Proteasome inhibitor PS-341 (VELCADE) induces stabilization of the TRAIL receptor DR5 mRNA through the 3'-untranslated region

Generation of TRAIL-resistant cell line models reveals distinct adaptive mechanisms for acquired resistance and re-sensitization

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces tumor cell-specific apoptosis, making it a prime therapeutic candidate. However, many tumor cells are either innately TRAIL-resistant, or they acquire resistance with adaptive mechanisms that remain poorly understood. In this study, we generated acquired TRAIL resistance models using multiple glioblastoma (GBM) cell lines to assess the molecular alterations in the TRAIL-resistant state. We selected TRAIL-resistant cells through chronic and long-term TRAIL exposure and noted that they showed persistent resistance both in vitro and in vivo. Among known TRAIL-sensitizers, proteosome inhibitor Bortezomib, but not HDAC inhibitor MS-275, was effective in overcoming resistance in all cell models. This was partly achieved through upregulating death receptors and pro-apoptotic proteins, and downregulating major anti-apoptotic members, Bcl-2 and Bcl-xL. We showed that CRISPR/Cas9 mediated silencing of DR5 could block Bortezomib-mediated re-sensitization, demonstrating its critical role. While overexpression of Bcl-2 or Bcl-xL was sufficient to confer resistance to TRAIL-sensitive cells, it failed to override Bortezomib-mediated re-sensitization. With RNA sequencing in multiple paired TRAIL-sensitive and TRAIL-resistant cells, we identified major alterations in inflammatory signaling, particularly in the NF-κB pathway. Inhibiting NF-κB substantially sensitized the most resistant cells to TRAIL, however, the sensitization effect was not as great as what was observed with Bortezomib. Together, our findings provide new models of acquired TRAIL resistance, which will provide essential tools to gain further insight into the heterogeneous therapy responses within GBM tumors. Additionally, these findings emphasize the critical importance of combining proteasome inhibitors and pro-apoptotic ligands to overcome acquired resistance.

Proteasome Inhibitor Drugs

Proteasomes are large, multicatalytic protein complexes that cleave cellular proteins into peptides. There are many distinct forms of proteasomes that differ in catalytically active subunits, regulatory subunits, and associated proteins. Proteasome inhibitors are an important class of drugs for the treatment of multiple myeloma and mantle cell lymphoma, and they are being investigated for other diseases. Bortezomib (Velcade) was the first proteasome inhibitor to be approved by the US Food and Drug Administration. Carfilzomib (Kyprolis) and ixazomib (Ninlaro) have recently been approved, and more drugs are in development. While the primary mechanism of action is inhibition of the proteasome, the downstream events that lead to selective cell death are not entirely clear. Proteasome inhibitors have been found to affect protein turnover but at concentrations that are much higher than those achieved clinically, raising the possibility that some of the effects of proteasome inhibitors are mediated by other mechanisms.

Proteasome Inhibitors Suppress ErbB Family Expression through HSP90-Mediated Lysosomal Degradation

Although dual EGFR/HER2 tyrosine kinase inhibitor lapatinib has provided effective clinical benefits for HER2-positive breast cancer patients, acquired resistance to this drug remains a major concern. Thus, the development of alternative therapeutic strategies is urgently needed for patients who failed lapatinib treatment. Proteasome inhibitors have been reported to possess high anti-tumor activity to breast cancer cells. Therefore, this study aims to examine whether and how proteasome inhibitor bortezomib can overcome lapatinib resistance. Treatments with several proteasome inhibitors, including Bortezomib, MG132, and proteasome inhibitor I (PSI), as well as the viabilities of both HER2-positive breast cancer cell lines and their lapatinib-resistant clones, were inhibited. Importantly, the expressions of ErbB family were downregulated at both transcriptional and translational levels. Also, our results further indicated that proteasome inhibitors decreased ErbB family expression through lysosomal degradation pathway in a heat shock protein 90 (HSP90)-dependent manner. In this study, our data supported a potential approach to overcome the acquired resistance of HER2-overexpressing breast cancer patients to lapatinib using proteasome inhibitors.

Pharmacological Targeting of Cell Cycle, Apoptotic and Cell Adhesion Signaling Pathways Implicated in Chemoresistance of Cancer Cells

Chemotherapeutic drugs target a physiological differentiating feature of cancer cells as they tend to actively proliferate more than normal cells. They have well-known side-effects resulting from the death of highly proliferative normal cells in the gut and immune system. Cancer treatment has changed dramatically over the years owing to rapid advances in oncology research. Developments in cancer therapies, namely surgery, radiotherapy, cytotoxic chemotherapy and selective treatment methods due to better understanding of tumor characteristics, have significantly increased cancer survival. However, many chemotherapeutic regimes still fail, with 90% of the drug failures in metastatic cancer treatment due to chemoresistance, as cancer cells eventually develop resistance to chemotherapeutic drugs. Chemoresistance is caused through genetic mutations in various proteins involved in cellular mechanisms such as cell cycle, apoptosis and cell adhesion, and targeting those mechanisms could improve outcomes of cancer therapy. Recent developments in cancer treatment are focused on combination therapy, whereby cells are sensitized to chemotherapeutic agents using inhibitors of target pathways inducing chemoresistance thus, hopefully, overcoming the problems of drug resistance. In this review, we discuss the role of cell cycle, apoptosis and cell adhesion in cancer chemoresistance mechanisms, possible drugs to target these pathways and, thus, novel therapeutic approaches for cancer treatment.

