An allosteric inhibitor of substrate recognition by the SCF(Cdc4) ubiquitin ligase

A patent review of SCF E3 ligases inhibitors for cancer：Structural design, pharmacological activities and structure-activity relationship

Currently, as the largest family of E3 ubiquitin ligases, Skp1-Cullin 1-F-box (SCF) E3 ligase complexes have attracted extensive attention. Among SCF complexes, Skp2, β-TrCP, and FBXW7 have undergone extensive research on their structures and functions. Previous studies suggest Skp2, β-TrCP, and FBXW7 are overexpressed in numerous cancers. Thus, the SCF E3 ligase complex has become a significant target for the development of anti-cancer drugs. Over the past few decades, a variety of anti-tumor inhibitors targeting the SCF E3 ligase complex have been attempted. However, since almost none of the SCF E3 ligase inhibitors passed clinical trials, the design and synthesis of the new inhibitors are needed. Here, we will introduce the structure and function of Skp2, β-TrCP, and FBXW7, their connections with cancer development, the relevant in vitro and in vivo activities, selectivity, structure-activity relationships, and the therapeutic or preventive application of small molecule inhibitors targeting these three F-box proteins reported in the patent (2010-present). This information will help develop drugs targeting the SCF E3 ubiquitin ligase, providing new strategies for future cancer treatments.

Molecular insights and clinical implications for the tumor suppressor role of SCFFBXW7 E3 ubiquitin ligase

FBXW7 is one of the most well-characterized F-box proteins, serving as substrate receptor subunit of SKP1-CUL1-F-box (SCF) E3 ligase complexes. SCFFBXW7 is responsible for the degradation of various oncogenic proteins such as cyclin E, c-MYC, c-JUN, NOTCH, and MCL1. Therefore, FBXW7 functions largely as a major tumor suppressor. In keeping with this notion, FBXW7 gene mutations or downregulations have been found and reported in many types of malignant tumors, such as endometrial, colorectal, lung, and breast cancers, which facilitate the proliferation, invasion, migration, and drug resistance of cancer cells. Therefore, it is critical to review newly identified FBXW7 regulation and tumor suppressor function under physiological and pathological conditions to develop effective strategies for the treatment of FBXW7-altered cancers. Since a growing body of evidence has revealed the tumor-suppressive activity and role of FBXW7, here, we updated FBXW7 upstream and downstream signaling including FBXW7 ubiquitin substrates, the multi-level FBXW7 regulatory mechanisms, and dysregulation of FBXW7 in cancer, and discussed promising cancer therapies targeting FBXW7 regulators and downstream effectors, to provide a comprehensive picture of FBXW7 and facilitate the study in this field.

Discovery of a First-in-Class Small-Molecule Ligand for WDR91 Using DNA-Encoded Chemical Library Selection Followed by Machine Learning

WD40 repeat-containing protein 91 (WDR91) regulates early-to-late endosome conversion and plays vital roles in endosome fusion, recycling, and transport. WDR91 was recently identified as a potential host factor for viral infection. We employed DNA-encoded chemical library (DEL) selection against the WDR domain of WDR91, followed by machine learning to predict ligands from the synthetically accessible Enamine REAL database. Screening of predicted compounds identified a WDR91 selective compound 1, with a KD of 6 ± 2 μM by surface plasmon resonance. The co-crystal structure confirmed the binding of 1 to the WDR91 side pocket, in proximity to cysteine 487, which led to the discovery of covalent analogues 18 and 19. The covalent adduct formation for 18 and 19 was confirmed by intact mass liquid chromatography-mass spectrometry. The discovery of 1, 18, and 19, accompanying structure-activity relationship, and the co-crystal structures provide valuable insights for designing potent and selective chemical tools against WDR91 to evaluate its therapeutic potential.

SCFβTrCP-mediated degradation of SHARP1 in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer associated with metastasis, high recurrence rate, and poor survival. The basic helix-loop-helix transcription factor SHARP1 (Split and Hairy-related Protein 1) has been identified as a suppressor of the metastatic behavior of TNBC. SHARP1 blocks the invasive phenotype of TNBC by inhibiting hypoxia-inducible factors and its loss correlates with poor survival of breast cancer patients. Here, we show that SHARP1 is an unstable protein that is targeted for proteasomal degradation by the E3 ubiquitin ligase complex SCFβTrCP. SHARP1 recruits βTrCP via a phosphodegron encompassing Ser240 and Glu245 which are required for SHARP1 ubiquitylation and degradation. Furthermore, mice injected with TNBC cells expressing the non-degradable SHARP1(S240A/E245A) mutant display reduced tumor growth and increased tumor-free survival. Our study suggests that targeting the βTrCP-dependent degradation of SHARP1 represents a therapeutic strategy in TNBC.

A robust yeast biocontainment system with two-layered regulation switch dependent on unnatural amino acid

Synthetic auxotrophy in which cell viability depends on the presence of an unnatural amino acid (unAA) provides a powerful strategy to restrict unwanted propagation of genetically modified organisms (GMOs) in open environments and potentially prevent industrial espionage. Here, we describe a generic approach for robust biocontainment of budding yeast dependent on unAA. By understanding escape mechanisms, we specifically optimize our strategies by introducing designed "immunity" to the generation of amber-suppressor tRNAs and developing the transcriptional- and translational-based biocontainment switch. We further develop a fitness-oriented screening method to easily obtain multiplex safeguard strains that exhibit robust growth and undetectable escape frequency (<~10-9) on solid media for 14 days. Finally, we show that employing our multiplex safeguard system could restrict the proliferation of strains of interest in a real fermentation scenario, highlighting the great potential of our yeast biocontainment strategy to protect the industrial proprietary strains.

