The Hsp90 chaperone machinery

Molecular chaperone dysfunction in neurodegenerative diseases and effects of curcumin

The intra- and extracellular accumulation of misfolded and aggregated amyloid proteins is a common feature in several neurodegenerative diseases, which is thought to play a major role in disease severity and progression. The principal machineries maintaining proteostasis are the ubiquitin proteasomal and lysosomal autophagy systems, where heat shock proteins play a crucial role. Many protein aggregates are degraded by the lysosomes, depending on aggregate size, peptide sequence, and degree of misfolding, while others are selectively tagged for removal by heat shock proteins and degraded by either the proteasome or phagosomes. These systems are compromised in different neurodegenerative diseases. Therefore, developing novel targets and classes of therapeutic drugs, which can reduce aggregates and maintain proteostasis in the brains of neurodegenerative models, is vital. Natural products that can modulate heat shock proteins/proteosomal pathway are considered promising for treating neurodegenerative diseases. Here we discuss the current knowledge on the role of HSPs in protein misfolding diseases and knowledge gained from animal models of Alzheimer's disease, tauopathies, and Huntington's diseases. Further, we discuss the emerging treatment regimens for these diseases using natural products, like curcumin, which can augment expression or function of heat shock proteins in the cell.

Heat shock protein 90 in Alzheimer's disease

Alzheimer's disease (AD) is the first most common neurodegenerative disease. Despite a large amount of research, the pathogenetic mechanism of AD has not yet been clarified. The two hallmarks of the pathology of AD are the extracellular senile plaques (SPs) of aggregated amyloid-beta (Aβ) peptide and the accumulation of the intracellular microtubule-associated protein tau into fibrillar aggregates. Heat shock proteins (HSPs) play a key role in preventing protein misfolding and aggregation, and Hsp90 can be viewed as a ubiquitous molecular chaperone potentially involved in AD pathogenesis. A role of Hsp90 regulates the activity of the transcription factor heat shock factor-1 (HSF-1), the master regulator of the heat shock response. In AD, Hsp90 inhibitors may redirect neuronal aggregate formation, and protect against protein toxicity by activation of HSF-1 and the subsequent induction of heat shock proteins, such as Hsp70. Therefore, we review here to further discuss the recent advances and challenges in targeting Hsp90 for AD therapy.

Tyrosine nitration as mediator of cell death

Nitrotyrosine is used as a marker for the production of peroxynitrite and other reactive nitrogen species. For over 20 years the presence of nitrotyrosine was associated with cell death in multiple pathologies. Filling the gap between correlation and causality has proven to be a difficult task. Here, we discuss the evidence supporting tyrosine nitration as a specific posttranslational modification participating in the induction of cell death signaling pathways.

Methylated phenanthrenes are more potent than phenanthrene in a bioassay of human aryl hydrocarbon receptor (AhR) signaling

Alkylated polycyclic aromatic hydrocarbons (APAHs) are abundant in petroleum, but data regarding their toxicological properties are limited. A survey of all monomethylated phenanthrene structures revealed that they were 2 times to 5 times more potent than phenanthrene for activation of human aryl hydrocarbon receptor in a yeast bioassay. Phenanthrenes with equatorial methyl groups had the greatest potency. The greater potency of the methylated phenanthrenes highlights the need for more toxicological data on APAHs.

UIM domain-dependent recruitment of the endocytic adaptor protein Eps15 to ubiquitin-enriched endosomes

BACKGROUND

Eps15 is an endocytic adaptor protein that stimulates clathrin-mediated endocytosis. Among other interactions, Eps15 binds ubiquitin via UIM domains, recruiting ubiquitinated cargo into clathrin-coated vesicles. In EGF-treated cells, Eps15 also localizes to endosomes. The basis of this localization is not known.

RESULTS

We show that accumulation of ubiquitinated cargo can recruit Eps15 to endosomes via UIM domain interactions. First, treatment of SK-Br-3 breast cancer cells, which overexpress the EGFR family member ErbB2, with geldanamycin to promote receptor ubiquitination and endosomal transport, recruited FLAG-Eps15 to endosomes. Two in-frame ubiquitin constructs, PM-GFP-Ub (retained in endosomes after endocytosis), and GFP-FYVE-UbΔGG (targeted directly to endosomes) also recruited Eps15 to endosomes, as did slowing endosome maturation with constitutively-active Rab5-Q79L. Endosomal recruitment required the UIM domains, but not the N-terminal EH domains or central coiled-coil domains, of Eps15. Silencing of the endosomal Eps15 binding partner Hrs did not affect recruitment of Eps15 to ubiquitin-enriched endosomes. In fact, Hrs silencing itself modestly recruited Eps15 to endosomes, probably by accumulating endogenous ubiquitinated cargo. Eps15 silencing did not affect lysosomal degradation of ubiquitinated ErbB2; however, GFP-FYVE-UbΔGG overexpression inhibited internalization of EGFR and transferrin receptor.

CONCLUSIONS

We show for the first time that ubiquitin is sufficient for Eps15 recruitment to endosomes. We speculate that Eps15 recruitment to ubiquitin-rich endosomes may reduce the level of Eps15 at the plasma membrane, slowing endocytosis to allow time for processing of ubiquitinated cargo in endosomes.

HSP90α plays an important role in piRNA biogenesis and retrotransposon repression in mouse

HSP90, found in all kingdoms of life, is a major chaperone protein regulating many client proteins. We demonstrated that HSP90α, one of two paralogs duplicated in vertebrates, plays an important role in the biogenesis of fetal PIWI-interacting RNAs (piRNA), which act against the transposon activities, in mouse male germ cells. The knockout mutation of Hsp90α resulted in a large reduction in the expression of primary and secondary piRNAs and mislocalization of MIWI2, a PIWI homolog. Whereas the mutation in Fkbp6 encoding a co-chaperone reduced piRNAs of 28–32 nucleotides in length, the Hsp90α mutation reduced piRNAs of 24–32 nucleotides, suggesting the presence of both FKBP6-dependent and -independent actions of HSP90α. Although DNA methylation and mRNA levels of L1 retrotransposon were largely unchanged in the Hsp90α mutant testes, the L1-encoded protein was increased, suggesting the presence of post-transcriptional regulation. This study revealed the specialized function of the HSP90α isofom in the piRNA biogenesis and repression of retrotransposons during the development of male germ cells in mammals.

