A liberal egalitarian framework

VEGF is known to play a critical role in embryonic vasculogenesis and angiogenesis. Loss of a single VEGF allele in mice is known to result in

embryonic lethality between day 11 and 12. The VEGF^+/-heterozygous embryos are growth retarded and exhibit a number of developmental abnormalities such as under-development of the fore-brain region, malformation of the outflow region of the heart, rudimentary dorsal aortae and reduced thickness of the ventricular wall. The yolk sac revealed a markedly reduced number of nucleated red blood cells within the blood islands. There were significant defects in the vasculature of a number of tissues and organs, including the placenta and nervous system

(Ferrara N et al., 1996;Carmeliet P et al., 1996). All these defects are believed to result directly or indirectly from the reduced levels of VEGF.

The effects of VEGF on post-natal development have also been assessed.

Partial inhibition of VEGF achieved by inducible Cre-loxP-mediated gene ablation resulted in increased mortality, stunted body growth, and impaired organ development, primarily of the liver (Gerber HP et al., 1999). Administration of a soluble VEGF receptor chimeric protein, mFlt(1-3)-IgG, which achieves a greater level of VEGF inhibition (than the conditional Cre-loxP-mediated gene ablation strategy), resulted in a nearly complete arrest of somatic growth and lethality within 4-6 days, when administered soon after birth. The primary cause of death in these mice appeared to be liver and renal failure. On isolation, endothelial cells from the liver exhibited increased apoptotic index, thereby suggesting that VEGF is required not only for proliferation but also survival of endothelial cells.

However, the dependence on VEGF was gradually lost after the fourth postnatal week, after which time VEGF inhibition did not result in any serious defects (Gerber HP et al., 1999).

In a fully grown animal, VEGF is primarily required for processes dependent on active angiogenesis, such as corpus luteum formation and wound healing. The development and endocrine functions of the ovarian corpus luteum are dependent on the growth of new capillary vessels (Bassett DL, 1943). VEGF mRNA is temporally and spatially upregulated during the proliferation of blood vessels in the rat, mouse and primate ovary and the rat uterus, suggesting that VEGF mediates the cyclical growth of blood vessels in the female reproductive tract (Ravindranath N et al., 1992;Cullinan-Bove K and Koos RD, 1993).

The angiogenic role of VEGF is also important for wound healing, as re-vascularisation of injured/damaged tissue is essential for repair. It is believed that VEGF also increases the permeability of capillaries surrounding wounds. This leads to fibrin deposition in the damaged tissue, which in turn acts as a substrate for tissue regrowth, including angiogenesis. The regulation of VEGF expression and angiogenesis is disturbed in abnormally healing wounds (Bates DO and Jones RO, 2003).

However, not all angiogenesis mediated by VEGF is beneficial. Many tumour cell lines secrete VEGF in vitro, suggesting that this molecule is an important mediator of tumour vascularisation. Without attracting blood vessel growth, many tumours are unable to grow to dangerous sizes due to the

limitations of obtaining nutrients via diffusion. In situ hybridisation studies have demonstrated that VEGF mRNA is up-regulated in many human tumours, including lung, breast, ovary, kidney and cancers of the gastrointestinal tract (Ferrara N, 1999). The discovery of the involvement of VEGF in tumour vascularisation and progression launched a huge effort to develop anti-VEGF therapies for the treatment of cancer, including anti-VEGF antibodies and VEGF receptor tyrosine kinase inhibitors. Bevacizumab (Avastin), a humanised anti-VEGF-A monoclonal antibody, received FDA approval in 2004 for the treatment of colorectal cancer. A phase III trial showed that Bevacizumab slowed tumour growth and improved survival when used in combination with other

chemotherapeutic agents (Hurwitz H et al., 2004), and other trials have shown varying degrees of efficacy of Bevacizumab with different cancers (Jain RK et al., 2006).

VEGF-mediated angiogenesis is also involved in eye disease.

Inappropriate blood vessel growth in the eye often interferes with vision, leading to visual impairment. A common cause of blindness in older people is wet age-related macular degeneration whereby blood vessels grow into the eye with advancing age, particularly in the retinal region around the optic nerve (the macula). VEGF is synthesized by retinal epithelial cells (Adamis AP et al., 1993), and is over-expressed in retinopathies in, for example, diabetes (Aiello LP et al., 1994). Anti-VEGF therapies Pegaptanib (Macugen) and Ranibizumab (Lucentis) are FDA-approved for the treatment of age-related macular degeneration, working in part by preventing VEGF-stimulated blood vessel growth in the retina (Ciulla TA and Rosenfeld PJ, 2009). In clinical trials, both Pegaptanib and Ranibizumab have been shown to slow the onset of macular degeneration and in some patients improve visual performance (Gragoudas ES et al., 2004;Rosenfeld PJ et al., 2006;Brown DM et al., 2006;Ciulla TA and Rosenfeld PJ, 2009).

VEGF-reducing therapies may suffer from a number of unwanted side-effects however, due to loss of VEGF-dependent vascular protection, including bleeding, oedema, clotting and hypertension (Kamba T and McDonald DM, 2007).

In some other pathologies, a pro-angiogenic strategy may be beneficial.

Atherosclerosis leads to build-up of plaques in artery walls, narrowing the lumen and increasing the likelihood of plaque rupture and thrombosis. Atherosclerosis of the coronary or cerebral arteries may lead to life-threatening events such as myocardial infarction or stroke. Stimulation of collateral vessel growth to bypass a narrowing vessel has been of major interest in treating conditions such as angina and peripheral vascular disease, and preventing future heart attacks. VEGF may have a role in collateral growth under normal and ischaemic conditions (Rissanen TT et al., 2005;Clayton JA et al., 2008) and adenovirus-mediated over-expression of VEGF has beneficial effects in animal models of peripheral and cardiac

ischaemia (Zachary I et al., 2000), though so far convincing therapeutic effects have not been demonstrated in clinical trials (Zachary I and Morgan RD, 2011).