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Figure 1 Schematic view of the pathology of pulmonary hypertension (PH). The figure summarizes the major pathological events that lead to the development of PH. In humans, PH is a complex and multifactorial, diffuse disorder of the pulmonary vasculature. However, for the purposes of research and inducing PH in animal models, the etiology of PH can be broadly distinguished into: (A) inflammatory; (B) high shear stress; and (C) hypoxia, even though none of these factors act alone during the progression of the disease. A common mechanism by which these insults provoke vascular dysfunction is by either increasing the production of reactive oxygen species (ROS) or by attenuating the ROS scavenging capability of the cells. Once produced, ROS can influence the growth and morphology of all three layers of the pulmonary vessels. In endothelial cells, ROS promote endothelial proliferation, decrease NO production, and increase the release of vasoactive mediators, leading to endothelial dysfunction. In smooth muscle cells (SMC), oxidative stress caused by ROS induces contraction and a switch to a synthetic phenotype. These SMC alterations are brought about by increasing the intracellular free Ca2+ and decreasing the expression of contractile phenotypic markers, while simultaneously enhancing the expression of proliferative phenotypic markers and growth factors. ROS can also augment the proliferation of fibroblasts in the adventitial layer and their expansion between the endothelium and the internal elastic lamina, known as the neointima (not depicted in the figure). Together, these changes result in vascular remodeling and the development of PH. Saurabh Aggarwal,1 Christine M. Gross,1 Shruti Sharma,1 Jeffrey R. Fineman,2,3 and Stephen M. Black*1 Compr Physiol DOI: /cphy. C Copyright © 2013 American Physiological Society 1
Figure 2 Reactive oxygen species (ROS) generation in pulmonary hypertension. The major sources of ROS in the vasculature include uncoupled endothelial nitric oxide synthase (eNOS), mitochondrial dysfunction, NADPH oxidase, and xanthine oxidase (XO). In pulmonary hypertension (PH), all of these sources contribute to the development of oxidative stress. eNOS uncoupling is mediated by limited L-arginine availability, as a result of increased degradation by arginase upregulation and attenuated synthesis by the downregulation of argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL), the enzymes responsible for the conversion of L-citrulline to L-arginine. Moreover, a sustained increase in asymmetric dimethyl-l-arginine (ADMA) levels, due to a decrease in dimethylarginine dimethylaminohydrolase 2 (DDAH2) activity, competes with L-arginine for binding to eNOS. In addition, in PH, eNOS function is impaired by a decrease in BH4, an essential cofactor for NO generation. GTP cyclohydrolase I (GCH1), the rate-limiting enzyme in BH4 biosynthesis, is ubiquinated and targeted for degradation by Hsp70/CHIP. Therefore, the low GCH1 levels, limit the production of BH4. Finally, the disruption of the zinc tetrathiolate (ZnS4) cluster by oxidative attack disrupts the eNOS dimer, which is accompanied by decreased NO generation and increased ROS production. Further, in PH, several markers of mitochondrial dysfunction are observed, including increased levels of uncoupling protein-2 (UCP-2), decreased levels of the mitochondrial antioxidant, superoxide dismutase-2 (SOD2), and the impaired function of complexes I, II, and III of the respiratory chain. Interestingly, ADMA appears to promote these changes in the mitochondria, and also augment mitochondrial ROS generation and decrease ATP production. In addition, several subunits of NADPH oxidase, including p47phox, p67phox, gp91phox, and Rac1, are upregulated in PH, increasing ROS production in the vasculature. Increased levels and activity of XO also contribute to oxidative stress and vascular dysfunction in the early stages of PH. Saurabh Aggarwal,1 Christine M. Gross,1 Shruti Sharma,1 Jeffrey R. Fineman,2,3 and Stephen M. Black*1 Compr Physiol DOI: /cphy. C Copyright © 2013 American Physiological Society 2
Figure 3 Reactive oxygen species (ROS) signaling in pulmonary hypertension. The vasoactive mediators that contribute to the development of pulmonary vascular remodeling and pulmonary hypertension (PH) can be broadly divided into two major categories: factors affecting vascular tone and factors influencing vessel wall thickening. ROS influence the generation of several of these factors. In PH, ROS decrease the expression and/or function of redox sensitive voltage gated K+ channels (Kv1.5 and Kv2.1), leading to elevations in intracellular K+ and Ca2+ influx through the activation of voltage-gated L-type Ca2+ channels. In turn, Ca2+ induces smooth muscle cell (SMC) contraction through the calmodulin-dependent activation of myosin light chain kinase (MLCK) and the subsequent phosphorylation of the myosin light chain (MLC). ROS also promote the activation of the small GTPase, RhoA, and its downstream effector, Rho kinase, which increase the phosphorylation of MLC through the inhibition of MLC phosphatase. In addition, ROS promote pulmonary vessel constriction by enhancing the production of endothelin-1 (ET-1) and thromboxane A2 (TXA2), while attenuating the levels of vasodilators, such as prostacyclin and peroxisome proliferator-activated receptors- (PPAR- ). The role of ROS in mediating pulmonary SMC proliferation and vessel thickening is more complex. Oxidative stress induces the expression of several growth factors, such as transforming growth factor-1 (TGF-1), vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), and platelet-derived growth factor (PDGF). In addition, ROS also activate p38MAPK and ERK1/2, which promote proliferation. The MAPK signaling pathway is also activated by pp60Src, through ROSmediated activation. Further, ROS stimulate Akt1, which promotes SMC proliferation through the upregulation of PGC-1. Together, the dysregulation of these constrictive and proliferative factors by ROS promotes the vascular remodeling seen in PH. Saurabh Aggarwal,1 Christine M. Gross,1 Shruti Sharma,1 Jeffrey R. Fineman,2,3 and Stephen M. Black*1 Compr Physiol DOI: /cphy. C Copyright © 2013 American Physiological Society 3
Figure 4 Reactive oxygen species (ROS) scavenging in pulmonary hypertension. ROS are scavenged by antioxidant systems to preserve the cellular redox homeostasis and to prevent oxidative damage to cellular proteins, lipids, and DNA. The major vascular enzymatic antioxidants are superoxide dismutase (SOD), catalase, and GPx. There are three types of SOD: cytoplasmic Cu/ZnSOD (SOD1), mitochondrial MnSOD (SOD2), and extracellular EC-SOD (SOD3) that catalyze the rapid conversion of O2– into H2O2. In PH, there is a decrease in the expression and/or activity of all three SOD isoforms, increasing O2– levels. The H2O2 that is produced by SOD can be enzymatically reduced in the cytoplasm by catalase and GPx. Catalase decomposes H2O2 to O2 and H2O, while GPx requires two molecules of glutathione (GSH) to reduce one molecule of H2O2 to two molecules of H2O and in the process GSH is oxidized to glutathione disulfide (GSSG). Catalase activity/expression is altered in PH, while the expression/activity of GPx is decreased in many forms of PH. The result of reduced ROS quenching by these antioxidants adds to the oxidative stress observed in PH. Saurabh Aggarwal,1 Christine M. Gross,1 Shruti Sharma,1 Jeffrey R. Fineman,2,3 and Stephen M. Black*1 Compr Physiol DOI: /cphy. C Copyright © 2013 American Physiological Society 4
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