HSP90s are required for NLR immune receptor accumulation in Arabidopsis

Heat shock proteins (HSPs) serve as molecular chaperones for diverse client proteins in many biological processes. In plant immunity, cytosolic HSP90s participate in the assembly, stability control and/or activation of immune receptor complexes. In this paper we report that in addition to the well-established positive roles that HSP90 isoforms play in plant immunity, they are also involved in the negative regulation of immune receptor accumulation. Point mutations in two HSP90 genes, HSP90.2 and HSP90.3, were identified from a forward genetic screen designed to isolate mutants with enhanced disease resistance. We found that specific mutations in HSP90.2 and HSP90.3 lead to heightened accumulation of immune receptors, including SNC1, RPS2 and RPS4. HSP90s may assist SGT1 in the formation of SCF E3 ubiquitin ligase complexes that target immune receptors for degradation. Such regulation is critical for maintaining appropriate levels of immune receptor proteins to avoid autoimmunity.

Synuclein gamma protects HER2 and renders resistance to Hsp90 disruption

Hsp90 is an important driver of stabilization and activation of several oncogenic proteins in many key pathways in oncogenesis, including HER2. The present study demonstrated that synuclein gamma (SNCG) prevents the protein degradation and protects the function of HER2 in the condition when the function of Hsp90 is blocked. Disruption of Hsp90 resulted in a significant degradation of HER2 and the loss of activity. However, SNCG completely recovered Hsp90 disruption-mediated losses of HER2 and the function. SNCG bound to HER2 in the presence and absence of Hsp90. Specifically, the C-terminal (Gln106-Asp127) of SNCG bound to the loop connecting αC helix and β4 sheet of the kinase domain of HER2. SNCG renders resistance to 17-AAG-induced tumor suppression in tumor xenograft. Crossing SNCG transgenic mice with HER2 mice stimulated HER2-induced tumor growth and rendered resistance to Hsp90 disruption. The present study indicates that SNCG protects Hsp90 client protein of HER2, and renders resistance to Hsp90 disruption.

Heat shock protein 90 and calcineurin pathway inhibitors enhance the efficacy of triazoles against Scedosporium prolificans via induction of apoptosis

Scedosporium prolificans is a pathogenic mold resistant to current antifungals, and infection results in high mortality. Simultaneous targeting of both ergosterol biosynthesis and heat shock protein 90 (Hsp90) or the calcineurin pathway in S. prolificans may be an important strategy for enhancing the potency of antifungal agents. We hypothesized that the inactive triazoles posaconazole (PCZ) and itraconazole (ICZ) acquire fungicidal activity when combined with the calcineurin inhibitor tacrolimus (TCR) or Hsp90 inhibitor 17-demethoxy-17-(2-propenylamino) geldanamycin (17AAG). PCZ, ICZ, TCR and 17AAG alone were inactive in vitro against S. prolificans spores (MICs > 128 μg/ml). In contrast, MICs for PCZ or ICZ in combination with TCR or 17AAG (0.125-0.50 μg/ml) were much lower compared with drug alone. In addition PCZ and ICZ in combination with TCR or 17AAG became fungicidal. Because apoptosis is regulated by the calcineurin pathway in fungi and is under the control of Hsp90, we hypothesized that this synergistic fungicidal effect is mediated via apoptosis. This observed fungicidal activity was mediated by increased apoptosis of S. prolificans germlings, as evidenced by reactive oxygen species accumulation, decreased mitochondrial membrane potential, phosphatidylserine externalization, and DNA fragmentation. Furthermore, induction of caspase-like activity was correlated with TCR or 17AAG + PCZ/ICZ-induced cell death. In conclusion, we report for the first time that PCZ or ICZ in combination with TCR or 17AAG renders S. prolificans exquisitely sensitive to PCZ or ICZ via apoptosis. This finding may stimulate the development of new therapeutic strategies for patients infected with this recalcitrant fungus.

SMYD2-dependent HSP90 methylation promotes cancer cell proliferation by regulating the chaperone complex formation

Heat shock protein 90 (HSP90) is a highly conserved molecular chaperone that facilitates the maturation of a wide range of proteins, and it has been recognized as a crucial facilitator of oncogene addiction and cancer cell survival. Although HSP90 function is regulated by a variety of post-translational modifications, the physiological significance of methylation has not fully been elucidated. Here we demonstrate that HSP90AB1 is methylated by the histone methyltransferase SMYD2 and that it plays a critical role in human carcinogenesis. HSP90AB1 and SMYD2 can interact through the C-terminal region of HSP90AB1 and the SET domain of SMYD2. Both in vitro and in vivo methyltransferase assays revealed that SMYD2 could methylate HSP90AB1 and mass spectrometry analysis indicated lysines 531 and 574 of HSP90AB1 to be methylated. These methylation sites were shown to be important for the dimerization and chaperone complex formation of HSP90AB1. Furthermore, methylated HSP90AB1 accelerated the proliferation of cancer cells. Our study reveals a novel mechanism for human carcinogenesis via methylation of HSP90AB1 by SMYD2, and additional functional studies may assist in developing novel strategies for cancer therapy.

