Optimization of fluorescently labeled Nrf2 peptide probes and the development of a fluorescence polarization assay for the discovery of inhibitors of Keap1-Nrf2 interaction

Development of a Fluorescence Probe for High-Throughput Screening of Allosteric Inhibitors Targeting TRAP1

Tumor necrosis factor receptor-associated protein 1 (TRAP1) is a molecular chaperone implicated in pro-tumorigenic pathways by regulating the folding of substrate proteins (clients) within cancer cells. Recent research has pinpointed a potentially druggable allosteric site within the client binding site (CBS) of TRAP1, suggesting this site might offer a more effective strategy for developing potent and selective TRAP1 inhibitors. However, the absence of reliable assay systems has hindered quantitative evaluation of inhibitors. In this study, we have developed a fluorescent probe, Rho6TPP, designed to target the CBS. Utilizing fluorescence polarization-based high-throughput screening assays, Rho6TPP exhibits excellent signal-to-noise ratio (>20), Z factor (>0.6), and Z' factor (>0.6). Additionally, it facilitates comparative analysis of existing small molecules and discovery of novel binders. MitoTam, a mitochondria-targeted tamoxifen, emerges as a potent CBS-targeting TRAP1 inhibitor. Our findings highlight the potential of Rho6TPP as a crucial tool for advancing the development of CBS-targeting TRAP1 inhibitors.

Novel and efficient techniques in the discovery of antioxidant peptides

As a research hotspot in food science and nutrition, antioxidant peptides can function by scavenging free radicals, inhibiting peroxides, and chelating metal ions. Therefore, how to efficiently discover and screen antioxidant peptides has become a key issue in research and production. Traditional discovery methods are time-consuming and costly, but also challenging to resolve the quantitative structure-activity relationship of antioxidant peptides. Several novel techniques, including artificial intelligence, molecular docking, bioinformatics, quantum chemistry, phage display, switchSENSE, surface plasmon resonance, and fluorescence polarization, are emerging rapidly as solutions. These techniques possess efficient capability for the discovery of antioxidant peptides, even with the potential for high-throughput screening. In addition, the quantitative structure-activity relationship can be resolved. Notably, combining these novel techniques can overcome the drawbacks of a single one, thus improving efficiency and expanding the discovery horizon. This review has summarized eight novel and efficient techniques for discovering antioxidant peptides and the combination of techniques. This review aims to provide scientific evidence and perspectives for antioxidant peptide research.

Bivalent inhibitors of the BTB E3 ligase KEAP1 enable instant NRF2 activation to suppress acute inflammatory response

Most BTB-containing E3 ligases homodimerize to recognize a single substrate by engaging multiple degrons, represented by E3 ligase KEAP1 dimer and its substrate NRF2. Inactivating KEAP1 to hinder ubiquitination-dependent NRF2 degradation activates NRF2. While various KEAP1 inhibitors have been reported, all reported inhibitors bind to KEAP1 in a monovalent fashion and activate NRF2 in a lagging manner. Herein, we report a unique bivalent KEAP1 inhibitor, biKEAP1 (3), that engages cellular KEAP1 dimer to directly release sequestered NRF2 protein, leading to an instant NRF2 activation. 3 promotes the nuclear translocation of NRF2, directly suppressing proinflammatory cytokine transcription. Data from in vivo experiments showed that 3, with unprecedented potency, reduced acute inflammatory burden in several acute inflammation models in a timely manner. Our findings demonstrate that the bivalent KEAP1 inhibitor can directly enable sequestered substrate NRF2 to suppress inflammatory transcription response and dampen various acute inflammation injuries.

Targeting kelch-like (KLHL) proteins: achievements, challenges and perspectives

Kelch-like proteins (KLHLs) are a large family of BTB-containing proteins. KLHLs function as the substrate adaptor of Cullin 3-RING ligases (CRL3) to recognize substrates. KLHLs play pivotal roles in regulating various physiological and pathological processes by modulating the ubiquitination of their respective substrates. Mounting evidence indicates that mutations or abnormal expression of KLHLs are associated with various human diseases. Targeting KLHLs is a viable strategy for deciphering the KLHLs-related pathways and devising therapies for associated diseases. Here, we comprehensively review the known KLHLs inhibitors to date and the brilliant ideas underlying their development.

Optimization Efforts for Identification of Novel Highly Potent Keap1-Nrf2 Protein-Protein Interaction Inhibitors

In research focused on protein-protein interaction (PPI) inhibitors, the optimization process to achieve both high inhibitory activity and favorable physicochemical properties remains challenging. Our previous study reported the discovery of novel and bioavailable Keap1-Nrf2 PPI inhibitor 8 which exhibited moderate in vivo activity in rats. In this work, we present our subsequent efforts to optimize this compound. Two distinct approaches were employed, targeting high energy water molecules and Ser602 as "hot spots" from the anchor with good aqueous solubility, metabolic stability, and membrane permeability. Through ligand efficiency (LE)-guided exploration, we identified two novel inhibitors 22 and 33 with good pharmacokinetics (PK) profiles and more potent in vivo activities, which appear to be promising chemical probes among the existing inhibitors.

Screening approaches for the identification of Nrf2-Keap1 protein-protein interaction inhibitors targeting hot spot residues

Protein-protein interactions (PPIs) play a crucial role in most biological processes and are important targets in the development of therapeutic agents. However, small molecule drug discovery that targets PPIs remains very challenging. Targeting hot spot residues is considered the best option for inhibiting such interactions, but there are few examples of how knowledge of hot spots can be used in high throughput screening to find hit compounds. A substrate adaptor protein for a ubiquitin ligase complex, Kelch-like ECH-associated protein 1 (Keap1), negatively modulates the expression of genes involved in cellular protection against oxidative stress. Here, we focused on three arginine hot spot residues in the Keap1 substrate binding pocket (Arg380, Arg415, and Arg483), and screened the carboxylic acid library owned by Japan Tobacco Inc. for compounds that interact with the arginine residues in differential scanning fluorescence assays. Furthermore, we identified several small molecule compounds that specifically bind to the Keap1 Kelch domain hot spots by comparing binding to alanine mutant proteins (R380A, R415A, and R483A) with binding to the wild-type protein using surface plasmon resonance (SPR) screening. These compounds inhibited the protein-protein interaction between the Keap1 Kelch domain and the nuclear factor erythroid 2-related factor 2 (Nrf2) peptide, and the ubiquitination of Nrf2 catalyzed by the Cul3/RINGBox 1 E3 ligase. In addition, the binding mode of one compound (Compound 4) was determined by X-ray crystallography after validation of binding by isothermal titration calorimetry, native mass spectrometry, and nuclear magnetic resonance. Compound 4 had favorable thermodynamic properties, and noncovalently bound to Keap1 with a stoichiometry of 1:1. Our results suggest that Compound 4 could potentially be developed into effective therapeutic or preventive agents for a variety of diseases and conditions such as oxidative stress response, inflammation, and carcinogenesis. We believe that the use of a set of complementary biophysical techniques including the SPR assay with single alanine mutant of hot spots provides opportunities to identify hit compounds for developing inhibitors of PPIs.

