Fas aggregation does not correlate with Fas-mediated apoptosis

Ouabain-induced perturbations in intracellular ionic homeostasis regulate death receptor-mediated apoptosis

Apoptosis is defined by specific morphological and biochemical characteristics including cell shrinkage (termed apoptotic volume decrease), a process that results from the regulation of ion channels and plasma membrane transporter activity. The Na(+)-K(+)-ATPase is the predominant pump that controls cell volume and plasma membrane potential in cells and alterations in its function have been suggested to be associated with apoptosis. We report here that the Na(+)-K(+)-ATPase inhibitor ouabain, potentiates apoptosis in the human lymphoma Jurkat cells exposed to Fas ligand (FasL) or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) but not other apoptotic agents such as H(2)O(2), thapsigargin or UV-C implicating a role for the Na(+)-K(+)-ATPase in death receptor-induced apoptosis. Interestingly, ouabain also potentiated perturbations in cell Ca(2+) homeostasis only in conjunction with the apoptotic inducer FasL but not TRAIL. Ouabain did not affect alterations in the intracellular Ca(2+) levels in response to H(2)O(2), thapsigargin or UV-C. FasL-induced alterations in Ca(2+) were not abolished in Ca(2+)-free medium but incubation of cells with BAPTA-AM inhibited both Ca(2+) perturbations and the ouabain-induced potentiation of FasL-induced apoptosis. Our data suggest that the impairment of the Na(+)-K(+)-ATPase activity during apoptosis is linked to perturbations in cell Ca(2+) homeostasis that modulate apoptosis induced by the activation of Fas by FasL.

Enhanced Fas-associated death domain recruitment by histone deacetylase inhibitors is critical for the sensitization of chronic lymphocytic leukemia cells to TRAIL-induced apoptosis

Chronic lymphocytic leukemia (CLL) is an incurable disease characterized by failure of mature lymphocytes to undergo apoptosis. CLL cells are inherently resistant to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Pretreatment with histone deacetylase inhibitors (HDACi) sensitizes CLL cells to TRAIL-mediated apoptosis primarily via TRAIL-R1 and offers a novel approach for the therapy of CLL and other malignancies. Depsipeptide (romidepsin), a HDACi, did not enhance TRAIL binding to TRAIL-R1, TRAIL-R1 aggregation, or internalization of TRAIL-R1, but it enhanced Fas-associated death domain protein (FADD) recruitment to TRAIL-R1 in the death-inducing signaling complex. Cotreatment with phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, dramatically inhibited the HDACi-mediated increase in FADD recruitment and sensitization to TRAIL-induced apoptosis and both of these were reversed by PKC inhibitors. Thus, enhanced FADD recruitment is a critical step in HDACi-mediated sensitization of CLL cells to TRAIL-induced apoptosis and this step is differentially affected by HDACi and phorbol 12-myristate 13-acetate. Using biotinylated TRAIL and streptactin-tagged TRAIL, we have identified several novel TRAIL receptor interacting proteins, including PKCbeta, lymphocyte-specific protease-1, Lyn, and Syk. These molecules may play an as yet unappreciated role in TRAIL signaling in CLL cells and inhibition of one or more of these kinases/phosphatases may provide a novel target to overcome TRAIL resistance.

CD95 activation in the liver: ion fluxes and oxidative signaling

Apoptosis is characterized by typical features as cell shrinkage, nuclear condensation, DNA fragmentation, and apoptotic body formation. Whereas some signs of apoptosis are cell type-and death signal-dependent, apoptotic cell volume decrease is an early and ubiquitous event and little is known about the signalling events, which are localized upstream of the plasma membrane transport steps leading to apoptotic cell volume decrease and the proapoptotic events, which are induced by osmolyte loss and cell shrinkage. Ion fluxes and oxidative signaling were recently shown to play an important role in signal transduction with respect to apoptotic cell death within the liver, as a ceramide-dependent activation of the NADPH oxidase was identified as the source of reactive oxygen species generation in rat hepatocytes upon treatment with CD95 ligand, hydrophobic bile salts or hyperosmolarity. The NADPH oxidase-derived ROS signal then allows via Yes, JNK, and EGFR activation for CD95 tyrosine phosphorylation as a prerequisite for CD95 targeting to the plasma membrane and formation of the death inducing signalling complex. Other covalent modifications such as CD95-tyrosine-nitration or CD95-serine/threonine-phosphorylation can interfere with the CD95 activation process. The findings not only provide a mechanistic explanation for the high susceptibility of dehydrated cells for apoptosis, but also give insight into the role of ion fluxes and oxidative signaling with respect to apoptotic cell death within the liver.

