A plastic SQSTM1/p62-dependent autophagic reserve maintains proteostasis and determines proteasome inhibitor susceptibility in multiple myeloma cells

NCX1/Ca2+ promotes autophagy and decreases bortezomib activity in multiple myeloma through non-canonical NFκB signaling pathway

Although bortezomib (BTZ) is the cornerstone of anti-multiple myeloma (MM) therapy, the inevitable primary and secondary drug resistance still seriously affects the prognosis of patients. New treatment strategies are in need. Sodium-calcium exchanger 1 (NCX1) is a calcium-permeable ion transporter on the membrane, and our previous studies showed that low NCX1 confers inferior viability in MM cells and suppressed osteoclast differentiation. However, the effect of NCX1 on BTZ sensitivity of MM and its possible mechanism remain unclear. In this study, we investigated the effect of NCX1 on BTZ sensitivity in MM, focusing on cellular processes of autophagy and cell viability. Our results provide evidence that NCX1 expression correlates with MM disease progression and low NCX1 expression increases BTZ sensitivity. NCX1/Ca2+ triggered autophagic flux through non-canonical NFκB pathway in MM cells, leading to attenuated the sensitivity of BTZ. Knockdown or inhibition of NCX1 could potentiate the anti-MM activity of BTZ in vitro and vivo, and inhibition of autophagy sensitized NCX1-overexpressing MM cells to BTZ. In general, this work implicates NCX1 as a potential therapeutic target in MM with BTZ resistance and provides novel mechanistic insights into its vital role in combating BTZ resistance.

Pharmacologic targeting of the p62 ZZ domain enhances both anti-tumor and bone-anabolic effects of bortezomib in multiple myeloma

Multiple myeloma (MM) is a malignancy of plasma cells whose antibody secretion creates proteotoxic stress relieved by the N-end rule pathway, a proteolytic system that degrades N-arginylated proteins in the proteasome. When the proteasome is inhibited, protein cargo is alternatively targeted for autophagic degradation by binding to the ZZ-domain of p62/ sequestosome-1. Here, we demonstrate that XRK3F2, a selective ligand for the ZZ-domain, dramatically improved two major responses to the proteasome inhibitor bortezomib (Btz) by increasing: i) killing of human MM cells by stimulating both Btz-mediated apoptosis and necroptosis, a process regulated by p62; and ii) preservation of bone mass by stimulating osteoblast differentiation and inhibiting osteoclastic bone destruction. Co-administration of Btz and XRK3F2 inhibited both branches of the bimodal N-end rule pathway exhibited synergistic anti-MM effects on MM cell lines and CD138+ cells from MM patients, and prevented stromal-mediated MM cell survival. In mice with established human MM, co-administration of Btz and XRK3F2 decreased tumor burden and prevented the progression of MM-induced osteolytic disease by inducing new bone formation more effectively than either single agent alone. The results suggest that p62-ZZ ligands enhance the anti- MM efficacy of proteasome inhibitors and can reduce MM morbidity and mortality by improving bone health.

GZ17-6.02 interacts with proteasome inhibitors to kill multiple myeloma cells

GZ17-6.02, a synthetically manufactured compound containing isovanillin, harmine and curcumin, has undergone phase I evaluation in patients with solid tumors (NCT03775525) with a recommended phase 2 dose (RP2D) of 375 mg PO BID. GZ17-6.02 was more efficacious as a single agent at killing multiple myeloma cells than had previously been observed in solid tumor cell types. GZ17-6.02 interacted with proteasome inhibitors in a greater than additive fashion to kill myeloma cells and alone it killed inhibitor-resistant cells to a similar extent. The drug combination of GZ17-6.02 and bortezomib activated ATM, the AMPK and PERK and inactivated ULK1, mTORC1, eIF2α, NFκB and the Hippo pathway. The combination increased ATG13 S318 phosphorylation and the expression of Beclin1, ATG5, BAK and BIM, and reduced the levels of BCL-XL and MCL1. GZ17-6.02 interacted with bortezomib to enhance autophagosome formation and autophagic flux, and knock down of ATM, AMPKα, ULK1, Beclin1 or ATG5 significantly reduced both autophagy and tumor cell killing. Knock down of BAK and BIM significantly reduced tumor cell killing. The expression of HDACs1/2/3 was significantly reduced beyond that previously observed in solid tumor cells and required autophagy. This was associated with increased acetylation and methylation of histone H3. Combined knock down of HDACs1/2/3 caused activation of ATM and the AMPK and caused inactivation of ULK1, mTORC1, NFκB and the Hippo pathway. HDAC knock down also enhanced ATG13 phosphorylation, increased BAK levels and reduced those of BCL-XL. Collectively, our present studies support performing additional in vivo studies with multiple myeloma cells.

Oncolytic Myxoma virus Increases Autophagy in Multiple Myeloma

Objective

Multiple myeloma, which affects plasma cells, is the second most common hematological malignancy. Despite the development of new drugs and treatment protocols, patient survival has not reached the desired level. In this study, we investigated the effects of Myxoma virus (MYXV), an oncolytic virus, on autophagy in myeloma cells.

Materials and Methods

We analyzed protein expressions of ATG-5, p62, Beclin-1, LC3B, and the apoptosis marker Bcl-2 as autophagy markers in human U-266 and mouse MOPC-315 myeloma cell lines subjected to different doses of MYXV. In addition, autophagic images of myeloma cells were investigated using transmission electron microscopy (TEM).

Results

In the first 24 h, which is the early stage of autophagy, ATG-5 and Beclin-1 expression levels were increased in the U-266 and MOPC-315 cell lines in the groups that had received MYXV at a multiplicity of infection of 15. At 48 h, a significant increase was detected in the expression of LC3B, which is a late indicator. Autophagosomes were observed in myeloma cells by TEM.

Conclusion

MYXV shows an antimyeloma effect by increasing autophagy in 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.

Modification of lysine-260 2-hydroxyisobutyrylation destabilizes ALDH1A1 expression to regulate bladder cancer progression

ALDH1A1 is one of the classical stem cell markers for bladder cancer. Lysine 2-hydroxyisobutyrylation (Khib) is a newfound modification to modulate the protein expression, and the underlying mechanisms of how ALDH1A1 was regulated by Khib modification in bladder cancer remains unknown. Here, ALDH1A1 showed a decreased K260hib modification, as identified by protein modification omics in bladder cancer. Decreasing ALDH1A1 expression significantly suppressed the proliferation, migration and invasion of bladder cancer cells. Moreover, K260hib modification is responsible for the activity of ALDH1A1 in bladder cancer, which is regulated by HDAC2/3. Higher K260hib modification on ALDH1A1 promotes protein degradation through chaperone-mediated autophagy (CMA), and ALDH1A1 K260hib could sensitize bladder cancer cells to chemotherapeutic drugs. Higher ALDH1A1 expression with a lower K260hib modification indicates a poor prognosis in patients with bladder cancer. Overall, we demonstrated that K260hib of ALDH1A1 can be used as a potential therapeutic target for bladder cancer treatment.

CD317 maintains proteostasis and cell survival in response to proteasome inhibitors by targeting calnexin for RACK1-mediated autophagic degradation

Unbalanced protein homeostasis (proteostasis) networks are frequently linked to tumorigenesis, making cancer cells more susceptible to treatments that target proteostasis regulators. Proteasome inhibition is the first licensed proteostasis-targeting therapeutic strategy, and has been proven effective in hematological malignancy patients. However, drug resistance almost inevitably develops, pressing for a better understanding of the mechanisms that preserve proteostasis in tumor cells. Here we report that CD317, a tumor-targeting antigen with a unique topology, was upregulated in hematological malignancies and preserved proteostasis and cell viability in response to proteasome inhibitors (PIs). Knocking down CD317 lowered Ca²⁺ levels in the endoplasmic reticulum (ER), promoting PIs-induced proteostasis failure and cell death. Mechanistically, CD317 interacted with calnexin (CNX), an ER chaperone protein that limits calcium refilling via the Ca²⁺ pump SERCA, thereby subjecting CNX to RACK1-mediated autophagic degradation. As a result, CD317 decreased the level of CNX protein, coordinating Ca²⁺ uptake and thus favoring protein folding and quality control in the ER lumen. Our findings reveal a previously unrecognized role of CD317 in proteostasis control and imply that CD317 could be a promising target for resolving PIs resistance in the clinic.

