Cell type-dependent release of nitric oxide and/or reactive nitrogenoxide species from intracellular SIN-1: effects on cellular NAD(P)H

Continuous Monitoring Reveals Protective Effects of N-Acetylcysteine Amide on an Isogenic Microphysiological Model of the Neurovascular Unit

Microphysiological systems mimic the in vivo cellular ensemble and microenvironment with the goal of providing more human-like models for biopharmaceutical research. In this study, the first such model of the blood-brain barrier (BBB-on-chip) featuring both isogenic human induced pluripotent stem cell (hiPSC)-derived cells and continuous barrier integrity monitoring with <2 min temporal resolution is reported. Its capabilities are showcased in the first microphysiological study of nitrosative stress and antioxidant prophylaxis. Relying on off-stoichiometry thiol-ene-epoxy (OSTE+) for fabrication greatly facilitates assembly and sensor integration compared to the prevalent polydimethylsiloxane devices. The integrated cell-substrate endothelial resistance monitoring allows for capturing the formation and breakdown of the BBB model, which consists of cocultured hiPSC-derived endothelial-like and astrocyte-like cells. Clear cellular disruption is observed when exposing the BBB-on-chip to the nitrosative stressor linsidomine, and the barrier permeability and barrier-protective effects of the antioxidant N-acetylcysteine amide are reported. Using metabolomic network analysis reveals further drug-induced changes consistent with prior literature regarding, e.g., cysteine and glutathione involvement. A model like this opens new possibilities for drug screening studies and personalized medicine, relying solely on isogenic human-derived cells and providing high-resolution temporal readouts that can help in pharmacodynamic studies.

Manganese porphyrin redox state in endothelial cells: Resonance Raman studies and implications for antioxidant protection towards peroxynitrite

Cationic manganese(III) ortho N-substituted pyridylporphyrins (MnP) act as efficient antioxidants catalyzing superoxide dismutation and accelerating peroxynitrite reduction. Importantly, MnP can reach mitochondria offering protection against reactive species in different animal models of disease. Although an LC-MS/MS-based method for MnP quantitation and subcellular distribution has been reported, a direct method capable of evaluating both the uptake and the redox state of MnP in living cells has not yet been developed. In the present work we applied resonance Raman (RR) spectroscopy to analyze the intracellular accumulation of two potent MnP-based lipophilic SOD mimics, MnTnBuOE-2-PyP⁵⁺ and MnTnHex-2-PyP⁵⁺ within endothelial cells. RR experiments with isolated mitochondria revealed that the reduction of Mn(III)P was affected by inhibitors of the electron transport chain, supporting the action of MnP as efficient redox active compounds in mitochondria. Indeed, RR spectra confirmed that MnP added in the Mn(III) state can be incorporated into the cells, readily reduced by intracellular components to the Mn(II) state and oxidized by peroxynitrite. To assess the combined impact of reactivity and bioavailability, we studied the kinetics of Mn(III)TnBuOE-2-PyP⁵⁺ with peroxynitrite and evaluated the cytoprotective capacity of MnP by exposing the endothelial cells to nitro-oxidative stress induced by peroxynitrite. We observed a preservation of normal mitochondrial function, attenuation of cell damage and prevention of apoptotic cell death. These data introduce a novel application of RR spectroscopy for the direct detection of MnP and their redox states inside living cells, and helps to rationalize their antioxidant capacity in biological systems.

