Targeting the Hsp90 C-terminal domain to induce allosteric inhibition and selective client downregulation

Repurposing Hsp90 inhibitors as antimicrobials targeting two-component systems identifies compounds leading to loss of bacterial membrane integrity

The discovery of antimicrobials with novel mechanisms of action is crucial to tackle the foreseen global health crisis due to antimicrobial resistance. Bacterial two-component signaling systems (TCSs) are attractive targets for the discovery of novel antibacterial agents. TCS-encoding genes are found in all bacterial genomes and typically consist of a sensor histidine kinase (HK) and a response regulator. Due to the conserved Bergerat fold in the ATP-binding domain of the TCS HK and the human chaperone Hsp90, there has been much interest in repurposing inhibitors of Hsp90 as antibacterial compounds. In this study, we explore the chemical space of the known Hsp90 inhibitor scaffold 3,4-diphenylpyrazole (DPP), building on previous literature to further understand their potential for HK inhibition. Six DPP analogs inhibited HK autophosphorylation in vitro and had good antimicrobial activity against Gram-positive bacteria. However, mechanistic studies showed that their antimicrobial activity was related to damage of bacterial membranes. In addition, DPP analogs were cytotoxic to human embryonic kidney cell lines and induced the cell arrest phenotype shown for other Hsp90 inhibitors. We conclude that these DPP structures can be further optimized as specific disruptors of bacterial membranes providing binding to Hsp90 and cytotoxicity are lowered. Moreover, the X-ray crystal structure of resorcinol, a substructure of the DPP derivatives, bound to the HK CheA represents a promising starting point for the fragment-based design of novel HK inhibitors.

IMPORTANCE

The discovery of novel antimicrobials is of paramount importance in tackling the imminent global health crisis of antimicrobial resistance. The discovery of novel antimicrobials with novel mechanisms of actions, e.g., targeting bacterial two-component signaling systems, is crucial to bypass existing resistance mechanisms and stimulate pharmaceutical innovations. Here, we explore the possible repurposing of compounds developed in cancer research as inhibitors of two-component systems and investigate their off-target effects such as bacterial membrane disruption and toxicity. These results highlight compounds that are promising for further development of novel bacterial membrane disruptors and two-component system inhibitors.

Covalent binding of withanolides to cysteines of protein targets

Withanolides represent an important category of natural products with a steroidal lactone core. Many of them contain an α,β-unsaturated carbonyl moiety with a high reactivity toward sulfhydryl groups, including protein cysteine thiols. Different withanolides endowed with marked antitumor and anti-inflammatory have been shown to form stable covalent complexes with exposed cysteines present in the active site of oncogenic kinases (BTK, IKKβ, Zap70), metabolism enzymes (Prdx-1/6, Pin1, PHGDH), transcription factors (Nrf2, NFκB, C/EBPβ) and other structural and signaling molecules (GFAP, β-tubulin, p97, Hsp90, vimentin, Mpro, IPO5, NEMO, …). The present review analyzed the covalent complexes formed through Michael addition alkylation reactions between six major withanolides (withaferin A, physalin A, withangulatin A, 4β-hydroxywithanolide E, withanone and tubocapsanolide A) and key cysteine residues of about 20 proteins and the resulting biological effects. The covalent conjugation of the α,β-unsaturated carbonyl system of withanolides with reactive protein thiols can occur with a large set of soluble and membrane proteins. It points to a general mechanism, well described with the leading natural product withaferin A, but likely valid for most withanolides harboring a reactive (electrophilic) enone moiety susceptible to react covalently with cysteinyl residues of proteins. The multiplicity of reactive proteins should be taken into account when studying the mechanism of action of new withanolides. Proteomic and network analyses shall be implemented to capture and compare the cysteine covalent-binding map for the major withanolides, so as to identify the protein targets at the origin of their activity and/or unwanted effects. Screening of the cysteinome will help understanding the mechanism of action and designing cysteine-reactive electrophilic drug candidates.

