Mi-Ra Cho

Antiplatelet Activity of Epigallocatechin Gallate Is Mediated by the Inhibition of PLCγ2 Phosphorylation, Elevation of PGD2 Production, and Maintaining Calcium-ATPase Activity

We have previously reported that green tea catechins displayed a potent antithrombotic effect by inhibition of platelet aggregation. In the present study, the antiplatelet and antithrombotic activities of epigallocatechin gallate (EGCG), the major catechin derived from green tea, were extensively investigated. EGCG inhibited arterial thrombus formation and U46619-, collagen-, and arachidonic acid (AA)-induced washed rabbit platelet aggregation in a concentration-dependent manner, with IC50 values of 61 ± 3, 85 ± 4, and 99 ± 4 μM, respectively. In line with the inhibition of collagen-induced platelet aggregation, EGCG revealed blocking of the collagen-mediated phospholipase (PL) Cγ2 and protein tyrosine phosphorylation, and it caused concentration-dependent decreases of cytosolic calcium mobilization, AA liberation, and serotonin secretion. In addition, the platelet aggregation, intracellular Ca2+ mobilization, and protein tyrosine phosphorylation induced by thapsigargin, a Ca2+-ATPase pump inhibitor, were completely blocked by EGCG. Contrary to the inhibition of AA-induced platelet aggregation, EGCG failed to inhibit cyclooxygenase and thromboxane (TX) A2 synthase activities, but it concentration-dependently elevated AA-mediated PGD2 formation. In contrast, epigallocatechin (EGC), a structural analogue of EGCG lacking a galloyl group in the 3′ position, slightly inhibited collagen-stimulated cytosolic calcium mobilization, but failed to affect other signal transductions as did EGCG in activated platelets and arterial thrombus formation. These results suggest that antiplatelet activity of EGCG may be attributable to its modulation of multiple cellular targets, such as inhibitions of PLCγ2, protein tyrosine phosphorylation and AA liberation, and elevation of cellular PGD2 levels, as well as maintaining Ca2+-ATPase activity, which may underlie its beneficial effect on the atherothrombotic diseases.

Inhibitory effects of J78, a newly synthesized 1,4-naphthoquinone derivative, on experimental thrombosis and platelet aggregation

Several compounds with the backbone of 1,4-naphthoquinone chemical structure have been reported to display antiplatelet and antithrombotic activities, indicating that this congener compound may be a new source in the antithrombotic drug development. In the present study, the possible antiplatelet activity and antithrombotic efficacy of J78 (2-chloro-3-[2′-bromo, 4′-fluoro- phenyl]-amino-8-hydroxy-1,4-naphthoquinone), a newly synthesized 1,4-naphthoquinone derivative, were examined. Orally administered J78 (50, 100 mg/kg) dose dependently protected mice against the collagen + epinephrine-induced thromboembolic death. Orally administered J78 also significantly inhibited the ADP- and collagen-induced rat platelet aggregation ex vivo, with inhibition values of 44 and 40%, respectively. J78 inhibited the collagen-, arachidonic acid- and thrombin-induced human platelet aggregation concentration dependently in vitro, with IC50 values of 7.8 ± 0.4, 10.1 ± 0.4 and 18.4 ± 2.0 µmol/l, respectively. It was also active in inhibiting Ca2+ ionophore, A23187-induced platelet aggregation, suggesting that J78 may have an inhibitory effect on Ca2+ mobilization. J78, however, did not alter coagulation parameters such as activated partial thromboplastin time and prothrombin time in human plasma. Taken together, these results suggest that J78 may be a promising antithrombotic agent, and its antithrombotic activity may be due to antiplatelet rather than anticoagulation activity.

Antiplatelet activity of [5-(2-methoxy-5-chlorophenyl)furan-2-ylcarbonyl]guanidine (KR-32570), a novel sodium/hydrogen exchanger-1 and its mechanism of action.

The antiplatelet effects of a novel guanidine derivative, KR-32570 ([5-(2-methoxy-5-chlorophenyl) furan-2-ylcarbonyl]guanidine), were investigated with an emphasis on the mechanisms underlying its inhibition of collagen-induced platelet aggregation. KR-32570 significantly inhibited the aggregation of washed rabbit platelets induced by collagen (10 μg/mL), thrombin (0.05 U/mL), arachidonic acid (100 μM), a thromboxane (TX) A2 mimetic agent U46619 (9,11-dideoxy-9,11-methanoepoxy-prostaglandin F2, 1 μM) and a Ca2+ ATPase inhibitor thapsigargin (0.5 μM) (IC50 values: 13.8±1.8, 26.3±1.2, 8.5±0.9, 4.3±1.7 and 49.8±1.4 μM, respectively). KR-32570 inhibited the collagen-induced liberation of [3H]arachidonic acid from the platelets in a concentration dependent manner with complete inhibition being observed at 50 μM. The TXA2 synthase assay showed that KR-32570 also inhibited the conversion of the substrate PGH2 to TXB2 at all concentrations. Furthermore, KR-32570 significantly inhibited the [Ca2+]i mobilization induced by collagen at 50 μM, which is the concentration that completely inhibits platelet aggregation. KR-32570 also decreased the level of collagen (10 μg/mL)-induced secretion of serotonin from the dense-granule contents of platelets, and inhibited the NHE-1-mediated rabbit platelet swelling induced by intracellular acidification. These results suggest that the antiplatelet activity of KR-32570 against collagen-induced platelet aggregation is mediated mainly by inhibiting the release of arachidonic acid, TXA2 synthase, the mobilization of cytosolic Ca2+ and NHE-1.

BMS-191095, a cardioselective mitochondrial katp opener, inhibits human platelet aggregation by opening mitochondrial katp channels

We evaluated the antiplatelet effects of two classes of ATP-sensitive potassium channel openers (KATP openers) on washed human platelets, and the study’s emphasis was on the role of mitochondrial KATP in platelet aggregation. Collagen-induced platelet aggregation was inhibited in a dose dependent manner by lemakalim and SKP-450, which are potent cardio-nonselective KATP openers, and also by cardioselective BMS-180448 and BMS-191095 (IC50: 1,130, >1,500, 305.3 and 63.9 μM, respectively), but a significantly greater potency was noted for the cardioselective KATP openers. The latter two KATP openers also inhibited platelet aggregation induced by thrombin, another important blood-borne platelet activator, with similar rank order of potency (IC50: 498.0 and 104.8 μM for BMS-180448 and BMS-191095, respectively). The inhibitory effects of BMS-191095 on collagen-induced platelet aggregation were significantly blocked by a 30-min pretreatment of platelets with glyburide (1 μM) or sodium 5-hydroxydecanoate (5-HD, 100 μM), a nonselective and selective mitochondrial KATP antagonist, respectively, at similar magnitudes; this indicates the role of mitochondrial KATP in the antiplatelet activity of BMS-191095. However, glyburide and 5-HD had no effect when they were added to the platelet cuvette immediately prior to the addition of BMS-191095. These findings indicate that cardioselective mitochondrial KATP openers like BMS-191095 are able to exert cardioprotective effects in cardiac ischemia/reperfusion injury via dual mechanisms directed at the inhibition of platelet aggregation and the protection of cardiomyocytes, and both these mechanisms are mediated by mitochondrial KATP.