Antidiabetic Effects and Underlying Mechanisms of Food-Derived Bioactive Peptides

Abstract

Type 2 diabetes mellitus (T2DM), characterized by progressive insulin secretion defect based on insulin resistance, is one of the leading public health problems with high morbidity and mortality rates. Uncontrolled hyperglycemia, which becomes a more severe indication with obesity in T2DM patients, increases the risk of cardiovascular disease, kidney damage, and retinal disorder. Effective diabetes treatment is possible with a comprehensive approach that includes controlling blood glucose levels, improving pancreatic β cell functions, and supporting insulin sensitivity through body weight management, nutritional therapy, and drug therapy. At this point, food-derived bioactive peptides and protein hydrolysates, which draw attention with their structural similarity to regulatory peptides in human metabolism, have great potential in treating T2DM and regulating glucose metabolism. Various glucoregulatory properties of bioactive peptides come to the fore through antidiabetic mechanisms such as the digestion of carbohydrates, the release of intestinal hormones, insulin function and secretion, glucose uptake, and adipose tissue modification. This review aims to evaluate the roles of food-derived bioactive peptides and protein hydrolysates in controlling glycemia and insulin sensitivity and their antidiabetic mechanisms of action and to examine the difficulties and opportunities related to the acquisition and research processes.

INTRODUCTION

Diabetes mellitus is a chronic metabolic disease identified by persistent hyperglycemia accompanied by a series of metabolic dysfunctions.¹ It is a significant public health problem worldwide, causing high mortality rates and reduced life expectancy.² Type 1 diabetes mellitus (insulin-dependent diabetes, T1DM) and type 2 diabetes mellitus (noninsulin-dependent diabetes, T2DM) are two basic types of diabetes. In T1DM, absolute insulin deficiency occurs as a result of the destruction of the beta cells of the pancreas due to ongoing autoimmune or nonautoimmune reasons.¹ T2DM, which accounts for 90% of all diabetic patients, is characterized by insulin resistance and inadequate insulin secretion.² The global diabetes prevalence in 2021 was estimated to be 10.5% (536.6 million) in 2021, and it was predicted that this rate would rise to 12.2% (783.2 million) in 2045.³

Uncontrolled hyperglycemia, which is the primary issue with type 2 diabetes and obesity, increases the risk of cardiovascular disease, kidney damage, and retinal disease. It has been reported that dementia, depression, sexual dysfunction, and lower extremity amputations are the main complications of chronic diabetes.²

Effective diabetes treatment necessitates a comprehensive approach that includes controlling blood glucose levels, enhancing pancreatic β cell function, and promoting insulin sensitivity through weight management, medical nutritional therapy, and medication.^2,4

Although drug therapy is successful in the treatment of diabetes, due to the side effects of drugs (antidiabetic peptides) such as hypoglycemia, edema, weight gain, gastrointestinal problems, lactic acidosis, liver toxicity, and the high cost of drug treatment, more cost-effective and reliable alternatives have begun to be investigated.⁵ At this point, food-derived bioactive compounds are attracting attention as there is a widespread view globally that many chronic diseases, including diabetes, can be controlled by changing lifestyle and nutritional habits.^6,7 Concordantly, protein hydrolysates and bioactive peptides have great potential as adjuvant agents in the treatment of T2DM and the regulation of glucose metabolism.⁸ Dietary bioactive peptides, which can be obtained from different animal and plant tissues, as well as edible insects, have increasingly gained recognition due to their potential positive effects in managing many chronic diseases.²

The review focuses on the roles of food-derived bioactive peptides and protein hydrolysates in regulating blood sugar and insulin sensitivity and their antidiabetic fundamental mechanisms of action, challenges, and opportunities related to their acquisition and research processes.

