Nitrite inhalation in rats elevates tissue NOS III expression and alters tyrosine nitration and phosphorylation

«Poppers maculopathy?» in Spain. A new ophthalmological disease

INTRODUCTION

The term «poppers» refers to products made of volatile alkyl nitrites used for recreational practices.

CLINICAL CASE

40 year old man reported blurred vision in both eyes and photophobia, and admitted regular use of «poppers». Best corrected visual acuity was 0.63/1 in both eyes. Bilateral yellowish vitelliform macular lesions were present in both eyes in the funduscopy, as well as a disruption of the IS/OS line in the optical coherence tomography.

DISCUSSION

This is the first case of «poppers maculopathy» registered in Spain. Clinical findings and symptoms are similar to previous reported cases.

Histopathological changes in the liver of rabbits exposed to high nitrate ingestion in drinking water

OBJECTIVES

In India, especially in Rajasthan, people drink water which contains high level of nitrates and the possibility of finding concentrations of up to 500 mg of nitrate ions per litre of water is not unusual. Excessive use of nitrate fertilisers and herbicides results in accumulation of nitrate in plants and methemoglobinaemia in cattle as consequences of nitrate poisoning. The ingested nitrate is converted to nitrite in the digestive system and it is absorbed in blood, thus causing methemoglobinaemia. Methaemoglobinaemia is not restricted to infants alone, but it is prevalent in higher age groups also.

METHODS

Therefore, an experimental study was conducted on 10 rabbits which were between three and a half months to four months of age, which had weights which ranged from 1.310 kg to 1.720 kg. Five groups A, B, C,D and E were formed, with two rabbits in each group. The control Group A was given water orally, which had 45 mg/litres of nitrate. Groups B to E (experimental groups) were administered water orally, which had concentrations of 100mg/litre, 200mg/litre, 400mg/litre and 500mg/litre of nitrate respectively, for 120 days. During experimental period, the differences in general behaviour of rabbits were noted. After this, rabbits were anaesthetised and sacrificed according to guidelines of ICMR and their livers were removed and processed for making paraffin sections,.Hematoxyllin and eosin staining was done for microscopic observations.

RESULTS

During experimental period, the animals were found to be lethargic on 75(th) day. Quantity of intake of food and water was not altered in the rabbits which were undergoing experiments in different groups. Rabbits of all groups i.e. A to E showed a continuous increase in heart rate (up to 218/minute in Group E) and respiration rate (up to 84/minute in Group E) respectively. The microscopic study showed mild necrosis of hepatocytes, with infiltration of inflammatory cells in between hepatocytes. In higher groups, the liver showed bridging necrosis and portal triditis. Dilatations of central vein with eosinophilic degeneration were observed in Group E only.

Adverse ophthalmic reaction in poppers users: case series of 'poppers maculopathy'

BACKGROUND

Poppers are a recreational substance of abuse belonging to the alkyl nitrite family of compounds. In the United Kingdom, where they are legal to purchase but illegal to sell for human consumption, 10% of the general population have tried them. They are considered low risk to physical and mental health. Two recent case series from France demonstrated foveal pathology in individuals associated with poppers use.

METHOD

A case series of seven patients presenting to four hospitals in the United Kingdom with visual impairment and maculopathy associated with inhalation of poppers.

RESULTS

All patients experienced visual symptoms associated with poppers use. The majority had impaired visual acuity, central scotomata, distortion, or phosphenes. Clinical signs on fundoscopy ranged from normal foveal appearance to yellow, dome-shaped lesions at the foveola. Spectral domain optical coherence tomography (SD-OCT) showed varying degrees of disruption of the presumed inner segment/outer segment (IS/OS) junction.

