Table 1.
In vitro studies on antidiabetic effect of flavan-3-ols and their microbial metabolites 1.
| In Vitro Test | Flavan-3-ol | Concentration/Dose | Results | Ref. |
|---|---|---|---|---|
| Glucose absorption in the gut | ||||
| Inhibition of α-glucosidase and α-amylase activity | GTE, GTP, EGCG | α-amylase: IC50 = 1370.812 ± 59.081–1849.612 ± 73.475 µg/mL α-glucosidase: IC50 = 4.421 ± 0.018–10.019 ± 0.017 µg/mL |
Inhibition of α-glucosidase by GTE was stronger than by acarbose (IC50 = 4822.783 ± 26.042 µg/mL) and the other compounds but had no effect on α-amylase. | [19] |
| Combination of GTE, GTP, EGCG, and acarbose at low concentrations had synergistic suppressive effects on α-glucosidase. | ||||
| α-amylase was inhibited at high concentrations of GTP and EGCG, but lower than that of acarbose (IC50 = 2715.654 ± 24.709 µg/mL). | ||||
| Inhibition of α-amylase and α-glucosidase activity | GSE, tea extracts, C, EC, EGC, EGCG, GCG, ECG | α-amylase: IC50 = 8.7 ± 0.8–378 ± 134 µg/L α-glucosidase: IC50 = 0.3 ± 0.1–31 µg/L |
α-amylase was only inhibited by GTE extract similarly to acarbose. | [20] |
| α-glucosidase was significantly inhibited by all compounds except C, EC following this order: Teavigo® > EGCG > GTE> GSE > GCG > WTE > ECG | ||||
| Inhibition of α-glucosidase activity | EGCG, ECG, EGCG3”Me, ECG3”Me | IC50 = 8.1−61.1 µM | Inhibition of α-glucosidase EGCG3”Me > EGCG > ECG3”Me > ECG | [21] |
| α-glucosidase inhibition assay | C | IC50 = 87.55 µg/mL | The α-glucosidase inhibitory potency was greater than acarbose (IC50 = 199.53 µg/mL). | [22] |
| Inhibition of α-glucosidase activity | Procyanidins B2, B5 and C1 | IC50 = 4.7 ± 0.2, 5.5 ± 0.1 and 3.8 ± 0.2 µg/mL | Trimeric procyanidin (C1) exerted the strongest inhibitory activity. Inhibitory effect was stronger than for acarbose (130.0 ± 20.0 µg/mL). | [23] |
| Insulin signaling pathways and glucose peripheral uptake | ||||
| Glucose uptake assay and insulin signaling pathway in HepG2 cells treated with PA | Theaflavin mixture (TF, TF-3-G, TF-3′-G, and TFDG) | 2.5–10 μg/mL | Increased 2-NBDG uptake. Increased membrane bound GLUT4 protein level and Akt phosphorylation. Decreased IRS-1 phosphorylation at Ser307. Increase of mtDNA copy number. Downregulation of PGC-1β mRNA level and increase of PRC mRNA expression. | [24] |
| GLUT1-mediated uptake of 3-O-methylglucose in human red blood cells | EGCG and ECG | - | Uptake of 0.1 mM 3MG was dose-dependently inhibited. | [25] |
| Glucose uptake, GLUT4 translocation, and JNK phosphorylation in insulin resistant 3T3-L1 adipocytes | EGCG | 0.1–5 µM | At 5 µM, increased glucose uptake. Dose-dependent reversion of Dex- and TNFα-induced JNK phosphorylation. At 1 µM, increased GLUT4 translocation. | [26] |
| GLUT4 translocation in L6 skeletal muscle cells | 5-(3,5-dihydroxyphenyl)-γ-VL | 1 and 3 µM | 3 µM promoted the strongest effect on GLUT4 translocation. AMPK phosphorylation increased. |
[27] |
| Glucose transport in human and murine 3T3-L1 adipocytes stimulated or not with insulin | PCA | 100 µmol/L | Reversion of oxLDL-induced decrease in glucose uptake and GLUT4 translocation. Reversion of oxLDL-induced decrease of adiponectin mRNA expression and secretion, and of PPARγ mRNA expression and activity. | [28] |
| Insulin signaling, glucose uptake, and glucose production in rat renal NRK-52E cells | EC, 2,3-DHB, 3,4-DHPA, 3-HPP and VA | 5–20 µM | Glucose uptake, glucose production, and PEPCK reduced after treatment with EC (5–20 µM) and 2,3-DBH (20 µM). | [29] |
