Angiopoietin-1 activates both anti- and proapoptotic mitogen-activated protein kinases

In this study, we identified whether mitogen-activated protein kinases (MAPKs) mediate the effects of angiopoietin-1 (Ang-1) on endothelial cell apoptosis. Exposure of human umbilical vein endothelial cells to Ang-1 (300 ng/ml) evoked within 15-30 min a 15-fold and a 5-fold increase in phosphorylation of ERK1/2 and p38 MAPKs, respectively. Inhibitors of the PI-3 kinase pathway attenuated Ang-1-induced ERK1/2 phosphorylation at a level up-stream from Raf and MEK1/2, but these inhibitors augmented Ang-1-induced p38 phosphorylation. When serum and growth supplements were withdrawn, the percentage of endothelial apoptosis tripled over 24 h compared with control cells. The presence of Ang-1 (300 ng/ml) significantly attenuated endothelial cell apoptosis and inhibited caspase-9, -7, and -3 activation. These antiapoptotic effects were augmented when a p38 inhibitor was combined with Ang-1, whereas inhibition of ERK1/2 eliminated the antiapoptotic properties of Ang-1. We conclude that both anti- (ERK1/2) and pro- (p38) apoptotic members of MAPKs are simultaneously activated by Ang-1 in endothelial cells and that activation of ERK1/2 by Ang-1 is mediated through the PI-3 kinase pathway. The strong antiapoptotic effects of the ERK and the PI-3 kinase pathways mask the proapoptotic function of p38 MAPKs resulting in net attenuation of apoptosis by Ang-1.