Activation of the endoplasmic reticulum stress pathway is associated with survival of myeloma cells

Integrative analysis of an endoplasmic reticulum stress-related signature in multiple myeloma

BACKGROUND

Multiple myeloma (MM) is a malignancy in which plasma cells proliferate abnormally, and it remains incurable. The cells are characterized by high levels of endoplasmic reticulum stress (ERS) and depend on the ERS response for survival. Thus, we aim to find an ERS-related signature of MM and assess its diagnostic value.

METHODS

We downloaded three datasets of MM from the Gene Expression Omnibus database. After identifying ERS-related differentially expressed genes (ERDEGs), we analyzed them using Gene Ontology enrichment analysis. A protein-protein interaction network, a transcription factor-mRNA network, a miRNA-mRNA network and a drug-mRNA network were constructed to explore the ERDEGs. The clinical application of these genes was identified by calculating the infiltration of immune cells and using receiver operating characteistic analyses. Finally, qPCR was performed to further confirm the roles of ERDEGs.

RESULTS

We obtained nine ERDEGs of MM. Gene Ontology enrichment indicated that the ERDEGs played a role in the endoplasmic reticulum membrane. Additionally, the protein-protein interaction network showed interaction among the ERDEGs, and there were 20 proteins, 107 transcription factors, 42 drugs or molecular compounds and 51 miRNAs which were likely to interact with the nine genes. In addition, immune cell infiltration analyses showed that there was a strong correlation between the nine genes and immune cells, and these potential biomarkers exhibited good diagnostic values. Finally, the expression of ERDEGs in MM cells was different from that in healthy donor samples.

CONCLUSION

The nine ERS-related genes, CR2, DHCR7, DNAJC3, KDELR2, LPL, OSBPL3, PINK1, VCAM1 and XBP1 are potential biomarkers of MM, and this supports further clinical development of the diagnosis and treatment of MM.

The PERK/ATF4/CHOP signaling branch of the unfolded protein response mediates cisplatin-induced ototoxicity in hair cells

Cisplatin is a widely used chemotherapeutic agent. However, its clinical application remains limited due to the high incidence of severe ototoxicity. It has been reported that the unfolded protein response (UPR) is involved in cisplatin-induced ototoxicity. However, the specific mechanism underlying its effect remains unclear. Therefore, the present study aimed to explore the sequential changes in the key UPR signaling branch and its potential pro-apoptotic role in cisplatin-induced ototoxicity. The hair cell-like OC-1 cells were treated with cisplatin for different periods and then the expression levels of the UPR- and apoptosis-related proteins were determined. The results showed that the apoptotic rate of cells was gradually increased with prolonged cisplatin treatment. Furthermore, the sequential changes in three UPR signaling branches were evaluated. The expression levels of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP) were gradually increased with up to 12 h of cisplatin treatment. The aforementioned expression profile was consistent with that observed for the apoptosis-related proteins. Subsequently, the proportion of apoptotic cells was notably decreased in CHOP-silenced hair cell-like OC-1 cells following treatment with cisplatin. Moreover, we found significant hair cells loss and a higher level of CHOP in cisplatin-treated cochlear explants in a time-dependent manner. Overall, the present study demonstrated that the protein kinase RNA‑like endoplasmic reticulum kinase (PERK)/ATF4/CHOP signaling branch could play an important role in cisplatin-induced cell apoptosis. Furthermore, the current study suggested that CHOP may be considered as a promising therapeutic target for cisplatin-induced ototoxicity.

Allosteric Inhibition of c-Abl to Induce Unfolded Protein Response and Cell Death in Multiple Myeloma

Endoplasmic reticulum stress activates inositol-requiring enzyme 1α (IRE1α) and protein kinase, R-like endoplasmic reticulum kinase (PERK), the two principal regulators of the unfolded protein response (UPR). In multiple myeloma, adaptive IRE1α signaling is predominantly activated and regulates cell fate along with PERK. Recently, we demonstrated that GNF-2, an allosteric c-Abl inhibitor, rheostatically enhanced IRE1α activity and induced apoptosis through c-Abl conformational changes in pancreatic β cells. Herein, we analyzed whether the pharmacological modulation of c-Abl conformation resulted in anti-myeloma effects. First, we investigated the effects of GNF-2 on IRE1α activity and cell fate, followed by an investigation of the anti-myeloma effects of asciminib, a new allosteric c-Abl inhibitor. Finally, we performed RNA sequencing to characterize the signaling profiles of asciminib. We observed that both GNF-2 and asciminib decreased cell viability and induced XBP1 mRNA splicing in primary human myeloma cells and myeloma cell lines. RNA sequencing identified the induction of UPR- and apoptosis-related genes by asciminib. Asciminib re-localized c-Abl to the endoplasmic reticulum, and its combination with a specific IRE1α inhibitor, KIRA8, enhanced cell death with the reciprocal induction of CHOP mRNA expression. Together, the allosteric inhibition of c-Abl-activated UPR with anti-myeloma effects; this could be a novel therapeutic target for multiple myeloma.

Looking into Endoplasmic Reticulum Stress: The Key to Drug-Resistance of Multiple Myeloma?

Multiple myeloma (MM) is the second most common hematologic malignancy, resulting from the clonal proliferation of malignant plasma cells within the bone marrow. Despite significant advances that have been made with novel drugs over the past two decades, MM patients often develop therapy resistance, especially to bortezomib, the first-in-class proteasome inhibitor that was approved for treatment of MM. As highly secretory monoclonal protein-producing cells, MM cells are characterized by uploaded endoplasmic reticulum stress (ERS), and rely heavily on the ERS response for survival. Great efforts have been made to illustrate how MM cells adapt to therapeutic stresses through modulating the ERS response. In this review, we summarize current knowledge on the mechanisms by which ERS response pathways influence MM cell fate and response to treatment. Moreover, based on promising results obtained in preclinical studies, we discuss the prospect of applying ERS modulators to overcome drug resistance in MM.

Structural and Ultrastructural Analysis of the Multiple Myeloma Cell Niche and a Patient-Specific Model of Plasma Cell Dysfunction

Multiple myeloma (MM) is a deadly, incurable malignancy in which antibody-secreting plasma cells (PCs) become neoplastic. Previous studies have shown that the PC niche plays a role cancer progression. Bone marrow (BM) cores from MM and a premalignant condition known as monoclonal gammopathy of unknown significance (MGUS) patients were analyzed with confocal and transmission electron microscopy. The BM aspirates from these patients were used to generate 3D PC cultures. These in vitro cultures were then assayed for the molecular, cellular, and ultrastructural hallmarks of dysfunctional PC at days 1 and 5. In vivo, evidence of PC endoplasmic reticulum stress was found in both MM and MGUS BM; however, evidence of PC autophagy was found only in MM BM. Analysis of in vitro cultures found that MM PC can survive and maintain a differentiated phenotype over an unprecedented 5 days, had higher levels of paraprotein production when compared to MGUS-derived cultures, and showed evidence of PC autophagy as well. Increased fibronectin deposition around PC associated with disease severity and autophagy dysregulation was also observed. 3D cultures constructed from BM aspirates from MGUS and MM patients allow for long-term culture of functional PC while maintaining their distinct morphological phenotypes.

Review of cancer cell resistance mechanisms to apoptosis and actual targeted therapies

The apoptosis pathway is a programmed cell death mechanism that is crucial for cellular and tissue homeostasis and organ development. There are three major caspase-dependent pathways of apoptosis that ultimately lead to DNA fragmentation. Cancerous cells are known to highly regulate the apoptotic pathway and its role in cancer hallmark acquisition has been discussed over the past decades. Numerous mutations in cancer cell types have been reported to be implicated in chemoresistance and treatment outcome. In this review, we summarize the mutations of the caspase-dependant apoptotic pathways that are the source of cancer development and the targeted therapies currently available or in trial.

