Impairment of the ubiquitin-proteasome system by cellular FLIP

Regulated cell death in glioma: promising targets for natural small-molecule compounds

Gliomas are prevalent malignant tumors in adults, which can be categorized as either localized or diffuse gliomas. Glioblastoma is the most aggressive and deadliest form of glioma. Currently, there is no complete cure, and the median survival time is less than one year. The main mechanism of regulated cell death involves organisms coordinating the elimination of damaged cells at risk of tumor transformation or cells hijacked by microorganisms for pathogen replication. This process includes apoptosis, necroptosis, autophagy, ferroptosis, pyroptosis, necrosis, parthanayosis, entosis, lysosome-dependent death, NETosis, oxiptosis, alkaliptosis, and disulfidaptosis. The main goal of clinical oncology is to develop therapies that promote the effective elimination of cancer cells by regulating cell death are the main goal of clinical oncology. Recently, scientists have utilized pertinent regulatory factors and natural small-molecule compounds to induce regulated cell death for the treatment of gliomas. By analyzing the PubMed and Web of Science databases, this paper reviews the research progress on the regulation of cell death and the role of natural small-molecule compounds in glioma. The aim is to provide help for the treatment of glioblastoma.

FLIP as a therapeutic target in cancer

One of the classic hallmarks of cancer is disruption of cell death signalling. Inhibition of cell death promotes tumour growth and metastasis, causes resistance to chemo- and radiotherapies as well as targeted agents, and is frequently due to overexpression of antiapoptotic proteins rather than loss of pro-apoptotic effectors. FLIP is a major apoptosis-regulatory protein frequently overexpressed in solid and haematological cancers, in which its high expression is often correlated with poor prognosis. FLIP, which is expressed as long (FLIP(L)) and short (FLIP(S)) splice forms, achieves its cell death regulatory functions by binding to FADD, a critical adaptor protein which links FLIP to the apical caspase in the extrinsic apoptotic pathway, caspase-8, in a number of cell death regulating complexes, such as the death-inducing signalling complexes (DISCs) formed by death receptors. FLIP also plays a key role (together with caspase-8) in regulating another form of cell death termed programmed necrosis or 'necroptosis', as well as in other key cellular processes that impact cell survival, including autophagy. In addition, FLIP impacts activation of the intrinsic mitochondrial-mediated apoptotic pathway by regulating caspase-8-mediated activation of the pro-apoptotic Bcl-2 family member Bid. It has been demonstrated that FLIP can not only inhibit death receptor-mediated apoptosis, but also cell death induced by a range of clinically relevant chemotherapeutic and targeted agents as well as ionizing radiation. More recently, key roles for FLIP in promoting the survival of immunosuppressive tumour-promoting immune cells have been discovered. Thus, FLIP is of significant interest as an anticancer therapeutic target. In this article, we review FLIP's biology and potential ways of targeting this important tumour and immune cell death regulator.

FLIP(L) is critical for aerobic glycolysis in hepatocellular carcinoma

Background

Tumor cells use aerobic glycolysis to rapidly generate ATP and growth substrate which expenses a large amount of glucose. However, how tumor cells take in enough glucose from the tumor microenvironment of insufficient blood supply remains poorly understood. The cellular FLICE-like inhibitory protein (FLIP), a cell apoptosis inhibiting molecule, is highly expressed in hepatocellular carcinoma (HCC) and is implicated in HCC development.

Methods

The effects of FLIP_L (the long form of FLIP) on aerobic glycolysis and glucose uptake were assessed in HCC cells and xenograft tumors. The correlations between FLIP_L expression and sodium/glucose cotransporter 1 (SGLT1) expression in clinical HCC tissues were analyzed. The consequences of FLIP_L-induced regulation of SGLT1 at the transcription and translation levels and the interaction between FLIP_L and SGLT1 were examined. FLIP_L-mediated tolerance upon glucose limitation and its mechanism were detected.

Results

We report a novel role for FLIP_L in promoting the aerobic glycolysis of HCC cells. FLIP_L overexpression induced a significant increase in cell aerobic glycolysis indexes including glucose uptake, glucose consumption, and lactate production. FLIP_L co-localized and interacted with SGLT1, a major active glucose transporter in HCC cells. FLIP_L increased the stability of SGLT1 protein by inhibiting its ubiquitination and degradation. The expression level of FLIP_L was positively correlated with the expression level of SGLT1 in 79 HCC tissues from surgical operation. Furthermore, FLIP_L increased cell tolerance ability and decreased cell apoptosis to low glucose by regulating SGLT1.