| IR and IRS-1 phosphorylated and total protein levels increased at 10 µM EC and 20 µM 2,3-DHB. Increased phosphorylation of Akt and GSK3. The inhibition of the PI3K/Akt pathway was restrained. | ||||
| Insulin signaling and glucose uptake and production in rat renal NRK-52E cells treated with high glucose | EC, 3,4-DHPA, 2,3-DHB and 3-HPP | 5–20 µM | The altered glucose uptake and production caused by high glucose was prevented by EC (5–20 µM) and 3,4-DHPA (10–20 µM). At 10 µM, tyrosine phosphorylated, and total levels of IR increased. The PI3K/Akt pathway and AMPK were activated and the PEPCK expression was reduced. | [30] |
| Beta cell viability and function | ||||
| GSIS in INS-1 cell. [Ca2+] oscillations induced by glucose in INS-1 cells | EGCG, GCG, EC, C, EGC, GC, ECG, CG | 10–100 μM | GSIS was decreased by 10 and 30 μM EGCG. GSIS was terminated by 100 μM EGCG and 100 μM GCG. EGC nearly abolished GSIS at 100 μM, GC and ECG partly inhibited it. EC, C, and CG did not show any effect. 100 μM EGCG decreased the oscillation of intracellular calcium. | [31] |
| GSIS in SFA-treated INS-1 cell; ROS production in high-glucose and H2O2-treated INS-1 cell | EC | 0.3 μmol/L 30 µmol/L | Increase of GSIS. Reversion of SFA-induced inhibition of CaMKII phosphorylation. Reduced ROS production. |
[32] |
| Insulin production in iron-loaded RINm5F pancreatic cells. Iron and ROS levels in RINm5F pancreatic cells | GTE | 1–20 µM EGCG 1–10 µM EGCG |
Dose-dependent increase of insulin secretion. | [33] |
| Dose-dependent decrease of iron and ROS levels. | ||||
| Cell viability and GSIS in PA- and H2O2-treated INS-1 pancreatic beta cells. H2O2-stimulated ROS production | Cinnam-tannin B1, procyanidin C1, cinnam-tannin D1 | 12.5–100 μmol/L | Dose-dependent increase of cell viability. | [34] |
| GSIS increase at 25 µmol/L. | ||||
| Decreased ROS production. | ||||
| Inhibition of hIAPP aggregation and molecular mechanism | EGCG | - | Blockage of inter-peptide hydrophobic/aromatic interactions and intra-peptide interactions. | [35] |
| Abolishment of β-hairpin-containing three-stranded β-sheet conformation. | ||||
| Shift of hIAPP dimer toward loosely packed coil-rich conformations. | ||||
| Amyloid formation by IAPP and disaggregation of amyloid fibrils with thioflavin-T binding assay and TEM. Cell viability in mixture IAPP:EGCG on rat INS-1 | EGCG | 3.2–32 µM | At 32 µM, inhibition of amyloid formation by IAPP. IAPP:EGCG (3.2 µM) complex did not seed amyloid formation by IAPP. Disaggregation of IAPP. Increased cell viability of INS-1 cells to 77%. | [36] |
| hIAPP fibrillation and aggregation | EGCG | 2–32 µM | Inhibition of hIAPP fibrillation. | [37] |
| Formation of amorphous aggregates instead of ordered fibrils. | ||||
| Beta cell function of rat INS-1E pancreatic beta cells and rat pancreatic islets | 3,4-DHPA, 2,3-DHB and 3-HPP | 1–5 µM | 3,4-DHPA and 3-HPP enhanced GSIS (5 and 1 µM, respectively). Under oxidative stress, 3,4-DHPA and 3-HPP reduced ROS and carbonyl group production, and GSIS returned to control levels. PKC and ERKs phosphorylation improved. | [38] |
| Beta cell function of Min6 pancreatic beta cells incubated with cholesterol | 3,4-DHPA | 10–250 µM | 3,4-DHPA reversed the diminished insulin secretion induced by cholesterol. It protected beta cells against apoptosis, oxidative stress, and mitochondrial dysfunction. | [39] |
