Compromising the unfolded protein response induces autophagy-mediated cell death in multiple myeloma cells

Autophagy-related mechanisms for treatment of multiple myeloma

Multiple myeloma (MM) is a type of hematological cancer that occurs when B cells become malignant. Various drugs such as proteasome inhibitors, immunomodulators, and compounds that cause DNA damage can be used in the treatment of MM. Autophagy, a type 2 cell death mechanism, plays a crucial role in determining the fate of B cells, either promoting their survival or inducing cell death. Therefore, autophagy can either facilitate the progression or hinder the treatment of MM disease. In this review, autophagy mechanisms that may be effective in MM cells were covered and evaluated within the contexts of unfolded protein response (UPR), bone marrow microenvironment (BMME), drug resistance, hypoxia, DNA repair and transcriptional regulation, and apoptosis. The genes that are effective in each mechanism and research efforts on this subject were discussed in detail. Signaling pathways targeted by new drugs to benefit from autophagy in MM disease were covered. The efficacy of drugs that regulate autophagy in MM was examined, and clinical trials on this subject were included. Consequently, among the autophagy mechanisms that are effective in MM, the most suitable ones to be used in the treatment were expressed. The importance of 3D models and microfluidic systems for the discovery of new drugs for autophagy and personalized treatment was emphasized. Ultimately, this review aims to provide a comprehensive overview of MM disease, encompassing autophagy mechanisms, drugs, clinical studies, and further studies.

7α,25-Dihydroxycholesterol-Induced Oxiapoptophagic Chondrocyte Death via the Modulation of p53-Akt-mTOR Axis in Osteoarthritis Pathogenesis

This study aimed to exploring the pathophysiological mechanism of 7α,25-dihydroxycholesterol (7α,25-DHC) in osteoarthritis (OA) pathogenesis. 7α,25-DHC accelerated the proteoglycan loss in ex vivo organ-cultured articular cartilage explant. It was mediated by the decreasing extracellular matrix major components, including aggrecan and type II collagen, and the increasing expression and activation of degenerative enzymes, including matrix metalloproteinase (MMP)-3 and -13, in chondrocytes cultured with 7α,25-DHC. Furthermore, 7α,25-DHC promoted caspase dependent chondrocytes death via extrinsic and intrinsic pathways of apoptosis. Moreover, 7α,25-DHC upregulated the expression of inflammatory factors, including inducible nitric oxide synthase, cyclooxygenase-2, nitric oxide, and prostaglandin E2, via the production of reactive oxygen species via increase of oxidative stress in chondrocytes. In addition, 7α,25-DHC upregulated the expression of autophagy biomarker, including beclin-1 and microtubule-associated protein 1A/1B-light chain 3 via the modulation of p53-Akt-mTOR axis in chondrocytes. The expression of CYP7B1, caspase-3, and beclin-1 was elevated in the degenerative articular cartilage of mouse knee joint with OA. Taken together, our findings suggest that 7α,25-DHC is a pathophysiological risk factor of OA pathogenesis that is mediated a chondrocytes death via oxiapoptophagy, which is a mixed mode of apoptosis, oxidative stress, and autophagy.

Negligible role of TRAIL death receptors in cell death upon endoplasmic reticulum stress in B-cell malignancies

Impairments in protein folding in the endoplasmic reticulum (ER) lead to a condition called ER stress, which can trigger apoptosis via the mitochondrial or the death receptor (extrinsic) pathway. There is controversy concerning involvement of the death receptor (DR)4 and DR5-Caspase-8 -Bid pathway in ER stress-mediated cell death, and this axis has not been fully studied in B-cell malignancies. Using three B-cell lines from Mantle Cell Lymphoma, Waldenström's macroglobulinemia and Multiple Myeloma origins, we engineered a set of CRISPR KOs of key components of these cell death pathways to address this controversy. We demonstrate that DR4 and/or DR5 are essential for killing via TRAIL, however, they were dispensable for ER-stress induced-cell death, by Thapsigargin, Brefeldin A or Bortezomib, as were Caspase-8 and Bid. In contrast, the deficiency of Bax and Bak fully protected from ER stressors. Caspase-8 and Bid were cleaved upon ER-stress stimulation, but this was DR4/5 independent and rather a result of mitochondrial-induced feedback loop subsequent to Bax/Bak activation. Finally, combined activation of the ER-stress and TRAIL cell-death pathways was synergistic with putative clinical relevance for B-cell malignancies.

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.

Autophagy and the Bone Marrow Microenvironment: A Review of Protective Factors in the Development and Maintenance of Multiple Myeloma

The role of the unfolded protein response (UPR) in plasma cells (PC) and their malignant multiple myeloma (MM) counterparts is a well described area of research. The importance of autophagy in these cells, as well as the interplay between autophagy and the UPR system, has also been well studied. In this review, we will discuss the relationship between these two cellular responses and how they can be utilized in MM to account for the high levels of monoclonal immunoglobulin (Ig) protein synthesis that is characteristic of this disease. Interactions between MM cells and the bone marrow (BM) microenvironment and how MM cells utilize the UPR/autophagy pathway for their survival. These interacting pathways form the foundation for the mechanism of action for bortezomib, a proteasome inhibitor used to modify the progression of MM, and the eventual drug resistance that MM cells develop. One important resistance pathway implicated in MM progression is caspase 10 which attenuates autophagy to maintain its prosurvival function and avoid cell death. We lay a groundwork for future research including 3D in vitro models for better disease monitoring and personalized treatment. We also highlight pathways involved in MM cell survival and drug resistance that could be used as new targets for effective treatment.

