Roles of Dietary Bioactive Peptides in Redox Balance and Metabolic Disorders

Bioactive peptides (BPs) are fragments of 2–15 amino acid residues with biological properties. Dietary BPs derived from milk, egg, fish, soybean, corn, rice, quinoa, wheat, oat, potato, common bean, spirulina, and mussel are reported to possess beneficial effects on redox balance and metabolic disorders (obesity, diabetes, hypertension, and inflammatory bowel diseases (IBD)). Peptide length, sequence, and composition significantly affected the bioactive properties of dietary BPs. Numerous studies have demonstrated that various dietary protein-derived BPs exhibited biological activities through the modulation of various molecular mechanisms and signaling pathways, including Kelch-like ECH-associated protein 1/nuclear factor erythroid 2-related factor 2/antioxidant response element in oxidative stress; peroxisome proliferator-activated-γ, CCAAT/enhancer-binding protein-α, and sterol regulatory element binding protein 1 in obesity; insulin receptor substrate-1/phosphatidylinositol 3-kinase/protein kinase B and AMP-activated protein kinase in diabetes; angiotensin-converting enzyme inhibition in hypertension; and mitogen-activated protein kinase and nuclear factor-kappa B in IBD. This review focuses on the action of molecular mechanisms of dietary BPs and provides novel insights in the maintenance of redox balance and metabolic diseases of human.


Introduction
Dietary food proteins contain short sequences of amino acids (AAs) that possess various biological activities. The short chains of AAs with biological properties are known as bioactive peptides (BPs). BPs usually contain 2-15 AA residues. The BP sequences present in the food proteins are generally hidden in the inner core of the parent proteins [1]. However, bioactive peptides can be released from food proteins with the help of several techniques including proteolysis, microbial fermentation, and gastrointestinal (GI) digestion [2][3][4]. Among these methods, in vitro enzymatic hydrolysis using commercial proteolytic enzymes is the widely employed method in recent times. The proteases cleave the specific sites on the dietary proteins and release the short chains of BPs [5]. Enzymatic hydrolysis of dietary proteins enhances the nutritional value, bioactivity, and functionality and reduces the allergenicity [6]. Several commercial proteases, namely, thermolysin, bromelain, trypsin, alcalase, papain, pepsin, neutrase, pancreatin, corolase, protamex, and pronase, have been employed to generate biologically active peptides from various dietary proteins. Among the above-mentioned proteases, alcalase, trypsin, pepsin, and pancreatin are the most widely used proteases for the preparation of peptides, with various health benefitting properties, from numerous food sources [7][8][9][10]. These enzymes are routinely used due to their broad specificity to produce smaller peptides with various bioactivities and easily availability.
Dietary BPs have been shown to positively affect the various systems of the human body including the immune, cardiovascular, GI, and nervous systems [11]. The BPs must cross the GI barrier and reach the target tissue or organ in order to exhibit health benefits. Dietary peptides exert bioactivities through the modulation (either enhance or decrease) of various molecular mechanisms and pathways. Bioactive properties of dietary peptides are affected by the peptide sequence, length, hydrophobicity, and composition [12][13][14]. Dietary peptides are easily absorbable across the intestinal border via peptide transport 1 (PepT1) and possess excellent functional properties (solubility, foaming, and emulsification properties) [15][16][17][18]. Additionally, dietary BPs are generally safer than synthetic drugs. Hence, BPs obtained from dietary sources could be used as health foods/nutraceuticals in the management/prevention of various diseases. Numerous BPs derived from whey, casein, milk, soybean, shark, bonita, pacific whiting, porcine, and bovine have been on the market in various countries for human use as functional foods/health foods/nutraceuticals [19].
Redox homeostasis (balance) is an important cellular process that plays a vital role in the maintenance of a normal physiological steady state [20]. Disturbance of balance between oxidants and antioxidants results in the oxidative stress. Recently, many BPs prepared from various food sources such as walnut, egg, fish, quinoa, soybean, millets, corn, wheat, rice, potato, milk, and spirulina have shown to possess beneficial effects in the maintenance of redox homeostasis and prevention/management of metabolic diseases [21][22][23][24][25][26]. However, reviews describing the role of dietary BPs in the modulation of molecular mechanisms of redox balance and metabolic diseases are scanty in the literature. Hence, the current review focuses on the recent literature related to the effects of dietary BPs on various molecular mechanisms and signaling pathways involved in the redox homeostasis and metabolic diseases (obesity, diabetes, hypertension, and inflammation), as shown in Figure 1.

