The network interaction of the human cytosolic 90 kDa heat shock protein Hsp90: A target for cancer therapeutics

Heat shock protein 90: biological functions, diseases, and therapeutic targets

Heat shock protein 90 (Hsp90) is a predominant member among Heat shock proteins (HSPs), playing a central role in cellular protection and maintenance by aiding in the folding, stabilization, and modification of diverse protein substrates. It collaborates with various co-chaperones to manage ATPase-driven conformational changes in its dimer during client protein processing. Hsp90 is critical in cellular function, supporting the proper operation of numerous proteins, many of which are linked to diseases such as cancer, Alzheimer's, neurodegenerative conditions, and infectious diseases. Recognizing the significance of these client proteins across diverse diseases, there is a growing interest in targeting Hsp90 and its co-chaperones for potential therapeutic strategies. This review described biological background of HSPs and the structural characteristics of HSP90. Additionally, it discusses the regulatory role of heat shock factor-1 (HSF-1) in modulating HSP90 and sheds light on the dynamic chaperone cycle of HSP90. Furthermore, the review discusses the specific contributions of HSP90 in various disease contexts, especially in cancer. It also summarizes HSP90 inhibitors for cancer treatment, offering a thoughtful analysis of their strengths and limitations. These advancements in research expand our understanding of HSP90 and open up new avenues for considering HSP90 as a promising target for therapeutic intervention in a range of diseases.

Unveiling the Role of Sorghum RPAP3 in the Function of R2TP Complex: Insights into Protein Assembly in Plants

The chaperone R2TP has multiple subunits that assist in the proper folding, assembly, and stabilization of various protein complexes in cells and its study can offer valuable insights into the regulation and maintenance of protein assemblies in plant systems. The 'T' component of R2TP is Tah1 in yeast, consisting of 111 residues, while its counterpart in humans is RPAP3, with 665 residues. RPAP3 acts as a co-chaperone of Hsp90 and facilitates interactions between RUVBL proteins and other complex components, enhancing the recruitment of client proteins by the R2TP complex. These facts further underscore the relevance of studying this complex in different organisms. The putative gene corresponding to the RPAP3 in Sorghum bicolor, a monocotyledon plant, was cloned, and the protein (396 residues) purified for biochemical characterization. SbRPAP3 exists as a folded monomer and has a RPAP3 domain, which is present in human RPAP3 but absent in yeast Tah1. SbRPAP3 retains its functional capabilities, including binding with RUVBLs, Hsp90, and Hsp70. By elucidating the role of RPAP3 in plant R2TP complex, we can further comprehend the molecular mechanisms underlying plant-specific protein assembly and contribute to advancements in plant biology and biotechnological applications.

Binding Mechanism of Inhibitors to Heat Shock Protein 90 Investigated by Multiple Independent Molecular Dynamics Simulations and Prediction of Binding Free Energy

The heat shock protein (HSP90) has been an import target of drug design in the treatment of human disease. An exploration of the conformational changes in HSP90 can provide useful information for the development of efficient inhibitors targeting HSP90. In this work, multiple independent all-atom molecular dynamics (AAMD) simulations followed by calculations of the molecular mechanics generalized Born surface area (MM-GBSA) were performed to explore the binding mechanism of three inhibitors (W8Y, W8V, and W8S) to HSP90. The dynamics analyses verified that the presence of inhibitors impacts the structural flexibility, correlated movements, and dynamics behavior of HSP90. The results of the MM-GBSA calculations suggest that the selection of GB models and empirical parameters has important influences on the predicted results and verify that van der Waals interactions are the main forces that determine inhibitor-HSP90 binding. The contributions of separate residues to the inhibitor-HSP90 binding process indicate that hydrogen-bonding interactions (HBIs) and hydrophobic interactions play important roles in HSP90-inhibitor identifications. Moreover, residues L34, N37, D40, A41, D79, I82, G83, M84, F124, and T171 are recognized as hot spots of inhibitor-HSP90 binding and provide significant target sites of for the design of drugs related to HSP90. This study aims to contribute to the development of efficient inhibitors that target HSP90 by providing an energy-based and theoretical foundation.

Selective Inhibition of the Hsp90α Isoform

The 90 kDa heat shock protein (Hsp90) is a molecular chaperone that processes nascent polypeptides into their biologically active conformations. Many of these proteins contribute to the progression of cancer, and consequently, inhibition of the Hsp90 protein folding machinery represents an innovative approach toward cancer chemotherapy. However, clinical trials with Hsp90 N-terminal inhibitors have encountered deleterious side effects and toxicities, which appear to result from the pan-inhibition of all four Hsp90 isoforms. Therefore, the development of isoform-selective Hsp90 inhibitors is sought to delineate the pathological role played by each isoform. Herein, we describe a structure-based approach that was used to design the first Hsp90α-selective inhibitors, which exhibit >50-fold selectivity versus other Hsp90 isoforms.

Purification and characterization of a novel and conserved TPR-domain protein that binds both Hsp90 and Hsp70 and is expressed in all developmental stages of Leishmania major

Heat shock proteins (Hsps) are involved in several important aspects of the cell proteostasis. Hsp90 interacts with at least a tenth of the cell proteome helping a large number of proteins to fold correctly. Hsp90 function is modulated by several co-chaperones having TPR (tetratricopeptide repeat) domains that allow for interaction with the C-terminal MEEVD motif of the chaperone. Another important chaperone, Hsp70, has a C-terminal EEVD motif that binds to TPR. Leishmania is a protozoan that causes leishmaniasis, a neglected disease in humans and other animals. There is still no effective treatment for leishmaniasis, however the study of structure and function of the proteins of the parasite may generate potential targets for future therapeutic intervention studies. In this work, the genome of Leishmania major was searched for a novel TPR-domain gene, which is conserved in Leishmania. The recombinant protein, LmTPR, was produced in pure and folded state and was characterized by biophysical tools as a monomer with an elongated conformation. Studies in Leishmania major were also preformed to complement these in vitro experiments. Splice Leader RNA-seq analysis and Western blot indicated that the protein was expressed in all developmental stages of the parasite. Binding assays confirmed that both Hsp90 and Hsp70 bind specifically to LmTPR. Finally, sequence and structural predictions indicated a C-terminal region as a RPAP3 domain. Altogether, this study identified a novel TPR-domain co-chaperone of Hsp90 that is conserved and expressed in all developmental stages of Leishmania major.

