Protective Roles of Apigenin Against Cardiometabolic Diseases: A Systematic Review

Apigenin is a flavonoid with antioxidant, anti-inflammatory, and anti-apoptotic activity. In this study, the potential effects of apigenin on cardiometabolic diseases were investigated in vivo and in vitro. Potential signaling networks in different cell types induced by apigenin were identified, suggesting that the molecular mechanisms of apigenin in cardiometabolic diseases vary with cell types. Additionally, the mechanisms of apigenin-induced biological response in different cardiometabolic diseases were analyzed, including obesity, diabetes, hypertension and cardiovascular diseases. This review provides novel insights into the potential role of apigenin in cardiometabolic diseases.


INTRODUCTION
Apigenin (4 ,5,7-trihydroxyflavone) is named after the genus Apium belonging to family Apiaceae (1). It is widely distributed in vegetables and fruits, such as celery, parsley, oranges and garlic (2), and is also found in herbs such as snow lotus and chamomile (3) (Figure 1A). As a secondary plant metabolite, apigenin is usually stored in plants in a water-soluble glycosylated form (4). Purified apigenin is a yellow powder with a low molecular weight (MW 270.24). It is nearly insoluble in water, moderately soluble in hot alcohol and soluble in dimethyl sulfoxide (DMSO) (5). Pure apigenin is chemically unstable and therefore stored in the dark at −20 • C (5).
The biosynthesis of apigenin occurs on the surface of the endoplasmic reticulum and requires four steps including intermediate synthesis, basic skeleton synthesis, precursor synthesis and generation of the apigenin structure (6) (Figure 1B). Current evidence indicates that the bioactivity of apigenin is dependent on its chemical structure, thus the structure-activity relationship of apigenin can be determined by extracting the molecular fragments associated with a specific biological activity. For example, double bonds in the two aromatic rings and hydroxyl groups on C-7 and C-4 induce the inhibition of α-glucosidase and α-amylase (7). The C-4 hydroxyl group in ring B is essential for immunomodulatory properties (8). The hydroxyl radicals at position 5, 7 and 4 are necessary for Liver X receptor activation (9).
Cardiometabolic disease links the metabolic syndrome disorders (abdominal adiposity, hypertension, dyslipidemia, hyperinsulinemia and glucose intolerance) that are predictive of cardiovascular disease and Type 2 diabetes (T2DM) (10). Recently, apigenin has been found to play a protective role in cardiometabolic diseases in vitro and in vivo. This systematic review summarizes the current perspective.

PROTECTIVE ROLES OF APIGENIN IN CARDIOMETABOLIC DISEASES Protective Role of Apigenin in Obesity and Lipid Metabolism
Obesity is attributed to chronic energy imbalance, including excessive energy intake and limited energy expenditure (11,12). Anti-obesity strategies focus on suppression of energy intake and stimulation of energy expenditure by regulating lipid metabolism, such as inhibiting pancreatic lipase activity and adipocyte differentiation (13). Studies suggest that apigenin controls energy intake by inhibiting appetite and stimulating energy expenditure by regulating lipid metabolism to alleviate obesity.
First, apigenin inhibits obesity by suppressing food consumption. In vitro and in vivo studies confirmed that apigenin upregulates the expression of anorexigenic neuropeptides pro-opiomelanocortin (POMC) and cocaine-and amphetaminerelated transcript (CART), resulting in inhibition of food intake. N-29-2 and SH-SY5Y cells transfected with pPOMC-Luc and pCART-Luc vectors were treated with 0.2-5 µM apigenin for 6 h, resulting in upregulation of pPOMC-Luc and pCART-Luc activity. In an in vivo study, 6-week-old C57BL/6J mice were injected (every 24 h) intraperitoneally with 1 or 10 mg/kg of apigenin in a short-term intervention. Male C57BL/6J mice fed with a high-fat diet (HFD) or a standard laboratory chow diet received 0.05% apigenin for 30 days to demonstrate that apigenin reduces food intake and visceral fat over a long-term period (14). POMC and CART neurons found in the retro-chiasmatic area and throughout the rostrocaudal span of the arcuate nucleus (ARC) play a role in appetite control (15). Increasing expression of POMC and CART induces the expression of leptin receptor B (LepRb) to facilitate leptin binding to LepRb, resulting in an anorexic effect and upregulation of insulin receptors to inhibit appetite.
