SR-A, belonging to the SR family, is expressed in VSMCs (Kodama et al., 1990; Daugherty et al., 1997; Li et al., 1995). It mainly interacts with ox-LDL and ac-LDL and changes with lipid internalization (Moore and Freeman, 2006). Suzuki et al. reported that knockout of SR-A1 in ApoE^−/− mice reduced cholesterol uptake and protected against atherosclerotic development (Suzuki et al., 1997). When cells are exposed to transforming growth factor-β 1 (TGF-β1) and ox-LDL in vitro, SR-AII and SR-AI levels are elevated in VSMCs (Li et al., 1995; Gong and Pitas, 1995; Yan et al., 2011). Krüppel-like factor 4 (KLF4) suppresses SR-A transcriptionally, and formononetin substantially attenuates the interplay between KLF4 and SR-A in VSMCs (Ma et al., 2020).

It is known that SR-BI is the core receptor for high-density lipoprotein (HDL) and mainly participates in cholesterol efflux (Phillips, 2014). Note that SR-BI also participates in the regulation of cholesterol influx. In endothelial cells (ECs), SR-BI is not only shown to inhibit the progress of AS by inducing the formation of nitric oxide but also promotes AS through enhancing LDL transcytosis (Mineo and Shaul, 2003; Yuhanna et al., 2001). However, whether SR-BI exhibits an atheroprotective effect in vascular tissue is unclear. Recent studies found SR-BI was a protective compensatory mechanism that maintained cholesterol homeostasis without ABCG1 (Oladosu et al., 2023).

CD36 was originally identified in the late 1980s and is in the SR family class B (Rao et al., 2014; Yu et al., 2013; Oquendo et al., 1989). Lesion development is significantly reduced in CD36^−/−/ApoE^−/− mice (Febbraio et al., 2000). CD36 expression increases after treatment with ox-LDL, high glucose, or oleic acid, which enhances lipid accumulation in human coronary VSMCs. (Ma et al., 2011; Schlich et al., 2015; Xue et al., 2010). Accumulated studies found triggering receptor expressed in myeloid cells 2 (TREM2) expressions were raised in plaques (Depuydt et al., 2020; Cochain et al., 2018; Willemsen and de Winther, 2020; Guo et al., 2023). Ox-LDL activated PI3K/AKT, p38 mitogen-activated protein kinases (MAPK), and c-Jun N-terminal kinase (JNK) signaling pathways and enhanced the expression of HDGF and TREM2, which upregulated the expression of LOX-1 and CD36 (Guo et al., 2023). Meanwhile, ox-LDL induced Wnt Family member 5A (WNT5A) autocrine signaling and then activated disheveled segment polarity protein 2 (DVL2)/JNK signaling, which raised the level of CD36 (Ackers et al., 2020). Urszula Rykaczewska et al. found that Bcl2-associated transcription factor 1-ribosomal protein S8-tetraspanin-9-CD36 participated in the regulation and fate of VSMCs transforming into foam cells (Rykaczewska et al., 2022).

LOX-1, which accelerates foam cell production, is expressed in VSMCs (Sawamura et al., 1997) and accounted for 5%–10% of ox-LDL uptake (Mehta et al., 2007; Schaeffer et al., 2009). Ox-LDL induces VSMC transformation to foam cells through mtROS/c-Fos/LOX-1, which promotes ox-LDL uptake (Miao et al., 2022). In contrast, Zhang W et al. found ox-LDL suppressed the activity of non-receptor tyrosine kinase on the alpha1 C subunit (CACNA1C) and the L-Ca channel subunit, which led to downregulation of LOX-1 and subsequently promoted the proliferation, migration, and foam cell formation. Importantly, genistein mitigated these effects (Zhang et al., 2022). Zheng et al. also suggested that ox-LDL promoted autophagic degradation of LOX-1 (Zheng Y. et al., 2023). Deng et al. found that LKB1 participated in the regulation of the expression of LOX-1, which activated sirtuin 6 (SIRT6) through direct phosphorylation and then histone deacetylation of the promoter of LOX-1 (Deng et al., 2023). Myotoxin III, which is consistent in structure and function with phospholipase A2, upregulated the expression of SR-A and LOX-1 through PPAR-γ and PPAR-β/δ and enhanced intake of acetylated low-density lipoprotein (ac-LDL) in VSMCs (Giannotti et al., 2019). Poria coco polysaccharides inhibited the expression of LOX-1 induced by ox-LDL (Zhao et al., 2020). However, LOX-1 activation promoted oxidative stress, yielding higher ox-LDL production, thereby enhancing cycle self-amplification (Xu et al., 2013).