Effects of invivo CXCR4 Blockade and Proteasome Inhibition on Bone Marrow Plasma Cells in HLA-Sensitized Kidney Transplant Candidates

To date, plasma cell (PC)-targeted therapies have been limited by suboptimal PC depletion and antibody rebound. We hypothesized this is partly because of PC residence in protective bone marrow (BM) microenvironments. The purpose of this proof-of-concept study was to examine the effects of the CXCR4 antagonist, plerixafor, on PC BM residence; its safety profile (alone and in combination with a proteasome inhibitor, bortezomib); and the transcriptional effect on BMPCs in HLA-sensitized kidney transplant candidates. Participants were enrolled into 3 groups: group A (n = 4), plerixafor monotherapy; and groups B (n = 4) and C (n = 4), plerixafor and bortezomib combinations. CD34⁺ stem cell and PC levels increased in the blood after plerixafor treatment. PC recovery from BM aspirates varied depending on the dose of plerixafor and bortezomib. Single-cell RNA sequencing on BMPCs from 3 group C participants pretreatment and posttreatment revealed multiple populations of PCs, with a posttreatment enrichment of oxidative phosphorylation, proteasome assembly, cytoplasmic translation, and autophagy-related genes. Murine studies demonstrated dually inhibiting the proteasome and autophagy resulted in greater BMPC death than did monotherapies. In conclusion, this pilot study revealed anticipated effects of combined plerixafor and bortezomib on BMPCs, an acceptable safety profile, and suggests the potential for autophagy inhibitors in desensitization regimens.

Autophagy is induced by swine acute diarrhea syndrome coronavirus through the cellular IRE1-JNK-Beclin 1 signaling pathway after an interaction of viral membrane-associated papain-like protease and GRP78

Autophagy plays an important role in the infectious processes of diverse pathogens. For instance, cellular autophagy could be harnessed by viruses to facilitate replication. However, it is still uncertain about the interplay of autophagy and swine acute diarrhea syndrome coronavirus (SADS-CoV) in cells. In this study, we reported that SADS-CoV infection could induce a complete autophagy process both in vitro and in vivo, and an inhibition of autophagy significantly decreased SADS-CoV production, thus suggesting that autophagy facilitated the replication of SADS-CoV. We found that ER stress and its downstream IRE1 pathway were indispensable in the processes of SADS-CoV-induced autophagy. We also demonstrated that IRE1-JNK-Beclin 1 signaling pathway, neither PERK-EIF2S1 nor ATF6 pathways, was essential during SADS-CoV-induced autophagy. Importantly, our work provided the first evidence that expression of SADS-CoV PLP2-TM protein induced autophagy through the IRE1-JNK-Beclin 1 signaling pathway. Furthermore, the interaction of viral PLP2-TMF451-L490 domain and substrate-binding domain of GRP78 was identified to activate the IRE1-JNK-Beclin 1 signaling pathway, and thus resulting in autophagy, and in turn, enhancing SADS-CoV replication. Collectively, these results not only showed that autophagy promoted SADS-CoV replication in cultured cells, but also revealed that the molecular mechanism underlying SADS-CoV-induced autophagy in cells.

Autophagy and Apoptosis: Current Challenges of Treatment and Drug Resistance in Multiple Myeloma

Over the past two decades, the natural history of multiple myeloma (MM) has evolved dramatically, owing primarily to novel agents targeting MM in the bone marrow microenvironment (BMM) pathways. However, the mechanisms of resistance acquisition remain a mystery and are poorly understood. Autophagy and apoptosis are tightly controlled processes and play a critical role in the cell growth, development, and survival of MM. Genetic instability and abnormalities are two hallmarks of MM. During MM progression, plasma malignant cells become genetically unstable and activate various signaling pathways, resulting in the overexpression of abnormal proteins that disrupt autophagy and apoptosis biological processes. Thus, achieving a better understanding of the autophagy and apoptosis processes and the proteins that crosslinked both pathways, could provide new insights for the MM treatment and improve the development of novel therapeutic strategies to overcome resistance. This review presents a sufficient overview of the roles of autophagy and apoptosis and how they crosslink and control MM progression and drug resistance. Potential combination targeting of both pathways for improving outcomes in MM patients also has been addressed.

Autophagy in Hematological Malignancies

Simple Summary

Autophagy is a dynamic and tightly regulated process that seems to have dual effects in cancer. In some contexts, it can induce carcinogenesis and promote cancer cell survival, whereas in others, it acts preventing tumor cell growth and tumor progression. Thus, autophagy functions seem to strictly depend on cancer ontogenesis, progression, and type. Here, we will dive into the current knowledge of autophagy in hematological malignancies and will highlight the main genetic components involved in each cancer type.

Abstract

Autophagy is a highly conserved metabolic pathway via which unwanted intracellular materials, such as unfolded proteins or damaged organelles, are digested. It is activated in response to conditions of oxidative stress or starvation, and is essential for the maintenance of cellular homeostasis and other vital functions, such as differentiation, cell death, and the cell cycle. Therefore, autophagy plays an important role in the initiation and progression of tumors, including hematological malignancies, where damaged autophagy during hematopoiesis can cause malignant transformation and increase cell proliferation. Over the last decade, the importance of autophagy in response to standard pharmacological treatment of hematological tumors has been observed, revealing completely opposite roles depending on the tumor type and stage. Thus, autophagy can promote tumor survival by attenuating the cellular damage caused by drugs and/or stabilizing oncogenic proteins, but can also have an antitumoral effect due to autophagic cell death. Therefore, autophagy-based strategies must depend on the context to create specific and safe combination therapies that could contribute to improved clinical outcomes. In this review, we describe the process of autophagy and its role on hematopoiesis, and we highlight recent research investigating its role as a potential therapeutic target in hematological malignancies. The findings suggest that genetic variants within autophagy-related genes modulate the risk of developing hemopathies, as well as patient survival.

Elevated SFXN2 limits mitochondrial autophagy and increases iron-mediated energy production to promote multiple myeloma cell proliferation

Human sideroflexin 2 (SFXN2) belongs to the SFXN protein family, which is a mitochondrial outer membrane protein involved in mitochondrial iron metabolism. Mitochondria are indispensable for cellular energy production and iron metabolism. However, it remains elusive how SFXN2 modulates mitochondrial homeostasis and cellular iron metabolism in multiple myeloma (MM). In this study, we first found that SFXN2 was significantly elevated and correlated to poor outcomes in MM patients from clinical datasets. SFXN2 overexpression promoted MM cell proliferation and suppressed starvation-induced autophagy/mitophagy, while SFXN2 knockdown aggravated mitochondria damage and autophagic processes in ARP1 and H929 MM cell lines. Furthermore, inhibition of SFXN2 exerted effectively anti-myeloma activity in vivo by using myeloma xenograft model. Mechanism studies indicated that heme oxygenase 1 (HO1) with anti-oxidant function contributed to the process of autophagy suppression and cellular proliferation mediated by SFXN2. Our study revealed the critical role of SFXN2 in regulating mitochondrial bioenergetics, mitophagy, cellular iron metabolism, and redox homeostasis in interconnected and intricate way. Collectively, these findings not only provide insights into the metabolic reprogramming of tumor cells, but also highlight the therapeutic potential of SFXN2 in combination with iron metabolism as target for prognosis and treatment in MM patients.

