The therapeutic potential of apigenin against atherosclerosis

Abstract

Apigenin is a natural flavonoid abundantly found in fruits, vegetables, and medicinal plants. It possesses protective effects against cancer, metabolic syndrome, dyslipidemia, etc. Atherosclerosis, a chronic immune-mediated inflammatory disease, is the underlying cause of coronary heart disease, stroke, and myocardial infarction. Numerous in vivo and in vitro studies have shown a protective effect of apigenin against atherosclerosis, attributed to its antioxidant and anti-inflammatory properties, as well as its antihypertensive effect and regulation of lipid metabolism. This study aimed to review the effects and mechanisms of apigenin against atherosclerosis for the first time. Apigenin displays encouraging results, and this review confirms the potential value of apigenin as a candidate medication for atherosclerosis.

1. Introduction

Atherosclerosis, a multifactorial disease characterized by the formation of fibrous plaques, is a major underlying cause of cardiovascular diseases. The pathogenesis of atherosclerosis is complex and involves oxidative stress, endothelial dysfunction, lipid accumulation, and chronic inflammation [[1], [2], [3], [4]]. Early prevention of atherosclerosis is among the top priorities of modern medicine owing to its high prevalence and huge medical burden. The medications currently used for atherosclerosis are lipid-lowering drugs including statins, niacin, and fibrates. However, their widespread use has been limited owing to a spectrum of serious adverse effects, such as hepatotoxicity, nephrotoxicity, and rhabdomyolysis. It is necessary to develop alternative drugs with high efficacy. Herbal compounds, such as flavonoid extracts, have been shown to be a new hope for patients with atherosclerotic cardiovascular diseases. Silibinin, luteolin and quercetin, several of the most studied and widely known bioactive flavonoids, exert noticeable anti-atherosclerotic actions [[5], [6], [7]].

Apigenin, a natural bioactive ingredient known as 4’,5,7-trihydroxyflavone, is one of the most investigated flavonoids. Apigenin is widely present in vegetables, fruits, herbs, and is most abundant in parsley [8,9]. Both in vitro and in vivo studies have shown that apigenin displays versatile biological effects, such as anti-inflammatory, antioxidant, antihypertensive effects, and regulation of lipid metabolism [8,[10], [11], [12], [13]]. Because of these biological effects, apigenin is considered to have therapeutic and preventive potential against atherosclerosis, and its dietary intake is recommended [14]. This review focuses on the therapeutic potential of apigenin against atherosclerosis as a key nutraceutical or therapeutic agent, as well as the mechanisms underlying its anti-atherogenic properties through in vivo and in vitro studies.

2. Sources of apigenin

Apigenin, mainly obtained from the genus Apium (Umbelliferae), is abundant in parsley, celery, and chamomile. The richest source is dried parsley, containing 45035 μg/g of apigenin. The dried flowers of chamomile contain 3000–5000 μg/g of apigenin, and chamomile tea, containing 0.8%–1.2 % apigenin, is popular worldwide for its soothing and calming effects [9,15]. Some dietary sources of apigenin and its contents are shown in Table 1. Other species, such as Perilla frutescens (L.) Britt.(Lamiaceae) [16], Chrysanthemum indicum L.(Asteraceae) [17], Scutellaria barbata D. Don (Lamiaceae) [18], Berberis vulgaris fruits (Berberidaceae) [19], Pseuderanthemum palatiferum (Nees) Radlk(Acanthaceae) [20] are the main natural sources of apigenin and are used as traditional medicines. In some other species, apigenin is present in the form of O-glycosides or C-glycosides, including apigenin-7-O-neohesperidoside in Mussaenda luteola [21], apigenin-7-O-β-D-glucuronide in Perilla frutescens (L.). Britt. [22], apigenin-7-O-β-D-glucopyranoside in Platycodon grandiflorus [23], apigenin-8-C-glucoside in Tetrastigma hemsleyanum Diels et Gilg. [24], apigenin-6-C-glucoside in Prunus avium L. and Prunus cerasus L [25]. Some apigenin biflavonoids, such as hinokiflavone, cupressuflavone, and amentoflavone, have also been isolated from natural sources [[26], [27], [28]]. The chemical structure, molecular formula, natural sources of apigenin, and its glycosides and biflavonoids are listed in Table 2.

Table 1.

