Potent activity of carfilzomib, a novel, irreversible inhibitor of the ubiquitin-proteasome pathway, against preclinical models of multiple myeloma

A comprehensive overview of daratumumab and carfilzomib and the recently approved daratumumab, carfilzomib and dexamethasone regimen in relapsed/refractory multiple myeloma

Introduction: Novel, effective regimens are needed in patients with relapsed and refractory myeloma (RRMM) who inevitably relapse after PI and IMID containing treatment. Areas covered: Pre-clinical data, early clinical and pivotal trials relevant to the development of the two backbone drugs of carfilzomib and daratumumab, and the two important recent trials, EQUULEUS and CANDOR leading to the FDA approval of the combination regimen of daratumumab, carfilzomib, and dexamethasone (DKd) for RRMM are detailed in this review. Expert opinion: EQUULEUS and CANDOR have established the efficacy of the DKd regimen in the landscape of bortezomib and lenalidomide refractory patients. The split dosing schedule of the first dose of daratumumab was approved by the FDA based on EQUULEUS, significantly improving patient convenience. Subcutaneous daratumumab is being evaluated in this combination to further improve tolerance and convenience. Further studies are needed to evaluate and optimally sequence the many effective and potent drugs available in RRMM.

Blockade of PLD1 potentiates the antitumor effects of bortezomib in multiple myeloma cells by inhibiting the mTOR/NF-κB signal pathway

OBJECTIVE

Phospholipase D1 (PLD1) is an enzyme of the phospholipase D (PLD) superfamily. It is involved in the occurrence of various tumors. However, its role in multiple myeloma (MM) remained undefined. This study aimed to investigate the mechanism of PLD1 in the therapy of myeloma disease.

MATERIAL AND METHODS

Cell lines U266 and H929 were treated with PLD1 specific inhibitor VU0359595 combined bortezomib, a proteasome inhibitor. Their effects on MM cell proliferation, apoptosis, and relevant signal pathways of apoptosis were determined by cell counting kit-8 (CCK-8), real-time polymerase reaction chain (RT-PCR), ATP assay, and western blot.

RESULTS

PLD1 was highly expressed in U266 and H929 cells. VU0359595 didn't affect the proliferation and apoptosis of MM cells. However, VU0359595 could enhance growth inhibition, decreasing mitochondrial membrane potentials (MMPs) and ATP levels of bortezomib treated MM cells. VU0359595 also strengthened bortezomib-induced apoptosis via activating caspase-8, caspase-9, caspase-3; and down-regulating the expressions of anti-apoptosis proteins BCL-2. In addition, the bortezomib-induced cytotoxicity on MM cells was significantly augmented by VU0359595 through efficient suppression of the mTOR/NF-κB signal pathway.

CONCLUSION

PLD1 inhibition can remarkably exert antitumor effects with bortezomib on MM, which is a novel potentially targeting therapeutic agent, especially for drug-resistant MM patients.

Ubiquitin-proteasome system (UPS) as a target for anticancer treatment

The ubiquitin-proteasome system (UPS) plays an important role in the cellular processes for protein quality control and homeostasis. Dysregulation of the UPS has been implicated in numerous diseases, including cancer. Indeed, components of UPS are frequently mutated or abnormally expressed in various cancers. Since Bortezomib, a proteasome inhibitor, received FDA approval for the treatment of multiple myeloma and mantle cell lymphoma, increasing numbers of researchers have been seeking drugs targeting the UPS as a cancer therapeutic strategy. Here, we introduce the essential component of UPS, including ubiquitinating enzymes, deubiquitinating enzymes and 26S proteasome, and we summarize their targets and mechanisms that are crucial for tumorigenesis. In addition, we briefly discuss some UPS inhibitors, which are currently in clinical trials as cancer therapeutics.

Characterization of PMI-5011 on the Regulation of Deubiquitinating Enzyme Activity in Multiple Myeloma Cell Extracts

Deubiquitinating enzyme (DUB)-targeted therapeutics have shown promise in recent years as alternative cancer therapeutics, especially when coupled with proteasome-based inhibitors. While a majority of DUB-based therapeutics function by inhibiting DUB enzymes, studies show that positive regulation of these enzymes can stabilize levels of protein degradation. Unfortunately, there are currently no clinically available therapeutics for this purpose. The goal of this work was to understand the effect of a botanical extract from Artemisia dracunculus L called PMI-5011 on DUB activity in cancer cells. Through a series of kinetic analyses and mathematical modeling, it was found that PMI-5011 positively regulated DUB activity in two model multiple myeloma cells line (OPM2 and MM.1S). This suggests that PMI-5011 interacts with the active domains of DUBs to enhance their activity directly or indirectly, without apparently affecting cellular viability. Similar kinetic profiles of DUB activity were observed with three bioactive compounds in PMI-5011 (DMC-1, DMC-2, davidigenin). Interestingly, a differential cell line-independent trend was observed at higher concentrations which suggested variances in inherent gene expressions of UCHL1, UCHL5, USP7, USP15, USP14, and Rpn11 in OPM2 and MM.1S cell lines. These findings highlight the therapeutic potential of PMI-5011 and its selected bioactive compounds in cancer.

TOP2A expression predicts responsiveness to carfilzomib in myeloma and informs novel combinatorial strategies for enhanced proteasome inhibitor cell killing

Microarray was utilized to determine if a genetic signature associated with resistance to carfilzomib (CFZ) could be identified. Twelve human myeloma (MM) cell lines (HMCLs) were treated with CFZ and a cell-viability profile was assessed categorizing HMCLs as sensitive or resistant to CFZ. The gene expression profiles (GEP) of untreated resistant versus sensitive HMCLs revealed 29 differentially expressed genes. TOP2A, an enzyme involved in cell cycle and proliferation, was overexpressed in carfilzomib-resistant HMCLs. TOP2A protein expression levels, evaluated utilizing trephine biopsy specimens acquired prior to treatment with proteasome inhibitors, were higher in patients failing to achieve a response when compared to responding patients. Logistic-regression analysis confirmed that TOP2A protein expression was a highly significant predictor of response to PIs (AUC 0.738). Further, the combination of CFZ with TOP2A inhibitors, demonstrated synergistic cytotoxic effects in vitro, providing a rationale for combining topoisomerase inhibitors with CFZ to overcome resistance in MM.

Therapeutic Targeting of MMP-12 for the Treatment of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a lung disorder characterized by progressive airflow obstruction associated with inflammation and emphysema, and it is currently one of the leading causes of death worldwide. Recent studies with genetically engineered mice reported that during pulmonary inflammation, basophil-derived interleukin-4 can act on lung-infiltrating monocytes causing aberrant expression of the matrix metalloproteinase-12 (MMP-12). MMP-12 activity in turn causes the destruction of alveolar walls leading to emphysema, making it potentially a valid target for pharmacological intervention. Using nuclear magnetic resonance (NMR)- and structure-based optimizations, the current study reports on the optimized novel, potent, and selective MMP-12 inhibitors with single-digit nanomolar affinity in vitro and in vivo efficacy. Using a murine model of elastase-induced emphysema we demonstrated that the most potent agents exhibited a significant decrease in emphysema-like pathology compared to vehicle-treated mice, thus suggesting that the reported agents may potentially be translated into novel therapeutics for the treatment of COPD.

