Repression of human heat shock factor 1 activity at control temperature by phosphorylation

Repression motif in HSF1 regulated by phosphorylation

The heat shock factor 1 (HSF1) is a transcription factor that itself is a sensor for stress and integrates various intrinsic or environmental stress sensing pathways. Thus HSF1 orchestrates the heat shock response (HSR) by translating these pathways into a distinct transcriptional program that aids the cells to cope with and adapt to proteotoxic stress. Although heavily researched the regulation of HSF1 activation is still not completely understood. A conserved reaction to stress is the hyperphosphorylation of the otherwise confined constitutive phosphorylated HSF1. Therefore, this stress specific phosphorylation is believed to be involved in the regulatory mechanism and hence, was and is focus of many studies, ascribing various effects to single phosphorylation sites. To gain additional insight into effects of phosphorylation, HSF1 carrying amino acid substitutions on up to 18 amino acids were tested for their transactivation potential on an HSR reporter plasmid. A pattern of eleven phosphor-mimicking and diminishing amino acid substitutions on well-known phosphorylation sites of HSF1 were introduced to produce transcriptional active [11 M(+)] or repressed [11 M(-)] phenotypes. It could be confirmed that heat activates HSF1 regardless of phosphorylation. Distinct cellular stress, obtained by chemical HSR inducers or mimicked by a constitutively active HSF1, showed clear differences in the activation potential of HSF1-11 M(+) and 11 M(-). Further refinement to the single amino acid level identified the S303/307 double-phosphorylation motif, wherein phosphorylation of S303 was sole responsible for the repressing effect. The effect could be reproduced in different cell lines and is not entirely based on degradation. A small repression motif could be dissociated from the HSF1 context, which is still capable of repressing the background transcription of a specifically designed reporter plasmid. Taken together these results indicate, that besides already described mechanisms of pS303/307 mediated repression of HSF1 activation, an additional mechanism repressing the transcriptional output of the entire HSE containing promoter is mediated by this small repressive motif.

Proteotoxic stress response in atherosclerotic cardiovascular disease: Emerging role of heat shock factor 1

Atherosclerosis is a major risk factor for cardiovascular diseases. Hypercholesterolemia has been both clinically and experimentally linked to cardiovascular disease and is involved in the initiation of atherosclerosis. Heat shock factor 1 (HSF1) is involved in the control of atherosclerosis. HSF1 is a critical transcriptional factor of the proteotoxic stress response that regulates the production of heat shock proteins (HSPs) and other important activities such as lipid metabolism. Recently, HSF1 is reported to directly interact with and inhibit AMP-activated protein kinase (AMPK) to promote lipogenesis and cholesterol synthesis. This review highlights roles of HSF1 and HSPs in critical metabolic pathways of atherosclerosis, including lipogenesis and proteome homeostasis.

Protein Kinase CK2 and Its Potential Role as a Therapeutic Target in Huntington's Disease

Huntington's Disease (HD) is a devastating neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene, for which no disease modifying therapies are currently available. Much of the recent research has focused on developing therapies to directly lower HTT expression, and while promising, these therapies have presented several challenges regarding administration and efficacy. Another promising therapeutic approach is the modulation of HTT post-translational modifications (PTMs) that are dysregulated in disease and have shown to play a key role in HTT toxicity. Among all PTMs, modulation of HTT phosphorylation has been proposed as an attractive therapeutic option due to the possibility of orally administering specific kinase effectors. One of the kinases described to participate in HTT phosphorylation is Protein Kinase CK2. CK2 has recently emerged as a target for the treatment of several neurological and psychiatric disorders, although its role in HD remains controversial. While pharmacological studies in vitro inhibiting CK2 resulted in reduced HTT phosphorylation and increased toxicity, genetic approaches in mouse models of HD have provided beneficial effects. In this review we discuss potential therapeutic approaches related to the manipulation of HTT-PTMs with special emphasis on the role of CK2 as a therapeutic target in HD.

HSF-1: Guardian of the Proteome Through Integration of Longevity Signals to the Proteostatic Network

Discoveries made in the nematode Caenorhabditis elegans revealed that aging is under genetic control. Since these transformative initial studies, C. elegans has become a premier model system for aging research. Critically, the genes, pathways, and processes that have fundamental roles in organismal aging are deeply conserved throughout evolution. This conservation has led to a wealth of knowledge regarding both the processes that influence aging and the identification of molecular and cellular hallmarks that play a causative role in the physiological decline of organisms. One key feature of age-associated decline is the failure of mechanisms that maintain proper function of the proteome (proteostasis). Here we highlight components of the proteostatic network that act to maintain the proteome and how this network integrates into major longevity signaling pathways. We focus in depth on the heat shock transcription factor 1 (HSF1), the central regulator of gene expression for proteins that maintain the cytosolic and nuclear proteomes, and a key effector of longevity signals.

Cellular stress response to extremely low-frequency electromagnetic fields (ELF-EMF): An explanation for controversial effects of ELF-EMF on apoptosis

Impaired apoptosis is one of the hallmarks of cancer, and almost all of the non‐surgical approaches of eradicating tumour cells somehow promote induction of apoptosis. Indeed, numerous studies have stated that non‐ionizing non‐thermal extremely low‐frequency magnetic fields (ELF‐MF) can modulate the induction of apoptosis in exposed cells; however, much controversy exists in observations. When cells are exposed to ELF‐EMF alone, very low or no statistically significant changes in apoptosis are observed. Contrarily, exposure to ELF‐EMF in the presence of a co‐stressor, including a chemotherapeutic agent or ionizing radiation, can either potentiate or inhibit apoptotic effects of the co‐stressor. In our idea, the main point neglected in interpreting these discrepancies is “the cellular stress responses” of cells following ELF‐EMF exposure and its interplay with apoptosis. The main purpose of the current review was to outline the triangle of ELF‐EMF, the cellular stress response of cells and apoptosis and to interpret and unify discrepancies in results based on it. Therefore, initially, we will describe studies performed on identifying the effect of ELF‐EMF on induction/inhibition of apoptosis and enumerate proposed pathways through which ELF‐EMF exposure may affect apoptosis; then, we will explain cellular stress response and cues for its induction in response to ELF‐EMF exposure; and finally, we will explain why such controversies have been observed by different investigators.

Targeting of HSP70/HSF1 Axis Abrogates In Vitro Ibrutinib-Resistance in Chronic Lymphocytic Leukemia

Simple Summary

The use of ibrutinib has changed the management and clinical history of patients with multiple-treated chronic lymphocytic leukemia (CLL). Nevertheless, an increasing number of patients develop resistance to treatment, with mechanisms still to be fully clarified. Since HSP70 plays a pivotal role in mediating the survival and the progression of CLL, we herein addressed the role of HSP70 and its regulator HSF1 in the development of ibrutinib-mediated resistance. We found an increase in both proteins when the treatment was failing, and thus the disease was progressing. This suggests the involvement of HSP70 in mechanisms of drug resistance. Moreover, we demonstrated that the use, at different levels, of HSP70/HSF1 axis inhibitors could represent a novel rational therapeutic approach to overcome ibrutinib resistance in those patients who relapsed after this type of treatment.

