Elevated expression of heat shock factor (HSF) 2A stimulates HSF1-induced transcription during stress

Interaction of HSF1 and HSF2 with the Hspa1b promoter in mouse epididymal spermatozoa

The Hspa1b gene is one of the first genes expressed after fertilization, with expression observed in the male pronucleus as early as the one-cell stage of embryogenesis. This expression can occur in the absence of stress and is initiated during the minor zygotic genome activation. There is a significant reduction in the number of embryos developing to the blastocyte stage when HSPA1B levels are depleted, which supports the importance of this protein for embryonic viability. However, the mechanism responsible for allowing expression of Hspa1b during the minor zygotic genome activation (ZGA) is unknown. In this report, we investigated the role of HSF1 and HSF2 in bookmarking Hspa1b during late spermatogenesis. Western blot results show that both HSF1 and HSF2 are present in epididymal spermatozoa, and immunofluorescence analysis revealed that some of the HSF1 and HSF2 proteins in these cells overlap the 4',6'-diamidino-2-phenylindole-stained DNA region. Results from chromatin immunoprecipitation assays showed that HSF1, HSF2, and SP1 are bound to the Hspa1b promoter in epididymal spermatozoa. Furthermore, we observed an increase in HSF2 binding to the Hspa1b promoter in late spermatids versus early spermatids, suggesting a likely period during spermatogenesis when transcription factor binding could occur. These results support a model in which the binding of HSF1, HSF2, and SP1 to the promoter of Hspa1b would allow the rapid formation of a transcription-competent state during the minor ZGA, thereby allowing Hspa1b expression.

Heat shock proteins and toll-like receptors

Researchers have only just begun to elucidate the relationship between heat shock proteins (HSP) and Toll-like receptors (TLR). HSP were originally described as an intracellular molecular chaperone of naïve, aberrantly folded, or mutated proteins and primarily implicated as a cytoprotective protein when cells are exposed to stressful stimuli. However, recent studies have ascribed novel functions to the Hsp70 protein depending on its localization: Surface-bound Hsp70 specifically activate natural killer (NK) cells, while Hsp70 released into the extracellular milieu specifically bind to Toll-like receptors (TLR) 2 and 4 on antigen-presenting cells (APC) and exerts immunoregulatory effects, including upregulation of adhesion molecules, co-stimulatory molecule expression, and cytokine and chemokine release-a process known as the chaperokine activity of Hsp70. This chapter discusses the most recent advances in the understanding of heat shock protein (HSP) and TLR interactions in general and highlights recent findings that demonstrate Hsp70 is a ligand for TLR and its biological significance.

Chaperone regulation of the heat shock protein response

The heat shock protein response appears to be triggered primarily by nonnative proteins accumulating in a stressed cell and results in increased expression of heat shock proteins (HSPs). Many heat shock proteins prevent protein aggregation and participate in refolding or elimination of misfolded proteins in their capacity as chaperones. Even though several mechanisms exist to regulate the abundance of cytosolic and nuclear chaperones, activation of heat shock transcription factor 1 (HSF1) is an essential aspect of the heat shock protein response. HSPs and co-chaperones that are assembled into multichaperone complexes regulate HSF1 activity at different levels. HSP90-containing multichaperone complexes appear to be the most relevant repressors of HSF1 activity. Because HSP90-containing multichaperone complexes interact not only specifically with client proteins including HSF1 but also generically with nonnative proteins, the concentration of nonnative proteins influences assembly on HSF1 of HSP90-containing complexes that repress activation, and may play a role in inactivation, of the transcription factor. Proteins that are unable to achieve stable tertiary structures and remain chaperone substrates are targeted for proteasomal degradation through polyubiquitination by co-chaperone CHIP. CHIP can activate HSF1 to regulate the protein quality control system that balances protection and degradation of chaperone substrates.

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.