Bortezomib-induced miRNAs direct epigenetic silencing of locus genes and trigger apoptosis in leukemia

MicroRNAs (miRNAs) have been suggested to repress transcription via binding the 3′-untranslated regions of mRNAs. However, the involvement and details of miRNA-mediated epigenetic regulation, particularly in targeting genomic DNA and mediating epigenetic regulation, remain largely uninvestigated. In the present study, transcription factor CCAAT/enhancer binding protein delta (CEBPD) was responsive to the anticancer drug bortezomib, a clinical and highly selective drug for leukemia treatment, and contributed to bortezomib-induced cell death. Interestingly, following the identification of CEBPD-induced miRNAs, we found that miR-744, miR-3154 and miR-3162 could target CpG islands in the 5′-flanking region of the CEBPD gene. We previously demonstrated that the Yin Yang 1 (YY1)/polycomb group (PcG) protein/DNA methyltransferase (DNMT) complex is important for CCAAT/enhancer binding protein delta (CEBPD) gene inactivation; we further found that Argonaute 2 (Ago2) interacts with YY1 and binds to the CEBPD promoter. The miRNA/Ago2/YY1/PcG group protein/DNMT complex linked the inactivation of CEBPD and genes adjacent to its 5′-flanking region, including protein kinase DNA-activated catalytic polypeptide (PRKDC), minichromosome maintenance-deficient 4 (MCM4) and ubiquitin-conjugating enzyme E2 variant 2 (UBE2V2), upon bortezomib treatment. Moreover, we revealed that miRNA binding is necessary for YY1/PcG group protein/DNMT complex-mediated epigenetic gene silencing and is associated with bortezomib-induced methylation on genomic DNA. The present study successfully characterized the interactions of the miRNA/Ago2/YY1/PcG group protein/DNMT complex and provided new insights for miRNA-mediated epigenetic regulation in bortezomib-induced leukemic cell arrest and cell death.

Intracellular localization of DR5 and related regulatory pathways as a mechanism of resistance to TRAIL in cancer

TNF-related apoptosis-inducing ligand (TRAIL) is a prominent cytokine capable of inducing apoptosis. It can bind to five different cognate receptors, through which diverse intracellular pathways can be activated. TRAIL's ability to preferentially kill transformed cells makes it a promising potential weapon for targeted tumor therapy. However, recognition of several resistance mechanisms to TRAIL-induced apoptosis has indicated that a thorough understanding of the details of TRAIL biology is still essential before this weapon can be confidently unleashed. Critical to this aim is revealing the functions and regulation mechanisms of TRAIL's potent death receptor DR5. Although expression and signaling mechanisms of DR5 have been extensively studied, other aspects, such as its subcellular localization, non-signaling functions, and regulation of its membrane transport, have only recently attracted attention. Here, we discuss different aspects of TRAIL/DR5 biology, with a particular emphasis on the factors that seem to influence the cell surface expression pattern of DR5, along with factors that lead to its nuclear localization. Disturbance of this balance apparently affects the sensitivity of cancer cells to TRAIL-mediated apoptosis, thus constituting an eligible target for potential new therapeutic agents.

Chloroquine enhances TRAIL-mediated apoptosis through up-regulation of DR5 by stabilization of mRNA and protein in cancer cells

Chloroquine (CQ), an anti-malarial drug, has immune-modulating activity and lysosomotropic activity. In this study, we investigated CQ sensitizes TRAIL-mediated apoptosis in human renal cancer Caki cells. Combination of CQ and TRAIL significantly induces apoptosis in human renal cancer Caki cells and various human cancer cells, but not in normal mouse kidney cells (TMCK-1) and human mesangial cells (MC). CQ up-regulates DR5 mRNA and protein expression in a dose- and time- dependent manner. Interestingly, CQ regulates DR5 expression through the increased stability in the mRNA and protein of DR5, rather than through the increased transcriptional activity of DR5. Moreover, we found that CQ decreased the expression of Cbl, an E3 ligase of DR5, and knock-down of Cbl markedly enhanced DR5 up-regulation. Other lysosomal inhibitors, including monensin and nigericin, also up-regulated DR5 and sensitized TRAIL-mediated apoptosis. Therefore, this study demonstrates that lysosomal inhibition by CQ may sensitize TRAIL-mediated apoptosis in human renal cancer Caki cells via DR5 up-regulation.

