A structural basis for the inhibition of collagen-stimulated platelet function by quercetin and structurally related flavonoids.
OBJECTIVE
Molecular mechanisms underlying the links between dietary intake of flavonoids and reduced cardiovascular disease risk are only partially understood. Key events in the pathogenesis of cardiovascular disease, particularly thrombosis, are inhibited by these polyphenolic compounds via mechanisms such as inhibition of platelet activation and associated signal transduction, attenuation of generation of reactive oxygen species, enhancement of nitric oxide production and binding to thromboxane A(2) receptors. In vivo, effects of flavonoids are mediated by their metabolites, but the effects and modes of action of these compounds are not well-characterized. A good understanding of flavonoid structure-activity relationships with regard to platelet function is also lacking.


METHODS
Inhibitory potencies of structurally distinct flavonoids (quercetin, apigenin and catechin) and plasma metabolites (tamarixetin, quercetin-3'-sulphate and quercetin-3-glucuronide) for collagen-stimulated platelet aggregation and 5-hydroxytryptamine secretion were measured in human platelets. Tyrosine phosphorylation of total protein, Syk and PLCgamma2 (immunoprecipitation and Western blot analyses), and Fyn kinase activity were also measured in platelets. Internalization of flavonoids and metabolites in a megakaryocytic cell line (MEG-01 cells) was studied by fluorescence confocal microscopy.


RESULTS
The inhibitory mechanisms of these compounds included blocking Fyn kinase activity and the tyrosine phosphorylation of Syk and PLCgamma2 following internalization. Principal functional groups attributed to potent inhibition were a planar, C-4 carbonyl substituted and C-3 hydroxylated C ring in addition to a B ring catechol moiety.


CONCLUSIONS
The structure-activity relationship for flavonoids on platelet function presented here may be exploited to design selective inhibitors of cell signalling.
Institute for Cardiovascular and Metabolic Research, School of Biological Sciences, University of Reading, Reading, Berkshire, UK.
Bernice
B
Wright
Leonardo A
LA
Moraes
Charles F
CF
Kemp
William
W
Mullen
Alan
A
Crozier
Julie A
JA
Lovegrove
Jonathan M
JM
Gibbins
D001792:Blood Platelets / Q000187:drug effects*; D015153:Blotting, Western; D002478:Cells, Cultured; D024042:Collagen Type I / Q000494:pharmacology*; D005419:Flavonoids / Q000737:chemistry / Q000494:pharmacology; D006801:Humans; D047468:Immunoprecipitation; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D051966:Phospholipase C gamma / Q000378:metabolism; D010974:Platelet Aggregation / Q000187:drug effects*; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D051076:Proto-Oncogene Proteins c-fyn / Q000378:metabolism; D011794:Quercetin / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D012701:Serotonin / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013329:Structure-Activity Relationship; D020349:Surface Plasmon Resonance; D000072377:Syk Kinase
D024042:Collagen Type I; D005419:Flavonoids; D047908:Intracellular Signaling Peptides and Proteins; D010980:Platelet Membrane Glycoproteins; C098341:platelet membrane glycoprotein VI; D012701:Serotonin; D011794:Quercetin; D011505:Protein-Tyrosine Kinases; D051076:Proto-Oncogene Proteins c-fyn; C000606896:SYK protein, human; D000072377:Syk Kinase; D051966:Phospholipase C gamma