Endocytosis-independent uptake of liposome-encapsulated superoxide dismutase prevents the killing of cultured hepatocytes by tert-butyl hydroperoxide

Oxidant stress, mitochondria, and cell death mechanisms in drug-induced liver injury: lessons learned from acetaminophen hepatotoxicity

Hepatotoxicity is a serious problem during drug development and for the use of many established drugs. For example, acetaminophen overdose is currently the most frequent cause of acute liver failure in the United States and Great Britain. Evaluation of the mechanisms of drug-induced liver injury indicates that mitochondria are critical targets for drug toxicity, either directly or indirectly through the formation of reactive metabolites. The consequence of these modifications is generally a mitochondrial oxidant stress and peroxynitrite formation, which leads to structural alterations of proteins and mitochondrial DNA and, eventually, to the opening of mitochondrial membrane permeability transition (MPT) pores. MPT pore formation results in a collapse of mitochondrial membrane potential and cessation of adenosine triphosphate synthesis. In addition, the release of intermembrane proteins, such as apoptosis-inducing factor and endonuclease G, and their translocation to the nucleus, leads to nuclear DNA fragmentation. Together, these events trigger necrotic cell death. Alternatively, the release of cytochrome c and other proapoptotic factors from mitochondria can promote caspase activation and apoptotic cell death. Drug toxicity can also induce an inflammatory response with the formation of reactive oxygen species by Kupffer cells and neutrophils. If not properly detoxified, these extracellularly generated oxidants can diffuse into hepatocytes and trigger mitochondrial dysfunction and oxidant stress, which then induces MPT and necrotic cell death. This review addresses the formation of oxidants and the defense mechanisms available for cells and applies this knowledge to better understand mechanisms of drug hepatotoxicity, especially acetaminophen-induced liver injury.

An increase in superoxide dismutase ameliorates oxygen-induced retinopathy in transgenic mice

PURPOSE

Oxygen therapy is a well-recognized risk factor for retinopathy of prematurity. We examined whether an increase in the naturally occurring enzyme copper-zinc superoxide dismutase (CuZnSOD), which controls oxygen, can reduce the severity of oxygen-induced retinopathy in a mouse model.

METHODS

Seven transgenic mice overexpressing CuZnSOD and six wild-type mice were exposed to 75% oxygen from postnatal day 7 to 12. Seven transgenic mice and five mice of the wild type were kept in room air and served as controls. Fluorescein-conjugated dextran angiography of retinal vasculature was performed and flat-mounted preparations were evaluated by scoring blood vessel growth, blood vessel tuft formation, extraretinal neovascularization, degree of central constriction, and tortuosity of vessels. In addition, quantification of the number of blood vessel tufts was performed in a masked fashion with haematoxylin and eosin staining of paraffin-embedded eye sections.

RESULTS

The mean retinal score+/-SD obtained by the wild-type mice was 9.4+/-2.0, whereas the transgenic mice overexpressing CuZnSOD obtained a value of 2.4+/-1.6 (P=0). The two control groups (wild type and transgenic) that were kept in room air, each obtained a score of 0. Significantly fewer extraretinal vascular tufts were seen in the transgenic mice (0.26+/-0.34) than in the wild-type mice (4.27+/-1.6) after both groups were exposed to oxygen (P<0.001).

CONCLUSIONS

The results suggest that high SOD activity protects neonatal mice against oxygen-induced retinopathy, and support the assumption that oxygen radicals are a major causative factor in oxygen-induced retinopathy.

Superoxide dismutase entrapped-liposomes restore the impaired endothelium-dependent relaxation of resistance arteries in experimental diabetes

Diabetes is associated with impaired endothelium-dependent relaxation. We questioned whether administration of superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) entrapped in long-circulating liposomes improves the vascular reactivity of the resistance arteries. Using the myograph technique, the vasodilation in response to acetylcholine was measured in mesenteric resistance arteries isolated from diabetic or normal hamsters treated for 3 days with superoxide dismutase entrapped in liposomes, with the same concentrations of free superoxide dismutase and plain liposomes, or untreated. Superoxide dismutase activity and nitric oxide (NO) levels were assayed by spectrophotometry, superoxide dismutase levels by Western blot and the role of N(pi)-nitro-L-arginine ethylester (L-NAME) on vasodilation by the myograph technique. Our data revealed that: (i) superoxide dismutase entrapped in liposomes restored to a great extent the endothelium-dependent relaxation of diabetic hamster resistance arteries; (ii) in superoxide dismutase entrapped in liposomes-treated diabetic animals, the activity and the level of superoxide dismutase in arterial homogenates as well as the serum nitrite levels were significantly higher than those in untreated hamsters or hamsters treated with free superoxide dismutase and plain liposomes: (iii) L-NAME inhibited the response of arteries to acetylcholine in superoxide dismutase entrapped in liposomes-treated diabetic hamsters. These results suggest that superoxide dismutase entrapped in liposomes is effective in scavenging superoxide anions, increases nitric oxide bioactivity and improves the vasorelaxation of resistance arteries in diabetic hamsters.

