Definition of extracellular localized epitopes of Hsp70 involved in an NK immune response

In order to define extracellular localized epitopes of Hsp70 on human tumor cells which are accessible to the immune system, six commercially available Hsp70-specific monoclonal antibodies (mAb) with different recognition sites were examined by immunological approaches. The recognition pattern of these antibodies was analyzed on purified recombinant Hsp70 proteins (rHsp70, Hsc70, DnaK), on lysates of Hsp70-expressing colon carcinoma cells (CX+) and on lysates of M21 rat-1 cells that overexpress human Hsp70 or Hsp70 fragments: ΔBgl (del 120–428) consisting of the C-terminal part and ΔSma (del 438–618) consisting of the N-terminal part of human Hsp70. All antibodies reacted equally well with rHsp70 and cytoplasmic Hsp70 derived from human tumor cells or M21 rat-1 cells. Only one antibody (MA3–007; Hsp70, Hsc70) detects a region localized within the ATPase domain of Hsp70 (amino acid 122–264) and reacts positively with the C-terminal deletion mutant ΔSma. All other antibodies, including RPN1197 are directed against the C-terminal peptide binding domain of Hsp70 and react positively with the N-terminal deletion mutant ΔBgl. Although all six antibodies detect full-length Hsp70 protein, derived from plasma membrane fractions of CX+ tumor cells, cell surface expressed Hsp70 on viable CX+ tumor cells, as determined by flowcytometry, is only recognized with the antibodies MA3–006 (Hsp70, Hsc70; 504–617), MA3–009 (Hsp70; 504–617) and RPN1197 (Hsp70). An estimation of the ratio of membrane-bound to cytoplasmic Hsp70 molecules revealed that 15–20% of total Hsp70 molecules are expressed on the plasma membrane. This tumor-selective cell surface expression of Hsp70 correlates with an increased sensitivity to lysis mediated by non-MHC restricted natural killer (NK) cells. We demonstrate that only antibodies directed against membrane-bound Hsp70 (MA3–006, MA3–009, RPN1197) inhibit NK-killing activity against Hsp70-expressing tumor cells. Taken together our data indicate that at least the C-terminal region 504–617, that contains at least one single α-helix (amino acid 512–536), has to be localized extracellularly and might be of importance for an NK-mediated anti-tumor immune response.     

Heat shock protein 104 inhibited the fibrillization of prion peptide 106-126 and disassembled prion peptide 106-126 fibrils in vitro

Amyloid-like fibrils have been associated with the pathogenesis of human prion diseases. Prion peptide of aa 106-126 (PrP106-126) exhibits many PrP(Sc)-like biochemical features, forming amyloid-like fibrils in vitro. Here, we found that the recombinant yeast-derived molecular chaperon Hsp104 inhibited significantly the fibril assembly of the synthetic PrP106-126 peptide by dynamic ThT assays in vitro. EM assays revealed almost no fibril-like structure after incubation of the synthetic PrP106-126 peptides with Hsp104 for 12h. Circular dichroism assays identified that treatment of Hsp104 shifted the secondary structure of PrP106-126 fibrils from β-sheet to a random coil. MTT tests confirmed that interaction of PrP106-126 with Hsp104 maintained the toxicity of PrP106-126 on human neuroblastoma cell line SK-N-SH. Additionally, Hsp104 was able to disassemble the mature PrP106-126 fibrils in vitro, leading to recovering the cytotoxicity of PrP106-126 on SK-N-SH cells. Our study provides the molecular evidences that the yeast-derived Hsp104 can interfere in the fibril assembly and disassembly of human PrP106-126 segment.     

