Dyslipidemia is a primary causative factor in the formation of AS lesions and constitutes a significant characteristic of AS. Consequently, regulating the concentration of plasma lipoprotein to return to normal levels is the primary method of AS treatment.

Lowering the level of AS-causing lipoprotein, especially serum low-density lipoprotein cholesterol (LDL-C), can effectively reduce the overall AS plaque load and change plaque composition (Kataoka et al., 2017). In clinical practice, potent lipid-lowering statin drugs are widely used to diminish lipid levels in AS patients, serving as a cornerstone in the treatment regimen. These drugs reduce the plasma cholesterol levels of patients by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA), a key enzyme in the cholesterol biosynthetic pathway, thereby exerting their anti-AS effect (Almeida and Budoff, 2019; Sarraju and Nissen, 2024). Since 2016, an increasing number of non-statin drugs with lipid-lowering and anti-AS effects have been discovered and are frequently utilized in clinical practice for anti-AS treatment, either in combination with statins or as standalone therapies (Masana et al., 2023; Nakano et al., 2023; Sucato et al., 2024), For instance, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors can diminish its binding to low-density lipoprotein receptors (LDLR) on the surface of hepatocytes by targeting PCSK9, thereby increasing the quantity of LDLR, facilitating the clearance of LDL-C in plasma, lowering LDL-C levels, and exerting a protective role against AS (Nicholls et al., 2022; Biccirè et al., 2023). Research has substantiated that the lipid-lowering efficacy of combined treatment with lipid-lowering drugs is significantly superior to that of statin monotherapy, and the impact on plaque regression is also more pronounced (Sucato et al., 2024).

Reducing the plasma lipid cholesterol level in patients with AS is a critical precursor to the effective treatment of AS and the regression of AS plaques. Lipid-lowering therapy has been shown to improve vascular endothelial function and increase plaque stability (Krone and Müller-Wieland, 1999; Tousoulis et al., 2002), making it a crucial strategy for reducing the risk factors associated with AS. Lipid-lowering agents, particularly statins and PCSK9 inhibitors, continue to play a pivotal role in therapeutic regimens due to their dual capacity for cholesterol reduction and their additional benefits, which include anti-inflammatory effects and the stabilization of atherosclerotic plaques. (Dimitriadis et al., 2024; Ueki et al., 2024). However, the onset of AS is driven by abnormally elevated blood lipid levels and endothelial dysfunction. Therefore, while reducing plasma lipoprotein concentrations, it is imperative to promote the regeneration and functional repair of vascular endothelial tissue in areas that have been damaged.

Normal arteries possess a well-developed three-layer structure consisting of the intima, the media, and the adventitia arranged from the innermost to the outermost layer. The vascular endothelium, the innermost component of the blood vessel, is composed of a single layer of endothelial cells that, together with the basement membrane, form the vascular intima (Pugsley and Tabrizchi, 2000; Krüger-Genge et al., 2019). The vascular endothelium serves not only as a mechanical barrier between blood and surrounding tissues but also as a crucial endocrine organ. By sensing hemodynamic changes and blood-borne signals, it regulates vasomotor function through the release of vasoactive substances, thereby maintaining vascular homeostasis (Krüger-Genge et al., 2019).

The surface of vascular endothelial cells is comprised of a layer of glycoproteins that play an important role in essential physiological processes, such as endothelial cell signal transduction, material transport, and immune response (Pugsley and Tabrizchi, 2000). The functional integrity of endothelial cells is vital for maintaining vascular homeostasis. Endothelial dysfunction arises when there is an imbalance between vasodilator and vasoconstrictor substances secreted by endothelial cells, coupled with chronic mechanical shear stress resulting from conditions such as dyslipidemia, hypertension, or diabetes (Nafisa et al., 2018; Le Master et al., 2020). This dysfunction leads to increased permeability of vascular endothelial cells and abnormal expression of inflammatory factors (Tribe and Poston, 1996). In the early stage of AS, substantial lipid accumulation beneath the vascular endothelium results in endothelial cell damage and subsequent dysfunction. Therefore, promoting the functional repair of damaged vascular endothelium is a crucial step in the treatment of anti-AS.

