The DNA-binding properties of two heat shock factors, HSF1 and HSF3, are induced in the avian erythroblast cell line HD6

Heat shock factor 1 mediates hemin-induced hsp70 gene transcription in K562 erythroleukemia cells

Transcriptional induction of the hsp70 gene is mediated by heat shock factor 1 (HSF1) rapidly activated upon heat and other stresses. HSF2 has been thought to be responsible for accumulation of HSP70 during hemin-induced differentiation of human K562 erythroleukemia cells because of accompanying acquisition of HSF2 DNA binding activity. However, there has not been any direct evidence for such a functional role of HSF2. The purpose of this study is to clarify the roles of HSF1 and HSF2 in HSP70 induction in hemin-treated K562 cells. We show here that a chimeric polypeptide of HSF2 and GAL4 DNA binding domain (GAL4-BD-HSF2) was unable to induce a GAL4 binding site-containing luciferase reporter gene in response to hemin and that exogenously overproduced HSF2 also failed to increase expression of a heat shock element-containing reporter. On the contrary, expression of a GAL4-BD-HSF1 chimeric protein responded to hemin treatment as well as to heat shock, and transiently overexpressed HSF1 caused hemin-responsive induction of the reporter gene in a dose-dependent manner. These results indicate that HSF1, rather than HSF2, primarily mediates the hemin-induced transcription of the hsp70 gene.

Heat shock response and protein degradation: regulation of HSF2 by the ubiquitin-proteasome pathway

Mammalian cells coexpress a family of heat shock factors (HSFs) whose activities are regulated by diverse stress conditions to coordinate the inducible expression of heat shock genes. Distinct from HSF1, which is expressed ubiquitously and activated by heat shock and other stresses that result in the appearance of nonnative proteins, the stress signal for HSF2 has not been identified. HSF2 activity has been associated with development and differentiation, and the activation properties of HSF2 have been characterized in hemin-treated human K562 erythroleukemia cells. Here, we demonstrate that a stress signal for HSF2 activation occurs when the ubiquitin-proteasome pathway is inhibited. HSF2 DNA-binding activity is induced upon exposure of mammalian cells to the proteasome inhibitors hemin, MG132, and lactacystin, and in the mouse ts85 cell line, which carries a temperature sensitivity mutation in the ubiquitin-activating enzyme (E1) upon shift to the nonpermissive temperature. HSF2 is labile, and its activation requires both continued protein synthesis and reduced degradation. The downstream effect of HSF2 activation by proteasome inhibitors is the induction of the same set of heat shock genes that are induced during heat shock by HSF1, thus revealing that HSF2 affords the cell with a novel heat shock gene-regulatory mechanism to respond to changes in the protein-degradative machinery.

Stress-inducible responses and heat shock proteins: new pharmacologic targets for cytoprotection

Molecular chaperones protect proteins against environmental and physiologic stress and from the deleterious consequences of an imbalance in protein homeostasis. Many of these stresses, if prolonged, result in defective development and pathologies associated with a diverse array of diseases due to tissue injury and repair including stroke, myocardial reperfusion damage, ischemia, cancer, amyloidosis, and other neurodegenerative diseases. We discuss the molecular nature of the stress signals, the mechanisms that underlie activation of the heat shock response, the role of heat shock proteins as cytoprotective molecules, and strategies for pharmacologically active molecules as regulators of the heat shock response.

Stress (heat shock) proteins: molecular chaperones in cardiovascular biology and disease

How a cell responds to stress is a central problem in cardiovascular biology. Diverse physiological stresses (eg, heat, hemodynamics, mutant proteins, and oxidative injury) produce multiple changes in a cell that ultimately affect protein structures and function. Cells from different phyla initiate a cascade of events that engage essential proteins, the molecular chaperones, in decisions to repair or degrade damaged proteins as a defense strategy to ensure survival. Accumulative evidence indicates that molecular chaperones such as the heat shock family of stress proteins (HSPs) actively participate in an array of cellular processes, including cytoprotection. The versatility of the ubiquitous HSP family is further enhanced by stress-inducible regulatory networks, both at the transcriptional and posttranscriptional levels. In the present review, we discuss the regulation and function of HSP chaperones and their clinical significance in conditions such as cardiac hypertrophy, vascular wall injury, cardiac surgery, ischemic preconditioning, aging, and, conceivably, mutations in genes encoding contractile proteins and ion channels.

Novel testis-specific protein that interacts with heat shock factor 2

Although heat shock factor 2 (HSF2) binds to heat shock element (HSE) constitutively during differentiation, development and spermatogenesis, little is known about the nature and mechanism of transcriptional control of heat shock genes by HSF2. We screened a human testis cDNA library for proteins that can associate with HSF2 by the yeast two-hybrid system, and isolated clones encoding a novel protein, designated HSF2 binding protein (HSF2BP), that associates with HSF2 in vitro and in vivo and is specifically expressed in testis. The interaction seemed to occur between the trimerization domain of HSF2 and the amino terminal hydrophilic region of HSF2BP that comprises two leucine zipper motifs. HSF2BP may therefore be involved in modulating HSF2 activation in testis.

