MicroRNA Regulation of Atherosclerosis

Abstract

Atherosclerosis and its attendant clinical complications, such as myocardial infarction, stroke, and peripheral artery disease, are the leading cause of morbidity and mortality in Western societies. In response to biochemical and biomechanical stimuli, atherosclerotic lesion formation occurs from the participation of a range of cell types, inflammatory mediators, and shear stress. Over the past decade, microRNAs (miRNAs) have emerged as evolutionarily conserved, noncoding small RNAs that serve as important regulators and fine-tuners of a range of pathophysiological cellular effects and molecular signaling pathways involved in atherosclerosis. Accumulating studies reveal the importance of miRNAs in regulating key signaling and lipid homeostasis pathways that alter the balance of atherosclerotic plaque progression and regression. In this review, we highlight current paradigms of miRNA-mediated effects in atherosclerosis progression and regression. We provide an update on the potential use of miRNAs diagnostically for detecting increasing severity of coronary disease and clinical events. Finally, we provide a perspective on therapeutic opportunities and challenges for miRNA delivery in the field.

Keywords: atherosclerosis; coronary artery disease; lipoproteins; microRNAs; vascular cell adhesion molecule.

Figure 1. microRNA orchestration of cholesterol homeostasis and macrophage activation in atherosclerosis

In the liver, miRNAs repress the expression of genes involved in lipoprotein packaging and secretion (eg. miR-30c, miR-27b), uptake (eg. miRNAs targeting LDLR and SRB1), and cholesterol efflux (eg. miRNA targeting ABCA1). miRNA repression of ABCA1 decreases cholesterol efflux to lipid-poor apolipoprotein A-I (apoA-I), and biogenesis of HDL. As the nascent HDL particle mediates free cholesterol (FC) uptake and remodels due to lecithin-cholesterol Acyltransferase (LCAT) conversion of FC to cholesterol ester (CE), secreted microRNAs, such as miR-223, miR-92a and miR-126 are detected on the mature HDL particles. These HDL carried and may mediate extracellular signaling by repressing genes in target tissues and HDL interaction with macrophages and endothelial cells may also result in miRNA exchange (ie. pick-up or delivery via scavenger receptor B1 (SR-B1). MiRNA targeting of SR-B1 and the ABC11 and ATP8B1 transporters reduce selective cholesterol uptake by the liver and excretion, respectively. In macrophages, miRNA targeting of ABCA1 reduces cholesterol efflux and reverse cholesterol transport back to the liver. In addition, miRNAs regulate the polarization of macrophages toward classical M1 (eg-33, miR-155) or alternative M2 (eg. miR-223, miR-27a) inflammatory activation, and also regulate lipoprotein uptake and foam cell formation. (Illustration Credit: Ben Smith).

Figure 2. Endothelial microRNAs regulate vascular inflammation

In response to biochemical and biomechanical stimuli, microRNAs regulate specific targets in endothelial cells (ECs) that alter the balance of pro- or anti-inflammatory signaling pathways. Biochemical stimuli such as tumor necrosis factor (TNF)-α reduces the expression of miR-181b, while it increases the expression of miR-146. MiR-181b targets importin-α3 in ECs, an effect that prevents cytoplasmic-to-nuclear translocation of nuclear factor κB (NF-κB) family members, p50 and p65, thereby reducing NF-κB-responsive gene expression such as adhesion molecules vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), or E-selectin. Induction of miR-146 targets the 3′-UTR of TNF-receptor-associated factor 6 (TRAF6), IRAK1, to regulate upstream NF-κB signaling and targets HuR which regulates endothelial nitric oxide synthase (eNOS) expression. The expression of miR-92a and miR-712 are increased in response to disturbed flow (d-flow), whereas miR-126-5p expression is reduced. miR-92a targets the transcription factors Kruppel-like factor 2 (KLF2) and 4 (KLF4), and suppressor of cytokine signaling (SOCS5) expression, an effect that decreases anti-inflammatory pathways and increases monocyte chemoattractant protein (MCP)-1 and IL-6 that further promote EC activation. miR-712 suppresses tissue inhibitor of metalloproteinase-3 (TIMP3) thereby increasing the expression of matrix metalloproteinases (MMPs) and A Disintegrin and MMP (ADAMs). In contrast, d-flow reduces expression of miR-126-5p thereby de-repressing its target gene delta-like 1 homolog (Dlk-1), a negative regulator of endothelial cell proliferation. Laminar flow induces the expression of the miR-143/miR-145 cluster, which may be packaged and released extracellularly in microvesicles and taken-up by neighboring vascular smooth muscle cells (VSMCs). Conversely, ECs may enable the passage of miR-143 and miR-145 released from VSMCs through specialized membrane protrusions known as tunneling nanotubes. HKII, hexokinase II. ITGβ8, integrin β 8.

Figure 3. MicroRNA regulation of vascular smooth muscle cell phenotype

In response to vascular wall injury or atherosclerosis, the expression of the miR-143/miR-145 cluster is markedly reduced in vascular smooth muscle cells (VSMCs). MiR-143 and miR-145 target the transcriptional regulators KLF4, KLF5, myocardin, and ELK-1 important for VSMC phenotypic switching from a contractile, mature, and differentiated cell type to a de-differentiated synthetic, and proliferative cell type. In addition, miR-143/miR-145 target genes important to the regulation of blood pressure such as angiotensin converting enzyme (ACE). In contrast, vascular wall injury increases expression of miR-221 and miR-222, an effect that decreases the expression of the cell cycle regulator c-Kit, p27(Kip1), and p57(Kip2). Induction of miR-21 expression targets phosphatase and tensin homolog (PTEN), thereby increasing the anti-apoptotic regulator B-cell lymphoma 2 (Bcl-2). Microvesicles or exosomes released by neighboring endothelial cells, and carrying microRNAs such as miR-143/miR-145 or miR-126 (bound to Argonaute2 (Ago2)), may be taken up by VSMCs enabling suppression of target genes and altering VSMC functional responses. FOXO3, Forkhead Box O3. IRS1, insulin receptor substrate 1.