TRAIL therapy in non-small cell lung cancer cells: sensitization to death receptor-mediated apoptosis by proteasome inhibitor bortezomib

Cap-dependent translation initiation factor eIF4E: an emerging anticancer drug target

Cancer cells tend to be more highly dependent on cap‐dependent translation than normal tissues. Thus, proteins involved in the initiation of cap‐dependent translation have emerged as potential anti‐cancer drug targets. Cap‐dependent translation is initiated by the binding of the factor eIF4E to the cap domain of mRNA. Detailed x‐ray crystal and NMR structures are available for eIF4E in association with cap‐analogs, as well as domains of other initiation factors. This review will summarize efforts to design potential antagonist of eIF4E that could be used as new pharmacological tools and anti‐cancer agents and. Insights drawn from these studies should aid in the design of future inhibitors of eIF4E dependent translation initiation. © 2012 Wiley Periodicals, Inc. Med Res Rev., 32, No. 4, 786‐814, 2012

Novel insights into the synergistic interaction of Bortezomib and TRAIL: tBid provides the link

The proteasome inhibitor Bortezomib has been identified as a potent enhancer of TRAIL-induced apoptosis in several human cancers. However, the identification of the underlying molecular mechanisms of this synergistic cell death induction has been ongoing over the last years. A recent study identifies a new mechanism of action for the synergism of TRAIL and Bortezomib.

Bortezomib sensitizes malignant human glioma cells to TRAIL, mediated by inhibition of the NF-{kappa}B signaling pathway

Previous studies have shown that the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has significant apoptosis-inducing activity in some glioma cell lines, although many lines are either moderately or completely resistant, which has limited the therapeutic applicability of this agent. Because our recent studies showed that inhibition of proteasomal function may be independently active as an apoptosis-inducing stimulus in these tumors, we investigated the sensitivity of a panel of glioma cell lines (U87, T98G, U373, A172, LN18, LN229, LNZ308, and LNZ428) to TRAIL alone and in combination with the proteasome inhibitor bortezomib. Analysis of these cell lines revealed marked differences in their sensitivity to these treatments, with two (LNZ308 and U373) of the eight cell lines revealing no significant induction of cell death in response to TRAIL alone. No correlation was found between sensitivity of cells to TRAIL and expression of TRAIL receptors DR4, DR5, and decoy receptor DcR1, caspase 8, apoptosis inhibitory proteins XIAP, survivin, Mcl-1, Bcl-2, Bcl-Xl, and cFLIP. However, TRAIL-resistant cell lines exhibited a high level of basal NF-κB activity. Bortezomib was capable of potentiating TRAIL-induced apoptosis in TRAIL-resistant cells in a caspase-dependent fashion. Bortezomib abolished p65/NF-κB DNA-binding activity, supporting the hypothesis that inhibition of the NF-κB pathway is critical for the enhancement of TRAIL sensitization in glioma cells. Moreover, knockdown of p65/NF-κB by shRNA also enhanced TRAIL-induced apoptosis, indicating that p65/NF-κB may be important in mediating TRAIL sensitivity and the effect of bortezomib in promoting TRAIL sensitization and apoptosis induction.

Up-regulation of the DR5 expression by proteasome inhibitor MG132 augments TRAIL-induced apoptosis in soft tissue sarcoma cell lines

Purpose

Current chemotherapeutics for treating locally advanced or metastatic soft tissue sarcomas (STS) are limited. Accordingly, the present in vitro study was conducted to evaluate the effects of treatment of STS cells with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) applied as a single agent or in combination with a proteasome inhibitor, MG132.

Materials and Methods

Sensitivity to TRAIL and activity of TRAIL-induced apoptotic pathways were analyzed in four STS cell lines: HTB-82 (rhabdomyosarcoma), HT-1080 (fibrosarcoma), HTB-93 (synovial sarcoma), and HTB-94 (chondrosarcoma). Reduction of the dye dimethylthiazolyl 2,5 diphenyltetrazolium bromide (MTT) was used to evaluate cytotoxic activity; western blots were used to evaluate TRAIL-induced apoptosis.

Results

TRAIL induced apoptosis in HTB-93 cells, but had little effect in HTB-82, HT-1080, or HTB-94 cells. Expression of TRAIL receptor-1 and -2 did not correlate with sensitivity to TRAIL. Co-incubation of cells with TRAIL and a proteasome inhibitor, MG132, augmented the apoptotic effect of TRAIL in both TRAIL-sensitive and TRAIL-resistant cells. This effect was due to up-regulation of TRAIL receptors and members of the pro-apoptotic BCL-2 family by MG132.

Conclusion

These data show that combining TRAIL with MG132 enhances apoptosis and overcomes TRAIL resistance. This restoration of TRAIL sensitivity occurs through an increase in the expression of death receptor 5 and of pro-apoptotic BCL-2 family members such as BAX.

