High-throughput screening for Hsp90 ATPase inhibitors

Heat Shock Proteins as Targets for Novel Antimalarial Drug Discovery

Plasmodium falciparum, the parasitic agent that is responsible for a severe and dangerous form of human malaria, has a history of long years of cohabitation with human beings with attendant negative consequences. While there have been some gains in the fight against malaria through the application of various control measures and the use of chemotherapeutic agents, and despite the global decline in malaria cases and associated deaths, the continual search for new and effective therapeutic agents is key to achieving sustainable development goals. An important parasite survival strategy, which is also of serious concern to the scientific community, is the rate at which the parasites continually develop resistance to drugs. Among the key players in the parasite's ability to develop resistance, maintain cellular integrity, and survives within an unusual environment of the red blood cells are the molecular chaperones of the heat shock proteins (HSP) family. HSPs constitute a novel avenue for antimalarial drug discovery and by exploring their ubiquitous nature and multifunctional activities, they may be suitable targets for the discovery of multi-targets antimalarial drugs, needed to fight incessant drug resistance. In this chapter, features of selected families of plasmodial HSPs that can be exploited in drug discovery are presented. Also, known applications of HSPs in small molecule screening, their potential usefulness in high throughput drug screening, as well as possible challenges are highlighted.

Assay design and development strategies for finding Hsp90 inhibitors and their role in human diseases

Heat shock protein 90 (Hsp90) is a molecular chaperone that facilitates the maturation of its client proteins including protein kinases, transcription factors, and steroid hormone receptors which are structurally and functionally diverse. These client proteins are involved in various cellular signaling pathways, and Hsp90 is implicated in various human diseases including cancer, inflammation, and diseases associated with protein misfolding; thus making Hsp90 a promising target for drug discovery. Some of its client proteins are well-known cancer targets. Instead of targeting these client proteins individually, however, targeting Hsp90 is more practical for cancer drug development. Efforts have been invested in recognizing potential drugs for clinical use that inhibit Hsp90 activity and result in the prevention of Hsp90 client maturation and dampening of subsequent signaling cascades. Here, we discuss current assays and technologies used to find and characterize Hsp90 inhibitors that include biophysical, biochemical, cell-based assays and computational modeling. This review highlights recent discoveries that N-terminal isoform-selective compounds and inhibitors that target the Hsp90 C-terminus that may offer the potential to overcome some of the detriments observed with pan Hsp90 inhibitors. The tools and assays summarized in this review should be used to develop Hsp90-targeting drugs with high specificity, potency, and drug-like properties that may prove immensely useful in the clinic.

In vitro methods for testing antiviral drugs

Despite successful vaccination programs and effective treatments for some viral infections, humans are still losing the battle with viruses. Persisting human pandemics, emerging and re-emerging viruses, and evolution of drug-resistant strains impose continuous search for new antiviral drugs. A combination of detailed information about the molecular organization of viruses and progress in molecular biology and computer technologies has enabled rational antivirals design. Initial step in establishing efficacy of new antivirals is based on simple methods assessing inhibition of the intended target. We provide here an overview of biochemical and cell-based assays evaluating the activity of inhibitors of clinically important viruses.

Non-benzoquinone geldanamycin analogs trigger various forms of death in human breast cancer cells

Background

Hsp90 proteins are important therapeutic targets for many anti-cancer drugs in clinical trials. Geldanamycin (GA) was identified as the first natural inhibitor of Hsp90, increasing evidence suggests that GA was not a good choice for clinical trials. In this study, we investigated two new non-benzoquinone geldanamycin analogs of Hsp90 inhibitors, DHQ3 and 17-demethoxy-reblastatin (17-DR), to explore the molecular mechanisms of their anti-cancer activity in vivo and vitro.

Methods

MTT and colony formation assays were used to measure cell viability. Flow cytometry, DAPI staining, ATP assay, electron microscopy, western blots, siRNAs transfection and immunofluorescence were used to determine the molecular mechanism of DHQ3- or 17-DR-induced different forms of death in human breast cancer MDA-MB-231 cells. Malachite green reagent was used to measure ATPase activity of the analogs.

