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-β-d-glucopyranose (β-PGG), one of the components of TA, as well as its natural anomer α-PGG possess activity. Mechanistic studies in adipocytes with α-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 α-PGG. In addition, α-PGG induces phosphorylation of the IR and Akt, activates PI 3-kinase, and stimulates membrane translocation of GLUT 4. Receptor binding studies indicate that α-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 α-PGG may bind to IR at a site located on the α-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.