Stoichiometry, abundance, and functional significance of the hsp90/hsp70-based multiprotein chaperone machinery in reticulocyte lysate

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.

The transportosome system as a model for the retrotransport of soluble proteins

The classic model of action of the glucocorticoid receptor (GR) sustains that its associated heat-shock protein of 90-kDa (HSP90) favours the cytoplasmic retention of the unliganded GR, whereas the binding of steroid triggers the dissociation of HSP90 allowing the passive nuclear accumulation of GR. In recent years, it was described a molecular machinery called transportosome that is responsible for the active retrograde transport of GR. The transportosome heterocomplex includes a dimer of HSP90, the stabilizer co-chaperone p23, and FKBP52 (FK506-binding protein of 52-kDa), an immunophilin that binds dynein/dynactin motor proteins. The model shows that upon steroid binding, FKBP52 is recruited to the GR allowing its active retrograde transport on cytoskeletal tracks. Then, the entire GR heterocomplex translocates through the nuclear pore complex. The HSP90-based heterocomplex is released in the nucleoplasm followed by receptor dimerization. Subsequent findings demonstrated that the transportosome is also responsible for the retrotransport of other soluble proteins. Importantly, the disruption of this molecular oligomer leads to several diseases. In this article, we discuss the relevance of this transport machinery in health and disease.

The chaperone system in cancer therapies: Hsp90

The chaperone system (CS) of an organism is composed of molecular chaperones, chaperone co-factors, co-chaperones, and chaperone receptors and interactors. It is present throughout the body but with distinctive features for each cell and tissue type. Previous studies pertaining to the CS of the salivary glands have determined the quantitative and distribution patterns for several members, the chaperones, in normal and diseased glands, focusing on tumors. Chaperones are cytoprotective, but can also be etiopathogenic agents causing diseases, the chaperonopathies. Some chaperones such as Hsp90 potentiate tumor growth, proliferation, and metastasization. Quantitative data available on this chaperone in salivary gland tissue with inflammation, and benign and malignant tumors suggest that assessing tissue Hsp90 levels and distribution patterns is useful for differential diagnosis-prognostication, and patient follow up. This, in turn, will reveal clues for developing specific treatment centered on the chaperone, for instance by inhibiting its pro-carcinogenic functions (negative chaperonotherapy). Here, we review data on the carcinogenic mechanisms of Hsp90 and their inhibitors. Hsp90 is the master regulator of the PI3K-Akt-NF-kB axis that promotes tumor cell proliferation and metastasization. We discuss pathways and interactions involving these molecular complexes in tumorigenesis and review Hsp90 inhibitors that have been tested in search of an efficacious anti-cancer agent. This targeted therapy deserves extensive investigation in view of its theoretical potential and some positive practical results and considering the need of novel treatments for tumors of the salivary glands as well as other tissues.

Recent advances toward the development of Hsp90 C-terminal inhibitors

Heat shock protein 90 (Hsp90) is a dynamic protein which serves to ensure proper folding of nascent client proteins, regulate transcriptional responses to environmental stress and guide misfolded and damaged proteins to destruction via ubiquitin proteasome pathway. Recent advances in the field of Hsp90 have been made through development of isoform selective inhibitors, Hsp90 C-terminal inhibitors and disruption of protein-protein interactions. These approaches have led to alleviation of adverse off-target effects caused by pan-inhibition of Hsp90 using N-terminal inhibitors. In this review, we provide an overview of relevant advances on targeting the Hsp90 C-terminal Domain (CTD) and the development of Hsp90 C-terminal inhibitors (CTIs) since 2015.

The disruption of protein-protein interactions with co-chaperones and client substrates as a strategy towards Hsp90 inhibition

The 90-kiloDalton (kD) heat shock protein (Hsp90) is a ubiquitous, ATP-dependent molecular chaperone whose primary function is to ensure the proper folding of several hundred client protein substrates. Because many of these clients are overexpressed or become mutated during cancer progression, Hsp90 inhibition has been pursued as a potential strategy for cancer as one can target multiple oncoproteins and signaling pathways simultaneously. The first discovered Hsp90 inhibitors, geldanamycin and radicicol, function by competitively binding to Hsp90's N-terminal binding site and inhibiting its ATPase activity. However, most of these N-terminal inhibitors exhibited detrimental activities during clinical evaluation due to induction of the pro-survival heat shock response as well as poor selectivity amongst the four isoforms. Consequently, alternative approaches to Hsp90 inhibition have been pursued and include C-terminal inhibition, isoform-selective inhibition, and the disruption of Hsp90 protein-protein interactions. Since the Hsp90 protein folding cycle requires the assembly of Hsp90 into a large heteroprotein complex, along with various co-chaperones and immunophilins, the development of small molecules that prevent assembly of the complex offers an alternative method of Hsp90 inhibition.

Differential regulation of the glucocorticoid receptor nucleocytoplasmic shuttling by TPR-domain proteins

A dimer of the heat-shock protein of 90-kDa (Hsp90) represents the critical core of the chaperone complex associated to the glucocorticoid receptor (GR) oligomer. The C-terminal end of the Hsp90 dimer shapes a functional acceptor site for co-chaperones carrying tetratricopeptide repeat (TPR) domains, where they bind in a mutually exclusive and competitive manner. They impact on the biological properties of the GR•Hsp90 complex and are major players of the GR transport machinery. Recently, we showed that the overexpression of a chimeric TPR peptide influences the subcellular distribution of GR. In this study, the functional role of endogenous proteins carrying TPR or TPR-like sequences on GR subcellular distribution was characterized. It is demonstrated that, contrarily to the positive influence of FKBP52 on GR nuclear accumulation, FKBP51 and 14-3-3 impaired this property. While SGT1α showed no significant effect, the overexpression of the Ser/Thr phosphatase PP5 resulted in a nearly equal nuclear-cytoplasmic redistribution of GR rather than its typical cytoplasmic localization in the absence of steroid. This observation led to analyse the influence of the phosphorylation status of GR, which resulted not linked to its nucleo-cytoplasmic shuttling mechanism. Nonetheless, it was evidenced that both PP5 and FKBP52 are related to the anchorage of the GR to nucleoskeleton structures. The influence of these TPR domain proteins on the steroid-dependent transcriptional activity of GR was also characterized. It is postulated that the pleiotropic actions of the GR in different cell types may be the consequence of the relative abundance of different TPR-domain interacting co-chaperones.

Structural Communication between the E. coli Chaperones DnaK and Hsp90

The 70 kDa and 90 kDa heat shock proteins Hsp70 and Hsp90 are two abundant and highly conserved ATP-dependent molecular chaperones that participate in the maintenance of cellular homeostasis. In Escherichia coli, Hsp90 (Hsp90Ec) and Hsp70 (DnaK) directly interact and collaborate in protein remodeling. Previous work has produced a model of the direct interaction of both chaperones. The locations of the residues involved have been confirmed and the model has been validated. In this study, we investigate the allosteric communication between Hsp90Ec and DnaK and how the chaperones couple their conformational cycles. Using elastic network models (ENM), normal mode analysis (NMA), and a structural perturbation method (SPM) of asymmetric and symmetric DnaK-Hsp90Ec, we extract biologically relevant vibrations and identify residues involved in allosteric signaling. When one DnaK is bound, the dominant normal modes favor biological motions that orient a substrate protein bound to DnaK within the substrate/client binding site of Hsp90Ec and release the substrate from the DnaK substrate binding domain. The presence of one DnaK molecule stabilizes the entire Hsp90Ec protomer to which it is bound. Conversely, the symmetric model of DnaK binding results in steric clashes of DnaK molecules and suggests that the Hsp90Ec and DnaK chaperone cycles operate independently. Together, this data supports an asymmetric binding of DnaK to Hsp90Ec.