Results

DHQ3 and 17-DR presented efficiently inhibitory effect in MDA-MB-231 cell lines, and DHQ3 induced necroptosis by activation of the RIP1-RIP3-MLKL necroptosis cascade. And DHQ3-induced cell death was inhibited by a necroptosis inhibitor, necrostatin-1 (Nec-1), but not by a caspase inhibitor z-VAD-fmk. On the other hand, 17-DR induced apoptosis in MDA-MB-231 cells, indicating a caspase-dependent killing mechanism. We further demonstrated that down-regulation of RIP1 and RIP3 by siRNA protected against DHQ3 but not 17-DR induced cell death. These results were confirmed by electron microscopy. DHQ3 and 17-DR induced the degradation of Hsp90 client proteins, and they showed strong antitumor effects in MDA-MB-231 cell-xenografted nude mice.

Conclusions

These findings supported that DHQ3 and 17-DR induce different forms of death in some cancer cell line via activation of different pathways. All of the results provided evidence for its anti-tumorigentic action with low hepatotoxicity in vivo, making them promising anti-breast cancer agents.

Diverging Novobiocin Anti-Cancer Activity from Neuroprotective Activity through Modification of the Amide Tail

Novobiocin is a natural product that binds the Hsp90 C-terminus and manifests Hsp90 inhibitory activity. Structural investigations on novobiocin led to the development of both anti-cancer and neuroprotective agents. The varied pharmacological activity manifested by these novobiocin analogs prompted the investigation of structure-function studies to identify these contradictory effects, which revealed that modifications to the amide side chain produce either anti-cancer or neuroprotective activity. Compounds that exhibit neuroprotective activity contain a short alkyl or cycloalkyl amide side chain. In contrast, anti-cancer agents contain five or more carbons, disrupt interactions between Hsp90α and Aha1, and induce the degradation of Hsp90-dependent client proteins.

Molecular docking and pharmacophore studies of heterocyclic compounds as Heat shock protein 90 (Hsp90) Inhibitors

Heat Shock Protein 90 was a key molecular chaperone involved in the proteome stability maintenance and its interference in many signaling networks associated with cancer progression, makes it of an important target for cancer therapeutics. The present study aimed to identify potential lead molecule among the selected heterocyclic compounds against Human Hsp90 (PDB: 1YET) through docking using GOLD 3.1 and pharmacophore studies using Discovery studio 2.1. On the basis of the GOLD Fitness scores, the compounds Q1G and T21 showed better binding affinity. Further the analyzed structure pharmacophore results are in consistence with the docking results indicating that both these compounds show antagonistic activity towards HSP90 respectively.

KU675, a Concomitant Heat-Shock Protein Inhibitor of Hsp90 and Hsc70 that Manifests Isoform Selectivity for Hsp90α in Prostate Cancer Cells

The 90-kDa heat-shock protein (Hsp90) assists in the proper folding of numerous mutated or overexpressed signal transduction proteins that are involved in cancer. Inhibiting Hsp90 consequently is an attractive strategy for cancer therapy as the concomitant degradation of multiple oncoproteins may lead to effective antineoplastic agents. Here we report a novel C-terminal Hsp90 inhibitor, designated KU675, that exhibits potent antiproliferative and cytotoxic activity along with client protein degradation without induction of the heat-shock response in both androgen-dependent and -independent prostate cancer cell lines. In addition, KU675 demonstrates direct inhibition of Hsp90 complexes as measured by the inhibition of luciferase refolding in prostate cancer cells. In direct binding studies, the internal fluorescence signal of KU675 was used to determine the binding affinity of KU675 to recombinant Hsp90α, Hsp90β, and Hsc70 proteins. The binding affinity (Kd) for Hsp90α was determined to be 191 μM, whereas the Kd for Hsp90β was 726 μM, demonstrating a preference for Hsp90α. Western blot experiments with four different prostate cancer cell lines treated with KU675 supported this selectivity by inducing the degradation of Hsp90α -: dependent client proteins. KU675 also displayed binding to Hsc70 with a Kd value at 76.3 μM, which was supported in cellular by lower levels of Hsc70-specific client proteins on Western blot analyses. Overall, these findings suggest that KU675 is an Hsp90 C-terminal inhibitor, as well as a dual inhibitor of Hsc70, and may have potential use for the treatment of cancer.

