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

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.

The role of XIAP in resistance to TNF-related apoptosis-inducing ligand (TRAIL) in Leukemia

The treatment for leukemic malignancies remains a challenge despite the wide use of conventional chemotherapies. Therefore, new therapeutic approaches are highly demanded. TNF-related apoptosis-inducing ligand (TRAIL) represents a targeted therapy against cancer because it induces apoptosis only in tumor cells. TRAIL is currently under investigation for the treatment of leukemia. Preclinical studies evaluated the potential therapeutic efficacy of TRAIL on cell lines and clinical samples and showed promising results. However, like most anti-cancer drugs, resistance to TRAIL-induced apoptosis may limit its clinical efficacy. It is critical to understand the molecular mechanisms of TRAIL. Therefore, rational therapeutic drug combinations for clinical trials of TRAIL-based therapies might be achieved. In a variety of leukemic cells, overexpression of X-linked inhibitor of apoptosis protein (XIAP), a negative regulator of apoptosis pathway, has been discovered. Implication of XIAP in the ineffective induction of cell death by TRAIL in leukemia has been explored in several resistant cell lines. XIAP inhibitors restored TRAIL sensitivity in resistant cells and primary leukemic blasts. Moreover, TRAIL resistance in leukemic cells could be overcome by the effects of several anti-leukemic agents via the mechanisms of XIAP downregulation. Here, we discuss targeting XIAP, a strategy to restore TRAIL sensitivity in leukemia to acquire more insights into the mechanisms of TRAIL resistance. The concluding remarks may lead to identify putative ways to resensitize tumors.

PPARγ activation by troglitazone enhances human lung cancer cells to TRAIL-induced apoptosis via autophagy flux

Members of the tumor necrosis factor (TNF) transmembrane cytokine superfamily, such as TNFα and Fas ligand (FasL), play crucial roles in inflammation and immunity. TRAIL is a member of this superfamily with the ability to selectively trigger cancer cell death but does not motive cytotoxicity to most normal cells. Troglitazone are used in the cure of type II diabetes to reduce blood glucose levels and improve the sensitivity of an amount of tissues to insulin. In this study, we revealed that troglitazone could trigger TRAIL-mediated apoptotic cell death in human lung adenocarcinoma cells. Pretreatment of troglitazone induced activation of PPARγ in a dose-dependent manner. In addition conversion of LC3-I to LC3-II and PPARγ was suppressed in the presence of GW9662, a well-characterized PPARγ antagonist. Treatment with troglitazone resulted in a slight increase in conversion rate of LC3-I to LC3-II and significantly decreased p62 expression levels in a dose-dependent manner. This indicates that troglitazone induced autophagy flux activation in human lung cancer cells. Inhibition of autophagy flux applying a specific inhibitor and genetically modified ATG5 siRNA enclosed troglitazone-mediated enhancing effect of TRAIL. These data demonstrated that activation of PPARγ mediated by troglitazone enhances human lung cancer cells to TRAIL-induced apoptosis via autophagy flux and also suggest that troglitazone may be a combination therapeutic target with TRAIL protein in TRAIL-resistant cancer cells.

Carfilzomib enhances natural killer cell-mediated lysis of myeloma linked with decreasing expression of HLA class I

Natural killer (NK) cell-based treatments are promising therapies for multiple myeloma (MM). Carfilzomib (CFZ), is a second-generation proteasome inhibitor, used to treat relapsed and refractory MM. In this study, we determined that CFZ treatment enhanced the sensitivity of MM cells to NK cell-mediated lysis. Here, we report that CFZ decreased the expression of human leukocyte antigen (HLA) class I in a time- and dose-dependent manner. CFZ also down-regulated the expression of newly formed HLA class I on MM cells. Treatment of MM with CFZ enhanced NK cell degranulation and significantly sensitized patients' MM cells to NK cell-mediated lysis. Furthermore, the enhancement of NK cell-mediated lysis was linked with the decreased expression of HLA class I. Our findings show a novel activity of CFZ as an immunomodulating agent and suggest a possible approach to therapeutically augment NK cell function in MM patients.

