**2.4. Other endothelial functions: involvement in hemostasis, inflammation, and angiogenesis**

Endothelium maintains blood fluidity, preventing intravascular coagulation and thrombus formation, respectively; endothelial cells express a variety of intraluminal surface proteins (such as thrombomodulin) and secrete molecules with anticoagulant and antithrombotic properties: ectonucleotidases and protein C and S as well as substances which inhibit platelet adhesion and aggregation (PGI<sup>2</sup> and NO).

On the other hand, endothelium enables *hemostasis* when needed through the expression of specific membrane glycoproteins which enable adhesion of platelets to the damaged vascular wall (i.e., subendothelial matrix); and secretion of diverse substances, such as vWF, thrombin, tissue factor, and many others. Additionally, endothelium plays a crucial role in the fibrinolytic system. Nevertheless, prothrombotic, procoagulation, and antifibrinolitic states are associated with thrombi formation and cardiovascular events.

period also occurs in many pathological conditions, such as cancerogenesis, tumor growth, and metastasis. To this end, the processes connected to angiogenesis represent an important therapeutic niche: on one side, antiangiogenic drugs are targeted against tumor growth [100], and on the other, proangiogenic drugs have to be designed to augment the angiogenic poten-

Endothelial cells also express enzymes for degradation of other autacoids, and enzymes

Endothelium, therefore, is a highly metabolic organ. Energy supply for its pleiotropic functions is derived mainly from glucose by anaerobic metabolism although endothelium is

transported to other cells may be conserved [102]. Fatty acids represent another potential fuel for endothelial cells; yet, as they have relatively few mitochondria, fatty acids have been proposed to only modestly contribute to total ATP generation [102]. Glucose is transported into endothelial cells via insulin-independent GLUT-1 transporter [103]; respectively, endothelial cells, particularly the microvascular ones [104], are susceptible to adverse effects of hyperglycemia which in multiple ways increases the level of oxydative stress. The mechanism involves mitochondrial hyperpolarization, which affects ROS production [105, 106]. However, ROS should not only be regarded as a foe: in recent years, they have been acknowledged as important players in endothelial homeostasis, modulating endothelium-dependent vasodilation, permeability, and angiogenesis [105, 106]. On the other hand, dysfunctional mitochondria have been implicated in endothelial dysfunction and vascular aging [107–109] and as such represent a potential therapeutic target [109]. Moreover, mitochondria might be regarded as oxygen sensors since in hypoxic conditions, the generation of ROS is increased and connected to hypoxia-mediated responses, such as increased permeability, changes in cell surface adhesion molecules, cell proliferation, and angiogenesis [110]. Exercise and diet have been shown to beneficially impact mitochondria dysfunction [108, 111]. Caloric restriction has also been connected with SIRT1: decreased ATP level activates AMP-activated protein kinase (AMPK),

); therefore, enough O<sup>2</sup>

Endothelium at a Glance

11

http://dx.doi.org/10.5772/intechopen.81286

to be

which convert humoral factors to attend their full activity, as, e.g., ACE.

directly exposed to blood with high oxygen partial pressure (pO<sup>2</sup>

the main cellular energy sensor which in turn activates SIRT1 [112].

**3. Endothelium and vascular tone regulation: endothelial** 

One of the main functions of endothelium is its involvement in vascular tone regulation. In response to mechanical and pharmacological (ACh, histamine, bradykinin, VEGF, various hormones as estrogen, CGRP, substance P, insulin, and platelets products (serotonin, ADP)) stimuli, endothelium releases a number of vasoactive mediators which, by affecting VSMC, regulate vascular tone and thus help to adjust blood flow to tissue demands. There is a considerable and complex interplay between endothelial and other humoral vasoactive substances, as well as the sympathetic nervous system. Mostly investigated endothe-

whereas the main constrictors include endothelin, thromboxane, AngII, the cytochrome P450

, and endothelium-derived hyperpolarizing factor(s) (EDHF),

tial [101].

