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

Cellular stress and RNA splicing

In response to physical and chemical stresses that affect protein folding and, thus, the execution of normal metabolic processes, cells activate gene-expression strategies aimed at increasing their chance of survival. One target of several stressing agents is pre-mRNA splicing, which is inhibited upon heat shock. Recently, the molecular basis of this splicing inhibition has begun to emerge. Interestingly, different mechanisms seem to be in place to block constitutive pre-mRNA splicing and to affect alternative splicing regulation. This could be important to modulate gene expression during recovery from stress. Thus, pre-mRNA splicing emerges as a central mechanism to integrate cellular and metabolic stresses into gene-expression profiles.

Novel high-throughput profiling of human transcription factors and its use for systematic pathway mapping

Transcription factors (TFs) are crucial components of regulatory networks that control gene transcription. Current TF assays are limited to the analysis of a single TF or require TF-specific antibodies. Here we report the Single Primer Amplification assisted Oligonucleotide Array-based Transcription Factor Assay (SPA-OATFA) which can directly analyze the binding activities of 240 human TFs simultaneously. Examining early events during serum-stimulation of HeLa cells as a model, we demonstrated the utility of SPA-OATFA combined with whole genome gene expression to systematically map the temporal activation of signaling pathways. Both TFs known to function in this stimulation response such as EGR1 and AP1 and new TFs such as HSF1 were identified. This information, combined with mRNA profiling, provided novel insights into the activities of regulatory pathways, and illustrates the potential of SPA-OATFA in detailed systems biology analysis of cell responses.

Expression of heat shock transcription factors and heat shock protein 72 in rat retina after intravitreal injection of low dose N-methyl-D-aspartate

The heat shock response is a genetically well-ordered process for cell to generate heat shock protein (HSP). Various stressors can trigger the response through heat shock transcriptional factor (HSF) regulation. Recent studies demonstrated that preconditioning of N-methyl-d-aspartate (NMDA) at non-lethal levels has neuroprotective effects, but the exact mechanisms are unclear. We hypothesize that the protective mechanisms of NMDA preconditioning could involve HSP expression. To understand the regulatory mechanisms of HSP under stress, we examined the expression of Hsp72, HSF1 and HSF2 in the adult rat retina after intravitreal injection of NMDA. Retinal ganglion cell (RGC) counting with retrograde labeling showed that 8 nmol, but not 0.8 nmol, of intravitreal NMDA reduced RGC survival. Western blotting and immunohistochemistry showed that non-lethal (0.8 nmol) doses of NMDA induced a time-dependent expression of HSF1 and HSF2, and that the expression of HSF1 and HSF2 in the RGC layer peaked between 9 and 18 h after injection. Parallel to the increased HSF expression, immunohistochemistry and in situ hybridization demonstrated that Hsp72 mRNA and protein expression increased 9 and 12 h after non-lethal NMDA injection, respectively. Our findings suggest that the expression of HSF1 and HSF2 is associated with the Hsp72-related stress response.

Alcohol regulates gene expression in neurons via activation of heat shock factor 1

Drinking alcohol causes widespread alterations in gene expression that can result in long-term physiological changes. Although many alcohol-responsive genes (ARGs) have been identified, the mechanisms by which alcohol alters transcription are not well understood. To elucidate these mechanisms, we investigated Gabra4, a neuron-specific gene that is rapidly and robustly activated by alcohol (10-60 mM), both in vitro and in vivo. Here we show that alcohol can activate elements of the heat shock pathway in mouse cortical neurons to enhance the expression of Gabra4 and other ARGs. The activation of Gabra4 by alcohol or high temperature is dependent on the binding of heat shock factor 1 (HSF1) to a short downstream DNA sequence, the alcohol response element (ARE). Alcohol and heat stimulate the translocation of HSF1 from the cytoplasm to the nucleus and the induction of HSF1-dependent genes, Hsp70 and Hsp90, in cultured neurons and in the mouse cerebral cortex in vivo. The reduction of HSF1 levels using small interfering RNA prevented the stimulation of Gabra4 and Hsp70 by alcohol and heat shock. Microarray analysis showed that many ARGs contain ARE-like sequences and that some of these genes are also activated by heat shock. We suggest that alcohol activates phylogenetically conserved pathways that involve intermediates in the heat shock cascade and that sequence elements similar to the ARE may mediate some of the changes in gene expression triggered by alcohol intake, which could be important in a variety of pathophysiological responses to alcohol.

Mechanism of small heat shock protein function in vivo: a knock-in mouse model demonstrates that the R49C mutation in alpha A-crystallin enhances protein insolubility and cell death

