Post-translational modification of heat-shock protein 90: impact on chaperone function

Mutations F352A and Y528A in human HSP90α reduce fibronectin association and fibrillogenesis in cell-derived matrices

HSP90 is a ubiquitously expressed chaperone protein that regulates the maturation of numerous substrate proteins called 'clients'. The glycoprotein fibronectin (FN) is an important protein of the extracellular matrix (ECM) and a client protein of HSP90. FN and HSP90 interact directly, and the FN ECM is regulated by exogenous HSP90 or HSP90 inhibitors. Here, we extend the analysis of the HSP90 - FN interaction. The importance of the N-terminal 70-kDa fragment of fibronectin (FN70) and FN type I repeat was demonstrated by competition for FN binding between HSP90 and the functional upstream domain (FUD) of the Streptococcus pyogenes F1 adhesin protein. Furthermore, His-HSP90α mutations F352A and Y528A (alone and in combination) reduced the association with full-length FN (FN-FL) and FN70 in vitro. Unlike wild type His-HSP90α, these HSP90 mutants did not enhance FN matrix assembly in the Hs578T cell line model when added exogenously. Interestingly, the HSP90 E353A mutation, which did not significantly reduce the HSP90 - FN interaction in vitro, dramatically blocked FN matrix assembly in Hs578T cell-derived matrices. Taken together, these data extend our understanding of the role of HSP90 in FN fibrillogenesis and suggest that promotion of FN ECM assembly by HSP90 is not solely regulated by the affinity of the direct interaction between HSP90 and FN.

Detecting Posttranslational Modifications of Hsp90 Isoforms

The molecular chaperone heat shock protein 90 (Hsp90) is essential in eukaryotes. Hsp90 chaperones proteins that are important determinants of multistep carcinogenesis. There are multiple Hsp90 isoforms including the cytosolic Hsp90α and Hsp90β as well as GRP94 located in the endoplasmic reticulum and TRAP1 in the mitochondria. The chaperone function of Hsp90 is linked to its ability to bind and hydrolyze ATP. Co-chaperones and posttranslational modifications (such as phosphorylation, SUMOylation, and ubiquitination) are important for Hsp90 stability and regulation of its ATPase activity. Both mammalian and yeast cells can be used to express and purify Hsp90 and TRAP1 and also detect post-translational modifications by immunoblotting.

Hsp90: From Cellular to Organismal Proteostasis

Assuring a healthy proteome is indispensable for survival and organismal health. Proteome disbalance and the loss of the proteostasis buffer are hallmarks of various diseases. The essential molecular chaperone Hsp90 is a regulator of the heat shock response via HSF1 and a stabilizer of a plethora of signaling proteins. In this review, we summarize the role of Hsp90 in the cellular and organismal regulation of proteome maintenance.

Drug design targeting active posttranslational modification protein isoforms

Modern drug design aims to discover novel lead compounds with attractable chemical profiles to enable further exploration of the intersection of chemical space and biological space. Identification of small molecules with good ligand efficiency, high activity, and selectivity is crucial toward developing effective and safe drugs. However, the intersection is one of the most challenging tasks in the pharmaceutical industry, as chemical space is almost infinity and continuous, whereas the biological space is very limited and discrete. This bottleneck potentially limits the discovery of molecules with desirable properties for lead optimization. Herein, we present a new direction leveraging posttranslational modification (PTM) protein isoforms target space to inspire drug design termed as "Post-translational Modification Inspired Drug Design (PTMI-DD)." PTMI-DD aims to extend the intersections of chemical space and biological space. We further rationalized and highlighted the importance of PTM protein isoforms and their roles in various diseases and biological functions. We then laid out a few directions to elaborate the PTMI-DD in drug design including discovering covalent binding inhibitors mimicking PTMs, targeting PTM protein isoforms with distinctive binding sites from that of wild-type counterpart, targeting protein-protein interactions involving PTMs, and hijacking protein degeneration by ubiquitination for PTM protein isoforms. These directions will lead to a significant expansion of the biological space and/or increase the tractability of compounds, primarily due to precisely targeting PTM protein isoforms or complexes which are highly relevant to biological functions. Importantly, this new avenue will further enrich the personalized treatment opportunity through precision medicine targeting PTM isoforms.

