Biological and biomedical functions of Penta-O-galloyl-D-glucose and its derivatives.

Penta-O-galloyl-D-glucose (PGG) is a simple hydrolysable tannin in plants. PGG exists in two anomeric forms, α-PGG and ß-PGG. While ß-PGG can be found in a wide variety of plants, α-PGG is rather rare in nature. Numerous studies with ß-PGG revealed a wide variety of biological activities, such as anti-microbial and anti-cancer functions. Until recently, studies with α-PGG were limited by the lack of its availability. Since the development of an efficient chemical synthesis of the compound, several investigations have revealed its anti-diabetic, anti-cancer, and anti-platelet-coagulation functions. Based on structure-activity-relationship (SAR) studies with α-PGG, a variety of α-PGG-related novel compounds were synthesized and some of them have been shown to possess promising therapeutic activities. In this review, the authors will survey and evaluate the biological functions of PGG with a focus on α-PGG and its derivatives.

Orally efficacious novel small molecule 6-chloro-6-deoxy-1,2,3,4-tetra-O-galloyl-α-D-glucopyranose selectively and potently stimulates insulin receptor and alleviates diabetes.

Type 2 diabetes (T2D) has become an epidemic worldwide while T1D remains a great medical challenge. Insulin receptor (IR) signaling activators could alleviate hyperglycemia, reduce the burden on the pancreas, and contribute to prevention and treatment of both types of diabetes. Previously, we reported the synthesis and identification of a natural antidiabetic compound α-penta-galloyl-glucose (α-PGG). Subsequent studies led to the identification of an α-P6GG derivative, 6-chloro-6-deoxy-1,2,3,4-tetra-O-galloyl-α-D-glucopyranose (6Cl-TGQ). Here, we report that 6Cl-TGQ not only induced rapid and long-lasting glucose uptake comparable to insulin in adipocytes but also reduced high blood glucose levels to near normal and significantly decreased plasma insulin levels and improved glucose tolerance performance in high-fat diet-induced T2D mice when administered orally at 5 mg/kg once every other day. Moreover, a single gavage of 6Cl-TGQ at 10 mg/kg induced rapid and sharp decline of blood glucose in streptozotocin-induced T1D mice. Our studies further indicated that 6Cl-TGQ activated IR signaling in cell models and insulin-responsive tissues of mice. 6Cl-TGQ-induced Akt phosphorylation was completely blocked by IR and PI3K inhibitors, while the induced glucose uptake was blocked by the same compounds and a Glut4 inhibitor. Receptor binding studies indicated that 6Cl-TGQ bound to IR with a higher affinity than α-PGG. Importantly, 6Cl-TGQ, unlike insulin, selectively induced phosphorylation of IR without activating IGF1R or its signaling and did not increase cancer cell proliferation. These results indicate that 6Cl-TGQ is a potent orally efficacious compound with low carcinogenic potential and may contribute to the prevention and treatment of T1D and T2D.

Functional identification of a hydroxyproline-o-galactosyltransferase specific for arabinogalactan protein biosynthesis in Arabidopsis.

