Sterol carrier protein 2: A promising target in the pathogenesis of atherosclerosis

Abstract

Atherosclerosis, the underlying pathophysiological basis of cardiovascular disease, has been recognized as a lipid-driven chronic inflammatory disease. Sterol carrier protein 2 (SCP-2) is a 13-kDa non-specific lipid-transfer protein expressed by various tissues and cells, such as liver, heart, vascular smooth muscle cells (VSMCs), and macrophages. SCP-2 has an extensive role in cardiovascular and metabolic diseases. Recently, SCP-2 was reported to promote the development of atherosclerosis by regulating lipid metabolism and peroxidation, endocannabinoid metabolism, vascular inflammation, and fatty acid metabolism. In this review, we summarized the recent advances regarding the role of SCP-2 in the pathogenesis of atherosclerosis and tried to provide a rationale for future investigation and a better understanding of the biological functions of SCP-2 in atherosclerotic cardiovascular disease.

Abbreviations

2-AG

2-arachinoylglicerol

3′UTR

3′ untranslated region

ABCA1

ATP binding cassette transporter A1

ac-LDL

acetylated-LDL

ACAT1

acyl-CoA: cholesterol acyltransferase 1

AEA

N-arachidonoylethanolamine

ATF-2

activating transcription factor-2

cAMP

cyclic adenosine monophosphate

ChOOH

Cholesterol hydroperoxide

FOXO3a

Forkhead box O3a

HDL-C

high-density lipoprotein cholesterol

HR-3

hormone receptor 3

LDL-C

low-density lipoprotein cholesterol

miR-15a

microRNA-15a

NKT

natural killer T

NPC1L1

Niemann-Pick C1-like 1

PLOOH

phospholipid hydroperoxide

RCT

reverse cholesterol transport

ROS

reactive oxygen species

SCP-2

sterol carrier protein 2

SF-1

steroidogenic factor 1

SR-A

scavenger receptor class A

THAP

thanatos-associated protein

TICE

transintestinal cholesterol efflux

VLDL

very low-density lipoprotein

VSMCs

vascular smooth muscle cells

WHO

World Health Organization

Introduction

Cardiovascular disease has long been regarded as the most common cause of disability and premature death across the world. Recently, the World Health Organization (WHO) reported that cardiovascular disease accounts for >17 million deaths per year and this figure will rise to >23 million by the year 2030.¹ Atherosclerosis is the major pathological basis of most cardiovascular diseases, such as peripheral artery disease, myocardial infarction, hypertension, and ischemic stroke.2, 3, 4 It is widely recognized that atherosclerosis is a lipid-driven chronic inflammatory disease, which is characterized by foam cell formation and atheromatous plaques in medium- and large-sized arteries.5, 6, 7 Although statins therapy is effective in improving plasma lipid profile in patients with cardiovascular disease, the residual cardiovascular risk is still representing a challenge worthy of attention.^8,9 Therefore, it is necessary to develop new promising therapeutic strategies aiming to target critical molecules involved in the pathogenesis of atherosclerosis.

Sterol carrier protein-2 (SCP-2), also called non-specific lipid-transfer protein, plays an important role in the intracellular transport and metabolism of lipids, such as phospholipids, fatty acids, and cholesterol.10, 11, 12 Several lines of evidence suggest that SCP-2 is involved in the development of myocardial infarction and diabetes mellitus.13, 14, 15 Of note, there is increasing evidence that SCP-2 is implicated in the occurrence and development of atherosclerosis through multiple mechanisms (Fig. 1). In the present review, we summarized the current knowledge about the pathophysiological and etiological role of SCP-2 in atherosclerosis to provide an important framework for future research and therapy.

Figure 1.