Some dietary sources of apigenin.

Dietary sources	Quantity (μg/g or μg/mL)	Reference
Dried parsley	45035	[15]
Chamomile	3000–5000
Parsley	2154.6
Celery seed	786.5
Vinespinach	622
Guava	579	[29]
Wolfberry leaves	547
Belimbi fruit	458
Bell pepper	272
Chinese celery	240.2	[15]
Kumquats	218.7
Garlic	217	[29]
Celery	191	[15]
Chinese cabbage	187	[29]
Dried Mexican oregano	177.1	[15]
French peas	176	[29]
Chinese violet	127	[30]
Artichoke	74.8	[15]
Juniper berries	72.6
Peppermint	53.9
Snake gourd	42.4	[29]
Rutabaga	38.5	[15]
Italian oregano	35	[9]
Common sage	24
Chili pepper	22	[30]
Weed passion flower	15
Virgin olive oil	11.7	[9]
Rosemary	5.5

Table 2.

The structure, molecular formula, natural sources of apigenin, and its glycosides and biflavonoids.

Compounds	Molecular formula (Molecular weight)	Structure	Source	Reference
Apigenin	C15H10O5 (270.24)		Apium L	[9]
Apigenin-7-O-neohesperidoside	C27H30O14 (578.52)		Mussaenda luteola	[21]
Apigenin-7-O-β-D-glucuronide	C21H18O11 (446.36)		Perilla frutescens (L.) Britt.	[22]
Apigenin-7-O-β-D-glucopyranoside	C21H20O10 (432.38)		Platycodon grandiflorus	[23]
Apigenin-8-C-glucoside	C21H20O10 (432.38)		Tetrastigma hemsleyanum Diels et Gilg.	[24]
Apigenin-6-C-glucoside	C21H20O10 (432.38)		Prunus avium L.	[25]
Hinokiflavone	C30H18O10 (538.46)		Juniperus rigida	[26]
Cupressuflavone	C30H18O10 (538.46)		Agathis microstachya J.F. Bailey & C.T. White	[28]
Amentoflavone	C30H18O10 (538.46)		Juniperus sabina L.	[27]

3. Pharmacological functions of apigenin

The molecular formula of apigenin is C₁₅H₁₀O₅, with a molecular weight of 270.24 g/mol. Apigenin has low aqueous solubility, 1.35 μg/mL in purified water and 2.16 μg/mL in phosphate buffers at pH 7.5, while it is freely soluble in dimethyl sulfoxide (>100 mg/mL) [31,32]. Apigenin is one of the most renowned compounds among the wide variety of flavonoids. It is a phenylbenzopyrone structure (C6-C3-C6) that allows apigenin to have a wide range of biological [6]. For example, the hydroxyl radicals at C-5, C-7 and C-4′ are essential for the activation of Liver X receptor [33]. The double bond between C-2 and C-3 in ring C as a glycon form and the C- 4′ hydroxyl group in ring B could enhance eNOS expression and induce vascular relaxation [34]. Apigenin possesses strong antioxidant activity by enhancing antioxidant enzymes (e.g. superoxide dismutase (SOD), catalase, glutathione peroxidase, and glutathione reductase) to counteract oxidative stress [[35], [36], [37]]. It is confirmed that apigenin can modulate various pro- and/or anti-inflammatory cytokines via nuclear factor kappa-light chain enhancer of activated B cells (NF-κB), c-Jun N-terminal kinases (JNK)/p38 mitogen-activated protein kinase pathway, as well as suppressing the nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome [[38], [39], [40], [41]]. Fu et al. [42] reported that apigenin effectively ameliorated ulcerative colitis by remodeling the gut microbiota to inhibit inflammation. Apigenin also inhibits cell proliferation by inducing cell cycle arrest at different proliferation stages by reducing the expression levels of cyclins and inactivation of cyclin-dependent kinases [[43], [44], [45]]. In addition, apigenin has been shown to exert anti-migratory properties by acting on regulatory substances. IL-6-linked downstream signaling pathways, including MAPK, Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3), and PI3K/Akt/mTOR pathways, are closely related to the anti-migratory properties of apigenin [46]. Apigenin has demonstrated a potent therapeutic effect in neurodegenerative diseases. Jameie et al.[47 [][][]revealed that Apigenin combined with β-estradiol could effectively reduce the number of β-amyloid plaques in the hippocampus and ameliorate symptoms of memory impairment and learning in ovariectomized rats. Apigenin also exerts anti-diabetic effects [48], lowers blood pressure [34], regulates lipid metabolism [49], and inhibits platelet function [50]both in vitro and in vivo. The pharmacological functions of apigenin are shown in Fig. 1.