Conclusions

Our results indicate that FLIP_L plays an essential role in HCC aerobic glycolysis and that SGLT1 is required for FLIP_L-modulated tumor proliferation under low glucose conditions. Targeting the actions of FLIP_L in cell glucose-dependent aerobic glycolysis may provide an attractive strategy for therapeutic intervention in HCC.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0358-3) contains supplementary material, which is available to authorized users.

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.

Cytoplasmic levels of cFLIP determine a broad susceptibility of breast cancer stem/progenitor-like cells to TRAIL

Background

The clinical application of TRAIL receptor agonists as a novel cancer therapy has been tempered by heterogeneity in tumour responses. This is illustrated in breast cancer, where TRAIL is cytotoxic in cell lines of mesenchymal origin but refractory in lines with an epithelial-like phenotype. However, it is now evident that intra-tumour heterogeneity includes a minority subpopulation of tumour-initiating stem/progenitor-like cells (CSCs) that possess mesenchymal characteristics. We hypothesised therefore that TRAIL may target these phenotypically distinct CSC-like cells that are common to most - if not all - breast cancers, thus impacting on the source of malignancy in a much broader range of breast tumour subtypes than previously envisaged.

Methods

We used colony formation, tumoursphere, flow cytometry and xenograft tumour initiation assays to observe the TRAIL sensitivity of CSC-like cells in a panel of two mesenchymal-like (TRAIL-sensitive) and four epithelial-like (TRAIL-resistant) breast cancer cell lines. Subcellular levels of the endogenous TRAIL inhibitor, cFLIP, were determined by western blot and immunofluorescence microscopy. The effect of the subcellular redistribution of cFLIP on TRAIL sensitivity and Wnt signalling was determined using cFLIP localisation mutants and the TOPFlash reporter assay respectively.

Results

TRAIL universally suppressed the clonal expansion of stem/progenitors in all six of the breast cancer cell lines tested, irrespective of their phenotype or overall sensitivity to TRAIL. A concomitant reduction in tumour initiation was confirmed in the TRAIL-resistant epithelial cell line, MCF-7, following serial dilution xenotransplantation. Furthermore TRAIL sensitivity of breast CSCs was inversely proportional to the relative cytoplasmic levels of cFLIP while overexpression of cFLIP in the cytosol using subcellular localization mutants of cFLIP protected these cells from cytotoxicity. The accumulation of nuclear cFLIP on the other hand did not influence TRAIL cytotoxicity but instead promoted Wnt-dependent signalling.

Conclusion

These data propose a novel role for TRAIL as a selective CSC agent with a broad specificity for both epithelial and mesenchymal breast tumour subtypes. Furthermore we identify a dual role for cFLIP in the maintenance of breast CSC viability, dependent upon its subcellular distribution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0478-y) contains supplementary material, which is available to authorized users.

Ursodeoxycholyl lysophosphatidylethanolamide inhibits cholestasis- and hypoxia-induced apoptosis by upregulating antiapoptosis proteins

An increase of toxic bile acids such as glycochenodeoxycholic acid occurs during warm ischemia reperfusion causing cholestasis and damage in hepatocytes and intrahepatic biliary epithelial cells. We aim to test antiapoptosis effects of ursodeoxycholyl lysophosphatidylethanolamide under cholestatic induction by glycochenodeoxycholic acid treatment of mouse hepatocytes and hypoxia induction by cobalt chloride treatment of intrahepatic biliary epithelial cancer Mz-ChA-1cell line. Such treatments caused marked increases in apoptosis as evidenced by activation of caspase 3, caspase 8 and poly (ADP-ribose) polymerase-1. Co-treatment with ursodeoxycholyl lysophosphatidylethanolamide significantly inhibited these increases. Interestingly, ursodeoxycholyl lysophosphatidylethanolamide was able to increase expression of antiapoptotic cellular FLICE-inhibitory protein in both cell types. Ursodeoxycholyl lysophosphatidylethanolamide also prevented the decreases of myeloid cell leukemia sequence-1 protein in both experimental systems, and this protection was due to ursodeoxycholyl lysophosphatidylethanolamide's ability to inhibit ubiquitination-mediated degradation of myeloid cell leukemia sequence-1, and to increase the phosphorylation of GSK-3β. In addition, ursodeoxycholyl lysophosphatidylethanolamide was able to prevent the decreased expression of another antiapoptotic cellular inhibitor of apoptosis 2 in cobalt chloride-treated Mz-ChA-1 cells. Hence, ursodeoxycholyl lysophosphatidylethanolamide mediated cytoprotection against apoptosis during toxic bile-acid and ischemic stresses by a mechanism involving accumulation of cellular FLICE-inhibitory protein, myeloid cell leukemia sequence-1 and cellular inhibitor of apoptosis 2 proteins. Ursodeoxycholyl lysophosphatidylethanolamide may thus be used as an agent to prevent hepatic ischemia reperfusion.

