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

Editing of HSF-1 and Na/K-ATPase α1 subunit by CRISPR/Cas9 reduces thermal tolerance of bovine skin fibroblasts to heat shock in vitro

A follow-up to our previous findings, the present study was planned to evaluate the role of Na/K-ATPase alpha1-subunit (ATP1A1) gene in heat shock tolerance. The primary fibroblast culture was established using ear pinna tissue samples of Sahiwal cattle (Bos indicus). The knockout cell lines of Na/K-ATP1A1 and HSF-1 (heat shock factor-1, as a positive control) genes were developed by CRISPR/Cas9 method and the gene-editing was confirmed by the genomic cleavage detection assay. The two knockout cell lines (ATP1A1 and HSF-1) and wild-type fibroblasts were exposed to heat shock at 42 °C in vitro and different cellular parameters viz., apoptosis, proliferation, mitochondrial membrane potential (ΔΨm), oxidative stress, along with expression pattern of heat-responsive genes were studied. The results showed that in vitro heat shock given to knockout fibroblast cells of both ATP1A1 and HSF-1 genes resulted in decreased cell viability, while increasing the apoptosis rate, membrane depolarization, and ROS levels. However, the overall impact was more in HSF-1 knockout cells as compared to ATP1A1 knockout cells. Taken together, these results indicated that the ATP1A1 gene plays a critical role as HSF-1 under heat stress and helps cells to cope with heat shock.

Heat-Induced Conformational Transition Mechanism of Heat Shock Factor 1 Investigated by Tryptophan Probe

A transcriptional regulatory system called heat shock response (HSR) has been developed in eukaryotic cells to maintain proteome homeostasis under various stresses. Heat shock factor-1 (Hsf1) plays a central role in HSR, mainly by upregulating molecular chaperones as a transcription factor. Hsf1 forms a complex with chaperones and exists as a monomer in the resting state under normal conditions. However, upon heat shock, Hsf1 is activated by oligomerization. Thus, oligomerization of Hsf1 is considered an important step in HSR. However, the lack of information about Hsf1 monomer structure in the resting state, as well as the structural change via oligomerization at heat response, impeded the understanding of the thermosensing mechanism through oligomerization. In this study, we applied solution biophysical methods, including fluorescence spectroscopy, nuclear magnetic resonance, and circular dichroism spectroscopy, to investigate the heat-induced conformational transition mechanism of Hsf1 leading to oligomerization. Our study showed that Hsf1 forms an inactive closed conformation mediated by intramolecular contact between leucine zippers (LZs), in which the intermolecular contact between the LZs for oligomerization is prevented. As the temperature increases, Hsf1 changes to an open conformation, where the intramolecular LZ interaction is dissolved so that the LZs can form intermolecular contacts to form oligomers in the active form. Furthermore, since the interaction sites with molecular chaperones and nuclear transporters are also expected to be exposed in the open conformation, the conformational change to the open state can lead to understanding the regulation of Hsf1-mediated stress response through interaction with multiple cellular components.

HSF-1: Guardian of the Proteome Through Integration of Longevity Signals to the Proteostatic Network

Discoveries made in the nematode Caenorhabditis elegans revealed that aging is under genetic control. Since these transformative initial studies, C. elegans has become a premier model system for aging research. Critically, the genes, pathways, and processes that have fundamental roles in organismal aging are deeply conserved throughout evolution. This conservation has led to a wealth of knowledge regarding both the processes that influence aging and the identification of molecular and cellular hallmarks that play a causative role in the physiological decline of organisms. One key feature of age-associated decline is the failure of mechanisms that maintain proper function of the proteome (proteostasis). Here we highlight components of the proteostatic network that act to maintain the proteome and how this network integrates into major longevity signaling pathways. We focus in depth on the heat shock transcription factor 1 (HSF1), the central regulator of gene expression for proteins that maintain the cytosolic and nuclear proteomes, and a key effector of longevity signals.

Hsf1 on a leash - controlling the heat shock response by chaperone titration

Heat shock factor 1 (Hsf1) is an ancient transcription factor that monitors protein homeostasis (proteostasis) and counteracts disturbances by triggering a transcriptional programme known as the heat shock response (HSR). The HSR is transiently activated and upregulates the expression of core proteostasis genes, including chaperones. Dysregulation of Hsf1 and its target genes are associated with disease; cancer cells rely on a constitutively active Hsf1 to promote rapid growth and malignancy, whereas Hsf1 hypoactivation in neurodegenerative disorders results in formation of toxic aggregates. These central but opposing roles highlight the importance of understanding the underlying molecular mechanisms that control Hsf1 activity. According to current understanding, Hsf1 is maintained latent by chaperone interactions but proteostasis perturbations titrate chaperone availability as a result of chaperone sequestration by misfolded proteins. Liberated and activated Hsf1 triggers a negative feedback loop by inducing the expression of key chaperones. Until recently, Hsp90 has been highlighted as the central negative regulator of Hsf1 activity. In this review, we focus on recent advances regarding how the Hsp70 chaperone controls Hsf1 activity and in addition summarise several additional layers of activity control.

Molecular Mechanisms of Heat Shock Factors in Cancer

Malignant transformation is accompanied by alterations in the key cellular pathways that regulate development, metabolism, proliferation and motility as well as stress resilience. The members of the transcription factor family, called heat shock factors (HSFs), have been shown to play important roles in all of these biological processes, and in the past decade it has become evident that their activities are rewired during tumorigenesis. This review focuses on the expression patterns and functions of HSF1, HSF2, and HSF4 in specific cancer types, highlighting the mechanisms by which the regulatory functions of these transcription factors are modulated. Recently developed therapeutic approaches that target HSFs are also discussed.

