S-Nitrosoglutathione Inhibits Inducible Nitric Oxide Synthase Upregulation by Redox Posttranslational Modification in Experimental Diabetic Retinopathy

Abstract

Purpose.: Diabetic retinopathy (DR) is associated with nitrosative stress. The purpose of this study was to evaluate the beneficial effects of S-nitrosoglutathione (GSNO) eye drop treatment on an experimental model of DR.

Methods.: Diabetes (DM) was induced in spontaneously hypertensive rats (SHR). Treated animals received GSNO eye drop (900 nM or 10 μM) twice daily in both eyes for 20 days. The mechanisms of GSNO effects were evaluated in human RPE cell line (ARPE-19).

Results.: In animals with DM, GSNO decreased inducible nitric oxide synthase (iNOS) expression and prevented tyrosine nitration formation, ameliorating glial dysfunction measured with glial fibrillary acidic protein, resulting in improved retinal function. In contrast, in nondiabetic animals, GSNO induced oxidative/nitrosative stress in tissue resulting in impaired retinal function. Nitrosative stress was present markedly in the RPE layer accompanied by c-wave dysfunction. In vitro study showed that treatment with GSNO under high glucose condition counteracted nitrosative stress due to iNOS downregulation by S-glutathionylation, and not by prevention of decreased GSNO and reduced glutathione levels. This posttranslational modification probably was promoted by the release of oxidized glutathione through GSNO denitrosylation via GSNO-R. In contrast, in the normal glucose condition, GSNO treatment promoted nitrosative stress by NO formation.

Conclusions.: In this study, a new therapeutic modality (GSNO eye drop) targeting nitrosative stress by redox posttranslational modification of iNOS was efficient against early damage in the retina due to experimental DR. The present work showed the potential clinical implications of balancing the S-nitrosoglutathione/glutathione system in treating DR.

Introduction

Diabetic retinopathy (DR) is the leading cause of blindness and visual disability in working-age adults. ¹ The pathogenesis of DR is complex and multifactorial, and includes molecular alterations to reactive oxygen species (ROS) and reactive nitrogen species (RNS), elevated nitric oxide (NO) and superoxide production, expression of different isoforms of nitric oxide synthase (NOS), nitrate and polyADP-ribosylate proteins (PARP), and downregulation of antioxidant enzymes. Therefore, better understanding of these mechanisms is a valuable tool for the pharmacologic treatment of DR. ²  

The NO formed by constitutive endothelial NOS (eNOS) and neuronal NOS (nNOS) has an important role in regulating physiologic functions from the cardiovascular system to the central and peripheral nervous systems. However, NO produced by inducible NOS (iNOS) in excessive amounts for long periods of time promotes nitrosative stress and results in cytotoxicities, such as apoptosis, inhibition of mitochondrial respiration, regulation of oxidative phosphorylation, neurodegeneration, and circulatory failure. ^3–6 This can be achieved through reaction with superoxide anions to yield peroxynitrite, which can produce toxic hydroxyl radicals and promote oxidative injury via the formation of peroxynitrous acid, a reactive nitrogen-containing species. Endogenous NO is unstable, and some of its main biological actions are mediated through S-nitrosylation, ⁷ that is, the covalent incorporation of nitric oxide moiety into thiol groups (C-SH or R-SH) to form S-nitrosothiol (SNO). The S-nitrosylation promotes posttranslational modification of certain proteins and affects their activities, such as transcription factors, enzymes, and structural proteins. Thus, S-nitrosylation demonstrates action in vasodilation, inflammation, and neurodegeneration. ^8,9  

In the diabetic setting, the increase in NO production as a result of iNOS induction is associated with inflammatory responses and oxidative stress, in retinas of experimental models. ¹⁰ Studies by our group showed an increase in iNOS protein expression in the retinas of animals with short duration of experimental diabetes. ^11,12 Retinas from donors with diabetes (DM) and nonproliferative DR (NPDR) showed higher iNOS immunoreactivity localized on the inner nuclear layer, probably on Müller glial cells, compared to subjects without DM and without ocular disease. ¹³ In addition, NO stable end product concentrations (nitrites and nitrates) in the vitreous were significantly elevated in patients with proliferative DR (PDR) compared to the control group. ¹⁴ These data suggested that high concentrations of NO mainly produced by iNOS might contribute to the pathogenesis of DR. 

The existence of more stable transport forms of endogenous NO has been postulated in view of the increased half-life of NO in vivo. ¹⁵ Low-molecular-weight thiols, such as cysteine, reduced glutathione (GSH), and penicillamine, are prime candidates for such carrier molecules, and they can form SNO on reaction with nitrogen oxides. ¹⁶ S-nitrosoglutathione (GSNO) is formed by the S-nitrosylation reaction of NO with GSH in the extracellular setting. Also, GSNO is the most abundant endogenous SNO and the most important form of nitric oxide in vivo, due to its ability to modulate cellular signaling through posttranslational modifications of redox-sensitive proteins by S-nitrosylation and/or S-glutathionylation. The intracellular stability of GSNO is regulated by chemically-driven degradation reactions, thiol, and metal-mediated decomposition and enzymatic reactions. ^17–19 The main enzymatic-dependent degradation described is the reduction of GSNO to oxidized glutathione (GSSG) and ammonia (NH₃) by glutathione-dependent formaldehyde dehydrogenase (or alcohol dehydrogenase III); also called GSNO reductase (GSNO-R). This enzyme uses the reducing power of NADH to convert GSNO to glutathione S-hydroxysulfenamide (GSNHOH), which, in turn, is converted into GSSG. The GSNO-R turnover significantly influences the whole-cell level of S-nitrosation. ^20,21 Its relative redox activities depend on substrate concentrations of nicotinamide adenine dinucleotide and its reduced form (NAD⁺/NADH) ratio. 

