Atherosclerosis is the leading cause of death in the world. It is a chronic inflammatory condition that is initiated by the intimal retention and modification of cholesterol-loaded lipoproteins and subsequent activations of endothelial cells and vascular smooth muscle cells (SMCs), recruitment of monocytes, and accumulation of inflammatory cells, extracellular matrix and lipids in the intima [1–3]. The origin of cellular components within the intimal lesion during atherosclerotic vascular remodelling remains highly controversial. Traditionally, it was believed that damaged endothelial cells can be replaced by proliferation and migration of neighbouring cells, whereas the cells in atherosclerotic lesions are contributed by monocyte-derived macrophages , proliferation of local sources such as medial SMCs and adventitial fibroblasts [5,6], as well as bone marrow-derived cells . Recent exciting advances in the field, however, have challenged this dogma and suggested that cells in the lesions could also be derived from resident vascular cells via cell transformations, including endothelial-to-mesenchymal transition (EndMT) to form SMC-like cells, and SMC transforming to macrophage-like cells. The pathological setting is further complicated by the identification of vascular resident stem/progenitor cells , which may contribute to all types of cells in the remodelling vessels during formation of atherosclerotic lesions . In this review, our focus will be on highlighting the recent reports on the mechanisms underlying cellular transformations of resident endothelial cells and SMCs, and how they might contribute to the pathogenesis of atherosclerosis.
Major morphological changes in tissues during embryonic development and in pathological processes are accompanied by dramatic transcriptional reprogramming that leads to significant cellular phenotypic transitions. One class of these changes is called epithelial-to-mesenchymal transition, which plays a key role during early embryonic development as well as in pathological processes such as tissue fibrosis . Endothelial cells, similar to epithelial cells, also display significant plasticity that can be controlled by a subcategory of such transition known as EndMT [11,12].
ROLE OF ENDOTHELIAL-TO-MESENCHYMAL TRANSITION IN ATHEROSCLEROSIS
EndMT is one of the key processes in the embryonic development of normal arterial intima , but it can also be reactivated postnatally to contribute to a number of pathological processes, as exemplified by a landmark study by the Kalluri's group that had firmly established EndMT's significant contribution in myocardial fibrosis using endothelial cell lineage tracing techniques . Studies using a similar approach have since established pathological EndMT in a number of other fibrotic diseases [15–20].
Strong evidences have emerged to suggest that EndMT also significantly contributes to the vascular remodelling process during atherosclerosis. For example, atheroprone ApoE−/− mice fed with high-fat diet with inflammatory stress have displayed signs of EndMT in their fibrotic heart tissues , whereas endothelial-conditional knockout of Wnt7b in mice on LDL receptor (LDLR)−/− background (another atheroprone model) has displayed increased levels of EndMT and calcification in their aortic tissues . These studies, however, lack definitive lineage tracing analyses. Using endothelial-specific Cdh5-Cre lineage reporter mice, Simons's group has reliably established EndMT as an important contributor in neointima formation through suppression of endothelial fibroblast growth factor (FGF) signalling that was triggered by the intense inflammatory responses in three vascular injury models, leading to a profound reduction of let-7 miRNA level and subsequent initiation of transforming growth factor-β (TGF-β)-dependent EndMT. These contribute to ≈10%, ≈5%, and ≈3% of SMC in the neointima of the vessels after 2–3 weeks in models of transplant arteriosclerosis, wire injury and vein-to-artery grafting respectively, and can be reversed by treatment with let-7b mimics . FGF receptor 1 (FGFR1) was subsequently found to be the key FGFR in the endothelial cells to regulate this process [24▪]. Cooley et al.[25▪▪] have further established that vascular neointimal SMCs comprised ≈50% endothelial-lineage cells after 5 weeks of murine vein graft remodelling that were dependent on the activation of TGF-β/Smad2/3 signalling in Tie2-Cre lineage tracing mice, and the neointima remodelling and the EndMT process could be reduced substantially by inhibition of TGF-β/Smad2/3 signalling. EndMT was further affirmed to contribute to the SMCs that underlie the neointimal thickening in chronically rejected human coronary arteries and failed human coronary bypass grafts [23–25▪▪], implicating its important role at least in the development of human transplant and vein graft atherosclerosis. The therapeutic potential in attenuating TGF-β signalling by administration of let-7b mimics or antibodies or small-molecule inhibitors of the TGF-β signalling pathway, as displayed in the animal models in these studies, could be conceivable clinical strategies in reducing or preventing the progression of vascular remodelling in human atherosclerosis.
