Flow cytometric HyPer-based assay for hydrogen peroxide

HyPer as a tool to determine the reductive activity in cellular compartments

A multitude of cellular metabolic and regulatory processes rely on controlled thiol reduction and oxidation mechanisms. Due to our aerobic environment, research preferentially focuses on oxidation processes, leading to limited tools tailored for investigating cellular reduction. Here, we advocate for repurposing HyPer1, initially designed as a fluorescent probe for H2O2 levels, as a tool to measure the reductive power in various cellular compartments. The response of HyPer1 depends on kinetics between thiol oxidation and reduction in its OxyR sensing domain. Here, we focused on the reduction half-reaction of HyPer1. We showed that HyPer1 primarily relies on Trx/TrxR-mediated reduction in the cytosol and nucleus, characterized by a second order rate constant of 5.8 × 102 M-1s-1. On the other hand, within the mitochondria, HyPer1 is predominantly reduced by glutathione (GSH). The GSH-mediated reduction rate constant is 1.8 M-1s-1. Using human leukemia K-562 cells after a brief oxidative exposure, we quantified the compartmentalized Trx/TrxR and GSH-dependent reductive activity using HyPer1. Notably, the recovery period for mitochondrial HyPer1 was twice as long compared to cytosolic and nuclear HyPer1. After exploring various human cells, we revealed a potent cytosolic Trx/TrxR pathway, particularly pronounced in cancer cell lines such as K-562 and HeLa. In conclusion, our study demonstrates that HyPer1 can be harnessed as a robust tool for assessing compartmentalized reduction activity in cells following oxidative stress.

Photosensitizing effects and physicochemical properties of chlorophyll a derivatives with hydrophilic oligoethylene glycol fragments at the macrocycle periphery

In this work, screening studies of the cytotoxic effect of chlorins with fragments of di-, tri-, and pentaethylene glycol at the macrocycle periphery in relation to HeLa, A549, and HT29 cells were performed. It is shown that, despite different hydrophobicity, all the compounds studied have a comparable photodynamic effect. The conjugate of chlorin e6 with pentaethylene glycol, which has the lowest tendency to association among the studied compounds with tropism for low density lipoproteins and the best characteristics of the formation of molecular complexes with Tween 80, has a significant difference in dark and photoinduced toxicity (ratio IC50(dark)/IC50(photo) approximately 2 orders of magnitude for all cell lines), which allows to hope for a sufficiently large "therapeutic window". A study of the interaction of this compound with HeLa cells shows that the substance penetrates the cell and, after red light irradiation induces ROS appearance inside the cell, associated, apparently, with the photogeneration of singlet oxygen. These data indicate that photoinduced toxic effects are caused by damage to intracellular structures as a result of oxidative stress. Programmed type of cell death characterized with caspase-3 induction is prevailing. So, the conjugate of chlorin e6 with pentaethylene glycol is a promising antitumor PS that can be successfully solubilized with Tween 80, which makes it suitable for further in vivo studies.

Inhibition of thioredoxin reductase activity reduces the antioxidant defense capacity of human pluripotent stem cells under conditions of mild but not severe oxidative stress

Pro-oxidative shift in redox balance, usually termed as "oxidative stress", can lead to different cell responses depending on its intensity. Excessive accumulation of reactive oxygen species ("oxidative distress") can cause DNA, lipid and protein damage. Physiological oxidative stimulus ("oxidative eustress"), in turn, can favor cell proliferation and differentiation - the processes of paramount importance primarily for stem cells. Functions of antioxidant enzymes in cells is currently a focus of intense research, however the role of different antioxidant pathways in pluripotent cell responses to oxidative distress/eustress is still under investigation. In this study, we assessed the contribution of the thioredoxin reductase (TrxR)-dependent pathways to maintaining the redox homeostasis in human induced pluripotent stem cells and their differentiated progeny cells under basal conditions and under conditions of oxidative stress of varying intensity. Employing the genetically encoded H₂O₂ biosensor cyto-HyPer and two inhibitors of thioredoxin reductase (auranofin and Tri-1), we show that the reduced activity of TrxR-dependent enzymatic systems leads to the non-cytotoxic disruption of thiol-disulfide metabolism in the cytoplasm of both pluripotent and differentiated cells under basal conditions. Quantifying the cytoplasmic concentrations of peroxide establishing in H₂O₂-stressed cells, we demonstrate that TrxR-dependent pathways contribute to the antioxidant activity in the cell cytoplasm under conditions of mild but not severe oxidative stress in both cell lines tested. The observed effects may testify about a conservative role of the TrxR-controlled enzymatic systems manifested as a response to physiological redox stimuli rather than a protection against the severe oxidative stress.