Short-term cold stress and heat shock proteins in the crustacean Artemia franciscana

In their role as molecular chaperones, heat shock proteins (Hsps) mediate protein folding thereby mitigating cellular damage caused by physiological and environmental stress. Nauplii of the crustacean Artemia franciscana respond to heat shock by producing Hsps; however, the effects of cold shock on Hsp levels in A. franciscana have not been investigated previously. The effect of cold shock at 1 °C followed by recovery at 27 °C on the amounts of ArHsp90, Hsp70, ArHsp40, and ArHsp40-2 mRNA and their respective proteins in A. franciscana nauplii was examined by quantitative PCR (qPCR) and immunoprobing of western blots. The same Hsp mRNAs and proteins were also quantified during incubation of nauplii at their optimal growth temperature of 27 °C. qPCR analyses indicated that the abundance of ArHsp90, Hsp70, and ArHsp40 mRNA remained relatively constant during both cold shock and recovery and was not significantly different compared with levels at optimal temperature. Western blotting revealed that ArHsp90, ArHsp40, and ArHsp40-2 were generally below baseline, but at detectable levels during the 6 h of cold shock, and persisted in early recovery stages before declining. Hsp70 was the only protein that remained constant in quantity throughout cold shock and recovery. By contrast, all Hsps declined rapidly during 6 h when nauplii were incubated continuously at 27 °C optimal temperature. Generally, the amounts of ArHsp90, ArHsp40, and ArHsp40-2 were higher during cold shock/recovery than those during continuous incubation at 27 °C. Our data support the conclusion that low temperature preserves Hsp levels, making them available to assist in protein repair and recovery after cold shock.

Mutations in the Hsp90 N Domain Identify a Site that Controls Dimer Opening and Expand Human Hsp90α Function in Yeast

Hsp90 is a highly conserved molecular chaperone important for the activity of many client proteins. Hsp90 has an N-terminal ATPase domain (N), a middle domain (M) that interacts with clients and a C-terminal dimerization domain (C). "Closing" of dimers around clients is regulated by ATP binding, co-chaperones, and post-translational modifications. ATP hydrolysis coincides with release of mature client and resetting the reaction cycle. Humans have two Hsp90s: hHsp90α and hHsp90β. Although 85% identical, hHsp90β supports Hsp90 function in yeast much better than hHsp90α. Determining the basis of this difference would provide important insight into functional specificity of seemingly redundant Hsp90s, and the evolution of eukaryotic Hsp90 systems and clientele. Here, we found host co-chaperones Sba1, Cpr6 and Cpr7 inhibited hHsp90α function in yeast, and we identified mutations clustering in the N domain that considerably improved hHsp90α function in yeast. The strongest of these rescuer mutations accelerated nucleotide-dependent lid closing, N-M domain docking, and ATPase. It also disrupted binding to Sba1, which prolongs the closed state, and promoted N-M undocking and lid opening. Our data suggest the rescuer mutations improve function of hHsp90α in yeast by accelerating return to the open state. Our findings imply hHsp90α occupies the closed state too long to function effectively in yeast, and define an evolutionarily conserved region of the N domain involved in resetting the Hsp90 reaction cycle.

Dynamically Shaping Chaperones. Allosteric Modulators of HSP90 Family as Regulatory Tools of Cell Metabolism in Neoplastic Progression

Molecular chaperones have recently emerged as fundamental regulators of salient biological routines, including metabolic adaptations to environmental changes. Yet, many of the molecular mechanisms at the basis of their functions are still unknown or at least uncertain. This is in part due to the lack of chemical tools that can interact with the chaperones to induce measurable functional perturbations. In this context, the use of small molecules as modulators of protein functions has proven relevant for the investigation of a number of biomolecular systems. Herein, we focus on the functions, interactions and signaling pathways of the HSP90 family of molecular chaperones as possible targets for the discovery of new molecular entities aimed at tuning their activity and interactions. HSP90 and its mitochondrial paralog, TRAP1, regulate the activity of crucial metabolic circuitries, making cells capable of efficiently using available energy sources, with relevant implications both in healthy conditions and in a variety of disease states and especially cancer. The design of small-molecules targeting the chaperone cycle of HSP90 and able to inhibit or stimulate the activity of the protein can provide opportunities to finely dissect their biochemical activities and to obtain lead compounds to develop novel, mechanism-based drugs.

Heat shock protein 90 inhibition and multi-target approach to maximize cardioprotection in ischaemic injury

Despite several advances in medicine, ischaemic heart disease remains a major cause of morbidity and mortality. The unravelling of molecular mechanisms underlying disease pathophysiology has revealed targets for pharmacological interventions. However, transfer of these pharmcological possibilities to clinical use has been disappointing. Considering the complexity of ischaemic disease at the cellular and molecular levels, an equally multifaceted treatment approach may be envisioned. The pharmacological principle of 'one target, one key' may fall short in such contexts, and optimal treatment may involve one or many agents directed against complementary targets. Here, we introduce a 'multi-target approach to cardioprotection' and propose heat shock protein 90 (HSP90) as a target of interest. We report on a member of a distinct class of HSP90 inhibitor possessing pleiotropic activity, which we found to exhibit potent infarct-sparing effects.