Preclinical evidence of a direct pro-survival role of arginine deprivation in multiple myeloma

Multiple myeloma grows by establishing multiple interactions with bone marrow cells. These include expansion of myeloid-derived suppressor cells, which drive immunoevasion via mechanisms that include arginase-1-driven depletion of L-arginine, thus indirectly promoting myeloma cell survival and tumor progression. The peculiar biology of malignant plasma cells postulates that arginine depletion may benefit their fitness also directly, e.g., by engaging the integrated stress response, or by stimulating autophagy through mTORC1 inhibition. We thus investigated the direct impact of arginine deprivation on myeloma cells and challenged its pathophysiological relevance in vitro and in vivo. First, we found that partial arginine depletion spared proliferation of human multiple myeloma cells at concentrations that arrest human T cells. Next, we asked if arginine shortage activates putative adaptive pathways in myeloma cells. Low arginine failed to activate the integrated stress response, as indicated by unmodified phosphorylation of the eukaryotic initiation factor 2α, but sizably inhibited mTORC1, as revealed by reduced phosphorylation of ribosomal protein S6. Notably, depressed mTORC1 activity was not sufficient to increase autophagy, as assessed by the lysosomal digestion rate of the autophagosome-associated protein, LC3-II. Rather, it stimulated mTORC2, resulting in increased phosphatidylinositol-3 kinase-dependent AKT phosphorylation and activity, leading to heightened inhibitory phosphorylation of the pro-apoptotic BAD protein. We then tested whether arginine depletion-activated AKT may protect malignant plasma cells from cell death. Indeed, culturing myeloma cells in low arginine medium significantly reduced the apoptotic effect of the first-in-class proteasome inhibitor, bortezomib, an outcome prevented by pharmacological inhibition of AKT phosphorylation. Finally, we challenged the relevance of the identified circuit in vivo. To gauge the pathophysiologic relevance of low arginine to myeloma growth independently of immunoevasion, we xenotransplanted human myeloma cells subcutaneously into T cell-deficient Rag2^–/–γc^–/– recipient mice and treated palpable tumor-bearing mice with the clinical-grade arginase inhibitor CB1158. Arginase inhibition significantly raised serum arginine concentration, reduced tumor growth by caliper assessment, and decreased intra-tumor AKT phosphorylation in vivo. Altogether, our results reveal a novel direct pro-survival effect of arginine deprivation on myeloma cells, with potential therapeutic implications.

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.

Contribution of the Tumor Microenvironment to Metabolic Changes Triggering Resistance of Multiple Myeloma to Proteasome Inhibitors

Virtually all patients with multiple myeloma become unresponsive to treatment with proteasome inhibitors over time. Relapsed/refractory multiple myeloma is accompanied by the clonal evolution of myeloma cells with heterogeneous genomic aberrations, diverse proteomic and metabolic alterations, and profound changes of the bone marrow microenvironment. However, the molecular mechanisms that drive resistance to proteasome inhibitors within the context of the bone marrow microenvironment remain elusive. In this review article, we summarize the latest knowledge about the complex interaction of malignant plasma cells with its surrounding microenvironment. We discuss the pivotal role of metabolic reprograming of malignant plasma cells within the tumor microenvironment with a subsequent focus on metabolic rewiring in plasma cells upon treatment with proteasome inhibitors, driving multiple ways of adaptation to the treatment. At the same time, mutual interaction of plasma cells with the surrounding tumor microenvironment drives multiple metabolic alterations in the bone marrow. This provides a tumor-promoting environment, but at the same time may offer novel therapeutic options for the treatment of relapsed/refractory myeloma patients.

Constitutive Activation of p62/Sequestosome-1-Mediated Proteaphagy Regulates Proteolysis and Impairs Cell Death in Bortezomib-Resistant Mantle Cell Lymphoma

Simple Summary

To decipher the molecular mechanism underlying the resistance of a significant fraction of mantle cell lymphoma (MCL) patients to the first-in-class proteasome inhibitor bortezomib (BTZ), we have characterized the ubiquitin-related proteome (i.e., ubiquitome) of a set of MCL cell lines with different degrees of sensitivity to the drug by coupling a tandem ubiquitin-binding entity (TUBE) approach to mass spectrometry, followed by phenotypic and functional validations in both in vitro and in vivo models of MCL. We identified an enrichment of autophagy–lysosome system (ALS) components in BTZ-resistant cells, which was associated with constitutive intracellular inactivation of proteasome subunits by a process called proteaphagy. Blockade of this phenomenon by the pharmacological or genetic inactivation of the autophagy receptor p62/SQSTM1 reactivated normal proteasomal activity and restored the BTZ antitumor effect in in vitro and in vivo models of BTZ resistance.

Abstract

Protein ubiquitylation coordinates crucial cellular events in physiological and pathological conditions. A comparative analysis of the ubiquitin proteome from bortezomib (BTZ)-sensitive and BTZ-resistant mantle cell lymphoma (MCL) revealed an enrichment of the autophagy–lysosome system (ALS) in BTZ-resistant cells. Pharmacological inhibition of autophagy at the level of lysosome-fusion revealed a constitutive activation of proteaphagy and accumulation of proteasome subunits within autophagosomes in different MCL cell lines with acquired or natural resistance to BTZ. Inhibition of the autophagy receptor p62/SQSTM1 upon verteporfin (VTP) treatment disrupted proteaphagosome assembly, reduced co-localization of proteasome subunits with autophagy markers and negatively impacted proteasome activity. Finally, the silencing or pharmacological inhibition of p62 restored the apoptosis threshold at physiological levels in BTZ-resistant cells both in vitro and in vivo. In total, these results demonstrate for the first time a proteolytic switch from the ubiquitin–proteasome system (UPS) to ALS in B-cell lymphoma refractory to proteasome inhibition, pointing out a crucial role for proteaphagy in this phenomenon and paving the way for the design of alternative therapeutic venues in treatment-resistant tumors.

Overcoming drug resistance by targeting protein homeostasis in multiple myeloma

Multiple myeloma (MM) is a plasma cell disorder typically characterized by abundant synthesis of clonal immunoglobulin or free light chains. Although incurable, a deeper understanding of MM pathobiology has fueled major therapeutical advances over the past two decades, significantly improving patient outcomes. Proteasome inhibitors, immunomodulatory drugs, and monoclonal antibodies are among the most effective anti-MM drugs, targeting not only the cancerous cells, but also the bone marrow microenvironment. However, de novo resistance has been reported, and acquired resistance is inevitable for most patients over time, leading to relapsed/refractory disease and poor outcomes. Sustained protein synthesis coupled with impaired/insufficient proteolytic mechanisms makes MM cells exquisitely sensitive to perturbations in protein homeostasis, offering us the opportunity to target this intrinsic vulnerability for therapeutic purposes. This review highlights the scientific rationale for the clinical use of FDA-approved and investigational agents targeting protein homeostasis in MM.

Sequestosome 1/p62: A multitasker in the regulation of malignant tumor aggression (Review)

Sequestosome 1 (SQSTM1)/p62 is an adapter protein mainly involved in the transportation, degradation and destruction of various proteins that cooperates with components of autophagy and the ubiquitin-proteasome degradation pathway. Numerous studies have shown that SQSTM1/p62 functions at multiple levels, including involvement in genetic stability or modification, post-transcriptional regulation and protein function. As a result, SQSTM1/p62 is a versatile protein that is a critical core regulator of tumor cell genetic stability, autophagy, apoptosis and other forms of cell death, malignant growth, proliferation, migration, invasion, metastasis and chemoradiotherapeutic response, and an indicator of patient prognosis. SQSTM1/p62 regulates these processes via its distinct molecular structure, through which it participates in a variety of activating or inactivating tumor-related and tumor microenvironment-related signaling pathways, particularly positive feedback loops and epithelial-mesenchymal transition-related pathways. Therefore, functioning as a proto-oncogene or tumor suppressor gene in various types of cancer and tumor-associated microenvironments, SQSTM1/p62 is capable of promoting or retarding malignant tumor aggression, giving rise to immeasurable effects on tumor occurrence and development, and on patient treatment and prognosis.