Roles of Dietary Bioactive Peptides in Maintaining Redox Balance
Redox reaction is a chemical reaction that involves the transfer of electrons from the reducing agent to the oxidizing agent [27]. Redox homeostasis (balance) plays a significant role in the maintenance of ordinary physiological functions of the human body. Redox balance is considerably affected by reactive oxygen species (ROS) that are generated during aerobic cellular metabolism [28]. Reactive oxygen species are unstable and highly reactive in the redox reactions due to the presence of unpaired electrons in the outer shells. Under normal physiological conditions, redox balance is maintained through careful regulation of ROS generation and elimination from the body [29]. However, excess production of ROS during oxidative stress conditions could alter the intracellular redox balance and promote the development of numerous diseases such as cancer, diabetes, atherosclerosis, cardiovascular, and neurodegenerative diseases. The body's natural antioxidant defense system, namely, superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT), plays a vital role in the maintenance of balance between ROS formation and elimination [30]. Several studies have demonstrated that a number of peptides identified from various dietary sources have shown the ability to suppress oxidative stress and maintain the redox balance through multiple molecular mechanisms such as scavenging free radicals; chelating transition metals; enhancing the production of endogenous antioxidant enzymes SOD, CAT, and GPx; and stimulating the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidant defense mechanism [12,15,17,26,[31][32][33][34]. The antioxidant mechanisms showed by the dietary peptides mainly depend on the peptide sequence, length, composition, and hydrophobicity [12,34,35]. The food-derived antioxidant peptides usually contain 3-15 AA residues [12,32]. Table 1 shows the AA sequences and molecular mechanisms of antioxidant peptides produced from various dietary proteins.
The Kelch-like ECH-associated protein 1-(Keap1-) Nrf2antioxidant response element (ARE) is the main antioxidant signaling pathway that prevents oxidative stress and helps maintain the optimum redox steady state in the body [36]. The Nrf2 is a vital leucine zipper transcription factor, which controls the expression of several antioxidant proteins in response to ROS stress. Keap1 is a suppressor protein for Nrf2, and under a normal ROS steady state, Keap1 binds with Nrf2 and helps proteasome to degrade the Keap1-Nrf2 complex. However, Nrf2 separates from Keap1 during oxidative stress and migrates to the nucleus where it attaches to ARE and thereby promotes the expression of several antioxidant enzymes/proteins [32,37,38] [39]. Hence, it is important to stimulate the Nrf2 antioxidant signaling pathway to suppress/prevent the oxidative stress in the body.
Recently, numerous novel antioxidant peptides that stimulate the Keap1-Nrf2-ARE antioxidant signaling pathway and antioxidant enzymes have been isolated from different dietary sources such as casein [40], milk protein concentrate [41], corn gluten [31], soybean [42], walnut [43], potato, Moringa oleifera seeds [25], watermelon seeds [39], Crassostrea rivularis [44], krill [38], turtle [45], Mytilus coruscus [46], Channa argus [3], and silver carp muscle [12]. Snakehead (Channa argus) soup was hydrolyzed using pepsin and pancreatin and identified four antioxidant peptides, IVLPDEGK, PGMLGGSPPGLLGGSPP, SDGSNIHFPN, and SVSIRADGGEGEVTVFT [3]. The authors performed molecular docking studies for the peptides and indicated that peptides could bind to the active site of Keap1 and thereby activate the cellular antioxidant Keap1-Nrf2 signaling pathway. A peptide RDPEER isolated from alcalase hydrolysate of watermelon seed reduced the oxidative stress by reducing ROS and increasing the antioxidant enzymes, SOD, CAT, and GSH-Px, in HepG2 cells [39]. Four casein-derived peptides, ARHPHPHLSFM, AVPYPQR, NPYVPR, and KVLPVPEK, were shown to decrease the oxidative stress in Caco-2 cells by enhancing antioxidant enzymes, namely, SOD1, Trx1, GR, TrxR1, and NQO1, through the activation of the Keap1-Nrf2 signaling mechanism. It was found that the peptides bound to the Nrf2 and prevented the binding between Keap1 and Nrf2 and thereby stimulated the increased expression of antioxidant enzymes [32]. In a study, a peptide, AMVDAIAR, isolated from pepsin hydrolysate of krill enhanced antioxidant enzymes SOD, CAT, and GPx and thereby suppressed the oxidative stress in H 2 O 2 -induced hepatocytes through increasing the expression of Nrf2 [38]. 2 Oxidative Medicine and Cellular Longevity Peptide EDYGA derived from soft-shelled turtle enhanced the Nrf2 level through the binding of the glutamate residue of the peptide to Arg 415 of the Kelch receptor pocket [45]. Dipeptide IF identified from potato exhibited antioxidant effects by increasing the antioxidant enzymes GPx 4, SOD1, HO-1, SOD2, and peroxiredoxin 2 in the kidney tissues of the spontaneously hypertensive rats (SHRs) [33]. The authors concluded that the dipeptide showed antioxidative activity through preventing Nrf2 degradation by protein kinase B (Akt) activation and GSK-3β phosphorylation.