Fluorescent Probes with Unnatural Amino Acids to Monitor Proteasome Activity in Real-Time

The proteasome is an essential protein complex that, when dysregulated, can result in various diseases in eukaryotic cells. As such, understanding the enzymatic activity of the proteasome and what can alter it is crucial to elucidating its roles in these diseases. This can be done effectively by using activity-based fluorescent substrate probes, of which there are many commercially available that target the individual protease-like subunits in the 20S CP of the proteasome. Unfortunately, these probes have not displayed appropriate characteristics for their use in live cell-based assays. In the work presented here, we have developed a set of probes which have shown improved fluorescence properties and selectivity toward the proteasome compared to other cellular proteases. By including unnatural amino acids, we have found probes which can be utilized in various applications, including monitoring the effects of small molecule stimulators of the proteasome in live cells and comparing the relative proteasome activity across different cancer cell types. In future studies, we expect the fluorescent probes presented here will serve as tools to support the discovery and characterization of small molecule modulators of proteasome activity.

Carfilzomib or bortezomib in combination with lenalidomide and dexamethasone for patients with newly diagnosed multiple myeloma without intention for immediate autologous stem-cell transplantation (ENDURANCE): a multicentre, open-label, phase 3, randomised, controlled trial

BACKGROUND

Bortezomib, lenalidomide, and dexamethasone (VRd) is a standard therapy for newly diagnosed multiple myeloma. Carfilzomib, a next-generation proteasome inhibitor, in combination with lenalidomide and dexamethasone (KRd), has shown promising efficacy in phase 2 trials and might improve outcomes compared with VRd. We aimed to assess whether the KRd regimen is superior to the VRd regimen in the treatment of newly diagnosed multiple myeloma in patients who were not being considered for immediate autologous stem-cell transplantation (ASCT).

METHODS

In this multicentre, open-label, phase 3, randomised controlled trial (the ENDURANCE trial; E1A11), we recruited patients aged 18 years or older with newly diagnosed multiple myeloma who were ineligible for, or did not intend to have, immediate ASCT. Participants were recruited from 272 community oncology practices or academic medical centres in the USA. Key inclusion criteria were the absence of high-risk multiple myeloma and an Eastern Cooperative Oncology Group performance status of 0-2. Enrolled patients were randomly assigned (1:1) centrally by use of permuted blocks to receive induction therapy with either the VRd regimen or the KRd regimen for 36 weeks. Patients who completed induction therapy were then randomly assigned (1:1) a second time to either indefinite maintenance or 2 years of maintenance with lenalidomide. Randomisation was stratified by intent for ASCT at disease progression for the first randomisation and by the induction therapy received for the second randomisation. Allocation was not masked to investigators or patients. For 12 cycles of 3 weeks, patients in the VRd group received 1·3 mg/m² of bortezomib subcutaneously or intravenously on days 1, 4, 8, and 11 of cycles 1-8, and day 1 and day 8 of cycles nine to twelve, 25 mg of oral lenalidomide on days 1-14, and 20 mg of oral dexamethasone on days 1, 2, 4, 5, 8, 9, 11, and 12. For nine cycles of 4 weeks, patients in the KRd group received 36 mg/m² of intravenous carfilzomib on days 1, 2, 8, 9, 15, and 16, 25 mg of oral lenalidomide on days 1-21, and 40 mg of oral dexamethasone on days 1, 8, 15, and 22. The coprimary endpoints were progression-free survival in the induction phase, and overall survival in the maintenance phase. The primary analysis was done in the intention-to-treat population and safety was assessed in patients who received at least one dose of their assigned treatment. The trial is registered with ClinicalTrials.gov, NCT01863550. Study recruitment is complete, and follow-up of the maintenance phase is ongoing.

FINDINGS

Between Dec 6, 2013, and Feb 6, 2019, 1087 patients were enrolled and randomly assigned to either the VRd regimen (n=542) or the KRd regimen (n=545). At a median follow-up of 9 months (IQR 5-23), at a second planned interim analysis, the median progression-free survival was 34·6 months (95% CI 28·8-37·8) in the KRd group and 34·4 months (30·1-not estimable) in the VRd group (hazard ratio [HR] 1·04, 95% CI 0·83-1·31; p=0·74). Median overall survival has not been reached in either group. The most common grade 3-4 treatment-related non-haematological adverse events included fatigue (34 [6%] of 527 patients in the VRd group vs 29 [6%] of 526 in the KRd group), hyperglycaemia (23 [4%] vs 34 [6%]), diarrhoea (23 [5%] vs 16 [3%]), peripheral neuropathy (44 [8%] vs four [<1%]), dyspnoea (nine [2%] vs 38 [7%]), and thromboembolic events (11 [2%] vs 26 [5%]). Treatment-related deaths occurred in two patients (<1%) in the VRd group (one cardiotoxicity and one secondary cancer) and 11 (2%) in the KRd group (four cardiotoxicity, two acute kidney failure, one liver toxicity, two respiratory failure, one thromboembolic event, and one sudden death).

INTERPRETATION

The KRd regimen did not improve progression-free survival compared with the VRd regimen in patients with newly diagnosed multiple myeloma, and had more toxicity. The VRd triplet regimen remains the standard of care for induction therapy for patients with standard-risk and intermediate-risk newly diagnosed multiple myeloma, and is a suitable treatment backbone for the development of combinations of four drugs.

FUNDING

US National Institutes of Health, National Cancer Institute, and Amgen.

Recent insights how combined inhibition of immuno/proteasome subunits enables therapeutic efficacy

The proteasome is a multicatalytic protease in the cytosol and nucleus of all eukaryotic cells that controls numerous cellular processes through regulated protein degradation. Proteasome inhibitors have significantly improved the survival of multiple myeloma patients. However, clinically approved proteasome inhibitors have failed to show efficacy against solid tumors, neither alone nor in combination with other therapies. Targeting the immunoproteasome with selective inhibitors has been therapeutically effective in preclinical models for several autoimmune diseases and colon cancer. Moreover, immunoproteasome inhibitors prevented the chronic rejection of allogeneic organ transplants. In recent years, it has become apparent that inhibition of one single active center of the proteasome is insufficient to achieve therapeutic benefits. In this review we summarize the latest insights how targeting multiple catalytically active proteasome subunits can interfere with disease progression in autoimmunity, growth of solid tumors, and allograft rejection.

