c-Abl Mediated Tyrosine Phosphorylation of Aha1 Activates Its Co-chaperone Function in Cancer Cells

Ordered ATP hydrolysis in the Hsp90 chaperone is regulated by Aha1 and a conserved post-translational modification

Hsp90 is a dimeric molecular chaperone that is important for the folding, stabilization, activation, and maturation of hundreds of protein substrates called "clients" in cells. Dozens of co-chaperones and hundreds of post-translational modifications (PTMs) regulate the ATP-dependent client activation cycle. The Aha1 co-chaperone is the most potent stimulator of the ATPase cycle of Hsp90 and phosphorylation of threonine 22 in Hsp90 can regulate the recruitment of Aha1 in cells. We report here that phosphorylation of threonine 22 regulates specific aspects of Aha1 function after recruitment occurs. The phosphomimetic substitution, T22E, neutralizes the action of the Aha1 NxNNWHW motif. Moreover, this substitution can exert this effect from only one protomer of the Hsp90 dimer. This work sheds light on how asymmetric modifications in the Hsp90 dimer can functionalize individual protomers and fine-tune the Hsp90 cycle.

The role of Aha1 in cancer and neurodegeneration

The 90 kDa Heat shock protein (Hsp90) is a family of ubiquitously expressed molecular chaperones responsible for the stabilization and maturation of >400 client proteins. Hsp90 exhibits dramatic conformational changes to accomplish this, which are regulated by partner proteins termed co-chaperones. One of these co-chaperones is called the activator or Hsp90 ATPase activity homolog 1 (Aha1) and is the most potent accelerator of Hsp90 ATPase activity. In conditions where Aha1 levels are dysregulated including cystic fibrosis, cancer and neurodegeneration, Hsp90 mediated client maturation is disrupted. Accumulating evidence has demonstrated that many disease states exhibit large hetero-protein complexes with Hsp90 as the center. Many of these include Aha1, where increased Aha1 levels drive disease states forward. One strategy to block these effects is to design small molecule disruptors of the Hsp90/Aha1 complex. Studies have demonstrated that current Hsp90/Aha1 small molecule disruptors are effective in both models for cancer and neurodegeration.

Molecular chaperones: Guardians of tumor suppressor stability and function

The term 'tumor suppressor' describes a widely diverse set of genes that are generally involved in the suppression of metastasis, but lead to tumorigenesis upon loss-of-function mutations. Despite the protein products of tumor suppressors exhibiting drastically different structures and functions, many share a common regulatory mechanism-they are molecular chaperone 'clients'. Clients of molecular chaperones depend on an intracellular network of chaperones and co-chaperones to maintain stability. Mutations of tumor suppressors that disrupt proper chaperoning prevent the cell from maintaining sufficient protein levels for physiological function. This review discusses the role of the molecular chaperones Hsp70 and Hsp90 in maintaining the stability and functional integrity of tumor suppressors. The contribution of cochaperones prefoldin, HOP, Aha1, p23, FNIP1/2 and Tsc1 as well as the chaperonin TRiC to tumor suppressor stability is also discussed. Genes implicated in renal cell carcinoma development-VHL, TSC1/2, and FLCN-will be used as examples to explore this concept, as well as how pathogenic mutations of tumor suppressors cause disease by disrupting protein chaperoning, maturation, and function.

Structural and functional complexity of HSP90 in cellular homeostasis and disease

Heat shock protein 90 (HSP90) is a chaperone with vital roles in regulating proteostasis, long recognized for its function in protein folding and maturation. A view is emerging that identifies HSP90 not as one protein that is structurally and functionally homogeneous but, rather, as a protein that is shaped by its environment. In this Review, we discuss evidence of multiple structural forms of HSP90 in health and disease, including homo-oligomers and hetero-oligomers, also termed epichaperomes, and examine the impact of stress, post-translational modifications and co-chaperones on their formation. We describe how these variations influence context-dependent functions of HSP90 as well as its interaction with other chaperones, co-chaperones and proteins, and how this structural complexity of HSP90 impacts and is impacted by its interaction with small molecule modulators. We close by discussing recent developments regarding the use of HSP90 inhibitors in cancer and how our new appreciation of the structural and functional heterogeneity of HSP90 invites a re-evaluation of how we discover and implement HSP90 therapeutics for disease treatment.

Saccharomyces cerevisiae as a tool for deciphering Hsp90 molecular chaperone function

Yeast is a valuable model organism for their ease of genetic manipulation, rapid growth rate, and relative similarity to higher eukaryotes. Historically, Saccharomyces cerevisiae has played a major role in discovering the function of complex proteins and pathways that are important for human health and disease. Heat shock protein 90 (Hsp90) is a molecular chaperone responsible for the stabilization and activation of hundreds of integral members of the cellular signaling network. Much important structural and functional work, including many seminal discoveries in Hsp90 biology are the direct result of work carried out in S. cerevisiae. Here, we have provided a brief overview of the S. cerevisiae model system and described how this eukaryotic model organism has been successfully applied to the study of Hsp90 chaperone function.

Lapatinib ditosylate rescues motor deficits in rotenone-intoxicated rats: Potential repurposing of anti-cancer drug as a disease-modifying agent in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor deficits induced by dopaminergic neuronal death in the substantia nigra (SN). Finding a successful neuroprotective therapy is still challenging despite improved knowledge of the etiology of PD and a variety of medications intended to reduce symptoms. Lapatinib (LAP), an FDA-approved anti-cancer medication, has been stated to exert its effect through the modulation of oxidative stress. Furthermore, recent studies display the neuroprotective effects of LAP in epilepsy, encephalomyelitis, and Alzheimer's disease in rodent models through the modulation of oxidative stress and ferroptosis. Nevertheless, it is questionable whether LAP exerts neuroprotective effects in PD. In the current study, administration of 100 mg/kg LAP in rotenone-treated rats for 21 days ameliorates motor impairment, debilitated histopathological alterations, and revived dopaminergic neurons by increasing tyrosine hydroxylase (TH) expression in SN, along with increased dopamine level. LAP remarkably restored the antioxidant defense mechanism system, GPX4/GSH/NRF2 axis, inhibiting oxidative markers, including iron, TfR1, PTGS2, and 4-HNE, along with suppression of p-EGFR/c-SRC/PKCβII/PLC-γ/ACSL-4 pathway. Moreover, LAP modulates HSP90/CDC37 chaperone complex, regulating many key pathological markers of PD, including LRRK2, c-ABL, and α-syn. It is concluded that LAP has neuroprotective effects in PD via modulation of many key parameters implicated in PD pathogenesis. Taken together, the current study offers insights into the potential repositioning of LAP as a disease-modifying drug in PD.

