Molecular chaperones as HSF1-specific transcriptional repressors

Stress-specific activation and repression of heat shock factors 1 and 2

Vertebrate cells express a family of heat shock transcription factors (HSF1 to HSF4) that coordinate the inducible regulation of heat shock genes in response to diverse signals. HSF1 is potent and activated rapidly though transiently by heat shock, whereas HSF2 is a less active transcriptional regulator but can retain its DNA binding properties for extended periods. Consequently, the differential activation of HSF1 and HSF2 by various stresses may be critical for cells to survive repeated and diverse stress challenges and to provide a mechanism for more precise regulation of heat shock gene expression. Here we show, using a novel DNA binding and detection assay, that HSF1 and HSF2 are coactivated to different levels in response to a range of conditions that cause cell stress. Above a low basal activity of both HSFs, heat shock preferentially activates HSF1, whereas the amino acid analogue azetidine or the proteasome inhibitor MG132 coactivates both HSFs to different levels and hemin preferentially induces HSF2. Unexpectedly, we also found that heat shock has dramatic adverse effects on HSF2 that lead to its reversible inactivation coincident with relocalization from the nucleus. The reversible inactivation of HSF2 is specific to heat shock and does not occur with other stressors or in cells expressing high levels of heat shock proteins. These results reveal that HSF2 activity is negatively regulated by heat and suggest a role for heat shock proteins in the positive regulation of HSF2.

The DNA-binding domain of yeast heat shock transcription factor independently regulates both the N- and C-terminal activation domains

The expression of heat shock proteins in response to cellular stresses is dependent on the activity of the heat shock transcription factor (HSF). In yeast, HSF is constitutively bound to DNA; however, the mitigation of negative regulation in response to stress dramatically increases transcriptional activity. Through alanine-scanning mutagenesis of the surface residues of the DNA-binding domain, we have identified a large number of mutants with increased transcriptional activity. Six of the strongest mutations were selected for detailed study. Our studies suggest that the DNA-binding domain is involved in the negative regulation of both the N-terminal and C-terminal activation domains of HSF. These mutations do not significantly affect DNA binding. Circular dichroism analysis suggests that a subset of the mutants may have altered secondary structure, whereas a different subset has decreased thermal stability. Our findings suggest that the regulation of HSF transcriptional activity (under both constitutive and stressed conditions) may be partially dependent on the local topology of the DNA-binding domain. In addition, the DNA-binding domain may mediate key interactions with ancillary factors and/or other intramolecular regulatory regions in order to modulate the complex regulation of HSF's transcriptional activity.

Adjusting the thermostat: the threshold induction temperature for the heat-shock response in intertidal mussels (genus Mytilus) changes as a function of thermal history

Spatio-temporal variation in heat-shock gene expression gives organisms the ability to respond to changing thermal environments. The temperature at which heat-shock genes are induced, the threshold induction temperature, varies as a function of the recent thermal history of an organism. To elucidate the mechanism by which this plasticity in gene expression is achieved, we determined heat-shock protein (Hsp) induction threshold temperatures in the intertidal mussel Mytilus trossulus collected from the field in February and again in August. In a separate experiment, threshold induction temperatures, endogenous levels of both the constitutive and inducible isoforms of Hsps from the 70 kDa family and the quantity of ubiquitinated proteins (a measure of cellular protein denaturation) were measured in M. trossulus after either 6 weeks of cold acclimation in the laboratory or acclimatization to warm, summer temperatures in the field over the same period. In addition, we quantified levels of activated heat-shock transcription factor 1 (HSF1) in both groups of mussels (HSF1 inducibly transactivates all classes of Hsp genes). Lastly, we compared the temperature of HSF1 activation with the induction threshold temperature in the congeneric M. californianus. It was found that the threshold induction temperature in M. trossulus was 23°C in February and 28°C in August. This agreed with the acclimation/acclimatization experiment, in which mussels acclimated in seawater tables to a constant temperature of 10–11°C for 6 weeks displayed a threshold induction temperature of 20–23°C compared with 26–29°C for individuals that were experiencing considerably warmer body temperatures in the intertidal zone over the same period. This coincided with a significant increase in the inducible isoform of Hsp70 in warm-acclimatized individuals but no increase in the constitutive isoform or in HSF1. Levels of ubiquitin-conjugated protein were significantly higher in the field mussels than in the laboratory-acclimated individuals. Finally, the temperature of HSF1 activation in M. californianus was found to be approximately 9°C lower than the induction threshold for this species.

Analysis of heat-shock transcription factor-DNA binding in Arabidopsis suspension cultures by UV laser crosslinking

Crosslinking techniques are important in examining protein-DNA interactions in living cells. Formaldehyde and UV light emitted by conventional lamps are the most commonly used crosslinking agents. The crosslinking step is followed by immunoprecipitation of specific protein-DNA adducts, and by analysis and quantification of the co-immunoprecipitated DNA. There are a few reported cases of fruitful in vivo protein-DNA crosslinking experiments, but not in plants. In this report, we analyse the binding of heat-shock transcription factor (HSF) to heat-shock promoters in intact Arabidopsis cells. Efficient protein-DNA crosslinking by irradiation of Arabidopsis suspension culture tissue was achieved using UV laser pulses. In addition, methods for immunoprecipitation and detection of the co-immunoprecipitated DNA are reported. Our results suggest that Arabidopsis HSFs immunoreactive for HSF1 antibodies bind constitutively to the HSP18.2 gene.

Analysis of heat shock transcription factor for suppression of polyglutamine toxicity

Individually over-expressed chaperones can interfere with cytotoxicity and aggregation of polyglutamine proteins in disease models. As chaperones cooperate, the analysis of suppression or reversal of polyglutamine pathology may require ways to up-regulate multiple chaperone coding genes. This condition might be achieved by exogenous expression of de-repressed forms of heat shock transcription factor 1 (HSF1), which mediates induction of several genes coding cytosolic and nuclear chaperones. Here we present the rationale behind this possible approach and the caveats, and employ a non-neuronal cell system to test whether Ataxin-1 aggregation can be modulated by de-repressed HSF1 mutants through augmented expression of chaperone coding genes. In our experiments, HSF1 mutants have induced heat shock protein 70 and Human DnaJ (HDJ)-1 to intermediate levels. Cells expressing such mutants also showed partial reduction of Ataxin-1 [31Q] aggregation. A consolidated positive outcome of these tests in cellular models would encourage experiments in transgenic mice and prospects for pharmacological modulation of HSF1 activity or delivery.

