**2.3.5 Vascular endothelial cells**

The ectopic endometrial cells require an accessible blood supply to proliferate and invade through the peritoneum after escaping immune surveillance. Greater angiogenic activity have been found in the peritoneal fluid of women with endometriosis, which are modulated by growth factors and cytokines such as VEGF & IL-8 secreted by ectopic endometrial cells and defected immune cells (Oosterlynck *et al.*, 1993). VEGF is a mitogen for endothelial cells and stimulates the proliferation of both vascular and lymphatic endothelial cells in vitro (Joukov *et al.*, 1997) and promotes angiogenesis or hyperplasia of lymphatic vessels in vivo (Jeltsch *et al.*, 1997). Increasing evidence indicate that VEGF plays an important role in the angiogenesis of peritoneal endometriosis (McLaren, 2000). VEGF is elevated in the peritoneal fluid and endometriotic lesion of women with endometriosis and correlated with the severity of this disease (McLaren *et al.*, 1996). Its expression is more pronounced around red endometriotic lesions as compared with the more inactive black implants (Donnez *et al.*, 1998).

#### **2.4 Molecular modulations**

462 Endometriosis - Basic Concepts and Current Research Trends

Natural killer cells (NK cells) constitute a major component of the innate immune system. They have two ways to take part in host defense by expressing different receptors binding to target cells. One receptor type binds immunoglobulin G (IgG). The other includes killeractivating receptors promoting cytotoxic activity and killer-inhibitory receptors (KIR) suppressing cytotoxic activity (Moretta *et al.*, 1995). Oosterlynck and Wilson have found that the cytotoxic activity of peripheral and peritoneal fluid NK cells from women with endometriosis was obviously decreased with the severity of endometriosis (Oosterlynck *et al.*, 1991; Wilson *et al.*, 1994). The decreased NK-mediated cytotoxicity in the peritoneal fluid might contribute to the establishment of endometriosis. The mechanisms that cause aberrant NK cell cytotoxicity are unclear, but seem to be involved in KIR expression (Wu *et al.*, 2000). In a recent study, Maeda et al. reported KIR2DL1 as the subclass of KIR overexpressed on

Several adhesion moleculars were found to be expressed in endometrium, which mediate the adhesion and invasion of endometrial cells to peritoneum. Koks found that endometrium preferentially adhere to the extracellular matrix (ECM) of the peritoneum mediated by integrin (Koks *et al.*, 2000). Endometrium expresses various integrins during menstruation shedding and the adhesion can be disrupted by blocking integrin. Integrins are cell-surface glycoproteins acting as receptors for ECM proteins. In normal eutopic endometrium, integrins are important in the interaction between glandular epithelial and stromal cells, and essential for implantation (Lessey *et al.*, 1992). After adherence of endometrial cells to the peritoneum, local degradation of the ECM is required for invasion and implantation. Metalloproteinases (MMPs) causing ECM breakdown, tissue collapse and menstruation was up-regulated in late secretory phase (Salamonsen *et al.*, 1996). This implies that the vital endometrial cells in peritoneal cavity during menstruation shedding already have the potential to invade into peritoneum. What's more, MMPs are present independent of the cycle phase in peritoneal and ovarian endometriosis (Salamonsen *et al.*, 1996), which promotes endometriotic cells to infiltrate into peritoneum further although endometriosis

The ectopic endometrial cells require an accessible blood supply to proliferate and invade through the peritoneum after escaping immune surveillance. Greater angiogenic activity have been found in the peritoneal fluid of women with endometriosis, which are modulated by growth factors and cytokines such as VEGF & IL-8 secreted by ectopic endometrial cells and defected immune cells (Oosterlynck *et al.*, 1993). VEGF is a mitogen for endothelial cells and stimulates the proliferation of both vascular and lymphatic endothelial cells in vitro (Joukov *et al.*, 1997) and promotes angiogenesis or hyperplasia of lymphatic vessels in vivo (Jeltsch *et al.*, 1997). Increasing evidence indicate that VEGF plays an important role in the angiogenesis of peritoneal endometriosis (McLaren, 2000). VEGF is elevated in the peritoneal fluid and endometriotic lesion of women with endometriosis and correlated with the severity of this disease (McLaren *et al.*, 1996). Its expression is more pronounced around red endometriotic

lesions as compared with the more inactive black implants (Donnez *et al.*, 1998).

NK cells in peripheral and peritoneal fluid of patients with endometriosis.

**2.3.3 Natural killer cells** 

**2.3.4 Peritoneal cells** 

has been established.

**2.3.5 Vascular endothelial cells** 

Cellular communications in endometriosis mediated by inflammatory cytokines and growth factors is mainly regulated by nuclear factor-κB (NF-κB) signaling pathway (Fig. 2) (Gonzalez-Ramos *et al.*). NF-κB mediated gene transcription promoting inflammation, invasion, angiogenesis, and cell proliferation and inhibiting apoptosis of endometriotic cells through p50/p65 dimers and NF-κB inhibitor IκBα has been found *in vitro* and *in vivo* studies (Gonzalez-Ramos *et al.*, 2010). Constitutive activation of NF-κB has been demonstrated in endometriotic lesions and peritoneal macrophages of patients with endometriosis (Laird *et al.*, 2000). Some drugs such as GnRH blocking NF-κB have been proven efficient at reducing endometriosis-associated symptoms in women (Han *et al.*, 2003). Overload iron produced by erythrocytes from menstruation shedding and cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) as well as oxidative stress stimulate NF-κB activation in macrophages and ectopic endometrial cells, which stimulates synthesis of proinflammatory cytokines, sending a positive feedback loop to the NF-κB signaling pathway. NF-κB activation enhances factors of anti-apoptosis, growth, invasion and angiogenesis as well as proinflammatory cytokines such as cyclooxygenase 2 (COX-2), vascular endothelial growth factor (VEGF), macrophage migration inhibitory factor (MIF), interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α), which promote the development of endometriosis. Intercellular adhesion molecule-1 (ICAM-1) and RANTES up-regulated by NF-κB activity could attract more macrophages to sites of inflammation.

Fig. 2. NF-κB signaling pathway in endometriosis