Polyethylene glycol-superoxide dismutase, a conjugate in search of exploitation

Without a doubt PEG-SOD has been the enzyme most studied in PEGylation. One can say that it represents the preferred model to assess chemistries for PEG activation, analytical procedures suitable for conjugate characterization, the influence of PEG size in conjugate removal from circulation and elimination of immunogenicity and antigenicity, and the effect of route of administration. The effect of PEG conjugation was studied in vitro and in vivo models in comparison with the free enzyme and the following conclusions may be drawn: (1) At the blood vessel level, PEG-SOD has been shown to provide a greater resistance to oxidant stress, to improve endothelium relaxation and inhibit lipid oxidation. (2) In the heart, PEG-SOD proved to be at least as effective as native SOD in treatment of reperfusion-induced arrhythmias and myocardial ischemia. (3) In the lung, PEG-SOD appeared to be able to reduce oxygen toxicity and E. coli-induced lung injury, but not in the treatment of lung physiopathology associated with endotoxin-induced acute respiratory failure and in the reduction of asbestos-induced cell damage. (4) On cerebral ischemia/reperfusion injuries the effect of PEG-SOD was uncertain, also due to the difficulty of cerebral cell penetration. (5) In kidney and liver ischemia both enzyme forms were found to ameliorate reperfusion damage. In view of so much positive research on PEG-SOD, it is surprising that no approved application in human therapy has been established and approved.

Oxygen toxicity in premature infants

Oxygen causes tissue injury through the formation of reactive oxygen intermediates and peroxidation of membrane lipids. Premature infants, who have severely reduced antioxidant defenses, are particularly sensitive to the toxic effects of oxygen. Supplemental oxygen in premature infants contributes to the development of chronic lung disease (bronchopulmonary dysplasia), characterized by dysregulated inflammation and altered expression of proteases and growth factors. This can result in fibrosis, asymmetric aeration, and respiratory insufficiency. Oxygen also induces aberrant physiologic responses that can be damaging in premature infants. For example, vasoconstriction in the retina is an early response to oxygen that can lead to vasoobliteration, neovascularization, and retinal traction (retinopathy of prematurity). Increasing knowledge of the mechanisms underlying oxygen toxicity in premature infants has suggested strategies to minimize tissue injury and to optimize long-term medical outcomes. These include limiting oxygen supplementation and light exposure, the use of antiinflammatory agents or antioxidants, and the use of room air in neonatal resuscitation.

The role of hepatocytes in the clearance of liposomes from the blood circulation

In this chapter we summarize literature and describe in more detail our own observations over a period of nearly two decennia on the role of hepatocytes in the hepatic clearance of intravenously administered liposomes. Evidence is presented indicating that, although size is an important parameter, it is not decisive in determining access of liposomes to the hepatocytes. Also lipid composition is an important parameter, including charge, rigidity and headgroup composition. The role of the fenestrated sinusoidal endothelial cells in determining liposome accessibility of hepatocytes is discussed as well as the involvement of opsonizing plasma proteins such as apolipoprotein E. Our observations led us to postulate the existence of at least four different mechanisms of interaction of liposomes with hepatocytes, i.e. an endocytic and a non-endocytic one for both neutral and negatively charged vesicles

Targeting of superoxide dismutase and catalase to vascular endothelium

Reactive oxygen species, such as superoxide anion (O2(-)) and H2O2, cause oxidative stress in endothelial cells, a condition implicated in the pathogenesis of many cardiovascular and pulmonary diseases. Antioxidant enzymes, superoxide dismutases (SOD, converting superoxide anion into H2O2) and catalase (converting H2O2 into water), are candidate drugs for augmentation of antioxidant defenses in endothelium. However, SOD and catalase undergo fast elimination from the bloodstream, which compromises delivery and permits rather modest, if any, protection against vascular oxidative stress. Coupling of polyethylene glycol (PEG) to the enzymes and encapsulating them in liposomes increases their bioavailability and enhances their protective effect. Chemical modifications and genetic manipulations of SOD and catalase have been proposed in order to provide more effective delivery to endothelium. For example, chimeric protein constructs consisting of SOD and heparin-binding peptides have an affinity for charged components of the endothelial glycocalix. However, the problem of developing a more effective and precise delivery of the drugs to endothelial cells persists. Endothelial surface antigens may be employed to provide targeting and subcellular addressing of drugs (vascular immunotargeting strategy). Thus, SOD and catalase conjugated to antibodies directed against the constitutively expressed endothelial antigens, angiotensin-converting enzyme (ACE) and adhesion molecules (ICAM-1 or PECAM-1), bind to endothelium in intact animals after intravascular administration, accumulate in the pulmonary vasculature, enter endothelial cells and augment their antioxidant defenses. Such immunotargeting strategies may provide secondary therapeutic benefits by inhibiting the function of target antigens. For example, blocking of ICAM-1 and PECAM-1 by carrier antibodies may attenuate inflammation and leukocyte-mediated vascular damage. Additional studies in animal models of vascular oxidative stress are necessary in order to more fully characterize potential therapeutic effects and limitations of targeting of antioxidant enzymes to endothelial cells.