Posttranslational modification and conformational state of heat shock protein 90 differentially affect binding of chemically diverse small molecule inhibitors

Heat shock protein 90 (Hsp90) is an essential molecular chaperone in eukaryotes that facilitates the conformational maturation and function of a diverse protein clientele, including aberrant and/or over-expressed proteins that are involved in cancer growth and survival. A role for Hsp90 in supporting the protein homeostasis of cancer cells has buoyed interest in the utility of Hsp90 inhibitors as anti-cancer drugs. Despite the fact that all clinically evaluated Hsp90 inhibitors target an identical nucleotide-binding pocket in the N domain of the chaperone, the precise determinants that affect drug binding in the cellular environment remain unclear, and it is possible that chemically distinct inhibitors may not share similar binding preferences. Here we demonstrate that two chemically unrelated Hsp90 inhibitors, the benzoquinone ansamycin geldanamycin and the purine analog PU-H71, select for overlapping but not identical subpopulations of total cellular Hsp90, even though both inhibitors bind to an amino terminal nucleotide pocket and prevent N domain dimerization. Our data also suggest that PU-H71 is able to access a broader range of N domain undimerized Hsp90 conformations than is geldanamycin and is less affected by Hsp90 phosphorylation, consistent with its broader and more potent anti-tumor activity. A more complete understanding of the impact of the cellular milieu on small molecule inhibitor binding to Hsp90 should facilitate their more effective use in the clinic.

Identification of mixed lineage leukemia 1(MLL1) protein as a coactivator of heat shock factor 1(HSF1) protein in response to heat shock protein 90 (HSP90) inhibition

Heat shock protein 90 (HSP90) inhibition inhibits cancer cell proliferation through depleting client oncoproteins and shutting down multiple oncogenic pathways. Therefore, it is an attractive strategy for targeting human cancers. Several HSP90 inhibitors, including AUY922 and STA9090, show promising effects in clinical trials. However, the efficacy of HSP90 inhibitors may be limited by heat shock factor 1 (HSF1)-mediated feedback mechanisms. Here, we identify, through an siRNA screen, that the histone H3 lysine 4 methyltransferase MLL1 functions as a coactivator of HSF1 in response to HSP90 inhibition. MLL1 is recruited to the promoters of HSF1 target genes and regulates their expression in response to HSP90 inhibition. In addition, a striking combination effect is observed when MLL1 depletion is combined with HSP90 inhibition in various human cancer cell lines and tumor models. Thus, targeting MLL1 may block a HSF1-mediated feedback mechanism induced by HSP90 inhibition and provide a new avenue to enhance HSP90 inhibitor activity in human cancers.

Targeting HSF1 sensitizes cancer cells to HSP90 inhibition

The molecular chaperone heat shock protein 90 (HSP90) facilitates the appropriate folding of various oncogenic proteins and is necessary for the survival of some cancer cells. HSP90 is therefore an attractive drug target, but the efficacy of HSP90 inhibitor may be limited by HSP90 inhibition induced feedback mechanisms. Through pooled RNA interference screens, we identified that heat shock factor 1(HSF1) is a sensitizer of HSP90 inhibitor. A striking combinational effect was observed when HSF1 knockdown plus with HSP90 inhibitors treatment in various cancer cell lines and tumor mouse models. Interestingly, HSF1 is highly expressed in hepatocellular carcinoma (HCC) patient samples and HCC is sensitive to combinational treatment, indicating a potential indication for the combinational treatment. To understand the mechanism of the combinational effect, we identified that a HSF1-target gene DEDD2 is involved in attenuating the effect of HSP90 inhibitors. Thus, the transcriptional activities of HSF1 induced by HSP90 inhibitors provide a feedback mechanism of limiting the HSP90 inhibitor's activity, and targeting HSF1 may provide a new avenue to enhance HSP90 inhibitors activity in human cancers.

Resolving hot spots in the C-terminal dimerization domain that determine the stability of the molecular chaperone Hsp90

Human heat shock protein of 90 kDa (hHsp90) is a homodimer that has an essential role in facilitating malignant transformation at the molecular level. Inhibiting hHsp90 function is a validated approach for treating different types of tumors. Inhibiting the dimerization of hHsp90 via its C-terminal domain (CTD) should provide a novel way to therapeutically interfere with hHsp90 function. Here, we predicted hot spot residues that cluster in the CTD dimerization interface by a structural decomposition of the effective energy of binding computed by the MM-GBSA approach and confirmed these predictions using in silico alanine scanning with DrugScore(PPI). Mutation of these residues to alanine caused a significant decrease in the melting temperature according to differential scanning fluorimetry experiments, indicating a reduced stability of the mutant hHsp90 complexes. Size exclusion chromatography and multi-angle light scattering studies demonstrate that the reduced stability of the mutant hHsp90 correlates with a lower complex stoichiometry due to the disruption of the dimerization interface. These results suggest that the identified hot spot residues can be used as a pharmacophoric template for identifying and designing small-molecule inhibitors of hHsp90 dimerization.

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

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

Nematode Sgt1-homologue D1054.3 binds open and closed conformations of Hsp90 via distinct binding sites

Heat shock protein 90 (Hsp90) is a highly conserved ATP-driven machine involved in client protein maturation, folding, and activation. The chaperone is supported by a set of cochaperones that confer client specificities. One of those proteins is the suppressor of G2 allele of skp1 (Sgt1), which participates together with Hsp90 in the immune responses of plants. Sgt1 consists of three domains: a TPR-, CS-, and SGS-domain, conserved in plants, yeast, and humans. The TPR-domain though is lacking in nematodes and insects. We observe that the Caenorhabditis elegans Sgt1 homologue D1054.3 binds to Hsp90 in the absence of nucleotides but much stronger in the presence of ATP and ATPγS. The latter binding mode is similar to p23, another CS-domain containing Hsp90 cofactor, even though binding is not observable for p23 in the absence of nucleotides. We use point mutations in Hsp90, which accumulate different conformations in the ATPase cycle, to differentiate between binding to open and closed Hsp90 conformations. These data support a strong contribution of the Hsp90 conformation to Sgt1 binding and highlight the ability of this cofactor to interact with all known Hsp90 conformations albeit with different affinities.