Epigenetic Alterations of Heat Shock Proteins (HSPs) in Cancer

Heat shock proteins (HSPs) are associated with various physiological processes (protein refolding and degradation) involved in the responses to cellular stress, such as cytotoxic agents, high temperature, and hypoxia. HSPs are overexpressed in cancer cells and play roles in their apoptosis, invasion, proliferation, angiogenesis, and metastasis. The regulation or translational modification of HSPs is recognized as a therapeutic target for the development of anticancer drugs. Among the regulatory processes associated with HSP expression, the epigenetic machinery (miRNAs, histone modification, and DNA methylation) has key functions in cancer. Moreover, various epigenetic modifiers of HSP expression have also been reported as therapeutic targets and diagnostic markers of cancer. Thus, in this review, we describe the epigenetic alterations of HSP expression in cancer cells and suggest that HSPs be clinically applied as diagnostic and therapeutic markers in cancer therapy via controlled epigenetic modifiers.

Redox Aspects of Chaperones in Cardiac Function

Molecular chaperones are stress proteins that allow the correct folding or unfolding as well as the assembly or disassembly of macromolecular cellular components. Changes in expression and post-translational modifications of chaperones have been linked to a number of age- and stress-related diseases including cancer, neurodegeneration, and cardiovascular diseases. Redox sensible post-translational modifications, such as S-nitrosylation, glutathionylation and phosphorylation of chaperone proteins have been reported. Redox-dependent regulation of chaperones is likely to be a phenomenon involved in metabolic processes and may represent an adaptive response to several stress conditions, especially within mitochondria, where it impacts cellular bioenergetics. These post-translational modifications might underlie the mechanisms leading to cardioprotection by conditioning maneuvers as well as to ischemia/reperfusion injury. In this review, we discuss this topic and focus on two important aspects of redox-regulated chaperones, namely redox regulation of mitochondrial chaperone function and cardiac protection against ischemia/reperfusion injury.

Detecting Posttranslational Modifications of Hsp90

The molecular chaperone Heat Shock Protein 90 (Hsp90) is essential in eukaryotes. Hsp90 chaperones proteins that are important determinants of multistep carcinogenesis. The chaperone function of Hsp90 is linked to its ability to bind and hydrolyze ATP. Co-chaperones as well as posttranslational modifications (phosphorylation, SUMOylation, and ubiquitination) are important for its stability and regulation of the ATPase activity. Both mammalian and yeast cells can be used to express and purify Hsp90 and also detect its posttranslational modifications by immunoblotting.

P2X7 receptor antagonism prevents IL-1β release from salivary epithelial cells and reduces inflammation in a mouse model of autoimmune exocrinopathy

Salivary gland inflammation is a hallmark of Sjögren's syndrome (SS), a common autoimmune disease characterized by lymphocytic infiltration of the salivary gland and loss of saliva secretion, predominantly in women. The P2X7 receptor (P2X7R) is an ATP-gated nonselective cation channel that induces inflammatory responses in cells and tissues, including salivary gland epithelium. In immune cells, P2X7R activation induces the production of proinflammatory cytokines, including IL-1β and IL-18, by inducing the oligomerization of the multiprotein complex NLRP3-type inflammasome. Here, our results show that in primary mouse submandibular gland (SMG) epithelial cells, P2X7R activation also induces the assembly of the NLRP3 inflammasome and the maturation and release of IL-1β, a response that is absent in SMG cells isolated from mice deficient in P2X7Rs (P2X7R^-/-). P2X7R-mediated IL-1β release in SMG epithelial cells is dependent on transmembrane Na⁺ and/or K⁺ flux and the activation of heat shock protein 90 (HSP90), a protein required for the activation and stabilization of the NLRP3 inflammasome. Also, using the reactive oxygen species (ROS) scavengers N-acetyl cysteine and Mito-TEMPO, we determined that mitochondrial reactive oxygen species are required for P2X7R-mediated IL-1β release. Lastly, in vivo administration of the P2X7R antagonist A438079 in the CD28^-/-, IFNγ^-/-, NOD.H-2^h4 mouse model of salivary gland exocrinopathy ameliorated salivary gland inflammation and enhanced carbachol-induced saliva secretion. These findings demonstrate that P2X7R antagonism in vivo represents a promising therapeutic strategy to limit salivary gland inflammation and improve secretory function.

