The proteasome inhibitor PS-341 overcomes TRAIL resistance in Bax and caspase 9-negative or Bcl-xL overexpressing cells

Journey of TRAIL from Bench to Bedside and its Potential Role in Immuno-Oncology

Induction of apoptosis in cancer cells has increasingly been the focus of many therapeutic approaches in oncology field. Since its identification as a TNF family member, TRAIL (TNF-related apoptosis-inducing ligand) paved a new path in apoptosis inducing cancer therapies. Its selective ability to activate extrinsic and intrinsic cell death pathways in cancer cells only, independently from p53 mutations responsible for conventional therapeutics resistance, spotted TRAIL as a potent cancer apoptotic agent. Many recombinant preparations of TRAIL and death receptor targeting monoclonal antibodies have been developed and being tested pre-clinically and clinically both as a single agent and in combinations. Of note, the monoclonal antibodies were not the only type of antibodies developed to target TRAIL receptors. Recent technology has brought forth several single chain variable domains (scFv) designs fused recombinantly to TRAIL as well. Also, it is becoming progressively more understandable that field of nanotechnology has revolutionized cancer diagnosis and therapy. The recent breakthroughs in materials science and protein engineering have helped considerably in strategically loading drugs into nanoparticles or conjugating drugs to their surface. In this review we aim to comprehensively highlight the molecular knowledge of TRAIL in the context of its pathway, receptors and resistance factors. We also aim to review the clinical trials that have been done using TRAIL based therapies and to review various scFv designs, the arsenal of nano-carriers and molecules available to selectively target tumor cells with TRAIL.

A Genome-Wide Loss-of-Function Screen Identifies SLC26A2 as a Novel Mediator of TRAIL Resistance

TRAIL is a potent death-inducing ligand that mediates apoptosis through the extrinsic pathway and serves as an important endogenous tumor suppressor mechanism. Because tumor cells are often killed by TRAIL and normal cells are not, drugs that activate the TRAIL pathway have been thought to have potential clinical value. However, to date, most TRAIL-related clinical trials have largely failed due to the tumor cells having intrinsic or acquired resistance to TRAIL-induced apoptosis. Previous studies to identify resistance mechanisms have focused on targeted analysis of the canonical apoptosis pathway and other known regulators of TRAIL receptor signaling. To identify novel mechanisms of TRAIL resistance in an unbiased way, we performed a genome-wide shRNA screen for genes that regulate TRAIL sensitivity in sublines that had been selected for acquired TRAIL resistance. This screen identified previously unknown mediators of TRAIL resistance including angiotensin II receptor 2, Crk-like protein, T-Box Transcription Factor 2, and solute carrier family 26 member 2 (SLC26A2). SLC26A2 downregulates the TRAIL receptors, DR4 and DR5, and this downregulation is associated with resistance to TRAIL. Its expression is high in numerous tumor types compared with normal cells, and in breast cancer, SLC26A2 is associated with a significant decrease in relapse-free survival.Implication: Our results shed light on novel resistance mechanisms that could affect the efficacy of TRAIL agonist therapies and highlight the possibility of using these proteins as biomarkers to identify TRAIL-resistant tumors, or as potential therapeutic targets in combination with TRAIL. Mol Cancer Res; 15(4); 382-94. ©2017 AACR.

Tanshinone IIA induces TRAIL sensitization of human lung cancer cells through selective ER stress induction

Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promised anticancer medicine targeting only the tumor, most cancers show resistance to TRAIL-induced apoptosis. For this reason, new therapeutic strategies to overcome the TRAIL resistance are required for more effective tumor treatment. In the present study, potential of tanshinone IIA as a TRAIL sensitizer was evaluated in human non-small cell lung cancer (NSCLC) cells. NSCLC cells showed resistance to TRAIL-mediated cell death, but combination treatment of Tanshinone IIA and TRAIL synergistically decreased cell viability and increased apoptosis in TRAIL-resistant NSCLC cells. Tanshinone IIA greatly induced death receptor 5 (DR5), but not death receptor 4 (DR4). Furthermore, DR5 knockdown attenuated the combination treatment of tanshinone IIA with TRAIL-mediated cell death in human NSCLC cells. Tanshinone IIA also increased CHOP and activated the PERK-ATF4 pathway suggesting that tanshinone IIA increased DR5 and CHOP by activating the PERK-ATF4 pathway. Tanshinone IIA also downregulated phosphorylation of STAT3 and expression of survivin. Taken together, these results indicate that tanshinone IIA increases TRAIL-induced cell death via upregulating DR5 and downregulating survivin mediated by, respectively, selective activation of PERK/ATF4 and inhibition of STAT3, suggesting combinatorial intervention of tanshinone IIA and TRAIL as a new therapeutic strategy for human NSCLC.

Decursin enhances TRAIL-induced apoptosis through oxidative stress mediated- endoplasmic reticulum stress signalling in non-small cell lung cancers

BACKGROUND AND PURPOSE

The TNF-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent due to its remarkable ability to selectively kill tumour cells. However, because most tumours exhibit resistance to TRAIL-induced apoptosis, the development of combination therapies to overcome resistance to TRAIL is required for effective cancer therapy.

EXPERIMENTAL APPROACH

Cell viability and possible synergy between the plant pyranocoumarin decursin and TRAIL was measured by MTT assay and calcusyn software. Reactive oxygen species (ROS) and apoptosis were measured using dichlorodihydrofluorescein and annexin/propidium iodide in cell flow cytometry. Changes in protein levels were assessed with Western blotting.

KEY RESULTS

Combining decursin and TRAIL markedly decreased cell viability and increased apoptosis in TRAIL-resistant non-small-cell lung cancer (NSCLC) cell lines. Decursin induced expression of the death receptor 5 (DR5). Inhibition of DR5 attenuated apoptotic cell death in decursin + TRAIL treated NSCLC cell lines. Interestingly, induction of DR5 and CCAAT/enhancer-binding protein homologues protein by decursin was mediated through selective induction of the pancreatic endoplasmic reticulum kinase (PERK)/activating transcription factor 4 (ATF4) branch of the endoplasmic reticulum stress response pathway. Furthermore, enhancement of PERK/ATF4 signalling by decursin was mediated by ROS generation in NSCLC cell lines, but not in normal human lung cells. Decursin also markedly down-regulated expression of survivin and Bcl-xL in TRAIL-resistant NSCLC cells.

