Activation of heat shock gene transcription by heat shock factor 1 involves oligomerization, acquisition of DNA-binding activity, and nuclear localization and can occur in the absence of stress

Heat shock response--pathophysiological implications

All organisms exposed to environmental stress conditions share a common molecular response characterized by a dramatic change in the pattern of gene expression followed by an elevated synthesis of heat shock or stress proteins. These proteins function as molecular chaperones to protect cells from environmental stress damage by binding to partially denatured proteins, dissociating protein aggregates, and regulating the correct folding and intracellular translocation of newly synthesized polypeptides. Accumulating evidence supports a role for heat shock proteins in a number of disease states of which inflammatory reactions and ischaema provide the best studied examples. The inducible heat shock response involves transcriptional gene activation mediated by specific regulatory proteins called heat shock transcription factors, which bind to the promoter of heat shock genes in a sequence-specific manner. However, the signalling pathways leading to the activation of these transcription factors need to be characterized in more detail to be able to understand the role, cause, or consequence, of heat shock proteins in human diseases. This review presents recent progress in unravelling the regulation of heat shock gene expression in cells subjected to heat or other forms of stress. By using inflammatory responses and myocardial ischaema as examples, the putative use of heat shock proteins are discussed as targets for future therapeutic applications.

Characterization of hypothermia-induced cellular stress response in mouse tissues

Cells respond to adverse environmental conditions by expressing heat shock proteins, which serve to protect cells from harmful effects of the stress conditions. In this study we demonstrated that mice subjected to whole body hypothermia induced the cellular stress response, resulting in the increased expression of hsp72 mRNA in brain, heart, kidney, liver, and lung. We performed a detailed analysis of the major parameters of the stress response and found that cold induction of hsp expression is mediated by heat shock factor 1 (HSF1), which is also responsible for heat induction of the cellular stress response. However, there are differences in the mechanisms of HSF1 activation by hypothermia versus hyperthermia, as hypothermia does not cause the hyperphosphorylation of HSF1 that is characteristic of heat-activated HSF1.

Proteolytic mapping of heat shock transcription factor domains

Heat shock transcription factors (HSFs) of higher eukaryotes respond to physical and cellular stress signals by trimerizing, binding to a specific site on DNA, and transactivating genes encoding the heat shock proteins. In this work, limited proteolysis was used as a biochemical probe of the domain organization of Drosophila HSF. Both unshocked monomeric and heat-shocked trimeric HSF possess an internal protease-sensitive region located between the amino-terminal and carboxyl-terminal hydrophobic heatad repeats, suggesting that this is a less structured region compared to those defined for DNA-binding, trimerization, and transactivation. For a few cleavage sites, the heat-shocked form of HSF is more accessible to proteases than the unshocked form, providing an additional diagnostic marker for inducible changes in conformation or modification between the latent and activated forms of HSF.

Sequential phosphorylation by mitogen-activated protein kinase and glycogen synthase kinase 3 represses transcriptional activation by heat shock factor-1

Mammalian heat shock genes are regulated at the transcriptional level by heat shock factor-1 (HSF-1), a sequence-specific transcription factor. We have examined the role of serine phosphorylation of HSF-1 in the regulation of heat shock gene transcription. Our experiments show that mitogen-activated protein kinases (MAPKs) of the ERK-1 family phosphorylate HSF-1 on serine residues and repress the transcriptional activation of the heat shock protein 70B (HSP70B) promoter by HSF-1 in vivo. These effects of MAPK are transmitted through a specific serine residue (Ser-303) located in a proline-rich sequence within the transcriptional regulatory domain of human HSF-1. However, despite the importance of Ser-303 in transmitting the signal from the MAPK cascade to HSP70 transcription, there was no evidence that Ser-303 could be phosphorylated by MAPK in vitro, although an adjacent residue (Ser-307) was avidly phosphorylated by MAPK. Further studies revealed that Ser-303 is phosphorylated by glycogen synthase kinase 3 (GSK3) through a mechanism dependent on primary phosphorylation of Ser-307 by MAPK. Secondary phosphorylation of Ser-303 by GSK3 may thus repress the activity of HSF-1, and its requirement for priming by MAPK phosphorylation of Ser-307 provides a potential link between the MAPK cascade and HSF-1. Our experiments thus indicate that MAPK is a potent inhibitor of HSF-1 function and may be involved in repressing the heat shock response during normal growth and development and deactivating the heat shock response during recovery from stress.

Sodium salicylate decreases intracellular ATP, induces both heat shock factor binding and chromosomal puffing, but does not induce hsp 70 gene transcription in Drosophila

Sodium salicylate has long been known to be an inducer of the heat shock puffs and presumably heat shock gene transcription in the polytene chromosomes of Drosophila salivary gland cells. Stress-induced transcription of the heat shock genes is mediated by the transcription factor known as Heat Shock Factor (HSF). In yeast, sodium salicylate has been reported to induce the DNA binding of HSF but not heat shock gene transcription itself, and similar findings have been reported in human cells. This apparent discrepancy in the induction of certain aspects of the heat shock response between these organisms prompted us to carefully reexamine the induction of the heat shock response in Drosophila salivary gland cells of third instar larvae and Drosophila tissue culture (SL2) cells. Sodium salicylate (3-30 mM) decreases intracellular ATP levels in SL2 cells and induces HSF binding activity in SL2 and salivary gland cells in a dose-dependent manner. Despite the induction of HSF binding and heat shock puffs in polytene chromosomes, we found no evidence for increased hsp 70 gene transcription suggesting that chromosomal puffing and gene transcription may be separable events. Salicylate did not induce the HSF hyperphosphorylation that is normally associated with HSF activation. Furthermore, salicylate (30 mM) prevented heat-induced hyperphosphorylation of HSF and hsp 70 gene transcription indicating that salicylate's inhibitory effect on hsp 70 transcription may be independent of its effect on HSF binding activity. We propose that the reduction in intracellular ATP caused by the addition of salicylate likely plays a role in the activation of HSF binding and the inhibition of both HSF hyperphosphorylation and hsp 70 gene transcription.