Unlocking DCAFs To Catalyze Degrader Development: An Arena for Innovative Approaches

Chemically induced proximity-based targeted protein degradation (TPD) has become a prominent paradigm in drug discovery. With the clinical benefit demonstrated by certain small-molecule protein degraders that target the cullin-RING E3 ubiquitin ligases (CRLs), the field has proactively strategized to tackle anticipated drug resistance by harnessing additional E3 ubiquitin ligases to enrich the arsenal of this therapeutic approach. Here, we endeavor to explore the collaborative efforts involved in unlocking a broad range of CRL4DCAF for degrader drug development. Throughout the discussion, we also highlight how both conventional and innovative approaches in drug discovery can be taken to realize this objective. Moving ahead, we expect a greater allocation of resources in TPD to pursue these high-hanging fruits.

RNF126, 168 and CUL1: The Potential Utilization of Multi-Functional E3 Ubiquitin Ligases in Genome Maintenance for Cancer Therapy

Ubiquitination is a post-translational modification (PTM) that is involved in proteolysis, protein-protein interaction, and signal transduction. Accumulation of mutations and genomic instability are characteristic of cancer cells, and dysfunction of the ubiquitin pathway can contribute to abnormal cell physiology. Because mutations can be critical for cells, DNA damage repair, cell cycle regulation, and apoptosis are pathways that are in close communication to maintain genomic integrity. Uncontrolled cell proliferation due to abnormal processes is a hallmark of cancer, and mutations, changes in expression levels, and other alterations of ubiquitination factors are often involved. Here, three E3 ubiquitin ligases will be reviewed in detail. RNF126, RNF168 and CUL1 are involved in DNA damage response (DDR), DNA double-strand break (DSB) repair, cell cycle regulation, and ultimately, cancer cell proliferation control. Their involvement in multiple cellular pathways makes them an attractive candidate for cancer-targeting therapy. Functional studies of these E3 ligases have increased over the years, and their significance in cancer is well reported. There are continuous efforts to develop drugs targeting the ubiquitin pathway for anticancer therapy, which opens up the possibility for these E3 ligases to be evaluated for their potential as a target protein for anticancer therapy.

A screen for MeCP2-TBL1 interaction inhibitors using a luminescence-based assay

Understanding the molecular pathology of neurodevelopmental disorders should aid the development of therapies for these conditions. In MeCP2 duplication syndrome (MDS)-a severe autism spectrum disorder-neuronal dysfunction is caused by increased levels of MeCP2. MeCP2 is a nuclear protein that binds to methylated DNA and recruits the nuclear co-repressor (NCoR) complex to chromatin via an interaction with the WD repeat-containing proteins TBL1 and TBLR1. The peptide motif in MeCP2 that binds to TBL1/TBLR1 is essential for the toxicity of excess MeCP2 in animal models of MDS, suggesting that small molecules capable of disrupting this interaction might be useful therapeutically. To facilitate the search for such compounds, we devised a simple and scalable NanoLuc luciferase complementation assay for measuring the interaction of MeCP2 with TBL1/TBLR1. The assay allowed excellent separation between positive and negative controls, and had low signal variance (Z-factor = 0.85). We interrogated compound libraries using this assay in combination with a counter-screen based on luciferase complementation by the two subunits of protein kinase A (PKA). Using this dual screening approach, we identified candidate inhibitors of the interaction between MeCP2 and TBL1/TBLR1. This work demonstrates the feasibility of future screens of large compound collections, which we anticipate will enable the development of small molecule therapeutics to ameliorate MDS.

Conservation and diversity in allosteric fingerprints of proteins for evolutionary-inspired engineering and design

Hand-in-hand work of physics and evolution delivered protein universe with diversity of forms, sizes, and functions. Pervasiveness and advantageous traits of allostery made it an important component of the protein function regulation, calling for thorough investigation of its structural determinants and evolution. Learning directly from nature, we explored here allosteric communication in several major folds and repeat proteins, including α/β and β-barrels, β-propellers, Ig-like fold, ankyrin and α/β leucine-rich repeat proteins, which provide structural platforms for many different enzymatic and signalling functions. We obtained a picture of conserved allosteric communication characteristic in different fold types, modifications of the structure-driven signalling patterns via sequence-determined divergence to specific functions, as well as emergence and potential diversification of allosteric regulation in multi-domain proteins and oligomeric assemblies. Our observations will be instrumental in facilitating the engineering and de novo design of proteins with allosterically regulated functions, including development of therapeutic biologics. In particular, results described here may guide the identification of the optimal structural platforms (e.g. fold type, size, and oligomerization states) and the types of diversifications/perturbations, such as mutations, effector binding, and order-disorder transition. The tunable allosteric linkage across distant regions can be used as a pivotal component in the design/engineering of modular biological systems beyond the traditional scaffolding function.

Identification and optimization of molecular glue compounds that inhibit a noncovalent E2 enzyme-ubiquitin complex

Pharmacological control of the ubiquitin-proteasome system (UPS) is of intense interest in drug discovery. Here, we report the development of chemical inhibitors of the ubiquitin-conjugating (E2) enzyme CDC34A (also known as UBE2R1), which donates activated ubiquitin to the cullin-RING ligase (CRL) family of ubiquitin ligase (E3) enzymes. A FRET-based interaction assay was used to screen for novel compounds that stabilize the noncovalent complex between CDC34A and ubiquitin, and thereby inhibit the CDC34A catalytic cycle. An isonipecotamide hit compound was elaborated into analogs with ~1000-fold increased potency in stabilizing the CDC34A-ubiquitin complex. These analogs specifically inhibited CDC34A-dependent ubiquitination in vitro and stabilized an E2~ubiquitin thioester reaction intermediate in cells. The x-ray crystal structure of a CDC34A-ubiquitin-inhibitor complex uncovered the basis for analog structure-activity relationships. The development of chemical stabilizers of the CDC34A-ubiquitin complex illustrates a general strategy for de novo discovery of molecular glue compounds that stabilize weak protein interactions.

