Sequential phosphorylation by mitogen-activated protein kinase and glycogen synthase kinase 3 represses transcriptional activation by heat shock factor-1

Proteasome inhibitors lactacystin and MG132 inhibit the dephosphorylation of HSF1 after heat shock and suppress thermal induction of heat shock gene expression

Recently, we have shown that two proteasome inhibitors, MG132 and lactacystin, induce hyperphosphorylation and trimerization of HSF1, and transactivate heat shock genes at 37 degrees C. Here, we examined the effects of these proteasome inhibitors and, in addition, a phosphatase inhibitor calyculin A (CCA) on the activation of HSF1 upon heat shock and during post-heat-shock recovery, with emphasis on HSF1 hyperphosphorylation and the ability of HSF1 to transactivate heat shock genes. When lactacystin, MG132, or CCA was present after heat shock, HSF1 remained hyperphosphorylated during post-heat-shock recovery at 37 degrees C. Failure of HSF1 to recover to its preheated dephosphorylated state correlated well with the suppression of the heat-induced hsp70 expression. In vitro, HSF1 from heat-shocked cells, when dephosphorylated, showed an increase in HSE-binding affinity. Taken together, these data suggest that phosphorylation of HSF1 plays an important role in the negative regulation of heat-shock response. Specifically, during post-heat-shock recovery phase, prolonged hyperphosphorylation of HSF1 suppresses heat-induced expression of heat shock genes.

The mammalian HSF4 gene generates both an activator and a repressor of heat shock genes by alternative splicing

The expression of heat shock genes is controlled at the level of transcription by members of the heat shock transcription factor family in vertebrates. HSF4 is a mammalian factor characterized by its lack of a suppression domain that modulates formation of DNA-binding homotrimer. Here, we have determined the exon structure of the human HSF4 gene and identified a major new isoform, HSF4b, derived by alternative RNA splicing events, in addition to a previously reported HSF4a isoform. In mouse tissues HSF4b mRNA was more abundant than HSF4a as examined by reverse transcription-polymerase chain reaction, and its protein was detected in the brain and lung. Although both mouse HSF4a and HSF4b form trimers in the absence of stress, these two isoforms exhibit different transcriptional activity; HSF4a acts as an inhibitor of the constitutive expression of heat shock genes, and hHSF4b acts as a transcriptional activator. Furthermore HSF4b but not HSF4a complements the viability defect of yeast cells lacking HSF. Moreover, heat shock and other stresses stimulate transcription of target genes by HSF4b in both yeast and mammalian cells. These results suggest that differential splicing of HSF4 mRNA gives rise to both an inhibitor and activator of tissue-specific heat shock gene expression.

Rapid and reversible relocalization of heat shock factor 1 within seconds to nuclear stress granules

Heat shock factor 1 (HSF1) is essential for the stress-induced expression of heat shock genes. On exposure to heat shock, HSF1 localizes within seconds to discrete nuclear granules. On recovery from heat shock, HSF1 rapidly dissipates from these stress granules to a diffuse nucleoplasmic distribution, typical of unstressed cells. Subsequent reexposure to heat shock results in the rapid relocalization of HSF1 to the same stress granules with identical kinetics. Although the appearance of HSF1 stress granules corresponds to the hyperphosphorylated, trimeric DNA-binding state of HSF1 and correlates temporally with the inducible transcription of heat shock genes, they are also present in heat-shocked mitotic cells that are devoid of transcription. This finding suggests a role for HSF1 stress granules as a nuclear compartment for the temporal regulation and spatial organization of HSF1 activity and reveals new features of the dynamics of nuclear organization.