Structure-based design and optimization of a new class of small molecule inhibitors targeting the P-stalk binding pocket of ricin

Ricin, a category-B agent for bioterrorism, and Shiga toxins (Stxs), which cause food poisoning bind to the ribosomal P-stalk to depurinate the sarcin/ricin loop. No effective therapy exists for ricin or Stx intoxication. Ribosome binding sites of the toxins have not been targeted by small molecules. We previously identified CC10501, which inhibits toxin activity by binding the P-stalk pocket of ricin toxin A subunit (RTA) remote from the catalytic site. Here, we developed a fluorescence polarization assay and identified a new class of compounds, which bind P-stalk pocket of RTA with higher affinity and inhibit catalytic activity with submicromolar potency. A lead compound, RU-NT-206, bound P-stalk pocket of RTA with similar affinity as a five-fold larger P-stalk peptide and protected cells against ricin and Stx2 holotoxins for the first time. These results validate the P-stalk binding site of RTA as a critical target for allosteric inhibition of the active site.

Structural modification of C2-substituents on 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acid derivatives as potent inhibitors of the Keap1-Nrf2 protein-protein interaction

The Keap1-Nrf2-ARE signaling pathway is an attractive therapeutic target for the prevention and treatment of oxidative stress-associated diseases by activating the cellular expression of cytoprotective enzymes and proteins. Small molecule inhibitors can directly disrupt the Keap1-Nrf2 protein-protein interaction (PPI), resulting in elevated levels of Nrf2 protein and subsequent stimulation of related antioxidant responses. Previously, we found that 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acid derivatives with an ether type C2-substituent on the benzene or naphthalene core exhibited potent inhibitory activities with IC50's in the submicromolar or nanomolar range. We here describe a more detailed structure-activity relationship study around the C2 substituents containing various polar linkers shedding new insight on their binding interactions with the Keap1 Kelch domain. The key observation from our findings is that the substituents at the C2-position of the benzene or naphthalene scaffold impact their inhibitory potencies in biochemical assays as well as activities in cell culture. The biochemical FP and TR-FRET assays revealed that the naphthalene derivatives 17b and 18 with an additional carboxylate at the C2 were the most active inhibitors against Keap1-Nrf2 PPI. In the cell-based assay, the two compounds were shown to be potent Nrf2 activators of the transcription of the Nrf2-dependent genes, such as HMOX2, GSTM3, and NQO1.

New Insights into Antioxidant Peptides: An Overview of Efficient Screening, Evaluation Models, Molecular Mechanisms, and Applications

Antioxidant peptides are currently a hotspot in food science, pharmaceuticals, and cosmetics. In different fields, the screening, activity evaluation, mechanisms, and applications of antioxidant peptides are the pivotal areas of research. Among these topics, the efficient screening of antioxidant peptides stands at the forefront of cutting-edge research. To this end, efficient screening with novel technologies has significantly accelerated the research process, gradually replacing the traditional approach. After the novel antioxidant peptides are screened and identified, a time-consuming activity evaluation is another indispensable procedure, especially in in vivo models. Cellular and rodent models have been widely used for activity evaluation, whilst non-rodent models provide an efficient solution, even with the potential for high-throughput screening. Meanwhile, further research of molecular mechanisms can elucidate the essence underlying the activity, which is related to several signaling pathways, including Keap1-Nrf2/ARE, mitochondria-dependent apoptosis, TGF-β/SMAD, AMPK/SIRT1/PGC-1α, PI3K/Akt/mTOR, and NF-κB. Last but not least, antioxidant peptides have broad applications in food manufacture, therapy, and the cosmetics industry, which requires a systematic review. This review introduces novel technologies for the efficient screening of antioxidant peptides, categorized with a new vision. A wide range of activity evaluation assays, encompassing cellular models, as well as rodent and non-rodent models, are provided in a comprehensive manner. In addition, recent advances in molecular mechanisms are analyzed with specific cases. Finally, the applications of antioxidant peptides in food production, therapy, and cosmetics are systematically reviewed.

Target-based drug discovery: Applications of fluorescence techniques in high throughput and fragment-based screening

Target-based discovery of first-in-class therapeutics demands an in-depth understanding of the molecular mechanisms underlying human diseases. Precise measurements of cellular and biochemical activities are critical to gain mechanistic knowledge of biomolecules and their altered function in disease conditions. Such measurements enable the development of intervention strategies for preventing or treating diseases by modulation of desired molecular processes. Fluorescence-based techniques are routinely employed for accurate and robust measurements of in-vitro activity of molecular targets and for discovering novel chemical molecules that modulate the activity of molecular targets. In the current review, the authors focus on the applications of fluorescence-based high throughput screening (HTS) and fragment-based ligand discovery (FBLD) techniques such as fluorescence polarization (FP), Förster resonance energy transfer (FRET), fluorescence thermal shift assay (FTSA) and microscale thermophoresis (MST) for the discovery of chemical probe to exploring target's role in disease biology and ultimately, serve as a foundation for drug discovery. Some recent advancements in these techniques for compound library screening against important classes of drug targets, such as G-protein-coupled receptors (GPCRs) and GTPases, as well as phosphorylation- and acetylation-mediated protein-protein interactions, are discussed. Overall, this review presents a landscape of how these techniques paved the way for the discovery of small-molecule modulators and biologics against these targets for therapeutic benefits.

Design of Tracers in Fluorescence Polarization Assay for Extensive Application in Small Molecule Drug Discovery

Development of fluorescence polarization (FP) assays, especially in a competitive manner, is a potent and mature tool for measuring the binding affinities of small molecules. This approach is suitable for high-throughput screening (HTS) for initial ligands and is also applicable for further study of the structure-activity relationships (SARs) of candidate compounds for drug discovery. Buffer and tracer, especially rational design of the tracer, play a vital role in an FP assay system. In this perspective, we provided different kinds of approaches for tracer design based on successful cases in recent years. We classified these tracers by different types of ligands in tracers, including peptide, nucleic acid, natural product, and small molecule. To make this technology accessible for more targets, we briefly described the basic theory and workflow, followed by highlighting the design and application of typical FP tracers from a perspective of medicinal chemistry.

Targeting Protein-Protein Interfaces with Peptides: The Contribution of Chemical Combinatorial Peptide Library Approaches

Protein-protein interfaces play fundamental roles in the molecular mechanisms underlying pathophysiological pathways and are important targets for the design of compounds of therapeutic interest. However, the identification of binding sites on protein surfaces and the development of modulators of protein-protein interactions still represent a major challenge due to their highly dynamic and extensive interfacial areas. Over the years, multiple strategies including structural, computational, and combinatorial approaches have been developed to characterize PPI and to date, several successful examples of small molecules, antibodies, peptides, and aptamers able to modulate these interfaces have been determined. Notably, peptides are a particularly useful tool for inhibiting PPIs due to their exquisite potency, specificity, and selectivity. Here, after an overview of PPIs and of the commonly used approaches to identify and characterize them, we describe and evaluate the impact of chemical peptide libraries in medicinal chemistry with a special focus on the results achieved through recent applications of this methodology. Finally, we also discuss the role that this methodology can have in the framework of the opportunities, and challenges that the application of new predictive approaches based on artificial intelligence is generating in structural biology.

Optimization of the C2 substituents on the 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid scaffold for better inhibition of Keap1-Nrf2 protein-protein interaction

Direct inhibition of the protein-protein interaction (PPI) between Kelch-like ECH-associated protein 1 (Keap1) and nuclear factor erythroid 2-related factor 2 (Nrf2) reduces the ubiquitination and subsequent degradation of Nrf2, leading to Nrf2 accumulation in the cytosol and the nuclear translocation of Nrf2. Once inside the nucleus, Nrf2 binds to and activates the expression of antioxidant response element (ARE) genes involved in redox homeostasis and detoxification. Herein, we report a series of 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid analogs with varying C2 substituents to explore the structure-activity relationships at this position of the central naphthalene core. The Keap1-binding activities were first screened with a fluorescence polarization (FP) assay followed by further evaluation of the more potent compounds using a more sensitive time-resolved fluorescence energy transfer (TR-FRET) assay. It was found that compound 24a with C2-phthalimidopropyl group was the most potent in this series showing an IC50 of 2.5 nM in the TR-FRET assay with a Ki value in the subnanomolar range. Our docking study indicated that the C2-phthalimidopropyl group in compound 24a provided an extra hydrogen bonding interaction with the key residue Arg415 that may be responsible for the observed boost in binding affinity. In addition, compounds 12b, 15, and 24a were shown to activate the Nrf2 signaling pathway in NCM460D cells resulting in elevated mRNA levels of GSTM3, HMOX1 and NQO1 by 2.4-11.7 fold at 100 μM as compared to the vehicle control.