DnaK chaperone-mediated control of activity of a sigma(32) homolog (RpoH) plays a major role in the heat shock response of Agrobacterium tumefaciens

RpoH (Escherichia coli sigma(32) and its homologs) is the central regulator of the heat shock response in gram-negative proteobacteria. Here we studied salient regulatory features of RpoH in Agrobacterium tumefaciens by examining its synthesis, stability, and activity while increasing the temperature from 25 to 37 degrees C. Heat induction of RpoH synthesis occurred at the level of transcription from an RpoH-dependent promoter, coordinately with that of DnaK, and followed by an increase in the RpoH level. Essentially normal induction of heat shock proteins was observed even with a strain that was unable to increase the RpoH level upon heat shock. Moreover, heat-induced accumulation of dnaK mRNA occurred without protein synthesis, showing that preexisting RpoH was sufficient for induction of the heat shock response. These results suggested that controlling the activity, rather than the amount, of RpoH plays a major role in regulation of the heat shock response. In addition, increasing or decreasing the DnaK-DnaJ chaperones specifically reduced or enhanced the RpoH activity, respectively. On the other hand, the RpoH protein was normally stable and remained stable during the induction phase but was destabilized transiently during the adaptation phase. We propose that the DnaK-mediated control of RpoH activity plays a primary role in the induction of heat shock response in A. tumefaciens, in contrast to what has been found in E. coli.

The loop domain of heat shock transcription factor 1 dictates DNA-binding specificity and responses to heat stress

Eukaryotic heat shock transcription factors (HSF) regulate an evolutionarily conserved stress-response pathway essential for survival against a variety of environmental and developmental stresses. Although the highly similar HSF family members have distinct roles in responding to stress and activating target gene expression, the mechanisms that govern these roles are unknown. Here we identify a loop within the HSF1 DNA-binding domain that dictates HSF isoform specific DNA binding in vitro and preferential target gene activation by HSF family members in both a yeast transcription assay and in mammalian cells. These characteristics of the HSF1 loop region are transposable to HSF2 and sufficient to confer DNA-binding specificity, heat shock inducible HSP gene expression and protection from heat-induced apoptosis in vivo. In addition, the loop suppresses formation of the HSF1 trimer under basal conditions and is required for heat-inducible trimerization in a purified system in vitro, suggesting that this domain is a critical part of the HSF1 heat-stress-sensing mechanism. We propose that this domain defines a signature for HSF1 that constitutes an important determinant for how cells utilize a family of transcription factors to respond to distinct stresses.

Comprehensive expression profile analysis of the Arabidopsis Hsp70 gene family

We isolated cDNA clones for two nuclear-encoded, organellar members of the Arabidopsis hsp70 gene family, mtHsc70-2 (AF217458) and cpHsc70-2 (AF217459). Together with the completion of the genome sequence, the hsp70 family in Arabidopsis consists of 14 members unequally distributed among the five chromosomes. To establish detailed expression data of this gene family, a comprehensive reverse transcriptase-polymerase chain reaction analysis for 11 hsp70s was conducted including analysis of organ-specific and developmental expression and expression in response to temperature extremes. All hsp70s showed 2- to 20-fold induction by heat shock treatment except cpHsc70-1 and mtHsc70-1, which were unchanged or repressed. The expression profiles in response to low temperature treatment were more diverse than those evoked by heat shock treatment. Both mitochondrial and all cytosolic members of the family except Hsp70b were strongly induced by low temperature, whereas endoplasmic reticulum and chloroplast members were not induced or were slightly repressed. Developmentally regulated expression of the heat-inducible Hsp70 in mature dry seed and roots in the absence of temperature stress suggests prominent roles in seed maturation and root growth for this member of the hsp70 family. This reverse transcriptase-polymerase chain reaction analysis establishes the complex differential expression pattern for the hsp70s in Arabidopsis that portends specialized functions even among members localized to the same subcellular compartment.

Heat shock factor-4 (HSF-4a) represses basal transcription through interaction with TFIIF

The heat shock transcription factors (HSFs) regulate the expression of heat shock proteins (hsps), which are critical for normal cellular proliferation and differentiation. One of the HSFs, HSF-4, contains two alternative splice variants, one of which possesses transcriptional repressor properties in vivo. This repressor isoform inhibits basal transcription of hsps 27 and 90 in tissue culture cells. The molecular mechanisms of HSF-4a isoform-mediated transcriptional repression is unknown. Here, we present evidence that HSF-4a inhibits basal transcription in vivo when it is artificially targeted to basal promoters via the DNA-binding domain of the yeast transcription factor, GAL4. By using a highly purified, reconstituted in vitro transcription system, we show that HSF-4a represses basal transcription at an early step during preinitiation complex assembly, as pre-assembled preinitiation complexes are refractory to the inhibitory effect on transcription. This repression occurs by the HSF-4a isoform, but not by the HSF-4b isoform, which we show is capable of activating transcription from a heat shock element-driven promoter in vitro. The repression of basal transcription by HSF-4a occurs through interaction with the basal transcription factor TFIIF. TFIIF interacts with a segment of HSF-4a that is required for the trimerization of HSF-4a, and deletion of this segment no longer inhibits basal transcription. These studies suggest that HSF-4a inhibits basal transcription both in vivo and in vitro. Furthermore, this is the first report identifying an interaction between a transcriptional repressor with the basal transcription factor TFIIF.

Sharp, an inducible cofactor that integrates nuclear receptor repression and activation

A yeast two-hybrid screen using the conserved carboxyl terminus of the nuclear receptor corepressor SMRT as a bait led to the isolation of a novel human gene termed SHARP (SMRT/HDAC1 Associated Repressor Protein). SHARP is a potent transcriptional repressor whose repression domain (RD) interacts directly with SMRT and at least five members of the NuRD complex including HDAC1 and HDAC2. In addition, SHARP binds to the steroid receptor RNA coactivator SRA via an intrinsic RNA binding domain and suppresses SRA-potentiated steroid receptor transcription activity. Accordingly, SHARP has the capacity to modulate both liganded and nonliganded nuclear receptors. Surprisingly, the expression of SHARP is itself steroid inducible, suggesting a simple feedback mechanism for attenuation of the hormonal response.

HSP70 and heat shock factor 1 cooperate to repress Ras-induced transcriptional activation of the c-fos gene

Heat shock protein 70 (HSP70) is a molecular chaperone involved in protein folding and resistance to the deleterious effects of stress. Here we show that HSP70 suppresses transcription of c-fos, an early response gene that is a key component of the ubiquitous AP-1 transcription factor complex. HSP70 repressed Ras-induced c-fos transcription only in the presence of functional heat shock factor1 (HSF1). This suggests that HSP70 functions as a corepressor with HSF1 to inhibit c-fos gene transcription. Therefore, besides its known function in the stress response, HSP70 also has the property of a corepressor and combines with HSF1 to antagonize Fos expression and may thus impact multiple aspects of cell regulation.

