Metastasis

Definition

The process by which cancer cells spread from a primary tumour site to another location in the body.

The multistep process of metastasis

1. Primary tumour: There are two schools of thought when it comes to the development of metastatic cells from primary tumours; the early and late dissemination models. In some tumours the primary tumour microenvironment selects for malignant cells.  The combination of hypoxia-inducible transcription factor (HIF), produced by tumours due to a poor blood supply, with the activation of the RAS signalling pathway, is associated with early metastasis. The late dissemination model hypothesizes that as the primary tumour progresses subsequent mutations give tumour cells the ability to metastasize.
2. Local invasion:
   1. Tumour cells that invade surrounding tissue have decreased intracellular adhesion via loss of function in E-cadherin, a molecule that links cells via their actin cytoskeleton. This loss of function is caused by loss of function mutations, gene silencing or gene downregulation caused by growth factors such as EGFR (epidermal growth factor receptor) and IGF-1 (insulin growth factor). 
   2. Tumour cells must then cross the basement membrane (BM), which serves as a barrier between epithelial tissue and the stroma, the latter of which is of mesenchymal origin. This step is facilitated through proteases, such as matrix metalloproteinases (MMPs), which are enzymes that cleave proteins. In addition, under hypoxic conditions tumour cells secrete lysyl oxidase (LOX), which creates a collagen framework via crosslinking for invasion. 
3. Intravasation: This is the process by which tumour cells enter the lumen of blood vessels or lymphatics by crossing the endothelium and associated pericytes. There are a number of growth factors associated with this process that are not fully understood. The leaky blood vessels formed via tumour-mediated angiogenesis are more permeable to intravasation of tumour cells than normal structurally intact microvasculature found elsewhere in the body.
4. Survival in the circulation: Cancer cells in the circulation are subject to immune attack and circulatory shear forces. In order to evade this cancer cells attach to platelets forming heterotypic clumps or attach to clotting factors such as thrombin and fibrinogen forming emboli known as homotypic clumps. These clumps may also help cancer cells evade anoikis, a normally occurring process in which apoptosis follows the loss of cell-to-cell adhesion. In addition, certain intestinal epithelial tumours have mutations in their tyrosine kinase receptors that enhance their ability to evade anoikis. 
5. Arrest: The organ at which cancer cells arrest is determined by anatomical and molecular factors. Organ microvasculature and its relation to the primary tumour influences metastasis. For instance the liver which has a large capillary bed is the site for metastasis for many primary tumours types, with colon cancer metastasis being most common given that the portal vein that carries blood back from the colon empties into the liver. Secondly, chemoattractants predispose arrest of circulating tumour cells. Endothelial cells express E- and P-selectins that bind tumour cells.
6. Extavasation: Extravasation differs from intravasation in that blood vessels at distant sites are generally structurally intact. Certain blood vessels in the liver and bone marrow are normally fenestrated and provide less of a barrier to extravasation of tumour cells. In other blood vessels, extravasation occurs through small gaps created by the normal turnover in endothelial cells or through vessel wall damage that attracts platelets and tumour cells that can be associated with them (homotypic clumps). Finally, primary tumours, through production of proteins, such as MMPs and VEGF, can disrupt endothelial barriers in distant organs. Breast cancer has been shown to use these mechanisms for pulmonary metastasis. 
7. Proliferation and angiogenesis: Proliferation in a metastatic site, much like the development of the primary tumour, requires uncontrolled cell replication. Many disseminated tumour cells undergo attrition, remain dormant or form microcolonies termed micrometastases and do not progress to gross metastases. Experimental models of metastatic cells have shown only 3% of extravasated tumour cells form an obvious metastasis. This low percentage is because the target organ microenvironment must be favourable to tumour cell proliferation as discussed in the models below. Metastatic proliferation requires the creation of new blood vessels, angiogenesis, which is triggered by hypoxia in the developing tumour. Tumour angiogenesis and lymphangiogenesis occur in a disorganized fashion and are promoted by vascular endothelial growth factors (VEGF), platelet-derived growth factor (PDGF) and transforming growth factor (TGF).

Determination of distant site for metastasis

1. Seed and soil hypothesis: The seed and soil hypothesis posits that certain organ tissues are predisposed to metastasis because their microenvironment is compatible with tumour cells. Stromal interaction with metastatic cells is vital to the proliferation of metastatic cells. For example breast cancer cells produce cytokines such as parathyroid hormone-related peptide (PTHrP) and interleukin-11 which favour osteoclast differentiation through the RANK-RANK ligand (RANKL) system, producing characteristic metastatic osteolytic lesions. The dissolved bone matrix in turn releases growth factors such as IGF-1 and transforming growth factor (TGF-β), which preferentially stimulate tumour growth as malignant cells overexpress receptors to these ligands. Prostate cancer cells on the other hand secrete PDGF and Wnt proteins that stimulate osteoblast differentiation, producing metastatic osteoblastic lesions.  Thus a dynamic interaction in which the seed (the circulating tumour cell) shapes the soil (the target organ microenvironment) to further stimulate tumour growth. However, this growth is only possible because bone has a responsive element, namely the RANK and RANKL axis, to facilitate the process.
2. Route of dissemination: Cancer cells disseminate through either a hematogenous or lymphatic route, with the two paths often being complementary. In hematogenous dissemination, tumour cells repeatedly spread from the primary tumour to distant sites with very few of these cells progressing to an overt metastasis. These metastatic cells develop early in tumour progression and do not pass through the lymphatic system. In lymphatic dissemination tumour cells spread via both the lymphatics to nearby lymph nodes to form solid metastases and through blood whereby they undergo attrition or lay dormant in distant organ sites. While in the lymph node it is hypothesized that tumour cells gain the ability to form solid distant metastasis via selection. These lymph node metastases then undergo secondary metastasis to distant organ sites. In general, carcinomas metastasize through lymphatics while sarcomas metastasize hematogenously.
3. Pre-metastatic niche: Recent studies have shown primary tumours secrete factors that mobilize bone marrow cells to future sites of metastasis. These bone marrow cells prime the organ microenvironment for metastasis by producing VEGF and other cytokines that promote angiogenesis.   

Signs and symptoms of metastasis

The symptoms of metastasis are often vague and a result of the damage done to the organ where the metastasis has occurred. It has been hypothesized that many of the constitutional symptoms of cancer such as weight loss, night sweats, fever, chills, decreased appetite and energy, are due to metastasis.

Therapeutic targets for metastasis

Effective anti-metastasis therapy has to limit the survival and proliferation of invasive tumour cells. Therapies such as MMP inhibitors have shown limited benefit in this regard. Two therapies, bevacizumab and trastuzumab have shown promise. Bevacizumab is a monoclonal antibody to VEGF and has been shown to be increase disease free survival in metastatic colon cancer when added to a chemotherapy regimen. Trastuzumab is a monoclonal antibody to the HER-2 antigen which is expressed by some early disseminating breast cancer cells and has been shown to be effective in metastatic breast cancer that is HER-2 positive. Other targets being studied are denosumab (an anti-RANK antibody) and bisphosphonate in the treatment of metastatic bone lesions. The evidence for both of these is equivocal. A future area of study is using antibodies against LOX, a protein that is involved in a number of steps in the metastatic cascade.