alphaA-crystallin (Cryaa/HSPB4) is a small heat shock protein and molecular chaperone that prevents nonspecific aggregation of denaturing proteins. Several point mutations in the alphaA-crystallin gene cause congenital human cataracts by unknown mechanisms. We took a novel approach to investigate the molecular mechanism of cataract formation in vivo by creating gene knock-in mice expressing the arginine 49 to cysteine mutation (R49C) in alphaA-crystallin (alphaA-R49C). This mutation has been linked with autosomal dominant hereditary cataracts in a four-generation Caucasian family. Homologous recombination in embryonic stem cells was performed using a plasmid containing the C to T transition in exon 1 of the cryaa gene. alphaA-R49C heterozygosity led to early cataracts characterized by nuclear opacities. Unexpectedly, alphaA-R49C homozygosity led to small eye phenotype and severe cataracts at birth. Wild type littermates did not show these abnormalities. Lens fiber cells of alphaA-R49C homozygous mice displayed an increase in cell death by apoptosis mediated by a 5-fold decrease in phosphorylated Bad, an anti-apoptotic protein, but an increase in Bcl-2 expression. However, proliferation measured by in vivo bromodeoxyuridine labeling did not decline. The alphaA-R49C heterozygous and homozygous knock-in lenses demonstrated an increase in insoluble alphaA-crystallin and alphaB-crystallin and a surprising increase in expression of cytoplasmic gamma-crystallin, whereas no changes in beta-crystallin were observed. Co-immunoprecipitation analysis showed increased interaction between alphaA-crystallin and lens substrate proteins in the heterozygous knock-in lenses. To our knowledge this is the first knock-in mouse model for a crystallin mutation causing hereditary human cataract and establishes that alphaA-R49C promotes protein insolubility and cell death in vivo.

Transcription of Satellite III non-coding RNAs is a general stress response in human cells

In heat-shocked human cells, heat shock factor 1 activates transcription of tandem arrays of repetitive Satellite III (SatIII) DNA in pericentromeric heterochromatin. Satellite III RNAs remain associated with sites of transcription in nuclear stress bodies (nSBs). Here we use real-time RT-PCR to study the expression of these genomic regions. Transcription is highly asymmetrical and most of the transcripts contain the G-rich strand of the repeat. A low level of G-rich RNAs is detectable in unstressed cells and a 10⁴-fold induction occurs after heat shock. G-rich RNAs are induced by a wide range of stress treatments including heavy metals, UV-C, oxidative and hyper-osmotic stress. Differences exist among stressing agents both for the kinetics and the extent of induction (>100- to 80.000-fold). In all cases, G-rich transcripts are associated with nSBs. On the contrary, C-rich transcripts are almost undetectable in unstressed cells and modestly increase after stress. Production of SatIII RNAs after hyper-osmotic stress depends on the Tonicity Element Binding Protein indicating that activation of the arrays is triggered by different transcription factors. This is the first example of a non-coding RNA whose transcription is controlled by different transcription factors under different growth conditions.

Anti-vpr activities of heat shock protein 27

HIV-1 Vpr plays a pivotal role in viral pathogenesis and is preferentially targeted by the host immune system. In this report, we demonstrate that a small heat shock protein, HSP27, exhibits Vpr-specific antiviral activity, as its expression is specifically responsive to vpr gene expression and increased levels of HSP27 inhibit Vpr-induced cell cycle G2 arrest and cell killing. We further show that overexpression of HSP27 reduces viral replication in T-lymphocytes in a Vpr-dependent manner. Mechanistically, Vpr triggers HSP27 expression through heat shock factor (HSF) 1, but inhibits prolonged expression of HSP27 under heat-shock conditions. Together, these data suggest a potential dynamic and antagonistic interaction between HIV-1 Vpr and a host cell HSP27, suggesting that HSP27 may contribute to cellular intrinsic immunity against HIV infection.

HDAC6 controls major cell response pathways to cytotoxic accumulation of protein aggregates

A cellular defense mechanism counteracts the deleterious effects of misfolded protein accumulation by eliciting a stress response. The cytoplasmic deacetylase HDAC6 (histone deacetylase 6) was previously shown to be a key element in this response by coordinating the clearance of protein aggregates through aggresome formation and their autophagic degradation. Here, for the first time, we demonstrate that HDAC6 is involved in another crucial cell response to the accumulation of ubiquitinated protein aggregates, and unravel its molecular basis. Indeed, our data show that HDAC6 senses ubiquitinated cellular aggregates and consequently induces the expression of major cellular chaperones by triggering the dissociation of a repressive HDAC6/HSF1 (heat-shock factor 1)/HSP90 (heat-shock protein 90) complex and a subsequent HSF1 activation. HDAC6 therefore appears as a master regulator of the cell protective response to cytotoxic protein aggregate formation.

Anti-vpr activities of heat shock protein 27

HIV-1 Vpr plays a pivotal role in viral pathogenesis and is preferentially targeted by the host immune system. In this report, we demonstrate that a small heat shock protein, HSP27, exhibits Vpr-specific antiviral activity, as its expression is specifically responsive to vpr gene expression and increased levels of HSP27 inhibit Vpr-induced cell cycle G2 arrest and cell killing. We further show that overexpression of HSP27 reduces viral replication in T-lymphocytes in a Vpr-dependent manner. Mechanistically, Vpr triggers HSP27 expression through heat shock factor (HSF) 1, but inhibits prolonged expression of HSP27 under heat-shock conditions. Together, these data suggest a potential dynamic and antagonistic interaction between HIV-1 Vpr and a host cell HSP27, suggesting that HSP27 may contribute to cellular intrinsic immunity against HIV infection.

