Downregulation of c-FLIP promotes caspase-dependent JNK activation and reactive oxygen species accumulation in tumor cells

TGR5 agonist inhibits intestinal epithelial cell apoptosis via cAMP/PKA/c-FLIP/JNK signaling pathway and ameliorates dextran sulfate sodium-induced ulcerative colitis

Excessive apoptosis of intestinal epithelial cell (IEC) is a crucial cause of disrupted epithelium homeostasis, leading to the pathogenesis of ulcerative colitis (UC). The regulation of Takeda G protein-coupled receptor-5 (TGR5) in IEC apoptosis and the underlying molecular mechanisms remained unclear, and the direct evidence from selective TGR5 agonists for the treatment of UC is also lacking. Here, we synthesized a potent and selective TGR5 agonist OM8 with high distribution in intestinal tract and investigated its effect on IEC apoptosis and UC treatment. We showed that OM8 potently activated hTGR5 and mTGR5 with EC50 values of 202 ± 55 nM and 74 ± 17 nM, respectively. After oral administration, a large amount of OM8 was maintained in intestinal tract with very low absorption into the blood. In DSS-induced colitis mice, oral administration of OM8 alleviated colitis symptoms, pathological changes and impaired tight junction proteins expression. In addition to enhancing intestinal stem cell (ISC) proliferation and differentiation, OM8 administration significantly reduced the rate of apoptotic cells in colonic epithelium in colitis mice. The direct inhibition by OM8 on IEC apoptosis was further demonstrated in HT-29 and Caco-2 cells in vitro. In HT-29 cells, we demonstrated that silencing TGR5, inhibition of adenylate cyclase or protein kinase A (PKA) all blocked the suppression of JNK phosphorylation induced by OM8, thus abolished its antagonizing effect against TNF-α induced apoptosis, suggesting that the inhibition by OM8 on IEC apoptosis was mediated via activation of TGR5 and cAMP/PKA signaling pathway. Further studies showed that OM8 upregulated cellular FLICE-inhibitory protein (c-FLIP) expression in a TGR5-dependent manner in HT-29 cells. Knockdown of c-FLIP blocked the inhibition by OM8 on TNF-α induced JNK phosphorylation and apoptosis, suggesting that c-FLIP was indispensable for the suppression of OM8 on IEC apoptosis induced by OM8. In conclusion, our study demonstrated a new mechanism of TGR5 agonist on inhibiting IEC apoptosis via cAMP/PKA/c-FLIP/JNK signaling pathway in vitro, and highlighted the value of TGR5 agonist as a novel therapeutic strategy for the treatment of UC.

ROS generation is involved in titanium dioxide nanoparticle-induced AP-1 activation through p38 MAPK and ERK pathways in JB6 cells

Titanium dioxide (TiO₂ ) is generally regarded as a nontoxic and nongenotoxic white mineral, which is mainly applied in the manufacture of paper, paint, plastic, sunscreen lotion and other products. Recently, TiO₂ nanoparticles (TiO₂ NPs) have been demonstrated to cause chronic inflammation and lung tumor formation in rats, which may be associated with the particle size of TiO₂ . Considering the important role of activator protein-1 (AP-1) in regulating multiple genes involved in the cell proliferation and inflammation and the induction of neoplastic transformation, we aimed to evaluate the potency of TiO₂ NPs (≤ 20 nm) on the activation of AP-1 signaling pathway and the generation of reactive oxygen species (ROS) in a mouse epidermal cell line, JB6 cells. MTT, electron spin resonance (ESR), AP-1 luciferase activity assay in vitro and in vivo, and Western blotting assay were used to clarify this problem. Our results indicated that TiO₂ NPs dose-dependently caused the hydroxyl radical (·OH) generation and sequentially increased the AP-1 activity in JB6 cells. Using AP-1-luciferase reporter transgenic mice models, an obvious increased AP-1 activity was detected in dermal tissue after exposure to TiO₂ NPs for 24 h. Interestingly, TiO₂ NPs increased the AP-1 activity via stimulating the expression of mitogen-activated protein kinases (MAPKs) family members, including extracellular signal-regulated protein kinases (ERKs), p38 kinase, and C-Jun N-terminal kinases (JNKs). Of note, the AP-1 activation induced by TiO₂ NPs could be blocked by specific inhibitors (SB203580, PD98059, and SP 600125, respectively) that inhibit ERKs and p38 kinase but not JNKs. These findings indicate that ROS generation is involved in TiO₂ NPs-induced AP-1 activation mediated by MAPKs signal pathway.

HBXIP induces anoikis resistance by forming a reciprocal feedback loop with Nrf2 to maintain redox homeostasis and stabilize Prdx1 in breast cancer

Anoikis resistance is an essential prerequisite for tumor metastasis, but the underlying molecular mechanisms remain unknown. Herein, we report that the oncoprotein hepatitis B X-interacting protein (HBXIP) is prominently upregulated in breast cancer cells following ECM detachment. Altering HBXIP expression can impair the anchorage-independent growth ability of tumor cells. Mechanistically, HBXIP, which binds to Kelch-like ECH-associated protein 1 (Keap1) to activate nuclear factor E2-related factor 2 (Nrf2), contains a cis-acting antioxidant response element (ARE) in the gene promoter and is a target gene of Nrf2. The HBXIP/Nrf2 axis forms a reciprocal positive feedback loop that reinforces the expression and tumor-promoting actions of each protein. In response to ECM detachment, Nrf2 reduces reactive oxygen species (ROS) accumulation, protects the mitochondrial membrane potential and increases cellular ATP, GSH and NADPH levels to maintain breast cancer cell survival. Meanwhile, the reinforcement of HBXIP induced by Nrf2 inhibits JNK1 activation by inhibiting ubiquitin-mediated degradation of Prdx1, which also plays an essential role in promoting ECM-detached cell survival. Furthermore, a strong positive correlation was identified between HBXIP expression and Prdx1 expression in clinical breast cancer tissues and TCGA Pan-Cancer Atlas clinical data of breast invasive carcinoma based on the cBioPortal cancer genomics database. Co-expression of HBXIP and Prdx1 predicts a poor prognosis for breast cancer patients. Collectively, our findings reveal a significant mechanism by which the HBXIP/Nrf2 feedback loop contributes to anoikis resistance by maintaining redox homeostasis and inhibiting JNK1 activation and support the likely therapeutic value of the HBXIP/Nrf2 axis in breast cancer patients.

