**3.1 Angiogenesis**

60 Liver Tumors

preponderance for cholangiocarcinoma in males (Tominaga and Kuroishi 1994) and the

Cholangiocarcinoma occurs with varying frequency in different regions of the world. This can be explained in part by the distribution of risk factors in geographic regions and ethnic groups (Ben-Menachem 2007). The common link between these regional risk factors seems

The prevalence of cholangiocarcinoma in Asian countries shares a relationship with infections such as liver flukes, Hepatitis B and Hepatitis C (Ben-Menachem 2007). In contrast, approximately 90% of patients diagnosed with cholangiocarcinoma in Western countries do not have any recognized risk factors (Ben-Menachem 2007). However, the remaining 10% of cases are associated with certain risk factors. Apart from factors related to chronic inflammation, both intra- and extrahepatic cholangiocarcinomas are well-known complications of primary sclerosing cholangitis (de Groen et al. 1999). Other known risk factors include obesity, hepatolithiasis, bacterial infection and/or bile stasis-related chronic

Neoplastic epithelial cells coexist with a biologically complex stroma composed of various types of stromal cells as well as the extracellular matrix, both of which create the complexity of the tumor microenvironment (Orimo and Weinberg 2006). Mouse models of tumorigenesis have revealed that stromal cells, in particular inflammatory cells, vascular endothelial cells and fibroblasts actively support tumor growth (Olumi et al. 1999; Tlsty 2001; Cunha et al. 2003; Bhowmick et al. 2004). In addition, the microenvironment is now well recognized as playing a role in neoplastic transformation, malignant progression and metastasis and invasion of cancer cells (Tlsty 2001; Bhowmick et al. 2004). Furthermore, the interaction between the cancer cells and the tumor microenvironment is a major factor influencing cancer treatment resistance to radiotherapy and chemotherapy (de Visser and Jonkers 2009; Shinohara and Maity 2009). Research indicates that the interplay between the cancer cells and the stromal cells of the microenvironment is bi-directional and dynamic. For example, neoplastic cells often secrete factors that work in a paracrine manner to recruit and activate a number of types of stromal cells into the tumor microenvironment as required (Rasanen and Vaheri 2010; Rojas et al. 2010; Onimaru and Yonemitsu 2011). Conversely, stromal cells, once recruited and activated, release factors into the extracellular milieu that can either stimulate or inhibit growth of the tumor (Rasanen and Vaheri 2010; Rojas et al. 2010; Onimaru and Yonemitsu 2011). The effects of the components of the tumor microenvironment on tumor growth are summarized in Figure 1. In particular, the proliferation and recruitment of vascular endothelial cells and subsequent formation of new blood vessels brings a nutrient supply thereby allowing growth and metastasis of the tumor. Cancer associated fibroblasts, on the other hand, can stimulate angiogenesis as well promote tumor growth and invasion. The presence of immune cells, in particular tumor-associated macrophages, in the microenvironment, confers resistance to toxic insults and also promotes growth. Lastly, proliferation of lymph endothelial cells and subsequent increase in

incidence in both sexes increases with age (Patel 2002).

to involve chronic inflammation and biliary irritation (Gores 2003).

cholangitis (Chen 1999; de Groen et al. 1999; Catalano et al. 2009).

lymphatic vessel density promotes tumor metastasis.

**3. Tumor microenvironment** 

**2.1 Risk factors** 

The physiological process of the formation of new blood vessels from pre-existing blood vessels is termed angiogenesis. Tumors require the formation of new blood vessels to supply oxygen and other essential nutrients, without which their growth would be severely restricted (McDougall et al. 2006). Generally, the process of angiogenesis involves a sequence of co-ordinated events that is initiated with the expression and release of various angiogenic factors from the tumor cells, such as vascular endothelial growth factor (VEGF), epidermal growth factor (EGF) and fibroblast growth factor (FGF). Once these angiogenic factors bind to their corresponding receptors on the cell surface of the endothelial cells, there is an increase in vascular permeability, leading to extravasation of plasma proteins and dissociation of pericyte coverage (Roberts and Palade 1997; Dvorak 2005). This is followed by proliferation and migration of the endothelial cells to initiate new vessel formation (Ausprunk and Folkman 1977). For new vessel formation to occur, there also needs to be localized degradation of the extracellular matrix, which is performed by the matrix metalloproteinases, cathepsin B and other degradation enzymes, as well as the expression of matrix proteins such as fibronectin and laminin (Mikkelsen et al. 1995; Gladson 1999; Ljubimova et al. 2006). The expression of these essential extracellular matrix proteins largely occurs in the tumor cells or cancer associated fibroblasts (Rasanen and Vaheri 2010), which then secrete them into the extracellular milieu.
