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

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.

Potential protective effect of NF-kappaB activity on the polymicrobial sepsis of rats preconditioning heat shock treatment

The present study was designed to investigate the role of NF-kappaB in influencing the outcome of sepsis modulated by previous heat shock treatment. Sepsis was induced in rats by cecum ligation and puncture (CLP) method, which manifests two distinct clinical phases: an initial hyperdynamic phase (9 h after CLP, early sepsis) followed by a hypodynamic phase (18 h after CLP, late sepsis). Rats of heated group were treated by whole body hyperthermia 24 h prior to the CLP operation. Lymphocytes were collected during the early and late sepsis phases. The expressions of Hsp72, p65 and I-kappa B were evaluated by Western blot and immunochemical analysis. NF-kappaB activity was detected by EMSA. The results showed that NF-kappaB activation was initiated during early sepsis and apparently suppressed during late stage of sepsis. Previously treated by heat shock, late-sepsis rats emerged with high preservation of p65 expression and NF-kappaB activity, while Hsp72 was over-expressed. In conclusion, down-regulation of NF-kappaB activity during late sepsis could be attenuated by pretreatment of heat shock through the preservation of p65 expression. The results may provide a mechanistic explanation for the improved outcome to polymicrobial sepsis of rats that are preconditioned with heat shock, as well as a novel highlight for therapeutic intervention of severe infection.

Rac1 regulates stress-induced, redox-dependent heat shock factor activation

The signaling pathway by which environmental stresses activate heat shock factors (HSFs) is not completely understood. We show that the small GTPase rac1, and Rac1-regulated reactive oxygen species (ROS) play an important role in stress-stimulated heat shock response. A dominant-negative allele of Rac1 (Rac1N17) inhibits the hypoxia/reoxygenation and sodium arsenite-induced transcriptional activity of HSF-1 and the transcription of heat shock protein 70. Rac1N17 also suppresses the production of intracellular ROS induced by hypoxia/reoxygenation or sodium arsenite. Moreover, direct suppression of intracellular ROS levels by antioxidants decreases stress-stimulated HSF activity. However, expression of a constitutively active mutant of Rac1 (Rac1V12) in the absence of extracellular stresses does not increase intracellular ROS levels or induce the heat shock response. These results show that Rac1 is a necessary but insufficient component of the stress-induced signaling pathway that leads to ROS production, activation of HSFs, and transcription of heat shock proteins.

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.

Phenylarsine oxide inhibits heat shock protein 70 induction in cultured guinea pig gastric mucosal cells

Phenylarsine oxide (PAO) forms a stable ring complex with vicinal dithiols that can be reversed with 2,3-dimercaptopropanol (DMP) but not by dithiothreitol (DTT) or 2-mercaptoethanol (2-ME). PAO at 2 microM or higher inhibited heat shock protein 70 (HSP70) induction within minutes in cultured guinea pig gastric mucosal cells exposed to heat (43 degrees C) for 30 min. PAO did not affect the nuclear translocation and phosphorylation of heat shock factor 1 (HSF1) induced by heat stress, but it completely blocked the binding activity of HSF1 to the heat shock element (HSE), leading to the block of expression of HSP70 mRNA and accumulation of HSP70 in the cells. These inhibitions were completely reversed with 2 microM DMP but not with 0.1 mM DTT or 1 mM 2-ME, suggesting specific interactions between PAO and vicinal dithiol-containing molecules. Thioredoxin (Trx) reversed the inhibition of the binding activity of HSF1 in whole cell extracts prepared from PAO-treated, heat-stressed cells. Our results suggest that PAO may react with vicinal-containing molecules including Trx and specifically block the interaction between HSF1 and HSE.

Fever and the heat shock response: distinct, partially overlapping processes

The heat shock response is an ancient and highly conserved process that is essential for surviving environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review studies showing that fever is beneficial in the infected host. We show that core temperatures achieved during fever can activate the heat shock response and discuss some of the biochemical consequences of such an effect. We present data suggesting 4 possible mechanisms by which fever might confer protection: (1) directly killing or inhibiting growth of pathogens; (2) inducing cytoprotective heat shock proteins (Hsps) in host cells; (3) inducing expression of pathogen Hsps, an activator of host defenses; and (4) modifying and orchestrating host defenses. Two of these mechanisms directly involve the heat shock response. We describe how heat shock factor-1, the predominant heat-induced transcriptional enhancer not only activates transcription of Hsps but also regulates expression of pivotal cytokines and early response genes. The relationship between fever and the heat shock response is an illuminating example of how a more recently evolved response might exploit preexisting biochemical pathways for a new function.

Acute pancreatitis results in induction of heat shock proteins 70 and 27 and heat shock factor-1

Heat shock proteins (HSPs) 70 and 27 are stress-responsive proteins that are important for cell survival after injury; the expression of these HSPs is regulated primarily by the transcription factor heat shock factor-1 (HSF-1). The purpose of this study was to determine the effect of acute pancreatitis on pancreatic HSPs (70, 27, 60, and 90) expression and to assess potential mechanisms for HSP induction using a murine model of cerulein-induced pancreatitis. We found an increase of both HSP70 and HSP27 levels with expression noted throughout the pancreas after induction of pancreatitis. HSP60 and HSP90 levels were constitutively expressed in the pancreas and did not significantly change with acute pancreatitis. HSF-1 DNA binding activity increased in accordance with increased HSP expression. We conclude that acute pancreatitis results in a marked increase in the expression of HSP70 and HSP27. Furthermore, the induction of HSP70 and HSP27 expression was associated with a concomitant increase in HSF-1 binding activity. The increased expression of both HSP70 and HSP27 noted with pancreatic inflammation may confer a protective effect for the remaining acini after acute pancreatitis.