Abstract

The Btk inhibitor ibrutinib has significantly changed the management of chronic lymphocytic leukemia (CLL) patients. Despite its clinical efficacy, relapses occur, and outcomes after ibrutinib failure are poor. Although BTK and PLCγ2 mutations have been found to be associated with ibrutinib resistance in a fair percentage of CLL patients, no information on resistance mechanisms is available in patients lacking these mutations. The heat shock protein of 70 kDa (HSP70) and its transcription factor heat shock factor 1 (HSF1) play a role in mediating the survival and progression of CLL, as well as taking part in drug resistance in various cancers. We demonstrated that resveratrol and related phenols were able to induce apoptosis in vitro in leukemic cells from CLL untreated patients by acting on the HSP70/HSF1 axis. The same was achieved in cells recovered from 13 CLL patients failing in vivo ibrutinib treatment. HSP70 and HSF1 levels decreased following in vitro treatment, correlating to apoptosis induction. We suggest an involvement of HSP70/HSF1 axis in controlling resistance to ibrutinib in CLL cells, since their inhibition is effective in inducing in vitro apoptosis in cells from ibrutinib refractory patients. The targeting of HSP70/HSF1 axis could represent a novel rational therapeutic strategy for CLL, also for relapsing patients.

Augmentation of the heat shock axis during exceptional longevity in Ames dwarf mice

How the heat shock axis, repair pathways, and proteostasis impact the rate of aging is not fully understood. Recent reports indicate that normal aging leads to a 50% change in several regulatory elements of the heat shock axis. Most notably is the age-dependent enhancement of inhibitory signals associated with accumulated heat shock proteins and hyper-acetylation associated with marked attenuation of heat shock factor 1 (HSF1)–DNA binding activity. Because exceptional longevity is associated with increased resistance to stress, this study evaluated regulatory check points of the heat shock axis in liver extracts from 12 months and 24 months long-lived Ames dwarf mice and compared these findings with aging wild-type mice. This analysis showed that 12M dwarf and wild-type mice have comparable stress responses, whereas old dwarf mice, unlike old wild-type mice, preserve and enhance activating elements of the heat shock axis. Old dwarf mice thwart negative regulation of the heat shock axis typically observed in usual aging such as noted in HSF1 phosphorylation at Ser307 residue, acetylation within its DNA binding domain, and reduction in proteins that attenuate HSF1–DNA binding. Unlike usual aging, dwarf HSF1 protein and mRNA levels increase with age and further enhance by stress. Together these observations suggest that exceptional longevity is associated with compensatory and enhanced HSF1 regulation as an adaptation to age-dependent forces that otherwise downregulate the heat shock axis.

Heat shock transcription factor 1 is SUMOylated in the activated trimeric state

The heat shock response is a transcriptional program of organisms to counteract an imbalance in protein homeostasis. It is orchestrated in all eukaryotic cells by heat shock transcription factor 1 (Hsf1). Despite very intensive research, the intricacies of the Hsf1 activation-attenuation cycle remain elusive at a molecular level. Post-translational modifications belong to one of the key mechanisms proposed to adapt the Hsf1 activity to the needs of individual cells, and phosphorylation of Hsf1 at multiple sites has attracted much attention. According to cell biological and proteomics data, Hsf1 is also modified by small ubiquitin-like modifier (SUMO) at several sites. How SUMOylation affects Hsf1 activity at a molecular level is still unclear. Here, we analyzed Hsf1 SUMOylation in vitro with purified components to address questions that could not be answered in cell culture models. In vitro Hsf1 is primarily conjugated at lysine 298 with a single SUMO, though we did detect low-level SUMOylation at other sites. Different SUMO E3 ligases such as protein inhibitor of activated STAT 4 enhanced the efficiency of in vitro modification but did not alter SUMO site preferences. We provide evidence that Hsf1 trimerization and phosphorylation at serines 303 and 307 increases SUMOylation efficiency, suggesting that Hsf1 is SUMOylated in its activated state. Hsf1 can be SUMOylated when DNA bound, and SUMOylation of Hsf1 does neither alter DNA-binding affinity nor affects heat shock cognate 71kDa protein (HSPA8)+DnaJ homolog subfamily B member 1-mediated monomerization of Hsf1 trimers and concomitant dislocation from DNA. We propose that SUMOylation acts at the transcription level of the heat shock response.

Neutralization of HSF1 in cells from PIK3CA-related overgrowth spectrum patients blocks abnormal proliferation

PIK3CA-related overgrowth spectrum is caused by mosaicism mutations in the PIK3CA gene. These mutations, which are also observed in various types of cancer, lead to a constitutive activation of the PI3K/AKT/mTOR pathway, increasing cell proliferation. Heat shock transcription factor 1 (HSF1) is the major stress-responsive transcription factor. Recent findings indicate that AKT phosphorylates and activates HSF1 independently of heat-shock in breast cancer cells. Here, we aimed to investigate the role of HSF1 in PIK3CA-related overgrowth spectrum. We observed a higher rate of proliferation and increased phosphorylation of AKT and p70S6K in mutant fibroblasts than in control cells. We also found elevated phosphorylation and activation of HSF1, which is directly correlated to AKT activation. Specific AKT inhibitors inhibit HSF1 phosphorylation as well as HSF1-dependent gene transcription. Finally, we demonstrated that targeting HSF1 with specific inhibitors reduced the proliferation of mutant cells. As there is currently no curative treatment for PIK3CA-related overgrowth spectrum, our results identify HSF1 as a new potential therapeutic target.

Molecular Mechanisms of Heat Shock Factors in Cancer

Malignant transformation is accompanied by alterations in the key cellular pathways that regulate development, metabolism, proliferation and motility as well as stress resilience. The members of the transcription factor family, called heat shock factors (HSFs), have been shown to play important roles in all of these biological processes, and in the past decade it has become evident that their activities are rewired during tumorigenesis. This review focuses on the expression patterns and functions of HSF1, HSF2, and HSF4 in specific cancer types, highlighting the mechanisms by which the regulatory functions of these transcription factors are modulated. Recently developed therapeutic approaches that target HSFs are also discussed.

Cell-Nonautonomous Regulation of Proteostasis in Aging and Disease

The functional health of the proteome is determined by properties of the proteostasis network (PN) that regulates protein synthesis, folding, macromolecular assembly, translocation, and degradation. In eukaryotes, the PN also integrates protein biogenesis across compartments within the cell and between tissues of metazoans for organismal health and longevity. Additionally, in metazoans, proteome stability and the functional health of proteins is optimized for development and yet declines throughout aging, accelerating the risk for misfolding, aggregation, and cellular dysfunction. Here, I describe the cell-nonautonomous regulation of organismal PN by tissue communication and cell stress-response pathways. These systems are robust from development through reproductive maturity and are genetically programmed to decline abruptly in early adulthood by repression of the heat shock response and other cell-protective stress responses, thus compromising the ability of cells and tissues to properly buffer against the cumulative stress of protein damage during aging. While the failure of multiple protein quality control processes during aging challenges cellular function and tissue health, genetic studies, and the identification of small-molecule proteostasis regulators suggests strategies that can be employed to reset the PN with potential benefit on cellular health and organismal longevity.

