Reduction of Radiation-Induced Vascular Nitrosative Stress by the Vitamin E Analog {gamma}-Tocotrienol: Evidence of a Role for Tetrahydrobiopterin
Purpose: The vitamin E analog {gamma}-tocotrienol (GT3) is a powerful radioprotector. GT3 reduces postradiation vascular peroxynitrite production, an effect dependent on inhibition of hydroxy-methylglutaryl-coenzyme A reductase. Hydroxy-methylglutaryl-coenzyme A reductase inhibitors mediate their pleiotropic effects via endothelial nitric oxide synthase that requires the cofactor tetrahydrobiopterin (BH4). This study investigated the effects of radiation on BH4 bioavailability and of GT3 on BH4 metabolism. Methods and Materials: Mice were exposed to 8.5 Gy of total body irradiation (TBI). Lung BH4 and total biopterin concentrations were measured 0, 3.5, 7, 14, and 21 days after TBI by use of differential oxidation followed by high-performance liquid chromatography. The effect of exogenous GT3 and BH4 treatment on postradiation vascular oxidative stress and bone marrow colony-forming units were assessed in vivo. The effect of GT3 on endothelial cell apoptosis and endothelial expression of guanosine triphosphate (GTP) cyclohydrolase 1 (GTPCH), GTPCH feedback regulatory protein (GFRP), GFRP transcription, GFRP protein levels, and GFRP-GTPCH protein binding was determined in vitro. Results: Compared with baseline levels, lung BH4 concentrations decreased by 24% at 3.5 days after TBI, an effect that was reversed by GT3. At 14 and 21 days after TBI, compensatory increases in BH4 (58% and 80%, respectively) were observed. Relative to vehicle-treated controls, both GT3 and BH4 supplementation reduced postirradiation vascular peroxynitrite production at 3.5 days (by 66% and 33%, respectively), and BH4 resulted in a 68% increase in bone marrow colony-forming units. GT3 ameliorated endothelial cell apoptosis and reduced endothelial GFRP protein levels and GFRP-GTPCH binding by decreasing transcription of the GFRP gene. Conclusions: BH4 bioavailability is reduced in the early postradiation phase. Exogenous administration of BH4 reduces postirradiation vascular oxidative stress. GT3 potently reduces the expression of GFRP, one of the key regulatory proteins in the BH4 pathway, and may thus exert some of its beneficial effects on postradiation free radical production partly by counteracting the decrease in BH4.