Antimicrobial, cytotoxic, and insulin-releasing activities of the amphibian host-defense peptide ocellatin-3N and its L-lysine-substituted analogs.

The host-defense peptide ocellatin-3N (GIFDVLKNLAKGVITSLAS.NH2 ), first isolated from the Caribbean frog Leptodactylus nesiotus, inhibited growth of clinically relevant Gram-positive and Gram-negative bacteria as well as a strain of the major emerging yeast pathogen Candida parapsilosis. Increasing cationicity while maintaining amphipathicity by the substitution Asp4 →Lys increased potency against the microorganisms by between 4- and 16-fold (MIC ≤3 µM) compared with the naturally occurring peptide. The substitution Ala18 →Lys and the double substitution Asp4 →Lys and Ala18 →Lys had less effects on potency. The [D4K] analog also showed 2.5- to 4-fold greater cytotoxic potency against non-small-cell lung adenocarcinoma A549 cells, breast adenocarcinoma MDA-MB-231 cells, and colorectal adenocarcinoma HT-29 cells (LC50 values in the range of 12-20 µM) compared with ocellatin-3N but was less hemolytic to mouse erythrocytes. However, the peptide showed no selectivity for tumor-derived cells [LC50 = 20 µM for human umbilical vein endothelial cells (HUVECs)]. Ocellatin-3N and [D4K]ocellatin-3N stimulated the release of insulin from BRIN-BD11 clonal ß-cells at concentrations ≥1 nM, and [A18K]ocellatin-3N, at concentrations ≥0.1 nM. No peptide stimulated the release of lactate dehydrogenase at concentrations up to 3 µM, indicating that plasma membrane integrity had been preserved. The three peptides produced an increase in intracellular [Ca2+ ] in BRIN-BD11 cells when incubated at a concentration of 1 µM. In view of its high insulinotropic potency and relatively low hemolytic activity, the [A18K] ocellatin analog may represent a template for the design of agents with therapeutic potential for the treatment of patients with type 2 diabetes.

Pharmacologically Active Phytomolecules Isolated from Traditional Antidiabetic Plants and Their Therapeutic Role for the Management of Diabetes Mellitus.

Diabetes mellitus is a chronic complication that affects people of all ages. The increased prevalence of diabetes worldwide has led to the development of several synthetic drugs to tackle this health problem. Such drugs, although effective as antihyperglycemic agents, are accompanied by various side effects, costly, and inaccessible to the majority of people living in underdeveloped countries. Medicinal plants have been used traditionally throughout the ages to treat various ailments due to their availability and safe nature. Medicinal plants are a rich source of phytochemicals that possess several health benefits. As diabetes continues to become prevalent, health care practitioners are considering plant-based medicines as a potential source of antidiabetic drugs due to their high potency and fewer side effects. To better understand the mechanism of action of medicinal plants, their active phytoconstituents are being isolated and investigated thoroughly. In this review article, we have focused on pharmacologically active phytomolecules isolated from medicinal plants presenting antidiabetic activity and the role they play in the treatment and management of diabetes. These natural compounds may represent as good candidates for a novel therapeutic approach and/or effective and alternative therapies for diabetes.

Anti-hyperglycaemic and insulin-releasing effects of Camellia sinensis leaves and isolation and characterisation of active compounds.

Anti-diabetic actions of Camellia sinensis leaves, used traditionally for type 2 diabetes (T2DM) treatment, have been determined. Insulin release, membrane potential and intra-cellular Ca were studied using the pancreatic ß-cell line, BRIN-BD11 and primary mouse pancreatic islets. Cellular glucose-uptake/insulin action by 3T3-L1 adipocytes, starch digestion, glucose diffusion, dipeptidyl peptidase-4 (DPP-IV) activity and glycation were determined together with in vivo studies assessing glucose homoeostasis in high-fat-fed (HFF) rats. Active phytoconstituents with insulinotropic activity were isolated using reversed-phase HPLC, LCMS and NMR. A hot water extract of C. sinensis increased insulin secretion in a concentration-dependent manner. Insulinotropic effects were significantly reduced by diazoxide, verapamil and under Ca-free conditions, being associated with membrane depolarisation and increased intra-cellular Ca2+. Insulin-releasing effects were observed in the presence of KCl, tolbutamide and isobutylmethylxanthine, indicating actions beyond K+ and Ca2+ channels. The extract also increased glucose uptake/insulin action in 3T3L1 adipocyte cells and inhibited protein glycation, DPP-IV enzyme activity, starch digestion and glucose diffusion. Oral administration of the extract enhanced glucose tolerance and insulin release in HFF rats. Extended treatment (250 mg/5 ml per kg orally) for 9 d led to improvements of body weight, energy intake, plasma and pancreatic insulin, and corrections of both islet size and ß-cell mass. These effects were accompanied by lower glycaemia and significant reduction of plasma DPP-IV activity. Compounds isolated by HPLC/LCMS, isoquercitrin and rutin (464·2 Da and 610·3 Da), stimulated insulin release and improved glucose tolerance. These data indicate that C. sinensis leaves warrant further evaluation as an effective adjunctive therapy for T2DM and source of bioactive compounds.

