Death receptors as targets for anti-cancer therapy

Panoramic dental X-ray exposure leads to oxidative stress, inflammation and apoptosis-mediated developmental defects in zebrafish embryos

Panoramic x-ray units are widely used in dental radiodiagnostics. Patients are exposed to relatively low radiation doses with panoramic imaging, but considering lifetime frequency of exposure, even a small risk can have serious health consequences. Our aim was to assess the effects of panoramic x-rays at two different exposure times on developing zebrafish embryos, focusing on oxidative stress, inflammation, apoptotic pathways, and development. Zebrafish embryos were divided into three groups: control, standard panoramic (SPE, 5.5 s exposure time) and pedodontic panoramic x-ray group (PPE, 4.8 s exposure time). Optically stimulated luminescence dosimeters were used to measure absorbed doses. Mean radiation doses for SPE and PPE were 7.83 mSv and 5.83 mSv respectively. At the end of 96 h post-fertilization, lipid peroxidation (LPO), nitric oxide (NO), reduced glutathione (GSH), glutathione S-transferase and superoxide dismutase were measured in the embryos. Expressions of genes related with inflammation (tnfα, il6, ill15, il21), immunoregulation (ifng) and apoptosis (p53, bax, casp2, casp3, casp8) were determined by RT-PCR. Even at reduced doses at high-speed mode, developmental toxicity was observed in both groups as evidenced by decreased pigmentation, yolk sac oedema, and spinal curvature. While deterioration of oxidant-antioxidant balance, suppression of immune response, induction of inflammation and apoptosis were observed through increased LPO, NO, decreased GSH, ifng, and increased expressions of genes related with inflammation and apoptosis, these effects were more pronounced in the SPE group. These results demonstrate the influence of exposure time and indicate the need for further consideration of optimal panoramic modes from a radiation-induced damage perspective.

Pro-apoptotic and anti-apoptotic regulation mediated by deubiquitinating enzymes

Although damaged cells can be repaired, cells that are considered unlikely to be repaired are eliminated through apoptosis, a type of predicted cell death found in multicellular organisms. Apoptosis is a structured cell death involving alterations to the cell morphology and internal biochemical changes. This process involves the expansion and cracking of cells, changes in cell membranes, nuclear fragmentation, chromatin condensation, and chromosome cleavage, culminating in the damaged cells being eaten and processed by other cells. The ubiquitin-proteasome system (UPS) is a major cellular pathway that regulates the protein levels through proteasomal degradation. This review proposes that apoptotic proteins are regulated through the UPS and describes a unique direction for cancer treatment by controlling proteasomal degradation of apoptotic proteins, and small molecules targeted to enzymes associated with UPS.

Intrinsic and extrinsic pathways of apoptosis: Role in cancer development and prognosis

Apoptosis, also named programmed cell death, is a fundament process required for morphogenetic homeostasis during early development and in pathophysiological conditions. It is come into existence in 1972 by work of Kerr, Wyllie and Currie and later on investigated during the research on development of the C. elegans. Trigger by several stimuli, apoptosis is necessary during the embryonic development and aging as homeostatic mechanism to control the cell population and also play a key role as defense mechanism against the immune responses and elimination of damaged cells. Cancer, a genetic disease, is a growing burden on the health and economy of both developing and developed countries. Every year there is tremendously increasing in the number of new cancer cases and mortality rate. Although, there is a significant improvement have been made in biotechnological and bioinformatic fields however, the therapeutic advantages and cancer etiology is still under explored. Several studies determined the deregulation of different apoptotic components during the cancer development and progression. Apoptosis relies on activation of distinct signaling pathways that are often deregulated in cancer. Thus, exploring the single or more than one apoptotic component underlying their expression in carcinogenesis could help to track the disease progression. Current book chapter will provide the several evidences supporting the use of different apoptotic components as prognosis and prediction markers in various human cancer types.

Apoptosis-Inducing TNF Superfamily Ligands for Cancer Therapy

Cancer is a complex disease with apoptosis evasion as one of its hallmarks; therefore, apoptosis induction in transformed cells seems a promising approach as a cancer treatment. TNF apoptosis-inducing ligands, which are naturally present in the body and possess tumoricidal activity, are attractive candidates. The most studied proteins are TNF-α, FasL, and TNF-related apoptosis-inducing ligand (TRAIL). Over the years, different recombinant TNF family-derived apoptosis-inducing ligands and agonists have been designed. Their stability, specificity, and half-life have been improved because most of the TNF ligands have the disadvantages of having a short half-life and affinity to more than one receptor. Here, we review the outlook on apoptosis-inducing ligands as cancer treatments in diverse preclinical and clinical stages and summarize strategies of overcoming their natural limitations to improve their effectiveness.

MicroRNAs in cancer cell death pathways: Apoptosis and necroptosis

To protect tissues and the organism from disease, potentially harmful cells are removed through programmed cell death processes, including apoptosis and necroptosis. These types of cell death are critically controlled by microRNAs (miRNAs). MiRNAs are short RNA molecules that target and inhibit expression of many cellular regulators, including those controlling programmed cell death via the intrinsic (Bcl-2 and Mcl-1), extrinsic (TRAIL and Fas), p53-and endoplasmic reticulum (ER) stress-induced apoptotic pathways, as well as the necroptosis cell death pathway. In this review, we discuss the current knowledge of apoptosis and necroptosis pathways and how these are impaired in cancer cells. We focus on how miRNAs disrupt apoptosis and necroptosis, thereby critically contributing to malignancy. Understanding which and how miRNAs and their targets affect cell death pathways could open up novel therapeutic opportunities for cancer patients. Indeed, restoration of pro-apoptotic tumor suppressor miRNAs (apoptomiRs) or inhibition of oncogenic miRNAs (oncomiRs) represent strategies that are currently being trialed or are already applied as miRNA-based cancer therapies. Therefore, better understanding the cancer type-specific expression of apoptomiRs and oncomiRs and their underlying mechanisms in cell death pathways will not only advance our knowledge, but also continue to provide new opportunities to treat cancer.

Aquaporin 1 suppresses apoptosis and affects prognosis in esophageal squamous cell carcinoma

Aquaporin 1 (AQP1) is a membrane protein whose main function is to transfer water across cellular membranes. Recent studies have described important roles for AQP1 in epithelial carcinogenesis and tumor behavior. The objectives of the present study were to investigate the role of AQP1 in the regulation of genes involved in tumor progression and the clinicopathological significance of its expression in esophageal squamous cell carcinoma (ESCC). An immunohistochemical analysis was performed on 50 primary tumor samples underwent esophagectomy. AQP1 was primarily located in the cytoplasm and/or the nuclear membrane of carcinoma cells. The 5-year survival rate of patients with the “cytoplasm dominant” expression of AQP1 (47.1%) was significantly lower than other patients (83.2%). The depletion of AQP1 using siRNA induced apoptosis in TE5 and TE15 cells. The results of microarray analysis revealed that Death receptor signaling pathway-related genes were changed in AQP1-depleted TE5 cells. In conclusion, the results of the present study suggested that the cytoplasm dominant expression of AQP1 is related to a poor prognosis in patients with ESCC, and that it activates tumor progression by affecting Death receptor signaling pathway. These results provide insights into the role of AQP1 as a mediator of and/or a biomarker for ESCC.

