The role of oxysterols in endothelial dysfunction

The endothelium is a thin monolayer of cells that covers the lumen of blood vessels from the heart to the smallest capillaries, creating a barrier between blood and the surrounding tissue. Currently, it is considered a real organ with the main function of modulating the vessel tone in response to humoral, nervous and mechanical stimuli. Furthermore, it plays an active role in vascular homeostasis mediating blood fluidity, regulation of inflammation, immune response and neovascularization. Endothelial dysfunction (ED) is a pathological condition characterized by reduced vasodilation, pro-oxidative state and procoagulant activity. ED has been identified as the main event in the pathogenesis of macrovascular disease including atherosclerosis. The oxidation of cholesterol in the low-density lipoprotein (LDL) produces a class of compounds called oxysterols. They are able to regulate many biological processes and exert several biochemical effects of potential pathophysiological relevance. Oxysterols are found in human LDL and in atherosclerotic plaques playing a key role in atheroma formation, thanks to their pro-inflammatory, pro-oxidant, pro-apoptotic, and fibrogenic properties. In the last decade, numerous studies have investigated the mechanisms and in vivo relevance of endoplasmic reticulum (ER) stress in the atherosclerosis process. Indeed, a close examination of ER stress and UPR pathways has demonstrated many links to major inflammatory and stress signaling networks. To this purpose, the present research is aimed at investigating the role of secosterol-B (SEC-B), a new oxysterol found in atherosclerotic plaques, as a possible inducer of endothelial dysfunction and damage affecting ER structure and function in endothelial cells. In detail, our results highlight that SEC-B is able to induce ER stress, as revealed by significant expansion and change of structure. At low doses cells try to cope with this stress by activating autophagy and the ubiquitin proteasome system in the attempt to restore ER function. However, at higher dose cell apoptosis occurs in a pathway that involves early phosphorylation of eIF2α and NF-kB activation, suggesting that the adaptive program fails and the cell activates the apoptotic program. Subsequently, to investigate the mechanism involved in SEC-B-induced ER stress, the study was focused on endothelial cell activation, evaluating inflammatory response induced by SEC-B. Our finding demonstrate that SEC-B is able to activate human microvascular endothelial cells by improving oxidative stress and affecting interleukin 1-β (IL1-β) and intracellular adhesion molecule-1 (ICAM-1) expression. In addition, our data suggest an involvement of SEC-B in TWEAK/Fn14 pathway regulation. The proinflammatory state leads to impairment of HMEC-1 function characterized by decrease of cell viability, down-modulation of intracellular NO bioavailability and enhancement in U937 recruitment on activated HMEC-1 cells. The results obtained in this study point to an important role of SEC-B in proinflammatory activation and ER damage of human endothelial cells that lead to ED. These findings provide additional insight about the role of oxysterols in ED and its potential implication in atherosclerotic disease.