Secondly, apigenin stimulates energy expenditure by regulating lipid metabolism, including adipogenesis, lipolysis, fatty acid oxidation, and cholesterol synthesis. Recent studies indicate that adipose tissues are generally targeted by apigenin eliciting the following effects: (1) Stimulation of PPARγ signaling. Several studies have demonstrated that apigenin inhibits adipocyte differentiation via STAT3 (the signal transducer and activator of the transcription 3)-CD36-PPARγ (peroxisome proliferator-activated receptorgamma) axis (16) and AMPK (5 -Adenosine monophosphateactivated protein kinase)/PPARγ axis (17). One study showed that 100 µM apigenin treatment inhibits the differentiation of 3T3-L1 preadipocytes to mature white adipocytes. Mouse models of diet-induced obesity receiving apigenin via subcutaneous injection for 13 days showed that apigenin reduced visceral fat mass (16). Apigenin binds to non-phosphorylated STAT3 to decrease STAT3 phosphorylation and nuclear translocation (18), followed by a decline in the expression of CD36, the downstream target gene involved in fatty acid transport (19). PPARγ is the transcript factor of central ligand-activated transcription factors. It inhibits adipogenesis and controls adipose tissue differentiation to regulate inflammation in obesity. PPARγ expression depends on CD36 expression and therefore apigenin treatment inhibits adipocyte differentiation via downregulation of PPARγ. Other studies reported that apigenin activates the phosphorylation of AMPK (5 -adenosine monophosphate-activated protein kinase) to downregulate adipogenesis via AMPK/PPARγ axis in 3T3-L1 cells treated with 10 µM apigenin for 1 h or 4 days (17) and in HFD mice treated with 200 mg/kg enzyme-treated celery extract (20). AMPK acts as a potential target against adipogenesis (21,22) and downregulates the expression of PPARγ. The adipogenous genes downstream of PPARγ, such as fatty acid-binding protein 4 and stearoyl-CoA desaturase, are also downregulated, thereby suppressing adipogenesis (17).
(2) Repression of enzyme activity. Guo et al. reported that 0.6 mM apigenin directly inhibits pancreatic lipase activity in vitro (23). Pancreatic lipase catalyzes the conversion of triglycerides to monoglycerides and fatty acids in the intestine. Obesity is alleviated by the suppression of pancreatic lipase, fatty acid synthesis and fat absorption. Gómez-Zorita et al. showed that treatment with 25 µM apigenin decreases the expression of fatty acid synthase (FAS), while increasing the expression of adipose triglyceride lipase (ATGL) in mature adipocytes derived from human mesenchymal stem cells (hMSCs), resulting in reduced adipogenesis (24).
Liver, in addition to adipose tissues, is essential for lipid metabolism. Abnormal lipid metabolism in the liver induced by obesity may cause hepatic steatosis. Apigenin also improves lipid metabolism in the liver to alleviate hepatic steatosis via following mechanisms: (1) Stimulation of PPARγ signaling. Apigenin modulates PPARγ expression in hepatic lipid metabolism via Nrf2-PPARγ axis. In Hep1-6 cells, apigenin activates nuclear factor erythroid 2-related factor 2 (Nrf2) via translocation into the nucleus to upregulate downstream antioxidant enzymes and downregulate lipid synthesis (26). Activation of Nrf2 by apigenin neutralizes the activation of PPARγ to regulate lipid metabolism in liver (27).
