Abstract 1493: Erythropoietin Stimulates Endothelial Biosynthesis of Tetrahydrobiopterin by Activation of Phosphatidylinositol 3-kinase/Protein Kinase B Signal Transduction Pathway

Abstract

Erythropoietin (EPO) has been recognized as a tissue protective cytokine. Recently, it has been shown that vascular protective effects of EPO are dependent on activation of endothelial nitric oxide synthase (eNOS). Tetrahydrobiopterin (BH 4 ) is an essential cofactor required for enzymatic activity of eNOS. Therefore, our objective was to characterize the effect of EPO on biosynthesis of BH 4 in vascular wall. Incubation of isolated wild-type (C57BL/6J) mouse aortas for 18 hours at 37°C in minimal essential medium supplemented with recombinant human EPO (1–50 U/ml) caused concentration-dependent increase in intracellular levels of BH 4 as determined by HPLC analysis. Maximum biosynthesis of BH 4 was detected at therapeutic concentrations of 5 U/mL (15.8±1.3 pmol/mg protein; P<0.05 vs control: 8.2±0.4 pmol/mg protein; n=6 – 8). Oxidative products of BH 4 , 7,8-dihydrobiopterin, were unaffected by EPO indicating that EPO does not affect oxidation of BH 4 . Removal of the endothelium abolished EPO-induced biosynthesis of BH 4 (P<0.05; n=5) demonstrating that the vascular endothelium is a major source of BH 4 . Treatment of intact isolated wild-type mouse aortas with a selective phosphatidylinositol (PI)3-kinase inhibitor wortmannin (1 μM) significantly reduced BH 4 biosynthesis by EPO (8.4±0.6 pmol/mg protein; P<0.05; n=6). Stimulatory effect of EPO on production of BH 4 in aorta was also detected in wild-type mice treated with recombinant human EPO (1000 U/kg, s.c. biweekly) for 14 days (P<0.05; n=6). This vascular effect was abolished in protein kinase B (Akt) 1-deficient mice treated with EPO (P<0.05; n=5). Furthermore, aortic GTP cyclohydrolase I (GTPCH I) enzymatic activity was augmented in EPO treated wild-type mice (0.48±0.12 pmol neopterin/mg protein; P<0.05 vs control: 0.23±0.05 pmol neopterin/mg; n=4) but not in EPO treated Akt 1-deficient mice (0.21±0.03 pmol neopterin/mg; n=4), indicating that the selective increase in BH 4 levels was caused by de-novo biosynthesis of BH 4 via Akt/GTPCH I pathway. Our results demonstrate that EPO stimulates biosynthesis of BH 4 in vascular endothelium. This effect is most likely designed to provide optimal intracellular concentration of cofactor necessary for EPO-induced elevation of eNOS activity.