Previous studies showed that GSNO administration provided protection in an experimental model of cerebral ischemia by downregulating the expression of iNOS and nuclear factor κB (NF-κB). ²² The therapeutic effects of GSNO have been demonstrated in experimental autoimmune uveitis, ²³ in which the oral administration of GSNO significantly suppressed the levels of inflammatory mediators associated with maintaining normal retinal histology and function. The RPE cells constitutes a site of immunosuppressive/inflammatory factor secretion inside the eye, ²⁴ such as iNOS and TNF-α. ^25,26 For this reason, human RPE cell line (ARPE-19) cells constitute an adequate model for assessing nitrosative stress in vitro. 

To our knowledge, no study has addressed the possible effects of GSNO in the development or progression of DR. Based on these observations, we hypothesized that diabetes increases iNOS protein expression and NO production, and that treatment with a GSNO compound could modulate iNOS expression/activity in the diabetic retina, and further determine the importance of the S-nitrosoglutathione/glutathione system in DR pathogenesis. The hypothesis was tested with an in vivo model of diabetes and through in vitro exposure of ARPE-19 cells to a high glucose (HG) condition. 

Materials and Methods

Animals Study

The animal study complied with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, in accordance with the local Committee for Ethics in Animal Research (1834-1/CEEA/IB/UNICAMP). Spontaneously hypertensive rats (SHR) 4 weeks old were provided by Taconic (Germantown, NY, USA) and bred in our animal facility. We have chosen to use SHR rats, because these animals display earlier and extensive retinal changes after streptozotocin (STZ, 50 mg/kg; Sigma-Aldrich, St. Louis, MO, USA) induction when compared to normotensive counterparts.^27,28 Enzymatic colorimetric GOD-PAP assay (Merck, Darmstadt, Germany) was used to measure plasma glucose levels 48 hours after the STZ or citrate buffer injection to confirm the induction; rats with plasma glucose values of ≥15 mM were considered diabetic for the present study. After the confirmation, the rats were randomized to be treated with either eye drop vehicle only, low-dose eye drop GSNO (900 nM), or high-dose eye drop GSNO (10 μM) twice daily in both eyes for 20 days. The preparation of GSNO eye drops was performed at the Chemistry Institute, State University of Campinas (UNICAMP). The GSNO was synthesized by the equimolar reaction between glutathione (GSH) and sodium nitrite, in the dark, and remained dormant for 40 minutes to allow complete nitrosation of thiol. After this period, it was precipitated with acetone, filtered, and lyophilized in the dark. To prepare the eye drops, the GSNO was dissolved in a phosphate buffer and added to a solution vehicle hydroxypropyl methylcellulose (HPMC) at a final concentration of 10 μm or 900 nm of GSNO and 2% (wt/wt) of HPMC. The basal and final systolic blood pressures (SBP) were obtained by noninvasive tail cuff blood pressure amplifier with a built-in automatic cuff pump (Model 229; IITC, Inc., Life Science, Woodland Hills, CA, USA). At the end of the experiment (20 days of treatment), the rats were submitted to full-flash electroretinography, blood collected for measurement of glycated hemoglobin (GHbA_1c) levels with colorimetric kit (Helena Glyco-Tek Affinity Column Method; Helena Laboratories, Beaumont, TX, USA), and then euthanized. Levels of NO estimated by Nitric Oxide Analyzer (NOA) method in aqueous humor and vitreous from control treated animals was performed to demonstrate the ability of GSNO in penetrating ocular tissues (see Supplementary Method and Fig. S1). 

ARPE-19 Cell Line Culture.

The ARPE-19 was obtained from the Federal University of Rio de Janeiro (RJCB Collection). Cells were cultured in Dulbecco's modified Eagle's medium and Ham's F12 (DMEM:F12) supplemented with 10% FBS and 1% penicillin/streptomycin. The ARPE-19 cell cultures were serum starved and then treated with normal glucose (NG), HG, with or without the following treatments. The cytotoxicity of the treatments with GSNO (from 10 μM to 10 nM) and NOS inhibitors (from 2 mM to 2 μM) after 24 hours in ARPE-19 cells was obtained by MTT cell viability assay. ²⁹ We considered no cytotoxicity if cell death was below 10% (data not shown). For NOS inhibitors, the cells were pretreated for one hour with a nonselective L-NAME (Sigma-Aldrich), and specific for iNOS, aminoguanidine (AG; Sigma-Aldrich) and N6-(1-iminoethyl)-lysine, hydrochloride (L-NIL; Cayman Chemical, Ann Arbor, MI, USA). 

Full-Flash Electroretinogram (ERG) Recording.

Retinal function was measured in SHR animals as described previously, with some modification. ³⁰ For retinal function analysis, we used −10 dB light stimulus for recordings of a- and b-waves, and 0 dB for c-wave in which better signal responses are evoked. 

Immunohistochemistry for Glial Fibrillary Acidic Protein (GFAP), Nitrotyrosine (NT), and Inducible Nitric Oxide Synthase (INOS) in Retinal Tissues and Immunofluorescence of Nitrotyrosine in ARPE-19 Cells.

