Therapeutic Potential of Seaweed-Derived Bioactive Compounds for Cardiovascular Disease Treatment

: Cardiovascular diseases are closely related to hypertension, type 2 diabetes mellitus, obesity, and hyperlipidemia. Many studies have reported that an unhealthy diet and sedentary lifestyle are critical factors that enhance these diseases. Recently, many bioactive compounds isolated from marine seaweeds have been studied for their beneﬁts in improving human health. In particular, several unique bioactive metabolites such as polyphenols, polysaccharides, peptides, carotene, and sterol are the most effective components responsible for these activities. This review summarizes the current in vitro, in vivo, and clinical studies related to the protective effects of bioactive compounds isolated from seaweeds against cardiovascular disorders, including anti-diabetic, anti-hypertensive, anti-hyperlipidemia, and anti-obesity effects. Therefore, this present review summarizes these concepts and provides a basis for further in-depth research.


Introduction
Cardiovascular disease (CVD) is known as the primary cause of death globally, and it is estimated that approximately 17.7 million people die from CVD, accounting for 31% of global deaths. In recent decades, the increasing prevalence of CVD has deteriorated children's and adults' physical and mental health, affecting their quality of life [1]. The main risk factors linked to CVD are hypertension (HTN), elevated blood low-density cholesterol, type 2 diabetes mellitus (T2DM), endothelial dysfunction, overweightness or obesity, high triglyceride levels, and dietary patterns [2].
Previous evidence has shown that hyperinsulinemia and obesity increase sympathetic nerve traffic, promoting salt reabsorption in the renal tubules and activating the renin-angiotensin system (RAS) [3,4]. Additionally, endothelial dysfunction and vascular oxidative stress were observed in the development of obesity. These reactions would amplify the reactive oxygen species (ROS), decrease the availability of nitric oxide (NO), and further affect the vascular tone [5,6]. The progression of the pathophysiological mechanisms of HTN and obesity-related T2DM are closely connected. Thus, it is necessary to find effective treatments to reduce risks and help decrease the epidemic levels of cardiovascular-related deaths.
Currently, multiple biological mechanisms underlying CVDs have been confirmed, providing evidence for the direct development of pharmacological tools and treatments. The most common medical treatments include calcium channel blockers, diuretics, and inhibitors of angiotensin-converting enzyme (ACE). ACE, which is a metalloproteinase, a strong vasoconstrictor implicated in the pathophysiology of hypertension, is important in regulating blood pressure by catalyzing the conversion of angiotensin I to angiotensin II [7,8]. As a result, the inhibition of ACE activity has become a prominent target for HTN management. Conversely, in vitro, the biochemical pathways between vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) are vital and are involved in regulating the vascular tone [9]. Therefore, we reviewed numerous in vitro and in vivo studies to summarize the newest therapeutic strategies.