Plasma cell biology: Foundations for targeted therapeutic development in transplantation

Solid organ transplantation is a life-saving procedure for patients with end-stage organ disease. Over the past 70 years, tremendous progress has been made in solid organ transplantation, particularly in T-cell-targeted immunosuppression and organ allocation systems. However, humoral alloimmune responses remain a major challenge to progress. Patients with preexisting antibodies to human leukocyte antigen (HLA) are at significant disadvantages in regard to receiving a well-matched organ, moreover, those who develop anti-HLA antibodies after transplantation face a significant foreshortening of renal allograft survival. Historical therapies to desensitize patients prior to transplantation or to treat posttransplant AMR have had limited effectiveness, likely because they do not significantly reduce antibody levels, as plasma cells, the source of antibody production, remain largely unaffected. Herein, we will discuss the significance of plasma cells in transplantation, aspects of their biology as potential therapeutic targets, clinical challenges in developing strategies to target plasma cells in transplantation, and lastly, novel approaches that have potential to advance the field.

Unique integrated stress response sensors regulate cancer cell susceptibility when Hsp70 activity is compromised

Molecular chaperones, such as Hsp70, prevent proteotoxicity and maintain homeostasis. This is perhaps most evident in cancer cells, which overexpress Hsp70 and thrive even when harboring high levels of misfolded proteins. To define the response to proteotoxic challenges, we examined adaptive responses in breast cancer cells in the presence of an Hsp70 inhibitor. We discovered that the cells bin into distinct classes based on inhibitor sensitivity. Strikingly, the most resistant cells have higher autophagy levels, and autophagy was maximally activated only in resistant cells upon Hsp70 inhibition. In turn, resistance to compromised Hsp70 function required the integrated stress response transducer, GCN2, which is commonly associated with amino acid starvation. In contrast, sensitive cells succumbed to Hsp70 inhibition by activating PERK. These data reveal an unexpected route through which breast cancer cells adapt to proteotoxic insults and position GCN2 and autophagy as complementary mechanisms to ensure survival when proteostasis is compromised.

The Interaction of the Tumor Suppressor FAM46C with p62 and FNDC3 Proteins Integrates Protein and Secretory Homeostasis

FAM46C is a non-canonical poly(A) polymerase uniquely mutated in up to 20% of multiple myeloma (MM) patients, implying a tissue-specific tumor suppressor function. Here, we report that FAM46C selectively stabilizes mRNAs encoding endoplasmic reticulum (ER)-targeted proteins, thereby concertedly enhancing the expression of proteins that control ER protein import, folding, N-glycosylation, and trafficking and boosting protein secretion. This role requires the interaction with the ER membrane resident proteins FNDC3A and FNDC3B. In MM cells, FAM46C expression raises secretory capacity beyond sustainability, inducing ROS accumulation, ATP shortage, and cell death. FAM46C activity is regulated through rapid proteasomal degradation or the inhibitory interaction with the ZZ domain of the autophagic receptor p62 that hinders its association with FNDC3 proteins via sequestration in p62⁺ aggregates. Altogether, our data disclose a p62/FAM46C/FNDC3 circuit coordinating sustainable secretory activity and survival, providing an explanation for the MM-specific oncosuppressive role of FAM46C and uncovering potential therapeutic opportunities against cancer.

Emerging Therapeutic Strategies to Overcome Drug Resistance in Multiple Myeloma

Simple Summary

Multiple myeloma is a deadly blood cancer, but fortunately drug development has substantially prolonged the lifespan of patients to average more than a decade after diagnosis with optimal therapy. As a result, the population of patients living with multiple myeloma has grown considerably. Through its course, patients suffer repeated relapses for which they require new lines of treatment. Currently, the key drug classes for treatment are immunomodulatory drugs, proteasome inhibitors, and monoclonal antibodies. The goal of this review is to summarize the understanding of the problem of resistance to these drugs, which is ultimately responsible for patient fatality. In addition, we will focus on how new agents that are promising in clinical trials overcome resistance.

Abstract

Multiple myeloma is a malignant plasma cell neoplasm that remains incurable and is ultimately fatal when patients acquire multi-drug resistance. Thus, advancing our understanding of the mechanisms behind drug resistance in multi-relapsed patients is critical for developing better strategies to extend their lifespan. Here, we review the understanding of resistance to the three key drug classes approved for multiple myeloma treatment: immunomodulatory drugs, proteasome inhibitors, and monoclonal antibodies. We consider how the complex, heterogenous biology of multiple myeloma may influence the acquisition of drug resistance and reflect on the gaps in knowledge where additional research is needed to improve our treatment approaches. Fortunately, many agents are currently being evaluated preclinically and in clinical trials that have the potential to overcome or delay drug resistance, including next-generation immunomodulatory drugs and proteasome inhibitors, novel small molecule drugs, chimeric antigen receptor T cells, antibody-drug conjugates, and bispecific antibodies. For each class, we discuss the potential of these strategies to overcome resistance through modifying agents within each class or new classes without cross-resistance to currently available drugs.

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.

Bifenthrin induces DNA damage and autophagy in Spodoptera frugiperda (Sf9) insect cells

Bifenthrin is one of the most commonly used synthetic pyrethroid insecticides. It targets the nervous system of insects, mainly acting on sodium channels in nerve cell membranes. The high use of bifrenthrin may lead to an increase in pest insect resistance. Additionally, there are only a few studies describing its cytotoxic action. A series of bioassays were carried out, and the results showed that bifenthrin has a significant ability to induce DNA damage and the inhibition of viability in Spodoptera frugiperda (Sf9) cells. Monodansylcadaverine staining and transmission electron microscope assays were used to observe significant levels of autophagosomes and mitochondrial dysfunction in the cytoplasm. Additionally, western blot analysis showed an upregulation in LC3-II and beclin-1 protein expression and a downregulation in p62 expression, which contributed to the cytotoxic effect of bifenthrin on Sf9 cells. Overall, bifenthrin significantly impacts the viability of Sf9 cells by inducing DNA damage and autophagy. These results provide a theoretical basis for understanding bifrenthin's mechanism of cytotoxicity.

Endoplasmic Reticulum Stress-Mediated p62 Downregulation Inhibits Apoptosis via c-Jun Upregulation

Cereblon (CRBN), a substrate receptor of cullin 4-RING E3 ligase (CRL4) regulates the ubiquitination and degradation of c-Jun, mediating the lipopolysaccharide-induced cellular response. However, the upstream signaling pathway that regulates this process is unknown. In this study, we describe how endoplasmic reticulum (ER) stress reversely regulates sequestosome-1 (p62)and c-Jun protein levels. Furthermore, our study reveals that expression of p62 attenuates c-Jun protein levels through the ubiquitin-proteasome system. Conversely, siRNA knockdown of p62 elevates c-Jun protein levels. Immunoprecipitation and immunoblotting experiments demonstrate that p62 interacts with c-Jun and CRBN to form a ternary protein complex. Moreover, we find that CRBN knockdown completely abolishes the inhibitory effect of p62 on c-Jun. Using brefeldin A as an inducer of ER stress, we demonstrate that the p62/c-Jun axis participates in the regulation of ER stress-induced apoptosis, and that CRBN is required for this regulation. In summary, we have identified an upstream signaling pathway, which regulates p62-mediated c-Jun degradation. Our findings elucidate the underlying molecular mechanism by which p62/c-Jun axis regulates the ER stress-induced apoptosis, and provide a new molecular connection between ER stress and apoptosis.

