The NADPH oxidase isoform NOX4 mainly produces H
2
O
2
. Previously we could show, that NOX4 is the major endothelial NOX isoform and constitutively active. Regulation of NOX4 and formation of reactive oxygen species are involved in NO release. The transcription factor NRF2 is a key mediator of cellular adaptation to redox stress. Therefore, regulation of NOX4 on the transcriptional level by NRF2 might be directly linked to NO release and endothelial function. Besides endothelial nitric oxide synthase (eNOS), a role of neuronal nitric oxide synthase (nNOS) in endothelial NO and H
2
O
2
release has been proposed. In this study, endothelial cells (HUVEC) were constantly exposed to high laminar shear stress (24 h, 30 dyn/cm
2
). Application of shear stress stimulated NO formation and induced elongation of endothelial cells in the direction of flow. Lentiviral overexpression of NOX4 strongly increased endothelial H
2
O
2
release, while downregulation using shNOX4 decreased H
2
O
2
release. Furthermore, application of shear stress caused downregulation of NOX4 as well as upregulation of eNOS and antioxidative response via NRF2 and its target genes NQO-1 and HMOX-1. H
2
O
2
can affect NO release by eNOS. We could show that downregulation of NOX4 leads to upregulation of eNOS mRNA and protein expression accompanied by attenuation of NRF2 pathway under flow conditions. Determination of NO release confirmed these results. This supports a compensatory mechanism maintaining a stable NO release after NOX4 inhibition in response to flow. Attenuation of NRF2 by shNRF2 inhibited shear stress-dependent induction of NRF2 mRNA and protein expression and its target genes. In addition, shNRF2 enhanced the shear stress-dependent downregulation of NOX4. Finally, we detected a stable mRNA and protein expression of nNOS in endothelial cells, which was unaffected by flow. Interestingly, downregulation of NOX4 resulted in induction of nNOS expression.
In conclusion, our data suggest an important role of H
2
O
2
production by NADPH oxidase 4 in maintaining NO release and endothelial function in response to flow. Inactivation of NADPH oxidase 4 attenuates NRF2 pathway and increases neuronal nitric oxide synthase as compensatory mechanisms.
Upregulation of Superoxide Dismutase and Nitric Oxide Synthase Mediates the Apoptosis-Suppressive Effects of Shear Stress on Endothelial Cells