HSP90 interacts with and regulates the activity of heat shock factor 1 in Xenopus oocytes

Heat-shock protein 90 inhibitors as novel cancer chemotherapeutics - an update

Heat-shock protein 90 (Hsp90) is a molecular chaperone whose association is required for stability and function of a growing number of signalling proteins that have been implicated in cancer cell survival, including several mutated proteins that are only found in specific cancers. Furthermore, a growing body of evidence suggests that cancer cells are particularly dependent on Hsp90 for their growth and survival, and, therefore, are more sensitive to the effects of its inhibition than are non-transformed cells and tissues. Several chemically distinct Hsp90 inhibitors have shown encouraging antitumour activity in multiple preclinical model systems, and one Hsp90 inhibitor, the benzoquinone ansamycin 17-allylamino, 17-demethoxygeldanamycin, has completed five Phase I clinical trials, with a number of Phase II trials soon to be underway or in progress. Other Hsp90 inhibitors are either in Phase I clinical trial or under development. This update will focus on how the latest developments in Hsp90 biology may better inform the clinical development of Hsp90 inhibitors.

Turning up the heat: The effects of thermal acclimation on the kinetics of hsp70 gene expression in the eurythermal goby, Gillichthys mirabilis

Most organisms respond to temperature fluctuations by altering the expression of an evolutionarily conserved family of proteins known as heat shock proteins (Hsps). Studies have shown Hsp expression and the activation of HSF1, one of the primary regulators of Hsp transcription, are highly malleable, varying with the recent thermal history of the organism; however, the mechanisms that confer plasticity to the regulation of this ubiquitous response are not well-understood. This study furthers our knowledge in this area by characterizing the activation kinetics of HSF1 and the corresponding transcription of hsp70 in the liver of the eurythermal goby, Gillichthys mirabilis, following a month-long acclimation at 13, 21 or 28 degrees C. Our data revealed HSF1 DNA-binding kinetics varied as a function of acclimation temperature and magnitude/duration of exposure, with gobies acclimated at 21 degrees C exhibiting the most robust response. Hsp70 mRNA followed a similar pattern with induction first occurring in the 13 and 21 degrees C fish, and then most robustly in the 28 degrees C group at 36 degrees C. The hsp70 mRNA induction pattern was corroborated by levels of HSF1 DNA-binding activity in each group and may have been lowest in the 28 degrees C group due to the 2-fold greater levels of hsp70 protein prior to thermal exposure. This study illustrates the integral role of HSF1 as a key regulator of Hsp induction and helps explain the plasticity of this response in ectothermic organisms.

Heat shock-independent induction of multidrug resistance by heat shock factor 1

The screening of two different retroviral cDNA expression libraries to select genes that confer constitutive doxorubicin resistance has in both cases resulted in the isolation of the heat shock factor 1 (HSF1) transcription factor. We show that HSF1 induces a multidrug resistance phenotype that occurs in the absence of heat shock or cellular stress and is mediated at least in part through the constitutive activation of the multidrug resistance gene 1 (MDR-1). This drug resistance phenotype does not correlate with an increased expression of heat shock-responsive genes (heat shock protein genes, or HSPs). In addition, HSF1 mutants lacking HSP gene activation are also capable of conferring multidrug resistance, and only hypophosphorylated HSF1 complexes accumulate in transduced cells. Our results indicate that HSF1 can activate MDR-1 expression in a stress-independent manner that differs from the canonical heat shock-activated mechanism involved in HSP induction. We further provide evidence that the induction of MDR-1 expression occurs at a posttranscriptional level, revealing a novel undocumented role for hypophosphorylated HSF1 in posttranscriptional gene regulation.

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.

Chaperoning oncogenes: Hsp90 as a target of geldanamycin

Heat shock protein 90 (Hsp90) is a molecular chaperone required for the stability and function of a number of conditionally activated and/or expressed signaling proteins, as well as multiple mutated, chimeric, and/or over-expressed signaling proteins, that promote cancer cell growth and/or survival. Hsp90 inhibitors, by interacting specifically with a single molecular target, cause the inactivation, destabilization, and eventual degradation of Hsp90 client proteins, and they have shown promising anti-tumor activity in preclinical model systems. One Hsp90 inhibitor, 17-AAG, has completed Phase I clinical trial and several Phase II trials of this agent are in progress. Hsp90 inhibitors are unique in that, although they are directed toward a specific molecular target, they simultaneously inhibit multiple signaling pathways that frequently interact to promote cancer cell survival. Further, by inhibiting nodal points in multiple overlapping survival pathways utilized by cancer cells, a combination of an Hsp90 inhibitor with standard chemotherapeutic agents may dramatically increase the in vivo efficacy of the standard agent. Hsp90 inhibitors may circumvent the characteristic genetic plasticity that has allowed cancer cells to eventually evade the toxic effects of most molecularly targeted agents. The mechanism-based use of Hsp90 inhibitors, both alone and in combination with other drugs, should be effective toward multiple forms of cancer.

Analysis of molecular chaperones using a Xenopus oocyte protein refolding assay

Heat shock proteins (Hsps) are molecular chaperones that aid in the folding and translocation of protein under normal conditions and protect cellular proteins during stressful situations. A family of Hsps, the small Hsps, can maintain denatured target proteins in a folding-competent state such that they can be refolded and regain biological activity in the presence of other molecular chaperones. Previous assays have employed cellular lysates as a source of molecular chaperones involved in folding. In this chapter, we describe the production and purification of a Xenopus laevis recombinant small Hsp, Hsp30C, and an in vivo luciferase (LUC) refolding assay employing microinjected Xenopus oocytes. This assay tests whether LUC can be maintained in a folding-competent state when heat denatured in the presence of a small Hsp or other molecular chaperone. For example, micro-injection of heat-denatured LUC alone into oocytes resulted in minimal reactivation of enzyme activity. However, LUC heat denatured in the presence of Hsp30C resulted in 100% recovery of enzyme activity after microinjection. The in vivo oocyte refolding system is more sensitive and requires less molecular chaperone than in vitro refolding assays. Also, this protocol is not limited to testing Xenopus molecular chaperones because small Hsps from other organisms have been used successfully.