Quantification of pyrophosphate as a universal approach to determine polymerase activity and assay polymerase inhibitors

The importance of DNA polymerases in biology and biotechnology, and their recognition as potential therapeutic targets, drives development of methods for deriving kinetic characteristics of polymerases and their propensity to perform polynucleotide synthesis over modified DNA templates. Among various polymerases, translesion synthesis (TLS) polymerases enable cells to avoid the cytotoxic stalling of replicative DNA polymerases at chemotherapy-induced DNA lesions, thereby leading to drug resistance. Identification of TLS inhibitors to overcome drug-resistance necessitates the development of appropriate high-throughput assays. Since polymerase-mediated DNA synthesis involves the release of inorganic pyrophosphate (PPi), we established a universal and fast method for monitoring the progress of DNA polymerases based on the quantification of PPi with a fluorescence-based assay that we coupled to in vitro primer extension reactions. The established assay has a nanomolar detection limit in PPi and enables the evaluation of single nucleotide incorporation and DNA synthesis progression kinetics. The results demonstrated that the developed assay is a reliable method for monitoring TLS and identifying nucleoside and nucleotide-based TLS inhibitors.

Improvement of inhibitor identification for heat shock protein 90α by utilizing a red-shifted fluorescence polarization probe

Heat shock protein-90 (HSP90) is an ATP-dependent molecular chaperone with intrinsic ATPase activity. HSP90 is required for the stability and function of client proteins, many of which are involved in oncogenesis. Thus, identification of HSP90 inhibitors would potentially lead to the discovery of cancer therapeutics. Here, we present a high-throughput screening campaign utilizing two geldanamycin (GM)-labeled probes in a fluorescence polarization (FP) assay. For the primary screen, a previously reported green BODIPY-labeled GM (GM-BODIPY) was used to evaluate a library collection of about 400,000 compounds. From this screen, 3058 compounds showed >30% inhibition. To distinguish true positives from compound interference, a confirmatory screen was deemed necessary. Accordingly, a red-shifted FP binding assay was developed using GM labeled with red BODIPY. This tool enabled reliable identification of promising HSP90α inhibitors.

Application of docking-based comparative intermolecular contacts analysis to validate Hsp90α docking studies and subsequent in silico screening for inhibitors

Heat shock protein (Hsp90α) has been recently implicated in cancer, prompting several attempts to discover and optimize new Hsp90α inhibitors. Towards this end, we docked 83 diverse Hsp90α inhibitors into the ATP-binding site of this chaperone using several docking-scoring settings. Subsequently, we applied our newly developed computational tool--docking-based comparative intramolecular contacts analysis (dbCICA)--to assess the different docking conditions and select the best settings. dbCICA is based on the number and quality of contacts between docked ligands and amino acid residues within the binding pocket. It assesses a particular docking configuration based on its ability to align a set of ligands within a corresponding binding pocket in such a way that potent ligands come into contact with binding site spots distinct from those approached by low-affinity ligands, and vice versa. The optimal dbCICA models were translated into valid pharmacophore models that were used as 3D search queries to mine the National Cancer Institute's structural database for new inhibitors of Hsp90α that could potentially be used as anticancer agents. The process culminated in 15 micromolar Hsp90α ATPase inhibitors.

The rapid and direct determination of ATPase activity by ion exchange chromatography and the application to the activity of heat shock protein-90

Adenosine nucleotides are involved as substrates or co-factors in several biochemical reactions, catalyzed by enzymes, which modulate energy production, signal transduction and cell proliferation. We here report the development and optimization of an ion exchange liquid chromatography (LC) method for the determination of ATP, ADP and AMP. This method is specifically aimed at the determination of the ATP-ase activity of human heat shock protein 90 (Hsp90), a molecular chaperone that has emerged as target enzyme in cancer therapy. Separation of the three nucleotides was achieved in a 15-min run by using a disk shaped monolithic ethylene diamine stationary phase of small dimensions (2mm×6mm i.d.), under a three-solvent gradient elution mode and UV detection at 256nm. The described direct LC method resulted highly specific as a consequence of the baseline separation of the three adenosine nucleotides and could be applied to the determination of the enzymatic activity of ADP/ATP generating or consuming enzymes (such as kinases). Furthermore, comparison of the LOD and LOQ values of the LC method with those obtained with the malachite green assay, which is one of the most used indirect screening methodologies for ATP-ase activity, showed that the LC method has a similar range of application without presenting the drawbacks related to contamination by inorganic phosphate ions and glycerol, which are present in Hsp90 commercial samples.