Regulation of multidrug resistance 1 (MDR1)/P-glycoprotein gene expression and activity by heat-shock transcription factor 1 (HSF1)

Infection of HeLa cells with adenovirus-carrying HSF1(+) cDNA, which encodes a mutated form of HSF1 with constitutive transactivation capacity, increased multidrug resistance 1 (MDR1) mRNA level and P-glycoprotein (P-gp) cell surface content and stimulated rhodamine 123 accumulation and vinblastine efflux activity. On the other hand, infection with adenovirus-carrying HSP70 and HSP27 cDNAs did not increase MDR1/P-gp expression. HSF1 regulates MDR1/P-gp expression at the transcriptional level, since HSF1(+) bound the heat-shock consensus elements (HSEs) in the MDR1 gene promoter and also activated the expression of an MDR1 promoter-driven reporter plasmid (pMDR1(-1202)). In addition, heat-shock increased pMDR1(-1202) promoter activity but not the activity of a similar reporter plasmid with point mutations at specific HSEs, and the heat-induced increase was totally inhibited by co-transfection with an expression plasmid carrying HSF1(-), a dominant negative mutant of HSF1. The stress inducers arsenite, butyrate, and etoposide also increased pMDR1(-1202) promoter activity, but the increase was not inhibited (in the case of butyrate) or was only partially inhibited (in the case of arsenite and etoposide) by HSF1(-). These results demonstrate that HSF1 regulates MDR1 expression, and that the HSEs present in the -315 to -285 region mediate the heat-induced activation of the MDR1 promoter. However, other factors may also participate in MDR1 induction by stressing agents.

Formation of nuclear HSF1 granules varies depending on stress stimuli

In concert with the stress-induced activation of human heat shock factor 1 (HSF1), the factor becomes inducibly phosphorylated and accumulates into nuclear granules. To date, these processes are not fully understood. Here, we show that although stress caused by the proteasome inhibitors MG132 and clasto-lactacystine beta-lactone induces the expression of Hsp70, the formation of HSF1 granules is affected differently in comparison to heat shock. Furthermore, proteasome inhibition increases serine phosphorylation on HSF1, but to a lesser extent than heat stress. Our results suggest that, depending on the type of stress stimulus, the multiple events associated with HSF1 activation might be affected differently.

Plants contain a novel multi-member class of heat shock factors without transcriptional activator potential

Based on phylogeny of DNA-binding domains and the organization of hydrophobic repeats, two families of heat shock transcription factors (HSFs) exist in plants. Class A HSFs are involved in the activation of the heat shock response, but the role of class B HSFs is not clear. When transcriptional activities of full-length HSFs were monitored in tobacco protoplasts, no class B HSFs from soybean or Arabidopsis showed activity under control or heat stress conditions. Additional assays confirmed the finding that the class B HSFs lacked the capacity to activate transcription. Fusion of a heterologous activation domain from human HSF1 (AD2) to the C-terminus of GmHSFB1-34 gave no evidence of synergistic enhancement of AD2 activity, which would be expected if weak activation domains were present. Furthermore, activity of AtHSFB1-4 (class B) was not rescued by coexpression with AtHSFA4-21 (class A) indicating that the class A HSF was not able to provide a missing function required for class B activity. The transcriptional activation potential of Arabidopsis AtHSFA4-21 was mapped primarily to a 39 amino acid fragment in the C-terminus enriched in bulky hydrophobic and acidic residues. Deletion mutagenesis of the C-terminal activator regions of tomato and Arabidopsis HSFs indicated that these plant HSFs lack heat-inducible regulatory regions analogous to those of mammalian HSF1. These findings suggest that heat shock regulation in plants may differ from metazoans by partitioning negative and positive functional domains onto separate HSF proteins. Class A HSFs are primarily responsible for stress-inducible activation of heat shock genes whereas some of the inert class B HSFs may be specialized for repression, or down-regulation, of the heat shock response.

Heat shock transcription factors and the hsp70 induction response in brain and kidney of the hyperthermic rat during postnatal development

Heat shock transcription factor (HSF) 1 levels increase in brain regions and decline in kidney during postnatal rat development. In both neonatal and adult rats, levels of HSF1 protein in brain and kidney are proportional to the levels of HSF DNA-binding activity and the magnitude of heat shock protein hsp70 induction after thermal stress. There appears to be more HSF1 protein in adult brain than is needed for induction of hsp70 after thermal stress, suggesting that HSF1 may have other functions in addition to its role as a stress-inducible activator of heat shock genes. HSF2 protein levels decline during postnatal rat development in brain regions and kidney. Gel mobility shift analysis shows that HSF2 is not in a DNA-binding form in the neonatal brain and kidney, suggesting that HSF2 may not be involved in the constitutive expression of hsps in early postnatal development. There is no apparent relationship between levels of HSF2 protein and basal levels of hsp90, hsp70, heat shock cognate protein hsc70, and hsp60.

Protein synthesis is required for stabilization of hsp70 mRNA upon exposure to both hydrostatic pressurization and elevated temperature

We have recently described that in chondrocytic cells high hydrostatic pressure (HP) causes a heat shock response via mRNA stabilization without a transcriptional activation of the hsp70 gene. In this study, we investigated whether this exceptional regulatory mechanism occurs more generally in different types of cells. Indeed, hsp70 mRNA and protein accumulated in HeLa, HaCat and MG-63 cells under 30 MPa HP, without DNA-binding of heat shock transcription factor 1 (HSF1) to the heat shock element of the hsp70 gene or formation of nuclear HSF1 granules, revealing a lack of transcriptional activation. Moreover, we observed that protein synthesis is needed for mRNA stabilization. Thus, high HP offers a model to study the mechanisms of hsp70 mRNA stabilization without HSF1-mediated induction of the heat shock gene response.

c-Jun NH2-terminal kinase targeting and phosphorylation of heat shock factor-1 suppress its transcriptional activity

The mammalian heat shock transcription factor HSF-1 regulates the expression of the heat shock proteins, molecular chaperones that are involved in cellular processes from higher order assembly to protein degradation. HSF-1 is a phosphorylated monomer under physiological growth conditions and is located mainly in the cytoplasm. Upon activation by a variety of environmental stresses, HSF-1 is translocated into the nucleus, forms trimers, acquires DNA binding activity, is hyperphosphorylated, appears as punctate granules, and increases transcriptional activity of target genes. As cells recover from stress, the punctate granules gradually disappear, and HSF-1 appears in a diffused staining pattern in the cytoplasm and nucleus. We have previously shown that the mitogen-activated protein kinase ERK phosphorylates and suppresses HSF-1-driven transcription. Here, we show that c-Jun NH(2)-terminal kinase (JNK) also phosphorylates and inactivates HSF-1. Overexpression of JNK facilitates the rapid disappearance of HSF-1 punctate granules after heat shock. Similar to ERK, JNK binds to HSF-1 in the conserved mitogen-activated protein kinases binding motifs and phosphorylates HSF-1 in the regulatory domain. The overexpression of an HSF-1-green fluorescent protein fusion construct lacking JNK phosphorylation sites causes this HSF-1 mutant to form nuclear granules that remain longer in the nucleus after heat shock. Taken together, these findings indicate that JNK phosphorylates HSF-1 and suppresses its transcriptional activity by rapidly clearing HSF-1 from the sites of transcription.

