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

Heat Shock Proteins and HSF1 in Cancer

Fitness of cells is dependent on protein homeostasis which is maintained by cooperative activities of protein chaperones and proteolytic machinery. Upon encountering protein-damaging conditions, cells activate the heat-shock response (HSR) which involves HSF1-mediated transcriptional upregulation of a group of chaperones - the heat shock proteins (HSPs). Cancer cells experience high levels of proteotoxic stress due to the production of mutated proteins, aneuploidy-induced excess of components of multiprotein complexes, increased translation rates, and dysregulated metabolism. To cope with this chronic state of proteotoxic stress, cancers almost invariably upregulate major components of HSR, including HSF1 and individual HSPs. Some oncogenic programs show dependence or coupling with a particular HSR factor (such as frequent coamplification of HSF1 and MYC genes). Elevated levels of HSPs and HSF1 are typically associated with drug resistance and poor clinical outcomes in various malignancies. The non-oncogene dependence ("addiction") on protein quality controls represents a pancancer target in treating human malignancies, offering a potential to enhance efficacy of standard and targeted chemotherapy and immune checkpoint inhibitors. In cancers with specific dependencies, HSR components can serve as alternative targets to poorly druggable oncogenic drivers.

Missense mutation of a class B heat shock factor is responsible for the tomato bushy root-2 phenotype

The bushy root-2 (brt-2) tomato mutant has twisting roots, and slower plant development. Here we used whole genome resequencing and genetic mapping to show that brt-2 is caused by a serine to cysteine (S75C) substitution in the DNA binding domain (DBD) of a heat shock factor class B (HsfB) encoded by SolycHsfB4a. This gene is orthologous to the Arabidopsis SCHIZORIZA gene, also known as AtHsfB4. The brt-2 phenotype is very similar to Arabidopsis lines in which the function of AtHsfB4 is altered: a proliferation of lateral root cap and root meristematic tissues, and a tendency for lateral root cap cells to easily separate. The brt-2 S75C mutation is unusual because all other reported amino acid substitutions in the highly conserved DBD of eukaryotic heat shock factors are dominant negative mutations, but brt-2 is recessive. We further show through reciprocal grafting that brt-2 exerts its effects predominantly through the root genotype even through BRT-2 is expressed at similar levels in both root and shoot meristems. Since AtHsfB4 is induced by root knot nematodes (RKN), and loss-of-function mutants of this gene are resistant to RKNs, BRT-2 could be a target gene for RKN resistance, an important trait in tomato rootstock breeding.Gene & accession numbersSolycHsfB4a - Solyc04g078770.

Structures of heat shock factor trimers bound to DNA

Heat shock factor 1 (HSF1) and 2 (HSF2) play distinct but overlapping regulatory roles in maintaining cellular proteostasis or mediating cell differentiation and development. Upon activation, both HSFs trimerize and bind to heat shock elements (HSEs) present in the promoter region of target genes. Despite structural insights gained from recent studies, structures reflecting the physiological architecture of this transcriptional machinery remains to be determined. Here, we present co-crystal structures of human HSF1 and HSF2 trimers bound to DNA, which reveal a triangular arrangement of the three DNA-binding domains (DBDs) with protein-protein interactions largely mediated by the wing domain. Two structural properties, different flexibility of the wing domain and local DNA conformational changes induced by HSF binding, seem likely to contribute to the subtle differential specificity between HSF1 and HSF2. Besides, two more structures showing DBDs bound to "two-site" head-to-head HSEs were determined as additions to the published tail-to-tail dimer-binding structures.

Determine genetic variations in heat shock factor gene family (HSFs) and study their effect on the functional and structural characterization of protein in Tali goat

In this study, the effect of genetic variations of four heat shock transcription factor genes (HSF1, HSF2, HSF4, and HSF5) on the 3 D protein structure and function were studied. We defined the breed-specific genetic variations of pooled DNA of Tali goat that differed from the goat reference sequence (CHI2.0). Disordered regions of HSF proteins were predicted using PONDR. Post-translation changes were studied by several predicted online servers. Then, the structure of the order region of proteins was anticipated by using the Swiss model. Tali goat HSF genes contain a total number of 181, 679, 91, and 301 SNPs for HSF1, 2, 4, and 5, respectively. Also, 5 and 3 variants were identified as nsSNPs in the coding region of HSF4 and HSF5, respectively. (r.145A/S), (r.322P/Y), (r.379T/C) in HSF4 and (r.300Q/P), (r.573E/Q) in HSF5 obtained the tolerant and high confidence (SIFT score) for nsSNPs. More than half of these proteins are predicted to be disordered (56, 50, 52, and 50%, respectively for HSF1, 2, 4, and 5). Phosphorylation, acetylation, glycosylation, and Sumoylation sites of HSFs were compared between Tali goat and reference goat. Three residues S145, S263, and S322 of HSF4 in Tali goat were phosphorylation sites, and in HSF5, the reference goat has a phosphorylation site in S593.

Stress-induced transcriptional memory accelerates promoter-proximal pause release and decelerates termination over mitotic divisions

Heat shock instantly reprograms transcription. Whether gene and enhancer transcription fully recover from stress and whether stress establishes a memory by provoking transcription regulation that persists through mitosis remained unknown. Here, we measured nascent transcription and chromatin accessibility in unconditioned cells and in the daughters of stress-exposed cells. Tracking transcription genome-wide at nucleotide-resolution revealed that cells precisely restored RNA polymerase II (Pol II) distribution at gene bodies and enhancers upon recovery from stress. However, a single heat exposure in embryonic fibroblasts primed a faster gene induction in their daughter cells by increasing promoter-proximal Pol II pausing and by accelerating the pause release. In K562 erythroleukemia cells, repeated stress refined basal and heat-induced transcription over mitotic division and decelerated termination-coupled pre-mRNA processing. The slower termination retained transcripts on the chromatin and reduced recycling of Pol II. These results demonstrate that heat-induced transcriptional memory acts through promoter-proximal pause release and pre-mRNA processing at transcription termination.

HSF1: Primary Factor in Molecular Chaperone Expression and a Major Contributor to Cancer Morbidity

Heat shock factor 1 (HSF1) is the primary component for initiation of the powerful heat shock response (HSR) in eukaryotes. The HSR is an evolutionarily conserved mechanism for responding to proteotoxic stress and involves the rapid expression of heat shock protein (HSP) molecular chaperones that promote cell viability by facilitating proteostasis. HSF1 activity is amplified in many tumor contexts in a manner that resembles a chronic state of stress, characterized by high levels of HSP gene expression as well as HSF1-mediated non-HSP gene regulation. HSF1 and its gene targets are essential for tumorigenesis across several experimental tumor models, and facilitate metastatic and resistant properties within cancer cells. Recent studies have suggested the significant potential of HSF1 as a therapeutic target and have motivated research efforts to understand the mechanisms of HSF1 regulation and develop methods for pharmacological intervention. We review what is currently known regarding the contribution of HSF1 activity to cancer pathology, its regulation and expression across human cancers, and strategies to target HSF1 for cancer therapy.