Effects of Spirulina platensis on insulin secretion, dipeptidyl peptidase IV activity and both carbohydrate digestion and absorption indicate potential as an adjunctive therapy for diabetes.

Spirulina platensis has been found to be useful in the treatment of type 2 diabetes. The present study aims to elucidate the effects of ethanol extract and butanol fraction of S. platensis on insulin release and glucose homoeostasis in type 2 diabetic rats, together with their mechanism of actions. In vitro and in vivo methods were used including cellular studies to determine potential role of ion channels and cAMP in the insulinotropic actions of the extracts. The ethanol extract and butanol fraction stimulated insulin release from mouse islets and pancreatic ß-cells in a concentration-dependent manner. The butanol fraction also similarly stimulated insulin release from perfused rat pancreas. The insulin-releasing action was augmented by glucose, isobutylmethylxanthine, tolbutamide and a depolarising concentration of KCl. The insulin secretory effect was attenuated with diazoxide and verapamil and by omission of extracellular Ca2+. Butanol fraction was found to significantly inhibit dipeptidyl peptidase IV enzyme activity. Moreover, butanol fraction improved glucose tolerance following oral glucose administration (2·5 g/kg body weight (b.w.)). The butanol fraction was tested on 24 h starved rats given an oral sucrose load (2·5 g/kg b.w.) to examine possible effects on carbohydrate digestion and absorption. S. platensis substantially decreased postprandial hyperglycaemia after oral sucrose load and increased unabsorbed sucrose content throughout the gut. During in situ intestinal perfusion with glucose, the butanol fraction reduced glucose absorption and promoted gut motility. Finally, chronic oral administration of butanol fraction for 28 d significantly decreased blood glucose, increased plasma insulin, pancreatic insulin stores, liver glycogen and improved lipid profile. The characterisation of active compounds from butanol fraction revealed the presence of p-coumaric acid, ß-carotene, catechin and other antioxidant polyphenols. In conclusion, S. platensis could be an adjunctive therapy for the management of type 2 diabetes.

Zinc-induced activation of GPR39 regulates glucose homeostasis through glucose-dependent insulinotropic polypeptide secretion from enteroendocrine K-cells.

The role of Zn2+-sensing receptor GPR39 on glucose homeostasis and incretin regulation was assessed in enteroendocrine L- and K-cells. Anti-hyperglycaemic, insulinotropic and incretin secreting properties of Zn2+ were explored in normal, diabetic and incretin receptor knockout mice. Compared to intraperitoneal injection, oral administration of Zn2+ (50 µmol/kg body weight) with glucose (18 mmol/kg) in lean mice reduced the glycaemic excursion by 25-34% (p < 0.05-p < 0.001) and enhanced glucose-induced insulin release by 46-48% (p < 0.05-p < 0.01). In diabetic mice, orally administered Zn2+ lowered glucose by 24-31% (p < 0.01) and augmented insulin release by 32% (p < 0.01). In glucagon like peptide-1 (GLP-1) receptor knockout mice, Zn2+ reduced glucose by 15-28% (p < 0.05-p < 0.01) and increased insulin release by 35-43% (p < 0.01). In contrast Zn2+ had no effect on responses of glucose-dependent insulinotropic polypeptide (GIP) receptor knockout mice. Consistent with this, Zn2+ had no effect on circulating total GLP-1 whereas GIP release was stimulated by 26% (p < 0.05) in lean mice. Immunocytochemistry demonstrated GPR39 expression on mouse enteroendocrine L- and K-cells, GLUTag cells and pGIP/Neo STC-1 cells. Zn2+ had a direct effect on GIP secretion from pGIPneo STC-1 cells, increasing GIP secretion by 1.3-fold. GPR39 is expressed on intestinal L- and K-cells, and stimulated GIP secretion plays an integral role in mediating enhanced insulin secretion and glucose tolerance following oral administration of Zn2+. This suggests development of potent and selective GPR39 agonists as a therapeutic approach for diabetes.

Insulinotropic, glucose-lowering, and beta-cell anti-apoptotic actions of peptides related to esculentin-1a(1-21).NH2.

