Cysteine reactivity distinguishes redox sensing by the heat-inducible and constitutive forms of heat shock protein 70

Nontoxic singlet oxygen generator as a therapeutic candidate for treating tauopathies

Methylene blue (MB) inhibits the aggregation of tau, a main constituent of neurofibrillary tangles. However, MB's mode of action in vivo is not fully understood. MB treatment reduced the amount of sarkosyl-insoluble tau in Drosophila that express human wild-type tau. MB concurrently ameliorated the climbing deficits of transgenic tau flies to a limited extent and diminished the climbing activity of wild-type flies. MB also decreased the survival rate of wild-type flies. Based on its photosensitive efficacies, we surmised that singlet oxygen generated through MB under light might contribute to both the beneficial and toxic effects of MB in vivo. We identified rose bengal (RB) that suppressed tau accumulation and ameliorated the behavioral deficits to a lesser extent than MB. Unlike MB, RB did not reduce the survival rate of flies. Our findings indicate that singlet oxygen generators with little toxicity may be suitable drug candidates for treating tauopathies.

Preventive methylene blue treatment preserves cognition in mice expressing full-length pro-aggregant human Tau

Introduction

Neurofibrillary tangles (NFT) composed of Tau are hallmarks of neurodegeneration in Alzheimer disease. Transgenic mice expressing full-length pro-aggregant human Tau (2N4R Tau-ΔK280, termed Tau^ΔK) or its repeat domain (TauRD-ΔK280, TauRD^ΔK) develop a progressive Tau pathology with missorting, phosphorylation, aggregation of Tau, loss of synapses and functional deficits. Whereas TauRD^ΔK assembles into NFT concomitant with neuronal death, Tau^ΔK accumulates into Tau pretangles without overt neuronal loss. Both forms cause a comparable cognitive decline (with onset at 10mo and 12mo, respectively), which is rescued upon switch-off of transgene expression. Since methylene blue (MB) is able to inhibit Tau aggregation in vitro, we investigated whether MB can prevent or rescue Tau-induced cognitive impairments in our mouse models. Both types of mice received MB orally using different preventive and therapeutic treatment protocols, initiated either before or after disease onset. The cognitive status of the mice was assessed by behavior tasks (open field, Morris water maze) to determine the most successful conditions for therapeutic intervention.

Results

Preventive and therapeutic MB application failed to avert or recover learning and memory deficits of TauRD^ΔK mice. Similarly, therapeutic MB treatment initiated after onset of cognitive impairments was ineffective in Tau^ΔK mice. In contrast, preventive MB application starting before onset of functional deficits preserved cognition of Tau^ΔK mice. Beside improved learning and memory, MB-treated Tau^ΔK mice showed a strong decrease of insoluble Tau, a reduction of conformationally changed (MC1) and phosphorylated Tau species (AT180, PHF1) as well as an upregulation of protein degradation systems (autophagy and proteasome). This argues for additional pleiotropic effects of MB beyond its properties as Tau aggregation inhibitor.

Conclusions

Our data support the use of Tau aggregation inhibitors as potential drugs for the treatment of AD and other tauopathies and highlights the need for preventive treatment before onset of cognitive impairments.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0204-4) contains supplementary material, which is available to authorized users.

Collaboration of geldanamycin-activated P70S6K and Hsp70 against beta-amyloid-induced hippocampal apoptosis: an approach to long-term memory and learning