| Beta cell function and glucose utilization in rat INS-1 beta cells and human skeletal muscle | EC, HA, HVA and 5-PVA | 5–100 µM | EC (10 and 25 µM), HA, and 5-PVA (25 µM) provoked glucose oxidation in skeletal muscle. After oxidative insult, skeletal mitochondrial function was conserved. In beta cells, EC (100 µM) and metabolites (5–100 µM) stimulated GSIS. | [40] |
| Endogenous glucose production | ||||
| Glucose production and PEPCK/G-6-Pase gene expression in H4IIE rat hepatoma cells incubated with pyruvate and lactate | EGCG | 12.5–100 µM | At 25 µM, glucose production was repressed comparable to that of insulin. | [41] |
| Dose-dependent reduction of PEPCK mRNA as well as G-6-Pase. PI3K inhibitor LY 294,002 reversed the repression of EGCG on PEPCK and G-6-Pase gene expression. NAC and SOD reversed the increased protein-tyrosine phosphorylation and reversed PEPCK and G-6-Pase gene repression. | ||||
| Gluconeogenesis and PEPCK/G-6-Pase gene expression in mouse cAMP-Dex-stimulated hepatocytes | EGCG | 0.25–1 µM | Dose-dependent attenuation of gluconeogenesis. Expression of PEPCK and G-6-Pase genes was blocked. | [42] |
| Activation of AMPK mediated by CaMKK and ROS-dependent. | ||||
| Gluconeogenesis pathway in palmitate-induced insulin resistant HepG2 cells | EGCG | 40 μM | Expression of PEPCK and G-6-Pase was reduced by 53% and 67%, respectively. Glucose production was reduced by 50%. | [43] |
| Incretin effect | ||||
| Plasma membrane potential and GLP-1 secretion in STC-1 cells under basal and nutrient-stimulated conditions | GSPE | 0.05–50 mg/L | At 0.05 and 0.5 mg/L, membrane depolarization. At 50 mg/L, hyperpolarization and suppression of GLP-1 secretion. | [44] |
| Under nutrient-stimulation, 50 mg/L limited membrane depolarization and reduced GLP-1 secretion. | ||||
| Insulin-stimulated glycogen synthesis and lipogenesis in high-glucose treated human hepatoma HepG2 cells | GTP (60% EGCG) | 0.1–10 µM | Enhanced glycogen synthesis, increased phosphorylation of Ser9 GSK3ß and Ser641 GS. | [45] |
| Inhibition of lipogenesis through enhanced expression of phosphorylated AMPKα and acetyl CoA carboxylase. | ||||
| Inflammation | ||||
| TNFα-induced activation of NF-κB, MAPKs, AP-1, and PPARγ in differentiated white 3T3-L1 adipocytes | EC | 0.5–10 µM | Dose-dependent decrease of JNK, ERK1/2, and p-38 phosphorylation, and nuclear AP-1-DNA binding. Inhibition of NF-κB signaling cascade activation, preventing p65 nuclear transport and nuclear NF-κB-DNA binding. Altered transcription of genes (MCP-1, IL-6, TNFα, resistin, PTP1B). Attenuation of decreased PPARγ expression. | [46] |
| Vasodilation | ||||
| Vasodilation of pre-contracted isolated rat aortic rings | 3-PP, 4-HPP, 3,4-DHPP, 4-HPA, 3,4-DHPA, HVA, 3-HB, PhG, 4-MC, m-CoA, 3-HPP and 3-HPA | 100 nM | 3-HPP had the strongest vasodilatory activity, which was NO and endothelium-dependent. | [47] |
| NO production by human aortic endothelial cells under glucotoxic conditions | 3-HPP | 1 µM | Insulin-stimulated increase in NO production was preserved, as well as phosphorylation of Akt and eNOS. The increase in ROS and RNS was prevented. | [48] |
| Endothelial function in human EA.hy926 endothelial cells | 3,4-DHPA, 2,3-DHB and 3-HPP | 10–12 µM | 3,4-DHPA and a mixture of the metabolites increased the NO generation and phosphorylation of eNOS, Akt, and AMPK. Under oxidative stress, cell viability was improved by the metabolites and reduced eNOS phosphorylation was reversed. ROS generation and phosphorylation of ERK and JNK were reversed. | [49] |