DNA-dependent protein kinase protects against heat-induced apoptosis

Purified heat shock transcription factor 1 (HSF1) binds to both the regulatory and catalytic components of the DNA-dependent protein kinase (DNA-PK). This observation suggests that DNA-PK may have a physiological role in the heat shock response. To investigate this possibility, we performed a comparison of cell lines that were deficient in either the Ku protein or the DNA-PK catalytic subunit versus the same cell lines that had been rescued by the introduction of a functional gene. DNA-PK-negative cell lines were up to 10-fold more sensitive to heat-induced apoptosis than matched DNA-PK-positive cell lines. There may be a regulatory interaction between DNA-PK and HSF1 in vivo, because constitutive overexpression of HSF1 sensitized the DNA-PK-positive cells to heat but had no effect in DNA-PK-negative cells. The initial burst of hsp70 mRNA expression was similar in DNA-PK-negative and -positive cell lines, but the DNA-PK-negative cells showed an attenuated rate of mRNA synthesis at later times and, in some cases, lower heat shock protein expression. These findings provide evidence for an antiapoptotic function of DNA-PK that is experimentally separable from its mechanical role in DNA double strand break repair.

Glycogen synthase phosphatase interacts with heat shock factor to activate CUP1 gene transcription in Saccharomyces cerevisiae

Upon heat shock, transcription of many stress-inducible genes is rapidly and dramatically stimulated by heat shock factor (HSF). A central region of the yeast HSF (designated HSFrr for "repression region") was previously identified and proposed to be involved in repressing the activation domain under non-heat-shock conditions. Here, we used the phage display system to isolate proteins that interact with HSFrr. This should identify factors that modulate HSF activity or directly participate in HSF-mediated transcriptional activation. We constructed a randomly sheared yeast genomic library to express yeast proteins on the surface of lambda phage. HSFrr binding phages were selected by cycles of affinity chromatography. DNA sequencing identified an HSFrr-interacting phage that contains the GAC1 gene. The GAC1 gene encodes the regulatory subunit for a type 1 serine/threonine phosphoprotein phosphatase, Glc7. Both gac1 and glc7 mutations had little effect on HSF activation of gene transcription of two heat shock genes, SSA4 and HSP82. In contrast, heat shock induction of CUP1 gene expression was completely abolished in a glc7 mutant and reduced in a gac1 mutant. The results demonstrate that the Glc7 phosphatase and its Gac1 regulatory subunit play positive roles in HSF activation of CUP1 transcription.

Non-steroidal anti-inflammatory drugs inhibit the expression of cytokines and induce HSP70 in human monocytes

Recent studies have shown that the non-steroidal anti-inflammatory drugs (NSAIDs) activate heat shock transcription factor (HSF1) from a latent cytoplasmic form to a nuclear, DNA binding state. As HSF1 can function as both an activator of heat shock genes and a repressor of non-heat shock genes such as IL1B and c- fos, we have examined the potential role of HSF1 in the effects of NSAIDs on gene expression in a human monocytic cell line THP-1. We found that two members of the NSAIDs, sodium salicylate and sulindac repress the IL1B promoter to similar degree to heat shock or HSF1 overexpression. In addition, sodium salicylate and additional NSAIDs used at concentrations that activate HSF1 also inhibited the expression of other monocytic genes (TNF-alpha, IL-1beta, IL-6, IL-8, IL-10, ICAM-1) activated by exposure to a pro-inflammatory stimulus (lipopolysaccharide, LPS). At least in the case of the IL1B promoter, repression did not seem to involve another factor whose activity is affected by the NSAIDs, NFkappaB as the IL1B promoter fragment used in our studies is not NFkappaB responsive and binds specifically to HSF1. Exposure to NSAIDs had a complex effect on HSP gene expression and while sulindac activated the stress responsive HSP70B promoter, sodium salicylate did not. In addition, only a subset of the NSAIDs induced HSP70 mRNA species. These findings reflect the properties of HSF1 which can be activated to at least two DNA binding forms only one of which activates heat shock promoters and suggest that individual NSAID family members may differentially induce one or other of these forms. Overall therefore, exposure to NSAIDs leads to a profound switch in gene expression in monocytic cells, with suppression of genes involved in macrophage activation and induction of stress genes and HSF1 appears to play a regulatory role in these effects.