The growing interest in natural products to promote human health has resulted in seaweeds becoming popular due to their high bioactive compound contents, especially those exhibiting effects related to cardiovascular protection and metabolism regulation. Because of their high nutritional value, seaweeds have been traditionally consumed as a healthy food in many Asian countries since ancient times [10]. Brown seaweeds (Ochrophyta), red seaweeds (Rhodophyta), and green seaweeds (Chlorophyta) are the three primary classifications or species of seaweeds [11,12]. Different types of brown seaweeds, such as Undaria, Laminaria, Gim, and Hiziki have been used in traditional Asian meals for a long time. They contain low levels of lipids and high levels of polysaccharides, fibers, and polyunsaturated fatty acids (PUFAs), which are considered high-quality nutritional components for food preparation. The consumption of seaweed in western countries is relatively low compared with Asian countries because of their food habits. However, over the last few decades, interest in seaweeds has gradually increased because of their various properties as food ingredients and the identification of their invaluable health effects [13]. Due to its chemical diversity and unique components, seaweed has been of interest in many studies and is widely used in the medical, nutraceutical, and cosmetic industries. The primary seaweed metabolites include proteins, polysaccharides, and lipids, whereas secondary metabolites contain phenolic compounds, halogenated compounds, sterols, terpenes, and small peptides among other bioactive compounds produced in seaweed tissues [12,14]. Seaweed not only contains various primary and secondary metabolites but can also be used as a latent material for bioactive oligopeptides and oligosaccharides via bioconversion processes such as fermentation and enzyme hydrolysis [15]. These bioactive substances derived from seaweeds have numerous therapeutic roles in metabolic disease prevention, with functional properties such as anti-oxidant, anti-bacterial, anti-cancer, antidiabetic, anti-tumor, anti-inflammatory, and cardiovascular protection activities. Recent studies have reported evidence of their effects on human health, and mechanisms of biological activity have been reported [13]. Seaweeds are photosynthetic eukaryotes that possess simple reproductive structures with diverse forms and sizes. Seaweeds exhibit rapid adaptabilities to survive due to the complex ocean environment, such as changes in salinity, temperature variations, nutrient-deficient habitats, and UV irradiation, which are attributed to their bioactive secondary metabolites which cannot be found in other terrestrial organisms [16]. In addition to its bioactive properties, other important advantages are their easy cultivation, rapid growth, and availability to produce high value-added bioactive compounds by manipulating environmental conditions, as well as through gene modification and mutagenesis.
Based on these viewpoints, we reviewed the available data from animal studies and clinical trials regarding the pathophysiological mechanisms under bioactive compound treatment and the links between cardiovascular and metabolic disorders ( Figure 1). Despite numerous efforts to improve the industrial use of bioactive seaweed compounds to prevent CVD, there have been many limitations. The nutraceutical industry demands more rigorous quality, standardization of components, and a clearer bioactivity mechanism than previously provided for seaweeds. To overcome this problem, it is necessary to secure information on the characteristics and diversity of seaweed's bioactive substances and systematically understand their overall biological activities in the metabolic disorders closely related to CVD. We believe this review can provide useful insights and open up great potential for many research groups considering the huge interest regarding seaweed's bioactive compounds in the prevention of CVD.

Seaweed-Derived Phlorotannins with Therapeutic Potential against CVD
Phlorotannins are polyphenol molecules generated as secondary metabolites by seaweed and consist of polymerization of phloroglucinol (1,3,5-tryhydroxybenzene) monomer units, which are biosynthesized via the acetate-malonate pathway, also known as the polyketide pathway [17]. In particular, some phlorotannins can be found exclusively in brown seaweeds, such as Ecklonia species [18]. Phlorotannins can be classified into four subclasses according to the type of linkage between the phloroglucinol units. Fuhalols and phlorethols, as well as phloroglucinol units, are linked by aryl ether bonds. Fucols are composed of phloroglucinol units linked with aryl-aryl bonds. Fucophloroethols are linked to ether and phenyl bonds. Eckols and carmalols are characterized by the presence of dibenzodioxin units and differ from carmalol in that they generally have a lower molecular weight and a phenoxyl moiety at C4 [19,20]. Many researchers have revealed various beneficial biological activities of phlorotannins (mainly brown seaweed such as Ecklonia and Ishige species), including anti-oxidant, anti-cancer, anti-bacterial, anti-allergic, anti-mutagenic, anti-diabetic, anti-inflammatiory, anti-proliferatiive, anti-hypertensive, and anti-obesity effects [18,[21][22][23][24]. Therefore, phlorotannins have been recognized for many years as promising bioactive compounds with health benefits for preventing and treating various human diseases.
In addition to ACE inhibition, vasodilators also contribute greatly to the anti-hypertensive properties. NO is a well-known vessel-relaxing factor produced from L-arginine by endothelial nitric oxide synthase (eNOS) in the presence of oxygen and the cofactors Ca 2+ and calmodulin (CaM) [30]. A previous study has indicated that genetically deficient eNOS mice are hypertensive with lower circulating NO levels, thus indicating the critical role of eNOS and NO in CVD [5,9]. It has also been reported that multiple mechanisms control NO production via eNOS activation [31].
