Heterogeneity in the phosphorylation of human death receptors by p42(mapk/erk2)

Structural determinants of DISC function: new insights into death receptor-mediated apoptosis signalling

Death receptors are members of the tumour necrosis factor (TNF) receptor superfamily characterised by an ~80 amino acid long alpha-helical fold, termed the death domain (DD). Death receptors diversified during early vertebrate evolution indicating that the DD fold has plasticity and specificity that can be easily adjusted to attain additional functions. Eight members of the death receptor family have been identified in humans, which can be divided into four structurally homologous groups or clades, namely: the p75(NTR) clade (consisting of ectodysplasin A receptor, death receptor 6 (DR6) and p75 neurotrophin (NTR) receptor); the tumour necrosis factor receptor 1 clade (TNFR1 and DR3), the CD95 clade (CD95/FAS) and the TNF-related apoptosis-inducing ligand receptor (TRAILR) clade (TRAILR1 and TRAILR2). Receptors in the same clade participate in similar processes indicating that structural diversification enabled functional specialisation. On the surface of nearly all human cells multiple death receptors are expressed, enabling the cell to respond to a plethora of external signals. Activation of different death receptors converges on the activation of three main signal transduction pathways: nuclear factor-κB-mediated differentiation or inflammation, mitogen-associated protein kinase-mediated stress response and caspase-mediated apoptosis. While the ability to induce cell death is true for nearly all DRs, the FAS and TRAILR clades have specialised in inducing cell death. Here we summarise recent discoveries about the molecular regulation and structural requirements of apoptosis induction by death receptors and discuss how this information can be used to better explain the biological functions, similarities and distinguishing features of death receptors.

Isolation, characterisation and reconstitution of cell death signalling complexes

Apoptosis and necroptosis are dependent on the formation/activation of distinct multi-protein complexes; these include the Death-Inducing Signalling Complex (DISC), apoptosome, piddosome, necrosome and ripoptosome. Despite intense research, the mechanisms that regulate assembly/function of several of these cell death signalling platforms remain to be elucidated. It is now increasingly evident that the composition and stoichiometry of components within these key signalling platforms not only determines the final signalling outcome but also the mode of cell death. Characterising these complexes can therefore provide new insights into how cell death is regulated and also how these cell death signalling platforms could potentially be targeted in the context of disease. Large multi-protein complexes can initially be separated according to their size by gel filtration or sucrose density gradient centrifugation followed by subsequent affinity-purification or immunoprecipitation. The advantage of combining these techniques is that you can assess the assembly of individual components into a complex and then assess the size and stoichiometric composition of the native functional signalling complex within a particular cell type. This, alongside reconstitution of a complex from its individual core components can therefore provide new insight into the mechanisms that regulate assembly/function of key multi-protein signalling complexes. Here, we describe the successful application of a range of methodologies that can be used to characterise the assembly of large multi-protein complexes such as the apoptosome, DISC and ripoptosome. Together with their subsequent purification and/or reconstitution, these approaches can provide novel insights into how cell death signalling platforms are regulated in both normal cell physiology and disease.

IL-6 cytoprotection in hyperoxic acute lung injury occurs via PI3K/Akt-mediated Bax phosphorylation

IL-6 overexpression protects mice from hyperoxic acute lung injury in vivo, and treatment with IL-6 protects cells from oxidant-mediated death in vitro. The mechanisms of protection, however, are not clear. We characterized the expression, localization, and regulation of Bax, a proapoptotic member of the Bcl-2 family, in wild-type (WT) and IL-6 lung-specific transgenic (Tg(+)) mice exposed to 100% O(2) and in human umbilical vein endothelial cells (HUVEC) treated with H(2)O(2) and IL-6. In control HUVEC treated with H(2)O(2) or in WT mice exposed to 100% O(2), a marked induction of Bax translocation and dimerization was associated with increased JNK and p38 kinase activity. In contrast, specific JNK or p38 kinase inhibitors or treatment with IL-6 inhibited Bax mitochondrial translocation and apoptosis of HUVEC. IL-6 Tg(+) mice exposed to 100% O(2) exhibited enhanced phosphatidylinositol 3-kinase (PI3K)/Akt kinase and increased serine phosphorylation of Bax at Ser(184) compared with WT mice. The PI3K-specific inhibitor LY-2940002 blocked this IL-6-induced Bax phosphorylation and promoted cell death. Furthermore, IL-6 potently blocked hyperoxia- or oxidant-induced Bax insertion into mitochondrial membranes. Thus IL-6 functions in a cytoprotective manner, in part, by suppressing Bax translocation and dimerization through PI3K/Akt-mediated Bax phosphorylation.