Tissue-specific expression of zebrafish (Danio rerio) heat shock factor 1 mRNAs in response to heat stress

All organisms respond to environmental, chemical and physiological stresses by enhanced synthesis of an evolutionarily conserved family of proteins known as heat shock proteins (HSPs) or stress proteins. Certain HSPs are also expressed constitutively during cell growth and development, and they function as molecular chaperones. The transcriptional regulation of hsp genes is mediated by the heat shock transcription factor (HSF). The stress response has been studied mostly in mammalian cell lines or organisms normally maintained under constant laboratory conditions. There is much less information on the regulation of the stress response of animals, such as fish, that have to tolerate large fluctuations in environmental and internal conditions. To characterize the regulation of the heat shock response in fish, we have cloned the first heat shock transcription factor from fish, zebrafish Danio rerio. Phylogenetic analysis confirms that the isolated zebrafish HSF belongs to the HSF1 family and is therefore designated zHSF1. Analysis by reverse transcriptase polymerase chain reaction (RT-PCR) shows the presence of two zHSF1 mRNA forms that are expressed in a tissue-specific fashion upon exposure to heat stress. Both forms are expressed in gonads under all conditions; in liver and to a lesser extent in the gills, the longer splice form of zHSF1 disappears upon heat shock. We present evidence for a unique tissue-specific regulation of HSF1 upon exposure to elevated temperature.

Canonical heat shock element in the alpha B-crystallin gene shows tissue-specific and developmentally controlled interactions with heat shock factor

Oligomerization of the heat shock factor (HSF) and its interaction with the heat shock element (HSE) are the hallmark of active transcriptional response to tangible physical or chemical stress. It is unknown if these interactions are subject to control and modulation by developmental cues and thus have tissue or stage specificity. By using promoter sequences containing a canonical HSE from the alphaB-crystallin gene, we demonstrate a tissue-specific transition from monomeric (in fetal and early neonatal stages that lack oligomeric HSF.HSE complexes) to oligomeric HSF-HSE interactions by postnatal day 10-21 in the ocular lens. Developmental control of these interactions is further demonstrated by induction of oligomeric HSF.HSE complexes in neonatal extracts by in vitro manipulations, interestingly, only in the lens and not in the brain, heart, or liver extracts. The exclusive presence of oligomeric HSF.HSE complexes in the postnatal/adult lens corresponds to known highly increased number of alphaB-crystallin transcripts in this tissue.

Sensitization of tumor cells to fas killing through overexpression of heat-shock transcription factor 1

Activation of the heat-shock or stress response is generally considered a cytoprotective response to heat or other proteotoxic stresses. In mammalian cells, stress-induced transcription of heat-shock genes is regulated by heat-shock transcription factor 1 (HSF1). We now show that activation of the Fas death receptor transactivates HSF1 in HeLa cells, a Fas-expressing cervical carcinoma line. Whereas HSF1 is constitutively expressed in a non-DNA-binding, transcriptionally inactive state, activation of Fas leads to enhanced transcription of a heat-shock reporter gene. The effects of Fas on heat-shock-gene transcription do not appear to be a consequence of cell death as they (1) precede apoptotic changes and (2) are not abrogated by YVAD-CMK, an inhibitor of Fas apoptosis that acts by blocking downstream effector proteases. Despite expressing Fas, HeLa cells are relatively insensitive to Fas-mediated killing, indicating that Fas expression alone, although necessary, is not sufficient for apoptosis. By overexpressing a constitutively activated form of HSF1, we sensitize HeLa cells to Fas-mediated killing. These findings shed new light on the interaction between two of the most evolutionarily conserved cell programs in nature, the Fas death pathway and the heat-shock response. Strategies designed to upregulate HSF1 in tumor cells, either through pharmacologic or gene-therapy approaches will hopefully provide a means with which to sensitize tumors to the killing effects of cancer therapies operating through the Fas receptor.

Molecular basis of competition between HSF2 and catalytic subunit for binding to the PR65/A subunit of PP2A

We recently identified the existence of a novel interaction between heat shock transcription factor 2 (HSF2) and the PR65/A subunit of protein phosphatase 2A (PP2A) and showed that HSF2 is able to compete with the PP2A catalytic subunit for binding to PR65. To elucidate the mechanistic basis of this competition between HSF2 and catalytic subunit at the molecular level we have sought to characterize sequences within PR65 that are important for interaction with HSF2. The results identify the intra-repeat loop within HEAT repeat 11 of PR65 as critical for interaction with HSF2. Analysis of point mutants within this loop region of PR65 identify lysine 416 as a residue critical for interaction with HSF2. Interestingly, this same lysine residue of PR65 is important for its binding to catalytic subunit. These results suggest that HSF2's ability to interfere with catalytic subunit binding to PR65 is due to competition between HSF2 and catalytic subunit for at least one amino acid residue of PR65, lysine 416. These data support the hypothesis that HSF2 represents a new type of PP2A regulatory protein.

Cloning and expression analysis of an inducible HSP70 gene from tilapia fish

We isolated and characterized the tilapia (Oreochromis mossambicus) HSP70 gene, highly homologous to other HSP70 genes. A dramatic increase of tilapia HSP70 mRNA levels was observed after heat shock of whole animals in all organs tested. Reporter constructs were tested for transient expression in carp cells and in microinjected zebrafish embryos. The entire isolated regulatory region (-851/+157) was able to mediate heat shock inducible expression of the reporter gene, with no preference for a particular tissue. Our studies represent the first transcriptional analysis of a HSP70 promoter from fish, revealing a powerful tool to direct controlled, tissue-independent gene expression in fish.

p53 suppresses the c-Myb-induced activation of heat shock transcription factor 3

Expression of heat shock proteins (HSPs) is controlled by heat shock transcription factors (HSFs). Vertebrates express multiple HSFs whose activities may be regulated by distinct signals. HSF3 is specifically activated in unstressed proliferating cells by direct binding to the c-myb proto-oncogene product (c-Myb), which plays an important role in cellular proliferation. This suggests that the c-Myb-induced HSF3 activation may contribute to the growth-regulated expression of HSPs. Here we report that the p53 tumor suppressor protein directly binds to HSF3 and blocks the interaction between c-Myb and HSF3. In addition, p53 stimulates the degradation of c-Myb through a proteasome-dependent mechanism, which is, at least partly, mediated by induction of Siah in certain types of cells. Induction of p53 by a genotoxic reagent in DT40 cells disrupts the HSF3-c-Myb interaction and down-regulates the expression of certain HSPs. Mutated forms of p53 found in certain tumors did not inhibit c-Myb-induced HSF3 activation. The regulation of HSF3 activity by c-Myb and p53 sheds light on the molecular events that govern HSP expression during cellular proliferation and apoptosis.