The phosphatidylinositol 3-kinase (PI3K) pathway members, including their downstream molecule, protein kinase B (Akt), are essential regulators. Activated Akt would directly phosphorylate ser1177 on eNOS, enhancing the [Ca 2+ ]/CaM complex [32]. Second, the concentration of intracellular Ca 2+ ([Ca 2+ ] i ) in ECs and VSMCs is closely related to the vascular tone and influences blood pressure [33]. Third, the L-type calcium channel is one of the critical ion channels that regulates vasoconstriction and vasodilation. Nitrodilators promote vasodilation by increasing soluble guanylyl cyclase (cGMP) and decreasing the [Ca 2+ ] i levels in VSMCs. These actions would reduce the phosphorylation of the Ca 2+sensitive myosin light chains, resulting in vasodilation [34]. One study demonstrated that Lu et al. used the EA.hy926 cells and zebrafish model to systematically establish the potential mechanisms of the vasodilation produced by diekol and diphlorethohydroxycarmalol isolated from Ecklonia cava (E. cava) and Ishige okamurae (I. okamurae) [35,36] ( Table 1). In addition, four major phlorotannins-dieckol, 2,7-phloroglucinol-6,6-bieckol, phlorofucofuroeckol A, and pyrogallol-phloroglucinol-6,6-bieckol-isolated from Ecklonia cava effectively inhibited monocyte-associated vascular inflammation and dysfunction by suppressing monocyte migration and protecting monocyte-associated endothelial cell death [37]. Moreover, pyrogallol-phloroglucinol-6,6 -bieckol and dieckol isolated from E. Cava have been shown to improve blood circulation in mice fed diets to induce obesity and hypertension [36,38]. Therefore, while numerous factors contribute to hypertension, it is well recognized that an increased vascular tone is always the ultimate goal.
The evidence mentioned above has great pharmaceutical potential, with some already being used in the clinical trial phase. Clinical studies have shown cardiovascular protection, supporting the benefits of polyphenols extracted from land plants [39]. Nevertheless, only a few epidemiological studies have discussed the association between the consumption of specific components of seaweed and blood pressure. Despite investigating the benefits of single compounds from seaweed, the general components, including minerals, fiber, and peptides, were more popular in clinical trials. This might be because high doses of these compounds have comparatively low side effects. A case control study demonstrated the anti-hypertensive effect of Undaria pinnatifida powder (5 g/capsule/day) by significantly reducing blood pressure in elderly Japanese patients following 8 weeks of administration [40]. However, Murray et al. reported that no reduction in blood pressure occurred in healthy adults who were administered the Fucus vesiculosus extract [41]. As mentioned above, several phlorotannins have been confirmed to possess effects against hypertension by ACE inhibition, calcium regulation, antagonism of L-type calcium channels, or activation of the critical pathway (PI3K/Akt/eNOS). Despite these mechanisms being verified, many other physiological signaling pathways in vascular regulation remain unclear. It will be helpful to explore the cardiovascular protection or anti-hypertension properties of phlorotannin via its use as a potassium channel opener and angiotensin receptor blocker.