An update on the status of HSP90 inhibitors in cancer clinical trials

The evolutionary conserved molecular chaperone heat shock protein 90 (HSP90) plays an indispensable role in tumorigenesis by stabilizing client oncoproteins. Although the functionality of HSP90 is tightly regulated, cancer cells exhibit a unique dependence on this chaperone, leading to its overexpression, which has been associated with poor prognosis in certain malignancies. While various strategies targeting heat shock proteins (HSPs) involved in carcinogenesis have been explored, only inhibition of HSP90 has consistently and effectively resulted in proteasomal degradation of its client proteins. To date, a total of 22 HSP90 inhibitors (HSP90i) have been tested in 186 cancer clinical trials, as reported by clinicaltrials.gov. Among these trials, 60 % have been completed, 10 % are currently active, and 30 % have been suspended, terminated, or withdrawn. HSP90 inhibitors (HSP90i) have been used as single agents or in combination with other drugs for the treatment of various cancer types in clinical trials. Notably, improved clinical outcomes have been observed when HSP90i are used in combination therapies, as they exhibit a synergistic antitumor effect. However, as single agents, HSP90i have shown limited clinical activity due to drug-related toxicity or therapy resistance. Recently, active trials conducted in Japan evaluating TAS-116 (pimitespib) have demonstrated promising results with low toxicity as monotherapy and in combination with the immune checkpoint inhibitor nivolumab. Exploratory biomarker analyses performed in various trials have demonstrated target engagement that suggests the potential for identifying patient populations that may respond favorably to the therapy. In this review, we discuss the advances made in the past 5 years regarding HSP90i and their implications in anticancer therapeutics. Our focus lies in evaluating drug efficacy, prognosis forecast, pharmacodynamic biomarkers, and clinical outcomes reported in published trials. Through this comprehensive review, we aim to shed light on the progress and potential of HSP90i as promising therapeutic agents in cancer treatment.

C-terminal domain dimerization in yeast Hsp90 is moderately modulated by the other domains

Heat shock protein 90 (Hsp90) serves as a crucial regulator of cellular proteostasis by stabilizing and regulating the activity of numerous substrates, many of which are oncogenic proteins. Therefore, Hsp90 is a drug target for cancer therapy. Hsp90 comprises three structural domains, a highly conserved amino-terminal domain (NTD), a middle domain (MD), and a carboxyl-terminal domain (CTD). The CTD is responsible for protein dimerization, is crucial for Hsp90's activity, and has therefore been targeted for inhibiting Hsp90. Here we addressed the question of whether the CTD dimerization in Hsp90, in the absence of bound nucleotides, is modulated by allosteric effects from the other domains. We studied full length (FL) and isolated CTD (isoC) yeast Hsp90 spin-labeled with a Gd(III) tag by double electron-electron resonance measurements to track structural differences and to determine the apparent dissociation constant (Kd). We found the distance distributions for both the FL and isoC to be similar, indicating that the removal of the NTD and MD does not significantly affect the structure of the CTD dimer. The low-temperature double electron-electron resonance-derived Kd values, as well as those obtained at room temperature using microscale thermophoresis and native mass spectrometry, collectively suggested the presence of some allosteric effects from the NTDs and MDs on the CTD dimerization stability in the apo state. This was evidenced by a moderate increase in the Kd for the isoC compared with the FL mutants. Our results reveal a fine regulation of the CTD dimerization by allosteric modulation, which may have implications for drug targeting strategies in cancer therapy.

Structural basis of the key residue W320 responsible for Hsp90 conformational change

The 90-kDa heat shock protein (Hsp90) is a homodimeric molecular chaperone with ATPase activity, which has become an intensely studied target for the development of drugs for the treatment of cancer, neurodegenerative and infectious diseases. The equilibrium between Hsp90 dimers and oligomers is important for modulating its function. In the absence of ATP, the passive chaperone activity of Hsp90 dimers and oligomers has been shown to stabilize client proteins as a holdase, which enhances substrate binding and prevents irreversible aggregation and precipitation of the substrate proteins. In the presence of ATP and its associated cochaperones, Hsp90 homodimers act as foldases with the binding and hydrolysis of ATP driving conformational changes that mediate client folding. Crystal structures of both wild type and W320A mutant Hsp90αMC (middle/C-terminal domain) have been determined, which displayed a preference for hexameric and dimeric states, respectively. Structural analysis showed that W320 is a key residue for Hsp90 oligomerization by forming intermolecular interactions at the Hsp90 hexameric interface through cation-π interactions with R367. W320A substitution results in the formation of a more open conformation of Hsp90, which has not previously been reported, and the induction of a conformational change in the catalytic loop. The structures provide new insights into the mechanism by which W320 functions as a key switch for conformational changes in Hsp90 self-oligomerization, and binding cochaperones and client proteins.Communicated by Ramaswamy H. Sarma.

