Cellular FLICE-like inhibitory protein (c-FLIP): a novel target for Taxol-induced apoptosis

Cancer stem cells: advances in knowledge and implications for cancer therapy

Cancer stem cells (CSCs), a small subset of cells in tumors that are characterized by self-renewal and continuous proliferation, lead to tumorigenesis, metastasis, and maintain tumor heterogeneity. Cancer continues to be a significant global disease burden. In the past, surgery, radiotherapy, and chemotherapy were the main cancer treatments. The technology of cancer treatments continues to develop and advance, and the emergence of targeted therapy, and immunotherapy provides more options for patients to a certain extent. However, the limitations of efficacy and treatment resistance are still inevitable. Our review begins with a brief introduction of the historical discoveries, original hypotheses, and pathways that regulate CSCs, such as WNT/β-Catenin, hedgehog, Notch, NF-κB, JAK/STAT, TGF-β, PI3K/AKT, PPAR pathway, and their crosstalk. We focus on the role of CSCs in various therapeutic outcomes and resistance, including how the treatments affect the content of CSCs and the alteration of related molecules, CSCs-mediated therapeutic resistance, and the clinical value of targeting CSCs in patients with refractory, progressed or advanced tumors. In summary, CSCs affect therapeutic efficacy, and the treatment method of targeting CSCs is still difficult to determine. Clarifying regulatory mechanisms and targeting biomarkers of CSCs is currently the mainstream idea.

Mechanisms of Drug Resistance in Ovarian Cancer and Associated Gene Targets

In the United States, over 100,000 women are diagnosed with a gynecologic malignancy every year, with ovarian cancer being the most lethal. One of the hallmark characteristics of ovarian cancer is the development of resistance to chemotherapeutics. While the exact mechanisms of chemoresistance are poorly understood, it is known that changes at the cellular and molecular level make chemoresistance challenging to treat. Improved therapeutic options are needed to target these changes at the molecular level. Using a precision medicine approach, such as gene therapy, genes can be specifically exploited to resensitize tumors to therapeutics. This review highlights traditional and novel gene targets that can be used to develop new and improved targeted therapies, from drug efflux proteins to ovarian cancer stem cells. The review also addresses the clinical relevance and landscape of the discussed gene targets.

Cell-penetrating peptide-mediated delivery of therapeutic peptides/proteins to manage the diseases involving oxidative stress, inflammatory response and apoptosis

OBJECTIVES

Peptides and proteins represent great potential for modulating various cellular processes including oxidative stress, inflammatory response, apoptosis and consequently the treatment of related diseases. However, their therapeutic effects are limited by their inability to cross cellular barriers. Cell-penetrating peptides (CPPs), which can transport cargoes into the cell, could resolve this issue, as would be discussed in this review.

KEY FINDINGS

CPPs have been successfully exploited in vitro and in vivo for peptide/protein delivery to treat a wide range of diseases involving oxidative stress, inflammatory processes and apoptosis. Their in vivo applications are still limited due to some fundamental issues of CPPs, including nonspecificity, proteolytic instability, potential toxicity and immunogenicity.

SUMMARY

Totally, CPPs could potentially help to manage the diseases involving oxidative stress, inflammatory response and apoptosis by delivering peptides/proteins that could selectively reach proper intracellular targets. More studies to overcome related CPP limitations and confirm the efficacy and safety of this strategy are needed before their clinical usage.

Loss of the MAF Transcription Factor in Laryngeal Squamous Cell Carcinoma

MAF is a transcription factor that may act either as a tumor suppressor or as an oncogene, depending on cell type. We have shown previously that the overexpressed miR-1290 influences MAF protein levels in LSCC (laryngeal squamous cell carcinoma) cell lines. In this study, we shed further light on the interaction between miR-1290 and MAF, as well as on cellular MAF protein localization in LSCC. We confirmed the direct interaction between miR-1290 and MAF 3′UTR by a dual-luciferase reporter assay. In addition, we used immunohistochemistry staining to analyze MAF protein distribution and observed loss of MAF nuclear expression in 58% LSCC samples, of which 10% showed complete absence of MAF, compared to nuclear and cytoplasmatic expression in 100% normal mucosa. Using TCGA data, bisulfite pyrosequencing and CNV analysis, we excluded the possibility that loss-of-function mutations, promoter region DNA methylation or CNV are responsible for MAF loss in LSCC. Finally, we identified genes involved in the regulation of apoptosis harboring the MAF binding motif in their promoter region by applied FIMO and DAVID GO analysis. Our results highlight the role of miR-1290 in suppressing MAF expression in LSCC. Furthermore, MAF loss or mislocalization in FFPE LSCC tumor samples might suggest that MAF acts as a LSCC tumor suppressor by regulating apoptosis.

YM155 sensitizes HeLa cells to TRAIL-mediated apoptosis via cFLIP and survivin downregulation

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-mediated apoptosis is a safe method for the treatment of various types of cancer. However, TRAIL therapy is less effective in certain types of cancer, including cervical cancer. To address this problem, a combinatorial approach was employed to sensitize cervical cancer at low dosages. YM155, a survivin inhibitor, was used at low dosages along with TRAIL to induce apoptosis in HeLa cells. The effects of the individual treatment with TRAIL and YM155 on apoptosis were assessed by propidium iodide assay. In addition, to validate the DNA damage exhibited by the combination treatment, the phosphorylation status of γH2A histone family member X was investigated by immunofluorescence and western blot analysis. TRAIL or YM155 alone had no significant effect on DNA damage and apoptosis. However, the TRAIL/YM155 combination triggered a synergistic pro-apoptotic stimulus in HeLa cells. The mRNA and protein levels of CASP8- and FADD-like apoptosis regulator (cFLIP), death receptor 5 (DR5) and survivin were monitored using RT-PCR and western blot analysis, respectively. This combinatorial approach downregulated both mRNA and protein expression levels of cFLIP and survivin. Further experimental results suggested that the combination treatment significantly reduced cell viability, invasion and migration of HeLa cells. Overall, the present findings indicated that the low dosage of YM155 sensitized HeLa cells to TRAIL-induced apoptosis via a mechanism involving downregulation of cFLIP and survivin. The results indicated the importance of combination drug treatment and reveal an effective therapeutic alternative for TRAIL therapy in human cervical cancer.