Palmitoylation of CD95 facilitates formation of SDS-stable receptor aggregates that initiate apoptosis signaling

Apoptosis signaling through CD95 (Fas/APO-1) involves aggregation and clustering of the receptor followed by its actin-dependent internalization. Internalization is required for efficient formation of the death-inducing signaling complex (DISC) with maximal recruitment of FADD, caspase-8/10 and c-FLIP occurring when the receptor has reached an endosomal compartment. The first detectable event during CD95 signaling is the formation of SDS-stable aggregates likely reflecting intense oligomerization of the receptor. We now demonstrate that these SDS-stable forms of CD95 correspond to very high molecular weight DISC complexes (hiDISC) and are the sites of caspase-8 activation. hiDISCs are found both inside and outside of detergent-resistant membranes. The formation of SDS-stable CD95 aggregates involves palmitoylation of the membrane proximal cysteine 199 in CD95. Cysteine 199 mutants no longer form SDS-stable aggregates, and inhibition of palmitoylation reduces internalization of CD95 and activation of caspase-8. Our data demonstrate that SDS-stable forms of CD95 are the sites of apoptosis initiation and represent an important early step in apoptosis signaling through CD95 before activation of caspases.

Increase of Fas-induced apoptosis by inhibition of extracellular phosphorylation of Fas receptor in Jurkat cell line

Apoptosis signalling through the Fas pathway requires several steps of aggregation of the Fas receptor in the membrane, including aggregation that may occur in the absence of Fas ligand. Association of Fas domains is determinant to signal transmission following Fas ligand binding to a specific domain. The domains involved in Fas aggregation are located in its extracellular region and contain three potential protein kinase C-binding motifs. We therefore studied the possibility that phosphorylation of the extracellular region of Fas might be implicated in the regulation of Fas-mediated apoptosis. Inhibition experiments of extracellular phosphorylation were performed in human Jurkat T leukemia cells with K252b, an impermeant protein-kinase inhibitor. Extracellular phosphorylation of Fas receptor was related to ecto-kinase, as assessed by the [gamma-(32)P] ATP labelling of Fas-116 kDa aggregates, suppressed by K252b inhibitor which significantly increased the sensitivity to Fas-mediated apoptosis. Ecto-PKC involvement was demonstrated by bisindolylmaleimide VIII, a selective inhibitor of protein kinase C which significantly increased both Fas aggregation in the membrane and Fas-mediated apoptosis and by the addition of the PKC pseudo-substrate 19-36 which inhibited the phosphorylation of 116 kDa Fas aggregates. These data support a role for Fas phosphorylation in the decreased sensitivity to apoptosis in the Jurkat T leukemia cell line.

RUNX3, a novel tumor suppressor, is frequently inactivated in gastric cancer by protein mislocalization

Loss of RUNX3 expression is suggested to be causally related to gastric cancer as 45% to 60% of gastric cancers do not express RUNX3 mainly due to hypermethylation of the RUNX3 promoter. Here, we examined for other defects in the properties of RUNX3 in gastric cancers that express RUNX3. Ninety-seven gastric cancer tumor specimens and 21 gastric cancer cell lines were examined by immunohistochemistry using novel anti-RUNX3 monoclonal antibodies. In normal gastric mucosa, RUNX3 was expressed most strongly in the nuclei of chief cells as well as in surface epithelial cells. In chief cells, a significant portion of the protein was also found in the cytoplasm. RUNX3 was not detectable in 43 of 97 (44%) cases of gastric cancers tested and a further 38% showed exclusive cytoplasmic localization, whereas only 18% showed nuclear localization. Evidence is presented suggesting that transforming growth factor-beta is an inducer of nuclear translocation of RUNX3, and RUNX3 in the cytoplasm of cancer cells is inactive as a tumor suppressor. RUNX3 was found to be inactive in 82% of gastric cancers through either gene silencing or protein mislocalization to the cytoplasm. In addition to the deregulation of mechanisms controlling gene expression, there would also seem to be at least one other mechanism controlling nuclear translocation of RUNX3 that is impaired frequently in gastric cancer.

