**3. Inhibitors of angiogenesis**

Angiogenesis is inhibited by thrombospondin-1 (TSP-1), a molecule produced by endothelial cells. Its secretion is induced by hypoxia and, experimentally, by NO inhibitors. In mesangial and smooth muscle cells, the cyclic guanosine monophosphate (GMPc) and kinase GMPc dependent protein, negatively regulate TSP-1 expression. CD36, a TSP-1 receptor, inhibits endothelial cell quimiotaxis (Isenberg el al., 2005). Low doses of NO stimulate endothelial cell proliferation, their motility and adhesion to collagen I matrixes, but the opposite is true for correspondent high doses. Exogenous TSP-1 treatment inhibits endothelial cell motility (Isenberg et al., 2005) and suppresses the angiogenic response provoked by low dose NO treatment (Ridnour et al., 2005).

Endogenous endostatin inhibits angiogenesis (Wickstrom et al., 2002; Paddenberg et al., 2006); it blocks the G1 phase of the cell cycle in endothelial cells, the attachment of VEGF165 and VEGF121 to the receptor VEGFR2 (Ribatti, 2009), and triggers endothelial cell apoptosis

(Dhanabal et al., 1999). Endostatin interferes with the assembly of the actin cytoskeleton of endothelial cells and inhibits their proliferation and participation in angiogenesis (Abdollahi et al., 2004; Skovseth et al., 2005). Endostatin expression on the surface of the smooth muscle cells of pulmonary arterioles in the mouse, is provoked by hypoxic exposure (Paddenberg et al., 2006). NO, in culture, induces endostatin production. Hence, endothelial apoptosis is also induced (Deininger et al., 2003). In human endothelial cell cultures, addition of endostatin reduces angiogenic activity by reducing HIF-1α, VEGF, VEGFR2, HGH and EGFR gene expression. Furthermore, expression of some anti-angiogenic genes, such HIF1An (a HIF antagonist), Kininogen, TSP-1 and a Vasostatin precursor is increased (Abdollahi et al., 2004). However, it appears that the mode of action of endostatin, in this context, needs to be fully elucidated (Deininger et al., 2003; Skovseth et al., 2005).

to healthy controls. The blood levels of (hypoxia inducible myeloid-activating factors such as erythropoietin, the stem cell factor (SCF), and hepatocyte growth factor (HGF) were higher than normal levels in diseased individuals, and related to PH severity. Similarly, Davie et al (2004) found that progenitor cells from bone marrow origin contribute to neo-vascularization in pulmonary arteries of hypoxic calves, based on quantitative morphometric analyses of lung tissue from normoxic and hypoxic individuals. Their results showed adventitial growth in *vasa vasorum* of pulmonary arteries. This change was attributed to the transformation of cells from the bone marrow, which would be mobilized into the circulation, and differentiate into endothelial and smooth muscle cells. This model was supposed to entail an increase in the

PH is linked to myeloid abnormalities, some of which may be related to increased production of HIF-inducible factors by a diseased pulmonary vasculature, but findings in non-affected subjects, suggest that myeloid abnormalities may be intrinsic to the disease process. Proangiogenic cell progenitors, and endothelial cells that have pathologic expression of hypoxiainducible factor 1 alpha (HIF-1 alpha), were shown to be quantitatively higher in PH bone marrow, blood, and pulmonary arteries than in healthy controls. Also, the HIF-inducible myeloid-activating factors erythropoietin, stem cell factor (SCF), and hepatocyte growth factor (HGF) showed higher than normal levels in blood of pulmonary hypertensive subjects, and related to disease severity (Samar et al., 2011). In addition, endothelial progenitor cells with angiogenic capacity were found in the pulmonary circulation (Yoder, 2011). Epidermal growth factor promotes both angiogenesis and vascular remodeling (Janakidevi et al., 1995; Toby et

Bone morphogenetic protein (BMP) is compromised in endothelial repair within the pulmo‐ nary microvessels, and its expression is reduced in hypoxic PH, due to an increment in endothelial cell-derived Gremlin 1 and Gremlin 2, which antagonize BMP 2 type receptor

Angiogenesis is inhibited by thrombospondin-1 (TSP-1), a molecule produced by endothelial cells. Its secretion is induced by hypoxia and, experimentally, by NO inhibitors. In mesangial and smooth muscle cells, the cyclic guanosine monophosphate (GMPc) and kinase GMPc dependent protein, negatively regulate TSP-1 expression. CD36, a TSP-1 receptor, inhibits endothelial cell quimiotaxis (Isenberg el al., 2005). Low doses of NO stimulate endothelial cell proliferation, their motility and adhesion to collagen I matrixes, but the opposite is true for correspondent high doses. Exogenous TSP-1 treatment inhibits endothelial cell motility (Isenberg et al., 2005) and suppresses the angiogenic response provoked by low dose NO

Endogenous endostatin inhibits angiogenesis (Wickstrom et al., 2002; Paddenberg et al., 2006); it blocks the G1 phase of the cell cycle in endothelial cells, the attachment of VEGF165 and VEGF121 to the receptor VEGFR2 (Ribatti, 2009), and triggers endothelial cell apoptosis

expression of c-kit(+), VEGF, fibronectin, and thrombin, as responses to hypoxia.

al., 2010).

106 Pulmonary Hypertension

expression (Cahill et al., 2012).

**3. Inhibitors of angiogenesis**

treatment (Ridnour et al., 2005).

Adiponectin (ADPN) reinforces the vasodilatory action of NO in pulmonary blood vessels (Summer et al, 2009), and as stated, if NO contributes to the control of angiogenesis, ADPN will be a potential inhibitor of vascularization. Also, Le Cras et al (2003) found that alpha transforming growth factor (TGF-α) overexpression induced a diminution in pulmonary vascular development, which was accompanied by severe PH and vascular remodeling.

Oxidative stress is higher in the lungs of pulmonary hypertensive chickens (Iqbal y col., 2001a, b), which has been associated, in another species, with decreasing numbers of blood vessels (Murfee and Schmid-Schonbein, 2008).Oxidative stress, occurring in chronically hypoxic mice, can enhance endostatin production (Deininger et al., 2003) (Paddenberg et al., 2006).