Evidence that a rapidly turning over protein, normally degraded by proteasomes, regulates hsp72 gene transcription in HepG2 cells

Heat shock protein 72/73 (Hsp70) is a cytosolic molecular chaperone that carries out fundamental roles under both normal and stress situations. There is great interest in delineating the mechanisms whereby Hsp70 levels are regulated. We observed that N-acetyl-leucyl-leucyl-norleucinal (ALLN), a synthetic aldehydic tripeptide that inhibits proteasomes, markedly induced Hsp70 levels (up to 30-fold above base line in HepG2 cells and human endothelial cells). Induction of Hsp70 by ALLN was dose-dependent and not related to cell toxicity. ALLN selectively increased Hsp70 levels without affecting Hsp25, Hsp27, Hsp60, Hsp86, Hsp90, Hsp104, or Bip (immunoglobulin heavy chain binding protein) in HepG2 cells. ALLN induced Hsp70 not only by stabilizing the protein but also by dramatically increasing its synthesis. The modulation of Hsp70 synthesis by ALLN resulted from a rapid and marked increase in transcription of the hsp72 gene, since the induction of hsp72 mRNA was blocked in cells co-treated with actinomycin D. hsp72 mRNA levels were affected in a time-dependent manner by exposure to ALLN; significant elevations occurred within 60 min of treatment, and a decline to background levels was observed by 7 h of recovery. The ALLN-induced increase in hsp72 gene expression was associated with trimerization of the heat shock transcriptional factor (HSF1). ALLN did not affect the steady-state level of HSF1 protein. The effects of ALLN appeared to require de novo protein synthesis, since the induction of both HSF1 trimerization and hsp72 transcription was blocked by co-treatment with cycloheximide. When we tested a series of protease inhibitors, only the related aldehydic tripeptides, N-acetyl-leucyl-leucyl-methioninal and the proteasome inhibitor, Cbz-leucyl-leucyl-leucinal, induced Hsp70 levels. The specific proteasome inhibitor, lactacystin, which has a different structure, also induced Hsp70 levels. Overall, our results suggest that a rapidly turning over protein that is normally degraded by proteasomes may be involved in the regulation of Hsp70 synthesis via effects on the hsp70 transcriptional factor, HSF1.

Identification and characterization of promoter elements responsible for the induction of the albumin gene by heat shock in early embryonic rat liver

We had reported earlier that the expression of albumin increases upon heat shock in embryonic rat liver cells at about 12-13 days of gestation. Here, we report on the identification of heat shock elements (HSEs) within -450 bp of the rat albumin promoter using chloramphenicol acetyl transferase (CAT) assays done with the extracts from H4II-E-C3 cells transfected with plasmids carrying the CAT reporter gene under the control of different deletion fragments of the rat albumin promoter. Gel retardation assays done with synthetic oligonucleotides representing putative HSEs in the rat albumin promoter and H4II-E-C3 cell extracts show that the heat shock factors bind this region in a sequence-specific and reversible manner. Super-shift assays demonstrated that the HSEs present in the rat albumin promoter are bound by HSF1 and not by HSF2. This effect of heat shock on the expression of rat serum albumin is seen only in the liver and is not observed in other tissues, suggesting that HSF-mediated activation of albumin gene cannot overcome the negative regulatory factors present in other tissues. In addition to the HSEs, we have identified a putative GAGA factor binding site in the rat albumin promoter at -228 bp to -252 bp position. These GAGA repeats are bound in a sequence-specific and reversible manner by two factors in a nonstressed cell, whereas only one of these two factors continues to bind the GAGA repeats under heat shock conditions. The physiological significance of these results is discussed.

Inhibition of HSP70 expression by calcium ionophore A23187 in human cells. An effect independent of the acquisition of DNA-binding activity by the heat shock transcription factor

Heat shock proteins (HSPs) are induced in mammalian cells in a variety of pathophysiological states and have an important role in cytoprotection in vitro and in vivo. In this study, we report that the calcium ionophore A23187, a glucose-regulated protein (GRP) inducer, dramatically inhibits HSP70 synthesis and HSP70 mRNA transcription after induction by heat shock, sodium arsenite, or prostaglandin A1 treatment in human K562 cells. A23187 does not suppress, and it actually prolongs, the DNA-binding activity of the human heat shock transcription factor (HSF), while it alters HSF1 phosphorylation in heat shock-treated cells. To inhibit HSP70 expression, A23187 needs to be present during heat shock, while treatment before or after heat shock does not affect HSP70 mRNA transcription. The GRP inducer thapsigargin, which specifically inhibits the endoplasmic reticulum Ca2+-ATPase, has no effect on heat-induced HSP70 synthesis, indicating that A23187 inhibitory activity is not due to depletion of intracellular calcium stores and is independent of the concomitant induction of GRP genes. Inhibition of HSP70 expression is correlated with alterations in HSF1 phosphorylation in heat-shocked cells, but not in sodium arsenite-treated cells, indicating that different mechanisms may be involved in mediating A23187 inhibitory activity.

The anti-cancer drug cisplatin induces H25 in Ehrlich ascites tumor cells by a mechanism different from transcriptional stimulation influencing predominantly H25 translation

Treatment of Ehrlich ascites tumor (EAT) cells with the anti-cancer drug cisplatin induces an increase of the intracellular level of the small heat shock protein Hsp25 without stimulating the general stress response. The mechanism of this induction process was investigated at the levels of gene transcription, protein synthesis and stability. We show that an increased synthesis of Hsp25 is predominantly responsible for the increased intracellular level of this protein. In addition, there is a slightly increased metabolic stability of Hsp25 in cisplatin-treated EAT cells. In contrast to the mechanism of Hsp25 induction by heat shock and other chemical stresses, stimulated synthesis of Hsp25 after treatment with cisplatin is not the result of increased transcription of the hsp25 gene. Cisplatin treatment does not significantly influence the oligomerization of heat shock transcription factors 1 and 2, hsp25 promoter activity or hsp25 mRNA stability, as judged by cross-linking experiments, reporter gene assay and Northern blot analysis. Hence, cisplatin specifically induces Hsp25 synthesis at the level of mRNA translation without any changes in hsp25 gene transcription.