MIND bomb 2 prevents RIPK1 kinase activity-dependent and -independent apoptosis through ubiquitylation of cFLIPL

Mind bomb 2 (MIB2) is an E3 ligase involved in Notch signalling and attenuates TNF-induced apoptosis through ubiquitylation of receptor-interacting protein kinase 1 (RIPK1) and cylindromatosis. Here we show that MIB2 bound and conjugated K48– and K63–linked polyubiquitin chains to a long-form of cellular FLICE-inhibitory protein (cFLIP_L), a catalytically inactive homologue of caspase 8. Deletion of MIB2 did not impair the TNF-induced complex I formation that mediates NF-κB activation but significantly enhanced formation of cytosolic death-inducing signalling complex II. TNF-induced RIPK1 Ser¹⁶⁶ phosphorylation, a hallmark of RIPK1 death-inducing activity, was enhanced in MIB2 knockout cells, as was RIPK1 kinase activity-dependent and -independent apoptosis. Moreover, RIPK1 kinase activity-independent apoptosis was induced in cells expressing cFLIP_L mutants lacking MIB2-dependent ubiquitylation. Together, these results suggest that MIB2 suppresses both RIPK1 kinase activity-dependent and -independent apoptosis, through suppression of RIPK1 kinase activity and ubiquitylation of cFLIP_L, respectively.

Nakabayashi et al find that the E3 ligase MIB2 ubiquitylates the apoptosis-inhibitor cFLIP and that deletion of MIB2 enhances both RIPK1 kinase-dependent and -independent apoptosis through an increase in RIPK1 kinase activity and impairment of ubiquitylation of cFLIP_L, respectively.

Regulation of distinct caspase-8 functions in retinal ganglion cells and astroglia in experimental glaucoma

Retinal ganglion cells (RGCs) expanding from the retina to the brain are primary victims of neurodegeneration in glaucoma, a leading cause of blindness; however, the neighboring astroglia survive the glaucoma-related stress and promote neuroinflammation. In light of diverse functions of caspase-8 in apoptosis, cell survival, and inflammation, this study investigated the importance of caspase-8 in different fates of glaucomatous RGCs and astroglia using two experimental approaches in parallel. In the first approach, cell type-specific responses of RGCs and astroglia to a caspase-8 cleavage-inhibiting pharmacological treatment were studied in rat eyes with or without experimentally induced glaucoma. The second approach utilized an experimental model of glaucoma in mice in which astroglial caspase-8 was conditionally deleted by cre/lox. Findings of these experiments revealed cell type-specific distinct processes that regulate caspase-8 functions in experimental glaucoma, which are involved in inducing the apoptosis of RGCs and promoting the survival and inflammatory responses of astroglia. Deletion of caspase-8 in astroglia protected RGCs against glia-driven inflammatory injury, while the inhibition of caspase-8 cleavage inhibited apoptosis in RGCs themselves. Various caspase-8 functions impacting both RGC apoptosis and astroglia-driven neuroinflammation may suggest the multi-target potential of caspase-8 regulation to provide neuroprotection and immunomodulation in glaucoma.

trans-Fatty acids facilitate DNA damage-induced apoptosis through the mitochondrial JNK-Sab-ROS positive feedback loop

trans-Fatty acids (TFAs) are unsaturated fatty acids that contain one or more carbon-carbon double bonds in trans configuration. Epidemiological evidence has linked TFA consumption with various disorders, including cardiovascular diseases. However, the underlying pathological mechanisms are largely unknown. Here, we show a novel toxic mechanism of TFAs triggered by DNA damage. We found that elaidic acid (EA) and linoelaidic acid, major TFAs produced during industrial food manufacturing (so-called as industrial TFAs), but not their corresponding cis isomers, facilitated apoptosis induced by doxorubicin. Consistently, EA enhanced UV-induced embryonic lethality in C. elegans worms. The pro-apoptotic action of EA was blocked by knocking down Sab, a c-Jun N-terminal kinase (JNK)-interacting protein localizing at mitochondrial outer membrane, which mediates mutual amplification of mitochondrial reactive oxygen species (ROS) generation and JNK activation. EA enhanced doxorubicin-induced mitochondrial ROS generation and JNK activation, both of which were suppressed by Sab knockdown and pharmacological inhibition of either mitochondrial ROS generation, JNK, or Src-homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1) as a Sab-associated protein. These results demonstrate that in response to DNA damage, TFAs drive the mitochondrial JNK-Sab-ROS positive feedback loop and ultimately apoptosis, which may provide insight into the common pathogenetic mechanisms of diverse TFA-related disorders.

c-FLIP, a Novel Biomarker for Cancer Prognosis, Immunosuppression, Alzheimer's Disease, Chronic Obstructive Pulmonary Disease (COPD), and a Rationale Therapeutic Target

