Cadmium-Mediated Activation of the HSP90/HSF1 Pathway Regulated by Reactive Persulfides/Polysulfides

Activation of HSP90/HSF1 Signaling as an Adaptive Response to an Electrophilic Metabolite of Morphine

Morphinone (MO) is an electrophilic metabolite of morphine that covalently binds to protein thiols, resulting in toxicity in vitro and in vivo. We have previously identified a variety of redox signaling pathways that are activated during electrophilic stress. However, the role of MO in such activation remains unknown. In this study, we examined whether MO could activate heat shock protein (HSP) 90/heat shock factor (HSF) 1 signaling in HepG2 cells. MO exposure caused S-modification of HSP90 (determined using biotin-PEAC₅-maleimide labeling) and nuclear translocation of transcription factor HSF1, thereby up-regulating its downstream genes encoding B-cell lymphoma 2-associated anthanogene 3 and heat shock 70 kDa protein 1. However, dihydromorphinone, a non-electrophilic metabolite of morphine, had little effect on HSF1 activation or upregulation of these genes, suggesting that covalent modification plays a role in this process and that the HSP90/HSF1 pathway is a redox-signaled adaptive response to morphine metabolism.

Cystine-dependent antiporters buffer against excess intracellular reactive sulfur species-induced stress

Reactive sulfur species (RSS) play a role in redox homeostasis; however, adaptive cell responses to excessive intracellular RSS are not well understood. Therefore, in this study, we generated transgenic (Tg) mice overexpressing cystathionine gamma-lyase (CSE) to produce excessive RSS. Contrary to expectations, tissue concentrations of RSS, such as cysteine persulfide (CysSSH), were comparable in both wild-type and CSE Tg mice, but the plasma concentrations of CysSSH were significantly higher in CSE Tg mice than in wild-type mice. This export of surplus intracellular RSS was also observed in primary hepatocytes of CSE Tg mice. Exposure of primary hepatocytes to the RSS generator sodium tetrasulfide (Na2S4) resulted in an initial increase in the intracellular concentration of RSS, which later returned to basal levels after export into the extracellular space. Interestingly, among all amino acids, cystine (CysSSCys) was found to be essential for CysSSH export from primary mouse hepatocytes, HepG2 cells, and HEK293 cells during Na2S4 exposure, suggesting that the cystine/glutamate transporter (SLC7A11) contributes, at least partially, to CysSSH export. We established HepG2 cell lines with knockout and overexpression of SLC7A11 and used them to confirm SLC7A11 as the predominant antiporter of CysSSCys and CysSSH. We observed that the poor efflux of excess CysSSH from the cell enhanced cellular stresses induced by Na2S4 exposure, such as polysulfidation of intracellular proteins, mitochondrial damage, and cytotoxicity. These results suggest the presence of a cellular response to excess intracellular RSS that involves the extracellular efflux of excess CysSSH by a cystine-dependent transporter to maintain intracellular redox homeostasis.

Alleviation of Cadmium Chloride-Induced Acute Genotoxicity, Mitochondrial DNA Disruption, and ROS Generation by Chocolate Coadministration in Mice Liver and Kidney Tissues

Increased human exposure to cadmium compounds through ingesting contaminated food, water, and medications causes negative long-term health effects, which has led to the focus of recent researches on finding natural antioxidants to mitigate cadmium-induced toxicity. Therefore, the current study was undertaken to estimate the possible ameliorative effect of chocolate coadministration on acute cadmium chloride (CdCl₂)-induced genomic instability and mitochondrial DNA damage in mice liver and kidney tissues. Concurrent administration of chocolate with CdCl₂ dramatically decreased the DNA damage level and the number of apoptotic and necrotic cells compared to mice given CdCl₂ alone. Extra-production of reactive oxygen species and increased expression of inducible nitric oxide synthase and heat shock proteins genes caused by CdCl₂ administration were also highly decreased after chocolate coadministration. Conversely, chocolate coadministration restored the integrity of the mitochondrial membrane potential disrupted by CdCl₂ administration, as well as the mitochondrial DNA copy number and expression level of heme oxygenase-1 gene were significantly upregulated after chocolate coadministration with CdCl₂. Thus, it was concluded that the coadministration of chocolate alleviated CdCl₂-induced genomic instability and mitochondrial DNA damage through its antioxidative and free radical scavenging capabilities, making chocolate a promising ameliorative product and recommended for inclusion in the daily human diet.