Role of Dietary Bioactive Peptides in Obesity
Obesity is a public health issue worldwide and characterized by excessive body fat accumulation due to the difference between energy intake and energy spending, which enhances the risk of metabolic disorders, namely, type 2 diabetes mellitus (T2DM), hypertension, and cardiovascular disorders [47,48]. It is estimated that globally, 18% men and 21% women will suffer from obesity by 2025 [49]. Adipocytes are fat cells and the main function of adipocytes is the storage of energy as fat. Preadipocytes differentiate into mature adipocytes through a process known as adipogenesis. Hypertrophy and hyperplasia of adipocytes are two mechanisms that contribute to the obesity and obesity-related metabolic disorders [50,51]. Several recent studies demonstrated that various dietary peptides from soybean, quinoa, common bean, camel whey, spirulina, blue mussel, skate, tuna, Alaska pollock, sardinella, hazelnut, and kefir exhibited antiobesity effects through modulation of multiple molecular mechanisms including the reduction of adipogenesis through downregulation of the expression of peroxisome proliferator-activated receptor-(PPAR-) γ, CCAAT/enhancer binding protein alpha (C/EBP-α), sterol regulatory element binding protein-(SREBP-) 1, and HMGCR, enhancing lipolysis, reducing body weight (BW) and food intake, inhibiting lipase activity, decreasing the accumulation of triglycerides, and blocking

Oxidative Medicine and Cellular Longevity
During maturation of preadipocytes into adipocytes, several transcriptional factors are involved. PPAR and C/EBP are two transcription factors that stimulate adipocyte differentiation [50]. Lipid and carbohydrate metabolism is regulated by the PPARs. SREBP1 is a lipogenic transcription factor upon activation by PPAR-γ which promotes the adipogenesis and lipogenesis. SREBP1 stimulates lipoprotein lipase and fatty acid synthase and thereby enhances the lipid accumulation in the adipocytes [50]. Therefore, inhibition of the PPARs, C/EBP, and SREBP1 transcriptional factors involved the adipogenesis and lipogenesis using dietaryderived BPs is an efficient approach in the prevention or treatment of obesity and related diseases. It has been revealed that numerous dietary peptides from soy bean, quinoa, hazelnut, canola, tuna, ark shell, and blue mussel showed antiobesity activities by inhibiting the expression of PPAR-γ, C/EBP, and SREBP1 transcriptional factors [10,51,54].
Additionally, the authors indicated that the hydrophobic AAs, proline, and leucine, of the peptide, might have contributed to the antiobesity effects of the peptides. Peptides with more hydrophobic AAs can easily penetrate the cell membrane and increase lipid solubility. In a study, peptides < 1 kDa from ark shell (Scapharca subcrenata) protein inhibited intracellular lipid buildup and enhanced the lipolysis [51]. The authors were also demonstrated that ark shell peptides inhibited adipogenesis by decreasing the expressions of PPAR-γ, C/EBP-α, SREBP-1c, lipoprotein lipase, and FAS in mouse mesenchymal stem cells. Moreover, the expression of PPAR-γ, C/EBP-α, and aP2 was decreased by tuna skin collagen-derived peptides in obese mice, which resulted in the decrease of adipocyte size [55].
