Erythropoietin promotes survival of primary human endothelial cells through PI3K-dependent, NF-kappaB-independent upregulation of Bcl-xL.

Erythropoietin (EPO) regulates the production of red blood cells primarily by preventing apoptosis of erythroid progenitors. More recently, however, EPO has emerged as a major cytoprotective cytokine in several nonhemopoietic tissues in the setting of stress or injury. The underlying mechanisms of the protective responses of EPO have not been fully defined. Here we show that EPO triggers a phosphatidylinositol 3-kinase-(PI3K)-dependent survival pathway that counteracts endothelial cell death. The protection conferred by PI3K relies on the subsequent induction of Bcl-x(L), a prosurvival member of the Bcl-2 protein family. In addition, EPO counteracts the upregulation of the pro-apoptotic BH3-only protein BIM, which is induced by serum withdrawal. EPO also activates extracellular signal-regulated kinase 1 and 2 (ERK1/2), which are involved in a Bcl-x(L)-independent cytoprotective pathway. EPO caused a prolonged activation of nuclear factor (NF)-kappaB, which was blocked by inhibition of PI3K, but not by inhibition of mitogen-activated protein (MAP)/ERK kinase (MEK), suggesting that EPO-activated NF-kappaB requires PI3K activity. However, the activation of the NF-kappaB pathway was not required for the ability of EPO to counteract endothelial apoptosis. Thus EPO promotes survival of endothelial cells through PI3K-dependent Bcl-x(L)-induction and BIM regulation, as well as through a separate mechanism involving the ERK pathway.