Endogenous and exogenous nitric oxide enhance the DNA strand scission induced by tert-butylhydroperoxide in PC12 cells via peroxynitrite-dependent and independent mechanisms, respectively

Cytotoxic and Antitumor Activity of Sulforaphane: The Role of Reactive Oxygen Species

According to recent estimates, cancer continues to remain the second leading cause of death and is becoming the leading one in old age. Failure and high systemic toxicity of conventional cancer therapies have accelerated the identification and development of innovative preventive as well as therapeutic strategies to contrast cancer-associated morbidity and mortality. In recent years, increasing body of in vitro and in vivo studies has underscored the cancer preventive and therapeutic efficacy of the isothiocyanate sulforaphane. In this review article, we highlight that sulforaphane cytotoxicity derives from complex, concurring, and multiple mechanisms, among which the generation of reactive oxygen species has been identified as playing a central role in promoting apoptosis and autophagy of target cells. We also discuss the site and the mechanism of reactive oxygen species' formation by sulforaphane, the toxicological relevance of sulforaphane-formed reactive oxygen species, and the death pathways triggered by sulforaphane-derived reactive oxygen species.

Sulforaphane induces DNA single strand breaks in cultured human cells

Sulforaphane (SFR), an isothiocyanate from cruciferous vegetables, possesses growth-inhibiting and apoptosis-inducing activities in cancer cell lines. Recently, SFR has been shown to promote the mitochondrial formation of reactive oxygen species (ROS) in human cancer cell lines. The present study was undertaken to see whether SFR-derived ROS might cause DNA damage in cultured human cells, namely T limphoblastoid Jurkat and human umbilical vein endothelial cells (HUVEC). 1-3 h treatments with 10-30 microM SFR elicited intracellular ROS formation (as assayed with dihydrorhodamine, DHR, oxidation) as well as DNA breakage (as assessed with fast halo assay, FHA). These effects lacked cell-type specificity, since could be observed in both Jurkat and HUVEC. Differential-pH FHA analysis of damaged DNA showed that SFR causes frank DNA single strand breaks (SSBs); no DNA double strand breaks (DSBs) were found within the considered treatment times (up to 3 h). SFR-derived ROS were formed at the mitochondrial respiratory chain (MRC) level: indeed rotenone or myxothiazol (MRC Complex I and III inhibitors, respectively) abrogated ROS formation. Furthermore ROS were not formed in Jurkat cells pharmacologically depleted of respiring mitochondria (MRC-/Jurkat). Formation of ROS was causally linked to the induction of SSBs: indeed all the experimental conditions capable of preventing ROS formation also prevented the damage of nuclear DNA from SFR-intoxicated cells. As to the toxicological relevance of SSBs, we found that their prevention slightly but significantly attenuated SFR cytotoxicity, suggesting that high-dose SFR toxicity is the result of a complex series of events among which GSH depletion seems to play a pivotal role. In conclusion, the present study identifies a novel mechanism contributing to SFR toxicity which - since DNA damage is a prominent mechanism underlying the cytotoxic activity of established antineoplastic agents - might help to exploit the therapeutic value of SFR in anticancer drug protocols.

Peroxynitrite Modulates Release of Inflammatory Mediators from Guinea Pig Lung Mast Cells Activated by Antigen-Antibody Reaction

BACKGROUND

Peroxynitrite (ONOO-), the product of the reaction between the superoxide anion (*O2-) and nitric oxide (NO), is produced during inflammatory disease and may be a major cytotoxic agent. No reports are available as to whether ONOO- generates or modulates inflammatory mediator release from activated guinea pig lung mast cells. In this study, we explored the modulatory role of intracellular ONOO- on inflammatory mediator release (histamine and leukotrienes) from activated mast cells.

METHODS

Guinea pig lung mast cells were purified by the enzyme digestion, and by using the rough and discontinuous Percoll density gradients. Mast cells were sensitized with IgG1 (anti-ovalbumin) antibody and challenged with ovalbumin (OVA). The intracellular ROS formation was determined by following the oxidative production of 2', 7'-dichlorofluorescein diacetate (DCFH-DA), dihydrorhodamine 123 (DHR), and anti-nitrotyrosine antibody immunofluorescence. Histamine was assayed using a fluorometric analyzer, leukotrienes by radioimmunoassay, intracellular Ca2+ levels by confocal scanning microscopy, and PLA(2) activity using prelabeling of [3H]arachidonic acid.