Hsp90 chaperone facilitates E2F1/2-dependent gene transcription in human breast cancer cells

The 90 kDa heat shock protein, Hsp90, is involved in the conformational stabilization and functional maturation of diverse cancer-promoting proteins. To date, more than 300 Hsp90 clients have identified, suggesting that Hsp90 plays a central role in deciding cancer cell fate. In this study, we present the nuclear functions of Hsp90 in regulating the E2F-dependent gene transcription. We show that the conformation specific Hsp90 inhibitor, 17AAG decreases the total cellular E2F levels more selectively in cancer cells than transformed cells. With the help of coimmunoprecipitation experiments, we show that Hsp90 interacts with E2F1 and E2F2 in cancer cells, whereas in transformed cells, only E2F1 interacts with Hsp90. Retention of E2F2 in the nucleus of cancer cells upon MG132 combination with 17AAG has suggested that Hsp90 is required for E2F2 stability and function. The HDAC6 inhibitor tubacin treatment did not interfere with E2F1/2 stability and nuclear accumulation. However, the HDAC3 inhibitor, RGFP966 treatment, decreased nuclear E2F1/2 and its target gene expression. The nuclear accumulation of E2F1 and E2F2 upon cell cycle inhibition correlated with decreased acetylated Hsp90. We expose the nuclear functions of Hsp90 in facilitating the cell cycle progression through stabilizing E2F1/2.

Are Hsp90 inhibitors good candidates against Covid-19?

Drug reposition, or repurposing, has become a promising strategy in therapeutics due to its advantages in several aspects of drug therapy. General drug development is expensive and can take more than 10 years to go through the designing, development, and necessary approval steps. However, established drugs have already overcome these steps and thus a potential candidate may be already available decreasing the risks and costs involved. Viruses invade cells, usually provoking biochemical changes, leading to tissue damage, alteration of normal physiological condition in organisms and can even result in death. Inside the cell, the virus finds the machinery necessary for its multiplication, as for instance the protein quality control system, which involves chaperones and Hsps (heat shock proteins) that, in addition to physiological functions, help in the stabilization of viral proteins. Recently, many inhibitors of Hsp90 have been developed as therapeutic strategies against diseases such as the Hsp90 inhibitors used in anticancer therapy. Several shreds of evidence indicate that these inhibitors can also be used as therapeutic strategies against viruses. Therefore, since a drug treatment for COVID-19 is urgently needed, this review aims to discuss the potential use of Hsp90 inhibitors in the treatment of this globally threatening disease.

Heat shock protein 90 kDa (Hsp90) from Aedes aegypti has an open conformation and is expressed under heat stress

Cellular proteostasis is maintained by a system consisting of molecular chaperones, heat shock proteins (Hsps) and proteins involved with degradation. Among the proteins that play important roles in the function of this system is Hsp90, which acts as a node of this network, interacting with at least 10% of the proteome. Hsp90 is ATP-dependent, participates in critical cell events and protein maturation and interacts with large numbers of co-chaperones. The study of Hsp90 orthologs is justified by their differences in ATPase activity levels and conformational changes caused by Hsp90 interaction with nucleotides. This study reports the characterization of Hsp90 from Aedes aegypti, a vector of several diseases in many regions of the planet. Aedes aegypti Hsp90, AaHsp90, was cloned, purified and characterized for its ATPase and chaperone activities and structural conformation. These parameters indicate that it has the characteristics of eukaryotic Hsp90s and resembles orthologs from yeast rather than from human. Finally, constitutive and increased stress expression in Aedes cells was confirmed. Taken together, the results presented here help to understand the relationship between structure and function in the Hsp90 family and have strong potential to form the basis for studies on the network of chaperone and Hsps in Aedes.

Regulation of FKBP51 and FKBP52 functions by post-translational modifications

FKBP51 and FKBP52 are two iconic members of the family of peptidyl-prolyl-(cis/trans)-isomerases (EC: 5.2.1.8), which comprises proteins that catalyze the cis/trans isomerization of peptidyl-prolyl peptide bonds in unfolded and partially folded polypeptide chains and native state proteins. Originally, both proteins have been studied as molecular chaperones belonging to the steroid receptor heterocomplex, where they were first discovered. In addition to their expected role in receptor folding and chaperoning, FKBP51 and FKBP52 are also involved in many biological processes, such as signal transduction, transcriptional regulation, protein transport, cancer development, and cell differentiation, just to mention a few examples. Recent studies have revealed that both proteins are subject of post-translational modifications such as phosphorylation, SUMOlyation, and acetylation. In this work, we summarize recent advances in the study of these immunophilins portraying them as scaffolding proteins capable to organize protein heterocomplexes, describing some of their antagonistic properties in the physiology of the cell, and the putative regulation of their properties by those post-translational modifications.

Construction of piperidine-2,4-dione-type azaheterocycles and their application in modern drug development and natural product synthesis

The review surveys the existing routes to piperidine-2,4-dione-type heterocycles including derivatives with the most vital types of biological activity. This heterocyclic platform is ideal for the construction of modern drugs and natural products.

Heat Shock Proteins Are Essential Components in Transformation and Tumor Progression: Cancer Cell Intrinsic Pathways and Beyond

Heat shock protein (HSP) synthesis is switched on in a remarkably wide range of tumor cells, in both experimental animal systems and in human cancer, in which these proteins accumulate in high levels. In each case, elevated HSP concentrations bode ill for the patient, and are associated with a poor outlook in terms of survival in most cancer types. The significance of elevated HSPs is underpinned by their essential roles in mediating tumor cell intrinsic traits such as unscheduled cell division, escape from programmed cell death and senescence, de novo angiogenesis, and increased invasion and metastasis. An increased HSP expression thus seems essential for tumorigenesis. Perhaps of equal significance is the pronounced interplay between cancer cells and the tumor milieu, with essential roles for intracellular HSPs in the properties of the stromal cells, and their roles in programming malignant cells and in the release of HSPs from cancer cells to influence the behavior of the adjacent tumor and infiltrating the normal cells. These findings of a triple role for elevated HSP expression in tumorigenesis strongly support the targeting of HSPs in cancer, especially given the role of such stress proteins in resistance to conventional therapies.