Deletion of Chromosomal Region 8p21 Confers Resistance to Bortezomib and Is Associated with Upregulated Decoy TRAIL Receptor Expression in Patients with Multiple Myeloma

Loss of the chromosomal region 8p21 negatively effects survival in patients with multiple myeloma (MM) that undergo autologous stem cell transplantation (ASCT). In this study, we aimed to identify the immunological and molecular consequences of del(8)(p21) with regards to treatment response and bortezomib resistance. In patients receiving bortezomib as a single first line agent without any high-dose therapy, we have observed that patients with del(8)(p21) responded poorly to bortezomib with 50% showing no response while patients without the deletion had a response rate of 90%. In vitro analysis revealed a higher resistance to bortezomib possibly due to an altered gene expression profile caused by del(8)(p21) including genes such as TRAIL-R4, CCDC25, RHOBTB2, PTK2B, SCARA3, MYC, BCL2 and TP53. Furthermore, while bortezomib sensitized MM cells without del(8)(p21) to TRAIL/APO2L mediated apoptosis, in cells with del(8)(p21) bortezomib failed to upregulate the pro-apoptotic death receptors TRAIL-R1 and TRAIL-R2 which are located on the 8p21 region. Also expressing higher levels of the decoy death receptor TRAIL-R4, these cells were largely resistant to TRAIL/APO2L mediated apoptosis. Corroborating the clinical outcome of the patients, our data provides a potential explanation regarding the poor response of MM patients with del(8)(p21) to bortezomib treatment. Furthermore, our clinical analysis suggests that including immunomodulatory agents such as Lenalidomide in the treatment regimen may help to overcome this negative effect, providing an alternative consideration in treatment planning of MM patients with del(8)(p21).

Trailing TRAIL Resistance: Novel Targets for TRAIL Sensitization in Cancer Cells

Resistance to chemotherapeutic drugs is the major hindrance in the successful cancer therapy. The tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor (TNF) family of ligands, which initiates apoptosis in cancer cells through interaction with the death receptors DR4 and DR5. TRAIL is perceived as an attractive chemotherapeutic agent as it specifically targets cancer cells while sparing the normal cells. However, TRAIL therapy has a major limitation as a large number of the cancer develop resistance toward TRAIL and escape from the destruction by the immune system. Therefore, elucidation of the molecular targets and signaling pathways responsible for TRAIL resistance is imperative for devising effective therapeutic strategies for TRAIL resistant cancers. Although, various molecular targets leading to TRAIL resistance are well-studied, recent studies have implicated that the contribution of some key cellular processes toward TRAIL resistance need to be fully elucidated. These processes primarily include aberrant protein synthesis, protein misfolding, ubiquitin regulated death receptor expression, metabolic pathways, epigenetic deregulation, and metastasis. Novel synthetic/natural compounds that could inhibit these defective cellular processes may restore the TRAIL sensitivity and combination therapies with such compounds may resensitize TRAIL resistant cancer cells toward TRAIL-induced apoptosis. In this review, we have summarized the key cellular processes associated with TRAIL resistance and their status as therapeutic targets for novel TRAIL-sensitizing agents.

Regulation of TRAIL-receptor expression by the ubiquitin-proteasome system

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand- receptor (TRAIL-R) family has emerged as a key mediator of cell fate and survival. Ligation of TRAIL ligand to TRAIL-R1 or TRAIL-R2 initiates the extrinsic apoptotic pathway characterized by the recruitment of death domains, assembly of the death-inducing signaling complex (DISC), caspase activation and ultimately apoptosis. Conversely the decoy receptors TRAIL-R3 and TRAIL-R4, which lack the pro-apoptotic death domain, function to dampen the apoptotic response by competing for TRAIL ligand. The tissue restricted expression of the decoy receptors on normal but not cancer cells provides a therapeutic rational for the development of selective TRAIL-mediated anti-tumor therapies. Recent clinical trials using agonistic antibodies against the apoptosis-inducing TRAIL receptors or recombinant TRAIL have been promising; however the number of patients in complete remission remains stubbornly low. The mechanisms of TRAIL resistance are relatively unexplored but may in part be due to TRAIL-R down-regulation or shedding of TRAIL-R by tumor cells. Therefore a better understanding of the mechanisms underlying TRAIL resistance is required. The ubiquitin-proteasome system (UPS) has been shown to regulate TRAIL-R members suggesting that pharmacological inhibition of the UPS may be a novel strategy to augment TRAIL-based therapies and increase efficacies. We recently identified b-AP15 as an inhibitor of proteasome deubiquitinase (DUB) activity. Interestingly, exposure of tumor cell lines to b-AP15 resulted in increased TRAIL-R2 expression and enhanced sensitivity to TRAIL-mediated apoptosis and cell death in vitro and in vivo. In conclusion, targeting the UPS may represent a novel strategy to increase the cell surface expression of pro-apoptotic TRAIL-R on cancer cells and should be considered in clinical trials targeting TRAIL-receptors in cancer patients.