Heat shock transcription factors regulate heat induced cell death in a rat histiocytoma

Heat shock response is associated with the synthesis of heat shock proteins (Hsps) which is strictly regulated by different members of heat shock transcription factors (HSFs). We previously reported that a rat histiocytoma, BC-8 failed to synthesize Hsps when subjected to typical heat shock conditions (42 degrees C, 60 min). The lack of Hsp synthesis in these cells was due to a failure in HSF1 DNA binding activity. In the present study we report that BC-8 tumor cells when subjected to heat shock at higher temperature (43 degrees C, 60 min) or incubation for longer time at 42 degrees C, exhibited necrosis characteristics; however,under mild heat shock (42 degrees C, 30 min) conditions cells showed activation of autophagy. Mild heat shock treatment induced proteolysis of HSF1, and under similar conditions we observed an increase in HSF2 expression followed by its enhanced DNA binding activity. Inhibiting HSF1 proteolysis by reversible proteasome inhibition failed to inhibit heat shock induced autophagy. Compromising HSF2 expression but not HSF1 resulted in the inhibition of autophagy, suggesting HSF2 dependent activation of autophagy. We are reporting for the first time that HSF2 is heat inducible and functions in heat shock induced autophagic cell death in BC-8 tumor cells.

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

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

Heat Shock 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.

Microevolution and ecotoxicology of metals in invertebrates

Risk assessment of metal-contaminated habitats based on responses in the field is complicated by the evolution of local, metal-resistant ecotypes. The unpredictability of occurrence of genetically determined adaptive traits, in terms of site-specific geochemistry, a population's inferred exposure history, and in the physiology of resistance, exacerbates the problem. Micro-evolutionary events warrant the attention of ecotoxicologists because they undermine the application of the bedrock of toxicology, the dose-response curve, to in situ field assessments. Here we survey the evidence for the existence of genetically differentiated, metal-resistant, invertebrate populations; we also describe some of the molecular mechanisms underpinning the adaptations. Quantitative changes in tissue-metal partitioning, and in the molecular and cellular responses (biomarkers)to alterations in internal bioreactive metal pools, are widely accepted as indicators of toxicity and/or exposure in free-living organisms. Both can be modulated by resistance. The understanding that all genomes are intrinsicallyflexible, with subtle sequence changes in promoter regions or epigenetic adjustments conferring significant phenotypic consequences, is deemed highly relevant. Equally relevant is the systems biology insight that genes and proteins are woven into networks. We advocate that biomarker studies should work toward assimilating and exploiting these biological realities through monitoring the activities of suites of genes (transcriptomics) and their expressed products (proteomics), as well as profiling the metabolite signatures of individuals (metabolomics) and by using neutral genetic markers to genotype populations. Ecotoxicology requires robust tools that recognize the imprint of evolution on the constitution of field populations, as well as sufficient mechanistic understanding of the molecular-genetic observations to interpret them in meaningful environmental diagnostic ways.

Targeting the heat shock factor 1 by RNA interference: a potent tool to enhance hyperthermochemotherapy efficacy in cervical cancer

Carcinoma of the uterine cervix is one of the highest causes of mortality in female cancer patients worldwide, and improved treatment options for this type of malignancy are highly needed. Local hyperthermia has been successfully used in combination with systemic administration of cisplatin-based chemotherapy in phase I/II clinical studies. Heat-induced expression of cytoprotective and antiapoptotic heat shock proteins (HSP) is a known complication of hyperthermia, resulting in thermotolerance and chemoresistance and hindering the efficacy of the combination therapy. Heat shock transcription factor 1 (HSF1) is the master regulator of heat-induced HSP expression. In the present report, we used small interfering RNA (siRNA) to silence HSF1 and to examine the effect of HSF1 loss of function on the response to hyperthermia and cisplatin-based chemotherapy in HeLa cervical carcinoma. We have identified the 322-nucleotide to 340-nucleotide HSF1 sequence as an ideal target for siRNA-mediated HSF1 silencing, have created a pSUPER-HSF1 vector able to potently suppress the HSF1 gene, and have generated for the first time human cancer cell lines with stable loss of HSF1 function. We report that, although it surprisingly does not affect cancer cell sensitivity to cisplatin or elevated temperatures up to 43 degrees C when administered separately, loss of HSF1 function causes a dramatic increase in sensitivity to hyperthermochemotherapy, leading to massive (>95%) apoptosis of cancer cells. These findings indicate that disruption of HSF1-induced cytoprotection during hyperthermochemotherapy may represent a powerful strategy to selectively amplify the damage in cancer cells and identify HSF1 as a promising therapeutic target in cervical carcinoma.

Co-expression of heat shock transcription factors 1 and 2 in rat retinal ganglion cells

Heat shock protein (HSP) plays an important role in the maintenance of neuronal survival during harmful conditions. Previously, we reported that metabolic stress induces HSP72 in retinal ganglion cells (RGCs) and protects against excitotoxicity, hypoxia and experimental glaucoma. To understand heat shock protein transcriptional mechanisms, we examined the cellular expression of heat shock factors 1 (HSF1) and 2 (HSF2) in the unstressed adult rat retina. Western blotting, immunohistochemistry and RT-PCR showed that mRNA and protein of HSF1 and HSF2 were present in the rat retina and predominantly expressed in RGC layer cells. Western blotting of dissociated RGC suspensions harvested with Thy-1 immuno-labeled magnetic beads confirmed that RGCs expressed HSF1, HSF2 and HSP72. Our findings suggest that both heat shock transcription factors 1 and 2 are linked to the heat shock response in retinal ganglion cells.

