IRE1α-XBP1 signaling pathway, a potential therapeutic target in multiple myeloma

Leukemia Research 49 (2016) 7–12

Contents lists available at ScienceDirect

Leukemia Research journal homepage: www.elsevier.com/locate/leukres

IRE1␣-XBP1 signaling pathway, a potential therapeutic target in multiple myeloma Lin Chen, Qian Li, Tiantian She, Han Li, Yuanfang Yue, Shuang Gao, Tinghui Yan, Su Liu, Jing Ma, Yafei Wang ∗ Department of Hematology and Blood and Marrow Transplantation, Tianjin Medical University Cancer Institute and Hospital, National Clinical Research Center for Cancer, Tianjin Key Laboratory of Cancer Prevention and Therapy, Huan-Hu-Xi Road, Ti-Yuan-Bei, He Xi District, Tianjin 300060, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 January 2016 Received in revised form 15 July 2016 Accepted 21 July 2016 Available online 22 July 2016 Keywords: Multiple myeloma UPR IRE1&alpha XBP1s Therapy Prognosis

a b s t r a c t Multiple myeloma (MM), which arises from the uncontrolled proliferation of malignant plasma cells, is the second most commonly diagnosed hematologic malignancy in the United States. Despite the development and application of novel drugs and autologous stem cell transplantation (ASCT), MM remains an incurable disease and patients become more prone to MM relapse and drug resistance. It is extremely urgent to find novel targeted therapy for MM. To date, the classic signaling pathways underlying MM have included the RAS/RAF/MEK/ERK pathway, the JAK-STAT3 pathway, the PI3K/Akt pathway and the NF-KB pathway. The IRE1␣-XBP1 signaling pathway is currently emerging as an important pathway involved in the development of MM. Moreover, it is closely associated with the effect of MM treatment and its prognosis. All these findings indicate that the IRE1␣-XBP1 pathway can be a potential treatment target. Herein, we investigate the relationship between the IRE1␣-XBP1 pathway and MM and discuss the functions of IRE1␣-XBP1-targeted drugs in the treatment of MM. © 2016 Elsevier Ltd. All rights reserved.

Contents 1. The unfolded protein response (UPR) and IRE1␣-XBP1 signaling pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2. IRE1-XBP1 signaling pathway and MM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. IRE1␣ inhibitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4. Proteasome inhibitors (PIs) induce ER stress and induce a terminal UPR in myeloma cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5. HSP inhibitors induce MM cell death by regulating UPR via the IRE1␣-XBP1 pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6. The adenosine moiety of toyocamycin inhibits IRE1a-induced splicing of XBP1 mRNA without affecting the activation of IRE1a, and its combination with bortezomib shows a synergistic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Resveratrol inhibits the transcriptional activity of XBP1s via sirtuin 1, and its combination with bortezomib and thalidomide might show a synergistic effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. The CDK inhibitor SCH727965 decreases the expression of XBP1 and nuclear accumulation of XBP1s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Reovirus exerts anti-myeloma effect by inducing ER stress and UPR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Immunotherapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11. Other key strategies to disrupt protein homeostasis could exploit the UPR/IRE1-XBP1 signaling pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 12. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Conflict of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

∗ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.leukres.2016.07.006 0145-2126/© 2016 Elsevier Ltd. All rights reserved.

8

L. Chen et al. / Leukemia Research 49 (2016) 7–12

1. The unfolded protein response (UPR) and IRE1␣-XBP1 signaling pathway Multiple stimuli and pathological conditions including hypoxia, oxidative injury, high-fat diet, hypoglycemia, inclusion body proteins and viral infections induce an accumulation of unfolded proteins in the endoplasmic reticulum (ER). To maintain ER homeostasis, cells initiate the activation of ER-associated protein degradation (ERAD), via cytosolic 26 s proteasomes, autophagy and the unfolded protein response (UPR). The UPR relieves the ER of the unfolded protein load by reducing protein synthesis and by restricting proteins from entering the ER and also accelerates protein folding by increasing the expression of ER stress-related molecular chaperones and folding enzymes [1–4]. During the differentiation of mature B cell to plasma cell, the significantly elevated production of immunoglobulin requires a massive expansion size of the ER. Herein, the efficient regulation of the ER is essential for plasma cell differentiation and cellular activities. Any conditions that interfere with ER function lead to an accumulation of unfolded proteins and ER stress. Persistent high-level antibody secretion in plasma cells and the inhibition of key apoptotic caspases in plasma cells temporarily act to block apoptotic signaling that is triggered by ER stress. If this ER stress persists or the adaptive response fails, cells initiate ER stress-induced apoptotic cell death through the activation of c-Jun amino-terminal kinases (JNKs), cellautonomous and UPR-controlled activation of death receptor 5 (DR5). The ER stress-related apoptotic mechanisms remain elusive. The apoptotic pathway through ER stress-mediated leakage of calcium into the cytoplasm may be directly activated by ER and leads to the activation of death effectors. ER stress activates Bim (a proapoptotic member of the Bcl-2 family which is essential for ER stress-induced apoptosis) and DR5 transcription through C/EBP homologous protein (CHOP)-mediated transcriptional induction. Persistent ER stress drives ligand-independent DR5 activation and cell apoptosis via caspase-8. Persistent ER stress also suppress the synthesis of anti-apoptotic Bcl-2 and Bcl-xL protein which is essential for protection from CHOP-dependent apoptosis during plasma cell differentiation [5–13]. MM is characterized by chronic ER stress induced by high production of monoclonal immunoglobulin [14]. Any strategies for MM cells returning to ER homeostasis are critical for the treatment of MM. Currently, the diagnosis of MM relies on serological or urine testing of monoclonal immunoglobulins or light chains [15]. The existing drugs for treating MM include vorinostat (HDAC inhibitor), bortezomib and clarithromycin. These drugs target the integrated networks of aggresome, proteasome and autophagy and induce efficient ER stress-mediated apoptosis in MM cells [16]. Myeloma cells comprise various subsets in differentiated phases and differentiation induction could be a potential therapeutic strategy for myeloma. Herein, UPR, which play crucial roles in terminal plasmacytic differentiation and maturation, can be an effective new therapeutics for myeloma [17]. In mammals, there are three UPR-related ER stress sensors, i.e., the ER transmembrane proteins inositol-requiring enzyme 1 (IRE1), PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6). These proteins function in response to ER stress through binding their ER-luminal domains to an ER chaperone 78 kDa glucose-regulated protein (GRP78), which is also called immunoglobulin binding protein (BiP). However, accumulated unfolded proteins also interact with GRP78, competitively inhibiting the interaction between GRP78 and the ER stress sensors, leading to dissociated but activated sensors [18]. Activated PERK inhibit the initiation step of mRNA translation by phosphorylating eukaryotic initiation factor 2␣ (eIF2␣). Activated ATF6 translocate to the Golgi complex, where it regulates the expression of molecules involved in protein quality control and ERAD. In addition, ATF6 promote IRE1a-mediated splicing of X-box binding