Modulation of Heat Shock Factor 1 Activity through Silencing of Ser303/Ser307 Phosphorylation Supports a Metabolic Program Leading to Age-Related Obesity and Insulin Resistance

Activation of the adaptive response to cellular stress orchestrated by heat shock factor 1 (HSF1), which is an evolutionarily conserved transcriptional regulator of chaperone response and cellular bioenergetics in diverse model systems, is a central feature of organismal defense from environmental and cellular stress. HSF1 activity, induced by proteostatic, metabolic, and growth factor signals, is regulated by posttranscriptional modifications, yet the mechanisms that regulate HSF1 and particularly the functional significance of these modifications in modulating its biological activity in vivo remain unknown. HSF1 phosphorylation at both Ser303 (S303) and Ser307 (S307) has been shown to repress HSF1 transcriptional activity under normal physiological growth conditions. To determine the biological relevance of these HSF1 phosphorylation events, we generated a knock-in mouse model in which S303 and S307 were replaced with alanine (HSF1^303A/307A). Our results confirmed that loss of phosphorylation in HSF1^303A/307A cells and tissues increases protein stability but also markedly sensitizes HSF1 activation under normal and heat- or nutrient-induced stress conditions. Interestingly, the enhanced HSF1 activation in HSF1^303A/307A mice activates a supportive metabolic program that aggravates the development of age-dependent obesity, fatty liver diseases, and insulin resistance. Thus, these findings highlight the importance of a posttranslational mechanism (through phosphorylation at S303 and S307 sites) of regulation of the HSF1-mediated transcriptional program that moderates the severity of nutrient-induced metabolic diseases.

The transcription factor SKN7 regulates conidiation, thermotolerance, apoptotic-like cell death and parasitism in the nematode endoparasitic fungus Hirsutella minnesotensis

The transcription factor SKN7 is a highly conserved protein among fungi and was initially recognized as a response regulator that protects cells from oxidative stress and maintains cell wall integrity in yeast. Orthologs of SKN7 are extensively present in biocontrol agents of plant pathogens, but they had not been functionally characterized. Here, we identified and characterized the transcription factor SKN7 in the nematode endoparasitic fungus Hirsutella minnesotensis. Null mutant lacking HIM-SKN7 (HIM_03620), which was generated by a gene disruption strategy, demonstrated reduced conidiation, increased sensitivity to high temperature, hydrogen peroxide, mannitol and ethanol, and reduced fungal resistance to farnesol. However, over-expression mutant showed increased conidial production, thermotolerance and resistance to farnesol, suggesting that HIM-SKN7 regulates antiapoptotic-like cell death in H. minnesotensis. Moreover, the results showed that in null mutant, H. minnesotensis had decreased endoparasitic ability as compared to wild type and over-expression strain. During the infection process, the relative expression of the HIM-SKN7 gene was significantly induced in the wild type and over-expression strain. The results of the present study advance our understanding of the functions of the SKN7 gene in biocontrol agents, in particular, nematode endoparasitic fungi.

Heat shock transcription factor 1 is activated as a consequence of lymphocyte activation and regulates a major proteostasis network in T cells critical for cell division during stress

Heat shock transcription factor 1 (HSF1) is a major transcriptional regulator of the heat shock response in eukaryotic cells. HSF1 is evoked in response to a variety of cellular stressors, including elevated temperatures, oxidative stress, and other proteotoxic stressors. Previously, we demonstrated that HSF1 is activated in naive T cells at fever range temperatures (39.5°C) and is critical for in vitro T cell proliferation at fever temperatures. In this study, we demonstrated that murine HSF1 became activated to the DNA-binding form and transactivated a large number of genes in lymphoid cells strictly as a consequence of receptor activation in the absence of apparent cellular stress. Microarray analysis comparing HSF1(+/+) and HSF1(-/-) gene expression in T cells activated at 37°C revealed a diverse set of 323 genes significantly regulated by HSF1 in nonstressed T cells. In vivo proliferation studies revealed a significant impairment of HSF1(-/-) T cell expansion under conditions mimicking a robust immune response (staphylococcal enterotoxin B-induced T cell activation). This proliferation defect due to loss of HSF1 is observed even under nonfebrile temperatures. HSF1(-/-) T cells activated at fever temperatures show a dramatic reduction in cyclin E and cyclin A proteins during the cell cycle, although the transcription of these genes was modestly affected. Finally, B cell and hematopoietic stem cell proliferation from HSF1(-/-) mice, but not HSF1(+/+) mice, were also attenuated under stressful conditions, indicating that HSF1 is critical for the cell cycle progression of lymphoid cells activated under stressful conditions.

Is thioredoxin reductase involved in the defense against DNA fragmentation in varicocele?

We aimed to investigate the role of thioredoxin reductase (TR) and inducible heat shock protein 70 (iHsp70) and their relationship with sperm quality in varicocele (VAR) patients. Semen samples were obtained from 16 subfertile men diagnosed as VAR and 10 fertile men who applied to the Andrology Laboratory of Istanbul Medical Faculty of Istanbul University. The sperm TR and iHsp 70 expression levels were determined using Western blot analysis. The TR activity of the sperm was assayed spectrophometrically. The sperm quality was evaluated both by conventional sperm analysis and by a terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) technique that assayed DNA-fragmented spermatozoa in semen samples. The percentage of TUNEL-positive spermatozoa in the VAR group (16.3%± 5.6%) was higher than that in the fertile group (5.5%± 1.9%). Significant inverse correlations were detected between the percentage of TUNEL-positive cells and both the concentration (r=-0.609; P=0.001) and motility (r=-0.550; P=0.004) of spermatozoa. Both the TR expression and activity were increased significantly in the VAR group (U=22.0; P=0.001 and U=33.5; P=0.012, respectively) as analyzed using the Mann-Whitney U Wilcoxon rank sum W test. Furthermore, significant positive correlations were found between TR expression and activity (r=0.406; P=0.040) and between TR expression and the percentage of TUNEL-positive cells (r=0.665; P=0.001). Sperm iHsp70 expression did not differ between the VAR and fertile groups. In conclusion, increased sperm TR expression might be a defense mechanism against apoptosis in the spermatozoa of men with VAR.