Expression of DR5 and c‑FLIP proteins as novel prognostic biomarkers for non‑small cell lung cancer patients treated with surgical resection and chemotherapy

TRAIL-R2 (DR5), one of the death receptors, can activate the extrinsic apoptosis pathway, while cellular FLICE-inhibitory protein (c-FLIP) can inhibit this pathway. Both of them play important roles in the occurrence and development of most tumors. To date, there is no relevant report concerning the relationship between expression of DR5 and c-FLIP protein and clinicopathological/prognostic implications in patients with non-small cell lung cancer (NSCLC) treated with surgical resection and chemotherapy. Thus, the aim of the present study was to investigate the potential prognostic significance of DR5 and c-FLIP in NSCLC patients and their predictive roles in the chemotherapeutic response. In the present study, DR5 and c-FLIP were detected by immunohistochemistry (IHC) in tissue microarrays of NSCLC. The results showed that the expression levels of DR5 and c-FLIP were significantly higher in lung squamous cell carcinoma (SCC) and lung adenocarcinoma (ADC) tissues compared with levels noted in the non-cancerous control lung tissues (all P<0.05). In addition, DR5 expression was significantly increased in lung ADC (P<0.001), whereas, c-FLIP was higher in lung SCC (P<0.001) and smoker patients with clinical stage III (P=0.019, P=0.016, respectively). In addition, NSCLC patients with overexpression of DR5 and loss of c-FLIP expression exhibited a higher overall survival (OS) rate as determined by Kaplan-Meier analysis (P=0.029, P=0.038, respectively). Multivariate analysis confirmed that high expression of DR5 and loss of c-FLIP expression were independent favorable prognostic factors for NSCLC patients (P=0.016, P=0.035, respectively). In conclusion, overexpression of DR5 and loss of c-FLIP expression may serve as novel favorable prognostic biomarkers for NSCLC patients treated with chemotherapy after radical resection and used as predictors for tumor response to chemotherapy drugs.

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.

Release of c-FLIP brake selectively sensitizes human cancer cells to TLR3-mediated apoptosis

Toll-like receptor 3 (TLR3) mediates innate immune responses by sensing viral dsRNA, but also induces apoptosis selectively in cancer cells. Our analysis by immunohistochemistry revealed that TLR3 is frequently overexpressed in 130 non-small cell lung cancer (NSCLC) patients' samples compared with normal bronchial epithelium (P < 0.0001, Mann-Whitney test), supporting the therapeutic potential of TLR3 ligand for this type of cancer. However, a proportion of TLR3-expressing cancer cell lines, including NSCLC, remain resistant to TLR3-mediated apoptosis, and the underlying mechanism of resistance remains unclear. We here investigated the molecular basis conferring resistance to non-transformed vs. transformed cells against TLR3-mediated cell death. In non-transformed epithelial cells cellular FLICE-like inhibitory protein (c-FLIP) and cellular Inhibitor of APoptosis (cIAPs) ubiquitin ligases exerted an efficient double brake on apoptosis signaling. In contrast, releasing only one of these two brakes was sufficient to overcome the resistance of 8/8 cancer cell lines tested. Remarkably, the release of the c-FLIP, but not cIAPs, brake only results in the sensitization of all human cancer cells to TLR3-mediated apoptosis. Taking advantage of the difference between transformed and non-transformed cells, we developed a rational strategy by combining the chemotherapeutic agent paclitaxel, which decreases c-FLIP expression, with TLR3 ligand. This combination was highly synergistic for triggering apoptosis in cancer cells but not in non-transformed cells. In vivo, the combination of paclitaxel with dsRNA delayed tumor growth and prolonged survival in a mouse xenograft lung tumor model. In conclusion, combining the release of the c-FLIP brake with TLR3 ligand synergizes to selectively kill cancer cells, and could represent an efficient and safe therapy against TLR3-expressing cancers such as NSCLC.

Mesenchymal Stem Cell Expressing TRAIL as Targeted Therapy against Sensitised Tumour

Tapping into the ability of engineered mesenchymal stem cells (MSCs) to mobilise into the tumour has expanded the scope of cancer treatment. Engineered MSCs expressing tumour necrosis factor (TNF)-related apoptosis inducing ligand (MSC-TRAIL) could serve as a platform for an efficient and targeted form of therapy. However, the presence of cancer stem cells (CSCs) that are resistant to TRAIL and apoptosis may represent a challenge for effective treatment. Nonetheless, with the discovery of small molecular inhibitors that could target CSCs and tumour signalling pathways, a higher efficacy of MSC-TRAIL mediated tumour inhibition can be achieved. This might pave the way for a more effective form of combined therapy, which leads to a better treatment outcome. In this review, we first discuss the tumour-homing capacity of MSCs, its effect in tumour tropism, the different approach behind genetically-engineered MSCs, and the efficacy and safety of each agent delivered by these MSCs. Then, we focus on how sensitisation of CSCs and tumours using small molecular inhibitors can increase the effect of these cells to either TRAIL or MSC-TRAIL mediated inhibition. In the conclusion, we address a few questions and safety concerns regarding the utilization of engineered MSCs for future treatment in patients.