FOXO3 on the Road to Longevity: Lessons From SNPs and Chromatin Hubs

Health span is driven by a precise interplay between genes and the environment. Cell response to environmental cues is mediated by signaling cascades and genetic variants that affect gene expression by regulating chromatin plasticity. Indeed, they can promote the interaction of promoters with regulatory elements by forming active chromatin hubs.

FOXO3 encodes a transcription factor with a strong impact on aging and age-related phenotypes, as it regulates stress response, therefore affecting lifespan. A significant association has been shown between human longevity and several FOXO3 variants located in intron 2. This haplotype block forms a putative aging chromatin hub in which FOXO3 has a central role, as it modulates the physical connection and activity of neighboring genes involved in age-related processes.

Here we describe the role of FOXO3 and its single-nucleotide polymorphisms (SNPs) in healthy aging, with a focus on the enhancer region encompassing the SNP rs2802292, which upregulates FOXO3 expression and can promote the activity of the aging hub in response to different stress stimuli. FOXO3 protective effect on lifespan may be due to the accessibility of this region to transcription factors promoting its expression. This could in part explain the differences in FOXO3 association with longevity between genders, as its activity in females may be modulated by estrogens through estrogen receptor response elements located in the rs2802292-encompassing region. Altogether, the molecular mechanisms described here may help establish whether the rs2802292 SNP can be taken advantage of in predictive medicine and define the potential of targeting FOXO3 for age-related diseases.

A promoter variant in ZNF804A decreasing its expression increases the risk of autism spectrum disorder in the Han Chinese population

Synaptic pathology may be one of the cellular substrates underlying autism spectrum disorder (ASD). ZNF804A is a transcription factor that can affect or regulate the expression of many candidate genes involved in ASD. It also localizes at synapses and regulates neuronal and synaptic morphology. So far, few reports have addressed possible associations between ZNF804A polymorphisms and ASD. This study aimed to investigate whether ZNF804A genetic variants contribute to ASD susceptibility and its possible pathological role in the disorder. We analyzed the relationship of two polymorphisms (rs10497655 and rs34714481) in ZNF804A promoter region with ASD in 854 cases versus 926 controls. The functional analyses of rs10497655 were then performed using real-time quantitative polymerase chain reaction, electrophoretic mobility shift assays, chromatin immunoprecipitation and dual-luciferase assays. The variant rs10497655 was significantly associated with ASD (P = 0.007851), which had a significant effect on ZNF804A expression, with the T risk allele homozygotes related with reduced ZNF804A expression in human fetal brains. HSF2 acted as a suppressor by down-regulating ZNF804A expression and had a stronger binding affinity for the T allele of rs10497655 than for the C allele. This was the first experiment to elucidate the process in which a disease-associated SNP affects the level of ZNF804A expression by binding with the upstream regulation factor HSF2. This result indicates that the rs10497655 allelic expression difference of ZNF804A during the critical period of brain development may have an effect on postnatal phenotypes of ASD. It reveals new roles of ZNF804A polymorphisms in the pathogenesis of psychiatric disorders.

Azadiradione ameliorates polyglutamine expansion disease in Drosophila by potentiating DNA binding activity of heat shock factor 1

Aggregation of proteins with the expansion of polyglutamine tracts in the brain underlies progressive genetic neurodegenerative diseases (NDs) like Huntington's disease and spinocerebellar ataxias (SCA). An insensitive cellular proteotoxic stress response to non-native protein oligomers is common in such conditions. Indeed, upregulation of heat shock factor 1 (HSF1) function and its target protein chaperone expression has shown promising results in animal models of NDs. Using an HSF1 sensitive cell based reporter screening, we have isolated azadiradione (AZD) from the methanolic extract of seeds of Azadirachta indica, a plant known for its multifarious medicinal properties. We show that AZD ameliorates toxicity due to protein aggregation in cell and fly models of polyglutamine expansion diseases to a great extent. All these effects are correlated with activation of HSF1 function and expression of its target protein chaperone genes. Notably, HSF1 activation by AZD is independent of cellular HSP90 or proteasome function. Furthermore, we show that AZD directly interacts with purified human HSF1 with high specificity, and facilitates binding of HSF1 to its recognition sequence with higher affinity. These unique findings qualify AZD as an ideal lead molecule for consideration for drug development against NDs that affect millions worldwide.

Olfactory experience primes the heat shock transcription factor HSF-1 to enhance the expression of molecular chaperones in C. elegans

Learning, a process by which animals modify their behavior as a result of experience, enables organisms to synthesize information from their surroundings to acquire resources and avoid danger. We showed that a previous encounter with only the odor of pathogenic bacteria prepared Caenorhabditis elegans to survive exposure to the pathogen by increasing the heat shock factor 1 (HSF-1)-dependent expression of genes encoding molecular chaperones. Experience-mediated enhancement of chaperone gene expression required serotonin, which primed HSF-1 to enhance the expression of molecular chaperone genes by promoting its localization to RNA polymerase II-enriched nuclear loci, even before transcription occurred. However, HSF-1-dependent chaperone gene expression was stimulated only if and when animals encountered the pathogen. Thus, learning equips C. elegans to better survive environmental dangers by preemptively and specifically initiating transcriptional mechanisms throughout the whole organism that prepare the animal to respond rapidly to proteotoxic agents. These studies provide one plausible basis for the protective role of environmental enrichment in disease.

Diversity, expansion, and evolutionary novelty of plant DNA-binding transcription factor families

Plant transcription factors (TFs) that interact with specific sequences via DNA-binding domains are crucial for regulating transcriptional initiation and are fundamental to plant development and environmental response. In addition, expansion of TF families has allowed functional divergence of duplicate copies, which has contributed to novel, and in some cases adaptive, traits in plants. Thus, TFs are central to the generation of the diverse plant species that we see today. Major plant agronomic traits, including those relevant to domestication, have also frequently arisen through changes in TF coding sequence or expression patterns. Here our goal is to provide an overview of plant TF evolution by first comparing the diversity of DNA-binding domains and the sizes of these domain families in plants and other eukaryotes. Because TFs are among the most highly expanded gene families in plants, the birth and death process of TFs as well as the mechanisms contributing to their retention are discussed. We also provide recent examples of how TFs have contributed to novel traits that are important in plant evolution and in agriculture.This article is part of a Special Issue entitled: Plant Gene Regulatory Mechanisms and Networks, edited by Dr. Erich Grotewold and Dr. Nathan Springer.