Disruption of heat shock factor 1 reduces the formation of conidia and thermotolerance in the mycoparasitic fungus Coniothyrium minitans

Coniothyrium minitans is a bio-control agent of Sclerotinia spp., and has the ability to produce abundant conidia to infect the host fungi. Mediation of heat shock factors (HSFs) is required to adapt to the acute temperatures, and to regulate the expression of heat shock proteins (HSPs) to function as molecular chaperones to assist in development, protein folding and stability. A heat shock factor 1 (HSF1) gene was identified from a T-DNA insertion mutant that lost the ability to form conidia in liquid culture as well as on solid media. Null mutants lacking CmHSF1 were constructed by gene disruption strategy. Mutants lacking CmHSF1 had reduced in conidial production and displayed decreased tolerance to heat and other abiotic stresses as compared to the wild type parent. Over-expression strains could recover faster from heat and abiotic stresses such as, ethanol, oxidative or osmotic stresses with or without heat shock. In over-expression strains, conidial germination was increased, and parasitic ability on sclerotia of Sclerotinia sclerotiorum was enhanced by 0.42-5.92% compared to the wild type strain. Increased expression levels in wild strain ZS-1 were observed when the fungus was grown at 37°C or 45°C with other abiotic stresses. CmHSF1 plays an important role in conidial production, conidial germination, and tolerance against heat and other abiotic stresses.

Inducible hsp70 in the regulation of cancer cell survival: analysis of chaperone induction, expression and activity

Understanding the mechanisms that control stress is central to realize how cells respond to environmental and physiological insults. All the more important is to reveal how tumour cells withstand their harsher growth conditions and cope with drug-induced apoptosis, since resistance to chemotherapy is the foremost complication when curing cancer. Intensive research on tumour biology over the past number of years has provided significant insights into the molecular events that occur during oncogenesis, and resistance to anti-cancer drugs has been shown to often rely on stress response and expression of inducible heat shock proteins (HSPs). However, with respect to the mechanisms guarding cancer cells against proteotoxic stresses and the modulatory effects that allow their survival, much remains to be defined. Heat shock proteins are molecules responsible for folding newly synthesized polypeptides under physiological conditions and misfolded proteins under stress, but their role in maintaining the transformed phenotype often goes beyond their conventional chaperone activity. Expression of inducible HSPs is known to correlate with limited sensitivity to apoptosis induced by diverse cytotoxic agents and dismal prognosis of several tumour types, however whether cancer cells survive because of the constitutive expression of heat shock proteins or the ability to induce them when adapting to the hostile microenvironment remains to be elucidated. Clear is that tumours appear nowadays more "addicted" to heat shock proteins than previously envisaged, and targeting HSPs represents a powerful approach and a future challenge for sensitizing tumours to therapy. This review will focus on the anti-apoptotic role of heat shock 70kDa protein (Hsp70), and how regulatory factors that control inducible Hsp70 synthesis, expression and activity may be relevant for response to stress and survival of cancer cells.

Regulation of HSF1 function in the heat stress response: implications in aging and disease

To dampen proteotoxic stresses and maintain protein homeostasis, organisms possess a stress-responsive molecular machinery that detects and neutralizes protein damage. A prominent feature of stressed cells is the increased synthesis of heat shock proteins (Hsps) that aid in the refolding of misfolded peptides and restrain protein aggregation. Transcriptional activation of the heat shock response is orchestrated by heat shock factor 1 (HSF1), which rapidly translocates to hsp genes and induces their expression. Although the role of HSF1 in protecting cells and organisms against severe stress insults is well established, many aspects of how HSF1 senses qualitatively and quantitatively different forms of stresses have remained poorly understood. Moreover, recent discoveries that HSF1 controls life span have prompted new ways of thinking about an old transcription factor. Here, we review the established role of HSF1 in counteracting cell stress and prospect the role of HSF1 as a regulator of disease states and aging.

Heat shock factors: integrators of cell stress, development and lifespan

Heat shock factors (HSFs) are essential for all organisms to survive exposures to acute stress. They are best known as inducible transcriptional regulators of genes encoding molecular chaperones and other stress proteins. Four members of the HSF family are also important for normal development and lifespan-enhancing pathways, and the repertoire of HSF targets has thus expanded well beyond the heat shock genes. These unexpected observations have uncovered complex layers of post-translational regulation of HSFs that integrate the metabolic state of the cell with stress biology, and in doing so control fundamental aspects of the health of the proteome and ageing.

Modulation of heat shock transcription factor 1 as a therapeutic target for small molecule intervention in neurodegenerative disease

A yeast-based small molecule screen identifies a novel activator of human HSF1 and protein chaperone expression and which appears to alleviate the toxicity of protein misfolding diseases.

Neurodegenerative diseases such as Huntington disease are devastating disorders with no therapeutic approaches to ameliorate the underlying protein misfolding defect inherent to poly-glutamine (polyQ) proteins. Given the mounting evidence that elevated levels of protein chaperones suppress polyQ protein misfolding, the master regulator of protein chaperone gene transcription, HSF1, is an attractive target for small molecule intervention. We describe a humanized yeast-based high-throughput screen to identify small molecule activators of human HSF1. This screen is insensitive to previously characterized activators of the heat shock response that have undesirable proteotoxic activity or that inhibit Hsp90, the central chaperone for cellular signaling and proliferation. A molecule identified in this screen, HSF1A, is structurally distinct from other characterized small molecule human HSF1 activators, activates HSF1 in mammalian and fly cells, elevates protein chaperone expression, ameliorates protein misfolding and cell death in polyQ-expressing neuronal precursor cells and protects against cytotoxicity in a fly model of polyQ-mediated neurodegeneration. In addition, we show that HSF1A interacts with components of the TRiC/CCT complex, suggesting a potentially novel regulatory role for this complex in modulating HSF1 activity. These studies describe a novel approach for the identification of new classes of pharmacological interventions for protein misfolding that underlies devastating neurodegenerative disease.

The misfolding of proteins into a toxic state contributes to a variety of neurodegenerative diseases such as Huntington, Alzheimer, and Parkinson disease. Although no known cure exists for these afflictions, many studies have shown that increasing the levels of protein chaperones, proteins that assist in the correct folding of other proteins, can suppress the neurotoxicity of the misfolded proteins. As such, increasing the cellular concentration of protein chaperones might serve as a powerful therapeutic approach in treating protein misfolding diseases. Because the levels of protein chaperones in the cell are primarily controlled by the heat shock transcription factor 1 [HSF1], we have designed and implemented a pharmacological screen to identify small molecules that can promote human HSF1 activation and increase the expression of protein chaperones. Through these studies, we have identified HSF1A, a molecule capable of activating human HSF1, increasing the levels of protein chaperones and alleviating the toxicity of misfolded proteins in both cell culture as well as fruit fly models of neurodegenerative disease.

