Molecular targets and mechanisms of bioactive peptides against metabolic syndromes

Shanshan Li ab, Ling Liu ab, Guoqing He b and Jianping Wu *bc
aCollege of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang Road, Hangzhou, 310058, China
bZJU-UA Joint Lab for Molecular Nutrition and Bioactive Peptides, College of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang Road, Hangzhou, 310058, China. E-mail:
cDepartment of Agricultural, Food and Nutritional Science, 4-10 Ag/For Building, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

Received 26th August 2017 , Accepted 8th November 2017

First published on 9th November 2017

Bioactive peptides are present in all living organisms and have critical roles ranging from protection against infection as the key element of innate immunity, regulating blood pressure and glucose levels, to reducing signs of ageing by killing senescent cells. Bioactive peptides are also encrypted within food protein sequences that can be released during proteolysis or food processing. These specific food protein fragments are reported to have potential for improving human health and preventing metabolic diseases through their impact on inflammation, blood pressure, obesity, and type-2 diabetes. This review mainly focuses on the molecular targets and the underlying mechanisms of bioactive peptides against various metabolic syndromes including inflammation, high blood pressure, obesity, and type-2 diabetes, to provide new insights and perspectives on the potential of bioactive peptides for management of metabolic syndromes.

1. Introduction

The prevalence of various chronic diseases, such as cardiovascular disease, chronic inflammation disease, cancers, diabetes, osteoporosis, etc., is increasing at an alarming rate worldwide.1 In addition to genetic factors, the onset and development of these chronic diseases are lifestyle-related, within which diet plays an important role.2–5 Many bioactive food components such as dietary fiber, omega-3 fatty acids, phytochemicals, and others are thought to have health-promoting properties.6 Food protein-derived bioactive peptides are increasingly recognized for their vast potential to improve human health and prevent chronic diseases via their impacts on various biochemical pathways.7

Numerous bioactive peptides have been identified from various kinds of food sources (as summarized in Table 1); soy, egg, milk, and fish are the most extensively studied proteins for preparing bioactive peptides. In addition to proteolysis, bioactive peptides can also be formed during food processing, in particular fermentation (i.e. His-His-Leu from soy paste,8 Met-Ala-Pro and Met-Lys-Pro from cheese9,10), or present naturally (i.e. lunasin, a 43-amino acid peptide identified from soy).11–13 Their diversified structures14 give bioactive peptides a broad scope of functions ranging from protection against infection as the key element of innate immunity, regulating blood pressure and glucose levels (Fig. 1), to reducing signs of ageing via killing senescent cells.15–17 The objective of this review is to provide an update on recent progress and perspectives regarding bioactive peptides against metabolic diseases and potential underlying mechanisms.