Inactivation of BRCA2 in human cancer cells identifies a subset of tumors with enhanced sensitivity towards death receptor-mediated apoptosis

Purpose

DNA repair defects due to detrimental BRCA2-mutations confer increased susceptibility towards DNA interstrand-crosslinking (ICL) agents and define patient subpopulations for individualized genotype-based cancer therapy. However, due to the side effects of these drugs, there is a need to identify additional agents, which could be used alone or in combination with ICL-agents. Therefore, we investigated whether BRCA2-mutations might also increase the sensitivity towards TRAIL-receptors (TRAIL-R)-targeting compounds.

Experimental design

Two independent model systems were applied: a BRCA2 gene knockout and a BRCA2 gene complementation model. The effects of TRAIL-R-targeting compounds and ICL-agents on cell viability, apoptosis and cell cycle distribution were compared in BRCA2-proficient versus-deficient cancer cells in vitro. In addition, the effects of the TRAIL-R2-targeting antibody LBY135 were assessed in vivo using a murine tumor xenograft model.

Results

BRCA2-deficient cancer cells displayed an increased sensitivity towards TRAIL-R-targeting agents. These effects exceeded and were mechanistically distinguishable from the well-established effects of ICL-agents. In vitro, ICL-agents expectedly induced an early cell cycle arrest followed by delayed apoptosis, whereas TRAIL-R-targeting compounds caused early apoptosis without prior cell cycle arrest. In vivo, treatment with LBY135 significantly reduced the tumor growth of BRCA2-deficient cancer cells in a xenograft model.

Conclusions

BRCA2 mutations strongly increase the in vitro- and in vivo-sensitivity of cancer cells towards TRAIL-R-mediated apoptosis. This effect is mechanistically distinguishable from the well-established ICL-hypersensitivity of BRCA2-deficient cells. Our study thus defines a new genetic subpopulation of cancers susceptible towards TRAIL-R-targeting compounds, which could facilitate novel therapeutic approaches for patients with BRCA2-deficient tumors.

Embelin-Induced Apoptosis of Human Prostate Cancer Cells Is Mediated through Modulation of Akt and β-Catenin Signaling

There is increasing evidence that embelin, an active component of Embelia ribes, induces apoptosis in human cancer cells, but the detailed mechanisms are still unclear. Here, we have investigated the effect of embelin on the growth of human prostate cancer cells. Embelin strongly inhibited cell growth especially in human prostate cancer cell lines, including PC3, DU145, LNCaP-LN3 and normal prostate epithelial cell, RWPE-1 compared to breast cancer (MDA-MB-231, MCF-7, and T47D), hepatoma (HepG2, Hep3B, and HuH-7), or choriocarcinoma (JEG-3). We observed that embelin induced apoptosis of PC3 cells in a time-dependent manner correlated with decreased expression of Bcl-2, Bcl-xL, and Mcl-1, increased translocation of Bax into mitochondria, and a reduction in the mitochondrial membrane potential. Furthermore, embelin induced voltage-dependent anion channel (VDAC) 1 expression and oligomerization, which may promote cytochrome c and AIF release. Because embelin was able to inhibit Akt activation and cyclooxygenase-2 expression, the effects on Wnt/ β-catenin signaling were determined. Embelin activated glycogen synthase kinase (GSK)-3β by preventing phosphorylation and suppressed β-catenin expression. Attenuation of β-catenin-mediated TCF transcriptional activity and gene transcription, such as cyclin D1, c-myc, and matrix metalloproteinase (MMP)-7, were shown in embelin-treated cells. The changes in β-catenin levels in response to embelin were blocked by lithium chloride, a GSK-3 inhibitor, indicating that embelin may decrease β-catenin expression via GSK-3β activation. Furthermore, exposure of PC3 cells to embelin resulted in a significant decrease in cell migration and invasion. In conclusion, these findings suggest that inhibition of Akt signaling and activation of GSK-3β partially contributes to the pro-apoptotic effect of embelin in prostate cancer cells.

Bortezomib sensitizes primary meningioma cells to TRAIL-induced apoptosis by enhancing formation of the death-inducing signaling complex

A meningioma is the most common primary intracranial tumor in adults. Here, we investigated the therapeutic potential of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in 37 meningiomas. Freshly isolated primary meningioma cells were treated with TRAIL with or without different sensitizing protocols, and apoptotic cell death was then quantified. Mechanisms of TRAIL sensitization were determined by a combination of Western blotting, flow cytometry, receptor complex immunoprecipitation, and siRNA-mediated knockdown experiments. Tumor necrosis factor-related apoptosis-inducing ligand receptor expression was analyzed using immunohistochemistry and quantified by an automated software-based algorithm. Primary tumor cells from 11 (29.7%) tumor samples were sensitive to TRAIL-induced apoptosis, 12 (32.4%) were intermediate TRAIL resistant, and 14 (37.8%) were completely TRAIL resistant. We tested synergistic apoptosis-inducing cotreatment strategies and determined that only the proteasome inhibitor bortezomib potently enhanced expression of the TRAIL receptors TRAIL-R1 and/or TRAIL-R2, the formation of the TRAIL death-inducing signaling complex, and activation of caspases; this treatment resulted in sensitization of all TRAIL-resistant meningioma samples to TRAIL-induced apoptosis. Bortezomib pretreatment induced NOXA expression and downregulated c-FLIP, neither of which caused the TRAIL-sensitizing effect. Native TRAIL receptor expression could not predict primary TRAIL sensitivity. This first report on TRAIL sensitivity of primary meningioma cells demonstrates that TRAIL/bortezomib cotreatment may represent a novel therapeutic option for meningiomas.

Isolation and chemopreventive evaluation of novel naphthoquinone compounds from Alkanna tinctoria

Botanically derived natural products have recently become an attractive source of new chemotherapeutic agents. To explore active anticolorectal cancer compounds, we carried out phytochemical studies on Alkanna tinctoria and isolated eight quinone compounds. Using different spectral methods, compounds were identified as alkannin (1), acetylalkannin (2), angelylalkannin (3), 5-methoxyangenylalkannin (4), dimethylacryl alkannin (5), arnebifuranone (6), alkanfuranol (7), and alkandiol (8). Compounds 4, 7, and 8 are novel compounds. The structures of the three novel compounds were elucidated on the basis of extensive spectroscopic evidence including high-resolution mass spectrometry and nuclear magnetic resonance spectra. The antiproliferative effects of these eight compounds on HCT-116 and SW-480 human colorectal cancer cells were determined using the MTS method. Cell cycle and apoptosis were determined using flow cytometry. Enzymatic activities of caspases were determined using a colorimetric assay, and interactions of compound 4 and caspase 9 were explored by docking analysis. Among the eight compounds, alkannin (1), angelylalkannin (3), and 5-methoxyangenylalkannin (4) showed strong antiproliferative effects, whereas compound 4 showed the most potent effects. Compound 4 arrested cancer cells in the S and G2/M phases, and significantly induced cell apoptosis. The apoptotic effects of compound 4 were supported by caspase assay and docking analysis. The structural-functional relationship assay suggested that to increase anticancer potential, future modifications on alkannin (1) should focus on the hydroxyl groups at C-5 and C-8.