Dysregulation of c-FLIP (cellular FADD-like IL-1β-converting enzyme inhibitory protein) has been shown in several diseases including cancer, Alzheimer's disease, and chronic obstructive pulmonary disease (COPD). c-FLIP is a critical anti-cell death protein often overexpressed in tumors and hematological malignancies and its increased expression is often associated with a poor prognosis. c-FLIP frequently exists as long (c-FLIPL) and short (c-FLIPS) isoforms, regulates its anti-cell death functions through binding to FADD (FAS associated death domain protein), an adaptor protein known to activate caspases-8 and -10 and links c-FLIP to several cell death regulating complexes including the death-inducing signaling complex (DISC) formed by various death receptors. c-FLIP also plays a critical role in necroptosis and autophagy. Furthermore, c-FLIP is able to activate several pathways involved in cytoprotection, proliferation, and survival of cancer cells through various critical signaling proteins. Additionally, c-FLIP can inhibit cell death induced by several chemotherapeutics, anti-cancer small molecule inhibitors, and ionizing radiation. Moreover, c-FLIP plays major roles in aiding the survival of immunosuppressive tumor-promoting immune cells and functions in inflammation, Alzheimer's disease (AD), and chronic obstructive pulmonary disease (COPD). Therefore, c-FLIP can serve as a versatile biomarker for cancer prognosis, a diagnostic marker for several diseases, and an effective therapeutic target. In this article, we review the functions of c-FLIP as an anti-apoptotic protein and negative prognostic factor in human cancers, and its roles in resistance to anticancer drugs, necroptosis and autophagy, immunosuppression, Alzheimer's disease, and COPD.

Cordycepin promotes apoptosis in renal carcinoma cells by activating the MKK7-JNK signaling pathway through inhibition of c-FLIPL expression

Cellular FLICE inhibitory protein (c-FLIP) is a key anti-apoptotic regulator that associates with the signaling complex downstream of NF-κB, negatively interfering with apoptotic signaling. The role of c-FLIP downregulation by negative regulation of NF-κB signaling during apoptosis is poorly understood. Here, we demonstrate that NF-κB-mediated c-FLIPL negatively regulates the JNK signaling pathway, and that cordycepin treatment of human renal cancer cells leads to apoptosis induction through c-FLIPL inhibition. TNF-α-induced inflammatory microenvironments stimulated NF-κB signaling and the c-FLIP long form (c-FLIPL) in TK-10 cells. Specifically, cordycepin inhibited TNF-α-mediated NF-κB activation, which induced renal cancer cell apoptosis. Cordycepin downregulated GADD45B and c-FLIPL, but upregulated MKK7 and phospho-JNK, by preventing nuclear mobilization of NF-κB. Furthermore, siRNA-mediated knockdown of GADD45B in cordycepin-treated TK-10 cells considerably increased MKK7 compared to cordycepin alone. siRNA-mediated knockdown of c-FLIPL prevented TNF-α-induced JNK inactivation, whereas c-FLIPL overexpression inhibited cordycepin-mediated JNK activation. The JNK inhibitor SP600125 strongly inhibited Bax expression. In nude mice, cordycepin significantly decreased tumor volume. Taken together, the results indicate that cordycepin inhibits TNF-α-mediated NF-κB/GADD45B signaling, which activates the MKK7-JNK signaling pathway through inhibition of c-FLIPL expression, thus inducing TK-10 cell apoptosis.

Suppression of c‑FLIPL promotes JNK activation in malignant melanoma cells

The up‑regulation of cellular Fas‑associated death domain‑like interleukin‑1β‑converting enzyme (FLICE)‑like inhibitory protein (c‑FLIP) has been reported in various tumor types, and has been previously shown to be associated with the clinicopathological features of melanoma. To assess its potential role in cancer therapy, the present study evaluated the effects of short hairpin (sh)RNAs of different c‑FLIP isoforms on cellular proliferation and c‑Jun N‑terminal kinase (JNK) signaling. Human c‑FLIP shRNA plasmids were constructed and transfected into the A875 melanoma cell line. It was observed that c‑FLIP shRNA exhibited strong inhibitory effects against the levels of phosphorylated‑JNK and inhibited cellular proliferation in A875 cells. Thus, this indicated that c‑FLIP long form shRNA serves a specific inhibitory role in cellular proliferation through inducing the activation of the JNK pathway in A875 cells. The present study provided insight into the development of RNAi based therapies for melanoma.

Inhibition of c-FLIP by RNAi enhances sensitivity of the human osteogenic sarcoma cell line U2OS to TRAIL-induced apoptosis

To study effects of cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) inhibition by RNA interference (RNAi) on sensitivity of U2OS cells to tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, plasmid pSUPER-c-FLIP-siRNA was constructed and then transfected into U2OS cells. A stable transfection cell clone U2OS/pSUPER-c-FLIP-siRNA was screened from the c-FLIP-siRNA transfected cells. RT-PCR and Western blotting were applied to measure the expression of c-FLIP at the levels of mRNA and protein. The results indicated that the expression of c-FLIP was significantly suppressed by the c-FLIP-siRNA in the cloned U2OS/pSUPER-c-FLIP-siRNA as compared with the control cells of U2OS/pSUPER. The cloned cell line of U2OS/pSUPER-c-FLIP-siRNA was further examined for TRAIL- induced cell death and apoptosis in the presence of a pan-antagonist of inhibitor of apoptosis proteins (IAPs) AT406, with or without 4 hrs pretreatment with rocaglamide, an inhibitor of c-FLIP biosynthesis, for 24 hrs. Cell death effects and apoptosis were measured by the methods of MTT assay with 3-(4,5-dimethylthiazol-2-yl)- 2,5-diphenyltetrazolium bromide and flow cytometry, respectively. The results indicated that TRAIL-induced cell death in U2OS/pSUPER-c-FLIP-siRNA was increased compared with control cells U2OS/pSUPER in the presence or absence of AT406. Flow cytometry indicated that TRAIL-induced cell death effects proceeded through cell apoptosis pathway. However, in the presence of rocaglamide, cell death or apoptotic effects of TRAIL were similar and profound in both cell lines, suggesting that the mechanism of action for both c-FLIP-siRNA and rocaglamide was identical. We conclude that the inhibition of c-FLIP by either c-FLIP-siRNA or rocaglamide can enhance the sensitivity of U2OS to TRAIL-induced apopotosis, suggesting that inhibition of c-FLIP is a good target for anti-cancer therapy.