image file: c7fo01323j-f1.tif
Fig. 1 Proposed mechanisms of bioactive peptides on anti-inflammatory, anti-obesity, and anti-diabetic activities. (A) Bioactive peptides exert their anti-diabetic activity mainly via promoting insulin signaling and AMPK signaling pathways. They may act as an insulin mimic to activate the insulin receptor (IR) and insulin receptor substrate (IRS), followed by recruitment of PI3K to IRS; subsequently, activation of PI3K phosphorylates PKB/AKT via PDK1, which in turn promote glycogen synthesis through GSK3, or promote glucose uptake through GLUT4, or inhibit gluconeogenesis through FOXO1. However, whether the feedback from excessive activation of mTORC1 signaling by over-nutrition in obesity could inhibit PI3K through PTEN or the interaction between IR and IRS or not requires further investigation. Bioactive peptides also may stimulate glucose uptake through activating the AMPK pathway and then promoting GLUT4 translocation to the cell membrane. (B) Bioactive peptides exert their anti-obesity activity via inhibiting expression of nuclear transcript factor PPARγ. (C) The anti-inflammatory activity of bioactive peptides is via inhibiting NF-κB, MAPK, and/or JAK-STAT pathways by inhibiting: (1) the stimulus (TNFα, IL-1, or LPS) and their corresponding receptor (TNFR, IL-1R, TLR)-mediated phosphorylation of MAPKKK and the downstream MAPK leading to inhibition of transcription factors (i.e. c-Myc, ATF-2 and c-Jun). (2) Activation of the IκB kinase (IKK) complex, which in turn limits phosphorylation of IκBα and its subsequent polyubiquitination, proteasomal degradation of IκBα as well as release of homo/heterodimers p50 and RelA (p65), resulting in inhibition of transcription of target genes for inflammatory response. (3) Cytokine-mediated phosphorylation of JAK and subsequent phosphorylation of STAT that in turn form the STAT-STAT dimer and translocation dimer to the nucleus, which need to be further validated. This figure was drawn using the Pathway Builder Tool (Version 2.0, Abbreviations: AMPK, AMP-activated protein kinase; FOXO1, forkhead box O1; PTEN, phosphatase and tensin homologue; GLUT4, glucose transporter 4; GSK3, glycogen synthase kinase; IL-1, interleukin-1; LPS, lipopolysaccharides; IR, insulin receptor; IRS, IR substrate; JAK-STAT, janus kinase-signal transducer and activator of transcription; MAPK, mitogen activated protein kinase; MAPKKK, MAPK kinase kinase; PDK1, 3-phosphoinositide-dependent kinase; PI3K, phosphoinositide 3-kinase; PKB/AKT, protein kinase B; PPARγ, peroxisome proliferator-activated receptor γ; NF-κB, nuclear factor-kappa B; TNFα, tumor necrosis factor.
Table 1 Examples of food proteins derived bioactive peptides and their mechanisms of action
Bioactivities Protein source Peptide identifies Mechanisms of action Cells or animals Ref.
Anti-inflammation peptides
Animal source Bovine casein Val-Pro-Pro (VPP); Ile-Pro-Pro (IPP) • Inhibited NF-κB pathway • 3T3-F442A preadipocytes 25 and 29
• Reduced adipokine levels • apolipoprotein E knockout mice
Bovine casein VPP • Inhibited either MAPK-JNK pathway or renin–angiotensin system • Leukocyte and endothelium 26 and 30
• Adipocyte and macrophage interactions
Egg Ile-Arg-Trp (IRW) • Inhibited NF-κB pathway • Human endothelial cells 31 and 32
Botanical source Marine algae Peptide from P. yezoensis • Inhibited MAPK pathways • RAW 264.7 35
Plant source Soy Val-Pro-Tyr • Reduced expression of TNF-α, IL-6, IL-1, and IL-17 • Caco-2 33
Soy lunasin • Reduced expression of Th2 cytokine • BALB/c mice 34
Anti-hypertensive peptides
Plant sources Fermented soybean His-His-Leu • Decreased ACE activity in aorta • SHRs 8
Sour milk VPP, IPP • Increased Mas receptor/NO mediated vasodilation • Endothelial cells 47 and 48
• Reduced ACE activity • Isolated aortic rings
Animal sources Ovotransferrin Ile-Gln-Trp, Leu-Lys-Pro • Increased NO-mediated vasodilation • Endothelial cells 49–52
• Reduced vascular inflammation • SHRs
Ovotransferrin IRW • Increased NO-mediated vasodilation • SHRs 56
• Reduced vascular inflammation
• Up-regulated ACE2 expression
Bonito Leu-Lys-Pro-Asn-Met • A pro-type ACE inhibitory peptide • SHRs 53
Sardine muscle Val-Tyr • Blocked the angiotensin II type 1 (AT1) receptor • Mild hypertensive subjects 55
Lactoferrin Leu-Ile-Trp-Lys-Leu, Arg-Pro-Tyr-Leu • Ang II type 1 (AT1) receptor antagonist • Rabbits 16
• Inhibition of angiotensin II-induced vasoconstriction
• ACE inhibition
Anti-obesity peptides
Animal sources Cheese whey Casein glycomacropeptides (GMP) • Inhibited proliferation and differentiation • Primary rat preadipocytes 72
Tuna Asp-Ile-Val-Asp-Lys-Ile-Glu-Ile • Inhibited C/EBPs and PPARγ expression • 3T3-L1 preadipocytes 73
• Inhibited adipocyte differentiation
Milk IPP and VPP • Upregulated PPARγ and adiponectin • 3T3-F442A preadipocytes 25
• Anti-inflammation
milk κ-casein Caseinomacropeptide • Stimulated cholecystokinin release • Male wistar rats 80
Plant sources Black soy Isoflavone-free peptide mixture (BSP) • Decreased appetite • Diet-induced obese (DIO) mice 77
• Activated AMPK and STAT3 phosphorylation • C2C12 myocytes
Soy protein β-conglycinin Val-Arg-Ile-Arg-Leu-Leu-Gln-Arg-Phe-Asn-Lys-Arg-Ser • Suppressed appetite • Male Sprague-Dawley rats 78
• Stimulated cholecystokinin (CCK) release
Anti-type 2 diabetes peptides
Plant sources Soybean Aglycin • Maintained IR and IRS1 expression • Type 2 diabetic mice 106
• Elevated phosphorylation of IR, IRS1, and AKT
• Increased translocation of GLUT4 to cell membrane
Soybean Trp-His • Activated AMPK in an insulin-independent way • L6 muscle cells 118 and 119
• Increased GLUT4 translocation to plasma membrane
Animal sources Bovine milk casein glycomacropeptide Ile-Pro-Pro-Lys-Lys-Asn-Gln-Asp-Lys-Thr-Glu • Increased phosphorylation of Akt and AMPK • HepG2 cells 117
• Activated both insulin and AMPK signaling pathways