Inhibition of the mitochondrial pyrimidine biosynthesis enzyme dihydroorotate dehydrogenase by doxorubicin and brequinar sensitizes cancer cells to TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising agent in selectively killing tumor cells. However, TRAIL monotherapy has not been successful as many cancer cells are resistant to TRAIL. Chemotherapeutic agents, such as doxorubicin have been shown to act synergistically with TRAIL, but the exact mechanisms of actions are poorly understood. In this study, we performed high-throughput small interfering RNA screening and genome-wide gene expression profiling on doxorubicin-treated U1690 cells to explore novel mechanisms underlying doxorubicin-TRAIL synergy. The screening and expression profiling results were integrated and dihydroorotate dehydrogenase (DHODH) was identified as a potential candidate. DHODH is the rate-limiting enzyme in the pyrimidine synthesis pathway, and its expression was downregulated by doxorubicin. We demonstrated that silencing of DHODH or inhibition of DHODH activity by brequinar dramatically increased the sensitivity of U1690 cells to TRAIL-induced apoptosis both in 2D and 3D cultures, and was accompanied by downregulation of c-FLIPL as well as by mitochondrial depolarization. In addition, uridine, an end product of the pyrimidine synthesis pathway was able to rescue the sensitization effects initiated by both brequinar and doxorubicin. Furthermore, several other cancer cell lines, LNCaP, MCF-7 and HT-29 were also shown to be sensitized to TRAIL by brequinar. Taken together, our findings have identified a novel protein target and its inhibitor, brequinar, as a potential agent in TRAIL-based combinatorial cancer therapy and highlighted for the first time the importance of mitochondrial DHODH enzyme and pyrimidine pathway in mediating TRAIL sensitization in cancer cells.

The TRAIL of oncogenes to apoptosis

Despite the significant advances in clinical research, surgical resection, radiotherapy and chemotherapy are still used as the primary method for cancer treatment. As compared to conventional therapies that often induce systemic toxicity and eventually contribute to tumor resistance, the TNF-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent that selectively triggers apoptosis in various cancer cells by interacting with its proapoptotic receptors DR4 and KILLER/DR5, while sparing the normal surrounding tissue. The intensive studies of TRAIL signaling pathways over the past decade have provided clues for understanding the molecular mechanisms of TRAIL-induced apoptosis in carcinogenesis and identified an array of therapeutic responses elicited by TRAIL and its receptor agonists. Analysis of its activity at the molecular level has shown that TRAIL improves survival either as monotherapies or combinatorial therapies with other mediators of apoptosis or anticancer chemotherapy. Combinatorial treatments amplify the activities of anticancer agents and widen the therapeutic window by overcoming tumor resistance to apoptosis and driving cancer cells to self-destruction. Although TRAIL sensitivity varies widely depending on the cell type, nontransformed cells are largely resistant to death mediated by TRAIL Death Receptors (DRs). Genetic alterations in cancer can contribute in tumor progression and often play an important role in evasion of apoptosis by tumor cells. Remarkably, RAS, MYC and HER2 oncogenes have been shown to sensitise tumor cells to TRAIL induced cell death. Here, we summarise the cross-talk of oncogenic and apoptotic pathways and how they can be exploited toward efficient combinatorial therapeutic protocols.

Delivery of sTRAIL variants by MSCs in combination with cytotoxic drug treatment leads to p53-independent enhanced antitumor effects

Mesenchymal stem cells (MSCs) are able to infiltrate tumor tissues and thereby effectively deliver gene therapeutic payloads. Here, we engineered murine MSCs (mMSCs) to express a secreted form of the TNF-related apoptosis-inducing ligand (TRAIL), which is a potent inducer of apoptosis in tumor cells, and tested these MSCs, termed MSC.sTRAIL, in combination with conventional chemotherapeutic drug treatment in colon cancer models. When we pretreated human colorectal cancer HCT116 cells with low doses of 5-fluorouracil (5-FU) and added MSC.sTRAIL, we found significantly increased apoptosis as compared with single-agent treatment. Moreover, HCT116 xenografts, which were cotreated with 5-FU and systemically delivered MSC.sTRAIL, went into remission. Noteworthy, this effect was protein 53 (p53) independent and was mediated by TRAIL-receptor 2 (TRAIL-R2) upregulation, demonstrating the applicability of this approach in p53-defective tumors. Consequently, when we generated MSCs that secreted TRAIL-R2-specific variants of soluble TRAIL (sTRAIL), we found that such engineered MSCs, labeled MSC.sTRAIL(DR5), had enhanced antitumor activity in combination with 5-FU when compared with MSC.sTRAIL. In contrast, TRAIL-resistant pancreatic carcinoma PancTu1 cells responded better to MSC.sTRAIL(DR4) when the antiapoptotic protein XIAP (X-linked inhibitor of apoptosis protein) was silenced concomitantly. Taken together, our results demonstrate that TRAIL-receptor selective variants can potentially enhance the therapeutic efficacy of MSC-delivered TRAIL as part of individualized and tumor-specific combination treatments.

Enhanced antitumor activity of the photosensitizer meso-Tetra(N-methyl-4-pyridyl) porphine tetra tosylate through encapsulation in antibody-targeted chitosan/alginate nanoparticles

meso-Tetra(N-methyl-4-pyridyl) porphine tetra tosylate (TMP) is a photosensitizer that can be used in photodynamic therapy (PDT) to induce cell death through generation of reactive oxygen species in targeted tumor cells. However, TMP is highly hydrophilic, and therefore, its ability to accumulate intracellularly is limited. In this study, a strategy to improve TMP uptake into cells has been investigated by encapsulating the compound in a hydrogel-based chitosan/alginate nanoparticle formulation. Nanoparticles of 560 nm in diameter entrapping 9.1 μg of TMP per mg of formulation were produced and examined in cell-based assays. These particles were endocytosed into human colorectal carcinoma HCT116 cells and elicited a more potent photocytotoxic effect than free drug. Antibodies targeting death receptor 5 (DR5), a cell surface apoptosis-inducing receptor up-regulated in various types of cancer and found on HCT116 cells, were then conjugated onto the particles. The conjugated antibodies further enhanced uptake and cytotoxic potency of the nanoparticle. Taken together, these results show that antibody-conjugated chitosan/alginate nanoparticles significantly enhanced the therapeutic effectiveness of entrapped TMP. This novel approach provides a strategy for providing targeted site-specific delivery of TMP and other photosensitizer drugs to treat colorectal tumors using PDT.