Modulation of Heat Shock Factor 1 Activity through Silencing of Ser303/Ser307 Phosphorylation Supports a Metabolic Program Leading to Age-Related Obesity and Insulin Resistance

Activation of the adaptive response to cellular stress orchestrated by heat shock factor 1 (HSF1), which is an evolutionarily conserved transcriptional regulator of chaperone response and cellular bioenergetics in diverse model systems, is a central feature of organismal defense from environmental and cellular stress. HSF1 activity, induced by proteostatic, metabolic, and growth factor signals, is regulated by posttranscriptional modifications, yet the mechanisms that regulate HSF1 and particularly the functional significance of these modifications in modulating its biological activity in vivo remain unknown. HSF1 phosphorylation at both Ser303 (S303) and Ser307 (S307) has been shown to repress HSF1 transcriptional activity under normal physiological growth conditions. To determine the biological relevance of these HSF1 phosphorylation events, we generated a knock-in mouse model in which S303 and S307 were replaced with alanine (HSF1^303A/307A). Our results confirmed that loss of phosphorylation in HSF1^303A/307A cells and tissues increases protein stability but also markedly sensitizes HSF1 activation under normal and heat- or nutrient-induced stress conditions. Interestingly, the enhanced HSF1 activation in HSF1^303A/307A mice activates a supportive metabolic program that aggravates the development of age-dependent obesity, fatty liver diseases, and insulin resistance. Thus, these findings highlight the importance of a posttranslational mechanism (through phosphorylation at S303 and S307 sites) of regulation of the HSF1-mediated transcriptional program that moderates the severity of nutrient-induced metabolic diseases.

Rethinking HSF1 in Stress, Development, and Organismal Health

The heat shock response (HSR) was originally discovered as a transcriptional response to elevated temperature shock and led to the identification of heat shock proteins and heat shock factor 1 (HSF1). Since then HSF1 has been shown to be important for combating other forms of environmental perturbations as well as genetic variations that cause proteotoxic stress. The HSR has long been thought to be an absolute response to conditions of cell stress and the primary mechanism by which HSF1 promotes organismal health by preventing protein aggregation and subsequent proteome imbalance. Accumulating evidence now shows that HSF1, the central player in the HSR, is regulated according to specific cellular requirements through cell-autonomous and non-autonomous signals, and directs transcriptional programs distinct from the HSR during development and in carcinogenesis. We discuss here these 'non-canonical' roles of HSF1, its regulation in diverse conditions of development, reproduction, metabolism, and aging, and posit that HSF1 serves to integrate diverse biological and pathological responses.

The Nuclear Proteome of White and Gray Matter from Schizophrenia Postmortem Brains

Schizophrenia (SCZ) is a serious neuropsychiatric disorder that manifests through several symptoms from early adulthood. Numerous studies over the last decades have led to significant advances in increasing our understanding of the factors involved in SCZ. For example, mass spectrometry-based proteomic analysis has provided important insights by uncovering protein dysfunctions inherent to SCZ. Here, we present a comprehensive analysis of the nuclear proteome of postmortem brain tissues from corpus callosum (CC) and anterior temporal lobe (ATL). We show an overview of the role of deregulated nuclear proteins in these two main regions of the brain: the first, mostly composed of glial cells and axons of neurons, and the second, represented mainly by neuronal cell bodies. These samples were collected from SCZ patients in an attempt to characterize the role of the nucleus in the disease process. With the ATL nucleus enrichment, we found 224 proteins present at different levels, and 76 of these were nuclear proteins. In the CC analysis, we identified 119 present at different levels, and 24 of these were nuclear proteins. The differentially expressed nuclear proteins of ATL are mainly associated with the spliceosome, whereas those of the CC region are associated with calcium/calmodulin signaling.

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.

Abnormal degradation of the neuronal stress-protective transcription factor HSF1 in Huntington's disease

Huntington's Disease (HD) is a neurodegenerative disease caused by poly-glutamine expansion in the Htt protein, resulting in Htt misfolding and cell death. Expression of the cellular protein folding and pro-survival machinery by heat shock transcription factor 1 (HSF1) ameliorates biochemical and neurobiological defects caused by protein misfolding. We report that HSF1 is degraded in cells and mice expressing mutant Htt, in medium spiny neurons derived from human HD iPSCs and in brain samples from patients with HD. Mutant Htt increases CK2α' kinase and Fbxw7 E3 ligase levels, phosphorylating HSF1 and promoting its proteasomal degradation. An HD mouse model heterozygous for CK2α' shows increased HSF1 and chaperone levels, maintenance of striatal excitatory synapses, clearance of Htt aggregates and preserves body mass compared with HD mice homozygous for CK2α'. These results reveal a pathway that could be modulated to prevent neuronal dysfunction and muscle wasting caused by protein misfolding in HD.

Regulatory Mechanisms of Hsp90

The ability of Hsp90 to activate a disparate clientele implicates this chaperone in diverse biological processes. To accommodate such varied roles, Hsp90 requires a variety of regulatory mechanisms that are coordinated in order to modulate its activity appropriately. Amongst these, the master-regulator heat shock factor 1 (HSF1) is critically important in upregulating Hsp90 during stress, but is also responsible, through interaction with specific transcription factors (such as STAT1 and Strap/p300) for the integration of a variety of biological signals that ultimately modulate Hsp90 expression. Additionally, transcription factors, such as STAT1, STAT3 (including STAT1-STAT3 oligomers), NF-IL6, and NF-kB, are known to influence Hsp90 expression directly. Co-chaperones offer another mechanism for Hsp90 regulation, and these can modulate the chaperone cycle appropriately for specific clientele. Co-chaperones include those that deliver specific clients to Hsp90, and others that regulate the chaperone cycle for specific Hsp90-client complexes by modulating Hsp90s ATPase activity. Finally, post-translational modification (PTM) of Hsp90 and its co-chaperones helps too further regulate the variety of different Hsp90 complexes found in cells.

CD30 Induces Heat Shock Protein 90 and Signal Integration in Classic Hodgkin Lymphoma Cells

Previous studies report deregulation of multiple signaling pathways in classic Hodgkin lymphoma (cHL) cells. However, the mechanisms of how these pathways are integrated are not fully understood. Herein, we show involvement of cHL hallmark antigen CD30 in this process. CD30 facilitates phosphorylation of heat shock factor 1, activates heat shock promoter element, and induces heat shock protein (HSP) 90. CD30 repression and subsequent inhibition of HSP90 suppresses NF-κB, extracellular signal-regulated kinase, AKT, and STAT pathways in cHL cell lines. Thus, CD30-mediated induction of HSP90 appears to serve as a central hub for integration of intracellular signaling in cHL cells. We also show that CD30 induces HSP90 through phosphorylation of heat shock factor 1 via c-Jun N-terminal kinase in cHL cells. Although anaplastic large-cell lymphoma (ALCL) also is associated with CD30 overexpression, our experiments reveal that HSP90 induction in ALCL-bearing nucleophosmin-anaplastic lymphoma kinase (ALK) does not depend on CD30 but instead on ALK via c-Jun N-terminal kinase. Together, these results highlight a novel role for CD30 in mediating integration of signaling pathways of cHL cells while being replaced in this function by ALK in ALCL cells.

