**1. Introduction**

The aim of this chapter is to present an overview of salient findings in human beings and animal models (particularly in the chicken), as related to known participating molecules in angiogenesis within the lung as a response to induced and natural environmental hypoxia, in the framework of the pathobiology of pulmonary hypertension (PH).

Hypoxic PH is now recognized as an important disease within the PH types. More than 140 million human beings are settled in geographical zones located 2500 m above sea level (Peñaloza and Arias, 2007). Animals which provide proteins for human consumption, have different degrees of susceptibility to develop PH, especially the commercial chicken, which is particularly prone to develop PH, either from hypoxia (Gómez et al., 2007, 2008; Vásquez and Hernández, 2011; Areiza et al, 2011 and others) or low temperatures (Pakdel et al., 2005; Pan et al., 2005). The chicken has been proposed as a model to study PH in humans (Al-Ruyabe et al., 2010; Wideman and Hamal, 2011).

Exposure to hypoxia has been reported to cause different effects on the pulmonary vasculature. These include increasing the extent of pulmonary vascular network, increased vascular tone, vascular remodeling and in some cases quantitative reduction of microvessels. Neo-formation of blood vessels in the lung, takes place through a mechanism not fully understood and often controversial. Remodeling of pulmonary arterioles has been thoroughly studied. It includes thickening of the medial muscle and adventitia layers. Many tasks have been directed to discover the role of the endothelium in maintaining pulmonary vascular tone and in the PH remodeling process. Endothelial dysfunction leads to chronically impaired production of vasodilators such as nitric oxide (NO) and prostacyclin along with prolonged over expression of vasoconstrictors such as endothelin-1 (ET-1). These changes affect vascular tone and promote vascular remodeling. Given that most of these mediators affect the growth of smooth muscle cells, an alteration in their production may facilitate the development of pulmonary

© 2013 Hernández and Areiza; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Hernández and Areiza; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

vascular hypertrophy and structural remodeling, which are characteristic of PH (Budhiraja et al., 2004; Humbert et al., 2004; reviewed by Hernández and Sandino de, 2011).

2009). Furthermore, induced ischemia results in pronounced cell proliferation and consequent

Angiogenesis and Pulmonary Hypertension http://dx.doi.org/10.5772/56054 105

If vascular growth in the lung is an adapting mechanism to hypoxia, genetically predisposed individuals will develop hypoxic PH, and have a diminished pulmonary vascular growth capacity. Hence, angiogenesis might be the expression of genetic adaptation or predisposition

The hypoxia inducible factor (HIF) system is oxygen sensitive. It includes HIF-1 and HIF-2. They bind and activate many of the same genes, but differ in their participation during hypobaric hypoxia. For instance, HIF-2 alone is responsible for the liberation of erythropoietin through stimulation of connective tissue cells in the kidney (Paliege et al., 2010) and other hematological reactions to hypoxia (reviewed by Tissot van Patot and Gassmann, 2011). HIF-1 is a key molecule in regulating the expression of several angiogenic molecules. Among the hypoxia-inducible genes which have important HIF-1 binding sites and are believed to participate in the pathogenesis of PH and angiogenesis as well, are those encoding VEGF (Liu et al., 1995), the VEGF receptor 1 (Gerber et al, 1997), and ET-1 (Hu et al., 1998). Expression of VEGF and its receptor Fetal Liver Kinase 1 receptor (FLK-1) was enhanced in hypoxic pulmo‐ nary hypertensive rats (Christou et al., 1998). Rats maintained under hypoxic hypoxia (exposure to 10% oxygen) showed augmented volume and total length of pulmonary blood vessels (Howell et al. 2002, 2003; Ooi et al., 2000). In another study, rats exposed to hypobaric hypoxia, exhibited reduced Angiopoietin-1/Tie2 and VEGF expression, together with a

diminished number of arterial blood vessels in the lung (Yamamoto et al., 2008).