Proteasome inhibition can induce an autophagy-dependent apical activation of caspase-8

Antiapoptotic Bcl-2 family proteins are often highly expressed in chemotherapy-resistant cancers and impair mitochondrial outer membrane permeabilisation (MOMP), an important requirement for caspase activation via the intrinsic apoptosis pathway. Interestingly, although Bcl-2 overexpression in HeLa cervical cancer cells abrogated caspase processing in response to intrinsic apoptosis induction by staurosporine, tunicamycin or etoposide, residual caspase processing was observed following proteasome inhibition by bortezomib ([(1R)-3-methyl-1-({(2S)-3-phenyl-2-[(pyrazin-2-ylcarbonyl)amino]propanoyl}amino)butyl]boronic acid), epoxomicin (N-acetyl-N-methyl-lisoleucyl-L-isoleucyl-N-[(1S)-3-methyl-1-[[(2R)-2-methyloxiranyl]carbonyl]butyl]-L-threoninamide) or MG-132 (N-(benzyloxycarbonyl)leucinylleucinylleucinal). Similar responses were found in Bcl-2-overexpressing H460 NSCLC cells and Bax/Bak-deficient mouse embyronic fibroblasts. Mild caspase processing resulted in low DEVDase activities, which were MOMP independent and persisted for long periods without evoking immediate cell death. Surprisingly, depletion of caspase-3 and experiments in caspase-7-depleted MCF-7-Bcl-2 cells indicated that the DEVDase activity did not originate from effector caspases. Instead, Fas-associated death domain (FADD)-dependent caspase-8 activation was the major contributor to the slow, incomplete substrate cleavage. Caspase-8 activation was independent of death ligands, but required the induction of autophagy and the presence of Atg5. Depletion of XIAP or addition of XIAP-antagonising peptides resulted in a switch towards efficient apoptosis execution, suggesting that the requirement for MOMP was bypassed by activating the caspase-8/caspase-3 axis. Combination treatments of proteasome inhibitors and XIAP antagonists therefore represent a promising strategy to eliminate highly resistant cancer cells, which overexpress antiapoptotic Bcl-2 family members.

Noxa/Bcl-2 protein interactions contribute to bortezomib resistance in human lymphoid cells

Previous studies have suggested that the BH3 domain of the proapoptotic Bcl-2 family member Noxa only interacts with the anti-apoptotic proteins Mcl-1 and A1 but not Bcl-2. In view of the similarity of the BH3 binding domains of these anti-apoptotic proteins as well as recent evidence that studies of isolated BH3 domains can potentially underestimate the binding between full-length Bcl-2 family members, we examined the interaction of full-length human Noxa with anti-apoptotic human Bcl-2 family members. Surface plasmon resonance using bacterially expressed proteins demonstrated that Noxa binds with mean dissociation constants (K(D)) of 3.4 nm for Mcl-1, 70 nm for Bcl-x(L), and 250 nm for wild type human Bcl-2, demonstrating selectivity but not absolute specificity of Noxa for Mcl-1. Further analysis showed that the Noxa/Bcl-2 interaction reflected binding between the Noxa BH3 domain and the Bcl-2 BH3 binding groove. Analysis of proteins expressed in vivo demonstrated that Noxa and Bcl-2 can be pulled down together from a variety of cells. Moreover, when compared with wild type Bcl-2, certain lymphoma-derived Bcl-2 mutants bound Noxa up to 20-fold more tightly in vitro, pulled down more Noxa from cells, and protected cells against killing by transfected Noxa to a greater extent. When killing by bortezomib (an agent whose cytotoxicity in Jurkat T-cell leukemia cells is dependent on Noxa) was examined, apoptosis was enhanced by the Bcl-2/Bcl-x(L) antagonist ABT-737 or by Bcl-2 down-regulation and diminished by Bcl-2 overexpression. Collectively, these observations not only establish the ability of Noxa and Bcl-2 to interact but also identify Bcl-2 overexpression as a potential mechanism of bortezomib resistance.