FLIP the Switch: Regulation of Apoptosis and Necroptosis by cFLIP

cFLIP (cellular FLICE-like inhibitory protein) is structurally related to caspase-8 but lacks proteolytic activity due to multiple amino acid substitutions of catalytically important residues. cFLIP protein is evolutionarily conserved and expressed as three functionally different isoforms in humans (cFLIP_L, cFLIP_S, and cFLIP_R). cFLIP controls not only the classical death receptor-mediated extrinsic apoptosis pathway, but also the non-conventional pattern recognition receptor-dependent apoptotic pathway. In addition, cFLIP regulates the formation of the death receptor-independent apoptotic platform named the ripoptosome. Moreover, recent studies have revealed that cFLIP is also involved in a non-apoptotic cell death pathway known as programmed necrosis or necroptosis. These functions of cFLIP are strictly controlled in an isoform-, concentration- and tissue-specific manner, and the ubiquitin-proteasome system plays an important role in regulating the stability of cFLIP. In this review, we summarize the current scientific findings from biochemical analyses, cell biological studies, mathematical modeling, and gene-manipulated mice models to illustrate the critical role of cFLIP as a switch to determine the destiny of cells among survival, apoptosis, and necroptosis.

Cellular FLICE-Inhibitory Protein Regulates Tissue Homeostasis

Cellular FLICE-inhibitory protein (cFLIP) is structurally related to caspase-8, but lacks its protease activity. Cflip gene encodes several splicing variants including short form (cFLIPs) and long form (cFLIP_L). cFLIP_L is composed of two death effector domains at the N terminus and a C-terminal caspase-like domain, and cFLIPs lacks the caspase-like domain. Our studies reveal that cFLIP plays a central role in NF-κB-dependent survival signals that control apoptosis and programmed necrosis. Germline deletion of Cflip results in embryonic lethality due to enhanced apoptosis and programmed necrosis; however, the combined deletion of the death-signaling regulators, Fadd and Ripk3, prevents embryonic lethality in Cflip-deficient mice. Moreover, tissue-specific deletion of Cflip reveals cFLIP as a crucial regulator that maintains tissue homeostasis of immune cells, hepatocytes, intestinal epithelial cells, and epidermal cells by preventing apoptosis and programmed necrosis.

Resistance to TRAIL in non-transformed cells is due to multiple redundant pathways

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine and a selective inducer of apoptosis in a range of tumour cells, but not in normal, untransformed cells. A large number of chemotherapeutics as well as biological agents are being tested for their potential to sensitise resistant tumour cells to TRAIL as a means to broaden the range of tumours treatable with TRAIL. However, because of the incomplete understanding of the mechanism(s) underlying TRAIL resistance in non-malignant cells, it is unpredictable whether the effect of these sensitisers will be restricted to tumour cells or they would also sensitise non-transformed cells causing unwanted toxicity. In this study, we carried out a systematic analysis of the mechanisms driving TRAIL resistance in non-transformed cells. We found that cellular FLICE-like inhibitory protein, anti-apoptotic B-cell lymphoma 2 proteins, and X-linked inhibitor of apoptosis protein were independently able to provide resistance to TRAIL. Deficiency of only one of these proteins was not sufficient to elicit TRAIL sensitivity, demonstrating that in non-transformed cells multiple pathways control TRAIL resistance and they act in a redundant manner. This is contrary to the resistance mechanisms found in tumour cell types, many of them tend to rely on a single mechanism of resistance. Supporting this notion we found that 76% of TRAIL-resistant cell lines (13 out of 17) expressed only one of the above-identified anti-apoptotic proteins at a high level (≥1.2-fold higher than the mean expression across all cell lines). Furthermore, inhibition or knockdown of the single overexpressed protein in these tumour cells was sufficient to trigger TRAIL sensitivity. Therefore, the redundancy in resistance pathways in non-transformed cells may offer a safe therapeutic window for TRAIL-based combination therapies where selective sensitisation of the tumour to TRAIL can be achieved by targeting the single non-redundant resistance pathway.

Mathematical modeling of apoptosis

Apoptosis is a form of programmed cell death, which is fundamental to all multicellular organisms. Deregulation of apoptosis leads to a number of severe diseases including cancer. Apoptosis is initiated either by extrinsic signals via stimulation of receptors at the cellular surface or intrinsic signals, such as DNA damage or growth factor withdrawal. Apoptosis has been extensively studied using systems biology which substantially contributed to the understanding of this death signaling network. This review gives an overview of mathematical models of apoptosis and the potential of systems biology to contribute to the development of novel therapies for cancer or other apoptosis-related diseases.

c-FLIP maintains tissue homeostasis by preventing apoptosis and programmed necrosis