An Hsp90 co-chaperone protein in yeast is functionally replaced by site-specific posttranslational modification in humans

Heat shock protein 90 (Hsp90) is an essential eukaryotic molecular chaperone. To properly chaperone its clientele, Hsp90 proceeds through an ATP-dependent conformational cycle influenced by posttranslational modifications (PTMs) and assisted by a number of co-chaperone proteins. Although Hsp90 conformational changes in solution have been well-studied, regulation of these complex dynamics in cells remains unclear. Phosphorylation of human Hsp90α at the highly conserved tyrosine 627 has previously been reported to reduce client interaction and Aha1 binding. Here we report that these effects are due to a long-range conformational impact inhibiting Hsp90α N-domain dimerization and involving a region of the middle domain/carboxy-terminal domain interface previously suggested to be a substrate binding site. Although Y627 is not phosphorylated in yeast, we demonstrate that the non-conserved yeast co-chaperone, Hch1, similarly affects yeast Hsp90 (Hsp82) conformation and function, raising the possibility that appearance of this PTM in higher eukaryotes represents an evolutionary substitution for HCH1.

The HSP90 chaperone machinery

The heat shock protein 90 (HSP90) chaperone machinery is a key regulator of proteostasis under both physiological and stress conditions in eukaryotic cells. As HSP90 has several hundred protein substrates (or 'clients'), it is involved in many cellular processes beyond protein folding, which include DNA repair, development, the immune response and neurodegenerative disease. A large number of co-chaperones interact with HSP90 and regulate the ATPase-associated conformational changes of the HSP90 dimer that occur during the processing of clients. Recent progress has allowed the interactions of clients with HSP90 and its co-chaperones to be defined. Owing to the importance of HSP90 in the regulation of many cellular proteins, it has become a promising drug target for the treatment of several diseases, which include cancer and diseases associated with protein misfolding.

Reactive electrophilic oxylipins trigger a heat stress-like response through HSFA1 transcription factors

Abiotic and biotic stresses are often characterized by an induction of reactive electrophile species (RES) such as the jasmonate 12-oxo-phytodienoic acid (OPDA) or the structurally related phytoprostanes. Previously, RES oxylipins have been shown massively to induce heat-shock-response (HSR) genes including HSP101 chaperones. Moreover, jasmonates have been reported to play a role in basal thermotolerance. We show that representative HSR marker genes are strongly induced by RES oxylipins through the four master regulator transcription factors HSFA1a, b, d, and e essential for short-term adaptation to heat stress in Arabidopsis. When compared with Arabidopsis seedlings treated at the optimal acclimation temperature of 37 °C, the exogenous application of RES oxylipins at 20 °C induced a much weaker induction of HSP101 at both the gene and protein expression levels which, however, was not sufficient to confer short-term acquired thermotolerance. Moreover, jasmonate-deficient mutant lines displayed a wild-type-like HSR and were not compromised in acquiring thermotolerance. Hence, the OPDA- and RES oxylipin-induced HSR is not sufficient to protect seedlings from severe heat stress but may help plants to cope better with stresses associated with protein unfolding by inducing a battery of chaperones in the absence of heat.

Calcyclin Binding Protein/Siah-1 Interacting Protein Is a Hsp90 Binding Chaperone

The Hsp90 chaperone activity is tightly regulated by interaction with many co-chaperones. Since CacyBP/SIP shares some sequence homology with a known Hsp90 co-chaperone, Sgt1, in this work we performed a set of experiments in order to verify whether CacyBP/SIP can interact with Hsp90. By applying the immunoprecipitation assay we have found that CacyBP/SIP binds to Hsp90 and that the middle (M) domain of Hsp90 is responsible for this binding. Furthermore, the proximity ligation assay (PLA) performed on HEp-2 cells has shown that the CacyBP/SIP-Hsp90 complexes are mainly localized in the cytoplasm of these cells. Using purified proteins and applying an ELISA we have shown that Hsp90 interacts directly with CacyBP/SIP and that the latter protein does not compete with Sgt1 for the binding to Hsp90. Moreover, inhibitors of Hsp90 do not perturb CacyBP/SIP-Hsp90 binding. Luciferase renaturation assay and citrate synthase aggregation assay with the use of recombinant proteins have revealed that CacyBP/SIP exhibits chaperone properties. Also, CacyBP/SIP-3xFLAG expression in HEp-2 cells results in the appearance of more basic Hsp90 forms in 2D electrophoresis, which may indicate that CacyBP/SIP dephosphorylates Hsp90. Altogether, the obtained results suggest that CacyBP/SIP is involved in regulation of the Hsp90 chaperone machinery.