protein 1 (XBP1) and increase its expression. In addition, activated ATF6 and XBP1 bind to the ER stress response element (ERSE) and the UPR element (UPRE), leading to up-regulated expression levels of target genes such as GRP78. IRE1P is the yeast homologue of human IRE1. In yeast, the transmembrane protein IRE1P activates HAC1 mRNA, and the product of HAC1 mRNA activates the UPR. In mammals, ATF6-induced IRE1 activation induce the splicing of XBP1, and only the spliced form of XBP1 (XBP1s) can activate the UPR effectively [19,20]. IRE1 is a highly conserved type I ER transmembrane protein that contains a kinase domain and an endoribonuclease domain. Misfolded protein-mediated dissociation between IRE1 and GRP78 during ER stress lead to the autophosphorylation of the cytoplasmic kinase domain of IRE1a and its subsequent oligomerization and ultimate activation of its RNase activity. Additionally, the kinase domain of IRE1a activate the JNK and NF-kB signaling pathways by recruiting different molecules, leading to apoptotic cell death [18]. The RNase activity of IRE1, however, can cut off an intron from the unsplicedX-box-binding protein 1 (XBP1u) mRNA with the help of an RNA ligase, resulting in spliced-X-box-binding protein 1 (XBP1s) — the active form of XBP1. XBP1s detaches from the membrane (XBP1u) and then transfers into the cytosol and nucleus of cells and acts as a transcription factor. XBP1s can regulate the transcription of genes involved in ER membrane biosynthesis, protein transportation, chaperoning, ERAD, secretory machinery of exocrine glands and hepatic lipogenesis [21–24]. Additionally, XBP1, together with the interferon regulatory factor 4 (IRF4) and the transcriptional repressor B lymphocyte-induced maturation protein 1(BLIMP1), plays an important role in plasmacytic differentiation. Signals involved in plasma cell differentiation, specifically interleukin-4, control the transcription of XBP1. Moreover, XBP1 regulates the expression of interleukin-6, a cytokine critical for driving B cells into immunoglobulin-secreting plasma cells and plasma cells survival [25–27]. Todd et al. [28] showed that XBP1CD19 mice (XBP1 deficiency mice) were protected from disease in an autoantibodymediated mouse lupus model. Cells lacking XBP1 and ATF6 showed an impaired ability to produce UPR target genes and activate ERSE. XBP1 and ATF6 might be directly downstream of XBP1 are ERdj3 and OBF-1[29]. XBP1 and ATF6 might have other redundant functions [30]. In addition to plasma and MM cells, XBP1s also be produced by bone marrow stromal cells (BMSCs), a key microenvironmental support for MM. High expression levels of XBP1s in healthy human BMSCs promoted MM cell growth and osteoclast formation in vitro and in vivo. Conversely, XBP1s deficiency in healthy donor BMSCs had no such effect. Therefore, knock-down of XBP1 in BMSCs of MM patients can be a good choice for the treatment of MM [31].

2. IRE1-XBP1 signaling pathway and MM One supervised analysis identified 263 genes to be differentially expressed between normal and monoclonal gammopathy of undetermined significance (MGUS) groups, 380 differentially expressed genes between normal and MM groups, and 197 genes overlapping between the groups. Only 74 genes were differentially expressed between the MGUS and MM groups, indicating a close association between the groups. XBP1s was one of those differentially expressed genes shared by MGUS and MM [32]. In addition, there were 34 up-regulated and 18 down-regulated genes in myeloma cells compared with non-myeloma cell lines. These genes included syndecan, BCMA, PIM2, MUM1/IRF4 and XBP1 [33]. However, IRE1␣ was expressed in all MM cell lines, although at different protein levels. XBP1u existed in all MM cell lines, whereas XBP1s could only be detected in a subset of MM cell lines. RT-PCR analysis demonstrated the presence of XBP1s in RPMI 8226 and LR5 cell lines. The

L. Chen et al. / Leukemia Research 49 (2016) 7–12

transfection of XBP1 shRNAs in RPMI 8226 cells could remarkably decrease both mRNA and protein levels of XBP1u and XBP1s, and the growth ability of MM cells was seriously crippled as a result. Moreover, XBP1 knockdown enhanced the sensitivity of MM cells to ER stressors including bortezomib and 17-AAG. These results indicate a crucial role of XBP1 in MM cell growth [14]. XBP1s levels can be used to predict the prognosis of MM and assess the effect of thalidomide therapy. In a study of 253 newly diagnosed MM cases, patients with low XBP1s/u ratios (≤1.33) had longer overall survival (OS) than patients with high XBP1s/u ratios (56 months vs. 40 months, P = 0.03) when independent risk factors including age, ␤2-MG and t (4;14) were excluded. However, the XBP1s/u ratio had no effect on treatment response or progression-free survival (PFS). Additionally, the XBP1s/u ratio can be used in combination with ␤2-MG and t (4;14) to separate high-risk patients from all MM patients [34]. Furthermore, this ratio shows a predictive value compared with the effects between thalidomide-based treatments and conventional treatments. Thalidomide was reported to suppress myeloma growth and to regulate the expression of survival-related cytokines within the bone marrow (BM) microenvironment, such as IL-6 and tumor necrosis factor (TNF) [35]. Thalidomide and other immunomodulatory drugs [IMiDs] could overcome the conventional drug resistance and improve treatment effects in MM patients because both MM cells and their microenvironment are targeted. Moreover, IMiDs can also induce apoptosis in MM cells resistant to such drugs as melphalan, doxorubicin and dexamethasone [36]. In patients with low XBP1s/u ratios, thalidomide therapy led to apparently longer OS than conventional therapy (P < 0.01). However, no such effect was observed in patients with a high XBP1s/u ratio. A similar result was obtained in the assessment of the effect of the XBP1s/u ratio on PFS of MM patients who received thalidomide therapy (P = 0.01). For relapsed MM, low XBP1s/u ratios indicated longer initial PFS and a better outcome than high XBP1s/u ratios (P = 0.001). Although XBP1s itself in MM cells can induce the expression of IL6 and IL-15, MM cells with low XBP1s/u could only turn to the BM for IL-6 to satisfy their needs for growth and survival due to the weak ability to induce IL-6 and IL-15 themselves. Thalidomide usage can aggravate this IL-6 reduction, leading to impaired growth and increased apoptosis of MM cells, favoring the survival of MM patients. MM cells with high XBP1s/u, however, were less dependent on the BM cytokines, and therefore, immunomodulatory drugs were less effective in these patients [34]. However, a study in 17 cases of relapsed or refractory MM indicated low XBP1 levels as a predictor of poor response to bortezomib treatment both in vitro and in vivo. Although the expression levels of XBP1 were low in bortezomib-resistant MM cells, recovery of the sensitivity of MM cells to bortezomib could not be achieved by simply increasing the expression levels of XBP1 [37]. Another study of 151 treatment-naïve MM cases showed that there was no correlation between XBP1 expression levels and the responses of patients to bortezomib-containing regimens. The authors found that patients with high XBP1 expression had a better outcome after bortezomib-based therapy [38].

