Atherosclerosis is the development of fatty plaques in blood vessel walls. It is a universal condition of aging, present to some degree in every older individual. Atherosclerosis contributes to many age-related diseases via narrowing of vessels and reduced blood flow on the one hand, and on the other causing more than a quarter of all human mortality via the stroke and heart attack that can follow rupture of an unstable plaque. We might think of atherosclerosis as a condition of macrophage dysfunction. Macrophages are innate immune cells responsible for clearing excess cholesterol from blood vessel walls. These cells ingest cholesterol and the LDL particles that transport cholesterol from the liver to the rest of the body. Then then hand off that cholesterol to HDL particles for transport back to the liver.
Considered at a high level, and skipping some of the complexities, atherosclerotic plaque develops when the influx of LDL-cholesterol exceeds the capacity of macrophages to clean it up. The research community is near entirely focused on the LDL-cholesterol side of the question, but as a result of decades of such work has now comprehensively demonstrated that even dramatic reductions in circulating LDL-cholesterol only modestly reduce the risk of heart attack and stroke, and cannot reliably or sizeably reverse established atherosclerotic plaque. It is past time to focus on the other side of the equation, the capacity of macrophages to remove cholesterol from blood vessel walls, and even survive and continue this work in the hostile environment of an established plaque in order to reduce its size. Today’s open access paper is an example of this sort of work, the search for proximate causes of macrophage dysfunction in the context of atherosclerosis that may form a basis for later drug development.
Immune cells, including the circulating monocytes that will transform into macrophages, are recruited to the vascular wall in atherogenesis and play a critical role in sustaining oxidative stress, inflammation, and extracellular matrix degradation. Atherosclerotic lesion macrophages can maintain several phenotypes, including classically activated (M1 or M[IFNγ+LPS]) pro-inflammatory macrophages and alternatively activated (M2 or M[IL4]) pro-resolving macrophages. Macrophage metabolic reprogramming, with pro-inflammatory cells relying on glycolysis and pro-resolving cells on oxidative phosphorylation for energy production, is closely related to the changes in atherosclerotic plaque environment and morphology. Nevertheless, the mechanisms of metabolic reprogramming of macrophages in atherosclerosis and its effects on plaque morphology are incompletely understood.
Mitochondrial dysfunction in macrophages in aging results in reduced ATP production, elevated reactive oxygen species (ROS) generation, and compromised mitochondrial quality control, features that are intricately linked to the shift in metabolism from oxidative phosphorylation to glycolysis and pro-inflammatory phenotype. Consequently, aging-associated atherosclerotic plaque mitochondrial oxidative stress and dysfunction result in increased lesion volume and vulnerable plaque features. Expression of mitochondria-localized NOX4 NADPH oxidase is increased with age in human and mouse vasculature and is associated with increased oxidative stress, vascular inflammation, aortic stiffness, and atherosclerotic lesion size and severity. Similarly, increased NOX4 expression in atherosclerotic plaque was associated with plaque instability and rupture, while direct inhibition, genetic downregulation of NOX4, or blockade of NOX4-dependent signaling pathways inhibited atherogenesis. In human coronary atherosclerotic lesions increased NOX4 expression was observed in nonphagocytic vascular cells, contributing to increased ROS levels, while increased NOX4-derived ROS in human monocytes was associated with higher metabolic priming, vascular recruitment, and atherosclerosis progression.
Targeting NOX4-dependent mitochondrial ROS holds promise in atherosclerosis management. However, the precise mechanisms of mitochondrial dysfunction in aging-associated atherosclerosis, its impact on plaque progression and phenotype, and therapeutic potential are not fully elucidated. Here, we tested the hypothesis that mitochondrial oxidative stress associated with increased NOX4 levels in aging results in metabolic priming in monocytes/macrophages to a pro-inflammatory phenotype switch, fostering atherosclerotic lesion progression. We used Apoe-/- mice as they have high cholesterol levels when fed a Western diet, leading to human-like atherosclerosis progression with similar lesion cellular composition, a prominent inflammatory profile, and aging-related phenotype useful for aging studies. Effects of aging were examined in 16-month-old mice, which represent the age equivalent of humans with exponentially increasing coronary heart disease incidence, making them a useful model to study the pathogenesis of atherosclerosis. Using aged Nox4-deficient Apoe-/- mice, mice we showed that reduced mitochondrial ROS in macrophages preserves mitochondrial function and is associated with pro-resolving phenotype attenuating atherosclerotic disease. We recapitulated our findings by inhibiting NOX4 activity in aged Apoe-/- mice.
Our findings suggest that increased NOX4 in aging drives macrophage mitochondrial dysfunction, glycolytic metabolic switch, and pro-inflammatory phenotype, advancing atherosclerosis. Inhibiting NOX4 or mitochondrial dysfunction could alleviate vascular inflammation and atherosclerosis, preserving plaque integrity.