Updates to the drug-resistant mechanism of proteasome inhibitors in multiple myeloma

Proteasome inhibitors (PIs) have been a kind of backbone therapies for newly diagnosed as well as relapsed or refractory myeloma patients in the last two decades. Bortezomib, the first-in-class PI, was approved by the United States Food and Drug Administration in 2003. The key roles of this class of agents are targeting at the overstressed 26S proteasome, which are involved in the pathogenesis of the disease. Despite recent advancements in clinical antimyeloma treatment, the acquisition of resistance is a major limitation in PI therapy. This review aims at a better understanding of the pathways and biomarkers involved in MM drug resistance.

Lysine-222 succinylation reduces lysosomal degradation of lactate dehydrogenase a and is increased in gastric cancer

Background

Lysine succinylation is an emerging posttranslational modification that has garnered increased attention recently, but its role in gastric cancer (GC) remains underexplored.

Methods

Proteomic quantification of lysine succinylation was performed in human GC tissues and adjacent normal tissues by mass spectrometry. The mRNA and protein levels of lactate dehydrogenase A (LDHA) in GC and adjacent normal tissues were analyzed by qRT-PCR and western blot, respectively. The expression of K222-succinylated LDHA was measured in GC tissue microarray by the K222 succinylation-specific antibody. The interaction between LDHA and sequestosome 1 (SQSTM1) was measured by co-immunoprecipitation (co-IP) and proximity ligation assay (PLA). The binding of carnitine palmitoyltransferase 1A (CPT1A) to LDHA was determined by co-IP. The effect of K222-succinylated LDHA on tumor growth and metastasis was evaluated by in vitro and in vivo experiments.

Results

Altogether, 503 lysine succinylation sites in 303 proteins were identified. Lactate dehydrogenase A (LDHA), the key enzyme in Warburg effect, was found highly succinylated at K222 in GC. Intriguingly, this modification did not affect LDHA ubiquitination, but reduced the binding of ubiquitinated LDHA to SQSTM1, thereby decreasing its lysosomal degradation. We demonstrated that CPT1A functions as a lysine succinyltransferase that interacts with and succinylates LDHA. Moreover, high K222-succinylation of LDHA was associated with poor prognosis in patients with GC. Finally, overexpression of a succinylation-mimic mutant of LDHA promoted cell proliferation, invasion, and migration.

Conclusions

Our data revealed a novel lysosomal pathway of LDHA degradation, which is mediated by the binding of K63-ubiquitinated LDHA to SQSTM1. Strikingly, CPT1A succinylates LDHA on K222, which thereby reduces the binding and inhibits the degradation of LDHA, as well as promotes GC invasion and proliferation. This study thus uncovers a new role of lysine succinylation and the mechanism underlying LDHA upregulation in GC.

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.

Impairment of Proteasome and Autophagy Underlying the Pathogenesis of Leukodystrophy

Impairment of the ubiquitin-proteasome-system (UPS) and autophagy causing cytoplasmic aggregation of ubiquitin andp62 have been implicated in the pathogenesis of most neurodegenerative disorders, yet, they have not been fully elucidated in leukodystrophies. The relationship among impairment of UPS, autophagy, and globoid cell leukodystrophy (GLD), one of the most common demyelinating leukodystrophies, is clarified in this study. We examined the ubiquitin and autophagy markers in the brains of twitcher mice, a murine model of infantile GLD, and in human oligodendrocytes incubated with psychosine. Immunohistochemical examinations showed spatiotemporal accumulation of ubiquitin- and p62-aggregates mainly in the white matter of brain and spinal cord at disease progression. Western blot analysis demonstrated a significant accumulation of ubiquitin, p62, and LC3-II in insoluble fraction in parallel with progressive demyelination and neuroinflammation in twitcher brains. In vitro study validated a dose- and time-dependent cytotoxicity of psychosine upon autophagy and UPS machinery. Inhibition of autophagy and UPS exacerbated the accumulation of insoluble ubiquitin, p62, and LC3-II proteins mediated by psychosine cytotoxicity as well as increased cytoplasmic deposition of ubiquitin- and p62-aggregates, and accumulation of autophagosomes and autolysosomes. Further, the subsequent accumulation of reactive oxygen species and reduction of mitochondrial respiration led to cell death. Our studies validate the impairment of proteasome and autophagy underlying the pathogenesis of GLD. These findings provide a novel insight into pathogenesis of GLD and suggest a specific pathomechanism as an ideal target for therapeutic approaches.

Molecular modelling and dynamic simulations of sequestosome 1 (SQSTM1) missense mutations linked to Paget disease of bone

The Paget disease (PDB; OMIM is 167250) is a chronic bone disease caused by pathogenic mutations in Sequestome1/p62 (SQSTM1) gene. This study has aimed to interpret the relationship of PDB linked SQSTM1 mutations with protein structure and its molecular dynamic features. The disease causative missense mutations were initially collected, and then analyzed for their, exonic and domain distribution, impact on secondary and tertiary structures, and their ability on protein-ligand interactions, using a combination of systems biology approaches. Our results show that most PDB linked SQSTM1 missense mutations affect amino acid residues clustered within or near the UBA domain (aa 389-434), which participates in the ubiquitination of substrates. We also report that the majority mutations occurred in α-helices over β-strands but their effects on the secondary structure were mostly neutral. Global tertiary structure deviations were minimal; however, at amino acid residue level minor structural changes were evident. The molecular dynamics simulation analysis showed that both PB1 and UBA domains were under constant structural fluctuations resulting in closed form conformation of SQSMT1 protein structure, when it is bound to PRKCI ligand. We also found salt bridge conformation changes in the UBA domain of SQSTM1 mutants when they bound to the PRKCI interactor protein. This finding suggests the possibility that mutations in SQSTM1 could impair its ability to ubiquitinate the substrates, eventually affecting autophagy and apoptosis, especially in mature osteoclasts. This study presents the additional insight into structure and function relationship between SQSTM1 mutations and PDB pathogenesis. Communicated by Ramaswamy H. Sarma.

Endoplasmic reticulum stress and the protein degradation system in ophthalmic diseases

Objective

Endoplasmic reticulum (ER) stress is involved in the pathogenesis of various ophthalmic diseases, and ER stress-mediated degradation systems play an important role in maintaining ER homeostasis during ER stress. The purpose of this review is to explore the potential relationship between them and to find their equilibrium sites.

Design

This review illustrates the important role of reasonable regulation of the protein degradation system in ER stress-mediated ophthalmic diseases. There were 128 articles chosen for review in this study, and the keywords used for article research are ER stress, autophagy, UPS, ophthalmic disease, and ocular.

Data sources

The data are from Web of Science, PubMed, with no language restrictions from inception until 2019 Jul.

Results

The ubiquitin proteasome system (UPS) and autophagy are important degradation systems in ER stress. They can restore ER homeostasis, but if ER stress cannot be relieved in time, cell death may occur. However, they are not independent of each other, and the relationship between them is complementary. Therefore, we propose that ER stability can be achieved by adjusting the balance between them.

Conclusion

The degradation system of ER stress, UPS and autophagy are interrelated. Because an imbalance between the UPS and autophagy can cause cell death, regulating that balance may suppress ER stress and protect cells against pathological stress damage.

