The phosphatidylinositol 3'-kinase (PI3K)/protein kinase B (Akt) signal transduction pathway plays a critical role in mediating endothelial cell survival during oxidative stress. The role of the PI3K/Akt pathway in promoting cell viability was studied in vascular endothelial cells treated with ionizing radiation. Western blot analysis showed that Akt was phosphorylated rapidly in response to radiation in primary culture human umbilical vein endothelial cells in the absence of serum or growth factors. Akt phosphorylation occurred after doses as low as 1 Gy. PI3K consists of p85 and p110 subunits, which play a central role in Akt activation in response to exogenous stimuli. A mutation within the Src homology region 2 domain of mutant p85 (Deltap85) prevented radiation-induced Akt phosphorylation, when overexpressed in endothelial cells. Vascular endothelial cells transduced with control vector were resistant to radiation-induced apoptosis, whereas endothelial cell transduction with adenovirus encoding the mutated p85 (Ad.Deltap85) reversed this resistance to apoptosis after treatment with intermediate radiation doses (2-6 Gy). Deltap85 overexpression alone had no effect on the viability or apoptosis of endothelial cells. However, irradiated endothelial cells overexpressing Deltap85 released cytochrome c into the cytosol fraction and activated proteolytic cleavage of caspases 3 and 9, thereby inducing the apoptotic response. Inhibition of caspase 3 blocked endothelial apoptosis induced by overexpression of Deltap85 and radiation. These findings suggest that growth factor-independent activation of Akt contributes, in part, to the inherent resistance of irradiated vascular endothelium to the activation of apoptotic response.