Oxidative stress and the development of endothelial dysfunction in congenital heart disease with increased pulmonary blood flow: lessons from the neonatal lamb

Abstract

Congenital heart diseases associated with increased pulmonary blood flow commonly leads to the development of pulmonary hypertension. However, most patients who undergo histological evaluation have advanced pulmonary hypertension, and therefore it has been difficult to investigate aberrations in signaling cascades that precede the development of overt vascular remodeling. This review discusses the role played by both oxidative and nitrosative stress in the lung and their impact on the signaling pathways that regulate vasodilation, vessel growth, and vascular remodeling in the neonatal lung exposed to increased pulmonary blood flow.

Copyright © 2010 Elsevier Inc. All rights reserved.

Figures

Fig. 1. Summary of ROS/RNS generation and the downstream targets involved in vascular remodeling in a neonatal lamb model (Shunt) of congenital heart disease (CHD) with pulmonary hypertension (PH) secondary to increased pulmonary blood flow

There are several sources of reactive oxygen species (ROS) and reactive nitrogen species (RNS) that contribute to the development of oxidative and nitrosative stress and subsequent vascular dysfunction in Shunt lambs with PH. Endothelial NOS (eNOS) uncoupling (1), which significantly generates both ROS and RNS, is mediated by limited L-arginine and tetrahydrobiopterin (BH4) availability. L-arginine levels are diminished due to increased degradation by arginase up-regulation and attenuated synthesis by argininosuccinate lyase (ASL) and argininosuccinate synthetase (ASS) down-regulation. Moreover, a sustained increase in asymmetric dimethylarginine (ADMA) levels prohibits L-arginine binding to eNOS. In addition, eNOS function is impaired by decreased GTP cyclohydrolase-1 (GCH1) enzyme. Low GCH1 levels limit the bioavailability of BH4, an essential co-factor for NO generation. Finally, the disruption of Hsp90-eNOS interaction potentiates the uncoupling of the enzyme. In Shunt lambs, ADMA also appears to promote mitochondrial dysfunction (2). ADMA increases mitochondrial ROS and decreases ATP levels. Several markers of mitochondrial dysfunction are observed, including increased levels of uncoupling protein-2 (UCP-2), decreased levels of the mitochondrial superoxide dismutase-2 (SOD2), and an increased lactate:pyruvate ratio. In addition, several subunits of NADPH oxidase (3), including p47phox and Rac1, are up-regulated, inducing ROS production in the pulmonary vasculature. In Shunts, there is also a significant increase in xanthine oxidase (XO) protein levels and XO derived O2− in the pulmonary vasculature (4). Further, altered endothelin-1 (ET-1) (5) signaling contributes to oxidative stress and vascular dysfunction in PH. Shunt lambs have elevated levels of ET converting enzyme-1 (ECE-1) and subsequent increased ET-1 in their peripheral lung tissue. The protein levels of the ET-1 receptors: ETA and ETB are altered. ETA, which predominantly mediates vasoconstriction in smooth muscle cells (SMC), is increased. However, ETB, which provokes a vasoconstrictive response in SMC and a vasodilatory response in endothelial cells (EC), is increased in pulmonary SMC and decreased in pulmonary EC. The role of ROS in mediating endothelial and smooth muscle proliferation is complex. Oxidative stress induces the expression of several growth factors, such as transforming growth factor-β1 (TGF-β1), vascular endothelial growth factor (VEGF), and fibroblast growth factor-2 (FGF-2) (6). In Shunts, there is a profound dysregulation of TGF-β1 receptors, ALK-5 and ALK-1. There is a down-regulation of the anti-angiogenic receptor, ALK-5, and an up-regulation of the pro-angiogenic receptor, ALK-1 (6). In addition, ROS influences the over-expression of VEGF and its receptors, Flt-1 and FlK-1, thereby adding to endothelial proliferation and migration (6). The up-regulation of FGF-2 by oxidative stress contributes to extracellular matrix deposition and smooth muscle wall thickening in Shunts (6). The down-regulation of NO signaling in PH is followed by a compensatory increase in vasodilatory molecules, such as B-type natriuretic peptide (BNP) and cGMP (7). However, vasodilation in PH is attenuated due to a nitration induced decrease in protein kinase G-1α (PKG-1α) activity (7).