Decreased levels of heat shock proteins in gut epithelial cells after exposure to plant lectins

BACKGROUND

The enterocytes of the intestinal epithelium are regularly exposed to potentially harmful substances of dietary origin, such as lectins. Expression of heat shock proteins (HSPs) by this epithelium may be part of a protective mechanism developed by intestinal epithelial cells to deal with noxious components in the intestinal lumen.

AIM

To investigate if the lectins PHA, a lectin from kidney beans (Phaseolus vulgaris) and WGA, a lectin from wheat germ (Triticum aestivum) could modify the heat shock response in gut epithelial cells and to establish the extent of this effect.

METHODS

Jejunal tissue sections from PHA and WGA fed rats were screened for expression of HSP70, HSP72, and HSP90 using monoclonal antibodies. Differentiated Caco-2 cells, the in vitro counterpart of villus enterocytes, were exposed to 100 microg/ml of PHA-E(4) or WGA for 48 hours and investigated for changes in DNA and protein synthesis by double labelling with [2-(14)C]thymidine and L-[methyl-(3)H]methionine. The relative concentrations of HSP60, HSP70, HSP72, and HSP90 and binding protein (BiP) in these cells exposed to lectins were analysed by polyacrylamide gel electrophoresis and immunoblotting. To establish if lectin exposed differentiated Caco-2 cells were still capable of producing a heat shock response, these cells received a heat shock (40 degrees C, 41 degrees C, and 42 degrees C) for one hour and were allowed to recover for six hours at 37 degrees C. During heat shock and recovery periods, lectin exposure was continued.

RESULTS

Constitutive levels of HSPs were measured in the intestinal cells of lactalbumin fed (control) rats, as may be expected from the function of this tissue. However, in PHA and WGA fed rats a marked decline in the binding of antibodies against several HSPs to the intestinal epithelium was found. These results were confirmed by in vitro experiments using differentiated Caco-2 cells exposed to PHA-E(4) and WGA. However, after exposure to lectins, these cells were still capable of heat induced heat shock protein synthesis, and total protein synthesis was not impaired indicating specific inhibition of HSP synthesis in non-stressed cells.

CONCLUSIONS

We conclude that PHA and WGA decrease levels of stress proteins in rat gut and enterocyte-like Caco-2 cells, leaving these cells less well protected against the potentially harmful content of the gut lumen.

Interaction of the Neurospora crassa heat shock factor with the heat shock element during heat shock and different developmental stages

The interaction of the heat shock factor (HSF) with the heat shock element (HSE) was determined by a non-radioactive electrophoretic mobility shift assay, in order to analyze HSF regulation in Neurospora crassa. HSF binds to HSE under normal, non-stress conditions and is thus constitutively trimerized. Upon heat shock, the HSF-HSE complex shows a retarded mobility. This was also observed in Saccharomyces cerevisiae, where this mobility shift was shown to be due to HSF phosphorylation [Sorger and Pelham (1988) Cell 54, 855-864]. In N. crassa, HSE-dependent electrophoretic mobility shift is temperature- and time-dependent. Under normal growth conditions, the HSF is located in the cytoplasm as well as in the nucleus. In germinating conidia the HSF shows a retarded mobility typical for heat shock even at normal growth temperatures. No HSF-dependent mobility shift was detectable in aerial hyphae.

Age-related alterations in the activation of heat shock transcription factor 1 in rat hepatocytes

The induction of hsp70 transcription by heat shock is significantly reduced in hepatocytes isolated from old rats compared to hepatocytes isolated from young/adult rats, and the decline in hsp70 transcription is correlated with a decrease in the induction of heat shock transcription factor 1 (HSF1) binding to the heat shock element. However, the decreased HSF1 binding activity to DNA is not due to reduced levels of HSF1 that are available for activation by heat shock. In fact, the levels of HSF1 are two- to threefold higher in hepatocytes from old rats, and the age-related increase in the levels of HSF1 protein in hepatocytes appears to arise from a decrease in the degradation of the HSF1 because HSF1 mRNA levels do not change and the synthesis of HSF1 decreases approximately 50% with age. No evidence was found for an impairment in HSF1 oligomerization in hepatocytes from old rats, e.g., the level of HSF1 trimers, the nuclear translocation of HSF1, and the phosphorylation of HSF1 after heat shock are similar in hepatocytes isolated from young/adult and old rats. However, the thermostability of the DNA binding activity of HSF1 was significantly reduced with age in a cell-free system as well as in isolated hepatocytes.

Inhibition of tumor necrosis factor-alpha transcription in macrophages exposed to febrile range temperature. A possible role for heat shock factor-1 as a negative transcriptional regulator

We previously reported that expression of tumor necrosis factor-alpha (TNFalpha) was attenuated in macrophages exposed to febrile range temperatures. In this study, we analyzed the influence of temperature on TNFalpha transcription in the Raw 264.7 macrophage cell line during incubation at 37 and 39.5 degrees C. The initial activation of TNFalpha transcription in response to endotoxin (LPS) was comparable in the 37 and 39.5 degrees C cell cultures, peaking within 10 min of LPS stimulation. However, the duration of transcriptional activation was markedly reduced in the 39.5 degrees C cells (30-60 min) compared with the 37 degrees C cells (2-4 h). Deletion mapping of the TNFalpha gene revealed that the proximal 85-nucleotide promoter sequence and the 5'-untranslated region were sufficient for temperature sensitivity. This sequence contains six heat shock response element (HRE) half-sites but no complete HREs. Electrophoretic mobility shift and immunoblot assays demonstrated that nuclear transclocation of heat shock factor (HSF) and its activation to a DNA-binding form occurred in the 39.5 degrees C cells in the absence of heat shock protein-70 gene activation. The proximal TNFalpha promoter/5'-untranslated region sequence competed for HSF binding to a classic HRE. Overexpression of HSF-1 reduced activity of the TNFalpha promoter. These data suggest that partial activation of HSF-1 during exposure to febrile, sub-heat shock temperatures may block TNFalpha transcription by binding to its proximal promoter or 5'-untranslated region.