SR-PSOX/CXCL16, a class G member of the SR family, is expressed in VSMCs in human AS plaques and primarily binds and internalizes the ox-LDL (Zanoni et al., 2018) (Wågsäter et al., 2004; PrabhuDas et al., 2017; Minami et al., 2001; Hofnagel et al., 2002). IFN-γ notably stimulates CXCL16 expression in human VSMCs and increases ox-LDL uptake. However, this process does not influence CD36 and LOX-1 levels (Wågsäter et al., 2004).

Belonging to the LDLR family, LRP1 is expressed in VSMCs and is responsible for LDL uptake (Boucher et al., 2003; Llorente-Cortés et al., 2000). In VSMCs, knockout of LRP1 makes individuals more prone to cholesterol-induced atherosclerosis via the signaling pathways of platelet-derived growth factor receptor (PDGFR) and TGF-β (Boucher et al., 2003; Boucher et al., 2007). LRP1 has a protective function against atherosclerosis through modulating ATP-binding cassette A1 (ABCA1) expression via extracellular regulated protein kinases 1/2 (ERK1/2)/cytosolic phospholipase A2 (cPLA2/)arachidonic acid (Graves et al., 1996; Lin et al., 1992), an inhibitor of ABCA1 expression driven by liver X receptors (LXR) (DeBose-Boyd et al., 2001). Gordts PL et al. suggested LRP1 NPxYxxL motif is essential in LRP1’s anti-atherosclerotic function (Gordts et al., 2009). Therefore, atherosclerosis development can be mitigated via LRP1. In contrast, higher levels of LRP1 and intracellular lipid deposition were found in VSMCs that originated in advanced human plaques (Llorente-Cortés and Badimon, 2005). In vivo, ag-LDL induces expression of LRP1 in VSMCs and, in turn, promotes ag-LDL internalization through cluster II CR9 domain’s C-terminal half (Llorente-Cortés et al., 2002; Costales et al., 2015). Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates the metabolism of LDL through LDLR and facilitates foam cell formation via SNHG16/EZH2/H3K27me3/TRAF5 in VSMCs (Liu et al., 2023; Glerup et al., 2017). Thus, LRP1 may exert opposing effects in VSMCs. The dual function of LRP1 in the metabolism of LDL in VSMCs remains unclear.

Lysosomal acid lipase (LAL), encoded by lipase A gene (LIPA), is the sole enzyme for hydrolyzing CEs to FC in late endosomes/lysosomes, which is called lipophagy, or selective autophagy (Ouimet et al., 2011). In both human and mouse atherosclerosis, LAL prevents the overload of intracellular lipids and sustains cholesterol balance, which is the core progress to regulate VSMC-derived foam cell formation (Dubland and Francis, 2015; Dubland et al., 2021). However, Dubland and Jerome found impaired lysosomal function or deficiency of LIPA/LAL is an inherent characteristic that is not related to species in SMCs rather than a gene deficiency that is specific to a subset of individuals. Dubland et al. also suggested that the upregulation of LIPA or increasing activity of VSMC is an effective strategy for enhancing the processing of intracellular accumulated cholesterol in VSMC-derived foam cells. FOXO1 or TFEB is the activator of LIPA expression and the function of LAL. However, further studies are required to understand how upregulating these activators inhibits VSMC-derived foam cell formation.

ACAT1 catalyzes FC to CEs in the ER and is stored in cytosolic LDs (Suckling and Stange, 1985; Rudel et al., 2001). Accumulative evidence supports that many CEs accumulate in VSMCs or macrophage-derived foam cells and accelerate plaque formation (Ross, 1993). Higashimori et al. identified toll-like receptor 4 (TLR4) deficiency inhibited ACAT1 expression and formation of VSMC-derived foam cells induced by free cholesterol (Higashimori et al., 2011). Y-W Yin et al. further found that PPARγ inhibited the expression of ACAT1 through TLR4/MyD88/NF-κB (Yin et al., 2014). Bethany J Bogan et al. reported fatty acid synthase (FASN), which synthesized fatty acids, was upregulated in atherosclerotic human coronary arteries and induced foam cells derived from SMCs when exposed to cholesterol, which not only induced CD68 expression through KLF4/sterol O-acyltransferase 1 (SOAT1) but also inhibited ABCA1 expression to limit cholesterol efflux. FASN, therefore, acts in the biosynthesizing of fatty acids and functions in cholesterol esterification’s inverse association with efflux (Bogan et al., 2024).

