Interplay between recombinant Hsp70 and proteasomes: proteasome activity modulation and ubiquitin-independent cleavage of Hsp70

The heat shock protein 70 (Hsp70, human HSPA1A) plays indispensable roles in cellular stress responses and protein quality control (PQC). In the framework of PQC, it cooperates with the ubiquitin-proteasome system (UPS) to clear damaged and dysfunctional proteins in the cell. Moreover, Hsp70 itself is rapidly degraded following the recovery from stress. It was demonstrated that its fast turnover is mediated via ubiquitination and subsequent degradation by the 26S proteasome. At the same time, the effect of Hsp70 on the functional state of proteasomes has been insufficiently investigated. Here, we characterized the direct effect of recombinant Hsp70 on the activity of 20S and 26S proteasomes and studied Hsp70 degradation by the 20S proteasome in vitro. We have shown that the activity of purified 20S proteasomes is decreased following incubation with recombinant human Hsp70. On the other hand, high concentrations of Hsp70 activated 26S proteasomes. Finally, we obtained evidence that in addition to previously reported ubiquitin-dependent degradation, Hsp70 could be cleaved independent of ubiquitination by the 20S proteasome. The results obtained reveal novel aspects of the interplay between Hsp70 and proteasomes.     

The ubiquitin-related BAG-1 provides a link between the molecular chaperones Hsc70/Hsp70 and the proteasome

The BAG-1 protein modulates the chaperone activity of Hsc70 and Hsp70 in the mammalian cytosol and nucleus. Remarkably, BAG-1 possesses a ubiquitin-like domain at its amino terminus, suggesting a link to the ubiquitin/proteasome system. Here we show that BAG-1 is indeed associated with the 26 S proteasome in HeLa cells. Binding of the chaperone cofactor to the proteolytic complex is regulated by ATP hydrolysis and is not mediated by Hsc70 and Hsp70. The presented findings reveal a role of BAG-1 as a physical link between the Hsc70/Hsp70 chaperone system and the proteasome. In fact, targeting of BAG-1 to the proteasome promotes an association of the chaperones with the proteolytic complex in vitro and in vivo. A regulatory function of the chaperone cofactor at the interface between protein folding and protein degradation is thus indicated.     

Hsp70 regulates the doxorubicin-mediated heart failure in Hsp70-transgenic mice

The aim of this study was to investigate the potential protective effect of the Hsp70 protein in the cardiac dysfunction induced by doxorubicin (DOX) and the mechanisms of its action. For this purpose, we used both wild-type mice (F1/F1) and Hsp70-transgenic mice (Tg/Tg) overexpressing human HSP70. Both types were subjected to chronic DOX administration (3 mg/kg intraperitoneally every week for 10 weeks, with an interval from weeks 4 to 6). Primary cell cultures isolated from embryos of these mice were also studied. During DOX administration, the mortality rate as well as weight reduction were lower in Tg/Tg compared to F1/F1 mice (P < 0.05). In vivo cardiac function assessment by transthoracic echocardiography showed that the reduction in left ventricular systolic function observed after DOX administration was lower in Tg/Tg mice (P < 0.05). The study in primary embryonic cell lines showed that the apoptosis after incubation with DOX was reduced in cells overexpressing Hsp70 (Tg/Tg), while the apoptotic pathway that was activated by DOX administration involved activated protein factors such as p53, Bax, caspase-9, caspase-3, and PARP-1. In myocardial protein extracts from identical mice with DOX-induced heart failure, the particular activated apoptotic pathway was confirmed, while the presence of Hsp70 appeared to inhibit the apoptotic pathway upstream of the p53 activation. Our results, in this DOX-induced heart failure model, indicate that Hsp70 overexpression in Tg/Tg transgenic mice provides protection from myocardial damage via an Hsp70-block in p53 activation, thus reducing the subsequent apoptotic mechanism.     

Hsp70 in oncology

Hsp70 classes of molecular chaperones are highly conserved in all organisms and play an essential role in the maintenance of cellular homeostasis. Hsp70s assist nascent chain protein folding and denatured proteins, as well as the import of proteins to the organelles, and solubilization of aggregated proteins. ATPase function is required for Hsp70 function. Hsp70s use ATP hydrolysis driven mechanism for substrate protein binding and release. Various Hsps are unregulated in cancers but their significance for tumor growth is poorly understood. Studies have linked Hsp70 to several types of carcinoma. Human Hsp70s allow proliferation of cancer cells and suppress apoptotic and senescence pathways. This review presents Hsp70s role for growth of transformed cells and the current state of Hsp70 as a drug target along with recent patents in humans in this particular area.     