The succinct synthesis of AT13387, a clinically relevant Hsp90 inhibitor

AT13387 is an orally bioavailable clinical candidate developed to inhibit theheat shock protein 90 (Hsp90). This article describes a modified synthetic route for the multi-gram production of AT13387 in 46% overall yield. The modified synthetic route is short, avoids stringent reaction conditions and difficult purifications, which led to increase in an overall yield.

Protein-Protein Interactions in the Molecular Chaperone Network

Molecular chaperones play a central role in protein homeostasis (a.k.a. proteostasis) by balancing protein folding, quality control, and turnover. To perform these diverse tasks, chaperones need the malleability to bind nearly any "client" protein and the fidelity to detect when it is misfolded. Remarkably, these activities are carried out by only ∼180 dedicated chaperones in humans. How do a relatively small number of chaperones maintain cellular and organismal proteostasis for an entire proteome? Furthermore, once a chaperone binds a client, how does it "decide" what to do with it? One clue comes from observations that individual chaperones engage in protein-protein interactions (PPIs)-both with each other and with their clients. These physical links coordinate multiple chaperones into organized, functional complexes and facilitate the "handoff" of clients between them. PPIs also link chaperones and their clients to other cellular pathways, such as those that mediate trafficking (e.g., cytoskeleton) and degradation (e.g., proteasome). The PPIs of the chaperone network have a wide range of affinity values (nanomolar to micromolar) and involve many distinct types of domain modules, such as J domains, zinc fingers, and tetratricopeptide repeats. Many of these motifs have the same binding surfaces on shared partners, such that members of one chaperone class often compete for the same interactions. Somehow, this collection of PPIs draws together chaperone families and creates multiprotein subnetworks that are able to make the "decisions" of protein quality control. The key to understanding chaperone-mediated proteostasis might be to understand how PPIs are regulated. This Account will discuss the efforts of our group and others to map, measure, and chemically perturb the PPIs within the molecular chaperone network. Structural biology methods, including X-ray crystallography, NMR spectroscopy, and electron microscopy, have all played important roles in visualizing the chaperone PPIs. Guided by these efforts and -omics approaches to measure PPIs, new advances in high-throughput chemical screening that are specially designed to account for the challenges of this system have emerged. Indeed, chemical biology has played a particularly important role in this effort, as molecules that either promote or inhibit specific PPIs have proven to be invaluable research probes in cells and animals. In addition, these molecules have provided leads for the potential treatment of protein misfolding diseases. One of the major products of this research field has been the identification of putative PPI drug targets within the chaperone network, which might be used to change chaperone "decisions" and rebalance proteostasis.

Heat Shock Proteins Revisited: Using a Mutasynthetically Generated Reblastatin Library to Compare the Inhibition of Human and Leishmania Hsp90s

Thirteen new reblastatin derivatives, with alkynyl, amino and fluoro substituents on the aromatic ring, were prepared by a chemo-biosynthetic approach using an AHBA(-) mutant strain of Streptomyces hygroscopicus, the geldanamycin producer. The inhibitory potencies of these mutaproducts and of an extended library of natural products and derivatives were probed with purified heat shock proteins (Hsps), obtained from Leishmania braziliensis (LbHsp90) as well as from human sources (HsHsp90). We determined the activities of potential inhibitors by means of a displacement assay in which fluorescence-labelled ATP competes for the ATP binding sites of Hsps in the presence of the inhibitor in question. The results were compared with those of cell-based assays and, in selected cases, of isothermal titration calorimetry (ITC) measurements. In essence, reblastatin derivatives are also able to bind effectively to the ATP-binding site of LbHsp90, and for selected derivatives, moderate differences in binding to LbHsp90 and HsHsp90 were encountered. This work demonstrates that parasitic heat shock proteins can be developed as potential pharmaceutical targets.

Revealing the interaction mode of the highly flexible Sorghum bicolor Hsp70/Hsp90 organizing protein (Hop): A conserved carboxylate clamp confers high affinity binding to Hsp90

Proteostasis is dependent on the Hsp70/Hsp90 system (the two chaperones and their co-chaperones). Of these, Hop (Hsp70/Hsp90 organizing protein), also known as Sti1, forms an important scaffold to simultaneously binding to both Hsp70 and Hsp90. Hop/Sti1 has been implicated in several disease states, for instance cancer and transmissible spongiform encephalopathies. Therefore, human and yeast homologous have been better studied and information on plant homologous is still limited, even though plants are continuously exposed to environmental stress. Particularly important is the study of crops that are relevant for agriculture, such as Sorghum bicolor, a C4 grass that is among the five most important cereals and is considered as a bioenergy feedstock. To increase the knowledge on plant chaperones, the hop putative gene for Sorghum bicolor was cloned and the biophysical and structural characterization of the protein was done by cross-linking coupled to mass spectroscopy, small angle X-ray scattering and structural modeling. Additionally, the binding to a peptide EEVD motif, which is present in both Hsp70 and Hsp90, was studied by isothermal titration calorimetry and hydrogen/deuterium exchange and the interaction pattern structurally modeled. The results indicate SbHop as a highly flexible, mainly alpha-helical monomer consisting of nine tetratricopeptide repeat domains, of which one confers high affinity binding to Hsp90 through a conserved carboxylate clamp. Moreover, the present insights into the conserved interactions formed between Hop and Hsp90 can help to design strategies for potential therapeutic approaches for the diseases in which Hop has been implicated.