Delivery of antioxidant enzyme proteins to the lung

Protection of alveolar epithelial cells (alveolocytes) and vascular endothelial cells against pulmonary oxidative stress is an important problem. An inadequate delivery to the target cells limits the protective utility of the antioxidant enzymes, superoxide dismutase (SOD) and catalase. SOD and catalase modifications, such as coupling with polyethylene glycol and encapsulation in liposomes, prolong the life span of the active enzymes in vivo. The airway administration of SOD and catalase protects alveolocytes against hyperoxic oxidative stress. Although pulmonary endothelium is poorly accessible from the airways, it is accessible from circulation. However, antioxidant enzymes and their derivatives display poor targeting to pulmonary endothelium. To improve the targeting and provide intracellular delivery to endothelium, the enzymes can be conjugated with antibodies against endothelial antigens, such as angiotensin-converting enzyme and adhesion molecules [intercellular adhesion molecule-1 (ICAM-1) or platelet-endothelial cell adhesion molecule-1 (PECAM-1)]. These immunoconjugates accumulate in the pulmonary vasculature in intact animals, enter endothelium, and augment the antioxidant defenses. The immunoconjugates directed against ICAM-1 and PECAM-1 may also provide a secondary therapeutic benefit by blocking of sequestration and infiltration of leukocytes in the lungs. Further investigations are necessary to evaluate the therapeutic effectiveness of the vascular immunotargeting of antioxidant enzymes and solve technical problems associated with production of safe, clinically useful conjugates.

Therapeutic effect of liposomal superoxide dismutase in an animal model of retinopathy of prematurity

A newborn rat model of retinopathy of prematurity was used to test the hypothesis that a lack of superoxide dismutase contributes to the retinal vaso-attenuation seen during exposure of the animals to hyperoxic conditions. To determine the endogenous superoxide dismutase activity of the retina under hyperoxic conditions, litters of albino rats were placed in either constant 80% ambient oxygen (constant hyperoxia), or placed in 21% oxygen (room air) immediately after birth. Every other day, for 14 days, several rat pups were sacrificed and their retinas removed for the determination of total superoxide dismutase (SOD) activity and manganese-associated SOD activity. An attempt was made to increase retinal SOD activity by intraperitoneal administration of exogenous SOD encapsulated in polyethylene glycol-modified liposomes. Additional litters were exposed to the same oxygen treatments and supplemented twice daily with either liposome-encapsulated superoxide dismutase in saline or liposomes containing saline without SOD. Animals were sacrificed at various time points for the determination of total superoxide dismutase activity and computer-assisted analysis of vessel density and avascular area. Animals raised in an atmosphere of constant 80% oxygen had significantly reduced levels of retinal superoxide dismutase activity through 6 days of life when compared to their room air-raised littermates. At 6 days of age, daily supplementation with liposome-encapsulated SOD had significantly increased retinal superoxide dismutase activity and reduced oxygen-induced vaso-attenuation as evidenced by increased vessel density and decreased avascular area, when compared to littermates exposed to constant hyperoxia that received control liposomes. Superoxide dismutase had no adverse effects on any of the animals regardless of treatment. Tracing experiments demonstrated that liposomes entered the retina and were found in cells morphologically resembling microglia. Delivery of SOD to the retina via long-circulating liposomes proved beneficial, suggesting that restoration and/or supplementation of endogenous antioxidants in oxygen-damaged retinal tissue is a potentially valuable therapeutic strategy.

Therapeutic effect of liposomal superoxide dismutase in an animal model of retinopathy of prematurity

A newborn rat model of retinopathy of prematurity was used to test the hypothesis that a lack of superoxide dismutase contributes to the retinal vaso-attenuation seen during exposure of the animals to hyperoxic conditions. To determine the endogenous superoxide dismutase activity of the retina under hyperoxic conditions, litters of albino rats were placed in either constant 80% ambient oxygen (constant hyperoxia), or placed in 21% oxygen (room air) immediately after birth. Every other day, for 14 days, several rat pups were sacrificed and their retinas removed for the determination of total superoxide dismutase (SOD) activity and manganese-associated SOD activity. An attempt was made to increase retinal SOD activity by intraperitoneal administration of exogenous SOD encapsulated in polyethylene glycol-modified liposomes. Additional litters were exposed to the same oxygen treatments and supplemented twice daily with either liposome-encapsulated superoxide dismutase in saline or liposomes containing saline without SOD. Animals were sacrificed at various time points for the determination of total superoxide dismutase activity and computer-assisted analysis of vessel density and avascular area. Animals raised in an atmosphere of constant 80% oxygen had significantly reduced levels of retinal superoxide dismutase activity through 6 days of life when compared to their room air-raised littermates. At 6 days of age, daily supplementation with liposome-encapsulated SOD had significantly increased retinal superoxide dismutase activity and reduced oxygen-induced vaso-attenuation as evidenced by increased vessel density and decreased avascular area, when compared to littermates exposed to constant hyperoxia that received control liposomes. Superoxide dismutase had no adverse effects on any of the animals regardless of treatment. Tracing experiments demonstrated that liposomes entered the retina and were found in cells morphologically resembling microglia. Delivery of SOD to the retina via long-circulating liposomes proved beneficial, suggesting that restoration and/or supplementation of endogenous antioxidants in oxygen-damaged retinal tissue is a potentially valuable therapeutic strategy.