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.

Role of an alpha-helical bulge in the yeast heat shock transcription factor

The heat shock transcription factor (HSF) is the master transcriptional regulator of the heat shock response. The identity of a majority of the genes controlled by HSF and the circumstances under which HSF becomes induced are known, but the details of the mechanism by which HSF is able to sense and respond to heat remains an enigma. For example, it is unclear whether HSF senses the heat shock directly or requires ancillary interactions from a heat-induced signaling pathway. We present the analysis of a series of mutations in an alpha-helical bulge in the DNA-binding domain of HSF. Deletion of residues in this bulged region increases the overall activity of the protein. Yeast containing the deletion mutant HSF are able to survive growth temperatures that are lethal to yeast containing wild-type HSF, and they are also constitutively thermotolerant. The increase in activity can be measured as an increase in both constitutive and induced transcriptional activity. The mutant proteins bind DNA more tightly than the wild-type protein does, but this is unlikely to account fully for the increase in transcriptional activity as yeast HSF is constitutively bound to its binding site in vivo. The stability of the mutant proteins to thermal denaturation is lower than wild-type, though their native-state structures are still well-folded. Therefore, the mutants may be structurally analogous to the heat-induced state of HSF, and suggest that the DNA-binding domain of HSF may be capable of sensing heat shock directly.

Heat shock transcription factor activation and hsp72 accumulation in aged skeletal muscle

Induction of the protective heat shock proteins (Hsps), and of Hsp72 in particular, has been reported to be decreased in certain tissues from aged animals. To determine if both fast and slow skeletal muscles from aged animals demonstrate an altered ability to induce and accumulate Hsp72, adult (age, 6 months) and aged (age, 20 months) Fischer 344 rats were subjected to heat stress. At selected times (0, 1, 3, and 24 hours) after a 10-minute, 41 degrees C heat stress, fast (white gastrocnemius [WG]) and slow (soleus) skeletal muscles were examined for either heat shock transcription factor (HSF) activation (trimerization and DNA-binding activity) or Hsp72 content using electrophoretic gel mobility shift assays and Western blotting, respectively. Immediately after heat stress, the level of HSF activation between aged and adult animals was similar for both muscles. HSF activation was undetectable at 1 and 3 hours after heat stress in all cases. Twenty-four hours after heat stress, Hsp72 content in the WG muscles from both aged and adult animals was significantly increased compared with unstressed, age-matched controls (P < 0.05). In contrast, perhaps because of their high constitutive Hsp72 levels, soleus muscles from both aged and adult animals did not demonstrate a significant increase in Hsp72 content after heat shock, but there was a trend toward increased levels. Hsp72 content in both the soleus and WG muscles demonstrated no significant differences between adult and aged animals in either the unstressed state (controls) or after heat shock. These results suggest that skeletal muscles from aged animals are capable of inducing the heat shock response and accumulating Hsp72.

Enhanced protein denaturation in indomethacin-treated cells

Indomethacin, a potent anti-inflammatory drug, activates the DNA-binding activity of human heat shock transcription factor 1 (HSF1), but this is insufficient to elevate heat shock gene expression. However, indomethacin pretreatment leads to a complete heat shock response at temperatures that are by themselves insufficient. Here, we showed that the heat-induced loss of enzymatic activity of a nuclear or a cytoplasmic luciferase expressed in murine cells was enhanced when cells had been pretreated with indomethacin. Additionally, in these cells the 70-kDa constitutive heat shock protein exhibited an enhanced aggregation in the presence of indomethacin. Similarly an increase in the aggregation of beta-galactosidase was observed. These data suggest that indomethacin at moderate temperatures accelerates the presence of denatured proteins in the cell, thus lowering the temperature threshold for a heat shock response.

Differential temperature dependency of chemical stressors in HSF1-mediated stress response in mammalian cells

Expression of stress proteins is generally induced by a variety of stressors. To gain a better understanding of the sensing and induction mechanisms of stress responses, we studied the effects of culture temperature on responses to various stressors, since the induction of hsp70 in mammalian cells by heat shock is somehow modulated by culture temperature. Hsp70 was not induced by treatment with sodium arsenite, azetidine-2-carboxylic acid, or zinc sulfate at the level of heat shock factor (HSF) 1 activation in cells incubated at low temperature, although these treatments induced hsp70 in cells incubated at 37 degrees C. The repression of sodium arsenite or zinc sulfate-induced HSF1 activation by low temperature was not simply due to the inhibition of protein synthesis. On the other hand, heat shock and iodoacetamide induced HSF 1 activation in cells incubated at either temperature. Thus, there seem to be two kinds of stressors that induce HSF1 activation independently of or dependent on culture temperature. Furthermore, the reduction of glutathione level seemed to be essential for HSF1 activation by chemical stressors.

Thermotolerance and cell death are distinct cellular responses to stress: dependence on heat shock proteins

We tested the hypothesis that heat shock protein (Hsp) induction and cell death are mutually exclusive responses to stress. Despite activation of heat shock transcription factor 1 at temperatures ranging from 40 to 46 degrees C, Hsp72 and Hsp27 were not induced above 42 degrees C. Moreover, cells underwent apoptosis at 44 degrees C and necrosis at 46 degrees C, with mitochondrial cytochrome c release at both temperatures. However, only apoptosis was associated with caspase activation. Treatment of cells with z-VAD-fmk prior to heat shock at 44 degrees C failed to restore Hsp induction despite inhibition of heat-induced apoptosis. Furthermore, accumulation of Hsps after incubation at 42 degrees C rendered the cells resistant to apoptosis. These results suggest that lack of Hsp induction is the cause rather than the consequence of cell death.

