An update on the phenotypic switching of vascular smooth muscle cells in the pathogenesis of atherosclerosis

Abstract

Vascular smooth muscle cells (VSMCs) are involved in phenotypic switching in atherosclerosis. This switching is characterized by VSMC dedifferentiation, migration, and transdifferentiation into other cell types. VSMC phenotypic transitions have historically been considered bidirectional processes. Cells can adopt a physiological contraction phenotype or an alternative "synthetic" phenotype in response to injury. However, recent studies, including lineage tracing and single-cell sequencing studies, have shown that VSMCs downregulate contraction markers during atherosclerosis while adopting other phenotypes, including macrophage-like, foam cell, mesenchymal stem-like, myofibroblast-like, and osteochondral-like phenotypes. However, the molecular mechanism and processes regulating the switching of VSMCs at the onset of atherosclerosis are still unclear. This systematic review aims to review the critical outstanding challenges and issues that need further investigation and summarize the current knowledge in this field.

Keywords: Atherosclerosis; Phenotypic switching; VSMCs.

Fig. 1

Overview of vascular smooth muscle cells (VSMC) phenotypic transition within media and atherosclerotic lesions in mice. Lineage tracing and scRNA-seq studies have shown that partially contractile VSMCs transform into transitional pluripotent cells (e.g. lgals3 + VSMCs, SEM cells and MSC-like SMCs) in response to environmental stimuli. SEM cells have the potential to differentiate into macrophage-like SMCs and fibrochondrocyte-like SMCs and even revert to their contractile phenotype. ATRA can inhibit the conversion of SMCs to SEM cells. KLF4-dependent Lgals3-activated VSMCs subsequently exhibited a shift to a variety of SMC phenotypes, including inflammatory cells, ECM-rich cells, osteogenic phenotypes and macrophage-like cells. Whether MSC-like SMCs can differentiate into other SMC phenotypes is currently unclear. There are two possible sources of SMC foam cells in atherosclerotic plaques. One is foam cells derived directly from SMCs and the other is from macrophage-like SMCs. It is unclear whether SMCs need to acquire a macrophage-like phenotype to become foam cells, or whether they become foam cells before expressing macrophage markers. OCT4 and SMC-specific knockout of TCF21 inhibit SMC phenotypic modulation in mice. ATRA indicates all-trans retinoic acid; KLF4, krüppel-like factor 4; TCF21, transcription factor 21; OCT4, octamer-binding transcription factor 4

Fig. 2

Influencing factors and transcription factors/co-factors of the phenotypic switching of VSMCs. The phenotypic switching of VSMCs in vivo is often influenced by the integration of a range of extracellular signals, signalling pathways, and transcription factors. The main extracellular signals (e.g. lipids, retinoic acid, inflammatory mediators, growth factors, reactive oxygen species) activate signalling pathways that converge on transcription factors (e.g. KLF4, NF-κB, SP-1, and TCF21) to regulate the transdifferentiation of VSMCs into various cell types such as macrophage-like cells, foam cells, osteochondrocytes, mesenchymal stem cell-like cells and myofibroblast-like cells in disease states. MYOCD is a cofactor of SRF that binds to the CArG-box element within the promoter of contraction-related genes to promote VSMC contractile gene expression. Many transcription factors (e.g. KLF4, NF-κB, SP-1, and TCF21) repress the expression of contraction-related genes by inhibiting SRF binding to CArG-boxes. TGFβ, transforming growth factor-β; IGF, insulin-like growth factor; ECM, extracellular matrix; Ang II, angiotensin II; ROS, reactive oxygen species; MYOCD, myocardin; SRF, serum response factor; KLF4, krüppel-like factor 4; NF-κB, nuclear factor-κB; SP-1, specificity protein-1; Elk1, Ets-like protein 1; Runx2, runt-related transcription factor 2; TCF21, transcription factor 21

Fig. 3

Signals mediating the phenotypic switching of VSMCs. The TGF-β signalling pathway is involved in the differentiation of SMC towards a contractile phenotype. Some growth factors (e.g., PDGF) exert their role in promoting VSMC dedifferentiation mainly through the Ras/Raf/MEK/ERK, PI3K/Akt, NF-κB, and JAK2/STAT3 pathways. Activation of Wnt/β-catenin protein signalling may promote the osteogenic transdifferentiation of VSMC. ECM binding to integrins causes activation of FAK. Phosphorylated FAK recruits Grb2-SoS complex, which then activates its downstream Ras and PI3K pathways. H2O2 is released from NOX4, which activates the PI3K and NF-κB pathways and mediates the VSMC phenotypic switch. In the nucleus, RA acts as a ligand for the RA receptor (RAR) and binds to the retinoic acid X receptor (RXR) near the target gene as a heterodimer with the RA response element (RARE) and thus regulates VSMC phenotype switching. Abbreviations: TGF-β, transforming growth factor β; SARA, Smad anchor for receptor activation; Grb2, growth factor receptor-bound protein 2; SoS, son of seven-less; Ras, rat sarcoma; Raf, Raf protein kinase; MEK, mitogen-activated ERK-regulating kinase; ERK, extracellular signal-regulated kinase; Elk1, Ets-like protein 1; PI3K, phosphatidylinositol 3-kinase; Akt/PKB, protein kinase B; mTOR, mammalian target of Rapamycin; FOXO4, forkhead box protein O; JAK, The Janus kinases; H2O2, hydrogen peroxide; NOX4, NADPH oxidase-4; ROS, reactive oxygen species; IKK, I kappa B kinase; NF-κB, nuclear factor kappa-B; ECM, extracellular matrix; FAK, focal adhesion kinase; RA, retinoic acid

Fig. 4

Epigenetic mechanisms regulating the phenotypic switching of VSMCs in response to vascular injury or atherosclerotic disease. The epigenetic processes include DNA methylation, histone modifications, non-coding RNA expression. During the phenotypic regulation of SMCs cultured in vitro, overall hypomethylation occurs. In the basal state, SMC contractile genes (e.g. SMα-Actin) are post-translationally modified through alterations such as acetylation of histones 3 and 4 (H3Ac and H4Ac) and dimethylation of H3K4 (H3K4diMe). These modifications are thought to remodel chromatin structure, allowing the SRF-Mycardin complex to bind to CArG-box elements and drive SMC-selective gene expression. In contrast, loss of previously activated histone modifications (e.g. H3/H4 Ac and H3K4diMe) and reduced ability of the SRF-Mycardin complex to bind to the CArG-box suppressed the expression of SMC marker genes. Non-coding RNAs (mainly microRNAs, long non-coding RNAs and circular RNAs) also play a key role in the VSMC phenotypic switch. The arrows and T-shaped ends represent promoting and inhibiting phenotype switch, respectively