Effects of the superoxide dismutase-mimic compound TEMPOL on oxidant stress-mediated endothelial dysfunction

Reactive Oxygen Species: Not Omnipresent but Important in Many Locations

Reactive oxygen species (ROS), such as the superoxide anion or hydrogen peroxide, have been established over decades of research as, on the one hand, important and versatile molecules involved in a plethora of homeostatic processes and, on the other hand, as inducers of damage, pathologies and diseases. Which effects ROS induce, strongly depends on the cell type and the source, amount, duration and location of ROS production. Similar to cellular pH and calcium levels, which are both strictly regulated and only altered by the cell when necessary, the redox balance of the cell is also tightly regulated, not only on the level of the whole cell but in every cellular compartment. However, a still widespread view present in the scientific community is that the location of ROS production is of no major importance and that ROS randomly diffuse from their cellular source of production throughout the whole cell and hit their redox-sensitive targets when passing by. Yet, evidence is growing that cells regulate ROS production and therefore their redox balance by strictly controlling ROS source activation as well as localization, amount and duration of ROS production. Hopefully, future studies in the field of redox biology will consider these factors and analyze cellular ROS more specifically in order to revise the view of ROS as freely flowing through the cell.

Functions of ROS in Macrophages and Antimicrobial Immunity

Reactive oxygen species (ROS) are a chemically defined group of reactive molecules derived from molecular oxygen. ROS are involved in a plethora of processes in cells in all domains of life, ranging from bacteria, plants and animals, including humans. The importance of ROS for macrophage-mediated immunity is unquestioned. Their functions comprise direct antimicrobial activity against bacteria and parasites as well as redox-regulation of immune signaling and induction of inflammasome activation. However, only a few studies have performed in-depth ROS analyses and even fewer have identified the precise redox-regulated target molecules. In this review, we will give a brief introduction to ROS and their sources in macrophages, summarize the versatile roles of ROS in direct and indirect antimicrobial immune defense, and provide an overview of commonly used ROS probes, scavengers and inhibitors.

ATM is required for SOD2 expression and homeostasis within the mammary gland

PURPOSE

ATM activates the NF-κB transcriptional complex in response to genotoxic and oxidative stress. The purpose of this study was to examine if the NF-κB target gene and critical antioxidant SOD2 (MnSOD) in cultured mammary epithelium is also ATM-dependent, and what phenotypes arise from deletion of ATM and SOD2 within the mammary gland.

METHODS

SOD2 expression was studied in human mammary epithelial cells and MCF10A using RNAi to knockdown ATM or the NF-κB subunit RelA. To study ATM and SOD2 function in mammary glands, mouse lines containing Atm or Sod2 genes containing LoxP sites were mated with mice harboring Cre recombinase under the control of the whey acidic protein promoter. Quantitative PCR was used to measure gene expression, and mammary gland structure was studied using histology.

RESULTS

SOD2 expression is ATM- and RelA-dependent, ATM knockdown renders cells sensitive to pro-oxidant exposure, and SOD mimetics partially rescue this sensitivity. Mice with germline deletion of Atm fail to develop mature mammary glands, but using a conditional knockout approach, we determined that Atm deletion significantly diminished the expression of Sod2. We also observed that these mice (termed Atm^Δ/Δ) displayed a progressive lactation defect as judged by reduced pup growth rate, aberrant lobulo-alveolar structure, diminished milk protein gene expression, and increased apoptosis within lactating glands. This phenotype appears to be linked to dysregulated Sod2 expression as mammary gland-specific deletion of Sod2 phenocopies defects observed in Atm^Δ/Δ dams.

CONCLUSIONS

We conclude that ATM is required to promote expression of SOD2 within the mammary epithelium, and that both ATM and SOD2 play a crucial role in mammary gland homeostasis.

Effect of tempol on peripheral neuropathy in diet-induced obese and high-fat fed/low-dose streptozotocin-treated C57Bl6/J mice

In this study, we sought to determine the efficacy of tempol on multiple neuropathic endpoints in a diet-induced obese mouse, a model of pre-diabetes, and a high-fat fed low-dose streptozotocin treated mouse, a model of type 2 diabetes. Tempol (4-hydroxy-2,2,6,6-tetramethylpiperdine -1-oxyl) is a low molecular weight, water soluble, membrane permeable, and metal-independent superoxide dismutase mimetic that has been widely used in cellular studies for the removal of intracellular and extracellular superoxide. This in vivo study was designed to be an early intervention. Fourteen weeks post-high-fat diet (6 weeks post-hyperglycemia) control, obese, and diabetic mice were divided into no treatment and treatment groups. The treated mice received tempol by gavage (150 mg/kg in water), while the untreated mice received vehicle. The diet-induced obese and the diabetic mice were maintained on the high-fat diet for the duration of the study, while the control group was maintained on the standard diet. Obesity and diabetes caused slowing of motor and sensory nerve conduction, reduction in intraepidermal nerve fiber density, thermal hypoalgesia, and mechanical allodynia. Treatment with tempol partially or completely protected obese and diabetic mice from these deficits. These studies suggest that tempol or other effective scavengers of reactive oxygen species may be a viable option for treating neural complications associated with obesity or type 2 diabetes.

