Inhibition of death receptor-mediated gene induction by a cycloheximide-sensitive factor occurs at the level of or upstream of Fas-associated death domain protein (FADD)

Role of caspases in CD95L- and TRAIL-induced non-apoptotic signalling in pancreatic tumour cells

The CD95 and TRAIL death receptors can potently stimulate proinflammatory signalling, especially in apoptosis resistant cells. Here, we show that caspases are of cell type-specific relevance for non-apoptotic death receptor signalling in pancreatic tumour cells. Inhibition of caspases by zVAD-fmk strongly enhanced the proinflammatory response in PancTuI, BxPc3 and Panc89 cells, but inhibited this response in Colo357 cells as well as in apoptosis-resistant Colo357-BclxL cells overexpressing BclxL. To characterize the role of caspases in non-apoptotic death receptor signalling, we analysed CD95L- and TRAIL-induced signalling pathways in Colo357-BclxL cells in comparison with PancTuI cells. Both death ligands induced NFkappaB, ERKs, JNK and p38 in Colo357-BclxL cells and except for ERKs also in PancTuI cells. However, inhibition of caspases with zVAD-fmk resulted in strong inhibition of all these signalling pathways in Colo357-BclxL, but enhanced NFkappaB and JNK signalling in PancTuI cells. Caspase-mediated activation of NFkappaB and ERKs were involved in CD95L- and TRAIL-induced up-regulation of proinflammatory genes in Colo357-BclxL cells. At the level of the DISC we did not observe any significant differences in recruitment or processing of FADD, caspase-8, FLIP, TRAF2 and RIP between PancTuI and Colo357-BclxL cells. Consequently, an NFkappaB and ERK stimulating, caspase-dependent factor must operate downstream of the DISC in Colo357-BclxL cells.

Rapid and profound potentiation of Apo2L/TRAIL-mediated cytotoxicity and apoptosis in thoracic cancer cells by the histone deacetylase inhibitor Trichostatin A: the essential role of the mitochondria-mediated caspase activation cascade

Apo2L/TRAIL is actively investigated as a novel targeted agent to directly induce apoptosis of susceptible cancer cells. Apo2L/TRAIL-refractory cells can be sensitized to the cytotoxic effect of this ligand by cytotoxic chemotherapeutics. The aim of this study was to evaluate the in vitro tumoricidal activity of the Apo2L/TRAIL + Trichostatin A in cultured thoracic cancer cells and to elucidate the molecular basis of the synergistic cytotoxicity of this combination. Concurrent exposure of cultured cancer cells to sublethal concentrations of Apo2L/TRAIL and Trichostatin A resulted in profound enhancement of Apo2L/TRAIL-mediated cytotoxicity in all cell lines regardless of their intrinsic susceptibility to this ligand. This combination was not toxic to primary normal cells. While Apo2L/TRAIL alone or Trichostatin A alone mediated < 20% cell death, 60 to 90% of cancer cells were apoptotic following treatment with TSA + Apo2L/TRAIL combinations. Complete translocation of Bax from the cytosol to the mitochondria compartment was mainly observed in combination-treated cells and this was correlated with robust elevation of caspase 9 proteolytic activity indicative of activation of the mitochondria apoptogenic effect. Profound TSA + Apo2L/TRAIL-mediated cytotoxicity and apoptosis were completely abrogated by either Bcl2 over-expression or by the selective caspase 9 inhibitor, highlighting the essential role of mitochondria-dependent apoptosis signaling cascade in this process. Moreover, increased caspase 8 activity observed in cells treated with the TSA + Apo2L/TRAIL combination was completely suppressed by Bcl-2 over-expression or by the selective caspase 9 inhibitor indicating that the elevated caspase 8 activity in combination-treated cells was secondary to a mitochondria-mediated amplification feedback loop of caspase activation. These finding form the basis for further development of HDAC inhibitors + Apo2L/TRAIL combination as novel targeted therapy for thoracic malignancies.

Testicular germ cell sensitivity to TRAIL-induced apoptosis is dependent upon p53 expression and is synergistically enhanced by DR5 agonistic antibody treatment

The ability of the TRAIL/DR5 signaling pathway to induce apoptosis has generally been limited to tumor cells. Here we report that in primary testis explants, addition of TRAIL (0.5 mug/ml) caused a three-fold increase in germ cell apoptosis. Furthermore, exposure of C57BL/6 mice to the testicular toxicant, mono-(2-ethylhexyl) phthalate (MEHP), caused an increased p53 stability and elevated DR5 mRNA levels coincident with increases in the levels of apoptosis in spermatocytes. To further assess the mechanisms responsible for the sensitivity of germ cells to undergo TRAIL/DR5-mediated apoptosis, we used the germ cell lines GC-1spg and GC-2spd(ts) (a temperature sensitive spermatocyte-like cell line that allows for p53 nuclear localization at 32 degrees C but not 37 degrees C). Addition of TRAIL and the anti-DR5 monoclonal antibody, MD5-1, triggered a robust synergistic increase of apoptosis in p53 permissive GC-2 cells (32 degrees C) but not in GC-1 cells. In addition, DR5 levels on the plasma membrane of permissive cells were considerably enhanced concomitant with p53 expression and after MD5-1 treatment. These data represent the first indication that testicular germ cells, specifically spermatocytes, can undergo TRAIL-mediated apoptosis and the clinically relevant observation that pretreatment with a DR5 monoclonal antibody can greatly sensitize their apoptotic response to TRAIL.

MG-132 Sensitizes TRAIL-Resistant Prostate Cancer Cells by Activating c-Fos/c-Jun Heterodimers and Repressing c-FLIP(L)

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent because it induces apoptosis in cancer cells but not in normal cells. Unfortunately, some cancer cells develop resistance to TRAIL-induced apoptosis. Therefore, it is clinically relevant to determine the molecular mechanisms that differentiate between TRAIL-sensitive and TRAIL-resistant tumors. Previously, we have shown that the antiapoptotic molecule cellular-FLICE-inhibitory protein long isoform [c-FLIP(L)] is necessary and sufficient to maintain resistance to TRAIL-induced apoptosis. We have found that c-FLIP(L) is transcriptionally regulated by the activator protein-1 (AP-1) family member protein c-Fos. Here, we report that MG-132, a small-molecule inhibitor of the proteasome, sensitizes TRAIL-resistant prostate cancer cells by inducing c-Fos and repressing c-FLIP(L). c-Fos, which is activated by MG-132, negatively regulates c-FLIP(L) by direct binding to the putative promoter region of the c-FLIP(L) gene. In addition to activating c-Fos, MG-132 activates another AP-1 family member, c-Jun. We show that c-Fos heterodimerizes with c-Jun to repress transcription of c-FLIP(L). Therefore, MG-132 sensitizes TRAIL-resistant prostate cancer cells by activating the AP-1 family members c-Fos and c-Jun, which, in turn, repress the antiapoptotic molecule c-FLIP(L).

TRAIL signalling: decisions between life and death

The TNF-related apoptosis-inducing ligand, TRAIL, has been shown to selectively kill tumour cells. This property has made TRAIL and agonistic antibodies against its death inducing receptors (TRAIL-R1 and TRAIL-R2) to some of the most promising novel biotherapeutic agents for cancer therapy. Here we review the signalling pathways initiated by the apoptosis- as well as the non-apoptosis-inducing receptors, TRAIL-R3 and TRAIL-R4. The TRAIL "death-inducing signalling complex" (DISC) transmits the apoptotic signal. DISC formation leads to activation of a protease cascade, finally resulting in cell death. The TRAIL death receptor-mediated "extrinsic" pathway and the "intrinsic" pathway, which is controlled by the interaction of members of the Bcl-2 family, interact with each other in the decision about life or death of a cell. Apoptotic and non-apoptotic signalling is influenced by the NF-kappaB, PKB/Akt and the MAPK signalling pathways. In this review we intend to summarise the most important findings on the TRAIL signalling network and the interplay in the decisions between life and death of a tumor cell.

Cellular caspase-8-like inhibitory protein (cFLIP) prevents inhibition of muscle cell differentiation induced by cancer cells

Cachexia is a frequent complication of cancer or other chronic diseases. To investigate the pathophysiology of cancer cachexia and pursue treatment options, we developed an in vitro assay of the effects of cancer cell-produced cytokines on primary muscle cells derived from murine skeletal muscle. These studies led to the novel observation that factors secreted by cell lines from prostate cancer and melanoma significantly inhibit differentiation of primary mouse muscle cells. The expression of interleukin (IL) -1beta, TNF-alpha, and proteolysis-inducing factor (PIF) by cancer cells used in this study suggested their role in preventing myogenic differentiation. Both NF-kappaB binding and transcriptional activity were enhanced in muscle cells treated with conditioned media from cancer cells or with proinflammatory cytokines. Stable expression of IKBSR, a known repressor of NF-kappaB activation, and cellular caspase-8-like inhibitory protein (cFLIP) inhibited activation of NF-kappaB in cancer cell media-treated muscle cells with an accompanying enhancement of myogenic protein expression and differentiation. In contrast, overexpression of antiapoptotic protein Bcl-xL did not protect myoblast cells exposed to the same treatment. Instead, we observed enhanced activation of NF-kappaB in Bcl-xL overexpressing cells. These studies show that the in vitro system recapitulates some of the molecular events causing muscle cachexia and provides the basis for new treatment approaches.