Synthesis of Reactive Sulfur Species in Cultured Vascular Endothelial Cells after Exposure to TGF-β1: Induction of Cystathionine γ-Lyase and Cystathionine β-Synthase Expression Mediated by the ALK5-Smad2/3/4 and ALK5-Smad2/3-ATF4 Pathways

Transforming growth factor-β₁ (TGF-β₁) occurs at high levels at damage sites of vascular endothelial cell layers and regulates the functions of vascular endothelial cells. Reactive sulfur species (RSS), such as cysteine persulfide, glutathione persulfide, and hydrogen persulfide, are cytoprotective factors against electrophiles such as reactive oxygen species and heavy metals. Previously, we reported that sodium trisulfide, a sulfane sulfur donor, promotes vascular endothelial cell proliferation. The objective of the present study was to clarify the regulation and significance of RSS synthesis in vascular endothelial cells after exposure to TGF-β₁. Bovine aortic endothelial cells in a culture system were treated with TGF-β₁ to assess the expression of intracellular RSS, the effect of RSS on cell proliferation in the presence of TGF-β₁, induction of RSS-producing enzymes by TGF-β₁, and intracellular signal pathways that mediate this induction. The results suggest that TGF-β₁ increased intracellular RSS levels to modulate its inhibitory effect on proliferation. The increased production of RSS, probably high-molecular-mass RSS, was due to the induction of cystathionine γ-lyase and cystathionine β-synthase, which are RSS-producing enzymes, and the induction was mediated by the ALK5-Smad2/3/4 and ALK5-Smad2/3-ATF4 pathways in vascular endothelial cells. TGF-β₁ regulates vascular endothelial cell functions such as proliferation and fibrinolytic activity; intracellular high-molecular-mass RSS, which are increased by TGF-β₁, may modulate the regulation activity in vascular endothelial cells.

Cadmium-Induced Kidney Injury: Oxidative Damage as a Unifying Mechanism

Cadmium is a nonessential metal that has heavily polluted the environment due to human activities. It can be absorbed into the human body via the gastrointestinal tract, respiratory tract, and the skin, and can cause chronic damage to the kidneys. The main site where cadmium accumulates and causes damage within the nephrons is the proximal tubule. This accumulation can induce dysfunction of the mitochondrial electron transport chain, leading to electron leakage and production of reactive oxygen species (ROS). Cadmium may also impair the function of NADPH oxidase, resulting in another source of ROS. These ROS together can cause oxidative damage to DNA, proteins, and lipids, triggering epithelial cell death and a decline in kidney function. In this article, we also reviewed evidence that the antioxidant power of plant extracts, herbal medicines, and pharmacological agents could ameliorate cadmium-induced kidney injury. Finally, a model of cadmium-induced kidney injury, centering on the notion that oxidative damage is a unifying mechanism of cadmium renal toxicity, is also presented. Given that cadmium exposure is inevitable, further studies using animal models are warranted for a detailed understanding of the mechanism underlying cadmium induced ROS production, and for the identification of more therapeutic targets.

Sodium trisulfide, a sulfane sulfur donor, stimulates bovine aortic endothelial cell proliferation in culture

Reactive sulfur species (RSS) include biological persulfide molecules that protect cells against oxidative stress and heavy metal toxicity. Vascular endothelial cells regulate blood coagulation and fibrinolytic activity, and prevent vascular disorders such as atherosclerosis. We hypothesized that RSS protect vascular endothelial cells not only from nonspecific cell damage but also from specific functional damage through regulation of specific cell functions. In the present study, cultured bovine aortic endothelial cells were treated with sodium trisulfide, a sulfane sulfur donor, and both [³H]thymidine incorporation and effects on cell cycle were analyzed. These results suggest that RSS stimulate vascular endothelial cell proliferation. RSS may reduce the functional cytotoxicity of antiproliferative agents.