Decoding the messaging of the ubiquitin system using chemical and protein probes

Post-translational modification of proteins by ubiquitin is required for nearly all aspects of eukaryotic cell function. The numerous targets of ubiquitylation, and variety of ubiquitin modifications, are often likened to a code, where the ultimate messages are diverse responses to target ubiquitylation. E1, E2, and E3 multiprotein enzymatic assemblies modify specific targets and thus function as messengers. Recent advances in chemical and protein tools have revolutionized our ability to explore the ubiquitin system, through enabling new high-throughput screening methods, matching ubiquitylation enzymes with their cellular targets, revealing intricate allosteric mechanisms regulating ubiquitylating enzymes, facilitating structural revelation of transient assemblies determined by multivalent interactions, and providing new paradigms for inhibiting and redirecting ubiquitylation in vivo as new therapeutics. Here we discuss the development of methods that control, disrupt, and extract the flow of information across the ubiquitin system and have enabled elucidation of the underlying molecular and cellular biology.

Targeting the Ubiquitin-Proteasome System for Cancer Therapeutics by Small-Molecule Inhibitors

The ubiquitin-proteasome system (UPS) is a critical regulator of cellular protein levels and activity. It is, therefore, not surprising that its dysregulation is implicated in numerous human diseases, including many types of cancer. Moreover, since cancer cells exhibit increased rates of protein turnover, their heightened dependence on the UPS makes it an attractive target for inhibition via targeted therapeutics. Indeed, the clinical application of proteasome inhibitors in treatment of multiple myeloma has been very successful, stimulating the development of small-molecule inhibitors targeting other UPS components. On the other hand, while the discovery of potent and selective chemical compounds can be both challenging and time consuming, the area of targeted protein degradation through utilization of the UPS machinery has seen promising developments in recent years. The repertoire of proteolysis-targeting chimeras (PROTACs), which employ E3 ligases for the degradation of cancer-related proteins via the proteasome, continues to grow. In this review, we will provide a thorough overview of small-molecule UPS inhibitors and highlight advancements in the development of targeted protein degradation strategies for cancer therapeutics.

Cullin-RING Ubiquitin Ligase Regulatory Circuits: A Quarter Century Beyond the F-Box Hypothesis

Cullin-RING ubiquitin ligases (CRLs) are dynamic modular platforms that regulate myriad biological processes through target-specific ubiquitylation. Our knowledge of this system emerged from the F-box hypothesis, posited a quarter century ago: Numerous interchangeable F-box proteins confer specific substrate recognition for a core CUL1-based RING E3 ubiquitin ligase. This paradigm has been expanded through the evolution of a superfamily of analogous modular CRLs, with five major families and over 200 different substrate-binding receptors in humans. Regulation is achieved by numerous factors organized in circuits that dynamically control CRL activation and substrate ubiquitylation. CRLs also serve as a vast landscape for developing small molecules that reshape interactions and promote targeted ubiquitylation-dependent turnover of proteins of interest. Here, we review molecular principles underlying CRL function, the role of allosteric and conformational mechanisms in controlling substrate timing and ubiquitylation, and how the dynamics of substrate receptor interchange drives the turnover of selected target proteins to promote cellular decision-making.

Targeting TRIM Proteins: A Quest towards Drugging an Emerging Protein Class

The ubiquitylation machinery regulates several fundamental biological processes from protein homeostasis to a wide variety of cellular signaling pathways. As a consequence, its dysregulation is linked to diseases including cancer, neurodegeneration, and autoimmunity. With this review, we aim to highlight the therapeutic potential of targeting E3 ligases, with a special focus on an emerging class of RING ligases, named tri-partite motif (TRIM) proteins, whose role as targets for drug development is currently gaining pharmaceutical attention. TRIM proteins exert their catalytic activity as scaffolds involved in many protein-protein interactions, whose multidomains and adapter-like nature make their druggability very challenging. Herein, we give an overview of the current understanding of this class of single polypeptide RING E3 ligases and discuss potential targeting options.

Major advances in targeted protein degradation: PROTACs, LYTACs, and MADTACs

Of late, targeted protein degradation (TPD) has surfaced as a novel and innovative chemical tool and therapeutic modality. By co-opting protein degradation pathways, TPD facilitates complete removal of the protein molecules from within or outside the cell. While the pioneering Proteolysis-Targeting Chimera (PROTAC) technology and molecular glues hijack the ubiquitin-proteasome system, newer modalities co-opt autophagy or the endo-lysosomal pathway. Using this mechanism, TPD is posited to largely expand the druggable space far beyond small-molecule inhibitors. In this review, we discuss the major advances in TPD, highlight our current understanding, and explore outstanding questions in the field.

Targeted modulation of E3 ligases using engineered ubiquitin variants

Ubiquitination plays an essential role in signal transduction to regulate most if not all cellular processes. Among the enzymes that are involved in the ubiquitin (Ub) signaling cascade, tremendous efforts have been focused on elucidating the roles of E3 Ub ligases as they determine the complexity and specificity of ubiquitination. Not surprisingly, the malfunction of E3 ligases is directly implicated in many human diseases, including cancer. Therefore, there is an urgent need to develop potent and specific molecules to modulate E3 ligase activity as intracellular probes for target validation and as pharmacological agents in preclinical research. Unfortunately, the progress has been hampered by the dynamic regulation mechanisms for different types of E3 ligases. Here, we summarize the progress of using protein engineering to develop Ub variant (UbV) inhibitors for all major families of E3 ligases and UbV activators for homologous with E6-associated protein C terminus E3s and homodimeric RING E3s. We believe that this provides a general strategy and a valuable toolkit for the research community to inhibit or activate E3 ligases and these synthetic molecules have important implications in exploring protein degradation for drug discovery.

