Inhibition of hepatic glucose 6-phosphatase system by the green tea flavanol epigallocatechin gallate

Procyanidin A2, an anti-diabetic condensed tannin extracted from Wendlandia glabrata, reduces elevated G-6-Pase and mRNA levels in diabetic mice and increases glucose uptake in CC1 hepatocytes and C1C12 myoblast cells

To reduce the global burden of diabetes in an affordable way great attention has been paid to the search for functional foods and herbal remedies. One of the most popularly used functional foods in the North Eastern region of India is tender shoots of Wendlandia glabrata DC. In the current study identification of active anti-diabetic constituent of the tender shoots of W. glabrata was guided through α-glucosidase inhibition and procyanidin A2 was identified with IC₅₀ 0.27 ± 0.01 μg mL⁻¹ making it potential source for postprandial management of DM type 2. The study has also demonstrated procyanidin A2 as a potent anti-diabetic agent that exhibits significant glucose-6-phosphatase inhibitory activities and downregulated mRNA level in diabetic mice as well as increases glucose uptake in hepatocytes and myoblast cells. This study revealed that easily available tender shoots of W. glabrata could be used to make specific dietary recommendations for consumption for affordable management of diabetes.

Wendlandia glabrata and procyanidin A2 isolated thereof are exhibited significant anti-diabetic effect.

Epigallocatechin-3-Gallate (EGCG) Promotes Autophagy-Dependent Survival via Influencing the Balance of mTOR-AMPK Pathways upon Endoplasmic Reticulum Stress

The maintenance of cellular homeostasis is largely dependent on the ability of cells to give an adequate response to various internal and external stimuli. We have recently proposed that the life-and-death decision in endoplasmic reticulum (ER) stress response is defined by a crosstalk between autophagy, apoptosis, and mTOR-AMPK pathways, where the transient switch from autophagy-dependent survival to apoptotic cell death is controlled by GADD34. The aim of the present study was to investigate the role of epigallocatechin-3-gallate (EGCG), the major polyphenol of green tea, in promoting autophagy-dependent survival and to verify the key role in connecting GADD34 with mTOR-AMPK pathways upon prolonged ER stress. Our findings, obtained by using HEK293T cells, revealed that EGCG treatment is able to extend cell viability by inducing autophagy. We confirmed that EGCG-induced autophagy is mTOR-dependent and PKA-independent; furthermore, it also required ULK1. We show that pretreatment of cells with EGCG diminishes the negative effect of GADD34 inhibition (by guanabenz or siGADD34 treatment) on autophagy. EGCG was able to delay apoptotic cell death by upregulating autophagy-dependent survival even in the absence of GADD34. Our data suggest a novel role for EGCG in promoting cell survival via shifting the balance of mTOR-AMPK pathways in ER stress.

Inhibition of microsomal cortisol production by (-)-epigallocatechin-3-gallate through a redox shift in the endoplasmic reticulum--a potential new target for treating obesity-related diseases

Conversion of cortisone to cortisol by 11β-hydroxysteroid dehydrogenase type 1 (11βHSD1) in the endoplasmic reticulum (ER) of the target cells is a major determinant of glucocorticoid action, and plays an important role in the development of obesity-related diseases. Inhibition of 11βHSD1 activity is, therefore, considered as a promising novel strategy for the treatment of metabolic syndrome and diabetes. Tea flavanols and their major representative, epigallocatechin gallate are known as antiobesity and antidiabetic agents. Their impacts on blood glucose level, hepatic glucose production, and insulin responsiveness resemble those observed on inhibition or depletion of 11βHSD1. We aimed to study the effect of epigallocatechin gallate on 11βHSD1 activity in ER-derived rat liver microsomes by measuring cortisone and cortisol with HPLC. Cortisol production was efficiently suppressed in a concentration dependent manner in intact microsomal vesicles. However, this effect was abolished by membrane permeabilization; and the three proteins involved in the overall process (11βHSD1, hexose 6-phosphate dehydrogenase, and glucose 6-phosphate transporter) were not or only mildly affected. Further investigation revealed the oxidation of luminal NADPH to NADP⁺, which attenuates cortisone reduction and favors cortisol oxidation in this compartment. Such a redox shift in the ER lumen might contribute to the beneficial health effects of tea flavanols and should be regarded as a promising strategy for the development of novel selective 11βHSD1 inhibitors to treat obesity-related diseases.