SCP-2-mediated regulation and the major cardiometabolic risk factors of atherosclerosis. Abbreviations: ACAT1, acyl-CoA: cholesterol acyltransferase 1; SR-A, scavenger receptor class A; ABCA/G1, ATP binding cassette transporter A/G1; NPC1L1, Niemann-Pick C1-like 1; TICE, transintestinal cholesterol efflux; LDLR, low-density lipoprotein receptor; PLOOH, phospholipid hydroperoxide; AEA, N-arachidonoylethanolamine; ChOOH, Cholesterol hydroperoxide; 2-AG, 2-arachinoylglicerol; CB1/2, cannabinoid receptor 1/2; AS, atherosclerosis. Arrows in red: promote; text in red: proatherogenic changes; text in green: antiatherogenic changes; text in black word: un-certain effects.

Structure and function of SCP-2

The SCP-2 enzyme gene has two start sites, which encode proteins sharing a common 13-kDa SCP-2 domain at the C-terminus: I) One site codes for a precursor 15-kDa pro-SCP-2 protein (no activity), which is post-translationally completely cleaved into the mature 13-kDa SCP-2 and a 2-kDa peptide.¹⁶ II) A second site codes for a full-length 58-kDa SCP-x protein that is partially post-translationally processed to yield a 46-kDa protein and a 13-kDa SCP-2¹⁶ (Fig. 2). Human SCP-2 is located on chromosome 1p32.3 and contains 123 amino acids and 18 exons. SCP-x/SCP-2 is comprised of at least four important domains that have been identified and associated with specific functions. First, the 46-kDa protein located at the N-terminal presequence of the 58-kDa SCP-x is a 3-ketoacyl-CoA thiolase functioning in the oxidation of branched chain fatty acids.^17,18 Also, this thiolase is most likely to split in the peroxisome during the transition between SCP-2 presequence and mature sequence. Second, the N-terminal presequence (20 amino acids) present in 15-kDa pro-SCP-2 significantly regulates the tertiary and secondary structure of SCP-2 and strengthens its intracellular targeting encoded by the C-terminal peroxisomal targeting sequence.^19,20 This may explain the observation that up to half of the total SCP-2 protein is located outside of peroxisomes.¹⁶ Third, the 32 amino acids at the N-terminal of 13-kDa SCP-2 protein form an amphipathic α-helix segment, one side of which is a membrane-binding domain. Of note, the amino acid residues with positive charges on one side of the amphiphilic α-helix enable SCP-2 to bind to the surface of membranes containing anionic phospholipids.²¹ Fourth, the helix D and β-strands 4,5 along with the hydrophobic faces of the amphipathic N-terminal α-helix in the 13-kDa SCP-2 protein form a ligand-binding cavity able to accommodate multiple types of lipids (e.g., cholesterol, isoprenoids, fatty acyl CoAs, phospholipids, and fatty acids).^20,22 Despite molecular details that comprise the fatty acid binding site in the SCP-2 binding pocket have been identified, the specific amino acid residues interacting with other bound ligands are still unknown.^23,24

Figure 2.

The SCP-x/pro-SCP-2 gene encodes two 13-kDa SCP-2 precursors. The 13-kDa SCP-2 gene can be produced by two sites. One site codes for a precursor 15-kDa pro-SCP-2 protein that undergoes completely post-translationally cleaved to the mature 13-kDa SCP-2 and a 2-kDa peptide. Another site codes for a full-length 58-kDa SCP-x protein, which is partially post-translationally processed into yield a 46-kDa protein and a 13-kDa SCP-2 protein.