FTY720 enhances TRAIL-mediated apoptosis by up-regulating DR5 and down-regulating Mcl-1 in cancer cells

FTY720, Fingolimod, is a functional antagonist to the sphingosine-1-phosphate (S1P) receptor and an inhibitor of sphingosine kinase 1. Here, we showed that a combination of FTY720 and TRAIL induced apoptosis in human renal, breast, and colon carcinoma cells. Most importantly, this combination had no effect on normal cells. Furthermore, the combined treatment with FTY720 and TRAIL reduced tumor growth in xenograft models. FTY720 up-regulated death receptor (DR)5 at post-translational level. Knockdown of DR5 markedly blocked apoptosis induced by the combined treatment. FTY720 also inhibited Mcl-1 expression at the post-translational level. Over-expression of Mcl-1 blocked apoptosis induced by FTY720 and TRAIL. Interestingly, phospho-FTY720 and inhibitors of sphingosine kinase failed to enhance TRAIL-induced apoptosis. Thus, FTY720 enables TRAIL-induced apoptosis through up-regulation of DR5 and down-regulation of Mcl-1 in human cancer cells.

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).

Selenocysteine derivative overcomes TRAIL resistance in melanoma cells: evidence for ROS-dependent synergism and signaling crosstalk

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), as one of the most promising targeted drug for new cancer therapeutics, is limited in clinical application by the evolution of resistance in many cancer cell lines, especially in malignant melanoma. Thus, it is urgently needed to identify chemosensitizers to enhance the apoptotic inducing efficacy of TRAIL and overcome resistance of malignant melanoma cells. Herein, we reported that 3,3'-diselenodipropionic acid (DSeA), a Selenocysteine derivative, could synergistically enhance the growth inhibitory effect of TRAIL on A375 melanoma cells though induction of ROS-dependent apoptosis with involvement of PTEN-mediated Akt inactivation and DNA damage-mediated p53 phosphorylation, which subsequently activated mitochondrial and death receptor apoptotic pathways. Moreover, silencing of p53 down-regulated the expression levels of p53-inducible genes, and effectively blocked the cell apoptosis. Suppression of PI3K significantly increased the apoptotic cell death. In contrast, antioxidants effectively reversed the cell apoptosis through regulation of Akt and p53 signaling pathways. Taken together, the combination of DSeA and TRAIL could be a novel strategy to overcome TRAIL resistance in malignant melanoma, and DSeA may be candidates for further evaluation as a chemosensitizer in clinical trails.

Expression of tumor necrosis factor-related apoptosis-inducing ligand death receptors DR4 and DR5 in human nonmelanoma skin cancer

Death receptors 4 and 5 (DR4 and DR5) are cell surface receptors that when activated by their ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) triggers apoptosis in most cancer cells but not in normal cells. Currently, it remains unclear whether DR4 and DR5 are involved in immune surveillance against nonmelanoma skin cancer (NMSC) progression. The aim of this study was to investigate the expression of DR4 and DR5 in NMSC and relate the results to the established clinicopathologic prognostic factors. This study was conducted on about 80 skin specimens from patients with NMSC (40 basal cell carcinoma and 40 squamous cell carcinoma) and diagnosed and confirmed by biopsy. Immunohistochemical analysis for DR4 and DR5 was carried out on formalin-fixed paraffin-embedded sections of skin tissues using avidin-biotin peroxidase method. Significant expression of both DR4 and DR5 was observed in NMSC cases. There was statistically significant association between DR4 and DR5 expression in squamous cell carcinoma and each of tumor site and lymph node metastasis. There was statistically significant association between DR4 expression in basal cell carcinoma and histopathologic subtypes (high expression in nodular type) and between DR5 expression and tumor site (high expression in sun-exposed area). In conclusion, expression of TRAIL receptors that mediate extrinsic apoptotic pathway in NMSC may be suggestive of a reassessment of the suitability of TRAIL-based strategy in future NMSC therapies.

Accumulation of autophagosomes in breast cancer cells induces TRAIL resistance through downregulation of surface expression of death receptors 4 and 5

TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis through death receptors (DRs) 4 and/or 5 expressed on the surface of target cells. We have previously shown that deficiency of DR4 and DR5 on the surface membrane is a critical mechanism of cancer cell resistance to the recombinant human TRAIL and its receptor agonistic antibodies, which are being evaluated clinically for treating cancers. In certain cancer cells, DR4 and DR5 were found to be mislocalized in intracellular compartments yet to be characterized. Here, we report a novel role of autophagy in the regulation of dynamics of TRAIL death receptors. We first assessed basal levels of autophagosomes in a panel of 11 breast cancer cell lines using complementary approaches (LC3 immunoblotting, RFP-LC3 fluorescence microscopy, and electron microscopy). We found high levels of basal autophagosomes in TRAIL resistant breast cancer cell lines (e.g. BT474 and AU565) and relevant mouse xenograft models under nutrition-rich conditions. Notably, DR4 and DR5 co-localized with LC3-II in the autophagosomes of TRAIL-resistant cells. Disruption of basal autophagosomes successfully restored the surface expression of the death receptors which was accompanied by sensitization of TRAIL-resistant cells to TRAIL induced apoptosis. By contrast, TRAIL-sensitive cell lines (MDA-MB-231) are characterized by high levels of surface DR4/DR5 and an absence of basal autophagosomes. Inhibition of lysosomal activity induced an accumulation of autophagosomes and a decrease in surface DR4 and DR5, and the cells became less sensitive to TRAIL-induced apoptosis. These findings demonstrate a novel role for the basal autophagosomes in the regulation of TRAIL death receptors. Further studies are warranted to explore the possibility of using autophagosome markers such as LC3-II/LC3-I ratios for prediction of tumor resistance to TRAIL related therapies. The results also provide a rationale for future non-clinical and clinical studies testing TRAIL agonists in combination with agents that directly inhibit autophagosome assembly.