Polyamino Acid Layer-by-Layer (LbL) Constructed Silica-Supported Mesoporous Titania Nanocarriers for Stimuli-Responsive Delivery of microRNA 708 and Paclitaxel for Combined Chemotherapy

Cellular Fas-associated protein with death domain-like interleukin-1β-converting enzyme-inhibitory protein (c-FLIP), often strongly expressed in numerous cancers, plays a pivotal role in thwarting apoptosis and inducing chemotherapy resistance in cancer. An integrated approach combining chemotherapy with suppression of c-FLIP levels could prove paramount in the treatment of cancers with c-FLIP overexpression. In this study, we utilized a polymeric layer-by-layer (LbL) assembly of silica-supported mesoporous titania nanoparticles (MTNst) to co-deliver paclitaxel (PTX) and microRNA 708 (miR708) for simultaneous chemotherapy and c-FLIP suppression in colorectal carcinoma. The resulting LbL miR708/PTX-MTNst showed dose-dependent cytotoxicity in HCT-116 and DLD-1 colorectal carcinoma cell lines, which was remarkably superior to that of free PTX or LbL PTX-MTNst. LbL miR708/PTX-MTNst strongly inhibited c-FLIP expression and resulted in increased expression of proapoptotic proteins. In DLD-1 xenograft tumor-bearing mice, the nanoparticles accumulated in the tumor, resulting in remarkable tumor regression, with the PTX and miR708-loaded nanoparticles showing significantly greater inhibitory effects than the free PTX or PTX-loaded nanoparticles. Immunohistochemical analyses of the tumors further confirmed the remarkable apoptotic and antiproliferative effects of the nanoparticles, whereas organ histology reinforced the biocompatibility of the system. Therefore, the LbL miR708/PTX-MTNst system, owing to its ability to deliver both chemotherapeutic drug and inhibitory miRNA to the tumor site, shows great potential to treat colorectal carcinoma in clinical settings.

Fungal 7-epi-10-deacetyltaxol produced by an endophytic Pestalotiopsis microspora induces apoptosis in human hepatocellular carcinoma cell line (HepG2)

BACKGROUND

Paclitaxel (taxol) is a potent anticancer drug that is used in the treatment of a wide variety of cancerous. In the present study, we identified a taxol derivative named 7-epi-10-deacetyltaxol (EDT) from the culture of an endophytic fungus Pestalotiopsis microspora isolated from the bark of Taxodium mucronatum. This study was carried out to investigate the effects of fungal EDT on cell proliferation, the induction of apoptosis and the molecular mechanisms of apoptosis in human hepatoma HepG2 cells in vitro.

METHODS

The endophytic fungus was identified by traditional and molecular taxonomical characterization and the fungal EDT was purified using column chromatography and confirmed by various spectroscopic and chromatographic comparisons with authentic paclitaxel. We studied the in vitro effects of EDT on HepG2 cells for parameters such as cell cycle distribution, DNA fragmentation, reactive oxygen species (ROS) generation and nuclear morphology. Further, western blot analysis was used to evaluate Bcl-2-associated X protein (Bax), B-cell lymphoma 2 (Bcl-2), p38-mitogen activated protein kinase (MAPK) and poly [ADP-ribose] polymerase (PARP) expression.

RESULTS

We demonstrate that the fungal EDT exhibited significant in vitro cytotoxicity in HepG2 cells. We investigated cytotoxicity mechanism of EDT in HepG2 cells. The results showed nuclear condensation and DNA fragmentation were observed in cells treated with fungal EDT. Besides, the fungal EDT arrested HepG2 cells at G2/M phase of cell cycle. Furthermore, fungal EDT induced apoptosis in HepG2 cells in a dose-dependent manner associated with ROS generation and increased Bax/Bcl-2 ratio, p38 MAPKs and PARP cleavage.

CONCLUSIONS

Our data show that EDT induced apoptotic cell death in HepG2 cells occurs through intrinsic pathway by generation of ROS mediated and activation of MAPK pathway. This is the first report for 7-epi-10-deacetyltaxol (EDT) isolated from a microbial source.

Resistance to Cell Death and Its Modulation in Cancer Stem Cells

Accumulating evidence has demonstrated that human cancers arise from various tissues of origin that initiate from cancer stem cells (CSCs) or cancer-initiating cells. The extrinsic and intrinsic apoptotic pathways are dysregulated in CSCs, and these cells play crucial roles in tumor initiation, progression, cell death resistance, chemo- and radiotherapy resistance, and tumor recurrence. Understanding CSC-specific signaling proteins and pathways is necessary to identify specific therapeutic targets that may lead to the development of more efficient therapies selectively targeting CSCs. Several signaling pathways-including the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR), maternal embryonic leucine zipper kinase (MELK), NOTCH1, and Wnt/Β-catenin&and expression of the CSC markers CD133, CD24, CD44, Oct4, Sox2, Nanog, and ALDH1A1 maintain CSC properties. Studying such pathways may help to understand CSC biology and lead to the development of potential therapeutic interventions to render CSCs more sensitive to cell death triggered by chemotherapy and radiation therapy. Moreover, recent demonstrations of dedifferentiation of differentiated cancer cells into CSC-like cells have created significant complexity in the CSCs hypothesis. Therefore, any successful therapeutic agent or combination of drugs for cancer therapy must eliminate not only CSCs but differentiated cancer cells and the entire bulk of tumor cells. This review article expands on the CSC hypothesis and paradigm with respect to major signaling pathways and effectors that regulate CSC apoptosis resistance. Moreover, selective CSC apoptotic modulators and their therapeutic potential for making tumors more responsive to therapy are discussed. The use of novel therapies, including small-molecule inhibitors of specific proteins in signaling pathways that regulate stemness, proliferation and migration of CSCs, immunotherapy, and noncoding microRNAs may provide better means of treating CSCs.

LCL161 increases paclitaxel-induced apoptosis by degrading cIAP1 and cIAP2 in NSCLC

BACKGROUND

LCL161, a novel Smac mimetic, is known to have anti-tumor activity and improve chemosensitivity in various cancers. However, the function and mechanisms of the combination of LCL161 and paclitaxel in non-small cell lung cancer (NSCLC) remain unknown.