Current and Emerging Treatments for Waldenström Macroglobulinemia

Waldenström macroglobulinemia (WM) is a rare lymphoplasmacytic lymphoma. The primary goal of therapy is to reduce symptoms related to direct infiltration of the bone marrow and decrease monoclonal IgM-associated complications. Active agents in the management of WM can be broadly classified as rituximab-alkylator combination therapy, proteasome inhibitor-based therapy, and Bruton's tyrosine kinase inhibitor-based therapy. MYD88L265P and CXCR4 genetic status are pivotal for tailoring treatment options. Ibrutinib is a suitable treatment option for both treatment-naïve and relapsing WM patients. Recent advances in the intracellular B cell and cytokine signaling pathways have contributed to the development of novel therapeutic strategies. Current clinical trials are promising and may further advance WM-directed therapy.

Concise Chemoenzymatic Total Synthesis and Identification of Cellular Targets of Cepafungin I

The natural product cepafungin I was recently reported to be one of the most potent covalent inhibitors of the 20S proteasome core particle through a series of in vitro activity assays. Here, we report a short chemoenzymatic total synthesis of cepafungin I featuring the use of a regioselective enzymatic oxidation to prepare a key hydroxylated amino acid building block in a scalable fashion. The strategy developed herein enabled access to a chemoproteomic probe, which in turn revealed the exceptional selectivity and potency of cepafungin I toward the β2 and β5 subunits of the proteasome. Further structure-activity relationship studies suggest the key role of the hydroxyl group in the macrocycle and the identity of the lipid tail in modulating the potency of this natural product family. This study lays the groundwork for further medicinal chemistry exploration to fully realize the anticancer potential of cepafungin I.

Characterization of platinum(II) complexes exhibiting inhibitory activity against the 20S proteasome

Proteasome inhibitors are useful for biochemical research and clinical treatment. In our previous study, we reported that the 4N-coordinated platinum complexes with anthracenyl ring and heterocycle exhibited proteasome-inhibitory activity. In the present study, the structure-activity relationships and characterization of these complexes were determined for the elucidation of the role of aromatic ligands. Lineweaver-Burk analysis revealed that the chemical structure of heterocycles affects the binding mode of platinum complexes. Platinum complexes with anthracenyl ring and pyridine showed competitive inhibition, although platinum complexes with anthracenyl ring and phenanthroline showed non-competitive inhibition. The structure-activity relationships demonstrated that anthracenyl moiety plays a crucial role in proteasome-inhibitory activity. The platinum complexes with naphthyl or phenyl rings exhibited lower inhibitory activities than the platinum complex with anthracenyl ring. The reactivity with N-acetylcysteine varied according to the chemical structure of complexes.

Role of the Ubiquitin Proteasome System in the Regulation of Blood Pressure: A Review

The kidney and the vasculature play crucial roles in regulating blood pressure. The ubiquitin proteasome system (UPS), a multienzyme process mediating covalent conjugation of the 76-amino acid polypeptide ubiquitin to a substrate protein followed by proteasomal degradation, is involved in multiple cellular processes by regulating protein turnover in various tissues. Increasing evidence demonstrates the roles of UPS in blood pressure regulation. In the kidney, filtered sodium is reabsorbed through diverse sodium transporters and channels along renal tubules, and studies conducted till date have provided insights into the complex molecular network through which ubiquitin ligases modulate sodium transport in different segments. Components of these pathways include ubiquitin ligase neuronal precursor cell-expressed developmentally downregulated 4-2, Cullin-3, and Kelch-like 3. Moreover, accumulating data indicate the roles of UPS in blood vessels, where it modulates nitric oxide bioavailability and vasoconstriction. Cullin-3 not only regulates renal salt reabsorption but also controls vascular tone using different adaptor proteins that target distinct substrates in vascular smooth muscle cells. In endothelial cells, UPS can also contribute to blood pressure regulation by modulating endothelial nitric oxide synthase. In this review, we summarize current knowledge regarding the role of UPS in blood pressure regulation, focusing on renal sodium reabsorption and vascular function.

Exploring the Proteolysis Mechanism of the Proteasomes

The proteasome is a key protease in the eukaryotic cells which is responsible for various important cellular processes such as the control of the cell cycle, immune responses, protein homeostasis, inflammation, apoptosis, and the response to proteotoxic stress. Acting as a major molecular machine for protein degradation, proteasome first identifies damaged or obsolete regulatory proteins by attaching ubiquitin chains and subsequently utilizes conserved pore loops of the heterohexameric ring of AAA+ (ATPases associated with diverse cellular activities) to pull and mechanically unfold and translocate the misfolded protein to the active site for proteolysis. A detailed knowledge of the reaction mechanism for this proteasomal proteolysis is of central importance, both for fundamental understanding and for drug discovery. The present study investigates the mechanism of the proteolysis by the proteasome with full consideration of the protein's flexibility and its impact on the reaction free energy. Major attention is paid to the role of the protein electrostatics in determining the activation barriers. The reaction mechanism is studied by considering a small artificial fluorogenic peptide substrate (Suc-LLVY-AMC) and evaluating the activation barriers and reaction free energies for the acylation and deacylation steps, by using the empirical valence bond method. Our results shed light on the proteolysis mechanism and thus should be important for further studies of the proteasome action.

Knowledge-based structural models of SARS-CoV-2 proteins and their complexes with potential drugs

The World Health Organization (WHO) has declared the coronavirus disease 2019 (COVID‐19) caused by the novel coronavirus SARS‐CoV‐2 a pandemic. There is, however, no confirmed anti‐COVID‐19 therapeutic currently. In order to assist structure‐based discovery efforts for repurposing drugs against this disease, we constructed knowledge‐based models of SARS‐CoV‐2 proteins and compared the ligand molecules in the template structures with approved/experimental drugs and components of natural medicines. Our theoretical models suggest several drugs, such as carfilzomib, sinefungin, tecadenoson, and trabodenoson, that could be further investigated for their potential for treating COVID‐19.

Proteomic analysis identifies mechanism(s) of overcoming bortezomib resistance via targeting ubiquitin receptor Rpn13

Our prior study showed that inhibition of 19S proteasome-associated ubiquitin receptor Rpn13 can overcome bortezomib resistance in MM cells. Here, we performed proteomic analysis of Rpn13 inhibitor (RA190)-treated MM cells and identified an antioxidant enzyme superoxide dismutase (SOD1) as a mediator of Rpn13 signaling. SOD1 levels are higher in MM patient cells versus normal PBMCs; and importantly, SOD1 expression correlates with the progression of disease and shorter survival. Functional validation studies show that RA190-induced cytotoxicity in bortezomib-sensitive and -resistant MM cells is associated with decrease in SOD1 levels; conversely, forced expression of SOD1 inhibits RA190-induced cell death. Genetic knockdown and biochemical blockade of SOD1 with LCS-1 sensitizes bortezomib-resistant MM cells to bortezomib. SOD1 inhibitor LCS-1 decreases viability in MM cell lines and patient cells. LCS-1-induced cell death is associated with: (1) increase in superoxide and ROS levels; (2) activation of caspases, and p53/p21 signaling; (3) decrease in MCL-1, BCL_xL, CDC2, cyclin-B1, and c-Myc; (4) ER stress response; and (5) inhibition of proteasome function. In animal model studies, LCS-1 inhibits xenografted bortezomib-resistant human MM cell growth and prolongs host survival. Our studies therefore show that targeting Rpn13 overcomes bortezomib resistance by decreasing cellular SOD1 levels, and provide the rationale for novel therapeutics targeting SOD1 to improve patient outcome in MM.