25-Hydroxycholesterol-Induced Oxiapoptophagy in L929 Mouse Fibroblast Cell Line

25-hydroxycholesterol (25-HC) is an oxysterol synthesized from cholesterol by cholesterol-25-hydroxylase during cholesterol metabolism. The aim of this study was to verify whether 25-HC induces oxiapoptophagy in fibroblasts. 25-HC not only decreased the survival of L929 cells, but also increased the number of cells with condensed chromatin and altered morphology. Fluorescence-activated cell sorting results showed that there was a dose-dependent increase in the apoptotic populations of L929 cells upon treatment with 25-HC. 25-HC-induced apoptotic cell death was mediated by the death receptor-dependent extrinsic and mitochondria-dependent intrinsic apoptosis pathway, through the cascade activation of caspases including caspase-8, -9, and -3 in L929 cells. There was an increase in the levels of reactive oxygen species and inflammatory mediators such as inducible nitric oxide synthase, cyclooxygenase-2, nitric oxide, and prostaglandin E2 in L929 cells treated with 25-HC. Moreover, 25-HC caused an increase in the expression of beclin-1 and microtubule-associated protein 1A/1B-light chain 3, an autophagy biomarker, in L929 cells. There was a significant decrease in the phosphorylation of protein kinase B (Akt) in L929 cells treated with 25-HC. Taken together, 25-HC induced oxiapoptophagy through the modulation of Akt and p53 cellular signaling pathways in L929 cells.

5,6-Epoxycholesterol Isomers Induce Oxiapoptophagy in Myeloma Cells

Simple Summary

As the second most frequent hematological malignancy, multiple myeloma remains incurable with recurrent patient relapse due to drug resistance. Therefore, the development of novel and potent therapies is urgently required. Herein, we demonstrated the anti-tumor activity of 5,6 α- and 5,6 β-epoxycholesterol isomers against human myeloma cells. Our results highlighted a striking anti-myeloma efficiency of these bioactive molecules and their added value in future potential treatments including combination therapy of multiple myeloma.

Abstract

Multiple myeloma (MM) is an incurable plasma cell malignancy with frequent patient relapse due to innate or acquired drug resistance. Cholesterol metabolism is reported to be altered in MM; therefore, we investigated the potential anti-myeloma activity of two cholesterol derivatives: the 5,6 α- and 5,6 β-epoxycholesterol (EC) isomers. To this end, viability assays were used, and isomers were shown to exhibit important anti-tumor activity in vitro in JJN3 and U266 human myeloma cell lines (HMCLs) and ex vivo in myeloma patients’ sorted CD138+ malignant cells. Moreover, we confirmed that 5,6 α-EC and 5,6 β-EC induced oxiapoptophagy through concomitant oxidative stress and caspase-3-mediated apoptosis and autophagy. Interestingly, in combination treatment a synergistic interaction was observed between 5,6 α-EC and 5,6 β-EC on myeloma cells. These data highlight a striking anti-tumor activity of 5,6 α-EC and 5,6 β-EC bioactive molecules against human myeloma cells, paving the way for their potential role in future therapeutic strategies in MM.

Possible Therapeutic Potential of Disulfiram for Multiple Myeloma

Multiple myeloma (MM) is a malignant disease of the plasma cells representing approximately 10% of all hemato-oncological diseases. Detection of the disease is most probable at around 65 years of age, and the average survival of patients is estimated to be 5–10 years, specifically due to frequent relapses and resistance to the therapy used. Thus, the search for new therapeutic approaches is becoming a big challenge. Disulfiram (DSF), a substance primarily known as a medication against alcoholism, has often been mentioned in recent years in relation to cancer treatment for its secondary anti-cancer effects. Recent studies performed on myeloma cell lines confirm high inhibition of the cell growth activity if a complex of disulfiram and copper is used. Its significant potential is now being seen in the cure of haematological malignities.

Crosstalk between endoplasmic reticulum stress and oxidative stress: a dynamic duo in multiple myeloma

Under physiological and pathological conditions, cells activate the unfolded protein response (UPR) to deal with the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum. Multiple myeloma (MM) is a hematological malignancy arising from immunoglobulin-secreting plasma cells. MM cells are subject to continual ER stress and highly dependent on the UPR signaling activation due to overproduction of paraproteins. Mounting evidence suggests the close linkage between ER stress and oxidative stress, demonstrated by overlapping signaling pathways and inter-organelle communication pivotal to cell fate decision. Imbalance of intracellular homeostasis can lead to deranged control of cellular functions and engage apoptosis due to mutual activation between ER stress and reactive oxygen species generation through a self-perpetuating cycle. Here, we present accumulating evidence showing the interactive roles of redox homeostasis and proteostasis in MM pathogenesis and drug resistance, which would be helpful in elucidating the still underdefined molecular pathways linking ER stress and oxidative stress in MM. Lastly, we highlight future research directions in the development of anti-myeloma therapy, focusing particularly on targeting redox signaling and ER stress responses.