Maintenance of Endoplasmic Reticulum Protein Homeostasis in Cancer: Friend or Foe

The endoplasmic reticulum, as the site of synthesis for proteins in the secretory pathway has evolved select machineries to ensure the correct folding and modification of proteins. However, sometimes these quality control mechanisms fail and proteins are misfolded. Other factors, such as nutrient deprivation, hypoxia or an increased demand on protein synthesis can also cause the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum. There are mechanisms that recognise and deal with this accumulation of protein through degradation and/or export. Many diseases are associated with aberrant quality control mechanisms, and among these, cancer has emerged as a group of diseases that rely on endoplasmic reticulum homeostasis to sustain development and growth. The knowledge of how protein quality control operates in cancer has identified opportunities for these pathways to be pharmacologically targeted, which could lead to newer or more effective treatments in the future.

Glucose-regulated protein 78 (GRP78) as a potential novel biomarker and therapeutic target in multiple myeloma

INTRODUCTION

Glucose-regulated protein 78 (GRP78) is a stress-inducible molecular chaperone expressed within the endoplasmic reticulum where it acts as a master regulator of the unfolded protein response (UPR) pathway. At times of ER stress, activation of the UPR, a multimolecular pathway, limits proteotoxicity induced by misfolded proteins. In malignancies, including multiple myeloma which is characterized by an accumulation of misfolded immunoglobulins, GRP78 expression is increased, with notable translocation of GRP78 to the cell surface. Studies suggest cell-surface GRP78 (csGRP78) to be of prognostic significance with emerging evidence that it interacts with a myriad of co-ligands to activate signaling pathways promoting cell proliferation and survival or apoptosis.

AREAS COVERED

This review focuses on the role of ER and csGRP78 in physiology and oncogenesis in multiple myeloma, addressing factors that shift the balance in GRP78 signaling from survival to apoptosis. The role of GRP78 as a potential prognostic biomarker is explored and current therapeutics in development aimed at targeting csGRP78 are addressed. We conducted a PubMed literature search using the keywords 'GRP78,' 'multiple myeloma' reviewing studies prior to 2020.

EXPERT OPINION

Cell-surface GRP78 expression is a potential novel prognostic biomarker in myeloma and targeting of csGRP78 is promising and requires further investigation.

Targeting Adaptive IRE1α Signaling and PLK2 in Multiple Myeloma: Possible Anti-Tumor Mechanisms of KIRA8 and Nilotinib

BACKGROUND

Inositol-requiring enzyme 1α (IRE1α), along with protein kinase R-like endoplasmic reticulum kinase (PERK), is a principal regulator of the unfolded protein response (UPR). Recently, the 'mono'-specific IRE1α inhibitor, kinase-inhibiting RNase attenuator 6 (KIRA6), demonstrated a promising effect against multiple myeloma (MM). Side-stepping the clinical translation, a detailed UPR phenotype in patients with MM and the mechanisms of how KIRA8 works in MM remains unclear.

METHODS

We characterized UPR phenotypes in the bone marrow of patients with newly diagnosed MM. Then, in human MM cells we analyzed the possible anti-tumor mechanisms of KIRA8 and a Food and Drug Administration (FDA)-approved drug, nilotinib, which we recently identified as having a strong inhibitory effect against IRE1α activity. Finally, we performed an RNA-sequence analysis to detect key IRE1α-related molecules against MM.

RESULTS

We illustrated the dominant induction of adaptive UPR markers under IRE1α over the PERK pathway in patients with MM. In human MM cells, KIRA8 decreased cell viability and induced apoptosis, along with the induction of C/EBP homologous protein (CHOP); its combination with bortezomib exhibited more anti-myeloma effects than KIRA8 alone. Nilotinib exerted a similar effect compared with KIRA8. RNA-sequencing identified Polo-like kinase 2 (PLK2) as a KIRA8-suppressed gene. Specifically, the IRE1α overexpression induced PLK2 expression, which was decreased by KIRA8. KIRA8 and PLK2 inhibition exerted anti-myeloma effects with apoptosis induction and the regulation of cell proliferation. Finally, PLK2 was pathologically confirmed to be highly expressed in patients with MM.

CONCLUSION

Dominant activation of adaptive IRE1α was established in patients with MM. Both KIRA8 and nilotinib exhibited anti-myeloma effects, which were enhanced by bortezomib. Adaptive IRE1α signaling and PLK2 could be potential therapeutic targets and biomarkers in MM.

DOT1L inhibition is lethal for multiple myeloma due to perturbation of the endoplasmic reticulum stress pathway

The histone 3 lysine 79 (H3K79) methyltransferase (HMT) DOT1L is known to play a critical role for growth and survival of MLL-rearranged leukemia. Serendipitous observations during high-throughput drug screens indicated that the use of DOT1L inhibitors might be expandable to multiple myeloma (MM). Through pharmacologic and genetic experiments, we could validate that DOT1L is essential for growth and viability of a subset of MM cell lines, in line with a recent report from another team. In vivo activity against established MM xenografts was observed with a novel DOT1L inhibitor. In order to understand the molecular mechanism of the dependency in MM, we examined gene expression changes upon DOT1L inhibition in sensitive and insensitive cell lines and discovered that genes belonging to the endoplasmic reticulum (ER) stress pathway and protein synthesis machinery were specifically suppressed in sensitive cells. Whole-genome CRISPR screens in the presence or absence of a DOT1L inhibitor revealed that concomitant targeting of the H3K4me3 methyltransferase SETD1B increases the effect of DOT1L inhibition. Our results provide a strong basis for further investigating DOT1L and SETD1B as targets in MM.

The multiple roles of the unfolded protein response regulator IRE1α in cancer

Cancer is associated with a number of conditions such as hypoxia, nutrient deprivation, cellular redox, and pH changes that result in accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) and trigger a stress response known as the unfolded protein response (UPR). The UPR is a conserved cellular survival mechanism mediated by the ER transmembrane proteins activating transcription factor 6, protein kinase-like endoplasmic reticulum kinase, and inositol-requiring enzyme 1α (IRE1α) that act to resolve ER stress and promote cell survival. IRE1α is a kinase/endoribonuclease (RNase) with multiple activities including unconventional splicing of the messenger RNA (mRNA) for the transcription factor X-Box Binding Protein 1 (XBP1), degradation of other mRNAs in a process called regulated IRE1α-dependent decay (RIDD) and activation of a pathway leading to c-Jun N-terminal kinase phosphorylation. Each of these outputs plays a role in the adaptive and cell death responses to ER stress. Many studies indicate an important role for XBP1 and RIDD functions in cancer and new studies suggest that these two functions of the IRE1α RNase can have opposing functions in the early and later stages of cancer pathogenesis. Finally, as more is learned about the context-dependent role of IRE1α in cancer development, specific small molecule inhibitors and activators of IRE1α could play an important role in counteracting the protective shield provided by ER stress signaling in cancer cells.

Immunogenic Cell Death and Immunotherapy of Multiple Myeloma

Over the past decades, immunotherapy has demonstrated a prominent clinical efficacy in a wide variety of human tumors. For many years, apoptosis has been considered a non-immunogenic or tolerogenic process whereas necrosis or necroptosis has long been acknowledged to play a key role in inflammation and immune-related processes. However, the new concept of “immunogenic cell death” (ICD) has challenged this traditional view and has granted apoptosis with immunogenic abilities. This paradigm shift offers clear implications in designing novel anti-cancer therapeutic approaches. To date, several screening studies have been carried out to discover bona fide ICD inducers and reveal the inherent capacity of a wide variety of drugs to induce cell death-associated exposure of danger signals and to bring about in vivo anti-cancer immune responses. Recent shreds of evidence place ER stress at the core of all the scenarios where ICD occur. Furthermore, ER stress and the unfolded protein response (UPR) have emerged as important targets in different human cancers. Notably, in multiple myeloma (MM), a lethal plasma cell disorder, the elevated production of immunoglobulins leaves these cells heavily reliant on the survival arm of the UPR. For that reason, drugs that disrupt ER homeostasis and engage ER stress-associated cell death, such as proteasome inhibitors, which are currently used for the treatment of MM, as well as novel ER stressors are intended to be promising therapeutic agents in MM. This not only holds true for their capacity to induce cell death, but also to their potential ability to activate the immunogenic arm of the ER stress response, with the ensuing exposure of danger signals. We provide here an overview of the up-to-date knowledge regarding the cell death mechanisms involved in situations of ER stress with a special focus on the connections with the drug-induced ER stress pathways that evoke ICD. We will also discuss how this could assist in optimizing and developing better immunotherapeutic approaches, especially in MM treatment.