Immunohistochemistry was performed as described previously by our group. ³¹ The retinal sections were incubated with goat polyclonal anti-GFAP (Santa Cruz Biotechnologies, Santa Cruz, CA, USA) or rabbit polyclonal anti-NT (Upstate Cell Signaling Solutions, Lake Placid, NY, USA) or rabbit polyclonal anti-iNOS (Santa Cruz Biotechnologies) overnight at 4°C. 

The immunofluorescence for ARPE-19 cells was performed as published previously. ³² Rabbit anti-NT (1:20) for overnight incubation at 4°C and secondary antibody Alexa 488 goat anti-rabbit (Invitrogen, San Diego, CA, USA) at 1:200 for 1 hour at room temperature were applied. 

Western Blotting Analysis for INOS or NF-κB in Whole Retinal Tissue and in ARPE-19 Cells.

The Western blotting was performed as described previously. ³¹ Membranes were incubated with rabbit polyclonal iNOS antibody (Cell Signaling Technology, Beverly, MA, USA) or rabbit polyclonal NFκB p65 (Santa Cruz Biotechnologies). 

Measuring Intracellular ROS Production in Cells by H₂DCFDA and NO^• Formation by DAF-2DA.

As previously published, ^31,33 we measured the total intracellular ROS production by 2′,7′-dichlorodihydrofluorescein diacetate (H₂DCFDA) and intracellular NO ^• levels by diaminofluorescein diacetate (DAF-2DA). 

Immunoprecipitation of GSNO-R and GSH/INOS.

The cells were lysed directly in a buffer containing 100 mM tris base, 10 mM sodium pyrophosphate, 100 mM sodium fluoride, 10 mM EDTA, 2 mM phenylmethylsulfonyl fluoride, 10 mM sodium ortovanadate, and 1% Triton X-100. Samples were incubated with rabbit anti-GSNO-R (ADH5 polyclonal antibody; Protein Tech Group, Chicago, IL, USA) or mouse monoclonal anti-glutathione (GSH) antibody (Virogen, Watertown, MA, USA) overnight, followed by the addition of protein A Sepharose for 1 hour. After centrifugation, the pellets were washed in buffer (100 mM Tris Base, 2 mM sodium ortovanadate, 1 mM EDTA, and 0.5% Triton X-100). The immune-precipitated samples were prepared under reducing or nonreducing conditions as necessary and loaded onto SDS polyacrylamide gels. The membranes were blocked in nonfat milk and incubated with anti-GSNO-R or rabbit polyclonal anti-iNOS (Cell Signaling Technology) and subsequently incubated with appropriate secondary antibodies. Equal loading and transfer were ascertained by ponceau for GSNO-R. 

Determination of Reduced Glutathione (GSH) Levels.

Retinal GSH level was measured using the method described by Beutler et al. ³⁴ with a few modifications. ²⁷  

Determination of GSNO by Ultra High-Performance Liquid Chromatography (UHPLC).

As described previously,³⁵ and with some adaptations, for the measurement of GSNO, the cells were washed with ice-cold PBS once before lysis with an extraction buffer (25 mM ammonia sulfamate dissolved in o-metaphosphoric acid 5%). The lysate was sonicated for 30 seconds. Samples were centrifuged, and the supernatant collected and filtrated. The GSNO measurement analyses were carried out on an Agilent 1290 Infinity UHPLC system (Agilent Technology, Waldbronn, Germany) by liquid chromatography with diode array detection (LC/DAD). Chromatographic separation was achieved on a 2.6 μm Kinetex-C18 column (50 × 2.1 mm; Phenomenex, Torrance, CA, USA), operating at 25°C. Mobile phases were constituted with 100% 20 mMKCl pH 2.5 (Ecibra, Curitiba, Brazil). The flow rate was 500 μL/min and injection volume was 3 μL. Chromatographic data were recorded and integrated using LCD ChemStation software (Agilent Technology). To confirm the GSNO identity in ARPE-19 cell samples, high-resolution electrospray ionization-MS analyses (MS/MS) were performed for GSNO in standard and in ARPE-19 cells exposed to normal glucose (see Supplementary Methods and Fig. S3). 

Statistical Analysis

The results were expressed as the means ± SD. The groups were compared by 1-way ANOVA, followed by the Fisher protected least-significant difference test. StatView statistics software (SAS Institute, Inc., Rockville, MD, USA) was used for all comparisons, with a significance value of P < 0.05. 

Results

In Vivo Study

The physiological characteristics of the study animals are shown in the Table. 

Table

 

Physiological Parameters of the Animals

Groups	Initial Body Weight, g	Final Body Weight, g	SBP, mm Hg	% HbA1c
CT, n = 5	170 ± 16	215 ± 18	191 ± 18	7.31 ± 0.44
CT-low dose, n = 6	178 ± 13	245 ± 10	184 ± 5	7.59 ± 0.30
CT-high dose, n = 5	163 ± 14	240 ± 8	182 ± 4	7.69 ± 0.29
DM, n = 6	166 ± 22	127 ± 13*	185 ± 8	11.67 ± 0.77*
DM-low dose, n = 5	169 ± 9	123 ± 14*	188 ± 11	11.30 ± 1.69*
DM-high dose, n = 6	173 ± 16	136 ± 14*	182 ± 6	10.45 ± 0.73*

 The DM groups had lower final body weight and higher glycated hemoglobin levels compared to the CT animals (*P < 0.01). The treatment did not change any parameters. CT, nondiabetic rats treated with vehicle of eye drop; CT-low dose, nondiabetic rats treated with GSNO 900 nM; CT-high dose, nondiabetic rats treated with GSNO 10 μM; DM, diabetic rats treated with vehicle of eye drop; DM-low dose, diabetic rats treated with GSNO 900 nM; DM-high dose, diabetic rats treated with GSNO 10 μM; SBP, systolic blood pressure; % HbA_1c, percentage of hemoglobin A_1c glycated.