An in vivo photo-cross-linking approach reveals a homodimerization domain of Aha1 in S. cerevisiae

Protein-protein interactions play an essential role in almost any biological processes. Therefore, there is a particular need for methods which describe the interactions of a defined target protein in its physiological context. Here we report a method to photo-cross-link interacting proteins in S. cerevisiae by using the non-canonical amino acid p-azido-L-phenylalanine (pAzpa). Based on the expanded genetic code the photoreactive non-canonical amino acid pAzpa was site-specifically incorporated at eight positions into a domain of Aha1 that was previously described to bind Hsp90 in vitro to function as a cochaperone of Hsp90 and activates its ATPase activity. In vivo photo-cross-linking to the cognate binding partner of Aha1 was carried out by irradiation of mutant strains with UV light (365 nm) to induce covalent intermolecular bonds. Surprisingly, an interaction between Aha1 and Hsp90 was not detected, although, we could confirm binding of suppressed pAzpa containing Aha1 to Hsp90 by native co-immunoprecipitation. However, a homodimer consisting of two covalently crosslinked Aha1 monomers was identified by mass spectrometry. This homodimer could also be confirmed using p-benzoyl-L-phenylalanine, another photoreactive non-canonical amino acid. Crosslinking was highly specific as it was dependent on irradiation using UV light, the exact position of the non-canonical amino acid in the protein sequence as well as on the addition of the non-canonical amino acid to the growth medium. Therefore it seems possible that an interaction of Aha1 with Hsp90 takes place at different positions than previously described in vitro highlighting the importance of in vivo techniques to study protein-protein interactions. Accordingly, the expanded genetic code can easily be applied to other S. cerevisiae proteins to study their interaction under physiological relevant conditions in vivo.

The regulatory mechanism of a client kinase controlling its own release from Hsp90 chaperone machinery through phosphorylation

It is believed that the stability and activity of client proteins are passively regulated by the Hsp90 (heat-shock protein 90) chaperone machinery, which is known to be modulated by its intrinsic ATPase activity, co-chaperones and post-translational modifications. However, it is unclear whether client proteins themselves participate in regulation of the chaperoning process. The present study is the first example to show that a client kinase directly regulates Hsp90 activity, which is a novel level of regulation for the Hsp90 chaperone machinery. First, we prove that PKCγ (protein kinase Cγ) is a client protein of Hsp90α, and, that by interacting with PKCγ, Hsp90α prevents PKCγ degradation and facilitates its cytosol-to-membrane translocation and activation. A threonine residue set, Thr¹¹⁵/Thr⁴²⁵/Thr⁶⁰³, of Hsp90α is specifically phosphorylated by PKCγ, and, more interestingly, this threonine residue set serves as a ‘phosphorylation switch’ for Hsp90α binding or release of PKCγ. Moreover, phosphorylation of Hsp90α by PKCγ decreases the binding affinity of Hsp90α towards ATP and co-chaperones such as Cdc37 (cell-division cycle 37), thereby decreasing its chaperone activity. Further investigation demonstrated that the reciprocal regulation of Hsp90α and PKCγ plays a critical role in cancer cells, and that simultaneous inhibition of PKCγ and Hsp90α synergistically prevents cell migration and promotes apoptosis in cancer cells.

The present study is the first example to show that a client directly regulates Hsp90 activity, which is a novel level of regulation for the Hsp90 chaperone machinery.

Structural insights into complexes of glucose-regulated Protein94 (Grp94) with human immunoglobulin G. relevance for Grp94-IgG complexes that form in vivo in pathological conditions

While the mechanism by which Grp94 displays its chaperone function with client peptides in the cell has been elucidated extensively, much less is known about the nature and properties of how Grp94 can engage binding to proteins once it is exposed on the cell surface or liberated in the extra-cellular milieu, as occurs in pathological conditions. In this work, we wanted to investigate the molecular aspects and structural characteristics of complexes that Grp94 forms with human IgG, posing the attention on the influence that glycosylation of Grp94 might have on the binding capacity to IgG, and on the identification of sites involved in the binding. To this aim, we employed both native, fully glycosylated and partially glycosylated Grp94, and recombinant, non-glycosylated Grp94, as well as IgG subunits, in different experimental conditions, including the physiological setting of human plasma. Regardless of the species and type, Grp94 engages a similar, highly specific and stable binding with IgG that involves sites located in the N-terminal domain of Grp94 and the hinge region of whole IgG. Grp94 does not form stable complex with Fab, F(ab)2 or Fc. Glycosylation turns out to be an obstacle to the Grp94 binding to IgG, although this negative effect can be counteracted by ATP and spontaneously also disappears in time in a physiological setting of incubation. ATP does not affect at all the binding capacity of non-glycosylated Grp94. However, complexes that native, partially glycosylated Grp94 forms with IgG in the presence of ATP show strikingly different characteristics with respect to those formed in absence of ATP. Results have relevance for the mechanism regulating the formation of stable Grp94-IgG complexes in vivo, in the pathological conditions associated with the extra-cellular location of Grp94.