Numerous studies demonstrated that SCP-2 promotes the transfer of lipid (e.g., cholesterol, phospholipid) from other intracellular membranes to mitochondria, as well as lipid transfer from the outer to the inner mitochondrial membrane.^25,26 SCP-2 can facilitate intermembrane lipid transfer by directly interacting with the lipid in the membrane and enhancing its desorption from the membrane.²⁷ Besides, SCP-2 also stimulates the exchange of oxidized derivatives,^28,29 glycosphingolipids and gangliosides,³⁰ and fatty acids^28,31 between vesicles or membranes. The underlying mechanism of SCP-2-mediated lipid transfer enhancement has been well described by a collisional model, which involves lipid binding and membrane interaction.³¹ The interaction of SCP-2 protein with vesicles has been studied by membrane surface pressure,³² rotational correlation time,³³ and circular dichroism.^31,34 Of note, SCP-2-mediated lipid transport and membrane interaction depend on vesicle composition, concentration, curvature, and charge, especially small highly-curved negatively charged vesicles.^20,35 Intriguingly, this characteristic has been found in SCP-2 from yeast to mammals, which may be related to the fact that the majority of SCP-2 protein possesses a positively charged surface patch.^36,37 Thus, electrostatic interaction is crucial for SCP-2 to interact with the membrane, as many other protein-membrane interactions have reported.³⁸ Taken together, understanding the structure and function of SCP-2 can help to provide a better understanding of its biological roles in lipid metabolism and atherogenesis.

Tissue and intracellular distribution of SCP-2

The 13-kDa SCP-2 protein is ubiquitously present in almost all mammalian tissues. However, the relative content of SCP-2 varies in different tissues. The 13-kDa SCP-2 protein was found in highest abundance in tissues involved in cholesterol transport and oxidation, such as liver, intestine, heart, testis, ovary, and adrenal.^14,39 Also, most SCP-2 in the vascular system is expressed in endothelial cells, macrophages, and vascular smooth muscle cells (VSMCs).40, 41, 42 Intracellular localization of SCP-2 has been well established by exploring the distribution of SCP-2/SCP-x gene products in transfected cells overexpressing the 13-kDa SCP-2 and 15-kDa pro-SCP-2.^19,43,44 Overexpression of 15-kDa pro-SCP-2 leads to a marked localization in peroxisomes, but nearly half are localized in extraperoxisomal sites.¹⁹ By contrast, overexpression of 13-kDa SCP-2 not only results in SCP-2 being predominantly extraperoxisomal in the cytoplasm, but also in other organelles such as endoplasmic reticulum, lysosomes, and mitochondria,^19,45 suggesting that SCP-2 may have a broad-scale lipid trafficking activity. The differential distribution of these gene products is owing to the stronger exposure of the C-terminal peroxisome targeting sequence (AKL) of 15-kDa pro-SCP-2 to aqueous buffers while that of the mature 13-kDa SCP-2 is slightly exposed.²⁰ Notably, the presence of the N-terminal presequence (20 amino acids) in 15-kDa pro-SCP-2 changes the conformation of the protein, thereby significantly enhancing the aqueous exposure of the C-terminal AKL peroxisome targeting sequence.^19,20 Therefore, 15-kDa pro-SCP-2 likely interacts with the peroxisomal membrane AKL receptor and then participates in the translocation of 15-kDa pro-SCP-2 into the peroxisome matrix. The relative amount of SCP-2 in cytoplasm vs. peroxisomes may be due to the extent of cleavage of the 20-amino acid N-terminal presequence of 15-kDa pro-SCP-2 before entering the peroxisomal matrix. However, the specific protease responsible for this cleavage and the intracellular localization of this protease remains unclear.

Regulation of SCP-2

SCP-2 expression can be positively and negatively regulated by numerous factors (Table 1). Forkhead box O3a (FOXO3a), an important transcription factor belonging to the Forkhead box O family, is closely implicated with neointima formation and atherogenesis.^46,47 It has been reported that FOXO3a positively regulates SCP-2 at the level of promoter activity in human colon carcinoma cells.⁴⁸ The orphan nuclear receptors, hormone receptor 3 (HR-3) and steroidogenic factor 1 (SF-1), have been identified as key transcription factors that promote SCP-2 expression by directly binding to its promoter regions.^49,50 SCP-2 is also transcriptionally upregulated by cyclic adenosine monophosphate (cAMP).⁵¹ Furthermore, SF-1 is required for cAMP-induced regulation of the SCP-2 gene.⁴⁹

Table 1.

