Aromatic-Participant Interactions Are Essential for Disulfide-Bond-Based Trimerization in Human Heat Shock Transcription Factor 1

Intermolecular Interactions between Cysteine and Aromatic Amino Acids with a Phenyl Moiety in the DNA-Binding Domain of Heat Shock Factor 1 Regulate Thermal Stress-Induced Trimerization

In this study, we investigated the trimerization mechanism and structure of heat shock factor 1 (HSF1) using western blotting, tryptophan (Trp) fluorescence spectroscopy, and molecular modeling. First, we examined the DNA-binding domains of human (Homo sapiens), goldfish (Carassius auratus), and walleye pollock (Gadus chalcogrammus) HSF1s by mutating key residues (36 and 103) that are thought to directly affect trimer formation. Human, goldfish, and walleye pollock HSF1s contain cysteine at residue 36 but cysteine (C), tyrosine (Y), and phenylalanine (F), respectively, at residue 103. The optimal trimerization temperatures for the wild-type HSF1s of each species were found to be 42, 37, and 20 °C, respectively. Interestingly, a mutation experiment revealed that trimerization occurred at 42 °C when residue 103 was cysteine, at 37 °C when it was tyrosine, and at 20 °C when it was phenylalanine, regardless of the species. In addition, it was confirmed that when residue 103 of the three species was mutated to alanine, trimerization did not occur. This suggests that in addition to trimerization via disulfide bond formation between the cysteine residues in human HSF1, trimerization can also occur via the formation of a different type of bond between cysteine and aromatic ring residues such as tyrosine and phenylalanine. We also confirmed that at least one cysteine is required for the trimerization of HSF1s, regardless of its position (residue 36 or 103). Additionally, it was shown that the trimer formation temperature is related to growth and survival in fish.

HSF1 and Its Role in Huntington's Disease Pathology

PURPOSE OF REVIEW

Heat shock factor 1 (HSF1) is the master transcriptional regulator of the heat shock response (HSR) in mammalian cells and is a critical element in maintaining protein homeostasis. HSF1 functions at the center of many physiological processes like embryogenesis, metabolism, immune response, aging, cancer, and neurodegeneration. However, the mechanisms that allow HSF1 to control these different biological and pathophysiological processes are not fully understood. This review focuses on Huntington's disease (HD), a neurodegenerative disease characterized by severe protein aggregation of the huntingtin (HTT) protein. The aggregation of HTT, in turn, leads to a halt in the function of HSF1. Understanding the pathways that regulate HSF1 in different contexts like HD may hold the key to understanding the pathomechanisms underlying other proteinopathies. We provide the most current information on HSF1 structure, function, and regulation, emphasizing HD, and discussing its potential as a biological target for therapy.

DATA SOURCES

We performed PubMed search to find established and recent reports in HSF1, heat shock proteins (Hsp), HD, Hsp inhibitors, HSF1 activators, and HSF1 in aging, inflammation, cancer, brain development, mitochondria, synaptic plasticity, polyglutamine (polyQ) diseases, and HD.

STUDY SELECTIONS

Research and review articles that described the mechanisms of action of HSF1 were selected based on terms used in PubMed search.

RESULTS

HSF1 plays a crucial role in the progression of HD and other protein-misfolding related neurodegenerative diseases. Different animal models of HD, as well as postmortem brains of patients with HD, reveal a connection between the levels of HSF1 and HSF1 dysfunction to mutant HTT (mHTT)-induced toxicity and protein aggregation, dysregulation of the ubiquitin-proteasome system (UPS), oxidative stress, mitochondrial dysfunction, and disruption of the structural and functional integrity of synaptic connections, which eventually leads to neuronal loss. These features are shared with other neurodegenerative diseases (NDs). Currently, several inhibitors against negative regulators of HSF1, as well as HSF1 activators, are developed and hold promise to prevent neurodegeneration in HD and other NDs.

CONCLUSION

Understanding the role of HSF1 during protein aggregation and neurodegeneration in HD may help to develop therapeutic strategies that could be effective across different NDs.