Sands et al (2012) gave insight on the interaction of members of the VEGF family in hypoxic PH. They found that VEGFB and the placental growth factor (PGF) inhibit or potentiate the actions of VEGFA, according to their relative concentrations, which change in the lungs of rats subjected to hypoxic hypoxia. The same authors stated that the abovementioned effects *in vivo* depend on specific concentrations of VEGF and PGF within the alveolar wall during adaptation to hypoxia. In this context, human subjects developing acute mountain sickness had higher levels of plasma VEGF on ascend to altitude as compared to their own values obtained under normoxic conditions. This variation did not occur in healthy subjects (Tissot

VEGF exerts its angiogenic effect through four different pathways, which include NO participation (proposed by Gramatikoff, 1999). In a previous study, adrenomedullin mRNA expression was greater in the lungs of chickens with pulmonary hypertension compared with those without pulmonary hypertension (Gómez et al., 2008) and Vadivel et al (2010) demon‐ strated that adrenomedullin promotes angiogenesis in the lung. Hence, two vasodilators of pulmonary blood vessels, NO and adrenomedullin, may also have angiogenic properties.

Fahra et al (2011) found that CD34(+) and CD133(+) progenitor cell numbers are higher in the bone marrow, blood, and pulmonary arteries of pulmonary hypertensive subjects as compared

lung vascularization in mice, within a period of 20 days (Wagner et al, 2006).

to PH inducers.

van Patot et al., 2005).

**2. Angiogenic promoting factors**

Both processes, angiogenesis and remodeling of pulmonary arterioles, appear to share the participation of similar molecules. In fact, some of them, known to increase pulmonary blood vessels tone during hypoxic exposure, are also involved in generating pulmonary blood vessels. In this framework, three main research cellular targets can be identified: the endothe‐ lium, the smooth muscle cells of the wall of pulmonary arterioles and, adventitial connective tissue cells.

PH has been related with malfunction of potassium channels and over-regulation of calcium channels on the cell membrane of vascular muscle cells as well as endothelial dysfunction. The latter, as it pertains with decreasing NO and prostacyclin (vasodilators) delivering and increment of vasocontrictors, such as endothelin-1 (ET-1) Inagami *et al.,* 1995; Voelkel and Tuder, 2000)

In adult subjects, angiogenesis could be taken as a compensatory process during hypoxic exposure. This process would be in addition to the well-known mechanisms of augmenting blood perfusion in less vascularized areas in the lung, hyperventilation and erythropoiesis. Those areas might be a potentially available area for *de novo* vascular formation. Human beings living in highlands, have developed a greater lung capacity through evolution, as an adaptive mechanism to hypobaric hypoxia (Frisancho, 1970). Also, high landers show increased lung capillary density than lowlanders, which was interpreted as an effect of hypoxia on angio‐ genesis (Bisschop et al, 2010).

Ooi et al (2000) and Howell et al (2002, 2003) determined that hypoxic hypoxia (10% oxygen) provoked augmentation in volume and total length in pulmonary blood vessels in rats, together with an increment in endothelial superficial area and cells numbers. In another work, chronically hypoxic rats, which exhibited a greater degree of vascular development through angiogenesis, had higher cardiac mass index values than the correspondent ones in animals subjected to hypoxia and hypercapnia or normoxia (Howell et al., 2004). In mice subjected to hypoxia, Pascaud et al (2003) gave evidence that inhibition of hypoxia-induced angiogenesis, enhanced the degree of PH.

Angiogenesis is regulated by angiogenic and anti-angiogenic molecules (Pascaud et al., 2003; Maharaj and D`Amore, 2007). Genetic and epigenetic factors permit blood vessels formation (Pauling and Vu, 2004). Taraseviciene-stewart et al (2001) found that plexiform lesions in idiopathic pulmonary hypertension (iPAH) were associated with disordered angiogenesis due to exaggerated mitosis of endothelial cells. Tuder and Voelkel (2002) reported endothelial cell proliferation in cases of primary PH, and, since these cells exhibited markers of angiogenesis, the authors named this process a "disordered or misguided angiogenesis". In this context, endothelium progenitor cells are now claimed to be therapeutic targets in PH due to their possible involvement in PH pathobiology. Nitric oxide synthases (eNOs), vascular endothelial growth factor (VEGF), angiopoietins and their receptor tie-2, which are associated with vascular remodeling, were all expressed in a monocrotaline induced model of PH (Cho et al., 2009). Furthermore, induced ischemia results in pronounced cell proliferation and consequent lung vascularization in mice, within a period of 20 days (Wagner et al, 2006).

If vascular growth in the lung is an adapting mechanism to hypoxia, genetically predisposed individuals will develop hypoxic PH, and have a diminished pulmonary vascular growth capacity. Hence, angiogenesis might be the expression of genetic adaptation or predisposition to PH inducers.