Dual role of Nrf2 in cancer: molecular mechanisms, cellular functions and therapeutic interventions

BACKGROUND

Nrf2 regulates oxidative stress, which is essential for cellular function. Fundamental initiation of Nrf2 in many malignancies increases prosurvival genes & endorses tumour cell propagation via metabolic reprogramming, suppression of tumour programmed cell death, & increased cancer stem cell self-renewal potential. More specifically, Nrf2 has been associated with cancer cell chemoresistance, radioresistance & inflammation-induced carcinogenesis.  METHODS AND RESULTS: Many Nrf2 inhibitors have been revealed for tumour treatment and targeting Nrf2 could be an effective cancer therapeutic method. Before spreading, cancer cells adapt to their surroundings. Cancer cells usually have mutations in tumor suppressor genes. In a variety of malignancies, somatic mutations & other anomalies in the Nrf2 genes, as well as renowned cancer suppressor genes including TP53, CDKN2A, PTEN & PIK3CA, have been found. In tumour cells, somatic mutations in the Nrf2 genes, as well as additional mechanisms that affect Nrf2 binding, and produce aberrant Nrf2 activation. Uncontrolled Nrf2 causes tumour cells to become resistant to antineoplastic drugs & reactive oxygen species (ROS), as well as guiding them toward metabolic reprogramming.  CONCLUSIONS: As a result, Nrf2 has been studied as potential malignancy treatment target. We covered the pathways, mechanisms, and dual characteristics of Nrf2 in malignancy in this article. We also discussed how Nrf2 inhibitors are targeted against cancer in this review.

Inhibitors of Keap1-Nrf2 protein-protein interaction reduce estrogen responsive gene expression and oxidative stress in estrogen receptor-positive breast cancer

Estrogen contributes to the development of breast cancer through estrogen receptor (ER) signaling and by generating genotoxic metabolites that cause oxidative DNA damage. To protect against oxidative stress, cells activate nuclear factor erythroid 2-related factor 2 (Nrf2) and its downstream cytoprotective genes that initiate antioxidant responses and detoxify xenobiotics. Nrf2 activation occurs by inhibiting the protein-protein interaction (PPI) between Nrf2 and its inhibitor Keap1, which otherwise targets Nrf2 for ubiquitination and destruction. In this study, we examined a series of novel direct inhibitors of Keap1-Nrf2 PPI in their role in promoting the availability of Nrf2 for antioxidant activity and attenuating estrogen-mediated responses in breast cancer. ER-positive human breast cancer cells MCF-7 were treated with 17β-estradiol (E2) in the presence or absence of selected Keap1-Nrf2 PPI inhibitors. Keap1-Nrf2 PPI inhibitors suppressed the mRNA and protein levels of estrogen responsive genes induced by E2 exposure, such as PGR. Keap1-Nrf2 PPI inhibitors caused significant activation of Nrf2 target genes. E2 decreased the mRNA and protein level of the Nrf2 target gene NQO1, and the Keap1-Nrf2 PPI inhibitors reversed this effect. The reversal of E2 action by these compounds was not due to binding to ER as ER antagonists. Further, a selected compound attenuated oxidative stress induced by E2, determined by the level of a biomarker 8-oxo-deoxyguanosine. These findings suggest that the Keap1-Nrf2 PPI inhibitors have potent antioxidant activity by activating Nrf2 pathways and inhibit E2-induced gene and protein expression. These compounds may serve as potential chemopreventive agents in estrogen-stimulated breast cancer.

An Approach to Evaluate the Effective Cytoplasmic Concentration of Bioactive Agents Interacting with a Selected Intracellular Target Protein

To compare the effectiveness of various bioactive agents reversibly acting within a cell on a target intracellular macromolecule, it is necessary to assess effective cytoplasmic concentrations of the delivered bioactive agents. In this work, based on a simple equilibrium model and the cellular thermal shift assay (CETSA), an approach is proposed to assess effective concentrations of both a delivered bioactive agent and a target protein. This approach was tested by evaluating the average concentrations of nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated-protein 1 (Keap1) proteins in the cytoplasm for five different cell lines (Hepa1, MEF, RAW264.7, 3LL, and AML12) and comparing the results with known literature data. The proposed approach makes it possible to analyze both binary interactions and ternary competition systems; thus, it can have a wide application for the analysis of protein-protein or molecule-protein interactions in the cell. The concentrations of Nrf2 and Keap1 in the cell can be useful not only in analyzing the conditions for the activation of the Nrf2 system, but also for comparing the effectiveness of various drug delivery systems, where the delivered molecule is able to interact with Keap1.

KEAP1-NRF2 protein-protein interaction inhibitors: Design, pharmacological properties and therapeutic potential

The transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) is considered the master regulator of the phase II antioxidant response. It controls a plethora of cytoprotective genes related to oxidative stress, inflammation, and protein homeostasis, among other processes. Activation of these pathways has been described in numerous pathologies including cancer, cardiovascular, respiratory, renal, digestive, metabolic, autoimmune, and neurodegenerative diseases. Considering the increasing interest of discovering novel NRF2 activators due to its clinical application, initial efforts were devoted to the development of electrophilic drugs able to induce NRF2 nuclear accumulation by targeting its natural repressor protein Kelch-like ECH-associated protein 1 (KEAP1) through covalent modifications on cysteine residues. However, off-target effects of these drugs prompted the development of an innovative strategy, the search of KEAP1-NRF2 protein-protein interaction (PPI) inhibitors. These innovative activators are proposed to target NRF2 in a more selective way, leading to potentially improved drugs with the application for a variety of diseases that are currently under investigation. In this review, we summarize known KEAP1-NRF2 PPI inhibitors to date and the bases of their design highlighting the most important features of their respective interactions. We also discuss the preclinical pharmacological properties described for the most promising compounds.

Design, synthesis and evaluation of novel small molecules acting as Keap1-Nrf2 protein-protein interaction inhibitors

Direct interference with Kelch-like ECH-associated protein 1 (Keap1)-Nrf2 protein-protein interaction (PPI) has recently been introduced as an attractive approach to control life-threatening diseases like myocarditis. The present study aimed to investigate the potential application in myocarditis of a series of novel non-naphthalene derivatives as potential Keap1-Nrf2 PPI inhibitors. Our results indicated that the optimal compound K22 displayed the highest metabolic stability and showed notable Keap1-Nrf2 PPI inhibitory activities in vitro. K22 effectively triggered Nrf2 activation and increased the protein and mRNA expression of Nrf2-regulated genes in H9c2 cells. Moreover, pre-treatment with K22 was shown to mitigate LPS-induced damage to H9c2 cells, causing a marked decrease in the levels of inflammatory factors as well as reactive oxygen species (ROS). Furthermore, K22 was also shown to be non-mutagenic in the Ames test. Overall, our findings suggest that K22 may be a promising drug lead as a Keap1-Nrf2 PPI inhibitor for myocarditis treatment.