Heat shock factors at a crossroad between stress and development

Organisms must be able to sense and respond rapidly to changes in their environment in order to maintain homeostasis and survive. Induction of heat shock proteins (Hsps) is a common cellular defense mechanism for promoting survival in response to various stress stimuli. Heat shock factors (HSFs) are transcriptional regulators of Hsps, which function as molecular chaperones in protecting cells against proteotoxic damage. Mammals have three different HSFs that have been considered functionally distinct: HSF1 is essential for the heat shock response and is also required for developmental processes, whereas HSF2 and HSF4 are important for differentiation and development. Specifically, HSF2 is involved in corticogenesis and spermatogenesis, and HSF4 is needed for maintenance of sensory organs, such as the lens and the olfactory epithelium. Recent evidence, however, suggests a functional interplay between HSF1 and HSF2 in the regulation of Hsp expression under stress conditions. In lens formation, HSF1 and HSF4 have been shown to have opposite effects on gene expression. In this chapter, we present the different roles of the mammalian HSFs as regulators of cellular stress and developmental processes. We highlight the interaction between different HSFs and discuss the discoveries of novel target genes in addition to the classical Hsps.

Heat Shock Factor 2 (HSF2) Contributes to Inducible Expression of hsp Genes through Interplay with HSF1

The heat shock response is a defense reaction activated by proteotoxic damage induced by physiological or environmental stress. Cells respond to the proteotoxic damage by elevated expression of heat shock proteins (Hsps) that function as molecular chaperones and maintain the vital homeostasis of protein folds. Heat shock factors (HSFs) are the main transcriptional regulators of the stress-induced expression of hsp genes. Mammalian HSF1 was originally identified as the transcriptional regulator of the heat shock response, whereas HSF2 has not been implicated a role in the stress response. Previously, we and others have demonstrated that HSF1 and HSF2 interact through their trimerization domains, but the functional consequence of this interaction remained unclear. We have now demonstrated on chromatin that both HSF1 and HSF2 were able to bind the hsp70 promoter not only in response to heat shock but also during hemin-induced differentiation of K562 erythroleukemia cells. In both cases an intact HSF1 was required in order to reach maximal levels of promoter occupancy, suggesting that HSF1 influences the DNA binding activity of HSF2. The functional consequence of the HSF1-HSF2 interplay was demonstrated by real-time reverse transcription-PCR analyses, which showed that HSF2 was able to modulate the HSF1-mediated expression of major hsp genes. Our results reveal, contrary to the predominant model, that HSF2 indeed participates in the transcriptional regulation of the heat shock response.

Overexpression of the heat-shock protein 70 is associated to imatinib resistance in chronic myeloid leukemia

Imatinib is an effective therapy for chronic myeloid leukemia (CML), a myeloproliferative disorder characterized by the expression of the recombinant oncoprotein Bcr-Abl. In this investigation, we studied an imatinib-resistant cell line (K562-r) generated from the K562 cell line in which none of the previously described mechanisms of resistance had been detected. A threefold increase in the expression of the heat-shock protein 70 (Hsp70) was detected in these cells. This increase was not associated to heat-shock transcription factor-1 (HSF-1) overexpression or activation. RNA silencing of Hsp70 decreased dramatically its expression (90%), and was accompanied by a 34% reduction in cell viability. Overexpression of Hsp70 in the imatinib-sensitive K562 line induced resistance to imatinib as detected by a large reduction in cell death in the presence of 1 muM of imatinib. Hsp70 level was also increased in blast cells of CML patients resistant to imatinib, whereas the level remained low in responding patients. Taken together, the results demonstrate that overexpression of Hsp70 can lead to both in vitro and in vivo resistance to imatinib in CML cells. Moreover, the overexpression of Hsp70 detected in imatinib-resistant CML patients supports this mechanism and identifies potentially a marker and a therapeutic target of CML evolution.