Rapid proteasomal degradation of posttranscriptional regulators of the TIS11/tristetraprolin family is induced by an intrinsically unstructured region independently of ubiquitination

The TIS11/tristetraprolin (TTP) CCCH tandem zinc finger proteins are major effectors in the destabilization of mRNAs bearing AU-rich elements (ARE) in their 3' untranslated regions. In this report, we demonstrate that the Drosophila melanogaster dTIS11 protein is short-lived due to its rapid ubiquitin-independent degradation by the proteasome. Our data indicate that this mechanism is tightly associated with the intrinsically unstructured, disordered N- and C-terminal domains of the protein. Furthermore, we show that TTP, the mammalian TIS11/TTP protein prototype, shares the same three-dimensional characteristics and is degraded by the same proteolytic pathway as dTIS11, thereby indicating that this mechanism has been conserved across evolution. Finally, we observed a phosphorylation-dependent inhibition of dTIS11 and TTP degradation by the proteasome in vitro, raising the possibility that such modifications directly affect proteasomal recognition for these proteins. As a group, RNA-binding proteins (RNA-BPs) have been described as enriched in intrinsically disordered regions, thus raising the possibility that the mechanism that we uncovered for TIS11/TTP turnover is widespread among other RNA-BPs.

DAPK2 is a novel modulator of TRAIL-induced apoptosis

Targeting molecules involved in TRAIL-mediated signalling has been hailed by many as a potential magic bullet to kill cancer cells efficiently, with little side effects on normal cells. Indeed, initial clinical trials showed that antibodies against TRAIL receptors, death receptor (DR)4 and DR5, are well tolerated by cancer patients. Despite efficacy issues in the clinical setting, novel approaches to trigger TRAIL-mediated apoptosis are being developed and its clinical potential is being reappraised. Unfortunately, as observed with other cancer therapies, many patients develop resistance to TRAIL-induced apoptosis and there is thus impetuous for identifying additional resistance mechanisms that may be targetable and usable in combination therapies. Here, we show that the death-associated protein kinase 2 (DAPK2) is a modulator of TRAIL signalling. Genetic ablation of DAPK2 using RNA interference causes phosphorylation of NF-κB and its transcriptional activity in several cancer cell lines. This then leads to the induction of a variety of NF-κB target genes, which include proapoptotic DR4 and DR5. DR4 and DR5 protein expression is correspondingly increased on the cell surface and this leads to the sensitisation of resistant cells to TRAIL-induced killing, in a p53-independent manner. As DAPK2 is a kinase, it is imminently druggable, and our data thus offer a novel avenue to overcome TRAIL resistance in the clinic.

RNA-binding protein-mediated post-transcriptional controls of gene expression: integration of molecular mechanisms at the 3' end of mRNAs?

Initially identified as an occasional and peculiar mode of gene regulation in eukaryotes, RNA-binding protein-mediated post-transcriptional control of gene expression has emerged, over the last two decades, as a major contributor in the control of gene expression. A large variety of RNA-binding proteins (RBPs) allows the recognition of very diverse messenger RNA sequences and participates in the regulation of basically all cellular processes. Nevertheless, the rapid outcome of post-transcriptional regulations on the level of gene expression has favored the expansion of this type of regulation in cellular processes prone to rapid and frequent modulations such as the control of the inflammatory response. At the molecular level, the 3'untranslated region (3'UTR) of mRNA is a favored site of RBP recruitment. RBPs binding to these regions control gene expression through two major modes of regulation, namely mRNA decay and modulation of translational activity. Recent progresses suggest that these two mechanisms are often interdependent and might result one from the other. Therefore, different RBPs binding distinct RNA subsets could share similar modes of action at the molecular level. RBPs are frequent targets of post-translational modifications, thereby disclosing numerous possibilities for pharmacological interventions. However, redundancies of the transduction pathways controlling these modifications have limited the perspectives to define RBPs as new therapeutic targets. Through the analysis of several examples of RBPs binding to 3'untranslated region of mRNA, we present here recent progress and perspectives regarding this rapidly evolving field of molecular biology.

HuR's post-transcriptional regulation of Death Receptor 5 in pancreatic cancer cells

Apoptosis is one of the core signaling pathways disrupted in pancreatic ductal adenocarcinoma (PDA). Death receptor 5 (DR5) is a member of the tumor necrosis factor (TNF)-receptor superfamily that is expressed in cancer cells. Binding of TNF-related apoptosis-inducing ligand (TRAIL) to DR5 is a potent trigger of the extrinsic apoptotic pathway, and numerous clinical trials are based on DR5-targeted therapies for cancer, including PDA. Human antigen R (HuR), an RNA-binding protein, regulates a select number of transcripts under stress conditions. Here we report that HuR translocates from the nucleus to the cytoplasm of PDA cells upon treatment with a DR5 agonist. High doses of DR5 agonist induce cleavage of both HuR and caspase 8. HuR binds to DR5 mRNA at the 5'-untranslated region (UTR) in PDA cells in response to different cancer-associated stressors and subsequently represses DR5 protein expression; silencing HuR augments DR5 protein production by enabling its translation and thus enhances apoptosis. In PDA specimens (n = 53), negative HuR cytoplasmic expression correlated with elevated DR5 expression (odds ratio 16.1, p < 0.0001). Together, these data demonstrate a feedback mechanism elicited by HuR-mediated repression of the key apoptotic membrane protein DR5.