METHODS

Cellular inhibitor of apoptotic protein 1 and 2 (cIAP1&2) expression in NSCLC tissues and adjacent non-tumor tissues were assessed by immunohistochemistry. The correlations between cIAP1&2 expression and clinicopathological characteristics, prognosis were analyzed. Cell viability and apoptosis were measured by MTT assays and Flow cytometry. Western blot and co-immunoprecipitation assay were performed to measure the protein expression and interaction in NF-kB pathway. siRNA-mediated gene silencing and caspases activity assays were applied to demonstrate the role and mechanisms of cIAP1&2 and RIP1 in lung cancer cell apoptosis. Mouse xenograft NSCLC models were used in vivo to determine the therapeutic efficacy of LCL161 alone or in combination with paclitaxel.

RESULTS

The expression of cIAP1 and cIAP2 in Non-small cell lung cancer (NSCLC) tumors was significantly higher than that in adjacent normal tissues. cIAP1 was highly expressed in patients with late TNM stage NSCLC and a poor prognosis. Positivity for both cIAP1 and cIAP2 was an independent prognostic factor that indicated a poorer prognosis in NSCLC patients. LCL161, an IAP inhibitor, cooperated with paclitaxel to reduce cell viability and induce apoptosis in NSCLC cells. Molecular studies revealed that paclitaxel increased TNFα expression, thereby leading to the recruitment of various factors and the formation of the TRADD-TRAF2-RIP1-cIAP complex. LCL161 degraded cIAP1&2 and released RIP1 from the complex. Subsequently, RIP1 was stabilized and bound to caspase-8 and FADD, thereby forming the caspase-8/RIP1/FADD complex, which activated caspase-8, caspase-3 and ultimately lead to apoptosis. In nude mouse xenograft experiments, the combination of LCL161 and paclitaxel degraded cIAP1,2, activated caspase-3 and inhibited tumor growth with few toxic effects.

CONCLUSION

Thus, LCL161 could be a useful agent for the treatment of NSCLC in combination with paclitaxel.

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.

Inhibition of cancer cell proliferation and apoptosis-inducing activity of fungal taxol and its precursor baccatin III purified from endophytic Fusarium solani

Background

Taxol (generic name paclitaxel), a plant-derived antineoplastic agent, used widely against breast, ovarian and lung cancer, was originally isolated from the bark of the Pacific yew, Taxus brevifolia. The limited supply of the drug has prompted efforts to find alternative sources, such as chemical synthesis, tissue and cell cultures of the Taxus species both of which are expensive and yield low levels. Fermentation processes with microorganisms would be the methods of choice to lower the costs and increase yields. Previously we have reported that F. solani isolated from T. celebica produced taxol and its precursor baccatin III in liquid grown cultures J Biosci 33:259-67, 2008. This study was performed to evaluate the inhibition of proliferation and induction of apoptosis of cancer cell lines by the fungal taxol and fungal baccatin III of F. solani isolated from T. celebica.

Methods

Cell lines such as HeLa, HepG2, Jurkat, Ovcar3 and T47D were cultured individually and treated with fungal taxol, baccatin III with or without caspase inhibitors according to experimental requirements. Their efficacy on apoptotic induction was examined.

Results

Both fungal taxol and baccatin III inhibited cell proliferation of a number of cancer cell lines with IC₅₀ ranging from 0.005 to 0.2 μM for fungal taxol and 2 to 5 μM for fungal baccatin III. They also induced apoptosis in JR4-Jurkat cells with a possible involvement of anti-apoptotic Bcl2 and loss in mitochondrial membrane potential, and was unaffected by inhibitors of caspase-9,-2 or -3 but was prevented in presence of caspase-10 inhibitor. DNA fragmentation was also observed in cells treated with fungal taxol and baccatin III.

Conclusions

The cytotoxic activity exhibited by fungal taxol and baccatin III involves the same mechanism, dependent on caspase-10 and membrane potential loss of mitochondria, with taxol having far greater cytotoxic potential.

Kaurene diterpene induces apoptosis in U87 human malignant glioblastoma cells by suppression of anti-apoptotic signals and activation of cysteine proteases

Gliomas are the most common and malignant primary brain tumors in humans. Studies have shown that classes of kaurene diterpene have anti-tumor activity related to their ability to induce apoptosis. We investigated the response of the human glioblastoma cell line U87 to treatment with ent-kaur-16-en-19-oic acid (kaurenoic acid, KA). We analyzed cell survival and the induction of apoptosis using flow cytometry and annexin V staining. Additionally, the expression of anti-apoptotic (c-FLIP and miR-21) and apoptotic (Fas, caspase-3 and caspase-8) genes was analyzed by relative quantification (real-time PCR) of mRNA levels in U87 cells that were either untreated or treated with KA (30, 50, or 70 µM) for 24, 48, and 72 h. U87 cells treated with KA demonstrated reduced viability, and an increase in annexin V- and annexin V/PI-positive cells was observed. The percentage of apoptotic cells was 9% for control cells, 26% for cells submitted to 48 h of treatment with 50 µM KA, and 31% for cells submitted to 48 h of treatment with 70 µM KA. Similarly, in U87 cells treated with KA for 48 h, we observed an increase in the expression of apoptotic genes (caspase-8, -3) and a decrease in the expression of anti-apoptotic genes (miR-21 and c-FLIP). KA possesses several interesting properties and induces apoptosis through a unique mechanism. Further experiments will be necessary to determine if KA may be used as a lead compound for the development of new chemotherapeutic drugs for the treatment of primary brain tumors.