The role of an inverted CCAAT element in transcriptional activation of the human DNA topoisomerase IIalpha gene by heat shock

Expression of the DNA topoisomerase IIalpha (topoIIalpha) gene is highly sensitive to various environmental stimuli including heat shock. The amount of topoIIalpha mRNA was increased 1.5-3-fold 6-24 h after exposure of T24 human urinary bladder cancer cells to heat shock stress at 43 degreesC for 1 h. The effect of heat shock on the transcriptional activity of the human topoIIalpha gene promoter was investigated by transient transfection of T24 cells with luciferase reporter plasmids containing various lengths of the promoter sequence. The transcriptional activity of the full-length promoter (nucleotides (nt) -295 to +85) and of three deletion constructs (nt -197 to +85, -154 to +85, and -74 to +85) was increased approximately 3-fold 24 h after heat shock stress. In contrast, the transcriptional activity of the minimal promoter (nt -20 to +85), which lacks the first inverted CCAAT element (ICE1), the GC box, and the heat shock element located between nt -74 and -21, was not increased by heat shock. Furthermore, the transcriptional activity of promoter constructs containing mutations in the GC box or heat shock element, but not that of a construct containing mutations in ICE1, was significantly increased by heat shock. Electrophoretic mobility shift assays revealed reduced binding of a nuclear factor to an oligonucleotide containing ICE1 when nuclear extracts were derived from cells cultured for 3-24 h after heat shock. No such change in factor binding was apparent with an oligonucleotide containing the heat shock element of the topoIIalpha gene promoter. Finally, in vivo footprint analysis of the topoIIalpha gene promoter revealed that two G residues of ICE1 that were protected in control cells became sensitive to dimethyl sulfate modification after heat shock. These results suggest that transcriptional activation of the topoIIalpha gene by heat shock requires the release of a negative regulatory factor from ICE1.

Characterization of HSE sequences in human Hsp40 gene: structural and promoter analysis

We have recently cloned a gene of Hsp40, a human homologue of bacterial DnaJ. Here we describe the structural and promoter analysis of human Hsp40 gene. Analysis of Hsp40 transcripts by 5' and 3' RACE suggested that they have different 3' ends, and primer extension studies revealed that the major transcription initiation site was localized 47 bp upstream of the ATG translation initiation codon. Promoter analysis using deletion derivatives defined a minimal region which was active in response to heat shock. The region contained the consensus heat shock element (HSE) sequences. The factor bound to these sequences was suggested to be a heat shock factor 1 (HSF1) by gel mobility supershift assay. In vivo footprinting and promoter analysis revealed that the HSEs in 5' upstream region of human Hsp40 gene were composed of eight contiguous (A/G)GAAN motifs and were essential for heat shock response. These results indicate that Hsp40 is a real heat shock protein. It is also shown that the HSE found in the first intron might not be the essential element for heat shock response.

The tomato Hsf system: HsfA2 needs interaction with HsfA1 for efficient nuclear import and may be localized in cytoplasmic heat stress granules

In heat-stressed (HS) tomato (Lycopersicon peruvianum) cell cultures, the constitutively expressed HS transcription factor HsfA1 is complemented by two HS-inducible forms, HsfA2 and HsfB1. Because of its stability, HsfA2 accumulates to fairly high levels in the course of a prolonged HS and recovery regimen. Using immunofluorescence and cell fractionation experiments, we identified three states of HsfA2: (i) a soluble, cytoplasmic form in preinduced cultures maintained at 25 degrees C, (ii) a salt-resistant, nuclear form found in HS cells, and (iii) a stored form of HsfA2 in cytoplasmic HS granules. The efficient nuclear transport of HsfA2 evidently requires interaction with HsfA1. When expressed in tobacco protoplasts by use of a transient-expression system, HsfA2 is mainly retained in the cytoplasm unless it is coexpressed with HsfA1. The essential parts for the interaction and nuclear cotransport of the two Hsfs are the homologous oligomerization domain (HR-A/B region of the A-type Hsfs) and functional nuclear localization signal motifs of both partners. Direct physical interaction of the two Hsfs with formation of relatively stabile hetero-oligomers was shown by a two-hybrid test in Saccharomyces cerevisiae as well as by coimmunoprecipitation using tomato and tobacco whole-cell lysates.

