Discovery and Development of Kelch-like ECH-Associated Protein 1. Nuclear Factor Erythroid 2-Related Factor 2 (KEAP1:NRF2) Protein-Protein Interaction Inhibitors: Achievements, Challenges, and Future Directions

Self-assembly of peptides in living cells for disease theranostics

The past few decades have witnessed substantial progress in biomedical materials for addressing health concerns and improving disease therapeutic and diagnostic efficacy. Conventional biomedical materials are typically created through an ex vivo approach and are usually utilized under physiological environments via transfer from preparative media. This transfer potentially gives rise to challenges for the efficient preservation of the bioactivity and implementation of theranostic goals on site. To overcome these issues, the in situ synthesis of biomedical materials on site has attracted great attention in the past few years. Peptides, which exhibit remarkable biocompability and reliable noncovalent interactions, can be tailored via tunable assembly to precisely create biomedical materials. In this review, we summarize the progress in the self-assembly of peptides in living cells for disease diagnosis and therapy. After a brief introduction to the basic design principles of peptide assembly systems in living cells, the applications of peptide assemblies for bioimaging and disease treatment are highlighted. The challenges in the field of peptide self-assembly in living cells and the prospects for novel peptide assembly systems towards next-generation biomaterials are also discussed, which will hopefully help elucidate the great potential of peptide assembly in living cells for future healthcare applications.

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.

Tiliroside attenuates acute kidney injury by inhibiting ferroptosis through the disruption of NRF2-KEAP1 interaction

BACKGROUND

Ferroptosis, an iron-dependent process that regulates cell death. Emerging evidences suggest that ferroptosis induces acute kidney injury (AKI) progression, and inhibiting ferroptosis provides an effect strategy for AKI treatment. The disruption of the NRF2-KEAP1 protein to protein interaction (PPI) induces NRF2 activation, which provides a promising strategy that can identify new ferroptosis inhibitors. A previous study revealed that tiliroside, a glycosidic flavonoid extracted from Edgeworthia chrysantha Lindl (buds), has anti-neuroinflammatory and neuroprotective effects via NRF2 activation. However, the mechanism through which tiliroside activates NRF2 is unknown, and it remains unclear whether it has protective effects against AKI.

PURPOSE

To investigate whether tiliroside has protective effects against AKI in mice and the associated mechanisms.

METHODS

Possible tiliroside substrates were analyzed using molecular docking. Cisplatin- and ischemia-reperfusion injury (IRI)-induced AKI mouse models and HK2 cells model were constructed to evaluate the protective effects of tiliroside. CRISPR/Cas9 mediated NRF2 knockout HK2 cells were used to verify whether NRF2 mediates tiliroside protective effects.

RESULTS

In vivo, our results showed that tiliroside treatment preserved kidney functions in AKI mice models, as showed by lower levels of serum creatinine (SCr), blood urea nitrogen (BUN), and renal injury markers, including neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule 1 (KIM1), compared with the mice in control groups. In vitro, tiliroside treatment greatly ameliorated cisplatin-induced ferroptosis through NRF2 activation in cultured HK2 cells, as evidenced by the protective effects of tiliroside being greatly blunted after the knockout of NRF2 in HK2 cells. Mechanistic studies indicated that tiliroside promoted NRF2/GPX4 pathway activation and ferroptosis inhibition, perhaps via the disruption of the NRF2-KEAP1 PPI.

CONCLUSION

Together, our results demonstrate that tiliroside may serve as a NRF2-KEAP1 PPI inhibitor and prevents ferroptosis-induced AKI, indicating its potential for clinical AKI treatment.

Anti-Osteoarthritis Mechanism of the Nrf2 Signaling Pathway

Osteoarthritis (OA) is a chronic degenerative disease and the primary pathogenic consequence of OA is inflammation, which can affect a variety of tissues including the synovial membrane, articular cartilage, and subchondral bone. The development of the intra-articular microenvironment can be significantly influenced by the shift of synovial macrophages between pro-inflammatory and anti-inflammatory phenotypes. By regulating macrophage inflammatory responses, the NF-κB signaling route is essential in the therapy of OA; whereas, the nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway appears to manage the relationship between oxidative stress and inflammation. Additionally, it has been demonstrated that under oxidative stress and inflammation, there is a significant interaction between transcriptional pathways involving Nrf2 and NF-κB. Studying how Nrf2 signaling affects inflammation and cellular metabolism may help us understand how to treat OA by reprogramming macrophage behavior because Nrf2 signaling is thought to affect cellular metabolism. The candidates for treating OA by promoting an anti-inflammatory mechanism by activating Nrf2 are also reviewed in this paper.

Medicinal Chemistry Insights into the Development of Small-Molecule Kelch-Like ECH-Associated Protein 1-Nuclear Factor Erythroid 2-Related Factor 2 (Keap1-Nrf2) Protein-Protein Interaction Inhibitors

Oxidative stress has been implicated in a wide range of pathological conditions. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) exerts a central role in regulating the cellular defense system against oxidative and electrophilic insults. Nonelectrophilic inhibition of the protein-protein interaction (PPI) between Kelch-like ECH-associated protein 1 (Keap1) and Nrf2 has become a promising approach to activate Nrf2. Recently, multiple drug discovery strategies have facilitated the development of small-molecule Keap1-Nrf2 PPI inhibitors with potent activity and favorable drug-like properties. In this Perspective, we summarize the latest progress of small-molecule Keap1-Nrf2 PPI inhibitors from medicinal chemistry insights and discuss future prospects and challenges in this field.