ROS Overproduction Sensitises Myeloma Cells to Bortezomib-Induced Apoptosis and Alleviates Tumour Microenvironment-Mediated Cell Resistance

Multiple myeloma (MM) is a plasma cell neoplasm that remains incurable due to innate or acquired resistance. Although MM cells produce high intracellular levels of reactive oxygen species (ROS), we hypothesised that they could remain sensitive to ROS unbalance. We tested if the inhibition of ROS, on one hand, or the overproduction of ROS, on the other, could (re)sensitise cells to bortezomib (BTZ). Two drugs were used in a panel of MM cell lines with various responses to BTZ: VAS3947 (VAS), an inhibitor of NADPH oxidase and auranofin (AUR), an inhibitor of thioredoxin reductase (TXNRD1), an antioxidant enzyme overexpressed in MM cells. We used several culture models: in suspension, on a fibronectin layer, in coculture with HS-5 mesenchymal cells, and/or in 3-D culture (or spheroids) to study the response of MM primary cells and cell lines. Several MM cell lines were sensitive to VAS but the combination with BTZ showed antagonistic or additive effects at best. By contrast, in all culture systems studied, the combined AUR/BTZ treatment showed synergistic effects on cell lines, including those less sensitive to BTZ and primary cells. MM cell death is due to the activation of apoptosis and autophagy. Modulating the redox balance of MM cells could be an effective therapy for refractory or relapse post-BTZ patients.

Characterization of a PERK Kinase Inhibitor with Anti-Myeloma Activity

Due to increased immunoglobulin production and uncontrolled proliferation, multiple myeloma (MM) plasma cells develop a phenotype of deregulated unfolded protein response (UPR). The eIF2-alpha kinase 3 [EIF2αK3, protein kinase R (PKR)-like ER kinase (PERK)], the third known sensor of endoplasmic reticulum (ER) stress, is a serine-threonine kinase and, like the other two UPR-related proteins, i.e., IRE1 and ATF6, it is bound to the ER membrane. MM, like other tumors showing uncontrolled protein secretion, is highly dependent to UPR for survival; thus, inhibition of PERK can be an effective strategy to suppress growth of malignant plasma cells. Here, we have used GSK2606414, an ATP-competitive potent PERK inhibitor, and found significant anti-proliferative and apoptotic effects in a panel of MM cell lines. These effects were accompanied by the downregulation of key components of the PERK pathway as well as of other UPR elements. Consistently, PERK gene expression silencing significantly increased cell death in MM cells, highlighting the importance of PERK signaling in MM biology. Moreover, GSK2606414, in combination with the proteasome inhibitor bortezomib, exerted an additive toxic effect in MM cells. Overall, our data suggest that PERK inhibition could represent a novel combinatorial therapeutic approach in MM.

Actionable Strategies to Target Multiple Myeloma Plasma Cell Resistance/Resilience to Stress: Insights From "Omics" Research

While the modern therapeutic armamentarium to treat multiple myeloma (MM) patients allows a longer control of the disease, this second-most-frequent hematologic cancer is still uncurable in the vast majority of cases. Since MM plasma cells are subjected to various types of chronic cellular stress and the integrity of specific stress-coping pathways is essential to ensure MM cell survival, not surprisingly the most efficacious anti-MM therapy are those that make use of proteasome inhibitors and/or immunomodulatory drugs, which target the biochemical mechanisms of stress management. Based on this notion, the recently realized discoveries on MM pathobiology through high-throughput techniques (genomic, transcriptomic, and other "omics"), in order for them to be clinically useful, should be elaborated to identify novel vulnerabilities in this disease. This groundwork of information will likely allow the design of novel therapies against targetable molecules/pathways, in an unprecedented opportunity to change the management of MM according to the principle of "precision medicine." In this review, we will discuss some examples of therapeutically actionable molecules and pathways related to the regulation of cellular fitness and stress resistance in MM.

Intracellular proliferation of Anaplasma phagocytophilum is promoted via modulation of endoplasmic reticulum stress signaling in host cells

Anaplasma phagocytophilum, an obligate intracellular bacterium that propagates within host granulocytes, is considered to modify the host intracellular environment for pathogenesis. However, the mechanism(s) underlying such host modifications remain unclear. Here, we aimed to investigate the relation between A. phagocytophilum and endoplasmic reticulum (ER) stress in THP-1 cells. A. phagocytophilum activated the three ER stress sensors: inositol-requiring enzyme-1 (IRE1), protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor-6 (ATF6). IRE1 activation occurred immediately after host cell invasion by A. phagocytophilum; however, the activated IRE1-induced splicing of X-box-binding protein 1 was not promoted during A. phagocytophilum infection. This suppression was sustained even after the doxycycline-mediated elimination of intracellular A. phagocytophilum. IRE1 knockdown accelerated A. phagocytophilum-induced apoptosis and decreased intracellular A. phagocytophilum. These data suggest that A. phagocytophilum utilizes IRE1 activation to promote its own intracellular proliferation. Moreover, PERK and ATF6 partially mediated A. phagocytophilum-induced apoptosis by promoting the expression of CCAAT/enhancer-binding protein homologous protein, which induces the transcription of several proapoptotic genes. Thus, A. phagocytophilum possibly manipulates the host ER stress signals to facilitate intracellular proliferation and infection of surrounding cells before/after host cell apoptosis.

