Signaling pathway for nitric oxide generation with simulated ischemia in flow-adapted endothelial cells

Ischemia in the intact ventilated lung (oxygenated ischemia) leads to endothelial generation of reactive oxygen species (ROS) and nitric oxide (NO). This study investigated the signaling pathway for NO generation with oxygenated ischemia in bovine pulmonary artery endothelial cells (BPAEC) that were flow adapted in vitro. BPAECs were cultured in an artificial capillary system and subjected to abrupt cessation of flow (ischemia) under conditions where cellular oxygenation was maintained. Immunoblotting and dichlorofluorescein/triazolofluorescein fluorescence were used to assess extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation and ROS/NO generation, respectively. ERK1/2 phosphorylation significantly increased during ischemia, whereas total ERK1/2 did not change. ERK1/2 phosphorylation was suppressed by an inhibitor of tyrosine phosphorylation (genestein), cholesterol-binding reagents (filipin or cyclodextrin), or inhibitors of ROS (diphenyleneiodonium, N-acetylcysteine, or catalase), suggesting a role for both membrane cholesterol and ROS in ERK1/2 activation. Ischemia resulted in a 1.8-fold increase in NO generation that was suppressed by inhibitors of ERK1/2 activation (PD-98059 or U-0126). A calmodulin inhibitor (calmidizolium) or removal of Ca2+ from the medium also blocked NO generation, indicating that endothelial NO synthase (eNOS) is the activated isoform. These results indicate ischemia induces NO generation (possibly through a membrane cholesterol-sensitive flow sensor), the ERK1/2 cascade mediates signaling from the sensor to eNOS, and ROS are required for ERK activation.