Transcriptional Induction of Cystathionine γ-Lyase, a Reactive Sulfur-Producing Enzyme, by Copper Diethyldithiocarbamate in Cultured Vascular Endothelial Cells

As toxic substances can enter the circulating blood and cross endothelial monolayers to reach parenchymal cells in organs, vascular endothelial cells are an important target compartment for such substances. Reactive sulfur species protect cells against oxidative stress and toxic substances, including heavy metals. Reactive sulfur species are produced by enzymes, such as cystathionine γ-lyase (CSE), cystathionine β-synthase, 3-mercaptopyruvate sulfurtransferase, and cysteinyl-tRNA synthetase. However, little is known about the regulatory mechanisms underlying the expression of these enzymes in vascular endothelial cells. Bio-organometallics is a research field that analyzes biological systems using organic-inorganic hybrid molecules (organometallic compounds and metal coordinating compounds) as molecular probes. In the present study, we analyzed intracellular signaling pathways that mediate the expression of reactive sulfur species-producing enzymes in cultured bovine aortic endothelial cells, using copper diethyldithiocarbamate (Cu10). Cu10 selectively upregulated CSE gene expression in vascular endothelial cells independent of cell density. This transcriptional induction of endothelial CSE required both the diethyldithiocarbamate scaffold and the coordinated copper ion. Additionally, the present study revealed that ERK1/2, p38 MAPK, and hypoxia-inducible factor (HIF)-1α/HIF-1β pathways mediate transcriptional induction of endothelial CSE by Cu10. The transcription factors NF-κB, Sp1, and ATF4 were suggested to act in constitutive CSE expression, although the possibility that they are involved in the CSE induction by Cu10 cannot be excluded. The present study used a copper complex as a molecular probe to reveal that the transcription of CSE is regulated by multiple pathways in vascular endothelial cells, including ERK1/2, p38 MAPK, and HIF-1α/HIF-1β. Bio-organometallics appears to be an effective strategy for analyzing the functions of intracellular signaling pathways in vascular endothelial cells.

Redox toxicology of environmental chemicals causing oxidative stress

Living organisms are surrounded with heavy metals such as methylmercury, manganese, cobalt, cadmium, arsenic, as well as pesticides such as deltamethrin and paraquat, or atmospheric pollutants such as quinone. Extensive studies have demonstrated a strong link between environmental pollutants and human health. Redox toxicity is proposed as one of the main mechanisms of chemical-induced pathology in humans. Acting as both a sensor of oxidative stress and a positive regulator of antioxidants, the nuclear factor erythroid 2-related factor 2 (NRF2) has attracted recent attention. However, the role NRF2 plays in environmental pollutant-induced toxicity has not been systematically addressed. Here, we characterize NRF2 function in response to various pollutants, such as metals, pesticides and atmospheric quinones. NRF2 related signaling pathways and epigenetic regulations are also reviewed.