9

localized ATF6 regulate the expression of downstream target genes via XBP1. Therefore, ATF6 and IRE1␣-mediated splicing of XBP1 mRNA are critical for later full activation of the UPR [40,41]. Salicylaldehydes were found to be a selective inhibitor of IRE1 because they could selectively inhibit the endoribonuclease activity of IRE1 in vitro and in vivo with no action on other RNases [42]. MKC-3946 is an inhibitor of the endoribonuclease domain of IRE1␣ and can block the splicing of XBP1 mRNA and induce moderate cytotoxicity in both primary MM cells and MM cell lines. The XBP1 splicing inhibition further activate the PERK pathway and resultantly enhance terminal UPR. However, MKC-3946 has no inhibitory effect on mononuclear cells from healthy donors, guaranteeing a cancer-specific role of MKC-3946. Previous work showed that MKC-3946 inhibited bortezomib- or 17-AAG-mediated splicing of XBP1 in RPMI 8226 cells and had no influence on the expression of IRE1␣, phopsho-IRE1␣ and molecular chaperone BiP/GRP78. Additionally, MKC-3946 significantly enhanced the anti-MM effect of bortezomib and 17-AAG in three primary MM cells. Moreover, MKC-3946 alone or with low-dose bortezomib significantly inhibited the growth of MM cells in vivo [14]. MKC-3946 enhance the cytotoxicity of ER stressors to MM cells, independently of the presence of BMSCs or exogenous IL-6. Furthermore, MKC-3946 in combination with ER stressors can antagonize the cytoprotective effect of the BM microenvironment on MM cells. STF-083010 is another inhibitor of the endoribonuclease domain of IRE1␣ with no influence on its kinase domain. STF-083010 lead to cytotoxicity in MM cells in a dose- and time-dependent manner. Furthermore, STF-083010 was selectively cytotoxic to CD138+ MM cells [43]. In addition to the above IRE1 inhibitors, there are other novel drugs targeting the IRE1-XBP1 signaling pathway. 4. Proteasome inhibitors (PIs) induce ER stress and induce a terminal UPR in myeloma cells Bortezomib is an effective inhibitor of the 26 S proteasome, a multisubunit protein complex responsible for the degradation of accumulated unfolded proteins [44]. Because the maturation and folding of the ER membrane and secretory proteins are precisely regulated inside the ER, those proteins failing to fold properly are ultimately degraded by the 26 S proteasome or ERAD. Previous reports have demonstrated that PIs lead to cytoplasmic sequestration of the transcription factor NF-␬B, therefore blocking the NF-␬B pathway [45]. Additionally, PIs lead to an accumulation of misfolded proteins in the ER and induce ER stress. PIs can inhibit the splicing of XBP1 by suppressing the endoribonuclease activity of IRE1␣ and also reduce the generation of XBP1s by enhancing the stability of XBP1u proteins. PIs rapidly induced components of the terminal UPR, including PERK, ATF4, an ER stress–induced transcription factor, and its target CHOP, leading to MM cell death. Because PIs promote monoclonal immunoglobulin-induced ER stress and related death in MM cells, PIs can be an effective treatment for MM [46–50]. However, bortezomib resistance occurred increasingly often, and the XBP1 c.499C >A mutation, which was located within the splicing region of the XBP1 mRNA, was found to underlie bortezomib resistance in MM [39].

3. IRE1␣ inhibitor Among the three major sensors of ER stress, IRE1␣ is the only highly conserved sensor from yeast to humans [39]. The endoribonuclease activity of IRE1␣ mediated the splicing of XBP1u mRNA to generate a new C-terminus, thereby converting XBP1u into XBP1s, a transcriptional activator of UPR. ATF6 can increase the levels of XBP1 mRNA, and IRE1␣ remove an unconventional 26-nucleotide intron off XBP1 mRNA, leading to the activation of XBP1. Additionally, nucleus-localized IRE1␣ and cytoplasm-

5. HSP inhibitors induce MM cell death by regulating UPR via the IRE1␣-XBP1 pathway Heat shock proteins (HSPs) are a set of molecular chaperones that play a critical role in the regulation of the folding and stability of proteins. In recent years, the proliferation-promoting and anti-apoptotic functions of HSP have been demonstrated in cancer cells. Because there are down-regulated expression levels of HSPs in MM cells [51–53], HSP could be taken as a potential therapeu-

