Mass spectrometry and 3-nitrotyrosine: strategies, controversies, and our current perspective

Posttranslational modification of Sirt6 activity by peroxynitrite

The mammalian sirtuin 6 (Sirt6) is a site-specific histone deacetylase that regulates chromatin structure and many fundamental biological processes. It inhibits endothelial cell senescence and inflammation, prevents development of cardiac hypertrophy and heart failure, modulates glucose metabolism, and represses tumor growth. The basic molecular mechanisms underlying regulation of Sirt6 enzymatic function are largely unknown. Here we hypothesized that Sirt6 function can be regulated via posttranslational modification, focusing on the role of peroxynitrite, one of the major reactive nitrogen species formed by excessive nitric oxide and superoxide generated during disease processes. We found that incubation of purified recombinant Sirt6 protein with 3-morpholinosydnonimine (SIN-1; a peroxynitrite donor that generates nitric oxide and superoxide simultaneously) increased Sirt6 tyrosine nitration and decreased its intrinsic catalytic activity. Similar results were observed in SIN-1-treated Sirt6, which was overexpressed in HEK293 cells, and in endogenous Sirt6 when human retinal microvascular endothelial cells were treated with SIN-1. To further investigate whether Sirt6 nitration occurs under pathological conditions, we determined Sirt6 nitration and activity in retina using a model of endotoxin-induced retinal inflammation. Our data showed that Sirt6 nitration was increased, whereas its activity was decreased, in this model. With mass spectrometry, we identified that tyrosine 257 in Sirt6 was nitrated after SIN-1 treatment. Mutation of tyrosine 257 to phenylalanine caused loss of Sirt6 activity and abolished SIN-1-induced nitration and decrease in its activity. Mass spectrometry analysis also revealed oxidation of methionine and tryptophan in Sirt6 after SIN-1 treatment. Our results demonstrate a novel regulatory mechanism controlling Sirt6 activity through reactive nitrogen species-mediated posttranslational modification under oxidative and nitrosative stress.

Loss of SOD3 (EcSOD) Expression Promotes an Aggressive Phenotype in Human Pancreatic Ductal Adenocarcinoma

PURPOSE

Pancreatic ductal adenocarcinoma (PDA) cells are known to produce excessive amounts of reactive oxygen species (ROS), particularly superoxide, which may contribute to the aggressive and refractory nature of this disease. Extracellular superoxide dismutase (EcSOD) is an antioxidant enzyme that catalyzes the dismutation of superoxide in the extracellular environment. This study tests the hypothesis that EcSOD modulates PDA growth and invasion by modifying the redox balance in PDA.

EXPERIMENTAL DESIGN

We evaluated the prognostic significance of EcSOD in a human tissue microarray (TMA) of patients with PDA. EcSOD overexpression was performed in PDA cell lines and animal models of disease. The impact of EcSOD on PDA cell lines was evaluated with Matrigel invasion in combination with a superoxide-specific SOD mimic and a nitric oxide synthase (NOS) inhibitor to determine the mechanism of action of EcSOD in PDA.

RESULTS

Loss of EcSOD expression is a common event in PDA, which correlated with worse disease biology. Overexpression of EcSOD in PDA cell lines resulted in decreased invasiveness that appeared to be related to reactions of superoxide with nitric oxide. Pancreatic cancer xenografts overexpressing EcSOD also demonstrated slower growth and peritoneal metastasis. Overexpression of EcSOD or treatment with a superoxide-specific SOD mimic caused significant decreases in PDA cell invasive capacity.

CONCLUSIONS

These results support the hypothesis that loss of EcSOD leads to increased reactions of superoxide with nitric oxide, which contributes to the invasive phenotype. These results allow for the speculation that superoxide dismutase mimetics might inhibit PDA progression in human clinical disease.

L-arginine/NO pathway is altered in children with haemolytic-uraemic syndrome (HUS)

The haemolytic uraemic syndrome (HUS) is the most frequent cause of acute renal failure in childhood. We investigated L-arginine/NO pathway in 12 children with typical HUS and 12 age-matched healthy control subjects. Nitrite and nitrate, the major NO metabolites in plasma and urine, asymmetric dimethylarginine (ADMA) in plasma and urine, and dimethylamine (DMA) in urine were determined by GC-MS and GC-MS/MS techniques. Urinary measurements were corrected for creatinine excretion. Plasma nitrate was significantly higher in HUS patients compared to healthy controls (P = 0.021), whereas urine nitrate was borderline lower in HUS patients compared to healthy controls (P = 0.24). ADMA plasma concentrations were insignificantly lower, but urine ADMA levels were significantly lower in the HUS patients (P = 0.019). Urinary DMA was not significantly elevated. In HUS patients, nitrate (R = 0.91) but not nitrite, L-arginine, or ADMA concentrations in plasma correlated with free haemoglobin concentration. Our results suggest that both NO production and ADMA synthesis are decreased in children with typical HUS. We hypothesize that in the circulation of children with HUS a vicious circle between the L-arginine/NO pathway and free haemoglobin-mediated oxidative stress exists. Disruption of this vicious circle by drugs that release NO and/or sulphydryl groups-containing drugs may offer new therapeutic options in HUS.