Definition of a multiple myeloma progenitor population in mice driven by enforced expression of XBP1s

Multiple myeloma (MM) is an incurable plasma cell malignancy with frequent treatment failures and relapses, suggesting the existence of pathogenic myeloma stem/progenitor populations. However, the identity of MM stem cells remains elusive. We used a murine model of MM with transgenic overexpression of the unfolded protein response sensor X-box binding protein 1 (XBP1s) in the B cell compartment to define MM stem cells. We herein report that a post-germinal center, pre-plasma cell population significantly expands as MM develops. This population has the following characteristics: (a) cell surface phenotype of B220+CD19+IgM-IgD-CD138-CD80+sIgG-AA4.1+FSChi; (b) high expression levels of Pax5 and Bcl6 with intermediate levels of Blimp1 and XBP1s; (c) increased expression of aldehyde dehydrogenase, Notch1, and c-Kit; and (d) ability to efficiently reconstitute antibody-producing capacity in B cell-deficient mice in vivo. We thus have defined a plasma cell progenitor population that resembles myeloma stem cells in mice. These results provide potentially novel insights into MM stem cell biology and may contribute to the development of novel stem cell-targeted therapies for the eradication of MM.

GRP78 modulates cell adhesion markers in prostate Cancer and multiple myeloma cell lines

Background

Glucose regulated protein 78 (GRP78) is a resident chaperone of the endoplasmic reticulum and a master regulator of the unfolded protein response under physiological and pathological cell stress conditions. GRP78 is overexpressed in many cancers, regulating a variety of signaling pathways associated with tumor initiation, proliferation, adhesion and invasion which contributes to metastatic spread. GRP78 can also regulate cell survival and apoptotic pathways to alter responsiveness to anticancer drugs. Tumors that reside in or metastasize to the bone and bone marrow (BM) space can develop pro-survival signals through their direct adhesive interactions with stromal elements of this niche thereby resisting the cytotoxic effects of drug treatment. In this study, we report a direct correlation between GRP78 and the adhesion molecule N-cadherin (N-cad), known to play a critical role in the adhesive interactions of multiple myeloma and metastatic prostate cancer with the bone microenvironment.

Methods

N-cad expression levels (transcription and protein) were evaluated upon siRNA mediated silencing of GRP78 in the MM.1S multiple myeloma and the PC3 metastatic prostate cancer cell lines. Furthermore, we evaluated the effects of GRP78 knockdown (KD) on epithelial-mesenchymal (EMT) transition markers, morphological changes and adhesion of PC3 cells.

Results

GRP78 KD led to concomitant downregulation of N-cad in both tumors types. In PC3 cells, GRP78 KD significantly decreased E-cadherin (E-cad) expression likely associated with the induction in TGF-β1 expression. Furthermore, GRP78 KD also triggered drastic changes in PC3 cells morphology and decreased their adhesion to osteoblasts (OSB) dependent, in part, to the reduced N-cad expression.

Conclusion

This work implicates GRP78 as a modulator of cell adhesion markers in MM and PCa. Our results may have clinical implications underscoring GRP78 as a potential therapeutic target to reduce the adhesive nature of metastatic tumors to the bone niche.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-5178-8) contains supplementary material, which is available to authorized users.

A Human Genome-Wide RNAi Screen Reveals Diverse Modulators that Mediate IRE1α-XBP1 Activation

Activation of the unfolded protein response (UPR) signaling pathways is linked to multiple human diseases, including cancer. The inositol-requiring kinase 1α (IRE1α)-X-box binding protein 1 (XBP1) pathway is the most evolutionarily conserved of the three major signaling branches of the UPR. Here, we performed a genome-wide siRNA screen to obtain a systematic assessment of genes integrated in the IRE1α-XBP1 axis. We monitored the expression of an XBP1-luciferase chimeric protein in which luciferase was fused in-frame with the spliced (active) form of XBP1. Using cells expressing this reporter construct, we identified 162 genes for which siRNA inhibition resulted in alteration in XBP1 splicing. These genes express diverse types of proteins modulating a wide range of cellular processes. Pathway analysis identified a set of genes implicated in the pathogenesis of breast cancer. Several genes, including BCL10, GCLM, and IGF1R, correlated with worse relapse-free survival (RFS) in an analysis of patients with triple-negative breast cancer (TNBC). However, in this cohort of 1,908 patients, only high GCLM expression correlated with worse RFS in both TNBC and non-TNBC patients. Altogether, our study revealed unidentified roles of novel pathways regulating the UPR, and these findings may serve as a paradigm for exploring novel therapeutic opportunities based on modulating the UPR.Implications: Genome-wide RNAi screen identifies novel genes/pathways that modulate IRE1α-XBP1 signaling in human tumor cells and leads to the development of improved therapeutic approaches targeting the UPR.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/16/5/745/F1.large.jpg Mol Cancer Res; 16(5); 745-53. ©2018 AACR.

Deptor transcriptionally regulates endoplasmic reticulum homeostasis in multiple myeloma cells

Multiple myeloma (MM) is a malignant disorder of plasma cells characterized by active production and secretion of monoclonal immunoglobulins (IgG), thus rendering cells prone to endoplasmic reticulum (ER) stress. For this reason, MM cell survival requires to maintain ER homeostasis at basal levels. Deptor is an mTOR binding protein, belonging to the mTORC1 and mTORC2 complexes. It was reported that Deptor is overexpressed in MM cells where it inhibits mTOR kinase activity and promotes cell survival by activating Akt signaling. Here we identify Deptor as a nuclear protein, able to bind DNA and regulate transcription in MM cells. In particular, we found that Deptor plays an important role in the maintenance of the ER network, sustaining the expression of several genes involved in this pathway. In agreement with this, Deptor depletion induces ER stress and synergizes the effect of the proteasome inhibitor bortezomib (Bz) in MM cells. These findings provide important new insights in the ER stress control in MM cells.

Homeostatic interplay between FoxO proteins and ER proteostasis in cancer and other diseases

Cancer cells are exposed to adverse conditions within the tumor microenvironment that challenge cells to adapt and survive. Several of these homeostatic perturbations insults alter the normal function of the endoplasmic reticulum (ER), resulting in the accumulation of misfolded proteins. ER stress triggers a conserved signaling pathway known as the unfolded protein response (UPR) to cope with the stress or trigger apoptosis of damaged cells. The UPR has been described as a major driver in the acquisition of malignant characteristics that ultimately lead to cancer progression. Although, several reports describe the relevance of the UPR in tumor growth, the possible crosstalk with other cancer-related pathways is starting to be elucidated. The Forkhead Box O (FoxO) subfamily of proteins has a major role in cancer progression, where chromosomal translocations and deregulated signaling lead to loss-of-function of FoxO proteins, contributing to tumor progression. Here we discuss the homeostatic connection between the UPR and FoxO proteins and its possible implications to tumor progression and the acquisition of several hallmarks of cancer. In addition, studies linking a crosstalk between the UPR and FoxO proteins in other diseases, including neurodegeneration and metabolic disorders is provided.

Endoplasmic reticulum stress in the development of multiple myeloma and drug resistance

Multiple myeloma (MM) is a haematological malignancy of mature antibody‐secreting plasma cells. Currently, MM is incurable, but advances in drug treatments have increased patient lifespan. One of the characteristics of MM is the excessive production of monoclonal immunoglobulin (also referred to as paraprotein). This high level of protein production induces endoplasmic reticulum (ER) stress, and proteasomal degradation of the paraprotein is required to avoid ER stress‐induced cell death. Consequently, proteasomal inhibitors such as bortezomib have been particularly effective therapies. Unfortunately development of resistance to bortezomib is common. In this review, we address how control of endoplasmic reticulum stress is important in the development of MM and how the unfolded protein response and its associated stress response pathways are involved in the development of bortezomib resistance.