10

L. Chen et al. / Leukemia Research 49 (2016) 7–12

tic target of MM. Thapsigargin and tunicamycin up-regulate the expression of CHOP and activate the UPR’s three branches. Bortezomib also activate CHOP and ATF6 but with a mild effect on XBP1 activation. Similarly, the HSP90 inhibitors 17-AAG and radicicol enhance the splicing of XBP1 mRNA and activate CHOP, JNK and ATF6. Therefore, HSP90 exerted an anti-myeloma effect by regulating UPR and ER stress. HSP90 inhibitors could also promote the expression of HSP70, which in turn inhibited the function of HSP90. This finding indicated that a combination of HSP70 inhibitor and HSP90 inhibitor might be a therapeutic choice for MM [48]. Additionally, the ability to bind with the cytoplasmic domains of kinases gives HSP90 a stabilizing effect on the endoribonuclease activity of IRE1␣ and subsequently an activating effect on XBP1. However, the HSP90-specific inhibitor 514 breaks the binding between HSP90 and the cytoplasmic domains of kinases, blocking the HSP90’s control on kinases and ultimately inhibiting MM. A combination of PI inhibitors and HSP inhibitors also achieved effective results in the treatment of MM [54]; one example is the combination of the HSP90 inhibitor TAS-116 and bortezomib. TAS-116 aggravate bortezomibinduced ER stress, resulting in the activation of terminal UPR and apoptosis. Additionally, TAS-116, which is available for oral administration, was stable during in vivo metabolism and had few side effects compared with other HSP90 inhibitors [55]. Currently, a clinical trial of the HSP90 inhibitor NVP-AUY922 monotherapy or in combination with bortezomib in the treatment of relapsed or refractory MM has entered phase I/IB [56]. Other HSP inhibitors can also be potential treatments for MM. The cytotoxic effect of the HSP70 inhibitor MLA3-101 in combination with HSP90 inhibitors or PI has been demonstrated. Additionally, a high concentration of MAL3-101 (30 ␮M) was observed to cause an increase in XBP1s generation in a timedependent manner [57]. An HSP27 inhibitor could overcome PI-induced drug resistance in lymphoma cells and enhance or restore their sensitivity to PI. However, the effect of HSP27 inhibitors on MM has not yet been investigated [58].

6. The adenosine moiety of toyocamycin inhibits IRE1a-induced splicing of XBP1 mRNA without affecting the activation of IRE1a, and its combination with bortezomib shows a synergistic effect Toyocamycin is a small-molecule inhibitor of XBP1 activation identified from the culture broth of an actinomycete strain. Toyocamycin selectively suppressed the drug-induced splicing of XBP1 mRNA in HeLa cells after cells were treated with thapsigargin, tunicamycin and 2-deoxyglucose without affecting the activation of PERK and ATF6. Moreover, this toyocamycin-induced splicing of XBP1 mRNA was demonstrated in MM cell lines and also in primary MM cells. Additionally, compared with high XBP1sexpressing RPMI8226 and XG7 cells, low XBP1s-expressing U266 cells showed relative insensitivity to the pro-apoptotic function of toyocamycin, indicating a partial XBP1s-dependent pro-apoptotic role of toyocamycin in MM cells. Toyocamycin act synergistically with bortezomib to induce dose-dependent apoptosis in bortezomib-resistant cells, indicating the importance of the combination of toyocamycin and bortezomib or toyocamycin in the treatment of MM [59]. PI was a representative successful treatment for MM in the past decade. However, despite its beneficial role in the treatment and prognosis of MM, PI resistance occurred. Nearly a third of MM patients had no response to bortezomib treatment, therefore indicating the critical role of combination therapy in the treatment of MM.

7. Resveratrol inhibits the transcriptional activity of XBP1s via sirtuin 1, and its combination with bortezomib and thalidomide might show a synergistic effect Resveratrol, namely trans-3,4 ,5,-trihydroxystilbene, inhibits the transcriptional activity of XBP1s via sirtuin 1, the latter suppressing the transcription of XBP1s and inducing ER stressinduced cell death through binding to the promoter region of XBP1 sequences. The results of chromatin immunoprecipitation indicated that resveratrol decreased the DNA binding capacity of XBP1 and increased the expression of sirtuin 1. The knock-down of the sirtuin 1 protein by shRNA abrogated the resveratrol-induced inhibitory effect on the transcriptional activity of XBP1s. Additionally, resveratrol dose-dependently promote the splicing of XBP1u into XBP1s [60,61]. Resveratrol can activate UPR, XBP1s, JNK and CHOP in MM. Reserveratrol also acts synergistically with bortezomib and thalidomide to increase the apoptosis of MM cells [62]. A clinical trial of SRT501 (resveratrol) in combination with bortezomib in relapsed and/or refractory MM has entered phase II [63]. 8. The CDK inhibitor SCH727965 decreases the expression of XBP1 and nuclear accumulation of XBP1s Cell cycle dysregulation is an important characteristic of neoplastic cells. For this reason, small molecule inhibitors targeting cyclin-dependent kinases (CDKs) have become the focus of cancer therapy [64]. SCH727965 is a CDK inhibitor that inhibits multiple CDKs including CDK2, CDK5, CDK1 and CDK9, among which CDK1 and CDK5 were found to play a central role in the post-transcriptional regulatory system. SCH727965 suppress the proliferation of MM cells in vitro and in vivo by down-regulating XBP–1s levels, which involved a post-transcriptional mechanism, not interrupting IRE1-induced splicing of XBP-1. Moreover, even at extremely low doses, SCH727965 still decreases the nuclear accumulation of XBP–1s and up-regulate the expression of Grp78 in response to ER stress. Furthermore, the anti-cancer function of SCH727965 was demonstrated in athymic nude mice. These findings indicated a potential link between CDK and the IRE1/XBP1s/Grp78 branch of UPR and highlighted CDK as a potential therapeutic target [65]. 9. Reovirus exerts anti-myeloma effect by inducing ER stress and UPR Reovirus-based therapy is a safe and effective anti-viral method. This method is found to show anti-cancer activity [66]. Oncolytic virotherapy induce ER stress and activate XBP1 splicing-mediated UPR, therefore inhibiting MM in vitro and in vivo. Furthermore, oncolytic virotherapy had no inhibitory effect on human hematopoietic stem cells (HHSC) when HHSCs were injected into NOD/SCID mice following exposure to live reovirus [67]. 10. Immunotherapy XBP1u- or XBP1s-derived peptide sequences including XBP1184-192, XBP1 SP196-204 and XBP1 SP367-375 showed a high affinity for HLA-A2, although their bond was unstable. YISPWILAV and YLFPQLISV are novel heteroclitic XBP1 peptides modified from XBP1184-192 and XBP1 SP367-375 peptides. These peptides showed a more stable binding with HLA-A2. Additionally, the YISPWILAV or YLFPQLISV peptides could activate the XBP1-specific clone of CD8 + T cells (XBP1-CTLs), leading to the death of HLA-A2+/XBP1+ MM cells by producing active molecules including granzyme, perforin and IFN-␥. This finding suggested XBP1-specific and HLA-restricted properties of XBP1-CTLs. More-