SUMO conjugation as regulator of the glucocorticoid receptor-FKBP51 cellular response to stress

In order to adequately respond to stressful stimuli, glucocorticoids (GCs) target almost every tissue of the body. By exerting a negative feedback loop in the hypothalamic-pituitary-adrenal (HPA) axis GCs inhibit their own synthesis and restore homeostasis. GCs actions are mostly mediated by the GC receptor (GR), a member of the nuclear receptor superfamily. Alterations of the GR activity have been associatedto different diseases including mood disorders and can lead to severe complication. Therefore, understanding the molecular complexity of GR modulation is mandatory for the development of new and effective drugs for treating GR-associated disorders. FKBP51 is a GR chaperone that has gained much attention because it is a strong inhibitor of GR activity and has a crucial role in psychiatric diseases. Both GR and FKBP51 activity are regulated by SUMOylation, a posttranslational (PTM). In this review, we focus on the impact of SUMO-conjugation as a regulator of this pathway.

The HSP90 Family: Structure, Regulation, Function, and Implications in Health and Disease

The mammalian HSP90 family of proteins is a cluster of highly conserved molecules that are involved in myriad cellular processes. Their distribution in various cellular compartments underlines their essential roles in cellular homeostasis. HSP90 and its co-chaperones orchestrate crucial physiological processes such as cell survival, cell cycle control, hormone signaling, and apoptosis. Conversely, HSP90, and its secreted forms, contribute to the development and progress of serious pathologies, including cancer and neurodegenerative diseases. Therefore, targeting HSP90 is an attractive strategy for the treatment of neoplasms and other diseases. This manuscript will review the general structure, regulation and function of HSP90 family and their potential role in pathophysiology.

Interaction of E. coli Hsp90 with DnaK Involves the DnaJ Binding Region of DnaK

The 90-kDa heat shock protein (Hsp90) is a widely conserved and ubiquitous molecular chaperone that participates in ATP-dependent protein remodeling in both eukaryotes and prokaryotes. It functions in conjunction with Hsp70 and the Hsp70 cochaperones, an Hsp40 (J-protein) and a nucleotide exchange factor. In Escherichia coli, the functional collaboration between Hsp90_Ec and Hsp70, DnaK, requires that the two chaperones directly interact. We used molecular docking to model the interaction of Hsp90_Ec and DnaK. The top-ranked docked model predicted that a region in the nucleotide-binding domain (NBD) of DnaK interacted with a region in the middle domain of Hsp90_Ec. We then made substitution mutants in DnaK residues suggested by the model to interact with Hsp90_Ec. Of the 12 mutants tested, 11 were defective or partially defective in their ability to interact with Hsp90_Ecin vivo in a bacterial two-hybrid assay and in vitro in a bio-layer interferometry assay. These DnaK mutants were also defective in their ability to function collaboratively in protein remodeling with Hsp90_Ec but retained the ability to act with DnaK cochaperones. Taken together, these results suggest that a specific region in the NBD of DnaK is involved in the interaction with Hsp90_Ec, and this interaction is functionally important. Moreover, the region of DnaK that we found to be necessary for Hsp90_Ec binding includes residues that are also involved in J-protein binding, suggesting a functional interplay among DnaK, DnaK cochaperones, and Hsp90_Ec.

A Conserved Tripeptide Sequence at the C Terminus of the Poxvirus DNA Processivity Factor D4 Is Essential for Protein Integrity and Function

Vaccinia virus (VACV) is a poxvirus, and the VACV D4 protein serves both as a uracil-DNA glycosylase and as an essential component required for processive DNA synthesis. The VACV A20 protein has no known catalytic function itself but associates with D4 to form the D4-A20 heterodimer that functions as the poxvirus DNA processivity factor. The heterodimer enables the DNA polymerase to efficiently synthesize extended strands of DNA. Upon characterizing the interaction between D4 and A20, we observed that the C terminus of D4 is susceptible to perturbation. Further analysis demonstrated that a conserved hexapeptide stretch at the extreme C terminus of D4 is essential for maintaining protein integrity, as assessed by its requirement for the production of soluble recombinant protein that is functional in processive DNA synthesis. From the known crystal structures of D4, the C-terminal hexapeptide is shown to make intramolecular contact with residues spanning the inner core of the protein. Our mutational analysis revealed that a tripeptide motif (²¹⁵GFI²¹⁷) within the hexapeptide comprises apparent residues necessary for the contact. Prediction of protein disorder identified the hexapeptide and several regions upstream of Gly²¹⁵ that comprise residues of the interface surfaces of the D4-A20 heterodimer. Our study suggests that ²¹⁵GFI²¹⁷ anchors these potentially dynamic upstream regions of the protein to maintain protein integrity. Unlike uracil-DNA glycosylases from diverse sources, where the C termini are disordered and do not form comparable intramolecular contacts, this feature may be unique to orthopoxviruses.

Anticancer Inhibitors of Hsp90 Function: Beyond the Usual Suspects

The 90-kDa heat-shock protein (Hsp90) is a molecular chaperone responsible for the stability and function of a wide variety of client proteins that are critical for cell growth and survival. Many of these client proteins are frequently mutated and/or overexpressed in cancer cells and are therefore being actively pursued as individual therapeutic targets. Consequently, Hsp90 inhibition offers a promising strategy for simultaneous degradation of several anticancer targets. Currently, most Hsp90 inhibitors under clinical evaluation act by blocking the binding of ATP to the Hsp90 N-terminal domain and thereby, induce the degradation of many Hsp90-dependent oncoproteins. Although, they have shown some promising initial results, clinical challenges such as induction of the heat-shock response, retinopathy, and gastrointestinal tract toxicity are emerging from human trials, which constantly raise concerns about the future development of these inhibitors. Novobiocin derivatives, which do not bind the chaperone's N-terminal ATPase pocket, have emerged over the past decade as an alternative strategy to inhibit Hsp90, but to date, no derivative has been investigated in the clinical setting. In recent years, a number of natural or synthetic compounds have been identified that modulate Hsp90 function via various mechanisms. These compounds not only offer new chemotypes for the development of future Hsp90 inhibitors but can also serve as chemical probes to unravel the biology of Hsp90. This chapter presents a synopsis of inhibitors that directly, allosterically, or even indirectly alters Hsp90 function, and highlights their proposed mechanisms of action.

Hsp70 and Hsp90 of E. coli Directly Interact for Collaboration in Protein Remodeling

Hsp90 is a highly conserved molecular chaperone that remodels hundreds of client proteins, many involved in the progression of cancer and other diseases. It functions with the Hsp70 chaperone and numerous cochaperones. The bacterial Hsp90 functions with an Hsp70 chaperone, DnaK, but is independent of Hsp90 cochaperones. We explored the collaboration between Escherichia coli Hsp90 and DnaK and found that the two chaperones form a complex that is stabilized by client protein binding. A J-domain protein, CbpA, facilitates assembly of the Hsp90Ec-DnaK-client complex. We identified E. coli Hsp90 mutants defective in DnaK interaction in vivo and show that the purified mutant proteins are defective in physical and functional interaction with DnaK. Understanding how Hsp90 and Hsp70 collaborate in protein remodeling will provide the groundwork for the development of new therapeutic strategies targeting multiple chaperones and cochaperones.

