Oxidative Stress Triggers Defective Autophagy in Endothelial Cells: Role in Atherothrombosis Development

Abstract

Atherothrombosis, a multifactorial and multistep artery disorder, represents one of the main causes of morbidity and mortality worldwide. The development and progression of atherothrombosis is closely associated with age, gender and a complex relationship between unhealthy lifestyle habits and several genetic risk factors. The imbalance between oxidative stress and antioxidant defenses is the main biological event leading to the development of a pro-oxidant phenotype, triggering cellular and molecular mechanisms associated with the atherothrombotic process. The pathogenesis of atherosclerosis and its late thrombotic complications involve multiple cellular events such as inflammation, endothelial dysfunction, proliferation of vascular smooth muscle cells (SMCs), extracellular matrix (ECM) alterations, and platelet activation, contributing to chronic pathological remodeling of the vascular wall, atheromatous plague formation, vascular stenosis, and eventually, thrombus growth and propagation. Emerging studies suggest that clotting activation and endothelial cell (EC) dysfunction play key roles in the pathogenesis of atherothrombosis. Furthermore, a growing body of evidence indicates that defective autophagy is closely linked to the overproduction of reactive oxygen species (ROS) which, in turn, are involved in the development and progression of atherosclerotic disease. This topic represents a large field of study aimed at identifying new potential therapeutic targets. In this review, we focus on the major role played by the autophagic pathway induced by oxidative stress in the modulation of EC dysfunction as a background to understand its potential role in the development of atherothrombosis.

Keywords: atherosclerosis; atherothrombosis; autophagy; endothelium; oxidative stress.

Figure 1

Platelet activation following exposure to collagen at the injury site and the production of free radicals. Protein tyrosine-phosphatases (PTPs) are the primary targets of ROS which, in turn, drive the upregulation of GPVI signaling. This event results in the overstimulation of platelets, inducing cytosolic calcium mobilization and the release of proaggregating factors from granules. Due to endothelial dysfunction, uncoupled eNOS uncoupling and NOX activation lead to the overproduction of ROS and RNS which are able to interact with platelets through the ADP receptor, triggering the same intracellular signaling pathway. Furthermore, nitrogen peroxide triggers the activation of fibrinogen and the stabilization of the fibrin clot. Thus, endothelial dysfunction and platelet activation in primary hemostasis, both consequences of a pro-oxidant state, converge and amplify each other, causing important prothrombotic effects. AA: Arachidonic Acid; COX-1: Cyclooxygenase-1; GP Ib: glycoprotein Ib; GPVI: glycoprotein VI; eNOS: endothelial Nitric Oxide Synthase; NOX: NADPH oxidase; P2Y: Peptide 2Y; RNS: Reactive Nitrogen Species; ROS: Reactive Oxygen Species; TXA2: Thromboxane A2.

Figure 2

The figure shows ROS and NOX modulation of different phases of secondary hemostasis and fibrinolysis through the upregulation of several proteases and the inhibition of antithrombotic factors in ECs. The imbalance generated between prothrombotic and antithrombotic factors leads to the stabilization of fibrin clot formation with subsequent development of the atherothrombotic process. ECs: Endothelial Cells; F V; Factor V; F Va; Factor Va; F VII; Factor VII; F VIIa: Factor VIIa; F VIII: Factor VIII; F VIIIa: Factor VIIIa; F IX: Factor IX; F IXa: Factor IXa; F X: Factor X; F Xa: Factor Xa; F XI: Factor XI; F XIa: Factor XIa; PAI-1: Plasminogen Activator Inhibitor-1; ROS: Reactive Oxygen Species; NOX: NADPH oxidase; TF: Tissue Factor.

Figure 3

Role of autophagy in endothelial cells during atherosclerosis. Different atherogenic stimuli, such as oxLDLs, AGEs, and saturated FA, can stimulate autophagy/mitophagy in ECs. Additionally, exposure to high shear stress can promote protective autophagy in ECs, playing antiapoptotic, antisenescent, anti-inflammatory, and antiatherogenic roles. Conversely, the exposure of ECs to low and disturbed levels of shear stress as well as oxLDL accumulation impairs autophagic flux as a result of inhibition of the AMPKα pathway and activation of the mTOR pathway as well as the blockade of fusion between autophagosomes and lysosomes. Defective autophagy leads to hyperactivation of inflammasomes, defective cholesterol efflux, and senescence and apoptotic cell death, thus exacerbating the atherosclerotic process. Solid lines indicate upregulated pathways. Dashed lines indicate downregulated pathways. AGEs: advanced glycation and products; ECs: endothelial cells; FA: fatty acids; MCP-1: monocyte chemoattractant protein-1; oxLDL: oxidized low-density lipoproteins; VCAM-1: vascular cell adhesion protein-1.