(2) Regulation of SREBP family. Apigenin treatment inhibits lipid homeostasis by the sterol regulatory element-binding protein (SREBP) family. Apigenin significantly decrease lipid accumulation, total intracellular cholesterol (TC), and intracellular triglyceride (TG) levels via the AMPK-SREBP-1/2 (sterol regulatory element-binding protein-1/2) axis in HepG2 cells. Apigenin-induced activation of AMPK downregulates the levels of SREBP-1 and SREBP-2 to reduce the synthesis of cholesterol, fatty acids, and triglycerides in the liver. The inhibition of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCR), which is the downstream target gene of SREBP-1 and FAS, the downstream target gene of SREBP-2 also regulates fatty acid and cholesterol synthesis (28).
Obesity-induced oxidative stress and inflammation also aggravate the symptoms of cardiometabolic diseases, leading to multiple cellular disorders (31,32). Current studies indicate that apigenin alleviates oxidative stress and inflammation by binding to PPARγ as an agonist to regulate M2 polarization with nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) inhibition. Apigenin-induced PPARγ activation blocks p65 nuclear translocation. NF-κB activation is inhibited in adipose tissue macrophages, leading to an increase in M2 macrophage polarization. The anti-inflammatory effects of M2 macrophages alleviate the metabolic disorder caused by obesity-related inflammation. Meanwhile, cytokines such as IL-6, IL-1β, and TNF-α are suppressed by the inhibition of NF-κB signaling (33) Frontiers in Nutrition | www.frontiersin.org following apigenin treatment. The protective effect of apigenin on adipocyte browning in the inflammatory environment is also mediated via p65/NF-κB pathway. The inflammatory environment suppresses adipocyte browning to reduce lipid metabolism (34). Apigenin suppresses p65 translocation into the nucleus to inhibit NF-κB activation and inflammatory markers in adipocytes to attenuate inflammation and suppress adipocyte browning (25,35). Apigenin also plays a protective role in the inhibition of white-to-brown adipose tissue differentiation. The inflammation induced via activation of uncoupling protein 1 and PGE2 receptor 4 (EP4) activates the cyclooxygenase 2 (COX2)/prostaglandin E2 (PGE2) axis, resulting in conversion of white to brown adipose tissue to generate heat by excessive energy expenditure (36).
Studies reported that obesity leads to many health complications. First, obesity has been associated with gastrointestinal disorders, such as gastroesophageal reflux disease, irritable bowel syndrome, and dyspepsia (37,38). Colon inflammation adversely affects enteric motor function, leading to gastrointestinal disorders (39). Apigenin reduces the levels of malondialdehyde (MDA), interleukin-6 (IL-6), and interleukin-1β (IL-1β), as well as eosinophil infiltration in colon tissue to alleviate inflammation. Further, apigenin regulates inducible nitric oxide synthase (iNOS) expression and substance P (SP) levels in high-fat-diet (HFD)-fed obese mice (40). SP is a neurotransmitter that stimulates the contraction of various intestinal tissues and is a neurokinin receptor 1 (41). Obesity induces the expression of SP leading to enhanced tachykinergic transmission in the enteric nervous system, resulting in abnormal colonic motor function. The suppression of SP by apigenin attenuates enteric motor dysfunctions (40). NO produced by iNOS may trigger inflammation and play a role in enteric nitrergic pathways (42). The downregulation of iNOS by apigenin attenuates inflammation and enteric motor dysfunction. The regulation of gut bacteria by apigenin also prevents colonic dysfunction in mice via modulation of NOD-like receptor family pyrin domain containing 6 (Nlrp6) (43). Nlrp6 is highly expressed in the intestine. Nlrp6 deficiency may lead to proliferation of Prevotellaceae, the gut bacteria found in patients with bowel diseases (44), by promoting Nlrp6 inflammasome, IL-18 secretion, and regulation of gut bacterial homeostasis. Further, apigenin improves intestinal dysbiosis via augmentation of Akkermansia and Incertae Sedis along with reduction of Faecalibaculum and Dubosiella at the genus level (45). Obesity has also been associated with sarcopenia (46). Obesity-induced muscle atrophy also contributes to impaired glucose and lipid homeostasis, proinflammatory responses, and inflammation-induced mitochondrial dysfunction (47). Apigenin ameliorates skeletal muscle atrophy by enhancing mitochondrial function in an obese mouse model exposed to HFD and in C2C12 cells. Apigenin treatment upregulated mitochondria-related genes, including peroxisome proliferatoractivated receptor-γ coactivator-1α (PGC1α), mt-TFAM (transcription factor of PGC1α), cytochrome C, and somatic cytochrome C (CyCs) following the activation of AMPK. Such upregulation is essential for initiation of mitochondrial biogenesis and improved mitochondrial function alleviate obesity-induced skeletal muscle atrophy (48,49).