Liquid droplet-associated proteins (LDAPs), which are situated on the exterior of lipid droplets (LDs), are crucial for their assembly and breakdown processes (Yuan et al., 2012). PLIN2, belonging to the PAT family, is considered a scaffold protein for LD formation and is expressed in foam cells (Paul et al., 2008; Becker et al., 2010). During the process of lipid accumulation, PLIN2 relocates from the cell’s periphery to the cytoplasmic region, where it aids in the sequestration of long-chain fatty acids and the genesis of lipid droplets. (Gao et al., 2000). Myotoxin III (MT-III) facilitates the translocation of PLIN2 and PLIN3 from their usual repositories, underscoring the significance of these proteins in the assembly of lipid droplets within VSMCs (Giannotti et al., 2019). MT-III, an Asp49 sPLA2, induces LD formation in VSMCs. This biological effect is contingent upon the activity of key enzymes such as diacylglycerol O-acyltransferase (DGAT) and acyl-CoA: cholesterol acyltransferase (ACAT), as well as a contribution by peroxisome proliferator-activated receptors (PPARs), specifically the γ and β/δ subtypes.

Neutral cholesteryl ester hydrolase (NCEH) hydrolyzes excess CEs to FC in LDs (Mulas et al., 2014; Yin et al., 2014; Dubland and Francis, 2015). This is the first step to start the reverse cholesterol transport (RCT). The FC from this process can be processed by two paths: one is re-esterification by ACAT1 in ER, and the other is efflux by ABC transporters (Sekiya et al., 2009; Escary et al., 1999). However, research related to the regulation of NCEH is limited in VSMCs. Additional research is necessary to determine the precise roles that NCEH plays in the level of FC as well as the process of cholesterol removal in VSMCs.

During conditions of excessive lipid burden, ABCG1 and ABCA1 as transporters, along with SR-B, are responsible for a significant portion of cholesterol removal, accounting for approximately 60%–70% of the total efflux capacity (Jin et al., 2015) (Pownall et al., 2021). The cholesterol transporters ABCA1 and ABCG1 are recognized for their protective role against atherosclerosis (Phillips, 2014; Favari et al., 2015). ABCG1 is a mediator in the delivery of unbound cholesterol molecules to HDL particles, thereby participating in cholesterol transport mechanisms (Ouimet et al., 2019). An elevated expression of ABCG1 results in the increased transfer of unbound HDL particles, indicating its significant role in cholesterol efflux (Frambach et al., 2020). Nevertheless, mutation of ABCA1 in ApoE^−/− mice demonstrates minimal impact on the development of atherosclerotic plaques, suggesting a complex interplay of these factors in AS (Aiello et al., 2003). ABCA1 is vital because it promotes unesterified cholesterol transfer to ApoA-1 molecules that are deficient in lipids, thereby initiating the formation of HDL particles (Yvan-Charvet et al., 2007). Accumulated studies found that expression of ABCA1 was impaired in foam cells from VSMCs compared to foam cells that are derived from leukocytes (Wang Y. et al., 2019; Robichaud et al., 2022). The diminished capacity to expel surplus cholesterol is a principal factor driving VSMCs to become the primary source of foam cells in lesions (Lin et al., 2024).

It is known that KLF4 influences the phenotypic transition of intimal SMCs within atherosclerotic lesions (Kim et al., 2023). However, there has always been controversy regarding the relationship between KLF4 and ABCA1. Shankman and others suggested that the relationship between KLF4 and ABCA1 might be indirect or mediated by other factors (Shankman et al., 2015). In contrast, Bethany J Bogan et al. found that ABCA1 expression was a target of KLF4 (Bogan et al., 2024). Dubland et al. reported that oxysterol 27-hydroxycholesterol (27-OHC) is translocated into the nucleus, where it interacts with liver X receptors (LXRs) situated on the regulatory regions of the ABCA1 and ABCG1 genes. This binding event triggers an increase in the expression of these transporters, which, in turn, enhances the cellular mechanism to remove surplus free cholesterol (Dubland et al., 2021). Nuclear factor of activated T-cells 5 (NFAT5), which functions independently of calcineurin, is a key component of the NFAT family that induced lipid metabolism-related (Sod1, Plin2) and cholesterol efflux gene expression (ABCA1) (Kappert et al., 2021). Mao X et al. found substrate stiffness regulates cholesterol efflux through ABCA1 in VSMC-derived foam cells (Mao et al., 2021). Ganoderma lucidum spore powder (GLSP) has been shown to mitigate the progression of atherosclerosis and the associated calcification of the aorta by enhancing the cholesterol transport processes mediated by ABCA1/G1, which are crucial for maintaining cellular lipid balance (Zheng G. et al., 2023).