Isolation and identification of a polypeptide in the Hsp 70 family that binds substance P.

During the course of an attempt to purify the substance P (SP) receptor from horse salivary glands by substance P-affinity chromatography, a polypeptide of Mr = 78,000 was isolated. The first fifteen amino acid residues at the amino terminus were determined and, unexpectedly, were found to be identical with the amino terminus of a glucose-regulated protein (GRP) of the same molecular weight, a protein that has been identified as a member of the heat shock protein family. This finding raises the intriguing possibility that SP may interact in vivo with GRPs and other members of the heat shock protein family and play a role in modulating their biological activities.     

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.     

All in the family: atypical Hsp70 chaperones are conserved modulators of Hsp70 activity

Divergent relatives of the Hsp70 protein chaperone such as the Hsp110 and Grp170 families have been recognized for some time, yet their biochemical roles remained elusive. Recent work has revealed that these "atypical" Hsp70s exist in stable complexes with classic Hsp70s where they exert a powerful nucleotide-exchange activity that synergizes with Hsp40/DnaJ-type cochaperones to dramatically accelerate Hsp70 nucleotide cycling. This represents a novel evolutionary transition from an independent protein-folding chaperone to what appears to be a dedicated cochaperone. Contributions of the atypical Hsp70s to established cellular roles for Hsp70 now must be deciphered.     

Defining the requirements for Hsp40 and Hsp70 in the Hsp90 chaperone pathway

The Hsp90 chaperoning pathway and its model client substrate, the progesterone receptor (PR), have been used extensively to study chaperone complex formation and maturation of a client substrate in a near native state. This chaperoning pathway can be reconstituted in vitro with the addition of five proteins plus ATP: Hsp40, Hsp70, Hop, Hsp90, and p23. The addition of these proteins is necessary to reconstitute hormone-binding capacity to the immuno-isolated PR. It was recently shown that the first step for the recognition of PR by this system is binding by Hsp40. We compared type I and type II Hsp40 proteins and created point mutations in Hsp40 and Hsp70 to understand the requirements for this first step. The type I proteins, Ydj1 and DjA1 (HDJ2), and a type II, DjB1 (HDJ1), act similarly in promoting hormone binding and Hsp70 association to PR, while having different binding characteristics to PR. Ydj1 and DjA1 bind tightly to PR whereas the binding of DjB1 apparently has rapid on and off rates and its binding cannot be observed by antibody pull-down methods using either purified proteins or cell lysates. Mutation studies indicate that client binding, interactions between Hsp40 and Hsp70, plus ATP hydrolysis by Hsp70 are all required to promote conformational maturation of PR via the Hsp90 pathway.     

Co-expression of human chaperone Hsp70 and Hsdj or Hsp40 co-factor increases solubility of overexpressed target proteins in insect cells

The insect-baculovirus expression system has proved particularly useful for producing recombinant proteins that are biologically active. Overexpression of foreign proteins using the recombinant baculovirus system is often accompanied by aggregation of the overexpressed protein, which is thought to be due to a limitation of the translated protein folding in the infected cells. Co-infection of a recombinant baculovirus capable of expressing the human chaperone Hsp70 slightly increased the solubility of the overexpressed Epstein-Barr virus replication protein, BZLF1. Co-expression of Hsp70 and its co-factor, Hsdj or Hsp40, was here found to improve the solubility of the target protein several fold. Thus, a baculovirus expression system producing these molecular chaperones may find application for improved production of target foreign gene products in insect cells.     

Binding of the maize cytosolic Hsp70 to calmodulin, and identification of calmodulin-binding site in Hsp70

Using a gel-overlay technique of biotinylated calmodulin (CaM), we showed that maize cytosolic Hsp70 protein could bind to CaM in the presence of 1 mM CaCl2. The purified maize cytosolic Hsp70 inhibited the activity of CaM-dependent NADK in a concentration-dependent manner. A synthetic peptide, which possesses the 21 amino acid sequence, PRALRRLRTACERAKRTLSST, at positions 261-281 in maize cytosolic Hsp70, could associate with CaM in the presence of 1 mM calcium. The synthetic peptide inhibited CaM-dependent NADK activity and PDE activity. This indicates that the 21-amino acid sequence at positions 261-281 is the CaM-binding site. The binding of CaM to Hsp70 inhibited the ATPase activity of Hsp70. The possible regulator function of Hsp70 in cell signaling events in response to heat stress is discussed.     