L. Chen et al. / Leukemia Research 49 (2016) 7–12

over, XBP1-CTLs were cytotoxic to primary HLA-A2+/CD138+ MM cells. These data suggested the effectiveness and feasibility of XBP1 peptides-based vaccination in the treatment of MM [68]. A phase I trial was conducted to evaluate the safety, tolerability and clinical activity of the GRP78 monoclonal immunoglobulin M antibody PAT-SM6 in the treatment of relapsed or refractory MM. Combined therapy of PAT-SM6 and other anti-myeloma drugs has also been planned [69]. 11. Other key strategies to disrupt protein homeostasis could exploit the UPR/IRE1-XBP1 signaling pathway Sulforaphane is a dietary isothiocyanate found in cruciferous vegetables. Sulforaphane was recently found to promote the splicing of XBP1 mRNA and to synergistically enhance ATO-mediated anti-myeloma effect [70]. ATPase valosin-containing protein (VCP; p97, which is an essential regulator of protein degradation and a new cancer therapeutic target) inhibitors blocked ERAD and activated eIF2␣ signaling (strong UPR reaction) in cancer cells derived from different tissues, including proteasome inhibitoradapted myeloma cells. CB-5083, an orally bioavailable inhibitor of p97 that induced a strong UPR is currently being evaluated in two phase I clinical trials in relapsed and refractory MM patients and in patients with advanced solid tumors [71–73]. 12. Conclusion MM remains an incurable malignancy of the hematologic system, and although the outcome of MM patients has been greatly improved recently, MM is likely to relapse or become drug-resistant [74]. Therefore, there is an urgency to find novel treatments for MM. The IRE1␣-XBP1 signaling pathway was closely associated with the pathogenesis of MM, and inhibitors of the IRE1␣-XBP1 pathway have shown certain therapeutic value. However, the link between the IRE1␣-XBP1 axis and MM is still not clear. Therefore, there are still numerous obstacles before inhibitors of IRE1␣-XBP1 can be clinically applied in the treatment of MM. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgments Grant Support: This work was supported in part by Key project supported by Tianjin Science and Technology Support Program (13ZCZCSY20300); Planned scientific research program of Tianjin Municipal Education Commission (20140112). References [1] I. Kim, W. Xu, J.C. Reed, Cell death and endoplasmic reticulum stress: disease relevance and therapeutic opportunities, Nat. Rev. Drug Discov. 7 (2008) 1013–1030. [2] C. Patil, P. Walter, Intracellular signaling from the endoplasmic reticulum to the nucleus: the unfolded protein response in yeast and mammals, Curr. Opin. Cell Biol. 13 (2001) 349–355. [3] Z. Kostova, D.H. Wolf, For whom the bell tolls: protein quality control of the endoplasmic reticulum and the ubiquitin-proteasome connection, EMBO J. 22 (2003) 2309–2317. [4] B. Tsai, Y. Ye, T.A. Rapoport, Retro-translocation of proteins from the endoplasmic reticulum into the cytosol, Nat. Rev. Mol. Cell Biol. 3 (2002) 246–255. [5] E. Szegezdi, S.E. Logue, A.M. Gorman, A. Samali, Mediators of endoplasmic reticulum stress-induced apoptosis, EMBO Rep. 7 (2006) 880–885. [6] J.W. Brewer, J.A. Diehl, PERK mediates cell-cycle exit during the mammalian unfolded protein response, Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 12625–12630.