Y-632 inhibits heat shock protein 90 (Hsp90) function by disrupting the interaction between Hsp90 and Hsp70/Hsp90 organizing protein, and exerts antitumor activity in vitro and in vivo

Heat shock protein 90 (Hsp90) stabilizes a variety of proteins required for cancer cell survival and has been identified as a promising drug target for cancer treatment. To date, several Hsp90 inhibitors have entered into clinical trials, but none has been approved for cancer therapy yet. Thus, exploring new Hsp90 inhibitors with novel mechanisms of action is urgent. In the present study, we show that Y-632, a novel pyrimidine derivative, inhibited Hsp90 in a different way from the conventional Hsp90 inhibitor geldanamycin. Y-632 induced degradation of diverse Hsp90 client proteins through the ubiquitin-proteasome pathway, as geldanamycin did; however, it neither directly bound to Hsp90 nor inhibited Hsp90 ATPase activity. Y-632 inhibited Hsp90 function mainly through inducing intracellular thiol oxidation, which led to disruption of the Hsp90-Hsp70/Hsp90 organizing protein complex and further induced cell adhesion inhibition, G0 /G1 cell cycle arrest, and apoptosis. Moreover, Y-632 efficiently overcame imatinib resistance mediated by Bcr-Abl point mutations both in vitro and in vivo. We believe that Y-632, acting as a novel small-molecule inhibitor of the Hsp90-Hsp70/Hsp90 organizing protein complex, has great potential to be a promising Hsp90 inhibitor for cancer therapy, such as for imatinib-resistant leukemia.

Contribution of chaperones to STAT pathway signaling

Aberrant STAT signaling is associated with the development and progression of many cancers and immune related diseases. Recent findings demonstrate that proteostasis modulators under clinical investigation for cancer therapy have a significant impact on STAT signaling, which may be critical for mediating their anti-cancer effects. Chaperones are critical for protein folding, stability and function and, thus, play an essential role in the maintenance of proteostasis. In this review we discuss the role of chaperones in STAT and tyrosine kinase (TK) protein folding, modulation of STAT and TK activity, and degradation of TKs. We highlight the important role of chaperones in STAT signaling, and how this knowledge has provided a framework for the development of new therapeutic avenues of targeting STAT signaling related pathologies.

Contributions of co-chaperones and post-translational modifications towards Hsp90 drug sensitivity

Hsp90 is a molecular chaperone and important driver of stabilization and activation of several oncogenic proteins that are involved in the malignant transformation of tumor cells. Therefore, it is not surprising that Hsp90 has been reported to be a promising target for the treatment of several neoplasias, such as non-small-cell lung cancer and HER2-positive breast cancer. Hsp90 chaperone function depends on its ability to bind and hydrolyze ATP and Hsp90 inhibitors have been shown to compete with nucleotides for binding to Hsp90. Multiple factors, such as co-chaperones and post-translational modification, are involved in regulating Hsp90 ATPase activity. Here, the impact of post-translational modifications and co-chaperones on the efficacy of Hsp90 inhibitors are reviewed.

Fungal Hsp90: a biological transistor that tunes cellular outputs to thermal inputs

Heat shock protein 90 (HSP90) is an essential, abundant and ubiquitous eukaryotic chaperone that has crucial roles in protein folding and modulates the activities of key regulators. The fungal Hsp90 interactome, which includes numerous client proteins such as receptors, protein kinases and transcription factors, displays a surprisingly high degree of plasticity that depends on environmental conditions. Furthermore, although fungal Hsp90 levels increase following environmental challenges, Hsp90 activity is tightly controlled via post-translational regulation and an autoregulatory loop involving heat shock transcription factor 1 (Hsf1). In this Review, we discuss the roles and regulation of fungal Hsp90. We propose that Hsp90 acts as a biological transistor that modulates the activity of fungal signalling networks in response to environmental cues via this Hsf1-Hsp90 autoregulatory loop.

