The chemistry and biology of nitroxide compounds

Protective effect of an antithyroid compound against γ-radiation-induced damage in human colon cancer cells

We have previously reported the radioprotective effect of propylthiouracil (PTU) on thyroid cells. The aim of the present study was to analyze whether tumor cells and normal cells demonstrate the same response to PTU. Human colon carcinoma cells were irradiated with γ-irradiation with or without PTU. We evaluated the clonogenic survival, intracellular reactive oxygen species levels, catalase, superoxide dismutase and glutathione peroxidase activities, and apoptosis by nuclear cell morphology and caspase-3 activity assays. Cyclic AMP (cAMP) levels were measured by radioimmunoassay. PTU treatment increased surviving cell fraction at 2 Gy (SF2) from 56.9 ± 3.6 in controls to 75.0 ± 3.5 (p < 0.05) and diminished radiation-induced apoptosis. In addition, we observed that the level of antioxidant enzymes' activity was increased in cells treated with PTU. Moreover, pretreatment with PTU increased intracellular levels of cAMP. Forskolin (p < 0.01) and dibutyryl cAMP (p < 0.05) mimicked the effect of PTU on SF2. Co-treatment with H89, an inhibitor of protein kinase A, abolished the radioprotective effect of PTU. PTU reduces the toxicity of ionizing radiation by increasing cAMP levels and also possibly through a reduction in apoptosis levels and in radiation-induced oxidative stress damage. We therefore conclude that PTU protects both normal and cancer cells during exposure to radiation in conditions mimicking the radiotherapy.

Systemic DNA damage accumulation under in vivo tumor growth can be inhibited by the antioxidant Tempol

Recently we found that mice bearing subcutaneous non-metastatic tumors exhibited elevated levels of two types of complex DNA damage, i.e., double-strand breaks and oxidatively-induced clustered DNA lesions in various tissues throughout the body, both adjacent to and distant from the tumor site. This DNA damage was dependent on CCL2, a cytokine involved in the recruitment and activation of macrophages, suggesting that this systemic DNA damage was mediated via tumor-induced chronic inflammatory responses involving cytokines, activation of macrophages, and consequent free radical production. If free radicals are involved, then a diet containing an antioxidant may decrease the distant DNA damage. Here we repeated our standard protocol in cohorts of two syngeneic tumor-bearing C57BL/6NCr mice that were on a Tempol-supplemented diet. We show that double-strand break and oxidatively-induced clustered DNA lesion levels were considerably decreased, about two- to three fold, in the majority of tissues studied from the tumor-bearing mice fed the antioxidant Tempol compared to the control tumor-bearing mice. Similar results were also observed in nude mice suggesting that the Tempol effects are independent of functioning adaptive immunity. This is the first in vivo study demonstrating the effect of a dietary antioxidant on abscopal DNA damage in tissues distant from a localized source of genotoxic stress. These findings may be important for understanding the mechanisms of genomic instability and carcinogenesis caused by chronic stress-induced systemic DNA damage and for developing preventative strategies.

Over 35% liquid-state 13C polarization obtained via dissolution dynamic nuclear polarization at 7 T and 1 K using ubiquitous nitroxyl radicals

The most versatile method to increase liquid-state (13)C NMR sensitivity is dissolution dynamic nuclear polarization. The use of trityl radicals is usually required to obtain very large (13)C polarization via this technique. We herein demonstrate that up to 35% liquid-state (13)C polarization can be obtained in about 1.5 h using ubiquitous nitroxyl radicals in (13)C-labeled sodium salts by partially deuterating the solvents and using a polarizer operating at 1 K and 7 T.

DNA double-strand break repair genes and oxidative damage in brain metastasis of breast cancer

BACKGROUND

Breast cancer frequently metastasizes to the brain, colonizing a neuro-inflammatory microenvironment. The molecular pathways facilitating this colonization remain poorly understood.

METHODS

Expression profiling of 23 matched sets of human resected brain metastases and primary breast tumors by two-sided paired t test was performed to identify brain metastasis-specific genes. The implicated DNA repair genes BARD1 and RAD51 were modulated in human (MDA-MB-231-BR) and murine (4T1-BR) brain-tropic breast cancer cell lines by lentiviral transduction of cDNA or short hairpin RNA (shRNA) coding sequences. Their functional contribution to brain metastasis development was evaluated in mouse xenograft models (n = 10 mice per group).

RESULTS

Human brain metastases overexpressed BARD1 and RAD51 compared with either matched primary tumors (1.74-fold, P < .001; 1.46-fold, P < .001, respectively) or unlinked systemic metastases (1.49-fold, P = .01; 1.44-fold, P = .008, respectively). Overexpression of either gene in MDA-MB-231-BR cells increased brain metastases by threefold to fourfold after intracardiac injections, but not lung metastases upon tail-vein injections. In 4T1-BR cells, shRNA-mediated RAD51 knockdown reduced brain metastases by 2.5-fold without affecting lung metastasis development. In vitro, BARD1- and RAD51-overexpressing cells showed reduced genomic instability but only exhibited growth and colonization phenotypes upon DNA damage induction. Reactive oxygen species were present in tumor cells and elevated in the metastatic neuro-inflammatory microenvironment and could provide an endogenous source of genotoxic stress. Tempol, a brain-permeable oxygen radical scavenger suppressed brain metastasis promotion induced by BARD1 and RAD51 overexpression.

CONCLUSIONS

BARD1 and RAD51 are frequently overexpressed in brain metastases from breast cancer and may constitute a mechanism to overcome reactive oxygen species-mediated genotoxic stress in the metastatic brain.