Physical interaction between bacterial heat shock protein (Hsp) 90 and Hsp70 chaperones mediates their cooperative action to refold denatured proteins

In eukaryotes, heat shock protein 90 (Hsp90) is an essential ATP-dependent molecular chaperone that associates with numerous client proteins. HtpG, a prokaryotic homolog of Hsp90, is essential for thermotolerance in cyanobacteria, and in vitro it suppresses the aggregation of denatured proteins efficiently. Understanding how the non-native client proteins bound to HtpG refold is of central importance to comprehend the essential role of HtpG under stress. Here, we demonstrate by yeast two-hybrid method, immunoprecipitation assays, and surface plasmon resonance techniques that HtpG physically interacts with DnaJ2 and DnaK2. DnaJ2, which belongs to the type II J-protein family, bound DnaK2 or HtpG with submicromolar affinity, and HtpG bound DnaK2 with micromolar affinity. Not only DnaJ2 but also HtpG enhanced the ATP hydrolysis by DnaK2. Although assisted by the DnaK2 chaperone system, HtpG enhanced native refolding of urea-denatured lactate dehydrogenase and heat-denatured glucose-6-phosphate dehydrogenase. HtpG did not substitute for DnaJ2 or GrpE in the DnaK2-assisted refolding of the denatured substrates. The heat-denatured malate dehydrogenase that did not refold by the assistance of the DnaK2 chaperone system alone was trapped by HtpG first and then transferred to DnaK2 where it refolded. Dissociation of substrates from HtpG was either ATP-dependent or -independent depending on the substrate, indicating the presence of two mechanisms of cooperative action between the HtpG and the DnaK2 chaperone system.

Down-regulation of lipid raft-associated onco-proteins via cholesterol-dependent lipid raft internalization in docosahexaenoic acid-induced apoptosis

Lipid rafts, plasma membrane microdomains, are important for cell survival signaling and cholesterol is a critical lipid component for lipid raft integrity and function. DHA is known to have poor affinity for cholesterol and it influences lipid rafts. Here, we investigated a mechanism underlying the anti-cancer effects of DHA using a human breast cancer cell line, MDA-MB-231. We found that DHA decreased cell surface levels of lipid rafts via their internalization, which was partially reversed by cholesterol addition. With DHA treatment, caveolin-1, a marker for rafts, and EGFR were colocalized with LAMP-1, a lysosomal marker, in a cholesterol-dependent manner, indicating that DHA induces raft fusion with lysosomes. DHA not only displaced several raft-associated onco-proteins, including EGFR, Hsp90, Akt, and Src, from the rafts but also decreased total levels of those proteins via multiple pathways, including the proteasomal and lysosomal pathways, thereby decreasing their activities. Hsp90 overexpression maintained its client proteins, EGFR and Akt, and attenuated DHA-induced cell death. In addition, overexpression of Akt or constitutively active Akt attenuated DHA-induced apoptosis. All these data indicate that the anti-proliferative effect of DHA is mediated by targeting of lipid rafts via decreasing cell surface lipid rafts by their internalization, thereby decreasing raft-associated onco-proteins via proteasomal and lysosomal pathways and decreasing Hsp90 chaperone function.

Dimerization hot spots in the structure of human Hsp90

Molecular dynamics and dimerization free energy analyses performed on the human Hsp90 dimer highlight dimerization hot spots and potential allosteric binding sites.

High-temperature protein G is an essential virulence factor of Leptospira interrogans

Leptospira interrogans is a global zoonotic pathogen and is the causative agent of leptospirosis, an endemic disease of humans and animals worldwide. There is limited understanding of leptospiral pathogenesis; therefore, further elucidation of the mechanisms involved would aid in vaccine development and the prevention of infection. HtpG (high-temperature protein G) is the bacterial homolog to the highly conserved molecular chaperone Hsp90 and is important in the stress responses of many bacteria. The specific role of HtpG, especially in bacterial pathogenesis, remains largely unknown. Through the use of an L. interrogans htpG transposon insertion mutant, this study demonstrates that L. interrogans HtpG is essential for virulence in the hamster model of acute leptospirosis. Complementation of the htpG mutant completely restored virulence. Surprisingly, the htpG mutant did not appear to show sensitivity to heat or oxidative stress, phenotypes common in htpG mutants in other bacterial species. Furthermore, the mutant did not show increased sensitivity to serum complement, reduced survival within macrophages, or altered protein or lipopolysaccharide expression. The underlying cause for attenuation thus remains unknown, but HtpG is a novel leptospiral virulence factor and one of only a very small number identified to date.

Expression of heat shock protein 90 genes during early development and infection in Megalobrama amblycephala and evidence for adaptive evolution in teleost

Heat shock protein 90 (HSP90) are highly conserved molecular chaperones, playing a pivotal role in cellular progress. In this study, we reported the characterization of the Hsp90α and Hsp90β genes in Megalobrama amblycephala, the expression profiling during early development, in various healthy tissues and in response to bacterial infection, and the assessment of their adaptive evolution. The Hsp90α cDNA contains an open reading frame (ORF) of 2193 bp encoding 731 amino acids and the Hsp90β cDNA has an ORF of 2184 bp encoding 728 amino acids. Using quantitative real-time PCR (qRT-PCR) analysis, the mRNA of both Hsp90α and Hsp90β reached the highest level at 15th day post-hatch. Using qRT-PCR and Western blot, both Hsp90α and Hsp90β were widely expressed in various healthy tissues and significantly higher in blood than in other tissues. Expression of both Hsp90α and Hsp90β were up-regulated upon bacterial infection and reached the peak level at 4 h post infection. Site model analysis indicated that one positive selection site (T717) in Hsp90α was found, while no positive selection site was observed in Hsp90β. Branch-site model test showed that there were adaptively evolutionary evidences in the branches of Salmoniformes and Gasterosteiformes for Hsp90α gene, and in the branch of Salmoniformes for Hsp90β gene.