| Antiglycative activity | ||||
| AGEs formation in BSA/glucose system and glyoxal trapping ability | PG, 3,4-DHPP, DHFA, 3-HPA, 3,4-DHPA and HVA | 2–50 µmol/L | DHFA at 10 μmol/L significantly inhibited albumin glycation. At 2 µmol/L, a mix of 3-HPA, 3,4-DHPA, and HVA inhibited glycation. PG, 3,4-DHPP, and 3,4-DHPA had a glyoxal trapping ability of 60%, 90%, and 65%, respectively. | [50] |
| AGEs formation in BSA/glucose and BSA/MGO systems | 3,4-DHPA, 3-HPA and HVA | 1 mM | The order of AGEs’ inhibition ability was: rutin > quercetin > 3,4-DHPA > aminoguanidine > 3-HPA > HVA | [51] |
1 2-NBDG: 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose; Akt: protein kinase B; AMPK: 5’ adenosine monophosphate-activated protein kinase; AP-1: activator protein 1; CaMK: Ca2+/calmodulin-dependent protein kinase; CaMKK: calcium/calmodulin-dependent protein kinase kinase; cAMP: cyclic adenosine monophosphate; Dex: dexamethasone; ERK: extracellular signal–regulated kinases; G-6-Pase: glucose-6-phosphatase; GLP-1: glucagon-like peptide-1; GLUT1: glucose transporter type 1; GLUT4: glucose transporter type 4; GSE: grape seed extract; GS: glycogen synthase; GSIS: glucose-stimulated insulin secretion; GSK3ß: glycogen synthase kinase 3 beta; GSPE: grape seed procyanidin extract; GTE: green tea extract; GTP: green tea polyphenol mixture; hIAPP: human islet amyloid polypeptide; IAPP: islet amyloid polypeptide; IC50: half maximal inhibitory concentration; IL: interleukin; IR: insulin receptor; IRS-1: insulin receptor substrate 1; JNK: c-Jun N-terminal kinases; MAPK: mitogen-activated protein kinase; MCP-1: monocyte chemoattractant protein 1; mtDNA: mitochondrial DNA; NAC: N-acetylcysteine; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; oxLDL: oxidized LDL; PA: palmitic acid; PEPCK: phosphoenolpyruvate carboxykinase; PGC-1: peroxisome proliferator-activated receptor coactivator-1; PI3K: phosphoinositide 3-kinase; PKC: protein kinase C; PPARγ: peroxisome proliferator-activated receptor-γ; PRC: PGC-1-related coactivator; PTP1B: protein-tyrosine phosphatase 1B; ROS: reactive oxygen species; SFA: saturated fatty acid; SOD: superoxide dismutase; STC: secretin tumor cell; TEM: transmission electron microscopy; TNFα: tumor necrosis factor; WTE: white tea extract. Flavan-3-ols and microbial metabolites: 2,3-DHB: 2,3-dihydroxybenzoic acid; 3-HB: 3-hydroxybenzoic acid; 3-HPA: 3-hydroxyphenylacetic acid; 3-HPP: 3-hydroxyphenyl propionic acid; 3-PP: 3-phenylpropionic acid; 3,4-DHPA: 3,4-dihydroxyphenylacetic-acid; 3,4-DHPP: 3,4-dihydroxyphenyl propionic acid; 4-HPA: 4-hydroxyphenylacetic acid; 4-MC: 4-methylcatechol; 5-PVA: 5-phenylvaleric acid; C: catechin; CG: catechin gallate; DHFA: dihydroferulic acid; EC: epicatechin; ECG: epicatechin gallate; ECG3”Me: epicatechin-3-O-(3-O-methyl) gallate; EGC: epigallocatechin; EGCG: epigallocatechin gallate; EGCG3”Me: epigallocatechin-3-O-(3-O-methyl) gallate; GC: gallocatechin; GCG: gallocatechin gallate; HA: hippuric acid; HVA: homovanillic acid; m-CoA: m-coumaric acid; PCA: protocatechuic acid; PhG: phloroglucinol; PG: pyrogallol; TF: theaflavin; TF-3-G: theaflavin-3-gallate; TF-3′-G: theaflavin-3′-gallate; TFDG: theaflavin-3,3′-digallate; VA: valeric acid; VL: valerolactone.