by Rehana Perveen, Kevin Funk, Jean Thuma, Shelli Wulf Ridge, Yanyan Cao, Jan Willem N. Akkerman, Xiaozhuo Chen, Huzoor Akbar
We have shown that 1,2,3,4,6-penta-O-galloyl-a-D-glucopyranose (a-PGG), an orally effective hypoglycemic small molecule, binds to insulin receptors and activates insulin-mediated glucose transport. Insulin has been shown to bind to its receptors on platelets and inhibit platelet activation. In this study we tested our hypothesis that if insulin possesses anti-platelet properties then insulin mimetic small molecules should mimic antiplatelet actions of insulin. Principal Findings
Incubation of human platelets with insulin or a-PGG induced phosphorylation of insulin receptors and IRS-1 and blocked ADP or collagen induced aggregation. Pre-treatment of platelets with a-PGG inhibited thrombin-induced release of P-selectin, secretion of ATP and aggregation. Addition of ADP or thrombin to platelets significantly decreased the basal cyclic AMP levels. Pre-incubation of platelets with a-PGG blocked ADP or thrombin induced decrease in platelet cyclic AMP levels but did not alter the basal or PGE1 induced increase in cAMP levels. Addition of a-PGG to platelets blocked agonist induced rise in platelet cytosolic calcium and phosphorylation of Akt. Administration of a-PGG (20 mg kg-1) to wild type mice blocked ex vivo platelet aggregation induced by ADP or collagen. Conclusions
These data suggest that a-PGG inhibits platelet activation, at least in part, by inducing phosphorylation of insulin receptors leading to inhibition of agonist induced: (a) decrease in cyclic AMP; (b) rise in cytosolic calcium; and (c) phosphorylation of Akt. These findings taken together with our earlier reports that a-PGG mimics insulin signaling suggest that inhibition of platelet activation by a-PGG mimics antiplatelet actions of insulin.