Autophagy and Protein Secretion

Autophagy - conventional for macroautophagy - is a major recycling strategy that ensures cellular homeostasis through the selective engulfment of cytoplasmic supramolecular cargos in double membrane vesicles and their rapid dispatch to the lysosome for digestion. As autophagy operates in the cytoplasm, its interference with secretory proteins, that is, proteins destined to the plasma membrane or the extracellular space, generally synthesized and routed within the endoplasmic reticulum (ER), has been relatively overlooked in the past. However, mounting evidence reveals that autophagy in fact heavily regulates protein secretion through diverse mechanisms. First, autophagy is closely involved in the unconventional secretion of leaderless proteins, a pool of proteins destined extracellularly, but lacking an ER-targeted leader sequence, and thus manufactured in the cytosol. Autophagy-related (ATG) genes now appear instrumental to the underlying pathways, hence the recently coined concept of secretory autophagy, or better ATG gene-dependent secretion. Indeed, ATG genes regulate unconventional protein secretion at multiple levels, ranging from intracellular inflammatory signaling, for example, through the control of mitochondrial health and inflammasome activity, to trafficking of leaderless proteins. Moreover, perhaps less expectedly, autophagy also participates in the control of conventional secretion, intersecting the secretory apparatus at multiple points, though with surprising differences among professional secretory cell types that disclose remarkable and unpredicted specificity. This review synopsizes the multiple mechanisms whereby autophagy interfaces with conventional and unconventional protein secretory pathways and discusses the relative teleology. Altogether, the diverse controls exerted on protein secretion broaden and deepen the homeostatic significance of autophagy within the cell.

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.

PTK2/FAK regulates UPS impairment via SQSTM1/p62 phosphorylation in TARDBP/TDP-43 proteinopathies

TARDBP/TDP-43 (TAR DNA binding protein) proteinopathies are a common feature in a variety of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal lobar degeneration (FTLD), and Alzheimer disease (AD). However, the molecular mechanisms underlying TARDBP-induced neurotoxicity are largely unknown. In this study, we demonstrated that TARDBP proteinopathies induce impairment in the ubiquitin proteasome system (UPS), as evidenced by an accumulation of ubiquitinated proteins and a reduction in proteasome activity in neuronal cells. Through kinase inhibitor screening, we identified PTK2/FAK (PTK2 protein tyrosine kinase 2) as a suppressor of neurotoxicity induced by UPS impairment. Importantly, PTK2 inhibition significantly reduced ubiquitin aggregates and attenuated TARDBP-induced cytotoxicity in a Drosophila model of TARDBP proteinopathies. We further identified that phosphorylation of SQSTM1/p62 (sequestosome 1) at S403 (p-SQSTM1 [S403]), a key component in the autophagic degradation of poly-ubiquitinated proteins, is increased upon TARDBP overexpression and is dependent on the activation of PTK2 in neuronal cells. Moreover, expressing a non-phosphorylated form of SQSTM1 (SQSTM1^S403A) significantly repressed the accumulation of insoluble poly-ubiquitinated proteins and neurotoxicity induced by TARDBP overexpression in neuronal cells. In addition, TBK1 (TANK binding kinase 1), a kinase that phosphorylates S403 of SQSTM1, was found to be involved in the PTK2-mediated phosphorylation of SQSTM1. Taken together, our data suggest that the PTK2-TBK1-SQSTM1 axis plays a critical role in the pathogenesis of TARDBP by regulating neurotoxicity induced by UPS impairment. Therefore, targeting the PTK2-TBK1-SQSTM1 axis may represent a novel therapeutic intervention for neurodegenerative diseases with TARDBP proteinopathies.Abbreviations: ALP: macroautophagy/autophagy lysosomal pathway; ALS: amyotrophic lateral sclerosis; ATXN2: ataxin 2; BafA1: bafilomycin A₁; cCASP3: cleaved caspase 3; CSNK2: casein kinase 2; FTLD: frontotemporal lobar degeneration; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; OPTN: optineurin; PTK2/FAK: PTK2 protein tyrosine kinase 2; SQSTM1/p62: sequestosome 1; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK binding kinase 1; ULK1: unc-51 like autophagy activating kinase 1; UPS: ubiquitin-proteasome system.

Measuring redox effects on the activities of intracellular proteases such as the 20S Proteasome and the Immuno-Proteasome with fluorogenic peptides

Proteolytic enzymes are often strongly affected by redox reactions, free radicals, oxidation, or oxidative stress. The 20S Proteasome and the Immuno-Proteasome are examples of major intracellular proteases whose concentration, transcription, translation, and proteolytic activity are all subject to redox regulation. Proteasomes are essential in maintaining overall protein homeostasis (or proteostasis), and their dysregulation results in detrimental phenotypes associated with various pathologies, including several common age-related diseases. Many studies have used Western blots to assess redox changes in Proteasome protein levels or RT-PCR to study RNA transcript levels, but actual measurements of proteolytic activity are far less common. Since each intact protein substrate exhibits a different proteolytic profile when incubated with proteasome or Immuno-Proteasome [± activators such as 19S or 11S (also called PA28)] and these proteolytic profiles are drastically altered if the protein substrate is denatured, for example by oxidation, heat, acetylation, or methylation. In an attempt to standardize proteasomal activity measurements small fluorogenic protein/peptide substrates were developed to test the three proteolytically active sites of the Proteasome and Immuno-Proteasome: trypsin-like, chymotrypsin-like, and caspase-like activities. Despite extensive use of fluorogenic peptide substrates to measure proteasome activity, there is an absence of a standardized set of best practices. In this study we analyze different parameters, such as sample concentration, AMC conjugated substrate concentration, duration of assay, and frequency of measurements, and examine how they impact the determination of Proteasome and Immuno-Proteasome activities using fluorogenic peptide substrates.

Identification of PIKfyve kinase as a target in multiple myeloma

The cellular cytotoxicity of APY0201, a PIKfyve inhibitor, against multiple myeloma was initially identified in an unbiased in vitro chemical library screen. The activity of APY0201 was confirmed in all 25 cell lines tested and in 40% of 100 ex vivo patient-derived primary samples, with increased activity in primary samples harboring trisomies and lacking t(11;14). The broad anti-multiple myeloma activity of PIKfyve inhibitors was further demonstrated in confirmatory screens and showed the superior potency of APY0201 when compared to the PIKfyve inhibitors YM201636 and apilimod, with a mid-point half maximal effective concentration (EC₅₀) at nanomolar concentrations in, respectively, 65%, 40%, and 5% of the tested cell lines. Upregulation of genes in the lysosomal pathway and increased cellular vacuolization were observed in vitro following APY0201 treatment, although these cellular effects did not correlate well with responsiveness. We confirm that PIKfyve inhibition is associated with activation of the transcription factor EB, a master regulator of lysosomal biogenesis and autophagy. Furthermore, we established an assay measuring autophagy as a predictive marker of APY0201 sensitivity. Overall, these findings indicate promising activity of PIKfyve inhibitors secondary to disruption of autophagy in multiple myeloma and suggest a strategy to enrich for likely responders.

Glioblastoma: Targeting the autophagy in tumorigenesis

Glioblastoma (GBM) is one of the most malignant and aggressive primary brain tumor, with a mean life expectancy of less than 15 months. The malignant nature of GBM prompts the need for further research on its tumorigenesis and novel treatments to improve its outcome. One of the promising research targets is autophagy, a fundamental metabolic process of degrading and recycling cellular components. Interventions to activate or inhibit autophagy have both been proposed as GBM therapies, suggesting a controversial, context-dependent role of autophagy in GBM tumorigenesis. In this review, we highlight the molecular links between GBM and autophagy with the focus on the effects of autophagy on the stemness maintenance, metabolism and proteostasis in GBM tumorigenesis. Understanding the molecular pathways involved in autophagy target is critical for GBM therapy.

Proteasome dysfunction in alveolar type 2 epithelial cells is associated with acute respiratory distress syndrome

Proteasomes are a critical component of quality control that regulate turnover of short-lived, unfolded, and misfolded proteins. Proteasome activity has been therapeutically targeted and considered as a treatment option for several chronic lung disorders including pulmonary fibrosis. Although pharmacologic inhibition of proteasome activity effectively prevents the transformation of fibroblasts to myofibroblasts, the effect on alveolar type 2 (AT2) epithelial cells is not clear. To address this knowledge gap, we generated a genetic model in which a proteasome subunit, RPT3, which promotes assembly of active 26S proteasome, was conditionally deleted in AT2 cells of mice. Partial deletion of RPT3 resulted in 26S proteasome dysfunction, leading to augmented cell stress and cell death. Acute loss of AT2 cells resulted in depletion of alveolar surfactant, disruption of the alveolar epithelial barrier and, ultimately, lethal acute respiratory distress syndrome (ARDS). This study underscores importance of proteasome function in maintenance of AT2 cell homeostasis and supports the need to further investigate the role of proteasome dysfunction in ARDS pathogenesis.