A Split Renilla Luciferase Complementation Assay for the Evaluation of Hsp90/Aha1 Complex Disruptors and Their Activity at the Aha1 C-Terminal Domain

Disruption of interactions between Hsp90 and the cochaperone protein, Aha1, has emerged as a therapeutic strategy to inhibit Aha1-driven cancer metastasis and tau aggregation in models of tauopathy. A combination of split Renilla luciferase assays was developed to screen and quantify the ability of small molecules to disrupt interactions between Hsp90 and both full length Aha1 protein (Aha1-FL) and the Aha1 C-terminal domain (Aha1-CTD). This luminescence-based approach was used to identify withaferin A and gedunin as disruptors of Hsp90/Aha1 interactions and provided insight into the binding regions for gambogic acid and gedunin on the Hsp90 homodimer. All compounds tested that disrupted Hsp90/Aha1-CTD interactions were found to disrupt interactions between Hsp90 and Aha1-FL, suggesting that interactions between Hsp90 and the Aha1-CTD play a key role in the stability of Hsp90/Aha1 complexes.

p23 and Aha1: Distinct Functions Promote Client Maturation

Hsp90 is a conserved molecular chaperone regulating the folding and activation of a diverse array of several hundreds of client proteins. The function of Hsp90 in client processing is fine-tuned by a cohort of co-chaperones that modulate client activation in a client-specific manner. They affect the Hsp90 ATPase activity and the recruitment of client proteins and can in addition affect chaperoning in an Hsp90-independent way. p23 and Aha1 are central Hsp90 co-chaperones that regulate Hsp90 in opposing ways. While p23 inhibits the Hsp90 ATPase and stabilizes a client-bound Hsp90 state, Aha1 accelerates ATP hydrolysis and competes with client binding to Hsp90. Even though both proteins have been intensively studied for decades, research of the last few years has revealed intriguing new aspects of these co-chaperones that expanded our perception of how they regulate client activation. Here, we review the progress in understanding p23 and Aha1 as promoters of client processing. We highlight the structures of Aha1 and p23, their interaction with Hsp90, and how their association with Hsp90 affects the conformational cycle of Hsp90 in the context of client maturation.

Impact of Co-chaperones and Posttranslational Modifications Toward Hsp90 Drug Sensitivity

Posttranslational modifications (PTMs) regulate myriad cellular processes by modulating protein function and protein-protein interaction. Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone whose activity is responsible for the stabilization and maturation of more than 300 client proteins. Hsp90 is a substrate for numerous PTMs, which have diverse effects on Hsp90 function. Interestingly, many Hsp90 clients are enzymes that catalyze PTM, demonstrating one of the several modes of regulation of Hsp90 activity. Approximately 25 co-chaperone regulatory proteins of Hsp90 impact structural rearrangements, ATP hydrolysis, and client interaction, representing a second layer of influence on Hsp90 activity. A growing body of literature has also established that PTM of these co-chaperones fine-tune their activity toward Hsp90; however, many of the identified PTMs remain uncharacterized. Given the critical role of Hsp90 in supporting signaling in cancer, clinical evaluation of Hsp90 inhibitors is an area of great interest. Interestingly, differential PTM and co-chaperone interaction have been shown to impact Hsp90 binding to its inhibitors. Therefore, understanding these layers of Hsp90 regulation will provide a more complete understanding of the chaperone code, facilitating the development of new biomarkers and combination therapies.

AHSA1 Promotes Proliferation and EMT by Regulating ERK/CALD1 Axis in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the major causes of cancer-related death worldwide. AHSA1 as a chaperone of HSP90 promotes the maturation, stability, and degradation of related cancer-promoting proteins. However, the regulatory mechanism and biological function of AHSA1 in HCC are largely unknown. Actually, we found that AHSA1 was significantly upregulated in HCC tissues and cell lines and was notably correlated with the poor clinical characteristics and prognosis of HCC patients in this study. Furthermore, both in vitro and in vivo, gain- and loss-of-function studies demonstrated that AHSA1 promoted the proliferation, invasion, metastasis, and epithelial-mesenchymal transition (EMT) of HCC. Moreover, the mechanistic study indicated that AHSA1 recruited ERK1/2 and promoted the phosphorylation and inactivation of CALD1, while ERK1/2 phosphorylation inhibitor SCH772984 reversed the role of AHSA1 in the proliferation and EMT of HCC. Furthermore, we demonstrated that the knockdown of CALD1 reversed the inhibition of proliferation and EMT by knocking AHSA1 in HCC. We also illustrated a new molecular mechanism associated with AHSA1 in HCC that was independent of HSP90 and MEK1/2. In summary, AHSA1 may play an oncogenic role in HCC by regulating ERK/CALD1 axis and may serve as a novel therapeutic target for HCC.

Emerging Link between Tsc1 and FNIP Co-Chaperones of Hsp90 and Cancer

Heat shock protein-90 (Hsp90) is an ATP-dependent molecular chaperone that is tightly regulated by a group of proteins termed co-chaperones. This chaperone system is essential for the stabilization and activation of many key signaling proteins. Recent identification of the co-chaperones FNIP1, FNIP2, and Tsc1 has broadened the spectrum of Hsp90 regulators. These new co-chaperones mediate the stability of critical tumor suppressors FLCN and Tsc2 as well as the various classes of Hsp90 kinase and non-kinase clients. Many early observations of the roles of FNIP1, FNIP2, and Tsc1 suggested functions independent of FLCN and Tsc2 but have not been fully delineated. Given the broad cellular impact of Hsp90-dependent signaling, it is possible to explain the cellular activities of these new co-chaperones by their influence on Hsp90 function. Here, we review the literature on FNIP1, FNIP2, and Tsc1 as co-chaperones and discuss the potential downstream impact of this regulation on normal cellular function and in human diseases.

Synthesis and Evaluation of Small Molecule Disruptors of the Aha1/Hsp90 Complex for the Reduction of Tau Aggregation

KU-177 was recently shown to disrupt interactions between Hsp90 and Aha1 in vitro. Subsequent studies in recombinant thioflavin T (ThT) assays demonstrated that KU-177 ablates Aha1-driven enhancement of Hsp90-dependent tau aggregation, which was confirmed by TEM. Using KU-177 as a lead compound, derivatives of KU-177 were synthesized and evaluated for their ability to disrupt Aha1/Hsp90 interactions and inhibit P301L tau aggregation. Preliminary structure-activity relationships were revealed, which led to the identification of a new lead compound that contains a cis-like amide bond. The new lead compounds retain the ability to disrupt Aha1/Hsp90 interactions in SH-SY5Y and SK-BR-3 cells without direct inhibition of Hsp90, providing a new scaffold for subsequent drug discovery efforts.

Heat shock proteins in cell signaling and cancer

Heat Shock Proteins (HSPs) and their co-chaperones have well-established roles in regulating proteostasis within the cell, the nature of which continues to emerge with further study. To date, HSPs have been shown to be integral to protein folding and re-folding, protein transport, avoidance of protein aggregation, and modulation of protein degradation. Many cell signaling events are mediated by the chemical modification of proteins post-translationally that can alter protein conformation and activity, although it is not yet known whether the changes in protein conformation induced by post-translational modifications (PTMs) are also dependent upon HSPs and their co-chaperones for subsequent protein re-folding. We discuss what is known regarding roles for HSPs and other molecular chaperones in cell signaling events with a focus on oncogenic signaling. We also propose a hypothesis by which Hsp70 and Hsp90 may co-operate to facilitate cell signaling events that may link PTMs with the cellular protein folding machinery.