APOLLON protein promotes early mitotic CYCLIN A degradation independent of the spindle assembly checkpoint

In the mammalian cell cycle, both CYCLIN A and CYCLIN B are required for entry into mitosis, and their elimination is also essential to complete the process. During mitosis, CYCLIN A and CYCLIN B are ubiquitylated by the anaphase-promoting complex/cyclosome (APC/C) and then subjected to proteasomal degradation. However, CYCLIN A, but not CYCLIN B, begins to be degraded in the prometaphase when APC/C is inactivated by the spindle assembly checkpoint (SAC). Here, we show that APOLLON (also known as BRUCE or BIRC6) plays a role in SAC-independent degradation of CYCLIN A in early mitosis. APPOLON interacts with CYCLIN A that is not associated with cyclin-dependent kinases. APPOLON also interacts with APC/C, and it facilitates CYCLIN A ubiquitylation. In APPOLON-deficient cells, mitotic degradation of CYCLIN A is delayed, and the total, but not the cyclin-dependent kinase-bound, CYCLIN A level was increased. We propose APPOLON to be a novel regulator of mitotic CYCLIN A degradation independent of SAC.

Targeting the Anti-Apoptotic Protein c-FLIP for Cancer Therapy

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor and critical anti-apoptotic regulator that inhibits tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis as well as chemotherapy-triggered apoptosis in malignant cells. c-FLIP is expressed as long (c-FLIP(L)), short (c-FLIP(S)), and c-FLIP(R) splice variants in human cells. c-FLIP binds to FADD and/or caspase-8 or -10 in a ligand-dependent and-independent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. Moreover, c-FLIP(L) and c-FLIP(S) are known to have multifunctional roles in various signaling pathways, as well as activating and/or upregulating several cytoprotective signaling molecules. Upregulation of c-FLIP has been found in various tumor types, and its downregulation has been shown to restore apoptosis triggered by cytokines and various chemotherapeutic agents. Hence, c-FLIP is an important target for cancer therapy. For example, small interfering RNAs (siRNAs) that specifically knockdown the expression of c-FLIP(L) in diverse human cancer cell lines augmented TRAIL-induced DISC recruitment and increased the efficacy of chemotherapeutic agents, thereby enhancing effector caspase stimulation and apoptosis. Moreover, small molecules causing degradation of c-FLIP as well as decreasing mRNA and protein levels of c-FLIP(L) and c-FLIP(S) splice variants have been found, and efforts are underway to develop other c-FLIP-targeted cancer therapies. This review focuses on (1) the functional role of c-FLIP splice variants in preventing apoptosis and inducing cytokine and drug resistance; (2) the molecular mechanisms that regulate c-FLIP expression; and (3) strategies to inhibit c-FLIP expression and function.