Development of nitroxide radicals-containing polymer for scavenging reactive oxygen species from cigarette smoke

We developed a nitroxide radicals-containing polymer (NRP), which is composed of poly(4-methylstyrene) possessing nitroxide radicals as a side chain via amine linkage, to scavenge reactive oxygen species (ROS) from cigarette smoke. In this study, the NRP was coated onto cigarette filters and its ROS-scavenging activity from streaming cigarette smoke was evaluated. The intensity of electron spin resonance signals of the NRP in the filter decreased after exposure to cigarette smoke, indicating consumption of nitroxide radicals. To evaluate the ROS-scavenging activity of the NRP-coated filter, the amount of peroxy radicals in an extract of cigarette smoke was measured using UV-visible spectrophotometry and 1,1-diphenyl-2-picrylhydrazyl (DPPH). The absorbance of DPPH at 517 nm decreased with exposure to cigarette smoke. When NRP-coated filters were used, the decrease in the absorbance of DPPH was prevented. In contrast, both poly[4-(cyclohexylamino)methylstyrene]- and poly(acrylic acid)-coated filters, which have no nitroxide radical, did not show any effect, indicating that the nitroxide radicals in the NRP scavenge the ROS in cigarette smoke. As a result, the extract of cigarette smoke passed through the NRP-coated filter has a lower cellular toxicity than smoke passed through poly[4-(cyclohexylamino)methylstyrene]- and poly(acrylic acid)-coated filters. Accordingly, NRP is a promising material for ROS scavenging from cigarette smoke.

Simple biological systems for assessing the activity of superoxide dismutase mimics

SIGNIFICANCE

Half a century of research provided unambiguous proof that superoxide and species derived from it-reactive oxygen species (ROS)-play a central role in many diseases and degenerative processes. This stimulated the search for pharmaceutical agents that are capable of preventing oxidative damage, and methods of assessing their therapeutic potential.

RECENT ADVANCES

The limitations of superoxide dismutase (SOD) as a therapeutic tool directed attention to small molecules, SOD mimics, that are capable of catalytically scavenging superoxide. Several groups of compounds, based on either metal complexes, including metalloporphyrins, metallocorroles, Mn(II) cyclic polyamines, and Mn(III) salen derivatives, or non-metal based compounds, such as fullerenes, nitrones, and nitroxides, have been developed and studied in vitro and in vivo. Very few entered clinical trials.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Development of SOD mimics requires in-depth understanding of their mechanisms of biological action. Elucidation of both molecular features, essential for efficient ROS-scavenging in vivo, and factors limiting the potential side effects requires biologically relevant and, at the same time, relatively simple testing systems. This review discuses the advantages and limitations of genetically engineered SOD-deficient unicellular organisms, Escherichia coli and Saccharomyces cerevisiae as tools for investigating the efficacy and mechanisms of biological actions of SOD mimics. These simple systems allow the scrutiny of the minimal requirements for a functional SOD mimic: the association of a high catalytic activity for superoxide dismutation, low toxicity, and an efficient cellular uptake/biodistribution.

SOD therapeutics: latest insights into their structure-activity relationships and impact on the cellular redox-based signaling pathways

SIGNIFICANCE

Superoxide dismutase (SOD) enzymes are indispensable and ubiquitous antioxidant defenses maintaining the steady-state levels of O2·(-); no wonder, thus, that their mimics are remarkably efficacious in essentially any animal model of oxidative stress injuries thus far explored.

RECENT ADVANCES

Structure-activity relationship (half-wave reduction potential [E1/2] versus log kcat), originally reported for Mn porphyrins (MnPs), is valid for any other class of SOD mimics, as it is dominated by the superoxide reduction and oxidation potential. The biocompatible E1/2 of ∼+300 mV versus normal hydrogen electrode (NHE) allows powerful SOD mimics as mild oxidants and antioxidants (alike O2·(-)) to readily traffic electrons among reactive species and signaling proteins, serving as fine mediators of redox-based signaling pathways. Based on similar thermodynamics, both SOD enzymes and their mimics undergo similar reactions, however, due to vastly different sterics, with different rate constants.

CRITICAL ISSUES

Although log kcat(O2·(-)) is a good measure of therapeutic potential of SOD mimics, discussions of their in vivo mechanisms of actions remain mostly of speculative character. Most recently, the therapeutic and mechanistic relevance of oxidation of ascorbate and glutathionylation and oxidation of protein thiols by MnP-based SOD mimics and subsequent inactivation of nuclear factor κB has been substantiated in rescuing normal and killing cancer cells. Interaction of MnPs with thiols seems to be, at least in part, involved in up-regulation of endogenous antioxidative defenses, leading to the healing of diseased cells.

FUTURE DIRECTIONS

Mechanistic explorations of single and combined therapeutic strategies, along with studies of bioavailability and translational aspects, will comprise future work in optimizing redox-active drugs.

Inhibition of myeloperoxidase- and neutrophil-mediated oxidant production by tetraethyl and tetramethyl nitroxides

The powerful oxidant HOCl (hypochlorous acid and its corresponding anion, (-)OCl) generated by the myeloperoxidase (MPO)-H2O2-Cl(-) system of activated leukocytes is strongly associated with multiple human inflammatory diseases; consequently there is considerable interest in inhibition of this enzyme. Nitroxides are established antioxidants of low toxicity that can attenuate oxidation in animal models, with this ascribed to superoxide dismutase or radical-scavenging activities. We have shown (M.D. Rees et al., Biochem. J. 421, 79-86, 2009) that nitroxides, including 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical), are potent inhibitors of HOCl formation by isolated MPO and activated neutrophils, with IC50 values of ~1 and ~6 µM respectively. The utility of tetramethyl-substituted nitroxides is, however, limited by their rapid reduction by biological reductants. The corresponding tetraethyl-substituted nitroxides have, however, been reported to be less susceptible to reduction. In this study we show that the tetraethyl species were reduced less rapidly than the tetramethyl species by both human plasma (89-99% decreased rate of reduction) and activated human neutrophils (62-75% decreased rate). The tetraethyl-substituted nitroxides retained their ability to inhibit HOCl production by MPO and activated neutrophils with IC50 values in the low-micromolar range; in some cases inhibition was enhanced compared to tetramethyl substitution. Nitroxides with rigid structures (fused oxaspiro rings) were, however, inactive. Overall, these data indicate that tetraethyl-substituted nitroxides are potent inhibitors of oxidant formation by MPO, with longer plasma and cellular half-lives compared to the tetramethyl species, potentially allowing lower doses to be employed.