Hypoxia induced altered expression of heat shock protein genes (Hsc71, Hsp90α and Hsp10) in Indian Catfish, Clarias batrachus (Linnaeus, 1758) under oxidative stress

Heat shock proteins (Hsps) are typically associated with stress response and tolerance. The Indian catfish, Clarias batrachus, is a freshwater air-breathing hypoxia tolerant teleost and is potentially important catfish species for aquaculture and for its economic value as food. The present study aimed at determining the transcriptional response of three Hsps, CbHsc71, CbHsp90α and CbHsp10 in hypoxia tolerant Indian catfish, C. batrachus, under experimental and natural hypoxia. The expression profile of above three genes were studied under different periods of hypoxia, through qRT-PCR. Primers were designed from ESTs obtained through SSH libraries constructed from hypoxia treated fishes. The Hsp10 ESTs and deduced protein was in silico characterized for its ORF and for its physical and chemical properties, respectively, using GeneScan, blastp, scanprosite, superfamily and other softwares. A phylogenetic tree was constructed based on deduced amino acid sequences of Hsc71, Hsp90α, Hsp90β of Homo sapiens and other fishes along with CbHsp10 protein in MEGA4. The deduced protein sequences of CbHsp10 was found to have characteristic Hsp10 family signatures, and it is proposed for inclusion of methionine in the consensus sequences of Hsp10 family signature, after the "proline" residue. At transcription level, these genes were found to be differentially regulated under hypoxia stress, in different tissues of C. batrachus. The CbHsc71 and CbHsp90α were up-regulated after short and long-term hypoxia, whereas CbHsp10 was significantly down-regulated after short-term hypoxia. The differential expression of these Hsps may play a role in protection and survival under hypoxia induced oxidative stress in C. batrachus.

Regulation of G protein signaling by the 70kDa heat shock protein

G protein-coupled receptors (GPCRs) transduce extracellular signals to the interior of the cell by activating membrane-bound guanine nucleotide-binding regulatory proteins (G proteins). An increasing number of proteins have been reported to bind to and regulate GPCRs. We report a novel regulation of the alpha(2A) adrenergic receptor (α(2A)-R) by the ubiquitous stress-inducible 70kDa heat shock protein, hsp70. Hsp70, but not hsp90, attenuated G protein-dependent high affinity agonist binding to the α(2A)-R in Sf9 membranes. Antagonist binding was unchanged, suggesting that hsp70 uncouples G proteins from the receptor. As hsp70 did not bind G proteins but complexed with the α(2A)-R in intact cells, a direct interaction with the receptor seems likely. In the presence of hsp70, α(2A)-R-catalyzed [(35)S]GTPγS binding was reduced by approximately 70%. In contrast, approximately 50-fold higher concentrations of hsp70 were required to reduce agonist binding to the stress-inducible 5-hydroxytryptamine(1A) receptor (5-HT(1A)-R). In heat-stressed CHO cells, the α(2A)-R was significantly uncoupled from G proteins, coincident with an increased localization of hsp70 at the membrane. The contrasting effect of hsp70 on the α(2A)-R compared to the 5-HT(1A)-R suggests that during stress, upregulation of hsp70 may attenuate signaling from specific GPCRs as part of the stress response to foster survival.

Biochemical reconstitution of steroid receptor•Hsp90 protein complexes and reactivation of ligand binding

Hsp90 is an essential and highly abundant molecular chaperone protein that has been found to regulate more than 150 eukaryotic signaling proteins, including transcription factors (e.g. nuclear receptors, p53) and protein kinases (e.g. Src, Raf, Akt kinase) involved in cell cycling, tumorigenesis, apoptosis, and multiple eukaryotic signaling pathways (1,2). Of these many 'client' proteins for hsp90, the assembly of steroid receptor•hsp90 complexes is the best defined (Figure 1). We present here an adaptable glucocorticoid receptor (GR) immunoprecipitation assay and in vitro GR•hsp90 reconstitution method that may be readily used to probe eukaryotic hsp90 functional activity, hsp90-mediated steroid receptor ligand binding, and molecular chaperone cofactor requirements. For example, this assay can be used to test hsp90 cofactor requirements and the effects of adding exogenous compounds to the reconstitution process. The GR has been a particularly useful system for studying hsp90 because the receptor must be bound to hsp90 to have an open ligand binding cleft that is accessible to steroid (3). Endogenous, unliganded GR is present in the cytoplasm of mammalian cells noncovalently bound to hsp90. As found in the endogenous GR•hsp90 heterocomplex, the GR ligand binding cleft is open and capable of binding steroid. If hsp90 dissociates from the GR or if its function is inhibited, the receptor is unable to bind steroid and requires reconstitution of the GR•hsp90 heterocomplex before steroid binding activity is restored (4) . GR can be immunoprecipitated from cell cytosol using a monoclonal antibody, and proteins such as hsp90 complexed to the GR can be assayed by western blot. Steroid binding activity of the immunoprecipitated GR can be determined by incubating the immunopellet with [(3)H]steroid. Previous experiments have shown hsp90-mediated opening of the GR ligand binding cleft requires hsp70, a second molecular chaperone also essential for eukaryotic cell viability. Biochemical activity of hsp90 and hsp70 are catalyzed by co-chaperone proteins Hop, hsp40, and p23 (5). A multiprotein chaperone machinery containing hsp90, hsp70, Hop, and hsp40 are endogenously present in eukaryotic cell cytoplasm, and reticulocyte lysate provides a chaperone-rich protein source (6). In the method presented, GR is immunoadsorbed from cell cytosol and stripped of the endogenous hsp90/hsp70 chaperone machinery using mild salt conditions. The salt-stripped GR is then incubated with reticulocyte lysate, ATP, and K(+), which results in the reconstitution of the GR•hsp90 heterocomplex and reactivation of steroid binding activity (7). This method can be utilized to test the effects of various chaperone cofactors, novel proteins, and experimental hsp90 or GR inhibitors in order to determine their functional significance on hsp90-mediated steroid binding (8-11).

Theiler's murine encephalomyelitis virus infection induces a redistribution of heat shock proteins 70 and 90 in BHK-21 cells, and is inhibited by novobiocin and geldanamycin

Theiler's murine encephalomyelitis virus (TMEV) is a positive-sense RNA virus belonging to the Cardiovirus genus in the family Picornaviridae. In addition to other host cellular factors and pathways, picornaviruses utilise heat shock proteins (Hsps) to facilitate their propagation in cells. This study investigated the localisation of Hsps 70 and 90 in TMEV-infected BHK-21 cells by indirect immunofluorescence and confocal microscopy. The effect of Hsp90 inhibitors novobiocin (Nov) and geldanamycin (GA) on the development of cytopathic effect (CPE) induced by infection was also examined. Hsp90 staining was uniformly distributed in the cytoplasm of uninfected cells but was found concentrated in the perinuclear region during late infection where it overlapped with the signal for non-structural protein 2C within the viral replication complex. Hsp70 redistributed into the vicinity of the viral replication complex during late infection, but its distribution did not overlap with that of 2C. Inhibition of Hsp90 by GA and Nov had a negative effect on virus growth over a 48-h period as indicated by no observable CPE in treated compared to untreated cells. 2C was detected by Western analysis of GA-treated infected cell lysates at doses between 0.01 and 0.125 μM, suggesting that processing of viral precursors was not affected in the presence of this drug. In contrast, 2C was absent in cell lysates of Nov-treated cells at doses above 10 μM, although CPE was evident 48 hpi. This is the first study describing the dynamic behaviour of Hsps 70 and 90 in TMEV-infected cells and to identify Hsp90 as an important host factor in the life cycle of this virus.

Heme-dependent activation of neuronal nitric oxide synthase by cytosol is due to an Hsp70-dependent, thioredoxin-mediated thiol-disulfide interchange in the heme/substrate binding cleft

We have reported that heme-dependent activation of apo-neuronal nitric oxide synthase (apo-nNOS) to the active holo-enzyme dimer is dependent upon factors present in reticulocyte lysate and other cytosols. Here, we find that both Hsp70 and thioredoxin are components of the activation system. The apo-nNOS activating activity of reticulocyte lysate is retained in a pool of fractions containing Hsp70 that elute from DE52 prior to Hsp90. All of the activating activity and 20-30% of the Hsp70 elute in the flow-through fraction upon subsequent ATP-agarose chromatography. Apo-nNOS activation by this flow-through fraction is inhibited by pifithrin-μ, a small molecule inhibitor of Hsp70, suggesting that a non-ATP-binding form of Hsp70 is involved in heme-dependent apo-nNOS activation. Previous work has shown that apo-nNOS can be activated by thiol-disulfide exchange, and we show substantial activation with a small molecule dithiol modeled on the active motifs of thioredoxin and protein disulfide isomerase. Further fractionation of the ATP-agarose flow-through on Sephacryl S-300 separates free thioredoxin from apo-nNOS activating activity, Hsp70, and a small amount of thioredoxin, all of which are eluted throughout the macromolecular peak. Incubation of apo-nNOS with the macromolecular fraction in combination either with the thioredoxin-containing fraction or with purified recombinant human thioredoxin restores full heme-dependent activating activity. This supports a model in which Hsp70 binding to apo-nNOS stabilizes an open state of the heme/substrate binding cleft to facilitate thioredoxin access to the active site cysteine that coordinates with heme iron, permitting heme binding and dimerization to the active enzyme.