Inhibitors of the Plasmodium falciparum Hsp90 towards Selective Antimalarial Drug Design: The Past, Present and Future

Malaria is still one of the major killer parasitic diseases in tropical settings, posing a public health threat. The development of antimalarial drug resistance is reversing the gains made in attempts to control the disease. The parasite leads a complex life cycle that has adapted to outwit almost all known antimalarial drugs to date, including the first line of treatment, artesunate. There is a high unmet need to develop new strategies and identify novel therapeutics to reverse antimalarial drug resistance development. Among the strategies, here we focus and discuss the merits of the development of antimalarials targeting the Heat shock protein 90 (Hsp90) due to the central role it plays in protein quality control.

AHA1 regulates cell migration and invasion via the EMT pathway in colorectal adenocarcinomas

The progression of colorectal cancer (CRC) has been well studied and understood with the development of molecular and genetic techniques. However, specific marker(s) that could be used to predict lymph node (LN) involvement, which is the most important prognostic factor for CRC, have not been identified so far. Our previous study, in which network analysis of LN(+) and LN(-) CRC gene expression was carried out with data obtained from the Cancer Genome Atlas, led to the identification of AHA1. AHA1 is a co-chaperone activator of the Hsp90 ATPase activity. However, the role of AHA1 expression in cancer cells is still unclear. To investigate how AHA1 expression regulates the cancer cell progression and/or metastasis of human CRC, the expression levels of AHA1 and Hsp90 were examined in 105 CRC tissue samples and compared with those in paired normal tissue. The RNA expression levels of AHA1 and Hsp90aa1, but not Hsp90ab, were significantly higher in cancer tissues than in adjacent paired normal tissues (p = 0.032 and p = 0.0002, respectively). In particular, AHA1, but not Hsp90aa1 and Hsp90ab, was closely associated with the TNM stage, LN stage, and tumor metastasis (p = 0.035, p = 0.012, and p = 0.0003, respectively). Moreover, the expression of AHA1 was not only higher in the CRC cell lines than in the normal colon fibroblast cell line but was also associated with the progression of these CRC cell lines. Overexpression of AHA1 in SW480 cells increased, whereas suppression of AHA1 expression in HCT116 cells reduced cell migration and invasion through the regulation of Snail, E-cadherin, pSRC, and pAKT, which are associated with EMT signaling. Taken together, our study suggests that AHA1 contributes to the metastatic advantage of human CRC.

Hsp90 Co-chaperones Form Plastic Genetic Networks Adapted to Client Maturation

Heat shock protein 90 (Hsp90) is a molecular chaperone regulating the activity of diverse client proteins together with a plethora of different co-chaperones. Whether these functionally cooperate has remained enigmatic. We analyze all double mutants of 11 Saccharomyces cerevisiae Hsp90 co-chaperones in vivo concerning effects on cell physiology and the activation of specific client proteins. We find that client activation is supported by a genetic network with weak epistasis between most co-chaperones and a few modules with strong genetic interactions. These include an epistatic module regulating protein translation and dedicated epistatic networks for specific clients. For kinases, the bridging of Hsp70 and Hsp90 by Sti1/Hop is essential for activation, whereas for steroid hormone receptors, an epistatic module regulating their dwell time on Hsp90 is crucial, highlighting the specific needs of different clients. Thus, the Hsp90 system is characterized by plastic co-chaperone networks fine-tuning the conformational processing in a client-specific manner.

Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Like Activity

Aha1 is the only co-chaperone known to strongly stimulate the ATPase activity of Hsp90. Meanwhile, besides the well-studied co-chaperone function, human Aha1 has also been demonstrated to exhibit chaperoning activity against stress-denatured proteins. To provide structural insights for a better understanding of Aha1's co-chaperone and chaperone-like activities, nuclear magnetic resonance (NMR) techniques were used to reveal the unique structure and internal dynamics features of full-length human Aha1. We then found that, in solution, both the two domains of Aha1 presented distinctive thermal stabilities and dynamics behaviors defined by their primary sequences and three-dimensional structures. The low thermal stability (melting temperature of Aha1^28-162: 54.45 °C) and the internal dynamics featured with slow motions on the µs-ms time scale were detected for Aha1's N-terminal domain (Aha1N). The aforementioned experimental results suggest that Aha1N is in an energy-unfavorable state, which would therefore thermostatically favor the interaction of Aha1N with its partner proteins such as Hsp90's middle domain. Differently from Aha1N, Aha1C (Aha1's C-terminal domain) exhibited enhanced thermal stability (melting temperature of Aha1^204-335: 72.41 °C) and the internal dynamics featured with intermediate motions on the ps-ns time scale. Aha1C's thermal and structural stabilities make it competent for the stabilization of the exposed hydrophobic groove of dimerized Hsp90's N-terminal domain. Of note, according to the NMR data and the thermal shift results, although the very N-terminal region (M1-W27) and the C-terminal relaxin-like factor (RLF) motif showed no tight contacts with the remaining parts of human Aha1, they were identified to play important roles in the recognition of intrinsically disordered pathological α-synuclein.

Hsp90 modulates human sperm capacitation via the Erk1/2 and p38 MAPK signaling pathways

BACKGROUND

Heat shock protein 90 (Hsp90) is a highly abundant eukaryotic molecular chaperone that plays important roles in client protein maturation, protein folding and degradation, and signal transduction. Previously, we found that both Hsp90 and its co-chaperone cell division cycle protein 37 (Cdc37) were expressed in human sperm. Hsp90 is known to be involved in human sperm capacitation via unknown underlying mechanism(s). As Cdc37 was a kinase-specific co-chaperone of Hsp90, Hsp90 may regulate human sperm capacitation via other kinases. It has been reported that two major mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase 1/2 (Erk1/2) and p38, are expressed in human sperm in the same locations as Hsp90 and Cdc37. Phosphorylated Erk1/2 has been shown to promote sperm hyperactivated motility and acrosome reaction, while phosphorylated p38 inhibits sperm motility. Therefore, in this study we explored whether Hsp90 modulates human sperm capacitation via the Erk1/2 and p38 MAPK signaling pathways.

METHODS

Human sperm was treated with the Hsp90-specific inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) during capacitation. Computer-assisted sperm analyzer (CASA) was used to detect sperm motility and hyperactivation. The sperm acrosome reaction was analyzed by using fluorescein isothiocyanate-conjugated Pisum sativum agglutinin (PSA-FITC) staining. The interactions between Hsp90, Cdc37, Erk1/2 and p38 were assessed using co-immunoprecipitation (Co-IP) experiments. Western blotting analysis was used to evaluate the levels of protein expression and phosphorylation.