CD95 is part of a let-7/p53/miR-34 regulatory network

The death receptor CD95 (APO-1/Fas) mediates apoptosis induction upon ligation by its cognate ligand CD95L. Two types of CD95 signaling pathways have been identified, which are characterized by the absence (Type I) or presence (Type II) of mitochondrial involvement. Micro(mi)RNAs are small noncoding RNAs that negatively regulate gene expression. They are important regulators of differentiation processes and are found frequently deregulated in many human cancers. We recently showed that Type I cells express less of the differentiation marker miRNA let-7 and, hence, likely represent more advanced tumor cells than the let-7 high expressing Type II cells. We have now identified miR-34a as a selective marker for cells that are sensitive to CD95-mediated apoptosis. Both CD95 and miR-34a are p53 target genes, and consequently, both the sensitivity of cancer cells to CD95-mediated apoptosis and the ability to respond to p53 mediated DNA genotoxic stress are linked. Interestingly, while miR-34a was found to positively correlate with the ability of cells to respond to genotoxic stress, let-7 was negatively correlated. The expression level of CD95 inversely correlated with the expression of let-7 suggesting regulation of let-7 expression by CD95. To test a link between p53 and miR-34a, we altered the expression of CD95. This affected the ability of cells to activate p53 and to regulate miR-34a. Our data point to a novel regulatory network comprising p53, CD95, let-7, and miR-34a that affects cancer cell survival, differentiation, and sensitivity to apoptotic signals. The possible relevance of this regulatory network for cancer stem cells is discussed.

Moutan Cortex Radicis inhibits inflammatory changes of gene expression in lipopolysaccharide-stimulated gingival fibroblasts

Moutan Cortex Radicis (MCR), the root bark of Paeonia suffruticosa Andrews (Paeoniaceae), is found in the traditional Chinese medicinal formulae which were used to treat periodontal diseases. This study investigated the changes in gene expression by MCR treatment when stimulated with lipopolysaccharide (LPS) in cultured human gingival fibroblasts (HGFs). A genome-wide expression GeneChip was used for the gene array analysis, and real-time reverse transcription polymerase chain reaction (RT-PCR) analysis was also performed to confirm the gene expression. It was shown that 42 of the 643 genes up-regulated by LPS, when compared to the control, were down-regulated by the MCR treatment. Of these 42 genes, the inflammation and immune response-related genes were especially noted, which indicates that MCR inhibits the induction of inflammation by LPS stimulation. In addition, 33 of the 519 genes down-regulated by LPS, when compared to the control, were up-regulated by the MCR treatment. The expression patterns of some representative genes by real-time RT-PCR correlated with those of the genes shown in the microarray. In addition, the MCR extract contained paeonol and paeoniflorin, which are known to have the anti-inflammatory effect as the major phenolic components of MCR. This study showed that the MCR extract could comprehensively inhibit a wide variety of activations of inflammation-related genes, which may be due to paeonol and paeoniflorin. It is, thus, suggested that MCR may be applied to alleviate the inflammation of periodontal diseases.

Glycogen synthase kinase-3 inhibition sensitizes pancreatic cancer cells to TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) induces apoptosis in a variety of cancer cell lines with little or no effect on normal cells. However, its effect is limited as some cancers including pancreatic cancer show de novo resistance to TRAIL induced apoptosis. In this study we report that GSK-3 inhibition using the pharmacologic agent AR-18, enhanced TRAIL sensitivity in a range of pancreatic and prostate cancer cell lines. This sensitization was found to be caspase-dependent, and both pharmacological and genetic knock-down of GSK-3 isoforms resulted in apoptotic features as shown by cleavage of PARP and caspase-3. Elevated levels of reactive oxygen intermediates and disturbance of mitochondrial membrane potential point to a mitochondrial amplification loop for TRAIL-induced apoptosis after GSK-3 inhibition. Consistent with this, overexpression of anti-apoptotic mitochondrial targets such as Bcl-XL, Mcl-1, and Bcl-2 rescued PANC-1 and PPC-1 cells from TRAIL sensitization. However, overexpression of the caspase-8 inhibitor CrmA also inhibited the sensitizing effects of GSK-3 inhibitor, suggesting an additional role for GSK-3 that inhibits death receptor signaling. Acute treatment of mice bearing PANC-1 xenografts with a combination of AR-18 and TRAIL also resulted in a significant increase in apoptosis, as measured by caspase-3 cleavage. Sensitization to TRAIL occurred despite an increase in β-catenin due to GSK-3 inhibition, suggesting that the approach might be effective even in cancers with dysregulated β-catenin. These results suggest that GSK-3 inhibitors might be effectively combined with TRAIL for the treatment of pancreatic cancer.

XIAP inhibition and generation of reactive oxygen species enhances TRAIL sensitivity in inflammatory breast cancer cells

We recently identified superoxide dismutase (SOD) overexpression and decreased induction of reactive oxygen species (ROS)-mediated apoptosis in models of inflammatory breast cancer (IBC) cells with acquired therapeutic resistance. This population of cells has high expression of X-linked inhibitor of apoptosis protein (XIAP), which inhibits both extrinsic and intrinsic apoptosis pathways. We therefore wanted to evaluate the effect of classical apoptosis-inducing agent TRAIL, a proapoptotic receptor agonist that selectively triggers death receptor (DR)-mediated apoptosis in cancer cells, in the IBC acquired resistance model. XIAP levels and subsequent inhibition of caspase activity inversely correlated with TRAIL sensitivity in our models of IBC. These include SUM149, a basal-type cell line isolated from primary IBC tumors and isogenic SUM149-derived lines rSUM149 and SUM149 wtXIAP, models of acquired therapeutic resistance with endogenous and exogenous XIAP overexpression, respectively. Inhibition of XIAP function using embelin, a plant-derived cell permeable small molecule, in combination with TRAIL caused a synergistic decrease in cell viability. Embelin treatment resulted in activation of extracellular signal-regulated kinase (ERK)1/2 and ROS accumulation, which correlated with downregulation of antioxidant protein SOD1 and consumption of redox modulator reduced glutathione in the XIAP-overexpressing cells. Simultaneous treatment with an SOD mimic, which protects against ROS accumulation, reversed the decrease in cell viability caused by embelin + TRAIL treatment. Embelin primes IBC cells for TRAIL-mediated apoptosis by its direct action on the anti-caspase activity of XIAP and by shifting the cellular redox balance toward oxidative stress-mediated apoptosis. Thus, ROS modulators represent a novel approach to enhance efficacy of TRAIL-based treatment protocols in IBC.

CD95 signaling in colorectal cancer

CD95 and its ligand (CD95L) are widely expressed in colorectal tumors, but their role in shaping tumor behavior is unclear. CD95 activation on tumor cells can lead to apoptosis, while CD95L attracts neutrophils, suggesting a function in tumor suppression. However, CD95 can also promote tumorigenesis, at least in part by activating non-apoptotic signaling pathways that stimulate tumor cell proliferation, invasion and survival. In addition, CD95 signaling in stromal cells and tumor-infiltrating inflammatory cells has to be taken into account when addressing the function of CD95 and its ligand in colorectal tumor biology. We present a model in which the tumor-suppressing and tumor-promoting activities of CD95/CD95L together determine colorectal tumor behavior. We also discuss how these multiple activities are changing our view of CD95 and CD95L as potential therapeutic targets in the treatment of colorectal cancer. We conclude that locking CD95 in apoptosis-mode may be a more promising anti-cancer strategy than simply inhibiting or stimulating CD95.

Effect of combined HCTB006 and 5-fluorouracil on the growth of human gastric cancer cell lines SGC-7901 and MKN28

AIM: To investigate the effect of combined anti-human death receptor 5 (DR5) monoclonal antibody HCTB006 and 5-fluorouracil (5-FU) on the growth of human gastric cancer cell lines SGC-7901 and MKN28 and to explore possible mechanisms involved.