Cloning of zebrafish (Danio rerio) heat shock factor 2 (HSF2) and similar patterns of HSF2 and HSF1 mRNA expression in brain tissues

The transcriptional activation of heat shock protein (HSP) genes is initiated by the binding of heat shock factors (HSFs) to heat shock elements (HSEs) located at the promotor regions. Multiple HSFs exist in larger eukaryotic organisms in order to sense various types of stress signals. Here we report the cloning of zebrafish (Danio rerio) HSF2 (zHSF2) cDNA (GenBank accession number AF412833 ) that has an open reading frame of 1470 nucleotides, encoding a polypeptide of 489 amino acids. Domain architecture analysis of the deduced zHSF2 sequence revealed the presence of a DNA-binding domain at the N-terminal end, an adjacent oligomerization domain and a vertebrate heat shock transcription factor domain. Amino acid alignment showed a 70% sequence identity between zHSF2 and human or mouse HSF2, while only a 45% identity was found between zHSF1a and zHSF2. Recombinant zHSF2 bound with a very high specificity to HSEs arranged as inverted arrays of 5'-nGAAn-3', as replacing one GAA with GTA almost abolished the formation of HSE-binding complex. Similar patterns of zHSF1a and zHSF2 mRNA expression in the brain regions of developing zebrafish were detected by whole mount in situ hybridization and paraffin sectioning, suggesting that most of the two HSF gene activities were controlled by a common mechanism during the embryonic development of zebrafish. The levels of both zHSF1a and zHSF2 mRNA in zebrafish tissues were moderately increased after heat stress.

STRESS PROTEIN EXPRESSION KINETICS

In all organisms there is an elevated synthesis of a select family of "stress proteins" in response to a broad array of environmentally driven stress vectors including elevated or depressed temperature, changes in pH, treatment with many classes of chemicals, ischemia, desiccation, and UV irradiation. The presence of stress proteins, often termed heat shock proteins (HSPs), has been recognized for more than four decades, and there is an extensive literature that addresses the structure and properties of HSPs, their function in normal and injured cells and tissues, and the molecular mechanisms of HSP expression in response to stress. Owing to this substantial aggregate of research, there is a growing appreciation of the potential for manipulating the magnitude and timing of elevated HSP expression to achieve targeted therapeutic objectives. The successful realization of this potential requires an understanding of the kinetics of the HSP expression process in response to sublethal stress regimens along with the ability to model the governing events in the process to design practical protocols that could be applied in therapeutic settings. Significant progress has been made in recent years in defining and developing capabilities in these two areas.

Up-regulation of the clusterin gene after proteotoxic stress: implication of HSF1-HSF2 heterocomplexes

Clusterin is a secreted protein chaperone up-regulated in several pathologies, including cancer and neurodegenerative diseases. The present study shows that accumulation of aberrant proteins, caused by the proteasome inhibitor MG132 or the incorporation of the amino acid analogue AZC (L-azetidine-2-carboxylic acid), increased both clusterin protein and mRNA levels in the human glial cell line U-251 MG. Consistently, MG132 treatment was capable of stimulating a 1.3 kb clusterin gene promoter. Promoter deletion and mutation studies revealed a critical MG132-responsive region between -218 and -106 bp, which contains a particular heat-shock element, named CLE for 'clusterin element'. Gel mobility-shift assays demonstrated that MG132 and AZC treatments induced the formation of a protein complex that bound to CLE. As shown by supershift and chromatin-immunoprecipitation experiments, CLE is bound by HSF1 (heat-shock factor 1) and HSF2 upon proteasome inhibition. Furthermore, co-immunoprecipitation assays indicated that these two transcription factors interact. Gel-filtration analyses revealed that the HSF1-HSF2 heterocomplexes bound to CLE after proteasome inhibition have the same apparent mass as HSF1 homotrimers after heat shock, suggesting that HSF1 and HSF2 could heterotrimerize. Therefore these studies indicate that the clusterin is a good candidate to be part of a cellular defence mechanism against neurodegenerative diseases associated with misfolded protein accumulation or decrease in proteasome activity.

Clustering of heat-shock factors

Clusterin is a ubiquitous glycoprotein found in most physiological fluids and tissues. Although not fully understood, the function of clusterin seems to be related to its ability to bind a wide variety of molecules. Since clusterin has been found associated with extracellular protein aggregates, a role as a molecular chaperone has been proposed. In this issue of the Biochemical Journal, Le Dréan and colleagues demonstrate an up-regulation of clusterin in neuronal cells exposed to proteotoxic stress that results in unfolded protein accumulation and proteasome impairment, both commonly associated with neurodegenerative diseases. Interestingly, expression of clusterin was found to be regulated by two members of the HSF (heat-shock factor) family, HSF1 and HSF2, which possibly form a trimeric complex on the clusterin promoter. The study proposes clusterin as a player in a cellular defence mechanism against harmful protein accumulation, and highlights the importance of elucidating further the exact role of clusterin and the intriguing interaction between HSF1 and HSF2.