A potent phosphodiester Keap1-Nrf2 protein-protein interaction inhibitor as the efficient treatment of Alzheimer's disease

The Keap1-Nrf2 pathway has been established as a therapeutic target for Alzheimer's disease (AD). Directly inhibiting the protein-protein interaction (PPI) between Keap1 and Nrf2 has been reported as an effective strategy for treating AD. Our group has validated this in an AD mouse model for the first time using the inhibitor 1,4-diaminonaphthalene NXPZ-2 with high concentrations. In the present study, we reported a new phosphodiester containing diaminonaphthalene compound, POZL, designed to target the PPI interface using a structure-based design strategy to combat oxidative stress in AD pathogenesis. Our crystallographic verification confirms that POZL shows potent Keap1-Nrf2 inhibition. Remarkably, POZL showed its high in vivo anti-AD efficacy at a much lower dosage compared to NXPZ-2 in the transgenic APP/PS1 AD mouse model. POZL treatment in the transgenic mice could effectively ameliorate learning and memory dysfunction by promoting the Nrf2 nuclear translocation. As a result, the oxidative stress and AD biomarker expression such as BACE1 and hyperphosphorylation of Tau were significantly reduced, and the synaptic function was recovered. HE and Nissl staining confirmed that POZL improved brain tissue pathological changes by enhancing neuron quantity and function. Furthermore, it was confirmed that POZL could effectively reverse Aβ-caused synaptic damage by activating Nrf2 in primary cultured cortical neurons. Collectively, our findings demonstrated that the phosphodiester diaminonaphthalene Keap1-Nrf2 PPI inhibitor could be regarded as a promising preclinical candidate of AD.

Optimization and evaluation of pyridinyl vinyl sulfones as Nrf2 activator for the antioxidant and anti-inflammatory effects

Many studies have reported that chalcone-based compounds exhibit biological activities such as anticancer, antioxidant, anti-inflammatory and neuroprotective effects. Among the published chalcone derivatives, (E)-1-(3-methoxypyridin-2-yl)-3-(2-(trifluoromethyl)phenyl)prop-2-en-1-one (VEDA-1209), which is currently undergoing preclinical study, was selected as a starting compound for the development of new nuclear factor erythroid 2-related factor 2 (Nrf2) activators. Based on our previous knowledge, we attempted to redesign and synthesize VEDA-1209 derivatives by introducing the pyridine ring and sulfone moiety to ameliorate its Nrf2 efficacy and drug-like properties. Among the synthesized compounds, (E)-3-chloro-2-(2-((3-methoxypyridin-2-yl)sulfonyl)vinyl) pyridine (10e) was found to have approximately 16-folds higher Nrf2 activating effects than VEDA-1209 (10e: EC₅₀ = 37.9 nM vs VEDA-1209: EC₅₀ = 625 nM) in functional cell-based assay. In addition, 10e effectively improved drug-like properties such as CYP inhibition probability and metabolic stability. Finally, 10e demonstrated excellent antioxidant and anti-inflammatory effects in BV-2 microglial cells and significantly restored spatial memory deficits in lipopolysaccharide (LPS)-induced neuroinflammatory mouse models.

A small-molecule inhibitor of Keap1-Nrf2 interaction attenuates sepsis by selectively augmenting the antibacterial defence of macrophages at infection sites

BACKGROUND

Macrophages at infection sites are considered as the promising therapeutic targets to prevent sepsis development. The Nrf2/Keap1 system acts as a critical modulator of the antibacterial activity of macrophages. Recently, Keap1-Nrf2 protein-protein interaction (PPI) inhibitors have emerged as safer and stronger Nrf2 activators; however, their therapeutic potential in sepsis remains unclear. Herein, we report a unique heptamethine dye, IR-61, as a Keap1-Nrf2 PPI inhibitor that preferentially accumulates in macrophages at infection sites.

METHODS

A mouse model of acute lung bacterial infection was used to investigate the biodistribution of IR-61. SPR study and CESTA were used to detect the Keap1 binding behaviour of IR-61 in vitro and in cells. Established models of sepsis in mice were used to determine the therapeutic effect of IR-61. The relationship between Nrf2 levels and sepsis outcomes was preliminarily investigated using monocytes from human patients.

FINDINGS

Our data showed that IR-61 preferentially accumulated in macrophages at infection sites, enhanced bacterial clearance, and improved outcomes in mice with sepsis. Mechanistic studies indicated that IR-61 potentiated the antibacterial function of macrophages by activating Nrf2 via direct inhibition of the Keap1-Nrf2 interaction. Moreover, we observed that IR-61 enhanced the phagocytic ability of human macrophages, and the expression levels of Nrf2 in monocytes might be associated with the outcomes of sepsis patients.

INTERPRETATIONS

Our study demonstrates that the specific activation of Nrf2 in macrophages at infection sites is valuable for sepsis management. IR-61 may prove to be a Keap1-Nrf2 PPI inhibitor for the precise treatment of sepsis.

FUNDING

This work was supported by the National Natural Science Foundation of China (Major program 82192884), the Intramural Research Project (Grants: 2018-JCJQ-ZQ-001 and 20QNPY018), and the Chongqing National Science Foundation (CSTB2022NSCQ-MSX1222).

Medicinal Chemistry Strategies for the Development of Inhibitors Disrupting β-Catenin's Interactions with Its Nuclear Partners

Dysregulation of the Wnt/β-catenin signaling pathway is strongly associated with various aspects of cancer, including tumor initiation, proliferation, and metastasis as well as antitumor immunity, and presents a promising opportunity for cancer therapy. Wnt/β-catenin signaling activation increases nuclear dephosphorylated β-catenin levels, resulting in β-catenin binding to TCF and additional cotranscription factors, such as BCL9, CBP, and p300. Therefore, directly disrupting β-catenin's interactions with these nuclear partners holds promise for the effective and selective suppression of the aberrant activation of Wnt/β-catenin signaling. Herein, we summarize recent advances in biochemical techniques and medicinal chemistry strategies used to identify potent peptide-based and small-molecule inhibitors that directly disrupt β-catenin's interactions with its nuclear binding partners. We discuss the challenges involved in developing drug-like inhibitors that target the interactions of β-catenin and its nuclear binding partner into therapeutic agents.