Basal expression of stress-inducible hsp70 mRNA detected in hippocampal and cortical neurons of normal rabbit brain

In response to stresses, such as elevated temperature, cells increase synthesis of a group of highly conserved proteins known as heat shock proteins (hsps). Here, we report detection of basal expression of the stress-inducible hsp70 mRNA species in neurons of the normal rabbit brain. By regional Northern blot analysis, basal levels of hsp70 mRNA were observed in control hippocampus, cortical layers, thalamus, and kidney. Using radioactive in situ hybridization, similar patterns of expression were noted for constitutive hsc70 mRNA and hsp70 mRNA in the unstressed rabbit forebrain. Non-radioactive (DIG) in situ hybridization allowed localization of both heat shock mRNA species to hippocampal neurons. In addition, a dual in situ hybridization protocol, which allowed colocalization of two mRNAs to a single cell, demonstrated that hsp70 and hsc70 mRNAs are expressed in the same hippocampal and cortical neurons.

Excess protein in nuclei isolated from heat-shocked cells results from a reduced extractability of nuclear proteins

An excellent correlation has been established between the quantity of protein associated with nuclei isolated from heat-shocked cells and the level of hyperthermic cell killing. However, controversy remains about whether increases in nuclear-associated protein result from a heat-induced migration of cytoplasmic proteins into the nucleus or because hyperthermia reduces the solubility of nuclear proteins in the detergent buffers commonly used to isolate nuclei. To address this controversy, the nuclear protein content was measured in whole and detergent-extracted cells before and following hyperthermia. It was found that hyperthermia caused no significant change in the nuclear protein content of whole, unextracted cells, and when fluorescently labeled proteins were microinjected into the cytoplasm no gross change in the selective permeability of the nuclear membrane to soluble proteins was observed during or following hyperthermia. Measurements in extracted cells showed that the detergent buffers removed protein from both the nucleus and cytoplasm of control, nonheated cells and that hyperthermia reduced the extractability of both nuclear and cytoplasmic proteins. The amount of protein found in nuclei isolated from heated cells approached that observed in nuclei within nonheated whole cells as the hyperthermic exposure was increased. Thus, the dose-dependent, two- to threefold increase in the protein content of nuclei isolated from heated cells represents a heat-induced reduction in the extractability of proteins normally present within cell nuclei and does not result from a mass migration of cytoplasmic proteins into the nucleus, although some specific proteins (e.g., the 70 KDa heat shock protein) do migrate to the nucleus following heat shock. Differential scanning calorimetry (DSC) measurements of whole cells, isolated nuclei, cytoplasts, and karyoplasts supported these conclusions and suggested that most of the detergent-insoluble proteins remaining in the nuclei and cytoplasm of heated cells are in their native state. Thus, a relatively small amount of denatured protein may be sufficient to initiate and sustain insoluble protein aggregates comprised of mostly native proteins. Analyses of the DSC data also implied that the previously identified critical target proteins, predicted to have a Tm of 46.0 degrees C, are present in both the nucleus and cytoplasm.

Modulation of protein phosphorylation and stress protein expression by okadaic acid on heat shock cells

We have demonstrated that pretreatment but not post-treatment with okadaic acid (OA) can aggravate cytotoxicity as well as alter the kinetics of stress protein expression and protein phosphorylation in heat shocked cells. Compared to heat shock, cells recovering from 1 hr pretreatment of OA at 200 nM and cotreated with heat shock at 45 degrees C for the last 15 min of incubation (OA-->HS treatment) exhibited enhanced induction of heat shock proteins (HSPs) 70 and 110. In addition to enhanced expression, the attenuation of HSC70 and HSP90 after the induction peaks was also delayed in OA-->HS-treated cells. The above treatment also resulted in the rapid induction of the 78 kDa glucose-regulated protein (GRP78), which expression remained constant in cells recovering from treatment with 200 nM OA for 1 hr, heat shocked at 45 degrees C for 15 min, or in combined treatment in reversed order (HS-->OA treatment). Enhanced phosphorylation of vimentin and proteins with molecular weights of 65, 40, and 33 kDa and decreased phosphorylation of a protein with a molecular weight of 29 kDa were also observed in cells recovering from OA-->HS treatment. Again, protein phosphorylation in cells recovering from HS-->OA treatment did not differ from those in cells treated only with heat shock. Since the alteration in the kinetics of stress protein expression and protein phosphorylation was tightly correlated, we concluded that there is a critical link between induction of the stress proteins and phosphorylation of specific proteins. Furthermore, the rapid induction of GRP78 under the experimental condition offered a novel avenue for studying the regulation of its expression.

Transcription regulation of alpha B-crystallin in astrocytes: analysis of HSF and AP1 activation by different types of physiological stress

The coordinated cellular responses to physiological stress are known to be effected in part by the activation of heat-shock factor 1, a transcriptional activator protein capable of binding to, and inducing transcription from genes containing heat shock elements. Other stress responsive signal transduction pathways also exist including the stress activated protein kinase cascade that regulates the activity of the transcription factor AP1. We have examined the expression of the low molecular stress proteins, heat shock protein 27 and alpha B-crystallin in astrocytes in response to physiological stress of different types and asked what component of this induction is effected at the transcriptional level and whether activation of heat shock factor 1 and AP1 might account for these events. We have found that stress regulated induction of alpha B-crystallin has a strong transcriptional component and that it may be effected by at least two different transcriptional mechanisms. In one set of phenomena, represented here by cadmium exposure, alpha B-crystallin and heat shock protein 27 are coordinately regulated and this occurs in the presence of activated heat shock factor 1. In the second series of phenomena, represented here by hypertonic stress, alpha B-crystallin is induced in the absence of heat shock factor activation and in the absence of any corresponding change in heat shock protein 27 expression. Although hypertonic stress does activate an AP1-like binding activity, the AP1 consensus binding site in the alpha B-crystallin promoter does not appear to be a target for this hypertonic stress inducible activity. These data suggest that the hypertonic stress response is effected through a heat shock factor independent mechanism and that hypertonic stress regulated induction of alpha B-crystallin does not directly depend on the SAPK pathway and AP1 activity.

Temperature-dependent increase in the DNA-binding activity of a heat shock factor in an extract of tobacco cultured cells

The DNA-binding activity of a tobacco heat shock factor (HSF) was induced by heat treatment (37-40 degrees C) of a cell-free extract that contained extra-nuclear fraction, but not in an extract of isolated nuclei. These observations suggest that an inactive form of HSF can directly recognize and transduce the heat shock signal and that such transduction requires components of the extranuclear fraction. Addition of ATP or of most other nucleoside triphosphates reduced the binding of the HSF to the heat shock element (HSE) in the same extract, and removal of ATP by dialysis from the extract restored the ability of the HSF to bind to DNA. The restored activity of the HSF could be eliminated again by a second addition of ATP. Our observations provide the first example of the involvement of ATP in the regulation of the reversible changes in HSF that control its ability to bind to HSEs in a cell-free extract.