2 Bioactive peptides on human health

2.1 Bioactive peptides against inflammation

Inflammation is one of the body's defensive responses to stimuli, such as pathogen invasion, injury, tissue damage, or infection.17 Controlled inflammation is essential for normal body function; however, persistent, chronic inflammation at a low intensity is a causative factor for a wide range of chronic diseases such as obesity, type 2 diabetes, asthma, osteoporosis, inflammatory bowel disease, cancer, central nervous system disease, and cardiovascular disease.17,18 Chronic inflammation is mediated mainly by the nuclear factor-kappa B (NF-κB) pathway, the mitogen activated protein kinase-c-jun N-terminal kinase (MAPK-JNK) pathway, and the janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway, usually under the stimuli of tumor necrosis factor α (TNFα), interleukin-1 (IL-1), and lipopolysaccharides (LPS).19–21 In the canonical NF-κB signaling pathway, the stimulus (TNFα, IL-1, or LPS) binds to its respective receptor (tumor necrosis factor receptor, TNFR, interleukin-1 receptor, IL-1R, or Toll-like receptors, TLRs) leading to activation of the IκB kinase (IKK) complex in the cytoplasm, which in turn phosphorylates IκBα followed by its subsequent polyubiquitination, proteasomal degradation of IκBα, and then release of the homo/heterodimers p50 and RelA (also known asp65); after translocation to the nucleus, these dimers activate transcription of target genes for the inflammatory response.22 In the MAPK pathway, phosphorylation and activation of MAPK kinase kinase (MAPKKK) leads to phosphorylation of MAPK kinase (MAPKK), which in turn mediates phosphorylation of MAPKs (including extracellular signal-regulated kinase, ERK, JNK, and p38). Activated MAPKs then phosphorylate a variety of transcription factors (i.e. c-Myc, ATF-2, and c-Jun) leading to diverse cellular activities such as cell proliferation, migration, differentiation, and survival for the ERK1/2 signaling pathway; autophagy, apoptosis stress, and inflammation for the JNK and p38 signaling pathways.23 Similarly, when cytokines bind to their corresponding receptors, JAKs are activated leading to phosphorylation of STAT and formation of STAT-STAT dimer. Subsequently, the dimer is translocated to the nucleus and regulates specific gene expression by directly binding to DNA resulting in the inflammatory response.24