Stool-fermented Plantago ovata husk induces apoptosis in colorectal cancer cells independently of molecular phenotype

Several studies have suggested that the partially fermentable fibre Plantago ovata husk (PO) may have a protective effect on colorectal cancer (CRC). We studied the potentially pro-apoptotic effect of PO and the implicated mechanisms in CRC cells with different molecular phenotypes (Caco-2, HCT116, LoVo, HT-29, SW480) after PO anaerobic fermentation with colonic bacteria as it occurs in the human colon. The fermentation products of PO induced apoptosis in all primary tumour and metastatic cell lines, independent of p53, adenomatous polyposis coli, β-catenin or cyclo-oxygenase-2 status. Apoptosis was caspase-dependent and both intrinsic and extrinsic pathways were implicated. The intrinsic pathway was activated through a shift in the balance towards a pro-apoptotic environment with an up-regulation of B-cell lymphoma protein 2 homologous antagonist killer (BAK) and a down-regulation of B-cell lymphoma-extra large (Bcl-xL) seen in HCT116 and LoVo cells. This resulted in mitochondrial membrane depolarisation, increased expression of caspase activators second mitochondria-derived activator of caspases (Smac)/Diablo, death effector apoptosis-inducing factor, apoptosome member apoptotic protease activating factor 1 and down-regulation of inhibitors of apoptosis Survivin and X-linked inhibitor of apoptosis in most cells. The extrinsic pathway was activated presumably through the up-regulation of death receptor (DR5). Some important differences were seen between primary tumour and metastatic CRC cells. Thus, metastatic PO-treated LoVo cells had a remarkable up-regulation of TNF-α ligand along with death-inducing signalling complex components receptor interacting protein and TNF-α receptor 1-associated death domain protein. The extrinsic pathway modulator FCICE-inhibitory protein (FLIP), an inhibitor of both spontaneous death ligand-independent and death receptor-mediated apoptosis, was significantly down-regulated after PO treatment in all primary tumour cells, but not in metastatic LoVo. These findings suggest that PO could potentially be a useful chemotherapy adjuvant.

β-Elemene piperazine derivatives induce apoptosis in human leukemia cells through downregulation of c-FLIP and generation of ROS

β-Elemene is an active component of the herb medicine Curcuma Wenyujin with reported antitumor activity. To improve its antitumor ability, five novel piperazine derivatives of β-elemene, 13-(3-methyl-1-piperazinyl)-β-elemene (DX1), 13-(cis-3,5-dimethyl-1-piperazinyl)-β-elemene (DX2), 13-(4-ethyl-1-piperazinyl)-β-elemene (DX3), 13-(4-isopropyl-1-piperazinyl)-β-elemene (DX4) and 13-piperazinyl-β-elemene (DX5), were synthesized. The antiproliferative and apoptotic effects of these derivatives were determined in human leukemia HL-60, NB4, K562 and HP100-1 cells. DX1, DX2 and DX5, which contain a secondary amino moiety, were more active in inhibiting cell growth and in inducing apoptosis than DX3 and DX4. The apoptosis induction ability of DX1 was associated with the generation of hydrogen peroxide (H₂O₂), a decrease of mitochondrial membrane potential (MMP), and the activation of caspase-8. Pretreatment with the antioxidants N-acetylcysteine and catalase completely blocked DX1-induced H₂O₂ production, but only partially its activation of caspase-8 and induction of apoptosis. HL-60 cells were more sensitive than its H₂O₂-resistant subclone HP100-1 cells to DX1-induced apoptosis. The activation of caspase-8 by these compounds was correlated with the decrease in the levels of cellular FLICE-inhibitory protein (c-FLIP). The proteasome inhibitor MG-132 augmented the decrease in c-FLIP levels and apoptosis induced by these derivatives. FADD- and caspase-8-deficient Jurkat subclones have a decreased response to DX1-induced apoptosis. Our data indicate that these novel β-elemene piperazine derivatives induce apoptosis through the decrease in c-FLIP levels and the production of H₂O₂ which leads to activation of both death receptor- and mitochondrial-mediated apoptotic pathways.

MicroRNA regulation of core apoptosis pathways in cancer

Recent research has demonstrated that microRNAs (miRNAs) are key regulators of many cell processes often deregulated in cancer, including apoptosis. Indeed, it is becoming clear that many miRNAs are anti-apoptotic and mediate this effect by targeting pro-apoptotic mRNAs or positive regulators of pro-apoptotic mRNAs. Conversely, many pro-apoptotic miRNAs target anti-apoptotic mRNAs or their positive regulators. We have reviewed the current knowledge in this area including evidence of miRNA involvement in cancer drug resistance.

Targeting c-FLIP in cancer

Cellular-FLICE inhibitory protein (c-FLIP) is a key anti-apoptotic regulator that inhibits cell death mediated by the death receptors Fas, DR4, DR5, and TNF-R1. Three splice variants of c-FLIP function at the DISC level by blocking the processing and activation of procaspase-8 and -10. Overexpression of c-FLIP has been identified in many different tumour types, and its downregulation in vitro has been shown to restore apoptosis mediated by CD95L and TRAIL. c-FLIP therefore represents a promising target for cancer therapy. This review focuses on the molecular mechanisms that control c-FLIP expression and current research into inhibitors of the protein. Increasing evidence supports the investigation of c-FLIP as a therapeutic target to restore an apoptotic response in cancer cells.