In addition to the downregulation of vital transcriptional factors (PPAR-γ, C/EBP-α, and SREBP-1c) of adipogenesis, several in vivo studies demonstrated that the antiobesity activity of dietary peptides is due to the decrease of BW and food consumption. Oral administration of peptides (10 mg/mL), produced from smooth hound (Mustelus mustelus) muscle by alkaline crude enzyme from M. mustelus intestines, for 21 days reduced the BW and food intake in rats [5]. The authors suggested that BW reduction was probably due to the regulation of appetite. The antiobesity effect of Alaska pollack-derived peptides was investigated, and it was found that peptide administration (100 or 300 mg/kg BW) to rats for 3 days decreased the weight of white adipose tissue and reduced the food intake [56]. The authors suggested that the decrease in food intake and white adipose tissue weight of rats after peptide treatment was due to downregulation/suppression of gene expressions of neuropeptide-Y and agoutirelated peptide in hypothalamus, which may reduce the     [129] ACC1: acetyl-CoA carboxylase 1; AMPK: AMP-activated protein kinase; aP2: adipocyte fatty acid-binding protein 2; BW: body weight; C/EBP-α: CCAAT/enhancer binding protein alpha; FAS: fatty acid synthase; FGFs: fibroblast growth factors; HFD: high-fat diet; PPAR-γ: peroxisome proliferator-activated receptor-γ; SREBP-1: sterol regulatory element-binding protein 1.

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Oxidative Medicine and Cellular Longevity appetite. In a study, it was found that antiobesity effects of sardinella-(Sardinella aurita-) derived peptides were mediated by reducing the BW gain, food intake, and the relative epididymal adipose tissue weight in Wistar rats after 10 weeks of peptide treatment [2]. It was demonstrated that Spirulina platensis-derived peptides exhibited antiobesity effects by reducing BW (39.8%) and lowering serum glucose (23.8%) through altering the gene expressions of Acadm, Retn, Fabp4, Ppard, and Slc27a1 in the brain and liver of mice fed with peptides (2 g/kg BW) for 4 weeks [57].
Recently, it was reported that walleye pollock skin collagenderived peptides considerably reduced the BW gain in obese mice after 8 weeks of peptide treatment [58]. The authors also pointed that peptides inhibited the growth of adipocytes and the accumulation of adipose tissue in obese mice. Furthermore, in a placebo-controlled, randomized clinical investigation, salmon fish-derived peptide supplementation (16 g/d) for 42 days notably decreased (5.6%) the BMI in obese humans [59]. Pancreatic lipase is an important enzyme that aids in the hydrolysis of dietary fat in the small intestine. Therefore, inhibition of pancreatic lipase is an efficient approach in the management and treatment of being overweight and obesity [2,24]. Apart from inhibition of adipogenesis and lipogenesis, some dietary peptides exhibited pancreatic lipase inhibitory activity. Peptides produced from camel milk by using alcalase-, bromelin-, and papain-inhibited porcine pancreatic lipase [24]. It was also found that sardinella peptide administration to rats for 10 weeks decreased the pancreatic lipase activity [2].

The Role of Dietary Bioactive Peptides in Diabetes Mellitus
Diabetes mellitus (DM) is a complex metabolic disorder with increased blood sugar levels and T2DM accounts for 90% of diabetes patients. T2DM results from insulin resistance (cells less responsive to the insulin actions) and/or insufficient insulin production from pancreatic beta cells. Obesity (BMI ≥ 30 kg/m 2 ) has been reported to greatly increase the risk of developing T2DM [60]. Uncontrolled T2DM can cause severe complications such as high blood pressure, stroke, heart attack, atherosclerosis, retinopathy, nephropathy, neuropathy, and dementia. Recently, several peptides derived from a variety of dietary protein sources including egg white, whey, casein, egg yolk, rice bran, quinoa, soybean, wheat, corn, black bean, oat globulin, walnut, potato, common bean, millets, spirulina, bovine, porcine, Atlantic cod, Atlantic salmon, halibut skin, Styela clava, boarfish, tilapia skin, largemouth bass, zebra blenny, blue whiting, and sea cucumber have shown antidiabetic effects by altering several molecular mechanisms of diabetes such as inhibition of enzymes including α-amylase, dipeptidyl peptidase-(DPP-) IV, and α-glucosidase; reduction of FBG and HbA1C; enhancement of HOMA-IR; stimulation of secretion of glucagon-like polypeptide-1 (GLP-1) and insulin levels; upregulation of phosphatidylinositol 3-kinase (PI3K), p-GSK-3β, p-Akt, and glucose transporter (GLUT)2/4 signaling pathways; blocking of glucose transporters GLUT2 and SGLT1; decreasing of the activation of p38 and c-Jun N-terminal kinase (JNK)1/2; enhancement of the stimulation of insulin receptor substrate-1 (IRS-1) tyrosine residue and Akt; and decreasing of gluconeogenesis through activation of IRS-1/PI3K/Akt and AMP-activated protein kinase (AMPK) [6,21,[61][62][63][64]. The isolated peptide sequences and molecular mechanisms of dietary antidiabetic peptides are shown in Table 3.
