Endothelial Nitric Oxide Synthase Phosphorylation at Threonine 495 and Mitochondrial Reactive Oxygen Species Formation in Response to a High H2O2 Concentration

Abstract

Background: Hydrogen peroxide (H₂O₂) is produced in vessels during ischemia/reperfusion and during inflammation, both leading to vascular dysfunction. We investigated cellular pathways involved in endothelial nitric oxide synthase (eNOS) phosphorylation at Threonine 495 (Thr⁴⁹⁵) in human umbilical vein endothelial cells (HUVECs) exposed to H₂O₂. Methods: HUVECs were exposed to 400 μ<smlcap>M</smlcap> H₂O₂ for 30 min. Phosphorylation at Thr⁴⁹⁵ was assessed by Western blotting and reactive oxygen species (ROS) monitored by flow cytometry. Protein kinase C (PKC) pathways were investigated by pretreatment with PKC-β inhibitor ruboxistaurin or pan-PKC inhibitor GF109203X. In addition, we investigated ROCK and ERK pathways by MEKK1/2 inhibitor U0126 and ROCK inhibitor Y27632. Results: H₂O₂ increased eNOS phosphorylation at Thr⁴⁹⁵ (to 176% vs. control (100%), p < 0.001) along with increased mitochondrial ROS formation (from 19.7 to 45.3%, p < 0.01). This rise in phosphorylation could be prevented by U0126 and Y27632 in a dose-dependent manner, but did not result in lowered mitochondrial ROS formation. Conversely, addition of the antioxidant N-acetyl-<smlcap>L</smlcap>-cysteine only prevented mitochondrial ROS formation but did not prevent phosphorylation of eNOS Thr⁴⁹⁵. Conclusion: H₂O₂-mediated phosphorylation of eNOS Thr⁴⁹⁵ is mediated by ROCK and ERK activity, but not by PKC, and is uncoupled from mitochondrial ROS signaling. Furthermore, ERK inhibition increased mitochondrial ROS formation.