Several food-derived bioactive peptides are reported to have anti-inflammatory activities both in in vitro cellular systems and in in vivo animal models.25–28 Milk-derived tripeptides Val-Pro-Pro (VPP) and Ile-Pro-Pro (IPP) showed anti-inflammation activity through inhibition of the NF-κB pathway, as well as increased release of adiponectin in adipocytes,25 contributing to their protective roles against atherosclerotic development in apolipoprotein E knockout mice.29 Furthermore, VPP's anti-inflammatory effect was reported to be mediated through inhibition of either the MAPK-JNK pathway in leukocyte and endothelium or the renin–angiotensin system (RAS) in adipocyte and macrophage interactions.26,30 Egg-derived tripeptide Ile-Arg-Trp (IRW)'s anti-inflammatory activity also acted through the NF-κB pathway.31,32 Other anti-inflammatory peptides are Val-Pro-Tyr (VPY) and lunasin from soy, and peptide from P. yezoensis (PPY1, a marine algae).33–35 Further information on anti-inflammatory peptides has been recently reviewed.17,25 Given the underlying role of chronic inflammation in many types of chronic diseases, anti-inflammatory peptides identified from food proteins may have a potential role against a wide range of chronic diseases. A number of anti-inflammatory functional foods have been marketed successfully (i.e. lactoferrin from milk whey protein and sesquiterpenoid from Inula plants), providing insight into developing novel peptides against chronic diseases.36–39 However, there is lack of understanding regarding specific peptide receptors and the associated signaling pathways that mediate their anti-inflammation activities; furthermore, crosstalk among RAS components and these signaling pathways requires further elucidation.

2.2 Bioactive peptides against high blood pressure

Most bioactive peptides have been characterized as inhibitors of angiotensin converting enzyme (ACE),8,40–44 a key enzyme for regulation of blood pressure, water, and salt balance in RAS.45 ACE is responsible for conversion of inactive decapeptide angiotensin I (Ang I) into a vasoconstrictive octapeptide, angiotensin II (Ang II), a major component in adverse blood pressure regulation. Ang II has two main receptors, angiotensin type 1 receptor (AT1) and angiotensin type 2 receptor (AT2).46 Binding of angiotensin II to the AT1 receptor causes vasoconstriction and high blood pressure. RAS has been extensively explored as the target for development of antihypertensive drugs.

Well-studied ACE inhibitory peptides include Val-Pro-Pro and Ile-Pro-Pro from sour milk,47–49 Ile-Arg-Trp, Ile-Gln-Trp, and Leu-Lys-Pro from egg protein ovotransferrin,50–52 and Leu-Lys-Pro-Asn-Met and Val-Tyr from fish.53–55 Although most of these peptides have been identified as ACE inhibitors, their mechanisms of action in vivo are more complicated than initially expected. It was revealed that lactoferrin-derived peptides Leu-Ile-Trp-Lys-Leu and Arg-Pro-Tyr-Leu can selectively inhibit angiotensin II-induced vasoconstriction by blocking AT1 receptors.16 Apart from binding to the AT1 receptor, Ang II can also be converted into Ang (1–7) by ACE2, a homologous enzyme to ACE; Ang-(1–7) binds to the G-protein-coupled receptor (GPCR)-MAS, showing a vasodilatory effect.45 Our group demonstrated for the first time that ACE2 is involved with the blood pressure-lowering activity of the ovotransferrin-derived ACE inhibitory peptide Ile-Arg-Trp in SHRs through upregulation of ACE2 gene expression.56 Molecular targets,57 as well as progress on understanding the mechanisms of ACE inhibitory peptides, have been recently reviewed.57,58 Given the complexity of the pathophysiology of hypertension, it seems that many ACE inhibitory peptides function beyond ACE inhibition in vivo.

2.3 Bioactive peptides against obesity

According to the World Health Organization (WHO), an individual with a body mass index (BMI, weight divided by height square) of 30.0 kg m−2 or greater is obese;59 BMI ranging from 25.0 to 29.9 kg m−2 is classified as overweight. Obese individuals often exhibit abdominal and upper body obesity and have larger waist circumferences (men ≥102 cm, women ≥88 cm); abdominal obesity is closely associated with disorders such as hyperglycemia, type 2 diabetes, hypertension, dyslipidemia, gallbladder disease, coronary heart disease (CHD), and cancer.60–62 Obesity develops when energy intake exceeds energy expenditure resulting from reduced physical exercise and sedentary lifestyle. The energy imbalance contributes to adipose dysfunction and development of obesity and relevant complications. Adipocyte dysfunction is manifest by an excessively increased number and size of adipocytes (hyperplasia and hypertrophy).63 Adipogenesis is regulated by peroxisome proliferator-activated receptor γ (PPARγ) (encoded by PPARG) and CCAAT/enhancer-binding protein α/β/δ (C/EBPα/β/δ);64 activation of PPARγ promotes differentiation of preadipocytes into mature adipocytes.65 Therefore, a potential target for obesity control is reducing adipogenesis by inhibiting the activity of PPARγ or reducing preadipocyte differentiation.66 A synthetic PPARγ antagonist, bisphenol A diglycidyl ether, has been demonstrated to inhibit adipogenesis in preadipocytes 3T3-L1 and 3T3-F442A, which may exert some positive effects in treatment of obesity.67 Other synthetic PPARγ antagonists, such as HX531,68 GW-9962, and LG100641,69 have also been investigated for their potential uses against diet-induced obesity.