11

[7] F. Urano, X. Wang, A. Bertolotti, Y. Zhang, P. Chung, H.P. Harding, D. Ron, Coupling of stress in the ER to activation of JNK protein kinases by transmembrane protein kinase IRE1, Science 287 (2000) 664–666. [8] M. Lu, D.A. Lawrence, S. Marsters, D. Acosta-Alvear, P. Kimmig, A.S. Mendez, A.W. Paton, J.C. Paton, P. Walter, A. Ashkenazi, Opposing unfolded-protein-response signals converge on death receptor 5 to control apoptosis, Science 345 (2014) 98–101. [9] B.T. Gaudette, N.N. Iwakoshi, L.H. Boise, Bcl-xL protein protects from C/EBP homologous protein (CHOP)-dependent apoptosis during plasma cell differentiation, J. Biol. Chem. 289 (2014) 23629–23640. [10] H.W. Auner, C. Beham-Schmid, N. Dillon, P. Sabbattini, The life span of short-lived plasma cells is partly determined by a block on activation of apoptotic caspases acting in combination with endoplasmic reticulum stress, Blood 116 (2010) 3445–3455. [11] Y. Ma, Y. Shimizu, M.J. Mann, Y. Jin, L.M. Hendershot, Plasma cell differentiation initiates a limited ER stress response by specifically suppressing the PERK-dependent branch of the unfolded protein response, Cell Stress Chaperones 15 (2010) 281–293. [12] H. Puthalakath, L.A. O’Reilly, P. Gunn, L. Lee, P.N. Kelly, N.D. Huntington, P.D. Hughes, E.M. Michalak, J. McKimm-Breschkin, N. Motoyama, T. Gotoh, S. Akira, P. Bouillet, A. Strasser, ER stress triggers apoptosis by activating BH3-only protein Bim, Cell 129 (2007) 1337–1349. [13] W. Rozpedek, D. Pytel, B. Mucha, H. Leszczynska, J.A. Diehl, I. Majsterek, The role of the PERK/eIF2alpha/ATF4/CHOP signaling pathway in tumor progression during endoplasmic reticulum stress, Curr. Mol. Med. (2016). [14] N. Mimura, M. Fulciniti, G. Gorgun, Y.T. Tai, D. Cirstea, L. Santo, Y. Hu, C. Fabre, J. Minami, H. Ohguchi, T. Kiziltepe, H. Ikeda, Y. Kawano, M. French, M. Blumenthal, V. Tam, N.L. Kertesz, U.M. Malyankar, M. Hokenson, T. Pham, Q. Zeng, J.B. Patterson, P.G. Richardson, N.C. Munshi, K.C. Anderson, Blockade of XBP1 splicing by inhibition of IRE1alpha is a promising therapeutic option in multiple myeloma, Blood 119 (2012) 5772–5781. [15] Criteria for the classification of monoclonal gammopathies, multiple myeloma and related disorders: a report of the International Myeloma Working Group, Br. J. Haematol. 121 (2003) 749–757. [16] S. Moriya, S. Komatsu, K. Yamasaki, Y. Kawai, H. Kokuba, A. Hirota, X.F. Che, M. Inazu, A. Gotoh, M. Hiramoto, K. Miyazawa, Targeting the integrated networks of aggresome formation, proteasome, and autophagy potentiates ER stressmediated cell death in multiple myeloma cells, Int. J. Oncol. 46 (2015) 474–486. [17] H. Jiang, J. Zou, H. Zhang, W. Fu, T. Zeng, H. Huang, F. Zhou, J. Hou, Unfolded protein response inducers tunicamycin and dithiothreitol promote myeloma cell differentiation mediated by XBP-1, Clin. Exp. Med. 15 (2015) 85–96. [18] L. Vincenz, R. Jager, M. O’Dwyer, A. Samali, Endoplasmic reticulum stress and the unfolded protein response: targeting the Achilles heel of multiple myeloma, Mol. Cancer Ther. 12 (2013) 831–843. [19] H. Yoshida, T. Matsui, A. Yamamoto, T. Okada, K. Mori, XBP1 mRNA is induced by ATF6 and spliced by IRE1 in response to ER stress to produce a highly active transcription factor, Cell 107 (2001) 881–891. [20] H. Yoshida, T. Matsui, N. Hosokawa, R.J. Kaufman, K. Nagata, K. Mori, A time-dependent phase shift in the mammalian unfolded protein response, Dev. Cell 4 (2003) 265–271. [21] J. Jurkin, T. Henkel, A.F. Nielsen, M. Minnich, J. Popow, T. Kaufmann, K. Heindl, T. Hoffmann, M. Busslinger, J. Martinez, The mammalian tRNA ligase complex mediates splicing of XBP1 mRNA and controls antibody secretion in plasma cells, EMBO J. 33 (2014) 2922–2936. [22] T. Iwawaki, R. Akai, K. Kohno, IRE1alpha disruption causes histological abnormality of exocrine tissues, increase of blood glucose level, and decrease of serum immunoglobulin level, PLoS One 5 (2010) e13052. [23] K. Yanagitani, Y. Imagawa, T. Iwawaki, A. Hosoda, M. Saito, Y. Kimata, K. Kohno, Cotranslational targeting of XBP1 protein to the membrane promotes cytoplasmic splicing of its own mRNA, Mol. Cell 34 (2009) 191–200. [24] H. Zhang, S. Nakajima, H. Kato, L. Gu, T. Yoshitomi, K. Nagai, H. Shinmori, S. Kokubo, M. Kitamura, Selective, potent blockade of the IRE1 and ATF6 pathways by 4-phenylbutyric acid analogues, Br. J. Pharmacol. 170 (2013) 822–834. [25] A.M. Reimold, N.N. Iwakoshi, J. Manis, P. Vallabhajosyula, E. Szomolanyi-Tsuda, E.M. Gravallese, D. Friend, M.J. Grusby, F. Alt, L.H. Glimcher, Plasma cell differentiation requires the transcription factor XBP-1, Nature 412 (2001) 300–307. [26] N.N. Iwakoshi, A.H. Lee, L.H. Glimcher, The X-box binding protein-1 transcription factor is required for plasma cell differentiation and the unfolded protein response, Immunol. Rev. 194 (2003) 29–38. [27] N.N. Iwakoshi, A.H. Lee, P. Vallabhajosyula, K.L. Otipoby, K. Rajewsky, L.H. Glimcher, Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1, Nat. Immunol. 4 (2003) 321–329. [28] D.J. Todd, L.J. McHeyzer-Williams, C. Kowal, A.H. Lee, B.T. Volpe, B. Diamond, M.G. McHeyzer-Williams, L.H. Glimcher, XBP1 governs late events in plasma cell differentiation and is not required for antigen-specific memory B cell development, J. Exp. Med. 206 (2009) 2151–2159. [29] Y. Shen, L.M. Hendershot, Identification of ERdj3 and OBF-1/BOB-1/OCA-B as direct targets of XBP-1 during plasma cell differentiation, J. Immunol. 179 (2007) 2969–2978. [30] A.H. Lee, N.N. Iwakoshi, L.H. Glimcher, XBP-1 regulates a subset of endoplasmic reticulum resident chaperone genes in the unfolded protein response, Mol. Cell. Biol. 23 (2003) 7448–7459.