An updated patent review of Nrf2 activators (2020-present)

INTRODUCTION

The nuclear factor erythroid 2-related factor 2 (Nrf2) is a pivotal transcription factor that controls the expression of numerous cytoprotective genes and regulates cellular defense system against oxidative insults. Thus, activating the Nrf2 pathway is a promising strategy for the treatment of various chronic diseases characterized by oxidative stress.

AREAS COVERED

This review first discusses the biological effects of Nrf2 and the regulatory mechanism of Kelch-like ECH-associated protein 1-Nrf2-antioxidant response element (Keap1-Nrf2-ARE) pathway. Then, Nrf2 activators (2020-present) are summarized based on the mechanism of action. The case studies consist of chemical structures, biological activities, structural optimization, and clinical development.

EXPERT OPINION

Extensive efforts have been devoted to developing novel Nrf2 activators with improved potency and drug-like properties. These Nrf2 activators have exhibited beneficial effects in in vitro and in vivo models of oxidative stress-related chronic diseases. However, some specific problems, such as target selectivity and brain blood barrier (BBB) permeability, still need to be addressed in the future.

NRF2 in dermo-cosmetic: From scientific knowledge to skin care products

The skin is the organ that is most susceptible to the impact of the exposome. Located at the interface with the external environment, it protects internal organs through the barrier function of the epidermis. It must adapt to the consequences of the harmful effects of solar radiation, the various chemical constituents of atmospheric pollution, and wounds associated with mechanical damage: oxidation, cytotoxicity, inflammation, and so forth. In this biological context, a capacity to adapt to the various stresses caused by the exposome is essential; otherwise, more or less serious conditions may develop accelerated aging, pigmentation disorders, atopy, psoriasis, and skin cancers. Nrf2-controlled pathways play a key role at this level. Nrf2 is a transcription factor that controls genes involved in oxidative stress protection and detoxification of chemicals. Its involvement in UV protection, reduction of inflammation in processes associated with healing, epidermal differentiation for barrier function, and hair regrowth, has been demonstrated. The modulation of Nrf2 in the skin may therefore constitute a skin protection or care strategy for certain dermatological stresses and disorders initiated or aggravated by the exposome. Nrf2 inducers can act through different modes of action. Keap1-dependent mechanisms include modification of the cysteine residues of Keap1 by (pro)electrophiles or prooxidants, and disruption of the Keap1-Nrf2 complex. Indirect mechanisms are suggested for numerous phytochemicals, acting on upstream pathways, or via hormesis. While developing novel and safe Nrf2 modulators for skin care may be challenging, new avenues can arise from natural compounds-based molecular modeling and emerging concepts such as epigenetic regulation.

Dehydroeburicoic Acid, a Dual Inhibitor against Oxidative Stress in Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a complicated disease which can lead to hepatocellular carcinoma; however, there is a lack of satisfactory therapeutics. Dehydroeburicoic acid (DEA) (1), a triterpenoid isolated from Antrodia cinnamomea, has been reported to act against ALD, but its mechanisms of action are still not clear. In this study, we report for the first time the use of DEA (1) as a dual inhibitor of the Keap1-Nrf2 protein-protein interaction (PPI) and GSK3β in an in vitro ALD cell model. DEA (1) engages Keap1 to disrupt the Keap1-Nrf2 PPI and inhibits GSK3β to restore Nrf2 activity in a Keap1-independent fashion. DEA (1) promotes Nrf2 nuclear translocation to activate downstream antioxidant genes. Importantly, DEA (1) restores the mitochondrial dysfunction induced by ethanol and generates antioxidant activity in the ALD cell model with minimal toxicity. We anticipate that DEA (1) could be a potential scaffold for the further development of clinical agents for treating ALD.

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Activation of the Nrf2 Antioxidant Pathway by Longjing Green Tea Polyphenols in Mice Livers

Previous studies have revealed that green tea polyphenol (GTP) could protect against liver injury due to oxidative stress. However, the mechanism underlying the bioactive actions of GTP in the liver has not been systematically evaluated. This study aimed to investigate the effect of GTP on the activation of the nuclear factor erythroid-2-related factor 2 (Nrf2)-Kelch-like ECH-associated protein 1 (keap1) pathway, using in silico and in vivo methods. Furthermore, the regulation of Nrf2 downstream target antioxidant response element (ARE) was also evaluated. The high-performance liquid chromatography analysis indicated that GTP includes 9 major compounds, and molecule docking analysis demonstrated that most of these polyphenols have a strong binding affinity with the keap1 Kelch domain, where keap1 binds to the Neh2 domain of Nrf2. Remarkably, the predominant compound of GTP, that is, epigallocatechin gallate, displayed the best binding affinity score, which can fully occupy all 3 polar subpockets of the keap1 Kelch domain. The Nrf2, keap1, and Nrf2 downstream target gene expression levels were changed in the livers compared to the control group. It showed that the Nrf2 expression level was significantly upregulated in GTP-induced mice liver across most treatments, while the keap1 expression level remained unchanged. Subsequently, we observed a significant increasing trend in the expression of the downstream ARE, including antioxidative enzymes, liver phase II enzymes, and liver efflux transporters in mice livers. The present study demonstrated that GTP could activate the Nrf2 signaling pathway by interrupting the Nrf2-keap1 protein–protein interaction

Exploring the target scope of KEAP1 E3 ligase-based PROTACs

Targeted protein degradation (TPD) uses small molecules to recruit E3 ubiquitin ligases into the proximity of proteins of interest, inducing ubiquitination-dependent degradation. A major bottleneck in the TPD field is the lack of accessible E3 ligase ligands for developing degraders. To expand the E3 ligase toolbox, we sought to convert the Kelch-like ECH-associated protein 1 (KEAP1) inhibitor KI696 into a recruitment handle for several targets. While we were able to generate KEAP1-recruiting degraders of BET family and murine focal adhesion kinase (FAK), we discovered that the target scope of KEAP1 was narrow, as targets easily degraded using a cereblon (CRBN)-recruiting degrader were refractory to KEAP1-mediated degradation. Linking the KEAP1-binding ligand to a CRBN-binding ligand resulted in a molecule that induced degradation of KEAP1 but not CRBN. In sum, we characterize tool compounds to explore KEAP1-mediated ubiquitination and delineate the challenges of exploiting new E3 ligases for generating bivalent degraders.