Antioxidant reactions of vitamin E in the perfused rat liver: product distribution and effect of dietary vitamin E supplementation

We have investigated the relationship between vitamin E (alpha-tocopherol, TH) oxidation and antioxidant protection in a perfused rat liver model. Perfusion of a male Sprague-Dawley rat liver with 2 mM tert-butylhydroperoxide (t-BuOOH) for 10 min resulted in lipid peroxidation and metabolic changes reflecting oxidative stress. Mitochondria isolated from the liver exhibited increases in state 3 and state 4 respiration and a decline in the respiratory control ratio. In livers from rats given supplementary vitamin E in the diet, TH content was 7- to 10-fold higher than in controls and lipid peroxidation and metabolic changes induced by t-BuOOH were decreased. In mitochondria from these vitamin E-supplemented livers, the t-BuOOH-induced increase in state 4 respiration was reduced and the respiratory control ratio was maintained. In livers from unsupplemented rats, t-BuOOH induced oxidation of TH to alpha-tocopherolquinone, alpha-tocopherolhydroquinone, 2,3-epoxy-alpha-tocopherolquinone, and 5,6-epoxy-alpha-tocopherolquinone, as determined by gas chromatography-mass spectrometry analysis. Yields of these products were approximately doubled by treatment of samples with dilute acid, which indicated the presence of tocopherone and epoxytocopherone precursors. Oxidation of TH in vitamin E-supplemented livers yielded the same products and the relative extent of TH oxidation appeared similar to that in unsupplemented livers. In livers from both unsupplemented and vitamin E-supplemented animals, the distribution of oxidation products was similar in whole liver and isolated mitochondria. These data provide the first simultaneous documentation of TH antioxidant reactions and antioxidant effects in an intact organ system during oxidative stress.

Lipid peroxidation in photodynamically stressed mammalian cells: use of cholesterol hydroperoxides as mechanistic reporters

Photodynamic action of merocyanine 540, an antileukemic sensitizing dye, on murine L1210 cells results in the formation of lipid hydroperoxides and loss of cell viability. High-performance liquid chromatography with mercury cathode electrochemical detection was used for determining lipid oxidation products, including the following cholesterol-derived hydroperoxides: 5 alpha-OOH, 6 alpha-OOH, 6 beta-OOH, and unresolved 7 alpha, 7 beta-OOH. Among these species, 5 alpha-, 6 alpha-, and 6 beta-OOH (singlet oxygen adducts) were predominant in the early stages of photooxidation, whereas 7 alpha- and 7 beta-OOH (products of free radical reactions) became so after prolonged irradiation or during dark incubation after exposure to a light dose. These mechanistic changes were studied in a unique way by monitoring shifts in the peroxide ratio, i.e., 7-OOH/5 alpha-OOH, or 7-OOH/6-OOH. When cells (10(7)/ml) were exposed to a visible light fluence of 0.6 J/cm2 in the presence of 10 microM merocyanine 540, 7-OOH/5 alpha-OOH increased by approximately 100% after 2 h of dark incubation at 37 degrees C. The increase was much larger (approximately 250%) when cells were photooxidized after treatment with 1 microM ferric-8-hydroxyquinoline, a lipophilic iron donor, whereas no increase was observed when cells were pretreated with 100 microM desferrioxamine, an avid iron chelator/redox inhibitor. Correspondingly, postirradiation formation of thiobarbituric acid-reactive material was markedly enhanced by ferric-8-hydroxyquinoline and suppressed by desferrioxamine, as was the extent of cell killing. When added to cells after a light dose, chain-breaking antioxidants such as butylated hydroxytoluene and alpha-tocopherol strongly protected against cell killing and slowed the increase in 7-OOH/5 alpha-OOH ratio. It is apparent from these results that (1) the 7-OOH/5 alpha-OOH or 7-OOH/6-OOH ratio can be used as a highly sensitive index of singlet oxygen vs. free radical dominance in photodynamically stressed cells; and (2) that postirradiation chain peroxidation plays an important role in photodynamically initiated cell killing.