4-(4-Chloro-2-methylphenoxy)-N-hydroxybutanamide (CMH) targets mRNA of the c-FLIP variants and induces apoptosis in MCF-7 human breast cancer cells

Cellular FLICE (FADD-like IL-1beta-converting enzyme)-inhibitory protein (c-FLIP) is a major resistance factor for the tumor necrosis factor-related apoptosis-inducing ligand TRAIL and in drug resistance in human malignancies. c-FLIP is an antagonist of caspases-8 and -10, which inhibits apoptosis and is expressed as long (c-FLIP(L)) and short (c-FLIP(S)) splice forms. c-FLIP is often overexpressed in various human cancers, including breast cancer. Several studies have shown that silencing c-FLIP by specific siRNAs sensitizes cancer cells to TRAIL and anticancer agents. However, systemic use of siRNA as a therapeutic agent is not practical at present. In order to reduce or inhibit c-FLIP expression, small molecules are needed to allow targeting c-FLIP without inhibiting caspases-8 and -10. We used a small molecule inhibitor of c-FLIP, 4-(4-chloro-2-methylphenoxy)-N-hydroxybutanamide (CMH), and show that CMH, but not its inactive analog, downregulated c-FLIP(L) and c-FLIP(S) mRNA and protein levels, caused poly(ADP-ribose) polymerase (PARP) degradation, reduced cell survival, and induced apoptosis in MCF-7 breast cancer cells. These results revealed that c-FLIP is a critical apoptosis regulator that can serve as a target for small molecule inhibitors that downregulate its expression and serve as effective targeted therapeutics against breast cancer cells.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and paclitaxel have cooperative in vivo effects against glioblastoma multiforme cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in conjunction with microtubule-targeting agents may be a promising novel anticancer treatment strategy. In vitro studies have suggested that relatively low concentrations of TRAIL enhance the lethality of paclitaxel (Taxol) against human cancer cells. The increased efficacy may be due to the triggering of caspase activation, resulting in mitotic checkpoint abrogation and catastrophe. We show here that wild-type p53 protects cells from caspase-dependent death induced by this therapeutic combination in vitro. We have now also developed an imaging-based model system to test the in vivo efficacy of combined TRAIL and Taxol, in which tumor growth and treatment response can be monitored noninvasively and in real-time. We further utilize bioluminescence, F18-fluorodeoxyglucose-positron emission tomography, and microscale computed tomography imaging to confirm the effects of combined treatment on tumors. These studies together provide the first in vivo confirmation that combined TRAIL plus paclitaxel results in better tumor control compared with either TRAIL or paclitaxel alone, and with no discernable increased normal tissue toxicity in the mouse. Interestingly, the in vivo antitumor response elicited by combined treatment was not affected by the p53 status of the tumor cells. These preclinical observations together suggest the therapeutic potential of combining TRAIL plus paclitaxel in cancer treatment, and support further preclinical and future clinical testing.

Autotaxin delays apoptosis induced by carboplatin in ovarian cancer cells

Drug resistance remains a barrier to the effective long term treatment of ovarian cancer. We have established an RNAi-based screen to identify genes which confer resistance to carboplatin or paclitaxel. To validate the screen we showed that siRNA interfering with the apoptosis regulators FLIP and Bcl-X(L) conferred sensitivity to paclitaxel and carboplatin respectively. The expression of 90 genes which have previously been shown to be over-expressed in drug-resistant ovarian cancer was inhibited using siRNA and the impact on sensitivity to carboplatin and paclitaxel was assessed. ENPP2 was identified as a candidate gene causing drug resistance. ENPP2 encodes autotaxin, a phospholipase involved in the synthesis of the survival factor lysophosphatidic acid. siRNA directed to ENPP2 resulted in earlier apoptosis following treatment with carboplatin. 2-carbacyclic phosphatidic acid (ccPA 16:1), a small molecule inhibitor of autotaxin, also accelerated apoptosis induced by carboplatin. Stable ectopic expression of autotaxin in OVCAR-3 cells led to a delay in apoptosis. When serum was withdrawn to remove exogenous LPA, ccPA caused a pronounced potentiation of apoptosis induced by carboplatin in cells expressing autotaxin. These results indicate that autotaxin delays apoptosis induced by carboplatin in ovarian cancer cells.

Resistance of cutaneous anaplastic large-cell lymphoma cells to apoptosis by death ligands is enhanced by CD30-mediated overexpression of c-FLIP

Death ligands, including TNF-alpha, CD95L/FasL, and TRAIL, mediate safeguard mechanisms against tumor growth and critically contribute to lymphocyte homeostasis. We investigated death receptor-mediated apoptosis and CD30/CD95 crosstalk in four CD30-positive cell lines of cutaneous anaplastic large-cell lymphoma (cALCL). Whereas CD95 stimulation strongly induced apoptosis in cALCL cells, the pro-apoptotic pathways of TNF-alpha and TRAIL were completely blocked at an early step. Expression of TNF receptor 1 was lost in three of four cell lines, providing an explanation for TNF-alpha unresponsiveness. TRAIL resistance may be explained by the consistent overexpression of cellular flice inhibitory protein (c-FLIP) (four of four cell lines) and frequent loss of the pro-apoptotic Bcl-2 protein Bid (three of four cell lines). Changes at the receptor-expression level were largely ruled out. CD30/CD95 crosstalk experiments showed that CD30 ligation leads to NF-kappaB-mediated c-FLIP upregulation in cALCL cells, which in turn conferred enhanced resistance to CD95-mediated apoptosis. Knockdown of c-FLIP by a lentiviral approach enhanced basic apoptosis rates in cALCL cells and diminished the CD30-mediated suppression of apoptosis, thus proving the significance of c-FLIP in this context. These in vitro findings may be indicative of the clinical situation of cALCL. Further clarifying the defects in apoptosis pathways in cutaneous lymphomas may lead to improved therapies for these disorders.

Human beta-galactoside alpha-2,3-sialyltransferase (ST3Gal III) attenuated Taxol-induced apoptosis in ovarian cancer cells by downregulating caspase-8 activity

Taxol triggers apoptosis in a variety of cancer cells, but it also upregulates cytoprotective proteins and/or pathways that compromise its therapeutic efficacy. In this report, we found that Taxol treatment resulted in caspase-8-dependent apoptosis in SKOV3 human ovarian cancer cells. Moreover, Taxol-induced apoptosis was associated with caspase-3 activation. Interestingly, Taxol treatment upregulated alpha-2,3-sialyltransferase (ST3Gal III) expression and forced expression of ST3Gal III attenuated Taxol-induced apoptosis. Furthermore, ST3Gal III overexpression inhibited Taxol-triggered caspase-8 activation, indicating that ST3Gal III upregulation produces cellular resistance to Taxol and hence reduces the efficacy of Taxol therapy.