In summary, apigenin alleviates obesity and its complications via a variety of mechanisms including inhibition of appetite, glucose signaling pathways and lipid metabolism. It also regulates the intestinal microbiome, enhances mitochondrial function and attenuates inflammation and oxidative stress. The aforementioned experimental approaches and mechanisms underlying the effects of apigenin on obesity are listed in Table 1. Dosages and duration of apigenin treatment in vivo and in vitro are also listed in Table 1.

Protective Role of Apigenin in Diabetes
Diabetes also plays an important role in cardiometabolic disease (50)(51)(52). Several studies investigating the effects of apigenin on type 2 diabetes mellitus (T2DM) report decreased insulin resistance, reduced abnormal glycolipid metabolism, and alleviation of oxidative stress (53,54).
Insulin resistance plays a significant role in the pathophysiology of T2DM (55). Insulin resistance adversely affects glycometabolism in insulin-targeted organs and tissues (54). Abnormal glycolipid metabolism is a typical clinical manifestation in patients with T2DM (56). Apigenin alleviates insulin resistance and glycolipid metabolic disorders via following mechanisms: (1) Inhibition of insulin receptor kinase. Apigenin inhibits tyrosine nitration of the insulin receptor kinase domain leading to alleviation of insulin resistance. Tyrosine nitration of IRβ (intracellular β subunits of the insulin receptor) may lead to decreased tyrosine phosphorylation, resulting in impaired insulin signal transduction in HFD mice (57). In vitro studies showed that apigenin decreases the Cu 2+ -catalyzed insulin receptor kinase domain fragment KK-1 and inhibits the formation of 3,3 -dityrosine (58).
(2) Regulation of miRNAs. Apigenin regulates miRNAs, which are associated with insulin resistance and glucose homeostasis.
In vitro experiments involving Huh7 cells and in vivo studies investigating miR103 transgenic mice validates apigeninmediated inhibition of the phosphorylation of transactivating response RNA-binding proteins (TRBP). Additionally, miRNAgenerating complexes inhibited, leading to suppression of precursor miRNA103 maturation expressed in liver and fat, resulting in insulin resistance and impaired glucose metabolism and homeostasis (59,60). Thus, apigenin-induced suppression of miRNA103 alleviates glucose intolerance (60).
(3) Upregulation of GLUT4/AMPK signaling. Apigenin extracted from Sophora davidii (Franch.) promotes glucose transporter 4 (GLUT4) expression and activates AMPK phosphorylation in L6 cells and insulin target tissues in KK-Ay mice (61). In insulin target tissues such as liver and fat, the upregulation of GLUT4 and the activation of AMPK facilitates glucose utilization to ameliorate insulin resistance (62,63).