A highly cytotoxic modified paullone ligand bearing a TEMPO free-radical unit and its copper(II) complex as potential hR2 RNR inhibitors

A new modified paullone ligand bearing a TEMPO free-radical unit (HL²) and its copper(ii) complex have been prepared. The compounds demonstrate high cytotoxicity in vitro and strongly inhibit cell-free hR2 RNR activity.

A new paullone–TEMPO conjugate and its copper(ii) complex inhibit RNR activity and show high antiproliferative activity in human cancer cell lines.

Microbiome remodelling leads to inhibition of intestinal farnesoid X receptor signalling and decreased obesity

The antioxidant tempol reduces obesity in mice. Here we show that tempol alters the gut microbiome by preferentially reducing the genus Lactobacillus and its bile salt hydrolase (BSH) activity leading to the accumulation of intestinal tauro-β-muricholic acid (T-β-MCA). T-β-MCA is an farnesoid X receptor (FXR) nuclear receptor antagonist, which is involved in the regulation of bile acid, lipid and glucose metabolism. Its increased levels during tempol treatment inhibit FXR signalling in the intestine. High-fat diet-fed intestine-specific Fxr-null (Fxr(ΔIE)) mice show lower diet-induced obesity, similar to tempol-treated wild-type mice. Further, tempol treatment does not decrease weight gain in Fxr(ΔIE) mice, suggesting that the intestinal FXR mediates the anti-obesity effects of tempol. These studies demonstrate a biochemical link between the microbiome, nuclear receptor signalling and metabolic disorders, and suggest that inhibition of FXR in the intestine could be a target for anti-obesity drugs.

Kinetics of glycoxidation of bovine serum albumin by methylglyoxal and glyoxal and its prevention by various compounds

The aim of this study was to compare several methods for measurement of bovine serum albumin (BSA) modification by glycoxidation with reactive dicarbonyl compounds (methylglyoxal ‒ MGO and glyoxal ‒ GO), for studies of the kinetics of this process and to compare the effects of 19 selected compounds on BSA glycation by the aldehydes. The results confirm the higher reactivity of MGO with respect to GO and point to the usefulness of AGE, dityrosine and N′-formylkynurenine fluorescence for monitoring glycation and evaluation of protection against glycation. Different extent of protection against glycation induced by MGO and GO was found for many compounds, probably reflecting effects on various stages of the glycation process. Polyphenols (genistein, naringin and ellagic acid) were found to protect against aldehyde-induced glycation; 1-cyano-4-hydroxycinnamic acid was also an effective protector.

Tempol protects cardiomyocytes from nucleoside reverse transcriptase inhibitor-induced mitochondrial toxicity

Nucleoside reverse transcriptase inhibitors (NRTIs), essential components of combinational therapies used for treatment of human immunodeficiency virus-1, damage heart mitochondria. Here, we have shown mitochondrial compromise in H9c2 rat cardiomyocytes exposed for 16 passages (P) to the NRTIs zidovudine (AZT, 50μM) and didanosine (ddI, 50μM), and we have demonstrated protection from mitochondrial compromise in cells treated with 200μM 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (Tempol) or 200μM 1-hydroxy-4-[2-triphenylphosphonio)-acetamido]-2,2,6,6-tetramethylpiperidine (Tempol-H), along with AZT/ddI, for 16P. Exposure to AZT/ddI caused a moderate growth inhibition at P3, P5, P7, and P13, which was not altered by addition of Tempol or Tempol-H. Mitochondrial oxidative phosphorylation capacity was determined as uncoupled oxygen consumption rate (OCR) by Seahorse XF24 Analyzer. At P5, P7, and P13, AZT/ddI-exposed cells showed an OCR reduction of 8.8-57.2% in AZT/ddI-exposed cells, compared with unexposed cells. Addition of Tempol or Tempol-H, along with AZT/ddI, resulted in OCR levels increased by about 300% above the values seen with AZT/ddI alone. The Seahorse data were further supported by electron microscopy (EM) studies in which P16 cells exposed to AZT/ddI/Tempol had less mitochondrial pathology than P16 cells exposed to AZT/ddI. Western blots of P5 cells showed that Tempol and Tempol-H upregulated expression of mitochondrial uncoupling protein-2 (UCP-2). However, Complex I activity that was reduced by AZT/ddI, was not restored in the presence of AZT/ddI/Tempol. Superoxide levels were increased in the presence of AZT/ddI and significantly decreased in cells exposed to AZT/3TC/Tempol at P3, P7, and P10. In conclusion, Tempol protects against NRTI-induced mitochondrial compromise, and UCP-2 plays a role through mild uncoupling.

The impact of ionizing radiation on placental trophoblasts

INTRODUCTION

Exposure to low-dose radiation is widespread and attributable to natural sources. However, occupational, medical, accidental, and terrorist-related exposures remain a significant threat. Information on radiation injury to the feto-placental unit is scant and largely observational. We hypothesized that radiation causes trophoblast injury, and alters the expression of injury-related transcripts in vitro or in vivo, thus affecting fetal growth.