In vivo activation of neural heat shock transcription factor HSF1 by a physiologically relevant increase in body temperature

Molecular mechanisms which underlie the heat shock response have commonly been analyzed using tissue culture systems, with less investigation of the intact mammal. In tissue culture, a temperature elevation of 5 degrees C is required to activate mammalian heat shock transcription factor 1 (HSF1) to the DNA-binding form. We demonstrate that a physiologically relevant increase in body temperature of 2.5 +/- 0.2 degrees C, similar to that attained during fever reactions, is sufficient to activate HSF1 in the rabbit nervous system. Maximal HSF activation, as measured by gel mobility shift assay, was attained at 1 hr with the cerebellum showing the strongest signal. Supershift experiments with antibodies specific to HSF1 and HSF2 demonstrated that the signal reflected activation of HSF1. Western blot analysis showed that cerebellum exhibited high levels of HSF1 protein.

Evidence for different mechanisms of induction of HSP70i: a comparison of cultured rat cortical neurons with astrocytes

This study is a follow-up of previous work which demonstrated that cultured cortical neurons did not synthesize HSP70i immediately after heat stress when compared with cultured cortical astrocytes. We have extended the period of observation for HSP70i induction of cultured cortical neurons and astrocytes up to 24 h after heat stress. Cultured rat cortical neurons derived from 16-day-old fetal rats respond differently to heat stress than cultured rat astrocytes derived from newborn rats. They showed a delayed HSP70i induction in the majority of cultured neurons and the response was heterogeneous and was absent in most smaller neurons. The delayed neuronal induction was accompanied by a prolonged activation of heat-shock transcription factor 1 (HSF-1) and prolonged transcription of HSP70i mRNA. In comparison astrocytes showed a marked early induction of HSP70i mRNA and protein. In addition the induction of HSP70i in astrocytes was followed by translocation of the protein into the nucleus, a finding which we failed to demonstrate in neurons. Immunostaining for HSP70i was more uniform in astrocytes than neurons. Many neurons did not stain for up to 24 h after heat shock in this study. Immunocytochemical staining of HSF-1 and 2 showed major differences between neurons and astrocytes. Astrocytes showed localization of HSF-1 to the nucleus before and after heat stress, while neurons showed HSF-1 localization to the cytoplasm and nucleus before and after heat stress. Finally HSF-2 was undetectable in neurons when compared with astrocytes by Western immunoblot analysis. However, astrocytes and neurons revealed weak immunostaining of HSF-2 in the cytoplasm and nucleus. The staining in the neurons was likely secondary to cross-reactivity to an unidentified protein. We conclude that HSP70i expression after heat shock is delayed in rat cortical neurons when compared with rat cortical astrocytes. In addition most small neurons did not synthesize HSP70i after heat shock. This difference in induction of HSP70i may be secondary to localization and activation of HSF-1 but not HSF-2. Neuronal susceptibility to injury may be related to the delayed induction of HSP70i and also the possible failure of newly synthesized HSP70i to translocate into the nucleus.

Activation of heat shock factor 1 DNA binding precedes stress-induced serine phosphorylation. Evidence for a multistep pathway of regulation

Exposure of mammalian cells in culture to the anti-inflammatory drugs sodium salicylate or indomethacin results in activation of heat shock factor 1 (HSF1) DNA binding activity. We have previously shown that the drug-induced HSF1 becomes associated with the heat shock elements of the hsp70 promoter, yet transcription of the hsp70 gene is not induced (Jurivich, D. A., Sistonen, L., Kroes, R. A., and Morimoto, R. I. (1992) Science 255, 1243-1245). In this study, we have examined the basis for uncoupling the heat shock transcriptional response. Comparison of heat shock and drug-induced forms of HSF1 has revealed that the transcriptionally inert drug-induced HSF1 is constitutively but not inducibly serine-phosphorylated, whereas heat shock-induced HSF1 is both constitutively and inducibly serine-phosphorylated. The transcriptionally inert intermediate represented by drug-induced HSF1 can be converted to the transcriptionally active state by a subsequent exposure to heat shock. The only detectable change in HSF1 is the acquisition of inducible serine phosphorylation. These data reveal that acquisition of the trimeric DNA binding state of HSF1 is independent of and precedes inducible phosphorylation and furthermore that inducible phosphorylation correlates with transcriptional activation.

Enhancement of stress-induced synthesis of hsp27 and alpha B crystallin by modulators of the arachidonic acid cascade

The regulation by intrinsic factors of responses to stress of two small stress proteins, hsp27 and alpha B crystallin, was examined in C6 rat glioma cells. Levels of hsp27 and alpha B crystallin were low in C6 glioma cells in confluent cultures. However, levels of the two proteins increased after exposure of cells to heat (42 degrees C for 30 min) or arsenite (50 microM for 1 h) stress. When cells were exposed to arsenite or hear in the presence of indomethacin (50 microM), an inhibitor of cyclooxygenase, or in the presence of nordihydroguaiaretic acid (NDGA; 50 microM), an inhibitor of lipoxygenase, induction of hsp27 and alpha B crystallin was markedly stimulated as detected by specific immunoassays, Western blot analysis, and Northern blot analysis. The presence of melittin (1 microM), an activator of phospholipase A2, during the stress period also stimulated the induction of the two proteins. The expression of hsp70 to each stress was also enhanced in the presence of indomethacin, NDGA, or melittin. The gel mobility shift assay revealed that these chemicals prolonged the arsenite-induced activation of heat shock element (HSE)-binding activity of heat shock transcriptional factor (HSF) in cells. Induction of hsp27 and alpha B crystallin in adrenal glands of heat-stressed (42 degrees C for 15 min) rats was also enhanced by prior injection of aspirin, another inhibitor of cyclooxygenase. These results indicate that the responses to stress of hsp27 and alpha B crystallin, as well as the response of hsp70, are coupled with the metabolic activity of the arachidonic acid cascade and the mechanism for regulation of stress responses observed in C6 cells is operative in tissues and organs in vivo.