Association of Hsp90 with p53 and Fizzy related homolog (Fzr) synchronizing Anaphase Promoting Complex (APC/C): An unexplored ally towards oncogenic pathway

The intricate molecular interactions leading to the oncogenic pathway are the consequence of cell cycle modification controlled by a bunch of cell cycle regulatory proteins. The tumor suppressor and cell cycle regulatory proteins work in coordination to maintain a healthy cellular environment. The integrity of this cellular protein pool is perpetuated by heat shock proteins/chaperones, which assist in proper protein folding during normal and cellular stress conditions. Among these versatile groups of chaperone proteins, Hsp90 is one of the significant ATP-dependent chaperones that aid in stabilizing many tumor suppressors and cell cycle regulator protein targets. Recently, studies have revealed that in cancerous cell lines, Hsp90 stabilizes mutant p53, 'the guardian of the genome.' Hsp90 also has a significant impact on Fzr, an essential regulator of the cell cycle having an important role in the developmental process of various organisms, including Drosophila, yeast, Caenorhabditis elegans, and plants. During cell cycle progression, p53 and Fzr coordinately regulate the Anaphase Promoting Complex (APC/C) from metaphase to anaphase transition up to cell cycle exit. APC/C mediates proper centrosome function in the dividing cell. The centrosome acts as the microtubule organizing center for the correct segregation of the sister chromatids to ensure perfect cell division. This review examines the structure of Hsp90 and its co-chaperones, which work in synergy to stabilize proteins such as p53 and Fizzy-related homolog (Fzr) to synchronize the Anaphase Promoting Complex (APC/C). Dysfunction of this process activates the oncogenic pathway leading to the development of cancer. Additionally, an overview of current drugs targeting Hsp90 at various phases of clinical trials has been included.

Structure, Function, and Inhibitors of the Mitochondrial Chaperone TRAP1

Tumor necrosis factor receptor-associated protein 1 (TRAP1) is a mitochondrial molecular chaperone modulating cellular metabolism and signaling pathways by altering the conformation, activity, and stability of numerous substrate proteins called clients. It exerts its chaperone function as an adaptive response to counter cellular stresses instead of maintaining housekeeping protein homeostasis. However, the stress-adaptive machinery becomes dysregulated to support the progression and maintenance of human diseases, such as cancers; therefore, TRAP1 has been proposed as a promising target protein for anticancer drug development. In this review, by collating recent reports on high-resolution TRAP1 structures and structure-activity relationships of inhibitors, we aimed to provide better insights into the chaperoning mechanism of the emerging drug target and to suggest an efficient strategy for the development of potent TRAP1 inhibitors.

Magnoflorine attenuates inflammatory responses in RA by regulating the PI3K/Akt/NF-κB and Keap1-Nrf2/HO-1 signalling pathways in vivo and in vitro

BACKGROUND

As a prolonged autoimmune disorder, rheumatoid arthritis (RA) is characterised by synovial hyperplasia and the erosion of bone and cartilage. Magnoflorine (MAG) is the main component purified from Clematis manshurica Rupr. Recent studies have shown that MAG has anti-inflammatory, antioxidant, and immunosuppressive effects, which are relevant to anti-RA activities.

OBJECTIVE

The current investigation was conducted to explore the anti-RA effects of MAG and to discover the possible molecular mechanisms.

METHODS

In vitro experiments, CCK-8, wound healing, and transwell assays were utilized to evaluate the anti-proliferative, anti-migratory, and anti-invasive activities of MAG, respectively. The rate of cell distribution and cell apoptosis were evaluated by flow cytometry. ROS generation was detected by DCFH-DA staining. Western blotting, quantitative real-time polymerase chain reaction assay, and immunofluorescent staining were employed to test the anti-RA effect of MAG as well as to explore the potential mechanisms by evaluating related gene and protein expression. For in vivo experiments, an adjuvant-induced arthritis (AIA) rat model was established. The related parameters were measured in rats. Then, rats were sacrificed, and ankle joints were collected for histopathological analysis and observation.