(4) Inhibition of α-amylase. Several studies reported that apigenin decreases the inhibition of α-amylase in Kunming mice, thus reducing the digestion of dietary carbohydrates (64). The digestive enzyme α-amylase hydrolyzes dietary carbohydrates into disaccharides and polysaccharides (65). Inhibition of the digestion of dietary carbohydrates delays glucose absorption and blocks the progression of T2DM. Therefore, the inhibition of α-amylase by apigenin ameliorates T2DM (64). Oxidative stress also triggers β-cell dysfunction, impaired glucose tolerance, and insulin resistance (66). The production of reactive oxygen species (ROS) in oxidative stress exacerbates the progression of T2DM and related complications. Apigenin treatment mitigates oxidative stress and intracellular ROS production via following mechanisms: (1) Decreased ROS production. Apigenin pre-treatment of streptozocin (STZ)-treated RINm5F pancreatic β cells ameliorates STZ-induced intracellular ROS production, as well as DNA damage, lipid peroxidation, and apoptosis. Apigenin pretreatment upregulates the expression of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) in RINm5F pancreatic β cells and diabetic rats (67). SOD catalyzes the conversion of superoxide radicals (O 2− ) to molecular oxygen (O 2 ) and hydrogen peroxide (H 2 O 2 ), resulting in a protective effect against ROS in cells (68). Catalase is a highly specific enzyme that catalyzes the decomposition of hydrogen peroxide into water and molecular oxygen (69). GSH-Px is a cytosolic enzyme that catalyzes the reduction of hydrogen peroxide and lipid peroxides by glutathione, releasing water, oxygen, and alcohol (70). These three enzymes are indispensable in defending against free radicals (71).
(2) Inhibition of AGE. Apigenin inhibits the formation of advanced glycation end products (AGEs) and thereby alleviates oxidative stress (72). AGE-mediated damage leads to altered protein structure and functions via cross-linking between molecules via the receptor for AGEs (RAGE). AGEs increase ROS formation and damage anti-oxidant systems (73). Apigenin treatment of human blood plasma proteins in vitro reduced the levels of AGEs (72).
(3) Regulation of Keap1-Nrf2 signaling. The anti-oxidant function of apigenin is mediated via the Kelch-like ECHassociated protein 1 (Keap1)-Nrf2 axis targeting liver tissues to alleviate oxidative stress (74). Nrf2 is a primary transcription factor interacting with the anti-oxidant response element (ARE) to regulate antioxidant protein expression. Keap1 is the specific repressor of Nrf2, which acts as an adaptor protein of the Cullin3based ubiquitin E3 ligase complex to facilitate the ubiquitination and subsequent proteolysis of Nrf2 (74), acting as a sensor of oxidative stress (75,76). Apigenin occupies the Nrf2-binding site to prevent the binding between Keap1 and Nrf2 and thereby promotes nuclear translocation of Nrf2, thus facilitating its antioxidant function (74).
Currently, the apigenin-mediated regulation of blood glucose homeostasis can be summarized as follows: regulating the key enzymes and improving oxidative stress as well as inflammation. A detailed summary of the studies discussed above and the proposed mechanisms of apigenin-mediated effects in diabetes are presented in Table 2. Dosages and duration of apigenin treatments in vivo and in vitro are also listed in Table 2.

Protective Role of Apigenin in Hypertension
Hypertension plays a central role in cardiometabolic diseases (79), which is prevalent in almost 80% of patients with metabolic syndrome (80). Recent studies reported that apigenin improves hypertension via attenuation of oxidative stress and recovery of mitochondrial dysfunction.
Apigenin plays a protective role in hypertension by alleviating oxidative stress. Apigenin can significantly restore normal blood pressure and reverse renal damage in cyclosporineinduced hypertensive Sprague-Dawley rats by decreasing lipid hydroperoxides and increasing anti-oxidant levels (81). Apigenin also controls elevated blood pressure in N-nitro-L-arginine methylester-induced hypertensive Sprague-Dawley rats by improving NO bioavailability, attenuating oxidative stress, and reducing vascular damage (82).
Apigenin also regulates pulmonary hypertension (PH). Mitochondrial dysfunction plays a vital role in PH, it may lead to the imbalance of ion homeostasis and downregulation of enzymes in apoptosis (83). Apigenin activates mitochondriadependent apoptosis via hypoxia-inducible factor 1α (HIF-1α)-KV1.5 channel pathway. The inhibition of HIF-1α by apigenin upregulates the expression of KV1.5 channels to restore mitochondrial function, thereby attenuating PH (84).
