A number of marine-derived bioactive compounds have been found to be involved in a wide range of biological processes (
Figure 5), including activation of signal transduction pathways, antioxidant defense, protein expression, and maintenance of mitochondrial integrity [
20,
21,
22]. TetraSOD, derived from
Tetraselmis chuii, promotes endogenous antioxidant defense mechanisms in the liver and modulates plasma markers of oxidative stress and inflammation, thereby ameliorating these effects associated with metabolic syndrome (MetS) [
23]. Collagen from the skin of
Sphyrna mokarran downregulates the expression of fatty acid synthase (FAS) and 3-hydroxy-3-methylglutaryl monoacyl-coenzyme A reductase (HMGCR), and upregulates the expression of lecithin-cholesterol acyltransferase (LCAT) in the liver to alleviate cholesterol accumulation [
24].
3.1. Marine Polysaccharides
Polysaccharides are divided into land-and sea-based types, which are mainly derived from marine organisms that live in a saline buffer system with a specific water pressure, high salinity, low temperature, insufficient light source, and low dissolved oxygen. These unique environmental factors lead to differences in synthesis pathways between land- and sea-derived substances, and marine polysaccharides have a novel structure and special biochemical mechanism because of the beneficial environmental factors [
25]. According to their sources, marine polysaccharides can be categorized as algal (classified as brown, red, and green algae according to pigment deposition), marine animal, and marine microbial polysaccharides. Algal polysaccharides are the main components of macroalgae and phytoplankton organisms [
26].
Furthermore, polysaccharides originating from algae have a complex structure, with highly polymerized branched chains and numerous reactive groups, which makes them show great potential bioactivity [
27]. Most marine animal polysaccharides are found in connective tissues such as the body wall of marine invertebrates such as echinoderms (sea urchins and sea cucumbers), including sulfated polysaccharides and fucoidan in sea cucumbers [
28]. The viscera of some marine animals are also a source of bioactive polysaccharides, such as sulfated polysaccharides extracted from abalone viscera with anticoagulant and hypolipidemic activities, and polysaccharides extracted from squid viscera with immunological activities [
29,
30,
31].
Although animals and plants in the ocean can also produce polysaccharides, marine microorganisms are more popular sources because of their rapid reproductive rate and production of abundant and easily isolated polysaccharides [
32]. Microorganisms are widely distributed on the seafloor and are highly adaptable to the environment. Furthermore, intracellular polysaccharides are mainly derived from the cell walls of marine fungi, bacteria, and actinomycetes. The secondary metabolites produced by microorganisms are referred to as extracellular polysaccharides, and include those produced by
Aspergillus versicolor N2bc from the deep-sea fungus, which have antioxidant activity [
33].
The development of purification and identification techniques has contributed to the gradual elucidation of the composition of polysaccharides, and their bioactivity has been shown to be closely related to their monosaccharide composition, the type of glycosidic bond, the number of hydroxyl groups, and the conformation of the polysaccharide chain [
34,
35]. Furthermore, natural polysaccharides with numerous glycoalkaloid acids are usually considered to have superior bioactivity [
34,
35]. Sulfated polysaccharides are able to interact with some biomolecules because of their sulfate functional groups and positive charges. Subsequent research led to the extraction of four types of sulfated polysaccharides from sea cucumbers (
Pearsonothuria graeffei and
Isostichopus badionotus) and investigated the relationship between structure and function [
36]. This study finally found that all four sulfated polysaccharides exhibited hypolipidemic effects in obese rats, and those with a stretchy linear conformation displayed a more pronounced activity [
36].
A marine-derived chitosan-oligosaccharide intervention significantly reduced plasma TC and increased HDL-C levels in hypercholesterolemic hamsters, and increased the relative abundance of
Bacteroidetes in the intestine [
37]. Wan et al. [
38] found that polysaccharides from
Chlorella pyrenoidosa (CPP) improved plasma and liver lipid metabolism and accelerated cecum total bile acid, short-chain fatty acid, and lipid metabolism. Furthermore, CPP also upregulated adenosine-monophosphate-activated protein kinase α (AMPKα) and downregulated the expression of acetyl-coenzyme A carboxylase, sterol regulatory element-binding protein 1c, and β-hydroxy-β-methylglutaryl coenzyme A [
38].