As a catalytically inactive homolog of caspase-8, a proapoptotic initiator caspase, c-FLIP blocks apoptosis by binding to and inhibiting caspase-8. The transcription factor nuclear factor κB (NF-κB) plays a pivotal role in maintaining the homeostasis of the intestine and the liver by preventing death receptor-induced apoptosis, and c-FLIP plays a role in the NF-κB-dependent protection of cells from death receptor signaling. Because c-Flip-deficient mice die in utero, we generated conditional c-Flip-deficient mice to investigate the contribution of c-FLIP to homeostasis of the intestine and the liver at developmental and postnatal stages. Intestinal epithelial cell (IEC)- or hepatocyte-specific deletion of c-Flip resulted in perinatal lethality as a result of the enhanced apoptosis and programmed necrosis of the IECs and the hepatocytes. Deficiency in the gene encoding tumor necrosis factor-α (TNF-α) receptor 1 (Tnfr1) partially rescued perinatal lethality and the development of colitis in IEC-specific c-Flip-deficient mice but did not rescue perinatal lethality in hepatocyte-specific c-Flip-deficient mice. Moreover, adult mice with interferon (IFN)-inducible deficiency in c-Flip died from hepatitis soon after depletion of c-FLIP. Pretreatment of IFN-inducible c-Flip-deficient mice with a mixture of neutralizing antibodies against TNF-α, Fas ligand (FasL), and TNF-related apoptosis-inducing ligand (TRAIL) prevented hepatitis. Together, these results suggest that c-FLIP controls the homeostasis of IECs and hepatocytes by preventing cell death induced by TNF-α, FasL, and TRAIL.

Ribosomal protein S3 interacts with TRADD to induce apoptosis through caspase dependent JNK activation

It has been reported that ribosomal protein S3 (rpS3) functions as a ribosomal protein, a DNA repair endonuclease, a proapoptotic protein, and an essential subunit of the native NF-κB complex. However, it is unknown how rpS3 induces apoptosis in response to extracellular stresses. We report here that rpS3 sensitizes genotoxic stress-induced apoptosis by activating JNK through a caspase dependent manner. This apoptotic effect was shown to result from the physical interaction between rpS3 and TRADD, as assessed by coimmunoprecipitation. Moreover, GFP-rpS3 colocalized with TRADD around the plasma membrane and in the cytoplasm during apoptosis. Thus, rpS3 appears to be recruited to the death-inducing signaling complex (DISC) to induce apoptosis by interacting TRADD in response to extracellular stresses. Based on the findings of this study, we concluded that rpS3 is recruited to the DISC and plays a critical role in both genotoxic stress and cytokine induced apoptosis.

Caspase-8 activity has an essential role in CD95/Fas-mediated MAPK activation

Stimulation of CD95/Fas/APO-1 results in the induction of both apoptotic and non-apoptotic signaling pathways. The processes regulating these two opposing pathways have not been thoroughly elucidated to date. In this study, using quantitative immunoblots, imaging, and mathematical modeling, we addressed the dynamics of the DED proteins of the death-inducing signaling complex (DISC), procaspase-8, and cellular FLICE inhibitory proteins (c-FLIPs) to the onset of CD95-mediated ERK1/2 and p38 mitogen-activated protein kinase (MAPK) activation. We found that CD95 DISC-induced caspase-8 activity is important for the initiation of ERK1/2 and p38 MAPK activation. The long c-FLIP isoform, c-FLIP(L), and the short c-FLIP isoform, c-FLIP(R), inhibited MAPK induction by blocking caspase-8 processing at the DISC. Furthermore, we built a mathematical model describing CD95 DISC-mediated MAPK activation and apoptosis. The model quantitatively defined the dynamics of DED proteins, procaspase-8, and c-FLIP, which lead to caspase-8 activation and induction of apoptotic and non-apoptotic signaling pathways. In conclusion, the combination of biochemical analysis with mathematical modeling provides evidence for an important role of caspase-8 in CD95-mediated activation of MAPKs, while c-FLIP exerts a regulatory function in this process.

Inhibition of SREBP1 sensitizes cells to death ligands

Evasion of death receptor ligand-induced apoptosis contributs to cancer development and progression. To better understand mechanisms conferring resistance to death ligands, we screened an siRNA library to identify sequences that sensitize resistant cells to fas activating antibody (CH-11). From this screen, we identified the Sterol-Regulatory Element-Binding Protein 1 (SREBP1), a transcription factor, which regulates genes involved in cholesterol and fatty acid synthesis including fatty acid synthase. Inhibition of SREBP1 sensitized PPC-1 and HeLa to the death receptor ligands CH-11 and TRAIL. In contrast, DU145 prostate cancer cells that are resistant to death ligands despite expressing the receptors on their cell surface remained resistant to CH-11 and TRAIL after knockdown of SREBP1. Consistent with the effects on cell viability, the addition of CH-11 activated caspases 3 and 8 in HeLa but not DU145 cells with silenced SREBP1. We demonstrated that knockdown of SREBP1 produced a marked decrease in fatty acid synthase expression. Furthermore, genetic or chemical inhibition of fatty acid synthase with shRNA or orlistat, respectively, recapitulated the effects of SREBP1 inhibition and sensitized HeLa but not DU145 cells to CH-11 and TRAIL. Sensitization to death receptor ligands by inhibition of fatty acid synthase was associated with activation of caspase 8 prior to caspase 9. Neither silencing of SREBP1 or fatty acid synthase changed basal expression of the core death receptor components Fas, caspase 8, FADD, caspase 3 or FLIP. Thus, inhibition of SREBP1 or its downstream target fatty acid synthase sensitizes resistant cells to death ligands.