P301S-hTau acetylates KEAP1 to trigger synaptic toxicity via inhibiting NRF2/ARE pathway: A novel mechanism underlying hTau-induced synaptic toxicities

BACKGROUND

Human Tau (hTau) accumulation and synapse loss are two pathological hallmarks of tauopathies. However, whether and how hTau exerts toxic effects on synapses remain elusive.

METHODS

Mutated hTau (P301S) was overexpressed in the N2a cell line, primary hippocampal neurons and hippocampal CA3. Western blotting and quantitative polymerase chain reaction were applied to examine the protein and mRNA levels of synaptic proteins. The protein interaction was tested by co-immunoprecipitation and proximity ligation assays. Memory and emotion status were evaluated by a series of behavioural tests. The transcriptional activity of nuclear factor-erythroid 2-related factor 2 (NRF2) was detected by dual luciferase reporter assay. Electrophoresis mobility shift assay and chromosome immunoprecipitation were conducted to examine the combination of NRF2 to specific anti-oxidative response element (ARE) sequences. Neuronal morphology was analysed after Golgi staining.

RESULTS

Overexpressing P301S decreased the protein levels of post-synaptic density protein 93 (PSD93), PSD95 and synapsin 1 (SYN1). Simultaneously, NRF2 was decreased, whereas Kelch-like ECH-associated protein 1 (KEAP1) was elevated. Further, we found that NRF2 could bind to the specific AREs of DLG2, DLG4 and SYN1 genes, which encode PSD93, PSD95 and SYN1, respectively, to promote their expression. Overexpressing NRF2 ameliorated P301S-reduced synaptic proteins and synapse. By means of acetylation at K312, P301S increased the protein level of KEAP1 via inhibiting KEAP1 degradation from ubiquitin-proteasome pathway, thereby decreasing NRF2 and reducing synapse. Blocking the P301S-KEAP1 interaction at K312 rescued the P301S-suppressed expression of synaptic proteins and memory deficits with anxiety efficiently.

CONCLUSIONS

P301S-hTau could acetylate KEAP1 to trigger synaptic toxicity via inhibiting the NRF2/ARE pathway. These findings provide a novel and potential target for the therapeutic intervention of tauopathies.

Corylin from Psoralea fructus (Psoralea corylifolia L.) protects against UV-induced skin aging by activating Nrf2 defense mechanisms

Oxidative stress damage can lead to premature skin aging or age-related skin disorders. Therefore, strategies to improve oxidative stress-induced aging are needed to protect the skin and to treat skin diseases. This study aimed to determine whether the flavonoid corylin derived from Psoralea corylifolia can prevent UV-induced skin aging and if so, to explore the potential molecular mechanisms. We found that corylin potently blocked UV-induced skin photoaging in mice by reducing oxidative stress and increasing the nuclear expression of nuclear factor-erythroid factor 2-related factor 2 Nrf2. We also found that corylin stimulated Nrf2 translocation into the nucleus and increased the delivery of its target antioxidant genes together with Kelch-like ECH-associated protein 1 (Keap1) to dissociate Nrf2. These findings indicate that corylin could prevent skin aging by inhibiting oxidative stress via Keap1-Nrf2 in mouse cells. Thus, Nrf2 activation might be a therapeutic target for preventing skin aging or skin diseases caused by aging. Our findings also provided evidence that warrants the further investigation of plant ingredients to facilitate the discovery of novel therapies targeting skin aging.

Self-Amplifying Assembly of Peptides in Macrophages for Enhanced Inflammatory Treatment

Enzyme-regulated in situ self-assembly of peptides represents one versatile strategy in the creation of theranostic agents, which, however, is limited by the strong dependence on enzyme overexpression. Herein, we reported the self-amplifying assembly of peptides precisely in macrophages associated with enzyme expression for improving the anti-inflammatory efficacy of conventional drugs. The self-amplifying assembling system was created via coassembling an enzyme-responsive peptide with its derivative functionalized with a protein ligand. Reduction of the peptides by the enzyme NAD(P)H quinone dehydrogenase 1 (NQO1) led to the formation of nanofibers with high affinity to the protein, thereby facilitating NQO1 expression. The improved NQO1 level conversely promoted the assembly of the peptides into nanofibers, thus establishing an amplifying relationship between the peptide assembly and the NQO1 expression in macrophages. Utilization of the amplifying assembling system as vehicles for drug dexamethasone allowed for its passive targeting delivery to acute injured lungs. Both in vitro and in vivo studies confirmed the capability of the self-amplifying assembling system to enhance the anti-inflammatory efficacy of dexamethasone via simultaneous alleviation of the reactive oxygen species side effect and downregulation of proinflammatory cytokines. Our findings demonstrate the manipulation of the assembly of peptides in living cells with a regular enzyme level via a self-amplification process, thus providing a unique strategy for the creation of supramolecular theranostic agents in living cells.

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.

