Protein phosphatase 2B inhibition promotes the secretion of von Willebrand factor from endothelial cells

Background:  Secretion of Weibel–Palade body (WPB) contents is regulated, in part, by the phosphorylation of proteins that constitute the endothelial exocytotic machinery. In comparison to protein kinases, a role for protein phosphatases in regulating endothelial exocytosis is undefined. Objective and method:  In this study, we investigated the role of protein phosphatase 2B (PP2B) in the process of endothelial exocytosis using pharmacological and gene knockdown approaches. Results:  We show that inhibition of protein phosphatase 2B (PP2B) activity by cyclosporine A (CsA), tacrolimus or a cell-permeable PP2B autoinhibitory peptide promotes the secretion of ultralarge von Willebrand factor (ULVWF) from human umbilical vein endothelial cells (HUVECs) in the absence of any other endothelial cell-stimulating agent. PP2B inhibitor-induced secretion and anchorage of ULVWF strings from HUVECs mediate platelet tethering. In support of a role for PP2B in von Willebrand factor (VWF) secretion, the catalytic subunit of PP2B interacts with the vesicle trafficking protein, Munc18c. Serine phosphorylation of Munc18c, which promotes granule exocytosis in other secretory cells, is increased in CsA-treated HUVECs, suggesting that this process may be involved in CsA-mediated WPB exocytosis. Furthermore, the plasma VWF antigen level is also enhanced in CsA-treated mice, and small interfering RNA-mediated knockdown of the α and β isoforms of the PP2B-A subunit in HUVECs enhanced VWF secretion. Conclusions:  These observations suggest that CsA promotes VWF release, in part by inhibition of PP2B activity, and are compatible with the clinically observed association of CsA treatment and increased plasma VWF levels in humans.