The regulatory factors of SCP-2.

Regulators	Cells/Tissues	Mechanisms	Functions	Ref.
Stimulators
FOXO3a	Human colon carcinoma cells	Binds SCP-2 promoter region	Protects against H2O2/Cu2+-induced oxidative damage	48
SF-1	Luteal cells	Binds SCP-2 promoter region	Promotes steroid hormone production	49
HR-3	Aag-2 cells	Binds SCP-2 promoter region	Promotes sterol trafficking	50
cAMP	Grs-21 cells, Immature female Sprague–Dawley rats	SF-1	Promotes steroid hormone production	51
Inhibitors
ATF-2	Aedes aegypti	Binds SCP-2 promoter region	Plays a negative role in development and growth	52
THAP	Aedes aegypti	Binds SCP-2 promoter region	Plays a negative role in development and growth	52
miR-15a	DF1 cells, Chicken breast muscle tissue	Targets the 3′UTR of SCP-2 mRNA	Promotes the differentiation of intramuscular preadipocytes	53
miR-1285-5p	Type 2 diabetes mellitus clinical samples	Promotes the hypermethylation of SCP-2 promoter region	A promising biomarkers and therapeutic target for type 2 diabetes mellitus	54
Gemfibrozil	Male Sprague–Dawley rats	Unknown	Plays a hypolipidemic role	55

In addition, a number of factors have been shown to negatively regulate SCP-2 expression. Activating transcription factor-2 (ATF-2) and thanatos-associated protein (THAP) were reported to antagonistically regulate SCP-2 transcriptional activity.⁵² SCP-2 also undergoes post-transcriptional modulation. Overexpression of microRNA-15a (miR-15a) significantly reduces SCP-2 expression in chicken intramuscular adipocytes.⁵³ Furthermore, luciferase assay showed that miR-15a inhibits SCP-2 translation by targeting the 3′ untranslated region (3′UTR) of SCP-2 mRNA.⁵³ SCP-2 is also identified as a direct target of miR-1285-5p.⁵⁴ MiR-1285-5p was reported to be involved in decreasing the expression of SCP-2 through hypermethylation of the promoter region.⁵⁴ Besides, SCP-2 gene expression is translationally or post-translationally downregulated by gemfibrozil, a widely used hypolipidemic drug.⁵⁵ However, the exact molecular mechanism of translational or post-translational regulation of SCP-2 remains poorly understand.

Roles of SCP-2 in the pathogenesis of atherosclerosis

Atherosclerosis is a complex and multifactorial disease involving numerous pathophysiological processes. Besides, atherosclerosis is also associated with obesity, type 2 diabetes and non-alcoholic fatty liver disease (NAFLD).^56,57 SCP-2 expression is significantly altered in mice models of high-fat diet-induced obesity and NAFLD as well as streptozotocin-treated diabetic rat,58, 59, 60, 61 suggesting a potential role of SCP-2 in these diseases despite the precise mechanisms are largely unknown. Importantly, SCP-2 has been shown to aggravate atherosclerosis by promoting lipid metabolic disorder, lipid peroxidation, vascular inflammation, endocannabinoid metabolism, and fatty acid metabolism.

SCP-2 in cholesterol metabolism

Lipid metabolic disorder, especially hyperlipidemia, is an important risk factor for atherosclerosis. Generally, high-density lipoprotein cholesterol (HDL-C) is thought to be atheroprotective, but very low-density lipoprotein (VLDL), triglyceride, and low-density lipoprotein cholesterol (LDL-C) are proatherogenic. It was estimated that a 1 mg/dL increment in circulating HDL-C levels is associated with a reduced risk of cardiovascular events in women (3%) and men (2%).⁶² Of note, deficiency of SCP-2 in LDLR^−/− mice exhibit a significant increase in HDL-C levels and a reduction in VLDL, LDL-C, and triglyceride levels,⁶³ suggesting an important role of SCP-2 in the development of hyperlipidemia.