Modulation of Drosophila heat shock transcription factor activity by the molecular chaperone DROJ1

Heat shock transcription factors (HSFs) play important roles in the cellular response to physiological stress signals. To examine the control of HSF activity, we undertook a yeast two-hybrid screen for proteins interacting with Drosophila HSF. DROJ1, the fly counterpart of the human heat shock protein HSP40/HDJ1, was identified as the dominant interacting protein (15 independent isolates from 58 candidates). Overexpression of DROJ1 in Drosophila SL2 cells delays the onset of the heat shock response. Moreover, RNA interference involving transfection of SL2 cells with double-stranded droj1 RNA depletes the endogenous level of DROJ1 protein, leading to constitutive activation of endogenous heat shock genes. The induction level, modest when DROJ1 was depleted alone, reached maximal levels when DROJ1 and HSP70/HSC70, or DROJ1 and HSP90, were depleted concurrently. Chaperone co-depletion was also correlated with strong induction of the DNA binding activity of HSF. Our findings support a model in which synergistic interactions between DROJ1 and the HSP70/HSC70 and HSP90 chaperones modulate HSF activity by feedback repression.

Activation of heat shock transcription factor in yeast is not influenced by the levels of expression of heat shock proteins

Heat shock transcription factor (HSF) transiently induces the expression of a universally conserved set of proteins, the heat shock proteins (Hsps), when cells are exposed to elevated temperatures as well as to a wide range of other environmental stresses. The tight control of heat shock gene expression has prompted a model, according to which HSF activity and 'free' heat shock protein levels are tied up in a regulatory loop. Other data have indicated that HSF senses stress directly. Here, we report that yeast cells in which the basal expression levels of Hsps have been significantly increased exhibit improved thermotolerance but display no detectable difference in the temperature required for transient activation of HSF. In a separate experiment, overexpression of SSA2, a member of the Hsp70 family and a prominent candidate for the feedback regulation of HSF, did not inhibit the heat shock response. Our findings challenge the dogma that relief of the suppression of HSF activity by Hsps can account for the acute heat shock response.

Heat shock and oxidative stress-induced exposure of hydrophobic protein domains as common signal in the induction of hsp68

The hypothesis of a common signal for heat shock (HS) and oxidative stress (OS) was analyzed in C6 cells with regard to the induction of heat shock proteins (Hsps). The synthesis rate and level of the strictly inducible Hsp68 was significantly higher after HS (44 degrees C) compared with OS (2 mm H2O2). This difference corresponded to higher and lower activation of the heat shock factor (HSF) by HS and OS, respectively. OS, on the other hand, showed stronger cytotoxicity compared with HS as indicated by drastic lipid peroxidation and inhibition of protein synthesis as well as of mitochondrial and endocytotic activity. Lactic dehydrogenase also revealed stronger inhibition of enzyme activity by OS than by HS as shown in cells and in vitro experiments. Conformational analysis of lactic dehydrogenase by the fluorophore 1-anilinonaphtalene-8-sulfonic acid, however, showed stronger exposure of hydrophobic domains after HS than after OS which correlates positively with the Hsp68 response. Treatment of cells with deoxyspergualin, which exhibits high affinity to Hsps, the putative inhibitors of HSF, strongly increased only OS-induced hsp68 expression. In conclusion, the results suggest that exposure of hydrophobic domains of cytosolic proteins represents the common first signal in the multistep activation pathway of HSF.

Dissociation of Kar2p/BiP from an ER sensory molecule, Ire1p, triggers the unfolded protein response in yeast

The unfolded protein response (UPR) is a signal transduction pathway induced by a variety of endoplasmic reticulum (ER) stresses and functions to maintain homeostasis of the cellular membrane in eukaryotes. Various ER stresses result in the accumulation of unfolded proteins in the ER, which is sensed by the transmembrane protein kinase/ribonuclease Ire1p that transmits a signal from the ER to the nucleus in Saccharomyces cerevisiae. Here we report that the yeast ER chaperone Kar2p/BiP, a member of the HSP70 family found in the ER, directly regulates the UPR by the interaction with Ire1p. In the absence of ER stress, Kar2p binds the lumenal domain of Ire1p and keeps Ire1p in an inactive unphosphorylated state. Upon exposure of cells to ER stresses, Kar2p is released from Ire1p, resulting in activation of Ire1p and signal transduction to the nucleus. Subsequently, KAR2 mRNA is induced and Kar2p accumulates in the ER in a time-dependent manner, restoring the system to the basal state. This negative autoregulation is similar to the regulation of mammalian cytosolic chaperone Hsp70 via its interaction with heat shock factor 1.

The HspR regulon of Streptomyces coelicolor: a role for the DnaK chaperone as a transcriptional co-repressordagger

The dnaK operon of Streptomyces coelicolor encodes the DnaK chaperone machine and HspR, the transcriptional repressor of the operon; HspR confers repression by binding to several inverted repeat sequences in the promoter region, dnaKp. Here, we demonstrate that HspR specifically requires the presence of DnaK protein to retard a dnaKp fragment in gel-shift assays. This requirement is independent of the co-chaperones, DnaJ and GrpE, and it is ATP independent. Furthermore the retarded protein-DNA complex can be 'supershifted' by anti-DnaK monoclonal antibody, demonstrating that DnaK forms an integral component of the complex. It was shown in DNase I footprinting experiments that refolding and specific binding of HspR to its DNA target does not require DnaK. We conclude that the formation of the stable DnaK-HspR-DNA ternary complex does not depend on the chaperoning activity of DnaK. In affinity chromatography experiments using whole-cell extracts, DnaK was shown to co-purify with HspR, providing additional evidence that the two proteins interact in vivo; it was not possible to purify HspR away from DnaK in any experiments unless a powerful denaturant was used. The level of heat shock induction of chromosomal DnaK could be partially suppressed by expressing dnaK extrachromosomally from a heterologous promoter. In addition, it is shown that DnaK confers enhanced HspR-mediated repression of transcription in vitro. Taken together, these results suggest that DnaK functions as a transcriptional co-repressor by binding to HspR at its operator sites. In this model, the DnaK-HspR system would represent a novel example of feedback regulation of gene expression by a molecular chaperone, in which DnaK directly activates a repressor, rather than inactivates an activator (as is the case in the DnaK-sigma32 and Hsp70-HSF systems of other organisms).