Higher induction of heat shock protein 72 by heat stress in cisplatin-resistant than in cisplatin-sensitive cancer cells

Induction of the heat shock proteins (HSPs) is involved in the increased resistance to cancer therapies such as chemotherapy and hyperthermia. We used two human ovarian cancer cell lines; a cisplatin (CDDP)-sensitive line A2780 and its CDDP-resistant derivative, A2780CP. The concentration of intracellular glutathione (GSH) is higher (2.7-fold increase) in A2780CP cells than in A2780 cells. A mild treatment with a heat stress (42 degrees C for 30 min) induced synthesis of both the heat shock protein 72 (Hsp72) mRNA and the HSP72 protein in A2780CP cells, but not in A2780 cells. In contrast, a severe heat stress (45 degrees C for 30 min) increased synthesis of the HSP72 protein in the two cell lines. The induced level of the HSP72 protein by the severe treatment was higher in A2780CP than in A2780 cells. The gel mobility shift assay showed that DNA binding activities of the heat shock factor (HSF) in the two cell lines were induced similarly and significantly by the mild heat stress. Immunocytochemistry using an anti HSF1 antibody also indicated that mild heat stress activated the HSF1 translocation from the cytosol to the nucleus similarly in the both cell lines. Pretreatment of CDDP-sensitive A2780 cells with N-acetyl-L-cysteine, a precursor of GSH, effectively enhanced induction of the Hsp72 mRNA by the mild heat stress. The present findings demonstrate that induction of the Hsp72 mRNA by the mild heat stress was more extensive in CDDP-resistant A2780CP cells. It is likely that the higher GSH concentration in A2780CP cells plays an important role in promoting Hsp72 gene expression induced by the mild heat stress probably through processes downstream of activation of HSF-DNA binding.

Proteasome inhibitors MG132 and lactacystin hyperphosphorylate HSF1 and induce hsp70 and hsp27 expression

MG132 and lactacystin, two 26S proteasome-specific protease inhibitors, can upregulate heat-shock gene transcription without heat shock. In this study, we showed that both of these inhibitors induce hyperphosphorylation and DNA-binding activity of HSF1 in the absence of heat shock (at 37 degreesC). Since trimerization of HSF1 is known to precede the acquisition of HSF1-DNA binding activity, it seems that MG132- and lactacystin-induced hyperphosphorylation of HSF1 causes conformational changes of HSF1 molecules at 37 degreesC and subsequently triggers its trimerization. Inhibition of protein synthesis by cycloheximide abolished the MG132- or lactacystin-induced hyperphosphorylation and DNA-binding activity of HSF1. These data suggest that the activity of a putative kinase(s) targeting HSF1 is upregulated in the presence of MG132 or lactacystin. The upregulation of the kinase activity requires de novo protein synthesis and is likely due to the inhibition of protein degradation of a short-lived, kinase(s) targeting HSF1 and/or the cofactor(s) for the kinases, through the ubiquitin-proteasome pathway.

Transcriptional activation of gonadotropin-releasing hormone (GnRH) receptor gene by GnRH: involvement of multiple signal transduction pathways

