Increased bleeding tendency and decreased susceptibility to thromboembolism in mice lacking the prostaglandin E receptor subtype EP3

Background-Among the prostanoids, thromboxane (TX) A(2) is a potent stimula tor of platelets, whereas prostaglandin (PG) I-2 inhibits their activation. The roles of PGE(2) in the regulation of platelet function have not been e stablished, however, and the contribution of PGE(2) in hemostasis and throm boembolism is poorly understood. The present study was intended to clarify these roles of PGE(2) by using mice lacking the PGE(2) receptor subtype 3 ( EP3-/- mice). Methods and Results-Expression of mRNAs for EP3 in murine platelets was con firmed by quantitative reverse transcription-polymerase chain reaction. PGE (2) and AE-248, a selective EP3 agonist, showed concentration-dependent pot entiation of platelet aggregation induced by U46619, a TXA(2) receptor agon ist, although PGE2 alone could not induce aggregation. PGE(2) and AE-248 in creased cytosolic calcium ion concentration ([Ca2+](i)), and AE-248 inhibit ed the forskolin-induced increase in cytosolic cAMP concentration ([cAMP]i) , suggesting G(i) coupling of EP3. The potentiating effects of PGE(2) and A E-248 on platelet aggregation along with their effects on [Ca2+](i) and [cA MP](i) were absent in EP3-/- mice. In vivo, the bleeding time was significa ntly prolonged in EP3-/- mice. Moreover, when mice were challenged intraven ously with arachidonic acid, mortality and thrombus formation in the lung w ere significantly reduced in EP3-/- mice. Conclusions-PGE(2) potentiated platelet aggregation induced by U46619 via E P3 by increasing [Ca2+](i), decreasing [cAMP](i), or both. This potentiatin g action of PGE(2) via EP3 is essential in mediating both physiological and pathological effects of PGE(2) in vivo.