Opposing actions of phosphatidylinositol 3-kinase and glycogen synthase kinase-3beta in the regulation of HSF-1 activity

Elevated temperatures activate the survival promoters Akt and heat shock factor-1 (HSF-1), a transcription factor that induces the expression of heat shock proteins (HSPs), such as HSP-70. Because neuronal mechanisms controlling these responses are not known, these were investigated in human neuroblastoma SH-SY5Y cells. Heat shock (45 degrees C) rapidly activated Akt, extracellular signal-regulated kinases 1 and 2 (ERK1/2), and p38, but only Akt was activated in a phosphatidylinositol 3-kinase (PI-3K)-dependent manner, as the PI-3K inhibitors LY294002 and wortmannin blocked Akt activation, but not ERK1/2 or p38 activation. Akt activation was not blocked by inhibition of p38 or ERK1/2, indicating the independence of these signaling systems. Heat shock treatment also caused a rapid increase in HSF-1 DNA binding activity that was partially dependent on PI-3K activity, as both the PI-3K inhibitors attenuated this response. Because Akt inhibits glycogen synthase kinase-3beta (GSK-3beta), an enzyme that facilitates cell death, we tested if GSK-3beta is a negative regulator of HSF-1 activation. Overexpression of GSK-3beta impaired heat shock-induced activation of HSF-1, and also reduced HSP-70 production, which was partially restored by the GSK-3beta inhibitor lithium. Thus, heat shock-induced activation of PI-3K and the inhibitory effect of GSK-3beta on HSF-1 activation and HSP-70 expression imply that Akt-induced inhibition of GSK-3beta contributes to the activation of HSF-1.

A nuclear localization signal is essential for stress-induced dimer-to-trimer transition of heat shock transcription factor 3

Expression of heat shock genes is regulated mainly at the level of transcription by heat shock transcription factors (HSFs). In avian, HSF1 and HSF3 are maintained in a cryptic monomer and dimer form, respectively, in the cytoplasm in the absence of stress. Upon heat stress, they undergo conformational change associated with the formation of a trimer and nuclear translocation. In this study, we identified regions that are necessary for the dimer-to-trimer transition of chicken HSF3 (cHSF3) upon stress conditions. One of these regions coincides with a functional nuclear localization signal (NLS) of cHSF3 that was recognized by a nuclear transport receptor importin alpha. Point mutations of basic amino acids in the NLS inhibit both nuclear translocation and trimer formation of cHSF3. These results demonstrate that the NLS acts positively on the trimer formation of cHSF3 upon stress conditions.

HSP70 and heat shock factor 1 cooperate to repress Ras-induced transcriptional activation of the c-fos gene

Heat shock protein 70 (HSP70) is a molecular chaperone involved in protein folding and resistance to the deleterious effects of stress. Here we show that HSP70 suppresses transcription of c-fos, an early response gene that is a key component of the ubiquitous AP-1 transcription factor complex. HSP70 repressed Ras-induced c-fos transcription only in the presence of functional heat shock factor1 (HSF1). This suggests that HSP70 functions as a corepressor with HSF1 to inhibit c-fos gene transcription. Therefore, besides its known function in the stress response, HSP70 also has the property of a corepressor and combines with HSF1 to antagonize Fos expression and may thus impact multiple aspects of cell regulation.

Cytoplasmic catalytic subunit of protein kinase A mediates cross-repression by NF-kappa B and the glucocorticoid receptor

Negative transcriptional regulation or cross-coupling between NF-kappa B (RelA) and the glucocorticoid receptor (GR) is proposed to play a regulatory role in human physiology and disease. Despite previous advances, the biochemical basis of this phenomenon remains a subject of controversy. We show here that the inhibition of GR activity by RelA does not require the RelA DNA binding, transactivation, or nuclear localization domains. Surprisingly, RelA repression of GR is abolished by mutation of the conserved protein kinase A (PKA) site at amino acid residue 276 of RelA. We show that GR associates in vivo and in vitro with the catalytic subunit of PKA (PKAc) in a ligand-independent manner and that GR transcription depends on PKA signaling. Indeed, we demonstrated that GR-mediated inhibition of NF-kappa B transactivation is PKAc-dependent. In contrast to previous models, we suggest that the cross-coupling requires a cytoplasmic step and is regulated by a PKAc-associated signaling.

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.

Stress-controlled transcription factors, stress-induced genes and stress tolerance in budding yeast

The transcriptional response to environmental changes is a major topic in both basic and applied research. From a basic point of view, to understand this response includes unravelling how the stress signal is sensed and transduced to the nucleus, to identify which genes are induced under each stress condition and, finally, to establish the phenotypic consequences of this induction in stress tolerance. The possibility of using genetic approaches has made the yeast Saccharomyces cerevisiae a compelling model to study stress response at a molecular level. Moreover, this information can be used to isolate and characterise stress-related proteins in higher eukaryotes and to design strategies to increase stress resistance in organisms of industrial interest. In this review the progress made in recent years is discussed.

Roles of molecular chaperones in the nervous system

Heat shock proteins (HSPs) are induced not only by heat shock but also by various other environmental stresses. HSPs such as Hsp90, Hsp70, Hsp60, Hsp40 and Hsp28 are also expressed constitutively at normal growth temperatures and have basic and indispensable functions in the life cycle of proteins as molecular chaperones, as well as playing a role in protecting cells from deleterious stresses. Recently, Hsc70 and Hsp40 were found to be localized to the synapse in the mammalian central nervous system, indicating a synaptic role for these HSPs. Molecular chaperones are able to inhibit the aggregation of partially denatured proteins and refold them. In addition, molecular chaperones, especially Hsp70, protect the brain and heart from severe ischemia. In these respects, there are expectations for the use of molecular chaperones for protection against and therapeutic treatment of inherited diseases caused by protein misfolding. In this study, we review Hsp70 and Hsp40, and refer to the roles of these molecules in the synapse and cytoprotective functions of HSPs in stress tolerance and neurodegenerative diseases.

Heat-shock protein expression is absent in the antarctic fish Trematomus bernacchii (family Nototheniidae)

The heat-shock response, the enhanced expression of one or more classes of molecular chaperones termed heat-shock proteins (hsps) in response to stress induced by high temperatures, is commonly viewed as a ‘universal’ characteristic of organisms. We examined the occurrence of the heat-shock response in a highly cold-adapted, stenothermal Antarctic teleost fish, Trematomus bernacchii, to determine whether this response has persisted in a lineage that has encountered very low and stable temperatures for at least the past 14–25 million years. The patterns of protein synthesis observed in in vivo metabolic labelling experiments that involved injection of (35)S-labelled methionine and cysteine into whole fish previously subjected to a heat stress of 10 degrees C yielded no evidence for synthesis of any size class of heat-shock protein. Parallel in vivo labelling experiments with isolated hepatocytes similarly showed significant amounts of protein synthesis, but no indication of enhanced expression of any class of hsp. The heavy metal cadmium, which is known to induce synthesis of hsps, also failed to alter the pattern of proteins synthesized in hepatocytes. Although stress-induced chaperones could not be detected under any of the experimental condition used, solid-phase antibody (western) analysis revealed that a constitutively expressed 70 kDa chaperone was present in this species, as predicted on the basis of requirements for chaperoning during protein synthesis. Amounts of the constitutively expressed 70 kDa chaperone increased in brain, but not in gill, during 22 days of acclimation to 5 degrees C. The apparent absence of a heat-shock response in this highly stenothermal species is interpreted as an indication that a physiological capacity observed in almost all other organisms has been lost as a result of the absence of positive selection during evolution at stable sub-zero temperatures. Whether the loss of the heat-shock response is due to dysfunctional genes for inducible hsps (loss of open reading frames or functional regulatory regions), unstable messenger RNAs, the absence of a functional heat-shock factor or some other lesion remains to be determined.