An Integrated Molecular Grafting Approach for the Design of Keap1-Targeted Peptide Inhibitors

Inhibiting the Nrf2:Keap1 interaction to trigger cytoprotective gene expression is a promising treatment strategy for oxidative stress-related diseases. A short linear motif from Nrf2 has the potential to directly inhibit this protein-protein interaction, but poor stability and limited cellular uptake impede its therapeutic development. To address these limitations, we utilized an integrated molecular grafting strategy to re-engineer the Nrf2 motif. We combined the motif with an engineered non-native disulfide bond and a cell-penetrating peptide onto a single multifunctionalizable and ultrastable molecular scaffold, namely, the cyclotide MCoTI-II, resulting in the grafted peptide MCNr-2c. The engineered disulfide bond enhanced the conformational rigidity of the motif, resulting in a nanomolar affinity of MCNr-2c for Keap1. The cell-penetrating peptide led to an improved cellular uptake and increased ability to enhance the intracellular expression of two well-described Nrf2-target genes NQO1 and TALDO1. Furthermore, the stability of the scaffold was inherited by the grafted peptide, which became resistant to proteolysis in serum. Overall, we have provided proof-of-concept for a strategy that enables the encapsulation of multiple desired and complementary activities into a single molecular entity to design a Keap1-targeted inhibitor. We propose that this integrated approach could have broad utility for the design of peptide drug leads that require multiple functions and/or biopharmaceutical properties to elicit a therapeutic activity.

E3 Ligase Ligands in Successful PROTACs: An Overview of Syntheses and Linker Attachment Points

Proteolysis-targeting chimeras (PROTACs) have received tremendous attention as a new and exciting class of therapeutic agents that promise to significantly impact drug discovery. These bifunctional molecules consist of a target binding unit, a linker, and an E3 ligase binding moiety. The chemically-induced formation of ternary complexes leads to ubiquitination and proteasomal degradation of target proteins. Among the plethora of E3 ligases, only a few have been utilized for the novel PROTAC technology. However, extensive knowledge on the preparation of E3 ligands and their utilization for PROTACs has already been acquired. This review provides an in-depth analysis of synthetic entries to functionalized ligands for the most relevant E3 ligase ligands, i.e. CRBN, VHL, IAP, and MDM2. Less commonly used E3 ligase and their ligands are also presented. We compare different preparative routes to E3 ligands with respect to feasibility and productivity. A particular focus was set on the chemistry of the linker attachment by discussing the synthetic opportunities to connect the E3 ligand at an appropriate exit vector with a linker to assemble the final PROTAC. This comprehensive review includes many facets involved in the synthesis of such complex molecules and is expected to serve as a compendium to support future synthetic attempts towards PROTACs.

An Overview of the Posttranslational Modifications and Related Molecular Mechanisms in Diabetic Nephropathy

Diabetic nephropathy (DN), a common diabetic microvascular complication, is characterized by its complex pathogenesis, higher risk of mortality, and the lack of effective diagnosis and treatment methods. Many studies focus on the diagnosis and treatment of diabetes mellitus (DM) and have reported that the pathophysiology of DN is very complex, involving many molecules and abnormal cellular activities. Given the respective pivotal roles of NF-κB, Nrf2, and TGF-β in inflammation, oxidative stress, and fibrosis during DN, we first review the effect of posttranslational modifications on these vital molecules in DN. Then, we describe the relationship between these molecules and related abnormal cellular activities in DN. Finally, we discuss some potential directions for DN treatment and diagnosis. The information reviewed here may be significant in the design of further studies to identify valuable therapeutic targets for DN.

Recent advances in β-catenin/BCL9 protein-protein interaction inhibitors

Wnt/β-catenin signaling is crucial both in normal embryonic development and throughout the life of an organism. Moreover, aberrant Wnt signaling has been associated with various diseases, especially cancer and fibrosis. Recent research suggests that direct targeting of the β-catenin/BCL9 protein-protein interaction (PPI) is a promising strategy to block the Wnt pathway. Progress in understanding the cocrystalline complex and mechanism of action of the β-catenin/BCL9 interaction facilitates the discovery process of its inhibitors, but only a few inhibitors have been reported. In this review, the discovery and development of β-catenin/BCL9 PPI inhibitors in the areas of drug design, structure-activity relationships and biological and biochemical properties are summarized. In addition, perspectives for the future development of β-catenin/BCL9 PPI inhibitors are explored.

Deconstructing Noncovalent Kelch-like ECH-Associated Protein 1 (Keap1) Inhibitors into Fragments to Reconstruct New Potent Compounds

Targeting the protein-protein interaction (PPI) between nuclear factor erythroid 2-related factor 2 (Nrf2) and Kelch-like ECH-associated protein 1 (Keap1) is a potential therapeutic strategy to control diseases involving oxidative stress. Here, six classes of known small-molecule Keap1-Nrf2 PPI inhibitors were dissected into 77 fragments in a fragment-based deconstruction reconstruction (FBDR) study and tested in four orthogonal assays. This gave 17 fragment hits of which six were shown by X-ray crystallography to bind in the Keap1 Kelch binding pocket. Two hits were merged into compound 8 with a 220-380-fold stronger affinity (K_i = 16 μM) relative to the parent fragments. Systematic optimization resulted in several novel analogues with K_i values of 0.04-0.5 μM, binding modes determined by X-ray crystallography, and enhanced microsomal stability. This demonstrates how FBDR can be used to find new fragment hits, elucidate important ligand-protein interactions, and identify new potent inhibitors of the Keap1-Nrf2 PPI.

Albicanol Alleviates D-Galactose-Induced Aging and Improves Behavioral Ability Via by Alleviating Oxidative Stress-Induced Damage

Albicanol is a natural terpenoid derived from Dryopteris fragrans. Herein, we assessed the ability of Albicanol to protect against oxidative stress-induced senescence. Using a murine model of D-galactose (D-gal)-induced aging, we determined that Albicanol treatment can reverse D-gal-mediated learning impairments and behavioral changes, while also remediating brain tissue damage in treated mice. We found that serum SOD, CAT, GSH-Px, and T-AOC levels were significantly decreased in aging mice, and that Albicanol treatment significantly increased the serum levels of these antioxidant enzymes. We additionally evaluated the impact of Albicanol treatment on the Keap1/Nrf2/ARE signaling pathway, and found that it was able to decrease Keap1 expression while increasing the expression of Nrf2, thereby activating this signaling pathway, suppressing oxidative damage, and enhancing the expression of downstream target genes including SOD, GSH, GST, HO-1, and NQO1 in this murine aging model system. Albicanol treatment also inhibited the secretion of inflammatory TNF-a and IL-1b. Together, these data indicated that Albicanol can activate Nrf2 pathway-related genes, thereby inhibition of delayed aging by alleviating oxidative stress-induced damage.