Purification, crystallization and X-ray diffraction analysis of the DNA-binding domain of human heat-shock factor 2

Cells respond to various proteotoxic stimuli and maintain protein homeostasis through a conserved mechanism called the heat-shock response, which is characterized by the enhanced synthesis of heat-shock proteins. This response is mediated by heat-shock factors (HSFs). Four genes encoding HSF1-HSF4 exist in the genome of mammals. In this protein family, HSF1 is the orthologue of the single HSF in lower eukaryotic organisms and is the major regulator of the heat-shock response, while HSF2, which shows low sequence homology to HSF1, serves as a developmental regulator. Increasing evidence has revealed biochemical properties and functional roles that are unique to HSF2, such as its DNA-binding preference and sumoylation patterns, which are distinct from those of HSF1. The structural basis for such differences, however, is poorly understood owing to the lack of available mammalian HSF structures. The N-terminal DNA-binding domain (DBD) is the most conserved functional module and is the only crystallizable domain in HSFs. To date, only HSF1 homologue structures from yeast and fruit fly have been determined. Along with extensive studies of the HSF family, more structural information, particularly from members with a remoter phylogenic relationship to the reported structures, e.g. HSF2, is needed in order to better understand the detailed mechanisms of HSF biology. In this work, the recombinant DBD (residues 7-112) from human HSF2 was produced in Escherichia coli and crystallized. An X-ray diffraction data set was collected to 1.32 Å resolution from a crystal belonging to space group P212121 with unit cell-parameters a = 65.66, b = 67.26, c = 93.25 Å. The data-evaluation statistics revealed good quality of the collected data, thus establishing a solid basis for the determination of the first structure at atomic resolution in this protein family.

protonation behavior of histidine during HSF1 activation by physiological acidification

The expression of eukaryotic molecular chaperones (heat shock proteins, HSPs) is triggered in response to a wide range of environmental stresses, including: heat shock, hydrogen peroxide, heavy metal, low-pH, or virus infection. Biochemical and genetic studies have clearly shown the fundamental roles of heat shock factor 1 (HSF1) in stress-inducible HSP gene expression, resistance to stress-induced cell death, carcinogenesis, and other biological phenomena. Previous studies show that acidic pH changes within the physiological range directly activate the HSF1 function in vitro. However, the detailed mechanism is unclear. Though computational pKa-predications of the amino acid side-chain, acidic-pH induced protonation of a histidine residue was found to be most-likely involved in this process. The histidine 83 (His83) residue, which could be protonated by mild decrease in pH, causes mild acidic-induced HSF1 activation (including in-vitro trimerization, DNA binding, in-vivo nuclear accumulation, and HSPs expression). His83, which is located in the loop region of the HSF1 DNA binding domain, was suggested to enhance the intermolecular force with Arginine 79, which helps HSF1 form a DNA-binding competent. Therefore, low-pH-induced activation of HSF1 by the protonation of histidine can help us better to understand the HSF1 mechanism and develop more therapeutic applications (particularly in cancer therapy). J. Cell. Biochem. 116: 977-984, 2015. © 2015 Wiley Periodicals, Inc.

Identification, tissue distribution and characterization of two heat shock factors (HSFs) in goldfish (Carassius auratus)

Heat shock proteins (HSPs) are synthesized rapidly in response to a variety of physiological or environmental stressors, whereas the transcriptional activation of HSPs is regulated by a family of heat shock factors (HSFs). In vertebrates, multiple HSFs (HSF1-4) have been reported to have different roles in response to a range of stresses. This paper reports the cDNA cloning of two goldfish (Carassius auratus) HSF gene families, HSF1 and three isoforms of HSF2. Both HSF1 and HSF2s showed high homology to the known HSFs from other organisms, particularly the zebrafish. Different patterns of HSF1 and HSF2 mRNA expression were detected in several goldfish tissues, highlighting their distinct roles. In cadmium (Cd)-treated tissues, the responses of HSP70 showed less difference. However, the increase in HSF1 and HSF2 in these tissues differs considerable. In particular, HSF2 was induced strongly in the heart and liver. On the other hand, in heart tissue, HSF1 showed the smallest increment. These results suggest the potential role of HSF2 in assisting HSF1 in these tissues. In another in vitro experiment of hepatocyte cultures, Cd exposure caused similar patterns of goldfish HSF1 and HSF2 mRNA expression and induction of the HSP70 protein. On the other hand, an examination of the characterization of recombinant proteins showed that HSF1 undergoes a conformation change induced by heat shock above 30 °C and approaches each other in the trimer, whereas HSF2 could not sense thermal stress directly. Furthermore, immune-blot analysis of HSFs showed that both monomers and trimmers of HSF1 were observed in cadmium-induced tissues, whereas HSF2 were all in monomeric. These results show that HSF1 and HSF2 play different roles in the transcription of heat shock proteins.

Genetic selection for constitutively trimerized human HSF1 mutants identifies a role for coiled-coil motifs in DNA binding

Human heat shock transcription factor 1 (HSF1) promotes the expression of stress-responsive genes and is a critical factor for the cellular protective response to proteotoxic and other stresses. In response to stress, HSF1 undergoes a transition from a repressed cytoplasmic monomer to a homotrimer, accumulates in the nucleus, binds DNA, and activates target gene transcription. Although these steps occur as sequential and highly regulated events, our understanding of the full details of the HSF1 activation pathway remains incomplete. Here we describe a genetic screen in humanized yeast that identifies constitutively trimerized HSF1 mutants. Surprisingly, constitutively trimerized HSF1 mutants do not bind to DNA in vivo in the absence of stress and only become DNA binding competent upon stress exposure, suggesting that an additional level of regulation beyond trimerization and nuclear localization may be required for HSF1 DNA binding. Furthermore, we identified a constitutively trimerized and nuclear-localized HSF1 mutant, HSF1 L189P, located in LZ3 of the HSF1 trimerization domain, which in response to proteotoxic stress is strongly compromised for DNA binding at the Hsp70 and Hsp25 promoters but readily binds to the interleukin-6 promoter, suggesting that HSF1 DNA binding is in part regulated in a locus-dependent manner, perhaps via promoter-specific differences in chromatin architecture. Furthermore, these results implicate the LZ3 region of the HSF1 trimerization domain in a function beyond its canonical role in HSF1 trimerization.