There is interest in exploring food components such as peptides or protein hydrolysates as potential alternatives for prevention of obesity. Whey protein isolate has been reported to inhibit expression of PPARγ and lipid accumulation in 3T3-L1 preadipocytes, and also to significantly reduce weight gain in rats fed high-fat diets.70 A soy protein hydrolysate71 and milk casein glycomacropeptides (GMP)72 have also been demonstrated to inhibit fat accumulation in primary and 3T3-L1 adipocytes, respectively. Recently, a peptide Asp-Ile-Val-Asp-Lys-Ile-Glu-Ile derived from boiled tuna was found to decrease glucose uptake and triglyceride (TG) levels, and inhibit C/EBPs and PPARγ levels in 3T3-L1 cells, indicating its potential use against obesity.73

As an endocrine agent, adipocytes secrete a variety of adipocytokines such as leptin, resistin, and adiponectin, which are important regulators of relevant metabolism.65 It has been reported that the plasma concentration of adiponectin in both men and women correlates negatively with BMI74 and visceral adiposity.75 Even though the specific modes of action for PPARγ activator thiazolidinediones (TZDs) such as rosiglitazone to elicit insulin sensitivity and weight gain remain under debate, moderate activation of PPARγ leading to increased adiponectin is considered to be a potential strategy for treating excessive visceral adiposity.61 Accordingly, the aforementioned lactotripeptides Val-Pro-Pro and Ile-Pro-Pro can also induce beneficial adipocyte differentiation, proper upregulation of PPARγ, and secretion of the protective lipid adipokine adiponectin in 3T3-F442A cells,25 indicating their potential role against obesity. A soy protein diet has been reported to increase adiponectin mRNA level in adipose tissue and adiponectin plasma concentration in obese mice,76 which may exert some positive effects to improve obesity. Yamauchi et al.68 indicated that either by reducing adipocyte differentiation and apoptosis or by upregulation of adiponectin, both moderate reduction of PPARγ activity and PPARγ activation could ameliorate insulin resistance, associated with decreased triglyceride content in muscle or liver and prevention of adipocyte hypertrophy. Thus, a crosstalk and coordinated role may exist between PPARγ and the various adipocytokines such as adiponectin to promote insulin sensitivity.

Apart from modulation of adipose biology to lower adiposity, bioactive peptides could also regulate energy balance via increasing satiety, reducing appetite, and increasing energy expenditure to lower food intake and weight gain. A black soybean protein-derived isoflavone-free peptide mixture was reported to reduce body weight and ameliorate hypercholesterolemia in high fat diet-induced obese rats via decreasing appetite and regulating whole-body energy balance.77 A peptide Val-Arg-Ile-Arg-Leu-Leu-Gln-Arg-Phe-Asn-Lys-Arg-Ser derived from soy protein β-conglycinin was demonstrated to decrease body fat and food intake via stimulated release of cholecystokinin (CCK) and thus suppression of appetite.78 Milk casein-derived casomorphins with opioid-like activities can interact with gastric opioid receptors to slow gastrointestinal motility to regulate appetite.79 Other milk proteins such as caseinoglycomacropeptide80 and whey protein81 also have been found to stimulate CCK release and activate the glucagon-like peptide-1(GLP-1) pathway, respectively, to increase satiety while suppressing appetite. Protein is the most satiating macronutrient. A high-protein diet particularly enriched with β-lactoglobulin has been reported to reduce body weight;82 it is not known, however, whether peptides released during gastrointestinal digestion might be responsible for the benefit.