Another strategy for the management and treatment of T2DM is to inhibit the DPP-IV that degrades and inactivates incretin hormones, namely, glucose-dependent insulinotropic peptide (GIP) and GLP-1. Therefore, inhibition of DPP-IV by dietary-derived BPs can enhance the half-life of GLP-1 and thereby increase the release of glucose-dependent insulin from pancreatic cells [61]. For inhibition of DPP-IV activity, several synthetic drugs such as saxagliptin, linagliptin, sitagliptin, and vildagliptin are presently used. However, side effects (diarrhea, nausea, stomach pain, headache, and sore throat) associated with these drugs have forced researches to search for DPP-IV inhibitors from natural dietary sources without any side effects [69]. Many recent studies have reported that dietary protein-derived peptides are an 11 Oxidative Medicine and Cellular Longevity 12 Oxidative Medicine and Cellular Longevity 13 Oxidative Medicine and Cellular Longevity  [67,[70][71][72]. It has been found that dietary peptides inhibit DPP-IV through attaching the peptides to the active sites of DPP-IV via hydrogen bonds and hydrophobic interactions and thereby prevent the enzyme action [14,21]. Two peptides NDWHTGPLS and TYPHQQPPILT derived from papain hydrolysates of millet proteins inhibited DPP-IV activity (75%) [21]. It was demonstrated that both the peptides inhibited DPP-IV activity by occupying the active sites of DPP-IV via hydrogen and pi bonds. In a recent study, Atlantic salmon skin was hydrolyzed by using trypsin and isolated a new DPP-IV inhibitory peptide, LDKVFR, with IC 50 of 128.7 μM [14]. Additionally, it was found that 6 H bonds and 8 hydrophobic interactions played a significant role in the inhibition of DPP-IV by LDKVFR. A peptide, LDQWLCEKL, obtained from trypsin hydrolysate of α-lactalbumin-rich whey proteins inhibited DPP-IV (IC 50 131 μM) through a noncompetitive mode of inhibition. The PI3K/Akt signaling pathway regulates the glucose uptake. When cells are resistant to insulin, glucose uptake is impaired in the liver and skeletal muscles. Insulin activates IRS after binding to IRS and the activated IRS phosphorylates IRS-1. Phosphorylation of IRS-1 results in the activation of PI3K. The activated PI3K phosphorylates Akt, and the activated Akt helps in the migration of intracellular GLUT2/4 to the plasma membrane and therefore increases the glucose absorption into cells. But insulin resistance weakens the PI3K/Akt signaling pathway [73,74]. Hence, stimulation of the PI3K/Akt molecular pathway is an efficient approach in the management of insulin resistance. In a study, the authors hydrolyzed walnut protein using alcalase and isolated an antidiabetic peptide LPLLR with a molecular weight of 610.4 Da. The authors reported that the identified peptide improved hepatic insulin resistance (IR) through enhancing glycogen synthesis and glucose uptake and reducing gluconeogenesis via activating the IRS-1/PI3K/Akt and AMPK signaling pathways in hepatic HepG2 cells [75]. Two hundred forty-two peptides, with a molecular weight ranging from 203 to 1907 Da, were isolated from the hydrolysate of sea cucumber and found that the peptides showed antidiabetic effects by upregulation of PI3K, p-Akt, p-GSK-3β, and GLUT2/4 signaling pathways, while decreasing p-IRS1 expression in diabetic rats [73].