Role of elevated pressure in TRAIL-induced apoptosis in human lung carcinoma cells

TNF-related apoptosis-inducing ligand (TRAIL, Apo2L) is a promising anticancer agent with high specificity for cancer cells. Many strategies have been proposed to enhance the sensitivity of cancer cells to TRAIL-mediated apoptosis, including the use of combination treatment with conventional cancer therapies. However, few reports have evaluated the effects of TRAIL in combination with mechanical stress, which can also cause apoptosis of cancer cells. In the present study, we describe a custom-designed culture system that delivers two atmospheres of elevated pressure (EP) by using compressed air, and which enhances the sensitivity of cancer cells to TRAIL-mediated apoptosis. The combination of TRAIL and EP significantly increased apoptosis of human H460 lung cancer cells more than hyperbaric normoxia or normobaric mild hyperoxia. EP-potentiating TRAIL-mediated apoptosis of H460 cells was accompanied by up-regulated death receptor 5 (DR5), activation of caspases, decreased mitochondrial membrane potential, and reactive oxygen species production. We also observed EP-induced sensitization of TRAIL-mediated apoptosis in other cancer cell types. In contrast, human normal cells showed no DNA damage or cell death when exposed to the combined treatment. In a chicken chorioallantoic membrane model, EP enhanced TRAIL-mediated apoptosis of tumors that developed from transplanted H460 cells. Collectively, EP enhanced TRAIL-induced apoptosis of human lung carcinoma cells in vitro and in vivo. These findings suggest that EP is a mechanical and physiological stimulus that might have utility as a sensitizing tool for cancer therapy.

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.

Mechanisms of drug sensitization to TRA-8, an agonistic death receptor 5 antibody, involve modulation of the intrinsic apoptotic pathway in human breast cancer cells

TRA-8, a monoclonal antibody to death receptor 5 induces apoptosis in various cancer cells; however, the degree of sensitivity varies from highly sensitive to resistant. We have previously shown that resistance to TRA-8 can be reversed by using chemotherapeutic agents, but the mechanism underlying this sensitization was not fully understood. Here, we examined the combination of TRA-8 with doxorubicin or bortezomib in breast cancer cells. In TRA-8-resistant BT-474 and T47D cells, both chemotherapy agents synergistically sensitized cells to TRA-8 cytotoxicity with enhanced activation of apoptosis shown by cleavage of caspases and PARP, reduced Bid, increased proapoptotic Bcl-2 proteins, and increased mitochondrial membrane depolarization. Doxorubicin or bortezomib combined with TRA-8 also reduced Bcl-XL and X-linked inhibitors of apoptosis (XIAP) in treated cells. Furthermore, targeting these proteins with pharmacologic modulators, AT-101, BH3I-2' and AT-406, produced sensitization to TRA-8. TRA-8 combined with AT-101 or BH3I-2', inhibitors of antiapoptotic Bcl-2 proteins, produced synergistic cytotoxicity against ZR-75-1, BT-474, and T47D cells. The IAP-targeting compound, AT-406, was synergistic with TRA-8 in BT-474 cells, and to a lesser extent T47D cells. Activation of the intrinsic apoptotic pathway was a common mechanism associated with sensitization of TRA-8-resistant breast cancer cell lines. Collectively, these studies show that the Bcl-2 and IAP families of proteins are involved in TRA-8 and chemotherapy resistance via their modulation of the intrinsic apoptotic pathway. Targeting these proteins with novel agents sensitized TRA-8-resistant breast cancer cells, suggesting this approach may represent a potent therapeutic strategy in the treatment of breast cancer.

NF-kappaB in lung cancer, a carcinogenesis mediator and a prevention and therapy target

Lung cancer ranks as the first malignant tumor killer worldwide. Despite the knowledge that carcinogens from tobacco smoke and the environment constitute the main causes of lung cancer, the mechanisms for lung carcinogenesis are still elusive. Cancer development and progression depend on the balance between cell survival and death signals. Common cell survival signaling pathways are activated by carcinogens as well as by inflammatory cytokines, which contribute substantially to cancer development. As a major cell survival signal, nuclear factor-kappaB (NF-kappaB) is involved in multiple steps in carcinogenesis and in cancer cell's resistance to chemo- and radio-therapy. Recent studies with animal models and cell culture systems have established the links between NF-kappaB and lung carcinogenesis, highlighting the significance of targeting NF-kappa signaling pathway for lung cancer treatment and chemoprevention. In this review, we summarize progresses in understanding the NF-kappaB pathway in lung cancer development as well as in modulating NF-kappaB for lung cancer prevention and therapy.

Targeting apoptosis pathways in lung cancer

Lung cancer is a devastating disease with a poor prognosis. Non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) represent different forms of lung cancer that are associated with distinct genetic causes and display different responses to therapy in the clinic. Whereas SCLC is often sensitive to chemotherapy at start of treatment, NSCLC are less chemo-sensitive. In NSCLC different histological subtypes are distinguished and increasing efforts are made to identify subtypes that respond to specific therapies, such as those harbouring epidermal growth factor receptor (EGFR) mutations that have benefit from treatment with EGFR inhibitors. Targeting of the apoptotic machinery represents another approach that aims to selectively kill cancer cells while sparing normal ones. Here we describe different ways that are currently explored to induce apoptosis in lung cancer cells, specifically pathways controlled by TNF-related apoptosis-inducing ligand (TRAIL), BCL-2 family members and apoptosis inhibitory proteins (IAPs). Preclinical studies are discussed and for some agents results from early clinical studies and future perspectives are considered.