Dynamic control of Hsf1 during heat shock by a chaperone switch and phosphorylation

Heat shock factor (Hsf1) regulates the expression of molecular chaperones to maintain protein homeostasis. Despite its central role in stress resistance, disease and aging, the mechanisms that control Hsf1 activity remain unresolved. Here we show that in budding yeast, Hsf1 basally associates with the chaperone Hsp70 and this association is transiently disrupted by heat shock, providing the first evidence that a chaperone repressor directly regulates Hsf1 activity. We develop and experimentally validate a mathematical model of Hsf1 activation by heat shock in which unfolded proteins compete with Hsf1 for binding to Hsp70. Surprisingly, we find that Hsf1 phosphorylation, previously thought to be required for activation, in fact only positively tunes Hsf1 and does so without affecting Hsp70 binding. Our work reveals two uncoupled forms of regulation - an ON/OFF chaperone switch and a tunable phosphorylation gain - that allow Hsf1 to flexibly integrate signals from the proteostasis network and cell signaling pathways.

DOI:http://dx.doi.org/10.7554/eLife.18638.001

Proteins are strings of amino acids that carry out crucial activities inside cells, such as harvesting energy and generating the building blocks that cells need to grow. In order to carry out their specific roles inside the cell, the proteins need to “fold” into precise three-dimensional shapes.

Protein folding is critical for life, and cells don’t leave it up to chance. Cells employ “molecular chaperones” to help proteins to fold properly. However, under some conditions – such as high temperature – proteins are more difficult to fold and the chaperones can become overwhelmed. In these cases, unfolded proteins can pile up in the cell. This leads not only to the cell being unable to work properly, but also to the formation of toxic “aggregates”. These aggregates are tangles of unfolded proteins that are hallmarks of many neurodegenerative diseases such as Alzheimer’s, Parkinson’s and amyotrophic lateral sclerosis (ALS).

Protein aggregates can be triggered by high temperature in a condition termed “heat shock”. A sensor named heat shock factor 1 (Hsf1 for short) increases the amount of chaperones following heat shock. But what controls the activity of Hsf1?

To answer this question, Zheng, Krakowiak et al. combined mathematical modelling and experiments in yeast cells. The most important finding is that the ‘on/off switch’ that controls Hsf1 is based on whether Hsf1 is itself bound to a chaperone. When bound to the chaperone, Hsf1 is turned ‘off’; when the chaperone falls off, Hsf1 turns ‘on’ and makes more chaperones; when there are enough chaperones, they once again bind to Hsf1 and turn it back ‘off’. In this way, Hsf1 and the chaperones form a feedback loop that ensures that there are always enough chaperones to keep the cell’s proteins folded.

Now that we know how Hsf1 is controlled, can we harness this understanding to tune the activity of Hsf1 without disrupting how the chaperones work? If we can activate Hsf1, we can provide cells with more chaperones. This could be a therapeutic strategy to combat neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.18638.002

Metabolic control of the proteotoxic stress response: implications in diabetes mellitus and neurodegenerative disorders

Proteome homeostasis, or proteostasis, is essential to maintain cellular fitness and its disturbance is associated with a broad range of human health conditions and diseases. Cells are constantly challenged by various extrinsic and intrinsic insults, which perturb cellular proteostasis and provoke proteotoxic stress. To counter proteomic perturbations and preserve proteostasis, cells mobilize the proteotoxic stress response (PSR), an evolutionarily conserved transcriptional program mediated by heat shock factor 1 (HSF1). The HSF1-mediated PSR guards the proteome against misfolding and aggregation. In addition to proteotoxic stress, emerging studies reveal that this proteostatic mechanism also responds to cellular energy state. This regulation is mediated by the key cellular metabolic sensor AMP-activated protein kinase (AMPK). In this review, we present an overview of the maintenance of proteostasis by HSF1, the metabolic regulation of the PSR, particularly focusing on AMPK, and their implications in the two major age-related diseases-diabetes mellitus and neurodegenerative disorders.

Bacterial endotoxin modifies heat shock factor-1 activity in RAW 264.7 cells: implications for TNF-α regulation during exposure to febrile range temperatures

Recent studies have identified heat shock factor (HSF)-1, the predominant heat/stress-stimulated transcriptional activator of heat shock protein genes as a repressor of certain cytokine genes, including TNF-α and IL-1β. We previously showed that exposing macrophages to febrile-range temperature (FRT; 39.5°C) activates HSF-1 to a DNA binding form that does not activate heat shock protein gene transcription, but apparently represses TNF-α and IL-1β transcription. Prewarming macrophages to 39.5°C for 30 min prior to stimulation with bacterial lipopolysaccharide (LPS) does not change the induction of TNF-α transcription, but markedly reduces its duration. This raised the question of how TNF-α transcription could occur at all in the presence of activated HSF-1. We used RAW 264.7 cells to test the hypothesis that macrophage activation triggers a transient reversal of HSF-1-mediated repression, thereby allowing induction of TNF-α transcription. Electrophoretic mobility shift assays revealed that LPS triggers a transient inactivation of HSF-1 that temporally correlates with TNF-α transcription and was associated with a transient increase in HSF-1 molecular weight, a decrease in its pI, and appearance of HSF-1 phosphorylating activity. The serine/threonine phosphatase inhibitor, calyculin A, blocked the inhibitory affect of FRT on LPS-induced TNF-α generation and prevented the re-activation of HSF-1. We propose that LPS stimulation of FRT-exposed macrophages stimulates a sequential phosphorylation and dephosphorylation of HSF-1, causing a cycle of inactivation and re-activation of HSF-1 repressor activity that allows a temporally-limited period of gene transcription.

Exercise-induced ROS in heat shock proteins response

Cells have evolved multiple and sophisticated stress response mechanisms aiming to prevent macromolecular (including proteins, lipids, and nucleic acids) damage and to maintain or re-establish cellular homeostasis. Heat shock proteins (HSPs) are among the most highly conserved, ubiquitous, and abundant proteins in all organisms. Originally discovered more than 50 years ago through heat shock stress, they display multiple, remarkable roles inside and outside cells under a variety of stresses, including also oxidative stress and radiation, recognizing unfolded or misfolded proteins and facilitating their restructuring. Exercise consists in a combination of physiological stresses, such as metabolic disturbances, changes in circulating levels of hormones, increased temperature, induction of mild to severe inflammatory state, increased production of reactive oxygen and nitrogen species (ROS and RNS). As a consequence, exercise is one of the main stimuli associated with a robust increase in different HSPs in several tissues, which appears to be also fundamental in facilitating the cellular remodeling processes related to the training regime. Among all factors involved in the exercise-related modulation of HSPs level, the ROS production in the contracting muscle or in other tissues represents one of the most attracting, but still under discussion, mechanism. Following exhaustive or damaging muscle exercise, major oxidative damage to proteins and lipids is likely involved in HSP expression, together with mechanically induced damage to muscle proteins and the inflammatory response occurring several days into the recovery period. Instead, the transient and reversible oxidation of proteins by physiological concentrations of ROS seems to be involved in the activation of stress response following non-damaging muscle exercise. This review aims to provide a critical update on the role of HSPs response in exercise-induced adaptation or damage in humans, focusing on experimental results where the link between redox homeostasis and HSPs expression by exercise has been addressed. Further, with the support of in vivo and in vitro studies, we discuss the putative molecular mechanisms underlying the ROS-mediated modulation of HSP expression and/or activity during exercise.

