Aim: In this study, we investigated the effects of caffeic acid (CAFA), a phenolic acid, on Ca
2+-antagonistic cyclic nucleotides associated with the phosphorylation of inositol 1,4,5-trisphosphate receptor (IP
3R) and vasodilator-stimulated phosphoprotein (VASP) and the thromboxane A
2 (TXA
2)-associated microsomal cyclooxygenase-1 (COX-1) activity in collagen (10
μg/mL)-stimulated platelet aggregation.
Methods: Washed platelets (10
8/mL) obtained from Sprague-Dawley rats (6-7 weeks old, male) were preincubated for 3 minutes at 37℃ in the presence of 2 mM exogenous CaCl
2 with or without CAFA or other materials, stimulated with collagen (10
μg/mL) for 5 minutes, then used to determine the levels of intracellular cytosolic Ca
2+ ([Ca
2+]
i), TXA
2, cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), COX-1 activity, VASP and IP
3R phosphorylation.
Results: CAFA dose-dependently inhibited collagen-induced platelet aggregation and suppressed the production of TXA
2, an aggregation-inducing autacoid associated with the strong inhibition of COX-1 in platelet microsomes exhibiting cytochrome C reductase activity. CAFA dose-dependently inhibited collagen-elevated [Ca
2+]
i mobilization, which was increased by a cAMP-dependent protein kinase (A-kinase) inhibitor, Rp-8-Br-cAMPS, but not a cGMP-dependent protein kinase (G-kinase) inhibitor, Rp-8-Br-cGMPS. In addition, CAFA significantly increased the formation of cAMP and cGMP, intracellular Ca
2+-antagonists that function as aggregation-inhibiting molecules. CAFA increased IP3R (320 kDa) phosphorylation, indicating the inhibition of IP
3-mediated Ca
2+ release from internal stores (i.e. the dense tubular system)
via the IP
3R on collagen-activated platelets. Furthermore, CAFA-induced IP
3R phosphorylation was strongly inhibited by an A-kinase inhibitor, Rp-8-Br-cAMPS, but only mildly inhibited by a G-kinase inhibitor, Rp-8-Br-cGMPS. These results suggest that the inhibition of [Ca
2+]i mobilization by CAFA is resulted from the cAMP/A-kinase-dependent phosphorylation of IP
3R. CAFA elevated the phosphorylation of VASP-Ser
157, an A-kinase substrate, but not the phosphorylation of VASP-Ser
239, a G-kinase substrate. We demonstrate that CAFA increases cAMP and subsequently phosphorylates both IP
3R and VASP-Ser
157 through A-kinase activation to inhibit [Ca
2+]
i mobilization and TXA
2 production
via the inhibition of the COX-1 activity.
Conclusions: These results strongly indicate that CAFA is a potent beneficial compound that elevates the level of cAMP-dependent protein phosphorylation in collagen-platelet interactions, which may result in the prevention of platelet aggregation-mediated thrombotic diseases.
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