Nucleic Acid programmable protein arrays: versatile tools for array-based functional protein studies

Protein microarrays offer a global perspective on the function of expressed gene products. However, technical issues related to the stability and dynamic range of microarrays printed with purified protein have hampered their widespread adoption. Taking an alternate approach, the Nucleic Acid Programmable Protein Array (NAPPA) is constructed by spotting protein-encoding plasmid DNA at high density, in addressable fashion, on an array surface. Proteins are subsequently generated in situ just prior to experimentation using cell-free expression systems. As such, the NAPPA platform offers a unique and viable alternative that circumvents many of the inherent limitations of spotted protein arrays, enabling diverse functional protein studies including protein-small molecule, protein-protein, antigen-antibody, and protein-nucleic acid interactions. It further offers a versatile and adaptable platform amenable to a variety of capture modalities and expression systems, and, most importantly, construction of the array is accessible to any lab with an array printer and laser slide scanner. This unit is intended to provide a reference for investigators wishing to generate arrays based on this platform, and details (1) the basic construction of cDNA-based protein microarrays from DNA isolation to printing and development, (2) quality-control efforts taken to ensure the usefulness and integrity of microarray data, and (3) a particular example of the application of self-assembling protein arrays to screen for blood-borne antibody biomarkers.

To fold or not to fold: modulation and consequences of Hsp90 inhibition

BACKGROUND

The 90-kDa heat-shock proteins (Hsp90) have rapidly evolved into promising therapeutic targets for the treatment of several diseases, including cancer and neurodegenerative diseases. Hsp90 is a molecular chaperone that aids in the conformational maturation of nascent polypeptides, as well as the rematuration of denatured proteins.

DISCUSSION

Many of the Hsp90-dependent client proteins are associated with cellular growth and survival and, consequently, inhibition of Hsp90 represents a promising approach for the treatment of cancer. Conversely, stimulation of heat-shock protein levels has potential therapeutic applications for the treatment of neurodegenerative diseases that result from misfolded and aggregated proteins.

CONCLUSION

Hsp90 modulation exhibits the potential to treat unrelated disease states, from cancer to neurodegenerative diseases, and, thus, to fold or not to fold, becomes a question of great value.

The stress of protein misfolding: from single cells to multicellular organisms

Organisms survive changes in the environment by altering their rates of metabolism, growth, and reproduction. At the same time, the system must ensure the stability and functionality of its macromolecules. Fluctuations in the environment are sensed by highly conserved stress responses and homeostatic mechanisms, and of these, the heat shock response (HSR) represents an essential response to acute and chronic proteotoxic damage. However, unlike the strategies employed to maintain the integrity of the genome, protection of the proteome must be tailored to accommodate the normal flux of nonnative proteins and the differences in protein composition between cells, and among individuals. Moreover, adult cells are likely to have significant differences in the rates of synthesis and clearance that are influenced by intrinsic errors in protein expression, genetic polymorphisms, and fluctuations in physiological and environmental conditions. Here, we will address how protein homeostasis (proteostasis) is achieved at the level of the cell and organism, and how the threshold of the stress response is set to detect and combat protein misfolding. For metazoans, the requirement for coordinated function and growth imposes additional constraints on the detection, signaling, and response to misfolding, and requires that the HSR is integrated into various aspects of organismal physiology, such as lifespan. This is achieved by hierarchical regulation of heat shock factor 1 (HSF1) by the metabolic state of the cell and centralized neuronal control that could allow optimal resource allocation between cells and tissues. We will examine how protein folding quality control mechanisms in individual cells may be integrated into a multicellular level of control, and further, even custom-designed to support individual variability and impose additional constraints on evolutionary adaptation.

Advances in the discovery and development of heat-shock protein 90 inhibitors for cancer treatment

INTRODUCTION: Over the last 15 - 20 years, targeted anticancer strategies have focused on therapies aimed at abrogating a single malignant protein. Agents that are directed towards the inhibition of a single oncoprotein have resulted in a number of useful drugs in the treatment of cancers (i.e., Gleevec, BCR-ABL; Tarceva and Iressa, EGFR). However, such a strategy relies on the notion that a cancer cell is dependent on a single signaling pathway for its survival. The possibility that a cancer cell may mutate or switch its dependence to another signaling pathway can result in the ineffectiveness of such agents. Recent advances in the biology of heat-shock protein 90 (Hsp90) have revealed intimate details into the complexity of the chaperoning process that Hsp90 is engaged in and, at the same time, have offered those involved in drug discovery several unique ways to interfere in this process. AREAS COVERED: This review provides the current understanding of the chaperone cycle of Hsp90 and presents the multifaceted approaches used by researchers in the discovery of potential Hsp90 drugs. It discusses the phenotypic outcomes in cancer cells on Hsp90 inhibition by these several approaches and also addresses several distinctions observed among direct Hsp90 ATP-pocket competitors providing commentary on the potential biological outcomes as well as the clinical relevance of such features. EXPERT OPINION: The significantly different phenotypic outcomes observed from Hsp90 inhibition by the many inhibitors developed suggest that the clinical development of Hsp90 inhibitors would be better served by careful consideration of the pharmacokinetic/pharmacodynamic properties of individual candidates rather than a generic approach directed towards the target.

Balance between folding and degradation for Hsp90-dependent client proteins: a key role for CHIP

Cells must regulate the synthesis and degradation of their proteins to maintain a balance that is appropriate for their specific growth conditions. Here we present the results of an investigation of the balance between protein folding and degradation for mammalian chaperone Hsp90-dependent client proteins. The central players are the molecular chaperones Hsp70 and Hsp90, the cochaperone HOP, and ubiquitin ligase, CHIP. Hsp70 and Hsp90 bind to HOP, thus forming a ternary folding complex whereas the binding of CHIP to the chaperones has previously been shown to lead to ubiquitination and ultimately to degradation of the client proteins as well as the chaperones. To understand the folding/degradation balance in more detail, we characterized the stoichiometries of the CHIP-Hsp70 and CHIP-Hsp90 complexes and measured the corresponding dissociation constants to be approximately 1 muM and approximately 4.5 muM, respectively. We quantified the rate of ubiquitination of various substrates by CHIP in vitro. We further determined that the folding and degradation machineries cannot coexist in one complex. Lastly, we measured the in vivo concentrations of Hsp70, Hsp90, HOP, and CHIP under normal conditions and when client proteins are being degraded due to inhibition of the folding pathway. These in vivo measurements along with the in vitro data allowed us to calculate the approximate cellular concentrations of the folding and degradation complexes under both conditions and formulate a quantitative model for the balance between protein folding and degradation as well as an explanation for the shift to client protein degradation when the folding pathway is inhibited.