RESULTS

ROS detected by DCFH-DA weakly increased in mast cells activated with OVA (1.0 g/ml), and the ROS so generated was inhibited by ebselen (50 microM). However, the ROS detected by DHR increased 3-fold under the same conditions. Peroxynitrite scavengers sL-MT, DMTU, and inhibitor FeTPPS inhibited ROS formation but the NADPH oxidase inhibitor diphenyleneiodonium (DPI) only partially inhibited this formation. Dimethyl thiourea (DMTU) and seleno-L-methionine (sL-MT) inhibited the tyrosine nitration of cytosolic proteins, the release of histamine and leukotrienes, Ca2+ influx, and the PLA(2) activity evoked by mast cell activation.

CONCLUSION

The data obtained suggests that the ROS generated by the antigen/antibody reaction activated mast cells is ONOO-, and that this modulates the release of inflammatory mediators via Ca2+ -dependent PLA(2) activity.

Different effects of tert-butylhydroperoxide-induced peroxynitrite-dependent and -independent DNA single-strand breakage on PC12 cell poly(ADP-ribose) polymerase activity

The short-chain lipid hydroperoxide analogue tert-butylhydroperoxide induces peroxynitrite-dependent and -independent DNA single strand breakage in PC12 cells. U937 cells that do not express constitutive nitric oxide synthase respond to tert-butylhydroperoxide treatment with peroxynitrite-independent DNA cleavage. Under experimental conditions leading to equivalent strand break frequencies, the analysis of poly(ADP-ribose) polymerase activity showed an increase in PC12 cells but not in U937 cells. The enhanced poly(ADP-ribose) polymerase activity observed in PC12 cells was paralleled by a significant decline in NAD+ content and both events were prevented by treatments suppressing formation of peroxynitrite. Although DNA breaks were rejoined at similar rates in the two cell lines, an inhibitor of poly(ADP-ribose) polymerase delayed DNA repair in PC12 cells but had hardly any effect in U937 cells. The results obtained using the latter cell type were confirmed with an additional cell line (Chinese hamster ovary cells) that does not express nitric oxide synthase. Collectively, our data suggest that tert-butylhydroperoxide-induced peroxynitrite-independent DNA strand scission is far less effective than the DNA cleavage generated by endogenous peroxynitrite in stimulating the activity of poly(ADP-ribose) polymerase.

tert-Butylhydroperoxide induces peroxynitrite-dependent mitochondrial permeability transition leading PC12 cells to necrosis

A short-term exposure of PC12 cells to tert-butylhydroperoxide, followed by recovery in fresh culture medium, causes cell death and the extent of this response progressively increases during the 120 min of post-treatment incubation. Morphological and biochemical analyses of these cells revealed that the mode of cell death was necrosis. Cell killing induced by the hydroperoxide seems to be in part mediated by peroxynitrite because the lethal response was markedly and similarly reduced by the nitric oxide synthase inhibitor N omega-nitro-L-arginine methylester and by scavengers of nitric oxide or peroxynitrite. This peroxynitrite-dependent mechanism of cytotoxicity was blunted by antioxidants and inhibitors of mitochondrial permeability transition and the onset of cell death was preceded by mitochondrial depolarization and loss of cellular ATP. We conclude that tert-butylhydroperoxide promotes peroxynitrite-dependent PC12 cell necrosis causally linked to peroxidation of membrane lipids and mitochondrial permeability transition.

Altered mitochondrial function and overgeneration of reactive oxygen species precede the induction of apoptosis by 1-O-octadecyl-2-methyl-rac-glycero-3-phosphocholine in p53-defective hepatocytes