Development of Noncovalent Small-Molecule Keap1-Nrf2 Inhibitors by Fragment-Based Drug Discovery

Targeting the protein-protein interaction (PPI) between the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and its repressor, Kelch-like ECH-associated protein 1 (Keap1), constitutes a promising strategy for treating diseases involving oxidative stress and inflammation. Here, a fragment-based drug discovery (FBDD) campaign resulted in novel, high-affinity (K_i = 280 nM), and cell-active noncovalent small-molecule Keap1-Nrf2 PPI inhibitors. We screened 2500 fragments using orthogonal assays─fluorescence polarization (FP), thermal shift assay (TSA), and surface plasmon resonance (SPR)─and validated the hits by saturation transfer difference (STD) NMR, leading to 28 high-priority hits. Thirteen co-structures showed fragments binding mainly in the P4 and P5 subpockets of Keap1's Kelch domain, and three fluorenone-based fragments featuring a novel binding mode were optimized by structure-based drug discovery. We thereby disclose several fragment hits, including their binding modes, and show how FBDD can be performed to find new small-molecule Keap1-Nrf2 PPI inhibitors.

Structure-activity relationships of 1,4-bis(arylsulfonamido)-benzene or naphthalene-N,N'-diacetic acids with varying C2-substituents as inhibitors of Keap1-Nrf2 protein-protein interaction

The Keap1-Nrf2-ARE pathway plays an important role in responding to oxidative stress and maintaining the redox homeostasis. Small molecule inhibitors targeting directly the Keap1-Nrf2 protein-protein interaction (PPI) can potentially be developed into effective preventive and therapeutic agents for numerous chronic inflammatory diseases. To improve the drug-like properties and inhibitory potency of these inhibitors, a series of 1,4-bis(arylsulfonamido)benzene or naphthalene-N,N'-diacetic acids with varying substituents at C-2 position of the benzene or naphthalene core were designed and synthesized. Among them, compound 12d with 2-(4-fluorobenzyloxy) group was the most potent direct inhibitor of Keap1-Nrf2 PPI with an IC₅₀ of 64.5 nM in the fluorescent polarization (FP) assay and 14.2 nM in a time-resolved fluorescence resonance energy transfer (TR-FRET) assay. Moreover, cell-based biological assay showed that 12d significantly increased the mRNA levels of Nrf2 downstream genes, GSTM3, HMOX2 and NQO1, through Nrf2 activation. The discovery of the new scaffolds possessing diverse O-linked fragments at the C2 position offers opportunities to further modify the chemical structures of Keap1-Nrf2 PPI inhibitors to improve their pharmacokinetic, efficacy and safety profiles.

Esculetin alleviates murine lupus nephritis by inhibiting complement activation and enhancing Nrf2 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Esculetin is a bioactive compound of medicinal herb Hydrangea paniculata, and has showed anti-oxidation and anti-inflammation bioactivities. Renal local oxidative stress and inflammation are import contributors for progression of lupus nephritis (LN).

AIM OF THE STUDY

In the present study, the renal protective effect of esculetin against LN was evaluated using MRL/lpr mice.

MATERIALS AND METHODS

MRL/lpr mice were orally administrated with esculetin (20 mg/kg and 40 mg/kg) from 10 to 20 weeks and then renal function and kidney pathology were analyzed.

RESULTS

Esculetin significantly attenuated renal impairment in MRL/lpr mice by reducing blood urea nitrogen (BUN), serum creatinine (Scr) and albuminuria, and ameliorated the glomerular hypertrophy, tubular interstitial fibrosis and mononuclear cell infiltration into interstitium. mRNA microarray suggested that esculetin could significantly down-regulate complement cascade, inflammation and fibrosis pathway, and up-regulate Nrf2-related anti-oxidation genes. Most surprising finding in the current study was that esculetin could inhibit the complement activation both in classical and alternative pathway using in vitro hemolysis assay, further enzyme assay suggested that esculetin blocked the C3 convertase (C4b2a) to exert this inhibitory capability. Molecular docking predicted that esculetin had four conventional hydrogen bonds interacting with C4b2a, and CDOCKER energy is relatively lower. Luciferase reporter gene demonstrated that esculetin could activate Nrf2 signaling pathway, and further flow cytometry confirmed that anti-oxidation bioactivity of esculetin was dependent on Nrf2 activation. On the other hand, esculetin could inhibit NFκB nuclear translocation and TGFβ-smad3 profibrosis pathway.

CONCLUSION

Esculetin shows beneficial effect on LN progression, and it may be a good natural leading compound for design of chemical compounds to treat LN.

Nrf2/Keap1/ARE signaling: Towards specific regulation

The Nrf2 transcription factor governs the expression of hundreds genes involved in cell defense against oxidative stress, the hallmark of numerous diseases such as neurodegenerative, cardiovascular, some viral pathologies, diabetes and others. The main route for Nrf2 activity regulation is via interactions with the Keap1 protein. Under the normoxia the Keap1 binds the Nrf2 and targets it to the proteasomal degradation, while the Keap1 is regenerated. Upon oxidative stress the interactions between Nrf2 and Keap1 are interrupted and the Nrf2 activates the transcription of the protective genes. Currently, the Nrf2 system activation is considered as a powerful cytoprotective strategy for treatment of different pathologies, which pathogenesis relies on oxidative stress including viral diseases of pivotal importance such as COVID-19. The implementation of this strategy is accomplished mainly through the inactivation of the Keap1 "guardian" function. Two approaches are now developing: the Keap1 modification via electrophilic agents, which leads to the Nrf2 release, and direct interruption of the Nrf2:Keap1 protein-protein interactions (PPI). Because of theirs chemical structure, the Nrf2 electrophilic inducers could non-specifically interact with others cellular proteins leading to undesired effects. Whereas the non-electrophilic inhibitors of the Nrf2:Keap1 PPI could be more specific, thereby widening the therapeutic window.

The Potential of Nrf2 Activation as a Therapeutic Target in Systemic Lupus Erythematosus

Inflammation and oxidative stress are well established in systemic lupus erythematosus (SLE) and are critical to the pathogenesis of autoimmune diseases. The transcription factor NF-E2 related factor 2 (Nrf2) is a central regulator of cellular anti-oxidative responses, inflammation, and restoration of redox balance. Accumulating reports support an emerging role for the regulation of Nrf2 in SLE. These include findings on the development of lupus-like autoimmune nephritis and altered immune cell populations in mice lacking Nrf2, as well as decreased Nrf2 abundance in the dendritic cells of patients with SLE. Nrf2-inducing agents have been shown to alleviate oxidative and inflammatory stress and reduce tissue injury in SLE mouse models. Since Nrf2 expression can be increased in activated T cells, the precise role of Nrf2 activation in different immune cell types and their function remains to be defined. However, targeting Nrf2 for the treatment of diseases associated with oxidative stress and inflammation, such as SLE, is promising. As investigation of Nrf2-inducing agents in clinical trials grows, defining the signaling and molecular mechanisms of action and downstream effects in response to different Nrf2-inducing agents in specific cells, tissues, and diseases, will be critical for effective clinical use.