RANK ligand expression in heat shock factor-2 deficient mouse bone marrow stromal/preosteoblast cells

Heat Shock Proteins (HSP) are molecular chaperones activated upon cellular stress/stimuli. HSP gene expression is regulated by Heat Shock Factors (HSF). We have recently demonstrated a functional role for heat shock factor-2 (HSF-2) in fibroblast growth factor-2 (FGF-2)-induced RANK ligand (RANKL), a critical osteoclastogenic factor expression on stromal/preosteoblast cells. In the present study, we show that FGF-2 treatment did not induce RANKL expression in HSF-2-/-stromal/preosteoblast cells. Interestingly, HSF-2 deficiency resulted in rapid induction of alkaline phosphatase (ALP) activity and osteocalcin mRNA expression in these cells. Furthermore, FGF-2 did not induce osteoclast formation in co-culture of normal mouse spleen cells and HSF-2-/-stromal/preosteoblast cells. Electron microscopy analysis demonstrated that osteoclasts from HSF-2-/-mice have poorly developed ruffled borders. These data further confirm that HSF-2 plays an important role in FGF-2-induced RANKL expression in stromal/preosteoblast cells. HSF-2 deficiency has pleotropic effects on gene expression during osteoblast differentiation and osteoclastogenesis in the bone microenvironment. Novel therapeutic agents that modulate HSF-2 activation may have therapeutic utility against increased levels of FGF-2 and bone destruction associated with pathologic conditions.

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.

The mechanism of heat-shock protein 70 gene expression abolition in gastric epithelium caused by Helicobacter pylori infection

BACKGROUND

The members of the family of heat shock factors coordinate the inducible transcription of heat shock genes in response to diverse stimuli. Any disturbances in signal transduction may lead to the attenuation of heat shock proteins synthesis and to cell death due to apoptosis. It has been shown by others that different nuclear factors, such as nuclear factor interleukin 6 or signal transducer and activator of transcription 3, co-operate with heat shock factors, mostly enhancing their activator effect on heat shock proteins genes expression. Therefore, we sought to determine whether apoptosis induced in the gastric epithelium exposed to live Helicobacter pylori might occur due to the elimination of HSP70 expression and deregulation of the heat shock response of the cell.

MATERIALS AND METHODS

Experiments were performed on KATO III gastric epithelial cells exposed to live cagA, vacA expressing Hp over different periods of time. Total cellular RNA, cytoplasmic and nuclear proteins were isolated for polymerase chain reaction, western-blot, electrophoretic mobility shift assay, decoy and co-immunoprecipitation studies.

RESULTS

We found that in human gastric epithelium exposed to Helicobacter pylori, heat shock factor 1 is bound and restrained in complexes by phosphorylated signal transducer and activator of transcription 3 protein. In consequence, heat shock factor 1 bound up with phosphorylated signal transducer and activator of transcription 3 protein is unable to activate HSP70 protein synthesis in KATO III cells under stress conditions. Helicobacter pylori also causes changes in bax/bcl-2 cellular equilibrium, leading to the induction of apoptosis.

CONCLUSIONS

The observed phenomenon might be the mechanism whereby gastric epithelium adapts to the infection of Helicobacter pylori, eliminating cells which are damaged or altered by bacterial cytotoxic products from the tissue.

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.

The Heat-Shock Factor is not Activated in Mammalian Cells Exposed to Cellular Phone Frequency Microwaves

There has been considerable interest in the biological effects of exposure to radiofrequency electromagnetic radiation, given the explosive growth of cellular telephone use, with the possible induction of malignancy being a significant concern. Thus the determination of whether nonthermal effects of radiofrequency electromagnetic radiation contribute to the process leading to malignancy is an important task. One proposed pathway to malignancy involves the induction of the stress response by exposures to cell phone frequency microwaves. The first step in the induction of the stress response is the activation of the DNA-binding activity of the specific transcription factor involved in this response, the heat-shock factor (HSF). The DNA-binding activity of HSF in hamster, mouse and human cells was determined after acute and continuous exposures to frequency domain multiple access (FDMA)- or code domain multiple access (CDMA)-modulated microwaves at low (0.6 W/kg) or high (approximately 5 W/kg) SARs at frequencies used for mobile communication. The DNA-binding activity of HSF was monitored using a gel shift assay; the calibration of this assay indicated that an increase of approximately 10% in the activation of the DNA-binding activity of HSF after a 1 degrees C increase in temperature could be detected. We failed to detect any increase in the DNA-binding ability of HSF in cultured mammalian cells as a consequence of any exposure tested, within the sensitivity of our assay. Our results do not support the notion that the stress response is activated as a consequence of exposure to microwaves of frequencies associated with mobile communication devices.

Heat shock protein 70 negatively regulates the heat-shock-induced suppression of the IκB/NF-κB cascade by facilitating IκB kinase renaturation and blocking its further denaturation

Heat shock (HS) treatment has been previously shown to suppress the IkappaB/nuclear factor-kappaB (NF-kappaB) cascade by denaturing, and thus inactivating IkappaB kinase (IKK). HS is characterized by the induction of a group of heat shock proteins (HSPs). However, their role in the HS-induced suppression of the IkappaB/NF-kappaB cascade is unclear. Adenovirus-mediated HSP70 overexpression was found not to suppress the TNF-alpha-induced activation of the IkappaB/NF-kappaB pathway, thus suggesting that HSP70 is unlikely to suppress this pathway. When TNF-alpha-induced activation of the IkappaB/NF-kappaB pathway was regained 24 h after HS, HSP70 was found to be highly up-regulated. Moreover, blocking HSP70 induction delayed TNF-alpha-induced IkappaBalpha degradation and the resolubilization of IKK. In addition, HSP70 associated physically with IKK, suggesting that HSP70 is involved in the recovery process via molecular chaperone effect. Adenovirus-mediated HSP70 overexpression prior to HS blocked the IkappaBalpha stabilizing effect of HS by suppressing IKK insolubilization. Moreover, the up-regulation of endogenous HSP70 by preheating, suppressed this subsequent HS-induced IKK insolubilization, and this effect was abrogated by blocking HSP70 induction. These findings indicate that HSP70 accumulates during HS and negatively regulates the HS-induced suppression of the IkappaB/NF-kappaB cascade by facilitating the renaturation of IKK and blocking its further denaturation.