RESULTS

Human sperm hyperactivation and acrosome reaction were inhibited by 17-AAG, suggesting that Hsp90 is involved in human sperm capacitation. In addition, Co-IP experiments revealed that 17-AAG reduced the interaction between Hsp90 and Cdc37, leading to the dissociation of Erk1/2 from the Hsp90-Cdc37 protein complex. Western blotting analysis revealed that levels of Erk1/2 and its phosphorylated form were subsequently decreased. Decreasing of Hsp90-Cdc37 complex also affected the interaction between Hsp90 and p38. Nevertheless, p38 dissociated from the Hsp90 protein complex and was activated by autophosphorylation.

CONCLUSIONS

Taken together, our findings indicate that Hsp90 is involved in human sperm hyperactivation and acrosome reaction. In particular, Hsp90 and its co-chaperone Cdc37 form a protein complex with Erk1/2 and p38 to regulate their kinase activity. These results suggest that Hsp90 regulates human sperm capacitation via the Erk1/2 and p38 MAPK signaling pathways.

The IMiD target CRBN determines HSP90 activity toward transmembrane proteins essential in multiple myeloma

The complex architecture of transmembrane proteins requires quality control (QC) of folding, membrane positioning, and trafficking as prerequisites for cellular homeostasis and intercellular communication. However, it has remained unclear whether transmembrane protein-specific QC hubs exist. Here we identify cereblon (CRBN), the target of immunomodulatory drugs (IMiDs), as a co-chaperone that specifically determines chaperone activity of HSP90 toward transmembrane proteins by means of counteracting AHA1. This function is abrogated by IMiDs, which disrupt the interaction of CRBN with HSP90. Among the multiple transmembrane protein clients of CRBN-AHA1-HSP90 revealed by cell surface proteomics, we identify the amino acid transporter LAT1/CD98hc as a determinant of IMiD activity in multiple myeloma (MM) and present an Anticalin-based CD98hc radiopharmaceutical for MM radio-theranostics. These data establish the CRBN-AHA1-HSP90 axis in the biogenesis of transmembrane proteins, link IMiD activity to tumor metabolism, and nominate CD98hc and LAT1 as attractive diagnostic and therapeutic targets in MM.

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CRBN functions as a transmembrane protein-specific co-chaperone of HSP90

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Disruption of CRBN-HSP90 interaction determines the anti-tumor activity of IMiDs

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The CD98hc/LAT1 complex is a central target of IMiDs in multiple myeloma

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CD98hc-Anticalin is a theranostic tool in multiple myeloma

AHA1 upregulates IDH1 and metabolic activity to promote growth and metastasis and predicts prognosis in osteosarcoma

Osteosarcoma (OS) is the most common primary malignant bone tumor in children and adolescents. Although activator of HSP90 ATPase activity 1 (AHA1) is reported to be a potential oncogene, its role in osteosarcoma progression remains largely unclear. Since metabolism reprogramming is involved in tumorigenesis and cancer metastasis, the relationship between AHA1 and cancer metabolism is unknown. In this study, we found that AHA1 is significantly overexpressed in osteosarcoma and related to the prognosis of osteosarcoma patients. AHA1 promotes the growth and metastasis of osteosarcoma both in vitro and in vivo. Mechanistically, AHA1 upregulates the metabolic activity to meet cellular bioenergetic needs in osteosarcoma. Notably, we identified that isocitrate dehydrogenase 1 (IDH1) is a novel client protein of Hsp90-AHA1. Furthermore, the IDH1 protein level was positively correlated with AHA1 in osteosarcoma. And IDH1 overexpression could partially reverse the effect of AHA1 knockdown on cell growth and migration of osteosarcoma. Moreover, high IDH1 level was also associated with poor prognosis of osteosarcoma patients. This study demonstrates that AHA1 positively regulates IDH1 and metabolic activity to promote osteosarcoma growth and metastasis, which provides novel prognostic biomarkers and promising therapeutic targets for osteosarcoma patients.

Human Hsp90 cochaperones: perspectives on tissue-specific expression and identification of cochaperones with similar in vivo functions

The Hsp90 molecular chaperone is required for the function of hundreds of different cellular proteins. Hsp90 and a cohort of interacting proteins called cochaperones interact with clients in an ATP-dependent cycle. Cochaperone functions include targeting clients to Hsp90, regulating Hsp90 ATPase activity, and/or promoting Hsp90 conformational changes as it progresses through the cycle. Over the last 20 years, the list of cochaperones identified in human cells has grown from the initial six identified in complex with steroid hormone receptors and protein kinases to about fifty different cochaperones found in Hsp90-client complexes. These cochaperones may be placed into three groups based on shared Hsp90 interaction domains. Available evidence indicates that cochaperones vary in client specificity, abundance, and tissue distribution. Many of the cochaperones have critical roles in regulation of cancer and neurodegeneration. A more limited set of cochaperones have cellular functions that may be limited to tissues such as muscle and testis. It is likely that a small set of cochaperones are part of the core Hsp90 machinery required for the folding of a wide range of clients. The presence of more selective cochaperones may allow greater control of Hsp90 activities across different tissues or during development.

The Hsp70-Hsp90 co-chaperone Hop/Stip1 shifts the proteostatic balance from folding towards degradation

Hop/Stip1/Sti1 is thought to be essential as a co-chaperone to facilitate substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Despite this proposed key function for protein folding and maturation, it is not essential in a number of eukaryotes and bacteria lack an ortholog. We set out to identify and to characterize its eukaryote-specific function. Human cell lines and the budding yeast with deletions of the Hop/Sti1 gene display reduced proteasome activity due to inefficient capping of the core particle with regulatory particles. Unexpectedly, knock-out cells are more proficient at preventing protein aggregation and at promoting protein refolding. Without the restraint by Hop, a more efficient folding activity of the prokaryote-like Hsp70-Hsp90 complex, which can also be demonstrated in vitro, compensates for the proteasomal defect and ensures the proteostatic equilibrium. Thus, cells may act on the level and/or activity of Hop to shift the proteostatic balance between folding and degradation.

Hop, also known as Stip1 or Sti1, facilitates substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Characterization of proteostasis-related pathways in STIP1 knock-out cell lines reveals that in eukaryotes Stip1 modulates the balance between protein folding and degradation.

Fumarate hydratase as a therapeutic target in renal cancer

INTRODUCTION

Renal cell carcinoma (RCC) is a heterogeneous group of cancers that can occur sporadically or as a manifestation of various inherited syndromes. Hereditary leiomyomatosis and renal cell carcinoma (HLRCC) is one such inherited syndrome that predisposes patients to HLRCC-associated RCC. These tumors are notoriously aggressive and often exhibit early metastases. HLRCC results from germline mutations in the FH gene, which encodes the citric acid cycle enzyme fumarate hydratase (FH). FH loss leads to alterations in oxidative carbon metabolism, necessitating a switch to aerobic glycolysis, as well as a pseudohypoxic response and consequent upregulation of various pro-survival pathways. Mutations in FH also alter tumor cell migratory potential, response to oxidative stress, and response to DNA damage.