Increased temperature and protein oxidation lead to HSP72 mRNA and protein accumulation in the in vivo exercised rat heart

Expression of myocardial heat shock protein 72 (HSP72), mediated by its transcription factor, heat shock factor 1 (HSF1), increases following exercise. However, the upstream stimuli governing exercise-induced HSF1 activation and subsequent Hsp72 gene expression in the whole animal remain unclear. Exercise-induced increases in body temperature may promote myocardial radical production, leading to protein oxidation. Conceivably, myocardial protein oxidation during exercise may serve as an important signal to promote nuclear HSF1 migration and activation of Hsp72 expression. Therefore, these experiments tested the hypothesis that prevention of exercise-induced increases in body temperature attenuates cardiac protein oxidation, diminishes HSF1 activation and decreases HSP72 expression in vivo. To test this hypothesis, in vivo exercise-induced changes in body temperature were manipulated by exercising male rats in either cold (4 degrees C) or warm ambient conditions (22 degrees C). Warm exercise increased both body temperature (+3 degrees C) and myocardial protein oxidation, whereas these changes were attenuated by cold exercise. Interestingly, exercise in both conditions did not significantly increase myocardial nuclear localized phosphorylated HSF1. Nonetheless, warm exercise elevated left-ventricular HSP72 mRNA by ninefold and increased myocardial HSP72 protein levels by threefold compared with cold-exercised animals. Collectively, these data indicate that elevated body temperature and myocardial protein oxidation promoted exercise-induced cardiac HSP72 mRNA expression and protein accumulation following in vivo exercise. However, these results suggest that exercise-induced myocardial HSP72 protein accumulation is not a result of nuclear-localized, phosphorylated HSF1, indicating that other transcriptional or post-transcriptional regulatory mechanisms are involved in exercise-induced HSP72 expression.

Effect of exercise with and without a thermal clamp on the plasma heat shock protein 72 response

The contribution of heat and exercise related stress to the release of heat shock protein 72 (HSP72) is currently unknown. The purpose of the present study was to determine the combined and independent effects of heat and exercise on the extracellular (e)HSP72 response. Eleven moderately trained male volunteers [means ± SD: age 21 ± 4 yr; body mass 75.7 ± 7.7 kg; maximal oxygen uptake (V̇o2 max) 57.8 ± 3.3 ml·kg−1·min−1] completed four 2-h, heat-manipulated, water-immersion trials. Trials were exercise-induced heat (EIH; rectal temperature change +2.2°C), clamped exercise (CEx; 0°C), passive heating (PHT; +2.3°C), and control (Con; 0°C). Exercise trials (EIH and CEx) comprised deep-water running at 58.5 ± 2.4 and 59.1 ± 1.7% v̇o2max. eHSP72 and catecholamine concentrations were determined by ELISA and HPLC, respectively, pre- and postimmersion. All trials induced an eHSP72 response ( P < 0.05) with postimmersion values significantly greater on EIH compared with other trials (6.0 ± 3.4; CEx 3.8 ± 2.6; PHT 2.7 ± 2.1; Con 2.2 ± 1.9 ng/ml). Exercising with a thermal clamp blunted the eHSP72 response, but postimmersion values were also greater than Con. PHT induced a large catecholamine response, but postimmersion eHSP72 values did not reach significance vs. Con. Given that exercising with a thermal clamp evoked a significant increase in plasma eHSP72 concentration, exercise-related stressors other than heat appeared influential in stimulating HSP72 release. Moreover, the catecholamine data from PHT suggest neither epinephrine nor norepinephrine was solely responsible for eHSP72 release.

Heat Shock Factor 1 as a Coordinator of Stress and Developmental Pathways

The transition from normal growth conditions to stressful conditions is accompanied by a robust upregulation of heat shock proteins, which dampen the cytotoxicity caused by misfolded and denatured proteins. The most prominent part of this transition occurs on the transcriptional level. In mammals, protein-damaging stress leads to the activation of heat shock factor 1 (HSF1), which binds to upstream regulatory sequences in the promoters of heat shock genes. The activation of HSF1 proceeds through a multi-step pathway, involving a monomer-to-trimer transition, nuclear accumulation and extensive posttranslational modifications. In addition to its established role as the main regulator of heat shock genes, new data link HSF 1 to developmental pathways. In this chapter, we examine the established stress-related functions and prospect the intriguing role of HSF 1 as a developmental coordinator.

Increased temperature, not cardiac load, activates heat shock transcription factor 1 and heat shock protein 72 expression in the heart

The expression of myocardial heat shock protein 72 (HSP72) postexercise is initiated by the activation of heat shock transcription factor 1 (HSF1). However, it remains unknown which physiological stimuli govern myocardial HSF1 activation during exercise. These experiments tested the hypothesis that thermal stress and mechanical load, concomitant with simulated exercise, provide independent stimuli for HSF1 activation and ensuing cardiac HSP72 gene expression. To elucidate the independent roles of increased temperature and cardiac workload in the exercise-mediated upregulation of left-ventricular HSP72, hearts from adult male Sprague-Dawley rats were randomly assigned to one of five simulated exercise conditions. Upon reaching a surgical plane of anesthesia, each experimental heart was isolated and perfused using an in vitro working heart model, while independently varying temperatures (i.e., 37°C vs. 40°C) and cardiac workloads (i.e., low preload and afterload vs. high preload and afterload) to mimic exercise responses. Results indicate that hyperthermia, independent of cardiac workload, promoted an increase in nuclear translocation and phosphorylation of HSF1 compared with normothermic left ventricles. Similarly, hyperthermia, independent of workload, resulted in significant increases in cardiac levels of HSP72 mRNA. Collectively, these data suggest that HSF1 activation and HSP72 gene transcriptional competence during simulated exercise are linked to elevated heart temperature and are not a direct function of increased cardiac workload.