Dynamics revelation of conformational changes and binding modes of heat shock protein 90 induced by inhibitor associations

Heat shock protein 90 (Hsp90) has been an attractive target of potential drug design for antitumor treatment. The current work integrates molecular dynamics (MD) simulations, calculations of binding free energy, and principal component (PC) analysis with scanning of inhibitor–residue interaction to probe the binding modes of inhibitors YK9, YKJ and YKI to Hsp90 and identify the hot spot of the inhibitor–Hsp90 binding. The results suggest that the introductions of two groups G1 and G2 into YKJ and YKI strengthen the binding ability of YKJ and YKI to Hsp90 compared to YK9. PC analysis based MD trajectories prove that inhibitor bindings exert significant effects on the conformational changes, internal dynamics and motion modes of Hsp90, especially for the helix α2 and the loops L1 and L2. The calculations of residue-based free energy decomposition and scanning of the inhibitor–Hsp90 interaction suggest that six residues L107, G108, F138, Y139, W162 and F170 construct the common hot spot of the inhibitor–residue interactions. Moreover the substitutions of the groups G1 and G2 in YKJ and YKI lead to two additional hydrogen bonding interactions and multiple hydrophobic interactions for bindings of YKJ and YKI to Hsp90. This work is also expected to contribute theoretical hints for the design of potent inhibitors toward Hsp90.

Heat shock protein 90 (Hsp90) has been an attractive target of potential drug design for antitumor treatment.

Proteomic interrogation of HSP90 and insights for medical research

INTRODUCTION

Heat shock protein 90 (HSP90) regulates protein homeostasis in eukaryotes. As a 'professional interactor', HSP90 binds to and chaperones many proteins and has both housekeeping and disease-related functions but its regulation remains in part elusive. HSP90 complexes are a target for therapy, notably against cancer, and several inhibitors are currently in clinical trials. Proteomic studies have revealed the vast interaction network of HSP90 and, in doing so, the extent of cellular processes the chaperone takes part in, especially in yeast and human cells. Furthermore, small-molecule inhibitors were used to probe the global impact of its inhibition on the proteome. Areas covered: We review here recent HSP90-related interactomics and total proteome studies and their relevance for research on cancer, neurodegenerative and pathogen diseases. Expert commentary: Proteomics experiments are our best chance to identify the context-dependent global proteome of HSP90 and thus uncover and understand its disease-specific biology. However, understanding the complexity of HSP90 will require multiple complementary, quantitative approaches and novel bioinformatics to translate interactions into ordered functional networks and pathways. Developing therapies will necessitate more knowledge on HSP90 complexes and networks with disease relevance and on total proteome changes induced by their perturbation. Most work has been done in cancer, thus a lot remains to be done in the context of other diseases.

Heat Shock Protein 70 (Hsp70) Suppresses RIP1-Dependent Apoptotic and Necroptotic Cascades

Hsp70 is a molecular chaperone that binds to "client" proteins and protects them from protein degradation. Hsp70 is essential for the survival of many cancer cells, but it is not yet clear which of its clients are involved. Using structurally distinct chemical inhibitors, we found that many of the well-known clients of the related chaperone, Hsp90, are not strikingly responsive to Hsp70 inhibition. Rather, Hsp70 appeared to be important for the stability of the RIP1 (RIPK1) regulators: cIAP1/2 (BIRC1 and BIRC3), XIAP, and cFLIP_S/L (CFLAR). These results suggest that Hsp70 limits apoptosis and necroptosis pathways downstream of RIP1. Consistent with this model, MDA-MB-231 breast cancer cells treated with Hsp70 inhibitors underwent apoptosis, while cotreatment with z-VAD.fmk switched the cell death pathway to necroptosis. In addition, cell death in response to Hsp70 inhibitors was strongly suppressed by RIP1 knockdown or inhibitors. Thus, these data indicate that Hsp70 plays a previously unrecognized and important role in suppressing RIP1 activity.Implications: These findings clarify the role of Hsp70 in prosurvival signaling and suggest IAPs as potential new biomarkers for Hsp70 inhibition. Mol Cancer Res; 16(1); 58-68. ©2017 AACR.

Effects of 17-DMAG on diffuse large B-cell lymphoma cell apoptosis

17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) is a water soluble, semisynthetic derivative of endotoxin that has anticancer effects. The aim of the present study was to determine whether 17-DMAG enhances the apoptosis of lymphoma cells in diffuse large B-cell lymphoma. Apoptosis was induced in SU-DHL-4 diffuse large B-cell lymphoma cells treated with 17-DMAG, as evaluated by MTT assay and flow cytometry analysis. Apoptosis-associated protein levels were assessed using western blotting, and the results indicated that B-cell lymphoma 2 (Bcl-2)-associated protein X (Bax) was upregulated, whereas heat shock protein family A member 5 (HSPA5) and Bcl-2 were downregulated. Additionally, staining with 5,5',6,6'-Tetrachloro-1,1',3,3'-tetraethyl-imidacarbocyanine iodide revealed that treatment with 17-DMAG decreased mitochondrial membrane potential in SU-DHL-4 diffuse large B-cell lymphoma cells. These results suggested that 17-DMAG is able to inhibit proliferation in diffuse large B-cell lymphoma cells in a concentration-dependent manner. The underlying mechanism may be that 17-DMAG induces oxidative stress, which inhibits the expression of HSPA5 and Bcl-2 and promotes the expression of Bax, leading to the apoptosis of SU-DHL-4 cells. Taken together, these results indicated that 17-DMAG may be an effective novel agent for the treatment of diffuse large B-cell lymphoma.

[Hsp90AB1 Protein is Overexpressed in Non-small Cell Lung Cancer Tissues and Associated with Poor Prognosis in Lung Adenocarcinoma Patients]

BACKGROUND

Heat shock protein 90 kDa alpha, class B member 1 (Hsp90AB1) is highly conserved ATP-dependent molecular chaperone, and over-expressed in a variety of tumor cells. Some molecules that play important roles in tumor development signaling pathways such as epidermal growth factor receptor (EGFR), human epidermal growth factor receptor-2 (HER2) are Hsp90AB1 client proteins. Hsp90AB1 interact with these client proteins and participate in a variety of pathophysiological processes of cells. The aim of this study is to detect the expression of Hsp90AB1 in non-small cell lung cancer (NSCLC) tissues, and explore its clinical significance.

METHODS

The expression of Hsp90AB1 in 213 NSCLC tissues and 147 normal lung tissues was detected by tissue microarray and immunohistochemical staining method, and the relationship of Hsp90AB1 expression with clinicopathological parameters and prognosis of NSCLC patients were analyzed.