Inhibition of heat shock transcription factor binding by a linear polyamide binding in an unusual 1:1 mode

Heat shock proteins (HSPs) are known to protect cells from heat, oxidative stress, and the cytotoxic effects of drugs, and thus can enhance cancer cell survival. As a result, HSPs are a newly emerging class of protein targets for chemotherapy. Among the various HSPs, the HSP70 family is the most highly conserved and prevalent. Herein we describe the development of a β-alanine rich linear polyamide that binds the GGA heat shock elements (HSEs) 3 and 4 in the HSP70 promoter in an unusual 1:1 mode and inhibits heat shock transcription factor 1 (HSF1) binding in vitro.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Deciphering human heat shock transcription factor 1 regulation via post-translational modification in yeast

Heat shock transcription factor 1 (HSF1) plays an important role in the cellular response to proteotoxic stresses. Under normal growth conditions HSF1 is repressed as an inactive monomer in part through post-translation modifications that include protein acetylation, sumoylation and phosphorylation. Upon exposure to stress HSF1 homotrimerizes, accumulates in nucleus, binds DNA, becomes hyper-phosphorylated and activates the expression of stress response genes. While HSF1 and the mechanisms that regulate its activity have been studied for over two decades, our understanding of HSF1 regulation remains incomplete. As previous studies have shown that HSF1 and the heat shock response promoter element (HSE) are generally structurally conserved from yeast to metazoans, we have made use of the genetically tractable budding yeast as a facile assay system to further understand the mechanisms that regulate human HSF1 through phosphorylation of serine 303. We show that when human HSF1 is expressed in yeast its phosphorylation at S303 is promoted by the MAP-kinase Slt2 independent of a priming event at S307 previously believed to be a prerequisite. Furthermore, we show that phosphorylation at S303 in yeast and mammalian cells occurs independent of GSK3, the kinase primarily thought to be responsible for S303 phosphorylation. Lastly, while previous studies have suggested that S303 phosphorylation represses HSF1-dependent transactivation, we now show that S303 phosphorylation also represses HSF1 multimerization in both yeast and mammalian cells. Taken together, these studies suggest that yeast cells will be a powerful experimental tool for deciphering aspects of human HSF1 regulation by post-translational modifications.

Novel aspects of heat shock factors: DNA recognition, chromatin modulation and gene expression

Heat shock factor (HSF) is an evolutionarily conserved stress-response regulator that activates the transcription of heat shock protein genes, whose products maintain protein homeostasis under normal physiological conditions, as well as under conditions of stress. The promoter regions of the target genes contain a heat shock element consisting of multiple inverted repeats of the pentanucleotide sequence nGAAn. A single HSF of yeast can bind to heat shock elements that differ in the configuration of the nGAAn units and can regulate the transcription of various genes that function not only in stress resistance, but also in a broad range of biological processes. Mammalian cells have four HSF family members involved in different, but in some cases similar, biological functions, including stress resistance, cell differentiation and development. Mammalian HSF family members exhibit differential specificity for different types of heat shock elements, which, together with cell type-specific expression of HSFs is important in determining the target genes of each HSF. This minireview focuses on the molecular mechanisms of DNA recognition, chromatin modulation and gene expression by yeast and mammalian HSFs.

An acylic polyisoprenoid derivative, geranylgeranylacetone protects against visceral adiposity and insulin resistance in high-fat-fed mice

Induction of heat shock protein (HSP)72 improves insulin resistance and obesity in diabetic animal models. Geranylgeranylacetone (GGA), known as an antiulcer drug, induces HSP72 and protects organs against several cellular stresses. This study investigated whether GGA administration would induce HSP72 in liver and render physiological protection against high-fat feeding in mice. A single and 4-wk oral administration of 200 mg/kg GGA was performed in high-fat diet (HFD)-fed mice. Metabolic parameters, cytokines, and gene expressions related to insulin signaling were evaluated. A single administration of GGA induced HSP72 in liver of normal chow-fed and HFD-fed mice. Insulin resistance after HFD was slightly ameliorated. Four weeks of GGA administration also increased HSP72 in liver and significantly improved insulin resistance and glucose homeostasis upon glucose challenge. Activation of c-jun NH₂-terminal kinase (JNK) was attenuated, and insulin signaling was improved in the liver of HFD mice. Visceral adiposity was decreased in GGA-treated mice, accompanied by reduced leptin and increased adiponectin levels. GGA can be a novel therapeutic approach to treat metabolic syndrome as well as type 2 diabetes by improving insulin signaling and reducing adiposity. These beneficial effects of GGA could be mediated through HSP72 induction and JNK inactivation in the liver.

Anaphase-promoting complex/cyclosome participates in the acute response to protein-damaging stress

The ubiquitin E3 ligase anaphase-promoting complex/cyclosome (APC/C) drives degradation of cell cycle regulators in cycling cells by associating with the coactivators Cdc20 and Cdh1. Although a plethora of APC/C substrates have been identified, only a few transcriptional regulators are described as direct targets of APC/C-dependent ubiquitination. Here we show that APC/C, through substrate recognition by both Cdc20 and Cdh1, mediates ubiquitination and degradation of heat shock factor 2 (HSF2), a transcription factor that binds to the Hsp70 promoter. The interaction between HSF2 and the APC/C subunit Cdc27 and coactivator Cdc20 is enhanced by moderate heat stress, and the degradation of HSF2 is induced during the acute phase of the heat shock response, leading to clearance of HSF2 from the Hsp70 promoter. Remarkably, Cdc20 and the proteasome 20S core α2 subunit are recruited to the Hsp70 promoter in a heat shock-inducible manner. Moreover, the heat shock-induced expression of Hsp70 is increased when Cdc20 is silenced by a specific small interfering RNA (siRNA). Our results provide the first evidence for participation of APC/C in the acute response to protein-damaging stress.

A 9 bp cis-element in the promoters of class I small heat shock protein genes on chromosome 3 in rice mediates L-azetidine-2-carboxylic acid and heat shock responses

In rice, the class I small heat shock protein (sHSP-CI) genes were found to be selectively induced by L-azetidine-2-carboxylic acid (AZC) on chromosome 3 but not chromosome 1. Here it is shown that a novel cis-responsive element contributed to the differential regulation. By serial deletion and computational analysis, a 9 bp putative AZC-responsive element (AZRE), GTCCTGGAC, located between nucleotides -186 and -178 relative to the transcription initiation site of Oshsp17.3 was revealed. Deletion of this putative AZRE from the promoter abolished its ability to be induced by AZC. Moreover, electrophoretic mobility shift assay (EMSA) revealed that the AZRE interacted specifically with nuclear proteins from AZC-treated rice seedlings. Two AZRE-protein complexes were detected by EMSA, one of which could be competed out by a canonical heat shock element (HSE). Deletion of the AZRE also affected the HS response. Furthermore, transient co-expression of the heat shock factor OsHsfA4b with the AZRE in the promoter of Oshsp17.3 was effective. The requirement for the putative AZRE for AZC and HS responses in transgenic Arabidopsis was also shown. Thus, AZRE represents an alternative form of heat HSE, and its interaction with canonical HSEs through heat shock factors may be required to respond to HS and AZC.