The therapeutic target Hsp90 and cancer hallmarks

Hsp90 is a major molecular chaperone that is expressed abundantly and plays a pivotal role in assisting correct folding and functionality of its client proteins in cells. The Hsp90 client proteins include a wide variety of signal transducing molecules such as protein kinases and steroid hormone receptors. Cancer is a complex disease, but most types of human cancer share common hallmarks, including self-sufficiency in growth signals, insensitivity to growth-inhibitory mechanism, evasion of programmed cell death, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis. A surprisingly large number of Hsp90-client proteins play crucial roles in establishing cancer cell hallmarks. We start the review by describing the structure and function of Hsp90 since conformational changes during the ATPase cycle of Hsp90 are closely related to its function. Many co-chaperones, including Hop, p23, Cdc37, Aha1, and PP5, work together with Hsp90 by modulating the chaperone machinery. Post-translational modifications of Hsp90 and its cochaperones are vital for their function. Many tumor-related Hsp90-client proteins, including signaling kinases, steroid hormone receptors, p53, and telomerase, are described. Hsp90 and its co-chaperones are required for the function of these tumor-promoting client proteins; therefore, inhibition of Hsp90 by specific inhibitors such as geldanamycin and its derivatives attenuates the tumor progression. Hsp90 inhibitors can be potential and effective cancer chemotherapeutic drugs with a unique profile and have been examined in clinical trials. We describe possible mechanisms why Hsp90 inhibitors show selectivity to cancer cells even though Hsp90 is essential also for normal cells. Finally, we discuss the "Hsp90-addiction" of cancer cells, and suggest a role for Hsp90 in tumor evolution.

The therapeutic target Hsp90 and cancer hallmarks

Hsp90 is a major molecular chaperone that is expressed abundantly and plays a pivotal role in assisting correct folding and functionality of its client proteins in cells. The Hsp90 client proteins include a wide variety of signal transducing molecules such as protein kinases and steroid hormone receptors. Cancer is a complex disease, but most types of human cancer share common hallmarks, including self-sufficiency in growth signals, insensitivity to growth-inhibitory mechanism, evasion of programmed cell death, limitless replicative potential, sustained angiogenesis, and tissue invasion and metastasis. A surprisingly large number of Hsp90-client proteins play crucial roles in establishing cancer cell hallmarks. We start the review by describing the structure and function of Hsp90 since conformational changes during the ATPase cycle of Hsp90 are closely related to its function. Many co-chaperones, including Hop, p23, Cdc37, Aha1, and PP5, work together with Hsp90 by modulating the chaperone machinery. Post-translational modifications of Hsp90 and its cochaperones are vital for their function. Many tumor-related Hsp90-client proteins, including signaling kinases, steroid hormone receptors, p53, and telomerase, are described. Hsp90 and its co-chaperones are required for the function of these tumor-promoting client proteins; therefore, inhibition of Hsp90 by specific inhibitors such as geldanamycin and its derivatives attenuates the tumor progression. Hsp90 inhibitors can be potential and effective cancer chemotherapeutic drugs with a unique profile and have been examined in clinical trials. We describe possible mechanisms why Hsp90 inhibitors show selectivity to cancer cells even though Hsp90 is essential also for normal cells. Finally, we discuss the "Hsp90-addiction" of cancer cells, and suggest a role for Hsp90 in tumor evolution.

HSP27 expression levels are associated with the sensitivity of hepatocellular carcinoma cells to 17-allylamino-17-demethoxygeldanamycin

AIMS, MATERIALS & METHODS: As heat-shock proteins are associated with tumor proliferation, differentiation, invasion and metastasis, we investigated whether targeting Hsp90 with the geldanamycin derivative 17-allylamino-17-demethoxygeldanamycin (17AAG) can inhibit the viability of hepatocellular carcinoma cell lines with various levels of metastatic potential. In addition, we investigated whether the use of Hsp27-siRNA can decrease resistance to 17AAG.

RESULTS

Although 17AAG upregulated the expression of heat-shock proteins, it did not affect the expression of Hsp90 client proteins in normal hepatocytes. Hsp90 inhibition by 17AAG degraded its client proteins in both low- and high-metastatic potential cell lines. siRNA inhibited Hsp27 expression in cell lines and improved the sensitivity of 17AAG.

CONCLUSION

17AAG inhibited the viability of hepatocellular carcinoma cells by degrading Hsp90 client proteins. The sensitivity of cells to 17AAG is associated with the level of Hsp27 expression.

Hsp90 inhibits α-synuclein aggregation by interacting with soluble oligomers

Aggregated α-synuclein is one of the main components of the pathological Lewy bodies associated with Parkinson's disease (PD). Many other proteins, including chaperones such as Hsp90 and Hsp70, have been found co-localized with Lewy bodies and the expression levels of Hsp90 have been found to be increased in brains of PD patients. Although the role of Hsp70 in the aggregation of α-synuclein has been extensively studied, relatively little is known about the effect of Hsp90 on this process. Here, we have investigated if Hsp90 can prevent the aggregation of the A53T pathological mutant of α-synuclein in vitro. A detailed study using many biophysical methods has revealed that Hsp90 prevents α-synuclein from aggregating in an ATP-independent manner and that it forms a strong complex with the transiently populated toxic oligomeric α-synuclein species formed along the aggregation pathway. We have also shown that, upon forming a complex with Hsp90, the oligomers are rendered harmless and nontoxic to cells. Thus, we have clear evidence that Hsp90 is likely to play an important role on these processes in vivo.

Opportunities and challenges for molecular chaperone modulation to treat protein-conformational brain diseases

A common pathological hallmark of protein-conformational brain diseases is the formation of disease-specific protein aggregates. In Alzheimer's disease, these are comprised of amyloid-β and Tau as opposed to α-synuclein in Parkinson's disease and N-terminal fragments of mutant huntingtin in Huntington's disease. Most aggregates also sequester molecular chaperones, a protein family that assists in the folding, refolding, stabilization, and processing of client proteins, including misfolded proteins in brain diseases. Molecular chaperone modulation has achieved remarkable therapeutic effects in some cellular and preclinical animal models of protein-conformational diseases. This has raised hope for chaperone-based strategies to combat these diseases. Here, we review briefly the functional diversity and medical significance of molecular chaperones, their therapeutic potential, and common and specific challenges towards clinical application.