Therapeutic potential of isatuximab in the treatment of multiple myeloma: Evidence to date

Management of multiple myeloma represents an ever changing paradigm with monoclonal antibodies adding the ability to treat patients with 3 and 4 drug regimens with acceptable toxicity profiles. In recent years, we have seen the FDA approve a number of regimens with both elotuzumab and daratumumab in combination with the standard approaches of immunomodulatory drugs, proteasome inhibitors, and steroids. Isatuximab is a naked, humanized IgG1 monoclonal antibody directed against CD38. With the recent FDA approval in March 2020, we seek to summarize the presented data to date and where this drug will fit into the future gestalt of myeloma therapy.

Proteasome inhibitors and Smac mimetics cooperate to induce cell death in diffuse large B-cell lymphoma by stabilizing NOXA and triggering mitochondrial apoptosis

Copy number gains and increased expression levels of cellular Inhibitor of Apoptosis protein (cIAP)1 and cIAP2 have been identified in primary diffuse large B-cell lymphoma (DLBCL) tissues. Second mitochondria-derived activator of caspases (Smac) mimetics were designed to antagonize IAP proteins. However, since their effect as single agents is limited, combination treatment represents a strategy for their clinical development. Therefore, we investigated the Smac mimetic BV6 in combination with proteasome inhibitors and analyzed the molecular mechanisms of action. We discovered that BV6 treatment sensitizes DLBCL cells to proteasome inhibition. We show a synergistic decrease in cell viability and induction of apoptosis by BV6/Carfilzomib (CFZ) treatment, which was confirmed by calculation of combination index (CI) and Bliss score. BV6 and CFZ acted together to trigger activation of BAX and BAK, which facilitated cell death, as knockdown of BAX and BAK significantly reduced BV6/CFZ-mediated cell death. Activation of BAX and BAK was accompanied by loss of mitochondrial membrane potential (MMP) and activation of caspases. Pretreatment with the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD.fmk) rescued BV6/CFZ-induced cell death, confirming caspase dependency. Treatment with CFZ alone or in combination with BV6 caused accumulation of NOXA, which was required for cell death, as gene silencing by siRNA or Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9-mediated NOXA inactivation inhibited BV6/CFZ-induced cell death. Together, these experiments indicate that BV6 and CFZ cooperatively induce apoptotic cell death via the mitochondrial pathway. These findings emphasize the role of Smac mimetics for sensitizing DLBCL cells to proteasome inhibition with important implications for further (pre)clinical studies.

Drug Development Targeting the Ubiquitin-Proteasome System (UPS) for the Treatment of Human Cancers

Cancer cells are characterized by a higher rate of protein turnover and greater demand for protein homeostasis compared to normal cells. In this scenario, the ubiquitin-proteasome system (UPS), which is responsible for the degradation of over 80% of cellular proteins within mammalian cells, becomes vital to cancer cells, making the UPS a critical target for the discovery of novel cancer therapeutics. This review systematically categorizes all current reported small molecule inhibitors of the various essential components of the UPS, including ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin ligases (E3s), the 20S proteasome catalytic core particle (20S CP) and the 19S proteasome regulatory particles (19S RP), as well as their mechanism/s of action and limitations. We also discuss the immunoproteasome which is considered as a prospective therapeutic target of the next generation of proteasome inhibitors in cancer therapies.

Role of Deubiquitinases in Human Cancers: Potential Targeted Therapy

Deubiquitinases (DUBs) are involved in various cellular functions. They deconjugate ubiquitin (UBQ) from ubiquitylated substrates to regulate their activity and stability. Studies on the roles of deubiquitylation have been conducted in various cancers to identify the carcinogenic roles of DUBs. In this review, we evaluate the biological roles of DUBs in cancer, including proliferation, cell cycle control, apoptosis, the DNA damage response, tumor suppression, oncogenesis, and metastasis. This review mainly focuses on the regulation of different downstream effectors and pathways via biochemical regulation and posttranslational modifications. We summarize the relationship between DUBs and human cancers and discuss the potential of DUBs as therapeutic targets for cancer treatment. This review also provides basic knowledge of DUBs in the development of cancers and highlights the importance of DUBs in cancer biology.

The role of ubiquitination in tumorigenesis and targeted drug discovery

Ubiquitination, an important type of protein posttranslational modification (PTM), plays a crucial role in controlling substrate degradation and subsequently mediates the "quantity" and "quality" of various proteins, serving to ensure cell homeostasis and guarantee life activities. The regulation of ubiquitination is multifaceted and works not only at the transcriptional and posttranslational levels (phosphorylation, acetylation, methylation, etc.) but also at the protein level (activators or repressors). When regulatory mechanisms are aberrant, the altered biological processes may subsequently induce serious human diseases, especially various types of cancer. In tumorigenesis, the altered biological processes involve tumor metabolism, the immunological tumor microenvironment (TME), cancer stem cell (CSC) stemness and so on. With regard to tumor metabolism, the ubiquitination of some key proteins such as RagA, mTOR, PTEN, AKT, c-Myc and P53 significantly regulates the activity of the mTORC1, AMPK and PTEN-AKT signaling pathways. In addition, ubiquitination in the TLR, RLR and STING-dependent signaling pathways also modulates the TME. Moreover, the ubiquitination of core stem cell regulator triplets (Nanog, Oct4 and Sox2) and members of the Wnt and Hippo-YAP signaling pathways participates in the maintenance of CSC stemness. Based on the altered components, including the proteasome, E3 ligases, E1, E2 and deubiquitinases (DUBs), many molecular targeted drugs have been developed to combat cancer. Among them, small molecule inhibitors targeting the proteasome, such as bortezomib, carfilzomib, oprozomib and ixazomib, have achieved tangible success. In addition, MLN7243 and MLN4924 (targeting the E1 enzyme), Leucettamol A and CC0651 (targeting the E2 enzyme), nutlin and MI-219 (targeting the E3 enzyme), and compounds G5 and F6 (targeting DUB activity) have also shown potential in preclinical cancer treatment. In this review, we summarize the latest progress in understanding the substrates for ubiquitination and their special functions in tumor metabolism regulation, TME modulation and CSC stemness maintenance. Moreover, potential therapeutic targets for cancer are reviewed, as are the therapeutic effects of targeted drugs.

