Atherosclerosis is a chronic inflammatory process that leads to the thickening of the intimal layer of large- and medium-sized arteries and results in the formation of plaques. It occurs due to an imbalance between lipid breakdown and the immune response, leading to a failure of inflammatory response resolution. The risk factors of atherosclerosis include hypertension, hyperglycemia, obesity, hyperlipidemia, and smoking.
Atherosclerosis is a major vascular disease that causes death and morbidity worldwide That means that 25% of cardiovascular disease deaths are derived from atherosclerosis. Around 75% of acute myocardial infarctions occur from plaque rupture; the highest incidence of plaque rupture was observed in men over 45 years, but rarely observed in women. Stroke from any cause represents the fifth leading cause of death and the major cause of serious long-term disability in adults.
It occurs due to an imbalance between lipid breakdown and the immune response, leading to a failure of inflammatory response resolution . An interplay of these risk factors can lead to endothelial activation and dysfunction. Endothelial dysfunction plays an important role in the pathogenesis of atherosclerosis and is a predictor of atherosclerotic risk . In addition, endothelial dysfunction promotes the expression of inflammatory cytokines and cell adhesion molecules and increases the permeability of the endothelium, promoting the transmigration of monocytes; this is followed by an accumulation of macrophages and oxidized lipids in the intimal space, which exacerbates the inflammatory process.
Atherosclerotic plaques narrow the lumen of vessels, obstructing blood flow, making organs susceptible to ischemia; additionally, plaques may rupture, leading to adverse effects such as myocardial ischemia or infarction, renal ischemia, and ischemic stroke due to the activated endothelial and smooth muscle cells, macrophages, lymphocytes, and large amounts of extracellular matrix during the process . The characteristics of the cells present in atherosclerotic plaques The resulting lesions may contain other immune cells, including rare dendritic cells (DCs), neutrophils and B-cells, and smooth muscle cells. Indeed, the typical feature of an atherosclerotic vessel is chronic vascular wall inflammation, which can be considered as an unresolved vascular inflammatory response .
During this process, the endothelial cells lose their apical-to-basal membrane polarity and cell-to-cell adhesion and acquire a migratory, fibroblast-like phenotype. Additionally, there is a suppression of endothelial cell markers including platelet endothelial cell adhesion molecule (PECAM), vascular endothelial cadherin (VE-cadherin), vascular endothelial growth factor 2 receptor (VEGFR-2), and endothelial nitric oxide synthase (eNOS). While EndMT is a normal physiologic process involved in cardiac embryogenesis/septate formation, pathologically its occurrence has been implicated in the initiation, progression, and stabilization of atherosclerotic plaques . In vivo endothelial cell lineage tracking systems provided evidence that the endothelial cells undergoing EndMT are major contributors to the pool of fibroblast-like cells seen in atherosclerotic plaques as there was co-expression of both endothelial- and mesenchymal-specific markers, and the degree of EndMT correlated with plaque instability .
EndMT is the increased expression of transcription factors, including Snail, Slug, Twist, LEF-1, ZEB1, and ZEB2, which inhibit the expression of endothelial genes and/or activate expression of mesenchymal genes . Additionally, TGF-β is known to control cell proliferation, cell migration, matrix synthesis, wound contraction, calcification, and the immune response, all major components of the atherosclerotic process . The resulting transcription factor complex translocates into the nucleus where it interacts with other transcription factors such as Snail, Twist, Snug, and Zeb1 to modulate the expression of genes involved in EndMT. Activation of the Snail, Twist, and Zeb proteins decreases the expression of endothelial cell-to-cell adhesion proteins, such as occludin, claudin, and cytokeratin, and upregulates mesenchymal markers .
TGF-β signaling is one of the primary drivers of atherosclerosis-associated vascular inflammation . The activation of TGF-β, through a Smad2/3-dependent process, promotes inflammation in vitro in cultured human endothelial cells by upregulating the expression of proinflammatory cytokines and chemokines and their receptors, such as CCL2 and CCR2, respectively, as well as adhesion molecules such as ICAM1 and VCAM1. However, TGF-β exhibited an anti-inflammatory effect on vascular smooth muscle cells. In vivo knockout of TGF-β abrogated inflammation promoted atherosclerotic plaque regression and prevented plaque development .