Smac mimetic LCL161 overcomes protective ER stress induced by obatoclax, synergistically causing cell death in multiple myeloma

Bcl2 and IAP families are anti-apoptotic proteins deregulated in multiple myeloma (MM) cells. Pharmacological inhibition of each of these families has shown significant activity only in subgroups of MM patients. Here, we have examined a broad-spectrum Bcl2 family inhibitor Obatoclax (OBX) in combination with a Smac mimetic LCL161 in MM cell lines and patient cells. LCL161/OBX combination induced synergistic cytotoxicity and anti-proliferative effects on a broad range of human MM cell lines. The cytotoxicity was mediated through inhibition of the IAPs, activation of caspases and up regulation of the pro-apoptotic proteins Bid, Bim, Puma and Noxa by the drug combination. In addition, we observed that OBX caused ER stress and activated the Unfolded Protein Response (UPR) leading to drug resistance. LCL161, however inhibited spliced Xbp-1, a pro-survival factor. In addition, we observed that OBX increased GRP78 localization to the cell surface, which then induced PI3K dependent Akt activation and resistance to cell death. LCL161 was able to block OBX induced Akt activation contributing to synergistic cell death. Our results support clinical evaluation of this combination strategy in relapsed refractory MM patients.

Heme oxygenase-1 nuclear translocation regulates bortezomibinduced cytotoxicity and mediates genomic instability in myeloma cells

Multiple myeloma (MM) is a clonal B-cell malignancy characterized by an accumulation of clonal plasma cells in the bone marrow leading to bone destruction and bone marrow failure. Several molecular mechanisms underlie chemoresistance among which heme oxygenase-1 (HO-1) could play a major role. The aim of the present research was to evaluate the impact of HO-1 in MM following bortezomib (BTZ) treatment and how HO-1 is implicated in the mechanisms of chemoresistance. MM cells were treated for 24h with BTZ (15 nM), a boronic acid dipeptide inhibitor of the 26S proteasome used in the treatment of patients with MM as first-line therapy. We evaluated cell viability, reactive oxygen species (ROS) formation, endoplasmic reticulum (ER) stress, HO-1 expression and compartmentalization and cellular genetic instability. Results showed that BTZ significantly reduced cell viability in different MM cell lines and induced ER-stress and ROS formation. Concomitantly, we observed a significant overexpression of both HO-1 gene and protein levels. This effect was abolished by concomitant treatment with 4-phenybutirric acid, a molecular chaperone, which is known to reduce ER-stress. Surprisingly, inhibition of HO activity with SnMP (10μM) failed to increase BTZ sensitivity in MM cells whereas inhibition of HO-1 nuclear translocation by E64d, a cysteine protease inhibitor, increased sensitivity to BTZ and decreased genetic instability as measured by cytokinesis-block micronucleus assay. In conclusion, our data suggest that BTZ sensitivity depends on HO-1 nuclear compartmentalization and not on its enzymatic activity and this finding may represent an important tool to overcome BTZ chemoresistance in MM patients.

The Heme Oxygenase System in Hematological Malignancies

SIGNIFICANCE

Several lines of evidence suggest that hematological malignancies exhibit an altered redox balance homeostasis that can lead to the activation of various survival pathways that, in turn, lead to the progression of disease and chemoresistance. Among these pathways, the heme oxygenase-1 (HO-1) pathway is likely to play a major role. HO catalyzes the enzymatic degradation of heme with the simultaneous release of carbon monoxide (CO), ferrous iron (Fe²⁺), and biliverdin. This review focuses on the role of HO-1 in various hematological malignancies and the possibility of exploiting such targets to improve the outcome of well-established chemotherapeutic regimens. Recent Advances and Critical Issues: Interestingly, the inhibition of the expression of HO-1 (e.g., with siRNA) or HO activity (with competitive inhibitors) contributes to the increased efficacy of chemotherapy and improves the outcome in animal models. Furthermore, some hematological malignancies (e.g., chronic myeloid leukemia and multiple myeloma) have served to explore the non-canonical functions of HO-1, such as the association between nuclear compartmentalization and genetic instability and/or chemoresistance.

FUTURE DIRECTIONS

The HO system may serve as an important tool in the field of hematological malignancies because it can be exploited to counteract chemoresistance and to monitor the outcome of bone marrow transplants and may be an additional target for combined therapies. Antioxid. Redox Signal. 27, 363-377.

Repression of Mcl-1 and disruption of the Mcl-1/Bak interaction in myeloma cells couple ER stress to mitochondrial apoptosis

As myeloma cells actively produce and secrete immunoglobulins, they are prone to ER stress, which if unresolved leads to apoptosis. We found that myeloma cell death induced by the ER stressor Thapsigargin was highly variable, ranging from 2 to 89%. Induction of ATF4 and CHOP was observed in myeloma cells under Thapsigargin independently of cell death. The decrease in Mcl-1 was associated with protein translation inhibition and identified as a crucial factor in Thapsigargin sensitivity, since it was the only Bcl-2 family protein differentially modified between sensitive and resistant myeloma cells. Bak but not Bax was found to contribute to Thapsigargin-induced apoptosis. Appropriately, a basal Mcl-1/Bak interaction was demonstrated in Thapsigargin-sensitive cells. Of note, the only pro-apoptotic protein freed from Mcl-1 under Thapsigargin was Bak, whereas Mcl-1/Noxa or Mcl-1/Bim complexes were simultaneously increased. Thus, the disruption of the basal Mcl-1/Bak complex in Thapsigargin-sensitive cells seemed to be an essential event in cell death induction, probably favored by the induced Noxa and Bim BH3-only proteins. These findings underscore the implication of the Mcl-1/Bak axis in myeloma cell death triggered by Thapsigargin.

Acridine Derivatives as Inhibitors of the IRE1α-XBP1 Pathway Are Cytotoxic to Human Multiple Myeloma

Using a luciferase reporter-based high-throughput chemical library screen and topological data analysis, we identified N-acridine-9-yl-N',N'-dimethylpropane-1,3-diamine (DAPA) as an inhibitor of the inositol requiring kinase 1α (IRE1α)-X-box binding protein-1 (XBP1) pathway of the unfolded protein response. We designed a collection of analogues based on the structure of DAPA to explore structure-activity relationships and identified N(9)-(3-(dimethylamino)propyl)-N(3),N(3),N(6),N(6)-tetramethylacridine-3,6,9-triamine (3,6-DMAD), with 3,6-dimethylamino substitution on the chromophore, as a potent inhibitor. 3,6-DMAD inhibited both IRE1α oligomerization and in vitro endoribonuclease (RNase) activity, whereas the other analogues only blocked IRE1α oligomerization. Consistent with the inhibition of IRE1α-mediated XBP1 splicing, which is critical for multiple myeloma cell survival, these analogues were cytotoxic to multiple myeloma cell lines. Furthermore, 3,6-DMAD inhibited XBP1 splicing in vivo and the growth of multiple myeloma tumor xenografts. Our study not only confirmed the utilization of topological data analysis in drug discovery but also identified a class of compounds with a unique mechanism of action as potent IRE1α-XBP1 inhibitors in the treatment of multiple myeloma. Mol Cancer Ther; 15(9); 2055-65. ©2016 AACR.

Endoplasmic Reticulum Stress and the Hallmarks of Cancer

Tumor cells are often exposed to intrinsic and external factors that alter protein homeostasis, thus producing endoplasmic reticulum (ER) stress. To cope with this, cells evoke an adaptive mechanism to restore ER proteostasis known as the unfolded protein response (UPR). The three main UPR signaling branches initiated by IRE1α, PERK, and ATF6 are crucial for tumor growth and aggressiveness as well as for microenvironment remodeling or resistance to treatment. We provide a comprehensive overview of the contribution of the UPR to cancer biology and the acquisition of malignant characteristics, thus highlighting novel aspects including inflammation, invasion and metastasis, genome instability, resistance to chemo/radiotherapy, and angiogenesis. The therapeutic potential of targeting ER stress signaling in cancer is also discussed.