GSNO Eye Drops Prevented DM Retinal Function Impairment and Early Markers of DR.

A significant retinal function impairment was observed in b-waves among control (CT) high-dose and DM rats compared to those treated with vehicle (P ≤ 0.02, Fig. 1B). To assess the RPE function, we acquired c-wave responses (Fig. 1C), and also a significant decrease in c-wave amplitude in the CT high-dose and DM rats compared to the CT group (P ≤ 0.02) was observed. Both doses of GSNO eye drops prevented this impairment in DM animals compared to the nontreated DM and CT high-dose groups (P < 0.01, Fig. 1D).To evaluate early structural marker of DR, we assessed GFAP immunoreactivity (Fig. 2A). There was a clear increase in retinal GFAP positivity in the DM rats in all layers of the retina compared to the CT group (P = 0.02). The treatment with GSNO in both doses significantly decreased the GFAP expression in the DM groups (P < 0.02; Figs. 2A, 2B). 

Figure 1

 

Retinal function evaluated by electroretinography. (A) Representative waveforms of the a- and b-waves in the CT and DM groups, which corresponds to the photoreceptor and inner retinal cell responses, respectively, in response to light stimulus intensity at −10 dB. (B) The error bars represent the mean ± SD of the b-wave amplitude in μvolts. *P ≤ 0.02 versus CT group; †P ≤ 0.05 versus DM and CT high-dose group. (C) Waveforms of c-waves in normal and diabetic groups in response to light stimulus intensity at 0 dB. (D) The error bars represent the mean ± SD of the c-wave amplitude in μvolts. *P ≤ 0.02 versus CT group; †P ≥ 0.01 versus DM and CT high-dose group. There was no significant difference in a-waves and implicit b-wave time between the studied groups (data not shown).

Figure 2

 

Early marker of diabetic retinopathy and nitrosative stress of the studied groups. (A) Photomicrograph representing immunolocalization of glial fibrillary acidic protein (GFAP) on retinal tissue. The GFAP in retinal tissue sections (5 μm) is shown in brown color (magnification ×400). (B) Error bars represent the mean ± SD of GFAP positivity analyses. The percentage of positivity per retinal field (mm²) was transformed to changes/fold in relation to the media of control in each experiment to compare independent experiments.*P = 0.02 versus CT group; †P ≤ 0.05versus DM group. The treatment in the CT low- and high-dose groups did not alter GFAP immunoreactivity compared to the CT group (P = 0.2). (C) Representative photomicrograph of NT immunoreactivity. The presence of NT is shown in brown on retinal tissue sections (5 μm) (magnification ×400). The positivity was widely expressed among all retinal layers, and especially in RPE. (D) The error bars represent the mean ± SD of the percentage of positivity per retinal field (mm²). *P ≤ 0.03 versus CT group; †P ≤ 0.03 versus DM group. At least 3 independent experiments were performed for each assay. RCL, rods and cones layer; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer.

GSNO Eye Drop Reestablished the Nitrosative Status in Retinas of DM Rats.

The nitrosative stress was estimated by nitrotyrosine expression, a product of tyrosine nitration mediated by RNS, such as peroxynitrite anion and nitrogen dioxide (Fig. 2C). We observed a GSNO-induced increase in NT expression in the CT-treated groups (P ≤ 0.03 versus the CT group). The DM group also presented higher levels of NT compared to the CT group (P = 0.007). Treatment with GSNO in the DM groups prevented the increase in NT production (P ≤ 0.03; Figs. 2C, 2D). 

To better understand the nitrosative stress mechanisms involved in producing NT in retinal tissue among different conditions, we evaluated the expression of iNOS (Fig. 3A). There was no difference in iNOS expression among the CT low- and high-dose compared to the CT group (P ≥ 0.1). Among the DM rats, there was a significant increase in iNOS expression compared to the CT groups (P = 0.03) and it was observed as an effective prevention in the DM high-dose group (P = 0.04; Figs. 3A, 3B). Immunoreactivity for iNOS localization showed diffuse positivity among all retinal layers, markedly in the RPE layer in the DM compared to the CT groups (P = 0.0005); in the DM high-dose group, the iNOS upregulation was prevented (P = 0.0004; Figs. 3C, 3D). 

Figure 3

 

Evaluation of nitrosative stress of studied groups. (A) Western blot for iNOS expression in total retinal lysate. (B) Equal loading and transfer were ascertained by reprobing the membranes for β-actin. The error bars represent mean ±SD of band densities expressed in arbitrary units of densitometry. *P = 0.03 versus CT group; †P = 0.04 versus DM group. (C) Representative photomicrograph of iNOS immunoreactivity and localization on retinal tissue (magnification ×400). The presence of iNOS is shown in brown in all layers of the retina especially in RPE layer. (D) The error bars represent the mean ± SD of the percentage of positivity per retinal field (mm²). *P = 0.0005 versus CT group; †P = 0.0004 versus DM group. At least 3 independent experiments were performed for each assay.

