Metabolite control of angiogenesis: angiogenic effect of citrate

Endothelial cells respond to hypoxic changes with resultant accumulation of several metabolites and switch over to angiogenic phenotype. Although certain intermediates of glycolytic and oxidative metabolic pathways have been known to affect angiogenesis, the effect of citrate, which accumulates in certain tumors, on angiogenesis is not known. Therefore, the effect of citrate on angiogenesis was studied using different model systems. Increased vascularization in chorioallantoic membrane assay, increased endothelial sprouting in rat aortic rings, and increased expression of CD31, E-selectin in endothelial cells suggested a possible proangiogenic effect of citrate. Upregulation of angiogenic factors such as vascular endothelial growth factor and fibroblast growth factor suggested that the effect of citrate involves modulation of expression of angiogenic growth factors. LY 294002, an inhibitor of PI3K-Akt pathway, and wortmannin, an inhibitor of Akt pathway, reversed the effect of citrate in human umbilical vein endothelial cells. Citrate induced significant upregulation and activation of Akt in endothelial cells. Rapamycin, an inhibitor of mTOR, also reversed the effect of citrate in human umbilical vein endothelial cells and sprouting of aortic rings suggesting that the angiogenic effect of citrate involves activation of PI3K-Akt-mTOR pathway.