Role of phosphatidylinositol-3 kinase/Akt pathway in bladder cancer cell apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand

TRAIL/Apo2L is a pro-apoptotic cytokine that is capable of inducing apoptosis in a wide variety of cancer cells but not in normal cells. Among various molecular strategies by which cancer cells evade apoptosis, PI3K/Akt signaling represents a dominant survival pathway. In this report, we investigated the role of PI3K/Akt pathway in TRAIL-induced apoptotic death in human bladder cancer cells. We observed that RT4 cells had very low level of constitutively active Akt and were sensitive to TRAIL, whereas UM-UC-3 and T24 cells had higher levels of constitutively active Akt and were resistant to TRAIL. Downregulation of constitutively active Akt by PI3K inhibitors, wortmannin and LY294002, reversed cellular resistance to TRAIL. However, transfecting constitutively active Akt into RT4 cells increased Akt activity and inhibited TRAIL-induced apoptosis. These results suggest that elevated Akt activity protects UM-UC-3 and T24 cells from TRAIL-induced apoptosis, and the PI3K/Akt signaling might inhibit apoptotic signals. Thus, the modulation of Akt activity by combining pharmacological drugs or genetic alterations of the Akt expression could induce cellular responsiveness to TRAIL and PI3K/Akt signaling pathway could serve as a novel target for therapeutic intervention in bladder cancer.

Valproic acid, an antiepileptic drug with histone deacetylase inhibitory activity, potentiates the cytotoxic effect of Apo2L/TRAIL on cultured thoracic cancer cells through mitochondria-dependent caspase activation

Inhibitors of histone deacetylases have been shown to enhance the sensitivity of cancer cells to tumor necrosis factor-related apoptosis-inducing ligand TRAIL-mediated cytotoxicity. Valproic acid (VA), a commonly used antiepileptic agent whose pharmacokinetics and toxicity profiles are well described, is a histone deacetylase inhibitor. This project aims to evaluate if VA can potentiate Apo2L/TRAIL-mediated cytotoxicity in cultured thoracic cancer cells and to elucidate the underlying molecular mechanism responsible for this effect. VA sensitized cultured thoracic cancer cells to Apo2L/TRAIL, as indicated by a 4-fold to a >20-fold reduction of Apo2L/TRAIL IC50 values in combination-treated cells. Although VA (0.5-5 mM) or Apo2L/TRAIL (20 ng/ml) induced less than 20% cell death, VA + Apo2L/TRAIL combinations caused 60% to 90% apoptosis of cancer cells. Moreover, substantial activation of caspases 8, 9, and 3, which was observed only in cells treated with the drug combination, was completely suppressed by Bcl2 overexpression or by the caspase 9 inhibitor. Both the caspase 9 inhibitor and Bcl2 completely abrogated the substantial cytotoxicity and apoptosis induced by this combination, thus highlighting the pivotal role of the type II pathway in this process. These findings provide a rationale for the development of VA and Apo2L/TRAIL combination as a novel molecular therapeutic for thoracic cancers.

Sensitivity of prostate cells to TRAIL-induced apoptosis increases with tumor progression: DR5 and caspase 8 are key players

BACKGROUND

As advanced prostate cancers are resistant to currently available chemotherapies, we evaluated the cytotoxic effect of TNF-related apoptosis-inducing ligand (TRAIL) and characterized the involvement of its five receptors DR4, DR5, DcR1, DcR2, and osteoprotegerin (OPG) and of the death-inducing signaling complex (DISC)-forming proteins caspase 8 and c-FLIP in prostate cell lines.

METHODS

We used six prostate cell lines, each corresponding to a particular stage in prostate tumorigenesis, and analyzed TRAIL sensitivity in relation to TRAIL receptors' expression.

RESULTS

TRAIL sensitivity was correlated with tumor progression and DR5 expression levels and apoptosis was exclusively mediated by DR5. DcR2 was significantly more abundant in tumor cells than in non-neoplastic ones and may contribute to partial resistance to TRAIL in some prostate tumor cells. Conversely, non-tumoral cells secreted high levels of OPG, which can protect them from apoptosis. Finally, caspase 8 expression levels were as DR5 directly correlated to TRAIL sensitivity in prostate tumor cells.

CONCLUSION

TRAIL-induced apoptosis is closely related to the balanced expression of its different receptors in prostate cancer cells and their modulation could be of potential clinical value for advanced tumor treatment.

Chemotherapy and TRAIL-mediated colon cancer cell death: the roles of p53, TRAIL receptors, and c-FLIP

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has recently attracted attention as a potential therapeutic agent in the treatment of cancer. We assessed the roles of p53, TRAIL receptors, and cellular Fas-associated death domain-like interleukin-1beta-converting enzyme inhibitory protein (c-FLIP) in regulating the cytotoxic effects of recombinant TRAIL (rTRAIL) alone and in combination with chemotherapy [5-fluorouracil (5-FU), oxaliplatin, and irinotecan] in a panel of colon cancer cell lines. Using clonogenic survival and flow cytometric analyses, we showed that chemotherapy sensitized p53 wild-type, mutant, and null cell lines to TRAIL-mediated apoptosis. Although chemotherapy treatment did not modulate mRNA or cell surface expression of the TRAIL receptors death receptor 4, death receptor 5, decoy receptor 1, or decoy receptor 2, it was found to down-regulate expression of the caspase-8 inhibitor, c-FLIP. Stable overexpression of the long c-FLIP splice form but not the short form was found to inhibit chemotherapy/rTRAIL-induced apoptosis. Furthermore, siRNA-mediated down-regulation of c-FLIP, particularly the long form, was found to sensitize colon cancer cells to rTRAIL-induced apoptosis. In addition, treatment of a 5-FU-resistant cell line with 5-FU down-regulated c-FLIP expression and sensitized the chemotherapy-resistant cell line to rTRAIL. We conclude that TRAIL-targeted therapies may be used to enhance conventional chemotherapy regimens in colon cancer regardless of tumor p53 status. Furthermore, inhibition of c-FLIP may be a vital accessory strategy for the optimal use of TRAIL-targeted therapies.

Sequencing, chromosomal mapping, and functional characterization of bovine FLICE-like inhibitory protein (FLIP)

FLICE-like inhibitory protein (FLIP) has been shown in both humans and mice to inhibit apoptosis and NF-kappaB activation induced by pro-inflammatory mediators. The activation of NF-kappaB and the induction of apoptosis are critical events in the pathogenesis of a variety of disease states in cattle, including mastitis. Since FLIP is known to moderate these events in other species, we mapped the bovine FLIP gene, sequenced bovine FLIP cDNA, and characterized its expression in cultured primary bovine endothelial cells. Sequencing of bovine FLIP revealed approximately 83, 74, and 68% amino acid sequence identity to its porcine, human, and murine orthologs, respectively. Bovine FLIP was mapped to chromosome 2 by radiation hybrid mapping. Interestingly the region to which bovine FLIP maps contains a putative quantitative trait locus for functional herd life which is an indicator of a cow's ability to survive involuntary culling due primarily to mastitis and infertility. In addition to sequencing and mapping, the function of bovine FLIP was studied. Over-expression of bovine FLIP protected against bacterial lipopolysaccharide (LPS)- and TNF-alpha-induced apoptosis in bovine endothelial cells consistent with previous studies of human FLIP. In addition, elevated expression of bovine FLIP blocked LPS- and TNF-alpha-induced upregulation of NF-kappaB-dependent gene products as assayed by E-selectin expression. Only the full-length bovine FLIP protein could inhibit NF-kappaB activation induced by LPS, whereas the death effector domain region alone was able to inhibit TNF-alpha-induced NF-kappaB activation. Together, these data demonstrate the conservation of FLIP's ability to inhibit apoptosis and to downregulate NF-kappaB activation across species.

Fas activation of a proinflammatory program in rheumatoid synoviocytes and its regulation by FLIP and caspase 8 signaling

OBJECTIVE

The expansion of an aggressive population of fibroblast-like synoviocytes (FLS) in rheumatoid arthritis (RA) synovium occurs despite their expression of functional death receptors and exposure to death receptor ligands. FLS can survive Fas challenge because of the constitutive expression of FLIP apoptosis inhibitor. We investigated whether Fas signaling plays a pathogenetic role by activating a nonapoptotic proinflammatory program in RA FLS.

METHODS

Cultured RA FLS were stimulated with an agonistic anti-Fas antibody in the presence or absence of the caspase inhibitor Z-VAD-FMK or after RNA interference with a short hairpin RNA expression plasmid directed against FLIP. NF-kappaB and activator protein 1 (AP-1) activation was studied by electrophoretic mobility shift assays and p65 immunofluorescence analysis, and expression of messenger RNA (mRNA) for monocyte chemoattractant protein 1, interleukin-8, IkappaB alpha, and matrix metalloproteinases (MMPs) 1, 9, and 13 was examined by reverse transcription-polymerase chain reaction. Chemotactic activity of Fas-activated FLS-conditioned media was studied in Transwell migration assays.

