Role of the CD95/CD95 ligand system in glucocorticoid-induced monocyte apoptosis

Glucocorticoids (GC) act as potent anti-inflammatory and immunosuppressive agents on a variety of immune cells. However, the exact mechanisms of their action are still unknown. Recently, we demonstrated that GC induce apoptosis in human peripheral blood monocytes. In the present study, we examined the signaling pathway in GC-induced apoptosis. Monocyte apoptosis was demonstrated by annexin V staining, DNA laddering, and electron microscopy. Apoptosis required the activation of caspases, as different caspase inhibitors prevented GC-induced cell death. In addition, the proteolytic activation of caspase-8 and caspase-3 was observed. In additional experiments, we determined the role of the death receptor CD95 in GC-induced apoptosis. CD95 and CD95 ligand (CD95L) were up-regulated in a dose- and time-dependent manner on the cell membrane and also released after treatment with GC. Costimulation with the GC receptor antagonist mifepristone diminished monocyte apoptosis as well as CD95/CD95L expression and subsequent caspase-8 and caspase-3 activation. In contrast, the caspase inhibitor N:-acetyl-Asp-Glu-Val-Asp-aldehyde suppressed caspase-3 activation and apoptosis, but did not down-regulate caspase-8 activation and expression of CD95 and CD95L. Importantly, GC-induced monocyte apoptosis was strongly abolished by a neutralizing CD95L mAb. Therefore, our data suggest that GC-induced monocyte apoptosis is at least partially mediated by an autocrine or paracrine pathway involving the CD95/CD95L system.     

The role of CD95 and CD95 ligand in cancer

CD95 (Fas/APO-1) and its ligand, CD95L, have long been viewed as a death receptor/death ligand system that mediates apoptosis induction to maintain immune homeostasis. In addition, these molecules are important in the immune elimination of virus-infected cells and cancer cells. CD95L was, therefore, considered to be useful for cancer therapy. However, major side effects have precluded its systemic use. During the last 10 years, it has been recognized that CD95 and CD95L have multiple cancer-relevant nonapoptotic and tumor-promoting activities. CD95 and CD95L were discovered to be critical survival factors for cancer cells, and were found to protect and promote cancer stem cells. We now discuss five different ways in which inhibiting or eliminating CD95L, rather than augmenting, may be beneficial for cancer therapy alone or in combination with standard chemotherapy or immune therapy.     

Fas/CD95 is associated with glucocorticoid-induced osteocyte apoptosis

Prolonged use of glucocorticoids is associated with decreased bone formation, increased resorption and osteonecrosis, through direct and indirect effects on the activity and viability of bone effector cells, osteoblasts and osteoclasts, and osteocytes. This study has investigated molecular pathways implicated in Dexamethasone-induced apoptosis of osteocytes, using a cell line and primary chicken cells. MLO-Y4 osteocytes were pre-treated with several bisphosphonates representing a range of anti-resorptive activities and conformation/structure relationships, and were subsequently challenged with Dexamethasone. Apoptotic cells were detected at various times after treatment using morphological and biochemical criteria. Dex was shown to induce apoptosis associated with the Fas/CD95 death receptor and in a caspase 8 dependent manner. The apoptotic response was inhibited by all variants of the BP molecules, including those with reduced anti-resorptive activity, indicating that Dex-induced apoptosis is independent of anti-osteoclastic activity. Dex-induced apoptosis was associated with a transient increase in phosphorylated ERK 1/2 and was blocked by the ERK inhibitor UO126. In addition, both UO126 and BPs decreased localization of Fas to the cell membrane. ERK activation by PMA did not induce death or Fas upregulation, suggesting that Fas may be important for the induction of apoptosis and the existence of an additional factor activated by Dex which enables the cooperation between the Dex-activated ERK and Fas pathways, during apoptosis of osteocytes. Furthermore, upregulation of death and Fas was not accompanied by upregulation of FasL, pointing to the possible existence of FasL-independent Fas-associated death in these cells.     

CD47 augments Fas/CD95-mediated apoptosis

Fas (CD95) mediates apoptosis of many cell types, but the susceptibility of cells to killing by Fas ligand and anti-Fas antibodies is highly variable. Jurkat T cells lacking CD47 (integrin-associated protein) are relatively resistant to Fas-mediated death but are efficiently killed by Fas ligand or anti-Fas IgM (CH11) upon expression of CD47. Lack of CD47 impairs events downstream of Fas activation including caspase activation, poly-(ADP-ribose) polymerase cleavage, cytochrome c release from mitochondria, loss of mitochondrial membrane potential, and DNA cleavage. Neither CD47 signaling nor raft association of CD47 is required to enable Fas apoptosis. CH11 induces association of Fas and CD47. Primary T cells from CD47-null mice are also protected from Fas-mediated killing relative to wild type T cells. Thus CD47 associates with Fas upon its activation and augments Fas-mediated apoptosis.     