**vasodilators**

lial vasodilators are NO, PGI<sup>2</sup>

Endothelium is importantly involved in *inflammatory processes* and tissue repair as it adjusts changes in vascular reactivity and permeability in response to various cytokines and autacoids, increasing plasma extravasation and larger molecules trafficking as well as the adhesion and recruitment of leucocytes across the vessel walls, mainly of the postcapillary venules to the site of injury/inflammation. It itself produces many substances involved in inflammation and subsequent tissue repair, i.e., VEGF and various cytokines (e.g., IL-1, TNF-α). Moreover, cytokines released from endothelium in an autocrine manner augment the inflammatory response by affecting intracellular signaling pathways which activate various transcription factors, especially the transcription factor NF-kB, to finally increase the expression of proadhesive and procoagulant genes. In addition, cytokines and VEGF also promote endothelial and VSMC proliferation [93]. In response to inflammation, endothelial cells increase the expression of a variety of adhesion molecules, belonging to three gene families, namely selectins, integrins, and the immunoglobulin (Ig) superfamily (comprising VCAM, intercellular adhesion molecules (ICAM), and PECAM-1) on their surface which enable leucocytes to recognize the affected sites, adhere to endothelium and cross the vessel wall. As the level of circulating adhesion molecules is increased in diseases, they have been used as a marker of endothelial dysfunction. Inappropriate regulation of inflammatory processes has been acknowledged as an early step in the development of atherosclerosis as well as other pathological processes [94].

Endothelium is crucial for vasculo- and *angiogenesis*. Postnatally, endothelial cells are relatively quiescent and the growth of new vessels (neoangiogenesis) in healthy adults only occurs in uterine cycle, reproduction (i.e., placenta formation), and wound healing, as well as in response to repeated exercise and endurance training in myocardium and skeletal muscles. Neoangiogenesis requires a fine orchestrated interplay between endothelial cells, VSMC, pericytes, and a variety of signaling molecules, including growth factors [95], chemokines, angiopoietins, semaphorins, angiogenic enzymes, adhesion molecules, ephrins, and MMPs [93]. Neoangiogenesis involves many interdependent processes; in brief, upon stimulation by various angiogenic growth factors, endothelial cells get activated and release protease to degrade and invade basal lamina and the underlaying extracellular matrix. Endothelial cells then migrate into the interstitial space, where they proliferate and differentiate to form solid sprouts connecting neighboring vessels [38, 93, 96]. The role of VEGF and angiopoietins and the involvement of caveolae have shortly been mentioned above. In addition, circulating EPCs have also been shown to play a role in angiogenesis as they can differentiate to mature endothelial cells and replace injured or senescent endothelial cells [41, 97]. The number of circulating EPCs has been shown to be increased in cancer [98]. One of the most important (patho)physiological stimuli for neoangiogenesis in adult period is hypoxia which modifies gene expression dependent on the activation of hypoxia-inducible factor-1 (HIF-1) and, among others, it triggers endothelial cells to get activated [99]. Excessive angiogenesis in adult period also occurs in many pathological conditions, such as cancerogenesis, tumor growth, and metastasis. To this end, the processes connected to angiogenesis represent an important therapeutic niche: on one side, antiangiogenic drugs are targeted against tumor growth [100], and on the other, proangiogenic drugs have to be designed to augment the angiogenic potential [101].

On the other hand, endothelium enables *hemostasis* when needed through the expression of specific membrane glycoproteins which enable adhesion of platelets to the damaged vascular wall (i.e., subendothelial matrix); and secretion of diverse substances, such as vWF, thrombin, tissue factor, and many others. Additionally, endothelium plays a crucial role in the fibrinolytic system. Nevertheless, prothrombotic, procoagulation, and antifibrinolitic states are