Long-standing Type 2 diabetes is associated with loss of both ß-cell function and ß-cell mass. Peptides derived from the frog-skin host-defense peptide esculentin-1 have been shown to exhibit potent, broad-spectrum antimicrobial activity. The aim of the present study is to determine whether such peptides also show insulinotropic and ß-cell protective activities. Esculentin-1a(1-21).NH2, esculentin-1b(1-18).NH2, and esculentin-1a(1-14).NH2 produced concentration-dependent stimulations of insulin release from BRIN-BD11 rat clonal ß-cells, 1.1B4 human-derived pancreatic ß-cells, and isolated mouse islets with no cytotoxicity at concentrations of up to 3 µM. The mechanism of insulinotropic action involved membrane depolarization and an increase in intracellular Ca2+ concentrations. The analogue [D-Lys14, D-Ser17]esculentin-1a(1-21).NH2 (Esc(1-21)-1c) was less potent in vitro than the all L-amino acid containing peptides and esculentin-1a(9-21) was inactive indicating that helicity is an important determinant of insulinotropic activity. However, intraperitoneal injection of Esc(1-21)-1c (75 nmol/kg body weight) together with a glucose load (18 mmol/kg body weight) in C57BL6 mice improved glucose tolerance with a concomitant increase in insulin secretion, whereas administration of esculentin-1a(1-21).NH2, esculentin-1b(1-18).NH2, and esculentin-1a(1-14) was without significant effect on plasma glucose levels. Esc(1-21)-1c (1 µM) protected BRIN-BD11 cells against cytokine-induced apoptosis (P < 0.01) and augmented proliferation of the cells (P < 0.01) to a similar extent as glucagon-like peptide-1. The data demonstrate that the multifunctional peptide Esc(1-21)-1c, as well as showing therapeutic potential as an anti-infective and wound-healing agent, may constitute a template for development of compounds for treatment of patients with Type 2 diabetes.

Assessment of the potential of temporin peptides from the frog Rana temporaria (Ranidae) as anti-diabetic agents.

Temporin A (FLPLIGRVLSGIL-NH2 ), temporin F (FLPLIGKVLSGIL-NH2 ), and temporin G (FFPVIGRILNGIL-NH2 ), first identified in skin secretions of the frog Rana temporaria, produced concentration-dependent stimulation of insulin release from BRIN-BD11 rat clonal ß-cells at concentrations ≥1 nM, without cytotoxicity at concentrations up to 3 µM. Temporin A was the most effective. The mechanism of insulinotropic action did not involve an increase in intracellular Ca2+ concentrations. Temporins B, C, E, H, and K were either inactive or only weakly active. Temporins A, F, and G also produced a concentration-dependent stimulation of insulin release from 1.1B4 human-derived pancreatic ß-cells, with temporin G being the most potent and effective, and from isolated mouse islets. The data indicate that cationicity, hydrophobicity, and the angle subtended by the charged residues in the temporin molecule are important determinants for in vitro insulinotropic activity. Temporin A and F (1 µM), but not temporin G, protected BRIN-BD11 cells against cytokine-induced apoptosis (P < 0.001) and augmented (P < 0.001) proliferation of the cells to a similar extent as glucagon-like peptide-1. Intraperitoneal injection of temporin G (75 nmol/kg body weight) together with a glucose load (18 mmol/kg body weight) in C57BL6 mice improved glucose tolerance with a concomitant increase in insulin secretion whereas temporin A and F administration was without significant effect on plasma glucose levels. The study suggests that combination therapy involving agents developed from the temporin A and G sequences may find application in Type 2 diabetes treatment.

Anti-diabetic actions of esculentin-2CHa(1-30) and its stable analogues in a diet-induced model of obesity-diabetes.