The mechanism of induction of apoptosis by the novel anti-cancer drug 1-O-octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) was investigated in p53-defective SV40 immortalized rat hepatocytes (CWSV1). Exposure to 12 microM ET-18-OCH3 for 36 h induced apoptosis as determined using classical morphological features and agarose gel electrophoresis of genomic DNA. Increased levels of reactive oxygen species (ROS) were detected spectrophotometrically using a nitroblue tetrazolium (NBT) assay in cells treated with ET-18-OCH3. Both the increased generation of ROS and the induction of apoptosis were inhibited when cells were treated concurrently with ET-18-OCH3 in the presence of the antioxidant alpha-tocopherol. Similar results were achieved when cells were switched acutely to choline-deficient (CD) medium in the presence of the antioxidant. The possible role of mitochondria in the generation of ROS was investigated. Both ET-18-OCH3 and CD decreased the phosphatidylcholine (PC) content of mitochondrial and associated membranes, which correlated with depolarization of the mitochondrial membrane as analyzed using 5,5',6,6'-tetramethylbenzimidazolcarbocyanine iodide (JC-1), a sensitive probe of mitochondrial membrane potential. Rotenone, an inhibitor of the mitochondrial electron transport chain, significantly reduced the intracellular level of ROS and prevented mitochondrial membrane depolarization, correlating with a reduction of apoptosis in response to either ET-18-OCH3 or CD. Taken together, these results suggest that the form of p53-independent apoptosis induced by ET-18-OCH3 is mediated by alterations in mitochondrial membrane PC, a loss of mitochondrial membrane potential, and the release of ROS, resulting in completion of apoptosis.

Eye instability induced by vestibular stimulation in rabbits

The slow compensatory phases of the vestibulo-ocular reflex (VOR) in the rabbit tend to drift and the drift reverses the direction. This periodic alternating drift (PAD) has two peculiar characteristics: (1) it is induced by sinusoidal vestibular stimulation in naive animals, being evoked immediately after stimulus onset and persisting after the end of stimulation; (2) the peak velocity and period of the drift are dependent on stimulus amplitude. PAD of the rabbit has strong similarities with PAN, a periodic alternating nystagmus observed in humans with cerbellar disorders and in monkeys after nodulo-uvulectomy, although its peak velocity is smaller. It is hypothesized that PAD is due to a slight instability, caused by vestibular stimulation in darkness, of the cerebellar adaptive loop, which exerts a variable gain control on the time constant of the velocity storage integrator.

Products of the phospholipase A(2) pathway mediate the dihydrorhodamine fluorescence response evoked by endogenous and exogenous peroxynitrite in PC12 cells

A short-term exposure of PC12 cells to tert-butylhydroperoxide promotes a rapid oxidation of dihydrorhodamine sensitive to nitric oxide synthase inhibitors and peroxynitrite scavengers. This response was not directly caused by peroxynitrite, but rather appeared to be mediated by peroxynitrite-dependent activation of phospholipase A(2). The following lines of evidence support this inference: (i) the peroxynitrite-dependent dihydrorhodamine fluorescence response was blunted by low concentrations of two structurally unrelated phospholipase A(2) inhibitors; (ii) under similar conditions, the phospholipase A(2) inhibitors prevented release of arachidonic acid; (iii) low levels of arachidonic acid restored the dihydrorhodamine fluorescence response in nitric oxide synthase- as well as phospholipase A(2)-inhibited cells; (iv) the dihydrorhodamine fluorescence response induced by authentic peroxynitrite was also blunted by phospholipase A(2) inhibitors and restored upon addition of reagent arachidonic acid. We conclude that endogenous, or exogenous, peroxynitrite does not directly oxidize dihydrorhodamine in intact cells. Rather, peroxynitrite appears to act as a signalling molecule promoting release of arachidonic acid, which in turn leads to formation of species causing the dihydrorhodamine fluorescence response.

Peroxynitrite-mediated release of arachidonic acid from PC12 cells

A short term exposure of PC12 cells to a concentration of tert-butylhydroperoxide (tB-OOH) causing peroxynitrite-dependent DNA damage and cytotoxiticity promoted a release of arachidonic acid (AA) that was sensitive to phospholipase A(2) (PLA(2)) inhibitors and insensitive to phospholipase C or diacylglycerol lipase inhibitors. The extent of AA release was also mitigated by nitric oxide synthase (NOS) inhibitors and peroxynitrite scavengers. Low levels (10 microM) of authentic peroxynitrite restored the release of AA mediated by tB-OOH in NOS-inhibited cells whereas concentrations of peroxynitrite of 20 microM, or higher, effectively stimulated a PLA(2) inhibitor-sensitive release of AA also in the absence of additional treatments. These results are consistent with the possibility that endogenous as well as exogenous peroxynitrite promotes activation of PLA(2).