c-Myb Is Critical for B Cell Development and Maintenance of Follicular B Cells

The c-Myb transcription factor is crucial during definitive hematopoiesis. However, the embryonic lethality of Myb traditional null mutations has precluded analysis of c-Myb function in lymphocytes. Using tissue-specific inactivation at the Myb locus, we demonstrate that loss of Myb causes a partial block during B cell development at the pro-B to pre-B cell transition, resulting in greatly decreased output of new B cells from the bone marrow. Furthermore, we demonstrate that Myb is not essential for the proliferation of splenic B cells, but that loss of c-Myb function prevents normal B cell homeostasis due to decreased splenic B cell survival. Decreased survival is accompanied by hyporesponsiveness to the B cell survival factor BLyS (also termed BAFF), decreased expression of the BLyS receptor 3 (BR3), and altered regulation of PKCdelta nuclear accumulation. Thus, c-Myb is important during multiple stages of B-lymphopoiesis.

Role of membrane sphingomyelin and ceramide in platform formation for Fas-mediated apoptosis

Engagement of the Fas receptor (CD95) initiates multiple signaling pathways that lead to apoptosis, such as the formation of death-inducing signaling complex (DISC), activation of caspase cascades, and the generation of the lipid messenger, ceramide. Sphingomyelin (SM) is a major component of lipid rafts, which are specialized structures that enhance the efficiency of membrane receptor signaling and are a main source of ceramide. However, the functions of SM in Fas-mediated apoptosis have yet to be clearly defined, as the responsible genes have not been identified. After cloning a gene responsible for SM synthesis, SMS1, we established SM synthase–defective WR19L cells transfected with the human Fas gene (WR/Fas-SM(−)), and cells that have been functionally restored by transfection with SMS1 (WR/Fas-SMS1). We show that expression of membrane SM enhances Fas-mediated apoptosis through increasing DISC formation, activation of caspases, efficient translocation of Fas into lipid rafts, and subsequent Fas clustering. Furthermore, WR/Fas-SMS1 cells, but not WR/Fas-SM(−) cells, showed a considerable increase in ceramide generation within lipid rafts upon Fas stimulation. These data suggest that a membrane SM is important for Fas clustering through aggregation of lipid rafts, leading to Fas-mediated apoptosis.

The extracellular domains of FasL and Fas are sufficient for the formation of supramolecular FasL-Fas clusters of high stability

Using fluorescent variants of Fas and FasL, we show that membrane FasL and Fas form supramolecular clusters that are of flexible shape, but nevertheless stable and persistent. Membrane FasL-induced Fas clusters were formed in caspase-8- or FADD-deficient cells or when a cytoplasmic deletion mutant of Fas was used suggesting that cluster formation is independent of the assembly of the cytoplasmic Fas signaling complex and downstream activated signaling pathways. In contrast, cross-linked soluble FasL failed to aggregate the cytoplasmic deletion mutant of Fas, but still induced aggregation of signaling competent full-length Fas. Moreover, membrane FasL-induced Fas cluster formation occurred in the presence of the lipid raft destabilizing component methyl-beta-cyclodextrin, whereas Fas aggregation by soluble FasL was blocked. Together, these data suggest that the extracellular domains of Fas and FasL alone are sufficient to drive membrane FasL-induced formation of supramolecular Fas-FasL complexes, whereas soluble FasL-induced Fas aggregation is dependent on lipid rafts and mechanisms associated with the intracellular domain of Fas.

Stimulation of Fas agonistic antibody-mediated apoptosis by heparin-like agents suppresses Hsp27 but not Bcl-2 protective activity

We report that in Jurkat T cells or freshly isolated T lymphocytes, physiological concentrations of high-molecular weight sulfated polysaccharides such as heparin, heparan sulfate, and dextran sulfate significantly increased the percentage of cell death induced by Fas IgM agonistic antibody. The phenomenon was caspase dependent and P53 independent and correlated with an increased accessibility of cell surface Fas receptors. We also observed that the Fas IgM agonistic antibody-dependent formation of sodium dodecyl sulfate (SDS)-resistant large structures containing Fas receptor was decreased in the presence of heparin-like agents. In contrast, the different agents had no effect when cell death was triggered by FasL, the natural ligand of Fas that does not generate SDS-resistant forms of Fas. Interestingly, the synergistic effect of heparin-like agents toward Fas IgM agonistic antibody-mediated cell death abolished Hsp27 antiapoptotic activity but did not alter much the protection generated by Bcl-2 expression.

