ANGIOGENESIS AND CANCER
B. S. Haldane, described by Peter Medawar as ‘the cleverest man I ever met’, observed in his essay On Being the Right Size that sheer size very often defines what bodily equipment an animal must have. Thus: ‘Insects, being so small, do not have oxygen-carrying bloodstreams. What little oxygen their cells require can be absorbed by simple diffusion of air through their bodies. But being larger means an animal must take on complicated oxygen pumping and distributing systems to reach all the cells.’
Cellular nutrients, along with oxygen, are delivered in mammals by the bloodstream. Oxygen diffuses from circulating erythrocytes to cells that must be within about ten cells distance (~100 µm) of a blood vessel to avoid hypoxia. Like normal cells, cancer cells require a supply of nutrients and oxygen if they are to grow. They are, nonetheless, a pathological growth that the body is n’t set up to accommodate and won’t provide with a blood supply in the way it would a tissue during normal development. This represents a major defence against cancer: even though a ‘micro-tumour’ – a small cluster of cancer cells – may have established itself, it cannot grow much beyond about one millimetre in diameter unless it can acquire a blood supply. Cancer cells achieve this by releasing chemical signals that stimulate the formation of new blood vessels – the process of angiogenesis (Fig. 1).
In switching on angiogenesis tumour cells are not doing anything novel as far as the organism is concerned. This process in which new capillaries sprout from pre-existing vessels is crucial for embryonic development although is in adult tissues. Transient, regulated angiogenesis does occur, however, as a repair process in adult tissues during the female reproductive cycle and also in wound healing. Much less usefully, angiogenesis occurs in over 70 diseases, including heart and vascular disease, rheumatoid arthritis, Crohn’s disease, psoriasis, proliferative retinopathy (a common consequence of diabetes). The critical feature of angiogenesis is that signals activate the proliferation of the endothelial cells that line the vessels of the circulatory system. Because this lining is just one cell thick it is quite fragile and therefore it can easily be damaged, thereby providing a route from the circulation to the surrounding tissues.
Tumour cells, if they are to grow significantly, thus need to establish an ‘angiogenic phenotype’ – i.e. acquire their own vascular system. This is thought to be a critical step in tumour development that must happen first in a primary tumour and then subsequently when cells from the primary have spread to secondary sites if those metastases are to expand. Angiogenesis is, therefore, an essential supporting component of most solid tumours. There is also a range of pre-malignant conditions (that includes, for example, some in situ carcinomas) within which both elevated levels of vascular endothelial growth factor (VEGF) and of microvessel density have been detected, indicating that, at least in some tumours, angiogenesis may be activated at a very early stage of development. Regardless of when it is initiated, the formation of a vascular network within a tumour creates a transport system waiting, so to speak, for metastatic cells to launch themselves once they are able to do so.
Like so many other facets of living systems, angiogenesis is a delicate balance: too much or not enough at any moment might be fatal. It will come as no surprise, therefore, to find that it is a multi-step process and that the equilibrium is under continuous regulation by both positive (pro-angiogenic) and negative (anti-angiogenic) factors (Fig. 2).
2. Angiogenic balance.
A vascular endothelial growth factor is the most potent angiogenic factor known and when tumours induce a local excess of VEGF angiogenesis is initiated. VEGF binds to specific receptors on the surface of endothelial cells and one of the responses is the release of specific proteases that cut collagen, a major component of the basement membrane that surrounds blood vessels. Once this border has been breached endothelial cells can proliferate and ‘sprout’ towards the micro-tumour. Sprouting blood vessels then infiltrate the micro-tumour, providing the oxygen and nutrients that permit it to grow beyond the avascular limit. The clinical importance of tumour vascularization is illustrated by the correlation between high levels of expression of the pro-angiogenic factor VEGF (that promotes the dense formation of tumour micro-vessels) and poor prognosis in a variety of human tumour types.
The first anti-angiogenic protein to be identified was angiostatin, so named because it stops the growth of endothelial cells in culture and can block the growth of human tumour cells when they are inoculated into mice. Over 20 angiogenesis inhibitors are now known, some of which become activated by enzymatic cleavage of larger molecules that have a quite different function. Thus angiostatin is the centre region of plasminogen (from which plasmin that breaks down the fibrin clots that protect wounds, is also made). Endostatin has similar properties to angiostatin and it is derived from a form of collagen. The gene that encodes this collagen, and hence endostatin, is on chromosome 21. Chromosome 21 is perhaps familiar because people with Down’s syndrome have three copies (trisomy 21) instead of the normal two. Strikingly, Down’s syndrome individuals have about half the normal lifetime risk of developing most cancers, although children have a 20- to 30-fold increased risk of leukaemia, and if they are diabetic never develop diabetic retinopathy. Endostatin may contribute to this protection although another chromosome 21 gene encodes a negative regulator of VEGF and the extra copy exerts an anti-angiogenic effect.