Ubiquitin biology in neurodegenerative disorders: From impairment to therapeutic strategies

The abnormal accumulation of neurotoxic proteins is the typical hallmark of various age-related neurodegenerative disorders (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis and Multiple sclerosis. The anomalous proteins, such as Aβ, Tau in Alzheimer's disease and α-synuclein in Parkinson's disease, perturb the neuronal physiology and cellular homeostasis in the brain thereby affecting the millions of human lives across the globe. Here, ubiquitin proteasome system (UPS) plays a decisive role in clearing the toxic metabolites in cells, where any aberrancy is widely reported to exaggerate the neurodegenerative pathologies. In spite of well-advancement in the ubiquitination research, their molecular markers and mechanisms for target-specific protein ubiquitination and clearance remained elusive. Therefore, this review substantiates the role of UPS in the brain signaling and neuronal physiology with their mechanistic role in the NDD's specific pathogenic protein clearance. Moreover, current and future promising therapies are discussed to target UPS-mediated neurodegeneration for better public health.

Advances in Deubiquitinating Enzyme Inhibition and Applications in Cancer Therapeutics

Since the discovery of the ubiquitin proteasome system (UPS), the roles of ubiquitinating and deubiquitinating enzymes (DUBs) have been widely elucidated. The ubiquitination of proteins regulates many aspects of cellular functions such as protein degradation and localization, and also modifies protein-protein interactions. DUBs cleave the attached ubiquitin moieties from substrates and thereby reverse the process of ubiquitination. The dysregulation of these two paramount pathways has been implicated in numerous diseases, including cancer. Attempts are being made to identify inhibitors of ubiquitin E3 ligases and DUBs that potentially have clinical implications in cancer, making them an important target in the pharmaceutical industry. Therefore, studies in medicine are currently focused on the pharmacological disruption of DUB activity as a rationale to specifically target cancer-causing protein aberrations. Here, we briefly discuss the pathophysiological and physiological roles of DUBs in key cancer-related pathways. We also discuss the clinical applications of promising DUB inhibitors that may contribute to the development of DUBs as key therapeutic targets in the future.

[Targeting Cullin-RING E3 ligases for anti-cancer therapy: efforts on drug discovery]

Cullin-RING E3 ligases (CRLs) are the major components of ubiquitin-proteasome system, responsible for ubiquitylation and subsequent degradation of thousands of cellular proteins. CRLs play vital roles in the regulation of multiple cellular processes, including cell cycle, cell apoptosis, DNA replication, signalling transduction among the others, and are frequently dysregulated in many human cancers. The discovery of specific neddylation inhibitors, represented by MLN4924, has validated CRLs as promising targets for anti-cancer therapies with a growing market. Recent studies have focused on the discovery of the CRLs inhibitors by a variety of approaches, including high through-put screen, virtual screen or structure-based drug design. The field is, however, still facing the major challenging, since CRLs are a large multi-unit protein family without typical active pockets to facilitate the drug design, and enzymatic activity is mainly dependent on undruggable protein-protein interactions and dynamic conformation changes. Up to now, most reported CRLs inhibitors are aiming at targeting the F-box family proteins (e.g., SKP2, β-TrCP and FBXW7), the substrate recognition subunit of SCF E3 ligases. Other studies reported few small molecule inhibitors targeting the UBE2M-DCN1 interaction, which specifically inhibits CRL3/CRL1 by blocking the cullin neddylation. On the other hand, several CRL activators have been reported, such as plant auxin and immunomodulatory imide drugs, thalidomide. Finally, proteolysis-targeting chimeras (PROTACs) has emerged as a new technology in the field of drug discovery, specifically targeting the undruggable protein-protein interaction. The technique connects the small molecule that selectively binds to a target protein to a CRL E3 via a chemical linker to trigger the degradation of target protein. The PROTAC has become a hotspot in the field of E3-ligase-based anti-cancer drug discovery.

Medicinal Chemistry of Inhibiting RING-Type E3 Ubiquitin Ligases

The ubiquitin proteasome system (UPS) presents many opportunities for pharmacological intervention. Key players in the UPS are E3 ubiquitin ligases, responsible for conjugation of ubiquitin to specific cognate substrates. Numbering more than 600 members, these ligases represent the most selective way to intervene within this physiologically important system. This Perspective highlights some of the dedicated medicinal chemistry efforts directed at inhibiting the function of specific single-protein and multicomponent RING-type E3 ubiquitin ligases. We present opportunities and challenges associated with targeting this important class of enzymes.

UbcH10 a Major Actor in Cancerogenesis and a Potential Tool for Diagnosis and Therapy

Malignant transformation is a multistep process in which several molecular entities become dysregulated and result in dysfunction in the regulation of cell proliferation. In past years, scientists have gradually dissected the pathways involved in the regulation of the cell cycle. The mitotic ubiquitin-conjugating enzymes UbcH10, has been extensively studied since its cloning and characterization and it has been identified as a constantly overexpressed factor in many types of cancer. In this paper, we have reviewed the literature about UbcH10 in human cancer, pointing out the association between its overexpression and exacerbation of cancer phenotype. Moreover, many recalled studied demonstrated how immunohistochemistry or RT-PCR analysis can distinguish normal tissues and benign lesions from malignant neoplasms. In other experimental studies, many of the consequences of UbcH10 overexpression, such as increased proliferation, metastasizing, cancer progression and resistance to anticancer drugs are reversed through gene silencing techniques. In recent years, many authors have defined UbcH10 evaluation in cancer patients as a useful tool for diagnosis and therapy. This opinion is shared by the authors who advertise how it would be useful to start using in clinical practice the notions acquired about this important moleculein the carcinogenesis of many human malignancies.