Prosurvival autophagy is regulated by protein kinase CK1 alpha in multiple myeloma

Multiple myeloma (MM) is a tumor of plasma cells (PCs). Due to the intense immunoglobulin secretion, PCs are prone to endoplasmic reticulum stress and activate several stress-managing pathways, including autophagy. Indeed, autophagy deregulation is maladaptive for MM cells, resulting in cell death. CK1α, a pro-survival kinase in MM, has recently been involved as a regulator of the autophagic flux and of the transcriptional competence of the autophagy-related transcription factor FOXO3a in several cancers. In this study, we investigated the role of CK1α in autophagy in MM. To study the autophagic flux we generated clones of MM cell lines expressing the mCherry-eGFP-LC3B fusion protein. We observed that CK1 inhibition with the chemical ATP-competitive CK1 α/δ inhibitor D4476 resulted in an impaired autophagic flux, likely due to an alteration of lysosomes acidification. However, D4476 caused the accumulation of the transcription factor FOXO3a in the nucleus, and this was paralleled by the upregulation of mRNA coding for autophagic genes. Surprisingly, silencing of CK1α by RNA interference triggered the autophagic flux. However, FOXO3a did not shuttle into the nucleus and the transcription of autophagy-related FOXO3a-dependent genes was not observed. Thus, while the chemical inhibition with the dual CK1α/δ inhibitor D4476 induced cell death as a consequence of an accumulation of ineffective autophagic vesicles, on the opposite, CK1α silencing, although it also determined apoptosis, triggered a full activation of the early autophagic flux, which was then not supported by the upregulation of autophagic genes. Taken together, our results indicate that the family of CK1 kinases may profoundly influence MM cells survival also through the modulation of the autophagic pathway.

Ubiquitin-activating enzyme inhibition induces an unfolded protein response and overcomes drug resistance in myeloma

Three proteasome inhibitors have garnered regulatory approvals in various multiple myeloma settings; but drug resistance is an emerging challenge, prompting interest in blocking upstream components of the ubiquitin-proteasome pathway. One such attractive target is the E1 ubiquitin-activating enzyme (UAE); we therefore evaluated the activity of TAK-243, a novel and specific UAE inhibitor. TAK-243 potently suppressed myeloma cell line growth, induced apoptosis, and activated caspases while decreasing the abundance of ubiquitin-protein conjugates. This was accompanied by stabilization of many short-lived proteins, including p53, myeloid cell leukemia 1 (MCL-1), and c-MYC, and activation of the activating transcription factor 6 (ATF-6), inositol-requiring enzyme 1 (IRE-1), and protein kinase RNA-like endoplasmic reticulum (ER) kinase (PERK) arms of the ER stress response pathway, as well as oxidative stress. UAE inhibition showed comparable activity against otherwise isogenic cell lines with wild-type (WT) or deleted p53 despite induction of TP53 signaling in WT cells. Notably, TAK-243 overcame resistance to conventional drugs and novel agents in cell-line models, including bortezomib and carfilzomib resistance, and showed activity against primary cells from relapsed/refractory myeloma patients. In addition, TAK-243 showed strong synergy with a number of antimyeloma agents, including doxorubicin, melphalan, and panobinostat as measured by low combination indices. Finally, TAK-243 was active against a number of in vivo myeloma models in association with activation of ER stress. Taken together, the data support the conclusion that UAE inhibition could be an attractive strategy to move forward to the clinic for patients with relapsed and/or refractory multiple myeloma.

Cigarette smoke induces ROS mediated autophagy impairment in human corneal epithelial cells

Cigarette smoke is an important indoor air pollutant which has deleterious effects on human health. Continued daily exposure to cigarette smoke has been attributed to the risk factor of ocular surface diseases. However, the mechanisms underlying the ocular surface damage are not fully elucidated. In this study, exposure to cigarette smoke extract (CSE) induced a dose- and time-dependent cytotoxicity in human corneal epithelial (HCE) cells, supported by the observation of reduced cell viability, increased apoptotic cells, elevated intracellular oxidative stress and loss of mitochondrial transmembrane potential. In addition, CSE exposure led to the impairment of proteostasis and autophagy, which resulted in the accumulation of ubiquitinated proteins as aggregates in peri-nuclear spaces. Furthermore, the autophagy inducer, cysteamine was shown to attenuate the CSE induced cell damage, oxidative stress and mitochondrial dysfunction in HCE cells. Moreover, cysteamine inhibited the formation of ubiquitin-positive aggregates around the peri-nuclear region, through regulating the autophagic activity of HCE cells. Similar to in vitro experiments, cigarette smoke induced proteostasis and autophagy impairment in corneal epithelial cells could be rescued by cysteamine in a cigarette smoke-exposed murine model. Therefore, this study may provide first evidence that dysfunction of autophagy contributes to the pathogenesis of ocular surface diseases associated with cigarette smoke exposure. Besides, it also suggests the potential therapeutic value of cysteamine in the prevention and treatment of cigarette smoke induced ocular surface injury. CSE induces cytotoxicity and accumulation of ubiquitinated proteins in HCE cells due to impairment of proteostasis and autophagy, which can be rescued by cysteamine.

Therapeutic Modulation of Autophagy in Leukaemia and Lymphoma. Cells. 2019;8. [PMID: 30704144 PMCID: PMC6406467 DOI: 10.3390/cells8020103]

Haematopoiesis is a tightly orchestrated process where a pool of hematopoietic stem and progenitor cells (HSPCs) with high self-renewal potential can give rise to both lymphoid and myeloid lineages. The HSPCs pool is reduced with ageing resulting in few HSPC clones maintaining haematopoiesis thereby reducing blood cell diversity, a phenomenon called clonal haematopoiesis. Clonal expansion of HSPCs carrying specific genetic mutations leads to increased risk for haematological malignancies. Therefore, it comes as no surprise that hematopoietic tumours develop in higher frequency in elderly people. Unfortunately, elderly patients with leukaemia or lymphoma still have an unsatisfactory prognosis compared to younger ones highlighting the need to develop more efficient therapies for this group of patients. Growing evidence indicates that macroautophagy (hereafter referred to as autophagy) is essential for health and longevity. This review is focusing on the role of autophagy in normal haematopoiesis as well as in leukaemia and lymphoma development. Attenuated autophagy may support early hematopoietic neoplasia whereas activation of autophagy in later stages of tumour development and in response to a variety of therapies rather triggers a pro-tumoral response. Novel insights into the role of autophagy in haematopoiesis will be discussed in light of designing new autophagy modulating therapies in hematopoietic cancers.

The Autophagy-Lysosomal Pathways and Their Emerging Roles in Modulating Proteostasis in Tumors

In normal physiological condition, the maintenance of cellular proteostasis is a prerequisite for cell growth, functioning, adapting to changing micro-environments, and responding to extracellular stress. Cellular proteostasis is maintained by specific proteostasis networks (PNs) to prevent protein misfolding, aggregating, and accumulating in subcellular compartments. Commonly, the PNs are composed of protein synthesis, molecular chaperones, endoplasmic reticulum (ER), unfolded protein response (UPR), stress response pathways (SRPs), secretions, ubiquitin proteasome system (UPS), and autophagy-lysosomal pathways (ALPs). Although great efforts have been made to explore the underlying detailed mechanisms of proteostasis, there are many questions remain to explore, especially in proteostasis regulated by the ALPs. Proteostasis out-off-balance is correlated with various human diseases such as diabetes, stroke, inflammation, hypertension, pulmonary fibrosis, and Alzheimer’s disease. Enhanced regulation of PNs is observed in tumors, thereby indicating that proteostasis may play a pivotal role in tumorigenesis and cancer development. Recently, inhibitors targeting the UPS have shown to be failed in solid tumor treatment. However, there is growing evidence showing that the ALPs play important roles in regulation of proteostasis alone or with a crosstalk with other PNs in tumors. In this review, we provide insights into the proteostatic process and how it is regulated by the ALPs, such as macroautophagy, aggrephagy, chaperone-mediated autophagy, microautophagy, as well as mitophagy during tumor development.