Overexpression of X-Box Binding Protein 1 (XBP1) Correlates to Poor Prognosis and Up-Regulation of PI3K/mTOR in Human Osteosarcoma

Increasing evidence demonstrates that dysregulation of XBP1 function contributes to tumorigenesis in some cancers. However, little is known about the role of XBP1 in the progression of osteosarcoma (OS). The expression of XBP1 in OS samples was measured by quantitative RT-PCR and Western blotting assays. Cell cycle analysis and cell counting kit 8 (CCK8) assays were performed to determine the effects of XBP1 expression on cells growth capacity. Cell apoptosis coassay was applied to determine cell survival. The expression of genes affected by XBP1 was examined by quantitative RT-RCR and validated by Western blotting assays. XBP1 was overexpressed in OS clinical samples compared with corresponding non-cancerous tissues. Overexpression of XBP1 was significantly associated with advanced clinical stages, high degree of malignancy and low tumor necrosis rate. Furthermore, hypoxia activated XBP1, and silencing XBP1 significantly enhanced OS cell apoptosis. Knock-down of XBP1 resulted in inhibition of OS growth. Most importantly, knockdown of XBP1 led to down-regulation of PIK3R3 and mTOR. Taken together, XBP1 is up-regulated and has a pro-tumor effect in OS with activation of PI3K/mTOR signaling. Thus, targeting XBP1 may provide a new potential therapeutic method for OS.

Long-Range Inhibitor-Induced Conformational Regulation of Human IRE1α Endoribonuclease Activity

Activation of the inositol-requiring enzyme-1 alpha (IRE1α) protein caused by endoplasmic reticulum stress results in the homodimerization of the N-terminal endoplasmic reticulum luminal domains, autophosphorylation of the cytoplasmic kinase domains, and conformational changes to the cytoplasmic endoribonuclease (RNase) domains, which render them functional and can lead to the splicing of X-box binding protein 1 (XBP 1) mRNA. Herein, we report the first crystal structures of the cytoplasmic portion of a human phosphorylated IRE1α dimer in complex with (R)-2-(3,4-dichlorobenzyl)-N-(4-methylbenzyl)-2,7-diazaspiro(4.5)decane-7-carboxamide, a novel, IRE1α-selective kinase inhibitor, and staurosporine, a broad spectrum kinase inhibitor. (R)-2-(3,4-dichlorobenzyl)-N-(4-methylbenzyl)-2,7-diazaspiro(4.5)decane-7-carboxamide inhibits both the kinase and RNase activities of IRE1α. The inhibitor interacts with the catalytic residues Lys599 and Glu612 and displaces the kinase activation loop to the DFG-out conformation. Inactivation of IRE1α RNase activity appears to be caused by a conformational change, whereby the αC helix is displaced, resulting in the rearrangement of the kinase domain-dimer interface and a rotation of the RNase domains away from each other. In contrast, staurosporine binds at the ATP-binding site of IRE1α, resulting in a dimer consistent with RNase active yeast Ire1 dimers. Activation of IRE1α RNase activity appears to be promoted by a network of hydrogen bond interactions between highly conserved residues across the RNase dimer interface that place key catalytic residues poised for reaction. These data implicate that the intermolecular interactions between conserved residues in the RNase domain are required for activity, and that the disruption of these interactions can be achieved pharmacologically by small molecule kinase domain inhibitors.

Voltage-gated calcium channel blockers deregulate macroautophagy in cardiomyocytes

Voltage-gated calcium channel blockers are widely used for the management of cardiovascular diseases, however little is known about their effects on cardiac cells in vitro. We challenged neonatal ventricular cardiomyocytes (CMs) with therapeutic L-type and T-type Ca(2+) channel blockers (nifedipine and mibefradil, respectively), and measured their effects on cell stress and survival, using fluorescent microscopy, Q-PCR and Western blot. Both nifedipine and mibefradil induced a low-level and partially transient up-regulation of three key mediators of the Unfolded Protein Response (UPR), indicative of endoplasmic (ER) reticulum stress. Furthermore, nifedipine triggered the activation of macroautophagy, as evidenced by increased lipidation of microtubule-associated protein 1 light chain 3 (LC3), decreased levels of polyubiquitin-binding protein p62/SQSTM1 and ubiquitinated protein aggregates, that was followed by cell death. In contrast, mibefradil inhibited CMs constitutive macroautophagy and did not promote cell death. The siRNA-mediated gene silencing approach confirmed the pharmacological findings for T-type channels. We conclude that L-type and T-type Ca(2+) channel blockers induce ER stress, which is divergently transduced into macroautophagy induction and inhibition, respectively, with relevance for cell viability. Our work identifies VGCCs as novel regulators of autophagy in the heart muscle and provides new insights into the effects of VGCC blockers on CMs homeostasis, that may underlie both noxious and cardioprotective effects.

Targeting the IRE1α-XBP1 branch of the unfolded protein response in human diseases

Accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) leads to ER stress, which is characteristic of cells with high level of secretory activity and implicated in a variety of disease conditions. In response to ER stress, the cell elicits an adaptive process called the unfolded protein response (UPR) to support cellular homeostasis and survival. However, prolonged and unsolvable ER stress also induces apoptosis. As the most conserved signaling branch of the UPR, the IRE1α-XBP1 pathway plays important roles in both physiological and pathological settings and its activity has profound effects on disease progression and prognosis. Recently, modulating this pathway with small molecule compounds has been demonstrated as a promising approach for disease therapy. In this review, we summarize a list of current investigational compounds targeting this pathway and their therapeutic features for treating human diseases.

Endoplasmic reticulum quality control in cancer: Friend or foe

Quality control systems in the endoplasmic reticulum (ER) mediated by unfolded protein response (UPR) and endoplasmic reticulum associated degradation (ERAD) ensure cellular function and organismal survival. Recent studies have suggested that ER quality-control systems in cancer cells may serve as a double-edged sword that aids progression as well as prevention of tumor growth in a context-dependent manner. Here we review recent advances in our understanding of the complex relationship between ER proteostasis and cancer pathology, with a focus on the two most conserved ER quality-control mechanisms--the IRE1α-XBP1 pathway of the UPR and SEL1L-HRD1 complex of the ERAD.

Shikonin, dually functions as a proteasome inhibitor and a necroptosis inducer in multiple myeloma cells

Shikonin (SHK), a natural small agent (MW 288.3), reportedly induces cell death in various tumor cells. We have found that SHK also exerts potent cytocidal effects on human multiple myeloma (MM) cells, but its anticancer mechanism in MM cells remains to be elucidated. SHK at 2.5–5 μM induced apoptosis in seven MM cell lines, including the bortezomib-resistant cell line KMS11/BTZ. The IC₅₀ value of SHK against KMS11/BTZ was comparable to that of a parental cell line KMS11 (1.1 and 1.56 μM, respectively). SHK induces accumulation of ubiquitinated proteins and activates XBP-1 in MM cells, suggesting that SHK functions as a proteasome inhibitor, eventually inducing ER stress-associated apoptosis. SHK increases levels of HSP70/72, which protects cells from apoptosis, and exerts greater cytocidal effects in combination with the HSP70/72 inhibitor VER-155008. At higher concentrations (10–20 μM), SHK induced cell death, which was completely inhibited by a necroptosis inhibitor, necrostatin-1 (Nec-1), while the cytocidal activity was unaffected by Z-VAD-FMK, strongly suggesting that cell death is induced by SHK at high concentrations through necroptosis. The present data show for the first time that SHK induces cell death in MM cells. SHK efficiently induces apoptosis and combination of heat shock protein inhibitor with low dose SHK enhances apoptosis, while high dose SHK induces necroptosis in MM cells. These findings together support the use of SHK as a potential therapeutic agent for MM.

