Endothelial Cell Dysfunction and the Pathobiology of Atherosclerosis

Abstract

Dysfunction of the endothelial lining of lesion-prone areas of the arterial vasculature is an important contributor to the pathobiology of atherosclerotic cardiovascular disease. Endothelial cell dysfunction, in its broadest sense, encompasses a constellation of various nonadaptive alterations in functional phenotype, which have important implications for the regulation of hemostasis and thrombosis, local vascular tone and redox balance, and the orchestration of acute and chronic inflammatory reactions within the arterial wall. In this review, we trace the evolution of the concept of endothelial cell dysfunction, focusing on recent insights into the cellular and molecular mechanisms that underlie its pivotal roles in atherosclerotic lesion initiation and progression; explore its relationship to classic, as well as more recently defined, clinical risk factors for atherosclerotic cardiovascular disease; consider current approaches to the clinical assessment of endothelial cell dysfunction; and outline some promising new directions for its early detection and treatment.

Keywords: atherosclerosis; endothelial cells; homeostasis; risk factors; thrombosis.

Figure 1. Endothelial-derived Nitric Oxide: Production and Biological Actions

Endothelial cells rapidly produce nitric oxide (NO) via a unique isoform of nitric oxide synthase (eNOS) in response to agonists (e.g., acetylcholine, bradykinin) and fluctuations in blood flow. Once generated, NO rapidly diffuses through the endothelial plasma membrane to activate guanylate cyclase in several cell types present in the blood (platelets, leukocytes) and also within the vessel wall (smooth muscle). Activation of guanylate cyclase in platelets results in inhibition of activation, adhesion and aggregation; in leukocytes, decreased adhesivity; in smooth muscle cells, dephosphorylation of myosin light chain and vasorelaxation. NO also reacts with hemoglobin in erythrocytes, enhancing oxygen delivery to tissues. Chronic exposure of endothelial cells to laminar flow results in transcriptional upregulation of eNOS, thus increasing their NO-forming capacity. (Illustration Credit: Ben Smith)

Figure 2. Endothelial Pro-Inflammatory Activation

In lesion-prone regions of the arterial vasculature, the actions of pro-inflammatory agonists (e.g., IL-1, TNF, endotoxin), oxidized lipoproteins (oxLDL) and advanced glycation end products (AGE), as well as biomechanical stimulation by disturbed blood flow, leads to endothelial activation. These biochemical and biomechanical stimuli signal predominantly via the pleiotropic transcription factor NFκB, resulting in a coordinated program of genetic regulation within the endothelial cell. This includes the cell surface expression of adhesion molecules (e.g., VCAM-1), secreted and membrane-associated chemokines (e.g., MCP-1, Fractalkine), and pro-thrombotic mediators (e.g., TF, vWF, PAI-1). These events foster the selective recruitment of monocytes and various types of T-lymphocytes, which become resident in the subendothelial space. The concerted actions of activated endothelial cells, smooth muscle cells, monocyte/macrophages and lymphocytes result in the production of a complex paracrine milieu of cytokines, growth factors and reactive oxygen species (ROS) within the vessel wall, which perpetuates a chronic pro-inflammatory state and fosters atherosclerotic lesion progression. (vWF, vonWillebrand Factor; PAI-1, Plasminogen Activator Inhibitor-1; RAGE, Receptor for AGE; OxLDL-R, Receptor for Oxidized LDL; IL-R, TNF-R, Receptor(s) for IL-1, TNF; and TLRs, Toll-like Receptors). (Illustration Credit: Ben Smith).

Figure 3. Hemodynamics and Endothelial Phenotypes

Computational analyses of in vivo blood flow patterns in the carotid artery bifurcation, in normal human subjects, yielded representative near-wall shear stress waveforms from two hemodynamically distinct, clinically relevant locations: the distal internal carotid (an atherosclerosis-resistant region) and the carotid sinus (an atherosclerosis-susceptible region). Exposure of cultured human endothelial monolayers to these two distinct biomechanical stimuli, resulted in markedly different cell morphologies (visualized here by cytoskeletal actin staining) and functional phenotypes. Pulsatile (unidirectional) laminar flow induced upregulation of key transcription factors (KLF2, KLF4, Nrf2), which orchestrated a multifunctional atheroprotective phenotype; in contrast, disturbed (oscillatory) flow resulted in enhanced expression of the pleiotropic transcription factor NFκB, resulting in a proinflammatory, atheroprone phenotype. (See Ref. for details.)

Figure 4. Endothelial Atheroprotective Genes and Vascular Homeostasis

The expression of atheroprotective genes in vascular endothelium is regulated by key transcriptional factors (e.g., KLF2, KLF4, Nrf2) in response to hemodynamic, hormonal and environmental stimuli. The coordination of this genetic program is further influenced by miRNAs, epigenetic modifications, and pharmacologic agents. The resultant vasoprotective endothelial phenotype supports a spectrum of functions critical to the maintenance of vascular homeostasis.