Fig. 1.

The pharmacological functions of apigenin.

4. Effect and mechanism of apigenin on atherosclerosis

There are three layers of an artery wall: (1) the intima, including a single layer of endothelial cells, a thin basal membrane, and a subendothelial layer of collagen fibers; (2) the media layer, consisting of smooth muscle cells (SMCs) and a fiber net of collagen and elastin; and (3) the adventitia, a loose connective tissue. Atherosclerosis is characterized by plaque formation in subendothelial arteries. Sites of bifurcation and inner curvature are particularly susceptible to the initiation of atherosclerosis. The exact causes of atherosclerosis are unknown; however, certain conditions, including high levels of cholesterol and low-density lipoprotein (LDL) in the blood, can promote atherosclerosis. Cholesterol is converted to LDL after esterification. Elevated circulating LDL levels are a key risk factor for atherosclerosis because the accumulation of these particles within the vessel wall may induce endothelial cell dysfunction, the key initiating step of atherogenesis [51]. LDL particles diffuse between endothelial cell junctions into the subendothelial space, where LDL is subsequently oxidized into oxidized LDL (oxLDL) predominantly mediated by ROS. OxLDL upregulates adhesion molecules and induces the secretion of chemokines and cytokines, eventually leading to the recruitment and migration of immune cells, particularly monocytes, into the vascular intima where oxLDL accumulates [1,52]. The recruited monocytes can differentiate into macrophages through a macrophage colony-stimulating factor (M-CSF)-dependent signal and then transform into cholesterol-rich ‘‘foam cells’’ via excessive ingestion of oxLDL [53]. Death of macrophage/smooth muscle foam cells via apoptosis, necrosis, and necroptosis releases their cellular contents, leads to the accumulation of cholesterol, and results in the formation of a lipid-rich necrotic core. Escalating inflammation, persistent foam cell recruitment, and a more necrotic environment within the atherosclerotic plaque further enlarge the lipid-rich necrotic core and thicken the fibrous cap. The fibrous cap provides lesion stability, but ultimately, this fibrous cap stability deteriorates due to the over-expressed matrix metalloproteinases produced mainly by macrophages in response to inflammatory mediators, subsequently causing plaque destabilization and rupture, and eventually resulting in thrombosis and clinical complications of atherosclerosis [1,52,53].

4.1. Anti-oxidative stress

Oxidative stress is a well-known risk factor for atherosclerosis. Oxidative stress indicates that the balance between oxidants and antioxidants is disrupted by abnormal production of ROS and reactive nitrogen species (RNS). ROS and RNS include superoxide anions (•O2−), hydrogen peroxide (H2O2), hydroxyl radicals (•OH), nitric oxide (•NO−), singlet oxygen (1/2 O2), and more complex species such as peroxynitrite (ONOO), hypochlorous acid (HOCl), and lipid peroxyl radicals [54,55]. The main sources of ROS/RNS in atherosclerosis include the mitochondrial electron transport chain, NADPH oxidases (NOX), xanthine oxidase (XOD), and dysfunctional uncoupled endothelial nitric oxide synthase (eNOS) [4]. ROS/RNS can promote atherosclerosis via oxidative modification of lipids, inflammation, as well as affecting the stability of the plaque fibrous cap [4].