A 14-mer Hsp70 peptide stimulates natural killer (NK) cell activity

Compared with normal cells, tumor cell lines exhibit an unusual plasma membrane localization of heat shock protein 70 (Hsp70). This tumor-selective Hsp70 membrane expression has been found to correlate with an increased sensitivity to lysis mediated by human natural killer (NK) cells that transiently adhere to plastic following cytokine stimulation. A human Hsp70-specific monoclonal antibody (mAb) detects membrane-bound Hsp70 on viable tumor cells and blocks the immune response of NK cells against Hsp70-expressing tumor cells. By peptide scanning (pep-scan) analysis, the epitope of this mAb was mapped as the C-terminal-localized 8-mer NLLGRFEL (NLL, amino acids [aa] 454-461). Most interestingly, similar to full-length Hsp70 protein, the N-terminal-extended 14-mer peptide TKDNNLLGRFELSG (TKD, aa 450-463) was able to stimulate the cytolytic and proliferative activity of NK cells at concentrations equivalent to full-length Hsp70 protein. Blocking studies revealed that an excess of the 14-mer peptide TKDNNLLGRFELSG inhibits the cytolytic activity of NK cells similar to that of Hsp70 protein. In comparison, other TKD-related peptides, including the 8-mer antibody epitope NLLGRFEL (aa 454-461), the 12-mer TKDNNLLGRFEL (aa 450-461), the 13-mer C-terminal-extended peptide NLLGRFELSGIPP (aa 454-466), the 14-mer TKD-equivalent sequences of Hsp70hom TKDNNLLGRFELTG (aa 450-463), Hsc70 TKDNNLLGKFELTG (aa 450-463), and DnaK AADNKSLGQFNLDG (aa 447-460) failed to activate NK activity.     

Binding specificity of an alpha-helical protein sequence to a full-length Hsp70 chaperone and its minimal substrate-binding domain

Hsp70 chaperones are involved in the prevention of misfolding, and possibly the folding, of newly synthesized proteins. The members of this chaperone family are capable of interacting with polypeptide chains both co- and posttranslationally, but it is currently not clear how different structural domains of the chaperone affect binding specificity. We explored the interactions between the bacterial Hsp70, DnaK, and the sequence of a model all-alpha-helical globin (apoMb) by cellulose-bound peptide scanning. The binding specificity of the full-length chaperone was compared with that of its minimal substrate-binding domain, DnaK-beta. Six specific chaperone binding sites evenly distributed along the apoMb sequence were identified. Binding site locations are identical for the full-length chaperone and its substrate-binding domain, but relative affinities differ. The binding specificity of DnaK-beta is only slightly decreased relative to that of full-length DnaK. DnaK's binding motif is known to comprise hydrophobic regions flanked by positively charged residues. We found that the simple fractional mean buried area correlates well with Hsp70's binding site locations along the apoMb sequence. In order to further characterize the properties of the minimal binding host, the stability of DnaK-beta upon chemical denaturation by urea and protons was investigated. Urea unfolding titrations yielded an apparent folding DeltaG degrees of 3.1 +/- 0.9 kcal mol-1 and an m value of 1.7 +/- 0.4 kcal mol-1 M-1.     

Chaperone-mediated hierarchical control in targeting misfolded proteins to aggresomes

Protein misfolding is a common event in living cells. Molecular chaperones not only assist protein folding; they also facilitate the degradation of misfolded polypeptides. When the intracellular degradative capacity is exceeded, juxtanuclear aggresomes are formed to sequester misfolded proteins. Despite the well-established role of chaperones in both protein folding and degradation, how chaperones regulate the aggregation process remains controversial. Here we investigate the molecular mechanisms underlying aggresome formation in mammalian cells. Analysis of the chaperone requirements for the fate of misfolded proteins reveals an unexpected role of heat shock protein 70 (Hsp70) in promoting aggresome formation. This proaggregation function of Hsp70 relies on the interaction with the cochaperone ubiquitin ligase carboxyl terminal of Hsp70/Hsp90 interacting protein (CHIP). Disrupting Hsp70-CHIP interaction prevents the aggresome formation, whereas a dominant-negative CHIP mutant sensitizes the aggregation of misfolded protein. This accelerated aggresome formation also relies on the stress-induced cochaperone Bcl2-associated athanogene 3. Our results indicate that a hierarchy of cochaperone interaction controls different aspects of the intracellular protein triage decision, extending the function of Hsp70 from folding and degradation to aggregation.     

The cytosolic chaperone Hsc70 promotes traffic to the cell surface of intracellular retained melanocortin-4 receptor mutants