Utilization of the cellular stress response to sensitize cancer cells to TRAIL-mediated apoptosis

INTRODUCTION

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) is a promising death ligand who has received significant attention due to its specific anti-cancer activity. Recently, a number of clinical trials involving either recombinant soluble TRAIL or agonistic death receptor (DR) antibodies have even been initiated. One major caveat in TRAIL-based anti-cancer therapies is that a considerable number of cancer cells are notorious resistant to apoptosis induction by TRAIL. Overcoming this primary or secondary evolved resistance is an utmost important goal of present cancer research. The current literature suggests that TRAIL resistance is mediated by a number of endogenous factors.

AREAS COVERED

According to recent research, stress-related transcription factors have acquired a pivotal role in the sensitization of highly resistant cancer cells, for example, pancreatic cancer and glioblastoma cells, to TRAIL-mediated cell death. Out of this transcription factor family, C/EBP-homologous protein (CHOP) is linked to the control of DR-mediated apoptosis by modulation of several apoptotic and anti-apoptotic factors. Stress responses in certain organelles, such as endoplasmic reticulum (ER) and mitochondria, are potent inductors of CHOP expression. This report focuses on the influence of stress responses on endogenous or acquired resistance to extrinsic apoptosis in tumor cells and summarizes recent findings and results. The Medline and ClinicalTrials database with key words were used for this review.

EXPERT OPINION

A potential novel treatment strategy for highly treatment-resistant tumors is the induction of a cellular stress response in cancer cells. The induction of an organelle-related stress response, such as nuclear, ER and mitochondrial stress, leads to a dramatic sensitization of a broad variety of cancer cells of different tumor entities to the apoptotic ligand, TRAIL. Importantly, non-neoplastic cells are not sensitized to TRAIL-mediated cell death through the unfolded protein response in most instances, suggesting that this treatment is not only of high efficacy, but even more less of unwanted toxicity in patients.

Bortezomib reverses the proliferative and antiapoptotic effect of neuropeptides on prostate cancer cells

OBJECTIVES

Neuropeptides are important signal initiators in advanced prostate cancer, partially acting through activation of nuclear factor kappa B. Central to nuclear factor kappa B regulation is the ubiquitin-proteasome system, pharmacological inhibition of which has been proposed as an anticancer strategy. We investigated the putative role of the proteasome inhibitor bortezomib in neuropeptides signaling effects on prostate cancer cells.

METHODS

Human prostate cancer cell lines, LNCaP and PC-3, were used to examine cell proliferation, levels of proapoptotic (caspase-3, Bad) and cell cycle regulatory proteins (p53, p27, p21), as well as total and phosphorylated Akt and p44/42 mitogen-activated protein kinase proteins. Furthermore, 20S proteasome activity, subcellular localization of nuclear factor kappa B and transcription of nuclear factor kappa B target genes, interleukin-8 and vascular endothelial growth factor, were assessed.

RESULTS

Neuropeptides (endothelin-1, bombesin) increased cell proliferation, whereas bortezomib decreased proliferation and induced apoptosis, an effect maintained after cotreatment with neuropeptides. Bad, p53, p21 and p27 were downregulated by neuropeptides in PC-3, and these effects were reversed with the addition of bortezomib. Neuropeptides increased proteasomal activity and nuclear factor kappa B levels in PC-3, and these effects were prevented by bortezomib. Interleukin-8 and vascular endothelial growth factor transcripts were induced after neuropeptides treatment, but downregulated by bortezomib. These results coincided with the ability of bortezomib to reduce mitogen-activated protein kinase signaling in both cell lines.

CONCLUSIONS

These findings are consistent with bortezomib-mediated abrogation of neuropeptides-induced proliferative and antiapoptotic signaling. Thus, the effect of the drug on the neuropeptides axis needs to be further investigated, as neuropeptide action in prostate cancer might entail involvement of the proteasome.

Apoptosis-modulating drugs for improved cancer therapy

Resistance to cell death induction has been recognized as a hallmark of cancer. Increasing understanding of the underlying molecular events regulating different cell death mechanisms like apoptosis, endoplasmic reticulum stress, autophagy, necroptosis and others has opened new possibilities for targeted interference with these pathways. While conventional chemotherapeutic agents usually inhibit cell cycle progression, DNA replication or mitosis execution, novel agents like small molecule kinase inhibitors also target survival-related kinases and signaling pathways and contribute to overcome resistance to chemotherapy and apoptosis. Additionally, antibodies targeting cellular death receptors have been described to specifically target tumor cells only. This review briefly highlights the pathways involved in (apoptotic) cell death and summarizes the current state of development of specific modulators of cell death and how they can help to improve the tolerability of chemotherapy regimens and increase survival rates in patients with advanced cancer diseases.

Immunomodulation using agonists and antagonists: potential clinical applications

INTRODUCTION

Extensive studies have gone into understanding the differential role of the innate and adaptive arms of the immune system in the context of various diseases. Receptor-ligand interactions are responsible for mediating cross-talk between the innate and adaptive arms of the immune system, so as to effectively counter the pathogenic challenge. While TLRs remain the best studied innate immune receptor, many other receptor families are now coming to the fore for their role in various pathologies. Research has focused on the discovery of novel agonists and antagonists for these receptors as potential therapeutics.