In addition,
Sargassum pallidum polysaccharides also improve hepatic lipid levels in the serum of mice induced by a high-fat diet (HFD) and significantly reduced fat accumulation in the liver and downregulated the expression levels of genes related to fat synthesis (FAS, SREBP-1c, and ACC) [
39]. Sea cucumber has shown outstanding activity in study of active substances [
35]. Liu et al. [
35] used
Apostichoru japonicus as raw material to prepare sea cucumber polysaccharide (AJP) using protease hydrolysis. AJP is mainly composed of aminoglucose, galactosamine, glucuronic acid, mannose, glucose, galactose, and fucose, with an average molecular weight of 36.2 kDa.
Studies revealed that treatment of hyperlipidemic Wistar rats with AJP significantly reduced their serum TC, TG, and LDL-C levels, whereas the HDL-C level was significantly increased. Liu et al. [
40] used a high voltage pulsed electric field to extract crude polysaccharides from
Haliotis discus hannai viscera and infrared spectroscopy analysis revealed the presence of sugar and sulfate groups. Consequently, abalone visceral polysaccharides were shown to reduce TC, TG, and LDL-C levels, while increasing HDL-C levels in the plasma of mice exposed to a HFD [
40]. Furthermore, the malondialdehyde (MDA) content decreased and superoxide dismutase (SOD) activity increased significantly [
40].
The lipid-lowering activity of marine-derived polysaccharides is highlighted by sulfated polysaccharides, which are mostly derived from seaweeds and other marine plants. However. few studies have investigated the lipid-lowering effects of marine microbial polysaccharides. In addition, the hypolipidemic activity of polysaccharides is related to their structures. Therefore, structural characterization of polysaccharides using chromatographic and spectroscopic techniques is useful for screening of active compounds and studying of their hypolipidemic molecular mechanisms.
3.2. Marine-Derived Unsaturated Fatty Acids
A typical feature of marine foods is that they are rich in docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Furthermore, dietary fatty acids from deep-sea fish such as salmon, cod, sardines, and Antarctic krill organisms have attracted much attention for their human health benefits [
41,
42,
43,
44,
45]. Deep-sea fish oils are rich in unsaturated fatty acids, including the Omega-3 family of fatty acids, represented by EPA and DHA, which have received much attention because of their nutritional health functions [
46]. Omega-3 polyunsaturated fatty acids in deep-sea fish oil improve hepatic lipid metabolism by regulating bile acid metabolism [
47]. Moreover, DHA is a natural endogenous ligand for peroxisome proliferator-activated receptors (PPARs), which it activates to enhance fatty acid β-oxidation in the mitochondria [
48,
49,
50]. This effect increases fatty acid catabolism, which in turn reduces plasma TG levels and, thus, consumption of EPA and DHA from deep-sea fish oils reduces the CVD-associated mortality rate [
48,
49,
50]. Current clinical guidelines recommend a combination of DHA and EPA for the treatment of severe hypertriglyceridemia [
51,
52,
53].
DHA activates AMPK, thereby inhibiting endoplasmic reticulum (ER) stress in the mitochondria [
20]. DHA supplementation in the grass carp diet was also significantly reduced hepatic TG, MDA, serum tumor necrosis factor-α (TNFα), and nuclear transcription factor κB (NFκB) levels [
20]. This effect inhibited palmitic acid (PA)-induced ER stress and lipid accumulation in vitro and
Ctenopharyngodon idella hepatocyte inflammation [
20]. Sabarinathan et al. [
54] evaluated the protective effects of DHA- and EPA-rich fish oil against atherosclerosis using a high cholesterol diet-induced zebrafish model. There results showed that the fish oil-fed zebrafish group accumulated 40% less cholesterol than the regular diet group did [
54].