AREAS COVERED

We review the mechanisms by which FH loss leads to HLRCC-associated RCC and how these mechanisms are being rationally targeted.

EXPERT OPINION

FH loss results in the activation of numerous salvage pathways for tumor cell survival in HLRCC-associated RCC. Tumor heterogeneity requires individualized characterization via next-generation sequencing, ultimately resulting in HLRCC-specific treatment regimens. As HLRCC-associated RCC represents a classic Warburg tumor, targeting aerobic glycolysis is particularly promising as a future therapeutic avenue.

Co-chaperones TIMP2 and AHA1 Competitively Regulate Extracellular HSP90:Client MMP2 Activity and Matrix Proteolysis

The extracellular molecular chaperone heat shock protein 90 (eHSP90) stabilizes protease client the matrix metalloproteinase 2 (MMP2), leading to tumor cell invasion. Although co-chaperones are critical modulators of intracellular HSP90:client function, how the eHSP90:MMP2 complex is regulated remains speculative. Here, we report that the tissue inhibitor of metalloproteinases-2 (TIMP2) is a stress-inducible extracellular co-chaperone that binds to eHSP90, increases eHSP90 binding to ATP, and inhibits its ATPase activity. In addition to disrupting the eHSP90:MMP2 complex and terminally inactivating MMP2, TIMP2 loads the client to eHSP90, keeping the protease in a transient inhibitory state. Secreted activating co-chaperone AHA1 displaces TIMP2 from the complex, providing a "reactivating" mechanism for MMP2. Gene knockout or blocking antibodies targeting TIMP2 and AHA1 released by HT1080 cancer cells modify their gelatinolytic activity. Our data suggest that TIMP2 and AHA1 co-chaperones function as a molecular switch that determines the inhibition and reactivation of the eHSP90 client protein MMP2.

Chemical Perturbation of Oncogenic Protein Folding: from the Prediction of Locally Unstable Structures to the Design of Disruptors of Hsp90-Client Interactions

Protein folding quality control in cells requires the activity of a class of proteins known as molecular chaperones. Heat shock protein-90 (Hsp90), a multidomain ATP driven molecular machine, is a prime representative of this family of proteins. Interactions between Hsp90, its co-chaperones, and client proteins have been shown to be important in facilitating the correct folding and activation of clients. Hsp90 levels and functions are elevated in tumor cells. Here, we computationally predict the regions on the native structures of clients c-Abl, c-Src, Cdk4, B-Raf and Glucocorticoid Receptor, that have the highest probability of undergoing local unfolding, despite being ordered in their native structures. Such regions represent potential ideal interaction points with the Hsp90-system. We synthesize mimics spanning these regions and confirm their interaction with partners of the Hsp90 complex (Hsp90, Cdc37 and Aha1) by Nuclear Magnetic Resonance (NMR). Designed mimics selectively disrupt the association of their respective clients with the Hsp90 machinery, leaving unrelated clients unperturbed and causing apoptosis in cancer cells. Overall, selective targeting of Hsp90 protein-protein interactions is achieved without causing indiscriminate degradation of all clients, setting the stage for the development of therapeutics based on specific chaperone:client perturbation.

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

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

Aha-type co-chaperones: the alpha or the omega of the Hsp90 ATPase cycle?

Heat shock protein 90 (Hsp90) is a dimeric molecular chaperone that plays an essential role in cellular homeostasis. It functions in the context of a structurally dynamic ATP-dependent cycle to promote conformational changes in its clientele to aid stability, maturation, and activation. The client activation cycle is tightly regulated by a cohort of co-chaperone proteins that display specific binding preferences for certain conformations of Hsp90, guiding Hsp90 through its functional ATPase cycle. Aha-type co-chaperones are well-known to robustly stimulate the ATPase activity of Hsp90 but other roles in regulating the functional cycle are being revealed. In this review, we summarize the work done on the Aha-type co-chaperones since the 1990s and highlight recent discoveries with respect to the complexity of Hsp90 cycle regulation.

A novel method for site-specific chemical SUMOylation: SUMOylation of Hsp90 modulates co-chaperone binding in vitro

SUMO is covalently attached to lysine side chains in target proteins by the action of a cascade of E1, E2, and E3 ligases. Unlike ubiquitin, SUMO does not target proteins for degradation but rather plays a regulatory role in activating target proteins or directing them to multiprotein complexes. Isolating SUMOylated proteins from native sources is challenging because of the low stoichiometry of SUMOylation that occurs for any given target protein in cells. Here we report a novel strategy to couple SUMO to the site of a target lysine for the purpose of in vitro study. Introduction of a single cysteine after the C terminal diglycine motif and a cysteine in place of a target lysine in a substrate protein allows for efficient and specific crosslinking of SUMO using a homo-bifunctional maleimide crosslinker. We demonstrate that SUMO can be crosslinked in this manner to amino acid position 178 in the dimeric molecular chaperone, Hsp90. Chemically SUMOylated Hsp90 has very similar ATPase activity compared to unmodified Hsp90 but displays preferential co-chaperone binding in vivo. Our novel strategy can easily be applied to other SUMOylated or ubiquitinated target protein in vitro.

Therapeutic Potential of the Hsp90/Cdc37 Interaction in Neurodegenerative Diseases

Alzheimer's, Huntington's, and Parkinson's are devastating neurodegenerative diseases that are prevalent in the aging population. Patient care costs continue to rise each year, because there is currently no cure or disease modifying treatments for these diseases. Numerous efforts have been made to understand the molecular interactions governing the disease development. These efforts have revealed that the phosphorylation of proteins by kinases may play a critical role in the aggregation of disease-associated proteins, which is thought to contribute to neurodegeneration. Interestingly, a molecular chaperone complex consisting of the 90 kDa heat shock protein (Hsp90) and Cell Division Cycle 37 (Cdc37) has been shown to regulate the maturation of many of these kinases as well as regulate some disease-associated proteins directly. Thus, the Hsp90/Cdc37 complex may represent a potential drug target for regulating proteins linked to neurodegenerative diseases, through both direct and indirect interactions. Herein, we discuss the broad understanding of many Hsp90/Cdc37 pathways and how this protein complex may be a useful target to regulate the progression of neurodegenerative disease.

Rapid deacetylation of yeast Hsp70 mediates the cellular response to heat stress

Hsp70 is a highly conserved molecular chaperone critical for the folding of new and denatured proteins. While traditional models state that cells respond to stress by upregulating inducible HSPs, this response is relatively slow and is limited by transcriptional and translational machinery. Recent studies have identified a number of post-translational modifications (PTMs) on Hsp70 that act to fine-tune its function. We utilized mass spectrometry to determine whether yeast Hsp70 (Ssa1) is differentially modified upon heat shock. We uncovered four lysine residues on Ssa1, K86, K185, K354 and K562 that are deacetylated in response to heat shock. Mutation of these sites cause a substantial remodeling of the Hsp70 interaction network of co-chaperone partners and client proteins while preserving essential chaperone function. Acetylation/deacetylation at these residues alter expression of other heat-shock induced chaperones as well as directly influencing Hsf1 activity. Taken together our data suggest that cells may have the ability to respond to heat stress quickly though Hsp70 deacetylation, followed by a slower, more traditional transcriptional response.

