Folic Acid Promotes Recycling of Tetrahydrobiopterin and Protects Against Hypoxia-Induced Pulmonary Hypertension by Recoupling Endothelial Nitric Oxide Synthase

Aims: Nitric oxide (NO) derived from endothelial NO synthase (eNOS) has been implicated in the adaptive response to hypoxia. An imbalance between 5,6,7,8-tetrahydrobiopterin (BH₄) and 7,8-dihydrobiopterin (BH₂) can result in eNOS uncoupling and the generation of superoxide instead of NO. Dihydrofolate reductase (DHFR) can recycle BH₂ to BH₄, leading to eNOS recoupling. However, the role of DHFR and eNOS recoupling in the response to hypoxia is not well understood. We hypothesized that increasing the capacity to recycle BH₄ from BH₂ would improve NO bioavailability as well as pulmonary vascular remodeling (PVR) and right ventricular hypertrophy (RVH) as indicators of pulmonary hypertension (PH) under hypoxic conditions. Results: In human pulmonary artery endothelial cells and murine pulmonary arteries exposed to hypoxia, eNOS was uncoupled as indicated by reduced superoxide production in the presence of the nitric oxide synthase inhibitor, L-(G)-nitro-L-arginine methyl ester (L-NAME). Concomitantly, NO levels, BH₄ availability, and expression of DHFR were diminished under hypoxia. Application of folic acid (FA) restored DHFR levels, NO bioavailability, and BH₄ levels under hypoxia. Importantly, FA prevented the development of hypoxia-induced PVR, right ventricular pressure increase, and RVH. Innovation: FA-induced upregulation of DHFR recouples eNOS under hypoxia by improving BH₄ recycling, thus preventing hypoxia-induced PH. Conclusion: FA might serve as a novel therapeutic option combating PH. Antioxid. Redox Signal. 23, 1076-1091.