Actions of esculentin-2CHa(1-30) (GFSSIFRGVAKFASKGLGKDLAKLGVDLVA) and its analogues, ([D-Arg7, D-Lys15, D-Lys23]-esculentin-2CHa(1-30) and [Lys15-octanoate]-esculentin-2CHa(1-30), were evaluated in high-fat fed NIH Swiss mice with impaired glucose tolerance and insulin resistance. Twice-daily i.p. administration of the esculentin-2CHa(1-30) peptides (75 nmol/kg body weight) or exendin-4 (25 nmol/kg) for 28 days reduced body weight, without altering cumulative energy intake. All peptides reduced blood glucose levels by 6-12 mmol/l concomitant with lower plasma insulin levels, with significance evident from day 6. All peptides improved glucose tolerance, insulin sensitivity, blood glucose profile over 24 h and decreased HbA1c to a similar extent as exendin-4. The peptides also reduced high fat diet-induced increases in plasma GLP-1 and glucagon. None of the peptides altered bone mineral density/content or lean mass but decreased fat mass. Islets isolated from peptide-treated mice exhibited improved glucose-, alanine- and GLP-1-stimulated insulin secretion. Islet morphometric analyses revealed that exendin-4 and the esculentin-2CHa(1-30) peptides significantly reduced islet, beta and alpha cell areas compared to high-fat controls. Esculentin-2CHa(1-30) peptides markedly reduced high fat diet-induced increase in beta cell proliferation and apoptosis. Peptide treatments had beneficial effects on expression of islet genes (Ins1, Slc2a2, Pdx1) and skeletal muscle genes involved in insulin action (Slc2a4, Pdk1, Irs1, Akt1). High-fat diet significantly increased LDL cholesterol which was reduced by the acylated esculentin-2CHa(1-30) analogue. Peptide treatments did not alter circulating concentrations of amylase and marker enzymes of liver function, indicating a lack of toxicity. These data indicate that esculentin-2CHa(1-30) and its analogues may be useful for improvement of blood glucose control and weight loss in type 2 diabetes.

Cytotoxic peptides with insulin-releasing activities from skin secretions of the Italian stream frog Rana italica (Ranidae).

Peptidomic analysis of norepinephrine-stimulated skin secretions from Italian stream frog Rana italica led to the purification and characterization of two host-defense peptides differing by a single amino acid residue belonging to the brevinin-1 family (brevinin-1ITa and -1ITb), a peptide belonging to the temporin family (temporin-ITa) and a component identified as prokineticin Bv8. The secretions contained relatively high concentrations of the methionine-sulphoxide forms of brevinin-1ITa and -1ITb suggesting that these peptides may have a role as antioxidants in the skin of this montane frog. Brevinin-1ITa (IVPFLLGMVPKLVCLITKKC) displayed potent cytotoxicity against non-small cell lung adenocarcinoma A549 cells (LC50  = 18 µM), breast adenocarcinoma MDA-MB-231 cells (LC50  = 8 µM) and colorectal adenocarcinoma HT-29 cells (LC50  = 18 µM), but the peptide was also strongly hemolytic against mouse erythrocytes (LC50  = 7 µM). Temporin-ITa (VFLGAIAQALTSLLGKL.NH2 ) was between three and fivefold less potent against these cells. Brevinin-1ITa inhibited growth of both Gram-positive Staphylococcus epidermidis and Gram-negative Escherichia coli as well as a strain of the opportunist yeast pathogen Candida parapsilosis, whereas temporin-ITa was active only against S. epidermidis and C. parapsilosis. Both peptides stimulated the release of insulin from BRIN-BD11 clonal ß-cells at concentrations ≥1 nM, but brevinin-1ITa was cytotoxic to the cells at concentrations ≥3 µM. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Metabolic effects of orally administered small-molecule agonists of GPR55 and GPR119 in multiple low-dose streptozotocin-induced diabetic and incretin-receptor-knockout mice.

AIMS/HYPOTHESIS: Abnormal cannabidiol (Abn-CBD) and AS-1269574 are potent selective agonists for GPR55 and GPR119, respectively. The present study evaluated the actions and ability of these small-molecule agonists to counteract experimental diabetes in mice. METHODS: Diabetes was induced in NIH Swiss mice by five consecutive daily intraperitoneal injections of 40 mg/(kg body weight) streptozotocin. Diabetic mice received daily oral administration of Abn-CBD or AS-1269574 (0.1 µmol/kg) or saline vehicle (0.9% wt/vol. NaCl) over 28 days. Body weight, food intake, fluid intake, plasma glucose, insulin, glucose tolerance, insulin release, lipid profile and pancreatic morphology were examined. Mechanism of action of agonists was assessed in acute studies using incretin-receptor-knockout mice. RESULTS: Abn-CBD and AS-1269574 decreased plasma glucose (20-26%, p < 0.05) and increased circulating insulin (47-48%, p < 0.05) by 10-28 days, compared with saline-treated diabetic controls. Food intake and polydipsia were reduced by both agonists (21-23%, p < 0.05 and 33-35%, p < 0.01, respectively). After 28 days of treatment, plasma glucagon concentrations were reduced (p < 0.01) and glucose tolerance was enhanced by 19-44% by Abn-CBD (p < 0.05 or p < 0.001) and AS-1269574 (p < 0.05 to p < 0.001). Plasma insulin responses were improved (p < 0.01) and insulin resistance was decreased (p < 0.05 or p < 0.01) in both Abn-CBD- and AS-1269574-treated groups. Triacylglycerols were decreased by 19% with Abn-CBD (p < 0.05) and 32% with AS-1269574 (p < 0.01) while total cholesterol was reduced by 17% (p < 0.01) and 15% (p < 0.05), respectively. Both agonists enhanced beta cell proliferation (p < 0.001) although islet area was unchanged. Acute studies in Gipr- and Glp1r-knockout mice revealed an important role for the glucagon-like peptide 1 (GLP-1) receptor in the actions of both agonists, with the glucose-lowering effects of Abn-CBD also partly mediated through the glucose-dependent insulinotropic peptide (GIP) receptor. CONCLUSIONS/INTERPRETATION: These data highlight the potential for fatty acid G-protein-coupled receptor-based therapies as novel insulinotropic and glucose-lowering agents acting partly through the activation of incretin receptors.