The accumulation of foam cells (cholesterol-laden macrophages) in the arterial walls plays a key role in the pathogenesis of atherosclerosis. Cholesterol accumulated in macrophages contributes to the formation of foam cells. In contrast, cholesterol efflux from macrophage foam cells to HDL particles is the first step in reverse cholesterol transport (RCT) that inhibits the development of hyperlipidemia and atherosclerosis.^64,65 It has been reported that acetylated-LDL (ac-LDL)-stimulated foam cells exhibit significantly increased SCP-2 mRNA and protein levels,^42,66 suggesting that SCP-2 plays a crucial role in foam cell formation. The formation of foam cells is attributed to decreased cholesterol efflux, enhanced cholesterol uptake, and increased cholesterol esterification. Acyl-CoA: cholesterol acyltransferase 1 (ACAT1) is a pivotal endoplasmic reticulum membrane-spanning enzyme that catalyzes the synthesis of cholesterol esters from free cholesterol in macrophages.^67,68 Of note, SCP-2 promotes ACAT1-catalyzed cholesterol esterification in mouse peritoneal macrophages and aorta.^42,69 Scavenger receptor class A (SR-A) and CD36 belong to the members of the scavenger receptor family, which are mainly responsible for macrophage cholesterol uptake. There is growing evidence that the knockdown of SR-A and CD36 dramatically inhibits foam cell formation and hypercholesterolemia.70, 71, 72 Importantly, numerous studies suggest that overexpression of SCP-2 enhances cholesterol uptake.^43,73 However, whether SCP-2 increases macrophage cholesterol uptake via regulating SR-A and CD36 expression is unknown. In addition, overexpression of SCP-2 was found to inhibit HDL-mediated cholesterol efflux.⁷⁴ Conversely, SCP-2 gene ablation strikingly increases cholesterol efflux to HDL.⁷⁵ However, the molecular mechanism by which SCP-2 suppresses macrophage cholesterol efflux is still largely unknown. ATP binding cassette (ABC) transporter A1 (ABCA1) and G1 (ABCG1), two important transmembrane proteins, reduce foam cell formation and protect against atherosclerosis by mediating cholesterol efflux to HDL particles.^76,77 Thus, it is highly possible that SCP-2 inhibits cholesterol efflux by regulating the expression of ABCA1 and/or ABCG1.

In addition to macrophages, VSMCs are another important source of foam cells in atherosclerosis. In human coronary atherosclerotic lesions, VSMCs contribute about 50% of foam cells and exhibit lower expression of ABCA1 in the intimal of early and advanced plaques.⁷⁸ Besides, in apoE^−/− mice fed a western diet, approximately 70% of atheroma foam cells are derived from VSMCs, accompanied by decreased ABCA1 expression.⁷⁹ Of note, the expression of SCP-2 mRNA and protein is markedly increased in VSMC-derived foam cells.⁴¹ Given that SCP-2 is a negative regulator of cholesterol efflux. These findings suggest that SCP-2 may inhibit cholesterol efflux from VSMC-derived foam cells by the regulation of ABCA1 expression.