RESULTS

Fas stimulation activated NF-kappaB and AP-1, and this response required caspase activity, since Z-VAD-FMK inhibitor precluded it. FLIP was processed to p43 protein after Fas stimulation in a caspase-dependent manner, and inhibition of FLIP expression resulted in reduced Fas-triggered NF-kappaB activation. Fas stimulation increased expression of mRNA for IkappaB alpha, MMPs, and chemokines, and Fas-activated RA FLS displayed increased chemotactic activity for monocytic cells.

CONCLUSION

Fas triggering may contribute to the proinflammatory features of RA FLS by activating NF-kappaB and AP-1 and by expression of relevant target genes, such as MMPs and chemokines. Fas proinflammatory signaling is dependent upon caspase activity and FLIP expression. These data implicate FLIP as a potentially important molecular switch that turns the Fas signaling away from apoptosis and toward induction of a proinflammatory phenotype in RA FLS.

Fas-associated death-domain protein inhibits TNF-α mediated NF-κB activation in cardiomyocytes

Fas-associated death-domain protein (FADD) is an adaptor molecule that links death receptors to caspase-8 in many cell types including cardiomyocytes (CMs). Although FADD has previously been reported to play an important role in CM apoptosis, the effect of FADD on CM NF-κB signaling, which is a proinflammatory pathway, has not been delineated. To investigate the role of FADD in CM NF-κB activation, we utilized adenoviral gene transfer of wild-type FADD and a truncation mutant that lacks the death-effector domain (FADD-DED) in rat CMs in vitro TNF-α activated NF-κB in CMs as demonstrated by phosphorylation and degradation of inhibitory-κB (IκB)-α-enhanced nuclear p65 and NF-κB DNA-binding activity as well as increased mRNA for the NF-κB-dependent adhesion molecule VCAM-1 (19 ± 4.1-fold) as measured by quantitative RT-PCR. Gene transfer of FADD inhibited TNF-α-induced IκB-α phosphorylation, decreased p65 nuclear translocation and NF-κB DNA-binding activity, and reduced VCAM-1 transcript levels by 53–65%. Interestingly, FADD-DED exhibited a similar but weaker inhibitory effect on NF-κB activation. The effects of FADD on NF-κB were cell-type specific. FADD expression also inhibited TNF-α-mediated NF-κB activation in human endothelial cells but not in rat pulmonary artery smooth muscle cells. In contrast, FADD expression actually activated NF-κB in human embryonic kidney (HEK)-293 cells. In CMs, FADD inhibited NF-κB activation as well as phosphorylation of IκB-α and IκB kinase (IKK)-β in response to cytokine stimulation or expression of the upstream kinases NF-κB-inducing kinase and IKK-β. These data demonstrate that FADD inhibits NF-κB activation in CMs, and this inhibition likely occurs at the level of phosphorylation and activation of IKK-β.

Molecular determinants of kinase pathway activation by Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand

Apo2 ligand/tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) mainly activates programmed cell death through caspases. By contrast, TNF primarily induces gene transcription through the inhibitor of kappaB kinase (IKK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase pathways. Apo2L/TRAIL also can stimulate these kinases, albeit less strongly; however, the underlying mechanisms of this stimulation and its relation to apoptosis are not well understood. Here we show that Apo2L/TRAIL activates kinase pathways by promoting the association of a secondary signaling complex, subsequent to assembly of a primary, death-inducing signaling complex (DISC). The secondary complex retained the DISC components FADD and caspase-8, but recruited several factors involved in kinase activation by TNF, namely, RIP1, TRAF2, and NEMO/IKKgamma. Secondary complex formation required Fas-associated death domain (FADD), as well as caspase-8 activity. Apo2L/TRAIL stimulation of JNK and p38 further depended on RIP1 and TRAF2, whereas IKK activation required NEMO. Apo2L/TRAIL induced secretion of interleukin-8 and monocyte chemoattractant protein-1, augmenting macrophage migration. Thus, Apo2L/TRAIL and TNF organize common molecular determinants in distinct signaling complexes to stimulate similar kinase pathways. One function of kinase stimulation by Apo2L/TRAIL may be to promote phagocytic engulfment of apoptotic cells.

Death receptor-induced signaling pathways are differentially regulated by gamma interferon upstream of caspase 8 processing

FasL and gamma interferon (IFN-gamma) are produced by activated T cells and NK cells and synergistically induce apoptosis. Although both cytokines can also elicit proinflammatory responses, a possible cross talk of these ligands with respect to nonapoptotic signaling has been poorly addressed. Here, we show that IFN-gamma sensitizes KB cells for apoptosis induction by facilitating death-inducing signaling complex (DISC)-mediated caspase 8 processing. Moreover, after protection against death receptor-induced apoptosis by caspase inhibition or Bcl2 overexpression, IFN-gamma also sensitized for Fas- and TRAIL death receptor-mediated NF-kappaB activation leading to synergistic upregulation of a variety of proinflammatory genes. In contrast, Fas-mediated activation of JNK, p38, and p42/44 occurred essentially independent from IFN-gamma sensitization, indicating that the apoptosis- and NF-kappaB-related FasL-IFN-gamma cross talk was not due to a simple global enhancement of Fas signaling. Overexpression of FLIP(L) and FLIP(S) inhibited Fas- as well as TRAIL-mediated NF-kappaB activation and apoptosis induction in IFN-gamma-primed cells suggesting that both responses are coregulated at the level of the DISC.

Functional expression of a proliferation-related ligand in hepatocellular carcinoma and its implications for neovascularization

AIM: To detect the expression of a proliferation-related ligand on human hepatocellular carcinoma (HCC) cell lines (SK-Hep1, HLE and HepG2) and in culture medium.

METHODS: APRIL expression was analyzed by Western blotting in HCC cell lines. Effects of APRIL to cell count and angiogenesis were analyzed, too.

RESULTS: Recombinant human APRIL (rhAPRIL) increased cell viability of HepG2 cells and, in HUVEC, rhAPRIL provided slight tolerance to cell death from serum starvation. Soluble APRIL (sAPRIL) from HLE cells increased after serum starvation, but did not change in SK-Hep1 or HepG2 cells. These cells showed down-regulation of VEGF after incubation with anti-APRIL antibody. Furthermore, culture medium from the HCC cells treated with anti-APRIL antibody treatment inhibited tube formation of HUVECs.

CONCLUSION: Functional expression of APRIL might contribute to neovascularization via an upregulation of VEGF in HCC.

Caspase-8 activity prevents type 2 cytokine responses and is required for protective T cell-mediated immunity against Trypanosoma cruzi infection

During Trypanosoma cruzi infection, T cells up-regulate caspase-8 activity. To assess the role of caspase-8 in T cell-mediated immunity, we investigated the effects of caspase-8 inhibition on T cells in viral FLIP (v-FLIP) transgenic mice. Compared with wild-type controls, increased parasitemia was observed in v-FLIP mice infected with T. cruzi. There was a profound decrease in expansion of both CD4 and CD8 T cell subsets in the spleens of infected v-FLIP mice. We did not find differences in activation ratios of T cells from transgenic or wild-type infected mice. However, the numbers of memory/activated CD4 and CD8 T cells were markedly reduced in v-FLIP mice, possibly due to defective survival. We also found decreased production of IL-2 and increased secretion of type 2 cytokines, IL-4 and IL-10, which could enhance susceptibility to infection. Similar, but less pronounced, alterations were observed in mice treated with the caspase-8 inhibitor, zIETD. Furthermore, blockade of caspase-8 by zIETD in vitro mimicked the effects observed on T. cruzi infection in vivo, affecting the generation of activated/memory T cells and T cell cytokine production. Caspase-8 is also required for NF-kappaB signaling upon T cell activation. Blockade of caspase-8 by either v-FLIP expression or treatment with zIETD peptide decreased NF-kappaB responses to TCR:CD3 engagement in T cell cultures. These results suggest a critical role for caspase-8 in the establishment of T cell memory, cell signaling, and regulation of cytokine responses during protozoan infection.

Does CD95 have tumor promoting activities?

CD95 (APO-1/Fas) is an important inducer of the extrinsic apoptosis signaling pathway and therapy induced apoptosis of many tumor cells has been linked to the activity of CD95. Changes in the expression of CD95 and/or its ligand CD95L are frequently found in human cancer. The downregulation or mutation of CD95 has been proposed as a mechanism by which cancer cells avoid destruction by the immune system through reduced apoptosis sensitivity. CD95 has therefore been viewed as a tumor suppressor. Furthermore, increased CD95L concentration in tumor patients has been linked to tumor cells killing infiltrating lymphocytes in a process called "the tumor counter-attack". Recent data have illuminated unknown activities of CD95 in tumor cells with downregulated or mutated CD95 in the presence of increased CD95L. Under these conditions the stimulation of CD95 signals nonapoptotic pathways, activating NF-kappaB and MAP kinases for example, which may result in the induction of tumorigenic or prosurvival genes. A new model of CD95 functions is proposed in which CD95 is converted from a tumor suppressor to a tumor promotor by a single point mutation in one of the CD95 alleles, a situation frequently found in advanced human cancer, resulting in apoptosis resistance and activation of tumorigenic pathways.