METHODS

Primary human trophoblasts (PHTs), BeWo or NCCIT cells were irradiated in vitro, and cell number and viability were determined. Pregnant C57Bl/6HNsd mice were externally irradiated on E13.5, and placentas examined on E17.5. RNA expression was analyzed using microarrays and RT-qPCR. The experiments were repeated in the presence of the gramicidin S (GS)-derived nitroxide JP4-039, used to mitigate radiation-induced cell injury.

RESULTS

We found that survival of in vitro-irradiated PHT cell was better than that of irradiated BeWo trophoblast cell line or the radiosensitive NCCIT mixed germ cell tumor line. Radiation altered the expression of several trophoblast genes, with a most dramatic effect on CDKN1A (p21, CIP1). Mice exposed to radiation at E13.5 exhibited a 25% reduction in mean weight by E17.5, and a 9% reduction in placental weight, which was associated with relatively small changes in placental gene expression. JP4-039 had a minimal effect on feto-placental growth or on gene expression in irradiated PHT cells or mouse placenta.

DISCUSSION AND CONCLUSION

While radiation affects placental trophoblasts, the established placenta is fairly resistant to radiation, and changes in this tissue may not fully account for fetal growth restriction induced by ionizing radiation.

Oxidative stress retards vascular development before neural degeneration occurs in retinal degeneration rd1 mice

PURPOSE

To investigate the role of reactive oxygen species (ROS) in retinal development during the early postnatal stage of rd1 mice.

METHODS

Development of the three retinal vascular layers of C57BL/6 J (WT) and C3H/HeN (rd1) mice was evaluated from 9th postnatal day (P9) to P21. Retinal ROS production was semi-quantitatively measured using dihydroethidium fluorescence. Mice were treated with intraperitoneal injections of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) at a dose of 275 mg/kg body weight, and PBS as the control from P3 to P8.

RESULTS

Rd1 mice showed retardation of retinal vascular development in the deep layer at P9. No significant difference was observed in the outer nuclear layer thickness of rd1 and WT mice. ROS production in the outer nuclear layer of rd1 mice was significantly higher than that in the outer nuclear layer of WT mice at P9, P13, and P17 (P < .05). TEMPOL facilitated the development of the deep vascular layer when compared with injection of PBS.

CONCLUSIONS

Retardation of retinal vascular development is observed in rd1 mice; ROS is partially responsible for this finding. When using rd1 mice, we should be aware of this difference in comparison to other retinal degeneration animal models and human pathophysiological changes.

Modulation of CEST images in vivo by T1 relaxation: a new approach in the design of responsive PARACEST agents

A novel approach for the design of responsive paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance imaging (MRI) agents has been developed where the signal is "turned on" by altering the longitudinal relaxation time (T1) of bulk water protons. To demonstrate this approach, a model Eu(DOTA-tetraamide) complex (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) containing two nitroxide free radical units was synthesized. The nitroxide groups substantially shortened the T1 of the bulk water protons which, in turn, resulted in quenching of the CEST signal. Reduction of paramagnetic nitroxide moieties to a diamagnetic species resulted in the appearance of CEST. The modulation of CEST by T1 relaxation provides a new platform for designing biologically responsive MRI agents.

The carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity is inhibited by the radical scavenger tempol

The nitroxide tempol inhibited the carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity. Tempol acted by scavenging the produced carbonate radical and by recombining with hSOD1-Trp32• radicals as indicated by MS/MS evidence.

Loss of TRPM2 function protects against irradiation-induced salivary gland dysfunction

Xerostomia as a result of salivary gland damage is a permanent and debilitating side effect of radiotherapy for head and neck cancers. Effective treatments for protecting, or restoring, salivary gland function are not available. Here we report that irradiation treatment leads to activation of the calcium-permeable channel, transient potential melastatin-like 2 (TRPM2), via stimulation of poly-ADP-ribose polymerase. Importantly, irradiation induced an irreversible loss of salivary gland fluid secretion in TRPM2+/+ mice while a transient loss was seen in TRPM2-/- mice with >60% recovery by 30 days after irradiation. Treatment of TRPM2+/+ mice with the free radical scavenger Tempol or the PARP1 inhibitor 3-aminobenzamide attenuated irradiation-induced activation of TRPM2 and induced significant recovery of salivary fluid secretion. Furthermore, TPL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) induced complete recovery of function in irradiated TRPM2-/- mice. These novel data demonstrate that TRPM2 is activated by irradiation, via PARP1 activation, and contributes to irreversible loss of salivary gland function.

Hyperpolarized 13C metabolic imaging using dissolution dynamic nuclear polarization

This article describes the basic physics of dissolution dynamic nuclear polarization (dissolution-DNP), and the impact of the resulting highly nonequilibrium spin states, on the physics of magnetic resonance imaging (MRI) detection. The hardware requirements for clinical translation of this technology are also presented. For studies that allow the use of externally administered agents, hyperpolarization offers a way to overcome normal magnetic resonance sensitivity limitations, at least for a brief T(1)-dependent observation window. A 10,000-100,000-fold signal-to-noise advantage provides an avenue for real-time measurement of perfusion, metabolite transport, exchange, and metabolism. The principles behind these measurements, as well as the choice of agent, and progress toward the application of hyperpolarized (13)C metabolic imaging in oncology, cardiology, and neurology are reviewed.