RESULTS

The expression level of Hsp90AB1 in lung cancer tissues (positive rate of 54.0%) was significantly higher than that in normal lung tissue (positive rate of 0.0%, P<0.001). The positive expression rate of Hsp90AB1 in lung adenocarcinoma tissues (61.2%) was significantly higher than that in lung squamous cell carcinoma tissues (37.9%)(P=0.002), and its over-expression was associated with poor prognosis in lung adenocarcinoma patients (P=0.032). The expression level of Hsp90AB1 had no significant correlation with clinical stage, lymph node metastasis, pathological grade or other factors (P>0.05).

CONCLUSIONS

Hsp90AB1 protein was over-expressed in NSCLC tissues, and was associated with lung cancer pathological type and overall survival in lung adenocarcinoma patients.

Heat Shock Protein 90 kDa (Hsp90) Has a Second Functional Interaction Site with the Mitochondrial Import Receptor Tom70

To accomplish its crucial role, mitochondria require proteins that are produced in the cytosol, delivered by cytosolic Hsp90, and translocated to its interior by the translocase outer membrane (TOM) complex. Hsp90 is a dimeric molecular chaperone and its function is modulated by its interaction with a large variety of co-chaperones expressed within the cell. An important family of co-chaperones is characterized by the presence of one TPR (tetratricopeptide repeat) domain, which binds to the C-terminal MEEVD motif of Hsp90. These include Tom70, an important component of the TOM complex. Despite a wealth of studies conducted on the relevance of Tom70·Hsp90 complex formation, there is a dearth of information regarding the exact molecular mode of interaction. To help fill this void, we have employed a combined experimental strategy consisting of cross-linking/mass spectrometry to investigate binding of the C-terminal Hsp90 domain to the cytosolic domain of Tom70. This approach has identified a novel region of contact between C-Hsp90 and Tom70, a finding that is confirmed by probing the corresponding peptides derived from cross-linking experiments via isothermal titration calorimetry and mitochondrial import assays. The data generated in this study are combined to input constraints for a molecular model of the Hsp90/Tom70 interaction, which has been validated by small angle x-ray scattering, hydrogen/deuterium exchange, and mass spectrometry. The resultant model suggests that only one of the MEEVD motifs within dimeric Hsp90 contacts Tom70. Collectively, our findings provide significant insight on the mechanisms by which preproteins interact with Hsp90 and are translocated via Tom70 to the mitochondria.

Identification of the key structural elements of a dihydropyrimidinone core driving toward more potent Hsp90 C-terminal inhibitors

Dramatic improvement in the biological activity of DHPM derivatives as a new class of Hsp90 C-terminal inhibitors for cancer therapy.

Adipogenesis is under surveillance of Hsp90 and the high molecular weight Immunophilin FKBP51

Adipose tissue plays a central role in the control of energy balance as well as in the maintenance of metabolic homeostasis. It was not until recently that the first evidences of the role of heat shock protein (Hsp) 90 and high molecular weight immunophilin FKBP51 have been described in the process of adipocyte differentiation. Recent reports describe their role in the regulation of PPARγ, a key transcription factor in the control of adipogenesis and the maintenance of the adipocyte phenotype. In addition, novel roles have been uncovered for FKBP51 in the organization of the architecture of the nucleus through its participation in the reorganization of the nuclear lamina. Therefore, the aim of this review is to integrate and discuss the recent advances in the field, with special emphasis on the roles of Hsp90 and FKBP51 in the process of adipocyte differentiation.

Disaggregases, molecular chaperones that resolubilize protein aggregates

The process of folding is a seminal event in the life of a protein, as it is essential for proper protein function and therefore cell physiology. Inappropriate folding, or misfolding, can not only lead to loss of function, but also to the formation of protein aggregates, an insoluble association of polypeptides that harm cell physiology, either by themselves or in the process of formation. Several biological processes have evolved to prevent and eliminate the existence of non-functional and amyloidogenic aggregates, as they are associated with several human pathologies. Molecular chaperones and heat shock proteins are specialized in controlling the quality of the proteins in the cell, specifically by aiding proper folding, and dissolution and clearance of already formed protein aggregates. The latter is a function of disaggregases, mainly represented by the ClpB/Hsp104 subfamily of molecular chaperones, that are ubiquitous in all organisms but, surprisingly, have no orthologs in the cytosol of metazoan cells. This review aims to describe the characteristics of disaggregases and to discuss the function of yeast Hsp104, a disaggregase that is also involved in prion propagation and inheritance.

Heat Shock Protein 90 Associates with the Per-Arnt-Sim Domain of Heme-free Soluble Guanylate Cyclase: IMplications for Enzyme Maturation

Heat shock protein 90 (hsp90) drives heme insertion into the β1 subunit of soluble guanylate cyclase (sGC) β1, which enables it to associate with a partner sGCα1 subunit and mature into a nitric oxide (NO)-responsive active form. We utilized fluorescence polarization measurements and hydrogen-deuterium exchange mass spectrometry to define molecular interactions between the specific human isoforms hsp90β and apo-sGCβ1. hsp90β and its isolated M domain, but not its isolated N and C domains, bind with low micromolar affinity to a heme-free, truncated version of sGCβ1 (sGCβ1(1-359)-H105F). Surprisingly, hsp90β and its M domain bound to the Per-Arnt-Sim (PAS) domain of apo-sGC-β1(1-359), which lies adjacent to its heme-binding (H-NOX) domain. The interaction specifically involved solvent-exposed regions in the hsp90β M domain that are largely distinct from sites utilized by other hsp90 clients. The interaction strongly protected two regions of the sGCβ1 PAS domain and caused local structural relaxation in other regions, including a PAS dimerization interface and a segment in the H-NOX domain. Our results suggest a means by which the hsp90β interaction could prevent apo-sGCβ1 from associating with its partner sGCα1 subunit while enabling structural changes to assist heme insertion into the H-NOX domain. This mechanism would parallel that in other clients like the aryl hydrocarbon receptor and HIF1α, which also interact with hsp90 through their PAS domains to control protein partner and small ligand binding interactions.