Combining trail with PI3 kinase or HSP90 inhibitors enhances apoptosis in colorectal cancer cells via suppression of survival signaling

TRAIL has been shown to induce apoptosis in cancer cells, but in some cases they fail to respond to this ligand. We explored the ability of representative phosphatidylinositol-3-kinase (PI3 Kinase)/mTOR and HSP90 inhibitors to overcome TRAIL resistance by increasing apoptosis in colorectal cancer models. We determined the sensitivity of 27 human colorectal cancer and 2 non-transformed colon epithelial cell lines to TRAIL treatment. A subset of the cancer cell lines with a range of responses to TRAIL was selected from the panel for treatment with TRAIL combined with the PI3 Kinase/mTOR inhibitor PI-103 or the HSP90 inhibitor 17-AAG (tanespimycin). Two TRAIL-resistant cell lines were selected for in vivo combination studies with TRAIL and 17-AAG. We found that 13 colorectal cancer cell lines and the 2 non-transformed colon epithelial cell lines were resistant to TRAIL. We demonstrated that co-treatment of TRAIL and PI-103 or 17-AAG was synergistic or additive and significantly enhanced apoptosis in colorectal cancer cells. This was associated with decreased expression or activity of survival protein biomarkers such as ERBB2, AKT, IKKα and XIAP. In contrast, the effect of the combination treatments in non-transformed colon cells was minimal. We show here for the first time that co-treatment in vivo with TRAIL and 17-AAG in two TRAIL-resistant human colorectal cancer xenograft models resulted in significantly greater tumor growth inhibition compared to single treatments. We propose that combining TRAIL with PI3 Kinase/mTOR or HSP90 inhibitors has therapeutic potential in the treatment of TRAIL-resistant colorectal cancers.

Mislocalization of death receptors correlates with cellular resistance to their cognate ligands in human breast cancer cells

Multiple clinical trials are ongoing to evaluate the potential antitumor activity of human TNF variants, Fas ligand (FasL), TNF-related apoptosis inducing ligand (TRAIL) and its agonistic antibodies. These drug products act through the death receptors (DRs) TNF receptor 1 (TNFR1), Fas/CD95, DR4 (TRAIL-R1) and/or DR5 (TRAIL-R2), respectively. Therefore, characterization of the level and localization of DR expression in cancer cells is important for DR-targeted therapy. In this study, we examined the subcellular distribution of the four DRs in a panel of 10 human breast cancer cell lines by western blots and flow cytometry and 50 human breast tumors by immunohistochemistry. Despite their total protein expressions, the DRs were found to be absent on the surface of some cell lines. Consistent with this result, all four DRs were found to be mostly expressed in the cytoplasm and/or the nucleus of primary breast tumors (n=50). We further determined the growth inhibition activity (GI50) of the death ligands, recombinant human TNFα, FasL and TRAIL, and found a correlation with the subcellular localization of the corresponding DRs. These results demonstrate an aberrant expression of the death receptors in breast cancer cells, and suggest that the lack of surface DRs appears to be predictive of tumor resistance to DR-targeted therapies.