Collectively, these data suggested that the nitrosative stress observed in the CT animals treated with eye drops was GSNO-mediated. However, in the DM animals the nitrosative stress was associated with iNOS upregulation. The following in vitro experiments were designed to better understand the different effects of GSNO treatment eye drop in CT and DM animals. To assess whether GSNO inhibits NF-κB in retinal tissue, we measured the expression of retinal NF-κB (see Supplementary Fig. S2). There was no difference among the studied groups. This evidence support that the protective effect of GSNO demonstrated in diabetic retinal is not through NF-κB inhibition. 

In Vitro Study

To better understand the role of the S-nitrosoglutathione/glutathione system in the diabetic setting, we conducted experiments with ARPE-19 cells, since RPE immunoreactivity for iNOS was highly expressed in diabetic tissue. 

GSNO Counteracted the Upregulation of ROS and RNS in Cells Exposed to HG, but Promoted Nitrosative Stress in NG.

We observed an increase in total ROS levels in response to HG compared to the NG condition (P = 0.03). The treatments with HG plus GSNO in nanomolar concentrations were effective in counteracting the upregulation of ROS production (P ≤ 0.02), but not in micromolar concentrations (P = 0.09; Figs. 4A, 4C). Since nanomolar concentrations of GSNO were more efficient in protecting the cells against increased ROS production, the subsequent experiments were conducted only at 1 and 100 nM of GSNO. At NG condition, treatment with GSNO did not alter the levels of ROS production compared to NG (P = 0.3; Figs. 4B, 4C). 

Figure 4

 

Total intracellular ROS production in ARPE-19 cells. Total ROS production was obtained by H₂DCFDA fluorescence. The ARPE-19 cell cultures at 80% of confluence were serum starved, then exposed to NG 5.5 mM; to NG+GSNO at 1 nM to 100 μM; to HG 30 mM; and to HG+GSNO at 1 nM–100 μM for 24 hours. NG + 24.5 mM of mannitol was used as an osmotic control. (A) Error bars represent the mean ± SD of fluorescence units obtained in ELISA reader and corrected by the number of cells at the end of each treatment. Mannitol was used for an osmotic control in this experiment to see if there is some effect of osmolarity. *P = 0.03 versus NG; †P ≤ 0.02 versus HG group. (B) Total ROS production under NG condition. (C) Representative photomicrographs of qualitative H₂DCFDA assay indicating the levels of total ROS production in ARPE-19 cells using fluorescence microscope (Zeiss Axio Observer.A1 Inverted; Carl Zeiss Meditec, Jena, Germany).

As detected in ROS production, the intracellular NO^• production was increased in HG compared to the NG condition (P = 0.0007); both doses of GSNO treatments prevented this increase in diabetic milieu condition (P ≤ 0.008, Fig. 5A). 

Figure 5

 

Intracellular NO^• production and iNOS expression in ARPE-19 cells. ARPE-19 cell cultures at 80% of confluence were serum starved, then exposed to NG 5.5 mM; to NG+GSNO at 100 nM; to HG 30 mM; and to HG+GSNO at 1 nM and 100 nM for 24 hours. NG + 24.5 mM of mannitol was used as an osmotic control. (A) Detection of intracellular NO^• by DAF-2DA fluorescence. Error bars represent the mean ± SD of the fluorescence units obtained via ELISA reader corrected by the number of cells at the end of each treatment. *P = 0.0007 versus NG condition; †P ≤ 0.008 versus HG treatment. (B) NO^• production in the presence of total NOS (L-NAME) and specific for iNOS (AG and L-NIL) inhibitors in cell culture. *P < 0.0001 versus NG; †P < 0.0001 versus HG. The mannitol treatment did not change the levels of NO^• production (P = 0.8). (C) Western blot for iNOS expression on total cell lysate. Exposed films were scanned with a densitometer (Bio-Rad Laboratories, Inc., Hercules, CA) and analyzed quantitatively with Multi-Analyst Macintosh Software for Image Analysis Systems. Equal loading and transfer were ascertained by reprobing the membranes for β-actin. The error bars represent mean ± SD of band densities expressed in arbitrary densitometric units. *P = 0.02 versus NG condition; †P ≤ 0.009 versus HG condition. At least 3 independent experiments were performed for each assay.

To investigate which isoform of NOS is the main source of the observed increased NO^• under the DM setting conditions, the cells cultured in the HG conditions were treated with L-NAME, a nonselective NOS inhibitor, and with specific blockers for the iNOS isoform, AG and L-NIL. ³⁶ All treatments with AG or L-NAME similarly prevented the increase in NO^• production observed in the HG condition (P < 0.0001). The L-NIL at 500 μM and 2 mM concentrations also prevented these increases (P < 0.0001, Fig. 5B), suggesting that the main source of NO production under HG conditions is iNOS, since blocking all isoforms of NOS with L-NAME added no further effect to that observed with iNOS selective blockers. In agreement, GSNO treatments in NG conditions did not alter the iNOS expression compared to control NG (P = 0.7). In the HG condition, there was a significant increase in iNOS expression compared to the NG conditions (P = 0.02). Treatment with GSNO at 1 and 100 nM concentrations prevented this increment (P ≤ 0.009, Fig. 5C). 