Application of a HyPer-3 sensor to monitor intracellular H2O2 generation induced by phenolic acids in differentiated Caco-2 cells

Intestinal epithelial cells (IECs) are an important point of contact between dietary food components consumed and subsequent whole-body utilization for body maintenance and growth. Selective bioactive phenolic acids, widely present in fruits, vegetables and beverages can generate hydrogen peroxide (H₂O₂) and contribute to the cellular redox balance, hence influencing well-known cellular antioxidant and pro-oxidant mechanisms. Our findings have showed that increasing extracellular H₂O₂ resulted in associated changes in intracellular H₂O₂ levels in Caco-2 cells (p < 0.05) which was facilitated by activity of a family of water channel membrane proteins, termed aquaporins (AQPs). To demonstrate this, a HyPer-3 genetically encoded fluorescent H₂O₂ sensitive indicator was used to enable fluorescent real-time imaging of intracellular H₂O₂ levels as a measure of changes occurring in extracellular H₂O₂ in differentiated Caco-2 cells exposed to different phenolic acids. The use of confocal microscopy and flow cytometry, respectively, captured visualization and quantification of H₂O₂ uptake in differentiated Caco-2 cells. DFP00173, an aquaporin 3 (AQP3) inhibitor was effective at inhibiting the intracellular uptake of H₂O₂ and was sensitive to varied levels of H₂O₂ generated when different phenolic acids were added to the culture media. In summary, HyPer-3 was shown to be an effective technique to demonstrate relative capabilities of structurally different dietary phenolic acids that have potential to alter intestinal redox balance by changing intracellular H₂O₂, and either antioxidant or pro-oxidant activity, respectively.

Senescent endometrial stromal cells transmit reactive oxygen species to the trophoblast-like cells and impair spreading of blastocyst-like spheroids

Successful implantation requires a fine-tuned dialog between the invading embryo and the maternal endometrium. Recently, we discovered that premature senescence of endometrial stromal cells (EnSC) might mediate improper decidual transformation of endometrial tissue and impair endometrial-blastocyst interaction. Here, we show that senescent EnSC are characterized by elevated intracellular reactive oxygen species (ROS) levels that originate from mitochondrial dysfunction and insufficient antioxidant defense. Decidualization of senescent EnSC is defective and is accompanied by the elevated intracellular and mitochondrial ROS levels. Antioxidant defense during decidualization is significantly less efficient in senescent EnSC compared to healthy ones. Senescent EnSC secrete increased amounts of ROS into the extracellular space. Elevated ROS released by senescent EnSC shift the redox balance and induce DNA damage in the neighboring trophoblast-like cells. In an in vitro implantation model, we observed impaired spreading of blastocyst-like spheroids into a monolayer of decidualizing senescent EnSC, which could be compensated by pretreatment of the senescent cells with the antioxidant, Tempol. Hence, we propose a possible mechanism that might be responsible, at least in part, for the defective embryo implantation realized via ROS transmitting from senescent EnSC to trophoblast cells. Such transmission results in the accumulation of ROS and subsequent DNA damage in trophoblastic cells, which might lead to improper migration and invasion of an embryo. In light of these findings, the application of antioxidants prior to implantation might be a promising strategy to improve implantation efficiency.

Resistance to H2O2-induced oxidative stress in human cells of different phenotypes

Application of genetically encoded biosensors of redox-active compounds promotes the elaboration of new methods for investigation of intracellular redox activities. Previously, we have developed a method to measure quantitatively the intracellular concentration of hydrogen peroxide (H₂O₂) in living cells using genetically encoded biosensor HyPer. In the present study, we refined the method and applied it for comparing the antioxidant system potency in human cells of different phenotypes by measuring the gradient between the extracellular and cytoplasmic H₂O₂ concentrations under conditions of H₂O₂-induced external oxidative stress. The measurements were performed using cancer cell lines (K-562 and HeLa), as well as normal human cells – all expressing HyPer in the cell cytoplasm. As normal cells, we used three isogenic lines of different phenotypes – mesenchymal stem/stromal cells (MSCs), induced pluripotent stem cells (iPSCs) derived from MSCs by reprogramming, and differentiated iPSC progenies with the phenotype resembling precursory MSCs. When exposing cells to exogenous H₂O₂, we showed that at low oxidative loads (<50 μM of H₂O₂) the gradient depended on extracellular H₂O₂ concentration. At high loads (>50 μM of H₂O₂), which caused the exhaustion of thioredoxin activity in the cell cytoplasm, the gradient stabilized, pointing out that it is the functional status of the thioredoxin-depended enzymatic system that drives the dependence of the H₂O₂ gradient on the oxidative load in human cells. At high H₂O₂ concentrations, the cytoplasmic H₂O₂ level in cancer cells was found to be several hundred times lower than the extracellular one. At the same time, in normal cells, extracellular-to-intracellular gradient amounted to thousands of times. Upon reprogramming, the potency of cellular antioxidant defense increased. In contrast, differentiation of iPSCs did not result in the changes in antioxidant system activity in the cell cytoplasm, assuming that intensification of the H₂O₂-detoxification processes is inherent to a period of early human development.

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HyPer can be used to compare the H₂O₂-detoxification activity of different cells.

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In H₂O₂-stressed cells, H₂O₂ gradient across the cell membrane depends on H₂O₂ load.

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In normal human cells, H₂O₂ gradient amounts to thousands of times.