Expression of antisense hsp70 is a major determining factor in heat-induced cell death of P-19 carcinoma cells

Overexpressed heat shock protein 70 (Hsp70) is known to be associated with thermoprotection in a number of cell lines and transgenic animals. We hypothesized that because overexpression of Hsp70 protects cells from lethal heat stress, inhibition of expression should make cells susceptible to heat stress. The model used for this study was a stably transfected P-19 carcinoma cell line expressing antisense hsp70 under the control of the hsp70b promoter. The results showed marked inhibition of Hsp70 expression after heat shock correlated with heat-induced cell death. Hsp90 and Hsc70 protein expression were not affected by the antisense construct. Unexpectedly, heme oxygenase (HO-1), another highly inducible heat shock protein, was not induced after heat shock in the antisense hsp70 cell line. Heat shock transcription factor-1 (HSF-1) was in a highly phosphorylated state in the antisense cell line before and after heat shock. This was in contrast to the untransfected control P-19 cells where HSF-1 was primarily highly phosphorylated after heat shock. A control cell line expressing only the vector, pMAMneo, without the antisense construct also showed partial loss of Hsp70 induction but not increased cell death after heat shock. The findings support the role of Hsp70 in thermoresistance.

Complex regulation of the yeast heat shock transcription factor

The yeast heat shock transcription factor (HSF) is regulated by posttranslational modification. Heat and superoxide can induce the conformational change associated with the heat shock response. Interaction between HSF and the chaperone hsp70 is also thought to play a role in HSF regulation. Here, we show that the Ssb1/2p member of the hsp70 family can form a stable, ATP-sensitive complex with HSF-a surprising finding because Ssb1/2p is not induced by heat shock. Phosphorylation and the assembly of HSF into larger, ATP-sensitive complexes both occur when HSF activity decreases, whether during adaptation to a raised temperature or during growth at low glucose concentrations. These larger HSF complexes also form during recovery from heat shock. However, if HSF is assembled into ATP-sensitive complexes (during growth at a low glucose concentration), heat shock does not stimulate the dissociation of the complexes. Nor does induction of the conformational change induce their dissociation. Modulation of the in vivo concentrations of the SSA and SSB proteins by deletion or overexpression affects HSF activity in a manner that is consistent with these findings and suggests the model that the SSA and SSB proteins perform distinct roles in the regulation of HSF activity.

Molecular chaperone function of mammalian Hsp70 and Hsp40--a review

Virtually all organisms respond to up-shifts in temperature (heat shock) by synthesizing a set of proteins called heat shock proteins (HSPs). The HSPs are induced not only by heat shock but also by various other environmental stresses. Induction of HSPs is regulated by the trans-acting heat shock factors (HSFs) and cis-acting heat shock element (HSE) present at the promoter region of each heat shock gene. Usually, HSPs are also expressed constitutively at normal growth temperatures and have basic and indispensable functions in the life cycle of proteins as molecular chaperones, as well as playing a role in protecting cells from the deleterious stresses. Molecular chaperones are able to inhibit the aggregation of partially denatured proteins and refold them using the energy of ATP. Recently, there are expectations for the use of molecular chaperones for the protection against and therapeutic treatment of inherited diseases caused by protein misfolding. In this review, the focus will be on the mammalian Hsp40, a homologue of bacterial DnaJ heat shock protein, and the beneficial functions of molecular chaperones.

The yeast heat shock transcription factor changes conformation in response to superoxide and temperature

In vitro DNA-binding assays demonstrate that the heat shock transcription factor (HSF) from the yeast Saccharomyces cerevisiae can adopt an altered conformation when stressed. This conformation, reflected in a change in electrophoretic mobility, requires that two HSF trimers be bound to DNA. Single trimers do not show this change, which appears to represent an alteration in the cooperative interactions between trimers. HSF isolated from stressed cells displays a higher propensity to adopt this altered conformation. Purified HSF can be stimulated in vitro to undergo the conformational change by elevating the temperature or by exposing HSF to superoxide anion. Mutational analysis maps a region critical for this conformational change to the flexible loop between the minimal DNA-binding domain and the flexible linker that joins the DNA-binding domain to the trimerization domain. The significance of these findings is discussed in the context of the induction of the heat shock response by ischemic stroke, hypoxia, and recovery from anoxia, all known to stimulate the production of superoxide.

Thermotolerant desert lizards characteristically differ in terms of heat-shock system regulation

We compare the properties and activation of heat-shock transcription factor (HSF1) and the synthesis of a major family of heat-shock proteins (HSP70) in lizard species inhabiting ecological niches with strikingly different thermal parameters. Under normal non-heat-shock conditions, all desert-dwelling lizard species studied so far differ from a northern, non-desert species (Lacerta vivipara) in the electrophoretic mobility and content of proteins constitutively bound to the regulatory heat-shock elements in the heat-shock gene promoter. Under these conditions, levels of activated HSF1 and of both HSP70 mRNA and protein are higher in the desert species than in the non-desert species. Upon heat shock, HSF1 aggregates in all species studied, although in desert species HSF1 subsequently disaggregates more rapidly. Cells of the northern species have a lower thermal threshold for HSP expression than those of the desert species, which correlates with the relatively low constitutive level of HSPs and high basal content of HSF1 in their cells.

SUMO-1 modification activates the transcriptional response of p53

The p53 tumour suppressor protein is regulated by ubiquitin-mediated proteasomal degradation. In normal cells p53 is constitutively ubiquitylated by the Mdm2 ubiquitin ligase. When the p53 response is activated by stress signals p53 levels rise due to inhibition of this degradative pathway. Here we show that p53 is modified by the small ubiquitin-like protein SUMO-1 at a single site, K386, in the C-terminus of the protein. Modification in vitro requires only SUMO-1, the SUMO-1 activating enzyme and ubc9. SUMO-1 and ubiquitin modification do not compete for the same lysine acceptor sites in p53. Overexpression of SUMO-1 activates the transcriptional activity of wild-type p53, but not K386R p53 where the SUMO-1 acceptor site has been mutated. The SUMO-1 modification pathway therefore acts as a potential regulator of the p53 response and may represent a novel target for the development of therapeutically useful modulators of the p53 response.