Heat shock response and acute lung injury

All cells respond to stress through the activation of primitive, evolutionarily conserved genetic programs that maintain homeostasis and assure cell survival. Stress adaptation, which is known in the literature by a myriad of terms, including tolerance, desensitization, conditioning, and reprogramming, is a common paradigm found throughout nature, in which a primary exposure of a cell or organism to a stressful stimulus (e.g., heat) results in an adaptive response by which a second exposure to the same stimulus produces a minimal response. More interesting is the phenomenon of cross-tolerance, by which a primary exposure to a stressful stimulus results in an adaptive response whereby the cell or organism is resistant to a subsequent stress that is different from the initial stress (i.e., exposure to heat stress leading to resistance to oxidant stress). The heat shock response is one of the more commonly described examples of stress adaptation and is characterized by the rapid expression of a unique group of proteins collectively known as heat shock proteins (also commonly referred to as stress proteins). The expression of heat shock proteins is well described in both whole lungs and in specific lung cells from a variety of species and in response to a variety of stressors. More importantly, in vitro data, as well as data from various animal models of acute lung injury, demonstrate that heat shock proteins, especially Hsp27, Hsp32, Hsp60, and Hsp70 have an important cytoprotective role during lung inflammation and injury.

Characterization of testis-specific serine-threonine kinase 3 and its activation by phosphoinositide-dependent kinase-1-dependent signalling

The family of testis-specific serine-threonine kinases (TSSKs) consists of four members whose expression is confined almost exclusively to testis. Very little is known about their physiological role and mechanisms of action. We cloned human and mouse TSSK3 and analysed the biochemical properties, substrate specificity and in vitro activation. In vitro TSSK3 exhibited the ability to autophosphorylate and to phosphorylate test substrates such as histones, myelin basic protein and casein. Interestingly, TSSK3 showed maximal in vitro kinase activity at 30 degrees C, in keeping with it being testis specific. Sequence comparison indicated the existence of a so-called 'T-loop' within the TSSK3 catalytic domain, a structure present in the AGC family of protein kinases. To test if this T-loop is engaged in TSSK3 regulation, we mutated the critical threonine residue within the T-loop to alanine (T168A) which resulted in inactivation of TSSK3 kinase. Furthermore, Thr168 is phosphorylated in vitro by the T-loop kinase phosphoinositide-dependent protein kinase-1 (PDK1). PDK1-induced phosphorylation increased in vitro TSSK3 kinase activity, suggesting that TSSK3 can be regulated in the same way as AGC kinase family members. Analysis of peptide sequences identifies the peptide sequence RRSSSY containing Ser5 that is a target for TSSK3 phosphorylation, as an efficient and specific substrate for TSSK3.

How do cells respond to their thermal environment?

Changes in growth temperature induce both activating and inactivating responses from cells, with the magnitude of the temperature change being among the factors that influence which type of response dominates. Aside from upregulated enzyme activity, induction of thermotolerance is the most widely studied and best understood activating response that cells exhibit following heat shock. Inactivating responses to heat shock that are of biomedical interest include heat radiosensitization and cytotoxicity. Interestingly, the activation energy for inducing thermotolerance, heat cytotoxicity, and radiosensitization all fall within a similar range of 120-146 kcal per mole. The relatively high activation energy for each of these responses suggests that they all involve a heat-induced molecular transition as a trigger, and several lines of research suggest strongly that protein denaturation is the common transition that triggers all three responses. Low levels of protein denaturation are sufficient to attract the 90 kDa heat shock protein (HSP90) such that it frees up heat shock factor 1, which then trimerizes to form an active transcription factor that upregulates expression of heat shock proteins. Upregulation of heat shock proteins and other heat-induced events result in the development of thermotolerance, which protects cells from subsequent exposure to heat shock and other stresses. A more severe heat shock increases protein denaturation proportionately and leads to aggregation of both denatured and native proteins. This results in inactivation of protein synthesis, cell cycle progression, and DNA repair processes such that cells either die or are sensitized to radiation and other cytotoxic events. The ultimate fate of cells following a heat shock depends upon the summation of the activation and inactivation events that are induced, which appears to be governed by the resultant magnitude of protein denaturation and aggregation. Treatments that stabilize cellular proteins against denaturation and aggregation reduce the magnitude of inactivating responses while increasing that of activating responses for a given heat shock (time at temperature), while treatments that sensitize proteins to denaturation and aggreation have the converse effect. These findings support the conclusion that the determinant of the cellular response to heat shock is the amount of heat-induced protein denaturation and aggregation and not the time at temperature.

Feedback regulation of the heat shock response

The heat shock response is triggered primarily by nonnative proteins accumulating in a stressed cell and results in increased expression of heat shock proteins (Hsps), i.e., of chaperones capable of participating in the refolding or elimination of nonnative proteins. Best known is the transcriptional part of this response that is mediated predominantly by heat shock factor 1 (HSF1). HSF1 activity is regulated at different levels by Hsps and co-chaperones and is modulated further by a number of mechanisms involving other stress-regulated aspects of cell metabolism.

CHIP interacts with heat shock factor 1 during heat stress

Heat shock factor 1 (HSF1) is a major transactivator of heat shock genes in response to stress and mediates cell protection against various harmful conditions. In this study, we identified the interaction of CHIP (carboxyl terminus of the heat shock cognate protein 70-interacting protein) with the N-terminus of HSF1. Using GST full-down assay, we found that CHIP directly interacts with C-terminal deleted HSF1 (a.a. 1-290) but not with full-length HSF1 under non-stressed conditions. Interestingly, interaction of CHIP with full-length HSF1 was induced by heat shock treatment. The structural change of HSF1 was observed under heat stressed conditions by CD spectra. These observations demonstrate the direct interaction between HSF1 and CHIP and this interaction requires conformational change of HSF1 by heat stress.

On mechanisms that control heat shock transcription factor activity in metazoan cells

Heat shock factor Hsf in nonvertebrate animals and homologous heat shock factor Hsf1 in vertebrate animals are key transcriptional regulators of the stress protein response. Hsf/Hsf1 is constitutively present in cells but is, typically, only active during periods during which cells are experiencing a physical or chemical proteotoxic stress. It has become increasingly clear that regulation of Hsf/Hsf1 activity occurs at multiple levels: the oligomeric status of Hsf/Hsf1, its DNA-binding ability, posttranslational modification, transcriptional competence, nuclear/ subnuclear localization, as well as its interactions with regulatory cofactors or other transcription factors all appear to be carefully controlled. This review emphasizes work reported over the past several years suggesting that regulation at several of these levels is mediated by repressive interactions of Hsp90-containing multichaperone complexes and/or individual chaperones and Hsf/Hsf1.

