Natural product inhibitors of Hsp90: potential leads for drug discovery

Natural Products Targeting Hsp90 for a Concurrent Strategy in Glioblastoma and Neurodegeneration

Glioblastoma multiforme (GBM) is one of the most common aggressive, resistant, and invasive primary brain tumors that share neurodegenerative actions, resembling many neurodegenerative diseases. Although multiple conventional approaches, including chemoradiation, are more frequent in GBM therapy, these approaches are ineffective in extending the mean survival rate and are associated with various side effects, including neurodegeneration. This review proposes an alternative strategy for managing GBM and neurodegeneration by targeting heat shock protein 90 (Hsp90). Hsp90 is a well-known molecular chaperone that plays essential roles in maintaining and stabilizing protein folding to degradation in protein homeostasis and modulates signaling in cancer and neurodegeneration by regulating many client protein substrates. The therapeutic benefits of Hsp90 inhibition are well-known for several malignancies, and recent evidence highlights that Hsp90 inhibitors potentially inhibit the aggressiveness of GBM, increasing the sensitivity of conventional treatment and providing neuroprotection in various neurodegenerative diseases. Herein, the overview of Hsp90 modulation in GBM and neurodegeneration progress has been discussed with a summary of recent outcomes on Hsp90 inhibition in various GBM models and neurodegeneration. Particular emphasis is also given to natural Hsp90 inhibitors that have been evidenced to show dual protection in both GBM and neurodegeneration.

Synthesis and Biological Activity of 3-(Heteroaryl)quinolin-2(1H)-ones Bis-Heterocycles as Potential Inhibitors of the Protein Folding Machinery Hsp90

In the context of our SAR study concerning 6BrCaQ analogues as C-terminal Hsp90 inhibitors, we designed and synthesized a novel series of 3-(heteroaryl)quinolin-2(1H), of types 3, 4, and 5, as a novel class of analogues. A Pd-catalyzed Liebeskind-Srogl cross-coupling was developed as a convenient approach for easy access to complex purine architectures. This series of analogues showed a promising biological effect against MDA-MB231 and PC-3 cancer cell lines. This study led to the identification of the best compounds, 3b (IC₅₀ = 28 µM) and 4e, which induce a significant decrease of CDK-1 client protein and stabilize the levels of Hsp90 and Hsp70 without triggering the HSR response.

Heat Shock Factors in Protein Quality Control and Spermatogenesis

Proper regulation of cellular protein quality control is crucial for cellular health. It appears that the protein quality control machinery is subjected to distinct regulation in different cellular contexts such as in somatic cells and in germ cells. Heat shock factors (HSFs) play critical role in the control of quality of cellular proteins through controlling expression of many genes encoding different proteins including those for inducible protein chaperones. Mammalian cells exert distinct mechanism of cellular functions through maintenance of tissue-specific HSFs. Here, we have discussed different HSFs and their functions including those during spermatogenesis. We have also discussed the different heat shock proteins induced by the HSFs and their activities in those contexts. We have also identified several small molecule activators and inhibitors of HSFs from different sources reported so far.

Natural compounds in the regulation of proteostatic pathways: An invincible artillery against stress, ageing, and diseases

Cells have different sets of molecules for performing an array of physiological functions. Nucleic acids have stored and carried the information throughout evolution, whereas proteins have been attributed to performing most of the cellular functions. To perform these functions, proteins need to have a unique conformation and a definite lifespan. These attributes are achieved by a highly coordinated protein quality control (PQC) system comprising chaperones to fold the proteins in a proper three-dimensional structure, ubiquitin-proteasome system for selective degradation of proteins, and autophagy for bulk clearance of cell debris. Many kinds of stresses and perturbations may lead to the weakening of these protective cellular machinery, leading to the unfolding and aggregation of cellular proteins and the occurrence of numerous pathological conditions. However, modulating the expression and functional efficiency of molecular chaperones, E3 ubiquitin ligases, and autophagic proteins may diminish cellular proteotoxic load and mitigate various pathological effects. Natural medicine and small molecule-based therapies have been well-documented for their effectiveness in modulating these pathways and reestablishing the lost proteostasis inside the cells to combat disease conditions. The present article summarizes various similar reports and highlights the importance of the molecules obtained from natural sources in disease therapeutics.

National Institute on Aging Workshop: Repurposing Drugs or Dietary Supplements for Their Senolytic or Senomorphic Effects: Considerations for Clinical Trials

BACKGROUND

Cell senescence is implicated in numerous age-related conditions. Drugs and nutritional supplements developed for a variety of purposes kill senescent cells (senolytics) or suppress their secretions (senomorphics). There is interest in repurposing such drugs to treat or prevent age-related diseases. To date, only small-scale preliminary trials have been conducted.

METHOD

At a workshop convened by the National Institute on Aging in August 2019, academic, industry, and government scientists reviewed issues for phase II trials of potentially repurposable drugs, or dietary supplements, to assess benefits and risks of their senolytic (killing senescent cells) or senomorphic (altering senescent cells' phenotypes) effects in treating or preventing age-related conditions.