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Upon cell reprogramming, the H₂O₂-detoxification activity in cells increases.

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High H₂O₂ loads compromise Trx-depended reduction of thiols in cell cytoplasm.

Physiological Signaling Functions of Reactive Oxygen Species in Stem Cells: From Flies to Man

Reactive oxygen species (ROS), superoxide anion and hydrogen peroxide, are generated as byproducts of oxidative phosphorylation in the mitochondria or via cell signaling-induced NADPH oxidases in the cytosol. In the recent two decades, a plethora of studies established that elevated ROS levels generated by oxidative eustress are crucial physiological mediators of many cellular and developmental processes. In this review, we discuss the mechanisms of ROS generation and regulation, current understanding of ROS functions in the maintenance of adult and embryonic stem cells, as well as in the process of cell reprogramming to a pluripotent state. Recently discovered cell-non-autonomous ROS functions mediated by growth factors are crucial for controlling cell differentiation and cellular immune response in Drosophila. Importantly, many physiological functions of ROS discovered in Drosophila may allow for deciphering and understanding analogous processes in human, which could potentially lead to the development of novel therapeutic approaches in ROS-associated diseases treatment.

Measuring intracellular concentration of hydrogen peroxide with the use of genetically encoded H2O2 biosensor HyPer

In this study, we propose a method for quantification of average hydrogen peroxide concentration within a living cell that is based on the use of genetically encoded H₂O₂ biosensor HyPer. The method utilizes flow cytometric measurements of HyPer fluorescence in H₂O₂-exposed cells to analyze the biosensor oxidation kinetics. Fitting the experimental curves with kinetic equations allows determining the rate constants of HyPer oxidation/reduction which are used further for the calculation of peroxide concentrations in the cells of interest both in the presence and absence of external H₂O₂. Applying this method to K562 cells, we have estimated the gradient as about 390-fold between the extracellular and intracellular level of exogenous H₂O₂ in cells exposed to the micromole doses of peroxide, as well as the average basal level of H₂O₂ in the cytosol of undisturbed cells ([H2O2]basal=2.2±0.4nM). The method can be extended to other H₂O₂-sensitive redox probes or to procedures in which, rather than adding external peroxide, intracellular production of peroxide is triggered, providing a tool to quantitate not only basal average H₂O₂ concentrations but also the concentration of peroxide build up in the vicinity of redox probes.

High doses of synthetic antioxidants induce premature senescence in cultivated mesenchymal stem cells

Stress-induced premature senescence program is known to be activated in cells by various genotoxic stressors, and oxidative stress is considered to be the main of those. To this end, many studies discover antioxidants as protective anti-aging agents. In the current study, we examined the effects of different antioxidants (Tempol, resveratrol, NAC, DPI) on the mesenchymal stem cells maintained in normal physiological conditions. We used high, but non-cytotoxic antioxidant doses which are widely used in laboratory practice to protect cells from oxidative damage. We show that these substances induce reversible block of cell proliferation and do not cause any genotoxic effects when applied to the quiescent cells. However, the same doses of the same substances, when applied to the proliferating cells, can induce irreversible cell cycle arrest, DNA strand breaks accumulation and DNA damage response activation. As a consequence, antioxidant-induced DNA damage results in the stress-induced premature senescence program activation. We conclude that high doses of antioxidants, when applied to the proliferating cells that maintain physiological levels of reactive oxygen species, can cause DNA damage and induce premature senescence which suggests to re-estimate believed unconditional anti-aging antioxidant properties.

Hydrogen peroxide in the reactions of cancer cells to cisplatin

BACKGROUND

Hydrogen peroxide (H₂O₂) is thought to be one of the key components involved in the responses of tumor cells to chemotherapy. The aim of this study was to reveal the pathways and the phases of cisplatin-induced cell death that are characterized by changes of H₂O₂ level.

METHODS

The genetically encoded cytosolic fluorescent sensor HyPer2 was used for flow cytometric analysis of the cisplatin-induced changes in H₂O₂ level in HeLa Kyoto cells. Using a vital dye and the apoptotic markers PE Annexin V or TMRE the pathways and stages of cell death were investigated simultaneously with HyPer2 response. The H₂O₂ level was studied separately in viable and early apoptotic cells after 12, 18, 24 h's incubation with cisplatin at several concentrations with or without the scavenger of reactive oxygen species NAC.

RESULTS

Cisplatin causes dose- and time-dependent increase of H₂O₂ level in TMRE-positive and PE Annexin V-negative cancer cells. The scavenging of ROS by NAC decreased the H₂O₂ level and restored cell viability.

CONCLUSION

Н₂О₂ generation begins in cells that have already lost mitochondrial membrane potential but have not yet externalized phosphatidylserine. Prevention of apoptosis by NAC confirmed the role of H₂O₂ in apoptosis induction.

GENERAL SIGNIFICANCE

This is the first time that the sensor HyPer2 has been used in parallel with apoptotic markers and vital dye to demonstrate the role of H₂O₂ in different stages and types of tumor cell death under chemotherapeutic action.