BIOACTIVE PEPTIDES

Proteins, one of the essential macromolecules in the human diet, consist of peptide bonds formed between an amino acid’s amino group and another amino acid’s carboxyl group in the same chain.⁹ Peptides, whose origins date back to before humanity, exist in all organisms.¹⁰ Since the end of the 20th century, scientists have emphasized the hydrolysis of proteins in foods because it has begun to be proven that some peptides that makeup proteins are bioactive and that they provide beneficial effects in the organism.^9,10

The fact that the peptides in the structure of proteins are ordinarily inactive prevents them from becoming bioactive by interacting with some other molecules and preventing their functionality.^8,9 These peptides can be liberated by (1) gastrointestinal digestion, (2) food processing, (3) microbial fermentation, and (4) proteases originating from animals, plants, or microorganisms.^2,6,8 In the human gastrointestinal tract, the digestion of proteins by proteases forms peptides that benefit health. However, the specificities of the food-derived proteases used in the studies included in this review are different. For instance, other peptides are obtained from the same substrate using various proteases, showing different functional properties.⁹ In addition, after the breakdown of peptide bonds during hydrolysis, peptide chains of different sizes are formed.¹¹ In this connection, the amino acid composition, sequence, and length/molecular weight of the peptides determine the functionality of the peptides.^8,9 Protein hydrolysate refers to the mixture of peptides that form the protein after hydrolysis.⁹

Bioactive peptides are protein fragments consisting of 2–50 amino acids embedded/encoded in the primary structure of dietary proteins. They determine the physiological impacts of food-derived proteins with their positive effects on body functions and/or human health beyond their nutritional value.^2,8 These fragments may show similar or completely different physiological effects compared with the parent protein.⁸ The physiological impacts of bioactive peptides include antihypertensive, antioxidant, anti-inflammatory, antiatherogenic, antidiabetic, antiobesity, anticancer, antithrombotic, antimicrobial, immunomodulatory, neuromodulatory, and mineral-binding properties.^2,7,8

Approximately 5000 bioactive peptides have been identified in the Bioactive Peptide Database (BioPepDB), which play a role in signal transduction regulation by binding to specific receptors on the cell surface.¹² Food-derived bioactive peptides, which draw attention with their structural similarity to regulatory peptides in human metabolism, can both interact with some enzymes and play a role as endogenous signaling molecules in some intracellular processes. With this aspect, bioactive peptides are presented as an essential resource in the design of unique drug molecules. In addition, their high pharmacological effect potential and safety profile in humans enable bioactive peptides to be investigated as important pharmaceutical agents.^8,9 Apart from their nutritional value, bioactive peptides are also considered functional nutrients with potential physiological effects on health or are seen as a crucial bioactive component to be used in functional food production.⁸ Various protein-rich foods of animal and plant origin are important sources of bioactive peptides. Besides eggs and milk, a variety of fish are the main foods used to acquire bioactive peptides from animal-based foods. In contrast, the main foods used to obtain bioactive peptides of plant origin are legumes, grains, pseudocereals, and oilseeds.^2,6,8,10

ANTIDIABETIC EFFECTS AND MECHANISMS OF FOOD-DERIVED BIOACTIVE PEPTIDES

Food-derived bioactive peptides manifest their antidiabetic activities through different mechanisms of action, depending on the sources from which they are obtained and their structural properties (amino acid composition, sequence, and molecular weight).^8,9

Dietary proteins have a significant satiety effect through stimulating intestinal hormone secretion, increased energy expenditure, and promotion of gluconeogenesis, whereas protein intake positively affects glycemia, insulin secretion, and body fatness. Although the positive outcomes of protein intake regarding energy homeostasis and glycemic control are mainly based on amino acid composition, the role of food-derived bioactive peptides as potential molecules responsible for these positive effects has been established over the years.^2,13

Various glucoregulatory properties of bioactive peptides become prominent through different mechanisms such as digestion of carbohydrates, the release of gut hormones, insulin secretion and function, glucose uptake, and adipose tissue modification.^2,10

α-Glucosidase and α-amylase inhibitory peptides

α-Glycosidase and α-amylase enzymes, which are involved in the digestion of carbohydrates, hydrolyze complex carbohydrates into monosaccharides that can be transported through the intestinal mucosa in the small intestines. While α-amylase breaks down long-chain carbohydrates, α-glycosidase, located on the brush border of the enterocytes lining the intestinal villi, turns into disaccharides and oligosaccharides into absorbable monosaccharides. α-Glucosidase and α-amylase inhibitors generally restrict the absorption of monosaccharides and prolong the digestion time of carbohydrates.^2,8,10 This situation could be helpful in reducing postprandial hyperglycemia. In contrast, the long-term use of these inhibitors could be beneficial in lowering fasting blood glucose levels and, therefore, in treating T2DM. Bloating, abdominal cramping, vomiting, and diarrhea are a few of the significant side effects of these inhibitors, whereas it is believed that natural α-glycosidase inhibitors will not cause these effects.^2,10