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.

HSFs in development

Heat shock transcription factors, as well as heat shock proteins, are involved in different steps in differentiation and development, in addition to their role in adaptation to stress. This has already been demonstrated in the case of the single heat shock factor present in Drosophila. Over the last 6 years, similar observations have accumulated from the progressive inactivation of the different hsf genes in mammals, the use of double-null animals, and the slow characterization of their complex phenotypes. Although these studies are not yet complete, the data so far can be used to draw some conclusions. All hsf genes contribute to development in mammals and to normal functions at the adult stage, by controlling the expression of Hsp and non-Hsp genes. Reproduction, the immune response and aging are the processes that are the most deeply affected. An attractive hypothesis would be that these new functions have been recruited during evolution in order to coordinate these processes: HSFs may occupy a central place in the trade off that organisms make between reproduction and maintenance, in response to the variations in the environment.

Heat shock response: lessons from mouse knockouts

Organisms are endowed with integrated regulatory networks that transduce and amplify incoming signals into effective responses, ultimately imparting cell death and/or survival pathways. As a conserved cytoprotective mechanism from bacteria to humans, the heat shock response has been established as a paradigm for inducible gene expression, stimulating the interests of biologists and clinicians alike to tackle fundamental questions related to the molecular switches, lineage-specific requirements, unique and/or redundant roles, and even efforts to harness the response therapeutically. Gene targeting studies in mice confirm HSF1 as a master regulator required for cell growth, embryonic development, and reproduction. For example, sterility of Hsf1-null female but not null male mice established strict requirements for maternal HSF1 expression in the oocyte. Yet Hsf2 knockouts by three independent laboratories have not fully clarified the role of mammalian HSF2 for normal development, fertility, and postnatal neuronal function. In contrast, Hsf4 knockouts have provided a consistent demonstration for HSF4's critical role during lens formation. In the future, molecular analysis of HSF knockout mice will bring new insights to HSF interactions, foster better understanding of gene regulation at the genome level, lead to a better integration of the HSF pathway in life beyond heat shock, the classical laboratory challenge.

Expression of heat shock proteins and heat shock protein messenger ribonucleic acid in human prostate carcinoma in vitro and in tumors in vivo

Heat shock proteins (HSPs) are thought to play a role in the development of cancer and to modulate tumor response to cytotoxic therapy. In this study, we have examined the expression of hsf and HSP genes in normal human prostate epithelial cells and a range of prostate carcinoma cell lines derived from human tumors. We have observed elevated expressions of HSF1, HSP60, and HSP70 in the aggressively malignant cell lines PC-3, DU-145, and CA-HPV-10. Elevated HSP expression in cancer cell lines appeared to be regulated at the post-messenger ribonucleic acid (mRNA) levels, as indicated by gene chip microarray studies, which indicated little difference in heat shock factor (HSF) or HSP mRNA expression between the normal and malignant prostate cell lines. When we compared the expression patterns of constitutive HSP genes between PC-3 prostate carcinoma cells growing as monolayers in vitro and as tumor xenografts growing in nude mice in vivo, we found a marked reduction in expression of a wide spectrum of the HSPs in PC-3 tumors. This decreased HSP expression pattern in tumors may underlie the increased sensitivity to heat shock of PC-3 tumors. However, the induction by heat shock of HSP genes was not markedly altered by growth in the tumor microenvironment, and HSP40, HSP70, and HSP110 were expressed abundantly after stress in each growth condition. Our experiments indicate therefore that HSF and HSP levels are elevated in the more highly malignant prostate carcinoma cells and also show the dominant nature of the heat shock-induced gene expression, leading to abundant HSP induction in vitro or in vivo.