Nitrosative Stress-Induced Disruption of Baroreflex Neural Circuits in a Rat Model of Hepatic Encephalopathy: A DTI Study

The onset of hepatic encephalopathy (HE) in liver failure is associated with high mortality; the underlying mechanism is undecided. Here we report that in an acute liver failure model employing intraperitoneal administration of thioacetamide in Sprague-Dawley rats, diffusion weighted imaging revealed a progressive reduction in apparent diffusion coefficient in the brain stem. Diffusion tensor imaging further showed that the connectivity between nucleus tractus solitarii (NTS), the terminal site of baroreceptor afferents in brain stem and rostral ventrolateral medulla (RVLM), the origin of sympathetic innervation of blood vessels, was progressively disrupted until its disappearance, coincidental with the irreversible cessation of baroreflex-mediated sympathetic vasomotor tone signifying clinically the occurrence of brain death. In addition, superoxide, nitric oxide, peroxynitrite and ammonia levels in the NTS or RVLM were elevated, alongside swelling of astroctytes. A scavenger of peroxynitrite, but not an antioxidant, delivered intracisternally reversed all these events. We conclude that nitrosative stress because of augmented peroxynitrite related to accumulation of ammonia and swelling of astrocytes in the NTS or RVLM, leading to cytotoxic edema in the brain stem and severance of the NTS-RVLM connectivity, underpins the defunct baroreflex-mediated sympathetic vasomotor tone that accounts for the high mortality associated with HE.

Different influences of extracellular and intracellular superoxide on relaxation through the NO/sGC/cGMP pathway in isolated rat iliac arteries

Superoxide production is increased in diseased blood vessels, which is considered to lead to impairment of the nitric oxide (NO)/soluble guanylate cyclase (sGC)/cGMP pathway. To investigate the respective influence of extracellular and intracellular superoxide on vascular function through the NO/sGC/cGMP pathway, mechanical responses of rat external iliac arteries without endothelium were studied under exposure to a superoxide-generating agent, pyrogallol, or menadione. Exposure to pyrogallol impaired the relaxation induced by acidified NaNO2 (exogenous NO) but not that by nitroglycerin (organic nitrate), BAY 41-2272 (sGC stimulator), BAY 60-2770 (sGC activator), or 8-Br-cGMP (cGMP analog). Superoxide dismutase (SOD) and tempol restored the impaired relaxation by acidified NaNO2. Superoxide production in the bathing solution, but not in artery segments, was significantly increased by exposure to pyrogallol, which was abolished in the presence of SOD or tempol. However, exposure to menadione impaired the relaxant response to acidified NaNO2, nitroglycerin, or BAY 41-2272, whereas it augmented that to BAY 60-2770. Also, this exposure had no effect on the 8-Br-cGMP-induced vasorelxation. Superoxide production in artery segments was dramatically enhanced by exposure to menadione, whereas that in the bathing solution was not affected. This increase in vascular superoxide production was normalized by tempol but not by SOD. These findings suggest that extracellular superoxide reacts with NO only outside the cell, whereas intracellular superoxide not only scavenges NO inside the cell but also shifts the sGC redox equilibrium.

Equine digital veins are more sensitive to superoxide anions than digital arteries

