Epac Is Involved in cAMP-Mediated Exocytosis of Weibel-Palade Bodies in Endothelial Cells through the Activation of the Small GTPase Rap1.

Vascular endothelial cells provide a dynamic interface between circulating blood and underlying tissues that is critically involved in maintaining vascular integrity and homeostasis. The endothelium provides a surface for adhesion and subsequent extravasation of leukocytes to sites of inflammation. In addition, vascular endothelial cells are involved in the regulation of vascular tone, contribute to neo-vascularisation and mediate the formation of a platelet plug in the event of vascular damage. Rapid recruitment of bio-active components from intracellular storage pools has been shown to contribute to the critical role of endothelial cells in maintaining vascular homeostasis. A significant number of haemostatic components and inflammatory mediators originate from endothelial cell-specific, cigar-shaped organelles called Weibel-Palade bodies (WPBs) that function as storage vesicles for von Willebrand factor (VWF), a multimeric adhesive glycoprotein crucial for platelet plug formation, the leukocyte receptor P-selectin and a number of bioactive compounds that include the chemoattractants IL-8 and eotaxin-3. Following stimulation with agonists that increase intracellular Ca²⁺ levels, such as thrombin or histamine, WPBs fuse with the plasma membrane, thereby releasing their content onto the cellular surface or into the circulation. More recently, agonists such as epinephrine and vasopressin that raise intracellular levels of cAMP have been shown to promote the release of WPBs. The physiological importance of this pathway is illustrated by the rise in VWF levels in patients with von Willebrand’s disease and mild hemophilia A following administration of the vasopressin analogue desmopressin (DDAVP) or epinephrine. We have previously shown that cAMP-mediated WPB release is dependent on protein kinase A (PKA) and involves activation of the small GTPase RalA. Here, we have investigated a possible role for another, PKA-independent cAMP-mediated signaling pathway in the regulation of WPB exocytosis, namely the exchange protein activated by cAMP (Epac) and its substrate, the small GTPase Rap1. SiRNA-mediated knockdown of the cAMP-GEF Epac1 results in decreased epinephrine-induced WPB exocytosis. Epinephrine stimulation of endothelial cells leads to activation of the small GTPase Rap1 in a PKA-independent fashion. In addition, siRNA-mediated knockdown of Epac1 completely abolished epinephrine-induced activation of Rap1 indicating that activation of Rap1 in response to epinephrine proceeds via Epac1. We subsequently addressed whether activation of Rap1 is crucial for WPB release. Downregulation of Rap1 activation through expression of Rap1GAP effectively inhibits epinephrine-induced WPB exocytosis. Taken together, these data uncover a new pathway by which endothelial cells can regulate WPB exocytosis in response to agonists that signal through cAMP.