One of the neuropathological hallmarks of Alzheimer's disease (AD) is the accumulation of beta-amyloid peptides (Aβ) in senile plaques. Aβ-induced oxidative stress is believed to be responsible for degeneration and apoptosis of neurons and consequent cognitive and memory deficits. Here, we investigated the possible neuroprotective effect of the heat shock protein 90 (Hsp90) inhibitor geldanamycin (GA) against amyloid pathogenesis in adult male Wistar rats. GA or vehicle was injected into the lateral cerebral ventricles of rats 24 h before injection of Aβ (1-42) in CA1 area of hippocampus. The learning and memory of the rats were assessed 7 days after injection of Aβ using passive avoidance (PA) task. As potential contributing factors in Aβ-induced memory decline, we evaluated apoptotic markers and also used terminal-transferase UTP nick end labeling (TUNEL) technique to detect apoptosis in the hippocampus of Aβ-injected rats. Our behavioral data suggest that GA pretreatment can significantly suppress memory deficits in Aβ-injected rats. There was also not only a marked increase in Hsp70 level but also upregulated 70 kDa ribosomal protein S6 kinase (p70S6K) in the hippocampus of GA-treated groups with a reduction in apoptotic factors including caspase-3, poly (ADP-ribose) polymerase, Bax/Bcl-2 ratio, and TUNEL-positive cells as well. Thus, we conclude that GA exerts its protective effects against Aβ (1-42) toxicity and memory deficits, at least in part, by upregulating of Hsp70 and P70S6K.

An RNA aptamer specific to Hsp70-ATP conformation inhibits its ATPase activity independent of Hsp40

The highly conserved and ubiquitous molecular chaperone heat shock protein 70 (Hsp70) plays a critical role in protein homeostasis (proteostasis). Controlled by its ATPase activity, Hsp70 cycles between two conformations, Hsp70-ATP and Hsp70-ADP, to bind and release its substrate. Chemical tools with distinct modes of action, especially those capable of modulating the ATPase activity of Hsp70, are being actively sought after in the mechanistic dissection of this system. Here, we report a conformation-specific RNA aptamer that binds only to Hsp70-ATP but not to Hsp70-ADP. We have refined this aptamer and demonstrated its inhibitory effect on Hsp70's ATPase activity. We have also shown that this inhibitory effect on Hsp70 is independent of its interaction with the Hsp40 co-chaperone. As Hsp70 is increasingly being recognized as a drug target in a number of age related diseases such as neurodegenerative, protein misfolding diseases and cancer, this aptamer is potentially useful in therapeutic applications. Moreover, this work also demonstrates the feasibility of using aptamers to target ATPase activity as a general therapeutic strategy.

Screening strategies to identify HSP70 modulators to treat Alzheimer's disease

Alzheimer’s disease, the most common type of dementia, is a progressive brain disease that destroys cognitive function and eventually leads to death. In patients with Alzheimer’s disease, beta amyloids and tau proteins form plaques/oligomers and oligomers/tangles that affect the ability of neurons to function properly. Heat shock protein 70 (HSP70) has the ability to prevent aggregation/oligomerization of beta amyloid/tau proteins, making it a potential drug target. To determine this potential, it is essential that we have appropriate in vitro and cell-based assays that help identify specific molecules that affect this aggregation or oligomerization through HSP70. Potential drug candidates could be identified through a series of assays, starting with ATPase assays, followed by aggregation assays with enzymes/proteins and cell-based systems. ATPase assays are effective in identification of ATPase modulators but do not determine the effect of the molecule on beta amyloid and tau proteins. Molecules identified through ATPase assays are validated by thioflavin T aggregation assays in the presence of HSP70. These assays help uncover if a molecule affects beta amyloid and tau through HSP70, but are limited by their in vitro nature. Potential drug candidates are further validated through cell-based assays using mammalian, yeast, or bacterial cultures. However, while these assays are able to determine the effect of a specific molecule on beta amyloid and tau, they fail to determine whether the action is HSP70-dependent. The creation of a novel, direct assay that can demonstrate the antiaggregation effect of a molecule as well as its action through HSP70 would reduce the number of false-positive drug candidates and be more cost-effective and time-effective.