In addition to the peptides described above, the antidiabetic activities of dietary peptides have also been investigated in human subjects. In a randomized and crossover clinical trial, whey peptide (<5000 Da) (1400 or 2800 mg/kg BW) administration to 21 prediabetic human subjects decreased under the curve (iAUC) of glucose as well as showed a minor insulinotropic effect and reduced HbA1c [76]. A doubleblind crossover clinical trial conducted using peptides (<2000 Da) derived from Atlantic cod showed that a single dose of 20 mg/kg BW considerably decreased the postprandial insulin in 41 healthy individuals [77]. Moreover, peptides derived from Styela clava have also been shown to decrease the hemoglobin A1c and plasma insulin levels after 4 weeks of administration in patients with T2DM [78].
It has been demonstrated that hydrogen bonds play a vital role in the binding of BPs to the ACE catalytic pocket and thereby facilitate ACE inhibition. Three ACE inhibitory peptides, LLSGTQNQPSFLSGF, NSLTLPILRYL, and TLEPNSVFLPVLLH, were isolated from lentil seeds with IC 50 of 44-120 μM, and it was found that the peptides inhibited ACE through interactions by hydrogen bonds with three residues of the ACE catalytic site [91]. A tripeptide YSK with ACE inhibition was identified from trypsin hydrolysate of rice bran, and ACE inhibition of YSK was due to the formation of hydrogen bonds with the binding site of ACE [1]. The molecular interactions between dipeptide, YV, obtained from ostrich egg white and ACE were studied and it demonstrated that YV inhibited ACE (IC 50 63.97 μg/mL) by binding to S1 and S2 pocket sites of ACE through hydrogen bonds [93]. A pentapeptide, ACKEP, purified from pistachio kernel hydrolysates was shown to inhibit ACE (IC 50 126 μM) by binding with seven AAs of the ACE catalytic site (His383, His387, Glu384, Arg522, Asp358, Ala356, and Asn70) and two atoms of ACKEP [88]. In a study, three ACE inhibitory peptides, AVKVL, YLVR, and TLVGR, were identified from alcalase hydrolysate of hazelnut. The authors found that all the three peptides exhibited ACE inhibition (IC 50 5.42-249.3 μM) via a noncompetitive inhibition through the formation of cation-pi interactions [89]. In another study, four ACE inhibitory peptides, EDEVSFSP, SRPFNL, RSPFNL, and ENPFNL, were purified from fermented soybean with IC 50 of 0.131-0.811 mg/mL [84]. It was suggested that the N-terminal sequence and the location of AAs in the peptides play an essential role in ACE inhibition. Recently, a peptide, VTPVGVPK, produced by α-chymotrypsin hydrolysis from black cumin seed was shown to inhibit ACE (IC 50 1.8 μM) through a noncompetitive inhibition [86]. A peptide, QHPHGLGALCAAPPST, identified from chymotrypsin hydrolysate of quinoa inhibited ACE by binding to the number of active hotspots of the ACE enzyme [62]. Moreover, two peptides, SAGGYIW and APATPSFW, were isolated from wheat gluten with ACE inhibition of IC 50 0.002-0.036 mg/mL [83]. The authors concluded that two peptides with proline and negatively charged residues inhibited ACE through the modulation of ionic and hydrophobic connections of the ACE active site.
In addition to ACE inhibition, several in vivo studies (animal and human) have reported the blood pressurelowering effects of numerous peptides isolated from various food sources. Peptides, SLVSPSAAAAAAPGGS and KKRSKKKSFG, generated from potato and rapeseed were found to reduce (154.7 mmHg) the mean arterial blood pressure of treated rats compared to the control (177 mmHg) group [85]. Two antihypertensive peptides, IQW and LKP, were identified from thermolysin and pepsin hydrolysates prepared from egg white ovotransferrin and demonstrated that tripeptide (IQW and LKP) administration reduced the mean blood pressure by 19 and 30 mmHg, respectively, compared to control SHRs [22]. Administration of a peptide, VELYP, produced from Sepia officinalis muscle, to SHR exhibited antihypertensive effects by decreasing SBP [96]. In a study, a beef myofibrillar protein-derived peptide LIV-GIIRCV at 400 and 800 mg/kg BW decreased SBP by 28 and 35 mmHg in SHRs, respectively [99]. A randomized and double-blind human trial conducted on level 1 hypertensive patients demonstrated that eight-week supplementation of casein-derived tripeptides, VPP and IPP, ameliorated cen-tral blood pressure and arterial stiffness [81]. In a recent randomized, double-blind clinical trial, the administration of shrimp-derived peptides (1200 mg/d) for eight weeks reduced the BP in mild-or moderate-hypertension patients [79]. Additionally, the authors suggested that the reduction of BP was probably due to the decrease of angiotensin II levels in hypertension patients.