Heat Shock Factor 1 Is a Substrate for p38 Mitogen-Activated Protein Kinases

Heat shock factor 1 (HSF1) monitors the structural integrity of the proteome. Phosphorylation at S326 is a hallmark for HSF1 activation, but the identity of the kinase(s) phosphorylating this site has remained elusive. We show here that the dietary agent phenethyl isothiocyanate (PEITC) inhibits heat shock protein 90 (Hsp90), the main negative regulator of HSF1; activates p38 mitogen-activated protein kinase (MAPK); and increases S326 phosphorylation, trimerization, and nuclear translocation of HSF1, and the transcription of a luciferase reporter, as well as the endogenous prototypic HSF1 target Hsp70. In vitro, all members of the p38 MAPK family rapidly and stoichiometrically catalyze the S326 phosphorylation. The use of stable knockdown cell lines and inhibitors indicated that among the p38 MAPKs, p38γ is the principal isoform responsible for the phosphorylation of HSF1 at S326 in cells. A protease-mass spectrometry approach confirmed S326 phosphorylation and unexpectedly revealed that p38 MAPK also catalyzes the phosphorylation of HSF1 at S303/307, previously known repressive posttranslational modifications. Thus, we have identified p38 MAPKs as highly efficient catalysts for the phosphorylation of HSF1. Furthermore, our findings suggest that the magnitude and persistence of activation of p38 MAPK are important determinants of the extent and duration of the heat shock response.

Heat Shock Proteins (HSP): Future Trends in Cancer Immunotherapy

Heat Shock Proteins (HSPs) are a large family of highly conserved proteins involved in assisting protein folding and unfolding in the cells. HSPs are expressed constitutively as well as inducibly and, interacting with antigen presenting cells, induce the expression of various cytokines and chemokines as well as the maturation and migration of dendritic cells, thus acting themselves as cytokines. HSP-chaperoned antigenic peptides are also generated within the tumor cells. Such chaperoned peptides are released in the extra cellular medium with an association of HSPs by cell stress, death or tumor cell lyses. HSP-peptide complexes from extra cellular medium are taken up by antigen presenting cells through CD91 receptor and are represented or cross-presented by their MHC class I molecules for specific anti-tumor immune response. In addition, HSPs expressed on the cell surface of tumor cells stimulate αβ T-cells and γδ T-cells as well as natural killer (NK) cells that are first-line defense mechanisms. In this manner, HSPs have the ability to stimulate both arms of the effecter mechanism of the immune system. These unique immunological attributes of HSPs are presently becoming the basis for tumor immunotherapy. Tumor-derived HSP-peptide complexes have been demonstrated to serve as anti-tumor vaccines. To date various approaches of vaccination using HSPs have been developed and tested clinically. These HSP-based vaccine approaches can be combined with hyperthermia and CTLA-4 blockade to enhance their anti-tumor potentiality.

In vitro evaluation of aspirin-induced HspB1 against heat stress damage in chicken myocardial cells

To understand the potential association of heat stress resistance with HspB1 induction by aspirin (ASA) in chicken myocardial cells, variations of HspB1 expression and heat stressed-induced damage of myocardial cells after ASA administration were studied in primary cultured myocardial cells. Cytopathological lesions as well as damage-related enzymes, such as creatine kinase-MB (CK-MB) and lactate dehydrogenase (LDH), indicated the considerable protective ability of ASA pre-treatment against acute heat stress. Immunostaining assays showed that heat stress caused HspB1 to relocate into the nucleus, while ASA did not. ELISA analysis, revealed that HspB1 expression induced by ASA averaged 45.62-fold higher than that of the control. These results indicated that the acute heat-stressed injuries were accompanied by comparatively lower HspB1 expression caused by heat stress in vitro. ASA pre-treatment induced a level of HspB1 presumed to be sufficient to protect myocardial cells from acute heat stress in the extracorporal model, although more detailed mechanisms will require further investigation.

Heat shock proteins in the retina: Focus on HSP70 and alpha crystallins in ganglion cell survival

Heat shock proteins (HSPs) belong to a superfamily of stress proteins that are critical constituents of a complex defense mechanism that enhances cell survival under adverse environmental conditions. Cell protective roles of HSPs are related to their chaperone functions, antiapoptotic and antinecrotic effects. HSPs' anti-apoptotic and cytoprotective characteristics, their ability to protect cells from a variety of stressful stimuli, and the possibility of their pharmacological induction in cells under pathological stress make these proteins an attractive therapeutic target for various neurodegenerative diseases; these include Alzheimer's, Parkinson's, Huntington's, prion disease, and others. This review discusses the possible roles of HSPs, particularly HSP70 and small HSPs (alpha A and alpha B crystallins) in enhancing the survival of retinal ganglion cells (RGCs) in optic neuropathies such as glaucoma, which is characterized by progressive loss of vision caused by degeneration of RGCs and their axons in the optic nerve. Studies in animal models of RGC degeneration induced by ocular hypertension, optic nerve crush and axotomy show that upregulation of HSP70 expression by hyperthermia, zinc, geranyl-geranyl acetone, 17-AAG (a HSP90 inhibitor), or through transfection of retinal cells with AAV2-HSP70 effectively supports the survival of injured RGCs. RGCs survival was also stimulated by overexpression of alpha A and alpha B crystallins. These findings provide support for translating the HSP70- and alpha crystallin-based cell survival strategy into therapy to protect and rescue injured RGCs from degeneration associated with glaucomatous and other optic neuropathies.

IER5 generates a novel hypo-phosphorylated active form of HSF1 and contributes to tumorigenesis

The transcription factors HSF1 and p53 both modulate the stress response, thereby protecting and facilitating the recovery of stressed cells, but both have the potential to promote tumor development. Here we show that a p53 target gene, IER5, encodes an activator of HSF1. IER5 forms a ternary complex with HSF1 and the phosphatase PP2A, and promotes the dephosphorylation of HSF1 at numbers of serine and threonine residues, generating a novel, hypo-phosphorylated active form of HSF1. IER5 is also transcriptionally upregulated in various cancers, although this upregulation is not always p53-dependent. The IER5 locus is associated with a so-called super enhancer, frequently associated with hyperactivated oncogenes in cancer cell lines. Enhanced expression of IER5 induces abnormal HSF1 activation in cancer cells and contributes to the proliferation of these cells under stressed conditions. These results reveal the existence of a novel IER5-mediated cancer regulation pathway that is responsible for the activation of HSF1 observed in various cancers.

Immediate-early response 5 (IER5) interacts with protein phosphatase 2A and regulates the phosphorylation of ribosomal protein S6 kinase and heat shock factor 1

Immediate-early response 5 (IER5) is a growth factor-inducible protein with homology to the N-terminus of IER2. Deletion analysis shows that a large region of IER5, including the N-terminal region, is involved in cell growth and stress resistance. The N-terminal region mediates IER5 oligomerization and binding to the B55 regulatory subunit of protein phosphatase 2A (PP2A). IER5 physically interacts with the PP2A target proteins ribosomal protein S6 kinase (S6K) and heat shock factor 1 (HSF1), and the interactions are essential for the reduced phosphorylation of S6K and HSF1. Our data indicate that oligomeric IER5 regulates PP2A activity and cell growth.

