Effects of expressing human Hsp70 and its deletion derivatives on heat killing and on RNA and protein synthesis

Hsp70 architecture: the formation of novel polymeric structures of Hsp70.1 and Hsc70 after proteotoxic stress

Heat induces Hsp70.1 (HSPA1) and Hsc70 (HSPA8) to form complex detergent insoluble cytoplasmic and nuclear structures that are distinct from the cytoskeleton and internal cell membranes. These novel structures have not been observed by earlier immunofluorescence studies as they are obscured by the abundance of soluble Hsp70.1/Hsc70 present in cells. While resistant to detergents, these Hsp70 structures display complex intracellular dynamics and are efficiently disaggregated by ATP, indicating that this pool of Hsp70.1/Hsc70 retains native function and regulation. Hsp70.1 promotes the repair of proteotoxic damage and cell survival after stress. In heated fibroblasts expressing Hsp70.1, Hsp70.1 and Hsc70 complexes are efficiently disaggregated before the cells undergo-heat induced apoptosis. In the absence of Hsp70.1, fibroblasts have increased rates of heat-induced apoptosis and maintain stable insoluble Hsc70 structures. The differences in the intracellular distribution of Hsp70.1 and Hsc70, combined with the ability of Hsp70.1, but not Hsc70, to promote the disaggregation of insoluble Hsp70.1/Hsc70 complexes, indicate that these two closely related proteins perform distinctly different cellular functions in heated cells.

Hsp72 chaperone function is dispensable for protection against stress-induced apoptosis

In addition to its role as a molecular chaperone, heat shock protein 72 (Hsp72) protects cells against a wide range of apoptosis inducing stresses. However, it is unclear if these two roles are functionally related or whether Hsp72 inhibits apoptosis by a mechanism independent of chaperone activity. The N-terminal adenosine triphosphatase domain, substrate-binding domain and the C-terminal EEVD regulatory motif of Hsp72 are all essential for chaperone activity. In this study, we show that Hsp72 mutants with a functional substrate-binding domain but lacking chaperone activity retain their ability to protect cells against apoptosis induced by heat and tumor necrosis factor alpha. In contrast, a deletion mutant lacking a functional substrate-binding domain has no protective capacity. The ability of the Hsp72 substrate-binding domain to inhibit apoptosis independent of the regulatory effects of the adenosine triphosphate-binding domain indicates that the inhibition of apoptosis may involve a stable binding interaction with a regulatory substrate rather than Hsp72 chaperone activity.

Hsp72 inhibits apoptosis upstream of the mitochondria and not through interactions with Apaf-1

Hsp72 protects cells against apoptosis in response to various stresses. By simultaneously measuring cytochrome c localization and nuclear morphology in mouse embryo fibroblasts, we have shown that Hsp72 blocks cytochrome c release from mitochondria in response to cytotoxic stress and that permeabilization of the outer mitochondrial membrane is the critical point in deciding the fate of the cell. Hsp72 did not inhibit apoptosis in mouse embryo fibroblasts once cytochrome c had been released from the mitochondria. Recent reports have claimed that Hsp72 can prevent caspase activation by inhibiting the oligomerization of Apaf-1 in the presence of cytochrome c and dATP. We now show that this apparent function of recombinant Hsp72 is due to the presence of salt in the Hsp72 preparation and that the same response can be achieved by the addition of heat-denatured Hsp72 in the same high salt buffer or by the high salt buffer alone. Hsp72 expressed in a range of different cell lines had no inhibitory effect on cytochrome c-stimulated caspase activity of cytosolic extracts. We conclude that the protective effect of Hsp72 occurs upstream of the mitochondria and not through the inhibition of the apoptosome.

Heat shock proteins and cardiovascular pathophysiology

In the eukaryotic cell an intrinsic mechanism is present providing the ability to defend itself against external stressors from various sources. This defense mechanism probably evolved from the presence of a group of chaperones, playing a crucial role in governing proper protein assembly, folding, and transport. Upregulation of the synthesis of a number of these proteins upon environmental stress establishes a unique defense system to maintain cellular protein homeostasis and to ensure survival of the cell. In the cardiovascular system this enhanced protein synthesis leads to a transient but powerful increase in tolerance to such endangering situations as ischemia, hypoxia, oxidative injury, and endotoxemia. These so-called heat shock proteins interfere with several physiological processes within several cell organelles and, for proper functioning, are translocated to different compartments following stress-induced synthesis. In this review we describe the physiological role of heat shock proteins and discuss their protective potential against various stress agents in the cardiovascular system.