Allogeneic transplantation, Fas signaling, and dysregulation of hepcidin

Hepatic iron overload is common in patients undergoing hematopoietic cell transplantation. We showed previously in a murine model that transplantation of allogeneic T cells induced iron deposition and down-regulation of hepcidin (Hamp) in hepatocytes. We hypothesized that hepatic injury was related to disrupted iron homeostasis triggered by the interaction of Fas-ligand, expressed on activated T cells, with Fas on hepatocytes. In the current study, we determined the effects of modified expression of the Flice inhibitory protein (FLIP long [FLIPL]), which interferes with Fas signaling, on the impact of Fas-initiated signals on the expression of IL-6 and Stat3 and their downstream target, Hamp. To exclude a possible contribution by other pathways, we used agonistic anti-Fas antibodies (rather than allogeneic T cells) to trigger Fas signaling. Inhibition of FLIPL by RNA interference resulted, as expected, not only in enhanced hepatocyte apoptosis in response to Fas signals, but also in decreased levels of IL-6, Stat3, and Hamp. In contrast, overexpression of FLIPL protected hepatocytes against agonistic anti-Fas antibody-mediated apoptosis and increased the levels of IL-6 and Stat3, thereby maintaining the expression of Hamp in an NF-κB-dependent fashion. In vivo overexpression of FLIPL in the liver via hydrodynamic transfection, similarly, interfered with Fas-initiated apoptosis and prevented down-regulation of IL-6, Stat3, and Hamp. These data indicate that Fas-dependent signals alter the regulation of iron homeostasis and suggest that signals initiated by Fas may contribute to peritransplantation iron accumulation.

The ubiquitin-proteasome system plays an important role during various stages of the coronavirus infection cycle

The ubiquitin-proteasome system (UPS) is a key player in regulating the intracellular sorting and degradation of proteins. In this study we investigated the role of the UPS in different steps of the coronavirus (CoV) infection cycle. Inhibition of the proteasome by different chemical compounds (i.e., MG132, epoxomicin, and Velcade) appeared to not only impair entry but also RNA synthesis and subsequent protein expression of different CoVs (i.e., mouse hepatitis virus [MHV], feline infectious peritonitis virus, and severe acute respiratory syndrome CoV). MHV assembly and release were, however, not appreciably affected by these compounds. The inhibitory effect on CoV protein expression did not appear to result from a general inhibition of translation due to induction of a cellular stress response by the inhibitors. Stress-induced phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha) generally results in impaired initiation of protein synthesis, but the sensitivity of MHV infection to proteasome inhibitors was unchanged in cells lacking a phosphorylatable eIF2alpha. MHV infection was affected not only by inhibition of the proteasome but also by interfering with protein ubiquitination. Viral protein expression was reduced in cells expressing a temperature-sensitive ubiquitin-activating enzyme E1 at the restrictive temperature, as well as in cells in which ubiquitin was depleted by using small interfering RNAs. Under these conditions, the susceptibility of the cells to virus infection was, however, not affected, excluding an important role of ubiquitination in virus entry. Our observations reveal an important role of the UPS in multiple steps of the CoV infection cycle and identify the UPS as a potential drug target to modulate the impact of CoV infection.

Modulation of Wnt signaling by the nuclear localization of cellular FLIP-L

Cellular FLIP (cFLIP) inhibits the apoptosis signaling initiated by death receptor ligation. We previously reported that a long form of cFLIP (cFLIP-L) enhances Wnt signaling via inhibition of beta-catenin ubiquitylation. In this report, we present evidence that cFLIP-L translocates into the nucleus, which could have a role in modulation of Wnt signaling. cFLIP-L has a functional bipartite nuclear localization signal (NLS) at the C-terminus. Wild-type cFLIP-L (wt-FLIP-L) localizes in both the nucleus and cytoplasm, whereas NLS-mutated cFLIP-L localizes predominantly in the cytoplasm. cFLIP-L also has a nuclear export signal (NES) near the NLS, and leptomycin B, an inhibitor of CRM1-dependent nuclear export, increases the nuclear accumulation of cFLIP-L, suggesting that it shuttles between the nucleus and cytoplasm. Expression of mutant cFLIP-L proteins with a deletion or mutations in the NLS and NES confers resistance to Fas-mediated apoptosis, as does wt-FLIP-L, but they do not enhance Wnt signaling, which suggests an important role of the C-terminus of cFLIP-L in Wnt-signaling modulation. When wt-FLIP-L is expressed in the cytoplasm by conjugation with exogenous NES (NES-FLIP-L), Wnt signaling is not enhanced, whereas the NES-FLIP-L increases cytoplasmic beta-catenin as efficiently as wt-FLIP-L. cFLIP-L physically interacts with the reporter plasmid for Wnt signaling, but not with the control plasmid. These results suggest a role for nuclear cFLIP-L in the modulation of Wnt signaling.