Impact of SIN-1-derived peroxynitrite flux on endothelial cell redox homeostasis and bioenergetics: protective role of diphenyl diselenide via induction of peroxiredoxins

Increased production of reactive nitrogen (RNS) and oxygen (ROS) species and its detrimental effect to mitochondria are associated with endothelial dysfunction. This study was designed to determine the effect of a peroxynitrite flux, promoted by 1,3-morpholinosydnonimine (SIN-1), in mitochondrial function and some redox homeostasis parameters in bovine aortic endothelial cells (BAEC). Moreover, the effect of diphenyl diselenide (PhSe)2, a simple organic selenium compound, in preventing peroxynitrite-mediated cytotoxicity was also investigated. Our results showed that overnight exposure to SIN-1 (250 μM) caused a profound impairment of oxygen consumption, energy generation and reserve capacity in mitochondria of BAEC. Mitochondrial dysfunction resulted in an additional intracellular production of peroxynitrite, amplifying the phenomenon and leading to changes in redox homeostasis. Moreover, we observed an extensive decline in mitochondrial membrane potential (ΔΨm) induced by peroxynitrite and this event was associated with apoptotic-type cell death. Alternatively, the pretreatment of BAEC with (PhSe)2, hindered peroxynitrite-mediated cell damage by preserving mitochondrial and endothelial function and consequently preventing apoptosis. The protective effect of (PhSe)2 was related to its ability to improve the intracellular redox state by increasing the expression of different isoforms of peroxiredoxins (Prx-1-3), efficient enzymes in peroxynitrite detoxification.

Bicarbonate plays a critical role in the generation of cytotoxicity during SIN-1 decomposition in culture medium

3-Morpholinosydnonimine (SIN-1) is used as a donor of peroxynitrite (ONOO⁻) in various studies. We demonstrated, however, that, the cell-culture medium remains cytotoxic to PC12 cells even after almost complete SIN-1 decomposition, suggesting that reaction product(s) in the medium, rather than ONOO⁻, exert cytotoxic effects. Here, we clarified that significant cytotoxicity persists after SIN-1 decomposes in bicarbonate, a component of the culture medium, but not in NaOH. Cytotoxic SIN-1-decomposed bicarbonate, which lacks both oxidizing and nitrosating activities, degrades to innocuous state over time. The extent of SIN-1 cytotoxicity, irrespective of its fresh or decomposed state, appears to depend on the total number of initial SIN-1 molecules per cell, rather than its concentration, and involves oxidative/nitrosative stress-related cell damage. These results suggest that, despite its low abundance, the bicarbonate-dependent cytotoxic substance that accumulates in the medium during SIN-1 breakdown is the cytotoxic entity of SIN-1.

PCB 9 exposure induces endothelial cell death while increasing intracellular calcium and ROS levels

Cultured porcine endothelial cells were used to determine the effects of several congeners of polychlorinated biphenyls (PCBs) on cell viability and changes induced by these congeners on levels of intracellular calcium and reactive oxygen species (ROS). Cultured endothelial cells were prepared as a single suspension and run on flow cytometry. Cell viability, intracellular calcium, and ROS concentrations were simultaneously determined by using propidium iodide, Fluo-3, and dihydrorodamine, respectively. The congeners tested included two coplanar tetrachlorinated congeners that have no ortho chlorines, PCBs 77 and 80, a tri-chloro congener which does not have any ortho chlorine, PCB 39, a di-ortho, tetrachlorinated congener, PCB 52, and a mono-ortho congener, PCB 9. PCB 9 was cytotoxic at 5 μM within 5 min of exposure, and the toxicity increased with time and concentration. None of the other congeners showed consistent cytotoxicity. The cytotoxicity was roughly correlated with elevations in cellular ROS levels, but not with changes in intracellular calcium. To the best of our knowledge, toxicity of lower chlorinated, more volatile congeners such as PCB 9 has not been previously studied. These observations may be taken to explain the elevated risk of cardiovascular disease previously reported among residents living near to hazardous waste sites containing PCBs.

