**4. Angiogenesis and HCC**

HCC progression requires the development and maintenance of adequate blood supply. This requires vascular endothelial cell proliferation and migration in order to establish a new vascular network. The transformation from a poorly vascular nodule to the hypervascular malignant phenotype of the disease has become a hallmark for diagnosis, treatment and possible future therapeutic approaches (Fernandez et al., 2009). Formation of new vessels with abnormal architecture is linked to fibrinogenesis and cirrhosis, processes that cause extensive cell damage and can lead to malignancy (Rosmorduc & Housset, 2010). The ability to form new vessels is critical not only for the emergence of HCC but also for the growth of metastatic nests and the transition from early to advanced stages of the disease. Indeed, impairing the blood supply that feeds the tumor with transarterial chemoembolization is an effective treatment for patients with advanced HCC and improves their survival (Llovet & Bruix, 2003).

Neoangiogenesis in HCC is a process stimulated by hypoxia, growth factors, oncogenes and nutrient concentration followed by the release of pro-angiogenic growth factors and activation of endothelial cells. The most prominent pro-angiogenic signal, as revealed in a number of studies, is vascular endothelial growth factor (VEGF) (Fernandez et al., 2009; Rosmorduc & Housset, 2010). Its expression can be induced as a result of hypoxia, oncogenic signaling and viral infection, involving MAPK pathway activation and transcriptional regulation by AP1 and HIF-1α (Hassan et al., 2009). Its significant role in HCC is shown by the fact that cells isolated from human tumors are able to produce VEGF by themselves. Hypoxia further enhances VEGF expression both by regulation at the transcriptional level and by stabilization of VEGF mRNA (von Marschall et al., 2001). Moreover, VEGF and its receptors (VEGFR-1 and VEGFR-2) are found over-expressed in patients' samples and their increased levels have been associated with the aggressive phenotype of the disease (Imura et al., 2004). In line with this observation, increased concentration of VEGF in serum samples is directly connected to poor patient outcome after resection or ablation (Poon, R.T. et al., 2004; Poon, R.T. et al., 2007).

Other factors that promote tumor angiogenesis in HCC are FGFs (Fibroblast growth factors), angiopoietins (Ang-2) and platelet-derived growth factor (PDGF). There is evidence that bFGF over-expression is involved in vascular endothelial cell proliferation during tumor angiogenesis in human HCC (Imura et al., 2004). Furthermore, Ang-2 levels as well as its activity through the Tie-2 receptor are increased in human samples and have been correlated with neovascularization and microvessel density (MVD) (Mitsuhashi et al., 2003). Finally, animal studies have shown that the involvement of PDGF-C in HCC development is linked to its role in the initiation of fibrosis (Campbell et al., 2005). Interestingly, it appears that all these factors team up with the VEGF signaling cascade in order to provoke normal or aberrant vessel formation in HCCs.

Since vascular formation is very important for tumor progression and transition to malignancy, it represents an intriguing therapeutic target for the treatment of HCC. At present, there are several drugs undergone investigation in experimental models or humans and some of them are already in use in various types of cancer. Most of the tested inhibitors target selectively the VEGF pathway and induce arrest of endothelial cell proliferation, regression of the existing vessels or suppress the mobilization of endothelial progenitor cells from bone marrow. These agents range from monoclonal antibodies (mAb) targeting VEGF-A, like bevacizumab (Avastin), to small molecules that inhibit autophosphorylation of VEGF receptors like sorafenib (Nexavar), the single agent that was approved for the treatment of advanced hepatocellular carcinoma in 2007 (Fernandez et al., 2009).
