Effect Of BFGF On Radiation-induced Endothelial Cell Apoptosis And Its Signaling Pathways

Vascular endothelial cells are quite sensitive to radiation injury. Radiation-induced endothelial cell apoptosis is involved in the development of many radiation injury diseases and complications (such as in radiation-induced skin ulcers, RSU or radiation-induced lung fibrosis), because of leading to dysfunction of blood vessels, hemorrhage and difficulty in angiogenesis. It is reported that radiation-induced apoptosis of endothelial cells in vitro and in vivo can be inhibited by bFGF. However, little is known about proangiogenic growth factor activation of cell survival pathways that inhibit radiation-induced endothelial cell apoptosis. Therefore studying the effect of bFGF on radiation-induced endothelial apoptosis and its signaling pathways may get a new insight into the mechanism and treatment of RSU as well as radiation-induced lung fibrosis. The study will also help to find potential therapeutic targets in tumor vascular endothelium to enhance the efficacy of radiation.In the present study, we investigated the survival role of bFGF on HUVEC exposed to irradiation. We found that bFGF potently prevents radiation-induced HUVEC apoptosis and promotes survival. Using several approaches, we demonstrated the critical role of Ras/MEK/MAPK/Rsk2/Bad (serine 112) pathway and PI3K/Akt/Bad (serine 136) pathway in such effects. We also show that bFGF induces CREB phosphorylation in the irradiated HUVEC.1. bFGF inhibits radiation-induced HUVEC apoptosis through Ras/MEK/MAPK/Rsk2/Bad (serine 112) pathway, and induces CREB phosphorylation at serine 133 in irradiated HUVECWe set up a cell apoptosis model in which primary cultured human umbilical vein endothelial cells (HUVEC) were irradiated with 60Co Y -rays. We found that bFGF obviously inhibited radiation-induced HUVEC apoptosis, and the effect was mediated in part by Ras/MEK/MAPK/Rsk2 (p90 ribosomal S6 kinase 2)/Bad pathway. This pathway was activated by irradiated HUVEC exposure to bFGF, involving phosphorylation of FGFR, MEK and p44/42 MAPK. The survival enhancing effect ofbFGF was partly inhibited by pretreatment with U0126 or PD98059. The fact that the anti-apoptotic effect of bFGF on irradiated HUVEC was not completely abrogated by U0126 or PD98059 suggests that other survival signaling may exist, perhaps via PI3K/Akt pathway. Transfection of a dominant-negative form of Rsk2 (DN Rsk2) in part blocked the anti-apoptosis effect of bFGF in irradiated HUVEC. Moreover, we firstly provided evidences that bFGF induced Bad phosphorylation (at serine 112) and CREB (cAMP response element-binding protein) phosphorylation (at serine 133) in the -ray irradiated HUVEC. bFGF-induced phosphorylation of BAD at serine 112 was diminished when the irradiated HUVEC were first incubated with the MEK inhibitor PD98059. In our model, stimulation with bFGF resulted in an immediate and dramatic loss of Bad-Bcl-XL association, and inhibition of MAPK signaling-dependent Bad phosphorylation at serine 112 promoted increased association with BC!-XL, suggests that MAPK pathway-dependent serine 112 phosphorylation of Bad is critical for the dissociation of Bad-Bcl-XL dimmers and the survival effect of bFGF. These results showed that Ras/MAPK/Bad pathway mediated the survival enhancing effect bFGF on HUVEC exposed to radiation. It is suggested that Ras/MAPK pathway in tumor vascular endothelium could be a potential therapeutic target to enhance the efficacy of ionizing radiation. Activation of Ras/MAPK pathway plays an important role in bFGF-induced endothelial survival during development of radiation-induced lung fibrosis or in treatment of RSU.2. bFGF inhibits radiation-induced HUVEC apoptosis via PI3K/Akt pathway, and by phosphorylation of Bad at serine 136PI3K/Akt signaling is an important pathway that promotes endothelial cell survival. In the present study, we found that PI3K/Akt signaling is more important than Ras/MAPK pathway in mediating bFGF-induced inhibiton of HUVEC apoptosis after radiation exposure. This pathway was activated by irradiated HUVEC exposure to bFGF, i...