α-Amylase inhibitor peptides retain the enzyme’s catalytic and substrate binding parts, preventing substrates (carbohydrate polymers) from binding to the enzyme and being hydrolyzed. Moreover, α-amylase inhibitor peptides can bind to starch and avoid starch digestion. As mentioned above, the effect of α-glucosidase inhibitor peptides is based on the hydrophobic interactions of these compounds with the active site of the enzyme.^2,14 So α-amylase and α-glucosidase inhibitory peptides show antidiabetic effects by decreasing postprandial blood glucose levels and increasing plasma insulin concentrations in normal and diabetic patients.¹⁵

Cow’s milk, camel’s milk, eggs, chickpeas, black beans, soybeans, quinoa, amaranth, cumin seeds, date seeds, and walnuts are among some foods from which α-glycosidase and/or α-amylase inhibitor peptides are obtained^{16

–26} (Table 1).

Table 1.

Some Dietary Bioactive Peptides with Antidiabetic Properties (Mostly in Vitro Models)

Source	Enzymes used/treatment	Substrate	Identified peptides (amino acid sequence)	Mechanism of action	References
Cow milk and camel milk	Alcalase, pronase, and simulated gastrointestinal digestion	Casein	FLWPEYGAL, ACGP, DGALHPPL	α-amylase inhibition in an in vitro model	25
Cow milk and camel milk	Alcalase, pronase, and simulated gastrointestinal digestion	Casein	LPTGWLM, MFE, GPAHCLL	α-glycosidase inhibition in an in vitro model	25
Goat milk	Flavourzyme® (peptidase)	Casein	SDIPNPIGSE, NPWDQVKR, SLSSSEESITH, QEPVLGPVRGPFP	Amelioration of insulin resistance in hepatic HepG2 cells in an in vitro model	27
Goat milk	Trypsin and chymotrypsin	Casein	MHQPPQPL, SPTVMFPPQSVL, VMFPPQSVL, INNQFLPYPY, AWPQYL	DPP-4 inhibition in an in vitro model	28
Donkey milk	Pepsin and pancreatin			Improvement in viability of damaged β cells in an in vitro modelIn an in vivo model, reduction in blood glucose levels and improvement of insulin resistance in diabetic Wistar rats	29
Camel milk	Pepsin	Whey	HLPGRG, QNVLPLH, PLMLP	DPP-4 inhibition in an in vitro and in a silico model	22
Camel milk	Pepsin	Whey	PAGNFLMNGLMHR, PAVACCLPPLPCHM, MLPLMLPFTMGY, PAGNFLPPVAAAPVM, CCGM, MFE, FCCLGPVPP	Inhibition of α-amylase and α-glucosidase in an in vitro and in a silico model	22
Egg	Alkalase	Egg white protein	RVPSLM, TPSPR	α-Glycosidase inhibition in an in vitro model	16
Egg	Pepsin	Egg yolk protein	YIEAVNKVSPRAGQF, YINQMPQKSRE, YINQMPQKSREA, VTGRFAGHPAAQ	DPP-4 inhibition in an in vitro model	30
Sardine	Subtilisin, trypsin, Flavourzyme® (peptidase)	Sardine muscle protein	Peptides <1400 Dalton, YACSVR	DPP-4 inhibition in an in vitro model	31
Salmon	Alkalase, Flavourzyme® (peptidase)	Salmon skin gelatin and protein in trimmings of salmon		Supporting insulin release from pancreatic BRIN-BD11 cells and GLP-1 secretion from enteroendocrine GLUTag cells in an in vitro modelDPP-4 inhibition in an in vitro model	32
Antarctic krill	Corolase PP		KVEPLP, PAL	DPP-4 inhibition in an in vitro model	33
Chickpea	Pepsin, pancreatin	Chickpea protein	PHPATSGGGL, YVDGSGTPLT, SPQSPPFATPLW, YVDGSGTPLT	Inhibition of DPP-4, α-amylase, and α-glycosidase in an in vitro model	23
Chickpea	Bromelain	Chickpea protein	GKAAPGSGGGTKA, KMTAGSGVTGLTQGASLAGSGAPSPLF		23
Black bean	Germination, alcalase hydrolysis, and simulated gastrointestinal digestion		RGPLVNPDPKPFL	Inhibition of DPP-4 and α-amylase in an in vitro model	17
Oat	Pepsin, trypsin, pancreatin, and alcalase	Globulin	LQAFEPLR, EFLLAGNNK	DPP-4 inhibition in an in vitro model	34
Soybean	Germination, pepsin, and pancreatin			Inhibition of DPP-4, α-amylase, and α-glycosidase in an in vitro model	24
Quinoa	Pepsin, pancreatin, simulated gastrointestinal digestion	11S globulin B (seed storage proteins)	GEHGSDGNV	Inhibition of DPP-4 and α-glycosidase in an in vitro model	18
			IQAEGGLT	Inhibition of DPP-4 and α-glycosidase in an in vitro model
			DKKYPK	α-glycosidase inhibition in an in vitro model
	Papain, ficin, bromelain	Seed storage proteins	MAF, NMF, HPF, MCG	DPP-4 inhibition in an in vitro model	35
Amaranth	Pepsin and pancreatin		FLISCLL, SVFDEELS, DFIILE, NRPET, HVIKPPS	Inhibition of DPP-4 and α-amylase in an in vitro model	19
Walnut	Alkalase		LPLLR	Inhibition of α-amylase and α-glucosidase in an in vitro modelIn an in vitro model, improvement in hepatic insulin resistance, increase in glycogen synthesis and glucose uptake, and decrease in gluconeogenesis through activation of IRS-1/PI3K/Akt and AMPK signaling pathways in hepatic HepG2 cells with insulin resistance.	36
Cumin seed	Protamex® (endoprotease)	Cumin seed protein	FRSKLLSDGAAAAKGALLPQYW	α-amylase inhibition in an in vitro model	20
Date seed	Alcalase, bromelain, papain, and protease	Date seed protein		Inhibition of DPP-4 and α-amylase in an in vitro model	21