Identification of an allosteric small-molecule inhibitor selective for the inducible form of heat shock protein 70

Inducible Hsp70 (Hsp70i) is overexpressed in a wide spectrum of human tumors, and its expression correlates with metastasis, poor outcomes, and resistance to chemotherapy in patients. Identification of small-molecule inhibitors selective for Hsp70i could provide new therapeutic tools for cancer treatment. In this work, we used fluorescence-linked enzyme chemoproteomic strategy (FLECS) to identify HS-72, an allosteric inhibitor selective for Hsp70i. HS-72 displays the hallmarks of Hsp70 inhibition in cells, promoting substrate protein degradation and growth inhibition. Importantly, HS-72 is selective for Hsp70i over the closely related constitutively active Hsc70. Studies with purified protein show HS-72 acts as an allosteric inhibitor, reducing ATP affinity. In vivo HS-72 is well-tolerated, showing bioavailability and efficacy, inhibiting tumor growth and promoting survival in a HER2+ model of breast cancer. The HS-72 scaffold is amenable to resynthesis and iteration, suggesting an ideal starting point for a new generation of anticancer therapeutics targeting Hsp70i.

A direct regulatory interaction between chaperonin TRiC and stress-responsive transcription factor HSF1

Heat shock transcription factor 1 (HSF1) is an evolutionarily conserved transcription factor that protects cells from protein-misfolding-induced stress and apoptosis. The mechanisms by which cytosolic protein misfolding leads to HSF1 activation have not been elucidated. Here, we demonstrate that HSF1 is directly regulated by TRiC/CCT, a central ATP-dependent chaperonin complex that folds cytosolic proteins. A small-molecule activator of HSF1, HSF1A, protects cells from stress-induced apoptosis, binds TRiC subunits in vivo and in vitro, and inhibits TRiC activity without perturbation of ATP hydrolysis. Genetic inactivation or depletion of the TRiC complex results in human HSF1 activation, and HSF1A inhibits the direct interaction between purified TRiC and HSF1 in vitro. These results demonstrate a direct regulatory interaction between the cytosolic chaperone machine and a critical transcription factor that protects cells from proteotoxicity, providing a mechanistic basis for signaling perturbations in protein folding to a stress-protective transcription factor.

Advances in therapeutics for neurodegenerative tauopathies: moving toward the specific targeting of the most toxic tau species

Neurodegenerative disease is one of the greatest health concerns today and with no effective treatment in sight, it is crucial that researchers find a safe and successful therapeutic. While neurofibrillary tangles are considered the primary tauopathy hallmark, more evidence continues to come to light to suggest that soluble, intermediate tau aggregates--tau oligomers--are the most toxic species in disease. These intermediate tau species may also be responsible for the spread of pathology, suggesting that oligomeric tau may be the best therapeutic target. Here, we summarize results for the modulation of tau by molecular chaperones, small molecules and aggregation inhibitors, post-translational modifications, immunotherapy, other techniques, and future directions.

Hsp70-Bag3 interactions regulate cancer-related signaling networks

Bag3, a nucleotide exchange factor of the heat shock protein Hsp70, has been implicated in cell signaling. Here, we report that Bag3 interacts with the SH3 domain of Src, thereby mediating the effects of Hsp70 on Src signaling. Using several complementary approaches, we established that the Hsp70-Bag3 module is a broad-acting regulator of cancer cell signaling by modulating the activity of the transcription factors NF-κB, FoxM1, Hif1α, the translation regulator HuR, and the cell-cycle regulators p21 and survivin. We also identified a small-molecule inhibitor, YM-1, that disrupts the Hsp70-Bag3 interaction. YM-1 mirrored the effects of Hsp70 depletion on these signaling pathways, and in vivo administration of this drug was sufficient to suppress tumor growth in mice. Overall, our results defined Bag3 as a critical factor in Hsp70-modulated signaling and offered a preclinical proof-of-concept that the Hsp70-Bag3 complex may offer an appealing anticancer target.

Redox signaling via the molecular chaperone BiP protects cells against endoplasmic reticulum-derived oxidative stress

Oxidative protein folding in the endoplasmic reticulum (ER) has emerged as a potentially significant source of cellular reactive oxygen species (ROS). Recent studies suggest that levels of ROS generated as a byproduct of oxidative folding rival those produced by mitochondrial respiration. Mechanisms that protect cells against oxidant accumulation within the ER have begun to be elucidated yet many questions still remain regarding how cells prevent oxidant-induced damage from ER folding events. Here we report a new role for a central well-characterized player in ER homeostasis as a direct sensor of ER redox imbalance. Specifically we show that a conserved cysteine in the lumenal chaperone BiP is susceptible to oxidation by peroxide, and we demonstrate that oxidation of this conserved cysteine disrupts BiP's ATPase cycle. We propose that alteration of BiP activity upon oxidation helps cells cope with disruption to oxidative folding within the ER during oxidative stress.