The liver is an important organ for cholesterol metabolism. It has been reported that hepatic overexpression of SCP-2 plays a detrimental role in lipid metabolism.⁸⁰ It can reduce circulating HDL-C and increase LDL-C concentrations, which contributes to the development of hyperlipidemia and atherosclerosis. Furthermore, these changes are linked to reduced hepatic LDLR, apoE, apoA-I, and apoB expression.⁸⁰ Moreover, SCP-2 overexpression in the mouse liver significantly enhances hepatobiliary cholesterol secretion.^61,81 In contrast, knockout of SCP-2 diminishes the ability to secrete cholesterol to bile acid for biliary excretion.82, 83, 84 It is well known that ABCG5 and ABCG8 are two important half-transporters mediating hepatobiliary elimination, contributing to 70%–90% of biliary cholesterol secretion.^85,86 However, several studies suggest that the knockout or overexpression of SCP-2 has no effect on hepatic levels of ABCG5/G8 protein or mRNA.^61,87,88 Recent studies have suggested that transintestinal cholesterol efflux (TICE) is an alternative nonbiliary pathway for cholesterol excretion.^89,90 In this process, hepatic cholesterol can be directly trafficked to the proximal small intestine via enterocytes. Thus, SCP-2 may promote hepatobiliary cholesterol secretion via the TICE pathway. Although SCP-2 has an active role in the regulation of hepatobiliary cholesterol secretion, the total bile acid pool size remains unchanged.⁸⁰ This can be explained by the enterohepatic circulation of bile acids and biliary cholesterol. Hepatic overexpression of SCP-2 significantly enhances intestinal cholesterol absorption, which leads to the recycling of cholesterol back into the blood.⁸⁰ Conversely, SCP2 deficiency inhibits the absorption of intestinal cholesterol and reduces circulating cholesterol levels.⁶³ Therefore, the increased hepatobiliary cholesterol secretion is in turn reabsorbed into the blood by the intestine. Of note, intestinal Niemann-Pick C1-like 1 (NPC1L1) is known as a pivotal transporter responsible for intestinal cholesterol absorption, and its expression is positively associated with the risk of hyperlipidemia and atherosclerosis.91, 92, 93 Thus, NPC1L1 may be a key target for SCP-2 to increase intestinal cholesterol absorption. Taken together, these studies suggest that SCP-2 promotes the development of hyperlipidemia by the regulation of hepatic cholesterol metabolism as well as intestinal cholesterol absorption.

SCP-2 in fatty acid metabolism

Fatty acids are identified as important components in cell membranes, and they are required for energy storage, generation of signaling molecules, and membrane proliferation. Accumulating evidence has suggested that abnormal fatty acid metabolism plays a key role in the etiology of atherosclerosis.^94,95 Increased fatty acid accumulation, especially in macrophages and endothelial cells, accelerates atherogenesis by significantly aggravating vascular dysfunction, promoting inflammation, and increasing lipid accumulation in the arterial walls.96, 97, 98 SCP-2 is closely associated with fatty acid metabolism. It has been reported that SCP-2 binds fatty acyl CoAs⁹⁹ and fatty acids^100,101 with high affinity. Furthermore, overexpression of SCP-2 enhances cellular fatty acid uptake and stimulates fatty acid oxidation.^11,102 In addition, SCP-2 also stimulates fatty acid esterification into the triacylglycerol fraction, which promotes cellular lipid dysfunction.¹⁰³ Taken together, these data suggest that SCP-2 may promote the development of atherosclerosis by regulating fatty acid metabolism.

SCP-2 in lipid peroxidation

It is well known that oxidative stress plays a significant role in the development of atherosclerosis and some of its effects are mediated via lipid oxidation.¹⁰⁴ In particular, lipid peroxidation is known as a free-radical process in which oxidants such as reactive oxygen species (ROS) attack unsaturated lipids, producing a variety of oxidation products.^105,106 Cholesterol hydroperoxide (ChOOH) and phospholipid hydroperoxide (PLOOH) are the major oxidation products of lipid peroxidation, which are actively involved in the inflammatory responses, apoptosis, and mitochondrial damage in atherosclerosis by interacting with endothelial cells, VSMCs, and macrophages.107, 108, 109 Notably, it has been reported that mouse fibroblast transfectant clones overexpressing SCP-2 are substantially more sensitive to apoptotic killing induced by ChOOH than vector controls.¹¹⁰ In striking contrast, two SCP-2 inhibitors, SCPI-3 and SCPI-1, significantly reduce ROS accumulation and protect against oxidative killing in ChOOH-treated cells.¹¹⁰ Furthermore, cellular SCP-2 promotes the intermembrane transfer of ChOOH and PLOOH under oxidative stress conditions, thereby greatly enhancing peroxide-induced mitochondrial damage and apoptosis.^26,110,111 Mechanistically, SCP-2 facilitates translocation via binding to donor/acceptor membranes and nonspecifically reducing the association/dissociation energy of lipid monomers.^26,32 The donor membrane has properties that contribute to SCP-2-enhanced PLOOH and ChOOH transfer, including increased net negative charge exerted by phosphatidylserine and increased phospholipid unsaturation. Thus, these findings suggest that SCP-2 acts as a crucial molecular regulator in the regulation of lipid peroxidation and may be a promising target in the pathogenesis of atherosclerosis.