In addition, compared to the normal diet fed group, the fatty acid synthesis (FAS) gene expression level in the livers of the fish oil fed group was significantly lower (p < 0.05) [
54]. Oral DHA may have unstable bioavailability and, therefore, Zhang et al. [
50] prepared a DHA liposome formulation as an injectable nanomedicine to avoid DHA degradation. This study found that DHA liposomes were easily phagocytosed by activated macrophages, exerted effective anti-inflammatory and antioxidant effects, and inhibited the formation of foam cells, further slowing down atherosclerosis development [
50]. Tian et al. [
21] isolated EPA-rich phosphatidylcholine (EPA-PC) and EPA-phosphatidylethanolamine (PE) from sea cucumber, and found that they both activated the transcription of PPARα/PPARγ. In addition, both compounds upregulated the expression of the target gene of lipid metabolism of PPARγ by dual-luciferase reporter in 3T3-L1 cells and inhibit the phosphorylation of PPARγ at Ser273. These effects, in turn, improved insulin resistance and abnormal lipid accumulation induced by a high-fat high-sucrose diet (HFSD) [
21].Krill oil also contains DHA and EPA, which when bound to phospholipids may have a higher bioavailability and absorption than that of omega-3 polyunsaturated fatty acids in fish oil and, thus, krill oil may have greater potential for the treatment of metabolic syndrome than fish oil does [
55,
56].
Krill oil supplementation decreased total TC, TG, and LDL-C levels in the liver and serum of hypercholesterolemic rats, as well as HMGCR activity [
57]. Furthermore, krill oil increased TC and bile acid levels in the feces of experimental rats, and promoted bile acid metabolism and cholesterol efflux [
57]. Liang et al. [
58] found that a combination intervention using krill oil and
Bifidobacterium animalis subsp.
Lactobacillus F1-7 significantly reduced the atherosclerotic plaque area, anti-inflammatory factor levels and modulated the cholesterol 7-alpha hydroxylase (CYP7A1) pathway to reduce lipid accumulation in mice.
The global abundance of marine biological resources provides an important guaranteed source of material for the development and application of unsaturated fatty acids. Currently, polyunsaturated fatty acids from deep-sea fish and shrimp have been developed into health products and medicines. Furthermore, although the fishy odor limits its scope of application, recent advances in technology such as microencapsulation, fish oil water-in-emulsions, and other delivery systems have emerged as potential solutions. These technologies not only remove the fishy odor, but they also improve the stability of the fish oil and bioavailability of active constituents. In addition, the advancement of such technologies to develop formulations for the application of fish oil in human health has created more possibilities and widened the scope of application of fish oil in human health management.
3.3. Marine Bioactive Peptides
Marine bioactive peptides have anti-inflammatory, antioxidant, anti-thrombotic, and other activities and have become a major research hotspot owing to their multiple bioactive functions, safety, and no toxicities. Marine bioactive peptide is a general term for different short peptides with complex linear and cyclic structures consisting of 2–16 amino acid residues in different combinations and arrangements obtained from marine organisms such as fish, sponges, sea squirts, seaweeds and mollusks. Conventional sources of marine bioactive peptides are generally the muscle or viscera, skin, shell, bone, and other by-products of marine organisms.
These substances are extracted using enzymes/hydrolysis (such as pepsin, trypsin, and alkaline protease), fermentation, acid-base extraction, and heat extraction followed by ultrafiltration and purification. The extracts are then analyzed to identify the components using chromatography in combination with the appropriate detector, to screen out the peptides with a high level of improved bioactivity. The activity of bioactive peptide is related to the protease used for extraction, the degree of hydrolysis, and the molecular weight of the product. Different proteases can cleave different sites, and the amino acid composition of the product will lead to different biological functions.