Regulation of thermotolerance and ischemic tolerance

Thermotolerance and ischemic tolerance are two major biological aspects where heat shock (stress) proteins exert essential roles for survival in cells as well as in various tissues. Bioflavonoids prevent the cells from acquiring thermotolerance after stresses through specific inhibition in the induction of heat shock proteins. The mechanism of this inhibition is revealed to be due to the prevention of the activation of heat shock factor 1 after heat shock. The induction of stress proteins during the ischemic stress is then described in global as well as focal cerebral ischemic model in rats. The activation of heat shock factor 1 after ischemia is first shown to induce various stress proteins in the central nervous system.

Heat shock factor-1 mediates the transcriptional activation of Alzheimer's beta-amyloid precursor protein gene in response to stress

Stress may be involved in the pathogenesis of Alzheimer's disease. There is a heat shock element located at position -317 bp on the beta-amyloid precursor protein (beta-APP) gene promoter. Recently we demonstrated [4] that stress, in the form of heat shock, ethanol and sodium arsenite treatment, transcriptionally activates the beta-APP gene. In this report we demonstrate that the nuclear factor that mediates this activation is heat shock factor-1.

The transcriptional regulation of heat shock genes: a plethora of heat shock factors and regulatory conditions

The inducible regulation of heat shock gene transcription is mediated by a family of heat shock factors (HSF) that respond to diverse forms of physiological and environmental stress including elevated temperature, amino acid analogs, heavy metals, oxidative stress, anti-inflammatory drugs, arachidonic acid, and a number of pathophysiological disease states. The vertebrate genome encodes a family of HSFs which are expressed ubiquitously, yet the DNA binding properties of each factor are negatively regulated and activated in response to specific conditions. This chapter will discuss the regulation of the HSF multi-gene family and the role of these transcriptional activators in the inducible expression of genes encoding heat shock proteins and molecular chaperones.

Effect of caloric restriction on the expression of heat shock protein 70 and the activation of heat shock transcription factor 1

The regulation of heat shock protein 70 (hsp70) expression is an excellent example of a cellular mechanism that has evolved to protect all living organisms from various types of physiological stresses; therefore, the reported age-related alterations in the ability of cells to express hsp70 in response to stress could seriously compromise the ability of a senescent organism in respond to changes in its environment. Because caloric restriction (CR) is the only experimental manipulation known to retard aging and increase the survival of rodents, it was of interest to analyze the effect of CR on the age-related alteration in the induction of hsp70 expression in rat hepatocytes. The effect of CR on the nuclear transcription of hsp70 gene in rat hepatocytes in response to various levels of heat shock was determined, and it was found that the age-related decline in the transcription of hsp70 at all temperatures studied was reversed by CR. Because the heat shock transcription factor (HSF) mediates the heat-induced transcription of hsp70, the effect of CR on the induction of HSF binding activity by heat shock was studied and found to arise from HSF1, which has been shown to be involved in the induction of HSF binding activity in other cell types. The age-related decrease in the induction of HSF1 binding activity in rat hepatocytes was reversed by CR, and did not appear to be due to an accumulation of inhibitory molecules with age. Interestingly, the level of HSF1 protein was significantly higher in hepatocytes isolated from old rats fed ad libitum compared to hepatocytes obtained from rats fed the CR diet even though the levels of HSF1 binding activity were lower for hepatocytes isolated from the old rats fed ad libitum. The levels of the mRNA transcript for HSF1 was not significantly altered by age or CR. Thus, the changes in HSF1 binding activity with age and CR do not arise from changes in the level of HSF1 protein available for activation.

Sensing stress and responding to stress

Heat shock protein gene expression is enhanced by proteotoxic stress, i.e., by conditions favoring protein unfolding. This upregulation of heat shock protein genes is mediated by heat shock transcription factor HSF1. A mechanism, the details of which are still elusive, senses adverse conditions and causes HSF1 to oligomerize and to acquire DNA-binding ability. The DNA-binding form of HSF1 then undergoes further conformational changes that render it transcriptionally competent. The current model in which heat shock protein 70 acts both as sensor of stress and as negative regulator of HSF1 oligomerization as well as alternative models involving additional protein factors are discussed.

Activation of heat shock factor 1 in rat brain during cerebral ischemia or after heat shock

Recently, many studies have demonstrated the induction of stress proteins in the mammalian nervous system under various pathological conditions. These altered genetic programs may function to protect individual cells against stressful conditions. However, little is known about the molecular mechanisms regulating these stress responses in animals. We report here the activation of a heat shock factor (HSF) in the rat brain during cerebral ischemia or after heat shock. Gel mobility shift assays revealed an increase in DNA binding activity to the heat shock element (HSE) during the early phases of ischemia. Supershift experiments using specific antisera against HSF1 and HSF2 showed that the ischemia-induced HSE-binding activity was mainly due to HSF1. In the heat-shocked brain, HSF1 was also activated, and the HSE-binding activity was higher in the cerebellum than in the cerebral cortex or hippocampus; Western blot analysis also showed that HSF1 was more abundant in the cerebellum than in the other two brain regions. Our results indicate that heat shock gene transcription is regulated by the activation of HSF1 in both cerebral ischemia and heat shock, and that different brain regions display differential sensitivities in their stress response. The cellular signals for heat shock gene transcription under in vivo pathological conditions will also be discussed.

Pharmacological induction of heat shock protein 68 synthesis in cultured rat astrocytes

The induction of the highly inducible 70-kDa heat shock protein (HSP 70) is associated with thermotolerance and survival from many other types of stress. This investigation studied the pharmacological induction of HSP 68 (HSP 68 is the rat homolog of human HSP 70) by 1,10-phenanthroline in cultured rat astrocytes under conditions that activated heat shock transcription factor-1 without inducing HSP 68 synthesis. Two conditions that activate heat shock transcription factor-1 and promote its binding to the heat shock element without subsequent transcription of HSP 68 mRNA, intracellular acidosis and exposure to salicylate, showed synthesis of HSP 68 when 1,10-phenanthroline was added to culture medium after the activation of heat shock transcription factor-1. 1,10-phenanthroline mimicked heat shock by inducing HSP 68 mRNA and protein under both conditions. 1,10-phenanthroline added alone to culture medium did not induce the synthesis of HSP 68 or activate heat shock transcription factor-1. These findings strongly suggest a multistep activation for HSP 68 synthesis and also demonstrate that the synthesis of HSP 68 can be pharmacologically regulated.

