Membrane photochemistry and photobiology

Antioxidant enzyme inhibitors enhance singlet oxygen-induced cell death in HL-60 cells

Singlet oxygen is a highly reactive form of molecular oxygen that may harm living systems by oxidizing critical cellular macromolecules and it also promotes deleterious processes such as cell death. The protective role of antioxidant enzymes against singlet oxygen-induced oxidative damage in HL-60 cells was investigated in control and cells pre-treated with diethyldithiocarbamic acid, aminotriazole and oxlalomalate, specific inhibitors of superoxide dismutase, catalase and NADP+-dependent isocitrate dehydrogenase, respectively. Upon exposure to rose bengal (20 microM)/light (15 min), which generates singlet oxygen, to HL-60 cells, the viability was lower and the lipid peroxidation and oxidative DNA damage were higher in inhibitor-treated cells as compared to control cells. We also observed the significant increase in the endogenous production of reactive oxygen species as well as the significant decrease in the intracellular GSH level in inhibitor-treated HL-60 cells exposed to singlet oxygen. Upon exposure to rose bengal (3 microM)/light (15 min), which induced apoptotic cell death, a clear inverse relationship was observed between the control and inhibitor-treated HL-60 cells in their susceptibility to apoptosis. These results suggest that antioxidant enzymes play an important role in cellular defense against singlet oxygen-induced cell death including necrosis and apoptosis.

Regulation of singlet oxygen-induced apoptosis by cytosolic NADP+-dependent isocitrate dehydrogenase

Singlet oxygen is a highly reactive form of molecular oxygen that may harm living systems by oxidizing critical cellular macromolecules and it also promotes deleterious processes such as cell death. Recently, we demonstrated that the control of redox balance and the cellular defense against oxidative damage are the primary functions of cytosolic NADP(+)-dependent isocitrate dehydrogenase (IDPc) through supplying NADPH for antioxidant systems. In this report, we demonstrate that modulation of IDPc activity in HL-60 cells regulates singlet oxygen-induced apoptosis. When we examined the protective role of IDPc against singlet oxygen-induced apoptosis with HL-60 cells transfected with the cDNA for mouse IDPc in sense and antisense orientations, a clear inverse relationship was observed between the amount of IDPc expressed in target cells and their susceptibility to apoptosis. The results suggest that IDPc plays an important protective role in apoptosis of HL-60 cells induced by singlet oxygen.

Inactivation of primary antioxidant enzymes in mouse keratinocytes by photodynamically generated singlet oxygen

Cellular antioxidant enzymes protect against damage caused by exposure to endogenous or exogenous prooxidants. Singlet oxygen ((1)O(2)) is a reactive form of oxygen that can be produced in vivo either in normal and pathophysiologic conditions or by photosensitizing chemicals, as during photodynamic treatment. We hypothesized that photodynamically generated (1)O(2) would decrease the enzymatic activities of cellular antioxidants. To test this hypothesis, we treated cultured mouse epidermal keratinocytes with the photosensitizer Photofrin plus visible light to produce (1)O(2), and then measured CuZnSOD, MnSOD, and catalase activities with both ingel and spectrophotometric enzyme activity assays. Our results demonstrated that the enzymatic activities of cellular CuZnSOD, MnSOD, and catalase were significantly decreased after keratinocytes were treated with Photofrin plus visible light. By contrast, the enzymatic activities of cellular CuZnSOD, MnSOD, and catalase were unaffected in control cells treated with Photofrin only or visible light only. Despite the decreased levels of enzymatic activities, the protein levels of all three primary antioxidant enzymes remained constant after photodynamic treatment, as determined by Western blotting. L-Histidine, a (1)O(2) quencher, protected against the inactivation of cellular CuZnSOD, MnSOD, and catalase enzymes induced by photodynamically generated (1)O(2). The conclusion from these experiments is that the primary cellular antioxidant enzymes CuZnSOD, MnSOD, and catalase can be inactivated by photodynamically generated (1)O(2) in nucleated mammalian cells. These findings may be useful in the future development of antineoplastic adjuvant therapies that use photodynamic generation of (1)O(2) to inactivate antioxidant defenses with a goal of sensitizing tumor cells to prooxidant-generating drugs.

Inactivation of NADP(+)-dependent isocitrate dehydrogenase by singlet oxygen derived from photoactivated rose bengal

Recently, we demonstrated that the control of cytosolic and mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of NADP(+)-dependent isocitrate dehydrogenase (ICDH) by supplying NADPH for antioxidant systems. When exposed to a singlet oxygen-producing system composed of rose bengal (RB) and visible light, ICDH was susceptible to oxidative modification and damage as indicated by the loss of activity and by the formation of carbonyl groups. The structural alterations of modified enzyme were indicated by the increase in susceptibility to proteases and the change in intrinsic fluorescence spectra. Upon exposure to photoactivated RB, a significant decrease in both cytosolic and mitochondrial ICDH activities was observed in HL-60 cells. The singlet oxygen-mediated damage to ICDH may result in the perturbation of cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant condition. When we examined the antioxidant role of cytosolic ICDH against singlet oxygen-induced damage with HL-60 cells transfected with the cDNA for mouse cytosolic ICDH in sense and antisense orientations, a clear inverse relationship was observed between the amount of cytosolic ICDH expressed in target cells and their susceptibility to singlet oxygen-mediated oxidative damage.