Glucocorticoid receptor and heat shock factor 1: novel mechanism of reciprocal regulation

Glucocorticoids control a host of bodily responses, ranging from carbohydrate metabolism in the liver to immunity and inflammation in the lymph system. In response to stress, glucocorticoid levels are known to rise-a response thought to provide a protective function against the stress event. It is now understood that the major function of glucocorticoids under stress is to protect not against the stress event itself but against overstimulation by host defenses (e.g., inflammation). Control of these responses is achieved by the glucocorticoid receptor, a member of the steroid receptor transcription factor family. The oldest, most conserved, and most ubiquitous of the stress responses is induced expression of heat shock proteins that act as chaperones against stress-induced denaturation of protein. Expression of heat shock protein genes is controlled by heat shock transcription factor 1. In this work, we review our observations and those of other laboratories demonstrating a relationship between the glucocorticoid and heat shock responses. We show that complex but reciprocal mechanisms of regulation occur between glucocorticoid receptor and heat shock transcription factor 1 and present a model of coordinated action that likely serves to fully reestablish homeostasis following stress.

HEAT SHOCK PROTEIN 90C is a bona fide Hsp90 that interacts with plastidic HSP70B in Chlamydomonas reinhardtii

We report on the molecular and biochemical characterization of HEAT SHOCK PROTEIN 90C (HSP90C), one of the three Hsp90 chaperones encoded by the Chlamydomonas reinhardtii genome. Fractionation experiments indicate that HSP90C is a plastidic protein. In the chloroplast, HSP90C was localized to the soluble stroma fraction, but also to thylakoids and low-density membranes containing inner envelopes. HSP90C is expressed under basal conditions and is strongly induced by heat shock and moderately by light. In soluble cell extracts, HSP90C was mainly found to organize into dimers, but also into complexes of high molecular mass. Also, heterologously expressed HSP90C was mainly found in dimers, but tetramers and fewer monomers were detected, as well. HSP90C exhibits a weak ATPase activity with a Km for ATP of approximately 48 microM and a kcat of approximately 0.71 min(-1). This activity was inhibited by the Hsp90-specific inhibitor radicicol. In coimmunoprecipitation experiments, we found that HSP90C interacts with several proteins, among them plastidic HSP70B. The cellular concentration of HSP70B was found to be 2.9 times higher than that of HSP90C, giving a 4.8:1 stoichiometry of HSP70B monomers to HSP90C dimers. The strong inducibility of HSP90C by heat shock implies a role of the chaperone in stress management. Furthermore, its interaction with HSP70B suggests that, similar to their relatives in cytosol and the endoplasmic reticulum, both chaperones might constitute the core of a multichaperone complex involved in the maturation of specific client proteins, e.g. components of signal transduction pathways.

Protein phosphatase 5 is a negative modulator of heat shock factor 1

The major stress protein transcription factor, heat shock factor (HSF1), is tightly regulated through a multilayered activation-deactivation process involving oligomerization, post-translational modification, and interaction with the heat shock protein (Hsp90)-containing multichaperone complex. Conditions of proteotoxic stress, such as heat shock, trigger reversible assembly of latent HSF1 monomers into DNA-binding homotrimers that bind with high affinity to cognate heat shock elements. Transactivation is a second and independently regulated function of HSF1 that is accompanied by hyperphosphorylation and appears to involve a number of signaling events. Association of HSF1 with Hsp90 chaperone complexes provides additional regulatory complexity, however, not all the co-chaperones have been identified, and the specific molecular interactions throughout the activation/deactivation pathway remain to be determined. Here we demonstrate that protein phosphatase 5 (PP5), a tetratricopeptide domain-containing component of Hsp90-steroid receptor complexes, functions as a negative modulator of HSF1 activity. Physical interactions between PP5 and HSF1-Hsp90 complexes were observed in co-immunoprecipitation and gel mobility supershift experiments. Overexpression of PP5 or activation of endogenous phosphatase activity resulted in diminished HSF1 DNA binding and transcriptional activities, and accelerated recovery. Conversely, microinjection of PP5 antibodies, or inhibition of its phosphatase activity in vivo, significantly delayed trimer disassembly after heat shock. Inhibition of PP5 activity did not activate HSF1 in unstressed cells. These results indicate that PP5 is a negative modulator of HSF1 activity.

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.

Characterization of goldfish heat shock protein-30 induced upon severe heat shock in cultured cells

Temperature-dependent changes of growth rate and protein components were investigated for primary cultured cells derived from goldfish caudal fin. When the culture temperature was shifted from 20 degrees C to 35 degrees C and 40 degrees C, the growth rate was increased at 35 degrees C as compared with that at 20 degrees C, but no cell growth was observed at 40 degrees C. The differential scanning calorimetry demonstrated the onset of the endothermic reaction for goldfish cellular components at 40 degrees C. Therefore, the temperature shift to 40 degrees C was found to be of severe heat shock for goldfish cultured cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that, although expression of 70-kDa components was slightly induced at 35 degrees C, the temperature shift to 40 degrees C markedly induced the expression of the 30-kDa component in addition to that of 70-kDa component. The N-terminal amino acid sequencing identified the 30- and 70-kDa components to be heat shock protein (Hsp)-30 and Hsp70, respectively. Northern blot analysis revealed that the enhanced Hsp30 messenger ribonucleic acid (mRNA) levels were only observed at 40 degrees C, whereas Hsp70 mRNA was slightly accumulated at 35 degrees C. These results indicated that Hsp30 might have important functions under severe heat stress condition.

Heat-shock protein 90 inhibitors as novel cancer chemotherapeutic agents

Heat-shock protein 90 (Hsp90) is a molecular chaperone whose association is required for the stability and function of multiple mutated, chimeric and overexpressed signalling proteins that promote cancer cell growth and/or survival. Hsp90 client proteins include mutated p53, Bcr-Abl, Raf-1, Akt, HER2/Neu (ErbB2) and hypoxia inducible factor-1alpha (HIF-1alpha). Through specific interaction with a single molecular target, Hsp90 inhibitors cause the destabilisation and eventual degradation of Hsp90 client proteins, and they have shown promising antitumour activity in preclinical model systems. One Hsp90 inhibitor, 17-allylamino-geldanamycin (17-AAG), is currently in Phase I clinical trials. Hsp90 inhibitors are unique in that, although they are directed towards a specific molecular target, they simultaneously inhibit multiple signalling pathways on which cancer cells depend for growth and survival. Further, because of the unique effect that Hsp90 inhibition has on cancer cells, combination of an Hsp90 inhibitor with standard chemotherapeutic agents may dramatically increase the in vivo efficacy of the standard agent.