Multiple Myeloma: Available Therapies and Causes of Drug Resistance

Multiple myeloma (MM) is the second most common blood cancer. Treatments for MM include corticosteroids, alkylating agents, anthracyclines, proteasome inhibitors, immunomodulatory drugs, histone deacetylase inhibitors and monoclonal antibodies. Survival outcomes have improved substantially due to the introduction of many of these drugs allied with their rational use. Nonetheless, MM patients successively relapse after one or more treatment regimens or become refractory, mostly due to drug resistance. This review focuses on the main drugs used in MM treatment and on causes of drug resistance, including cytogenetic, genetic and epigenetic alterations, abnormal drug transport and metabolism, dysregulation of apoptosis, autophagy activation and other intracellular signaling pathways, the presence of cancer stem cells, and the tumor microenvironment. Furthermore, we highlight the areas that need to be further clarified in an attempt to identify novel therapeutic targets to counteract drug resistance in MM patients.

Proteasome Inhibitor Drugs

Proteasomes are large, multicatalytic protein complexes that cleave cellular proteins into peptides. There are many distinct forms of proteasomes that differ in catalytically active subunits, regulatory subunits, and associated proteins. Proteasome inhibitors are an important class of drugs for the treatment of multiple myeloma and mantle cell lymphoma, and they are being investigated for other diseases. Bortezomib (Velcade) was the first proteasome inhibitor to be approved by the US Food and Drug Administration. Carfilzomib (Kyprolis) and ixazomib (Ninlaro) have recently been approved, and more drugs are in development. While the primary mechanism of action is inhibition of the proteasome, the downstream events that lead to selective cell death are not entirely clear. Proteasome inhibitors have been found to affect protein turnover but at concentrations that are much higher than those achieved clinically, raising the possibility that some of the effects of proteasome inhibitors are mediated by other mechanisms.

Lessons Learned from Proteasome Inhibitors, the Paradigm for Targeting Protein Homeostasis in Cancer

Targeting aberrant protein homeostasis (proteostasis) in cancer is an attractive therapeutic strategy. However, this approach has thus far proven difficult to bring to clinical practice, with one major exception: proteasome inhibition. These small molecules have dramatically transformed outcomes for patients with the blood cancer multiple myeloma. However, these agents have failed to make an impact in more common solid tumors. Major questions remain about whether this therapeutic strategy can be extended to benefit even more patients. Here we discuss the role of the proteasome in normal and tumor cells, the basic, preclinical, and clinical development of proteasome inhibitors, and mechanisms proposed to govern both intrinsic and acquired resistance to these drugs. Years of study of both the mechanism of action and modes of resistance to proteasome inhibitors reveal these processes to be surprisingly complex. Here, we attempt to draw lessons from experience with proteasome inhibitors that may be relevant for other compounds targeting proteostasis in cancer, as well as extending the reach of proteasome inhibitors beyond blood cancers.

Proteasome Inhibitors in Cancer Therapy and their Relation to Redox Regulation

Redox homeostasis is important for the maintenance of cell survival. Under physiological conditions, redox system works in a balance and involves activation of many signaling molecules. Regulation of redox balance via signaling molecules is achieved by different pathways and proteasomal system is a key pathway in this process. Importance of proteasomal system on signaling pathways has been investigated for many years. In this direction, many proteasome targeting molecules have been developed. Some of them are already in the clinic for cancer treatment and some are still under investigation to highlight underlying mechanisms. Although there are many studies done, molecular mechanisms of proteasome inhibitors and related signaling pathways need more detailed explanations. This review aims to discuss redox status and proteasomal system related signaling pathways. In addition, cancer therapies targeting proteasomal system and their effects on redox-related pathways have been summarized.

Bioinformatics Analysis of Potential Key Genes in Trastuzumab-Resistant Gastric Cancer

Background

This study was performed to identify genes related to acquired trastuzumab resistance in gastric cancer (GC) and to analyze their prognostic value.

Methods

The gene expression profile GSE77346 was downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were obtained by using GEO2R. Functional and pathway enrichment was analyzed by using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG). Search Tool for the Retrieval of Interacting Genes (STRING), Cytoscape, and MCODE were then used to construct the protein-protein interaction (PPI) network and identify hub genes. Finally, the relationship between hub genes and overall survival (OS) was analyzed by using the online Kaplan-Meier plotter tool.

Results

A total of 327 DEGs were screened and were mainly enriched in terms related to pathways in cancer, signaling pathways regulating stem cell pluripotency, HTLV-I infection, and ECM-receptor interactions. A PPI network was constructed, and 18 hub genes (including one upregulated gene and seventeen downregulated genes) were identified based on the degrees and MCODE scores of the PPI network. Finally, the expression of four hub genes (ERBB2, VIM, EGR1, and PSMB8) was found to be related to the prognosis of HER2-positive (HER2+) gastric cancer. However, the prognostic value of the other hub genes was controversial; interestingly, most of these genes were interferon- (IFN-) stimulated genes (ISGs).

Conclusions

Overall, we propose that the four hub genes may be potential targets in trastuzumab-resistant gastric cancer and that ISGs may play a key role in promoting trastuzumab resistance in GC.

Isatuximab plus carfilzomib/dexamethasone versus carfilzomib/dexamethasone in patients with relapsed/refractory multiple myeloma: IKEMA Phase III study design

Although the treatment of relapsed/refractory multiple myeloma has improved dramatically over the past decade, the disease remains incurable; therefore, additional therapies are needed. Novel combination therapies incorporating monoclonal antibodies have shown significant promise. Here we describe the design of a Phase III study (NCT03275285, IKEMA), which is evaluating isatuximab plus carfilzomib and low-dose dexamethasone, versus carfilzomib/dexamethasone in relapsed/refractory multiple myeloma. The primary end point is progression-free survival. Responses are being determined by an independent review committee using 2016 International Myeloma Working Group criteria, and safety will be assessed throughout. The first patient was recruited in November 2017, and the last patient was recruited in March 2019; 302 patients have been randomized, and the study is ongoing. Clinical trial registration: NCT03275285.

A dual inhibitor of the proteasome catalytic subunits LMP2 and Y attenuates disease progression in mouse models of Alzheimer's disease

The immunoproteasome (iP) is a variant of the constitutive proteasome (cP) that is abundantly expressed in immune cells which can also be induced in somatic cells by cytokines such as TNF-α or IFN-γ. Accumulating evidence support that the iP is closely linked to multiple facets of inflammatory response, eventually leading to the development of several iP inhibitors as potential therapeutic agents for autoimmune diseases. Recent studies also found that the iP is upregulated in reactive glial cells surrounding amyloid β (Aβ) deposits in brains of Alzheimer’s disease (AD) patients, but the role it plays in the pathogenesis of AD remains unclear. In this study, we investigated the effects of several proteasome inhibitors on cognitive function in AD mouse models and found that YU102, a dual inhibitor of the iP catalytic subunit LMP2 and the cP catalytic subunit Y, ameliorates cognitive impairments in AD mouse models without affecting Aβ deposition. The data obtained from our investigation revealed that YU102 suppresses the secretion of inflammatory cytokines from microglial cells. Overall, this study indicates that there may exist a potential link between LMP2/Y and microglia-mediated neuroinflammation and that inhibition of these subunits may offer a new therapeutic strategy for AD.