Inducible heat shock protein 70 kD and inducible nitric oxide synthase in hemorrhage/resuscitation-induced injury

Inducible head shock protein 70 kD (HSP-70i) has been shown to protect cells, tissues, and organs from harmful assaults in vivo and in vitro experimental models. Hemorrhagic shock followed by resuscitation is the principal cause of death among trauma patients and soldiers in the battlefield. Although the underlying mechanisms are still not fully understood, it has been shown that nitric oxide (NO) overproduction and inducible nitric oxide synthase (iNOS) overexpression play important roles in producing injury caused by hemorrhagic shock including increases in polymorphonuclear neutrophils (PMN) infiltration to injured tissues and leukotriene B(4) (LTB(4)) generation. Moreover, transcription factors responsible for iNOS expression are also altered by hemorrhage and resuscitation. It has been evident that either up-regulation of HSP-70i or down-regulation of iNOS can limit tissue injury caused by ischemia/reperfusion or hemorrhage/resuscitation. In our laboratory, geldanamycin, a member of ansamycin family, has been shown to induce HSP- 70i overexpression and then subsequently to inhibit iNOS expression, to reduce cellular caspase-3 activity, and to preserve cellular ATP levels. HSP-70i is found to couple to iNOS and its transcription factor. Therefore, the complex formation between HSP-70i and iNOS may be a novel mechanism for protection from hemorrhage/resuscitation-induced injury.

SUMO wrestling with type 1 diabetes

Post-translational modification of proteins by phosphorylation, methylation, acetylation, or ubiquitylation represent central mechanisms through which various biological processes are regulated. Reversible covalent modification (i.e., sumoylation) of proteins by the small ubiquitin-like modifier (SUMO) has also emerged as an important mechanism contributing to the dynamic regulation of protein function. Sumoylation has been linked to the pathogenesis of a variety of disorders including Alzheimer's disease (AD), Huntington's disease (HD), and type 1 diabetes (T1D). Advances in our understanding of the role of sumoylation suggested a novel regulatory mechanism for the regulation of immune responsive gene expression. In this review, we first update recent advances in the field of sumoylation, then specifically evaluate its regulatory role in several key signaling pathways for immune response and discuss its possible implication in T1D pathogenesis.

Characterization of the genomic structures and selective expression profiles of nine class I small heat shock protein genes clustered on two chromosomes in rice (Oryza sativa L.)

The cytosolic class I small heat shock proteins (sHSP-CI) represent the most abundant sHSP in plants. Here, we report the characterization and the expression profile of nine members of the sHSP-CI gene family in rice (Oryza sativa Tainung No.67), of which Oshsp16.9A, Oshsp16.9B, Oshsp16.9C, Oshsp16.9D and Oshsp17.9B are clustered on chromosome 1, and Oshsp17.3, Oshsp17.7, Oshsp17.9A and Oshsp18.0 are clustered on chromosome 3. Oshsp17.3 and Oshsp18.0 are linked by a 356-bp putative bi-directional promoter. Individual gene products were identified from the protein subunits of a heat shock complex (HSC) and from in vitro transcription/ translation products by two-dimensional gel electrophoreses (2-DE). All sHSP-CI genes except Oshsp17.9B were induced strongly after a 2-h heat shock treatment. The genes on chromosome 3 were induced rapidly at 32 and 41 degrees C, whereas those on chromosome 1 were induced slowly by similar conditions. Seven of these genes, except Oshsp16.9D and Oshsp17.9B, were induced by arsenite (As), but only genes on chromosome 3 were strongly induced by azetidine-2-carboxylic acid (Aze, a proline analog) and cadmium (Cd). A similar expression profile of all sHSP-CI genes at a lower level was evoked by ethanol, H2O2 and CuCl2 treatments. Transient expression assays of the promoter activity by linking to GUS reporter gene also supported the in vivo selective expression of the sHSP-CI genes by Aze treatment indicating the differential induction of rice sHSP-CI genes is most likely regulated at the transcriptional level. Only Oshsp16.9A abundantly accumulated in mature dry seed also suggested additionally prominent roles played by this HSP in development.