Alternative approaches to Hsp90 modulation for the treatment of cancer

Hsp90 is responsible for the conformational maturation of newly synthesized polypeptides (client proteins) and the re-maturation of denatured proteins via the Hsp90 chaperone cycle. Inhibition of the Hsp90 N-terminus has emerged as a clinically relevant strategy for anticancer chemotherapeutics due to the involvement of clients in a variety of oncogenic pathways. Several immunophilins, co-chaperones and partner proteins are also necessary for Hsp90 chaperoning activity. Alternative strategies to inhibit Hsp90 function include disruption of the C-terminal dimerization domain and the Hsp90 heteroprotein complex. C-terminal inhibitors and Hsp90 co-chaperone disruptors prevent cancer cell proliferation similar to N-terminal inhibitors and destabilize client proteins without induction of heat shock proteins. Herein, current Hsp90 inhibitors, the chaperone cycle, and regulation of this cycle will be discussed.

Cucurbitacin D Is a Disruptor of the HSP90 Chaperone Machinery

Heat shock protein 90 (Hsp90) facilitates the maturation of many newly synthesized and unfolded proteins (clients) via the Hsp90 chaperone cycle, in which Hsp90 forms a heteroprotein complex and relies upon cochaperones, immunophilins, etc., for assistance in client folding. Hsp90 inhibition has emerged as a strategy for anticancer therapies due to the involvement of clients in many oncogenic pathways. Inhibition of chaperone function results in client ubiquitinylation and degradation via the proteasome, ultimately leading to tumor digression. Small molecule inhibitors perturb ATPase activity at the N-terminus and include derivatives of the natural product geldanamycin. However, N-terminal inhibition also leads to induction of the pro-survival heat shock response (HSR), in which displacement of the Hsp90-bound transcription factor, heat shock factor-1, translocates to the nucleus and induces transcription of heat shock proteins, including Hsp90. An alternative strategy for Hsp90 inhibition is disruption of the Hsp90 heteroprotein complex. Disruption of the Hsp90 heteroprotein complex is an effective strategy to prevent client maturation without induction of the HSR. Cucurbitacin D, isolated from Cucurbita texana, and 3-epi-isocucurbitacin D prevented client maturation without induction of the HSR. Cucurbitacin D also disrupted interactions between Hsp90 and two cochaperones, Cdc37 and p23.

The HSP90 inhibitor ganetespib: A potential effective agent for Acute Myeloid Leukemia in combination with cytarabine

HSP90 is a multi-client chaperone involved in regulating a large array of cellular processes and is commonly overexpressed in many different cancer types including hematological malignancies. Inhibition of HSP90 holds promise for targeting multiple molecular abnormalities and is therefore an attractive target for heterogeneous malignancies such as Acute Myeloid Leukemia (AML). Ganetespib is a highly potent second generation HSP90 inhibitor which we show is significantly more effective against primary AML blasts at nanomolar concentrations when compared with cytarabine (p<0.001). Dose dependant cytotoxicity was observed with an apoptotic response coordinate with the loss of pro-survival signaling through the client protein AKT. Combination treatment of primary blasts with ganetespib and cytarabine showed good synergistic interaction (combination index (CI): 0.47) across a range of drug effects with associated reduction in HSP70 feedback and AKT signaling levels. In summary, we show ganetespib to have high activity in primary AMLs as a monotherapy and a synergistic relationship with cytarabine when combined. The combination of cytotoxic cell death, suppression of cytoprotective/drug resistance mechanisms such as AKT and reduced clinical toxicity compared to other HSP90 inhibitors provide strong rationale for the clinical assessment of ganetespib in AML.

Glucose-regulated protein 78 mediates the therapeutic efficacy of 17-DMAG in colon cancer cells

Glucose-regulated protein 78 (GRP78) is expressed as part of the molecular response to endoplasmic reticulum (ER) stress and mediates protein folding within the cell. GRP78 is also an important biomarker of cancer progression and the therapeutic response of patients with different cancer types. However, the role of GRP78 in the cytotoxic effect of 17-DMAG in colon cancer cells remains unclear. GRP78 expression was knocked down by small interfering RNA (siRNA). The anticancer effects of 17-DMAG were assessed by an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, a flow cytometric cell-cycle analysis, and an Annexin V-propidium iodide (PI) apoptotic assay. We found that HT-29 cells expressed a lower level of GRP78 compared with DLD-1 cells. The MTT assay revealed that HT-29 cells were more sensitive to 17-DMAG treatment than DLD-1 cells. GRP78 knock down (GRP78KD) cells demonstrated an increased sensitivity to 17-DMAG treatment compared with the scrambled control cells. Based on the cell-cycle analysis and Annexin V-PI apoptotic assay, apoptosis dramatically increased in GRP78KD cells compared with scrambled control DLD-1 cells after these cells were treated with 17-DMAG. Finally, we observed a decrease in the level of Bcl-2 and an increase in the levels of Bad and Bax in GRP78KD cells treated with 17-DMAG. These results are consistent with an increased sensitivity to 17-DMAG after knock down of GRP78. The level of GRP78 expression may determine the therapeutic efficacy of 17-DMAG against colon cancer cells.

Stability of the human Hsp90-p50Cdc37 chaperone complex against nucleotides and Hsp90 inhibitors, and the influence of phosphorylation by casein kinase 2

The molecular chaperone Hsp90 is regulated by co-chaperones such as p50^Cdc37, which recruits a wide selection of client protein kinases. Targeted disruption of the Hsp90-p50^Cdc37 complex by protein–protein interaction (PPI) inhibitors has emerged as an alternative strategy to treat diseases characterized by aberrant Hsp90 activity. Using isothermal microcalorimetry, ELISA and GST-pull down assays we evaluated reported Hsp90 inhibitors and nucleotides for their ability to inhibit formation of the human Hsp90β-p50^Cdc37 complex, reconstituted in vitro from full-length proteins. Hsp90 inhibitors, including the proposed PPI inhibitors gedunin and H2-gamendazole, did not affect the interaction of Hsp90 with p50^Cdc37in vitro. Phosphorylation of Hsp90 and p50^Cdc37 by casein kinase 2 (CK2) did not alter the thermodynamic signature of complex formation. However, the phosphorylated complex was vulnerable to disruption by ADP (IC₅₀ = 32 µM), while ATP, AMPPNP and Hsp90 inhibitors remained largely ineffective. The differential inhibitory activity of ADP suggests that phosphorylation by CK2 primes the complex for dissociation in response to a drop in ATP/ADP levels. The approach applied herein provides robust assays for a comprehensive biochemical evaluation of potential effectors of the Hsp90-p50^Cdc37 complex, such as phosphorylation by a kinase or the interaction with small molecule ligands.