Redox regulation of mammalian heat shock factor 1 is essential for Hsp gene activation and protection from stress

The activation of eukaryotic heat shock protein (Hsp) gene expression occurs in response to a wide variety of cellular stresses including heat shock, hydrogen peroxide, uncoupled oxidative phosphorylation, infection, and inflammation. Biochemical and genetic studies have clearly demonstrated critical roles for mammalian heat shock factor 1 (HSF1) in stress-inducible Hsp gene expression, resistance to stress-induced programmed cell death, extra-embryonic development, and other biological functions. Activation of mammalian Hsp gene expression involves the stress-inducible conversion of HSF1 from the inactive monomer to the DNA-binding competent homotrimer. Although Hsp activation is a central conserved process in biology, the precise mechanisms for stress sensing and signaling to activate HSF1, and the mechanisms by which many distinct stresses activate HSF1, are poorly understood. In this report we demonstrate that recombinant mammalian HSF1 directly senses both heat and hydrogen peroxide to assemble into a homotrimer in a reversible and redox-regulated manner. The sensing of both stresses requires two cysteine residues within the HSF1 DNA-binding domain that are engaged in redox-sensitive disulfide bonds. HSF1 derivatives in which either or both cysteines were mutated are defective in stress-inducible trimerization and DNA binding, stress-inducible nuclear translocation and Hsp gene trans-activation, and in the protection of mouse cells from stress-induced apoptosis. This redox-dependent activation of HSF1 by heat and hydrogen peroxide establishes a common mechanism in the stress activation of Hsp gene expression by mammalian HSF1.

Interspecific- and acclimation-induced variation in levels of heat-shock proteins 70 (hsp70) and 90 (hsp90) and heat-shock transcription factor-1 (HSF1) in congeneric marine snails (genusTegula): implications for regulation ofhspgene expression

In our previous studies of heat-shock protein (hsp) expression in congeneric marine gastropods of the genus Tegula, we observed interspecific and acclimation-induced variation in the temperatures at which heat-shock gene expression is induced (Ton). To investigate the factors responsible for these inter- and intraspecific differences in Ton, we tested the predictions of the ‘cellular thermometer’ model for the transcriptional regulation of hsp expression. According to this model, hsps not active in chaperoning unfolded proteins bind to a transcription factor, heat-shock factor-1 (HSF1), thereby reducing the levels of free HSF1 that are available to bind to the heat-shock element, a regulatory element upstream of hsp genes. Under stress, hsps bind to denatured proteins, releasing HSF1, which can now activate hsp gene transcription. Thus, elevated levels of heat-shock proteins of the 40, 70 and 90 kDa families (hsp 40, hsp70 and hsp90, respectively) would be predicted to elevate Ton. Conversely, elevated levels of HSF1 would be predicted to decrease Ton. Following laboratory acclimation to 13, 18 and 23°C, we used solid-phase immunochemistry (western analysis) to quantify endogenous levels of two hsp70 isoforms (hsp74 and hsp72), hsp90 and HSF1 in the low- to mid-intertidal species Tegula funebralis and in two subtidal to low-intertidal congeners, T. brunnea and T. montereyi. We found higher endogenous levels of hsp72 (a strongly heat-induced isoform) at 13 and 18°C in T. funebralis in comparison with T. brunnea and T. montereyi. However, T. funebralis also had higher levels of HSF1 than its congeners. The higher levels of HSF1 in T. funebralis cannot, within the framework of the cellular thermometer model, account for the higher Ton observed for this species, although they may explain why T. funebralis is able to induce the heat-shock response more rapidly than T. brunnea. However, the cellular thermometer model does appear to explain the cause of the increases in Ton that occurred during warm acclimation of the two subtidal species, in which warm acclimation was accompanied by increased levels of hsp72, hsp74 and hsp90, whereas levels of HSF1 remained stable. T. funebralis, which experiences greater heat stress than its subtidal congeners, consistently had higher ratios of hsp72 to hsp74 than its congeners, although the sum of levels of the two isoforms was similar for all three species except at the highest acclimation temperature (23°C). The ratio of hsp72 to hsp74 may provide a more accurate estimate of environmental heat stress than the total concentrations of both hsp70 isoforms.

Dynamic association of transcriptional activation domains and regulatory regions in Saccharomyces cerevisiae heat shock factor

In Saccharomyces cerevisiae, the heat shock transcription factor (HSF) is thought to be a homotypic trimer that is bound to the promoters of heat shock protein (HSP) genes at both normal and heat shock temperatures. Exposure to heat shock greatly and rapidly induces HSF transcriptional activity without further increasing DNA-binding affinity. It is believed that HSF is under negative regulation at normal growth temperatures, but the detailed mechanism by which HSF is activated is still not clear. We report the analysis of mutations in a conserved arginine (residue 274) at the C-terminal end of the DNA-binding domain (DBD). Two mutations significantly increase both basal activity of HSF at normal temperatures and induced activity on heat shock. We demonstrate by coimmunoprecipitation experiments that the mutations reduce the association between the DNA-binding domain/oligomerization domain and the transcription activation domains. Our studies suggest that the DNA-binding domain of HSF can interact with activation domains directly, and this interaction is important for the repression of HSF activity under normal growth conditions. Destabilizing this interaction by heat or by mutations results in HSF transcriptional activation. We propose that Arg-274 is critical for intramolecular repression of HSF activity in normally growing cells.

Heat shock proteins and cardiovascular pathophysiology

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

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.