Optimization of 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acids as inhibitors of Keap1-Nrf2 protein-protein interaction to suppress neuroinflammation

The protein-protein interaction (PPI) between kelch-like ECH-associated protein 1 (Keap1) and nuclear factor erythroid 2-related factor 2 (Nrf2) is recognized as a promising target for the prevention and treatment of oxidative stress-related inflammatory diseases. Herein, a series of novel 1,4-bis(arylsulfonamido)naphthalene-N,N'-diacetic acid analogs (7p-t and 8c) were designed to further explore the structure-activity relationships of the series. Their activities were measured first with a fluorescence polarization (FP) assay and more potent compounds were further evaluated using a more sensitive time-resolved fluorescence energy transfer (TR-FRET) assay, demonstrating IC₅₀ values between 7.2 and 31.3 nM. In cytotoxicity studies, the naphthalene derivatives did not show noticeable toxicity to human HepG2-C8 and mouse brain BV-2 microglia cells. Among them, compound 7q bearing oxygen-containing fused rings was shown to significantly stimulate the cellular Nrf2 signaling pathway, including activation of antioxidant response element (ARE)-controlled expression of Nrf2 target genes and proteins. More importantly, 7q suppressed up-regulation of several pro-inflammatory cytokines in lipopolysaccharide (LPS)-challenged BV-2 microglial cells, representing a potential therapeutic application for controlling neuroinflammatory disorders.

Targeting NRF2 to treat cancer

NRF2 is a basic leucine zipper (bZip) transcription factor that is the master regulator of redox homeostasis. Under basal conditions, the cellular level of NRF2 is low due to a posttranslational regulation by the ubiquitin proteasome system (UPS). But, when an organism is challenged with oxidative or xenobiotic stress, the NRF2 pathway is activated by inhibition of the E3 ubiquitin ligase complex that normally marks NRF2 for destruction. For several decades, researchers have searched for molecules that can intentionally activate NRF2, as this was shown to be a means to prevent certain diseases, at least in animal models. In the present era, there are many compounds known to activate the NRF2 pathway including natural products and synthetic compounds, covalent and non-covalent compounds, and others. However, it was also revealed that like many protective pathways, the NRF2 pathway has a dark side. Just as NRF2 can protect normal cells from damage, it can protect malignant cells from damage. As cells transform, they are exposed to many stressors and aberrant upregulation of NRF2 can facilitate transformation and it can help cancer cells to grow, to spread, and to resist treatment. For this reason, researchers are also interested in the discovery and development of NRF2 inhibitors. In the present review, we will begin with a general discussion of NRF2 structure and function, we will discuss the latest in NRF2 non-covalent activators, and we will discuss the current state of NRF2 inhibitors.

Competitive and noncompetitive fluorescence polarization immunoassays for the detection of benzothiostrobin using FITC-labeled dendrimer-like peptides

Peptides obtained from phage display libraries are valuable reagents for small-molecule immunoassays. However, their application in fluorescence polarization immunoassays (FPIAs) is limited by phage particles. Here, monomer, dendrimer-like dimer, tetramer peptidomimetic and anti-immunocomplex tracers were designed and synthesized using lysine as special scaffolds and spacers to develop competitive and noncompetitive FPIAs for benzothiostrobin. The affinity between tracers and monoclonal antibodies or immunocomplexes increased with the tracer valence. A higher signal-to-noise ratio and sensitivity could be generated in the FPIAs based on tetramer tracers. The sensitivities of competitive (50% inhibitory concentration) and noncompetitive (50% saturation concentration) FPIAs were 19.71 ± 4.65 and 40.43 ± 2.73 ng mL^-1, respectively. The spiked recoveries were 78.3%-105.2% with relative standard deviations (RSDs) of 0.7%-15.4% for the competitive FPIA, while 78.7%-115.3% with RSDs of 0.7%-12.5% for the noncompetitive FPIA. The amounts of benzothiostrobin in rice detected by the FPIAs were consistent with those detected by high performance liquid chromatography.

Ginnalin A Binds to the Subpockets of Keap1 Kelch Domain To Activate the Nrf2-Regulated Antioxidant Defense System in SH-SY5Y Cells

Ginnalin A (GA), a polyphenol from the red maple, was reported to be a potential ROS scavenger or an activator of nuclear factor erythroid-2 related factor 2 (Nrf2) in cancer cells. However, whether GA could activate Nrf2 in neuronal cells and the exact mode of action are unknown. We performed molecular docking calculations, which revealed that GA fits well into the five subpockets of the Kelch-like ECH-associated protein1 (Keap1) Kelch domain via hydrogen bonding and hydrophobic interaction. Our cytotoxicity assays demonstrate that pretreating SH-SY5Y cells with 20 μM GA effectively prevents cells from oxidative assault by 6-hydroxydopamine (6-OHDA). Fluorescence imaging indicates that upon the GA pretreatment, Nrf2 dissociates from the Keap1-Nrf2 complex and translocates into nucleus to activate the cellular antixodant system. Real-time qPCR quantification and Western blotting verified that the GA pretreatment elevates NAD(P)H quinone oxidoreductase-1 (NQO1) by more than 4.6-fold, heme oxygenase (HO-1) by about 1.2-fold, and the glutamate-cysteine ligase catalytic (GCLC) subunit by 0.7-fold. The higher antixidant protein levels, along with increased glutathione concentration, decrease intracellular reactive oxygen species and alleviate the 6-OHDA-induced oxidative damage. Silence of Nrf2 abrogates the cytoprotection of the GA pretreatment, confirming that the Keap1/Nrf2-ARE (antioxidant response element) pathway is solely responsible for the GA's biological effects. GA is a promising natural compound for sensitizing neuronal cells' antioxidative defense system to offset oxidative stress, a condition closely linked to the pathogenesis of Parkinson's disease.

Perspectives on the Clinical Development of NRF2-Targeting Drugs

The transcription factor NRF2 (nuclear factor erythroid 2-related factor 2) triggers homeostatic responses against a plethora of environmental or endogenous deviations in redox metabolism, inflammation, proteostasis, etc. Therefore, pharmacological activation of NRF2 is a promising therapeutic strategy for several chronic diseases that are underlined by low-grade oxidative inflammation and dysregulation of redox metabolism, such as neurodegenerative, cardiovascular, and metabolic diseases. While NRF2 activation is useful in inhibiting carcinogenesis, its inhibition is needed in constituted tumors where NRF2 provides a survival advantage in the challenging tumor niche. This review describes the electrophilic and non-electrophilic NRF2 activators with clinical projection in various chronic diseases. We also analyze the status of NRF2 inhibitors, which are for the moment in a proof-of-concept stage. Advanced in silico screening and medicinal chemistry are expected to provide new or repurposing small molecules with increased potential for fostering the development of targeted NRF2 modulators. The nuclear factor erythroid 2 (NFE2)-related factor 2 (NRF2) is rapidly degraded by proteasomes under a basal condition in a Keap1-dependent manner. ROS oxidatively modifies Keap1 to release NRF2 and allow its nuclear translocation. Here it binds to the antioxidant response element to regulate gene transcription. An alternative mechanism controlling NRF2 stability is glycogen synthase kinase 3 (GSK-3)-induced phosphorylation. Indicated in blue are NRF2-activating and NRF2-inhibiting drugs.