In vivo UVB irradiation induces clustering of Fas (CD95) on human epidermal cells

In vitro studies with human cell lines have demonstrated that the death receptor Fas plays a role in ultraviolet (UV)-induced apoptosis. The purpose of the present study was to investigate the relation between Fas expression and apoptosis as well as clustering of Fas in human epidermis after a single dose of UVB irradiation. Normal healthy individuals were irradiated with three minimal erythema doses (MED) of UVB on forearm or buttock skin. Suction blisters from unirradiated and irradiated skin were raised, and Fas, FasL, and apoptosis of epidermal cells quantified by flow cytometry. Clustering of Fas was from skin biopsied. Soluble FasL in suction blister fluid was quantified by ELISA. Flow cytometric analysis demonstrated increased expression intensity of Fas after irradiation, with 1.6-,2.2- and 2.7-fold increased median expression at 24, 48 and 72 h after irradiation, respectively (n=4). Apoptosis was demonstrated by the TUNEL reaction, and the maximum of apoptotic cells was detected at 48 h after irradiation. Double-staining of Fas and TUNEL showed that apoptosis was restricted to the Fas-positive epidermal subpopulation, but there was no correlation between the intensities of Fas expression and TUNEL reaction. Median expression intensity of FasL-positive cells transiently decreased to 0.9- and 0.8-fold of the preirradiation respective level after 24 h and 48 h, respectively, and returned to the respective preirradiation level at 72 h after irradiation (n=4). Concentrations of soluble FasL in suction blister fluid from UVB-irradiated skin did not differ from those in unirradiated skin (n=5). Confocal laser scanning microscopy showed a rapid clustering of Fas within 30 min after irradiation. A simultaneous clustering of the adapter signalling protein FADD suggested that Fas clustering has a functional significance. Our results ar in accordance with previous findings from in vitro studies, and suggest that Fas is activated in vivo in human epidermis after UVB exposure.

Cutting edge: SDS-stable Fas microaggregates: an early event of Fas activation occurring with agonistic anti-Fas antibody but not with Fas ligand

The 45 kDa Fas or CD95 receptor triggers apoptosis via the caspase cascade when stimulated by its ligand FasL or by agonistic Abs. Activated Fas receptors seem to oligomerize very early into SDS-stable and reducing agent-resistant microaggregates of 200-250 kDa on SDS-PAGE. However, these microaggregates have so far only been reported using agonistic anti-Fas Abs, and no results have been reported using FasL. Here, we demonstrate that the microaggregates do not form in response to FasL, while they always appear in response to the agonistic Ab, in four different cell lines and in normal lymphocytes from human blood. Therefore, the Fas microaggregates are not required for the induction of apoptosis via FasL. These results also suggest that subtle differences exist in the apoptotic pathways triggered by anti-Fas agonistic Abs and by FasL.

Disabling of Receptor Activator of Nuclear Factor-κB (RANK) Receptor Complex by Novel Osteoprotegerin-like Peptidomimetics Restores Bone Loss in Vivo

The tumor necrosis factor family ligand, tumor necrosis factor-related activation-induced cytokine (TRANCE), and its receptors, receptor activator of nuclear factor-kappaB (RANK) and osteoprotegerin (OPG), are known to be regulators of development and activation of osteoclasts in bone remodeling. Sustained osteoclast activation that occurs through TRANCE-RANK causes osteopenic disorders such as osteoporosis and contributes to osteolytic metastases. Here, we report a rationally designed small molecule mimic of osteoprotegerin to inhibit osteoclast formation in vitro and limit bone loss in an animal model of osteoporosis. One of the mimetics, OP3-4, significantly inhibited osteoclast formation in vitro (IC(50) = 10 microm) and effectively inhibited total bone loss in ovariectomized mice at a dosage of 2 mg/kg/day. Unlike soluble OPG receptors, which preclude TRANCE binding to RANK, OP3-4 shows the ability to modulate RANK-TRANCE signaling pathways and alters the biological functions of the RANK-TRANCE receptor complex by facilitating a defective receptor complex. These features suggest that OPG-derived small molecules can be used as a probe to understand complex biological functions of RANK-TRANCE-OPG receptors and also can be used as a platform to develop more useful therapeutic agents for inflammation and bone disease.