RESULTS

Participants reviewed mechanisms and effects of cellular senescence, senolytics, and senomorphics of several classes and their potential role in treating or preventing disease, modulators of the senescence-associated secretory phenotype, needs for senescence markers, data and specimen resources, infrastructure for planning trials, and potential effects on outcomes in older patients with multimorbidity and polypharmacy.

CONCLUSIONS

Participants noted the importance of considering potential effects of candidate drugs on multiple aging outcomes. It is important to assess drugs' specificity for killing senescent cells and the balance between senolytic and cytotoxic effects. Markers of specific senescent cell types are needed to assess intervention responses. There are potential interactions with coexisting diseases and their treatments in older persons. Standardized measures could enhance comparisons and pooling of data. Additional characterization of human cell senescent phenotypes is needed for developing better and more specific senolytics and senomorphics.

Cell Surface GRP94 as a Novel Emerging Therapeutic Target for Monoclonal Antibody Cancer Therapy

Glucose-regulated protein 94 (GRP94) is an endoplasmic reticulum (ER)-resident member of the heat shock protein 90 (HSP90) family. In physiological conditions, it plays a vital role in regulating biological functions, including chaperoning cellular proteins in the ER lumen, maintaining calcium homeostasis, and modulating immune system function. Recently, several reports have shown the functional role and clinical relevance of GRP94 overexpression in the progression and metastasis of several cancers. Therefore, the current review highlights GRP94’s physiological and pathophysiological roles in normal and cancer cells. Additionally, the unmet medical needs of small chemical inhibitors and the current development status of monoclonal antibodies specifically targeting GRP94 will be discussed to emphasize the importance of cell surface GRP94 as an emerging therapeutic target in monoclonal antibody therapy for cancer.

Pharmacological Properties of Chalcones: A Review of Preclinical Including Molecular Mechanisms and Clinical Evidence

Chalcones are among the leading bioactive flavonoids with a therapeutic potential implicated to an array of bioactivities investigated by a series of preclinical and clinical studies. In this article, different scientific databases were searched to retrieve studies depicting the biological activities of chalcones and their derivatives. This review comprehensively describes preclinical studies on chalcones and their derivatives describing their immense significance as antidiabetic, anticancer, anti-inflammatory, antimicrobial, antioxidant, antiparasitic, psychoactive, and neuroprotective agents. Besides, clinical trials revealed their use in the treatment of chronic venous insufficiency, skin conditions, and cancer. Bioavailability studies on chalcones and derivatives indicate possible hindrance and improvement in relation to its nutraceutical and pharmaceutical applications. Multifaceted and complex underlying mechanisms of chalcone actions demonstrated their ability to modulate a number of cancer cell lines, to inhibit a number of pathological microorganisms and parasites, and to control a number of signaling molecules and cascades related to disease modification. Clinical studies on chalcones revealed general absence of adverse effects besides reducing the clinical signs and symptoms with decent bioavailability. Further studies are needed to elucidate their structure activity, toxicity concerns, cellular basis of mode of action, and interactions with other molecules.

Identification of the naphthoquinone derivative inhibitors binding site in heat shock protein 90: an induced-fit docking, molecular dynamics and 3D-QSAR study

The combination of molecular modeling methods to identify the putative binding site of inhibitors constitutes an important tool in drug discovery. In this work, we used these analyses to understand the potent inhibitory effect of naphthoquinone derivatives on heat shock protein 90 (Hsp90), one of the proteins involved in many types of cancer. Molecular docking results indicated that some favorable interactions of key amino acid residues at the binding site of Hsp90 with these quinones would be responsible for the inhibition of Hsp90 activity. Molecular docking and molecular dynamics simulation were carried out to further understand the binding modes and the interactions between the protein and these inhibitors. The main residues of the internal cavity were Val136, Phe138, Tyr139, Val150, Trp162 and Val186. The high concordance between the docking results and 3D-QSAR contour maps gives us helpful information about the environment of the binding site. Our results provide the bases for a rational modification of new molecules based in quinone scaffold, in order to design more potent Hsp90 inhibitors, which would exhibit highly potent antitumor activity.Communicated by Ramaswamy H. Sarma.

A Chemical Biology Approach to the Chaperome in Cancer-HSP90 and Beyond

Cancer is often associated with alterations in the chaperome, a collection of chaperones, cochaperones, and other cofactors. Changes in the expression levels of components of the chaperome, in the interaction strength among chaperome components, alterations in chaperome constituency, and in the cellular location of chaperome members, are all hallmarks of cancer. Here we aim to provide an overview on how chemical biology has played a role in deciphering such complexity in the biology of the chaperome in cancer and in other diseases. The focus here is narrow and on pathologic changes in the chaperome executed by enhancing the interaction strength between components of distinct chaperome pathways, specifically between those of HSP90 and HSP70 pathways. We will review chemical tools and chemical probe-based assays, with a focus on HSP90. We will discuss how kinetic binding, not classical equilibrium binding, is most appropriate in the development of drugs and probes for the chaperome in disease. We will then present our view on how chaperome inhibitors may become potential drugs and diagnostics in cancer.