Hsf1 on a leash - controlling the heat shock response by chaperone titration

Heat shock factor 1 (Hsf1) is an ancient transcription factor that monitors protein homeostasis (proteostasis) and counteracts disturbances by triggering a transcriptional programme known as the heat shock response (HSR). The HSR is transiently activated and upregulates the expression of core proteostasis genes, including chaperones. Dysregulation of Hsf1 and its target genes are associated with disease; cancer cells rely on a constitutively active Hsf1 to promote rapid growth and malignancy, whereas Hsf1 hypoactivation in neurodegenerative disorders results in formation of toxic aggregates. These central but opposing roles highlight the importance of understanding the underlying molecular mechanisms that control Hsf1 activity. According to current understanding, Hsf1 is maintained latent by chaperone interactions but proteostasis perturbations titrate chaperone availability as a result of chaperone sequestration by misfolded proteins. Liberated and activated Hsf1 triggers a negative feedback loop by inducing the expression of key chaperones. Until recently, Hsp90 has been highlighted as the central negative regulator of Hsf1 activity. In this review, we focus on recent advances regarding how the Hsp70 chaperone controls Hsf1 activity and in addition summarise several additional layers of activity control.

Recombinant buckwheat glutaredoxin intake increases lifespan and stress resistance via hsf-1 upregulation in Caenorhabditis elegans

Glutaredoxin (Grx) is a polypeptide with low molecular weight, which has been extracted from buckwheat and has been suggested to have multiple functions revolving around oxidative stress responses and cell signaling. Here, we report the antioxidant activity of recombinant buckwheat Grx (rbGrx) to reduce aging effects in Caenorhabditis elegans (C. elegans) as well as the mechanism involved. Our results showed that rbGrx beneficially affected the health span of C. elegans, including pharyngeal-pumping rate, locomotion, and lipofuscin accumulation. Furthermore, stress assay showed that rbGrx could extend the lifespan under both oxidative and heat stress. Further studies indicated that the longevity-extending effects of rbGrx could be attributed to its in vitro and in vivo antioxidant activities. After treatment with rbGrx, SOD activity, CAT activity, GSH content, and GSH/GSSG ratio were increased, while MDA content was decreased, which led to low intracellular levels of reactive oxygen species in C. elegans. Moreover, rbGrx up-regulated hsf-1 mRNA level and could not expand the lifespan of the hsf-1 mutant C. elegans (sy441); however, this had no effect on the transcription of daf-16 and skn-1 and could expand the lifespan of both daf-16 and skn-1 mutants. These results suggested dependency of the rbGrx effect on the heat shock transcription factor (HSF-1) and independency on both DAF-16 and SKN-1. In summary, our results demonstrated the anti-aging activity of rbGrx, which increased resistance to cellular stress and improved the health span of C. elegans. These results are very important for the use of rbGrx in anti-aging research.

Mechanisms of Hsp90 regulation

Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in the activation of disparate client proteins. This implicates Hsp90 in diverse biological processes that require a variety of co-ordinated regulatory mechanisms to control its activity. Perhaps the most important regulator is heat shock factor 1 (HSF1), which is primarily responsible for upregulating Hsp90 by binding heat shock elements (HSEs) within Hsp90 promoters. HSF1 is itself subject to a variety of regulatory processes and can directly respond to stress. HSF1 also interacts with a variety of transcriptional factors that help integrate biological signals, which in turn regulate Hsp90 appropriately. Because of the diverse clientele of Hsp90 a whole variety of co-chaperones also regulate its activity and some are directly responsible for delivery of client protein. Consequently, co-chaperones themselves, like Hsp90, are also subject to regulatory mechanisms such as post translational modification. This review, looks at the many different levels by which Hsp90 activity is ultimately regulated.

Regulation of the mammalian heat shock factor 1

Living organisms are endowed with the capability to tackle various forms of cellular stress due to the presence of molecular chaperone machinery complexes that are ubiquitous throughout the cell. During conditions of proteotoxic stress, the transcription factor heat shock factor 1 (HSF1) mediates the elevation of heat shock proteins, which are crucial components of the chaperone complex machinery and function to ameliorate protein misfolding and aggregation and restore protein homeostasis. In addition, HSF1 orchestrates a versatile transcriptional programme that includes genes involved in repair and clearance of damaged macromolecules and maintenance of cell structure and metabolism, and provides protection against a broad range of cellular stress mediators, beyond heat shock. Here, we discuss the structure and function of the mammalian HSF1 and its regulation by post-translational modifications (phosphorylation, sumoylation and acetylation), proteasomal degradation, and small-molecule activators and inhibitors.