CONCLUSIONS AND IMPLICATIONS

ROS generation by decursin selectively activated the PERK/ATF4 axis of the endoplasmic reticulum stress signalling pathway, leading to enhanced TRAIL sensitivity in TRAIL-resistant NSCLC cell lines, partly via up-regulation of DR5.

Morusin Induces TRAIL Sensitization by Regulating EGFR and DR5 in Human Glioblastoma Cells

Glioblastoma is one of the most malignant primary tumors, and the prognosis for glioblastoma patients remains poor. Tumor-necrosis-factor-related apoptosis-inducing ligand (TRAIL) is considered a promising anticancer agent due to its remarkable ability to selectively kill tumor cells. However, since many cancers are resistant to TRAIL, strategies to overcome resistance are required for the successful use of TRAIL in the clinic. In the present study, the potential of morusin as a TRAIL sensitizer in human glioblastoma cells was evaluated. Treatment with TRAIL or morusin alone showed weak cytotoxicity in human glioblastoma cells. However, combination treatment of TRAIL with morusin synergistically decreased cell viability and increased apoptosis compared with single treatment. Morusin induced expression of death receptor 5 (DR5), but not DR4 or decoy receptors (DcR1 and DcR2). Furthermore, morusin significantly decreased anti-apoptotic molecules survivin and XIAP. In addition, morusin reduced expression of EGFR and PDFGR as well as phosphorylation of STAT3, possibly mediating down-regulation of survivin and XIAP. Together these results suggest that morusin enhances TRAIL sensitivity in human glioblastoma cells through regulating expression of DR5 and EGFR. Therefore, the combination treatment of TRAIL and morusin may be a new therapeutic strategy for malignant glioma patients.

Bortezomib Improves Adoptive T-cell Therapy by Sensitizing Cancer Cells to FasL Cytotoxicity

Cancer immunotherapy shows great promise but many patients fail to show objective responses, including in cancers that can respond well, such as melanoma and renal adenocarcinoma. The proteasome inhibitor bortezomib sensitizes solid tumors to apoptosis in response to TNF-family death ligands. Because T cells provide multiple death ligands at the tumor site, we investigated the effects of bortezomib on T-cell responses in immunotherapy models involving low-avidity antigens. Bortezomib did not affect lymphocyte or tissue-resident CD11c(+)CD8(+) dendritic cell counts in tumor-bearing mice, did not inhibit dendritic cell expression of costimulatory molecules, and did not decrease MHC class I/II-associated antigen presentation to cognate T cells. Rather, bortezomib activated NF-κB p65 in CD8(+) T cells, stabilizing expression of T-cell receptor CD3ζ and IL2 receptor-α, while maintaining IFNγ secretion to improve FasL-mediated tumor lysis. Notably, bortezomib increased tumor cell surface expression of Fas in mice as well as human melanoma tissue from a responsive patient. In renal tumor-bearing immunodeficient Rag2(-/-) mice, bortezomib treatment after adoptive T-cell immunotherapy reduced lung metastases and enhanced host survival. Our findings highlight the potential of proteasome inhibitors to enhance antitumor T-cell function in the context of cancer immunotherapy.

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.

Elevated pressure enhanced TRAIL-induced apoptosis in hepatocellular carcinoma cells via ERK1/2-inactivation

The high frequency of intrinsic resistance to TNF-related apoptosisinducing ligand (TRAIL) in tumor cell lines has necessitated the development of strategies to sensitize tumors to TRAIL-induced apoptosis. We previously showed that elevated pressure applied as a mechanical stressor enhanced TRAIL-mediated apoptosis in human lung carcinoma cells in vitro and in vivo. This study focused on the effect of elevated pressure on the sensitization of TRAIL-resistant cells and the underlying mechanism. We observed elevated pressure-induced sensitization to TRAIL-mediated apoptosis in Hep3B cells, accompanied by the activation of several caspases and the mitochondrial signaling pathway. Interestingly, the enhanced apoptosis induced by elevated pressure was correlated with suppression of extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) phosphorylation and CREB without any change to other MAPKs. Phosphorylation of Bcl-2-associated death promoter (BAD) also decreased, leading to inhibition of the mitochondrial pathway. To confirm whether the activation of pERK1/2 plays a key role in the TRAIL-sensitizing effect of elevated pressure, Hep3B cells were pre-treated with the ERK1/2-specific inhibitor PD98059 instead of elevated pressure. Co-treatment with PD98059 and TRAIL augmented TRAIL-induced apoptosis and decreased BAD phosphorylation. The inhibition of ERK1/2 activation by elevated pressure and PD98059 also reduced BH3 interacting-domain death agonist (BID), thereby amplifying apoptotic stress at the mitochondrial level. Our results suggest that elevated pressure enhances TRAIL-induced apoptosis of Hep3B cells via specific suppression of ERK1/2 activation among MAPKs.