Epigallocatechin-3-O-gallate (EGCG) protects the insulin sensitivity in rat L6 muscle cells exposed to dexamethasone condition

The tea polyphenol epigallocatechin-3-O-gallate (EGCG) displays some antidiabetic effects; however the mechanisms are incompletely understood. In the present study, the investigation of the effects of EGCG on insulin resistance was performed in rat L6 cells treated with dexamethasone. We found that dexamethasone increased Ser307 phosphorylation of insulin receptor substrate-1 (IRS-1) and reduced phosphorylation of AMPK and Akt. Furthermore, glucose uptake and glucose transporter (GLUT4) translocation were inhibited by dexamethasone. However, the treatment of EGCG improved insulin-stimulated glucose uptake by increasing GLUT4 translocation to plasma membrane. Furthermore, we also demonstrated these EGCG effects essentially depended on the AMPK and Akt activation. Together, our data suggested that EGCG inhibited dexamethasone-induced insulin resistance through AMPK and PI3K/Akt pathway.

Cancer chemoprevention through dietary antioxidants: progress and promise

It is estimated that nearly one-third of all cancer deaths in the United States could be prevented through appropriate dietary modification. Various dietary antioxidants have shown considerable promise as effective agents for cancer prevention by reducing oxidative stress which has been implicated in the development of many diseases, including cancer. Therefore, for reducing the incidence of cancer, modifications in dietary habits, especially by increasing consumption of fruits and vegetables rich in antioxidants, are increasingly advocated. Accumulating research evidence suggests that many dietary factors may be used alone or in combination with traditional chemotherapeutic agents to prevent the occurrence of cancer, their metastatic spread, or even to treat cancer. The reduced cancer risk and lack of toxicity associated with high intake of fruits and vegetables suggest that specific concentrations of antioxidant agents from these dietary sources may produce cancer chemopreventive effects without causing significant levels of toxicity. This review presents an extensive analysis of the key findings from studies on the effects of dietary antioxidants such as tea polyphenols, curcumin, genistein, resveratrol, lycopene, pomegranate, and lupeol against cancers of the skin, prostate, breast, lung, and liver. This research is also leading to the identification of novel cancer drug targets.

Silencing of the MT1-MMP/ G6PT axis suppresses calcium mobilization by sphingosine-1-phosphate in glioblastoma cells

The contributions of membrane type-1 matrix metalloproteinase (MT1-MMP) and of the glucose-6-phosphate transporter (G6PT) in sphingosine-1-phosphate (S1P)-mediated Ca(2+) mobilization were assessed in glioblastoma cells. We show that gene silencing of MT1-MMP or G6PT decreased the extent of S1P-induced Ca(2+) mobilization, chemotaxis, and extracellular signal-related kinase phosphorylation. Chlorogenic acid and (-)-epigallocatechin-3-gallate, two diet-derived inhibitors of G6PT and of MT1-MMP, respectively, reduced S1P-mediated Ca(2+) mobilization. An intact MT1-MMP/G6PT signaling axis is thus required for efficient Ca(2+) mobilization in response to bioactive lipids such as S1P. Targeted inhibition of either MT1-MMP or G6PT may lead to reduced infiltrative and invasive properties of brain tumor cells.

[Effect of green tea flavonols on the function of the endoplasmic reticulum]

The various (e.g. anti-tumor and anti-diabetic) health effects of green tea attributed to its flavonols, primarily to epigallocatechin-gallate, got into the focus of interest. The endoplasmic reticulum, which plays key role in the metabolism of carcinogens, in the synthesis of secreted or cell surface proteins as well as in the glucose production, might be a potential target for anti-tumor and anti-diabetic agents. Therefore, it is an important question how the flavonols affect its functions. Experiments carried out in microsomes and hepatoma cells revealed that flavonols inhibit glucuronide transport in the endoplasmic reticulum, which may reduce the reactivation of carcinogens; they inhibit glucosidase II, which may cause endoplasmic reticulum stress and apoptosis in hepatoma cells; and they hinder glucose efflux, which may decrease hepatic glucose production and blood glucose level. These observations are useful for further investigation of the relevant transport processes and transporters and also contribute to the better understanding of the mechanisms of flavanol effects.