Methylation-associated transcriptional silencing of the major histocompatibility complex-linked hsp70 genes in mouse cell lines

The MHC-linked hsp70 locus consists of duplicated genes, hsp70.1 and hsp70.3, which in primary mouse embryo cells are highly heat shock-inducible. Several mouse cell lines in which hsp70 expression is not activated by heat shock have been described previously, but the basis for the deficiency has not been identified. In this study, genomic footprinting analysis has identified a common basis for the deficient response of the hsp70.1 gene to heat shock in four such cell lines, viz., the promoter is inaccessible to transcription factors, including heat shock transcription factor. Southern blot analyses reveal extensive CpG methylation of a 1.2-kilobase region spanning the hsp70.1 transcription start site and hypermethylation of the adjacent hsp70.3 gene, which is presumably also inaccessible to regulatory factors. Of four additional, randomly chosen mouse cell lines, three show no or minimal hsp70.3 heat shock responsiveness and CpG methylation of both hsp70 genes, and two of the three lines exhibit a suboptimal hsp70.1 response to heat shock as well. In all three lines, the accessibility of the hsp70.1 promoter to transcription factors is detectable but clearly diminished (relative to that in primary mouse cells). Our results suggest that the tandem hsp70 genes are concomitantly methylated and transcriptionally repressed with high frequency in cultured mouse cells.

Salicylate triggers heat shock factor differently than heat

Sodium salicylate has the unusual property of partially inducing the human heat shock response (Jurivich, D. A., Sistonen, L., Kroes, R., and Morimoto, R. I. (1992) Science 255, 1243-1245). Salicylate induces the DNA binding state of the human heat shock transcription factor (HSF), but this is insufficient to elevate heat shock gene expression. Because it is not known how HSF enhances heat shock gene expression, further analysis of the transcriptionally inert, salicylate-induced HSF was undertaken to potentially identify components of the heat shock response that are necessary for full transcriptional induction. Like thermal stress, exposure of HeLa cells to salicylate led to the induction of HSF1 into a DNA-bound state. Despite continued exposure of cells to salicylate, HSF1.DNA binding attenuated much more rapidly than a continuous heat shock. Western blot analysis revealed that the salicylate-induced form of HSF1 was not hyperphosphorylated like the heat-induced form. Furthermore, supershifts of the HSF1 bound to an heat shock element (HSE) oligonucleotide by monoclonal antibodies to phosphoamino acids revealed that salicylate induced threonine phosphorylation of HSF1, whereas heat led to a predominance of HSF1 serine phosphorylation. These data suggest that salicylate-independent signals are necessary to convert HSF1 into a transactivator of heat shock gene expression and that brief acquisition of DNA binding by this factor is insufficient to maximally enhance transcription.

Stable overexpression of human HSF-1 in murine cells suggests activation rather than expression of HSF-1 to be the key regulatory step in the heat shock gene expression

Transcription of the heat shock genes is regulated by the activation of the heat shock transcription factor (HSF-1). After heat shock, HSF-1 forms oligomers and binds to the heat shock element (HSE), which consists of several repeats of NGAAN located in the promoter region of the heat shock genes. HSF-1 is then phosphorylated, leading to the enhanced transcription of the heat shock genes likely by transactivation. We have stably overexpressed the human heat shock transcription factor-1 (HSF-1) in murine cells to investigate whether the regulation of the expression of the heat shock genes may partly reside at the level of HSF-1 expression. Human HSF-1 cDNA was cloned into a retroviral vector (pvhhsf-1) and was overexpressed in a murine fibroblast cell line. The overexpressed human HSF-1 is found in both the cytoplasm and nucleus of control cells but is translocated into the nucleus upon heat shock. Electrophoretic mobility shift analysis suggests that the human HSF-1 has constitutive DNA binding ability and its DNA binding ability is increased upon heat shock. Cross-linking experiments indicate that the overexpressed human HSF-1 is mainly a monomer under control conditions and forms oligomers upon heat shock. Immunoblotting shows that the human HSF-1 is phosphorylated upon heat shock and its apparent molecular weight is shifted up by at least 10 kDa. In spite of both the DNA binding ability and phosphorylation, the overexpression of human HSF-1 does not increase the transcription of murine HSP-70 mRNA or increase the synthesis of other HSPs after heat shock beyond that observed in control untransfected cells. An exception is the enhanced synthesis of a 47-50 kDa protein after heat shock and an apparent lack of induction of one HSP-70 kDa species when the protein pattern is analyzed by isoelectric focusing. Interestingly, cells overexpressing human HSF-1 show a 4-fold increase in the basal expression of luciferase when the plasmids containing the human HSP-70 promoter ligated to the luciferase reporter gene are transiently expressed in these cells. Murine cells overexpressing human HSF-1 are more resistant to the cytotoxic effects of heat when compared to the control untransfected cells, but the kinetics of thermotolerance development and decay is similar between HSF-1 transfected and untransfected cells. In conclusion, human HSF-1 protein in murine fibroblasts is modified in a similar fashion as the endogenous mouse HSF-1 after heat shock. However, the overexpression of HSF-1 does not result in overproduction of heat shock proteins after heat shock, perhaps because these cells contain abundant amounts of endogenous HSF-1.

Modulation of HSP68 gene expression after heat shock in thermosensitized and thermotolerant cells is not solely regulated by binding of HSF to HSE

Induction of heat shock proteins (HSP) is generally regarded as a consequence of binding of the heat shock transcription factor (HSF) to heat shock elements (HSE), i.e. to be a single hit induction. The activation of HSF and the induction of HSP68 mRNA were studied in non pretreated Reuber H35 rat hepatoma cells in a thermosensitized and in a thermotolerant state. It was found that HSF in Reuber H35 hepatoma cells already acquires maximum DNA binding activity at temperatures that are too low to induce HSP68 mRNA. Directly following heat shock cells are in a transient thermosensitized state. In this state a second stress of lower impact leads to even higher production of HSP68, which corresponds with a decreased decay rate HSF-HSE binding. Directly following the thermosensitized state cells become refractory. In this period a second stress of the same impact does lead to HSF-HSE binding but the production of HSP68 mRNA is lowered, while only higher-impact stresses lead to high inductions of the said mRNA. The results indicate that regulation of HSP68 gene transcription involves at least one additional event outside the acquisition of DNA-binding activity by HSF and that this process can thus be described as a multiple-hit occurrence.