A mitochondria-targeted macrocyclic Mn(II) superoxide dismutase mimetic

Superoxide (O(2)(·-)) is the proximal mitochondrial reactive oxygen species underlying pathology and redox signaling. This central role prioritizes development of a mitochondria-targeted reagent selective for controlling O(2)(·-). We have conjugated a mitochondria-targeting triphenylphosphonium (TPP) cation to a O(2)(·-)-selective pentaaza macrocyclic Mn(II) superoxide dismutase (SOD) mimetic to make MitoSOD, a mitochondria-targeted SOD mimetic. MitoSOD showed rapid and extensive membrane potential-dependent uptake into mitochondria without loss of Mn and retained SOD activity. Pulse radiolysis measurements confirmed that MitoSOD was a very effective catalytic SOD mimetic. MitoSOD also catalyzes the ascorbate-dependent reduction of O(2)(·-). The combination of mitochondrial uptake and O(2)(·-) scavenging by MitoSOD decreased inactivation of the matrix enzyme aconitase caused by O(2)(·-). MitoSOD is an effective mitochondria-targeted macrocyclic SOD mimetic that selectively protects mitochondria from O(2)(·-) damage.

Nitroxide TEMPO: a genotoxic and oxidative stress inducer in cultured cells

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) is a low molecular weight nitroxide and stable free radical. In this study, we investigated the cytotoxicity and genotoxicity of TEMPO in mammalian cells using the mouse lymphoma assay (MLA) and in vitro micronucleus assay. In the absence of metabolic activation (S9), 3mM TEMPO produced significant cytotoxicity and marginal mutagenicity in the MLA; in the presence of S9, treatment of mouse lymphoma cells with 1-2mM TEMPO resulted in dose-dependent decreases of the relative total growth and increases in mutant frequency. Treatment of TK6 human lymphoblastoid cells with 0.9-2.3mM TEMPO increased the frequency of both micronuclei (a marker for clastogenicity) and hypodiploid nuclei (a marker of aneugenicity) in a dose-dependent manner; greater responses were produced in the presence of S9. Within the dose range tested, TEMPO induced reactive oxygen species and decreased glutathione levels in mouse lymphoma cells. In addition, the majority of TEMPO-induced mutants had loss of heterozygosity at the Tk locus, with allele loss of ⩽34Mbp. These results indicate that TEMPO is mutagenic in the MLA and induces micronuclei and hypodiploid nuclei in TK6 cells. Oxidative stress may account for part of the genotoxicity induced by TEMPO in both cell lines.

Outcome of periapical lesions in a rat model of type 2 diabetes: refractoriness to systemic antioxidant therapy

INTRODUCTION

This study evaluated the development of periapical lesions in a rat model of type 2 diabetes and assessed the potential actions of the antioxidant agent tempol in this model.

METHODS

Male Wistar rats were used; they received tap water (N = 5) or a 20% glucose solution (N = 15) during a period of 9 weeks. At the sixth week, periapical lesions were induced on the first mandibular molars, and the animals were subdivided into 4 groups. The subgroup 1 was composed of nondiabetic rats orally receiving saline solution (10 mL/kg). Chronically glucose-fed rats were divided into the following subgroups: (2) saline-treated animals (10 mL/kg by oral route), and animals treated with tempol by gavage at doses of (3) 50 mg/kg or (4) 100 mg/kg. The body weight was monitored thoroughly. After 21 days of apical periodontitis induction, the animals were killed, and the mandibles were collected and submitted to radiographic and histologic analysis. The livers were collected to determine free radicals, and the blood plasma was used to measure insulin levels.

RESULTS

Type 2 diabetic rats displayed a significant decrease of body weight gain and a slight increase of insulin levels, which were allied to reduced levels of the antioxidant components catalase and reduced glutathione; these alterations were reversed by tempol. Concerning the periapical lesions, neither radiographic nor histologic analysis revealed any significant difference between control and type 2 diabetic rats. In diabetic rats, the apical periodontitis was refractory to tempol treatment.

CONCLUSIONS

The extent and cellularity of periapical lesions in glucose-fed type 2 diabetic rats were similar to those seen in control rats. Despite affecting other parameters related to diabetes, tempol failed to improve the outcome of endodontic lesions in type 2 diabetic animals.

Tempol moderately extends survival in a hSOD1(G93A) ALS rat model by inhibiting neuronal cell loss, oxidative damage and levels of non-native hSOD1(G93A) forms

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive dysfunction and death of motor neurons by mechanisms that remain unclear. Evidence indicates that oxidative mechanisms contribute to ALS pathology, but classical antioxidants have not performed well in clinical trials. Cyclic nitroxides are an alternative worth exploring because they are multifunctional antioxidants that display low toxicity in vivo. Here, we examine the effects of the cyclic nitroxide tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) on ALS onset and progression in transgenic female rats over-expressing the mutant hSOD1(G93A) . Starting at 7 weeks of age, a high dose of tempol (155 mg/day/rat) in the rat´s drinking water had marginal effects on the disease onset but decelerated disease progression and extended survival by 9 days. In addition, tempol protected spinal cord tissues as monitored by the number of neuronal cells, and the reducing capability and levels of carbonylated proteins and non-native hSOD1 forms in spinal cord homogenates. Intraperitoneal tempol (26 mg/rat, 3 times/week) extended survival by 17 days. This group of rats, however, diverted to a decelerated disease progression. Therefore, it was inconclusive whether the higher protective effect of the lower i.p. dose was due to higher tempol bioavailability, decelerated disease development or both. Collectively, the results show that tempol moderately extends the survival of ALS rats while protecting their cellular and molecular structures against damage. Thus, the results provide proof that cyclic nitroxides are alternatives worth to be further tested in animal models of ALS.