Targeted disruption of heat shock transcription factor 1 abolishes thermotolerance and protection against heat-inducible apoptosis

Heat shock transcription factor 1 (HSF1) is a member of the vertebrate HSF family that regulates stress-inducible synthesis of heat shock proteins (HSPs). Although the synthesis of the constitutively expressed and inducible members of the heat shock family of stress proteins correlates with increased cellular protection, their relative contributions in acquired cellular resistance or "thermotolerance" in mammalian cells is presently unknown. We report here that constitutive expression of multiple HSPs in cultured embryonic cells was unaffected by disruption of the murine HSF1 gene. In contrast, thermotolerance was not attainable in hsf1(-/-) cells, and this response was required for protection against heat-induced apoptosis. We conclude that 1) constitutive and inducibly expressed HSPs exhibit distinct physiological functions for cellular maintenance and adaptation, respectively, and 2) other mammalian HSFs or distinct evolutionarily conserved stress response pathways do not compensate for HSF1 in the physiological response to heat shock.

Disruption of the HSF3 gene results in the severe reduction of heat shock gene expression and loss of thermotolerance

The vertebrate genome encodes a family of heat shock factors (HSFs 1-4) of which the DNA-binding and transcriptional activities of HSF1 and HSF3 are activated upon heat shock. HSF1 has the properties of a classical HSF and exhibits rapid activation of DNA-binding and transcriptional activity upon exposure to conditions of heat shock and other stresses, whereas HSF3 typically is activated at higher temperatures and with distinct delayed kinetics. To address the role of HSF3 in the heat shock response, null cells lacking the HSF3 gene were constructed by disruption of the resident gene by somatic recombination in an avian lymphoid cell line. Null cells lacking HSF3, yet expressing normal levels of HSF1, exhibited a severe reduction in the heat shock response, as measured by inducible expression of heat shock genes, and did not exhibit thermotolerance. At intermediate heat shock temperatures, where HSF1 oligomerizes to an active trimer in wild-type cells, HSF1 remained as an inert monomer in the HSF3 null cell line. HSF3 null cells were restored to a nearly normal heat shock-responsive state by reintroduction of an exogenous HSF3 gene. These results reveal that HSF3 has a dominant role in the regulation of the heat shock response and directly influences HSF1 activity.

Cooperative and competitive protein interactions at the hsp70 promoter

Drosophila heat shock factor (HSF) binds to specific sequence elements of heat shock genes and can activate their transcription 200-fold. Though HSF has an acidic activation domain, the mechanistic details of heat shock gene activation remain undefined. Here we report that HSF interacts directly with the general transcription factor TBP (TATA-box binding protein), and these two factors bind cooperatively to heat shock promoters. A third factor that binds heat shock promoters, GAGA factor, also interacts with HSF and further stabilizes HSF binding to heat shock elements (HSEs). The interaction of HSF and TBP is explored in some detail here and is shown to be mediated by residues in both the amino- and carboxyl-terminal portions of HSF. This HSF/TBP interaction can be specifically disrupted by competition with the potent acidic transcriptional activator VP16. We further show that the acidic domain of the largest subunit of Drosophila RNA polymerase II (Pol II) associates with TBP in vitro and is specifically displaced from TBP upon addition of HSF. The region of TBP that mediates both HSF and Pol II acidic domain binding maps to the conserved carboxyl-terminal repeats and depends on at least one of the TBP residues known to be contacted by VP16 and to be critical for transcription activation. We discuss these findings in the context of a model in which HSF triggers hsp70 transcription by freeing the hsp70 promoter-paused Pol II from the constraints on elongation caused by the affinity of Pol II for general transcription factors.

The trimerization domain of human heat shock factor 2 is able to interact with nucleoporin p62

Heat shock factor 2 (HSF2) acquires DNA binding activity during hemin-induced differentiation of human K562 erythroleukemia cells. To investigate the mechanisms responsible for the regulation of HSF2 activity, we searched for proteins that can associate with HSF2 by the yeast two-hybrid system. Nucleoporin p62, a major component of the nuclear pore complex, was cloned from cDNA libraries of K562 cells. We demonstrated physical interaction between HSF2 and p62 both by a glutathione S-transferase (GST) pull-down assay in vitro and by a two-hybrid assay in K562 cells. HSF1 is also able to interact with p62 on a GST pull-down assay, but not on a mammalian two-hybrid system. Furthermore, it was shown that this interaction occurred between the trimerization domain of HSF2 and the C-terminal alpha-helical coiled-coil domain of p62. These data suggest the possibility that p62 is involved in the activation or regulation of HSF2.