Natural compounds as potential Hsp90 inhibitors for breast cancer-Pharmacophore guided molecular modelling studies

Breast cancer is one of the major impediments affecting women globally. The ATP-dependant heat shock protein 90 (Hsp90) forms the central component of molecular chaperone machinery that predominantly governs the folding of newly synthesized peptides and their conformational maturation. It regulates the stability and function of numerous client proteins that are frequently upregulated and/or mutated in cancer cells, therefore, making Hsp90 inhibition a promising therapeutic strategy for the development of new efficacious drugs to treat breast cancer. In the present in silico investigation, a structure-based pharmacophore model was generated with hydrogen bond donor, hydrogen bond acceptor and hydrophobic features complementary to crucial residues Ala55, Lys58, Asp93, Ile96, Met98 and Thr184 directed at inhibiting the ATP-binding activity of Hsp90. Subsequently, the phytochemical dataset of 3210 natural compounds was screened to retrieve the prospective inhibitors after rigorous validation of the model pharmacophore. The retrieved 135 phytocompounds were further filtered by drug-likeness parameters including Lipinski's rule of five and ADMET properties, then investigated via molecular docking-based scoring. Molecular interactions were assessed using Genetic Optimisation for Ligand Docking program for 95 drug-like natural compounds against Hsp90 along with two clinical drugs as reference compounds - Geldanamycin and Radicicol. Docking studies revealed three phytochemicals are better than the investigated clinical drugs. The reference and hit compounds with dock scores of 48.27 (Geldanamycin), 40.90 (Radicicol), 73.04 (Hit1), 72.92 (Hit2) and 68.12 (Hit3) were further validated for their binding stability through molecular dynamics simulations. We propose that the non-macrocyclic scaffolds of three identified phytochemicals might aid in the development of novel therapeutic candidates against Hsp90-driven cancers.

Targeting hsp90 family members: A strategy to improve cancer cell death

A crucial process in biology is the conversion of the genetic information into functional proteins that carry out the genetic program. However, a supplementary step is required to obtain functional proteins: the folding of the newly translated polypeptides into well-defined, three-dimensional conformations. Proteins chaperones are crucial for this final step in the readout of genetic information, which results in the formation of functional proteins. In this review, a special attention will be given to the strategies targeting hsp90 family members in order to increase cancer cell death. We argue that disruption of hsp90 machinery and the further client protein degradation is the main consequence of hsp90 oxidative cleavage taking place at the N-terminal nucleotide-binding site. Moreover, modulation of Grp94 expression will be discussed as a potential therapeutic goal looking for a decrease in cancer relapses.

Screening of Neem extracts for microbial anti-chaperone activity by employing in vitro enzyme refolding assay

Microbial heat shock proteins (Hsps) play an important role in pathogenesis and development of resistance to existing drugs. New compounds that target microbial molecular chaperones have the potential of combating the challenge of anti-microbial resistance. The present study was aimed at assessing the employment of in vitro enzyme refolding assay to detect anti-chaperone activity of Neem (Azadirachta indica) extracts. Protein extracts of thermotolerant Escherichia coli cells were used as a source of Hsps or chaperones. Thermotolerance was found to be induced by pre-treating E. coli cells at 47 °C before subjecting them to a lethal temperature of 55 °C. This thermotolerance correlated with over-expression of specific proteins and reduced aggregation as evident from the SDS-PAGE profiles. Refolding assays of denatured enzymes exhibited 45% activity regain in presence of cell protein extracts containing chaperones compared to less than 5% regain in BSA negative controls. The chaperone activity was found to be ATP dependent. Addition of Neem extracts to refolding reaction mixtures distinctly reduced the activity regain (20%) in a dose dependent manner (500 and 1000 ppm). The negative influence of plant extract on refolding of the enzyme in the presence of chaperones gives evidence to its anti-chaperone activity. We propose that the employment of in vitro enzyme refolding assays will help not only to analyze the activity of known and putative chaperones but also to screen natural compounds for anti-microbial-Hsp activity.

Modulation of expression of heat shock proteins and apoptosis by Flueggea leucopyrus (Willd) decoction in three breast cancer phenotypes

Background

During the past few years, there has been an increasing interest among the Traditional and Folk medical practitioners of Sri Lanka in the use of a decoction prepared from Flueggea leucopyrus (Willd.) for treating various cancers including breast cancer. In the present study, the cytotoxicity of this decoction and its effects on Heat Shock Protein (HSP) expression and apoptosis were compared in three breast cancer phenotypes, to scientifically evaluate if a decoction prepared from F. leucopyrus (Willd.) is useful for the treatment of breast cancer.

Methods

Cytotoxic potential of the F. leucopyrus decoction was determined by evaluating its effects in MCF-7, MDA-MB-231 and SKBR-3 breast cancer cell lines, and MCF-10A (non-cancerous) breast cell line, by use of the Sulphorhodamine (SRB) assay. The effect of the decoction on HSP gene expression in the above cells was evaluated by (a) Real time reverse transcription PCR (RT-PCR) and (b) Immunofluorescence analysis of HSP protein expression. Effects of the decoction on apoptosis were evaluated by (a) fluorescent microscopic examination of apoptosis related morphological changes and (b) DNA fragmentation (c) Caspase 3/7 assay.