12

L. Chen et al. / Leukemia Research 49 (2016) 7–12

[31] G. Xu, K. Liu, J. Anderson, K. Patrene, S. Lentzsch, G.D. Roodman, H. Ouyang, Expression of XBP1s in bone marrow stromal cells is critical for myeloma cell growth and osteoclast formation, Blood 119 (2012) 4205–4214. [32] F.E. Davies, A.M. Dring, C. Li, A.C. Rawstron, M.A. Shammas, S.M. O’Connor, J.A. Fenton, T. Hideshima, D. Chauhan, I.T. Tai, E. Robinson, D. Auclair, K. Rees, D. Gonzalez, A.J. Ashcroft, R. Dasgupta, C. Mitsiades, N. Mitsiades, L.B. Chen, W.H. Wong, N.C. Munshi, G.J. Morgan, K.C. Anderson, Insights into the multistep transformation of MGUS to myeloma using microarray expression analysis, Blood 102 (2003) 4504–4511. [33] J.O. Claudio, E. Masih-Khan, H. Tang, J. Goncalves, M. Voralia, Z.H. Li, V. Nadeem, E. Cukerman, O. Francisco-Pabalan, C.C. Liew, J.R. Woodgett, A.K. Stewart, A molecular compendium of genes expressed in multiple myeloma, Blood 100 (2002) 2175–2186. [34] T. Bagratuni, P. Wu, D.C.D. Gonzalez, E.L. Davenport, N.J. Dickens, B.A. Walker, K. Boyd, D.C. Johnson, W. Gregory, G.J. Morgan, F.E. Davies, XBP1s levels are implicated in the biology and outcome of myeloma mediating different clinical outcomes to thalidomide-based treatments, Blood 116 (2010) 250–253. [35] D. Gupta, S.P. Treon, Y. Shima, T. Hideshima, K. Podar, Y.T. Tai, B. Lin, S. Lentzsch, F.E. Davies, D. Chauhan, R.L. Schlossman, P. Richardson, P. Ralph, L. Wu, F. Payvandi, G. Muller, D.I. Stirling, K.C. Anderson, Adherence of multiple myeloma cells to bone marrow stromal cells upregulates vascular endothelial growth factor secretion: therapeutic applications, Leukemia 15 (2001) 1950–1961. [36] T. Hideshima, D. Chauhan, Y. Shima, N. Raje, F.E. Davies, Y.T. Tai, S.P. Treon, B. Lin, R.L. Schlossman, P. Richardson, G. Muller, D.I. Stirling, K.C. Anderson, Thalidomide and its analogs overcome drug resistance of human multiple myeloma cells to conventional therapy, Blood 96 (2000) 2943–2950. [37] S.C. Ling, E.K. Lau, A. Al-Shabeeb, A. Nikolic, A. Catalano, H. Iland, N. Horvath, P.J. Ho, S. Harrison, S. Fleming, D.E. Joshua, J.D. Allen, Response of myeloma to the proteasome inhibitor bortezomib is correlated with the unfolded protein response regulator XBP-1, Haematologica 97 (2012) 64–72. [38] M. Gambella, A. Rocci, R. Passera, F. Gay, P. Omede, C. Crippa, P. Corradini, A. Romano, D. Rossi, M. Ladetto, M. Boccadoro, A. Palumbo, High XBP1 expression is a marker of better outcome in multiple myeloma patients treated with bortezomib, Haematologica 99 (2014) e14–e16. [39] S.Y. Hong, T. Hagen, Multiple myeloma Leu167Ile (c.499C > A) mutation prevents XBP1 mRNA splicing, Br. J. Haematol. 161 (2013) 898–901. [40] K. Lee, W. Tirasophon, X. Shen, M. Michalak, R. Prywes, T. Okada, H. Yoshida, K. Mori, R.J. Kaufman, IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response, Genes Dev. 16 (2002) 452–466. [41] F. Urano, A. Bertolotti, D. Ron, IRE1 and efferent signaling from the endoplasmic reticulum, J. Cell Sci. 113 (Pt 21) (2000) 3697–3702. [42] K. Volkmann, J.L. Lucas, D. Vuga, X. Wang, D. Brumm, C. Stiles, D. Kriebel, A. Der-Sarkissian, K. Krishnan, C. Schweitzer, Z. Liu, U.M. Malyankar, D. Chiovitti, M. Canny, D. Durocher, F. Sicheri, J.B. Patterson, Potent and selective inhibitors of the inositol-requiring enzyme 1 endoribonuclease, J. Biol. Chem. 286 (2011) 12743–12755. [43] I. Papandreou, N.C. Denko, M. Olson, H. Van Melckebeke, S. Lust, A. Tam, D.E. Solow-Cordero, D.M. Bouley, F. Offner, M. Niwa, A.C. Koong, Identification of an Ire1alpha endonuclease specific inhibitor with cytotoxic activity against human multiple myeloma, Blood 117 (2011) 1311–1314. [44] R.C. Gardner, S.J. Assinder, G. Christie, G.G. Mason, R. Markwell, H. Wadsworth, M. McLaughlin, R. King, M.C. Chabot-Fletcher, J.J. Breton, D. Allsop, A.J. Rivett, Characterization of peptidyl boronic acid inhibitors of mammalian 20 S and 26 S proteasomes and their inhibition of proteasomes in cultured cells, Biochem. J 346 (Pt 2) (2000) 447–454. [45] T. Hideshima, D. Chauhan, P. Richardson, C. Mitsiades, N. Mitsiades, T. Hayashi, N. Munshi, L. Dang, A. Castro, V. Palombella, J. Adams, K.C. Anderson, NF-kappa B as a therapeutic target in multiple myeloma, J. Biol. Chem. 277 (2002) 16639–16647. [46] A.H. Lee, N.N. Iwakoshi, K.C. Anderson, L.H. Glimcher, Proteasome inhibitors disrupt the unfolded protein response in myeloma cells, Proc. Natl. Acad. Sci. U. S. A. 100 (2003) 9946–9951. [47] K.L. Calame, K.I. Lin, C. Tunyaplin, Regulatory mechanisms that determine the development and function of plasma cells, Annu. Rev. Immunol. 21 (2003) 205–230. [48] E.L. Davenport, H.E. Moore, A.S. Dunlop, S.Y. Sharp, P. Workman, G.J. Morgan, F.E. Davies, Heat shock protein inhibition is associated with activation of the unfolded protein response pathway in myeloma plasma cells, Blood 110 (2007) 2641–2649. [49] E.A. Obeng, L.M. Carlson, D.M. Gutman, W.J. Harrington, K.P. Lee, L.H. Boise, Proteasome inhibitors induce a terminal unfolded protein response in multiple myeloma cells, Blood 107 (2006) 4907–4916. [50] H.W. Auner, S. Cenci, Recent advances and future directions in targeting the secretory apparatus in multiple myeloma, Br. J. Haematol. 168 (2015) 14–25. [51] L. Zhang, J.H. Fok, F.E. Davies, Heat shock proteins in multiple myeloma, Oncotarget 5 (2014) 1132–1148. [52] S. Bustany, J. Cahu, G. Descamps, C. Pellat-Deceunynck, B. Sola, Heat shock factor 1 is a potent therapeutic target for enhancing the efficacy of treatments for multiple myeloma with adverse prognosis, J. Hematol. Oncol. 8 (2015) 40. [53] E.T. Soo, G.W. Yip, Z.M. Lwin, S.D. Kumar, B.H. Bay, Heat shock proteins as novel therapeutic targets in cancer, In Vivo 22 (2008) 311–315.

