2.1. Experimental Studies
A recent systematic review and meta-analysis described all the pre-clinical data performed in rodents and published before July 15, 2022 [
33]. This paper concluded that intake of protein from fish muscles or fish by-products significantly decrease circulating total cholesterol (TC) concentration when compared to their control group. Of note, the authors highlighted that the stronger effect of fish-derived proteins’ intake was observed in the subgroup comprising genetically modified rodent models, which spontaneously develop hypertension after birth, and rodents fed diets enriched with cholesterol alone or in combination with cholate (added to exacerbate hypercholesterolemia [
50,
51,
52]. This data, indicating that the potency for preventing an increase of TC was higher than that for lowering TC plasma levels, may have relevant clinical application, albeit not directly transferable to human. Furthermore, the authors analysed diverse mechanisms of action to justify the hypocholesterolemic effects exerted by the dietary intake of fish or fish proteins (
Figure 1).
They found that in almost half of the analysed studies a lower TC concentration was associated with higher faecal excretion of cholesterol and/or bile acids. Furthermore, in two papers [
53,
54] the above-described effects were also combined with higher mRNA expression levels of cholesterol 7-alpha-hydroxylase (CYP7A1) [
55], which is the first and rate-liming enzyme in the cholesterol metabolism, such as the bile acid synthesis. On the contrary, the impact of fish or fish-proteins on the expression of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMG-CoA reductase), LDL-receptor (LDL-R), and acyl-CoA:cholesterol acyltransferase (ACAT2) was difficult to assess, because it was marked influenced by the rodent model used. However, an elegant and recent work demonstrated that the hypocholesterolemic effect exerted by Alcalase-silver carp hydrolysate (Alcalase-SCH) was associated with an up-regulation of LDL-R expression and a down-regulation of Niemann-Pick C1-like 1 (NPC1L1) and ACAT2 [
56]. In addition, these authors identified novel peptides, present in the Alcalase-SCH, as main contributors to the hypocholesterolemic activity of Alcalase-SCH [
56]. Of note, in line with data obtained with soya, potatoes and rice proteins, lower methionine/glycine and lysine/arginine ratios were also observed in fish proteins compared with casein, together with a lower TC plasma level. In addition, salmon protamine is a strongly alkaline polycationic low-molecular-weight protein, in which nearly two-thirds of the amino acid composition is arginine [
57,
58,
59]. It is well known that arginine, being a precursor for nitric oxide (NO) synthesis, may positively affect vascular function [
60]. Indeed, arginine supplementation has been shown to decrease neointimal formation in animal models [
61,
62], and to improve flow-mediated vasodilation in humans [
63].
These data were subsequently confirmed by diverse experiments. Oral administration of jellyfish collagen hydrolysate (JCH) was able to prevent the increase of serum glucose, TC and TG levels, together with the body gain weight in a mouse model of obesity, aka mice fed a high-fat diet (HFD) [
64]. Additionally, JCH administration modulated oxidative stress and inflammatory response, crucial factors implicated in obesity-related pathologies, and helped recover the alteration on microbiota composition induced by high-fat diet, specifically by contrasting the lowering of
Romboutsia’s abundance [
64]. Similar data were published by Shi et al, in healthy mice fed a chow diet and treated with Half-fin Anchovy hydrolysate (HAHp) or with its Maillard reaction products (HAHp-MRPs) by oral gavage [
65]. Significantly, the glycation process or Maillard reaction, aka the chemical process involving proteins and sugars during food processing, can enhance protein and peptide functionalities, including antioxidant and antihypertensive activity. The glycated proteins or peptides may resist digestion and undergo fermentation in the colon, potentially benefiting gut health. Studies have shown that glycated proteins, such as those from pea [
66] and milk, can exhibit similar probiotic effects as Galactooligosaccharides (GOS) alone [
67]. GOS are a type of prebiotic that support beneficial intestinal bacteria and produce SCFAs that have a variety of biological functions, hence promoting gut health [
68]. GOS ferments quickly, producing gas and bloating. This has raised interest in prebiotics that affect the distal colon and are linked to a lower risk of colon cancer [
69]. Glycated peptides' effect on the gut microbiota is yet unclear, though [
70]. Jin et al. [
71] investigated the effects of GOS glycated with fish peptides on GM of rats using the Maillard reaction. The composition of the gut microbiota and colonic fermentation were affected by the new glycoconjugates, offering the first
in vivo proof of these prebiotic effects. Additionally, Han et al. [
72] explored the chemical characteristics of glycoconjugates of myofibrillar proteins from grass carp that were conjugated with glucose via the Maillard reaction during dry heating. Glycation increased furosine levels, promoted structural changes in the proteins and reduced protein digestibility. The butyrate production during fermentation was influenced by glycation and showed positive correlation with
Mitsuokella, Lachnospiraceae_UCG-004, Sutterella, Salinimicrobium, Fodinibius and
Nitriliruptor, but anti-correlation with
Enterococcus, Dorea, Escherichia-Shigella, and
Phascolarctobacterium. These findings demonstrated that the glycation of myofibrillar proteins could have positive outcomes on gut health [
72].