Results

F. leucopyrus decoction can mediate significant cytotoxic effects in all three breast cancer cells phenotypes (IC₅₀ values: 27.89, 99.43, 121.43 μg/mL at 24 h post incubation periods, for MCF-7, MDA-MB-231, SKBR-3 respectively) with little effect in the non-cancerous breast cell line MCF-10A (IC₅₀: 570.4 μg/mL). Significant (*P <0.05) inhibitions of HSP 90 and HSP 70 expression were mediated by the decoction in MCF-7 and MDA-MB-231, with little effect in the SKBR-3 cells. Clear apoptotic morphological changes on Acridine orange/Ethidium bromide staining and DNA fragmentation were observed in all three breast cancer cell lines. Caspase 3/7 were significantly (*P <0.05) activated only in MDA-MB-231 and SKBR-3 cells indicating caspase dependent apoptosis in these cells and caspase independent apoptosis in MCF-7 cells.

Conclusions

Modulation of HSP 90 and HSP 70 expressions is a possible mechanism by which the decoction of F. leucopyrus mediates cytotoxic effects MCF-7 and MDA-MB-231 cells. This effect appears to correlate with enhanced apoptosis in these cells. In SKBR-3 cells, mechanisms other than HSP inhibition may be utilized to a greater extent by the decoction to mediate the observed cytotoxic effects. Overall findings suggest that the decoction has the potential to be exploited further for effective treatment of breast cancer.

Natural Product Inspired N-Terminal Hsp90 Inhibitors: From Bench to Bedside?

The 90 kDa heat shock proteins (Hsp90) are responsible for the conformational maturation of nascent polypeptides and the rematuration of denatured proteins. Proteins dependent upon Hsp90 are associated with all six hallmarks of cancer. Upon Hsp90 inhibition, protein substrates are degraded via the ubiquitin-proteasome pathway. Consequentially, inhibition of Hsp90 offers a therapeutic opportunity for the treatment of cancer. Natural product inhibitors of Hsp90 have been identified in vitro, which have served as leads for the development of more efficacious inhibitors and analogs that have entered clinical trials. This review highlights the development of natural product analogs, as well as the development of clinically important inhibitors that arose from natural products.

Hsp90 inhibitor as a sensitizer of cancer cells to different therapies (review)

Hsp90 is a molecular chaperone that maintains the structural and functional integrity of various client proteins involved in signaling and many other functions of cancer cells. The natural inhibitors, ansamycins influence the Hsp90 chaperone function by preventing its binding to client proteins and resulting in their proteasomal degradation. N- and C-terminal inhibitors of Hsp90 and their analogues are widely tested as potential anticancer agents in vitro, in vivo as well as in clinical trials. It seems that Hsp90 competitive inhibitors target different tumor types at nanomolar concentrations and might have therapeutic benefit. On the contrary, some Hsp90 inhibitors increased toxicity and resistance of cancer cells induced by heat shock response, and through the interaction of survival signals, that occured as side effects of treatments, could be very effectively limited via combination of therapies. The aim of our review was to collect the data from experimental and clinical trials where Hsp90 inhibitor was combined with other therapies in order to prevent resistance as well as to potentiate the cytotoxic and/or antiproliferative effects.

Identifying a C-terminal ATP binding sites-based novel Hsp90-Inhibitor in silico: a plausible therapeutic approach in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a progressive brain disorder, which gradually and irreversibly destroys the intellectual and cognitive abilities of the brain. Heat shock protein 90 (Hsp90α) is a molecular chaperone which was found to regulate the function of number of client proteins including tau that is involved in the cause of the AD. Inhibition of Hsp90α by C-Terminal domain (CTD) ATP binding-site blockage might be used as an effective treatment strategy against the disease via degradation of tau proteins that are involved in the progression of the disease. Till date, a variety of drugs have been identified as Hsp90α inhibitors, which include Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone. However, which drug among the four binds to the CTD ATP binding site strongly and what are the specific residue responsible for such binding, have not been reported so far.

HYPOTHESIS

We hypothesize that binding site for ATP of Hsp90α CTD contains multiple ATP binding sites. We also hypothesize that a drug which can bind to the ATP binding site of CTD strongly can inhibit Hsp90α function which is in turn redirects towards the proteasomal degradation of diseased client protein like tau in AD. Such inhibition will find a novel therapeutic approach in the treatment of AD.

EXPERIMENTAL DESIGN

The identification of ATP binding site of Hsp90α CTD was done using various software tools like Hex 6.3, CastP, protein Hydrophobicity plots, ATPint and LigPlot+ v.1.4.5. Docking experiments were conducted between Hsp90αCTD and its inhibitors at these ATP binding site using the Autodock 4.0. The docking energies were further compared to obtain the most effective Hsp90α inhibitor of CTD.