Proteasome inhibition in multiple myeloma: lessons for other cancers

Cellular protein homeostasis (proteostasis) depends on the controlled degradation of proteins that are damaged or no longer required by the ubiquitin-proteasome system (UPS). The 26S proteasome is the principal executer of substrate-specific proteolysis in eukaryotic cells and regulates a myriad of cellular functions. Proteasome inhibitors were initially developed as chemical tools to study proteasomal function but rapidly became widely used anticancer drugs that are now used at all stages of treatment for the bone marrow cancer multiple myeloma (MM). Here, we review the mechanisms of action of proteasome inhibitors that underlie their preferential toxicity to MM cells, focusing on endoplasmic reticulum stress, depletion of amino acids, and effects on glucose and lipid metabolism. We also discuss mechanisms of resistance to proteasome inhibition such as autophagy and metabolic rewiring and what lessons we may learn from the success and failure of proteasome inhibition in MM for treating other cancers with proteostasis-targeting drugs.

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.

Protein Quality Control in the Endoplasmic Reticulum and Cancer

The endoplasmic reticulum (ER) is an essential compartment of the biosynthesis, folding, assembly, and trafficking of secretory and transmembrane proteins, and consequently, eukaryotic cells possess specialized machineries to ensure that the ER enables the proteins to acquire adequate folding and maturation for maintaining protein homeostasis, a process which is termed proteostasis. However, a large variety of physiological and pathological perturbations lead to the accumulation of misfolded proteins in the ER, which is referred to as ER stress. To resolve ER stress and restore proteostasis, cells have evolutionary conserved protein quality-control machineries of the ER, consisting of the unfolded protein response (UPR) of the ER, ER-associated degradation (ERAD), and autophagy. Furthermore, protein quality-control machineries of the ER play pivotal roles in the control of differentiation, progression of cell cycle, inflammation, immunity, and aging. Therefore, severe and non-resolvable ER stress is closely associated with tumor development, aggressiveness, and response to therapies for cancer. In this review, we highlight current knowledge in the molecular understanding and physiological relevance of protein quality control of the ER and discuss new insights into how protein quality control of the ER is implicated in the pathogenesis of cancer, which could contribute to therapeutic intervention in cancer.

Implications of aging and the endoplasmic reticulum unfolded protein response on the molecular modality of breast cancer

The endoplasmic reticulum (ER) is an important subcellular organelle that is involved in numerous activities required to achieve and maintain functional proteins in addition to its role in the biosynthesis of lipids and as a repository of intracellular Ca²⁺. The inability of the ER to cope with protein folding beyond its capacity causes disturbances that evoke ER stress. Cells possess molecular mechanisms aimed at clearing unwanted cargo from the ER lumen as an adaptive response, but failing to do so navigates the system towards cell death. This systemic approach is called the unfolded protein response. Aging insults cells through various perturbations in homeostasis that involve curtailing ER function by mitigating the expression of its resident chaperones and enzymes. Here the unfolded protein response (UPR) cannot protect the cell due to the weakening of its protective arm, which exacerbates imbalanced homeostasis. Aging predisposed breast malignancy activates the UPR, but tumor cells maneuver the mechanistic details of the UPR, favoring tumorigenesis and thereby eliciting a treacherous condition. Tumor cells exploit UPR pathways via crosstalk involving various signaling cascades that usher tumor cells to immortality. This review aims to present a collection of data that can delineate the missing links of molecular signatures between aging and breast cancer.

The novel autophagy inhibitor elaiophylin exerts antitumor activity against multiple myeloma with mutant TP53 in part through endoplasmic reticulum stress-induced apoptosis

Elaiophylin is a natural compound and a novel and potent inhibitor of late stage autophagy with outstanding antitumor activity in human ovarian cancer cells. However, the possible biologic effects and functional linkage between elaiophylin and multiple myeloma (MM) have not been explored. This study aimed to assess the effect of elaiophylin on MM cells with mutant TP53 and the possible molecular mechanism. The results suggested that elaiophylin exerted anti-myeloma activity by inducing apoptosis and proliferation arrest. As expected, elaiophylin blocked autophagy flux in MM cells. Subsequently, persistent activation of endoplasmic reticulum (ER) stress was induced. Moreover, the apoptotic effect was to some extent attenuated by the ER stress inhibitor tauroursodeoxycholic acid (TUDCA). Further studies indicated that elaiophylin effectively suppressed MM cell growth without obvious side effects in zebrafish embryo and mouse xenograft models. Taken together, our data are the first to demonstrate that exposure of human MM cells with mutant TP53 to elaiophylin blocked autophagy flux and thus induced cell death, which partially involved ER stress-associated apoptosis. Targeted disruption of the cellular protein handling system by elaiophylin is therefore a promising therapeutic strategy for overcoming incurable MM, even when TP53 mutations are present.