Direct Keap1-kelch inhibitors as potential drug candidates for oxidative stress-orchestrated diseases: A review on In silico perspective

The recent outcry in the search for direct keap1 inhibitors requires a quicker and more effective drug discovery process which is an inherent property of the Computer Aided Drug Discovery (CADD) to bring drug candidates into the clinic for patient's use. This Keap1 (negative regulator of ARE master activator) is emerging as a therapeutic strategy to combat oxidative stress-orchestrated diseases. The advances in computer algorithm and compound databases require that we highlight the functionalities that this technology possesses that can be exploited to target Keap1-Nrf2 PPI. Therefore, in this review, we uncover the in silico approaches that had been exploited towards the identification of keap1 inhibition in the light of appropriate fitting with relevant amino acid residues, we found 3 and 16 other compounds that perfectly fit keap1 kelch pocket/domain. Our goal is to harness the parameters that could orchestrate keap1 surface druggability by utilizing hotspot regions for virtual fragment screening and identification of hotspot residues.

Curcumin ameliorates mercuric chloride-induced liver injury via modulating cytochrome P450 signaling and Nrf2/HO-1 pathway

Environmental mercury is a concern for coastal ecosystem health, and exerts adverse effects on human health. Despite the growing body of evidence showing the hepatoprotective roles of curcumin on mercury, the knowledge between the macroscopic descriptions and the actual mechanism(s) underlying these processes is getting larger remains elusive. Herein, mice received single injection of mercuric chloride (HgCl₂) (5 mg/kg body weight) and/or curcumin (50 mg/kg, body weight, p.o.). Firstly, the results showed curcumin could decline HgCl₂-induced up-regulated the levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Additionally, we also found that curcumin could suppress inflammatory damage, unbalance of trace elements (including sodium, magnesium, kalium, calcium overload), oxidative burst induced by HgCl₂, which could be associated with cytochrome P450 (CYP450) signaling. Secondly, we found that curcumin could prevent HgCl₂-induced cell death both in vivo and in vitro. Furthermore, curcumin significantly increased the nuclear translocation of nuclear factor E2-related factor 2 (Nrf2) and consequently upregulated the expression of heme oxygenase 1 (HO-1) under HgCl₂ treatment. Meanwhile, inhibition of HO-1 by zinc protoporphyria could abolish the cytoprotective effects of curcumin in HgCl₂-treated L02 hepatocytes. In conclusion, our data identify that curcumin could enhance Nrf2-mediated HO-1 to upregulate antioxidant ability, which might be associate with CYP450 signaling to suppress liver damage induced by HgCl₂. The present study further enriches and perfects the mechanism theory of HgCl₂ toxicity and suggest that the CYP450 signaling and Nrf2/HO-1 pathway is important in shedding light on curcumin's hepatoprotective effects in HgCl₂ toxicity.

Targeting oxidative stress in disease: promise and limitations of antioxidant therapy

Oxidative stress is a component of many diseases, including atherosclerosis, chronic obstructive pulmonary disease, Alzheimer disease and cancer. Although numerous small molecules evaluated as antioxidants have exhibited therapeutic potential in preclinical studies, clinical trial results have been disappointing. A greater understanding of the mechanisms through which antioxidants act and where and when they are effective may provide a rational approach that leads to greater pharmacological success. Here, we review the relationships between oxidative stress, redox signalling and disease, the mechanisms through which oxidative stress can contribute to pathology, how antioxidant defences work, what limits their effectiveness and how antioxidant defences can be increased through physiological signalling, dietary components and potential pharmaceutical intervention.

Mining Natural Products for Macrocycles to Drug Difficult Targets

Lead generation for difficult-to-drug targets that have large, featureless, and highly lipophilic or highly polar and/or flexible binding sites is highly challenging. Here, we describe how cores of macrocyclic natural products can serve as a high-quality in silico screening library that provides leads for difficult-to-drug targets. Two iterative rounds of docking of a carefully selected set of natural-product-derived cores led to the discovery of an uncharged macrocyclic inhibitor of the Keap1-Nrf2 protein–protein interaction, a particularly challenging target due to its highly polar binding site. The inhibitor displays cellular efficacy and is well-positioned for further optimization based on the structure of its complex with Keap1 and synthetic access. We believe that our work will spur interest in using macrocyclic cores for in silico-based lead generation and also inspire the design of future macrocycle screening collections.

Generation of potent Nrf2 activators via tuning the electrophilicity and steric hindrance of vinyl sulfones for neuroprotection

Oxidative stress is constantly involved in the etiopathogenesis of an ever-widening range of neurodegenerative diseases. As a consequence, effective repression of cellular oxidative stress to a redox homeostatic condition is a promising and feasible strategy to treat, or at least retard the progression of, such disorders. Nrf2, a primary orchestrator of cellular antioxidant response machine, is responsible for detoxifying and compensating for deleterious oxidative stress via transcriptional activation of a diverse array of antioxidant biomolecules. In the framework of our persistent interest in disclosing small molecules that interfere with cellular redox-regulating machinery, we report herein the synthesis, optimization, and biological assessment of 47 vinyl sulfone scaffold-bearing small molecules, most of which exhibit robust neuroprotective effect against H₂O₂-mediated lesions to PC12 cells. After initial screening, the most potent neuroprotective compounds 9b and 9c with marginal cytotoxicity were selected for the follow-up studies. Our results demonstrate that their neuroprotective effects are attributed to the up-regulation of a panel of antioxidant genes and corresponding gene products. Further mechanistic studies indicate that Nrf2 is indispensable for the cellular performances of 9b and 9c, arising from the fact that silence of Nrf2 gene drastically nullifies their protective action. Taken together, 9b and 9c discovered in this work merit further development as neuroprotective candidates for the treatment of oxidative stress-mediated pathological conditions.