An optical and non-invasive method to detect the accumulation of ubiquitin chains

The accumulations of excess amounts of polyubiquitinated proteins are cytotoxic and frequently observed in pathologic tissue from patients of neurodegenerative diseases. Therefore, optical and non-invasive methods to detect the increase of the amounts of polyubiquitinated proteins in living cells is a promising strategy to find out symptoms and environmental cause of neurodegenerative diseases, also for identifying compounds that could inhibit gathering of polyubiquitinated proteins. Therefore, we generated a pair of fluorescent protein [Azamigreen (Azg) and Kusabiraorange (Kuo)] tagged ubiquitin on its N-terminus (Azg-Ub and Kuo-Ub) and developed an Azg/Kuo-based Fluorescence Resonance Energy Transfer (FRET) assay to estimate the amount of polyubiquitin chains in vitro and in vivo. The FRET intensity was attenuated in the presence of ubiquitin-activating enzyme inhibitor, PYR-41, indicating that both fluorescent ubiquitin is incorporated into ubiquitin chains likewise normal ubiquitin. The FRET intensity was enhanced by the addition of the proteasome inhibitor, MG-132, and was reduced in the presence of the autophagy activator Rapamycin, designating that ubiquitin chains with fluorescent ubiquitin act as the degradation signal equally with normal ubiquitin chains. In summary, the above optical methods provide powerful research tools to estimate the amounts of polyubiquitin chains in vitro and in vivo, especially non-invasively in living cells.

Biological Background of Resistance to Current Standards of Care in Multiple Myeloma

A high priority problem in multiple myeloma (MM) management is the development of resistance to administered therapies, with most myeloma patients facing successively shorter periods of response and relapse. Herewith, we review the current knowledge on the mechanisms of resistance to the standard backbones in MM treatment: proteasome inhibitors (PIs), immunomodulatory agents (IMiDs), and monoclonal antibodies (mAbs). In some cases, strategies to overcome resistance have been discerned, and an effort should be made to evaluate whether resensitization to these agents is feasible in the clinical setting. Additionally, at a time in which we are moving towards precision medicine in MM, it is equally important to identify reliable and accurate biomarkers of sensitivity/refractoriness to these main therapeutic agents with the goal of having more efficacious treatments and, if possible, prevent the development of relapse.

The promise of chimeric antigen receptor (CAR) T cell therapy in multiple myeloma

A cure for multiple myeloma (MM), a malignancy of plasma cells, remains elusive. Nearly all myeloma patients will eventually relapse and develop resistance to currently available treatments. There is an unmet medical need to develop novel and effective therapies that can induce sustained responses. Early phase clinical trials using chimeric antigen receptor (CAR) T cell therapy have shown great promise in the treatment of relapsed and/or refractory MM. In this review article, we provide an overview of the CAR constructs, the gene transfer vector systems, and approaches for T cell activation and expansion. We then summarize the outcomes of several early phase clinical trials of CAR T cell therapy in MM and the novel CAR T targets that are under development. Finally, we explore the potential mechanisms that result in disease relapse after CAR T therapy and propose future directions in CAR T therapy in MM.

Establishment of a novel human CIC-DUX4 sarcoma cell line, Kitra-SRS, with autocrine IGF-1R activation and metastatic potential to the lungs

Approximately 60–70% of EWSR1-negative small blue round cell sarcomas harbour a rearrangement of CIC, most commonly CIC-DUX4. CIC-DUX4 sarcoma (CDS) is an aggressive and often fatal high-grade sarcoma appearing predominantly in children and young adults. Although cell lines and their xenograft models are essential tools for basic research and development of antitumour drugs, few cell lines currently exist for CDS. We successfully established a novel human CDS cell line designated Kitra-SRS and developed orthotopic tumour xenografts in nude mice. The CIC-DUX4 fusion gene in Kitra-SRS cells was generated by t(12;19) complex chromosomal rearrangements with an insertion of a chromosome segment including a DUX4 pseudogene component. Kitra-SRS xenografts were histologically similar to the original tumour and exhibited metastatic potential to the lungs. Kitra-SRS cells displayed autocrine activation of the insulin-like growth factor 1 (IGF-1)/IGF-1 receptor (IGF-1R) pathway. Accordingly, treatment with the IGF-1R inhibitor, linsitinib, attenuated Kitra-SRS cell growth and IGF-1-induced activation of IGF-1R/AKT signalling both in vitro and in vivo. Furthermore, upon screening 1134 FDA-approved drugs, the responses of Kitra-SRS cells to anticancer drugs appeared to reflect those of the primary tumour. Our model will be a useful modality for investigating the molecular pathology and therapy of CDS.

20S Proteasome as a Drug Target in Trichomonas vaginalis

Trichomoniasis is a sexually transmitted disease with hundreds of millions of annual cases worldwide. Approved treatment options are limited to two related nitro-heterocyclic compounds, yet resistance to these drugs is an increasing concern. New antimicrobials against the causative agent, Trichomonas vaginalis, are urgently needed. We show here that clinically approved anticancer drugs that inhibit the proteasome, a large protease complex with a critical role in degrading intracellular proteins in eukaryotes, have submicromolar activity against the parasite in vitro and on-target activity against the enriched T. vaginalis proteasome in cell-free assays. Proteomic analysis confirmed that the parasite has all seven α and seven β subunits of the eukaryotic proteasome although they have only modest sequence identities, ranging from 28 to 52%, relative to the respective human proteasome subunits. A screen of proteasome inhibitors derived from a marine natural product, carmaphycin, revealed one derivative, carmaphycin-17, with greater activity against T. vaginalis than the reference drug metronidazole, the ability to overcome metronidazole resistance, and reduced human cytotoxicity compared to that of the anticancer proteasome inhibitors. The increased selectivity of carmaphycin-17 for T. vaginalis was related to its >5-fold greater potency against the β1 and β5 catalytic subunits of the T. vaginalis proteasome than against the human proteasome subunits. In a murine model of vaginal trichomonad infection, proteasome inhibitors eliminated or significantly reduced parasite burden upon topical treatment without any apparent adverse effects. Together, these findings validate the proteasome of T. vaginalis as a therapeutic target for development of a novel class of trichomonacidal agents.