Uncoupling Stress-Inducible Phosphorylation of Heat Shock Factor 1 from Its Activation

In mammals the stress-inducible expression of genes encoding heat shock proteins is under the control of the heat shock transcription factor 1 (HSF1). Activation of HSF1 is a multistep process, involving trimerization, acquisition of DNA-binding and transcriptional activities, which coincide with several posttranslational modifications. Stress-inducible phosphorylation of HSF1, or hyperphosphorylation, which occurs mainly within the regulatory domain (RD), has been proposed as a requirement for HSF-driven transcription and is widely used for assessing HSF1 activation. Nonetheless, the contribution of hyperphosphorylation to the activity of HSF1 remains unknown. In this study, we generated a phosphorylation-deficient HSF1 mutant (HSF1Δ∼PRD), where the 15 known phosphorylation sites within the RD were disrupted. Our results show that the phosphorylation status of the RD does not affect the subcellular localization and DNA-binding activity of HSF1. Surprisingly, under stress conditions, HSF1Δ∼PRD is a potent transactivator of both endogenous targets and a reporter gene, and HSF1Δ∼PRD has a reduced activation threshold. Our results provide the first direct evidence for uncoupling stress-inducible phosphorylation of HSF1 from its activation, and we propose that the phosphorylation signature alone is not an appropriate marker for HSF1 activity.

FBXW7 modulates cellular stress response and metastatic potential through ​HSF1 post-translational modification

Heat-shock factor 1 (HSF1) orchestrates the heat-shock response in eukaryotes. Although this pathway has been evolved to help cells adapt in the presence of challenging conditions, it is co-opted in cancer to support malignancy. However, the mechanisms that regulate HSF1 and thus cellular stress response are poorly understood. Here we show that the ubiquitin ligase FBXW7 α interacts with HSF1 through a conserved motif phosphorylated by GSK3β and ERK1. FBXW7α ubiquitylates HSF1 and loss of FBXW7α results in impaired degradation of nuclear HSF1 and defective heat-shock response attenuation. FBXW7α is either mutated or transcriptionally downregulated in melanoma and HSF1 nuclear stabilization correlates with increased metastatic potential and disease progression. FBXW7α deficiency and subsequent HSF1 accumulation activates an invasion-supportive transcriptional program and enhances the metastatic potential of human melanoma cells. These findings identify a post-translational mechanism of regulation of the HSF1 transcriptional program both in the presence of exogenous stress and in cancer.

HSF1 transcriptional activity is modulated by IER5 and PP2A/B55

Heat shock factor 1 (HSF1) is the master transcriptional regulator of chaperone genes. HSF1 regulates the expression of the immediate-early response gene IER5, which encodes a protein that has roles in the stress response and cell proliferation. Here, we have shown that IER5 interacts with protein phosphatase 2A (PP2A) and its B55 regulatory subunits. Expression of IER5 and B55 in cells leads to HSF1 dephosphorylation and activation of HSF1 target genes. The B55 subunits directly bind to HSF1. These results suggest that IER5 functions as a positive feedback regulator of HSF1 and that this process involves PP2A/B55 and HSF1 dephosphorylation.

Multi-Level Interactions Between Heat Shock Factors, Heat Shock Proteins, and the Redox System Regulate Acclimation to Heat

High temperature has become a global concern because it seriously affects the growth and reproduction of plants. Exposure of plant cells to high temperatures result in cellular damage and can even lead to cell death. Part of the damage can be ascribed to the action of reactive oxygen species (ROS), which accumulate during abiotic stresses such as heat stress. ROS are toxic and can modify other biomacromolecules including membrane lipids, DNA, and proteins. In order to protect the cells, ROS scavenging is essential. In contrast with their inherent harms, ROS also function as signaling molecules, inducing stress tolerance mechanisms. This review examines the evidence for crosstalk between the classical heat stress response, which consists of heat shock factors (HSFs) and heat shock proteins (HSPs), with the ROS network at multiple levels in the heat response process. Heat stimulates HSF activity directly, but also indirectly via ROS. HSFs in turn stimulate the expression of HSP chaperones and also affect ROS scavenger gene expression. In the short term, HSFs repress expression of superoxide dismutase scavenger genes via induction of miRNA398, while they also activate scavenger gene expression and stabilize scavenger protein activity via HSP induction. We propose that these contrasting effects allow for the boosting of the heat stress response at the very onset of the stress, while preventing subsequent oxidative damage. The described model on HSFs, HSPs, ROS, and ROS scavenger interactions seems applicable to responses to stresses other than heat and may explain the phenomenon of crossacclimation.

Fever, immunity, and molecular adaptations

The heat shock response (HSR) is an ancient and highly conserved process that is essential for coping with environmental stresses, including extremes of temperature. Fever is a more recently evolved response, during which organisms temporarily subject themselves to thermal stress in the face of infections. We review the phylogenetically conserved mechanisms that regulate fever and discuss the effects that febrile-range temperatures have on multiple biological processes involved in host defense and cell death and survival, including the HSR and its implications for patients with severe sepsis, trauma, and other acute systemic inflammatory states. Heat shock factor-1, a heat-induced transcriptional enhancer is not only the central regulator of the HSR but also regulates expression of pivotal cytokines and early response genes. Febrile-range temperatures exert additional immunomodulatory effects by activating mitogen-activated protein kinase cascades and accelerating apoptosis in some cell types. This results in accelerated pathogen clearance, but increased collateral tissue injury, thus the net effect of exposure to febrile range temperature depends in part on the site and nature of the pathologic process and the specific treatment provided.

Transmembrane signaling in Saccharomyces cerevisiae as a model for signaling in metazoans: state of the art after 25 years

In the very first article that appeared in Cellular Signalling, published in its inaugural issue in October 1989, we reviewed signal transduction pathways in Saccharomyces cerevisiae. Although this yeast was already a powerful model organism for the study of cellular processes, it was not yet a valuable instrument for the investigation of signaling cascades. In 1989, therefore, we discussed only two pathways, the Ras/cAMP and the mating (Fus3) signaling cascades. The pivotal findings concerning those pathways undoubtedly contributed to the realization that yeast is a relevant model for understanding signal transduction in higher eukaryotes. Consequently, the last 25 years have witnessed the discovery of many signal transduction pathways in S. cerevisiae, including the high osmotic glycerol (Hog1), Stl2/Mpk1 and Smk1 mitogen-activated protein (MAP) kinase pathways, the TOR, AMPK/Snf1, SPS, PLC1 and Pkr/Gcn2 cascades, and systems that sense and respond to various types of stress. For many cascades, orthologous pathways were identified in mammals following their discovery in yeast. Here we review advances in the understanding of signaling in S. cerevisiae over the last 25 years. When all pathways are analyzed together, some prominent themes emerge. First, wiring of signaling cascades may not be identical in all S. cerevisiae strains, but is probably specific to each genetic background. This situation complicates attempts to decipher and generalize these webs of reactions. Secondly, the Ras/cAMP and the TOR cascades are pivotal pathways that affect all processes of the life of the yeast cell, whereas the yeast MAP kinase pathways are not essential. Yeast cells deficient in all MAP kinases proliferate normally. Another theme is the existence of central molecular hubs, either as single proteins (e.g., Msn2/4, Flo11) or as multisubunit complexes (e.g., TORC1/2), which are controlled by numerous pathways and in turn determine the fate of the cell. It is also apparent that lipid signaling is less developed in yeast than in higher eukaryotes. Finally, feedback regulatory mechanisms seem to be at least as important and powerful as the pathways themselves. In the final chapter of this essay we dare to imagine the essence of our next review on signaling in yeast, to be published on the 50th anniversary of Cellular Signalling in 2039.