Superoxide-mediated clastogenesis and anticlastogenic effects of exogenous superoxide dismutase

Superoxide-mediated clastogenesis is characteristic for various chronic inflammatory diseases with autoimmune reactions and probably plays a role in radiation-induced clastogenesis and in the congenital breakage syndromes. It is consistently prevented by exogenous superoxide dismutase (SOD), but not by heat-inactivated SOD, indicating that the anticlastogenic effect is related to the catalytic function of the enzyme. Increased superoxide production by activated monocytes/macrophages is followed by release of more long-lived metabolites, so-called clastogenic factors, which contain lipid peroxidation products, unusual nucleotides of inosine, and cytokines such as tumor necrosis factor alpha. Since these components are not only clastogenic, but can stimulate further superoxide production by monocytes and neutrophils, the genotoxic effects are self-sustaining. It is shown here that anticlastogenic effects of exogenous SOD are preserved despite extensive washing of the cells and removal of all extracellular SOD. Using flow cytometry and confocal laser microscopy, rapid adherence of the fluorescently labeled enzyme to the cell surface could be observed with slow uptake into the cell during the following hours. The degree of labeling was concentration and time dependent. It was most important for monocytes, compared with lymphocytes, neutrophils, and fibroblasts. The cytochrome c assay showed significantly diminished O2- production by monocytes, pretreated with SOD and washed thereafter. The preferential and rapid binding of SOD to monocytes may be of importance not only for the superoxide-mediated genotoxic effects, described above, but also from a therapeutic standpoint. It can explain the observation that beneficial effects of injected SOD lasted for weeks and months despite rapid clearance of the enzyme from the blood stream according to pharmacodynamic studies.

Age-related changes in the binding and uptake of Cu, Zn superoxide dismutase in rat liver cells

The present paper reports the effect of aging on receptor mediated endocytosis of Cu, Zn superoxide dismutase in rat liver cells. The fate of bovine Cu, Zn superoxide dismutase conjugated to colloidal gold was followed by electron microscopy in young (2 months) and old (24 months) rats in situ, in vivo and in vitro experiments. The use of different models for the study of the binding and internalization of the enzyme allowed to discriminate the contribution of each different liver cell type. The data obtained demonstrate that aging of the liver affects binding and uptake of this enzyme. In particular both the number of binding sites and the rate of internalization were depressed in old rats. Therefore, the hypothesis of therapeutic application of superoxide dismutase for age-related diseases needs to be revalued in view of the fact that receptor-mediated endocytosis of this protein is a mechanism affected by senescence.

Protection of cultured rat gastric cells against oxidant stress by iron chelation. Role of lipid peroxidation

Reactive oxygen metabolites may be involved in the pathogenesis of ethanol-, nonsteroidal antiinflammatory drug-, Helicobacter pylori-, or ischemia/reoxygenation-induced gastric mucosal injury in vivo. Iron plays a critical role in mediating oxidant injury in vitro. The present study examined a possible role of lipid peroxidation in inducing oxidant damage by determining the effect of iron chelation on cytotoxicity and on lipid peroxidation in cultured rat gastric cells. Cytotoxicity was quantified by 51Cr release from prelabeled cells that were exposed to tert-butyl hydroperoxide (tBHP) so as to overwhelm the glutathione redox cycle. Lipid peroxidation was assessed by measuring malondialdehyde (MDA) production. tBHP caused a time-related and dose-dependent increase of 51Cr release. The presence of phenanthroline (a chelator of Fe2+) during tBHP exposure and pretreatment with deferoxamine (a chelator of Fe3+) reduced tBHP-induced 51Cr release dose dependently. The generation of MDA increased as the concentrations of tBHP increased, but in a time course study, such generation preceded cytolysis. Both iron chelators attenuated MDA production in a dose-dependent fashion. Oxidant stress causes lipid peroxidation in cultured gastric cells, which is then followed by cytolysis. Iron plays a critical role in inducing lipid peroxidation as well as in mediating cytolysis. Iron chelation protects these cells from oxidant stress presumably through inhibition of lipid peroxidation.

Inhibition of glycogen synthesis by epidermal growth factor in hepatocytes. The role of cell density and pertussis toxin-sensitive GTP-binding proteins

Epidermal growth factor (EGF) counteracts the stimulation of glycogen synthesis by insulin in hepatocytes, but it is not known whether this is due to inhibition of glycogen synthesis or to inhibition of the insulin-signalling mechanism. This study investigates the mechanisms by which EGF affects the basal rate and the insulin stimulation of glycogen synthesis. The basal rate of glycogen synthesis is higher at low than at high cell density. EGF inhibits the basal rate of glycogen synthesis at low cell density but not in confluent cultures and abolishes the difference due to density. However, EGF inhibits the stimulation of glycogen synthesis by insulin irrespective of cell density. Increasing glycogen synthesis by increasing the [glucose] does not abolish the difference in rates of glycogen synthesis due to cell density, neither does it induce responsiveness to EGF at high cell density, establishing that responsiveness to EGF is a function of cell density and not of the basal rate and that inhibition of the insulin stimulation also cannot be accounted for by the higher rate of glycogen synthesis. Cytochalasin D and phalloidin, which alter cell morphology through interactions with the microfilament cytoskeleton, mimic the cell-density-dependent inhibition of glycogen synthesis by EGF. The inhibition of glycogen synthesis by EGF and cytochalasin D is additive and cytochalasin D potentiates the inhibition of glycogen synthesis by EGF, suggesting involvement of a cytoskeletal mechanism. Exogenous phospholipase C inhibits glycogen synthesis at both low and high cell density and the inhibition at low cell density is not additive with that caused by either EGF or cytochalasin D, suggesting that these agonists inhibit glycogen synthesis through changes in Ca2+ and/or diacylglycerol. The inhibition of glycogen synthesis by EGF in the absence of insulin stimulation is blocked by neomycin, which inhibits Ca2+ release from intracellular stores but not by antagonists of protein kinase C. It was also inhibited by pertussis toxin (50%), suggesting that it may involve GTP-binding-protein-mediated release of Ca2+ from intracellular stores. The inhibition of the stimulation of glycogen synthesis by insulin was not affected by neomycin and was only marginally inhibited by pertussis toxin or guanosine 5'-O-[3-thio]triphosphate (GTP[S]). We infer from these findings that the inhibition by EGF of the basal rate of glycogen synthesis and of the insulin stimulation are mediated by different mechanisms. The latter is pertussis toxin insensitive and independent of cell density, whereas the former is expressed only at low cell density, it is potentiated by cytochalasin D and inhibited by pertussis toxin.