DOI:http://dx.doi.org/10.7554/eLife.03496.001

The endoplasmic reticulum is the cellular compartment where approximately one third of the cell's proteins are made. Inside, chaperone molecules bind to newly made protein chains and help them to fold into the three-dimensional structure required for the protein to work correctly. A chaperone called Ero1 helps to facilitate this folding process by catalyzing a reaction that forms strong chemical bonds, which help stabilize the final protein structures. However, this help from Ero1 comes at a cost: forming a stabilizing bond this way also produces a peroxide molecule as a byproduct.

Peroxide is a ‘reactive oxygen species’: a chemical that can oxidize and damage proteins and DNA, which can potentially kill the cell. Three other enzymes in the endoplasmic reticulum can convert peroxide into water, to protect the cells from reactive oxygen species build-up. However, not all cells that use Ero1 have these other enzymes, suggesting that other pathways must exist to manage reactive oxygen species.

Wang et al. took advantage of yeast cells containing a hyperactive mutant version of the Ero1 enzyme to look for alternative detoxifying mechanisms that occur when the cell is stressed by an excess of reactive oxygen species. In these cells, Wang et al. observed that the high levels of reactive oxygen species caused part of a chaperone molecule called BiP to oxidize. This modification of BiP acts like a switch that the reactive oxygen species flip on. When activated by the reactive oxygen species, BiP enhances its activity as a folding molecular chaperone, keeping proteins apart. This is thought to allow BiP to minimize the protein misfolding that may otherwise occur in the wake of the damage caused by the building levels of peroxide. Wang et al. created a mutant BiP chaperone that mimics the oxidized form, and found that it also protects cells from the damage inflicted by the excess of reactive oxygen species.

Wang et al. propose that the BiP chaperone may be an important sensor of reactive oxygen species that changes its activity when these harmful chemicals are present and helps to protect the cell from damage. The success in mimicking the protective effects of oxidized BiP with a mutant BiP suggest that in the future one may be able to design small molecule drugs that bind to BiP to produce the activity of the modified form.

DOI:http://dx.doi.org/10.7554/eLife.03496.002

Celastrol targets proteostasis and acts synergistically with a heat-shock protein 90 inhibitor to kill human glioblastoma cells

Glioblastoma multiforme is a devastating disease of the central nervous system and, at present, no effective therapeutic interventions have been identified. Celastrol, a natural occurring triterpene, exhibits potent anti-tumor activity against gliomas in xenograft mouse models. In this study, we describe the cell death mechanism employed by celastrol and identify secondary targets for effective combination therapy against glioblastoma cell survival. In contrast to the previously proposed reactive oxygen species (ROS)-dependent mechanism, cell death in human glioblastoma cells is shown here to be mediated by alternate signal transduction pathways involving, but not fully dependent on, poly(ADP-ribose) polymerase-1 and caspase-3. Our studies indicate that celastrol promotes proteotoxic stress, supported by two feedback mechanisms: (i) impairment of protein quality control as revealed by accumulation of polyubiquitinated aggregates and the canonical autophagy substrate, p62, and (ii) the induction of heat-shock proteins, HSP72 and HSP90. The Michael adduct of celastrol and N-acetylcysteine, 6-N-acetylcysteinyldihydrocelastrol, had no effect on p62, nor on HSP72 expression, confirming a thiol-dependent mechanism. Restriction of protein folding stress with cycloheximide was protective, while combination with autophagy inhibitors did not sensitize cells to celastrol-mediated cytotoxicity. Collectively, these findings imply that celastrol targets proteostasis by disrupting sulfyhydryl homeostasis, independently of ROS, in human glioblastoma cells. This study further emphasizes that targeting proteotoxic stress responses by inhibiting HSP90 with 17-N-Allylamino-17-demethoxygeldanamycin sensitizes human glioblastoma to celastrol treatment, thereby serving as a novel synergism to overcome drug resistance.

