Nitrated Lipids in a Murine Model of Sickle Cell Disease at Baseline.

Introduction

Sickle cell disease (SCD) is increasingly recognized as a disorder of inflammatory homeostasis. One major focus in recent years has been understanding the role of Nitric Oxide (NO) in the pathophysiology of SCD. NO is a critical mediator of inflammatory pathways and current evidence supports the precept that NO bioavailability is decreased in SCD, resulting in normal concentrations of NO at baseline but an inability to increase NO during stress. Nitrated fatty acids, such as Nitrolinoleate (LNO₂), have recently been reported as potent and abundant anti-inflammatory signaling mediators with the ability to cause vasorelaxation and inhibition of platelet and neutrophil activation. Evidence supports their anti-inflammatory signaling is mediated through the release of NO and NO-related products. LNO₂ has not previously been described in patients with SCD and our objective was to quantify LNO₂ in a murine model of SCD at baseline.

Methods

Whole blood was obtained from transgenic sickle cell and wild type mice (n = 5 and 6, respectively). Blood was centrifuged and separated into plasma and packed red blood cells (RBCs). These biological samples were prepared for lipid analysis by the method of Bligh and Dyer; care was taken so that the pH of the extraction milieu was consistently maintained at 7 so as to avoid artifactual nitration. Samples were analyzed for free LNO₂ content by electrospray ionization tandem mass spectrometry. Using a hybrid triple quadrupole ion trap mass spectrometer, MRM transitions were monitored that specifically identified nitrated linoleic acid species; these species were concomitantly confirmed by the qualitative analytical abilities of the ion trap. The presence of nitrated linoleic acid was confirmed by HPLC chromatographic retention times, MS/MS “fingerprints” and was quantitated by the inclusion of a known quantity of ¹³C-labeled LNO₂.

Results

LNO₂ concentration was calculated as a function of the ratio of analyte to internal standard peak areas by using an internal standard curve linear over five orders of magnitude. Free LNO₂ in the RBCs and plasma of 5 transgenic sickle cell mice were 3.97 ± 2.56 nM and 12.37 ± 9.83 nM, respectively. Free LNO₂ in the RBCs and plasma of 6 wild type mice were 9.49 ± 8.32 nM and 14.91 ± 10.08 nM, respectively. There were no significant differences in LNO₂ concentration between any of the groups.

Conclusions

LNO₂ is present in both transgenic sickle cell mice and wild type mice in comparable concentrations at baseline. As a mediator of NO anti-inflammatory signaling, this is consistent with human studies showing comparable concentrations of NO metabolites at baseline between sickle cell patients and healthy controls. Further study of LNO₂ in sickle cell disease is warranted to better understand its role in the inflammatory process associated with acute stress, such as vaso-occlusive pain crisis and acute chest syndrome, when NO bioavailability is decreased.