Stable isotope- and mass spectrometry-based metabolomics as tools in drug metabolism: a study expanding tempol pharmacology

The application of mass spectrometry-based metabolomics in the field of drug metabolism has yielded important insights not only into the metabolic routes of drugs but has provided unbiased, global perspectives of the endogenous metabolome that can be useful for identifying biomarkers associated with mechanism of action, efficacy, and toxicity. In this report, a stable isotope- and mass spectrometry-based metabolomics approach that captures both drug metabolism and changes in the endogenous metabolome in a single experiment is described. Here the antioxidant drug tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) was chosen because its mechanism of action is not completely understood and its metabolic fate has not been studied extensively. Furthermore, its small size (MW = 172.2) and chemical composition (C(9)H(18)NO(2)) make it challenging to distinguish from endogenous metabolites. In this study, mice were dosed with tempol or deuterated tempol (C(9)D(17)HNO(2)) and their urine was profiled using ultraperformance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry. Principal component analysis of the urinary metabolomics data generated a Y-shaped scatter plot containing drug metabolites (protonated and deuterated) that were clearly distinct from the endogenous metabolites. Ten tempol drug metabolites, including eight novel metabolites, were identified. Phase II metabolism was the major metabolic pathway of tempol in vivo, including glucuronidation and glucosidation. Urinary endogenous metabolites significantly elevated by tempol treatment included 2,8-dihydroxyquinoline (8.0-fold, P < 0.05) and 2,8-dihydroxyquinoline-β-d-glucuronide (6.8-fold, P < 0.05). Urinary endogenous metabolites significantly attenuated by tempol treatment including pantothenic acid (1.3-fold, P < 0.05) and isobutrylcarnitine (5.3-fold, P < 0.01). This study underscores the power of a stable isotope- and mass spectrometry-based metabolomics in expanding the view of drug pharmacology.

Synthesis and mechanistic studies of novel spin-labeled combretastatin derivatives as potential antineoplastic agents

Two series (14a-d and 21a-h) of novel spin-labeled combretastatin derivatives were synthesized and evaluated for cytotoxicity against four tumor cell lines (K562, SGC-7901, Hela and HepG-2). Simultaneously, a representative compound 21a was selected to investigate the antitumor mechanisms of these synthetic compounds. The results indicated that some of the compounds showed significant cytotoxicity against four tumor cell lines in vitro and were more active than etoposide, a clinically available anticancer drug. Among the newly synthesized compounds, 21a, 21b and 21c displayed the greatest cytotoxicity against three tested tumor cell lines (HEPG-2, BGC-832 and Hela), with IC(50) values ranging from 0.15 to 1.05 μM, compared with values of 0.014-0.403 μM for 3-amino-deoxycombretastatin A-4 (3). In addition, the mechanistic analysis revealed that compound 21a effectively interfered with tubulin dynamics to prevent mitosis in cancer cells, leading to cell cycle arrest and, eventually, dose dependent apoptosis.

Hepatic reduction of carbamoyl-PROXYL in ferric nitrilotriacetate induced iron overloaded mice: an in vivo ESR study

Reduction of a nitroxyl radical, carbamoyl-PROXYL in association of free radical production and hepatic glutathione (GSH) was investigated in iron overloaded mice using an in vivo L-band electron spin resonance (ESR) spectrometer. Significant increases in hepatic iron, lipid peroxidation and decrease in hepatic GSH were observed in mice intraperitoneally (i.p.) administrated with ferric nitrilotriacetate (Fe(III)-NTA, a total 45 µmol/mouse over a period of 3 weeks). Free radical production in iron overloaded mice was evidenced by significantly enhanced rate constant of ESR signal decay of carbamoyl-PROXYL, which was slightly reduced by treatment with iron chelator, deferoxamine. Moreover, the rate constant of ESR signal decay was negatively correlated with hepatic GSH level (r=-0.586, p<0.001). On the other hand, hepatic GSH-depletion (>80%) in mice through daily i.p. injection and drinking water supplementation of L-buthionine-[S,R]-sulfoximine (BSO) significantly retarded ESR signal decay, while there were no changes in serum aspartate aminotransferase and liver thiobarbituric acid-reactive substances levels. In conclusion, GSH plays two distinguish roles on ESR signal decay of carbamoyl-PROXYL, as an antioxidant and as a reducing agent, dependently on its concentration. Therefore, it should be taken into account in the interpretation of free radical production in each specific experimental setting.

Dual action molecules: bioassays of combined novel antioxidants and angiotensin II receptor antagonists

In this study we have investigated the in vitro angiotensin II receptor antagonist and antioxidant activity of a series of compounds in which the antioxidant pharmacophores (selenium, phenol, benzothiophene, ebselen or nitroxide) have been incorporated into the AT(1) receptor antagonist (sartan) milfasartan. Activity of these compounds was assessed in tissue-based assays. The novel molecules (30nM), nitrasartan or phenol-milfasartan, retained AT(1) receptor antagonist potency in rat isolated right atria. Antioxidant capacity of the substituted sartans was examined in an AAPH (2,2'-azobis (2-amidinopropane) hydrochloride)-induced haemolysis assay (mouse C57/BL6 isolated erythrocytes). Each of the antioxidant pharmacophores (10μM), except benzothiophene, protected against radical-mediated lysis. Of the novel sartans, only analogues incorporating selenium, phenol or nitroxide (nitrasartan) protected against radical-induced haemolysis. In the tissue-based assay using mouse isolated paced left atria, the free radical generator doxorubicin (30μM) resulted in a decrease in left atrial force over 90min. In this assay the phenol, nitroxide or ebselen antioxidant pharmacophores protected against doxorubicin-induced negative inotropy but selenocystine and benzothiophene did not. Nitrasartan (10μM) was the only novel analogue to protect against radical-induced negative inotropy. Nitrasartan also antagonised angiotensin II responses and decreased superoxide production in a concentration-dependent manner in rat isolated carotid arteries and aortae, respectively. In conclusion, nitrasartan is a dual action molecule demonstrating both AT(1) receptor antagonist potency and antioxidant properties in vitro.