Chaperone heat shock protein 70 in nucleus accumbens core: a novel biological target of behavioural sensitization to morphine in rats

Drug addiction is a major public health issue, yet the underlying adaptation of neural networks by drugs of abuse is not fully understood. We have previously linked chaperone heat shock protein 70 (Hsp70) to drug-induced adaptations. Focusing on the NAc core and shell, the present study aims to provide further findings for our understanding of the relation between behavioural sensitization to morphine and Hsp70 at transcriptional and functional levels in rats. Firstly, we delineated the characteristics of behavioural sensitization induced by a single morphine exposure (1-10 mg/kg, s.c.). Secondly, Hsp70 protein expression in the NAc core was time- and dose-relatedly induced during the development of behavioural sensitization to a single morphine exposure in rats, and Pearson analysis indicated a positive correlation between behavioural sensitization and Hsp70 expression in NAc core. Thirdly, at the transcriptional level, intra-NAc core injection of the specific heat shock factor-I (HSF-I) inhibitor N-Formyl-3,4-methylenedioxy-benzylidine-γ-butyrolactam (KNK437) suppressed Hsp70 expression and the development of behavioural sensitization, while the HSF-I specific inducer geranylgeranylacetone (GGA) promoted both of them. Interestingly, intra-NAc shell injection of KNK437 or GGA did not affect the development of behavioural sensitization. Finally, both the functional inhibition of Hsp70 ATPase activity by methylene blue (MB), and the antagonism of Hsp70 substrate binding site (SBD) activity by pifithrin-μ (PES) impaired the development of behavioural sensitization when they were microinjected into the NAc core. Taken together, the critical involvement of chaperone Hsp70 in behavioural sensitization to morphine identifies a biological target for long-lasting adaptations with relevance to addiction.

Methylene blue upregulates Nrf2/ARE genes and prevents tau-related neurotoxicity

Methylene blue (MB, methylthioninium chloride) is a phenothiazine that crosses the blood brain barrier and acts as a redox cycler. Among its beneficial properties are its abilities to act as an antioxidant, to reduce tau protein aggregation and to improve energy metabolism. These actions are of particular interest for the treatment of neurodegenerative diseases with tau protein aggregates known as tauopathies. The present study examined the effects of MB in the P301S mouse model of tauopathy. Both 4 mg/kg MB (low dose) and 40 mg/kg MB (high dose) were administered in the diet ad libitum from 1 to 10 months of age. We assessed behavior, tau pathology, oxidative damage, inflammation and numbers of mitochondria. MB improved the behavioral abnormalities and reduced tau pathology, inflammation and oxidative damage in the P301S mice. These beneficial effects were associated with increased expression of genes regulated by NF-E2-related factor 2 (Nrf2)/antioxidant response element (ARE), which play an important role in antioxidant defenses, preventing protein aggregation, and reducing inflammation. The activation of Nrf2/ARE genes is neuroprotective in other transgenic mouse models of neurodegenerative diseases and it appears to be an important mediator of the neuroprotective effects of MB in P301S mice. Moreover, we used Nrf2 knock out fibroblasts to show that the upregulation of Nrf2/ARE genes by MB is Nrf2 dependent and not due to secondary effects of the compound. These findings provide further evidence that MB has important neuroprotective effects that may be beneficial in the treatment of human neurodegenerative diseases with tau pathology.

Detection of protein S-sulfhydration by a tag-switch technique

Protein S-sulfhydration (forming -S-SH adducts from cysteine residues) is a newly defined oxidative posttranslational modification and plays an important role in H2 S-mediated signaling pathways. In this study we report the first selective, "tag-switch" method which can directly label protein S-sulfhydrated residues by forming stable thioether conjugates. Furthermore we demonstrate that H2 S alone cannot lead to S-sulfhydration and that the two possible physiological mechanisms include reaction with protein sulfenic acids (P-SOH) or the involvement of metal centers which would facilitate the oxidation of H2 S to HS(.) .