Death receptor-3, a new E-Selectin counter-receptor that confers migration and survival advantages to colon carcinoma cells by triggering p38 and ERK MAPK activation

E-selectin-mediated adhesion of colon cancer cells to endothelial cells is a key event in metastasis. However, the signaling mechanisms that confer metastatic advantages to cancer cells adhering to E-selectin are ill defined. By using affinity column chromatography and pull-down assays on purified membrane extracts of HT29 and LoVo cells coupled to mass spectrometry analysis, we obtained the first evidence indicating that E-selectin binds to death receptor-3 (DR3) expressed by the cancer cells. Thereafter, we accumulated several results, suggesting that DR3 is an E-selectin receptor on colon cancer cells and that its activation by E-selectin triggers the activation of p38 and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) and confers migration and survival advantages. First, by Western blotting, we found that the E-selectin-binding protein, identified as DR3, is recognized by two anti-DR3 antibodies. Second, the neutralization of DR3 with an antibody and its knockdown by small interfering RNA decrease the adhesion of colon cancer cells to E-selectin and E-selectin-expressing human umbilical vein endothelial cells. Third, inhibiting DR3 and knocking down its expression impair transendothelial migration of HT29 cells and block the activation of p38 and ERK by E-selectin. Fourth, high molecular weight isoforms of DR3 are expressed in samples of primary human colon carcinoma but not in samples from normal colon tissue. Intriguingly, DR3 is a death receptor but its activation by E-selectin does not induce apoptosis in colon cancer cells, except when ERK is inhibited. Our findings identify novel signaling and functional roles of DR3 activated in response to E-selectin and highlight the potential link between DR3 and metastasis.

Protective Effects of Neurotrophic Factors on Tumor Necrosis Factor-related Apoptosis-inducing Ligand (TRAIL)-mediated Apoptosis of Murine Adrenal Chromaffin Cell Line tsAM5D

We previously established the murine adrenal chromaffin cell line tsAM5D, which was immortalized with the temperature-sensitive simian virus 40 large T-antigen. tsAM5D cells have the capacity to differentiate into neuron-like cells in response to neurotrophic factors when the culture temperature is shifted from 33 to 39 degrees C. In this model system, the temperature shift in the absence of neurotrophic factors led to cell death. Hoechst staining analysis revealed that typical apoptotic nuclei appeared in a time-dependent manner after the temperature shift. Upon shifting to 39 degrees C, the degradation of T-antigen was accompanied by the transcriptional activation of p53 protein. Among the p53 target genes, death receptor 5 (DR5), which is the receptor for tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), showed the highest level of induction. Interestingly, TRAIL-neutralizing antibody protected tsAM5D cells from the temperature shift-induced apoptotic cell death by blocking the activation of caspase-8 and -3, indicating the involvement of TRAIL-mediated death signaling in the temperature shift-induced apoptosis. Glial cell line-derived neurotrophic factor (GDNF) inhibited the TRAIL-mediated activation of caspase-8 in tsAM5D cells exposed to 39 degrees C and cooperated with basic fibroblast growth factor and ciliary neurotrophic factor. Interestingly, the temperature shift induced oligomerization of DR5, which is the earliest process necessary for transduction of the death signal. This oligomerization was inhibited by treatment with GDNF plus ciliary neurotrophic factor but not by that with GDNF alone or GDNF plus basic fibroblast growth factor. These results are discussed with respect to the intracellular mechanism underlying the protective function of neurotrophic factors against TRAIL-mediated death signaling.