Diabetes mellitus (DM) is a prime risk factor for dramatically increasing CVD, contributing to more than 3 million cardiovascular deaths worldwide each year [42]. Type 2 diabetes mellitus (T2DM) is a complex metabolic disorder involving insulin resistance, impaired insulin signaling and β-cell dysfunction, and abnormal glucose and lipid metabolism. Hyperglycemia is the most important criterion for all types of diabetes and is the cause of diabetic complications such as CVD. Therefore, it is important to prevent or delay the onset of diabetes by controlling blood glucose levels in diabetic patients, as hyperglycemia increases the risk of developing CVD long before clinical diabetes begins [43]. In humans, α-glucosidase and α-amylase in the small intestine play an important role in the digesting dietary carbohydrates into glucose. Therefore, reducing postprandial hyperglycemia by delaying the absorption of glucose in the body through the inhibition of α-amylase and α-glucosidase is an important approach for treating T2DM [44,45]. The enzyme inhibitors used for this action suppress the digestion of carbohydrates and consequently slow the postprandial plasma glucose rise, thereby delaying the rate of glucose absorption [46]. The inhibition of starch-digesting enzymes using synthetic drugs exhibiting antidiabetic effects with α-glucosidase inhibitory properties, such as acarbose, voglibose, miglitol, and emiglitate, is an important clinical strategy for controlling postprandial hyperglycemia. However, these synthetic drugs have been reported to effectively lower postprandial blood glucose levels but cause serious side effects, such as liver disorders [45,47]. The use of enzyme inhibitors derived from natural products (terrestrial plants or seaweeds) with a lower risk of the potential side effects caused by synthetic enzyme inhibitors is recommended. Therefore, to avoid or reduce the side effects caused by currently used synthetase inhibitors, the use of enzyme inhibitors (α-amylase and α-glucosidase) from natural products is considered the best alternative. It has been reported that minor phlorotannin derivatives such as eckol, 2-phloroeckol, 8,8 -bieckol, 6,8 -bieckol, and 2-O-(2,4,6-trihydroxyphenyl)-6,6 -bieckol isolated from E. cava showed α-glucosidase inhibitory activity, with an IC 50 value ranging from 2.3 to 59.8 µM (Table 1). Among the minor phlorotannin derivatives, 8,8 -bieckol and 6,8 -bieckol exhibited the strongest α-glucosidase inhibitory activity. In addition, molecular docking studies revealed that both phlorotannins act as α-glucosidase inhibitors by competitive inhibition [48]. In addition, several researchers have found that phlorotannins such as fucodiphloroethol G, dieckol, 6,6 -bieckol, 7-phloroeckol, phlorofucofuroeckol A, and 2,7 -phloroglucinol-6,6 -bieckol isolated from Ecklonia cava have inhibitory activity against α-glucosidase and α-amylase [49][50][51]. Moon et al. revealed that phlorotannins, such as phloroglucinol, dioxinodehydroeckol, eckol, phlorofucofuroeckol-A, dieckol, and 7-phloroeckol isolated from Ecklonia stolinifera (E. stolinifera) have anti-diabetic effects by inhibiting α-glucosidase. In addition, it was confirmed that these phlorotannins effectively inhibit protein tyrosine phosphatase 1 B, an enzyme that plays an important role in the development of insulin resistance, thereby preventing a rapid increase in postprandial blood glucose levels [52]. In addition, diphlorethohydroxycarmalol (DPHC), ishophloroglucin A (IPA), and octaphlorethol A (OPA) were isolated from I. okamurae and Ishige foliacea (I. foliacea) from the Ishigeaceae family and showed α-glucosidase and α-amylase inhibitory activity [53,54]. In particular, oral administration of 100 mg/kg DPHC significantly suppressed the postprandial blood glucose levels in streptozotocin-induced diabetic mice [55]. According to Lee et al., molecular docking analysis revealed that OPA interacts with amino acid residues in the region close to the active site of α-glucosidase. Hence, OPA has the potential to be used as a non-competitive inhibitor with a high-affinity binding site for α-glucosidase [54].