RESULTS

MAG significantly decreased the proliferation, migration, invasion, and reactive oxygen species levels in IL-1β-treated MH7A cells. Furthermore, MAG promoted cell apoptosis by increasing Bax levels and decreasing Bcl-2 levels. MAG also induced cell cycle arrest. Inflammatory cytokines (iNOS, COX-2, IL-6, and IL-8) and MMPs (MMP-1, 2, 3, 9, and 13) were reduced by MAG treatment. Molecular analysis revealed that MAG exerted anti-RA effects by partly inhibiting the PI3K/Akt/NF-κB signalling axis and activating the Keap1-Nrf2/HO-1 signalling pathway. In vivo studies have revealed that MAG treatment substantially improved severe symptoms in AIA rats, and these curative effects were linked to the attenuation of inflammatory responses.

CONCLUSION

These results first suggested that MAG exhibits anti-arthritic effects in IL-1β-treated MH7A cells and AIA rat models. Thus, MAG may be used as a new drug to treat RA clinically.

Development of a First-in-Class Small-Molecule Inhibitor of the C-Terminal Hsp90 Dimerization

Heat shock proteins 90 (Hsp90) are promising therapeutic targets due to their involvement in stabilizing several aberrantly expressed oncoproteins. In cancerous cells, Hsp90 expression is elevated, thereby exerting antiapoptotic effects, which is essential for the malignant transformation and tumor progression. Most of the Hsp90 inhibitors (Hsp90i) under investigation target the ATP binding site in the N-terminal domain of Hsp90. However, adverse effects, including induction of the prosurvival resistance mechanism (heat shock response or HSR) and associated dose-limiting toxicity, have so far precluded their clinical approval. In contrast, modulators that interfere with the C-terminal domain (CTD) of Hsp90 do not inflict HSR. Since the CTD dimerization of Hsp90 is essential for its chaperone activity, interfering with the dimerization process by small-molecule protein-protein interaction inhibitors is a promising strategy for anticancer drug research. We have developed a first-in-class small-molecule inhibitor (5b) targeting the Hsp90 CTD dimerization interface, based on a tripyrimidonamide scaffold through structure-based molecular design, chemical synthesis, binding mode model prediction, assessment of the biochemical affinity, and efficacy against therapy-resistant leukemia cells. 5b reduces xenotransplantation of leukemia cells in zebrafish models and induces apoptosis in BCR-ABL1⁺ (T315I) tyrosine kinase inhibitor-resistant leukemia cells, without inducing HSR.

Crystal structure of the middle and C-terminal domains of Hsp90α labeled with a coumarin derivative reveals a potential allosteric binding site as a drug target

The 90 kDa heat-shock protein (Hsp90) is an abundant molecular chaperone that is essential to activate, stabilize and regulate the function of a plethora of client proteins. As drug targets for the treatment of cancer and neurodegenerative diseases, Hsp90 inhibitors that bind to the N-terminal ATP-binding site of Hsp90 have shown disappointing efficacy in clinical trials. Thus, allosteric regulation of the function of Hsp90 by compounds that interact with its middle and C-terminal (MC) domains is now being pursued as a mechanism to inhibit the ATPase activity and client protein-binding activity of Hsp90 without concomitant induction of the heat-shock response. Here, the crystal structure of the Hsp90αMC protein covalently linked to a coumarin derivative, MDCC {7-diethylamino-3-[N-(2-maleimidoethyl)carbamoyl]coumarin}, which is located in a hydrophobic pocket that is formed at the Hsp90αMC hexamer interface, is reported. MDCC binding leads to the hexamerization of Hsp90, and the stabilization and conformational changes of three loops that are critical for its function. A fluorescence competition assay demonstrated that other characterized coumarin and isoflavone-containing Hsp90 inhibitors compete with MDCC binding, suggesting that they could bind at a common site or that they might allosterically alter the structure of the MDCC binding site. This study provides insights into the mechanism by which the coumarin class of allosteric inhibitors potentially disrupt the function of Hsp90 by regulating its oligomerization and the burial of interaction sites involved in the ATP-dependent folding of Hsp90 clients. The hydrophobic binding pocket characterized here will provide new structural information for future drug design.