RESULTS

From our experiments, Leucine (Leu) 665, Leu 666 and Leu 694 were predicted to be located in CTD ATP binding site. Furthermore, docking studies were performed of various Hsp90α inhibitors like Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone with the previously recognized ATP binding residues of CTD i.e. Leu 665, Leu 666 and Leu 694. The docking results of Derrubone showed the highest binding energy at all the three sites of ATP interaction. Additionally, Derrubone showed the best binding energy at Leu 666 (-7.53kcal/mol) compared to Leu 665 (-7.20kcal/mol) and Leu 694 (-6.67kcal/mol).

CONCLUSION

Based on our findings, we propose that the recognized sites i.e. Leu665, Leu 666 and Leu694 could possibly be the binding sites of Hsp90α CTD for ATP and the Hsp90 inhibitors. It was predicted that Derrubone could bind with CTD of Hsp90α strongly and resulted tau protein degradation which might be considered to be a therapeutic approach in AD.

Microarray-based screening of heat shock protein inhibitors

Based on the importance of heat shock proteins (HSPs) in diseases such as cancer, Alzheimer's disease or malaria, inhibitors of these chaperons are needed. Today's state-of-the-art techniques to identify HSP inhibitors are performed in microplate format, requiring large amounts of proteins and potential inhibitors. In contrast, we have developed a miniaturized protein microarray-based assay to identify novel inhibitors, allowing analysis with 300 pmol of protein. The assay is based on competitive binding of fluorescence-labeled ATP and potential inhibitors to the ATP-binding site of HSP. Therefore, the developed microarray enables the parallel analysis of different ATP-binding proteins on a single microarray. We have demonstrated the possibility of multiplexing by immobilizing full-length human HSP90α and HtpG of Helicobacter pylori on microarrays. Fluorescence-labeled ATP was competed by novel geldanamycin/reblastatin derivatives with IC50 values in the range of 0.5 nM to 4 μM and Z(*)-factors between 0.60 and 0.96. Our results demonstrate the potential of a target-oriented multiplexed protein microarray to identify novel inhibitors for different members of the HSP90 family.

Targeting heat-shock-protein 90 (Hsp90) by natural products: geldanamycin, a show case in cancer therapy

Covering 2005 to 2013. In this review recent progress in the development of heat shock proteins (Hsp90) in oncogenesis is illuminated. Particular emphasis is put on inhibitors such as geldanamycin and analogues that serve as a natural product show case. Hsp90 has emerged as an important target in cancer therapy and/or against pathogenic cells which elicit abnormal Hsp patterns. Competition for ATP by geldanamycin and related compounds abrogate the chaperone function of Hsp90. In this context, this account pursues three topics in detail: a) Hsp90 and its biochemistry, b) Hsp90 and its role in oncogenesis and c) strategies to create compound libraries of structurally complex inhibitors like geldanamycin on which SAR studies and the development of drugs that are currently in different stages of clinical testing rely.

Potent antitrypanosomal activities of heat shock protein 90 inhibitors in vitro and in vivo

African sleeping sickness, caused by the protozoan parasite Trypanosoma brucei, is universally fatal if untreated, and current drugs are limited by severe toxicities and difficult administration. New antitrypanosomals are greatly needed. Heat shock protein 90 (Hsp90) is a conserved and ubiquitously expressed molecular chaperone essential for stress responses and cellular signaling. We investigated Hsp90 inhibitors for their antitrypanosomal activity. Geldanamycin and radicicol had nanomolar potency in vitro against bloodstream-form T. brucei; novobiocin had micromolar activity. In structure-activity studies of geldanamycin analogs, 17-AAG and 17-DMAG were most selective against T. brucei as compared to mammalian cells. 17-AAG treatment sensitized trypanosomes to heat shock and caused severe morphological abnormalities and cell cycle disruption. Both oral and parenteral 17-DMAG cured mice of a normally lethal infection of T. brucei. These promising results support the use of inhibitors to study Hsp90 function in trypanosomes and to expand current clinical development of Hsp90 inhibitors to include T. brucei.

Spotlight on the microbes that produce heat shock protein 90-targeting antibiotics

Heat shock protein 90 (Hsp90) is a promising cancer drug target as a molecular chaperone critical for stabilization and activation of several of the oncoproteins that drive cancer progression. Its actions depend upon its essential ATPase, an activity fortuitously inhibited with a very high degree of selectivity by natural antibiotics: notably the actinomycete-derived benzoquinone ansamycins (e.g. geldanamycin) and certain fungal-derived resorcyclic acid lactones (e.g. radicicol). The molecular interactions made by these antibiotics when bound within the ADP/ATP-binding site of Hsp90 have served as templates for the development of several synthetic Hsp90 inhibitor drugs. Much attention now focuses on the clinical trials of these drugs. However, because microbes have evolved antibiotics to target Hsp90, it is probable that they often exploit Hsp90 inhibition when interacting with each other and with plants. Fungi known to produce Hsp90 inhibitors include mycoparasitic, as well as plant-pathogenic, endophytic and mycorrhizal species. The Hsp90 chaperone may, therefore, be a prominent target in establishing a number of mycoparasitic (interfungal), fungal pathogen–plant and symbiotic fungus–plant relationships. Furthermore the Hsp90 family proteins of the microbes that produce Hsp90 inhibitor antibiotics are able to reveal how drug resistance can arise by amino acid changes in the highly conserved ADP/ATP-binding site of Hsp90.

