**1. Introduction**

The endothelium is a continuous cellular monolayer lining the interior of the blood vessels and heart. Classically considered to exert its actions only as a mechanical barrier, also plays important roles in vascular physiology. Endothelium participates in numerous metabolic and regulatory functions such as the control of primary hemostasis, blood coagulation and fibrinolysis, interaction with lipoprotein metabolism, platelet and leukocyte interactions with the vessel wall, presentation of histocompatibility antigens, regulation of vascular tone and growth, and of blood pressure. The endothelium exerts these actions through the release of such vasoactive compounds as prostacyclin, thromboxane A2, nitric oxide (NO), bradykinin, endothelin, angiotensin, and free radicals that control the functions of vascular smooth muscle cells and of circulating blood cells (Ross, 1999; Vapaatalo & Mervaala, 2001).

The integrity and functionality of the arterial endothelium play a crucial role in the physiology of circulation (Filipe *et al.*, 2008) and, as a consequence, in preventing the development of cardiovascular diseases, whose genesis is currently considered a consequence of the anatomical and functional disruption of the endothelium (Spyridopoulos *et al.*, 1997). When the ability of the endothelial cells to release relaxing is reduced, and in particular if the propensity to produce contractile factors is enhanced, endothelial dysfunction appears as a first step in the sequence of events that leads to atherosclerosis and coronary disease. Thus, no single mechanism is responsible for all endothelium dependent responses and their modulation by pathophysiological states leads to endothelial dysfunction characterized by an imbalance in endothelial regulators (Vanhoutte *et al.*, 2009).

Clinical and experimental data support the consideration of endothelium as a target for estradiol and other sexual hormones. A number of studies have demonstrated a favourable profile for estrogens in both experimental animal, as well as in *in vitro* models (Turgeon *et al.*, 2006). However, the protective effect detected in a considerable number of observational clinical studies (Barrett-Connor & Grady, 1998) has not been confirmed by randomized placebo-controlled trials (Hulley *et al.*, 1998; Grady *et al.*, 2002), which described clinical complications, such as thrombosis in veins and coronary arteries, developed in postmenopausal women during the administration of exogenous hormones (Cano *et al.*, 2007).

Estradiol Regulation of Prostanoids Production in Endothelium 325

species, the activity and the pattern of gene expression of these enzymes are differentially

**Membrane phospholipids**

**Arachidonic acid**

**Prostaglandin H2**

**Prostaglandins (D2, E2, F2a)**

COX-1 has been considered to be the constitutively expressed protein, while COX-2 is induced at sites of inflammation. Following that hypothesis, COX-1 would generate prostaglandins for physiological, housekeeping functions like gastrointestinal mucosal integrity and regulation of renal blood flow, while COX-2 would form the prostaglandins

But this separation of functions is so not clear. For instance, COX-2 is constitutively expressed in some regions of the central nervous system, and in renal and uterus tissues, suggesting it may play a role under physiologic conditions (Kim *et al.*, 1999; FitzGerald, 2002; Cheng & Harris, 2004). In fact, both COX-1 and COX-2 are involved not only in physiological, but also in pathological processes. The importance of this topic has impelled many outstanding reviews (Vane *et al.*, 1998; Parente & Perretti, 2003; Cipollone *et al.*, 2008;

Regarding the vascular system, both isoenzymes are expressed in endothelium and smooth muscle cells. However, endothelial cells contain up to 20 times more COX than smooth muscle cells (DeWitt *et al.*, 1983). As mentioned before for other organs, COX-1 has usually been considered in endothelium as the constitutive isoform, while COX-2 is induced by a number of cardiovascular risk factors, such as cytokines, cholesterol, lipoproteins, and hypoxia. Actually, both COX isoenzymes share characteristics of constitutive and inducible enzymes in endothelium. Shear stress induces COX-1 gene expression in human umbilical vein endothelial cells (HUVEC) (Doroudi *et al.*, 2000), while clinical studies with a selective

responsible for inflammatory symptoms (Smith *et al.*, 2000; Parente & Perretti, 2003).

Fig. 1. Biosynthesis of prostaglandins and thromboxanes.

**Prostaglandin G2**

**Thromboxanes (TXA2)**

*Phospholipase A2*

*Cyclooxygenase-1 or Cyclooxygenase-2*

*Prostaglandins and thromboxanes synthases and isomerases*

regulated (Davidge, 2001).

**Prostacyclin (PGI2)**

Vanhoutte, 2009).

A number of studies have demonstrated that estrogens exert significant benefits on the cardiovascular system, and at least a part of these benefits are attributed to the direct effect of estradiol on vascular endothelial cells (Mendelsohn & Karas, 1999; Rubanyi *et al.*, 2002; Sader & Celermajer, 2002). Estradiol is able to stimulate endothelial NO production through several mechanisms, including increased expression of NO synthases (mainly endothelial NO synthase), increased L-arginine availability, non-genomic activation of second messengers (Simoncini, 2009), translocation to intracellular sites, modulation of NO degrading sites (Tostes *et al.*, 2003), and modulation of endogenous antagonist levels (Monsalve *et al.*, 2007).

Additionally, estradiol is able to exert antioxidant actions on endothelium (Shwaery *et al.*, 1998; Hermenegildo *et al.*, 2002), to modulate the renin-angiotensin system (Farhat *et al.*, 1996; Alvarez *et al.*, 2002), and to decrease endothelin-1 production (Mikkola *et al.*, 1995; Akishita *et al.*, 1998). Furthermore, estradiol regulates endothelial cell expression of adhesion molecules (Caulin-Glaser *et al.*, 1996; Abu-Taha *et al.*, 2009).

Estradiol has also been implicated on the regulation of prostanoids production in endothelial cells. Two main vascular prostanoids, prostacyclin and thromboxane A2, play an essential role in the maintenance of vascular homeostasis. Prostacyclin is a vasodilator and an inhibitor of platelet aggregation; in contrast, thromboxane A2 is a vasoconstrictor and a promoter of platelet aggregation. As a consequence of their opposing roles, an imbalance in prostacyclin or thromboxane production has been implicated in the physiopathology of many thrombotic and cardiovascular disorders. In this chapter, we will discuss clinical and experimental data that document the endothelial effects of estradiol on prostanoid production and regulation, and their vascular consequences.