Herpes simplex virus type 1 (HSV-1)-induced apoptosis in human dendritic cells as a result of downregulation of cellular FLICE-inhibitory protein and reduced expression of HSV-1 antiapoptotic latency-associated transcript sequences

Herpes simplex virus type 1 (HSV-1) is one of the most frequent and successful human pathogens. It targets immature dendritic cells (iDCs) to interfere with the antiviral immune response. The mechanisms underlying apoptosis of HSV-1-infected iDCs are not fully understood. Previously, we have shown that HSV-1-induced apoptosis of iDCs is associated with downregulation of the cellular FLICE-inhibitory protein (c-FLIP), a potent inhibitor of caspase-8-mediated apoptosis. In this study, we prove that HSV-1 induces degradation of c-FLIP in a proteasome-independent manner. In addition, by using c-FLIP-specific small interfering RNA (siRNA) we show for the first time that downregulation of c-FLIP expression is sufficient to drive uninfected iDCs into apoptosis, underlining the importance of this molecule for iDC survival. Surprisingly, we also observed virus-induced c-FLIP downregulation in epithelial cells and many other cell types that do not undergo apoptosis after HSV-1 infection. Microarray analyses revealed that HSV-1-encoded latency-associated transcript (LAT) sequences, which can substitute for c-FLIP as an inhibitor of caspase-8-mediated apoptosis, are much less abundant in iDCs as compared to epithelial cells. Finally, iDCs infected with an HSV-1 LAT knockout mutant showed increased apoptosis when compared to iDCs infected with the corresponding wild-type HSV-1. Taken together, our results demonstrate that apoptosis of HSV-1-infected iDCs requires both c-FLIP downregulation and diminished expression of viral LAT.

Expression and biological significance of c-FLIP in human hepatocellular carcinomas

BACKGROUND

c-FLIP can be considered as a tumor-progression factor in regard to its anti-apoptotic functions. In the present study, we intended to investigate the expression of c-FLIP in human HCC tissues, and its relation with drug-induced cell apoptosis through the specific inhibition of c-FLIP expression by siRNA in 7721 cells.

METHODS

c-FLIP expression was quantified immunohistochemically in HCC tissues(eighty-six cases), and corresponding noncancerous tissues (fifty-seven cases). Patients with HCC were followed up for cancer recurrence. Then, the c-FLIP gene was silenced with specific siRNA in 7721 HCC cells. c-FLIP expression was detected by RT-PCR, Western Blot and immunocytochemical staining. The cellular viability and cell apoptosis were assayed in vitro with cells treated with doxorubicin.

RESULTS

Positive immunostaining was detected for c-FLIP in 83.72% (72/86) human HCC tissues, 14.81% (4/27) hepatic cirrhosis, 11.11% (2/18) hepatic hemangioma tissues, and absent in normal hepatic tissues. The overexpression(more than 50%) of c-FLIP in HCC adversely affected the recurrence-free survival. Through c-FLIP gene silencing with siRNA, the expressions of c-FLIP mRNA and protein were remarkably down-regulated in 7721 HCC cells. And doxorubicin showed apparent inhibition on cell proliferations, and induced more apoptosis.

CONCLUSION

These results indicate that c-FLIP is frequently expressed in human HCCs, and its overexpression implied a lesser probability of recurrence-free survival. The specific silencing of c-FLIP gene can apparently up-regulate drug-induced HCC cell apoptosis, and may have therapeutic potential for the treatment of human HCC.

Identifying fragilities in biochemical networks: robust performance analysis of Fas signaling-induced apoptosis

Proper control of apoptotic signaling is critical to immune response and development in multicellular organisms. Two tools from control engineering are applied to a mathematical model of Fas ligand signaling-induced apoptosis. Structured singular value analysis determines the volume in parameter space within which the system parameters may exist and still maintain efficacious signaling, but is limited to linear behaviors. Sensitivity analysis can be applied to nonlinear systems but is difficult to relate to performance criteria. Thus, structured singular value analysis is used to quantify performance during apoptosis rejection, ensuring that the system remains sensitive but not overly so to apoptotic stimuli. Sensitivity analysis is applied when the system has switched to the death-inducing, apoptotic steady state to determine parameters significant to maintaining the bistability. The analyses reveal that the magnitude of the death signal is fragile to perturbations in degradation parameters (failures in the ubiquitin/proteasome mechanism) while the timing of signal expression can be tuned by manipulating local parameters. Simultaneous parameter uncertainty highlights apoptotic fragility to disturbances in the ubiquitin/proteasome system. Sensitivity analysis reveals that the robust signaling characteristics of the apoptotic network is due to network architecture, and the apoptotic signaling threshold is best manipulated by interactions upstream of the apoptosome.