Apigenin has also been reported to diminish the complications induced by hypertension, such as renal damage and fibrosis due to abnormal collagen accumulation in kidneys (85). Apigenin significantly attenuated hypertension and renal fibrosis in deoxycorticosterone acetate (DOCA)-salt-induced hypertensive rats (86). Apigenin activates transient receptor potential vanilloid 4 (TRPV4), a non-selective cation channel widely expressed in the kidney. Ca 2+ influx is then promoted in vascular endothelium and smooth muscle to induce vasodilation (87) and activation of the AMPK/SIRT1 signaling pathway to inhibit the TGF-β1 and Smad-2/3 signaling pathway (Sma and Mad proteins from Caenorhabditis elegans and Drosophila, respectively). This inhibition stimulates cellular transformation into fibroblasts and increases the synthesis of matrix proteins to induce renal fibrosis (86,88,89). Thus, apigenin alleviates renal fibrosis and structural and functional damage.
Current evidence suggests that apigenin decreases blood pressure mainly via improved NO bioactivity and oxidative stress, regulation of apoptosis-related mitochondrial genes and promotion of vasodilation in vascular endothelium. Experimental studies and mechanisms of action involving apigenin in hypertension are listed in Table 3. Experimental dosages and durations of apigenin treatment in vivo and in vitro are listed in Table 3.

Protective Role of Apigenin in Cardiovascular Diseases
Apigenin prevents cardiovascular diseases via antioxidant and anti-apoptotic mechanisms in vascular endothelial cells and cardiomyocytes.
Vascular endothelial dysfunction is a major mediator in cardiovascular diseases (90). Abnormal glucose metabolism He et al. (84) In vitro experiment HBZY-1 cells, M1CCD cells and oxidative stress in vascular endothelial cells may lead to vascular endothelial dysfunction. Several studies have discussed the ameliorative effect of apigenin on endothelial dysfunction. First, apigenin increases apelin expression to rescue endothelial dysfunction. Apelin is an endogenous ligand for the G-proteincoupled APJ receptor expressed in the cardiovascular system. It increases glucose uptake and SOD activity, reversing the impaired glucose metabolism and homeostasis and the severe oxidative stress in human endothelial cells (91)(92)(93)(94). Oxidative stress in endothelial cells leads to endothelial dysfunction and angiogenesis (92). The suppression of apelin in human endothelial cells can be reversed by apigenin treatment. Second, apigenin inhibits NF-κB-associated signaling pathways and suppresses intercellular adhesion molecule-1 (ICAM-1) expression in vascular endothelial dysfunction. ICAM-1 is a cell surface receptor that binds lymphocyte functionassociated antigen 1 (LFA-1), mediating the interaction between keratinocytes and leukocytes (95). ICAM-1 plays an essential role in controlling abnormal inflammatory infiltration, adhesion, and migration (96). Apigenin inhibits NF-κB activation to improve NO production and SOD activity in endothelial cells and suppress ICAM-1 expression in human vascular endothelial cells (HUVECs) (97). Third, apigenin inactivates the PI3K (phosphoinositide-3-kinase) Akt (protein kinase B) axis in HUVECs during vascular endothelial dysfunction. The PI3K/Akt axis is an essential pathway in the pathogenesis of cardiovascular complications in T2DM (98). Apigenin treatment inhibited the phosphorylation of Akt-residues Ser473 and Thr308 to prevent vascular endothelial dysfunction (99). Finally, apigenin decreased ROS and improved NO levels to alleviate vascular endothelial dysfunction induced by mitochondriadependent apoptosis via inhibition of protein kinase C βII (PKCβII) phosphorylation. PKCβII promotes oxidative stress, ROS production and mitochondria-dependent apoptosis in vascular endothelial dysfunction (100,101). Apigenin treatment upregulated the expression of the