HSF1 regulation of β-catenin in mammary cancer cells through control of HuR/elavL1 expression

There is now compelling evidence to indicate a place for heat shock factor 1 (HSF1) in mammary carcinogenesis, tumor progression and metastasis. Here we have investigated a role for HSF1 in regulating the expression of the stem cell renewal factor β-catenin in immortalized human mammary epithelial and carcinoma cells. We found HSF1 to be involved in regulating the translation of β–catenin, by investigating effects of gain and loss of HSF1 on this protein. Interestingly, although HSF1 is a potent transcription factor, it was not directly involved in regulating levels of β-catenin mRNA. Instead, our data suggest a complex role in translational regulation. HSF1 was shown to regulate levels of the RNA binding protein HuR that controlled β-catenin translation. An extra complexity was added to this scenario when it was shown that the long non-coding RNA molecule lincRNA-p21, known to be involved in β-catenin mRNA (CTNNB1) translational regulation, was controlled by HSF1 repression. We have shown previously that HSF1 was positively regulated through phosphorylation by mTOR kinase on a key residue, serine 326 essential for transcriptional activity. In this study we found that mTOR knockdown not only decreased HSF1-S326 phosphorylation in mammary cells, but also decreased β-catenin expression through a mechanism requiring HuR. Our data point to a complex role for HSF1 in the regulation of HuR and β-catenin expression that may be significant in mammary carcinogenesis.

ANG II promotes IGF-IIR expression and cardiomyocyte apoptosis by inhibiting HSF1 via JNK activation and SIRT1 degradation

Hypertension-induced cardiac hypertrophy and apoptosis are major characteristics of early-stage heart failure. Our previous studies found that the activation of insulin-like growth factor receptor II (IGF-IIR) signaling was critical for hypertensive angiotensin II (ANG II)-induced cardiomyocyte apoptosis. However, the detailed mechanism by which ANG II regulates IGF-IIR in heart cells remains elusive. In this study, we found that ANG II activated its downstream kinase JNK to increase IGF-IIR expression through the ANG II receptor angiotensin type 1 receptor. JNK activation subsequently led to sirtuin 1 (SIRT1) degradation via the proteasome, thus preventing SIRT1 from deacetylating heat-shock transcription factor 1 (HSF1). The resulting increase in the acetylation of HSF1 impaired its ability to bind to the IGF-IIR promoter region (nt -748 to -585). HSF1 protected cardiomyocytes by acting as a repressor of IGF-IIR gene expression, and ANG II diminished this HSF1-mediated repression through enhanced acetylation, thus activating the IGF-IIR apoptosis pathway. Taken together, these results suggest that HSF1 represses IGF-IIR gene expression to protect cardiomyocytes. ANG II activates JNK to degrade SIRT1, resulting in HSF1 acetylation, which induces IGF-IIR expression and eventually results in cardiac hypertrophy and apoptosis. HSF1 could be a valuable target for developing treatments for cardiac diseases in hypertensive patients.

PP2A-AMPKα-HSF1 axis regulates the metal-inducible expression of HSPs and ROS clearance

Metals such as cadmium and arsenic are ubiquitous toxicants that cause a variety of adverse health effects. Heat shock proteins (HSPs) response to metal-induced stress and protect cells from further damage. However, the intracellular signalling pathways responsible for activation of HSPs expression are not fully understood. Here, we demonstrate that protein phosphatase 2A (PP2A) regulates expression of HSP70 and HSP27 via dephosphorylation of an AMP-activated protein kinase α subunit (AMPKα) at Thr172. Dephosphorylated AMPKα phosphorylates heat shock factor 1 (HSF1) at Ser303, leading to significant transcriptional suppression of HSP70 and HSP27 in CdCl2- or NaAsO2-treated cells. Suppression of PP2A regulatory B56δ subunit resulted in the sustained phosphorylation of AMPKα upon CdCl2 treatment, subsequent reduction in expression of HSP70 and HSP27, and thereby dramatic reduction of reactive oxygen species (ROS) clearance. We further revealed that PP2A B56δ physically interacted with AMPKα, providing evidence that PP2A B56δ-AMPKα-HSF1 signalling pathway participated in regulating the inducible expression of HSPs and ROS clearance. Taken together, we identified a novel PP2A-dependent signalling pathway involved in regulation of HSPs expression in response to metal stress.

Plasma membranes as heat stress sensors: from lipid-controlled molecular switches to therapeutic applications

The classic heat shock (stress) response (HSR) was originally attributed to protein denaturation. However, heat shock protein (Hsp) induction occurs in many circumstances where no protein denaturation is observed. Recently considerable evidence has been accumulated to the favor of the "Membrane Sensor Hypothesis" which predicts that the level of Hsps can be changed as a result of alterations to the plasma membrane. This is especially pertinent to mild heat shock, such as occurs in fever. In this condition the sensitivity of many transient receptor potential (TRP) channels is particularly notable. Small temperature stresses can modulate TRP gating significantly and this is influenced by lipids. In addition, stress hormones often modify plasma membrane structure and function and thus initiate a cascade of events, which may affect HSR. The major transactivator heat shock factor-1 integrates the signals originating from the plasma membrane and orchestrates the expression of individual heat shock genes. We describe how these observations can be tested at the molecular level, for example, with the use of membrane perturbers and through computational calculations. An important fact which now starts to be addressed is that membranes are not homogeneous nor do all cells react identically. Lipidomics and cell profiling are beginning to address the above two points. Finally, we observe that a deregulated HSR is found in a large number of important diseases where more detailed knowledge of the molecular mechanisms involved may offer timely opportunities for clinical interventions and new, innovative drug treatments. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Regulation of the heat stress response in Arabidopsis by MPK6-targeted phosphorylation of the heat stress factor HsfA2

So far little is known on the functional role of phosphorylation in the heat stress response of plants. Here we present evidence that heat stress activates the Arabidopsis mitogen-activated protein kinase MPK6. In vitro and in vivo evidence is provided that MPK6 specifically targets the major heat stress transcription factor HsfA2. Activation of MPK6 results in complex formation with HsfA2. MPK6 phosphorylates HsfA2 on T249 and changes its intracellular localisation. Protein kinase and phosphatase inhibitor studies indicate that HsfA2 protein stability is regulated in a phosphorylation-dependent manner, but this mechanism is independent of MPK6. Overall, our data show that heat stress-induced targeting of HsfA2 by MPK6 participates in the complex regulatory mechanism how plants respond to heat stress.