TNF-alpha sensitizes normal and fibrotic human lung fibroblasts to Fas-induced apoptosis

Pulmonary accumulation of fibroblasts and myofibroblasts in idiopathic pulmonary fibrosis/usual interstitial pneumonia (IFP/UIP) has been linked to (1) increased migration of a circulating pool of fibrocytes, (2) cell proliferation, and (3) resistance to apoptosis. The mechanism of physiologic apoptosis of lung fibroblasts is poorly understood. Using normal and fibrotic human lung fibroblasts and the human lung fibroblast cell line, MRC-5, we examined the regulation of Fas-induced apoptosis by the proinflammatory cytokines TNF-alpha and IFN-gamma. Herein, we show that the basal resistance of lung fibroblasts and myofibroblasts to Fas-induced apoptosis is overcome by sensitization with TNF-alpha. IFN-gamma did not sensitize cells to Fas-induced apoptosis, but exhibited synergistic activity with TNF-alpha. Sensitization by TNF-alpha was observed in MRC-5 cells and in fibroblasts and myofibroblasts from normal and fibrotic human lung, suggesting that this represents a conserved mechanism to engage Fas-induced apoptosis. The mechanism of sensitization was localized at the level of recruitment of the adapter protein, FADD, to the cytoplasmic domain of Fas. Collectively, these findings suggest that fibroblast apoptosis involves two steps, sensitization and induction, and that inadequate pulmonary inflammation in IPF/UIP may favor fibroblast accumulation by reducing sensitization to apoptosis.

Instant decisions: transcription-independent control of death-receptor-mediated apoptosis

Transcription-independent modulation of signaling mediated by death receptors (DRs) has emerged as an important determinant of cell survival during both development and cellular homeostasis. Frequently, a given DR signal must be redirected rapidly either to inhibit or to potentiate the apoptotic response. This process requires immediate, protein-synthesis-independent modifications of the regulatory molecules involved. Numerous mechanisms have been shown to regulate DR responses without engaging the apoptosis-directing transcription machinery. These mechanisms involve key posttranslational modifications such as phosphorylation, ubiquitination and proteolytic degradation, all of which affect the activities of proteins at different levels in the DR signaling pathways. Changes in the organization of regulatory molecules and in their interactions with other factors also affect the DR signaling pathways. The balance between these modulatory signals rapidly decides the fate of a cell.

×Phosphorylation of Bax Ser184 by Akt Regulates Its Activity and Apoptosis in Neutrophils

Although important for apoptosis, the mechanism of Bax regulation is poorly understood. This study demonstrates that phosphorylation of Ser(184) regulates Bax activity. The phosphorylation required phosphatidylinositol 3-kinase/Akt activation and appeared to be mediated by Akt itself. In the serine-phosphorylated form, Bax was detected in the cytoplasm, could not be immunoprecipitated with the activation-specific antibody 6A7, and promoted heterodimerization with Mcl-1, Bcl-x(L), and A1. Apoptotic neutrophils possessed reduced levels of serine-phosphorylated Bax correlating with an increase in activated Bax as well as an increase in the amount of Bax found translocated to the mitochondria. We suggest that Bax is regulated by phosphorylation of Ser(184) in an Akt-dependent manner and that phosphorylation inhibits Bax effects on the mitochondria by maintaining the protein in the cytoplasm, heterodimerized with antiapoptotic Bcl-2 family members.