Previous studies have shown that GnRH activates transcriptional activity of its own receptor (GnRHR) gene in part through the cAMP signal transduction pathway. In the present study we explored the possible involvement of multiple signal transduction pathways in GnRH regulation of GnRHR gene transcription; these studies relied upon a luciferase reporter gene vector (GnRHR-pXP2) containing a 1226-bp promoter fragment (-1164 to +62, relative to the major transcription start site) of the mouse GnRHR gene in GGH3 cells (GH3 cells stably expressing rat GnRHR). Activation of protein kinase C (PKC) by phorbol myristic acid significantly stimulated GnRHR-luciferase reporter gene (GnRHR-Luc) activity, but did not potentiate the stimulation of GnRHR-Luc activity by the GnRH agonist, buserelin (GnRH-A). Inhibition of PKC by PKC inhibitor (GF 109203X) or depletion of PKC blocked phorbol myristic acid- or GnRH-A-stimulated GnRHR-Luc activity, but did not affect (Bu)2cAMP-stimulated GnRHR-Luc activity. In addition, GnRH-A-stimulated GnRHR-Luc activity was inhibited by preventing external Ca2+ influx with the external Ca2+ chelator EGTA or the Ca2+ ion channel antagonist, D600. Surprisingly, overexpression of the mitogen-activated protein kinase (MAPK) kinase kinase (Raf-1) inhibited GnRHR-Luc activity and partially blocked GnRH-A-stimulated GnRHR-Luc activity. In contrast, inhibition of MAPK activity by MAPK kinase inhibitor (PD 98059) or by overexpression of kinase-deficient MAPKs activated basal and GnRH-A-stimulated GnRHR-Luc activity. These results suggested that PKC- and Ca2+-dependent signal transduction pathways participate in the GnRH activation of GnRHR promoter activity, and that the MAPK cascade is involved in the negative regulation of basal and GnRH-stimulated GnRHR transcriptional activity conferred by the 1226-bp promoter fragment.

Structural organization and promoter analysis of murine heat shock transcription factor-1 gene

Heat shock factor-1 (HSF-1) activates transcription of heat shock proteins in eukaryotes. Several overlapping genomic clones containing the murine HSF-1 gene were isolated from a phage genomic library. Results indicate that the HSF-1 gene contains 13 exons that span at least 30 kilobase pairs. Sequence analysis of the 5'-untranslated region of HSF-1 suggests that it contains sequences of a recently described Bop1 gene in reverse orientation within its first 331 base pairs (bp) upstream of the translation initiation site. The minimal promoter sequence required for HSF-1 basal expression was identified by deletion analysis from -4 kilobase pairs to -331 bp of the promoter fused to a luciferase reporter gene using transient transfection assays. Results indicate that 331 bp upstream of the HSF-1 translation start site is required for maximal basal expression in NIH3T3 and F9 cells. This fragment also results in high levels of luciferase activity in the reverse orientation, that is, 5' to the Bop1 gene, suggesting that this segment is bidirectional and could be utilized for basal expression of both HSF-1 and Bop1 genes. This segment of the promoter contains recognition elements for Sp1 and CCAAT-box binding transcription factors, which when mutated in either sense or antisense orientations to the HSF-1 gene results in a reduction of basal expression by 50-75% relative to wild type, suggesting that these sites are critical for basal expression of both HSF-1 and Bop1 genes.

Differential activation of p38 mitogen-activated protein kinase and extracellular signal-regulated protein kinases confers cadmium-induced HSP70 expression in 9L rat brain tumor cells

We have reported that treatment with CdCl2 at 40-100 microM induces the heat shock proteins (HSPs) in 9L rat brain tumor cells, during which the activation of heat shock factor (HSF) is essentially involved. By exploiting protein kinase inhibitors, we further analyzed the possible participation of specific protein kinases in the above processes. It was found that induction of HSP70 in cells treated with a high concentration of cadmium (i.e. 100 microM) is preceded by the phosphorylation and activation of p38 mitogen-activated protein kinase (p38(MAPK)), while that in cells treated with a low concentration (60 microM) is accompanied by the phosphorylation and activation of extracellular-regulated protein kinases 1 and 2 (ERK1/2). In 100 microM cadmium-treated cells, both HSP70 induction and HSF1 activation are eliminated in the presence of SB203580, a specific inhibitor of p38(MAPK). By contrast, in 60 microM cadmium-treated cells, the processes are not affected by SB203580 but are significantly suppressed by PD98059, which indirectly inhibits ERK1/2 by acting on MAPK-ERK kinase. Taken together, we demonstrate that p38(MAPK) and ERK1/2 can be simultaneously or independently activated under different concentrations of cadmium and that the signaling pathways participate in the induction of HSP70 by acting on the inducible phosphorylation of HSF1. We thus provide the first evidence that both p38(MAPK) and ERK signaling pathways can differentially participate in the activation of HSF1, which leads to the induction of HSP70 by cadmium.