Insulin resistance and impaired glucose metabolism are the most common factors promoting the development of type 2 diabetes mellitus. Type 2 diabetes mellitus is typically caused by two factors: impaired insulin production by pancreatic β cells and a failure of insulin-sensitive tissues to respond appropriately to insulin [56]. Signaling pathways involved in insulin secretion in β-cells under physiological conditions can be divided into a few stages. First, insulin release is primarily triggered by high glucose concentrations, mainly via glucose transporter 2 (GLUT2). When glucose catabolism is stimulated, intracellular ATP levels increase and close the cell membrane potassium channels, increasing intracellular Ca 2+ concentrations and amplifying insulin secretion [57,58]. Furthermore, the AMP-activated protein kinase (AMPK) system simulates the effect of insulin on glucose transport in the muscle and glucose production in the liver. Because the AMPK system plays an important role in glucose homeostasis, it is a key target for discovering anti-diabetic agents [59]. Some evidence has demonstrated that 2,7"-phloroglucinol-6,6 -bieckol isolated from E. cava protects against high glucose-induced glucotoxicity and apoptotic cell death in INS-1 cells [60] (Table 1). In addition, in C57BL/KsJ-db/db mice, the oral administration of 20 mg/kg dieckol isolated from E. cava significantly reduced blood glucose levels, serum insulin levels, and body weight [61]. Yang et al. demonstrated that oral administration of ishophloroglucin A (IPA) and oxtaphlorethol A (OPA) isolated from I. okamurae and I. foliacea significantly ameliorated glucose intolerance and the fasting glucose levels in high-fat diet (HFD)-fed mice, thereby reducing fasting and 2 h blood glucose levels, as well as stimulated GLUT4 in HFD mouse muscle [62,63]. Furthermore, phlorotannins such as DPHC and IPA isolated from I. okamurae treatment exhibited an anti-angiogenic effect by interfering with the VEGFR-2 signaling pathway [64,65]. These phlorotannin compounds isolated from various seaweeds exhibit anti-diabetic efficacy through various mechanisms in vivo and in vitro, demonstrating the potential as natural agents that can replace synthetic anti-diabetic treatments. Despite the evidence suggesting that type 2 diabetes mellitus might be caused by more complex molecular pathways implicated in cell biology, the main concepts of treatments still mainly focus on the pathway we mentioned above, and clinical trials using single compounds isolated from seaweeds have not been reported.

Seaweed-Derived Polysaccharides with Therapeutic Potential against CVD
In terms of economic value, seaweed polysaccharides are the most important products derived from seaweeds. Several studies have demonstrated the pharmacological activities of polysaccharides obtained from seaweeds. They are the multi-component mixtures made up of one or more monosaccharides linked by one, three, or four glycosidic linkages distributed between and inside seaweed cells [89]. In recent years, the most famous polysaccharides such as ulvan (sulfated glucuronoxylorhamnans), fucoidan (L-fucose and sulfate ester groups), and carrageenan (sulfated galactans) extracted from green, brown, and red seaweeds, respectively, have been extensively investigated for their health-promoting effects [90][91][92][93].
Among the risk factors for cardiovascular disease, seaweed-derived polysaccharides have been the focus of anti-diabetic research, and studies using fucoidan are the most widely reported. Fucoidan derived from Sargassum wightii, Sargassum thumbergii, Sragassum honeri, Sargassum ringgoldianum, Sargassum, siliquastrum, Sargassum graminifolium, Sargassum kjellmanianum, Fucus vesiculosus, Ascophyllum nodosum, Saccharina longicruris, Cystoseira crinite, Ecklonia maxima, and Turbinaria conoides exhibit anti-diabetic activity through the inhibition of α-amylase and α-glucosidase alone or both enzymes [94][95][96][97][98][99][100][101][102] (Table 2). Interestingly, Kim et al. revealed that the α-amylase and α-glucosidase inhibition of fucoidan differed depending on the species from which the fucoidan was extracted and by the month and year of collection [98]. In addition, the oral administration of Sargassum fusiforme-derived fucoidan decreased the fasting blood