A highly selective Hsp90 affinity chromatography resin with a cleavable linker

Over 200 proteins have been identified that interact with the protein chaperone Hsp90, a recognized therapeutic target thought to participate in non-oncogene addiction in a variety of human cancers. However, defining Hsp90 clients is challenging because interactions between Hsp90 and its physiologically relevant targets involve low affinity binding and are thought to be transient. Using a chemo-proteomic strategy, we have developed a novel orthogonally cleavable Hsp90 affinity resin that allows purification of the native protein and is quite selective for Hsp90 over its immediate family members, GRP94 and TRAP 1. We show that the resin can be used under low stringency conditions for the rapid, unambiguous capture of native Hsp90 in complex with a native client. We also show that the choice of linker used to tether the ligand to the insoluble support can have a dramatic effect on the selectivity of the affinity media.

Post-translational modifications of Hsp90 and their contributions to chaperone regulation

Molecular chaperones, as the name suggests, are involved in folding, maintenance, intracellular transport, and degradation of proteins as well as in facilitating cell signaling. Heat shock protein 90 (Hsp90) is an essential eukaryotic molecular chaperone that carries out these processes in normal and cancer cells. Hsp90 function in vivo is coupled to its ability to hydrolyze ATP and this can be regulated by co-chaperones and post-translational modifications. In this review, we explore the varied roles of known post-translational modifications of cytosolic and nuclear Hsp90 (phosphorylation, acetylation, S-nitrosylation, oxidation and ubiquitination) in fine-tuning chaperone function in eukaryotes. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Hsp90: structure and function

Hsp90 is a highly abundant and ubiquitous molecular chaperone which plays an essential role in many cellular processes including cell cycle control, cell survival, hormone and other signalling pathways. It is important for the cell's response to stress and is a key player in maintaining cellular homeostasis. In the last ten years, it has become a major therapeutic target for cancer, and there has also been increasing interest in it as a therapeutic target in neurodegenerative disorders, and in the development of anti-virals and anti-protozoan infections. The focus of this review is the structural and mechanistic studies which have been performed in order to understand how this important chaperone acts on a wide variety of different proteins (its client proteins) and cellular processes. As with many of the other classes of molecular chaperone, Hsp90 has a critical ATPase activity, and ATP binding and hydrolysis known to modulate the conformational dynamics of the protein. It also uses a host of cochaperones which not only regulate the ATPase activity and conformational dynamics but which also mediate interactions with Hsp90 client proteins. The system is also regulated by post-translational modifications including phosphorylation and acetylation. This review discusses all these aspects of Hsp90 structure and function.

Casein kinase 2 phosphorylation of Hsp90 threonine 22 modulates chaperone function and drug sensitivity

The molecular chaperone Heat Shock Protein 90 (Hsp90) is essential for the function of various oncoproteins that are vital components of multiple signaling networks regulating cancer cell proliferation, survival, and metastasis. Hsp90 chaperone function is coupled to its ATPase activity, which can be inhibited by natural products such as the ansamycin geldanamycin (GA) and the resorcinol radicicol (RD). These compounds have served as templates for development of numerous natural product Hsp90 inhibitors. More recently, second generation, fully synthetic Hsp90 inhibitors, based on a variety of chemical scaffolds, have also been synthesized. Together, 18 natural product and synthetic Hsp90 inhibitors have entered clinical trial in cancer patients. To successfully develop Hsp90 inhibitors for oncology indications it is important to understand the factors that influence the susceptibility of Hsp90 to these drugs in vivo. We recently reported that Casein Kinase 2 phosphorylates a conserved threonine residue (T22) in helix-1 of the yeast Hsp90 N-domain both in vitro and in vivo. Phosphorylation of this residue reduces ATPase activity and affects Hsp90 chaperone function. Here, we present additional data demonstrating that ATP binding but not N-domain dimerization is a prerequisite for T22 phosphorylation. We also provide evidence that T22 is an important determinant of Hsp90 inhibitor sensitivity in yeast and we show that T22 phosphorylation status contributes to drug sensitivity in vivo.

The Hsp90 chaperone machinery: conformational dynamics and regulation by co-chaperones

Hsp90 is a dimeric molecular chaperone required for the activation and stabilization of numerous client proteins many of which are involved in essential cellular processes like signal transduction pathways. This activation process is regulated by ATP-induced large conformational changes, co-chaperones and posttranslational modifications. For some co-chaperones, a detailed picture on their structures and functions exists, for others their contributions to the Hsp90 system is still unclear. Recent progress on the conformational dynamics of Hsp90 and how co-chaperones affect the Hsp90 chaperone cycle significantly increased our understanding of the gearings of this complex molecular machinery. This article is part of a Special Issue entitled: Heat Shock Protein 90 (Hsp90).

