Endothelial to Mesenchymal Transition: Role in Physiology and in the Pathogenesis of Human Diseases

Abstract

Numerous studies have demonstrated that endothelial cells are capable of undergoing endothelial to mesenchymal transition (EndMT), a newly recognized type of cellular transdifferentiation. EndMT is a complex biological process in which endothelial cells adopt a mesenchymal phenotype displaying typical mesenchymal cell morphology and functions, including the acquisition of cellular motility and contractile properties. Endothelial cells undergoing EndMT lose the expression of endothelial cell-specific proteins such as CD31/platelet-endothelial cell adhesion molecule, von Willebrand factor, and vascular-endothelial cadherin and initiate the expression of mesenchymal cell-specific genes and the production of their encoded proteins including α-smooth muscle actin, extra domain A fibronectin, N-cadherin, vimentin, fibroblast specific protein-1, also known as S100A4 protein, and fibrillar type I and type III collagens. Transforming growth factor-β1 is considered the main EndMT inducer. However, EndMT involves numerous molecular and signaling pathways that are triggered and modulated by multiple and often redundant mechanisms depending on the specific cellular context and on the physiological or pathological status of the cells. EndMT participates in highly important embryonic development processes, as well as in the pathogenesis of numerous genetically determined and acquired human diseases including malignant, vascular, inflammatory, and fibrotic disorders. Despite intensive investigation, many aspects of EndMT remain to be elucidated. The identification of molecules and regulatory pathways involved in EndMT and the discovery of specific EndMT inhibitors should provide novel therapeutic approaches for various human disorders mediated by EndMT.

Graphical abstract

FIGURE 1.

Diagram illustrating the endothelial to mesenchymal transition (EndMT) process. The diagram illustrates the morphological, phenotypic, and gene expression program changes occurring during the process of endothelial to mesenchymal cell transition leading to the phenotypic conversion of endothelial cells into activated myofibroblasts displaying increased production of various mesenchymal-specific macromolecules including α-smooth muscle actin (α-SMA), COL1, COL3, fibronectin (FN), FN-extra domain A (EDA)/vimentin, N-cadherin, and fibroblast-specific protein-1 (FSP-1). These events are accompanied by the loss of endothelial cell-specific gene products such as CD31/platelet-endothelial cell adhesion molecule-1 (CD31/PECAM-1), vascular-endothelial cadherin (VE-cadherin), COL4, vascular epidermal growth factor receptor (VEGFR), and von Willebrand factor.

FIGURE 2.

Canonical transforming growth factor (TGF)-β and noncanonical pathways in endothelial to mesenchymal transition (EndMT). The canonical TGF-β pathway is triggered upon binding of TGF-β homodimers to a transmembrane heterodimeric TGF-β I/II receptor complex in the cell surface. The binding of TGF-β to the heterodimeric receptor complex leads to the phosphorylation-mediated activation of the TGF-β receptor I/II complex. The active TGF-β receptor I/II complex phosphorylates cytoplasmic Smad2 and Smad3 proteins resulting in their activation. Activated Smad2 and Smad3 form a complex with Smad4 that translocates to the nucleus. Inside the nucleus, the Smad2/Smad3/Smad4 complex interacts with Smad binding elements (SBE) of TGF-β-responsive genes including those encoding profibrotic extracellular matrix (ECM) macromolecules and transcription factors such as Snail1 and 2 and Twist and stimulates their transcription. Another intracellular Smad protein, the inhibitory Smad7, abrogates TGF-β-induced signaling cascades by inhibitory effects on the activated TGF-β receptors and is a potent negative regulator of TGF-β-induced gene expression responses. Noncanonical TGF-β pathways include the mitogen-activated protein kinase (MAPK) family of serine/threonine-specific protein kinases and numerous other kinases such as phosphatidylinositol 3-kinase (PI3K), RhoA, Rac, c-Abl, and protein kinase C (PKC)-δ. TGF-β can activate all three known MAPK pathways: extracellular signal-regulated kinase (ERK), p38 MAPK, and c-Jun NH₂-terminal kinases (JNK). Signaling through these pathways mediates Smad2/3-independent TGF-β responses. These pathways result in either reduced transcription of endothelial-specific genes or increased the transcription of mesenchymal-specific genes effects mediated by EndMT-related transcription factors such as Snail1 and 2 and TWIST1.

FIGURE 3.

Diagrammatic representation of other pathways involved in endothelial to mesenchymal transition (EndMT) regulation. Molecular signaling pathways beside the transforming growth factor (TGF)-β pathways that induce or participate in the EndMT process. These include endothelin (ET)-1, NOTCH, caveolin (CAV)-1, Wnt, high glucose, and hypoxia inducible factor (HIF)-1α pathways. The morphogen pathways NOTCH and Wnt are important modulators of EndMT. CAV-1 exerts an inhibitory effect owing to the internalization of TGF-β receptors and their subsequent degradation. ET-1 induces a synergistic stimulation of TGF-β-induced EndMT involving the canonical Smad2/3 pathway. Hypoxia induces EndMT through the effects of HIF-1α activation of Snail1. High glucose concentrations have been shown to induce EndMT involving extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. Shear stress forces (represented by undulating arrows) induce EndMT through several different molecular mechanisms. One mechanism is initiated by the mechanical force-induced release and liberation of TGF-β from the latency associated peptide (LAP) followed by activation of the TGF-β canonical pathway. Other mechanisms include reactive oxygen species (ROS) generation and activation of NFκB followed by the activation of NOX1/4 oxidases resulting in increasing production and accumulation of ROS.

FIGURE 4.

Diagrammatic representation of endothelial to mesenchymal transition (EndMT) induction by cytokines and other inflammatory mediators. The cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-13 have been show to induce EndMT either directly or through molecular interactions with other pathways. IL-1β, IL-6, and TNF-α induction is mediated by IRAk and is followed by activation of NFκB. IL-13-induced EndMT involves an alternate pathway mediated by STAT6 and the AKT molecular cascade.

FIGURE 5.

Potential sources of transforming growth factor (TGF)-β for initiation of endothelial to mesenchymal transition (EndMT). The most important cellular and noncellular sources of TGF-β available to endothelial cells for initiation of EndMT are shown. Among the cellular sources, the inflammatory and immunologic cells and the autogenous endothelial cells following their stimulation are the most important. The TGF-β bound to extracellular matrix (ECM) molecules is the most important noncellular source.