Inherited modifications in protein structure frequently cause a loss-of-function by interfering with protein synthesis, transport, or stability. For the obesity-linked melanocortin-4 receptor (MC4R) and other G protein-coupled receptors, many mutants are intracellular retained. The biogenesis and trafficking of G protein-coupled receptors are regulated by multiple factors, including molecular chaperone networks. Here, we have investigated the ability of the cytosolic cognate 70-kDa heat-shock protein (Hsc70) chaperone system to modulate cell surface expression of MC4R. Clinically occurring MC4R mutants S58C, P78L, and D90N were demonstrated to have reduced trafficking to the plasma membrane and to be retained at the endoplasmic reticulum (ER). Analyses by fluorescence recovery after photobleaching revealed that the mobility of MC4R mutant protein at the ER was reduced, implying protein misfolding. In cells expressing MC4R, overexpression of Hsc70 resulted in increased levels of wild-type and mutant receptors at the cell surface. MC4R and Hsc70 coimmunoprecipitated, and fluorescence recovery after photobleaching analyses showed that increasing cellular levels of Hsc70 promoted the mobility of ER retained MC4R. Moreover, expression of HSJ1b, a cochaperone that enhances degradation of Hsc70 clients, reduced cellular levels of MC4R. Hsp70 and Hsp90 chaperone systems collaborate in the cellular processing of clients. For MC4R, inhibition of endogenous Hsp90 by geldanamycin reduced receptor levels. By contrast, expression of the Hsp90 cochaperone Aha1 (activator of Hsp90 ATPase) increased cellular levels of MC4R. Finally, we demonstrate that signaling of intracellular retained MC4R mutants is increased in cells overexpressing Hsc70. These data indicate that cytosolic chaperone systems can facilitate rescue of intracellular retained MC4R by improving folding. They also support proteostasis networks as a potential target for MC4R-linked obesity.     

The balance between Hsp70 and its cochaperones Hdj1 and Bag1 determines its substrate-binding activity

Heat shock protein Hsp70 presents one of the most effective cell protective systems. Its protective activity is mostly due to the fact that Hsp70 is able to restore native conformation of newly synthesized or damaged proteins. Two other proteins. Hdj and Bag 1, are involved in the process, allowing Hsp70 to perform binding-release cyclec of target proteins. The aim of this study was to investigate interactions between cochaperones Hdj 1 and Bag 1, and the major cell chaperone Hsp in vitro. The accumulation of Hsp70 and Hdj 1 in human erythroleukemia K562 cells was stimulated by heat stress (43 degrees C, 60 min). Cells were collected at certain time periods after heat stress, and amounts of cell chaperones were measured using Western blotting and ELISA assay. The level of Hsp70 chaperone activity in cell extracts was estimated using original technique. The effects of exogenous cochaperones and of their parts on this activity were also investigated. The results of the study indicate that Hsp70 chaperone activity is regulated by the level of its cochaperones, especially Hdj 1. At the same time the amount of ATP appears to be critical for functional activity of Hsp70. Hdj 1 and Bag 1 peptides, which bind to Hsp70 with high affinity, are able to significally reduce its chaperone activity. This finding confirms the possibility of using peptide approach for regulation of Hsp70 function at the cellular and organismal levels.     

Hsp70 chaperone-based gel composition as a novel immunotherapeutic anti-tumor tool

The recent advances in designing Hsp70-based anti-cancer vaccines and the ability of the chaperone to penetrate inside a living cell prompted us to develop a non-invasive method for the treatment of surface tumors. We designed hydrogel-containing gel-forming substances and human recombinant Hsp70 and applied them on the surface of a 7-day-old B16F10 melanoma tumor. According to the results of histochemistry, Hsp70 diffused through skin layer inside the B16 tumor, and this transport was proved by biochemical data. The application of Hsp70 gel reduced the rate of tumor growth by 64 % and prolonged the life of animals by 46 %. Increased survival was correlated with the enhancement of B16-specific cytotoxicity and up-regulation of gamma–interferon production. Taken together, the data confirm the anti-tumor effect of pure recombinant Hsp70 delivered intratumorally and demonstrate the relevance of a novel non-invasive technology of Hsp70-based therapy.     

Exogenous heat shock protein HSP70 reduces response of human neuroblastoma cells to lipopolysaccharide

The effect of exogenous heat shock protein HSP70 and lipopolysaccharide (LPS) on the production of reactive oxygen species (ROS), TNFα secretion, and mRNA expression by human neuroblastoma SK-N-SH cells. It was shown that exogenous HSP70 protects neuroblastoma cells from the action of LPS. The protection mechanism of HSP70 includes a reduction in the production of ROS and TNFα and a decrease in the expression of TLR4 and IL-1β mRNA in SK-N-SH cells induced by LPS.     