The Unfolded Protein Response in Homeostasis and Modulation of Mammalian Immune Cells

The endoplasmic reticulum (ER) plays important roles in eukaryotic protein folding and lipid biosynthesis. Several exogenous and endogenous cellular sources of stress can perturb ER homeostasis leading to the accumulation of unfolded proteins in the lumen. Unfolded protein accumulation triggers a signal-transduction cascade known as the unfolded protein response (UPR), an adaptive mechanism which aims to protect cells from protein aggregates and to restore ER functions. Further to this protective mechanism, in immune cells, UPR molecular effectors have been shown to participate in a wide range of biological processes such as cell differentiation, survival and immunoglobulin and cytokine production. Recent findings also highlight the involvement of the UPR machinery in the maturational program and antigen presentation capacities of dendritic cells. UPR is therefore a key element in immune system homeostasis with direct implications on both adaptive and innate immune responses. The present review summarizes the knowledge on the emerging roles of UPR signaling cascades in mammalian immune cells as well as the consequences of their dysregulation in relation to the pathogenesis of several diseases.

Alpha Particles Induce Autophagy in Multiple Myeloma Cells

OBJECTIVES

Radiation emitted by the radionuclides in radioimmunotherapy (RIT) approaches induce direct killing of the targeted cells as well as indirect killing through the bystander effect. Our research group is dedicated to the development of α-RIT, i.e., RIT using α-particles especially for the treatment of multiple myeloma (MM). γ-irradiation and β-irradiation have been shown to trigger apoptosis in tumor cells. Cell death mode induced by (213)Bi α-irradiation appears more controversial. We therefore decided to investigate the effects of (213)Bi on MM cell radiobiology, notably cell death mechanisms as well as tumor cell immunogenicity after irradiation.

METHODS

Murine 5T33 and human LP-1 MM cell lines were used to study the effects of such α-particles. We first examined the effects of (213)Bi on proliferation rate, double-strand DNA breaks, cell cycle, and cell death. Then, we investigated autophagy after (213)Bi irradiation. Finally, a coculture of dendritic cells (DCs) with irradiated tumor cells or their culture media was performed to test whether it would induce DC activation.

RESULTS

We showed that (213)Bi induces DNA double-strand breaks, cell cycle arrest, and autophagy in both cell lines, but we detected only slight levels of early apoptosis within the 120 h following irradiation in 5T33 and LP-1. Inhibition of autophagy prevented (213)Bi-induced inhibition of proliferation in LP-1 suggesting that this mechanism is involved in cell death after irradiation. We then assessed the immunogenicity of irradiated cells and found that irradiated LP-1 can activate DC through the secretion of soluble factor(s); however, no increase in membrane or extracellular expression of danger-associated molecular patterns was observed after irradiation.

CONCLUSION

This study demonstrates that (213)Bi induces mainly necrosis in MM cells, low levels of apoptosis, and autophagy that might be involved in tumor cell death.

Novel therapeutic strategies for multiple myeloma

Multiple myeloma (MM) is a plasma-cell malignancy which remains incurable despite the recent emergence of multiple novel agents. Importantly, recent genetic and molecular analyses have revealed the complexity and heterogeneity of this disease, highlighting the need for therapeutic strategies to eliminate all clones. Moreover, the bone marrow microenvironment, including stromal cells and immune cells, plays a central role in MM pathogenesis, promoting tumor cell growth, survival, and drug resistance. New classes of agents including proteasome inhibitors, immunomodulatory drugs, monoclonal antibodies, and histone deacetylase inhibitors have shown remarkable efficacy; however, novel therapeutic approaches are still urgently needed to further improve patient outcomes. In this review, we discuss the recent advances and future strategies to ultimately develop MM therapies with curative potential.

Recent advances and future directions in targeting the secretory apparatus in multiple myeloma

Multiple myeloma is a genetically heterogeneous tumour of transformed plasma cells, terminally differentiated effectors of the B cell lineage specialized in producing large amounts of immunoglobulins. The uniquely well-developed secretory apparatus that equips normal and transformed plasma cells with the capacity for high-level protein secretion constitutes a distinctive therapeutic target. In this review we discuss how fundamental cellular processes, such as the unfolded protein response (UPR), endoplasmic reticulum (ER)-associated degradation and autophagy, maintain intracellular protein homeostasis (proteostasis) and regulate plasma cell ontogeny and malignancy. We summarize our current understanding of the cellular effects of proteasome inhibitors and the molecular bases of resistance to them. Furthermore, we discuss how improvements in our understanding of the secretory apparatus and of the complex interactions between intracellular protein synthesis and degradation pathways can disclose novel drug targets for multiple myeloma, defining a paradigm of general interest for cancer biology and disorders of altered proteostasis.