CD95 and TRAF2 promote invasiveness of pancreatic cancer cells

Pancreatic adenocarcinoma represents a tumor type with extremely poor prognosis. High apoptosis resistance and a strong invasive and early metastatic potential contribute to its highly malignant phenotype. Here we identified the death receptor adaptor molecule TRAF2 as a key player in pancreatic cancer pathophysiology. Using immunohistochemistry and Western blot analysis we found TRAF2 overexpressed in 34 of 36 pancreatic tumor samples as well as in pancreatic tumor cell lines resistant to CD95-mediated apoptosis. The high TRAF2 protein level was not related to chromosomal changes, as monitored by FISH analysis. Instead, the NF-kappaB- and MEK-signaling pathways were involved. Introduction of a TRAF2 expression vector in CD95-sensitive Colo357 cells resulted in (i) resistance to CD95-induced apoptosis; (ii) increased constitutive NF-kappaB and AP-1 activity; and (iii) higher basal secretion of matrix metalloproteinases (MMPs), urokinase-type plasminogen activator (uPA), and IL-8, leading to increased invasiveness. High apoptosis resistance and uPA secretion could be reverted by TRAF2-specific siRNA. Stimulation of TRAF2-overexpressing cells with CD95 ligand led to induction of NF-kappaB and AP-1, enhanced IL-8- and uPA-secretion, and a further increased invasiveness. Thus, TRAF2 overexpression does not only block apoptosis induction by CD95 but also converts this death receptor into a mediator of invasiveness.

Modifications and intracellular trafficking of FADD/MORT1 and caspase-8 after stimulation of T lymphocytes

The adaptor protein FADD/MORT1 is essential for apoptosis induced by 'death receptors', such as Fas (APO-1/CD95), mediating aggregation and autocatalytic activation of caspase-8. Perhaps surprisingly, FADD and caspase-8 are also critical for mitogen-induced proliferation of T lymphocytes. We generated novel monoclonal antibodies specific for mouse FADD and caspase-8 to investigate whether cellular responses, apoptosis or proliferation, might be explained by differences in post-translational modification and subcellular localisation of these proteins. During both apoptosis signalling and mitogenic activation, FADD and caspase-8 aggregated in multiprotein complexes and formed caps at the plasma membrane but they did not colocalise with lipid rafts. Interestingly, mitogenic stimulation, but not Fas ligation, induced a unique post-translational modification of FADD. These different modifications may determine whether FADD and caspase-8 induce cell death or proliferation.

Receptor-selective Mutants of Apoptosis-inducing Ligand 2/Tumor Necrosis Factor-related Apoptosis-inducing Ligand Reveal a Greater Contribution of Death Receptor (DR) 5 than DR4 to Apoptosis Signaling

Apoptosis-inducing ligand 2 (Apo2L), also called tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), triggers programmed cell death in various types of cancer cells but not in most normal cells. Apo2L/TRAIL is a homotrimeric protein that interacts with five receptors: death receptor 4 (DR4) and DR5 mediate apoptosis activation, whereas decoy receptor 1 (DcR1), DcR2, and osteoprotegerin counteract this function. Many cancer cell lines express both DR4 and DR5, and each of these receptors can initiate apoptosis independently of the other. However, the relative contribution of DR4 and DR5 to ligand-induced apoptosis is unknown. To investigate this question, we generated death receptor-selective Apo2L/TRAIL variants using a novel approach that enables phage display of mutated trimeric proteins. Selective binding to DR4 or DR5 was achieved with three to six-ligand amino acid substitutions. The DR4-selective Apo2L/TRAIL variants examined in this study showed a markedly reduced ability to trigger apoptosis, whereas the DR5-selective variants had minimally decreased or slightly increased apoptosis-inducing activity. These results suggest that DR5 may contribute more than DR4 to Apo2L/TRAIL-induced apoptosis in cancer cells that express both death receptors.

Influence of expressed TRAIL on biophysical properties of the human leukemic cell line Jurkat

The cDNA fragment of human TRAIL (TNF-related apoptosis inducing ligand) was cloned into RevTet-On, a Tet-regulated and high-level gene expression system. The gene expression system was constructed in a human leukemic cell line: Jurkat. By using RevTet-On TRAIL gene expression system in Jurkat as a cell model, we studied the influence of TRAIL gene on the changes of cellular apoptosis before and after the TRAIL gene expression, which was induced by adding tetracycline derivative doxycycline (Dox). The results indicated that the cellular apoptosis ratio was largely dependent on the trail gene expression level. Moreover, it was found that the apoptosis-inducing TRAIL could cause significant changes in the biophysical properties of Jurkat cells. The cell surface charge density decreased, the membrane fluidity declined, the elastic coefficients K1 increased, and the proportion of a-helix in membrane protein secondary structure decreased. Thus, the apoptosis-inducing TRAIL gene caused significant changes on the biomechanic properties of Jurkat cells.

Enhancement of therapeutic potential of TRAIL by cancer chemotherapy and irradiation: mechanisms and clinical implications

Activation of cell surface death receptors by their cognate ligands triggers apoptosis. Several human death receptors (Fas, TNF-R1, TRAMP, DR4, DR5, DR6, EDA-R and NGF-R) have been identified. The most promising cytokine for anticancer therapy is TRAIL/APO-2L, which induces apoptosis in cancer cells by binding to death receptors TRAIL-R1/DR4 and TRAIL-R2/DR5. The cytotoxic activity of TRAIL is relatively selective to cancer cells compared to normal cells. Signaling by TRAIL and its receptors is tightly regulated process essential for key physiological functions in a variety of organs, as well as the maintenance of immune homeostasis. Despite early promising results, recent studies have identified several TRAIL-resistant cancer cells of various origins. Based on molecular analysis of death-receptor signaling pathways several new approaches have been developed to increase the efficacy of TRAIL. Resistance of cancer cells to TRAIL appears to occur through the modulation of various molecular targets. They may include differential expression of death receptors, constitutively active Akt and NFkappaB, overexpression of cFLIP and IAPs, mutations in Bax and Bak genes, and defects in the release of mitochondrial proteins in resistant cells. Conventional chemotherapeutic and chemopreventive drugs, and irradiation can sensitize TRAIL-resistant cells to undergo apoptosis. Thus, these agents enhance the therapeutic potential of TRAIL in TRAIL-sensitive cells and sensitize TRAIL-resistant cells. TRAIL and TRAIL-receptor antibodies may prove to be useful for cancer therapy, either alone or in association with conventional approaches such as chemotherapy or radiation therapy. This review discusses intracellular mechanisms of TRAIL resistance and various approaches that can be taken to sensitize TRAIL-resistant cancer cells.

Fas Ligand Induces Cell-autonomous NF-κB Activation and Interleukin-8 Production by a Mechanism Distinct from That of Tumor Necrosis Factor-α

Fas ligand (FasL) has been well characterized as a death factor. However, recent studies revealed that FasL possesses inflammatory activity. Here we found that FasL induces production of the inflammatory chemokine IL-8 without inducing apoptosis in HEK293 cells. Reporter gene assays involving wild-type and mutated IL-8 promoters and NF-kappaB- and AP-1 reporter constructs indicated that an FasL-induced NF-kappaB and AP-1 activity are required for maximal promoter activity. FasL induced NF-kappaB activation with slower kinetics than did TNF-alpha, yet this response was cell autonomous and not mediated by secondary paracrine factors. The death domain of Fas, FADD, and caspase-8 were required for NF-kappaB activation by FasL. A dominant-negative mutant of IKKgamma inhibited the FasL-induced NF-kappaB activation. However, TRADD and RIP, which are essential for the TNF-alpha-induced NF-kappaB activation, were not involved in the FasL-induced NF-kappaB activation. Moreover, CLARP/FLIP inhibited the FasL- but not the TNF-alpha-induced NF-kappaB activation. These results show that FasL induces NF-kappaB activation and IL-8 production by a novel mechanism, distinct from that of TNF-alpha. In addition, we found that mouse FADD had a dominant-negative effect on the FasL-induced NF-kappaB activation in HEK293 cells, which may indicate a species difference between human and mouse in the FasL-induced NF-kappaB activation.

p38 MAPK mediates TNF-induced apoptosis in endothelial cells via phosphorylation and downregulation of Bcl-x(L)

The role of p38 mitogen-activated protein kinase (MAPK) in apoptosis is a matter of debate. Here, we investigated the involvement of p38 MAPK in endothelial apoptosis induced by tumor necrosis factor alpha (TNF). We found that activation of p38 MAPK preceded activation of caspase-3, and the early phase of p38 MAPK stimulation did not depend on caspase activity, as shown by pretreatment with the caspase inhibitors z-Val-Ala-Asp(OMe)-fluoromethylketone (zVAD-fmk) and Boc-Asp(OMe)-fluoromethylketone (BAF). The p38 MAPK inhibitor SB203580 significantly attenuated TNF-induced apoptosis in endothelial cells, suggesting that p38 MAPK is essential for apoptotic signaling. Furthermore, we observed a time-dependent increase in active p38 MAPK in the mitochondrial subfraction of cells exposed to TNF. Notably, the level of Bcl-x(L) protein was reduced in cells undergoing TNF-induced apoptosis, and this reduction was prevented by treatment with SB203580. Immunoprecipitation experiments revealed p38 MAPK-dependent serine-threonine phosphorylation of Bcl-x(L) in TNF-treated cells. Exposure to lactacystin prevented both the downregulation of Bcl-x(L) and activation of caspase-3. Taken together, our results suggest that TNF-induced p38 MAPK-mediated phosphorylation of Bcl-x(L) in endothelial cells leads to degradation of Bcl-x(L) in proteasomes and subsequent induction of apoptosis.