DISCUSSION

Although poppers have been in use for several decades, in 2007, following legislative changes, there was a change in the most commonly used compound from isobutyl nitrite to isopropyl nitrite. There were no reports of 'poppers maculopathy' before this. Poppers maculopathy may be missed if patients are not directly questioned about their use. The disruption or loss of the presumed IS/OS junction on SD-OCT are a characteristic feature. Further study of maculopathy in poppers users is now needed. Raising public awareness of the ocular risks associated with their use may be necessary.

Protein tyrosine nitration: selectivity, physicochemical and biological consequences, denitration, and proteomics methods for the identification of tyrosine-nitrated proteins

Protein tyrosine nitration (PTN) is a post-translational modification occurring under the action of a nitrating agent. Tyrosine is modified in the 3-position of the phenolic ring through the addition of a nitro group (NO2). In the present article, we review the main nitration reactions and elucidate why nitration is not a random chemical process. The particular physical and chemical properties of 3-nitrotyrosine (e.g., pKa, spectrophotometric properties, reduction to aminotyrosine) will be discussed, and the biological consequences of PTN (e.g., modification of enzymatic activity, sensitivity to proteolytic degradation, impact on protein phosphorylation, immunogenicity and implication in disease) will be reviewed. Recent data indicate the possibility of an in vivo denitration process, which will be discussed with respect to the different reaction mechanisms that have been proposed. The second part of this review article focuses on analytical methods to determine this post-translational modification in complex proteomes, which remains a major challenge.

Inactivation of hepatic enzymes by inhalant nitrite--in vivo and in vitro studies

We examined the effects of acute isobutyl nitrite (ISBN) exposure on the activity of several hepatic enzymes. Two strains of adult male mice (Balb/c and C57BL/6) were exposed to 900 ppm ISBN or ambient air for 45 minutes. The enzyme activity of hepatic cytochrome P450 (CYP)-mediated deethylation, glutathione S-transferase (GST), and carboxylesterase (CBE) was monitored through the substrates 3-cyano-7-ethoxycoumarin (CEC), 1-chloro-2,4-dinitrobenzene, and p-nitrophenyl acetate, respectively. Acute ISBN exposure led to a significant reduction in hepatic CYP-mediated CEC deethylation, GST, and CBE activity in Balb/c mice (of 81.5%, 74.7%, and 25.2%, respectively, vs control mice, each at P < .05) when livers were harvested immediately after inhalant exposure. The corresponding decreases in C57BL/6 mice were smaller (with reductions of 21.8%, 18.8%, and 13.3%, respectively, each at P < .05). This enzyme activity, tested in C57BL/6 mice only, returned to control values after a 24-hour period of nonexposure. Follow-up mechanistic investigations using rat liver GST indicated that ISBN-mediated enzyme inactivation was not caused by its metabolites: inorganic nitrite ion (NO2-) or nitric oxide. This inactivation could be prevented, but not reversed, by added glutathione, suggesting irreversible protein oxidation. Using different NO donors as comparative agents, we found that GST inactivation by ISBN was not associated with protein S-nitrosylation or disulfide formation, but with tyrosine nitration. Inhalant nitrite exposure, therefore, led to a significant reduction in hepatic enzyme activity in mice, possibly through tyrosine nitration of hepatic proteins. This effect raises the possibility of drug-drug metabolic interactions from inhalant nitrite abuse. However, determining the applicability of these findings to humans will require further study.

Effects of repeated in vivo inhalant nitrite exposure on gene expression in mouse liver and lungs