glucose levels in high-fat diet/streptozotocin-induced diabetic mice model and reduced diabetes-related intestinal bacteria, which may potentially aid diabetes [103][104][105]. Many studies have demonstrated cardioprotection by polysaccharides. Sulfated polysaccharides from Padina tetrastromatica exerted cardioprotective effects by activating the PI3K/Akt/Nrf2 signaling pathway in in vitro and Sprague Dawley rat models [106]. Fucoidan is a well-known L-fucose-enriched sulfate. Low molecular weight fucoidan (MW = 7000 Da) produced from Laminaria japonica (L. japonica) could induce endothelium-dependent vasodilation via the eNOS expression upregulation in cerebral microvascular endothelial cells and effectively improve blood pressure and local blood flow in rats [107]. Cystoseira crinite-derived fucoidan exhibits ACE inhibitory activity in vitro and in vivo [108]. In addition, Li et al. suggested that fucoidan (100 mg/kg/day) from Undaria pinnatifida prevents vascular dysfunction through PI3K/Akt/eNOS-dependent mechanisms in a hypertensive rat model [109]. Furthermore, continuous feeding with kappa-carrageenan produced from Kappaphycus alvarezii or iota-carrageenan from Sarconema filiform to high-fat diet-fed rats attenuated the parameters defining cardiovascular and metabolic health such as body weight, abdominal and liver fat, systolic blood pressure, plasma total cholesterol concentrations, and the plasma activities of alanine transaminase and aspartate transaminase [110]. In green seaweeds, ulvan is one of the major polysaccharide structures isolated from Ulva pertusa which had anti-hyperlipidemic effects via the modulation of lipid levels and the mRNA expression of FXR, LXR, and PPARγ in an in vivo rat model [111]. However, the anti-hypertensive effect of ulvan is not clear. Thus, further detailed investigations are required.
These functional compounds from seaweed protect the muscle and platelets and stimulate blood vessel dilatation [112]. In human trials, one previous report demonstrated that daily fucoidan consumption from L. japonica could remarkably suppress thrombus formation and further protect cardiovascular health [113]. However, there have been limited studies on the anti-hypertensive effects in human subjects. Most of the related research is closely associated with cardioprotective effects, such as anticoagulant polysaccharides in in vitro and in vivo models [114][115][116]. Therefore, due to the bioactivity of polysaccharides, more detailed human trials are important for verifying and developing alternative therapies soon. -Administration of fucoidan decreased fasting blood glucose levels, dietary and water intake, and alleviated pathological changes in the heart and liver -Fucoidan supplements altered the composition of gut microbiota in STZ-induced diabetic mice [103] Diabetes mellitus/inhibition of hyperglycemia, altered the composition of gut microbiota Fucoidan in vivo -Administration of fucoidan decreased fasting blood glucose, food consumption, water intake, and serum lipid levels in high-fat diet/STZ-induced diabetic mice -Administration of fucoidan altered the composition of gut microbiota and increased the levels of carnitine and choline in the colon [105] Diabetes mellitus/improved insulin sensitivity, altered the composition of gut microbiota Fucoidan in vivo -Administration of fucoidan reduced fasting blood glucose and insulin resistance indexes and improved glucose tolerance -Administration of fucoidan increased the abundance and diversity of gut microbiota in obese mice and improved intestinal integrity [104] Sargassum hemiphyllum Diabetes mellitus/prevention of pancreatic β cell damage and dysfunction Fucoidan in vitro and in vivo -Fucoidan treatment attenuated pancreatic β cell death, pancreatic islet mass loss, and dysfunction -Fucoidan treatment increased insulin synthesis via activation of Sirt-1-dependent upregulation of PDX and GLP-1R [117] Undaria pinnatifida Diabetes mellitus/regulation of blood glucose homeostasis Fucoidan in vivo -Serum insulin (µIU/mL): db/db mice (41.6) → db/db mice + fucoidan (37.7) -Fasting blood glucose (mg/dL): db/db mice (445) → db/db mice + fucoidan (257) [118] Diabetes mellitus/improved insulin-stimulated glucose uptake Fucoidan in vitro -Fucoidan treatment stimulated glucose uptake and