Immunohistochemical distribution of proteins belonging to the receptor-mediated and the mitochondrial apoptotic pathways in human placenta during gestation

The balance between cell death and cell proliferation and its regulation are essential features of many physiological processes and are particularly important in fetal morphogenesis and adult tissue homeostasis. Apoptosis is a type of cell suicide that is activated in two main ways: through a receptor-mediated pathway or through a mitochondrial pathway. We have investigated the immunohistochemical distribution of proteins belonging to these two pathways in human placenta during gestation by comparing their expression levels between the first and third trimester of gestation. In the first trimester, the receptor-mediated pathway prevails over the mitochondrial pathway with a moderate/intense expression of its three components, viz., Fas ligand (FasL), Fas, and caspase-8, and weak positivity of anti-apoptotic FLIP, these proteins being mainly localized in the cytotrophoblast compartment. In the third trimester of gestation, there is an increased expression of mitochondrial pathway proteins, viz., Apaf-1 and caspase-9. We have also investigated the expression level of caspase-3, the primary effector caspase of both pathways, and have observed that it is moderately expressed during gestation, being mainly localized in the cytotrophoblast during the first trimester and in both placental compartments during the third trimester of gestation. Thus, both pathways actively function in human placenta to execute cell death. By means of immunoelectron microscopy, we have further shown that, in human placenta, the two proteins of the mitochondrial pathway together with caspase-3 are localized both in the cytoplasm and in the nucleus. In particular, Apaf-1 and caspase-9 are distributed near to the nuclear envelope suggesting an important role for these two proteins in disrupting the nuclear-cytoplasmic barrier.

cFLIPL inhibits tumor necrosis factor-related apoptosis-inducing ligand-mediated NF-kappaB activation at the death-inducing signaling complex in human keratinocytes

Human keratinocytes undergo apoptosis following treatment with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) via surface-expressed TRAIL receptors 1 and 2. In addition, TRAIL triggers nonapoptotic signaling pathways including activation of the transcription factor NF-kappaB, in particular when TRAIL-induced apoptosis is blocked. The intracellular protein cFLIP(L) interferes with TRAIL-induced apoptosis at the death-inducing signaling complex (DISC) in many cell types. To study the role of cFLIP(L) in TRAIL signaling, we established stable HaCaT keratinocyte cell lines expressing varying levels of cFLIP(L). Functional analysis revealed that relative cFLIP(L) levels correlated with apoptosis resistance to TRAIL. Surprisingly, cFLIP(L) specifically blocked TRAIL-induced NF-kappaB activation and TRAIL-dependent induction of the proinflammatory target gene interleukin-8. Biochemical characterization of the signaling pathways involved showed that apoptosis signaling was inhibited at the DISC in cFLIP(L)-overexpressing keratinocytes, although cFLIP(L) did not significantly impair enzymatic activity of the receptor complex. In contrast, recruitment and modification of receptor-interacting protein was blocked in cFLIP(L)-overexpressing cells. Taken together, our data demonstrate that cFLIP(L) is not only a central antiapoptotic modulator of TRAIL-mediated apoptosis but also an inhibitor of TRAIL-induced NF-kappaB activation and subsequent proinflammatory target gene expression. Hence, cFLIP(L) modulation in keratinocytes may not only influence apoptosis sensitivity but may also lead to altered death receptor-dependent skin inflammation.

NFkappaB activation by Fas is mediated through FADD, caspase-8, and RIP and is inhibited by FLIP

Fas (APO-1/CD95) is the prototypic death receptor, and the molecular mechanisms of Fas-induced apoptosis are comparably well understood. Here, we show that Fas activates NFκB via a pathway involving RIP, FADD, and caspase-8. Remarkably, the enzymatic activity of the latter was dispensable for Fas-induced NFκB signaling pointing to a scaffolding-related function of caspase-8 in nonapoptotic Fas signaling. NFκB was activated by overexpressed FLIP_L and FLIP_S in a cell type–specific manner. However, in the context of Fas signaling both isoforms blocked FasL-induced NFκB activation. Moreover, down-regulation of both endogenous FLIP isoforms or of endogenous FLIP_L alone was sufficient to enhance FasL-induced expression of the NFκB target gene IL8. As NFκB signaling is inhibited during apoptosis, FasL-induced NFκB activation was most prominent in cells that were protected by Bcl2 expression or caspase inhibitors and expressed no or minute amounts of FLIP. Thus, protection against Fas-induced apoptosis in a FLIP-independent manner converted a proapoptotic Fas signal into an inflammatory NFκB-related response.

Death receptor-induced cell killing

Apoptosis pathways activated by death receptors of the tumour necrosis factor (TNF) family such as Fas, TNFR1, or the TRAIL receptors DR4 and DR5 are implicated in diverse diseases. These are also the best-understood apoptosis pathways and many of our ideas about apoptosis regulation come from studying these pathways. Cell killing from such receptors occurs because of recruitment to the receptor of the adaptor protein FADD, which in turn recruits the pro form of caspase-8. Aggregation of pro-caspase-8 leads to its auto-activation and subsequent activation of effector caspases such as caspase-3. The apoptotic signal can be amplified through the mitochondria and inhibited through the action of competing molecules such as the inhibitor c-FLIP, which binds to the receptor complex in place of caspase-8. This simple mechanism explains much of the cell death that is induced by death receptors. However, recent studies indicate that we must incorporate new information into this model. Some examples that add new layers of complexity will be discussed in this review.

Cellular FLIP long form-transgenic mice manifest a Th2 cytokine bias and enhanced allergic airway inflammation

Cellular FLIP long form (c-FLIP(L)) is a caspase-defective homologue of caspase-8 that blocks apoptosis by death receptors. The expression of c-FLIP(L) in T cells can also augment extracellular signal-regulated kinase phosphorylation after TCR ligation via the association of c-FLIP(L) with Raf-1. This contributes to the hyperproliferative capacity of T cells from c-FLIP(L)-transgenic mice. In this study we show that activated CD4(+) T cells from c-FLIP(L)-transgenic mice produce increased amounts of Th2 cytokines and decreased amounts of Th1 cytokines. This correlates with increased serum concentrations of the Th2-dependent IgG1 and IgE. The Th2 bias of c-FLIP(L)-transgenic CD4(+) T cells parallels impaired NF-kappa B activity and increased levels of GATA-3, which contribute, respectively, to decreased IFN-gamma and increased Th2 cytokines. The Th2 bias of c-FLIP(L)-transgenic mice extends to an enhanced sensitivity to OVA-induced asthma. Taken together, these results show that c-FLIP(L) can influence cytokine gene expression to promote Th2-driven allergic reaction, in addition to its traditional role of blocking caspase activation induced by death receptors.

TNF-related apoptosis-inducing ligand is involved in neutropenia of systemic lupus erythematosus

Neutropenia is a common laboratory finding in systemic lupus erythematosus (SLE). However, the molecular mechanism of SLE neutropenia has not been fully explained. In this study, we examined whether TNF-related apoptosis-inducing ligand (TRAIL) is involved in the pathogenesis of SLE neutropenia using samples from SLE patients. Serum TRAIL levels in SLE patients with neutropenia were significantly higher than those of SLE patients without neutropenia and healthy volunteers. Serum TRAIL levels showed a significant negative correlation with neutrophil counts in SLE patients. The expression of TRAIL receptor 3 was significantly lower in SLE patients with neutropenia than in patients without neutropenia or in healthy volunteers. Treatment with glucocorticoids negated the decrease of TRAIL receptor 3 expression on neutrophils of SLE patients. TRAIL may accelerate neutrophil apoptosis of neutrophils from SLE patients, and autologous T cells of SLE patients, which express TRAIL on surface, may kill autologous neutrophils. Interferon gamma and glucocorticoid modulated the expression of TRAIL on T cells of SLE patients and also modulated the expression of cellular Fas-associating protein with death domain–like interleukin-1β–converting enzyme (FLICE)–inhibitory protein (cFLIP), an inhibitor of death receptor signaling, in neutrophils. Thus, our results provide a novel insight into the molecular pathogenesis of SLE neutropenia.

The anticancer drug mithramycin A sensitises tumour cells to apoptosis induced by tumour necrosis factor (TNF)

In this report we show that mithramycin considerably increases the direct cytotoxic effect of tumour necrosis factor (TNF) on tumour cells in vitro. Sensitisation to TNF-induced apoptosis was prevented by the broad caspase inhibitor zVAD-fmk, whereas overexpression of Bcl-2 had no effect. Mithramycin also potentiated cell death induced by Fas agonistic antibodies. In contrast, mithramycin reduced the percentage of cells undergoing apoptosis due to factor withdrawal. TNF-induced activation of NF-kappaB (NF-κB)-dependent gene expression was not modulated by mithramycin treatment. Concomitantly with the increased sensitivity, the protein level of the short-spliced cFLIP variant was downregulated. These results indicate that mithramycin enhances TNF-induced cell death in an NF-κB-independent manner, and suggest that the Fas-associated death domain protein plays a crucial role in the TNF-sensitising effect of mithramycin.