Cosuppression of the chloroplast localized molecular chaperone HSP90.5 impairs plant development and chloroplast biogenesis in Arabidopsis

BACKGROUND

HSP90.5 is a chloroplast localized HSP90 family molecular chaperone in Arabidopsis, and it has been implicated in plant abiotic stress resistance, photomorphogenesis and nuclear-encoded protein import into the chloroplast. However, how these processes are controlled by HSP90 is not well understood. To understand the role of HSP90.5 in chloroplast function and biogenesis, in this study, we generated transgenic Arabidopsis plants that overexpress a C-terminally FLAG-tagged HSP90.5. By characterizing three HSP90.5 cosuppression lines, we demonstrated the essential role of HSP90.5 in plant growth and chloroplast biogenesis.

RESULTS

Immunoblotting and quantitative PCR analyses revealed three independent HSP90.5 cosuppressing transgenic lines. All three cosuppression lines displayed a certain degree of variegated phenotype in photosynthetic tissues, and the cosuppression did not affect the expression of cytosolic HSP90 isoforms. HSP90.5 cosuppression was shown to be developmentally regulated and occurred mostly at late developmental stage in adult leaves and inflorescence tissues. HSP90.5 cosuppression also caused significantly reduced rosette leaf growth, transient starch storage, but did not affect rosette leaf initiation or inflorescence production, although the fertility was reduced. Isolation of chloroplasts and size exclusion chromatography analysis indicated that the FLAG at the HSP90.5 C-terminus does not affect its proper chloroplast localization and dimerization. Finally, transmission electron microscopy indicated that chloroplast development in HSP90.5 cosuppression leaves was significantly impaired and the integrity of chloroplast is highly correlated to the expression level of HSP90.5.

CONCLUSION

We thoroughly characterized three HSP90.5 cosuppression lines, and demonstrated that properly controlled expression of HSP90.5 is required for plant growth and development in many tissues, and especially essential for chloroplast thylakoid formation. Since the homozygote of HSP90.5 knockout mutant is embryonically lethal, this study provides transgenic lines that mimic the conditional knockout line or siRNA line of the essential HSP90.5 gene in Arabidopsis.

The effect of celastrol, a triterpene with antitumorigenic activity, on conformational and functional aspects of the human 90kDa heat shock protein Hsp90α, a chaperone implicated in the stabilization of the tumor phenotype

BACKGROUND

Hsp90 is a molecular chaperone essential for cell viability in eukaryotes that is associated with the maturation of proteins involved in important cell functions and implicated in the stabilization of the tumor phenotype of various cancers, making this chaperone a notably interesting therapeutic target. Celastrol is a plant-derived pentacyclic triterpenoid compound with potent antioxidant, anti-inflammatory and anticancer activities; however, celastrol's action mode is still elusive.

RESULTS

In this work, we investigated the effect of celastrol on the conformational and functional aspects of Hsp90α. Interestingly, celastrol appeared to target Hsp90α directly as the compound induced the oligomerization of the chaperone via the C-terminal domain as demonstrated by experiments using a deletion mutant. The nature of the oligomers was investigated by biophysical tools demonstrating that a two-fold excess of celastrol induced the formation of a decameric Hsp90α bound throughout the C-terminal domain. When bound, celastrol destabilized the C-terminal domain. Surprisingly, standard chaperone functional investigations demonstrated that neither the in vitro chaperone activity of protecting against aggregation nor the ability to bind a TPR co-chaperone, which binds to the C-terminus of Hsp90α, were affected by celastrol.

CONCLUSION

Celastrol interferes with specific biological functions of Hsp90α. Our results suggest a model in which celastrol binds directly to the C-terminal domain of Hsp90α causing oligomerization. However, the ability to protect against protein aggregation (supported by our results) and to bind to TPR co-chaperones are not affected by celastrol. Therefore celastrol may act primarily by inducing specific oligomerization that affects some, but not all, of the functions of Hsp90α.

GENERAL SIGNIFICANCE

To the best of our knowledge, this study is the first work to use multiple probes to investigate the effect that celastrol has on the stability and oligomerization of Hsp90α and on the binding of this chaperone to Tom70. This work provides a novel mechanism by which celastrol binds Hsp90α.

Regulation of lead toxicity by heat shock protein 90 (daf-21) is affected by temperature in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, stress resistance can be regulated by dauer formation (daf) genes. In the present study, regulation of heavy metal lead (Pb) toxicity by the 90-kDa heat shock proteins (Hsp90; daf-21) was investigated in both wild-type C. elegans and daf-21/Hsp90 mutants by focusing on the effects of varied temperatures below (15°C) or above (25 and 30°C) the presumptive optimum growth temperature (20°C). More acute toxicity of Pb, indicated by the 24-h median lethal concentrations (LC50), was observed in wild-type adults than in the daf-21 mutant adults at 15, 20 and 25°C; however, the daf-21 mutant adults showed more sensitivity at 30°C. Enhanced Pb sensitivity (e.g., decrease LC50) in both types of C. elegans was observed with both increased and decreased temperatures when compared to that at 20°C. Additional examined endpoints included time course of toxicity at LC50s, pharyngeal pumping, reproduction, life span, and Hsp90 expression. Collective results showed that temperatures both above and below 20°C exacerbated Pb toxicity, and that the protein level of daf-21/Hsp90 was one of the most sensitive indicators of Pb toxicity in wild-type C. elegans, while pharyngeal pumping was more Pb sensitive in daf-21 mutants. Therefore, the expression of daf-21/Hsp90 has apparent utility for the prediction and assessment of Pb-induced toxicity in nematodes. Further, the stress responses related to Hsp90 expression in C. elegans may have considerable potential as sensitive biomarkers for the monitoring of environmental Pb contamination.