The extent of EndMT contribution in the disease processes discussed needed to be interpreted with care however, as the use of Tie1, Tie2, or Cdh-5 promoter as endothelial lineage markers in these studies could have also marked hematopoietic cells [26–28]. Further investigations using labelled bone marrow cell transplant or parabiosis to exclude involvement of hematopoietic cells in the disease process, with more advanced lineage tracing strategies (such as Col1A1-Cre reporter mice to unequivocally exclude involvement of cells with fibroblast origin) in other atheroprone animal models [29▪▪], in addition to direct histological evidences in plaque formation in human native atherosclerosis, are all necessary to elucidate the definitive role EndMT plays in the pathogenesis before establishing it as a useful therapeutic target in human atherosclerosis.
MOLECULAR MECHANISM GOVERNING ENDOTHELIAL-TO-MESENCHYMAL TRANSITION
During EndMT, endothelial cells undergo a series of gross changes including loss of the endothelial cell apico-basal polarity because of the downregulation of endothelial cell intercellular adhesions complexes such as CD31 and VE-cadherin, a dramatic cytoskeletal remodelling and gene expression changes, and acquisition of spindle-like morphology with mesenchymal/SMC markers such as α-smooth muscle actin (α-SMA), fibroblast-specific protein 1 (or S100A4), and vimentin (Fig. 1). TGF-β superfamily of signalling molecules is the widely regarded master regulator of these processes. Several TGF-β superfamily members signal through Smad-dependent (mainly Smad2/3) and Smad-independent pathways [mitogen-activated protein kinase (MAPK), extracellular-signal-regulated kinase, phosphinositide 3-kinase (PI3K) and p38 MAPK signalling] in modulating transcription factors including Snail1, Snail2 (or Slug), Twist and HEY1/2, further triggering a cascade of signalling pathways that suppress endothelial cell marker expression and induce mesenchymal marker expression [30,31]. Crucially, in line with the paradoxical effects of activating Smad1/5/8 and Smad2/3 in endothelial cell functions [31,32], bone morphogenetic protein 7 (BMP7) antagonizes TGF-β signalling through Smad1/5/8 and inhibits EndMT in mouse models of cardiac [14,19,33▪] and renal fibrosis . Interestingly, TGF-β1 was recently reported to induce aberrant DNA promoter methylation and subsequent transcriptional silencing of Ras-GTP activity inhibitor RASAL1 in driving EndMT in human coronary endothelial cells, an effect that can be reversed by BMP7 through 10–11 translocation 3-mediated demethylation [33▪]. Further studies have established that TGF-β-induced EndMT through non-Smad pathway required protein kinase C-delta (PKC-δ) and c-Abl . We have recently shown that a novel mouse HDAC3 splicing isoform, HDAC3α, can directly drive endothelial cells to undergo EndMT through PI3K/Akt and TGF-β2/Smad2/Snail2 pathways .