Elaborate ligand-based modeling reveals new nanomolar heat shock protein 90α inhibitors

Heat shock protein (Hsp90α) has been recently implicated in cancer prompting several attempts to discover and optimize new Hsp90α inhibitors. Toward this end, we explored the pharmacophoric space of 83 Hsp90α inhibitors using six diverse sets of inhibitors to identify high-quality pharmacophores. Subsequently, genetic algorithm and multiple linear regression analysis were employed to select an optimal combination of pharmacophoric models and 2D physicochemical descriptors capable of accessing a self-consistent quantitative structure activity relationship (QSAR) of optimal predictive potential (r(67)(2)=0.811, F 42.8, r(LOO)(2)=0.748, r(PRESS)(2) (against 16 external test inhibitors) = 0.619). Three orthogonal pharmacophores emerged in the QSAR equation suggesting the existence of at least three binding modes accessible to ligands within the Hsp90α binding pocket. Receiver operating characteristic (ROC) curves analysis established the validity of QSAR-selected pharmacophores. We employed the pharmacophoric models and associated QSAR equation to screen the national cancer institute (NCI) list of compounds and our in-house-built drugs and agrochemicals database (DAC). Twenty-five nanomolar and low micromolar Hsp90α inhibitors were identified. The most potent were formoterol, amodaquine, primaquine, and midodrine with IC(50) values of 3, 5, 6, and 20 nM, respectively.

Some sulfonamide drugs inhibit ATPase activity of heat shock protein 90: investigation by docking simulation and experimental validation

Eight selected sulfonamide drugs were investigated as inhibitors of heat shock protein 90 (Hsp90). The investigation included simulated docking experiments to fit the selected compounds within the binding pocket of Hsp90. The selected molecules were found to readily fit within the ATP-binding pocket of Hsp90 in low-energy poses. The sulfonamides torsemide, sulfathiazole, and sulfadiazine were found to inhibit the ATPase activity of Hsp90 with IC(50) values of 1.0, 2.6, and 1.5 μM, respectively. Our results suggest that these well-established sulfonamides can be good leads for subsequent optimization into potent Hsp90 inhibitors.

Hsp90 (heat shock protein 90) inhibitor occupancy is a direct determinant of client protein degradation and tumor growth arrest in vivo

Several Hsp90 (heat shock protein 90) inhibitors are currently under clinical evaluation as anticancer agents. However, the correlation between the duration and magnitude of Hsp90 inhibition and the downstream effects on client protein degradation and cancer cell growth inhibition has not been thoroughly investigated. To investigate the relationship between Hsp90 inhibition and cellular effects, we developed a method that measures drug occupancy on Hsp90 after treatment with the Hsp90 inhibitor IPI-504 in living cells and in tumor xenografts. In cells, we find the level of Hsp90 occupancy to be directly correlated with cell growth inhibition. At the molecular level, the relationship between Hsp90 occupancy and Hsp90 client protein degradation was examined for different client proteins. For sensitive Hsp90 clients (e.g. HER2 (human epidermal growth factor receptor 2), client protein levels directly mirror Hsp90 occupancy at all time points after IPI-504 administration. For insensitive client proteins, we find that protein abundance matches Hsp90 occupancy only after prolonged incubation with drug. Additionally, we investigate the correlation between plasma pharmacokinetics (PK), tumor PK, pharmacodynamics (PD) (client protein degradation), tumor growth inhibition, and Hsp90 occupancy in a xenograft model of human cancer. Our results indicate Hsp90 occupancy to be a better predictor of PD than either plasma PK or tumor PK. In the nonsmall cell lung cancer xenograft model studied, a linear correlation between Hsp90 occupancy and tumor growth inhibition was found. This novel binding assay was evaluated both in vitro and in vivo and could be used as a pharmacodynamic readout in the clinic.

