Nitric oxide-dependent generation of reactive species in sickle cell disease: Actin tyrosine nitration induces defective cytoskeletal polymerization

The intermittent vascular occlusion occurring in sickle cell disease (SCD) leads to ischemia-reperfusion injury and activation of inflammatory processes including enhanced production of reactive oxygen species and increased expression of inducible nitric-oxide synthase (NOS2). Appreciating that impaired nitric oxide-dependent vascular function and the concomitant formation of oxidizing and nitrating species occur in concert with increased rates of tissue reactive oxygen species production, liver and kidney NOS2 expression, tissue 3-nitrotyrosine (NO₂Tyr) formation and apoptosis were evaluated in human SCD tissues and a murine model of SCD. Liver and kidney NOS2 expression and NO₂Tyr immunoreactivity were significantly increased in SCD mice and humans, but not in nondiseased tissues. TdT-mediated nick end-label (TUNEL) staining showed apoptotic cells in regions expressing elevated levels of NOS₂ and NO₂Tyr in all SCD tissues. Gas chromatography mass spectrometry analysis revealed increased plasma protein NO₂Tyr content and increased levels of hepatic and renal protein NO₂Tyr derivatives in SCD (21.4 ± 2.6 and 37.5 ± 7.8 ng/mg) versus wild type mice (8.2 ± 2.2 and 10 ± 1.2 ng/mg), respectively. Western blot analysis and immunoprecipitation of SCD mouse liver and kidney proteins revealed one principal NO₂Tyr-containing protein of 42 kDa, compared with controls. Enzymatic in-gel digestion and MALDI-TOF mass spectrometry identified this nitrated protein as actin. Electrospray ionization and fragment analysis by tandem mass spectrometry revealed that 3 of 15 actin tyrosine residues are nitrated (Tyr⁹¹, Tyr¹⁹⁸, and Tyr²⁴⁰) at positions that significantly modify actin assembly. Confocal microscopy of SCD human and mouse tissues revealed that nitration led to morphologically distinct disorganization of filamentous actin. In aggregate, we have observed that the hemoglobin point mutation of sickle cell disease that mediates hemoglobin polymerization defects is translated, via inflammatory oxidant reactions, into defective cytoskeletal polymerization.