Identification of an allosteric pocket on human hsp70 reveals a mode of inhibition of this therapeutically important protein

Hsp70s are important cancer chaperones that act upstream of Hsp90 and exhibit independent anti-apoptotic activities. To develop chemical tools for the study of human Hsp70, we developed a homology model that unveils a previously unknown allosteric site located in the nucleotide binding domain of Hsp70. Combining structure-based design and phenotypic testing, we discovered a previously unknown inhibitor of this site, YK5. In cancer cells, this compound is a potent and selective binder of the cytosolic but not the organellar human Hsp70s and has biological activity partly by interfering with the formation of active oncogenic Hsp70/Hsp90/client protein complexes. YK5 is a small molecule inhibitor rationally designed to interact with an allosteric pocket of Hsp70 and represents a previously unknown chemical tool to investigate cellular mechanisms associated with Hsp70.

Allosteric heat shock protein 70 inhibitors rapidly rescue synaptic plasticity deficits by reducing aberrant tau

BACKGROUND

The microtubule-associated protein tau accumulates in neurodegenerative diseases known as tauopathies, the most common being Alzheimer's disease. One way to treat these disorders may be to reduce abnormal tau levels through chaperone manipulation, thus subverting synaptic plasticity defects caused by tau's toxic accretion.

METHODS

Tauopathy models were used to study the impact of YM-01 on tau. YM-01 is an allosteric promoter of triage functions of the most abundant variant of the heat shock protein 70 (Hsp70) family in the brain, heat shock cognate 70 protein (Hsc70). The mechanisms by which YM-01 modified Hsc70 activity and tau stability were evaluated with biochemical methods, cell cultures, and primary neuronal cultures from tau transgenic mice. YM-01 was also administered to acute brain slices of tau mice; changes in tau stability and electrophysiological correlates of learning and memory were measured.

RESULTS

Tau levels were rapidly and potently reduced in vitro and ex vivo upon treatment with nanomolar concentrations of YM-01. Consistent with Hsc70 having a key role in this process, overexpression of heat shock protein 40 (DNAJB2), an Hsp70 co-chaperone, suppressed YM-01 activity. In contrast to its effects in pathogenic tauopathy models, YM-01 had little activity in ex vivo brain slices from normal, wild-type mice unless microtubules were disrupted, suggesting that Hsc70 acts preferentially on abnormal pools of free tau. Finally, treatment with YM-01 increased long-term potentiation in tau transgenic brain slices.

CONCLUSIONS

Therapeutics that exploit the ability of chaperones to selectively target abnormal tau can rapidly and potently rescue the synaptic dysfunction that occurs in Alzheimer's disease and other tauopathies.

The redox biochemistry of protein sulfenylation and sulfinylation

Controlled generation of reactive oxygen species orchestrates numerous physiological signaling events (Finkel, T. (2011) Signal transduction by reactive oxygen species. J. Cell Biol. 194, 7-15). A major cellular target of reactive oxygen species is the thiol side chain (RSH) of Cys, which may assume a wide range of oxidation states (i.e. -2 to +4). Within this context, Cys sulfenic (Cys-SOH) and sulfinic (Cys-SO2H) acids have emerged as important mechanisms for regulation of protein function. Although this area has been under investigation for over a decade, the scope and biological role of sulfenic/sulfinic acid modifications have been recently expanded with the introduction of new tools for monitoring cysteine oxidation in vitro and directly in cells. This minireview discusses selected recent examples of protein sulfenylation and sulfinylation from the literature, highlighting the role of these post-translational modifications in cell signaling.