Transcription factor NRF2 as a promising therapeutic target for Alzheimer's disease

Oxidative stress is considered as one of the pathogenesis of Alzheimer's disease (AD) and plays an important role in the occurrence and development of AD. Nuclear factor erythroid 2 related factor 2 (NRF2) is a key regulatory of oxidative stress defence. There is growing evidence indicating the relationship between NRF2 and AD. NRF2 activation mitigates multiple pathogenic processes involved in AD by upregulating antioxidative defense, inhibiting neuroinflammation, improving mitochondrial function, maintaining proteostasis, and inhibiting ferroptosis. In addition, several NRF2 activators are currently being evaluated as AD therapeutic agents in clinical trials. Thus, targeting NRF2 has been the focus of a new strategy for prevention and treatment of AD. In this review, the role of NRF2 in AD and the NRF2 activators advanced into clinical and preclinical studies will be summarized.

Discovery of polypodiside as a Keap1-dependent Nrf2 activator attenuating oxidative stress and accumulation of extracellular matrix in glomerular mesangial cells under high glucose

Diabetic nephropathy (DN) is a severe microvascular complication of diabetes mellitus. High glucose has resulted in oxidative stress and following renal fibrosis as the crucial nodes of this disease. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor regulating transcription of many antioxidant genes and suppressing synthesis of extracellular matrix. To discover Nrf2 activators targeting DN, we have evaluated polypodiside using cell-based assays. The results showed polypodiside inhibited the high glucose-induced self-limited proliferation of glomerular meangial cells. Activation of Nrf2 and enhanced transcription to antioxidant response elements were observed in the presence of polypodiside. Oxidative stress and accumulation of extracellular matrix induced by high glucose in glomerular meangial cells have been ameliorated by polypodiside. Further investigations revealed the effects of polypodiside on glomerular meangial cells were associated with activation of Nrf2. Co-immunoprecipitation of Nrf2 disclosed polypodiside disrupted the Kelch-like ECH-associated protein-1 (Keap1)-Nrf2 interaction. Molecular docking elucidated polypodiside could enter the Nrf2 binding cavity of Keap1 via interacting with the residues encompassing that cavity. These findings indicate polypodiside is a Keap1-dependent Nrf2 activator affording the catabatic effects against oxidative stress and accumulation of extracellular matrix in glomerular meangial cells under high glucose.

Optimization of linear and cyclic peptide inhibitors of KEAP1-NRF2 protein-protein interaction

Inhibition of KEAP1-NRF2 protein-protein interaction is considered a promising strategy to selectively and effectively activate NRF2, a transcription factor which is involved in several pathologies such as Huntington's disease (HD). A library of linear peptides based on the NRF2-binding motifs was generated on the nonapeptide lead Ac-LDEETGEFL-NH₂ spanning residues 76-84 of the Neh2 domain of NRF2 with the aim to replace E78, E79 and E82 with non-acidic amino acids. A deeper understanding of the features and accessibility of the T80 subpocket was also targeted by structure-based design. Approaches to improve cell permeability were investigated using both different classes of cyclic peptides and conjugation to cell-penetrating peptides. This insight will guide future design of macrocycles, peptido-mimetics and, most importantly, small neutral brain-penetrating molecules to evaluate whether NRF2 activators have utility in HD.

Discovery of 2-oxy-2-phenylacetic acid substituted naphthalene sulfonamide derivatives as potent KEAP1-NRF2 protein-protein interaction inhibitors for inflammatory conditions

Nuclear factor erythroid 2-related factor 2 (NRF2) is a pleiotropic transcription factor which regulates the constitutive and inducible transcription of a wide array of genes and confers protection against a variety of pathologies. Directly disrupting Kelch-like ECH-associated protein 1 (KEAP1)-NRF2 protein-protein interaction (PPI) has been explored as a promising strategy to activate NRF2. We reported here the first identification of a series of 2-oxy-2-phenylacetic acid substituted naphthalene sulfonamide derivatives as potent KEAP1-NRF2 inhibitors. Our efforts led to the potent small molecule KEAP1-NRF2 inhibitor, 20c, which exhibited a K_d of 24 nM to KEAP1 and an IC₅₀ of 75 nM in disrupting KEAP1-NRF2 interaction. Subsequent biological studies provided consistent evidence across mouse macrophage cell-based and in vivo models that 20c induced NRF2 target gene expression and enhanced downstream antioxidant and anti-inflammatory activities. Our study not only demonstrated that small molecule KEAP1-NRF2 PPI inhibitors can be potential preventive and therapeutic agents for diseases and conditions involving oxidative stress and inflammation but also enriched the chemical diversity of the KEAP1-NRF2 inhibitors.

Discovery of a coumarin derivative as Nrf2 activator mitigating oxidative stress and fibrosis in mesangial cells under high glucose

Diabetic nephropathy (DN) is a severe microvascular complication of diabetes mellitus. Oxidative stress and fibrosis largely contribute to the progression of DN. Recently, Nrf2 was found to be a potential target preventing DN. In the discovery of novel Nrf2 activators for the treatment of DN, we have evaluated coumarin derivatives from Wikstroemi indiaca. Molecular docking results have shown compound 4 could bind to Keap1 and activate Nrf2 significantly. Cell-based assays have revealed compound 4 activated Nrf2 and attenuated oxidative stress and fibrosis induced by high glucose in mesangial cells. Meanwhile, it was validated that disruption of the interaction between Keap1 and Nrf2 was involved in the activation of Nrf2 by compound 4 in mesangial cells under high glucose.