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.

Discovery and Optimization of Salicylic Acid-Derived Sulfonamide Inhibitors of the WD Repeat-Containing Protein 5-MYC Protein-Protein Interaction

The treatment of tumors driven by overexpression or amplification of MYC oncogenes remains a significant challenge in drug discovery. Here, we present a new strategy toward the inhibition of MYC via the disruption of the protein-protein interaction between MYC and its chromatin cofactor WD Repeat-Containing Protein 5. Blocking the association of these proteins is hypothesized to disrupt the localization of MYC to chromatin, thus disrupting the ability of MYC to sustain tumorigenesis. Utilizing a high-throughput screening campaign and subsequent structure-guided design, we identify small-molecule inhibitors of this interaction with potent in vitro binding affinity and report structurally related negative controls that can be used to study the effect of this disruption. Our work suggests that disruption of this protein-protein interaction may provide a path toward an effective approach for the treatment of multiple tumors and anticipate that the molecules disclosed can be used as starting points for future efforts toward compounds with improved drug-like properties.

Prevention of cancer dormancy by Fbxw7 ablation eradicates disseminated tumor cells

Dormant cancer cells known as disseminated tumor cells (DTCs) are often present in bone marrow of breast cancer patients. These DTCs are thought to be responsible for the incurable recurrence of breast cancer. The mechanism underlying the long-term maintenance of DTCs remains unclear, however. Here, we show that Fbxw7 is essential for the maintenance of breast cancer dormancy. Genetic ablation of Fbxw7 in breast cancer cells disrupted the quiescence of DTCs, rendering them proliferative, in mouse xenograft and allograft models. Fbxw7-deficient DTCs were significantly depleted by treatment with paclitaxel, suggesting that cell proliferation induced by Fbxw7 ablation sensitized DTCs to chemotherapy. The combination of Fbxw7 ablation and chemotherapy reduced the number of DTCs even when applied after tumor cell dissemination. Mice injected with Fbxw7-deficient cancer cells survived longer after tumor resection and subsequent chemotherapy than did those injected with wild-type cells. Furthermore, database analysis revealed that breast cancer patients whose tumors expressed FBXW7 at a high level had a poorer prognosis than did those with a low FBXW7 expression level. Our results suggest that a wake-up strategy for DTCs based on Fbxw7 inhibition might be of value in combination with conventional chemotherapy for the treatment of breast cancer.

A patent review of the ubiquitin ligase system: 2015-2018

INTRODUCTION

Ubiquitin-proteasome system (UPS) has been validated as a novel anticancer drug target in the past 20 years. The UPS contains two distinct steps: ubiquitination of a substrate protein by ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and ubiquitin ligase (E3), and substrate degradation by the 26S proteasome complex. The E3 enzyme is the central player in the ubiquitination step and has a wide range of specific substrates in cancer cells, offering great opportunities for discovery and development of selective drugs. Areas covered: This review summarizes the recent advances in small molecule inhibitors of E1s, E2s, and E3s, with a focus on the latest patents (from 2015 to 2018) of E3 inhibitors and modulators. Expert opinion: One strategy to overcome limitations of current 20S proteasome inhibitors is to discover inhibitors of the upstream key components of the UPS, such as E3 enzymes. E3s play important roles in cancer development and determine the specificity of substrate ubiquitination, offering novel target opportunities. E3 modulators could be developed by rational design, natural compound or library screening, old drug repurposes, and application of other novel technologies. Further understanding of mechanisms of E3-substrate interaction will be essential for discovering and developing next-generation E3 inhibitors as effective anticancer drugs.

A Structure-Based Strategy for Engineering Selective Ubiquitin Variant Inhibitors of Skp1-Cul1-F-Box Ubiquitin Ligases

Skp1-Cul1-F-box (SCF) E3 ligases constitute the largest and best-characterized family of the multisubunit E3 ligases with important cellular functions and numerous disease links. The specificity of an SCF E3 ligase is established by one of the 69 human F-box proteins that are recruited to Cul1 through the Skp1 adaptor. We previously reported generation of ubiquitin variants (UbVs) targeting Fbw7 and Fbw11, which inhibit ligase activity by binding at the F-box-Skp1 interface to competitively displace Cul1. In the present study, we employed an optimized engineering strategy to generate specific binding UbVs against 17 additional Skp1-F-box complexes. We validated our design strategy and uncovered the structural basis of binding specificity by crystallographic analyses of representative UbVs bound to Skp1-Fbl10 and Skp1-Fbl11. Our study highlights the power of combining phage display with structure-based design to develop UbVs targeting specific protein surfaces.

Acute Respiratory Distress Syndrome: Bench-to-Bedside Approaches to Improve Drug Development

Despite 50 years of extensive research, no definite drug is currently available to treat acute respiratory distress syndrome (ARDS), and the supportive therapies remain the mainstay of treatment. To improve drug development for ARDS, researchers need to deeply analyze the “omics” approaches, reevaluate the suitable therapeutic targets, resolve the problems of inadequate animal modeling, develop the strategies to reduce the heterogeneity, and reconsider new therapeutic and analytical approaches for better designs of clinical trials.

WD40 repeat domain proteins: a novel target class?