Heat shock factors and the control of the stress response

Living cells are continually challenged by conditions which cause acute and chronic stress. To adapt to environmental changes and survive different types of injuries, eukaryotic cells have evolved networks of different responses which detect and control diverse forms of stress. One of these responses, known as the heat shock response, has attracted a great deal of attention as a universal fundamental mechanism necessary for cell survival under a variety of unfavorable conditions. In mammalian cells, the induction of the heat shock response requires the activation and translocation to the nucleus of one or more heat shock transcription factors which control the expression of a specific set of genes encoding cytoprotective heat shock proteins. The discovery that the heat shock response is turned on under several pathological conditions and contributes to establish a cytoprotective state in a variety of human diseases, including ischemia, inflammation, and infection, has opened new perspectives in medicine and pharmacology, as molecules activating this defense mechanism appear as possible candidates for novel cytoprotective drugs. This article focuses on the regulation and function of the heat shock response in mammalian cells and discusses the molecular mechanisms involved in its activation by stress and bioactive cyclopentenone prostanoids, as well as its interaction with nuclear factor kappaB, a stress-regulated transcription factor with a pivotal role in inflammation and immunity.

Multiple components of the HSP90 chaperone complex function in regulation of heat shock factor 1 In vivo

Rapid and transient activation of heat shock genes in response to stress is mediated in eukaryotes by the heat shock transcription factor HSF1. It is well established that cells maintain a dynamic equilibrium between inactive HSF1 monomers and transcriptionally active trimers, but little is known about the mechanism linking HSF1 to reception of various stress stimuli or the factors controlling oligomerization. Recent reports have revealed that HSP90 regulates key steps in the HSF1 activation-deactivation process. Here, we tested the hypothesis that components of the HSP90 chaperone machine, known to function in the folding and maturation of steroid receptors, might also participate in HSF1 regulation. Mobility supershift assays using antibodies against chaperone components demonstrate that active HSF1 trimers exist in a heterocomplex with HSP90, p23, and FKBP52. Functional in vivo experiments in Xenopus oocytes indicate that components of the HSF1 heterocomplex, as well as other components of the HSP90 cochaperone machine, are involved in regulating oligomeric transitions. Elevation of the cellular levels of cochaperones affected the time of HSF1 deactivation during recovery: attenuation was delayed by immunophilins, and accelerated by HSP90, Hsp/c70, Hip, or Hop. In immunotargeting experiments with microinjected antibodies, disruption of HSP90, Hip, Hop, p23, FKBP51, and FKBP52 delayed attenuation. In addition, HSF1 was activated under nonstress conditions after immunotargeting of HSP90 and p23, evidence that these proteins remain associated with HSF1 monomers and function in their repression in vivo. The remarkable similarity of HSF1 complex chaperones identified here (HSP90, p23, and FKBP52) and components in mature steroid receptor complexes suggests that HSF1 oligomerization is regulated by a foldosome-type mechanism similar to steroid receptor pathways. The current evidence leads us to propose a model in which HSF1, HSP90 and p23 comprise a core heterocomplex required for rapid conformational switching through interaction with a dynamic series of HSP90 subcomplexes.

The mammalian HSF4 gene generates both an activator and a repressor of heat shock genes by alternative splicing

The expression of heat shock genes is controlled at the level of transcription by members of the heat shock transcription factor family in vertebrates. HSF4 is a mammalian factor characterized by its lack of a suppression domain that modulates formation of DNA-binding homotrimer. Here, we have determined the exon structure of the human HSF4 gene and identified a major new isoform, HSF4b, derived by alternative RNA splicing events, in addition to a previously reported HSF4a isoform. In mouse tissues HSF4b mRNA was more abundant than HSF4a as examined by reverse transcription-polymerase chain reaction, and its protein was detected in the brain and lung. Although both mouse HSF4a and HSF4b form trimers in the absence of stress, these two isoforms exhibit different transcriptional activity; HSF4a acts as an inhibitor of the constitutive expression of heat shock genes, and hHSF4b acts as a transcriptional activator. Furthermore HSF4b but not HSF4a complements the viability defect of yeast cells lacking HSF. Moreover, heat shock and other stresses stimulate transcription of target genes by HSF4b in both yeast and mammalian cells. These results suggest that differential splicing of HSF4 mRNA gives rise to both an inhibitor and activator of tissue-specific heat shock gene expression.

The hsp70-associating protein Hap46 binds to DNA and stimulates transcription

We investigated the ubiquitously expressed hsp70-associating protein Hap46, which is also called RAP46 and is homologous to BAG-1, for activities independent of hsp70 interactions. We observed in vitro binding to various DNA fragments but detected no apparent sequence specificity. Deletion of the amino-terminal decapeptide, which contains two clusters of three basic amino acids each, abolished the DNA-binding ability of Hap46. Similarly, exchange of either of these positively charged clusters for three alanines resulted in loss of DNA binding. Using a fusion of Hap46 and green fluorescent protein, we found preferential accumulation in cell nuclei on heat stress as compared with unstressed cells. The repressive effect of heat shock on overall transcriptional activity in human DU145 carcinoma cells was largely prevented when Hap46 was overexpressed by transfection. Such overproduction of Hap46 also resulted in enhanced expression of specific reporter gene constructs and in increased levels of mRNAs specific for hsp70 and hsp40 after temperature stress. In vitro transcription with nuclear extracts was stimulated greatly by Hap46. Like DNA binding, transcriptional enhancement required amino-terminally located basic amino acid residues but not the carboxyl-terminal portion of Hap46 known to participate in hsp70 interaction. Our results show that Hap46 is a bifunctional protein that can interact with both hsp70s and DNA, employing different portions of the molecule. They also suggest that Hap46 is involved in temperature-sensitive regulation of transcription, acting as a general transcriptional activator.

Rapid activation of heat shock factor-1 DNA binding by H2O2 and modulation by glutathione in human neuroblastoma and Alzheimer's disease cybrid cells

Because cellular signaling systems are critical mediators of responses to oxidative stress, a condition associated with neurodegenerative disorders, the redox-dependent regulation of heat shock factor-1 (HSF-1) was investigated in human neuroblastoma SH-SY5Y cells. Exposure of cells to 200 microM H2O2 caused a rapid increase in HSF-1 DNA binding that was evident within 10 min, and caused a robust increase that reached levels 8-fold the basal activity. In comparison, the transcription factors, activator protein-1 (AP-1) and early growth response-1 (EGR-1), were activated more slowly and to a lesser extent. Activation of HSF-1 DNA binding activity was associated with a cytosolic to nuclear translocation of HSF-1 protein, and was detected with concentrations of H2O2 of 100 microM and greater. Intracellular glutathione modulated H2O2-induced HSF-1 DNA binding activity, as depletion of glutathione caused HSF-1 to be activated with lower concentrations of H2O2 (25 microM) and supplementation of glutathione blocked HSF-1 activation by 100 to 400 microM H2O2. Alzheimer's disease (AD) and control cybrid cells (SH-SY5Y cells in which the mitochondria were replaced with platelet mitochondria from AD or matched control subjects) were used to test the effects of the chronic oxidative stress caused by the excessive production of reactive oxygen intermediates (ROIs) in AD cybrids on HSF-1 activity. Basal and maximal (induced by H2O2 in glutathione-depleted cells) HSF-1 DNA binding activity were lower in AD than control cybrids, suggesting that the cells had compensated for excessive ROIs. These results indicate that the activation of HSF-1 is highly sensitive to oxidative stress and is regulated by endogenous antioxidant mechanisms.