The L279P mutation of nuclear distribution gene C (NudC) influences its chaperone activity and lissencephaly protein 1 (LIS1) stability

LIS1, a gene mutated in classical lissencephaly, plays essential roles in cytoplasmic dynein regulation, mitosis and cell migration. However, the regulation of LIS1 (lissencephaly protein 1) protein remains largely unknown. Genetic studies in Aspergillus nidulans have uncovered that the Nud (nuclear distribution) pathway is involved in the regulation of cytoplasmic dynein complex and a temperature-sensitive mutation in the nudC gene (L146P) greatly reduces the protein levels of NudF, an Aspergillus ortholog of LIS1. Here, we showed that L146 in Aspergillus NudC and its flanking region were highly conservative during evolution. The similar mutation in human NudC (L279P) obviously led to reduced LIS1 and cellular phenotypes similar to those of LIS1 down-regulation. To explore the underlying mechanism, we found that the p23 domain-containing protein NudC bound to the molecular chaperone Hsp90, which is also associated with LIS1. Inhibition of Hsp90 chaperone function by either geldanamycin or radicicol resulted in a decrease in LIS1 levels. Ectopic expression of Hsp90 partially reversed the degradation of LIS1 caused by overexpression of NudC-L279P. Furthermore, NudC was found to regulate the ATPase activity of Hsp90, which was repressed by the mutation of L279P. Interestingly, NudC itself was shown to possess a chaperone function, which also was suppressed by the L279P mutation. Together, these data suggest that NudC may be involved in the regulation of LIS1 stability by its chaperone function.

HOP is a monomer: investigation of the oligomeric state of the co-chaperone HOP

The co-chaperone Hsp70-Hsp90 organizing protein (HOP) plays a central role in protein folding in vivo, binding to both Hsp70 and Hsp90 and bringing them together in a functional complex. Reports in the literature concerning the oligomeric state of HOP have been inconsistent-is it a monomer, dimer, or higher order oligomer? Knowing the oligomeric state of HOP is important, because it places limits on the number and types of multiprotein complexes that can form during the folding cycle. Thus, the number of feasible models is simplified. Here, we explicitly investigate the oligomeric state of HOP using three complementary methods: gel filtration chromatography, sedimentation equilibrium analytical ultracentrifugation (AUC), and an in vivo coexpression assay. We find that HOP does not behave like a monomeric globular protein on gel filtration. Rather its behavior is consistent with it being either an elongated monomer or a dimer. We follow-up on these studies using sedimentation equilibrium AUC, which separates on the basis of molecular weight (MW), independent of shape. Sedimentation equilibrium AUC clearly shows that HOP is a monomer, with no indication of higher MW species. Finally, we use an in vivo coexpression assay that also supports the conclusion that HOP is a monomer.

Nestin modulates glucocorticoid receptor function by cytoplasmic anchoring

Nestin is the characteristic intermediate filament (IF) protein of rapidly proliferating progenitor cells and regenerating tissue. Nestin copolymerizes with class III IF-proteins, mostly vimentin, into heteromeric filaments. Its expression is downregulated with differentiation. Here we show that a strong nestin expression in mouse embryo tissue coincides with a strong accumulation of the glucocorticoid receptor (GR), a key regulator of growth and differentiation in embryonic development. Microscopic studies on cultured cells show an association of GR with IFs composed of vimentin and nestin. Cells lacking nestin, but expressing vimentin, or cells expressing vimentin, but lacking nestin accumulate GR in the nucleus. Completing these networks with an exogenous nestin, respectively an exogenous vimentin restores cytoplasmic anchoring of GR to the IF system. Thus, heteromeric filaments provide the basis for anchoring of GR. The reaction pattern with phospho-GR specific antibodies and the presence of the chaperone HSC70 suggest that specifically the unliganded receptor is anchored to the IF system. Ligand addition releases GR from IFs and shifts the receptor into the nucleus. Suppression of nestin by specific shRNA abolishes anchoring of GR, induces its accumulation in the nucleus and provokes an irreversible G1/S cell cycle arrest. Suppression of GR prior to that of nestin prevents entry into the arrest. The data give evidence that nestin/vimentin specific anchoring modulates growth suppression by GR. We hypothesize that expression of nestin is a major determinant in suppression of anti-proliferative activity of GR in undifferentiated tissue and facilitates activation of this growth control in a precise tissue and differentiation dependent manner.

HSP90alpha and HSP90beta isoforms selectively modulate MHC class II antigen presentation in B cells

Two isoforms of heat shock protein (HSP) 90, alpha and beta, are abundantly expressed in the cytoplasm of cells, yet only HSP90alpha serves as a chaperone to potentiate epitope presentation in the context of MHC class I molecules. By contrast, the role of HSP90 isoforms in MHC class II presentation of exogenous and endogenous Ags remains less clear. Studies here using human B lymphoblasts demonstrate the importance of HSP90alpha and HSP90beta isoforms in selectively regulating class II presentation of the diabetes autoantigen glutamic acid decarboxylase (GAD). Inactivation of HSP90 function using geldanamycin or radicicol inhibited MHC class II presentation of exogenous and endogenous GAD, but did not perturb the presentation of several other intra- and extracellular Ags. Treatment of human B cells with geldanamycin and radicicol did not alter cellular MHC class II expression, but did induce a stress response in these APCs. Yet, cell stress alone failed to perturb MHC class II presentation of GAD. HSP90 was found to associate with select Ags such as GAD in cells and ex vivo. Knockdown of HSP90alpha or HSP90beta expression using small interfering RNA decreased the abundance of each isoform, respectively, but did not affect MHC class II expression or induce a stress response. Notably, disruption of HSP90alpha or HSP90beta expression specifically inhibited class II presentation of the exogenous and endogenous GAD Ag. Precomplexing HSP90 with GAD Ag enhanced exogenous GAD Ag presentation. These results demonstrate a requirement for HSP90alpha and HSP90beta in regulating class II presentation of select Ags.

Cisplatin differently affects amino terminal and carboxyl terminal domains of HSP90

The 90-kDa heat shock protein (HSP90) is a molecular chaperone that assists in the folding and assembly of proteins in the cytosol. We previously demonstrated that the antineoplastic reagent, cisplatin, inhibits the aggregation prevention activity of mammalian HSP90. We now show that cisplatin binds both the amino terminal and carboxyl terminal domains of the human HSP90 and differently affects these two domains. Cisplatin blocks the aggregation prevention activity of HSP90C, but not HSP90N. In contrast, cisplatin induces a conformational change in HSP90N, but not HSP90C. These results indicate that cisplatin modulates the HSP90 activities through two different mechanisms using the two distinct binding sites of the HSP90 molecule.

Interactions of S100A2 and S100A6 with the tetratricopeptide repeat proteins, Hsp90/Hsp70-organizing protein and kinesin light chain

S100A2 and S100A6 interact with several target proteins in a Ca2+-regulated manner. However, the exact intracellular roles of the S100 proteins are unclear. In this study we identified Hsp70/Hsp90-organizing protein (Hop) and kinesin light chain (KLC) as novel targets of S100A2 and S100A6. Hop directly associates with Hsp70 and Hsp90 through the tetratricopeptide (TPR) domains and regulates Hop-Hsp70 and Hop-Hsp90 complex formation. We have found that S100A2 and S100A6 bind to the TPR domain of Hop, resulting in inhibition of the Hop-Hsp70 and Hop-Hsp90 interactions in vitro. Although endogenous Hsp70 and Hsp90 interact with Hop in resting Cos-7 cells, but not with S100A6, stimulation of these cells with ionomycin caused a Hop-S100A6 interaction, resulting in the dissociation of Hsp70 and Hsp90 from Hop. Similarly, glutathione S-transferase pulldown and co-immunoprecipitation experiments revealed that S100A6 binds to the TPR domain of KLC, resulting in inhibition of the KLC-c-Jun N-terminal kinase (JNK)-interacting protein 1 (JIP-1) interaction in vitro. The transiently expressed JIP-1 interacts with KLC in resting Cos-7 cells but not with S100A6. Stimulation of these cells with ionomycin also caused a KLC-S100A6 interaction, resulting in dissociation of JIP-1 from KLC. These results strongly suggest that the S100 proteins modulate Hsp70-Hop-Hsp90 multichaperone complex formation and KLC-cargo interaction via Ca2+-dependent S100 protein-TPR protein complex formation in vivo as well as in vitro. Moreover, we have shown that S100A2 and S100A6 interact with another TPR protein Tom70 and regulate the Tom70-ligand interaction in vitro. Thus, our findings suggest a new intracellular Ca2+-signaling pathway via S100 proteins-TPR motif interactions.