Lin et al, found that small-molecule peptides from the bone collagen of
Harpadon nehereus (HNCP) exerted antidiabetic effects in Streptozotocin induced diabetes mice [
73]. Specifically, HNCP administration significantly decreased the plasma levels of glucose, TC, TG, LDL-C and increased HDL-C concentration and insulin secretion. Moreover, HNCP improved glucose metabolism and showed remarkable antioxidant activity in this type 1 diabetic mouse model by regulating the expression levels of glycosynthesis and gluconeogenesis-related [i.e., glucokinase (GK), phosphoenolpyruvate carboxikinase1 (PEPCK1) and glucose-6-phosphate (G6Pase)] and antioxidant enzymes [i.e., catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and quinone oxidoreductase 1 (NQO1)], respectively. Additionally, the same authors demonstrated that this latter effect, aka the antioxidant activity, was mediated by the activation of the nuclear factor-erythroid 2-related factor 2 (Nrf2) pathway [
73]. It has been established the crucial role played by Nrf2 in redox balance, inflammation, cytotoxicity and cellular metabolism, and its involvement in many oxidative stress-based diseases [
74]. Similar results were obtained in Streptozotocin-induced diabetes rats treated with the small peptide (<1kDa) fraction from
Takifugu bimaculatus skin hydrolysate (TBP) [
75]. Specifically, TBP was chosen because in an
in vitro assay exhibited the strongest dipeptidyl peptidase-IV (DPP-IV) inhibitory activity. DPP-IV inhibition hinders the degradation of glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) that are released post-prandially, increasing their half-life and amplifying the insulin effect on glucose homeostasis [
76]. In the i
n vivo experiment, TBS diminished weight loss, lowered fasting blood glucose concentrations, increased insulin secretion, improved irregular hormonal fluctuations and lipid metabolism, and mitigated histopathological damage in the pancreas and liver. Additionally, the relative abundance of Firmicutes decreased, alongside the increase in Bacteroidetes, significant modifications were observed at the genus level, and two metabolites, hippuric acid and ergosta-5,7,22,24(28)-tetraen-3beta-al were identified following TBP administration [
75]. In line with these data, a salmon peptide fraction (SPF), containing low-molecular-weight peptides, was able to prevent the development of obesity and metabolic disorders, dampening inflammation in both hepatic and intestinal tissues, and to modulate thrombosis risk factors in high-fat-high-sugar-fed vitamin D-deficient dyslipidemic mice [
77]. Interestingly, Fang et al. applied a multistage strategy, in detail, a molecular docking-based virtual screening within a small library of marine-derived natural products with follow-up
in vitro and
in vivo phenotypic assays, aiming at discovery new lipid lowering molecules [
78].
Hypertension, as above already mentioned, is a major risk factor for CVDs. One pharmacological approach aiming at reducing blood pressure is represented by the angiotensin-I-converting enzyme (ACE) inhibitors. ACE is a key enzyme that catalyzes the conversion of angiotensin I (an inactive decapeptide) to angiotensin II (octapeptide), a potent vasoconstrictor, which stimulates the release of aldosterone, and eventually increases the blood pressure. Fish-derived bioactive peptides have been widely investigated for their anti-hypertensive effects, such as ACE inhibition. All the studies published before 2020, have been collected in reviews [
32,
79,
80]. According to the results obtained, the bio-efficacy and bioavailability of the final peptide products are marked affected by the used extraction processes (the enzymatic hydrolysis as well as isolation/purification techniques). Additionally, size and chain length together with the presence of some amino acids (tyrosine, tryptophan, proline, and phenylalanine) at the C-terminal of the fish-derived peptide’ structures are crucial for ACE inhibition and antihypertensive effects [
32,
79,
80].