Molecular mechanisms mediating the beneficial metabolic effects of [Arg4]tigerinin-1R in mice with diet-induced obesity and insulin resistance.

The frog skin host-defense peptide tigerinin-1R stimulates insulin release in vitro and improves glucose tolerance and insulin sensitivity in animal models of type 2 diabetes. This study extends these observations by investigating the molecular mechanisms of action underlying the beneficial metabolic effects of the analogue [Arg4]tigerinin-1R in mice with diet-induced obesity, glucose intolerance and insulin resistance. The study also investigates the electrophysiological effects of the peptide on KATP and L-type Ca2+ channels in BRIN-BD11 clonal ß cells. Non-fasting plasma glucose and glucagon concentrations were significantly (p<0.05) decreased and plasma insulin increased by twice daily treatment with [Arg4]tigerinin-1R (75 nmol/kg body weight) for 28 days. Oral and intraperitoneal glucose tolerance were significantly (p<0.05) improved accompanied by enhanced secretion and action of insulin. The peptide blocked KATP channels and, consistent with this, improved beta cell responses of isolated islets to a range of secretagogues. Peptide administration resulted in up-regulation of key functional genes in islets involved insulin secretion (Abcc8, Kcnj11, Cacna1c and Slc2a2) and in skeletal muscle involved with insulin action (Insr, Irs1, Pdk1, Pik3ca, and Slc2a4). These observations encourage further development of tigerinin-1R analogues for the treatment of patients with type 2 diabetes.

In vitro and in vivo insulinotropic properties of the multifunctional frog skin peptide hymenochirin-1B: a structure-activity study.

Hymenochirin-1b (Hym-1B; IKLSPETKDNLKKVLKGAIKGAIAVAKMV.NH2) is a cationic, α-helical amphibian host-defense peptide with antimicrobial, anticancer, and immunomodulatory properties. This study investigates the abilities of the peptide and nine analogues containing substitutions of Pro(5), Glu(6), and Asp(9) by either L-lysine or D-lysine to stimulate insulin release in vitro using BRIN-BD11 clonal ß cells or isolated mouse islets and in vivo using mice fed a high-fat diet to produce obesity and insulin resistance. Hym-1B produced a significant and concentration-dependent increase in the rate of insulin release from BRIN-BD11 cells without cytotoxicity at concentrations up to 1 µM with a threshold concentration of 1 nM. The threshold concentrations for the analogues were: [P5K], [E6K], [D9K], [P5K, E6K] and [E6K, D9k] 0.003 nM, [E6K, D9K] and [D9k] 0.01 nM, [P5K, D9K] 0.1 nM and [E6k] 0.3 nM. All peptides displayed cytotoxicity at concentrations ≥1 µM except the [P5K] and [D9k] analogues which were non-toxic at 3 µM. The potency and maximum rate of insulin release from mouse islets produced by the [P5K] peptide were significantly greater than produced by Hym-1B. Neither Hym-1B nor the [P5K] analogue at 1 µM concentration had an effect on membrane depolarization or intracellular Ca(2+). The [P5K] analogue (1 µM) produced a significant increase in cAMP concentration in BRIN-BD11 cells and stimulated GLP-1 secretion from GLUTag cells. Down-regulation of the protein kinase A pathway by overnight incubation with forskolin completely abolished the insulin-releasing effects of [P5K]hym-1B. Intraperitoneal administration of the [P5K] and [D9k] analogues (75 nmol/kg body weight) to high-fat-fed mice with insulin resistance significantly enhanced glucose tolerance with a concomitant increase in insulin secretion. We conclude that [P5K]hym-1B and [D9k]hym-1B show potential for development into anti-diabetic agents.