[54] M.G. Marcu, M. Doyle, A. Bertolotti, D. Ron, L. Hendershot, L. Neckers, Heat shock protein 90 modulates the unfolded protein response by stabilizing IRE1alpha, Mol. Cell. Biol. 22 (2002) 8506–8513. [55] R. Suzuki, T. Hideshima, N. Mimura, J. Minami, H. Ohguchi, S. Kikuchi, Y. Yoshida, G. Gorgun, D. Cirstea, F. Cottini, J. Jakubikova, Y.T. Tai, D. Chauhan, P.G. Richardson, N.C. Munshi, T. Utsugi, K.C. Anderson, Anti-tumor activities of selective HSP90alpha/beta inhibitor, TAS-116, in combination with bortezomib in multiple myeloma, Leukemia 29 (2015) 510–514. [56] R. Seggewiss-Bernhardt, R.C. Bargou, Y.T. Goh, A.K. Stewart, A. Spencer, A. Alegre, J. Blade, O.G. Ottmann, C. Fernandez-Ibarra, H. Lu, S. Pain, M. Akimov, S.P. Iyer, Phase 1/1 B trial of the heat shock protein 90 inhibitor NVP-AUY922 as monotherapy or in combination with bortezomib in patients with relapsed or refractory multiple myeloma, Cancer Am. Cancer Soc. (2016) 2015. [57] M.J. Braunstein, S.S. Scott, C.M. Scott, S. Behrman, P. Walter, P. Wipf, J.D. Coplan, W. Chrico, D. Joseph, J.L. Brodsky, O. Batuman, Antimyeloma effects of the heat shock protein 70 molecular chaperone inhibitor MAL3-101, J. Oncol. 2011 (2011) 232037. [58] D. Chauhan, G. Li, R. Shringarpure, K. Podar, Y. Ohtake, T. Hideshima, K.C. Anderson, Blockade of Hsp27 overcomes Bortezomib/proteasome inhibitor PS-341 resistance in lymphoma cells, Cancer Res. 63 (2003) 6174–6177. [59] M. Ri, E. Tashiro, D. Oikawa, S. Shinjo, M. Tokuda, Y. Yokouchi, T. Narita, A. Masaki, A. Ito, J. Ding, S. Kusumoto, T. Ishida, H. Komatsu, Y. Shiotsu, R. Ueda, T. Iwawaki, M. Imoto, S. Iida, Identification of Toyocamycin, an agent cytotoxic for multiple myeloma cells, as a potent inhibitor of ER stress-induced XBP1 mRNA splicing, Blood Cancer J. 2 (2012) e79. [60] F.M. Wang, Y.J. Chen, H.J. Ouyang, Regulation of unfolded protein response modulator XBP1s by acetylation and deacetylation, Biochem. J. 433 (2011) 245–252. [61] F.M. Wang, D.L. Galson, G.D. Roodman, H. Ouyang, Resveratrol triggers the pro-apoptotic endoplasmic reticulum stress response and represses pro-survival XBP1 signaling in human multiple myeloma cells, Exp. Hematol. 39 (2011) 999–1006. [62] A. Bhardwaj, G. Sethi, S. Vadhan-Raj, C. Bueso-Ramos, Y. Takada, U. Gaur, A.S. Nair, S. Shishodia, B.B. Aggarwal, Resveratrol inhibits proliferation, induces apoptosis, and overcomes chemoresistance through down-regulation of STAT3 and nuclear factor-kappaB-regulated antiapoptotic and cell survival gene products in human multiple myeloma cells, Blood 109 (2007) 2293–2302. [63] R. Popat, T. Plesner, F. Davies, G. Cook, M. Cook, P. Elliott, E. Jacobson, T. Gumbleton, H. Oakervee, J. Cavenagh, A phase 2 study of SRT501 (resveratrol) with bortezomib for patients with relapsed and or refractory multiple myeloma, Br. J. Haematol. 160 (2013) 714–717. [64] Y. Dai, S. Grant, Cyclin-dependent kinase inhibitors, Curr. Opin. Pharmacol. 3 (2003) 362–370. [65] T.K. Nguyen, S. Grant, Dinaciclib (SCH727965) inhibits the unfolded protein response through a CDK1- and 5-dependent mechanism, Mol. Cancer Ther. 13 (2014) 662–674. [66] L. Vidal, H.S. Pandha, T.A. Yap, C.L. White, K. Twigger, R.G. Vile, A. Melcher, M. Coffey, K.J. Harrington, J.S. DeBono, A phase I study of intravenous oncolytic reovirus type 3 Dearing in patients with advanced cancer, Clin. Cancer Res. 14 (2008) 7127–7137. [67] C.M. Thirukkumaran, Z.Q. Shi, J. Luider, K. Kopciuk, H. Gao, N. Bahlis, P. Neri, M. Pho, D. Stewart, A. Mansoor, D.G. Morris, Reovirus as a viable therapeutic option for the treatment of multiple myeloma, Clin. Cancer Res. 18 (2012) 4962–4972. [68] J. Bae, R. Carrasco, A.H. Lee, R. Prabhala, Y.T. Tai, K.C. Anderson, N.C. Munshi, Identification of novel myeloma-specific XBP1 peptides able to generate cytotoxic T lymphocytes: a potential therapeutic application in multiple myeloma, Leukemia 25 (2011) 1610–1619. [69] L. Rasche, J. Duell, I.C. Castro, V. Dubljevic, M. Chatterjee, S. Knop, F. Hensel, A. Rosenwald, H. Einsele, M.S. Topp, S. Brandlein, GRP78-directed immunotherapy in relapsed or refractory multiple myeloma − results from a phase 1 trial with the monoclonal immunoglobulin M antibody PAT-SM6, Haematologica 100 (2015) 377–384. [70] N.A. Doudican, S.Y. Wen, A. Mazumder, S.J. Orlow, Sulforaphane synergistically enhances the cytotoxicity of arsenic trioxide in multiple myeloma cells via stress-mediated pathways, Oncol. Rep. 28 (2012) 1851–1858. [71] D.J. Anderson, R. Le Moigne, S. Djakovic, B. Kumar, J. Rice, S. Wong, J. Wang, B. Yao, E. Valle, V.S.S. Kiss, A. Madriaga, F. Soriano, M.K. Menon, Z.Y. Wu, M. Kampmann, Y. Chen, J.S. Weissman, B.T. Aftab, F.M. Yakes, L. Shawver, H.J. Zhou, D. Wustrow, M. Rolfe, Targeting the AAA ATPase p97 as an approach to treat cancer through disruption of protein homeostasis, Cancer Cell 28 (2015) 653–665. [72] R.J. Deshaies, Proteotoxic crisis, the ubiquitin-proteasome system, and cancer therapy, BMC Biol. 12 (2014) 94. [73] K. Parzych, T.M. Chinn, Z. Chen, S. Loaiza, F. Porsch, G.N. Valbuena, M.F. Kleijnen, A. Karadimitris, E. Gentleman, H.C. Keun, H.W. Auner, Inadequate fine-tuning of protein synthesis and failure of amino acid homeostasis following inhibition of the ATPase VCP/p97, Cell. Death. Dis. 6 (2015) e2031. [74] S.K. Kumar, S.V. Rajkumar, A. Dispenzieri, M.Q. Lacy, S.R. Hayman, F.K. Buadi, S.R. Zeldenrust, D. Dingli, S.J. Russell, J.A. Lust, P.R. Greipp, R.A. Kyle, M.A. Gertz, Improved survival in multiple myeloma and the impact of novel therapies, Blood 111 (2008) 2516–2520.