Atherosclerosis is a chronic inflammatory disease of the vascular system and is the leading cause of cardiovascular diseases worldwide. Excessive generation of reactive oxygen species (ROS) leads to a state of oxidative stress which is a major risk factor for the development and progression of atherosclerosis. ROS are important for maintaining vascular health through their potent signalling properties. However, ROS also activate pro-atherogenic processes such as inflammation, endothelial dysfunction and altered lipid metabolism. As such, considerable efforts have been made to identify and characterise sources of oxidative stress in blood vessels. Major enzymatic sources of vascular ROS include NADPH oxidases, xanthine oxidase, nitric oxide synthases and mitochondrial electron transport chains. The production of ROS is balanced by ROS-scavenging antioxidant systems which may become dysfunctional in disease, contributing to oxidative stress. Changes in the expression and function of ROS sources and antioxidants have been observed in human atherosclerosis while in vitro and in vivo animal models have provided mechanistic insight into their functions. There is considerable interest in utilising antioxidant molecules to balance vascular oxidative stress, yet clinical trials are yet to demonstrate any atheroprotective effects of these molecules. Here we will review the contribution of ROS and oxidative stress to atherosclerosis and will discuss potential strategies to ameliorate these aspects of the disease.

Objective—: Mitochondrial DNA (mtDNA) damage is present in murine and human atherosclerotic plaques. However, whether endogenous levels of mtDNA damage are sufficient to cause mitochondrial dysfunction and whether decreasing mtDNA damage and improving mitochondrial respiration affects plaque burden or composition are unclear. We examined mitochondrial respiration in human atherosclerotic plaques and whether augmenting mitochondrial respiration affects atherogenesis. Approach and Results—: Human atherosclerotic plaques showed marked mitochondrial dysfunction, manifested as reduced mtDNA copy number and oxygen consumption rate in fibrous cap and core regions. Vascular smooth muscle cells derived from plaques showed impaired mitochondrial respiration, reduced complex I expression, and increased mitophagy, which was induced by oxidized low-density lipoprotein. Apolipoprotein E–deficient (ApoE ^−/− ) mice showed decreased mtDNA integrity and mitochondrial respiration, associated with increased mitochondrial reactive oxygen species. To determine whether alleviating mtDNA damage and increasing mitochondrial respiration affects atherogenesis, we studied ApoE ^−/− mice overexpressing the mitochondrial helicase Twinkle (Tw ⁺ /ApoE ^−/− ). Tw ⁺ /ApoE ^−/− mice showed increased mtDNA integrity, copy number, respiratory complex abundance, and respiration. Tw ⁺ /ApoE ^−/− mice had decreased necrotic core and increased fibrous cap areas, and Tw ⁺ /ApoE ^−/− bone marrow transplantation also reduced core areas. Twinkle increased vascular smooth muscle cell mtDNA integrity and respiration. Twinkle also promoted vascular smooth muscle cell proliferation and protected both vascular smooth muscle cells and macrophages from oxidative stress–induced apoptosis. Conclusions—: Endogenous mtDNA damage in mouse and human atherosclerosis is associated with significantly reduced mitochondrial respiration. Reducing mtDNA damage and increasing mitochondrial respiration decrease necrotic core and increase fibrous cap areas independently of changes in reactive oxygen species and may be a promising therapeutic strategy in atherosclerosis.

Mitochondria are often regarded as the cellular powerhouses through their ability to generate ATP, the universal fuel for metabolic processes. However, in recent years mitochondria have been recognised as critical regulators of cell death, inflammation, metabolism, and the generation of reactive oxygen species (ROS). Thus, mitochondrial dysfunction directly promotes cell death, inflammation, and oxidative stress and alters metabolism. These are key processes in atherosclerosis and there is now evidence that mitochondrial DNA (mtDNA) damage leads to mitochondrial dysfunction and promotes atherosclerosis directly. In this review we discuss the recent evidence for and mechanisms linking mtDNA defects and atherosclerosis and suggest areas of mitochondrial biology that are potential therapeutic targets.

Atherosclerotic lesions show significant mitochondrial dysfunction but the underlying mechanisms and consequences remain unknown. Cardiolipin is a phospholipid found exclusively in the mitochondrial inner membrane, the site of oxidative phosphorylation. Tafazzin is a trans-acylase that acylates immature monolysocardiolipin to mature cardiolipin. Tafazzin mutations can result in Barth’s Syndrome, which is characterised by dilated cardiomyopathy, skeletal myopathy and impaired growth. However, a role for tafazzin in atherosclerosis development has not been previously identified. Here we show that tafazzin expression is decreased in atherosclerotic lesions and specifically in plaque vascular smooth muscle cells (VSMCs). MicroRNA 125a-5p expression is increased in plaques, downregulates tafazzin expression and is induced by oxidised low-density lipoprotein in a NFκB-dependent manner. Silencing tafazzin or overexpression of mutant tafazzin decreases VSMC cardiolipin content and mitochondrial respiration, and promotes apoptosis and atherosclerosis. In contrast tafazzin overexpression increases respiration, protects against apoptosis and increases features of plaque stability. Tafazzin therefore has important effects on VSMC mitochondrial function and atherosclerosis, and is a potential therapeutic target in atherosclerotic disease.