Advances in the role of heat shock protein 90 in prostate cancer

Prostate cancer is one of the most common tumours in adult men and heat shock proteins play an important biological function in prostate cancer as molecular chaperones involved in the pathogenesis, diagnosis, treatment and prognosis of a wide range of tumours. Among them, increased expression of HSP90, a member of the heat shock protein family, is associated with resistance to prostate cancer denervation and can promote tumour resistance, invasion and bone metastasis, thus making prostate cancer more difficult to treat. Therefore, targeting HSP90 in prostate cancer could be a promising strategy for oncology treatment. This paper reviews the structure and function of HSP90, HSP90-mediated denudation resistance in prostate cancer and HSP90-targeted antitumor therapy, with the aim of providing a new theoretical basis for prostate cancer treatment options in the clinical setting.

Synthesis and antiproliferative activity of 6BrCaQ-TPP conjugates for targeting the mitochondrial heat shock protein TRAP1

A series of 6BrCaQ-C_n-TPP conjugates 3a-f and 5 was designed and synthesized as a novel class of TRAP1 inhibitors. Compound 3a displayed an excellent anti-proliferative activity with mean GI₅₀ values at a nanomolar level in a diverse set of human cancer cells (GI₅₀ = 0.008-0.30 μM) including MDA-MB231, HT-29, HCT-116, K562, and PC-3 cancer cell lines. Moreover, the best lead compound 6BrCaQ-C₁₀-TPP induces a significant mitochondrial membrane disturbance combined to a regulation of HSP and partner protein levels as a first evidence that his mechanism of action involves the TRAP-1 mitochondrial Hsp90 machinery.

Development of a High-throughput NanoBRET Screening Platform to Identify Modulators of the RAS/RAF Interaction

Activating mutations in RAS are found in approximately 30% of human cancers, resulting in the delivery of a persistent signal to critical downstream effectors that drive tumorigenesis. RAS-driven malignancies respond poorly to conventional cancer treatments and inhibitors that target RAS directly are limited; therefore, the identification of new strategies and/or drugs to disrupt RAS signaling in tumor cells remains a pressing therapeutic need. Taking advantage of the live-cell bioluminescence resonance energy transfer (BRET) methodology, we describe the development of a NanoBRET screening platform to identify compounds that modulate binding between activated KRAS and the CRAF kinase, an essential effector of RAS that initiates ERK cascade signaling. Using this strategy, libraries containing synthetic compounds, targeted inhibitors, purified natural products, and natural product extracts were evaluated. These efforts resulted in the identification of compounds that inhibit RAS/RAF binding and in turn suppress RAS-driven ERK activation, but also compounds that have the deleterious effect of enhancing the interaction to upregulate pathway signaling. Among the inhibitor hits identified, the majority were compounds derived from natural products, including ones reported to alter KRAS nanoclustering (ophiobolin A), to impact RAF function (HSP90 inhibitors and ROS inducers) as well as some with unknown targets and activities. These findings demonstrate the potential for this screening platform in natural product drug discovery and in the development of new therapeutic agents to target dysregulated RAS signaling in human disease states such as cancer.

Structure-Activity Relationships of Benzothiazole-Based Hsp90 C-Terminal-Domain Inhibitors

Heat shock protein 90 (Hsp90) is a chaperone responsible for the maturation of many cancer-related proteins, and is therefore an important target for the design of new anticancer agents. Several Hsp90 N-terminal domain inhibitors have been evaluated in clinical trials, but none have been approved as cancer therapies. This is partly due to induction of the heat shock response, which can be avoided using Hsp90 C-terminal-domain (CTD) inhibition. Several structural features have been shown to be useful in the design of Hsp90 CTD inhibitors, including an aromatic ring, a cationic center and the benzothiazole moiety. This study established a previously unknown link between these structural motifs. Using ligand-based design methodologies and structure-based pharmacophore models, a library of 29 benzothiazole-based Hsp90 CTD inhibitors was prepared, and their antiproliferative activities were evaluated in MCF-7 breast cancer cells. Several showed low-micromolar IC₅₀, with the most potent being compounds 5g and 9i (IC₅₀, 2.8 ± 0.1, 3.9 ± 0.1 μM, respectively). Based on these results, a ligand-based structure-activity relationship model was built, and molecular dynamics simulation was performed to elaborate the binding mode of compound 9i. Moreover, compound 9i showed degradation of Hsp90 client proteins and no induction of the heat shock response.