Apigenin is a strong antioxidant, and its atheroprotective activity is mainly ascribed to its antioxidant properties, especially towards the oxidation of LDL [56,57]. This finding is consistent with in vitro experiments on the susceptibility of LDL to oxidative modification [50,58]. A study conducted by Clayton et al. [59] showed that apigenin can reverse vascular endothelial dysfunction and large elastic artery stiffening in old (27 months) C57BL/6 mice. The molecular mechanisms involved in these effects include the amelioration of oxidative stress via inhibition of NOX and upregulation of SOD1 and SOD2, with subsequent reduced production of ROS. In addition, 40 μM apigenin attenuates oxLDL-induced endothelial dysfunction by inhibiting intracellular ROS generation [60]. Apigenin, along with betulinic acid and skimmianine at a 2:2:1 ratio, can protect rabbits from oxidative damage by decreasing the activities of LOX and MPO and enhancing the antioxidant activity of SOD [61]. The accumulation of advanced glycation end products (AGEs) may induce oxidative stress, leading to endothelial dysfunction and atherosclerotic plaque formation. A previous study showed that pretreatment of human umbilical vein endothelial cells with 10 μM apigenin can significantly alleviate AGE-induced oxidative stress by downregulating NOX activity with subsequent reduced production of ROS and upregulation of heme oxygenase-1(HO-1), glutamate-cysteine ligase catalytic subunit (GCLC), and glutamate-cysteine ligase modifier subunit (GCLM) via nuclear factor erythroid-2-related factor 2 (Nrf2) pathway [62]. Similar anti-oxidative effects of apigenin have also been reported by Dou. et al. [63]. Their studies demonstrated that apigenin isolated from Gentiana veitchiorum flower could reverse high-fat diet-induced oxidative stress in rats when administered orally for 28 days by enhancing SOD levels and decreasing free radicals due to accelerated cholesterol elimination. The molecular mechanism underlying the anti-oxidative stress of apigenin in atherosclerosis is shown in Fig. 2.

Fig. 2.

Detailed mechanisms involved in anti-oxidative stress of apigenin in atherosclerosis.

4.2. Anti-inflammatory effect

The development of atherosclerosis can be characterized into five stages: (1) LDL retention in arteries, (2) LDL modification (mainly oxidation), (3) aggravating inflammation, (4) foam cell formation and fibrous plaques, and (5) calcification, plaque rupture, and thrombosis [64]. Chronic inflammation is a key factor that contributes to the initiation and progression of atherosclerosis. Endothelial cells, monocytes, lymphocytes, and various cytokines and chemokines are involved in the inflammatory process in atherosclerosis and play important roles in all stages of this disease [65,66].

Apigenin, along with betulinic acid and skimmianine, is involved in the downregulating CD36 mediated TLR2/NF-kB/NLRP3 signaling pathways, eventually leading to enhanced TGF-β and reduced IL-1β and IL-18 in the aortic tissue of rabbits fed a high cholesterol diet [61]. Another study by Kumar et al. [67] showed that apigenin, isolated from Justicia gendarussa, inhibits inflammation in oxLDL-induced hPBMCs by reducing pro-inflammatory cytokines (COX-2, PGE2, IL-1β, and TNF-α) and enhancing anti-inflammatory cytokine (IL-10). In addition, apigenin can markedly suppress the secretion of MCP-1, ICAM-1, VCAM-1, and P-selectin [60,62]. The effects of apigenin on macrophage apoptosis, endothelial inflammation and dysfunction, and leukocyte adhesion, which are involved in the inflammatory process in atherosclerosis, have been proven in both in vitro and in vivo studies [59,[67], [68], [69]]. The molecular mechanism underlying the anti-inflammatory effect of apigenin on atherosclerosis is shown in Fig. 3.

Fig. 3.

Anti-inflammatory mechanisms of apigenin in atherosclerosis.

4.3. Regulation of lipid metabolism

Atherosclerosis is a lipid driven disease. There are five types of lipoproteins, including chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), LDL, and high-density lipoprotein (HDL). Of these lipoproteins, LDL is the most prominent that performs the function of carrying cholesterol from the liver to peripheral tissues. High plasma LDL concentrations cause the modification of lipoprotein particles. When modified lipoprotein particles are present in excess, the early stage of atherosclerosis is triggered. Among all the therapeutic targets involved in the pathogenesis of atherosclerosis, major success has mainly emerged from agents that control lipid homeostasis, such as statins. Apigenin has been observed to play a role in regulating lipid metabolism. A study conducted by Wong et al. [49] showed that co-administration of apigenin can target the low-density lipoprotein receptor (LDLR) and cholesterol transporter Niemann-Pick C1-like 1 (NPC1L1) to block cholesterol absorption and promote its elimination, thereby suppressing hypercholesterolemia and preventing aortic plaque formation in golden hamsters. A similar result from Dou.et al.[63 [][][]demonstrated that apigenin can accelerate cholesterol elimination via low-density lipoprotein receptor (LDLR) and lecithin-cholesterol acyltransferase (LCAT) signaling pathway. Apigenin also plays an important role in promoting lipid catabolism, suppressing cholesterol biosynthesis, enhancing cholesterol efflux via AMP-activated protein kinase (AMPK) pathways and up-regulating ABCA1and ABCG1 [11,[70], [71], [72], [73], [74]].