In general, when the body’s intrinsic repair mechanisms are functioning effectively, endothelial damage can be repaired, and vascular integrity can be restored. These mechanisms typically entail cell replication and the replacement of non-functional endothelial cells (Yang et al., 2018). Current research suggests that endothelial cell repair involves two main mechanisms. On the one hand, existing mature endothelial cells may undergo mitosis, but their proliferation capacity is limited as most of them are terminally differentiated cells. The second mechanism involved in endothelial repair is via circulating endothelial progenitor cells (EPCs) (Woywodt et al., 2002). EPCs represent a circulating subset of vascular endothelial cells that are crucial for maintaining the stability of vascular endothelial structure and function. These cells are capable of differentiating and producing new endothelial cells, which helps in repairing damaged endothelial tissue. EPCs play a significant role in preserving the homeostasis of the vascular environment (Woywodt et al., 2002; Komici et al., 2023). These immature progenitor cells possess robust proliferation and differentiation capacities. In instances of endothelial tissue damage, they migrate to the affected site requiring repair, facilitating the proliferation and regeneration of endothelial cells within the damaged area or differentiate into mature endothelial cells via utilizing paracrine signaling pathways to support endothelial repair and preserve vascular homeostasis (Bai et al., 2010; Yao et al., 2013; Zhang et al., 2014).

Preserving endothelial barrier integrity is essential for upholding vascular homeostasis, regulating the fluid balance between circulating substances and surrounding tissues, and preventing the occurrence of vascular diseases. Repairing damaged endothelial cells to ameliorate endothelial dysfunction, along with lipid-lowering therapy, is critical for blocking factors that contribute to AS. However, while these strategies are essential, they are undoubtedly far from sufficient for the stability and regression of plaques. A comprehensive treatment for the regression of AS plaques should also focus on the processes involved in plaques regression and modifications in their composition.

VSMCs are important cellular components in the pathological progression of AS. It can undergo transdifferentiates into various types of cells under pathological conditions, such as macrophage-like cells, foam cells, osteoblast-like cells, mesenchymal stem cell-like cells, and myofibroblast-like cells. This transdifferentiation significantly influences the development and process of AS (Qi et al., 2019; Wang et al., 2023).

The phenotypic transformation of VSMCs in vivo is often affected by integrating molecular and environmental factors. Transcription factors such as myocardin (MYOCD), serum response factor (SRF), Krüppel-like factor 4 (KLF4), and octamer-binding transcription factor (OCT4) are notably significant in governing the phenotypic transition of VSMCs. These transcription factors are recognized as key regulators of the phenotypic transformation of VSMCs (Ackers-Johnson et al., 2015; Kansakar et al., 2021). MYOCD acts as a cofactor of SRF, binding to the CArG-box element in the promoters of contraction-related genes, thereby promoting the expression of VSMC contraction genes (Xia et al., 2017). Furthermore, the upregulation of MYOCD expression has protective effects on AS lesions by inhibiting plaque growth. Xia et al. (Xia et al., 2021) indicated that MYOCD can modulate the expression of ATP-binding cassette transporter A1 (ABCA1) in human aortic vascular smooth muscle cells (HAVSMCs) via an SRF-dependent mechanism to impact cholesterol transport processes. Knocking out MYOCD in ApoE^−/− mice resulted in reduced cholesterol efflux and increased intracellular cholesterol content due to the downregulation of ABCA1 expression. This dysregulation contributed to the exacerbation of atherosclerosis and facilitated plaque formation.