Several other signalling pathways, including Wnt and Notch, also modulate EndMT. Canonical Wnt signalling was shown to activate EndMT in an experimentally induced myocardial infarction model using canonical Wnt lineage tracing mice . Conversely, Wnt7b was recently shown to preserve endothelial cells, whereas its inhibitor DKK1 promotes EndMT . The role of Notch signalling in EndMT has also been reported in heart valve and atrioventricular cushion development and vascular smooth muscle differentiation [37,38]. Notch-induced EndMT could be mediated through Runx3 and EndMT transcription factor HEY1/2 . FGF signalling plays a part in decreasing EndMT , possibly through its inhibition of TGF-β signalling [23,24▪], or through Ras and MAPK signalling . Endothelin-1 [42,43], angiotensin II [44,45], nicotinamide adenine dinucleotide phosphate oxidase-2 , advanced glycation products , and the deficiency of plasminogen activator inhibitor-1 , vascular endothelial growth factor (VEGF) , Tie1  and caveolin-1  have all been shown to play a role in initiating the EndMT process. Aside from the major signalling pathways, the role of miRNAs, epigenetic regulation and histone modification in regulating EndMT have recently emerged. TGF-β-induced EndMT could be partly dependent on miR-21 , whereas miR-155 , miR-200b , miR-302c  and let-7  that inhibit EndMT were all downregulated during the process. Recently, histone acetylation [36,52] and hypermethylation of EndMT inhibitors such as BMP7 and RASAL1 [19,33▪] were also implicated in modulating the EndMT process. All these have collectively demonstrated the complexity of this transformation process and further research efforts are necessary to improve our understanding of the underlying mechanisms.
SMOOTH MUSCLE CELL TRANSFORMATION IN ATHEROGENESIS
SMCs are the key cell type in early arterial intimal thickenings and are one of the major components throughout the development of human atherosclerosis [54,55]. Conventional belief is that medial SMCs can only undergo dedifferentiation from a quiescent, contractile phenotype towards a synthetic, proliferative state upon inflammatory triggers to participate in atherosclerotic lesional plaque development. In fact, the hypothesis of phenotypic modulation of SMCs in the plaque has gained force and is recently supported by studies identifying substantial amount of intimal cells with SMC origin within the atherosclerotic lesions using a tamoxifan-inducible Myh11 (gene encoding for the highly specific mature SMC marker smooth muscle myosin heavy chain [SM-MHC])-driven-LacZ lineage tracing mice on the atheroprone ApoE−/− background [56,57]. Most crucially, most of these cells lacked detectable expression of the traditional SMC markers α-SMA and SM-MHC, indicating phenotypic differentiation of SMCs is unequivocally taking place during atherosclerotic lesion formation . Far from being one dimensional in dedifferentiating to the synthetic SMCs during atherogenesis, intimal SMC lineage cells were suggested to be differentiating into macrophage foam cells as supported by previous studies revealing intimal cells coexpressing both SMC (α-SMA) and macrophage markers (CD68) in human atherosclerotic lesions [58,59▪▪], and 40% of CD68+ cells in advanced human atherosclerotic plaque are likely to be of SMC origin [59▪▪]. Indeed, cholesterol loading of arterial SMCs has resulted in markedly decreasing SMC markers expression while acquiring markers and functions of macrophages, which raises the possibility that foam cells in the plaque originally presumed to be macrophages may, in fact, be derived from SMCs [58,60,61] (Fig. 2a). Recently, downregulation of miR-143/145-myocardian axis has been identified to regulate the cholesterol-induced transition of SMCs to macrophage-like cells [62▪▪] (Fig. 2b). These lipid-laden SMC-derived foam cells display diminished type I collagen and fibronectin assembly  and dysfunctional macrophage-like activities (phagocytosis and efferocytosis) [62▪▪], which can potentially expand the size of plaque, curtailing their abilities to synthesize a stable fibrous cap and intensify plaque vulnerability. The role these dysfunctional SMC-derived macrophages might play in atherogenesis is corroborated by the identification of substantial reduced expression in the reverse cholesterol transporter ATP-binding cassette transporter A1 in SMC-derived cells compared with intimal myeloid-derived cells in the human atherosclerotic lesion [59▪▪].
A recent study using conditional lineage tracing mice on ApoE−/− background has in fact identified that more than half of the SMC-derived cells in the lesion also express the macrophage markers Mac2 and CD68, although the low labelling efficiency (≈10%) in this study has hindered assessment of the overall contribution of these cells within the lesion [63▪▪]. Nonetheless, concerns were raised in regarding the true identities of the SMC-lineage marked cells in the lesion as the immunohistochemical analyses used in these studies were not rigorous enough, whereas the choice of macrophage markers used is also expressed in neutrophils, natural killer cells, CD8+ dendritic cell, and CD8+ T-cells [64▪]. This highlights the importance of looking at a comprehensive list of macrophage markers in these lineage-marked cells in flow cytometry in order to discern their true cell fates.