AREAS COVERED

In this review, we present an overview of the recent advances in the discovery of drugs targeting important receptors such as G-protein coupled receptors, TRAIL-R, IL-1β receptor, PPARs, etc. All these receptors play a critical role in the modulation of the immune response. We focus on the recent paradigms applied for the generation of specific and effective therapeutics for these receptors and their status in clinical trials.

EXPERT OPINION

Non-specific activation by antagonist/agonist is a difficult problem to dodge. This demands innovation in ligand designing with the use of strategies such as allosterism and dual-specific ligands. Rigorous preclinical and clinical studies are required in transforming a compound to a therapeutic.

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.

Targeting the extrinsic apoptosis signaling pathway for cancer therapy

The extrinsic apoptosis pathway is triggered by the binding of death ligands of the tumor necrosis factor (TNF) family to their appropriate death receptors (DRs) on the cell surface. One TNF family member, TNF-related apoptosis-inducing ligand (TRAIL or Apo2L), seems to preferentially cause apoptosis of transformed cells and can be systemically administered in the absence of severe toxicity. Therefore, there has been enthusiasm for the use of TRAIL or agonist antibodies to the TRAIL DR4 and DR5 in cancer therapy. Nonetheless, many cancer cells are very resistant to TRAIL apoptosis in vitro. Therefore, there is much interest in identifying compounds that can be combined with TRAIL to amplify its apoptotic effects. In this review, I will provide a brief overview of apoptosis signaling by TRAIL and discuss apoptosis-sensitizing agents, focusing mainly on the proteasome inhibitor bortezomib (VELCADE) and some novel sensitizers that we have recently identified. Alternative ways to administer TRAIL or DR agonist antibodies as therapeutic agents will also be described. Finally, I will discuss some of the gaps in our understanding of TRAIL apoptosis signaling and suggest some research directions that may provide additional information for optimizing the targeting of the extrinsic apoptosis pathway for future cancer therapy.

Naturally occurring, tumor-specific, therapeutic proteins

The emerging approach to cancer treatment known as targeted therapies offers hope in improving the treatment of therapy-resistant cancers. Recent understanding of the molecular pathogenesis of cancer has led to the development of targeted novel drugs such as monoclonal antibodies, small molecule inhibitors, mimetics, antisense and small interference RNA-based strategies, among others. These compounds act on specific targets that are believed to contribute to the development and progression of cancers and resistance of tumors to conventional therapies. Delivered individually or combined with chemo- and/or radiotherapy, such novel drugs have produced significant responses in certain types of cancer. Among the most successful novel compounds are those which target tyrosine kinases (imatinib, trastuzumab, sinutinib, cetuximab). However, these compounds can cause severe side-effects as they inhibit pathways such as epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor, which are also important for normal functions in non-transformed cells. Recently, a number of proteins have been identified which show a remarkable tumor-specific cytotoxic activity. This toxicity is independent of tumor type or specific genetic changes such as p53, pRB or EGFR aberrations. These tumor-specific killer proteins are either derived from common human and animal viruses such as E1A, E4ORF4 and VP3 (apoptin) or of cellular origin, such as TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) and MDA-7 (melanoma differentiation associated-7). This review aims to present a current overview of a selection of these proteins with preferential toxicity among cancer cells and will provide an insight into the possible mechanism of action, tumor specificity and their potential as novel tumor-specific cancer therapeutics.

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.

Tumor Necrosis Factor-related apoptosis-inducing ligand (TRAIL) Receptor-1 and Receptor-2 agonists for cancer therapy

IMPORTANCE OF THE FIELD

Agents that activate the TNF-related apoptosis-inducing ligand death receptors, TRAIL-R1 and TRAIL-R2, have attracted substantial attention and investment as potential anti-cancer therapies. Preclinical studies of TRAIL-R agonists indicate that they may be efficacious in a wide range of tumor types, especially when combined with chemotherapeutic agents.

AREAS COVERED IN THIS REVIEW

The rationale for clinical development of TRAIL-R agonists is described, including the basis for combining these agents with other agents that modulate the 'checks and balances' of the apoptotic pathways. Accruing data that highlight differences between TRAIL-R1 and TRAIL-R2 that could affect the clinical significance of their specific agonists are described. The clinical experience to date with each of the agonists is summarized.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the rationale for the clinical development of TRAIL-R agonists, as well as the current status of clinical trials of these interesting new agents.

TAKE HOME MESSAGE

Ongoing clinical trials will provide important information regarding the future development of TRAIL-R agonists.

The ubiquitin-proteasome system in prostate cancer and its transition to castration resistance

Prostate cancer is the most common carcinoma in the male population. In its initial stage, the disease is androgen-dependent and responds therapeutically to androgen deprivation treatment but it usually progresses after a few years to an androgen-independent phase that is refractory to hormonal manipulations. The proteasome is a multi-unit protease system that regulates the abundance and function of a significant number of cell proteins, and its inhibition results in cancer cell growth inhibition and apoptosis and is already exploited in the clinic with the use of proteasome inhibitor bortezomib in multiple myeloma. In order to be recognized by the proteasome, a target protein needs to be linked to a chain of the small protein ubiquitin. In this paper, we review the role of ubiquitin-proteasome system (UPS) in androgen receptor-dependent transcription as well as in the castration resistant stage of the disease, and we discuss therapeutic opportunities that UPS inhibition offers in prostate cancer.