Although plants contain substantial amounts of arabinogalactan proteins (AGPs), the enzymes responsible for AGP glycosylation are largely unknown. Bioinformatics indicated that AGP galactosyltransferases (GALTs) are members of the carbohydrate-active enzyme glycosyltransferase (GT) 31 family (CAZy GT31) involved in N- and O-glycosylation. Six Arabidopsis GT31 members were expressed in Pichia pastoris and tested for enzyme activity. The At4g21060 gene (named AtGALT2) was found to encode activity for adding galactose (Gal) to hydroxyproline (Hyp) in AGP protein backbones. AtGALT2 specifically catalyzed incorporation of [(14)C]Gal from UDP-[(14)C]Gal to Hyp of model substrate acceptors having AGP peptide sequences, consisting of non-contiguous Hyp residues, such as (Ala-Hyp) repetitive units exemplified by chemically synthesized (AO)7 and anhydrous hydrogen fluoride-deglycosylated d(AO)51. Microsomal preparations from Pichia cells expressing AtGALT2 incorporated [(14)C]Gal to (AO)7, and the resulting product co-eluted with (AO)7 by reverse-phase HPLC. Acid hydrolysis of the [(14)C]Gal-(AO)7 product released (14)C-radiolabel as Gal only. Base hydrolysis of the [(14)C]Gal-(AO)7 product released a (14)C-radiolabeled fragment that co-eluted with a Hyp-Gal standard after high performance anion-exchange chromatography fractionation. AtGALT2 is specific for AGPs because substrates lacking AGP peptide sequences did not act as acceptors. Moreover, AtGALT2 uses only UDP-Gal as the substrate donor and requires Mg(2+) or Mn(2+) for high activity. Additional support that AtGALT2 encodes an AGP GALT was provided by two allelic AtGALT2 knock-out mutants, which demonstrated lower GALT activities and reductions in ß-Yariv-precipitated AGPs compared with wild type plants. Confocal microscopic analysis of fluorescently tagged AtGALT2 in tobacco epidermal cells indicated that AtGALT2 is probably localized in the endomembrane system consistent with its function.

Synthesis and Antitumor Activity of Ellagic Acid Peracetate.

Ellagic acid (1) was synthesized for the first time from methyl gallate through 3-pentagalloylglucose (α-PGG), and ellagic acid peracetate (3,4,3',4'-tetra-O-acetylellagic acid, 2) was derived from 1 by acetylation. Oral administration of 2 suppressed melanoma growth significantly in C7BL/6 immunocompetent mice without having any effect on natural killer (NK) cell activity. Comparison of the immunoenhancing activities of 1 and 2 indicated that the latter compound increased white blood cell quantities in peripheral blood and immune cells enriched from the bone marrow and liver of mice. Therefore, both the antitumor efficacy and the immunity enhancement by 2 were greater than those by 1. In addition, on oral administration neither 1 nor 2 resulted in whole body, liver, or spleen weight changes of normal, tumor-free mice, indicating that these compounds are potentially non-toxic to mice. It was shown that ellagic acid peracetate (2) inhibits B16 melanoma cell growth in vitro, and induces B16 cell apoptosis, corresponding to BCL-2 down-regulation. Collectively, the present data imply that 2 can suppress tumor growth by enhancing mouse immunity and inducing tumor cell apoptosis without apparent side effects.

Anomeric selectivity in the synthesis of galloyl esters of D-glucose.

The anomeric selectivity of the ester formation between d-glucopyranose and gallic acid was investigated under a variety of conditions. A new protocol was established that allows performing the reaction under conditions where mutarotation is very slow. Highly alpha- or beta-selective transformations are possible when starting with alpha- or beta-d-glucopyranose, respectively. Due to the kinetic anomeric effect, a high alpha-selectivity is more difficult to achieve than a high beta-selectivity. The new methodology presented in this article was compared with established procedures for the synthesis of gallotannins. In addition to the advantages of a high yield and an easy purification protocol, the new procedure uniquely allowed for a highly selective synthesis of alpha-products.

Antidiabetes and Anti-obesity Activity of Lagerstroemia speciosa.

The leaves of Lagerstroemia speciosa (Lythraceae), a Southeast Asian tree more commonly known as banaba, have been traditionally consumed in various forms by Philippinos for treatment of diabetes and kidney related diseases. In the 1990s, the popularity of this herbal medicine began to attract the attention of scientists worldwide. Since then, researchers have conducted numerous in vitro and in vivo studies that consistently confirmed the antidiabetic activity of banaba. Scientists have identified different components of banaba to be responsible for its activity. Using tumor cells as a cell model, corosolic acid was isolated from the methanol extract of banaba and shown to be an active compound. More recently, a different cell model and the focus on the water soluble fraction of the extract led to the discovery of other compounds. The ellagitannin Lagerstroemin was identified as an effective component of the banaba extract responsible for the activity. In a different approach, using 3T3-L1 adipocytes as a cell model and a glucose uptake assay as the functional screening method, Chen et al. showed that the banaba water extract exhibited an insulin-like glucose transport inducing activity. Coupling HPLC fractionation with a glucose uptake assay, gallotannins were identified in the banaba extract as components responsible for the activity, not corosolic acid. Penta-O-galloyl-glucopyranose (PGG) was identified as the most potent gallotannin. A comparison of published data with results obtained for PGG indicates that PGG has a significantly higher glucose transport stimulatory activity than Lagerstroemin. Chen et al. have also shown that PGG exhibits anti-adipogenic properties in addition to stimulating the glucose uptake in adipocytes. The combination of glucose uptake and anti-adipogenesis activity is not found in the current insulin mimetic drugs and may indicate a great therapeutic potential of PGG.