As a vital transcription factor, PPARγ regulates several genes involved in metabolizing lipids, glucose, and inflammation (Kamon et al., 2003; Tavares et al., 2007). As opposed to control wild-type (WT) mice, PPARγ/ApoE double knockout mice obtained more severe lesions associated with atherosclerosis, indicating PPARγ′s role in AS (Gao et al., 2020). Recent studies found that PPARγ inhibited inflammatory cytokines induced by LPS and ACAT1 expression via the TLR4/MyD88/NF-κB signaling pathway, which inhibited the VSMC-derived foam cell formation (Yin et al., 2014; Zhang et al., 2011). In addition, PARP1 facilitates the translocation of PLIN2 and PLIN3 and disrupts the interaction between ABCG1 and PLIN1, which leads to the accumulation of LDs (Lin et al., 2024). Myotoxin III regulates the relocation of lipid droplet metabolism-related proteins, including PLIN2 and PLIN3, and SR-A and LOX-1 expression through PPAR-γ in VSMCs (Giannotti et al., 2019). When VSMCs were changed into adipocytes, the expression of CCAAT enhancer-binding proteins (C/EBPα) and PPARγ increased (Davies et al., 2005; Porse et al., 2001). A recent study suggests C/EBPα acetylation induces PPARγ transactivation in macrophage-derived foam cells, suggesting that C/EBPα/PPARγ may participate in VSMC-derived foam cells, but this needs to be further confirmed (Gao et al., 2020).

In cultured SMCs, TLR4 regulates the expression of inflammation induced by ox-LDL. Yin et al. discovered that activation of TLR4/MyD88/NF-κB by ox-LDL increased ACAT1 expression and promoted VSMC foam cell formation (Yin et al., 2014). Once TLR4 is knocked out, atherosclerotic plaque and foam cells derived from VSMCs induced by high lipids are inhibited. Xiao He et al. further found that MgCl₂ inhibited the activation of the TLR4/NF-κB signaling pathway induced by ox-LDL, which attenuated pyroptosis of VSMC-derived foam cells (He et al., 2023). Following further study, Zhiyang Han et al. pointed out that HOXA1 may contribute to the positive feedback relationship between TLR4 and NF-κB (Han et al., 2023). Recently, Chen Z et al. showed that TLR4 regulated ox-LDL-induced VSMC foam cell production via the Sirt1/3 and Src pathways (Chen et al., 2021; Yang et al., 2015). TLR4 also participates in the regulation of ROS recruitment in VSMC-derived foam cells (Moldogazieva et al., 2019). Zhen Sun et al. suggested that macrophage CD36 can help TLR4 bind to foam cells (Sun et al., 2022). The above results suggest that TLR4 is involved in VSMC foam cell formation with ox-LDL inducement, which may be targeted in future research to develop effective therapeutic medications for AS.

A P2 receptor family member, P2RY12 includes P2RX/P2X ion channels and P2RY/P2Y G protein-coupled receptors (Walker et al., 2020). Recent studies showed levels of P2RY12 receptors in VSMCs were elevated during AS, as most VSMCs that are P2RY12-positive are situated proximate to foam cells and the plaque area (Pi et al., 2021). Conversely, in advanced AS, P2RY12 receptor activation mitigated the efflux of cholesterol as well as autophagy inhibition, which was through PI3K-AKT-mTOR in vivo and in vitro (Pi et al., 2021; Bao et al., 2024). Recent studies found that Gualou-Xiebai (GLXB), Geniposide, and Huanglian Jiedu, herbs used in Traditional Chinese medicine to treat AS, raised the levels of lipophagy via the P2RY12/PI3K/AKT signaling pathway (Lin et al., 2024; Pi et al., 2021; Bao et al., 2024).