RIP1 comes back to life as a cell death regulator in TNFR1 signaling

Cell death induction by tumor necrosis factor has been an intensively studied area for the last two decades. Although it may appear that the skeleton should have been picked clean by now, new secrets about tumor necrosis factor death signaling are still being uncovered. In particular, the recent evidence that ubiquitination of the death kinase receptor-interacting protein 1 regulates its participation in apoptotic and necrotic cell death is opening up unexplored avenues in the catacombs of tumor necrosis factor death signaling. In this minireview, we focus on two major cell-death checkpoints that determine whether receptor-interacting protein 1 functions as a pro-survival or pro-death molecule.

Reactive oxygen species in TNFalpha-induced signaling and cell death

TNFalpha is a pleotropic cytokine that initiates many downstream signaling pathways, including NF-kappaB activation, MAP kinase activation and the induction of both apoptosis and necrosis. TNFalpha has shown to lead to reactive oxygen species generation through activation of NADPH oxidase, through mitochondrial pathways, or other enzymes. As discussed, ROS play a role in potentiation or inhibition of many of these signaling pathways. We particularly discuss the role of sustained JNK activation potentiated by ROS, which generally is supportive of apoptosis and "necrotic cell death" through various mechanisms, while ROS could have inhibitory or stimulatory roles in NF-kappaB signaling.

Butyric acid induces apoptosis via oxidative stress in Jurkat T-cells

Reactive oxygen species (ROS) are essential for the induction of T-cell apoptosis by butyric acid, an extracellular metabolite of periodontopathic bacteria. To determine the involvement of oxidative stress in apoptosis pathways, we investigated the contribution of ROS in mitochondrial signaling pathways, death-receptor-initiated signaling pathway, and endoplasmic reticulum stress in butyric-acid-induced T-cell apoptosis. N-acetyl-L-Cysteine (NAC) abrogated mitochondrial injury, cytochrome c, AIF, and Smac release, and Bcl-2 and Bcl-xL suppression and Bax and Bad activation induced by butyric acid. However, the decrease in cFLIP expression by butyric acid was not restored by treatment with NAC; increases in caspase-4 and -10 activities by butyric acid were completely abrogated by NAC. NAC also affected the elevation of GRP78 and CHOP/GADD153 expression by butyric acid. These results suggest that butyric acid is involved in mitochondrial-dysfunction- and endoplasmic reticulum stress-mediated apoptosis in human Jurkat T-cells via a ROS-dependent mechanism.

Cystatin B inhibition of TRAIL-induced apoptosis is associated with the protection of FLIP(L) from degradation by the E3 ligase itch in human melanoma cells

Past studies have identified a number of distinct mechanisms that contribute to the resistance of melanoma cells against apoptosis induced by TNF-related apoptosis-inducing ligand (TRAIL). In this report we show that cystatin B is another endogenous inhibitor of TRAIL-induced apoptosis. Cystatin B-deficient melanoma cell lines established by shRNA knockdown displayed increased apoptosis that was associated with enhanced activation of caspase-8 induced by TRAIL. This was not related to the inhibitory effect of cystatin B on the lysosomal cysteine proteases, cathepsin B and L, as they did not have a role in TRAIL-induced apoptosis in most melanoma cell lines even when cystatin B was inhibited. Instead, sensitization of melanoma cells to TRAIL-induced apoptosis by inhibition of cystatin B appeared associated with decreased stability of FLIP(L) as the levels of FLIP(L) were reduced because of shortened half-life time in melanoma cells deficient in cystatin B. In contrast, over-expression of cystatin B increased the levels of FLIP(L), decreased the amount of the E3 ligase Itch associated with FLIP(L), and reduced FLIP(L) ubiquitination. Inhibition of Itch by siRNA restored the levels of FLIP(L) and blocked sensitization to TRAIL-induced apoptosis associated with deficiency in cystatin B. Taken together, these results indicate that cystatin B regulates Itch-mediated degradation of FLIP(L) and thereby TRAIL-induced apoptosis in melanoma cells.

Chrysin sensitizes tumor necrosis factor-alpha-induced apoptosis in human tumor cells via suppression of nuclear factor-kappaB

Chrysin (5,7-dihydroxyflavone) is a natural flavonoid commonly found in many plants. The anti-cancer property of chrysin has been demonstrated although the molecular mechanisms remain to be further elucidated. In the present study, we found that, pretreatment with chrysin greatly sensitized various human cancer cells to tumor necrosis factor-alpha (TNFalpha)-induced apoptosis. In the search of the molecular mechanisms responsible for the sensitization effect of chrysin, we discovered that such sensitization is closely associated with the inhibitory effect of chrysin on TNFalpha-mediated nuclear transcription factor-kappaB (NF-kappaB) activation. Pretreatment with chrysin inhibited TNFalpha-induced degradation of Inhibitor of kappaB (IkappaB) protein and subsequent nuclear translocation of p65. As a result, chrysin suppressed the expression of NF-kappaB-targeted anti-apoptotic gene, c-FLIP-L. The role of c-FLIP-L was further confirmed by its ectopic expression, which significantly protected cell death induced by combined treatment with chrysin and TNFalpha. Data from this study thus reveal a novel function of chrysin and enhance the value of chrysin as an anti-cancer agent.

Early growth response-1 is a regulator of DR5-induced apoptosis in colon cancer cells

Background:

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) induces tumour cell apoptosis by binding to death receptor 4 (DR4) and DR5. DR4 and DR5 activation however can also induce inflammatory and pro-survival signalling. It is not known how these different cellular responses are regulated and what the individual role of DR4 vs DR5 is in these processes.

Methods:

DNA microarray study was carried out to identify genes differentially expressed after DR4 and DR5 activation. RT–PCR and western blotting was used to examine the expression of early growth response gene-1 (Egr-1) and the proteins of the TRAIL signalling pathway. The function of Egr-1 was studied by siRNA-mediated knockdown and overexpression of a dominant-negative version of Egr-1.