Antagonism of protein-protein interactions (PPIs) with small molecules is becoming more feasible as a therapeutic approach. Successful PPI inhibitors tend to target proteins containing deep peptide-binding grooves or pockets rather than the more common large, flat protein interaction surfaces. Here, we review one of the most abundant PPI domains in the human proteome, the WD40 repeat (WDR) domain, which has a central peptide-binding pocket and is a member of the β-propeller domain-containing protein family. Recently, two WDR domain-containing proteins, WDR5 and EED, as well as other β-propeller domains have been successfully targeted by potent, specific, cell-active, drug-like chemical probes. Could WDR domains be a novel target class for drug discovery? Although the research is at an early stage and therefore not clinically validated, cautious optimism is justified, as WDR domain-containing proteins are involved in multiple disease-associated pathways. The druggability and structural diversity of WDR domain binding pockets suggest that understanding how to target this prevalent domain class will open up areas of disease biology that have so far resisted drug discovery efforts.

PWP1 Mediates Nutrient-Dependent Growth Control through Nucleolar Regulation of Ribosomal Gene Expression

Ribosome biogenesis regulates animal growth and is controlled by nutrient-responsive mTOR signaling. How ribosome biogenesis is regulated during the developmental growth of animals and how nutrient-responsive signaling adjusts ribosome biogenesis in this setting have remained insufficiently understood. We uncover PWP1 as a chromatin-associated regulator of developmental growth with a conserved role in RNA polymerase I (Pol I)-mediated rRNA transcription. We further observed that PWP1 epigenetically maintains the rDNA loci in a transcription-competent state. PWP1 responds to nutrition in Drosophila larvae via mTOR signaling through gene expression and phosphorylation, which controls the nucleolar localization of dPWP1. Our data further imply that dPWP1 acts synergistically with mTOR signaling to regulate the nucleolar localization of TFIIH, a known elongation factor of Pol I. Ribosome biogenesis is often deregulated in cancer, and we demonstrate that high PWP1 levels in human head and neck squamous cell carcinoma tumors are associated with poor prognosis.

Identifying novel small molecule antagonists for mLST8 protein using computational approaches

Mammalian lethal with SEC13 protein 8 (mLST8), is an indispensable protein subunit of mammalian target of rapamycin (mTOR) signaling pathway that interacts with the kinase domain of mTOR protein, thereby stabilizing its active site. Experimental studies reported the over expression of mLST8 in human colon and prostate cancers by activation of both mTORC1/2 complexes and subsequent downstream substrates leading to tumor progression. Considering its role, targeting mLST8 protein would be a therapeutic approach against tumor progression in colon and prostate cancers. Hence, using in silico structure based drug design approach, the comparative binding patterns of 1,1'-binapthyl-2,2'diol (BINOL), 1-(2-carboxynaphth-1yl)-2-naphthoic acid (SCF-12) and their analogs in the cavity of mLST8 were explored. ADME and binding energy calculations led to the identification of five compounds with favorable Glide (G) scores and implicated the importance of Asn132 and Gln225 as key binding residues. Molecular dynamics (MD) simulations and free energy landscape (FEL) approaches helped in elucidating the binding mechanism and suggested the possibility of ligands 1-3 namely, ZINC01765622, ZINC62723702 and ZINC02576980 to be promising antagonists for mLST8. Thus, this study substantiates the prospect of targeting mLST8 protein using potent hits which could hinder tumor progression in colon and prostate cancers.

Pathophysiology and Management of Acute Respiratory Distress Syndrome in Children

Acute respiratory distress syndrome (ARDS) is a syndrome of noncardiogenic pulmonary edema and hypoxia that accompanies up to 30% of deaths in pediatric intensive care units. Pediatric ARDS (PARDS) is diagnosed by the presence of hypoxia, defined by oxygenation index or Pao₂/Fio₂ ratio cutoffs, and new chest infiltrate occurring within 7 days of a known insult. Hallmarks of ARDS include hypoxemia and decreased lung compliance, increased work of breathing, and impaired gas exchange. Mortality is often accompanied by multiple organ failure. Although many modalities to treat PARDS have been investigated, supportive therapies and lung protective ventilator support remain the mainstay.

Ubiquitin Ligases: Structure, Function, and Regulation

Ubiquitin E3 ligases control every aspect of eukaryotic biology by promoting protein ubiquitination and degradation. At the end of a three-enzyme cascade, ubiquitin ligases mediate the transfer of ubiquitin from an E2 ubiquitin-conjugating enzyme to specific substrate proteins. Early investigations of E3s of the RING (really interesting new gene) and HECT (homologous to the E6AP carboxyl terminus) types shed light on their enzymatic activities, general architectures, and substrate degron-binding modes. Recent studies have provided deeper mechanistic insights into their catalysis, activation, and regulation. In this review, we summarize the current progress in structure-function studies of ubiquitin ligases as well as exciting new discoveries of novel classes of E3s and diverse substrate recognition mechanisms. Our increased understanding of ubiquitin ligase function and regulation has provided the rationale for developing E3-targeting therapeutics for the treatment of human diseases.

Induced protein degradation: an emerging drug discovery paradigm

Small-molecule drug discovery has traditionally focused on occupancy of a binding site that directly affects protein function, and this approach typically precludes targeting proteins that lack such amenable sites. Furthermore, high systemic drug exposures may be needed to maintain sufficient target inhibition in vivo, increasing the risk of undesirable off-target effects. Induced protein degradation is an alternative approach that is event-driven: upon drug binding, the target protein is tagged for elimination. Emerging technologies based on proteolysis-targeting chimaeras (PROTACs) that exploit cellular quality control machinery to selectively degrade target proteins are attracting considerable attention in the pharmaceutical industry owing to the advantages they could offer over traditional small-molecule strategies. These advantages include the potential to reduce systemic drug exposure, the ability to counteract increased target protein expression that often accompanies inhibition of protein function and the potential ability to target proteins that are not currently therapeutically tractable, such as transcription factors, scaffolding and regulatory proteins.

