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Lung injury in children often causes abnormal pulmonary arterial vasoreactivity and muscularization. Through processes that are incompletely understood, many children with lung injury develop progressive and irreversible pulmonary hypertension, intra- and extra-pulmonary shunting of deoxygenated blood, and severe hypoxemia. The long-term goals of my laboratory are to explore the fundamental mechanisms of lung injury and to develop novel therapies for pulmonary vascular disease.
We have discovered that inhaled nitric oxide (NO) gas modulates pulmonary vasoconstriction associated with pulmonary vascular disease. In the lung, NO is produced by endothelial cells diffuses into subjacent smooth muscle cells (SMC), where it increases cGMP levels and causes vasorelaxation. Because endogenous NO-cGMP signaling is decreased in patients with pulmonary vascular disease, we tested whether or not exogenous NO decreases pulmonary hypertension. In newborn lambs with pulmonary hypertension, we observed that low levels of inhaled NO rapidly cause pulmonary vasodilatation. Furthermore, the dilator effect of inhaled NO was limited to the lungs since it did not cause systemic vasodilatation. After evaluating the dose-response to inhaled NO in the laboratory and developing a safe NO delivery system, we performed the first clinical trials of inhaled NO in pediatric patients with pulmonary hypertension. Low levels of inhaled NO were observed to safely decrease hypoxemia and pulmonary hypertension in critically ill newborns with pulmonary vascular disease and intrapulmonary shunt. Subsequently, I lead a prospective, randomized, placebo controlled, multicenter study that demonstrated that inhaled NO treatment decreases hypoxemia and the requirement for extracorporeal membrane oxygenation (ECMO) in newborns with pulmonary hypertension. These studies stimulated investigations of inhaled NO in the pediatric lung through out the world and were pivotal in the acceptance of inhaled NO by the Federal Drug Administration of the United States as a therapy for pulmonary hypertension and hypoxemia in newborns. We also were the first to perform studies examining whether or not inhaled NO ameliorates pulmonary hypertension in patients with structural heart lesions. In the cardiac catheterization laboratory, we demonstrated that inhaled NO safely and selectively decreases pulmonary vasoconstriction in infants and children with congenital heart disease and pulmonary hypertension. These later observations were the basis for several clinical trials that were performed demonstrating that inhaled NO prevents malignant pulmonary hypertension in many pediatric patients following cardiac surgery.
Our investigations also revealed that inhaled NO prevents abnormal pulmonary vascular remodeling in the injured lung. Previous studies indicate that NO signaling regulates cell proliferation. Our previous studies indicated inhalation NO is selectively delivered to the lung and has minimal systemic effects. Therefore, we tested whether or not inhaled NO decreases pulmonary artery cell proliferation in newborn animals with lung injury. We observed that inhaled NO attenuates abnormal pulmonary artery remodeling in newborn animals. Furthermore, we determined that inhaled NO protects the lung against abnormal remodeling by directly inhibiting the proliferation of pulmonary artery smooth muscle cell precursors. These fundamental discoveries have recently stimulated the formation of several clinical investigations that are testing whether or not inhaled NO prevents pulmonary vascular disease in newborns with lung injury.
Our recent studies indicate that NO regulates vascular smooth muscle cell proliferation through the activation of cGMP-dependent protein kinase (PKG). The antiproliferative mechanisms of NO are incompletely understood. Studies suggest that NO directly decreases the proliferation of cultured cells. However, NO has been observed to nonspecifically nitrosylate proteins and DNA. Therefore, it is possible that the direct antiproliferative effect of NO observed in vitro is nonphysiologic. In smooth muscle cells, NO increases cGMP by stimulating soluble guanylate cyclase. Through this second messenger, NO has been observed to stimulate PKG activity. We hypothesized that the antiproliferative effect of NO in smooth muscle cells is through the modulation of PKG activity. However, to test this mechanism in vitro is difficult since cultured vascular smooth muscle cells do not express PKG. Therefore, we first constructed an adenovirus that encodes an inducible isoform of human PKG and demonstrated that it causes the expression of PKG in infected, cultured smooth muscle cells that is the same as what is observed in arteries and in freshly dispersed pulmonary artery cells. We then demonstrated that stimulation of PKG in these cells inhibits their proliferation. Additionally, the stimulation of PKG was observed to completely account for the antiproliferative effect of NO. These investigations demonstrate that NO-PKG signaling importantly regulates vascular smooth muscle cell proliferation . Furthermore, they indicate that the investigation of PKG phosphoryation targets may be important in the development of novel therapies that prevent the abnormal smooth muscle cell proliferation observed in pulmonary vascular disease.
In summary, our studies reveal that inhaled NO is an important therapy for pulmonary vascular disease in newborns and children. Additionally, NO-PKG signaling is an important antiproliferative mechanism in vascular smooth muscle cells.
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