The role of PKN in the regulation of alphaB-crystallin expression via heat shock transcription factor 1

We previously reported that PKN, a fatty acid-activated serine/threonine protein kinase, translocates from the cytosol to the nucleus by stresses such as heat shock, sodium arsenite, and serum starvation. To clarify the role of PKN under heat stress, we examined whether PKN regulates the expression of heat shock proteins. Co-expression of heat shock transcription factor 1 (HSF1) and the catalytically active fragment of PKN induced the accumulation of alphaB-crystallin but not HSP27 and HSP70 in HeLa S3 cells. The expression of the reporter gene for alphaB-crystallin promoter was activated by co-expression of HSF1 and the catalytically active fragment of PKN, and this activation was dependent on the protein kinase activity of PKN. Deletion analysis of the alphaB-crystallin promoter region revealed that both the proximal and the distal heat shock elements were necessary for the transactivation. These results raise the possibility that there is a signal transduction pathway mediating stress signals for the accumulation of alphaB-crystallin by HSF1 and PKN.

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

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

Modulation of the heat-induced activation of mitogen-activated protein (MAP) kinase by quercetin

Effects of quercetin, a bioflavonoid compound, on heat-induced activation of mitogen-activated protein (MAP) kinase in rat hepatoma (H4) cells were examined. Quercetin decreased cell viability and induced DNA fragmentation in heat-shocked H4 cells. MAP kinase in heat-shocked cells was activated and reached a peak at 1 hr after the heat shock, and then gradually decreased. Quercetin inhibited the heat-induced activation of MAP kinase observed at 1 hr after heat shock, but markedly stimulated MAP kinase activity at 4 hr after heat shock. Thus, quercetin modulated the heat-induced activation of MAP kinase in a biphasic manner. Present observations indicate that quercetin modulates protein phosphorylation, especially that controled by MAP kinase, in early events of heat shock response.

Glycogen synthase kinase 3beta and extracellular signal-regulated kinase inactivate heat shock transcription factor 1 by facilitating the disappearance of transcriptionally active granules after heat shock

Heat shock transcription factor 1 (HSF-1) activates the transcription of heat shock genes in eukaryotes. Under normal physiological growth conditions, HSF-1 is a monomer. Its transcriptional activity is repressed by constitutive phosphorylation. Upon activation, HSF-1 forms trimers, acquires DNA binding activity, increases transcriptional activity, and appears as punctate granules in the nucleus. In this study, using bromouridine incorporation and confocal laser microscopy, we demonstrated that newly synthesized pre-mRNAs colocalize to the HSF-1 punctate granules after heat shock, suggesting that these granules are sites of transcription. We further present evidence that glycogen synthase kinase 3beta (GSK-3beta) and extracellular signal-regulated kinase mitogen-activated protein kinase (ERK MAPK) participate in the down regulation of HSF-1 transcriptional activity. Transient increases in the expression of GSK-3beta facilitate the disappearance of HSF-1 punctate granules and reduce hsp-70 transcription after heat shock. We have also shown that ERK is the priming kinase for GSK-3beta. Taken together, these results indicate that GSK-3beta and ERK MAPK facilitate the inactivation of activated HSF-1 after heat shock by dispersing HSF-1 from the sites of transcription.

Interaction between the Arabidopsis thaliana heat shock transcription factor HSF1 and the TATA binding protein TBP

The heat shock factor (HSF1) is the central regulator of the heat stress (hs) response and is required for stimulating the transcription of the hs genes and consequently the expression of heat shock proteins. To promote the polymerase II-dependent transcription of the hs genes, HSF has to communicate with the basal transcription machinery. Here, we report that the Arabidopsis thaliana HSF1 interacts directly with TBP, the general TATA box binding transcription factor, as shown by affinity chromatography and electrophoretic mobility shift analyses in vitro. An in vivo interaction between AtHSF1 and AtTBP1 was suggested by results employing the yeast two-hybrid system.