Caspase-8 as a therapeutic target in cancer

Caspase-8 is an apical caspase which initiates programmed cell death following death receptor ligation. This central role in apoptosis has prompted significant clinical interest in regulating caspase-8 expression and proteolytic activity. However, caspase-8 has also been found to play a number of non-apoptotic roles in cells, such as promoting activation NF-κB signaling, regulating autophagy and altering endosomal trafficking, and enhancing cellular adhesion and migration. Therefore, depending upon the specific cellular context, caspase-8 may either potentiate or suppress tumor malignancy. Accordingly, a marked heterogeneity exists in the expression patterns of caspase-8 among different tumor types. Therapeutics have been developed which can increase caspase-8 expression, yet it remains unclear whether this approach will be beneficial in all cases. Care is warranted, and the role of caspase-8 should be addressed on a case by case basis.

Overexpression of Par-4 sensitizes TRAIL-induced apoptosis via inactivation of NF-kappaB and Akt signaling pathways in renal cancer cells

The prostate-apoptosis-response-gene-4 (Par-4) is up-regulated in prostate cells undergoing programmed cell death. Furthermore, Par-4 protein has been shown to function as an effector of cell death in response to various apoptotic stimuli that trigger mitochondria and membrane receptor-mediated cell death pathways. In this study, we investigated how Par-4 modulates TRAIL-mediated apoptosis in TRAIL-resistant Caki cells. Par-4 overexpressing cells were strikingly sensitive to apoptosis induced by TRAIL compared with control cells. Par-4 overexpressing Caki cells treated with TRAIL showed an increased activation of the initiator caspase-8 and the effector caspase-3, together with an enforced cleavage of XIAP and c-FLIP. TRAIL-induced reduction of XIAP and c-FLIP protein levels in Par-4 overexpressing cells was prevented by z-VAD pretreatment. In addition, the surface DR5 protein level was increased in TRAIL-treated Par-4 overexpressing cells. Interestingly, even though a deletion of leucine zipper domain in Par-4 recovered Bcl-2 level to basal level induced by wild type Par-4, it partly decreased sensitivity to TRAIL in Caki cells. In addition, exposure of Caki/Par-4 cells to TRAIL led to reduction of phosphorylated Akt levels, but deletion of leucine zipper domain of Par-4 did not affect these phosphorylated Akt levels. In conclusion, we here provide evidence that ectopic expression of Par-4 sensitizes Caki cells to TRAIL via modulation of multiple targets, including DR5, Bcl-2, Akt, and NF-kappaB.

Bortezomib sensitizes human esophageal squamous cell carcinoma cells to TRAIL-mediated apoptosis via activation of both extrinsic and intrinsic apoptosis pathways

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive human cancers, and novel treatment modalities are required. We investigated the therapeutic potential of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) in combination with the proteasome inhibitor bortezomib (Velcade) on human ESCC cell lines. Bortezomib enhanced the susceptibility to TRAIL in 12 of the 15 ESCC cell lines tested, although most showed low sensitivity to TRAIL as a single agent. The enhancement of TRAIL-induced apoptosis by bortezomib was caspase dependent. Increased processing of caspase-8 often accompanied enhancement of TRAIL-induced apoptosis by bortezomib. However, the increased cell surface expression of death receptors observed on bortezomib treatment did not seem to be crucial for this effect. For some ESCC, bortezomib treatment resulted in a more efficient recruitment of caspase-8 and the Fas-associated death domain to the death-inducing signaling complex. Additional downregulation of the cellular FLICE-inhibitory protein long isoform [c-FLIP(L)] could cooperate in the activation of the extrinsic pathway in some cases. For other ESCC, the crucial effect of bortezomib treatment seemed to be increased signaling via the intrinsic apoptotic pathway on subsequent exposure to TRAIL. Thus, bortezomib could sensitize ESCC to TRAIL apoptosis by multiple molecular mechanisms of action. Therefore, the combination of bortezomib and TRAIL might be a novel therapeutic strategy for ESCC patients who fail to respond to standard chemoradiotherapy that predominantly targets the mitochondrial apoptotic pathway.

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.

Bortezomib restores stroma-mediated APO2L/TRAIL apoptosis resistance in multiple myeloma

OBJECTIVES

Hematopoietic stroma promotes resistance to immune control by APO2L/TRAIL in multiple myeloma (MM) cells in part by increasing synthesis of the anti-apoptotic protein c-FLIP. Here, we tested whether bortezomib can reverse the APO2L/TRAIL environmental mediated-immune resistance (EM-IR).