Induction of heat shock protein 72 synthesis by endogenous tumor necrosis factor via enhancement of the heat shock element-binding activity of heat shock factor 1

Endogenous tumor necrosis factor (enTNF) acts as a resistance factor against cytotoxicity caused by heat by inducing manganous superoxide dismutase (MnSOD), thereby scavenging reactive oxygen free radicals. On the other hand, it is also well known that heat shock proteins (HSP) which are induced by heat stress behave as cytoprotective factor against this stress. However, the relationship of these two resistance factors is not elucidated yet. In the present study, we therefore proposed the possibility that enTNF enhances HSP72 expression. Heat-sensitive L-M (mouse tumorigenic fibroblast) cells, which normally do not express enTNF, were transfected with a nonsecretory-type human TNF-alpha expression vector to produce enTNF. Stable transfectants showed resistance to heat treatment and an increase of HSP72 expression. Conversely, when HeLa (human uterine cervical cancer) cells, which normally produce an appreciable amount of enTNF, were transfected with an antisense TNF-alpha mRNA expression vector to inhibit enTNF synthesis, their heat sensitivity was enhanced and HSP72 expression was reduced by half. Although enTNF caused no difference in the level of heat shock factor (HSF) 1 in these cells, enTNF expression correlated well with the binding activity of HSF-1 to a 32P-labeled synthetic oligonucleotide containing the human heat shock element (HSE). These results indicate that enTNF participates not only in intrinsic resistance against heat via induction of MnSOD but also via enhancement of the HSE-binding activity of HSF 1 followed by augmentation of HSP72 expression.

Electromagnetic field exposure induces rapid, transitory heat shock factor activation in human cells

Stimulation of human promyelocytic HL60 cells by a 60Hz magnetic field at normal growth temperatures results in heat shock factor 1 activation and heat shock element binding, a sequence of events that mediates the stress-induced transcription of the stress gene HSP70 and increased synthesis of the stress response protein hsp70kD. Thus, the events mediating the electromagnetic field-stimulated stress response appear to be similar to those reported for other physiological stresses (e.g., hyperthermia, heavy metals, oxidative stress) and could well be the general mechanism of interaction of electromagnetic fields with cells.

Different thresholds in the responses of two heat shock transcription factors, HSF1 and HSF3

Avian cells express three HSF genes encoding a unique factor, HSF3, as well as homologues of mammalian HSF1 and HSF2. HSF1 is the major factor that mediates the heat shock signal in mammalian cells. We reported previously that cHSF3, as well as cHSF1, is activated by heat shock in chicken cells. In this study, we examined the functional differences between cHSF1 and cHSF3. Comparison of the heat-inducible DNA binding activity of cHSF1 with cHSF3 at various temperatures revealed that the latter was activated at higher temperatures than the former. At a mild heat shock, such as 41 degrees C, only cHSF1 was activated, whereas both cHSF1 and cHSF3 were activated following a severe heat shock at 45 degrees C. Heat-inducible nuclear translocation and trimerization were accompanied by DNA binding activity. We also observed that cHSF3 was activated by treating cells with higher concentrations of sodium arsenite compared to cHSF1. The DNA binding activity of cHSF3 by severe heat shock lasted for a longer period than that of cHSF1. Interestingly, the total amount of cHSF3 increased only upon severe heat shock, whereas that of HSF1 decreased. Substantial amounts of cHSF3 remained in the soluble fraction under severe heat shock, whereas cHSF1 rapidly moved to the insoluble fractions in that conditions. Comparison of transcriptional activity of the activation domains of cHSF1 and cHSF3 revealed that the activity of cHSF3 was as strong as that of cHSF1. These findings indicate that there are different thresholds for cHSF1 and cHSF3 and that cHSF3 is involved in the persistent and burst activation of stress genes upon severe stress in chicken cells. Pretreatment of cycloheximide elevated the threshold concentrations of arsenite of both factors. This suggests that denaturation of nascent polypeptides could be the first trigger for the activation of both factors, and the pathways for activation of cHSF1 and cHSF3 may be identical, or at least share some common mechanisms.

Evidence for the involvement of mouse heat shock factor 1 in the atypical expression of the HSP70.1 heat shock gene during mouse zygotic genome activation

The mouse HSP70.1 gene, which codes for a heat shock protein (hsp70), is highly transcribed at the onset of zygotic genome activation (ZGA). This expression, which occurs in the absence of stress, is then repressed. It has been claimed that this gene does not exhibit a stress response until the blastocyst stage. The promoter of HSP70.1 contains four heat shock element (HSE) boxes which are the binding sites of heat shock transcription factors (HSF). We have been studying the presence and localization of the mouse HSFs, mHSF1 and mHSF2, at different stages of embryo development. We show that mHSF1 is already present at the one-cell stage and concentrated in the nucleus. Moreover, by mutagenizing HSE sequences and performing competition experiments (in transgenic embryos with the HSP70.1 promoter inserted before a reporter gene), we show that, in contrast with previous findings, HSE boxes are involved in this spontaneous activation. Therefore, we suggest that HSF1 and HSE are important in this transient expression at the two-cell stage and that the absence of typical inducibility at this early stage of development results mainly from the high level of spontaneous transcription of this gene during the ZGA.

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

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