HSP70-mediated acceleration of translational recovery after stress is independent of ribosomal RNA synthesis

HSP70 is known to protect cells against stressful events. In the present study, the hypothesis was investigated that elevated HSP70 levels protect RNA polymerase I during stress, leading to decreased inhibition of ribosomal RNA (rRNA) synthesis and accelerated recovery of protein translation after stress. To this end, transcriptional and translational activity was studied in H9c2 cells during recovery after a severe heat treatment (SHT, 1 h 45 degrees C) in the presence of elevated HSP70 levels. The latter was achieved by heat pretreatment or by adenovirus-mediated hsp70 gene transfer. Rates of transcription and translation were determined by measuring cellular 3H-labelled uridine and leucine incorporation, respectively. The two types of pretreatment did not affect basal rates of transcription and translation, immediately before SHT. During SHT, both transcriptional and translational rates dropped to less than 10% of basal levels in pretreated as well as non-pretreated cells. Two and four h after SHT, both transcriptional and translational rates were significantly higher in HSP70-overexpressing cells compared to non-pretreated cells. However, immediately after SHT, transcription rates were similarly depressed in non-pretreated and pretreated cells, showing that increased levels of HSP70 did not protect RNA polymerase I activity during SHT. Thus, the HSP70-mediated acceleration of translational recovery is not preceded in time by an enhanced recovery of rRNA synthesis. Therefore, the HSP70-mediated early recovery of protein synthesis after heat stress is independent of rRNA synthesis.

The chaperone function of hsp70 is required for protection against stress-induced apoptosis

Cellular stress can trigger a process of self-destruction known as apoptosis. Cells can also respond to stress by adaptive changes that increase their ability to tolerate normally lethal conditions. Expression of the major heat-inducible protein hsp70 protects cells from heat-induced apoptosis. hsp70 has been reported to act in some situations upstream or downstream of caspase activation, and its protective effects have been said to be either dependent on or independent of its ability to inhibit JNK activation. Purified hsp70 has been shown to block procaspase processing in vitro but is unable to inhibit the activity of active caspase 3. Since some aspects of hsp70 function can occur in the absence of its chaperone activity, we examined whether hsp70 lacking its ATPase domain or the C-terminal EEVD sequence that is essential for peptide binding was required for the prevention of apoptosis. We generated stable cell lines with tetracycline-regulated expression of hsp70, hsc70, and chaperone-defective hsp70 mutants lacking the ATPase domain or the C-terminal EEVD sequence or containing AAAA in place of EEVD. Overexpression of hsp70 or hsc70 protected cells from heat shock-induced cell death by preventing the processing of procaspases 9 and 3. This required the chaperone function of hsp70 since hsp70 mutant proteins did not prevent procaspase processing or provide protection from apoptosis. JNK activation was inhibited by both hsp70 and hsc70 and by each of the hsp70 domain mutant proteins. The chaperoning activity of hsp70 is therefore not required for inhibition of JNK activation, and JNK inhibition was not sufficient for the prevention of apoptosis. Release of cytochrome c from mitochondria was inhibited in cells expressing full-length hsp70 but not in cells expressing the protein with ATPase deleted. Together with the recently identified ability of hsp70 to inhibit cytochrome c-mediated procaspase 9 processing in vitro, these data demonstrate that hsp70 can affect the apoptotic pathway at the levels of both cytochrome c release and initiator caspase activation and that the chaperone function of hsp70 is required for these effects.

Production and characterization of a monoclonal antibody specific to the human 70-kDa heat shock protein

Heat shock protein 70 (hsp 70) plays major roles in apoptosis prevention and thermotolerance as well as molecular chaperoning. It is also expressed on the surface of human tumor cells, but not on normal cells, suggesting that hsp70 may be some tumor-associated antigen. To investigate the diverse functions of the protein species, various types of transgenic mice or cell models overexpressing human hsp70 have been made. In these models a monoclonal antibody (MAb) specific for the human hsp70 is highly desirable to distinguish the human from the endogenous mouse hsp70. It proved difficult to make this species-specific MAb, because the hsp70 homologues are members of a family of highly conserved, abundant, and ubiquitous proteins expressed in organisms ranging from bacteria to humans. In the present study, we prepared four MAbs against human hsp70. Three, HD 5, HD 7 and HD 11, recognize human and mouse hsp70. One, though, HD 8, recognizes human hsp70, but not mouse hsp70. By Western blot analysis of hsp70 deletion mutants, the epitope of the HD 8 MAb was determined as the 585-616 amino acid region of the human hsp70, a region with relatively low homology to mouse hsp70.