Interactions between extracellular signal-regulated protein kinase 1, 14-3-3epsilon, and heat shock factor 1 during stress

Cytoprotection during the heat shock response is a complex phenomenon involving multiple inducible mechanisms. We have examined the interaction of two key molecular components in the response, heat shock transcription factor 1 (HSF1) and extracellular signal regulated protein kinase (ERK). Whereas both HSF1 and ERK are required to protect cells against apoptosis, ERK activation is paradoxically antagonistic to trans-activation of hsp promoters by HSF1 and HSP accumulation during heat shock. We have found that the two pathways interact directly and that heat shock causes the physical association of ERK1 with HSF1, an interaction that promotes the kinase activity of ERK in heat-shocked cells. ERK activation results in the recruitment of the phosphoserine binding protein 14-3-3epsilon in a manner dependent on previous HSF1 phosphorylation by ERK. The effects of 14-3-3epsilon binding on HSF1 were complex, however, depending on extracellular conditions, in that HSF1-14-3-3 binding at 37 degrees C led to the cytoplasmic sequestration and repression of HSF1, whereas heat shock overrode these effects and caused quantitative nuclear localization of HSF1. Although the effects of 14-3-3epsilon binding to HSF1 were overridden acutely by stress, during recovery from heat shock, 14-3-3epsilon association again led to enhanced cytoplasmic localization of HSF1, implicating a role for ERK/14-3-3epsilon in HSF1 deactivation in recovering cells. Association of HSF1 with ERK and 14-3-3epsilon during heat shock may thus modulate the amplitude of the response and lead to efficient termination of HSP expression on resumption of growth conditions.

Expression of a Dominant Negative Heat Shock Factor-1 Construct Inhibits Aneuploidy in Prostate Carcinoma Cells*

Recent studies have implicated heat shock proteins (HSP) and heat shock transcription factor 1 (HSF1) in tumor progression. We have examined the role of HSF1 in the malignant phenotype of PC-3 prostate carcinoma cells. We have developed a dominant negative construct of HSF1 that antagonizes transcription from HSP promoters and results in the depletion of intracellular HSP 70. Our studies indicate that expression of DN-HSF1 dramatically alters the DNA content of PC-3 cells (derived from p53 null prostatic carcinoma) and inhibits aneuploidy in these cells. This effect is due to prolonged expression of DN-HSF1, and transient expression of the dominant negative factor from an inducible promoter failed to cause the effect. Inhibition of aneuploidy in p53 null PC-3 cells by DN-HSF1 expression was recapitulated by expression within the cells of wild type p53. Furthermore, cells expressing DN-HSF1 showed a profound inhibition in the development of aneuploidy when exposed to chemical agents that disrupt the mitotic spindle and prevent progression through metaphase. Inhibition of aneuploidy in PC-3 cells expressing DN-HSF1 was associated with delayed breakdown of cyclin B1 compared with controls, consistent with a role for wild type HSF1 in the regulation of cyclin B1 degradation, a key step in the control of mitosis. Our experiments therefore demonstrate that HSF1 plays a functional role in cancer cells under nonstress conditions and influences cell cycle behavior and progression through mitosis and promotes the development of the aneuploid state.

Intracellular trafficking of heat shock factor 2

HSF2 is an enigmatic member of the heat shock factor family, identified in the homeotherm classes of birds and mammals. We report the characterization of HSF2 from an evolutionary ancient vertebrate, the fish rainbow trout (rtHSF2). rtHSF2 appears closely related to its mammalian counterparts at structural and functional levels. The conservation of the distinctive features of HSF2 from fish to human suggests that it should ensure important biological functions, not redundant with those of HSF1. Proteasome inhibition, reported as a potent stimulator of HSF2, leads to the stabilization and to a striking nuclear trafficking of rtHSF2-GFP fusion protein. Upon treatment with the proteasome inhibitor MG132, rtHSF2-GFP accumulates into PML nuclear bodies (NBs) independently of its sumoylation and, if expressed at moderate level, moves to nucleoli. The translocation of rtHSF2-GFP from NBs to nucleoli is greatly favored by overexpression of the heat shock protein Hsp70. The mammalian counterpart mouse HSF2 (mHSF2) also exhibited changes in intracellular distribution upon MG132 treatment. mHSF2 partitioned between a juxtanuclear area that we characterized as an aggresome and the nucleoli. These relocalizations are likely to reflect common structural changes of mouse and trout HSF2 upon activation.