4.4. Effect of apigenin on programmed cell death

Programmed cell death, including apoptosis, autophagy, pyroptosis, ferroptosis and necroptosis, is closely related to the development of atherosclerosis [75]. Studies proved that apoptosis is the typical programmed cell death modality underlying the anti-atherogenic effects of apigenin. Endothelial cell apoptosis promotes the formation and development of atherosclerosis [76]. Qin et al. [77]reported that apigenin suppressed pro-apoptotic proteins, Bax and Caspase-3, and increased anti-apoptotic protein, Bcl-2, in high glucose treated endothelial cells, resulting in attenuation of apoptosis in endothelial cells. Macrophage apoptosis plays a particularly critical role in atherosclerosis. According to previous studies, macrophage apoptosis has a dual role in atherosclerosis: it decreases lesion cellularity and progression the early stages of atherosclerosis but accelerates plaque necrosis in more-advanced lesions [75]. In the study by Zeng et al. [69], apigenin induced the apoptosis of oxLDL-induced murine peritoneal macrophages by regulating the expression of Bax, Caspase-3 and Bcl-2 through downregulation of plasminogen activator inhibitor 2. Similarly, another report by Xu et al. [78] also showed that intragastric administration of apigenin (40 and 80 mg/kg) noticeably upregulated the Bcl-2/Bax ratio in hyperlipidemic rats, thereby preventing atherosclerosis.

4.5. Effect of apigenin on scavenger receptors and ATP-binding cassette transporters

Scavenger receptors are a group of endocytic pattern recognition receptors that perform a wide range of cellular functions, such as fatty acid metabolism and endocytosis of modified lipoproteins. Scavenger receptors is of critical importance in atherosclerosis [53]. Scavenger receptor-A (SR-A), cluster of differentiation 36 (CD36), and scavenger receptor for phosphatidylserine and oxidized lipoprotein (SR-PSOX) are involved in lipoprotein uptake and can internalize the majority of oxLDL. ApoE, ATP-binding cassette (ABC) transporter G1(ABCG-1), ABCG-4, and ABCA-1 are closely related to cholesterol efflux [53,79].

Apigenin is active as a substance on scavenger receptors. Su et al.[70 [][][]tests the effects of apigenin on CD36 in 3T3-L1 cells and diet-induced obesity mouse model. The results showed that CD36 expression was significantly reduced in apigenin-treated 3T3-L1 cells and adipose tissues of diet-induced obese mice. Other evidence emerges from the study of Ren et al. [80], who reported that apigenin obviously promoted ABCA1 expression and enhanced ABCA1-mediated cholesterol efflux, both in a concentration-and time-dependent manner, in oxLDL induced RAW264.7 macrophages. Additionally, treatment with 50 μM apigenin significantly reduced the expression of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), one of the main scavenger receptors, in endothelial cells [68]. Similar findings had also been reported by Xu. et al. [78]. The effects of apigenin on lipid metabolism and scavenger receptors in atherosclerosis are shown in Fig. 4.

Fig. 4.

Effects of apigenin on lipid metabolism and scavenger receptor in atherosclerosis.

4.6. Blood-pressure-lowering effect

Hypertension is an important risk factor of atherosclerosis. Studies have shown that the antiatherogenic role of apigenin may also be attributed to its antihypertensive activity. Apigenin can manage blood pressure by acting as an angiotensin-converting enzyme inhibitor [81,82]. A rat model of arterial hypertension induced by L-NAME also confirmed that apigenin reduced elevated blood pressure levels by ameliorating vascular reactivity to vasoconstrictors and sodium retention, and enhancing vasodilator ability via increased production of NO [34]. Similarly, apigenin (50 mg/kg) administered to rats by gavage also decreased the elevated systolic blood pressure induced by the excessive ingestion of fructose from 146 mmHg to 123 mmHg [72]. Moreover, apigenin exerts a hypotensive effect by attenuating contractions induced by various vasoconstrictors [83].