Oligomerization of Hsp70: Current Perspectives on Regulation and Function

The Hsp70 molecular chaperone in conjunction with Hsp90 and a suite of helper co-chaperones are required for the folding and subsequent refolding of a large proportion of the proteome. These proteins are critical for cell viability and play major roles in diseases of proteostasis which include neurodegenerative diseases and cancer. As a consequence, a large scientific effort has gone into understanding how chaperones such as Hsp70 function at the in vitro and in vivo level. Although many chaperones require constitutive self-interaction (dimerization and oligomerization) to function, Hsp70 has been thought to exist as a monomer, especially in eukaryotic cells. Recent studies have demonstrated that both bacterial and mammalian Hsp70 can exist as a dynamic pool of monomers, dimer, and oligomers. In this mini-review, we discuss the mechanisms and roles of Hsp70 oligomerization in Hsp70 function, as well as thoughts on how this integrates into well-established ideas of Hsp70 regulation.

Structure, Function, and Regulation of the Hsp90 Machinery

Heat shock protein 90 (Hsp90) is a molecular chaperone involved in the maturation of a plethora of substrates ("clients"), including protein kinases, transcription factors, and E3 ubiquitin ligases, positioning Hsp90 as a central regulator of cellular proteostasis. Hsp90 undergoes large conformational changes during its ATPase cycle. The processing of clients by cytosolic Hsp90 is assisted by a cohort of cochaperones that affect client recruitment, Hsp90 ATPase function or conformational rearrangements in Hsp90. Because of the importance of Hsp90 in regulating central cellular pathways, strategies for the pharmacological inhibition of the Hsp90 machinery in diseases such as cancer and neurodegeneration are being developed. In this review, we summarize recent structural and mechanistic progress in defining the function of organelle-specific and cytosolic Hsp90, including the impact of individual cochaperones on the maturation of specific clients and complexes with clients as well as ways of exploiting Hsp90 as a drug target.

Not quite the SSAme: unique roles for the yeast cytosolic Hsp70s

The Heat Shock Protein 70s (Hsp70s) are an essential family of proteins involved in folding of new proteins and triaging of damaged proteins for proteasomal-mediated degradation. They are highly conserved in all organisms, with each organism possessing multiple highly similar Hsp70 variants (isoforms). These isoforms have been previously thought to be identical in function differing only in their spatio-temporal expression pattern. The model organism Saccharomyces cerevisiae (baker's yeast) expresses four Hsp70 isoforms Ssa1, 2, 3 and 4. Here, we review recent findings that suggest that despite their similarity, Ssa isoforms may have unique cellular functions.

Novel insights into molecular chaperone regulation of ribonucleotide reductase

The molecular chaperones Hsp70 and Hsp90 bind and fold a significant proportion of the proteome. They are responsible for the activity and stability of many disease-related proteins including those in cancer. Substantial effort has been devoted to developing a range of chaperone inhibitors for clinical use. Recent studies have identified the oncogenic ribonucleotide reductase (RNR) complex as an interactor of chaperones. While several generations of RNR inhibitor have been developed for use in cancer patients, many of these produce severe side effects such as nausea, vomiting and hair loss. Development of more potent, less patient-toxic anti-RNR strategies would be highly desirable. Inhibition of chaperones and associated co-chaperone molecules in both cancer and model organisms such as budding yeast result in the destabilization of RNR subunits and a corresponding sensitization to RNR inhibitors. Going forward, this may form part of a novel strategy to target cancer cells that are resistant to standard RNR inhibitors.

Adapting to stress - chaperome networks in cancer

In this Opinion article, we aim to address how cells adapt to stress and the repercussions chronic stress has on cellular function. We consider acute and chronic stress-induced changes at the cellular level, with a focus on a regulator of cellular stress, the chaperome, which is a protein assembly that encompasses molecular chaperones, co-chaperones and other co-factors. We discuss how the chaperome takes on distinct functions under conditions of stress that are executed in ways that differ from the one-on-one cyclic, dynamic functions exhibited by distinct molecular chaperones. We argue that through the formation of multimeric stable chaperome complexes, a state of chaperome hyperconnectivity, or networking, is gained. The role of these chaperome networks is to act as multimolecular scaffolds, a particularly important function in cancer, where they increase the efficacy and functional diversity of several cellular processes. We predict that these concepts will change how we develop and implement drugs targeting the chaperome to treat cancer.

Protein Tyrosine Phosphatase Receptor Type J (PTPRJ) Regulates Retinal Axonal Projections by Inhibiting Eph and Abl Kinases in Mice

Eph receptors play pivotal roles in the axon guidance of retinal ganglion cells (RGCs) at the optic chiasm and the establishment of the topographic retinocollicular map. We previously demonstrated that protein tyrosine phosphatase receptor type O (PTPRO) is specifically involved in the control of retinotectal projections in chicks through the dephosphorylation of EphA and EphB receptors. We subsequently revealed that all the mouse R3 subfamily members (PTPRB, PTPRH, PTPRJ, and PTPRO) of the receptor protein tyrosine phosphatase (RPTP) family inhibited Eph receptors as their substrates in cultured mammalian cells. We herein investigated the functional roles of R3 RPTPs in the projection of mouse retinal axon of both sexes. Ptpro and Ptprj were expressed in mouse RGCs; however, Ptprj expression levels were markedly higher than those of Ptpro Consistent with their expression levels, Eph receptor activity was significantly enhanced in Ptprj-knock-out (Ptprj-KO) retinas. In Ptprj-KO and Ptprj/Ptpro-double-KO (DKO) mice, the number of retinal axons that projected ipsilaterally or to the contralateral eye was significantly increased. Furthermore, retinal axons in Ptprj-KO and DKO mice formed anteriorly shifted ectopic terminal zones in the superior colliculus (SC). We found that c-Abl (Abelson tyrosine kinase) was downstream of ephrin-Eph signaling for the repulsion of retinal axons at the optic chiasm and in the SC. c-Abl was identified as a novel substrate for PTPRJ and PTPRO, and the phosphorylation of c-Abl was upregulated in Ptprj-KO and DKO retinas. Thus, PTPRJ regulates retinocollicular projections in mice by controlling the activity of Eph and c-Abl kinases.SIGNIFICANCE STATEMENT Correct retinocollicular projection is a prerequisite for proper vision. Eph receptors have been implicated in retinal axon guidance at the optic chiasm and the establishment of the topographic retinocollicular map. We herein demonstrated that protein tyrosine phosphatase receptor type J (PTPRJ) regulated retinal axonal projections by controlling Eph activities. The retinas of Ptprj-knock-out (KO) and Ptpro/Ptprj double-KO mice exhibited significantly enhanced Eph activities over those in wild-type mice, and their axons showed defects in pathfinding at the chiasm and retinocollicular topographic map formation. We also revealed that c-Abl (Abelson tyrosine kinase) downstream of Eph receptors was regulated by PTPRJ. These results indicate that the regulation of the ephrin-Eph-c-Abl axis by PTPRJ plays pivotal roles in the proper central projection of retinal axons during development.