Several transcription factors, including KLF4, NF-κB, specific protein-1 (SP-1), and transcription factor 21 (TCF21), exert inhibitory control over the expression of genes associated with smooth muscle contraction. Their mechanisms involve disrupting the interaction between SRF and CArG-box, thereby initiating the dedifferentiation process of contractile smooth muscle cells (SMCs). Conversely, the maintenance of the contractile phenotype is positively governed by transforming growth factor β (TGFβ), miR143, and miR145. Notably, these molecules can cooperatively contribute to the partial downregulation of KLF4, thereby counteracting its influence in promoting smooth muscle cells dedifferentiation. This underscores their critical function in modulating the phenotype of smooth muscle cells (Grootaert and Bennett, 2021; Kansakar et al., 2021).

KLF4, a pivotal transcription factor involved in the phenotypic transformation of VSMCs, comprises three distinct domains: transcriptional repression, transcriptional activation, and DNA binding domains (Yang et al., 2021; Yap et al., 2021). The DNA-binding domain features a highly conserved cysteine 2/histidine two zinc finger structure, which facilitates specific recognition and binding to GC-rich DNA sequences within the promoter regions of target genes. This domain is essential for regulating critical cellular functions such as proliferation, differentiation, and apoptosis. Additionally, the presence of a transcriptional repression domain in conjunction with an N-terminal transcription activation domain enables KLF4 to switch between activating and inhibiting the transcription of target genes (Yoshida and Hayashi, 2014). VSMCs-specific knockout of KLF4 has been shown to significantly reduce plaque size and enhance plaque stability by impeding the transformation of smooth muscle cells into macrophage-like cells. This finding highlights the critical role of KLF4 in maintaining VSMC phenotype and its potential implications in atherosclerosis progression and plaque vulnerability (Shankman et al., 2015; Alencar et al., 2020).

OCT4, another pivotal pluripotency factor, plays a significant role in regulating phenotypic transitions and exerts an antagonistic effect on KLF4 (Cherepanova et al., 2016). The study conducted by Shin et al. (Shin et al., 2023) used an Oct4-IRES-GFP reporter mouse model and a tamoxifen-inducible Myh11-CreERT2 Oct4 knockout mouse model and found that the targeted knockout of OCT4 in VSMCs was observed to reduce the expression levels of key contractile markers including ACTA2, MYH11, and tagln. This reduction was associated with enhanced migration and excessive proliferation of VSMCs. The results indicate that OCT4 can be promptly upregulated in response to vascular injury, effectively modulating the expression of ACTA2 to impede phenotypic transitions and mitigate the excessive proliferation of VSMCs, ultimately preventing aberrant thickening of the vascular wall. In addition to the aforementioned regulatory factors, recent studies have revealed that VSMCs can autonomously release low levels of adenosine triphosphate (ATP) to modulate vasoconstriction. Chen et al. (Chen et al., 2024) observed that autocrine ATP release hinders the transition of VSMCs to a synthetic phenotype. This was evidenced by experiments inhibiting ATP release in human aortic smooth muscle cells, which showcased that the activation of P2Y2 receptors (P2Y2R) through basal ATP release is crucial for preserving the contractile phenotype of VSMCs. Furthermore, the role of mitochondria as central players in energy metabolism and their involvement in the phenotypic transformation of SMCs has become increasingly recognized. The research conducted by Sun et al. (Sun et al., 2024) discovered that the mitochondrial PKA anchoring protein A-kinase anchoring protein 1 (AKAP1) can inhibit PDGF-BB-mediated phenotypic transformation, proliferation, and migration of VSMCs by promoting Dynamin-related protein 1 (Drp1) phosphorylation (Ser637) via rat carotid artery balloon injury model and HAVSMCs treated with PDGF-BB in vivo and in vitro. In parallel, Paredes et al. (Paredes et al., 2024) employed primary human aortic smooth muscle cells (HASMCs) and a mouse model of elastase-induced aneurysm formation to demonstrate that caseinolytic protease proteolytic protein (ClpP), a mitochondrial protease, can govern the phenotype of VSMCs by elevating the NAD⁺/NADH ratio and activating the NAD⁺-dependent deacetylase Sirtuin1 (SIRT1). This action aids in upholding the contractile phenotype and differentiation state while diminishing the expression of VSMC dedifferentiation markers. These findings underscore the critical role of energy metabolism and mitochondrial homeostasis in modulating the VSMC phenotype. Consequently, fine-tuning mitochondrial homeostasis and energy metabolism may offer a promising preventive strategy against atherosclerosis by regulating the phenotypic transition of VSMCs. (Figure 2).