The Rpd3L HDAC complex is essential for the heat stress response in yeast

To ensure cell survival and growth during temperature increase, eukaryotic organisms respond with transcriptional activation that results in accumulation of proteins that protect against damage and facilitate recovery. To define the global cellular adaptation response to heat stress, we performed a systematic genetic screen that yielded 277 yeast genes required for growth at high temperature. Of these, the Rpd3 histone deacetylase complex was enriched. Global gene expression analysis showed that Rpd3 partially regulated gene expression upon heat shock. The Hsf1 and Msn2/4 transcription factors are the main regulators of gene activation in response to heat stress. RPD3-deficient cells had impaired activation of Msn2/4-dependent genes, while activation of genes controlled by Hsf1 was deacetylase-independent. Rpd3 bound to heat stress-dependent promoters through the Msn2/4 transcription factors, allowing entry of RNA Pol II and activation of transcription upon stress. Finally, we found that the large, but not the small Rpd3 complex regulated cell adaptation in response to heat stress.

Arylamine N-acetyltransferase 1 gene regulation by androgens requires a conserved heat shock element for heat shock factor-1

Human arylamine N-acetyltransferase 1 (NAT1) is a widely distributed protein that has been implicated in a number of different cancers including breast and prostate. Previously, NAT1 gene expression was shown to be androgen dependent, although the effect of androgens was not due to direct activation of the NAT1 promoter. Here, we show that heat shock factor (HSF)1 is induced by androgen in androgen receptor-positive prostate 22Rv1 cells. It also binds to a heat shock element (HSE) in the NAT1 promoter located 776 bp upstream of the transcriptional start site. Mutation of the HSE inhibited androgen responsiveness and prevented direct upregulation of the NAT1 promoter by HSF1. Although HSF2 also bound to the HSE, it did not increase promoter activity. HSF1 induced endogenous NAT1 activity in this cell line in the absence of androgen. This could be attenuated by pretreating cells with HSF1-directed small interfering RNA but not by a scrambled sequence. Our results show that HSF1 is an important transcription factor for induction of NAT1 in human cells and is required for androgen activation of the NAT1 promoter.

Mutational analysis of human heat-shock transcription factor 1 reveals a regulatory role for oligomerization in DNA-binding specificity

HSF (heat-shock transcription factor) trimers bind to the HSE (heat-shock element) regulatory sequence of target genes and regulate gene expression. A typical HSE consists of at least three contiguous inverted repeats of the 5-bp sequence nGAAn. Yeast HSF is able to recognize discontinuous HSEs that contain gaps in the array of the nGAAn sequence; however, hHSF1 (human HSF1) fails to recognize such sites in vitro, in yeast and in HeLa cells. In the present study, we isolated suppressors of the temperature-sensitive growth defect of hHSF1-expressing yeast cells. Intragenic suppressors contained amino acid substitutions in the DNA-binding domain of hHSF1 that enabled hHSF1 to regulate the transcription of genes containing discontinuous HSEs. The substitutions facilitated hHSF1 oligomerization, suggesting that the DNA-binding domain is important for this conformational change. Furthermore, other oligomerization-prone derivatives of hHSF1 were capable of recognizing discontinuous HSEs. These results suggest that modulation of oligomerization is important for the HSE specificity of hHSF1 and imply that hHSF1 possesses the ability to bind to and regulate gene expression via various types of HSEs in diverse cellular processes.

Alpha-helix 1 in the DNA-binding domain of heat shock factor 1 regulates its heat-induced trimerization and DNA-binding

Heat shock factor 1 (HSF1) primarily regulates various cellular stress responses. The role of alpha-helix1 (H1) in its DNA-binding domain (DBD) during HSF1 activation remains unknown. Here, HSF1 lacking H1 loses its heat-induced activity, suggesting the importance of the latter. Furthermore, the CD spectra and AMBER prediction show that this H1 deficiency does not change the structure of HSF1 monomer, but does impact its heat-induced trimerization. Point mutation showed that Phe18 in H1 interacts with Tyr60, and that Trp23 interacts with Phe104 by an aromatic-aromatic interaction. Thus, the presence of H1 stabilizes the DBD structure, which facilitates the heat-induced trimerization and DNA-binding of HSF1.

Hsp70 and cardiac surgery: molecular chaperone and inflammatory regulator with compartmentalized effects

Open heart surgery is a unique model to study the interplay between cellular injury, regulation of inflammatory responses and tissue repair. Stress-inducible heat shock protein 70-kDa (Hsp70) provides a molecular link between these events. In addition to molecular chaperoning, Hsp70 exerts modulatory effects on endothelial cells and leukocytes involved in inflammatory networks. Hsp70 residing in the intracellular compartment is part of an inhibitory feedback loop that acts on nuclear factor kappaB (NF-kappaB). In contrast, extracellular Hsp70 is recognized by multiple germline-encoded immune receptors, e.g., Toll-like receptor (TLR) 2, TLR4, LOX-1, CD91, CD94, CCR5 and CD40. Hsp70 is thereby able to enhance chemotaxis, phagocytosis and cytolytic activity of innate immune cells and stimulate antigen-specific responses. These apparent contradictory pro- and anti-inflammatory effects of endogenous Hsp70 in the context of cardiac surgery are still not fully understood. An all-embracing model of the compartmentalized effects of endogenous Hsp70 in the orchestration of inflammatory responses in cardiac surgery is proposed.

Heterotrimerization of heat-shock factors 1 and 2 provides a transcriptional switch in response to distinct stimuli

Organisms respond to circumstances threatening the cellular protein homeostasis by activation of heat-shock transcription factors (HSFs), which play important roles in stress resistance, development, and longevity. Of the four HSFs in vertebrates (HSF1-4), HSF1 is activated by stress, whereas HSF2 lacks intrinsic stress responsiveness. The mechanism by which HSF2 is recruited to stress-inducible promoters and how HSF2 is activated is not known. However, changes in the HSF2 expression occur, coinciding with the functions of HSF2 in development. Here, we demonstrate that HSF1 and HSF2 form heterotrimers when bound to satellite III DNA in nuclear stress bodies, subnuclear structures in which HSF1 induces transcription. By depleting HSF2, we show that HSF1-HSF2 heterotrimerization is a mechanism regulating transcription. Upon stress, HSF2 DNA binding is HSF1 dependent. Intriguingly, when the elevated expression of HSF2 during development is mimicked, HSF2 binds to DNA and becomes transcriptionally competent. HSF2 activation leads to activation of also HSF1, revealing a functional interdependency that is mediated through the conserved trimerization domains of these factors. We propose that heterotrimerization of HSF1 and HSF2 integrates transcriptional activation in response to distinct stress and developmental stimuli.