This work was designed to investigate (i) the effect of superoxide dismutase (SOD) inhibition on endothelial function and (ii) the free radical-induced endothelial dysfunction in equine digital veins (EDVs) and equine digital arteries (EDAs) isolated from healthy horses. EDV and EDA rings were suspended in a 5 ml organ bath containing Krebs solution. After a 60 min equilibration period, EDV and EDA rings were contracted with phenylephrine. Then, cumulative concentration-response curves (CCRCs) to acetylcholine were performed. In both EDVs and EDAs, acetylcholine (1 nM to 10 µM) produced concentration-dependent relaxation. We investigated the influence of SOD inhibition by diethyldithiocarbamate (DETC; 100 µM), a CuZnSOD inhibitor, on EDAs and EDVs relaxant responses to acetylcholine. Acetylcholine -mediated relaxation was impaired by DETC only in EDVs. SOD activity assayed by a xanthine-xanthine oxidase method was higher in EDAs compared with EDVs (P<0.05). CCRCs to acetylcholine established in the presence of pyrogallol (30 µM) or homocysteine (20 µM), two superoxide anions generating systems showed that in both EDVs and EDAs, the acetylcholine-mediated relaxation was significantly impaired by pyrogallol and homocysteine. This impairment was more pronounced in EDVs than in EDAs. Moreover, the pyrogallol-induced impairment of acetylcholine-mediated relaxation was potentiated by DETC to a greater extent in EDVs. We concluded that due to the lower activity of SOD, EDVs are more sensitive to superoxide anions than EDAs. So, any alteration of superoxide anions metabolism is likely to have a more important impact on venous rather than arterial relaxation.

Regulation of acid-base transporters by reactive oxygen species following mitochondrial fragmentation

Mitochondrial morphology is determined by the balance between the opposing processes of fission and fusion, each of which is regulated by a distinct set of proteins. Abnormalities in mitochondrial dynamics have been associated with a variety of diseases, including neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and dominant optic atrophy. Although the genetic determinants of fission and fusion are well recognized, less is known about the mechanism(s) whereby altered morphology contributes to the underlying pathophysiology of these disease states. Previous work from our laboratory identified a role for mitochondrial dynamics in intracellular pH homeostasis in both mammalian cell culture and in the genetic model organism Caenorhabditis elegans. Here we show that the acidification seen in mutant animals that have lost the ability to fuse their mitochondrial inner membrane occurs through a reactive oxygen species (ROS)-dependent mechanism and can be suppressed through the use of pharmacological antioxidants targeted specifically at the mitochondrial matrix. Physiological approaches examining the activity of endogenous mammalian acid-base transport proteins in rat liver Clone 9 cells support the idea that ROS signaling to sodium-proton exchangers contributes to acidification. Because maintaining pH homeostasis is essential for cell function and viability, the results of this work provide new insight into the pathophysiology associated with the loss of inner mitochondrial membrane fusion.

Role of free radicals in vascular dysfunction induced by high tidal volume ventilation

OBJECTIVE

To demonstrate that increased formation of reactive oxygen (ROS) and nitrogen species (RNS) is involved in VILI-induced vascular dysfunction.

METHODS

Male Sprague-Dawley anesthetized rats were ventilated for 60 min using low V(T) ventilation [V(T) 9 ml/kg, positive end-expiratory pressure (PEEP) 5 cmH(2)O, n = 18], and high V(T) ventilation (V(T) 35 ml/kg, zero PEEP, n = 18). Arterial pressure and respiratory system mechanics were monitored. Blood samples for the determination of arterial blood gases and lactate concentration were drawn. Vascular rings from the thoracic aortae were mounted in organ baths for isometric tension recording. We studied endothelium-dependent relaxation in norepinephrine-precontracted rings (acetylcholine, 10 nM-10 microM) and contraction induced by norepinephrine (1 nM-10 microM) in resting vessels. Vascular rings were preincubated for 30 min with Zn-Mn-SOD (100 u/ml) or tempol (10(-4) M) (extracellular and intracellular superoxide scavengers, respectively) or MnTMPyP (10(-5) M) (a superoxide and peroxynitrite scavenger). The presence of superoxide and nitrotyrosine in aortic rings was evaluated by immunofluorescence.

RESULTS

High V(T) ventilation induced hypotension, systemic acidosis, hypoxemia and hyperlactatemia, as well as impairment in acetylcholine and norepinephrine-induced responses in vitro. Responses to acetylcholine were improved by tempol (P = 0.004) and completely corrected (P < 0.001) by MnTMPyP. Responses to norepinephrine were also improved by treatment with tempol (P < 0.001) and MnTMPyP (P < 0.001). However, Zn-Mn-SOD did not improve acetylcholine- or norepinephrine-induced responses. Immunostaining for both superoxide and nitrotyrosine was increased in aortic rings from the high V(T) group.