Transcriptional activation of mouse cytosolic chaperonin CCT subunit genes by heat shock factors HSF1 and HSF2

The chaperonin containing TCP-1 (CCT) is a eukaryotic molecular chaperone consisting of eight subunit species and assists in the folding of cytosolic proteins. We show here that all eight mouse CCT subunit genes contain sequences called heat shock elements for binding heat shock transcription factors (HSFs) by electrophoretic mobility shift assays and that these genes are transcriptionally activated by HSFs in reporter gene assays using HeLa cells transiently overexpressing HSFs. These results suggest that HSF1 and/or HSF2 play a role in Cct gene expression.

Induction of stress proteins in human endothelial cells by heavy metal ions and heat shock

In the present study, we compared the induction of heat shock proteins (HSPs) by heat and heavy metal ions in three different endothelial cell types, namely, human umbilical vein endothelial cells, human pulmonary microvascular endothelial cells, and the cell line EA.hy 926. Our results show that especially Zn(2+) and Cd(2+) are inducers of 70-kDa (HSP70), 60-kDa (HSP60), 32-kDa (HSP32), and 27-kDa (HSP27) HSPs. The strength of inducibility is specific for each HSP. Ni(2+) and Co(2+) only show an inducible effect at very high concentrations, that is, in the clearly cytotoxic range. Furthermore, we investigated the time course of HSP expression and the involvement of heat shock factor-1. Our study demonstrates that the three endothelial cell types that were under investigation show comparable stress protein expression when treated with heavy metal ions or heat shock. The expression of stress proteins may be used as an early marker for the toxic damage of cells. This damage can be an inducer of acute respiratory distress syndrome in which microvascular endothelial lesions occur early. Our study provides evidence that human umbilical vein endothelial cells or EA.hy 926 cells, which are much more easily isolated and/or cultivated than pulmonary microvascular endothelial cells, could be used as alternative cell culture systems for studies on cellular dysfunction in the lung caused by toxic substances, certainly with respect to the expression of HSPs.

Proteasome inhibitors lactacystin and MG132 inhibit the dephosphorylation of HSF1 after heat shock and suppress thermal induction of heat shock gene expression

Recently, we have shown that two proteasome inhibitors, MG132 and lactacystin, induce hyperphosphorylation and trimerization of HSF1, and transactivate heat shock genes at 37 degrees C. Here, we examined the effects of these proteasome inhibitors and, in addition, a phosphatase inhibitor calyculin A (CCA) on the activation of HSF1 upon heat shock and during post-heat-shock recovery, with emphasis on HSF1 hyperphosphorylation and the ability of HSF1 to transactivate heat shock genes. When lactacystin, MG132, or CCA was present after heat shock, HSF1 remained hyperphosphorylated during post-heat-shock recovery at 37 degrees C. Failure of HSF1 to recover to its preheated dephosphorylated state correlated well with the suppression of the heat-induced hsp70 expression. In vitro, HSF1 from heat-shocked cells, when dephosphorylated, showed an increase in HSE-binding affinity. Taken together, these data suggest that phosphorylation of HSF1 plays an important role in the negative regulation of heat-shock response. Specifically, during post-heat-shock recovery phase, prolonged hyperphosphorylation of HSF1 suppresses heat-induced expression of heat shock genes.

HSF3 is a major heat shock responsive factor duringchicken embryonic development

The expression of heat shock genes in vertebrates is regulated mainly at the level of transcription by four heat shock transcription factors (HSFs 1-4). Avian cells express at least three HSFs (HSFs 1-3). HSF1 is rapidly activated by even mild heat shock, while HSF3 is activated only by severe heat shock. In contrast, HSF2 is not activated by heat stress and has been speculated to have developmental functions. Here, we examined the temporal and spatial profiles of changes in the levels of these three HSFs in various tissues during chicken development. We found that HSF3 was almost constantly expressed in various tissues during early to late chicken development. The expression of HSF1 was equally high in most tissues early in development and thereafter declined to different levels in a tissue-dependent manner. Thus, HSF3 became the dominant heat-responsive factor in all tissues examined. The magnitude of heat shock response determined by Northern blotting did not always correlate with the level of HSF1 expression, suggesting that not only HSF1 but also HSF3 may be a major factor mediating stress signals to heat shock gene expression in the chicken. In addition, the high-level and ubiquitous expression of HSF2 as well as HSF1 and HSF3 in early embryogenesis suggested the involvement of these factors in all developmental processes.

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.

Involvement of c-Jun NH(2)-terminal kinase pathway in differential regulation of heat shock proteins by anticancer drugs

In the present study, we examined the modulation of heat shock factor 1 (HSF1) activity and expression of heat shock proteins (HSPs) after exposure to anticancer drugs. Anticancer drugs induced HSF1 DNA-binding activity, and this was followed by an increase of mitochondrial HSP75 and HSP60 levels and concurrent decrease of cytoplasmic HSP70 levels. Unlike heat shock-induced full phosphorylation, HSF1 was partially phosphorylated after exposure to vincristine, and this result was tightly correlated with the kinetics of JNK/SAPK activation, and up-regulation of mitochondrial HSP75 level and concurrent down-regulation of HSP70. Furthermore, the dominant-negative mutant of SEK1 blocked the phosphorylation of HSF1 and up-regulation of mitochondrial HSP75 in response to vincristine or vinblastine. These data suggest that anticancer drugs regulate the HSF1 transcriptional activity differently from heat shock, and JNK/SAPK pathway appears to be involved in anticancer drug-induced HSF1 phosphorylation and consequently differential regulation of mitochondrial HSP75 and HSP60 and cytoplasmic HSP70.