Development of anaplastic lymphoma kinase (ALK) small-molecule inhibitors for cancer therapy

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) involved in the genesis of several human cancers; indeed, ALK was initially identified in constitutively activated and oncogenic fusion forms--the most common being nucleophosmin (NPM)-ALK--in a non-Hodgkin's lymphoma (NHL) known as anaplastic large-cell lymphoma (ALCL) and subsequent studies identified ALK fusions in the human sarcomas called inflammatory myofibroblastic tumors (IMTs). In addition, two recent reports have suggested that the ALK fusion, TPM4-ALK, may be involved in the genesis of a subset of esophageal squamous cell carcinomas. While the cause-effect relationship between ALK fusions and malignancies such as ALCL and IMT is very well established, more circumstantial links implicate the involvement of the full-length, normal ALK receptor in the genesis of additional malignancies including glioblastoma, neuroblastoma, breast cancer, and others; in these instances, ALK is believed to foster tumorigenesis following activation by autocrine and/or paracrine growth loops involving the reported ALK ligands, pleiotrophin (PTN) and midkine (MK). There are no currently available ALK small-molecule inhibitors approved for clinical cancer therapy; however, recognition of the variety of malignancies in which ALK may play a causative role has recently begun to prompt developmental efforts in this area. This review provides a succinct summary of normal ALK biology, the confirmed and putative roles of ALK fusions and the full-length ALK receptor in the development of human cancers, and efforts to target ALK using small-molecule kinase inhibitors.

Unc45b forms a cytosolic complex with Hsp90 and targets the unfolded myosin motor domain

Myosin folding and assembly in striated muscle is mediated by the general chaperones Hsc70 and Hsp90 and a myosin specific co-chaperone, UNC45. Two UNC45 genes are found in vertebrates, including a striated muscle specific form, Unc45b. We have investigated the role of Unc45b in myosin folding. Epitope tagged murine Unc45b (Unc45b^Flag) was expressed in muscle and non-muscle cells and bacteria, isolated and characterized. The protein is a soluble monomer in solution with a compact folded rod-shaped structure of ∼19 nm length by electron microscopy. When over-expressed in striated muscle cells, Unc45b^Flag fractionates as a cytosolic protein and isolates as a stable complex with Hsp90. Purified Unc45b^Flag re-binds Hsp90 and forms a stable complex in solution. The endogenous Unc45b in muscle cell lysates is also found associated with Hsp90. The Unc45b^Flag/Hsp90 complex binds the partially folded myosin motor domain when incubated with myosin subfragments synthesized in a reticulocyte lysate. This binding is independent of the myosin rod or light chains. Unc45b^Flag does not bind native myosin subfragments consistent with a chaperone function. More importantly, Unc45b^Flag enhances myosin motor domain folding during de novo motor domain synthesis indicating that it has a direct role in myosin maturation. Thus, mammalian Unc45b is a cytosolic protein that forms a stable complex with Hsp90, selectively binds the unfolded conformation of the myosin motor domain, and promotes motor domain folding.

Unc45 activates Hsp90-dependent folding of the myosin motor domain

Myosin folding and assembly in striated muscle are mediated by the general chaperones Hsc70 and Hsp90 and involve a myosin-specific co-chaperone related to the Caenorhabditis elegans gene unc-45. Two unc-45 genes are found in vertebrates, a general cell isoform, unc45a, and a striated muscle-specific isoform, unc45b. We have investigated the role of both isoforms of mouse Unc45 in myosin folding using an in vitro synthesis and folding assay. A smooth muscle myosin motor domain (MD) fused to green fluorescent protein (GFP) (MD::GFP) was used as substrate, and folding was measured by native gel electrophoresis and functional assays. In the absence of Unc45, the MD::GFP chimera folds poorly. Addition of either Unc45a or Unc45b dramatically enhances the folding in a reaction that is dependent on Hsp90 ATPase activity. Unc45a is more effective than Unc45b with a higher apparent affinity and greater extent of folding. The Unc45-Hsp90 chaperone complex acts late in the folding pathway and promotes motor domain maturation after release from the ribosome. Unc45a behaves kinetically as an activator of the folding reaction by stimulating the rate of the Hsp90-dependent folding by >20-fold with an apparent K(act) of 33 nm. This analysis of vertebrate Unc45 isoforms clearly demonstrates a direct role for Unc45 in Hsp90-mediated myosin motor domain folding and highlights major differences between the isoforms in substrate specificity and mechanism.

cAMP-dependent protein kinase regulates the mitochondrial import of the nuclear encoded NDUFS4 subunit of complex I

The subunits of complex I encoded by the mammalian nuclear genes NDUFS4 (AQDQ protein) and NDUFB11 (ESSS protein) contain serine/threonine consensus phosphorylation sequences (CPS) in their presequence, the first also in the C-terminus. We have studied the impact of PKA mediated phosphorylation on the mitochondrial import of in vitro and in vivo synthesized NDUFS4 protein. The intramitochondrial accumulation of the mature form of in vitro synthesized NDUFS4 protein, but not that of ESSS protein, was promoted by PKA and depressed by alkaline phosphatase (AP). In HeLa cells, control or transfected with the NDUFS4 cDNA construct, the mitochondrial level of mature NDUFS4 protein was promoted by 8-Br-cAMP and depressed by H89. Ser173Ala mutagenesis in the C-terminus CPS abolished the appearance in mitochondria of the mature form of NDUFS4 protein. The promoting effect of PKA on the mitochondrial accumulation of mature NDUFS4 protein appears to be due to inhibition of its retrograde diffusion into the cytosol.

Novobiocin and additional inhibitors of the Hsp90 C-terminal nucleotide-binding pocket

The 90 kDa heat shock proteins (Hsp90), which are integrally involved in cell signaling, proliferation, and survival, are ubiquitously expressed in cells. Many proteins in tumor cells are dependent upon the Hsp90 protein folding machinery for their stability, refolding, and maturation. Inhibition of Hsp90 uniquely targets client proteins associated with all six hallmarks of cancer. Thus, Hsp90 has emerged as a promising target for the treatment of cancer. Hsp90 exists as a homodimer, which contains three domains. The N-terminal domain contains an ATP-binding site that binds the natural products geldanamycin and radicicol. The middle domain is highly charged and has high affinity for co-chaperones and client proteins. Initial studies by Csermely and co-workers suggested a second ATP-binding site in the C-terminus of Hsp90. This C-terminal nucleotide binding pocket has been shown to not only bind ATP, but cisplatin, novobiocin, epilgallocatechin-3-gallate (EGCG) and taxol. The coumarin antibiotics novobiocin, clorobiocin, and coumermycin A1 were isolated from several streptomyces strains and exhibit potent activity against Gram-positive bacteria. These compounds bind type II topoisomerases, including DNA gyrase, and inhibit the enzyme-catalyzed hydrolysis of ATP. As a result, novobiocin analogues have garnered the attention of numerous researchers as an attractive agent for the treatment of bacterial infection. Novobiocin was reported to bind weakly to the newly discovered Hsp90 C-terminal ATP binding site ( approximately 700 M in SkBr3 cells) and induce degradation of Hsp90 client proteins. Structural modification of this compound has led to an increase of 1000-fold in activity in anti-proliferative assays. Recent studies of structure-activity relationship (SAR) by Renoir and co-workers highlighted the crucial role of the C-4 and/or C-7 positions of the coumarin and removal of the noviose moiety, which appeared to be essential for degradation of Hsp90 client proteins. Unlike the N-terminal ATP binding site, there is no reported co-crystal structure of Hsp90 C-terminus bound to any inhibitor. The Hsp90 C-terminal domain, however, is known to contain a conserved pentapeptide sequence (MEEVD) which is recognized by co-chaperones. Cisplatin is a platinum-containing chemotherapeutic used to treat various types of cancers, including testicular, ovarian, bladder, and small cell lung cancer. Most notably, cisplatin coordinates to DNA bases, resulting in cross-linked DNA, which prohibits rapidly dividing cells from duplicating DNA for mitosis. Itoh and co-workers reported that cisplatin decreases the chaperone activity of Hsp90. This group applied bovine brain cytosol to a cisplatin affinity column, eluted with cisplatin and detected Hsp90 in the eluent. Subsequent experiments indicated that cisplatin exhibits high affinity for Hsp90. Moreover Csermely and co-workers determined that the cisplatin binding site is located proximal to the C-terminal ATP binding site. EGCG is one of the active ingredients found in green tea. EGCG is known to inhibit the activity of many Hsp90-dependent client proteins, including telomerase, several kinases, and the aryl hydrocarbon receptor (AhR). Recently Gasiewicz and co-workers reported that EGCG manifests its antagonistic activity against AhR through binding Hsp90. Similar to novobiocin, EGCG was shown to bind the C-terminus of Hsp90. Unlike previously identified N-terminal Hsp90 inhibitors, EGCG does not appear to prevent Hsp90 from forming multiprotein complexes. Studies are currently underway to determine whether EGCG competes with novobiocin or cisplatin binding. Taxol, a well-known drug for the treatment of cancer, is responsible for the stabilization of microtubules and the inhibition of mitosis. Previous studies have shown that taxol induces the activation of kinases and transcription factors, and mimics the effect of bacterial lipopolysaccharide (LPS), an attribute unrelated to its tubulin-binding properties. Rosen and co-workers prepared a biotinylated taxol derivative and performed affinity chromatography experiments with lysates from both mouse brain and macrophage cell lines. These studies led to identification of two chaperones, Hsp70 and Hsp90, by mass spectrometry. In contrast to typical Hsp90-binding drugs, taxol exhibits a stimulatory response. Recently it was reported that the geldanamycin derivative 17-AAG behaves synergistically with taxol-induced apoptosis. This review describes the different C-terminal inhibitors of Hsp90, with specific emphasis on structure-activity relationship studies of novobiocin and their effects on anti-proliferative activity.