MATERIAL AND METHODS

MM cell lines (RPMI 8226 and U266) and CD138+ patient's MM cells were directly adhered to HS5 stroma exposed to HS5 or bone marrow stroma of patients with MM released soluble factors in a transwell system. Cells were treated with either APO2L/TRAIL (10 ng/mL), bortezomib (10 nm) or both.

RESULTS

Pretreatment with bortezomib effectively overcomes APO2L/TRAIL apoptosis resistance in myeloma cell lines and in CD138+ cells while directly adhered or in transwell assay. Bortezomib was not cytotoxic to HS5 stroma cells and only altered monocyte chemotactic protein-2-3 and IL-10 levels in the stroma-myeloma milieu. Factors released by HS5 stroma increased expression of c-FLIP, induced STAT-3 and ERK phosphorylation and reduced DR4 receptor expression in MM cells. HS5 stroma-released factor(s) induced NF-kappaB activation after 20 h exposure in association with an enhanced c-FLIP transcription. Bortezomib effectively reduced c-FLIP protein expression without affecting other proteins. Bortezomib also increased DR4 and DR5 expression in the presence of stroma.

CONCLUSIONS

These findings provide the rationale to combine bortezomib and APO2L/TRAIL to disrupt the influence of the stroma microenvironment on MM cells.

TRAIL-induced apoptosis: between tumor therapy and immunopathology

The death ligand members of the tumor necrosis factor (TNF) family are potent inducers of apoptosis in a variety of cell types. In particular, TNF-related apoptosis-inducing ligand (TRAIL) has recently received much scientific and commercial attention because of its potent tumor cell-killing activity while leaving normal untransformed cells mostly unaffected. Furthermore, TRAIL strongly synergizes with conventional chemotherapeutic drugs in inducing tumor cell apoptosis, making it a most promising candidate for future cancer therapy. Increasing evidence indicates, however, that TRAIL may also induce or modulate apoptosis in primary cells. A particular concern is the potential side effect of TRAIL-based tumor therapies in the liver. In this review we summarize some of the recent findings on the role of TRAIL in tumor cell and hepatocyte apoptosis.

Paclitaxel promotes a caspase 8-mediated apoptosis through death effector domain association with microtubules

Microtubule-perturbing drugs have become front line chemotherapeutics, inducing cell cycle crisis as a major mechanism of action. However, these agents exhibit pleiotropic effects on cells, and can induce apoptosis via other means. Paclitaxel, a microtubule-stabilizing agent, induces a caspase-dependent apoptosis, though the precise mechanism(s) remain unclear. Here, we used genetic approaches to evaluate the role of caspase 8 in paclitaxel-mediated apoptosis. We observed that caspase 8-expressing cells are more sensitive to paclitaxel than caspase 8-deficient cells. Mechanistically, caspase 8 was found associated with microtubules, and this interaction increased following paclitaxel-treatment. The prodomains (DEDs) of caspase 8 were sufficient for interaction with microtubules, but the caspase 8 holoprotein was required for apoptosis. DED-only forms of caspase 8 were found in both primary and tumor cell lines, associating with perinuclear microtubules and the centrosome. Microtubule-association, and paclitaxel-sensitivity, depends upon a critical lysine (K156) within a microtubule-binding motif (KLD) in DED-b of caspase 8. The results reveal an unexpected pathway of apoptosis mediated by caspase 8.

Sensitization of human hepatic stellate cells to tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis by leflunomide

During the resolution phase of hepatic fibrosis, a crucial mechanism is the apoptosis of activated hepatic stellate cells (HSCs). It is necessary to find more anti-fibrosis drugs that would modulate HSCs to be more susceptible to apoptotic stimuli. Here we showed that A771726, the active metabolite of leflunomide, markedly enhanced tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in the human hepatic stellate cell line LX-2. A771726 could increase caspase activity in LX-2 cells in a dose-dependent manner. A771726 did not increase the expression of TRAIL receptors in LX-2 cells but could inhibit activation of the c-Jun NH2-terminal kinase (JNK) pathway through decreasing TRAIL-induced JNK and c-Jun phosphorylation. Moreover, A771726 could accelerate TRAIL-induced apoptosis via inhibiting nuclear factor-kappaB (NF-kappaB) activation in LX-2 cells. In conclusion, our results indicated leflunomide could enhance the sensitivity of LX-2 cells to TRAIL-induced apoptosis via inhibiting the survival pathways and provided a promising approach to anti-fibrotic therapy with leflunomide.

NFkappaB signaling in carcinogenesis and as a potential molecular target for cancer therapy

It has become increasingly clear that deregulation of the NFkappaB signaling cascade is a common underlying feature of many human ailments including cancers. The past two decades of intensive research on NFkappaB has identified the basic mechanisms that govern the functioning of this pathway but uncovering the details of why this pathway works differently in different cellular contexts or how it interacts with other signaling pathways remains a challenge. A thorough understanding of these processes is needed to design better and more efficient therapeutic approaches to treat complex diseases like cancer. In this review, we summarize the literature documenting the involvement of NFkappaB in cancer, and then focus on the approaches that are being undertaken to develop NFkappaB inhibitors towards treatment of human cancers.