Development of a Homogeneous Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) Assay for the Inhibition of Keap1-Nrf2 Protein-Protein Interaction

The transcription factor, nuclear factor erythroid 2-related factor 2 (Nrf2), plays a major role in regulating the antioxidant defense system through the Kelch-like ECH-associated protein 1-Nrf2-antioxidant response element (Keap1-Nrf2-ARE) pathway. Small-molecule inhibitors targeting Keap1-Nrf2 protein-protein interaction (PPI) decrease the rate of Nrf2 degradation by the 26S proteasome and thus increase the intracellular level of Nrf2, which translocates into the nucleus, leading to upregulated expression of cytoprotective and antioxidant enzymes. Such inhibitors can be developed into potential preventive and therapeutic agents of diseases caused by oxidative damage. To more effectively identify promising Nrf2 activators through the inhibition of Keap1-Nrf2 PPI, a homogeneous time-resolved fluorescence resonance energy transfer (TR-FRET) assay was developed in this work by indirectly labeling the Keap1 Kelch domain protein with Tb-anti-His antibody as the donor and using, as the acceptor, fluorescein isothiocyanate (FITC)-labeled 9mer Nrf2 peptide amide, the same fluorescent probe that was used in an earlier fluorescence polarization (FP) assay. Assay conditions, including concentrations of the various components, buffer type, and incubation time, were optimized in the TR-FRET competition assay with known small-molecule inhibitors of Keap1-Nrf2 PPI. Under the optimized conditions, the Keap1-Nrf2 TR-FRET assay exhibited great sensitivity with a high dynamic range and considerable stability for as long as 5 h. The Z' factor was determined to be 0.82, suggesting that the assay is suitable for high-throughput screening and lead optimization of inhibitors of Keap1-Nrf2 PPI. Furthermore, the TR-FRET assay is capable of differentiating potent inhibitors of Keap1-Nrf2 PPI down to the subnanomolar inhibition constant (Ki) range.

Recent advances in the development of protein-protein interactions modulators: mechanisms and clinical trials

Protein-protein interactions (PPIs) have pivotal roles in life processes. The studies showed that aberrant PPIs are associated with various diseases, including cancer, infectious diseases, and neurodegenerative diseases. Therefore, targeting PPIs is a direction in treating diseases and an essential strategy for the development of new drugs. In the past few decades, the modulation of PPIs has been recognized as one of the most challenging drug discovery tasks. In recent years, some PPIs modulators have entered clinical studies, some of which been approved for marketing, indicating that the modulators targeting PPIs have broad prospects. Here, we summarize the recent advances in PPIs modulators, including small molecules, peptides, and antibodies, hoping to provide some guidance to the design of novel drugs targeting PPIs in the future.

Beyond repression of Nrf2: An update on Keap1

Nrf2 (NFE2L2 - nuclear factor (erythroid-derived 2)-like 2) is a transcription factor, which is repressed by interaction with a redox-sensitive protein Keap1 (Kelch-like ECH-associated protein 1). Deregulation of Nrf2 transcriptional activity has been described in the pathogenesis of multiple diseases, and the Nrf2/Keap1 axis has emerged as a crucial modulator of cellular homeostasis. Whereas the significance of Nrf2 in the modulation of biological processes has been well established and broadly discussed in detail, the focus on Keap1 rarely goes beyond the regulation of Nrf2 activity and redox sensing. However, recent studies and scrutinized analysis of available data point to Keap1 as an intriguing and potent regulator of cellular function. This review aims to shed more light on Keap1 structure, interactome, regulation and non-canonical functions, thereby enhancing its significance in cell biology. We also intend to highlight the impact of balance between Keap1 and Nrf2 in the maintenance of cellular homeostasis.

Recent progress in Keap1-Nrf2 protein-protein interaction inhibitors

Therapeutic targeting the protein-protein interaction (PPI) of Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and its main regulator, Kelch-like ECH-Associating protein 1 (Keap1) has been emerged as a feasible way to combat oxidative stress related diseases, due to the key role of Nrf2 in oxidative stress regulation. In recent years, many efforts have been made to develop potent Keap1-Nrf2 inhibitors with new chemical structures. Various molecules with diverse chemical structures have been reported and some compounds exhibit high potency. This review summarizes peptide and small molecule Keap1-Nrf2 inhibitors reported recently. We also highlight the pharmacological effects and discuss the possible therapeutic application of Keap1-Nrf2 inhibitors.

Development of an enzyme-linked immunosorbent assay for Keap1-Nrf2 interaction inhibitors identification

Development of Keap1–Nrf2 interaction inhibitors is a promising strategy for the discovery of therapeutic agents against oxidative stress-mediated diseases. Two motifs of Nrf2, ETGE and DLG motif, are responsible for Keap1-Nrf2 binding. Previously, ETGE peptide or ETGE-derived peptide-based approaches were used to detect Keap1-Nrf2 interaction; however, these approaches are not able to monitor Keap1-DLG motif binding. We first report here a novel Enzyme-linked Immunosorbent Assay (ELISA) approach to detect the protein-protein interaction of full length Keap1 and Nrf2. In our assay, the test compounds can target either ETGE or DLG binding site, therefore facilitating the exploration of diverse Keap1-Nrf2 inhibitors. Three FDA-approved drugs, zafirlukast, dutasteride and ketoconazole, were found to inhibit the Keap1-Nrf2 interaction with IC₅₀ of 5.87, 2.81 and 1.67 μM, respectively. Additionally, these three drugs also activated Nrf2 pathway in neuroblasts and lipopolysaccharide (LPS)-challenged mice. The results presented here indicate that the ELISA approach has the capacity to identify Keap1-Nrf2 inhibitors.

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We established a novel Enzyme-linked Immunosorbent Assay for Keap1-Nrf2 interaction inhibitors identification.

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This ELISA is the first approach to detect the protein-protein interaction of full length Keap1 and Nrf2.

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In this assay, the test compounds can target either ETGE or DLG binding site, therefore facilitating the exploration of diverse Keap1-Nrf2 inhibitors.

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Zafirlukast, dutasteride and ketoconazole have been identified as inhibitors of the Keap1-Nrf2 interaction.

Nrf2 activation through the inhibition of Keap1-Nrf2 protein-protein interaction

Activation of the transcription factor Nrf2 via the Keap1-Nrf2-ARE signaling system regulates the transcription and subsequent expression of cellular cytoprotective proteins and plays a crucial role in preventing pathological conditions exacerbated by the overproduction of oxidative stress. In addition to electrophilic modulators, direct non-covalent inhibitors that interrupt the Keap1-Nrf2 protein-protein interaction (PPI) leading to Nrf2 activation have attracted a great deal of attention as potential preventive and therapeutic agents for oxidative stress-related diseases. Structural studies of Keap1-binding ligands, development of biochemical and cellular assays, and new structure-based design approaches have facilitated the discovery of small molecule PPI inhibitors. This perspective reviews the Keap1-Nrf2-ARE system, its physiological functions, and the recent progress in the discovery and the potential applications of direct inhibitors of Keap1-Nrf2 PPI.

Synthesis and Evaluation of Noncovalent Naphthalene-Based KEAP1-NRF2 Inhibitors

The oxidative stress response, gated by the protein-protein interaction of KEAP1 and NRF2, has garnered significant interest in the past decade. Misregulation in this pathway has been implicated in disease states such as multiple sclerosis, rheumatoid arthritis, and diabetic chronic wounds. Many of the known activators of NRF2 are electrophilic in nature and may operate through several biological pathways rather than solely through the activation of the oxidative stress response. Recently, our lab has reported a nonelectrophilic, monoacidic, naphthalene-based NRF2 activator which exhibited good potency in vitro. Herein, we report a detailed structure-activity relationship of naphthalene-based NRF2 activators, an X-ray crystal structure of our monoacidic KEAP1 inhibitor, and identification of an underexplored area of the NRF2 binding pocket of KEAP1.

