No abstract

Summary. Background: Quercetin, a flavonoid present in the human diet, which is found in high levels in onions, apples, tea and wine, has been shown previously to inhibit platelet aggregation and signaling in vitro. Consequently, it has been proposed that quercetin may contribute to the protective effects against cardiovascular disease of a diet rich in fruit and vegetables. Objectives: A pilot human dietary intervention study was designed to investigate the relationship between the ingestion of dietary quercetin and platelet function. Methods: Human subjects ingested either 150 mg or 300 mg quercetin-4¢-O-b-D-glucoside supplement to determine the systemic availability of quercetin. Platelets were isolated from subjects to analyse collagen-stimulated cell signaling and aggregation. Results: Plasma quercetin concentrations peaked at 4.66 lM (± 0.77) and 9.72 lM (± 1.38) 30 min after ingestion of 150-mg and 300-mg doses of quercetin-4¢-O-b-Dglucoside, respectively, demonstrating that quercetin was bioavailable, with plasma concentrations attained in the range known to affect platelet function in vitro. Platelet aggregation was inhibited 30 and 120 min after ingestion of both doses of quercetin-4¢-O-b-D-glucoside. Correspondingly, collagen-stimulated tyrosine phosphorylation of total platelet proteins was inhibited. This was accompanied by reduced tyrosine phosphorylation of the tyrosine kinase Syk and phospholipase Cc2, components of the platelet glycoprotein VI collagen receptor signaling pathway. Conclusions: This study provides new evidence of the relatively high systemic availability of quercetin in the form of quercetin-4¢-O-b-D-glucoside by supplementation, and implicates quercetin as a dietary inhibitor of platelet cell signaling and thrombus formation.

No abstract

Formation and rearrangement of disulfide bonds during the correct folding of nascent proteins is modulated by a family of enzymes known as thiol isomerases, which include protein disulfide isomerase (PDI), endoplasmic reticulum protein 5 (ERP5), and ERP57. Recent evidence supports an alternative role for this family of proteins on the surface of cells, where they are involved in receptor remodeling and recognition. In platelets, blocking PDI with inhibitory antibodies inhibits a number of platelet activation pathways, including aggregation, secretion, and fibrinogen binding. Analysis of human platelet membrane fractions identified the presence of the thiol isomerase protein ERP5. Further study showed that ERP5 is resident mainly on platelet intracellular membranes, although it is rapidly recruited to the cell surface in response to a range of platelet agonists. Blocking cell-surface ERP5 using inhibitory antibodies leads to a decrease in platelet aggregation in response to agonists, and a decrease in fibrinogen binding and P-selectin exposure. It is possible that this is based on the disruption of integrin function, as we observed that ERP5 becomes physically associated with the integrin ␤ 3 subunit during platelet stimulation. These results provide new insights into the involvement of thiol isomerases and regulation of platelet activation. IntroductionIn classical terms, reduction/oxidation systems within a cell have been represented very simply. The cytoplasmic environment is hypoxic and reducing, whereas the extracellular environment is normoxic and oxidizing. The generation of a disulfide bond from 2 cysteine residues is an oxidation reaction. To correctly generate these bonds inside the cell, there are, therefore, a group of enzymes known as the thiol isomerases. These are capable of the formation, reduction, and rearrangement of the disulfide-bonding patterns of proteins, often as part of folding of nascent proteins. The thiol isomerase enzymes are anchored to the endoplasmic reticulum via KDEL-receptor proteins. [1][2][3] Recent studies have suggested additional functions for thiol isomerase enzymes: on the surface of cells, where they participate in receptor activation and remodeling, and substrate processing. [4][5][6] Protein disulfide isomerase (PDI) is the best-characterized thiol isomerase to demonstrate this dual functionality. A number of cell types, including bovine aortic endothelial cells, 7 rat hepatocytes, 8,9 and human B cells, 5,10 have been shown to secrete PDI, which associates with the cell surface. Cell-surface PDI has been implicated in the reduction of the disulfide-linked diptheria toxin heterodimer 11 and events triggering the entry of HIV into lymphoid cells. 6,12 On the basis of a series of investigations, initially by Detweiller and coworkers, a role for PDI in platelet physiology is now established. 4,[13][14][15][16] Early studies demonstrated PDI was present on the external membrane of activated and resting platelets, and proteins with thiol isomerase activity were secreted f...