Understanding the Functional Interplay between Mammalian Mitochondrial Hsp70 Chaperone Machine Components

Mitochondria biogenesis requires the import of several precursor proteins that are synthesized in the cytosol. The mitochondrial heat shock protein 70 (mtHsp70) machinery components are highly conserved among eukaryotes, including humans. However, the functional properties of human mtHsp70 machinery components have not been characterized among all eukaryotic families. To study the functional interactions, we have reconstituted the components of the mtHsp70 chaperone machine (Hsp70/J-protein/GrpE/Hep) and systematically analyzed in vitro conditions for biochemical functions. We observed that the sequence-specific interaction of human mtHsp70 toward mitochondrial client proteins differs significantly from its yeast counterpart Ssc1. Interestingly, the helical lid of human mtHsp70 was found dispensable to the binding of P5 peptide as compared with the other Hsp70s. We observed that the two human mitochondrial matrix J-protein splice variants differentially regulate the mtHsp70 chaperone cycle. Strikingly, our results demonstrated that human Hsp70 escort protein (Hep) possesses a unique ability to stimulate the ATPase activity of mtHsp70 as well as to prevent the aggregation of unfolded client proteins similar to J-proteins. We observed that Hep binds with the C terminus of mtHsp70 in a full-length context and this interaction is distinctly different from unfolded client-specific or J-protein binding. In addition, we found that the interaction of Hep at the C terminus of mtHsp70 is regulated by the helical lid region. However, the interaction of Hep at the ATPase domain of the human mtHsp70 is mutually exclusive with J-proteins, thus promoting a similar conformational change that leads to ATPase stimulation. Additionally, we highlight the biochemical defects of the mtHsp70 mutant (G489E) associated with a myelodysplastic syndrome.     

Molecular chaperone function of stress inducible Hsp70 is critical for intracellular multiplication of Toxoplasma gondii

Intracellular pathogens like Toxoplasma gondii often target proteins and pathways critical for host cell survival and stress response. Molecular chaperones encoded by the evolutionary conserved Heat shock proteins (Hsps) maintain proteostasis and are vital to cell survival following exposure to any form of stress. A key protein of this family is Hsp70, an ATP-driven molecular chaperone, which is stress inducible and often indiscernible in normal cells. Role of this protein with respect to intracellular survival and multiplication of protozoan parasite like T. gondii remains to be examined. We find that T. gondii infection upregulates expression of host Hsp70. Hsp70 selective inhibitor 2-phenylethynesulfonamide (PES) attenuates intracellular T. gondii multiplication. Biotinylated PES confirms selective interaction of this small molecule inhibitor with Hsp70. We show that PES acts by disrupting Hsp70 chaperone function which leads to dysregulation of host autophagy. Silencing of host Hsp70 underscores its importance for intracellular multiplication of T. gondii, however, attenuation achieved using PES is not completely attributable to host Hsp70 indicating the presence of other intracellular targets of PES in infected host cells. We find that PES is also able to target T. gondii Hsp70 homologue which was shown using PES binding assay. Detailed molecular docking analysis substantiates PES targeting of TgHsp70 in addition to host Hsp70. While establishing the importance of protein quality control in infection, this study brings to the fore a unique opportunity of dual targeting of host and parasite Hsp70 demonstrating how structural conservation of these proteins may be exploited for therapeutic design.     

Interaction of BAG1 and Hsp70 mediates neuroprotectivity and increases chaperone activity

It was recently shown that Bcl-2-associated athanogene 1 (BAG1) is a potent neuroprotectant as well as a marker of neuronal differentiation. Since there appears to exist an equilibrium within the cell between BAG1 binding to heat shock protein 70 (Hsp70) and BAG1 binding to Raf-1 kinase, we hypothesized that changing BAG1 binding characteristics might significantly alter BAG1 function. To this end, we compared rat CSM14.1 cells and human SHSY-5Y cells stably overexpressing full-length BAG1 or a deletion mutant (BAGDeltaC) no longer capable of binding to Hsp70. Using a novel yellow fluorescent protein-based foldase biosensor, we demonstrated an upregulation of chaperone in situ activity in cells overexpressing full-length BAG1 but not in cells overexpressing BAGDeltaC compared to wild-type cells. Interestingly, in contrast to the nuclear and cytosolic localizations of full-length BAG1, BAGDeltaC was expressed exclusively in the cytosol. Furthermore, cells expressing BAGDeltaC were no longer protected against cell death. However, they still showed accelerated neuronal differentiation. Together, these results suggest that BAG1-induced activation of Hsp70 is important for neuroprotectivity, while BAG1-dependent modulation of neuronal differentiation in vitro is not.     