Resistance of short term activated T cells to CD95-mediated apoptosis correlates with de novo protein synthesis of c-FLIPshort

In the early phase of an immune response, T cells are activated and acquire effector functions. Whereas these short term activated T cells are resistant to CD95-mediated apoptosis, activated T cells in prolonged culture are readily sensitive, leading to activation-induced cell death and termination of the immune response. The translation inhibitor, cycloheximide, partially overcomes the apoptosis resistance of short term activated primary human T cells. Using this model we show in this study that sensitization of T cells to apoptosis occurs upstream of mitochondria. Neither death-inducing signaling complex formation nor expression of Bcl-2 proteins is altered in sensitized T cells. Although the caspase-8 inhibitor c-FLIP(long) was only slightly down-regulated in sensitized T cells, c-FLIP(short) became almost undetectable. This correlated with caspase-8 activation and apoptosis. These data suggest that c-FLIP(short), rather than c-FLIP(long), confers resistance of T cells to CD95-mediated apoptosis in the context of immune responses.

Fas-disabling small exocyclic peptide mimetics limit apoptosis by an unexpected mechanism

Fas ligand- (FasL) mediated apoptosis is an important element of tissue-specific organ damage. We have developed biologically active small exocyclic peptide mimetics that disable apoptotic functions of Fas. The most effective mimetic binds to both its receptor and FasL with comparable affinity. In vitro, the most effective antagonist blocked FasL-induced cytotoxicity completely and specifically. In vivo, the antagonistic mimetic also prevented Concanavilin A (Con A) induced hepatitis, a CD4(+) T cell-mediated animal model of liver injury. Although current approaches prevent Fas receptor signaling by excluding FasL binding to Fas, the small molecule mimetics reported here disable Fas by promoting a defective Fas-FasL receptor complex. This event desensitizes FasL-mediated apoptosis by inhibiting extracellular signal regulated kinase activity and up-regulating NF-kappaB.

Intracellular regulation of Fas-induced apoptosis in human fibroblasts by extracellular factors and cycloheximide

Fibroblasts play an important role in reparative and inflammatory processes by synthesizing extracellular matrix components and releasing growth factors and cytokines. Fibroblast apoptosis has been observed at the termination phase of reparative or fibrotic responses, but its regulation in this context is poorly known. We investigated the susceptibility of human dermal fibroblasts (DF) to Fas-induced apoptosis and its regulation by extracellular factors potentially involved in immune-mediated inflammation and repair. DF expressed all components of the Fas apoptotic pathway: surface Fas, Fas-associated protein with death domain, and caspase-8 proteins. However, Fas activation resulted in caspase-8 activation and apoptosis only in the presence of cycloheximide (CHX). DF constitutively expressed Fas-associated death domain-like IL-1-converting enzyme-like inhibitory protein (FLIP) that was drastically down-regulated by CHX. Exogenous growth factors, cytokines, and adherence to the extracellular matrix shifted the balance of FLIP-caspase-8 proteins and modified the susceptibility of DF to Fas- or Fas-CHX-induced apoptosis. Short-term serum deprivation, suspension culture, and pretreatment with IFN-gamma or TNF-alpha increased, whereas long-term serum-free culture and pretreatment with TGF-beta or IL-10 decreased the apoptotic susceptibility of DF. Surface Fas expression was only modified by TNF-alpha and IFN-gamma, whereas all studied factors modified FLIP-caspase-8 protein expression, consistently with their pro- or antiapoptotic effects. Antisense FLIP oligonucleotides prevented resistance to Fas-induced apoptosis in DF. FLIP-caspase-8 balance seems tightly regulated in fibroblasts by extracellular factors that determine their susceptibility to Fas- or Fas-CHX-induced apoptosis. Th1 and Th regulatory cytokines display opposite effects on fibroblast apoptosis that suggest that their pro- or antifibrotic effects involve direct effects on fibroblast survival.

Stimulation of human Toll-like receptor (TLR) 2 and TLR6 with membrane lipoproteins of Mycoplasma fermentans induces apoptotic cell death after NF-kappa B activation

Mycoplasmal membrane diacylated lipoproteins not only initiate proinflammatory responses through Toll-like receptor (TLR) 2 and TLR6 via the activation of the transcriptional factor NF-kappaB, but also initiate apoptotic responses. The aim of this study was to clarify the apoptotic machineries. Mycoplasma fermentans lipoproteins and a synthetic lipopeptide, MALP-2, showed cytocidal activity towards HEK293 cells transfected with a TLR2-encoding plasmid. The activity was synergically augmented by co-expression of TLR6, but not by co-expression of other TLRs. Under the condition of co-expression of TLR2 and TLR6, the lipoproteins could induce maximum NF-kappa B activation and apoptotic cell death in the cells 6 h and 24 h after stimulation respectively. Dominant-negative forms of MyD88 and FADD, but not IRAK-4, reduced the cytocidal activity of the lipoproteins. In addition, both dominant-negative forms also downregulated the activation of both NF-kappa B and caspase-8 in the cells. Additionally, the cytocidal activity was sufficiently attenuated by a selective inhibitor of p38 MAPK. These findings suggest that mycoplasmal lipoproteins can trigger TLR2- and TLR6-mediated sequential bifurcate responses: NF-kappa B activation as an early event, which is partially mediated by MyD88 and FADD; and apoptosis as a later event, which is regulated by p38 MAPK as well as by MyD88 and FADD.

Investigation of the susceptibility of human cell lines to bovine herpesvirus 4 infection: demonstration that human cells can support a nonpermissive persistent infection which protects them against tumor necrosis factor alpha-induced apoptosis

Bovine herpesvirus 4 (BoHV-4) is a gammaherpesvirus that has a worldwide distribution in the population of cattle. Many factors make human contamination by BoHV-4 likely to occur. In this study, we performed in vitro experiments to assess the risk and the consequences of human infection by BoHV-4. First, by using a recombinant BoHV-4 strain expressing enhanced green fluorescent protein under the control of the human cytomegalovirus immediate-early gene promoter, we tested 21 human cell lines for their sensitivity and their permissiveness to BoHV-4 infection. These experiments revealed that human cell lines from lymphoid and myeloid origins were resistant to infection, whereas epithelial cells, carcinoma cells, or adenocarcinoma cells isolated from various organs were sensitive but poorly permissive to BoHV-4 infection. Second, by using the HeLa cell line as a model of human cells sensitive but not permissive to BoHV-4 infection, we investigated the resistance of infected cells to apoptosis and the persistence of the infection through cellular divisions. The results obtained can be summarized as follows. (i) BoHV-4 nonpermissive infection of HeLa cells protects them against tumor necrosis factor alpha-induced apoptosis. (ii) BoHV-4 infection of HeLa cells persists in cell culture; however, the percentage of infected cells decreases with time due to erratic transmission of the viral genome through cell division. (iii) BoHV-4 infection has no effect on the rate of HeLa cell division. Altogether, these data suggest that BoHV-4 could infect humans. This study also stresses the importance of considering the insidious effects of nonpermissive infection when the biosafety of animal gammaherpesviruses for humans is being considered.

Enhancement of death-receptor induced caspase-8-activation in the death-inducing signalling complex by uncoupling of oxidative phosphorylation

Signalling through the death receptor CD95 induces apoptosis by formation of a signalling complex at the cell membrane and subsequent caspase-8 and caspase-3-activation. Treatment of Jurkat T cells with protonophores across the mitochondrial membrane such as 2,4-dinitrophenol (DNP) enhances the death-inducing capacity of CD95. In this study, we show that this enhancement is due to the specific acceleration of caspase-8-processing and activation at the CD95-receptor. DNP-treatment did not affect NF-kappaB-induction by CD95. Immunoprecipitation experiments showed that the amounts of the adapter FADD/MORT1 and pro-caspase-8 at the CD95-receptor were not altered by DNP. Subcellular fractionation studies revealed that the amount of mature caspase-8 but not pro-caspase at the membrane was increased following CD95-stimulation in the presence of DNP. As a consequence of caspase-activation, c-FLIP-levels in the cytosol decreased. In Jurkat cells overexpressing c-FLIPS, DNP was still able to enhance caspase-activation. The enhancing capacity of DNP was seen in some cell lines (Jurkat, CEM and HeLa) but not in SKW6 cells and was also found in mitogen-stimulated human T cells. Furthermore, the enhancement extended to TRAIL-induced caspase-activation. Thus, a mechanism exists by which caspase-8-activation can be accelerated at death receptors and this mechanism can be triggered by targeting mitochondrial oxidative phosphorylation.