Involvement of the TRAP-1 homologue, Dd-TRAP1, in spore differentiation during Dictyostelium development

Tumor necrosis factor receptor-associated protein 1 (TRAP1) is a member of the molecular chaperone HSP90 (90-kDa heat shock protein) family. We have previously demonstrated that Dictyostelium discoideum TRAP1 (Dd-TRAP1) synthesized at the vegetative growth phase is retained during the whole course of D. discoideum development, and that at the multicellular slug stage, it is located in prespore-specific vacuoles (PSVs) of prespore cells as well as in the cell membrane and mitochondria. Thereupon, we examined the function of Dd-TRAP1 in prepore and spore differentiation, using Dd-TRAP1-knockdown cells (TRAP1-RNAi cells) produced by the RNA interference method. As was expected, Dd-TRAP1 contained in the PSV was found to be exocytosed during sporulation to constitute the outer-most layer of the spore cell wall. In the TRAP1-RNAi cells, PSV formation and therefore prespore differentiation were significantly impaired, particularly under a heat stress condition. Although the TRAP1-RNAi cells formed apparently normal-shaped spores with a cellulosic wall, the spores were less resistant to heat and detergent treatments, as compared with those of parental MB35 cells derived from Ax-2 cells. These findings strongly suggest that Dd-TRAP1 may be closely involved in late development including spore differentiation, as well as in early development as realized by its induction of prestarvation response.

Celastrols as inducers of the heat shock response and cytoprotection

Alterations in protein folding and the regulation of conformational states have become increasingly important to the functionality of key molecules in signaling, cell growth, and cell death. Molecular chaperones, because of their properties in protein quality control, afford conformational flexibility to proteins and serve to integrate stress-signaling events that influence aging and a range of diseases including cancer, cystic fibrosis, amyloidoses, and neurodegenerative diseases. We describe here characteristics of celastrol, a quinone methide triterpene and an active component from Chinese herbal medicine identified in a screen of bioactive small molecules that activates the human heat shock response. From a structure/function examination, the celastrol structure is remarkably specific and activates heat shock transcription factor 1 (HSF1) with kinetics similar to those of heat stress, as determined by the induction of HSF1 DNA binding, hyperphosphorylation of HSF1, and expression of chaperone genes. Celastrol can activate heat shock gene transcription synergistically with other stresses and exhibits cytoprotection against subsequent exposures to other forms of lethal cell stress. These results suggest that celastrols exhibit promise as a new class of pharmacologically active regulators of the heat shock response.

Radicicol activates heat shock protein expression and cardioprotection in neonatal rat cardiomyocytes

Heat shock proteins (HSPs) constitute an endogenous cellular defense mechanism against environmental stresses. In the past few years, studies have shown that overexpression of HSPs can protect cardiac myocytes against ischemia-reperfusion injury. In an attempt to increase the HSPs in cardiac tissue, we used the compound radicicol that activates HSP expression by binding to the HSP 90 kDa (HSP90). HSP90 is the main component of the cytosolic molecular chaperone complex, which has been implicated in the regulation of the heat shock factor 1 (HSF1). HSF1 is responsible for the transcriptional activation of the heat shock genes. In the present study, we show that radicicol induces HSP expression in neonatal rat cardiomyocytes, and this increase in HSPs confers cardioprotection to these cardiomyocytes. We also show that radicicol induction of the HSP and cardioprotection is dependent on the inhibition of HSP90 in cardiomyocytes. These results indicate that modulation of the active HSP90 protein level plays an important role in cardioprotection. Therefore, compounds, such as radicicol and its possible derivatives that inhibit the function of HSP90 in the cell may represent potentially useful cardioprotective agents.

Heat shock proteins in the regulation of apoptosis: new strategies in tumor therapy: a comprehensive review

Heat shock proteins (Hsp) form the most ancient defense system in all living organisms on earth. These proteins act as molecular chaperones by helping in the refolding of misfolded proteins and assisting in their elimination if they become irreversibly damaged. Hsp interact with a number of cellular systems and form efficient cytoprotective mechanisms. However, in some cases, wherein it is better if the cell dies, there is no reason for any further defense. Programmed cell death is a widely conserved general phenomenon helping in many processes involving the reconstruction of multicellular organisms, as well as in the elimination of old or damaged cells. Here, we review some novel elements of the apoptotic process, such as its interrelationship with cellular senescence and necrosis, as well as bacterial apoptosis. We also give a survey of the most important elements of the apoptotic machinery and show the various modes of how Hsp interact with the apoptotic events in detail. We review caspase-independent apoptotic pathways and anoikis as well. Finally, we show the emerging variety of pharmacological interventions inhibiting or, just conversely, inducing Hsp and review the emergence of Hsp as novel therapeutic targets in anticancer protocols.

Role of Hsp17.4-CII as coregulator and cytoplasmic retention factor of tomato heat stress transcription factor HsfA2

HsfA2 is a heat stress (hs)-induced Hsf in peruvian tomato (Lycopersicon peruvianum) and the cultivated form Lycopersicon esculentum. Due to the high activator potential and the continued accumulation during repeated cycles of heat stress and recovery, HsfA2 becomes a dominant Hsf in thermotolerant cells. The formation of heterooligomeric complexes with HsfA1 leads to nuclear retention and enhanced transcriptional activity of HsfA2. This effect seems to represent one part of potential molecular mechanisms involved in its activity control. As shown in this paper, the activity of HsfA2 is also controlled by a network of nucleocytoplasmic small Hsps influencing its solubility, intracellular localization and activator function. By yeast two-hybrid interaction and transient coexpression studies in tobacco (Nicotiana plumbaginifolia) mesophyll protoplasts, we found that tomato (Lycopersicon esculentum) Hsp17.4-CII acts as corepressor of HsfA2. Given appropriate conditions, both proteins together formed large cytosolic aggregates which could be solubilized in presence of class CI sHsps. However, independent of the formation of aggregates or of the nucleocytoplasmic distribution of HsfA2, its transcriptional activity was specifically repressed by interaction of Hsp17.4-CII with the C-terminal activator domain. Although not identical in all aspects, the situation with the highly expressed, heat stress-inducible Arabidopsis HsfA2 was found to be principally similar. In corresponding reporter assays its activity was repressed in presence of AtHsp17.7-CII but not of AtHsp17.6-CII or LpHsp17.4-CII.

