Endothelial permeability, LDL deposition, and cardiovascular risk factors-a review

Abstract

Early atherosclerosis features functional and structural changes in the endothelial barrier function that affect the traffic of molecules and solutes between the vessel lumen and the vascular wall. Such changes are mechanistically related to the development of atherosclerosis. Proatherogenic stimuli and cardiovascular risk factors, such as dyslipidaemias, diabetes, obesity, and smoking, all increase endothelial permeability sharing a common signalling denominator: an imbalance in the production/disposal of reactive oxygen species (ROS), broadly termed oxidative stress. Mostly as a consequence of the activation of enzymatic systems leading to ROS overproduction, proatherogenic factors lead to a pro-inflammatory status that translates in changes in gene expression and functional rearrangements, including changes in the transendothelial transport of molecules, leading to the deposition of low-density lipoproteins (LDL) and the subsequent infiltration of circulating leucocytes in the intima. In this review, we focus on such early changes in atherogenesis and on the concept that proatherogenic stimuli and risk factors for cardiovascular disease, by altering the endothelial barrier properties, co-ordinately trigger the accumulation of LDL in the intima and ultimately plaque formation.

Keywords: Atherosclerosis; Cardiovascular risk factors; Endothelium; Vascular permeability.

Figure 1

The entrance of LDL in the arterial intima of healthy vessels is determined by the glycocalyx and endothelial vesicles (1A). In vessels where the glycocalyx is compromised, the entrance of LDL in the arterial intima is determined by endothelial vesicles (1B) and open endothelial junctions (1C). The efflux occurs through the IEL (2) and the backward diffusion (3A and 3B), which may be of minor relevance as it goes against hydraulic conductivity. LDL can be trapped by binding to the extracellular matrix (ECM) (4). Adapted from Fry.

Figure 2

Caveolae-/Cav-1-/cavin-1-mediated endocytosis and exocytosis of LDL particles in endothelial cells. The vesicular transport of LDL mediated by the caveolae/Cav-1/cavin-1 complex is triggered by PKC or Src, with the contribution of dynamin, a target protein for Src and PKC. Dynamin acts as a guanosine triphosphatase in the fixing of caveolae, and enables signals for the vesicle internalization. LDL endocytosis is further influenced by signalling pathways that further regulate LDL uptake. In turn, exocytosis is mediated by an integral membrane protein family, called SNARE, which comprises SNAP receptors and promotes the docking and fusion of vesicles with the cell membrane in order to release their contents through its interaction with N-Ethyl Maleimide (NEM)-Sensitive Factor, which is activated after the recruitment by its SNAP receptor.

Figure 3

Proteins involved in endothelial junctions. AJ are represented by the cadherin–catenin complex and the nectin–afadin complex. VE-cadherin binds to β-catenin and plakoglobin (probably these two different molecules both connect to the actin cytoskeleton by a still uncertain mechanism that probably involves alpha-actinin and eplin, as it does for E-cadherin), and p120 through its cytoplasmic tail. β-Catenin and plakoglobin in turn link α-catenin, a filamentous (F) actin-binding protein, which is the primary link between the AJ and the actin cytoskeleton. α-Catenin can bind vinculin and α-actinin, stabilizing the anchorage to F-actin microfilaments (for a review see⁷¹). When α-catenin is stretched (upon tension on the junctions), a latent vinculin binding site becomes available and initiates a second interaction of the cadherin–catenin complex with the F-actin cytoskeleton. Nectin binds afadin, which in turn binds ponsin, connecting nectin to the F-actin cytoskeleton. TJ are formed by occludin, claudin, JAMs, and by many intracellular components, such as ZO-1, which assemble molecular complexes and connect junctional structures to the cytoskeleton. GJ are formed by three Cx: Cx43, Cx40, and Cx37 (see also Table 1).

Figure 4

Increased oxidative stress as the final common pathway of the effects of cardiovascular risk factors, and its effects on the endothelial permeability in endothelial cells. ROS stimulate a NF-κB-dependent upregulation of intercellular adhesion molecule (ICAM)-1 expression. ICAM-1 binds the β₂ integrins of neutrophils, inducing activation of endothelial NADPH oxidase (NOX) via Src, and enhancing ROS production. ROS release from neutrophils and NOX activates the redox sensitive Ca²⁺ permeability transient receptor potential cation channel subfamily M member 2 (TRPM2), leading to Ca²⁺ entry into endothelial cells (ECs) and promoting PKC-α activation, which, in turn, may regulate EC permeability via the phosphorylation of p120 and its dissociation from VE-cadherin, and the activation of Src, leading to tyrosine phosphorylation of VE-cadherin and β-catenin, eventually resulting in AJ destabilization. Ca²⁺ entry (bound to calmodulin) activates MLC kinase, which phosphorylates MLC, mechanism through which myosin starts to move along the actin fibres and eliciting a co-ordinated spatial activation of RhoA and a global reorganization of the actin cytoskeleton, resulting in endothelial barrier dysfunction. RhoA, via activation of Rho kinase (ROCK), inhibits MLC phosphatase, making the effect of MLC kinase activity longer lasting. A loss of endothelial nitric oxide (NO) bioavailability is caused by its reaction with ${\mathrm{O}}_2^{\text{*–}}$, produced by uncoupled endothelial nitric oxide synthase (eNOS) (ueNOS), yielding ONOO^–production, which contributes to vary the cellular redox state.