Drug Targeting Organ-Specific Strategies

proliferation to up to 20–2000 times faster than in healthy adult endothelium [7]. In all types
of angiogenesis, either under physiological or pathologicaal conditions, endothelial cell activation
is followed by matrix degradation, cellular migration, proliferation, and ultimately
neovasculature maturation (Figure 9.1).
9.1.2.1 Role of Growth Factors VEGF and FGF-2
9.1 Introduction 235
More than 20 cytokines from various sources have now been reported to be involved in the
processes taking place during angiogenesis [8]. Vascular endothelial growth factor (VEGF)
and basic Fibroblast Growth Factor (bFGF or FGF-2) are the two growth factors whose roles
in angiogenesis have been most extensively studied to date.VEGF (also known as VEGF-A)
isoforms VEGF-121, 145, 165, 183, 189 and 205 are produced through alternative splicing
from a single VEGF gene located on chromosome 6 [9]. The isoforms differ in their molecular
composition and weight, and biological properties. The larger forms of VEGF differ from
VEGF-121 by the presence of a heparin-binding domain, which is encoded by the exon-7.
VEGF is abundantly produced by hypoxic tumour cells, macrophages and other cells of the
immune system [10]. It induces vasodilatation via endothelial nitric oxide production and increases
endothelial cell permeability [11]. This allows plasma proteins to enter the tissue to
form a fibrin-rich provisional network [12]. VEGF also induces the expression of various
proteases and receptors important in cellular invasion and tissue remodelling, it activates cellular
proliferation and prevents endothelial cell apoptosis [13,14].The two VEGF-specific tyrosine
kinase receptors,VEGFR-1 (Flt-1) and VEGFR-2 (KDR/flk-1), are expressed on vascular
endothelium, and to a lesser extent on monocytes/macrophages and certain tumour cell
types. Interaction of VEGF with VEGFR-2 is a critical requirement to induce the full spectrum
of VEGF biological responses. In addition to the two VEGF receptors, VEGF-165 has
been found to bind neuropilin-1, which is also expressed on endothelial cells [15]. A recent
study using genetic deletion methods has determined that neuropilin-1 is important for embryonic
vessel formation [16].
Endothelial cells exploit various proteases such as matrix metalloproteinases to penetrate
into new areas of the body by degrading the basement membrane. Furthermore, urokinaseplasminogen
activator and tissue type-plasminogen activator convert the ubiquitous plasma
protein plasminogen to plasmin. Plasmin is believed to be the most important protease for
the mobilization of FGF-2 from the ECM pool. FGF-2 induces endothelial cell motility, proliferation
and proteinase activity, and modulates integrin levels [17,18].The cellular effects of
FGFs are mediated via specific binding to high-affinity tyrosine kinase receptors [17]. In addition,
low affinity FGF receptors consist of polysaccharide components of heparan sulfate
proteoglycans on cell surfaces and ECM. Binding to these components present in the ECM
has been proposed as a mechanism to stabilize and protect FGF from inactivation. Heparan
sulfate on cell surfaces, on the other hand, plays a more active role in displacing ECM-bound
FGF-2 and its subsequent presentation to the high affinity signal transducing receptors [19].
Angiogenesis seems exquisitely sensitive to small changes in factors such as VEGF and FGF-
2, which may have important therapeutic implications in treatment of angiogenesis-driven
disorders [20,21].