Magnitude and Duration of Thermal Stress Determine Kinetics ofhspGene Regulation in the GobyGillichthys mirabilis

The stress-induced transcription of heat shock genes is controlled by heat shock transcription factor 1 (HSF1), which becomes activated in response to heat and other protein denaturants. In previous research on the eurythermal goby Gillichthys mirabilis, thermal activation of HSF1 was shown to vary as a function of acclimation temperature, suggesting the mechanistic importance of HSF1 activation to the plasticity of heat shock protein (Hsp) induction temperature. We examined the effect of season on the thermal activation of HSF1 in G. mirabilis, as well as the relative kinetics of HSF1 activation and Hsp70 mRNA production at ecologically relevant temperatures. There was no predictable seasonality in the thermal activation of HSF1, perhaps due to the existence of stressors, in addition to heat, acting in the field. Concentrations of Hsp70, a negative regulator of HSF1, as well as those of HSF1, varied with collection date. The rapidity of HSF1 activation and of Hsp70 mRNA synthesis increased with laboratory exposure temperature. Furthermore, Hsp70 mRNA production was more sustained at 35 degrees C than at 30 degrees C. Therefore, both the magnitude and the duration of a heat shock are important in determining the intensity of heat shock gene induction.

DAXX interacts with heat shock factor 1 during stress activation and enhances its transcriptional activity

DAXX, a modulator of apoptosis and a repressor of basal transcription, was identified in a two-hybrid screen as a protein capable of interacting with a trimeric form of human heat shock factor 1 (HSF1). In human cells, DAXX interacted with HSF1 essentially only during stress, i.e., when factor trimerization occurred. Several lines of experimentation suggested that DAXX is an important mediator of HSF1 activation: (i) overexpression of DAXX enhanced basal transactivation competence of HSF1 in the absence of a stress; (ii) a DAXX fragment exerted dominant-negative effects on HSF1 activation by different types of stress; (iii) induction of heat shock or stress protein (HSP)70 by heat stress was defective in a cell line lacking functional DAXX; and (iv) RNA interference depletion of DAXX also substantially reduced heat induction of HSF1 activity and HSP70 expression. HSF1 transactivation competence is repressed by an HSP90-containing multichaperone complex that interacts with trimeric factor. Overexpressed HSF1, known to be largely trimeric, only marginally increased HSF1 activity on its own but potentiated the activating effect of DAXX overexpression. Expression of a nonnative protein capable of competing for multichaperone complex also synergistically enhanced activation of HSF1 by DAXX. These observations suggest a model in which DAXX released from its nuclear stores during stress opposes repression of HSF1 transactivation competence by multichaperone complex through its interaction with trimerized HSF1. Our identification of DAXX as a mediator of HSF1 activation raises the question whether DAXX produces some of its pleiotropic effects through modulation of HSP levels.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Coactivator ASC-2 mediates heat shock factor 1-mediated transactivation dependent on heat shock

Upon exposure to elevated temperatures, mammalian cells increase the expression of the heat shock proteins (HSP) through activation of the heat shock factor 1 (HSF1). Since most transcription factors require coactivators for efficient transcriptional activity, we tried to identify the coactivator(s) that interacts with and modulates the activities of HSF1. In vitro glutathione S-transferase (GST) pull-down assay revealed that HSF1 strongly interacts with activating signal cointegrator (ASC)-2 and weakly with cyclic adenosine monophosphate responsive element binding protein (CBP). We also show that cotransfection of ASC-2, but not CBP, potentiates HSF1-mediated transactivation based on its cognate element (heat shock element, HSE) linked to luciferase reporter. The molecular interaction of HSF1 and ASC-2 was stimulated by heat shock in cells and the overexpression of HSF1-interacting domain of ASC-2 inhibited the specific induced protein association and HSF1-mediated transactivation. Taking these results together, we suggest that ASC-2 in a novel coactivator for HSF1 and heat shock stress may contribute the strong active transcription complex through sequential recruitment of HSF1 and ASC-2.

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).

Geldanamycin reduces necrotic and apoptotic injury due to oxygen-glucose deprivation in astrocytes

Recent data show that geldanamycin can protect the brain against stroke in vivo, and this may be due to induction of heat shock proteins. Our previous results show that heat shock protein 70 expression by retroviral transfection protects astrocytes from necrotic injury by combined oxygen-glucose deprivation, an in vitro model of ischemia. This study tested the ability of geldanamycin to protect astrocytes from either necrotic or apoptotic injury induced by oxygen-glucose deprivation. Astrocytes were pre-treated with 0.1 microgram ml-1 geldanamycin in the medium 4, 8, or 16 h before as well as during oxygen-glucose deprivation. Increased protein levels of heat shock protein 70 were observed after 8 h pre-treatment with geldanamycin and increased further at 16 h pre-treatment, as detected by immunoblotting. Geldanamycin pre-treatment protected mature astrocytes from necrotic cell death and young astrocytes from apoptotic death. Geldanamycin protection of astrocytes against in vitro ischemia is likely due to upregulation of heat shock protein 70.

Formation of nuclear stress granules involves HSF2 and coincides with the nucleolar localization of Hsp70

The heat-shock response is characterized by the activation of heat-shock transcription factor 1 (HSF1), followed by increased expression of heat-shock proteins (Hsps). The stress-induced subnuclear compartmentalization of HSF1 into nuclear stress granules has been suggested to be an important control step in the regulation of stress response and cellular homeostasis in human cells. In this study, we demonstrate that the less-well characterized HSF2 interacts physically with HSF1 and is a novel stress-responsive component of the stress granules. Based on analysis of our deletion mutants, HSF2 influences to the localization of HSF1 in stress granules. Moreover, our results indicate that the stress granules are dynamic structures and suggest that they might be regulated in an Hsp70-dependent manner. The reversible localization of Hsp70 in the nucleoli strictly coincides with the presence of HSF1 in stress granules and is dramatically suppressed in thermotolerant cells. We propose that the regulated subcellular distribution of Hsp70 is an important regulatory mechanism of HSF1-mediated heat shock response.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.

Between genotype and phenotype: protein chaperones and evolvability

Protein chaperones direct the folding of polypeptides into functional proteins, facilitate developmental signalling and, as heat-shock proteins (HSPs), can be indispensable for survival in unpredictable environments. Recent work shows that the main HSP chaperone families also buffer phenotypic variation. Chaperones can do this either directly through masking the phenotypic effects of mutant polypeptides by allowing their correct folding, or indirectly through buffering the expression of morphogenic variation in threshold traits by regulating signal transduction. Environmentally sensitive chaperone functions in protein folding and signal transduction have different potential consequences for the evolution of populations and lineages under selection in changing environments.

Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery

Nearly 100 proteins are known to be regulated by hsp90. Most of these substrates or "client proteins" are involved in signal transduction, and they are brought into complex with hsp90 by a multiprotein hsp90/hsp70-based chaperone machinery. In addition to binding substrate proteins at the chaperone site(s), hsp90 binds cofactors at other sites that are part of the heterocomplex assembly machinery as well as immunophilins that connect assembled substrate*hsp90 complexes to protein-trafficking systems. In the 5 years since we last reviewed this subject, much has been learned about hsp90 structure, nucleotide-binding, and cochaperone interactions; the most important concept is that ATP hydrolysis by an intrinsic ATPase activity results in a conformational change in hsp90 that is required to induce conformational change in a substrate protein. The conformational change induced in steroid receptors is an opening of the steroid-binding cleft so that it can be accessed by steroid. We have now developed a minimal system of five purified proteins-hsp90, hsp70, Hop, hsp40, and p23- that assembles stable receptor*hsp90 heterocomplexes. An hsp90*Hop*hsp70*hsp40 complex opens the cleft in an ATP-dependent process to produce a receptor*hsp90 heterocomplex with hsp90 in its ATP-bound conformation, and p23 then interacts with the hsp90 to stabilize the complex. Stepwise assembly experiments have shown that hsp70 and hsp40 first interact with the receptor in an ATP-dependent reaction to produce a receptor*hsp70*hsp40 complex that is "primed" to be activated to the steroid-binding state in a second ATP-dependent step with hsp90, Hop, and p23. Successful use of the five-protein system with other substrates indicates that it can assemble signal protein*hsp90 heterocomplexes whether the substrate is a receptor, a protein kinase, or a transcription factor. This purified system should facilitate understanding of how eukaryotic hsp70 and hsp90 work together as essential components of a process that alters the conformations of substrate proteins to states that respond in signal transduction.

Thermal acclimation changes DNA-binding activity of heat shock factor 1(HSF1) in the gobyGillichthys mirabilis: implications for plasticity in the heat-shock response in natural populations

The intracellular build-up of thermally damaged proteins following exposure to heat stress results in the synthesis of a family of evolutionarily conserved proteins called heat shock proteins (Hsps) that act as molecular chaperones, protecting the cell against the aggregation of denatured proteins. The transcriptional regulation of heat shock genes by heat shock factor 1(HSF1) has been extensively studied in model systems, but little research has focused on the role HSF1 plays in Hsp gene expression in eurythermal organisms from broadly fluctuating thermal environments. The threshold temperature for Hsp induction in these organisms shifts with the recent thermal history of the individual but the mechanism by which this plasticity in Hsp induction temperature is achieved is unknown. We examined the effect of thermal acclimation on the heat-activation of HSF1 in the eurythermal teleost Gillichthys mirabilis. After a 5-week acclimation period (at 13, 21 or 28°C) the temperature of HSF1 activation was positively correlated with acclimation temperature. HSF1 activation peaked at 27°C in fish acclimated to 13°C, at 33°C in the 21°C group, and at 36°C in the 28°C group. Concentrations of both HSF1 and Hsp70 in the 28°C group were significantly higher than in the colder acclimated fish. Plasticity in HSF1 activation may be important to the adjustable nature of the heat shock response in eurythermal organisms and the environmental control of Hsp gene expression.

Mutation or deletion of the C-terminal tail affects the function and structure of Xenopus laevis small heat shock protein, hsp30

Small heat shock proteins (shsps) act as molecular chaperones by preventing heat-induced aggregation and unfolding of cellular proteins by a mechanism that is still unclear. Previously we found that the C-terminal end of Xenopus shsp, hsp30C (30C), was essential for optimal chaperone activity. Examination of the C-terminal tail of 30C revealed that it had a net negative charge. Involvement of this negative charge in chaperone activity was assessed by the creation of two mutants, D209G (Asp converted to the more neutrally charged and less polar Gly at position 209) and D209/213G (Asp to Gly at position 209 and 213). Compared to 30C and D209G, D209/213G was impaired in inhibiting heat-induced citrate synthase aggregation. In rabbit reticulocyte lysate and Xenopus oocyte microinjection refolding assays the mutants were not as efficient as 30C in maintaining heat-treated luciferase in a folding competent state. Circular dichroism analysis revealed that D209G was similar in secondary structure to 30C whereas D209/213G displayed a loss of alpha-helical-like and beta-sheet structure. Also, C-terminal truncation of 30C or 30D (an hsp30 isoform) resulted in a loss of secondary structure and function. This study clearly shows that mutation of aspartic acid residues in the C-terminal end of hsp30 or its truncation disrupts secondary structure and impairs its chaperone activity.

Heat shock induces mini-Cajal bodies in the Xenopus germinal vesicle

Cajal bodies are evolutionarily conserved nuclear organelles that are believed to play a central role in assembly of RNA transcription and processing complexes. Although knowledge of Cajal body composition and behavior has greatly expanded in recent years, little is known about the molecules and mechanisms that lead to the formation of these organelles in the nucleus. The Xenopus oocyte nucleus or germinal vesicle is an excellent model system for the study of Cajal bodies, because it is easy to manipulate and it contains 50-100 Cajal bodies with diameters up to 10 μm. In this study we show that numerous mini-Cajal bodies (less than 2 μm in diameter) form in the germinal vesicle after oocytes recover from heat shock. The mechanism for heat shock induction of mini-Cajal bodies is independent of U7 snRNA and does not require transcription or import of newly translated proteins from the cytoplasm. We suggest that Cajal bodies originate by self-organization of preformed components, preferentially on the surface of B-snurposomes.