anti-apoptotic gene, B-cell lymphoma-2 (Bcl-2), while the pro-apoptotic gene, Bcl-2 associated X (Bax), was downregulated, resulting in attenuation of mitochondria-dependent apoptosis in endothelial cells (102). Further, cardiac hypertrophy is another manifestation of cardiovascular diseases (103). Current evidence suggests that abnormal glycolipid metabolism and overexpression of HIF-1α in cardiac cells causes cardiac hypertrophy (104). In vivo and in vitro experimental data suggest that apigenin alleviates cardiac hypertrophy via suppression of HIF-1α, thereby reversing the expression of PPARα/γ and target genes including glycerol-3-phosphate acyltransferase (GPAT), glucose transporter 4 (GLUT-4), carnitine palmitoyltransferase 1 (CPT-1) and pyruvate dehydrogenase kinase 4 (PDK-4). Downregulation of GLUT4 and upregulation of PDK-4 can inhibit excessive glucose intake and oxidation, preventing abnormal glucose metabolism. Downregulation of GPAT and upregulation of CPT-1 decreases the rate of triglyceride synthesis and augments fatty acid oxidation, thereby improving lipid metabolism (105,106). Thus, the hypoxic myocardial energy utilization (107)(108)(109)(110) can be reversed. Ren et al. (97) In summary, apigenin can ameliorate cardiovascular diseases via reduction of oxidative stress and mitochondria-dependent apoptosis in vascular endothelial cells as well as regulation of glucose and lipid metabolism in cardiomyocytes. The experimental studies and mechanisms of action involving apigenin in hypertension are presented in Table 4. Experimental dosages and duration of apigenin treatment in vivo and in vitro are also listed in Table 4.

Apigenin Analogs and Their Effects on Alleviating Cardiometabolic Diseases
Apigenin analogs are derived from the basic flavonoid skeleton via hydroxyl group substitution, glycosylation, hydroxylation, and methylation (111,112). In plants, apigenin is stored as glycosides such as apigenin 7-O-apioglucoside in celery and parsley (113,114) and apigenin 8-C-glucoside isolated from bamboo leaves (115). Several apigenin analogs carry the basic flavonoid skeleton similar to apigenin and exhibit biological activity in cardiometabolic diseases.

POTENTIAL SIGNALING PATHWAYS MEDIATED BY APIGENIN FOR AMELIORATION OF CARDIOMETABOLIC DISEASE IN DIFFERENT CELL TYPES
In summary, the potential signaling pathways mediated by apigenin resulting in alleviation of cardiometabolic diseases in different cell types are illustrated in Figures 2A-E. Apigenin alleviates cardiometabolic diseases mainly by regulating glycolipid metabolism, oxidative stress, and oxidative stressinduced inflammation and apoptosis. Notably, apigenin plays contrasting roles in different types of cells. Apigenin acts as an agonist of PPARγ in adipose tissue macrophages. PPARγ binds to p65 to inhibit nuclear translocation to block NF-κB signaling pathway resulting in attenuation of inflammation (Figure 2A) (33). However, apigenin inhibits PPARγ expression in adipocytes, hepatocytes, and cardiomyocytes by acting as an antagonist (Figures 2B-D).

Apigenin in Adipocytes
In adipocytes, apigenin acts as a functional regulator of lipid metabolism to reduce fat accumulation. Apigenin downregulates PPARγ expression by inhibiting the STAT3/CD36 axis (16) and the activation of AMPK (17). Apigenin directly induces p65 phosphorylation to prevent its nuclear translocation to ensure continued inhibition of NF-κB signaling in the absence of PPARγ as a mediator (25,35).

Apigenin in Hepatocytes
Since the liver is an important site of energy metabolism, apigenin is a potential mediator of glycolipid metabolism in hepatocytes. Apigenin acts as a PPARγ antagonist via direct activation of Nrf2 and indirect activation of Nrf2 via the Keap1-Nrf2 pathway (126). Additionally, apigenin activates AMPK to inhibit SREBP-1 and SPEBP-2 to regulate hepatic fatty acid oxidation and cholesterol synthesis (28).