2.4 Bioactive peptides against type-2 diabetes (T2D)

According to Diabetes Canada, “Type 2 diabetes is a disease in which your pancreas does not produce enough insulin, or your body does not properly use the insulin it makes” ( Type 2 diabetes (T2D) affects 422 million adults worldwide and this number is expected to reach 592 million by 2035.83,84 In addition to genetic factors, modern sedentary lifestyle and overnutrition are closely associated with the onset and development of T2D.85 As discussed above, obesity is a key contributor to the development of T2D because of prolonged and chronic exposure to excessive nutrition.85 In normal glucose homeostasis, an increase in plasma glucose concentration after a meal stimulates insulin release, thus enhancing uptake of glucose by the liver, gut, and peripheral tissues (i.e. muscle), and also suppressing hepatic glucose production; however, this dynamic balance is disturbed in T2D.86 Pathogenesis of T2D is characterized by impaired insulin secretion and/or insulin resistance.87 Impaired insulin secretion resulting from β-cell loss and dysfunction may occur because of amyloid deposition in pancreatic islets, β-cell differentiation, glucotoxicity, and lipotoxicity,87 while insulin resistance is an impaired response of target organs (adipose tissue, skeletal muscle, and liver) to insulin-mediated actions, such as inadequate suppression of hepatic glucose production, and reduced insulin-mediated glucose uptake and utilization in skeletal muscle and adipose tissue.88,89

The cellular and molecular mechanisms of T2D are associated with insulin signaling, mammalian target of rapamycin (mTOR), and AMP-activated protein kinase (AMPK) signaling pathways.90,91 Under physiological conditions, insulin stimulates phosphorylation of the insulin receptor (IR) and IR substrate (IRS), followed by recruitment of phosphoinositide 3-kinase (PI3K) to IRS. Subsequently, activation of PI3K phosphorylates protein kinase B (PKB/AKT) and atypical protein kinase C isoforms (aPKCs) via 3-phosphoinositide-dependent kinase (PDK1).92 However, in a state of chronic hyperglycemia, a defect of IR or post-receptor (i.e. IRS1, IRS2, and PI3K) may lead to a series of metabolic disorders including (1) decreased glucose transporter 4 (GLUT4)-dependent glucose uptake,90 (2) reduced glycogen synthesis through glycogen synthase kinase (GSK3) or salt-inducible kinase 2 (SIK2),90 (3) increased gluconeogenesis through forkhead box O1 (FOXO1),90 and (4) limited protein synthesis through mTORC1-S6K1 axis in diabetes.90 Furthermore, increased activity of extracellular signal-regulated kinase (ERK) in adipocytes and adipose tissue is associated with T2D pathogenesis.93 Feedback to the insulin pathway also contributes to regulation of glucose homeostasis. Albeit AKT-mTORC1 signaling is limited in diabetes, excessive activation of mTORC1 signaling by over-supplied nutrients in obesity could (1) inhibit IRS and its interaction with IR through growth factor receptor-bound protein 10 (GRB10) phosphorylation;90,94,95 and (2) inhibit phosphatidylinositol 3,4,5-trisphosphate (PIP3) via hypoxia inducible factor 1α (HIF1α) and phosphatase and tensin homologue (PTEN).90,96

It appears that a number of peptides (including proteins) and phytochemicals can enhance insulin sensitivity via multiple signaling pathways.97,98 For instance, Momordica charantia (bitter melon, a traditional medicine used for treatment of T2D) alone and in its powder form can reduce insulin resistance through suppression of NF-κB and JNK/p38 MAPKs pathways in liver, muscle, and adipose tissues in obese and diabetic rats.99,100 A cucurbitane-type triterpenoid enhanced the insulin signaling pathway, GLUT4 expression, and MAPK activation.101 Momordicosides increase glucose uptake through stimulating translocation of GLUT4 to the cell membrane.102Momordica charantia insulin receptor-binding protein (mcIRBP)-derived peptides, mcIRBP-19 and mcIRBP-68, enhance insulin binding to IR, stimulating phosphorylation of AKT and expression of GLUT4, resulting in increased glucose uptake in adipocytes and glucose clearance in diabetic mice.103–105 Soy protein also shows anti-diabetic activities. Aglycin isolated from soybean can improve glucose uptake via elevated phosphorylation of IR, IRS1, and AKT, as well as translocation of GLUT4 to cell membrane.106 Recent progress has been reviewed on the potential of anti-diabetic plants on improving insulin sensitivities.107 However, the link between food-derived bioactive peptides and mTOR signaling, as well as their feedback to insulin signaling, has not been explored.