A-ring oxygenation modulates the chemistry and bioactivity of caged Garcinia xanthones

Natural products of the caged Garcinia xanthones (CGX) family are characterized by a unique chemical structure, potent bioactivities and promising pharmacological profiles. We have developed a Claisen/Diels-Alder reaction cascade that, in combination with a Pd(0)-catalyzed reverse prenylation, provides rapid and efficient access to the CGX pharmacophore, represented by the structure of cluvenone. To further explore this pharmacophore, we have synthesized various A-ring oxygenated analogues of cluvenone and have evaluated their bioactivities in terms of growth inhibition, mitochondrial fragmentation, induction of mitochondrial-dependent cell death and Hsp90 client inhibition. We found that installation of an oxygen functionality at various positions of the A-ring influences significantly both the site-selectivity of the Claisen/Diels-Alder reaction and the bioactivity of these compounds, due to remote electronic effects.

Cruentaren A binds F1F0 ATP synthase to modulate the Hsp90 protein folding machinery

The molecular chaperone Hsp90 requires the assistance of immunophilins, co-chaperones, and partner proteins for the conformational maturation of client proteins. Hsp90 inhibition represents a promising anticancer strategy due to the dependence of numerous oncogenic signaling pathways upon Hsp90 function. Historically, small molecules have been designed to inhibit ATPase activity at the Hsp90 N-terminus; however, these molecules also induce the pro-survival heat shock response (HSR). Therefore, inhibitors that exhibit alternative mechanisms of action that do not elicit the HSR are actively sought. Small molecules that disrupt Hsp90-co-chaperone interactions can destabilize the Hsp90 complex without induction of the HSR, which leads to inhibition of cell proliferation. In this article, selective inhibition of F₁F₀ ATP synthase by cruentaren A was shown to disrupt the Hsp90-F₁F₀ ATP synthase interaction and result in client protein degradation without induction of the HSR.

Identification of a new series of potent diphenol HSP90 inhibitors by fragment merging and structure-based optimization

Heat shock protein 90 (HSP90) is a molecular chaperone to fold and maintain the proper conformation of many signaling proteins, especially some oncogenic proteins and mutated unstable proteins. Inhibition of HSP90 was recognized as an effective approach to simultaneously suppress several aberrant signaling pathways, and therefore it was considered as a novel target for cancer therapy. Here, by integrating several techniques including the fragment-based drug discovery method, fragment merging, computer aided inhibitor optimization, and structure-based drug design, we were able to identify a series of HSP90 inhibitors. Among them, inhibitors 13, 32, 36 and 40 can inhibit HSP90 with IC50 about 20-40 nM, which is at least 200-fold more potent than initial fragments in the protein binding assay. These new HSP90 inhibitors not only explore interactions with an under-studied subpocket, also offer new chemotypes for the development of novel HSP90 inhibitors as anticancer drugs.

Role of activating transcription factor 3 protein ATF3 in necrosis and apoptosis induced by 5-fluoro-2'-deoxyuridine

Necrosis and apoptosis are the two major forms of cell death. We have studied the mechanisms that regulate the cell death observed during treatment of mouse cancer cell line FM3A with the anticancer drug 5-fluoro-2'-deoxyuridine (FUdR). To detect causal differences between necrosis and apoptosis, we exploited the necrosis in original clone F28-7 and the apoptosis in its variant F28-7-A that occur on treatment with FUdR. Activating transcription factor 3 (ATF3) was strongly induced during necrosis but not apoptosis. In addition, we found that ATF3 expression is regulated by heat shock protein 90 (HSP90) at the mRNA stage. Knockdown of Atf3 by siRNA in the F28-7 cells resulted in apoptotic morphology rather than necrotic morphology. These results suggest that ATF3 is a cell-death regulator in necrosis and apoptosis.

Geldanamycin-mediated inhibition of heat shock protein 90 partially activates dendritic cells, but interferes with their full maturation, accompanied by impaired upregulation of RelB

Background

The chaperon heat shock protein 90 (HSP90) constitutes an important target for anti-tumor therapy due to its essential role in the stabilization of oncogenes. However, HSP90 is ubiquitously active to orchestrate protein turnover, chemotherapeutics that target HSP90 may affect immune cells as a significant side effect. Therefore, we asked for potential effects of pharmacological HSP90 inhibition at a therapeutically relevant concentration on human dendritic cells (DCs) as main inducers of both cellular and humoral immune responses, and on human CD4⁺ T cells as directly activated by DCs and essential to confer B cell help.

Methods

Unstimulated human monocyte-derived DCs (MO-DCs) were treated with the prototypical HSP90 inhibitor geldanamycin (GA). Based on dose titration studies performed to assess cytotoxic effects, GA was applied at a rather low concentration, comparable to serum levels of clinically used HSP90 inhibitors. The immuno-phenotype (surface markers, cytokines), migratory capacity, allo T cell stimulatory and polarizing properties (proliferation, cytokine pattern) of GA-treated MO-DCs were assessed. Moreover, effects of GA on resting and differentially stimulated CD4⁺ T cells in terms of cytotoxicity and proliferation were analysed.

Results

GA induced partial activation of unstimulated MO-DCs. In contrast, when coapplied in the course of MO-DC stimulation, GA prevented the acquisition of a fully mature DC phenotype. Consequently, this MO-DC population exerted lower allo CD4⁺ T cell stimulation and cytokine production. Furthermore, GA exerted no cytotoxic effect on resting T cells, but abrogated proliferation of T cells stimulated by MO-DCs at either state of activation or by stimulatory antibodies.

Conclusion

HSP90 inhibitors at clinically relevant concentrations may modulate adaptive immune responses both on the level of DC activation and T cell proliferation. Surprisingly, unstimulated DCs may be partially activated by that agent. However, due to the potent detrimental effects of HSP90 inhibitors on stimulated CD4⁺ T cells, as an outcome a patients T cell responses might be impaired. Therefore, HSP90 inhibitors most probably are not suitable for treatment in combination with immunotherapeutic approaches aimed to induce DC/T cell activation.