DPP-4, dipeptidyl peptidase-IV; GLP-1, glucagon like peptide-1; IRS-1, insulin receptor substrate-1; PI3K, phosphatidylinositol 3-kinase; Akt, protein kinase B; AMPK, AMP-activated protein kinase.

Insulin mimetic peptides

A glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) and the best-known incretins are primarily released from intestinal K and L cells, respectively, following the intake of nutrients. These peptidic hormones regulate the postprandial insulin secretion.^2,10 In patients with type 2 diabetes, the destruction of pancreatic islet cells causes the effect of incretins to decrease. In patients with diabetes, administration of incretin has been shown to increase β cell proliferation and provide an insulinotropic effect without causing hypoglycemia.²

Increasing endogenous GLP-1 release through dietary management is a promising alternative treatment strategy to control hyperglycemia in T2DM. In humans, amino acids, especially glutamine, stimulate GLP-1 secretion, whereas it increases GLP-1, GIP, and insulin concentrations.² Salmon, yogurt, cheese, blue whiting, and wheat are among some foods that support GLP-1 secretion in in vitro and/or in vivo models^32,37
–39 (Tables 1 and 2).

Dipeptidyl peptidase-IV inhibitor peptides

Activation of dipeptidyl peptidase-IV (DPP-4) causes rapid degradation and inactivation of incretins. DPP-4 inhibitors are oral antidiabetic drugs that can prolong the half-life of endogenous GLP-1 and GIP and subsequently sustain the insulin response.^2,10

It is stated that the structure of DPP-4 inhibitor peptides contains high amounts of hydrophobic amino acids such as alanine, glycine, isoleucine, leucine, and phenylalanine. It is thought that the hydrophobic pockets located in the active part of DPP-4 play a vital role in the inhibition of the enzyme by inhibitory peptides.⁹ Furthermore, peptides containing proline residues in the N-terminal region have been reported to be effective in DPP-4 inhibition. In contrast, the position of residues can also determine the activity of peptides. For instance, Pro-Ile dipeptide does not inhibit DPP-4 activation, whereas Ile-Pro dipeptide inhibits DPP-4 activation.^8,55 Peptides containing proline at the penultimate position in the amino acid sequence have also been shown to inhibit DPP-4. In quantitative structure–activity relationship studies, the hydrophobicity of two amino acids in the N-terminal region of the peptide showed a positive correlation with the DPP-4 inhibition potential of the peptide.⁵⁵