4.7. Anti-hyperglycaemia effect

Hyperglycaemia is another risk factor for atherosclerosis due to an increased risk of endothelial cell injury and dysfunction caused by it. Apigenin has shown anti-hyperglycaemia effects by inhibiting glucosidase, α-amylase, and maltase activities, as well as increasing insulin sensitivity and secretion. The extract of Passiflora edulis Sims leaves, a rich source of apigenin present in the form of C-glycosides, was investigated in an alloxan-induced diabetic rat model as a natural agent against diabetes. The results showed that treatment with the extract of Passiflora edulis Sims leaf at a dose of 20 mg/kg for 90 days produced a decrease in blood glucose and fructosamine levels [48]. Apigenin and its precursor, apigenin-7-O-glucoside, both inhibited α-amylase activity and D-[U-¹⁴C]-glucose and sucrose transport, suggesting their potential to attenuate postprandial hyperglycaemia [84]. Apigenin can also downregulate glucose transporter 1, which is closely related to glucose uptake, and ameliorates insulin resistance in rats with high fructose-induced metabolic disturbances [72].

4.8. Anti-platelet effect

Platelets participate in atherosclerosis. They play a pivotal role in the initial steps of atherosclerosis by triggering leukocyte infiltration into the arterial wall. Evidence also indicates that platelets can traverse the endothelium and act as direct mediators in the progression of atherosclerotic plaque [[85], [86], [87]]. Studies have reported that the extract isolated from parsley (Petroselinum crispum) leaves, including the presence of apigenin, has anti-aggregating activity and decreases the adhesion of human platelets to collagen [88]. Platelet hyper-reactivity plays a major role in atherothrombosis. Guerrero et al. [89]reported apigenin can inhibit platelet function through Thromboxane A2-mediated signal pathway. Apigenin also possess strong inhibition on arachidonic acid- and adenosine diphosphate-induced platelet aggregation [48,50].

5. Limitation of apigenin used as a therapeutic agent

The most challenging problem related to apigenin is how to increase its solubility and achieve effective bioavailability when used as a therapeutic agent. Apigenin is classified as a class II biopharmaceutical because of its low solubility and high intestinal permeability [31,32]. Recently, innovative techniques, such as microwave-assisted extraction, ultrasound-assisted extraction, and enzymatic techniques have been used to recover apigenin from natural sources [6,8]. New therapeutic formulations and drug delivery systems such as micelles, liposomes, nanoparticles, as well as carbon nanopowder solid dispersions and self-microemulsifying drug delivery systems that contain apigenin could be used to improve its solubility and bioavailability in the future [32,[90], [91], [92], [93]].

Apigenin is a flavonoid with low toxicity. However, some studies have reported that acute exposure to apigenin at doses more than 100 mg/kg may induce hepatocyte degeneration and liver toxicity in male Swiss mice [94]. Although apigenin is considered safe and has a high safety threshold, further studies are still required to gain deeper insights into the toxicity and ascertain a safe range of this compound because available reports about its potential toxicity are scarce and inconsistent. On the other hand, apigenin also has potential side effects, including muscle relaxation, upset stomach, skin irritation, and a mild sedative effect when intentionally consumed at high doses [95,96].

Identification of interactions with other drugs is another problem for apigenin. Apigenin intake as a dietary supplement has been steadily increasing; however, there is little awareness of the potential for drug interactions with conventional drugs. A few reports have demonstrated that there is a significant chance of drug interactions when concurrent use of apigenin with aspirin, raloxifene, venlafaxine, and anti-tumor drugs, characterized by changing plasma levels, altering pharmacokinetic parameters including AUC and T_max, and gaining synergies from co-administered drugs [32,55,95,97,98]. Moreover, apigenin is a competitive inhibitor of CYP2C9 and can interact with drugs that act as substrates for CYP2C9. This may therefore enhance the risk of toxicity of warfarin, tolbutamide, and phenytoin, which are CYP2C9 substrates with narrow therapeutic indices [99]. More attention should be paid when these medications are administered in combination with apigenin, and it is necessary to contact the healthcare provider before taking apigenin supplements if a person is taking any prescription medication.