Isolation of heat shock factor HsfA1a-binding sites in vivo revealed variations of heat shock elements in Arabidopsis thaliana

The information about DNA-binding sites of regulatory protein is important to understanding the regulatory network of DNA-protein interactions in the genome. In this report we integrated chromatin immunoprecipitation with DNA cloning to isolate genomic sites bound in vivo by heat shock factor HsfA1a in Arabidopsis thaliana. Plantlets were subjected to formaldehyde crosslinking, followed by immunoprecipitation of chromatin. The immunoprecipitated DNA was amplified by PCR and cloned. From a library enriched in putative HsfA1a-binding sites, 21 different genomic fragments were identified (65-332 bp). Six fragments contained known HsfA1a-binding motif (perfect heat shock element). Six fragments contained novel HsfA1a-binding motifs: (1) gap-type, (2) TTC-rich-type, (3) stress responsive element (STRE). Representatives of each were verified by in vitro electrophoretic mobility shift assay. About 81% of the isolated fragments contained the HsfA1a-binding motifs, and/or could be bound by HsfA1a, demonstrating that the method is efficient in the isolation of genomic binding sites of a regulatory protein. The nearest downstream genes to the HsfA1a-binding fragments, which were considered as potential HsfA1a target genes, include a set of classical heat shock protein genes: Hsp17.4, Hsp18.2, Hsp21, Hsp81-1, Hsp101, and several novel genes encoding a non-race specific disease resistance protein and a transmembrane CLPTM1 family protein.

Different Mechanisms Are Involved in the Transcriptional Activation by Yeast Heat Shock Transcription Factor through Two Different Types of Heat Shock Elements

The hydrophobic repeat is a conserved structural motif of eukaryotic heat shock transcription factor (HSF) that enables HSF to form a homotrimer. Homotrimeric HSF binds to heat shock elements (HSEs) consisting of three inverted repeats of the sequence nGAAn. Sequences consisting of four or more nGAAn units are bound cooperatively by two HSF trimers. We show that in Saccharomyces cerevisiae cells oligomerization-defective Hsf1 is not able to bind HSEs with three units and is not extensively phosphorylated in response to stress; it is therefore unable to activate genes containing this type of HSE. Several lines of evidence indicate that oligomerization is a prerequisite for stress-induced hyperphosphorylation of Hsf1. In contrast, oligomerization and hyperphosphorylation are not necessary for gene activation via HSEs with four units. Intragenic suppressor screening of oligomerization-defective hsf1 showed that an interface between adjacent DNA-binding domains is important for the binding of Hsf1 to the HSE. We suggest that Saccharomyces cerevisiae HSEs with different structures are regulated differently; HSEs with three units require Hsf1 to be both oligomerized and hyperphosphorylated, whereas HSEs with four or more units do not require either.

Interaction between heat shock transcription factors (HSFs) and divergent binding sequences: binding specificities of yeast HSFs and human HSF1

The target genes of the heat shock transcription factor (HSF) contain a cis-acting sequence, the heat shock element (HSE), which consists of multiple inverted repeats of the sequence 5'-nGAAn-3'. Using data acquired in this and a previous study, we have identified the HSEs in 59 of 62 target genes of Saccharomyces cerevisiae Hsf1. The Hsf1 protein recognizes continuous and discontinuous repeats of the nGAAn unit; the nucleotide sequences and configuration of the units diverge slightly among functional HSEs. When Schizosaccharomyces pombe HSF was expressed in S. cerevisiae cells, heat shock induced S. pombe HSF to bind to various HSE types, which properly activated transcription from almost all target genes, suggesting that the S. pombe genome also contains divergent HSEs. Human HSF1 induced the heat shock response via HSEs with continuous units in S. cerevisiae cells but failed to do so via HSEs with discontinuous units. Binding of human HSF1 to the discontinuous type of HSE was observed in vitro but was significantly inhibited in vivo. These results show that human HSF1 recognizes HSEs in a slightly different way than yeast HSFs and suggest that the configuration of the unit is an important determinant for HSF-HSE interactions.

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.

Cytotoxicity and cell signalling induced by continuous mild hyperthermia in freshly isolated mouse hepatocytes

An increasing body of data has been demonstrating that mammalian cells have elaborate networks of molecular signalling in counteracting heat shock and in developing adaptation to oxidative stress to avoid cell death. However, the precise mechanisms linking heat shock, oxidative stress and cell survival/cell death mechanisms are not yet clearly understood. The purpose of this study was thus to study the time course of hyperthermia-induced oxidative stress and cellular signalling through the activation of heat shock factor 1 (HSF1) and heat shock protein 70 (HSP70), using freshly isolated mouse hepatocytes. The results accomplished in this work demonstrated that mild continuous hyperthermia (41 degrees ) leads to oxidative stress and loss of cellular viability in a time-dependent manner, with significant effects already observed at the first hour of incubation. These toxic effects developed concomitantly with activation of HSF1 and emerged before the formation of HSP70 levels. Thus, although cell signalling was triggered through the transcriptional activation of HSP70 via HSF1, this putative protective process did not modify the trend of hepatotoxic effects mediated by this type of hyperthermic challenging.

HSF1 and constitutively active HSF1 improve vascular endothelial function (heat shock proteins improve vascular endothelial function)

We have been examining the role of heat shock factor 1 (HSF1) in the pleiotropic effects of statins. In parallel studies, we found that statin induces the nuclear translocation of HSF1 and that a decoy oligonucleotide encoding the heat shock element inhibits the statin-induced expression of heat shock protein 70, endothelial nitric oxide synthase (eNOS) and thrombomodulin. Also, in vascular endothelial cells, increases in the expression of human HSF1 corresponded with elevated steady-state levels of eNOS and thrombomodulin and reduced levels of endothelin-1 and plasminogen activator inhibitor-1. We also found that heat shock proteins induced eNOS and thrombomodulin expression and reduced PAI-1 and ET-1 expression. In particular, a combination of HSP70 and HSP90 strongly induced eNOS expression and reduced PAI-1 expression. In the current studies, we generated a constitutively active form of HSF1 and found that it is more effective than the wild-type HSF at inducing thrombomodulin and eNOS expression and decreasing endothelin-1 and plasminogen activator inhibitor-1 expression. These results show that the wild-type and constitutively active forms of HSF1 induce anticoagulation and relaxation factors in vascular endothelial cells and could therefore be used to treat cardiovascular disease.

