Hydrogen peroxide stimulates tetrahydrobiopterin synthesis through activation of the Jak2 tyrosine kinase pathway in vascular endothelial cells

Tetrahydrobiopterin (BH4) is an essential cofactor for nitric oxide synthase (NOS). We previously described that hydrogen peroxide (H2O2) increases BH4 levels through the induction of GTP-cyclohydrolase I (GTPCH), which is the rate-limiting enzyme for the synthesis of BH4, in vascular endothelial cells. The aim of this study was to examine the underlying mechanism of H2O2-induced BH4 synthesis in vascular endothelial cells. The increases in BH4 levels induced by H2O2 were strongly reduced by a Janus kinase-2 (Jak2) inhibitor, AG490. The H2O2-induced increases in GTPCH mRNA expression and GTPCH activity were also blocked by treatment with AG490. H2O2 elicited an increase in the level of phosphorylated Jak2, suggesting that the induction of BH4 by H2O2 was mediated by the Jak2 pathway. Signal transducers and activators of transcription (Stats) are the best-known substrates for Jak2. The H2O2-induecd increases in BH4 levels were reduced by treatment with fludarabine, which is shown to cause a specific depletion of Stat1 protein but not of other Stats. Moreover, H2O2 caused the DNA binding of Stat1, and this was inhibited by AG490. Stat1 phosphorylation was enhanced by H2O2 treatment, and the phosphorylation was attenuated by AG490. These findings suggest that the stimulation of BH4 synthesis through the induction of GTPCH is mediated at least in-part by the Jak2-Stat1 pathway.