Bortezomib sensitises TRAIL-resistant HPV-positive head and neck cancer cells to TRAIL through a caspase-dependent, E6-independent mechanism

Human papillomavirus (HPV) is causative for a new and increasing form of head and neck squamous cell carcinomas (HNSCCs). Although localised HPV-positive cancers have a favourable response to radio-chemotherapy (RT/CT), the impact of HPV in advanced or metastatic HNSCC remains to be defined and targeted therapeutics need to be tested for cancers resistant to RT/CT. To this end, we investigated the sensitivity of HPV-positive and -negative HNSCC cell lines to TRAIL (tumour necrosis factor-related apoptosis-inducing ligand), which induces tumour cell-specific apoptosis in various cancer types. A clear correlation was observed between HPV positivity and resistance to TRAIL compared with HPV-negative head and neck cancer cell lines. All TRAIL-resistant HPV-positive cell lines tested were sensitised to TRAIL-induced cell death by treatment with bortezomib, a clinically approved proteasome inhibitor. Bortezomib-mediated sensitisation to TRAIL was associated with enhanced activation of caspase-8, -9 and -3, elevated membrane expression levels of TRAIL-R2, cytochrome c release and G2/M arrest. Knockdown of caspase-8 significantly blocked cell death induced by the combination therapy, whereas the BH3-only protein Bid was not required for induction of apoptosis. XIAP depletion increased the sensitivity of both HPV-positive and -negative cells to TRAIL alone or in combination with bortezomib. In contrast, restoration of p53 following E6 knockdown in HPV-positive cells had no effect on their sensitivity to either single or combination therapy, suggesting a p53-independent pathway for the observed response. In summary, bortezomib-mediated proteasome inhibition sensitises previously resistant HPV-positive HNSCC cells to TRAIL-induced cell death through a mechanism involving both the extrinsic and intrinsic pathways of apoptosis. The cooperative effect of these two targeted anticancer agents therefore represents a promising treatment strategy for RT/CT-resistant HPV-associated head and neck cancers.

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.

Targeting death receptors for TRAIL by agents designed by Mother Nature

Selective killing of cancer cells is one of the major goals of cancer therapy. Although chemotherapeutic agents are being used for cancer treatment, they lack selectivity toward tumor cells. Among the six different death receptors (DRs) identified to date, DR4 and DR5 are selectively expressed on cancer cells. Therefore, unlike chemotherapeutic agents, these receptors can potentially mediate selective killing of tumor cells. In this review we outline various nutraceuticals derived from 'Mother Nature' that can upregulate DRs and thus potentiate apoptosis. These nutraceuticals increase tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-induced apoptosis of cancer cells through different mechanisms. First, nutraceuticals have been found to induce DRs through the upregulation of various signaling molecules. Second, nutraceuticals can downregulate tumor cell-survival pathways. Third, nutraceuticals alone have been found to activate cell-death pathways. Although both TRAIL and agonistic antibodies against DR4 and DR5 are in clinical trials, combination with nutraceuticals is likely to boost their anticancer potential.

sTRAIL coupled to liposomes improves its pharmacokinetic profile and overcomes neuroblastoma tumour resistance in combination with Bortezomib

Neuroblastoma (NB), the most common and deadly extracranial solid tumour of childhood, represents a challenging in paediatric oncology. Soluble tumour necrosis factor (TNF)-related apoptosis-inducing ligand (sTRAIL) is a cancer cell-specific molecule exerting remarkable anti-tumour activities against paediatric malignancies both in vitro and in preclinical settings. However, due to its too fast elimination and to the undesired related side effects, the improvement of sTRAIL in vivo bioavailability and the specific delivery to the tumour is mandatory for increasing its therapeutic efficacy. In this manuscript, we developed an innovative pegylated liposomal formulation carrying the sTRAIL at the outer surface (sTRAIL-SL) with the intent to improve its serum half-life and increase its efficacy in vivo, while reducing side effects. Furthermore, the possibility to combine sTRAIL-SL with the proteasome inhibitor Bortezomib (BTZ) was investigated, being BTZ able to sensitize tumour cells toward TRAIL-induced apoptosis. We demonstrated that sTRAIL preserved and improved its anti-tumour activity when coupled to nanocarriers. Moreover, sTRAIL-SL ameliorated its PK profile in blood allowing sTRAIL to exert a more potent anti-tumour activity, which led, upon BTZ priming, to a statistically significant enhanced life spans in two models of sTRAIL-resistant NB-bearing mice. Finally, mechanistic studies indicated that the combination of sTRAIL with BTZ sensitized sTRAIL-resistant NB tumour cells to sTRAIL-induced cell death, both in vitro and in vivo, through the Akt/GSK3/β-catenin axis-dependent mechanism. In conclusion, our results suggest that sTRAIL-SL might be an efficient vehicle for sTRAIL delivery and that its use in clinic, in combination with BTZ, might represent an adjuvant strategy for the treatment of stage IV, sTRAIL-resistant, NB patients.

HSP27, 70 and 90, anti-apoptotic proteins, in clinical cancer therapy (Review)

Among the heat shock proteins (HSP), HSP27, HSP70 and HSP90 are the most studied stress-inducible HSPs, and are induced in response to a wide variety of physiological and environmental insults, thus allowing cells to survive to lethal conditions based on their powerful cytoprotective functions. Different functions of HSPs have been described to explain their cytoprotective functions, including their most basic role as molecular chaperones, that is to regulate protein folding, transport, translocation and assembly, especially helping in the refolding of misfolded proteins, as well as their anti-apoptotic properties. In cancer cells, the expression and/or activity of the three HSPs is abnormally high, and is associated with increased tumorigenicity, metastatic potential of cancer cells and resistance to chemotherapy. Associating with key apoptotic factors, they are powerful anti-apoptotic proteins, having the capacity to block the cell death process at different levels. Altogether, the properties suggest that HSP27, HSP70 and HSP90 are appropriate targets for modulating cell death pathways. In this review, we summarize the role of HSP90, HSP70 and HSP27 in apoptosis and the emerging strategies that have been developed for cancer therapy based on the inhibition of the three HSPs.