Antiestrogen-binding site ligands induce autophagy in myeloma cells that proceeds through alteration of cholesterol metabolism

Multiple myeloma (MM) is a malignancy characterized by the accumulation of clonal plasma cells in the bone marrow. Despite extensive efforts to design drugs targeting tumoral cells and their microenvironment, MM remains an incurable disease for which new therapeutic strategies are needed. We demonstrated here that antiestrogens (AEs) belonging to selective estrogen receptor modulators family induce a caspase-dependent apoptosis and trigger a protective autophagy. Autophagy was recognized by monodansylcadaverin staining, detection of autophagosomes by electronic microscopy, and detection of the cleaved form of the microtubule-associated protein light chain 3. Moreover, autophagy was inhibited by drugs such as bafilomycin A1 and 3-methyladenosine. Autophagy was mediated by the binding of AEs to a class of receptors called the antiestrogen binding site (AEBS) different from the classical estrogen nuclear receptors. The binding of specific ligands to the AEBS was accompanied by alteration of cholesterol metabolism and in particular accumulation of sterols: zymostenol or desmosterol depending on the ligand. This was due to the inhibition of the cholesterol-5,6-epoxide hydrolase activity borne by the AEBS. We further showed that the phosphoinositide 3-kinase/AKT/mammalian target of rapamycin pathway mediated autophagy signaling. Moreover, AEBS ligands restored sensitivity to dexamethasone in resistant MM cells. Since we showed previously that AEs arrest MM tumor growth in xenografted mice, we propose that AEBS ligands may have a potent antimyeloma activity alone or in combination with drugs used in clinic.

Autophagy and radiosensitization in cancer

Autophagy is a natural self-degradative process by which cells eliminate misfolded proteins and damaged organelles. Autophagy has been shown to have multiple functions in tumor cells that may be dependent on the tumor type and the treatment conditions. Autophagy can have a cytoprotective role and be thought of as a survival mechanism or be cytotoxic in nature and mediate cell death. Radiation, one of the primary treatments for many different types of cancer, almost uniformly promotes autophagy in tumor cells. While autophagy produced in response to radiation is often considered to be cytoprotective, radiation-induced autophagy has also been shown to mediate susceptibility to radiation. This review addresses the complexity of autophagy in response to radiation treatment in three different cancer models, specifically lung cancer, breast cancer and glioblastoma. A deeper understanding of the different roles played by autophagy in response to radiation should facilitate the development of approaches for enhancing the therapeutic utility of radiation by providing strategies for combination treatment with unique radiosensitizers as well as preventing the initiation of strategies which are likely to attenuate the effectiveness of radiation therapy.

Regulated in DNA damage and development 1 (REDD1) promotes cell survival during serum deprivation by sustaining repression of signaling through the mechanistic target of rapamycin in complex 1 (mTORC1)

Regulated in DNA damage and development 1 (REDD1) functions to repress signaling through the mechanistic target of rapamycin (mTOR) protein kinase in complex 1 (mTORC1) in response to diverse stress conditions. In the present study, we investigated the role of REDD1 in the response of cells to growth cessation induced by serum deprivation. REDD1 expression was induced within 2h of depriving cells of serum, with the induction being mediated through ER stress, as evidenced by activation of PERK, enhanced eIF2α phosphorylation, and ATF4 facilitated transcription of the REDD1 gene. In wild-type cells, signaling through mTORC1 was rapidly (within 30min) repressed in response to serum deprivation and the repression was sustained for at least 10h. In contrast, in REDD1 knockout cells mTORC1 signaling recovered toward the end of the 10h-deprivation period. Interestingly, Akt phosphorylation initially declined in response to serum deprivation and then recovered between 2 and 4h in wild-type but not REDD1 knockout cells. The recovery of mTORC1 signaling and the failure of Akt phosphorylation to do so in the REDD1 knockout cells were accompanied by a dramatic increase in caspase-3 cleavage and cell death, both of which were blocked by rapamycin. Furthermore, overexpression of constitutively active Akt rescued REDD1 knockout cells from serum deprivation induced cell death. Overall, the results implicate REDD1 as a key regulatory checkpoint that coordinates growth signaling inputs to activate pro-survival mechanisms and reduce susceptibility to cell death.

Intracellular NAD⁺ depletion enhances bortezomib-induced anti-myeloma activity

We recently demonstrated that Nicotinamide phosphoribosyltransferase (Nampt) inhibition depletes intracellular NAD⁺ content leading, to autophagic multiple myeloma (MM) cell death. Bortezomib has remarkably improved MM patient outcome, but dose-limiting toxicities and development of resistance limit its long-term utility. Here we observed higher Nampt messenger RNA levels in bortezomib-resistant patient MM cells, which correlated with decreased overall survival. We demonstrated that combining the NAD⁺ depleting agent FK866 with bortezomib induces synergistic anti-MM cell death and overcomes bortezomib resistance. This effect is associated with (1) activation of caspase-8, caspase-9, caspase-3, poly (ADP-ribose) polymerase, and downregulation of Mcl-1; (2) enhanced intracellular NAD⁺ depletion; (3) inhibition of chymotrypsin-like, caspase-like, and trypsin-like proteasome activities; (4) inhibition of nuclear factor κB signaling; and (5) inhibition of angiogenesis. Furthermore, Nampt knockdown significantly enhances the anti-MM effect of bortezomib, which can be rescued by ectopically overexpressing Nampt. In a murine xenograft MM model, low-dose combination FK866 and Bortezomib is well tolerated, significantly inhibits tumor growth, and prolongs host survival. Taken together, these findings indicate that intracellular NAD⁺ level represents a major determinant in the ability of bortezomib to induce apoptosis in MM cells and provide proof of concept for the combination with FK866 as a new strategy to enhance sensitivity or overcome resistance to bortezomib.