Furthermore, different degrees of hydrolysis generate peptides of different lengths, and a hydrolyzed product is more easily absorbed than a single amino acid or unhydrolyzed protein is. Recently, some researchers are of the opinion that shorter peptides with smaller molecular weight are more easily absorbed and more active [
59,
60]. Previously, peptides with hypolipidemic activity were obtained from marine organisms such as
Rhopilema esculentum [
61],
Corbicula fluminea, Muller [
62], and
Sardinella aurita [
63]. Subsequently, an increasing number of researchers began to focus on developing bioactive peptides from marine organisms.
Wergedahl et al. [
64] used lipid-free enzyme digest (a mixture of small peptides and free amino acids) prepared using enzymatic hydrolysis of
Salmo salar, L. with Protamex
TM protease as a diet and found that it increased the HDL-C/TC ratio in rat plasma. Furthermore, the process further lowered rat plasma cholesterol by decreasing the activity of hepatic HMGCR in Zucker (
fa/fa) obese rats [
64]. Fish skin collagen peptides also have the ability to enhance cholesterol metabolism, and
Sphyrna mokarran skin collagen peptides can alleviate HLP induced by a HFD with alcohol [
24].
Lee et al. [
65] investigate the effects of enzymatically obtained tuna skin collagen peptides from 3T3-L1 preadipocytes in a HFD-induced obese mouse model, and found that subcritical hydrolysis of fish collagen peptide significantly inhibited lipid accumulation during 3T3-L1 cell differentiation. In addition, this substance also significantly reduced serum TC, TG, and LDL levels, whereas it increased serum HDL levels in the obese mice [
65]. Collagen peptides from the skin of
Raja kenojei have also demonstrated an anti-obesity effect on lipid metabolism in mice fed a HFD [
22].
Furthermore, collagen peptides significantly inhibited lipid accumulation and increased serum HDL levels in obese mice through downregulation of fatty acid synthesis (sterol regulatory element-binding protein-1 [SREBP-1], FAS, and acetyl coenzyme A carboxylase [ACC]), sterol regulatory element-binding protein 2 (SREBP-1), SREBP-2, and HMGCR in the liver of mice [
22]. It also upregulated the expression levels of proteins used for β-oxidation (PPAR-α and carnitine palmitoyltransferase 1 [CPT1]) to inhibit fat accumulation [
22]. Recently, to better define the molecular mechanism of bioactive proteins, studies have used purification methods such as chromatography to identify the peptide sequences in the active proteins.
Glycine (Gly) was abundant in the enzymatic digest of
Zosterissessor ophiocephalus under the action of alkaline protease, which reduced the enzyme activity of serum HMGCR. Furthermore, this effect downregulated the expression of the LDL receptor (LDLR) in HFD rats, resulting in a blockade of cholesterol synthesis [
44]. The hypolipidemic activity of two pentapeptides (VIAPW and IRWWW) identified from the muscle digests of
Miichthys miiuy on oleic acid (OA)-induced lipid accumulation in HepG2 cells, was exhibited by their significant dose-dependent inhibition of OA-induced lipid accumulation and reduction of intracellular levels of TG and TC [
66]. Both pentapeptides downregulated the expression levels of
SREBP-1c,
SREBP-2,
FAS,
ACC, and
HMGCR genes in lipid synthesis and upregulated the expression levels of
PPARα,
ACOX-1, and
CPT-1 genes in lipid oxidation [
66].
With the existence of hundreds of peptides, rapidly screen out target peptides using traditional evaluation methods is difficult and, therefore, computer-aided drug design plays an important role in facilitating the process. Target identification is the first step in modern drug development, because most important physiological processes in organisms, such as cell cycle regulation, anabolism, signal transduction, and transmission of genetic information are closely dependent on the identification of proteins and ligands and their interactions. Some computer software such as Discovery studio, Autodock, Pymol, and MOE can predict the affinity and binding stability of both.