The DeISGylase USP18 limits TRAIL-induced apoptosis through the regulation of TRAIL levels: Cellular levels of TRAIL influences responsiveness to TRAIL-induced apoptosis

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a promising molecule for anti-cancer therapies. Unfortunately, cancer cells frequently acquire resistance to rhTRAIL. Various co-treatments have been proposed to overcome apoptosis resistance to TRAIL. Here we show that downregulation of the deISGylase USP18 sensitizes cancer cells to rhTRAIL, whereas, elevate levels of USP18 inhibit TRAIL-induced apoptosis, in a deISGylase-independent manner. USP18 influences TRAIL signaling through the control of the IFN autocrine loop. In fact, cells with downregulated USP18 expression augment the expression of cellular TRAIL. Downregulation of cellular TRAIL abrogates the synergism between TRAIL and USP18 siRNA and also limits cell death induced by rhTRAIL. By comparing the apoptotic responsiveness to TRAIL in a panel of cancer cell lines, we have discovered a correlation between TRAIL levels and the apoptotic susceptibility to rhTRAIL, In cells expressing high levels of TRAIL-R2 susceptibility to rhTRAIL correlates with TRAIL expression. In conclusion, we propose that cellular TRAIL is an additional factor that can influence the apoptotic response to rhTRAIL.

Manipulating the apoptotic pathway: potential therapeutics for cancer patients

This review summarizes the current state of scientific understanding of the apoptosis pathway, with a focus on the proteins involved in the pathway, their interactions and functions. This forms the rationale for detailing the preclinical and clinical pharmacology of drugs that modulate the pivotal proteins in this pathway, with emphasis on drugs that are furthest advanced in clinical development as anticancer agents. There is a focus on describing drugs that modulate three of the most promising targets in the apoptosis pathway, namely antibodies that bind and activate the death receptors, small molecules that inhibit the anti-apoptotic Bcl-2 family proteins, and small molecules and antisense oligonucleotides that inactivate the inhibitors of apoptosis, all of which drive the equilibrium of the apoptotic pathway towards apoptosis. These structurally different yet functionally related groups of drugs represent a promising novel approach to anticancer therapeutics whether used as monotherapy or in combination with either classical cytotoxic or other molecularly targeted anticancer agents.

BAK and NOXA are critical determinants of mitochondrial apoptosis induced by bortezomib in mesothelioma

Based on promising preclinical efficacy associated with the 20S proteasome inhibitor bortezomib in malignant pleural mesothelioma (MPM), two phase II clinical trials have been initiated (EORTC 08052 and ICORG 05–10). However, the potential mechanisms underlying resistance to this targeted drug in MPM are still unknown. Functional genetic analyses were conducted to determine the key mitochondrial apoptotic regulators required for bortezomib sensitivity and to establish how their dysregulation may confer resistance. The multidomain proapoptotic protein BAK, but not its orthologue BAX, was found to be essential for bortezomib-induced apoptosis in MPM cell lines. Immunohistochemistry was performed on tissues from the ICORG-05 phase II trial and a TMA of archived mesotheliomas. Loss of BAK was found in 39% of specimens and loss of both BAX/BAK in 37% of samples. However, MPM tissues from patients who failed to respond to bortezomib and MPM cell lines selected for resistance to bortezomib conserved BAK expression. In contrast, c-Myc dependent transactivation of NOXA was abrogated in the resistant cell lines. In summary, the block of mitochondrial apoptosis is a limiting factor for achieving efficacy of bortezomib in MPM, and the observed loss of BAK expression or NOXA transactivation may be relevant mechanisms of resistance in the clinic.

Modulating cell-to-cell variability and sensitivity to death ligands by co-drugging

TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) holds promise as an anti-cancer therapeutic but efficiently induces apoptosis in only a subset of tumor cell lines. Moreover, even in clonal populations of responsive lines, only a fraction of cells dies in response to TRAIL and individual cells exhibit cell-to-cell variability in the timing of cell death. Fractional killing in these cell populations appears to arise not from genetic differences among cells but rather from differences in gene expression states, fluctuations in protein levels and the extent to which TRAIL-induced death or survival pathways become activated. In this study, we ask how cell-to-cell variability manifests in cell types with different sensitivities to TRAIL, as well as how it changes when cells are exposed to combinations of drugs. We show that individual cells that survive treatment with TRAIL can regenerate the sensitivity and death-time distribution of the parental population, demonstrating that fractional killing is a stable property of cell populations. We also show that cell-to-cell variability in the timing and probability of apoptosis in response to treatment can be tuned using combinations of drugs that together increase apoptotic sensitivity compared to treatment with one drug alone. In the case of TRAIL, modulation of cell-to-cell variability by co-drugging appears to involve a reduction in the threshold for mitochondrial outer membrane permeabilization.

Oppositional regulation of Noxa by JNK1 and JNK2 during apoptosis induced by proteasomal inhibitors

Proteasome inhibitors (PIs) potently induce apoptosis in a variety of tumor cells, but the underlying mechanisms are not fully elucidated. Comparing PI-induced apoptosis susceptibilities of various mouse embryonic fibroblast (MEF) lines differing in their c-jun N-terminal kinase (JNK) 1 and 2 status, we show that several hallmarks of apoptosis were most rapidly detectable in JNK2-/- cells, whereas they appeared only delayed and severely reduced in their intensities in cells expressing JNK2. Consistent with our finding that PI-induced apoptosis requires de novo protein synthesis, the proteasomal inhibitor MG-132 induced expression of the BH3-only protein Noxa at the transcriptional level in a JNK1-dependent, but JNK2-opposing manner. As the knockdown of Noxa blocked only the rapid PI-induced apoptosis of JNK2-/- cells, but not the delayed death occurring in JNK1-/- and JNK1+/+ cells, our data uncover a novel PI-induced apoptosis pathway that is regulated by the JNK1/2-dependent expression of Noxa. Furthermore, several transcription factors known to modulate Noxa expression including ATF3, ATF4, c-Jun, c-Myc, HIF1α, and p53 were found upregulated following MG-132 exposure. From those, only knockdown of c-Myc rescued JNK2-/- cells from PI-induced apoptosis, however, without affecting expression of Noxa. Together, our data not only show that a rapid execution of PI-induced apoptosis requires JNK1 for upregulation of Noxa via an as yet unknown transcription factor, but also that JNK2 controls this event in an oppositional manner.