Finally, apigenin is mainly administered orally in the form of dietary supplements, but not as a therapeutic drug used alone. Human clinical trials are a prerequisite to consider the potential use of apigenin in various diseases; however, nearly all the available conclusions on the preventive and/or therapeutic potential of apigenin come from cellular or animal experiments. Human clinical trials to comprehend the role of apigenin as a therapeutic drug are still in their infancy, and its pharmacodynamic/pharmacokinetic analyses in humans have not yet been well elucidated. In conclusion, further research is needed before apigenin can be developed as a therapeutic drug.

6. Conclusions

Apigenin is abundant in vegetables, fruits, and herbs and possesses many biological activities. Both in vitro and in vivo evidence has demonstrated that apigenin is a promising bioactive agent for the prevention and treatment of atherosclerosis (as summarized in Table 3). Available trials suggest that apigenin alleviates atherosclerosis mainly via regulating oxidative stress, inflammation, as well as lipid metabolism, apoptosis, blood-pressure, and hyperglycaemia. A summary of the studies about the apigenin-mediated regulation in atherosclerosis is illustrated in Fig. 5. However, the few available clinical trials mainly supported the potential protective effects of apigenin (or extracts rich in apigenin) on insomnia, anxiety disorders and depression, knee osteoarthritis, and Alzheimer's disease [[100], [101], [102], [103], [104], [105]], the clinical trials about its contribution to atherosclerosis are lacking. There is a need to characterize the safety and efficacy of apigenin in future clinical trials to determine its potential as an anti-atherogenic agent.

Table 3.

Preclinical studies on the atheroprotective properties of apigenin.