Human heat shock factor 1 is predominantly a nuclear protein before and after heat stress

The induction of the heat shock genes in eukaryotes by heat and other forms of stress is mediated by a transcription factor known as heat shock factor 1 (HSF1). HSF1 is present in unstressed metazoan cells as a monomer with low affinity for DNA, and upon exposure to stress it is converted to an ‘active’ homotrimer that binds the promoters of heat shock genes with high affinity and induces their transcription. The conversion of HSF1 to its active form is hypothesized to be a multistep process involving physical changes in the HSF1 molecule and the possible translocation of HSF1 from the cytoplasm to the nucleus. While all studies to date have found active HSF1 to be a nuclear protein, there have been conflicting reports on whether the inactive form of HSF is predominantly a cytoplasmic or nuclear protein. In this study, we have made antibodies against human HSF1 and have reexamined its localization in unstressed and heat-shocked human HeLa and A549 cells, and in green monkey Vero cells. Biochemical fractionation of heat-shocked HeLa cells followed by western blot analysis showed that HSF1 was mostly found in the nuclear fraction. In extracts made from unshocked cells, HSF1 was predominantly found in the cytoplasmic fraction using one fractionation procedure, but was distributed approximately equally between the cytoplasmic and nuclear fractions when a different procedure was used. Immunofluorescence microscopy revealed that HSF1 was predominantly a nuclear protein in both heat shocked and unstressed cells. Quantification of HSF1 staining showed that approximately 80% of HSF1 was present in the nucleus both before and after heat stress. These results suggest that HSF1 is predominantly a nuclear protein prior to being exposed to stress, but has low affinity for the nucleus and is easily extracted using most biochemical fractionation procedures. These results also imply that HSF1 translocation is probably not part of the multistep process in HSF1 activation for many cell types.

Cell-specific expression of heat shock transcription factors 1 and 2 in unstressed rat spinal cord

We investigated the intracellular distribution of heat shock factors 1 and 2 (HSF1, HSF2) in rat spinal cord by immunoblotting and immunohistochemistry using selective policlonal antibodies. Our results showed that both HSF1 and HSF2 were expressed in spinal cord cells (both neurons and glia) but at different intensity and cell localization. HSF1 was unusually distributed in the perinuclear compartment of selected neurons of the gray matter while astrocytes, oligodendrocytes and ependymal cells were predominantly stained in the nucleus. HSF2 was expressed at lower levels than HSF1 and was scattered in both nucleus and cytoplasm of the motoneurons of the ventral horns while glial cells again showed a nuclear positivity. This study suggested that the different ability of neurons vs. glial cells to react against adverse conditions might well be correlated with the different constitutive localization of HSFs.

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.

Differentiation lineage-specific expression of human heat shock transcription factor 2

Differentiation of multipotential hematopoietic cells into lineage-committed precursors involves the selection and maintenance of appropriate programs of gene expression, regulated by specific transcription factors. Using human K562 erythroleukemia cells capable of differentiating along erythroid and megakaryocytic lineages, we explore the differentiation-related role of heat shock transcription factor 2 (HSF2), which belongs to a family of transcription factors generally known to regulate heat shock gene expression. We demonstrate that enhanced HSF2 expression and the acquisition of HSF2 DNA binding activity are strictly specific for erythroid characteristics of K562 cells. Our results reveal a multistep regulatory process of HSF2 gene expression. In K562 cells undergoing hemin-mediated erythroid differentiation, the increase in HSF2 protein levels is preceded by transcriptional induction of the HSF2 gene, accompanied by increased HSF2 mRNA stability. In contrast, during megakaryocytic differentiation induced by the phorbol ester TPA, expression of HSF2 is rapidly down-regulated, leading to a complete loss of the HSF2 protein. These results indicate that the determination of HSF2 expression occurs at the early stages of lineage commitment. Taken together, our data suggest that HSF2 could function as a lineage-restricted transcription factor during differentiation of K562 cells along either the erythroid or the megakaryocytic pathway.

Regulation of protein phosphatase 2A activity by heat shock transcription factor 2

Heat shock transcription factor (HSF) mediates the stress-induced expression of heat shock protein genes (hsp). However, HSF is required for normal cell function even in the absence of stress and is important for cell cycle progression, but the mechanism that mediates these effects of HSF is unknown. Here, it is shown that a member of the HSF family, HSF2, interacts with the PR65 (A) subunit of protein phosphatase 2A (PP2A). HSF2 binding to PR65 blocks its interaction with the catalytic subunit, due to competition between HSF2 and catalytic subunit for the same binding site in PR65. In addition, overexpression of HSF2 stimulates PP2A activity in cells, indicating the relevance of HSF2 as a regulator of PP2A in vivo. These results identify HSF2 as a dual function protein, capable of regulating both hsp expression and PP2A activity. This could function as a mechanism by which hsp expression is integrated with the control of cell division or other PP2A-regulated pathways.

Higher induction of heat shock protein 72 by heat stress in cisplatin-resistant than in cisplatin-sensitive cancer cells

Induction of the heat shock proteins (HSPs) is involved in the increased resistance to cancer therapies such as chemotherapy and hyperthermia. We used two human ovarian cancer cell lines; a cisplatin (CDDP)-sensitive line A2780 and its CDDP-resistant derivative, A2780CP. The concentration of intracellular glutathione (GSH) is higher (2.7-fold increase) in A2780CP cells than in A2780 cells. A mild treatment with a heat stress (42 degrees C for 30 min) induced synthesis of both the heat shock protein 72 (Hsp72) mRNA and the HSP72 protein in A2780CP cells, but not in A2780 cells. In contrast, a severe heat stress (45 degrees C for 30 min) increased synthesis of the HSP72 protein in the two cell lines. The induced level of the HSP72 protein by the severe treatment was higher in A2780CP than in A2780 cells. The gel mobility shift assay showed that DNA binding activities of the heat shock factor (HSF) in the two cell lines were induced similarly and significantly by the mild heat stress. Immunocytochemistry using an anti HSF1 antibody also indicated that mild heat stress activated the HSF1 translocation from the cytosol to the nucleus similarly in the both cell lines. Pretreatment of CDDP-sensitive A2780 cells with N-acetyl-L-cysteine, a precursor of GSH, effectively enhanced induction of the Hsp72 mRNA by the mild heat stress. The present findings demonstrate that induction of the Hsp72 mRNA by the mild heat stress was more extensive in CDDP-resistant A2780CP cells. It is likely that the higher GSH concentration in A2780CP cells plays an important role in promoting Hsp72 gene expression induced by the mild heat stress probably through processes downstream of activation of HSF-DNA binding.

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.