VSMCs demonstrate remarkable plasticity. Vessel injury induces a phenotypic transformation from differentiated to dedifferentiated VSMCs, which involves reduced expression of contractile proteins and increased production of extracellular matrix and inflammatory cytokines (Liang et al., 2024). This pathological phenotype switching plays a major role in the development of atherosclerosis and plaques (Li et al., 2013). Preserving the contractile phenotype of VSMCs and impeding their transition to a synthetic phenotype is crucial in halting vascular intimal thickening in the progression of AS. This approach can mitigate the formation and build-up of foam cells, thereby averting the advancement of AS.

The deposition of lipoproteins beneath the endothelium plays a crucial role in the development of atherosclerosis, with foam cells abundant in cholesterol esters emerging as a distinctive feature of atherosclerotic lesions. Reverse cholesterol transport (RCT), the mechanism of ferrying surplus cholesterol from peripheral tissue cells (including foam cells within atherosclerotic plaques) to the liver for breakdown and elimination from the body (Ohashi et al., 2005), has garnered significant interest as a potential strategy for halting or potentially reversing atherosclerotic lesions.

Lipid-bound cholesterol internalized by macrophages undergoes hydrolysis within lysosomes, yielding free cholesterol that is subsequently shuttled to and integrated into the plasma membrane. Surplus membrane cholesterol and a fraction of free cholesterol derived from low-density lipoprotein are directed to the endoplasmic reticulum, where they undergo re-esterification by acyl-CoA: cholesterol acyltransferase-1 (ACAT1) to form cholesterol ester (CE), which is then stored in the cytoplasm as lipid droplets to avert cytotoxicity stemming from excess free cholesterol. However, excessive accumulation of CE promotes foam cell formation in macrophages, subsequently accelerating the progression of AS (Ghosh, 2011). Hence, fostering the reverse transport of cholesterol within macrophages is imperative in thwarting foam cell formation and the progression of atherosclerotic lesions.

However, only free cholesterol can undergo efflux to facilitate reverse cholesterol transport (RCT). Therefore, promoting the hydrolysis of CE is not only the initial step in the occurrence of RCT but also the rate-limiting step in this process (Ouimet and Marcel, 2012; Ghosh, 2012; Sakai et al., 2014; Hafiane et al., 2019; Ouimet et al., 2019). The hydrolysis of intracellular CE can be classified into neutral CE hydrolysis and acidic CE hydrolysis due to the distinct pH levels in the reaction milieu (Jeong et al., 2017). Key enzymes participating in neutral CE hydrolysis include hormone-sensitive lipase (Lipe), carboxylesterase 3 (Ces3), and neutral CE hydrolase 1 (NCEH1), with NCEH1 assuming a primary function (Sakai et al., 2014). Furthermore, studies indicate that lysosomal acid lipase (LAL) also possesses the capability to hydrolyze CE. In the context of lipid accumulation, autophagy facilitates the transport of cytoplasmic lipid droplets to lysosomes within macrophages. Within the lysosomal lumen, LAL catalyzes the breakdown of lipid droplet CE, yielding free cholesterol for elimination (Jeong et al., 2017; Ouimet et al., 2019; Korbelius et al., 2023).