Nitroxides attenuate carrageenan-induced inflammation in rat paws by reducing neutrophil infiltration and the resulting myeloperoxidase-mediated damage

Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) and other cyclic nitroxides have been shown to inhibit the chlorinating activity of myeloperoxidase (MPO) in vitro and in cells. To examine whether nitroxides inhibit MPO activity in vivo we selected acute carrageenan-induced inflammation on the rat paw as a model. Tempol and three more hydrophobic 4-substituted derivatives (4-azido, 4-benzenesulfonyl, and 4-(4-phenyl-1H-1,2,3-triazol-1-yl)) were synthesized, and their ability to inhibit the in vitro chlorinating activity of MPO and carrageenan-induced inflammation in rat paws was evaluated. All of the tested nitroxides inhibited the chlorinating activity of MPO in vitro with similar IC(50) values (between 1.5 and 1.8 μM). In vivo, the attenuation of carrageenan-induced inflammation showed some correlation with the lipophilicity of the nitroxide at early time points but the differences in the effects were small (<2-fold) compared with the differences in lipophilicity (>200-fold). No inhibition of MPO activity in vivo was evident because the levels of MPO activity in rat paws correlated with the levels of MPO protein. Likewise, paw edema, levels of nitrated and oxidized proteins, and levels of plasma exudation correlated with the levels of MPO protein in the paws of the animals that were untreated or treated with the nitroxides. The effects of the nitroxides in vivo were compared with those of 4-aminobenzoic hydrazide and of colchicine. Taken together, the results indicate that nitroxides attenuate carrageenan-induced inflammation mainly by reducing neutrophil migration and the resulting MPO-mediated damage. Accordingly, tempol was shown to inhibit rat neutrophil migration in vitro.

Potential implication of the chemical properties and bioactivity of nitrone spin traps for therapeutics

Nitrone therapeutics has been employed in the treatment of oxidative stress-related diseases such as neurodegeneration, cardiovascular disease and cancer. The nitrone-based compound NXY-059, which is the first drug to reach clinical trials for the treatment of acute ischemic stroke, has provided promise for the development of more robust pharmacological agents. However, the specific mechanism of nitrone bioactivity remains unclear. In this review, we present a variety of nitrone chemistry and biological activity that could be implicated for the nitrone's pharmacological activity. The chemistries of spin trapping and spin adduct reveal insights on the possible roles of nitrones for altering cellular redox status through radical scavenging or nitric oxide donation, and their biological effects are presented. An interdisciplinary approach towards the development of novel synthetic antioxidants with improved pharmacological properties encompassing theoretical, synthetic, biochemical and in vitro/in vivo studies is covered.

Heparin-polynitroxides: synthesis and preliminary evaluation as cardiovascular EPR/MR imaging probes and extracellular space-targeted antioxidants

We report here the synthesis of heparin-polynitroxide derivatives (HPNs) in which nitroxide moieties are linked either to uronic acid or glycosamine residues of the heparin macromolecule. HPNs have low anticoagulant activity, possess superoxide scavenging properties, bind to the vascular endothelium/extra-cellular matrix and can be detected by EPR and MRI techniques. As the vascular wall-targeted redox-active paramagnetic compounds, HPNs may have both diagnostic (molecular MRI) and therapeutic (ecSOD mimics) applications.

Oxidized phospholipids as biomarkers of tissue and cell damage with a focus on cardiolipin

Oxidized phospholipid species are important, biologically relevant, lipid signaling molecules that usually exist in low abundance in biological tissues. Along with their inherent stability issues, these oxidized lipids present themselves as a challenge in their detection and identification. Often times, oxidized lipid species can co-chromatograph with non-oxidized species making the detection of the former extremely difficult, even with the use of mass spectrometry. In this study, a normal-phase and reverse-phase two dimensional high performance liquid chromatography (HPLC)-mass spectrometric system was applied to separate oxidized phospholipids from their non-oxidized counterparts, allowing unambiguous detection in a total lipid extract. We have utilized bovine heart cardiolipin as well as commercially available tetralinoleoyl cardiolipin oxidized with cytochrome c (cyt c) and hydrogen peroxide as well as with lipoxygenase to test the separation power of the system. Our findings indicate that oxidized species of not only cardiolipin, but other phospholipid species, can be effectively separated from their non-oxidized counterparts in this two dimensional system. We utilized three types of biological tissues and oxidative insults, namely rotenone treatment of lymphocytes to induce mitochondrial damage and cell death, pulmonary inhalation exposure to single walled carbon nanotubes, as well as total body irradiation, in order to identify cardiolipin oxidation products, critical to the cell damage/cell death pathways in these tissues following cellular stress/injury. Our results indicate that selective cardiolipin (CL) oxidation is a result of a non-random free radical process. In addition, we assessed the ability of the system to identify CL oxidation products in the brain, a tissue known for its extreme complexity and diversity of CL species. The ability of the two dimensional HPLC-mass spectrometric system to detect and characterize oxidized lipid products will allow new studies to be formulated to probe the answers to biologically important questions with regard to oxidative lipidomics and cellular insult. This article is part of a Special Issue entitled: Oxidized phospholipids - their properties and interactions with proteins.