Substrate transfer from the chaperone Hsp70 to Hsp90

Hsp90 is an essential chaperone protein in the cytosol of eukaryotic cells. It cooperates with the chaperone Hsp70 in defined complexes mediated by the adaptor protein Hop (Sti1 in yeast). These Hsp70/Hsp90 chaperone complexes play a major role in the folding and maturation of key regulatory proteins in eukaryotes. Understanding how non-native client proteins are transferred from one chaperone to the other in these complexes is of central importance. Here, we analyzed the molecular mechanism of this reaction using luciferase as a substrate protein. Our experiments define a pathway for luciferase folding in the Hsp70/Hsp90 chaperone system. They demonstrate that Hsp70 is a potent capture device for unfolded protein while Hsp90 is not very efficient in this reaction. When Hsp90 is absent, in contrast to the in vivo situation, Hsp70 together with the two effector proteins Ydj1 and Sti1 exhibits chaperone activity towards luciferase. In the presence of the complete chaperone system, Hsp90 exhibits a specific positive effect only in the presence of Ydj1. If this factor is absent, the transferred luciferase is trapped on Hsp90 in an inactive conformation. Interestingly, identical results were observed for the yeast and the human chaperone systems although the regulatory function of human Hop is completely different from that of yeast Sti1.     

Transgenic mice expressing the human inducible Hsp70 have hippocampal neurons resistant to ischemic injury

Using transgenic mice constitutively expressing the human inducible Hsp70, we examined the role of Hsp70 on cell survival after focal cerebral oschemia. Twenty-four hours after premanent occlusion of the middle cerebral artery, no difference in infarct area was detected between Hsp70-transgenic and non-transgenic mice. In the non-transgenic mice, many pyramidal neurons of the ipsilateral hippocampus were observed to be pykontic. However, in all Hsp70-transgenic mice, hippocampal pyramidal neurons showed normal morphology and no evidence of pyknosis. This suggests that constitutive expression of Hsp70 reduces the extent of damage following permanent middle cerebral artery occlusion.     

Structural studies of the Hsp70/Hsp90 organizing protein of Plasmodium falciparum and its modulation of Hsp70 and Hsp90 ATPase activities

HOP is a cochaperone belonging to the foldosome, a system formed by the cytoplasmic Hsp70 and Hsp90 chaperones. HOP acts as an adapter protein capable of transferring client proteins from the first to the second molecular chaperone. HOP is a modular protein that regulates the ATPase activity of Hsp70 and Hsp90 to perform its function. To obtain more detailed information on the structure and function of this protein, we produced the recombinant HOP of Plasmodium falciparum (PfHOP). The protein was obtained in a folded form, with a high content of α-helix secondary structure. Unfolding experiments showed that PfHOP unfolds through two transitions, suggesting the presence of at least two domains with different stabilities. In addition, PfHOP primarily behaved as an elongated dimer in equilibrium with the monomer. Small-angle X-ray scattering data corroborated this interpretation and led to the reconstruction of a PfHOP ab initio model as a dimer. Finally, the PfHOP protein was able to inhibit and to stimulate the ATPase activity of the recombinant Hsp90 and Hsp70–1, respectively, of P. falciparum. Our results deepened the knowledge of the structure and function of PfHOP and further clarified its participation in the P. falciparum foldosome.     

Cooperative interaction of Hsp40 and TPR1 with Hsp70 reverses Hsp70-HspBp1 complex formation

Hsp40 and TPR1 are chaperone adaptors that regulate Hsp70-dependent folding processes by interacting with the amino terminal and carboxy terminal domains of Hsp70, respectively. In this study, we report cooperative interactions involving Hsp70, Hsp40, and TPR1 that enhance Hsp70-dependent folding of chemically denatured substrates. Hsp40 and Hsp70 dependent folding of chemically denatured luciferase was enhanced by up to 80% when TPR1 was also present. HspBp1, a negative modulator of Hsp70, completely inhibited Hsp70-dependent folding in the presence of Hsp40. However, when TPR1 was included in the reaction, the inhibitory effect of HspBp1 was reversed. To analyze the interactions, Kd analysis and competition assays were carried out. The Kds of the interactions of Hsp40, TRP1, and HspBp1 with Hsp70 were 0.5, 0.6, and 0.04 mM, respectively. Interestingly, the Hsp70/HspBp1 complex could only be dissociated in the presence of both Hsp40 and TPR1, suggesting cooperative interaction between Hsp70, Hsp40 and TPR1. To examine these interactions in vivo, we established a tetracycline-regulatable Hela cell line that expresses Hsp70 in the absence of doxycycline. Expression of HspBp1 inhibited Hsp70-dependent folding of heat-denatured luciferase, and this effect was only reversed in the presence of Hsp40 and TPR1. Our findings reveal a novel mechanism of positive regulation of Hsp70-dependent folding.     