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.

Transcriptional regulation and binding of heat shock factor 1 and heat shock factor 2 to 32 human heat shock genes during thermal stress and differentiation

Transcription of mammalian heat shock genes can be regulated by heat shock factors (HSF) 1 and 2. Although it has been shown previously that these factors respond to distinct stimuli, a broad analysis of the induction and function of these factors in living cells has not been performed. In our study, we assayed binding of human HSF1 and HSF2 at the promoters of 32 genes identified through LocusLink as heat shock genes in response to elevated temperature and hemin-induced differentiation in human K562 erythroleukemic cells using the chromatin immunoprecipitation technique. We also measured the induced expression of these genes under these 2 conditions. We found that 17 of the 32 genes were transcriptionally induced during heat shock, and HSF1 binding was detected at 15 of the 17 promoters. Nearly all the genes induced by heat shock were also induced to a lesser degree during hemin treatment. However, some genes were induced significantly more during hemin treatment than during heat shock. A new finding is that HSF1 and HSF2 bind to the same targets, but HSF1 binding is activated more by heat than by hemin treatment, and HSF2 binding is only activated by hemin treatment and not by heat. This technology also identified previously unknown HSF1 binding sites near genes that were previously shown to be heat inducible that may contribute to gene-specific regulation.

Identification of genes responsive to UV-A radiation in human lens epithelial cells using complementary DNA microarrays

UV-A radiation produces cataract in animals, enhances photoaging of the lens and skin and increases the phototoxicity of drugs. However, the nature of genes that are activated or repressed after cellular exposure to UV-A radiation remains enigmatic. Because lens epithelial cells exposed to UV-A radiation undergo apoptosis 4 h after exposure to the stress, we sought to establish the change in gene expression in cells by UV-A radiation using gene expression profiling using complementary DNA microarrays containing about 12 000 genes. We identified 78 genes abnormally expressed in UV-A-irradiated cells (showing >2.5-fold change at P < 0.05). These genes are implicated in various biological processes, including signal transduction and nucleic acid binding, and genes encoding enzymes. A majority of the genes were downregulated. Our analysis revealed that the expression of genes for the transcription factors ATF-3 and Pilot increased four-fold, whereas the gene for the apoptosis regulator NAPOR-1 decreased five-fold. These changes were confirmed by real-time quantitative reverse transcriptase-polymerase chain reaction. The calpain large polypeptide 3 (CANP3) gene also increased nine-fold after UV-A radiation. In addition, peroxisomal biogenesis factor 7, glucocorticoid receptor-alpha and tumor-associated calcium signal transducer genes decreased three- to eight-fold. Western blot analysis further confirmed the increase in protein expression of ATF-3 and CANP3 and decreased expression of glucocorticoid receptor-alpha in the irradiated cells. Surprisingly, most of these genes had not been previously shown to be modulated by UV-A radiation. Our results show that human lens epithelial cells respond to a single dose of UV-A radiation by enhancing or suppressing functionally similar sets of genes, some of which have opposing functions, around the time at which apoptosis occurs. These studies support the intriguing concept that activation of competing pathways favoring either cell survival or death is a means to coordinate the response of cells to UV-A stress.

The heat-shock-induced suppression of the IkappaB/NF-kappaB cascade is due to inactivation of upstream regulators of IkappaBalpha through insolubilization

Heat shock (HS) was found to suppress the IkappaB/NF-kappaB cascade via the inhibition of IkappaB kinase (IKK) activity; however, the mechanism has not been clear. This study was undertaken to elucidate the detail of the mechanism involved. TNF-alpha-induced activation of IKK was suppressed by HS in human bronchial epithelial cells, and this was associated with the absence of IKK in the immunoprecipitates. It was not due to a degradation of IKK, but due to a conversion of IKK from a soluble to an insoluble form. IKK lost its activity rapidly upon exposure to HS in vitro. The time course of the insolubilization of IKK coincided with the decrease in IKK activity. However, inhibition of IKK insolubilization by the induction of thermotolerance did not reverse the HS-induced suppression of IKK activation and IkappaBalpha degradation. Upstream activators of IKK, such as NF-kappaB-inducing kinase (NIK) and IL-1R-associated kinase (IRAK) were also insolubilized by HS. The HS-induced insolubilization of NIK was not blocked by the induction of thermotolerance. Overexpression of NIK resumed TNF-alpha-induced activation of IKK in thermotolerant cells. These results indicate that the loss of activity of NIK, IRAK, and IKK through insolubilization is responsible for the HS-induced suppression of IkappaB/NF-kappaB pathway.