The Role of Dietary Bioactive Peptides in Inflammatory Bowel Diseases (IBD)
Inflammation is a complex and natural response of the body in an attempt to resolve harmful stimuli such as pathogens, tissue injuries, infections, or toxins. However, uncontrolled and chronic inflammation has been reported to be linked with several diseases such as T2DM, metabolic syndrome, IBD, cardiovascular disease, cancer, asthma, arthritis, and chronic obstructive lung disease [101]. IBD symptoms such as diarrhea, abdominal pain, fever, vomiting, BW loss, and rectal bleeding affect the quality of life of patients [102]. Crohn's disease (DC) and ulcerative colitis (UC) are two major forms of IBD [103]. UC is the inflammation of colon. Chronic inflammation in the intestine produces excessive and uncontrolled proinflammatory cytokines including tumor necrosis factor-alpha (TNF-α), interleukin-(IL-) 1β, IL-6, IL-8, IL-12, interferon-gamma (IFN-γ), and IL-17 [102,103]. The mechanism of anti-inflammation by food proteinderived BPs is that they can inhibit the phosphorylation of signaling pathways including nuclear factor-kappa B (NF-κB), mitogen-activated protein kinase (MAPK), Janus kinase-signal transducer and activator of transcription (JAK-STAT), and peptide transporter PepT1 as shown in Figure 2 [26,104,105]. The NF-κB pathway contains IKKs, IκBs, and p65/p50, while the MAPK pathway contains p38, JNK, and extracellular signal-regulated kinases (ERK). BPs can inhibit the NF-κB receptor, resulting in the inhibition of the activation of inhibitory κB kinases (IKKα/β/γ), which can lead to phosphorylation of cytoplasmic transcription factor (IκBα/β/γ) and IκBα degradation. BPs can inhibit the MAPK receptor and inhibit MAP3K phosphorylation, which can mediate the phosphorylation of the downstream MAP2K and MAPK. The inhibition of phosphorylation of MAPK and JAK2-STATs by BPs can alleviate the release of cytokines. The BP inhibition of translocations of the above transcription factors in nucleus (ATF-2, AP-1, and c-Jun) can cause the gene change, reducing the productions of proinflammatory cytokines, such as IL-1β, IL-6, IL-8, TNF-α, and IFNγ, resulting in the inflammation suppression ( Figure 2). In addition, the PepT1 can transport small BPs to the bloodstream; therefore, the role of PepT1 is vital to the bioactivity of BPs and needs further investigation [15,26].
Peptides isolated from a variety of dietary protein sources (e.g., soy bean, common bean, corn, egg white, whey, casein, salmon, and crucian carp) have shown to inhibit intestinal inflammation through multiple molecular mechanisms. These include downregulation of the expression of IL-8, IL-1β, IL-6, TNF-α, IFN-γ, IL-12, and IL-17; upregulation of IL-10; and inhibition of activation of the NF-κB and MAPK 16 Oxidative Medicine and Cellular Longevity YV [93] 17 Oxidative Medicine and Cellular Longevity pathways via suppression of phosphorylation of p65, ERK1/2, p38, JNK1/2, and Syk signaling molecules [102,103,106,107]. The isolated peptides and their molecular mechanisms of anti-intestinal inflammatory effects are presented in Table 5.