Inhibition of DNA binding by differential sumoylation of heat shock factors

Covalent modification of proteins by the small ubiquitin-related modifier SUMO regulates diverse biological functions. Sumoylation usually requires a consensus tetrapeptide, through which the binding of the SUMO-conjugating enzyme Ubc9 to the target protein is directed. However, additional specificity determinants are in many cases required. To gain insights into SUMO substrate selection, we have utilized the differential sumoylation of highly similar loop structures within the DNA-binding domains of heat shock transcription factor 1 (HSF1) and HSF2. Site-specific mutagenesis in combination with molecular modeling revealed that the sumoylation specificity is determined by several amino acids near the consensus site, which are likely to present the SUMO consensus motif to Ubc9. Importantly, we also demonstrate that sumoylation of the HSF2 loop impedes HSF2 DNA-binding activity, without affecting its oligomerization. Hence, SUMO modification of the HSF2 loop contributes to HSF-specific regulation of DNA binding and broadens the concept of sumoylation in the negative regulation of gene expression.

Heat-shock cognate 70 is required for the activation of heat-shock factor 1 in mammalian cells

HSF1 (heat-shock factor 1) plays an essential role in mediating the appropriate cellular response to diverse forms of physiological stresses. However, it is not clear how HSF1 is regulated by interacting proteins under normal and stressful conditions. In the present study, Hsc70 (heat-shock cognate 70) was identified as a HSF1-interacting protein using the TAP (tandem affinity purification) system and MS. HSF1 can interact with Hsc70 in vivo and directly in vitro. Interestingly, Hsc70 is required for the regulation of HSF1 during heat stress and subsequent target gene expression in mammalian cells. Moreover, cells transfected with siRNAs (small interfering RNAs) targeted to Hsc70 showed greatly decreased HSF1 activation with expression of HSF1 target genes being dramatically reduced. Finally, loss of Hsc70 expression in cells resulted in an increase in stress-induced apoptosis. These results indicate that Hsc70 is a necessary and critical regulator of HSF1 activities.

Mutated Yeast Heat Shock Transcription Factor Activates Transcription Independently of Hyperphosphorylation

The homotrimeric heat shock transcription factor (HSF) binds to the heat shock element of target genes and regulates transcription in response to various stresses. The Hsf1 protein of Saccharomyces cerevisiae is extensively phosphorylated upon heat shock; a modification that is under positive regulation by its C-terminal regulatory domain (CTM). Hyperphosphorylation has been implicated in gene-specific transcriptional activation. Here, we surveyed genes whose heat shock response is reduced by a CTM mutation. The CTM is indispensable for transcription via heat shock elements bound by a single Hsf1 trimer but is dispensable for transcription via heat shock elements bound by Hsf1 trimers in a cooperative manner. Intragenic mutations located within or near the wing region of the winged helix-turn-helix DNA-binding domain suppress the temperature-sensitive growth phenotype associated with the CTM mutation and enable Hsf1 to activate transcription independently of hyperphosphorylation. Deletion of the wing partially restores the transcriptional defects of the unphosphorylated Hsf1. These results demonstrate a functional link between hyperphosphorylation and the wing region and suggest that this modification is involved in a conformational change of a single Hsf1 trimer to an active form.

The anticancer activity of the fungal metabolite terrecyclic acid A is associated with modulation of multiple cellular stress response pathways

Tumors are dependent on cellular stress responses, in particular the heat shock response, for survival in their hypoxic, acidotic, and nutrient-deprived microenvironments. Using cell-based reporter assays, we have identified terrecyclic acid A (TCA) from Aspergillus terreus, a fungus inhabiting the rhizosphere of Opuntia versicolor of the Sonoran desert, as a small-molecule inducer of the heat shock response that shows anticancer activity. Further characterization suggested that TCA also affects oxidative and inflammatory cellular stress response pathways. The presence of an alpha-methylene ketone moiety suggested that TCA may form adducts with sulfhydryl groups of proteins. Reaction with labile intracellular cysteines was supported by our finding that the glutathione precursor N-acetyl-cysteine protected tumor cells from the cytotoxic effects of TCA whereas the glutathione-depleting agent buthionine sulfoximine enhanced its activity. Related sesquiterpenes have been shown to increase levels of reactive oxygen species (ROS) and to inhibit nuclear factor kappaB (NF-kappaB) transcriptional activity. To assess whether TCA could have similar activities, we used a ROS-sensitive dye and flow cytometry to show that TCA does indeed increase ROS levels in 3LL cells. When tested in cells carrying NF-kappaB reporter constructs, TCA also exhibited concentration-dependent inhibition of cytokine-induced NF-kappaB transcriptional activity. These findings suggest that TCA modulates multiple stress pathways-the oxidative, heat shock, and inflammatory responses-in tumor cells that promote their survival. Small-molecule natural products such as TCA may serve as useful probes for understanding the relationships between these pathways, potentially providing leads for the design of novel and effective anticancer drugs.

Constitutive nuclear import and stress-regulated nucleocytoplasmic shuttling of mammalian heat-shock factor 1

Inducible expression of major cytosolic and nuclear chaperone proteins is mediated by the heat-shock transcription factor HSF1 that is activated by derepressive mechanisms triggered by transient heat stress and sustained proteotoxicity. Despite progress in defining essential aspects of HSF1 regulation, little is known about the cellular dynamics enabling this factor to mediate gene responses to cytosolic stress signals. We report that the inactive, stress-responsive form of HSF1 accumulates in the nucleus due to a relatively potent import signal, which can be recognized by importin-alpha/beta, and simultaneously undergoes continuous nucleocytoplasmic shuttling due to a comparatively weak, nonetheless efficient, export activity not involving the classical exportin-1 pathway. Strikingly, experimental stresses at physiological or elevated temperature reversibly inactivate the export competence of HSF1. Likewise, mutations mimicking stress-induced derepression impair export but not import. These findings are consistent with a dynamic process whereby exported molecules that are derepressed in an inductive cytosolic environment are recollected and pause in the nucleoplasm, replacing progressively the inactive pool. While steady-state nuclear distribution of the bulk of HSF1 ensures a rapid gene response to acute heat stress, our results suggest that the capture in the nucleus of molecules primed for activation in the cytosol may underlie responses to sustained proteotoxicity.

Polo-like kinase 1 phosphorylates heat shock transcription factor 1 and mediates its nuclear translocation during heat stress

Heat shock transcription factor 1 (HSF1) is activated by pathophysiologic stresses and activation leads to an increased cellular level of heat shock proteins (Hsp(s)). Although the activation of HSF1 occurs via multiple stress-induced processes such as hyperphosphorylation, the exact cellular mechanism of HSF1 activation is still unclear. Here we show polo-like kinase 1 (PLK1) and HSF1 interact in vivo using the tandem affinity purification system. Although the interaction between HSF1 and PLK1 is increased by thermal stress, overexpression of PLK1 did not affect HSF1 trimerization or DNA binding activity. This interaction results in the phosphorylation of HSF1 on serine 419 by PLK1. Interestingly, mutation of serine 419 to alanine inhibited heat-stress induced HSF1 nuclear translocation. Our results suggest that the phosphorylation of HSF1 by PLK1 is an essential step for HSF1 nuclear translocation by heat stress.