Male germ cell-specific alteration in temperature set point of the cellular stress response

Heat shock factor (HSF), a transcriptional regulator with heat-activatable DNA binding ability, mediates the stress-induced expression of eukaryotic heat shock protein genes. Previous results from this laboratory demonstrated that a preparation of mixed male germ cell types from mouse testis exhibited a lower temperature threshold for activation of HSF1 DNA binding relative to other mouse cell types (Sarge, K.D., Bray, A.E., and Goodson, M.L. (1995) Nature 374, 126). The purpose of the present study was to determine whether the phenomenon of reduced HSF1 activation temperature is common to all testis cell types, both somatic and germ cell types, or whether it is a special property of male germ cells. The results show that a purified population of pachytene spermatocytes, one of the male germ cell types, exhibits a profile of reduced HSF1 activation temperature identical to that observed for the mixed germ cell preparation, with a threshold HSF1 activation temperature of 35 degrees C. Activation of HSF1 DNA binding in male germ cells by incubation at 38 degrees C is accompanied by the classic cellular stress response parameters of heat-induced HSF1 phosphorylation and increased expression of the hsp72 stress protein. In contrast, a preparation of somatic testis cell types exhibits HSF1 activation only at temperatures of 42 degrees C and above, a profile identical to that observed for mouse liver cells and mammalian cell lines. These results demonstrate that the phenomenon of reduced HSF1 activation temperature is a unique property of male germ cell types within the mammalian testis and demonstrate that HSF1 activated at this lower temperature threshold is fully capable of mediating a productive cellular stress response in these cell types.

The cDNA encoding Xenopus laevis heat-shock factor 1 (XHSF1): nucleotide and deduced amino-acid sequences, and properties of the encoded protein

We have isolated a cDNA encoding Xenopus laevis (Xl) heat-shock factor 1 (XHSF1). XHSF1, translated from the mRNA synthesized in vitro, will bind specifically to the Xl hsp70 promoter (hsp70). Microinjection of XHSF1 mRNA into Xl oocytes leads to synthesis of XHSF1 which accumulates in the nucleus and selectively activates Xl phsp70p activity at 18 degrees C.

Myocardial self-preservation: absence of heat shock factor activation and heat shock proteins 70 mRNA accumulation in the human heart during cardiac surgery

Following myocardial ischemia, heat shock proteins (HSPs) have been found to be associated with a reduction in infarct size and enhanced postischemic functional recovery. Stress-induced regulation of the HSPs is mediated by the activation and binding of the heat shock transcription factor (HSF) to a specific DNA sequence located in front of all HSP genes, known as the heat shock element (HSE). To determine whether HSPs were induced in the human heart following the ischemic stress experienced during cardiac surgery, biopsies were performed of the right atrium at three sequential times: prior to establishing cardiopulmonary bypass; immediately after aortic declamping; and following termination of bypass. These samples from the atria of patients undergoing coronary bypass surgery were assessed for HSF activation using mobility shift gels, and analyzed for HSP 72 mRNA by Northern blot. Although a high level of the HSP 72 protein was noted at all intervals, no HSF activation was detected, nor was an accumulation of HSP 72 mRNA observed at any time during surgery. These data suggest that HSPs are not induced during cardiac surgery and that the high "constitutive" level of the HSP 72 protein detected in these hearts may not be secondary to an HSF-HSE interaction, but rather, the result of other transcription factors acting at alternative regions of the HSP 70 promoter.

A constitutive heat shock element-binding factor is immunologically identical to the Ku autoantigen

Analysis of the heat shock element (HSE)-binding proteins in extracts of rodent cells, during heat shock and their post-heat shock recovery, indicates that the regulation of heat shock response involves a constitutive HSE-binding factor (CHBF), in addition to the heat-inducible heat shock factor HSF1. We purified the CHBF to apparent homogeneity from HeLa cells using column chromatographic techniques including an HSE oligonucleotide affinity column. The purified CHBF consists of two polypeptides with apparent molecular masses of 70 and 86 kDa. Immunoblot and gel mobility shift analysis verify that CHBF is identical or closely related to the Ku autoantigen. The DNA binding characteristics of CHBF to double-stranded or single-stranded DNA are similar to that of Ku autoantigen. In gel mobility shift analysis using purified CHBF and recombinant human HSF1, CHBF competes with HSF1 for the binding of DNA sequences containing HSEs in vitro. Furthermore, when Rat-1 cells were co-transfected with human Ku expression vectors and the hsp70-promoter-driven luciferase reporter gene, thermal induction of luciferase is significantly suppressed relative to cells transfected with only the hsp70-luciferase construct. These data suggest a role of CHBF (or Ku protein) in the regulation of heat response in vivo.

Activation of heat-shock transcription factor in rat heart after heat shock and exercise

Stress-induced transcriptional regulation of the heat-shock proteins (HSP) is mediated by activation and binding of the heat-shock transcription factors (HSF) to the heat-shock element (HSE). Given the similarities between the stressors known to activate the HSF in cultured cells and the physiological stresses known to occur during exercise, HSF activation was examined in the hearts from exercising animals. Sprague-Dawley rats (5 rats/group) were run on a treadmill (24 m/min) for either 0, 20, 40, or 60 min or to exhaustion (102 +/- 7 min). Protein extracts were assessed for HSF activation by mobility-shift gels. Extracts from the hearts of nonrunning rats demonstrated no HSF activation, whereas HSF activation was detected in 80% of the hearts from animals that run for at least 40 min. These results demonstrate that treadmill running is capable of activating the HSF and increasing 70-kDa HSP mRNA in the rat myocardium.

Estrogen dependent expression of heat shock transcription factor: implications for uterine synthesis of heat shock proteins

Transcriptional induction of heat shock protein genes is generally mediated by binding of heat shock transcription factor(s) to the heat shock element present in the promoters of heat shock genes. Although the steady-state levels of heat shock factor mRNAs vary among different tissues, at present virtually nothing is known regarding the cellular signals responsible for their synthesis and hence the observed variations. In this report we demonstrate that the heat shock transcription factor (HSTF or HSF) is under positive regulation by estrogen. The effect of estrogen was observed with both types of heat shock factors (HSF-1 and HSF-2) and occurred at both the mRNA and protein level. Immunolocalization studies emphasized the potential biological importance of these observations whereby the increase in uterine HSF-1 and HSF-2 due to estrogen was found to be associated with the endometrium, the primary tissue component which is targeted for estrogen action. This is the first demonstration of a cellular factor which can regulate HSF-1 and HSF-2 gene expression. The implications of these findings to uterine heat shock protein gene expression are discussed.