Combination of a selective activator of the glucocorticoid receptor Compound A with a proteasome inhibitor as a novel strategy for chemotherapy of hematologic malignancies

Glucocorticoids are widely used for the treatment of hematological malignancies; however, their chronic use results in numerous metabolic side effects. Thus, the development of selective glucocorticoid receptor (GR) activators (SEGRA) with improved therapeutic index is important. GR regulates gene expression via (1) transactivation that requires GR homodimer binding to gene promoters and is linked to side effects and (2) transrepression-mediated via negative GR interaction with other transcription factors. Novel GR modulator Compound A (CpdA) prevents GR dimerization, retains glucocorticoid anti-inflammatory activity and has fewer side effects compared with glucocorticoids in vivo. Here we tested CpdA anticancer activity in human T- and B-lymphoma and multiple myeloma cells expressing GR and their counterparts with silenced GR. We found that CpdA in GR-dependent manner strongly inhibited growth and viability of human T-, B-lymphoma and multiple myeloma cells. Furthermore, primary leukemia cell cultures from T-ALL patients appeared to be equally sensitive to glucocorticoid dexamethasone and CpdA. It is known that GR expression is controlled by proteasome. We showed that pretreatment of lymphoma CEM and NCEB cells with proteasome-inhibitor Bortezomib resulted in GR accumulation and enhanced ligand properties of CpdA, shifting GR activity toward transrepression evaluated by inhibition of NFкB and AP-1 transcription factors. We also revealed remarkable GR-dependent cooperation between CpdA and Bortezomib in suppressing growth and survival of T- and B-lymphoma and multiple myeloma MM.1S cells. Overall, our data provide the rationale for novel GR-based therapy for hematological malignancies based on combination of SEGRA with proteasome inhibitors.

Targeted ovarian cancer treatment: the TRAILs of resistance

Ovarian cancer (OC) is the leading cause of death from gynecological malignancies. Although most patients respond to the initial therapy when presenting with advanced disease, only 10-15% maintain a complete response following first-line therapy. Recurrence defines incurable disease in most cases. Despite improvements with conventional chemotherapy combinations, the overall cure rate remained mostly stable over the years. Increased long-term survival in OC patients will only be achieved through a comprehensive understanding of the basic mechanisms of tumor cell resistance. Such knowledge will translate into the development of new targeted strategies. In addition, because OC is considered to be a heterogeneous group of diseases with distinct gene expression profiles, it is likely that different approaches to treatment for distinct sub-types will be required to optimize response. One of the new promising anti-cancer therapies is the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL has the ability to selectively induce apoptosis in tumor cells with little toxicity to normal cells. Death receptor ligands such as TRAIL rely on the activation of the apoptotic signaling pathway to destroy tumor cells. TRAIL induces the formation of a pro-apoptotic death-inducing signaling complex (DISC) via its death receptors, TRAIL receptor 1 (TRAIL R1) and TRAIL receptor 2 (TRAIL R2). The formation of the DISC activates caspase-8 which requires further signal amplification through the mitochondrial pathway for an efficient activation of effector caspases in OC cells. The initial enthusiasm for TRAIL has been hampered by accumulating data demonstrating TRAIL resistance in various tumor types including OC cells. There is, therefore, a need to identify markers of TRAIL resistance, which could represent new hits for targeted therapy that will enhance TRAIL efficacy. In addition, the identification of patients that are more likely to respond to TRAIL therapy would be highly desirable. In this review, we discuss the different molecular and cellular mechanisms leading to TRAIL resistance in OC. In particular, we address the mechanisms involved in intrinsic, acquired and environment-mediated TRAIL resistance, and their potential implication in the clinical outcome.

Proteasome inhibitors suppress expression of NPM and ARF proteins

Proteasome inhibitors stabilize numerous proteins by inhibiting their degradation. Previously we have demonstrated that proteasome inhibitors thiostrepton, MG132 and bortezomib paradoxically inhibit transcriptional activity and mRNA/protein expression of FOXM1. Here we demonstrate that, in addition to FOXM1, the same proteasome inhibitors also decrease mRNA and protein expression of NPM and ARF genes. These data suggest that proteasome inhibitors may suppress gene expression by stabilizing their transcriptional inhibitors.

FoxM1 knockdown sensitizes human cancer cells to proteasome inhibitor-induced apoptosis but not to autophagy

Apoptosis has been widely accepted as the primary mechanism of drug-induced cell death. Recently, a second type of cell death pathway has been demonstrated: autophagy, also called programmed type II cell death. Autophagy is a highly regulated process, by which selected components of a cell are degraded. It primarily functions as a cell survival mechanism under stress. However, persistent stress can also promote extensive autophagy leading to cell death. Forkhead box M1 (FoxM1), an oncogenic transcription factor that is abundantly expressed in a wide range of human cancers. Here we evaluated the role of FoxM1 in sensitivity of human cancer cells to proteasome inhibitor-induced apoptosis and autophagy. We found that FoxM1 knockdown sensitized the human cancer cells to apoptotic cell death induced by proteasome inhibitors, such as, MG132, bortezomib and thiostrepton, while it did not affect the levels of autophagy following treatment with these drugs.