Bortezomib sensitizes human renal cell carcinomas to TRAIL apoptosis through increased activation of caspase-8 in the death-inducing signaling complex

Bortezomib (VELCADE) could sensitize certain human renal cell carcinoma (RCC) lines to the apoptotic effects of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Analysis of seven human RCC showed a clear increase in the sensitivity of four of the RCC to TRAIL cytotoxicity following bortezomib (5-20 nmol/L) treatment, whereas the remaining three remained resistant. Tumor cell death following sensitization had all the features of apoptosis. The enhanced antitumor activity of the bortezomib and TRAIL combination was confirmed in long-term (6 days) cancer cell outgrowth assays. The extent of proteasome inhibition by bortezomib in the various RCC was equivalent. Following bortezomib treatment, neither changes in the intracellular protein levels of various Bcl-2 and IAP family members, nor minor changes in expression of TRAIL receptors (DR4, DR5), correlated well with the sensitization or resistance of RCC to TRAIL-mediated apoptosis. However, enhanced procaspase-8 activation following bortezomib pretreatment and subsequent TRAIL exposure was only observed in the sensitized RCC in both cell extracts and death-inducing signaling complex immunoprecipitates. These data suggest that the molecular basis for bortezomib sensitization of RCC to TRAIL primarily involves early amplification of caspase-8 activity. In the absence of this increased caspase-8 activation, other bortezomib-induced changes are not sufficient to sensitize RCC to TRAIL-mediated apoptosis.

Proteasome inhibition and allogeneic hematopoietic stem cell transplantation: a review

The proteasome and its associated ubiquitin protein modification system have proved to be an important therapeutic target in the treatment of multiple myeloma and other cancers. In addition to direct antitumor effects, proteasome inhibition also exerts strong effects on nonneoplastic immune cells. This indicates that proteasome inhibition, through the use of agents like bortezomib, could be used therapeutically to modulate immune responses. In this review we explore the emerging data, both preclinical and clinical, highlighting the importance of proteasome targeting of immunologic responses, primarily in the context of allogeneic hematopoietic stem cell transplantation (HSCT), both for the control of transplant-related toxicities like acute and chronic graft-versus-host disease (aGVHD, cGHVHD), and for improved malignant disease control after allogeneic HSCT.

Proteasome inhibition: a new therapeutic strategy to cancer treatment

The ubiquitin-proteasome system is a major pathway for protein degradation. Targeting this pathway using proteasome inhibitors represents a novel approach for the treatment of cancer. Proteasome inhibitors lower cell proliferation and induce apoptosis in solid and hematologic malignancies through multiple mechanisms, including stabilization of cell cycle regulators and pro-apoptotic factors, stimulation of bone morphogenetic protein signaling, inhibition of protein translation, and sensitization to ligand-induced apoptosis. In this connection, proteasome inhibition activates macroautophagy, a compensatory protein degradation system, as well as other pro-survival signaling pathways. Inhibition of these auto-protective responses sensitizes cancer cells to the anti-proliferative effects of proteasome inhibitors.

Constitutive down-regulation of Osterix in osteoblasts from myeloma patients: in vitro effect of Bortezomib and Lenalidomide

Bortezomib and Lenalidomide have been shown to be effective in the control of multiple myeloma (MM) progression. We have investigated their role in the in vitro expression of Osterix by primary osteoblast cultures from MM patients and found that Osterix RNA was constitutively down-regulated in these cells. Treatment of osteoblasts with Bortezomib resulted in an increase of Osterix RNA and in enhanced activity of both BMP-2 and Runx2. Instead, Lenalidomide was unable to modify Osterix transcription. These findings provide additional evidence suggesting that, at least in vitro, Bortezomib promotes the osteoblast maturation whereas Lenalidomide is ineffective.

Quercetin enhances TRAIL-induced apoptosis in prostate cancer cells via increased protein stability of death receptor 5

AIMS

Quercetin has been shown to enhance tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis of prostate cancer cells via mechanisms that include upregulation of death receptor (DR) 5, a protein reported to play an important role in sensitizing cancer cells to apoptosis. We aimed to determine the specific mechanisms underlying quercetin-induced DR5 expression.

MAIN METHODS

Human prostate cancer cells were exposed to quercetin and TRAIL. Trypan blue assays and terminal transferase dUTP nick-end labeling (TUNEL) assays evaluated changes in TRAIL resistance after quercetin treatment, and flow cytometry examined quercetin-induced death receptor expression in DU-145 cells. Western blotting, reverse transcription-polymerase chain reaction (RT-PCR) and transiently transfection were utilized to confirm apoptotic patterns of prostate cancer cells.