This is achieved by analyzing the intermolecular electrostatic interactions between the active molecule and the receptor amino acid residues, and the effects of interactive forces such as ionic bonding, hydrogen bonding, and van der Waals' forces. Zhao et al. [
67] used ultrafiltration and molecular exclusion chromatography to isolate
Ostrea rivularis Gould protein, which was followed by purification using ultrafiltration and molecular exclusion chromatography [
67]. Subsequently, three novel peptides with strong xanthine oxidase (XO) inhibitory activity (ALSGSW, GGYGIF, and MAIGLW) were screened using liquid chromatography-tandem mass spectrometry (LC-MS/MS) identification and molecular docking techniques [
67].
Furthermore, the mechanism of the peptide–XO interaction was revealed using molecular docking techniques, the structures of the peptides were rationally designed based on this information, and the results showed that replacing the GGYGIF peptide with Trp Gly at the N-terminus significantly improve its XO inhibition rate [
67]. In addition, the peptide composed of simple structural amino acids connected with aromatic amino acids exhibited better inhibitory activity than the others did.
The specificity of the protease cleavage site has led to the discovery that the activity is linked to the composition of amino acids and the structure of the peptide. Therefore, enzymatic cleavage using targeted cleavage techniques to obtain peptides with the intended activity is also an attractive future research direction. Moreover, considering the need for adequate oral bioavailability and bioactivity of peptides, future prospects for marine peptide research should focus on developing separation and purification techniques with higher selectivity and resolution than conventional methods. In addition, the development of embedding techniques such as nanoemulsions and nanoliposomes would be extremely useful to identify and obtain more novel peptides at higher yields and lower costs.
3.4. Others
It is worth noting that seaweed contains a high proportion of secondary metabolites such as polyphenols, which are a good source of lipid-lowering bioactive substances [
68]. The highest proportions of phenolic compounds in green and red algae are bromophenol, phenolic acids, and flavonoids. Phenyltannins are a composite polymer of phloroglucinol (1,3,5-trihydroxybenzene), which is the main secondary metabolite of polyphenols found only in marine brown algae. Polyphenol extracts of brown algae can activate AMPK signal transduction, thereby reducing lipid accumulation in the organism [
69,
70,
71]. Fourteen compounds were isolated from the fermentation broth of
Streptomyces nitrospororus YBH10-5 in the Arctic, and compound 12, farnesyl, significantly increased the expression of key proteins in Hep-G2 cells (PPAR α) and their downstream genes (
CPT-1), acyl CoA oxidase 1 (
ACOX), malonyl CoA decarboxylase 1 (
MCD1), and the expression level of cholesterol 7 α hydroxylase (CYP7A1) [
72].
Marine fish and shellfish species also contain a substance called taurine, which is a sulfur-containing nonessential amino acid that is likely widely involved in the metabolism of living organisms, especially regulation of abnormal lipid metabolism [
73]. Animal and in vitro experiments have shown that taurine supplementation significantly reduces the level of blood lipid (such as TG, TC, LDL, and HDL) [
74,
75]. In addition, clinical and epidemiological studies have found that taurine inhibits the process of HLP and atherosclerosis caused by HFD [
74,
75]. As a research hotspot, astaxanthin not only has significant antioxidant effects, but its role in regulating metabolic syndrome cannot be ignored. As early as in 2010, a clinical study demonstrated the lipid-lowering effect of astaxanthin in subjects, for the first time [
76]. The results showed that astaxanthin reduce TG levels in patient serum, while HDL-C and serum adiponectin levels were significantly increase [
76].
The oceans are rich in lipid-lowering bioactive substances, but most are obtained as extracted mixtures. The method for mixture extraction is simple but not conducive to an in-depth discussion of the conformational relationship of the active substances. Recently, scientists have used spectroscopy, chromatography-MS, energy spectrometry, and other techniques for purification and structural characterization in studying the molecular structure of the compounds contained in active substances. These techniques have also been used to elucidate the relationship between the chemical bonding and functional groups and the activity.