The HSP70 family and cancer

The HSP70 family of heat shock proteins consists of molecular chaperones of approximately 70kDa in size that serve critical roles in protein homeostasis. These adenosine triphosphatases unfold misfolded or denatured proteins and can keep these proteins in an unfolded, folding-competent state. They also protect nascently translating proteins, promote the cellular or organellar transport of proteins, reduce proteotoxic protein aggregates and serve general housekeeping roles in maintaining protein homeostasis. The HSP70 family is the most conserved in evolution, and all eukaryotes contain multiple members. Some members of this family serve specific organellar- or tissue-specific functions; however, in many cases, these members can function redundantly. Overall, the HSP70 family of proteins can be thought of as a potent buffering system for cellular stress, either from extrinsic (physiological, viral and environmental) or intrinsic (replicative or oncogenic) stimuli. As such, this family serves a critical survival function in the cell. Not surprisingly, cancer cells rely heavily on this buffering system for survival. The overwhelming majority of human tumors overexpress HSP70 family members, and expression of these proteins is typically a marker for poor prognosis. With the proof of principle that inhibitors of the HSP90 chaperone have emerged as important anticancer agents, intense focus has now been placed on the potential for HSP70 inhibitors to assume a role as a significant chemotherapeutic avenue. In this review, the history, regulation, mechanism of action and role in cancer of the HSP70 family are reviewed. Additionally, the promise of pharmacologically targeting this protein for cancer therapy is addressed.

Phenotypic identification of the redox dye methylene blue as an antagonist of heat shock response gene expression in metastatic melanoma cells

Repurposing approved and abandoned non-oncological drugs is an alternative developmental strategy for the identification of anticancer therapeutics that has recently attracted considerable attention. Due to the essential role of the cellular heat shock response in cytoprotection through the maintenance of proteostasis and suppression of apoptosis, small molecule heat shock response antagonists can be harnessed for targeted induction of cytotoxic effects in cancer cells. Guided by gene expression array analysis and a phenotypic screen interrogating a collection of 3,7-diamino-phenothiazinium derivatives, we have identified the redox-drug methylene blue (MB), used clinically for the infusional treatment of methemoglobinemia, as a negative modulator of heat shock response gene expression in human metastatic melanoma cells. MB-treatment blocked thermal (43 °C) and pharmacological (celastrol, geldanamycin) induction of heat shock response gene expression, suppressing Hsp70 (HSPA1A) and Hsp27 (HSPB1) upregulation at the mRNA and protein level. MB sensitized melanoma cells to the apoptogenic activity of geldanamycin, an Hsp90 antagonist known to induce the counter-regulatory upregulation of Hsp70 expression underlying cancer cell resistance to geldanamycin chemotherapy. Similarly, MB-cotreatment sensitized melanoma cells to other chemotherapeutics (etoposide, doxorubicin). Taken together, these data suggest feasibility of repurposing the non-oncological redox drug MB as a therapeutic heat shock response antagonist for cancer cell chemosensitization.

The Role of Sulfhydryl Reactivity of Small Molecules for the Activation of the KEAP1/NRF2 Pathway and the Heat Shock Response

The KEAP1/NRF2 pathway and the heat shock response are two essential cytoprotective mechanisms that allow adaptation and survival under conditions of oxidative, electrophilic, and thermal stress by regulating the expression of elaborate networks of genes with versatile protective functions. The two pathways are independently regulated by the transcription factor nuclear factor-erythroid 2 p45-related factor 2 (NRF2) and heat shock factor 1 (HSF1), respectively. The activity of these transcriptional master regulators increases during conditions of stress and also upon encounter of small molecules (inducers), both naturally occurring as well as synthetically produced. Inducers have a common chemical property: the ability to react with sulfhydryl groups. The protein targets of such sulfhydryl-reactive compounds are equipped with highly reactive cysteine residues, which serve as sensors for inducers. The initial cysteine-sensed signal is further relayed to affect the expression of large networks of genes, which in turn can ultimately influence complex cell fate decisions such as life and death. The paper summarizes the multiple lines of experimental evidence demonstrating that the reactivity with sulfhydryl groups is a major determinant of the mechanism of action of small molecule dual activators of the KEAP1/NRF2 pathway and the heat shock response.