The role of endoplasmic reticulum stress in maintaining and targeting multiple myeloma: a double-edged sword of adaptation and apoptosis

Increased cellular protein production places stress on the endoplasmic reticulum (ER), because many of the nascent proteins pass through the ER for folding and trafficking. Accumulation of misfolded proteins in the ER triggers the activation of three well-known pathways including IRE1 (inositol requiring kinase 1), ATF6 (activating transcription factor 6), and PERK (double stranded RNA-activated protein kinase-like ER kinase). The activity of each sensor modulates the overall ER strategy for managing protein quality control as cellular needs change due to growth, differentiation, infection, transformation, and host of other possible physiological states. Here we review the role of ER stress in multiple myeloma (MM), an incurable plasma cell neoplasm. MM is closely linked to dysregulated unfolded protein response in the ER due to the heightened production of immunoglobulin and the metabolic demands of malignant uncontrolled proliferation. Together, these forces may mean that myeloma cells have an “Achilles heel” which can be exploited as a treatment target: their ER stress response must be constitutively active at a remarkably high level to survive their unique metabolic needs. Therefore, inhibition of the ER stress response is likely to injure the cells, as is any further demand on an already over-worked system. Evidence for this vulnerability is summarized here, along with an overview of how each of the three ER stress sensors has been implicated in myeloma pathogenesis and treatment.

Autophagy in multiple myeloma: what makes you stronger can also kill you

Autophagy, a process for recycling cellular constituents, is normally associated with cell survival and is thought to be beneficial for tumor maintenance. However, in this issue of Cancer Cell, Lamy and colleagues report that multiple myeloma utilizes caspase-10 to restrain autophagy and undergoes autophagic cell death upon its removal or inhibition.

Suppression of autophagy enhanced growth inhibition and apoptosis of interferon-β in human glioma cells

Interferon-beta (IFN-β) is a cytokine with anti-viral, anti-proliferative, and immunomodulatory effects. In this study, we investigated the effects of IFN-β on the induction of autophagy and the relationships among autophagy, growth inhibition, and apoptosis induced by IFN-β in human glioma cells. We found that IFN-β induced autophagosome formation and conversion of microtubule associated protein 1 light chain 3 (LC3) protein, whereas it inhibited cell growth through caspase-dependent cell apoptosis. The Akt/mTOR signaling pathway was involved in autophagy induced by IFN-β. A dose- and time-dependent increase of p-ERK 1/2 expression was also observed in human glioma cells treated with IFN-β. Autophagy induced by IFN-β was suppressed when p-ERK1/2 was impaired by treatment with U0126. We also demonstrated that suppression of autophagy significantly enhanced growth inhibition and cell apoptosis induced by IFN-β, whereas inhibition of caspase-dependent cell apoptosis impaired autophagy induced by IFN-β. Collectively, these findings indicated that autophagy induced by IFN-β was associated with the Akt/mTOR and ERK 1/2 signaling pathways, and inhibition of autophagy could enhance the growth inhibitory effects of IFN-β and increase apoptosis in human glioma cells. Together, these findings support the possibility that autophagy inhibitors may improve IFN-β therapy for gliomas.

Bortezomib/proteasome inhibitor triggers both apoptosis and autophagy-dependent pathways in melanoma cells

Generally, both endoplasmic reticulum (ER) stress and mitochondrial dysregulation are a potential therapeutic target of anticancer agents including bortezomib. The treatment of melanoma cells with bortezomib was found to induce apoptosis together with the upregulation of Noxa, Mcl-1, and HSP70 proteins, and the cleavage of LC3 and autophagic formation. Also, bortezomib induced ER-stress as evidenced by the increase of intracellular Ca(2+) release. In addition, bortezomib enhanced the phosphorylation of inositol-requiring transmembrane kinase and endonuclease 1α (IRE1α), apoptosis signal-regulating kinase 1 (ASK1), c-jun-N-terminal kinase (JNK) and p38, and the activation of the transcription factors AP-1, ATF-2, Ets-1, and HSF1. Bortezomib-induced mitochondrial dysregulation was associated with the accumulation of reactive oxygen species (ROS), the release of both apoptosis inducing factor (AIF) and cytochrome c, the activation of caspase-9 and caspase-3, and cleavage of Poly (ADP-ribose) polymerase (PARP). The pretreatment of melanoma cells with the inhibitor of caspase-3 (Ac-DEVD-CHO) was found to block bortezomib-induced apoptosis that subsequently led to the increase of autophagic formation. In contrast, the inhibition of ASK1 abrogated bortezomib-induced autophagic formation and increased apoptosis induction. Furthermore, the inhibition of JNK, of HSP70 also increased apoptosis induction without influence of bortezomib-induced autophagic formation. Based on the inhibitory experiments, the treatment with bortezomib triggers the activation of both ER-stress-associated pathways, namely IRE1α-ASK1-p38-ATF-2/ets-1-Mcl-1, and IRE1α-ASK1-JNK-AP-1/HSF1-HSP70 as well as mitochondrial dysregulation-associated pathways, namely ROS-ASK1-JNK-AP-1/HSF1-HS70, and AIF-caspase-3-PARP and Cyt.c, and caspase-9-caspase-3-PARP. Taken together, our data demonstrates for the first time the molecular mechanisms, whereby bortezomib triggers both apoptosis and autophagic formation in melanoma cells.