inhibited basal lipolysis in hypertrophied insulin resistance [119] Hypertension/vascular dysfunction prevention Fucoidan in vivo -Fucoidan treatment induce NO release and eNOS activation -In L -NAME-induced hypertensive rats, administration of fucoidan attenuated elevated blood pressure, increased endothelium-dependent vasodilation, and improved vascular elasticity [109] Undaria pinnatifida Obesity/inhibition of adipocyte differentiation Fucoidan in vitro -Fucoidan treatment suppressed adipogenesis by inhibiting proliferator-activated receptor γ, CCAAR/enhancer-binding protein α, adipocyte protein 2, and lipid accumulation in 3T3-L1 cells [120] Saccharina longicruris Diabetes mellitus/α-amylase inhibition Fucoidan in vitro -α-amylase activity (%):fucoidan (1 mg/mL) inhibited α-amylase activity by 80.3% [97] Diabetes mellitus/α-amylase inhibition

Padina tetrastromatica
Hypertension/cardioprotective effect Sulfated polysaccharides in vitro and in vivo -Treatment of sulfated polysaccharides isolated from Padina tetrastromatica reduced isoproterenol-induced cardiac damage via activation of PI3K/Akt/Nrf2 signaling pathway [106] Atherosclerosis/reduced hyperlipidemia, endothelial dysfunction Sulfated polysaccharides in vitro and in vivo -Sulfated polysaccharides treatment maintained lipid homeostasis by regulating the expressions of SREBP-2 and LDL-R -Administration of sulfated polysaccharides normalized ISO-induced oxidative damage, hyperlipidemia, endothelial dysfunction, and inflammation in a rat model. [122] Sarconema filiforme Modulation of cardiovascular and metabolic health parameters Carrageenan in vivo -Administration of 5% carrageenan in high-fat diet-fed rats attenuated cardiovascular diseases and metabolic health parameters [110] Ulva Pertusa Hyperlipidemia/modulating hyperlipidemia related parameters Ulvan in vivo -Ulvan decreased total cholesterol and low-density lipoprotein levels in high cholesterol-fed rats -Ulvan treatment improved lipid profiles via regulating FXR, PPARγ, and LXR expression levels [111] ACE, angiotensin I-converting enzyme; STZ, streptozotocin.

Seaweed-Derived Peptides with Therapeutic Potential against CVD
As the importance of marine organisms as sources of novel bioactive substances is growing, marine bioactive peptides have received much attention recently. Bioactive peptides are usually 2−20 amino acid residues [123]. Depending on the amino acid sequence, they may be involved in various biological functions such as antioxidant, anti-cancer, opioid agonists or antagonists, immunomodulatory, anti-thrombotic, anti-atherosclerotic, and antimicrobial activities, in addition to nutrient utilization [123]. A pepsin-hydrolyzed peptide (VECYGPNRPQF) from Chlorella vulgaris protein waste possessed potent antioxidant activity against various free radicals and exhibited gastrointestinal enzyme resistance. Still, no cytotoxicity was observed in human lung fibroblast cell lines (WI-38) [124]. The antitumor polypeptide Y2 was obtained from the trypsin digest of Spirulina platensis proteins [125]. Cian et al. found that enzymatic hydrolysates from a phycobili protein byproduct of Porphyra columbina exhibited immunosuppressive effects on rat splenocytes by enhancing IL-10 production and inhibiting the production of TNF-α and IFN-γ [126].

Seaweed-Derived Carotenoids and Other Components with Therapeutic Potential against CVD
All carotenoids, including fucoxanthin, carotene, lycopene, and siphonaxanthin, are bioactive substances from seaweeds. Green seaweed extracts rich in carotenoids exhibit significant antigenotoxic activities [147]. Fucoxanthin is a recognized secondary metabolite found in macroalgae, and its biological properties are well established [23]. Fucoxanthin from brown seaweed exerts prebiotic and anti-inflammatory activities in human intestinal epithelial cells [148]. Fucoxanthin upregulates the expression of UCP1 in white adipose tissue (WAT) in KK-Ay mice [149]. Fucoxanthin and siphonaxanthin inhibit angiogenesis by downregulating the FGF-2-mediated intracellular signals in vascular endothelial cells [150]. Siphonaxanthin has been reported to have anti-angiogenic and anti-inflammatory effects [151]. Siphonaxanthin induces apoptosis by decreasing Bcl-2 expression and activating caspase-3 in human leukemia (HL-60) cells [152].