TRAIL promotes membrane blebbing, detachment and migration of cells displaying a dysfunctional intrinsic pathway of apoptosis

Recently, tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/Apo2L) has been shown to be a potential candidate for cancer therapy. TRAIL induces apoptosis in various cancer cells but not in normal tissues. Here we show that HCT116 and SW480 cells with a deficient mitochondrial apoptotic pathway were resistant to TRAIL-induced apoptosis, whereas HCT116 and SW480 cells with a functional mitochondrial apoptotic pathway underwent apoptosis upon exposure to TRAIL. Surprisingly, TRAIL induced phenotypic changes in cells with a dysfunctional mitochondrial apoptotic pathway, including membrane blebbing and a transient loss of adhesion properties to the substratum. Accordingly, TRAIL stimulated the ability of these cells to migrate. This behavior was the consequence of a transient TRAIL-induced ROCK1 cleavage. In addition, we report that Bax-deficient HCT116 cells exposed to TRAIL for a prolonged period lost their sensitivity to TRAIL as a result of downregulation of TRAIL receptor expression, and became resistant to combination of TRAIL and other drugs such as MG-132 and bortezomib. These findings may have important consequences for TRAIL anti-cancer therapy.

Bortezomib enhances antigen-specific cytotoxic T cell responses against immune-resistant cancer cells generated by STAT3-ablated dendritic cells

Dendritic cell (DC)-based vaccines have received attention as a new therapeutic modality against cancer. However, increased STAT3 activity in the tumor microenvironment makes DCs tolerogenic and suppresses their antitumor activity. In this study, we explored the effects of a combination treatment consisting of a proteasome inhibitor, bortezomib, and an antigen specific STAT3-ablated (STAT3⁻/⁻) DC-based vaccine on the control of TC-1(P3) tumors, a p53-degraded immune resistant cancer cells. We found that E7-antigen expressing STAT3⁻/⁻ DC (E7-DC-1STAT3⁻/⁻) vaccination enhanced generation of E7-specific CD8⁺ T cells, but was not enough to control TC-1(P3) cancer cells. Therefore, we investigated whether bortezomib could create a synergistic effect with E7-DC-1STAT3⁻/⁻ vaccination. We found that apoptosis via down-regulation of STAT3 and NF-κB and up-regulation of Fas and death receptor 5 (DR5) expression in TC-1(P3) induced by bortezomib was independent of p53 status. We also observed that TC-1(P3) cells pretreated with bortezomib had markedly enhanced anti-tumor effects on E7-specific CD8⁺ T cells through a Fas/DR5-mediated mechanism. In addition, TC-1(P3) tumor-bearing mice treated with bortezomib prior to vaccination with E7-DC-1STAT3⁻/⁻ demonstrated enhanced generation of E7-specific CD8⁺ T cells and prolonged survival compared to those treated with monotherapy. These results suggest that the anti-tumor effects against a p53-degraded immune resistant variant generated by antigen-expressing STAT3-ablated mature DCs may be enhanced by bortezomib via death receptor-mediated apoptosis.

The HSP70 and autophagy inhibitor pifithrin-μ enhances the antitumor effects of TRAIL on human pancreatic cancer

TRAIL and agonistic death receptor-specific antibodies can induce apoptosis in cancer cells with little cytotoxicity to normal cells. To improve TRAIL-induced antitumor effects, we tested its effectiveness in combination with pifithrin (PFT)-μ, which has the potential to inhibit HSP70 function and autophagy, both of which participate in TRAIL resistance in cancer cells. Among the four human pancreatic cancer cell lines tested, MiaPaca-2, Panc-1, and BxPC-3 cells showed varying sensitivities to TRAIL. In MiaPaca-2 and Panc-1 cells, knockdown of HSP70 or beclin-1, the latter an autophagy-related molecule, by RNA interference augmented TRAIL-induced antitumor effects, decreasing cell viability, and increasing apoptosis. On the basis of these findings, we next determined whether the TRAIL-induced antitumor effects could be augmented by its combination with PFT-μ. The combination of TRAIL plus PFT-μ significantly decreased the viability and colony-forming ability of MiaPaca-2 and Panc-1 cells compared with cells treated with either agent alone. When applied alone, PFT-μ increased Annexin V(+) cells in both caspase-dependent and -independent manners. It also promoted TRAIL-induced apoptosis and arrested cancer cell growth. Furthermore, PFT-μ antagonized TRAIL-associated NF-κB activation in cancer cells. In a xenograft mouse model, combination therapy significantly inhibited MiaPaca-2 tumor growth compared with treatment with either agent alone. The results of this study suggest protective roles for HSP70 and autophagy in TRAIL resistance in pancreatic cancer cells and suggest that PFT-μ is a promising agent for use in therapies intended to enhance the antitumor effects of TRAIL.

Bortezomib and TRAIL: a perfect match for apoptotic elimination of tumour cells?

Tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is a cytokine that selectively eradicates tumour cells via specific cell surface receptors and is intensively explored for use as a novel anticancer approach. To enhance the efficacy of TRAIL receptor agonists the proteasome inhibitor bortezomib is one of the most potent sensitizers. Here we review the main mechanisms underlying bortezomib-dependent TRAIL sensitization, including stimulation of apoptosis by increasing expression of TRAIL receptors, reduction of cFLIP and enhancement of caspase 8 activation, and modulation of Bcl-2 family proteins and inhibitor of apoptosis proteins (IAPs). Concomitantly, pro-survival signals are suppressed such as elicited by NF-κB and Akt. The different preclinical tumour models explored with this combination, including primary tumour (stem) cells, stroma co-culture and mice models, are discussed, as well as possible hurdles for clinical activity. Collectively, anticipating a solid rationale for bortezomib-TRAIL combination and very promising preclinical results, its clinical activity remains to be demonstrated.