A M55V polymorphism in a novel SUMO gene (SUMO-4) differentially activates heat shock transcription factors and is associated with susceptibility to type I diabetes mellitus

Three SUMO (small ubiquitin-related modifier) genes have been identified in humans, which tag proteins to modulate subcellular localization and/or enhance protein stability and activity. We report the identification of a novel intronless SUMO gene, SUMO-4, that encodes a 95-amino acid protein having an 86% amino acid homology with SUMO-2. In contrast to SUMO-2, which is highly expressed in all of the tissues examined, SUMO-4 mRNA was detected mainly in the kidney. A single nucleotide polymorphism was detected in SUMO-4, substituting a highly conserved methionine with a valine residue (M55V). In HepG2 (liver carcinoma) cells transiently transfected with SUMO-4 expression vectors, Met-55 was associated with the elevated levels of activated heat shock factor transcription factors as compared with Val-55, whereas the levels of NF-kappaB were suppressed to an identical degree. The SUMO-4M (Met) variant is associated with type I diabetes mellitus susceptibility in families (p = 4.0 x 10(-4)), suggesting that it may be involved in the pathogenesis of type I diabetes.

Phenotypic characterization of mouse embryonic fibroblasts lacking heat shock factor 2

In murine cells, the heat shock response is regulated by a transcription factor, HSF1, which triggers the transcription of heat shock genes. HSF2 has been shown to be involved in meiosis and mouse brain development. We characterized the effects of the absence of HSF2 in mouse embryonic fibroblasts (MEFs). The temperature threshold of the heat shock response appeared lowered in Hsf2(-/-) MEFS as monitored by the synthesis of heat shock protein HSP70. In contrast to unstressed wild type MEFS, HSP70 and HSF1 are localized in the nucleus of unstressed Hsf2(-/-) MEFS, a characteristic of stressed cells. HSF1 is not activated for DNA-binding at unstressed temperature in Hsf2(-/-) MEFS. Therefore, the absence of HSF2 induces some but not all of the characteristics of the stress response. In addition, Hsf2(-/-) MEFS exhibited proliferation defects, altered morphology, remodeling of the fibronectin network.

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.

Transcriptional Regulation of the Metazoan Stress Protein Response

This review provides an updated account of the regulation of the metazoan stress protein response. Where indicated, observations made with yeasts are also included. However, a discussion of the plant stress protein response is intentionally omitted (for a review, see 1). The stress protein response, as discussed hereafter, is understood to relate to the response by virtually all cells to heat and other stressors that results in the induced expression of so-called heat shock or stress genes. The protein products of these genes localize largely to the cytoplasm, nucleus, or organelles. An analogous response controls the expression of related genes, whose products reside in the endoplasmic reticulum. The response, termed ER stress response or unfolded protein response, is mediated by a separate regulation system that is not discussed in this review. Note, however, that recent work suggests the existence of commonalities between the regulatory systems controlling the stress protein and ER stress responses (2).

Identification of Xenopus heat shock transcription factor-2: conserved role of sumoylation in regulating deoxyribonucleic acid-binding activity of heat shock transcription factor-2 proteins

Heat shock transcription factor (Hsf)-1 and Hsf2 are members of the heat shock factor (HSF) protein family involved in heat shock protein (hsp) gene regulation, a regulation that is critical for the ability of cells to survive exposure to stress conditions. Although the role of Hsf1 in binding and activating transcription of hsp gene promoters in response to cell stress is well established, how Hsf2 enhances stress-induced hsp expression is not understood. To gain an insight into the critical conserved features of the regulation and function of Hsf2, we have identified and characterized the Hsf2 protein from Xenopus laevis. We found that, similar to its human counterpart, Xenopus Hsf2 is sumoylated at lysine 82 and that, as it does in human Hsf2, the modification event of the small ubiquitin-related modifier 1 functions to increase the deoxyribonucleic acid-binding activity of this transcription factor in Xenopus. These results indicate that sumoylation is an evolutionarily conserved modification of Hsf2 proteins, supporting the position of this modification as a critical regulator of Hsf2 function.