Nitric Oxide (NO) Upregulates NO Synthase Expression in Fetal Intrapulmonary Artery Endothelial Cells

Abstract

Endothelium-derived nitric oxide (NO) generated by endothelial NO synthase (eNOS) is critically involved in pulmonary vasodilation during cardiopulmonary transition at birth. Inhaled NO therapy has recently been considered for patients with persistent pulmonary hypertension of the newborn (PPHN). To better understand the mechanisms regulating NO synthesis in the developing pulmonary circulation and the possible ramifications of NO therapy, studies were performed with early passage ovine fetal intrapulmonary artery endothelial cells (PAEC) to determine whether NO directly modulates eNOS expression. To examine the effects of exogenous NO, PAEC were treated with the NO donor spermine NONOate or the parent compound spermine. Exogenous NO caused increases in eNOS protein expression and NOS enzymatic activity that were detectable within 16 h of exposure. In contrast, the inhibition of endogenous NO production with nitro-L-arginine-methyl ester (L-NAME) caused a reduction in eNOS protein expression that was evident within 8 h. Paralleling the changes in eNOS protein, eNOS messenger RNA (mRNA) abundance was upregulated by exogenous NO and downregulated by L-NAME, suggesting that NO modulation of eNOS expression involves processes at the level of gene transcription or mRNA stability. Thus, in fetal PAEC there is positive-feedback regulation of eNOS expression by both exogenous and endogenous NO. These findings suggest that difficulties with transient effectiveness or prolonged requirements for NO therapy in certain PPHN patients are not due to declines in eNOS expression. Further, conditions such as fetal hypoxemia that impair PAEC NO production may attenuate eNOS expression through this mechanism, thereby contributing to the pathogenesis of PPHN.

Inhaled Nitric Oxide Effects on Lung Structure and Function in Chronically Ventilated Preterm Lambs

“…Our findings that eNOS protein abundance remained at control levels in both arteries and airways of preterm lambs that received iNO during mechanical ventilation, however, are consistent with the previous report that NO donors increased NOS expression in cultured pulmonary artery endothelial cells derived from fetal sheep (47). Preservation of vascular eNOS, however, yielded no significant benefit with respect to either PVR or arterial smooth muscle abundance.…”

Utility of large-animal models of BPD: chronically ventilated preterm lambs

“…The latter result surprised us because diminished eNOS is sometimes used to explain the apparent elevation of pulmonary vascular resistance that frequently occurs when iNO therapy is discontinued. On the other hand, our surprising finding is consistent with the results of in vitro studies, which showed that NO donors increase NOS expression in cultured pulmonary artery endothelial cells derived from fetal sheep (202).…”

Intrauterine Growth Retardation Is Associated With Reduced Activity and Expression of the Cationic Amino Acid Transport Systems y + /hCAT-1 and y + /hCAT-2B and Lower Activity of Nitric Oxide Synthase in Human Umbilical Vein Endothelial Cells

“…42 However, incubation of lamb fetal intrapulmonary artery endothelium with L-NAME decreased, and exogenous NO increased, eNOS activity, protein, and mRNA levels. 43 Despite the high L-NAME concentration used in the latter study (2 mmol/L), these contradictory findings could result from the different cell types studied. Our results of higher eNOS mRNA and protein and lower eNOS activity could represent an adaptive response of HUVECs from IUGR pregnancies to a reduced NO level.…”