This proposal will test the hypothesis that radiation-induced late lung and heart toxicities are a consequence of endothelial dysfunction defined as uncoupled nitric oxide synthetase activity and decreased nitric oxide bioavailability establishing a state of chronic inflammation driving a persistent pro-fibrotic process associated with abnormal wound repair and late normal tissue injury. For radiation a sub-lethal dose that induces the hematopoietic syndrome (5 Gy) immediately followed by thorax only “top-up” of 6.5 Gy will be used as the radiation protocol. The proposal will test whether sepiapterin, a metabolic precursor of tetrahydrobiopterin, or induction of the inflammatory protein, heme oxygenase-1, 24 hours post IR re-establishes a normal wound repair mechanism removing the driving force for chronic inflammation and fibrosis. The proposed experiments also test whether exosomes shed from irradiated endothelial cells provide potential biomarkers for the late effects of radiation. AIM 1 Hypothesis: radiation uncouples nitric oxide synthetase activity by reducing tetrahydrobiopterin in cardiac and lung endothelial cells in vitro and this results endothelial cell dysfunction as evaluated by markers of inflammation, senescence and endothelial-mesenchymal transition. AIM 2 Hypothesis: oral administration of sepiapterin or induction of heme oxygenase-1 expression with hemin 24 hours after a radiation exposure, enhances lung and cardiac function as evidenced by decreased breathing frequency, decreased inflammation, enhanced contractile reserve, improved relaxation and diastolic function, reduced fibrosis and enhanced survival. AIM 3 Hypothesis: exosomes and their cargo purified from the plasma of irradiated mice in Aim 2 and from the plasma of patients treated by radiotherapy for lung and breast cancer stimulate endothelial cell dysfunction and represent a potential source of biomarkers for lung and cardiac injury following radiation.