METHODS: The ability of HCTB006 and 5-FU, alone or in combination, to inhibit the proliferation of SGC-7901 and MKN28 cells was measured by ATP-Lite assay. The expression of DR5 on the surface of gastric cancer cells was examined by flow cytometry. The level of X-linked inhibitor of apoptosis (XIAP) and caspase 3 in treated cells was detected by Western blot.

RESULTS: SGC-7901 and MKN28 cells were less sensitive to HCTB006. Concentration- and time-dependent cytotoxicity of 5-FU was exhibited in SGC-7901 and MKN28 cells. The combination of 5-FU and HTCB006 exhibited a synergistic effect on the proliferation of SGC 7901 and MKN28 cells. The positive rates of DR5 expression on the surface of SGC-7901 and MKN28 cells were 93.8% and 87.7%, respectively. Western blot analysis revealed that combined HCTB006 and 5-FU induced XIAP degradation and caspase 3 activation.

CONCLUSION: The combination of 5-FU and HTCB006 exhibited a synergistic effect on the growth of SGC-7901 and MKN28 cells possibly via a mechanism associated with XIAP degradation. The in vitro sensitivity of gastric cancer cells to HCTB006 has no direct association with DR5 expression.

Type I IFNs signaling and apoptosis resistance in glioblastoma cells

Deletion of type I IFN genes and resistance to apoptosis induced by type I IFNs are common in glioblastoma. Here we have investigated the importance of the constitutive weak IFN-signaling in the apoptotic response to IFN-α in glioblastoma cells. U87MG cells hold a deletion of type I IFN genes, whereas in T98G cells the spontaneous IFN signaling is intact. In response to IFN-α U87MG cells produce much less TRAIL, while other IFN-inducible genes were efficiently up-regulated. Alterations in TRAIL promoter sequence and activity were not observed. DNA methylation can influence TRAIL transcription but without overt differences between the two cell lines. We also discovered that TRAIL mRNA stability is influenced by IFN-α, but again no differences can be appreciated between the two cell lines. By silencing IFNAR1 we provide evidences that the spontaneous IFN signaling loop is required to sustain elevated levels of TRAIL expression, possibly through the regulation of IRF-1. Despite the presence/absence of the constitutive IFN signaling, both cell lines were resistant to IFN-α induced apoptosis. Targeting the deisgylase USP18 can overcome resistance to IFN-induced apoptosis only in T98G cells. Alterations in elements of the extrinsic apoptotic pathway, such as Bid and c-FLIP contribute to apoptotic resistance of U87MG cells. Down-regulation of USP18 expression together with the induction of ER-stress efficiently restored apoptosis in U87MG cells. Finally, we demonstrated that the BH3-only protein Noxa provides an important contribution in the apoptotic response to ER-stress in USP18 silenced cells.

Conatumumab (AMG 655) coated nanoparticles for targeted pro-apoptotic drug delivery

Colloidal nanoparticle drug delivery systems have attracted much interest for their ability to enable effective formulation and delivery of therapeutic agents. The selective delivery of these nanoparticles to the disease site can be enhanced by coating the surface of the nanoparticles with targeting moieties, such as antibodies. In this current work, we demonstrate that antibodies on the surface of the particles can also elicit key biological effects. Specifically, we demonstrate the induction of apoptosis in colorectal HCT116 cancer cells using PLGA nanoparticles coated with Conatumumab (AMG 655) death receptor 5-specific antibodies (DR5-NP). We show that DR5-NP preferentially target DR5-expressing cells and present a sufficient density of antibody paratopes to induce apoptosis via DR5, unlike free AMG 655 or non-targeted control nanoparticles. We also demonstrate that DR5-targeted nanoparticles encapsulating the cytotoxic drug camptothecin are effectively targeted to the tumour cells, thereby producing enhanced cytotoxic effects through simultaneous drug delivery and apoptosis induction. These results demonstrate that antibodies on nanoparticulate surfaces can be exploited for dual modes of action to enhance the therapeutic utility of the modality.

Harnessing tumor necrosis factor receptors to enhance antitumor activities of drugs

Cancer is the second-leading cause of death in the U.S. behind heart disease and over stroke. The hallmarks of cancer comprise six biological capabilities acquired during the multistep development of human tumors. The inhibition of cell death pathways is one of these tumor characteristics which also include sustained proliferative signaling, evading growth suppressor signaling, replicative immortality, angiogenesis, and promotion of invasion and metastasis. Cell death is mediated through death receptor (DR) stimulation initiated by specific ligands that transmit signaling to the cell death machinery or through the participation of mitochondria. Cell death involving DR is mediated by the superfamily of tumor necrosis factor receptor (TNF-R) which includes TNF-R type I, CD95, DR3, TNF-related apoptosis-inducing ligand (TRAIL) receptor-1 (TRAIL-R1) and -2 (TRAIL-R2), DR6, ectodysplasin A (EDA) receptor (EDAR), and the nerve growth factor (NGF) receptor (NGFR). The expression of these receptors in healthy and tumor cells induces treatment side effects that limit the systemic administration of cell death-inducing therapies. The present review is focused on the different therapeutic strategies such as targeted antibodies or small molecules addressed to selective stimulated DR-mediated apoptosis or reduce cell proliferation in cancer cells.

Differential effects of alpha-particle radiation and x-irradiation on genes associated with apoptosis

This study examined differential effects of alpha-(α-) particle radiation and X-rays on apoptosis and associated changes in gene expression. Human monocytic cells were exposed to α-particle radiation and X-rays from 0 to 1.5 Gy. Four days postexposure, cell death was measured by flow cytometry and 84 genes related to apoptosis were analyzed using real-time PCR. On average, 33% of the cells were apoptotic at 1.5 Gy of α-particle radiation. Transcript profiling showed statistical expression of 15 genes at all three doses tested. Cells exposed to X-rays were <5% apoptotic at ~1.5 Gy and induced less than a 2-fold expression in 6 apoptotic genes at the higher doses of radiation. Among these 6 genes, Fas and TNF-α were common to the α-irradiated cells. This data suggests that α-particle radiation initiates cell death by TNF-α and Fas activation and through intermediate signalling mediators that are distinct from X-irradiated cells.

Targeting of XIAP combined with systemic mesenchymal stem cell-mediated delivery of sTRAIL ligand inhibits metastatic growth of pancreatic carcinoma cells

Disseminating tumors are one of the gravest medical problems. Here, we combine the tumor-specific apoptosis-inducing activity of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) with the ability of mesenchymal stem cells (MSCs) to infiltrate both tumor and lymphatic tissues to target primary tumors as well as disseminated cancer cells in a human pancreatic cancer mouse model. Furthermore, we targeted X-linked inhibitor of apoptosis protein (XIAP) by RNA interference (RNAi) inside the cancer cells to make use of the apoptosis sensitization as well the antimetastatic effect that is afforded by XIAP silencing. We generated MSCs, termed MSC.sTRAIL, that express and secrete a trimeric form of soluble TRAIL (sTRAIL). MSC.sTRAIL triggered limited apoptosis in human pancreatic carcinoma cells that were resistant to soluble recombinant TRAIL, which is most likely due to the enhanced effect of the direct, cell-mediated delivery of trimeric TRAIL. MSC.sTRAIL-mediated cell death was markedly increased by concomitant knockdown of XIAP by RNAi in the cancer cells. These findings were confirmed in xenograft models, in which tumors from the parental pancreatic carcinoma cells showed only growth retardation on treatment with MSC.sTRAIL, whereas tumors with silenced XIAP that were treated with MSC.sTRAIL went into remission. Moreover, animals with XIAP-negative xenografts treated with MSC.sTRAIL were almost free of lung metastasis, whereas animals treated with control MSCs showed substantial metastatic growth in the lungs. In summary, this is the first demonstration that a combined approach using systemic MSC-mediated delivery of sTRAIL together with XIAP inhibition suppresses metastatic growth of pancreatic carcinoma.