In vitro experiments demonstrated that protein hydrolysates from fish by-products exerted competitive, non-competitive and mixed inhibition modes against ACE. In line with these data,
in vivo experiments, mainly performed in spontaneously hypertensive rats (SHRs), proved the strong antihypertensive activity of protein hydrolysate from diverse marine organisms. Of note, grass carp peptides, rich in phenylalanine, leucine, aspartic acid, and glycine, significantly reduced the systolic blood pressure compared to the control group treated with captopril, the drug of choice for hypertensive patients [
81]. Similar data were obtained by Chen et al, by administrating the Leu-Ser-Gly-Tyr-Gly-Pro peptide [
82] from tilapia skin gelatine to SHRs [
83]. Moreover, these authors via molecular docking comparison identified four connecting residues of the ACE active site, which may justify the mechanism of inhibition [
83]. Recently, an experimental study demonstrated that both intact and hydrolysed blue whiting proteins reduced blood pressure in an obese rat model, inhibiting renin activity but not showing ACE inhibitory effect [
84]. Whereas, a peptide composed of 13 amino acid residues, DPALATEPDPMPF, obtained from Nile tilapia (
Orechromis niloticus) exhibited potent ACE inhibitory and radical scavenging activities, suggesting a potential it use in functional foods [
85]. Indeed, ZBPHs administration to rats fed a high-cholesterol/cholic acid containing diet attenuated cholesterol-caused cardiac injury, testified by biochemical and histological improvement as well as significantly protecting heart genomic DNA’s oxidative damage induced by Fenton’s reagent [
86]. Finally, Maneesai et al, investigated the impact of tuna protein hydrolysate (TPH) on CV remodelling and dysfunction in a rat model of metabolic syndrome (MS) [
87]. The results of this study demonstrated that TPH supplementation improved all the metabolic parameters, including dyslipidemia, hyperglycemia, obesity, hypertension, cardiac hypertension, endothelial dysfunction, oxidative stress and inflammation, in the in a dose-dependent manner. These effects were related to the TPH ability at modulating angiotensin II receptor type 1 (AT
1R)/NADPH oxidase 2 (NOX2), endothelial nitric oxide (eNOS), Nfr2/heme oxygenase 1 (HO-1) and peroxisome proliferator-activated receptor (PPAR)gamma/nuclear factor kappa B (NF-kB) protein expression in heart and aorta [
87].
Various experimental studies have investigated the impact of fish protein hydrolysates (FPHs), from salmon or anchovy by products (spine, viscera, collagen), on atherosclerosis development. All the studies were performed on genetically modified mice, aka apoE-deficient mice, fed high-fat [
88,
89] or high-fat/high cholesterol diet [
90,
91]. Altogether, the results demonstrated that these FPHs reduced plaque area and lipid accumulation in the aorta as well as in the aortic sinus. Conversely, no differences in extracellular matrix, macrophages and T-lymphocytes were observed in the plaque area of FPHs-fed mice compared to control animals. Of note, these effects were associated with lower levels of pro-inflammatory cytokines in the serum and aorta [
88,
89,
90,
91,
92]. Interestingly, two studies showed that taurine proved efficacy in reducing the atherosclerosis development in both apoE-deficient mice fed chow diet with or without TMAO [
93,
94]. Furthermore, the authors demonstrated that dietary taurine exerted its anti-atherosclerotic effects via increasing the hepatic gene expression of conjugated bile acid synthesis and eventually, increasing the conjugated BA to unconjugated BA ratio in the liver as well as serum. Meanwhile, taurine improved the TMAO-induced abnormal bile acid profile in the gallbladder. Moreover, taurine increased bile acid deconjugation, by enhancing the genera
Ruminiclostridium level, and excretion of fecal neutral sterols. In line with the data obtained with the FPHs, taurine positively modified the TMAO-induced inflammation in both serum and aorta [
93,
94].