Regulation of colorectal cancer cell apoptosis by the n-3 polyunsaturated fatty acids Docosahexaenoic and Eicosapentaenoic

Several studies have suggested that the n-3 fatty acids Docosahexaenoic (DHA) and Eicosapentaenoic (EPA) have an important protective effect on colorectal cancer, and this could be at least partly due to their proapoptotic activity. It is unclear, however, how this phenomenon is triggered and what mechanisms are implicated. Here, we show that both DHA and EPA have an important proapoptotic effect on colorectal cancer cells with different molecular phenotypes but not in noncancerous cells. Apoptosis is caspase dependent, and both intrinsic and extrinsic pathways are implicated. The dimerization of Bax and Bak, the depolarization of the mitochondrial membrane, and the subsequent release of cytochrome c and Smac/Diablo to the cytosol evidence the activation of the intrinsic pathway. The implication of the extrinsic pathway is shown by the activation of caspase-8, along with the down-regulation of FLIP. The timing of caspase-8 activation, and the oligomerization of Bid with Bax, suggest a cross-talk with the intrinsic pathway. None of the death receptors that commonly initiate the extrinsic pathway: FAS, TNF-R1, and TRAIL-R2 are found to be responsible for triggering the apoptosis cascade induced by DHA and EPA. Neither PPARgamma nor cyclooxygenase-2, two likely candidates to regulate this process, play a significant role. Our findings suggest that the down-regulation of two key regulatory elements of the extrinsic and intrinsic pathways, FLIP and XIAP, respectively, is determinant in the induction of apoptosis by DHA and EPA. These fatty acids could potentially be useful adjuvant anticancer agents in combination with other chemotherapeutic elements.

Over-expression of c-FLIP confers the resistance to TRAIL-induced apoptosis on gallbladder carcinoma

Gallbladder carcinoma (GBC) is an aggressive malignancy with high mortality, mainly due to the reduced chance of curative resection and the resistance to chemotherapeutic drugs. Here, we showed that cellular Fas-associated death domain-like interleukin-1 converting enzyme inhibitory protein (c-FLIP), an anti-apoptotic protein, was over-expressed in the most of gallbladder carcinoma tissues, as judged by immunohistochemistry. Semi-quantitation was performed by determining the percentage of c-FLIP-positive cells: no positive cells (-), approximately 1% positive cells (+), approximately 30% positive cells (++), and >70% positive cells (+++). Out of the 35 tissue specimens of gallbladder carcinoma, positive c-FLIP expression was found in 26 samples (6/positive+++, 13/++, 7/+), whereas negative or weak c-FLIP staining was detected in normal (1/+, 9/-) and adenomatous (2/+, 8/-) gallbladder tissues. Then, we used a small interference RNA (siRNA), which can substantially down-regulate the expression levels of c-FLIP mRNA and protein in GBC-SD and SGC-996 human gallbladder carcinoma cells, as confirmed by real-time PCR and western blot analyses. Furthermore, the combined treatment with the c-FLIP siRNA and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) significantly induced apoptosis in gallbladder carcinoma cells, as judged by the increases in pyknosis, caspase-3/7 activities, and Annexin V-propidium iodide labeling, a marker for chromatin condensation. Thus, the siRNA-mediated down-regulation of c-FLIP profoundly enhances the sensitivity to TRAIL-induced apoptosis. In conclusion, c-FLIP expression is up-regulated in gallbladder carcinoma and the down-regulation of c-FLIP sensitizes TRAIL-induced apoptosis. The present study provides a potent strategy for the treatment of gallbladder carcinoma by targeting the c-FLIP.

Activation-induced degradation of FLIP(L) is mediated via the phosphatidylinositol 3-kinase/Akt signaling pathway in macrophages

Cellular FLIP (Flice-like inhibitory protein) is critical for the protection against death receptor-mediated cell apoptosis. In macrophages, FLIP long (FLIP(L)) and FLIP short (FLIP(S)) mRNA was induced by tumor necrosis factor (TNF) alpha, mediated through NF-kappaB. However, we observed TNFalpha reduced the protein level of FLIP(L), but not FLIP(S), at 1 and 2 h. Similar results were observed with lipopolysaccharide. The reduction of FLIP(L) by TNFalpha was not mediated by caspase 8, or through JNK or Itch, but was suppressed by inhibition of the phosphatidylinositol 3-kinase/Akt pathway employing chemical inhibitors, a dominant negative Akt-1, or Akt-1 small interfering RNA. The reduction of FLIP(L) resulted in the short term induction of caspase 8-like activity, which augmented NF-kappaB activation. A co-immunoprecipitation assay demonstrated that Akt-1 physically interacts with FLIP(L). Moreover, TNFalpha enhanced FLIP(L) serine phosphorylation, which was increased by activated Akt-1. Serine 273, a putative Akt-1 phosphorylation site in FLIP(L), was critical for the activation-induced reduction of FLIP(L). Thus, these observations document a novel mechanism where by TNFalpha facilitates the reduction of FLIP(L) protein, which is dependent on the phosphatidylinositol 3-kinase/Akt signaling.