H2S, Polysulfides, and Enzymes: Physiological and Pathological Aspects

We have been studying the general aspects of the functions of H₂S and polysulfides, and the enzymes involved in their biosynthesis, for more than 20 years. Our aim has been to elucidate novel physiological and pathological functions of H₂S and polysulfides, and unravel the regulation of the enzymes involved in their biosynthesis, including cystathionine β-synthase (EC 4.2.1.22), cystathionine γ-lyase (EC 4.4.1.1), thiosulfate sulfurtransferase (rhodanese, EC 2.8.1.1), and 3-mercaptopyruvate sulfurtransferase (EC 2.8.1.2). Physiological and pathological functions, alternative biosynthetic processes, and additional functions of H₂S and polysulfides have been reported. Further, the structure and reaction mechanisms of related enzymes have also been reported. We expect this issue to advance scientific knowledge regarding the detailed functions of H₂S and polysulfides as well as the general properties and regulation of the enzymes involved in their metabolism. We would like to cover four topics: the physiological and pathological functions of H₂S and polysulfides, the mechanisms of the biosynthesis of H₂S and polysulfides, the properties of the biosynthetic enzymes, and the regulation of enzymatic activity. The knockout mouse technique is a useful tool to determine new physiological functions, especially those of H₂S and polysulfides. In the future, we shall take a closer look at symptoms in the human congenital deficiency of each enzyme. Further studies on the regulation of enzymatic activity by in vivo substances may be the key to finding new functions of H₂S and polysulfides.

Nrf2 Activation and Its Coordination with the Protective Defense Systems in Response to Electrophilic Stress

Molecular responses mediated by sensor proteins are important for biological defense against electrophilic stresses, such as xenobiotic electrophile exposure. NF-E2-related factor 2 (Nrf2) has an essential function as a master regulator of such cytoprotective molecular responses along with sensor protein Kelch-like ECH-associated protein 1. This review focuses on Nrf2 activation and its involvement with the protective defense systems under electrophilic stresses integrated with our recent findings that reactive sulfur species (RSS) mediate detoxification of electrophiles. The Nrf2 pathway does not function redundantly with the RSS-generating cystathionine γ-lyase pathway, and vice versa.

Adaptive Responses to Electrophilic Stress and Reactive Sulfur Species as their Regulator Molecules

We are exposed to numerous xenobiotic electrophiles on a daily basis through the environment, lifestyle, and dietary habits. Although such reactive species have been associated with detrimental effects, recent accumulated evidence indicates that xenobiotic electrophiles appear to act as signaling molecules. In this review, we introduce our findings on 1) activation of various redox signaling pathways involved in cell proliferation, detoxification/excretion of electrophiles, quality control of cellular proteins, and cell survival during exposure to xenobiotic electrophiles at low concentrations through covalent modification of thiol groups in sensor proteins, and 2) negative regulation of reactive sulfur species (RSS) in the modulation of redox signaling and toxicity caused by xenobiotic electrophiles.

Reference gene selection for quantitative gene expression analysis in black soldier fly (Hermetia illucens)

Hermetia illucens is an important resource insect for the conversion of organic waste. Quantitative PCR (qPCR) is the primary tool of gene expression analysis and a core technology of molecular biology research. Reference genes are essential for qPCR analysis; however, a stability analysis of H. illucens reference genes has not yet been carried out. To find suitable reference genes for normalizing gene expression data, the stability of eight housekeeping genes (including ATP6V1A, RPL8, EF1, Tubulin, TBP, GAPDH, Actin and RP49) was investigated under both biotic (developmental stages, tissues and sex) and abiotic (heavy metals, food, antibiotics) conditions. Gene expression data were analysed by geNorm, NormFinder, BestKeeper, and ΔCt programs. A set of specific reference genes was recommended for each experimental condition using the results of RefFinder synthesis analysis. This study offers a solid foundation for further studies of the molecular biology of H. illucens.

RNA processing errors triggered by cadmium and integrator complex disruption are signals for environmental stress

BACKGROUND

Adaptive responses to stress are essential for cell and organismal survival. In metazoans, little is known about the impact of environmental stress on RNA homeostasis.

RESULTS

By studying the regulation of a cadmium-induced gene named numr-1 in Caenorhabditis elegans, we discovered that disruption of RNA processing acts as a signal for environmental stress. We find that NUMR-1 contains motifs common to RNA splicing factors and influences RNA splicing in vivo. A genome-wide screen reveals that numr-1 is strongly and specifically induced by silencing of genes that function in basal RNA metabolism including subunits of the metazoan integrator complex. Human integrator processes snRNAs for functioning with splicing factors, and we find that silencing of C. elegans integrator subunits disrupts snRNA processing, causes aberrant pre-mRNA splicing, and induces the heat shock response. Cadmium, which also strongly induces numr-1, has similar effects on RNA and the heat shock response. Lastly, we find that heat shock factor-1 is required for full numr-1 induction by cadmium.

