The role of PI3K/Akt/mTOR signaling in dose-dependent biphasic effects of glycine on vascular development.

Glycine, a non-essential amino acid, exerts concentration-dependent biphasic effects on angiogenesis. Low-doses of glycine promote angiogenesis, whereas high-doses cause anti-angiogenesis. The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling participates in angiogenesis of both physiological development, and pathological events including tumor and inflammation. We assessed the role of PI3K/Akt/mTOR signaling in vascular development, and the interaction with glycine, using transgenic zebrafish Tg(fli1a:Myr-mCherry) embryos expressing fluorescent proteins in vascular endothelial cells. Treatment with inhibitors of mTORC1 (rapamycin and everolimus), mTORC1/mTORC2 (KU0063794), PI3K (LY29400), and Akt (Akt inhibitor) decreased the development of intersegmental vessels (ISVs). These inhibitors cancelled the angiogenic effects of a low-dose of glycine, while acted synergistically with a high-dose of glycine in anti-angiogenesis. mTOR signaling regulates the gene expression of vascular endothelial growth factor (VEGF), a major angiogenic factor, and nitric oxide (NO) synthase (NOS), an enzyme for the synthesis of an angiogenic mediator NO. Expressions of VEGF and NOS were consistent with the vascular features induced by glycine and an mTOR inhibitor. Our results suggest that PI3K/Akt/mTOR signaling may interact with dose-dependent biphasic effects of exogenous glycine on in vivo angiogenesis. mTOR signaling is a key target for cancer therapy, thus, the combining mTOR inhibitors with glycine may be a potential approach for controlling angiogenesis.