The long isoform of cellular FLIP is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway

Although the long isoform of cellular FLIP (c-FLIP(L)) has been implicated in TCR-mediated signaling, its role in T cell proliferation remains controversial. Some studies have demonstrated that overexpression of c-FLIP(L) promotes T cell proliferation and NF-kappaB activation, whereas others have reported that c-FLIP(L) overexpression has no effect or even inhibits T cell proliferation. To establish the role of c-FLIP(L) in T lymphocyte proliferation, we have generated a conditional knockout mouse strain specifically lacking c-FLIP(L) in T lymphocytes. c-FLIP(L)(-/-) mice exhibit severely impaired effector T cell development after Listeria monocytogenes infection in vivo and c-FLIP(L)-deficient T cells display defective TCR-mediated proliferation in vitro. However, c-FLIP(L)(-/-) T cells exhibit normal NF-kappaB activity upon TCR stimulation. These results demonstrate that c-FLIP(L) is essential for T lymphocyte proliferation through an NF-kappaB-independent pathway.

Cellular FLICE-like inhibitory protein (C-FLIP): a novel target for cancer therapy

Cellular FLICE-like inhibitory protein (c-FLIP) has been identified as a protease-dead, procaspase-8-like regulator of death ligand-induced apoptosis, based on observations that c-FLIP impedes tumor necrosis factor-alpha (TNF-alpha), Fas-L, and TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis by binding to FADD and/or caspase-8 or -10 in a ligand-dependent fashion, which in turn prevents death-inducing signaling complex (DISC) formation and subsequent activation of the caspase cascade. c-FLIP is a family of alternatively spliced variants, and primarily exists as long (c-FLIP(L)) and short (c-FLIP(S)) splice variants in human cells. Although c-FLIP has apoptogenic activity in some cell contexts, which is currently attributed to heterodimerization with caspase-8 at the DISC, accumulating evidence indicates an anti-apoptotic role for c-FLIP in various types of human cancers. For example, small interfering RNAs (siRNAs) that specifically knocked down expression of c-FLIP(L) in diverse human cancer cell lines, e.g., lung and cervical cancer cells, augmented TRAIL-induced DISC recruitment, and thereby enhanced effector caspase stimulation and apoptosis. Therefore, the outlook for the therapeutic index of c-FLIP-targeted drugs appears excellent, not only from the efficacy observed in experimental models of cancer therapy, but also because the current understanding of dual c-FLIP action in normal tissues supports the notion that c-FLIP-targeted cancer therapy will be well tolerated. Interestingly, Taxol, TRAIL, as well as several classes of small molecules induce c-FLIP downregulation in neoplastic cells. Efforts are underway to develop small-molecule drugs that induce c-FLIP downregulation and other c-FLIP-targeted cancer therapies. In this review, we assess the outlook for improving cancer therapy through c-FLIP-targeted therapeutics.

Ubiquitin-proteasome system as a modulator of cell fate

The ubiquitin-proteasome system is the major non-lysosymal system for degrading proteins in the cell; the work leading to its discovery was awarded the Nobel Prize in Chemistry in 2004. In addition to small ubiquitin-like modifiers (e.g. Sumo and Nedd8), ubiquitin is involved in the complex regulation of the levels and function of many proteins and signaling pathways involved in determining cell fate. The cell death regulatory proteins, such as Bcl-2 family proteins and caspases are targeted for degradation by the ubiquitin proteasome system (UPS). In addition to mediating the degradation of proteins, the UPS regulates function and translocation of proteins, many of which play a role in the determination of cell fate. For example the UPS can regulate the activity of transcription factors, such as P53, NF-kappaB and HIF-1 alpha, which control the expression of protein mediators of cell death. Aberrant UPS function has been reported in multiple neuropathologies including Parkinson's diseases and ischemia. With the number of ubiquitin conjugating and de-conjugating enzymes reaching close to the levels of protein kinases and phosphatases, it is clear that ubiquitination is an important biological regulatory step for proteins.