Selective regulation by delta-PKC and PI 3-kinase in the assembly of the antiapoptotic TNFR-1 signaling complex in neutrophils

TNF is implicated in the attenuation of neutrophil constitutive apoptosis during sepsis. Antiapoptotic signaling is mediated principally through the TNF receptor-1 (TNFR-1). In adherent neutrophils, when beta-integrin signaling is activated, TNF phosphorylates TNFR-1 and activates prosurvival and antiapoptotic signaling. Previously, we identified the delta-PKC isotype and phosphatidylinositol (PI) 3-kinase as critical regulators of TNF signaling in adherent neutrophils. Both kinases associate with TNFR-1 in response to TNF and are required for TNFR-1 serine phosphorylation, NF-kappaB activation, and inhibition of apoptosis. The purpose of this study was to examine the role of delta-PKC and PI 3-kinase in the assembly of TNFR-1 signaling complex that regulates NF-kappaB activation and antiapoptotic signaling. Coimmunoprecipitation studies established that PI 3-kinase, delta-PKC, and TNFR-1 formed a signal complex in response to TNF. delta-PKC recruitment required both delta-PKC and PI 3-kinase activity, whereas PI 3-kinase recruitment was delta-PKC independent, suggesting that PI 3-kinase acts upstream of delta-PKC. An important regulatory step in control of antiapoptotic signaling is the assembly of the TNFR-1-TNFR-1-associated death domain protein (TRADD)-TNFR-associated factor 2 (TRAF2)-receptor interacting protein (RIP) complex that controls NF-kappaB activation. Inhibition of either delta-PKC or PI 3-kinase decreased TNF-mediated recruitment of RIP and TRAF2 to TNFR-1. In contrast, TRADD recruitment was enhanced. Thus delta-PKC and PI 3-kinase are positive regulators of TNF-mediated association of TRAF2 and RIP with TNFR-1. Conversely, these kinases are negative regulators of TRADD association. These results suggest that delta-PKC and PI 3-kinase regulate TNF antiapoptotic signaling at the level of the TNFR-1 through control of assembly of a TNFR-1-TRADD-RIP-TRAF2 complex.

Protein kinase C modulates tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis by targeting the apical events of death receptor signaling

We have further examined the mechanism by which phorbol ester-mediated protein kinase C (PKC) activation protects against tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced cytotoxicity. We now report that activation of PKC targets death receptor signaling complex formation. Pre-treatment with 12-O-tetradecanoylphorbol-13-acetate (PMA) led to inhibition of TRAIL-induced apoptosis in HeLa cells, which was characterized by a reduction in phosphatidylserine (PS) externalization, decreased caspase-8 processing, and incomplete maturation and activation of caspase-3. These effects of PMA were completely abrogated by the PKC inhibitor, bisindolylmaleimide I (Bis I), clearly implicating PKC in the protective effect of PMA. TRAIL-induced mitochondrial release of the apoptosis mediators cytochrome c and Smac was blocked by PMA. This, together with the observed decrease in Bid cleavage, suggested that PKC activation modulates apical events in TRAIL signaling upstream of mitochondria. This was confirmed by analysis of TRAIL death-inducing signaling complex formation, which was disrupted in PMA-treated cells as evidenced by a marked reduction in Fas-associated death domain protein (FADD) recruitment, an effect that could not be explained by any change in FADD phosphorylation state. In an in vitro binding assay, the intracellular domains of both TRAIL-R1 and TRAIL-R2 bound FADD: activation of PKC significantly inhibited this interaction suggesting that PKC may be targeting key apical components of death receptor signaling. Significantly, this effect was not confined to TRAIL, because isolation of the native TNF receptor signaling complex revealed that PKC activation also inhibited TNF receptor-associated death domain protein recruitment to TNF-R1 and TNF-induced phosphorylation of IkappaB-alpha. Taken together, these results show that PKC activation specifically inhibits the recruitment of key obligatory death domain-containing adaptor proteins to their respective membrane-associated signaling complexes, thereby modulating TRAIL-induced apoptosis and TNF-induced NF-kappaB activation, respectively.

Okadaic-acid-induced apoptosis in malignant glioma cells

OBJECT

Okadaic acid (OA), a potent protein phosphatase inhibitor, has been known to induce apoptosis in a variety of cell types. The authors attempted to characterize further this model by identifying proteins involved in this form of programmed cell death.