Methods to validate Hsp90 inhibitor specificity, to identify off-target effects, and to rethink approaches for further clinical development

The molecular chaperone Hsp90 is one component of a highly complex and interactive cellular proteostasis network (PN) that participates in protein folding, directs misfolded and damaged proteins for destruction, and participates in regulating cellular transcriptional responses to environmental stress, thus promoting cell and organismal survival. Over the last 20 years, it has become clear that various disease states, including cancer, neurodegeneration, metabolic disorders, and infection by diverse microbes, impact the PN. Among PN components, Hsp90 was among the first to be pharmacologically targeted with small molecules. While the number of Hsp90 inhibitors described in the literature has dramatically increased since the first such small molecule was described in 1994, it has become increasingly apparent that not all of these agents have been sufficiently validated for specificity, mechanism of action, and lack of off-target effects. Given the less than expected activity of Hsp90 inhibitors in cancer-related human clinical trials, a re-evaluation of potentially confounding off-target effects, as well as confidence in target specificity and mechanism of action, is warranted. In this commentary, we provide feasible approaches to achieve these goals and we discuss additional considerations to improve the clinical efficacy of Hsp90 inhibitors in treating cancer and other diseases.

Endogenous epitope tagging of heat shock protein 70 isoform Hsc70 using CRISPR/Cas9

Heat shock protein 70 (Hsp70) is an evolutionarily well-conserved molecular chaperone involved in several cellular processes such as folding of proteins, modulating protein-protein interactions, and transport of proteins across the membrane. Binding partners of Hsp70 (known as "clients") are identified on an individual basis as researchers discover their particular protein of interest binds to Hsp70. A full complement of Hsp70 interactors under multiple stress conditions remains to be determined. A promising approach to characterizing the Hsp70 "interactome" is the use of protein epitope tagging and then affinity purification followed by mass spectrometry (AP-MS/MS). AP-MS analysis is a widely used method to decipher protein-protein interaction networks and identifying protein functions. Conventionally, the proteins are overexpressed ectopically which interferes with protein complex stoichiometry, skewing AP-MS/MS data. In an attempt to solve this issue, we used CRISPR/Cas9-mediated gene editing to integrate a tandem-affinity (TAP) epitope tag into the genomic locus of HSC70. This system offers several benefits over existing expression systems including native expression, no requirement for selection, and homogeneity between cells. This cell line, freely available to chaperone researchers, will aid in small and large-scale protein interaction studies as well as the study of biochemical activities and structure-function relationships of the Hsc70 protein.

Extracellular Phosphorylation of TIMP-2 by Secreted c-Src Tyrosine Kinase Controls MMP-2 Activity

The tissue inhibitor of metalloproteinases 2 (TIMP-2) is a specific endogenous inhibitor of matrix metalloproteinase 2 (MMP-2), which is a key enzyme that degrades the extracellular matrix and promotes tumor cell invasion. Although the TIMP-2:MMP-2 complex controls proteolysis, the signaling mechanism by which the two proteins associate in the extracellular space remains unidentified. Here we report that TIMP-2 is phosphorylated outside the cell by secreted c-Src tyrosine kinase. As a consequence, phosphorylation at Y90 significantly enhances TIMP-2 potency as an MMP-2 inhibitor and weakens the catalytic action of the active enzyme. TIMP-2 phosphorylation also appears to be essential for its interaction with the latent enzyme proMMP-2 in vivo. Absence of the kinase or non-phosphorylatable Y90 abolishes TIMP-2 binding to the latent enzyme, ultimately hampering proMMP-2 activation. Together, TIMP-2 phosphorylation by secreted c-Src represents a critical extracellular regulatory mechanism that controls the proteolytic function of MMP-2.

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c-Src tyrosine kinase phosphorylates TIMP-2

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Secreted c-Src phosphorylates TIMP-2 extracellularly

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TIMP-2 Y90 phosphorylation promotes extracellular interaction with proMMP-2

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Tyrosine phosphorylation of TIMP-2 regulates proMMP-2 processing and MMP-2 activity

Phosphorylation induced cochaperone unfolding promotes kinase recruitment and client class-specific Hsp90 phosphorylation

During the Hsp90-mediated chaperoning of protein kinases, the core components of the machinery, Hsp90 and the cochaperone Cdc37, recycle between different phosphorylation states that regulate progression of the chaperone cycle. We show that Cdc37 phosphorylation at Y298 results in partial unfolding of the C-terminal domain and the population of folding intermediates. Unfolding facilitates Hsp90 phosphorylation at Y197 by unmasking a phosphopeptide sequence, which serves as a docking site to recruit non-receptor tyrosine kinases to the chaperone complex via their SH2 domains. In turn, Hsp90 phosphorylation at Y197 specifically regulates its interaction with Cdc37 and thus affects the chaperoning of only protein kinase clients. In summary, we find that by providing client class specificity, Hsp90 cochaperones such as Cdc37 do not merely assist in client recruitment but also shape the post-translational modification landscape of Hsp90 in a client class-specific manner.

The Hsp90 chaperone cycle is influenced by multiple phosphorylation events but their regulatory functions are poorly understood. Here, the authors show that phosphorylation and unfolding of cochaperone Cdc37 tailors the Hsp90 chaperone cycle by recruiting kinases that promote distinct phosphorylation patterns.

Evidence for Hsp90 Co-chaperones in Regulating Hsp90 Function and Promoting Client Protein Folding

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes such as protein translation, translocation, and disassembly of protein complexes. Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facilitates folding of kinase and non-kinase clients

The tumor suppressors Tsc1 and Tsc2 form the tuberous sclerosis complex (TSC), a regulator of mTOR activity. Tsc1 stabilizes Tsc2; however, the precise mechanism involved remains elusive. The molecular chaperone heat‐shock protein 90 (Hsp90) is an essential component of the cellular homeostatic machinery in eukaryotes. Here, we show that Tsc1 is a new co‐chaperone for Hsp90 that inhibits its ATPase activity. The C‐terminal domain of Tsc1 (998–1,164 aa) forms a homodimer and binds to both protomers of the Hsp90 middle domain. This ensures inhibition of both subunits of the Hsp90 dimer and prevents the activating co‐chaperone Aha1 from binding the middle domain of Hsp90. Conversely, phosphorylation of Aha1‐Y223 increases its affinity for Hsp90 and displaces Tsc1, thereby providing a mechanism for equilibrium between binding of these two co‐chaperones to Hsp90. Our findings establish an active role for Tsc1 as a facilitator of Hsp90‐mediated folding of kinase and non‐kinase clients—including Tsc2—thereby preventing their ubiquitination and proteasomal degradation.