Resveratrol Downregulates Biomarkers of Sepsis Via Inhibition of Proteasome's Proteases

Lipopolysaccharide (LPS) is the main agonist of gram-negative bacteria and initiates inflammation. We recently reported that plasmas from sepsis patients revealed increased levels of following group of biomarkers; VCAM-1, ICAM1, CRP, resistin, and proteasome LMP subunits. Our objective here was to compare effects of resveratrol (shown to be a nonspecific proteasome inhibitor by us) and a known LMP7 inhibitor (ONX-0914, specific inhibitor) on proteasome's activities, as well as on inflammatory markers mentioned above in human blood monocytes. Using fluorescence-based assays on blood monocytes purified proteasomes, resveratrol (0-100 μM) inhibited all three protease activities, predominantly LMP7. Similarly, resveratrol inhibited all three protease activities using cell-based luminescence assay. In contrast, ONX-0914 was more selective and potent for LMP7 activity. Resveratrol and ONX-0914, both significantly inhibited expression of LPS-induced biomarkers mentioned above in CD14 monocytes. Moreover, resveratrol itself, as well as in combination with LPS, accumulated pIκBα in CD14 monocytes. Collectively, our data suggest that resveratrol is a less potent inhibitor of all three; CT-like (predominantly LMP7), T-like and PA protease activities and is less toxic to human monocytes than ONX-0914 (a selector inhibitor of only LMP7) as observed by an autophagy detection kit. Also, resveratrol reduces LPS-induced inflammatory cytokine expression by decreasing the translocation of NF-κB due to an increase in inhibitor pIκBα. Therefore, resveratrol can be used to curb inflammation in diseased states like sepsis and other disorders.

Rational Targeting of Cdc42 Overcomes Drug Resistance of Multiple Myeloma

Multiple myeloma (MM) drug resistance highlights a need for alternative therapeutic strategies. In this study, we show that CASIN, a selective inhibitor of cell division cycle 42 (Cdc42) GTPase, inhibited proliferation and survival of melphalan/bortezomib-resistant MM cells more profoundly than that of the sensitive cells. Furthermore, CASIN was more potent than melphalan/bortezomib in inhibiting melphalan/bortezomib-resistant cells. In addition, CASIN sensitized melphalan/bortezomib-resistant cells to this drug combination. Mechanistically, Cdc42 activity was higher in melphalan/bortezomib-resistant cells than that in the sensitive cells. CASIN inhibited mono-ubiquitination of Fanconi anemia (FA) complementation group D2 (FANCD2) of the FA DNA damage repair pathway in melphalan-resistant but not melphalan-sensitive cells, thereby sensitizing melphalan-resistant cells to DNA damage. CASIN suppressed epidermal growth factor receptor (EGFR), signal transducer and activator of transcription 3 (STAT3), and extracellular signal-regulated kinase (ERK) activities to a larger extent in bortezomib-resistant than in melphalan-sensitive cells. Reconstitution of ERK activity partially protected CASIN-treated bortezomib-resistant cells from death, suggesting that CASIN-induced killing is attributable to suppression of ERK. Importantly, CASIN extended the lifespan of mouse xenografts of bortezomib-resistant cells and caused apoptosis of myeloma cells from bortezomib-resistant MM patients. Finally, CASIN had negligible side effects on peripheral blood mononuclear cells (PBMC) from healthy human subjects and normal B cells. Our data provide a proof of concept demonstration that rational targeting of Cdc42 represents a promising approach to overcome MM drug resistance.

Safe Use of Carfilzomib in a Patient with Multiple Myeloma and Intermittent Type 1 Brugada ECG Pattern: A Case Report

Cardiovascular adverse events (CVAEs) are of considerable importance in patients with multiple myeloma (MM), given the significant prevalence of coexisting cardiovascular risk factors and the potential treatment-induced toxicity. Brugada syndrome is a rare cardiological disease responsible for arrhythmia and potentially fatal cardiac arrest. Brugada phenocopies (BrP) are clinical entities which show an identical ECG patterns, but prompt resolution after treatment of the trigger event. A 65-year-old female newly diagnosed MM patient treated with a carfilzomib-based chemotherapy developed a type 1 Brugada ECG pattern during a hospitalization course for sepsis. As fever and the septic event resolved, further ECGs showed no abnormalities and carfilzomib-based treatment could be resumed with no further CVAEs. Though fever-induced BrP is a universally known phenomenon, to our knowledge this is the first case of BrP in a patient with MM during active treatment with carfilzomib.

Targeting the Proteasome in Refractory Pediatric Leukemia Cells: Characterization of Effective Cytotoxicity of Carfilzomib

BACKGROUND

Leukemia accounts for 30% of all childhood cancers and although the survival rate for pediatric leukemia has greatly improved, relapse is a major cause of treatment failure. Therefore, the development and introduction of novel therapeutics to treat relapsed pediatric leukemia is urgently needed. The proteasome inhibitor bortezomib has been shown to be effective against adult hematological malignancies such as multiple myeloma and lymphoma, but is frequently associated with the development of resistance. Carfilzomib is a next-generation proteasome inhibitor that has shown promising results against refractory adult hematological malignancies.

OBJECTIVE

Carfilzomib has been extensively studied in adult hematological malignancies, providing the rationale for evaluating proof-of-concept activity of carfilzomib in pediatric leukemia.

METHODS

The effects of carfilzomib on pediatric leukemia cell lines and primary pediatric leukemia patient samples were investigated in vitro using the alamar blue cytotoxicity assay, western blotting, and a proteasome activity assay. Synergy with commonly used anticancer drugs was determined by calculation of combination indices.

RESULTS

In vitro preclinical data show pharmacologically relevant concentrations of carfilzomib are cytotoxic to pediatric leukemia cell lines and primary pediatric leukemia cells. Target modulation studies validate the effective inhibition of the proteasome and induction of apoptosis. We also identify agents that have effective synergy with carfilzomib in these cells.

CONCLUSIONS

Our data provide pre-clinical information that can be incorporated into future early-phase clinical trials for the assessment of carfilzomib as a treatment for children with refractory hematological malignancies.

Targeted drug combination therapy design based on driver genes

Targeted therapies against cancer types with more than one driver gene hold bright but elusive promise, since approved drugs are not available for all driver mutations and monotherapies often result in resistance. Targeting multiple driver genes in different pathways at the same time may provide an impact extensive enough to fight resistance. Our goal was to find synergistic drug combinations based on the availability of targeted drugs and their biological activity profiles and created an associated compound library based on driver gene-related protein targets. In this study, we would like to show that driver gene pattern based customized combination therapies are more effective than monotherapies on six cell lines and patient-derived primary cell cultures.

We tested 55–102 drug combinations targeting driver genes and driver pathways for each cell line and found 25–85% of these combinations highly synergistic. Blocking 2–5 cancer pathways using only 2–3 targeted drugs was sufficient to reach high rates of tumor cell eradication at remarkably low concentrations.