tert.-butyl hydroperoxide-mediated DNA base damage in cultured mammalian cells

tert.-Butyl hydroperoxide has been utilized to study the effect of oxidative stress on living cells; however, its effect on DNA bases in cells has not been characterized. In the present work, we have investigated DNA base damage in mammalian cells exposed to this organic hydroperoxide. SP2/0 derived murine hybridoma cells were treated with 4 concentrations of tert.-butyl hydroperoxide for varying periods of time. Chromatin was isolated from treated and control cells and subsequently analyzed by gas chromatography-mass spectrometry with selected-ion monitoring for DNA base damage. Quantification of damaged DNA bases was achieved by isotope-dilution mass spectrometry. The amounts of 8 products were significantly higher than control levels in cells treated with tert.-butyl hydroperoxide at a concentration range of 0.01-0.1 mM. At concentrations from 1.0 to 10 mM, product formation was inhibited and the amounts of products were similar to those in control cells. The bimodal nature of the dose-response may be qualitatively analogous to previous reports of bimodal killing of E. coli bacteria by hydrogen peroxide. The nature of the identified DNA base lesions suggests the involvement of the hydroxyl radical in their formation. tert.-Butyl hydroperoxide is known to produce the tert.-butoxyl radical in reactions with metal ions. However, it is unlikely that the tert.-butoxyl radical produces these DNA lesions. It is suggested that DNA base damage arises from tert.-butyl hydroperoxide-mediated oxidative stress in cells, resulting in formation of hydroxyl radicals in close proximity to DNA. The inhibition of product formation at high concentrations of tert.-butyl hydroperoxide may be explained by the scavenging of tert.-butoxyl radical by tert.-butyl hydroperoxide resulting in inhibition of oxidative stress. The plausibility of the scavenging mechanism was evaluated with a mathematical simulation of the dose-response for DNA damage in solutions containing hydrogen peroxide. The simulation model predicted a bimodal dose-response which agreed qualitatively with the results in this study and with other in vivo and in vitro studies reported in the literature.

Cytoprotection against merocyanine 540-sensitized photoinactivation of the Na+,K(+)-adenosine triphosphatase in leukemia cells: glutathione and selenoperoxidase involvement

When irradiated with broad-band visible light in the presence of merocyanine 540 (MC540), murine leukemia L1210 cells grown under selenium-deficient conditions (Se(-) cells) accumulated lipid hydroperoxides and lost viability more rapidly than selenium-satisfied (Se(+) cells). These findings suggest that cytoprotection against photoperoxidation and photokilling is mediated at least in part by selenoperoxidase (SePX) action. Similar protection against photoinactivation of an intrinsic membrane enzyme, the Na+,K(+)-ATPase, has been observed. Thus, irradiation of MC540-sensitized Se(-) cells resulted in an immediate and progressive inactivation of ouabain-sensitive Na+,K(+)-ATPase; by contrast, activity loss in Se(+) cells was preceded by a prominent lag. Enzyme photo-inactivation in Se(-) cells was inhibited by ebselen, an SePX mimetic, confirming that SePX(s) is (are) involved in natural protection. Desferrioxamine treatment (iron sequestration/inactivation) resulted in higher hydroperoxide levels and slower Na+,K(+)-ATPase inactivation during MC540/light exposure, whereas ferric-8-hydroxyquinoline treatment (iron supplementation) had the opposite effect. Thus, iron appears to play an important role in both of these processes. In contrast, photoinactivation of another intrinsic enzyme in L1210 cells, acetylcholinesterase (AChE), was unaffected by selenium or iron manipulation. On the basis of these findings, we propose that lipid peroxidation plays an important role in the photoinactivation of Na+,K(+)-ATPase, but not AChE. This is consistent with the fact that Na+,K(+)-ATPase's active site lies within the membrane bilayer, whereas AChE's active site lies outside the bilayer.