Recent progress in the development of small molecule Nrf2 activators: a patent review (2017-present)

Introduction: The transcription factor Nrf2 (nuclear factor erythroid 2-related factor 2) is the first line of defense against a plethora of environmental or endogenous deviations in redox metabolism, proteostasis, inflammation, etc. Therefore, pharmacological activation of Nrf2 is a potential therapeutic approach for several diseases related to oxidative stress and inflammation, such as cancer, cardiovascular, and neurodegenerative diseases.Areas covered: The authors first describe the biological function of Nrf2 and the molecular regulatory mechanism of Keap1-Nrf2-ARE ((Kelch-like ECH-Associating protein 1)-Nrf2-(antioxidant response element)). Then, they review recent progress of covalent activators and non-covalent Keap1-Nrf2 protein-protein interaction (PPI) inhibitors from patents and publications in 2017-present, consisting of new chemical molecules, structure optimization of reported activators and progress in preclinical or clinical trials.Expert opinion: Despite significant achievements in the development of Nrf2 activators, the selectivity is the primary consideration. Due to reacting with redox-sensitive cysteines in proteins except for Keap1, electrophilic activators often exhibit off-target effects. For Keap1-Nrf2 PPI inhibitors, how to enhance in vivo efficacy and/or penetrate blood-brain barrier (BBB) to reach central nervous system (CNS) is also challenging. Fragment-based drug discovery (FBDD), carboxylic acid bioisosteric replacement and prodrug approach might be used to circumvent this challenge. Moreover, the possibility of cancer risk caused by Nrf2 activation needs to be considered carefully.

Interaction Energetics and Druggability of the Protein-Protein Interaction between Kelch-like ECH-Associated Protein 1 (KEAP1) and Nuclear Factor Erythroid 2 Like 2 (Nrf2)

Development of small molecule inhibitors of protein-protein interactions (PPIs) is hampered by our poor understanding of the druggability of PPI target sites. Here, we describe the combined application of alanine-scanning mutagenesis, fragment screening, and FTMap computational hot spot mapping to evaluate the energetics and druggability of the highly charged PPI interface between Kelch-like ECH-associated protein 1 (KEAP1) and nuclear factor erythroid 2 like 2 (Nrf2), an important drug target. FTMap identifies four binding energy hot spots at the active site. Only two of these are exploited by Nrf2, which alanine scanning of both proteins shows to bind primarily through E79 and E82 interacting with KEAP1 residues S363, R380, R415, R483, and S508. We identify fragment hits and obtain X-ray complex structures for three fragments via crystal soaking using a new crystal form of KEAP1. Combining these results provides a comprehensive and quantitative picture of the origins of binding energy at the interface. Our findings additionally reveal non-native interactions that might be exploited in the design of uncharged synthetic ligands to occupy the same site on KEAP1 that has evolved to bind the highly charged DEETGE binding loop of Nrf2. These include π-stacking with KEAP1 Y525 and interactions at an FTMap-identified hot spot deep in the binding site. Finally, we discuss how the complementary information provided by alanine-scanning mutagenesis, fragment screening, and computational hot spot mapping can be integrated to more comprehensively evaluate PPI druggability.

Discovery of disubstituted xylylene derivatives as small molecule direct inhibitors of Keap1-Nrf2 protein-protein interaction

The Keap1-Nrf2-ARE system represents a crucial antioxidant defense mechanism that protects cells against reactive oxygen species. Targeting Keap1-Nrf2 protein-protein interaction (PPI) has become a promising drug target for several oxidative stress-related and inflammatory diseases including pulmonary fibrosis, chronic obstructive pulmonary disorder (COPD) and cancer chemoprevention. For the development of a potential therapeutic agent, drug-like properties and potency are important considerations. In this work, we focused on the modification of 4 as a lead through a molecular dissection strategy in an effort to improve its metabolic stability, leading to the discovery of a series of new disubstituted xylylene derivatives. The preliminary SAR of 9a indicated that compound 21a containing S-methylated acetate moieties exhibited comparable potency to the lead compound 4 in a fluorescent polarization assay but with improved metabolic stability in the presence of human liver microsomes.

Isoquinoline Kelch-like ECH-Associated Protein 1-Nuclear Factor (Erythroid-Derived 2)-like 2 (KEAP1-NRF2) Inhibitors with High Metabolic Stability

Pharmacological activation of NRF2 (nuclear factor erythroid 2-related factor 2) arises from blocking the interaction of NRF2 with its negative regulator, KEAP1 (Kelch-like ECH-associated protein 1). We previously reported an isoquinoline-based NRF2 activator, but this compound showed negative logD_7.4 and a -2 charge at physiological pH, which may have limited its membrane permeability. In this work, we report potent, metabolically stable analogs that result from replacing a carboxymethyl group at the 4-position with a fluoroalkyl group.

Development of a Cell-Permeable Cyclic Peptidyl Inhibitor against the Keap1-Nrf2 Interaction

Macrocyclic peptides have proven to be highly effective inhibitors of protein-protein interactions but generally lack cell permeability to access intracellular targets. We show herein that macrocyclic peptides may be rendered highly cell-permeable and biologically active by conjugating them with a cyclic cell-penetrating peptide (CPP). A previously reported cyclic peptidyl inhibitor against the Kelch-like ECH-associated protein 1 (Keap1)-nuclear factor erythroid-2 (Nrf2) interaction (K_D = 18 nM) was covalently attached to a cyclic CPP through a flexible linker. The resulting bicyclic peptide retained the Keap1-binding activity, resisted proteolytic degradation, readily entered mammalian cells, and activated the transcriptional activity of Nrf2 at nanomolar to low micromolar concentrations in cell culture. The inhibitor provides a useful tool for investigating the biological function of Keap1-Nrf2 and a potential lead for further development into a novel class of anti-inflammatory and anticancer agents. Our data suggest that other membrane-impermeable cyclic peptides may be similarly rendered cell-permeable by conjugation with a cyclic CPP.

A Comparative Assessment Study of Known Small-Molecule Keap1-Nrf2 Protein-Protein Interaction Inhibitors: Chemical Synthesis, Binding Properties, and Cellular Activity

Inhibiting the protein-protein interaction (PPI) between the transcription factor Nrf2 and its repressor protein Keap1 has emerged as a promising strategy to target oxidative stress in diseases, including central nervous system (CNS) disorders. Numerous non-covalent small-molecule Keap1-Nrf2 PPI inhibitors have been reported to date, but many feature suboptimal physicochemical properties for permeating the blood-brain barrier, while others contain problematic structural moieties. Here, we present the first side-by-side assessment of all reported Keap1-Nrf2 PPI inhibitor classes using fluorescence polarization, thermal shift assay, and surface plasmon resonance-and further evaluate the compounds in an NQO1 induction cell assay and in counter tests for nonspecific activities. Surprisingly, half of the compounds were inactive or deviated substantially from reported activities, while we confirm the cross-assay activities for others. Through this study, we have identified the most promising Keap1-Nrf2 inhibitors that can serve as pharmacological probes or starting points for developing CNS-active Keap1 inhibitors.

A Case Study on the Keap1 Interaction with Peptide Sequence Epitopes Selected by the Peptidomic mRNA Display

Many protein-protein and peptide-protein interactions (PPIs) play key roles in the regulation of biological functions, and therefore, the modulation of PPIs has become an attractive target of new drug development. Although a number of PPIs have already been identified, over 100 000 unknown PPIs are predicted to exist. To uncover such unknown PPIs, it is important to devise a conceptually distinct method from that of currently available methods. Herein, an mRNA display by using a total RNA library derived from various human tissues, which serves as a unique method to physically isolate peptide epitopes that potentially bind to a target protein of interest, is reported. In this study, selection was performed against Kelch-like ECH-associated protein (Keap1) as a model target protein, leading to a peptide epitope originating from astrotactin-1 (ASTN1). It turned out that this ASTN1 peptide was able to interact with Keap1 more strongly than that with a known peptide derived from Nrf2; a well-known, naturally occurring Keap1 binder. This case study demonstrates the applicability of peptidomic mRNA display for the rapid exploration of consensus binding peptide motifs and the potential for the discovery of unknown PPIs with other proteins of interest.