Characterization and expression of the mouse Hsc70 gene

A genomic clone encoding the mouse Hsc70 gene has been isolated and characterized by DNA sequence analysis. The gene is approximately 3. 9 kb in length and contains eight introns, the fifth, sixth and eighth of which encode the three U14 snoRNAs. The gene has been located on Chr 9 in the order Fli1-Itm1-Olfr7-Hsc70(Rnu14)-Cbl by genetic analysis. Expression of Hsc70 is universal in all tissues of the mouse, but is slightly elevated in liver, skeletal muscle and kidney tissue, while being depressed in testes. In cultured mouse NIH 3T3 cells or human HeLa cells, Hsc70 mRNA levels are low under normal conditions, but can be induced 8-fold higher in both lines by treatment with the amino acid analog azetidine. A similar induction is seen in cells treated with the proteosome inhibitor MG132 suggesting that elevated Hsc70 expression may be coupled to protein degradation. Surprisingly, expression of the human Hsc70 gene is also regulated by cell-cycle position being 8-10-fold higher in late G1/S-phase cells as opposed to the levels in early G1-phase cells.

Cellular localization of the heat shock transcription factors HSF1 and HSF2 in the rat brain during postnatal development and following hyperthermia

The heat shock transcription factor HSF1 mediates the induction of heat shock genes in response to temperature elevation and other traumatic events. The induced hsps play roles in cellular repair and protective mechanisms. Immunocytochemistry revealed that in the unstressed rat, HSF1 was already prepositioned in the nucleus at abundant levels in both neuronal and glial cell types. Following a fever-like temperature, glial cells rapidly induced hsp70 whereas populations of large neurons did not. The lack of hsp70 induction in these neurons in vivo did not appear to be due to deficiencies in levels of nuclear HSF1. During postnatal development of the cerebellum, levels of HSF1 increased progressively from day 1 to 30. Members of the hsp gene set are also constitutively expressed in the unstressed animal and play roles as molecular chaperones. HSF2, which has been proposed as a developmental regulator of constitutive heat shock gene expression, demonstrated a developmental alteration in cellular localization, namely a nuclear distribution in neurons at postnatal day 2 and a cytoplasmic localization at day 30. During postnatal development the overall levels of neural HSF2 declined. This profile showed no obvious correlation with previously observed levels of constitutive hsp expression during postnatal neural development.

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.

Proteasome inhibitors MG132 and lactacystin hyperphosphorylate HSF1 and induce hsp70 and hsp27 expression

MG132 and lactacystin, two 26S proteasome-specific protease inhibitors, can upregulate heat-shock gene transcription without heat shock. In this study, we showed that both of these inhibitors induce hyperphosphorylation and DNA-binding activity of HSF1 in the absence of heat shock (at 37 degreesC). Since trimerization of HSF1 is known to precede the acquisition of HSF1-DNA binding activity, it seems that MG132- and lactacystin-induced hyperphosphorylation of HSF1 causes conformational changes of HSF1 molecules at 37 degreesC and subsequently triggers its trimerization. Inhibition of protein synthesis by cycloheximide abolished the MG132- or lactacystin-induced hyperphosphorylation and DNA-binding activity of HSF1. These data suggest that the activity of a putative kinase(s) targeting HSF1 is upregulated in the presence of MG132 or lactacystin. The upregulation of the kinase activity requires de novo protein synthesis and is likely due to the inhibition of protein degradation of a short-lived, kinase(s) targeting HSF1 and/or the cofactor(s) for the kinases, through the ubiquitin-proteasome pathway.

Stress and the cell nucleus: dynamics of gene expression and structural reorganization

A growing number of experimental observations reveal that the cell nucleus is functionally compartmentalized yet organized to ensure a dynamic response to events that influence nuclear activities. The cellular and molecular response to physiological and environmental stress induces a rapid and transient change in gene expression associated with major changes in nuclear architecture that impacts on signals involved in cell growth. In this review, we will address the effects of stress on the functional compartmentation of the cell nucleus and the dynamic reorganization of nuclear structures and function.

Aggregation of hsp70 and hsc70 in vivo is distinct and temperature-dependent and their chaperone function is directly related to non-aggregated forms

We used non-denaturing gradient analysis of cell extracts before and after heat treatment of the cells and showed that hsp70 and hsc70 aggregate in vivo in a temperature-dependent fashion. Their aggregation profiles were found to be clearly distinguishable and sensitive to ATP depletion. Pore exclusion limit electrophoresis showed that these two proteins are mainly found in autoaggregated forms including dimers, trimers and oligomers. The addition of denatured luciferase to the cell extracts reversed the aggregation of both proteins towards their non-aggregated forms. Immunoprecipitation and Western-blot analysis showed that the non-aggregated form is the only one bound to denatured luciferase. Our results suggest that aggregated hsp70 and hsc70 represent predominantly self-associated molecules unable to exert chaperone activity. The cochaperone hsp40 was also found to be aggregated and, on addition of denatured luciferase, its aggregation was reversed to a non-aggregated state. Immunoprecipitation analysis indicated that hsp40 forms a complex with the non-aggregated form of hsc70 and denatured luciferase. These results confirm previous in vitro studies and support the suggestion that in vivo cytosolic hsp70 and hsc70 exist mainly in an oligomer-monomer equilibrium which is dependent on the environmental temperature, the levels of ATP and the presence of denatured proteins.

Structural organization and promoter analysis of murine heat shock transcription factor-1 gene

Heat shock factor-1 (HSF-1) activates transcription of heat shock proteins in eukaryotes. Several overlapping genomic clones containing the murine HSF-1 gene were isolated from a phage genomic library. Results indicate that the HSF-1 gene contains 13 exons that span at least 30 kilobase pairs. Sequence analysis of the 5'-untranslated region of HSF-1 suggests that it contains sequences of a recently described Bop1 gene in reverse orientation within its first 331 base pairs (bp) upstream of the translation initiation site. The minimal promoter sequence required for HSF-1 basal expression was identified by deletion analysis from -4 kilobase pairs to -331 bp of the promoter fused to a luciferase reporter gene using transient transfection assays. Results indicate that 331 bp upstream of the HSF-1 translation start site is required for maximal basal expression in NIH3T3 and F9 cells. This fragment also results in high levels of luciferase activity in the reverse orientation, that is, 5' to the Bop1 gene, suggesting that this segment is bidirectional and could be utilized for basal expression of both HSF-1 and Bop1 genes. This segment of the promoter contains recognition elements for Sp1 and CCAAT-box binding transcription factors, which when mutated in either sense or antisense orientations to the HSF-1 gene results in a reduction of basal expression by 50-75% relative to wild type, suggesting that these sites are critical for basal expression of both HSF-1 and Bop1 genes.