Serum starvation raises turnover of phosphorylated p62/SQSTM1 (Serine 349), reveals expression of proteasome and N-glycanase1 interactive protein RAD23B and sensitizes human synovial fibroblasts to BAY 11-7085-induced cell death

Phosphorylation of p62/SQSTM1 (p62) on Serine 349 (P-Ser349 p62) as well as proteasome dysfunction have been shown to activate the cell protective Keap1/Nrf2 pathway. We showed previously that BAY 11-7085-induced human synovial fibroblast cell death includes autophagy and p62 downregulation. In this work, we have studied expression of P-Ser349 p62 in human synovial fibroblasts. Results showed that P-Ser349 p62 was not detected in synovial cell extracts unless cells were cultured in the presence of proteasome inhibitor (MG132). MG132 revealed P-Ser349 p62 turnover, that was further increased by concomitant autophagy inhibition and markedly enhanced in serum starved cells. Starvation sensitized synovial fibroblasts to BAY 11-7085 while MG132 protected both non-starved and starved cells from BAY 11-7085-induced cell death. Lentivirus mediated overexpression of phosphorylation-mimetic p62 mutant S349E markedly protected synovial fibroblasts from BAY 11-7085. Inhibitor of Keap1-P-S349 p62 interaction, K67, had synergistic effect with MG132. Starvation increased p62 molecular weight, that was reversed by serum and bovine serum albumin re-feeding. Furthermore, starvation markedly induced RAD23B. Increased endo-β-N-acetylglucosaminidase (ENGase) turnover was detected in starved synovial fibroblasts. PNGase F treatment produced faster migration p62 form in human synovial tissue extracts but starvation-like p62 form of higher molecular weight in synovial cell extracts. Co-transfection of NGLY1, with p62 or p62 mutants S349A and S349E markedly stabilized p62 expressions in HEK293 cells. Tunicamycin upregulated p62 and protected synovial fibroblasts from BAY 11-7085-induced cell death. These results showed that P-Ser349 p62 has pro-survival role in human synovial fibroblasts and that de-glycosylation events are involved in p62 turnover.

The Ins and Outs of Autophagy and Metabolism in Hematopoietic and Leukemic Stem Cells: Food for Thought

Discovered over fifty years ago, autophagy is a double-edged blade. On one hand, it regulates cellular energy sources by "cannibalization" of its own cellular components, feeding on proteins and other unused cytoplasmic factors. On the other, it is a recycling process that removes dangerous waste from the cytoplasm keeping the cell clean and healthy. Failure of the autophagic machinery is translated in dysfunction of the immune response, in aging, and in the progression of pathologies such as Parkinson disease, diabetes, and cancer. Further investigation identified autophagy with a protective role in specific types of cancer, whereas in other cases it can promote tumorigenesis. Evidence shows that treatment with chemotherapeutics can upregulate autophagy in order to maintain a stable intracellular environment promoting drug resistance and cell survival. Leukemia, a blood derived cancer, represents one of the malignancies in which autophagy is responsible for drug treatment failure. Inhibition of autophagy is becoming a strategic target for leukemic stem cell (LSC) eradication. Interestingly, the latest findings demonstrate that LSCs show higher levels of mitochondrial metabolism compared to normal stem cells. With this review, we aim to explore the links between autophagy and metabolism in the hematopoietic system, with special focus on primitive LSCs.

Compensatory increases of select proteostasis networks after Hsp70 inhibition in cancer cells

Cancer cells thrive when challenged with proteotoxic stress by inducing components of the protein folding, proteasome, autophagy and unfolded protein response (UPR) pathways. Consequently, specific molecular chaperones have been validated as targets for anti-cancer therapies. For example, inhibition of Hsp70 family proteins (hereafter Hsp70) in rhabdomyosarcoma triggers UPR induction and apoptosis. To define how these cancer cells respond to compromised proteostasis, we compared rhabdomyosarcoma cells that were sensitive (RMS13) or resistant (RMS13-R) to the Hsp70 inhibitor MAL3-101. We discovered that endoplasmic reticulum-associated degradation (ERAD) and autophagy were activated in RMS13-R cells, suggesting that resistant cells overcome Hsp70 ablation by increasing misfolded protein degradation. Indeed, RMS13-R cells degraded ERAD substrates more rapidly than RMS cells and induced the autophagy pathway. Surprisingly, inhibition of the proteasome or ERAD had no effect on RMS13-R cell survival, but silencing of select autophagy components or treatment with autophagy inhibitors restored MAL3-101 sensitivity and led to apoptosis. These data indicate a route through which cancer cells overcome a chaperone-based therapy, define how cells can adapt to Hsp70 inhibition, and demonstrate the value of combined chaperone and autophagy-based therapies.This article has an associated First Person interview with the first author of the paper.

Autophagy mediates epithelial cancer chemoresistance by reducing p62/SQSTM1 accumulation

To cope with intrinsic and environmental stress, cancer cells rely on adaptive pathways more than non-transformed counterparts. Such non-oncogene addiction offers new therapeutic targets and strategies to overcome chemoresistance. In an attempt to study the role of adaptive pathways in acquired drug resistance in carcinoma cells, we devised a model of in vitro conditioning to three standard chemotherapeutic agents, cisplatin, 5-fluorouracil, and docetaxel, from the epithelial cancer cell line, HEp-2, and investigated the mechanisms underlying reduced drug sensitivity. We found that triple-resistant cells suffered from higher levels of oxidative stress, and showed heightened anti-stress responses, including the antioxidant Nrf2 pathway and autophagy, a conserved pleiotropic homeostatic strategy, mediating the clearance of aggregates marked by the adapter p62/SQSTM1. As a result, re-administration of chemotherapeutic agents failed to induce further accumulation of reactive oxygen species and p62. Moreover, autophagy proved responsible for chemoresistance through the avoidance of p62 accumulation into toxic protein aggregates. Indeed, p62 ablation was sufficient to confer resistance in parental cells, and genetic and pharmacological autophagic inhibition restored drug sensitivity in resistant cells in a p62-dependent manner. Finally, exogenous expression of mutant p62 lacking the ubiquitin- and LC3-binding domains, required for autophagic engulfment, increased chemosensitivity in TDR HEp-2 cells. Altogether, these findings offer a cellular system to investigate the bases of acquired chemoresistance of epithelial cancers and encourage challenging the prognostic and antineoplastic therapeutic potential of p62 toxicity.

Multiple Myeloma and Bone: The Fatal Interaction

Multiple myeloma (MM) is the second-most-common hematologic malignancy and the most frequent cancer to involve bone. MM bone disease (MMBD) has devastating consequences for patients, including dramatic bone loss, severe bone pain, and pathological fractures that markedly decrease the quality of life and impact survival of MM patients. MMBD results from excessive osteoclastic bone resorption and persistent suppressed osteoblastic bone formation, causing lytic lesions that do not heal, even when patients are in complete and prolonged remission. This review discusses the cellular and molecular mechanisms that regulate the uncoupling of bone remodeling in MM, the effects of MMBD on tumor growth, and potential therapeutic approaches that may prevent severe bone loss and repair damaged bone in MM patients.