The mammalian tRNA ligase complex mediates splicing of XBP1 mRNA and controls antibody secretion in plasma cells

The unfolded protein response (UPR) is a conserved stress-signaling pathway activated after accumulation of unfolded proteins within the endoplasmic reticulum (ER). Active UPR signaling leads to unconventional, enzymatic splicing of XBP1 mRNA enabling expression of the transcription factor XBP1s to control ER homeostasis. While IRE1 has been identified as the endoribonuclease required for cleavage of this mRNA, the corresponding ligase in mammalian cells has remained elusive. Here, we report that RTCB, the catalytic subunit of the tRNA ligase complex, and its co-factor archease mediate XBP1 mRNA splicing both in vitro and in vivo. Depletion of RTCB in plasma cells of Rtcb(fl/fl) Cd23-Cre mice prevents XBP1s expression, which normally is strongly induced during plasma cell development. RTCB-depleted plasma cells show reduced and disorganized ER structures as well as severe defects in antibody secretion. Targeting RTCB and/or archease thus represents a promising strategy for the treatment of a growing number of diseases associated with elevated expression of XBP1s.

Involvement of ER stress in apoptosis induced by sialic acid-binding lectin (leczyme) from bullfrog eggs

Sialic-acid binding lectin (SBL) isolated from bullfrog (Rana catesbeiana) oocytes is a multifunctional protein which has lectin activity, ribonuclease activity and cancer-selective antitumor activity. It has been reported that SBL induces apoptosis accompanied by rigid mitochondrial perturbation, which indicates mediation of the intrinsic pathway. However, the mechanism of the antitumor effect of SBL has not been fully elucidated. We report, here, that ER stress is evoked in SBL-treated cells. We show that caspase-4, an initiator caspase of ER stress-mediated apoptosis was activated, and inhibition of caspase-4 resulted in significant attenuation of apoptosis induced by SBL. We analyzed the precise mechanism of activation of the caspase cascade induced by SBL, and found that caspase-9 and -4 are activated upstream of activation of caspase-8. Further study revealed that SBL induces the mitochondrial and ER stress-mediated pathways independently. It is noteworthy that SBL can induce cancer-selective apoptosis by multiple apoptotic signaling pathways, and it can serve as a candidate molecule for anticancer drugs in a novel field.

Endoplasmic reticulum stress and the unfolded protein response: targeting the Achilles heel of multiple myeloma

Multiple myeloma is characterized by the malignant proliferating antibody-producing plasma cells in the bone marrow. Despite recent advances in therapy that improve the survival of patients, multiple myeloma remains incurable and therapy resistance is the major factor causing lethality. Clearly, more effective treatments are necessary. In recent years it has become apparent that, as highly secretory antibody-producing cells, multiple myeloma cells require an increased capacity to cope with unfolded proteins and are particularly sensitive to compounds targeting proteostasis such as proteasome inhibitors, which represent one of the most prominent new therapeutic strategies. Because of the increased requirement for dealing with secretory proteins within the endoplasmic reticulum, multiple myeloma cells are heavily reliant for survival on a set of signaling pathways, known as the unfolded protein response (UPR). Thus, directly targeting the UPR emerges as a new promising therapeutic strategy. Here, we provide an overview of the current understanding of the UPR signaling in cancer, and outline its important role in myeloma pathogenesis and treatment. We discuss new therapeutic approaches based on targeting the protein quality control machinery and particularly the IRE1α/XBP1 axis of the UPR.

Therapeutic enhancement of ER stress by insulin-like growth factor I sensitizes myeloma cells to proteasomal inhibitors

PURPOSE

Multiple myeloma is a clonal plasma cell disorder in which growth and proliferation are linked to a variety of growth factors, including insulin-like growth factor type I (IGF-I). Bortezomib, the first-in-class proteasome inhibitor, has displayed significant antitumor activity in multiple myeloma.

EXPERIMENTAL DESIGN

We analyzed the impact of IGF-I combined with proteasome inhibitors on multiple myeloma cell lines in vivo and in vitro as well as on fresh human myeloma cells.

RESULTS

Our study shows that IGF-I enhances the cytotoxic effect of proteasome inhibitors against myeloma cells. The effect of bortezomib on the content of proapoptotic proteins such as Bax, Bad, Bak, and BimS and antiapoptotic proteins such as Bcl-2, Bcl-XL, XIAP, Bfl-1, and survivin was enhanced by IGF-I. The addition of IGF-I to bortezomib had a minor effect on NF-κB signaling in MM.1S cells while strongly enhancing reticulum stress. This resulted in an unfolded protein response (UPR), which was required for the potentiating effect of IGF-I on bortezomib cytotoxicity as shown by siRNA-mediated inhibition of GADD153 expression.

CONCLUSIONS

These results suggest that the high baseline level of protein synthesis in myeloma can be exploited therapeutically by combining proteasome inhibitors with IGF-I, which possesses a "priming" effect on myeloma cells for this family of compounds.

TRIM13 (RFP2) downregulation decreases tumour cell growth in multiple myeloma through inhibition of NF Kappa B pathway and proteasome activity

Multiple myeloma (MM) is an incurable neoplasm caused by proliferation of malignant plasma cells in the bone marrow (BM). MM is characterized frequently by a complete or partial deletion of chromosome 13q14, seen in more than 50% of patients at diagnosis. Within this deleted region the tripartite motif containing 13 (TRIM13, also termed RFP2) gene product has been proposed to be a tumour suppressor gene (TSG). Here, we show that low expression levels of TRIM13 in MM are associated with chromosome 13q deletion and poor clinical outcome. We present a functional analysis of TRIM13 using a loss-of-function approach, and demonstrate that TRIM13 downregulation decreases tumour cell survival as well as cell cycle progression and proliferation of MM cells. In addition, we provide evidence for the involvement of TRIM13 downregulation in inhibiting the NF kappa B pathway and the activity of the 20S proteasome. Although this data does not support a role of TRIM13 as a TSG, it substantiates important roles of TRIM13 in MM tumour survival and proliferation, underscoring its potential role as a novel target for therapeutic intervention.

Blockade of XBP1 splicing by inhibition of IRE1α is a promising therapeutic option in multiple myeloma

Multiple myeloma (MM) cells are characterized by high protein synthesis resulting in chronic endoplasmic reticulum (ER) stress, which is adaptively managed by the unfolded protein response. Inositol-requiring enzyme 1α (IRE1α) is activated to splice X-box binding protein 1 (XBP1) mRNA, thereby increasing XBP1s protein, which in turn regulates genes responsible for protein folding and degradation during the unfolded protein response. In this study, we examined whether IRE1α-XBP1 pathway is a potential therapeutic target in MM using a small-molecule IRE1α endoribonuclease domain inhibitor MKC-3946. MKC-3946 triggered modest growth inhibition in MM cell lines, without toxicity in normal mononuclear cells. Importantly, it significantly enhanced cytotoxicity induced by bortezomib or 17-AAG, even in the presence of bone marrow stromal cells or exogenous IL-6. Both bortezomib and 17-AAG induced ER stress, evidenced by induction of XBP1s, which was blocked by MKC-3946. Apoptosis induced by these agents was enhanced by MKC-3946, associated with increased CHOP. Finally, MKC-3946 inhibited XBP1 splicing in a model of ER stress in vivo, associated with significant growth inhibition of MM cells. Taken together, our results demonstrate that blockade of XBP1 splicing by inhibition of IRE1α endoribonuclease domain is a potential therapeutic option in MM.

Differentiation induction enhances bortezomib efficacy and overcomes drug resistance in multiple myeloma

AIM

It is of clinical importance to find methods to overcome bortezomib resistance. In the current study, we clarified the relationship between resistance to bortezomib and the differentiation status of myeloma cells, and explored the feasibility of induction of differentiation in overcoming bortezomib resistance in myeloma.

METHODS

Cell morphology, immunoglobulin light-chain protein secretion levels, and XBP-1 expression were used to evaluate the differentiation status of myeloma cells. Low dose 2-ME2 alone or in combination with ATRA was used to induce differentiation in myeloma cells.