Tubocapsenolide A, a novel withanolide, inhibits proliferation and induces apoptosis in MDA-MB-231 cells by thiol oxidation of heat shock proteins

Tubocapsenolide A (TA), a novel withanolide-type steroid, exhibits potent cytotoxicity against several human cancer cell lines. In the present study, we observed that treatment of human breast cancer MDA-MB-231 cells with TA led to cell cycle arrest at G(1) phase and apoptosis. The actions of TA were correlated with proteasome-dependent degradation of Cdk4, cyclin D1, Raf-1, Akt, and mutant p53, which are heat shock protein 90 (Hsp90) client proteins. TA treatment induced a transient increase in reactive oxygen species and a decrease in the intracellular glutathione contents. Nonreducing SDS-PAGE revealed that TA rapidly and selectively induced thiol oxidation and aggregation of Hsp90 and Hsp70, both in intact cells and in cell-free systems using purified recombinant proteins. Furthermore, TA inhibited the chaperone activity of Hsp90-Hsp70 complex in the luciferase refolding assay. N-Acetylcysteine, a thiol antioxidant, prevented all of the TA-induced effects, including oxidation of heat shock proteins, degradation of Hsp90 client proteins, and apoptosis. In contrast, non-thiol antioxidants (trolox and vitamin C) were ineffective to prevent Hsp90 inhibition and cell death. Taken together, our results demonstrate that the TA inhibits the activity of Hsp90-Hsp70 chaperone complex, at least in part, by a direct thiol oxidation, which in turn leads to the destabilization and depletion of Hsp90 client proteins and thus causes cell cycle arrest and apoptosis in MDA-MB-231 cells. Therefore, TA can be considered as a new type of inhibitor of Hsp90-Hsp70 chaperone complex, which has the potential to be developed as a novel strategy for cancer treatment.     

More than folding: localized functions of cytosolic chaperones

Compared with other chaperone systems, heat shock proteins Hsp70 and Hsp90 interact with a larger variety of co-chaperone proteins that regulate their activity or aid in the folding of specific substrate proteins. Although many co-chaperones are soluble cytosolic proteins, co-chaperone domains are also found in modular adaptor proteins, which are often localized to intracellular membranes or elements of the cytoskeleton. These specialized co-chaperones include auxilin, cysteine string protein, Tom70, UNC-45 and homologs of Bag-1. The localized co-chaperones can harness the ATP-dependent mechanisms of Hsp70 and Hsp90 to do conformational work in diverse functional contexts, including vesicle secretion and recycling, protein transport and the regulated assembly and/or disassembly of protein complexes. Such flexibility is unique to the cytosolic Hsp70 and Hsp90 chaperone system.     

Heat shock proteins, substrate specificity and modulation of function

Hsp70 is a universally conserved essential protein chaperone. In addition to its roles in many cellular process, Hsp70 protects cells from stress by binding partially unfolded proteins. Therefore, Hsp70 prevents protein aggregation and prion formation. Prions are infectious agents and are responsible for several fatal neurodegenerative diseases. Eukaryotic cells have several cytosolic Hsp70 isoforms, some constitutively expressed (Hsc70s), and others expressed only when cells are exposed to stress (Hsp70s). To determine which factors conferred functional specificity, we constructed hybrid Hsc/Hsp chaperones. All hybrids supported growth except those that contained the ATPase domain derived from inducible Hsp70. Thus, regulation of peptide binding by ATP hydrolysis must differ significantly between Hsc- and Hsp70 isoforms. In this work, nucleotide and peptide binding domain communication of Hsp70 proteins during their interaction with nucleotides and peptide substrates were investigated in vitro by using hybrid constructs.     

Hsp70 localizes differently from chaperone Hsc70 in mouse mesoangioblasts under physiological growth conditions

Mouse A6 mesoangioblasts express Hsp70 even in the absence of cellular stress. Its expression and its intracellular localization were investigated under normal growth conditions and under hyperthermic stress. Immunofluorescence assays indicated that without any stress a fraction of Hsp70 co-localized with actin microfilaments, in the cell cortex and in the contractile ring of dividing cells, while the Hsc70 chaperone did not. Hsp70 immunoprecipitation assays confirmed that a portion of Hsp70 binds actin. Immunoblot assays showed that both proteins were present in the nucleus. After heat treatment Hsp70 and actin continued to co-localize in the leading edge of A6 cells but not on microfilaments. Although Hsp70 and Hsc70 are both basally synthesized they showed different cellular distribution, suggesting an Hsp70 different activity respect to the Hsc70 chaperone. Moreover, we found Hsp70 in the culture medium as it has been described in other cell types.