Okadaic acid potentiates heat-induced activation of erk2

Subjecting exponentially growing HeLa cells to heat shock at 45 degrees C for 30 min leads to retarded migration of erk1 and erk2, as revealed on immunoblots indicating their activation. Renaturation gels confirmed activation of erk2 but not erk1. Treatment of cells with okadaic acid (OA) alone marginally upregulated erk1 and erk2, whereas simultaneous exposure to heat shock and OA led to a considerably augmented response for erk2 which was approximately 3-fold higher than the sum of heat- and OA-induced activation. Chronic treatment of cells with 12-O-tetradecanoyl-phorbol 13-acetate marginally diminished the extent of erk2 stimulation, but had no influence on the OA-induced potentiation of heat-induced erk2 activity.

Potential involvement of a constitutive heat shock element binding factor in the regulation of chemical stress-induced hsp70 gene expression

It was reported that chemical stresses such as arsenite, cadmium or salicylate fail to induce synthesis of the inducible form of HSP70 (HSP70i). We report here that exposure of cells to higher doses of these chemical treatments induced significant synthesis of HSP70i in CHO cells as well as other cell lines. The synthesis of HSP70i is primarily regulated at the transcriptional level. Although all tested chemical treatments induced heat shock factor (HSF) binding to the heat shock element (HSE), HSP70i synthesis appears to be regulated by an alternative factor (CHBF) which constitutively binds to the HSE at 37 degrees C. The treatments, which dissociate the HSE-CHBF complex, induced significant HSP70i synthesis. The treatments, which failed to induce HSP70i synthesis, still activated HSF binding to HSE but the HSE-CHBF complex remained as that of untreated control cells.

Effect of an alkaline shift on induction of the heat shock response in human fibroblasts

The simultaneous exposure of WI-38 human fibroblasts (HF) to a heat shock (45 degrees C, 30 min) and an alkaline shift (> or = pH 8.0) in the incubation medium increased and extended the expression of heat shock proteins (hsps). Hsp70 was the most prominent inducible hsp synthesized during the recovery phase after the double shock, and the increase in synthesis depended on the degree of alkalinization during the heat shock. The accumulation of inducible hsp70, which was shown by Western blotting to occur in the late part of the recovery period, was more pronounced in the cells exposed to alkaline medium during the heat shock. Northern blotting did not reveal any increase in hsp70 mRNA, although time course studies following the double shock indicated a more prolonged presence of mRNA. Hsp70 gene activation was evaluated by a gel retardation assay using a 32P-labelled DNA oligonucleotide containing the heat shock consensus element (HSE) and a heat shock-induced specific binding protein (heat shock transcription factor, HSTF) from the cell extract. Heat shock activated HSTF-DNA binding and induced hsp70 mRNA expression as well as the synthesis and accumulation of hsp70. Alkaline shift, which by itself did not induce hsps expression, activated HSTF DNA-binding. However, in combination with heat shock, alkaline shift enhanced and prolonged HSTF-HSE complex association and hsp expression at both mRNA and protein levels. Since the alkaline shift-induced activation of hsp gene does not allow full transcription, these results provide further support for the multistep nature of the heat shock transcriptional response.

Heat-inducible DNA binding of purified heat shock transcription factor 1

The heat-induced expression of heat shock proteins, called the cellular stress response, is mediated by heat shock transcription factor 1 (HSF1). HSF1 exists in unstressed cells in an inactive form, which is converted to the DNA binding from upon exposure of cells to elevated temperature. We have developed a protocol for isolation of the non-DNA binding form of recombinant mouse HSF1, involving expression and affinity purification of HSF1 as a fusion with the glutathione S-transferase protein in Escherichia coli, followed by specific protease cleavage to release pure HSF1 protein. We report here that the purified inactive HSF1 can be converted to the DNA binding form by heat treatment in vitro. Chemical cross-linking analysis demonstrates that this conversion is accompanied by oligomerization of HSF1 from a monomeric to a trimeric native structure, similar to that observed for HSF1 in heat-shocked cells. These results indicate that elements residing in the HSF1 polypeptide are sufficient both for maintenance of this factor in the non-DNA binding from and for its heat-induced conversion to the DNA binding form and support a role for HSF1 as the "molecular thermostat" in eukaryotic cells, which senses adverse environmental conditions and activates the cellular stress response.

Stimulation of the DNA-dependent protein kinase by RNA polymerase II transcriptional activator proteins

The DNA-dependent protein kinase (DNA-PK) phosphorylates RNA polymerase II and a number of transcription factors. We now show that the activity of DNA-PK is directly stimulated by certain transcriptional activator proteins, including the human heat shock transcription factor 1 (HSF1) and a transcriptionally active N-terminal 147 amino acid GAL4 derivative. Stimulation of DNA-PK activity required specific sequences in the activator proteins outside the minimal DNA binding domains. The stimulation of DNA-PK activity also required DNA and was greater with DNA containing relevant activator binding sites. Comparison of different HSF binding fragments showed that optimal stimulation occurred when two HSF binding sites were present. Stimulation with HSF and GAL4 was synergistic with Ku protein, another regulator of DNA-PK activity. DNA-PK is tightly associated with the transcriptional template, and an increase in its activity could potentially influence transcription through the phosphorylation of proteins associated with the transcription complex.

Hsp70 and aging

An alteration in the ability of cells to express heat shock proteins could be physiologically important in aging because all living organisms show a reduced ability to respond to stress with increasing age. Using hepatocytes freshly isolated from young adult and old rats, we have shown that the induction of hsp70 expression by heat shock is reduced approximately 50% with age. The decrease in hsp70 expression occurs at the level of transcription and appears to arise from a defect in the heat shock transcription factor. Other investigators have also shown that the induction of hsp70 expression by heat shock as well as other stresses declines significantly with age in a variety of tissues from rats as well as mononuclear cells from human subjects. In addition, a decrease in the inducibility of hsp70 is observed with cell senescence in cultured cells. Therefore, it appears that a reduced ability to express hsp70 in response to stress may be a common phenomenon underlying the aging process.