Molecular targets of TRAIL-sensitizing agents in colorectal cancer

Tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL), a member of the TNF superfamily, interacts with its functional death receptors (DRs) and induces apoptosis in a wide range of cancer cell types. Therefore, TRAIL has been considered as an attractive agent for cancer therapy. However, many cancers are resistant to TRAIL-based therapies mainly due to the reduced expression of DRs and/or up-regulation of TRAIL pathway-related anti-apoptotic proteins. Compounds that revert such defects restore the sensitivity of cancer cells to TRAIL, suggesting that combined therapies could help manage neoplastic patients. In this article, we will focus on the TRAIL-sensitizing effects of natural products and synthetic compounds in colorectal cancer (CRC) cells and discuss the molecular mechanisms by which such agents enhance the response of CRC cells to TRAIL.

Cellular signal transduction can be induced by TRAIL conjugated to microcapsules

The extracellular agent tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can induce apoptosis in tumor cells but spare normal cells. Ligation of TRAIL to a nanoparticle would serve to facilitate targeting to an extravascular site. Polymeric ultrasound contrast agents (UCA) (microencapsulated gas bubbles) can be tracked by ultrasound imaging, and fragmented into nanoparticles by focused ultrasound. This tumor-targeted delivery system has been shown to deliver more efficiently than solid nanoparticles. Additionally, small molecule inhibitors such as bortezomib, shown to sensitize TRAIL-resistant cells, could be co-administered within these UCA. In this pilot study, TRAIL was conjugated to UCA while preserving the agent's sensitivity to ultrasound. Human cancer cell lines, OVCAR-3 and A2058, were bathed with the TRAIL-UCA with and without the addition of bortezomib. Apoptosis was quantified using flow cytometry. OVCAR-3 treated with TRAIL-UCA exhibit significant (p < 0.05) apoptotosis compared to unmodified UCA, equal to positive controls, but no synergistic effect when combined with bortezomib. A2058 cells treated with TRAIL-UCA also exhibited significant apoptosis (p < 0.01) compared to unmodified UCA, similar to positive controls and bortezomib significantly increased apoptosis in combination with TRAIL-UCA. We conclude that TRAIL-ligated UCA show exciting potential as a new therapy.

Proteasome inhibition can impair caspase-8 activation upon submaximal stimulation of apoptotic tumor necrosis factor-related apoptosis inducing ligand (TRAIL) signaling

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) can induce extrinsic apoptosis, resulting in caspase-8 activation, but may also initiate transcription-dependent prosurvival signaling. Proteasome inhibitors were suggested to promote TRAIL signal transduction through the death-inducing signaling complex (DISC) by modulating the relative abundance of core DISC components, thereby enhancing caspase-8 activation and apoptosis. To test this hypothesis, we quantified the changes in DISC protein levels as an early consequence of proteasome inhibition in HeLa cervical cancer cells and, based on these data, mathematically modeled the proapoptotic TRAIL signaling toward caspase-8 activation. Modeling results surprisingly suggested that caspase-8 activation might be delayed in presence of proteasome inhibitors, in particular at submaximal TRAIL doses. Subsequent FRET-based single cell time-lapse imaging at conditions where transcription dependent prosurvival signaling was blocked confirmed this hypothesis: caspase-8 activity was delayed by hours in the presence of proteasome inhibitors epoxomicin or bortezomib. Corresponding delays were detected for effector caspase processing and cell death. Contrary to current models, we therefore provide evidence that synergies between TRAIL and proteasome inhibitors do not result from changes in the levels of core DISC signaling proteins.

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.

Phase 1 trial of gemcitabine with bortezomib in elderly patients with advanced solid tumors

INTRODUCTION

Bortezomib, a proteasome inhibitor, has synergistic antitumor activity with gemcitabine, an antimetabolite, in preclinical and clinical studies. The safety of this combination has not yet been established in elderly patients; therefore, this dose-escalation study was designed to assess the maximum-tolerated dose of bortezomib and gemcitabine in patients aged 70 years or older with advanced-stage solid tumors.

PATIENTS AND METHODS

Gemcitabine was administered intravenously (800 to 1000 mg/m) over 30 minutes on days 1 and 8, followed 60 minutes later by bortezomib administered as an intravenous push over 3 to 5 seconds (1.0 to 1.8 mg/m) on a 21-day cycle. This study used a standard phase 1 dose-escalation design with 3 or 6 patients per dose level.

RESULTS

Seventeen patients with stage IV solid tumors were treated. Median age was 73 years (range: 70 to 87 y). All patients had an Eastern Cooperative Oncology Group (ECOG) performance status less than 2. Median number of earlier chemotherapy regimens was 2 (range: 0 to 6). Dose-limiting toxicities were seen in 2 of 8 patients enrolled at the second dose level of gemcitabine (1000 mg/m) and bortezomib (1.0 mg/m), which consisted of grade ≥3 lower extremity edema, thrombocytopenia, fatigue, and dehydration. The most common grade ≥3 toxicities included thrombocytopenia (n=9), neutropenia (n=6), and anemia (n=5). Partial response (n=3) or disease stabilization (n=3) was seen in 6 of 14 evaluable patients.

CONCLUSIONS

Concurrent weekly gemcitabine (800 mg/m) and bortezomib (1 mg/m) is the recommended schedule for future phase 2 trials in elderly patients with stage IV solid tumors.

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.

Inhibition of protein degradation induces apoptosis through a microtubule-associated protein 1 light chain 3-mediated activation of caspase-8 at intracellular membranes

The accumulation of damaged or misfolded proteins, if unresolved, can lead to a detrimental consequence within cells termed proteotoxicity. Since cancerous cells often display elevated protein synthesis and by-product disposal, inhibition of the protein degradation pathways is an emerging approach for cancer therapy. However, the molecular mechanism underlying proteotoxicity remains largely unclear. We show here that inhibition of proteasomal degradation results in an increased oligomerization and activation of caspase-8 on the cytosolic side of intracellular membranes. This enhanced caspase-8 oligomerization and activation are promoted through its interaction with the ubiquitin-binding protein SQSTM1/p62 and the microtubule-associated protein light chain 3 (LC3), which are enriched at intracellular membranes in response to proteotoxic stress. Silencing LC3 by shRNA, or the LC3 mutants defective in membrane localization or p62 interaction fail to induce caspase-8 activation and apoptosis. Our results unveiled a previously unknown mechanism through which disruption of protein homeostasis induces caspase-8 oligomerization, activation, and apoptosis.