Activation of GPR119 by fatty acid agonists augments insulin release from clonal ß-cells and isolated pancreatic islets and improves glucose tolerance in mice.

G-protein coupled receptor 119 (GPR119) is emerging as a potential target for the treatment of type 2 diabetes with beneficial effects on glucose homeostasis. This study assessed the insulin-secreting properties of various GPR119 agonists and the distribution of GPR119 in pancreatic islets. Endogenous ligands [oleoylethanolamide (OEA), palmitoylethanolamine (PEA)] and chemically synthetic analogues (AS-1269574, PSN-375963) were investigated in clonal BRIN-BD11 cells and mouse pancreatic islets. Secondary messenger assays such as intracellular Ca²âº and cAMP in response to agonists at normoglycaemic and hyperglycaemic conditions were assessed. Cytotoxicity was assessed by LDH release. AS-1269574 was the most potent and selective agonist tested in isolated islets, with an EC50 value of 9.7×10⁻7 mol/l, enhancing insulin release maximally by 63.2%. Stimulation was also observed with GPR119 ligands; OEA (3.0×10⁻6 mol/l; 37.5%), PSN-375963 (2.4×10⁻6 mol/l; 28.7%) and PEA (1.2×10⁻6 mol/l; 22.2%). Results were corroborated by studies using BRIN-BD11 cells, which revealed augmentation of intracellular Ca²âº and cAMP. Both OEA and AS-1269574 enhanced insulin release and improved glucose tolerance in vivo in NIH Swiss mice. These results demonstrate the cellular localisation of GPR119 on islet cells (ß and pancreatic polypeptide cells), its activation of the ß-cell stimulus-secretion coupling pathway and glucose lowering effects in vivo.

Amphibian host-defense peptides with potential for Type 2 diabetes therapy - an updated review.

Investigations conducted since 2018 have identified several host-defense peptides present in frog skin secretions whose properties suggest the possibility of their development into a new class of agent for Type 2 diabetes (T2D) therapy. Studies in vitro have described peptides that (a) stimulate insulin release from BRIN-BD11 clonal ß-cells and isolated mouse islets, (b) display ß-cell proliferative activity and protect against cytokine-mediated apoptosis and (c) stimulate production of the anti-inflammatory cytokine IL-10 and inhibit production of the pro-inflammatory cytokines TNF-α and IL-1ß. Rhinophrynin-27, phylloseptin-3.2TR and temporin F are peptides with therapeutic potential. Studies in vivo carried out in db/db and high fat-fed mice have shown that twice-daily administration of [S4K]CPF-AM1 and [A14K]PGLa-AM1, analogs of peptides first isolated from the octoploid frog Xenopus amieti, over 28 days lowers circulating glucose and HbA1c concentrations, increases insulin sensitivity and improves glucose tolerance and lipid profile. Peptide treatment produced potentially beneficial changes in the expression of skeletal muscle genes involved in insulin signaling and islet genes involved in insulin secretion in these murine models of T2D. Lead compounds uncovered by the study of frog HDPs may provide a basis for the design of new types of agents that can be used, alone or in combination with existing therapies, for the treatment of T2D.

Protective Effects of Medicinal Plant-Based Foods against Diabetes: A Review on Pharmacology, Phytochemistry, and Molecular Mechanisms.

Diabetes mellitus (DM) comprises a range of metabolic disorders characterized by high blood glucose levels caused by defects in insulin release, insulin action, or both. DM is a widespread condition that affects a substantial portion of the global population, causing high morbidity and mortality rates. The prevalence of this major public health crisis is predicted to increase in the forthcoming years. Although several drugs are available to manage DM, these are associated with adverse side effects, which limits their use. In underdeveloped countries, where such drugs are often costly and not widely available, many people continue to rely on alternative traditional medicine, including medicinal plants. The latter serves as a source of primary healthcare and plant-based foods in many low- and middle-income countries. Interestingly, many of the phytochemicals they contain have been demonstrated to possess antidiabetic activity such as lowering blood glucose levels, stimulating insulin secretion, and alleviating diabetic complications. Therefore, such plants may provide protective effects that could be used in the management of DM. The purpose of this article was to review the medicinal plant-based foods traditionally used for the management of DM, including their therapeutic effects, pharmacologically active phytoconstituents, and antidiabetic mode of action at the molecular level. It also presents future avenues for research in this field.

Insulin secretory actions of ethanolic extract of Acacia arabica bark in high fat-fed diet-induced obese Type 2 diabetic rats.

