Endothelial cell loss, survival and regeneration are important aspects of the response to vascular injury leading to neointimal hyperplasia and accelerated atherosclerosis. Nitric oxide (NO), produced by endothelial nitric oxide synthase (eNOS), is a key regulator of endothelial cell biology and has been shown to prevent endothelial cell apoptosis. The cofactor tetrahydrobiopterin (BH4) is essential for eNOS catalytic activity but its impact on endothelial cell survival and regeneration remains unclear. We investigated the effect of BH4 on endothelial cell survival and vascular remodelling using ApoE-KO mice with transgenic endothelial-targeted overexpression of GTP cyclohydrolase 1 (GCH), the rate limiting enzyme of BH4 synthesis, and with endothelial specific transgenic expression of the LacZ reporter gene. Using venous bypass grafts as an in vivo model of acute vascular injury, we observed that endothelial-specific augmentation of BH4 in GCH/ApoE-KO mice improved survival of vein graft-derived endothelial cells and reduced neointimal hyperplasia. To address the hypothesis that augmentation of BH4 increases the capacity of endothelial cells from GCH/ApoE-KO mice to survive vascular injury, we cultured primary lung endothelial cells from mice expressing the GCH transgene and wild type littermates. Endothelial cells, isolated by immunomagnetic beads, were positive for CD31, CD102 and Tie2. Protein levels of eNOS were not different between wild type and GCH mice. BH4 levels were selectively increased in pulmonary endothelial cells from GCH mice, > 10-fold, compared with a 3-fold increase in total lung tissue BH4. There was no difference in total lung endothelial cell content or amount of isolated cells between ApoE-KO and GCH/ApoE-KO mice, determined by Tie2-driven β-galactosidase activity. However, after 3 days of culture both total endothelial cell number and number of endothelial cell colonies in GCH/ApoE-KO were significantly increased (236% ± 47 p=0.028 and 195% ± 28 p=0.014 respectively, n=5) whereas mean colony size remained unchanged. These observations indicate an important role for BH4 in endothelial cell survival and endothelial regeneration, and identify BH4 as a potential therapeutic target in vascular injury states.
Nitric oxide inhibits neointimal hyperplasia following vascular injury via differential, cell-specific modulation of SOD-1 in the arterial wall
