Angiogenesis is a process involving the growth of new capillaries from pre-existing blood vessels. Angiogenesis repairs damaged tissue when wounds are healing; therefore, normal cells can switch on the growth of blood vessel by releasing angiogenic factors (Folkman, 2002). In contrast, they also can produce antiangiogenic factors which switch blood vessel growth off (Carmeliet & Jain, 2000). Tumour angiogenesis is defined as: cancer cells stimulate the growth of hundreds of capillaries from the nearby blood vessels which grow around and/or into the tumour to establish an independent supply of nutrients and oxygen. Angiogenesis is not only important to tumour development but also a critical step in tumour metastasis. The immature, highly permeable blood vessels, which have little basement membrane and few intercellular junctional complexes, provide efficient route of exit for tumour cells to leave the primary site, enter the blood stream, travel to another part of the body and begin to grow there.
1.8.1 Vascular endothelial growth factor (VEGF)
There are at least five members of the vascular endothelial growth factor (VEGF) family which exist in man: VEGF-A (usually referred to as VEGF), VEGF-B, VEGF-C and VEGF-D and a structurally related molecule, placental growth factor (PlGF). All members of VEGF family are crucial important angiogenic factors which primarily target vascular endothelial cells. They promote new blood vessel formation
by binding tyrosine kinase receptors (also known as VEGF receptors) located on cell surface. Three VEGF receptors have been recognized: VEGFR-1 or fms-like tyrosyl kinase-1(Flt-1), VEGFR-2 or Kinase insert domain receptor (KDR/Flk-1) and VEGFR-3 or Flt-4. Both VEGF and VEGF-B bind to VEGFR-1. VEGFR-1 is involved in the organization of development of blood vessel, haematopoiesis and enhances VEGF-induced VEGF-2 signaling during abnormal angiogenesis. VEGF but not VEGF-2 binds to VEGFR-2 results in promoting cell proliferation, mitosis, vascular permeability and angiogenesis, whereas VEGF-C and VEGF-D are ligands for VEGFR-3, which stimulates lymphangiogenesis (Ferrara, 2004).
1.8.2 The isoforms of VEGF
VEGF is a ~45kDa homodimeric heparin-binding glycoprotein which was firstly identified by Senger et al. in 1983. VEGF gene consists of 8 exons separated by 7 introns and is located at chromosome 6p21.3 (Aragon-Ching & Dahut, 2009). So far, there are 12 VEGF isoforms which have been divided into two families according to their terminal exon (exon 8) splice site: the proximal splice site, designated as VEGFxxx or distal splice site, designated as VEGFxxxb (Nowak et al, 2008). The identified multiple proteins of VEGF expressed in human tissues and cells are named as VEGF121, VEGF121b, VEGF145, VEGF145b, VEGF165, VEGF165b, VEGF189, VEGF189b and VEGF206 according to the different number of amino acids. VEGF121 is thought to be the most diffusible isoform due to the absence of a heparin-binding domain. VEGF165 is the predominant VEGF isoform secreted by different types of cells. VEGF165 not only exists in diffusible location but also remains bound to the cell surface and the extra cellular matrix (ECM). However, the larger isoforms such
asVEGF189 and VEGF206 with high affinity to heparin remain localized within ECM (Ferrara, 2004).
1.8.3 VEGF and prostate cancer
Most of the tumour cells secrete VEGF in vitro indicating that VEGF may play a crucial role in tumour angiogenesis. This has been confirmed by in situ hybridization studies which demonstrated that VEGF mRNA is expressed in the majority of human carcinomas such as breast cancer, lung cancer, bladder cancer and ovary cancer (Ferrara, 2004). In prostate, VEGF is expressed differently between normal, benign and prostate cancer cells. It has been reported that the expression of VEGF mRNA was detected in PIN, and poorly differentiated tissues, but not in normal prostate tissue (Huss et al, 2001). Further studies also showed that the microvessel density (MVD) was increased significantly in metastatic prostate cancer samples when compared with non-metastatic prostate cancer tissues (Kitagawa et al, 2005). In addition, higher expression of hypoxia-inducible factor (HIF), a key regulator of VEGF expression, was detected in malignant prostate cancer compared with the adjacent normal and benign prostate tissue (Du et al, 2003). Clinical studies revealed that the level of VEGF expression in serum, plasma or urine was correlated with higher Gleason grade, metastasis and disease-specific survival (Bok et al, 2001; Duque et al, 1999). On the other hand, inhibition of VEGFR-1 and VEGFR-2 using AZD-2171 (Cediranib, AstraZeneca) induced tumour shrinkage in 56.5% of patients (13 out of 23 patients with measurable disease) with 4 meeting the criteriafor partial response (Aragon-Ching & Dahut, 2009). These results indicated that VEGF played a dual role in prostate cancer at both the early initiating stage and the later stage for tumour progression and metastasis. VEGF interacts with VEGF-2 to stimulate
endothelial cell proliferation through the mitogen activated protein kinase (MAPK) pathway and promote vascular permeability, and subsequently with VEGFR-1 to assist the organization of new capillary tubes.
1.8.4 Other growth factors in Prostate cancer
Several growth factors, such as insulin-like growth factor (IGF) and epidermal growth factor (EGF) were found to increases the trans-activation potential of AR in prostate cancer. Epidemiological studies suggested that there is a correlation between the increased risk of developing prostate cancer and the high serum level of IGF-1 or low levels of IGFBP-3, a serum protein that regulates the binding of free IGF-1 to IGF receptor (IGFR) (Chan et al, 1998). However, these findings have not been successfully demonstrated by some other studies (Chen et al, 2005).
The EGFR is expressed in 40-80% of malignant prostate cancer cells and this increased expression was correlated with high Gleason score and tumour progression from an androgen-dependent to an androgen-independent state (Syed & Tolcher, 2003). EGF was shown to activate the transcriptional activity of AR by either increasing the expression levels or stimulating the activity of the AR co-activators in prostate cancer cells. Therefore, EGF is thought to promote malignant progression and metastasis of advanced prostate cancer (Reddy et al, 2006).