DPP-4 inhibitor peptides have great potential in providing glycemic control by supporting GLP-1 and insulin secretion.² In in vitro models, eggs, goat’s milk, sardines, salmon, blue whiting, Antarctic krill, chickpeas, black beans, oats, soybeans, quinoa, amaranth, and date seed supply DPP-4 inhibitory peptides,^{17
–19,21,23,24,28,30

–35,37} whereas camel milk provides in an in silico model²² (Tables 1 and 2).

Peptides inhibiting glucose transporters

Glucose and galactose, products of carbohydrate digestion, are transported from enterocytes through the intestinal brush border membrane. This process occurs in two stages that require active sodium-glucose cotransporter 1 (SGLT1) and facilitative glucose transporter 2 (GLUT2) proteins. Thirty minutes after starting a meal, digestive products reach the apical membrane of the jejunum, where SGLT1 mainly mediates their absorption from epithelial cells. In contrast, GLUT2 ensures the basolateral exit of these hexoses from epithelial cells into the circulation. For this reason, parameters affecting SGLT1 and GLUT2 activities also dictate glucose absorption and metabolism. SGLT1 has also been shown to be important in intestinal glucose absorption and incretin secretion. However, food-derived bioactive peptides targeting transporters in glucose absorption have been investigated less frequently.²

High-molecular weight (14 kDa) and low-molecular weight (2 kDa) peptides, which are tryptic hydrolysis products of rice albumin, alleviated the increment in postprandial blood glucose levels by inhibiting the transport of glucose through intestinal epithelial cells in healthy Wistar rats. The underlying mechanism is as follows: while high-molecular weight fractions function like dietary fiber, low-molecular weight fractions inhibit SGLT1 expression.⁵⁴ Bioactive peptides obtained from black beans provided the blockade of GLUT2 and SGLT1 in the in silico model⁴⁹ (Table 2).

Table 2.

Some Dietary Bioactive Peptides with Antidiabetic Properties (Mostly in Vivo and Clinical Models)