Identification of a novel class of target genes and a novel type of binding sequence of heat shock transcription factor in Saccharomyces cerevisiae

In response to hyperthermia, heat shock transcription factor (HSF) activates transcription of a set of genes encoding heat shock proteins (HSPs). The promoter regions of HSP genes contain the HSF binding sequence called the heat shock element (HSE), which consists of contiguous inverted repeats of the sequence 5'-nGAAn-3' (where n is any nucleotide). We have constructed an hsf1 mutant of Saccharomyces cerevisiae and analyzed genome-wide changes in heat shock response in the mutant cells. The results have revealed that Hsf1 is necessary for heat-induced transcription of not only HSP but also genes encoding proteins involved in diverse cellular processes such as protein degradation, detoxification, energy generation, carbohydrate metabolism, and maintenance of cell wall integrity. Approximately half of the Hsf1-regulated genes lacked the typical HSE in their promoter regions. Instead, several of these genes have a novel Hsf1 binding sequence that contains three direct repeats of nTTCn (or nGAAn) interrupted by 5 bp. The number and spacing of the repeating units are critical determinants for heat-induced transcription as well as for recognition by Hsf1. In the yeast genome, the presence of the sequence is enriched in Hsf1-regulated genes, suggesting that it is generally used as an HSE in the Hsf1 regulon.

Phosphorylation of the yeast heat shock transcription factor is implicated in gene-specific activation dependent on the architecture of the heat shock element

Heat shock transcription factor (HSF) binds to the heat shock element (HSE) and regulates transcription, where the divergence of HSE architecture provides gene- and stress-specific responses. The phosphorylation state of HSF, regulated by stress, is involved in the activation and inactivation of the transcription activation function. A domain designated as CTM (C-terminal modulator) of the Saccharomyces cerevisiae HSF is required for the activation of genes containing atypical HSE but not typical HSE. Here, we demonstrate that CTM function is conserved among yeast HSFs and is necessary not only for HSE-specific activation but also for the hyperphosphorylation of HSF upon heat shock. Moreover, both transcription and phosphorylation defects due to CTM mutations were restored concomitantly by a set of intragenic suppressor mutations. Therefore, the hyperphosphorylation of HSF is correlated with the activation of genes with atypical HSE but is not involved in that of genes with typical HSE. The function of CTM was circumvented in an HSF derivative lacking CE2, a yeast-specific repression domain. Taken together, we suggest that CTM alleviates repression by CE2, which allows HSF to be heat-inducibly phosphorylated and presume that phosphorylation is a prerequisite for the activator function of HSF when it binds to an atypical HSE.

Genome-wide analysis of the biology of stress responses through heat shock transcription factor

Heat shock transcription factor (HSF) and the promoter heat shock element (HSE) are among the most highly conserved transcriptional regulatory elements in nature. HSF mediates the transcriptional response of eukaryotic cells to heat, infection and inflammation, pharmacological agents, and other stresses. While HSF is essential for cell viability in Saccharomyces cerevisiae, oogenesis and early development in Drosophila melanogaster, extended life span in Caenorhabditis elegans, and extraembryonic development and stress resistance in mammals, little is known about its full range of biological target genes. We used whole-genome analyses to identify virtually all of the direct transcriptional targets of yeast HSF, representing nearly 3% of the genomic loci. The majority of the identified loci are heat-inducibly bound by yeast HSF, and the target genes encode proteins that have a broad range of biological functions including protein folding and degradation, energy generation, protein trafficking, maintenance of cell integrity, small molecule transport, cell signaling, and transcription. This genome-wide identification of HSF target genes provides novel insights into the role of HSF in growth, development, disease, and aging and in the complex metabolic reprogramming that occurs in all cells in response to stress.

Molecular characterization of heat shock-like factor encoded on the human Y chromosome, and implications for male infertility

Azoospermia and oligospermia are major causes of male infertility. Some genes located on the Y chromosome are suggested as candidates. Recently, HSFY, which is similar to the HSF (heat shock transcription factor) family, has been mapped on the human Y chromosome as multicopies. However, newly available sequence data deposited at NCBI shows that only the HSFY gene located on Yq has a long open reading frame containing a HSF-type DNA-binding domain. HSFY is similar to LW-1 on the human X chromosome and a murine HSFY-like sequence (mHSFYL), 4933413G11Rik, on the mouse chromosome 1. LW-1 and mHSFYL have 53% and 70% homology to HSFY for amino acid sequences of their presumed DNA-binding domains, respectively. Comparison of the presumed DNA-binding domains unveiled that the three HSF-like factors, HSFY, LW-1, and mHSFYL, belong to a different class than conventional HSFs. When we screened for deletions on the Yq of males suffering from infertility, we found that HSFY was involved in interstitial deletions on the Y chromosomes for two azoospermic males who had DBY, USP9Y, and DAZ but did not have RBMY located on the AZFb. Expression analysis of HSFY, LW-1, and mHSFYL unveiled that they are expressed predominantly in testis. Furthermore, immunhistochemistry of HSFY in testis showed that its expression is restricted to both Sertoli cells and spermatogenic cells and that it exhibits a stage-dependent translocation from the cytoplasm to the nucleus in spermatogenetic cells during spermatogenesis. These results may suggest that deletion of HSFY is involved in azoospermia or oligospermia.

Activation of heat shock genes is not necessary for protection by heat shock transcription factor 1 against cell death due to a single exposure to high temperatures

Heat shock response, which is characterized by the induction of a set of heat shock proteins, is essential for induced thermotolerance and is regulated by heat shock transcription factors (HSFs). Curiously, HSF1 is essential for heat shock response in mammals, whereas in avian HSF3, an avian-specific factor is required for the burst activation of heat shock genes. Amino acid sequences of chicken HSF1 are highly conserved with human HSF1, but those of HSF3 diverge significantly. Here, we demonstrated that chicken HSF1 lost the ability to activate heat shock genes through the amino-terminal domain containing an alanine-rich sequence and a DNA-binding domain. Surprisingly, chicken and human HSF1 but not HSF3 possess a novel function that protects against a single exposure to mild heat shock, which is not mediated through the activation of heat shock genes. Overexpression of HSF1 mutants that could not bind to DNA did not restore the susceptibility to cell death in HSF1-null cells, suggesting that the new protective role of HSF1 is mediated through regulation of unknown target genes other than heat shock genes. These results uncover a novel role of vertebrate HSF1, which has been masked under the roles of heat shock proteins.