CD95(Fas/APO-1) signals ceramide generation independent of the effector stage of apoptosis

Although numerous studies document caspase-independent ceramide generation preceding apoptosis upon environmental stress, the molecular ordering of ceramide generation during cytokine-induced apoptosis remains uncertain. Here, we show that CD95-induced ceramide elevation occurs during the initiation phase of apoptosis. We titrated down the amount of FADD transfected into HeLa and 293T cells until it was insufficient for apoptosis, although cycloheximide (CHX) still triggered the effector phase. Even in the absence of CHX, ceramide levels increased rapidly, peaking at 2.7 +/- 0.2-fold of control 8 h post-transfection. Dominant negative FADD failed to confer ceramide generation or CHX-mediated apoptosis. Ceramide generation induced by FADD was initiator caspase-dependent, being blocked by crmA. Limited pro-caspase 8 overexpression also increased ceramide levels 2.7 +/- 0.2-fold, yet failed, without CHX, to initiate apoptosis. Expression of membrane-targeted oligomerized CD-8 caspase 8 induced apoptosis without CHX, yet elevated ceramide only to a level equivalent to limited pro-caspase 8 transfection. Ceramide elevations were detected concurrently by diacylglycerol kinase and electrospray tandem mass spectrometry. These investigations provide evidence that ceramide generation is initiator caspase-dependent and occurs prior to commitment to the effector phase of apoptosis, definitively ordering ceramide as proximal in CD95 signaling.     

CD95/CD95 ligand-mediated counterattack does not block T cell cytotoxicity

Expression of CD95 ligand on parenchymal, epithelial, or tumor cells has been suggested to downregulate the immune response and to control lymphocyte activation. Suppression might be mediated by induction of apoptosis or by inhibition of Ca(2+) channels upon CD95 triggering. We, therefore, aimed to employ this model to modify the immune response to an antigen presented to cytotoxic T cells by antigen-presenting MC57 cells. This model would be very useful to specifically downregulate the immune response to autoantigens in autoimmune situations. However, cytotoxic T cell lines tested in the present study were resistant to CD95 ligand expression on antigen-presenting MC57 cells. In addition, coincubation of the lymphocytes with antigen presenting cells failed to block cytotoxicity mediated by the T lymphocytes. We, therefore, conclude that single expression of CD95 ligand on antigen-presenting cells is insufficient to specifically downregulate an immune response by CD8(+-)triggered immune response.     

The CD95 receptor: apoptosis revisited

CD95 is the quintessential death receptor and, when it is bound by ligand, cells undergo apoptosis. Recent evidence suggests, however, that CD95 mediates not only apoptosis but also diverse nonapoptotic functions depending on the tissue and the conditions.     

Erk-dependent cytosolic phospholipase A2 activity is induced by CD95 ligand cross-linking in the mouse derived Sertoli cell line TM4 and is required to trigger apoptosis in CD95 bearing cells

In the present study we demonstrated that CD95L cross-linking generated reverse signalling in the mouse derived Sertoli cell line TM4. Treatment of TM4 cells with mAb anti-CD95L induced activation of the cytosolic phospholipase A2 (cPLA2). Cytosolic PLA2 activation was controlled by the MAPK pathway as indicated by the ability of the specific MEK inhibitor, PD098059, to abolish cPLA2 activation. In addition, Western blot experiments showed a rapid increase in phosphorylated Erk1/2 following CD95L cross-linking, while no effect on the phosphorylation of other MAPK, p38 or JNK, was observed. CD95L cross-linking by mAb increased the levels of soluble CD95L and apoptotic activity of TM4 cell supernatants, which was blocked by co-incubation with the PLA2 inhibitor, AACOCF3 or PD098059. Finally, pre-treatment of TM4 cells with AACOCF3 or PD098059 completely abolished TM4-induced apoptosis of Jurkat T cells, thus indicating that the Erk/cPLA2 pathway is required for CD95L-induced apoptosis.     

Tumor regression after adoptive transfer of effector T cells is independent of perforin or Fas ligand (APO-1L/CD95L).