TRAIL receptor signaling and therapeutics

IMPORTANCE OF THE FIELD

TNF-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family of cytokines, which can induce apoptotic cell death in a variety of tumor cells by engaging specific death receptors, TRAIL-R1 and TRAIL-R2, while having low toxicity towards normal cells. There is interest in cancer therapy inducing cell death by activation of the death-receptor-mediated apoptotic pathway while avoiding decoy-receptor-mediated neutralization of the signal. This has led to the development of a number of receptor-specific TRAIL-variants and agonistic antibodies. Some of these soluble recombinant TRAIL and agonist antibodies targeting TRAIL-R1 and/or TRAIL-R2 are progressing in clinical trials. In addition, TRAIL-resistant tumors can be sensitized to TRAIL by a combination of TRAIL or agonistic antibodies with chemotherapeutic agents, targeted small molecules or irradiation.

AREAS COVERED IN THIS REVIEW

Recent advances in developing TRAIL or its agonist receptor antibodies in cancer therapy. We also discuss combination therapies in overcoming TRAIL resistance in cancer cells.

WHAT THE READER WILL GAIN

Knowledge of current clinical trials, the promise and obstacles in the future development of therapies affecting TRAIL signaling pathways.

TAKE HOME MESSAGE

Cancer therapeutics targeting the TRAIL/TRAIL receptor signaling pathway hold great promise for molecularly targeted pro-apoptotic anti-cancer therapy.

APO010, a synthetic hexameric CD95 ligand, induces human glioma cell death in vitro and in vivo

Death receptor targeting has emerged as one of the promising novel approaches of cancer therapy. The activation of one such prototypic death receptor, CD95 (Fas/APO-1), has remained controversial because CD95 agonistic molecules have exhibited either too strong toxicity or too little activity. The natural CD95 ligand (CD95L) is a cytokine, which needs to trimerize to mediate a cell death signal. Mega-Fas-Ligand, now referred to as APO010, is a synthetic hexameric CD95 agonist that exhibits strong antitumor activity in various tumor models. Here, we studied the effects of APO010 in human glioma models in vitro and in vivo. Compared with a cross-linked soluble CD95L or a CD95-agonistic antibody, APO010 exhibited superior activity in glioma cell lines expressing CD95 and triggered caspase-dependent cell death. APO010 reduced glioma cell viability in synergy when combined with temozolomide. The locoregional administration of APO010 induced glioma cell death in vivo and prolonged the survival of tumor-bearing mice. A further exploration of APO010 as a novel antiglioma agent is warranted.

Anoikis mediators in oral squamous cell carcinoma

Anoikis - apoptotic cell death triggered by loss of extracellular matrix (ECM) contacts - is dysregulated in many chronic debilitating and fatal diseases. Mechanisms rendering tumor cells resistant to anoikis, although not completely understood, possess significant therapeutic promise. In death receptor-mediated anoikis mechanisms, focal adhesion kinase (FAK) and receptor-interacting protein (RIP) dissociate, leading to association of RIP with Fas, formation of the death-inducing signaling complex (DISC), activation of caspase-3, and propagation of anoikis. In contrast, anoikis resistance is accomplished through constitutive activation of survival pathways that include integrin-dependent activation of FAK and extracellular-signal-regulated kinase (ERK). In addition, FAK and RIP association confers anoikis resistance by inhibiting the association of RIP with Fas and formation of the death signaling complex, which allows cells to escape anoikis. Up-regulation of CD44 also contributes to survival signals and promotes anoikis resistance. This review will focus on the roles of death receptors, prosurvival pathways, and the molecular players involved in anoikis escalation and resistance in oral squamous cell carcinoma.

Drugs targeting Bcl-2 family members as an emerging strategy in cancer

Inhibiting apoptosis is widely accepted as a necessary step in the transition from normal to cancer cells, and most cancer therapies exert their effects by indirectly reversing this process. Commitment to apoptosis is caused by permeabilisation of the outer mitochondrial membrane – a process regulated by the binding between different members of the Bcl-2 family. Furthermore, Bcl-2 family members also bind to the endoplasmic reticulum, where they modify processes such as the unfolded-protein response and autophagy that also cause or modify different types of cell death. With the growing understanding of the importance of the Bcl-2 family as crucial regulators of the decision to initiate apoptosis, much effort has been directed at developing small molecules that modify function by directly binding to Bcl-2 proteins. Preclinical experiments have confirmed that these agents kill cancer cells and overcome chemotherapy resistance. Two of these drugs are in the initial stages of clinical development (ABT-263 and obatoclax), and early results show clinical efficacy at tolerable doses. Important questions for the future include the role of these drugs as monotherapy versus combination therapy with other anticancer drugs, and the related issue of the relative toxicity to cancerous versus normal cells.

Quercetin induced apoptosis in association with death receptors and fludarabine in cells isolated from chronic lymphocytic leukaemia patients

Background:

Quercetin is a flavonoid naturally present in food and beverages belonging to the large class of phytochemicals with potential anti-cancer properties. Here, we investigated the ability of quercetin to sensitise primary cells from chronic lymphocytic leukaemia (CLL) to death receptor (DR) agonists, recombinant TNF-related-apoptosis-inducing ligand (rTRAIL) and anti-CD95, and to fludarabine, a widely used chemotherapeutic drug against CLL.

Methods:

Peripheral white blood cells were isolated from patients and incubated with medium containing 50 ng ml anti-CD95 agonist antibody; 10 ng ml recombinant TRAIL; 10–25 μM quercetin and 3.5–14 μM fludarabine. Neutral Red assay was used to measure cell viability, where as apoptosis was assessed by determining caspase-3 activity, exposure to Annexin V and PARP fragmentation.

Results:

Quercetin significantly enhanced anti-CD95- and rTRAIL-induced cell death as shown by decreased cell viability, increased caspase-3 and -9 activities, and positivity to Annexin V. In addition, association of quercetin with fludarabine increases the apoptotic response in CLL cells of about two-fold compared with quercetin monotreatment.

Conclusion:

This work shows that resistance to DR- and fludarabine-induced cell death in leukaemic cells isolated from CLL patients can be ameliorated or bypassed by the combined treatment with quercetin. Considering the low toxicity of the molecule, our study results are in favour of a potential use of quercetin in adjuvant chemotherapy in combination with other drugs.