Proteasome inhibition increases HuR level, restores heat-inducible HSP72 expression and thermotolerance in WI-38 senescent human fibroblasts

At the end of their replicative potential in vitro, late passage WI-38 human diploid fibroblasts (HDF) have a low basal expression of heat shock protein 72 (HSP72) and an attenuated ability to induce it in response to heat shock. The transient exposure to the specific and reversible proteasome inhibitor MG132 during a mild heat shock induced late passage HDF to synthesize and accumulate high levels of HSP72. This HSP72 expression was long-lasting and appeared to result from both increased cytoplasmic levels and enhanced translation of HSP72 mRNA. The level of HuR, a stabilizing mRNA-binding protein, increased following the MG132 treatment. This result is consistent with the proposed role of HuR in assisting mRNA export to the cytoplasm and in antagonizing its degradation. Furthermore, the previous exposure of late passage HDF to a mild heat shock in the presence of MG132 protected these cells against the otherwise lethal effect of a subsequent severe heat shock. This acquisition of thermotolerance appeared to be correlated with the level of HSP72.

Nuclear translocation of papillomavirus minor capsid protein L2 requires Hsc70

Minor capsid protein L2 of papillomaviruses plays an essential role in virus assembly by recruiting viral components to PML bodies, the proposed sites of virus morphogenesis. We demonstrate here that the function of L2 in virus assembly requires the chaperone Hsc70. Hsc70 was found dispersed in naturally infected keratinocytes and cultured cells. A dramatic relocation of Hsc70 from the cytoplasm to PML bodies was induced in these cells by L2 expression. Hsc70-L2 complex formation was confirmed by coimmunoprecipitation. The complex was modulated by the cochaperones Hip and Bag-1, which stabilize and destabilize Hsc70-substrate complexes, respectively. Cytoplasmic depletion of Hsc70 caused retention of wild-type and N-terminally truncated L2, but not of C-terminally truncated L2, in the cytoplasm. This retention was partially reversed by overexpression of Hsc70 fused to green fluorescent protein but not by ATPase-negative Hsc70. Hsc70 associated with L1-L2 virus-like particles (VLPs) but not with VLPs composed either of L1 alone or of L1 and C-terminally truncated L2. Moreover, displacement of Hsc70 from L1-L2 VLPs by encapsidation of DNA, generating pseudovirions, was found. These data indicate that Hsc70 transiently associates with viral capsids during the integration of L2, possibly via the L2 C terminus. Completion of virus assembly results in displacement of Hsc70 from virions.

Molecular chaperones and the stress of oncogenesis

Protein-damaging stresses induce the expression of 'heat-shock proteins', which have essential roles in protecting cells from the potentially lethal effects of stress and proteotoxicity. These stress-protective heat-shock proteins are often overexpressed in cells of various cancers and have been suggested to be contributing factors in tumorigenesis. An underlying basis of oncogenesis is the acquisition and accumulation of mutations that provide the transformed cell with the combined characteristics of deregulated cell proliferation and suppressed cell death. Heat-shock proteins with dual roles as regulators of protein conformation and stress sensors may therefore have intriguing and central roles in both cell proliferation and apoptosis. It has been established that heat-shock proteins exhibit specificity to particular classes of polypeptide substrates and client proteins in vivo, and that chaperones can stabilize mutations that affect the folded conformation. Likewise, overexpression of chaperones has also been shown to protect cells against apoptotic cell death. The involvement of chaperones, therefore, in such diverse roles might suggest novel anticancer therapeutic approaches targeting heat-shock protein function for a broad spectrum of tumor types.

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.

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.

Functional Role for Heat Shock Factors in the Transcriptional Regulation of Human RANK Ligand Gene Expression in Stromal/Osteoblast Cells

RANK Ligand (RANKL) is a critical osteoclastogenic factor that is expressed on stromal cells and osteoblasts. Most resorption stimuli induce osteoclast formation by modulating RANKL gene expression in marrow stromal/osteoblast cells. However, it is unclear how these stimuli modulate RANKL gene expression in the bone microenvironment. To characterize the transcriptional control of human RANKL gene expression in stromal/osteoblast cells, we PCR-amplified and cloned a 2-kb 5'-flanking sequence of the RANKL gene, using normal human osteoblast derived genomic DNA as a template. Sequence analysis identified the presence of several potential Heat Shock Factor (HSF) responsive elements (HSE) in the human RANKL gene promoter region. Co-expression of HSF-1 or HSF-2 with the RANKL gene promoter-luciferase reporter plasmid in human osteoblastic cells (NOBC) demonstrated a 2-fold and 4.5-fold increase in promoter activity, respectively. RT-PCR analysis for HSF-1 and 2 mRNA expression in human bone marrow-derived stromal cells (SAKA-T) and osteoblast cells detected only HSF-2 expression. As evident from EMSA analysis, in contrast to 1,25(OH)(2)D(3) SAKA-T cells treated with b-FGF demonstrated increased levels of HSF-2 binding to the HSE present in the RANKL gene promoter region. Immunocytochemical staining further confirmed nuclear localization of HSF-2 in both SAKA-T transformed stromal cells and human bone marrow derived primary stromal/preosteoblastic cells in response to b-FGF treatment. Furthermore, b-FGF treatment of SAKA-T cells transfected with the luciferase reporter plasmid containing the hRANKL HSE region (-2 kb to -1275 bp) upstream to a heterologous promoter showed increased levels of transactivation. Western blot analysis further demonstrated enhanced levels of RANKL expression and HSP-27 phosphorylation in SAKA-T cells treated with b-FGF. In addition, overexpression of HSF-2 in SAKA-T cells resulted in a 5-fold increase in the levels of RANKL expression in these cells. These data further suggest that HSF-2 is a downstream target of b-FGF to induce RANKL expression in stromal/osteoblast cells, and that HSF may play an important role in modulating RANKL gene expression in the bone microenvironment.