SIN-1 cytotoxicity to PC12 cells is mediated by thiol-sensitive short-lived substances generated through SIN-1 decomposition in culture medium

As a generator of peroxynitrite (ONOO(-)), 3-morpholinosydnonimine (SIN-1) is widely used in the study of oxidative/nitrosative stress in cultured cells, although controversy exists regarding active species responsible for cytotoxicity. In this study, we report that unstable thiol-sensitive substances, generated from the reaction of SIN-1 with components in culture medium, play a crucial role in SIN-1 cytotoxicity in PC12 cells. Exposure of cells to culture medium obtained after almost complete SIN-1 decomposition at 37 degrees C for 2h demonstrated almost the same degree of cytotoxicity as did fresh SIN-1. The cytotoxicity of SIN-1-decomposed medium largely depended on serum, decayed with time, and could be completely abolished by the addition of thiols. Degradation of synthetic ONOO(-) in the culture medium did not reproduce the unstable cytotoxicity. The presence of superoxide dismutase (SOD) during SIN-1 decomposition prevented the formation of the cytotoxic substances, whereas SOD had no protection against the cytotoxicity itself, suggesting a crucial role of simultaneously generated superoxide and nitric oxide in the formation of the toxicants, but not in their cytotoxic action. The cytotoxicity of fresh SIN-1 is dramatically suppressed in a basal medium (Hanks balanced salt), suggesting that the cytotoxicity of fresh SIN-1 also requires components of culture medium. These results suggest that SIN-1 cytotoxicity in PC12 cells is mediated via the generation of cytotoxic substances in the medium during its decomposition.

Effects of the Cdc2-like kinase-family and DNA topoisomerase I on the alternative splicing of eNOS in TNF-α-stimulated human endothelial cells

Nitric oxide (NO) is synthesized by endothelial nitric oxide synthase (eNOS) and plays an important role in vascular homeostasis and cardiovascular diseases. It has recently been shown that increased expression of alternatively spliced eNOS isoforms eNOS 13A, B and C and heterodimerization with ‘full-length’ eNOS is associated with a decreased eNOS activity. The regulatory pathways enabling this phenomenon are completely unknown. This study examined the effect of Cdc2-like kinases and DNA topoisomerase I on eNOS splicing in TNF-α-induced human umbilical vein endothelial cells (HUVECs). We found that inhibition of DNA topoisomerase I, but not Cdc2-like kinases, prevents the TNF-α-induced increase in eNOS isoform expression and NO reduction in HUVEC. Moreover, we show that the inhibition of DNA topoisomerase I or the Cdc2-like kinases differently modulates the phosphorylation of the serine/arginine-rich proteins SRp75 and SRp55. Our results demonstrate, for the first time, that DNA topoisomerase I but not Cdc2-like kinases serves as an important regulator of the differential eNOS isoform expression in endothelial cells, thereby modulating the TNF-α-induced eNOS activity switch.

Sodium as the major mediator of NO-induced cell death in cultured hepatocytes

NO has been shown to induce cellular injury via inhibition of the mitochondrial respiratory chain and/or oxidative/nitrosative stress. Here, we studied which mechanism and downstream mediator is responsible for NO toxicity to hepatocytes. When cultured rat hepatocytes were incubated with spermineNONOate (0.01-2 mM) at 2, 5, 21 and 95% O(2) in Krebs-Henseleit buffer (37 degrees C), spermineNONOate caused concentration-dependent hepatocyte death (lactate dehydrogenase release, propidium iodide uptake) with morphological features of both apoptosis and necrosis. Increasing O(2) concentrations protected hepatocytes from NO-induced injury. Steady-state NO concentrations were lower at higher O(2) concentrations, suggesting formation of reactive nitrogen oxide species. Despite this, the scavenger ascorbic acid was hardly protective. In contrast, at equal NO concentrations loss of viability was higher at lower O(2) concentrations and inhibitors of hypoxic injury, fructose and glycine (10 mM), strongly decreased NO-induced injury. Upon addition of spermineNONOate, the cytosolic Na(+) concentration rapidly increased. The increase in sodium depended on the NO/O(2) ratio and was paralleled by hepatocyte death. Sodium-free Krebs-Henseleit buffer strongly protected from NO-induced injury. SpermineNONOate also increased cytosolic calcium levels but the Ca(2+) chelator quin-2-AM did not diminish cell injury. These results show that - in analogy to hypoxic injury - a sodium influx largely mediates the NO-induced death of cultured hepatocytes. Oxidative stress and disturbances in calcium homeostasis appear to be of minor importance for NO toxicity to hepatocytes.