Exposure to inhalant organic nitrites (drugs of abuse commonly known as "poppers") has been reported to enhance tumor growth in mice, but the mechanism is not fully defined. This study examined the effect of repeated in vivo nitrite exposures on gene expression in the mouse liver and lungs using a gene array panel of 94 cancer- and angiogenesis-related genes. Using 2-fold change as a threshold criterion, repeated nitrite exposure was found to alter the expression of 65 and 23 genes in the liver and lungs, respectively. Six genes were significantly upregulated (p<or=0.05), viz., those encoding VEGF, VEGFD (vascular endothelial growth factor A and D, respectively) in the lungs and FGF1, FGF4 (fibroblast growth factor 1 and 4, respectively), Hsp70 (heat shock 70kDa protein 4), and PF4 (platelet factor 4) in the liver. mRNA encoding HO-1 (heme oxygenase-1) and Smad7 were marginally (p=0.057) stimulated in the liver. Follow-up studies in the liver revealed significant nitrite-induced expression of VEGF protein and mRNA. Immuno-staining of liver slices revealed that the increased hepatic VEGF expression resided mainly in hepatocytes. Stimulation of hepatic VEGF expression by ISBN was not different in endothelial nitric oxide synthase (eNOS) knockout vs. wild-type mice. In conclusion, multiple exposures to inhalant nitrite appeared to cause alteration in the expression of a number of genes relating to cancer and angiogenesis, including VEGF. eNOS presence did not appear to be essential for nitrite-induced VEGF expression. These studies demonstrate that in vivo exposure to inhalant nitrites results in changes in the angiogenesis cascade.

Effects of flavonoids extracted from Scutellaria baicalensis Georgi on hemin–nitrite–H2O2 induced liver injury

Hemin-nitrite-H2O2 system may play a role in liver oxidative injury in some pathological events. In this paper, the effects of the three active components of the root of Scutellaria baicalensis Georgi, i.e. baicalin, baicalein and wogonin, on hemin-nitrite-H2O2 induced liver injury were studied in liver homogenate, liver microsome and human hepatoblastoma cell line HepG2 cells. It was found that hemin-nitrite-H2O2 could induce liver homogenate protein nitration, lipid peroxidation and liver microsome protein oxidation; it also caused a decrease of HepG2 cells viability. Baicalein, baicalin and wogonin could inhibit protein nitration and lipid peroxidation in liver homogenate as well as in HepG2 cells in a dose-dependent manner, the inhibition order was baicalein>baicalin>>wogonin. These three flavonoids also inhibited the oxidation of protein in liver microsome, the decrease of cell viability and the content of GSH in HepG2 cells, among which baicalin represented the most inhibitory effect. Besides, hemin-H2O2 induced cell injury could be augmented with the existence of nitrite, indicating protein nitration involved in hemin-nitrite-H2O2 induced liver injury. These results demonstrated hemin-nitrite-H2O2 could induce liver injury through oxidizing or nitrating different biomolecules. Baicalein, baicalin and wogonin could inhibit hemin-nitrite-H2O2 induced liver injury in dose-dependent manners by inhibiting oxidation and nitration.

Inhalant nitrite exposure alters mouse hepatic angiogenic gene expression

Inhalant nitrites are drugs of abuse that have been shown to enhance tumor growth rate in mice and are epidemiologically linked to an increased risk of Kaposi's sarcoma. Because nitrites produce nitric oxide, we hypothesized that their toxicological effects might be partly mediated via regulation of angiogenic factors such as vascular endothelial growth factor (VEGF). Preliminary studies showed that isobutyl nitrite (ISBN) incubation stimulated VEGF protein expression in J774 macrophage cells. C57BL/6 mice exposed to ISBN in air exhibited significant up-regulation of VEGF protein and mRNA in the liver, but not in the lung. Liver mRNA expression of VEGF receptor 2 (VEGFR-2), VEGFR-3, Smad5, and Smad7 was also significantly altered. These results demonstrate that in vivo exposure to an inhalant nitrite results in altered tissue expression of VEGF and its receptors, suggesting that some of its toxicological effects may be mediated partly through a mechanism involving angiogenesis.

Signal transduction by protein tyrosine nitration: competition or cooperation with tyrosine phosphorylation-dependent signaling events?