Sanguinarine-induced apoptosis: generation of ROS, down-regulation of Bcl-2, c-FLIP, and synergy with TRAIL

Sanguinarine is a benzophenanthridine alkaloid derived from the root of Sanguinaria canadensis and other poppy-fumaria species, possessing potent antibacterial, antifungal, and anti-inflammatory activities. In this study, we investigated the underling mechanisms by which sanguinarine induce apoptosis in human breast cancer MDA-231 cells. Treatment of MDA-231 cells with sanguinarine induced remarkable apoptosis accompanying the generation of ROS. Consistently, sanguinarine-induced apoptosis was mediated by the increased reproductive cell death. Pretreatment with NAC or GSH attenuated sanguinarine-induced apoptosis, suggesting the involvement of ROS in this cell death. During sanguinarin-induced apoptosis, protein levels of pro-caspase-3, Bcl-2, cIAP2, XIAP, and c-FLIPs were reduced. Sanguinarine-mediated apoptosis was substantially blocked by ectopic expression of Bcl-2 and cFLIPs. Additionally, we found that sub-lethal doses of sanguinarine remarkably sensitized breast cancer cells to TRAIL-mediated apoptosis, but the cell death induced by sanguinarine and TRAIL in combination was not blocked by overexpression of Bcl-2 or Akt. Therefore, combinatory treatment of sanguinarine and TRAIL may overcome the resistance of breast cancer cells due to overexpression of Akt or Bcl-2.

c-FLIP knockdown induces ligand-independent DR5-, FADD-, caspase-8-, and caspase-9-dependent apoptosis in breast cancer cells

Cellular-FLICE inhibitory protein (c-FLIP) is an inhibitor of apoptosis downstream of the death receptors Fas, DR4, and DR5, and is expressed as long (c-FLIP(L)) and short (c-FLIP(S)) splice forms. We found that the knockdown of c-FLIP using small interfering RNA (siRNA) triggered ligand-independent caspase-8- and -9-dependent spontaneous apoptosis and decreased the proliferation of MCF-7 breast cancer cells. Further analysis revealed that an apoptotic inhibitory complex (AIC) comprised of DR5, FADD, caspase-8, and c-FLIP(L) exists in MCF-7 cells, and the absence of c-FLIP(L) from this complex induces DR5- and FADD-mediated caspase-8 activation in the death inducing signaling complex (DISC). c-FLIP(S) was not detected in the AIC, and using splice form-specific siRNAs we showed that c-FLIP(L) but not c-FLIP(S) is required to prevent spontaneous death signaling in MCF-7 cells. These results clearly show that c-FLIP(L) prevents ligand-independent death signaling and provides direct support for studying c-FLIP as a relevant therapeutic target for breast cancers.

The Fas death signaling pathway connecting reactive oxygen species generation and FLICE inhibitory protein down-regulation

Fas-mediated apoptosis plays an important role in normal tissue homeostasis, and disruption of this death pathway contributes to many human diseases. Induction of apoptosis via Fas activation has been associated with reactive oxygen species (ROS) generation and down-regulation of FLICE inhibitory protein (FLIP); however, the relationship between these two events and their role in Fas-mediated apoptosis are unclear. We show herein that ROS are required for FLIP down-regulation and apoptosis induction by Fas ligand (FasL) in primary lung epithelial cells. ROS mediate the down-regulation of FLIP by ubiquitination and subsequent degradation by proteasome. Inhibition of ROS by antioxidants or by ectopic expression of ROS-scavenging enzymes glutathione peroxidase and superoxide dismutase effectively inhibited FLIP down-regulation and apoptosis induction by FasL. Hydrogen peroxide is a primary oxidative species responsible for FLIP down-regulation, whereas superoxide serves as a source of peroxide and a scavenger of NO, which positively regulates FLIP via S-nitrosylation. NADPH oxidase is a key source of ROS generation induced by FasL, and its inhibition by dominant-negative Rac1 expression or by chemical inhibitor decreased the cell death response to FasL. Taken together, our results indicate a novel pathway of FLIP regulation by an interactive network of reactive oxygen and nitrogen species that provides a key mechanism of apoptosis regulation in Fas-induced cell death and related apoptosis disorders.

Mechanistic insight into taxol-induced cell death

We analysed the involvement of proteases during taxol-mediated cell death of human A549 non-small-cell lung carcinoma cells using a proteomics approach that specifically targets protein N termini and further detects newly formed N termini that are the result of protein processing. Our analysis revealed 27 protease-mediated cleavages, which we divided in sites C-terminal to aspartic acid (Asp) and sites C-terminal to non-Asp residues, as the result of caspase and non-caspase protease activities, respectively. Remarkably, some of the former were insensitive to potent pancaspase inhibitors, and we therefore suggest that previous inhibitor-based studies that report on the caspase-independent nature of taxol-induced cell death should be judged with care. Furthermore, many of the sites C-terminal to non-Asp residues were also uniquely observed in a model of cytotoxic granule-mediated cell death and/or found by in vitro cataloging human mu-calpain substrates using a similar proteomics technique. This thus raises the hypothesis that killing tumor cells by chemotherapy or by immune cells holds similar non-Asp-specific proteolytic components with strong indications to calpain activity.

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.

Apaf-1 and caspase-9 deficiency prevents apoptosis in a Bax-controlled pathway and promotes clonogenic survival during paclitaxel treatment

Taxane derivatives such as paclitaxel elicit their antitumor effects at least in part by induction of apoptosis, but the underlying mechanisms are incompletely understood. Here, we used different cellular models with deficiencies in key regulators of apoptosis to elucidate the mechanism of paclitaxel-induced cell death. Apoptosis by paclitaxel was reported to depend on the activation of the initiator caspase-10; however, we clearly demonstrate that paclitaxel kills murine embryonic fibroblasts (MEFs) devoid of caspase-10 as well as human tumor cell lines deficient in caspase-10, caspase-8, or Fas-associating protein with death domain. In contrast, the lack of Apaf-1 or caspase-9, key regulators of the mitochondrial pathway, not only entirely protected against paclitaxel-induced apoptosis but could even confer clonogenic survival, depending on the cell type and drug concentration. Thus, paclitaxel triggers apoptosis not through caspase-10, but via caspase-9 activation at the apoptosome. This conclusion is supported by the fact that Bcl-2-overexpressing cells and Bax/Bak doubly-deficient MEFs were entirely resistant to paclitaxel-induced apoptosis. Interestingly, also the single knockout of Bim or Bax, but not that of Bak or Bid, conferred partial resistance, suggesting a particular role of these mediators in the cell-death pathway activated by paclitaxel.