The function of HSP72 in suppression of c-Jun N-terminal kinase activation can be dissociated from its role in prevention of protein damage

Activation of the c-Jun N-terminal kinase (JNK) by a variety of stimuli is critical for regulation of many cellular processes including apoptosis. The major inducible heat shock protein Hsp72 has previously been demonstrated to inhibit activation of JNK in cells exposed to heat shock and other protein-damaging agents, thus suppressing apoptosis. Hsp72 can protect proteins from stress-induced damage. To test if this protective function of Hsp72 is involved in JNK suppression, we investigated whether Hsp72 can avert activation of JNK by stimuli that do not cause protein damage. We show that Hsp72 suppresses activation of JNK induced by non-protein-damaging stimuli, interleukin-1 and UV irradiation, as well as by constitutively active components of the JNK signaling cascade Cdc42 and MEKK1. Furthermore, Hsp72 strongly reduced activation of JNK by phosphatase inhibitors. We also demonstrate that an Hsp72 mutant that lacks the ATPase domain is still capable of JNK suppression, thus indicating that the protein refolding activity of Hsp72 is not critical for inhibition of JNK activation. Taken together these data suggest that Hsp72 plays a regulatory role in JNK signaling and that the function of Hsp72 in protein protection or refolding is not involved in JNK regulation.

Intron-independent association of splicing factors with active genes

The cell nucleus is organized as discrete domains, often associated with specific events involved in chromosome organization, replication, and gene expression. We have examined the spatial and functional relationship between the sites of heat shock gene transcription and the speckles enriched in splicing factors in primary human fibroblasts by combining immunofluorescence and fluorescence in situ hybridization (FISH). The hsp90alpha and hsp70 genes are inducibly regulated by exposure to stress from a low basal level to a high rate of transcription; additionally the hsp90alpha gene contains 10 introns whereas the hsp70 gene is intronless. At 37 degrees C, only 30% of hsp90alpha transcription sites are associated with speckles whereas little association is detected with the hsp70 gene, whose constitutive expression is undetectable relative to the hsp90alpha gene. Upon exposure of cells to heat shock, the heavy metal cadmium, or the amino acid analogue azetidine, transcription at the hsp90alpha and hsp70 gene loci is strongly induced, and both hsp transcription sites become associated with speckles in >90% of the cells. These results reveal a clear disconnection between the presence of intervening sequences at specific gene loci and the association with splicing factor-rich regions and suggest that subnuclear structures containing splicing factors are associated with sites of transcription.

Modulation of human heat shock factor trimerization by the linker domain

Heat shock transcription factors (HSFs) are stress-responsive proteins that activate the expression of heat shock genes and are highly conserved from bakers' yeast to humans. Under basal conditions, the human HSF1 protein is maintained as an inactive monomer through intramolecular interactions between two coiled-coil domains and interactions with heat shock proteins; upon environmental, pharmacological, or physiological stress, HSF1 is converted to a homotrimer that binds to its cognate DNA binding site with high affinity. To dissect regions of HSF1 that make important contributions to the stability of the monomer under unstressed conditions, we have used functional complementation in bakers' yeast as a facile assay system. Whereas wild-type human HSF1 is restrained as an inactive monomer in yeast that is unable to substitute for the essential yeast HSF protein, mutations in the linker region between the DNA binding domain and the first coiled-coil allow HSF1 to homotrimerize and rescue the viability defect of a hsfDelta strain. Fine mapping by functional analysis of HSF1-HSF2 chimeras and point mutagenesis revealed that a small region in the amino-terminal portion of the HSF1 linker is required for maintenance of HSF1 in the monomeric state in both yeast and in transfected human 293 cells. Although linker regions in transcription factors are known to modulate DNA binding specificity, our studies suggest that the human HSF1 linker plays no role in determining HSF1 binding preferences in vivo but is a critical determinant in regulating the HSF1 monomer-trimer equilibrium.

Rapid and reversible relocalization of heat shock factor 1 within seconds to nuclear stress granules

Heat shock factor 1 (HSF1) is essential for the stress-induced expression of heat shock genes. On exposure to heat shock, HSF1 localizes within seconds to discrete nuclear granules. On recovery from heat shock, HSF1 rapidly dissipates from these stress granules to a diffuse nucleoplasmic distribution, typical of unstressed cells. Subsequent reexposure to heat shock results in the rapid relocalization of HSF1 to the same stress granules with identical kinetics. Although the appearance of HSF1 stress granules corresponds to the hyperphosphorylated, trimeric DNA-binding state of HSF1 and correlates temporally with the inducible transcription of heat shock genes, they are also present in heat-shocked mitotic cells that are devoid of transcription. This finding suggests a role for HSF1 stress granules as a nuclear compartment for the temporal regulation and spatial organization of HSF1 activity and reveals new features of the dynamics of nuclear organization.

A nuclear export signal prevents Saccharomyces cerevisiae Hsp70 Ssb1p from stimulating nuclear localization signal-directed nuclear transport

Hsp70 has been implicated in nuclear localization signal (NLS)-directed nuclear transport. Saccharomyces cerevisiae contains distinct SSA and SSB gene families of cytosolic Hsp70s. The nucleocytoplasmic localization of Ssa1p and Ssb1p was investigated using green fluorescent protein (GFP) fusions. Whereas GFP-Ssa1p localized both to the nucleus and cytoplasm, GFP-Ssb1p appeared only in the cytosol. The C-terminal domain of Ssb1p contains a leucine-rich nuclear export signal (NES) that is necessary and sufficient to direct nuclear export. The accumulation of GFP-Ssb1p in the nuclei of xpo1-1 cells suggests that Ssb1p shuttles across the nuclear envelope. Elevated levels of SSA1 but not SSB1 suppressed the NLS-GFP nuclear localization defects of nup188-Delta cells. Studies with Ssa1p/Ssb1p chimeras revealed that the Ssb1p NES is sufficient and necessary to inhibit the function of Ssa- or Ssb-type Hsp70s in nuclear transport. Thus, NES-less Ssb1p stimulates nuclear transport in nup188-Delta cells and NES-containing Ssa1p does not. We conclude that the differential function of Ssa1p and Ssb1p in nuclear transport is due to the NES-directed export of the Ssb1p and not to functional differences in their ATPase or peptide binding domains.