Oncogenic K-Ras turns death receptors into metastasis-promoting receptors in human and mouse colorectal cancer cells

BACKGROUND & AIMS

Death receptors expressed on tumor cells can prevent metastasis formation by inducing apoptosis, but they also can promote migration and invasion. The determinants of death receptor signaling output are poorly defined. Here we investigated the role of oncogenic K-Ras in determining death receptor function and metastatic potential.

METHODS

Isogenic human and mouse colorectal cancer cell lines differing only in the presence or absence of the K-Ras oncogene were tested in apoptosis and invasion assays using CD95 ligand and tumor necrois factor-related apoptosis-inducing ligand (TRAIL) as stimuli. Metastatic potential was assessed by intrasplenic injections of green fluorescent protein- or luciferase-expressing tumor cells, followed by intravital fluorescence microscopy or bioluminescence imaging, and confocal microscopy and immunohistochemistry. Ras-effector pathway control of CD95 output was assessed by an RNA-interference and inhibitor-based approach.

RESULTS

CD95 ligand and TRAIL stimulated invasion of colorectal tumor cells and liver metastases in a K-Ras-dependent fashion. Loss of mutant K-Ras switched CD95 and TRAIL receptors back into apoptosis mode and abrogated metastatic potential. Raf1 was essential for the switch in CD95 function, for tumor cell survival in the liver, and for K-Ras-driven formation of liver metastases. K-Ras and Raf1 suppressed Rho kinase (ROCK)/LIM kinase-mediated phosphorylation of the actin-severing protein cofilin. Overexpression of ROCK or LIM kinase allowed CD95L to induce apoptosis in K-Ras-proficient cells and prevented metastasis formation, whereas their suppression protected K-Ras-deficient cells against apoptosis.

CONCLUSIONS

Oncogenic K-Ras and its effector Raf1 convert death receptors into invasion-inducing receptors by suppressing the ROCK/LIM kinase pathway, and this is essential for K-Ras/Raf1-driven metastasis formation.

Inhibition of nuclear factor-kappa B differentially affects thyroid cancer cell growth, apoptosis, and invasion

Background

Nuclear factor-κB (NF-κB) is constitutively activated in many cancers and plays a key role in promoting cell proliferation, survival, and invasion. Our understanding of NF-κB signaling in thyroid cancer, however, is limited. In this study, we have investigated the role of NF-κB signaling in thyroid cancer cell proliferation, invasion, and apoptosis using selective genetic inhibition of NF-κB in advanced thyroid cancer cell lines.

Results

Three pharmacologic inhibitors of NF-κB differentially inhibited growth in a panel of advanced thyroid cancer cell lines, suggesting that these NF-κB inhibitors may have off-target effects. We therefore used a selective genetic approach to inhibit NF-κB signaling by overexpression of a dominant-negative IκBα (mIκBα). These studies revealed decreased cell growth in only one of five thyroid cancer cell lines (8505C), which occurred through a block in the S-G2/M transition. Resistance to TNFα-induced apoptosis was observed in all cell lines, likely through an NF-κB-dependent mechanism. Inhibition of NF-κB by mIκBα sensitized a subset of cell lines to TNFα-induced apoptosis. Sensitive cell lines displayed sustained activation of the stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) pathway, defining a potential mechanism of response. Finally, NF-κB inhibition by mIκBα expression differentially reduced thyroid cancer cell invasion in these thyroid cancer cell lines. Sensitive cell lines demonstrated approximately a two-fold decrease in invasion, which was associated with differential expression of MMP-13. MMP-9 was reduced by mIκBα expression in all cell lines tested.

Conclusions

These data indicate that selective inhibition of NF-κB represents an attractive therapeutic target for the treatment of advanced thyroid. However, it is apparent that global regulation of thyroid cancer cell growth and invasion is not achieved by NF-κB signaling alone. Instead, our findings suggest that other important molecular processes play a critical role in defining the extent of NF-κB function within cancer cells.

TRAIL and triptolide: an effective combination that induces apoptosis in pancreatic cancer cells

INTRODUCTION

An emerging therapy in oncology is the induction of apoptotic cell death through anti-death receptor therapy. However, pancreatic cancer is resistant to apoptosis including anti-death receptor therapy. We have previously described how triptolide decreases resistance to apoptosis in pancreatic cancer cells in vitro and in vivo. We hypothesized that triptolide decreases tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) resistance in pancreatic cancer cells. The aim of this study was to evaluate the effects that combined therapy with TRAIL and triptolide have on different parameters of apoptosis.

METHODS

Four different pancreatic cancer cell lines were exposed to triptolide, TRAIL, or a combination of both drugs. We assessed the effects that combined therapy with TRAIL and triptolide has on cell viability, apoptosis, caspase-3 and caspase-9 activities, and poly(ADP)-ribose polymerase cleavage.

RESULTS

Pancreatic cancer cells were resistant to TRAIL therapy; however, combined therapy with triptolide and TRAIL significantly decreased the cell viability in all the cell lines and increased apoptotic cell death as a result of caspase-3 and caspase-9 activation.

CONCLUSIONS

Pancreatic cancer is highly resistant to anti-death receptor therapy, but combined therapy with TRAIL and triptolide is an effective therapy that induces apoptotic cell death in pancreatic cancer cells.

Molecules that modulate Apaf-1 activity

Programmed cell death, apoptosis, is a highly regulated cellular pathway, responsible for the elimination of cells in the organism that are no longer needed or extensively damaged. Defects in the regulation of apoptosis could be at the molecular basis of different diseases, either when it is insufficient or excessive. The formation of the macromolecular complex, apoptosome, is a key event in this pathway, which has also been defined as the intrinsic apoptosis pathway. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated apoptotic protease-activating factor (Apaf-1), dATP, and procaspase-9. Recent studies have produced a wealth of information about the regulation and functions of Apaf-1, but additional studies aimed at elucidating its role as a signaling device at the crosstalk between different signaling pathways are needed to take advantage for the development of modulators of apoptosis pathways and possible therapeutic applications.

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.

Clusterin and chemoresistance

Resistance to anticancer agents is one of the primary impediments to effective cancer therapy. Chemoresistance occurs not only to clinically established therapeutic agents but also to novel targeted therapeutics. Both intrinsic and acquired mechanisms have been implicated in drug resistance but it remains controversial which mechanisms are responsible that lead to failure of therapy in cancer patients. Recent focus has turned to clusterin (CLU) as a key contributor to chemoresistance to anticancer agents. Its role has been documented in prostate cancer for paclitaxel/docetaxel resistance as well as in renal, breast, and lung tumor cells. Moreover, it is abnormally upregulated in numerous advanced stage and metastatic cancers spanning prostate, renal, bladder, breast, head and neck, colon, cervical, pancreatic, lung carcinomas, melanoma, and lymphoma. It is noteworthy that only the cytoplasmic/secretory clusterin form (sCLU), and not the nuclear form, is expressed in aggressive late stage tumors, which is in line with its antiapoptotic function. Most significantly, sCLU expression is documented to lead to broad-based resistance to other unrelated chemotherapeutic agents such as doxorubicin, cisplatin, etoposide, and camphothecin. Resistance to targeted death-inducing molecules, tumor necrosis factor, Fas and TRAIL, or histone deacetylase inhibitors can also be mediated by sCLU. Expression of sCLU may be an adaptive response to genotoxic and oxidative stresses but this adaptive response could pose a threat in malignant cells being treated with cytotoxic agents by enhancing their survival potential. The actual mechanisms for sCLU induction are unclear but STAT1 is required for its constitutive upregulation in docetaxel-resistant tumor cells. Known as a protein chaperone, sCLU appears to stabilize Ku70/Bax complexes, sequestering Bax from its ability to induce mitochondrial release of cytochrome c that triggers cell apoptosis. Thus, sCLU has a key role in preventing apoptosis induced by cytotoxic agents and has the potential to be targeted for cancer therapy.