Natural and semisynthetic azaphilones as a new scaffold for Hsp90 inhibitors

A series of mold metabolites of Ascomycetes, structurally belonging to the class of azaphilones, were found to inhibit the heat shock protein Hsp90. In particular, bulgarialactone B was tested for its binding to Hsp90 using surface plasmon resonance and limited proteolysis assays and for its effects on Hsp90 client proteins expression in a series of human tumor cell lines. This compound showed high affinity for Hsp90, interacting with the 90-280 region of the N-terminal domain and down-regulated the Hsp90 client proteins Raf-1, survivin, Cdk4, Akt, and EGFR. Bulgarialactone B and other natural azaphilones showed antiproliferative activity in a panel of human tumor cell lines; their conversion into semisynthetic derivatives by reaction with primary amines increased the antiproliferative activity. Preliminary results indicated in vivo activity of bulgarialactone B against an ascitic ovarian carcinoma xenograft, thus supporting the therapeutic potential of this novel series of Hsp90 inhibitors.

Detection of the ATPase activity of the molecular chaperones Hsp90 and Hsp72 using the TranscreenerTM ADP assay kit

The molecular chaperone heat shock protein 90 (Hsp90) is required for the correct folding and stability of a number of client proteins that are important for the growth and maintenance of cancer cells. Heat shock protein 72 (Hsp72), a co-chaperone of Hsp90, is also emerging as an attractive cancer drug target. Both proteins bind and hydrolyze adenosine triphosphate (ATP), and ATPase activity is essential for their function. Inhibition of Hsp90 ATPase activity leads to the degradation of client proteins, resulting in cell growth inhibition and apoptosis. Several small-molecule inhibitors of the ATPase activity of Hsp90 have been described and are currently being evaluated clinically for the treatment of cancer. A number of methods for the measurement of ATPase activity have been previously used, but not all of these are ideally suited to screening cascades in drug discovery projects. The authors have evaluated the use of commercial reagents (Transcreener ADP) for the measurement of ATPase activity of both yeast and human Hsp90 (ATP K(m) approximately 500 microM) and human Hsp72 (ATP K(m) ~1 microM). The low ATPase activity of human Hsp90 and its stimulation by the co-chaperone Aha1 was measured with ease using reduced incubation times, generating robust data (Z' = 0.75). The potency of several small-molecule inhibitors of both Hsp90 and Hsp72 was determined using the Transcreener reagents and compared well to that determined using other assay formats.

KU135, a novel novobiocin-derived C-terminal inhibitor of the 90-kDa heat shock protein, exerts potent antiproliferative effects in human leukemic cells

The 90-kDa heat shock protein (Hsp90) assists in the proper folding of numerous mutated or overexpressed signal transduction proteins that are involved in cancer. Consequently, there is considerable interest in developing chemotherapeutic drugs that specifically disrupt the function of Hsp90. Here, we investigated the extent to which a novel novobiocin-derived C-terminal Hsp90 inhibitor, designated KU135, induced antiproliferative effects in Jurkat T-lymphocytes. The results indicated that KU135 bound directly to Hsp90, caused the degradation of known Hsp90 client proteins, and induced more potent antiproliferative effects than the established N-terminal Hsp90 inhibitor 17-allylamino-demethoxygeldanamycin (17-AAG). Closer examination of the cellular response to KU135 and 17-AAG revealed that only 17-AAG induced a strong up-regulation of Hsp70 and Hsp90. In addition, KU135 caused wild-type cells to undergo G(2)/M arrest, whereas cells treated with 17-AAG accumulated in G(1). Furthermore, KU135 but not 17-AAG was found to be a potent inducer of mitochondria-mediated apoptosis as evidenced, in part, by the fact that cell death was inhibited to a similar extent by Bcl-2/Bcl-x(L) overexpression or the depletion of apoptotic protease-activating factor-1 (Apaf-1). Together, these data suggest that KU135 inhibits cell proliferation by regulating signaling pathways that are mechanistically different from those targeted by 17-AAG and as such represents a novel opportunity for Hsp90 inhibition.

Synthesis and evaluation of Hsp90 inhibitors that contain the 1,4-naphthoquinone scaffold

High-throughput screening of a library of diverse molecules has identified the 1,4-naphthoquinone scaffold as a new class of Hsp90 inhibitors. The synthesis and evaluation of a rationally-designed series of analogues containing the naphthoquinone core scaffold has provided key structure-activity relationships for these compounds. The most active inhibitors exhibited potent in vitro activity with low micromolar IC(50) values in anti-proliferation and Her2 degradation assays. In addition, 3g, 12, and 13a induced the degradation of oncogenic Hsp90 client proteins, a hallmark of Hsp90 inhibition. The identification of these naphthoquinones as Hsp90 inhibitors provides a new scaffold upon which improved Hsp90 inhibitors can be developed.