The adoptive transfer of tumor-specific effector T cells can result in complete regression and cure mice with systemic melanoma, but the mechanisms responsible for regression are not well characterized. Perforin- and Fas ligand (APO-1/CD95 ligand)-mediated cytotoxicity have been proposed as mechanisms for T cell-mediated tumor destruction. To determine the role of perforin and Fas ligand (FasL) in T cell-mediated tumor regression in a murine melanoma model, B16BL6-D5 (D5), we generated D5-specific effector T cells from tumor vaccine-draining lymph nodes of wild type (wt), perforin knock out (PKO), or FasL mutant (gld) mice and treated established D5 metastases in mice with the same genotype. Effector T cells from wt, PKO and gld mice induced complete regression of pulmonary metastases and significantly prolonged survival of the treated animals regardless of their genotype. Complete tumor regression induced by PKO effector T cells was also observed in a sarcoma model (MCA-310). Furthermore, adoptive transfer of PKO and wt effector T cells provided long-term immunity to D5. Therapeutic T cells from wt, PKO, or gld mice exhibit a tumor-specific type 1 cytokine profile; they secrete IFN-gamma, but not IL-4. In these models, T cell-mediated tumor regression and long-term antitumor immunity are perforin and FasL independent.     

Modulation of the CD95-induced apoptosis: the role of CD95 N-glycosylation

Protein modifications of death receptor pathways play a central role in the regulation of apoptosis. It has been demonstrated that O-glycosylation of TRAIL-receptor (R) is essential for sensitivity and resistance towards TRAIL-mediated apoptosis. In this study we ask whether and how glycosylation of CD95 (Fas/APO-1), another death receptor, influences DISC formation and procaspase-8 activation at the CD95 DISC and thereby the onset of apoptosis. We concentrated on N-glycostructure since O-glycosylation of CD95 was not found. We applied different approaches to analyze the role of CD95 N-glycosylation on the signal transduction: in silico modeling of CD95 DISC, generation of CD95 glycosylation mutants (at N136 and N118), modulation of N-glycosylation by deoxymannojirimycin (DMM) and sialidase from Vibrio cholerae (VCN). We demonstrate that N-deglycosylation of CD95 does not block DISC formation and results only in the reduction of the procaspase-8 activation at the DISC. These findings are important for the better understanding of CD95 apoptosis regulation and reveal differences between apoptotic signaling pathways of the TRAIL and CD95 systems.     

Induction of CD95 ligand and apoptosis by doxorubicin is modulated by the redox state in chemosensitive- and drug-resistant tumor cells.

Induction of CD95 ligand (CD95-L) may contribute to drug-induced apoptosis in chemosensitive leukemias and solid tumors. Here we report that induction of CD95-L and apoptosis by doxorubicin in leukemic and neuroblastoma cells is regulated by the redox state and reactive oxygen species (ROS). Preincubation of chemosensitive cells with antioxidants such as N-acetyl-cysteine (NAC) or glutathione (GSH), significantly reduced doxorubicin-induced apoptosis, hyperexpression of ROS, loss of mitochondrial membrane potential (DeltaPsim) and upregulation of CD95-L expression. Doxorubicin-resistant cells exhibited higher levels of GSH in comparison to chemosensitive cells and were deficient in hyperproduction of ROS, loss of DeltaPsim and upregulation of CD95-L in response to cytotoxic drugs. Downregulation of intracellular GSH concentrations reversed deficient drug-induced hyperproduction of ROS and CD95-L upregulation. In addition, overexpression of Bcl-XL in CEM cells blocked doxorubicin-triggered ROS and CD95-L expression. These findings suggest that induction of CD95-L by cytotoxic drugs is modulated by the cellular redox state and mitochondria derived ROS.     

p53-mediated up-regulation of CD95 is not involved in genotoxic drug-induced apoptosis of human breast tumor cells

Induction of CD95 (Fas/APO-1) and CD95 ligand during chemotherapeutic treatment may contribute to the death by apoptosis of some tumor cells. In this study, we have analyzed the role of the CD95 system in genotoxic drug-induced death of human breast tumor cells. Incubation of the breast tumor cell lines MCF-7 and EVSA-T with doxorubicin or methotrexate caused apoptosis after 48 h of treatment. These drugs induced a marked increase in the level of CD95 mRNA and protein in wild-type p53-expressing MCF-7 cells. On the contrary, the breast cancer cell line EVSA-T that expresses high levels of an inactive form of p53, did not up-regulate CD95 upon drug treatment. Elevation of CD95 expression by DNA-damaging drugs was notably blocked in MCF-7 cells expressing the human papillomavirus type 16 E6 protein (E6 cells) which prevented p53 accumulation upon DNA damage. However, E6 cells were still killed by the drugs. Furthermore, the genotoxic drugs did not induce the expression of CD95 ligand in MCF-7 cells at doses that caused apoptosis in these breast tumor cells. Moreover, drug-induced apoptosis of breast tumor cells was not prevented in the presence of either a CD95 antagonistic antibody or a CD95 ligand blocking antibody. We also observed a strong synergism between lower doses of DNA-damaging drugs and CD95 agonistic antibody in the induction of apoptosis in MCF-7 cells. In summary, our data indicate that drug-induced apoptosis of breast tumor cells occurs by a CD95/CD95L-independent mechanism although by elevating the tumor suppressor proteins p53 and CD95, genotoxic drugs may sensitize breast tumor cells to CD95-mediated apoptosis.     