In Ae's study, fucoxanthin extracted from Undaria pinnatifida (U. pinnatifida) increased the serum HDL. It decreased the triglyceride levels in high-fat diet-fed rats at a 0.2% diet dose for 4 weeks [153] (Table 4). In addition, the α-amylase and α-glucosidase inhibitory activities of fucoxanthin were observed in Kawee-Ai's study [154]. The administration of a 3-g/kg 9-cis β-carotene diet disrupted the increases in plasma cholesterol and LDL in LDL-receptor knockout mice [155]. Feeding 8% 9-Cis β-carotene feed for 23 days disrupted triglyceride elevation in 5-week-old female db/db mice [156]. Other carotenoids have also been shown to have anti-atherogenic effects. Zhuo et al. showed that siphonaxanthin extracted from Codium fragile disrupts the elevation of the serum total cholesterol, triglyceride, and HDL in KK-Ay mice [157]. In addition, (all-E)-lutein and (9-Z)-zeaxanthin displayed anti-diabetes mellitus potency via α-glucosidase inhibition [158].
Seaweeds have a variety of physiologically active ingredients in addition to phlorotannins, polysaccharides, peptides, and carotenoids. It has been reported that seaweeds contain various sterol compounds [159]. Sterols are an important family of lipids present in most eukaryotic cells. They are categorized into the steroid group, which contains the same fused four-ring core structure and has different biological roles than hormones and signaling molecules. The search for natural bioactive sterols as safe alternatives from marine seaweed is important in the food industry. Fucosterol isolated from the marine seaweed Pelvetia siliquosa causes a significant elevation of free radical scavenging enzyme activities, such as superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSHpx) [160]. Moreover, 3 g,28 ξ-dihydroxy-24-ethylcholesta-5,23 Z-dien and 24 ξ-hydroperoxy-24-vinylcholesterol isolated from the brown seaweed Sargassum carpophyllum showed cytotoxic activity against human promyelocytic leukemia cells [161].
These physiologically active dietary components derived from seaweeds have been actively studied at the in vitro and in vivo levels to inhibit or alleviate the symptoms of diabetes, obesity, hypertension, and hyperglycemia and hyperlipidemia, which are closely related to the pathogenesis of CVD. However, research on the clinical trial stage using the seaweeds and bioactive dietary substances is insufficient. Bioactive dietary components derived from food are known to affect epigenetic gene expression in humans through various mechanisms, such as alteration of the chromatin structure or non-coding RNA, activation of transcription factors by signaling cascades, and direct ligand binding to nuclear receptors. Studies on the influence of polyphenols, PUFAs, and vitamins, which are bioactive dietary components contained in terrestrial plant resources, on epigenetic gene expression in humans through epigenetic change in the genome, miRNA level change, IncRNA level change, and transcription factor activity change are continuously being conducted [165]. By clearly analyzing the mechanisms of interaction between these dietary components and human genome structure and gene activity, a better understanding of these relationships will help design dietary guidelines that can help maintain good health by providing more effective dietary recommendations for individuals. Therefore, it is considered necessary to study not only the various bioactive dietary substances contained in seaweeds but also their influence on epigenetic changes in parallel. Diabetes mellitus/α-glucosidase inhibition 18α-glycyrrhetinic acid,18βglycyrrhetinic acid, FAs C20:4 (∆ 7,9,11,13  TG, triglyceride; LXR, liver X receptor.

Conclusions
The active compounds extracted from seaweed have been widely used for a long time, owing to their many benefits for improving human health. Increasing evidence has indicated that increasing the consumption of seaweeds helps decrease the risks of CVD and other related diseases by modulating different biological signaling pathways in vivo and in vitro. Further detailed human research is necessary to verify the clinical relevance of bioactive compounds isolated from seaweeds. In summary, this review lists the bioactive compounds isolated from different seaweeds to ameliorate CVD. This review will help summarize these concepts and promote further in-depth research.