Geldanamycin induces heat shock proteins in brain and protects against focal cerebral ischemia

Geldanamycin (GA), a benzoquinone ansamycin, binds Hsp90 in vitro, releases heat shock factor (HSF1) and induces heat shock proteins (Hsps). Because viral and transgenic overexpression of Hsps protects cells against ischemia in vitro, we hypothesized that GA would protect brain from focal ischemia by inducing Hsps in vivo. Adult male Sprague-Dawley rats were subjected to 2-hour middle cerebral artery occlusions (MCAO) using the suture technique followed by 22-h reperfusions. GA or vehicle was injected into the lateral cerebral ventricles (i.c.v) 24 h before ischemia. Geldanamycin at 1 microg/kg decreased infarct volumes by 55.7% (p < 0.01) and TUNEL-positive cells by 30% in cerebral cortex. GA also improved behavioral outcomes (p < 0.01) and reduced brain edema (p < 0.05). Western blots showed that the 1 microg/kg GA dose induced Hsp70 and Hsp25 protein 8.2-fold and 2.7-fold, respectively, by 48 h following administration. Immunocytochemistry showed that GA induced Hsp70 in neurons and Hsp25 in glia and arteries in cortex, hippocampus, hypothalamus, and other brain regions. GA reduced co-immunoprecipitation of HSF1 with Hsp90 in brain tissue homogenates, promoted HSE-binding of HSF in brain nuclear extracts using gel shift assays, and increased luciferase reporter gene transcription for the Hsp70 promoter in PC12 cells. The data show that geldanamycin protects brain from focal ischemia and that this may be due, at least in part, to geldanamycin stimulation of heat shock gene transcription.

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

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

Suppressive effects of ansamycins on inducible nitric oxide synthase expression and the development of experimental autoimmune encephalomyelitis

The production of nitric oxide by the inflammatory isoform of nitric oxide synthase (NOS2) in brain glial cells is thought to contribute to the causes and development of neurological diseases and trauma. We previously demonstrated that activation of a heat shock response (HSR) by hyperthermia reduced NOS2 expression in vitro, and in vivo attenuated the clinical and histological symptoms of the demyelinating disease experimental autoimmune encephalomyelitis (EAE; Heneka et al. [2001] J. Neurochem. 77:568-579). Benzoquinoid ansamycins are fungal-derived antibiotics with tyrosine kinase inhibitory properties, and which also induce a HSR by allowing activation of HS transcription factor HSF1. We now show that two members of this class of drugs (geldanamycin and 17-allylamino-17-demethoxygeldanamycin) also induce a HSR in primary rat astrocytes and rat C6 glioma cells. Both drugs dose-dependently reduced nitrite accumulation, NOS2 steady-state mRNA levels, and the cytokine-dependent activation of a rat 2.2-kB NOS2 promoter construct stably expressed in C6 cells. These inhibitory effects were partially reversed by quercetin, a bioflavonoid which prevents HSF1 binding to DNA and thus attenuates the HSR. Ansamycins increased mRNA levels of the inhibitory IkappaBalpha protein, suggesting that inhibition of NFkappaB activation could contribute to their suppressive effects. Finally, in C57BL/6 mice actively immunized to develop EAE, a single injection of geldanamycin at 3 days after immunization reduced disease onset by over 50%. These results indicate that ansamycins can exert potent anti-inflammatory effects on brain glial cells which may provide therapeutic benefit in neuroinflammatory diseases.

Xenopus small heat shock proteins, Hsp30C and Hsp30D, maintain heat- and chemically denatured luciferase in a folding-competent state

In this study we characterized the chaperone functions of Xenopus recombinant Hsp30C and Hsp30D by using an in vitro rabbit reticulocyte lysate (RRL) refolding assay system as well as a novel in vivo Xenopus oocyte microinjection assay. Whereas heat- or chemically denaturated luciferase (LUC) did not regain significant enzyme activity when added to RRL or microinjected into Xenopus oocytes, compared with native LUC, denaturation of LUC in the presence of Hsp30C resulted in a reactivation of enzyme activity up to 80-100%. Recombinant Hsp30D, which differs from Hsp30C by 19 amino acids, was not as effective as its isoform in preventing LUC aggregation or maintaining it in a folding-competent state. Removal of the first 17 amino acids from the N-terminal region of Hsp30C had little effect on its ability to maintain LUC in a folding-competent state. However, deletion of the last 25 residues from the C-terminal end dramatically reduced Hsp30C chaperone activity. Coimmunoprecipitation and immunoblot analyses revealed that Hsp30C remained associated with heat-denatured LUC during incubation in reticulocyte lysate and that the C-terminal mutant exhibited reduced affinity for unfolded LUC. Finally, we found that Hsc70 present in RRL interacted only with heat-denatured LUC bound to Hsp30C. These findings demonstrate that Xenopus Hsp30 can maintain denatured target protein in a folding-competent state and that the C-terminal end is involved in this function.

Evidence for a mechanism of repression of heat shock factor 1 transcriptional activity by a multichaperone complex

In the absence of stress, human heat shock factor 1 (hHSF1) is in its unactivated form. hHSF1 polypeptide is in a dynamic heterocomplex with Hsp90 and is incapable of specifically binding DNA. When cells are stressed, heterocomplex assembly is disrupted. Unbound hHSF1 homotrimerizes, acquires DNA binding activity, and concentrates in the nucleus, but remains transcriptionally inactive. A subsequent reaction converts this inactive, trimeric form into the active, hyperphosphorylated transcription factor. Subsequent to the stressful event, hHSF1 is deactivated and eventually returned to its unactivated form. Evidence is presented herein that trimeric hHSF1 has the propensity to dynamically associate with an Hsp90-immunophilin-p23 complex through its regulatory domain. Formation of this heterocomplex results in repression of the transcriptional activity of trimeric hHSF1. Stress-denatured proteins effectively compete with trimeric hHSF1 for Hsp90-immunophilin-p23 complex, counteracting assembly of the heterocomplex and repression of hHSF1 transcriptional activity. This repression mechanism may be required for a proportional transcriptional response to stress. Formation of the heterocomplex may also represent the first step toward returning the hHSF1 to its unactivated form.

Microhabitats, thermal heterogeneity, and patterns of physiological stress in the rocky intertidal zone

Thermal stress has been considered to be among the most important determinants of organismal distribution in the rocky intertidal zone. Yet our understanding of how body temperatures experienced under field conditions vary in space and time, and of how these temperatures translate into physiological performance, is still rudimentary. We continuously monitored temperatures at a site in central California for a period of two years, using loggers designed to mimic the thermal characteristics of mussels, Mytilus californianus. Model mussel temperatures were recorded on both a horizontal and a vertical, north-facing microsite, and in an adjacent tidepool. We periodically measured levels of heat shock proteins (Hsp70), a measure of thermal stress, from mussels at each microsite. Mussel temperatures were consistently higher on the horizontal surface than on the vertical surface, and differences in body temperature between these sites were reflected in the amount of Hsp70. Seasonal peaks in extreme high temperatures ("acute" high temperatures) did not always coincide with peaks in average daily maxima ("chronic" high temperatures), suggesting that the time history of body temperature may be an important factor in determining levels of thermal stress. Temporal patterns in body temperature during low tide were decoupled from patterns in water temperature, suggesting that water temperature is an ineffective metric of thermal stress for intertidal organisms. This study demonstrates that spatial and temporal variability in thermal stress can be highly complex, and "snapshot" sampling of temperature and biochemical indices may not always be a reliable method for defining thermal stress at a site.