Molecular chaperones Hsp90 and Hsp70 deliver preproteins to the mitochondrial import receptor Tom70

The role of cytosolic factors in protein targeting to mitochondria is poorly understood. Here, we show that in mammals, the cytosolic chaperones Hsp90 and Hsp70 dock onto a specialized TPR domain in the import receptor Tom70 at the outer mitochondrial membrane. This interaction serves to deliver a set of preproteins to the receptor for subsequent membrane translocation dependent on the Hsp90 ATPase. Disruption of the chaperone/Tom70 recognition inhibits the import of these preproteins into mitochondria. In yeast, Hsp70 rather than Hsp90 is used in import, and Hsp70 docking is required for the formation of a productive preprotein/Tom70 complex. We outline a novel mechanism in which chaperones are recruited for a specific targeting event by a membrane-bound receptor.     

Independent regulation of Hsp70 and Hsp90 chaperones by Hsp70/Hsp90-organizing protein Sti1 (Hop1)

Hsp70 and Hsp90 protein chaperones cooperate in a protein-folding pathway required by many "client" proteins. The co-chaperone Sti1p coordinates functions of Hsp70 and Hsp90 in this pathway. Sti1p has three tetratricopeptide repeat (TPR) domains. TPR1 binds Hsp70, TPR2a binds Hsp90, and the ligand for TPR2b is unknown. Although Sti1p is thought to be dedicated to the client folding pathway, we earlier showed that Sti1p regulated Hsp70, independently of Hsp90, in a way that impairs yeast [PSI+] prion propagation. Using this prion system to monitor Sti1p regulation of Hsp70 and an Hsp90-inhibiting compound to monitor Hsp90 regulation, we identified Sti1p mutations that separately affect Hsp70 and Hsp90. TPR1 mutations impaired Sti1p regulation of Hsp70, but deletion of TPR2a and TPR2b did not. Conversely, TPR2a and TPR2b mutations impaired Sti1p regulation of Hsp90, but deletion of TPR1 did not. All Sti1p mutations variously impaired the client folding pathway, which requires both Hsp70 and Hsp90. Thus, Sti1p regulated Hsp70 and Hsp90 separately, Hsp90 is implicated as a TPR2b ligand, and mutations separately affecting regulation of either chaperone impair a pathway that is dependent upon both. We further demonstrate that client folding depended upon bridging of Hsp70 and Hsp90 by Sti1p and find conservation of the independent regulation of Hsp70 and Hsp90 by human Hop1.     

Alteration of familial ALS-linked mutant SOD1 solubility with disease progression: its modulation by the proteasome and Hsp70

Accumulation of misfolded Cu/Zn superoxide dismutase (SOD1) occurs in patients with a subgroup of familial amyotrophic lateral sclerosis (fALS). To identify the conversion of SOD1 from a normally soluble form to insoluble aggregates, we investigated the change of SOD1 solubility with aging in fALS-linked H46R SOD1 transgenic mice. Mutant SOD1 specifically altered to insoluble forms, which were sequentially separated into Triton X-100-insoluble/sodium dodecyl sulfate (SDS)-soluble and SDS-insoluble/formic acid-soluble species. In spinal cords, the levels of SDS-dissociable soluble SOD1 monomers and SDS-stable soluble dimers were significantly elevated before motor dysfunction onset. In COS-7 cells expressing H46R SOD1, treatment with proteasome inhibitors recapitulated the alteration of SOD1 solubility in transgenic mice. In contrast, overexpression of Hsp70 reduced accumulation of mutant-specific insoluble SOD1. SDS-soluble low molecular weight species of H46R SOD1 may appear as early misfolded intermediates when their concentration exceeds the capacity of the proteasome and molecular chaperones.     

Induction of heat shock protein 70 (Hsp70) by proteasome inhibitor MG 132 protects articular chondrocytes from cellular death in vitro and in vivo

The aim of this work was to determine whether Hsp70 overexpression via proteasome inhibitor MG132 was able to protect chondrocytes towards mono-iodoacetate (MIA) cytotoxicity both in vitro and in vivo. In vitro, overexpression of Hsp70 via MG132 was significantly able to protect chondrocytes from MIA toxicity (MTT/LDH analyses). Hsp70 essentially mediated this chondroprotective effect as demonstrated by antisense strategy. In vivo, chondrocytic overexpression of Hsp70, after a preventive intra-articular injection of MG132 in rat knee, was sufficient to decrease the severity of OA-induced MIA lesions, as demonstrated histologically and biochemically. In conclusion, intracellular overexpression of Hsp70, through proteasome inhibition, could be an interesting tool in protecting chondrocytes from cellular injuries, either necrotic or apoptotic in nature, and thus might be a novel chondroprotective modality in rat experimental OA.