Transcriptional activity of heat shock factor 1 at 37 degrees C is repressed through phosphorylation on two distinct serine residues by glycogen synthase kinase 3 and protein kinases Calpha and Czeta

Heat shock factor 1 (HSF1) is the key transcriptional regulator of the heat shock genes that protect cells from environmental stress. However, because heat shock gene expression is deleterious to growth and development, we have examined mechanisms for HSF1 repression at growth temperatures, focusing on the role of phosphorylation. Mitogen-activated protein kinases (MAPKs) of the ERK family phosphorylate HSF1 and represses transcriptional function. The mechanism of repression involves initial phosphorylation by MAP kinase on serine 307, which primes HSF1 for secondary phosphorylation by glycogen synthase kinase 3 on a key residue in repression (serine 303). In vivo expression of glycogen synthase kinase 3 alpha or beta thus represses HSF1 through phosphorylation of serine 303. HSF1 is also phosphorylated by MAPK in vitro on a second residue (serine 363) adjacent to activation domain 1, and this residue is additionally phosphorylated by protein kinase C. In vivo, HSF1 is repressed through phosphorylation of this residue by protein kinase Calpha or -zeta but not MAPK. Regulation at 37 degrees C, therefore, involves the action of three protein kinase cascades that repress HSF1 through phosphorylation of serine residues 303, 307, and 363 and may promote growth by suppressing the heat shock response.

Novel testis-specific protein that interacts with heat shock factor 2

Although heat shock factor 2 (HSF2) binds to heat shock element (HSE) constitutively during differentiation, development and spermatogenesis, little is known about the nature and mechanism of transcriptional control of heat shock genes by HSF2. We screened a human testis cDNA library for proteins that can associate with HSF2 by the yeast two-hybrid system, and isolated clones encoding a novel protein, designated HSF2 binding protein (HSF2BP), that associates with HSF2 in vitro and in vivo and is specifically expressed in testis. The interaction seemed to occur between the trimerization domain of HSF2 and the amino terminal hydrophilic region of HSF2BP that comprises two leucine zipper motifs. HSF2BP may therefore be involved in modulating HSF2 activation in testis.

Function of the C-terminal transactivation domain of human heat shock factor 2 is modulated by the adjacent negative regulatory segment

DNA binding of heat shock factor 2 (HSF2) is induced during hemin-induced differentiation of human erythroleukemia cell line K562. To identify the transcriptional activation and the regulatory domains of HSF2, we constructed a series of deletion derivatives fused to the yeast GAL4 DNA binding domain and analyzed their transactivation activity. A minimal transactivation domain of HSF2 was localized to the C-terminus (residues 472-536), as in HSF1, although amino acid sequence similarity for these regions was rather limited and the potential transactivation ability was about 25% that of HSF1. The transactivation mediated by this region of HSF2 was found to be negatively regulated by the adjacent 18 amino acid segment (residues 428-445) under normal conditions. Furthermore, the latter segment, when fused to the GAL4 activation domain, markedly inhibited GAL4 activity. Extract containing most derivatives of HSF2 retaining this segment exhibited doublet or triplet bands in gel mobility shift assays with heat shock element-containing DNA, suggesting possible involvement of some factors interacting with that segment in the negative regulation. Another putative transactivation domain and two negative regulatory regions were also localized within the internal region.

Transcriptional activation of heat shock factor HSF1 probed by phosphopeptide analysis of factor 32P-labeled in vivo