Discovery of (2S)-1-[4-(2-{6-amino-8-[(6-bromo-1,3-benzodioxol-5-yl)sulfanyl]-9H-purin-9-yl}ethyl)piperidin-1-yl]-2-hydroxypropan-1-one (MPC-3100), a purine-based Hsp90 inhibitor

Modulation of Hsp90 (heat shock protein 90) function has been recognized as an attractive approach for cancer treatment, since many cancer cells depend on Hsp90 to maintain cellular homeostasis. This has spurred the search for small-molecule Hsp90 inhibitors. Here we describe our lead optimization studies centered on the purine-based Hsp90 inhibitor 28a containing a piperidine moiety at the purine N9 position. In this study, key SAR was established for the piperidine N-substituent and for the congeners of the 1,3-benzodioxole at C8. These efforts led to the identification of orally bioavailable 28g that exhibits good in vitro profiles and a characteristic molecular biomarker signature of Hsp90 inhibition both in vitro and in vivo. Favorable pharmacokinetic properties along with significant antitumor effects in multiple human cancer xenograft models led to the selection of 28g (MPC-3100) as a clinical candidate.

Probing molecular mechanisms of the Hsp90 chaperone: biophysical modeling identifies key regulators of functional dynamics

Deciphering functional mechanisms of the Hsp90 chaperone machinery is an important objective in cancer biology aiming to facilitate discovery of targeted anti-cancer therapies. Despite significant advances in understanding structure and function of molecular chaperones, organizing molecular principles that control the relationship between conformational diversity and functional mechanisms of the Hsp90 activity lack a sufficient quantitative characterization. We combined molecular dynamics simulations, principal component analysis, the energy landscape model and structure-functional analysis of Hsp90 regulatory interactions to systematically investigate functional dynamics of the molecular chaperone. This approach has identified a network of conserved regions common to the Hsp90 chaperones that could play a universal role in coordinating functional dynamics, principal collective motions and allosteric signaling of Hsp90. We have found that these functional motifs may be utilized by the molecular chaperone machinery to act collectively as central regulators of Hsp90 dynamics and activity, including the inter-domain communications, control of ATP hydrolysis, and protein client binding. These findings have provided support to a long-standing assertion that allosteric regulation and catalysis may have emerged via common evolutionary routes. The interaction networks regulating functional motions of Hsp90 may be determined by the inherent structural architecture of the molecular chaperone. At the same time, the thermodynamics-based "conformational selection" of functional states is likely to be activated based on the nature of the binding partner. This mechanistic model of Hsp90 dynamics and function is consistent with the notion that allosteric networks orchestrating cooperative protein motions can be formed by evolutionary conserved and sparsely connected residue clusters. Hence, allosteric signaling through a small network of distantly connected residue clusters may be a rather general functional requirement encoded across molecular chaperones. The obtained insights may be useful in guiding discovery of allosteric Hsp90 inhibitors targeting protein interfaces with co-chaperones and protein binding clients.

Loss of HDAC6, a novel CHIP substrate, alleviates abnormal tau accumulation

The abnormal accumulation of the microtubule-binding protein tau is associated with a number of neurodegenerative conditions, and correlates with cognitive decline in Alzheimer's disease. The ubiquitin ligase carboxy terminus of Hsp70-interacting protein (CHIP) and the molecular chaperone Hsp90 are implicated in protein triage decisions involving tau, and have consequently been targeted for therapeutic approaches aimed at decreasing tau burden. Here, we present evidence that CHIP binds, ubiquitinates and regulates expression of histone deacetylase 6 (HDAC6). As the deacetylase for Hsp90, HDAC6 modulates Hsp90 function and determines the favorability of refolding versus degradation of Hsp90 client proteins. Moreover, we demonstrate that HDAC6 levels positively correlate with tau burden, while a decrease in HDAC6 activity or expression promotes tau clearance. Consistent with previous research on Hsp90 clients in cancer, we provide evidence that a loss of HDAC6 activity augments the efficacy of an Hsp90 inhibitor and drives client degradation, in this case tau. Therefore, our current findings not only identify HDAC6 as a critical factor for the regulation of tau levels, but also indicate that a multi-faceted treatment approach could more effectively arrest tau accumulation in disease.

Heat shock response and insulin-associated neurodegeneration

Dysfunctional insulin and insulin-like growth factor-I (IGF-I) signaling contributes to the pathological progression of diabetes, diabetic peripheral neuropathy (DPN), Alzheimer's (AD), Parkinson's (PD) and Huntington's diseases (HD). Despite their prevalence, there are limited therapeutic options available for the treatment of these neurodegenerative disorders. Therefore, establishing a link between insulin/IGF-I and the pathoetiology of these diseases may provide alternative approaches toward their management. Many of the heat shock proteins (Hsps) are well-known molecular chaperones that solubilize and clear damaged proteins and protein aggregates. Recent studies suggest that modulating Hsps may represent a promising therapeutic avenue for improving insulin and IGF-I signaling. Pharmacological induction of the heat shock response (HSR) may intersect with insulin/IGF-I signaling to improve aspects of neurodegenerative phenotypes. Herein, we review the intersection between Hsps and the insulin/IGF systems under normal and pathological conditions. The discussion will emphasize the potential of non-toxic HSR inducers as viable therapeutic agents.