Roles of the heat shock transcription factors in regulation of the heat shock response and beyond

The heat shock response, characterized by increased expression of heat shock proteins (Hsps) is induced by exposure of cells and tissues to extreme conditions that cause acute or chronic stress. Hsps function as molecular chaperones in regulating cellular homeostasis and promoting survival. If the stress is too severe, a signal that leads to programmed cell death, apoptosis, is activated, thereby providing a finely tuned balance between survival and death. In addition to extracellular stimuli, several nonstressful conditions induce Hsps during normal cellular growth and development. The enhanced heat shock gene expression in response to various stimuli is regulated by heat shock transcription factors (HSFs). After the discovery of the family of HSFs (i.e., murine and human HSF1, 2, and 4 and a unique avian HSF3), the functional relevance of distinct HSFs is now emerging. HSF1, an HSF prototype, and HSF3 are responsible for heat-induced Hsp expression, whereas HSF2 is refractory to classical stressors. HSF4 is expressed in a tissue-specific manner; similar to HSF1 and HSF2, alternatively spliced isoforms add further complexity to its regulation. Recently developed powerful genetic models have provided evidence for both cooperative and specific functions of HSFs that expand beyond the heat shock response. Certain specialized functions of HSFs may even include regulation of novel target genes in response to distinct stimuli.

Arrest of spermatogenesis in mice expressing an active heat shock transcription factor 1

In mammals, testicular temperature is lower than core body temperature, and the vulnerable nature of spermatogenesis to thermal insult has been known for a century. However, the primary target affected by increases in temperature is not yet clear. We report here that male mice expressing an active form of heat shock transcription factor 1 (HSF1) in the testis are infertile due to a block in spermatogenesis. The germ cells entered meiotic prophase and were arrested at pachytene stage, and there was a significant increase in the number of apoptotic germ cells in these mice. In wild-type mice, a single heat exposure caused the activation of HSF1 and similar histological changes such as a stage-specific apoptosis of pachytene spermatocytes. These results suggest that male infertility caused by thermal insult is at least partly due to the activation of HSF1, which induces the primary spermatocytes to undergo apoptosis.

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.

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.

Polyunsaturated fatty acids enhance the heat induced stress response in rainbow trout (Oncorhynchus mykiss) leukocytes

The heat shock response has been studied extensively, yet the molecular signals that trigger the response remain elusive. The dogma of the heat shock response contends that denatured proteins initiate the response, but evidence is accumulating to point to a more complex system in which at least more than one signal is involved in this process. Thermal stress initiates changes in cellular phospholipid membrane physical state, which when acted upon by phospholipases may release lipid mediators that could serve as triggering signals during the heat shock response. We have examined the heat shock response in freshly isolated leukocytes from the pronephros of rainbow trout (Oncorhynchus mykiss). In this study, we show that leukocytes isolated from rainbow trout acclimated to 5 or 19 degrees C express elevated levels of heat shock protein 70 (hsp70) mRNA when heat shocked at 5 degrees C above their respective acclimation temperature and supplementation with exogenous docosahexaenoic acid or arachidonic acid followed by heat shock enhanced levels of hsp70 mRNA. The time course for docosahexaenoic acid induced enhancement of hsp70 mRNA was accelerated compared with heat shock alone, and staurosporine inhibited the docosahexaenoic acid induced increase of hsp70 mRNA. We also provide evidence that phospholipase A2 is involved in the heat shock response.

Heat-shock proteins, molecular chaperones, and the stress response: evolutionary and ecological physiology

Molecular chaperones, including the heat-shock proteins (Hsps), are a ubiquitous feature of cells in which these proteins cope with stress-induced denaturation of other proteins. Hsps have received the most attention in model organisms undergoing experimental stress in the laboratory, and the function of Hsps at the molecular and cellular level is becoming well understood in this context. A complementary focus is now emerging on the Hsps of both model and nonmodel organisms undergoing stress in nature, on the roles of Hsps in the stress physiology of whole multicellular eukaryotes and the tissues and organs they comprise, and on the ecological and evolutionary correlates of variation in Hsps and the genes that encode them. This focus discloses that (a) expression of Hsps can occur in nature, (b) all species have hsp genes but they vary in the patterns of their expression, (c) Hsp expression can be correlated with resistance to stress, and (d) species' thresholds for Hsp expression are correlated with levels of stress that they naturally undergo. These conclusions are now well established and may require little additional confirmation; many significant questions remain unanswered concerning both the mechanisms of Hsp-mediated stress tolerance at the organismal level and the evolutionary mechanisms that have diversified the hsp genes.

Cooperative binding of heat shock factor to the yeast HSP82 promoter in vivo and in vitro

Previous work has shown that heat shock factor (HSF) plays a central role in remodeling the chromatin structure of the yeast HSP82 promoter via constitutive interactions with its high-affinity binding site, heat shock element 1 (HSE1). The HSF-HSE1 interaction is also critical for stimulating both basal (noninduced) and induced transcription. By contrast, the function of the adjacent, inducibly occupied HSE2 and -3 is unknown. In this study, we examined the consequences of mutations in HSE1, HSE2, and HSE3 on HSF binding and transactivation. We provide evidence that in vivo, HSF binds to these three sites cooperatively. This cooperativity is seen both before and after heat shock, is required for full inducibility, and can be recapitulated in vitro on both linear and supercoiled templates. Quantitative in vitro footprinting reveals that occupancy of HSE2 and -3 by Saccharomyces cerevisiae HSF (ScHSF) is enhanced approximately 100-fold through cooperative interactions with the HSF-HSE1 complex. HSE1 point mutants, whose basal transcription is virtually abolished, are functionally compensated by cooperative interactions with HSE2 and -3 following heat shock, resulting in robust inducibility. Using a competition binding assay, we show that the affinity of recombinant HSF for the full-length HSP82 promoter is reduced nearly an order of magnitude by a single-point mutation within HSE1, paralleling the effect of these mutations on noninduced transcript levels. We propose that the remodeled chromatin phenotype previously shown for HSE1 point mutants (and lost in HSE1 deletion mutants) stems from the retention of productive, cooperative interactions between HSF and its target binding sites.

Protective mechanisms in germ cells: stress proteins in spermatogenesis

A wide range of environmental exposures trigger protective mechanisms in reproductive tissues which are mediated by stress or heat shock proteins (HSPs). These stress proteins maintain normal cellular functions such as protein synthesis, as well as assist in resisting and recovering from toxicant-induced cellular damage. Over the past decade a number of laboratories have examined the expression and potential functions of these stress proteins during gametogenesis (reviewed in Dix, 1997a) and in reproductive toxicology (Dix, 1997b). This paper reviews the expression of HSPs in testes, presents a detailed analysis of the function of Hsp70-2 during the meiotic phase of spermatogenesis, and concludes with a discussion of stress-inducible HSPs and putative protective mechanisms.