Model	Dose	Administration Route &duration	Mechanism	Reference
C57BL/6 mice	0.5 mg/mL	Drinking water/6 weeks	NO Bioavailability↑, ROS↓, SOD1↑, SOD2↑, aortic AGEs↓, Uptake of oxLDL↓, IL-1β↓, IL-6↓, IFN-γ↓, TNFα↓	[59]
RAW 264.7 cells	1, 2 μM	24 h
EA.hy 926 cells	40 mM	2h	ROS↓, O2ˉ↓, p38MAPK↓, NF-kB↓, ICAM-1↓, VCAM-1↓, E-selectin↓, MMPs↓	[60]
Rabbits	5 mg/kg (2:2:1)	Orally/90 days	LOX↓, MPO↓, SOD↑, TBARS↓, TLR2↓, NOS↓, MyD88↓, TRAF6↓, NFκB-p65↓, NLRP3↓, IL-1β↓, IL-18↓, TGF-β↑, COX↓, 15-LOX↓	[61]
HPBMCs	25 μM (2:2:1)	24 h
HUVECs	10 μM	1 h	ROS↓, NADPH oxidase↓, RAGE ↓, Nrf2↑, GCLC↑, GCLM↑, HO-1↑, IL-6↓, MCP-1↓, ICAM1↓, TGF-β1↑, p-ERK 1/2↓, NF-κB↓	[62]
SD rats	50–200 mg/kg	Orally/28 days	Radical scavenging rates↑, MDA↓, SOD↑, Lipid vacuoles↓, LDLR↑, LCAT↑, TG↓, TC↓	[63]
Cu2+-induced LDL oxidation	3.13–25 μg/mL	–	LDL oxidation↓	[50]
Cu2+-induced LDL oxidation	0–200 mmol/l	–	Susceptibility of LDL to oxidative modification↓	[58]
hPBMCs	25 μM	24 h	TLR4↓, MyD88↓, TRIF↓, TRAF 6↓, NF-κB ↓, COX-2↓, PGE2↓, TNF-α↓, IL-1β↓, IL-10↑	[67]
ApoE−/− mice	100 mg/kg	Orally/8 weeks	MOMA-2↓, Macrophage-derived foam cells↓, Mcl-1↓, Bcl-2↓, Bax↑, PAI-2↓, p-Akt Ser473↓,	[69]
MPMs	50 μM	48 h
Endothelial cells	30, 50 μM	8, 24 h	ICAM-1↓, VCAM-1↓, CCL2↓, PYCARD↓, LOX-1↓, NLPR3↓, TXNIP↓, OLR1↓, SCARF1↓, CXCL16↓,	[68]
Huh7, Huh7.5, HepG, AML12 cells	0–20 μM	18h	Beclin 1↑, ATG5↑, ATG7↑, LC3-I↑, LC-3-II↑, SQSTM1↑, β-oxidation↑, ACSL1↑, CTP1α↑, ACOX1↑, PPARα↑, PGC1α↑, MTCO1↓, OPA1↑, Mfn2↑, Mitochondrial ROS↓	[11]
Golden hamsters	60, 300 ppm	Food intake/9 weeks	TC↓, TG↓, Npc1l1↓, Lxrα↓, Ldlr↑, Srebp-2↑	[49]
C57BL/6 mice	0.04 % (w/w)	Food intake/12 weeks	PPARγ↓, LPL↓, AP2↓, FASN↓, SCD1↓, HSL↑, FOXO1↑, SIRT1↑, p-AMPK↑, p-ACC↑, UCP-1↑	[71]
SD rats	50 mg/kg	Orally/12 weeks	TG↓, TC↓, HDL-C↑, sensitivity of insulin↑, ADMA↓, PI3K↓, AKT↓, GLUT1↓	[72]
C57BL/6 mice	50, 250 ppm	Food intake/8 weeks	TC↓, TG↓, non-HDL-C↓, Hmgcr↓, Srebp-2↓, P-AMPK↑	[73]
hMSCs	1–25 μM	2, 3 or 8 days	TG↓, acc↓, perilipin↓, atgl↑, fasn↓	[74]
C57BL/6 mice	15, 30 mg/kg	Injection/14 days	VAT↓, p-STAT3↓, CD36↓, PPARγ↓	[70]
3T3-L1 cells	10–100 μM	2–10 days
HUVECs, HAECs	3, 30 μM	48, 72 h	p-NF-κBp65↓, p-Akt↑, Bcl-2↑, Bax↓, caspase-3↓, p-PKCβII↓, ROS↓, NO↑	[77]
MPMs	50 μM	48 h	Mcl-1↓, Bcl-2↓, Bax↑, Caspase-3↑, PAI-2↓	[69]
SD rats	20–80 mg/kg	Orally/6 weeks	TC↓, TG↓, LDL-c↓, HDL-c↑, LOX-1↓, Bcl-2↑, Bax↑, Bcl-2/Bax ratio↑	[78]
ApoE−/− mice	10 mg/kg	Orally/8 weeks	CD68 and α-SMA in aortic plaques↓, TC↓, TG↓, LDL↓, HDL↑, ABCA1↑, TLR-4↓, MyD88↓, NF-κB p65↓, p-IκB-α↓, IL-1β↓, IL-6↓, TNF-α↓	[80]
RAW264.7 cells	10–40 μM	6–24 h
SD rats	1.44 mg/kg	Drinking water/6 weeks	Nitrite urinary excretion↑, Sodium retention↓, Relaxation to Ach↑, Vasodilator ability↑	[34]
ACE inhibitory assay	50–200 μg/mL	–	ACE activity↓, IC50 = 28.91 ± 13.42 μg/mL	[81]
ACE inhibitory assay	20–330 μg/mL	–	ACE activity↓, IC50 = 280 ± 3.2 μM	[82]
RIRAs, ASMCs	10–100 μM	–	CaCC currents↓, Kv currents↑, Depolarization-, U46619-, PE-, VP-induced RIRA contractions↓	[83]
Caco-2, TC7 cells	12, 50 μM	–	D-[U-14C]-glucose and sucrose transport↓, α-amylase activity↓	[84]
Wistar Rats	20 mg/kg	Orally/90 days	Blood glucose levels↓, fructosamine↓, ROS in platelets↓, urea values↓, atherogenic particles↓, TC↓	[48]
Platelet aggregation assay	3.13–100 μg/mL	–	AA-induced platelet aggregation↓, ADP-induced platelet aggregation↓	[50]
Platelet aggregation assay	0.1–0.5 mg/ml	–	Thrombin-, collagen-, ADP-induced platelet aggregation↓	[88]
Platelets, HEK293T cells	0–100 μM	–	Platelet calcium flux↓, p-ERK 1/2 in platelets↓, Platelet tyrosine phosphorylation↓	[89]

Fig. 5.

Mechanisms involved in the anti-atherogenic effects of apigenin.

CRediT authorship contribution statement

Xueqiang Jiang: Writing – original draft, Conceptualization. Huimin Huang: Writing – review & editing, Visualization, Conceptualization.

Declaration of competing interest

The authors declare no conflicts of interest.