The crystal structure of the Sgt1-Skp1 complex: the link between Hsp90 and both SCF E3 ubiquitin ligases and kinetochores

The essential cochaperone Sgt1 recruits Hsp90 chaperone activity to a range of cellular factors including SCF E3 ubiquitin ligases and the kinetochore in eukaryotes. In these pathways Sgt1 interacts with Skp1, a small protein that heterodimerizes with proteins containing the F-box motif. We have determined the crystal structure of the interacting domains of Saccharomyces cerevisiae Sgt1 and Skp1 at 2.8 Å resolution and validated the interface in the context of the full-length proteins in solution. The BTB/POZ domain of Skp1 associates with Sgt1 via the concave surface of its TPR domain using residues that are conserved in humans. Dimerization of yeast Sgt1 occurs via an insertion that is absent from monomeric human Sgt1. We identify point mutations that disrupt dimerization and Skp1 binding in vitro and find that the interaction with Skp1 is an essential function of Sgt1 in yeast. Our data provide a structural rationale for understanding the phenotypes of temperature-sensitive Sgt1 mutants and for linking Skp1-associated proteins to Hsp90-dependent pathways.

An allosteric conduit facilitates dynamic multisite substrate recognition by the SCFCdc4 ubiquitin ligase

The ubiquitin ligase SCF^Cdc4 mediates phosphorylation-dependent elimination of numerous substrates by binding one or more Cdc4 phosphodegrons (CPDs). Methyl-based NMR analysis of the Cdc4 WD40 domain demonstrates that Cyclin E, Sic1 and Ash1 degrons have variable effects on the primary Cdc4^WD40 binding pocket. Unexpectedly, a Sic1-derived multi-CPD substrate (pSic1) perturbs methyls around a previously documented allosteric binding site for the chemical inhibitor SCF-I2. NMR cross-saturation experiments confirm direct contact between pSic1 and the allosteric pocket. Phosphopeptide affinity measurements reveal negative allosteric communication between the primary CPD and allosteric pockets. Mathematical modelling indicates that the allosteric pocket may enhance ultrasensitivity by tethering pSic1 to Cdc4. These results suggest negative allosteric interaction between two distinct binding pockets on the Cdc4^WD40 domain may facilitate dynamic exchange of multiple CPD sites to confer ultrasensitive dependence on substrate phosphorylation.

The deubiquitinase USP21 maintains the stemness of mouse embryonic stem cells via stabilization of Nanog

Nanog is a master pluripotency factor of embryonic stem cells (ESCs). Stable expression of Nanog is essential to maintain the stemness of ESCs. However, Nanog is a short-lived protein and quickly degraded by the ubiquitin-dependent proteasome system. Here we report that the deubiquitinase USP21 interacts with, deubiquitinates and stabilizes Nanog, and therefore maintains the protein level of Nanog in mouse ESCs (mESCs). Loss of USP21 results in Nanog degradation, mESCs differentiation and reduces somatic cell reprogramming efficiency. USP21 is a transcriptional target of the LIF/STAT3 pathway and is downregulated upon differentiation. Moreover, differentiation cues promote ERK-mediated phosphorylation and dissociation of USP21 from Nanog, thus leading to Nanog degradation. In addition, USP21 is recruited to gene promoters by Nanog to deubiquitinate histone H2A at K119 and thus facilitates Nanog-mediated gene expression. Together, our findings provide a regulatory mechanism by which extrinsic signals regulate mESC fate via deubiquitinating Nanog.

Nanog regulates embryonic stem cell (ESC) pluripotency but what controls Nanog protein stability is unclear. Here, the authors show that in mouse ESCs, Nanog protein is ubiquitinated and stabilized by the deubiquitinase USP21, which in turn is regulated by extrinsic signals, STAT3 and ERK.

Specific targeting of the deubiquitinase and E3 ligase families with engineered ubiquitin variants

The ubiquitin proteasome system (UPS) has garnered much attention due to its potential for the development of therapeutics. Following a successful clinical application of general proteasome inhibitors much effort has been devoted to targeting individual UPS components including E3 enzymes and deubiquitinases that control specificity of ubiquitination. Our group has developed a novel approach for targeting the UPS proteins using engineered ubiquitin variants (Ubvs). These drug‐like proteins can serve as valuable tools to study biological function of UPS components and assist in the development of small molecules for clinical use. In this review, we summarize studies of Ubvs targeting members of three major families, including deubiquitinases, HECT E3 ligases, and CRL E3 ligases. In particular, we focus on Ubv binding mechanisms, structural studies, and effects on enzyme function. Furthermore, new insights gained from the Ubvs are discussed in the context of small molecule studies.

NRF2-targeted therapeutics: New targets and modes of NRF2 regulation

Pharmacological activation of the transcription factor nuclear factor-erythroid derived 2-like 2 (NRF2), the key regulator of the cellular antioxidant response, has been recognized as a feasible strategy to reduce oxidative/electrophilic stress and prevent carcinogenesis or other chronic illnesses, such as diabetes and chronic kidney disease. In contrast, due to the discovery of the "dark side" of NRF2, where prolonged activation of NRF2 causes tissue damage, cancer progression, or chemoresistance, efforts have been devoted to identify inhibitors. Currently, only one NRF2 activator has been approved for use in the clinic, while no specific NRF2 inhibitors have been discovered. Future development of NRF2-targeted therapeutics should be based on our current understanding of the regulatory mechanisms of this protein. In addition to the KEAP1-dependent mechanisms, the recent discovery of other pathways involved in the degradation of NRF2 have opened up new possibilities for the development of safe and specific therapeutics. Here, we review available and putative NRF2-targeted therapeutics and discuss their modes of action as well as their potential for disease prevention and treatment.

