Citrulline Plus Arginine Induces an Angiogenic Response and Increases Permeability in Retinal Endothelial Cells via Nitric Oxide Production.

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We previously observed elevated plasma levels of citrulline and arginine in diabetic retinopathy patients compared to diabetic controls. We tested our hypothesis that citrulline plus arginine induces angiogenesis and increases permeability in retinal endothelial cells. Human retinal microvascular endothelial cells (HRMECs) were treated with citrulline, arginine, or citrulline + arginine, and angiogenesis was measured with cell proliferation, migration, and tube formation assays. Permeability was measured in HRMEC monolayers via trans-endothelial electrical resistance (TEER) and FITC-labeled dextran. We also measured arginase activity, arginase-1 and arginase-2 expression, protein expression and phosphorylation of endothelial nitric oxide synthase (eNOS), and nitric oxide (NO) production. Citrulline + arginine induced endothelial cell proliferation (p = 0.018), migration (p = 0.011), and tube formation (p = 0.0042). Citrulline + arginine also increased FITC-dextran flow-through (p = 1.5 × 10^-5) and decreased TEER (p = 0.010). Citrulline + arginine had no effect on arginase activity, but it increased eNOS (p = 6.3 × 10^-4) and phosphorylated eNOS (p = 0.029), as well as NO production (p = 0.025). Inhibiting eNOS prevented the increase in NO (p = 0.0092), inhibited citrulline + arginine-induced cell migration (p = 0.0080) and tube formation (p = 0.0092), and blocked citrulline + arginine-related alterations in FITC-dextran flow-through (p = 3.6 × 10^-4) and TEER (p = 3.9 × 10^-4). These data suggest that citrulline + arginine treatment induces angiogenesis and increases permeability in retinal endothelial cells by activating eNOS and increasing NO production.