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.

Conventional and novel PKC isoenzymes modify the heat-induced stress response but are not activated by heat shock

In mammalian cells, the heat-induced stress response is mediated by the constitutively expressed heat shock transcription factor 1 (HSF1). Upon exposure to elevated temperatures, HSF1 undergoes several post-translational modifications, including inducible phosphorylation or hyperphosphorylation. To date, neither the role of HSF1 hyperphosphorylation in regulation of the transcriptional activity of HSF1 nor the signaling pathways involved have been characterized. We have previously shown that the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol 13-acetate (TPA), markedly enhances the heat-induced stress response, and in the present study we elucidate the mechanism by which PKC activation affects the heat shock response in human cells. Our results show that several conventional and novel PKC isoenzymes are activated during the TPA-mediated enhancement of the heat shock response and that the enhancement can be inhibited by the specific PKC inhibitor bisindolylmaleimide I. Furthermore, the potentiating effect of TPA on the heat-induced stress response requires an intact heat shock element in the hsp70 promoter, indicating that PKC-responsive pathways are able to modulate the activity of HSF1. We also demonstrate that PKC is not activated by heat stress per se. These results reveal that PKC exhibits a significant modulatory role of the heat-induced stress response, but is not directly involved in regulation of the heat shock response.

Direct sensing of heat and oxidation by Drosophila heat shock transcription factor

The heat shock transcription factor HSF activates expression of its target genes in response to elevated temperatures and chemical or physiological stress. A key step in the activation process involves the formation of HSF homotrimers, leading to high-affinity DNA binding. The mechanism by which HSF trimerization and DNA binding is regulated by stress signals has remained elusive. Here, we report that trimerization and DNA binding of purified Drosophila HSF can be directly and reversibly induced in vitro by heat shock temperatures in the physiological range and by an oxidant, hydrogen peroxide. Other inducers of the heat shock response, including salicylate, dinitrophenol, ethanol, and arsenite, have no effect on HSF trimerization in vitro, indicating that these inducers act by indirect mechanisms.

Regulation of HSF1 activation and Hsp expression in mouse tissues under physiological stress conditions

The studies described above illustrate several important points concerning the study of the cellular stress response. First, they indicate that there are important differences between the nature of the stress response observed in tumor cell lines and that of primary cells of mouse tissues. Therefore, caution should be exercised to avoid overextending and overinterpreting the results of studies using these cell lines until evidence can be obtained from in vivo studies on intact animals or primary cells of tissues. Second, they reveal the existence of cell-type-dependent differences between cell types of the same organism, thus potentially calling into question the supposed universal cytoprotective nature of the cellular stress response. Future studies can address this question by determining the underlying mechanisms that regulate these cell-type-dependent differences in the stress response, and by determining whether there are biological reasons that some cell types exhibit differences in either the temperature set-point or magnitude of the stress 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.

Intramolecular repression of mouse heat shock factor 1

The pathway leading to transcriptional activation of heat shock genes involves a step of heat shock factor 1 (HSF1) trimerization required for high-affinity binding of this activator protein to heat shock elements (HSEs) in the promoters. Previous studies have shown that in vivo the trimerization is negatively regulated at physiological temperatures by a mechanism that requires multiple hydrophobic heptad repeats (HRs) which may form a coiled coil in the monomer. To investigate the minimal requirements for negative regulation, in this work we have examined mouse HSF1 translated in rabbit reticulocyte lysate or extracted from Escherichia coli after limited expression. We show that under these conditions HSF1 behaves as a monomer which can be induced by increases in temperature to form active HSE-binding trimers and that mutations of either HR region cause activation in both systems. Furthermore, temperature elevations and acidic buffers activate purified HSF1, and mild proteolysis excises fragments which form HSE-binding oligomers. These results suggest that oligomerization can be repressed in the monomer, as previously proposed, and that repression can be relieved in the apparent absence of regulatory proteins. An intramolecular mechanism may be central for the regulation of this transcription factor in mammalian cells, although not necessarily sufficient.

Heat shock transcription factor 1 binds selectively in vitro to Ku protein and the catalytic subunit of the DNA-dependent protein kinase

Heat shock transcription factor 1 (HSF1) functions as the master regulator of the heat shock response in eukaryotes. We have previously shown that, in addition to its role as a transcription factor, HSF1 stimulates the activity of the DNA-dependent protein kinase (DNA-PK). DNA-PK is composed of two components: a 460-kDa catalytic subunit and a 70- and 86-kDa heterodimeric regulatory component, also known as the Ku protein. We report here that HSF1 binds specifically to each of the two components of DNA-PK. Binding occurs in the absence of DNA. The complex with the Ku protein is stable and forms at a stoichiometry close to unity between the Ku protein heterodimer and the active HSF1 trimer. The binding is blocked by antibodies against HSF1. Our results show that HSF1 also binds directly, but more weakly, to the catalytic subunit of DNA-PK. Both interactions are dependent on a specific region within the HSF1 regulatory domain. This sequence is necessary but not sufficient for HSF1 stimulation of DNA-PK activity. The ability of HSF1 to interact with both components of DNA-PK provides a potential mechanism for the activation of DNA-PK in response to heat and other forms of stress.

A proteinaceous gene regulatory thermometer in Salmonella

Novel utilization of the coiled-coil motif is presented that enables TlpA, an autoregulatory repressor protein in Salmonella, to sense temperature shifts directly and thereby to modulate the extent of transcription repression. Salmonella cells shifted to higher temperatures, such as those encountered at host entry, showed derepressed tlpA activity. tlpA::lacZ fusions indicated that the promoter itself is insensitive to thermal shifts and that transcription control was exerted by the autorepressor TlpA only. In vitro studies with highly purified TlpA showed concentration and temperature dependence for both fully folded conformation and function, indicating that the thermosensing in TlpA is based on monomer-to-coiled-coil equilibrium.