Early transcriptional activation of the hsp70.1 gene by osmotic stress in one-cell embryos of the mouse

In fertilized mouse eggs, de novo transcription of embryonic genes is first observed during the S phase of the one-cell stage. This transcription, however, is mostly limited to the male pronucleus and possibly uncoupled from translation, making the functional meaning obscure. We found that one-cell mouse embryos respond to the osmotic shock of in vitro isolation with migration of HSF1, the canonical stress activator of mammalian heat shock genes, to pronuclei and by transient transcription of the hsp70.1, but not hsp70.3 and hsp90, heat shock genes. Isolated growing dictyate oocytes also display a nuclear HSF1 localization, but, in contrast with embryos, they transcribe both hsp70.1 and hsp70.3 genes only after heat shock. Intranuclear injection of double-stranded oligodeoxyribonucleotides containing HSE, GAGA box or GC box consensus sequences, and antibodies raised to transcription factors HSF1, HSF2, Drosophila melanogaster GAGA factor, or Sp1 demonstrated that hsp70.1 transcription depends on HSF1 in both oocytes and embryos and that Sp1 is dispensable in oocytes and inhibitory in the embryos. Hsp70.1 thus represents the first endogenous gene so far identified to be physiologically activated and tightly regulated after fertilization in mammals.

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.

Changes in the number of HSF1 positive granules in the nucleus reflects heat shock semiquantitatively

PURPOSE

The purpose of this study is to examine the changes in the number of HSF1 granules in the nucleus caused by different degree of heat stress.

MATERIALS AND METHODS

A human esophageal cancer cell line, TE-2, was used. HSF1 granules were examined in an immunofluorescence study, and the changes in the average number of HSF1 granules after heat alone or heat in combination with KNK437, Hsp inhibitor, were evaluated. A band shift of HSF1 was also determined by western blot.

RESULTS

HSF1 granules appeared soon after the start of heating at 43 degrees C and reached a peak at 60 min and gradually disappeared after discontinuation of heat. In the fractionated heat treatment, preheating (43 degrees C, 30 min) suppressed the increase in the number of the granules during the second heating, but suppression of Hsp72 by KNK437 resulted in increase in the number of granules. Continued heating at 43 degrees C with or without KNK437 maintained the number of the granules at the peak level during heat treatment. The band shift of HSF1 examined by western blot correlated with the changes in the number of granules. The number of granules also reflected the degree of stress according to different temperature.

CONCLUSION

The number of HSF1 granules in the nucleus well reflected heat stress, and was almost consistent with phosphorylation of HSF1. The number of HSF1 granules would be a useful tool for evaluating different degrees of heat stress semiquantitatively.

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.

Identification and characterization of a novel enhancer for the human MCT-1 oncogene promoter

Cloning and characterization of the promoter region for the MCT-1 oncogene is described. We used luciferase assays to identify cis-acting elements responsible for human MCT-1 promoter function. The MCT-1 promoter is TATA-less with a consensus initiator element located at the transcription start site and facilitated by two Sp1 sites that directs basal transcription. Deletion of a region of the MCT-1 promoter (-133 to -122) resulted in significant decrease in luciferase activity, suggesting that this region contains a positive cis-acting element. Using mobility shift assays with a 26-mer oligonucleotide, which contains this fragment and its flanking regions, we demonstrated the presence of sequence-specific DNA-binding protein in both Jurkat and Hela nuclear extracts that we designated as LMBF (for lymphoid MCT-1 binding factor). This 26-mer oligonucleotide containing the LMBF binding site is required for maximum transcriptional activity of the MCT-1 promoter. Although the 26-mer oligonucleotide contains a sequence with strong homology to a heat-shock factor consensus, competitive electrophoretic mobility shift assay (EMSA) analysis demonstrated that the binding protein is not a known member of heat shock family. Furthermore, this sequence when placed in reverse orientation downstream of the luciferase gene was able to enhance luciferase activity driven by a minimal promoter. These data are consistent with this sequence behaving as an enhancer. Finally, Southwestern blot analysis revealed a 96-kDa protein capable of binding a probe containing the LMBF binding site.

Heat shock proteins as regulators of the immune response

Until recently, heat shock proteins (also known as heat stress proteins) have mostly been regarded as intracellular molecules that mediate a range of essential housekeeping and cytoprotective functions. However, interest in their role as intercellular signalling molecules has been fuelled by the observations that these molecules can be released and are present in the extracellular environment under physiological conditions. They can elicit cytokine production by, and adhesion molecule expression of, a range of cell types, and they can deliver maturation signals and peptides to antigen presenting cells through receptor-mediated interactions. These functions suggest that heat shock proteins could be immunoregulatory agents with potent and widely-applicable therapeutic uses. Furthermore, the induction of self heat shock protein immune reactivity can attenuate autoimmunity and delay transplant rejection, and heat shock proteins derived from tumours and pathogens can elicit specific, protective immunity. This review will focus on this rapidly evolving area of heat shock protein biology.