METHODS

Cellular proliferation was assessed using a colorimetric nonradioactive proliferation assay and cell counts. Apoptosis was determined by fluorescent microscopy. Activation of the mitogen-activated protein kinase (MAPK) pathways was determined by immunoprecipitation of extracellular signal-regulated kinase (ERK), c-Jun-N-terminal kinase (JNK), and p38 followed by in vitro kinase assays. Western blot analyses were conducted to show inhibitory-kappaB (IkappaB) phosphorylation and degradation as well as Bax upregulation. The binding of nuclear factor-kappaB (NFkappaB) was shown by electrophoretic mobility shift assay. Okadaic acid induced cell death in T98G human malignant cell lines (50% inhibiting concentration = 20-25 nM). In T98G cells YO-PRO fluorescent staining was identified, thus indicating an apoptotic mechanism with a smaller percentage of cells undergoing necrotic cell death. Additionally OA induced JNK and MAPK activities in a time-dependent manner, increased the expression of Bax, and increased IkappaB phosphorylation and NFkappaB activation. There was a temporal correlation between these subcellular events and the detection of apoptosis morphology in glioma cells.

CONCLUSIONS

The authors believe that OA acts by blocking dephosphorylation events, thus activating apoptotic pathways through ERK and JNK activity. Additionally Bax, IkappaB and NFkappaB may also play a role in regulating these pathways.

All in the family: evolutionary and functional relationships among death receptors

Over the last decade, significant progress has been made towards identifying the signaling pathways within mammalian cells that lead to apoptosis mediated by death receptors. The simultaneous expression of more than one death receptor in many, if not all, cell types suggests that functional innovation has driven the divergence of these receptors and their cognate ligands. To better understand the physiological divergence of the death receptors, a phylogenetic analysis of vertebrate death receptors was conducted based upon amino-acid sequences encoding the death domain regions of currently known and newly identified members of the family. Evidence is presented to indicate an ancient radiation of death receptors that predates the emergence of vertebrates, as well as ongoing divergence of additional receptors both within several receptor lineages as well as modern taxonomic lineages. We speculate that divergence among death receptors has led to their functional specialization. For instance, some receptors appear to be primarily involved in mediating the immune response, while others play critical roles during development and tissue differentiation. The following represents an evolutionary approach towards an understanding of the complex relationship among death receptors and their proposed physiological functions in vertebrate species.

Mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in activated T cells abrogates TRAIL-induced apoptosis upstream of the mitochondrial amplification loop and caspase-8

Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) induce apoptosis in many different cell types. Jurkat T cells die rapidly by apoptosis after treatment with either ligand. We have previously shown that mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) can act as a negative regulator of apoptosis mediated by the Fas receptor. In this study we examined whether MAPK/ERK can also act as a negative regulator of apoptosis induced by TRAIL. Activated Jurkat T cells were efficiently protected from TRAIL-induced apoptosis. The protection was shown to be MAPK/ERK dependent and independent of protein synthesis. MAPK/ERK suppressed TRAIL-induced apoptosis upstream of the mitochondrial amplification loop because mitochondrial depolarization and release of cytochrome c were inhibited. Furthermore, caspase-8-mediated relocalization and activation of Bid, a proapoptotic member of the Bcl family, was also inhibited by the MAPK/ERK signaling. The protection occurred at the level of the apoptotic initiator caspase-8, as the cleavage of caspase-8 was inhibited but the assembly of the death-inducing signaling complex was unaffected. Both TRAIL and Fas ligand have been suggested to regulate the clonal size and persistence of different T cell populations. Our previous results indicate that MAPK/ERK protects recently activated T cells from Fas receptor-mediated apoptosis during the initial phase of an immune response before the activation-induced cell death takes place. The results of this study show clearly that MAPK/ERK also participates in the inhibition of TRAIL-induced apoptosis after T cell activation.