SIRT3 interactions with FOXO3 acetylation, phosphorylation and ubiquitinylation mediate endothelial cell responses to hypoxia.

The endothelial cells (ECs) that line the vascular lumen are exposed to a wide variety of environmental stresses, such as hypoxia. Maladaptation to stress in ECs is a key event in the development of cardiovascular disease. Sirtuin 3 (SIRT3) is an NAD+-dependent protein deacetylase that modulates various proteins to control mitochondrial function and metabolism. We found that hypoxia elicits an increase in SIRT3 mRNA and protein expression in ECs. Under the same hypoxic conditions, the forkhead box class O transcription factor FOXO3 is deacetylated by SIRT3. The SIRT3-mediated deacetylation of FOXO3 further reduces FOXO3 phosphorylation, ubiquitination and degradation, thereby stabilizing FOXO3 proteins. As a result, the level of FOXO3 protein is increased during hypoxia. Moreover, a set of FOXO3-dependent mitochondrial antioxidant enzymes, including manganese superoxide dismutase (MnSOD), peroxiredoxin 3 (Prx3), Prx5 and thioredoxin 2 (Trx2), are up-regulated in ECs to facilitate ROS detoxification in response to hypoxia. The SIRT3-mediated deacetylation of FOXO3 preserves mitochondrial bioenergetic function and increases cell survival under hypoxic conditions. These results indicate that SIRT3 stabilizes FOXO3 via deacetylation, which enhances the mitochondrial antioxidant defence system to increase the adaptive capacity of ECs during hypoxia. This finding provides a direction for ameliorating the development of cardiovascular diseases.

Shear-induced endothelial mechanotransduction: the interplay between reactive oxygen species (ROS) and nitric oxide (NO) and the pathophysiological implications.

Hemodynamic shear stress, the blood flow-generated frictional force acting on the vascular endothelial cells, is essential for endothelial homeostasis under normal physiological conditions. Mechanosensors on endothelial cells detect shear stress and transduce it into biochemical signals to trigger vascular adaptive responses. Among the various shear-induced signaling molecules, reactive oxygen species (ROS) and nitric oxide (NO) have been implicated in vascular homeostasis and diseases. In this review, we explore the molecular, cellular, and vascular processes arising from shear-induced signaling (mechanotransduction) with emphasis on the roles of ROS and NO, and also discuss the mechanisms that may lead to excessive vascular remodeling and thus drive pathobiologic processes responsible for atherosclerosis. Current evidence suggests that NADPH oxidase is one of main cellular sources of ROS generation in endothelial cells under flow condition. Flow patterns and magnitude of shear determine the amount of ROS produced by endothelial cells, usually an irregular flow pattern (disturbed or oscillatory) producing higher levels of ROS than a regular flow pattern (steady or pulsatile). ROS production is closely linked to NO generation and elevated levels of ROS lead to low NO bioavailability, as is often observed in endothelial cells exposed to irregular flow. The low NO bioavailability is partly caused by the reaction of ROS with NO to form peroxynitrite, a key molecule which may initiate many pro-atherogenic events. This differential production of ROS and RNS (reactive nitrogen species) under various flow patterns and conditions modulates endothelial gene expression and thus results in differential vascular responses. Moreover, ROS/RNS are able to promote specific post-translational modifications in regulatory proteins (including S-glutathionylation, S-nitrosylation and tyrosine nitration), which constitute chemical signals that are relevant in cardiovascular pathophysiology. Overall, the dynamic interplay between local hemodynamic milieu and the resulting oxidative and S-nitrosative modification of regulatory proteins is important for ensuing vascular homeostasis. Based on available evidence, it is proposed that a regular flow pattern produces lower levels of ROS and higher NO bioavailability, creating an anti-atherogenic environment. On the other hand, an irregular flow pattern results in higher levels of ROS and yet lower NO bioavailability, thus triggering pro-atherogenic effects.