To estimate the oxidative/nitrosative damage in ARPE-19 cells, we assessed NT (Fig. 6). Under NG conditions, the GSNO treatments (either 1 or 100 nM) did not significantly change the positivity of NT compared to NG (P > 0.05). However, in the highest dose of the GSNO treatment there was a strong tendency to increase compared to NG (P = 0.07), suggesting an action as a nitrosating inducer. Higher positivity was observed clearly in cells exposed to HG compared to the NG condition (P = 0.0006), and the presence of GSNO treatments counteracted this increase (P ≤ 0.003; Figs. 6A, 6B). 

Figure 6

 

Immunofluorescence of nitrotyrosine in ARPE-19 cell lines. ARPE-19 cell cultures at 80% of confluence were serum starved, then exposed to NG 5.5 mM; to NG+GSNO at 1 and 100 nM; to HG 30 mM; and to HG+GSNO at 1 and 100 nM for 24 hours. (A) Confocal images showing NT positivity and localization. The positivity of NT is shown in green (localized on the cytoplasm) and the nucleus is indicated with nuclear dye (DAPI) under a confocal laser scanning microscope (×630; Carl Zeiss Meditec). (B) The error bars represent the mean ± SD of the score of positivity, from 0 for no positivity to 4 for ≥80% of positivity by blindness. *P = 0.0006 versus NG treatment; †P ≤ 0.003 versus HG treatment. At least 3 independent experiments were performed.

These findings suggested that, under HG condition, there is an increase in RNS accompanied by upregulation of iNOS expression. The NO^• upregulation in HG was mediated by iNOS isoform and GSNO counteracted this effect. Based on this, these data indicated that GSNO under DM milieu is not a nitrosating agent but, instead, prevented RNS. 

The Dual Effect of GSNO.

To better understand whether GSNO itself can generate NO under NG conditions independently of iNOS, and whether in HG conditions GSNO can inhibit NO production via NOS system, we assessed NO^•, GSNO, and GSH levels under NG and HG conditions. 

In the NG+GSNO condition, the NO levels increased when compared to NG alone (P = 0.01). In the presence of iNOS inhibitor, the NO levels did not decrease compared to NG+GSNO (P = 0.2). These observations indicated that in NG, GSNO acts as an NO donor inducing nitrosative stress. Under the HG conditions, there was a marked increase in the NO levels compared to the NG condition (P = 0.001). In the presence of GSNO alone or associated with AG, we observed similar decreases in the NO^• intracellular levels compared to the HG conditions (P = 0.002). The combination of AG in the presence of GSNO treatment did not further decrease NO^• levels, demonstrating that in HG conditions GSNO counteracts NO^• upregulation through iNOS inhibition (Fig. 7A). 

Figure 7

 

Effects of GSNO under NG and HG conditions in ARPE-19 cells. ARPE-19 cell cultures at 80% of confluence were serum starved, then exposed to NG 5.5 mM; to NG+GSNO at 100 nM; to HG 30 mM; and to HG+GSNO at 100 nM for 24 hours. (A) Detection of intracellular NO^• by DAF-DA fluorescence method in cells exposed to NG and HG in the presence or absence of specific iNOS inhibitor (AG). Error bars represent the mean ± SD of the fluorescence units corrected by the cell number. *P ≥ 0.001 versus NG treatment; †P = 0.002 versus HG condition. (B) Measurement of GSH by colorimetric assay. Absorbance was read at 412 nm. GSH was used as an external standard for preparation of a standard curve. Error bars represent the mean ± SD of μmol of GSH corrected by protein cell lysate concentration (μg). *P < 0.04 versus NG; †P < 0.03 versus NG. (C) Measurement of endogenous GSNO by UHPLC method. Chromatography analyses of standard GSNO and levels of GSNO under different treatments, retention time = 0.6 minutes. Error bars represent the mean ± SD of μmol of GSNO levels corrected by protein cell lysate concentration (μg) under different conditions. *P ≤ 0.02 versus NG condition. At least 3 independent experiments were performed.

The redox state of the GSH/GSSG combination is an important indicator of the redox environment, ³⁷ and glutathione dysregulation is linked with the etiology and progression of human diseases. ³⁸ We quantified the GSH levels in ARPE-19 cells, and in the NG plus GSNO condition, we observed an increase in GSH levels compared to the NG condition (P = 0.04). In the HG condition, the GSH levels were lower when compared to NG (P = 0.04). The presence of GSNO in the HG conditions did not prevent this effect (P = 0.9, Fig. 7B). In the NG conditions, the presence of GSNO resulting in GSH increase might be due to an increase in its synthesis through increased expression of γ-glutamylcysteine synthetase ³⁹ promoted by NO^• generated by the GSNO+GSH = GSSG+NO^• reaction, ¹⁸ or might be metal-catalyzed ¹⁷ or thioredoxin-catalyzed ⁴⁰ degradation-dependent. 

We also evaluated the levels of endogenous GSNO in ARPE-19 cells with the UHPLC method. Curiously, in the NG condition treated with GSNO, the levels of GSNO decreased compared to the NG condition (P = 0.02). This intriguing observation indicated that exogenous GSNO is catalyzed rapidly by either by an enzymatic process or chemically reacting with thiol groups as GSH leading to total GSNO intracellular pool decreasing. The levels of GSNO are lower in the HG compared to NG condition (P = 0.005), and the treatment with GSNO under HG condition did not lead to an increase in GSNO levels (P = 0.2, Fig. 7C). In the HG condition, GSNO improved nitrosative stress, not through the reestablishment of GSH and/or GSNO levels. 

GSNO Decreases NO^• Levels in HG by S-Glutathionylation of INOS.