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

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

HSF1 protects neurons through a novel trimerization- and HSP-independent mechanism

Heat shock factor 1 (HSF1) protects neurons from death caused by the accumulation of misfolded proteins. It is believed that this protective effect is mediated by the transcriptional stimulation of genes encoding heat shock proteins (HSPs), a family of chaperones that refold or degrade misfolded proteins. Whether HSF1 is protective when neuronal death is not caused by protein misfolding has not been studied. Here, we report that HSF1 expression is necessary for the survival of rat neurons and that HSF1 mRNA and protein expression is reduced in neurons primed to die. Knock-down of HSF1 induces death of otherwise healthy neurons, whereas reestablishment of elevated levels of HSF1 protects neurons even when death is not due to accumulation of misfolded proteins. Neuroprotection by HSF1 does not require its trimerization, an event obligatory for the binding of HSF1 to heat shock elements within HSP gene promoters. Moreover, knock-down of HSP70 or blockade of HSP90 signaling does not reduce neuroprotection by HSF1. Although several neuroprotective molecules and signaling pathways, including CaMK, PKA, Casein kinase-II, and the Raf-MEK-ERK and PI-3K-Akt pathways, are not required for HSF1-mediated neuroprotection, protection is abrogated by inhibition of classical histone deacetylases (HDACs). We report that the novel mechanism of neuroprotection by HSF1 involves cooperation with SIRT1, an HDAC with well documented neuroprotective effects. Using a cell culture model of Huntington's disease, we show that HSF1 trimerization is not required for protection against mutant huntingtin-induced neurotoxicity, suggesting that HSF1 can protect neurons against both proteinopathic and nonproteinopathic death through a noncanonical pathway.

Small molecule activators of the heat shock response: chemical properties, molecular targets, and therapeutic promise

All cells have developed various mechanisms to respond and adapt to a variety of environmental challenges, including stresses that damage cellular proteins. One such response, the heat shock response (HSR), leads to the transcriptional activation of a family of molecular chaperone proteins that promote proper folding or clearance of damaged proteins within the cytosol. In addition to its role in protection against acute insults, the HSR also regulates lifespan and protects against protein misfolding that is associated with degenerative diseases of aging. As a result, identifying pharmacological regulators of the HSR has become an active area of research in recent years. Here, we review progress made in identifying small molecule activators of the HSR, what cellular targets these compounds interact with to drive response activation, and how such molecules may ultimately be employed to delay or reverse protein misfolding events that contribute to a number of diseases.

Functional HSF1 requires aromatic-participant interactions in protecting mouse embryonic fibroblasts against apoptosis via G2 cell cycle arrest

The present study highlighted the aromatic-participant interactions in in vivo trimerization of HSF1 and got an insight into the process of HSF1 protecting against apoptosis. In mouse embryonic fibroblasts (MEFs), mutations of mouse HSF1 (W37A, Y60A and F104A) resulted in a loss of trimerization activity, impaired binding of the heat shock element (HSE) and lack of heat shock protein 70 (HSP70) expression after a heat shock. Under UV irradiation, wild-type mouse HSF1 protected the MEFs from UV-induced apoptosis, but none of the mutants offered protection. We found that normal expression of HSF1 was essential to the cell arrest in G2 phase, assisting with the cell cycle checkpoint. The cells that lack normal HSF1 failed to arrest in the G2 phase, resulting in the process of cell apoptosis. We conclude that the treatment with UV or heat shock stresses appears to induce the approach of HSF1 monomers directly via aromatic-participant interactions, followed by the formation of a HSF1 trimer. HSF1 protects the MEFs from the stresses through the expression of HSPs and a G2 cell cycle arrest.

Regulation of HSF1 function in the heat stress response: implications in aging and disease

To dampen proteotoxic stresses and maintain protein homeostasis, organisms possess a stress-responsive molecular machinery that detects and neutralizes protein damage. A prominent feature of stressed cells is the increased synthesis of heat shock proteins (Hsps) that aid in the refolding of misfolded peptides and restrain protein aggregation. Transcriptional activation of the heat shock response is orchestrated by heat shock factor 1 (HSF1), which rapidly translocates to hsp genes and induces their expression. Although the role of HSF1 in protecting cells and organisms against severe stress insults is well established, many aspects of how HSF1 senses qualitatively and quantitatively different forms of stresses have remained poorly understood. Moreover, recent discoveries that HSF1 controls life span have prompted new ways of thinking about an old transcription factor. Here, we review the established role of HSF1 in counteracting cell stress and prospect the role of HSF1 as a regulator of disease states and aging.

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

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

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

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

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

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