Targeting NAD+ salvage pathway induces autophagy in multiple myeloma cells via mTORC1 and extracellular signal-regulated kinase (ERK1/2) inhibition

Malignant cells have a higher nicotinamide adenine dinucleotide (NAD(+)) turnover rate than normal cells, making this biosynthetic pathway an attractive target for cancer treatment. Here we investigated the biologic role of a rate-limiting enzyme involved in NAD(+) synthesis, Nampt, in multiple myeloma (MM). Nampt-specific chemical inhibitor FK866 triggered cytotoxicity in MM cell lines and patient MM cells, but not normal donor as well as MM patients PBMCs. Importantly, FK866 in a dose-dependent fashion triggered cytotoxicity in MM cells resistant to conventional and novel anti-MM therapies and overcomes the protective effects of cytokines (IL-6, IGF-1) and bone marrow stromal cells. Nampt knockdown by RNAi confirmed its pivotal role in maintenance of both MM cell viability and intracellular NAD(+) stores. Interestingly, cytotoxicity of FK866 triggered autophagy, but not apoptosis. A transcriptional-dependent (TFEB) and independent (PI3K/mTORC1) activation of autophagy mediated FK866 MM cytotoxicity. Finally, FK866 demonstrated significant anti-MM activity in a xenograft-murine MM model, associated with down-regulation of ERK1/2 phosphorylation and proteolytic cleavage of LC3 in tumor cells. Our data therefore define a key role of Nampt in MM biology, providing the basis for a novel targeted therapeutic approach.

EGFR inhibitor BIBU induces apoptosis and defective autophagy in glioma cells

The importance of aberrant EGFR signaling in glioblastoma progression and the promise of EGFR-specific therapies, prompted us to determine the efficacy of novel EGFR inhibitor BIBU-1361 [(3-chloro-4-fluoro-phenyl)-[6-(4-diethylaminomethyl-piperidin-1-yl)-pyrimido [5,4-d]pyrimidin-4-yl]-amine] in affecting glioma survival. BIBU induced apoptosis in a caspase-dependent manner and induced cell cycle arrest in glioma cells. Apoptosis was accompanied by decreased EGFR levels and its increased distribution towards caveolin rich lipid raft microdomains. BIBU inhibited pro-survival pathways Akt/mTOR and gp130/JAK/STAT3; and decreased levels of pro-inflammatory cytokine IL-6. BIBU caused increased LC3-I to LC3-II conversion and triggered the internalization of EGFR within vacuoles along with its increased co-localization with LC3-II. BIBU caused accumulation of p62 and increased levels of cleaved forms of Beclin-1 in all the cell lines tested. Importantly, BIBU failed to initiate execution of autophagy as pharmacological inhibition of autophagy with 3-Methyladenine or Bafilomycin failed to rescue BIBU mediated death. The ability of BIBU to abrogate Akt and STAT3 activation, induce apoptosis and prevent execution of autophagy warrants its investigation as a potent anti-glioma target.

DangER: protein ovERload. Targeting protein degradation to treat myeloma

Myeloma is a malignancy of the antibody-producing plasma cells and, as such, these cells synthesize large quantities of unfolded or misfolded immunoglobulin. The build-up of excess protein triggers a number of downstream signal transduction cascades, including endoplasmic reticulum stress and autophagy. As a result, myeloma cells are uniquely reliant on these and other protein handling pathways for their survival. Strategies aimed at targeting this vulnerability have proved successful with the proteasome inhibitor, bortezomib, already licensed for clinical use. In addition to the proteasome, various other points within the protein handling pathways are also the subject of drug discovery projects, with some already progressing into clinical trials. These include compounds directed against heat shock proteins, the unfolded protein response and pathways both upstream and downstream of the proteasome. More recently, the role of autophagy has been recognized in myeloma. In this review, we discuss the various pathways used by myeloma cells for survival, with particular emphasis on the emerging role and conundrum of autophagy, as well as highlighting pre-clinical research on novel inhibitors targeting protein handling pathways.

Unfolded protein stress in the endoplasmic reticulum and mitochondria: a role in neurodegeneration

Protein-folding occurs in several intracellular locations including the endoplasmic reticulum and mitochondria. In normal conditions there is a balance between the levels of unfolded proteins and protein folding machinery. Disruption of homeostasis and an accumulation of unfolded proteins trigger stress responses, or unfolded protein responses (UPR), in these organelles. These pathways signal to increase the folding capacity, inhibit protein import or expression, increase protein degradation, and potentially trigger cell death. Many aging-related neurodegenerative diseases involve the accumulation of misfolded proteins in both the endoplasmic reticulum and mitochondria. The exact participation of the UPRs in the onset of neurodegeneration is unclear, but there is significant evidence for the alteration of these pathways in the endoplasmic reticulum and mitochondria. Here we will discuss the involvement of endoplasmic reticulum and mitochondrial stress and the possible contributions of the UPR in these organelles to the development of two neurodegenerative diseases, Parkinson's disease (PD) and Alzheimer's disease (AD).