Flavonoids inhibit platelet function through binding to the thromboxane A2 receptor

Background. Dietary flavonoids are known for their antiplatelet activity resulting in cardiovascular protection, although the specific mechanisms by which this inhibition occurs has not been fully established. Objective: The aim of this study was to investigate the interaction of nine flavonoids representative of various chemical classes, with platelet responses dependent on thromboxane A(2) (TxA(2)) generation and on receptor antagonism, and to analyze the structural requirements for such effects. Methods: The effect of several types of flavonoids on platelet aggregation, serotonin release, and TxA2 generation was investigated. Competitive radioligand binding assays were used to screen for affinity of these compounds to TxA2 receptors. Results: Flavones (apigenin and luteolin) and isoflavones (genistein) abrogated arachidonic acid and collagen-induced platelet responses, such as aggregation and secretion, with a less substantial effect on TxA2 synthesis. These compounds were identified as specific ligands of the TxA(2) receptor in the mumol L-1 range, this effect accounting for antiplatelet effects related to stimulation with those agonists. Tight binding of flavonoids to the human TxA2 receptor relies on structural features such as the presence of the double bond in C2-C3, and a keto group in C4. Conclusions: The inhibition by specific flavonoids of in vitro platelet responses induced by collagen or arachidonic acid seems to be related, to a great extent, to their ability to compete for binding to the TxA2 receptor. Therefore, antagonism of this TxA2 receptor may represent an additional mechanism for the inhibitory effect of these compounds in platelet function.