Apigenin in Cardiomyocytes
Apigenin treatment of cardiomyocytes regulates glucose and lipid metabolism to maintain normal cellular function. HIF-1α activation via apigenin regulates the PPAR family, leading to the appropriate regulation of downstream target genes related to glycolipid metabolism. Apigenin suppresses PPARγ expression via the activation of HIF-1α as an antagonist (105,106). Meanwhile, the upregulation of HIF-1α following apigenin treatment increases PPARα expression (105,106).

Apigenin in Endothelial Cells
Apigenin plays a protective role in vascular endothelial dysfunction by regulating several signaling pathways in endothelial cells to alleviate oxidative stress, inflammation, and mitochondria-dependent apoptosis. The inhibition of the NF-κB signaling pathway with apigenin treatment also ameliorates inflammatory response in endothelial cells and increases NO production (97). NF-κB inhibition also suppresses the expression of ICAM-1 to improve abnormal inflammatory adhesion, migration, and infiltration, resulting in the alleviation of vascular endothelial dysfunction (97). In addition to NF-κB pathways, apigenin also activates the PI3K-Akt pathway and inhibits PKCβII activation by reducing oxidative stress and oxidative stress-related apoptosis in mitochondria (102). The expression of anti-apoptotic gene, Bcl-2, and the pro-apoptotic gene, Bax, in these two pathways reduces abnormal apoptosis. The activation of the PI3K-Akt pathway also promotes eNOS activity to restore NO levels and thereby attenuates oxidative stress (102). Apigenin treatment also mitigates angiogenesis induced by inflammation. HIF-1α inhibition by apigenin directly reduces the expression of vascular endothelial growth factor (VEGF) in angiogenesis, thus alleviating the angiogenesis induced by vascular dysfunction (127).

CONCLUSION AND PERSPECTIVES
A review of studies investigating apigenin suggests critical biological mechanisms, including reducing oxidative stress and oxidative stress-induced inflammation and apoptosis, and improving glycolipid metabolism. Figure 3 summarizes the potential signaling pathways of apigenin underlying the protection against cardiometabolic diseases.
The molecular structure of apigenin suggests poor water solubility, chemical instability and moderate permeability, which prevent maximum bioavailability. Therefore, new delivery and design strategies have been formulated including the development of apigenin glycosides and acylated derivatives to enhance water solubility (128,129). Apigenin-AuNP complex can be developed at room temperature at pH 10 to enhance the stability of apigenin in the body (130). Nano-apigenin using poly (lactic-co-glycolide) (PLGA) can also improve the bioactivity of apigenin (131). Pharmacokinetic and pharmacodynamic profiles of apigenin in rats and mice have been studied. The peak plasma concentration C max and the time to reach the peak plasma concentration T max were 1.07 ng/mL and 1 h, respectively, and the area under the concentration-time curve (AUC 0−24 ) was 3.9 ng h/mL in mice (132). However, the bioavailability of apigenin in humans is still unknown. Further studies are needed to confirm the bioavailability and safety profile in humans.
In summary, the extensive review and validation of in vitro and in vivo evidence suggests that apigenin is a natural compound that can be used to protect against cardiometabolic diseases. Environment-wide association studies (EWAS) also indicate that apigenin is one of the protective factors in cardiovascular diseases at the population level (133). Further studies are required to establish the optimum dose of apigenin in alleviating cardiometabolic diseases in humans, developing a novel approach for clinical management of the disease.

AUTHOR CONTRIBUTIONS
XL designed this review, helped with writing and revising of the manuscript, and provided critical feedback. YX contributed to collecting and screening the literature, as well as summarizing the data, and then composed and revised the manuscript. HW reviewed the manuscript. All authors were involved in final approval of the submitted version.