One possible mechanism by which GSNO displays different effects under NG or HG conditions is through GSNO-R, which reduces GSNO to GSSG. To address whether GSNO-R has a role in decreasing NO levels under the HG+GSNO treatment, we evaluated the expression of GSNO-R. There was no difference between NG and NG+GSNO treatment (P = 0.8); however, under HG, GSNO-R protein expression was markedly decreased (P = 0.05). The HG+GSNO treatment increased GSNO-R protein expression, but did not reach conventional statistical significance (P = 0.09, Fig. 8A). This increase in GSNO-R in cells exposed to HG treated with GSNO contributes to denitrosylation of GSNO leading to GSSG release. The decrease of endogenous GSNO under NG+GSNO treatment (Fig. 7C) also may be explained by the denitrosylation promoted by GSNO-R (Fig. 8A). 

Figure 8

 

GSNO promotes S-glutathionylation of iNOS. ARPE-19 cell cultures at 80% of confluence were serum starved, then exposed to NG 5.5 mM; to NG+GSNO at 100 nM; to HG 30 mM; and to HG+GSNO at 100 nM for 24 hours. (A) Immunoprecipitation of cell lysate with GSNO-R antibody incubated with GSNO-R antibody. The GSNO-R protein expression was measured by Western blot. Equal loading protein and transfer were confirmed by Ponceau. Error bars represent the mean ± SD expressed in arbitrary units of densitometry. *P = 0.05 versus NG. At least 3 independent experiments were performed for each assay. (B) Cell lysate was immunoprecipitated with the GSH protein complex antibody and immunoblotted against iNOS. The GSH-protein complexes were blotted against iNOS. Equal loading protein was ascertained by reprobing the membranes for total GSH complex proteins. (C) Controls for S-glutathionylation of iNOS were done by 0.5 mM GSSG (positive control) or GSSG plus the reduced agent DTT 0.25 mM (negative control) to reverse the reaction. The detection of S-glutathionylated iNOS at 130 kDa was present in cells exposed to HG treated with GSNO; in cells exposed to NG in the presence of GSSG, the expression of S-glutathionylated iNOS was equally increased as compared to HG plus GSNO, and DTT reversed this posttranslational iNOS modification.

It was demonstrated previously that S-glutathionylation of eNOS regulates its activity. ⁴¹ To investigate whether similar posttranslational modification could be involved in inhibition of iNOS by GSNO in HG conditions, we addressed the S-glutathionylation of iNOS. We observed that there was no expression of S-glutathionylated iNOS in NG alone or in NG+GSNO. Under HG conditions, we observed a faint signal that was markedly increased in the presence of GSNO. This finding indicates that GSNO treatment promotes S-glutathionylation of iNOS (Fig. 8B). To verify and confirm the specificity of iNOS glutathionylation immunoblotting, we treated the cells with GSSG 0.5 mM to induce S-glutathionylation or with the reducing agent dithiothreitol (DTT) 0.25 mM to reverse this reaction. ⁴¹ We observed that GSSG in NG conditions promoted S-glutathionylation of iNOS similar to that observed in the HG plus GSNO treatment; as expected, the presence of DTT reverses the S-glutathionylation of iNOS protein (Fig. 8C). Taken together, these results suggested that nitrosative stress was prevented by GSNO treatment through iNOS inhibition by S-glutathionylation. The posttranslational modification probably was promoted by the release of GSSG through GSNO denitrosylation via GSNO-R. In contrast, in the NG condition, GSNO treatment promoted nitrosative stress through NO formation. These findings showed the potential clinical implications of balancing the S-nitrosoglutathione/glutathione system in treating DR. 

Discussion

In this innovative study, we described that GSNO eye drop mitigated nitrosative stress and slowed the early structural changes present in the retina, thus improving retinal function in an experimental model of diabetes. Of interest, iNOS upregulation in the RPE layer in the DM animals may reveal the role of RPE in the pathogenesis of DR In the ARPE-19 cells exposed to HG, treatment with GSNO alleviated oxidative and nitrosative stress by decreasing iNOS protein expression, thus reducing intracellular NO^• levels. In addition, GSNO-R expression was improved. Therefore, posttranslational modification (S-glutathionylation) of iNOS by GSNO in ARPE-19 cells under HG condition suggested its inhibition. This evidence further explains the protective mechanism of GSNO. Rosenfeld et al. ⁴² already have identified a GSH binding site adjacent to the N-NO-pterin of iNOS. However, under NG conditions, where the intracellular GSH pool is high, the GSNO compound acts as a donor of free NO. 

Nitrosative stress is caused by overproduction of RNS. Diabetic stimuli may trigger generation of excess superoxide, which is converted rapidly to peroxynitrite (reaction of O₂ ^•- with NO^•), hydroxyl radicals (Fenton reaction or the iron-catalyzed Haber–Weiss reaction), and hydrogen peroxide (reaction catalyzed by superoxide dismutase). Peroxynitrite can modify tyrosine residues in proteins to form nitrotyrosine. Nitrotyrosine is a well-accepted indicator of RNS generation, and this stable end product is involved in inactivating mitochondrial and cytosolic proteins, resulting in damage to cellular constituents. Moreover, nitrotyrosine can initiate lipid peroxidation, increase DNA damage, deplete intracellular GSH levels, and induce overexpression of proinflammatory factors and adhesion molecules. Therefore, nitration is being increasingly proposed as a contributor to tissue injury in human diseases. ^43,44  