Bortezomib pre-treatment prolongs interferon-alpha-induced STAT1 phosphorylation in melanoma cells

Bortezomib is a proteasome inhibitor that can synergize with interferon-alpha (IFN-alpha) to induce apoptosis in melanoma cells in vitro and inhibit tumor growth in vivo. We hypothesized that proteasome inhibition may be an effective means to sensitize melanoma cells to the direct effects of IFN-alpha. Pre-treatment of human melanoma cells with bortezomib led to significantly increased transcription of interferon-stimulated genes as determined by real-time PCR. Flow cytometric and immunoblot analyses indicated that the enhanced direct actions of IFN-alpha on melanoma cells were the result of prolonged phosphorylation of STAT1 (P-STAT1) on both the Tyrosine(701) and Serine(727) residues. In contrast, the enhanced IFN-alpha-induced P-STAT1 was not observed in peripheral blood mononuclear cells that were pre-treated with bortezomib. These data suggest that proteasome inhibition represents a mechanism to enhance the direct effects of IFN-alpha on melanoma cells thereby complementing its immunostimulatory properties.

Bortezomib overcomes tumor necrosis factor-related apoptosis-inducing ligand resistance in hepatocellular carcinoma cells in part through the inhibition of the phosphatidylinositol 3-kinase/Akt pathway

Hepatocellular carcinoma (HCC) is one of the most common and aggressive human malignancies. Recombinant tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anti-tumor agent. However, many HCC cells show resistance to TRAIL-induced apoptosis. In this study, we showed that bortezomib, a proteasome inhibitor, overcame TRAIL resistance in HCC cells, including Huh-7, Hep3B, and Sk-Hep1. The combination of bortezomib and TRAIL restored the sensitivity of HCC cells to TRAIL-induced apoptosis. Comparing the molecular change in HCC cells treated with these agents, we found that down-regulation of phospho-Akt (P-Akt) played a key role in mediating TRAIL sensitization of bortezomib. The first evidence was that bortezomib down-regulated P-Akt in a dose- and time-dependent manner in TRAIL-treated HCC cells. Second, LY294002, a PI3K inhibitor, also sensitized resistant HCC cells to TRAIL-induced apoptosis. Third, knocking down Akt1 by small interference RNA also enhanced TRAIL-induced apoptosis in Huh-7 cells. Finally, ectopic expression of mutant Akt (constitutive active) in HCC cells abolished TRAIL sensitization effect of bortezomib. Moreover, okadaic acid, a protein phosphatase 2A (PP2A) inhibitor, reversed down-regulation of P-Akt in bortezomib-treated cells, and PP2A knockdown by small interference RNA also reduced apoptosis induced by the combination of TRAIL and bortezomib, indicating that PP2A may be important in mediating the effect of bortezomib on TRAIL sensitization. Together, bortezomib overcame TRAIL resistance at clinically achievable concentrations in hepatocellular carcinoma cells, and this effect is mediated at least partly via inhibition of the PI3K/Akt pathway.

TRAIL receptor signalling and modulation: Are we on the right TRAIL?

Tumour necrosis factor-related apoptosis-inducing ligand or Apo2 ligand (TRAIL/Apo2L) is a member of the tumour necrosis factor (TNF) superfamily of cytokines that induces apoptosis upon binding to its death domain-containing transmembrane receptors, death receptors 4 and 5 (DR4, DR5). Importantly, TRAIL preferentially induces apoptosis in cancer cells while exhibiting little or no toxicity in normal cells. To date, research has focused on the mechanism of apoptosis induced by TRAIL and the processes involved in the development of TRAIL resistance. TRAIL-resistant tumours can be re-sensitized to TRAIL by a combination of TRAIL with chemotherapeutics or irradiation. Studies suggest that in many cancer cells only one of the two death-inducing TRAIL receptors is functional. These findings as well as the aim to avoid decoy receptor-mediated neutralization of TRAIL led to the development of receptor-specific TRAIL variants and agonistic antibodies. These molecules are predicted to be more potent than native TRAIL in vivo and may be suitable for targeted treatment of particular tumours. This review focuses on the current status of TRAIL receptor-targeting for cancer therapy, the apoptotic signalling pathway induced by TRAIL receptors, the prognostic implications of TRAIL receptor expression and modulation of TRAIL sensitivity of tumour cells by combination therapies. The mechanisms of TRAIL resistance and the potential measures that can be taken to overcome them are also addressed. Finally, the status of clinical trials of recombinant TRAIL and DR4-/DR5-specific agonistic antibodies as well as the pre-clinical studies of receptor-selective TRAIL variants is discussed including the obstacles facing the use of these molecules as anti-cancer therapeutics.