Synthesis and structure-activity relationship study of antidiabetic penta-O-galloyl-D-glucopyranose and its analogues.

The rapid increase of obesity-associated diabetes has created urgent demands for more effective antidiabetic therapies and pharmaceuticals that are able to address the problems of hyperglycemia and weight gain simultaneously. Our previous studies indicated that the alpha- and beta-anomers of penta-O-galloyl-D-glucopyranose (PGG), 2 and 3, act as insulin mimetics that bind to and activate the insulin receptor, stimulate glucose transport in adipocytes, and reduce blood glucose and insulin levels in diabetic and obese animals. In addition, they inhibit differentiation of preadipocytes into adipocytes. These activities suggest that 2 and 3 may reduce blood glucose without increasing adiposity. To investigate the structure-activity relationship of 2 and 3, four series of novel compounds were prepared and their glucose transport stimulatory activities were measured using a radioactive glucose uptake bioassay. The assay results indicate that both the glucose and the galloyl groups are critical to the activity of 2 and 3. It appears that the glucose core provides an optimal scaffold to present the galloyl groups with the correct spatial orientation to induce activity. Moreover, the galloyl groups linked to the 1, 2, 3, and 4 positions of glucose are essential, while the galloyl group connected to the 6 position of 2 is unnecessary for the induction of activity. The discovery that two related novel compounds, 6-deoxytetra-O-galloyl-alpha-D-glucopyranose (43) and tetra-O-galloyl-alpha-D-xylopyranose (59), also possess glucose transport stimulatory activity suggests that 2 may be further modified around position 6 to modulate and enhance its efficacy. To test this hypothesis, we developed a new synthetic method that allows for the stereoselective preparation of derivatives of 2 that are modified on C-6. We found that 6-chloro-6-deoxy-1,2,3,4-tetra-O-galloyl-alpha-D-glucopyranose (80) exhibits a significantly higher glucose transport stimulatory activity than 2. Its activity is comparable to that of insulin.

Natural anti-diabetic compound 1,2,3,4,6-penta-O-galloyl-D-glucopyranose binds to insulin receptor and activates insulin-mediated glucose transport signaling pathway.

Insulin mimetics from natural sources are potential therapeutics that can act alone or supplement insulin and other anti-diabetic drugs in the prevention and treatment of diabetes. We recently reported the insulin-like glucose transport stimulatory activity of tannic acid (TA) in 3T3-L1 adipocytes. In this study, we find that chemically synthesized 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose (beta-PGG), one of the components of TA, as well as its natural anomer alpha-PGG possess activity. Mechanistic studies in adipocytes with alpha-PGG, the more potent of the two anomers, reveal that inhibitors that block the insulin-mediated glucose transport, including one that inhibits the insulin receptor (IR), also completely abolish the glucose transport activated by alpha-PGG. In addition, alpha-PGG induces phosphorylation of the IR and Akt, activates PI 3-kinase, and stimulates membrane translocation of GLUT 4. Receptor binding studies indicate that alpha-PGG binds to the IR and affects the binding between insulin and IR by reducing the maximum binding of insulin to IR without significantly altering the binding affinity of insulin to IR. Western blotting analysis of the products of a cross-linking reaction suggests that alpha-PGG may bind to IR at a site located on the alpha-subunit of the receptor. Animal studies demonstrate that PGG reduces blood glucose levels and improves glucose tolerance in diabetic and obese animals. Our results suggest that PGG may serve as a model for the development of new types of anti-diabetic and anti-metabolic syndrome therapeutics.