Accumulated studies revealed the mechanistic target of rapamycin (mTOR), a downstream effector of PPARγ and P2RY12, participates in nutrient sensing and regulates autophagy in VSMC-derived foam cell formation (Kim et al., 2023; Pi et al., 2021; Zuo et al., 2023). Pi et al. suggested that the P2RY12 receptor inhibited autophagosomes through PI3K-AKT-mTOR in VSMCs, which were inhibited by inhibition of Cx43 (Pi et al., 2021; Qin et al., 2022). Kim et al. found PPARγ/mTORC2/FOXO3a-autophagy mitigated VSMC senescence (Kim et al., 2023). Wingless/Int-1 signaling further downregulates mTORC1/ribosomal protein S6 kinase 1 (p-P70S6) and limits VSMC cholesterol buildup in humans and atherosclerosis progression in mice (Awan et al., 2022). Bisdemethoxycurcumin (BDMC), possessing anti-oxidation, anti-inflammation, and other multi-pharmacological functions, promotes autophagy and mitigates lipid droplets via the PDK1/AKT/mTOR signaling pathway, which alleviates the accumulation of lipids in VSMCs. Therefore, it can be utilized in atherosclerosis treatment and avoidance as a therapeutic agent (Zuo et al., 2023).

TFEB is a core transcription factor and regulates lysosome biogenesis and autophagy (Settembre et al., 2011). In VSMCs, activation of TFEB can modulate the migration and propagation of VSMCs, as well as reduce the formation of VSMC-derived foam cells (Pi et al., 2019; Wang YT. et al., 2019). Chen et al. found the sulfhydration of TFEB Cys212 increased nuclear translocation; LAPTM5 (biogenesis of lysosomes), LDLRAP1 (an autosomal recessive hypercholesterolemic gene known as LDL receptor cytoplasm tail binding protein), and Atg9A (associated with autophagosome formation) are its target genes (Chen Z. et al., 2022). TFEB also participates in the regulation of lysosomal acidification in macrophages. Recovery of lysosomal acidification is another challenge that VSMCs must face in lipid metabolism processes. Therefore, recovery of lysosomal acidification will be a strategy to rescue macrophage lysosomal dysfunction, and this strategy’s applicability to VSMC-derived foam cells remains to be investigated.

Epsin1 and Epsin2 are members of the ubiquitin-binding endocytic adapter protein family. In macrophages, epsins are responsible for regulating EC reprograming to mesenchymal lipid uptake and cholesterol efflux in macrophages coming from AS plaque (Dong et al., 2023; Dong et al., 2020; Cui et al., 2023). Epsins are a positive regulatory factor in atherosclerotic lesion formation. Epsins promote the development of atherosclerosis by preventing the ubiquitination and degradation of LRP-1 in macrophages and regulating calcium release from the ER by promoting the proteasomal degradation of IP3R1 through its ubiquitin interaction motif (UIM) in endothelial cells (Dong et al., 2020; Brophy et al., 2019). Recent studies found that lack of Epsins induced VSMC transfer into myofibroblasts and ECs, which benefits the repair of vascular injury. This process was regulated through OCT4 and KLF4 (Wang et al., 2024). Nevertheless, whether epsins are involved in the disruption of lipid metabolism in VSMCs and VSMC-derived foam cell transformation and, if so, their regulatory mechanisms, has not been explored.

As a serine protease and a PCSK family member, proprotein convertase subtilisin/PCSK9 has an involvement in cardiovascular disease and dyslipidemia progression caused by atherosclerosis (Ragusa et al., 2021; Katsuki et al., 2022). A recent study revealed PCSK9 increased the lesions in mice fed with high-fat content and foam cell production in ox-LDL-treated VSMCs through lncRNA small nucleolar RNA host gene 16 (SNHG16)/histone-lysine N-methyltransferase enzyme (EZH2)/tumor necrosis factor receptor-associated factor 5 (TRAF5) (Liu et al., 2023). Homocysteine induces DNA hypermethylation in the promoter region of C1q/TNF-related protein 9 (CTRP9) via upregulated expression of DNA methyltransferase 1 (DNMT1) while also adversely regulating the deposition of lipids in VSMCs as mediated by ERs (Wang et al., 2023). Methyl-CpG binding protein 2 (MECP2)/ transcription factor homeobox A9 (HOXA9)/E3 ubiquitin ligase Peli1 participated in VSMC phenotypic switching induced by ox-LDL (Fu et al., 2023; Burger et al., 2022). A protein that binds to β-galactoside, galectin-1 (Gal-1) serves as an abdominal aortic aneurysm (AAA) and atherosclerosis therapeutic target. It enables more foam cells to be formed and increase mitochondrial dysfunction in VSMCs; such effects can be avoided via recombinant Gal-1 treatment (Roldán-Montero et al., 2022) (Figure 3).