Activators and Inhibitors of NRF2: A Review of Their Potential for Clinical Development

The transcription factor NRF2 (nuclear factor erythroid 2-related factor 2) triggers the first line of homeostatic responses against a plethora of environmental or endogenous deviations in redox metabolism, proteostasis, inflammation, etc. Therefore, pharmacological activation of NRF2 is a promising therapeutic approach for several chronic diseases that are underlined by oxidative stress and inflammation, such as neurodegenerative, cardiovascular, and metabolic diseases. A particular case is cancer, where NRF2 confers a survival advantage to constituted tumors, and therefore, NRF2 inhibition is desired. This review describes the electrophilic and nonelectrophilic NRF2 activators with clinical projection in various chronic diseases. We also analyze the status of NRF2 inhibitors, which at this time provide proof of concept for blocking NRF2 activity in cancer therapy.

Drug screening assay based on the interaction of intact Keap1 and Nrf2 proteins in cancer cells

BACKGROUND

The Nrf2-Keap1 interaction is the major regulatory pathway for cytoprotective responses against oxidative and electrophilic stresses. Keap1, a substrate protein of a Cul3-dependent E3 ubiquitin ligase complex, is a negative regulator of Nrf2. The use of chemicals to regulate the interaction between Keap1 and Nrf2 has been proposed as a strategy for the chemoprevention of degenerative diseases and cancers.

RESULTS

The interactions between Keap1 and Nrf2 in vitro and in vivo were investigated using fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC) strategies in our study. Nrf2 with its N-terminal fused to eGFP and Keap1 with its C-terminal fused to mCherry were expressed and purified in vitro. When purified eGFP-Nrf2 and Keap1-mChrry proteins were mixed together, a strong FRET signal could be detected, indicating an efficient energy transfer from eGFP to mCherry. Moreover, the FRET was detected in vivo using confocal microscopy in colon cancer HCT-116 cells that were co-transfected with eGFP-Nrf2 and Keap1-mCherry. Finally, using an eGFP BiFC approach, the Keap1-Nrf2 interaction was also detected in MCF7 cells by transfecting eGFP N-terminal fused to Nrf2 (eN158-Nrf2) and eGFP C-terminal fused to Keap1 (eC159-Keap1). Using the BiFC and FRET systems, we demonstrated that the prototypical Nrf2-activiting compound tBHQ and the antitumor drug F-dUrd might interfere with the intracellular interaction between Keap1 and Nrf2 whereas the 5-Fu have little role in activating the protective response of Nrf2 pathway in cancer cells.

CONCLUSIONS

By analyzing the perturbation of the energy transfer between the donor and acceptor fluorophores and the bimolecular fluorescence complementation of eGFP, we can screen potential inhibitors for the interaction between Keap1 and Nrf2.

Rational cyclization-based minimization of entropy penalty upon the binding of Nrf2-derived linear peptides to Keap1: A new strategy to improve therapeutic peptide activity against sepsis

Nrf2 is a critical regulator of innate immune response and survival during sepsis, which is constitutively degraded through binding to the Keap1 adapter protein of E3 ubiquitin ligase. Two linear peptides DLG and ETG derived from, respectively, the low-affinity and high-affinity motifs of Nrf2 binding site exhibit self-binding affinity to Keap1 central hole (active pocket); they can be exploited as therapeutic self-inhibitory peptides to disrupt the Nrf2-Keap1 interaction. Molecular dynamics simulation and binding energetics decomposition reveal that the two peptides possess large flexibility and intrinsic disorder in unbound free state, and thus would incur a considerable entropy penalty upon binding to Keap1. In order to improve Keap1-peptide binding affinity (or free energy ΔG), instead of traditionally increasing favorable enthalpy contribution (ΔH) we herein describe a rational peptide cyclization strategy to minimize unfavorable entropy penalty (ΔS) upon the binding of Nrf2-derived linear peptides to Keap1. Crystal structure analysis impart that the native active conformations of DLG and ETG peptides bound with Keap1 are folded into U-shape and hairpin configurations, respectively, and adopt their turning head to insert into the central hole of Keap1. Here, cyclization is designed by adding a disulfide bond across the two arms of DLG U-shape or ETG hairpin, which would not influence the direct intermolecular interaction between Keap1 and peptide as well as desolvation effect involved in the interaction, but can effectively constrain the conformational flexibility and disorder of the two peptides in free state, thus largely minimizing entropy penalty upon the binding. Both free energy calculation and binding affinity assay substantiate that the cyclization, as might be expected, can moderately or considerably enhance peptide binding potency to Keap1, with affinity (dissociation constant K_d) increase by 1.4-7.5-fold for designed cyclic peptides relative to their linear counterparts.

Development of a High-Throughput Cul3-Keap1 Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) Assay for Identifying Nrf2 Activators

Nrf2, a master regulator of the phase II gene response to stress, is kept at low concentrations in the cell through binding to Keap1, an adaptor protein for the Cul3 ubiquitin ligase complex. To identify Nrf2 activators, two separate time-resolved fluorescence resonance energy transfer (TR-FRET) assays were developed to monitor the binding of Nrf2-Keap1 and Cul3-Keap1, respectively. The triterpenoid, 1-[2-cyano-3-,12-dioxooleana-1,9(11)-dien-28-oyl] imidazole (CDDO-Im) and its analogs, exhibited approximately 100-fold better potency in the Cul3-Keap1 assay than in the Nrf2-Keap1 assay, and this difference was more profound at 37 °C than at room temperature in the Nrf2-Keap1 assay, but this phenomenon was not observed in the Cul3-Keap1 assay. A full diversity screen of approximately 2,200,000 GSK compounds was run with the Cul3-Keap1 TR-FRET assay and multiple chemical series were identified and characterized.

Design and development of Nrf2 modulators for cancer chemoprevention and therapy: a review

A major cell defense mechanism against oxidative and xenobiotic stress is mediated by the Nrf2/Keap1 signaling pathway. The Nrf2/Keap1 pathway regulates gene expression of many cytoprotective and detoxifying enzymes, thus playing a pivotal role in maintaining redox cellular homeostasis. Many diseases including cancer have been closely related to impaired Nrf2 activity. Targeting Nrf2 and modulating its activity represents a novel modern strategy for cancer chemoprevention and therapy. In this review, different design strategies used for the development of Nrf2 modulators are described in detail. Moreover, the main focus is on important and recently developed Nrf2 activators and inhibitors, their in vitro and in vivo studies, and their potential use as chemopreventive agents and/or cancer therapeutics.

Replacement of a Naphthalene Scaffold in Kelch-like ECH-Associated Protein 1 (KEAP1)/Nuclear Factor (Erythroid-derived 2)-like 2 (NRF2) Inhibitors

Activators of nuclear factor-erythroid 2-related factor 2 (NRF2) could lead to promising therapeutics for prevention and treatment of oxidative stress and inflammatory disorders. Ubiquitination and subsequent degradation of the transcription factor NRF2 is mediated by Kelch-like ECH-associated protein-1 (KEAP1). Inhibition of the KEAP1/NRF2 interaction with small molecules leads to NRF2 activation. Previously, we and others described naphthalene-based NRF2 activators, but the 1,4-diaminonaphthalene scaffold may not represent a drug-like scaffold. Paying particular attention to aqueous solubility, metabolic stability, potency, and mutagenicity, we modified a previously known, naphthalene-based nonelectrophilic NRF2 activator to give a series of non-naphthalene and heterocyclic scaffolds. We found that, compared to previously reported naphthalene-based compounds, a 1,4-isoquinoline scaffold provides a better mutagenic profile without sacrificing potency, stability, or solubility.