Mapping of tryptic phosphopeptides of heat shock factor 1 (HSF1) from non-stressed or moderately heat-stressed HeLa cells, labeled in vivo by [32P]orthophosphate, revealed four major phosphopeptides A to D. Heat stress drastically increased phosphopeptide signals. To identify target peptides and amino acids and to correlate phosphorylation and transactivation function, phosphopeptide maps were produced of LexA-human HSF1 chimeras and mutant derivatives thereof, and transactivation activities of original and mutant chimeras were compared. LexA-HSF1 chimeras were previously shown to be regulated identically to HSF1, except that they transactivate promoters with LexA-binding sites instead of hsp promoters. The patterns of phosphopeptides of LexA-HSF1 and endogenous HSF1 were similar. Analysis of single residue substitutions suggested that phosphopeptide C is peptide VKEEPPSPPQSPR (297-309) phosphorylated on Ser-307 but not Ser-303. Substitution of Ser-307 but not Ser-303 caused deregulation of factor activity. Mapping of several constitutively active chimeras associated unphosphorylated peptide C with the transcriptionally active HSF1 conformation, suggesting that dephosphorylation of this peptide (at Ser-307) may either be an integral step in the activation process or serve to maintain the active conformation of HSF1. Exploiting this correlation, indirect evidence was obtained that activation domains of HSF1 interact with the distantly located regulatory domain to maintain the factor in an inactive state.

Disruption of the HSF3 gene results in the severe reduction of heat shock gene expression and loss of thermotolerance

The vertebrate genome encodes a family of heat shock factors (HSFs 1-4) of which the DNA-binding and transcriptional activities of HSF1 and HSF3 are activated upon heat shock. HSF1 has the properties of a classical HSF and exhibits rapid activation of DNA-binding and transcriptional activity upon exposure to conditions of heat shock and other stresses, whereas HSF3 typically is activated at higher temperatures and with distinct delayed kinetics. To address the role of HSF3 in the heat shock response, null cells lacking the HSF3 gene were constructed by disruption of the resident gene by somatic recombination in an avian lymphoid cell line. Null cells lacking HSF3, yet expressing normal levels of HSF1, exhibited a severe reduction in the heat shock response, as measured by inducible expression of heat shock genes, and did not exhibit thermotolerance. At intermediate heat shock temperatures, where HSF1 oligomerizes to an active trimer in wild-type cells, HSF1 remained as an inert monomer in the HSF3 null cell line. HSF3 null cells were restored to a nearly normal heat shock-responsive state by reintroduction of an exogenous HSF3 gene. These results reveal that HSF3 has a dominant role in the regulation of the heat shock response and directly influences HSF1 activity.

Heat shock activates c-Src tyrosine kinases and phosphatidylinositol 3-kinase in NIH3T3 fibroblasts

There is increasing evidence that cellular responses to stress are in part regulated by protein kinases, although specific mechanisms are not well defined. The purpose of these experiments was to investigate potential upstream signaling events activated during heat shock in NIH3T3 fibroblasts. Experiments were designed to ask whether heat shock activates p60 c-Src tyrosine kinase or phosphatidylinositol 3-kinase (PI 3-kinase). Using in vitro protein kinase activity assays, it was demonstrated that heat shock stimulates c-Src and PI 3-kinase activity in a time-dependent manner. Also, there was increased PI 3-kinase activity in anti-phosphotyrosine and anti-c-Src immunoprecipitated immunocomplexes from heated cells. Heat shock activated mitogen-activated protein kinase (MAPK) and p70 S6 kinase (S6K) in these cells. The role of PI 3-kinase in regulating heat shock activation of MAPK and p70 S6K was investigated using wortmannin, a specific pharmacological inhibitor of PI 3-kinase. The results demonstrated that wortmannin inhibited heat shock activation of p70 S6K but only partially inhibited heat activation of MAPK. A dominant negative Raf mutant inhibited activation of MAPK by heat shock but did not inhibit heat shock stimulation of p70 S6K. Genistein, a tyrosine kinase inhibitor, and suramin, a growth factor receptor inhibitor, both inhibited heat shock stimulation of MAPK activity and tyrosine phosphorylation of MAPK. Furthermore, a selective epidermal growth factor receptor (EGFR) inhibitor, tryphostin AG1478, and a dominant negative EGFR mutant also inhibited heat shock activation of MAPK. Heat shock induced EGFR phosphorylation. These results suggest that early upstream signaling events in response to heat stress may involve activation of PI 3-kinase and tyrosine kinases, such as c-Src, and a growth factor receptor, such as EGFR; activation of important downstream pathways, such as MAPK and p70 S6K, occur by divergent signaling mechanisms similar to growth factor stimulation.