Source	Enzymes used/treatment	Substrate	Identified peptides (amino acid sequence)	Mechanism of action	References
Yoghurt, cheese	Chymotrypsin, trypsin	Whey		Increased GLP-1 secretion in intestinal enteroendocrine cells in an in vitro model	38
Cow milk	Trypsin	Cow milk protein		Decrease in blood glucose level in diabetic albino rats in an in vivo model.	40
Cow milk	Pepsin and pancreatin	Casein	PPQSVLSLSQSK, HQPHQPLPPTVMFPPQSVL	The dose-dependent insulinotropic effect of casein hydrolysate on pancreatic BRIN-BD11 cells in an in vitro modelDecrease in blood glucose level during glucose tolerance test in an in vivo model (male ob/ob mice) and faster response to glucose in glucose-stimulated insulin secretion in an ex vivo model.In a clinical model, a decrease in blood glucose levels in the postprandial period in healthy adults with a BMI >25 kg/m², increased insulin secretion	41
Egg	Thermolysin and pepsin	Egg white protein	WEKAFKDED, QAMPFRVTEQE, ERYPIL, VFKGL	In an in vitro model, supporting adipocyte differentiation in 3T3-F442A preadipocytes (e.g., PPARγ upregulation), insulin-sensitizing and insulin-mimetic effects (e.g., increase in Akt and ERK1/2 phosphorylation)	42,43
	Thermolysin and pepsin	Egg white protein	Egg white protein hydrolysate	In the in vivo model, in rats with insulin resistance, increased insulin sensitivity, improvement in oral glucose tolerance, decrease in systemic inflammation, supporting insulin-induced Akt phosphorylation in muscles and adipose tissue, reduction in adipocyte size, increase in PPARγ2 abundance	44
	Alcalase	Lysozyme		In a clinical model, acute decrease in plasma glucose concentration and serum insulin level in adults with impaired glucose tolerance or type 2 diabetes (BMI = 25–35 kg/m²)	45
Blue whiting	Alcalase, Flavourzyme® (peptidase), and simulated gastrointestinal digestion (pepsin and corolase PP)			DPP-4 inhibition in an in vitro model; increased insulin release, increased membrane potential, increased intracellular calcium concentration, and cAMP concentration in pancreatic BRIN-BD11 cells; increased GLP-1 release in enteroendocrine GLUTag cells.In an in vivo model, decrease in blood glucose concentration and AUC value during the glucose tolerance test in healthy male NIH Swiss mice.	37
Atlantic cod	Protamex® (endoprotease)			In the clinical model, in adults (40–65 years of age and BMI = 20–30 kg/m²), decrease in insulin secretion in the postprandial period without any change in blood glucose response and serum GLP-1 levels.	46
				In the clinical model, dose-dependent decrease in serum glucose and insulin levels in the postprandial period in adults (60–80 years old and BMI = 20–30 kg/m²)	47
				In a clinical model, no change in fasting insulin, postprandial insulin, glucose, and GLP-1 levels in adults with metabolic syndrome (40–70 years, 27–35 kg/m²)	48
Black bean	Alcalase		AKSPLF, ATNPLF, FEELN, LSVSVL	In silico model, blockade of GLUT2 and SGLT1 glucose transportersDecreased glucose absorption in colorectal Caco-2 cells in an in vitro modelIn an in vivo model, decreased postprandial blood glucose levels during the oral glucose tolerance test in male Wistar ratsIn an in vivo model, a dose-dependent decrease in blood glucose levels in hyperglycemic rats	49
Soybean		β-conglycinin	IAVPGEVA, IAVPTGVA, LPYP	Activation of the AMPK signaling pathway in hepatic HepG2 cells in an in vitro model	50
Pea			VLP, LLP, LL, LL	Increased hepatic glucose absorption and utilization through the effect on IRS-1/PI3K/Akt and p38MAPK pathways in insulin-resistant hepatic HepG2 cells in an in vitro model	51
Pea	Alcalase and neutrase		ALP, VLP, LLP, SP	In an in vivo model, in diabetic mice fed with a high-fat diet and induced with streptozotocin, decrease in blood glucose level, improvement in glucose intolerance, increase in insulin secretion, and glycogen synthesis	52
Pea	Serine protease		NRT_N0G5IJ (Peptides consisting of 7–16 amino acids, most of which contain 40% hydrophobic residues, obtained from pea protein using artificial intelligence)	Increased glucose uptake in human skeletal muscle cells in an in vitro modelIn an in vivo model, a decrease in fasting blood glucose level and HbA1c level in db/db diabetic male miceIn the clinical model, decrease in HbA1c level in adults (HbA1c level: 5.7–6.4% and BMI = 20–35 kg/m²)	53
Wheat	Bacterial protease	Gluten	Wheat protein hydrolysate	Increased GLP-1 secretion in enteroendocrine GLUTag cells in an in vitro modelIn an in vivo model, improvement in hyperglycemia in healthy male Sprague–Dawley rats during the glucose tolerance test process through stimulation of GLP-1 secretion and subsequent insulin secretion	39
Rice	Trypsin	Albumin	High-molecular weight peptides (14 kDa) and low-molecular weight peptides (2 kDa)	In an in vitro model, the acting of HMP-like dietary fiber in glucose absorption; in STC-1 cells, suppression of SGLT-1 expression by LMPIn an in vivo model, suppression of increment of postprandial blood glucose level in healthy Wistar rats	54
Walnut	Neutrase and alcalase		Peptide fractions of intermediate molecular weight (3–10 KDa)	In an in vitro model, α-glycosidase inhibition and increased extracellular glucose utilization in insulin-resistant hepatic HepG2 cellsIn an in vivo model, a decrease in fasting blood glucose levels increase in insulin secretion, liver glucokinase, and glycogen levels in streptozotocin-induced diabetic mice	26

BMI, body mass index; HMP, high-molecular weight peptides; LMP, low-molecular weight peptides; SGLT1, sodium glucose cotransporter-1; PPARγ, peroxisome proliferator-activated receptor gamma; ERK1/2, extracellular signal-regulated kinase 1/2; cAMP, cyclic adenosine monophosphate; AUC, area under the curve; GLUT2, glucose transporter-2; AMPK, AMP-activated protein kinase; p38MAPK, p38 mitogen-activated protein kinase.