Over-expression of heat shock factor 1 phenocopies the effect of chronic inhibition of TOR by rapamycin and is sufficient to ameliorate Alzheimer's-like deficits in mice modeling the disease

Rapamycin, an inhibitor of target-of-rapamycin, extends lifespan in mice, possibly by delaying aging. We recently showed that rapamycin halts the progression of Alzheimer's (AD)-like deficits, reduces amyloid-beta (Aβ) and induces autophagy in the human amyloid precursor protein (PDAPP) mouse model. To delineate the mechanisms by which chronic rapamycin delays AD we determined proteomic signatures in brains of control- and rapamycin-treated PDAPP mice. Proteins with reported chaperone-like activity were overrepresented among proteins up-regulated in rapamycin-fed PDAPP mice and the master regulator of the heat-shock response, heat-shock factor 1, was activated. This was accompanied by the up-regulation of classical chaperones/heat shock proteins (HSPs) in brains of rapamycin-fed PDAPP mice. The abundance of most HSP mRNAs except for alpha B-crystallin, however, was unchanged, and the cap-dependent translation inhibitor 4E-BP was active, suggesting that increased expression of HSPs and proteins with chaperone activity may result from preferential translation of pre-existing mRNAs as a consequence of inhibition of cap-dependent translation. The effects of rapamycin on the reduction of Aβ, up-regulation of chaperones, and amelioration of AD-like cognitive deficits were recapitulated by transgenic over-expression of heat-shock factor 1 in PDAPP mice. These results suggest that, in addition to inducing autophagy, rapamycin preserves proteostasis by increasing chaperones. We propose that the failure of proteostasis associated with aging may be a key event enabling AD, and that chronic inhibition of target-of-rapamycin may delay AD by maintaining proteostasis in brain. Read the Editorial Highlight for this article on doi: 10.1111/jnc.12098.

Forkhead box M1 is regulated by heat shock factor 1 and promotes glioma cells survival under heat shock stress

The forkhead box M1 (FoxM1) is a key transcription factor regulating multiple aspects of cell biology. Prior studies have shown that FoxM1 is overexpressed in a variety of human tumors, including brain tumor, and plays a critical role in cancer development and progression. In this study we found that FoxM1 was up-regulated by heat shock factor 1 (HSF1) under heat shock stress condition in multiple cell lines. Knockdown of HSF1 with HSF1 siRNA or inhibition of HSF1 with a HSF1 inhibitor abrogated heat shock-induced expression of FoxM1. Genetic deletion of HSF1 in mouse embryo fibroblast cells also abolished heat shock stress-induced FoxM1 expression. Moreover, we showed that HSF1 directly bound to FoxM1 promoter and increased FoxM1 promoter activity. Furthermore, we demonstrated that FoxM1 was required for the G(2)-M phase progression through regulating Cdc2, Cdc20, and Cdc25B under a mild heat shock stress but enhanced cell survival under lethal heat shock stress condition. Finally, in human glioblastoma specimens, FoxM1 overexpression correlated with elevated HSF1 expression. Our results indicate that FoxM1 is regulated by HSF1 and is critical for HSF1-mediated heat shock response. We demonstrated a novel mechanism of stress resistance controlled by HSF1 and a new HSF-FoxM1 connection that mediates cellular thermotolerance.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

Glutamine-mediated dual regulation of heat shock transcription factor-1 activation and expression

BACKGROUND

Regulation of transcriptional activity of heat shock factor-1 (HSF1) is widely thought to be the main point of control for heat shock protein (Hsp) expression.

RESULTS

Glutamine increases Hsf1 gene transcription in a C/EBPβ-dependent manner and up-regulates HSF1 activity.

CONCLUSION

Glutamine is an activator for both HSF1 expression and transactivation.

SIGNIFICANCE

Glutamine-induced HSF1 expression provides a novel mechanistic frame for HSF1-Hsp axis regulation. Heat shock transcription factor-1 (HSF1) is the master regulator for cytoprotective heat shock protein (Hsp) expression. It is widely thought that HSF1 expression is non-inducible, and thus the key control point of Hsp expression is regulation of the transactivation activity of HSF1. How HSF1 expression is regulated remains unknown. Herein we demonstrate that glutamine (Gln), a preferred fuel substrate for the gut, enhanced Hsp expression both in rat colonic epithelium in vivo and in cultured non-transformed young adult mouse colonic epithelial cells. This was associated with up-regulation of the transactivation activity of HSF1 via increased HSF1 trimerization, nuclear localization, DNA binding, and relative abundance of activating phosphorylation at Ser-230 of HSF1. More intriguingly, Gln enhanced HSF1 protein and mRNA expression and Hsf1 gene promoter activity. Within the -281/-200 region of the Hsf1 promoter, deletion of the putative CCAAT enhancer-binding protein (C/EBP) binding site abolished the HSF1 response to Gln. C/EBPβ was further shown to bind to this 82-bp sequence both in vitro and in vivo. Gln availability strikingly altered the ratio of C/EBPβ inhibitory and active isoforms, i.e. liver-enriched inhibitory protein and liver-enriched activating protein. Liver-enriched inhibitory protein and liver-enriched activating protein were further shown to be an independent repressor and activator, respectively, for Hsf1 gene transcription, and the relative abundance of these two C/EBPβ isoforms was demonstrated to determine Hsf1 transcription. We show for the first time that Gln not only enhances the transactivation of HSF1 but also induces Hsf1 expression by activating its transcription in a C/EBPβ-dependent manner.

Specificity of linear motifs that bind to a common mitogen-activated protein kinase docking groove

Mitogen-activated protein kinases (MAPKs) have a docking groove that interacts with linear "docking" motifs in binding partners. To determine the structural basis of binding specificity between MAPKs and docking motifs, we quantitatively analyzed the ability of 15 docking motifs from diverse MAPK partners to bind to c-Jun amino-terminal kinase 1 (JNK1), p38α, and extracellular signal-regulated kinase 2 (ERK2). Classical docking motifs mediated highly specific binding only to JNK1, and only those motifs with a sequence pattern distinct from the classical MAPK binding docking motif consensus differentiated between the topographically similar docking grooves of ERK and p38α. Crystal structures of four complexes of MAPKs with docking peptides, representing JNK-specific, ERK-specific, or ERK- and p38-selective binding modes, revealed that the regions located between consensus positions in the docking motifs showed conformational diversity. Although the consensus positions in the docking motifs served as anchor points that bound to common MAPK surface features and mostly contributed to docking in a nondiscriminatory fashion, the conformation of the intervening region between the anchor points mostly determined specificity. We designed peptides with tailored MAPK binding profiles by rationally changing the length and amino acid composition of intervening regions located between anchor points. These results suggest a coherent structural model for MAPK docking specificity that reveals how short linear motifs binding to a common kinase docking groove can mediate diverse interaction patterns and contribute to correct MAPK partner selection in signaling networks.

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.

Post-translational modification of human heat shock factors and their functions: a recent update by proteomic approach

Heat shock factors (HSFs) are vital for modulating stress and heat shock-related gene expression in cells. The activity of HSFs is controlled largely by post-translational modifications (PTMs). For example, basal phosphorylation of HSF1 on three serine sites suppresses the heat shock response, and hyperphosphorylation of HSF1 on several other serine and threonine sites by stress-activated kinases results in its activation, while acetylation on K80 inhibits its DNA-binding ability. Sumoylation of HSF2 on K82 regulates its DNA-binding ability, whereas sumoylation of HSF4B on K293 represses its transcriptional activity. With the advancement of proteomic technology, novel PTM sites on various HSFs have been identified with the use of tandem mass spectrometry (MS/MS), but the functions of many of these PTMs are still unclear. Yet, it should be noted that the discovery of these novel PTM sites provided the necessary evidence for the existence of these PTM marks in vivo. Followed by subsequent functional analysis, this would ultimately lead to a better understanding of these PTM marks. MS/MS-based proteomic approach is becoming a gold standard in PTM validation in the field of life science. Here, the recent literature of all known PTMs reported on human HSFs and the resulting functions will be discussed.