Control of singlet oxygen-induced oxidative damage in Escherichia coli

Singlet oxygen ((1)O(2)) is a highly reactive form of molecular oxygen that may harm living systems by oxidizing critical cellular macromolecules. The oxyR gene product regulates the expression of the enzymes and proteins that are needed for cellular protection against oxidative stress. In this study, the role of oxyR in cellular defense against a singlet oxygen was investigated using Escherichia coli oxyR mutant strains. Upon exposure to methylene blue and visible light, which generates singlet oxygen, the oxyR overexpression mutant was much more resistant to singlet oxygen-mediated cellular damage when compared to the oxyR deletion mutant in regard to growth kinetics, viability and protein oxidation. Induction and inactivation of major antioxidant enzymes, such as superoxide dismutase and catalase, were observed after their exposure to a singlet oxygen generating system in both oxyR strains. However, the oxyR overexpression mutant maintained significantly higher activities of antioxidant enzymes than did the oxyR deletion mutant. These results suggest that the oxyR regulon plays an important protective role in singlet oxygen-mediated cellular damage, presumably through the protection of antioxidant enzymes.

Curcumin (diferuloylmethane), a singlet oxygen ((1)O(2)) quencher

Curcumin (diferuloylmethane) is a major component of food flavoring turmeric (Curcuma longa), and has been reported to be anticarcinogenic and anti-inflammatory. Although curcumin was shown to have antioxidant properties, its exact antioxidant nature has not been fully investigated. In this report we have investigated the possible antioxidant properties of curcumin using EPR spectroscopic techniques. Curcumin was found to inhibit the (1)O(2)-dependent 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) formation in a dose-dependent manner. (1)O(2) was produced in a photosensitizing system using rose bengal as sensitizer, and was detected as TEMP-(1)O(2) adducts by electron paramagnetic resonance (EPR) spectroscopic techniques using TEMP as a spin-trap. Curcumin at 2.75 microM caused 50% inhibition of TEMP-(1)O(2) adduct formation. However, curcumin only marginally inhibited (24% maximum at 80 microM) reduction of ferricytochrome c in a xanthine-xanthine oxidase system demonstrating that it is not an effective superoxide radical scavenger. Additionally, there was minor inhibition of DMPO-OH adduct formation by curcumin (solubilized in ethanol) when an ethanol control was included in the EPR spin-trapping study, suggesting that curcumin may not be an effective hydroxyl radical scavenger. Together these data demonstrate that curcumin is able only to effectively quench singlet oxygen at very low concentration in aqueous systems.

Inactivation of NADP(+)-dependent isocitrate dehydrogenase by reactive oxygen species

Recently, we demonstrated that the control of cytosolic and mitochondrial redox balance and the cellular defense against oxidative damage is one of the primary functions of NADP(+)-dependent isocitrate dehydrogenase (ICDH) through supply of NADPH for antioxidant systems. When exposed to various reactive oxygen species such as hydrogen peroxide, singlet oxygen generated by photoactivated dye, superoxide anion, and hydroxyl radical produced by metal-catalyzed Fenton reactions, ICDH was susceptible to oxidative modification and damage, which was indicated by the loss of activity, fragmentation of the peptide as well as by the formation of carbonyl groups. Oxidative damage to ICDH was inhibited by antioxidant enzymes, free radical scavengers, and spin-trapping agents. The structural alterations of modified enzymes were indicated by the increase in thermal instability and binding of the hydrophobic probe 8-anilino-1-naphthalene sulfonic acid (ANSA). The reactive oxygen species-mediated damage to ICDH may result in the perturbation of cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant condition.

Inactivation of catalase and superoxide dismutase by singlet oxygen derived from photoactivated dye

Both superoxide dismutase (SOD) and catalase are key enzymes in the antioxidant system of the cells that work to maintain low steady-state concentrations of the reactive oxygen species. When exposed to a singlet oxygen-producing system composed of dye, such as methylene blue or rose bengal, and visible light both SOD and catalase were susceptible to oxidative modification and damage as indicated by the loss of activity, fragmentation and aggregation of peptide as well as by the formation of carbonyl groups. Histidine, a powerful quenching agent for singlet oxygen, and the polyamines, such as spermine and spermidine, were effective at protecting the activity loss mediated by illuminated dye, whereas spin traps were only mildly effective. The structural alterations of modified enzymes were indicated by the increase in susceptibility to proteases, the change in absorption spectra and in fluorescence spectra. The singlet oxygen-mediated damage to SOD and catalase may result in the perturbation of cellular antioxidant defense mechanisms and subsequently lead to a pro-oxidant condition.