This review article is an attempt to stimulate a discussion on the significance of protein tyrosine nitration to cellular signaling and its relationships with protein tyrosine phosphorylation. Initially, it provides basic information on growth factor and oxidants as modulators/mediators of tyrosine phosphorylation-dependent signal transduction pathways. The effects of exogenous and endogenous tyrosine nitration on such pathways were examined by reviewing published and unpublished observations. From an initial perspective that tyrosine nitration was a toxic manifestation of nitric oxide, the concept evolved to a protein modification that could also function in cellular signaling events, possibly cooperating with tyrosine phosphorylation.

Cytotoxicity by nitrite inhalants is not related to peroxynitrite formation

Nitrite inhalant abuse has been correlated with HIV and Kaposi's sarcoma. Mouse models of inhalant exposure show immunosuppression and loss of immune cells. In the present study, isobutyl nitrite caused a dose-dependent loss of viability of a macrophage cell line. In the absence of cells, isobutyl nitrite reacted with hydrogen peroxide to form peroxynitrite. However, assays of mitochondrial respiration and nitration that detect peroxynitrite indicated that very little was present in cell cultures following exposure to the inhalants. Isobutyl, isoamyl, and butyl nitrites inhibited mitochondrial respiration, but only at high concentrations. Similarly, the nitrating activity of isobutyl nitrite occurred only at high concentrations and was not affected by the presence of hydrogen peroxide. Western blots showed that the inhalant did not increase nitrotyrosine formation in RAW cells or in peritoneal exudate macrophages (PEM) from exposed mice. Thus, the toxicity induced by isobutyl nitrite was probably not due to peroxynitrite formation.

Lack of critical involvement of endothelial nitric oxide synthase in vascular nitrate tolerance in mice

We examined the direct involvement of endothelial nitric oxide (eNOS) in nitrate tolerance using eNOS knockout (eNOS (-/-)) and wild-type (eNOS (+/+)) mice. Animals were treated with either nitroglycerin (NTG, 20 mg kg(-1)s.c. 3 x daily for 3 days) or vehicle (5% dextrose, D5W), and nitrate tolerance was assessed ex vivo in isolated aorta by vascular relaxation studies and cyclic GMP accumulation. Western blot was performed to determine NOS expression after NTG treatment. In both the eNOS (-/-) and (+/+) mice, the EC(50) from NTG concentration-response curve was increased by approximately 3 fold, and vascular cyclic GMP accumulation was similarly decreased after NTG pretreatment. Vascular tolerance did not lead to changes in eNOS protein expression in eNOS (+/+) mice. These results indicate that vascular nitrate tolerance was similarly induced in eNOS (-/-) and (+/+) mice, suggesting that eNOS may not be critically involved in nitrate tolerance development in mice.

Systemic biochemical effects of inhaled NO in rats: increased expressions of NOS III, nitrotyrosine-, and phosphotyrosine-immunoreactive proteins in liver and kidney tissues

Inhaled nitric oxide (iNO) has been shown to reduce pulmonary hypertension associated with several disease states. The effects of iNO are thought to be restricted to the pulmonary vasculature because of its rapid inactivation by hemoglobin. Recent data have suggested, however, that iNO can form nitrosothiols, which can be carried throughout the circulation, thus increasing the half life and bioactivity on NO. Other studies have shown that iNO can affect intestinal ischemia and renal hemodynamics. In this study, rats were exposed to 49 +/- 4 ppm or 107 +/- 13 ppm NO for 4 h and the lung, spleen, liver, and kidney tissues were removed and measured for NOS II and NOS III protein, nitrotyrosine (NT), and phosphotyrosine (PT) immunoreactivity. Following 107 ppm iNO, increases in NOS III protein expression, NT, and PT were observed in the liver and kidney, but not in the lung or spleen. No such increases were noted after the lower dose of iNO. These results paralleled those shown for isobutyl nitrite that we reported earlier and indicated that iNO can cause changes in protein chemistry in organs and tissues beyond the lungs. Since iNO produced little systemic hemodynamic effects, it is unlikely that the observed biochemical alterations were derived secondarily from physiological changes.