PERSPECTIVES AND CONCLUDING REMARKS
As described above, there is a clear indication that EndMT could contribute significantly to the neointima formation in the pathogenesis of atherosclerosis. The substantial proportion of luminal endothelial cells that were undergoing EndMT during vascular remodelling [23,25▪▪] could be expected to secrete large amounts of collagen and other matrix proteins causing vascular fibrosis, as well as a plentiful source of SMCs that might subsequently be derived into other cell types to contribute to the atherosclerotic lesion formation. Luminal endothelial cells transformation to a mesenchymal phenotype could also likely to lead to endothelial dysfunction, breakdown of intercellular junctions, leading to increased vascular permeability and encourages leukocyte infiltration during atherosclerosis (Fig. 1). The impact of EndMT on the initiation and perpetuation of atherosclerosis is, however, not fully clarified yet. Further investigation to provide direct evidence of the EndMT contributing to atherosclerosis in humans is imperatively necessary.
The cellular transition activities in the plaque are, in fact, even more complicated than the phenotypic modulation of SMCs, as shown by the ability of macrophages undergoing transformation to SMCs upon TGF-β or thrombin stimuli in vitro, and lineage tracing studies have shown that hematopoietic-derived cells express early but not differentiated SMC markers in ApoE−/− lesions (reviewed in ). Collectively, these complex cellular phenotypic transitions highlight the potential in the misclassification of the lesional cell origins based on traditional SMC and macrophage histological markers. It is likely that SMC marker-positive cells in atherosclerotic lesions might be derived from cells other than SMCs, whereas macrophage marker-positive cells might not be derived from monocyte or macrophages. In fact, an epigenetic mark, dimethylation of lysine 4 of histone h3 (h3K4me2) at the MYH11 locus, was recently identified to be restricted to the SMC lineage in human and mouse tissue and persisted even in phenotypically modulated SMCs with no detectable expression of SMC markers in the atherosclerotic lesions, and this represents a very reliable SMC lineage marker to use in any future histological examination of atherosclerotic tissues .
Most importantly, the recent discovery of vascular resident stem/progenitor cells provides direct evidence that they might be a main source of SMCs, macrophages and foam cells within atherosclerotic lesions [8,66]. These cells possess high proliferative capacity and potential to generate endothelial, smooth muscle, hematopoietic or mesenchymal cell progeny [67▪]. Accumulating data indicate the involvement of some of these stem/progenitor cells in the pathogenesis of atherosclerosis, raising potential implications that lesional cells might be derived from resident stem cells. In the vessel wall, Bearzi et al. demonstrated that human coronary arteries contain small clusters of clonogenic c-kit+VEGFR2+CD45−tryptase− progenitor cells that can differentiate toward endothelial cells, SMCs and, in part, cardiomyocytes. The functional capacity of these human cells was illustrated by their integration into newly formed small, intermediate and large-sized blood vessels after in-vivo transfer in a model of coronary artery stenosis in immunosuppressed dogs. This indicates a possibility that smooth muscle-like cells derived from EndMT observed by the investigators may be, in fact, derived from endothelial progenitor cells. Similarly, macrophages/foam cells identified to be of SMCs origin might indeed be a result from stem cell differentiation.
The traditional view on cell origins and accumulation in atherosclerotic lesions has been challenged by the recent findings of cell transformation and transdifferentiation, i.e., one type of mature cells differentiating into another type of mature cells. The discovery of vascular resident stem/progenitor cells provides a potential source of all types of vascular cells within atherosclerotic lesions. Further study to investigate the relationship among all types of cells using specific cell lineage tracing techniques and elucidating their underlying mechanisms would help to provide some basic information for developing effective, novel therapies for patients with atherosclerosis.
Financial support and sponsorship
Vascular stem cell research in the Xu laboratory is supported by the British Heart Foundation and Oak Foundation.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
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