Results:

We show that the immediate early gene, Egr-1, regulates TRAIL sensitivity. Egr-1 is constitutively expressed in colon cancer cells and further induced upon activation of DR4 or DR5. Our results also show that DR4 mediates a type II, mitochondrion-dependent apoptotic pathway, whereas DR5 induces a mitochondrion-independent, type I apoptosis in HCT15 colon carcinoma cells. Egr-1 drives c-FLIP expression and the short splice variant of c-FLIP (c-FLIP_S) specifically inhibits DR5 activation.

Conclusion:

Selective knockdown of c-FLIP_S sensitises cells to DR5-induced but not DR4-induced apoptosis and Egr-1 exerts an effect as an inhibitor of the DR5-induced apoptotic pathway, possibly by regulating the expression of c-FLIP_S.

Cellular FLICE-inhibitory protein (c-FLIP) signalling: a key regulator of receptor-mediated apoptosis in physiologic context and in cancer

Cellular FLICE-inhibitory protein (c-FLIP) is a catalytically inactive procaspase-8/10 homologue that associates with the signalling complex downstream of death-receptors negatively interfering with apoptotic signalling. Three c-FLIP splice variants have been identified: c-FLIP(L), c-FLIP(S) and c-FLIP(R), with all three functioning as apoptosis inhibitors involved in modulation of caspase-8/10 activity in both physiologic and pathologic contexts. Furthermore, a cell-type specific pro-apoptotic role, depending on caspase-8 to c-FLIP(L) ratio, has also been described for the long isoform. The present review summarizes recent findings concerning c-FLIP proteins' function and regulation, with a main focus on the c-FLIP(L) deregulated expression in cancer. The role of c-FLIP(L) as anti-apoptotic pro-survival factor in tumors and the potential utility of this molecule as a possible alternative therapeutic target are discussed.

Camptothecin and khat (Catha edulis Forsk.) induced distinct cell death phenotypes involving modulation of c-FLIPL, Mcl-1, procaspase-8 and mitochondrial function in acute myeloid leukemia cell lines

Background

An organic extract of the recreational herb khat (Catha edulis Forsk.) triggers cell death in various leukemia cell lines in vitro. The chemotherapeutics camptothecin, a plant alkaloid topoisomerase I inhibitor, was tested side-by-side with khat in a panel of acute myeloid leukemia cell lines to elucidate mechanisms of toxicity.

Results

Khat had a profound effect on MOLM-13 cells inducing mitochondrial damage, chromatin margination and morphological features of autophagy. The effects of khat on mitochondrial ultrastructure in MOLM-13 correlated with strongly impaired routine respiration, an effect neither found in the khat-resistant MV-4-11 cells nor in camptothecin treated cells. Enforced expression of anti-apoptotic Bcl-2 protein provided protection against camptothecin-induced cell death and partly against khat toxicity. Khat-induced cell death in MOLM-13 cells included reduced levels of anti-apoptotic Mcl-1 protein, while both khat and camptothecin induced c-FLIP_L cleavage and procaspase-8 activation.

Conclusion

Khat activated a distinct cell death pathway in sensitive leukemic cells as compared to camptothecin, involving mitochondrial damage and morphological features of autophagy. This suggests that khat should be further explored in the search for novel experimental therapeutics.

Mind bomb 1 regulation of cFLIP interactions

Mind bomb 1 (Mib1) is a multidomain E3 ligase that directs ubiquitination of the Notch ligands Delta and Jagged to promote their endocytosis. Here we examine Notch-independent functions of Mib1 and find that its activities are linked to the initiation of the extrinsic cell death pathway. Expression of Mib1 induces a spontaneous, caspase-dependent cell death. Consistent with this, depletion of endogenous Mib1 decreases tumor-necrosis factor (TNF)-induced cell death. Mib1 was found to bind to cellular Fas-associated death domain (FADD)-like IL-1b converting enzyme (FLICE)-like inhibitory proteins (cFLIP-L and cFLIP-S), whereas only cFLIP-s can inhibit Mib1-induced cell death. The interaction between Mib1 and cFLIP decreases the association of caspase-8 with cFLIP, which activates caspase-8 and induces cell death. Collectively, these results suggest that in addition to a central role in Notch signaling, Mib1 has an important role in regulating the extrinsic cell death pathway.

Induction of apoptosis in human leukemia cells by grape seed extract occurs via activation of c-Jun NH2-terminal kinase

PURPOSE

To characterize the functional role of c-Jun NH(2)-terminal kinase (JNK) and other apoptotic pathways in grape seed extract (GSE)-induced apoptosis in human leukemia cells by using pharmacologic and genetic approaches.

EXPERIMENTAL DESIGN

Jurkat cells were treated with various concentrations of GSE for 12 and 24 h or with 50 microg/mL GSE for various time intervals, after which apoptosis, caspase activation, and cell signaling pathways were evaluated. Parallel studies were done in U937 and HL-60 human leukemia cells.

RESULTS

Exposure of Jurkat cells to GSE resulted in dose- and time-dependent increase in apoptosis and caspase activation, events associated with the pronounced increase in Cip1/p21 protein level. Furthermore, treatment of Jurkat cells with GSE resulted in marked increase in levels of phospho-JNK. Conversely, interruption of the JNK pathway by pharmacologic inhibitor (e.g., SP600125) or genetic (e.g., small interfering RNA) approaches displayed significant protection against GSE-mediated lethality in Jurkat cells.