Bcl-2 family proteins contribute to apoptotic resistance in lung cancer multicellular spheroids

Combinatorial therapies using the proteasome inhibitor, bortezomib, have been found to induce synergistic apoptosis in cancer cells grown as monolayers; however, three-dimensional spheroid culture may be a better model for the multicellular resistance found in solid tumors, such as lung cancer. We tested the combinatorial apoptotic strategy of using bortezomib together with TNF-related apoptosis-inducing ligand (TRAIL), both in monolayers and in spheroids of A549 lung cancer cells. Indeed, bortezomib plus TRAIL induced synergistic apoptosis in A549 cells grown as monolayers, but had little effect on A549 cells grown as three-dimensional multicellular spheroids. The acquired resistance of spheroids was not due to a limitation of diffusion, to survival pathways, such as NF-kappaB or PI3K/Akt/mTOR, or to the up-regulation of FLIP(S) (Fas-associated death domain-like IL-1 beta-converting enzyme inhibitory protein, short). We then investigated a role for the Bcl-2 family of anti- and proapoptotic proteins. When cells formed spheroids, antiapoptotic Bcl-2 increased, whereas antiapoptotic Mcl-1 decreased. ABT-737, a small molecule that inhibits Bcl-2, but not Mcl-1, abolished the multicellular resistance of A549 spheroids to bortezomib plus TRAIL. In another lung cancer cell line, H1299, acquisition of multicellular resistance in spheroids was also accompanied by an increase in Bcl-2 and decrease in Mcl-1. In H1299 spheroids compared with those of A549, however, Mcl-1 remained higher, and Mcl-1 knockdown was more effective than ABT-737 in removing multicellular resistance. Our study suggests that the balance of Bcl-2 family proteins contributes to the acquired multicellular resistance of spheroids, and suggests a possible target for improving the response of lung cancer to bortezomib therapies.

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

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

Proteasome inhibitors enhance TRAIL-induced apoptosis through the intronic regulation of DR5: involvement of NF-kappa B and reactive oxygen species-mediated p53 activation

Manipulation of TRAIL receptor 2 (DR5) pathway is a promising therapeutic strategy to overcome TRAIL-resistant lung cancer cells. Preclinical studies have shown that proteasome inhibitors enhance TRAIL-induced apoptosis in lung cancer cells, but the underlying mechanism has not been fully elucidated. In this study, we demonstrated the enhancement of TRAIL-mediated apoptosis in human alveolar epithelial cells by proteasome inhibitors that up-regulate DR5 expression. This effect was blocked by DR5-neutralizing Ab. Using reporter assay, we demonstrated that the p53 and NF-kappaB elements on the DR5 first intron region were involved in proteasome inhibitor-induced DR5 expression. Both p53 small interfering RNA and NF-kappaB inhibitor suppressed DR5 expression, strengthening the significance of p53 and NF-kappaB in DR5 transcription. The protein stability, Ser(392) phosphorylation and Lys(373)/Lys(382) acetylation of p53 were enhanced by MG132. In addition to p53, IkappaBalpha degradation and NF-kappaB translocation was also observed. Moreover, the binding of p53 and p65 to the first intron of DR5 was demonstrated by DNA affinity protein-binding and chromatin immunoprecipitation assays. Intracellular reactive oxygen species (ROS) generation after MG132 treatment contributed to p53, but not p65 nuclear translocation and DNA-binding activity. ROS scavenger dramatically inhibited the apoptosis induced by proteasome inhibitors plus TRAIL. The p53-null H1299 cells were resistant to proteasome inhibitor-induced DR5 up-regulation and enhancement of TRAIL-induced apoptosis. These findings reveal that proteasome inhibitor-mediated NF-kappaB and ROS-dependent p53 activation are contributed to intronic regulation of DR5 transcription, and resulted in the subsequent enhancement of TRAIL-induced apoptosis in human lung cancer cells.

