Mitochondria-derived reactive oxygen species mediate heme oxygenase-1 expression in sheared endothelial cells ^s

Bovine aortic endothelial cells (ECs) respond to nitric oxide (NO) donors by activating the redox-sensitive NF-E2-related factor 2/antioxidant response element pathway and up-regulating heme oxygenase (HO)-1 expression. EC exposure to steady laminar shear stress causes a sustained increase in NO, a transient increase in reactive oxygen species (ROS), and activation of the HO-1 gene. Because ste.ady laminar flow increases the mitochondrial superoxide (O ₂ ^- production, we hypothesized that mitochondria-derived ROS play a role in shear-induced HO-1 expression. Flow (10 dynes/cm ², 6 h)- induced expression of HO-1 protein was abolished when BAECs were preincubated and sheared in the presence of either N ^G-nitro-L- arginine methyl ester or N-acetyl-L-cysteine, suggesting that either NO or ROS up-regulates HO-1. Ebselen and diphenylene iodonium blocked HO-1 expression, and uric acid had no effect. The mitochondrial electron transport chain inhibitors, myxothiazol, rotenone, or antimycin A, and the mi-tochondria-targeted ant.ioxidant peptide, Szeto-Schiller (SS)- 31, which scavenges O ₂ ^-, hydrogen peroxide (H ₂O ₂), peroxyni- trite, and hydroxyl radicals, markedly inhibited the increase in HO-1 expression. These data collectively suggest that mito- chondrial H ₂O ₂ mediates the HO-1 induction. MitoSOX and 2',7'-dichlorofluorescin (DCF) fluorescence showed that mitochondrial O ₂ ^- levels and intracellular peroxides, respectively, are higher in sheared ECs compared with static controls and, in part, dependent on NO. SS-31 significantly inhibited both the shear-induced MitoSOX and DCF fluorescence signals. Either phosphatidylinositol 3-kinase or mitogen-activated protein ki- nase cascade inhibitors blocked the HO-1 induction. In conclusion, under shear, EC mitochondria-derived H2O2 diffuses to the cytosol, where it initiates oxidative signaling leading to HO-1 up-regulation and maintenance of the atheroprotective EC status.