Erythropoietin increases endothelial biosynthesis of tetrahydrobiopterin by activation of protein kinase Bα/Akt1

Tetrahydrobiopterin (BH4) is an essential cofactor required for enzymatic activity of endothelial NO synthase. Recently, it has been shown that vascular protective effects of erythropoietin (EPO) are dependent on activation of endothelial NO synthase. Therefore, our objective was to characterize the effect of EPO on the biosynthesis of BH4 in the vascular wall. Incubation of isolated C57BL/6J mouse aortas for 18 hours with recombinant human EPO (1 to 50 U/mL) caused a concentration-dependent increase in intracellular BH4 levels and activity of GTP-cyclohydrolase I. Maximal biosynthesis of BH4 was detected at therapeutic concentrations of 5 U/mL. Removal of the endothelium abolished EPO-induced biosynthesis of BH4 demonstrating that the vascular endothelium is a major source of BH4. Treatment with a selective phosphatidylinositol 3-kinase inhibitor wortmannin significantly reduced BH4 biosynthesis stimulated by EPO. The stimulatory effect of EPO on vascular GTP-cyclohydrolase I activity, BH4 production, and phosphorylation of endothelial NO synthase was also detected in vivo in mice treated with recombinant human EPO. These effects of EPO were abolished in protein kinase Bα/Akt1-deficient mice. In addition, EPO significantly increased systolic blood pressure and the number of circulating platelets in Akt1-deficient mice. Our results demonstrate that EPO stimulates biosynthesis of BH4 in vascular endothelium and that the increase in BH4 levels is caused by de novo biosynthesis of BH4 via the phosphatidylinositol 3-kinase/Akt1 pathway. This effect is most likely designed to provide optimal intracellular concentration of the cofactor necessary for EPO-induced elevation of endothelial NO synthase activity.