Low resolution structural studies indicate that the activator of Hsp90 ATPase 1 (Aha1) of Leishmania braziliensis has an elongated shape which allows its interaction with both N- and M-domains of Hsp90

The Hsp90 molecular chaperone is essential for protein homeostasis and in the maturation of proteins involved with cell-cycle control. The low ATPase activity of Hsp90 is critical to drive its functional cycle, which is dependent on the Hsp90 cochaperones. The Activator of Hsp90 ATPase-1 (Aha1) is a protein formed by two domains, N- and C-terminal, that stimulates the Hsp90 ATPase activity by several folds. Although the relevance of Aha1 for Hsp90 functions has been proved, as well as its involvement in the desensitization to inhibitors of the Hsp90, the knowledge on its overall structure and behavior in solution is limited. In this work we present the functional and structural characterization of Leishmania braziliensis Aha1 (LbAha1). This protozoan is the causative agent of cutaneous and mucocutaneous leishmaniasis, a neglected disease. The recombinant LbAha1 behaves as an elongated monomer and is organized into two folded domains interconnected by a flexible linker. Functional experiments showed that LbAha1 interacts with L. braziliensis Hsp90 (LbHsp90) with micromolar dissociation constant in a stoichiometry of 2 LbAha1 to 1 LbHsp90 dimer and stimulates 10-fold the LbHsp90 ATPase activity showing positive cooperativity. Furthermore, the LbHsp90::LbAha1 complex is directed by enthalphy and opposed by entropy, probably due to the spatial freedom restrictions imposed by the proteins' interactions. Small-angle X-ray scattering data allowed the reconstruction of low resolution models and rigid body simulations of LbAha1, indicating its mode of action on LbHsp90. Western blot experiments allowed Aha1 identification (as well as Hsp90) in three Leishmania species at two temperatures, suggesting that Aha1 is a cognate protein. All these data shed light on the LbAha1 mechanism of action, showing that it has structural dimensions and flexibility that allow interacting with both N-terminal and middle domains of the LbHsp90.

The association of SNPs in Hsp90β gene 5' flanking region with thermo tolerance traits and tissue mRNA expression in two chicken breeds

Thermo stress induces heat shock proteins (HSPs) expression and HSP90 family is one of them that has been reported to involve in cellular protection against heat stress. But whether there is any association of genetic variation in the Hsp90β gene in chicken with thermo tolerance is still unknown. Direct sequencing was used to detect possible SNPs in Hsp90β gene 5' flanking region in 3 chicken breeds (n = 663). Six mutations, among which 2 SNPs were chosen and genotypes were analyzed with PCR-RFLP method, were found in Hsp90β gene in these 3 chicken breeds. Association analysis indicated that SNP of C.-141G>A in the 5' flanking region of the Hsp90β gene in chicken had some effect on thermo tolerance traits, which may be a potential molecular marker of thermo tolerance, and the genotype GG was the thermo tolerance genotype. Hsp90β gene mRNA expression in different tissues detected by quantitative real-time PCR assay were demonstrated to be tissue dependent, implying that different tissues have distinct sensibilities to thermo stress. Besides, it was shown time specific and varieties differences. The expression of Hsp90β mRNA in Lingshan chickens in some tissues including heart, liver, brain and spleen were significantly higher or lower than that of White Recessive Rock (WRR). In this study, we presume that these mutations could be used in marker assisted selection for anti-heat stress chickens in our breeding program, and WRR were vulnerable to tropical thermo stress whereas Lingshan chickens were well adapted.

Applying chaperones to protein-misfolding disorders: molecular chaperones against α-synuclein in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the accumulation of a protein called α-synuclein (α-syn) into inclusions known as lewy bodies (LB) within neurons. This accumulation is also due to insufficient formation and activity of dopamine produced in certain neurons within the substantia nigra. Lewy bodies are the pathological hallmark of the idiopathic disorder and the cascade that allows α-synuclein to misfold, aggregate and form these inclusions has been the subject of intensive research. Targeting these early steps of oligomerization is one of the main therapeutic approaches in order to develop neurodegenerative-modifying agents. Because the folding and refolding of alpha synuclein is the key point of this cascade, we are interested in this review to summarize the role of some molecular chaperones proteins such as Hsp70, Hsp90 and small heat shock proteins (sHsp) and Hsp 104. Hsp70 and its co-chaperone, Hsp70 and small heat shock proteins can prevent neurodegeneration by preventing α-syn misfolding, oligomerization and aggregation in vitro and in Parkinson disease animal models. Hsp104 is able to resolve disordered protein aggregates and cross beta amyloid conformers. Together, these chaperones have a complementary effect and can be a target for therapeutic intervention in PD.

Modified Clp protease complex in the ClpP3 null mutant and consequences for chloroplast development and function in Arabidopsis

The plastid ClpPRT protease consists of two heptameric rings of ClpP1/ClpR1/ClpR2/ClpR3/ClpR4 (the R-ring) and ClpP3/ClpP4/ClpP5/ClpP6 (the P-ring) and peripherally associated ClpT1/ClpT2 subunits. Here, we address the contributions of ClpP3 and ClpP4 to ClpPRT core organization and function in Arabidopsis (Arabidopsis thaliana). ClpP4 is strictly required for embryogenesis, similar to ClpP5. In contrast, loss of ClpP3 (clpp3-1) leads to arrest at the hypocotyl stage; this developmental arrest can be removed by supplementation with sucrose or glucose. Heterotrophically grown clpp3-1 can be transferred to soil and generate viable seed, which is surprising, since we previously showed that CLPR2 and CLPR4 null alleles are always sterile and die on soil. Based on native gels and mass spectrometry-based quantification, we show that despite the loss of ClpP3, modified ClpPR core(s) could be formed, albeit at strongly reduced levels. A large portion of ClpPR subunits accumulated in heptameric rings, with overaccumulation of ClpP1/ClpP5/ClpP6 and ClpR3. Remarkably, the association of ClpT1 to the modified Clp core was unchanged. Large-scale quantitative proteomics assays of clpp3-1 showed a 50% loss of photosynthetic capacity and the up-regulation of plastoglobules and all chloroplast stromal chaperone systems. Specific chloroplast proteases were significantly up-regulated, whereas the major thylakoid protease (FtsH1/FtsH2/FtsH5/FtsH8) was clearly unchanged, indicating a controlled protease network response. clpp3-1 showed a systematic decrease of chloroplast-encoded proteins that are part of the photosynthetic apparatus but not of chloroplast-encoded proteins with other functions. Candidate substrates and an explanation for the differential phenotypes between the CLPP3, CLPP4, and CLPP5 null mutants are discussed.