Free cholesterol is extricated from macrophages within the endothelium of the blood vessel wall via various mechanisms, including the action of ATP-binding cassette (ABC) transporters A1 and G1 (ABCA1/G1), passive diffusion, scavenger receptor B1 (SR-B1), and caveolins, among others. Subsequently, the liberated free cholesterol is ensnared by high-density lipoproteins (HDL) and apolipoprotein A1 (ApoA1) for conveyance to the liver for excretion (Ouimet and Marcel, 2012; Favari et al., 2015; Ouimet et al., 2019). ABCA1/G1 serves as pivotal receptors in the initial phase of RCT within atherosclerotic plaques (Yvan-Charvet et al., 2007), actively facilitating the transfer of cellular cholesterol from lipid-laden macrophages to extracellular receptors. Augmenting the expression of HDL or ApoA1 has been shown to mitigate the necrotic build-up of foam cells (Kluck et al., 2023). (Figure 3)

Macrophages uptake excessive lipids, leading to their transformation into foam cells. The buildup of lipid-laden foam cells beneath the vascular endothelium stands as a primary factor driving the expansion and progression of AS plaques. Facilitating the reverse transport of cholesterol within foam cells and diminishing the lipid burden of macrophages are intricately linked to fostering plaque regression. A recent study suggested that targeting Epsins, a family of endocytic adaptors, in lesional macrophages may offer therapeutic benefits for advanced atherosclerosis by reducing CD36-mediated lipid uptake and increasing ABCG1-mediated cholesterol efflux (Cui et al., 2023). Additionally, injection of recombinant HDL or apoA-I mimetic peptides accelerates cholesterol efflux from peripheral tissues, improves vascular endothelial state, and leads to regression of atherosclerotic plaque (Chiesa and Sirtori, 2003; Poteryaeva and Usynin, 2022). These studies indicate that enhancing reverse cholesterol transport within plaques may hold significant potential for facilitating plaque regression.

In the context of apoptotic foam cells or necrotic cores, timely removal of apoptotic cell remnants holds particular significance. Concurrently with enhancing cholesterol efflux from foam cells, the organism’s clearance mechanisms, and phagocytic capacities should be actively engaged to forestall the accumulation of apoptotic cells and the further enlargement of necrotic cores.

The necrotic core formation in AS lesions is attributed to the accumulation of cellular debris resulting from apoptotic necrosis of lipid- and cholesterol-laden foam cells. Under physiological conditions, apoptotic cell fragments are typically engulfed by phagocytic macrophages via efferocytosis, thus averting the release of apoptotic cellular contents and accumulation of debris (Westman et al., 2020), However, during the progression of AS, macrophages adopt a foam cell phenotype due to excessive lipid accumulation, which impairs efferocytic and phagocytic functions. This reduction in clearance capacity for apoptotic cells culminates in the accumulation of apoptotic cells and subsequent secondary necrosis. Consequently, the accumulation of cellular debris and release of cellular contents may exacerbate the progression, enlargement, and potential rupture of AS plaques (Kasikara et al., 2021; Li et al., 2022; Puylaert et al., 2022). Thus, enhancing the phagocytic clearance capacity of macrophages represents a crucial strategy for preventing the development of AS lesions and promoting plaque regression.

Macrophages migrate toward the vascular endothelium and phagocytose lipid particles, ultimately transforming into foam cells. The accumulation of these foam cells serves as a prerequisite for the development and progression of atherosclerotic plaques. Notably, macrophages exhibit substantial heterogeneity in their behavior and function during the development of atherosclerotic lesions. As the microenvironment within the plaque undergoes dynamic changes, the phenotypic profile of macrophages adapts accordingly. Distinct macrophage subtypes occupy dominant positions at various stages of atherosclerosis, reflecting their specific roles and contributions (Tabas and Bornfeldt, 2016). These diverse macrophage phenotypes play crucial roles in either promoting or inhibiting disease progression, thereby significantly impacting the overall development and evolution of atherosclerotic lesions.