Glycogen synthase phosphatase interacts with heat shock factor to activate CUP1 gene transcription in Saccharomyces cerevisiae

Upon heat shock, transcription of many stress-inducible genes is rapidly and dramatically stimulated by heat shock factor (HSF). A central region of the yeast HSF (designated HSFrr for "repression region") was previously identified and proposed to be involved in repressing the activation domain under non-heat-shock conditions. Here, we used the phage display system to isolate proteins that interact with HSFrr. This should identify factors that modulate HSF activity or directly participate in HSF-mediated transcriptional activation. We constructed a randomly sheared yeast genomic library to express yeast proteins on the surface of lambda phage. HSFrr binding phages were selected by cycles of affinity chromatography. DNA sequencing identified an HSFrr-interacting phage that contains the GAC1 gene. The GAC1 gene encodes the regulatory subunit for a type 1 serine/threonine phosphoprotein phosphatase, Glc7. Both gac1 and glc7 mutations had little effect on HSF activation of gene transcription of two heat shock genes, SSA4 and HSP82. In contrast, heat shock induction of CUP1 gene expression was completely abolished in a glc7 mutant and reduced in a gac1 mutant. The results demonstrate that the Glc7 phosphatase and its Gac1 regulatory subunit play positive roles in HSF activation of CUP1 transcription.

Thermotolerant cells show an attenuated expression of Hsp70 after heat shock

Expression of heat shock proteins (hsps) results in the protection of cells from subsequent stresses. However, hsps are also toxic when present within cells for a prolonged time period. Thus, the expression of hsps should be tightly regulated. In the present study, the expression of Hsp70 after heat shock was compared between thermotolerant cells, which contain a large concentration of Hsp70, and nonthermotolerant cells (naive). Accumulation of Hsp70, assessed by Western blotting, was negligible when thermotolerant cells were heat-shocked a second time. Hsp70 transcription was similar between thermotolerant and naive cells during heat shock. However, Hsp70 transcription was attenuated more rapidly in thermotolerant than naive cells immediately upon return to non-heat shock conditions. In addition, Hsp70 mRNA stability was reduced in thermotolerant cells as compared with naive cells following the stress. New synthesis of Hsp70 and the efficiency of Hsp70 mRNA translation were similar between thermotolerant and naive cells during the post-stress period. These results suggest that thermotolerant cells limit Hsp70 expression by transcriptional and pretranslational mechanisms, perhaps to avoid the potential cytotoxic effect of these proteins.

Regulatory domain of human heat shock transcription factor-2 is not regulated by hemin or heat shock

Heat shock transcription factor 2 (HSF-2) activates transcription of heat shock proteins in response to hemin in the human erythroleukemia cell line, K562. To understand the regulation of HSF-2 activation, a series of deletion mutants of HSF-2 fused to the GAL-4 DNA binding domain were generated. We have found that human HSF-2 has a regulatory domain located in the carboxyl-terminal portion of the protein which represses the activity of its activation domain under normal physiological conditions. The repressive effects of this domain can be eliminated by its deletion in GAL4-HSF-2 fusion constructs. The regulatory domain of HSF-2 can also repress a heterologous chimeric activator that contains a portion of the VP16 activation domain. The activation domain of HSF-2 is a segment of approximately 77 amino acids located proximal to the carboxyl-terminal hydrophobic heptad repeat (leucine zipper 4) of the molecule. Interestingly, the GAL4-HSF-2 fusion protein and the 77 amino acids activation domain are inactive and are not activated by pretreatment of cells with either hemin or elevated temperature. Our data suggest that regulation of HSF-2 differs from HSF-1 in that its regulatory domain is not responsive to hemin or heat directly.

Gene expression and chromatin organization during mouse oocyte growth

Mouse oocytes can be classified according to their chromatin organization and the presence [surrounded nucleolus (SN) oocytes] or absence [nonsurrounded nucleolus (NSN) oocytes] of a ring of Hoechst-positive chromatin around the nucleolus. Following fertilization only SN oocytes are able to develop beyond the two-cell stage. These studies indicate a correlation between SN and NSN chromatin organization and the developmental competence of the female gamete, which may depend on gene expression. In the present study, we have used the HSP70.1Luc transgene (murine HSP70.1 promoter + reporter gene firefly luciferase) to analyze gene expression in oocytes isolated from ovaries of 2-day- to 13-week-old females. Luciferase was assayed on oocytes after classification as SN or NSN type. Our data show that SN oocytes always exhibit a higher level of luciferase activity, demonstrating a higher gene expression in this category. Only after meiotic resumption, metaphase II oocytes derived from NSN or SN oocytes acquire the same level of transgene expression. We suggest that the limited availability of transcripts and corresponding proteins, excluded from the cytoplasm until GVBD in NSN oocytes, could explain why these oocytes have a lower ability to sustain embryonic development beyond the two-cell stage at which major zygotic transcription occurs. With this study we have furthered our knowledge of epigenetic regulation of gene expression in oogenesis.

Sensitivity of Drosophila heat shock transcription factor to low pH

The heat shock transcription factor (HSF) mediates the induction of heat shock gene expression. The activation of HSF involves heat shock-induced trimerization, binding to its cognate DNA sites, and the acquisition of transcriptional competence. In this study, the oligomeric properties of Drosophila HSF were analyzed by equilibrium analytical ultracentrifugation and gel filtration chromatography. Previous findings showed that trimerization of purified Drosophila HSF was directly sensitive to heat and oxidation (1). Here we report that low pH, in the physiological range, also directly induces HSF trimerization and DNA binding in vitro. Furthermore, the induction of HSF trimerization by low pH is synergistic with the actions of heat and oxidation. Since heat or chemical stress leads to a moderate decrease of intracellular pH, we suggest that intracellular acidification may contribute to activating the heat shock response in vivo.

Intranuclear targeted delivery of functional NF-kappaB by 70 kDa heat shock protein

The 70 kDa heat shock protein (Hsp70) is a highly conserved, ubiquitous protein involved in chaperoning proteins to various cellular organelles. Here we show that when added exogenously to cells, Hsp70 is readily imported into both cytoplasmic and nuclear compartments in a cell-type-specific fashion. We exploited this ability of Hsp70 to deliver NF-kappaB, a key transcriptional regulator of inflammatory responses. We demonstrate that a fusion protein composed of a C-terminal Hsp70 peptide and the p50 subunit of NF-kappaB was directed into the nucleus of cells, could bind DNA specifically, and activated Igkappa expression and TNFalpha production. We therefore propose that Hsp70 can be used as a vehicle for intracytoplasmic and intranuclear delivery of proteins or DNA to modulate gene expression and thereby control immune responses.