SCP-2 in endocannabinoid metabolism

The endocannabinoid system consists of endocannabinoids, such as 2-arachinoylglicerol (2-AG) and N-arachidonoylethanolamine (AEA), and types 1 and 2 G-protein coupled cannabinoid receptors (CB1 and CB2). Endocannabinoids are arachidonic acid derivatives that play a crucial physiological role in atherosclerotic cardiovascular disease by CB1 and CB2 activation and signaling.^112,113 A growing number of studies have demonstrated that enhanced endocannabinoid system activation promotes atherogenesis through multiple effects, including regulation of macrophage cholesterol metabolism, vascular inflammation, leukocyte recruitment, and consequently atherosclerotic plaque stability.114, 115, 116, 117 Moreover, elevated levels of circulating endocannabinoids are also prevalent in human and mouse atherosclerosis.^117,118 Of note, SCP-2 has been identified as an important binding protein of AEA and 2-AG, which can promote their cellular uptake and accumulation.^119,120 In contrast, loss of SCP-2 was reported to decrease serum levels of free arachidonic acid, thereby attenuating the availability of arachidonic acid uptake by cells and downstream synthesis of 2-AG and AEA.¹²¹ Thus, these studies suggest that SCP-2 promotes atherosclerosis by regulating endocannabinoid metabolism. However, whether SCP-2 regulates endocannabinoid metabolism by activating CB1 and/or CB2 is still largely unknown.

SCP-2 in vascular inflammation

Atherosclerosis has long been regarded as a progressive chronic inflammatory disease of the vascular wall. Vascular inflammation not only promotes lipid metabolism dysregulation but also contributes to plaque rupture and the onset of cardiovascular events.^122,123 Type II natural killer T (NKT) cells are CD1d-restricted and immunoregulatory T cells, and the disorder of these cells is involved in vascular inflammation and atherosclerosis-associated immune response.^124,125 More recently, proinflammatory CD1d-restricted type II NKT cells reactive with the endogenous SCP-2 peptide have been reported to be implicated in vascular inflammation in rats.^126,127 The SCP-2 peptide can activate CD1d-restricted type II NKT cells to produce pro-inflammatory cytokines and thereby enhance vascular inflammation.^126,127 Of note, endothelial cells, macrophages, and VSMCs are the major effector cells involving vascular inflammation and atherosclerosis.128, 129, 130 Thus, further studies are necessary to investigate whether SCP-2 promotes vascular inflammation by directly regulating these effector cells.

Conclusions and future directions

SCP-2 is an important lipid binding protein with multiple biological functions and has attracted growing interest in recent years. SCP-2 plays a critical role in the development of atherosclerosis through its involvement in the regulation of lipid metabolic disorder, vascular inflammation, lipid peroxidation, endocannabinoid metabolism, and fatty acid metabolism (Table 2).

Table 2.

The potential role of SCP-2 in the pathogenesis of atherosclerosis.