Acacia arabica commonly known as 'babul' has been widely used for the treatment of numerous diseases, including diabetes due to their potential pharmacological actions. The aim of the present study was to investigate the insulinotropic and antidiabetic properties of ethanol extract of Acacia arabica (EEAA) bark through in vitro and in vivo studies in high fat-fed (HFF) rats. EEAA at 40-5000 µg/ml significantly increased (P<0.05-0.001) insulin secretion with 5.6 and 16.7 mM glucose, respectively, from clonal pancreatic BRIN BD11 ß-cells. Similarly, EEAA at 10-40 µg/ml demonstrated a substantial (P<0.05-0.001) insulin secretory effect with 16.7 mM glucose from isolated mouse islets, with a magnitude comparable to 1 µM glucagon-like peptide-1 (GLP-1). Diazoxide, verapamil, and calcium-free conditions decreased insulin secretion by 25-26%. The insulin secretory effect was further potentiated (P<0.05-0.01) with 200 µM isobutylmethylxanthine (IBMX; 1.5-fold), 200 µM tolbutamide (1.4-fold), and 30 mM KCl (1.4-fold). EEAA at 40 µg/ml, induced membrane depolarization and elevated intracellular Ca2+ as well as increased (P<0.05-0.001) glucose uptake in 3T3L1 cells and inhibited starch digestion, glucose diffusion, dipeptidyl peptidase-IV (DPP-IV) enzyme activity, and protein glycation by 15-38%, 11-29%, 15-64%, and 21-38% (P<0.05, 0.001), respectively. In HFF rats, EEAA (250 mg/5 ml/kg) improved glucose tolerance, plasma insulin, and GLP-1 levels, and lowered DPP-IV enzyme activity. Phytochemical screening of EEAA revealed the presence of flavonoids, tannins and anthraquinone. These naturally occurring phytoconstituents may contribute to the potential antidiabetic actions of EEAA. Thus, our finding suggests that EEAA, as a good source of antidiabetic constituents, would be beneficial for Type 2 diabetes patients.

Antihyperglycaemic activity of Asparagus racemosus roots is partly mediated by inhibition of carbohydrate digestion and absorption, and enhancement of cellular insulin action.

Asparagus racemosus roots have been shown to enhance insulin secretion in perfused pancreas and isolated islets. The present study investigated the effects of ethanol extracts of A. racemosus roots on glucose homeostasis in diabetic rats, together with the effects on insulin action in 3T3 adipocytes. When administered orally together with glucose, A. racemosus extract improved glucose tolerance in normal as well as in two types of diabetic rats. To investigate the possible effects on carbohydrate absorption, the sucrose content of the gastrointestinal tract was examined in 12 h fasted rats after an oral sucrose load (2.5 g/kg body weight). The extract significantly suppressed postprandial hyperglycaemia after sucrose ingestion and reversibly increased unabsorbed sucrose content throughout the gut. The extract also significantly inhibited the absorption of glucose during in situ gut perfusion with glucose. Furthermore, the extract enhanced glucose transport and insulin action in 3T3-L1 adipocytes. Daily administration of A. racemosus to type 2 diabetic rats for 28 d decreased serum glucose, increased pancreatic insulin, plasma insulin, liver glycogen and total oxidant status. These findings indicate that antihyperglycaemic activity of A. racemosus is partly mediated by inhibition of carbohydrate digestion and absorption, together with enhancement of insulin secretion and action in the peripheral tissue. Asparagus racemosus may be useful as a source of novel antidiabetic compounds or a dietary adjunct for the management of diabetes.

In vitro and in vivo antihyperglycemic activity of the ethanol extract of Heritiera fomes bark and characterization of pharmacologically active phytomolecules.

OBJECTIVE: This study aimed to demonstrate the mechanistic basis of Heritiera fomes, which has traditionally been used to treat diabetes. METHODS: Clonal pancreatic ß-cells and primary islets were used to measure insulin release. 3T3-L1 cells were used to analyse insulin action, and in vitro systems were used to measure further glucose-lowering activity. In vivo assessment was performed on streptozotocin (STZ)-induced type-2 diabetic rats and reversed-phase-HPLC followed by liquid chromatography mass spectrometry (LC-MS) to detect bioactive molecules. KEY FINDINGS: Ethanol extract of Heritiera fomes (EEHF) significantly increased insulin release with stimulatory effects comparable to 1 µM glucagon-like peptide 1, which were somewhat reduced by diazoxide, verapamil and calcium-free conditions. Insulin release was stimulated by tolbutamide, isobutyl methylxanthine and KCl. EEHF induced membrane depolarization and increased intracellular Ca2+ levels. EEHF enhanced glucose uptake in 3T3L1 cells and decreased protein glycation. EEHF significantly inhibited postprandial hyperglycaemia following sucrose loading and inversely elevated unabsorbed sucrose concentration in the gut. It suppressed glucose absorption during in situ gut perfusion. Furthermore, EEHF improved glucose tolerance, plasma insulin and gut motility, and decreased plasma dipeptidyl peptidase IV activity. Procyanidins, epicatechin and proanthocyanidins were some of the identified bioactive constituents that may involve in ß-cell actions. CONCLUSIONS: This study provides some evidence to support the use of H. fomes as an antidiabetic traditional remedy.