Geldanamycin restores a defective heat shock response in vivo

Induced expression of heat shock proteins (Hsps) plays a central role in promoting cellular survival after environmental and physiological stress. We have previously shown that scrapie-infected mouse neuroblastoma (ScN2a) cells fail to induce the expression of Hsp72 and Hsp28 after various stress conditions. Here we present evidence that this impaired stress response is due to an altered regulation of HSF1 activity. Upon stress in ScN2a cells, HSF1 was converted into hyperphosphorylated trimers but failed to acquire transactivation competence. A kinetic analysis of HSF1 activation revealed that in ScN2a cells trimer formation after stress was efficient, but disassembly of trimers proceeded much faster than in the uninfected cell line. Geldanamycin, a Hsp90-binding drug, significantly delayed disassembly of HSF1 trimers after a heat shock and restored stress-induced expression of Hsp72 in ScN2a cells. Heat-induced Hsp72 expression required geldanamycin to be present; following removal of the drug ScN2a cells again lost their ability to mount a stress response. Thus, our studies show that a defective stress response can be pharmacologically restored and suggest that the HSF1 deactivation pathway may play an important role in the regulation of Hsp expression.

Heat shock protein 90 homeostasis controls stage differentiation in Leishmania donovani

The differentiation of Leishmania parasites from the insect stage, the promastigote, toward the pathogenic mammalian stage, the amastigote, is triggered primarily by the rise in ambient temperature encountered during the insect-to-mammal transmission. We show here that inactivation of heat shock protein (Hsp) 90, with the use of the drugs geldanamycin or radicicol, mimics transmission and induces the differentiation from the promastigote to the amastigote stage. Geldanamycin also induces a growth arrest of cultured promastigotes that can be forestalled by overexpression of the cytoplasmic Hsp90. Moreover, we demonstrate that Hsp90 serves as a feedback inhibitor of the cellular heat shock response in Leishmania. Our results are consistent with Hsp90 homeostasis serving as cellular thermometer for these primitive eukaryotes, controlling both the heat shock response and morphological differentiation.

Stress-specific activation and repression of heat shock factors 1 and 2

Vertebrate cells express a family of heat shock transcription factors (HSF1 to HSF4) that coordinate the inducible regulation of heat shock genes in response to diverse signals. HSF1 is potent and activated rapidly though transiently by heat shock, whereas HSF2 is a less active transcriptional regulator but can retain its DNA binding properties for extended periods. Consequently, the differential activation of HSF1 and HSF2 by various stresses may be critical for cells to survive repeated and diverse stress challenges and to provide a mechanism for more precise regulation of heat shock gene expression. Here we show, using a novel DNA binding and detection assay, that HSF1 and HSF2 are coactivated to different levels in response to a range of conditions that cause cell stress. Above a low basal activity of both HSFs, heat shock preferentially activates HSF1, whereas the amino acid analogue azetidine or the proteasome inhibitor MG132 coactivates both HSFs to different levels and hemin preferentially induces HSF2. Unexpectedly, we also found that heat shock has dramatic adverse effects on HSF2 that lead to its reversible inactivation coincident with relocalization from the nucleus. The reversible inactivation of HSF2 is specific to heat shock and does not occur with other stressors or in cells expressing high levels of heat shock proteins. These results reveal that HSF2 activity is negatively regulated by heat and suggest a role for heat shock proteins in the positive regulation of HSF2.

Heat shock proteins and cardiovascular pathophysiology

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

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

Eukaryotic heat shock transcription factors (HSF) regulate an evolutionarily conserved stress-response pathway essential for survival against a variety of environmental and developmental stresses. Although the highly similar HSF family members have distinct roles in responding to stress and activating target gene expression, the mechanisms that govern these roles are unknown. Here we identify a loop within the HSF1 DNA-binding domain that dictates HSF isoform specific DNA binding in vitro and preferential target gene activation by HSF family members in both a yeast transcription assay and in mammalian cells. These characteristics of the HSF1 loop region are transposable to HSF2 and sufficient to confer DNA-binding specificity, heat shock inducible HSP gene expression and protection from heat-induced apoptosis in vivo. In addition, the loop suppresses formation of the HSF1 trimer under basal conditions and is required for heat-inducible trimerization in a purified system in vitro, suggesting that this domain is a critical part of the HSF1 heat-stress-sensing mechanism. We propose that this domain defines a signature for HSF1 that constitutes an important determinant for how cells utilize a family of transcription factors to respond to distinct stresses.

Expression, activity, and subcellular localization of the Yin Yang 1 transcription factor in Xenopus oocytes and embryos

Yin Yang 1 (YY1) is a multifunctional transcription factor that acts as an activator, repressor, or initiator of transcription of numerous cellular and viral genes. Previous studies in tissue culture model systems suggest YY1 plays a role in development and differentiation in multiple cell types, but the biological role of YY1 in vertebrate oocytes and embryos is not well understood. Here we analyzed expression, activity, and subcellular localization profiles of YY1 during Xenopus laevis development. Abundant levels of YY1 mRNA and protein were detected in early stage oocytes and in all subsequent stages of oocyte and embryonic development through to swimming larval stages. The DNA binding activity of YY1 was detected only in early oocytes (stages I and II) and in embryos after the midblastula transition (MBT), which suggested that its potential to modulate gene expression may be specifically repressed in the intervening period of development. Experiments to determine transcriptional activity showed that addition of YY1 recognition sites upstream of the thymidine kinase promoter had no stimulatory or repressive effect on basal transcription in oocytes and post-MBT embryos. Although the apparent transcriptional inactivity of YY1 in oocytes could be explained by the absence of DNA binding activity at this stage of development, the lack of transcriptional activity in post-MBT embryos was not expected given the ability of YY1 to bind its recognition elements. Subsequent Western blot and immunocytochemical analyses showed that YY1 is localized in the cytoplasm in oocytes and in cells of developing embryos well past the MBT. These findings suggest a novel mode of YY1 regulation during early development in which the potential transcriptional function of the maternally expressed factor is repressed by cytoplasmic localization.