Our results demonstrate that the efficiency of cancer treatment may be significantly improved by combining drugs against multiple tumor specific drivers.

Carfilzomib with immunomodulatory drugs for the treatment of newly diagnosed multiple myeloma

Carfilzomib, a selective proteasome inhibitor (PI), is approved for the treatment of patients with relapsed or refractory multiple myeloma (MM). Combination regimens incorporating a PI and immunomodulatory drug (IMiD) have been associated with deep responses and extended survival in patients with newly diagnosed MM (NDMM). Carfilzomib-based combinations with immunomodulators are being extensively studied in the frontline setting. The objective of this review was to describe efficacy and safety data for carfilzomib-based, PI/immunomodulatory combinations in NDMM. Information sources were articles indexed in PubMed and abstracts from key hematology/oncology congresses published between January 2012 and December 2018. PubMed and congresses were searched for prospective clinical studies assessing the combination of carfilzomib with an IMiD for NDMM treatment. Retrospective and preclinical reports, case reports/series, reviews, and clinical studies not evaluating carfilzomib-immunomodulator combinations in NDMM were excluded based on review of titles and abstracts. A total of nine articles and 72 abstracts were deemed relevant and included in the review. A total of six distinct carfilzomib-based, PI/immunomodulator combination regimens have been evaluated in 12 clinical trials. Overall, treatment with these regimens has resulted in deep responses, including high rates of negativity for minimal residual disease. These deep responses have translated to long progression-free survival and overall survival rates. Efficacy results for these regimens have generally been consistent across subgroups defined by age, transplant eligibility, and cytogenetic risk. The safety profile of carfilzomib in NDMM is consistent with that observed in the relapsed-refractory MM setting. Clinical studies have found that carfilzomib-based combinations with immunomodulators are highly active with a favorable safety profile in NDMM. The carfilzomib, lenalidomide, and dexamethasone (KRd) drug backbone is a promising foundation for treatment strategies aimed at achieving long-term, deep responses (functional cures) in the frontline setting. Several ongoing studies are evaluating KRd, with or without anti-CD38 monoclonal antibodies.

Limited Chemical Structural Diversity Found to Modulate Thyroid Hormone Receptor in the Tox21 Chemical Library

BACKGROUND

Thyroid hormone receptors (TRs) are critical endocrine receptors that regulate a multitude of processes in adult and developing organisms, and thyroid hormone disruption is of high concern for neurodevelopmental and reproductive toxicities in particular. To date, only a small number of chemical classes have been identified as possible TR modulators, and the receptors appear highly selective with respect to the ligand structural diversity. Thus, the question of whether TRs are an important screening target for protection of human and wildlife health remains.

OBJECTIVE

Our goal was to evaluate the hypothesis that there is limited structural diversity among environmentally relevant chemicals capable of modulating TR activity via the collaborative interagency Tox21 project.

METHODS

We screened the Tox21 chemical library (8,305 unique structures) in a quantitative high-throughput, cell-based reporter gene assay for TR agonist or antagonist activity. Active compounds were further characterized using additional orthogonal assays, including mammalian one-hybrid assays, coactivator recruitment assays, and a high-throughput, fluorescent imaging, nuclear receptor translocation assay.

RESULTS

Known agonist reference chemicals were readily identified in the TR transactivation assay, but only a single novel, direct agonist was found, the pharmaceutical betamipron. Indirect activation of TR through activation of its heterodimer partner, the retinoid-X-receptor (RXR), was also readily detected by confirmation in an RXR agonist assay. Identifying antagonists with high confidence was a challenge with the presence of significant confounding cytotoxicity and other, non-TR-specific mechanisms common to the transactivation assays. Only three pharmaceuticals-mefenamic acid, diclazuril, and risarestat-were confirmed as antagonists.

DISCUSSION

The results support limited structural diversity for direct ligand effects on TR and imply that other potential target sites in the thyroid hormone axis should be a greater priority for bioactivity screening for thyroid axis disruptors. https://doi.org/10.1289/EHP5314.

Potent and Preferential Degradation of CDK6 via Proteolysis Targeting Chimera Degraders

A focused PROTAC library hijacking cancer therapeutic target CDK6 was developed. A design principle as "match/mismatch" was proposed for understanding the degradation profile differences in these PROTACs. Notably, potent PROTACs with specific and remarkable CDK6 degradation potential were generated by linking CDK6 inhibitor palbociclib and E3 ligase CRBN recruiter pomalidomide. The PROTAC strongly inhibited proliferation of hematopoietic cancer cells including multiple myeloma and robustly degraded copy-amplified/mutated forms of CDK6, indicating future potential clinical applications.

Small-Molecule Inhibitors of the Proteasome's Regulatory Particle

Cells need to synthesize and degrade proteins consistently. Maintaining a balanced level of protein in the cell requires a carefully controlled system and significant energy. Degradation of unwanted or damaged proteins into smaller peptide units can be accomplished by the proteasome. The proteasome is composed of two main subunits. The first is the core particle (20S CP), and within this core particle are three types of threonine proteases. The second is the regulatory complex (19S RP), which has a myriad of activities including recognizing proteins marked for degradation and shuttling the protein into the 20S CP to be degraded. Small-molecule inhibitors of the 20S CP have been developed and are exceptional treatments for multiple myeloma (MM). 20S CP inhibitors disrupt the protein balance, leading to cellular stress and eventually to cell death. Unfortunately, the 20S CP inhibitors currently available have dose-limiting off-target effects and resistance can be acquired rapidly. Herein, we discuss small molecules that have been discovered to interact with the 19S RP subunit or with a protein closely associated with 19S RP activity. These molecules still elicit their toxicity by preventing the proteasome from degrading proteins, but do so through different mechanisms of action.

The CUL5 ubiquitin ligase complex mediates resistance to CDK9 and MCL1 inhibitors in lung cancer cells

Overexpression of anti-apoptotic proteins MCL1 and Bcl-xL are frequently observed in many cancers. Inhibitors targeting MCL1 are in clinical development, however numerous cancer models are intrinsically resistant to this approach. To discover mechanisms underlying resistance to MCL1 inhibition, we performed multiple flow-cytometry based genome-wide CRISPR screens interrogating two drugs that directly (MCL1i) or indirectly (CDK9i) target MCL1. Remarkably, both screens identified three components (CUL5, RNF7 and UBE2F) of a cullin-RING ubiquitin ligase complex (CRL5) that resensitized cells to MCL1 inhibition. We find that levels of the BH3-only pro-apoptotic proteins Bim and Noxa are proteasomally regulated by the CRL5 complex. Accumulation of Noxa caused by depletion of CRL5 components was responsible for re-sensitization to CDK9 inhibitor, but not MCL1 inhibitor. Discovery of a novel role of CRL5 in apoptosis and resistance to multiple types of anticancer agents suggests the potential to improve combination treatments.