Hypoxia-induced decoy receptor 2 gene expression is regulated via a hypoxia-inducible factor 1alpha-mediated mechanism

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising candidate for anti-tumor therapy because of its high selectivity towards cancer cells. TRAIL has four major distinct receptors: DR4 and DR5 can recruit Fas-associated death domain protein to induce extrinsic death signal, while DcR1 and DcR2 are decoy receptors that can neutralize TRAIL toxicity by binding to TRAIL. Hypoxia is an important feature of solid tumors that renders tumor cells resistant to some chemotherapeutic agents, including TRAIL, and we therefore investigated the role of hypoxia in TRAIL receptor expression in human colon cancer cells. Hypoxia upregulated DcR2 protein expression in five different human colon cancer cell lines (HCT116, HT29, SW480, SW620, and WiDr). Flow cytometry analysis indicated that the increased DcR2 protein was expressed on the cell surface membrane. In contrast, hypoxia had no effect on DR4, DR5, or DcR1 protein levels. RT-PCR analysis suggested that this protein increase was the result of DcR2 gene transcription. Transcription factors were investigated using p53-null cells, pharmacological inhibitors, and a small interfering RNA approach. Our results demonstrated that hypoxia-inducible factor 1alpha played a crucial role in regulating the transcription of DcR2, but that neither p53 nor NF-kappaB contributed to this regulation. Moreover, TRAIL-induced, but not agonistic DR5 antibody-induced cell death was attenuated under hypoxic conditions. These results suggest that increased DcR2 protein levels might play a role in TRAIL resistance in solid tumors.

Signaling activated by the death receptors of the TNFR family

BACKGROUND

The fine balance in cellular life and death is affected by a number of tightly regulated, direct signals that can help to turn the balance either in favor of or against the ultimate fate. Among the most prominent players in the field of the extracellular signals leading to cell death, preferentially through induction of apoptosis belong several receptors from so-called Death Receptors group of the Tumour Necrosis Factors Receptors (TNFR) family.

METHODS AND RESULTS

Over 15 years of the research on activation and regulation of the most prominent member of this group - receptors for the ligands TRAIL, FasL and TNFalpha brought not only a detail (and still refining) mechanism of these receptors activation and downstream signaling, but also connected them with the ultimate apoptotic gatekeeper - mitochondria. Mitochondria are, in addition to their essential role as the energy factories also repositories of a cavalry of apoptosis-inducing as well as regulatory proteins. However, in addition to the pro-death signaling, these receptors were also shown under certain circumstances to activate an opposite, pro-proliferative signaling as well as to participate in pro-inflammatory responses.

CONCLUSIONS

Thus despite the concerned effort of a number of groups and thousands of published papers, novel roles for the intriguing group of these receptors and their ligands and fine tuning of their signaling still await to be uncovered. This cut-through review will be mainly focused on the prominent death-inducing members of this group - TNFR1, Fas/CD95 and TRAIL receptors.

Acquired activation of the Akt/cyclooxygenase-2/Mcl-1 pathway renders lung cancer cells resistant to apoptosis

Acquired apoptosis resistance plays an important role in acquired chemoresistance in cancer cells during chemotherapy. Our previous observations demonstrated that acquired tumor necrosis factor-related apoptosis-inducing ligand resistance in lung cancer cells was associated with Akt-mediated stabilization of cellular FLICE-like inhibitory protein (c-FLIP) and Mcl-1. In this report, we determined that these cells also have acquired resistance to apoptosis induced by chemotherapeutics such as cisplatin and doxorubicin (Adriamycin), which was detected in vitro in cell cultures and in vivo in xenografted tumors. We further found that cyclooxygenase-2 (COX-2) is dramatically overexpressed in cells with acquired apoptosis resistance. COX-2 seems to be a crucial mediator in acquired apoptosis resistance because suppressing COX-2 activity with a chemical inhibitor or reducing COX-2 protein expression level with COX-2 small interfering RNA dramatically alleviated resistance to therapeutic-induced apoptosis. Inhibiting Akt markedly suppressed COX-2 expression, suggesting COX-2 is a downstream effector of this cell survival kinase-mediated apoptosis resistance. Furthermore, the expression of Mcl-1 but not c-FLIP was significantly reduced when COX-2 was suppressed, and knockdown of Mcl-1 substantially sensitized the cells to apoptosis. Our results establish a novel pathway that consists of Akt, COX-2, and Mcl-1 for acquired apoptosis resistance, which could be a molecular target for circumventing acquired chemoresistance in lung cancer.

Novel therapeutic approaches to the treatment of metastatic breast cancer

Metastatic breast cancer is ultimately an incurable disease, although recent data have shown that its incidence is decreasing and that patients with metastatic breast cancer live longer. This improvement in survival seems to be linked with the introduction of new therapeutic agents, novel combinations of existing therapies and targeted therapies. Our increasing understanding of the molecular biology of metastatic disease has allowed the development of therapies aimed at specific molecular targets. Some of these have already been approved for the treatment of metastatic breast cancer in combination with cytotoxics, and others have shown promising results regarding disease-free survival, overall response rates and time to disease progression. Given the enormous amount of information about drug discovery in cancer, it is important to be familiar with the present state of the treatment of metastatic breast cancer. The purpose of this review is to provide an update on the development of some of the most promising novel agents and treatment strategies in metastatic breast cancer.

Humanization of an agonistic anti-death receptor 4 single chain variable fragment antibody and avidity-mediated enhancement of its cell death-inducing activity

Development of agonistic monoclonal antibodies (mAbs) against the pro-apoptotic molecule death receptor 4 (DR4) [or tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) receptor 1] is an attractive anti-cancer strategy because of their potential for inducing tumor-specific cell death. In this study, we humanized an agonistic anti-DR4 AY4 scFv raised in mice (mAY4) by grafting the complementarity-determining regions (CDRs) onto a fixed human framework, while preserving the so-called Vernier zone residues, a group of framework (FR) residues directly underneath the CDRs, with the murine residues in the humanized antibody, hAY4. The humanized hAY4 scFv maintained the antigen binding affinity and epitope specificity of mAY4. To investigate how the valence of hAY4 scFv affects DR4-mediated cell death, bivalent and trivalent forms of hAY4 scFv were generated by linking a hinge region to the coiled-coil domain of a dimerizing leucine zipper and trimerizing isoleucine zipper, respectively. Compared to the monovalent and bivalent forms, the trivalent hAY4 scFv induced more potent caspase-dependent apoptotic cell death as evidenced by increased activation of caspase-8 and downstream pro-apoptotic molecules. Our results suggest that like other TNF family receptors, avidity-mediated oligomerization of DR4 augments the receptor-mediated apoptotic cell death by promoting intracellular cell death signaling.