Proteasome inhibitors sensitize glioma cells and glioma stem cells to TRAIL-induced apoptosis by PKCε-dependent downregulation of AKT and XIAP expressions

In this study we examined the effects of proteasome inhibitors on cell apoptosis in TRAIL-resistant glioma cells and glioma stem cells (GSCs). Treatment with proteasome inhibitors and TRAIL induced apoptosis in all the resistant glioma cells and GSCs, but not in astrocytes and neural progenitor cells. Since PKCε has been implicated in the resistance of glioma cells to TRAIL, we examined its role in TRAIL and proteasome inhibitor-induced apoptosis. We found that TRAIL did not induce significant changes in the expression of PKCε, whereas a partial decrease in PKCε expression was obtained by proteasome inhibitors. A combined treatment of TRAIL and proteasome inhibitors induced accumulation of the catalytic fragment of PKCε and significantly and selectively decreased its protein and mRNA levels in the cancer but not in normal cells. Overexpression of PKCε partially inhibited the apoptotic effect of the proteasome inhibitors and TRAIL, and the caspase-resistant PKCεD383A mutant exerted a stronger inhibitory effect. Silencing of PKCε induced cell apoptosis in both glioma cells and GSCs, further supporting its role in cell survival. TRAIL and the proteasome inhibitors decreased the expression of AKT and XIAP in a PKCε-dependent manner and overexpression of these proteins abolished the apoptotic effect of this treatment. Moreover, silencing of XIAP sensitized glioma cells to TRAIL. Our results indicate that proteasome inhibitors sensitize glioma cells and GSCs to TRAIL by decreasing the expression of PKCε, AKT and XIAP. Combining proteasome inhibitors with TRAIL may be useful therapeutically in the treatment of gliomas and the eradication of GSCs.

Individual caspase-10 isoforms play distinct and opposing roles in the initiation of death receptor-mediated tumour cell apoptosis

The cysteine protease caspase-8 is an essential executioner of the death receptor (DR) apoptotic pathway. The physiological function of its homologue caspase-10 remains poorly understood, and the ability of caspase-10 to substitute for caspase-8 in the DR apoptotic pathway is still controversial. Here, we analysed the particular contribution of caspase-10 isoforms to DR-mediated apoptosis in neuroblastoma (NB) cells characterised by their resistance to DR signalling. Silencing of caspase-8 in tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-sensitive NB cells resulted in complete resistance to TRAIL, which could be reverted by overexpression of caspase-10A or -10D. Overexpression experiments in various caspase-8-expressing tumour cells also demonstrated that caspase-10A and -10D isoforms strongly increased TRAIL and FasL sensitivity, whereas caspase-10B or -10G had no effect or were weakly anti-apoptotic. Further investigations revealed that the unique C-terminal end of caspase-10B was responsible for its degradation by the ubiquitin-proteasome pathway and for its lack of pro-apoptotic activity compared with caspase-10A and -10D. These data highlight in several tumour cell types, a differential pro- or anti-apoptotic role for the distinct caspase-10 isoforms in DR signalling, which may be relevant for fine tuning of apoptosis initiation.

Distinct TRAIL resistance mechanisms can be overcome by proteasome inhibition but not generally by synergizing agents

One impediment to the use of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor-targeted agents as antitumor drugs is the evolution of resistance, a common problem in cancer. On the contrary, many different kinds of drugs synergize with TRAIL in TRAIL-sensitive tumor cells, raising the question whether one can overcome resistance with the same drugs producing synergy. This is an important question, because recent clinical trials suggest that combination treatments with cytotoxic drugs and TRAIL receptor-targeted agents do not provide additional benefit compared with cytotoxic agents on their own. Such results might be expected if drug combinations that synergize in sensitive tumor cells but cannot overcome TRAIL resistance are used in patients whose tumors were not selected for retention of TRAIL sensitivity. We tested this idea by creating isogenic tumor cells with acquired TRAIL resistance or defined mechanisms of resistance that occur in human tumors and then comparing them to the TRAIL-sensitive parental cell line. Although diverse classes of anticancer drugs were all able to synergize with TRAIL in sensitive cells, most agents were unable to overcome resistance and there was no relationship between the amount of synergy seen with a particular agent and its ability to overcome acquired resistance. An important exception was proteasome inhibitors, which were, however, able to overcome diverse resistance mechanisms. Our findings suggest that one should select drugs for TRAIL receptor agonist combination therapy based not just on their ability to synergize, but rather on their ability to overcome resistance as well as synergize.

Antibody-based fusion proteins to target death receptors in cancer

Ideally, an immunotoxin should be inactive 'en route', acquire activity only after tumor cell surface binding and have no off-target effects towards normal cells. In this respect, antibody-based fusion proteins that exploit the tumor-selective pro-apoptotic death ligands sFasL and sTRAIL appear promising. Soluble FasL largely lacks receptor-activating potential, whereas sTRAIL is inactive towards normal cells. Fusion proteins in which an anti-tumor antibody fragment (scFv) is fused to sFasL or sTRAIL prove to be essentially inactive when soluble, while gaining potent anti-tumor activity after selective binding to a predefined tumor-associated cell surface antigen. Importantly, off-target binding by scFv:sTRAIL to normal cells showed no signs of toxicity. In this review, we highlight the rationale and perspectives of scFv:TRAIL/scFv:sFasL based fusion proteins for cancer therapy.

Novel agents to improve outcome of allogeneic transplantation for patients with multiple myeloma

Over the last few decades therapy for multiple myeloma has improved remarkably. In particular, the introduction of novel agents has allowed improved response rates prior to, and after, stem cell transplantation with extension of progression-free survival in high-risk patients. Nevertheless, most patients relapse, leaving multiple myeloma an incurable disease. Despite being the only treatment option that has real curative potential, allogeneic transplantation has not shown its superiority to autologous transplantation due to its high morbidity and mortality rates. This review highlights how novel agents might help to reduce treatment-related mortality and to improve tumor control prior to and post-allogeneic stem cell transplant, which will hopefully result in significantly improved long-term disease control, and maybe a cure following this treatment modality.