Tubocapsenolide A targets C-terminal cysteine residues of HSP90 to exert the anti-tumor effect

Heat shock protein 90 (HSP90) is a chaperone protein that has been shown to regulate cancer progression. As a result, HSP90 has emerged as an attractive target for cancer therapy. Tubocapsenolide A (TA) is an anti-tumor component isolated from Tubocapsicum anomalum. Although the anti-tumor activity of TA was considered to be related to HSP90, the binding site and deep anti-tumor mechanisms still need to be elucidated. In this study, we found that TA is a covalent inhibitor of HSP90, which inhibits HSP90 ATPase activity without blocking ATP binding. Further studies indicated that TA targets the C-terminal Cys521 site, which led to HSP90 partial oligomerization and hindered its anti-aggregation and refolding activity. The damage of the chaperone activity disrupted the interaction between HSP90 and its cochaperone CDC37 as well as its client proteins, thereby inducing cell cycle arrest and apoptosis. Moreover, TA was found to have therapeutic effects on the xenograft tumor model by inducing the degradation of HSP90 client proteins. Together, our results identified HSP90 as the direct target of TA for mediating the anti-tumor activity. TA could serve as a lead compound for developing novel HSP90 C-terminal covalent inhibitors with binding site different from the ATP-binding domain.

Exploring the Lapse in Druggability: Sequence Analysis, Structural Dynamics and Binding Site Characterization of K-RasG12C Variant, a Feasible Oncotherapeutics Target

BACKGROUND

The difficulty in druggability of K-Ras variant has presented a challenge in the treatment of cancer diseases associated with its dysfunctionality. Despite the identification of different binding sites, limited information exists in the literature about their characteristics. Therefore, identification, crossvalidation and characterization of its druggable sites would aid the design of chemical compounds that will arrest its dysfunctionality related oncogenesis.

OBJECTIVE

This study entails the identification, cross-validation and characterization of K-Ras G12C variant's binding sites for potential druggability, coupled with the elucidation of alterations in 3D conformations and dynamics.

METHOD

Molecular dynamics simulation was carried out on the inactive, the active and the hyperactive K-RasG12Cvariant using the amber software package. The SiteMap software was employed in identifying and characterizing the druggable binding sites while the validation of the binding sites was carried out with the SiteHound and MetaPocket servers.

RESULTS

Four druggable binding sites were identified, validated and characterized based on physicochemical attributes such as size, volume, degree of enclosure or exposure, degree of contact, hydrophobic/hydrophilic character, hydrophobic/hydrophilic balance and hydrogen-bonding features. Conformational studies also revealed that the K-Ras variant exhibited notable structural instability, increased flexibility and a strongly anticorrelated movement compared to the inactive and active wildtype forms.

CONCLUSION

The attributes of the characterized druggable sites will be useful in designing site-specific K-Ras inhibitors for the treatment of K-Ras variant associated cancer diseases.

Role of Hsp90 and ATP in modulating apyrase activity and firefly luciferase kinetics

The present manuscript describes a novel bioassay consisting of apyrase and heat shock protein 90 (Hsp90) without additional co-chaperone supplementation; intended for high-throughput screening of anti-cancer drugs and prognosis of stress. In this regard, Hsp90 and adenosine 5'-triphosphate (ATP) mediated firefly luciferase (FLuc) kinetics was investigated using apyrase and FLuc as client proteins. Bioluminescent assay containing Hsp90, ATP, and apyrase led to complete loss of luminescence at 50 °C which indicates the protective role of Hsp90 against thermal denaturation. Similarly, the assay sample comprising Hsp90, ATP, and FLuc showed 2 fold increments in luminescence than their counterparts. Introduction of bovine serum albumin (BSA) to the pre-incubated assay mixture led to an initial rise in the luminescence (28%) in comparison to the sample containing Hsp90, ATP and FLuc. Therefore, FLuc based HTS assays are not suitable for clinical samples which may contain stabilizing agents. However, thermally denatured FLuc and apyrase could not regain their active conformation even when Hsp90 and ATP were introduced in the assay system. This observation justifies the role of Hsp90 to be protective rather than a reparation agent when acts without co-chaperones.