Response of larval barnacle proteome to CO(2)-driven seawater acidification

The majority of benthic marine invertebrates have a complex life cycle, during which the pelagic larvae select a suitable substrate, attach to it, and then metamorphose into benthic adults. Anthropogenic ocean acidification (OA) is postulated to affect larval metamorphic success through an altered protein expression pattern (proteome structure) and post-translational modifications. To test this hypothesis, larvae of an economically and ecologically important barnacle species Balanus amphitrite, were cultured from nauplius to the cyprid stage in the present (control) and in the projected elevated concentrations of CO(2) for the year 2100 (the OA treatment). Cyprid response to OA was analyzed at the total proteome level as well as two protein post-translational modification (phosphorylation and glycosylation) levels using a 2-DE based proteomic approach. The cyprid proteome showed OA-driven changes. Proteins that were differentially up or down regulated by OA come from three major groups, namely those related to energy-metabolism, respiration, and molecular chaperones, illustrating a potential strategy that the barnacle larvae may employ to tolerate OA stress. The differentially expressed proteins were tentatively identified as OA-responsive, effectively creating unique protein expression signatures for OA scenario of 2100. This study showed the promise of using a sentinel and non-model species to examine the impact of OA at the proteome level.

Reproducible enrichment of extracellular heat shock proteins from blood serum using monomeric avidin

Extracellular heat shock proteins (eHsps) in blood circulation have been associated with various diseases, including cancer. However, the lack of methods to enrich eHsps from serum samples has hampered the characterization of eHsps. This Letter presents our serendipitous finding that the monomeric avidin resin can serve as an affinity resin to enrich eHsps from blood serum. Biochemical mechanism of this eHsp enrichment as well as implications in biomarker discovery is discussed.

Pnck induces ligand-independent EGFR degradation by probable perturbation of the Hsp90 chaperone complex

We have recently described a novel role for pregnancy-upregulated non-ubiquitous calmodulin kinase (Pnck) in the induction of ligand-independent epidermal growth factor receptor (EGFR) degradation (Deb TB, Coticchia CM, Barndt R, Zuo H, Dickson RB, and Johnson MD. Am J Physiol Cell Physiol 295: C365-C377, 2008). In the current communication, we explore the probable mechanism by which Pnck induces ligand-independent EGFR degradation. Pnck-induced EGFR degradation is calcium/calmodulin independent and is regulated by cell density, with the highest EGFR degradation observed at low cell density. Pnck is a novel heat shock protein 90 (Hsp90) client protein that can be co-immunoprecipitated with Hsp90. Treatment of Pnck-overexpressing cells with the pharmacologic Hsp90 inhibitor geldanamycin results in enhanced EGFR degradation, and destruction of Pnck. In cells in which Pnck is inducing EGFR degradation, we observed that Hsp90 exhibits reduced electrophoretic mobility, and through mass spectrometric analysis of immunopurified Hsp90 protein we demonstrated enhanced phosphorylation at threonine 89 and 616 (in both Hsp90-α and -β) and serine 391 (in Hsp90-α). Kinase-active Pnck protein is degraded by the proteasome, concurrent with EGFR degradation. A Pnck mutant (T171A) protein with suppressed kinase activity induced EGFR degradation to essentially the same level as wild-type (WT) Pnck, suggesting that Pnck kinase activity is not required for the induction of EGFR degradation. Although EGFR is degraded, overexpression of WT Pnck paradoxically promoted cellular proliferation, whereas cells expressing mutant Pnck (T171A) were growth inhibited. WT Pnck promoted S to G(2) transition, but cells expressing the mutant exhibited higher residency time in S phase. Basal MAP kinase activity was inhibited by WT Pnck but not by mutant T171A Pnck protein. Cyclin-dependent kinase (Cdk) inhibitor p21/Cip-1/Waf-1 was transcriptionally suppressed downstream to MAP kinase inhibition by WT Pnck, but not the mutant protein. Collectively, these data suggest that 1) Pnck induces ligand-independent EGFR degradation most likely through perturbation of Hsp90 chaperone activity due to Hsp90 phosphorylation, 2) EGFR degradation is coupled to proteasomal degradation of Pnck, and 3) modulation of basal MAP kinase activity, p21/Cip-1/Waf-1 expression, and cellular growth by Pnck is independent of Pnck-induced ligand-independent EGFR degradation.

Detecting HSP90 phosphorylation

Heat-shock protein 90 (HSP90) is an essential molecular chaperone in eukaryotes. It is important for chaperoning proteins that are important determinants of multistep carcinogenesis. HSP90's ATPase activity is associated with its chaperone function. Co-chaperones as well as posttranslational modifications (phosphorylation, acetylation, and S-nitrosylation) are important for regulating its ATPase activity. Yeast can be used to express and purify HSP90 and also detect its phosphorylation by pan-phosphoserine or phosphothreonine antibodies.