Synthesis and SAR study of N-(4-hydroxy-3-(2-hydroxynaphthalene-1-yl)phenyl)-arylsulfonamides: heat shock protein 90 (Hsp90) inhibitors with submicromolar activity in an in vitro assay

Heat shock protein 90 is emerging as an important target in cancer chemotherapy. In a program directed toward identifying novel chemical probes for Hsp90, we found N-(4-hydroxy-3-(2-hydroxynaphthalene-1-yl)phenyl)benzene sulfonamide as an Hsp90 inhibitor with very weak activity. In this report, we present a new and general method for the synthesis of a variety of analogs around this scaffold, and discuss their structure-activity relationships.

Discovery of aminoquinolines as a new class of potent inhibitors of heat shock protein 90 (Hsp90): Synthesis, biology, and molecular modeling

The molecular chaperone Hsp90 plays important roles in maintaining malignant phenotypes. Recent studies suggest that Hsp90 exerts high-affinity interactions with multiple oncoproteins, which are essential for the growth of tumor cells. As a result, research has focused on finding Hsp90 probes as potential and selective anticancer agents. In a high-throughput screening exercise, we identified quinoline 7 as a moderate inhibitor of Hsp90. Further hit identification, SAR studies, and biological investigation revealed several synthetic analogs in this series with micromolar activities in both fluorescent polarization (FP) assay and a cell-based Western blot (WB) assay. These compounds represent a new class of Hsp90 inhibitors with simple chemical structures.

High-throughput screening fluorescence polarization assay for tumor-specific Hsp90

Heat shock protein 90 (Hsp90) is a molecular chaperone that has emerged as an important target in cancer and several other diseases, such as neurodegenerative diseases, nerve injuries, inflammation, and infection. Discovery of novel agents that inhibit Hsp90 and have druglike properties is therefore a major focus in several academic and industrial laboratories. In this study, the authors describe the development and optimization in a 384-well format of a novel assay for the identification of Hsp90 inhibitors using fluorescence polarization, which measures competitive binding of red-shifted fluorescently labeled geldanamycin (GM-cy3B) to Hsp90 found in the NCI-N417 small-cell lung carcinoma cells. The authors demonstrate that GMcy3B binds with high affinity and specificity to cellular Hsp90. The assay results in excellent signal-to-noise ratios (>10) and Z' values (>0.75) at tracer concentrations greater than 4 nM and 1 microg/well of total NCI-N417 protein, indicating a robust assay. It also equilibrates after 5 h of incubation at room temperature and remains stable for up to 24 h. Furthermore, it is a simple mix-and-read format that is cost-effective and uses only low amounts of fluorophore and cell lysates. A study using more than 15,000 compounds from the National Institutes of Health Molecular Libraries Screening Center Network was performed to validate its performance in a high-throughput screening format.

High-throughput screen for small molecules that modulate the ATPase activity of the molecular chaperone DnaK

DnaK is a molecular chaperone of Escherichia coli that belongs to a family of conserved 70-kDa heat shock proteins. The Hsp70 chaperones are well known for their crucial roles in regulating protein homeostasis, preventing protein aggregation, and directing subcellular traffic. Given the complexity of functions, a chemical method for controlling the activities of these chaperones might provide a useful experimental tool. However, there are only a handful of Hsp70-binding molecules known. To build this area, we developed a robust, colorimetric, high-throughput screening (HTS) method in 96-well plates that reports on the ATPase activity of DnaK. Using this approach, we screened a 204-member focused library of molecules that share a dihydropyrimidine core common to known Hsp70-binding leads and uncovered seven new inhibitors. Intriguingly, the candidates do not appear to bind the hydrophobic groove that normally interacts with peptide substrates. In sum, we have developed a reliable HTS method that will likely accelerate discovery of small molecules that modulate DnaK/Hsp70 function. Moreover, because this family of chaperones has been linked to numerous diseases, this platform might be used to generate new therapeutic leads.