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.

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.

TNF-related apoptosis-inducing ligand (TRAIL): a new path to anti-cancer therapies

Since its discovery in 1995, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a member of the tumor necrosis factor super family, has been under intense focus because of its remarkable ability to induce apoptosis in malignant human cells while leaving normal cells unscathed. Consequently, activation of the apoptotic signaling pathway from the death-inducing TRAIL receptors provides an attractive, biologically-targeted approach to cancer therapy. A great deal of research has focused on deciphering the TRAIL receptor signaling cascade and intracellular regulation of this pathway, as many human tumor cells possess mechanisms of resistance to TRAIL-induced apoptosis. This review focuses on the current state of knowledge regarding TRAIL signaling and resistance, the preclinical development of therapies targeted at TRAIL receptors and modulators of the pathway, and the results of clinical trials for cancer treatment that have emerged from this base of knowledge. TRAIL-based approaches to cancer therapy vary from systemic administration of recombinant, soluble TRAIL protein with or without the combination of traditional chemotherapy, radiation or novel anti-cancer agents to agonistic monoclonal antibodies directed against functional TRAIL receptors to TRAIL gene transfer therapy. A better understanding of TRAIL resistance mechanisms may allow for the development of more effective therapies that exploit this cell-mediated pathway to apoptosis.

Protein kinase Cbeta modulates ligand-induced cell surface death receptor accumulation: a mechanistic basis for enzastaurin-death ligand synergy

Although treatment with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) is known to protect a subset of cells from induction of apoptosis by death ligands such as Fas ligand and tumor necrosis factor-alpha-related apoptosis-inducing ligand, the mechanism of this protection is unknown. This study demonstrated that protection in short term apoptosis assays and long term proliferation assays was maximal when Jurkat or HL-60 human leukemia cells were treated with 2-5 nm PMA. Immunoblotting demonstrated that multiple PKC isoforms, including PKCalpha, PKCbeta, PKCepsilon, and PKC, translocated from the cytosol to a membrane-bound fraction at these PMA concentrations. When the ability of short hairpin RNA (shRNA) constructs that specifically down-regulated each of these isoforms was examined, PKCbeta shRNA uniquely reversed PMA-induced protection against cell death. The PKCbeta-selective small molecule inhibitor enzastaurin had a similar effect. Although mass spectrometry suggested that Fas is phosphorylated on a number of serines and threonines, mutation of these sites individually or collectively had no effect on Fas-mediated death signaling or PMA protection. Further experiments demonstrated that PMA diminished ligand-induced cell surface accumulation of Fas and DR5, and PKCbeta shRNA or enzastaurin reversed this effect. Moreover, enzastaurin sensitized a variety of human tumor cell lines and clinical acute myelogenous leukemia isolates, which express abundant PKCbeta, to tumor necrosis factor-alpha related apoptosis-inducing ligand-induced death in the absence of PMA. Collectively, these results identify a specific PKC isoform that modulates death receptor-mediated cytotoxicity as well as a small molecule inhibitor that mitigates the inhibitory effects of PKC activation on ligand-induced death receptor trafficking and cell death.

Bortezomib sensitizes primary human esthesioneuroblastoma cells to TRAIL-induced apoptosis

TNF-related apoptosis-inducing ligand (TRAIL), a promising novel anti-cancer cytokine of the TNF superfamily, and Bortezomib, the first-in-class clinically used proteasome inhibitor, alone or in combination have been shown to efficiently kill numerous tumor cell lines. However, data concerning primary human tumor cells are very rare. Using primary esthesioneuroblastoma cells we analyzed the anti-tumor potential and the mechanism employed by Bortezomib in combination with TRAIL for the treatment of this rare but aggressive tumor. Expression of components of the TRAIL pathway was analyzed in tumor specimens and isolated primary tumor cells at the protein level. Cells were treated with TRAIL, Bortezomib, and a combination thereof, and apoptosis induction was quantified. Clonogenicity assays were performed to elucidate the long-term effect of this treatment. Despite expressing all components of the TRAIL pathway, freshly isolated primary esthesioneuroblastoma cells were completely resistant to TRAIL-induced apoptosis. They could, however, be very efficiently sensitized by subtoxic doses of Bortezomib. The influence of Bortezomib on the TRAIL pathway was analyzed and showed upregulation of TRAIL death receptor expression, enhancement of the TRAIL death-inducing signaling complex (DISC), and downregulation of anti-apoptotic proteins of the TRAIL pathway. Of clinical relevance, TRAIL-resistant primary tumor cells could be repeatedly sensitized by Bortezomib, providing the basis for repeated clinical application schedules. This is the first report on the highly synergistic induction of apoptosis in primary esthesioneuroblastoma cells by Bortezomib and TRAIL. This combination, therefore, represents a promising novel therapeutic option for esthesioneuroblastoma.

Combination of bortezomib, thalidomide, and dexamethasone in the treatment of relapsed, refractory IgD multiple myeloma

The use of the proteasome inhibitor bortezomib has been recently introduced into the treatment of relapsed, refractory multiple myeloma (MM). We here demonstrate a case of a patient with IgD MM who was successfully treated with a combination of bortezomib, thalidomide, and dexamethasone. Relapse of the disease occurred following tandem autologous transplantation and was refractory to a salvage therapy consisting of thalidomide, cyclophosphamide, etoposide, and dexamethasone. While the administration of thalidomide was continued, the addition of bortezomib and dexamethasone led to a complete remission after 2 cycles of therapy. The feasibility of this treatment is supported by a recently reported phase I/II trial that used a lower dose of thalidomide in combination with an equal dose of bortezomib. This is the first report of a patient with IgD MM treated accordingly, suggesting that this entity is highly sensitive to the novel therapy.