The CD99 signal enhances Fas-mediated apoptosis in the human leukemic cell line, Jurkat

The CD99 antigen has been implicated in various cellular processes, including apoptosis in T cells. Previously, we reported two monoclonal antibodies that recognize different epitopes of the CD99 molecule, named DN16 and YG32. In this study, we investigated the role of each CD99 epitope in T cell apoptosis. Unlike the DN16 epitope, CD99 ligation via the YG32 epitope failed to induce T cell death. Surprisingly, however, the YG32 signal enhanced Fas-mediated apoptosis in Jurkat T cells. Augmentation of Fas-mediated apoptosis by YG32 ligation was inhibited by treatment with either of the caspase inhibitors z-VAD-fmk or z-IETD-fmk, and YG32 ligation appeared to induce Fas oligomerization. These results suggest that each CD99 epitope plays a distinct role in T cell biology, especially in T cell apoptosis.

Homeostasis of naive and memory CD4+ T cells: IL-2 and IL-7 differentially regulate the balance between proliferation and Fas-mediated apoptosis

Cytokines play a crucial role in the maintenance of polyclonal naive and memory T cell populations. It has previously been shown that ex vivo, the IL-7 cytokine induces the proliferation of naive recent thymic emigrants (RTE) isolated from umbilical cord blood but not mature adult-derived naive and memory human CD4(+) T cells. We find that the combination of IL-2 and IL-7 strongly promotes the proliferation of RTE, whereas adult CD4(+) T cells remain relatively unresponsive. Immunological activity is controlled by a balance between proliferation and apoptotic cell death. However, the relative contributions of IL-2 and IL-7 in regulating these processes in the absence of MHC/peptide signals are not known. Following exposure to either IL-2 or IL-7 alone, RTE, as well as mature naive and memory CD4(+) T cells, are rendered only minimally sensitive to Fas-mediated cell death. However, in the presence of the two cytokines, Fas engagement results in a high level of caspase-dependent apoptosis in both RTE as well as naive adult CD4(+) T cells. In contrast, equivalently treated memory CD4(+) T cells are significantly less sensitive to Fas-induced cell death. The increased susceptibility of RTE and naive CD4(+) T cells to Fas-induced apoptosis correlates with a significantly higher IL-2/IL-7-induced Fas expression on these T cell subsets than on memory CD4(+) T cells. Thus, IL-2 and IL-7 regulate homeostasis by modulating the equilibrium between proliferation and apoptotic cell death in RTE and mature naive and memory T cell subsets.

Hyperosmolarity triggers CD95 membrane trafficking and sensitizes rat hepatocytes toward CD95L-induced apoptosis

The effect of hyperosmolarity on CD95 membrane targeting and CD95 ligand (CD95L)-induced apoptosis was studied in rat hepatocytes. CD95 showed a predominant intracellular localization in normoosmotically exposed rat hepatocytes, whereas hyperosmotic exposure induced, within 1 hour, CD95 trafficking to the plasma membrane followed by activation of caspase-3 and -8. Hyperosmotic CD95 membrane targeting was sensitive to inhibition of c-Jun-N-terminal kinase (JNK), protein kinase C (PKC), and cyclic adenosine monophosphate, but not to inhibition of extracellular regulated kinases (Erks) or p38 mitogen activated protein kinase (p38(MAPK)). Hyperosmotic CD95 targeting to the plasma membrane was dose-dependently diminished by glutamine or taurine, probably caused by an augmentation of volume regulatory increase. Despite CD95 trafficking to the plasma membrane and caspase activation, hyperosmolarity per se did not induce apoptosis. Hyperosmolarity, however, sensitized hepatocytes toward CD95L-induced apoptosis, as assessed by annexin V staining and terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling (TUNEL) assay. This sensitization was abolished when hyperosmotic CD95 membrane trafficking was prevented by cyclic adenosine monophosphate, PKC, or JNK inhibition, whereas these effectors had no effect on CD95L-induced apoptosis in normoosmotically exposed hepatocytes. CD95L addition under normoosmotic conditions caused CD95 membrane trafficking, which was sensitive to JNK inhibition, but not to cyclic adenosine monophosphate or inhibition of PKC, Erks, and p38(MAPK). In conclusion, multiple signaling pathways are involved in CD95 membrane trafficking. Hyperosmotic hepatocyte shrinkage induces CD95 trafficking to the plasma membrane, which involves JNK-, PKA-, and PKC-dependent mechanisms and sensitizes hepatocytes toward CD95L-mediated apoptosis.