KEY FINDINGS

After stimulation with quercetin, DU-145 cells exhibited stronger sensitization to TRAIL. Quercetin treatment enhanced TRAIL-induced activation proteins in the caspase pathway, such as poly (ADP-ribose) polymerase (PARP), caspase-3, and caspase-9. Quercetin dose-dependently increased DR5 levels in prostate cancer cells, which was mediated by increased transcription and protein stability, but not mRNA stability. Ectopic expression of DR5 dose-dependently increased TRAIL-induced apoptosis.

SIGNIFICANCE

Our results showed that the role of quercetin and TRAIL combination therapy may provide a novel strategy for treating prostate cancer by overcoming critical mechanisms of apoptosis resistance.

TRAILing death in cancer

The observation that certain types of cancer express death receptors on their cell surface has triggered heightened interest in exploring the potential of receptor ligation as a novel anti-cancer modality, and since the expression is somewhat restricted to cancer cells the therapeutic implications are very promising. One such death receptor ligand belonging to the tumor necrosis receptor (TNF) superfamily, TNF-related apoptosis-inducing ligand (TRAIL), has been in the limelight as a tumor selective molecule that transmits death signal via ligation to its receptors (TRAIL-R1 and TRAIL-R2 or death receptors 4 and 5; DR4 and DR5). Interestingly, TRAIL-induced apoptosis exhibits hallmarks of extrinsic as well as intrinsic death pathways, and, therefore, is subject to regulation both at the cell surface receptor level as well as more downstream at the post-mitochondrial level. Despite the remarkable selectivity of DR expression on cancer cell surface, development of resistance to TRAIL-induced apoptosis remains a major challenge. Therefore, unraveling the cellular and molecular mechanisms of TRAIL resistance as well as identifying strategies to overcome this problem for an effective therapeutic response remains the cornerstone of many research endeavors. This review aims at presenting an overview of the biology, function and translational relevance of TRAIL with a specific view to discussing the various regulatory mechanisms and the current trends in reverting TRAIL resistance of cancer cells with the obvious implication of an improved clinical outcome.

The histone deacetylase inhibitor suberoylanilide hydroxamic acid sensitises human hepatocellular carcinoma cells to TRAIL-induced apoptosis by TRAIL-DISC activation

This paper shows that the histone deacetylase inhibitor SAHA sensitised at sub-toxic doses human hepatocellular carcinoma cells (HepG2, Hep3B and SK-Hep1) to TRAIL-induced apoptosis, while it was ineffective in primary human hepatocytes (PHHs). In particular in HCC cells SAHA increased the expression of death receptor 5 (DR5) and caused a decrement of c-Flip. These two modifications provoked in the presence of TRAIL the rapid production of TRAIL-DISC and the activation of caspase-8. Consequently SAHA/TRAIL combination induced many apoptotic events, such as a cleavage of Bid into tBid, dissipation of mitochondrial membrane potential, activation of caspase-3 with the consequent cleavage of both NF-kB and Akt. The decrease in NF-kB level seemed to be responsible for the reduction in the content of IAP family antiapoptotic proteins while the decrease in Akt level caused a reduction in phospho-Bad. These events led to the activation of caspase-9, which contributed to the strong apoptotic activity of TRAIL. Sensitisation of human hepatocellular carcinoma cells to TRAIL-induced apoptosis by SAHA may suggest new strategies for the treatment of liver tumours.

Down-regulation of vascular endothelial growth factor receptor 2 is a major molecular determinant of proteasome inhibitor-mediated antiangiogenic action in endothelial cells

The ubiquitin-proteasome system is the major pathway for intracellular protein degradation in eukaryotic cells. This system controls a wide range of cellular regulatory proteins, including transcription factors and cell cycle regulatory proteins. Recent evidence also established the importance of the proteasome in tumor development, showing antitumor and antiangiogenic actions by using selective inhibitors in vivo. As signaling via the vascular endothelial growth factor receptor 2 (VEGFR2) pathway is critical for angiogenic responses to occur, we explored whether antiangiogenic effects due to proteasome inhibition were partly mediated through decreased endothelial VEGFR2 expression. This study shows that different proteasome inhibitors blocked VEGFR2 expression in a time-dependent and concentration-dependent manner. This blockade was paralleled by the respective inhibition of the formation of capillary-like structures and endothelial cell migration. In contrast, neither tie-2 nor VEGFR1 expression was significantly affected by proteasome inhibitor treatment. The suppressive effects on VEGFR2 expression were not conveyed by increased shedding or a decrease in protein half-life, suggesting that transcriptional mechanisms accounted for the observed effects. In line with this conclusion, proteasome inhibition significantly suppressed VEGFR2 mRNA accumulation. In addition, inhibitor treatment considerably decreased the transcriptional activity of 5' deletional VEGFR2 promoter gene constructs. Proteasome inhibition-mediated repression was controlled by a GC-rich region that harbored one consensus Sp1-binding site. Subsequent EMSA analyses showed decreased constitutive Sp1-dependent DNA binding in response to proteasome inhibition. In addition, we could show that proteasome inhibitors reduced VEGFR2 mRNA stability. Therefore, VEGFR2 expression may constitute a critical molecular target of proteasome inhibitors that may mediate their antiangiogenic effects in vivo.