Use of Fluorescence Probes for Detection of Reactive Nitrogen Species: A Review

The biological and toxicological effects that have been attributed to reactive nitrogen species (RNS) are increasingly stirring the scientific inquisitiveness about the molecular mechanisms involved. However, RNS present some characteristics that complicate their detection, namely their short lifetime and the normal presence of a variety of endogenous compounds capable of reacting with these reactive species, when the studies are performed in biological matrices. The development of methodologies capable of circumvent these difficulties is thus of fundamental importance. Fluorescence probes are particularly important due to their high sensibility and usefulness in temporal and spatial monitoring of RNS, particularly in microanalysis conditions in biological media akin to cells or tissues. In the present review is given an account of the fluorescence probes that have been used for detection of nitric oxide (*NO), peroxynitrite anion (ONOO-), as well as of some of its derivatives in biological and nonbiological media.

Initiation of a superoxide-dependent chain oxidation of lactate dehydrogenase-bound NADH by oxidants of low and high reactivity

In cells, NADH and NADPH are mainly bound to dehydrogenases such as lactate dehydrogenase (LDH). In cell-free systems, the binary LDH-NADH complex has been demonstrated to produce reactive oxygen species via a chain oxidation of NADH initiated and propagated by superoxide. We studied here whether this chain radical reaction can be initiated by oxidants other than LDH largely increased the oxidation of NADH (but not of NADPH) by O(2), H(2)O(2) and during the intermediacy of HNO(2). LDH also increased the oxidation of NADH by peroxynitrite. The increases in NADH oxidation were completely prevented by superoxide dismutase (SOD). In contrast, the nitrogen dioxide-dependent oxidation of NADH and NADPH was decreased by LDH in a SOD-independent manner. These experimental data strongly indicate that oxidation of LDH-bound NADH can be induced from reaction of either weak oxidants with LDH-bound NADH or of strong oxidants with free NADH thus yielding which is highly effective to propagate the chain. Our results underline the importance of SOD in terminating superoxide-dependent chain reactions in cells under oxidative stress.

A comparative study of the effects of molsidomine and 3-morpholinosydnonimine on the redox status of rat erythrocytes and reticulocytes

After enzymic biotransformation, molsidomine (MO) acts via the metabolite 3-morpholinosydnonimine (SIN-1) through spontaneous liberation of nitric oxide (NO) and superoxide (O(2)(.-)). The aim of this study was to compare the effects of MO and its active metabolite SIN-1 on the redox status of rat erythrocytes and reticulocytes. Rat erythrocyte as well as reticulocyte-rich red blood cell (RBC) suspensions were aerobically incubated (2 h, 37 degrees C) without (control) or in the presence of different concentrations of MO or SIN-1. In rat erythrocytes, biotransformation of MO resulted in the production of NO and nitroxyl (NO(-)). Endogenous superoxide anion (O(2)(.-)) participated in peroxynitrite generation. SIN-1 simultaneously liberated NO and O(2)(.-), which formed peroxynitrite (at least in part), but the liberated NO predominantly reacted with haemoglobin, forming methaemoglobin in erythrocytes. In reticulocytes, MO and SIN-1 caused an increase in the levels of both nitrite and 3-nitrotyrosine (an indicator of peroxynitrite), whereas they decreased the level of O(2)(.-). In reticulocytes, MO was metabolized into SIN-1 which led to the generation of NO, which reacted with O(2)(.-) (endogenous or exogenous) forming reactive nitrogen species. In conclusion, there are two metabolic pathways for MO biotransformation: one causing NO and NO(-) generation predominantly in erythrocytes and the other, via SIN-1 metabolism, in reticulocytes. The main difference between the action of MO and SIN-1 was that the latter caused oxidative damage in RBCs.