CONCLUSION

Our results are consistent with a model in which disruption of integrator processing of RNA acts as a molecular damage signal initiating an adaptive stress response mediated by heat shock factor-1. When numr-1 is induced via this pathway in C. elegans, its function in RNA metabolism may allow it to mitigate further damage and thereby promote tolerance to cadmium.

Environmental Electrophile-Mediated Toxicity in Mice Lacking Nrf2, CSE, or Both

BACKGROUND

Transcription factor Nrf2 (nuclear factor-erythroid 2-related factor 2) plays a key role in detoxification of electrophiles via formation of glutathione (GSH) adducts and subsequent excretion into extracellular spaces. We found that reactive sulfur species (RSS), such as cysteine persulfides produced by cystathionine [Formula: see text] (CSE), capture environmental electrophiles through formation of sulfur adducts. However, contributions of Nrf2 and CSE to the blockage of environmental electrophile-mediated toxicity remain to be evaluated.

OBJECTIVES

The aim of this study was to clarify roles that CSE and Nrf2 play in the protection against various environmental electrophiles. We also wished to clarify the molecular basis of the developmental window of toxicity through investigating expression levels of Nrf2, RSS-producing enzymes, and sulfur nucleophiles during developmental stages of mice.

METHODS

Wild-type (WT), CSE knockout (KO), Nrf2 KO, Nrf2/CSE double KO (DKO) mice, and their primary hepatocytes were analyzed in this study. Cadmium (Cd), methylmercury (MeHg), 1,4-naphthoquinone, crotonaldehyde, and acrylamide were used. We conducted Western blotting, real-time polymerase chain reaction (PCR), 3-(4,5-dimethylthiazol-2-yl)-2,5-triphenyl tetrazolium bromide (MTT) assays, liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) analysis, alanine transaminase (ALT) activity, histopathological analysis, and rotarod test.

RESULTS

Primary hepatocytes from DKO mice were significantly more sensitive to the environmental electrophiles than each single KO counterpart. Both Nrf2 and CSE single KO mice were highly susceptible to Cd and MeHg, and such sensitivity was further exacerbated in the DKO mice. Lower-level expressions of CSE and sulfur nucleophiles than those in adult mice were observed in a window of developmental stage.

CONCLUSIONS

Our mouse model provided new insights into the response to environmental electrophiles; while Nrf2 is recognized as a key transcription factor for detoxification of environmental electrophiles, CSE is crucial factor to repress their toxicity in a parallel mode. In addition, the sensitivity of fetuses to MeHg appears to be, at least in part, associated with the restricted production of RSS due to low-level expression of CSE. https://doi.org/10.1289/EHP4949.

Biological hydropersulfides and related polysulfides - a new concept and perspective in redox biology

The chemical biology of thiols (RSH, e.g., cysteine and cysteine-containing proteins/peptides) has been a topic of extreme interest for many decades due to their reported roles in protein structure/folding, redox signaling, metal ligation, cellular protection, and enzymology. While many of the studies on thiol/sulfur biochemistry have focused on thiols, relatively ignored have been hydropersulfides (RSSH) and higher order polysulfur species (RSS_n H, RSS_n R, n > 1). Recent and provocative work has alluded to the prevalence and likely physiological importance of RSSH and related RSS_n H. RSSH of cysteine (Cys-SSH) has been found to be prevalent in mammalian systems along with Cys-SSH-containing proteins. The RSSH functionality has not been examined to the extent of other biologically relevant sulfur derivatives (e.g., sulfenic acids, disulfides, etc.), whose roles in cell signaling are strongly indicated. The recent finding of Cys-SSH biosynthesis and translational incorporation into proteins is an unequivocal indication of its fundamental importance and necessitates a more profound look into the physiology of RSSH. In this Review, we discuss the currently reported chemical biology of RSSH (and related species) as a prelude to discussing their possible physiological roles.