Chaperone Hsp27 inhibits translation during heat shock by binding eIF4G and facilitating dissociation of cap-initiation complexes

Inhibition of protein synthesis during heat shock limits accumulation of unfolded proteins that might damage eukaryotic cells. We demonstrate that chaperone Hsp27 is a heat shock-induced inhibitor of cellular protein synthesis. Translation of most mRNAs requires formation of a cap-binding initiation complex known as eIF4F, consisting of factors eIF4E, eIF4A, eIF4E kinase Mnk1, poly(A)-binding protein, and adaptor protein eIF4G. Hsp27 specifically bound eIF4G during heat shock, preventing assembly of the cap-initiation/eIF4F complex and trapping eIF4G in insoluble heat shock granules. eIF4G is a specific target of Hsp27, as eIF4E, eIF4A, Mnk1, poly(A)-binding protein, eIF4B, and eIF3 were not bound by Hsp27 and were not recruited into insoluble complexes. Dissociation of eIF4F was enhanced during heat shock by ectopic overexpression of Hsp25, the murine homolog of human Hsp27. Overexpression of Hsc70, a constitutive homolog of Hsp70, prevented loss of cap-initiation complexes and maintained eIF4G solubility. Purified Hsp27 specifically bound purified eIF4G in vitro, prevented in vitro translation, eliminated eIF4G interaction with protein binding factors, and promoted eIF4G insolubilization. These results therefore demonstrate that Hsp27 is a heat-induced inhibitor of eIF4F-dependent mRNA translation.

ATPase activity of the heat shock protein hsp72 is dispensable for its effects on dephosphorylation of stress kinase JNK and on heat-induced apoptosis

A major inducible heat shock protein, Hsp72, has previously been found to stimulate dephosphorylation (inactivation) of stress kinase JNK in heat-shocked cells and protect them from apoptosis. Using Rat-1 fibroblasts with constitutive expression of a human Hsp72 or its deletion mutant lacking an ATPase domain (C-terminal fragment (CTF)), we tested whether ATPase activity of Hsp72 is necessary for these effects. We found that expression of CTF markedly increased, similarly to the intact protein, JNK dephosphorylation in heat-shocked cells. As a result, JNK inactivation following heat shock occurred much faster in cells expressing either full-length or mutant Hsp72 than in parental cells and this was accompanied by suppression of heat-induced apoptosis. Thus, protein refolding activity of Hsp72 appears to be dispensable for its effect on JNK inactivation and apoptosis.

Heat shock protein 70 overexpression protects LLC-PK1 tubular cells from heat shock but not hypoxia

BACKGROUND

Overexpression of the 70 kDa heat shock protein (Hsp70) protects myocytes and neural cells from hypoxic injury. In contrast, Hsp70 induction in the kidney after ischemic or thermal preconditioning does not correlate well with protection from hypoxic injury. Herein, we directly tested if Hsp70 overexpression protects LLC-PK1 porcine tubular epithelial cells from hypoxic or thermal injury.

METHODS

LLC-PK1 cells were either cotransfected with an Hsp70 and a luciferase expression vector or singly transfected with the luciferase expression vector. Loss of intracellular luciferase activity was used to assess injury after exposure to hypoxia or hyperthermia and after recovery under normal growth conditions.

RESULTS

Overexpression of Hsp70 decreased loss of and improved restoration of intracellular luciferase activity in LLC-PK1 cells exposed to hyperthermia. In contrast, Hsp70 overexpression did not decrease the loss of or improve restoration of luciferase activity in cells exposed to hypoxia.

CONCLUSIONS

These results suggest that Hsp70 overexpression is sufficient to protect LLC-PK1 proximal tubular cells from hyperthermia but is not sufficient for protection from hypoxia.

Reduction of heat-shock protein-70 after prolonged treatment with retinoids: biological and clinical implications

Heat shock proteins (HSPs) are a group of highly conserved polypeptides involved in cellular response to heat or other physical or chemical stresses. It has been recently reported that HSPs could play a role in cellular differentiation. In this study we have evaluated, by a cytofluorimetric method, the presence of HSP-70 in HL-60 cells during treatment with all-trans retinoic acid (ATRA), 9-cis retinoic acid (9-cis RA), and 13-cis retinoic acid (13-cis RA). After 1 and 3 days of incubation at 10(-7) M, HSP-70 did not show any variation compared to control; prolonging the exposure, together with the appearance of cellular differentiation along the granulocytic pathway and apoptosis, a progressive decrease of HSP-70 was observed that, after 8 days of treatment, was reduced by 40% with ATRA and by 28% with 9-cis RA compared to untreated samples, while only minimal changes were evident by incubating the cells with 13-cis RA. Reduction of HSP-70 was not associated with decreased protein synthesis, as demonstrated by [3H] leucine incorporation. Double labeling with propidium iodide showed a decrease in HSP-70 in all the phases of the cell cycle concomitant with a reduced percentage of cycling cells in ATRA-treated samples. Dot blot and Northern blot analysis demonstrated no change in HSP-70 mRNA after retinoid treatment, thus suggesting a post-transcriptional regulation of the phenomenon. This reduced production of HSP-70 caused by ATRA and by 9-cis RA, though to a lesser extent, could render the cells more sensitive to cytotoxic agents and could provide the rationale for the efficacy of ATRA + chemotherapy combinations.