M´etodo de reacci´on qu´ımica de estado s´olido

1.2.1 The Hallmarks of Cancer

In their seminal paper ‘The Hallmarks of Cancer’ published in Nature in 2000, Hanahan and Weinberg identified a number of characteristics shared by cancer cells (76). These

characteristics are (i) self-sufficiency in growth signals, (ii) insensitivity to antigrowth signals, (iii) evasion of apoptosis, (iv) limitless replicative potential, (v) sustained

angiogenesis and (vi) tissue invasion and metastasis (76). As previously discussed, outcomes in patients with colorectal cancer are closely related to stage of disease, which is defined by the degree of local tumour invasion and the presence of local and distant metastases, and the majority of cancer-related deaths occur as a consequence of metastatic disease.

1.2.2 The molecular basis of cancer progression

1.2.2.1 Epithelial-mesenchymal transition

Enhanced cancer cell motility plays an important role in both invasion and metastasis (77). Firstly, primary tumour cells invade local tissues in a process known as epithelial-

mesenchymal transition (EMT). EMT is seen during normal foetal and postnatal development and is regulated by transcription factors such as Twist1, Snail1 and Slug, which inhibit E- cadherin expression, zonaoccludens 1 (ZO1) and Beta4-integrin (78). It results in the

reorganisation of cytoskeletal structures and the upregulation of fibroblast markers and genes such as vimentin, N-cadherin, Twist and α-smooth muscle actin (α-SMA). Cells consequently become more mobile, plastic, isolated and resistant to apoptosis. Several molecules have been

implicated in EMT, including TGF-β, which has been shown to induce EMT in embryonic epithelial cells.

1.2.2.2 Invasion

Tumour cells invade through the basement membrane and degrade the extracellular matrix to invade into blood and lymphatic vessels in a process controlled by the matrix

metalloproteinases. These are endopeptidases that are involved in normal tissue remodelling during growth and wound healing. Altered expression of MMPs has been identified in colorectal cancer (79). Normally, cells detaching from the extracellular matrix lose cell- matrix interactions and consequently undergo programmed cell death in a process known as anoikis but successfully metastasising cancer cells are able to evade anoikis.

1.2.2.3 Migration

Migration is stimulated by scatter factors including hepatocyte growth factor (HGF). Initially recognised as a promoter of hepatocyte cell growth, HGF is now understood to play an important role in cell growth and cell motility for a range of epithelial cells (80). The activity of hepatocyte growth factor increases after injury, suggesting an important role in normal wound healing and organ regeneration and higher levels of the HGF receptor (cMet) have been found in metastatic lesions compared with primary tumours (80). As the tumour progresses, it will encounter normal blood vessels and new vessel growth is induced by angiogenic factors. Tumour cells invade the blood vessels and are transported through the circulatory system until they reach an appropriate microenvironment. Angiogenesis is important in order for tumour cells to become established at this metastatic site and to enable

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1.2.2.4 Haematogenous and lymphatogenous spread

As they spread, tumour cells migrate and invade into local tissues including local blood or lymphatic vessels (intravasation) and the development of new blood and lymphatic vessels (angiogenesis and lymphangiogenesis) is induced by angiogenic and lymphangiogenic factors. Tanigawa et al (1997) found that the degree of new vessel formation around colorectal tumour was the most important predictor of haematogenous metastatic development (81).

Angiogenesis is the biological process by which new capillaries are formed (82). It is essential in normal embryological development and post-embryonic angiogenesis is minimal in adult life though certain sites retain angiogenic activity, such as the corpus lutea in the ovaries, the endometrium of the uterus and there are high levels of angiogenic activity during the proliferative phase of wound healing (83, 84). However, inappropriate angiogenesis may occur in pathological states such as in rheumatoid arthritis, diabetic retinopathy and in cancer.

Angiogenic factors are secreted by tumours, resulting in the stimulation of new vessel growth, improving the delivery of nutrients and oxygen to the growing tumour (85, 86). These new vessels provide nutrients to the growing tumour, are poorly developed compared to normal vessels facilitating penetration and embolization increasing the metastatic potential of the tumour, and support the survival and growth of metastatic tumour deposits.

The development of the embryological vascular system involves a complex process of differentiation, proliferation, migration and maturation resulting in the formation of an organized network of vessels (87). Several molecules are involved in the regulation of angiogenesis including the VEGF family, Delta/Notch, Slit/Robo, neuropilin and HGF. Once the vascular tree is complete, the balance of stimulating and inhibiting factors stabilizes and

angiogenesis is usually limited to normal physiological processes like reproduction and wound healing. Post-embryonic angiogenesis is also induced by fibroblast growth factor (FGF), angiogenin, TGF-α, TGF-β, platelet derived growth factor (PDGF), TNF-α, interleukins, chemokines and angiopoietins (85).

Bevacizumab (Avastin®) is a humanized variant of a murine anti-human monoclonal antibody targeting VEGF-A; a prime angiogenic molecule involved in the development of blood supply for both the primary tumour and secondary tumour deposits. The FDA has approved bevacizumab for the first- and second-line treatment of metastatic colorectal cancer in combination with chemotherapy (88). However, current NICE guidelines do not

recommend bevacizumab for patients with metastatic colorectal cancer who have not been previously treated with the drug.

Lymphangiogenesis is the process of lymphatic system development. The lymphatic system comprises a network of thin-walled, blind-ending vessels lines with a single layer of

lymphatic endothelial cells. They originate from the same embryological tissue as the arteriovenous system. They collect extravasated fluid, proteins and electrolytes from extracellular fluid, returning this lymphatic fluid back to the arteriovenous system. The lymphatic system is also part of the immunosurveillance system, presenting foreign antigens to lymphocytes in lymph nodes scattered along the lymphatic vascular chain.

In 1902, Sabin proposed a mechanism of lymphangiogenesis whereby lymphatic vessel development is subsequent to blood vessel development and which involved the sprouting of lymphatic vessels from venous endothelium of nearby veins. Subsequent work found that the expression of Prox-1 by venous endothelial cells within the embryo reflects the site of budding and sprouting of lymphatic cells that give rise to the primary lymphatic system.

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1.2.2.5 Development of distant metastases

Once the cancer cell reaches a suitable metastatic site, they leave the blood/lymphatic system (extravasation) and lodge in the target organ. They may then enter a period of metastatic dormancy for months or even years before subsequent growth and progression.

There are two major theories relating to tumour invasion and the development of metastases. Firstly, Paget’s seed and soil theory, which states that a specific organ microenvironment is required for the development of metastases. Secondly, Ewing’s anatomical theory states that invasion and metastases depend on the regional lymphovascular system. In practice, both of these factors play a role. Regional metastases usually follow the anatomy of the regional blood and lymphatic vessels whilst distant metastases often show a preference for certain organ microenvironments. Furthermore, the ability of the cancer cell to establish metastases depends not only on the tumour cell itself but also on factors relating to the microenvironment of the potential metastatic site. For example, the release of chemokines by host tissues may aid tumour cell migration.