It is becoming increasingly clear that iNOS activity is induced in rats and humans with DR. ^13,45,46 Previous studies showed that iNOS^−/−or the inhibition of iNOS in the ischemic retina prevented angiogenesis locally in the avascular retina, mediated at least in part by downregulation of VEGF receptor 2 (VEGFR2). At the same time, pathological retinal neovascularization was considerably stronger in iNOS-expressing animals, showing that iNOS plays a crucial role in retinal neovascular disease. ⁴⁷ It also was shown that iNOS^−/− mice display protection from retinal cell apoptosis in an ischemic proliferative retinopathy model. ⁴⁸ Diabetic iNOS^−/−mice showed downregulation of several inflammatory factors, nitration of proteins, superoxide production, and leucostasis, thus preventing the formation of acellular capillaries and pericyte ghosts. ⁴⁹ Our data showed that the upregulation of iNOS expression in diabetes or HG conditions led to excessive NO generation. Furthermore, treatment with GSNO was highly associated with S-glutathionylation of iNOS, thus decreasing NO levels. When NO generation by iNOS is pharmacologically inhibited by the iNOS-specific inhibitor, AG, NO levels were significantly decreased in the presence or absence of GSNO. The use of AG in this present study used as a selective iNOS inhibitor aimed to assess the mechanism by which GSNO exerted the protective effects. By doing this, we could demonstrate that this protective mechanism is through iNOS inhibition. 

In NG situations, treatment with GSNO evoked an increase in nitrosative stress in vitro with higher levels of NO, although the GSH levels were upregulated in the ARPE-19 cells. Therefore, the presence of GSNO leads to GSH and NO production. This might be either spontaneous or metal-catalyzed, ¹⁷ or thioredoxin-catalyzed ⁴⁰ degradation-dependent. Under HG conditions, we observed an increase in oxidative/nitrosative stress and higher levels of NO^• accompanied by decreases in the GSH and GSNO levels. The low GSH content in these cells may contribute to these effects. Cells exposed to HG showed decreased levels of GSNO-R protein expression, and the supplementation of GSNO at 100 nM improved GSNO-R protein expression and possibly increased the denitrosylation of GSNO, leading to GSSG generation (Fig. 9). A previous work reported that GSSG induces S-glutathionylation of eNOS protein under oxidative stress, uncoupling it and, thus, altering its function, ⁴¹ and other investigators demonstrated that this process was described in streptozotocin-induced animals. ⁵⁰ The S-glutathionylation of NOS isoforms shows to be protective or not, dependent of the physiology function of the isoform. In this present work, S-glutathionylation of iNOS isoform is retinal protective. 

Figure 9

 

Schematic representation of possible mechanisms involved in nitrosative stress under normal or high glucose conditions in ARPE-19 cells. In NG+GSNO: The high content of intracellular GSH can react with exogenous GSNO generating NO, thus promoting nitrosative stress. The exogenous and/or endogenous GSNO may be converted by GSNO-R or metal or other enzyme-catalyzed, leading to low levels of GSNO. In HG condition: iNOS generates high levels ofNO accompanied by decrease of endogenous GSH, GSNO, and GSNO-R levels, resulting in nitrosative stress. The low content of GSH can explain the low levels of endogenous GSNO. In HG+GSNO: The effect of treatment was not due to endogenous GSH and GSNO reestablishment levels.GSNO-R expression is improved, which denitrosylates exogenous GSNO generating GSSG+NH₃ formation; GSSG S-glutathionylates iNOS, reducing NO production, thus, preventing nitrosative stress.

The GSNO-R acts only on GSNO, meaning that SNO proteins are not substrates, and it controls protein S-nitrosylation by influencing the cellular equilibrium between SNO proteins and GSNO. ⁵¹ Others enzymatic systems, such as human carbonyl reductase 1 (hCBR1), an NADPH-dependent short chain dehydrogenase/reductase, has been demonstrated to reduce GSNO. ⁵² Previous studies using GSNO-R^−/− mice showed increased levels of SNO proteins and decreased survival in mice when exposed to endotoxin, and these effects are attenuated by an inhibitor of iNOS. ⁵³ Subsequent studies demonstrated that GSNO-R deficiency is linked to S-nitrosylation of the DNA repair enzyme. ⁷ In our in vitro model we described decreased expression of GSNO-R accompanied by reduction of GSNO levels in HG condition. 

The present study showed, for the first time to our knowledge, the therapeutic effect of GSNO eye drop in counteracting nitrosative stress in an experimental model of DR, with consequent improved retinal function. The GSNO supplementation prevented nitrosative stress by reducing NO generation through iNOS inhibition by S-glutathionylation under diabetic milieu conditions. The regulation of the S-nitrosoglutathione/glutathione system with RNS-based signaling pathways might be potential therapeutic targets in ocular diabetic complications. 

Supplementary Materials

doc S1 

ppt S2 

ppt S3 

ppt S4 

Acknowledgments

The authors thank the staff of the Life Sciences Core Facility from the UNICAMP for support with confocal microscop, and the personnel from the Renal Pathophysiology Laboratory, FCM, UNICAMP, for their invaluable help with this work. 

Supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (Grants 2008/57560-0 and 2011/06719-1), and by a scholarship from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (MABR). 

Disclosure: M.A.B. Rosales, None; K.C. Silva, None; D.A. Duarte, None; M.G. de Oliveira, None; G.F.P. de Souza, None; R.R. Catharino, None; M.S. Ferreira, None; J.B. Lopes de Faria, None; J.M. Lopes de Faria, None