CONCLUSIONS

Our data support a role for intracellular ROS and peroxynitrite in the high V(T) ventilation-induced vascular dysfunction.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Oxidative stress-induced renal angiotensin AT1 receptor upregulation causes increased stimulation of sodium transporters and hypertension

Reactive oxygen species have emerged as important molecules in cardiovascular dysfunction such as diabetes and hypertension. Recent work has shown that oxidative stress and angiotensin II signaling mutually regulate each other by multiple mechanisms and contribute to the development of hypertension. Most of the known biological actions of angiotensin II can be attributed to AT1 receptors. The present study was carried out to investigate the role of renal AT1 receptor signaling in oxidative stress-mediated hypertension. Male Sprague-Dawley rats received tap water (control) or 30 mM L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mM tempol (an antioxidant) for 2 wk. Compared with control rats, BSO-treated rats exhibited increased oxidative stress and reduced antioxidant levels and developed hypertension. BSO treatment also caused increased renal proximal tubular AT1 receptor protein abundance, message levels, and ligand binding. In these rats, angiotensin II caused significantly higher accumulation of inositol trisphosphate (IP3) and phospholipase C (PLC) activation which was sensitive to blockade by AT1 but not to AT2 antagonist. Also, angiotensin II-mediated, AT1-dependent MAP kinase, Na-K-ATPase, and Na/H exchanger 3 activation was higher in BSO-treated rats than in control rats. Tempol supplementation of BSO-treated rats restored redox status, normalized AT1 receptor expression, and decreased blood pressure. Tempol also normalized the angiotensin II-mediated, AT1-dependent IP3 accumulation and PLC, MAP kinase, Na-K-ATPase, and Na/H exchanger 3 stimulation. These data suggest that oxidative stress leads to AT1 receptor upregulation, which in turn causes overstimulation of sodium transporters and subsequently contributes to sodium retention and hypertension. Tempol, while reducing oxidative stress, normalizes AT1 receptor signaling and decreases blood pressure.

Vascular oxidative stress precedes high blood pressure in spontaneously hypertensive rats

This study examines whether longitudinal antioxidant treatment initiated in prehypertensive spontaneously hypertensive rats (SHR) can attenuate vascular oxidant stress and prevent blood pressure elevation during development. Male SHR and age-matched Wistar-Kyoto rats (WKY) were treated from 6 to 11 weeks of age with Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl) (1 mmol/l in drinking water), a membrane-permeable superoxide dismutase mimetic. Mean systolic blood pressures (SBPs) were measured by tail-cuff Agonist-induced and basal O2- production was measured in thoracic aortas of 6- and 11-week-old SHR and WKY by lucigenin-derived chemiluminescence and oxidative fluorescent microscopy, respectively. SBP of 6-week-old SHR (131 +/- 5 mmHg) and WKY (130 +/- 4 mmHg) were not different; however, 11-week-old SHR SBP (171 +/- 4 mmHg) was significantly greater (p = .0001) than 11-week-old WKY SBP (143 +/- 5 mmHg). Tempol treatment completely, but reversibly, prevented this age-related rise in SHR SBP (SHR + Tempol: 137 +/- 4 mmHg; p < .0001 versus untreated SHR). Agonist-induced vascular O2- was increased in 6- (p = .03) and 11-week-old SHR (p < .0001) and 11-week-old WKY (p = .03) but not in 6-week-old WKY. Long-term Tempol treatment significantly lowered O2- production in both strains. Basal O2- measurements in both 6- and 11-week-old SHR were qualitatively increased compared with age-matched WKY; this increase in SHR was inhibited with in vitro Tempol treatment. These data show that antioxidant treatment to reduce oxidative stress prevents the age-related development of high blood pressure in an animal model of genetic hypertension.

Cell permeable ROS scavengers, Tiron and Tempol, rescue PC12 cell death caused by pyrogallol or hypoxia/reoxygenation

The role of superoxide anion (O(2)*-) in neuronal cell injury induced by reactive oxygen species (ROS) was examined in PC12 cells using pyrogallol (1,2,3-benzenetrior), a donor to release O(2)*-. Pyrogallol induced PC12 cell death at concentrations, which evidently increased intracellular O(2)*-, as assessed by O(2)(*-)-sensitive fluorescent precursor hydroethidine (HEt). Caspase inhibitors, Z-VAD-FMK and Z-Asp-CH(2)-DCB, failed to protect cells from injury caused by elevation of intracellular O(2)*-, although these inhibitors had effects on hypoxia- or hydrogen peroxide (H(2)O(2))-induced PC12 cell death. Two known O(2)*- scavengers, Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid) and Tempol (4-hydroxy-2,2,6,6-tetramethylpiperydine-1-oxyl) rescued PC12 cells from pyrogallol-induced cell death. Hypoxia/reoxygenation injury of PC12 cells was also blocked by Tiron and Tempol. Further understanding of the underlying mechanism of the protective effects of these radical scavengers reducing intracellular O(2)*- on neuronal cell death may lead to development of new therapeutic treatments for hypoxic/ischemic brain injury.