SCP-2 expression	Cells/tissues	Mechanisms	Effects	AS	Ref.
Knockout	Global	Intestinal cholesterol absorption↓, hepatic triglyceride/VLDL secretion↓	Hyperlipidemia↓	↓	63
Overexpression	Rat peritoneal macrophages	ACAT1↑	Cholesterol esterification↑	↑	42,66
Overexpression	Rat aorta	ACAT1↑	Cholesterol esterification↑	↑	69
Overexpression	Mouse L-cell fibroblasts	–	Cholesterol uptake↑	↑	43,73
Overexpression	Mouse L-cell fibroblasts	–	Cholesterol efflux↓	↑	74
Knockdown	Primary mouse hepatocytes	–	Cholesterol efflux↑	↓	75
Overexpression	VSMCs	–	Lipid accumulation↑	↑	41
Overexpression	Liver	LDLR, apoE, apoA-I, and apoB↓, Intestinal cholesterol absorption↑	Hyperlipidemia↑	↑	80
Overexpression	Liver	–	Hepatobiliary cholesterol secretion↑	↑	61,81
Overexpression	Liver	PLOOH and ChOOH transfer↑	Lipid peroxidation↑	↑	26
Suppression	Mouse L-cell fibroblasts	ChOOH transfer and uptake↓	Lipid peroxidation↓	↓	110
Overexpression	Rat hepatoma cells	ChOOH transfer↑	Lipid peroxidation↑	↑	111
Overexpression	HEK 293 Cells	AEA and 2-AG uptake/accumulation↑	Endocannabinoid accumulation↑	↑	119,120
Knockout	Global	AEA and 2-AG accumulation↓	Endocannabinoid accumulation↓	↓	121
Overexpression	Vascular endothelial cells	CD1d-restricted type II NKT cells↑	Vascular inflammation↑	↑	126,127
Overexpression	Mouse L-cell fibroblasts	–	fatty acid uptake, oxidation, and esterification↑	↑	11,102

–, Not determined; AS, atherosclerosis.

Ongoing studies of SCP-2 will be needed to elucidate its exact role in atherosclerosis. Although many studies have established the harmful effects of enhanced expression, it is currently difficult to speculate on the most effective strategy to inhibit the expression of SCP-2 in atherosclerosis. Moreover, more work is required to clarify how to most effectively target SCP-2, either by transcriptional/post-transcriptional regulation, or by post-translational modification. Currently, most of our knowledge about the role of SCP-2 in the pathogenesis of atherosclerosis has been learned from studies performed in vitro and rodent models with genetic modifications and/or dietary treatments. Nevertheless, rodent models are insufficient to reflect the complex heterogeneity of pathological changes that occur during the progression of atherosclerosis in humans. In contrast, large-animal models can offer striking advantages, including their marked metabolic, physiological, biochemical, and genetic similarities to humans. Thus, they may be better models and helpful for bridging the gap between basic research and prudent clinical usage. In addition, there are also several crucial questions that require to be answered in future research. 1) Pyroptosis, a pro-inflammatory programmed cell death, plays a crucial role in the development of atherogenesis.131, 132, 133 Does SCP-2 promote atherosclerosis by regulating pyroptosis? 2) Macrophage polarization is also known as a pivotal determinant of inflammation and is closely related to the progression of atherosclerosis.^134,135 However, there is very little known about the role of SCP-2 in the regulation of macrophage polarization. 3) Does a specific and efficient method to counteract the detrimental actions of SCP-2 on atherosclerosis exist in vivo? Finally, it is of importance to explore whether SCP-2 inhibits the whole-body RCT process. In summary, understanding these sorts of questions will undoubtedly provide insightful knowledge about the underlying mechanisms and accelerate the development of SCP-2-targeted therapy.

Author contributions

Conception and design: all authors; Manuscript writing: Can Xu; Collection and assembly of data: Can Xu and Heng li; Final approval of manuscript: all authors.

Conflict of interests

Authors declare no conflict of interests.

Funding

This work was supported by the National Natural Sciences Foundation of China (No. 81770461) and the Science and Technology Project of Hengyang City, China (No. hkf201947206).