Insulin Secretory Actions of Ethanol Extract of Eucalyptus citriodora Leaf, including Plasma DPP-IV and GLP-1 Levels in High-Fat-Fed Rats, as Well as Characterization of Biologically Effective Phytoconstituents.

Due to the numerous adverse effects of synthetic drugs, researchers are currently studying traditional medicinal plants to find alternatives for diabetes treatment. Eucalyptus citriodora is known to be used as a remedy for various illnesses, including diabetes. This study aimed to explore the effects of ethanol extract of Eucalyptus citriodora (EEEC) on in vitro and in vivo systems, including the mechanism/s of action. The methodology used involved the measurement of insulin secretion from clonal pancreatic ß-cells, BRIN BD11, and mouse islets. Other in vitro systems further examined EEEC's glucose-lowering properties. Obese rats fed a high-fat-fed diet (HFF) were selected for in vivo evaluation, and phytoconstituents were detected via RP-HPLC followed by LC-MS. EEEC induced insulin secretion in a concentration-dependent manner with modulatory effects, similar to 1 µM glucagon-like peptide 1 (GLP-1), which were partly declined in the presence of Ca2+-channel blocker (Verapamil), KATP-channel opener (Diazoxide), and Ca2+ chelation. The insulin secretory effects of EEEC were augmented by isobutyl methylxanthine (IBMX), which persisted in the context of tolbutamide or a depolarizing concentration of KCl. EEEC enhanced insulin action in 3T3-L1 cells and reduced glucose absorption, and protein glycation in vitro. In HFF rats, it improved glucose tolerance and plasma insulin, attenuated plasma DPP-IV, and induced active GLP-1 (7-36) levels in circulation. Rhodomyrtosone B, Quercetin-3-O-ß-D-glucopyranoside, rhodomyrtosone E, and quercitroside were identified as possible phytoconstituents that may be responsible for EEEC effects. Thus, these findings revealed that E. citriodora could be used as an adjunct nutritional supplement to manage type 2 diabetes.

Insulin secretory and antidiabetic actions of Heritiera fomes bark together with isolation of active phytomolecules.

In folklore, Heritiera fomes (H. fomes) has been extensively used in treatment of various ailments such as diabetes, cardiac and hepatic disorders. The present study aimed to elucidate the antidiabetic actions of hot water extract of H. fomes (HWHF), including effects on insulin release from BRIN BD11 cells and isolated mouse islets as well as glucose homeostasis in high-fat-fed rats. Molecular mechanisms underlying anti-diabetic activity along with isolation of active compounds were also evaluated. Non-toxic concentrations of HWHF stimulated concentration-dependent insulin release from isolated mouse islets and clonal pancreatic ß-cells. The stimulatory effect was potentiated by glucose and isobutyl methylxanthine (IBMX), persisted in presence of tolbutamide or a depolarizing concentration of KCl but was attenuated by established inhibitors of insulin release such as diazoxide, verapamil, and Ca2+ chelation. HWHF caused depolarization of the ß-cell membrane and increased intracellular Ca2+. The extract also enhanced glucose uptake and insulin action in 3T3-L1 differentiated adipocytes cells and significantly inhibited in a dose-dependent manner starch digestion, protein glycation, DPP-IV enzyme activity, and glucose diffusion in vitro. Oral administration of HWHF (250 mg/5ml/kg b.w.) to high-fat fed rats significantly improved glucose tolerance and plasma insulin responses and it inhibited plasma DPP-IV activity. HWHF also decreased in vivo glucose absorption and intestinal disaccharidase activity while increasing gastrointestinal motility and unabsorbed sucrose transit. Compounds were isolated from HWHF with similar molecular weights to quercitrin (C21 H20 O11) ranging from 447.9 to 449.9 Da which stimulated the insulin release in vitro and improved both glucose tolerance and plasma insulin responses in mice. In conclusion, H. fomes and its water-soluble phytochemicals such as quercitrin may exert antidiabetic actions mediated through a variety of mechanisms which might be useful as dietary adjunct in the management of type 2 diabetes.