Dietary anti-intestinal inflammatory peptides are short chains of AAs that generally contain 2-10 AAs. The common AAs of these peptides are alanine, valine, leucine, serine, methionine, tyrosine, and phenylalanine [102,103,108]. Pepsin and pancreatin are the two most commonly employed

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Oxidative Medicine and Cellular Longevity proteolytic enzymes to produce anti-inflammatory peptides from various food proteins [103,109,110]. The TNF-αtreated Caco-2 cell is a widely used human intestinal cell model for the investigation of the anti-inflammatory property of dietary peptides. TNF-α activates the both NF-κB and MAPK signaling pathways in Caco-2 cells and thereby produces large quantities of proinflammatory mediators [110]. Excess and uncontrolled production of proinflammatory cytokines plays a vital role in the progression of intestinal inflammation [103]. Numerous recent studies reported that dietary BPs could inhibit the intestinal inflammation through the reduction of proinflammatory mediators. Four peptides, DEDT-QAMPFR, MLGATSL, SLSFASR, and MSYSAGF, isolated from egg white exerted anti-inflammatory activities in colitis mouse by inhibiting the production of TNF-α and IL-6 as well as reducing the mRNA-expressions TNF-α, IL-6, IL-17, IL-1β, IFN-γ, and MCP-1 [111]. Tripeptide VPY from soybean inhibited IL-8 secretion in Caco-2 cells [108]. The peptide was also found to decrease the mRNA expressions of inflammatory mediators TNF-α, IL-6, IL-1β, IFN-γ, and IL-17 in the peptide-treated mice colon. Dipeptides (CR, FL, HC, lLL, and MK) produced from egg white ovotransferrin, by using pepsin and trypsin hydrolysis, decreased the gene expression of TNF-α, IL-8, IL-6, IL-1β, and IL-12, while enhancing IL-10 expression, in Caco-2 cells [103]. Crucian carp-derived 178 peptides (<1500 Da) at 50, 100, and 150 μg/mL considerably reduced the secretion of TNF-α, IL-6, and IL-1β in IEC-6 small intestine cells as well as in dextran sodium sulfate-induced ulcerative colitis mice [106].
Several recent reports demonstrated the role of NF-κB in the pathogenesis of IBD [106]. Activation of NF-κB has been shown to be involved in the IBD patients [112]. The NF-κB and MAPK pathways are two vital proinflammatory signaling pathways that majorly regulate cellular inflammatory responses by secreting various cytokines after activation by various inflammatory stimuli [113]. Transcription factor NF-κB regulates the inflammatory responses by stimulating the production of various proinflammatory cytokines and chemokines. Phosphorylation of IκB by inflammatory stimuli (LPS and TNF-α) releases the NF-κB that migrates to the nucleus and activates expression of the numerous genes connected with inflammation [110,112]. The MAPK family contains three components such as p38 MAPK, ERK1/2, and JNK/SAPK. Phosphorylation of MAPK components stimulates the other kinases and migrates to the nucleus where they induce the transcription of several inflammatory genes and thereby enhance the secretion of proinflammatory mediators [110].

Conclusions and Further Perspectives
Numerous dietary peptides showed beneficial effects on redox balance and metabolic disorders (obesity, T2D, hypertension, and inflammation). Dietary peptides modulated several molecular mechanisms (e.g., Keap1-Nrf2-ARE signaling pathway in oxidative stress, PPAR-γ, C/EBP-α, SREBP1 pathway in obesity, IRS-1/PI3K/Akt and AMPK signaling pathways in T2D, ACE inhibition in hypertension, and MAPK in IBD) and thereby exerted positive effects in redox balance and metabolic disorders. Most of the studies are conducted using cell and animal models. Although substantial evidence from cell and animal investigations is available for the BPs as described in this review, scientific evidence from clinical studies is still meager. Hence, more clinical investigations are needed to get in-depth knowledge about the BP's efficacy, absorption, distribution, metabolism, excretion, toxicity, and effect on gut microbiome in the human body in the future. In the future, the benefits and risks of longterm and large-quantity consumption of BPs on human health need to be addressed. The interaction of BPs with other drugs in the human body has to be investigated comprehensively. Additionally, newer technologies are needed to produce BPs cost effectively from dietary sources. The BPs should be produced with consumer-acceptable taste, quality, and stability. Although there are several challenges for future growth, the dietary BPs could be used as health foods in the management/prevention of metabolic disorders (obesity, T2DM, hypertension, and inflammation) and oxidative stress-related diseases (e.g., cancer and IBD). We hope that the BP's industry will have a bright future in the coming years as people are increasingly aware of health benefits of dietary BPs.

Data Availability
No data were used to support this review article.

Conflicts of Interest
The authors declare no conflict of interest.