Engineering an antibiotic to fight cancer: optimization of the novobiocin scaffold to produce anti-proliferative agents

Development of the DNA gyrase inhibitor, novobiocin, into a selective Hsp90 inhibitor was accomplished through structural modifications to the amide side chain, coumarin ring, and sugar moiety. These species exhibit ∼700-fold improved anti-proliferative activity versus the natural product as evaluated by cellular efficacies against breast, colon, prostate, lung, and other cancer cell lines. Utilization of structure-activity relationships established for three novobiocin synthons produced optimized scaffolds, which manifest midnanomolar activity against a panel of cancer cell lines and serve as lead compounds that manifest their activities through Hsp90 inhibition.

Hsp90 regulates tau pathology through co-chaperone complexes in Alzheimer's disease

Alzheimer's disease is a tauopathy which involves the deposition of microtubule-associated tau proteins into neurofibrillary tangles. Post-translational modifications, in particular site-specific phosphorylations, affect the conformation of tau protein which is an intrinsically disordered protein. These structural changes significantly increase the affinity of tau protein for certain molecular chaperones. Hsp90 is a major cellular chaperone which assembles large complexes with a variety of co-chaperones. The main function of Hsp90 complexes is to maintain protein quality control and assist in protein degradation via proteasomal and autophagic-lysosomal pathways. Tau protein is a client protein for these Hsp90 complexes. If the tau protein is in an abnormal or modified form, then it can trigger the recruitment of CHIP protein, a co-chaperone with E3 activity, to the complex which induces the ubiquitination of tau protein and activates its downstream degradation processes. Large immunophilins, FKBP51 and FKBP52 are also co-chaperones of Hsp90-tau complexes. These proteins contain peptidylprolyl cis/trans isomerase activity which catalyzes phosphorylation-dependent rotation in pSer/Thr-Pro peptide bond. The proline switch in the tau conformation triggers dephosphorylation of Ser/Thr residues phosphorylated, e.g. by two well-known tau kinases Cdk5 and GSK-3β. Binding of PP5 protein phosphatase to Hsp90 complex, can also dephosphorylate tau protein. Subsequently, dephosphorylated tau protein can be shuttled back to the microtubules. It seems that high-affinity binding of abnormal tau to Hsp90 complexes may have some counteracting effects on the aggregation process, since Hsp90 inhibitors can ameliorate the aggregation process in several neurodegenerative diseases. We will review the role of Hsp90 chaperone network in the regulation of tau biology and pathology in Alzheimer's disease.

A systematic protocol for the characterization of Hsp90 modulators

Several Hsp90 modulators have been identified including the N-terminal ligand geldanamycin (GDA), the C-terminal ligand novobiocin (NB), and the co-chaperone disruptor celastrol. Other Hsp90 modulators elicit a mechanism of action that remains unknown. For example, the natural product gedunin and the synthetic anti-spermatogenic agent H2-gamendazole, recently identified Hsp90 modulators, manifest biological activity through undefined mechanisms. Herein, we report a series of biochemical techniques used to classify such modulators into identifiable categories. Such studies provided evidence that gedunin and H2-gamendazole both modulate Hsp90 via a mechanism similar to celastrol, and unlike NB or GDA.

HSP90 inhibitor, celastrol, arrests human monocytic leukemia cell U937 at G0/G1 in thiol-containing agents reversible way

Background

Because some of heat shock protein 90's (HSP90) clients are key cell cycle regulators, HSP90 inhibition can affect the cell cycle. Recently, celastrol is identified both as a novel inhibitor of HSP90 and as a potential anti-tumor agent. However, this agent's effects on the cell cycle are rarely investigated. In this study, we observed the effects of celastrol on the human monocytic leukemia cell line U937 cell cycle.

Results

Celastrol affected the proliferation of U937 in a dose-dependent way, arresting the cell cycle at G0/G1 with 400 nM doses and triggering cell death with doses above 1000 nM. Cell cycle arrest was accompanied by inhibition of HSP90 ATPase activity and elevation in HSP70 levels (a biochemical hallmark of HSP90 inhibition), a reduction in Cyclin D1, Cdk4 and Cdk6 levels, and a disruption of the HSP90/Cdc37/Cdk4 complex. The observed effects of celastrol on the U937 cell cycle were thiol-related, firstly because the effects could be countered by pre-loading thiol-containing agents and secondly because celastrol and thiol-containing agents could react with each other to form new compounds.

Conclusions

Our results disclose a novel action of celastrol-- causing cell cycle arrest at G0/G1 phase based upon thiol-related HSP90 inhibition. Our work suggests celastrol's potential in tumor and monocyte-related disease management.