Modulation of Human Hsp90α Conformational Dynamics by Allosteric Ligand Interaction at the C-Terminal Domain

Recent years have seen heat shock protein 90 kDa (Hsp90) attract significant interest as a viable drug target, particularly for cancer. To date, designed inhibitors that target the ATPase domain demonstrate potent anti-proliferative effects, but have failed clinical trials due to high levels of associated toxicity. To circumvent this, the focus has shifted away from the ATPase domain. One option involves modulation of the protein through allosteric activation/inhibition. Here, we propose a novel approach: we use previously obtained information via residue perturbation scanning coupled with dynamic residue network analysis to identify allosteric drug targeting sites for inhibitor docking. We probe the open conformation of human Hsp90α for druggable sites that overlap with these allosteric control elements, and identify three putative natural compound allosteric modulators: Cephalostatin 17, 20(29)-Lupene-3β-isoferulate and 3'-Bromorubrolide F. We assess the allosteric potential of these ligands by examining their effect on the conformational dynamics of the protein. We find evidence for the selective allosteric activation and inhibition of Hsp90's conformational transition toward the closed state in response to ligand binding and shed valuable insight to further the understanding of allosteric drug design and Hsp90's complex allosteric mechanism of action.

Dihydropyridines Allosterically Modulate Hsp90 Providing a Novel Mechanism for Heat Shock Protein Co-induction and Neuroprotection

Chaperones play a pivotal role in protein homeostasis, but with age their ability to clear aggregated and damaged protein from cells declines. Tau pathology is a driver of a variety of neurodegenerative disease and in Alzheimer's disease (AD) it appears to be precipitated by the formation of amyloid-β (Aβ) aggregates. Aβ-peptide appears to trigger Tau hyperphosphorylation, formation of neurofibrillary tangles and neurotoxicity. Recently, dihydropyridine derivatives were shown to upregulate the heat shock response (HSR) and provide a neuroprotective effect in an APPxPS1 AD mouse model. The HSR response was only seen in diseased cells and consequently these compounds were defined as co-inducers since they upregulate chaperones and co-chaperones only when a pathological state is present. We show for compounds tested herein, that they target predominantly the C-terminal domain of Hsp90, but show some requirement for its middle-domain, and that binding stimulates the chaperones ATPase activity. We identify the site for LA1011 binding and confirm its identification by mutagenesis. We conclude, that binding compromises Hsp90's ability to chaperone, by modulating its ATPase activity, which consequently induces the HSR in diseased cells. Collectively, this represents the mechanism by which the normalization of neurofibrillary tangles, preservation of neurons, reduced tau pathology, reduced amyloid plaque, and increased dendritic spine density in the APPxPS1 Alzheimer's mouse model is initiated. Such dihydropyridine derivatives therefore represent potential pharmaceutical candidates for the therapy of neurodegenerative disease, such as AD.

Identification of Hsp90 Inhibitors with Anti-Plasmodium Activity

Malaria remains a global health burden partly due to Plasmodium parasite resistance to first-line therapeutics. The molecular chaperone heat shock protein 90 (Hsp90) has emerged as an essential protein for blood-stage Plasmodium parasites, but details about its function during malaria's elusive liver stage are unclear. We used target-based screens to identify compounds that bind to Plasmodium falciparum and human Hsp90, which revealed insights into chemotypes with species-selective binding. Using cell-based malaria assays, we demonstrate that all identified Hsp90-binding compounds are liver- and blood-stage Plasmodium inhibitors. Additionally, the Hsp90 inhibitor SNX-0723 in combination with the phosphatidylinositol 3-kinase inhibitor PIK-75 synergistically reduces the liver-stage parasite load. Time course inhibition studies with the Hsp90 inhibitors and expression analysis support a role for Plasmodium Hsp90 in late-liver-stage parasite development. Our results suggest that Plasmodium Hsp90 is essential to liver- and blood-stage parasite infections and highlight an attractive route for development of species-selective PfHsp90 inhibitors that may act synergistically in combination therapies to prevent and treat malaria.