AUTHOR=Gaspar Renato Simões , da Silva Samira Abdalla , Stapleton Jennifer , Fontelles João Lucas de Lima , Sousa Hiran Reis , Chagas Vinicyus Teles , Alsufyani Shuruq , Trostchansky Andrés , Gibbins Jonathan M. , Paes Antonio Marcus de Andrade 

TITLE=Myricetin, the Main Flavonoid in Syzygium cumini Leaf, Is a Novel Inhibitor of Platelet Thiol Isomerases PDI and ERp5

JOURNAL=Frontiers in Pharmacology

VOLUME=10

YEAR=2020

URL=https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2019.01678

DOI=10.3389/fphar.2019.01678

ISSN=1663-9812

ABSTRACT=<sec><title>Background</title><p>Flavonoids have been characterized as a prominent class of compounds to treat thrombotic diseases through the inhibition of thiol isomerases. <italic>Syzygium cumini</italic> is a flavonoid-rich medicinal plant that contains myricetin and gallic acid. Little is known about the potential antiplatelet properties of <italic>S. cumini</italic> and its constituent flavonoids.</p></sec><sec><title>Objective</title><p>To evaluate the antiplatelet effects and mechanism of action of a polyphenol-rich extract (PESc) from <italic>S. cumini</italic> leaf and its most prevalent polyphenols, myricetin and gallic acid.</p></sec><sec><title>Methods</title><p>PESc, myricetin, and gallic acid were incubated with platelet-rich plasma and washed platelets to assess platelet aggregation and activation. <italic>In vitro</italic> platelet adhesion and thrombus formation as well as <italic>in vivo</italic> bleeding time were performed. Finally, myricetin was incubated with recombinant thiol isomerases to assess its potential to bind and inhibit these, while molecular docking studies predicted possible binding sites.</p></sec><sec><title>Results</title><p>PESc decreased platelet activation and aggregation induced by different agonists. Myricetin exerted potent antiplatelet effects, whereas gallic acid did not. Myricetin reduced the ability of platelets to spread on collagen, form thrombi <italic>in vitro</italic> without affecting hemostasis <italic>in vivo</italic>. Fluorescence quenching studies suggested myricetin binds to different thiol isomerases with similar affinity, despite inhibiting only protein disulfide isomerase (PDI) and ERp5 reductase activities. Finally, molecular docking studies suggested myricetin formed non-covalent bonds with PDI and ERp5.</p></sec><sec><title>Conclusions</title><p>PESc and its most abundant flavonoid myricetin strongly inhibit platelet function. Additionally, myricetin is a novel inhibitor of ERp5 and PDI, unveiling a new therapeutic perspective for the treatment of thrombotic disorders.</p></sec>