In addition, AMPK activation was recently proposed as a potential therapeutic target for prevention and amelioration of insulin resistance and T2D.91,108 A Ca2+-dependent pathway mediated by Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ) and a low energy level (high AMP/ATP or ADP/ATP ratio) mediated by serine/threonine kinase II (LKB1), phosphorylated and then activated AMPK.109–111 Activation of AMPK phosphorylates AS160 (a 160 kDa Akt substrate), resulting in reduced activity of Rab guanosine triphosphatase (GTPase) but increased GTP load on the storage sites of GLUT4, therefore promoting GLUT4 translocation to cell membrane, which ultimately stimulates glucose uptake in skeletal muscle and adipocytes.112–116 Some bioactive peptides or protein hydrolysates have been demonstrated to improve insulin sensitivity through activating insulin and/or AMPK signaling pathways. For instance, a peptide Ile-Pro-Pro-Lys-Lys-Asn-Gln-Asp-Lys-Thr-Glu derived from casein glycomacropeptide prevented high glucose-induced insulin resistance in hepG2 cells by activating both insulin and AMPK signaling pathways via phosphorylation of Akt and AMPK.117 A dipeptide Trp-His (WH) and a soybean hydrolysate fraction prepared by electrodialysis enhanced glucose uptake in L6 muscle cells through activation of AMPK in an insulin-independent way and enhanced GLUT4 translocation to the plasma membrane, respectively.118,119 Soy β-conglycinin and soybean peptides improved glucose uptake and insulin sensitivity through activation of AMPK in skeletal muscle in Goto-Kakizaki rats.118,120,121

For T2D patients, a normal plasma glucose level either in the fasting or postprandial states is of great importance. Postprandial plasma glucose level is considered to be a better marker of glycemic control than fasting plasma glucose levels.122 Postprandial plasma level is regulated mainly by α-glucosidase and ubiquitous enzyme dipeptidyl peptidase (DPP)-IV. Inhibitors of α-glucosidase and DPP-IV have been explored extensively for potential for management of T2D.122 Research progress in this field has been reviewed.123

Given the complex pathophysiology of T2D, there are multiple targets for bioactive peptides, such as enhancing insulin secretion, potentiating insulin and/or AMPK signaling, and/or inhibiting inflammation, and improved GLUT4 expression and translocation to membrane.87,93,117,124–127 In addition to these targets, the crosstalk among insulin, AMPK, and mTOR signaling pathway is also a key treatment aspect for T2D management in response to anti-diabetes peptides.

3 Conclusions

Bioactive peptides have beneficial effects on blood pressure, inflammation, obesity, and T2D, indicating great promise as functional foods/nutraceuticals against metabolic syndromes. Bioactive peptides such as Leu-Lys-Pro-Asn-Met and Val-Tyr have been commercialized.54 However, use of bioactive peptides has been criticized because their susceptibility to gastrointestinal proteases and peptidase could render them inactive prior to reaching sites of action, despite many bioactive peptides being proved bioavailable and bioactive in vivo. Recent progress on characterization of various target pathways indicates that bioactive peptides might function locally through receptors on cell membranes. This signifies the importance of further research to explore their mechanisms of action in vivo. Integration of advanced omics technologies with bioinformatic analysis is expected to shed light on the novel pathways of action of the peptides in vivo. It would also be worthwhile studying the effects of bioactive peptides on “master regulators” or “central targets” (i.e. MAPK) and “specific targets” (i.e. IR for T2D, PPAR for obesity) to understand the multifunctional nature of many bioactive peptides and the crosstalk that may be involved. There is an urgent need for translational research so that beneficial bioactive peptides can be made commercially available.

Conflicts of interest

No conflicts of interest.


This work was supported by National Key R&D Program of China (2017YFD0400600) and Natural Sciences and Engineering Research Council of Canada.


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