p38 inhibitor, DU-145 cells were made resistant to Daxx501-625-induced apoptosis. Capase-3 deficiency, Bax deficiency, or overexpression of a dominant-negative caspase-9 mutant prevented apoptosis, even though the Daxx501-625 fragment still activated the ASK1-MEK-MAPK pathway. Interestingly, Daxx501-625-induced Bcl-2 interacting domain (Bid) cleavage was suppressed in the dominant-negative caspase-9 mutant cells, whereas Bim was still phosphorylated in these cells. These results suggest that cleavage of Bid occurs downstream of caspase-9 activation. In contrast, phosphorylation of Bim is upstream of caspase-9 activation. Taken together, our results suggest that Daxx501-625-induced apoptosis is mediated through the ASK1-MEK-JNK/p38-Bim-Bax-dependent caspase pathway.