Mild heat and proteotoxic stress promote unique subcellular trafficking and nucleolar accumulation of RGS6 and other RGS proteins. Role of the RGS domain in stress-induced trafficking of RGS proteins

RGS proteins comprise a large family of proteins named for their ability to negatively regulate heterotrimeric G protein signaling. RGS6 is a member of the R7 subfamily of RGS proteins possessing DEP (disheveled/Egl-10/pleckstrin) homology and GGL (G protein gamma-subunit-like) domains in addition to the semiconserved RGS domain. Our previous study documented unusual complexity in splicing of the human RGS6 gene, and we demonstrated localization of various RGS6 splice forms at sites other than the plasma membrane, including the cytoplasm and nucleus, where G proteins are not localized (Chatterjee, T. K., Liu, Z., and Fisher, R. A. (2003) J. Biol. Chem. 278, 30261-30271). Here we provide new evidence that mild heat stress, proteasome-mediated proteotoxic stress, and HSF1 expression induces dramatic relocalization of RGS6 proteins from such sites to nucleoli. This response was observed in COS-7 cells expressing various splice forms of RGS6, was not elicited by other forms of cellular stress and was observed in cells treated with various protein kinase inhibitors or co-expressing a dominant-negative kinase inactive SAPK. The RGS domain of RGS6 was identified as a primary structural module providing support for its stress-induced nucleolar trafficking and various other RGS proteins or their isolated RGS domains similarly undergo nucleolar migration in response to heat or proteotoxic stress or during co-expression of HSF1. The atypical RGS domains of axin and AKAP10 also underwent stress-induced nucleolar trafficking while structural domains outside of the RGS domain of some RGS proteins can override nucleolar trafficking in response to stress. Inhibition of rDNA transcription also promoted nucleolar migration of RGS6, a response previously observed in a subset of nucleolar proteins. The DEP domain of RGS6, but not its RGS domain, conferred structural support for its transcription-linked nucleolar migration. RGS6 exhibited trafficking from subnuclear dots to nucleoli in response to heat-, proteotoxic- or transcription-linked stress. These results provide new evidence that mammalian RGS proteins undergo unique subcellular trafficking in response to specific forms of cellular stress and implicate the RGS family of proteins in cellular stress signaling pathways.

Heat shock proteins, cellular chaperones that modulate mitochondrial cell death pathways

Stress or heat shock proteins (HSPs) are ubiquitous and highly conserved proteins whose expression is induced in response to a wide variety of physiological and environmental insults. They allow the cells to survive to otherwise lethal conditions. Various mechanisms have been proposed to account for the cytoprotective functions of HSPs. These proteins play an essential role in intracellular "house-keeping" by assisting the correct folding of nascent and stress-accumulated misfolded proteins and preventing their aggregation. Several HSPs have also demonstrated to directly interact with various components of the tightly regulated programmed cell death machinery, upstream, and downstream of the mitochondrial events. Finally, HSPs could play a role in the proteasome-mediated degradation of selected proteins under stress conditions. Altogether, these properties could make HSPs appropriate targets for modulating cell death pathways.

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

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

Heat shock transcription factor 2 is not essential for embryonic development, fertility, or adult cognitive and psychomotor function in mice

Members of the heat shock factor (HSF) family are evolutionarily conserved regulators that share a highly homologous DNA-binding domain. In mammals, HSF1 is the main factor controlling the stress-inducible expression of Hsp genes while the functions of HSF2 and HSF4 are less clear. Based on its developmental profile of expression, it was hypothesized that HSF2 may play an essential role in brain and heart development, spermatogenesis, and erythroid differentiation. To directly assess this hypothesis and better understand the underlying mechanisms that require HSF2, we generated Hsf2 knockout mice. Here, we report that Hsf2(-/-) mice are viable and fertile and exhibit normal life span and behavioral functions. We conclude that HSF2, most probably because its physiological roles are integrated into a redundant network of gene regulation and function, is dispensable for normal development, fertility, and postnatal psychomotor function.

Determination of the consensus binding sequence for the purified embryonic heat shock factor 2

Heat shock transcription factors (HSFs) are characterized by their ability, upon activation, to bind to heat shock response elements (HSE) present in the promoter of their target genes. HSE are composed of inverted repeats of the pentamer nGAAm. In this study, we compare the embryonic HSF2 protein, purified from F9 embryonal carcinoma cells tumor, and the in vitro synthesized HSF2. We show that the context of HSF2 synthesis influences its thermosensitivity and DNA-binding properties. Therefore, we determined the consensus binding sequence for the purified embryonic HSF2 by the technique of systematic evolution of ligands by exponential enrichment (SELEX). We show that embryonic HSF2 prefers sites containing three or four nGAAm inverted pentamers and that its optimal binding sequence contains the 8-mer palindromic core 5'-TTCTAGAA-3'. The consensus binding sequence for the embryonic HSF2 will be very helpful to identify new targets for this factor, during developmental and differentiation processes.

Invited review: Effects of heat and cold stress on mammalian gene expression

This review examines the effects of thermal stress on gene expression, with special emphasis on changes in the expression of genes other than heat shock proteins (HSPs). There are approximately 50 genes not traditionally considered to be HSPs that have been shown, by conventional techniques, to change expression as a result of heat stress, and there are <20 genes (including HSPs) that have been shown to be affected by cold. These numbers will likely become much larger as gene chip array and proteomic technologies are applied to the study of the cell stress response. Several mechanisms have been identified by which gene expression may be altered by heat and cold stress. The similarities and differences between the cellular responses to heat and cold may yield key insights into how cells, and by extension tissues and organisms, survive and adapt to stress.