Endothelium is importantly involved in *inflammatory processes* and tissue repair as it adjusts changes in vascular reactivity and permeability in response to various cytokines and autacoids, increasing plasma extravasation and larger molecules trafficking as well as the adhesion and recruitment of leucocytes across the vessel walls, mainly of the postcapillary venules to the site of injury/inflammation. It itself produces many substances involved in inflammation and subsequent tissue repair, i.e., VEGF and various cytokines (e.g., IL-1, TNF-α). Moreover, cytokines released from endothelium in an autocrine manner augment the inflammatory response by affecting intracellular signaling pathways which activate various transcription factors, especially the transcription factor NF-kB, to finally increase the expression of proadhesive and procoagulant genes. In addition, cytokines and VEGF also promote endothelial and VSMC proliferation [93]. In response to inflammation, endothelial cells increase the expression of a variety of adhesion molecules, belonging to three gene families, namely selectins, integrins, and the immunoglobulin (Ig) superfamily (comprising VCAM, intercellular adhesion molecules (ICAM), and PECAM-1) on their surface which enable leucocytes to recognize the affected sites, adhere to endothelium and cross the vessel wall. As the level of circulating adhesion molecules is increased in diseases, they have been used as a marker of endothelial dysfunction. Inappropriate regulation of inflammatory processes has been acknowledged as an early step in the development of atherosclerosis as well as other pathological processes [94]. Endothelium is crucial for vasculo- and *angiogenesis*. Postnatally, endothelial cells are relatively quiescent and the growth of new vessels (neoangiogenesis) in healthy adults only occurs in uterine cycle, reproduction (i.e., placenta formation), and wound healing, as well as in response to repeated exercise and endurance training in myocardium and skeletal muscles. Neoangiogenesis requires a fine orchestrated interplay between endothelial cells, VSMC, pericytes, and a variety of signaling molecules, including growth factors [95], chemokines, angiopoietins, semaphorins, angiogenic enzymes, adhesion molecules, ephrins, and MMPs [93]. Neoangiogenesis involves many interdependent processes; in brief, upon stimulation by various angiogenic growth factors, endothelial cells get activated and release protease to degrade and invade basal lamina and the underlaying extracellular matrix. Endothelial cells then migrate into the interstitial space, where they proliferate and differentiate to form solid sprouts connecting neighboring vessels [38, 93, 96]. The role of VEGF and angiopoietins and the involvement of caveolae have shortly been mentioned above. In addition, circulating EPCs have also been shown to play a role in angiogenesis as they can differentiate to mature endothelial cells and replace injured or senescent endothelial cells [41, 97]. The number of circulating EPCs has been shown to be increased in cancer [98]. One of the most important (patho)physiological stimuli for neoangiogenesis in adult period is hypoxia which modifies gene expression dependent on the activation of hypoxia-inducible factor-1 (HIF-1) and, among others, it triggers endothelial cells to get activated [99]. Excessive angiogenesis in adult

associated with thrombi formation and cardiovascular events.

10 Endothelial Dysfunction - Old Concepts and New Challenges

Endothelial cells also express enzymes for degradation of other autacoids, and enzymes which convert humoral factors to attend their full activity, as, e.g., ACE.

Endothelium, therefore, is a highly metabolic organ. Energy supply for its pleiotropic functions is derived mainly from glucose by anaerobic metabolism although endothelium is directly exposed to blood with high oxygen partial pressure (pO<sup>2</sup> ); therefore, enough O<sup>2</sup> to be transported to other cells may be conserved [102]. Fatty acids represent another potential fuel for endothelial cells; yet, as they have relatively few mitochondria, fatty acids have been proposed to only modestly contribute to total ATP generation [102]. Glucose is transported into endothelial cells via insulin-independent GLUT-1 transporter [103]; respectively, endothelial cells, particularly the microvascular ones [104], are susceptible to adverse effects of hyperglycemia which in multiple ways increases the level of oxydative stress. The mechanism involves mitochondrial hyperpolarization, which affects ROS production [105, 106]. However, ROS should not only be regarded as a foe: in recent years, they have been acknowledged as important players in endothelial homeostasis, modulating endothelium-dependent vasodilation, permeability, and angiogenesis [105, 106]. On the other hand, dysfunctional mitochondria have been implicated in endothelial dysfunction and vascular aging [107–109] and as such represent a potential therapeutic target [109]. Moreover, mitochondria might be regarded as oxygen sensors since in hypoxic conditions, the generation of ROS is increased and connected to hypoxia-mediated responses, such as increased permeability, changes in cell surface adhesion molecules, cell proliferation, and angiogenesis [110]. Exercise and diet have been shown to beneficially impact mitochondria dysfunction [108, 111]. Caloric restriction has also been connected with SIRT1: decreased ATP level activates AMP-activated protein kinase (AMPK), the main cellular energy sensor which in turn activates SIRT1 [112].