Antioxidant properties of the vasoactive endocannabinoid, 2-arachidonoyl glycerol (2-AG)

Reactive oxygen species (ROS) were shown to play a role in altering blood-brain barrier (BBB) permeability and formation of brain edema induced by trauma and/or ischemia. 2-arachidonoyl glycerol (2-AG), a novel, potent vasodilatory and cytoprotective endocannabinoid has been implicated to act as an antioxidative agent. This study examines: 1) the possible 2-AG modulation of BBB injury and edema formation induced by closed head injury (CHI); and 2) comparable effects between 2-AG and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TPL), a known antioxidant nitroxide on endothelial Ca2+ and cytoskeletal responses to H2O2 (ROS). 2-AG treatment reduced the CHI-induced increase in BBB permeability and brain edema. The endothelial H2O2-stimulated Ca2+ mobilization and cytoskeleton (vimentin) rearrangement was modified by either 2-AG or TPL. These findings provide evidence of 2-AG antioxidant activity and are consistent with the involvement of ROS in the pathomechanism of CHI-induced BBB injury and brain edema.

Tempol selectively attenuates angiotensin II evoked vasoconstrictor responses in spontaneously hypertensive rats

OBJECTIVE

To assess whether superoxide anions mediate vasoconstrictor responses to agonists in blood vessels of spontaneously hypertensive rats (SHRs).

METHODS

The effect of the superoxide dismutase mimetic, 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (tempol), on responses to angiotensin II (Ang II), endothelin-1, phenylephrine and potassium chloride was determined in aortic rings and perfused mesenteric vascular beds (MVB) of adult male rats of the Sprague-Dawley, Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) strains. The effect of tempol on Ang II-evoked superoxide production was assessed in aortic rings.

RESULTS

There were no differences in the maximum tension (Emax) attained in response to agonists, but the negative logarithm of the concentration required to produce 50% of the maximal response (EC50) for Ang II was lower (P < 0.05) in aortic rings of SHRs. In the MVBs of SHRs, the Emax but not the EC50 values attained in response to Ang II, endothelin-1 and phenylephrine were greater. Tempol significantly and selectively reduced the Emax of Ang II in both aorta and MVB preparations with intact endothelium. The reduction in Emax attained in response to Ang II was more pronounced in SHRs (P < 0.01) than in WKY rats (P < 0.05) or Sprague-Dawley rats (P < 0.05). The inhibitory effect of tempol was absent when a nitric oxide synthase inhibitor was included or endothelium was denuded. A significant increase in lucigenin chemiluminescence evoked by Ang II in both intact and endothelium-denuded aortic rings of SHRs was abolished when tempol was included in the buffer.

CONCLUSIONS

These data suggest that increased superoxide anions mediate vasoconstrictor responses to Ang II, but not to other agonists, in an endothelium-dependent manner, by quenching vasodilatory mediator, nitric oxide. This may account for the exaggerated vasoconstrictor responses to Ang II in SHRs.

Effects of the superoxide dismutase-mimetic compound tempol on endothelial dysfunction in streptozotocin-induced diabetic rats

Evidence exists to support the beneficial effects of superoxide dismutase on endothelial dysfunction induced by hyperglycemia in vitro. In vivo, however, studies of the effects of native superoxide dismutase preparations on the vascular complications accompanying diabetes are limited, and their therapeutic application potential has so far been disappointing. The objective of this study was to evaluate, for the first time in vivo, the effects of long-term administration of tempol, a stable superoxide dismutase-mimic compound, on diabetes-induced endothelial dysfunction in rats. Diabetes was induced by streptozotocin and rats were monitored for 8 weeks with or without treatment with tempol (100 mg/kg, s.c., b.i.d). Diabetic rats showed increased vascular levels of superoxide, which was accompanied by increased levels of the oxidative stress markers malondialdehyde and 8-epi-prostaglandin F(2alpha). In addition, the vasorelaxant as well as the cGMP-producing effects of acetylcholine and glyceryl trinitrate were reduced in diabetic rats. Treatment with tempol abolished not only the differences in the vascular content of superoxide, malondialdehyde and 8-epi-prostaglandin F(2alpha), but also the differences in the relaxation and cGMP responses of aortic rings to both acetylcholine and glyceryl trinitrate between control and diabetic rats. These results support the involvement of reactive oxygen species in mediation of hyperglycemia-induced endothelial dysfunction in vivo, and provide the rationale for potential utilization of stable superoxide dismutase-mimic nitroxides for the prevention of the vascular complications accompanying diabetes.