Alternate strategies of Hsp90 modulation for the treatment of cancer and other diseases

The 90 kDa heat shock protein (Hsp90) has become a validated target for the development of anti-cancer agents. Several Hsp90 inhibitors are currently under clinical trial investigation for the treatment of cancer. All of these agents inhibit Hsp90's protein folding activity by binding to the N-terminal ATP binding site of the Hsp90 molecular chaperone. Administration of these investigational drugs elicits induction of the heat shock response, or the overexpression of several Hsps, which exhibit antiapoptotic and pro-survival effects that may complicate the application of these inhibitors. To circumvent this issue, alternate mechanisms for Hsp90 inhibition that do not elicit the heat shock response have been identified and pursued. After providing background on the structure, function, and mechanism of the Hsp90 protein folding machinery, this review describes several mechanisms of Hsp90 modulation via small molecules that do not induce the heat shock response.

Modulation of heat shock transcription factor 1 as a therapeutic target for small molecule intervention in neurodegenerative disease

A yeast-based small molecule screen identifies a novel activator of human HSF1 and protein chaperone expression and which appears to alleviate the toxicity of protein misfolding diseases.

Neurodegenerative diseases such as Huntington disease are devastating disorders with no therapeutic approaches to ameliorate the underlying protein misfolding defect inherent to poly-glutamine (polyQ) proteins. Given the mounting evidence that elevated levels of protein chaperones suppress polyQ protein misfolding, the master regulator of protein chaperone gene transcription, HSF1, is an attractive target for small molecule intervention. We describe a humanized yeast-based high-throughput screen to identify small molecule activators of human HSF1. This screen is insensitive to previously characterized activators of the heat shock response that have undesirable proteotoxic activity or that inhibit Hsp90, the central chaperone for cellular signaling and proliferation. A molecule identified in this screen, HSF1A, is structurally distinct from other characterized small molecule human HSF1 activators, activates HSF1 in mammalian and fly cells, elevates protein chaperone expression, ameliorates protein misfolding and cell death in polyQ-expressing neuronal precursor cells and protects against cytotoxicity in a fly model of polyQ-mediated neurodegeneration. In addition, we show that HSF1A interacts with components of the TRiC/CCT complex, suggesting a potentially novel regulatory role for this complex in modulating HSF1 activity. These studies describe a novel approach for the identification of new classes of pharmacological interventions for protein misfolding that underlies devastating neurodegenerative disease.

The misfolding of proteins into a toxic state contributes to a variety of neurodegenerative diseases such as Huntington, Alzheimer, and Parkinson disease. Although no known cure exists for these afflictions, many studies have shown that increasing the levels of protein chaperones, proteins that assist in the correct folding of other proteins, can suppress the neurotoxicity of the misfolded proteins. As such, increasing the cellular concentration of protein chaperones might serve as a powerful therapeutic approach in treating protein misfolding diseases. Because the levels of protein chaperones in the cell are primarily controlled by the heat shock transcription factor 1 [HSF1], we have designed and implemented a pharmacological screen to identify small molecules that can promote human HSF1 activation and increase the expression of protein chaperones. Through these studies, we have identified HSF1A, a molecule capable of activating human HSF1, increasing the levels of protein chaperones and alleviating the toxicity of misfolded proteins in both cell culture as well as fruit fly models of neurodegenerative disease.

Physiological regulation of Akt activity and stability

The serine/threonine protein kinase B (PKB, also known as Akt) constitutes an important node in diverse signaling cascades downstream of growth factor receptor tyrosine kinases. Akt plays an essential role in cell survival, growth, migration, proliferation, polarity, and metabolism (lipid and glucose); cell cycle progression; muscle and cardiomyocyte contractility; angiogenesis; and self-renewal of stem cells. Altered Akt activity has been associated with cancer and other disease conditions, such as diabetes mellitus, neurodegenerative diseases, and muscle hypotrophy. In the past decade, the upstream signals that lead to Akt activation, the downstream substrates that exert the effects of Akt, and the secondary binding proteins that regulate Akt activation have been well documented. Recent reports from our group and others have revealed how the stability of Akt protein is regulated through phosphorylation on its Thr-Pro motifs. This literature review details findings of those reports and others relevant to the regulation of Akt activation by its upstream kinases, with a focus on mammalian target of rapamycin complexes (mTORCs) and inactivation by PHLDA3 and the protein phosphatases PP2A and pleckstrin homology domain leucine-rich repeat protein phosphatase (PHLPP). Reports on ubiquitin-dependent Akt degradation, caspase-dependent cleavage, and the roles of molecular chaperone heat shock protein 90 (Hsp90) in the regulation of Akt stability are summarized. The highlight will be on the role of "turn motif" phosphorylation and an isomerase, Pin1, in the regulation of Akt stability. We also discuss issues related to the intricate mTORC2-AktmTORC1 loop and the contradictory regulation of Akt phosphorylation and stabilization of Akt by mTORC2. Finally, we offer perspective on potential future directions for investigation, particularly on translating the knowledge we learned on the regulation of Akt stability into therapeutic intervention on human cancer with Akt alteration.