Targeting the dynamic HSP90 complex in cancer

The molecular chaperone heat shock protein 90 (HSP90) has been used by cancer cells to facilitate the function of numerous oncoproteins, and it can be argued that cancer cells are 'addicted' to HSP90. However, although recent reports of the early clinical efficacy of HSP90 inhibitors are encouraging, the optimal use of HSP90-targeted therapeutics will depend on understanding the complexity of HSP90 regulation and the degree to which HSP90 participates in both neoplastic and normal cellular physiology.

Hsp90 phosphorylation, Wee1 and the cell cycle

Heat Shock Protein 90 (Hsp90) is an essential molecular chaperone in eukaryotic cells, and it maintains the functional conformation of a subset of proteins that are typically key components of multiple regulatory and signaling networks mediating cancer cell proliferation, survival, and metastasis. It is possible to selectively inhibit Hsp90 using natural products such as geldanamycin (GA) or radicicol (RD), which have served as prototypes for development of synthetic Hsp90 inhibitors. These compounds bind within the ADP/ATP-binding site of the Hsp90 N-terminal domain to inhibit its ATPase activity. As numerous N-terminal domain inhibitors are currently undergoing extensive clinical evaluation, it is important to understand the factors that may modulate in vivo susceptibility to these drugs. We recently reported that Wee1Swe1-mediated, cell cycle-dependent, tyrosine phosphorylation of Hsp90 affects GA binding and impacts cancer cell sensitivity to Hsp90 inhibition. This phosphorylation also affects Hsp90 ATPase activity and its ability to chaperone a selected group of clients, comprised primarily of protein kinases. Wee1 regulates the G2/M transition. Here we present additional data demonstrating that tyrosine phosphorylation of Hsp90 by Wee1Swe1 is important for Wee1Swe1 association with Hsp90 and for Wee1Swe1 stability. Yeast expressing non-phosphorylatable yHsp90-Y24F, like swe1∆ yeast, undergo premature nuclear division that is insensitive to G2/M checkpoint arrest. These findings demonstrate the importance of Hsp90 phosphorylation for proper cell cycle regulation.

Swe1Wee1-dependent tyrosine phosphorylation of Hsp90 regulates distinct facets of chaperone function

Saccharomyces WEE1 (Swe1), the only "true" tyrosine kinase in budding yeast, is an Hsp90 client protein. Here we show that Swe1(Wee1) phosphorylates a conserved tyrosine residue (Y24 in yeast Hsp90 and Y38 in human Hsp90alpha) in the N domain of Hsp90. Phosphorylation is cell-cycle associated and modulates the ability of Hsp90 to chaperone a selected clientele, including v-Src and several other kinases. Nonphosphorylatable mutants have normal ATPase activity, support yeast viability, and productively chaperone the Hsp90 client glucocorticoid receptor. Deletion of SWE1 in yeast increases Hsp90 binding to its inhibitor geldanamycin, and pharmacologic inhibition/silencing of Wee1 sensitizes cancer cells to Hsp90 inhibitor-induced apoptosis. These findings demonstrate that Hsp90 chaperoning of distinct client proteins is differentially regulated by specific posttranslational modification of a unique subcellular pool of the chaperone, and they provide a strategy to increase the cellular potency of Hsp90 inhibitors.

The complex dance of the molecular chaperone Hsp90

Hsp90 chaperone function requires traversal of a nucleotide-dependent conformational cycle, but the slow and variable rate of Hsp90-mediated ATP hydrolysis is difficult to envision as a determinant of conformational change. A recent study solves this dilemma by showing that Hsp90 samples multiple conformational states in the absence of nucleotides, which serve to influence, but not direct, the cycle. The conformational program of Hsp90 is conserved from bacteria to humans, although the population dynamics are species specific.

Molecular chaperones in pathogen virulence: emerging new targets for therapy

Infectious organisms have to cope with demanding and rapidly changing environments during establishment in the host. This is particularly relevant for pathogens that utilize different hosts to complete their life cycle. In addition to homeotic environmental challenges, other stressful factors, such as oxidative bursts, are often triggered in response to infection. It is not surprising that many successful pathogens have developed robust chaperone systems to conquer the stressful environments in the host. In addition to discussing ingenious ways by which pathogens have utilized chaperones, the potential of exploiting pathogen chaperones as drug targets is also discussed.