Peptides assisting peripheral glucose uptake

Peptides activating the phosphatidylinositol 3-kinase/protein kinase B pathway

Insulin directs the signaling processes that control the metabolic fate of nutrients. The phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways are the central signaling cascades that mediate most metabolic and transcriptional effects of insulin. While the MAPK pathway basically controls mitogenic processes, growth, and cell differentiation in cells, in this pathway, activation of extracellular signal-regulated kinase 1/2 (ERK1/2) plays a direct role in cell proliferation and differentiation. The PI3K pathway is another of the major signaling cascades responsible for most metabolic effects of insulin. In this pathway, activation of protein kinase B (Akt) provides the translocation of glucose transporters from the intracellular region to the plasma membrane for glucose uptake. Glucose transporters simplify the passive transport of glucose across the cell membrane in peripheral cells. It was reported that peripheral glucose uptake collapsed in insulin resistance and T2DM.²

The bioactive peptide obtained from walnuts activated the insulin receptor substrate-1 (IRS-1)/PI3K/Akt signaling pathway in insulin-resistant hepatic HepG2 cells,³⁶ whereas the bioactive peptide obtained from peas activated the IRS-1/PI3K/Akt and p38MAPK pathways in insulin-resistant hepatic HepG2 cells in an in vitro model.⁵¹ Bioactive peptides derived from egg white protein increased Akt and ERK1/2 phosphorylation in 3T3-F442A preadipocytes in an in vitro model.^42,43 In contrast, in an in vivo model, they promoted Akt phosphorylation in insulin-resistant rats⁴⁴ (Tables 1 and 2).

Activating AMP-activated protein kinase pathway

Adenosine monophosphate-activated protein kinase is an evolutionarily preserved serine/threonine kinase known as a critical regulator of metabolism.⁵⁶ When the cellular energy level is low, the activation of this nutrient-sensitive kinase ensures the maintenance of normal energy levels by stimulating the processes that enable ATP formation (such as fatty acid oxidation) and inhibiting the processes that cause ATP usage (such as triglyceride and protein synthesis). Activators of the AMP-activated protein kinase (AMPK) pathway, impaired in type 2 diabetes, can support insulin sensitivity by stimulating glucose uptake in skeletal muscles, supporting fatty acid oxidation in adipose tissue, and reducing hepatic glucose production.²

Some protein hydrolysates and bioactive peptides have been reported to activate the AMPK pathway. Bioactive peptides derived from walnuts activated the AMPK signaling pathway in insulin-resistant hepatic HepG2 cells.³⁶ Moreover, peptides obtained from soybean β-conglycinin protein also ensured the activation of the AMPK signaling pathway in hepatic HepG2 cells⁵⁰ (Tables 1 and 2).

Peptides promoting adipocyte differentiation

Adipose tissue controls whole-body glucose and lipid homeostasis by sequestering fat and producing various hormones and cytokines in healthy people and disease states. Chronic excessive caloric intake and inability to form new adipocytes induce ectopic fat storage, resulting in peripheral insulin resistance, especially in skeletal muscles.⁵⁷ Insulin resistance refers to a reduced response to the binding of insulin to its receptor in peripheral tissues.⁵⁸ There is plenty of evidence that excess adiposity and adipose tissue inflammation contribute to insulin resistance [reviewed in References^59,60]. The relationship between adipose tissue and insulin resistance is elucidated through several hypotheses. These include the production of pro-inflammatory cytokines by adipocytes and adipose tissue macrophages, excess free fatty acids, decreased adiponectin, increased resistin and retinol-binding protein, ceramide accumulation, and ectopic fat accumulation in the liver and skeletal muscle.^60,61 It has also been reported that there is a reduction in the expression of adipogenic genes in obese individuals with type 2 diabetes.²

Adipocyte differentiation enables the formation of new adipocytes with greater capacity for fat storage. For insulin sensitivity and glucose homeostasis to be expected, an appropriate proportion of functional adipose tissue must be compared to body size. Peroxisome proliferator-activated receptor-gamma (PPARγ) is one of the fundamental transcription factors that play a role in this process. It provides activation of genes involved in adipocyte differentiation and fatty acid sequestration.²

Several dietary bioactive peptides have been reported to enhance adipocyte differentiation in in vitro and in vivo models.² Bioactive peptides obtained from egg white protein hydrolysate supported adipocyte differentiation in 3T3-F442A preadipocytes and upregulated PPARγ in an in vitro model.^42,43 In contrast, in an in vivo model, egg white protein hydrolysate reduced adipocyte size and increased PPARγ2 abundance in insulin-resistant rats⁴⁴ (Table 2).