Prevention of TNF-alpha-induced apoptosis in polyamine-depleted IEC-6 cells is mediated through the activation of ERK1/2

It has been documented that polyamines play a critical role in the regulation of apoptosis in intestinal epithelial cells. We have recently reported that protection from TNF-alpha/cycloheximide (CHX)-induced apoptosis in epithelial cells depleted of polyamines is mediated through the inactivation of a proapoptotic mediator, JNK. In this study, we addressed the involvement of the MAPK pathway in the regulation of apoptosis after polyamine depletion of IEC-6 cells. Polyamine depletion by alpha-difluromethylornithine (DFMO) resulted in the sustained activation of ERK in response to TNF-alpha/CHX treatment. Pretreatment of polyamine-depleted IEC-6 cells with a cell membrane-permeable MEK1/2 inhibitor, U-0126, significantly inhibited TNF-alpha/CHX-induced ERK phosphorylation and significantly increased DNA fragmentation, JNK activity, and caspase-3 activity in response to TNF-alpha/CHX. Moreover, the dose dependency of U-0126-mediated inhibition of TNF-alpha/ CHX-induced ERK phosphorylation correlated with the reversal of the antiapoptotic effect of DFMO. IEC-6 cells expressing constitutively active MEK1 had decreased TNF-alpha/CHX-induced JNK phosphorylation and were significantly protected from apoptosis. Conversely, a dominant-negative MEK1 resulted in high basal activation of JNK, cytochrome c release, and spontaneous apoptosis. Polyamine depletion of the dominant-negative MEK1 cells did not prevent JNK activation or cytochrome c release and failed to confer protection from both TNF-alpha/CHX and camptothecin-induced apoptosis. Finally, expression of a dominant-negative mutant of JNK significantly protected IEC-6 cells from TNF-alpha/CHX-induced apoptosis. These data indicate that polyamine depletion results in the activation of ERK, which inhibits JNK activation and protects cells from apoptosis.

Regulation of phorbol ester-mediated TRAF1 induction in human colon cancer cells through a PKC/RAF/ERK/NF-κB-dependent pathway

Tumor necrosis factor (TNF) receptor-associated factors (TRAFs) are cytoplasmic adapter proteins that link a wide variety of cell surface receptors to the apoptotic signaling cascade. The purpose of this study was to delineate the signaling pathways and TRAF1 promoter elements responsible for phorbol ester-mediated TRAF1 induction in human colon cancers. Here, we found that the PKC activators, phorbol 12-myristate 13-acetate (PMA) and bryostatin I, induced TRAF1 mRNA expression; pretreatment with actinomycin D blocked PMA-mediated TRAF1 expression suggesting induction at the transcriptional level. In contrast, expression of other TRAFs (TRAF2, 3 and 4) was minimally altered by PMA. Various PKC isoform-selective inhibitors blocked PMA-mediated TRAF1 mRNA and promoter stimulation; rottlerin, a selective PKCdelta inhibitor, had no effect suggesting that Ca(2+)-dependent PKC isoforms (e.g., PKCalpha and betaI) play a role in TRAF1 regulation. In addition, the MEK/ERK inhibitors, PD98059 and UO126, suppressed PMA-stimulated TRAF1 promoter activity indicating a role for ERK in TRAF1 induction. Moreover, cotransfection of a dominant-negative Raf-1 (Raf-C4) significantly reduced PMA-stimulated TRAF1 promoter activity whereas transfection of dominant-negative Ras or treatment with Ras inhibitors had minimal to no effect on TRAF1 induction suggesting dependence on Raf, but not Ras, activation. Finally, site-specific mutagenesis of functional NF-kappaB sites (particularly the most proximal site) in the TRAF1 promoter significantly decreased PMA-mediated promoter activity. In conclusion, our results demonstrate selective induction of TRAF1 in human colon cancer cells through a Ca(2+)-dependent PKC/Raf-1/ERK/NF-kappaB-dependent pathway.

Human MUC1 carcinoma-associated protein confers resistance to genotoxic anticancer agents

The MUC1 transforming protein is overexpressed by most human carcinomas. The present studies demonstrate that the MUC1 C-terminal subunit (MUC1 C-ter) localizes to mitochondria in HCT116/MUC1 colon carcinoma cells and that heregulin stimulates mitochondrial targeting of MUC1 C-ter. We also show that MUC1 attenuates cisplatin-induced (1) release of mitochondrial apoptogenic factors, (2) activation of caspase-3, and (3) induction of apoptosis. Moreover, knockdown of MUC1 expression in A549 lung and ZR-75-1 breast carcinoma cells by MUC1siRNA was associated with increased sensitivity to genotoxic drugs in vitro and in vivo. These findings indicate that MUC1 attenuates the apoptotic response to DNA damage and that this oncoprotein confers resistance to genotoxic anticancer agents.

TRAIL and NFkappaB signaling--a complex relationship

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) or Apo2L is a ligand of the TNF family interacting with five different receptors of the TNF receptor superfamily, including two death receptors. It has attracted wide interest as a potential anticancer therapy because some recombinant soluble forms of TRAIL induce cell death predominantly in transformed cells. The nuclear factor-kappaB (NFkappaB)?Rel family of proteins are composed of a group of dimeric transcription factors that have an outstanding role in the regulation of inflammation and immunity. Control of transcription by NFkappaB proteins can be of relevance to the function of TRAIL in three ways. First, induction of antiapoptotic NFkappaB dependent genes critically determines cellular susceptibility toward apoptosis induction by TRAIL-R1, TRAIL-R2, and other death receptors. Each of the multiple of known NFkappaB inducers therefore has the potential to interfere with TRAIL-induced cell death. Second, TRAIL and some of its receptors are inducible by NFkappaB, disclosing the possibility of autoamplifying TRAIL signaling loops. Third, the TRAIL death receptors can activate the NFkappaB pathway. This chapter summarizes basic knowledge regarding the understanding of the NFkappaB pathway and focuses on its multiple roles in TRAIL signaling.

Upregulation of FLIP(S) by Akt, a possible inhibition mechanism of TRAIL-induced apoptosis in human gastric cancers

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in some, but not all cancer cells. To assess the regulation of TRAIL-resistance in the human gastric cancer cells, we examined TRAIL sensitivity, TRAIL receptor expression, and intracellular signaling events induced by TRAIL. All the gastric cancer cell lines tested were susceptible to TRAIL to some extent, except for SNU-216 cell line, which was completely resistant. TRAIL receptor expression was not related to the TRAIL-sensitivity. Of the cell lines tested, SNU-216 showed the highest level of constitutively active Akt and the short form of FLICE inhibitory protein (FLIP(S)). Treatment with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or with the protein synthesis inhibitor cycloheximide induced a suppression of constitutive Akt activation in SNU-216 cells and a concomitant decrease in the expression of FLIP(S). The reduction of Akt activity by LY294002 affected the transcriptional level of FLIP(S), but not the mRNA stability. As a result, LY294002 or cycloheximide significantly enhanced TRAIL-induced apoptosis. Moreover, the overexpression of constitutively active Akt in the TRAIL-sensitive cell line, SNU-668, rendered the cell line resistant to TRAIL. In addition, infection of the same cell line with retrovirus expressing FLIP(S) completely inhibited TRAIL-induced apoptosis by blocking the activation of caspase-8. Therefore, our results suggest that Akt activity promotes human gastric cancer cell survival against TRAIL-induced apoptosis via upregulation of FLIP(S), and that the cytotoxic effect of TRAIL can be enhanced by modulating the Akt/FLIP(S) pathway in human gastric cancers.

Fas-associated factor-1 inhibits nuclear factor-kappaB (NF-kappaB) activity by interfering with nuclear translocation of the RelA (p65) subunit of NF-kappaB

Fas-associated factor-1 (FAF1) is a Fas-binding pro-apoptotic protein that is a component of the death-inducing signaling complex in Fas-mediated apoptosis. Here, we show that FAF1 is involved in negative regulation of NF-kappaB activation. Overexpression of FAF1 decreased the basal level of NF-kappaB activity in 293 cells. NF-kappaB activation induced by tumor necrosis factor (TNF)-alpha, interleukin-1beta, and lipopolysaccharide was also inhibited by FAF1 overexpression. Moreover, FAF1 suppressed NF-kappaB activation induced by transducers of diverse NF-kappaB-activating signals such as TNF receptor-associated factor-2 and -6, MEKK1, and IkappaB kinase-beta as well as NF-kappaB p65, one of the end point molecules in the NF-kappaB activation pathway, suggesting that NF-kappaB p65 might be a target molecule upon which FAF1 acts. Subsequent study disclosed that FAF1 physically interacts with NF-kappaB p65 and that the binding domain of FAF1 is the death effector domain (DED)-interacting domain (amino acids 181-381), where DEDs of the Fas-associated death domain protein and caspase-8 interact. The NF-kappaB activity-modulating potential of FAF1 was also mapped to the DED-interacting domain. Finally, overexpression of FAF1 prevented translocation of NF-kappaB p65 into the nucleus and decreased its DNA-binding activity upon TNFalpha treatment. This study presents a novel function of FAF1, in addition to the previously known function as a component of the Fas death-inducing signaling complex, i.e. NF-kappaB activity suppressor by cytoplasmic retention of NF-kappaB p65 via physical interaction.