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

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

The heat shock response reduces myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis in mice

The stress response (SR) can block inflammatory gene expression by preventing activation of transcription factor nuclear factor-kappa B (NF-kappaB). As inflammatory gene expression contributes to the pathogenesis of demyelinating diseases, we tested the effects of the SR on the progression of the demyelinating disease experimental autoimmune encephalomyelitis (EAE). EAE was actively induced in C57BL/6 mice using an encephalitogenic myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide. Whole body hyperthermia was used to induce a heat shock response (HSR) in immunized mice 2 days after the booster MOG(35-55) peptide injection. The HSR reduced the incidence of EAE by 70%, delayed disease onset by 6 days, and attenuated disease severity. The HSR attenuated leukocyte infiltration into CNS assessed by quantitation of perivascular infiltrates, and by reduced staining for CD4 and CD25 immunopositive T-cells. T-cell activation, assessed by the production of interferon gamma (IFNgamma) in response to MOG(35-55), was also decreased by the HSR. The HSR reduced inflammatory gene expression in the brain that normally occurs during EAE, including the early increase in RANTES (regulated on activation of normal T-cell expressed and secreted) expression, and the later expression of the inducible form of nitric oxide synthase. The early activation of transcription factor NF-kappaB was also blocked by the HSR. The finding that the SR reduces inflammation in the brain and the clinical severity of EAE opens a novel therapeutic approach for prevention of autoimmune diseases.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

The distribution of heat shock proteins in the nervous system of the unstressed mouse embryo suggests a role in neuronal and non-neuronal differentiation

Heat shock proteins (Hsps) act as molecular chaperones and are generally constitutively expressed in the absence of stress. Hsps are also inducible by a variety of stressors whose effects could be disastrous on the brain. It has been shown previously that Hsps are differentially expressed in glial and neuronal cells, as well as in the different structures of the brain. This differential expression has been related to specific functions distinct from their general chaperone function, such as intracellular transport. We investigated here the constitutive expression of 5 Hsps (the small Hsp, Hsp25, the constitutive Hsc70 and Hsp90beta, the mainly inducible Hsp70 and Hsp90alpha), and of a molecular chaperone, TCP-1alpha during mouse nervous system development. We analyzed, by immunohistochemistry, their distribution in the central nervous system and in the ganglia of the peripheral nervous system from day 9.5 (E9.5) to day 17.5 (E17.5) of gestation. Hsps are expressed in different cell classes (neuronal, glial, and vascular). The different proteins display different but often overlapping patterns of expression in different regions of the developing nervous system, suggesting unique roles at different stages of neural maturation. Their putative function in cell remodeling during migration or differentiation and in protein transport is discussed. Moreover we consider Hsp90 function in cell signaling and the role of Hsp25 in apoptosis protection.

Modulation of Drosophila heat shock transcription factor activity by the molecular chaperone DROJ1

Heat shock transcription factors (HSFs) play important roles in the cellular response to physiological stress signals. To examine the control of HSF activity, we undertook a yeast two-hybrid screen for proteins interacting with Drosophila HSF. DROJ1, the fly counterpart of the human heat shock protein HSP40/HDJ1, was identified as the dominant interacting protein (15 independent isolates from 58 candidates). Overexpression of DROJ1 in Drosophila SL2 cells delays the onset of the heat shock response. Moreover, RNA interference involving transfection of SL2 cells with double-stranded droj1 RNA depletes the endogenous level of DROJ1 protein, leading to constitutive activation of endogenous heat shock genes. The induction level, modest when DROJ1 was depleted alone, reached maximal levels when DROJ1 and HSP70/HSC70, or DROJ1 and HSP90, were depleted concurrently. Chaperone co-depletion was also correlated with strong induction of the DNA binding activity of HSF. Our findings support a model in which synergistic interactions between DROJ1 and the HSP70/HSC70 and HSP90 chaperones modulate HSF activity by feedback repression.

Remodeling of yeast genome expression in response to environmental changes

We used genome-wide expression analysis to explore how gene expression in Saccharomyces cerevisiae is remodeled in response to various changes in extracellular environment, including changes in temperature, oxidation, nutrients, pH, and osmolarity. The results demonstrate that more than half of the genome is involved in various responses to environmental change and identify the global set of genes induced and repressed by each condition. These data implicate a substantial number of previously uncharacterized genes in these responses and reveal a signature common to environmental responses that involves approximately 10% of yeast genes. The results of expression analysis with MSN2/MSN4 mutants support the model that the Msn2/Msn4 activators induce the common response to environmental change. These results provide a global description of the transcriptional response to environmental change and extend our understanding of the role of activators in effecting this response.

Implication of a small GTPase Rac1 in the activation of c-Jun N-terminal kinase and heat shock factor in response to heat shock

Heat shock induces c-Jun N-terminal kinase (JNK) activation as well as heat shock protein (HSP) expression through activation of the heat shock factor (HSF), but its signal pathway is not clearly understood. Since a small GTPase Rac1 has been suggested to participate in the cellular response to stresses, we examined whether Rac1 is involved in the heat shock response. Here we show that moderate heat shock (39-41 degrees C) induces membrane translocation of Rac1 and membrane ruffling in a Rac1-dependent manner. In addition, Rac1N17, a dominant negative mutant of Rac1, significantly inhibited JNK activation by heat shock. Since Rac1V12 was able to activate JNK, it is suggested that heat shock may activate JNK via Rac1. Similar inhibition by Rac1N17 of HSF activation in response to heat shock was observed. However, inhibitory effects of Rac1N17 on heat shock-induced JNK and HSF activation were reduced as the heat shock temperature increased. Rac1N17 also inhibited HSF activation by l-azetidine-2-carboxylic acid, a proline analog, and heavy metals (CdCl)), suggesting that Rac1 may be linked to HSF activation by denaturation of polypeptides in response to various proteotoxic stresses. However, Rac1N17 did not prevent phosphorylation of HSF1 in response to these proteotoxic stresses. Interestingly, a constitutively active mutant Rac1V12 did not activate the HSF. Therefore, Rac1 activation may be necessary, but not sufficient, for heat shock-inducible HSF activation and HSP expression, or otherwise a signal pathway(s) involving Rac1 may be indirectly involved in the HSF activation. In sum, we suggest that Rac1 may play a critical role(s) in several aspects of the heat shock response.