High-throughput assay for the identification of Hsp90 inhibitors based on Hsp90-dependent refolding of firefly luciferase

Previously, we have demonstrated that the renaturation of heat denatured firefly luciferase is dependent upon the activity of Hsp90 in rabbit reticulocyte lysate. Here, we demonstrate that this assay may identify inhibitors that obstruct the chaperone activity of Hsp90 either by direct binding to its N-terminal or C-terminal nucleotide binding sites or by interference with the ability of the chaperone to switch conformations. The assay was adapted and optimized for high-throughput screening. Greater than 20,000 compounds were screened to demonstrate the feasibility of using this assay on a large scale. The assay was reproducible (av Z-factor=0.62) and identified 120 compounds that inhibited luciferase renaturation by greater than 70% at a concentration of 12.5 microg/mL. IC50 values for twenty compounds with varying structures were determined for inhibition of luciferase refolding and in cell-based assays for Hsp90 inhibition. Several compounds had IC50 values <10 microM and represent a number of new lead structures with the potential for further development and optimization as potent Hsp90 inhibitors.

Novobiocin: Redesigning a DNA Gyrase Inhibitor for Selective Inhibition of Hsp90

Novobiocin is a member of the coumermycin family of antibiotics and is a well-established inhibitor of DNA gyrase. Recent studies have shown that novobiocin binds to a previously unrecognized ATP-binding site at the C-terminus of Hsp90 and induces degradation of Hsp90-dependent client proteins at approximately 700 microM. In an effort to develop more efficacious inhibitors of the C-terminal binding site, a library of novobiocin analogues was prepared and initial structure-activity relationships revealed. These data suggested that the 4-hydroxy moiety of the coumarin ring and the 3'-carbamate of the noviose appendage were detrimental to Hsp90 inhibitory activity. In an effort to confirm these findings, 4-deshydroxy novobiocin (DHN1) and 3'-descarbamoyl-4-deshydroxynovobiocin (DHN2) were prepared and evaluated against Hsp90. Both compounds were significantly more potent than the natural product, and DHN2 proved to be more active than DHN1. In an effort to determine whether these moieties are important for DNA gyrase inhibition, these compounds were tested for their ability to inhibit DNA gyrase and found to exhibit significant reduction in gyrase activity. Thus, we have established the first set of compounds that clearly differentiate between the C-terminus of Hsp90 and DNA gyrase, converted a well-established gyrase inhibitor into a selective Hsp90 inhibitor, and confirmed essential structure-activity relationships for the coumermycin family of antibiotics.

Modulation of Hsp90 function in neurodegenerative disorders: a molecular-targeted therapy against disease-causing protein

Abnormal accumulation of disease-causing protein is a commonly observed characteristic in chronic neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and polyglutamine (polyQ) diseases. A therapeutic approach that could selectively eliminate would be a promising remedy for neurodegenerative disorders. Spinal and bulbar muscular atrophy (SBMA), one of the polyQ diseases, is a late-onset motor neuron disease characterized by proximal muscle atrophy, weakness, contraction fasciculations, and bulbar involvement. The pathogenic gene product is polyQ-expanded androgen receptor (AR), which belongs to the heat shock protein (Hsp) 90 client protein family. 17-Allylamino-17-demethoxygeldanamycin (17-AAG), a novel Hsp90 inhibitor, is a new derivative of geldanamycin that shares its important biological activities but shows less toxicity. 17-AAG is now in phase II clinical trials as a potential anti-cancer agent because of its ability to selectively degrade several oncoproteins. We have recently demonstrated the efficacy and safety of 17-AAG in a mouse model of SBMA. The administration of 17-AAG significantly ameliorated polyQ-mediated motor neuron degeneration by reducing the total amount of mutant AR. 17-AAG accomplished the preferential reduction of mutant AR mainly through Hsp90 chaperone complex formation and subsequent proteasome-dependent degradation. 17-AAG induced Hsp70 and Hsp40 in vivo as previously reported; however, its ability to induce HSPs was limited, suggesting that the HSP induction might support the degradation of mutant protein. The ability of 17-AAG to preferentially degrade mutant protein would be directly applicable to SBMA and other neurodegenerative diseases in which the disease-causing proteins also belong to the Hsp90 client protein family. Our proposed therapeutic approach, modulation of Hsp90 function by 17-AAG treatment, has emerged as a candidate for molecular-targeted therapies for neurodegenerative diseases. This review will consider our research findings and discuss the possibility of a clinical application of 17-AAG to SBMA and other neurodegenerative diseases.