In vivo gene delivery by embryonic-stem-cell-derived astrocytes for malignant gliomas

The treatment of malignant gliomas with current therapies remains a challenge in neurooncology. Our recent work showed that embryonic stem cell (ESC)-derived astrocytes conditionally expressing genes can be used to induce apoptosis in malignant glioma cells in vitro. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) gene has been shown to induce apoptosis in a variety of tumor cells, including gliomas. The aim of this study was to assess the proapoptotic effects of transgenic TRAIL delivered by ESC-derived astrocytes on malignant gliomas in vivo. Malignant glioma A172 cells were used to induce heterotopic xenografts in nude mice. ESC-derived astrocytes conditionally expressing TRAIL were injected into the xenografts. TRAIL expression was documented in the malignant glioma xenografts by reverse transcription PCR and immunohistochemistry after external gene induction. A significant reduction in tumor volume occurred 48 h after a single injection (14%) and double injections (31%) in the experimental groups. Terminal dUTP nick end labeling (TUNEL) revealed abundant apoptotic tumor cells in the experimental groups. Seven days after injection, the tumor had undergone severe necrosis, with only scattered residual tumor cells at the periphery. Death receptor DR4 expression increased significantly in the experimental groups compared with controls. Our data suggest that ESC-derived astrocytes conditionally expressing TRAIL should be considered as vectors to deliver gene therapy for malignant gliomas.

Analysis of changes in the proteome of HL-60 promyeloid leukemia cells induced by the proteasome inhibitor PSI

Proteasome inhibitors display potent anti-neoplastic and anti-angiogenic properties both in vitro and in vivo. The mechanisms, however, by which proteasome inhibitors kill tumor cells are still fairly elusive as is the molecular basis of resistance to treatment. To address these questions, we employed a high-throughput Western blotting procedure to analyze changes in a subproteome of approximately 800 proteins in the promyelocytic leukemia cell line HL-60 upon treatment with the proteasome inhibitor PSI (Z-Ile-Glu(OtBu)-Ala-Leu-aldehyde) and correlated the changes of selected target proteins with the changes in two multidrug-resistant HL-60 variants. In total, 105 proteins were upregulated more than 1.5-fold after PSI treatment, while 79 proteins were downregulated. Activation of caspases-3 and -8, modulation of members of the Bcl-2 family as well as stimulation of stress signaling pathways was prominent during HL-60 apoptosis. We also identified changes in the abundance of proteins previously not known to be affected by proteasome inhibitors. In contrast, two multidrug-resistant HL-60 cell lines, overexpressing either MRP1 or P-glycoprotein were largely resistant to PSI-induced apoptosis and could not be resensitized by the pharmacological inhibitors of the drug efflux pumps MK571 or PSC833. Drug resistance was also independent of the upregulation of Bad. Overexpression of multidrug resistance proteins, P-glycoprotein and MRP-1 is thus not sufficient to explain resistance of HL-60 cells to treatment with proteasome inhibitor PSI, which remains more closely related to a low level of Bax expression and to the inability to activate JNK. Alternative routes to the acquisition of resistance to PSI have therefore to be considered.

TRAIL and cancer therapy

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors are promising targets for the selective eradication of tumor cells while sparing normal cells. Currently, both recombinant TRAIL proteins and TRAIL receptor agonistic antibodies are being tested in the clinic, showing encouraging antitumor activities and mild side effects. Unfortunately, resistance to TRAIL therapy is frequently encountered requiring combined treatments with sensitizing agents. Standard chemotherapeutics can enhance TRAIL sensitivity; however, more specific and less toxic agents are needed to exploit the full antitumor potential of TRAIL. Here, a brief overview of the TRAIL signaling pathway is given together with a short description of early results obtained with TRAIL therapy in the clinic. Mechanisms of TRAIL resistance and ways to overcome these by targeted agents that either neutralize apoptotic blockades or suppress prosurvival signals also triggered by TRAIL are highlighted, such as inhibitors of IAPs, Bcl-2 family members, HDACi, and modulators of NF-kappaB, Raf and EGFR signaling.

The proteasomal and apoptotic phenotype determine bortezomib sensitivity of non-small cell lung cancer cells

Bortezomib is a novel anti-cancer agent which has shown promising activity in non-small lung cancer (NSCLC) patients. However, only a subset of patients respond to this treatment. We show that NSCLC cell lines are differentially sensitive to bortezomib, IC50 values ranging from 5 to 83 nM. The apoptosis-inducing potential of bortezomib in NSCLC cells was found to be dependent not only on the apoptotic phenotype but also on the proteasomal phenotype of individual cell lines. Upon effective proteasome inhibition, H460 cells were more susceptible to apoptosis induction by bortezomib than SW1573 cells, indicating a different apoptotic phenotype. However, exposure to a low dose of bortezomib did only result in SW1573 cells, and not in H460 cells, in inhibition of proteasome activity and subsequent apoptosis. This suggests a different proteasomal phenotype as well. Additionally, overexpression of anti-apoptotic protein Bcl-2 in H460 cells did not affect the proteasomal phenotype of H460 cells but did result in decreased bortezomib-induced apoptosis. In conclusion, successful proteasome-inhibitor based treatment strategies in NSCLC face the challenge of having to overcome apoptosis resistance as well as proteasomal resistance of individual lung cancer cells. Further studies in NSCLC are warranted to elucidate underlying mechanisms.