Crosstalk of other signaling pathways (Notch and Wnt) with the TGF-β pathway is also involved in the induction of EndMT. The extent of atherosclerotic lesions is indirectly proportional to the expression of fibroblast growth factors (FGFs) In addition, there was a marked reduction in expression of fibroblast growth factor receptor 1 (FGFR1) and an upregulation of phosphorylated Smads, a marker of TGF-β/Smad pathway activation, in the coronary artery of patients with coronary artery disease compared to patients without coronary artery disease . Thereby, therapeutic strategies targeting the FGF signaling pathway, such as inhibition of fibroblast growth factor receptor signaling, attenuated atherosclerosis in apolipoprotein E-deficient mice .
Multifunctional inflammatory cytokines are produced by many inflammatory cells including blood leukocytes, macrophages, smooth muscle cells, and platelets . In vitro studies show that the exposure of primary endothelial cells (ECs) with inflammatory cytokines, including IFN-γ, TNF-α, and IL-1β, leads to reduced FGFR1 expression. However, these reports suggest that inhibition of individual inflammatory cytokines is not enough to be effective in treating atherosclerosis. They also found that inflammatory stress, including shear stress, oxidative stress and oxidized lipoproteins, exacerbates the progression of cardiac fibrosis in high-fat-fed ApoE KO mice via EndMT, suggesting EndMT and inflammation act synergistically to redistribute plasma lipids to cardiac tissues and accelerate the progression of cardiac fibrosis .
Furthermore, in vivo studies have implicated numerous inflammatory mediators during the initiation and continued development of atherosclerosis . Inflammatory signaling synergizes with the induction of EndMT and inflammatory stress exacerbates cardiac fibrosis progression in high-fat-fed ApoE KO mice . Therefore, inhibiting both inflammation and EndMT may provide a new method for treating atherosclerosis. In addition, chronic inflammation can cause endothelium dysfunction, which is another critical element accounting for atherosclerosis.
In straight sections of a blood vessel, vascular endothelial cells typically align and elongate in the direction of fluid flow. The multiple functions of vascular endothelial cells are illustrated. Activation of endothelial MMP-2 can induce endothelial loss of integrity and dysfunction . Recruited vascular wall cells can remodel the surrounding extracellular matrix through MMPs that affect migration, proliferation, and apoptosis of endothelial cells and vascular smooth muscle cells (VSMCs) .
However, the pathogenesis of atherosclerosis is very complicated and includes endothelial dysfunction, inflammatory response, oxidative stress, smooth muscle cell activation, and thrombosis. However, endothelial dysfunction was considered as the initial factor in inducing atherosclerosis . Experimental and clinical studies show that ECs can be regenerated by bone marrow-derived circulating endothelial progenitor cells (EPCs), which repair endothelial cell dysfunction and prevent atherosclerosis . Moreover, many studies have revealed an association between endothelial dysfunction and inflammatory stress in vascular biology.
Other mechanisms related to oxidative stress can induce endothelial damage indirectly by reducing PPARγ activity or adiponectin levels. Other studies have shown a protective role by inhibiting endothelial dysfunction via the activation of PPARγ . Xu et al. reported that in rat microvascular EC culture, PPARγ agonists reversed oxLDL-induced endothelial dysfunction by stimulating AMP-activated protein kinase (AMPK), which is a serine/threonine-protein kinase that upregulates the Akt/eNOS/NO pathway, enhancing eNOS activity. Some AMPK activators, such as statins, improve endothelial function and have shown antiatherogenic properties.
Chronic stimulation of ECs by various factors, including pro-inflammatory cytokines and hypoxia, cause ECs to develop an imbalance of endothelial homeostasis, which results in endothelial dysfunction. Therefore, conditions causing endothelial cell activation toward EndMT, including the expression of mesenchymal genes, are detrimental. If this activation happens persistently, over time it may progress to endothelial dysfunction and eventually to a morphological change in cells . Hence, knowing the pathways implicated in this pathological process of atherosclerosis, including endothelial dysfunction, inflammatory response, and EndMT, helps to develop drugs against incipient atherosclerosis.