CHALLENGES, OPPORTUNITIES, AND CONSEQUENCES

In the classical approach, the discovery and production of bioactive peptides involve several steps and are usually time-consuming and costly. Moreover, commercialization of the discovered peptide is difficult due to the small amount and low purity obtained after extensive fractionation and isolation.⁶² Bioinformatics, based on applied computational methods used in managing, organizing, and interpreting information in biological systems, is gaining increasing importance by enabling researchers to overcome difficulties by offering a more applicable workflow and reducing the application steps. Artificial intelligence, including machine learning approaches, has also recently been used to discover food-derived bioactive peptides.² The effectiveness and safety of NRT_N0G5IJ (PeptiForce™), produced with artificial intelligence and which has glycoregulatory properties, have been confirmed in human skeletal muscle cells. In the ongoing process, the antidiabetic property of the peptide has been clinically proven in diabetic mice and, finally, in prediabetic individuals.⁵³

It is critical to evaluate the biological activities of bioactive peptides discovered and produced by both in silico and classical approaches in the in vivo model. Most studies in this field rely on in vitro and cell-based assays to identify peptides and evaluate their biological activities. Although in vivo evaluations of bioactive peptides are being performed, these studies are uncommon. Furthermore, there are a limited number of clinical studies investigating the effectiveness of bioactive peptides. The scarcity of data on the bioavailability and metabolic processes of bioactive peptides in in vivo models is a primary challenge in the research field of bioactive peptides.^2,9

Moreover, knowledge of bioactive peptides’ absorption, distribution, metabolism, and excretion processes is also critically important. The sensitivity of bioactive peptides to degradation in the gastric and intestinal microenvironment is an issue often not considered in the research of peptides that show their bioactivity, primarily through systemic circulation.² Various physical and chemical methods can improve the bioavailability and stability of peptides. These methods include adding organic acids such as citric acid or supporting the pH balance of intestinal enzymes through enzyme inhibitors, enteric coating using polyacrylic polymers, and nanoparticles. The stability of peptides can also be enhanced by various chemical approaches, such as cyclization and conjugation.⁸ Considering the interactions between the food matrix and dietary components (especially between peptides and polyphenols), as well as the harsh conditions of the gastrointestinal tract, it is also likely that bioactive peptides will enter the systemic circulation in amounts that cannot induce biological activity.⁶³

Due to the bitter taste of peptides, the successful use of bioactive peptides as functional food ingredients depends on improving the organoleptic properties of the peptides.² The combination of size exclusion chromatography and reverse-phase HPLC with e-tongue was constructed by partial least squares regression models to estimate the bitterness of dairy protein hydrolysates.⁶⁴

Suppose the scientific perception and clinical effectiveness of food-derived bioactive peptides are further supported. In that case, these peptides may stand out as a unique biofunctional peptide-based nutritional strategy to be used in managing T2DM and other metabolic diseases. In contrast, it is assumed that antidiabetic bioactive peptides can be taken together with other available antidiabetic drugs. However, further clinical studies are recommended to examine the possible presence of undesirable reactivities and the route of administration that would enable bioactive peptides to exert their physiological functionality in the body.¹⁰ Consequently, bioactive peptides and protein hydrolysates hold great promise as valuable functional ingredients in medical nutritional therapy to fight the global epidemic of type 2 diabetes.

Footnotes

AUTHORS’ CONTRIBUTIONS

H.O.I.: Conceptualization, methodology, validation, formal analysis, investigation, resources, writing—original draft, writing—review and editing, visualization, and project administration. G.S.: Conceptualization, methodology, validation, and writing—review and editing.

AUTHOR DISCLOSURE STATEMENT

The authors report that there are no competing interests to declare.

FUNDING INFORMATION

This work was not supported by any institution.

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