Hsp90 inhibitors: small molecules that transform the Hsp90 protein folding machinery into a catalyst for protein degradation

The 90 kDa heat shock proteins (Hsp90) are responsible for the conformational maturation of nascent polypeptides and the renaturation of denatured proteins. In transformed cells, numerous mutated and overexpressed proteins rely on the Hsp90 protein folding machinery for tumor progression. The Hsp90-mediated protein folding process is dependent upon ATP, and when inhibitors of ATP are present, the Hsp90 machinery is unable to fold client proteins into their biologically active form, which results in the degradation of protein substrates via the ubiquitin-proteasome pathway. Consequently, Hsp90 has evolved into a promising anti-cancer target because multiple oncogenic proteins can be simultaneously degraded as a consequence of Hsp90 inhibition. This review serves to explain the Hsp90 protein folding process, the impact of Hsp90 inhibition, the identification of natural product inhibitors, and the development of rationally designed inhibitors of the Hsp90 protein folding machinery.

The biochemical role of the heat shock protein 90 chaperone complex in establishing human telomerase activity

Telomerase is a ribonucleoprotein complex that synthesizes the G-rich DNA found at the 3'-ends of linear chromosomes. Human telomerase consists minimally of a catalytic protein (hTERT) and a template-containing RNA (hTR), although other proteins are involved in regulating telomerase activity in vivo. Several chaperone proteins, including hsp90 and p23, have demonstrable roles in establishing telomerase activity both in vitro and in vivo, and previous reports indicate that hsp90 and p23 are required for the reconstitution of telomerase activity from recombinant hTERT and hTR. Here we report that hTERT and hTR associate in the absence of a functional hsp90-p23 heterocomplex. We also report that hsp90 inhibitors geldanamycin and novobiocin inhibit recombinant telomerase even after telomerase is assembled. Inhibition by geldanamycin could be overcome by allowing telomerase to first bind its primer, suggesting a role for hsp90 in loading telomerase onto the telomere. Inhibition by novobiocin could not similarly be overcome but instead resulted in destabilization of the hTERT polypeptide. We propose that the hsp90-p23 complex fine tunes and stabilizes a functional telomerase structure, allowing primer loading and extension.

Glucocorticoid sensitivity: pathology, mutations and clinical implications

Glucocorticoids exert diverse effects on virtually all cell types and tissues. Subtle changes in sensitivity may be generalized and congenital or acquired in a tissue-specific manner. Such changes may lead to altered susceptibility to metabolic diseases, such as ischemic heart disease, or to insensitivity to the therapeutic actions of synthetic glucocorticoids such as in inflammatory disease. This review will cover current theories of how glucocorticoids exert genetic and other congenital effects on glucocorticoid sensitivity, and acquired changes in glucocorticoid sensitivity seen principally in inflammatory and malignant disease. Recent important developments in the field include the impact of genetic variation within the glucocorticoid receptor gene, the effects of early life experience on long-term glucocorticoid sensitivity, studies identifying the role of nuclear factor κB in modulating glucocorticoid sensitivity in vitro and in vivo, and the action of macrophage migration inhibitory factor in modulating the anti-inflammatory effects of glucocorticoids. The role of chromatin organization in regulating glucocorticoid action on proinflammatory genes is discussed, as is the regulation of glucocorticoid sensitivity in human malignancy in the context of pathogenesis and treatment response.

Chaperoning of glucocorticoid receptors

A multiprotein hsp90/hsp70-based chaperone machinery functions as a 'cradle-to-grave' system for regulating the steroid binding, trafficking and turnover of the glucocorticoid receptor (GR). In an ATP-dependent process where hsp70 and hsp90 act as essential chaperones and Hop, hsp40, and p23 act as nonessential co-chaperones, the machinery assembles complexes between the ligand binding domain of the GR and hsp90. During GR-hsp90 heterocomplex assembly, the hydrophobic ligand-binding cleft is opened to access by steroid, and subsequent binding of steroid within the cleft triggers a transformation of the receptor such that it engages in more dynamic cycles of assembly/disassembly with hsp90 that are required for rapid dynein-dependent translocation to the nucleus. Within the nucleus, the hsp90 chaperone machinery plays a critical role both in GR movement to transcription regulatory sites and in the disassembly of regulatory complexes as the hormone level declines. The chaperone machinery also plays a critical role in stabilization of the GR to ubiquitylation and proteasomal degradation. The initial GR interaction with hsp70 appears to be critical for the triage between hsp90 heterocomplex assembly and preservation of receptor function vs CHIP-dependent ubiquitylation and proteasomal degradation. The hsp90 chaperone machinery is ubiquitous and functionally conserved among eukaryotes, and it is possible that all physiologically significant actions of hsp90 require the hsp70-dependent assembly of client protein-hsp90 heterocomplexes.

Inhibition of Hsp72-mediated protein refolding by 4-hydroxy-2-nonenal

A proteomic approach was applied to liver cytosol from rats fed a diet consisting of high fat and ethanol to identify 4-hydroxy-2-nonenal (4-HNE)-modified proteins in vivo. Cytosolic Hsp72, the inducible variant of the Hsp70 heat shock protein family, was consistently among the proteins modified by 4-HNE. Despite 1.3-fold induction of Hsp72 in the livers of ethanol-fed animals, no increase in Hsp70-mediated luciferase refolding in isolated heptocytes was observed, suggesting inhibition of this process by 4-HNE. A 50% and 75% reduction in luciferase refolding efficiency was observed in rabbit reticulocyte lysate (RRL) supplemented with recombinant Hsp72 which had been modified in vitro with 10 and 100 microM 4-HNE, respectively. This observation was accompanied by a 25% and 50% decrease in substrate binding by the chaperone following the same treatment; however, no effect on complex formation between Hsp72 and its co-chaperone Hsp40 was observed. Trypsin digest and mass spectral analysis of Hsp72 treated with 10 and 100 microM 4-HNE consistently identified adduct formation at Cys267 in the ATPase domain of the chaperone. The role of this residue in the observed inhibition was demonstrated through the use of DnaK, a bacterial Hsp70 variant lacking Cys267. DnaK was resistant to 4-HNE inactivation. Additionally, Hsp72 was resistant to inactivation by the thiol-unreactive aldehyde malondialdehyde (MDA), further supporting a role for Cys in Hsp72 inhibition by 4-HNE. Finally, the affinity of Hsp72 for ATP was decreased 32% and 72% following treatment of the chaperone with 10 and 100 microM 4-HNE, respectively. In a model of chronic alcoholic liver injury, induction of Hsp72 was not accompanied by an increase in protein refolding ability. This is likely the result of 4-HNE modification of the Hsp72 ATPase domain.