Polysulfide Na2S4 regulates the activation of PTEN/Akt/CREB signaling and cytotoxicity mediated by 1,4-naphthoquinone through formation of sulfur adducts

Electrophiles can activate redox signal transduction pathways, through actions of effector molecules (e.g., kinases and transcription factors) and sensor proteins with low pKa thiols that are covalently modified. In this study, we investigated whether 1,4-naphthoquinone (1,4-NQ) could affect the phosphatase and tensin homolog (PTEN)–Akt signaling pathway and persulfides/polysulfides could modulate this adaptive response. Simultaneous exposure of primary mouse hepatocytes to Na₂S₄ and 1,4-NQ markedly decreased 1,4-NQ-mediated cell death and S-arylation of cellular proteins. Modification of cellular PTEN during exposure to 1,4-NQ was also blocked in the presence of Na₂S₄. 1,4-NQ, at up to 10 µM, increased phosphorylation of Akt and cAMP response element binding protein (CREB). However, at higher concentrations, 1,4-NQ inhibited phosphorylation of both proteins. These bell-shaped dose curves for Akt and CREB activation were right-shifted in cells treated with both 1,4-NQ and Na₂S₄. Incubation of 1,4-NQ with Na₂S₄ resulted in formation of 1,4-NQ–S–1,4-NQ-OH. Unlike 1,4-NQ, authentic 1,4-NQ-S-1,4-NQ-OH adduct had no cytotoxicity, covalent binding capability nor ability to activate PTEN-Akt signaling in cells. Our results suggested that polysulfides, such as Na₂S₄, can increase the threshold of 1,4-NQ for activating PTEN–Akt signaling and cytotoxicity by capturing this electrophile to form its sulfur adducts.

1,4-Naphthoquinone activates the HSP90/HSF1 pathway through the S-arylation of HSP90 in A431 cells: Negative regulation of the redox signal transduction pathway by persulfides/polysulfides

The current consensus is that environmental electrophiles activate redox signal transduction pathways through covalent modification of sensor proteins with reactive thiol groups at low concentrations, while they cause cell damage at higher concentrations. We previously exposed human carcinoma A431 cells to the atmospheric electrophile 1,4-naphthoquinone (1,4-NQ) and found that heat shock protein 90 (HSP90), a negative regulator of heat shock factor 1 (HSF1), was a target of 1,4-NQ. In the study presented here, we determined whether 1,4-NQ activates HSF1. We also examined whether such redox signaling could be regulated by nucleophilic sulfur species. Exposure of A431 cells to 1,4-NQ covalently modified cellular HSP90, resulting in repression of the association between HSF1 with HSP90, thereby enhancing HSF1 translocation into the nuclei. Liquid chromatography-tandem mass spectrometry analysis with recombinant HSP90 revealed that the modifications site were Cys412 and Cys564. We found that HSF1 activation mediated by 1,4-NQ upregulated downstream genes, such as HSPA6. HSF1 knockdown accelerated 1,4-NQ-mediated cytotoxicity in the cells. While simultaneous treatment with reactive persulfide and polysulfide, Na₂S₂ and Na₂S₄, blocked 1,4-NQ-dependent protein modification and HSF1 activation in A431 cells, the knockdown of Cys persulfide producing enzymes cystathionine β-synthase (CBS) and/or cystathionine γ-lyase (CSE) enhanced these phenomena. 1,4-NQ-thiol adduct and 1,4-NQ-S-1,4-NQ adduct were produced during the enzymatic reaction of recombinant CSE in the presence of 1,4-NQ. The results suggest that activation of the HSP90-HSF1 signal transduction pathway mediated by 1,4-NQ protects cells against 1,4-NQ and that per/polysulfides can diminish the reactivity of 1,4-NQ by forming sulfur adducts.