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.

NRF2 Regulation Processes as a Source of Potential Drug Targets against Neurodegenerative Diseases

NRF2 acts by controlling gene expression, being the master regulator of the Phase II antioxidant response, and also being key to the control of neuroinflammation. NRF2 activity is regulated at several levels, including protein degradation by the proteasome, transcription, and post-transcription. The purpose of this review is to offer a concise and critical overview of the main mechanisms of NRF2 regulation and their actual or potential use as targets for the treatment of neurodegenerative diseases.

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.

Exploiting the Indole Scaffold to Design Compounds Binding to Different Pharmacological Targets

Several indole derivatives have been disclosed by our research groups that have been collaborating for nearly 25 years. The results of our investigations led to a variety of molecules binding selectively to different pharmacological targets, specifically the type A γ-aminobutyric acid (GABA_A) chloride channel, the translocator protein (TSPO), the murine double minute 2 (MDM2) protein, the A_2B adenosine receptor (A_2B AR) and the Kelch-like ECH-associated protein 1 (Keap1). Herein, we describe how these works were conceived and carried out thanks to the versatility of indole nucleus to be exploited in the design and synthesis of drug-like molecules.

A hydrogen peroxide responsive prodrug of Keap1-Nrf2 inhibitor for improving oral absorption and selective activation in inflammatory conditions

Transcription factor Nuclear factor erythroid 2-related factor 2 (Nrf2) and its negative regulator, the E3 ligase adaptor Kelch-like ECH-associated protein 1 (Keap1), control the redox and metabolic homeostasis and oxidative stress. Inhibitors of Keap1-Nrf2 interaction are promising in oxidative stress related inflammatory diseases but now hit hurdles. By utilizing thiazolidinone moiety to shield the key carboxyl pharmacophore in Keap1-Nrf2 inhibitor, a hydrogen peroxide (H₂O₂)-responsive prodrug pro2 was developed. The prodrug modification improved the physicochemical properties and cell membrane permeability of the parent drug. Pro2 was stable and stayed inactive under various physiological conditions, while became active by stimulation of H₂O₂ or inflammation derived reactive oxygen species. Moreover, pro2 exhibited proper pharmacokinetic profile suitable for oral administration and enhanced anti-inflammatory efficiency in vivo. Thus, this novel prodrug approach may not only provide an important advance in the therapy of chronic inflammatory diseases with high level of H₂O₂, but also offer a fresh solution to improve the drug-like and selectivity issues of Keap1-Nrf2 inhibitors.

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Pro2 was developed by utilizing H₂O₂-responsive thiazolidinone moiety to shield carboxyl group in Keap1-Nrf2 inhibitor.

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Pro2 was stable and inactive under various physiological conditions, while became active under inflammatory conditions.

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Pro2 exhibited proper pharmacokinetic profile for oral administration and enhanced anti-inflammatory efficiency in vivo.

Identification and optimization of piperine analogues as neuroprotective agents for the treatment of Parkinson's disease via the activation of Nrf2/keap1 pathway

Parkinson's disease (PD) is a slowly progressive and complex neurodegenerative disorder. Up to date, there are no approved drugs that could slow or reverse the neurodegenerative process of PD. Here, we reported the synthesis of series of piperine analogues and the evaluation of their neuroprotective effects against hydrogen peroxide (H₂O₂) induced damage in the neuron-like PC12 cells. Among these analogues, 3b exhibited the most potent protection effect and its underlying mechanism was further investigated. Further results indicated that the ROS scavenging and cytoprotection effect of 3b might be related to the Nrf2 activation and upregulation of related phase II antioxidant enzymes, such as HO-1 and NQO1. In in vivo study, oral administration (100 mg/kg) of 3b significantly attenuated PD-associated behavioral deficits in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD and protected tyrosine hydroxylase-immunopositive dopaminergic neurons. Our results provided evidence that 3b might be a promising candidate for Parkinson's disease treatment.

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.

BRUP-1, an intracellular bilirubin modulator, exerts neuroprotective activity in a cellular Parkinson's disease model

Bilirubin, the end product of heme redox metabolism, has cytoprotective properties and is an essential metabolite associated with cardiovascular disease, inflammatory bowel disease, type 2 diabetes, and neurodegenerative diseases including Parkinson's disease (PD). PD is characterized by progressive degeneration of nigral dopaminergic neurons and is associated with elevated oxidative stress due to mitochondrial dysfunction. In this study, using a ratiometric bilirubin probe, we revealed that the mitochondrial inhibitor, rotenone, which is widely used to create a PD model, significantly decreased intracellular bilirubin levels in HepG2 cells. Chemical screening showed that BRUP-1 was a top hit that restored cellular bilirubin levels that were lowered by rotenone. We found that BRUP-1 up-regulated the expression level of heme oxygenase-1 (HO-1), one of the rate-limiting enzyme of bilirubin production via nuclear factor erythroid 2-related factor 2 (Nrf2) activation. In addition, we demonstrated that this Nrf2 activation was due to a direct inhibition of the interaction between Nrf2 and Kelch-like ECH-associated protein 1 (Keap1) by BRUP-1. Both HO-1 up-regulation and bilirubin restoration by BRUP-1 treatment were significantly abrogated by Nrf2 silencing. In neuronal PC12D cells, BRUP-1 also activated the Nrf2-HO-1 axis and increased bilirubin production, resulted in the suppression of neurotoxin-induced cell death, reactive oxygen species production, and protein aggregation, which are hallmarks of PD. Furthermore, BRUP-1 showed neuroprotective activity against rotenone-treated neurons derived from induced pluripotent stem cells. These findings provide a new member of Keap1-Nrf2 direct inhibitors and suggest that chemical modulation of heme metabolism using BRUP-1 may be beneficial for PD treatment.