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

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

Analysis of the phosphorylation of human heat shock transcription factor-1 by MAP kinase family members

The activation of heat shock transcription factor-1 (HSF-1) after treatment of mammalian cells with stresses such as heat shock, heavy metals, or ethanol induces the synthesis of heat shock proteins. HSF-1 is phosphorylated at normal growth temperature and is hyperphosphorylated upon stress. We recently presented evidence that HSF-1 can be phosphorylated by the mitogen activated protein kinase, ERK1, and that such phosphorylation appears to negatively regulate the activity of HSF-1. In this report, we have tested the ability of ERK1 to phosphorylate various HSF-1 deletion mutants. Our results show that ERK1 phosphorylation is dependent on a region of HSF-1 extending from amino acids 280 to 308. This region contains three serine residues that are potential ERK1 phosphorylation sites. The region falls within a previously defined regulatory domain of HSF-1. The possibility of protein kinases other than ERK1 phosphorylating HSF-1 was also examined using in-gel kinase assays. The results show that HSF-1 can be phosphorylated in a ras-dependent manner by other members of the MAP kinase family such as JNK and p38 protein kinases and possibly others.

Ultraviolet light attenuates heat-inducible gene expression

Ultraviolet light (UV) induces a stress response mediated through transcription factors such as NF-kB and AP-1, yet little is known about its effect on other transactivators of stress gene expression such as heat shock factor (HSF1). Analysis of UV-treated HeLa cells unexpectedly revealed uncoupling of the heat shock response. UV weakly induced HSF1 into its DNA bound state and markedly attenuated heat-inducible gene expression. HSF1 was further analyzed as a potential target for the uncharacteristic uncoupling of the thermal stress response by another type of stress. Heat-inducible multimerization and nuclear translocation of HSF1 were found to be intact in UV-treated cells; however, the monomeric rather than the multimeric form of HSF1 become hyperphosphorylated by UV. This effect could be partially abolished by the antioxidant N-acetyl cysteine with partial reconstitution of hs gene expression. The reported role of a MAP kinase blockade of HSF1 transactivating properties could not be confirmed by an inhibitor of the MAP kinase pathway. Fibroblasts defective in SAP kinase activity also did not exhibit resistance to UV-inducible phosphorylation of HSF1. Two-dimensional phosphopeptide mapping of HSF1 revealed a single tryptic peptide to be affected by UV, but no new pattern of phosphorylation was evident relative to tryptic phosphopeptide profile observed in control cells. These data suggest that UV uncoupling of the hs response possibly involves steps in addition to those associated with phosphorylation the monomeric form of HSF1.

Repression of the heat shock factor 1 transcriptional activation domain is modulated by constitutive phosphorylation

Heat shock transcription factor 1 (HSF1) is constitutively expressed in mammalian cells and negatively regulated for DNA binding and transcriptional activity. Upon exposure to heat shock and other forms of chemical and physiological stress, these activities of HSF1 are rapidly induced. In this report, we demonstrate that constitutive phosphorylation of HSF1 at serine residues distal to the transcriptional activation domain functions to repress transactivation. Tryptic phosphopeptide analysis of a collection of chimeric GAL4-HSF1 deletion and point mutants identified a region of constitutive phosphorylation encompassing serine residues 303 and 307. The significance of phosphorylation at serines 303 and 307 in the regulation of HSF1 transcriptional activity was demonstrated by transient transfection and assay of a chloramphenicol acetyltransferase reporter construct. Whereas the transfected wild-type GAL4-HSF1 chimera is repressed for transcriptional activity and derepressed by heat shock, mutation of serines 303 and 307 to alanine results in derepression to a high level of constitutive activity. Similar results were obtained with mutation of these serine residues in the context of full-length HSF1. These data reveal that constitutive phosphorylation of serines 303 and 307 has an important role in the negative regulation of HSF1 transcriptional activity at control temperatures.