A chemical compound inhibiting the Aha1-Hsp90 chaperone complex

The eukaryotic Hsp90 chaperone machinery comprises many co-chaperones and regulates the conformation of hundreds of cytosolic client proteins. Therefore, it is not surprising that the Hsp90 machinery has become an attractive therapeutic target for diseases such as cancer. The compounds used so far to target this machinery affect the entire Hsp90 system. However, it would be desirable to achieve a more selective targeting of Hsp90-co-chaperone complexes. To test this concept, in this-proof-of-principle study, we screened for modulators of the interaction between Hsp90 and its co-chaperone Aha1, which accelerates the ATPase activity of Hsp90. A FRET-based assay that monitored Aha1 binding to Hsp90 enabled identification of several chemical compounds modulating the effect of Aha1 on Hsp90 activity. We found that one of these inhibitors can abrogate the Aha1-induced ATPase stimulation of Hsp90 without significantly affecting Hsp90 ATPase activity in the absence of Aha1. NMR spectroscopy revealed that this inhibitory compound binds the N-terminal domain of Hsp90 close to its ATP-binding site and overlapping with a transient Aha1-interaction site. We also noted that this inhibitor does not dissociate the Aha1-Hsp90 complex but prevents the specific interaction with the N-terminal domain of Hsp90 required for catalysis. In consequence, the inhibitor affected the activation and processing of Hsp90-Aha1-dependent client proteins in vivo We conclude that it is possible to abrogate a specific co-chaperone function of Hsp90 without inhibiting the entire Hsp90 machinery. This concept may also hold true for other co-chaperones of Hsp90.

Mechanisms of Hsp90 regulation

Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in the activation of disparate client proteins. This implicates Hsp90 in diverse biological processes that require a variety of co-ordinated regulatory mechanisms to control its activity. Perhaps the most important regulator is heat shock factor 1 (HSF1), which is primarily responsible for upregulating Hsp90 by binding heat shock elements (HSEs) within Hsp90 promoters. HSF1 is itself subject to a variety of regulatory processes and can directly respond to stress. HSF1 also interacts with a variety of transcriptional factors that help integrate biological signals, which in turn regulate Hsp90 appropriately. Because of the diverse clientele of Hsp90 a whole variety of co-chaperones also regulate its activity and some are directly responsible for delivery of client protein. Consequently, co-chaperones themselves, like Hsp90, are also subject to regulatory mechanisms such as post translational modification. This review, looks at the many different levels by which Hsp90 activity is ultimately regulated.

Regulatory Mechanisms of Hsp90

The ability of Hsp90 to activate a disparate clientele implicates this chaperone in diverse biological processes. To accommodate such varied roles, Hsp90 requires a variety of regulatory mechanisms that are coordinated in order to modulate its activity appropriately. Amongst these, the master-regulator heat shock factor 1 (HSF1) is critically important in upregulating Hsp90 during stress, but is also responsible, through interaction with specific transcription factors (such as STAT1 and Strap/p300) for the integration of a variety of biological signals that ultimately modulate Hsp90 expression. Additionally, transcription factors, such as STAT1, STAT3 (including STAT1-STAT3 oligomers), NF-IL6, and NF-kB, are known to influence Hsp90 expression directly. Co-chaperones offer another mechanism for Hsp90 regulation, and these can modulate the chaperone cycle appropriately for specific clientele. Co-chaperones include those that deliver specific clients to Hsp90, and others that regulate the chaperone cycle for specific Hsp90-client complexes by modulating Hsp90s ATPase activity. Finally, post-translational modification (PTM) of Hsp90 and its co-chaperones helps too further regulate the variety of different Hsp90 complexes found in cells.

Multifunctional Abl kinases in health and disease

The Abelson tyrosine kinases were initially identified as drivers of leukemia in mice and humans. The Abl family kinases Abl1 and Abl2 regulate diverse cellular processes during development and normal homeostasis, and their functions are subverted during inflammation, cancer and other pathologies. Abl kinases can be activated by multiple stimuli leading to cytoskeletal reorganization required for cell morphogenesis, motility, adhesion and polarity. Depending on the cellular context, Abl kinases regulate cell survival and proliferation. Emerging data support important roles for Abl kinases in pathologies linked to inflammation. Among these are neurodegenerative diseases and inflammatory pathologies. Unexpectedly, Abl kinases have also been identified as important players in mammalian host cells during microbial pathogenesis. Thus, the use of Abl kinase inhibitors might prove to be effective in the treatment of pathologies beyond leukemia and solid tumors. In this Cell Science at a Glance article and in the accompanying poster, we highlight the emerging roles of Abl kinases in the regulation of cellular processes in normal cells and diverse pathologies ranging from cancer to microbial pathogenesis.

The Mechanism of Hsp90 ATPase Stimulation by Aha1

Hsp90 is a dimeric molecular chaperone responsible for the folding, maturation, and activation of hundreds of substrate proteins called ‘clients’. Numerous co-chaperone proteins regulate progression through the ATP-dependent client activation cycle. The most potent stimulator of the Hsp90 ATPase activity is the co-chaperone Aha1p. Only one molecule of Aha1p is required to fully stimulate the Hsp90 dimer despite the existence of two, presumably identical, binding sites for this regulator. Using ATPase assays with Hsp90 heterodimers, we find that Aha1p stimulates ATPase activity by a three-step mechanism via the catalytic loop in the middle domain of Hsp90. Binding of the Aha1p N domain to the Hsp90 middle domain exerts a small stimulatory effect but also drives a separate conformational rearrangement in the Hsp90 N domains. This second event drives a rearrangement in the N domain of the opposite subunit and is required for the stimulatory action of the Aha1p C domain. Furthermore, the second event can be blocked by a mutation in one subunit of the Hsp90 dimer but not the other. This work provides a foundation for understanding how post-translational modifications regulate co-chaperone engagement with the Hsp90 dimer.

The FNIP co-chaperones decelerate the Hsp90 chaperone cycle and enhance drug binding

Heat shock protein-90 (Hsp90) is an essential molecular chaperone in eukaryotes involved in maintaining the stability and activity of numerous signalling proteins, also known as clients. Hsp90 ATPase activity is essential for its chaperone function and it is regulated by co-chaperones. Here we show that the tumour suppressor FLCN is an Hsp90 client protein and its binding partners FNIP1/FNIP2 function as co-chaperones. FNIPs decelerate the chaperone cycle, facilitating FLCN interaction with Hsp90, consequently ensuring FLCN stability. FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins. Lastly, downregulation of FNIPs desensitizes cancer cells to Hsp90 inhibitors, whereas FNIPs overexpression in renal tumours compared with adjacent normal tissues correlates with enhanced binding of Hsp90 to its inhibitors. Our findings suggest that FNIPs expression can potentially serve as a predictive indicator of tumour response to Hsp90 inhibitors.