Physiological consequences of mutations in the htpG heat shock gene of Escherichia coli

Mutation of the heat shock gene, htpG, causes severe defects of several cellular functions in Escherichia coli. A null htpG mutant constructed by gene replacement was impaired in the biosynthesis and secretion of several enzymes, and in biofilm formation and proteolysis. A significant decrease in the activity of β-lactamase in the ΔhtpG mutant was observed at 42°C. The alkaline phosphatase activity in sonicates of cells propagated at this raised temperature was lower in the ΔhtpG mutant than in the wild-type strain. The ability of the ΔhtpG mutant to degrade abnormal proteins was also impaired compared with the wild-type, but was increased at 42°C. Assays based on bioluminescence and crystal violet staining demonstrated that biofilm formation was diminished in the ΔhtpG mutant at the elevated temperature. All these defects can be complemented upon introducing htpG wild allele.

Heat shock protein 90 functions to stabilize and activate the testis-specific serine/threonine kinases, a family of kinases essential for male fertility

Spermiogenesis is characterized by a profound morphological differentiation of the haploid spermatid into spermatozoa. The testis-specific serine/threonine kinases (TSSKs) comprise a family of post-meiotic kinases expressed in spermatids, are critical to spermiogenesis, and are required for male fertility in mammals. To explore the role of heat shock protein 90 (HSP90) in regulation of TSSKs, the stability and catalytic activity of epitope-tagged murine TSSKs were assessed in 293T and COS-7 cells. TSSK1, -2, -4, and -6 (small serine/threonine kinase) were all found to associate with HSP90, and pharmacological inhibition of HSP90 function using the highly specific drugs 17-AAG, SNX-5422, or NVP-AUY922 reduced TSSK protein levels in cells. The attenuation of HSP90 function abolished the catalytic activities of TSSK4 and -6 but did not significantly alter the specific activities of TSSK1 and -2. Inhibition of HSP90 resulted in increased TSSK ubiquitination and proteasomal degradation, indicating that HSP90 acts to control ubiquitin-mediated catabolism of the TSSKs. To study HSP90 and TSSKs in germ cells, a mouse primary spermatid culture model was developed and characterized. Using specific antibodies against murine TSSK2 and -6, it was demonstrated that HSP90 inhibition resulted in a marked decrease of the endogenous kinases in spermatids. Together, our findings demonstrate that HSP90 plays a broad and critical role in stabilization and activation of the TSSK family of protein kinases.

Cdc37 (cell division cycle 37) restricts Hsp90 (heat shock protein 90) motility by interaction with N-terminal and middle domain binding sites

The ATPase-driven dimeric molecular Hsp90 (heat shock protein 90) and its cofactor Cdc37 (cell division cycle 37 protein) are crucial to prevent the cellular depletion of many protein kinases. In complex with Hsp90, Cdc37 is thought to bind an important lid structure in the ATPase domain of Hsp90 and inhibit ATP turnover by Hsp90. As different interaction modes have been reported, we were interested in the interaction mechanism of Hsp90 and Cdc37. We find that Cdc37 can bind to one subunit of the Hsp90 dimer. The inhibition of the ATPase activity is caused by a reduction in the closing rate of Hsp90 without obviously bridging the two subunits or affecting nucleotide accessibility to the binding site. Although human Cdc37 binds to the N-terminal domain of Hsp90, nematodal Cdc37 preferentially interacts with the middle domain of CeHsp90 and hHsp90, exposing two Cdc37 interaction sites. A previously unreported site in CeCdc37 is utilized for the middle domain interaction. Dephosphorylation of CeCdc37 by the Hsp90-associated phosphatase PPH-5, a step required during the kinase activation process, proceeds normally, even if only the new interaction site is used. This shows that the second interaction site is also functionally relevant and highlights that Cdc37, similar to the Hsp90 cofactors Sti1 and Aha1, may utilize two different attachment sites to restrict the conformational freedom and the ATP turnover of Hsp90.

The Hsp90-Sti1 interaction is critical for Leishmania donovani proliferation in both life cycle stages

The heat shock protein 90 plays a pivotal role in the life cycle control of Leishmania donovani promoting the fast-growing insect stage of this parasite. Equally important for insect stage growth is the co-chaperone Sti1. We show that replacement of Sti1 is only feasible in the presence of additional Sti1 transgenes indicating an essential role. To better understand the impact of Sti1 and its interaction with Hsp90, we performed a mutational analysis of Hsp90. We established that a single amino acid exchange in the Leishmania Hsp90 renders that protein resistant to the inhibitor radicicol (RAD), yet does not interfere with its functionality. Based on this RAD-resistant Hsp90, we established a combined chemical knockout/gene complementation (CKC) approach. We can show that Hsp90 function is required in both insect and mammalian life stages and that the Sti1-binding motif of Hsp90 is crucial for proliferation of insect and mammalian stages of the parasite. The Sti1-binding motif in Leishmania Hsp90 is suboptimal - optimizing the motif increased initial intracellular proliferation underscoring the importance of the Hsp90-Sti1 interaction for this important parasitic protozoan. The CKC strategy we developed will allow the future analysis of more Hsp90 domains and motifs in parasite viability and infectivity.