Regulation of Hsp70 function by HspBP1: structural analysis reveals an alternate mechanism for Hsp70 nucleotide exchange

HspBP1 belongs to a family of eukaryotic proteins recently identified as nucleotide exchange factors for Hsp70. We show that the S. cerevisiae ortholog of HspBP1, Fes1p, is required for efficient protein folding in the cytosol at 37 degrees C. The crystal structure of HspBP1, alone and complexed with part of the Hsp70 ATPase domain, reveals a mechanism for its function distinct from that of BAG-1 or GrpE, previously characterized nucleotide exchange factors of Hsp70. HspBP1 has a curved, all alpha-helical fold containing four armadillo-like repeats unlike the other nucleotide exchange factors. The concave face of HspBP1 embraces lobe II of the ATPase domain, and a steric conflict displaces lobe I, reducing the affinity for nucleotide. In contrast, BAG-1 and GrpE trigger a conserved conformational change in lobe II of the ATPase domain. Thus, nucleotide exchange on eukaryotic Hsp70 occurs through two distinct mechanisms.

FK506-binding proteins 51 and 52 differentially regulate dynein interaction and nuclear translocation of the glucocorticoid receptor in mammalian cells

We used a cellular system to elucidate the molecular determinants of the large immunophilin FK506-binding proteins (FKBP)51 and -52 for their action on the glucocorticoid receptor in mammalian cells. Increasing the levels of FKBP51 reduced the transcriptional activity of the receptor, as reported. Elevated levels of FKBP52 per se showed no effect but mitigated the inhibition of the receptor induced by FKBP51. We discovered that nuclear translocation of the glucocorticoid receptor was delayed by FKBP51. This correlates with the reduced interaction of FKBP51 with the motor protein dynein compared with FKBP52. From mutational analyses, we concluded that three features of the immunophilins are required for efficient receptor signaling in mammalian cells: hsp90 interaction, dynein association, and peptidylprolyl isomerase (PPIase) enzyme activity. The relevance of dynein for receptor function was substantiated by several experiments: 1) coexpression of dynamitin, which disrupts the transport complex and reduces receptor activity; 2) coexpression of the PPIase domain fragment of FKBP52, which is known to disrupt interaction of the receptor to dynein and reduce glucocorticoid receptor function, in contrast to the corresponding fragment of FKBP51; and 3) swapping of the PPIase domains FKBP51 and FKBP52, which reverses the respective activity. We concluded from our results that the mechanisms of the regulatory system FKBP51/FKBP52 discovered in yeast also operate in mammals to modulate hormone binding of the receptor. In addition, differential regulation of dynein association and nuclear translocation contributes to the effects of the two immunophilins on the glucocorticoid receptor in mammals.

Hsp90: an emerging target for breast cancer therapy

Rapidly evolving insights into the specific molecular genetic abnormalities that drive the growth and metastasis of breast cancer have led to the development of targeted therapeutics that do not rely on the generalized disruption of DNA metabolism and cell division for activity. Of particular interest are inhibitors of cellular signal transduction pathways involving tyrosine kinases as well as selective modulators of steroid hormone signaling, histone acetylation, angiogenesis and tumor cell apoptosis. Unique within this array of promising new agents, however, are compounds that target heat shock protein 90 (Hsp90). This molecular chaperone associates with a distinct, but surprisingly diverse, set of proteins that are referred to as Hsp90 client proteins. Hsp90 binds to these clients, and plays a key role in regulating their stability and function. Many of the proteins chaperoned by Hsp90 are involved in breast cancer progression and resistance to therapy, including the estrogen receptor, receptor tyrosine kinases of the erbB family, Akt, and mutant p53. Several small molecule inhibitors of Hsp90 have been identified that can deplete cellular levels of multiple oncogenic client proteins simultaneously by enhancing their ubiquitination and proteasome-mediated degradation. The activity of Hsp90 inhibitors has been well validated in preclinical breast cancer models, both in single-agent studies and in combination with conventional chemotherapy. One of these inhibitors, 17-allylamino, 17-demethoxygeldanamycin (17-AAG, NSC 330507) has recently completed phase I testing. The agent was well tolerated at drug exposures that were shown to cause modulation of Hsp90 client protein levels. Given the redundancy and complexity of the molecular abnormalities present in most breast cancers, the ability of Hsp90 inhibitors to alter the activity of multiple oncogenic targets may prove of unique therapeutic benefit.

Caspase-dependent, geldanamycin-enhanced cleavage of co-chaperone p23 in leukemic apoptosis

Co-chaperone p23 is a component of the heat-shock protein (Hsp)90 multiprotein-complex and is an important modulator of Hsp90 activity. Hsp90 client proteins involved in oncogenic survival signaling are frequently mutated in leukemia, and the integrity of the Hsp90 complex could therefore be important for leukemic cell survival. We demonstrate here that p23 is cleaved to a stable 17 kDa fragment in leukemic cell lines treated with commonly used chemotherapeutic drugs. The cleavage of p23 paralleled the activation of procaspase-7 and -3 and was suppressed by the caspase-3/-7 inhibitor DEVD-FMK. In vitro translated 35S-p23 (in reticulocyte lysate) was cleaved at D142 and D145 by caspase-7 and -3. Cleavage of p23 occurred in caspase-3-deficient MCF-7 cells, suggesting a role for caspase-7 in intact cells. The Hsp90 inhibitor geldanamycin enhanced caspase-dependent p23 cleavage both in vitro and in intact cells. Geldanamycin also enhanced anthracycline-induced caspase activation and apoptosis. We conclude that p23 is a prominent target in leukemic cell apoptosis. Geldanamycin enhanced p23 cleavage both by rendering p23 more susceptible to caspases and by enhancing chemotherapy-induced caspase activation. These findings underscore the importance of the Hsp90-complex in antileukemic treatment, and suggest that p23 may have a role in survival signaling.

Anaplastic lymphoma kinase proteins in growth control and cancer

The normal functions of full-length anaplastic lymphoma kinase (ALK) remain to be completely elucidated. Although considered to be important in neural development, recent studies in Drosophila also highlight a role for ALK in gut muscle differentiation. Indeed, the Drosophila model offers a future arena for the study of ALK, its ligands and signalling cascades. The discovery of activated fusion forms of the ALK tyrosine kinase in anaplastic large cell lymphoma (ALCL) has dramatically improved our understanding of the pathogenesis of these lymphomas and enhanced the pathological diagnosis of this subtype of non-Hodgkin's lymphoma (NHL). Likewise, the realisation that a high percentage of inflammatory myofibroblastic tumours express activated-ALK fusion proteins has clarified the causation of these mesenchymal neoplasms and provided for their easier discrimination from other mesenchymal-derived inflammatory myofibroblastic tumour (IMT) mimics. Recent reports of ALK expression in a range of carcinoma-derived cell lines together with its apparent role as a receptor for PTN and MK, both of which have been implicated in tumourigenesis, raise the possibility that ALK-mediated signalling could play a role in the development and/or progression of a number of common solid tumours. The therapeutic targeting of ALK may prove to have efficacy in the treatment of many of these neoplasms.