Death receptor Fas/Apo-1/CD95 expressed by human placental cytotrophoblasts does not mediate apoptosis

Trophoblasts, the fetal cells that line the villous placenta and separate maternal blood from fetal tissue, express both Fas antigen and the tumor necrosis factor (TNF) receptor p55 (TNFRp55), two members of the TNF receptor family that contain a cytoplasmic "death domain" that mediates apoptotic signals. We show that Fas mRNA expressed by cultured villous cytotrophoblasts isolated from term placentas encodes transmembrane sequences and that the protein is full-length (approximately 45 kDa), suggesting that the product is an active plasma membrane-anchored receptor. Its location on the cell surface was confirmed by cellular ELISA analysis of live cells. Although cytotrophoblast apoptosis was induced by TNFalpha, and both anti-Fas antibody (CH11) and FasL-expressing T lymphocyte hybridoma (activated A1.1) cells induced HeLa cell apoptosis, neither CH11 antibody nor activated A1.1 cells stimulated apoptosis in term or first-trimester cytotrophoblasts or in term syncytiotrophoblasts. We conclude that Fas- but not TNFRp55-mediated apoptosis is blocked in primary villous trophoblasts. These data suggest that the Fas response is specifically inactivated by unknown mechanisms to avoid autocrine or paracrine killing by Fas ligand constitutively expressed on neighboring cyto- or syncytiotrophoblasts.     

Fas ligand (CD95 ligand) controls angiogenesis beneath the retina

A principal cause of blindness is subretinal neovascularization associated with age-related macular degeneration. Excised neovascular membranes from patients with age-related macular degeneration demonstrated a pattern of Fas+ new vessels in the center of the vascular complex, surrounded by FasL+ retinal pigment epithelial cells. In a murine model, Fas (CD95)-deficient (Ipr) and FasL-defective (gld) mice had a significantly increased incidence of neovascularization compared with normal mice. Furthermore, in gld mice there is massive subretinal neovascularization with uncontrolled growth of vessels. We found that cultured choroidal endothelial cells were induced to undergo apoptosis by retinal pigment epithelial cells through a Fas-FasL interaction. In addition, antibody against Fas prevented vascular tube formation of choroidal endothelial cells derived from the eye in a three-dimensional in vitro assay. Thus, FasL expressed on retinal pigment epithelial cells may control the growth and development of new subretinal vessels that can damage vision.     

Induction of apoptosis by IFNgamma in human neuroblastoma cell lines through the CD95/CD95L autocrine circuit.

The CD95 (APO-1/Fas) system can mediate apoptosis in immune cells as well as in tumour cells, where it may contribute to tumour immune-escape. On the other hand, its induction by anticancer drugs may lead to tumour reduction. Interferongamma (IFNgamma) increases the sensitivity of tumour cell lines to anti-CD95 antibody-mediated apoptosis. We describe induction of apoptosis by IFNgamma through the expression of CD95 and its ligand (CD95L) in human neuroblastoma cell lines. Neuroblastoma cells showed low constitutive expression of CD95 and CD95L. Subsequent to IFNgamma-modulated increase in CD95 and CD95L mRNA as well as protein levels, apoptosis was observed. Our results demonstrated that cytokine-mediated apoptosis was mediated through the activation of the CD95/CD95L autocrine circuit since: (i) cell death occurred following CD95/CD95L expression and correlated with CD95 and CD95L expression levels, (ii) failed to occur in a clone which weakly upregulated CD95 and lacked CD95L induction after IFNgamma stimulation, (iii) was at least partially inhibited by using blocking F(ab')2 anti-CD95 antibody fragments and the recombinant Fas-Fc protein, that prevented the interaction between CD95 and CD95L. The intracellular molecular mechanisms elicited by IFNgamma are clearly highly complex, with several signalling pathways being activated, including the CD95 system. These findings suggest that IFNgamma may have a significant potential in the therapy of neuroblastoma in vivo.