Thrombopoietin stimulates endothelial cell motility and neoangiogenesis bya platelet-activating factor-dependent mechanism

In this study, we demonstrate that human umbilical cord vein-derived endoth elial cells (HUVECs) expressed c-Mpl, the thrombopoietin (TPO) receptor, an d that TPO activates HUVECs in vitro, as indicated by directional migration , synthesis of 1-alkyl-/1-acyl-platelet-activating factor (PAF) and interle ukin-8 (IL-8), and phosphorylation of the signal transducers and activators of transcription (STAT) STAT1 and STAT5B. The observation that WEB 2170 an d CV3988, 2 structurally unrelated PAF receptor antagonists, prevented the motility of HUVECs induced by TPO suggests a role of PAF as secondary media tor. Moreover, kinetic analysis of TPO-induced tyrosine phosphorylation of STAT demonstrated that STAT5B activation temporally correlated with the syn thesis of PAF. PAF, in turn, induced a rapid tyrosine phosphorylation of ST AT5B and PAF receptor blockade, by WEB 2170, preventing both TPO- and PAF-m ediated STATSE activation. The in vivo angiogenic effect of TPO, studied in a mouse model of Matrigel implantation, demonstrated that TPO induced a do se-dependent angiogenic response that required the presence of heparin. Mor eover, the in vivo angiogenic effect of TPO was inhibited by the PAF recept or antagonist WEB 2170 but not by the anti-basic fibroblast growth factor n eutralizing antibody. These results indicate that the effects of TPO are no t restricted to cells of hematopoietic lineages, because TPO is able to act ivate endothelial cells and to induce an angiogenic response in which the r ecruitment of endothelial cells is mediated by the synthesis of PAF. Moreov er, biochemical analysis supports the hypothesis that STATSE may be involve d in the signaling pathway leading to PAF-dependent angiogenesis.