RESULTS

The differentiation status of myeloma cells was related to myeloma sensitivity to bortezomib. After successful induction of differentiation, the myeloma cells were more sensitive to bortezomib with decreased growth and an increased rate of apoptosis. Induction of differentiation increased the proteasome workload in myeloma cells by increasing immunoglobulin secretion, while reducing proteasome capacity by decreasing proteasome activity. The imbalance between increased proteasome workload and decreased proteasome capacity is a possible mechanism by which induction of differentiation overcomes myeloma resistance to bortezomib.

CONCLUSION

The current study demonstrated, for the first time, that myeloma differentiation status is associated with myeloma sensitivity to bortezomib and that induction of differentiation can overcome myeloma resistance to bortezomib.

Antimyeloma Effects of the Heat Shock Protein 70 Molecular Chaperone Inhibitor MAL3-101

Multiple myeloma (MM) is the second most common hematologic malignancy and remains incurable, primarily due to the treatment-refractory/resistant nature of the disease. A rational approach to this compelling challenge is to develop new drugs that act synergistically with existing effective agents. This approach will reduce drug concentrations, avoid treatment resistance, and also improve treatment effectiveness by targeting new and nonredundant pathways in MM. Toward this goal, we examined the antimyeloma effects of MAL3-101, a member of a new class of non-ATP-site inhibitors of the heat shock protein (Hsp) 70 molecular chaperone. We discovered that MAL3-101 exhibited antimyeloma effects on MM cell lines in vitro and in vivo in a xenograft plasmacytoma model, as well as on primary tumor cells and bone marrow endothelial cells from myeloma patients. In combination with a proteasome inhibitor, MAL3-101 significantly potentiated the in vitro and in vivo antimyeloma effects. These data support a preclinical rationale for small molecule inhibition of Hsp70 function, either alone or in combination with other agents, as an effective therapeutic strategy for MM.

Regulation of unfolded protein response modulator XBP1s by acetylation and deacetylation

XBP1 (X-box-binding protein 1) is a key modulator of the UPR (unfolded protein response), which is involved in a wide range of pathological and physiological processes. The mRNA encoding the active spliced form of XBP1 (XBP1s) is generated from the unspliced form by IRE1 (inositol-requiring enzyme 1) during the UPR. However, the post-translational modulation of XBP1s remains largely unknown. In the present study, we demonstrate that XBP1s is a target of acetylation and deacetylation mediated by p300 and SIRT1 (sirtuin 1) respectively. p300 increases the acetylation and protein stability of XBP1s, and enhances its transcriptional activity, whereas SIRT1 deacetylates XBP1s and inhibits its transcriptional activity. Deficiency of SIRT1 enhances XBP1s-mediated luciferase reporter activity in HEK (human embryonic kidney)-293 cells and the up-regulation of XBP1s target gene expression under ER (endoplasmic reticulum) stress in MEFs (mouse embryonic fibroblasts). Consistent with XBP1s favouring cell survival under ER stress, Sirt1-/- MEFs display a greater resistance to ER-stress-induced apoptotic cell death compared with Sirt1+/+ MEFs. Taken together, these results suggest that acetylation/deacetylation constitutes an important post-translational mechanism in controlling protein levels, as well as the transcriptional activity, of XBP1s. The present study provides a novel insight into the molecular mechanisms by which SIRT1 regulates UPR signalling.

TRAIL-induced apoptosis of human melanoma cells involves activation of caspase-4

Although it is conventionally regarded as an inflammatory caspase, recent studies have shown that caspase-4 plays a role in induction of apoptosis by endoplasmic reticulum (ER) stress. We report here that activation of caspase-4 is also involved in induction of apoptosis by TNF-related apoptosis-inducing ligand (TRAIL) in human melanoma cells. Treatment with TRAIL resulted in activation of caspase-4. This appeared to be mediated by caspase-3, in that caspase-4 was activated later than caspase-8, -9, and -3, and that inhibition of caspase-3 blocked TRAIL-induced caspase-4 activation. Notably, TRAIL triggered ER stress in melanoma cells as shown by up-regulation of the GRP78 protein and the spliced form of XBP-1 mRNA. This seemed to be necessary for activation of caspase-4, as activation of caspase-3 by agents that did not trigger ER stress did not cause activation of caspase-4. Importantly, inhibition of caspase-4 also partially blocked caspase-3 activation, suggesting that activation of caspase-4 may be positive feed-back mechanism to further enhance caspase-3 activation. Collectively, these results show that activation of caspase-4 contributes to TRAIL-induced apoptosis and is associated with induction of ER stress by TRAIL in melanoma cells, and may have important implications for improving therapeutic efficacies of TRAIL in melanoma.

Tetraspanin-induced death of myeloma cell lines is autophagic and involves increased UPR signalling

Background:

Multiple myeloma (MM) therapy is hindered by the interaction of the heterogeneous malignant plasma cells with their microenvironment and evolving drug resistance. We have previously shown that the membranal tetraspanins, CD81 and CD82, are under-expressed in MM cells and that their reintroduction causes massive non-apoptotic death. In this study, we aimed to characterise the tetraspanin-induced MM death.

Methods:

Multiple myeloma cell lines were transiently transfected with eGFP–CD81N1/CD82N1 fusion proteins and assessed for death mode by flow cytometry (propidium iodide, ZVAD-fmk, 3MA), activation of unfolded protein response (UPR), and autophagy (immunoblot, RT–PCR).

Results:

Cell death induced by CD81N1 and CD82N1 in MM cell lines was autophagic and involved endoplasmic reticulum (ER)-stress manifested by activation of UPR pathways, PERK (protein kinase-like ER kinase) and IRE1 (inositol-requiring 1). We also established the relative X-box binding protein 1 baseline expression levels in a panel of MM cell lines and their general dependence on autophagy for survival. Timeline of UPR cascades and cell fate supported our results.

Interpretation:

This is the first publication implicating tetraspanins in UPR signalling pathways, autophagy, and autophagic death. Integration of our findings with published data highlights the unifying dependence of MM cells on ER–Golgi homoeostasis, and underscores the potential of tetraspanin complexes and ER-stress as leverage for MM therapy.

Activation of the endoplasmic reticulum stress-associated transcription factor x box-binding protein-1 occurs in a subset of normal germinal-center B cells and in aggressive B-cell lymphomas with prognostic implications

X box-binding protein 1 (Xbp-1) is a transcription factor that is required for the terminal differentiation of B lymphocytes into plasma cells. The Xbp-1 gene is activated in response to endoplasmic reticulum stress signals, which generate a 50-kDa nuclear protein that acts as a potent transactivator and regulates the expression of genes related to the unfolded protein response. Activated Xbp-1 is essential for cell survival in plasma-cell tumors but its role in B-cell lymphomas is unknown. We analyzed the expression of activated Xbp-1 in reactive lymphoid tissues, 411 lymphomas and plasma-cell neoplasms, and 24 B-cell lines. In reactive tissues, Xbp-1 was only found in nuclear extracts. Nuclear expression of Xbp-1 was observed in occasional reactive plasma cells and in a subpopulation of Irf-4(+)/Bcl-6(-)/Pax-5(-) B cells in the light zones of reactive germinal centers, probably representing cells committed to plasma-cell differentiation. None of the low-grade lymphomas showed evidence of Xbp-1 activation; however, Xbp-1 activation was found in 28% of diffuse large B-cell lymphomas, independent of germinal or postgerminal center phenotype, as well as in 48% of plasmablastic lymphomas and 69% of plasma-cell neoplasms. Diffuse large B-cell lymphomas with nuclear Xbp-1 expression had a significantly worse response to therapy and shorter overall survival compared with negative tumors. These findings suggest that Xbp-1 activation may play a role in the pathogenesis of aggressive B-cell lymphomas.