Polyamines are required for activation of c-Jun NH2-terminal kinase and apoptosis in response to TNF-alpha in IEC-6 cells

Intracellular polyamine homeostasis is important for the regulation of cell proliferation and apoptosis and is necessary for the balanced growth of cells and tissues. Polyamines have been shown to play a role in the regulation of apoptosis in many cell types, including IEC-6 cells, but the mechanism is not clear. In this study, we analyzed the mechanism by which polyamines regulate the process of apoptosis in response to tumor necrosis factor-alpha (TNF-alpha). TNF-alpha or cycloheximide (CHX) alone did not induce apoptosis in IEC-6 cells. Significant apoptosis was observed when CHX was given along with TNF-alpha, as indicated by a significant increase in the detachment of cells, caspase-3 activity, and DNA fragmentation. Polyamine depletion by treatment with alpha-difluoromethylornithine significantly reduced the level of apoptosis, as judged by DNA fragmentation and the caspase-3 activity of attached cells. Apoptosis in IEC-6 cells was accompanied by the activation of upstream caspases-6, -8, and -9 and NH2-terminal c-Jun kinase (JNK). Inhibition of JNK activation prevented caspase-9 activation. Polyamine depletion prevented the activation of JNK and of caspases-6, -8, -9, and -3. SP-600125, a specific inhibitor of JNK activation, prevented cytochrome c release from mitochondria, JNK activation, DNA fragmentation, and caspase-9 activation in response to TNF-alpha/CHX. In conclusion, we have shown that polyamine depletion delays and decreases TNF-alpha-induced apoptosis in IEC-6 cells and that apoptosis is accompanied by the release of cytochrome c, the activation of JNK, and of upstream caspases as well as caspase-3. Polyamine depletion prevented JNK activation, which may confer protection against apoptosis by modulation of upstream caspase-9 activation.

Fas and Fas-associated death domain protein regulate monocyte chemoattractant protein-1 expression by human smooth muscle cells through caspase- and calpain-dependent release of interleukin-1alpha

We previously reported that treatment of human vascular smooth muscle cells (SMCs) with proapoptotic stimuli, including Fas ligand plus cycloheximide (FasL/Chx), or overexpression of Fas-associated death domain protein (FADD) result in increased expression of monocyte chemoattractant protein-1 (MCP-1) and other proinflammatory genes. In this study, we demonstrate that Fas/FADD-induced MCP-1 upregulation is driven by an autocrine/paracrine signaling loop in which interleukin (IL)-1alpha synthesis and release are activated through caspase- and calpain-dependent processes. Untreated SMCs contain very little IL-1alpha protein or transcript. Both were increased greatly in response to Fas/FADD activation, primarily through an autocrine/paracrine pathway in which secreted IL-1alpha stimulated additional IL-1alpha synthesis and release. Caspase 8 (Csp8) activity increased in response to FasL/Chx treatment, and Csp8 inhibitors markedly reduced IL-1alpha release and MCP-1 upregulation. In contrast, Csp8 activity was not significantly increased in response to FADD overexpression and caspase inhibitors did not effect FADD-induced MCP-1 upregulation. Both FasL/Chx treatment and FADD overexpression increased the activity of calpains. Calpain inhibitors reduced IL-1alpha release and MCP-1 upregulation in both FADD-overexpressing SMCs and FasL/Chx-treated SMCs without blocking Csp8 activity. This indicates that calpains are not required for activation of caspases and that caspase activation is not sufficient for IL-1alpha release and MCP-1 upregulation. These data suggest that calpains play a dominant role in Fas/FADD-induced IL-1alpha release and MCP-1 upregulation and that caspase activation may function to amplify the effects of calpain activation.

Differential recruitment of caspase 8 to cFlip confers sensitivity or resistance to Fas-mediated apoptosis in a subset of familial lymphoma patients

Resistance to Fas-mediated apoptosis (FMA) has been implicated in the pathogenesis of hematologic malignancies. Recently, a collaborative study showed that germline Fas mutations represent a genetic risk factor for the development of Hodgkin's and non-Hodgkin lymphoma. Here, we report that transformed B cell lines from familial lymphoma patients show a range of sensitivity to Fas-mediated apoptosis with lymphocytes from two patients with a marked resistance to Fas-, but not p53-mediated cell death. Fas resistance in these cells was associated with reduced recruitment of the initiator caspase 8 compared to cFlip, an inhibitor of apoptosis, to the death-inducing signaling complex (DISC). A decreased ratio of caspase 8 to cFlip in total cell extracts as well as in the DISC was associated with a profound disturbance of the Fas signaling cascade. We propose here that the relative reduction in caspase 8 to cFlip in the Fas DISC confers a survival advantage to lymphocytes and predisposes to the development of malignancy in some familial lymphoma patients.

The proteasome inhibitor PS-341 overcomes TRAIL resistance in Bax and caspase 9-negative or Bcl-xL overexpressing cells

We demonstrate that PS-341, a small molecule inhibitor of the proteasome, markedly sensitizes resistant prostate, colon, and bladder cancer cells to TNF-like apoptosis-inducing ligand (TRAIL)-induced apoptosis irrespective of Bcl-xL overexpression. PS-341 treatment by itself does not affect the levels of Bax, Bak, caspases 3 and 8, c-Flip or FADD, but elevates levels of TRAIL receptors DR4 and DR5. This increase in receptor protein levels is associated with the ubiquitination of the DR5 protein. When PS-341 is combined with TRAIL, the levels of activated caspase 8 and cleaved Bid are substantially increased. In Bax-negative TRAIL-resistant HC-4 colon cancer cells, the combination of PS-341 and TRAIL overcomes the block to activation of the mitochondrial pathway and causes SMAC and cytochrome c release followed by apoptosis. Similarly, murine embryonic fibroblasts lacking Bax undergo apoptosis when exposed to the combination of PS-341 and TRAIL; however, fibroblasts lacking Bak are significantly resistant. Taken together, these findings indicate that PS-341 enhances TRAIL-induced apoptosis by increasing the cleavage of caspase 8, causing Bak-dependent release of mitochondrial proapoptotic proteins.

Cellular FLICE/caspase-8-inhibitory protein as a principal regulator of cell death and survival in human hepatocellular carcinoma

Human hepatocellular carcinomas (HCCs) show resistance to apoptosis mediated by several death receptors. Because cellular FLICE/caspase-8-inhibitory protein (cFLIP) is a recently identified intracellular inhibitor of caspase-8 activation that potently inhibits death signaling mediated by all known death receptors, including Fas, TNF-receptor (TNF-R), and TNF-related apoptosis-inducing ligand receptors (TRAIL-Rs), we investigated the expression and function of cFLIP in human HCCs. We found that cFLIP is constitutively expressed in all human HCC cell lines and is expressed more in human HCC tissues than in nontumor liver tissues. Metabolic inhibitors, actinomycin D (ActD) or cycloheximide (CHX), dramatically rendered HCC cells sensitive to Fas-mediated apoptosis. Neither caspase-8 nor caspase-3 was activated by agonistic anti-Fas antibody alone, but both caspases were activated by Fas stimulation in the presence of ActD or CHX, indicating the importance of caspase-8 inhibitors that are sensitive to metabolic inhibitors. Actually, cFLIP expression was decreased in ActD or CHX treatment. cFLIP down-regulation induced by cFLIP antisense oligodeoxynucleotides sensitized HLE cells to Fas, TNF-R, and TRAIL-R-mediated apoptosis. Furthermore, cFLIP over-expression activated nuclear factor (NF)-kappaB and cFLIP down-regulation attenuated NF-kappaB activation induced by TNF-alpha or TRAIL. Pretreatment with pan-caspase-inhibitor, benzyloxycarbonyl-Val-Ala-Asp (OMe) fluoromethyl ketone (Z-VAD-fmk), restored NF-kappaB activity attenuated by cFLIP down-regulation. cFLIP expression was increased by TNF-alpha, TRAIL, or vascular endothelial growth factor but decreased by wortmannin, indicating that cFLIP expression is regulated by both the NF-kappaB and phosphatidylinostiol-3 kinase (PI-3)/Akt pathways. These results suggest that cFLIP plays an important role in cell survival not simply by inhibiting death-receptor-mediated apoptosis but also by regulating NF-kappaB activation in human HCCs.

TRAIL-induced apoptosis and gene induction in HaCaT keratinocytes: differential contribution of TRAIL receptors 1 and 2

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) exerts a potent cytotoxic activity especially against many tumor cell types such as transformed keratinocytes. The specific role of the different TRAIL receptors in this process, however, is unknown. In this report we examine the role the TRAIL receptors play in both the apoptotic and nonapoptotic responses of HaCaT keratinocytes to leucine zipper TRAIL (LZ-TRAIL). By employing receptor-specific blocking antibodies we demonstrate that TRAIL receptor 1 plays the primary role in mediating caspase activation and apoptosis in HaCaT cells. Furthermore, we show that this receptor mainly mediates nuclear factor kappaB activation and expression of the pro-inflammatory cytokine interleukin-8 and that nuclear factor kappaB activation is critically required for the induction of pro-inflammatory cytokines in response to LZ-TRAIL. Taken together, our data suggest that beside its potent pro-apoptotic role, LZ-TRAIL leads to pro-inflammatory responses that are mainly mediated by TRAIL receptor 1 in HaCaT keratinocytes.