Differing effects of the inhibition of poly(ADP-ribose) polymerase on the course of oxidative cell injury in hepatocytes and fibroblasts

The effects of the two inhibitors of poly(ADP-ribose) polymerase, 3-aminobenzamide (ABA) and benzamide (BA), on the oxidative killing of L929 mouse fibroblasts and primary cultures of rat hepatocytes were studied. The killing of L929 cells by tert-butyl hydroperoxide (TBHP) occurred by two mechanisms, one sensitive and the other insensitive to the antioxidant N,N'-diphenylphenylene diamine (DPPD). Cell killing by either mechanism was prevented by the ferric iron chelator deferoxamine. ABA and BA prevented the killing of L929 cells that occurred in the presence, but not in the absence, of DPPD. ABA and BA inhibited the activity of poly(ADP-ribose) polymerase by 85%. Protection was accompanied by the sparing of the depletion of both NAD and ATP, but there was no effect of either ABA or BA on the iron-dependent appearance of single-strand breaks in DNA. Depletion of ATP by treating the fibroblasts with 2-deoxyglucose and sodium azide did not result in any loss of viability. H2O2 similarly killed the L929 cells by a mechanism that depended on a source of ferric iron. However, DPPD had no effect on the cell killing, and ABA and BA completely protected the cells in the presence or absence of DPPD. H2O2 caused the appearance of single-strand breaks that were prevented by deferoxamine, but again not by ABA or BA. ABA and deferoxamine reduced, but did not prevent, the depletion of both NAD and ATP occurring with H2O2. With the cultured hepatocytes, ABA and BA inhibited poly(ADP-ribose) polymerase at concentrations that were without effect on either the extent of cell killing or the depletion of NAD occurring with either TBHP, H2O2, or menadione. These data indicate that the relationship between oxidative DNA damage and the genesis of lethal injury is very different in the two types of cells. In the fibroblasts, the appearance of single strand breaks in DNA was accompanied by depletion of NAD and ATP and subsequently by the death of the cells. These events were mediated by the activity of poly(ADP-ribose) polymerase, as inhibition of the enzyme prevented their development. In the hepatocytes, inhibition of poly(ADP-ribose) polymerase was without effect on the oxidative death of the cells.

Selenoperoxidase-mediated cytoprotection against the damaging effects of tert-butyl hydroperoxide on leukemia cells

Murine leukemia L1210 cells grown for 5-7 d in the presence of 1% serum without added selenium [Se(-) cells] expressed < 5% of the glutathione peroxidase (GPX) activity of selenium-supplemented controls [Se(+) cells]. Clonogenic survival assays indicated that t-butyl hydroperoxide (t-BuOOH) is much more toxic to Se(-) cells (LC50 approximately 10 microM) than to Se(+) or selenium-repleted [Se(-/+)] cells (LC50 approximately 250 microM). Hypersensitivity of Se(-) cells to t-BuOOH was partially reversed by treating them with Ebselen, a selenoperoxidase mimetic; thus, selenoperoxidase insufficiency was probably the most serious defect of Se deprivation. Cytotoxicity of t-BuOOH was inhibited by desferrioxamine and by alpha-tocopherol, indicating that redox iron and free radical intermediates are involved. Elevated sensitivity of Se(-) cells to t-BuOOH was accompanied by an increased susceptibility to free radical lipid peroxidation, which became even more pronounced in cells that had been grown in arachidonate (20:4, n-6) supplemented media. That glutathione (GSH) is required for cytoprotection was established by showing that Se(+) cells are less resistant to t-BuOOH after exposure to buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, or 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. Coupled enzymatic assays indicated that Se(+) or Se(-/+) cells metabolize t-BuOOH 20-25 times more rapidly than Se(-), consistent with the measured difference in GPX activities of these cells. Correspondingly, when challenged with t-BuOOH, Se(+) cells showed an initial loss of GSH and elevation of GSSG that exceeded that of Se(-) cells. It was further shown that like Se(-) cells, BSO- or BCNU-treated Se(+) cells metabolize t-BuOOH more slowly than nontreated controls. These results clearly indicate that selenoperoxidase action in the glutathione cycle is a vital element in cellular defense against toxic hydroperoxides.

Cyclosporin A protects hepatocytes subjected to high Ca2+ and oxidative stress

Hepatocytes incubated with 0.8 mM t-butylhydroperoxide are protected by cyclosporin A when the medium Ca2+ concentration is 10 mM, but not when it is 2.5 mM. The highest Ca2+ level is associated with an inhibition of t-butylhydroperoxide-dependent malondialdehyde accumulation and with mitochondrial Ca2+ loading within the cells. These findings are new evidence that t-butylhydroperoxide can kill cells by peroxidation-dependent and -independent mechanisms, and suggest that the mitochondrial permeability transition and the resultant de-energization are components of the peroxidation-independent mechanism. Cyclosporin A may have considerable utility for the protection of cells subjected to oxidative stress.

Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): lethal peroxidative membrane injury

Oxidative stress induced by hydrogen peroxide (H2O2) may contribute to the pathogenesis of ischemic-reperfusion injury in the heart. For the purpose of investigating directly the injury potential of H2O2 on heart muscle, a cellular model of H2O2-induced myocardial oxidative stress was developed. This model employed primary monolayer cultures of intact, beating neonatal-rat cardiomyocytes and discrete concentrations of reagent H2O2 in defined, supplement-free culture medium. Cardiomyocytes challenged with H2O2 readily metabolized it such that the culture content of H2O2 diminished over time, but was not depleted. The consequent H2O2-induced oxidative stress caused lethal sarcolemmal disruption (as measured by lactate dehydrogenase release), and cardiomyocyte integrity could be preserved by catalase. During oxidative stress, a spectrum of cellular derangements developed, including membrane phospholipid peroxidation, thiol oxidation, consumption of the major chain-breaking membrane antiperoxidant (alpha-tocopherol), and ATP loss. No net change in the protein or phospholipid contents of cardiomyocyte membranes accompanied H2O2-induced oxidative stress, but an increased turnover of these membrane constituents occurred in response to H2O2. Development of lethal cardiomyocyte injury during H2O2-induced oxidative stress did not require the presence of H2O2 itself; a brief "pulse" exposure of the cardiomyocytes to H2O2 was sufficient to incite the pathogenic mechanism leading to cell disruption. Cardiomyocyte disruption was dependent upon an intracellular source of redox-active iron and the iron-dependent transformation of internalized H2O2 into products (e.g., the hydroxyl radical) capable of initiating lipid peroxidation, since iron chelators and hydroxyl-radical scavengers were cytoprotective. The accelerated turnover of cardiomyocyte-membrane protein and phospholipid was inhibited by antiperoxidants, suggesting that the turnover reflected molecular repair of oxidized membrane constitutents. Likewise, the consumption of alpha-tocopherol and the oxidation of cellular thiols appeared to be epiphenomena of peroxidation. Antiperoxidant interventions coordinately abolished both H2O2-induced lipid peroxidation and sarcolemmal disruption, demonstrating that an intimate pathogenic relationship exists between sarcolemmal peroxidation and lethal compromise of cardiomyocyte integrity in response to H2O2-induced oxidative stress. Although sarcolemmal peroxidation was causally related to cardiomyocyte disruption during H2O2-induced oxidative stress, a nonperoxidative route of H2O2 cytotoxicity was also identified, which was expressed in the complete absence of cardiomyocyte-membrane peroxidation. The latter mode of H2O2-induced cardiomyocyte injury involved ATP loss such that membrane peroxidation and cardiomyocyte disruption on the one hand and cellular de-energization on the other could be completely dissociated.(ABSTRACT TRUNCATED AT 400 WORDS)

Lethal damage to murine L1210 cells by exogenous lipid hydroperoxides: protective role of glutathione-dependent selenoperoxidases

The effect of selenium deprivation on the viability of murine L1210 cells exposed to various exogenous lipid hydroperoxides has been investigated. Selenoperoxidase activities of cells grown for longer than 1 week in 1% serum with no added selenium [Se(-) cells] were less than 10% of the activities of selenium-satisfied controls [Se(+) cells] or selenium-repleted counterparts [Se(-/+) cells]. The enzymes measured were classical glutathione peroxidase (GPX) and phospholipid hydroperoxide glutathione peroxidase (PHGPX). Se(-) cells exhibited a compensatory increase in catalase activity. Dye exclusion and clonal survival assays indicated that Se(-) and Se(+) cells were relatively insensitive to photochemically generated phospholipid hydroperoxides in liposomal form. However, both cell types were sensitive to liposomal cholesterol hydroperoxides, e.g., 7-hydroperoxycholesterol (7-OOH), Se(-) being much more so (LD50 approximately 10 microM) than Se(+) (LD50 approximately 75 microM). By contrast, 7-hydroxycholesterol over a comparable concentration range was minimally toxic to Se(-) and Se(+) cells. Cell killing by 7-OOH was inhibited by desferrioxamine and by butylated hydroxytoluene, suggesting that iron-mediated free radical reactions are involved. The involvement of glutathione in cytoprotection was confirmed by showing that Se(+) cells were more sensitive to 7-OOH after treating with buthionine sulfoximine, an inhibitor of GSH synthesis. Cellular detoxification of 7-OOH is provisionally attributed to PHGPX rather than GPX, since 7-OOH and other cholesterol hydroperoxides were found to be good substrates for PHGPX in a cell free system, but were unreactive with GPX.

DNA damage by oxygen-derived species. Its mechanism and measurement in mammalian systems

When cells are exposed to oxidative stress, DNA damage frequently occurs. The molecular mechanisms causing this damage may include activation of nucleases and direct reaction of hydroxyl radicals with the DNA. Several oxygen-derived species can attack DNA, producing distinctive patterns of chemical modification. Observation of these patterns and measurement of some of the products formed has been used to determine the role of different oxygen-derived species in DNA cleavage reactions, to assess the extent of oxidative damage to DNA in vivo and to investigate the mechanism of DNA damage by ionizing radiation and chemical carcinogens.