The proteasome load versus capacity balance determines apoptotic sensitivity of multiple myeloma cells to proteasome inhibition

Proteasome inhibitors (PIs) are effective against multiple myeloma (MM), but the mechanisms of action and bases of individual susceptibility remain unclear. Recent work linked PI sensitivity to protein synthesis and proteasome activity, raising the question whether different levels of proteasome expression and workload underlie PI sensitivity in MM cells (MMCs). Exploiting human MM lines characterized by differential PI sensitivity, we report that highly sensitive MMCs express lower proteasome levels and higher proteasomal workload than relatively PI-resistant MMCs, resulting in the accumulation of polyubiquitinated proteins at the expense of free ubiquitin (proteasome stress). Manipulating proteasome expression or workload alters apoptotic sensitivity to PI, demonstrating a cause-effect relationship between proteasome stress and apoptotic responses in MMCs. Intracellular immunostaining in primary, patient-derived MMCs reveals that polyubiquitinated proteins hallmark neoplastic plasma cells, in positive correlation with immunoglobulin (Ig) content, both intra- and interpatient. Moreover, overall proteasome activity of primary MMCs inversely correlates with apoptotic sensitivity to PI. Altogether, our data indicate that the balance between proteasome workload and degradative capacity represents a critical determinant of apoptotic sensitivity of MMCs to PI, potentially providing a framework for identifying indicators of responsiveness and designing novel combination therapies.

Inhibition of eIF2alpha dephosphorylation maximizes bortezomib efficiency and eliminates quiescent multiple myeloma cells surviving proteasome inhibitor therapy

The proteasome inhibitor bortezomib (Velcade) effectively eradicates multiple myeloma (MM) cells, partly by activating endoplasmic reticulum (ER) stress apoptotic signaling. However, MM recurrences in bortezomib-treated patients are invariable. We have shown that ER stress signaling can also induce growth arrest and survival in cancer cells. Thus, we hypothesized that bortezomib therapy could induce quiescence and survival of residual MM cells, contributing to disease recurrence. Here, we report that in MM cells, proteasome inhibition with MG-132 or bortezomib results in a surviving cell fraction that enters a prolonged quiescent state (G(0)-G(1) arrest). Mechanism analysis revealed that bortezomib-surviving quiescent cells attenuate eIF2alpha phosphorylation and induction of the ER stress proapoptotic gene GADD153. This occurs independently of the eIF2alpha upstream kinases PERK, GCN2, and PKR. In contrast, the prosurvival ER-chaperone BiP/Grp78 was persistently induced. The bortezomib-surviving quiescent fraction could be eradicated by a simultaneous or sequential combination therapy with salubrinal, an inhibitor of GADD34-PP1C phosphatase complex, and, in consequence, eIF2alpha dephosphorylation. This effect was mimicked by expression of a phosphorylated mimetic eIF2alpha-S51D mutant. Our data indicate that bortezomib can induce growth arrest in therapy-surviving MM cells and that attenuation of eIF2alpha phosphorylation contributes to this survival. Most importantly, this survival mechanism can be blocked by inhibiting eIF2alpha dephosphorylation. Thus, strategies that maintain eIF2alpha in a hyperphosphorylated state may be a novel therapeutic approach to maximize bortezomib-induced apoptosis and reduce residual disease and recurrences in this type of cancer.

Fatty acid synthase is a novel therapeutic target in multiple myeloma

This study investigated the biological significance of the inhibition of fatty acid synthase (FAS) in multiple myeloma (MM) using the small molecule inhibitor Cerulenin. Cerulenin triggered growth inhibition in both MM cell lines and MM patient cells, and overcame the survival and growth advantages conferred by interleukin-6, insulin-like growth factor-1, and bone marrow stromal cells. It induced apoptosis in MM cell lines with only modest activation of caspase -8, -9, -3 and PARP; moreover, the pan-caspase inhibitor Z-VAD-FMK did not inhibit Cerulenin-induced apoptosis and cell death. In addition, treatment of MM cells with Cerulenin primarily up-regulated apoptosis-inducing factor/endonuclease G, mediators of caspase-independent apoptosis. Importantly, Cerulenin induced endoplasmic reticulum stress response via up-regulation of the Grp78/IRE1alpha/JNK pathway. Although the C-Jun-NH(2)-terminal kinase (JNK) inhibitor SP600215 blocked Cerulenin-induced cytotoxicity, it did not inhibit apoptosis and caspase cleavage. Furthermore, Cerulenin showed synergistic cytotoxic effects with various agents including Bortezomib, Melphalan and Doxorubicin. Our results therefore indicate that inhibition of FAS by Cerulenin primarily triggered caspase-independent apoptosis and JNK-dependent cytotoxicity in MM cells. This report demonstrated that inhibition of FAS has anti-tumour activity against MM cells, suggesting that it represents a novel therapeutic target in MM.

Inhibition of MEK sensitizes human melanoma cells to endoplasmic reticulum stress-induced apoptosis

Past studies have shown that activation of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK is a common cause for resistance of melanoma cells to death receptor-mediated or mitochondria-mediated apoptosis. We report in this study that inhibition of the MEK/ERK pathway also sensitizes melanoma cells to endoplasmic reticulum (ER) stress-induced apoptosis, and this is mediated, at least in part, by caspase-4 activation and is associated with inhibition of the ER chaperon glucose-regulated protein 78 (GRP78) expression. Treatment with the ER stress inducer tunicamycin or thapsigargin did not induce significant apoptosis in the majority of melanoma cell lines, but resistance to these agents was reversed by the MEK inhibitor U0126 or MEK1 small interfering RNA (siRNA). Induction of apoptosis by ER stress when MEK was inhibited was caspase dependent with caspase-4, caspase-9, and caspase-3 being involved. Caspase-4 seemed to be the apical caspase in that caspase-4 activation occurred before activation of caspase-9 and caspase-3 and that inhibition of caspase-4 by a specific inhibitor or siRNA blocked activation of caspase-9 and caspase-3, whereas inhibition of caspase-9 or caspase-3 did not inhibit caspase-4 activation. Moreover, overexpression of Bcl-2 inhibited activation of caspase-9 and caspase-3 but had minimal effect on caspase-4 activation. Inhibition of MEK/ERK also resulted in down-regulation of GRP78, which was physically associated with caspase-4, before and after treatment with tunicamycin or thapsigargin. In addition, siRNA knockdown of GRP78 increased ER stress-induced caspase-4 activation and apoptosis. Taken together, these results seem to have important implications for new treatment strategies in melanoma by combinations of agents that induce ER stress and inhibitors of the MEK/ERK pathway.

IPI-504, a novel and soluble HSP-90 inhibitor, blocks the unfolded protein response in multiple myeloma cells

BACKGROUND

Inhibitors of heat shock protein (Hsp) 90 induce apoptosis in multiple myeloma (MM) cells, but the molecular mechanisms underlying this cytotoxic outcome are not clear. Here, we investigate the effect of IPI-504, a novel and highly soluble inhibitor of the Hsp90 ATPase activity, on the unfolded protein response (UPR) in MM cells. The UPR is a stress response pathway triggered by sensors located at the endoplasmic reticulum (ER) membrane whose function is to reduce an excessive accumulation of misfolded protein in the ER. During normal development of B-lymphocytes to antibody-producing plasma cells, a partial UPR has been described, where IREalpha and ATF-6 are stimulated, whereas the third sensor, PERK, is not induced.

METHODS

Levels of the activated forms of the three main UPR sensors ATF-6, XBP-1 and PERK/eIF-2 were monitored in two different MM cells lines and one non-MM cell lines under various experimental conditions including incubation with increasing concentration of IPI-504. Also, MM cells were incubated with IPI-504 and several apoptosis markers were monitored.

RESULTS

We show here that a partial UPR is constitutively activated in plasma cell-derived MM cells and that IPI-504 can potently inhibit this pathway. IPI-504 achieves this by inactivating the transcription factors XBP1 and ATF6. In addition, IPI-504 also blocks the tunicamycin-induced phosphorylation of eIF2 by PERK. Dose-response and time course experiments reveal that IPI-504's inhibitory effect on the UPR parallels its cytotoxic and pro-apoptotic effects on MM cells.

CONCLUSION

The results presented here suggest that the IPI-504-induced apoptosis might be, in part, mediated by the inhibition of the partial UPR. Other malignancies that rely on intact and efficient UPR to survive could be considered as new indications for Hsp90 inhibitors.