The antioxidant tempol reduces carcinogenesis and enhances survival in mice when administered after nonlethal total body radiation

There is significant interest in the development of agents that can ameliorate radiation damage after exposure to radiation has occurred. Here we report that chronic supplementation of the antioxidant Tempol in the diet of mice can reduce body weight without toxicity, decrease cancer, and extend survival when administered after nonlethal total body radiation (TBI). These effects were apparent in two different strains of mice (C3H, CBA) exposed to TBI (3 Gy). Notably, delaying administration of the Tempol diet one month after TBI could also enhance survival. Tempol reduced the incidence of hematopoietic neoplasms (lymphomas) in both strains, whereas both the onset and incidence of nonhematopoietic neoplasms were reduced in CBA mice. These results encourage further study of Tempol as a chemopreventive, to reduce the incidence of radiation-induced second malignancies after a course of definitive radiation therapy. Tempol may also find applications to reduce the risk of cancers in populations exposed to nonlethal radiation due to nuclear accidents or terrorist attacks.

Sonochemical degradation of cyclic nitroxides in aqueous solution

The sonochemical degradation of eight five- and six-membered nitroxides has been studied by EPR spectroscopy after exposure to ultrasound at a frequency of 354 kHz in argon-saturated aqueous solution. Concentration vs. time profiles do not follow a simple rate law. Octanol/water partition functions have been determined for all eight nitroxides, and an excellent linear correlation has been found between initial decomposition rates and hydrophobicity (log K(octanol/water)). Variation of initial rate with concentration was investigated for one compound (TEMPONE) and is largely consistent with an equilibrium distribution of substrate between bulk solution and the gas/liquid interface.

Measurement of deoxyinosine adduct: Can it be a reliable tool to assess oxidative or nitrosative DNA damage?

Adenosine deaminases (ADA) are key enzymes that deaminate adenosine (A) or deoxyadenosine (dA) and produce inosine or deoxyinosine (dI), respectively. While ADA only deaminates free dA, reactive nitrogen species (RNS) or reactive oxygen species (ROS) deaminate adenine base on the DNA and leave dI, which is a pre-mutagenic lesion. Therefore, dI adduct in the genomic DNA has been considered a biomarker of DNA damage caused by RNS or by ROS. In the presented study, genomic DNA was isolated from frozen calf thymus in low or room temperature, with or without an addition of antioxidant. The number of dI in the DNA was measured using liquid chromatography-tandem mass spectrometry. While low temperature (LT) work-up with an addition of antioxidant in reagents helped to prevent artifactual formation of oxidative DNA lesions in the calf thymus DNA (CTD), it also significantly inhibited activities of proteinase, which in turn resulted in significant ADA contamination in the final DNA samples. ADA remained in LT-CTD completely deaminated most dA when the DNA was subjected to enzymatic hydrolysis to single nucleosides. The ADA contamination in the DNA was significantly reduced when DNA was isolated from pre-isolated nuclear fraction rather than from entire tissue homogenates. However, enzymes used for DNA hydrolysis were confirmed to contain significant amounts of ADA. Therefore, these enzymes would increase deamination of dA during DNA hydrolysis. Artifactual dI production by contaminated ADA was dramatically reduced by an addition of EHNA (erythro-9-(2-hydroxy-3-nonyl)adenine), which is a potent inhibitor of ADA. However, time- and temperature-dependent dI production from dA in phosphate buffer solution was observed. More importantly, TEMPO, an antioxidant commonly used to prevent DNA oxidation, was found to deaminate dA independent to ADA. Overall, these findings indicate that assay methods measuring dI or other dA DNA adducts in genomic DNA should be carefully validated to minimize artificial errors caused by dA deamination. Recommendations to overcome those technical challenges were discussed in this presentation.

Unique oxidation of imidazolidine nitroxides by potassium ferricyanide: strategy for designing paramagnetic probes with enhanced sensitivity to oxidative stress

Potassium ferricyanide (PF), routinely employed for the oxidation of sterically-hindered hydroxylamines to nitroxides, is considered to be chemically inert towards the latter. In the present study, we report on an unexpected oxidative fragmentation of the imidazolidine nitroxides containing hydrogen atom in the 4-position of the heterocycle (HIMD) by PF resulting in the loss of the EPR signal. The mechanistic EPR, spectrophotometric, electrochemical and HPLC-MS studies support the assumption that the HIMD fragmentation is facilitated by the proton abstraction from the 4-position of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. Increase in steric hindrance around the radical fragment by introducing ethyl substituents decreased the rate of ascorbate-induced HIMD reduction by more than 20 times, but did not affect the rate of ferricyanide-induced HIMD oxidation. This preferential sensitivity of HIMDs to oxidative processes has been used to detect peroxyl radicals in the presence of high concentration of the reducing agent, ascorbate. HIMD-based EPR probes capable to discriminate oxidative and reductive processes might find application in biomedicine and related fields for monitoring the oxidative stress and reactive radical species in biological systems.

The heart-protective mechanism of nitronyl nitroxide radicals on murine viral myocarditis induced by CVB3

Our previous researches showed that nitronyl nitroxyl derivatives, NNP and NNVP were good anti-oxidants and provided radioprotective effects in C6 cells. The objective of the present study is to investigate the possible antiviral effects and underlying pharmacological of the two nitronyl nitroxide radicals against CVB3 in vitro and in vivo. The results showed that NNP and NNVP were some of the most potent compounds in terms of their antiviral effects by protecting myocardial cells against oxidative damage of free radicals. Treatment with NNP or NNVP could decrease the intracellular ROS level in vitro. They could lead to a significant decrease in activities of biochemical markers AST, CK and LDH in infected murine serum and could increase SOD and CAT activities and decreased MDA activities compared with infected control in vivo. NNP and NNVP could reduce NO production in infected mice by reacting with NO to produce the imino nitroxides which was confirmed by ESR spectrometry. In addition, NNP and NNVP could both decrease the mRNA expression of proinflammatory cytokines, TNF-α, IL-2 and IL-6. In conclusion, nitronyl nitroxide radicals NNP and NNVP were shown to have antiviral activities against CVB3 and they may represent potential therapeutic agents for viral myocarditis.