Targeted Protein Degradation by Small Molecules

Protein homeostasis networks are highly regulated systems responsible for maintaining the health and productivity of cells. Whereas therapeutics have been developed to disrupt protein homeostasis, more recently identified techniques have been used to repurpose homeostatic networks to effect degradation of disease-relevant proteins. Here, we review recent advances in the use of small molecules to degrade proteins in a selective manner. First, we highlight all-small-molecule techniques with direct clinical application. Second, we describe techniques that may find broader acceptance in the biomedical research community that require little or no synthetic chemistry. In addition to serving as innovative research tools, these new approaches to control intracellular protein levels offer the potential to develop novel therapeutics targeting proteins that are not currently pharmaceutically vulnerable.

RACK1 stabilises the activity of PP2A to regulate the transformed phenotype in mammary epithelial cells

Conflicting reports implicate the scaffolding protein RACK1 in the progression of breast cancer. RACK1 has been identified as a key regulator downstream of growth factor and adhesion signalling and as a direct binding partner of PP2A. Our objective was to further characterise the interaction between PP2A and RACK1 and to advance our understanding of this complex in breast cancer cells. We examined how the PP2A holoenzyme is assembled on the RACK1 scaffold in MCF-7 cells. We used immobilized peptide arrays representing the entire PP2A-catalytic subunit to identify candidate amino acids on the C subunit of PP2A that might be involved in binding of RACK1. We identified the RACK1 interaction sites on PP2A. Stable cell lines expressing PP2A with FR69/70AA, R214A and Y218F substitutions were generated and it was confirmed that the RACK1/PP2A interaction is essential to stabilise PP2A activity. We used Real-Time Cell Analysis and a series of assays to demonstrate that disruption of the RACK1/PP2A complex also reduces the adhesion, proliferation, migration and invasion of breast cancer cells and plays a role in maintenance of the cancer phenotype. This work has significantly advanced our understanding of the RACK1/PP2A complex and suggests a pro-carcinogenic role for the RACK1/PP2A interaction. This work suggests that approaches to target the RACK1/PP2A complex are a viable option to regulate PP2A activity and identifies a novel potential therapeutic target in the treatment of breast cancer.

Small-molecule binding sites to explore protein-protein interactions in the cancer proteome

The Cancer Genome Atlas (TCGA) offers an unprecedented opportunity to identify small-molecule binding sites on proteins with overexpressed mRNA levels that correlate with poor survival. Here, we analyze RNA-seq and clinical data for 10 tumor types to identify genes that are both overexpressed and correlate with patient survival. Protein products of these genes were scanned for binding sites that possess shape and physicochemical properties that can accommodate small-molecule probes or therapeutic agents (druggable). These binding sites were classified as enzyme active sites (ENZ), protein-protein interaction sites (PPI), or other sites whose function is unknown (OTH). Interestingly, the overwhelming majority of binding sites were classified as OTH. We find that ENZ, PPI, and OTH binding sites often occurred on the same structure suggesting that many of these OTH cavities can be used for allosteric modulation of enzyme activity or protein-protein interactions with small molecules. We discovered several ENZ (PYCR1, QPRT, and HSPA6) and PPI (CASC5, ZBTB32, and CSAD) binding sites on proteins that have been seldom explored in cancer. We also found proteins that have been extensively studied in cancer that have not been previously explored with small molecules that harbor ENZ (PKMYT1, STEAP3, and NNMT) and PPI (HNF4A, MEF2B, and CBX2) binding sites. All binding sites were classified by the signaling pathways to which the protein that harbors them belongs using KEGG. In addition, binding sites were mapped onto structural protein-protein interaction networks to identify promising sites for drug discovery. Finally, we identify pockets that harbor missense mutations previously identified from analysis of TCGA data. The occurrence of mutations in these binding sites provides new opportunities to develop small-molecule probes to explore their function in cancer.

Inhibition of SCF ubiquitin ligases by engineered ubiquitin variants that target the Cul1 binding site on the Skp1-F-box interface

Skp1-Cul1-F-box (SCF) E3 ligases play key roles in multiple cellular processes through ubiquitination and subsequent degradation of substrate proteins. Although Skp1 and Cul1 are invariant components of all SCF complexes, the 69 different human F-box proteins are variable substrate binding modules that determine specificity. SCF E3 ligases are activated in many cancers and inhibitors could have therapeutic potential. Here, we used phage display to develop specific ubiquitin-based inhibitors against two F-box proteins, Fbw7 and Fbw11. Unexpectedly, the ubiquitin variants bind at the interface of Skp1 and F-box proteins and inhibit ligase activity by preventing Cul1 binding to the same surface. Using structure-based design and phage display, we modified the initial inhibitors to generate broad-spectrum inhibitors that targeted many SCF ligases, or conversely, a highly specific inhibitor that discriminated between even the close homologs Fbw11 and Fbw1. We propose that most F-box proteins can be targeted by this approach for basic research and for potential cancer therapies.

Bacteria-host relationship: ubiquitin ligases as weapons of invasion

Eukaryotic cells utilize the ubiquitin (Ub) system for maintaining a balanced functioning of cellular pathways. Although the Ub system is exclusive to eukaryotes, prokaryotic bacteria have developed an armory of Ub ligase enzymes that are capable of employing the Ub systems of various hosts, ranging from plant to animal cells. These enzymes have been acquired through the evolution and can be classified into three main classes, RING (really interesting new gene), HECT (homologous to the E6-AP carboxyl terminus) and NEL (novel E3 ligases). In this review we describe the roles played by different classes of bacterial Ub ligases in infection and pathogenicity. We also provide an overview of the different mechanisms by which bacteria mimic specific components of the host Ub system and outline the gaps in our current understanding of their functions. Additionally, we discuss approaches and experimental tools for validating this class of enzymes as potential novel antibacterial therapy targets.