CONCLUSIONS

The result of the present study showed that GSE induces apoptosis in Jurkat cells through a process that involves sustained JNK activation and Cip1/p21 up-regulation, culminating in caspase activation.

TNF-alpha mediates pseudorabies virus-induced apoptosis via the activation of p38 MAPK and JNK/SAPK signaling

PRV infection causes apoptosis in vitro and in vivo. However, the significance of PRV-induced apoptosis and its signaling pathways is still unknown. This work investigates the role of MAPK pathways in mediating PRV-induced apoptosis. Flow cytometry, apoptosis ELISA and western blotting using antibodies against cleaved caspase-3, -6 and PARP demonstrated that PRV induces apoptosis in a time- and dose-dependent manner. p38 and JNK/SAPK inhibitors significantly protected cells from PRV-induced apoptosis. Inhibitor treatment did not affect Us3a gene transcription and progeny virus production. Western blotting revealed that PRV activates p38 and JNK/SAPK signaling. Inhibition of NF-kappaB had no effect on PRV-mediated apoptosis. Non-replicative PRV failed to activate p38 and JNK/SAPK or induce apoptosis. PRV infection increases TNF-alpha transcription, translation and secretion, as well as TNF-alpha receptor expression. Inhibition of p38 and JNK/SAPK reduced PRV-induced TNF-alpha up-regulation. Neutralization assay confirmed that TNF-alpha is a key mediator involved in PRV-induced apoptosis.

Increased susceptibility to tumor necrosis factor-alpha in butyric acid-induced apoptosis is caused by downregulation of cFLIP expression in Jurkat T cells

Butyric acid is one of the major extracellular metabolites of periodontopathic Gram-negative bacteria. We previously demonstrated that butyric acid induced apoptosis in human T cells. In the present study, we examined the interaction between butyric acid and TNF-alpha in Jurkat T-cell apoptosis. Simultaneous treatment with TNF-alpha enhanced butyric acid-induced apoptosis by promoting caspase activity more than was achieved by either reagent alone. We examined which genes were associated with the increased susceptibility to TNF-alpha caused by butyric acid, and revealed that expression of cFLIP decreased with increased concentrations of butyric acid. Furthermore, exogenous expression of cFLIP protein suppressed the enhancing effect by TNF-alpha in the apoptosis. These results suggest that butyric acid downregulates cFLIP expression and increases the susceptibility to TNF-alpha by activating caspases via the death receptor signal.

Mitochondrial Extrusion through the cytoplasmic vacuoles during cell death

Under various conditions, noxious stimuli damage mitochondria, resulting in mitochondrial fragmentation; however, the mechanisms by which fragmented mitochondria are eliminated from the cells remain largely unknown. Here we show that cytoplasmic vacuoles originating from the plasma membrane engulfed fragmented mitochondria and subsequently extruded them into the extracellular spaces in undergoing acute tumor necrosis factor alpha-induced cell death in a caspase-dependent fashion. Notably, upon fusion of the membrane encapsulating mitochondria to the plasma membrane, naked mitochondria were released into the extracellular spaces in an exocytotic manner. Mitochondrial extrusion was specific to tumor necrosis factor alpha-induced cell death, because a genotoxic stress-inducing agent such as cisplatin did not elicit mitochondrial extrusion. Moreover, intact actin and tubulin cytoskeletons were required for mitochondrial extrusion as well as membrane blebbing. Furthermore, fragmented mitochondria were engulfed by cytoplasmic vacuoles and extruded from hepatocytes of mice injected with anti-Fas antibody, suggesting that mitochondrial extrusion can be observed in vivo under pathological conditions. Mitochondria are eliminated during erythrocyte maturation under physiological conditions, and anti-mitochondrial antibody is detected in some autoimmune diseases. Thus, elucidating the mechanism underlying mitochondrial extrusion will open a novel avenue leading to better understanding of various diseases caused by mitochondrial malfunction as well as mitochondrial biology.

Mitogen-activated protein kinase phosphatase-1 represses c-Jun NH2-terminal kinase-mediated apoptosis via NF-kappaB regulation

The mechanism regulating radiation-induced anti-apoptotic response, a limiting factor in improving cell radiosensitivity, remains elusive. Mitogen-activated protein kinase (MAPK) phosphatase (MKP)-1 is the major member of MKPs that dephosphorylates and inactivates MAPK. Here we provide the evidence that MKP-1 was negatively bridging between NF-κB-mediated prosurvival pathway and c-Jun N-terminal kinase (JNK)-mediated proapoptotic response. MKP-1 was induced by γ-radiation and repressed radiation-induced pro-apoptotic status. NF-κB RelA/p50 heterodimer was recruited to MKP-1 gene promoter to induce MKP-1 transcription. Deletion of the NF-κB-binding site or inactivation of NF-κB by its small interfering RNA significantly decreased the radiation-induced MKP-1 promoter activity. In addition, MKP-1-deficient mouse embryonic fibroblasts exhibited a prolonged activation of JNK but not p38 or extracellular signal-regulated kinase subfamilies of MAPKs. The prolonged activation of JNK was not induced by treatment with tumor necrosis factor α or interleukin-6, and inactivation of JNK but not p38 or ERK abolished radiation-induced proapoptotic status, indicating that JNK is specifically inhibited by radiation-induced MKP-1. Three MKP-1 wild type human tumor cell lines treated with MKP-1 small interfering RNA showed an increased proapoptotic response that can be rescued by overexpression of wild type mouse MKP-1. Together, these results suggest that MKP-1 is a NF-κB-mediated prosurvival effector in attenuating JNK-mediated pro-apoptotic response; NF-κB/MKP-1-mediated negative JNK regulation represents a potential therapeutic target for adjusting cell radiosensitivity.